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The availability and reliability of the high data rate space-to-ground optical coherent communication links are critically challenged because the wave front of the signal beam is distorted and impaired when propagating through atmospheric turbulence. Based on the free-space interference of two successive data bits delayed by an unequal-arm-length Mach-Zehnder interferometer (MZI), a pupil-matching time-delay self-homodyne interference differential phase-shift keying (DPSK) optical receiver with 2 × 4 90-deg optical hybrid is designed for the high data rate space-to-ground optical communication links due to its immunity from wave front impairment. The delayed optical path difference (OPD) in the unequal-arm-length MZI corresponds to the duration of one bit and can be stabilized to below 1000th of the wavelength by the closed-loop feedback control. The maximum system insertion loss is <1 dB. The measurement accuracy of OPD in unequal-armlength MZI is 0.01 mm by the deramping method from a chirped laser. The double-bit-rate 2.5-/5-Gbps DPSK optical receiver has been presented. Parallel and separate atmospheric measurement along the optical communication link is also performed simultaneously. The 2.5-and 5-Gbps optical communication links have already been verified with a ϕ300-mm receiving telescope between two buildings in downtown over a distance of 2.4 km in the worst-case atmospheric conditions. The measured bit-error-rate is better than 10-6. Without wave front compensation of the adaptive optics, local oscillator, and optical phase-locked loops, the pupil-matching time-delay self-homodyne interference DPSK optical receiver has great significance for future developments of space-to-ground optical communication links. [ABSTRACT FROM AUTHOR]

In this paper is described a novel technique for producing an electro-optical intensity synthesizer which can generate different periodic time domain waveforms through only sine or cosine wave applied-voltages. The synthesizer presented here consists of a series of stages between two polarizers, with each stage consisting of an electro-optic element and a compensator. Every electro-optical element has the same applied-voltage function but different azimuth angles and ratios between the longitudinal and transverse lengths. The main principle is the synthesis of an electro-optic effect and a polarization interference effect in the time domain. This technique is based on an expanded Fourier positive-direction searching algorithm, which can not only simplify the calculation process but also produces many choices of structural parameters for different waveforms generation. A three-stage synthesis of an electro-optical birefringent system for continuous square waveform is undertaken to prove the principle. [ABSTRACT FROM AUTHOR]

The phase contrast across the crystal thickness induced by the internal field is measured by the digital holographic interferometry just after the congruent lithium niobate crystal is partially poled. The direction of applied external field is antiparallel to that of internal field, and the measured phase contrast varies linearly with the applied external field. A new internal field is obtained by this method and named effective internal field. The distinct discrepancy between effective and equivalent internal fields is observed. The authors attribute this effect to the new macroscopic representation of elastic dipole components of defect complex in the crystal. [ABSTRACT FROM AUTHOR]

Laser beam angular scan accuracy measurement plays an important role in many fields. A sub-microradian laser beam scan precision measurement method that utilizes the wavefront interference method with a Zygo interferometer is proposed. The experimental results show that the measurement accuracy is approximately 0.04 rad, and the new method is very effective and feasible. [ABSTRACT FROM AUTHOR]