an infrared radiation detector for laser diagnosticsAbstractA liquid biased, liquid crystal, infrared radiation detector has been devised for the purpose of determining the time integrated output spectrum and power spatial distribution of infrared radiation. The device has been specifically used to measure the spectrum and spatial power distribution of the output of a quasi-cw carbon monoxide gas lase匚The detector recording medium is composed of thin mylaroated with temperature sensitive, encapsulated,cholesteric, liquid crystals and energy absorbing black paint.A uniform spatial color distribution of the liquid crystals is established and maintained by immersion in a controlled temperature liquid bath.The resulting uniform color distribution is modified, at various spatial points, by the addition of laser energy obtained either indirectly from the laser output coupler to yield the spatial dependence of the near or far field laser output or from a spectrometer output to yield the spectrum of the laser output. The addition of less than 15 mJ /cm2 of incident energy is adequate to modify the color distribution. The uniformity and precise control of the liquid crystal temperature and hence color, by the use of a liquid bath, makes it possible to compare colors, and hence laser output powers, at various spatial points.This permits the mapping of the laser near field output spatial distribution and the distribution of the laser spectrum with greater sensitivity than has been possible with more conventional techniques.Results showing the near field energy distribution and output spectrum of a supersonic flow,electric discharge CO laser obtained using the detection technique will be presented.IntroductionA supersonic flow quasi -cw carbon monoxide gas laser has recently been constructed. (1) Among the relevant data necessary for characterizing lasers of this type are the temporal, spatial, and spectral energy distribution of the infrared laser output. Temporal behavior can be determined using standard infrared detectors. However, spatial and spectral determinations require special diagnostic considerations.The duty cycle of the laser of approximately thirty minutes and the relatively short run time of 2 ms preclude the use of a single detector or linear array for spatial scanning of the laser output.In addition, the large output beam divergence inherent with this laser and the stable resonator used result in an excessive focussed spot size (approximately 1 cm x 2 cm). Because of this large spot size,only a small portion of the laser output can be focussed through the entrance slit of an infrared. spectrometenThe amplitude of the first order dispersed output of the spectrometer is, therefore, very small and difficult to record using conventional methods such as fluorescent screen recording.lt has, therefore,been necessary to develop a liquid crystal recording technique for the purpose of determining the spatial and spectral energy distribution of the infrared laser output.The optical characteristics of liquid crystals are modified by heat, pressure, electric fields, and magnetic fields.(2) In particular, the wavelengths of light reflected from the surface of cholesteric liquid crystals is temperature dependent. This fact makes it possible to detect and measure electromagnetic radiation absorbed by the liquid crystals by observing the resulting color distribution on the liquid crystal surface. This in fact has been done to observe microwave fields and interference patterns and the mode structure of the output of a CW CO2 gas laser.(3 -6) Liquid crystals have also been used as a recordingmedium for infrared holography and Mach Zehnder interferometry. (7 9)This paper describes a liquid biased liquid crystal detector and the results obtained using this detector to analyze the spatial and spectral energy distribution of the output of a large aperture, quasi cwinfrared lase匸 The measurement technique, including construction details of the liquid crystal detector,and the total experimental arrangement, will be presented together with a brief description of the laser to be analyzed. The experimental spatial and spectral measurement results obtained using the liquid crystal detector will then be presented and compared with data obtained using other techniques.Measurement TechniqueThe liquid crystal detector is shown in Figure 1 and consists of a liquid crystal recording medium and a liquid bias mechanism. Infrared radiation to be recorded is incident upon a . 0008 cm thick transparent Mylar sheet which has been coated with a thin film of cholesteric liquid crystals and a film of absorbing black paint. The radiation is absorbed by the black surface and conducted to the liquid, crystals. The color distribution of the liquid crystals is consequently modified according to the incident radiant energy.The color distribution is photographically recorded through the Mylar sheet.The commercially available liqu