An increasing number of electro-optical systems are being used by pilots in tactical aircraft. This means
that the afore mentioned systems must operate through the aircrafts canopy, unfortunately the canopy
functions as a less than ideal lens element in the electro-optical sensor optical path. The canopy serves first
and foremost as an aircraft structural component, considerations like minimizing the drag co-efficient and
the ability to survive bird strikes take precedence over achieving optimal optical characteristics. This paper
describes how the authors characterized the optical characteristics of an aircraft canopy. Families of
modulation transfer functions were generated, for various viewing geometries through the canopy and for
various electro-optical system entrance pupil diameters. These functions provided us with the means to
significantly reduce the effect of the canopy "lens" on the performance of a representative electro-optical
system, using an Astigmatic Corrector Lens. A comparison of the electro-optical system performance with
and without correction is also presented.
Electro-optical laser systems that use astigmatic semi-conductor laser diodes frequently use beam shaping
mechanisms to eliminate the astigmatism and improve the beam uniformity. Conventional beam shaping
solutions that use asymmetric diffractive or refractive optical systems are expensive and add additional
opto-mechanical complexity. An effective alternative approach to using either the conventional refractive
or diffractive methods is to intentionally introduce specific optical aberrations into the transmitted beam.
The intentional introduction of optical aberrations is not a universal alternative, but in many cases the
introduced aberrations minimize the astigmatic characteristics of the emitted laser diode radiation
sufficiently to make further reduction unnecessary. The technique has been used by the authors, in a
number of laser designator and LIDAR transceiver designs. This paper describes how this technique has
been successfully used to produce lower cost, high performance laser systems. A simple design example is
presented together with a description of the modeling techniques employed and measured field test results.
A temperature insensitive silicon photoconductor has been designed and produced, which significantly improves the
detector performance over a wide operational temperature range. The detector Responsivity in conventional silicon
photodetectors is a function of temperature. The Responsivity and the associated detector sensitivity can vary by as much
as 600% over the normal operating temperature ranges of many commercial and military products. The temperature
controlled photoconductor described in this paper incorporates a sheet resistance heater that is integrated into the silicon
wafer structure. A closed loop control circuit operates the detector at an optimized temperature. This is done in a way so
as to maximize the system signal-to-noise ratio, regardless of the temperature and background illumination level of the
environment. The measured detector sensitivity improvement is approximately 6 times higher at the low end of the
operational temperature range of an electro-optical sensor employing the heated detector and 1.7 to 2.0 times higher for
an electro-optic sensor operating at 20 degrees C, compared to a sensor employing a standard silicon photo-conductor.
An electro-optical system incorporating the new device can realize a significant performance increase and/or realize a
significant reduction in the system aperture size (and related packaging parameters like weight, volume, etc.) while
maintaining parity performance with similar systems that do not use the new detector. The paper describes the device
and presents laboratory and field test results that testify as to the performance improvement that was achieved. These test
results, for devices operating between -54 deg C and +100 deg C show that the heated detectors have significantly higher
performance than conventional silicon detectors operating in the same environments. The advantages of using these
devices in place of conventional detectors are also covered.
Designers of medium and high performance optical systems often overlook a very simple technique that can improve the system transmission and image contrast, as well as reduce scattering within the system. The resulting improvement in the optical collection efficiency can be used to increase performance or be traded off to realize improvements in other areas (i.e. aperture size, weight, etc.). The technique is based on the observation that many (if not most) anti-reflection coatings specified for lens surfaces, are specified at a normal angle of incidence. Since most of the energy incident on a typical lens impinges at angles other than the normal, the efficiency of an anti-reflection coating at any surface might be improved by using an approach based on weighted average angles of the incident radiation. This paper describes one approach to calculate weighted average coating angles for a optical systems. The optical transmissions are estimated, when the respective coatings are specified at the normal angle of incidence and at an angle based on the incident ray geometry. The measured transmission of two (otherwise identical) aspheric lenses, one coated using a standard SLAR coating specified at a normal incidence angle and the other coated using a standard SLAR coating specified at optimized incidence angles are presented.
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