28

CHAPTER 3 REVIEW OF LITERATURE

29

CHAPTER 3 REVIEW OF LITERATURE

3.0 INTRODUCTION

This chapter is devoted to describe the progress in the optical logic

devices and optical bistable devices and circuits. In the first instance,

optical logic devices have been reviewed. In the end, an account of optical

bistable devices and circuits will be given.

Logic gates and flip flops are the basic building blocks of any

digital system. Digital circuits consist of combinational and/or

sequential logic circuits. The basic building blocks of combinational and

sequential circuits are the logic gates. A logic gate is a combinational

circuit in which the output depends on the present inputs only and the

sequential circuit is one in which the output depends on the present

inputs and past output. As the logic gates are used in building digital

systems, it is necessary for each logic block to have the capacity to drive

the following stages. This means that they must be cascadable. The

requirement for achieving cascadability is that the input logic levels and

the output logic levels should be the same or at least they should be

within tolerable limits, so that there is no degradation in the cascaded

system. Conventional digital logic families are well known. There are

several conventional logic families evolved due to the special

requirements of application areas. No single logic family can be best

suited for all the applications. Each logic family has got its own

application area, advantages and limitations.

There are several approaches for implementing optical logic

devices. Most of the optical logic devices seem to be based on the property

of a material or a device, which gives an output light depending upon the

magnitude of the input light. The input light may be divided into multiple

signals and the output is interpreted as an AND or an OR logic operation

of the input light signals. Optical logic devices may be broadly classified

into two categories. They are 1)All optical logic devices and

30

2)Optoelectronic logic devices. All optical logic devices use nonlinear

optical material to realize optical logic gates and are discussed in the

following.

3.1 ALL OPTICAL LOGIC DEVICES

Several research groups have been working for building optical

devices. A wide variety of optical components are required in order to

perform every function currently carried out by electronic circuits. These

include polarization splitters [22-24], polarization rotators [25], TE/TM

mode converters [26-28], straight and bent wave guides [29-32],

Semiconductor Optical Amplifiers [33-35], side coupled resonator filters

[36-37], tunable Fabry Perot filters [38-39], isolators [40-42], time delay

components [43-47], phase shifters [48-49]. As evidenced by an

abundance of literature describing design, building and operation of the

above components, significant progress has been achieved for future

optical domain applications. However, for various reasons, successes in

this area have been limited to special-purpose applications, and the

general-purpose optical computer has been evasive. The major problem

with optical transistors, logic gates, switches, or other nonlinear devices

serving similar purposes, is the large amount of heat that would be

generated by the high energy consumption of optical devices. For

developing all optical systems, the ultimate practical devices are required

to be small (micron or sub micron), fast (ps), low switching energy (10-15

J), operable at room temperatures, and incorporable into an integrated

system. At present no optical device has achieved all of these

requirements.

All optical logic devices generally use both fiber-based and

semiconductor based nonlinear elements. In the former case, the physical

nonlinearity is the Kerr nonlinearity of silica glass. In the latter case, the

nonlinearity results from a variety of ultra fast mechanisms in

semiconductor gain media, including carrier heating, cross phase

modulation and cross gain modulation [50]. Many researchers have

reported all optical logic gates like NOR using a Semiconductor Optical

31

Amplifier (SOA) [51], OR with NOR using an Ultrafast Nonlinear

Interferometer (UNI) [52], XOR using a Terahertz Optical Asymmetric

Demultiplexer (TOAD) [53] and so on. Even though logic gates using the

UNI and the TOAD have the advantage of high speed, they are very

complex and difficult to integrate with other logic gates.

3.1.1 All Optical switching based on Cross Phase Modulation (XPM)

Optical switching using a nonlinear interferometer makes it

possible for one optical signal to control and switch another optical signal

through the nonlinear interaction in a material. The input signal to be

switched is split between the arms of the interferometer. The

interferometer is balanced so that, in the absence of a control signal, the

input signal emerges from the output port. The presence of a strong

control pulse changes the refractive index of the medium given by

Δn= n2I ⋅⋅⋅Eq.(3-1)

where, Δn is the change in the refractive index of the medium, n2 is the

nonlinear refraction coefficient and I is the intensity of light incident on

the medium. A change in the index adds a phase shift between the two

arms of the interferometer, so that the input signal is switched over to the

output port. This method of switching based on cross phase modulation

(XPM) is schematically shown in Fig.3.1.

medium Nonlinear

Linear medium

Control pulse

Signal pulse

Pout

n = n0 + n2 I

Fig.3.1 Block diagram of all optical switching using cross phase

modulation (XPM) in nonlinear interferometer.

32

3.1.2 Nonlinear Optical Loop Mirror (NLOM)

One way to implement cross phase modulation (XPM) based

switching is to use a Sagnac interferometer, where one of the output ports

also serves as the input port for the signal to be switched. This

configuration is commonly referred to as the Nonlinear Optical Loop

Mirror (NOLM) [54-55] and is shown in Fig.3.2.

Fig.3.2 Block diagram of Nonlinear Optical Loop Mirror (NLOM).

The input coupler splits the input signal pulse into two counter-

propagating pulses, which subsequently combine again at the coupler,

each having traveled around the loop. A strong control pulse is then

introduced into the nonlinear loop as a unidirectional beam. The high

energy control pulse is to induce a refractive index change, Δn, which is

experienced fully by a co-propagating signal pulse. This refractive index

change results in a differential phase shift (Δφ) between the counter-

propagating signal pulses as they arrive back at the input coupler and is

given by,

Δφ = k.Δn.L ⋅⋅⋅Eq.(3-2)

where, k is the wave vector and L is the path length over which the

induced index change (Δn) is effective. The path length is chosen such

that complete switching occurs, i.e., the phase shift is π radians.

33

In a fiber interferometer, the physical mechanism is the intrinsic

Kerr nonlinearity of glass. Since the optical nonlinearity in glass is very

small (n2 ≈ 3x10-20 m2 W-1 for silica), the power-length product of the

device requires typically 1W-km for a phase shift of π. In practice, fiber

interferometer path lengths of several kilometers are needed to keep the

average control power to below 100mW. The long path lengths make it

difficult to build stable and compact devices.

3.1.3 SOA based Terahertz Optical Asymmetric Demultiplexer (TOAD)

A Semiconductor Optical Amplifier (SOA) is similar to a

semiconductor laser diode, except that the reflectivity of the end faces is

deliberately minimized to suppress lasing action. Thus, the SOA acts as a

one-pass device for a light wave with a population inversion is created by

electrical pumping. The conduction and valence bands in a

semiconductor can be modeled as an ensemble of two level atom-like

systems, which are coupled through various scattering mechanisms. For

photons that are resonant with the transition energy levels of the states

that are inverted, stimulated emission can occur, i.e., photons at these

frequencies produce a gain. As the intensity of light increases, the gain

saturates from the depopulation of the conduction band due to stimulated

emission. The change in the gain due to saturation is associated with a

refractive index change.

The refractive nonlinearity of the SOA is ~108 times larger than an

equivalent length of silica fiber. The relaxation time associated with the

relaxation of the refractive index to its equilibrium value is governed by

the inter band carrier lifetime which is typically 100-500 picoseconds.

Since the inter band carrier lifetimes are very slow, switching at data-

rates much higher than 1Gbit/s based on inter-band carrier relaxation

did not seem possible. This limitation was overcome by placing the SOA

asymmetrically with respect to the center of the loop in the Sagnac

interferometer, as shown in Fig.3.3. This is called the Semiconductor

Laser Amplifier in a Loop Mirror (SLALOM) [56] or Terahertz Optical

34

Asymmetric Demultiplexer (TOAD) [57]. Since the rise time associated

with the change in refractive index is less than a picosecond [58], it is

possible to obtain a switching window which is shorter than the recovery

time limited by the inter-band carrier relaxation. It has been

demonstrated that a window width of ≤10ps can be created which allows

demultiplexing a 50GHz pulse train down to a base rate of 1GHz [59]. By

operating the SOA in strong saturation, the carrier relaxation time can be

modified to ~25 picoseconds [60]. The cross phase modulation (XPM)

bandwidth also increases with the device length due to traveling wave

effects [61-62] and demonstrations of wavelength conversion at 100

Gbit/s using XPM have been reported [63].

Fig.3.3 Block diagram of Terahertz Optical Asymmetric Demultiplexer.

There have been successful demonstrations of different optical

functionalities using configurations based on the NOLM and TOAD. These

include demultiplexers and the implementation of simple boolean

functions such as AND, NOT and Exclusive-OR (XOR) for address

recognition [64-65]. More sophisticated boolean functions such as 3-bit

adders [66-67] and parity bit generators [68] have been demonstrated

using a combination of several TOAD-based gates.

35

3.1.4 Ultrafast Nonlinear Interferometer (UNI)

Ultra fast Nonlinear Interferometer (UNI) is a balanced, single-arm

interferometer that does not require any external stabilization of the

interferometer arms. A schematic of the UNI gate is shown in Fig.3.4. The

signal pulse that is to be switched is split into two orthogonal polarization

components with a time delay (typically equal to half the bit period) by

passing it through highly birefringent fiber. The two orthogonal pulses

then pass through a SOA and are temporally recombined after passing

through a second birefringent fiber and interfered. The State of

Polarization of the signal pulse after recombining is determined by the

induced phase changes from the time-dependent refractive index changes

in the presence of a control pulse (which could be co propagating or

counter-propagating) that is aligned temporally with one of the orthogonal

pulses in the SOA. The signal pulse then passes through a fiber polarizer

that is adjusted such that the signal pulse is orthogonal to the polarizer

in the presence of the control pulse and parallel to the polarizer when the

control pulse is not present.

Fig.3.4 Block diagram of Ultrafast Nonlinear Interferometer (UNI).

100Gbit/s bit wise switching has been demonstrated using the UNI

gate [69]. For dual rail logic, such as XOR, the phase of each polarization

state can be accessed and changed independently by two separate control

pulses. All-optical XOR gate based on the UNI has been demonstrated on

36

a 40GHz clock pulse [70] and at 20Gbit/s [71]. A 40GHz all-optical shift-

register with an inverter has also been demonstrated using 2mm SOAs in

the UNI gate configuration [72].

3.1.5 Limitations of all optical logic devices

In general, all optical logic devices based on interferometric

switches are more complicated to fabricate, difficult to stabilize, and they

require three input beams, the two logical inputs and a clock stream.

Also, these devices are not, in general, cascadable. Recently, Mach-

Zehnder interferometer and UNI configurations have been demonstrated

that are cascadable. Still, without the use of optical amplifiers and filters,

the fan out for these gates is low, limited by device loss and by the

accumulation of amplified spontaneous emission noise. The recovery time

can be as short as the duration of the pulses doing the switching, as in

the case of fiber-based switches, or as long as the carrier temperature

equilibration time (approx. 1ps) as in the case of active semiconductor-

based switches. Semiconductor-based switches are attractive because

they are ultimately integrable and have large nonlinear coefficients,

facilitating smaller device dimensions. However, due to the nonlinear

absorption effects accompanying the nonlinear index changes, it is

unlikely that nonlinear wave guide lengths (using conventional

semiconductor materials) can be reduced much below a hundred microns.

Therefore, optical logic gates will not compete with electronic gates in

terms of density on a chip. Also, without careful design, the

semiconductor-based switching devices can be limited by carrier

population and thermal effects to operation speeds of a few gigahertz. In

summary, at present no method or technique has the low power, small

size and cascadable combinations for all-optical monolithic circuit logic.

3.2 OPTICAL BISTABILITY

An optical bistable device can be thought of as an optical non

linearity (to provide a gate or switching function) combined with positive

feedback (to provide two stable states). Any optical system that possesses

37

two different steady state transmission states for the same input intensity

can be said to exhibit optical bistability.

Optical bistable devices are the most basic components of all

optical systems [73], because these devices are extremely versatile and

have practical potential for optical logic elements, memory elements,

limiter, oscillator, gate modulator and discriminator, laser pulse shaping,

optical switching, signal processing applications [74-75].

3.2.1 Materials for optical bistability

Different types of materials and techniques have been used to

demonstrate the Optical Bistability (OB). The method of achieving OB that

has been investigated experimentally involves combining a medium which

displays intrinsic nonlinear refraction (intensity dependent refractive

index) with a Fabry Perot cavity. Szoke et al proposed that a saturable

absorber inside a Fabry Perot optical resonator could exhibit two bistable

states of transmission for the same input intensity. But later, Gibbs et al,

demonstrated optical bistability with sodium vapor in an interferometer.

OB due to nonlinear absorption alone in a Fabry Perot cavity is much

more difficult to observe [76]. They deduced that the effect was caused by

nonlinear refraction rather than saturable absorption. Many of OB

experiments involve gases [77-78] or liquids [79] as the nonlinear

medium, but there are obvious practical advantages, from the point of

view of device fabrication, in using a solid, and the first solid to exhibit OB

was ruby [80]. Semiconductors, while known to exhibit comparatively

large passive nonresonant χ3 [81], received serious attention as potential

materials for OB after the discoveries of strong nonlinear refraction in the

region just below the optical band gap in InSb [82-83] and GaAs [84-87].

3.2.2 Types of optical bistable devices

There are two types of optical bistable devices. They are the

intrinsic (or all-optical type) and the hybrid types. Most of the bistable

devices are Fabry-Perot etalons containing a nonlinear material whose

refractive changes with applied light intensity. Consequently, such devices

38

are referred as all optical or intrinsic optical bistable devices: intrinsic

because the feedback occurs via the etalon material itself, possibly

assisted by mirrors. In hybrid bistable devices, light interacting with a

nonlinear optical material was detected by a photo detector and

electrically fedback to change the transmission of the material [88].

The hybrid bistable device does not require large optical power since

its non linearity is artificial. In this type of device, the speed of operation

is limited by the photon-electron conversion. In all optical bistable device,

the nonlinear coefficient is small hence large power is required.

3.2.3 Basic structure of all optical bistable device

The basic structure of optical bistable device is shown in Fig.3.5. It

simply consists of two partially reflecting mirrors, placed parallel to each

other. The nonlinear material used in the cavity is a compound

semiconductor, for example, indium antimonide. Each mirror partially

reflects and partially transmits the light that falls on it. This might be the

basis for an optical bistability. If a beam of light strikes the first mirror,

some percentage of the light is reflected, and some goes through. The

same happens at the other mirror.

d

Nonlinear Medium transmitted intensity

Partially transmitting

incident intensity

reflected intensity

mirrors Fig.3.5 Basic structure of all optical bistable device.

3.2.4 Basic characteristic of all optical bistable device

If the intensity of the transmitted radiation is plotted against the

intensity of the incident beam, the resonator exhibits optical hysteresis

cycle as shown in Fig.3.6. An optical device exhibiting such a cycle is said

39

to be optically bistable. This is because it has two stable regions where

transmitted intensity varies little with changes in the incident intensity.

Fig.3.6 Basic characteristics of optical bistable device.

The hysteresis cycle described earlier can be used as a memory

element. Hysteresis is not needed if an optical switch is to be used in

processing information instead storing information. P.W.Smith and

W.J.Tomlinson [89] mentioned the possibility of tuning an optical bistable

device as shown in Fig.3.7. The hysteresis loop can be altered by changing

the length of the interferometer and the wavelength of the incident beam

of the material in the cavity.

Fig.3.7 Characteristics of hysteresis loop.

It is possible to reduce the area of the hysteresis loop. It is even

possible to reduce it to zero, resulting a single valued transmission curve,

in which each level of incident intensity is associated with only one level

Tran

smitt

ed p

ower

Incident power

Hysteresis loop

Incident intensity Incident intensity Incident intensity

Single valuedloop

Narrow hysteresis loop

Broad hysteresis loop

Tran

smitt

ed in

tens

ity

Tran

smitt

ed in

tens

ity

Tran

smitt

ed in

tens

ity

40

of transmitted intensity. The shape of the single valued function itself can

be manipulated. In one of the most useful shapes, transmission is low

and almost constant at low incident intensity. Then at threshold, the

transmission rises steeply reaching a high level and almost remains

constant as the incident intensity increases further.

3.2.5 Optical logic gates based on OB characteristics of etalon device

The nonlinear characteristics of all optical bistable device are

intelligently used to arrive at optical logic functions like OR, AND which

are shown in Fig.3.8. The optical nonlinearity can be used to implement

an AND gate by having several inputs cooperate to contribute the required

threshold intensity as shown in Fig.3.8. The optical AND logic function is

obtained such that the light from several sources is required to achieve

threshold and to produce high output level [90]. Both incident beams

must be present for high transmission.

The optical nonlinearity can also be used to implement an OR logic

function by arranging it such that the light from any of several sources is

sufficient to achieve threshold and produce a high output level. The OR

gate yields a high output if any one of its inputs are high.

Fig.3.8 Optical AND, OR logic functions obtained using optical nonlinear

characteristics.

The AND and OR gates can be organized to form a functional logic

block which can be customized to mimic the function of any of the sixteen

A and BIncident power

AND gate

Tran

smitt

ed p

ower

ON

Tran

smitt

ed p

ower

Incident power

OR gate

A or B

ON

OFF

41

Boolean connectives such as a NOR, NAND, OR etc., [91]. For optical

applications, the ultimate bistable device will be small (micron or sub

micron), fast (ps), require very little energy (10-15 Joules) and low holding

power (<mW), will operate at room temperature and can be incorporated

into an integrated system. At present, no optical bistable device has

achieved all of these goals. 3.3 OPTO-ELECTRONIC LOGIC GATES

Opto-electronic logic gates are based on conversion of light into

electrical signal and logic operation is performed and finally converting

electrical signal into light output. Some of the devices reported are: (1)

Self Electro-optic Effect Device (SEED), and (2) Light Amplifying Optical

Switch (LAOS) devices. A brief account of these devices are given below.

3.3.1 Self Electro-optic Effect Device (SEED)

Self Electro-optic Effect Device (SEED), essentially combines a

Multiple Quantum Well Modulator (MQWM) with a PIN photodetector. The

structure of simple multiple quantum well modulator is shown in Fig.3.9

The MQWM is placed within the intrinsic region of a PIN diode. A

quantum well region typically consists of 50 to 100 quantum wells, is

grown as the undoped intrinsic region in the p-i-n diode. The physical

mechanism exploited by the SEED’s is the quantum confined stark effect

(QCSE) [92-94], which is an electro absorption mechanism observed in

quantum wells. The MQW PIN diode structure uses the quantum confined

stark effect (QCSE) to shift the optical absorption spectrum as a function

of the applied electric field. Fig.3.10 shows three typical absorption curves

for the SEED’s for three different bias voltages. The combination of light

modulation mechanism with photo detection makes it an optically

controlled optical device. The MQW modulators can be operated in two

different modes, as shown in Fig.3.10. In the so called λ0 mode, the

absorption decreases with increasing reverse bias voltage. In the λ1 mode,

the absorption increases with increasing reverse bias.

42

P

N

MQW Intrinsic

Output light

Input light

Fig.3.9 Structure of Self Electro-optic Effect Device (SEED).

Fig.3.10 Absorption characteristics of SEED.

The simplest configuration of SEED is resistor based SEED (R-

SEED). The R-SEED consists of a SEED connected with a resistor as

shown in Fig.3.11. It is operated at λ0, where the absorption of the device

is such that it decreases with increasing reverse bias voltage (or,

equivalently, increases with decreasing reverse bias voltage). The

bistability of a simple R-SEED is produced by a positive feedback

mechanism and is shown in Fig.3.12 in terms of the incident power

versus transmitted power curve. If the light incident on the MQW PIN

diode is of low intensity, there is little photocurrent and therefore little

voltage drop across the resistor. Essentially all of the voltage drop occurs

across the diode, and the absorption by the diode is quite low. Increasing

incident light gives increasing photocurrent, leading to a larger voltage

drop across the series resistor and hence to a larger absorption in the

diode and consequently, even a larger photo current. The input-output

characteristic of R-SEED exhibits hysteresis characteristic. The width of

43

the hysteresis loop is controlled by several factors including the voltage

required to move the absorption peak to λ0, the value of the series

resistance, and the difference between the absorption maximum and

minimum.

P

N

MQW Intrinsic

Output light

Input light

R

V

Fig.3.11 Schematic of R-SEED diode.

Fig.3.12 Input-Output characteristics of R-SEED diode.

The nonlinear bistable effect of SEED can be utilized to form optical

logic gates such as NOR, OR, NAND and AND along with various memory

functions [95-96].

Bistable region

Discontinuous “Switching”

Decreasing diode voltage

Transmitted Power

Incident Power

44

3.4 LIGHT AMPLIFYING OPTICAL SWITCH (LAOS) DEVICE

The device structure of LAOS and its characteristics has been

discussed in the following.

3.4.1 LAOS device structure

The initial work on LAOS devices was reported by Beneking and

Saski [97-98]. The device consists of a Heterojunction Phototransistor

(HPT) vertically integrated with either an LED or a LASER diode. This

device works both as detector of light as well as emitter of light. The HPT

in the LAOS device acts as an optical detector and provides gain. Input

light falling on the device will turn the HPT ON, which in turn drives the

LED or LASER diode to provide the optical output signal. These devices

are made using compounds of GaAs. The LAOS device is fabricated by

growing thin layers of InP and InGaAs on InP substrate as shown in

Fig.3.13 along with its equivalent circuit.

Fig.3.13 Structure of LAOS device and its equivalent circuit.

3.4.2 Characteristics of LAOS device

The basic characteristics of LAOS is explained as follows. For small

applied voltages, the electrical characteristics of the LAOS are very similar

to those of the series connected discrete components. In this case, the

HPT characteristics are simply shifted by the voltage drop across LED as

Inp Emitter, 0.5 μm, n = 5×1017 InGaAs Base, 0.1 μm, p = 1×1018 InGaAs Collector, 1.5 μm, n = 2×1016 InP Cladding, 0.2 μm, n = 2×1018 InGaAs Active, 0.2 μm, undoped InP Cladding, 0.2 μm, p = 2×1018

InP Substrate, Semi-Insulating

Output Light

Input Light

VBias

HPTT

HPT

VB

LAOS

Light Output

Light Input

45

shown in Fig.3.14. The LAOS device is designed to provide positive

fedback so that some light from the LED is re-absorbed by the HPT. As

the applied voltage increases, the optical and electrical feedback between

the LED and HPT also increases [99]. This positive feedback along with

the device nonlinearities causes the LAOS to switch from a low

current/high resistance mode to a high current/low resistance via a

negative differential resistance. The current-voltage characteristics of the

LAOS are now similar to those of a SCR as shown in Fig.3.15. The LAOS

can be switched into a high current state with either an optical or an

electrical pulse. When this positive feedback mechanism exceeds a

threshold, the LAOS switches to a low impedance state in which a large

optical output is generated.

Fig.3.14 Low voltage characteristics of LAOS device.

Fig.3.15 High voltage characteristics of LAOS device which are similar to a SCR.

V(Volts)

0.30

0.20

0.10

0.00

Increasing input light

0 1 2 3

I (m

A)

Increasing input light

V (Volts) 4 Vcr 62VH0 0.00

Icr

0.50

1.00

1.50

2.00

IH

I (m

A)

46

C.W.Wilmsen et al [100] have extended the work to the fabrication

of optical logic circuits namely optical inverter, optical NOR, optical AND

and optical OR gates which are discussed in the following.

3.4.3 Integrated optical inverter

The equivalent circuit of an optical inverter [101] fabricated from

LAOS device is shown in Fig.3.16. The optical inversion is achieved by

biasing the LAOS device ON and OFF with an input light signal to the

parallel HPT. The LAOS is biased by setting the voltage across the LAOS

device above the break over voltage (VBO). With this bias voltage, the LAOS

is latched in a low impedance/high light output state. To turn OFF the

LAOS, an optical input signal is applied to the parallelly connected HPT,

which increases the current through the HPT and decreases the voltage

drop across the LAOS. If the input signal is large enough, the biasing

voltage V is reduced below VBO and the LAOS turns off. Thus, the optical

inversion logic function is achieved.

LAOS

Optical Input

PT

RB

-VB

Optical Output

Fig.3.16 Equivalent circuit of optical inverter fabricated from LAOS device.

3.4.4 Integrated optical NOR gate

Adding additional phototransistor in parallel with the optical

inverter gives an optical NOR gate [102]. The equivalent circuit of 2-input

optical NOR gate is shown in Fig.3.17. With no light input signal, the

LAOS is in the conducting ON state and the LED emits an output light

signal. When the input light signal is applied to either A or B HPT, the

47

LAOS is in the nonconducting OFF state and no light is emitted. Note that

the output is low, when either A or B is high. The output is high only

when both A and B are low. Thus, the optical NOR logic function is

demonstrated.

Optical Input2

LAOS

Optical Output

PT2

RB

VB

Optical Input1

PT1

Fig.3.17 Optical NOR gate fabricated using LAOS device and its equivalent

circuit.

3.4.5 Integrated optical NAND gate

An optical NAND gate [103] is realized by integrating the LAOS

device with two HPT’s. The two HPT’s serve as the light detectors and are

connected in series with each other and in parallel to the LAOS device.

The equivalent circuit of the optical NAND gate is given in Fig.3.18.

R

LAOS

-VB

Optical Input2

Optical Input1

Optical Output

PT1

PT2

Fig.3.18 Equivalent circuit of 2-input optical NAND gate.

48

To obtain the NAND logic function, a bias voltage V greater than

the break over voltage VBO must be applied to the circuit. This bias

condition causes the LAOS to latch into the high current/high light

output ON state. This operating condition remains unchanged if only one

of the HPT’s remain OFF. If both input light signals are incident on the

HPT’s, then the LAOS current will be flowing through the HPT’s and the

LAOS will switch to the low current/low light output OFF state. It can be

seen that the output is LOW only when both of the input light signals are

HIGH. Thus, the 2-input optical NAND function is realized.

3.4.6 Integrated optical AND gate

Placing the phototransistors in series with the LAOS yields an

optical AND gate [104]. The equivalent circuit of planar integrated optical

AND gate is shown in Fig.3.19.

Fig.3.19 Equivalent circuit of optical AND gate fabricated from LAOS

devices.

3.4.7 Optoelectronic EX-OR gate

Fig.3.20 shows the equivalent circuit of optoelectronic XOR gate

which generates the AB min term [105]. The HPT connected in series with

the LAOS device forms the noninverting input (A input for the AB min

VB

PT1

PT2

OpticalInput1

OpticalInput2

RB

OpticalOutput

49

term). When this input is OFF, only the HPT leakage current can flow into

the parallel combination of a LAOS and HPT. When the input is ON, the

HPT serves as a current limit for the HPT/LAOS parallel circuit. The HPT

connected in parallel with the LAOS device provides the inverting input (B

input for the AB min term). When this input is ON, the HPT current

flowing into the HPT/LAOS parallel circuit is shunted through the

parallel HPT, turning OFF the LAOS device, and hence the light output.

However, if the series input is ON and the parallel input is OFF, a current

greater than the LAOS critical current flows through the LAOS device,

generating an optical output.

Due to the separate realization of the XOR min terms, the outputs

of the XOR gate are spatially separated. This spatial separation is useful

in systems, such as data base filters and comparators, where both A>B

and B>A information is needed. If, the XOR function is only needed as in

the case of a binary adder, then both outputs can be directed onto the

same detector.

LAOS

Optical Input2

Optical Output

Optical Input1

VB

PT1

R

PT2

Fig.3.20 Equivalent circuit which generates the AB min term for

optoelectronic EX-OR gate.

3.5 OPTOELECTRONIC SEQUENTIAL CIRCUITS

Some of the optoelectronic flip flops and memory cells reported in

the literature have been discussed in the following section.

50

3.5.1 Optical flip flop using discrete components

An optical flip flop with a very simple structure based on

optoelectronic feedback using discrete components has been reported

[106]. The discrete version of optical flip flop consists of two

phototransistors and two LED’s. Inputs and outputs of this flip flop can

take the form of optical and/or electrical signals. It neither consists of

logic gates nor includes bistable elements.

An integrated Opto-Electronic RS flip-flop based on optically

coupled inverters has also been reported [107]. The RS flip flop consists of

two optical inverters each of which consists of the parallel connection of a

LED and an HPT that have a load resistor connected in series as shown in

Fig.3.21 along with function table. The RS flip flop consists of two

inverters which are coupled with optical interconnections. In the RS FF,

the output of one inverter is input to the other.

Fig.3.21 Equivalent circuit of integrated optoelectronic RS flip-flop and its

function table.

3.5.2 Integrated Optical set-reset memory cell

Previously, Matsuda and coworkers have reported photodetector-

emitter type memory devices [108-110]. Highly compact integrated optical

set-reset memory is reported based on a photodetector-emitter type of

integrated device i.e., a light amplifying optical switch (LAOS) [111].

Equivalent circuit of optical set-reset memory cell is shown in Fig.3.22.

FUNCTION TABLE

S R Qn+1

LOW LOW Qn

LOW HIGH LOW

HIGH LOW HIGH

HIGH HIGH Not Allowed

VB

R S

R1 R2

QQ

51

The memory cell is composed of two Heterojunction phototransistors

(HPT's) vertically integrated with a Light Emitting Diode (LED). This

configuration has a single light input window for both the set and reset

inputs. The HPT's serve as input light detectors and the LED provides the

output light. The lower HPT-LED pair forms an optical latching device i.e.,

the LAOS. The top HPT is connected in parallel with the LAOS by

grounding the emitters of the two parts. The set function is realized by

applying an input pulse of wavelength λ2, which latches the LAOS into the

ON state. Reset is accomplished by applying an input pulse of wavelength

λ1, which turns ON the top HPT thus increasing the voltage across the

load resistor and reducing the voltage across the LAOS to below its

holding voltage.

Reset Input

Set Input

LAOS Optical

Output

PT1

R

VB

PT2

Fig.3.22 Equivalent circuit of optical set-reset memory cell.

The integrated device has to be biased with V, smaller than the

break over voltage of the LAOS (VBO), for memory operation. Without any

input light, the memory cell gives a LOW output. The set input causes the

LAOS to switch to yield a HIGH optical output. The reset input turns the

upper HPT ON and resets the LAOS back to a LOW state. The output is

triggered ON by the set pulse and remains ON until the reset beam goes

HIGH. Thus, the optical set-reset memory function is realized.

52

3.6 INTRODUCTION TO ELECTRO-OPTICAL HYBRID LOGIC CIRCUITS

So far logic gates which responds to only one stimulus namely

optical or electrical signal have been described. There have been proposals

for logic circuits which can respond to both electrical and optical signals.

One such investigation is reported by M.K.Ravishankar & M.Satyam

“Optically/Electrically (Symmetrically) Triggerable Bistable Multivibtrator”

[112]. Since the bistable multivibrator can be triggered either by electrical

or optical inputs and gives both electrical and optical outputs, the

authors named such circuits as Electro-Optical Hybrid Circuits or simply

Hybrid Circuits.

The hybrid bistable multivibrator consists of two cross-coupled

inverter circuits, each inverter is made up of one phototransistor with its

collector terminal connected to a resistor and a light emitting diode to

provide electrical and optical outputs respectively as shown in Fig.3.23.

The state of the circuit can be changed by applying either by electrical

triggering or by optical triggering pulse to the base of the

phototransistors. The electrical and optical triggering is done by using

the circuits shown in Fig.3.24 and Fig.3.25.

XPT2

Q

PT1

Q

R2

Q

output

R1 68KB

Rc

Electrical

5V

C=0.01uF220 Ω

C=0.01uF 220 Ω

47K

output

R2

LED2 LED1

R1 68K

Rc

Fig3.23 Hybrid bistable multivibrator.

Y

Q optical

47K

A

53

3.7 Summary

Most of the research groups have studied logic gates which

responds to only one stimulus namely either electrical or optical signals.

Some of these investigations are only a sort of interpretations for the

optical logic. Most of them are not cascadable. In view of this, it is thought

that there is a need to develop logic gates which can respond to both

optical and electrical signals and are also cascadable so that complex

hybrid opto-electronic systems can be built. The thought process behind

visualizing these hybrid logic gates and demonstration of these ideas

through building logic circuits using discrete components forms the

content of this thesis. The thesis aims at explaining the importance of

Electro-Optical hybrid concept and then attempts at how this new branch

of hybrid opto-electronics has to be developed.

The work comprises of realizing the basic hybrid building blocks

like, hybrid logic gates and hybrid flip flops. In the next chapter, hybrid

logic gates like, hybrid inverter, hybrid NOR and hybrid NAND gates etc.,

are described. Hybrid combinational circuits like hybrid half adder, full

adder and 4-bit adder have also been discussed.

0 A

C=0.01uF

LEDLED

Input

YVD2

V

D1

1KΩ

Input

X

R0

B

R1=100 Ω

Fig.3.24 Electrical triggering circuit.

Fig.3.25 Optical triggering circuit.