This PDF file contains the front matter associated with SPIE Proceedings Volume 10798, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

The continuous development on the field of laser sources and laser applications in the last decade has not only driven the capabilities in the industry domain, it also changed the way the military sees and uses laser systems. With the introduction of a high energy laser (HEL) system onboard the USS Ponce in 2014 the practical use of laser weapons started into a new phase. Future Navy ships will be equipped with laser weapons; plans for other applications like C-UAS or C-RAM are making progress. Other nations will most likely follow, they do already have plans or even prototypes. Having started as a support tool on the battlefield the laser will become a new effector with multiple applications. This paper describes the rationale, the tasks and the status of the Test Facility for High Energy Laser (TuV-HEL) at the Bundeswehr Technical Center for Weapons and Ammunition (WTD 91) in Meppen (DEU).

New designs of a defense system using a chemical oxygen iodine laser (COIL) are presented to realize a boost-phase interception of a ballistic missile. Although a space-based laser (SBL), in which the Hydrogen-Fluoride chemical laser is the primary candidate, can realize such a defense system, many SBLs are necessary to cover even a single missile site because they need to continuously go around the earth. This is an expensive system if the potential enemy is a small country. Meanwhile a high energy laser (HEL) carried by a high-altitude airship (HAA) can realize a geostationary defense system if the HEL is quite lightweight. A chemical oxygen-iodine laser (COIL) is suitable since it does not require a heavy electric power supply. But since the COIL should be as light as possible, it would be more advantageous if it can operate without a vacuum pump that requires a large electric-power supply and cooling water. Rate-equation based simulations have been performed to see if it can operate without a vacuum pump by filling a buffer gas at the pressure higher than the outside. The simulation results indicate that it can operate continuously at an altitude of 20 km where the atmospheric pressure becomes ~5,400 Pa (~40 Torr). Moreover, since atmospheric turbulence is greatly reduced at that altitude, adaptive optics is also not necessary for focusing the beam after a long propagation. A simple focusing mirror can focus the beam tightly enough to destroy the target of >100 km away.

The development of efficient, low-cost and robust high peak power pulsed lasers in the ~ 2 – 2.1 µm spectral region is required for many application areas in the mid-infrared (mid-IR) photonics sector. In particular, such laser sources can be used to efficiently access the deeper mid-IR region through optical parametric frequency conversion techniques, utilizing nonlinear crystals such as ZGP and OP-GaAs, or supercontinuum gener¬ation in highly nonlinear fibers. Such mid-IR frequency comb systems are of particular interest for laser countermeasures, remote sensing, high precision spec¬troscopy, and environmental monitoring. Compact and efficient ultrafast 2 µm lasers can also be used as seed sources for developing high energy amplifier systems operating in the 2 – 7 µm region which will benefit many applications from the areas of laser material pro-cessing, strong-field phys¬ics, as well as the development of tabletop X-ray coherent sources. So far, the work on the development of pulsed lasers that operate in the 2-2.1 µm spectral region is rather limited and based predominantly on Ho-doped gain media which require relatively expensive and bulky Tm-laser pump sources. Tm3+-doped cubic sesquioxides RE2O3 (RE=Lu, Sc, and Y) occupy a prominent position amongst other Tm3+-doped gain media. They possess advanta¬geous thermo-mechanical properties and spectroscopic features that make them ideal for high power lasers development in the 2 – 2.1 µm region using a low-cost laser diode pump platform around 800 nm. In particular, their thermal conductivities are in range of 13-17 W/m·K (compared to that of YAG which is 11 W/m·K) and in the case of Lu2O3 it decreases negligibly when the rare-earth-ion doping concentration is increased allowing high-power operation under direct diode pumping. In contrast to most Tm3+-doped crystalline and amorphous gain media, their broadband emission spectra extend well beyond 2 µm allowing efficient operation close to 2.1 µm reaching atmospheric transparency window. The attractive characteristics of rare-earth ion doped crystalline sesquioxides gain media have also led them to being studied extensively as ceramics. Currently, high optical quality sesquioxide ceramics can be produced by nanocrystalline and vacuum-sintering technologies. Such ceramic gain media possess stronger fracture toughness than single crystals and afford a high potential for size scalability thereby offering practical advantages in high-power laser implementations. Here we report on our recent progress in the development of a diode-pumped Tm-doped sesquioxide class ceramic ultrafast lasers operating around 2.1 µm region. In particular, a diode-pumped femtosecond Tm:Lu2O3 laser is demonstrated generating directly transform-limited <500 fs pulses with an average output power in excess of 1 W and a peak power of >30 kW at a center wavelength of 2070 nm. Both semiconductor saturable absorber mirror and Ker-lens mode-locking techniques were investigated. The perspectives for further power scaling during ultrashort pulse generation under direct diode pumping around 800 nm and in-band pumping at 1.6 µm region will be discussed.

There have been concerted efforts to develop high-energy diode-pumped alkali vapor lasers (DPAL). These hybrid gas phase / solid-state laser systems offer possibilities for constructing high-powered lasers that have high beam quality. DPAL's utilize excitation of the alkali metal 2P3/22S1/2 transition, followed by collisional relaxation and lasing on the 2P1/2-2S1/2 line. Considerable progress has been made, but there are technical challenges associated with the reactivity of the metal atoms. Rare gas atoms (Rg) excited to the np5(n+1)s 3P2 configuration are metastable and have spectral properties that are closely similar to those of the alkali metals. In principle, optically pumped lasers can be constructed using excitation of the np5(n+1)p  np5(n+1)s transitions. We have demonstrated gain and lasing for optically pumped Ne*, Ar*, Kr* and Xe*. Three-level lasing schemes were used, with He as the collisional energy transfer agent that established the population inversion. These laser systems have the advantage using inert reagents that are gases at room temperature, with excellent potential for closed-cycle, multi-wavelength operation. The primary technical difficulty for the rare gas laser is the discharge production of sufficient Rg* metastables in the presence of >200 Torr of He. We have developed a high frequency pulsed discharge that yields >10^13 /cm^3 Ar* in the presence of He at pressures up to 730 Torr. Using this discharge, a diode pumped Ar* laser providing 4.1 W of continuous wave output has been demonstrated, with an optical conversion efficiency of 31%. Further development of the pulsed discharge system, lasing demonstrations with Xe* and preliminary experiments with dielectric barrier discharges will be discussed.

Experiments on power scaling of Diode Pumped Alkali Lasers (DPALs) revealed some limiting effects, which cause output power degradation in time, alkali cell windows and gain medium contamination and damage, 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. This paper (based on the talk presented at the SPIE Security + Defence Conference, Berlin, Germany, 10-13 September 2018) presents our results on the study of limiting effects causing lasing degradation. We performed contactless measurements of temperature rise in the gain medium of an operating DPAL based on Cs and K atoms with different buffer gases including hydrocarbons and noble gases and measured critical for degradation temperatures. In these experiments we also observed side fluorescence from the lasing gain medium, which allows studying excitation of higher energy levels because of alkali atoms ionization and recombination.

Experimental and theoretical parametric study of static and flowing-gas diode-pumped Cs lasers is reported. In the static case dependence of the output laser power and the beam quality factor M² on the power and spatial shape of the pump beam is studied. An optical model of multi-transverse mode operation of alkali vapor lasers [Auslender et al, Opt. Express 25, 19767 (2017)] is applied to the experimental results. The values of the laser power and M² predicted by the model are in good agreement with the experimental results for different shapes and powers of the pump beam We also report, briefly, on our recently published work [Yacoby et al, Opt. Express 26, 17814 (2018)] on flowing-gas Cs-DPAL where the output power and gas temperature rise in the laser cell at different flow velocities were studied and the results analyzed by our three-dimensional computational fluid-dynamics) model.

The semi-analytical model for evaluation of partial coherent combining of 2D laser beams was developed. The 2D arrays of laser beams ordered in rectangular or hexagonal lattice architecture were analyzed. The far field intensity distributions were calculated based on partial coherent summation of individual Fourier images. The partial coherence coefficients matrix based on the geometry of the array and Gaussian-Schell coherence function with a priori defined coherence radius was implemented. To define metrics of combining efficiency, Power In Bucket (PIB) distributions were calculated for each case. The more dense hexagonal geometry has shown the advantages over rectangular one, mainly because of better filling factor. The two opposite cases (fully coherent combing vs incoherent combining) were analyzed in the first steps. It was found that taking the criterion of 86.5% of PIB we obtained the same beam diameter in both cases for rectangular geometry. In a case of hexagonal geometry more than 2x beam area in far field was obtained for the incoherent combining w.r.t coherent combining for ‘top-hat’ beam evidencing the important role of the compactation and beam profile shaping. The worst case of profile is the untruncated Gaussian one for which the power content in main diffraction lobe is below 40% and more than 60% bigger beam area at 86.5% PIB comparing to ‘top-hat’ beam array with the same lattice architecture.

Environmental particles are ubiquitous in all but the cleanest laboratory environments. Whether these particles are suspended in air or deposited on surfaces, they can greatly degrade the performance of practical laser systems. However, they also create radically different physical situations than what are reported in traditional laser damage testing, which uses ultra-short pulse lasers focused to microscopic spots on pristine dielectric surfaces. When a surface contaminated with absorbing particles is exposed to a high power laser, the particles heat to their vaporization temperatures, many migrate over the surface, some evaporate away, and some coalesce over time periods on the order of a millisecond. During this time, the particles act as both a conductive heat source and an absorption point for the optical material underneath. If sufficient heat energy is transferred to the optic, then free carriers will be thermally generated across bandgap, and catastrophic breakdown will result if a critical concentration is reached. Thus there is a strong bandgap dependence to contamination-induced breakdown, and despite the common misconceptions about “random failures”, resistance to dirt can be built into an optical system at the design stage. Particles in air can also nucleate breakdown. The details of this process are still obscure but it appears to have much in common with particles on surfaces. One major difference is the process of laser acceleration of absorbing particles. When the particle is laser-heated to extreme temperature, it begins to evaporate, and each evaporated atom contributes a small amount of momentum to the parent particle. There is a small temperature gradient across the particle due to the directionality of laser heating; therefore one side of the particle evaporates faster than the other. The sum of all of the evaporation events gives the particle a substantial velocity. Interestingly, the heat transfer within the particle cannot be explained by conduction, convection, or radiation, but rather appears to be driven by photon diffusion, a process normally dominant in the photospheres of stars.

The effects of laser irradiation on materials include thermal ablation, shock and radiation, where the thermal ablation is the major one in industrial application. When the laser beam irradiates the target, the temperature rises rapidly from the outside in until reaching a certain temperature. The material is melted even gasification. The steam expands and splashes, while washing away the molten material in liquid or solid state and forming pits or perforation. The effect of thermal ablation is related to the parameters of laser source, the external environment parameters and the material parameters. The parameters of the laser source include the wavelength, power density, irradiation time, CW or pulse and the pulse length. The short pulse laser mainly ablated the material by reaching the threshold of power density, while the long pulse laser by reaching the threshold of energy density. In this paper, a dual pulse length method is discussed and theoretically analyzed. A dual pulse length laser with nanosecond and microsecond pulse length is used. The experiment is carried on in three situations: only microsecond laser, only nanosecond laser and both. Experiment results show that the short pulse laser is much better than the long pulse laser under the same average power condition. When the dual pulse width laser is irradiated and the exposure time is accurately matched, the effect is greatly improved and the damage threshold is decreased by one order of magnitude.

A growing number of applications are calling for the compact high energy laser sources. In the last decade, significant progress has been made in the area of solid state lasers especially fiber lasers. The solid state laser is widely used in the processing industry, telecommunication systems at present. However, thermal effect, nonlinear effect and the damage of optical components limits the output power. We present a laser coherent combining technique based on heterodyne method in all-fiber feedback format. In this technique the feedback signals are coupled and transferred by fiber to simplify the system, while all factors of the signals such as the wavefront distribution, polarization state, power ratio of the sub-beams and the reference beam need to be considered and strictly controlled. Compared with the previous system, this technique brings another important advantage: Only the coupling side should meet stringent tolerance toward collimation. Phase detection for laser interferometry, phase control of sub-lasers is theoretically analysis and simulated to reveal the system control bandwidth, phase precision. A high speed phase detection circuit and a phase control circuit are developed. Proof of concept of this technique is experimentally demonstrated at 1.06μm. The experiment setup is shown. Stable results are obtained. The peak power rises up while the theoretical result is 2. Experiments reveal the validity of the technique in nanosecond pulse laser coherent combining.

We demonstrated a large face pumped double-sided liquid cooling Nd:YAG slab laser. The pump light incident from the large surface of the crystal, which are cooled by high-speed flowing cooling water, while the laser beam incident on to the end face, and travels in ZigZag path along the long direction in the crystal. The flat-flat resonant cavity was built, and the output coupler transmission was 30%. The Nd:YAG slab crystal with trapezoidal shape was used as the gain medium, the size of which was 190mm×12mm×4mm, one of the surfaces of 190mm×12mm was coated with antireflection film for 808nm and another was coated with reflection film for 808nm, and the end faces of 4mm×12mm were coated with antireflection film for 1064nm, the doping concentration of Nd³⁺ ion was 1.0at.%. The CW LD array and QCW LD array were used as the pump source to pump the slab crystal, the light emitting surfaces of which have the same size, and the pumping light passed through the pump windows made off used silica and incident into the crystal. Under CW LD pump, the maximum of 420W laser output was gotten, and under QCW LD pump, the maximum of 502W laser out put was gotten. Due to the much higher peak power of QCW laser diode, the small-signal gain was much higher, and induced the optical efficiency of QCW pumping system was much higher, and its thermal effect was relatively smaller because of the high extraction efficiency.