An Optical 2×4 90° Hybrid Based on a Birefringent Crystal for a Coherent Receiver in a Free-Space Optical Communication System

The design and fabrication of an optical 2×490°hybrid based on birefringent crystals for a coherent receiver in a free−space optical communication system are presented.For the quadrature receiver,two pairs of 180°phase−shift outputs are obtained and one pair has a phase difference of 90°with respect to the other.The 90°hybrid comprises two pairs of stacked birefringent plates,a phase retardation plate and an analyser birefringent plate.The testing results measured by the heterodyne method verify that the 2×4 optical 90°hybrid can work correctly and effectively.The phase compensation and further optimization schemes are also proposed.

Experimental research of circular incoherently synthetic aperture imaging ladar using chirped-laser and heterodyne detection

Synthetic aperture imaging ladar （SAIL） technique belongs fully coherent processing in both the time domain and space domain and has a rather high implement difficulty. To solve this problem, the concept of circular incoherently SAIL is introduced. A speckle version image of a two-dimensional （2D） letter ＇E＇ target is reconstructed from E-field projection data detected by a circular incoherently SAIL system. The experimental system is constructed by three subsystems using chirped-pulse laser as the light source and heterodyne detection to get the range information of the target. The reconstruction of the image and the noise effect are also discussed in detail.

Experimental study on minimum resolvable velocity for heterodyne laser Doppler vibrometry

A high spatial resolution, high velocity resolution all-fiber laser Doppler vibrometry （LDV） based on heterodyne detection for vibration measurements is reported. A linewidth of 1-kHz single-mode continuous fiber laser, polarization-preserving fiber, and a telescope with 30-ram aperture are used in this LDV. With the inphase-quadrature circuit and the digital differentiating discriminator, a high velocity resolution of 96.9 nm/s and a high displacement resolution of 2.5 pm are obtained simultaneously with a glass attached to a piezoceramic transducer. These values correspond to the measurement uncertainties of vibration velocity and displacement within 4.14% and 4.6%, respectively.

Application of phase retrieval algorithm in reflective tomography laser radar imaging

We apply phase retrieval method to align projection data for tomographic reconstruction in reflective tomography laser radar imaging. In our experiment, the target is placed on a spin table with an unknown, but fixed, axis. The oscillatory motion of the target in the incident direction of the laser pulse is added at each view to simulate the real satellites random motion. The experimental simulation results demonstrate the effectiveness of this method to improve image reconstruction quality. Future research also includes the development of projection registration based on phase retrieval for targets with more complicated structure.

Semiconductor saturable-absorber mirror passively Q-switched Yb:YAG microchip laser

A compact passively Q-switched Yb：YAG microchip laser is demonstrated. Featuring a semiconductor saturable-absorber mirror （SESAM）, the laser yields pulses of 219 ps when the length of the microchip Yb：YAG crystal is 100 #m and the beam quality is M2 〈 1.3. To the best of our knowledge, pulses from the proposed laser are the shortest Q-switching pulses obtained from Yb：YAG microchip lasers currently available.

Dual-wavelength f iber grating laser in linear overlapping cavity

We report a switchable dual-wavelength fiber grating laser in linear overlapping cavity. The laser features two overlapping cavities sharing a single Yb-doped gain medium fiber and two sets of fiber Bragg gratings. A coiling fiber setup is inserted into the 1035 nm laser cavity. Given that the bending loss is inversely related to the bending radius, the cavity loss of 1035-nm can be modulated. Modulating the bending loss facilitates the switching of the fiber grating laser to a single-or dual-wavelength output at 1030 or 1035 nm and convenient tuning of the power ratio of the two wavelengths. An approximately 152.6-mW output power and up to 38-dB polarization extinction ratio are observed. The simultaneous lasing at I030 and 1035 nm is a qualified seeder source for amplification to high power scale and can be applied to difference frequency generation of a terahertz signal. This dual-wavelength fiber grating laser is a potential pump source for generating terahertz radiation using a novel approach.