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Expressions are obtained for the probability of error of a binary intensity-modulated direct-detection optical communication system consisting of an ideal photodetector followed by a nonideal photoelectron counter, in this case an RC low-pass filter. Calculation of probability of error requires the determination of the output statistics of the nonideal counter. Numerical techniques are used to evaluate the expression for probability of error and optimum thresholds as a function of signal level. The optimum filter bandwidth required to simulate an ideal counter is also evaluated numerically. Results indicate that, for the particular case considered, the nonideal receiver requires an additional 1 dB of signal power to achieve performance equivalent to the ideal receiver.

A pulse-position modulated (PPM) optical communication system using narrow pulses of light for data transmission requires accurate time synchronization between transmitter and receiver. The presence of signal energy in the form of optical pulses suggests the use of a pulse edge-tracking method of maintaining the necessary timing. In this report the edge-tracking operation in a binary PPM system is examined, taking into account the quantum nature of the optical transmissions. Consideration is given first to "pure" synchronization using a periodic pulsed intensity, then extended to the case where position modulation is present and auxiliary bit decisioning is needed to aid the tracking operation. Performance analysis is made in terms of timing error and its associated statistics. Timing error variances are shown as a function of system signal-to-noise ratio.

Digital baseband and digital subcarrier noncoherent optical communication systems are considered in this paper. Expressions are developed for the bit error probability of optical subcarrier systems which use a nonliner polarization modulator and either a one-detector or a two-detector receiver, The expressions are evaluated numerically, and the results are used to compare the one- and two-detector subcarrier systems with the corresponding one- and two-detector baseband systems for a fLxed bit error probability.

A model that is sufficiently general to describe the predominant statistical characteristics of the output of many real optical detectors is formulated. This model is used to study the optimum receiver processing for direct-detection optical communication systems. In particular, the structures of detectors and estimators for randomly filtered doubly stochastic Poisson processes observed in additive white Gaussian noise are considered. Representations for the posterior statistics of a vector-valued Markov process that modulates the intensity of the doubly stochastic Poisson process are obtained. Quasi-optimum estimators and detectors are specified in general terms and specialized for several important applications. These include a demodulator for subcarrier angle modulation, a detector structure for binary signaling with known intensities, and a detector structure for binary signaling in the turbulent atmosphere.

The application of high-radiance electroluminescent diodes in optical communication systems will require efficient, high-current, high-speed modulation circuits capable of driving the low-impedance diode load. A technique which has proved to ho useful in driving such diodes at a 100 Mb/s rate, using a two-stage transistor emitter-follower circuit and a suitable matching network, is described.

Based on a model for the calculation of the variance of relaxation noise we show that it is possible in principle to modulate aminiature solid state laser at data rates (random) of 50 Mbit s−1 and higher by modulating the cavity loss, while keeping the relaxation oscillation noise small enough to permit a 10−9 error rate. Because of the small transit time through the miniature cavity a large modulation index (M ∼ 45) can be achieved, and although the laser power output suffers this can be compensated for in data transmission by the narrow output pulse shape that characterizes our intra-cavity modulation scheme.

A survey of the history and the development of laser communication technology in Germany is given. Fibers, lasers, diodes, and repeaters are treated.

The increasing sophistication of optical (and infrared) components and techniques, combined with rapidly expanding communication requirements, suggests that optical systems operating partly or entirely within the atmosphere may soon represent desirable solutions to real communications problems. The design of such systems will be strongly influenced by considerations of atmospheric turbulence, molecular absorption, and aerosol scattering. The most promising alternatives are heterodyne systems operating at the 10.6 µ wavelength, and direct-detection systems at near-infrared or visible wavelengths. Feasible links include horizontal, ground-to-satellite, and satellite-to-ground (or aircraft) geometries. System requirements and atmospheric effects are reviewed. Components, signaling, and diversity techniques which will partially overcome atmospheric limitations are discussed. Specific representative systems designs are presented.

The effect of weak, random inhomogeneities on wave propagation is studied. Of particular concern is the case of long distance propagation where the nature of the wave is significanfly affected by the inhomogeneities. Conventional perturbation techniques such as geometrical optics and the Born and Rytov approximations cannot be applied in this realm. The approximation technique employed is basically a selective summation technique of the type utilized in other areas of physics such as quantum electrodynamics and the theory of many-body interactions. Results are obtained for the average value and two-point correlation function of an arbitrary, initial wave. A physical interpretation of the results in terms of coherent and incoherent scattering and the classical theory of dielectrics is given. Wave statistics and the application of the results to the problem of determining the effect of the atmosphere on coherent optical communication are discussed.

High-speed digital repeaters are being considered in the Bell System and elsewhere for both long- and short-haul communication systems. This paper describes two devices which were built to serve as prototypes of the IF portion of a millimeter guided-wave communication system but which might serve equally well as the IF portion of the repeaters for an optical communication system or for microwave radio systems. These two prototypes, designed for a differentially coherent phase-shift-keyed (DCPSK) signal, have been built and operated at a bit rate of 160 Mb/s using an 11.2-GHz IF signal. Both models operated with error rates very close to those predicted theoretically. One of the models seems to be particularly suitable for such systems; it consists of comparatively simple circuitry, and its operation is within 0.5 dB of the theoretical ideal behavior for a DCPSK system.

This paper, an introduction to this first Special Issue on Optical Communications, follows some of the arguments that have prevailed for and against optics as a viable communications medium for terrestrial and space applications, and assesses the present state of the art of optics devices, systems, and theory, based on discussions with scientists and engineers who have been specializing in the optical field. It also attempts in a small way to give some appreciation of the special problems the traditional communications engineer faces in readjusting his thinking and his "physical intuition" in going from the world of electrons to the world of photons.

A novel and promising technique for the direct detection of wide-band microwave-frequency-modulated light has been analyzed and demonstrated. The system consists of an optical dispersing element followed by a photocathode and transverse-field interaction circuit. The disperser causes the light beam to swing back and forth in accordance with the instantaneous optical frequency, resulting in generation of a transversely modulated electron beam at the photocathode, which modulation is then detected by the circuit. Results obtained with an experimental FM phototube confirm the analysis for small modulation indices, including a spurious effect due to aperturing of the swinging light beam. Improved versions of the FM phototube involving electron multiplication should provide shot noise limited operation at VHF or microwave frequencies. The ability of the FM phototube to utilize large-index modulation is unique and may lead to superior incoherent detection systems. In particular the signal to shot noise ratio is nearly that of an ideal heterodyne receiver, and the FM phototube system inherently discriminates against background light.

This paper reviews research and developmental activities in optical communications currently under way in Japan. Three major systems: 1) atmospheric optical communication systems, 2) optical-cable systems, and 3) lens-waveguide systems, using various types of lasers and newly developed optical devices and components, have been extensively studied at the Electrical Communication Laboratories of Nippon Telegraph and Telephone (NTT) and other electronic manufacturing companies in Japan.

When optical communication over great distances in space becomes feasible, its utility may depend on mutual acquisition or reacquisition of responding stations. Various factors can impede this process, one of which is the problem of distinguishin g an optical transmitter against a starlit background. It is reasoned that this compels the use of a scanning method and therefore the concept of introducing a spatial filter to assist acquisition is examined. Account is taken of relative velocity of the stations (velocity aberration), significant propagation times, and possible false acquisition of a bright star. The paper discusses in detail the fundamental bases of the problem; particular attention is given to probabilistic aspects. It shows that a spatial filter can be effective potentially; this is illustrated by analysis of an imaginary but feasible situation. It also indicates that the full potential cannot be realized when propagation times approach or exceed the scanning interval. Finally, the paper emphasizes the need for dependability in optical communication, and the fact that this is a service, and should not impose undue constraints on a mission. The influence of these considerations on system design is discussed.

Several modulated light sources of historical nature are described which have been used in transmitting intelligence to distant points. Some of the characteristics of cesium-vapor lamps and high-intensity short-are xenon lamps are described. The phenomenon of acoustical resonance in enclosed arcs is discussed.

If a laser's energy gap is adjusted spatially so that it varies with position in a plane transverse to the direction of light propagation then each position in the transverse plane will amplify a different part of the spectrum. A simple optical technique is shown for dispersing an optical beam so that each frequency component passes through the appropriate part of a transversely adjusted gap (TAG) laser. A TAG laser can be constructed by subjecting a GaAs injection laser to a transverse thermal gradient. Calculations indicate that such a device would have a bandwidth of 20 \times 10^{12} Hz for a 25-fold increase compared to a conventional GaAs injection laser. An analysis of optical ray angle and spatial-dispersion alignment tolerances is presented. Two TAG-laser subsystems-a multiplex modulator and an amplifier-are proposed and discussed to indicate how the TAG laser's larger bandwidth and spatial dispersion may simplify optical communication systems. A preliminary-test TAG-laser oscillator has been constructed and tested. It verified the basic concept and exhibited a bandwidth of 2.45 \times 10^{12} Hz as predicted by the model.

The performance of a maximum likelihood receiver whose input is the photoelectric current created by a modulated light beam is studied for a number of discrete modulation schemes. The performance is compared with that of a classical microwave receiver, i.e., one designed to optimize detection in a Gaussian channel. Such a receiver has the advantages of availability, simplicity, ease of implementation, and greater tolerance to slow photodetector response and is an obvious candidate for suboptimum detection. Procedures for calculating receiver performance in the Poisson regime by the tilted probability density technique are described and applied specifically to modulation schemes employing one or more subcarriers that are biphase or quadriphase modulated. Results indicate that the classical receiver falls noticeably short of the optimum only at or near 100 percent modulation, where it may require at worst up to 30 percent more light power in order to equal the maximum likelihood receiver. When modulation is incomplete, or when background radiation is significant, the difference in performance is generally much smaller.

Previously published results on the performance of optical direct detection digital receivers using avalanche detectors are extended to the case where incoherent noise due to quantum amplifiers in the transmission medium is present at the detector. These calculations are applied to determine the usefulness of quantum amplifiers in simple digital transmission systems where the optical source instability results in a required amplifier bandwidth which may be orders of magnitude greater than the modulation bandwidth. It is concluded that practical applications exist where quantum amplifiers can be used in analog repeaters between regenerating repeaters in a hybrid digital system; and also as front ends of regenerating repeaters to increase their sensitivities.

Lower bounds are given on the attainable mean-square performance in causally tracking the phase and frequency of a subcarrier that modulates an optical carrier in a direct-detection opticalcommunication system.

A reasonable model for the atmosphere as an optical communication channel is presented and then reduced, with some approximation, to an equal-strength diversity system. By using the realistic assumption of a very large variance for the channel's random phase, the operations that the maximum-likelihood receiver must perform are determined. This resulting receiver is also a min-max receiver. Functional bounds to the error probability that results with orthogonal waveforms are determined. The variation of these bounds with the energy-to-noise ratio per diversity path and with the amount of atmospheric turbulence is discussed, and numerical results are presented for the low-rate (union-bound) performance. These results demonstrate that the error probability depends strongly on the amount of turbulence. However, if the amount of diversity can be controlled, the minimum energy-to-noise ratio per information bit for which the error probability can be made to vanish is independent of the turbulence. This minimum is just that which would exist in the absence of turbulence.

Techniques are described for optically multiplexing and demultiplexing individual pulse code modulation channels in order to develop the terminal capability for a high-capacity optical communication system. It is shown that, using these techniques, an information capacity in excess of 1010bit/s can be achieved on the output beam of a single laser.

The choice of the focal length of the transmitter in an optical system is analyzed, using the formula for the transferred power as a basis.

Interest in fibre-optic communication systems was aroused by Kao and Hockham in 1966. This survey endeavours to relate how the concept has grown within the United Kingdom from that inception. The work is reviewed from the standpoint of a systems engineer, but briefly covers the components vital to fibre-optic communication systems and mentions the more fundamental studies on which all other aspects have been based.

Indoor wireless optical communication is a good alternative to existing mature RF technology. However various challenges in indoor optical wireless technology are due to free space loss, ambient light, and multi path dispersion causing inter symbol interference (ISI). The degradation in performance due to these facts is very much influenced by the channel topology. So in this paper the performance of indoor optical configuration has been analyzed using three types of channel topologies viz., directed (LOS), non-directed (LOS), and multi beam diffused link for various transmitter and receiver design parameters. The analysis has been carried using Optiwave simulation tools.

The characteristics of high-sensitivity photodetectors suitable for wide bandwidth optical communication systems are summarized. Photodiodes, photomultipliers, and photoconductive detectors for wavelengths from 0.3 µm to 10.6 µm are covered. The use of internal current gain by means of avalanche and electron multiplication and by means of optical heterodyne detection to increase sensitivity of high speed photodetectors is discussed. The application to visible and infrared laser communication systems is reviewed.

In this paper a generalised result for theN-fold joint photoelectron counting distribution for independently modulated radiation is given. We extend the recent results of Diament and Teich, for the one-fold photoelectron counting distribution for light propagated through an atmosphere characterised by log-normal irradiance fluctuations, to theN-fold joint photoelectron counting distribution. An approximate solution for thisN-fold distribution is obtained, for detection intervals {Ti} «τa whereτa is the characteristic time of the atmospheric turbulence. We present specifically the two-fold joint photocounting distribution for amplitude-stabilised laser radiation passing through such an atmosphere for several levels of turbulence and degrees of correlation. Cases including additive, independent, non-interfering Poisson noise are considered. Computer generated plots of the photocounting distribution are presented. For noise-free detection, the otherwise narrow-peaked photocounting distribution is seen to broaden markedly and shift its peak to lower counts as the turbulence level increases. Furthermore, a non-singular counting distribution is obtained for fully correlated detection. In the presence of additive noise and varying only the signal-to-noise ratioγ, the probability surface is intermediate between that of the Poisson and that of the noise-free log-normal fading counting distribution. The peak, however, is observed to decrease and then again increase in magnitude asγ → ∞, for correlated detection only. These results are expected to be of use in the study of atmospheric turbulence, as well as in the evaluation of certain stochastic functionals that occur in optical communication theory for the turbulent atmospheric channel.

Subnanosecond pulses, having rise and fall times not exceeding 300 psec, have been selected from the output of a single mode ruby laser. The pulse width, ≈ 700 psec, agrees well with the observed broadening of the spectrum of the pulse.

The main properties of laser radiation are reviewed and the paper discusses how well practical devices approach the ideal with examples of the latest results which have been achieved. Various applications are then outlined and specific examples in optical communications, holographic stores, spatial filtering and pattern recognition are described.

The main reasons for the present interest in the potentialities of the laser for communication system applications lie in the coherent nature of the radiation and the short wavelength. The property of coherence, as considered in this article, is the ability of an electromagnetic wave to interfere with itself. The phenomenon is characterized by the ability to form a highly collimated beam, which leads to the ability to achieve communications over great distances. However, the perturbing effects of the earth's atmosphere on coherent optical radiation raise a serious obstacle to the use of lasers in earthbound communication systems. Space and interplanetary communications applications appear more promising as atmospheric effects are eliminated.

Frequency-division multiplexing techniques offer means for circumventing the bandwidth limitations of optical modulators and detector. Practical broad-band long-distance optical communication system of this type appear feasible and some possible embodiments are described. Calculations are made to determine the approximate information capacity and other characteristics of several 4000-mile repeaterad systems.

Experiments in short-pulse optical communications have been performed at high data rates (107bit/s) with M-ary modulation formats and, using a subnanosecond internally gated receiver, at very high data rates (2 × 108bit/s) with binary modulation formats to experimentally verify the potential of short-pulse low-duty-cycle direct-detection formats. High resolution TV pictures and error rate data are given for two M-ary formats, pulse interval modulation and pulse position modulation, and error rate data and gated optical receiver characteristics are given for the 200-Mbit/s pulsed gated binary modulation. Results demonstrate the capability of short-pulse laser-communication systems to discriminate against background light and to efficiently convey information. Subnanosecond gating of the receiver was achieved in the 200-Mbit/s experiments. In the M-ary experiments, as many as 12 bit/pulses were transmitted, using 4095 digitally selected 1 -ns time slots. High-quality TV pictures were transmitted at average detected signal to background ratios of less than 0.01.

Operating over a frequency range from audio to ultraviolet, the modern laser may be the most widely applicable active device at the disposal of today's engineer, enabling him to generate, amplify, modulate, transmit, and detect signals at frequencies more than 10000 times higher than was previously possible. Yet despite the almost unlimited potential for new and exotic applications offered by these shorter wavelengths, innovations have not lived up to their rave notices. Reasons for this relative inertia vary according to the application intended but, in general, it can be predicted that progress in the laser art will accelerate with the improvement of laser efficiency, development of auxiliary systems, and an updating of engineering thinking in terms of finding the ``problems'' that lasers can solve. This first article in a series deals with the laser devices themselves?their advantages and their deficiencies?and will summarize what is happening in the laser art today. Other parts, scheduled to appear in future issues of IEEE SPECTRUM, will cover applications in materials processing, instrumentation and measurement, optical communication, medicine, holography, data processing and storage, displays, and safety.

Analysis of the signal-to-noise ratio of optical point detectors under conditions of large background radiation (BLIP detection) gives relations describing the background limit to the sensitivity of point-to-point optical communication systems. In narrowband applications, noise-equivalent-power (NEP) has a utility for study of the point BLIP detector which is equivalent to that of the detectivity D*for large area BLIP detectors; for wide bandwidths, a more suitable parameter is the retrieval efficiency β of the receiver. Each of these is expressed in terms of D BLIP *, a quantity graphed in references on infrared detectors. A numerical exmple shows that even on the surface of the earth, broadband point-to-point communications may well be receiver limited rather than background limited.

Previous results on the statistics of avalanche detectors are generalized to the case where electrons and holes suffer collision ionizations with unequal probability. It is assumed here that the ratio of collision ionization probabilities per unit length of weaker-to-stronger carrier is a constant k independent of position in the high-field region. The moment-generating function of the random avalanche gain G is obtained as a function of k and the average gain G, and is used to obtain Chernov bounds on error rates of digital optical receivers employing avalanche detectors. It is shown the required energy per pulse to achieve a given error rate decreases as k decreases for fixed G. For each k > 0, there is an optimal mean gain G opt resulting in minimum required energy per pulse. At k = 0.1 G opt ≈ 100 and the required energy is within 10 dB of that required with very high gains (a few thousand) at k = 0.

A series of papers which appeared in the September 1969 issue of the Bell System Technical Journal treated the theory of dielectric waveguides and stressed the potential use of such media for optical communication circuits.1−4 Here we report on the realization of low-loss, thin glass films which can be used for circuit fabrication. Methods of preparing planar films and waveguides having rectangular cross section are described along with the techniques used in evaluating their optical characteristics.

The theory and observations of the optical propagation effects of the clear turbulent atmosphere are reviewed, with particular attention to those characteristics most important to the designer of an optical communication system. Among the phenomena considered are the variance, probability distribution, spatial covariance, aperture smoothing, and temporal power spectrum of intensity fluctuations, and similar quantities for phase fluctuations and angle of arrival.

Characteristics and potentialities of incoherent and lasing optical semiconductor radiators for space communication

A wide-band optical communication system that modulates the envelope of the carrier of a high-speed optical pulse having no relation to the PCM signals, was developed. This optical pulse is supplied from a He-Ne gas laser oscillator that is controlled to generate a single transversal mode. The transmission bit rate of this optical communication system is 123 Mbits/s, and this system has a transmission capacity of three monochrome video signals, three program signals, and one channel of 48-kHz high-speed facsimile signal or one National Television System Commission (NTSC) color TV signal and one program signal. When this optical communication system is used to propagate through the atmosphere for a distance of 2-3 km, reliability is expected to be 99 percent.

A sophisticated laser communication system has been built and tested both in the laboratory and over field distances. It is envisioned that laser systems using the same techniques might someday return real time television and telemetry from manned interplanetary spacecraft. In the breadboard system, commercial rate television, sound, and a data channel are multiplexed into a thirty million bit per second pulse code modulation (PCM) format, and this signal is modulated onto the laser beam using polarization modulation (PL). We believe this is the first application of a high power Argon ion laser in a communication system. This paper contains an illustrated description of the system, a brief discussion of problems associated with the integration of the high power laser and the wide band modulator, and a summary of experimental results obtained under field conditions. Photographs of the TV monitor, transmitted under various atmospheric conditions, are included.

In this review paper we discuss the scope of the Liquid Phase Epitaxy (LPE) for the growth of the semiconductor materials required for optical communication devices. LPE is non-hazardous comparatively cheaper technology for the material growth and growth processing laboratory could be set up easily. LPE offers the selective and equilibrium growth which is suitable for simple structures. LPE set up could be easily installed for the research and academic institutes as well as in the industries. Key words: Liquid Phase Epitaxy (LPE), Impurities, Optical Communications, Multilayer, Heterostructures, Light Emitting Diodes (LED), Photodetectors, Solar cells and Photodiodes. DOI: 10.3329/bjsir.v44i3.4402 Bangladesh J. Sci. Ind. Res. 44(3), 297-302, 2009

A major problem in deep-space communication systems is that of obtaining high data rates (of the order of 107 bits per second). This article proposes some design concepts that indicate the probable feasibility of achieving wide-band communications by means of the laser. The example selected here is a hypothetical mission to Venus, chosen because of its great brightness and, hence, high background-noise level. Since no earth satellite relay is assumed, the communication channel includes the atmosphere. The down-link is the one considered because of its high-information-rate requirement.

Passive linear optical components play a primary role in an optical communications system for use in space applications. The important functions of acquisition, tracking, transmit-beam off-set, and background-noise discrimination may be efficiently implemented using passive optical elements. These specific system functions are discussed with reference to a representative system in order to identify the required features of particular optical components. This is followed by a detailed discussion of the optical components including: a 1/2-A Fabry-Perot filter; a transfer-lens/optical-image-divider combination capable of 0.1-second of arc tracking accuracy; variable deviation wedges for transmit-beam offset; dichroic beamsplitters for isolation between the transmit and receive beams; and an axicon for redistributing the Gaussian energy profile from a TEM 00 -mode laser to match the obscured circular aperture of a conventional Cassegrainian telescope.

Linear photodiode-amplifier combinations will be required for optical communication systems, in particular for those using fibre optics as the transmission medium. The noise performance of photodiode-amplifier combinations is derived and presented, and some parameters affecting photodiode and first stage amplifier design are reviewed. It is shown that, of foreseen devices, silicon JFETs will be quietest as amplifier first stages for base bandwidths of up to 10 MHz, GaAs FETs up to 100 MHz and bipolar transistors will be preferred for broader bandwidths. In tuned applications the JFET is to be preferred up to 8 MHz and GaAs FETs will be the best devices above that frequency.

Measurements were made using a 200 X 256 element solid-state imaging system to determine and characterize the behavior of XY interconnected pulse-biased phototransistors in the presence of high light levels such as can occur when the sun or other discrete sources are focused on a part of the mosaic. Wavefom photographs depicting operation under these conditions are presented, along with the conclusions reached from their analysis. Operation was found to agree with previously published results over a wide range of light levels, but at unusually high light levels, a photovoltaic mode becomes significant. Under these conditions desensitization of the mosaic occurs along the affected emitter bus, while parts of the mosaic in which photovoltaic action does not occur are unaffected. At very low image rates, mosaic operation changes from frame integration to line integration. This mode of operation appears applicable to some special imaging conditions of potential interest-most notably, applications in which it is desirable to image very bright scenes with transmission of the signal over very narrow bandwidths.

Techniques for constructing a stable mode-locked Nd:YAlG laser source suitable for long term unattended operation are described. These techniques involve the use of two types of electronic feedback loops-the oscillator loop and the servo loop. An additional auxiliary loop to provide for long term operation is described and demonstrated. The detuning properties of the mode-locked laser, using both intracavity loss and phase modulation, are obtained experimentally for the open loop. It was found that the loss-modulated mode-locked laser was noisy due to relaxation oscillations. However, the use of a phase-modulated mode-locked laser presented other problems due to the possibility of a spontaneous 180 ° switch in envelope phase of the pulses. A technique for quenching this phase switching is described. The results of applying these techniques led to the experimental demonstration of a mode-locked Nd : YAlG laser that operated for long times (48 h), with an error rate (dropping a pulse or mistiming of a pulse) of 1.4 \times 10^{-10} , with and rms variation of 8.2 percent in the second harmonic at 0.53 μm and with an absence of relaxation oscillation.

The theoretical possibilities of communicating through scattering channels are considered in the context of digital communication. Direct-detection, heterodyne, and optimum receivers are discussed and the near optimality of direct-detection systems is noted. It is found that the error probability depends only upon the total received signal energy if certain noise thresholds are exceeded; below these thresholds the performance deteriorates rapidly. It is often difficult to exceed the thresholds with present-day technology, but it should become possible to do so as the technology evolves.

Optical communication is tending today to mean a rather special method of data transmission but if one is unbiased by that knowledge the term will probably be interpreted with a much wider meaning. Almost certainly the visual process will be associated with such communication and, in a sense, this is the most significant aspect of it since the ability to 'see' something is by far the most powerful method of communicating it with the human brain.

This paper reviews the field of high-speed small-aperture modulators for applications in optical communications, with emphasis on electrooptic modulation. The capabilities and limitations of electrooptic modulators are discussed based on a review of the physical origin of the electrooptic effect. Thermal and photoconduction phenomena, which may severely limit the operation of practical devices, are emphasized. The modulation power and bandwidth limitations using various schemes of electrooptic interaction are derived and compared. It is shown that lumped modulators are capable of efficient modulation for bandwidths up to about 1 GHz for visible wavelengths and are also attractive for their simplicity. For broader bandwidth capability the traveling wave or zigzag types of interaction become more efficient but with added complexity. Finally, acoustooptic and magnatooptic modulators are briefly discussed and compared with electrooptic modulators.