# Pursuing High Quality Phase-Only Liquid Crystal on Silicon (LCoS) Devices
## 5 Phase Precision
The non-uniform voltage occurs mainly on the analog drive's LCoS panel because the analog drive uses a dynamic random access memory (DRAM) capacitive method to supply different voltages for different gray levels. By contrast, different gray levels in the LCoS panel with digital drive are generated by the static random access memory (SRAM) to determine the voltage difference (\(\Delta\)V = \(V_{w}-V_{b}\), in which \(V_{w}\) is \(V_{white}\), and \(V_{b}\) is \(V_{black}\)) in conjunction with different time sequences (namely, pulse width) driving design. As a result, the digital driving scheme has the DC balance characteristic, which has fewer problems with voltage non-uniformity. In addition, the data addressing speed of the digital drive chip is very fast (ns \(\sim\) us). This will not be the main cause of the unevenness and inaccuracy of the spatial phase, even if the digital driver adopts the time sequential driving scheme. The major problem associated with digital time sequential driving design is \"phase flicker\", which will be discussed in more detail in Section 6, which is devoted to phase stability. The default setting may be off or not satisfied for the design applications after receiving the SLM panel from the venders. As a result, phase linearity calibration and phase precision compensation have been hot topics for SLM research. Various reported phase compensation methods can be roughly divided into two categories: one is by electronic compensation method, and the other is by optical compensation method. Most of the LCoS-SLM driver board settings are not open to users. As a result,  
Figure 5: The scheme of eight different sessions (mxy) over the entire active area in the Equation (4). the electronic compensation method may only be available to original manufacturers. Most SLM users applied optical algorithms to achieve high-quality wavefronts by correcting wavefront aberration using the Zernike polynomials algorithm. The algorithm can correct the most serious wavefront distortions, but is limited to a circular aperture due to the principle of Zernike polynomials. The compensation is inappropriate for square or rectangular apertures, such as the LCoS-SLM panel. For example, P512 series products from Boulder Nonlinear Systems (BNS), have wavefront distortion value of 1.2 \(\lambda\) before compensation. They can be optically compensated to \(\lambda/4\)[58] but not up to the high phase precision standard (<\(\lambda/50\)). Their latest product, HSP1920, adopted electronic compensation for square sub-apertures in 2018. The wavefront distortion value can be greatly enhanced from \(\lambda/7\) to \(\lambda/20\) (RMS). Even though Zernike polynomials are not fully applicable to a square aperture to correct high-end aberrations, other research teams continue to improve or seek alternative optical algorithms for phase compensation. [PERSON] et al. proposed to use the Zernike polynomials method plus least mean square fitting to adjust the LCoS uniformity of 1 \(\times\) 12,288 pixels. Their result suggested that the RMS value of the inherent wave-front distortion can be suppressed approximately to \(\lambda/34\)[59]. [PERSON] et al. proposed using the principle of 7 th-order 3D polynomials to compensate for the phase uniformity of the HSPDM512 product of BNS in the form of 64 \(\times\) 64 pixels. It uses the peak-to-valley value as an evaluation method for wavefront distortion. It was shown that the maximum error amount can be corrected from 0.6-0.8 \(\pi\) to 0.3 \(\pi\)[46]. This method effectively corrects the error, but increases the overall computation time for the CGH phase compensation. The computation time can be accelerated up to 0.13 ms per CGH by using the Compute Unified Device Architecture (CUDA) encoding program. This result may not affect real-time operation on its panel, but will significantly prolong the computation time when using higher resolution (>4K2K) SLM with a high frame rate (>240 Hz).