| Why use optics for radio-frequency spectrum analysis, especially when electronic spectrum analyzers are so good? There are several reasons: optical processing is inherently parallel; coherent light Fourier transforms as it propagates; and the processing speed is usually determined by the time it takes light to propagate through the system. Of course, there are disadvantages to optical processing, namely the difficulty in generating long time delays using optics and the (relatively) small dynamic range of optical detectors. Optical systems are good for analyzing pulsed or hopping signals, and electronic systems for weak continuous-wave signals.; Traditionally, optical processors use spatial parallelism to monitor many channels simultaneously. Exploiting this parallelism requires converting time-domain signals into spatial modulation. Coordinate transformations, then, make domain transformations possible. The systems based on tapped delay lines described in Chapters 1 and 2 all use spatial coordinate transformations for spectrum analysis, while the spectral-hole-burning spectrum analyzers discussed in Chapters 3 and 4 use spectral parallelism for spectrum analysis. Spectrum analyzers that use more than one dimension, such as the spatial-spectral processor in Chapter 5, could potentially operate with time-bandwidth products of up to 108, something far beyond the reach of electronic spectrum analyzers for the foreseeable future. |
