Phase Characteristics Measurement Of Liquid Crystal Spatial Light Modulator

The phase modulation depth of twist nematic liquid-crystal spatial light modulator(TN-LCSLM) is affected by the angles of polarizer before and after LC device. Before using it the related parameters need to be experimentally measured. A transmission TN-LCSLM from Daheng company is used as measuring object in this paper. Some works are done, which include that determining the twist angle, measuring the phase modulation depth by setting up Mach-Zehnder interferometer, and measuring the phase and intensity modulation in the maximal phase modulation degree depth position. The experimental result verifies that this SLM can obtain the phase-only modulation with the modulation depth of 1.3 waves.Many methods have been proposed to measure the phase modulation characteristics of zero twist nematic liquid-crystal spatial light modulator(ZTN-LCSLM), but all of them include some defects. The most heavy problem be that it is susceptible to be affected from environment vibration and the solving procedure is complicated. Based on the polarization beam splitting principle of ZTN-LCSLM, the paper proposes a common-path interferometry to measure its phase modulation characteristic curve. The incident linearly polarized beam is divided into two orthogonal polarized beams by the LCSLM. Then they transmit along the same optical path and come to the WP. Two beams from the WP generate the lateral shearing and a tilt modulation will be introduced, and hence the carrier-frequency fringe is generated through a analyzer polarizer. The relative phase-shift amount between two frames of fringes is calculated by using the Fourier-transform algorithm. Furthurmore, the global phase modulation characteristics of LCSLM can be obtained.The consistent degree between the generated(or fitting) aberration and the theoretical aberration loaded onto LCSLM decides the phase mudulation precision of the device. It also influences the device application scope greatly. For example, it can be whether used as high-precision diffractive optical element or not, such as diffractive lens. For the measurement of generated aberration, commercial digital interferometer and set-up interferomrter by one are generally used. In the case, the inconveniently measuring process and un-objective measuing result including system error will be introduced to these methods. It is not benefit for scientific research. An common-path interferometer is proposed to measure the generated aberration of the ZTN-LCSLM. The phase distribution extracted by four phase-stepped reconstruction algorithm is the result to be measured, and so it is an absolute measurement method with no systerm error. The LC-SLM is not only used as a measured object but also a polarization beam splitter in this interferometer. Through placing a polarizer with its transmission axis at 45° before the LCSLM, reference and probe waves with common optical path are obtained. This optical configuration is simple, compact and easy to be built. Detailed theoretical analysis and experimental results are presented.