Through careful design of the various anti-reflection coatings, the overall performance at 1060nm is optimized in such a way that when both the silicon and InGaAs chips respond to the incident laser, its wavelength is most likely in the transition region. This fairly coarse wavelength detection still yields a good indication of the technology being used, be it lower-end red or 905nm (near-IR) lasers or higher-end, higher-power 1060nm (YAG) or newer 1550nm (IR, eye-safe) equipment.
The current HARLID™ module configuration uses 6-bits, and three reference channels (not shown in Figure 5) allowing the determination of the corresponding bit-level (logical “0” or “1”) through direct comparison to neighboring reference channel which remain illuminated for all AOA and helps offset any change in the atmospheric background illumination. The careful design of the 6-bit digital Gray code pattern allows equal angular steps for 64 intervals, which encodes the module’s ~90 degrees field of view (FoV) with a resolution of ±0.8 degrees.
Since both the high- and low-sensitivity channels are illuminated through a single mask, different binary codes are generated respectively for the same incident collimated beam. High- and lowsensitivity specific look-up tables (LUT) are used to determine the AOA (Figure 7). The highlighted bit in each AOAspecific digital code clearly shows that a single bit will vary as the incident laser moves towards normal incidence or viceversa through each of the 64 intervals.
Deployment Strategies and Electronics
Since each HARLID™ is meant to encode in a single axis at a time, most LWS systems use at least two HARLID™ modules, one to encode azimuth and the other elevation, by orienting them at 90 degrees from each other. One can envision using four HARLID™ modules for 360 degrees of azimuth coverage (with limited elevation data) or even eight modules to create a hemispherical detection dome structure set atop a vehicle.
Signal processing can be handled through an analog back-end, which converts each individual photocurrent into voltages through trans-impedance amplifiers (TIAs) and then monitors the signal levels of the reference channels to ease the discrimination of the “1” or “0” status of each bit. High-pass filtering allows the rejection of DC light sources and background light conditions. The electronics must detect individual pulses for typical PRF used in the field; a system designed with a slow response rate will distort and soften the rising edges of detected pulses, making the discrimination of logical state more complex and more prone to errors. Furthermore, the HARLID™’s multiple reference detectors ease discrimination in non-uniform illumination conditions (common for long–range illumination) and offsets the baseline environmental illumination level.
Comparator circuits (CCs) are used to set the logical state of each individual bit; it is highly dependent on the performance of a pulse detection circuit (PDC). The PDC synchronizes the output of the electronics with the arrival of each laser pulse and therefore properly latch the individual CC. Using a summing amplifier to average each of the reference channels, the PDC can produce a “detection pulse” which is fed to another CC with its threshold set above typical noise levels. The output of this last CC can then be used to correctly latch all the other CCs and, therefore, synchronize the reported binary code/AOA to the incoming laser pulses.
Future-Proof Design and Conclusions
The HARLID™ is truly a unique component that offers lots of opportunities for LWR system designers with little tradeoffs and can be customized and modified to follow new trends, such as new laser technologies, in the military market. While highly-integrated components such as the HARLID™ may seem daunting when first evaluated, the benefits far outweigh the required efforts needed for a thorough evaluation. Alternative AOAdetection strategies typically require multiple detectors positioned at specific locations across the LWS-equipped vehicle, more complex processing of signal that must be routed through the mainframe of said vehicle and triangulation algorithms that aim to effectively mimic the AOA-digitalization performed directly by the HARLID™ module.
This article was written by Éric Desfonds, Eng., Application Engineer, High Performance Sensors & Defense (Vaudreuil-Dorion, Canada). For more information, visit http://info.hotims.com/40432-500.
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