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This paper is intended to present firstly the current status at AIM on quantum well (QWIP) and antimonide superlattices (SL) detection modules for multi spectral ground and airborne applications in the high performance range i.e. for missile approach warning systems and secondly presents possibilities with long linear arrays i.e. 576x7 MCT to measure spectral selective in the 2 - 11μm wavelength range.
QWIP and antimonide based superlattice (SL) modules are developed and produced in a work share between AIM and the Fraunhofer Institute for Applied Solid State Physics (IAF). The sensitive layers are manufactured by the IAF, hybridized and integrated to IDCA or camera level by AIM. In case of MCT based modules, all steps are done by AIM.
QWIP dual band or dual color detectors provide good resolution as long as integration times in the order of 5-10ms can be tolerated. This is acceptable for all applications where no fast motions of the platform or the targets are to be expected.
For spectral selective detection, a QWIP detector combining 3-5 μm (MWIR) and 8-10 μm (LWIR) detection in each pixel with coincident integration has been developed in a 384x288x2 format with 40 μm pitch. Excellent thermal resolution with NETD < 30 mK @ F/2, 6.8 ms for both peak wavelengths (4.8 μm and 8.0 μm) has been achieved. Thanks to the well established QWIP technology, the pixel outage rates even in these complex structures are well below 0.5% in both bands. The spectral cross talk between the two wavelength bands is equal or less than 1%. The substrate on the sensitive layer of the FPA was completely removed in this case and as a consequence the optical crosstalk in the array usually observed in QWIP arrays resulting in low MTF values was suppressed resulting in sharp image impression.
For rapidly changing scenes - like e.g. in case of missile warning applications for airborne platforms - a material system with higher quantum efficiency is required to limit integration times to typically 1ms. AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The type II SL technology provides - similar to QWIPs - an accurate engineering of sensitive layers by MBE with very good homogeneity and potentially good yield and resistivity against high temperature application i.e. under processing or storage. While promising results on single SL pixels have been reported since many years, so far no SL based detection module could be realized with reasonable performances. IAF and AIM last year managed to realize first most promising SL based detectors. Fully integrated IDCAs with a MWIR SL single color device with 256x256 pixels in 40 μm pitch have been integrated and tested. In the next step the pitch was reduced to 24μm in a 384x288 pixel configuration. With this design and further improved technology a very good pixel operabilities with very low cluster sizes (≤ 4 pixel) and performances with quantum efficiencies as high as known from MCT is reached in the meantime.
A dual color device based on SL technology on the existing 384x288 read-out circuit (ROIC) as used in the dual band QWIP device is available. It combines spectral selective detection in the 3-4.1 μm wavelength range and 4.1-5 μm wavelength range in each pixel with coincident integration in a 384x288x2 format and 40 μm pitch. Excellent thermal resolution with NETD < 17 mK @ F/2, 2.8 ms for the longer wavelength range (red band) and NETD < 30 mK @ F/2, 2.8 ms for the shorter wavelength range (blue band) has been achieved. The pixel outage rates remains below 1% in both colors. The spectral cross talk of the red band to the blue band is estimated below 1%o which is important to reduce significantly the false alarm rate in missile approach warning systems as the primarily intended use of the dual color detector is.
Real time analysis of gases, i.e. the detection of toxic or agent gases, by multi spectral detection in the IR used the characteristic infrared emission or absorption lines of different gas types. Spectroscopic systems consisting of a spectrometer with the need for large linear MCT array with small pixel sizes are used in this case. Possibilities are outlined to use long linear arrays, such as the 576x7 MCT detector, to perform spectral selective measurements in the 2-11μm wavelength range. For these applications a 576x7 MCT FPA is integrated in an open dewar cooler assy without window able to operate directly coupled in an evacuated and cooled spectrometer. The sensitivity of the array is consequently not limited by the transmission of a window for vacuum conservation in the full sensitive wavelength range of MCT up to the cut-off of 10.5 μm.
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