We report laser operation of spinning mirror mechanically Q-switched (MQS), flashlamp pumped 2.94 μm Er:YAG laser depending on the angular speed of the mirror, repetition rate, size and temperature of the gain element, pulse duration and jitter of Q-switch pulses, as well as pump pulse energy. The highest output energy of 260 mJ with a pulse duration of 150 ns was realized with the use of 7×120 mm Er(50%):YAG at 5 Hz repetition rate and 4200 rad/s angular speed of the spinning mirror. The efficiency of Q-switched operation was ~50% with respect to free-running regime. Using optical triggering, the pulse jitter was measured to be smaller than 10 ns for 160 ns Q-switched pulses. Optical triggering could be used for synchronization with mode-locked laser in chirp-pulse and regenerative amplifiers. We also report on development of room temperature gain-switched Fe:ZnSe laser pumped by a radiation of MQS Er:YAG laser. The maximum output energy of 9 mJ from Fe:ZnSe laser was demonstrated using MQS Er:YAG laser as pump source.
There is strong demand for effective gain materials for the 3.0-3.9 μm spectral range not nicely covered by Cr:ZnSe and Fe:ZnSe amplification bands. We characterized, Fe:ZnAl2O4 ceramic sample, Fe:MgAl2O4 and Fe:InP single crystals as promising laser materials for this mid-IR spectral range. In all crystals, the absorption bands corresponding to 5E↔5T2 transition of Fe2+ ions in the tetrahedral sites were measured. In addition, absorption band of Fe2+ ions in the octahedral sites were observed in Fe:ZnAl2O4 sample with maximum absorption cross-section at ~1.0 μm. From the absorption measurements, the radiative lifetime of Fe:MgAl2O4 was calculated to be 60 μs. Saturation absorption of Fe2+ ions in Fe:ZnAl2O4 was studied using Ho:YAG@2.09 μm and Er:YAG@2.94 μm lasers. Saturation measurements were taken up to energy density of 2 J/cm2 and showed no saturation of absorption. This can be explained by a fast non-radiative (<100 ps) relaxation time from the 5T2 level of Fe2+ ions in the Fe:ZnAl2O4 sample at RT. A strong mid-IR photoluminescence (PL) signal in Fe:InP crystal was detected under the direct excitation of the 5E↔5T2 transition of Fe2+ ions using Er:YAG@2.94 μm laser as well as excitation using photo-ionization process under radiation from Nd:YAG@1.064 μm laser. This indicates that Fe:InP crystals could become promising mid-IR laser media with optimization of fabrication technology.
The neutral vacancy (GR1) centers are the intrinsic lattice defects in diamond with interesting optical properties. In the present work, we performed optical characterization of the GR1 centers in CVD-grown bulk diamond to explore their laser feasibility. The non-linear optical measurements reveal efficient saturable absorption of ground level of the centers under ns-pulsed excitation at 632 nm. The transmission changes from 55% at small incident intensity to 84% at the saturated level. The experimentally measured data was fitted with equations representing two limiting cases and obtained the saturation parameters, namely saturation fluence (6.8 mJ/cm2) and saturation intensity (6.4 MW/cm2), at 633 nm for ground state absorption. The intensity of saturation 6.4 MW/cm2 obtained after fitting the experimental data is two orders of magnitude smaller than that at 694 nm documented in the literature for GR1 centers in natural diamond. The calculation gives essentially the same value of absorption cross-section (⁓4.5 × 10-17 cm2) at 633 nm for both approximations. We also estimated the radiative lifetime and quantum yield as about 8.5 ns and 13%, correspondingly. The cross-sections were estimated for transition 1E ↔ 1T2. The emission cross-section was obtained to be ⁓9 × 10-17 cm2 near the maximum of emission band. Such a high absorption cross-section and fast recovery of ground state in combination with high concentration of the centers can provide high optical gain.
In this paper, prospects of using diamond with NV− centers as a gain medium have been studied. Spectroscopic characterization of NV− centers in diamond as well as absorption saturation and pump-probe experiments have been carried out. Absorption and emission cross-sections were estimated to be 2.8 × 10-17 cm2 and 4.3 × 10-17 cm2 at the maximum of absorption and emission bands, respectively. It was observed from emission spectra under pulse excitation that some NV− are photoionized to NV0 centers with ZPL at 575 nm. Room temperature luminescence lifetime of NV− centers was measured to be 12ns, which is close to the previously reported lifetime in bulk diamond (~13ns). Saturated transmission was only about 11% of calculated values even at energy fluence much higher than the saturation flux. Two excited state absorptions (ESAs) with different relaxation times (“fast-decay” and “slow-decay with relaxation times of ~500 ns and several tens of microseconds, respectively) were revealed in transmission decay kinetics at 632 nm. Kinetics of transmission at 670 nm was dominated by “slow-decay” ESA process. Kinetics of dk/k0in shorter wavelength were strongly dominated by “fast-decay” ESA process. These results definitively indicate that stimulated emission of NV− centers is suppressed by photoionization and ESAs and the possibility of diamond lasers based on NV− centers is low.