Crossed Bragg cell implementation of an interferometric spectrum analyzer

Crossed Bragg Cell Implementationof an Interferometric Spectrum AnalyzerJeny B. WoodHarris Corporation Electronic Systems SectorP0 Box 98000, MS 13-7747, Melbourne Fl. 32902AbstractAcousto-optic interferometric spectrum analyzers are important for signal detection and signal processingapplications due to their dynamic range and their coherent detection capabilities. The inability to fabricate a photodetectorwhich matches the available dynamic range of the optical subassembly often limits the performance of an interferometricspectrum analyzer. The photodetector fabrication can be simplified by designing the interferometric spectrum analyzer in away that a common IF is present at each pixel location. The conventional approach to generating a constant IF in aninterferometric spectrum analyzer is to mix the incoming signal with a reference signal on the photodetector element. Whenthis technique is used to produce the IF, an interferometric spectrum analyzer contains spurious signals in its output with noinput signal as a result of the overlap of the reference signals on the photodetectors. An alternate approach to producing theconstant IF desired for the photodetectors is discussed in this paper. The described approach does not generate spurious signalswhen no input signal is present.1 IntroductionAcousto-optic spectrum analyzers have become a competitive approach in implementing channelized receivers. Theprincipal advantage of the acousto-optic technology over competing technologies is the combination of the parallel processingrequired for a dense signal environment with comparatively small size, low weight and low power consumption. Acousto-optic spectrum analyzers can be divided into two classes; power spectrum analyzers and interferometric spectrum analyzers.The interferometric spectrum analyzer has three main advantages over the power spectrum analyzer: increased dynamic range,complex signal detection, and sharper filter shapes. The two primary disadvantages of the mterferometric spectrum analyzerare the increase in fabrication complexity and spurious signals that have been generated by the reference signal used forcoherent detection. The interferometric spectrum analyzer will be more complicated to implement than the power spectrumanalyzer due to the increased processing necessary to coherently detect the signal of interest (501). The unwanted spurioussignals can be eliminated, however, which removes one of the disadvantages of using an interferometric spectrum analyzer.The technique described in the following paper eliminates unwanted spurious signals from an interferometricspectrum analyzer by avoiding deleterious combinations of signals at the detector plane. Specifically, the reference signalused to produce the IF out of the photodetector is the unmodulated optical beam and the constant IF is produced by using thetwice diffracted beam from sequential Bragg cells. The first Bragg cell contains the 501 and provides a downshift on thediffracted beam. The second cell is rotated 90 with respect to the first cell. It contains a reference signal, such as a chirp, andprovides an upshift on the diffracted optical beam. The optical beam diffracted by both the first and second Bragg cells formsa line of beads in the Fourier plane for each SOl. The temporal offset of each bead, as compared to the unmodulated light, isthe difference between the frequency of the 501 and the frequency component of the reference signal that formed the bead.Mixing only the beads with the sample temporal offset with the unmodulated light on a photodetector array produces aconstant IF signal out of each pixel. The remainder of this paper will discuss an implementation of a crossed Bragg cellinterferomeiric spectrum analyzer.2 Crossed Bragg Cell Interferometric Spectrum AnalyzerA schematic representation of the crossed Bragg cell interferometric spectrum analyzer (CISA) is shown in Figure 2-1. The symbol C represents a cylinder lens, S a spherical lens, BC a Bragg cell, M a mirror, BS a beamsplitter, ii thetransform lens, and PDA is a photodetector array. The CISA uses a laser for a stable, coherent source. The laser beam iscollimated and expanded by the laser collimating lens (LCL) prior to entering a Mach-Zehnder interferometer. One path of theinterferometer contains two crossed Bragg cells and several lenses while the other path only contains a lens. The key to theCISA is the leg of the interferometer with the Bragg cells. The first Bragg cell (BC1) has the optical beam from the LCLfocused through its acoustic column by a cylinder lens, Cl. The electrical signal applied to the transducer of BC1 is the