Background introduction
As a dynamic programmable optical element, liquid crystal spatial light modulator (LC-SLM) plays a very important role in precision optical modulation applications such as wavefront shaping and beam control. A typical phase-only SLM works by inducing phase delay at each LCD pixel by loading voltage control to achieve regulation of the wavefront of the incident light.
With the development of optical field control technology towards fine direction, the modulation accuracy of phase type LC-SLM is required to be higher. For example, in the field of ultrafast intelligent processing, phase SLM is required to achieve fine regulation of wave front with high phase modulation accuracy; In the field of microscopic imaging, phase SLM is required to achieve high signal-to-noise ratio and high resolution imaging. In the field of contact-free optical micromanipulation, phase SLM is required to achieve high-precision and efficient particle capture. However, phase distortion usually occurs in commercial SLM, which leads to many problems in the practical application of wavefront control of LCOS devices, such as low light utilization efficiency, poor modulation accuracy, and finally unable to realize the corresponding functions.
Following its origin, the phase distortion is mainly caused by the non-linearity and non-uniformity of the phase modulation of SLM physical structure and environmental conditions, which can be attributed to two factors:
1. The error driving electrical signal applied to the liquid crystal (LC);
Figure 1 Dynamic modulation LUT modulation error
2. Distortion caused by the curvature of SLM substrate or backplane and uneven thickness of LC layer;
Figure 2 Introduction of SLM hardware distortion
The former belongs to the dynamic phase response error caused by drive module control error, which can be corrected by LUT. The latter is an inherent characteristic of the product device, which will affect the efficiency and wavefront quality, and the precision of the modulated phase profile is relatively low, which directly affects the precision of the phase modulation. In order to solve this problem, the SLM hardware (light valve) needs to be measured and corrected.
SLM profile test and calibration principles
In order to respond to the application requirements based on high-precision phase modulation and improve the performance of SLM products, CMSI has developed a static wave front error and correction technology for measuring SLM based on the Tyman-Green interferometry. The system optical path diagram is shown in Figure 3. The specific working principle is as follows: The plane wave is formed after the laser beam expansion and collimation, which is divided into two beams by the beam splitter (BS). One beam is irradiated to SLM by BS transmission, and then reflected by SLM modulation. Another beam of light is irradiated to the reference mirror by BS reflection, and by reference reflection, the reflected light modulated by SLM interferes with the light reflected by the flat mirror (M) when passing through BS, and then the CCD can collect and record the interference fringe through the 4f system configured at its front end.
Figure 3. Theyman Green interferometric optical path
The SLM is placed at the position shown in FIG. 3, and the interference fringe is collected by CCD in the optical path system. FIG. 4 shows the measured SLM interference fringe.
FIG. 4 Interference fringe pattern collected
Then the initial SLM profile image and data can be obtained by the interference fringe processing algorithm. The profile data and images after SLM correction can be obtained by the surface profile correction algorithm, and are quantitatively represented by the commonly used profile evaluation indexes PV and RMS. Table 1 below lists the test and correction images and data of the three SLMS tested and corrected.
Table 1 Different types of SLM initial and modified rear types
It can be seen from the above table that compared with the initial profile, the modified rear profile tends to be flat basically, and the RMS accuracy of the profile can basically reach 1/35λ@632.8nm.
The influence of surface calibration on modulation effect
In order to better compare the modulation performance of SLM before and after correction, the modulation effect of three common beams (Gaussian beam, Airy beam and vortex beam) was tested by building a real light field test system.
Table 2 Debugging results of actual light field before and after SLM surface correction
From the above test results, it can be seen that the modified SLM modulation effect will be more perfect and closer to the theoretical effect.
Product launch
The company's latest 3.0 software has reserved a profile compensation interface to support phase SLM (for 532nm and 635nm) profile detection and correction calibration, configure the corresponding profile correction file according to different working wavelengths, and realize the compensation of profile correction through supporting software.