Sergey B. Mirov

Efficient tunable LiF: crystal lasers

Optimization of the technology of preparation of active elements made of LiF crystals containing color centers ensured high values of the parameters of wide-band generation of light by the color centers in a nonselective resonator pumped with YAG:Nd3+ laser radiation (λ = 1.064 μ). The efficiency measured relative to the incident pump energy was 25%, the efficiency of energy absorption was 32%, and the differential efficiency was 50%. The lasing band had a maximum at λ =1.17 μ and the half-width was Δλ >1000 cm−1. Stimulated emission of radiation by the centers was observed when the pump laser was operated under free-running conditions (smooth pulses) and the efficiency measured relative to the incident pump energy was then 0.5%. An efficient LiF: laser with a spectrally narrow (<0.05 nm) and continuously tunable (within the 1.09–1.23 μ range) emission line was constructed. The efficiency at the maximum of the tuning curve (λ = 1.17 μ) was 10% measured relative to the incident energy and 12% measured relative to the absorbed energy; the differential efficiency was 23%. The operating temperature of the active elements of the laser was the room value and the repetition frequency of the output pulses was up to 100 Hz.

12-Watt CW Polycrystalline Cr 2+ :ZnSe Laser Pumped by Tm-Fiber Laser

We demonstrate high-power (12 W, 43% efficient) polycrystalline Cr2+:ZnSe CW laser, operating at 2425 nm, pumped by Tm-fiber laser at 1908 nm. The output power shows no roll-off indicating that much higher output-powers are achievable.

Output characteristics of strongly excited LiF:F/sup -//sub 2/ crystal lasers

An investigation was made of the dependences of the energy characteristics of lasing and of the efficiency of conversion of pump radiation on the pump power density up to 200 MW/cm/sup 2/ supplied to LiF:F/sup -//sub 2/ lasers. It was established that the dependences in question had maxima. This made it possible to optimize the lasers in respect of their output parameters. The output energy of an LiF:F/sup -//sub 2/ laser with a nonselective resonator reached 0.225 J and the efficiency of conversion of the pump energy was 38%. Preliminary experiments were also made on tuning of high-energy (in excess of 0.1 J) radiation from an LiF:F/sup -//sub 2/ laser and of conversion of this radiation to the second harmonic.

Passive laser Q-switches based on MeF2:Nd2+ (Me-Ca, Sr, Ba) crystals

We present the results of investigations of nonlinear absorption in alkaline-earth fluoride crystals containing divalent neodymium ions, and also the results of utilizing these crystals for passive Q-switching and mode-locking in solid state lasers radiating in the wavelength range (1.3 - 1.6) micrometers . Spectroscopic parameters of the studied materials such as the effective absorption cross-section and the relaxation time of the state of self-induced transparency (SIT) were estimated. The nature of the residual absorption was determined in these crystals and a simple spectroscopic model describing the SIT process in MeF 2 :Nd 2+ -crystals is offered. It is demonstrated that MeF 2 :Nd 2+ -crystals can be used as Q- switches for the lasers of the near IR wavelength range. The results of Q-switching of neodymium and erbium lasers generating in the ranges of 1.3 and 1.54 micrometers , respectively, are presented. The single-peak generation regime and generation of a train of modulated pulses are investigated. The use of SrF 2 :Nd 2+ -crystals allowed us to realize the mode-locked regime in YAG:Nd-crystal laser, operating on the transition 4 F 3/2 yields 4 I 13/2 . Evaluation of the duration of peaks gives the time less than 400 psec.

Phase conjugation in LiF and NaF color center crystals

A detailed experiment on phase conjugation via degenerate four-wave mixing in LiF:F2- and NaF (F2+)* crystals pumped by pulse-periodic Nd:YAG laser is described. The dependence of the efficiency of phaser conjugation on the energies and polarizations of the interacting beams and on the concentration of F2- CC is described. The mechanism of phase conjugation is connected with the saturation of the absorption in the color center crystals. Phase conjugation was investigated in LiF:F2- crystals that worked simultaneously as a passive Q-switch of a Nd:YAG laser. This allows use of the LiF:F2- crystal as a phase conjugated mirror of a Nd:YAG laser.

Technological Nd-lasers with passive Q-switches based on LiF:F2 crystals

A new class of high power solid state lasers on neodymium containing media with passive Q- switches on LiF:F2- crystals was investigated. The generation regimes and the technological possibilities of such lasers were studied.

Continuous-wave ring LiF: laser

A method for reducing thermooptic deformations of the active element in a laser with an LiF crystal containing color centers was proposed and implemented. Continuous-wave emission was achieved at λ = 1.1–1.2 μm at T = 313 K. The output power was 6 mW when the pump power was 3 W.

Grain growth and abnormal Fe diffusion in polycrystalline ZnS at elevated temperatures and pressures

Middle-infrared (mid-IR) lasers enabling a wide range of scientific, medical, technological, and defense related applications continue to enjoy a strong demand. Transition metal (TM) doped II-VI chalcogenides are appealing mid-IR gain medial providing direct access to 1.8-6 μm spectral range. . II-VI chalcogenides are available in single crystal and in polycrystalline forms. With respect to single crystals, polycrystalline gain elements fabricated by postgrowth thermal diffusion of TM impurities in II-VI hosts feature better optical quality and enable superior laser characteristics. Despite significant progress in post-growth thermal diffusion technology, there are still some difficulties associated with the diffusion of certain TM ions in certain II-VI hosts. Specifically, the diffusion length Fe in ZnS during ~1 month annealing at 950°C is of the order of 0.1 mm. In this work, enhancement of diffusion coefficient under Hot Isostatic Pressing, at temperature and pressure of 1350°C and 2000 atm, and effect of these extreme conditions on the overall optical quality of the crystal were studied. The high temperature was applied to increase the diffusion rate, and the high pressure was needed to suppress strong sublimation and sphalerite - wurtzite phase transition at elevated temperatures. Under these conditions, the diffusion coefficient Fe in ZnS was enhanced by 5500 times as compared to standard diffusion processes carried out at 950°C. It was also demonstrated that the grain size had grown from ~30μm to ~5.5mm, which is believed to be another reason for efficient diffusion besides the elevation of temperature. The XRD patterns were measured such that the X-ray beam falls on a single grain. The XRD patterns showed only peaks characteristic to single crystals with zinc blende structure. Lasing characterization was performed to investigate the optical quality of the crystal. Slope efficiencies of 23.2% and 15.4% were obtained for TM11 and TM00 modes of operation, respectively. The emission of the laser was demonstrated to be in the 3840-3920 nm.

Laser spectroscopy of highly doped NV- centers in diamond

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/k0 in 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.

Massively parallel sensing of trace molecules and their isotopologues with broadband mid-IR frequency combs produced via optical subharmonic generation

We present a new platform for mid-infrared (MIR) dual-comb spectroscopy, based on a pair of ultra-broadband subharmonic optical parametric oscillators (OPOs) pumped by two phase-locked thulium-fiber combs. Our system provides fast (7 ms for a single interferogram), moving-parts-free, simultaneous acquisition of 350,000 spectral data points, spaced by 115-MHz intermodal interval over 3.1–5.5 μm spectral range. Parallel detection of 22 trace molecular species in a gas mixture, including isotopologues containing such isotopes as 13C, 18O, 17O, 15N, 34S, 33S and 2H (deuterium), with part-per-billion sensitivity and sub-Doppler resolution has been demonstrated. We also show that by utilizing Kerr-lens mode-locked Cr:ZnS lasers operating at λ≈2.35 μm one can create MIR frequency combs spanning almost two octaves in wavelength.