We demonstrated the efficient generation of a linearly polarized Ho:YAG laser pumped by a thulium fiber laser. Independent of the pumping power and without utilizing any dispersive optical element, the output radiation remained stable with a high degree of linear polarization. The output with 2044 nm wavelength and 0.12 nm bandwidth was measured. In the free-running generation, with respect to the incident pump power, we recorded more than 9 W of output power with 59% slope efficiency. In addition, in the pulsed generation, we obtained more than 2.2 mJ pulses with 50 ns time duration, and 44 kW peak power.
A detailed study of a Tm:YAP laser in continuous-wave (CW), single-pass end-pumped by a 793 nm diode laser is presented. The laser based on c-cut 3 at. % Tm:YAP crystal was experimentally examined and presented in the dependence on transmittance and radius of curvature of output coupling mirrors. A detailed spectral analysis was presented. The influence of a heat-sink cooling water temperature on the laser performance was studied. At room temperature, for an output coupling transmission of 19.5%, the maximum CW output power of 4.53 W was achieved, corresponding to a slope efficiency of 41.5% and an optical-to-optical conversion efficiency of 25.7% with respect to the incident pump power, respectively. We have shown that the output spectrum at a certain wavelength (e.g. 1940 nm) for a given pump power can be realized via the change of resonator parameters (OC transmittance, mode size).
To homogenize inversion profiles and mitigate thermo-optic effects in high-power, end-pumped lasers the optimal spatial profile of pump beam should be close to ‘top-hat’ one, whereas typical pumping beams have Gaussian-like profiles. The aim of work was to examine feasibility of laser beam transformation with the use of a beam shaper consisted of a pair of aspheric refractive elements. Two beam shapers (with magnification m = 0.4, 0.8 respectively) for transformation of Gaussian profile to Super-Gaussian were designed and fabricated applying Magneto-Rheological Finishing technology. Both elements were experimentally verified for diffraction limited and partially coherent laser beams. The analytical model based on Fourier transform plane wave decomposition was applied for verification of experiments and to determine the performance of fabricated elements.
Passively Q-switched, closed-loop, self-adaptive resonator with a Nd:YAG as an active medium is presented. For maximal pump energy of 840 mJ Q-switched generation provided 5 pulse series with total energy of 120 mJ. Single pulse width was 24 ns. The beam quality parameter M2 was 1.6. Four-wave mixing and linear resonators were compared.
We report on free-running operation in a side-pumped Yb:YAG slab laser. For maximum available pump pulse energy of
850 mJ at 967.7 nm delivered by a 2D laser diode stack with fast-axis-collimation, the output pulses with energy of 150
mJ at 1.03 μm were obtained. The laser system operated in room temperature providing a slope efficiency of 26.9%. The
performance of the laser is described.
The novel scheme of self-adaptive, closed-loop resonator of diode-side-pumped Nd:YAG slab laser was presented. The
dynamic holography principle was exploited to spatial cleaning of the laser mode. The phase conjugate mirror was
created inside the gain medium as a result of four-wave mixing of resonator standing waves intersecting at a small angle.
The output beam was extracted from the cavity as a 1-st diffraction order of laser mode interacting with the dynamic gain
gratings created inside active medium. The near diffraction limited (parameter M2=1.2) output beam with 250 mJ of
energy at repetition rate up to 25 Hz in free running regime was achieved. Any spectral narrowing effects were not
observed.
Pulsed thulium-doped silica fiber laser operating at a wavelength of 1994.8 nm was developed. The laser was fast gain-switched
by 1.55-μm radiation generated from a fiber amplifier system seeded by a directly modulated distributed
feedback semiconductor laser. The Tm-doped fiber laser delivered stable nanosecond pulses at a repetition rate ranging
from 50 kHz to 300 kHz. The maximum average output power as high as 1.08 W with a slope efficiency of 49% for the
repetition rate of 100 kHz was reported. The shortest stable pulses recorded had a width of (20-25) ns and the energy of
<15 μJ. The laser system was developed in all-fiber architecture, thus providing robustness, compactness, high
insensitivity to atmospheric conditions (dust, vibrations, humidity).
The development of high energy and high power lasers based on solid state technology is mostly limited by thermal effects that occur inside the laser cavity under high heat loads and intensities. The thermo-optical effects emerging inside cavity mirrors, output couplers and windows can significantly degrade beam quality of such lasers. The knowledge on transient thermal effects occurring inside bulk laser elements exposed on laser intensities of several dozens of kW/cm2 is of special interest for some specific applications (e.g. heat capacity lasers). The goal of this paper were theoretical analysis and experimental verification of these effects. Tips for best materials choice for cavity mirrors, laser windows and output couplers were shown. Simple theoretical thermo-optical model was presented. The special laboratory setup allowing simultaneous registration of thermo-optical effects applying shearing interferometer and wavefront sensor (Shack-Hartmann test) was elaborated. The non-stationary and stationary thermo-optical effects emerging inside tested mirrors can be observed, be measured and resolved as result of surface absorption in coating layers and volume absorption in the material. The resolution of measurements: less than 0.1 K and thermally induced optical power of about 0.1 D were demonstrated.
An efficient high-peak-power Ho:YAG hybrid laser resonantly pumped by a 20 W linearly polarized Tm:fiber laser
at the wavelength of 1908 nm was developed. At room temperature a maximum continuous output power of 10.7 W
with a slope efficiency of over 55% with respect to the incident pump power was achieved. In Q-switching regime
an acousto-optic modulators were applied. The research was conducted for normal and Brewster's angle Q-switches
respectively. In CW pumping regime the repetition rate was changed from 500 Hz to 5000 Hz. For the best case, for
5 kHz repetition rate, pulses of 1.6 mJ energy and 123 kW peak-power were achieved at the wavelength of 2090.2 nm
with an M2 ≈ 1.6.
The most important limitations in development of high energy and high power lasers based on solid state technology are
thermal effects occurring under high intensity and high heat loads. The thermo-optical effects occurring inside output
couplers, folding mirrors, output windows can significantly diminish the beam quality of high power lasers and therefore
have to be investigated. The knowledge on transient thermal effects occurring inside bulk laser elements exposed on
laser intensities of several dozens of kW/cm2 is of special interest for some specific applications (e.g. heat capacity
lasers). The aims of work were theoretical analysis of those effects occurring inside the laser mirrors and its experimental
verification. The hints for choice of the best materials (from the point of view of thermal limitations) for laser windows
and output couplers were pointed out. The special laboratory setup enabling simultaneous registration of thermo-optical
effects applying shearing interferometry and wavefront sensing by means of Shack-Hartmann test was worked out. The
transient as well as averaged in time thermal-optical effects occurring inside the volume of examined element as a result
of surface absorption in the coatings and bulk absorption in the material can be resolved and measured. The resolution of
measurements: less than 0.1 K temperature difference and thermally induced optical power of about 0.1 D were
demonstrated.
The 1.5 μm pulsed 3-stage all-fiber MOPA source seeded by a directly modulated DFB laser was developed. It operated
at the repetition rate ranging from 400 kHz to 2 MHz and delivered up to 6.5 μJ in 1-ns pulse. The total signal gain of up
to 57 dB and the maximum average output power of over 2.8 W (at 2 MHz) were demonstrated when the total pump
power of the MOPA was 10.64 W. The amplified pulses were very stable and did not reveal either duration and shape
change in relation to input pulses generated by the seed.
The aim of work is to develop efficient theoretical model enabling analysis and optimization of Q-switched quasi-threelevel
lasers. The model consists of two parts: pumping part and Q-switched part, which can be separated in a case of
active Q-switching regime. For the pumping of quasi-three-level gain medium the semi-analytical model was developed,
enabling the calculations for average occupation of upper laser level for given pump power and pump duration, spatial
pump beam profile, length and dopant level of gain medium. Moreover, ground-state-depletion, up-conversion parasitic
relaxation and temperature effects were considered in the model. The new approach for optimization of CW regime of
quasi-three-level lasers was developed for Q-switched lasers operating with high repetition rates. Moreover, for long
pump durations comparable to laser upper level lifetimes, the optimization procedure based on Lagrange multiplier
technique was developed. The simple analytical formulae for effective pump duration needed to achieve the quasistationary
inversion for given pump power density and up-conversion parameter were derived. The model enables the
optimization of gain medium length and absorbance, average pump area and out-coupling losses for wide class of quasithree-
level lasers.
The aims of paper were theoretical analysis of thermo-optic effects occurring inside laser elements under high heat load
and its experimental verification for two particular cases: dichroic window and Nd:YAG ceramics disk. Transient
thermal effects in dichroic mirrors and ceramic gain media were modeled applying COMSOL Multiphysics software and
verified experimentally. Moreover, thermally induced distortions, thermally induced stresses and birefringence were
calculated for gain elements of rod and disk shapes applying analytical, stationary model based on linear thermoelasticity
theory. The 100-mm diameter dichroic mirrors made of BK7 and fused silica and gain disks made of Nd:YAG
ceramics of 15-mm diameter and 3-mm thickness were prepared for experimental verification of the theoretical models.
The special laboratory set-up enabling simultaneous registration of thermally induced birefringence and wavefront
distortions was worked out. We have investigated the thermo-optical effects for different heat densities in range of
0.1 kW/cm2 up to 50 kW/cm2 changing the pump power , beam diameter or duty cycle. The experiments were carried
out in lasing and non lasing conditions. The new method of measurement of heat conversion efficiency and absorption in
mirrors based on threshold shearing interferometry was proposed and verified for dichroic mirror and ceramic Nd:YAG
disk.
The study describes the efficient, acousto-optic Q-switching of the Er:YAG laser at the 1645 nm 'eye-safe' wavelength. For longitudinal pumping at the wavelength of 1532 nm, a linearly-polarized 10 W erbium fiber laser radiation was used. The investigated Er:YAG crystal was 40 mm long and its erbium concentration was 0.25 %. The active crystal was mounted in a copper heat-sink maintaining a 15°C temperature of coolant water. For giant pulse generation, the fused-silica acousto-optic modulator was inserted inside the Er:YAG laser oscillator near the output mirror of the resonator.
Laser output characteristics were performed depending on the parameters of output coupler reflectance (R= 95%, 90%, 85%) and the repetition rate (from 0.1 to 10 kHz). In free running experiments almost 2.8 W of output power with 55% slope efficiency with respect to incident pump power was obtained. In Q-switching regime the high peak power generation was demonstrated. For maximum incident pump power of 7.8 W, pulse energy up to 4 mJ was generated with a pulse duration less than 40 ns at a 500-Hz repetition rate, which corresponds to nearly 110 kW of peak power. This laser source can find application as a transmitter in 'eye-safe' rangefinders, ladars etc.
The numerical model of Q-switching regime for quasi-III-level laser including thermal effects was developed and
verified with experiment. The room-temperature, high repetition rate, actively Q-switched Tm:YLF oscillator operating
near 2-μm wavelength was constructed and examined. The 25-W fiber coupled laser diode with emission spectrum
centered at 792-nm wavelength was deployed as a pump source. We have demonstrated in a short linear cavity above
5 W of output power for CW pumping and above 7 W for quasi-CW pumping regime (10 Hz repetition rate, 10 ms pump
duration). The divergence angle was about 2.6 mrad, beam quality parameter M2 < 1.1 for higher pump level. Applying
2-plates Lyot's filter, the continuous tuning in the range of 1845-1935 nm was achieved. The linewidth of tunable
spectrum was less than 0.7 nm for all pump power range. The acousto-optic modulator made of fused silica was
deployed for Q-switching. For the quasi-CW regime of pumping, the maximum Q-switched energy was 10.5 mJ with
pulse duration of 22 ns corresponding to near 450 kW of peak power. For CW pumping regime we also demonstrated
above 10 mJ for the repetition rate of 133 Hz, however with longer, 46-ns pulse duration as a result of significant
increase in temperature of crystal. The output energy and peak power were limited in Q-switching regime by damages of
applied laser elements.
The Nd:YAG ceramics of nominal 1% and 2% Nd dopant were produced by a solid-state reaction of high-purity (4N)
nanometric oxides powders i.e. Al2O3, Y2O3 and Nd2O3. Yttrium oxide nanopowder (XRD crystallite size of 79 nm) was
produced by precipitation from water solution of high-purity hydrated nitrate, by means of ammonia hydro-carbonate. The
cold isostatic pressing method was applied to densify granulated powder. Further the sintering and annealing processes were
deployed to produce the final Nd:YAG ceramic samples. Mean grain sizes of about 20 µm and grain boundaries less than
10 nm of elaborated Nd:YAG ceramic samples were evaluated in SEM and TEM measurements. Thus, in microscale the
quality of obtained ceramic was quite satisfactory. However, in macroscale, the samples had unacceptable level of voids and
pores observed in visual inspection, which resulted in low 76% transmission, corresponding to extinction coefficient of
0.32 cm-1 at 1064-nm wavelength. The sizes of defects, pores and inclusions were of several dozens of micrometer. The
active elements of rod and slab shape were fabricated and characterized in two diode pumping laser set ups. In end pumping
configuration as a pump source 20-W fiber coupled laser diode was deployed. For the low duty cycle pumping (1 ms/20 Hz)
above 30% slope efficiency was achieved. In the best case, 3.7 W of output power for 18 W of pump power, with M2 <1.4
were demonstrated for uncoated ceramics rod of φ4x3mm size. We have used uncoated elements, thus we can expect
increase in laser parameters for optimized pumping conditions and samples with anti-reflective coatings. The parameters of
elaborated ceramic material at this stage of work are unsatisfactory for laser application. However, the results of TEM
measurements and laser characterizations seems to be quite promising..
We report on continuously tunable operation of a diode pumped lasers based on Tm-doped materials, emitting
in the 1.8 - 2.μ1 m spectral band. In our study we compare results obtained with three various single crystals
doped by Tm3+ ions: Yttrium Aluminum perovskite YAP (YAlO3), Gadolinium orthovanadate GdVO4, and
Yttrium Lithium Fluoride YLF (YLiF4). Following samples were available: the 3mm long a-cut crystal rod of
Tm:YAP with 4% at. Tm/Y (diameter 3 mm); the 8mm long b-cut crystal rod of Tm:YLF with 3.5% at. Tm/Y
(diameter 3 mm); the 2.7mm long a-cut crystal block of Tm:GdVO4 with 2% at. Tm/Gd (crystal face 5×3 mm).
For active medium pumping, the laser diode radiation was used. Because the tested samples differs significantly
in absorption spectra, two fibre-coupled (core diameter 400 µm) temperature-tuned laser diodes were used: first
operating at wavelength 793nm was used for Tm:YAP and Tm:YLF; the second operating at wavelength 802nm
was used for Tm:GdVO4. In both cases, the continuous power up to 20W was available for pumping. The diode
radiation was focused into the active crystal by two achromatic doublet lenses with the focal length f = 75 mm.
The measured radius of pumping beam focus inside the crystal was 260 µm. The longitudinally diode pumped
crystals were tested in linear, 80mm long, hemispherical laser cavity. The curved (radius 150mm) output coupler
reflectivity was ~ 97 % in range from 1.8 up to 2.1 μm. The pumping flat mirror had maximal reflectivity in this
range and it had high transmission around 0.8 μm. A 1.5mm thick birefringent plate made from quartz (Lyot
filter) inserted under a Brewster's angle was used as a tuning element. This plate was placed inside the resonator
between the crystal and the output coupler. Using Tm:YAP crystal, the maximal output power of 2.8W in
this set-up was obtained. The laser could be tuned from 1865nm up to 2036nm with a maximum at 1985 nm.
Laser based on Tm:YLF crystal was tunable from 1835nm up to 2010nm with a maximum at 1928 nm (3.0W
was reached). Using the Tm:GdVO4 tunable operation with greater that 1W output at 1920nm and 130nm
tuning range (1842-1972 nm) was demonstrated. The overall reached tuning range of over 200nm covers many
important atmospheric absorption lines and contains also the local absorption peak of liquid water, making them
attractive for applications such as high resolution spectroscopy, atmospheric remote sensing, laser radar, and
laser microsurgery.
We report on a passively Q-switched end pumped Nd:YLF laser including a noncritically phase-matched KTP singly
resonant intracavity optical parametric oscillator (IOPO-KTP). For the Q-switching operation we have used Cr:YAG
saturable absorber. The optimized passively Q-switched Nd:YLF laser without IOPO generated linearly polarized pulses
of 11.5 ns and 1.07 mJ at 1047 nm. The conversion efficiency of the optimized Q-switched pulse energy at 1047 nm to
1547 nm of signal approached about 47%. For optimizing both Nd:YLF laser and IOPO we have numerically solved
theoretical model. We have achieved 1.6-ns duration pulses at 1547 nm with energy of 0.5 mJ and peak power above of
300 kW. The beam quality was excellent (M2≈1).
The Tm:YLF laser end-pumped by a fiber coupled laser diode bar, actively Q-switched with tunability option was
worked out and examined for low duty factor and true cw pumping regimes. The numerical model of such a laser
including thermo-optic effects was developed to examine properties of Q-switching regimes. Above 7-W and near 30%
slope efficiency was demonstrated in free-running mode for low duty factor pumping for the best case. Above 2-W
output power was obtained in cw pumping regime for 220-mm long cavity and 15% transmission of output mirror. The
cw output power was limited here by reabsorption losses and thermal lensing. The Lyot's filter tuning in 1845-1935 nm
range with 3-nm linewidth was achieved for free-running regime. The divergence angle was about 4.3 mrad and
estimated parameter M2 < 1.3. For Q-switching regime, the acousto-optic modulator made of fused silica was deployed.
In the best case of low duty factor pumping (10-ms pump duration, 10-Hz repetition rate) pulse energy was 10.5 mJ,
pulse duration was about 20 ns corresponding to near 0.5 MW of peak power. In case of cw pumping for maximum
incident pump power of 19 W we have demonstrated 10-mJ pulses of 220 kW peak power for 133 Hz of repetition rate.
The output energy and peak power was limited here by damages of applied laser elements. The peak power of 12 kW
and 1.7 W of average output power were achieved for the highest rep. rate of 1 kHz.
We report on a passively Q-switched end pumped Nd:YLF laser including a noncritically phase-matched KTP singly
resonant intracavity optical parametric oscillator (IOPO-KTP). For the Q-switching operation we have used Cr:YAG
saturable absorber. The optimized passively Q-switched Nd:YLF laser without IOPO generated linearly polarized pulses
of 11.5 ns and 1.07 mJ at 1047 nm. The conversion efficiency of the optimized Q-switched pulse energy at 1047 nm to
1547 nm of signal approached about 47%. For optimizing both Nd:YLF laser and IOPO we have numerically solved
theoretical model. We have achieved 1.6-ns duration pulses at 1547 nm with energy of 0.5 mJ and peak power above of
300 kW. The beam quality was excellent (M2≈1).
The properties of several Tm doped crystals regarding application in Q-switched tunable laser were analyzed theoretically. The acousto-optically Q-switched Tm:YLF laser is described in experimental part. The fiber coupled (0.4-mm core diameter) laser diode bar at 792-nm wavelength was deployed for pumping. The uncoated, with 3.5% dopant of thulium YLF rod of &slasho;3x8mm size wrapped with indium foil was mounted in copper heat-sink maintaining 293 K temperature of coolant water. Above 6 W of output power and near 50% slope efficiency with respect to absorbed pump power was demonstrated in free-running mode for a short, 40-mm long resonator. The quartz acousto-optic modulator with above 80% diffraction efficiency for 25-W power of RF was taken as the Q-switch for such a laser. In the best case of Q-switching mode up to 2-mJ output energy with 15-ns pulse duration corresponding to 100-kW peak power was demonstrated for rep. rate of a few Hz. Emission wavelength was around 1904 nm for both free-running and Q-switching regimes
Operation of Nd:YAG triangle slab laser side pumped by 600 W quasiicontinuous laser diode passively mode locked using semiconductor saturable absorber is reported. Pulse trains with energy up to 2 mJ and pulse duration of 65 ps were generated and pulse shortening by passive negative feedback introduced due to the beam defocusing in GaAs saturable absorber substrate was measured.
The main properties of acousto-optic modulators (AOMs) applied in laser technology are presented and discussed. The critical review of application of AOMs in several types of diode-pumped solid-state lasers (DPSSLs) is given. A short description of few DPSSLs developed in our group is presented. The parameters of a simple AO Q-switched Nd:YVO4 laser (peak power up to 60 kW, pulse duration 5 to 15 ns, repetition rate 10 to 100 kHz, with average power above 5 W) are satisfactory for various applications. The achieved brightness of 1017 W/m2/sr is comparable to that of the strongest technological Q-switched lasers of kilowatt average power. The main aim of paper is to present novel type of laser with acousto-optic modulation, namely, the AO Q-switched and mode locked (QML) laser. We have designed a 3.69-m-long Z-type cavity with resonance frequency matched to the rf of the AOM. As a gain medium an Nd:YVO4 crystal end-pumped by a 20-W laser diode was applied. The envelope energy of the QML pulse train was up to 130 µJ with subnanosecond mode-locked pulses of 30-µJ maximum energy.
Two 3x3x10 mm3 0.3% at. Nd:YVO4 rods, end pumped by a pair of 25-W CW high power fiber coupled laser diodes,
were employed in the efficient laser based on the V-type resonator scheme. A high slope efficiency, the mitigation of the
thermo-optic effects as well as optimization of energetic and spatial parameters of the laser beam for the whole range of
pump power were demonstrated in such a laser operating in acousto-optic Q-switching regime. The pulses of 180-&mgr;J
energy and 20-ns pulse duration up to 100-kHz repetition rate have been obtained. The pulse energy decreases and
duration grows up with the increase in pulse repetition rate. The average output power of 14 W for CW regime and 10 W
for Q -switching mode were accomplished for the maximum pump power. Brightness and high peak power of this laser
are satisfactory to achieve efficient nonlinear conversions (generation of higher harmonics, pumping of optical
parametric oscillators, etc.). The electronically controlled, wide range of pulse energy, high peak power in a near TEM00
mode characterizes the constructed laser. Due to energetic and spatial parameters, obtained in CW and Q-switched
regimes, this laser can be used in the following disciplines: medicine, marking and material processing, laser metrology,
range-finding, pointing and illuminating of targets, etc.
The main properties of acousto-optic modulators (AOM) applied in laser technology are presented and discussed in the
paper. The critical review of application of AOMs in several types of diode pumped solid state lasers (DPSSL) is given.
The short description of few DPSSLs developed in our group is presented in the following chapters of the paper. The
parameters of a simple AO-Q-switched Nd:YVO4 laser (peak power up to 60 kW, pulse duration of 5-15 ns, repetition
rate in the range 10-100 kHz, with average power above 5 W) are satisfactory for different application as follows: higher
harmonic generation, pumping of 'eye-safe' OPOs etc. The achieved brightness of 1017 W/m2/srd is comparable to the
strongest technological Q-switched lasers of kW class of average power. The main aim of paper is to present novel type
of lasers with acousto-optic modulation namely: AO-q-switched and mode locked (AO-QML) lasers. We have designed
the 3.69-m long Z-type resonator of the frequency matched to the RF frequency of AOM. As a gain medium the
Nd:YVO4 crystal end pumped by 20 W laser diode was applied. The energy of envelope of QML pulse train was up to
130 &mgr;J with sub-nanosecond mode locked pulse of maximum 30-&mgr;J energy.
The Q-switched laser with triangle slab made of Nd:YAG crystal side pumped by 600-W quasi-cw diode laser stack has
been designed. The multimode (M2≈2.6) output energy of about 42 mJ was demonstrated in free running mode for
110-mJ pump energy. In Q-switch experiments, the KDDP Pockels cell was placed between the slab and rear mirror in
plane-plane cavity with output coupler of 84% transmission. The energy of 8 mJ in 2.1-ns pulse duration was obtained
for near TEM00 output beam. For passive Q-switching by means of Cr:YAG crystal of 12.6% unsaturated transmission
the energy of 5.1 mJ in 2.5-ns pulse duration was obtained for output beam close to TEM00 mode.
Review of thermo-optic effects in gain media of diode pumped lasers is presented. Methods of modeling and
compensation of such effects are discussed. Results of characterization of end-pumped lasers by means of energetic and
caustics measurements techniques are presented and discussed. Application of Wigner distribution method for
characterization of aberrated laser beams is presented and discussed. Thermally induced astigmatism was observed for
the anisotropic rods at high heat load.
Triangular slabs made of Nd:YAG or Nd:YAP crystals were tested as active media for diode-pumped lasers. The resonator arrangements for the Q-switched regime were prepared for emission corresponding to the Nd3+ ion transition 4F3/24I13/2 in each crystal (=1318 nm for Nd:YAG, and =1342 nm for Nd:YAP). Optical pumping was accomplished by a fast-axis collimated quasi-cw diode laser stack with peak power of 600 W delivering up to 150 mJ of pump energy. In free-running regime, the maximum reached energies were 24 and 27.5 mJ for Nd:YAG and Nd:YAP crystals, respectively. The corresponding slope efficiencies were 19.9% and 23.7% for Nd:YAG and Nd:YAP laser oscillators, respectively. The passive Q switching for 1.3-µm wavelength was accomplished by means of a saturable absorber made of V3+:YAG crystal, whose initial transmission was optimized for the shortest possible pulse length. The 6-ns-duration giant pulses were obtained with energies of 740 and 432 µJ for Nd:YAG and Nd:YAP, respectively. These results correspond to attained peak powers of 125 kW (Nd:YAG) and 77 kW (Nd:YAP) in the fundamental TEM00 mode, which allows this laser to be used as an efficient source for further nonlinear conversion or other applications.
The efficient, eye-safe, high repetition rate, intracavity optical parametric oscillator (IOPO) inside acousto-optic Q-switched Nd:YVO4 laser end pumped by 15-W fiber coupled diode was demonstrated. The pumping Q-switched laser gives 3-W average output power at 1064-nm wavelength and 40-kHz repetition rate. The additional separating mirror, 'x-cut' KTP crystal and the output coupler, highly reflective at 1064-nm and partially transparent at 1572-nm wavelength, form flat-flat IOPO resonator of 35-mm length. We have achieved 3-ns duration pulses for 20-mm long KTP and 4-ns duration pulses for 30-mm KTP length, respectively. Above 10-kW peak power pulses with the average power of 1.5 W at the signal wavelength for 40 kHz repetition rate were demonstrated. Due to intracavity gain guiding effect, diffraction limited signal beam was achieved. Conversion efficiency of 50% with respect to Q-switched output at
1064-nm wavelength and 11% with respect to diode pump power were achieved.
Nd:YAG and Nd:YAP crystals in form of triangle which makes possible to realize a slab side-pumped configuration with one total internal reflection were tested as an active media for diode-pumped laser. The resonator arrangements for Q-switched regime were prepared for the emission corresponding to Nd3+ ion transition 4F3/2→4I13/2 referring to each crystal (λ= 1318 nm Nd:YAG and λ= 1342 nm Nd:YAP). Optical pumping was accomplished by a fast axis collimated quasi-CW diode DILAS E7Y1-808.3-600Q-H175V with peak power 600 W. Pumping radiation was focused by two plan-convex lenses into an active medium. The parameters of the pumping radiation were: wavelength 806nm, maximum pumping energy was 150mJ, pulse length 250μs, repetition rate up to 14 Hz. In free running regime the maximum reached energy was 24 mJ and 27.5 mJ for Nd:YAG and Nd:YAP, respectively. The corresponding obtained slope efficiency was 19.9 % and 23.7 % for Nd:YAG and Nd:YAP laser oscillator, respectively. Proper Q-switching for 1.3 μm was realized with saturable absorber V:YAG which initial transmission was optimized for shortest possible pulse length. For that obtained pulses were 6 ns with the energies 740 μJ and 432 μJ for Nd:YAG and Nd:YAP, respectively. This results correspond to peak power reached 125 kW (Nd:YAG), and 77 kW (Nd:YAP) in fundamental TEM00 mode which allows this laser to be used as an efficient source for further nonlinear conversion or other applications.
The compact picosecond, high power lasers are demanded in several areas e.g. micromachining, ophthalmology etc. One of the promising methods enabling generation of picosecond pulses trains with energies of a few μJ each in a simple resonator is the simultaneous Q-Switching and Mode Locking (QML) regime. We have demonstrated the efficient QML regime applying acousto-optic modulator in double role as an active Q-switch and mode locker. The pumping beam, emitted by 20-W laser diode bar with beam shaper forms the caustics of 0.8-mm width inside the 0.3 at.% Nd3+ doped 10-mm-long YVO4 crystal located in the close vicinity to the rear flat mirror of first arm of Z-type resonator. The acousto-optic Q-switch with 40.7 MHz radio-frequency was located near flat output coupler. The two folding mirrors were mounted on the translation stages to enable matching the resonance frequency of the cavity to the modulation frequency of acousto-optic cell. Due to weak prelassing at 40.7 MHz frequency, the Q-switched pulse train has started to build up from ordered mode locked radiation. The QML pulses with envelope durations of 100-150 ns and near 100% modulation depth were observed for wide range of pump powers and repetition rates. Above 3 W of output average power, 0.130 mJ of the envelope energy having approximately 5-8 mode locked pulses were achieved. The maximum energy of mode locked pulse was about 0.03 mJ with pulse durations much less than 1 ns.
Growing application of solid state lasers brings the requirements
of new wavelengths generation together with a compact simple
construction of laser system. We have demonstrated compact, diode
pumped Nd:YVO4/LiF:F2- nanosecond laser source operating simultaneously at 1064 and 1143 nm wavelengths. The laser based on Nd:YVO4 crystal gives an efficient output and offers various options for wavelength conversion. The LiF:F2- crystal inserted inside the resonator plays dual role of passive Q-switch for Nd:YVO4 laser, and gain medium of color center oscillator. The 52 mm long cavity consisted of 0.25% Nd3+ doped YVO4 and LiF:F2- crystal of 40% initial transmission. The resonator output coupler with curvature radius of 150 mm has 5% and 15% transmission at 1064 and 1150 nm wavelengths, respectively. The rear mirror was HR for both wavelengths and AR for 810 nm diode pumping (20 W diode bar with beam shaper). The linear polarized at both wavelengths output pulses with the energy 6 and 2 μJ at 1064 nm and 1143 nm, respectively, were detected. The corresponding pulse length (FWHM) was 6.5 ns and 4.5 ns. The pulse frequency at each wavelength was about 75 kHz. The 15 nm width of spectrum centered at 1143 nm wavelength was measured. The multimode output with divergence angles of 5.2×3.5 mrad was evidenced. The pump threshold was about 6 W and maximum mean power was 0.145 W for 18 W pump power with duty factor of 25%. This laser could be used for spectroscopic and other applications.
The simultaneous Q-Switching and Mode Locking (QML) regime provides the generation of relatively high peak power picosecond pulses train with energies of a few μJ each in a simple resonator. The critical review of QML methods and results including our investigations is given in the first part of presentation. The application of several types of saturable crystalline absorbers (Cr4+:YAG, V3+:YAG, LiF, GaAs) leads to chaotic, partial QML effect, with less than 100% modulation depth in principle. The fully modulated efficient QML laser was demonstrated in the next part. The acousto-optic cell playing a double role of Q-switch and Mode Locker was located near flat output coupler. The two folding mirrors were mounted on the translation stages for matching the resonance frequency of the cavity to the radio frequency of acousto-optic modulator. The QML pulses with envelope durations of 100-150 ns and 100% modulation depth were observed for wide range of pump powers and repetition rates. In the preliminary experiments up to 3 W of output average power, 100μJ of the envelope energy, having approximately 5-8 mode locked pulses were achieved.
Review of thermo-optic effects in gain media of diode end pumped lasers is presented. Degradation in beam quality due to these effects is analyzed theoretically and verified in experiments. The slit scan method was implemented for registration of intensity profiles along the caustics of laser beam. The M2 parameter, coherence degree, and thermal lens power of gain medium in dependence on pumping rate, were determined using Wigner transform and ABCD methods. Investigations of neodymium lasers (Nd:YVO4, Nd:YAP) operating in free running or Q-switching regimes were carried out. Detrimental role of up-conversion processes for Nd:YAP laser was shown.
The efficient, eye-safe, high repetition rate, intracavity optical parametric oscillator (IOPO) inside acousto-optic Q-switched Nd:YVO4 laser end pumped by 15-W fiber coupled diode was demonstrated. The pumping Q-switched laser gives 3-W average output power at 1064-nm wavelength and 40-kHz repetition rate. The additional separating mirror, 'x-cut' KTP crystal and the output coupler, highly reflective at 1064-nm and partially transparent at 1572-nm wavelength, form flat-flat IOPO resonator of 35-mm length. We have achieved 3-ns duration pulses for 20-mm long KTP and 4-ns duration pulses for 30-mm KTP length, respectively. Above 10-kW peak power pulses with the average power of 1.5 W at the signal wavelength for 40 kHz repetition rate were demonstrated. Due to intracavity gain guiding effect, diffraction limited signal beam was achieved. Conversion efficiency of 50% with respect to Q-switched output at 1064-nm wavelength and 11% with respect to diode pump power were achieved.
The efficient cw Mode Locking (cw-ML) regime was demonstrated in Nd:YVO4 laser by means of saturable absorber mirror (SAM). The 0.3-at.% Nd3+ doped 10-mm-long YVO4 crystal end pumped by 20 W diode module with beam shaper was applied as a gain medium located in the close vicinity to the rear flat mirror of first arm of Z-type resonator of 316 cm total length with two curved mirrors of 100-cm curvature radii. The SAM of 2%-saturable absorptance and saturation fluence of 0.050 μJ/cm2 was mounted at the opposite end of resonator. The developed "dynamically stable" cavity design mitigates detrimental role of thermal aberration in gain medium, enforcing clean perfect mode locking even for highest pump densities. The cw-ML pulses with 47.5 MHz repetition rate and pulse durations in the range of 15-30 ps were observed for wide range of pump powers and output coupler losses. In the best case for 32% of output coupler transmission, up to 6.2 W of average power with near 35% slope efficiency were achieved. The thresholds for Q-switched ML, cw-ML regimes were 2.67 W and 6.13 W of pump power, respectively. For the maximum pump power of 20 W we obtained 133 nJ of pulse energy with 16-ps pulse duration, resulting in peak power higher than 8 kW. The threshold energy density at SAM giving the QML regime was estimated to be about 0.030 μJ/cm2, threshold of cw-ML regime was 0.220 μJ/cm2.
Nd:YAG and Nd:YAP slab crystal in the form of triangle with the
Brewster-angle-cut polished input faces was used as an active
medium for diode-side-pumped laser. A horizontal projection of the
active medium form is a triangle with 19.22 mm long base, 5 mm
height, and thickness of 4 mm. This active crystal shape is one
from the simplest form which makes possible to realise a slab
side-pumped configuration with a total internal reflection.
Optical pumping was accomplished by a quasi-cw diode ARR18P400
with peak power 400 W closely attached to the active crystal
without any coupling optics. Both material were operated for most
known Nd3+ ion transition 4F3/2→4I11/2 (1 μm) as well as for transition 4F3/2→4I13/2 which leads to the emission at 1.3 μm. The systems were tested in free running and Q switch regime. This system is enough compact to be useful tools for direct medical application.
We present in this paper the highest, to our knowledge, average output power and conversion efficiency at 1572-nm signal wavelength for intracavity optical parametric oscillator (IOPO) inside acousto-optic Q-switched Nd:YVO4 laser end pumped by 15-W power fiber coupled diode. The Nd:YVO4 laser of the 147-mm long cavity gives 3-W average output power at 1064-nm wavelength at repetition rate 40 kHz. The additional separating mirror and x-cut KTP crystal form, with the output coupler high reflective at 1064-nm and partially transparent at 1572-nm wavelength flat-flat IOPO resonator of 35-mm length. We have achieved 3-ns duration pulses for 20-mm long KTP and 4-ns duration pulses for 30-mm KTP length, respectively. Up to 1.5 W of an average power at the signal wavelength for 40 kHz repetition rate of acousto-optic Q-switch was demonstrated. Due to intracavity gain guiding effect, diffraction limited signal beam was achieved. Maximum peak power of 10 kW was demonstrated. Conversion efficiency of 50% with respect to Q-switched output at 1064-nm wavelength and 11% with respect to diode pump power were achieved.
The saturable absorbers (Cr4+:YAG, GaAs and LiF crystals for 1064-nm wavelength, V3+:YAG crystals for 1340-nm respectively) were examined as passive Mode Lockers and Q-switches in diode pumped Nd:YVO4 lasers in the Z-type resonators. In each case, partially modulated long trains of QML pulses were observed. As a rule, envelopes with about 1 μs duration and more than 50% depth of modulation were observed. For stabilization of the mode locking trains nonlinear crystals (KTP or LBO) as negative feedback elements were inserted. The fully modulated QML trains for intracavity II harmonic conversion at 670-nm wavelength in V3+:YAG Q-switched Nd:YVO4 laser with LBO crystal were demonstrated.
Numerical model of side pumped gain elements of rod or slab shape enabling estimation of overlapping efficiency and threshold was developed. This model was applied for analysis and optimization of laser heads side pumped by single diode bar. The diode side pumped lasers heads for rod and slab active elements were elaborated and characterized under real pumping conditions with 40W diode bar. It was shown in experiments that the optical performance for both types
of heads is delimited by transversal, asymmetrical thermal gradients resulting in high level of depolarization and diffraction losses. The fundamental mode limit is about 20 W per 1 cm for a head side pumped by single diode bar.
Two models of gain and thermal guiding effects were derived. In the first one the complex ABCD matrix for a crystal under gain and thermal guiding was applied to describe the operation of microchip near threshold. In the second one, the simple iterative procedure was proposed to calculate effective fundamental mode parameters of a cavity under thermal and gain guiding for given bare cavity ABCD matrix and pumping parameters, including gain saturation, passive cavity losses and reabsorption ones. The influence of gain guiding effects causes changes of waist width in the range up to 50% comparing to expectations derived from thermal guiding theory. Application of such method for resonators of passively Q switched lasers was proposed. Results of calculations for microchips were verified with experiment.
Two models of gain and thermal guiding effects were derived. In the first one the complex ABCD matrix for a crystal under gain and thermal guiding was applied to describe the operation of microchip near threshold. In the second one, the simple iterative procedure was proposed to calculate effective fundamental mode parameters of a cavity under thermal and gain guiding for given bare cavity ABCD matrix and pumping parameters, including gain saturation, passive cavity losses and reabsorption ones. The influence of gain guiding effects causes changes of waist width in the range up to 50% comparing to expectations derived from thermal guiding theory. Application of such method for resonators of passively Q-switched lasers was developed. Results of calculations for microchips were verified with experiment.
The peak power of 0.65 MW and energy of 1.9 mJ at 1572 nm-wavelength was demonstrated in a singly resonant diode pumped intracavity optical parametric oscillator (IOPO). The energy of 4.3 mJ in fundamental mode at 1064 nm-wavelength with 15 ns-pulse duration in passively Q-switched regime was achieved in the arranged for IOPO Nd:YAG slab laser pumped by 300 W quasi-cw diode array. The KTP crystal, size of 20×5×5 mm3, 'x-cut’ for non-critically phase-matched parametric generation at the signal wavelength of 1572 nm was used in experiments. In optimized, with respect to single pulse energy, IOPO cavity with output coupler of 50% transmission at signal wavelength the five-fold shortening of signal pulse duration with respect to 1064 nm pump radiation was observed. The twice-higher level of signal peak power of 650 kW, compared to pump laser at 1064 nm in the same cavity without OPO, was achieved. The conversion efficiency of 44% with respect to 1064 nm pump beam and 3.8% with respect to diode pump energy was demonstrated.
The diode side pumped lasers heads for rod and slab active elements were elaborated and characterized under real pumping conditions. Model of side pumped laser elements of rod or slab shape enabling estimation of overlapping efficiency and threshold was developed. It was shown in experiments that the optical performance for both types of heads is delimited by transversal, asymmetrical thermal gradients resulting in high level of depolarization and diffraction losses. The fundamental mode limit is about 20 W per 1 cm for a head side pumped by single diode bar.
The peak power of 0.65 MW and energy of 1.9 mJ at 1572 nm-wavelength was demonstrated in a singly resonant diode pumped intracavity optical parametric oscillator (IOPO). The energy of 4.3 mJ in fundamental mode at 1064 nm-wavelength with 15 ns-pulse duration in passively Q-switched regime was achieved in the arranged for IOPO Nd:YAG slab laser pumped by 300 W quasi-cw diode array. The KTP crystal, size of 20×5×5 mm3, 'x-cut' for non-critically phase-matched parametric generation at the signal wavelength of 1572 nm was used in experiments. In optimized, with respect to single pulse energy, IOPO cavity with output coupler of 50% transmission at signal wavelength the five-fold shortening of signal pulse duration with respect to 1064 nm pump radiation was observed. The twice-higher level of signal peak power of 650 kW, compared to pump laser at 1064nm in the same cavity without OPO, was achieved. The conversion efficiency of 44% with respect to 1064 nm pump beam and 3.8% with respect to diode pump energy was demonstrated.
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