Tech Briefs

Sensitivity improvements may enable low-power, eye-safe ladars for camouflage penetration, target identification, unmanned vehicle navigation, and face recognition.

This work is a follow-on to an effort to develop a method using Geiger-mode avalanche photodiode (GM-APD) photon-counting detectors in the U.S. Army Research Laboratory’s chirped amplitude modulation (AM) ladar receiver to yield sensitivities approaching the shot noise limit. Such sensitivities represent about four orders of magnitude improvement over the sensitivities of the currently used unity-gain, optoelectronic mixing (OEM) metal-semiconductor-metal (MSM) detectors. A variant of the chirped AM ladar has been experimentally assembled and tested, and new single photon-counting detector products were evaluated in terms of their benefits to the chirped AM ladar.

Figure 1. The original Photon-Counting Chirped AM Ladar architecture with a GM detector.
Although for a single photon detection, the output voltage of a GM-APD single photon-counting module (SPCM) is a count pulse of constant amplitude that is not proportional to the light power, the AM waveform can be recovered since the mean arrival rate of photons at the detector is proportional to the light power, even though individual photon arrivals are randomly distributed. Thus, the mean photon arrival rate and, therefore, the photon count rate output by a GM-APD SPCM will be modulated by an amplitude modulation of the light power. This process is akin to the use of pulse position modulation to convert analog amplitude signals to digital data streams in digital telecommunications systems.

Figure 2. The alternate configuration of the Ladar With OEM.
The constant amplitude pulse from a GM-APD photon-counting module has a duration equal to the quenching time of the quenching circuit following the GM-APD; this usually dominates the GM-APD dead time. Typically, the dead time can be from tens of nanoseconds to several microseconds, although shorter dead times are attainable with specially designed quenching circuits. The rise time of the count pulse, however, is typically sub-nanosecond. This sets the upper limit of the photon counting receiver bandwidth and, therefore, the minimum achievable timing/range resolution. The inverse of the dead time sets the upper limit on the photon arrival rate since subsequent photons incident on the receiver in times less than the dead time from the arrival of the previous photon will not produce a count pulse. This results in errors in the measurement of the arrival rate modulation.