fabrication of a multichannel wavelength-division multiplexing-passive optical net demultiplexer...

$: Fabrication of a Multichannel Wavelength-Division Multiplexing-Passive Optical Net Demultiplexer with Arrayed-Waveguide Gratings and Diffractive Optical Elements$
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Fabrication of a multichannel wavelength-divisionmultiplexing–passive optical net demultiplexerwith arrayed-waveguide gratings anddiffractive optical elements

Edgar Pawlowski, Margit Ferstl, Heik Hellmich, Berndt Kuhlow, Carsten Warmuth, andJose R. Salgueiro

A novel, to our knowledge, integrated wavelength-division multiplexing–passive optical net demulti-plexer that uses an arrayed-waveguide grating and diffractive optical elements is presented. Thedemultiplexer is used to distribute 1.3-mm wavelength signals and to multiplex an eight-channelwavelength-division multiplexer spectrum at a 1.55-mm wavelength. The device shows high function-ality and good optical performance. The measured cross talk was less than 221 dB, and the 3-dBbandwidth was determined to be 97 GHz, which is close to the theoretical value of 93 GHz. Averagelosses of 4.5 and 8 dB were measured for the 1.3- and the 1.55-mm signals, respectively. © 1999 OpticalSociety of America

OCIS codes: 050.1380, 050.1950, 050.1970, 060.1810, 130.3120, 160.6030.

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1. Introduction

To increase the capacity of optical transmission sys-tems, it is advantageous to use the high parallel ca-pability of optical wavelength-division multiplexing,1~WDM! techniques. Key devices for WDM commu-

ication systems are optical demultiplexers. A va-iety of different devices such as Mach–Zehnderlters,2 diffractive optical elements3 ~DOE’s!,cousto-optical filters,4 transversal filters,5,6 and

arrayed-waveguide gratings7–9 ~AWG’s! have alreadybeen developed. However, to realize a device withhigh functionality, it is useful to integrate waveguidedevices with multifunctional elements like DOE’s.As a special case of WDM functionality upgrading, weconsidered a 1.55-mm WDM extension of an existingpassive optical net10 ~PON!.

In this paper we report the fabrication of a fully

E. Pawlowski [email protected]!, M. Ferstl, H. Hellmich, B.Kuhlow, and C. Warmuth are with the Heinrich-Hertz-Institut furNachrichtentechnik Berlin GmbH, Einsteinufer 37, 10587, Berlin,Germany. J. R. Salgueiro is with the Universidad de Santiago deCompostela, Campus Sur, 15706 Santiago de Compostela, Spain.

Received 5 August 1998; revised manuscript received 10 Decem-ber 1998.

0003-6935y99y143039-07$15.00y0© 1999 Optical Society of America

integrated multichannel WDM–PON demultiplexer.The device is used to distribute 1.3-mm wavelengthsignals ~1yN power! and to multiplex an eight-channel WDM spectrum at a 1.55-mm wavelengthwith a 200-GHz channel spacing. The new device isa combination of a star coupler and a demultiplexer.Similar elements have been proposed in the past,11,12

but in this study we realize a more compact devicewith higher transmission characteristics in the1.3-mm window.

The fabrication process of the multiplexer isbased on planar light-wave circuit ~PLC! technologyhat is based on silica waveguides.13 PLC technol-

ogy is very attractive for our applications. It al-lows high reliability and low-cost productionbecause of PLC’s suitability for integrating opticalcomponents. Furthermore, the silica waveguidesshow low optical losses ~,0.1 dBycm! and highhemical and mechanical stability. It should beoted that the waveguide–fiber coupling losses forhe silica waveguides were nearly negligible ~,0.3B!, reflecting no modal field mismatch between thebers and the waveguides. For simple processing,e fabricated the elements, including the DOE-

oupling elements, with conventional photolithog-aphy processing techniques.14 The design of the

WDM–PON demultiplexer is based on beam-propagation method ~BPM! simulations and on

10 May 1999 y Vol. 38, No. 14 y APPLIED OPTICS 3039

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coupled-mode equations that are solved by thetransfer-matrix method. The device has potentialapplications in the optical telecommunicationsfield.

2. Design

To take advantage of existing optical networks andto avoid the need for additional fibers within theWDM access network, we used a WDM overlay on aPON. Figure 1 shows a special WDM–PON trans-mission setup in which an existing PON operatingat a 1.3-mm wavelength with a 1-to-N power splitters upgraded for use in the 1.55-mm window. There-ore some 1.3 mmy1.55 mm WDM splitters werenstalled in the central office, in front of and behindhe remote node, and in front of the optical networknits. An AWG was installed in the remote node.he AWG demultiplexes the incoming signal combnd routes the individual wavelengths to the corre-

Fig. 1. WDM–PON transmission path in a WDM network. CO,EMUX, multiplexer–demultiplexer.

Fig. 2. Schematic diagram

040 APPLIED OPTICS y Vol. 38, No. 14 y 10 May 1999

ponding optical network units. The new proposedDM–PON demultiplexer integrates all the func-

ions of the remote node without the first WDMplitter. The demultiplexer was realized as a com-ination of an AWG and a DOE coupler, as shown inig. 2.An AWG is a high-order optical grating that is

ealized with waveguides. An AWG consists of in-ut waveguides, output waveguides, and two equal-ocusing slab regions that are combined with Naveguides to form a phase grating between the two

lab regions. The purpose of the grating is to pro-ide a constant path-length difference between adja-ent paths connecting the two couplers. Thisonstant distance causes a difference in the phase-roducing peaks of maximum transmission at theavelengths for which the phase is an integer mul-

iple of 2p. In the vicinity of the slab region theaveguides are closely spaced ~20 mm!. Mutual cou-

ral office; RN, remote node; ONU’s, optical network units; MUXy

WDM–PON demultiplexer.

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Table 1. Optimized Demultiplexer Parameters Used in the Experiments

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pling between adjacent waveguides was optimized byuse of BPM simulations.15–17
The AWG was designed on the basis of single-modesilica waveguides and comprises 60 regularly ar-ranged waveguides with a constant path-length dif-ference @DL 5 m~lyneff!, where l is the centralwavelength, neff is the effective refractive index of thewaveguides, and m is the order of the grating#. Theight at the end of the AWG, which is arranged in aircle with a radius of f 5 2.48 mm, radiates as awo-dimensional spherical wave into the second slab.he spherical phase front converges to a focal point,here the eight outgoing waveguides are located.ingle-mode waveguides are necessary for exacthase control, and this control technique is wellnown.7–9 Here we show that it is possible to dis-

tribute with near uniformity a 1.3-mm signal to alleight output waveguides by using a DOE couplerwith rectangular or blazed grooves.

The DOE coupler was designed as a high-indexgrating. The design is based on coupled-mode equa-tions that are solved by the transfer-matrix meth-od.18,19 To obtain a condition such that only a singleeam that radiates into the planar waveguide is cou-led, the pitch of the DOE coupler must satisfy theell-known expression L , 2ly~neff 1 ncla!, where neff

is the effective refractive index of the guided mode,ncla is the refractive index of the cladding, and L ishe grating period. Figure 3 shows a sketch of theaveguide and the DOE coupler configuration in the

econd slab of the WDM–PON demultiplexer. TheOE was designed as a special grating. High radi-tion efficiencies can be obtained ~.80%! for binaryratings at the Bragg incidence of sin f 5 ly~2nd!nd if 0.5 , dyl . 1.5, where f 5 90° 2 a ~Ref. 20!.

To simplify the fiber-mounting process, we coupledthe incoming light vertically to the grating surface ~a5 0°!. For this case it is necessary to use blazedgratings to achieve high efficiencies. The optimizeddemultiplexer parameters used in our experimentsare listed in Table 1.

Figure 4 shows a BPM simulation of the electric-

Fig. 3. Waveguide and grating-coupler configuration of a WDM–PON demultiplexer.

field waveform when the light from a fiber is coupledthrough the grating coupler into the second AWGslab and into the eight output waveguides at a1.3-mm wavelength. It can clearly be seen thatnearly all the light is coupled into the waveguides.Special efforts were made to design a device with auniform field distribution by use of the coupling-element distance to the output waveguides, the grat-ing design, and the size of the waveguides asparameters. The optimized electric-field distribu-tion in the output waveguides was better than 610%.Calculations with grating couplers of more complexbeam-shaping characteristics will be realized infuture-generation devices. The different fabricationprocesses of the elements are described indepen-dently in Sections 3 and 4.

Fig. 4. Waveform when light is coupled from the grating into theoutput demultiplexer waveguides.

Parameter Value

WaveguideWavelength l 1.3–1.55 mmRefractive index of cladding ncla 1.454Refractive index of core nc 1.444Refractive index of waveguide neff 1.449Waveguide dimensions 6 mm 3 6 mmThickness of cladding d 20 mm

WGWavelength l 1.55 mmNumber of channels N 8Channel spacing Dl 1.6 nm ~200 GHz!Diffraction order m 35Number of arrayed waveguides 60Slab arc length f 2.48 mm

ratingWavelength l 1.3 mmProfile BlazePeriod 760 nmDepth d 0.67 mmMaterial Silicon amorphGrating areas 40 mm 3 40 mm

and 800 mm 3 800 mmRefractive index n 3.4


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3. Fabrication of the Arrayed-Waveguide Grating andthe Waveguides

The AWG and the waveguides were fabricated byflame hydrolysis deposition ~FHD! of SiO2–GeO2 lay-ers by optical lithography and reactive-ion etching~RIE!. The complete manufacturing process con-ists of different technological stages. The firsttage of fabrication was to deposit two successivelass-particle layers ~buffer and core! on the silicon

substrate by FHD. Fine glass particles synthesizedby flame hydrolysis are deposited onto silicon waferson a turntable.21 The refractive index is controlledby the GeCl4 flow ratio. After FHD the wafers withporous glass layers are heated to 1300 °C in an elec-tric furnace for consolidation. The core ridges de-sired for the waveguides are defined by conventionaltechnology. Second, the substrate with the two sil-ica layers was cleaned, coated with resist ~AZ5218 at700 rpm!, and prebaked at 90 °C for 5 min on a hotlate. Third, the patterns from the mask wereransferred into the resist ~image reversal! by opticalontact lithography. This process involves exposurend reversal baking ~120 °C for 3 min on a hot plate!.

Fig. 5. View of the interface between the input waveguides, thefirst slab region, and the output waveguides.

Fig. 6. SEM photographs of ~a! the e


he exposure was carried out on a mask alignerSuss, Model MA 100M! at a wavelength of 310 nm.

Figure 5 shows a section of the mask with an inter-face between the input waveguides, the first slab re-gion, and the output waveguides. The waveguideshave a width of 6 mm and were placed 20 mm apartrom each other. After the lithography process theore layer was processed by RIE with CHF3–H2 as the

etching gas. The etching ratio of SiO2:resist was inthe range of 2 to 3. The RIE rate of SiO2 was chosento be 3 mmyh for single-mode waveguide formation.

igure 6~a! shows a scanning electron microscopeSEM! photograph of etched core ridges in the inputWG slab, and Fig. 6~b! shows a SEM photograph of

he arrayed waveguides. There are slight side-wallrregularities, but it can also be seen that the sidealls are almost vertical. Finally, the core ridgesre covered with a thick SiO2 cladding to bury the

waveguides. The SiO2 cladding-layer thickness isapproximately 20 mm. The core sizes of thewaveguides are 6 mm 3 6 mm, and the relativeefractive-index difference between the cladding andhe core layers is 0.7%. The complete device size is2 mm 3 26 mm. The clear and simple waveguidesroduced with these techniques make it possible toabricate the AWG’s with high quality and excellenteproducibility.

4. Fabrication of the Diffractive Optical ElementCoupler

The DOE coupler is fabricated directly on the clad-ding layer by UV contact lithography in combinationwith an amplitude–phase mask,14 different subse-quent ion-beam-sputtering deposition ~IBSD! pro-cesses, and a dry-etch step. After defining thewaveguide core ridges by RIE techniques ~Section 3!,

e coated the coupling area with silicon by ISBD.ilicon layers produced by IBSD have good adhesion,high packing density, and an amorphous structure.he transmission spectrum ranges from 1.1 to 10 mm.he thickness of the silicon layer is controlled to

input slab region and ~b! the AWG’s.
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within 0.33 mm. After ISBD the element was coatedith a 1.2-mm-thick resist ~AZ5214 at 6000 rpm! andrebaked at 90 °C for 180 s on a hot plate. A criticalrocess is transferring the pattern from the masknto the resist by optical contact lithography. Forroper linewidth control, we used an attenuatedhase-shifting mask in which the conventionalpaque part of the mask was replaced with an ab-orbing p-phase shifter. The p modulation of the

phase is due to negative electric amplitudes, highercontrasts, and nearly no stray light at the imageedges. The improvement in imaging can besignificant.22–24 Figure 7~a! shows a representativeSEM photograph of a grating transferred by opticallithography with an attenuated phase mask into a1.2-mm-thick resist ~AZ 5214!. The resist shows ver-tical side slopes with low roughness and a measuredline:space ratio of approximately 0.5.

The transfer of the resist patterns into the siliconlayer on top of the slab waveguide material was re-alized by use of ion-beam etching with Ar1 ions.The ability to etch in a preshaped photoresist re-quires anisotropic etching and a constant resist-to-substrate selectivity during the entire etchingprocess. Ion-beam etching with Ar1 or N2

1 ions ac-celerated to an energy of 700 eV provides good etch-ing results at pressures of 4 3 1025 mbar. Thestructure depth is defined by the etch selectivity be-tween the resist and the substrate material and canbe monitored by in situ mass spectrometry. Figure7~b! shows a silicon grating fabricated on top of a slabwaveguide. The grating period is L 5 760 nm. Theelement size is 800 mm 3 800 mm. Also, smallerlements are realized for different test purposes.or a high coupling efficiency it is necessary to con-entrate all the light into one diffraction order. Tobtain such a coupling condition, we fabricatedlazed grating couplers. The blazed structure is re-lized by IBSD of silicon at different angles. A sim-le computer-simulation routine was used toptimize the sputtering process. In comparisonith other techniques the ISBD technique is veryttractive because of the simple fabrication processnd the realization of smooth grating profiles. Fig-re 7~c! shows a SEM photograph of a blazed grating

abricated with this oblique sputtering method. Therating period was L 5 760 nm, and the depth wasetermined to be 0.62 mm, which is close to the the-retical value of 0.67 mm. Finally, we coated theoupling element, the AWG structure, and theaveguides with a 20-mm-thick SiO2 cladding layer

by FHD. The simple fabrication techniques ensurelow-cost production of the AWG–PON demultiplexer.

5. Experimental Results

The experimental WDM–PON demultiplexer was in-vestigated by the coupling of a 1.3-mm signal from apigtailed semiconductor laser into the device. First,we measured the efficiency of coupling into filmwaveguides. The measured efficiencies are shownin Fig. 8. Coupling efficiencies of 42% ~3.8 dB! and8% ~1.6 dB! were measured for a binary and a blazed

rating coupler, respectively. Consequently, thelazed grating shows a higher coupling efficiency intolm waveguides. The measured efficiency is compa-

Fig. 7. SEM photographs of gratings ~a! transferred by opticalphase-mask lithography into a 1.2-mm-thick resist ~AZ 5214!, ~b!etched into silicon on top of a slab waveguide, ~c! blazed by obliquesputtering. The grating spacing is L 5 760 nm.


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rable with other experimental values obtained frompolymer coupling elements, which are fabricated withmore expensive electron-beam direct-writing tech-niques.25 This comparability indicates good opticalperformance. An average loss of 4.5 dB, which in-cludes fiber–waveguide coupling losses, is measuredin addition to the 9-dB theoretical splitting loss forthe AWG–PON demultiplexer at the 1.3-mm wave-ength. The additional losses are caused by lightpilling over into side slopes ~1 dB!, light imaged into

gaps between the output waveguides ~0.5 dB!, andwaveguide propagation losses of 0.5–0.8 dB.

The 1.55-mm transmission spectrum of the multi-plexer was measured at the eight output ports by useof an IR detector ~Hewlett-Packard, Model HP8153A! when light from a tunable laser source ~Model

P 8168B! is coupled to the main input port. Figureshows the measured transmission spectrum of theDM–PON demultiplexer with a 200-GHz ~1.6-nm!

hannel spacing and a central wavelength of 1.55 mm.he measured next-neighbor cross talk is lower than21 dB. The 3-dB bandwidth was determined toithin 97 GHz, which is close to the theoretical value

rom BPM calculations of 93 GHz. The experimental

Fig. 8. Diffraction efficiencies of binary and blazed grating cou-plers.

Fig. 9. Measured transmission spectrum of the 8 3 8 AWG–PONith a 200-GHz channel spacing.


esults agree quite well with the theoretical values.herefore it is confirmed that an AWG and a DOEan be combined to realize more complex devices.

6. Concluding Remarks

We have proposed and fabricated a novel integratedmultichannel WDM–PON demultiplexer by using anAWG and a DOE. The basic functions of distribut-ing and multiplexing the light at wavelengths of 1.3mm and 1.55 mm have been demonstrated. Blazedratings with submicrometer periods have been real-zed with optical lithography and an oblique sputter-ng process. The fabrication process represents aromising direction for planar waveguide technology.ow-cost integration of these different optical compo-ents into complex functional circuits is possible.he demonstrated fabrication techniques are com-only used in microelectronics technology, so no ad-

itional production equipment needs to be developed.he device shows high functionality and good opticalerformance. It is expected that this new device willnd wide application in WDM systems.

The authors thank W. Furst for data generation, G.rzyrembel for help with the design of the AWG’s, R.teingruber for writing the masks, and G. Urmann

or the SEM measurements. This study was sup-orted financially by the Federal Ministry for Educa-ion, Science, Research and Technology ~BMBF!

within national project 01BS609 and by the city ofBerlin.

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fabrication of a multichannel wavelength-division multiplexing-passive optical net demultiplexer...

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