Hideki Okamura

Light-Driven Actuator with Shape Memory Alloy for Manipulation of Macroscopic Objects

With the aim of manipulating macroscopic objects, the possibility of improving energy conversion efficiency of light-driven actuators (LDAs) was investigated. Thermal expansion is one of the most efficient mechanisms reported so far but a rough estimation revealed that its upper limit in efficiency is ∼10−4%. Thermodynamics suggests several percent can be achieved in theory, so by using a suitable mechanism there is room for improvement. Based on those considerations we propose a LDA using a pre-tensioned wire of shape memory alloy. This design reduces the energy loss in converting the input light into mechanical motion. We demonstrated that a force larger than 1 Newton and an energy conversion efficiency of 1 percent can be achieved.

Light-Driven Actuator with Energy Conversion Efficiency in the Order of 1%

We propose a light-driven actuator capable of actuating macroscopic objects using pre-tensioned wire of NiTi shape memory alloy of a diameter of 50 µm. The use of pre-tensioned wire made it possible to avoid concentration of stress, and enabled large load to be applied. With 1.0 W Argon-ion laser beam, this actuator produced work of 3.7 mJ with 0.5 s response time. The efficiency was 0.97% when a load of 0.98 N was applied. High efficiency can be ascribed to use of shape memory alloy, suitable coupling of a materials state change with the actuating mechanism, and low heat dissipation.

Shift lens external-cavity diode laser for broad wavelength tuning and switching

We propose an external-cavity laser diode that uses an electromagnetically actuated collimating lens for wavelength tuning. Because the wavelength in the grating equation is highly sensitive to the incident angle, a slight (approximately 1 microm) transverse displacement of the collimating lens induces a wavelength shift of approximately 1 nm. We experimentally demonstrate a single-mode, stable, reproducible, and cw tuning over a range of 8 nm. The wavelength is proportional to the current applied to the lens actuator, and no hysteresis is observed within the experimental error. By changing the current, we achieve fast and reproducible wavelength switching.

Evaluation of tracking ability of a phase conjugate mirror using a CCD array and spatial light modulator for optical energy transmission

We investigate the tracking ability of an optical phase conjugator using a commercial CCD array and a projector LCD panel. This system allows one to use two separate laser oscillators for capturing interference patterns and generating phase conjugate light. Since a long coherence length is not required for the latter part, amplification of the phase conjugate light can be easily attained by using a laser oscillator for high-power applications such as machining. The wavelengths of the two laser oscillators can be independently chosen. For our experimental configuration an amplification factor of 7.8×10(4) is theoretically possible. Also, a formula for the maximum tracking range is derived. The proposed system is particularly suitable for power transmission by light.

Laser irradiation induced vibrations in solids

Pulsed Laser beam irradiation induced nm peak to peak resonant vibrations in solids were generated, detected and analyzed. For the evaluation of the induced vibrations, transducers and optical methods were used. The enhancement effect of vibrational amplitude by resonance and other methods was confirmed. The existence of vibrations of picometer amplitude induced by mechanical means in solids are visualized by use of synchronous illumination and optical manipulation.

Simultaneous optical tracking of multiple targets in a field of view greater than 20

A multi-target tracking system using a self-starting optical phase conjugator was developed in this study. This system generates phase conjugate light (PCL) in a Nd:YAG resonator. Accurate tracking capability with a beam wander of 120 μrad and constant PCL generation were confirmed over a field of view greater than 20°. This field of view was expanded by means of collector optics positioned in front of the phase conjugator. The developed scheme enables automatic and simultaneous optical energy transfer to multiple distant targets by utilizing the unique properties of optical phase conjugation of automatic target tracking and pointing.

Optical phase conjugation by four-wave mixing in Nd:YAG laser oscillator for optical energy transfer to a remote target

A self-starting phase conjugator was designed for optical energy transfer to a remote target. Saturable-gain four-wave mixing in a laser resonator was achieved using a flash-lamp pumped Nd:YAG crystal and phase-conjugate light (PCL) generation were verified. Wavefront correction experimentation revealed that beam wander caused by air turbulence is compensated. Tracking capability was demonstrated in the range of 9 mrad with tracking accuracy of ±0.04 mrad. The maximum field of view was measured to be 4.7°. Dependence of phase-conjugate light energy on reference light energy was investigated. The maximum output of 320 mJ was obtained. The temporal behavior of PCL is discussed based on the four-wave mixing mechanism. Unlike a conventional loop resonator type phase conjugator, this system is applicable for wireless energy transfer to a remote target.

Optimization of Photothermal Oscillators Based on Thermal Diffusion Analysis

The effect of material parameters on the oscillation amplitude of a photothermal actuator/oscillator was discussed, and a criterion to maximize the amplitude was derived. Bending stress is inversely proportional to the thickness of an irradiated portion, while a certain thickness is necessary for a temperature gradient to be created. Numerical simulations suggested the bending effect becomes the largest when the thickness is 1.1039 times the thermal diffusion length of the substance. An experiment using Cu samples with thickness ranging from 0.05 to 0.5 mm confirmed the theoretical predictions.

On the efficiency of heat engines by pulsed laser

The purpose of this paper is to present a possibility of improving the efficiency of light driven actuators by using pulsed laser and to investigate theoretical limit of its efficiency, thereby providing an insight for the design of an efficient light driven actuator in the future. Many light driven actuators based on the heat deposit can be regarded as heat engines, and consequently their efficiencies are subjected to the limitation from Carnots theory, which dictates that the maximum efficiency is larger for larger temperature difference between the high- and the low-temperature reservoirs. If one uses focused laser pulses, an extremely high temperature difference can be created for a short fraction of time, therefore, there is a possibility of achieving a higher efficiency. In the theoretical treatment, a cycle in which heat is given instantaneously in an adiabatic condition was considered. The formula for the maximum efficiency was derived, which gives about the half that of Carnot cycle efficiency at an ambient temperature of 300K. In an ideal case, for the instantaneous temperature increase of 200K the maximum efficiency can be about 20 %.

Retrodirective tracking of a moving target using phase conjugate light generated in a Fabry-Perot Nd:YAG laser

This report describes the retrodirective tracking of a target moving at a constant speed using a Fabry–Perot laser resonator-type self-starting phase conjugator. Three-dimensional pointing ability was confirmed with this system: the generated phase conjugate light can focus at a target position. To track the moving targets, target-reflected light is Doppler-shifted, which causes wash-out of the gain grating. To address this problem, numerical analysis and experiments showed that shortening the pulse width is an efficient means of eliminating the gain grating wash-out effect. For the measured target speeds of 0–80 mm/s, the phase conjugate light output is unaffected when the pulse width of 18 ns was used, in contrast to the case of 400 μs pulse width, where the phase conjugate light output rapidly decreased as the target speed approached 10 mm/s. Numerical simulation revealed that phase conjugate light can even be generated for fast-moving targets such as space debris in low-earth-orbit with pulses of seve...