KEYWORDS: Polarization, Laser processing, Spatial light modulators, Laser systems engineering, Digital Light Processing, Internet of things, Holography, Phase shift keying, Optical signal processing, Manufacturing
The arbitrary dynamic control of both amplitude and polarization distributions is attracting strong interest in laser processing field to manage the quality and to collect valuable polarization characteristics of processing materials in smart manufacturing. We present a holographic method to generate arbitrary polarization state of multiple beams by synchronizing two phase-only liquid crystal spatial light modulators (SLMs) with imaging feedback system for hologram designing of each polarization state. This research work will help to accelerate the use of liquid crystal SLMs for high-throughput and optimized additive manufacturing.
We developed a liquid-crystal spatial light modulator having a 30 mm active area and a multilayered dielectric mirror for industrial infrared lasers to establish an innovative manufacturing and fabrication technique in the smart-manufacturing post-pandemic era. The reconstruction of computer-generated holograms was achieved to demonstrate the concept of this device in the IR region. The incident phase performance characteristics of this device under high-power laser irradiation were obtained using a 1030 nm ultrashort pulse laser. The work presented here will accelerate the use of liquid-crystal SLMs in high-precision laser processing of the process-resistant materials and high-throughput processing for additive manufacturing.
We proposed an optimization method of a hologram in holographic laser processing. The laser beam was diffracted by a
designed computer-generated hologram (CGH) displayed on a liquid-crystal-on-silicon (SLM), and then formed
spatially shaped three-dimensional optical pattern which detected by a CCD image sensor at different focal position for
continuously optimizing with the weighted iterative Fourier transform (WIFT) algorithm. The uniformity of the pattern
was increased from 11% to 95%, which was also well proved by the corresponding 3D processing results. This method
provides the holographic laser processing system with high-stability, that is, the ability to dynamically compensate for
system imperfections, and has the ability to be suitable for a wide range of high-precision, high-throughput applications
in the field of 3D manufacturing.
A method that realizes the three-dimensional holographic laser reconstruction and processing by using an optimized computer-generated hologram (CGH) was demonstrated. A pre-designed CGH that contains three-dimensional (3D) information displayed on a liquid-crystal-on-silicon spatial light modulator (LCOS-SLM) can obtain the desired 3D diffraction pattern in the optical system, called 3D holographic laser reconstruction, and when it is reduced by an objective lens and incident on a sample, it can realize the 3D holographic laser processing. Using an optimization method, called in-system optimization, to compensate for the imperfections that exist in the actual laser system, and obtain the high-quality CGH. Consequently, a high uniformity 3D holographic laser reconstruction was achieved, and the consistent scale of the corresponding processing holes proved the feasibility and significance of the CGH optimization.
In this study, we developed a liquid-crystal spatial light modulator with high laser power capacity for industrial ultrafast pulse lasers to demonstrate innovative manufacturing and fabrication techniques using a cyber-physical system. The incident phase performance characteristic of this device was obtained with a 60 W, 1035 nm ultrafast laser. This research work will help to accelerate the use of liquid crystal spatial light modulators for high-precision laser processing of resistant materials and high-throughput for additive manufacturing.
Laser dicing with tightly focused nanosecond pulsed laser light inside a semiconductor wafer is a dry, debris-free dicing method achieved by the generation of thermal microcracks. This method has two practical issues: a dicing speed that is limited by the repetition rate of the pulsed laser and potential damage to integrated circuits on the wafer from excessive laser intensity due to insufficient beam divergence. By correcting aberrations and generating multiple beams via wavefront modulation, multiple focused beams inside the wafer will become sufficiently divergent to avoid undesirable potential laser damage. We confirmed these improvements by dicing sapphire wafers with a pulsed laser and a high-numerical-aperture objective lens.
We attempted to observe deep regions in biological samples through two-photon excitation microscopy adopting a spatial light modulator (SLM). The SLM is used for correcting spherical aberration (SA) caused by the refractive-index mismatch between the immersion medium and sample. In the observation of fluorescent beads in transparent epoxy resin, the fluorescence intensity from a scan with SA correction was 50 times that from a scan without SA correction. After that, we observed blood vessels in a mouse brain, which became transparent with a clearing agent.
We report here a holographic high speed accessing microscope of sensory-driven synaptic activity across all inputs to single living neurons in the context of the intact cerebral cortex. This system is based on holographic multiple beam generation with spatial light modulator, we have demonstrated performance of the holographic excitation efficiency in several in vitro prototype system. 3D weighted iterative Fourier Transform method using the Ewald sphere in consideration of calculation speed has been adopted; multiple locations can be patterned in 3D with single hologram. Standard deviation of intensities of spots are still large due to the aberration of the system and/or hologram calculation, we successfully excited multiple locations of neurons in living mouse brain to monitor the calcium signals.
Conventional methods of compensating for self-distortion in liquid-crystal-on-silicon spatial light modulators (LCOS-SLM) are based on aberration correction, where the wavefront of the incident beam is modulated to compensate for aberrations caused by the imperfect optical flatness of the LCOS-SLM surface. Previously, we proposed an effective method to compensate for the distortion by displaying a compensation phase pattern obtained from interferometry However, the phase distribution of an LCOS-SLM varies with changes in ambient temperature and requires additional correction. The ambient temperature of LCOS-SLMs can vary under certain circumstances, i.e. equipped inside systems for field use or long-term operations. In this presentation, we discussed a novel phase compensation method under temperature-varying conditions based on an orthonormal Legendre series expansion of the phase distribution from viewpoint of multiple beam holographic generation. We found several Legendre coefficients that follow quadratic functions of ambient temperature. This prompted us to propose an algorithm for correcting the temperature dependency by displaying a phase pattern using two simple steps: an initializing step and a temperature correction step. We investigated the temperature dependency by controlling the ambient temperature with an incubator and successfully corrected for self-distortion in a temperature range of approximately 68°F to 122°F, giving an optical flatness of <λ /10. Our approach has the potential to be adopted in tight-focusing applications which require wavefront modulation with very high accuracy. Additionally, the concept of this method is extensible to the thermal behavior of other optical devices, such as lenses and mirrors, which have the possibility of causing unexpected aberrations.
The low intensity regions existed around the singular point of optical vortex (OV) beams influence the position detection accuracy of phase singular points, especially for OVs of large topological charge. A new method using Shack-Hartmann wavefront sensor (SH-WFS) is proposed to solve this problem. In the method, we combine several phase slopes obtained at individual lenses of a lens array into one to avoid the influence of the low intensity region and use correlation matching with pre-calculated simulation data to get the accurate singular point position. The experimental results show that the position detection precision of this method is maintained less than 0.2 in unit of lens-size for OVs of charges -20 to 20.
A high performance 2D Spatial Light Phase Modulator named LCOS-SLM has been developed for wide range usefulness from basic researches to practical applications. We show the fundamental characteristics of LCOS-SLM device we developed, and introduce the activity examples such as the scientific researches of singular optics, fluorescence microscopy and adaptive optics. We have also applied the device for industrial applications in laser processing and machining, and medical application using adaptive optics system in scanning laser opthalmoscope.
We propose a simple method of realizing an accurate position detection of phase singularities in an optical vortex (OV) beam using a Shack–Hartmann wavefront sensor (SH-WFS). The method calculates circulations which are the discrete contour integrals of phase slope vectors measured by the SH-WFS and then determines the accurate positions of the singular points by calculating the centers-of-gravity with a fixed window size around the local peak of the circulation distribution. We use closed paths that connect the centers of eight-connected, instead of 2×2-neighboring lenslet apertures for calculating the circulations. Both the numerical analysis and proof-of-principle experiment were performed to confirm the measurement accuracy. In experiments, the positions of singular points in OV beams generated by a liquid-crystal-on-silicon spatial light modulator were measured. The root-mean-square error of the position measurement was approximately 0.09 in units of the lens size of the lenslet array used in the SH-WFS. We also estimated the topological charges of the singular points being detected based on the peak circulations, and the results agreed well with theoretical ones. The method achieves both rapid implementation and sublens-size spatial resolution detection and is suitable for applications that require real-time control of OV beams.
We propose a new method, based on Shack-Hartmann wavefront sensor (SH-WFS), to achieve high-accuracy position detection of phase singular points of optical vortex (OV) beam. The method calculates evaluation values related to phase slopes of incoming wavefront from Hartmanngram recorded by SH-WFS, and then determines precisely the position of the singular points by calculating the centroid of the 3x3-evaluation-value distribution centered at peak position. A main point is that, in evaluation-value calculation, we use a closed contour connecting the centers of 8-connected, instead of 2x2, lenslet apertures. Theoretical analysis shows that the measurement errors can be greatly reduced in comparison to that of 2x2 closed contour. Proof experiments were performed to confirm its accuracy by measuring singular points of OV beams generated by a liquid crystal on silicon spatial light modulator. The root-mean-square error of the measured position of singular points was approximately 0.052, in units of the lens size of lenslet array used in the SH-WFS. The method achieves fast-speed and sub-lens size spatial resolution detection, is suitable for real-time control applications.
Dynamics of micrometer-sized dielectric objects can be controlled by optical tweezers with scanning light, however, the trapped objects fail to track the scan when drag exceeds the trapping by too quick movement. On the other hand, optical vortices (OVs), which have a property of carrying angular momenta, can directly control torque on objects rather than their position. Laguerre-Gaussian (LG) beams are the most familiar examples of OV and have been studied extensively so far. Revolution of the objects trapped by the LG beams provides typical models of nonequilibrium statistical system, but stable mid-water trapping by the LG beams becomes essential to evaluate physical properties of the system without extrinsic hydrodynamic effects,. Nevertheless, off-axis revolutions of small objects trapped in mid-water by the LG beams have not yet been established with secure evidences. Here we report stable off-axis trapping of dielectric spheres in mid-water using high-quality LG beams generated by a holographic complex-amplitude modulation method. Direct evidence of the three-dimensional off-axis LG trapping was established via estimating the trapping position by measuring the change of revolution radii upon pressing the spheres onto glass walls. Resultantly, the axial trapping position was determined as about half the wavelength behind the beam waist position. This result provides a direct scientific evidence for possibility of off-axis three-dimensional trapping with a single LG beam, moreover, suggests applications as powerful tools for studying energy-conversion mechanisms and nonequilibrium nature in biological molecules under torque.
KEYWORDS: Image processing, Parallel processing, Sensors, Personal digital assistants, Field programmable gate arrays, Control systems, Transistors, Imaging systems, Photonics, Detection and tracking algorithms
We have designed and constructed a column parallel vision (CPV) system to realize an intelligent and general purpose image processing system with higher frame rate within 1 millisecond. The system consists of an original designed photo detector array (PDA) with high frame rate, a parallel processing unit with fully parallel processing elements (PEs), and a controller for PDA and PEs. The column parallel architecture enables the PDA 1 millisecond frame rate with 256 analog levels which is required for industrial image processing and measurements. The parallel processing unit has been fabricated by using FPGAs and has 128 X 128 PEs to perform the fully parallel image processing. The PEs have the S3PE architecture which has SIMD type parallel processing flow, and was constructed only 500 transistors suitable for integration in future. Since the PEs are operated by control signal from the controller, we can achieve desired image processing task to the system by changing the software. We have demonstrated that the system can be worked as a tracking system. The experimental results show the system was realized high speed feedback loop as 1000 frame/s including the tracking operation with noise reduction, matching (self-window algorithm) and moment calculation.
Optical interconnections and integrated optoelectronic devices are expected to be promising candidates that expand interconnection bandwidth between large-scale integrated circuits (LSIs). We have constructed an optoelectronic parallel computing system that has a reconfigurable free- space parallel optical interconnection module called OCULAR- II. It has a multi-layer architecture that eliminates the data transfer bottleneck between optoelectronic processing modules by reconfigurable free-space optical interconnections. An optoelectronic processing module is composed of a two-dimensional processing element array where each pixel has its own optical output channel by a VCSEL and optical input channel. The optical interconnection is integrated into a compact module where an optically addressable phase only spatial light modulator and an imaging optical system are compactly fabricated. Each component of the OCULAR-II system has been designed to be modular and compact. Therefore, just cascading optoelectronic processing modules and optical interconnection modules makes a pipelined parallel processing system. In the optical interconnection module, a custom designed Fourier Transform lens has been used to reduce the working distance of the lens system. A computer generated hologram (CGH) is written on a liquid crystal display (LCD) that is coupled by a fiber optic plate (FOP) to the optically addressable SLM. The interconnection topology between optoelectronic chips is controlled by changing the CGH patterns, which is calculated in advance. A global interconnectivity among processor arrays is also achievable since the communication channels are constructed via optical path in free space. The data broadcasting between processors that are located spatially far away can be efficiently implemented by free-space optical links in OCULAR-II's optical interconnection module.
In order to overcome the limitation of the image processing speed of the conventional video camera system, we have designed and constructed a column parallel vision (CPV) system. The CPV system realizes an intelligent and general purpose image processing capability with higher frame rate within 1 millisecond (i.e. 1000 frames/s). The system mainly consists of three parts; a sensing unit, a parallel processing unit and a control unit. The sensing unit has a 128x128 photodiode (PD) array, 128-column parallel amplifier array and A/D converter array to achieve high-speed frame rate and has been integrated in a CMOS chip. The parallel processing unit is composed of fully 128x128 processing elements (PEs) to perform a real-time image processing. The control unit has functions of PD/PE control and interface to the host computer. We have confirmed that the CPV system can process general image processing functions within a few ten microseconds. We have also demonstrated that the system can be worked as a tracking system with high-speed feedback loop and have evaluated the accuracy of the position calculation for the desired target.
A multi-layered optoelectronic parallel processing system, which is called Optoelectronic Computer Using Laser Arrays with Reconfiguration is shown. This system consists of layers of processing modules, which are composed of electronic programmable processing element array each having parallel optical input/output connected by optical interconnection modules. Every module is designed to be modular and cascadable. The algorithms for this system are also shown which exploit the aggregate bandwidth supplied by optics and the computation versatility given by electronic processors.
A compact electrically-addressable spatial light phase modulator module is described. The module consists of an electrically-addressed liquid crystal display (LCD), an optically-addressed phase-only spatial light modulator (SLM), one of which substrates is a fiber optic plate (FOP), a laser diode, and collimating optics for it. The module size is 95 mm long, 55 mm wide, and 90 mm high. The module had a nearly 100% reflectivity and a diffraction efficiency close to the theoretical maximum. Surplus diffraction light caused by the pixelized structure was reduced to approximately 3%, almost 50% of which was in the LCD alone. A reflection type of SLM would cause another power loss of the readout light by a half mirror, which was set up so as to separate the incident and reflected lights. An oblique incidence readout method was evaluated in the module, instead of the half mirror scheme, for the readout. We have found adequate alignments among the polarization and incident directions of the readout light, and the corresponding liquid crystal orientation. Consequently, almost no degradation in diffraction efficiency was observed for the incident angle within 45 degrees.
Recently, there has been considerable interest in the application of combined phase/amplitude encryption to the field of optical pattern recognition based security systems, due to the high level of security that it offers1. We have developed a new phase-only encryption technique based in principle on a phase-coding method which uses a generalization of the Zernike phase contrast technique, but breaks the small-phase-angle limitation of Zernike's method2. This phase-coding technique for image formation, in which the spatial average value of an input phase modulated image is combined with a pre-estimated phase retardation produced by a phase contrast filter, has been demonstrated experimentally3. In this paper we describe an extension of our phase-coding method4 and its application in an experimental system for the decryption and visualization of phase encrypted images using commercially available Parallel-Aligned Liquid Crystal Spatial Light Modulators (PAL-SLM) supplied by Hamamatsu Photonics5.
An electrically addressed spatial light modulator (SLM), which is non-pixellated and has a high diffraction efficiency, has been developed by combining an optically addressed liquid crystal SLM with a CRT. It has been confirmed that the designed SLM has excellent performance for CGH (computer generated hologram) reconstruction.
We have developed a compact joint transform correlator by using the parallel aligned nematic liquid crystal spatial light modulator as a real time hologram device, and have applied the correlator to fingerprint identification. The identification performance was evaluated using 2400 data from 600 fellow workers. The test results showed that the false-reject rate was 0.2 percent with 0.2 percent false acceptance. The response time of the system was less than 0.1 seconds as a result of parallel processing in the optical system.
A new method is presented for synthesizing arbitrary grey level intensity patterns based on phase contrast imaging. The concept is grounded on an extension of the Zernike phase contrast method into the domain of full range phase modulation. By controlling the average value of the input phase function and by choosing appropriate phase retardation at the phase contrast filter, a pure phase to intensity imaging is accomplished. The method presented is also directly applicable in dark field image synthesis. Preliminary experimental results are demonstrated for binary-only phase modulation.
Parallel aligned liquid crystal-spatial light modulator (PAL-SLM) has been applied to an optical interconnection system by a CGH technique. In the system, the PAL-SLM is expected to have electrically addressed capability. We have developed a CRT coupled PAL-SLM that can be electrically addressed. For that purpose, the CRT have been designed and evaluated on a fluorescence spectrum and an afterglow time of a phosphor to suit PAL-SLM features. And we have also evaluated two coupling methods between the CRT and the PAL- SLM ; one is a relay lens coupling method and the other is a fiber optic plate (FOP) coupling method. Consequently, it can be concluded that a combination of a red phosphor with a long afterglow time and the FOP coupling method is suitable for the CGH application. We can get a fine and noiseless CGH reconstruction image by using this module.
A new method is presented for synthesizing arbitrary grey level intensity patterns based on phase contrast imaging. The concept is grounded on an extension of the Zernike phase contrast method into the domain of full range phase modulation. By controlling the average value of the input phase function and by choosing appropriate phase retardation at the phase contrast filter, a pure phase to intensity imaging is accomplished. The method presented is also directly applicable in dark field image synthesis. Preliminary experimental results are demonstrated for binary-only phase modulation.
An optically addressed liquid crystal phase only spatial light modulator has been developed. The transfer characteristics and the diffraction efficiency of the device are discussed theoretically and experimentally.
A new method for optical microscopy with super-resolution exceeding the Rayleigh limit was developed to observe the shape of particles contaminating the LSI patterned wafer in the far- field. The principle of the new method is as follows. The optical spot being smaller than the measured particles has to be scanned on the particle to measure the shape of particles on the wafer. The phase-shifting mask making small optical spot consists of the checkered mask which has been coated with a transparent layer patterned to ensure that the optical phases of the nearest apertures are opposite. The destructive interference between the two opposite phases of light waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution, so that the size and resolution of the coherent laser spots can exceed the size limit of the Airy disk and resolution of Rayleigh limit. The new method for optical microscope with super resolution was examined theoretically and experimentally. The coherent beam generated from 15 mW He-Ne laser projected to the phase-shifting checkered mask forms small laser spots of which size are under 0.1 micrometers in diameter. It is clarified that in the far-field, the scattered light from two spots with opposite phases on the wafer can be observed in exceeding with Rayleigh limit.
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