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.

Photon counting chirped AM Ladar Architecture.
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.

An alternate configuration for the proof-of-principle (POP) ladar 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.

To test the OEM configuration the laboratory setup shown in the figure was modified by removing the sub-nanosecond pulse generator and microwave mixer. The sinusoidal LO signal was then put into the trigger input of a pulse/signal generator to produce a chirped square wave between 0 and 5 V amplitude. This square wave LO was applied to the gate input of the SPCM. Since the minimum gate duration for the SPCM is 50 ns, the maximum chirp frequency that can be used in this setup is about 10 MHz. Most of the microwave components in the setup have a frequency response that starts to roll off below 10 MHz. Thus, the useful chirp bandwidth is very limited for this setup in the OEM configuration.

Based on the chirp bandwidth, chirp duration, and delay time, the predicted frequency of the peak in the IF spectrum is 145.5 Hz. For this experiment, the number of signal counts is 265,000 and the number of background counts is 700. The predicted electrical power SNR for these parameters is 42.18 dB, and the measured electrical power S/N is 36.42 dB, which differs from the theoretical prediction by –5.76 dB. This corresponds to a factor of about 4× lower electrical power S/N and about 2× lower electrical current/ voltage S/N than theoretically predicted. The source of S/N loss in this experiment is currently unknown but may be due to incomplete modulation of the laser, excess noise caused by gating the SPCM at high frequencies, excess noise from the IF amplifier, excess noise during the "flyback" time between chirps, and/or peak spreading due to anomalies in the chirp.

The two prior ladar embodiments demonstrated the sensitivity advantage of using the GM-APD over the unity gain MSM detectors. A problem with using the GM-APD is that the dead time limits the bandwidth of the chirp modulation which, in turn, limits the ladar range resolution. For most of the applications considered for ladar, such as imaging objects the size of a military vehicle, a range resolution of 0.25 m is desirable. This resolution requires a chirp bandwidth of 600 MHz. To obtain these higher chirp bandwidths, placing an electro-optical modulator (EOM) driven by the LO signal in the light path before the GM-APD is suggested.

This will convert the incoming microwave modulated light into a light signal containing an IF component that can be "sampled" by the GM-APD. Because the IF in most chirped AM ladar applications will be below 1 MHz, the dead time of the GM-APD is not usually an issue. Quantum-well EOMs are built at the near and short wave IR bands with several gigahertz of bandwidth, and Mach-Zender EOMs are built at 1.55 μm with up to 40 GHz of bandwidth. Use of these devices can lead to range resolutions in the low millimeter regime.

This work was done by Brian C. Redman and Barry L. Stann of the Army Research Laboratory.