Laser-Writing and Characterization of Single-Mode Rib
Waveguides on Planar Germanosilicate Wafers
A Thesis Presented to
The Department of Electkal and Computer Engineering
University of Toronto
Jianhao Yang
In partial fulfillment of the requirements
for the degree of
Master of Applied Science
January, 1997
O Jianhao Yang, 1997
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DEDICATED TO MY MOTHER
Laser-Writing and Characterizatioo of Single-Mode Rib Waveguides
on Planar Germanosilicate Wafers
Jianhao Yang
M.A.Sc., 1997
Department of Electrical and Computer Engineering
University of Toronto
For the first tirne, a laser-writing technique has bcen applied to planar
Germanosilicate wafers to fabricate rib waveguides. This approach is an extension of
other laser-witinç processes in polymers and in III-V semiconductors. Single-mode rib
waveçuides with large cross-section size of 8prnx8pm have been fabricated using a
157nm F? excimer laser. The strong absorption of silica materials to 157nm radiation is
key to providing smoothly etched surfaces. Surface scattering !oss of the waveguides
was IdBkm at 635nrn wavclcngth, whilc thc singlc-modc coupling cficicncy from a
single-mode pigrailed fiber was 11%. Computer simulation bascd on the Beam
Propagation Method (BPM) has been applied to guide the waveguide design. The single-
mode conditions and beam profile in the rib waveguide obtained by the simulation are in
csccllcnt agreement with thc expcrimcntal observations.
The thesis presents a comprehensive discussion of optimizïng the waveguide
quality based on several criteria including single-mode guuiding, scattering loss, coupling
rfficiency and confinement factor. Cornparison of this work with that of other research
groups shows that our rib waveguides are attractive in large cross-section size, single-
mode guiding, relativety large coupling efficiency and single-step fabrication.
The thesis demonstrates that the laser ablation technique can be developed into a
process competing with the Reactive [on Etching ( R E ) technique in fabricating silica rib
waveguides for applications to photonic circuits. Future improvernents of this technique
are also addressed.
First of ail I deeply thank Professor Peter R. Herman for his numerous valuable
suggestions and financial support during the course of this work.
f would also like to thank: Professor Jirnmy Xu for allowing me using the Bearn
Propagation Method (BPM) software, a ginding machine, and a optical power meter in
his lab; Professor S. Zukotynski for allowing access to a stylus profilorneter and optical
microscope; Dr. Robin Tarn of OLLRC for Iending an XYZ-translater and allowing
access to several fiber-handling equipment; Mr. Fred Neud of the University of Toronto
for allowing me using a polishing machine.
1 was geatly enjoying the fiendship with Keith Beckley. From him, I not only
learned how to operate the Fz excimer laser in the early stage of this thesis, but also
learned the Canadian culture from many of our interesting conversations.
Financial support from the University of Toronto in from OP the University of
Toronto Open Fellowship is acknowledged. I also appreciate Ms. Sarah Cherien for her
coordination of the requirement of this program.
Lastly but not the least, [ thank my wife, Yuhui, for her encouragement and
support during this thesis work.
TABLE OF CONTENTS
Chapter 1 Introduction ....................................................................................... 1
................................... 1 . I Introduction to Optical Waveguide Technology 1
1.2 OveMew of Waveguide Fabrication Techniques ................................. 3
.............................................................. 1.2.1 Reactive Ion Etching 3
1.2.2 Ion Exchanging Method ......................................................... 4
1 2.3 Direct Wntingj Process .......................................................... .5
............... 1.3 Direct Writing oFSilica Rib Waveguides by Laser Ablation 7
............ 1.4 Purpose and Structure of This Thesis .................................... 9
Chapter 2 Theoretical Studies and Computer Simulation
......................................................................... of Silica Rib Waveguide 17
......................................................................... 1.1 Single-Mode Analysis 12
2.1.1 Theoretical Approximation ............................................... 12
2.1.2 Cornputer Simulation ...... ,. ................ .. ............................. 16
2.2 Optirnization of Confinement Factor ............................ ,., ................... 2 1
Chapter 3 Experirneotal Setup ......................... .. ............................................ 25
3.1 Wavepide Fabrication ...................................................................... 2 5
3.1.1 Expenmental Setup for Laser Ablation ............................ 2 5
3.1.2 Optimization of Edge Resolution .... .. .................................. 28
...................................... 3.1 -3 Waveguide Fabrication Procedure 3 1
3.2 Waveguide Characterization ............................................................... 34
.................................. 3.2.1 Coupling Light into Rib Waveguides 31
3.2.2 The Loss Measurement and B e m Profile Mcasurement .... 37
.................................................................... Chapter 4 Results and Discussions 40
4.1 Results ................................................................................................. 40
.................................................... 4.1.1 Wavcguidc Cross-Section 40
........................................................ 4.1.2 Surface Scattering Loss 43
41.3 Single-Mode Coupling ........................ ... ......................... 49
4.3 Discussions ................... ,.., ............................................................ 53
4.2.1 Bcam Profiles .................................................................... - 3 3
................. 4.2.2 Optimized Waveguide Width and Etched Depth 55
...................................... 4.2.3 Theory of Surface Scattering Loss -56
........... .......................... 4.3.4 Cornpanson cvith Other Work ... 59
Chapter 5 Conclusions ......................................................................................... 64
5.1 Surnmary ............................... .... ....................................................... 64
........................................................................................ 5.2 Future Work 66
References ................................................ ...,.. ... 68
Chapter 1
Introduction
1.1 Introduction to Optical Waveguide Technology
Data transmission rates of 40Gbps will soon be required in the telecommunication
industry to serve numerous applications including multimedia entertainment. Fiber optic
networks are the best choice for transporting such a high volume of data [Gree93].
Currently, Optical Carrier-192 is being deployed in the Synchronous Optical Network
(SONET) to provide lOGbps data transmission rate for long haul communication, while
alternative fiber systems are under developed to penetrate into the home from Local Area
Networks &AN) and Wide Area Networks (WAN). The rapidly increasing use of optical
technology also demands the development of new optical technology, such as
Wavelength Division Multiplexing, optical switching, and optical interconnection.
Optics not only provides high transmission speed in telecommunication, but also shows
advantages in parallel processing and opticai computing. Many of these areas are based
on the optical waveguide technology. In this sense, optical waveguide technology is
greatly affecting Our daily life.
Low-loss, high-qudity optical waveguides are the basic building block for this
technology, especially for the many photonic devices and photonic circuits under
development. Although optical fiber is a low loss waveguide, it is not suitable to be
packaged in compact photonic chips. Channel waveguides or rib waveguides in planar
optical matenal are a more practicai choice in packaging photonic devices. However,
existing fiber fabrication technology can not be readily borrowed to fabncate channel
waveguides since a channel or rib waveguide is very different from a fiber. The demands
of photonic packaging and the lack of Iow-cost fabrication technology for optical channel
waveyides has been the basic driver behind the development of new waveguide
fabrication process during the last ten years [Lado96].
The difficulty in developing Iow-cost fabrication process for low loss channelfrib
waveguide is that the fabrication technique usually introduces surface roughness during
the process, causing the transmitted light to suffer higher loss cornpared with standard
single-mode fiber. Surface roughness is the biggest challenge in defining a new
fabrication process.
It is also not clear which material is the best choice for the optical channel or nb
waveguides that could be packaged into photonic or opto-electronic circuits. Materials
such as lithium niobate are impractical as a substrate for board Ievel electronics and
suffers frorn its high dielectric constant and high-cost mart89]. Waveguides based on
polymer materials usuaily provide a lower loss, but advanced devices that are based on
the photosensitivity effect or many opto-electronic effects cannot be fabricated on
polymer materials. Serniconductor materials produce a small guiding cross-section
(compared with standard telecommunication fiber) for channeVrib waveguides due to
their large refractive indices. This mismatch leads to large coupling loss with optical
fiben. Silica material is a natural choice for channeVrib waveguides because it is the
basic material for many optical devices and compatible with fiber devices. However,
silica is difficult to process because of its large melting temperature and energy bandgap
(8eV). A simple process which can fabricate channeYnb waveguide in silica material and
provides low optical loss is highly desirable.
Some common waveguide fabrication techniques are described in next section.
1.2 Overview of Waveguide Fabrication Techniques
1.2.1 Reactive Ion Etching ( RIE )
Currently, the most successful industrial technique to fabricate silica channel
waveguides is Reactive Ion Etching (RIE). The etching process is a physical-chernical
process whereby silica or doped silica is removed from a surface through bombardment
by ions excited by a plasma. By using a predesigned mask, or pattern, which defines the
unetched area, a channel waveguide or any other complex layout of waveguides and
devices can be produced. Typically, silica reacts strongly with elements like fluorine
under ion bombardment. Ions of the reactive element are created in the plasma, and an
electncal field is used to direct the ions toward the surface to be etched [Lado96].
The RIE process involves several compiicated steps. A typical procedure for
fabricating silica waveguide is as follows: 1.) A silicon layer or an optically flat surface
(quartz, for example), typically a few rnillimeters thick, is used as a wafer. The silicon
wafer is sometimes pre-heated in order to oxidize a layer several microns thick and
facilitate the adhesion of deposited silica layer. 2.) A uniform buffer of pure silica is
deposited on the surface of the wafer to a thickness of about 10jm. 3.) A guiding layer of
glas with slightly higher refractive index is then deposited on the buffer. Idealiy, the
index difference must match the core-cladding index difference of a standard fiber,
typically 0.3-0.8%. 4.) With a contact mask fabricated by photolithography, the parts of
guiding layer not covered by the mask are etched away by the RIE process descnbed in
the previous paragraph. 5.) The mask is then removed chemicdly to leave a rectangular
cross-sectioned, rib-like pattern on the top of the buffer layer. 6.) Usually a final layer of
silica is deposited as the upper cladding. This is preferred to match the index of the upper
cladding with that of the buffer layer so as to produce symmetric waveguides.
The contact mask is typically fonned by a lithographie processes, borrowed
directly from similar processes in electronics industry. This well-developed technology is
one of the principal advantages of RIE. The mask dimensions are defined by theoretical
analysis for the particular layout of waveguide, and, using computer aided design (CAD)
techniques, the actual mask is fabricated to sub-micron accuracy.
The main disadvantage of RIE is that it is a complex process which involves many
steps and incorporates toxic chernical elements. The process is also costly since the
fabrication plants are very expensive. Further, the method is less flexible than a wnting
process in producing complex waveguide layouts.
1.2.2 Ion-Exchange Technique
The ion-exchange method relies on replacing the ions in a glass substrate with
different ions by a diffusion process. The result is an increase in refractive index without
seriously disrupting the lattice structure. Typically a sodium-doped glas is used as the
substrate material. A complement mask is pattemed onto the surface by
photolithography. This mask is the reverse of the mask used in RIE as the area not
covered by the mask defines the cores of the waveguide and devices. The sarnple is then
immersed in a liquid salt bath of potassium nitrate at high temperature (-300°C) for a
sufficient time. Potassium ions migrate into the glas rnatrix and replace sodium ions,
which migrate out of the glass into the liquid. This results in an increase of the refractive
index in the uncovered area which forms a core guiding layer. After the ion exchangc
process, the mask is dissolved, leaving a buned channel waveguide in the silica substrate.
The ion-exchange technique is simpler, overall, than RIE. However, the diffusion
process yields a semi-circular cross-sectional guiding profile with the highest index of
refraction near the surface. This non-circular index profile imposes a significant
limitation for many applications due to poor matching with the circula index of
refraction profiles of fi bers [Lad096].
1.2.3 Direct Writing Process
Channel waveguides cm be fabricated without the need for a contact mask by
locally changing the index of the material through its molecular response to focused
particle [Town941 or ultraviolet laser bearns. The direct writing techniques are very
attractive because of their sirnplicity and flexibility for writing complex computer-
controlled patterns.
Particle beam direct writing: In this technique, a proton beam is focused ont0 a
serniconductor such as GaAs and to generate lattice damage, resulting in a region with
reduced carrier concentration [Huns9 11. The refractive index is slightly larger in the low
carrier concentration region, leading to a buried channel waveguide. The waveguides
produced by proton bombardment usually have large loss, typically 200dB/cm, but the
loss can be reduced to 3dWcm after annealing at temperature below 500°C [Huns9 11.
Currently, laser-based direct writing has drawn a great deal of research effort
[Hart89, Mukh94, Bozh92, Osgo92, Maxw95, Sva1951 to write channel or rib
waveguides. This technique c m be cataioged into several types depending on the laser
wavelength and the waveguide material:
Laser writing of rib wavemiide in polymer: Common polymer materials such as
polyrnethylmethacrylate (PMMA) are widely used and c m produce low-loss rib
waveguides [Hart89, Mukh941. Laser ablation of PMMA films using excimer and Ar ion
lasers have been used to pattern integrated waveguides with typical loss of IdBfcm
[Mukh94, Bozh92]. It was ais0 reported that after filtering the PMMA solution with a
O. lpm pore size, and controlling the laser exposure to below the ablation threshold (laser
ablation usually produces rough surface), the optical loss of PMMA channel waveguide
was reduced to O.OBdB/cm [Mukh94]. This is the lowest loss ever reported for channel
waveguides and represents the state of the art of the polymer waveguide fabrication
technology .
Laser assisted chemicai et ch in^: Laser-assisted chernical etching is an attractive
method for etching smooth profiles in serniconductor materiais such as GaAs Pavi881
and InP [M00fi4]. This technique has been extended to etch single-mode rib waveguides
in GaAs/AiGaAs wafers[Osgo92] with waveguide loss of IdBkm. Passive waveguide
devices such as a Y-branch was also successfully fabncated using the same techniques
[Osg092].
Laser writine in silica material: For silica material, wavelengths available from
any continuous wave (CW) lasers are not suitable for writing waveguides because the
photon energy is too low compared with the 8eV energy bandgap of silica to significantly
alter the material. Only pulsed ultraviolet lasers are capable of writing waveguides in
silica material. If the laser fluence is below the ablation threshold of silica, for certain
types of doped silica and certain laser wavelengths, the material exposed to laser beam
will change its refractive index [Hi1178]. This photosensitivity effect is a nonlinear
photon material interaction process, which involves permanent modification of point
defects in certain types of silica materials. That induces a permanent change in the
refractive index. Currently, the mechanism of photosensitivity is still not fully
understood. However, the ability of pemanently increasing the refractive index of the
laser exposed area in Ge-doped silica has been used to fabricate novel silica-based
devices such as fiber gratings [Hi1193]. There is also a strong trend to apply this effect to
fabncate silica channel waveguides. For example, single mode channel waveguides have
recently been wntten directly by point-by-point techniques using a focused, 244nm
wavelength laser beam [Svd94].
1.3 Direct Writing of Silica Rib Waveguide by Laser Ablation
If the laser fluence is above the ablation threshold of silica, the materials exposed
to laser bearn will be removed. Laser ablation of optical materials is attractive for many
applications such as fabricating optical interconnects or diffractive optics, and repainng
photonic circuits. The advantages of laser ablation include simplicity, flexibility, low-
cost and easy control of etching parameters.
This laboratory has studied laser ablation on diverse materials such as polymer,
quartz, and semiconductor at both 193-nm and 157-nm radiation [Herm92]. Laser
ablation of silica material at 157-nm radiation is of particular interest for the following
reason: the bandgap for hised silica is about 8eV, and the bandgap for the crystal siiica is
about 9eV. This large bandgap makes it difficult to ablate silica by typical commercial
excimer laser such as ArF laser (193-nm) or KrF laser (244-nm). On the other hand, FI
excimer laser radiation, with 7.9-eV photon energy close to the silica bandgap, is strongly
coupled into ultraviolet-grade hsed silica through defects generated by the 7.9-eV
photons Werm921. This Iowers the ablation threshold fluence, produces srnoothly-etched
surfaces, avoiding microcracks and laser debris, and provides easy control of etched depth
by the number of laser shots. Recent work [Hem961 shows the ablation threshold of the
interested Ge-doped silica is about 0.36~lcm'.
These attractive properties make 157-nm laser ablation an effective tool in micro-
machining silica materials. It is of panicular interest to apply the technique to define a
single-step fabrication process for silica nb waveguides. In this approach, the UV laser
beam firstly passes through a rectangular aperture mask with a wire in the rniddle, and is
projected by a lens, forming an image in the sarnple surface. The imaged laser beam. with
fluence above the ablation threshold, removes the exposed material. Two trenches are
produced in the sarnple, leaving the middle unetched regions as a rib waveguide.
Cornputer-controlled target positioning then permits fabrication of a guiding path.
Such a laser-ablation scheme offers several advantages: 1 .) The process is single
step; 2.) No mask is required; 3.) The layout of the waveguide or devices can be
cornputer-controlled by translating to pnnt a programmable photonic circuit.
There is no known report on using the laser ablation technique to write silica
channel or rib waveguides. This is partly because the availability of the 157nm F2
excimer laser, which is the most effective tool to etch silica, is very limited. Although
ultrafast laser pulse is also capable of etching silica [Liu94], there is also no known report
on using the ultrafast laser to write silica nb waveguides. It is the goal of this thesis to
pnnt rib waveguides in silica material using the F2 laser. If such a technique can produce
good quaiity silica waveguides, it could be a competitive fabrication method to the
standard RIE technique.
Although the photosensitivity writing technique is also capable of writing buried
channel waveguides in germanosilicate [Svd94], laser ablation offers additional
advantages such as ease of control of etch depth. More importantly, these two techniques
are indeed complementary. There is a potentiai to combine both laser ablation technique
and photosensitivity technique to fabricate more complicated silica based devices in a
single silica chip.
1.4 Purpose and Structure of This Thesis
As mentioned earlier, silica material is a natural choice for fabricating channel or
rib waveguides because it is the basic matenal for many optical devices, and because of
its compatibility with fiber and fiber devices. The current standard silica waveguide
fabrication technique (RIE) suffers from disadvantages of numerous processing steps,
high-cost, and use of toxic chernical materiais. Cumntly, there is a great deal of research
work [Hart89, Mukh94, Bozh92, Osgo92, Maxw95, Svd951 in defining simpler
fabrication techniques based on direct laser writing. However, there is no report on
fabricating rib waveguides using laser ablation. The purpose of this thesis project is to
develop a low-cost, single-step fabrication technique for silica rib waveguides based on
laser ablation. Our goals are also to optimize the overall quality of the silica rib
waveguides based on critena such as scattering loss, coupling efficiency from standard a
fiber, and single-mode confinement.
The structure of this thesis is as follows: Chapter 2 provides a theoretical
analysis to define waveguide parameters in meeting requirements of single mode
confinement. Since there is no analytical results for waveguide with trapezoid cross-
section shape, computer simulation is the only tool used to study such waveguide. In
chapter 2, intensive computer simulation will be performed, and the results are then
compared with the theoretical cdculation results.
Chapter 3 describes the experimental setup for the waveguide fabrication. We
start by describing the basic ablation experimental setup. The edge resolution due to
diffraction and lens aberration effect is also optirnized. For the waveguide
characterization, the most difficult part is the coupling of appreciable light into the
waveguides. A tapered fiber is used to solve this problem. We aiso describe a simple
method to measure surface scattering loss.
Chapter 4 presents the experimental results and compares them with the
theoretical anaiysis in chapter 2. The optimization of waveguide parameters such as
waveguide width and etch depth is important in designing good quality waveguides. We
will discuss the optical physics behind the citena in optimizing these parameten. Also, a
discussion on the mechanism of the surface scattering loss is also included. We will also
compare the beam profiles obtained from both cornputer simulations and experimental
measurement. The chapter is concluded by a cornparison of this thesis work with related
research efforts.
Chapter 5 summarizes the results and achievements of this thesis and provides
suggestions to extend this thesis project.
Chapter 2
Theoretical Studies And Cornputer Simulations of Silica Rib Waveguide
Detail theoretical studies of silica nb waveguide are carricd out in order to define
the single mode conditions. However, due to the nature of laser matenal processing, the
cross-sectional shape of the nb waveguide fabkated by laser ablation technique is not
rectangular. Instead, the cross-section is more similar to a trapezoid shape. This imposes
a difficulty in performing the theoretical analysis. The approach in this chapter is to first
mode1 the laser-ablated waveguide as a rectangular waveguide by analytical means. Then
we perform a computer simulation on the trapezoid waveguide, and compare the resutts
with the rectangular approximation. This approach is used in both defining single-mode
conditions (section 2.1) and optimizing the confinement factor (section 2.2).
2.1 Single Mode Analysis
2.1.1 Theoretical Approximation
Although the theory of a dieiectric waveguide is more than a century old,
application of this theory to a rectangular rib channel was not studied in detailed until the
1980s. It is often assumed that the cross-section dimensions of a three-dimension rib
waveguide must be similar to the dimensions of a single-mode slab waveguide (made
from the sarne material) in order to support single-mode propagation. This assumption is
incorrect as pointed out by Peterman [Petegl]. The single-mode condition of a rib
waveguide is more restrictive than that of a slab waveguide. For exarnple, for a Ge-doped
silica core layer on a silica subtract with 0.3% index difference, the slab waveguide
thickness must be less than 2 p in order to be single mode at 6 3 5 ~ . This dimension is
rnuch less than the core diameter of a single-mode fiber. Hence the coupling loss from
the fiber to the waveguide is very high. On the other hand, as will be shown in this
section, it is possible to design a large 8pmx8pm cross-section size for a single-rnode rib
waveguide in the same material.
Figure 2.1 shows the cross-section of the rib waveguide used in the theoretical
approximation. The ri5 width is defined as w, the guiding layer thickness is H, and the
etched depth is (H-h).
air I T germanosilicate
silica substrate
silicon wafer
Figure 2.1 ~ectangular rib waveguide for analytical model.
The refractive indices of the air, the guiding layer, and the substrate layer are no, nl and
nz, respectively, for a specified wavelength. The three-dimensional rib-guide modes are
denoted as TE,, or TM,,, where n = O, 1, 2, . . . , and m = O, 1, 2, . . . . The single-
mode condition is derived by the effective index method, as described below.
First, the wave propagation in a slab waveguide is considered as if the lateral
direction is infinite and the vertical direction is asymmetric. The dispersion relation for
the propagation constant Psiab of this type of waveguide is well-known and can be written
in terms of the effective index of refraction of the guide Nh = P s i a h as [Osgo891
Here, TEo modes are assumed, k = 2irlh is the free space propagation constant at
wavelength h, and h is the thickness of the slab waveguide. Solving Equation (2.1) gives
the effective index of refraction, Nb, as a function of thickness h.
The next step in the effective index method is to view the waveguide structure in
Figure 2.1 from the top and to consider the regions of different thickness in Figure 2.1 as
regions of different effective index of refraction as shown in Figure 2.2.
Figure 2.2 Top view of a channel waveguide with effective indices Nh and NH.
The stmcture in Figure 2.2 is simply a 1-D syrnmetric waveguide and the anaiytical
results are well-known. The propagation constant Pchmnel and effective index of refraction
ner = Pehmcl/k can be found by:
The single mode condition is defined as [Osgo92]:
Combination of Equations (2.1) and (2.3) can in principle define the single mode
condition by numencal means. Peterman pete9 11 also gives an approximate analytical
expression derived from the effective index method. The results show that the single-
mode condition for the % mode is
where = l / J ~ + l l , / ~ .
The waveguides studied in the thesis are based on the planar waveguides
fabricated by Photonic Integrated Research Inc. (PIEU, SMPWL). The guiding layer is
Ge-doped silica with thickness H = 8 p and refractive index nl = 1.46 12 at h = 0.635 ym.
The substrate layer is silica with thickness of 2 0 p , and refractive index of n2 = 1.4568.
Assuming no = 1.0 for air, the single-mode conditions of Equation (2.3) define the
maximum waveguide width plotted as a function of thickness, H, in Figure 2.3. Regions
above the curves support more than one propagation mode.
Figure 2.3 shows the mutual dependence of waveguide (w) width and etched
depth (H-h) in maintaining single-mode conditions. A wide rib waveguide can be single-
mode if the etch depth is suficiently shallow. On the other hand, a deeply etched rib can
also be single-mode if the width is sufficiently narrow. For an 8 p thick waveguide. rib
width of less than IOpm are recommended for a 4p.m of etched depth.
Figure 2.3 Single-mode boundary for silica channel waveguides: each . . curve corresponds to different etched depth and defines the maximum wavebide width, W, as a function of thickness, H. Larger waveguide widths support multimode propagation.
2.1.2 Cornputer Simulation
A more realistic profile for the cross-section of the rib waveguide fabncated by
laser ablation is a trapezoid shape. A trapezoid shape that closely reflects the shape of
laser-etched ribs produced in this study is shown in Figure 2.4. Here width w is defined
as the FWHM of the etched depth, and W, is the width of the sloped surface on the sides
of the trapezoid shape.
H= 8 p f h=5 pm Ge-doped silica
/ 31 20- silica substrate
Figure 2.4 A trapezoid mode1 of a laser etched silica rib waveguide.
To study this waveguide, a computer simulation based on Beam Propagation
Method (BPM) is employed (BPMCAD, National institute of Optics, Canada. 1991).
This version of BPM can not directly simulate a trapezoid waveguide. Therefore, we
divided a 4pm high trapezoid layer into 100 thin layers of 0.04pm thickness as shown in
Figure 2.5. Each layer is a rectangular shape with linearly increasing length of 0.04prn
for each adjacent layer. Thus, the result (Figure 2.5 in next page) closely matches the
trapezoid shape in Figure 2.4.
To snidy how various waveguide cross-sections affect the wave propagation in the
rib waveguide, two BMP simulations on both waveguides with rectangular cross-sections
and waveguides with trapezoid cross-sections were carried out, each having identical
germanosilicate
Figure 2.5 The trapezoid waveguide is modeled as many rectangular thin layers for the BPM simulation.
values of w = 6 p (FWHM), h=5pnun, and H = 8 p . For die trapezoid waveguide,
wp2km. The propagation step for this simulation is 0.25j~m, the wavelength is 0.635p,
and the propagation distance is 4mrn for both types of waveguides. The input electncai
field is a Gaussian function with 3 p n width (FWHM) and (0.3p.111, 0 . 3 ~ ) off the
waveguide center in order to excite higher modes. Figure 2.6a and 2.6b show the
evolution of the electrical field durhg the wave propagation for both waveguide types.
Figure 2.7 shows the cornparison of the intensity profiles after a 4mm propagation iength.
Figure 2.6 Comparison of wave propagation in rectangular and trapezoid waveguides. a. Electrical field distribution for rectangular waveguide; b. Electrical field distribution for trapezoid waveguide.
-15 -10 -5 O 5 10 15
Distance (pm)
A Y 1 .I
VI E al
(c) = 0 . 8 - u
2 00
Figure 2.7 Comparison of intensity profiles after 4mm of propagation in rectangular and trapezoidal rib waveguides.
2 0.6 - T3
a Q) - N -
.LI - 0.4 - E O O - - a
0.2 1 - O a O
O
- 1 - - O
- O O
0 Rectangular Waveguide Trapezoid Waveguide
- a 1
- - 0 - - O O
*
From Figure 2.7, we find that the impact of different waveguide cross-section
shape on the lowest-mode wave propagation is very smail. The output intensity profiles
from both types of waveguides are very similar. The 6 . 4 ~ width (FWHM) of the
intensity profiles after propagating 4mm distance are identical for both waveguides. This
is expected since the fields of the lowest-mode is mostly distributed in the center of
waveguides, and the distortion from the waveguide boundary is small. Hence, the wave
propagation of the lowest mode only weakly depends on the cross-sectionai shape of the
waveguides.
In order to test the single-mode boundary by BPM, a second-mode field
distribution generated from a rectangular waveguide was used as an input field to the
trapezoid waveguide to excite a higher mode. If the trapezoid waveguide does not
support the second-mode, the propagating field distribution must then gradually
diminishes with propagation distance. For a given etch depth of the trapezoid waveguide,
the maximum width at which the second-mode is supported defines the single-mode
boundary. For example, for a fixed etch depth of 3pm and a thickness (H) of 6pm,
variation of the waveguide width from 4pm to I O p n showed propagation of a second-
mode for widths exceeding -8f lpm. Therefore, for single-mode propagation, the
waveguide width should be less than -8klp.m. The uncertainty arises because of the
finite propagation distance (-4mm). This BPM simulation was repeated for other etched
depths. Results are shown in Figure 2.7, and compared with the analyticai results in
section 2.1.1. From Figure 2.7, the single-mode condition for a trapezoid waveguide is
very similar to that of a rectangular waveguide.
O 2 4 6 8 1 O 12
Thickness of Guiding Layer, H (p)
Figure 2.8 Maximum waveguide width for single-mode propagation piotted as a function of waveguide thickness, H, for rectangular waveguides (open circles) and trapezoid waveguides (solid circles). Both rib waveguides are assumed to be etched to one-half of the waveguide thickness. The error bars for the trapezoid rib waveguides anse from the finite propagation distance used in the BPM simulation.
2.2 Optimization of Confinement Factor
The light intensity scattered from the surface of the waveguide due to surface
roughness is proportional to the light intensity at the surface. Therefore, the field
distribution in the optical waveguide affects the surface scattering loss. Intuitively, if
most of the light is confined inside the waveguide with less light near the surface, then
the scattering loss is reduced. Therefore, we need to optirnize the waveguide parameters
such that they not only satisS> the single-mode conditions, but also provide a large
confinement factor.
Analytical calculation of the confinement factor of a trapezoid waveguide is
difficult. From Figure 2.7, the output fields of the lowest mode after propagation of I mm
distance are very similar for both rectangular and trapezoid waveguides. Therefore, it is
reasonable that the analytical results of the confinement factor derived from a rectangular
waveguide is a good approximation for the confinement factor of a trapezoid waveguide.
The confinement factor r is defined as
where I is the Iight intensity. Iin refers to the intensity within the waveguide and I.., refers
to the iight intensity outside the waveguide. For a rectangular waveguide, the
confinement factor I' is given by [Che0901 :
based on the effective index method described in the previous section, where
y= k( ne$ - N ~ ' ) ' ~
= k wH2 -
Nh and NH are deterrnined from Equation (2.1), and nea is calculated from Equation (2.3).
Numerical solutions of Nh and nerf were obtained by Matlab. Values of Nh for the PIRI
waveguide is listed in Table 2.1 for various ratios of MH, and for H = 8pm, no = 1 .O. ni =
1.46 12, nz = 1.4568. and h = 0.63pm.
Table 2.1 Effective index of refraction for PIRI waveguide with different etch depths
Figures 2.9 shows the confinement factor plotted as a function of waveguide
width and etched depth. The confinement factor increases rapidly with increasing values
of both parameters. For a fixed etch depth of 4 p , the confinement factor reaches more
than 90% when the waveguide width is 8pm, and increases much more slowly with
further increasing of width. Similarly, for a given waveguide width of IOpm, the
confinement factor is more than 90% when the etched depth is 4pm, and increases much
more slowiy with further increasing of depth. These two plots show that an 8pm of
waveguide width and a 4pm of etched depth are suficient to achieve a 90% confinement
factor. Further increment of both parameters provides little increment of the confinement
factor but makes it more difficult to maintain single-mode conditions.
In conclusion, this chapter provided theoreticai background on choosing
waveguide width and etched depth to achieve single-mode propagation and large
confinement factor. However, other factors such as loss and coupling efficiency are also
important in optimizing the waveguide width and etched depth. Chapter 4 will present a
more complete discussion on choosing these two parameters based on the optimization of
loss. coupling efficiency, confinement factor and single-mode propagation.
O 1 2 3 4 5 6 7
Etched Depth (pm)
Waveguide Width (p)
Figure 2.8 Confinement factor ploned as a function of waveguide width for an etch depth of 4pm (top), and plotted as a function of etch depth for a waveguide width of 8pm (bottom). Values were determined by the effective index method.
Chapter 3
Experimental Setup and Procedure
In this chapter, the experimental arrangement and procedure for fabricating silica
rib waveguides will be descnbed in detail. We will also describe the experimental setup
for characterizing the nb waveguide, including the coupling of light from a pigtailed fiber
to the rib waveguide, measurement of the scattering loss from the surface of the
waveguide and measurement of the beam profiles of the waveguide output.
3.1 Waveguide Fabrication
3.1.1 Experimental Setup for Laser Micromachining
A carefui study of laser ablation of germanosilicate provides essential information
of etched rates and ablation threshold for controlling etch depth and smcothness of the
rib waveguide. We will briefly describe the experimental setup for the laser ablation
studies and summarize the relevant results in this section.
Figure 3.1 shows a schematic of the experimental setup for the laser ablation and
waveguide fabrication (For laser ablation, there was no wire in the rectangular aperture).
The laser source is a home-built high pressure F2 excimer laser with the following
parameters: the laser wavelength is 157nrn with the spectral linewidth of 0.005nm
[Herm93]; the duration of laser pulse is about lSns (FWHM); and a typical pulse energy
is 40d. The excimer laser was operated at a 1Hz repetition rate in this expenment.
vacuum chamber
sliding aperture
Rotatable Mirror waveguide 1 (MgF,) sample I 1574 laser beam
wire with 75pm diameter
Figure 3.1 Experimental setup for laser ab1 ation and v
stage
laveguide fabrication.
The laser beam has a typical area of 8mmx 12mm and a 2x 1 O" radian of divergent
angle. The laser beam passes through a rectangular aperture (2mmx3mm) which is
imaged ont0 the germanosilicate target sample by a biconvex lens made of W grade
MgFz (focus length, fi57nm = 86rnm). The laser fluence was controlled by moving the
aperture dong a vacuum sealed glas tube, altering the demagnification factor in this
experiment in a ranges of 6 to 9 times.
The optical system was contained in a vacuum chamber descnbed in detail by
Chen [Chengl]. The vacuum chamber was pumped to 1 0 ' ~ Torr with a turbomolecular
pump (Leybold W / N T 50) before operation of the laser. During experiment, the
chamber was purged with argon gas at lOSCFH flow rate to rninimize contamination of
the system by air and eliminate optical darnage due to VW photochemical reactions with
hydrocarbon contarninants.
The laser energy was measured with a scintillator plate and diode detector
(Startech VHR-0020-1295, SN-0200) by detecting the refiected W light from a
rotatable mirror as shown in Figure 3.1. The sensitivity of this detector is several micro-
Joule and was cross-calibrated with a Molectron ID500 pyrometer with J25 detector
head. The calibration procedure was straightforward, but required correction for a weak
(-10%) red component in the laser beam and elimination of laser-discharge noise. For a
detailed description, please refer to a technical report in this laboratory [YanggS].
The etching target was placed on a XY-translator (Oriel 16928) offering
transverse motion over a range of O S " with O. l p n resolution.
F? laser ablation of Ge-doped silica was carefully studied in this laboratory
Figure 3.2 Laser ablation etched rate of Ge-doped silica (PIRI SMPWL) at 157nm wavelength plotted as a hnction of laser fluence.
[Herm96], yielding the etched rates shown in Figure 3.2 as a function of the logarithm of
laser fiuence. A linear representation of the data provides an ablation threshold for the
PIRI waveguide of -0.36 k m t . The etched rates provided by this graph is important in
controlling the etched depth of rib waveguides. For example, to produce a 3pm deep of
trench at 31/cmt, -50 laser shots are needed. Note also that much higher laser fluences
are required to etch germanosilicate with 193nm or longer wavelength lasers, and
surfaces are much rough than that produced by the 157nm laser [Herrn96].
3.1.2 Optimization of Edge Resolution
The experimental setup for fabricating the n b waveguides was similar to the one
shown in Figure 3.1. The only modification was to the aperture. A 75pm diameter
copper wire was added to the rniddle of a 2.5mmx5mrn aperture. The image of this wire
prevents etching of the slab waveguide, leaving a rib sandwiched by two wide trenches.
We usually oriented the wire vertically, and moved the waveguide sample vertically to
produce a Icm long rib waveguide.
Due to diffraction and lens aberration, the side-wall of the rib waveguide was not
sharply defined, resulting in a trapezoid cross-section shape. We define the width of the
sloped edge (W. in Figure 2.4) of the trapezoid shape as the edge resolurion. The
challenge is to minimize diffraction and aberration of lens to increase the edge resolution.
Figure 3.3 shows the schematic arrangement of the optical system. The focal
length of the lens is f , the wire diarneter is Do, and the 157m beam divergent angle is 8.
Other lengths are defined as shown in Figure 3.3. The beam divergent angle after the
aperture le& waveguide aperture
Figure 3.3 Optical system (left figure) used in the waveguide fabrication expenment. The aperture (right figure) was made of four knife-edges. A copper wire with diameter of 751m was glued to the middle of the rectangular aperture. The aperture was imaged by the lens ont0 the waveguide sample to etch a rib-Iike optical guide.
aperture is hn +8 where h/L includes aperture diffraction. The spherical aberration at the
circle of least confusion in the image plane is then given by Nala941:
Here, a depends on the shape of lens and is 0.404 for the biconvex lens used in this
experimen t.
For diffraction, we first estimate the beam size projected from the mask to the lens
surface as L+(W+8)So. This gives a minimum diffraction feature size of:
To illustrate the dependence of both W, and Wd on the demagnification factor, M, we
substitute S, = MS, and Si = (I+i&Qf in Equations (3.1) and (3.2) and add both Wu and
Wd together to represent the worst case. This results in a theoretical expression for the
edge resolution defining the slope-side-wall width of the trapezoid waveguide:
The edge resolution depends on the demagnification factor M and the aperture size L, two
parameters to be optirnized. Other parameters in Equation (3.3) are not adjustable. From
equation (3.3), it is easy to see that increasing L will reduce W,. The dependence on M is
more cornplex. Figure 3.4 shows a plot of the edge resolution, W,, as a function of
aperture size for typical demagnification factor in this experiment, with f =86mm, and O=
2x lo5. It can be seen that i t is possible to achieve a 41~m of edge width for M=8 and a
full aperture size of 2L=Smm.
O i 1 I 1 1 1 1 1 1 1 I
O 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Aperture Size (mm)
Figure 3.4 Edge resolution as a function of the full aperture size (2L) for typicai demagnification factors in the waveguide fabrication experiment.
3.1.2 Waveguide Fabrication Procedure
There are at least three technical problems needed to be carefully solved in the
waveguide fabrication procedure. Before the waveguide fabrication, the rib width and
etch depth must be specified. Many criteria must be considered in choosing these two
numbers as will be discussed in Chapter 4. The final selection then dictates the optimal
demagnification factor and the lens positions. The second step is to make sure the
shadow of the wire in the aperture is exactly aligned in the center of the imaging lem.
Otherwise, lens aberration will make the edge resolution very large and the waveguide nb
will not be symmetnc. A PMMA sample was placed behind the aperture mask and
exposed to the F2 laser radiation, which provided etched patterns that helped position the
mask in a uniform portion of the 157nm beam. The lens was centered in the bearn using
the same technique. Several iterations were needed to adjust the relative positions of wire
and the lens. This was accomplished by observing the sürface features of the rib
waveguides in PMMA samples by a microscope and profilorneter, and selecting the most
symmeuic cross-section profile of the rib.
The third issue is to make sure the image of the wire was precisely on the
waveguide surface (referred to Figure 3.1). In Our experimental setup, the lens
longinidinai position was controlled by a micrometer with O.ûû1" resolution. To find the
best focus position, it was necessary to translate the lens by <0.005" step-size, while
exposing PMMA or waveguide sarnples to 157nm radiation. After several iterations, the
optimal lens position was found when the side-wail widths of the rib waveguides were
rninirnized.
The waveguide sarnple was attached in the XY-translator by double-sided tape
and moved vertically during laser operation. This translation produced two Icm long
trenches on the surface of slab waveguide resulting a rib waveguide in the middle region.
In order to minimize the surface roughness, the sample was translated each laser shot,
leaving a small ripple at each edge position of the imaged mask.
The step size of each translation on the waveguide of sample was detemined in
following way. Given N laser shots to etch to a required depth, and the wire length, L,
reduced to L, = VM on the sample, the step size is LJV which we denote as Ay. N
depends on the laser fluence and the required etch depth, and determined by the method
described on page 27. The required etch depth must support the single-mode conditions
as described in Chapter 2. Selection of the laser fluence is a trade-off. A large fluence
etches deeper holes each shot, reducing N, but increasing the edge-effect surface
roughness. A lower fluence near the threshold also leads to increasing surface roughness
due to the shot-to-shot variation in laser fluence (-10%) which greatly affects the shot-to-
shot etch depth near the threshold (see Figure 3.1). Selection of the laser fluence then
dictates the lens position (or dernagnification factor) and the nurnber of laser shots, N.
It is necessary to compensate for the slightly different orientation of the vertical
wire and the direction of motion of the translation motor which otherwise leads to a
notched waveguide pattern. The angular offset, typically $ 410°, is compensated by
stepping the horizontal motor position, Ax, ezch time a vertical step is made. Smooth rib
waveguides result when tane = M A Y .
Before rnicromachining the slab waveguides, the facets of the waveguide sarnpies
were ground by S i c paper, in sequence of 14pm and 5 p grain size, then polished by
A1D3 powder of lpm. To hold the waveguides steady during grinding and polishing, r
holder was used. Typical polishing time is -10 minutes. Longer polishing times lead to
rounded edges on the facets.
The waveguide fabrication then proceeds as follows:
a.) Clean the waveguide facets and surfaces with optical methanol;
b.) Mount the waveguide ont0 the XY-translater such that the laser beam just
misses the beginning facet of the waveguide;
c.) Tum on the excimer laser (1Hz);
d.) Measure the laser energy with the Startech energy meter to confirm that the
appropriate fluence reaches the sample target;
e.) Slowly step the sample until the laser beam is very close (-200km) to rhe
beginning facet of the waveguide. Turn off the excimer laser;
t) Move the sample by Ay verticdiy (typically 10-15pm) and Ax (typically 1 .O-
1 Spm) horizontally;
g.) Fire the laser for only one shot;
h.) Go back to step f, and repeat until the laser beam leaves the opposite facet of
the waveguide;
i.) Measure the laser energy in the middle and at the end of the whole procedure.
The number of repeated translations is the waveguide length divided by Ay, and is
typically 103 for fabricating a k m long waveguide. The motor translation was done each
step since high precision was required. It typically took three houn to fabricate a lcm
long rib waveguide therefore it is desirable to let a cornputer to do this job in the future.
The laser energy was assumed to be the average of the three measurements taken
during the fabrication process. There was less than a 10% drop in laser energy during the
fabrication, producing uniform rib profiles dong the length of the waveguides.
3.2 Waveguide Characterization
3.2.1 Coupling Light into Wb Waveguide
The rib waveguide studied in this thesis has a cross-section size of about
8pmx8pm, and a numericai aperture of about 0.12 in the vertical direction. It is a
challenge to focus a laser bearn into this small cross-section size while also rnatching the
small numerical aperture.
Figure 3.5 shows the schematic of the expenmental setup for waveguide
characterization. The light coupling is the main concem of this section. The key
component for the coupling is a pigtailed fiber, as shown in the enlarged graph and also in
Figure 3.6. This fiber is the 3M single-mode fiber for 635nm, with a 4p.m core diameter
and 1 2 5 ~ ccldding diarneter (3M FS-SC-3224). The reason for working at 635nm is
simply the convenience of using visible light. In principle, it is straightforward to extend
this work into the 1550nm regime. The pigtailed fiber is about 1.5 meter long, with a
7rnm stripped length at the end. The stripped part of fiber is rnetailized in order to protect
the bared fiber. At the end of the fiber, the facet is rounded with a 10pm radius of
curviiiure. This results in a semi-sphericül lens ended fiber (SLEF). Figure 3.6 is il
picturc of this pigtailed fiber
v The pigtaiied fibcr
diode Iaser \ waveguide sample
beam splitter i
\ &3b f dete~tor--~+ &
n eyes
Figure 3.5 Schematic of waveguide characterization experirnental setup. Expanded view of pigtailed fiber is shown at top. See text for further details.
Figure 3.6 The pigtailed fiber under a microscope with 20x objective and 10x eye- piece. The diameter of the fiber is 125pm, and the curvature of the end-facet is IOpm.
The laser source was a AlGaInP laser diode with a multi-quantum well structure
(PDLD L63-3 12-0.5-PH6-lFa). The laser diode was packaged with another piece of 3M
fiber ( FS-SC-3224) so that the laser output could be coupled into the pigtailed fiber.
Some of the key parameters of this fiber coupled laser are summarized here:
wavelength: h = 635nm, single longitudinal mode
optical laser power output: 5mW CW
coupling efficiency from laser diode to fiber: 16%
operation current: 65mA, (85mA maximum)
The output optical power from the fiber was -0.8mW. This fiber was connected to the
pigtailed fiber by a fiber adapter (FCIAPC in Figure 3.5). This introduces about 3dB Ioss,
yielding -0.4mW power after the pigtailed fiber.
The pigtailed fiber was placed in a fiber holder (Thorlab MDT711-125) which
was mounted on a precision XYZ translater (Thorlab MDT602) with -0.5pm position
sensitivity. This high sensitivity is necessary for efficient single-mode coupling between
the pigtailed fiber and the rib waveguide of the small core diameter of -8ym.
The typicai distance from the pigtailed fiber ta the rib waveguide was 10-15pm
when coupling was optimized. In order not to damage the pigtailed fiber, the fiber-to-
waveguide distance was observed by an optical microscope, usuaily before the laser was
turned on.
Three criteria were used to determine when optirnized coupling was achieved by
transIating the pigtailed fiber. a.) At optimized coupling, the scattered light from the
waveguide surface was maximized as observed by a microscope; b.) The far field
intensity pattern of the rib waveguide output could also indicate the optimum coupling
efficiency. c.) The light coupling was also optimized when the bnghtness of scattered
light from the output facet of the nb waveguide was maximized as observed by a
microscope.
In order to record the near-field intensity profile at the n b waveguide output facet,
we used a lûûx micro-objective just after the waveguide end lacet, and a CCD camera
(Connectix QuickCam) as shown in Figure 3.5. The camera image was sent to a
computer via the serial port and captured with a commercial software (Connetix
QuickPict). Precise positioning of the CCD carnera to image the waveguide end facet
was obtained with the laser source tumed off and the end facet brightly illuminated;
precise XYZ positioning of the micro-objective lens then produced a sharp image of the
waveguide facet. Keeping the same carnera position, we turned on the laser and captured
the optical output intensity profile at the waveguide facet.
3.2.2 Loss Measurement and Beam Profile Measurement
The detection system used to measure the surface scattering loss of the waveguide
is shown in Figure 3.5. A microscope was used to collect light scattering from a smal!
waveguide segment. The objective was typically 50x. and the eye piece was 10x. The
observed waveguide segment was 0.5mrn long when using a SOx micro-objective. This
segment length, denoted as Ad, was the unit length in measuring the scattering loss. The
power of scattering light from this waveguide segment is P, = P u , where P is the
optical power confined in the segment of the nb waveguide. Assuming the waveguide
loss obeys the exponential law with absorption coefficient cc, and denoting x as the
distance from the front facet to the waveguide segment, one obtains
P,. " p*r\e- (U) ,
where Po is the optical power from the pigtailed fiber, and 7 is the coupling coefficient
from the pigtailed fiber into the single mode light of the nb waveguide. Assuming
isotropic scattering, the actual light power collected by the detector can be wntten as
Pd = P,,@2 / 27r = P,qc e'm(crM)R / 21r (3.5)
where R is the solid angle in which the micro-objective collects light and determined by
the numerical aperture, and 5 represents the percentage of this collected light that reaches
the detector through the prisrn and optical lenses.
The detector (8 18-SL) and the optical power meter (1830-C) are frorn Newport
Corporation. The sensitivity of this power meter is about IOpW. The active area of the
detector is 1.5cmxl.6cm which is relatively large compared with the bearn size
(0.5cmx0.5cm) after the eye piece. Since the collected scattering light is rather weak
(-nW), the detector and the beam paths were sealed from any background light.
Between the detector and the eyepiece, a pnsm (Edmund Scientific, M32,504)
served as a bearn splitter. About 50% of the coilected scattering light passed through the
pnsm and was detected, yielding @ 5 O I . This prism permitted simultaneous detection of
the scattering light power while observing the waveguide segment by eyes. This way, the
microscope-to-waveguide distance can be readily corrected to within the depth of focus
(2.8)c = 1 . 8 ~ for the 50x micro-objective). Simple ray tracing of the microscope system
showed that deviation of the working distance in such a range (< 3h) did not affect the
optical power of detected light due to blur. This was also confirrned expenmentally.
A field stop was used to filter out scattering light from surface other than
originating from the rib waveguide surface as shown in Figure 3.5. The field stop filter
was placed at the focal plane of eye-piece lens inside the microscope tube. Figure 3.7
shows the shape of the field stop filter.
rib,waveguide (-8 pm wide)
icroscope field of view (0.5mrn 0)
E open field stop - 12pm wide
Figure 3.7 Diagram of the microscope field stop ( 5 0 ~ objective), together with image of a 8pm width rib waveguide. Sizes have been transformed to dimensions on the waveguide surface (50x demagnification). Only scattered light originating within the 12p.m wide field of view was passed by the filter.
Chapter 4
Results and Discussion
4.1 Results
4.1.1 The Waveguide Cross-Section
In this thesis, four waveguide sarnples are studied in detail. For sample A, the
waveguide width and etched depth are 8pm4pm which is the optimized shape as will be
discussed in detail later. For sample B, C, and D, the width x etched-depths are
12prnx4pm. IOpmx7p1, and 8 p m ~ 6 p , respectively. Laser parameters such as laser
fluence, demagnification factor, and step size for al1 the four rib waveguides, are
surnmarized in Table 4.1 of page 49. The aperture mask used to fabricate these rib
waveguides was described in page 29.
Figure 4.1 shows the endview and topview of a rib waveguide fabricated on a
PIRI slab waveguide (sample A) and observed with a lOOx microscope. The waveguide
width (FWHM) and the etch depth were measured using a calibrated ruler to be 8pm and
4pm, respectively.). The waveguide (sarnple A) was fabricated at 2.8 k m 2 of laser
fluence which, according to Figure 3.2, gives a O.O6Clm/pulse etch rate. The etched
surfaces were exposed to 40 laser pulses for a total etch depth of 2 . 4 p . Diffraction
effects at the rib edge provided deeper trenches resulting in a total etch depth of 4 p .
- 'ma -
ri b waveguide
1
silicon wafer
Figure 4.3 The top picture is an endview of waveguide sample A. The gray color shows the silica (24pm) coating which aiso includes a 4pm thick germanosilicate guiding layer at the top. The white color represents the silicon wafer. At the top of the germanosilicate layer is the trapezoid-shaped rib waveguide of -8pm FWHM and 3pm etch depth. The bottom edge of the rib is deeper due to edge diffraction of the wire. The bottom picture is the topview of part of the waveguide sample A with 8pm width.
Figure 4.2 Surface profiles of the rib waveguide sample A. Waveguide width ( F m ) was measured to be IOpm and the etch depth was 4ym.
Figure 4.2 shows the surface profiles of this waveguide recorded by a stylus
profilorneter, yielding a 4pm etch depth, a 10ym waveguide width (FWHM) and a 2 p
edge resolution (defined as the width of the sloped side-wall from 10% to 90% of the etch
depth. in Chapter 3, we have calculated the theoretical edge resolution due to diffraction
and lens aberration resolution to be - 4 p when working at a demagnification factor of 8.
which was different from the measured 2p-n in Figure 4.2. There are several reasons
contributing to this discrepancy. Firstly, the calculation in Chapter 3 simply added W.
and Wd (in page 28) which likely over-estimated the resolution feature size; Secondly, the
calculation in Chapter 3 was based on laser bearn intensity profiles and not on etch depth
profiles which follow a logarithm response of fluences (Figure 3.2). The non-linear
response for fiuences (2.8Ucm2) which was at 8x ablation threshold of PIRI waveguides
favors a sharper feature size; Lastly, surface receiving a laser fluence above the threshold
of PIRI waveguide can remove materiais. Al1 the three factors lead the calculation in
Chapter 3 to over-estimate the side-wall width. This reasoning also implies that the edge
resolution depends on matenals. For PMMA, the edge resolution is about 3pm at the
same demagnification factor. However, the edge resolution is independent of the number
of laser shots; larger number of laser shots etches the deeper trenches, resulting in steeper
slope.
The waveguide width measured by the profilometer (10pm) was larger than that
measured from Figure 4.1 (-8pm). The difference was due to the steep slope of
waveguide side-wall that usually exceeds -45' angle. The profilorneter tip can not touch
the side-wall and gives a wider width than the actual width.
4.1.2 Surface Scattering Loss
Due to the nature of laser ablation, the surface of ablated germanosilicate is not
optically smooth, typically resulting in a 50nm R M S (root of mean square) surface
roughness as determined by the profilometer for a 4 p deep hole. Furthemore, the
fabrication procedure requires a stepping of the sample across the image of the aperture
for each laser shot. This produces a ripple with penod of the stepping size, typicaily 10-
1 5 p . Figure 4.3 is the surface profile of waveguide sample A at the bottom of the
trench near the rib waveguide, and taken parallel to the rib. The ripple period is lOpm,
and the height is -0 .06p, matching the etched rate at 2.81/cm2 laser fluence according to
Figure 4.3 Surface profile at the bottom of trench and near the rib of waveguide sample A. The ripple height is about 0.06pm wwhh is close to the surface roughness intrinsic to laser ablation, making it difficult to distinguish both types of roughness.
Figure 4.4 Topview of waveguide sample D. The surface ripple has a period of 1Opm and a height of about -O. lpm as determined by a stylus profilorneter.
Figure 3.2. This ripple height is close to the RMS surface roughness due to laser
ablation. Therefore it is difficult to distinguish the surface roughness due to laser ablation
from that due to the stepping process. Figure 4.4 shows a top view of waveguide sample
D for a 2 0 ~ microscope objective. The surface ripple is clearly visible and with a period
of -lOpm, which matches the step-size used for translating the motor.
Figure 4.5 shows the scattered optical power from the nb waveguide surfaces
measured as a function of waveguide position for the four lOmm long waveguides.
Typically from the beginning waveguide facet to 3 or 4mm, the scattering light consists of
both lowest mode and higher mode losses, yielding a steep drop in intensity for this
region; from 3 or 4mm to 8mm, the scattering light is typically due only to the lowest
order mode, if the waveguide is a single-mode waveguide. The power fall-off is slower
in this region, and yields the single-mode scattering loss as shown by the solid lines in
each of the graphs in Figure 4.5. The intensity rise in the last -2mm is due to light
scattered by the waveguide facet, and scattered by backwards propagating light reflected
by the end facet. This allows the microscope to collect part of the facet-scattered or
reflected light even when the waveguide edge is still not within the observed field of the
microscope. The lowest scattering loss was measured to be -4dBkm from sample A.
We repeated the loss measurement four times, and found the uncertainty of the
measurement to be -20%. Several factors contribute to this uncertainty: 1.) surface
debris produced by laser pulses cm causes larger light scattering; 2.) power of the diode
laser can slightly drop during the loss measurement; 3.) coupling between the pigtailed
fiber and the iib waveguide can also drift during the loss measurement.
O 2 4 6 8 10 12
Distance (mm)
Figure 4.5a Scattering power loss as a function of waveguide position for sarnple A. See the text for a detailed description of the graph.
I I I l u 1 I I , . I I I I I
4 6 8 IO
Distance (mm)
Figure 4.5b Scattenng power loss as a function of waveguide position for sarnple B
Distance (mm)
Figure 4 . 5 ~ Scattering power loss as a function of waveguide position for sarnple D.
The loss results obtained in Figure 4.5 are summarized in Table 4.1. The lowest
loss of 3.6dBkm was found for sarnple A. The waveguide width of sample D was the
same as that for sarnple Ay but the etched depth is increased by 2pm. Cornparison of the
losses of sarnple A and D shows that deeper etch depth for a rib waveguide has higher
scattenng losses. We will present the explanation in section 4.2.3.
For sample By the loss was particularly high due to the fact that the guide supports
multimode. A careful examination of Figure 4.33 for sample B shows that leveling off of
scattered light power associated with single-mode propagation does not occur. Higher
modes encounter larger loss since the field distributions place larger portion of field at the
rough waveguide boundaries. Hence, it is not recommended to fabricate a waveguide
with widths larger than - 10pm for a 4 p deep rib, and nsk multi-mode propagation
(also see Figure 2.7).
From the above discussion, when choosing the waveguide parameters to keep the
scattering loss low, the waveguide width should be no larger than IO pm and etched depth
should be no larger than 4 pm. Smaller etched depth may produce lower scattering
losses. This trades against a low coupling efficiency which we will discuss in next
section.
Sample C2 differs from Ci (see Table 4.1) in the number of laser shots per moving
step. Sample C2 was fabricated by five laser shots per step, while CI was fabricated in the
sarne way except by one laser shot per step. The loss for CI was 7.7 dB/crn while the loss
for Cz was 11.2 dB/crn. This reveals a relationship between scattering loss and the
periodical surface ripple caused by stepping process. The ripple height was measured by
a profilonieter to be 0.06pm for Ci and 0 . 3 p for Ct. Therefore, reduction of the ripple
height should reduce the scattering loss. One might think that working at Iow laser
Fiuence will reduce the ripple height. However, lower laser fiuence increases the nurnber
of laser shots required to produce the expected trench depth, and also increasing the etch
depth uncertainty due to +IO% laser energy fluctuations. Optimization is reached when
the final trench depth uncertainty is comparable with the single-shot edge rate that defines
the ripple amplitude.
To make this point clear, supposed Ge work at 2.5~/cm', which yields an etch rate
of -0 .058p depth per laser shot from Figure 3.2. To produce a 4 p etch depth, about
70 laser shots is required. The uncertainty of the etch depth for N laser shots is
AD, = a ( A F / F) where Do is defined in D = Do log(F I 4 ) (solid line in Figure
3.2) and is 0.072j~m for the PIRI waveguide. The fluctuation of the laser fluence,
LW / F , is about 10%. Putting al1 the numbers together, one gets AD,, = 0.059 p. This
theoretical value already exceeds Our observed O.OSpm (RMS) of surface roughness,
dernonstrating that there is no reason to work below 2.51/cm2 if a 4p.m of etch depth is
required. To reduce the ripple amplitude, other ideas should be developed.
Table 4.
results ol
sample
A
B
Cl
c2
D
4.1.3 Single Mode Coupling
Figure 4.6 shows the optical output at the end facet of waveguide sample A. The
detailed procedure of capturing this picture was described in section 3.2.1. In Figure 4.6,
the output beam profile is an elliptical shape with FWHM dimensions of 6 p m ~ 4 p . No
I Summary of wîveguide pararneters, fabrication pararneters, and rneasured
the four waveguides (M in column 6 is the demagnification factor).
ri b ' width
8p-n
1 2 p
l O p
1 0 p
8 p
etch
depth
4 p
4pm
7pm
7pm
6pm
laser
fluence
2.8J/cm2
2.5J/cm2
2.5Ucrn2
22.~/crn~
2.81/cm2
single
mode
Yes
no
Yes
Yes
Yes
step
size Ay
lOpm
1 2 p
5p.m
2 0 p
l 0 p
' M
8
8
8
8
9
loss
3.6 dB/cm
1 1.4 dB/cm
7.7 dB/cm
1 1.2 &/cm
7.6 dB/cm
coupling
efficiency
1 1.3%
-
35%
-
6%
node is observed in the beam profile, which indicates single mode confinement. This
observation agrees with the BPM simulation results in Figure 2.7 which predicted that a
rib waveguide with 8pm width and 4pm etched depth supported the single-mode
propagation. Further discussion of the beam profiles is provided in Section 4.2.1.
Figure 4.6 Output beam profile of waveguide sample A. The FWHM of intensity profile was measured to be - 6 . 0 ~ horizontally and - 4 . 0 ~ m verticdly. (A cornparison of the intensity profile to BPM simulation results is given in Figure 4.10.)
The ratio of optical power which is coupled into the single mode of this rib
waveguide to the optical power for the pigtailed fiber was measured to be 1 1%. This is
relatively low. Figure 4.7 shows a Gaussian bearn mode1 to help understand the light
propagation from the fiber tip. Assurning the output from the fiber is a Gaussian beam,
the facet of fiber serves as a lens with radius of curvature R.
radius of curvanire R Wb
Figure 4.7 A Gaussian beam mode1 for the beam propagation from the pigtailed fiber.
The beam waist is denoted as Wb which is related to the beam divergent angle by
cp = fiAhwb. Figure 4.8 shows the output beam from the pigtailed fiber at the best
focus point. The beam waist (1/e of the peak optical intensity) is estimated to be about
3.5pm, which gives cp = 0.1 16 radians. The beam divergence is controlled by the radius
of curvature R (see Figure 4.7). Larger R gives smaller cp. In principle, a single-mode
fiber with flat end-facet (R = p.) can sufficiently couple the optical power into the rib
waveguide if it is directly attached ont0 the waveguide facet. However, such an
arrangement is not practical since it prevents translating the fiber to achieve optimal
coupling. A finite radius of curvature (R = l O p in our case) is needed so that the focus
point is at some distance (-15pm in our case) from the fiber end, allowing the fiber io be
freely translated while keeping the focus point at the waveguide facet. This eases the
coupling difficulty and permits optimal single-mode coupling.
Figure 4.8 Output beam profiles from the pigtailed fiber at the best focus point. The profile width (l/e of the peak intensity) was estimated to be 3.5prn.
The low coupling efficiency is not due to the beam size, but due to mismatches in
the bearn divergent angle cp and the numeficd aperture (NA.) of the rib waveguide. The
nb waveguides have a vertical N.A. of O. 11 which is very close to <p. But the laterai N.A.
is ,/N; - N: . From Table 2.2, for an etch depth of 4pm, this N.A. is only 0.0503
which is only one haif of the N.A. apemire, resulting in an asymmetric rib waveguide.
Therefore the ratio of optical power coupled into the rib waveguide is less than
0.0503/0.113 = 44.5%. The coupling effïciency is written as q= q l q z q 3 where
ql is due to the mismatch of lateral numericai aperture and is 44.5%;
qz is the transmission coefficient at the waveguide facet; and
q3 is the percentage of single mode light among the coupled light.
Here, qz is given by (n, - u2 = 95%, where nz is the refractive index of Ge-doped silica. (n, + 1)'
q 3 is less than 50% from the graph in Figure 3.5a. Then the overall coupling efficiency q
for sample A is less than 20%.
To increase ql, a new pigtailed fiber should be designed with larger radius of
curvature such that the divergent angle is small and matches the lateral numerical aperture
of the waveguide. Altematively, we can increase the lateral numerical aperture by
increasing the etched depth. If the etched depth is about 7 p , the conesponding lateral
numericai aperture is 0.105 which is very close to the divergent angle of the present
pigtailed fiber. Consequently the coupling efficiency is substantially improved. This is
shown in Table 4.1, where the coupling efficiency for sample Cl is 35%. Unfortunately,
the scattering loss increases substantially as also shown in Table 4.1.
4.2 Discussion
4.2.1 Beam Profiles
From the output bearn of the rib waveguide shown in Figure 4.5. one obtains the
intensity profile. It is worthy to compare this measured beam profile with the BPM result
to mess the accuracy of cornputer simulation.
The BPM simulation was based on the trapezoid waveguide model with the
parameters selected to represent sample A. The waveguide width is 8pm, the etch depth
is 4pm, and the edge resolution is 3pm. Figure 4.9 shows the wave propagation along
this waveguide for 4mm length. Note that Figure 4.9 gives the distribution of electrical
field, instead of optical intensity.
We calculated the normalized intensity profile based on the field distribution at
the end of the wave propagation in Figure 4.9, and compared it with the normalized
intensity profile obtained from Figure 4.6. The result is shown in Figure 4.10. The
agreement is very good. The widths (FWHM) are 6.Opm and 6.3pm for the measured
profile and the BPM calculation respectively. Exponential curve fitting of the tail of the
beam profile gives the decay constant y outside the waveguide. We found that y was
0.47pf1 from both the BPM results and the measured profiled. This value can also be
computed analytically from the rectangular waveguide model, as defined by Equation
(2.6). For a rectangular rib waveguide witii 8 p width and 4 p depth, y is calculated to
be 0 . 4 4 ~ " . The agreement is fairiy good.
The consistency among the rectangular waveguide calculation, the BPM
simulation, and the rneasured results in this cornparison of output beam profiles indicates
thüi tlic BPM simulation givcs vcry rcliable resulis, and can bc a powerlul tool in guiding
tlic design of silica rib waveguidcs.
Figure 4.9 Electrical field distributions obtained from BPM simulation of a trapezoid waveguide mode1 for sample A. Single-mode propagation is clear after 2mm of propagation dis tance.
Distance (pm)
Figure 4.10 Cornparison of beam intensity profile for sample A. The normalized intensity for BPM was obtained by squaring the values of the electrical field obtained in Figure 4.9 at 4.1 mm propagation distance.
4.2.2 Optimized Waveguide Width and Etch Depth
Based on the results in this chapter, we gain more insight in choosing the
waveguide width and etched depth. This section will summarize the guidelines for
choosing these two parameters. At least four criteria should be kept in mind in
determining these two pararneters:
a. Single mode conditions;
b. Confinement factor;
c. Coupling efficiency (related to lateral numerical aperture);
d. Waveguide loss.
Table 4.2 shows how changes to the etch depth or waveguide width affects the
above four pararneters. Increasing etch depth will increase the coupling efficiency while
also increasing the scattering losses, as shown in Table 4.1. There is a trade-off. Which
critena is more important depends on the type of application. For example, scattering
loss is not a big concem for a relatively short waveguide, then a deeper etch depth is
preferred. Otherwise, the etch depth should not be larger than 4pm for a PIRi waveguide.
Although in this case the mismatch of the laterd N.A. of the rib waveguide to the N.A. of
pigtailed fiber exists, in principle, it can be elirninated if the pigtailed fiber is properly
designed by adjusting the radius of curvature (Figure 4.7). If4pm depth is a good choice,
then to maintain single-mode propagation, the waveguide width should be no larger than
9pn. Further, the width should exceed 7 p for a >80% of confinement factor. This
leads to an optimum choice of 8p.m rib width (FWZIM) and 4pm etch depth.
Table 4.2 Relationship between four waveguide pararneters and the waveguide width
and etch depth.
4.2.2 Theory of Surface Scattering Loss
parameter
increasing
width
increasing
depth
For silica material based waveguides, surface scattering loss is the dominant loss.
This is in contrast with semiconductor-based waveguides where absorption loss is the
rnost important. A very smooth waveguide boundary of silica-based waveguide can
reduce the loss down to 0.5 dBkm such as in a standard optical fiber. However, silica rib
waveguides show much higher loss that fiber.
Our results shows a 4dB/cm loss of the rib waveguide fabricated in this thesis.
Industry standard for waveguide loss in photonics circuit is about ldB/cm before the
waveguide is considered for serious applications Wart891. To further reduce the loss, we
need theoretical guidance and a better understanding of the mechanism of surface
scattering loss. Unfortunately, there is no simple theory of surface scattenng loss for a ri6
waveguide. Ladouceur and Love [Lad0961 present a theory for scattering loss of buried
channei waveguides that assumes weak guiding where the index difference between the
guiding materid and the cladding material is less than 1%. This is not the case for the rib
single mode
more
difficult
more
difficult
confinement
factor
increasing
increasing
coupling
efficiency
no effect
increasing
loss
increasing
increasing
waveguides in this thesis because the index difference between the air and the rib is 45%.
Marcuse [Marc821 derived a very general and cornplex theory for surface scattering loss
for slab waveguides. A more simplified version, based on the Rayleigh criterion, was
derived by Tien [Tien'll]. The Rayleigh criterium applies only to the case of long
correlation length of surface roughness. in this section, we attempt to gain some insight
into the scattering loss from Tien's theory.
In Our case, the surface roughness in the sidewall of the rib is much higher than
the surface roughness between the Ge-silica and silica interface or between the Ge-silica
and air interface. If the model of rectangular waveguide is used, then the loss due to the
roughness of the sidewall is estimated by [Tien7 1,Osgo92]:
, cos3 8 / sin 8 a, = 2n,'k2a- w + 2 / y
where sine = N,, / n, and y is the decay constant outside the sidewall. o is the statistical
variance (RMS) of surface roughness, which was measured to be -50nm for waveguide
sample A. The meaning of 8 is shown in figure 4.1 1. Using this model, we obtained
y-0.21m and 0=88.S0. The resulting loss was calculated to be only 0.2dB/cm. This is in
poor agreement with the measured loss of 4dl3lcm for sarnple A.
air 4 *
8p.m Ge-silica *
1
8 air
Figure 4.1 1 Top view of the rib waveguide showing the intemal reflection angle 0.
The poor agreement can be attributed to the fact that Equation (4.1) only applies
for a slab waveguide. There is no simple way to extend it to calculate the scattering loss
of channel waveguide. To firther complicate the situation, in the vertical direction of the
rib waveguide in this thesis, half of sidewdl is very rough from laser etching and exposed
to air, while the other half is the thin guiding Ge-doped silica layer. It is difficult to
denve a theory of scattering loss from an inhomogeneous sidewall, and probably requires
the effort of a new research project.
Nevertheless, from Equation (4.1) we can still get some qualitative understanding
on the scattering loss. The loss is proportional to d. If the etched depth is increased,
then more of the guiding light will reflect in the sidewall where surface roughness is high.
Effectively, $ increases. Etching deeper also decreases 0 (since the effective index in the
etched region decreases, as seen in Table 2.1) and y. Al1 these changes lead to an
increase of loss, as observed experimentally in section 4.1.2. Furthemore, 0 is the
incident angle in the sidewall and its value depends on the propagating mode. Higher
modes have smaller 8, thereby yielding larger loss according to Equation (4.1). This was
also observed in section 4.1.2.
It is also important to note that Equation (4.1) shows the 1/X2 dependency of
scattering loss since the loss measurement in this thesis was performed at 0.635ym
wavelength. This wavelength scding will reduce the 4dBkm loss at 0.635~ to less than
ldB/cm at the 1.55p.m telecommunication wavelength. In fact, Takato, Yasu and
kawachi [Taka861 have shown that the surface scattenng loss of a rectangular silica
waveguide fabricated by RIE was 0.3dBkm at 0.6pm and O. ldB/cm at 1 S p .
It is possible to further reduce the scattering loss by reducing etched depth, but the
difference between the lateral numerical aperture and vertical numencal aperture
increases. This will degrade the quality of waveguide since the coupling loss from
standard fiber to such waveguide will increase. Effort to reduce scattering loss should be
directed into reduction of surface roughness, or coating an upper cladding on the
waveguide, as will be further discussed in Chapter 5.
4.2.3 Comparison With Other Work
The drive of this thesis work is to define a simple laser-based waveguide
fabrication technique for silica material. Currently there are a number of research groups
which are also actively developing new fabrication techniques for various optical material
[Hart89, Osgo92, Rich94, Mukh941. Most of them employed direct laser writing, which,
as mentioned earlier, is promising in defining a single-step waveguide fabrication
process. It is valuable to compare our work with the works of other groups.
Krchnavek, Lalk, and Hartman Bart891 used an Ar-ion laser (h = 350nm) to
write channel waveguides using spinsn polymer (Norland 61). The polymer was
pattemed by exposure to the laser. After exposure, the pattern was developed by rinsing
the film in acetone, leaving the unexposed region as a channel waveguide. Strictly
speaking, the process is not single-step. One of the advantage of this approach is that the
writing speed was fast, at 10-400Cun/s writing rate. This is related to that fact that Ar-ion
laser is a continue-wave laser. The waveguide cross-section was approximately 8x4pm.
The scattering loss was less than IdB/cm after optirnization of the cross-section area.
However, the fabncated waveguides in this report were multimode. Since the report also
showed that the loss decreases as the cross-sectional area increases, it is not clear whether
the loss is as low as ldB/cm for single mode waveguide.
Channel waveguides were also fabricated by Mukhejee, Eapen and Baral
[Mukh94] in polymethylmethacrylate (PMMA) using an intracavity doubled Ar ion laser
(h = 257nm). The loss was greatly reduced to about 0.08dB/cm when the PMMA
solution used was filtered through a OJmm pore. Waveguides based on unfiltered
PMMA solution typically yielded 2dB/cm loss. Reducing the polyrner size of PMMA
decreased the bulk scattering loss to achieve a very low waveguide loss. However, these
waveguides [Mukh94] were also multimode, and the fabrication process required special
treatment of the waveguide material, defining an overall complex laser process.
Although polymer material draws much attention as the guiding material for
channel waveguides, there are some advantages in silica material which can not be
achieved by polymer materiai. For example, the photosensitivity effect is only observed
in silica material. Combining the photosensitivity effect and simple waveguide writing
techniques provides a potential of fabricating complicated photonic circuit in a single
silica chip. This can not be achieved with polymer materîals.
Serniconductor material aiso draws attention as a waveguide material because of
its use as lasers, detectors, and also because it affords a strong electro-optic effect.
Osgood et al. [Osgo921 extended the laser assisted chernical etching technique to
fabricate rib waveguides in GaAs (refractive index 3.4049) on AlGaAs (refractive index
3.3566) substrate. The laser wavelength was 275nm, and the etchant was
HCL:HN03:H20::4: 1 :50. The resulting waveguide cross-section was 4 p x 1.31m. and
was single mode with a loss of 0.9dB/crn. Several drawbacks existed in these
waveguides. Firstly the cross-section size was too small which causes larger loss in
coupling light from fiber to the waveguide. Secondly the difference between the lateral
numerical aperture and vertical numerical aperture was much higher than the silica rîb
waveguides produced in this thesis work. There were two reasons for this: 1.) the
guiding GaAs layer, has more than twice the refractive index of germanosilicate; 2.) the
etch depth was only 0.25pm and 1 6 of the thickness of the GaAs layer.
Table 4.3 summarizes the cornparison of this work with other work in defining
single-step waveguide fabrication by laser writing techniques. The cornparison shows
that, as the first reported silica rib waveguide fabricated by laser ablation technique, the
rib waveguide in this thesis has already shown advantages such as single-step fabrication,
single mode confinement with large cross-section size, and relatively small difference of
numericd apertures in lateral and vertical direction. The limitation is in the relatively
large surface scattering loss. It is possible to further reduce the loss by reducing the etch,
depth, reducing the surface roughness, and improving the laser energy fluctuation, as will
be discussed in next Chapter.
Table 4.3 Cornparison of this thesis work with other results
1 Report 1 Report 1 Report 1 RIE ( This
Guiding Material
Cross-Section
[Osgo921 GaAs
Single Mode
Single Step
4x 1.3pm
Loss
Yes
[Hart891 Polymer
Yes
0.9dBkm at
1.3pm
Writing Speed I N'A
4 x 8 ~
Table 4.3 also includes a cornparison of this work with the RIE techniques
[Taka861 in fabricating silica channel waveguide. The channel waveguide in Ref.
[Taka861 was a buried channel waveguide with an upper silica cladding which helped to
reduce the scattering loss. The channel waveguide fabricated by RIE process had much
lower loss (O.ldB/cm at 1.5p-n) but suffered from many complex fabrication steps and
high-cost. On the other hand, the rib waveguide fabrication technique in this thesis offers
fast writing speed and is single-step, but suffers from high loss. Both techniques
produced single-mode waveguides with 8prn~8jun of cross-section sizes which were
compatible with those of standard single-mode optical fiber. This property was not
achieved by other three fabrication techniques listed in Table 4.3.
Nukh94J Polymer
No
8 x 3 ~
[Talca861 silica
No
Thesis Ge-silica
8 ~ 8 p m 8~8prn
Yes Yes
RIE is a large area process and is good for replication of identical photonic
circuits. However, different circuits requires different contact masks which are also
complex to fabricate. Direct laser-writing, while slow, does not require contact masks.
The positioning of sample targets can be cornputer-controlled, offenng much greater
flexibility for printing complex photonic circuits which is especiaily important for small
size applications. The laser-writing speed cm be improved dramatically since the current
laser pulse repetition rate of 1Hz can be increased to 100Hz, yielding 1000pm/s of
writing speed. Construction of a new Ft excimer laser is underway in this laboratory.
In conciusion, the waveguide fabrication technique developed in this thesis has
s hown promising potential to produce high-quality silica rib waveguides, and offers
advantages of single-step and fast writing speed. We are optimistic that the surface
scattering loss cm be brought down to ldB/cm if working at 1.55prn and alter furthet
improvement of surface roughness, as will be funher discussed in next chapter. This
allows small scale applications which only require short waveguide length (several
centirneters) Wart891. In this sense, this laser-ablation-based waveguide writing
technique imposes a practical impact to RIE technique in providing silica channel
waveguides for short-distance applications.
Chapter 5
Conclusions
5.1 Summary of This Thesis
Single-mode rib waveguides have been successfully fabricated on planar
germanosilicate, for the first time, by a laser ablation technique. The laser ablation
technique offers a single-step, loss-cost fabrication process for single-mode silica
waveguide with large cross-section size compatible with standard single-mode fibers.
The waveguide loss was -4dB/cm at 0.63pm wavelength. It is optimistic that the loss c m
be brought down to less than ldB/crn at 1 . 5 5 ~ with further improvement of surface
roughness. This pioneering work on silica rib waveguide fabrication demonstrates that
the laser-ablation-based waveguide wnting technique cm practically compete with the
RIE technique in providing silica channel waveguides for short-distance photonic
applications.
In this study, the edge resolution of the waveguide side walls was optirnized to be
-2pm. This is notable since only a single lens was used in this system. Computer
simulations were carried out to study the impact of the trapezoid waveguide cross-section
on the single mode conditions. We found that the lowest mode propagation is not
seriously effected by the geometric shape of the waveguide cross-section. The single-
mode condition for a trapezoid waveguide was very sirnilar to that of a rectangular
waveguide. Agreement on the bearn profiles obtained by BPM simulation and
experimental measurement was excellent.
Coupling of light into the rib waveguide was facilitated by a serni-spherical lens
ended fiber (SLEF). The coupling efficiency was found to be only -1 1% for the lowest
loss waveguide owning to the mismatch of the bearn divergent angle and the lateral
numerical aperture of the rib waveguide. The mismatch of numerical aperture can be
eliminated as long as the radius of curvature of the pigtailed fiber is properly designed.
This probiem existed in rnany rib waveguides reponed in literature and is an important
factor to access the overall waveguide quality.
The thesis presenü a comprehensive discussion of optimizing the waveguide
quality based on many criteria including single-mode guiding, scattering loss, coupling
efficiency and confinement factor. Cornparison of this work with other research groups
shows that the rib waveguides fabricated in this thesis are attractive in large cross-section
size, single-mode guiding, relatively large coupling effïciency and single-step fabrication.
The limitation of this thesis work was the relatively large scattering Ioss. Further
research effort is required in order to make the laser-ablation-based waveguide writing
technique compete with RIE in fabricating low loss waveguides.
5.2 Future work
Future work on laser-writing of rib waveguides should focus on reducing the
optical scattering loss. Several techniques can be employed, as described below.
Redesign of the aperture shown in Figure 3.3, such that the wire is displaced a
srnall distance in front of the rectangular aperture can create a sharp image of the wire on
the waveguide surface while blurring the hard edges of the aperture. This reduces the
surface ripple height due to the stepping process.
To reduce the surface roughness due to laser ablation, two improvement could be
taken. The first is to increase the unifonnity of laser beam. Currently, a new Fz laser is
being constmcted in this laboratory. We expect this laser can produce a more uniform
laser beam to help reduce the surface roughness. The second improvement is to reduce
the laser energy fluctuation from pulse to pulse. Computer control could also help to
compensate for such fluctuation by automatically adjusting the laser energy or the
translating speed of the waveguide sample.
The above approaches still keep the waveguide fabrication as a single-step
process. Additional steps to hirther reduce the surface roughness include chemically
treatment of the silica waveguide after fabrication to srnoothen the roughness; or coating
the waveguide surface with a polyrner materid to reduce the index difference between the
core layer and cladding layer. One advantage for the later case is that the scattering loss
theory is available Eabo96].
Measuring the waveguide loss at 1 . 5 5 ~ wavelength should also be carried out in
the future, because of the particular importance of this wavelength and the expected
reduction in scattering loss for longer wavelength light [Taka86].
Computer control of stepping process is another improvement recommended in
the future. This will not only reduce the fabrication time, increase the accurecy, but will
also allow more complicated waveguide devices to be printed.
Extension of the fabrication technique developed in this thesis to fabricate devices
such as directional couplers in germanosilicate should be straightfomard, although the
scattering loss need to be reduced first.
Photosensitivity writing is also attractive for future work since there two potential
advantages it can offer: the first is to solve the asyrnmetric vertical and horizontal
numerical aperture problem because the waveguide will be buned and the typical index
change is 0.3% [Maxw94] in Ge-doped silica which matches the index difference
between the core layer and substrate layer in PIRI waveguide (0.3%). Secondly,
photosensitivity may produce a smoother waveguide boundary further reducing the
scattering loss.
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[Moo 1941
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