Hiroaki Furuse

Generation of 500-mJ nanosecond pulses from a diode-pumped Yb:YAG TRAM laser amplifier

Amplification of 10-ns laser pulses to an energy of 500 mJ at a 10-Hz repetition rate in a cryogenic multi-pass multi-total-reflection-active-mirror (multi-TRAM) amplifier was achieved. By using a multi-TRAM, which is a YAG ceramic composite with three Yb:YAG active layers, a maximum single-pass gain of 12 and a total storage energy of 1.5 J were obtained.

High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics

The temperature dependence of the Verdet constant of a 0.8 at % Ti-doped terbium aluminum garnet (TAG) ceramics was investigated using lasers with wavelengths of 632.8 and 1064 nm. A high value of the Verdet constant was obtained at 296 K – namely, 184 and 53 rad/Tm for 632.8 and 1064 nm, respectively. The Verdet constant of the Ti:TAG ceramics at 1064 nm is about 1.5 times higher than that of the terbium gallium garnet (TGG) ceramics. The transmittance of this sample was about 75% at the wavelength of 1 μm. This material represents a possible candidate for next-generation devices that utilize the magneto-optic effect.

Thermo-optic properties of ceramic YAG at high temperatures

The thermo-optic coefficient dn/dT at 632.8 nm and thermal expansion coefficient α of transparent ceramic yttrium aluminum garnet (YAG) were measured between room temperature and 600 K. The data showed that dn/dT increases with temperature and α is in good agreement with that of single-crystal YAG. To the best of our knowledge, these are the first experimental data of the thermo-optic properties of highly transparent ceramic YAG above room temperature. We also present, using previously reported values measured below room temperature, fitting parameters for dn/dT that are valid over a wide temperature range (70–600 K) with an average error of 2.0%.

Amplification characteristics of a cryogenic Yb 3+ :YAG total-reflection active-mirror laser

We have studied the amplification characteristics of a cryogenically cooled Yb³⁺:YAG total-reflection active-mirror (TRAM) ceramic laser including wavefront distortion, birefringence loss, small signal gain (SSG), and temperature rise for developing high-performance master oscillator power amplifier (MOPA) systems. A 0.6 mm thick Yb³⁺:YAG ceramic sample was used, and maximum pump intensity ~10u2009u2009kW/cm² was reached. The transmitted wavefront was measured by using a Shack-Hartmann wavefront sensor, and we evaluated the thermal lens focal length and Strehl ratio for different pump conditions. We have also observed a butterfly-like leakage profile of thermally induced birefringence loss at the maximum pump intensity. From SSG measurements, we obtained moderate laser gain of G=3 for one bounce with a near aberration-free wavefront. Gain calculations, which included also temperature dependence of the emission cross section and reabsorption of Yb³⁺:YAG, were in good agreement with the experiments. These experimental results will be useful as benchmark data for numerical simulations of temperature distribution in TRAM and for designing multikilowatt-class high-performance MOPA systems.

Magneto-optical characteristics of holmium oxide (Ho 2 O 3 ) ceramics

Transparent Ho2O3 ceramics are fabricated and their magneto-optical characteristics are reported for the first time, to the best of our knowledge. The value of the Verdet constant was measured in the 560–1064 nm wavelength range, and the value at 1064 nm is 46.3 rad/Tm. This corresponds to the Verdet constant for terbium aluminum garnet (TAG), which is ~1.3 times higher than that of terbium gallium garnet (TGG). The in-line transmittance at 1 μm wavelength is only ~60%, but the optical properties can be further improved by optimizing sintering conditions. This is a new potential magneto-optic material that can be applicable for high-average-power lasers.

Cryogenically cooled CeF3 crystal as media for high-power magneto-optical devices

The thermally induced depolarization and Verdet constant of CeF3 crystals-their most important characteristics-have been studied in the 79-293 K temperature range. It has been found that thermal effects reduce substantially upon cooling down to 79 K and the Verdet constant grows in inverse proportion to the temperature. It was shown that CeF3 crystals are not inferior to TGG as a medium for Faraday isolators, including cryogenic ones.

Thermo-optic effects of ceramic TGG in the 300–500 K temperature range

The thermal expansion coefficient α and the thermo-optic coefficient dn/dT were measured using a He-Ne laser (λ = 632.8 nm) between 300 K and 500 K for ceramic and single-crystal terbium gallium garnet (TGG). For ceramic TGG, the experimental values of α and dn/dT at 300 K were 7.0×10−6 K−1 and 17.5×10−6 K−1, respectively. We proposed fitting relations regarding the temperature dependence of α and dn/dT. Both α and dn/dT increased with temperature, and the temperature dependencies for ceramic TGG were in agreement with those of single-crystal TGG with orientation. We then evaluated thermally induced depolarization for an average laser power of 1 kW. The depolarization was evaluated to be larger than 0.01 at room temperature. Moreover, we simulated the thermal lens focal length and found that the temperature dependence is very slight compared to its dependence on the beam radius.

Boiling effect in liquid nitrogen directly cooled Yb 3+ :YAG laser

Liquid nitrogen (LN2) behavior on the surface of excited Yb(3+):YAG is investigated using fluorometry. From the time-resolved temperature variations and integrated fluorescence spectra intensity on this directly cooled Yb(3+):YAG surface, we observe a phase transition of LN2 from nucleate boiling to film boiling. As a result of this pool boiling, good beam quality should occur when the temperature and heat flux at an excited surface of Yb(3+):YAG are below 95 K and 15.8u2009u2009W/cm2, respectively. That is, the LN2 should remain in a steady state of nucleate boiling to produce good beam quality using pool boiling.

Cryogenic Multi-TRAM Amplifier Producing Energy of 500 mJ at 10 Hz Repetition Rate

Experimental amplification of 10 ns pulses to energy of 500 mJ at 10 Hz repetition rate in cryogenic multi-pass multi-TRAM (Total-Reflection-Active-Mirror) amplifier was achieved for the first time.