Bo Liu

Characteristics of recording and thermal fixing in lithium niobate

We present a theoretical model in which the band-transport equations and the coupled-wave equations are considered to study the two thermal-fixing methods (simultaneous fixing and postfixing) in Fe:LiNbO(3). We found that, in simultaneous fixing, the existing ionic-grating affects the writing of the electronic grating by reduction of the coupling gain, and the grating envelope of the fixed-index grating is quite uniform inside the photorefractive crystal in comparison with the method of postfixing. The resulting diffraction efficiency of the fixed-volume grating is dependent mainly on the initial intensity modulation of the two writing beams. A set of experiments is also presented.

Photorefractive miniaturized integration of optical three-dimensional systems

A scheme using a CO 2 laser beam to thermally fix selective layers or lumps of photorefractive holograms to integrate and miniaturize an optical three-dimensional system into a single block of LiNbO 3 crystal is suggested. Theoretical and experimental investigations on laser local thermal lixing are given, and a stage of the modified gamma network is demonstrated experimentally.

HIGH-EFFICIENCY VOLUME HOLOGRAM RECORDING WITH A PULSED SIGNAL BEAM

We present an efficient photorefractive volume hologram recording technique with a pulsed signal beam and continuous reference-beam illumination. The grating envelope can be simply controlled by manipulation of the duty cycle of the signal beam. Thus, for any grating coupling strength and different initial reference-signal intensity ratios, the diffraction efficiency can be maximized with this technique and can be greatly increased in comparison with that of the conventional recording technique.

Matrix–vector multiplication in a photorefractive crystal

Abstract We propose and demonstrate a new method to implement parallel matrix–vector multiplication with every element binary encoded in a photorefractive crystal. In this method, we use some different wavelength beams, as much as the number of bits of the maximum element, to produce the shift steps of multiplication. The principle of operation and preliminary experimental results are presented.

Single-block optical system using layer laser thermal fixed photorefractive holograms

Abstract A scheme using CO 2 laser beams to thermally fix layers of photorefractive holograms to implement a solid-state optical system in a single block of LiNbO 3 crystal is suggested. To limit the heating region with shape similar to the heating beam, two CO 2 laser beams of narrow width are used to laterally illuminate a layer of the crystal, and additional absorbers are attached to the crystal surfaces outside the heating region to guide the heating flows. Analytical discussions based on the heat-transfer equations are given. As a proof-of-principle example, a modified gamma network is adapted for demonstration, and a stage of such a network is constructed in a block of crystal. It can thus be seen that it is possible to package an optical system into a miniaturized single-block of photorefractive crystal.

Photorefractive miniaturized integration of optical 3D systems

A scheme using CO2 laser beam to thermally fix selective layers or lumps of photorefractive holograms to integrate and miniaturize an optical 3D system into a single block of LiNbO3 crystal is suggested. Theoretical and experimental investigations on laser local thermal fixing are given, and a stage of the modified gamma network is experimentally demonstrated.

Double two-beam mixing in photorefractive materials

The mixing of two sets of mutually coherent beams, termed double two-beam mixing, is considered. At low modulation depth, combination gratings of the primaries are analyzed theoretically and show that the internal electric fields in photorefractive media also have cross components with wave vectors K1 ± K2 in addition to the primary components K1 and K2 and higher orders of 2K1 and 2K2, where K1 and K2 are wave vectors of two fringes. Each component is given in the form of an analytical solution, and the coupling behavior is studied for all relations between K1 and K2. The results show that the coupling behavior depends not only on the primary gratings but also on the combination grating K1 and K2 and the high-order grating 2K1 or 2K2. These features can be employed theoretically in phase-intensity transform and wavelength conversion.

Reconfigurable interconnection using fixed regional angular volume holograms in photorefractive materials

We demonstrate a scheme of reconfigurable optical interconnections using angular multiple holograms in a photorefractive medium. The conjugation light paths of three regular interconnect configurations are stored and fixed. Real-time two-wave mixing with the forward read-out technique combined with a special optical imaging system is employed to achieve the incident angle of the input beam as changeable but the position has no shift. Issues about ability of reconfiguration, cross-over and energy efficiency are discussed theoretically. A proof-of-principle experiment is also presented.

AN OPTICAL MULTIPLICATION BASED ON A QUADRATIC RESIDUE NUMBER SYSTEM

Based on the quadratic residue number system (QRNS), a novel optical matrix-vector multiplication is proposed. Complex number multiplication has higher parallelism than other algorithms. If the residue number is binary encoded, this multiplication can be easily implemented with a multichannel incoherent optical correlator.

Crystal substrate integration of free-space optical computing systems

This paper summarized our developments on the integration of miniaturized optical 3D systems using crystal substrates, which include a birefringence-customized stacking technique with the building blocks of double refraction and a photorefractive integration technique with the photorefractive local holograms thermally fixed by CO2 laser heating. The principles are introduced and the corresponding configurations are given, and the examples of various integrated digital optical computing systems such as logic processor, morphological image processor, and interconnection networks are shown.