Using the spatial light modulator as a binary optical element: application to spatial beam shaping for high-power lasers.

Nearly every new large-scale laser system application at LLNL has requirements for beam control which exceed the current level of available technology. For applications such as inertial confinement fusion, laser isotope separation, laser machining, and laser the ability to transport significant power to a target while maintaining good beam quality is critical. There are many ways that laser wavefront quality can be degraded. Thermal effects due to the interaction of high-power laser or pump light with the internal optical components or with the ambient gas are common causes of wavefront degradation. For many years, adaptive optics based on thing deformable glass mirrors with piezoelectric or electrostrictive actuators have be used to remove the low-order wavefront errors from high-power laser systems. These adaptive optics systems have successfully improved laser beam quality, but have also generally revealed additional high-spatial-frequency errors, both because the low-order errors have been reduced and because deformable mirrors have often introduced some high-spatial-frequency components due to manufacturing errors. Many current and emerging laser applications fall into the high-resolution category where there is an increased need for the correction of high spatial frequency aberrations which requires correctors with thousands of degrees of freedom. The largest Deformable Mirrors currently availablemore » have less than one thousand degrees of freedom at a cost of approximately $1M. A deformable mirror capable of meeting these high spatial resolution requirements would be cost prohibitive. Therefore a new approach using a different wavefront control technology is needed. One new wavefront control approach is the use of liquid-crystal (LC) spatial light modulator (SLM) technology for the controlling the phase of linearly polarized light. Current LC SLM technology provides high-spatial-resolution wavefront control, with hundreds of thousands of degrees of freedom, more than two orders of magnitude greater than the best Deformable Mirrors currently made. Even with the increased spatial resolution, the cost of these devices is nearly two orders of magnitude less than the cost of the largest deformable mirror.« less

Liquid crystal spatial light modulators are used in a wide range of modern problems in science and technology. These modulators are used to control the amplitude, phase, and direction of propagation of coherent optical radiation in optical information processing systems. However, the influence of the characteristics of the temporal dynamics of liquid crystal spatial light modulators on the performance of information optical systems, including diffractive neural networks, has not been sufficiently studied. The article presents the results of a study of the temporal dynamics of phase modulation of the liquid crystal spatial light modulator SLM-200 (Santec, Japan). Computer-synthesized binary phase diffractive optical elements were used in the experiments, and the characteristics of the temporal dynamics of the optical modulator were measured: 125 ms is the rise time of the diffraction efficiency when displaying diffractive optical elements on the screen; 61.9 ms is the decay time when switching frames. With these characteristics, it is possible to ensure the formation of a variable optical field at a frame display frequency of 2 Hz with an interference level of –17.1 dB. Increasing the frame display frequency leads to the appearance of unavoidable interframe interference, which in turn limits the effective performance of the information system. The results obtained can be useful in designing high-performance optical information processing systems and diffraction neural networks

Liquid crystal spatial light modulators for precise dynamic manipulation of coherent light fields, used in diffractive optoelectronic optical data processing systems, are considered. This paper presents the results of a study of the temporal dynamics of the HoloEye PLUTO-2 VIS-016 liquid crystal spatial light modulator for analysis of light fields rate modulation. Experiments using binary phase computer generated holograms and binary focusing phase diffractive optical elements were conducted. Based on experimental data, the time characteristics of the modulator response were determined. It was found that when the rise time of the diffraction efficiency was 146 ms after the hologram displaying onto the SLM, and when switching to a new hologram, the decay time was 97 ms. These results allowed the dynamic generation of an alternating holograms at a refresh rate of 2 Hz with an interference level of –16 dB. Increasing the frequency of fringe pattern updates increases the level of interframe noise in the generated holograms, and when updated at the specification frequency, the generated distributions cannot be separated. Determining the actual frame rate based on the rise and decay times of the diffraction efficiency makes it possible to correctly calculate the minimum operating time of an information optical system containing a liquid crystal spatial light modulator.

Fiber lasers provide an attractive means of reaching high output laser power because of their advantages in terms of compactness, reliability, efficiency and beam quality. In order to obtain much higher output power than it is possible from a single fiber, beam-combining techniques have been investigated. In this communication, we present a new technique of coherent fiber combining, based on self adaptive digital holography that does not require any phase error measurement. A low power plane reference beam is first launched into the fiber amplifier array. The interference pattern between the beams with phase φ(x,y) issued from the fiber array and a plane reference beam is recorded on a digital camera and directly transferred to a Spatial Light Modulator (SLM) which acts as a programmable digital hologram. This hologram is read out simultaneously and a phase conjugate beam with phase -φ(x,y) is generated in order -1 of the diffraction pattern. This beam is then injected in the fiber amplifier array. At the output of the fiber amplifier array, the phase of each elementary beam are locked. Experimental demonstration of coherent beam combining by digital holography is demonstrated with polarization maintaining fibers operating at 1 μm. Digital holography is realized thanks to a CCD/CMOS camera and a liquid crystal SLM. Owing to the high resolution of existing SLMs and cameras, this technique could be applied to phase lock a large number of fiber amplifiers.

Synthesis and reconstruction of computer generated holograms by a double pass technique on a twisted nematic-based liquid crystal spatial light modulator

A versatile, scalable wavefront control approach based upon proven liquid crystal (LC) spatial light modulator (SLM) technology was extended for potential use in high-energy near-infrared laser applications. The reflective LC SLM module demonstrated has a two-inch diameter active aperture with 812 pixels. Using an ultra-low absorption transparent conductor in the LC SLM, a high laser damage threshold was demonstrated. Novel dual frequency liquid crystal materials and addressing schemes were implemented to achieve fast switching speed (<1ms at 1.31 microns). Combining this LCSLM with a novel wavefront sensing method, a closed loop wavefront controller is being demonstrated. Compared to conventional deformable mirrors, this non-mechanical wavefront control approach offers substantial improvements in speed (bandwidth), resolution, power consumption and system weight/volume.

Recent advances in the shaping of ultrafast optical waveforms using liquid crystal (LC) spatial light modulators (SLM) are presented. Two LC SLMs are used in a novel arrangement to produce programmable waveforms with specified time-dependent amplitude and temporal phase profiles with the greatest fidelity and complexity to date. The apparatus is also used to demonstrate the generation of an ultrafast waveform with a programmable time-dependent polarization profile. A general theoretical result that describes the space-time electric field profile of waveforms shaped by the spectral filtering of spatially separated frequency components is also presented. The main result is that diffraction gives rise to a translational spatial shift in the electric field profile that varies linearly with time along the shaped waveform.

Binary optical elements are finding increased use in a wide range of applications. Fabrication of binary optical elements generally remains a time consuming and expensive process. Even in high volume, costs can be prohibitive, especially for elements larger than a few millimeters in diameter. Replications techniques, such as injection molding, have recently begun to show promise as a means of manufacturing binary optical elements, and doing so at a significant cost reduction over conventional photolithographic means. We report on recent improvements in the application of injection molding techniques to the replication of binary optical elements. Several examples of binary elements fabricated by these techniques, as well as present process capability, will be discussed.

At present time methods of optical encryption are actively developed. The majority of existing methods of optical encryption use not only light intensity distribution, easily registered with photosensors, but also its phase distribution which require application of complex holographic schemes in conjunction with spatially coherent monochromatic illumination. This leads to complex optical schemes and low decryption quality. To eliminate these disadvantages it is possible to implement optical encryption using spatially incoherent monochromatic illumination which requires registration of light intensity distribution only. Encryption is accomplished by means of optical convolution of image of scene to be encrypted and encryption diffractive optical element (DOE) point spread function (PSF) which serves as encryption key. Encryption process is described as follows. Scene is illuminated with spatially-incoherent monochromatic light. In the absence of encryption DOE lens forms image of scene in photosensor plane. DOE serves as encryption element, its PSF - encryption key. Light passing through DOE forms convolution of object image and DOE PSF. Registered by photosensor convolution is encrypted image. Decryption was conducted numerically on computer by means of inverse filtration with regularization. Kinoforms were used as encryption DOE because they have single diffraction order. Two liquid crystal (LC) spatial light modulators (SLM) were used to implement dynamic digital information input and dynamic encryption key change. As input scene amplitude LC SLM HoloEye LC2002 with 800×600 pixels 32×32 &mu;m² and 256 gray levels was used. To image synthesized encryption kinoforms phase LC SLM HoloEye PLUTO VIS with 1920×1080 pixels 8×8 &mu;m² and 256 phase levels was used. Set of test images was successfully optically encrypted and then numerically decrypted. Encrypted images contents are hidden. Decrypted images despite quite high noise levels are positively recognizable. Results of optical encryption and numerical decryption are presented.

Spatial beam shaping can be achieved using wavefront modulators to increase ultrafast laser processing efficiency. These modulators can display a pre-calculated phase mask on the beam path in order to shape the laser intensity distribution following a user defined target in the processing plane or volume. Due to the non-perfect optical response of wavefront modulators, the experimental distribution may differ from the target. We investigate the use of electrically addressed and optically addressed liquid crystal spatial light modulators with ultrafast laser pulses. Applications in parallelized surface and bulk processing are achieved with both modulators showing the advantages and drawbacks of these technologies. In particular, we focus on the limitations of these devices in terms of spatial and phase delay resolution, showing the consequences on the shaped beam distribution. Calculation strategies to overcome these limitations are discussed.

Holographic stereograms are an important way of generating 3-D images from photographs of real objects or of computer-generated 3-D objects. We have made holographic stereograms using several commercially available liquid crystal spatial light modulators, which have been modified in some cases, to produce transmission masks for a video image. The spatial light modulator is used to form the image in producing holographic stereograms. The image source has been either a video camera (or VCR) or the video signal from a computer graphics terminal. The use of the liquid crystal spatial light modulator eliminates the photographic intermediary in the process of forming the holographic stereograms. This technique speeds the process of forming holographic stereograms, increases automation of the process, and reduces the time required in creating the hologram. The major limitations of this technique are the contrast and resolution of the spatial light modulators which are commercially available and of low cost. Since the spatial light modulator technology is rapidly advancing, better performance in both of these areas is expected. Therefore, we believe that this technique offers an attractive means of producing holographic stereograms, especially from computer graphics, in situations where a short cycle time is important.

This work introduces three rectangular quasi-flat-top spots, which are provided by binary optical elements (BOEs) and utilized for the illumination in a microflow cytometer. The three spots contain, respectively, one, two, and three rectangles (R1, R2, and R3). To test the performance of this mechanism, a microflow cytometer is established by integrating the BOEs and a three-dimensional hydrodynamic focusing chip. Through the experiments of detecting fluorescence microbeads, the three spots present good fluorescence coefficients of variation in comparison with those derived from commercial instruments. Benefiting from a high spatial resolution, when using R1 spot, the micro flow cytometer can perform a throughput as high as 20 000 events per second (eps). Illuminated by R2 or R3 spot, one bead emits fluorescence twice or thrice, thus the velocity can be measured in real time. Besides, the R3 spot provides a long-time exposure, which is conducive to improving fluorescence intensity and the measurement stability. In brief, using the spots shaped and homogenized by BOEs for illumination can increase the performance and the functionality of a micro flow cytometer.

High-power laser plays an important role in many fields, such as directed energy weapon, optoelectronic contermeasures, inertial confinement fusion, industrial processing and scientific research. The uniform nearfield and wavefront are the important part of the beam quality for high power lasers, which is conducive to maintaining the high spatial beam quality in propagation. We demonstrate experimentally that the spatial intensity and wavefront distribution at the output is well compensated simultaneously in the complex high-power solid-state laser system by using the small-aperture spatial light modulator (SLM) and deformable mirror (DM) in the front stage. The experimental setup is a hundred-Joule-level Nd:glass laser system operating at three wavelengths at 1053 nm (1ω), 527 nm (2ω) and 351 nm (3ω) with 3 ns pulse duration with the final output beam aperture of 60 mm. While the clear arperture of the electrically addressable SLM is less than 20 mm and the effective diameter of the 52-actuators DM is about 15 mm. In the beam shaping system, the key point is that the two front-stage beam shaping devices needs to precompensate the gain nonuniform and wavefront distortion of the laser system. The details of the iterative algorithm for improving the beam quality and the strategy of achiving high beam quality on spatial intensity and wavefront simultaneously are presented. Experimental results show that the output wavefront RMS value is 0.06, and simultaneously the output near-field modulation is 1.38:1 and the fluence contrast is 10.5% at 3 ns at 1053nm with 40-Joule-level energy.

A simulation platform is set up to study the ship-borne laser communications. A liquid crystal spatial light modulator is used as a non-mechanical beam steering control device to simulate beam motions of moving platforms. Because of its light weight, low power and compact form, the liquid crystal spatial light modulator is superior to its mechanical counterparts. The theory of beam steering is introduced firstly. And then the performance of the simulation platform is researched and evaluated. The steering angles range from -2.89° to 2.89° with the control precision of ~ 0.02°. The simulated sinusoidal frequency of the spatial light modulator can reach 4 Hz in maximum. Finally, the beam motions of the ship platforms with different natural rolling periods are simulated based on the platform. The correlation coefficients between the theoretical and the simulated motion curves are greater than 0.9805. Results show that it is feasible to realize the beam motions simulation of specific moving platforms using the liquid crystal spatial light modulator.

Phase-only liquid crystal spatial light modulator (SLM) has been widely used for accurate phase-manipulation in holographic display, optical tweezers, lithography, etc. Due to the nonlinear optical response of liquid crystal and manufacturing defects, the grayscale-phase response could be different for every single SLM device. The calibration for phase modulation is needed for improving the performance of the SLM involved system. The principles of phase-only SLM are reviewed. The main phase calibration methods for SLM are discussed and compared.