The performance of laser weapon system depends among others on harnessing or mitigation of transient thermal optics effects (trans-TOE’s) occurring inside High Energy Laser (HEL) and in laser optics beam forming train as well. In the developed in last year at MUT laser effector based on of 10-kW fiber HEL the optical train consists of about ten lenses, mirrors and windows including the most critical fiber endcap. We have measured the transient 2D temperature distributions in these optical elements under 10-kW laser beam exposition and compared results to numerical modeling in COMSOL Multiphysics. Applying such experimental / numerical approach the effective absorption in dielectric layers of typical mirrors and in volume of transmissive elements under high laser power were determined. The layer absorption was determined to 20 − 50 ppm for High Reflective (HR) mirrors and less than 10 ppm for Anti Reflecting (AR) coatings. The idea of dynamic self-compensation of trans-TOE’s by means of tailored design of the following transmissive and reflecting elements was proposed. The numerical model of this concept for the simplest combination consisted of 2 HR mirrors and single AR coated lens was presented.
The laboratory model of 10-kW class laser effector was designed and assembled. The special laboratory setup for characterization of its parameters and research on interaction with materials was developed. As a result of dynamic thermaloptic phenomena inside laser source and measurement setup the observed laser beam distributions in far field present features of 4D spatial-temporal, non-stationary stochastic process, thus averaging in given plane over long exposures times was not justified. Measurement of laser beam parameters directly in far field and Wavefront Sensing Measurement by Shack-Hartmann method were applied in experiment. To analyze the experimental data of distorted wavefront measurements Wigner Transform method was applied. Beam quality and brightness determined via Wigner approach was changed in the same way as the direct measurements of beam parameters in far field. The deterministic aberration as a result of dynamic thermal-optic effects depending on averaged laser power was found, which can explain non-Gaussian profiles in the vicinity of focal plane.
Aperture losses and thermo-optic effects (TOE) inside optics as well as the effective beam width in far field should be taken into account in the analysis of the most appropriate laser beam profile for high-power applications. We have theoretically analyzed such a problem for a group of super-Gaussian beams taking first only diffraction limitations. Furthermore, we have investigated TOE on far-field parameters of such beams to determine the influence of absorption in optical elements on beam quality degradation. The best compromise gives the super-Gaussian profile of index p = 5, for which beam quality does not decrease noticeably and the thermo-optic higher order aberrations are compensated. The simplified formulas were derived for beam quality metrics (parameter M2 and Strehl ratio), which enable estimation of the influence of heat deposited in optics on degradation of beam quality. The method of dynamic compensation of such effect was proposed.
The relations between range of operation and aperture of laser weapon system were investigated, taking into account diffraction and technical limitations as beam quality, accuracy of point tracking, technical quality of optical train, etc. As a result for the medium ranges of 1 - 2 km we restricted the analysis to apertures not wider than 150 mm and the optical system without adaptive optics. To choose the best laser beam shape, the minimization of aperture losses and thermooptical effects inside optics as well as the effective width of laser beam in far field should be taken into account. We have analyzed theoretically such a problem for the group of a few most interesting from that point of view profiles including for reference two limiting cases of Gaussian beam and ‘top hat’ profile. We have found that the most promising is the SuperGaussian profile of index p = 2 for which the surfaces of beam shaper elements can be manufactured in the acceptable cost-effective way and beam quality does not decrease noticeably. Further, we have investigated the thermo-optic effects on the far field parameters of Gaussian and ‘top hat’ beams to determine the influence of absorption in optical elements on beam quality degradation. The simplified formulae were derived for beam quality measures (parameter M2 and Strehl ratio) which enables to estimate the influence of absorption losses on degradation of beam quality.
We demonstrate partially athermal Nd:YAG transmitter. Laser generates pulses with energy of 55 mJ with duration of 15 ns. It corresponds to 3.7 MW of pulse peak power. The full-angle beam divergence is 2.5 mrad. 18% of pulse energy stability is maintained over temperature range of 20-36°C.
A single and double-bounce grazing-incidence Nd:YVO4 laser is presented. The output pulse energy of ~20 mJ with slope efficiency reaching up to 24.5% was achieved. The beam quality parameter M2 was 1.25.
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.
Efficient generation of diode pumped Tm:YLF laser end-pumped by 25-W laser diode bar was demonstrated. Above 5 W of output power has been obtained. The output spectrum was centered at 1908-nm with <15 nm linewidth. In active Q-switching mode, for 20-Hz repetition rate, up to 5.5 mJ output energy with a pulse duration of 11 ns was achieved. Further pulse energy scaling up was limited by the damage of laser elements. The divergence angle was about 3.5 mrad and estimated parameter M2 < 1.15. We also report mid-infrared laser generation at 2488 nm with a linewidth of ~60 nm in a polycrystalline Cr:ZnSe active medium. In a free-running regime, for 80 mJ incident pump energy, 3 mJ of output pulse energy was achieved.
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.
A compact and efficient, passively cooled, diode pumped Tm:YLF laser is presented. The power of over 15 W
for 10% duty cycle and 55-W of pump power was achieved. When applying the pump power of 25 W and using
elongated 120-mm long optical cavity, the output power as high as 5 W with a slope efficiency of 50% was also
presented. The maximum pulse peak power and pulse duration achieved, recorded for 20 Hz of repetition rate and
10% pump duty factor was near 0.5 MW and 11 ns, respectively. The output laser beam was characterized by very good
quality with divergence angle of 3.5 mrad and M2 < 1.15.
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
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.
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
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 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-
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
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 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.