De'an Liu

A prototype of intersatellite laser communications terminals

In this paper, a prototype of intersatellite laser communication terminal for a principle demonstration is reported, and the corresponding ground support equipments are described, too. The terminal has two main subsystems, the first is one for the laser communication and the second for the pointing, acquisition and tracking. Laser diodes are used for the communication link, and with the average laser power of more than 200mW and the data-rate up to 600Mbps. The PAT unit consists of a fine pointing mechanism and a coarse pointing assemble, which reaches a tracking accuracy of ~5μrad. The on-ground test equipments are included in a communication test bed with a long-distance beam propagation simulator, a PAT test bed with an optical satellite trajectory simulator, and a wavefront test bed with deferent lateral-shearing interferometers.

Design of a dynamically configurable OADM device

A dynamically configurable optical add/drop multiplexer (OADM) device was proposed based on volume holographic gratings in doubly doped lithium niobate crystals and the principles of optical crystallography. The device consists of a wavelength demultiplexer module and an array of 2×2 electro-optic switches based on the internal reflection on the surface of the crystal. Two design schemes were presented: OADM based on discrete crystals with unifunctional integration and OADM based on monolithic crystal with multifunctional integration. This device can dynamically select signal channels that need to be added or dropped and simultaneously add/drop arbitrary signal channels. The suggested OADM has superior properties of simple and compact construction, convenient manipulation, lowered insertion losses and resistance to environmental perturbation.

Multi-beam coupling in doubly-doped photorefractive LiNbO3:Fe:Mn crystals

A theoretical simulation of multi-beam coupling in LiNbO3:Fe:Mn crystals is given based on jointly solving the two-center material equations and the coupled-wave equations. Within a single crystal, multiple signals are amplified through coupling process from the pump light. The coupling gain of each signal results from coupling both between the pump and the signal and between different signals. The gain of each signal receives is dependent on the intensity of the signal light. And a competition is found between the various signals.

Study of photorefractive recording in Ce:Cu:LiNbO 3 on the wavelengths dependence

Two-center holographic recordings are performed in a strongly oxidized Ce:Cu:LiNbO3 with three wavelengths of red, green and blue of recording beams, respectively. Three different photorefractive phenomena are observed and a critical strong nonvolatile hologram is optimally generated with the green beams and the optimal switching technique could be jointly used to obtain a nearly 100% high diffraction. Theoretical verification is given, and a prescription on optical parameters to match the green recording-wavelength for high diffraction is given.

Improvement of diffraction efficiency for 90-degree holographic recording geometry by using external electric fields in Fe-doped LiNbO 3 crystals

Significant improvements of photorefractive properties such as diffraction efficiency with 90-degree holographic recording geometry have been obtained by the application of external electric fields. Experimental results show that the highest diffraction efficiency can be obtained at least 3 times than without external electric field in Fe-doped LiNbO3 crystals. The vectorial Kukhtarev equations are modified in single center model taking into account the directions of optical axis and external electric field. The photorefractive properties of single doped LiNbO3:Fe crystal using 90-degree storage geometry are theoretically investigated by jointly solving the vectorial material equations and the two dimensional coupled wave equations. The variation trends of the theoretical results are similar to the experimental results.

The sinusoidal oscillatory diffraction induced by strong refractive index change in reduced Fe:LiNbO3 crystals

In this paper two-beam interference experiments are performed to investigate the diffraction characteristics of volume gratings in Fe:LiNbO3 crystals (0.15wt% Fe2O3), the results show that the dependence of the diffraction characteristics on the oxidization-reduction state and the oscillatory diffraction can be found in the reduced crystals. The theoretical analysis shows that the strongest space charge field can be formed in the reduced crystals, which induce the strong refractive index change, and because of the sin2 relation between refractive index change and diffraction efficiency, the oscillatory diffraction can be found in the reduced crystals. In application for high diffraction efficiency the oxidized treatment should be performed for Fe:LiNbO3 crystals so that the depth integral of saturated refractive index change is equal to π/2.

Study on two-color holographic storage in doubly doped Fe:Ni:LiNbO3 crystals

Holographic recording experiments of doubly-doped LiNbO3:Fe:Ni crystals were conducted by three kinds of different two-color recording schemes. The results show that the saturation diffraction efficiency, the fixing diffraction efficiency, and the recording sensitivity of oxidized LiNbO3:Fe:Ni crystal are higher than those of other reported doubly-doped LiNbO3 crystals. Based on the doped energy-band diagram, the effect of microcosmic optical parameter of the deep trap center on holographic recording properties of doubly-doped LiNbO3 is analyzed theoretically. LiNbO3:Fe:Ni has the potential of being a new highly efficient nonvolatile holographic recording material.

The effect of the bandwidth of an ultrashort pulsed beam to its properties diffracted by volume holographic grating

For an ultrashort pulsed beam (UPB) with different polarization states diffracted by a volume holographic grating (VHG), the effects of the bandwidth to its diffraction properties are studied. By developing the coupled wave theory of Kogelnik, the formulas of the spectral and temporal distributions of the transmission and diffraction pulse beams are analytically given, with considering the effect of the dispersion of the grating media further. Then the bandwidths of the VHG for the UPB with different polarization states are investigated. We show the changes of the spectral and temporal distributions of the transmission and diffraction pulsed beams with different ratios of the bandwidth of the UPB to the bandwidth of the VHG. Finally, the varieties of the diffraction efficiency of the VHG for the UPB with different polarization states are studied.

Global investigation of doping systems in doubly doped LiNbO3:Fe crystals for nonvolatile holographic recording

Dopants in doubly doped lithium niobate crystals are crucial for properties of nonvolatile holographic recording. In our experiments, a series of possible doubly doped congruent LiNbO3:Fe:X (X=Mn, Cu, Rh, Ru, Ni) are proposed and investigated for nonvolatile holographic recording. The experimental results demonstrate that the dopants with different distances of energy band to conduction band have different recording efficiency. Further analysis approve that the dopants which have moderate energy-band distance to conduction band can perform recording with both high diffraction efficiency and long lifetime. The compared results show that LiNbO3:Fe:Ni is promising for nonvolatile holographic recording, and its recording conditions are optimized.

Improvement of the nonvolatile holographic recording by application of external electric field

Based on jointly solving the two-center material equations with nonzero external electric field and the coupled-wave equations, we have numerically calculated the variations of the depth of refractive-index change and the spatial phase shift (between the grating and the light interference pattern) in the steady state versus various external electric fields. Different effects are found in the recording and the fixing phases of the nonvolatile holographic recording, and consequently, external electric fields applied in the positive direction along c axis (or large one in the negative direction of c axis) in the recording phase and that applied in the negative direction of c axis in the fixing phase are proved to benefit strong photorefractive performance. Experimental verifications are given with small external electric field.