Modulation-Demodulation Techniques

Abstract

This chapter, divided in six sections, begins with the naturally modulated sources: lasers that are modulated due to principle of operation—pulsed modulated solid-state lasers and sinusoidally modulated Zeeman two-frequency lasers. The Zeeman laser presents an exceptional possibility of creating monochromatic sinusoidally modulated light source. With some straightforward mathematics, the conditions of most efficient modulation are derived and experimental oscilloscope traces are presented. This forms the basis for the technology of measuring the cutoff frequency of photoreceivers. Mechanical modulators with perforated rotating wheel are simple devices that are suitable for modulation of any optical beams. But even they can be further optimized by some simple practical tricks. Electro-optical modulators (EOMs) described in the third section are compact, reliable, and surprisingly versatile devices being used for amplitude, polarization, phase and frequency modulation of laser beams. However, their performance can only be fully understood with detailed mathematics, explaining interaction of laser beams with electrically active birefringent crystals. Various types of artifacts may compromise modulation, and this section shows how to avoid it. Numerical computations help to realize these phenomena. Another widely used and also physically complicated device is the acousto-optical modulators (AOMs). The nature of interaction between light and acoustic wave can only be understood with differential equations and special functions, describing periodical solutions for the diffracted beam. Nonetheless, even with this theoretical complexity, it is possible to explain the principle of acousto-optical diffraction in simple terms of reflection from a periodical structure. In this section, deep theory is followed by very practical explanation of the design features of a typical AOM. The next section describes simple and efficient practical solutions for modulating LEDs and LDs, supporting the conclusions by real oscilloscope traces. It is not widely known that even white-light LEDs can be modulated at high frequencies, and temporal evolution of their spectrum is presented, obtained with the help of the gated spectrometer outlined in Chap. 9. The last section of this chapter guides the reader through basics of demodulation techniques and filtering: LC filters, crystal filters, diode rectifiers, active rectifiers, synchronous demodulation (lock-in amplifiers). Along with necessary theoretical explanation, practical circuits and schemes are presented, ready for implementation.

Not everything is possible in optics, and optical modulation has many limitations. This chapter advises on how to choose the proper technique for the specific application.