Toshihiro Somekawa

Terahertz generation by optical rectification in lithium niobate crystal using a shadow mask

A simple approach to generate high energy, frequency and bandwidth tunable multicycle THz pulses by optical rectification (OR) of spatially shaped femtosecond laser pulses in the lithium niobate (LN) crystal is proposed and demonstrated. A one dimensional binary shadow mask is used as a laser beam shaper. By building the masks image in the bulk LN crystal with various demagnifications, the frequency of THz generation was tuned in the range of 0.3 - 1.2 THz. There exist also an opportunity to tune the bandwidth of THz generation from 20 GHz to approximately 1 THz by changing the optical beam size on the crystal. The energy spectral density of narrowband THz generation is almost independent of the bandwidth and is typically 0.18 μJ/THz for ~1 W pump power at 1 kHz repetition rate.

Temperature dependence of laser-induced damage threshold of optical coatings at different pulse widths

The temperature dependence of the laser-induced damage threshold on optical coatings was studied in detail for laser pulses from 123 K to 473 K at different temperature. For pulses longer than a few picoseconds, the laser-induced damage threshold of coated substrates increased with decreasing temperature. This temperature dependence was reversed for pulses shorter than a few picoseconds. We describe the physics models to explain the observed scaling. The electron avalanche is essential to explain the differences in the temperature dependence.

Direct Absorption Spectroscopy of CO2 Using a Coherent White Light Continuum

A white light continuum was generated by focusing the terawatt 800 nm femtosecond laser pulse in Kr gas. The white light spectrum spanned in a broad wavelength range from 300 to more than 2200 nm. The white light continuum was used for the first time to perform direct absorption spectroscopy of CO2 at around 2000 nm in a laboratory absorption cell. The present system can be used for measuring the CO2 concentrations with an accuracy of 1–2 ppm (ca. 0.5%) in atmosphere for around 5.5 km propagation lengths through the air.

Wafer-to-wafer selective flip-chip transfer by sticky silicone bonding and laser debonding for rapid and easy integration test

Wafer-bonding-based integration can be rapidly and easily tested between different types of devices by wafer-to-wafer flip-chip transfer technology described in this paper. Devices to be tested (e.g. MEMS) on a support wafer are bonded and electrically connected with a target wafer (e.g. LSI) using sticky silicone bumps, and then any of the devices are selectively debonded from the support wafer by backside laser irradiation. After transferred, the device is temporary sealed with a silicone ring, and underfill polymer can be used for permanent bonding. Because silicone bonding is made just by physical contact at room temperature, and the elasticity of silicone absorbs mismatch in thermal expansion, integration between different materials of wafer is possible. For practical demonstration, LiNbO3-based SAW resonators were transferred to an LSI wafer.

Remote Detection and Identification of CO2 Dissolved in Water Using a Raman Lidar System

We demonstrated the first range-resolved detection and identification of CO2 dissolved in water by Raman lidar. A frequency doubled Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (532 nm) is used as the lidar light source, and the Raman signals at ~1273 and ~1380 cm-1 from CO2 dissolved in water were detected. In lidar application, CO2 dissolved in water was identified in glass bottles 20 m away by using the CO2 Raman band at ~1380 cm-1.

Furfural analysis in transformer oils using laser raman spectroscopy

The concentration of furfural in transformer oils is a well known indicator for decomposition of insulating paper. Using laser Raman spectroscopy, we performed detection experiments of furfural dissolved in insulating oils without complex and time consuming preprocessing procedures. Furfural was characterized by Raman signal at ~1707 cm-1, where no spectral interferences caused by oil-derived Raman signals occur. The results show that laser Raman spectroscopy is a useful alternative method for transformer health diagnosis.

Differential optical absorption spectroscopy measurement of CO 2 using a nanosecond white light continuum

We built a differential optical absorption spectroscopy system to measure near-surface CO2 absorption in the atmosphere using a nanosecond white light continuum. The white light laser can cover wavelengths ranging from 420 to 2400 nm, where the CO2 and H2O absorption lines are located. At an optical path length of 568 m, it was possible to evaluate atmospheric CO2 concentration from absorption bands of CO2 and H2O in the vicinity of 2000 nm detected by broadband white light simultaneously.

Nondestructive Sensor Using Microwaves From Laser Plasma by Subnanosecond Laser Pulses

Conventional ground-penetrating radar (GPR) requires large-aperture antennas or long-span measurements to survey a remote location precisely. We propose a laser-driven GPR (LGPR) as a new detection method. LGPR uses microwaves from laser-produced plasmas as remote transmitters and can survey a remote location using a compact instrument. We performed numerical simulations to investigate the radiation mechanism of microwaves from laser plasmas and confirmed the pulsewidth of the laser suitable for LGPR. Experiments with subnanosecond pulse lasers clarified the feasibility and detection performance of LGPR.

Depolarization Light Detection and Ranging Using a White Light LIDAR System

We carried out depolarization light detection and ranging (LIDAR) measurements for the first time using a coherent white light continuum with multi-wavelength backscatter measurements. The white light continuum from the UV to the IR region was generated in Kr gas by a terawatt femtosecond laser system. The LIDAR system consisted of a depolarization channel at 450 nm and the five-wavelength Mie scattering channels at 350, 450, 550, 700, and 800 nm. This technique can potentially open a promising avenue toward simultaneous depolarization LIDAR at multi-wavelength.

Analysis of dissolved C 2 H 2 in transformer oils using laser Raman spectroscopy

We have developed a laser Raman spectroscopy technique for assessing the working conditions of transformers by measuring dissolved C2H2 gas concentrations present in transformer oils. A frequency doubled Q-switched Nd:YAG laser (532 nm) was used as a laser source, and Raman signals at ~1972 cm(-1) originating from C2H2 gas dissolved in oil were detected. The results show that laser Raman spectroscopy is a useful alternative method for detecting transformer faults.