Frederic M. Davidson

Gaussian approximation versus nearly exact performance analysis of optical communication systems with PPM signaling and APD receivers

A 25 Mbit/s direct-detection optical communication system that used Q=4 PPM signaling was constructed and its performance measured under laboratory conditions. The system used a single mode AlGaAs laser diode ( lambda =834 nm) and low-noise silicon avalanche photodiode (APD). A procedure is given to numerically compute system performance which uses the nearly exact Webbs approximation of the true Conradi distribution for the APD output that does not require excessive amounts of computer time (a few CPU minutes on VAX 8600 per system operating point). Comparison revealed that modeling the APD output as a Gaussian process under conditions of negligible background radiation and low (less than 10/sup -12/A) APD bulk leakage currents leads to substantial underestimates of optimal APD gain and overestimates of system bit error probability. Examples are given which illustrate the breakdown of the Gaussian approximation in assessing system performance. The measured performance of the system was found to be in excellent agreement with the performance predicted by the nearly exact computational procedure. This system achieved a bit error probability of 10/sup -6/ at a received signal energy corresponding to an average of 60 absorbed photons/bit and optimal APD gain of 700. >

Fiber-coupling efficiency for free-space optical communication through atmospheric turbulence

High-speed free-space optical communication systems have recently used fiber-optic components. The received laser beam in such a system must be coupled into a single-mode fiber at the input of the receiver module. However, propagation through atmospheric turbulence degrades the spatial coherence of a laser beam and limits the fiber-coupling efficiency. We numerically evaluate the fiber-coupling efficiency for laser light distorted by atmospheric turbulence. We also investigate the use of a coherent fiber array as a receiver structure and find that a coherent fiber array that consists of seven subapertures would significantly increase the fiber-coupling efficiency.

Photon counting with silicon avalanche photodiodes

Avalanche photodiodes (APDs) are studied for use as photon-counting detectors. The APD may be biased slightly above (Geiger mode) or slightly below its voltage breakdown point. In the latter case, if the photon absorption rate is low enough, each individual photoelectron current pulse may be resolved with the use of a discriminator. APDs used in this photon-counting mode are shown to give the best performance at low light levels. Experimentally, overall photon detection probabilities of 5.0 and 0.33% were obtained at lambda =820 nm and lambda =1.064 mu m, respectively, with a photon counter dead time as low as 15 ns and a dark current counting rate of 7000/s. The APD photon counter exhibited an exponential photon interarrival time probability density and a near-Poissonian photon-counting probability. >

Interleaved convolutional coding for the turbulent atmospheric optical communication channel

The coding gain of a constraint-length-three, rate one-half convolutional code over a long clear-air atmospheric direct-detection optical communication channel using binary pulse-position modulation signalling is directly measured as a function of interleaving delay for both hard- and soft-decision Viterbi decoding. Maximum coding gains theoretically possible for this code with perfect interleaving and physically unrealizable perfect-measurement decoding were about 7 dB under conditions of weak clear-air turbulence, and 11 dB at moderate turbulence levels. The time scale of the fading (memory) of the channel was directly measured to be tens to hundreds of milliseconds, depending on turbulence levels. Interleaving delays of 5 ms between transmission of the first and second channel bits output by the encoder yield coding gains within 1.5 dB of theoretical limits with soft-decision Viterbi decoding. Coding gains of 4-5 dB were observed with only 100 mu s of interleaving delay. Soft-decision Viterbi decoding always yielded 1-2 dB more coding gain than hard-decision Viterbi decoding. >

Slot clock recovery in optical PPM communication systems with avalanche photodiode photodetectors

Slot timing recovery in a direct-detection optical PPM communication system can be achieved by processing the photodetector output waveform with a nonlinear device whose output forms the input to a phase-locked loop. The choice of a simple transition detector as the nonlinearity is shown to give satisfactory synchronization performance. The RMS phase error of the recovered slot clock and the effect of slot timing jitter on the bit error probability were directly measured. The experimental system consisted of an AlGaAs laser diode ( lambda =834 nm) and a silicon avalanche photodiode photodetector. The system used Q=4 PPM signaling and operated at a source data rate of 25 Mb/s. The mathematical model developed to compute the RMS phase error of the recovered clock is shown to be in good agreement with results of actual measurements of phase errors. The use of the recovered slot clock in the receiver resulted in no significant degradation in receiver sensitivity compared to a system with perfect slot timing. The system achieved a bit error probability of 10/sup -6/ at a received optical signal energy of 55 detected photons per information bit. >

Homodyne detection using photorefractive materials as beamsplitters

The use of two-wave mixing in photorefractive (PR) materials in a homodyne detection receiver structure is studied. The interaction between a strong local oscillator optical field and a weak signal optical field in the PR material results in the formation of a volume index of refraction phase grating. The grating automatically aligns such that it coherently couples the signal and local oscillator fields just like a fixed beamsplitter in a conventional homodyne receiver structure. The slow response of the PR material automatically compensates for slow changes in the angle of arrival of the signal beam but acts as a fixed grating with respect to rapid phase modulation of the signal beam. The performance of a homodyne receiver that uses a PR material as a beamsplitter, is shown to be up to 3 dB superior to the performance of a conventional homodyne receiver that uses a fixed 50-50 beamsplitter. Furthermore, a PR receiver can adjust and retain its optimal performance when changes in the received signal power occur, whereas a conventional receiver cannot.

Simple laser velocimeter that uses photoconductive semiconductors to measure optical frequency differences.

The dc photocurrents generated by steady-state moving space-charge fields inside photoconductive semiconductors containing deep level donors and traps can be used to determine the relative frequency differences between the two interfering optical fields that establish the space-charge fields. A simple laser velocimeter that uses a semi-insulating GaAs:Cr sample to detect the Doppler frequency shift between two laser beams is demonstrated.

Word timing recovery in direct detection optical PPM communication systems with avalanche photodiodes using a phase lock loop

A technique for word timing recovery in a direct detection optical pulse position modulation (PPM) communication system is described. It tracks on back-to-back pulse pairs in the received random PPM data sequences with the use of a phase locked loop. The experimental system consisted of an AlGaAs laser diode transmitter ( lambda =833 nm) and a silicon avalanche photodiode photodetector, and its used Q=4 PPM signaling at a source data rate of 25 Mb/s. The mathematical model developed to characterize system performance is shown to be in good agreement with the experimental measurements. Use of this recovered PPM word clock, along with a slot clock recovery system described previously, caused no measurable penalty in receiver sensitivity when compared to a receiver which used common transmitter/receiver clocks. The completely self-synchronized receiver was capable of acquiring and maintaining both slot and word synchronizations for input optical signal levels as low as 20 average detected photons per information bit. The receiver achieved a bit error probability of 10/sup -6/ at less than 60 average detected photons per information bit. >

Coherent optical detection through two-wave mixing in photorefractive materials.

Photorefractively induced index-of-refraction phase gratings are shown to combine coherently the optical fields of a strong pump and a suitably amplitude- or phase-modulated signal beam in such a way that an apparent amplification of the modulating waveform appears as intensity modulation of the transmitted signal and pump beam intensities. The source of the signal gain is shown to be the square-law (intensity) detection of the coherently combined pump and modulated signal beams, just as in coherent optical communication systems in which a strong local-oscillator field is coherently added with a weak optical signal field by a beam splitter.

Photorefractive two-wave mixing in the presence of high-speed optical phase modulation

Equations that describe the steady-state dependence of the coherent-coupling properties of photorefractively induced refractive-index gratings on high-speed periodic biphase, sinusoidal, and triangular phase modulation impressed on one of the input optical beams are found and solved for both depleted and undepleted pump conditions. The period of the phase modulation wave form was kept short compared with the grating-formation time but did not cause significant spectral broadening. The results obtained were verified with data obtained from measurements of two-wave mixing in the photorefractive material, InP:Fe.