Excerpt

The choice of available infrared (IR) detectors for insertion into modern IR systems is both large and confusing. The purpose of this volume is to provide a technical database from which rational IR detector selection criteria can evolve and thus clarify the options open to the modern IR system designer. Emphasis will be mainly on high-performance IR systems operating in a tactical environment, although there will be limited discussion of both strategic environments and low- to medium-performance system requirements.

Early IR imaging systems utilized extrinsically doped Ge as the detecting material and operated at 28 K. However, the development in the 1960s of the semiconductor alloys HgCdTe [1] and PbSnTe [2], with their tunable bandgaps that covered the complete IR spectrum from 1 to 20 µm, led to the birth of the first generation of modern high-performance IR systems in the early 1970s, with the advent of the so-called Common Module, first developed by Texas Instruments. The heart of this system was a simple 180-element linear parallel-scan HgCdTe photoconductive array mounted on a cold finger operating at approximately 77 K. The IR image formed at the focal plane of the system was scanned across the simple linear array of detectors by a rotating mirror, thus generating one line of the IR scene at a time, with an available integration time (or noise bandwidth) determined by the system line time, and the image was formatted by the subsequent off-focal plane electronics. This first-generation imaging system was thus mechanically complex, but geometrically simple from a detector, and hence array producibility, point of view. The array bias and amplifier electronics were mounted off the cold finger at an elevated temperature. HgCdTe had proved to be the material of choice at this time because of issues with PbSnTe associated with its high dielectric constant and inferior mechanical properties. The use of the direct bandgap alloy HgCdTe resulted in high absorption coefficients for the IR, and for the first time enabled the use of thin material, and hence the deployment of standard semiconductor photolithographic techniques in IR focal plane array fabrication. The Common Module has survived in one form or another for some 30 years and is still in production today.

The 1990s fostered the evolution of the second generation of IR systems, in which the detector bias and signal electronics were incorporated onto the cold finger itself.

© 2007 Society of Photo-Optical Instrumentation Engineers

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