Optoelectronic Devices and Optical Imaging Techniques

Optoelectronic devices are those which convert light into electrical energy or vice versa. In some cases the same device may perform both functions. In the past few years, a host of new devices has been developed for commercial applications — LEDs, photodiodes, solar cells, laser diodes, and many more.

The sources of optical radiation between approximately 0.1 μm and 1 μm wavelength (1 μm = 10⁻⁶ m) may be classed into two types according to their spectral line width. The communication engineer is aware of the desirability of a signal source which is monochromatic. Such a single frequency source may be modulated and can act as the carrier of a high concentration of information. On the other hand, a wide bandwidth source would not be suitable for communication applications, since the modulation signal would become irretrievably scrambled in the transmission and detection process. Thus, the most important question we may ask about an optical source is whether or not its emission has a narrow spectral width.* The coherent sources, the gas, liquid or solid state lasers, have a spectral width of the order of 0.01 to 0.1 nm giving a relative bandwidth of the order of 10⁻⁵ to 10⁻⁴ (the line width of a He-Ne laser emitting red light at λ₀ = 632.8 nm is Δf = 7.5 GHz). The incoherent sources, the electroluminescent devices such as the light emitting diode, have a spectral width of the order of 10 nm with a relative bandwidth of the order of 10⁻². The applications of electroluminscent devices are mainly in optical coupling, and optical display and illumination.

In this chapter we shall discuss the main detector of infrared illumination, the photoconductor. This device is of importance because it is compact, operates on a low bias voltage, and responds over a wide range of wavelengths depending on the material chosen. The photoconductor has the potential of high current response since its gain factor may be larger than unity, provided material parameters are chosen carefully. Besides infrared detection, the photoconductor finds application as a detector of intense light, in the laser power meter, and in radiometric and photometric calibration of light sources such as the LED.

This chapter discusses the two types of photodetector found most commonly in optoelectronic systems, the photodiode and phototransistor. Both devices are compact light detectors requiring no more than a few tens of volts for bias, and if made of silicon are responsive to light in-the visible and near infrared-wavelengths. Improved device technology has increased the sensitivity and bandwidth of the photodiode to the point where its performance is comparable with that of the photomultiplier tube, which is sensitive and fast enough to detect individual photons. The avalanche photodiode is particularly similar in performance to the photomultiplier tube. The photodiode finds application in the optically coupled isolator, optical data links, and in optical communications, where a fast response time is required for the very wide bandwidth expected of an optical communication system. The phototransistor, although somewhat more limited in bandwidth than the photodiode, finds numerous applications as a high-current response photodetector.

It has long been recognised that a system in thermal equilibrium exhibits internal fluctuations which result from the kinetic energy of molecules and electrons. For example, an isolated resistor at room temperature has internal charge fluctuations which give rise to a randomly fluctuating voltage at its terminals. Even though it may be electrically neutral in the sense that its average voltage is zero, it contains a very large number of atoms whose outer electrons may occasionally gain enough thermal energy to break free and move about, causing local fluctuations in the internal charge density. An electron that has broken free from its particular host atom executes a random walk through the resistor until being recaptured, and its average kinetic energy while free is given by \(\tfrac{1}{2}m{\bar V^2} = \tfrac{3}{2}kT\) where m is its mass and \({\bar V^2}\) its mean square velocity. Since the mass of an electron is very small its mean square velocity is very large, and thermal fluctuations in charge density in a resistor give rise to a very rapid variation in terminal voltage. This type of noise is often referred to as ‘white’ noise, since it has a frequency spectrum which is flat out beyond any frequencies of interest in electrical systems. It is more properly called thermal noise.

The technology of solar cells has been developed as a result of the need for long-lasting power supplies for satellites and space vehicles. The device is based on the photovoltaic effect in the photodiode. With no external bias a photodiode will deliver power to a load resistance if irradiated by light with photons of sufficient energy. By making the collection area as large as possible, the solar cell will deliver a large photocurrent from direct illumination by sunlight. A solar cell with a collection area of 30 cm² and a power conversion efficiency of 10 per cent will deliver 300 mW to a load if illuminated by bright sunlight.

The light emitting diode of chapter 2 can be modulated to act as the source in an optical communication system. The primary disadvantage of using an LED in such a system is that it emits light over a relatively wide range of wavelengths — the relative width of an LED’s emission is typically around 5 per cent. One effect of this rather broad spectral emission is that because of dispersion in the optical transmission path the modulation bandwidth must be limited, so that different portions of the modulation spectrum do not arrive at different times at the receiver. If an LED which has a peak emission at 600 nm with a 5 per cent spectral width is used as the source in an optical communication system, the bandwidth of the source is 25 000 GHz. Trying to modulate such a source is rather like superimposing modulation on noise.

The purpose of this chapter is to discuss optical imaging from the point of view of dynamic range, resolution and information capacity. Although photographic film has no proper place in a book on optoelectronic devices, we shall begin with a discussion of the properties of film in order to establish the basic principles of optical imaging. This is followed by a brief discussion of holography, an exciting new method of recording three-dimensional images. The remainder of the chapter deals with a new solid state imaging device, the charge coupled device (CCD) area image sensor. The resolution, dynamic range and information capacity of this device is considered in comparison with that of photographic film. The author concludes that although the density of sensing elements of the CCD area image sensor is low, its information capacity is reasonably high — because of the large dynamic range of silicon photodetectors — and that the problem is finding a method of encoding the optical image that can utilise this information capacity.