LASER RADAR Imaging

A prototype active imaging laser-radar receiver incorporates an array of fiber-coupled multichannel receivers to acquire images from a single laser pulse. Developed for USAF Wright Laboratory Armament Directory (Eglin AFB, FL) by Buck Burns and Steve Yun at H. M. Burns Engineering Corp. (Orlando, FL), the receiver can acquire images at frame rates of 1 kHz, an order of magnitude faster than previously available systems.

Jun 1st, 1997

LASER RADAR Imaging

Receiver yields images from a single pulse

A prototype active imaging laser-radar receiver incorporates an array of fiber-coupled multichannel receivers to acquire images from a single laser pulse. Developed for USAF Wright Laboratory Armament Directory (Eglin AFB, FL) by Buck Burns and Steve Yun at H. M. Burns Engineering Corp. (Orlando, FL), the receiver can acquire images at frame rates of 1 kHz, an order of magnitude faster than previously available systems.

Conventional scanned laser-radar imaging receivers require multiple pulses to assemble a full image, and they suffer from jitter and image tearing caused by platform/target instability and environmental effects. The single-pulse approach eliminates these distortions, providing high-quality, high-speed range-based images.

The receiver consists of a focal-plane array formed of end-polished multimode fibers; each fiber acts as a light bucket, capturing the optical signal and relaying it to the detectors (see figure on p. 54). A multichannel-optical-receiver-photonic-hybrid (MORPH) circuit board incorporating an array of avalanche photodiodes (APDs) performs the actual optical detection and processing. Each column of fibers connects to a MORPH board, which carries an APD detector for each pixel in the column (see figure inset).

In operation, a laser transmitter--operating at 1.5 µm, in this case--sends out an optical pulse, starting the time-of-flight clock. The receiver`s optical system captures the return signal and relays it to the fiberoptic focal-plane array, which transmits the signal to the detectors. Multichannel application-specific integrated circuits (ASICs) clock time of flight for each channel, and the data are ported out to a computer, where they are converted to a range-based image.

Fiber coupling

To create the fiberoptic focal-plane array, columns formed of 105-µm-core-diameter, 125-µm-cladding-diameter multimode silica optical fiber laid side by side on 250-µm centers are ribbonized and laminated in Kapton to make a vertical fiber stack. These stacks are then mounted side by side in a focal-plane fiber-alignment block to create an array.

The optical ribbon cable is linked to the MORPH board by an injection-molded, glass-filled composite optical connector with losses of less than 1 dB. Each fiber in the ribbon connects to a fiber on the board that carries the optical signal to the corresponding APD. The 200-µm-diameter indium gallium arsenide (InGaAs) APDs are sensitive to 1.5-µm input, with a responsivity of 9.4 A/W. A custom ceramic ferrule and microcage assembly permit fiber-to-detector alignment while maintaining the proper spacing between the detector and the fiber endface. A transimpedance amplifier converts the APD photocurrent into a voltage, and a threshold comparator registers detection to sto¥the time-of-flight counter.

To demonstrate the approach, Burns Engineering is building a 1 ¥ 16-array prototype. The 16-pixel focal-plane column connects to a single 16-channel MORPH board, and a pair of eight-channel ASICs record time-of-flight data for the detectors. As configured, the dual-clock-speed system features a maximum range of 5 km with a down-range resolution of just greater than 7.5 cm; the clock can be reconfigured to yield a 10-km range with 15-cm resolution.

The fore optics in the current system consist of an f/3 achromat with a 75-mm clear aperture and a 150-mm focal length. This yields a single-pixel field of view of approximately 400 µrad and an interpixel angular subtense of 1 mrad, giving a transverse spatial resolution of approximately 5 m at the maximum range of 5 km.

The modular design can be easily expanded to a 25 ¥ 25 or larger array, while the 3 ¥ 5-in. MORPH boards hel¥kee¥the system compact. In addition to the InGaAs APDs, the receiver is also compatible with silicon APD or PIN diode detectors and can be used with any suitable laser transmitter.

This work was presented in paper #3065-04 at the SPIE AeroSense meeting (April 20-25, Orlando, FL).

Kristin Lewotsky

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