Yasunori Igasaki

LCOS spatial light modulator controlled by 12-bit signals for optical phase-only modulation

We developed a liquid-crystal-on-silicon (LCOS) spatial light modulator (SLM) for phase-only modulation. The SLM was designed mainly for wavefront control in adaptive optics, optical manipulation, laser processing, etc. A dielectric multilayer mirror was incorporated into the device to enhance the reflectivity. The number of pixels was 792 x 612 and their size was 20 x 20 microns square. The range of the phase modulation exceeded one wavelength, and the light-utilization efficiency for monochromatic light was approximately 90%. The silicon backplane of the SLM was mechanically weak and its surface was not flat. The poor flatness degraded the output wavefront from the SLM. The device was driven by electronics composed of a digital-visual-interface (DVI) receiver, a field programmable gate array, and 12-bit digital-to-analog converters (DACs). The converted analog voltage signals from the DACs were transmitted to the pixels of the SLM and created phase changes. The driver had several kinds of control modes for the device, according to the level of flatness compensation. In one of the modes, the driver received 12-bit data and transferred them directly to the DACs. This 12-bit control mode enabled highly flexible control of the device characteristics. In the presentation, we report details of the device and experimental results on compensation of distortion in the output wavefront from the device.

Formation of a doughnut laser beam for super-resolving microscopy using a phase spatial light modulator

The formation of a doughnut-shaped laser beam is presented. To generate the beam, we use an optically addressed parallel-aligned nematic liquid-crystal phase spatial light modulator (PAL-SLM), and observe the shape of the focused beam. By using a compensating technique for wave aberration, the beam has a symmetric doughnut shape with a hole size of 1 µm on the focal plane. The experimental result shows that the generated beam can be expected to be applicable to super-resolving microscopy based on the fluorescence depletion process.

Compact high-efficiency electrically addressable phase-only spatial light modulator

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.

Advanced a-Se film with high sensitivity and heat-resistance for X-ray detectors

Amorphous selenium (a-Se) is well known to provide superior spatial resolution and low dark current when used as a direct conversion X-ray photoconductor in flat panel detectors (FPD). However, a-Se properties are also known to fluctuate at higher environmental temperatures, so the temperature has to be carefully controlled. To overcome this problem we developed a newly modified a-Se photoconductor with electrical and X-ray characteristics that remain constant at temperatures up to 70 degrees C. On the other hand, in terms of a-Se dark current levels, the higher the electrical field, the higher the dark current level. For this reason, conventional a-Se photoconductors are used at a comparatively low electric field of 10 V/μm. We also investigated the electrical characteristics of film compositions containing a-Se that provide high gain and low dark current. Experiments were made with sandwich cells and then with CMOS (50 μm pixel pitch) readout panels. Our new a-Se photoconductor operated at 40 V/μm delivers sensitivity 3 to 4 times higher than the conventional a-Se operated at 10 V/μm, while keeping the dark current density at 5 pA/mm2. This a-Se photoconductor will prove effective for low-dose X-ray imaging including mammography and tomosynthesis.

Four-Channel, 8 x 8 Bit, Two-Dimensional Parallel Transmission by use of Space-Code-Division Multiple-Access Encoder and Decoder Modules.

We experimentally demonstrate four-channel multiplexing of 64-bit (8 x 8) two-dimensional (2-D) parallel data links on the basis of optical space-code-division multiple access (CDMA) by using new modules of optical spatial encoders and a decoder with a new high-contrast 9-m-long image fiber with 3 x 10(4) cores. Each 8 x 8 bit plane (64-bit parallel data) is optically encoded with an 8 x 8, 2-D optical orthogonal signature pattern. The encoded bit planes are spatially multiplexed and transmitted through an image fiber. A receiver can recover the intended input bit plane by means of an optical decoding process. This result should encourage the application of optical space-CDMA to future high-throughput 2-D parallel data links connecting massively parallel processors.

Image fiber optic space-CDMA parallel transmission experiment using 8 x 8 VCSEL/PD arrays.

We experimentally demonstrate space-code-division multiple access (space-CDMA) based twodimensional (2-D) parallel optical interconnections by using image fibers and 8 x 8 vertical-cavity surface-emitting laser (VCSEL)/photo diode (PD) arrays. Two spatially encoded four-bit (2 x 2) parallel optical signals were emitted fiom 2-D VCSEL arrays and transmitted through image fibers. The encoded signals were multiplexed by an image-fiber coupler and detected by a 2-D PD array on the receiver side. The receiver recovered the intended parallel signal by decoding the signal. The transmission speed was 64 Mbps/ch (total throughput: 512 Mbps). Bit-error-rate (BER) measurement with a laterally misaligned PD array showed the array had a misalignment tolerance of 25 microm for a BER performance of 10(-9).

High speed parallel aligned liquid crystal spatial light modulator operated at 180 Hz

We made a parallel aligned liquid crystal spatial light modulator (PAL-SLM) respond within 6 ms to meet requirements for a single-SLM color projection system. This was done by adjusting thicknesses of its optical addressing layer and its light modulating layer.

Electrically addressed spatial light phase modulator

A nonpixelized electrically addressable spatial light phase- only modulator has been developed. The device consists of an optically addressed parallel aligned nematic liquid crystal spatial light modulator (PAL-SLM), coupling optics, an XGA liquid crystal display (LCD) which serves as an accurate addressable mask for the PAL-SLM, a laser diode (LD) for illuminating the LCD and collimating optics for the LD. The device has a phase modulation capability of over 2(pi) radians and a high diffraction efficiency of greater than 35% at a spatial frequency of 10 lp/mm of binary (0,(pi) ) grating. Also when a multilevel (0, 0.5(pi) , (pi) , 1.5(pi) ) grating was written in the device, a diffraction efficiency of greater than 70% at a spatial frequency of 10 lp/mm was obtained. Moreover, surplus diffraction light (diffraction noise) caused by the pixelized structure was reduced to less than 3% by the coupling optics, compared with almost 50% occurring in the LCD by itself.

Four multiplexed, 8/spl times/8-bit 2-D parallel transmission based upon space-CDMA

4 channel multiplexing for 64 bit two dimensional (2-D) parallel optical data link based upon optical space-CDMA is experimentally demonstrated by using newly developed modules of optical spatial encoder and decoder. Each 8/spl times/8 bit-plane (64 bit parallel data) is optically encoded with an 8/spl times/8 2-D optical orthogonal signature pattern. The encoded bit-planes are spatially multiplexed, and it is transmitted through an image fiber having 10/sup 5/ cores. Each receiver can recover the intended input bit-plane via the optical decoding process. Total error-rate is 2.73%. This result will encourage the application of optical space-CDMA to future high-throughput 2-D parallel data links connecting massively parallel processors.

Visual alignment using image fiber micro-optics for long-distance, ultra-high throughput Space-CDMA based 2-D parallel optical data link

We experimentally demonstrate 100-m-long image fiber transmission of four-channel multiplexed two-dimensional signals. To upgrade the system throughput, we study several hundred Gb/s-class 2-D optical parallel data link using an image fiber for the transmission and 2-D arrays of vertical-cavity surface-emmitting laser diodes (VCSELs) and p-i-n photo-diodes (PDs) as the transmitter and the receiver respectively. This system employs space code division multiple access (Space-CDMA) to multiplex 2-D optical parallel signals. To establish multi-channel optical link between 2-D VCSEL array and PD array with a high alignment precision and a good repeatability, we develop a novel visual alignment technique using a micro-optic image fiber coupler, which consists of miniature cube beamsplitter and graded index (GRIN) rod lenses. The effectiveness of the visual alignment with the image fiber coupler is experimentally demonstrated. This result will encourage the application of optical space-CDMA using an image fiber and 2-D arrays of VCSELs and PDs to future high-throughput 2-D parallel data links connecting massively parallel processors.