Multi-Stage Acousto-Optic Modulator

Abstract

Multi -Stage Acousto -Optic ModulatorD.R. SUHREWestinghouse R&D CenterPittsburgh, PA 15235ABSTRACTBulk acousto -optic modulation was investigated as a technique for generating highbandwidth linear chirp waveforms for ladar applications. The high modulation bandwidth wasachieved by cascading the Bragg cells, in which each cell produces only a fraction of thetotal bandwidth, and large optical beam diameters and high optical energies can be used.Both germanium and thallium arsenic selenide (TAS) were investigated as acousto -opticmaterials, and comparisons were made between the two materials. Cooled germanium was alsostudied as a means of lowering the acoustic attenuation, and the thermal conductivity ofcryogenically cooled Ge was measured.1. INTRODUCTIONThe basic approach of this study uses multiple Bragg scattering from a series of Braggcells, with each pair of cells having opposite propagation directions for the acousticwaves. With each Bragg interaction, the optical beam scatters through twice the Braggangle, and gains the acoustic frequency. The succession of Bragg cells will increase thebandwidth available from a single cell, while the opposing acoustic waves will give acancellation of the frequency gradients across a single Bragg cell when using linearlychirped waveforms. The gradients are due to the finite acoustic velocity, which willproduce a frequency spread in the output. With the counter -propagating acoustic fields, thefrequency gradients will cancel, giving the proper modulation waveform.The chirp is reset through the use of a flyback period. With this design, the acousticchirp length is much smaller than the optical aperture dimension, and the linear frequencymodulation waveform is discontinued during the flyback period, which is equal to theacoustic transit time of the aperture. Since the acoustic transit time across a cm apertureis about a µsec, this method should work well for chirp periods on the order of a msec,where the discontinuity will be only a small fraction of the chirp period.2. MODULATOR DESIGNA block diagram of the modulator system is shown in Figure 1, in which N units eachproducing a bandwidth of Bt /N are cascaded in series, where Bt is the total systembandwidth. An even number of Bragg cells must be used, since the first cell of the pairproduces a given beam steering angle, while the second cell corrects the steering angle.Focussing of the optical beam to a divergence angle larger than the steering angle istherefore not required, as with the standard AO modulator, and large optical apertures arepossible. With the multi -stage approach, the Bragg cells are operated as scanners ratherthan as modulators, with the scanners being used in pairs to correct for beam steering.Figure 1 Block diagram of system.