This PDF file contains the front matter associated with SPIE Proceedings Volume 8381, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Yttrium aluminium garnet (YAG) has been widely used as a solid-state laser host because of its superior optical, thermal, mechanical properties, as well as its plurality in hosting active ions with a wide range of ionic radii. Drawing YAG into single crystalline fiber has the potential to further scale up the attainable power level with high mode quality. The recent advancement on the codrawing laser-heated pedestal growth (CDLHPG) technique can produce glass-clad YAG crystalline fibers for laser applications. The drawing speed can reach 10 cm/min for mass production. The CDLHPG technique has shown advantages on transition-metal ion doped YAG and short-fluorescent-lifetime ion doped YAG host. Compared to silica fiber lasers, the crystalline core offers high emission cross section for transition metal ions because of the unique local matrix. The challenges on the development of glass-clad YAG fibers, including core crystallinity, diameter uniformity, dopant segregation, residual strain, post-growth thermal treatment, and the thermal expansion coefficient mismatch between the crystalline core and glass clad are discussed. Chromium, ytterbium, and neodymium ions doped YAG fiber lasers have been successfully achieved with high efficiency and low threshold power. Power scaling with a clad-pump/side-coupling scheme using single clad or double clad YAG fibers is also discussed.

A rectangular-core fiber that guides and amplifies a higher-order-mode can potentially scale to much higher average powers than what is possible in traditional circular core large-mode-area fibers. Such an amplifier would require mode-conversion at the input and output to enable interfacing with TEM₀₀ mode seed sources and generate diffraction-limited radiation for various applications. We discuss the simulation and experimental results of a mode conversion technique that uses two diffractive-optic-elements in conjugate Fourier planes to convert a diffraction limited TEM₀₀ mode to the higher-order-mode of a ribbon core fiber. Our experiments show that the mode-conversion-efficiency exceeds 84% and can theoretically approach 100%.

Optical fibers that support single mode operation while achieving large mode areas are key elements for scaling up optical powers and pulse energies of fiber lasers. Here we report on a study of the modal properties of a new-generation of polarization maintaining large-mode-area photonic crystal fibers based on the spectrally and spatially resolved (S²) imaging technique. A fiber designed for Tm fiber laser system single mode operation in the 2μm spectral range is demonstrated for coiling diameters smaller than 40cm. At shorter wavelengths in the 1.3μm range, efficient higher order mode suppression requires tide coiling to about 20cm diameters.

There are 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 the wide range of nonlinearities, the fundamental solution is scaling 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. There are two basic approaches, lower refractive index contrast to counter the increase of waveguide dimension or/and introduction of additional losses to suppress higher order modes. Lower index contrast leads to weak waveguides, resulting in fibers no longer being coil-able. Our research has been focused on designs for significant higher mode suppression. In conventional waveguides, modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple the modes out to suppress them. This severely limits the scalability of all designs based conventional fibers. In an all-solid photonic bandgap fiber, modes are guided due to anti-resonance of cladding photonic crystal lattice. This leads strongly mode-dependent guidance. Our theoretical study has shown that it can have some of the highest differential mode losses among all designs with equivalent mode areas. Our design and experimental works have shown the potential of this approach for all-glass fibers with >50μm core which can be coiled for high power applications.

The hydrothermal crystal growth technique is developed for the growth of epitaxial single crystal layers on YAG. High quality epitaxial layers of functionalized layers can be grown using Y₂O₃ doped with the desired ion and Al₂O₃ as feedstocks in water between 600-650°C at 1 kbar pressure, with a mineralizer of 1-4M K₂CO₃.The epitaxial layers are doped with a variety of doping ions that enable a number of optical functionalities. These include undoped regions to serve as endcaps, Q-switching regions, ASE suppression cladding layers, waveguide layers and a number of other applications. Different layers can be grown sequentially on the same crystal to create multifunctional single crystals. Epitaxial layers have been grown on both {111} and {100} faces with rates of growth being {100}>{111}>{110}. Growth rates range typically from 25 to 100 microns per day but faster and slower rates have been observed. The technique is not restricted to YAG and can be extended to any oxide hosts that can be grown hydrothermally. Work is being extended to LuAG, YVO₄ and M₂O₃ (M = Lu, Sc).The techniques presented here can address long-standing performance issues associated with solid-state laser materials; when combined with crystal joining technologies, new crystal functionalities emerge that can further improve solid-state laser performance; we refer to this new generation of laser crystals as "smart".

As pump intensity in Diode Pumped Alkali Lasers (DPAL) is scaled to more than 100 times threshold, several nonlinear optical processes are encountered including two photon absorption and stimulated Raman scattering. A pulsed, optically pumped potassium laser with pump intensities exceeding 1 MW/cm² has been demonstrated with output intensities exceeding 100 kW/cm², requiring helium buffer gas pressures above 3 atm. At low pressure Stimulated Electronic Raman Scattering (SERS) has been observed in the same system. Indeed, second and third order SERS has been observed from the DPAL upper laser level. Two-photon absorption at wavelengths near then DPAL pump transition has also been observed and used to demonstrate lasing in the blue and mid infrared. Lasing in the blue has also been achieved by direct excitation of the second excited ²P_3/2 level in Cs.

The performance of Diode Pumped Alkali Lasers (DPAL) depends critically on both collisionally broadened linehapes and rates for fine structure mixing. The first four potential surfaces for K, Rb, and Cs interactions with rare gases have been computed at the MCSCF/MR SOCI level. These surfaces are then used to compute scattering matrix elements for the spin-orbit relaxation, yielding temperature dependent cross-sections. Theoretical predictions are compared to recent experimental results. The observed fine structure mixing rates for rare gas collisions are interpreted in terms of collision adiabaticity. For molecular partners, ro-vibrational energy appears to dominate the mechanism.

Several different quantum-cascade (QC) laser designs spanning the wavelength range between ~3.8 and ~4.8 microns were grown, and devices were fabricated and tested. The active regions of these designs consist of strained layers of (In,Ga)As and (In,Al)As. For several of these designs, we varied design parameters including injector doping, sectioncoupling strength, and the number of QC laser periods. Lasers were tested near room temperature under both quasi-cw and low-duty-cycle conditions. Device performance is compared with theoretical expectations, and conclusions are reached on the relative merit of various design modifications.

Mid-infrared lasers find interesting applications in laser-based countermeasure technologies, remote sensing, maritime/terrestrial awareness and so on. However, the development of laser sources in this spectral region is limited. We present here an alternative solution to the mid-infrared laser which is based on difference-frequency generation (DFG) in a nonlinear crystal pumped by synchronized and tunable near-infrared fiber lasers that are commercially available. This idea is not new and has been explored by other groups, but the latest innovations in near-infrared fiber lasers have enabled the creation of fast-scanning picosecond fiber lasers. One such picosecond system is the synchronized programmable laser from Genia Photonics that can combine two picosecond fiber laser systems in which both output pulses are synchronized at the DFG crystal. The first laser was continuously tunable from 1525 nm to 1600 nm and one million different wavelengths can be scanned within one second. For the second fiber laser, its wavelength was fixed at 1080 nm. In principle, the DFG in a PPLN crystal could produce a tunable mid-infrared source spanning from 3.32 μm up to 3.7 μm. Other and wider tuning ranges are possible with different choices of pump wavelengths. For the PPLN crystal used in this work, the DFG phase-matching window for a fixed temperature was 2.6 nm wide and was broad enough for our 25 ps pulse train having a spectral width of 0.25 nm. The quantum efficiency achieved for the DFG was 44% at the maximum power available.

Different type-II InGaAs/GaAsSb quantum well design structures on InP substrate for mid-infrared emission has been modeled by six band k•p method. The dispersion relations, optical matrix element, optical gain and spontaneous emission rate are calculated. The effects of the parameters of quantum wells (thickness, composition) and properties of cladding layers were investigated. For injected carrier concentration of 5×10¹² cm^-2, peak gain values around 2.6-2.7 μm wavelengths of the order of 1000 cm^-1 can be achieved, which shows that type-II InGaAs/GaAsSb quantum wells are suitable for infrared laser operation beyond 2μm at room temperature.

This paper reports on the latest advancements in vertical high-power diode laser stacks using micro-channel coolers, which deliver the most compact footprint, power scalability and highest power/bar of any diode laser package. We present electro-optical (E-O) data on water-cooled stacks with wavelengths ranging from 7xx nm to 9xx nm and power levels of up to 5.8kW, delivered @ 200W/bar, CW mode, and a power-conversion efficiency of >60%, with both-axis collimation on a bar-to-bar pitch of 1.78mm. Also, presented is E-O data on a compact, conductively cooled, hardsoldered, stack package based on conventional CuW and AlN materials, with bar-to-bar pitch of 1.8mm, delivering average power/bar >15W operating up to 25% duty cycle, 10ms pulses @ 45C. The water-cooled stacks can be used as pump-sources for diode-pumped alkali lasers (DPALs) or for more traditional diode-pumped solid-state lasers (DPSSL). which are power/brightness scaled for directed energy weapons applications and the conductively-cooled stacks as illuminators.

We report on high brightness diode laser at 1.5 μm with wavelength stabilized output. 22W are delivered from a uncoated 100μm fiber with 0.15 NA at 1532nm with a bandwidth of 2 nm. InP diode lasers emitting at 1.5 μm show much lower power than GaAs based diodes emitting around 900 nm due to the low electro-optical efficiency of 1.5 μm diodes of about 35%, compared to about 65% of GaAs diodes. Single emitters allow the highest power from given size broad area emitter due to optimized cooling. Up to 6W (15W) are available from a 95 μm broad area single emitter at 1.5 μm (9xx nm). At 1.5 μm the maximum power is typically limited by thermal roll over and efficient heat dissipation from the diode is essential for power scaling. Optical stacking is deployed for power scaling thus symmetrizing the beam quality in fast and slow axis for efficient fiber coupling. Typically, 65% efficiency for optical stacking and fiber coupling are achieved resulting in more than 22W from a 100 μm fiber of 0.15 NA. Resonant pumping of Er lasers requires a 2nm linewidth centered at 1530nm. The free running diodes show about 10nm linewidth (96% power content) and about 2.5nm/A tuning coefficient with varying drive current depending on heatsinking. Frequency stabilization is achieved with external Volume Bragg Gratings. More than 85% power is confined within a 2nm bandwidth up to 8A drive current resulting in 17W from the uncoated 100 μm fiber. The diodes are emitting at 1546nm at 8A without VBG and 20W from the fiber are possible with the proper lower wavelength diode.

Vertical-cavity surface-emitting lasers can be processed in large two-dimensional arrays of single devices to scale up the power for solid-state laser pumping. These arrays emit in a circular, uniform beam, with a narrow and stable emission spectrum that is well suited to the absorption spectra of solid-state gain media. kW-class 808 nm QCW VCSEL pump modules were developed to pump compact Nd:YAG lasers. An end-pumped Nd:YAG laser was constructed that produced 7.1 W average IR power, as well as a dual side-pumped passively Q-switched frequency-quadrupled Nd:YAG laser that generated 0.8 mJ UV pulses at a 240 Hz repetition rate.

The National Research Council's (NRC) Decadal Survey (DS) of Earth Science and Applications from Space has identified the Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) as an important atmospheric science mission. NASA Langley Research Center, working with its partners, is developing fiber laser architecture based intensity modulated CW laser absorption spectrometer for measuring XCO2 in the 1571 nm spectral band. In support of this measurement, remote sensing of O2 in the 1260 nm spectral band for surface pressure measurements is also being developed. In this paper, we will present recent progress made in the development of advanced transmitter modules for CO₂ and O₂ sensing. Advanced DFB seed laser modules incorporating low-noise variable laser bias current supply and low-noise variable temperature control circuit have been developed. The 1571 nm modules operate at >80 mW and could be tuned continuously over the wavelength range of 1569-1574nm at a rate of 2 pm/mV. Fine tuning was demonstrated by adjusting the laser drive at a rate of 0.7 pm/mV. Heterodyne linewidth measurements have been performed showing linewidth ~200 kHz and frequency jitter ~75 MHz. In the case of 1260 nm DFB laser modules, we have shown continuous tuning over a range of 1261.4 - 1262.6 nm by changing chip operating temperature and 1261.0 - 1262.0 nm by changing the laser diode drive level. In addition, we have created a new laser package configuration which has been shown to improve the TEC coefficient of performance by a factor of 5 and improved the overall efficiency of the laser module by a factor of 2.

We report the development of a fiber-coupled diode laser module with high spatial and spectral brightness. Four arrays of diode laser bars are multiplexed using polarization and narrow-band wavelength combination. The module achieves 500 W of output power from a 200 μm, 0.2 NA fiber. The output spectrum, composed of contributions from more than 150 emitters, is narrowed using VBGs and has nearly 100% content within +/- 1.5 nm of 975 nm at full power.

Advances in both diode laser design and packaging technology, particularly thermal management, are needed to enhance the brightness of fiber coupled diode lasers while maintaining the small size and light weight required for defense applications. The principles of design for high efficiency fiber coupling are briefly covered. Examples are provided of fielded and demonstrated 100 and 200 micron diameter fiber coupled packages ranging in output from a few hundred to kW-class units in fibers, to include sub-kg/kW capabilities. The demand for high-power and high-brightness fiber coupled diode laser devices is mainly driven by applications for solid-state and fiber laser pumping. The ongoing power scaling of fiber lasers requires scalable fiber-coupled diode laser devices with increased power and brightness. A modular diode laser concept combining high power, high brightness, wavelength stabilization and low weight, which is considerable concern in the SWaP trades needed to field defense systems, has been developed. In particular the defense technology requires robust but lightweight high-power diode laser sources in combination with high brightness. The heart of the concept is a specially tailored diode laser bar, with the epitaxial and lateral structures designed such that only standard fast- and slow-axis collimator lenses in combination with appropriate focusing optics are required to couple the beam into a fiber with a core diameter of 200 μm and a numerical aperture (NA) of 0.22. The spectral quality, which is an important issue especially for fiber laser pump sources, is ensured by means of Volume Holographic Gratings (VHG) for wavelength stabilization. This paper presents a detailed characterization of different diode laser sources based on the scalable modular concept. The optical output power is scaled from 180 W coupled into a 100 μm NA 0.22 fiber up to 800W coupled into a 400 μm NA 0.22 fiber. In addition we present a lightweight laser unit with an output power of more than 300 W for a 200 μm NA 0.22 fiber with a weight vs. power ratio of only 0.9 kg/kW.

Selections from our recent developments in passive phase locking and coherent combining of lasers are presented. These include the principles of our approaches, lasers configurations, experimental procedures and results with solid state lasers and fiber lasers.

This paper discusses the experimental study and provides the analysis of an innovative approach to the passive-coherent combining of a spectrally-broadband laser channel array. Conventional coherent laser beam combining methods use a single-step technique based on phase-control of radiation in the combining channels with the goal of reducing the output beam to its diffraction limits. The novelty of our method is a two-stage approach in beam combining that enables (1) to couple multiple channels into a single beam with a stable mode structure, and (2) to convert this coupled beam in to a diffraction-limited state by using a stationary wavefront corrector. By using this method with the laser cavity in a stable configuration we effectively demonstrated our approach. This resulted in an efficient coupling of multiple channels by oscillating a single high-index Hermit-Gaussian mode. The cavity-based beam combining technique facilitates phase locking of multiple spectrally-broadband channels and therefore allows the suppression of nonlinear effects in individual fiber channels.

We present a broadband, all-dielectric, diffractive optical element (DOE) for spectral beam combining with optimized efficiency. We achieve maximal efficiency and polarization insensitivity for the sum of incident wavelengths by varying grating etch depth and duty cycle of a rectangular profile grating realized with the precision of ebeam mask definition. Design and fabrication considerations that maximize efficiency are quantified, including material options, e-beam defined lithographic parameters such as grating periods and aspect ratios, tailored wavelength dispersion, and polarization independence. These results are compared to published efficiency values of >95% diffraction efficiency for a single polarization and single wavelength and polarization-independent efficiency values of >98% also for a single wavelength.

Ti:Sapphire and optical parametric chirped-pulse amplifier ultrafast laser systems are currently limited in average power to < 100 watts by relatively low average power pump sources. In order to achieve both high peak and average powers concomitantly, we describe the design and operation of picosecond and nanosecond Yb:YAG cryogenic laser systems currently being developed at Snake Creek Lasers (SCL) with a goal of initially producing 1 J/pulse output at 1029 nm and green output at 515 nm with energy/pulse > 0.5 J, and with a repetition rate up to 1 kHz. Yb based lasers are particularly promising because of a long upper state radiative lifetime, reducing the diode pump power needed to produce a target energy/pulse, as well as very favorable high average power scaling properties at liquid nitrogen temperature. A comparison of a number of Yb based materials including Yb:YAG, Yb:Lu₂O₃, and others will be presented. Using a recently developed kinetics model as well as new system design codes, we describe the average and peak power scaling of cryogenic Yb:YAG lasers as well as the limitations imposed by optically induced damage, nonlinear phase accumulation, and amplified spontaneous emission.

Cryogenic Yb:YAG lasers operating at 1029 nm have been demonstrated at Snake Creek Lasers with high average power CW and ultrafast output powers, and provide near diffraction-limited output beams that are ideal for applications in harmonic generation. We describe experiments that have produced high average power green output power at 515 nm as well as preliminary experiments producing UV output power at 257.25 nm. Frequency doubling experiments used a 20 mm long non-critically phase-matched LBO crystal mounted in a constant temperature oven. A mode-locked Yb fiber laser operating at 50 MHz was used to drive a two Yb:YAG cryogenic amplifier system, producing hundreds of watts of average power output with a FWHM pulsewidth of 12 ps. Doubling efficiencies of > 50 % have been observed. For frequency quadrupling, we have used hydrothermally grown KTTP crystals grown at Clemson University and Advanced Photonic Crystals. KBBF offers unprecedented UV transmission down to 155 nm, and was used in a Type I phasematching configuration. The properties of KBBF will be discussed, as well as the experimental results observed and conversion efficiency.

We describe our approach and latest results for Raytheon's RELI (Robust Electric Laser Initiative) program. Our architecture leverages a slab-based, Master Oscillator / Power Amplifier (MOPA) architecture based on Raytheon's unique planar waveguide amplifier. Technical objectives for this effort are to demonstrate > 25 kW output with excellent beam quality and an electrical to optical efficiency > 30%. The planar waveguide architecture provides compact packaging and is inherently scalable to 100 kW or greater in a single beam line. We report on the latest progress and test results for the program.

NASA is currently developing several Earth science laser missions that were recommended by the US National Research Council (NRC) Earth Science Decadal Report. The Ice Cloud and Land Elevation Satellite-2 (ICESat-2) will carry the Advanced Topographic Laser Altimeter System (ATLAS) is scheduled for launch in 2016. The Active Sensing of CO₂ Emissions over Nights, Days, and Seasons (ASCENDS) mission and will measure column atmospheric CO₂ concentrations from space globally. The Gravity Recovery And Climate Experiment (GRACE) Follow-On (GRACEFO) and GRACE-2 missions measure the spatially resolved seasonal variability in the Earth's gravitational field. The objective of the Lidar Surface Topography (LIST) mission is to globally map the topography of the Earth's solid surface with 5 m spatial resolution and 10 cm vertical precision, as well as the height of overlying covers of vegetation, water, snow, and ice. This paper gives an overview of the laser transmitter and receiver approaches and technologies for several future missions that are being investigated by the NASA Goddard Space Flight Center.

We have developed an integrated Tm:fiber master oscillator power amplifier (MOPA) system producing 100 W output power, with sub-nm spectral linewidth at -10 dB level, >10 dB polarization extinction ratio, and diffraction-limited beam quality. This system consists of polarization maintaining fiber, spliced together with fiberized pump combiners, isolators and mode field adaptors. Recent advances in PM fibers and components in the 2 μm wavelength regime have enabled the performance of this integrated high power system; however further development is still required to provide polarized output approaching kilowatt average power.

Yttrium aluminum garnet (YAG) is a candidate host material for high power lasers, and the advantages of polycrystalline forms of YAG relative to single crystal (in terms of production cost, mechanical/optical properties, and feasible dopant concentrations) have been demonstrated. In addition, novel gain media in fiber form have attracted attention because of the potential for enhanced cooling efficiency, vibration resistance, and elimination of free space optics. Consequently, our work focuses on the development and characterization of optical-quality polycrystalline YAG fibers. Utilizing robust ceramic processes, we can readily produce highly transparent YAG fibers of ~30 μm diameter. We have recently begun characterizing both the mechanical properties and optical loss of these polycrystalline YAG fibers. In this report, the experimental setups for these measurements will be described and the effects of processing variables on the properties of polycrystalline YAG fibers will be discussed.

We report on initial testing of an edge-pumped Yb:YAG disk laser having a composite ceramic construction with undoped perimetral edge. The edge is designed to channel pump light while efficiently outcoupling amplified spontaneous emission (ASE). Edge-pumping allows for reduced doping of crystals with laser ions, which translates to a lower lasing threshold in Yb:YAG material and much reduced waste heat flux. Uniform gain and stable lasing were achieved.

There is a critical demand for high quality, transparent ceramic YAG fibers for high powered fiber lasers. The production of laser quality ceramic fibers hinges on advanced ceramic processing technology, along with the availability of highly sinterable powder with high phase and chemical purity. These two fundamental technologies have been successfully developed at UES. Nd (1.1 a/o) and Yb (1.0 a/o)-doped yttrium aluminum garnet (YAG) fibers with high optical quality were produced by combining UES's tailored powders with advanced consolidation processes including fiber extrusion and vacuum sintering. The as-sintered and as-annealed fibers, approximately 30 microns in diameter, appeared transparent and successfully transmitted laser beams; further development will allow for the production of doped ceramic YAG fiber lasers for advanced high power and high energy fiber laser systems.

A fiber ring laser which generates ~ 570 ps wide pulse train at 40 GHz has been demonstrated using a photonic crystal fiber as a nonlinear optical loop mirror (NOLM). Theoretical simulation of the NOLM transmission has been carried out using the split-step Fourier method.

5W peak power at 911 nm is demonstrated with a pulsed Neodymium (Nd) doped fiber master oscillator power amplifier (MOPA). This result is the first reported high gain (16dB) fiber amplifier operation at 911nm. Pulse repetition frequency (PRF) and duty-cycle dependence of the all fiber system is characterized. Negligible performance degreadation is observed down to 1% duty cycle and 10 kHz PRF, where 2.5μJ of pulse energy is achieved. Continuous wave (CW) MOPA experiments achieved 55mW average power and 9dB gain with 15% optical to optical (o-o) efficiency. Excellent agreement is established between dynammic fiber MOPA simulation tool and experimental results in predicting output amplified spontaneous emission (ase) and signal pulse shapes. Using the simulation tool robust Stimulated Brillion Scattering (SBS) free operation is predicted out of a two stage all fiber system that generates over 10W's of peak power with 500 MHz line-width. An all fiber 911 nm pulsed laser source with >10W of peak power is expected to increase reliability and reduce complexity of high energy 455 nm laser system based on optical parametric amplification for udnerwater applications. The views expressed are thos of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government.

A kW-class fiber-amplifier based laser illuminator system at 1030nm is demonstrated. At 125 kHz pulse repetition rate, 1.9mJ energy per pulse (235W average power) is achieved for 100nsec pulses with >72% optical conversion efficiency, and at 250kHz repetition, >350W average power is demonstrated, limited by the available pumps. Excellent agreement is established between the experimental results and dynamic fiber amplifier simulation, for predicting the pulse shape, spectrum and ASE accumulation throughout the fiber-amplifier chain. High pulse-energy, high power fiber-amplifier operation requires careful engineering - minimize ASE content throughout the pre-amplifier stages, use of large mode area gain fiber in the final power stage for effective pulse energy extraction, and pulse pre-shaping to compensate for the laser gain-saturation induced intra-pulse and pulse-pattern dependent distortion. Such optimization using commercially available (VLMA) fibers with core size in the 30-40μm range is estimated to lead to >4mJ pulse energy for 100nsec pulse at 50kHz repetition rate. Such waveform agile high-power, high-energy pulsed fiber laser illuminators at λ=1030nm satisfies requirements for active-tracking/ranging in high-energy laser (HEL) weapon systems, and in uplink laser beacon for deep space communication.

Based on the experience gained developing our market leading visible spectrum supercontinuum sources NKT Photonics has built the first mid-infrared supercontinuum source based on modelocked picosecond fiber lasers. The source is pumped by a ≈ 2 um laser based on a combination of erbium and thulium and use ZBLAN fibers to generate a 1.75-4.4 μm spectrum. We will present results obtained by applying the source for mid-infrared microscopy where absorption spectra can be used to identify the chemical nature of different parts of a sample. Subsequently, we discuss the possible application of a mid-IR supercontinuum source in other areas including infrared countermeasures.

We present detailed studies of the effect of sinusoidal phase modulation on stimulated Brillouin scattering (SBS) in ytterbium-doped fiber amplifiers. Based on a time-dependent SBS model, SBS enhancement factor versus pump linewidth for different modulation depths ranging from 0 to π , and modulation frequencies ranging from 30 MHz to 500 MHz were analyzed. In addition, experimental validation of SBS suppression via sinusoidal phase modulation is presented with experimental results agreeing well with the model and simulations. Furthermore, narrow linewidth coherent beam combining (CBC) of 5 sinusoidal phase modulated lasers is demonstrated via LOCSET.

We report on the progress towards the development and performance of Photonic Crystal Fiber (PCF) based multi-kW fiber amplifiers that are both narrow linewidth and have near diffraction limited beam quality for government and industrial applications that are both reliable and highly affordable. These amplifiers based on the foundation of Photonic Crystal Fibers along with a novel multi fiber-coupled laser diode stack and a monolithic 6+ l×l large fiber pump/signal multiplexer. The pixilated nature of the PCF fabrication process allows for the inclusion of precise index of refraction control so designs to control SBS, prevent modal instabilities and integrate large quantities of pump light can be realized, The precisely aligned 2-D laser diode emitter array found in laser diode stacks is utilized by way of a simple in-line imaging process with no mirror reflections to process a 2-D array of 380-450 elements into 3 400/440μm 0.22NA pump delivery fibers. The fiber combiner is an etched air taper design that transforms low numerical aperture (NA), large diameter pump radiation into a high NA, small diameter format for pump injection into an air-clad large mode area PCF, while maintaining a constant core size through the taper for efficient signal coupling and throughput. A completely monolithic amplifier utilizing these components has been demonstrated at 967W of output power at 1085nm.

We demonstrate single-mode, highly polarized output from a thulium-doped photonic crystal fiber (PCF) with 50 μm core diameter and an ultra-large mode area >1000 μm². To our knowledge, this is the largest mode area of any flexible PCF and is capable of enabling the generation of high peak powers. In a Q-switched oscillator configuration, this fiber produces peak powers as high as 8.9 kW with 435 μJ, 49 ns pulses, >15 dB polarization extinction ratio and quasi diffraction-limited beam quality. The pulse energy was scaled to >1 mJ in amplifier configuration.

First operation of the ⁴F _{9 / 2} → ⁶H _{13 / 2} laser transition in dysprosium doped yttrium aluminum garnet is reported. Room temperature operation at 582.7nm was obtained using 447nm GaN diode lasers pumps. Gaussian single-mode operation was demonstrated with a non-optimized slope efficiency of 12%. Millisecond pulsed operation generated 150mW with power limited by the pump diodes brightness. An emission cross section of 4.1E-21cm² at 582.7nm was determined by laser threshold analysis.

Performance of spectral beam combining depends on thermal effects on the optical components like volume Bragg gratings used for spectral selectivity of the beams combined. These thermal effects are results of absorption of laser radiation and in case of high power lasers can lead to reduction of efficiency of combining and losses. For example the Gaussian intensity distribution of laser beam leads to higher temperature in the central part of a grating and, hence, changing its operating specifications. Homogenizing of the temperature profile over the working field of a volume Bragg grating would increase efficiency of its operation. This can be realized through applying the beam shaping optics, for example refractive field mapping beam shapers providing high flexibility in building various optical setups due to their unique features: almost lossless intensity profile transformation, providing flattop, super Gauss or inverse Gauss profiles with the same beam shaper, saving of the beam consistency, high transmittance and flatness of output beam profile, extended depth of field, capability to adapt to real intensity profiles of TEM00 and multimode laser sources. This paper will describe some design basics of refractive beam shapers of the field mapping type, with emphasis on the features important for building and applications of high-power laser sources. There will be presented results of applying the refractive beam shapers in real research installations.

This study demonstrates the development of a high energy laser tactical decision aid (HELTDA) by the AFIT/CDE for mission planning High Energy Laser (HEL) weapon system engagements as well as centralized, decentralized, or hybrid predictive avoidance (CPA/DPA/HPA) assessments. Analyses of example HEL mission engagements are described as well as how mission planners are expected to employ the software. Example HEL engagement simulations are based on geographic location and recent/current atmospheric weather conditions. The atmospheric effects are defined through the AFIT/CDE Laser Environmental Effects Definition and Reference (LEEDR) model or the High Energy Laser End-to-End Operational Simulation (HELEEOS) model upon which the HELTDA is based. These models enable the creation of vertical profiles of temperature, pressure, water vapor content, optical turbulence, and atmospheric particulates and hydrometeors as they relate to line-by-line layer extinction coefficient magnitude at wavelengths from the UV to the RF. Seasonal and boundary layer variations (summer/winter) and time of day variations for a range of relative humidity percentile conditions are considered to determine optimum efficiency in a specific environment. Each atmospheric particulate/hydrometeor is evaluated based on its wavelength-dependent forward and off-axis scattering characteristics and absorption effects on the propagating environment to and beyond the target. In addition to realistic vertical profiles of molecular and aerosol absorption and scattering, correlated optical turbulence profiles in probabilistic (percentile) format are included. Numerical weather model forecasts are incorporated in the model to develop comprehensive understanding of HEL weapon system performance.

Solution presented in this article is a system using image acquisition time gating method. The time-spatial framing method developed by authors was used to build Laser Photography System (LPS). An active vision system for open space monitoring and terrorist threats detection is being built as an effect of recent work lead in the Institute of Optoelectronics, MUT. The device is destined to prevent and recognize possible terrorist threats in important land and marine areas. The aim of this article is to discuss the properties and hardware configuration of the Laser Photography System.

Ultrashort laser pulses (~100 fs duration) are known to generate charge separation in solid, liquid and gas targets through a variety of nonlinear mechanisms. This process results in the emission of a broadband electromagnetic pulse (EMP) in the microwave and terahertz (THz) regions of the electromagnetic spectrum. Possible applications of this phenomenon include remote RF and THz generation for material detection and diagnostics. We investigate the energy and spectrum of the EMP emitted from copper and glass targets irradiated by single 800 nm, 38 fs duration pulses with varying energy. The detector is two feet from the target and the detection bandwidth is 2-18 GHz. We also demonstrate our ability to enhance the emitted EMP energy from a copper target by more than an order of magnitude by irradiating the target with a 1064 nm, 14 ns duration pulse at a specific time delay relative to the ultrashort pulse. We attribute the increased optical to RF energy conversion to enhanced absorption of the ultrashort pulse by the nanosecond pulse-generated plasma at the surface of the target.

The issue of heat transfer in high energy lasers has been a serious problem for years. One valid method of mitigating this problem is the use of low quantum defect solid-state materials operated at cryogenic temperatures¹. A significant problem exists due to mismatch of coefficient of thermal expansion (CTE) and repeatedly cycling through a temperature range of ~200 K. Other groups, T.Y. Fan et al at MIT Lincoln Laboratory, have used ingenious crystal holders to overcome this problem. In this paper, we suggest the use of silicon/aluminum (Si/Al) alloys produced by Sandvik Osprey Ltd. that can have their CTE altered easily to match the CTE of whatever crystal material is chosen and still have a thermal transfer coefficient suitable for large heat transfer. We show the results of testing three different Si/Al alloys for CTE and thermal conductivity. We further test the material in a flow boil-off cryogenic cooling system that shows that the CE6 alloy material is capable of heat transfer of 21.5KW/m²K , with cold plate temperatures maintained below 110 K. The CE6 material has a CTE that almost exactly matches YAG from 90--300K.

Practical, large-area, high-power diode pumps for one micron (Nd, Yb) as well as eye-safer wavelengths (Er, Tm, Ho) are critical to the success of any high energy diode pumped solid state laser. Diode efficiency, brightness, availability and cost will determine how realizable a fielded high energy diode pumped solid state laser will be. 2-D Vertical-Cavity Surface-Emitting Laser (VCSEL) arrays are uniquely positioned to meet these requirements because of their unique properties, such as low divergence circular output beams, reduced wavelength drift with temperature, scalability to large 2-D arrays through low-cost and high-volume semiconductor photolithographic processes, high reliability, no catastrophic optical damage failure, and radiation and vacuum operation tolerance. Data will be presented on the status of FLIR-EOC's VCSEL pump arrays. Analysis of the key aspects of electrical, thermal and mechanical design that are critical to the design of a VCSEL pump array to achieve high power efficient array performance will be presented.