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.

A block diagram of one embodiment of the chirped AM ladar with a GM detector is shown in Figure 1. 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. As shown in Figure 1, 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.

In the follow-on work, an alternate configuration for the POP ladar, shown in Figure 2, was assembled. Here, the LO modulates the excess bias voltage above and below the GM-APD’s breakdown bias voltage to cause OEM with the LO. In this OEM configuration, the detector’s minimum gate duration must be less than one-half of the reciprocal of the highest frequency in the chirp waveform, and the maximum gate repetition rate must be at least equal to the highest frequency in the chirp waveform. In the OEM configuration, the output of the SPCM will have an envelope that is modulated with the IF waveform recovered by low-pass or bandpass filtering.

As usual, digitizing the IF waveform and taking the magnitude of the 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.

Modifying the ladar architecture is suggested by placing an OEM in the receive light path to down-convert the microwave modulated light signal returning from the target into a low-frequency IF signal, thus eliminating the problem caused by dead time. Also noted is a new photon-counting module that will move the operating wavelength into the eye-safe band, simplify the design, and most likely improve the performance by allowing the ladar to operate over a high dynamic range not possible with the existing SPCMs.

This work was done by Brian C. Redman of Lockheed Martin Coherent Technologies and Barry L. Stann of the Army Research Laboratory. ARL-0045

This Brief includes a Technical Support Package (TSP).
Photon-Counting Chirped Amplitude Modulation Ladar

(reference ARL-0045) is currently available for download from the TSP library.

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