Design of a Cryogenic IR Detector with Integrated Optics
Semiconductor Devices, P O Box 2250, Haifa 31021, ISRAEL
Israeli Ministry of Defense
Cryogenically cooled IR detectors, which are used in applications such as situational awareness, search & track, missile
launch and approach warning, typically use wide angle, single field of view optical systems. We describe a complete IR
imaging optical assembly for such applications, which is mounted inside a cold shield and is maintained at a stabilized
cryogenic temperature inside the dewar.
A typical system houses two to four lenses and a cold filter, and weighs 5
grams or less. Despite this integration and added complexity, the resulting Detector-Dewar-Cooler Assembly (DDCA)
has overall dimensions similar to those of equivalent-performing DDCAs without integrated optics. Moreover, Compact
designs integrating wide-angle optics and a warm, high-magnification, telescope module for narrow FOV applications
are seen as a straightforward extension of our system.
We conclude with an in-depth, technical overview describing the
design considerations for a typical wide-field imaging system.
IR detectors, IR imaging.
In the conventional approach to IR imaging, the imaging optics is located outside the detecting DDCA (Detector Dewar
This is because the imaging optics typically has a relatively high thermal mass. A typical imaging
optics arrangement includes a few imaging lenses and/or mirrors, as well as a focus correction system permitting the
system to remain in focus at a range of ambient temperatures. Such imaging optics is associated with mechanical
assemblies and electronics.
With the conventional approach, an IR imaging system suffers from thermal noise reducing the system performance.
On the one hand, uncooled imaging optics, located outside a DDCA, is highly sensitive to temperature changes, because
the refractive index of the optics in the IR spectral range is highly dependent on its temperature.
A change in the
refractive index unavoidably introduces optical aberrations and focus changes, which require focal correction, and
which in turn needs the use of focus control and focus adjustment mechanisms.
On the other hand, imaging optics (e.g.
lenses and mechanical components), as any other object, emit thermal energy in the IR, which presents a noise
component in the detected IR signal thus reducing the signal to noise ratio of such systems. Some thermal noise effects
associated with temperature changes in the imaging optics might be compensated by utilizing the non-uniformity
correction (NUC) procedure for calibration and correction of the readout signal collected from the FPA. However,
during the use of the NUC procedure for calibration of the IR imaging system, the system is put in an inoperative state
(during which the system is "blind"). It is, therefore, preferable to minimize the number of NUC procedures that are
required during the operation of the system.