Abstract:
We demonstrate a photon-counting communications link using Geiger-Mode avalanche photodiodes. High-efficiency pulse-position modulation and forward error correcting codes are used to demonstrate a receiver sensitivity of 1.5 incident photons/bit.

Introduction
Future deep-space optical communications links, such as the NASA Mars Laser Communications Demonstration [1], will require optical receivers capable of reliably receiving multiple bits per detected photon [2]. Such performance greatly exceeds the capabilities of conventional telecommunications optical receivers which typically require more than 10 photons/bit (PPB). Recently, low duty-cycle modulation formats, such as pulse-position modulation (PPM), have been used together with moderate-overhead forward error correction coding to demonstrate an optically-preamplified direct-detection receiver with 4 PPB sensitivity [3]. Even better performance is possible using a photon-counting receiver.
Until recently, use of photon-counting receivers for optical communications has been limited due to the poor detection efficiency of photon-counting detectors. However, recent advances in Geiger-mode avalanche photodiodes (GM-APDs) have made them promising candidates for photon-counting detectors. GM-APDs operating at 1.06 µm and 1.55 µm are being developed with detection efficiencies greater than 50%, timing resolution less than 1 ns, and dark count rates less than 100 kHz [4]. One limitation which still impacts performance when GM-APDs are used for optical communications is the relatively long reset time requirement, which requires the detector to be turned off for a period of time before it can be re-armed after a detection event. During the reset time, the detector is inactive and incident photons are not detected. GM-APDs operating at infrared wavelengths typically require reset times of tens of microseconds. Using conventional pulsed modulation formats, such as on-off keying, each detection event can represent at most 1 bit. Thus, using a single GM-APD detector with a 30 µs reset time would limit the achievable data rate to ~ 33 kbit/s. However, using higher-order modulation formats, such as PPM, multiple bits can be received for each detection event. In the present work, we demonstrate more than 2 received bits per detected photon using 64-ary PPM.

Figure 1: Experimental setup for demonstration of high-efficiency photon-counting communications using GM-APDs.

Figure 2: Measured communications performance.

Experiment
Figure 1 shows the experimental setup for demonstrating high-efficiency optical communications using a gated-mode PPM transmitter with a GM-APD-based receiver. The transmitter encodes the source data using a 1/2-rate turbo code for forward-error correction [5]. The coded data stream is used to intensity-modulate the output of a 1.55 µm DFB laser using 64-ary pulse-position modulation at 200 Mslot/s. Thus, each pulse conveys 6 channel bits (3 source bits) of information. For gated-mode operation, one pulse is transmitted for each GM-APD reset time (30 µs), yielding a source data rate of 100 kbit/s. Due to the extremely low duty cycle of the modulation waveform (~1 pulse every 3200 slots), two intensity modulators are cascaded in order to provide the high extinction-ratio necessary to ensure that most of the power at the output of the transmitter is in the pulsed slots. At the output of the modulator cascade, less than 0.1 dB of the total power is in the empty slots.
The receiver consists of a single commercially-available fiber-coupled 1.55 µm GM-APD. At the output of the GM-APD, the detected signal is PPM demodulated. The output of the PPM demodulator may either be input directly to a bit-error rate tester for uncoded performance measurement or captured and decoded using a software turbo decoder to demonstrate the performance of the forward error correcting code.

Results
Figure 2 shows an example of the measured bit-error rate performance of the GM-APD-based receiver as a function of the incident photon flux, measured at the input fiber facet of the fiber-coupled APD. For this demonstration, the GM-APD was biased at 47V at a temperature of -40C, giving a dark count rate of 428 kHz and a detection efficiency of 28%. An error floor of ~4 x 10-2 is observed in the uncoded data. This error floor is a consequence of the noise characteristics of the detector—for gated-mode PPM, there is a finite probability that the detector will register a dark count before the incident pulse arrives. Such detection events will cause errors that cannot be mitigated by increasing the optical power incident on the receiver. No such error floor is observed when the 1/2-rate turbo code is employed. In this case, the receiver output is error-free for received powers in excess of 1.8 dB PPB (1.5 PPB). This performance represents a 6.9 dB penalty from the optimal capacity for this channel. The 5.5-dB penalty introduced by the 28% detection efficiency of the GM-APD accounts for most of measured penalty in performance. The remaining 1.4-dB penalty is due to coding and implementation losses. Note that after accounting for losses due to detection efficiency, the receiver provides error-free performance with -3.7 dB detected PPB, or, 2.34 bits per detected photon.

Summary
In summary, we have demonstrated a photon counting optical communication link using commercially-available Geiger-mode APDs operating at a wavelength of 1.55 µm. A receiver sensitivity of 1.5 incident PPB was demonstrated using high-efficiency PPM modulation together with a 1/2-rate turbo code. To the best of our knowledge, this is the most sensitive optical receiver reported to date. Moreover, this demonstration shows that even with current limitations in the detection efficiency of photon-counting detectors, photon-counting receivers can perform better than competing technologies in terms of overall receiver sensitivity.

References
[1] B. L. Edwards, et al., “Overview of the Mars laser communications demonstration project,” American Institute of Aeronautics and Astronautics, Space 2003 Conference and Exposition, Sept. 2003.
[2] D. M. Boroson, R. S. Bondurant, J. J. Scozzafava, “Overview of high rate deep space laser communications options,” SPIE Proceedings, vol. 5338: Free-Space Laser Communications Technologies, pp. 37-49, June 2004.
[3] D. O. Caplan, B. S. Robinson, R. J. Murphy, and M. L. Stevens, “Demonstration of 2.5-Gslot/s optically-preamplified M-PPM with 4 photons/bit receiver sensitivity,” Optical Fiber Communications Conference, PDP32, 2005.
[4] K. A. McIntosh, et al., “Arrays of III-V Semiconductor Geiger-mode Avalanche Photodiodes,” LEOS Annual Mtg., WZ1, 2003.
[5] B. Moision, J. Hamkins, “Low complexity serially concatenated codes for the deep space optical channel,” IEEE International Symposium on Information Theory, June 2003.
1 This work was sponsored by the National Aeronautics and Space Administration under Air Force Contract #F19628-00-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government.