Photon Counting Chirped Amplitude Modulation Ladar

Army Research Laboratory

Monday, December 01 2008

Ladar applications include camouflage penetration, target ID, manned and unmanned ground and air vehicle navigation, and 3D face recognition.

This work is a follow-up to prior efforts to develop a method using Geiger-mode avalanche photodiode (GM-APD) photon counting detectors in chirped amplitude modulation (AM) ladar receivers 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, opto- electronic mixing (OEM) metal-semiconductor-metal (MSM) detectors. These sensitivity improvements may enable compact, low-power, eye-safe, and/or long-range ladar with low-cost, low-bandwidth readout integrated circuits for foliage and camouflage penetration, target ID, manned and unmanned ground and air vehicle navigation, 3D face recognition, battle damage assessment, and change detection.

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.

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.

A block diagram of one embodiment of the chirped AM ladar with a GM detector is shown in the figure. Chirped modulated laser light is transmitted toward the target where some of the light is reflected back to the ladar. On the return path, the chirped AM waveform is preserved, with a round-trip time shift, so that the mean photon arrival rates at the receiver are modulated with the time shifted chirp waveform. The GM-APD’s output count pulse edge triggers a short pulse generator to output a short pulse of a duration that is less than or equal to 1/(4.fchirp_max), where fchirp_max equals the maximum frequency in the chirp waveform. The resulting arrival rate modulated short pulses are mixed with a radio-frequency local oscillator (LO) having the same chirp waveform as the transmitter to produce a series of random pulses with mean arrival rates modulated by the product of the LO and received light modulation waveforms, i.e., the intermediate frequency (IF) waveform. Low-pass (or band pass) filtering the mixer output yields a sinusoid with a frequency proportional to the round-trip time between the ladar transceiver and the target. Digitizing the IF waveform and taking the magnitude of the fast Fourier transform (FFT) of the data produces the IF magnitude spectrum for which there is a peak at a frequency proportional to the round-trip time with an amplitude proportional to the mean return signal.