EC- 6702

OPTICAL COMMUNICATION

AND NETWORKS

UNIT – III

FIBER OPTICAL SOURCES AND COUPLING

Injection LASER diode

Optical output vs Drive Current

DH Injection LASER diode

• Number of photons emitted per radiative electron-hole pair

recombination above threshold, gives us the external quantum

efficiency.

• Note that:

)mA(

)mW(]m[8065.0

)(

dI

dP

dI

dP

E

q

g

g

g

th

thiext

%40%15 %;70%60 exti

External Quantum Efficiency

External Quantum Efficiency

• Efficient operation of a laser diode requires reducing the # of

lateral modes, stabilizing the gain for lateral modes as well as

lowering the threshold current.

• These are met by structures that confine the optical wave,

carrier concentration and current flow in the lateral direction.

• The important types of laser diodes are: gain-induced, positive

index guided, and negative index guided.

Injection LASER diode structures

Injection LASER diode structures

(a) gain-induced guide (b)positive-index waveguide (c)negative-index waveguide

• Gain – Guided Lasers

– Fabrication of multimode injection lasers with a single or small number

of lateral modes is achieved by the use of stripe geometry.

– These devices are often called gain – guided lasers.

Index Guided Laser - Buried Heterosturucture device

Injection LASER diode structures

Injection LASER diode structures

Injection LASER diode structures

Modulation of Laser Diodes

• Internal Modulation: Simple but suffers from non-linear

effects.

• External Modulation: for rates greater than 2 Gb/s, more

complex, higher performance.

• Most fundamental limit for the modulation rate is set by

the photon life time in the laser cavity:

• Another fundamental limit on modulation frequency is the

relaxation oscillation frequency given by:

th

ph

gn

c

RRLn

c

21

1ln

2

11

2/1

11

2

1

thphspI

If

Relaxation oscillation peak

Temperature variation of the

threshold current0/

)(TT

zth eITI

POWER LAUNCHING AND COUPLING

LENSING SCHEMES

• If the source emitting area is larger than the fiber core area,

then the resulting optical power coupled in to the fiber is the

maximum that can be achieved.

• If the emitting area of the source is smaller than the core area,

a miniature lens may be placed between the source and the

fiber to improve the power coupling efficiency.

• The function of the microlens is to magnify the emitting area

of the source to match exactly the core area of the fiber end

face.

POWER LAUNCHING AND COUPLING

LENSING SCHEMES

LENSING SCHEMES

Nonimaging Microsphere

• One of the most efficient lensing methods is the use of a non

imaging microsphere.

• The focal point can be found from the gaussian lens formula:

Fiber to Fiber joints

• A permanent bond between two ends of fiber is generallyreferred to as a splice.

• A demountable joint between two fiber ends is known as aconnector.

• The optical power loss at the joints depends on parameterssuch as– the input power distribution to the joint,

– the length of the fiber between the optical source and the joint,

– the geometric and waveguide characteristics of the two fiber ends at thejoint and

– the fiber end face qualities.

• The optical power that can be coupled from one fiber to

another is limited by the number of modes that can propagate

in each fiber.

• In general, any two fibers that are to be joined will have

– varying degrees of differences in their radii a,

– axial numerical apertures NA(0) and

– index profiles α.

• Thus the fraction of energy coupled from one fiber to another

is proportional to the common mode volume Mcomm

Fiber to Fiber joints

• The fiber to fiber coupling efficiency ηf is given by

• The fiber to fiber coupling loss LF is given in terms of ηf as

LF = - 10 log ηf

Fiber to Fiber joints

Fiber to Fiber joints

• Mechanical alignment is a major problem when joining two fibers,owing to their microscopic size.

• A standard multimode GI fiber core is 50 – 100μm in diameter,which is roughly the thickness of a human hair, whereas single modefiber have diameters on the order of 9 μm.

• Radiation losses result from mechanical misalignments, because theradiation cone of the emitting fiber does not match the acceptancecone of the receiving fiber.

• The magnitude of the radiation loss depends on the degree ofmisalignment.

Fiber to Fiber jointsMechanical Misalignments

• The three fundamental types of misalignment between the

fibers are shown in fig:

Longitudinal Separation

Lateral (Axial) Misalignment

Angular Misalignment

Fiber to Fiber jointsMechanical Misalignments

• In addition to mechanical misalignments, differences in the

geometrical and waveguide characteristics of any two

waveguides being joined can have a profound effect on fiber to

fiber coupling loss.

• These include variations in core diameter, core area ellipticity,

numerical aperture, refractive index profile and core cladding

concentricity of each fiber.

Fiber to Fiber jointsFiber Related Losses

• One of the first steps that must be followed before fibers are

connected or spliced to each other is to prepare the fiber end

faces properly.

• In order not to have light deflected or scattered at the joint, the

fiber ends must be flat, perpendicular to the fiber axis and

smooth.

• End preparation techniques that have been extensively used

include sawing, grinding and polishing and controlled

fracture.

Fiber to Fiber jointsFiber End Face Preparation

• Conventional grinding and polishing techniques can produce a

very smooth surface that is perpendicular to the fiber axis.

However this method is quite time consuming and requires a

fair amount of operator skill.

• Controlled fracture techniques are based on score and break

methods for cleaving fibers.

• In this operation, the fiber to be cleaved is first scratched to

create a stress concentration at the surface.

Fiber to Fiber jointsFiber End Face Preparation

• The fiber is then bent over a curved form while tension issimultaneously applied as shown below:

• This action produces a stress distribution across the fiber.

• The maximum stress occurs at the scratch point so that a crackstarts to propagate through the fiber.

Fiber to Fiber jointsFiber End Face Preparation

FIBER SPLICING

• A fiber splice is a permanent or semipermanent joint between

two fibers. These are typically used to create long optical links

or in situations where frequent connection and disconnection

are not needed.

• In making and evaluating such splices, one must take in to

account the geometrical differences in two fibers, fiber mis

alignments at the joint and the mechanical strength of the

splice.

• Splices may be divided into two broad categories depending upon the splicing technique utilized. These are

– fusion splicing or welding and

– mechanical splicing.

• Fusion splicing is accomplished by applying localized heating (e.g.by a flame or an electric arc) at the interface between two butted,pre aligned fiber ends causing them to soften and fuse.

• Mechanical splicing, in which the fibers are held in alignment bysome mechanical means, may be achieved by various methodsincluding the use of tubes around the fiber ends (tube splices) orV-grooves into which the butted fibers are placed (groove splices).

FIBER SPLICING

• The fusion splicing of single fibers involves the heating of the two preparedfiber ends to their fusing point with the application of sufficient axialpressure between the two optical fibers.

• It is therefore essential that the stripped (of cabling and buffer coating)fiber ends are adequately positioned and aligned in order to achieve goodcontinuity of the transmission medium at the junction point.

• Hence the fibers are usually positioned and clamped with the aid of aninspection microscope.

• Flame heating sources such as microplasma torches (argon and hydrogen) andoxhydric microburners (oxygen, hydrogen and alcohol vapor) have beenutilized with some success. However, the most widely used heating source is anelectric arc.

FIBER SPLICING

Fusion Splicing

FIBER SPLICING

Fusion Splicing

• Fusion splicing of single-mode fibers with typical core diameters between 5

and 10 μm presents problems of more critical fiber alignment (i.e. lateral

offsets of less than 1 μm are required for low-loss joints).

• However, splice insertion losses below 0.3 dB may be achieved due to a

self-alignment phenomenon which partially compensates for any lateral

offset.

• Self-alignment, illustrated in Figure below, is caused by surface tension

effects between the two fiber ends during fusing.

FIBER SPLICING

Fusion Splicing

• A possible drawback with fusion splicing is that the heat necessary to fuse

the fibers may weaken the fiber in the vicinity of the splice.

• It has been found that even with careful handling, the tensile strength of the

fused fiber may be as low as 30% of that of the uncoated fiber before

fusion.

• The fiber fracture generally occurs in the heat affected zone adjacent to the

fused joint.

FIBER SPLICING

Fusion Splicing

FIBER SPLICING

Fusion Splicing

• A number of mechanical techniques for splicing individual optical fibers

have been developed.

• A common method involves the use of an accurately produced rigid

alignment tube into which the prepared fiber ends are permanently bonded.

This snug tube splice is illustrated in Figure below and may utilize a glass

or ceramic capillary with an inner diameter just large enough to accept the

optical fibers.

FIBER SPLICING

Mechanical Splicing

• Transparent adhesive (e.g. epoxy resin) is injected through a transverse bore in the capillary togive mechanical sealing and index matching of the splice.

• Average insertion losses as low as 0.1 dB have been obtained with multimode graded indexand single-mode fibers using ceramic capillaries.

• However, in general, snug tube splices exhibit problems with capillary tolerancerequirements.

• Hence as a commercial product they may exhibit losses of up to 0.5 dB

• This loose tube splice uses an oversized square-section metal tube which easily accepts theprepared fiber ends.

• Transparent adhesive is first inserted into the tube followed by the fibers.

• The splice is self-aligning when the fibers are curved in the same plane, forcing the fiber endssimultaneously into the same corner of the tube.

FIBER SPLICING

Mechanical Splicing

• The splice is made permanent by securing the fibers in the V-groove with epoxy

resin. Jigs for producing Vgroove splices have proved quite successful, giving joint

insertion losses of around 0.1 dB

• V-groove splices formed by sandwiching the butted fiber ends between a V-groove

glass substrate and a flat glass retainer plate, as shown in Figure below, have also

proved very successful in the laboratory.

FIBER SPLICING

Mechanical Splicing

• Splice insertion losses of less than 0.01 dB when coupling

single-mode fibers have been reported using this technique.

FIBER SPLICING

Mechanical Splicing

Photo Detectors

• Optical receivers convert optical signal (light) to electrical

signal (current/voltage)

– Hence referred ‘O/E Converter’

• Photodetector is the fundamental element of optical receiver,

followed by amplifiers and signal conditioning circuitry

• There are several photodetector types:

– Photodiodes, Phototransistors, Photon multipliers, Photo-

resistors etc.

Photodetector Requirements

• Good sensitivity (responsivity) at the

desired wavelength and poor responsivity

elsewhere wavelength selectivity

• Fast response time high bandwidth

• Compatible physical dimensions

• Low noise

• Insensitive to temperature variations

• Long operating life and reasonable cost

Photodiodes

• Due to above requirements, only photodiodes are used as photodetectors in optical communication systems

• Positive-Intrinsic-Negative (pin) photodiode

– No internal gain

• Avalanche Photo Diode (APD)

– An internal gain of M due to self multiplication

• Photodiodes are sufficiently reverse biased during normaloperation no current flow, the intrinsic region is fullydepleted of carriers

Signal to Noise Ratio

2 2

22 ( ) ( ) 2 4 /

p

p D L B L

i MSNR

q I I M F M B qI B k TB R

Detected current = AC (ip) + DC (Ip)

Signal Power = <ip2>M2

Typically not all the noise terms will have equal weight.

Often the average signal current is much larger than the

leakage and dark currents

SNR vs. Received Power

Response Time in pin photodiode

Transit time, td and carrier drift velocity vd are related by

/d dt w v For a high speed Si PD, td = 0.1 ns

Rise and fall times

Photodiode has uneven rise and fall times depending on:

1. Absorption coefficient s() and

2. Junction Capacitance Cjo r

j

AC

w

Junction Capacitance

o rj

AC

w

εo = 8.8542 x 10(-12) F/m; free space permittivity

εr = the semiconductor dielectric constant

A = the diffusion layer (photo sensitive) area

w = width of the depletion layer

Large area photo detectors have large junction capacitance hence

small bandwidth (low speed)

A concern in free space optical receivers

Various pulse responses

( )

0( ) (1 )s xP x P e

Absorbed optical power at

distance x exponentially decays

depending on s

Comparisons of pin Photodiodes

NOTE: The values were derived from various vendor data sheets and from

performance numbers reported in the literature. They are guidelines for comparison

purposes. Detailed values on specific devices for particular applications can be

obtained from photodetector and receiver module suppliers.