This PDF file contains the front matter associated with SPIE Proceedings Volume 8547, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The paper will give an overview over the laser weapon activities at RWM (Rheinmetall Waffe Munition) over the last years. Starting from the actual scenarios for laser weapon applications as: CRAM (Counter Rocket Artillery Mortar), Air Defence and UXO (unexploded ordnance) clearing. The basic requirements of a future laser weapon as beam diameter, beam quality, tracking capability, adaptive optics were deduced.

For the UXO scenario a mobile directed energy laser demonstrator for humanitarian mine and UXO clearing based on fiber lasers is presented. Based on the parameters the system concept including the cooling system, power supply and the integration into the armoured vehicle TM 170 are explained. The contribution show first experiments of UXO and IED clearing.

Different technical approaches to achieve laser power in the 100 kW regime combined with very good beam quality are discussed to fulfil the requirements of the CRAM and Air Defence scenario. Spectral coupling and the beam superimposing both are performed by Rheinmetall Waffe Munition.

At the spectral coupling the basic technology parameters for the fiber laser and the dielectric grating as the latest results were put into context with the power levels reached at other groups.

For the beam super imposing technology the basic experiments regarding the tracking capability and compensation of the atmosphere on the test range at Unterlüß will be explained.

A generic 10 kW Laser Weapon Demonstrator based on 2 Laser Weapon Modules (LWM) from RWM each 5 kW fiber Laser with beam forming and tracking integrate by the team of RWM and RAD (Rheinmetall Air Defense) into a Ground based Air Defend system consisting of Skyguard and Millenium turret are presented.

The flight path of the UAV within the valley of the life firing range at Ochsenboden Switzerland is shown. Selected results of the successful tests against UAV’s are presented. It shows the capability of the generic 10 kW Laser Weapon Demonstrator to track and to destroy the target.

From these results the next steps of Rheinmetall Waffe Munition for a 100 kW class laser weapon are explained.

The Laser Weapons System (LaWS) testbed has been a demonstrator of many High Energy Laser (HEL)-related technologies for the Navy. LaWS employs a sharedaperture design with incoherent beam combining, and makes extensive use of Commercial Off-The-Shelf (COTS) technologies in the areas of inertial sensing, fiber lasers, sensors, and video trackers. Since the system has demonstrated the art of the possible with COTS elements, it is useful to examine where performance can be increased with the greatest possible effect, as well as which modifications are necessary to support better integration with ships’ systems. A review of past test events, architectures, and further plans will be provided, while emphasizing the past and future evolution of the LaWS system. These evolutions will relate within the context of technical drivers that most affected system development.

Spectrally Beam Combined fiber laser provide a superior architecture for power scaling laser to high power. We present experimental results where we achieved < 3 kW, M2 = 1.35 with < 39% E-O efficiency by combining 12 individual fiber lasers into a single high brightness beam.

Due to the enormous progress in the field of high-power fiber lasers during the last years commercial industrial fiber lasers are now available, which deliver a near-diffraction limited beam with power levels up to10kW. For the realization of a future laser weapon system, which can be used for Counter-RAM or similar air defence applications, a laser source with a beam power at the level of 100kW or more is required. At MBDA Germany the concept for a high-energy laser weapon system is investigated, which is based on such existing industrial laser sources as mentioned before. A number of individual high-power fiber laser beams are combined together, using one common beam director telescope. By this ‛geometric‛ beam coupling scheme, sufficient laser beam power for an operational laser weapon system can be achieved. The individual beams from the different lasers are steered by servo-loops, using fast tip-tilt mirrors. This principle enables the concentration of the total laser beam power at the common focal point on a distant target, also allowing fine tracking of target movements and first order compensation of turbulence effects on laser beam propagation. The proposed beam combination concept was demonstrated using several experimental set-ups. Different experiments were performed, to investigate laser beam target interaction and target fine tracking also at large distances. Content and results of these investigations are reported. An example for the lay-out of an Air Defence High Energy Laser Weapon (ADHELW ) is given. It can be concluded, that geometric high-power beam combining is an important step for the realization of a laser weapon system in the near future.

A three level analytic model for optically pumped alkali metal vapor lasers is developed by considering the steady state rate equations for the longitudinally averaged number densities of the ground ²S _1/2 and first excited ²P_3/2, and ²P_1/2 states. The threshold pump intensity includes both the requirements to fully bleach the pump transition and exceed optical losses, typically about 200 Watts/cm². Slope efficiency depends critically on the fraction of incident photons absorbed. For efficient operation, the collisional relaxation between the two upper levels should be fast to prevent bottle-necking. By assuming a statistical distribution between the upper two levels, the limiting analytic solution for the quasi-two level system is achieved. The highly saturated pump limit of the recently developed three-level model for Diode Pumped Alkali Lasers (DPAL) is also developed. The model is anchored to several recent laser demonstrations. A rubidium laser pumped on the 5 ²S_1/2 – 5 ²P_3/2 D₂ transition by a pulsed dye laser at pump intensities exceeding 3.5 MW/cm² (< 1000 times threshold) has been demonstrated. Output energies as high as 12 μJ/pulse are limited by the rate for collision relaxation of the pumped ²P_3/2 state to the upper laser ²P_1/2 state. More than 250 photons are available for every rubidium atom in the pumped volume during each pulse. For modest alkali atom and ethane spin-orbit relaxer concentrations, the gain medium can only process about 50 photons/atom during the 2 – 8 ns pump pulse. At 110° C and 550 Torr of ethane, the system is bottlenecked. The system efficiency based on absorbed photons approaches 36% even for these extreme pump conditions. Furthermore, at 320°C with 2500 torr of helium, a pulsed potassium laser with 1.15 MW/cm² peak intensity and 9.3% slope efficiency has been demonstrated.

A mathematical method is described to compute the pressure dependent spectrum of the D₁ and D₂ lines of atomic cesium in the presence of argon. The method is based on the Anderson Tallman unified theory of pressure broadening in which the spectrum is determined form the Fourier transform of the auto-correlation function. The method uses modified potential energy surfaces of the ground and excited states that correlate to the ²S_1/2 ground state and the ²P_1/2 and ²P_3/2excited states at large inter-nuclear separation. These surfaces are used to form interaction difference potentials to determine the auto-correlation function. In addition to being able to compute pressure dependent spectra that exhibit symmetry and far wing structure the method also allows us to compute the low pressure shift and broadening rates of the Lorentzian line core.

In the past decade, the Thin Disk laser design was very successful as a high power laser design for cw lasers with good beam quality and high efficiency. Also pulsed lasers based on Thin Disk amplifiers achieved comparable high average power, but mostly with medium pulse energies. Latest results show that the thin disk still has not reached the scaling limits, neither in cw operation nor in pulsed operation.

This paper highlights the latest advances of disk laser technology at Trumpf. The disk laser combines unique properties, especially high output brilliance (at the lowest pump brilliance requirements of any high power platform), power scalability and broad applicability from cw to ps systems. In the new generation of cw disk lasers, 6kW are extracted from one disk in an industrial product at beam qualities suitable for welding. Moreover, scaling laser power to 10 kW per disk and resonators with higher brilliance are discussed. These advances are enabled by a combination of power scaling and increase of optical-to-optical efficiency. In addition, applications of the disk laser principle to pulsed operation, from ns to ps duration, at infrared and green wavelengths are discussed. Finally, an outlook on the capabilities of disk lasers towards highest cw power and ultra-high peak powers of petawatts and beyond is given.

Boeing has been developing solid state lasers for high energy applications since 2004 using Yb:YAG thin disk lasers as pioneered by A. Giesen¹ and commercialized by Trumpf Laser GmbH.² In this paper, we report results of our second generation design and status of a third generation we are currently developing, which will produce 35 kW and a beam quality <2.

The thin disk laser is a successful concept for high output power and/or high pulse energy, high efficiency and good beam quality in the 1 μm range. Holmium-doped materials are a promising approach to transform this success to the 2 μm range. Ho:YAG is especially interesting for high pulse energies due to the long fluorescence lifetime (~ 8 ms) which provides good energy storage capabilities. We have realized a Ho:YAG thin-disk laser with a cw output power of 15 W at 2.09 μm and a maximum optical-to-optical efficiency of 37%. The laser was pumped with a Tm-fiber laser. Numerical simulations of the Ho:YAG thin disk laser show the potential for further scaling. As broadly tunable alternative, also a Cr:ZnSe thin disk laser was investigated. A Tm-fiber laser and a fiber coupled diode stack were tested as pump sources. A laser power of 3.5 W was achieved with diode pumping.

Non-self-sustained electric discharge and electric breakdown were triggered and guided by a train of ultrashort sub-TW ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire - KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only. When transporting the laser radiation over the long distance, UV filaments of ~ 100 m length were observed.

This paper reviews the experimental component of the US Air Force-supported Multi-University Research Initiative (MURI) on femtosecond laser filamentation in transparent dielectric media. The program comprises a coordinated effort by research groups from 6 US universities and from the multi-terawatt laser laboratory at the Kirtland Air Force Base. The goal of the program is to conduct fundamental basic research on femtosecond laser filamentation, to develop new first-principle-based models of ultrashort and ultraintense pulse propagation in transparent dielectrics and to validate these models though a comprehensive experimental campaign.

We report on our research in power scaling OPSL around 1 μm to exceed 100W per chip by combining a rigorous quantum design of an optimized MQW epitaxial structure, highly accurate and reproducible wafer growth and an efficient thermal management strategy. Recently we have utilized these state-of-the-art optimized OPSL chips to achieve a new record for a mode-locked OPSL with an intra-cavity SESAM. The average output power of the laser in the optimum operation point of mode-locked operation was 5.1W while being pumped with 25W of net pump power. This corresponds to a pulse energy of 3 nJ and a pulse peak power of 3.8 kW.

There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an all-solid photonic bandgap fiber, modes are guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly modedependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of ~800m2 can be made with excellent higher order mode suppression.

In this paper, we present our recent results in developing cladded-single crystal fibers for high power single frequency fiber lasers significantly exceeding the capabilities of existing silica fiber based lasers. This fiber laser would not only exploit the advantages of crystals, namely their high temperature stability, high thermal conductivity, superior environmental ruggedness, high propensity for rare earth ion doping and low nonlinearity, but will also provide the benefits from an optical fiber geometry to enable better thermal management thereby enabling the potential for high laser power output in short lengths. Single crystal fiber cores with diameters as small as 35m have been drawn using high purity rare earth doped ceramic or single crystal feed rods by Laser Heated Pedestal Growth (LHPG) process. The mechanical, optical and morphological properties of these fibers have been characterized. The fibers are very flexible and show good overall uniformity. We also measured the optical loss as well as the non-radiative loss of the doped crystal fibers and the results show that the fibers have excellent optical and morphological quality. The gain coefficient of the crystal fiber matches the low quantum defect laser model and it is a good indication of the high quality of the fibers.

A new passive technique approach to phase lock a laser array is proposed and analyzed. It improves the laser combining efficiency with a large number of emitters. Our architecture combines selective coupling based on phase-contrast filter and resonant phase nonlinearity of amplifiers. A numerical study predicts that with 20 fiber lasers the architecture leads to a phase locking efficiency which is twice the value observed up to now in published experiments.

We report on the performance of monolithic, polarization maintaining, Er-doped photonic crystal fibers (PCF) and amplifiers operating in the eye-safer wavelength regime from 1.55-1.6 um. As part of this effort, we have developed novel 6x1+1 pump/signal combiners for air-clad photonic crystal fibers with six 0.22 NA, 200/220 um pump input fibers and a 25/250 PM signal fiber that allow efficient pump and signal coupling to the air-clad Er-doped PCF. These etched air taper combiners have been demonstrated at the kilowatt level under 976 nm pumping and perform an efficient brightness transformation from 0.19 NA, 1532 nm fiber coupled diode pumps into the 0.6 NA air-clad Er-doped PCF with a measured pump throughput efficiency of 88-92% and a signal throughput of 65-80% with a PER of <18 dB. These novel combiners have been efficiently spliced to 40 um core, 200 um pump cladding Er-doped PCFs providing high efficiency resonantly pumped, monolithic, eye-safer PCF fiber lasers and amplifiers. Using grating stabilized 1532 nm pump diodes, our current experiments have demonstrated single transverse mode operation of both monolithic eyesafer PCF lasers and amplifiers at the multi-Watt level with slope efficiencies of over 55%.

We review methods of combining beams using the Brillouin interaction. We analyze a two cell geometry that provides both high gain amplification (G~300) simultaneously with high efficiency power conversion (η=90%). We consider some configurations that could demonstrate this performance but note that their complexity makes them relatively unattractive at this point in time.

We had demonstrated the advantage of using Coherent Polarization Locking on thermal-sensitive Ho:YAG laser cavity. We overcome several thermal issues related to Ho:YAG laser by distributing the gain over a large volume. We passively coherent locked two orthogonal polarized lasers, achieving 9.6W of output power with near perfect (greater than 99%) combining efficiency. The resultant laser produced near diffraction-limited beam quality of M² ~1.1 and excellent power stability. As compared to conventional laser cavity, we had shown the increased in single-pass absorption, suppression of output power saturation and improvement in beam quality using Coherent Polarization Locking.

In order to address the question of the possibility of a high energy laser with an emission in the “eye-safe” wavelength range, various architectures may be considered. To provide a truly scalable and efficient laser source, however, only bulk solid-state lasers using resonantly diode-pumped erbium show the necessary properties, when coupled with the solid-state heat-capacity (SSHCL) principle of operation. Although seen as nearly being impossible to realize, such a quasi-three-level laser medium can be used in heat-capacity operation. In this operation mode, the laser medium is not cooled during lasing in order to avoid the thermal lensing occurring in bulk lasers, which, in actively cooled operation, would result in a low beam quality, destabilize the laser cavity or would even cause crystal fracture. In heat-capacity mode, the laser medium will substantially heat up during operation, which will cause an increase in re-absorption for a quasi-three-level laser medium, resulting in a general drop in output power over time. However, theoretical and experimental investigations have proven that this effectis of no concern for an Er³⁺:YAG SSHCL. This paper presents an overview on the theoretical background of the Er³⁺:YAG SSHCL, experimental results including recent advances in output power and efficiency, an investigation on the laser scaling properties and recent results on intra-cavity adaptive optics for beam-quality enhancement. The effect of fluorescence re-absorption on the laser properties, especially on threshold and laser efficiency will also be discussed. Up to 4.65 kW and 440 J in less than 800 ms are achieved using optimized doping levels for upconversion reduction in this resonantly-diode-pumped Er³⁺:YAG SSHCL, representing the current world record in eye-safe diode-pumped solid-state laser technology. Optical-to-optical efficiencies of over 41% and slope efficiencies of over 51% are obtained with respect to the incident pump power.

The analytic solution to the problem of time-dependent non-homogeneous heat equation is derived for a temporally pulsed and spatially non-homogeneous source term. For pulsed pumping with an inverse repetition rate much less than system thermal relaxation time, the problem of heat flow in a laser medium is typically studied within the approximation of time-independent heat equation. When the condition fails, due for instance to a short relaxation time or to a low repetition rate, transient analysis of thermal effects becomes necessary. Moreover the time-independent formalism fails in predicting both the focusing properties of the active material and any beam bending inside the resonator, while the transient analysis of thermal effects allows to finely predict the temperature distribution and to still apply, locally, the matrix formalism. In the paper we apply the formalism to a double side pumped Er:Yb glass slab laser with a non symmetric cooling scheme at several repetition rates. By evaluating the temporal evolution of the local temperature in the slab cross section, the difference with the stationary spatial temperature distribution turns out to be not negligible at repetition rates below 10 Hz. We observe that the lack of symmetry in the temperature profile reduces thermal focusing effects, but leads to a dynamic drift of mode laser axis which can make unstable the resonator cavity. We validated the model by comparing the theoretical values of slab focal length and of modal axis drift with experimental measurements at several repetition rates, proving also that the thermal focusing becomes a secondary effect in comparison with modal axis drift at increasing repetition rates.

Diode lasers have a broad wavelength range, from the visible to beyond 2.2μm. This allows for various applications in the defense sector, ranging from classic pumping of DPSSL in range finders or target designators, up to pumping directed energy weapons in the 50+ kW range. Also direct diode applications for illumination above 1.55μm, or direct IR countermeasures are of interest. Here an overview is given on some new wavelengths and applications which are recently under discussion. In this overview the following aspects are reviewed: • High Power CW pumps at 808 / 880 / 940nm • Pumps for DPAL – Diode Pumped Alkali Lasers • High Power Diode Lasers in the range < 1.0 μm • Scalable Mini-Bar concept for high brightness fiber coupled modules • The Light Weight Fiber Coupled module based on the Mini-Bar concept Overall, High Power Diode Lasers offer many ways to be used in new applications in the defense market.

Optically pumped atomic gas lasers are currently being developed in several laboratories. The objective is to construct high-powered lasers that also exhibit excellent beam quality. This is achieved by using the gas laser medium to phase combine the outputs from multiple solid state lasers. To date, the focus has been on optically pumped alkali vapor lasers. 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 could be constructed using excitation of the np⁵(n+1)p ← np⁵(n+1)s transitions. We have demonstrated this potential by observing gain and lasing for optically pumped Ne*, Ar*, Kr* and Xe*. Three-level lasing schemes were used, with He or Ar 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.

A simple, semi-analytical model of diode pumped alkali lasers (DPALs), applicable to both static and flowing-gas devices, is reported. Unlike other models, assuming a 3-level scheme of the laser and neglecting influence of the temperature on the lasing power, it takes into account temperature rise and losses of alkali atoms due to ionization and chemical reactions, resulting in a decrease of the pump absorption and slope efficiency. The applicability of the model is demonstrated by (1) obtaining good agreement with measurements in a static DPAL [B.V. Zhdanov, J. Sell and R.J. Knize, Electron. Lett. 44, 582 (2008)], (2) predicting the dependence of power on the flow velocity in flowing-gas DPALs and (3) checking the effect of using a buffer gas with high molar heat capacity and large relaxation rate constant between the ²P_3/2 and ²P_1/2 fine-structure levels of the of the alkali atom. It is found that ionization processes have a small effect on the laser operation, whereas chemical reactions of alkali atoms with hydrocarbons strongly affect the lasing power. The power strongly increases with flow velocity and by replacing, e.g., ethane by propane as a buffer gas the power may be further increased by up to 30%. 8 kW is achievable for 20 kW pump at flow velocity of 20 m/s.

Different sorts of CO laser systems such as a compact slab RF discharge overtone CO laser (λ~3.0 – 4.0 μm) with output power up to ~2.0 W, a master oscillator - power amplifier fundamental band CO laser facility emitting nanosecond pulses with peak power up to ~0.4 MW in the spectral range of ~5.0 – 7.5 μm, an efficient (~25%) second harmonic emitter based on the latter system, and difference frequency generation (4.3 – 4.9 μm) under nonlinear mixing of fundamental, second harmonic and sum frequency CO laser spectral lines are discussed.

This paper presents the results obtained in analyzing a quasi-cw flash-lamp pumped high power Nd:YAG slab laser oscillator/two stages amplifier operated in passive optical Q-switching regime using LiF:F₂^- crystals with an improved design. A numerical simulation method based on laser rate equation is developed for theoretical analysis of a passively Q-switched Nd:YAG slab laser system. A comparison of simulation results with experimental data is also presented to certify the viability of the developed theoretical analysis method. Laser pulses output energy of 330 mJ at a FWHM time duration of 50-75 ns and at a repetition frequency of 10 – 25 kHz (pps) are reported and numerical simulation of these experimental results are presented pointing to the output parameters stability.