Application of new nonlinear-optical crystals for development of novel methods for nonlinear-optical conversion of solid-state laser radiation into mid-infrared range presents an important task of modern infrared photonics. Significance of this challenge is caused not only by limited choice of solid-state sources of coherent radiation in mid-infrared range, but also by potential applications of such sources in science, technology, medicine, and biology. Efficient method of optical frequency down-conversion is the difference-frequency generation (DFG) allowing the single-pass conversion of the pump and signal optical frequencies lying in the near-IR range into the mid-IR idler wave. The narrowband, frequency stable signal wave for the DFG is generated in our setup by the stimulated Raman scattering (SRS) in a cubically nonlinear crystal (CaCO3, BaWO4, or diamond). In order to present a comparative study, the LiGaSe2 and LiGaS2 crystals with the equal length of 8 mm were used. Narrowband idler waves at the discrete wavelengths of 4.6 / 5.4 / 7.5 / 9.2 μm and high pulse energies in the range 10 - 50 μJ were generated. The measured linewidths were close to the monochromator resolution limit of < 2 cm-1 (~10 nm @ 7.5 μm) and they can be even narrower. It can be supposed that the idler wave linewidth should be comparable with the Raman mode linewidth (ΔνR = 1.2 2.7 cm-1). Generation at 10.8 μJ was achieved in LiGaSe2 only and the output energy was at in the order of ~100 μJ (close to the measuring probe resolution limit).
Parametric four-wave mixing of frequency components in a crystalline Raman laser allowed generation of a collimated beam of not only Stokes, but also anti-Stokes components of the Raman radiation. Recently, to widen the angular tolerance of four-wave mixing and to obtain high conversion efficiency into the anti-Stokes wave, we have developed new schemes of the parametric Raman anti-Stokes lasers at 503 nm and 954 nm with tangentially phase-matched collinear beam interaction of orthogonally-polarized Raman components in a CaCO3 crystal under 532 nm and 1064 nm laser pumping. Now we use not only the CaCO3 crystal, but also other Raman-active crystals with different birefringence for the tangentially phase-matched parametric Raman laser under green (532 nm) pumping. We have theoretically and experimentally studied characteristics of tangential phase matching of Stokes <–< anti-Stokes interaction for different negative and positive uniaxial crystals with high and low birefringence. We have developed and experimentally realized the extracavity parametric Raman anti-Stokes lasers based not only on highly-birefringent uniaxial negative CaCO3 (1086 cm–1, 503 nm) and positive GdVO4 (882 cm–1, 508 nm) crystals, but also on a low-birefringent uniaxial positive SrWO4 crystal (921 cm–1, 507 nm). Cyan anti-Stokes radiation was generated from green (532 nm), 5-ns, 1-mJ pump radiation. While high-birefringent crystals require probe-pump technique of double beam excitation, the low-birefringent crystal parametric Raman laser can be developed in the simplest system of single beam excitation. The green-to-cyan anti-Stokes conversion efficiency higher 1% was achieved in all the laser schemes.
In this paper, the characteristics of synchronously pumped picosecond Raman laser based on the 36 mm long SrWO4 crystal at the shifts of ν1 = 921 cm–1 and ν2 = 336 cm–1 under 36-ps extracavity pumping are presented. The first Stokes at the wavelength of 1178 nm, corresponding to the Raman shift of ν1, with the slope efficiency of 45.1%, output energy of up to 40 nJ, and pulse duration of 33 ps, respectively, was achieved for the output coupler reflectivity of 89%. In the case of setup with the higher cavity Q-factor at 1178 nm, we obtained not only usual ν1- shifted Raman radiation at 1178 nm, but also the unusual line at the wavelength of 1227 nm (ν1 + ν2). This additional line was achieved with the slope efficiency of 18.1 % and the output energy of 15 nJ with the output coupler reflectivity of 96.5% at 1227 nm. Strong pulse shortening from 36 ps down to 1.4 ps at the wavelength of 1227 nm was observed at +50 μm positive detuning of the external cavity length. This value is close to the inverse value of the ν2 line.
New possibilities to use the zircon-type YVO4 and GdVO4 Raman-active crystals for extreme SRS-radiation pulse shortening higher than 30 times down to the inverse value of the vibrational Raman line width in a synchronouslypumped crystalline Raman laser with combined long (ν1) and short (ν2) shift Raman conversion have been found. It is caused by strong broadening of the short-shift bending vibration ν2 line in the spontaneous Raman spectrum of these crystals. We report characteristics of all-solid-state extracavity Raman lasers based on the 16-mm long a-cut YVO4 (ν1 = 889 cm–1, ν2 = 376 cm–1) and GdVO4 (ν1 = 882 cm–1, ν2 = 382 cm–1) crystals under synchronous pumping by a 1063-nm 35-picosecond Nd:GdVO4 laser. Lasing was obtained in the YVO4 (GdVO4) Raman laser at not only the ν1- shifted first Stokes wavelength of 1173 nm (1174 nm), but also at the (ν1 + ν2)-shifted Stokes wavelength of 1228 nm (1228 nm) with slope efficiency of 5.8 % (5.0 %) and output pulse energy up to 11 nJ (10 nJ) at 1228 nm. At 50 μm positive detuning of the external cavity length the strongest 30-fold and 42-fold shortening of the (ν1+ν2)-shifted Raman radiation pulse down to 1.2 ps and 850 fs in the YVO4 and GdVO4 crystals, respectively, has been achieved. These values are close to the inverse values of the ν2 line widths of 11 cm–1 and 24 cm–1, respectively.
A novel, all-solid-state, self-mode-locked, collinearly phase-matched, parametric Raman Nd:YAG/CaCO3 laser at
1168 nm anti-Stokes wavelength is reported. We have achieved parametric Raman conversion into the 1565 nm Stokes
and 1168 nm anti-Stokes components and self-mode-locking using single Kerr-lens and Raman-active CaCO3 nonlinear
crystal inside the cavity of the diode side-pumped Nd:YAG laser generating at 1338 nm. Collinear phase matching of
equally polarized Stokes-anti-Stokes coupling was self-organized due to zero dispersion of the CaCO3 crystal at the
fundamental laser wavelength of 1338 nm. We demonstrate possibilities of the Stokes and anti-Stokes picosecond pulse
shortening and separation of few and even only one 120 ps ultra-short 1168 nm anti-Stokes pulse from the self-modelocked
laser pulse train because of fast and spatially uniform depletion of pumping of intracavity Raman conversion
without using any electro-optical device. We have obtained high energy output of up to 1.1 μJ in the single intensive
anti-Stokes 120 ps ultra-short pulse which was up to 6.8 % from overall output radiation pulse train energy.
A lot of attention is currently focused on synchronously pumped, extra-cavity crystalline Raman lasers
generating one or two Stokes Raman components in KGW or diamond Raman-active crystals, and also
generating additional components of stimulated polariton scattering in lithium niobate crystal having both cubic
and quadratic nonlinearities.
In this contribution we report on generation of more than two Stokes components of stimulated Raman
scattering with different Raman shifts in the all-solid-state, synchronously pumped, extra-cavity Raman laser
based on the Raman-active a-cut BaWO4 crystal excited by a mode-locked, 220 nJ, 36 ps, 150 MHz diode sidepumped
Nd:GdVO4 laser generating at the wavelength of 1063 nm. Excitation by the pumping radiation
polarized along the BaWO4 crystal optical axis resulted in the Raman generation with not only usual (925cm – 1),
but also additional (332cm – 1) Raman shift. Besides the 1180-nm first and 1323 nm second Stokes components
with the Raman shift of 925cm – 1 from the 1063nm fundamental laser wavelength, we have achieved generation
of the additional 1227 nm Raman component with different Raman shift of 332cm – 1 from the 1180nm
component. At the 1227 nm component the strongest 12-times pulse shortening from 36ps down to 3ps was
obtained due to shorter dephasing time of this additional Raman line (3ps for the 332-cm – 1 line instead of 6.5ps
for the 925cm – 1 line). It has to be also noted that the 1225 nm generation is intracavity pumped by the 1179 nm
first Stokes component resulting in the strongest pulse shortening close to the 332cm -1 line dephasing time (3ps).
Slope efficiency of three Stokes components generation exceeded 20%.
Lasers based on stimulated-Raman-scattering process can be used for the frequency-conversion to the wavelengths that are not readily available from solid-state lasers. Parametric Raman lasers allow generation of not only Stokes, but also anti-Stokes components. However, practically all the known crystalline parametric Raman anti-Stokes lasers have very low conversion efficiencies of about 1 % at theoretically predicted values of up to 40 % because of relatively narrow angular tolerance of phase matching in comparison with angular divergence of the interacting beams. In our investigation, to widen the angular tolerance of four-wave mixing and to obtain high conversion efficiency into the antiStokes wave we propose and study a new scheme of the parametric Raman anti-Stokes laser at 503 nm with phasematched collinear beam interaction of orthogonally polarized Raman components in calcite under 532 nm 20 ps laser pumping. We use only one 532-nm laser source to pump the Raman-active calcite crystal oriented at the phase matched angle for orthogonally polarized Raman components four-wave mixing. Additionally, we split the 532-nm laser radiation into the orthogonally polarized components entering to the Raman-active calcite crystal at the certain incidence angles to fulfill the tangential phase matching compensating walk-off of extraordinary waves for collinear beam interaction in the crystal with the widest angular tolerance of four-wave mixing. For the first time the highest 503-nm anti-Stokes conversion efficiency of 30 % close to the theoretical limit of about 40 % at overall optical efficiency of the parametric Raman anti-Stokes generation of up to 3.5 % in calcite is obtained due to realization of tangential phase matching insensitive to the angular mismatch.
A high-energy, high-beam-quality laser based on a single Nd:YAG rod with laser diode side-pumping and multiloop self-adaptive reciprocal cavity is presented. The optimized laser cavity geometry allowed to change the laser oscillation from single 200-ns self-Q-switched pulse followed by low-intensity free-running lasing to repetitive high-intensive pulses by periodically writing and erasing holographic gain gratings in the active Nd:YAG medium. The intensified self-Q-switching pulse train oscillation was realized by the interaction with a diffusely reflecting target placed in the focal plane of a lens at the laser output. Effects of shortening of the repetitive laser pulses down to 70-ns duration, stabilization of the pulse repetition period (~ 5 μs), and mode-locking temporal modulation of the pulses were observed. The use of a passive F2–:LiF Q-switch resulted in stable giant pulse train oscillation with total output energy of up to 0.43 J, individual pulse energy of 50 mJ, and its peak power of 4 MW at the individual pulse duration of 12 ns.
The joule-range high-beam-quality Nd3+:YAG laser oscillation in the multiloop self-pumped phase-conjugate cavity
without an output coupling mirror at four-wave mixing of intracavity beams directly in the 110-mm long Nd3+:YAG rod
with 12.6 kW diode side-pumping is developed and studied. The oscillation efficiency of up to 18 % and average power
of up to 22 W were achieved in the optimized geometry of the laser cavity at the 13.9 kW pumping with repetition rate of
up to 20 Hz and pulse duration of up to 475 μs. The laser generated a single high-intensive 200-ns giant laser pulse (or
series of such pulses with microsecond period at increased pumping) oscillated at self-Q-switch on gain gratings with
relatively narrow linewidth of about 2 GHz followed by low-intensive free-running wideband lasing. We also realized a
passive Q-switch by the F2-:LiF and Cr4+:YAG crystals with oscillation of pulse trains and also 30-ns single-pulse
oscillation with an individual pulse energy of up to 60 mJ and peak power of up to 4 MW at narrowed laser spectra down
to 1.7 GHz.
The phase-locked multichannel Nd:YAG laser systems with the long- and short-range coupling via the
holographic gain gratings in the active elements are developed. The phase locking of various lamp- and diodepumped
loop Nd:YAG lasers with an interference contrast of the laser channels of up to 0.87 and a singlemode
lasing efficiency of up to 20% is experimentally demonstrated. The experimental results for the twoand
three-channel laser systems are generalized on multichannel laser systems using the numerical simulation.
It is demonstrated that the maximum number of the short-range-coupled laser channels can be increased
owing to the leveling of the parameters of laser channels to a value that is greater than that the maximum
number of the channels in the presence of the long-range coupling, which is limited by the damage threshold
of the active element of the interchannel coupling. Use of the multi-loop configuration of the laser resonator
allows compensating not only phase distortions but also a gradient of gain in the diode side-pumped active
elements.
Use of the self-adaptive Nd:YAG laser with dynamically adjustable passive Q-switch allowed to drill high-aspect-ratio
holes deeper than 7 mm in steel, aluminium, Ni-based alloy, and ceramics Al2O3, AlN, SiC at the average drilling rate
exceeding 1 mm per laser pulse, which is close to the initial drilling rates.
We present a new microtechnology of hole drilling based on Nd:YAG laser with self-adaptive loop resonator and scanned passive LiF:F2- Q-switch. It allowed us to provide a high-productive laser drilling of 100-micron holes deeper than 10 mm at average drilling rate higher then one micron per laser pulse.
A phase-locked three-channel Nd:YAG laser system based on phase-conjugate loop oscillators is designed and studied.
The Nd:YAG laser system had three laser outputs as the self-Q-switched laser 100-mJ monopulses interfering with an
interference pattern contrast tending to unity giving a sixfold interferential increase of the laser brightness. A new opportunity
to apply the Nd:YAG laser systems based on phase-conjugate loop oscillators for drilling of the super-deep (up to
27 mm deep) holes of small 100-micron diameter for the most perspective metals, alloys and ceramics with different
composition is shown.
Phase-locked lasing of a three-channel holographic laser system based on self-pumped phase-conjugate loop oscillators
is experimentally demonstrated in accordance with a transient numerical modeling. The Nd:YAG laser system had three
laser outputs as the self-Q-switched laser monopulses interfering with a fringe pattern visibility tending to unity.
Stimulated Raman scattering in BaWO4 crystal under high-energy pumping of a self-phase-conjugated LiF:F2- Q-switched Nd:GGG laser is investigated. The output Stokes radiation obtained has the pulse train energy of up to 2 J, the peak power of up to 1 MW, and the conversion efficiency of up to 20%.
A laser scheme for phase locking of radiation of two self-phase-conjugated Nd:YAG lasers by coupling in active rod gain gratings is proposed, theoretically studied, and experimentally tested. A single-mode, single-frequency radiation of the proposed system has the peak power equal to 15 MW, which exceeds the summed peak power of two initial lasers. (Summary only available)
Self-Q-switching of the Nd:crystal (Nd:YAG, Nd:GGG) and the Nd:glass multiloop lasers (phosphorus GLS-22) with self- pumped phase conjugation (SPC) in active media is investigated experimentally and theoretically. A possibility to realize single-mode single-frequent self-Q-switched laser radiation as repetitive giant pulse trains of 50 W average and 100 kW peak power with no Q-switcher is shown. Temporal dynamics of self-Q-switching and the mechanism of its occurrence on gain gratings are investigated with a help of the rate equations.
The powerful neodymium lasers with the self-phase-conjugation (SPC) and passive Q-swither are submitted. It is shown, that the application of a LiF:F2- crystal as Q-swither and SPC-mirror simultaneously, and also a Sagnac interferometer as end reflector of the cavity allows to increase efficiency and quality SPC-radiation. High power and spatial laser characteristics are obtained: Nd:YAG laser -- 114 W average power at 0.5 mrad beam divergence; Nd:YAP laser -- 51 W average power at 1.2 mrad beam divergence; Nd:Glass laser -- 18 J in pulse train at 1 mrad beam divergence.
The numerical and experimental investigation of a self- starting Nd:YAG laser oscillator with a cavity completed by population gratings induced in the laser crystal by generating beams themselves are reported. The spatio- temporal characteristics of the laser comprising three Nd:YAG amplifiers and a saturable absorber were investigated. The generation of single mode beam with an average power of as large as 100 W and high quality was achieved.
We report the experimental and numerical investigation of a self-starting Nd:YAG laser oscillator with cavity completed by population gratings which are induced in the laser crystal by generating beams themselves. The spatio-temporal characteristics of the laser including three Nd:YAG amplifiers and the saturable absorber (LiF:F2- or Cr4+:YAG) have been investigated. The laser oscillator with a nonlinear mirror has displayed self-adaptivity to strong thermally induced intracavity distortions. The generation of single mode beams with an average power of as large as 100 W and near-diffraction limited quality has been achieved.
Self-pumped phase-conjugate multiloop Nd:YAG, Nd:YAP, and Nd:Glass lasers are investigated and developed. The parametric feedback is realized by dynamic holographic gratings in active and passive LiF:F2- Q-switcher medium. High power and spatial laser characteristics are obtained: Nd:YAG laser - - 114 W average power at 0.5 mrad beam divergence; Nd:YAP laser -- 51 W average power at 1.2 mrad beam divergence; Nd:Glass laser --18 J in pulse train at 1 mrad beam divergence.
Optimization and adaptation of 15 - 200 micrometer small dia and 2 - 20 mm deep holes drilling with the hole shape variation less than plus or minus 10% are discussed. For this purpose, the single-frequency laser system with self-phase- conjugation and passive Q-switching by a scanned gradiently colored LiF:F2- crystal is used. Steels, hard- facing and aluminum alloys, and ceramics are used as samples.
The repetitively pulsed Nd:YAG laser with dynamic cavity formed with participation of dynamic holographic gratings induced in Nd:YAG laser crystal and Li:F absorber has been investigated. The generation regimes and technological opportunities of this Nd:YAG laser were studied. High spatial brightness of radiation at diffraction limited divergence has allowed to carry out effective punching of calibrate holes in different materials with a diameter up to 15 micrometers and maximal depth up to 20 mm.
Q-switched regimes of the nanosecond pulse-periodic Nd:YAG laser with dynamic cavity formed with participation of dynamics holographic gratings in laser elements have been investigated. A Sagnac interferometer was applied as a laser cavity mirror for angular selection of initial radiation. As a passive Q-switch, we used saturable absorber crystal LiF:F2-, which increased total intracavity diffraction efficiency of dynamic gratings completing the cavity. Self-pumped phase conjugation in Nd:YAG amplifier and LiF:F2- absorber provided adaptive properties of the cavity. The peak power of generated beam with diffraction quality exceeded 17.5 MW and the average power achieved 50 W.
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