Download INFRA-RED OPTICAL COMMUNICATION SYSTEMS*

infrared Physics,
1968.Vol. 8, pp. 123429.Pergamon
Press.Printedin GreatBritain
INFRA-RED
OPTICAL COMMUNICATION
J. LYTOLLIS, C. K. KAO
Standard Telecommunication
Laboratories
and G. I.
SYSTEMS*
TURNER
Limited, London Road, Harlow, Essex
(Received 13 July 1967)
Abstract-This
paper presents a survey of the present position of the technology of optical
communication systems. The gallium arsenide laser and gas lasers are compared as sources.
The Pockel effect, acoustic and Faraday effect modulators are discussed. The advantages and
disadvantages of various transmission systems are also discussed. Brief comments are made
about various types of detectors. It is concluded that the information capacity is limited by the
bandwidth of the source, modulator or detector. No satisfactory transmission system for long
distance telecommunications
use has yet been perfected, but guided systems of the type
illustrated by a dielectric fibre guide, seem to offer the most promise. Despite the advances
made in sources and modulators, many problems still remain before optical frequencies can
be used for long distance telecommunication
systems. Short range and outer space systems
are feasible with the existing techniques.
INTRODUCTION
OPTICAL communication systems using infra-red radiation are by no means a new idea. Many
countries in the Second World War developed optical communication systems for use on the
battlefield using beams of i.r. radiation modulated at audio frequencies. The advantages
and disadvantages of infra-red communication systems are shown in Table I.
TABLE
1. INFRA-RED
COMMUNICATION
SYSTEMS
Disadvantages
Advantages
Atmospheric
Secure
Cannot be jammed
Causes no interference
Equipment smaller, lighter and cheaper than radio
equipment
Wide bandwidth available
and weather conditions
Line of sight
affect range
World War II military systems fulfilled the purpose for which they were designed,
namely short range voice channels, but they did not use the wide-band capabilities of the
high optical frequencies. Most of the systems relied on mechanical methods of modulation,
and such systems are necessarily slow. However, as Table 2 illustrates, the wide bandwidth
capability is there waiting to be used if only the right techniques of transmission, modulation and detection can be developed. It has been estimated that all the telephone systems
in existence in the world could be impressed on one optical communication channel.
This paper presents a survey of the present position of the technology of optical communication systems. The aspects to be considered are sources, modulation techniques,
transmission, detection and information capacity.
*@ 1967 Standard Telecommunication
Laboratories
Limited.
123
124
J.LYTOLLIS,C.
K. KAO
and G. ~TURNER
TABLE 2. CHANNEL CAPACITY OF VARIOUS SYSTEMS
Communication
type
Radio
Microwave
Optical
Frequency
(Hz)
4.2 ir IO*
4.2 x 10”
4.2 v lOI1
Usable
bandwidth
(%I
IO
I
0.1
Bandwidth
(Hz)
Channel
bandwidth
(Hz)
4.2 x lo3
4.2 x 10s
4.2 x 101’
4.2 x lo3
4.2 ;’ to”
4.2 x 10”
Number of
channels
1
10000
I oo,ooo,ooo
SOURCES
The invention
of the laser was the beginning of renewed interest in the use of optical
frequencies for communication
purposes. Here was a source which was extremely bright
and which emitted light that was coherent, just as radio waves are coherent. What one could
do at radio frequencies was now possible at optical frequencies,
at least in theory. The
emitted beam was well collimated at source and so all the power output of the source could
be collected into a simple optical system and projected as a parallel beam of high power
density. A ruby laser was used by American scientists to project a beam of light from
Earth to Moon(l) using a 30 cm telescope in the transmitter and, despite the diffusing nature
of the Moon’s surface, the return signal was successfully picked up and detected using a
120 cm telescope in the receiver.
There are three classes of laser. (1) Solid state crystal lasers, of which ruby and neodymium doped lasers are the best known examples; (2) gas lasers, of which neon-helium
is the most used, but the new powerful carbon dioxide laser is now attracting great attention; (3) semi-conductor
lasers, of which GaAs is the best known and most highly developed.
The GaAs laser is operated by passing a current through the p-n junction in the forward
direction. The output of the laser can be modulated by simply modulating
this current. It
therefore looks attractive as a source for optical communications.
Its advantages and disadvantages are listed in Table 3. The GaAs laser is a good source for communication
systems in so far as it is an efficient, intensely bright source capable of being modulated
at
TABLE 3. GAAS LASER AS A SOURCE FOR OPTICAI. COMMUNICATIONS
Disadvantages
Advantages
I.
2.
3.
4.
5.
6.
Rugged, small device.
Simple pumping process [d.c. current at 1 i
volts]
Short time constant [ < 1O-g set]
... Easily modulated to 1000 MHz bandwidth
High efficiency [lo0 % internal, 40 % external]
High brightness [lMW/cmz]
High average power [watts at 77”K]
1. Refrigeration
2.
8.
4.
required for high duty cycles
Poor spatial coherence [beam > 10” wide]
Multimode
. Heterodyne detection is not possible
Low impedance [ < 191
... Impedance matching difficult
GHz bandwidths
because of its short time constant.
It is small, robust, has a long life, and
can be produced cheaply in quantity. Its main disadvantages
are that it requires cooling to
liquid nitrogen temperature
if continuous
operation is required, and that it can only be
operated at its full bandwidth capability if overall efficiency is sacrificed to achieve a good
impedance match. Subsidiary disadvantages are that it does not have the good beam collimation normally associated with lasers, and that multimoding
increases the linewidth.
Infra-red optical communication
systems
125
Improved and new fabrication
techniques have steadily improved the characteristics
of
the laser at room temperature,
but still only pulsed operation at low duty cycles is possible.
The best reported performance
is 105 pulses/set with 3 W peak power output using drive
pulses of 50A, 70 nsec wide, However, continuous
operation at room temperature
is the
real requirement.
This can be obtained from the GaAs diode in its non-lasing form. Such a
diode is a less efficient source than the laser, has no coherence, and a smaller bandwidth.
However, continuous
output powers of 0.1 W at room temperature
and 1 W at liquid
nitrogen temperature have been achieved.@ The bandwidth obtainable is up to several tens
of megahertz. We have successfully transmitted
television pictures using GaAs diodes. A
pulse-width-modulated
data transmission
link with a 3 MHz bandwidth
has been built
and demonstrated.
Figure 1 shows the transmitter
in schematic form. Figure 2 shows the
receiver layout. We have also built intruder alarms using GaAs diodes as sources and silicon diodes as detectors. Figure 3 shows a diagram of the system. The beam is modulated
diode
r-l
7 MC/S
sowtooth
generotor
Optical
system
FIG. I. STL optical link transmitter.
Frc. 2. STL optical link receiver.
up
to 300
m
FIG. 3. STL intruder detector.
electrically at a fixed frequency, and a narrow band amplifier is used in the detector circuit.
Any interruption
of the beam can be used to sound an alarm. The range is up to 300 metres
in clear weather.
126
J. LYTOLLIS,C. K. KAO and G. I. TURNER
MODULATION
TECHNIQUES
There was a time when only the GaAs laser could be modulated at high frequencies with
a wide bandwidth.
However, great progress has been made in the last few years in producing modulation
methods which enable any class of laser to be so modulated.
The gas
laser was always attractive as a source for communication
purposes because of its room
temperature
operation,
good stability and high coherence but, until recently, it lacked
efficiency and high power. Now, however, tens of watts power can be produced by gas
lasers, and carbon dioxide lasers can operate with 10% efficiency to produce up to + kW
of power.c3) A gas laser combined with one of the new modulation
methods therefore now
looks more attractive than the GaAs laser.
The Pockel effect in certain electro-optic
crystals such as potassium di-hydrogen
phosphate, ammonium
di-hydrogen
phosphate
and potassium tantalate
niobate is the most
widely used modulation
method. These materials are commonly known as KDP, ADP and
KTN. The Pockel effect is the solid state equivalent of the Kerr effect in liquids. Table 4
shows the performance
of a cavity modulator
and of a travelling wave modulator.(4) The
latter gives the better performance.
Bandwidths of GHz are seen to be possible at low power
levels. More recently, lithium tantalate has been used, and this requires only l/20 power of
KDP.@,
TABLE 4. POCKEL EFFECT MODULATORS
Type
Cavity
Travelling wave
Material
Modulation
frequency
Bandwidth
KDP
KDP
3GHz
Baseband
at microwave
frequencies
4MHz
5GHz
Modulation
power
WI
1.5
12
Modulation
index
0.13
-1
-
Another modulation
method is based on electro-acoustic
effects in piezo-electric crystals
such as quartz or barium titanate, or electro-magnetic
effects in electro-optic
crystals of
high refractive index. High frequency signals are applied to transducers
attached to the
crystals, and a standing wave field pattern established within the crystal. This periodically
varies the refractive index of the crystal to establish a grating effect, so that the emergent
light forms a diffraction pattern. Suitable positioning of a slit and detector enables modulation of the light to be detected. Microwave carrier frequencies can be used, and only low
voltage drive is required. The bandwidth
is 10% of the carrier frequency, and depths of
modulation
of 100 o/0can be approached.(4) Significantly, a recent paper reports that tellurium
has been used to modulate light from a carbon dioxide laser at 10.6 p wavelength at frequencies of 20-300 MHz with good efficiency.cG)
Other possible modulation
techniques are free carrier absorption
effects in semiconductors, absorption
edge shift by electric fields and the Faraday magneto-optical
effect. The
first two methods are useful at low frequencies only. The third method can be used at
microwave frequencies. Recently, using gallium doped yttrium iron garnet, modulators have
been produced with 200 MHz bandwith and 40% depth of modulation
requiring
only
l/10 watt driving power. With l/3 watt driving power, a bandwidth
of 400 MHz was
achieved. (5)
Infra-red optical comn~unication systems
127
TRANSMISSION
The atmosphere is not an ideal medium for the transmission
of optical signals. Selective
absorption
occurs due to presence of water vapour and carbon dioxide (Fig. 4). Smoke,
dust, mist, haze, fog, rain and snow, all cause attenuation
of the beam, which may be severe
and frequency independent.
However, point-to-point
systems can be operated over lO20 km ranges using only milliwatts of power in clear weather. Thermal gradients cause
refractive index changes within the air mass, and this deviates the beam randomly, causing
scintillation
effects.
Wavelength,
FIG. 4. Smoothed
vertical transmission
/A
curve of the earth’s atmosphere.
For long distance communication
systems, transmission
which does not depend on the
atmosphere must be used. Several proposed systems(‘-“) rely on a hollow pipe which contains optical refracting and reflecting components,
is sealed, and is filled with a dry gas
under pressure. The disadvantages
of such systems are their lack of physical flexibility, the
close engineering tolerances with which they must comply, and the high cost of installation.
We are currently exploring the possibility
of using dielectric waveguides to transmit
optical signals. The theory of the method has been written up in the Proceedings of the
IEE(rO), and will not be dealt with further here. An optical fibre with a high refractive
index core and low refractive index cladding is the guide, the core having a diameter of
several times the wavelength of light, and the cladding making up a diameter of say, 50100 I”. The light is launched into this fibre, and is guided by the core, but most of the light
travels in the cladding (see Fig. 5). The great advantage of such a guide is that it can be
bent round sharp curves without severe attenuation,
and so it could be used in existing
cable ducts if necessary. We have demonstrated
single and multimode
transmission
at
optical frequencies using such fibres (Fig. 6). Light from gas lasers and from GaAs lasers
has been launched into the fibres using suitable optics. Losses in such fibres are at present
high because of the high absorption
of even good optical quality glass due to certain impurities which are difficult to remove-ferrous
and ferric ions being the chief offenders.
IP-I
128
J. LYTOLLIS,C. K. KAO and G. I. TURNER
Medium
3
FIG. 5. Propagation
cj=l
in single-mode optical fibre.
However, it is possible to produce lower loss glasses with present
aim is to bring losses down to 20 dB/km.
day knowledge,
and our
DETECTORS
Below 1~ wavelength, photo-emissive
detectors can be used, and no real difficulty exists
in detecting even microwave modulation
frequencies using travelling wave photo-detectors
or specially based photo-emissive
diodes. For longer wavelengths, only p-n junction diodes
of specially low capacity can detect at frequencies in excess of 10 MHz. Extrinsic photoconductive detectors can operate out to 40 p wavelength, but are limited to time constants
of lo-’ set or more, and need refrigeration.
INFORMATION
CAPACITY
The source, modulator
and detector generally govern the information
capacity of a
communication
system. A reasonable
capacity would appear to be around
100 MHz
for an infra-red system at wavelengths longer than 1 p, with somewhat
higher capacity
below this wavelength. Of course, many different wavelengths can be transmitted
along
the system simultaneously,
and dispersive optics used to feed-in and feed-out each channel
at either end. Using fibre optical guides, bundles of fibres could form multi-channel
transmission systems with small cross-sectional
areas.
CONCLUSION
In conclusion
one can say that advances have been made towards the achievement
of
practical wideband communication
systems at optical frequencies, but that problems still
remain. Any system developed must be cost competitive with existing systems on a cost
per channel basis if it is to be commercially
acceptable for telecommunication
purposes.
Short range systems have been improved greatly by the invention
of lasers and the developments which have taken place in modulators.
Of course, in outer space, where there
are no atmosphere problems, optical communication
systems are easier to realise, and have
already been used.
Acknowledgement-The
authors thank the management
Limited for permission to publish this paper.
of Standard
Telecommunication
Laboratories
FIG. 6(a).
FIG.6(b).
Fucing page 128
Fro. 6(c).
FIG. 6(d).
FIG. 6. Photographs
of light patterns
produced
by modes
Infra-red optical communication
systems
REFERENCES
I. BOWNESS,C., D. MISSIOand T. ROGALA, Proc. Insf. Radio Engrs 50, 1703 (1962).
2.
3.
4.
5.
6.
7.
8.
9.
10.
CARR, W. N. and G. E. PIITMAN, Appl. Phys. Lett. 3, 173 (1963).
PATEL, C. K. N., Appl. Phys. Left. 7, 15 (1965).
ANDERSON,L. K., Microwaves 4, 46, January (1965).
Bells Lab. Rec. 45, 20, January (1967).
DIXON, R. W. and A. N. CHESTER, Appl. Phys. Lett. 9, 190 (1966).
EAGLESFIELD,C. C., Proc. In&n. elect. Engrs 109, 26 (1962).
BERREMAN,D. W., Be/l Syst. tech. J. 43, Part 1, 1469 (1964).
MARCATILI, E. A. J. and R. A. SCHMELTZER,Bell Syst. tech. J. 43, Part 2, 1783 (1964).
KAO, C. K. and G. A. HOCKHAM,Proc. Instn. elect. Engrs 113, 1151 (1966).
129