Laser Detection System Technology

April 1, 2012

While most ordnance is now laserguided, there is still much work to be done on the defensive detection of laser designators. Hence the growing need for advanced laser-warning systems (LWS). While multiple techniques can be used to detect and triangulate the position of an incident laser designator, one of the current front-runners in the race for positional accuracy is Excelitas’ High Angular Resolution Laser Irradiance Detector (HARLID™).

Figure 1. LWS-equipped helicopter responding with flares and smoke screen after AOA detection of incident laser beam designators.

HARLID is able to precisely pinpoint incoming threats in real-time allowing for the quasi-instantaneous deployment of semi- and automated-threat response mechanisms such as smoke screens or flares (Figure 1). Its unique digitalencoding of the angle of arrival (AOA) of well-collimated laser designators operating between 500 and 1650nm has received positive feedback from the men and women who depend on its operation on the front-line of modern-day armed conflicts.

How Does It Work?

Figure 2. HARLID™ TO-8 can dimensions (Gray code mask not shown, dimensions in inches).

Close-proximity combat remains the last resort for most strategic missions. Through the use of laser-guided artillery, soldiers can gain the upper hand in a combat situation while remaining, hopefully, out of harm’s way. The days of dropping large loads of “dumb” ordnance with the hope of inflicting sufficient damage to the target are over. Over the years, the LWS and Laser Range Finder (LRF) markets have thus taken the lion’s share of the military electro-optical (EO) market.

The HARLID™ effectively encodes digitally the AOA of incident laser beams from laser guidance systems, designators, beam riders and range finders; furthermore it enables the detection of “friend-or-foe” of said laser designator through Pulse Repetition Frequency (PRF) detection and laser technology used through coarse wavelength band detection.

Figure 3. HARLID™ Silicon and InGaAs sandwich-detector configuration.

Originally conceptualized by the Defence R&D Canada – Valcartier team and designed for manufacturing by Excelitas Canada Inc, the HARLID™ is often described as a low-weight, lowpower, self-contained LWS building block. Enclosed within a custom 20- pin TO-8 can (Figure 2), it can detect light from 500 to 1650nm using a Silicon-InGaAs sandwich-chip design (Figure 3).

Each PIN chip has eighteen-pixels, half of which are high-sensitivity channels, meant to detect lower incident powers, while the other half are low-sensitivity channels with smaller apertures and built-in optical attenuation of about 15dB , meant to detect higher incident laser power with reduced risk of saturating the back-end electronics. The detectors are designed with a guard ring to insure good signal-to-noise ratios (SNR) and a dynamic range of up to about 6 decades of incident power densities through overlapped coverage of both the high- and low-sensitivity channels.

Figure 4. HARLID™ TO-8 pin out details showing separation of high- and low-sensitivity channels, Hx and Lx respectively.

Each reference, high- and low-sensitivity channel can be accessed through individual pins (Figure 4). Both chips are metalized and laid out so that a single voltage bias is required to power up both chips. The PIN design is optimized to insure low capacitance and fast riseand fall-time of about 2ns, allowing the detection of the actual pulse shape and designator PRF. This compact rugged assembly was designed to meet a full range of military applications as validated by several MIL-STD-883 qualification efforts; amongst which the acceleration testing (Method 2001), mechanical shock (Meth od 2002), sine vibration (Method 2007), leak testing (Meth od 1014) and temperature cycling (Method 1010) were completed and exceeded successfully.

The projection of the incident laser beam through the HARLID™’s digital Gray code mask onto multiple channels in parallel, isolated from each other using individual “light guides”, depending on the angular displacement of the incident laser beam in a plane perpendicular to the longitudinal axis of the detector array, produces a linear shift of the representation of the apertures of the Gray code mask upon the individual pixels of the detector array and thus a different AOA-specific binary code (Figure 5). In other words, the HARLID™ module directly encodes and measures the AOA subtended by the projection through the shadowing Gray code mask of the incident collimated laser beam.

Figure 5. HARLID™ schematic (without reference channels) showing binary code 0001 for normal incidence AOA and 0110 for a different AOA.

The photocurrents generated from the silicon and InGaAs chips are merged through parallel wirebonding; therefore the combined responsivity curve of Figure 6 is expected for each channel, except the high-sensitivity reference channel 3 where the silicon and InGaAs photocurrents are routed to pins H10 and H8 respectively. The silicon chip is responsive from 500 to about 1100nm and the InGaAs chip from about 800 to 1650nm, with a transition region around 1060nm or the YAG-region.

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.

Figure 6. Combined responsivity of Silicon and InGaAs sandwich detector as a function of wavelength.

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

Figure 7. Subset of HARLID™ look-up tables (LUT) for high- and low-sensitivity channels, and alwaysilluminated reference channels.

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, Click Here .