Masaki Tsunekane

Analytical and experimental studies on the characteristics of composite solid-state laser rods in diode-end-pumped geometry

In this paper, we report analytical and experimental studies on the characteristics of end-pumped composite laser rods with undoped end, using mainly Nd:YAG rods as an example. It is found that the peak temperature rise in a composite rod decreases to <70% of that in a noncomposite crystal. Thermal stress is dramatically reduced to <60% by employing the composite rod structure, We also demonstrate high-power operation of the diode-end-pumped composite Nd:YVO/sub 4/ rod and a maximum CW output power of 9.3 W was achieved, which is about 1.5 times higher than that in the noncomposite rod. This high-power performance of the composite rod is primarily attributed to the reduction of thermal stress inside the rod.

Composite, all-ceramics, high-peak power Nd:YAG/Cr 4+ :YAG monolithic micro-laser with multiple-beam output for engine ignition

A passively Q-switched Nd:YAG/Cr(4+):YAG micro-laser with three-beam output was realized. A single active laser source made of a composite, all-ceramics Nd:YAG/Cr(4+):YAG monolithic cavity was pumped by three independent lines. At 5 Hz repetition rate, each line delivered laser pulses with ~2.4 mJ energy and 2.8-MW peak power. The M(2) factor of a laser beam was 3.7, and stable air breakdowns were realized. The increase of pump repetition rate up to 100 Hz improved the laser pulse energy by 6% and required ~6% increase of the pump pulse energy. Pulse timing of the laser-array beams can by adjusted by less than 5% tuning of an individual line pump energy, and therefore simultaneous multi-point ignition is possible. This kind of laser can be used for multi-point ignition of an automobile engine.

High Peak Power, Passively

The compact (electric spark plug size), diode-pumped, passively Q-switched Nd:YAG/Cr4+:YAG microlaser was developed for ignition of engines. Output energy of 2.7 mJ per pulse and 11.7 mJ per four-pulse train with a pulsewidth of 600 ps and an M 2 value of 1.2 were obtained at a pump duration of 500 ¿s. The optical-to-optical conversion efficiency was 19%. Brightness of the microlaser was calculated as 0.3 PW/ sr-cm2 and optical power intensity was calculated as 5 TW/cm2 at the focal point of ignition. The enhanced combustion by the microlaser ignition was successfully demonstrated in a constant-volume chamber at room temperature and atmospheric pressure. The cross section area of a flame kernel generated by laser ignition is 3 times larger than that by a conventional spark plug at 6 ms after ignition in a stoichiometric mixture (A/F 15.2) of C 3H 8/air, even though ignition energy of the laser is 1/3 of that of the spark plug. Hundred percent ignition was successfully demonstrated in a lean mixture of A/F 17.2 by laser ignition, where electric spark plug ignition failed.

Q

A solid-state laser material of composite all-ceramic Yb:Y3Al5O12 is applied as a source of a high-power, diode edge-pumped microchip laser. 520W quasi-continuous-wave and 414W continuous-wave (cw) output powers were obtained from the 3.7-mm-diameter, Yb doped ceramic core with a 200μm thickness. The cw output power densities of 3.9kW∕cm2 and 0.19MW∕cm3 in the core area and volume, respectively, are the highest for an active-mirror solid-state laser. The maximum thermal stress in the ceramic core is estimated to be 384MPa at the noncooled surface and is twice the tensile strength of single-crystal Y3Al5O12.

-switched Microlaser for Ignition of Engines

300 W continuous-wave operation of a diode edge-pumped, hybrid (single-crystal/ceramic) composite,Yb3+:YAG microchip laser with a 5 mm diameter and 300 microm thickness single-crystal core uniformly bonded to a water-cooled heat sink by a new Au-Sn soldering system has been demonstrated. The beam quality factor M2 follows the mode mismatch between the core and the fundamental mode and was improved to 17 with a maximum output power of 230 W. A thermally induced convex mirror with a spherical radius of curvature ranging from -2.5 to -1.5 m was observed; the radius of curvature decreases through thermal deformation of the microchip as the pump power increases.

High-power operation of diode edge-pumped, composite all-ceramic Yb:Y3Al5O12 microchip laser

We present comparative studies on laser performance of diode end-pumped, composite Nd:YAG rods with undoped ends at 946 nm. Efficient heat removal in a composite rod can reduce the peak of rise in temperature in the active segment to 61% by theoretical calculation. The maximum, continuous-wave output power of 1.5 W, which is three times higher than a conventional noncomposite rod, was obtained at an absorbed pump power of 8 W because of the 55 deg reduction in temperature rise that was estimated experimentally. The performance and characteristics of composite rods in quasi-three-level laser operation are analyzed and discussed in detail, taking into account the reduction in reabsorption loss, which is strongly temperature dependent.

300 W continuous-wave operation of a diode edge-pumped, hybrid composite Yb:YAG microchip laser

We present comparative studies of the laser performance of a diode-end-pumped, composite Tm:YAG rod with undoped ends. Efficient heat removal in a composite rod can reduce the peak of the rise in temperature in the 2-mm-long active segment by 19% compared with that in a noncomposite rod by theoretical calculation. The threshold pump power was reduced by 10%, and the focal power of the thermal lens was improved by 15%. These improvements permit high output performance and flexible design of laser cavities in diode-end-pumped Tm:YAG lasers.

EFFICIENT 946-NM LASER OPERATION OF A COMPOSITE ND:YAG ROD WITH UNDOPED ENDS

We present what is to our knowledge the first theoretical and experimental estimation of the thermal characteristics of a composite Nd:YAG rod with an Al2O3 (sapphire) end in diode-end-pumped geometry. The peak temperature rise in the active segment is calculated to be reduced to 66% and the focal length of the thermal lens was measured to be reduced by 20% compared with a noncomposite Nd:YAG rod. By using a composite rod we successfully demonstrated the improvement of high-power performance that is due to reduction of thermally induced birefringence, which has not been observed in a composite rod with an undoped YAG end.

Improvement of thermal effects in a diode-end-pumped, composite Tm:YAG rod with undoped ends

We present a new stability mechanism in an intracavity-doubled Nd:YAG laser oscillating in a multi-fundamental-longitudinal mode. Self-mode-locked oscillation, in which phases of the modes give the destructive interference between the sum and doubling components, leads to the low-noise operation of the diode-pumped, >6-W cw green laser, which we previously demonstrated [Opt. Lett. 21 1912 (1996)]. This phase-locked mode of operation would reduce the nonlinear output coupling and ultimately eliminate the chaotic amplitude fluctuation from the so-called green problem. The internally reflected green light at the end mirror has a significant effect on phase locking and elimination of noise through parametric processes.

Reduction of thermal effects in a diode-end-pumped, composite Nd:YAG rod with a sapphire end

A high peak power (>MW), passively Q-switched Yb:YAG/Cr:YAG micro-laser end-pumped by fiber-coupled 120-W QCW LDs (Repetition rate <; 100 Hz) is developed. The convex output coupler with a curvature of -2 m successfully enlarges the fundamental mode size in the micro-laser cavity, and the output pulse energy increases to 3.6 mJ at a Cr:YAG initial transmission of 89% without optical damage. The TEM00 transverse mode and the single-frequency oscillation are confirmed. The pulse duration is 1.3 ns, and then the peak power is estimated as 2.8 MW. To our knowledge, these are the highest pulse energy and peak power ever reported in Yb:YAG/Cr:YAG micro-lasers. The Yb:YAG rod with 5 at.% Yb concentration and 4.2-mm thickness keeps more than 80% single-pass pump absorption ranging from 15°C to 45 °C in LD temperature. On the other hand, the characteristic temperatures of Yb:YAG/Cr:YAG and Nd:YAG/Cr:YAG lasers were estimated as 110 and 278 K, respectively. This means that the ratio of the increase in threshold pump energy to temperature of a Yb:YAG/Cr:YAG laser is twice or higher than that of a Nd:YAG/Cr:YAG laser due to a quasi-3-level system of Yb:YAG.