In modern optical processing and display applications, there are increased needs of a real-time device and such a device is called a spatial light modulator (SLM). Typical examples of SLMs are acousto-optics modulators (AOMs), electro-optic modulators (EOMs) and liquid crystal displays. In this chapter, we will concentrate on these types of modulators and discuss their uses, such as phase modulation and intensity modulation, in information processing.

Pulsar radio emission is variable on many time scales, from nanosecond structure in single pulses to intermittency on timescales of many years. Isolated single pulses may be intense individual pulses from a regular sequence, or individual fast radio bursts. The excitation of radio emission varies both in its location and its timing.

Based on Fourier analysis, a theoretical description is given of the harmonics arising from current modulation of a DFB laser with its wavelength scanned through a gas absorption line. It is shown that each harmonic consists of a primary component from the wavelength modulation and two secondary components arising from the mixing of the intensity and wavelength modulations, with additional components if the laser light-current characteristic is non-linear. The importance of the lock-in detection phase is discussed and the need for calibration-free, consistent operation in the face of possible drift of laser parameters with time or with aging. Two methods are examined for extraction of gas parameters, one based on the effect of gas absorption on the laser intensity modulation, with correction factors applied at high modulation indices, and the other based on measurement of the second harmonic signal normalised through the first harmonic. It is shown that both methods can give similar sensitivities, but the harmonic ratio method is much superior in noise performance at the expense of increased complexity in signal processing and uncertainty if the laser parameters are prone to drift.

A detailed account of the electronic, thermal and optical properties of DFB lasers is presented relevant to the requirements of wavelength modulation spectroscopy. An RC thermal model is first used to provide a simple analytical description of the thermal scanning and wavelength tuning properties of the laser and then a one-dimensional solution of the heat conduction equation is derived for a better description of thermal tuning. Perturbationanalysis of the laser rate equations is used to determine the effect of current modulation on the carrier, intensity and wavelength modulation, taking into account both spectral and spatial hole-burning effects. Theoretical relationships are derived for the overall tuning coefficient, combining both thermal and carrier contributions, with expressions given for its magnitude and phase as a function of the modulation frequency relative to the intensity modulation. This is important for the interpretation of the harmonic components that arise with wavelength modulation spectroscopy. It is shown how a fibre-optic ring resonator may be employed for the experimental determination of the tuning coefficient and for calibration of wavelength scans.