Seiji Shimizu

Liquid-cooled 24 W mid-infrared Er:ZBLAN fiber laser

A 24 W liquid-cooled CW 3 microm fiber laser with a multimode-core Er-doped ZBLAN fiber has been developed. The output power of 24 W and an optical-to-optical efficiency of 14.5% (with respect to incident pump power) were obtained with 975 nm diode pumping. Efficient cooling was implemented by a combination of fluid cooling over the entire length of the fiber and conductive cooling at both end faces of the fiber. Consequently, stable high-power operation was demonstrated. To our knowledge, this is the highest output power obtained by a 3 microm fiber laser. Furthermore, the high power can be further scaled up, since the output power in the present work is limited only by the available pump power.

12 WQ-switched Er:ZBLAN fiber laser at 28 μm

A diode-pumped, actively Q-switched 2.8 μm fiber laser oscillator with an average output power of more than 12 W has been realized through the use of a 35 μm core erbium-doped ZBLAN fiber and an acousto-optic modulator; to our knowledge, this is the first 3 μm pulsed fiber laser in the 10 W class. Pulse energy up to 100 μJ and pulse duration down to 90 ns, corresponding to a peak power of 0.9 kW, were achieved at a repetition rate of 120 kHz.

Stable 10 W Er:ZBLAN fiber laser operating at 2.71-2.88 μm.

We have developed a diode-pumped tunable 3 μm fiber laser with a cw output power of the order of 10 W with the use of an erbium-doped ZBLAN fiber. A tunability range of 110 nm (2770 to 2880 nm) with an output power between 8 and 11 W was demonstrated. As the pump power was increased, the center of the wavelength range was shifted toward longer wavelengths, but the width of the wavelength range was largely unaffected. The total tunability range for various pump power levels was 170 nm (2710 to 2880 nm). To our knowledge, this is the highest performance (output power and tunability) obtained from a tunable 3 μm fiber laser.

X-ray polarization spectroscopy for measurement of anisotropy of hot electrons generated with ultraintense laser pulse

Polarization spectroscopy is a useful diagnostic method to measure directly the anisotropy of hot electron velocity distribution function inside ultrahigh intense laser produced plasmas. Polarization of a Cl–Heu2002α line (2.79u2002keV) from double-layer targets, which consist of polystyrene and chlorinated polystyrene, was observed with a toroidally curved crystal spectrograph. It was found that the line from the target surface is polarized more parallel to the surface, whereas that from a deeper region is more perpendicular to it. The polarization of emitted x rays corresponds to the shape of velocity distribution function of hot electrons. The anisotropic shape of hot electron velocity distribution function depending on depth of a target was clarified for the first time using this diagnostic method.

Ultrafast Single-Shot Optical Oscilloscope based on Time-to-Space Conversion due to Temporal and Spatial Walk-Off Effects in Nonlinear Mixing Crystal

A simple method for single-shot sub-picosecond optical pulse diagnostics has been demonstrated by imaging the time evolution of the optical mixing onto the beam cross section of the sum-frequency wave when the interrogating pulse passes over the tested pulse in the mixing crystal as a result of the combined effect of group-velocity difference and walk-off beam propagation. A high linearity of the time-to-space projection is deduced from the process solely dependent upon the spatial uniformity of the refractive indices. A snap profile of the accidental coincidence between asynchronous pulses from separate mode-locked lasers has been detected, which demonstrates the single-shot ability.

Efficient continuous wave and quasi-continuous wave operation of a 28 μm Er:Lu_2O_3 ceramic laser

We have demonstrated a highly efficient 2.8 μm Er-doped Lu2O3 ceramic laser and investigated the lasing dynamics by time-resolved spectroscopy. During room-temperature continuous wave operation, a slope efficiency of 22% was achieved with a high-quality transparent ceramic. To our knowledge, this is the highest slope efficiency obtained by an Er:Lu2O3 ceramic laser. In addition, an output peak power of 1.2 W was obtained during quasi-continuous wave operation. Time-resolved spectroscopy showed that the emission wavelengths exhibited a red shift from 2715 to 2845 nm, which indicated that continuous wave operation may be possible at 2740 and 2845 nm.

Energetic ion generation by Coulomb explosion in cluster gas and a low-density plastic foam with an intense femtosecond laser

The energy distributions of protons emitted from the Coulomb explosion of hydrogen clusters by an intense femtosecond laser have been experimentally obtained. Ten thousand hydrogen clusters were exploded, emitting 8.1-keV protons under laser irradiation of intensity 6 × 1016W/cm2. The energy distributions are interpreted well by a spherical uniform cluster analytical model. The maximum energy of the emitted protons can be characterized by cluster size and laser intensity. The laser intensity scale for the maximum proton energy, given by a spherical cluster Coulomb explosion model, is in fairly good agreement with the experimental results obtained at a laser intensity of 1016–1017 W/cm2 and also when extrapolated with the results of three-dimensional particle simulations at 1020–1021 W/cm2. Energetic proton generation in low-density plastic (C5H10) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm3. The foam target was irradiated by 100-fs pulses of a laser with intensity 1 × 1018 W/cm2. A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape explained well by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.

Characterization of Er:Lu 2 O 3 ceramic lasers for efficient emission at 2.8 μm

High-power mid-IR lasers at 3 μm wavelength have many potential applications in medical and industrial fields because of the strong absorption by water at that wavelength region. Recently, erbium-doped cubic rare-earth sesquioxide crystals (e.g. Lu<inf>2</inf>O<inf>3</inf>, Y<inf>2</inf>O<inf>3</inf>, and Sc<inf>2</inf>O<inf>3</inf>) are gaining attention as laser sources due to their lower phonon energies and higher thermal conductivities compared with YAG. In particular, Er:Lu<inf>2</inf>O<inf>3</inf> is known as suitable source because the thermal conductivity stays high even at a high doping level [1]. The high quality polycrystalline transparent ceramics of Lu<inf>2</inf>O<inf>3</inf> which became available in recent years will open up the possibility of high-power mid-IR lasers because of the advantages such as mechanical strength and manufacturability of large volume.

High Power 3 μm Erbium Fiber Lasers

Recent breakthrough in mid-infrared fluoride-fiber lasers has enabled generation of high average power of a few tens of watts. We will review the recent advances in high power 3 μm Er fiber lasers.