Off-focal-plane time delay and integration (OTDI) using charge coupled devices

The signal-to-noise ratios (SNR) of mechanically scanned imaging sensors can be improved by scanning the same image point over N detectors and adding the resulting detector signals in phase, a process referred to as time delay and integration (TDI). In principle, this allows the signal level associated with a single resolution element to be increased by a factor N, while SNR increases by √N. TDI has previously been demonstrated by using ch.arge coupled devices (CCDs) to delay and integrate the signals from a variety of infrared detectors, including PbSnTe, InSb, and HgCdTe. In these cases, signals from a linear array of detectors were read out and integrated by a CCD register that was located directly on the focal plane (see Figure 1a). Use of an on-focal-plane CCD register to multiplex the individual detector signals for subsequent demultiplexing and TDI off the focal plane (see Figure 1b) is an attractive approach under the following circumstances: . When long dwell times and/or high backgrounds result in a prohibitively large on-focal-plane CCD TDI register. . When access to individual detector signals can be used to advantage in performing signal processing functions before TDI. An OTDI device using CCDs has been fabricated and used to perform OTDI of signals from a 32-element linear array of InSb detectors. Peak D* of 8 x 10¹² cm√Hz/w was achieved at a background of 3.4 x 10¹³ photons/cm²-sec, which is approximately 80 percent of the enhanced BLIP D* at this background. In addition, overload protection and background subtraction circuits were tested, and the compatibility of the OTDI device with a bidirectional image scan was demonstrated.