Modulation of Light

the modulation of electromagnetic radiation in the optical region (visible light and ultraviolet and infrared radiation). In modulation of light the amplitude (and therefore the intensity), phase, frequency, or polarization of the radiated oscillations is changed. For any of these cases, in the final analysis, the set of frequencies that characterizes the radiation—its harmonic composition—is changed.

Modulation of light makes possible “loading” of the luminous flux with information, which is then transmitted by the light and can be extracted and used. In principle, the quantity of information that can be transmitted by modulation of oscillations of any type increases with increasing frequency of the oscillations (particularly because, as the frequency of the modulated oscillations, called the carrier frequency, increases, an increase in the frequency bandwidth of the modulating signals becomes possible; the frequencies of the modulating oscillations must be lower than those of the carrier). The frequency of visible light is 10¹⁵–10¹⁶ hertz (Hz), and that of the entire range of optical radiation is 10¹²–10²⁰ Hz (that is, considerably higher than for any other oscillations that are modulated to transmit information). This factor, and frequently also the impossibility of solving a technical or scientific problem without the use of optical radiation, determines the importance and prospects of modulation of light.

In many technical applications the frequency of the modulating signal is so low compared to the frequency of the opticajl radiation that the variation in the harmonic composition of the latter is negligible, and the modulation is taken to mean a periodic or nonperiodic change of only the radiation intensity. The simplest example of such modulation, which has been known since ancient times, is signaling by interruption of a luminous flux. In such light modulation in modern engineering, the type of optical signal chosen as the best for solving a specific problem is of importance. The signals may be short light pulses or signals that are nearly rectangular or harmonic.

“Natural” modulation of light takes place even during the emission of light by elementary radiators, such as atoms, molecules, and ions. The finite de-excitation time of such radiators (of the order of 10⁻⁸ to 10⁻⁹ sec) results in some frequency spread of the emitted radiation. Natural modulation of light also takes place during scattering of light and during interaction of radiators with one another. The processes may be studied both in individual radiators and in systems of radiators.

However, in many cases natural light radiation may be regarded with sufficient accuracy as monochromatic (as harmonic oscillations of only one frequency) and may experience forced modulation. A distinction is made between internal modulation, which is accomplished within the radiation source, and external modulation, which is produced by means of special devices called light modulators. (These terms are also applied to the “coarse” modulation of nonmonochromatic light mentioned above, in which a variation of the spectral composition of the radiation does not play an important part.) Light detectors of all types react only to variations in light intensity—that is, in the amplitude of its oscillations. Consequently, modulation of the frequency, phase, or polarization are converted in practice to amplitude modulation by one means or another—either directly in the modulator circuit or before the photodetector (called heterodyne detection). Here the harmonic composition of the amplitude-modulated light depends on the initial form of modulation and the method of conversion to amplitude modulation.

The main parameters that characterize amplitude modulation of light are the fundamental frequency and the frequency band-width of the modulating signal; the depth of modulation m = (I/_max − I_min/(I/_max +_min), where I is the luminous flux; and the absolute value of the modulation amplitude and the transmittance of the modulator (the power of the signal registered by the detector depends on these factors). Internal amplitude modulation of light is achieved, for example, by varying the supply voltage and current of an artificial source of radiation in accordance with a specific law. This method is most efficient for gasdischarge light sources and semiconductor radiators. Internal light modulation is also used extensively in lasers (see below).

The simplest light modulators are mechanical devices that are able to interrupt the luminous flux for certain specific time intervals. Among them are rotating disks with holes (shutters), gratings, vibrating or rotating vanes, mirrors, and prisms, as well as devices in which controlled disturbance of optical contact by a modulating (nonoptical) signal takes place. Another class of devices used for external amplitude modulation of light consists of modulators that operate on the principle of controlling the light absorption in semiconductors. The absorption is a function of the concentration and mobility of free charge carriers in the semiconductor (free electrons and holes), and they may be controlled by varying the voltage and current. Studies of the properties of transparent ferrites and antiferromagnets, which began only in the 1960’s, indicate that they are also promising for modulators.

Mechanical modulators provide the maximum transmittance and depth of modulation, but they operate at frequencies no higher than 107 Hz for the modulating signal and do not permit quick frequency retuning (their bandwidth is small). Theoretically, semiconductor modulators can modulate light at frequencies up to about 10¹⁰−10₁₁ Hz with a bandwidth that is restricted only by the parameters of the electronic circuit, but the depth of modulation and overall efficiency of such modulators are small as a result of the great absorption of light in the semiconductors and the low electric strength of the materials.

The effects most frequently used for light modulation, which cause variation of the index of refraction of an optical medium under the influence of an external field (the modulating signal), are electro-optical (the Kerr and Pockels effects), magneto-optical (the Faraday effect), and acousto-optical. In modulators operating on the basis of these effects, phase modulation of the light takes place (with subsequent conversion into amplitude modulation); consequently, they are also called phase cells. The frequency of the modulating signals in most optical media used to fill phase cells may be as high as 10¹¹ Hz.

When the electro-optical effect is used, a scheme like that shown in Figure l,a may be used, in which the amplitude modulation results from interference of two or more phase-modulated light beams; or a polarization scheme like that shown in Figure l,b, in which the phase modulation of two mutually perpendicular components of linearly polarized light results in polarization modulation, which is transformed into amplitude modulation by an analyzer, may be used.

In the case of use of the Faraday effect (rotation of the plane of polarization of light in a magnetic field), amplitude modulation of light is achieved according to a scheme similar to that shown in Figure l,b. The frequency and bandwidth for light modulation by means of electro-optical or magneto-optical cells depend mainly on the parameters of the circuit that controls their action and can be fairly large.

The acousto-optical effect consists in the change of the index of refraction of a medium under the action of elastic stresses produced by ultrasonic and hypersonic acoustic waves in the medium. In solids, unlike liquids and gases, double refraction also takes place. The periodic variation of the direction of propagation of light in a fluid when low-frequency ultrasonic waves are passed through it causes scanning of the light beam. In a high-frequency acoustic wave field, the microperiodic variations in the index of refraction create a structure that represents a phase diffraction grating for light. The diffraction of light in a medium by a traveling or standing acoustic wave can produce amplitude modulation of the light according to the scheme in Figure l,c. In solids light may also be amplitude-modulated by acoustic waves in polarizing schemes of the kind in Figure l,b (because of double refraction). The frequency region of the modulating signals in acousto-optical methods of modulation is broad (up to the superhigh-frequency range), but because of the low velocity of sound compared with the velocity of light, the width of the frequency band is small—not more than (1–2) × 10⁶ Hz.

The overall efficiency of modulation of light depends to a considerable extent on the parameters of the light beam. The advent of lasers, with the high degree of monochromaticity inherent in their radiation and with their small divergence and very powerful luminosity, made possible the construction of economical and efficient modulators with schemes that are completely unsuitable for use with noncoherent light sources. The use of some methods of external modulation for the internal modulation of lasers has proved possible (through modulation of the quality factor of their rods or, in semiconductor and gas lasers, by means of a pulsed power supply). Modulation of light is used in lasers not only for information input but also to increase

Figure 1. Schemes of light modulators: (1₀) luminous input flux, (l) modulated output luminous flux, (a) Interference modulator. The effect of the control (modulating) voltage U on the phase cell (1) causes a shift of the interference maximum in the output flux l as a result of a change in the index of refraction of the medium filling the cell. Correspondingly, there is a change in the light intensity at the modulator output (the rays reflected from mirrors [2] and [3] interfere; [4] semitransparent dichroic mirror, [5] light output window), (b) Polarizing modulator. A polarizer (1) and an analyzer (3) initially are crossed and pass no light. Under the influence of a modulating signal U the plane of polarization of the light in an electro-optical or magneto-optical cell (2) is rotated (or its linear polarization is converted into elliptical polarization), and a light signal appears at the output, (c) Diffraction modulator. The vibrations of an electroacoustic transducer (a piezoelectric crystal or piezoceramic plate [1]) at a frequency f create in an acousto-optical medium (2) an ultrasonic wave that acts on the input luminous flux like a diffraction grating. In the focal plane of an objective (4) a diffraction pattern periodically appears and disappears (at the moment when the standing wave passes through zero or during modulation by the traveling acoustic wave), and at each maximum of the pattern (for example, at the zero maximum, which is singled out by slot [5]) the light intensity is modulated with a frequency of 2f, or the frequency of the traveling wave; (3) ultrasound reflector (or absorber).

the power of the radiation, in some cases by several orders of magnitude. The shortest known light pulses, with a duration of the order of 10⁻¹¹ to 10⁻¹² sec, which corresponds to a frequency band of 10¹¹− 10¹² Hz, are produced in solid-state lasers in which the resonators are Q-spoiled by cells (“gates”) filled with fluids that become translucent upon irradiation by a powerful light beam.

Modulation of light is used extensively in research, particularly in studying processes in a substance that are stimulated by light, such as luminescence, photoconductivity, and photochemical reactions. It is also used in optical ranging, which is used for measurements of the range and speed of moving objects; in optical communications systems, optical sound recording, optoelectronics, facsimile transmission, and television; for measurement and comparison of luminous fluxes, and for the measurement of short and subshort (down to l0⁻¹² to 10⁻¹³ sec) time intervals. The coding, decoding, and recording of information by means of modulation of light are used in computer technology, and acoustic light-modulation methods are used in analog computers.