Hirofumi Miyajima

Cesium vapor laser pumped by a volume-Bragg-grating coupled quasi-continuous-wave laser-diode array

The experimental demonstration of a broad-area Quasi-continuous-wave (QCW) laser-diode array pumped cesium vapor laser is reported in this letter according to our experimental results. We used a volume-Bragg grating to narrow the spectral linewidth for acquirement of enough resonance transitions. A glass cylindrical cell filled with cesium/buffer gas was set in an end-pumped flat-concave cavity and the gas pressure was set lower than 1atm. Using the QCW drive mode with the repetitive rate and pulse width of 1kHz and 50μs, respectively, 13.45μJ pulse energy has been achieved with the slope efficiency of about ∼1.8%.

360-W average power operation with a single-stage diode-pumped Nd:YAG amplifier at a 1- kHz repetition rate

We report a high-average-power laser-diode-pumped Nd:YAG master oscillator power amplifier system that has a minimum number of elements in the single multipass zigzag-slab amplifier stage and is used to pump a high-peak-power and high-average-power Ti:sapphire laser system. This phase-conjugated system produces an average power of 362 W at 1 kHz in a 30-ns pulse with an optical-to-optical conversion efficiency of 14%. With an external KTP doubler this system generates 132 W of green average output power at 1 kHz with a conversion efficiency of 60% when pumped at a power level of 222 W. To the best of our knowledge these results represent the highest average output power at both infrared and green wavelengths achieved in a single amplifier stage.

Jet-type, water-cooled heat sink that yields 255-W continuous-wave laser output at 808 nm from a 1-cm laser diode bar

A newly designed jet-type, water-cooled heat sink (the funryu heat sink, meaning fountain flow in Japanese) yielded 255-W cw laser output at 808 nm from a 1-cm bar made from InGaAsP/InGaP quantum-well active layers with a 67% fill factor [70 quantum-well laser diode (LD) array along the 1-cm bar]. A funryu heat sink measuring 1.1 mm in thickness gave the LD 0.25 degrees C/W thermal resistance, one of the lowest values achieved with a 1-cm LD bar. Over a short period of operation, the device reached a maximum cw power of 255 W. To the best of our knowledge, this is the highest power ever achieved in 808-nm LD operation. In the future, the funryu heat sink may be capable of 80-W cw operation over an extended lifetime of several thousand hours.

High-Brightness Narrow-Bandwidth High-Power Laser-Diode Array Based on an External-Cavity Technique

We describe an off-axis external-cavity designed to yield a high-brightness, narrow-bandwidth emission from a high-power laser-diode array (LDA). This off-axis external cavity consists of a beam-transformation system and a volume Bragg grating. Using this external-cavity technique, a high-power broad-area LDA operating at a drive current of 25 A achieved a divergence angle of 2.3° and a bandwidth of 0.67 nm with an output power of 14.9 W.

High-Efficiency 894-nm Laser Emission of Laser-Diode-Bar-Pumped Cesium-Vapor Laser

We report a high-efficiency cesium-vapor laser with a high-gas-pressure (~3-atm helium and 0.49-atm ethane) cell pumped by a high-power external-cavity laser-diode bar. Peak laser power of 12.1 W at 894 nm was obtained, when the absorbed peak pump power was 23.1 W. The achieved slope efficiencies with the incident pump power and the absorbed pump power were 33 and 81.7%, respectively.

Single-wavelength 5.6 kW direct diode laser with a high-efficiency beam combination

Our group developed a new optical system equipped with a slit mirror to combine the output power from two LD stack modules (30 bars each) and focus them into a single beam. The new device realized a CW output power of more than 3 kW with a transfer efficiency of 92%. Furthermore, by combining beams from two added stack modules to the main optical path using polarization coupling, we have achieved a CW output power of 5.6 kW with a single wavelength of 808 nm. The focused spot size was 2.4 mm×0.5 mm (the power intensity was 467 kW/cm2) at a practical focal length of 100 mm. The transfer efficiency from the LD output to the focal spot was estimated to be 86.5%. The total energy efficiency against the input electricity was approximately 45%. This value is the highest ever reported in a high-power LD applied to material processing, and a full threefold higher than that of the diode-pumped Nd:YAG laser, a device heretofore known to be highly efficient (at around 15%).

Divergence-narrowed external-cavity broad-area laser-diode array

We demonstrate a divergence-narrowed broad-area laser-diode array (LDA) with an external-cavity configuration that consists of a beam-transformation system and an off-axis reflecting mirror. Sixteen off-axis external-cavity laser diodes are formed for the high-power LDA. At a drive current of 21.2 A (four times the threshold current), the divergence angle full width at half maximum of the slow axis is reduced from 7.26° without the external cavity to 1.45° with the external cavity. The external-cavity LDA achieves a cw power output of 9.8 W, which is 60% of the free-running LDA.

Advanced devices for near-infrared time-resolved spectroscopy and optical computed tomography: high-sensitive/fast PMT, high-power PLP, miniaturized CFD/TAC module, and high-speed multichannel signa

We have developed a 64-channel time-resoled spectroscopy (TRS) system based on a time-correlated single photon counting (TCPC) method to achieve near infrared spectroscopy and/or optical computed tomography (CT) for clinical applications. This system employs advanced devices such as a high power picosecond light pulser (PLP), a high sensitive/fast photomultiplier tube (PMT) and a high speed signal processing circuit. The PLP offers an average optical power of around 0.25 mW, and the PMT has a quantum efficiency greater than 2% at 800 nm. The signal processing circuit is composed of miniaturized CFD/TAC modules and signal acquisition unit with 1 MHz ADC, and provides 64 independent TCPC circuits for time- of-flight measurement. System performance was estimated by measuring transmitted and reflected light passing through tissue-like phantom models simulating the human breast and infant head. We concluded that this system has the sufficient performance for optical CT utilizing time-of-flight measurement.

High power density vertical-cavity surface-emitting lasers with ion implanted isolated current aperture

We report on GaAs-based high power density vertical-cavity surface-emitting laser diodes (VCSELs) with ion implanted isolated current apertures. A continuous-wave output power of over 380 mW and the power density of 4.9 kW/cm2 have been achieved at 15 °C from the 100-μm-diameter aperture, which is the highest output characteristic ever reported for an ion implanted VCSEL. A high background suppression ratio of over 40 dB has also been obtained at the emission wavelength of 970 nm. The ion implantation technique provides an excellent current isolation in the apertures and would be a key to realize high power output from a VCSEL array.

Efficient and Reliable High-Power Laser Diode Bars With Low-Smile Implementation

We report on our recent development of high power 940 nm laser diodes (LDs) bars. The optimization of the laser structure and thermal management allow over 200-W continuous wave (CW) operation with 58% conversion efficiency from the 1-cm LD bar. We have also developed an assembly technique, and have suppressed the smile of LD bars mounted even on Cu heatsinks with hard solder to less than 1 μm. Long term stability have also be confirmed under 200-W CW operation.