Masaki Yumoto

Thermo-optical modeling of high power operation of 2 μm codoped Tm,Ho solid-state lasers

The results of coupled thermo-optical modeling of Tm,Ho solid-state laser operation are reported. A rate dynamics model integrated together with a TEM00 distribution for the total number of stimulated photons is coupled with a two-dimensional time-dependent heat transfer model. The heat transfer model includes absorption, heat release, and transfer inside the crystal as well as the thermal effect of the spontaneously emitted infrared radiation. In water cooled crystal operation this radiation is shown to be absorbed within the water boundary layer, producing significant inhibition of dissipation of the heat released inside the crystal. This effect leads to crystal superheating and significant inhibition of laser energy output. Absorption loss, in particular, due to water vapor present in the cavity is found to decrease significantly laser pulse energy. Numerical results are compared to previous (Tm,Ho:yttrium lithium fluoride) and new (Tm,Ho:ceramic yttrium aluminum garnet) experimental data and discussed...

Electronic Wavelength Tuning of Tunable Laser with Acousto-Optic Tunable Filter

We investigated the design of an acousto-optic tunable filter (AOTF) as a wavelength-tuning element of a tunable laser in terms of the dependences of the diffraction efficiency, bandwidth and stability of the wavelength on the internal temperature of the AOTF. In a simulation, the diffraction efficiency reached more than 90% in the wavelength region from 700 to 1000 nm, and the variation of diffraction efficiency was less than ±1.0% at temperatures from 20 to 30 °C in the AOTF. In an experiment, we demonstrated that the internal temperature of the AOTF induces a wavelength shift of 0.12 nm/°C. Most experimental results for the output characteristics of the tunable laser with the AOTF agreed with those predicted numerically.

128 mJ/Pulse, Laser-Diode-Pumped, Q-Switched Tm:YAG Laser

We demonstrated a high-pulse-energy, laser-diode-pumped Tm:YAG laser in AO Q-switched operation. In laser experiments, we introduced a composite-type Tm:YAG laser rod with undoped endcaps. The rod was pumped by 785-nm quasi-continuous-wave laser diodes in a side-pumping configuration. To obtain a high energy and high beam quality, we designed a Tm:YAG laser cavity utilizing the thermal lens effect of the laser rod. A Q-switched output energy of 128 mJ at 10 Hz with a pulse width of 160 ns was obtained from the laser oscillator at room temperature, and the slope efficiency reached 14%. A high beam quality in the TEM00 mode was also realized.

Mid-infrared tunable optical parametric oscillator pumped by a Q-switched Tm,Ho:YAG ceramic laser

Since resonant absorption of light caused by a variety of different molecular bond occurs in the mid-infrared (MIR) wavelength region, many applications using tunable MIR lasers have been reported. However, the applicable fields of the MIR tunable lasers have been restricted by their large size and high cost equipments. Therefore, we are developing a compact tunable MIR laser using an optical parametric oscillator (OPO). To obtain a long term stability and a high conversion efficiency, a diode-pumped and Q-switched Tm,Ho:YAG ceramic laser with a wavelength of 2.1 μm was adopted for the pump source. A maximum output energy of 40 mJ was obtained with the Tm,Ho:YAG ceramic laser at a pulse width of 150 ns and a repetition rate of 10 Hz. An experiment was performed using a singly-resonant OPO with a ZnGeP2 crystal pumped by another OPO with a wavelength of 2.1 μm. A threshold pump fluence of 0.2 J/cm2 and a slope efficiency of 60% were obtained at a signal and idler wavelengths of 3.3 and 5.6 μm, respectively. Using these results and a theoretical model calculation, the maximum output energy of MIR-OPO pumped with the Tm,Ho:YAG ceramic laser was estimated to be about 20 mJ.

All-solid-state rapidly tunable coherent 6-10 μm light source for lidar environmental sensing

We report on an all-solid-state rapidly tunable pulsed coherent 6-10 μm light source achieved in an optical parametric oscillator (OPO) pumped with an electronically tuned Cr:ZnSe laser and its application to lidar remote sensing for environmental detection. We designed a lidar system using the 6-10 μm light source and a telescope with a primary mirror of 50 cm and a high-efficient HgCdTe detector. The lidar system would be a valuable system in the measurement of chemical agents in the 100-300 m.

Q-switch operation of thulium and holmium-doped YAG ceramic laser at room temperature

Polycrystalline ceramic with activator ions has already become popular material as laser medium. In this study, composite type rod which consisted with doped- and undoped-YAG sections was prepared for the laser experiments. By the improvement of pumping chamber, pulse energy of 930 mJ was obtained from oscillator with TEM00 in normal pulse mode at 10 Hz at room temperature. Giant pulse generation was carried out by inserting an AO Q-switch into the laser cavity. Because of the limitation of surface damage thresholds on the laser rod and the mirrors, the maximum output energy up to 21 mJ was obtained.

Multi-Wavelength Spectroscopic Application Using Rapid and Random Wavelength-Tuned Mid-Infrared Light Source

We report a rapid and random wavelength-tuned mid-infrared (IR) light source with difference-frequency generation, using an electronically tuned dual-wavelength-oscillated Ti:Al2O3 laser. A tuning range from 5.4 to 8.2 µm is achieved. The wavelength stability of the mid-IR radiation is measured to be less than ±1.1 ×10-3 µm, and we realize rapid and random wavelength tuning with the switching speed of 1 ms. Using the light source, the real-time monitoring of acetone vaporization is demonstrated with absorption at 7.33 µm, compared with water with absorption at 6.25 µm. The results show that our mid-IR light source is attractive for multi-wavelength spectroscopic applications.

Electronically tuned Cr:ZnSe laser pumped with Q-switched Tm:YAG laser

We demonstrated a Q-switched, Tm:YAG-laser-pumped electronically tuned Cr:ZnSe laser, which was equipped with an acousto-optic tunable filter as a wavelength-tuning element. A tuning range from 2.17 to 2.71 μm and a maximum output energy of 7.9 mJ at 2.41 μm were realized. The energy conversion efficiency reached 34.1% at 2.41 μm. In addition, the Cr:ZnSe laser produced a high-quality beam in the TEM(00) mode.

50 mJ/pulse, electronically tuned Cr:ZnSe master oscillator power amplifier

We have developed an electronically tuned Cr:ZnSe master oscillator power amplifier with nanosecond pulse operation. The amplifier system produced a broad tuning range from 2.26 to 2.66 μm and an output energy exceeding 50 mJ was obtained over a tuning range of 100 nm. The maximum energy conversion reached 46.6% at a wavelength of 2.41 μm when the pump fluence was 1.4 J/cm2. In addition, we show that Cr:ZnSe is an excellent gain medium for pulsed laser amplifiers to obtain high pulse energy in the broad mid-IR region and high extraction efficiency under low input fluence of 100 mJ/cm2 order.

Development of a mid-infrared tunable optical parametric oscillator pumped by a Q-switched Tm,Ho:YAG laser

A compact and high-energy pulsed mid-infrared laser using an optical parametric oscillator (OPO) has been developed using a diode-pumped and Q-switched Tm,Ho:YAG ceramic laser with a wavelength of 2.09 μm as a pump source. A singly-resonant OPO with a 20 mm long AgGaSe2 crystal was used, and the crystal was set at an angle normal to the pump beam. The output idler pulse energy was up to about 200 μJ with the pump energy of about 6 mJ for both the type I and type II phase matching conditions. The wavelength of the idler pulses was 5.97 and 6.37 μm for type I and type II, respectively. The output characteristics predicted using a model calculation were in good agreement with the experimental results. It is suggested that the output idler pulse energy in the experiment is limited by the surface damage threshold of the AgGaSe2 crystal. By increasing the pump beam diameter from 1 to 3 mm (3-fold) and the pump energy from 6 to 54 mJ (9-fold), the idler pulse energy of 1.8 mJ (= 200 μJ × 9) will be obtained without increasing the pump intensity and without saturation of the output idler pulse energy.