Sensitivity of IR imaging systems based on a planar semiconductor-gas discharge (SGD) cell is considered. These systems feature conversion of IR images into visible ones with the response time on the microsecond scale. A converted (visible) image at the cell output is captured by an image sensor coupled to the cell either with a lens or with a fiber optic taper. Comparison of both methods shows that fiber tapers can provide much higher ultimate coupling efficiency but has less flexibility in usage and bring in a high additional heat load on the cooling unit. Obtained equations allow calculation of sensitivity for the whole system, taking into account such parameters of its constituents as the conversion efficiency and dark current density of the IR converter cell, readout noise of the image sensor, light transfer efficiency from the cell to the sensor, as well as the equivalent pixel area in the gas discharge plane and the exposure duration. The equations are applied to evaluate sensitivity of the IR imaging system utilizing a SGD cell filled with argon, where a Si:Zn semiconductor sensitive in a spectral range of 1.1 - 3.5 μm is used. The results demonstrate the possibility of achieving quite a high sensitivity performance of considered systems.
The imager consists of a planar semiconductor-gas discharge (SGD) cell allowing the ultra fast IR-to-visible conversion with response time on the microsecond scale. The semiconductor wafer is made of Si:Zn providing the spectral range of 1.1 - 3.5 micrometers . The 100 micrometers discharge gap is filled with Ar under the pressure of 100 hPa. The cell is cooled down approximately to 90 K. Among studied properties are noise, both in time and space domains, detectivity, noise equivalent irradiance and, when applying the imager in a thermal imaging system, noise equivalent temperature difference (NETD). Investigations of the spatial noise and NETD have been carried out by using a low-noise CCD camera capturing output images of the SGD cell. For measuring the temporal noise, a low-noise photomultiplier is used to detect gas discharge radiation from the area of about one resolved pixel. The own noise of the SGD cell is found by comparing signal-noise dependencies obtained at acquiring outgoing light of the cell, on the one hand, with those at observing a thermal radiation source with well describable photon noise, on the other hand. The results indicate that the imager has surprisingly low noise which is very close to the photon-noise limit.
The infrared (IR) image converter is based on a planar semiconductor-gas discharge structure operating at a temperature of about 100 K in the spectral range of 1.1 to 3.5 micrometers . Semiconductor material and gas are Si:Zn and Ar respectively. The conversion of input IR images into the visible is characterized by a time constant in the order of a few microseconds, the dynamic range is at least 104 and good linearity is observed. Together with the Hamamatsu framing camera C4187 the IR converter has been applied to investigate in the microsecond range the spatio-temporal dynamics of radiation of 1.318 micrometers Nd:YAG laser. The second application was the study of the mode evolution of an Er:YSGG laser operating at a wavelength of 2.79 micrometers and an Er:YAG laser with a wavelength of 2.94 micrometers using a pulse length of 100 to 150 microsecond(s) , and a pulse energy of about 40 mJ. In this case, the IR converter is combined with an intensified CCD camera, of which the exposure time is down to 10 microsecond(s) . In the third application, the IR converter is used in combination with a fast CMOS camera to monitor Nd:YAG laser welding of stainless steel samples at the rate of 320 frames/s.
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