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.

Recently some of us have shown that the use of a correct ab initio approach to nonlinear pulse propagation simulations during nonlinear optical device designing can result in threefold efficiency increase with respect to the efficiency of existing solutions [1]. In that work we have focused on small beam size, and thus, high divergence regime where the effects of diffraction, spatial and temporal walk-off are difficult to separate and thus require numerical approach. Our collinear model of pulse propagation enabled modelling and optimization of a cascade third harmonic generation in a single element tripler. Herewith, we present the results of expanding our model to treatment of non-collinear optical configurations and the model application to our OPCPA design [2,3]. To the best of our knowledge this is a first propagation model that enables non-collinear configurations while in minimal assumptions regime: unidirectionality and paraxial approximation. By definition the non-collinear propagation is required when interaction of two or more beams is considered. In this case the use of not necessarily slowly varying envelope for each interacting beam is justified from both physical and computational reasons. In case of 3D non-collinear propagation the above conclusion leads to the concept of reference wavevector. Our model is based on unidirectional pulse propagation equation (UPPE [4]) in a rotated frame of reference. Rotation by a different angle has to be, however, performed separately for each of the interacting beams. Apparently even for quite high mutual beam angles it is enough to solve scalar, rather than vectorial, version of UPPE and sustain accuracy. Finally the initial conditions (rotated optical pulses) can be prepared through arbitrary 3D rotation through Fourier Transform shear operations. The idea, realization and advantages of the above mentioned, novel concepts: reference wavevector, rotated UPPE and arbitrary Fourier rotation will be discussed in the presentation. The real life examples of model results for the usage will be presented: an LBO based chirped pulse non-collinear optical parametric amplifier working in “exotic” (off major plane) phase matching conditions and the redesigned multiterawatt BiBO based OPCPA system [2]. 1. T. M. Kardaś, M. Nejbauer, P. Wnuk, B. Resan, C. Radzewicz, and P. Wasylczyk, "Full 3D modelling of pulse propagation enables efficient nonlinear frequency conversion with low energy laser pulses in a single-element tripler," Scientific Reports 7, 42889 (2017). 2. P Wnuk, Y Stepanenko, C Radzewicz “Multi-terawatt chirped pulse optical parametric amplifier with a time-shear power amplification stage”, Optics Express 17(17), 15264 (2009) 3. Y. Stepanenko, "On the efficiency of a multiterawatt optical parametric amplifier: numerical model and optimization," JOSA B 28, 2337–2346 (2011). 4 M. Kolesik and J. V. Moloney, "Nonlinear optical pulse propagation simulation: From Maxwell’s to unidirectional equations," Phys. Rev. E 70, (2004).

We developed a tunable-line-width 101 W average-power all-solid-state 589nm double spectral line sodium beacon laser. The laser was based on the technical route of 1064nm and 1319nm Nd:YAG laser extra cavity sum frequency generation. The laser contained two spectral lines: 589.1591 nm and 589.1571 nm. The former line was matched to the sodium D_2a absorption line with the average power of 81W, while the other line was matched to the sodium D_2b absorption line with the average power of 20W. The beam quality of the two spectral line lasers was both less than 1.3. The two lasers were polarized-combined to transmit coaxially. The initial line width of the laser was about 0.3GHz, which was in the comb-like discrete structure of about three longitudinal modes. We used a white noise generator to modulate the 1064nm single frequency seed laser in frequency domain. The line width’s tunability was accomplished by tuning the driving power of the white noise generator. The final line width tuning range of the 589nm laser was ~0.3GHz to ~1.1GHz.

Quadriwave lateral shearing interferometry (QWLSI) could acquire the phase and intensity distribution of laser beam with remarkable resolution in only one measurement. It’s a potential method to measure the phase step of multimode laser. However, the discontinuity point of intensity will reduce the quality of interferogram and lead to wrong wavefront retrieve result. In this paper, analysis and simulations have been conducted on the measuring ability of the QWLSI for the higher-order modes (HOM) in large mode area (LMA) fiber. A set of interferogram image of ideal linearly polarized (LP) mode are calculated and analyzed by a commercial retrieve software. The results show that the proper ratio of the shearing distance to the gap size between HOM petals is the critical parameter to retrieve wavefront correctly. To study the influence of coherence on wavefront retrieving, we calculated the interferogram of multimode laser with different modal phase fluctuation. The results indicate that the partially coherent beam will introduce significant ambiguity into the retrieved wavefront. Finally, the feasibility of wavefront correction of multimode laser with LSI and spatial light modulator is demonstrated with simulations.

Diode Pumped Alkali Laser (DPAL) is one of the main candidates for development of a high power directed energy system producing laser beam from a single aperture with high spatial quality. Currently, several groups in the US and abroad demonstrated DPAL systems with kW level output power and efficiency higher than 50%. At the same time, the DPAL power scaling experiments revealed some limiting effects, which require detailed study to understand the nature of these effects and ways to mitigate them. Examples of such effects are output power degradation in time, alkali cell windows and gain medium contamination and damage that causes lasing efficiency decrease or even lasing termination. These problems can be connected to thermal effects, ionization, chemical interactions between the gain medium components and alkali cells materials. Study of all these and, possibly, other limiting effects and ways to mitigate them is very important for high power DPAL development. In this paper we present our new results of experiments on measurements of the temperature rise in the gain medium of operating DPAL leading to the output power degradation even before visible damage in the gain cell occurs. This degradation can be both recoverable and non-recoverable, depending on operation conditions and the system design.

The pump-to-laser beam overlap and the cell length of static diode-pumped Cs lasers are crucial parameters for optimization of these lasers. In a previous publication we modeled the influence of the pump-to-laser beam overlap on the performance of Ti:Sapphire pumped cesium vapor laser (T. Cohen, E. Lebiush, I. Auslender, B.D. Barmashenko and S. Rosenwaks, Opt. Exp. 24, 14374 (2016)). In the present paper we report on experiments and modeling in progress on diode pumped cesium vapor laser.

Comprehensive analysis of the performance and beam quality of subsonic flowing-gas K diode-pumped alkali lasers (DPALs) with different pumping geometries, using 3D computational fluid dynamics model, is reported. The model is first applied to a K DPAL with transverse pumping and parameters similar to those of the 1.5 kW K DPAL [Pitz et al, Proc. SPIE 9729, 972902 (2016)] and the calculated results are in satisfactory agreement with the measurements. To study the possibility of scaling up the K DPAL the model is then applied to 100-kW class device with transverse and end pumping geometry. Dependence of the output power on the flow velocity and the pumping geometry is studied. Comparison between end and transverse pumping schemes shows that the output power is almost unaffected by the pumping geometry. However, the spatial intensity distribution of the output laser beam depends on the pumping geometry: it is uniform for the end pumping, whereas for the transverse pumping it is strongly non-uniform at high gas temperature (corresponding to large density of K atoms), becoming more uniform with temperature reduction. The model is applied to evaluation of the beam quality of flowing-gas K DPALs which strongly depends on the refractive index distribution in the gain medium. The beam divergence and the width of the intensity profile in the far field for the end pumping appear to be much smaller than for the transverse pumping. Wave front corrections of the transversely pumped device using cylindrical lens results in substantial reduction of the laser beam divergence and improvement of its quality which becomes comparable with that of the end pumped laser.

In the present paper we use a simple optical model to describe multi-transverse mode operation of alkali lasers. The model is based on calculations of the pump and laser beam intensities in the gain medium, where the laser beam intensity is a linear combination of the azimuthally-symmetric Laguerre-Gaussian modes. The model was applied to optically pumped cesium vapor laser studied experimentally and theoretically previously [Cohen, T., Lebiush, E., Auslender, I., Barmashenko B.D., and Rosenwaks, S., Opt. Exp. 24, 14374 (2016)]. It was found in our calculations that for low pump power and small pump beam radii, only fundamental lasing mode oscillates, just as shown experimentally in this study. However, for higher pump powers and larger pump beam diameters, several transverse modes participate in oscillation. The number and intensities of the oscillating modes as a function of the pump beam power and radius are found. In order to check the validity of the model, it was applied to pulsed static Cs DPAL [Zhdanov, B. et al, Electron. Lett. 44, 582(2008)] with the pump beam radius much larger than that of the fundamental laser mode and constant gas temperature. The model predicts linear dependence of the laser power on the pump power, the values of the former being in agreement with the experimental results.

Analysis of beam propagation, kinetic and fluid dynamic processes in Cs diode pumped alkali lasers (DPALs), using wave optics model and gasdynamic code, is reported. The analysis is based on a three-dimensional, time-dependent computational fluid dynamics (3D CFD) model. The Navier-Stokes equations for momentum, heat and mass transfer are solved by a commercial Ansys FLUENT solver based on the finite volume discretization technique. The CFD code which solves the gas conservation equations includes effects of natural convection and temperature diffusion of the species in the DPAL mixture. The DPAL kinetic processes in the Cs/He/C₂H₆ gas mixture dealt with in this paper involve the three lowest energy levels of Cs, (1) 6²S_1/2, (2) 6²P_1/2 and (3) 6²P_3/2. The kinetic processes include absorption due to the 1→3 D₂ transition followed by relaxation the 3 to 2 fine structure levels and stimulated emission due to the 2→1 D₁ transition. Collisional quenching of levels 2 and 3 and spontaneous emission from these levels are also considered. The gas flow conservation equations are coupled to fast-Fourier-transform algorithm for transverse mode propagation to obtain a solution of the scalar paraxial propagation equation for the laser beam. The wave propagation equation is solved by the split-step beam propagation method where the gain and refractive index in the DPAL medium affect the wave amplitude and phase. Using the CFD and beam propagation models, the gas flow pattern and spatial distributions of the pump and laser intensities in the resonator were calculated for end-pumped Cs DPAL. The laser power, DPAL medium temperature and the laser beam quality were calculated as a function of pump power. The results of the theoretical model for laser power were compared to experimental results of Cs DPAL.

We report on a novel method that shows the potential to provide real-time, standoff forensic analysis of samples being irradiated by a high energy laser (HEL). The interaction of the HEL beam with matter produces specific optical signatures that can be detected from the location of the HEL system. A spectroscopic analysis of these signals can then provide useful information to the operator including the impact the laser has on the sample as well as providing data about the its structure and composition.

In this paper we present the results of systematic investigations of interaction between high power near-IR laser beam and composite material samples composed of sandwich structures of structural composites of Fiber Reinforced Polymer. Sandwiches of carbon and glass fabric woven|/epoxy resin shells and foam core as well as carbon reinforced/epoxy laminate have been tested. The experiments are performed by means of high speed 3D digital image correlation and thermographic methods. The studies allow to estimate the symptoms and results of damage effects against composite material targets such as an UAV’s airframe shell.

We present a high-efficiency mid-infrared optical parametric amplifier (OPA) pumped by a Nd:YAG slab laser with rectangular beam distribution. To improve the conversion efficiency of OPA, we used an approximate uniform pump beam, which helped most of the pump area maintain the optimal intensity to reduce the back conversion effect. The uniform pump distribution without any peak intensity also reduced the damage chances of the nonlinear crystal of PPMgOLN and increased its pump power capability in power-scaling operations. To make sufficient usage of the narrow and small interface of PPMgOLN, we chose a rectangular pump shape whose size was adjusted to match the maximum effective interface of PPMgOLN. The idler laser of 3.82 μm from an optical parametric oscillator (OPO) was powerscaled in the following OPA system. We used two 1.064 μm lasers to pump the OPO and OPA separately. The pulsewidth adjustment and pulse synchronization of the 1 μm pump laser and 3.82 μm seed laser were realized by changing the parameters of the two acoustic-optical Q-switches in the two pump lasers. With the input pump power of 293.4 W, the amplified 3.82 μm laser power was 40.3 W deducting the injected seed laser power of 2.9 W from OPO. The corresponding conversion efficiency from the pump to the idler was 13.7% for the PPMgOLN OPA.

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.

We demonstrated photodarkening induced core laser leakage phenomenon in the long term operation of a 3 kW copumping fiber amplifier. Output power perturbation of 4~6 minutes period was observed in the 6 hours maximum power operation. Despite the perturbation, the maximum power also drops from 3 kW to 2.65 kW due to the leakage of the core laser. This indicates that the co-pumping scheme may not be suitable to achieve high power fiber laser with long term output stability. We also provide a solution by adopting the counter-pumping scheme that shows very stable output power in the 1-hour maximum power operation test.

In this work, the spectral broadening and the stimulated Raman scattering (SRS) properties of fiber oscillators with different temporal and spectral characteristics were analyzed theoretically. We propose a spectral model for the amplification process in fiber amplifiers and a qualitative model for fiber oscillators through the superposition of multi-longitudinal modes. It is revealed that there are there are clearly linear and nonlinear broadening regions along with the power scaling in the MOPA structure, while the growth rate and the SRS threshold are all closely related to the temporal and spectral characteristics of the fiber oscillator. The preliminary analysis of the simulation results shows that both the noise floor and the temporal fluctuations in fiber oscillators will significantly impact the SRS threshold of the MOPA structure, while only the temporal fluctuations in fiber oscillators would impact the spectral broadening properties. The developed model represents a powerful tool for the optimization of a fiber oscillator through its structural parameters.

This PDF file contains the front matter associated with SPIE Proceedings Volume 10436, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.