optical tdm

8/6/2019 Optical Tdm

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CONTENTS


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Chapters Page

nos.

1. INTRODUCTION 1

1.1 Different types multiplexing techniques 2

1.1.1 Frequency Division Multiplexing 2

1.1.2 Time Division Multiplexing3

1.1.2.1 Synchronous Time Division Multiplexing 3

1.1.2.2 Statistical Time Division 3

1.1.3 Wavelength Division Multiplexing (WDM) 4

1.1.4 Code Division multiplexing(CDM) 4

2. PRINCIPLES OF OPTICAL TIME-DIVISION 5

MULTIPLEXING AND DEMULTIPLEXING

2.1Electrical and Optical Multiplexed Systems 5

2.2. Optical Time-Division Multiplexing 7

2.3. Optical Switching and Demultiplexing 12

2.4 Timing Recovery 19

3 .EXPERIMENTS 20

3.1Multiple-Laser Systems 21

3.2 Single-Laser System 31

4 CONCLUSIONS AND OUTLOOK 32

5.REFERENCES 35


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Chapetr-1

1. Introduction

Progress in very high bit-rate light wave systems has been stimulated by consistent demands for

expanded transmission capacity. These demands have led to increased interest in multi giga bit

per-second pulse-code modulated (PCM) systems, and have put an emphasis on the need for

high-speed and wide-band electronics in light wave transmitters and receivers. To date, it has

generally been possible to meet these needs with high-speed Si and GaAs circuits, but at gigabit-

per-second bit rates it becomes increasingly difficult to develop the necessary digital electronic

circuits. One method to relieve this electronic speed bottleneck is to extend the well-known

techniques of electrical multiplexing into the optical domain. The two main approaches to optical

multiplexing are optical wavelength-division (or frequency-division) multiplexing and optical

time-division multiplexing. This presentation concentrates on optical time division multiplexing.

In optical time-division multiplexing (OTDM), a high bit-rate data stream is constructed directly

by time-multiplexing several lower bit-rate optical streams. Similarly, at the receiver end of the

system, the very high bit-rate optical signal is demultiplexed to several lower bit-rate optical

signals before detection and conversion to the electrical domain. This approach to optical time-

division multiplexing and demultiplexing moves the demand for high-speed performance away

from electronic devices such as transistors, and places it on optical and optoelectronic devices

such as pulsed semiconductor lasers and optical switches. The time-division multiplexing

approach is a purely digital technique and is therefore compatible with the concept of an

all-digital network that combines switching and transmission. In addition, optical time-

division multiplexing offers system design flexibility, including the possibility of adjustable

bandwidth allocation in different baseband channels and the possibility of simple system

hardware in which only a single transmitter laser is required for all channels. The potential of

optical t ime division multiplexing and demultiplexing for very high bit-rate PCM systems

has been recognized for more than two decades but until recently there have been few

system-level demonstrations of the technique at multi gigabit-per- second bit rates. The

implementation of very high bit-rate. OTDM systems has been slow because electronic

multiplexing has usually served adequately and because the necessary hardware, such as high-


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speed optical switches and compact pulsed semiconductor lasers, has only recently reached a

sufficient state of refinement. This paper describes recent experiments in optical time-division

multiplexing and demultiplexing that have been made possible by improvements in lasers and

switch /modulators. Our emphasis is on very high bit-rate point-to-point transmission systems

but many of the concepts are also relevant to multiuser systems and time-multiplexed photonic

switching networks. We review system architectures, describe the requirements on individual

system components, and give examples of transmission system experiments operating at bit rates

up to 16 Gbit /s. One of the key factors affecting the performance of multiplexed systems is

crosstalk between baseband channel . In presentation explore sources of crosstalk in optical time-

division multiplexing and demultiplexing, and describe how the main system components affect

the overall crosstalk performance. Under the simplest conditions, a medium can carry only one

signal at any moment in time. For multiple signals to share one medium, the medium must

somehow be divided, giving each signal a portion of the total bandwidth.The current techniques

that can accomplish this include

1.1 Different types multiplexing techniques

1.frequency division multiplexing (FDM)

2.Time division multiplexing (FDM)

3.wavelength division multiplexing (WDM)

4.code division multiplexing (CDM)

1.1.1Frequency Division Multiplexing

Assignment of non-overlapping frequency ranges to each user or signal on a medium. Thus,

all signals are transmitted at the same time, each using different frequencies.A multiplexor

accepts inputs and assigns frequencies to each device. The multiplexor is attached to a high-

speed communications line.A corresponding multiplexor, or demultiplexor, is on the end of the


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high-speed line and separates the multiplexed signals.

1.1.2Time Division Multiplexing:

Sharing of the signal is accomplished by dividing available transmission time on a medium

among users.Digital signaling is used exclusively.

Time division multiplexing comes in two basic forms:

1. Synchronous time division multiplexing, and

2. Statistical, or asynchronous time division multiplexing

1.1.2.1 Synchronous Time Division Multiplexing

The original time division multiplexing.The multiplexor accepts input from attached devices in a

round-robin fashion and transmit the data in a never ending pattern.T-1 and ISDN telephone

lines are common examples of synchronous time division multiplexing.

1.1.2.2 Statistical Time Division:

MultiplexingA statistical multiplexor transmits only the data from active workstations (or why

work when you dont have to).If a workstation is not active, no space is wasted on the


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multiplexed stream.A statistical multiplexor accepts the incoming data streams and creates a

frame containing only the data to be transmitted.A statistical multiplexor does not require a lineover as high a speed line as synchronous time division multiplexing since STDM does not assume all

sources will transmit all of the time!Good for low bandwidth lines (used for LANs).

1.1.3Wavelength Division Multiplexing (WDM)

Give each message a different wavelength (frequency)Easy to do with fiber optics and optical

sources, Dense wavelength division multiplexing is often called just wavelength divisionmultiplexing .Dense wavelength division multiplexing multiplexes multiple data streams onto a

single fiber optic line. Different wavelength lasers (called lambdas) transmit the multiple

signals.Each signal carried on the fiber can be transmitted at a different rate from the

othersignals.Dense wavelength division multiplexing combines many (30, 40, 50, 60,

more?)onto one fiber.

1.1.4 Code Division Multiplexing (CDM)

Also known as code division multiple access (CDMA),An advanced technique that allows

multiple devices to transmit on the same frequencies at the same time using different codes .Used

for mobile communications. time. Each mobile device is assigned a unique 64-bit code (chip

spreading code)


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Chapter-2

2. PRINCIPLES OF OPTICAL TIME-DIVISION

M UL TIPLEXING AND DEMUL TIPLEXING

2.1Electrical and Optical Multiplexed Systems

The basic principle of time-division multiplexing and demuItiplexing is that each of the

baseband data streams is allocated a series of time slots on the multiplexed channel.

A multiplexer (MUX) assembles the h higher bit-rate bit stream from the baseband streams and

a demultiplexer(DEMUX) reconstructs bit streams at the original lower bit rate by separating bits

in the multiplexed stream . The techniques for this process are well established for electrical

time-division multiplexing and demultiplexing but are only now emerging in optical systems

.Fig. I highlights the similarities and differences between electrically time-multiplexed and

optically time multiplexed light wave systems . In this figure and in subsequent figures , thick

lines are used for optical (fiber) signal paths and thin lines are used for electrical signal paths .In

an electrically time-multiplexed system , Fig . lea),multiplexing is carried out in the electrical

domain , before the electrical-to-optical (E/O) conversion . Demultiplexing is carried out after

the optical-to-electrical (0 / E) conversion. For n baseband channels, each of bit rate B, the

multiplexed bit rate is nB. Potential electronic bottle necks occur in the MUX and the E /0

converter, and in the 0/ E converter and the DEMUX , where the electronics must operate at the

full multiplexed bit rate . These bottlenecks arise from a) speed limitations of digital integrated

circuits, b) speed limitations of high-power and low-noise linear amplifiers used to drive the

laser or modulator in the E/O converter and in the O/E converter , c) limited modulation

bandwidths of lasers and modulators , and d)the fact that the receiver sensitivity offered by an

avalanche photodiode degrades by more than 3 dB for every octave increase in receiver

bandwidth. These problems have so far limited the maximum b it rate for electricallymultiplexed systems to 10 Gbit/s. In the optically multiplexed system, Fig. l (b) , the electronic

bottlenecks are removed by moving the E / 0 and o/ E converters (i.e, the transmitters and

receivers) into the baseband channels. Multiplexing is carried out after the E /0 conversion and

demultiplexing is carried out before the O/E conversion. All electronics associated with signal

processing operate only at the baseband bit rate .Note that a control signal is needed to drive the


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demultiplexer. In general , this control signal could be either electrical or optical , depending on

the demultiplexer technology. At p resent the most practical optical demultiplexers are based on

electrooptic switches, which use electrical control signals . It will be shown later that the

bandwidth of this electrical control signal need not be large for demultiplexing in an OTDM

system. An important difference between electrically multiplexed and optically multiplexed

systems is that in electrical systems the multiplexing and demultiplexing can be carried out at

points in the system where the signal has been amplified to large levels . The signal-to-noise

ratio is determined by the receiver and its associated low-noise front end,and is not affected by

loss in the multiplexing or demultiplexing operations. In an OTDM system, on the other hand ,

the multiplex ing and demultiplexing is carried out on the optical signal, Thus optical losses

reduce the signal level relative to the receiver noise, and losses must be kept small . It may be

feasible, however, to place optical amplifers at selected points in the system to compensate for

some of these losses .


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2.2. Optical Time-Division Multiplexing

In this section we consider optical waveforms and timing requirements for optical multiplexing,

and examine topologies for transmitters and multiplexers. The operation of time-multiplexing

several lower bitrate baseband channels onto a higher bit-rate channel can be divided into three

sub functions: sampling, timing, and combining. The sampling function takes samples of the

incoming baseband data stream, thereby identifying the value of each incoming bit. The timing

function ensures that the samples are available at the correct time slots on the multiplexed


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channel. The combining function assembles all the sampled baseband data streams to generate

the higher bit-rate multiplexed data stream. In multi gigabit-per-second electrically multiplexed

systems it is convenient to sample each of the input data streams using short sampling pulses that

are timed to correspond to the appropriate time slots on the multiplexed bit stream. If the

sampling pulse widths are less than one bit-period of the high bit-rate multiplexed signal, the

combiner can be a simple summing circuit. A similar strategy is a preferred approach to optical

time-division multiplexing because it can capitalize on mode-locked and gain switched

semiconductor lasers, which are capable of generating pulses more than ten times shorter than

electrical pulses. In this approach to multiplexing the sampling function is carried out in the E /0

converters (i.e., the system transmitters). Consequently, the mux in Fig. l(b) is required only to

do the combining function. The following concentrates on this method of multiplexing. Fig. 2

shows the schematic of an E /O converter (transmitter) that can be used to sample the input data

before optical combining.

Short optical pulses from a laser are incident on an optical modulator, which is driven by

an input electrical data stream. The electrical data stream could be either in the return-to-zero

(RZ) or the non-retum-to-zero (NRZ) format, but NRZ is usually preferable because it minimizes

the bandwidth requirements of the baseband digital electronics, the modulator, and its drive

amplifiers. The optical pulse train from the laser samples the electrical input data via the

modulator, thereby converting it from NRZ in the electrical domain to RZ in the optical domain.

An important feature of the laser-modulator combination in Fig. 2 is that when the laser and

input data are correctly timed, the modulator is either fully "on" or fully "off" when the optical

pulse passes through it. This means that the modulation process does not cause the optical signal

to chirp. Sampling the baseband data in the E / 0 converter enables the optical combining

function to be carried out using a passive device such as a fiber directional coupler power

combiner. Another advantage of this approach is that independent of the type of combiner used

in the system, the RZ output format from the E / 0 converters leads to low multiplexing crosstalk

(see below). Furthermore, the optical power from the lasers in the E /0 converters is used

efficiently since the signal in each baseband channel is zero during time slots to be occupied by

other channels in the multiplexed data stream. This is a significant practical consideration

because semiconductor lasers are average-power limited devices. The timing scheme for a

general n-channel optical time division multiplexed system is shown in Fig. 3. The n optical


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signals incident on the combiner are RZ pulse trains with repetition rates B and with pulsewidths

T(measured at the baseline). The incoming bit streams are temporally offset from one another by

delays D. Data are encoded on each pulse train, before combining, so in general some of the

individual pulses will have zero amplitude. However, in Fig. 3 all bits are shown as "ones" for

clarity. When the pulse spacing in Fig. 3 is adjusted for maximum multiplexed hit rate, each

pulse in the multiplexed bit stream just comes into contact with its nearest neighbors. Under

these circumstances D = T, and the multiplexed bit rate is 1 / T. With laser pulses that are 10 ps

wide at the baseline, for example, the multiplexed bit rate could be as high as 100 Gbit / s.

Fig. 3 shows that the RZ signal format provides low system crosstalk. Since each baseband

signal is always nominally zero except in its allotted time slot on the multiplexed bit stream, it

cannot interfere with other channels. In practice, the pulse stream will have a finite on / off ratio

and the resulting baseline light signal between pulses will cause a component of crosstalk.

Furthermore, leading and trailing tails on the pulses will also cause cross talk if the pulses are

spaced such that tails overlap on adjacent pulses.

To avoid possible overlap problems it is usually necessary to ensure that the pulses are somewhat

shorter than the one bit period of the multiplexed bit stream. It is shown in Section II-C below

that shorter pulses also help to reduce crosstalk in the demultiplexer. Since shorter pulses occupy

a wider optical spectrum, reducing the pulse width may increase the dispersion penalty, even if

the lasers operate at or near the wavelength of zero first order chromatic dispersion in the fiber.

Thus, choosing the optimum pulse width may entail a compromise between system crosstalk and

pulse spreading caused by fiber dispersion. For example, in local applications requiring very

high bit rates over short lengths of fiber, optimum system performance would be obtained by

minimizing crosstalk. This could be achieved using optical pulses that are significantly shorter

than the multiplexed bit period. In longer distance applications, on the other hand, it may be

necessary to put more emphasis on minimizing the dispersion penalty rather than crosstalk. Thus

a long distance system might achieve optimum performance with longer laser pulses. Block

schematics showing two possible configurations for the E /0 converters (transmitters) and

combiner for an n-channel OTDM system are presented in Fig. 4. The first configuration, Fig.

4(a), uses n optical pulse generators, all driven by the same master clock. These pulse generators

could be mode-locked or gain-switched [semiconductor lasers. The pulse streams are delayed

with respect to one another using delay elements either in the electrical clock paths (as shown


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here) or in the optical signal paths. Electrical delays would usually be preferred because they are

easily made adjustable. Data encoding and sampling can bc carried out using optical modulators

such as waveguide electro

optic devices at the outputs of the pulse generators. An alternative to using external modulators

would be to directly encode data on gain-switched lasers. The data-encoded pulse streams in Fig.

4(a) are brought together in the combiner. A potential disadvantage of the transmitter- combiner

arrangement in Fig . 4(a) is that the wavelengths of all lasers need to be closely matched to avoid

pulse overlap at the receiver end of the fiber caused by different propagation times.


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Fig. 4. Schematics of two configurations for E/O converters and combiner.

(a) Multiple optical pulse generators. (b) Single optical pulse generator.

One approach to circumvent this problem is to use an active control mechanism that detects

pulse overlap at the receiver end of the system and continuously adjusts the timing of the

transmitter pulses accordingly . This type of control scheme could also be used to adjust the

timing of bit streams originating in different locations.The second OTDM transmitter

configuration, Fig. 4(b),uses a single optical pulse generator . The output of the generator is split

passively into 11 channels, which are then encoded with data and properly delayed with respect

to one another. This arrangement requires only one laser a feature that cannot be achieved with

other optical multiplexing methods such as wavelength-division multiplexing. In addition, all

channels operate at precisely the same optical frequency and will therefore propagate through a

fiber with the same delay. The main disadvantage of the scheme in Fig. 4(b) is that the total


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transmitted power will be less than in Fig. 4(a) because there is only one laser. However, the

transmitted power level could be increased using optical amplification .Up to this point, we have

assumed that the baseband modulation in both systems in Fig. 4 is synchronous with the pulse

stream from the lasers. This is desirable for best performance in very high bit-rate systems but

the requirement of synchronous operation can be removed if the modulation bit rate is

substantially less than the repetition rate of the lasers . Thus, one can trade off capacity for

increased system flexibility. Asynchronous operation is particularly attractive for systems where

the baseband channels originate in different locations. The optical combiner in Fig . 4(a) and (b)

can, in general, be either passive or active. A passive combiner would incorporate devices such

as fiber directional couplers, while active combining would use active devices such as optical

switches .

The passive combiner option has the advantage of simplicity but its losses can become

large because each 3-dB directional coupler introduces 3-dB combining loss. The losses in an

active combiner should, in principle, be smaller because the loss per device is potentially low.

An integrated array of switches on a single chip promises the lowest combining loss for systems

with more than two baseband channels. In either case, optical amplification may counter the

power penalties incurred. An active combiner has the potential of reducing crosstalk, by

decreasing pulse overlap caused by finite on I off ratios and leading and trailing tails on the

pulses .In a passive combiner, a tail extending into the time slot of a neighboring bit would be un

attenuated, but in an active combiner it would be reduced by the (time-dependent)switching

function . In addition to combining the baseband signals, an active combiner could also perform

the sampling function required in optical multiplexing. An active multiplexer of this type could,

in principle, eliminate the need for pulsed lasers. However, CW laser operations would place

very stringent requirements on the switch speed and extinction if low crosstalk is to be achieved.

In addition, CW operation would not be efficient because of the average power limit of

semiconductor lasers and the low duty cycle of each baseband optical signal.

2.3. Optical Switching and Demultiplexing

The demultiplexer is the most critical element of an OTDM system. Its purpose is to

direct each bit of the arriving multiplexed bit stream to the appropriate O/E converter, as shown


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in F ig . l (b).For good system sensitivity , it is necessary that most photons from each in coming

bit are transferred to the appropriate 0 /E converter. Thus the optical demultiplexer should switch

the entire bit rather than just sample part of it. In addition, it is important that crosstalk between

channels is small. The basic building block of an optical demultiplexer is a 1 x 2 switch.

Typically, optical switches such as directional coupler devices are fabricated as2 x 2 cross points,

which can serve as I X 2 switches by simply terminating one port. However, waveguide devices

specifically designed as 1 X 2 switches also are feasible. We consider here a model of analog

1X2 optical switch and the functional switching properties that influence the design of the

demultiplexer. In Section III we will give specific details of the switches used in our system

experiments. Fig. 5 shows the optical and electrical connections to a 1x 2 switch and the form of

the switching characteristic as a function of the applied electrical control voltage V. For

simplicity, the excess loss of the switch is taken to be zero but nonzero loss can be included by

scaling the output power axes. When used as a demultiplexer, the input power PI is applied at

port I and the output powers P2 and P3 appear at ports 2 and 3 . In general, the output powers are

nonlinear functions of the control voltage V. The characteristics shown here are for a reactive

switch, such as a directional coupler switch, where all the input power is transmitted to the

outputs and is not intentionally dissipated in the device. The switching characteristics can be

written as

where f( V) is the (nonlinear) switching function. The switch is an analog device, which means

that the optical power at each output port changes smoothly and continuously with control

voltage. There is significant power at both outputs for most of the control voltage range of

interest and the output powers are equal when V = V-3dB We show below that this analog

switching characteristic strongly influences crosstalk. The switching function in Fig. 5 reaches its

maximum and minimum values at V +and V _, respectively; for optimum switch performance

the control voltage should swing dynamically between these voltage limits. Note that, in general,


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the switch extinction is not perfect, even at V = V+ and V = V_ : the output power at ports 2 and

3 have minima given by the extinction parameters - and +, respectively. Crosstalk in the

demultiplexer is influenced by the residual nonzero extinction parameters - and of the

switch, the pulse width of the bits on the multiplexed stream, and by the shape of the waveform

of the switching function f ( V). The pulse widths and the switching function waveform are

important because they both affect crosstalk that occurs while the switch is in transition from one

state to another. The optical receiver following the demultiplexer will generally be designed with

limited bandwidth (to maximize sensitivity) and will integrate all photo electrons detected in a

baseband bit period, including those detected while the switch is in transition between states.

Fig. 6 shows the multiplexed bit stream at the input to the demultiplexer and the control voltage

used to separate it into two bit streams each at half the bit rate of the input stream. The switch is

driven by a periodic switch control voltage V with a frequency equal to half of the input bit rate.

The control voltage swings between the values V_and V+ in each cycle of the drive signal. To

minimize crosstalk caused by the nonzero extinctions - and +, the phase shift of the control

voltage V is adjusted such that the switching function f( V) reaches its maximum and minimum

values at the instants the input optical pulses reach their peak powers. However, even under these

conditions, a small amount of crosstalk is inevitable because of the limited extinction. This is

illustrated in Fig. 6(d)and (e) as small unwanted pulses between the main output pulses on each

of the two output channels.


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Fig. 5. (a) 1 X 2 optical switch. (b)Switching characteristic of 1 X 2optical switch.

The waveform of the switching function affects crosstalk because the switch can deliver

significant power to both output ports over a range of values of f( V). To minimize crosstalk

caused by this property of the switch, it is necessary, in principle, to use a square-wave switching

function f( V). Since the f( V) is monotonic between V = V_ and V = V+, a square-wave f( V)

implies a square-wave control voltage V, as shown in Fig. 6(b). For multi gigabit-per-second

multiplexed bit rates it is difficult to generate a square voltage waveform, because of the finitebandwidth of the drive electronics. A practical and desirable solution is to use a sinusoidal V

instead of the square waveform and to use optical pulses that are significantly shorter than a bit

period on the multiplexed stream. A sinusoidal control voltage waveform requires high

frequency electronics, but only with narrow bandwidth.


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Fig. 6. Timing scheme for demultiplexing with a 1 X 2 switch.

The resulting crosstalk is poorer than would be with a square-wave voltage, but this problem is

mitigated to a degree because the inherent nonlinearity off ( V) tends to make the f ( V)

waveform more square than sinusoidal. This squaring of the switching function waveform is

illustrated in Fig. 7, which shows the calculated power atone output port of a I-em-long reversed

. switch the input port of the switch is driven by CW light and the control voltage is a sinusoid

at 4 GHz with an amplitude of 1.4 times the de switching voltage. For systems with more than

two baseband channels, larger demultiplexer switching networks can be constructed by

interconnecting 1 x 2 building blocks .There are two main classes of demultiplexer network: the

binary tree and the linear bus. These are shown in Fig. 8.The binary tree demultiplexer, Fig. 8(a),

uses (n-1) switches. Its first stage is a high-speed switch which demultiplexes the data stream to

two data streams, each at half the multiplexed bit rate. This switch needs to operate at a high

frequency but, as explained earlier, it can be driven by a sinusoid from narrow-band electronics.


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Subsequent switches in the binary tree structure of Fig. 8(a) also can be driven by sinusoids, but

operate at sub harmonics of the drive frequency of the first switch. These lower frequency

switches are synchronized to the timing of the first switch. In some applications it may be

feasible to use a configuration in which all switches operate at the same frequency r 181. The

binary tree demultiplexer would find application where all baseband receivers are located

together, such as in a point-to-point trunk system. Note that in order to achieve low

demultiplexing crosstalk on all output ports of the binary tree it is necessary that each switch in

the tree provides low crosstalk on both of its outputs . This means that both extinction parameters

(- and + ) should be small in each switch. The linear bus arrangement, Fig. 8(b), uses switches

connected serially, and enables the demultiplexers and base band receivers to be placed in

separate locations such as in a local loop. The demultiplexed bit rates can be made different in

each baseband by appropriate selection of the switching rate for the switches. There are two main

disadvantages of the linear bus demultiplexer arrangement. First, the switches should ideally be

driven by pulses rather than sinusoids. Wide-band drive electronics and switching characteristics

are required, since higher harmonics of the pulse repetition rate are needed to obtain pulse-like

characteristics. Second, data from outputs towards the end of the bus suffer the accumulated

insertion loss of many devices. The loss for the linear bus increases with n, while the loss for the

binary tree increases at the slower rate of 2 logz n. This problem of increased loss could be

alleviated using in-line optical amplifiers, providing spontaneous emission noise does not

degrade the receiver sensitivity . An advantage of the linear bus demultiplexer is that it places

less stringent requirements on one extinction parameter of each of the 1 x 2 switches. For low

crosstalk, good extinction is required only on the demultiplexed output of each switch. Crosstalk

onto the bus from a demultiplexed channel is reduced by subsequent switches. If all switches in

the bus have good extinction in both switch state s then (n - I) switches are required, as shown in

Fig. 8(b). However, if the extinction is good only on the demultiplexed outputs, the "bus" output

on the final switch will be unusable because of crosstalk and a total of n switches will be

required in the demultiplexer.


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2.4 Timing Recovery

As explained earlier, successful demultiplexing depends on correct timing of the demultiplexer

switches. In an optically time-division multiplexed system there is generally no electrical signalavailable at the multiplexed bit rate. It is therefore necessary, in systems using electrically driven

demultiplexer switches, to generate an electrical clock signal for the demultiplexer. One solution

to this problem would be to tap off a small component of the multiplexed optical signal at the

input to the demultiplexer and to feed this signal to a separate receiver. The output of this

receiver would then be used to drive a microwave phase-locked loop that generates a phase-

stable clock signal. The receiver could be narrow-band and optimized for the frequency

component of interest.

An alternative to the above scheme would be to obtain a signal for the

timing recovery circuit from a demultiplexed electrical signal at a receiver output. Such a method

has been used in the experiments described in Section III. For this technique to produce low

crosstalk demultiplexing, it is important that timing information contained in the pulse position

of the incoming multiplexed bits is conveyed without distortion to the timing recovery circuit.

Any pulse shaping caused by the demultiplexer will tend to corrupt this timing information. This

requirement of demultiplexing with minimum pulse distort ion of the optical data bits reinforces

the need for optical pulse widths that are significantly shorter than a bit period on the

multiplexed data channel. It also re in forces the need for a switching function with a waveform

that approximates a square wave.


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Chaper-3

3 EXPERIMENTS

In this section we describe a series of system experiments we have carried out to illustrate

some of the principles presented in the previous sections. The various experiments differ from

one another in the number of channel s multiplexed, the type of lasers used, and in the

multiplexing architecture. In all experiments the baseband electrical data at both the transmitter

and receiver ends of the system are at 4 Gbit/s and are in the NRZ format. Key components in

the experiments are actively mode locked and gain-switched semiconductor lasers, which

generate optical pulses for the transmitters, and Ti: LiNb03 waveguide electro optic

switch/modulators, which are used in the transmitters and demultiplexer. All were designed for

operation at 1.3-p,m wavelength, where the experiments were carried out . Before giving details

of the experiments we briefly describe these key components. The actively mode-locked

semiconductor lasers use antireflection coated CSBH laser chips and 5-cm-longfiber extended

cavities having microwave resonance frequencies of 2 GHz. To general pulses at 4 GHz (the

baseband bit rate), the lasers were mode locked at the second harmonic of their microwave

resonance frequency. Up to four lasers of this kind were operated simultaneously from a

common clock. It was necessary to control the cavity lengths of all four lasers to within 0.5 mm

to ensure that all mode-locking bandwidths overlapped. The pulse widths of the fiber extended

cavity lasers are - 15ps FWHM, or - 30 ps at the baseline. The on/off ratio of the pulses is better

than 20 dB and the output power delivered to the fiber is in the range 5-7 dBm. If we were to

place the pulses with minimum spacing, such that D =T, the multiplexed bit rate would be

greater than 30Gbit/s. In the pre sent experiments, we have increased the spacing to larger values

than this to ensure low crosstalk, and have achieved bit rates up to 16 Gbit/s.

The fiber extended-cavity laser structure has no frequency-selective element and

consequently each laser operates at a slightly different wavelength. In addition, each laser has an

optical spectrum that is wider than the transform-limited bandwidth determined by the pulse

width. This no optimum spectrum is mainly caused by residual short cavity modes of the

antireflection coated laser chips. The overall spectral width of each laser was - 5 nm. In some of


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our experiments we have used gain-switched DFB lasers in place of the mode-locked lasers. The

gain-switched laser s were also operated at 4 GHz by driving them with a 4-GHz sinusoid. The

pulse widths of these lasers were - 25 ps FWH M, which is not as short as the mode-locked

lasers, but the gain-switched laser offers the advantage of simplicity since no external cavity is

required. The spectral widths of the gain-switched pulse trains were - I nm. We were able to

select DFB lasers which operated within a few nanometers of the wave length of zero first-order

fiber dispersion.

The optical modulators and demultiplexing switches used in the experiments are high-

speed waveguide electro optic directional coupler devices. Uniform traveling-wave electrodes

were used for the modulators, which are similar in design and performance to the modulators

used in previous fiber transmission experiments. Tn the demultiplexer, high- speed -reversal

switches are used to en sure low crosstalk in both switch states. To obtain a low drive voltage ,

these devices use a polarization dependent design. Thu s, a fiber in-line compensator was used to

adjust the polarization state at the input to the switches.

3.1Multiple-Laser Systems

We present data obtained from two experimental systems using the multiple-laser

architecture of Fig. 4(a).One of these systems uses two baseband channels and one uses four

baseband channels. Tn both systems the baseband data rate is 4 Gb it /s. A block diagram of the

four channel system is shown in Fig. 9. A common 4-GHz clock drives the four transmitters viaa series of microwave delay lines that are adjusted to provide correct timing of the optical

pulses. In the two-channel system, only two transmitters were used. A schematic of the optical

transmitters is shown in Fig. 10. The clock drives both the word generator and the laser, thereby

ensuring that the electrical data is synchronized to the optical pulses. The lasers used in these

experiments were of the fiber extended-cavity type described above . All lasers were within 8

nm of the wavelength of zero first-order chromatic dispersion in the fiber . The total spectral

width off our multiplexed lasers was 10 nm . For reasons of economy, only one channel was

modulated with pseudorandom data, Data on the other three channels were simulated using

various combinations of periodic 2-GHz and4-GHz pulse trains.


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Fig. 9, Block schematic of 4-channel aTDM system, The electrical time

delays 7 are 62.5 ps, corresponding to one bit period at 16 Gbit/s .

The combiner architecture used in Fig. 9 was a binary tree arrangement of passive 3-dB

fiber directional couplers. The demultiplexer was a binary tree of active Ti: LiNb03 direction

coupler switches . A block diagram of the demultiplexer and associated electronics is shown in

Fig. 1 1. The first switch is a 5-mm-long high-speed fiber-pigtailed traveling-wave reversal

directional coupler switch , and is driven by a sinusoidal control voltage at 8 GHz. The two

outputs from this switch are further de multiplexed in two additional fiber-pigtailed switches,

each 10 mm long and driven at 4GHz. Microwave delay liners TI through T3are used to obtain

the correct timing for all switches. The Ti: LiNb03switches used in t he demultiplexer are

discrete devices that are interconnected using fiber pigtails. For the present prototype

experiments, the emphasis was on switching speed and no special care was taken to obtain

minimum insertion loss. As a result, the excess loss of the complete1 x 4 demultiplexer was 12

dB. This loss could be reduced to - 5 dB using an integrated 1 X 4 demultiplexer on a single

chip. For the two-channel system, only one switch was necessary. Its control signal was a


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sinusoid at 4 GHz . The receivers in both systems used an avalanche photodiode coupled to a

low-noise GaAs FET .

The four-channel 16-Gbit/s system in Fig. 9 serves as a good example of how optical

multiplexing and demultiplexing can remove t he need for very wideband electronics. This

system transmits data over 8 km of fiber at16 Gbit/s, but uses a maximum electronic bandwidth

of only 2.6 GHz (in the drive electronics for the modulator and in the receiver). Since the

demultiplexer switches are driven by sinusoids, There is negligible electrical bandwidth required

in the demultiplexer.. Examples of bit patterns for the two-channel 8-Gbit/s system are shown in

Fig. 12. Fig. 1 2(a) shows the RZ bit pattern "0100110" at the output of transmitter 1, while Fig.

1 2(b) is the RZ bit pattern" 11111111" at the output of transmitter 2. Fig. 12(c) is the

corresponding multiplexed8-Gbit/s RZ signal at the input to the transmission fiber and Fig. 12(d)

is the demultiplexed and re-shaped 4Gbit/s NRZ "01001100" at the output of receiver 1.Fig. 13

illustrates the operation of the four-channel 16-Gbit/s system. The multiplexed 16 Gbit/s RZ

bitpattern"100111111001111" at the output of the multiplexer is shown in Fig. 13(a) . Note that

the bits in this oscilloscope trace are not completely resolved because of the limited bandwidth of

the photo detector (25 GHz) and sampling oscilloscope ( 14 GHz) used to monitor the

multiplexed1 6-Gbit/s signal. The demultiplexed and received 4-Gbit/s NRZ eye pattern

corresponding to Fig. 13(a) is shown in Fig. 1 3(b). This skewing is visible in Fig. 15(c)-(e) for

different levels of intentionally induced crosstalk, obtained by incorrect adjustment of the bias

point and timing of the demultiplexer control voltage away from their optimum values

The pseudorandom modulation on channel 1 combined with the "101010" bit

pattern on channel 2 is a convenient combination of signal formats for assessing and minimizing

crosstalk. It provides a graphic illustration of the effects of crosstalk, it enables the

demultiplexer timing and drive conditions to be adjusted for minimum crosstalk, and it enables

an upper limit on the crosstalk to be directly measured using a microwave spectrum analyzer

.Fig. 16 shows the microwave spectrum of the signals on channel l and channel 2 when the bias

and timing is adjusted for minimum skewing and hence minimum crosstalk. The 2-GHz

components of the two signals differ by-23 dB (electrical), indicating that the crosstalk is - 23dB

or better. This translates to a crosstalk of - ll. 5 dB or better in the optical domain. Note that this

crosstalk is an overall system crosstalk, which reflects contributions both from multiplexing and


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Fig. 11. Block schematic of demultiplexer and associated electronics for

4-channel system.


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.

demultiplexing. In the present system the multiplexing crosstalk is estimated to be less than -20

dB (optical). Therefore, the system crosstalk is dominated by crosstalk in the demultiplexer.


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From above measured bit error rate curves for the two-channel 8-Gbit/s system and the four-

channel 16-Gbit/s system in Fig. 17. These curves show the error rate as a function of the

received 4-Gbit/s power at the input to receiver 1. Four curves are shown: two for 8Gbit/s and

two for 16 Gbit/s. At each bit rate, one curve is presented for transmission through 8 km of fiber

and one curve is presented for the system with the fiber removed ( 0 km ). This pennits an

assessment of various sources of system penalty.


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Fig. 14. Ey e patterns in 8-Gbit/s system

Fig. 15. Effect of crosstalk on received eye patterns in 8-Gbit/s system

The best receiver sensitivity in Fig. 17 was obtained at8 Gbit/s with no fiber. Under these

conditions there is no fiber dispersion penalty. it shown separately that there is negligible

crosstalk penalty with no fiber at 8Gbit/s by observing that the receiver sensitivity is unchanged

if one of the two transmitters is switched off . The data therefore show that there is negligible

crosstalk caused by the laser pulse on/off ratio or the extinction of the demultiplexer switch.

When the fiber is included in the 8-Gbit /s system, The receiver sensitivity for 10-9

error rate


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degrades by 2.5 dB. This is a dispersion penalty attributed to the non optimum spectrum of the

mode-locked lasers. The system margin (i.e., the power available at the receiver above the

receiver sensitivity) is 13.9 dB for the8-Gbit/s system, estimate that the maximum transmission

distance for this dispersion-limited system would be about 16 km. A transmission distance of

over 40 km is possible using lasers with improved spectra (see Section III-B below).The receiver

sensitivity at 10-9

error rate for the fourchannel16-Gbit/s system with no fiber is 2.9 dB poorer

than that of the 8-Gbit/s system, indicating that there is a 2. 9-dB crosstalk penalty at 16 Gbit / s.

Fig. 16. Measurement of crosstalk in 8-Gbit/s demultiplexer. The horizontal

scale on the microwave spectrum analyzer is 10 MHz/div.


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Fig. 17. Measured error rates of 8-Gbit/s and 16-Gbit/s systems against

4-Gbit/s power level at input to receiver

This crosstalk is dominated by the contribution of the high-speed ( 8 GHz)reverse switch at

the input of the demultiplexer tree. It is partly caused by increased effective values of the

extinction parameters - and + which is, in turn, caused by under modulation of the switch at

high frequencies. In addition to the above crosstalk penalty, a dispersion penalty of 3.6 dB is

introduced when the transmission fiber is included in the system. The dispersion penalty arises

from two separate effects. The first effect is the well known pulse-broadening associated with the

spectral width of each laser. The second effect is an overlap between adjacent pulses, caused by

the small fiber propagation time differences associated with wavelength differences between

lasers. The dispersion penalty can be viewed as an additional crosstalk penalty but we use the

tern "dispersion penalty" here to separate clearly the crosstalk penalty inherent in the multiplexer


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and demultiplexer from the additional penalty caused by the dispersive characteristics of the

fiber.

Fig. 18 traces the losses and power levels of a single 4-Gbit/s signal as it passes through

the 16-Gbit/s system from the laser to the receiver APD. Losses and penalties are shown at the

top of the figure and optical power levels are shown at the bottom.

The penalties include a 3-dBduty cycle penalty that occurs with external modulation, the 3. 6-dB

dispersion penalty associated with the 16-Gbit/s signal in the 8-km length of fiber, and the 2.9-

dBcrosstalk penalty associated with the demultiplexer. It can be seen in Fig. 18 that most of the

losses in the system are in the modulator, the multiplexer, and the demultiplexer. As pointed out

earlier, we expect that the demultiplexer loss could be reduced to 5 dB by monolithic integration

on a single chip . Similarly, the modulators could be integrated with an active multiplexer togive a combined loss also of 5 dB-an improvement of almost 7 dB on the present system . The

system is dispersion-limited in its present form, but with reduced switch losses and improved

laser spectra, we estimate that the transmission distance could be increased to more than 40km

.The receiver sensitivities given in the preceding paragraphs were measured after the

demultiplexer , at the input to the 4-Gbit/s baseband receiver . It is also of interest to consider the


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overall receiver sensitivity of the 8-Gbit/s system at the input to the demultiplexer , and to

compare this number with the best reported receiver sensitivity for a conventional 8-Gbit/s direct

detection system with electrical demultiplexing . The two sensitivities are - 24. 1dBm for the

OTOM system and - 2 5.8 dBm for a state of-the-art 8-Gbit/s APO-FET receiver [4 1 ] . The

present OTOM system is therefore only 1.7 dB less sensitive than the APO-FET receiver . This

difference could easily be eliminated by optimizing the demultiplexer switch and reducing its

loss to a value of 3.0 dB. The overall 1 6-Gbit/s receiver sensitivity of the 16-Gbit/s system is -

10 . 9dBm. We cannot compare this with a conventional direct detection receiver because there

are no published reports of receivers operating at bit rates as high as 16 Gbit/s. If the

demultiplexer loss in the 16-Gbit/s system was reduced to 5.0 dB by integration and if the

crosstalk penalty was made negligible by improving the h igh-frequency switch extinction , the

overall 16-Gbit/s receiver sensitivity would become -20.8 dBm.

3.2 Single-Laser System

To demonstrate that the fiber dispersion penalty can be reduced using lasers with

improved spectra , we briefly describe a two -channel 8-Gbit/s system experiment using the gain-

switched lasers described earlier. The spectral width of the gain-switched lasers is - 1 nm , and

the emission wavelength is within a few nanometers of the wavelength I)f zero first-order fiber

dispersion. In addition, we have ensured that both baseband channels operate at exactly the same

wavelength by using the single-laser topology of F ig . 4(b). The laser output is split into two

using a fiber directional coupler and the optical delays in Fig .

4(b) are introduced using short lengths of fiber . The two channels are combined using a second

fiber directional coupler . The improved spectrum has enabled us to increase the transmission

fiber length to 40 km . The demultiplexer and receiver section of the system is identical to the

multiple-laser 8-Gbit/s system described above. Fig. 19 shows measured error-rate curves for the

8-Gbit/s two-channel single-laser system. A baseline curve is given for 0 km of fiber and with

only one of the two 4-Gbit/s channels operating . The full 8-Gbit/s system error rate curve is

given for a fiber transmission distance of 40 km .The offset between the two curves is 1 .2 dB,

indicating a total penalty (crosstalk plus dispersion) of 1.2 dB.


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Chaper-4

4 CONCLUSIONS AND OUTLOOK

The system experiments described here have illustrated the potential of optical time-

division multiplexing for multigigabit-per- second point-to point transmission systems. We have

shown that multiplexing, demultiplexing, And timing recovery can be achieved without the need

for very wide bandwidth electronics, and have demonstrated a transmission bit rate that is higher

than in any previous time-multiplexed system. The experiments have also served to highlight the

limitations of OTOM, and help to identify areas in which device improvement s are needed

before the full potential of optical time-division multiplexing can be realized. The two key

limitations in very high bit-rate OTOM are pulse-spreading caused by fiber dispersion and

crosstalk in the demultiplexer. It is desirable, therefore, to optimize those components which

influence these factors. To minimize the dispersion penalty, the semiconductor laser(s) in the

transmitters should have minimum optical spectral width ( i.e. , be Fourier-transform limited). It

is also desirable that the laser wavelength can be adjusted to a value that is close to the

wavelength of the other transmitters (so that all pulses experience the same propagation time),

and close to the zero first-order dispersion wavelength of the fiber (to minimize pulse spreading).

For a four-channel system with transform limited pulses and laser wavelengths within 2 nm of

the zero first-order dispersion wavelength , we estimate that the dispersion penalty will be

sufficiently small to permit loss-limited operation and to allow transmission distances of more

than50 km at 16 Gbit/s. A bit rate of 16 Gbit/s is about the maximum speed at which the present

demultiplexer can operate without significant increases in crosstalk penalty. To improve the

demultiplexer crosstalk performance at 16 Gbit/ s and to extend OTDM methods to higher bit

rates it will be necessary to introduce new approaches to switch design. It will be necessary to

reduce the required drive power to take advantage of the inherent speed capability of technology.

Several complementary avenues to be explored include electrical/optical velocity matching

techniques, optimization tradeoffs to minimize RF attenuation, and resonant microwave


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techniques. Optoelectronic integration in -V semiconductors of the functions and devices

employed here may also permit further optimization of OTDM systems. In addition, all-optical

techniques may lead to the achievement of unforeseen bit rates.

Fig . 1 9 . Measured error rate of single-laser 8-Gbit/s system against

4-Gbit/s power level at input to receiver

In this presentation briefly touched on the possibility of exploiting optical amplifiers in

OTDM systems. Optical amplifiers promise exciting opportunities for improving system

performance and flexibility by compensating for losses. They appear to be particularly attractive

for increasing the transmitted power in systems using single-laser transmitters with a split-and-

recombine architecture to multiplex many baseband channels. Optical amplifiers are also


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attractive for compensating the accumulated losses in demultiplexers using the linear bus

configuration. It has been shown that traveling-wave optical amplifiers can amplify high-

repetition-rate pico second pulses without distortion, but there are several areas that need to be

investigated before the full potential and limitations of optical amplifiers in OTDM systems can

be gauged. In particular, it is necessary to consider possible degradation of crosstalk performance

caused by spontaneous noise generated in the amplifiers , and the introduction of pattern effects

into high bit-rate optical data streams by carrier storage effects in the optical amplifier. These

effects may introduce some limitations, but there seems no doubt that optical amplifiers will

greatly enhance the design flexibility of OTDM systems and will lead to improved system

performance.


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Chaper-5

5.REFERENCES

[ 1 ] H. Kobrinski, R. M. Bulky, M. S. Goodman, M. P. Vecchl. C. A .Brackett, L. Curtis, and 1.

L. Gimlet, " Demonstration o f high capacity in the LAMBDANET architecture: A

multiwavelength optical network . " Electron. Lell. , vol . 2 3 , pp. 824-826, 1 987.

[21 O. E. De Lange, " Wide-band optical communications systems: PartII-Frequency division

multiplexing," Proc. IEEE. , vol . 58, pp.1 683- 1 690, 1970; see also: B . Glance, 1. Stone. M. A

. Pollock, P .J . Fitzgerald, C . A . Burrus, B . L. Kasper, and L . w. Stulz, "DenselyspacedFDM

coherent star network with optical signals confined toequally-spaced frequencies , " 1.Lightwave Technol. , this issue, pp.1770-1786.

[3] T. S. Kinsel and R . T . Denton, "Terminals for a high-speed opticalpulse code modulation

communication system: II. Optical multiplexingand demu1tiplexing , " Proc. IEEE. , vol . 56, pp.

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[4] F . S. Chen, "Demultiplexers for high-speed optical PCM , " IEEE 1.Quantum Electron. , vol.

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[5] S . 1 . Buchsbaum and R . Kompfner, "Time-division multiplex opticaltransmission system ,

" U . S . Patent 3 506 834 , 1970.

[6] T. S. Kinsel and F . S . Chen, "Experimental evaluation of an opticaltime division

demultiplexer for twenty-four channel s , " Appl. Opt. ,vol. 1 1 , pp. 1 4 1 1 - 14 1 8, 1 972.

[71 M . Thewalt, "Time domain multiplexing of signals on an optical fiberusing mode-locked

laser pulses , " IBM Tech. Disclosure Bull. ,vol. 24, pp. 2473-2475, 1 98 1 .

[8] A . Alping, T . Andersson, R . Tell, and S . T . Eng. "20-Gbit/s opticaltime multiplexing with

TJS GaA1As lasers , " Electron. Lett. , vol. 1 8 ,pp. 422-424. 1982.

[9] S . K . Korotky , G. Eisenstein, R . C . Alferness, 1. J. Veselka, L . L .Buhl, G . T . Harvey,

and P. H . Read, "Fully connectorized highspeedTi : LiNbO, switch/modulator for time-

divisionmultiplexing anddata encoding," 1. Lightwave Technol . . vol. LT-3, pp. 1 -6, 1985.

[ 1 0] P. R. Prucnal . M . A. Santoro, S . K . Sehgal, and I . P. Kaminow ," TDMA fiber-optic

network with optical processing," Electron.


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