Electrooptic tunable filters for infrared wavelengths

This paper describes the performance characteristics of an electronically tunable infrared filter based on the electrooptic effect. Like the acoustooptic tunable filter, this device operates by selectively coupling principal polarizations in a birefringent crystal at a Phase-matched wavelength by means of a spatially periodic refractive index perturbation. Instead of a traveling acoustic wave, however, the electrooptic tunable filter employs a temporally static electric field. The main advantages of this filter are its very low power consumption and its versatility of passband programming by virtue of separately addressable voltages under microprocessor control. Two experimental embodiments of the filter are described, one using longitudinal fields, collinear with the light beam, the other using transverse fields in a "thick waveguide" configuration. Experimental results of filter characteristics in the wavelength range2-10 mum are presented. We also present examples of passband synthesis that are achieved by apodizing the amplitude of the applied electric field perturbations. These include passband broadening and sidelobe suppression. The experimental transfer characteristics exhibit good agreement with the theoretical computer plots. Typical driver voltages are ∼ 500 V.     

Modal dimensionality and dephasing effects in slab waveguide electrooptic filters

The voltage-dependent transfer characteristics of multi-mode slab waveguide electrooptic filters have been reexamined analytically in considerable detail, with special attention to differential modal phase delays, modal dimensionality, and attenuation. An important determination was the minimum number of modes (i.e., minimum dimensionality) needed for a reliable representation of the model. We find that a dimensionality of 10 is essentially adequate for the device modeled, having thickness approximately 130 μm, operating near a 0.475-μm wavelength. In addition, we find that inclusion of an initial dead length, with the attendant depbasing of modes, is an important factor in achieving good agreement with experimental data. Suitable allowance for mode-dependent attenuation also aids in improving this match.     

Improved electrode geometry for electrooptic frequency translation in a channel waveguide

The technique of optical frequency translation by a rotating electric field in a trigonal electrooptic crystal has been adapted to a buried channel waveguide structure. To implement the field rotation under the constraints of a planar geometry, an electrode configuration was adopted consisting of three coplanar, collinear metal strips driven in phase quadrature, with the light guide under the central strip. Elimination of a cyclic modulation of field amplitude, characterizing the results of a previous study by the author, was achieved by appropriately widening the outer electrodes relative to the central one and by choosing a suitable channel depth. Spurious sidebands previously generated by an effective counter-rotating field component are thus removed. It is shown that with a residual modal birefringence equivalent tolambda/20-differential phase retardation, the device here described can achieve 98 percent translation into the single first-order sideband with less than 0.5 percent combined spurious carrier and second-order upper sideband.     

CO2laser preamplifier capabilities for low-level 10.6-µm direct-detection receivers

We examine the potential of CO2laser preamplifiers for sensitivity enhancement in low-level, direct-detection 10.6-μm receivers. For the condition in which a gain-dependent competition exists between the background noise and amplifier spontaneous emission noise (assuming negligible thermal noise), the analysis predicts an optimum useful SNR enhancement of only 6 dB for a blackbody background field of 300 K and 4.1 dB for a background of 260 K, when the amplifier gain bandwidth perp-line is 100 MHz and the infrared (IR) filter bandwidth is 0.10 μm. Based on preselected choice of gain and bandwidth, a two-stage, water-cooled, flowing-gas amplifier of optimized design was constructed. A maximum gain of 3.12 dB was attained forP(20)with a He : CO2: N2mixture of 5.0 : 1.0 : 0.6 at a coolant temperature of 285 K and a slow gas refresh rate of 0.2 volumes/s. Using a fast-flow system with 12-volume/s refresh rate, we measured an amplifier gain of 3.9 dB, close to the design estimate of 4.1 dB. With a calibrated HgCdTe detector,f/4cold shield, and narrow-band (0.25 μm) cold filter, a spontaneous emission flux density ofsim 1.0 times 10^{14}photons/ cm². s was measured at the 3.12-dB gain level, in close agreement with the theoretical estimate. Excess noise resulting from amplifier discharge was undetectable above the basic detector noise.     

Electrooptic light-beam deflection

The impetus given display, recording, computer, and space technologies by the revolutionary discovery of the laser has created an ever-increasing need for controlled optical-beam deflection techniques. Those that have been developed thus far, usually employing mechanical or acoustical methods, have had either high insertion loss or limitation on bandwidth. Electrooptic methods have not only overcome these problems, but have successfully met the demands of systems requiring aperiodic and rapid-access modes of operation. After the basic theory of optical deflection, resolution, and control are outlined, two experimental models of electrooptic beam deflectors, as well as the two major difficulties that have been encountered, are described.