Enwen Dai

Phase mapping of domain kinetics in lithium niobate by digital holographic interferometry

The phase mapping of domain kinetics under the uniform steady-state electric field is achieved and investigated in the LiNbO3 crystals by digital holographic interferometry. We obtained the sequences of reconstructed three-dimensional and two-dimensional wave-field phase distributions during the electric poling in the congruent and near stoichiometric LiNbO3 crystals. The phase mapping of individual domain nucleation and growth in the two crystals are obtained. It is found that both longitudinal and lateral domain growths are not linear during the electric poling. The phase mapping of domain wall motions in the two crystals is also obtained. Both the phase relaxation and the pinning-depinning mechanism are observed during the domain wall motion. The residual phase distribution is observed after the high-speed domain wall motion. The corresponding analyses and discussions are proposed to explain the phenomena.

An aperture-matched phase-compensated differential phase shift keying receiver with a 90° hybrid

Space to ground laser communication is limited by the effects of atmosphere. Differential phase shift keying (DPSK) is suited for high data rate space to ground communication links due to its immunity of the wave front of a beam passing atmospheric turbulence, which carry the information in optical phase changes between bits. The benefit of DPSK over OOK is the 3 dB lower optical signal to noise ratio to reach a given bit error rate (BER) using a balanced receiver. An aperture-matched phase-compensated DPSK receiver with a 90° hybrid is proposed here. The receive optics are based on free-space optics. A Mach-Zehnder delay interferometer is used for differential delay which is equal to the bit period. The input pupil is imaged onto the same place by two afocal systems composed of 4f imaging lenses. The optical path difference is stabilized to a faction of the wavelength with a fine phase adjustment which is measured by a 2×4 90° optical hybrid and closed loop electric circuit. The design and experiments are given in this paper. optical hybrid and closed loop electric circuit. The design and experiments are given in this paper. optical hybrid and closed loop electric circuit. The design and experiments are given in this paper.

Control analysis of acquirement and locking in inter-satellite laser communications

Process of acquirement and locking-up of compound axis system in inter-satellite laser communication has been studied. The effect of different condition parameters on the process of acquirement and locking-up have also been researched and simulated. Simulation results show that when the system with appropriate bandwidth has been adopted, both fine pointing system and coarse pointing system can satisfy three requirements of step response, stability criterion and dwell time and then finish the acquirement and locking-up of beacon laser beam. The simulation results provide the suited condition parameters for both fine pointing system and coarse pointing system, which is very helpful to the subsequent point and tracking processes.

A new method for acquiring the complex hologram in optical scanning holography

In this paper, we propose a new method that can acquire the complex hologram of a 3D object for optical scanning holography (OSH) by using polarized optical elements and a 90 degrees 2×4 optical hybrid. The 3D object is scanned by a wave front combined by a plane and spherical wave with orthogonal polarization. The transmitted/reflected signal light is divided into two beams by a polarized beamsplitter (PBS). Then after the polarization control, the two separated beams are heterodyne detected by a 90 degrees 2×4 optical hybrid with two balanced detectors. The 3D object can be reconstructed from the complex hologram acquired from the balanced detectors. Compared with the conventional OSH, the acousto-optic modulator and complex electronic system are not needed in this method. Another advantage of this method lies in its high efficiency since there is almost no energy loss in the generation of the scanning beam by using the polarized optical elements. This method should be used for 3D holographic microscopy and 3D holographic television.

Laboratory demonstration of Fresnel telescope imaging ladar

In this paper, we present a laboratory demonstration of Fresnel telescope imaging ladar system for imaging the faraway objects with high resolution. Two concentric and coaxial quadratic wavefront with orthogonal polarization are used as scanning beams to illuminate the target. The scattered light from the target is heterodyne detected by a 90 degree 2×4 optical hybrid with two balanced receivers. The target image can be reconstructed by digital processing of the output signals of the balanced receivers. Point targets 4.3m away are reconstructed with high resolution in experiments.

Efficient beam combining with high brightness of a phase-locked laser array

We proposed a novel beam combining technique for coherent laser arrays that uses a conjugate Dammann grating (CDG) and a phase plate. By using this technique, a coherent laser array can be not only coherently combined to a single beam but also aperture filled to a far field single central lobe beam by means of phase compensation in theory. The aperture size and power density of the combined beam can be conveniently changed. Moreover, the technique applies for both binary and continuous amplitude profiles emitted from an individual laser of laser arrays, especially Gaussian beam. An verified experiment with a simulated 5×5 coherent laser array using an aperture mask was performed that effectively verified the feasibility of the proposed concept. This new method has the potential for scaling to much higher output powers with good beam quality by coherent combination and aperture filling of coherent laser arrays in practical applications.

Large-optics shearing interferometer for the wavefront sensing of widely tunable laser

There is large aperture and diffraction-limited laser beam in the space laser applications such as laser communication. Laser wavefront can be measured by shearing interferometer or Shark-Hartmann sensor etc. Large-Optics shearing interferometer based on Mach-Zehnder plate structure has been manufactured to differentially analyze the laser wavefront from two parts of aperture-divided fringes. One of six optical plates of the interferometer is divided to the up part and the down. The precision of measurement is higher than the full aperture design. It is suitable for the diameter below 290mm with the changeable shear amount from 1mm to 150mm. There are two sets of collimators used for the parallel of the plates. Another single-mode 635nm laser collimator which is measured through double-shearing plate before is serving as a standard wavefront of 150mm diameter. One path of the interference is changed with precise adjustment unit in several microns that the interference may be happened between equal optical path reflection and the other. It can be used for widely tunable laser and other laser system which has short coherent length. The apparatus and the experiments are explained in detail in this paper. Many systems of different quality and diameter and coherent length are measured by the large-optics shearing interferometer. The experimental wavefront results are fitted to Zernike polynomial and the Zernike coefficients are derived.

Study of nonlinear phase error correction technique for synthetic aperture ladar

Based on Fourier transform of heterodyne signal and linear fitting of phase time-evolution, a nonlinearity-induced phase error correction technique for synthetic aperture ladar is experimentally studied. The problem of beat frequency matching in the conventional method for nonlinearity-induced phase error correction in target channel by designing a reference channel and phase shift equations is practically analyzed, and its limitation on application under the condition of large time delay in target channel is verified. According to this problem, a new method of directly nonlinearity-induced phase error correction by linear fitting of phase time-evolution in target channel is proposed. In experiments, the rang resolution of 2 mm is gained at 1.5 μm wavelength, which is coincident well with theory. The results show that this technique can overcome the problem of beat frequency matching in two channels and is effective for practical applications with different time delay in target channel.

Phase-mapping of domain wall kinetics in lithium niobate by digital holographic interferometry

The phase-mapping of domain wall kinetics under the uniform steady-state electric field is achieved and investigated in the LiNbO3 crystals by the digital holographic interferometry. We obtained the sequences of reconstructed three-dimensional and two-dimensional wave-field phase distributions during the electric poling in the congruent and near stoichiometric LiNbO3 crystals. The phase-mapping of domain wall motions in the two crystals are obtained. Both the phase relaxation and the pinning-depinning mechanism are observed during the domain wall motion. The residual phase distribution is observed after the high-speed domain wall motion. The corresponding analyses and discussions are proposed to explain the phenomena.