The spatial light modulator (SLM) based on liquid crystal on silicon (LCoS) technology has been widely used in applications such as holography and wavelength selective switches (WSS). Although there is some progress in terms of larger filling factor, improved surface process engineering, the concern lies in the light efficiency of SLM. For the reflectivity, various dielectric layer design have been proposed. This paper presents experimental results on the reflectivity of different filling factors and surface treatments. On the diffraction efficiency, factors such as liquid crystal material, voltage control and fringe field effect are also discussed. Previous research has focus on the phase depth and phase linearity of the SLM, but this paper emphasizes the importance of cell design and voltage control in achieving higher efficiency. The efficiency of blazed gratings with different numbers of steps are compared and intentionally increasing the driving voltage for gratings with a small pitch to compensate for the decrease in sharpness in the fly-back zone are suggested. Then, to estimate the influence of the fringe field effect, four simplified model are analyzed and the calculation result with two panels applied in optical communication in different grating cases, including small pitch grating and larger pitch grating are compared with real test results. The conclusion is valuable for guiding cell design and providing insight into the small pitch large angle blazed grating. Additionally, the diffraction efficiency of a two pi blazed grating is compared for different voltage ranges, starting from the lowest to the highest, with four types of liquid crystal materials filled into identical LCoS cells. Although the changing slope and efficiency values differ, the overall trend remains similar. In conclusion, both material selection and driving voltage control are crucial. A 1550nm LCoS based SLM is designed with more than 80% reflectivity, and the diffraction efficiency is higher than 50% with 4 step single pixel grating and 83% with 16 step single pixel grating.
In this paper, we focus on improving the performance of the SLM based on Liquid Crystal on Silicon, like frame rate, color control, phase linearity, surface flatness and surface process. The colorfulness is realized by lighting different laser light source with color sequential driving method. With this method, the color could be recovered simply using compact three color light sources, like in the present AR glasses. The lights are collimated and illuminate the SLM panel in pre-set time sequence, synchronized with the display content. Light efficiency has always been a great concern. It is important to increase the light efficiency while not decrease the diffraction efficiency at the same time.
For augmented reality applications, the micro display panels are generally used, as a straightforward method for display engines. But there are the light efficiency and vergence accommodation conflict problems, which affect the performance and application of the AR glasses, HMD and HUDs. One way to solve the problem is to use holography display instead of direct display, which exploit the diffraction light instead of reflection light of the liquid crystal panel. One of the big problems is the form factor of holographic systems which use SLM. Most of current SLMs are composed of a driver board with FPGA and a liquid crystal on silicon panel. The FPGA driver board is big and very power consuming, which makes it better for general SLM systems that cares more about changing the driving algorithm flexibly rather than make it a wearable gadget. A compact SLM system is designed specifically for AR applications, which replaced the normally used FPGA system with a driver IC and greatly reduce they whole system size and power consumption while realize the same function for holographic display. The driver IC could receive the RGB signal or MIPI signal and transform the standard picture signal to a high frame rate color sequential signal, also the gray scale could be tuned through on chip registers either beforehand or instantly, which makes it good for liquid crystals working in different wavelength. The driver board is 2*6 cm2 with a type-C connector which could easily transmit image data from a laptop or a cellphone. This form factor is almost the same with monocular glasses. The power consumption of the driver IC is around 250mW for the driver IC comparing to the normally 5W power of FPGAs. Also, the frame rate could be from 60 to 120 Hz. A simple Fourier holography optical system is used to demonstrate the holographic display, with a lens tear apart from a smart glass on the market. The 3D holographic is displayed in the lens.
Liquid crystal on silicon achieves the corresponding gray scale by outputting the corresponding voltage value. The gamma 2.2 curve is just complementary to the human eye's color sensitivity, that means, the human eye has the best viewing effect on the display device with gamma 2.2 curve. On the one hand, in order to fit the response curve of the display device better to the gamma 2.2 curve to improve the viewing effect, gamma value with analog correction can not meet the fitting requirements;on the other hand,under the condition of ensuring the low power consumption,it is more difficult for designers to redesign a higher precision DAC.Improving the display accuracy of silicon-based liquid crystal has become an important research direction.In this paper, the frame ratio control algorithm to improve the display accuracy of silicon-based liquid crystal is proposed. The experimental results show that, the display accuracy is 7 bit without processing,there will be a "ladder" curve segment in the gamma curve of silicon-based liquid crystal. After using the frame ratio control algorithm, the display accuracy is increased to 10 bit,the measured gamma curve eliminates the "ladder". Moreover, because the more frames are divided, the smoother curvature of measured gamma curve is, the higher display accuracy is. This conclusion proves that the frame ratio control algorithm can improve the display accuracy in silicon-based liquid crystal display, and make the image display effect finer and smoother, which has a wide range of application scenarios and value。
High resolution and accuracy phase modulation could help to improve the diffraction efficiency of spatial light modulator. For spatial light modulator with analog driving pixels, the phase accuracy is mainly decided by the DAC design. The conventional circuit design method to improve the DAC accuracy is using a large chip area or using expensive advanced manufacturing technology. In this paper, a calibration method with both analog and digital algorithm is presented. By adding digital calibration and improving the frame frequency, the liquid crystal response characteristic is improved without changing the on chip DAC design. An 8 bit DAC LCoS panel is used for verification. The experiment result shows, the phase error is reduced from 1.49% to 0.44%, and the diffraction efficiency is improved from 72% to 75%. And a holography picture is projected to verify the effect.
Freeform surfaces are optical surfaces without linear or rotational symmetry. Their high degrees of design freedom liberate designers and engineers from restrictions on optical surface geometry, yielding compact and lightweight imaging systems with excellent optical performance. Freeform optics have become a competitive tool in the design of optical seethrough head-mounted display (OST-HMD) systems. In this paper, we present two different OST-HMD systems which have different optical different configurations and both employ freeform optics to correct and balance optical aberrations. In the optimization design of the two OST-HMD systems, we start from a spherical imaging system with an on-axis configuration, and then tilt and decenter each optical surface to find a starting point. In the final state of optimization, the order of each XY polynomial used to represent the freeform surface is gradually increased. The modulation transfer functions of the two OST-HMD systems are evaluated and the three-dimensional models of the two systems are also presented.
Spectral confocal technology is an important three-dimensional measurement technology with high accuracy and non-contact; however, traditional spectral confocal system usually consists of prisons and several lens whose volume and weight is enormous and heavy, besides, due to the chromatic aberration characteristics of ordinary optical lenses, it is difficult to perfectly focus light in a wide bandwidth. Meta-surfaces are expected to realize the miniaturization of conventional optical element due to its superb abilities of controlling phase and amplitude of wavefront of incident at subwavelength scale, and in this paper, an efficient spectral confocal meta-lens (ESCM) working in the near infrared spectrum (1300nm-2000nm) is proposed and numerically demonstrated. ESCM can focus incident light at different focal lengths from 16.7 to 24.5μm along a perpendicular off-axis focal plane with NA varying from 0.385 to 0.530. The meta-lens consists of a group of Si nanofins providing high polarization conversion efficiency lager than 50%, and the phase required for focusing incident light is well rebuilt by the resonant phase which is proportional to the frequency and the wavelength-independent geometric phase, PB phase. Such dispersive components can also be used in implements requiring dispersive device such as spectrometers.
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