Norihiro Fukuchi

Generation of high-quality higher-order Laguerre-Gaussian beams using liquid-crystal-on-silicon spatial light modulators

We report on the high-quality holographic generation of higher-order Laguerre-Gaussian (LG) beams using a liquid-crystal-on-silicon spatial light modulator. The effects of the input beam pattern on the output LG beam quality are investigated in detail through theoretical discussions and experiments. Correlation analyses between observed beam patterns and theoretical mode profiles reveal that higher beam quality is achieved for output LG beams generated from a top-hat input beam than from a Gaussian input beam.

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.

Mode purities of Laguerre-Gaussian beams generated via complex-amplitude modulation using phase-only spatial light modulators.

We investigate output mode purities of Laguerre-Gaussian (LG) beams generated from four typical simultaneous amplitude and phase modulation methods with phase-only spatial light modulators (SLMs). Numerical simulations supposing the practical SLM, i.e., stepwise phase modulation with a pixelated device, predict an output mode purity of beyond 0.969 for the LG beams of less than radially and azimuthally fifth order. Experimental results of generating LG beams are also shown to demonstrate the effects of the simultaneous phase and amplitude modulation.

Universal generation of higher-order multiringed Laguerre-Gaussian beams by using a spatial light modulator.

Laguerre-Gaussian (LG) beams of various higher-order radial modes are generated by using a reflective phase-only liquid crystal on silicon (LCOS) spatial light modulator (SLM). Because of the LCOS SLMs phase-modulation characteristic of a wide spatial bandwidth, a phase modulation scheme effectively generates higher-order LG beams of up to the fifth-order radial mode. We also perform correlation analyses between the observed and the theoretical two-dimensional mode profiles to universally obtain correlation coefficients of more than 0.946, which suggest mode generations of high quality.

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.

Lensless Vanderlugt optical correlator using two phase-only spatial light modulators

A lensless Vanderlugt optical correlator using two phase-only spatial light modulators (SLMs) is proposed. The SLMs are used for displaying input and filter patterns respectively. The SLMs are also used as programmable lenses in order to realize the lensless construction. This lensless system is simple and its alignment adjustment is easy. The performance of the SLMs as programmable lenses is also described.

Shaping tight-focusing patterns of linearly polarized beams through elliptic apertures

We propose elliptic deformation of apertures for shaping focusing patterns of tightly focused linearly polarized beams. Numerical integration of the vectorial Rayleigh-Sommerfeld diffraction formula predicts the formation of symmetric focusing patterns by shrinking the aperture shape in the direction perpendicular to the polarization of the beams. The elliptic deformation is also applied to a focused linearly polarized beam through an annular aperture to demonstrate the formation of smaller symmetric focal spot than that through a simple oval aperture.

Highly stable wavefront control using a hybrid liquid-crystal spatial light modulator

We have developed a hybrid liquid-crystal spatial light modulator (LC-SLM) in which a reflection-type optically addressed (OA) LC-SLM is combined with a liquid crystal display (LCD) via coupling optics. The LCD is controlled by 8-bit video signals. This construction allows us to eliminate diffraction artifacts due to the pixellated structure of the LCD from the modulated light of the hybrid LC-SLM and enables the hybrid device to be electrically addressed. Nematic liquid crystal molecules in the OA LC-SLM are homogeneously aligned to create pure phase modulation having a variation of one wavelength. These features make the hybrid LC-SLM suitable for wavefront control. Wavefront control with large phase stroke and high stability is desirable to realize high-quality adaptive optics, high-quality optical manipulation, and so forth. Therefore, we experimentally investigated the stroke and stability of the phase modulation of the hybrid LC-SLM. We used the wrapped-phase representation to expand the phase stroke virtually. The results show that the hybrid LC-SLM could produce a phase stroke of more than 20 wavelengths and a phase instability of less than 0.001 wavelengths. We also conducted an experiment to compensate for the nonlinearity of the phase modulation. The results suggest that nonlinearity could be suppressed to less than 1%, and that approximately 200 gray levels over one wavelength of phase variation were available, even after compensation.

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.

High-quality generation of higher-order Laguerre-Gaussian beams using liquid crystal on silicon spatial light modulator

We report holographic generation of higher-order Laguerre-Gaussian (LG) beams using a liquid crystal on silicon spatial light modulator (LCOS-SLM) device. In our experimental set-up, a flat-top light beam was projected on the LCOS-SLM to generate LG beams of various mode indices without changes of the optical system. Additionally, the size of the holographic phase pattern was optimized for each beam to maximize the mode purity of the obtained beam. Holographic generation of LG beams is easily influenced by a distortion of the optical system and deviation of the phase setting from an ideal one. Nevertheless, we obtained high-quality LG beams with an additional phase pattern on the LCOS-SLM for canceling the distortion of the optical system and with calibration of the phase control voltage for precise expression of the phase patterns. Numerical analyses are also performed for two-dimensional beam profiles to verify the quality of the obtained beams. Through fitting the obtained profiles to theoretical ones, we calculate the correlation coefficients R between the observed and fitted profiles to find that R > 0.95 for all beams and that the correlation coefficients behave similarly to the theoretically estimated mode purities, facts indicating that the quality of the obtained LG beams is close to the theoretical limit in our experiments.