Christopher T. Field

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.

Photocurrents in photoconductive semiconductors generated by a moving space-charge field

The behavior of photocurrents generated in the semiconductors InP:Fe, GaAs:Cr, and undoped GaAs inoptical two-wave mixing experiments as a function of the frequency difference between the two waves is characterized and verified experimentally. The sign, the concentration, and the ratio gammaR/micro of the dominant charge carriers and the effective concentration of trapping centers can be found from this dependence.

50-Mbps free-space direct detection laser diode optical communication system with Q=4PPM signaling

A 50 Mbps direct detection optical communication system for use in an intersatellite link was constructed with an AlGaAs laser diode transmitter and a silicon avalanche photodiode photodetector. The system used a Q = 4 PPM format. The receiver consisted of a maximum likelihood PPM detector and a timing recovery subsystem. The PPM slot clock was recovered at the receiver by using a transition detector followed by a PLL. The PPM word clock was recovered by using a second PLL whose input was derived from the presence of back-to-back PPM pulses contained in the received random PPM pulse sequences. The system achieved a bit error rate of 0.000001 at less than 50 detected signal photons/information bit. The receiver was capable of acquiring and maintaining slot and word synchronization for received signal levels greater than 20 photons/information bit, at which the receiver bit error rate was about 0.01.

Coherent homodyne optical communication receivers with photorefractive optical beam combiners

The principles of operation and the results of performance measurements are reported of a new type of coherent optical receiver that used a dynamic volume index of refraction grating formed inside a photorefractive material to coherently combine signal and local oscillator light prior to photodetection. Because the refractive index grating is formed by the interference pattern generated where mutually coherent optical beams overlap, the receiver can automatically adjust to changes in angle of arrival or optical wavefront profiles which occur on time scales longer than the grating formation time. The grating appears stationary to high-speed phase modulation imposed on the signal beam and coherently diffracts local oscillator light into the signal beam direction. Performance measurements are reported for a prototype system that used two independent Nd:YAG lasers at 1.064 /spl mu/m, an iron-doped indium phosphide photorefractive crystal, and a four-slot phase modulation signal format. A receiver BER of 10/sup -6/ was obtained at received signal powers that corresponded to an average of 70 detected signal photons per bit at a source data rate of 50 Mb/s, 130 detected signal photons/bit at 220 Mb/s, and about 400 detected signal photons/bit at a 325 Mb/s source data rate. Quantum-limited operation corresponds to an average of six detected signal photons per transmitted information bit for this signal format. >

50 Mbps optical homodyne communication receiver with a photorefractive optical beam combiner

Performance measurements are reported for a coherent homodyne optical communication receiver that contained an iron-doped indium phosphide photorefractive beam combiner, rather than a conventional optical beam splitter. The system attained a bit error probability of 10/sup -6/ at received signal powers corresponding to about 75 detected photons per bit. No precision beam alignment or mode-matching optics were required, and tracking of slow phase changes between signal and local-oscillator light was performed automatically by the dynamic nature of the photorefractively formed refractive-index grating. The system used phase-modulated Nd:YAG laser light at lambda =1.06 mu m.<<ETX>>

Optical phase lock loop with a photorefractive optical beam combiner

Two narrow linewidth Nd:YAG unidirectional nonplanar ring oscillator lasers were electronically phase locked using photorefractive two-wave mixing in an iron-doped indium phosphide (InP:Fe) crystal as an optical phase detector. The phase lock loop phase error signal was derived from a low-amplitude sinusoidal phase modulation signal impressed on the strong local oscillator beam at a frequency large compared to the inverse of the photorefractive-grating formation time. A 60-mW pump laser was phase locked to a 10-nW signal laser beam with an rms phase error less than 0.1 rad. No spatial mode matching, phase conjugate mirror configuration or optical coupling between the two laser cavities were required.<<ETX>>

Frequency acquisition with photorefractive two-wave mixing

We describe how two-wave mixing of light scattered at the entrance face of a photorefractive crystal can be used to measure the frequency difference between two independent lasers. This method can be used to accomplish initial frequency acquisition in coherent optical homodyne receivers.

Coherent optical homodyne receiver performance with an iron-doped indium phosphide photorefractive beam combiner

Performance measurements are reported of a coherent homodyne optical communication receiver that contained an iron doped indium phosphide photorefractive beam combiner, rather than a conventional optical beam splitter. The system attained a bit error probability of 10 exp -6 at received signal powers corresponding to less than 100 detected photons per bit. The system used phase modulated Nd:YAG laser light at a wavelength of 1.06 micron.