This PDF file contains the front matter associated with SPIE Proceedings Volume 10637, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

To advance the science of high power fiber lasers, in-house drawn specialty optical fibers are investigated. Ongoing research involves the fabrication and testing of Yb- and Tm-doped fibers at 1μm and 2μm. Using specialized fiber and pump mixing geometries, dopant profiles and system configurations, the performance of our in-house drawn active fibers has been examined. Results on a highly multi-mode, high average power pulsed Raman fiber amplifier pumped by a thin disc laser are presented. The Raman fiber is a large mode-area graded index fiber, also drawn in house. Finally, the development of capabilities for kilometer range propagation experiments of kW-level CW and TW-level pulsed lasers at the TISTEF laser range is reported.

Coherent | Nufern has a long history of high power Yb-doped fiber amplifier and component development. This ranges from amplifiers in the >1.5kW regime up to demonstrations of >3kW output power. In this paper, we discuss our latest advancements in the performance of high power, narrow linewidth, kW-class, monolithic Yb-doped fiber amplifiers, as well as the packaging of our two newest amplifier systems. Our lightest amplifier offers 1.6kW output power at <4.5kg/kW. This is an OEM version which requires external power, but offers a significant (nearly 1.5x) decrease in overall mass compared to our previous generation of amplifiers. Our 2.1kW amplifier is our smallest and highest power amplifier. It contains integrated electronics to offer a complete stand-alone amplifier, requiring only DC voltage input, external cooling, and software control. This has nearly a 2x reduction in volume compared to our previous generation. We discuss the performance and specifications of these two amplifier packages. This includes power scaling with narrow linewidth, as well as a significant and simultaneous reduction in volume and mass compared to our previous generations. We discuss the packaging challenges with these architectures, as well as the diode technologies which have enabled such a reduction in our packaging volume and mass. We also discuss experimental results in the power scaling of these fiber amplifiers.

Holmium doped fiber lasers (HoDFL) are attractive candidates for high energy lasers used in directed energy applications because they operate at wavelengths that are safer to the eye. The common solution-doping technique for making HoDFs can result in the incorporation of hydroxyl (OH) impurity in the active fiber core. The HoDFL operational wavelength of ~2.0 µm is near the 2.2 µm combination absorption band of the OH fundamental mode and the SiO4 tetrahedron vibration, so the OH concentration must be <1 ppm to prevent degradation of the laser performance. We have routinely fabricated HoDF with [OH] < 0.5 ppm. We have developed an ultralow OH processing technique based on both atmospheric exclusion from the silica core soot preform and careful, extensive drying. We report a resonantly-pumped solution doped Ho3+ fiber laser with a slope efficiency of 74%, and an output power of 96W. We are also investigating nanoparticle (NP) doping, where the holmium ions are encaged in a nanoparticle host selected for properties such as low phonon energy, where they are shielded from the Silica lattice. By optimizing variables such as precursor concentrations, NP ripening times, and surfactant selection during synthesis we have been able to increase the Ho NP concentration levels in Silica fiber cores. This has also allowed us to increase concentrations of otherwise incompatible low phonon energy host materials into the cores of the fibers. Cores comprising Ho doped LaF3 and Lu2O3 nanoparticles exhibited slope efficiencies as high as 85% at 2.06 µm in a MOPA configuration.

A review of recent results on new schemes and regimes of Raman fiber lasers (RFL) is presented. The key feature of RFLs is generation in new wavelength ranges, which are not covered by existing fiber lasers based on rare-earth doped active fibers. New high-brightness 9хх nm all-fiber source based on directly diode pumped passive GRIN multimode fiber with in-core fiber Bragg gratings (FBGs) enables high-efficiency Raman generation of high-quality (M2≤2.6) beam at powers up to ~100 W with further scaling capabilities. Mirrorless scheme based on Rayleigh-backscattering random distributed feedback (RDFB) in a singlemode polarization-maintaining (PM) fiber provides ultimate pump-to-Stokes conversion efficiency for the first order as well as cascaded generation of higher orders in 1-2 μm range. Single-frequency Raman lasing in a short (~10 m) PM fiber with RDFB based on random FBG array has been also demonstrated. Pulsed operation of RFL at active Q-switching and mode-locking in passive fibers is realized with pulse energies and durations of up to 30 μJ and ~50 ps, correspondingly. New regime of Raman dissipative solitons (of the first and second Stokes orders) synchronously generated in a common or external resonant cavity and their nonlinear mixing in PCF provide generation of <300 femtosecond pulses with ~10 nJ energy at new wavelengths in 900-1300 nm range.

We report the performance of a two stage single clad (SC) Thulium-doped fiber amplifier (TDFA), delivering an output power of 5 W at 1952 nm without stimulated Brillouin scattering (SBS) for a single-frequency input signal. A slope efficiency greater than 60 %, a signal gain greater than 60 dB and an input dynamic range > 30 dB are achieved. The amplifier topology was optimized with a modelization tool of the SC TDFA performance: experimental results and simulations are in good agreement.

High Power multi-kW class fiber lasers have become a leading technology in Directed Energy applications. With Direct Energy weapons and countermeasures moving closer to a deployable technology, industry players are now looking to ensure the components within their systems can withstand the harsh environments in which they will be used. With limited power available in the field, efficiency is a key criterion for these systems and there is a careful balance for diode laser pumps as a piece of the overall system. Increasing the cooling capacity delivered to the diode pumps will increase their Electrical-to-Optical efficiency, but requires more energy be consumed in the cooling loop through lowering the coolant temperature or increasing the pump speed to increase flow rate. In this paper, Coherent|DILAS aims to map these uncharted waters for its Low SWaP diode lasers by exploring trade space for the parameters that are critical to the overall system efficiency. By changing coolant types from water to glycol mixes, coolant freezing can be eliminated while the effects of coolant viscosity are explored. Additionally, direct changes to the coolant temperature and flow rates further explore cooling/efficiency trade space. Experiments are then repeated with an external grating to lock the center wavelength at the 976nm absorption band. The range at which locking is maintained and the efficiency change will be explored for various coolant, flow, and temperature configurations. With a large web of interacting processes being explored, Coherent|DILAS aims to enable further overall system optimization within Directed Energy community.

We have developed a versatile pulsed laser system for high precision magnetometry. The operating wavelength of the system can be configured to optically pump alkali vapors such as rubidium and cesium. The laser system consists of an auto-locked, interference filter stabilized, external cavity diode laser (ECDL), a tapered waveguide amplifier, and a pulsing module. The auto-locking controller can be used by an untrained operator to stabilize the laser frequency with respect to a library of atomic, molecular, and solid-state spectral markers. The ECDL output can be amplified from 20 mW to 2 W in continuous wave (CW) mode. The pulsing module, which includes an acousto-optic modulator (AOM), can generate pulses with durations of 20 ns and repetition rates of several MHz. Accordingly, the laser system is well suited for applications such as gravimetry, magnetometry, and differential absorption lidar. In this work, we focus on magnetometric applications and demonstrate that the laser source is suitable for optically pumping rubidium vapor. We also describe numerical simulations of optical pumping relevant to the rubidium D1 and D2 transitions at 795 nm and 780 nm respectively. These studies are relevant to the design and construction of a new generation of portable, rubidium, spin exchange relaxation-free (SERF) magnetometers, capable of sensitivities of 1 fT Hz^{-1/2 1}.

The proliferation of man-portable air-defense systems (MANPADS) is extremely wide. MANPADS are responsible for over 60% of total aircraft casualties since 1960’s. It is estimated that over 500,000 of these systems are deployed worldwide with a large number being out of governmental control. Directional infrared countermeasure (DIRCM) systems have been deployed in order to counter the threat. Laser based DIRCM system requires laser sources which can operate in bands I, II and IV. Up to day bands II and IV are covered by compact and lightweight quantum cascade lasers (QCLs), but for wavelength generation in band I, bulky and expensive solid-state or fiber laser solutions are used. Recent development of GaSb laser diode technology at Brolis Semiconductors greatly improved optical output powers and efficiency of laser diodes working in 1900 - 2450 nm range (band I). In this work we present a laser diode module which is based on incoherent beam combining of two high-power GaSb laser diode emitters working in 2.1-2.3 μm spectral band. This laser module is capable of providing directional beam with radiant intensity value of more than 30 kW/str. Module is extremely compact and lightweight (<50 g). E-O efficiency of the module is 15% and it can be operated in CW or pulsed operation modes replicating any waveform required for DIRCM application.

The two-dimensional (2D) temperature profile of a high-power junction-down broad-area diode laser facet subject to back-irradiance (BI) is studied via CCD-based thermoreflectance (TR) imaging and finite element modeling. The temperature rise in the active region (ΔΤ_AR) is determined at different diode laser optical powers, back-irradiance reflectance levels, and back-irradiance spot locations. Interestingly, our study shows that ΔΤ_AR rises sharpest not when the back-irradiance is boresight-aligned with the active region but rather when it is centered in the absorbing substrate approximately 5 μm away from the active region, a distance roughly equal to half of the back-irradiance spot FWHM (9 μm). At this critical location, ΔΤ_AR is found to increase by nearly a factor of three compared to its increase without back-irradiance. This provides insight on an important location for back-irradiance that may be correlated with catastrophic optical damage (COD) for diode lasers fabricated on absorbing substrates, and also suggests a thermal basis for truncated lifetime and deegraded performance for diode lasers experiencing backirradiance.

Kilowatt-class fiber lasers and amplifiers are becoming increasingly important building blocks for power-scaling laser systems in various architectures for directed energy applications. Currently, state-of-the-art Yb-doped fiber lasers operating near 1060 nm operate with optical-to-optical power-conversion efficiency of about 66%. State-of-the-art fiber-coupled pump diodes near 975 nm operate with about 50% electrical-to-fiber-coupled optical power conversion efficiency at 25C heatsink temperature. Therefore, the total system electrical-to-optical power conversion efficiency is about 33%. As a result, a 50-kW fiber laser will generate 75 kW of heat at the pump module and 25 kW at the fiber laser module with a total waste heat of 100 kW. It is evident that three times as much waste heat is generated at the pump module. While improving the efficiency of the diodes primarily reduces the input power requirement, increasing the operating temperature primarily reduces the size and weight for thermal management systems. We will discuss improvement in diode laser design, thermal resistance of the package as well as improvement in fiber-coupled optical-to-optical efficiency to achieve high efficiency at higher operating temperature. These factors have a far-reaching implication in terms of significantly improving the overall SWAP requirements thus enabling DEW-class fiber lasers on airborne and other platforms.

Many military applications require reliable laser diode operation across a wide range of elevated temperatures. Size, Weight and Power (SWaP) restrictions often limit cooling options and necessitate that high-power emission at the desired pump wavelength is maintained across wide temperature ranges. A family of QCW diode arrays has been developed for operation in these harsh environments. The arrays may be constructed with either multi-wavelength diode bars or wavelength locked with Volume Bragg Gratings (VBGs) to optimize absorption across a wide range of temperatures. The arrays have been designed to withstand the mechanical shock and vibration requirements common to military environments. This paper includes a comparison of the multi-wavelength arrays and VBG-locked arrays at high temperatures. Both sets of arrays were characterized across a broad temperature range and exposed to MIL-STD shock and vibration testing. VBG locked arrays are shown to provide >90% locking across a 15° operating range whereas multi-wavelength arrays allowed power absorption to be maintained across incredibly wide ranges (e.g. -40° to 70°C). Life test results from arrays operated at 80oC, 250A, 60Hz for over 600 million pulses are also presented. These arrays demonstrate excellent high-temperature reliability over a pulse count well in excess of the requirement of many military applications (e.g. range finders / target designators).

Maximizing brightness and peak power out of the laser diode array is essential in the development of high powered solid state and fiber laser systems. Recent developments have enabled single 1 cm laser diode bars capable of producing over 500W peak power at wavelengths between 770nm and 1100nm. New technologies in bar cooling and optical beam shaping have allowed scaled laser pump diodes to achieve peak powers over 1MW. Novel manifold designs have allowed 100kW to 1MW stacks maintaining a brightness of over 11kW/cm². The latest performance from high brightness pump diodes operating under a variety of pulse conditions will be discussed. Additionally, discussion will be provided regarding a novel method of powering and controlling diodes with megawatt-class powers in MIL applications.

Second harmonic generation efficiency of a pulsed Er fiber laser with periodically poled lithium niobate (PPLN) is optimized by varying input pulsewidth. The Er-doped fiber amplifier was a 3-stage, 1550 nm amplifier with 1-10 ns variable pulsewidth, 180 kHz pulse repetition frequency, and 5.5 W of output power. The laser was focused into a single, 10 mm long piece of PPLN to convert to 775 nm through second harmonic generation. The pulsewidth was varied and we observed a correlation between pulsewidth and conversion efficiency. At the minimum pulsewidth, τ = 2 ns, we achieved 31% conversion efficiency, and as we increased the pulsewidth we saw an increase in second harmonic generation conversion efficiency. At τ = 10 ns, our maximum pulsewidth, we saw a conversion efficiency of 68%, which was the highest conversion efficiency achieved in this experiment. The increase of efficiency with reduced pump intensity is attributed to the decrease in spectral width of the laser at longer pulsewidths. Measuring the spectrum of the laser verified the presence of self-phase modulation at the shorter pulsewidths.

We report on a 2 μm master oscillator power amplifier (MOPA) fiber laser system capable of producing 700 μJ pulse energies from a single 1.5 m long amplifier. The oscillator is a single-mode, thulium-doped fiber that is Q-switched by an acousto-optic modulator. The oscillator seeds the amplifier with 1 W average power at 20 kHz repetition rate. The power amplifier is a polarization-maintaining, large mode area thulium-doped fiber cladding pumped by a 793 nm fiber-coupled diode. The fiber length is minimized to avoid nonlinearities during amplification while simultaneously enabling high energy extraction. The system delivers 700 μJ pulse energies with 114 ns pulse duration and 14 W average power at 1977 nm center wavelength.

A dual channel high energy solid state laser using intracavity volume Bragg gratings is presented. CW operation showed a maximum output power of 5.7 W with a slope efficiency of 48%. Implementation of a Cr:ZnSe saturable absorber with 80% transmission achieved q-switching regime with 160 ns long pulses and combined energy of 4 mJ. Each channel had less than 1 nm spectral width in the range between 1880 and 1908 nm. The spectral difference between the two channels was tuned from 5 to 20 nm, corresponding to 0.4 to 1.7 THz, providing an alternative method for generating narrowband terahertz radiation.

Single crystal fiber composed of rare earth doped YAG offers the potential for high power scaling of fiber lasers due to it lower intrinsic stimulated Brillouin cross-sections and higher thermal conductivity. The use of rare earth doped YAG fibers also mitigates issues associated with photodarkening, as well as issues associated with the silica multiphonon edge absorption and OH- quenching observed in Ho doped silica fiber lasers operating at eye safer wavelengths. To date, small diameter single crystal YAG fibers as small as 17 µm have been grown at Naval Research Laboratory to achieve a core architecture. Recent work has focused on development of cladding structures on the single crystal core material through sputtering, liquid phase epitaxy, and hydrothermal crystal growth to achieve a true core/clad all- crystalline wave guiding structure. Crystalline claddings have been grown with all three approaches with varying degree of quality and crystallinity. In this paper, we will report on our progress in fabricating crystal claddings on YAG single crystal core material.

Yttrium Iron Garnet (YIG) is an important material for high power infrared isolators as well as in microwave applications. When processed in bulk polycrystalline form, the transmission of this cubic material is greatly affected by non-stoichiometry and porosity content. This work investigates the effect of sintering additives, such as MgO, on the fabrication of YIG ceramics by way of conventional solid-state reactive (CSSR) sintering at 1380C for differing soak times. Specifically, we will be discussing the effect of these additives on the microstructure and densification kinetics and show that IR transparent bulk sized ceramics can be obtained with a grain size below 3 um.

Publisher’s Note: This paper, originally published on 5/4/2018, was replaced with a corrected/revised version on 3/7/2019. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

Avalanche photodiodes (APDs) are a promising detector technology for light detection and ranging (LIDAR) systems needed for a variety of DoD and commercial applications. However, a new material that is sensitive to 1.55 μm and has low “excess noise” is needed to achieve the required signal-to-noise. The main issue for improving APD signal-to-noise is to reduce excess noise. Excess noise is inevitable in APDs because impact ionization must occur to obtain a high multiplication gain. One solution to reduce the excess noise is to develop a new material system with favorable impact ionization coefficients. The ratio of electron (α) and hole (β) impact ionization coefficients, defined as k value, is intrinsically defined by the material and is a dominant factor for the APD’s excess noise. In this work, we investigate InAs/AlSb type-II superlattice (T2SL) APD. The superlattices provide us with additional degrees of freedom to engineer the electronic band structure. Our work is building on previous, promising results with the quaternary system AlInAsSb. We have theoretically modeled an InAs/AlSb type II superlattice (T2SL) system that can provide flexibility to engineer the electronic band structure to achieve single carrier impact ionization and reduce the excess noise. The simulation of this T2SL predicts that InAs/AlSb has higher absorption and would work as an electron- APD with low k. We will discuss design, growth, fabrication and IV characterization of this photodiodes.

Defense sensing systems must be both productive and robust to accomplish their mission. Active infrared sensing devices consist of many components such as the active medium, mirrors, beam-splitters, modulators, gratings, detectors, etc. Each of these components is subject to damage by the laser beam itself or environmental factors. Misalignment of these components due to vibration and temperatures changes can also reduce performance. The result is a complex and expensive system subject to multiple points of degradation or complete failure. However, beam confinement or “no free-space optics” via fiber transmission and even component assembly within the fiber itself can achieve reliability and low cost for sensing systems with reduced component count and less susceptibility to misalignment. We present measurements of high-power infrared laser beam transmission in chalcogenide fibers. The fiber compositions were As39S61 for the core and As38.5S61:5 for the cladding, resulting in a numerical aperture of 0.2. A polyetherimide jacket provided structural support. Multiwatt CW transmission was demonstrated in near single-mode 12 micron core fiber. Efficient coupling of quantum cascade lasing into anti-reflection coated chalcogenide fiber was also demonstrated. Efficient beam transport without damage to the fiber required careful coupling only into core modes. Beams with M2 ≥ 1.4 and powers higher than 1 W produced damage at the fiber entrance face. This was most likely due to heating of the highly absorptive polymer jacket by power not coupled into core modes. We will discuss current power limitations of chalcogenide fiber and schemes for significantly increasing power handling capabilities.

Laser sources operating near a wavelength of four microns are important for a broad range of applications that require power scaling beyond the state-of-the-art. The highest power demonstrated in the spectral region from a solid-state laser source is based upon nonlinear optical (NLO) conversion using the NLO crystal ZnGeP₂ (ZGP). High-power operation in ZGP is known to be limited by thermal lensing. By comparing the figure of merit for thermal lensing in ZGP with other NLO crystal candidates, CdSiP₂ (CSP) particularly offers significant advantages. However as was the case with ZGP during its early development, the physics of observed crystal defects, and their relevance to power scaling, was not at first sufficiently understood to improve the crystal’s characteristics as a NLO wavelength conversion element. During the past decade, significant progress has been made (1) with the first reported growth of a large CSP crystals, (2) in understanding the crystal’s characteristics and its native defects, (3) in improving growth and processing techniques for producing large, low-loss crystals, and (4) in demonstrating CSP’s potential for generating high-power mid-infrared laser light. The paper will summarize this progress.

II-VI chalcogenides (e.g. ZnSe/S) doped with transition metal (TM) ions such as Cr, and Fe are arguably the materials of choice for fabrication of effective mid-IR gain media. TM:II-VI materials feature a favorable blend of laser spectroscopic parameters: a four-level energy structure, absence of excited state absorption, close to 100% quantum efficiency of fluorescence (for Cr doped II-VI media), broad mid-IR vibronic absorption and emission bands. This talk summarizes progress in fabrication of high quality Cr:ZnS/Se and Fe:ZnS/Se by cation vacancy and cation interstitial enhanced post growth thermal diffusion. We also describe recent breakthrough on recrystallization and effective doping of ZnS ceramics under hot isostatic pressing resulting in a large cm-scale monocrystalline domains formation and an increase of the Fe diffusion coefficient by three orders of magnitude. We report recent advances in high-power Cr:ZnS/Se and Fe:ZnSe laser systems, enabling a wide range of tunability (1.8-5.0µm) with output power levels of up to 140 W near 2500 nm, 32 W at 2940 nm, and 35 W at 4300 nm with corresponding optical efficiencies of 62%, 29%, and 35%. Current improvements of output characteristics of polycrystalline Cr:ZnS/Se oscillators in Kerr-Lens-Mode-Locked (KLM) regime are reported: up to 2 W output power at 75-1200 MHz repetition rate, up to 2 cycle pulse duration (16 fs) with efficiency of 20-25% with regards to Er-fiber laser pump power. The effects of efficient up-conversion of mid-IR fs pulses in the laser medium as well as supercontinuum generation are demonstrated. Further extension of mid-IR spectral coverage to 3-8 m is demonstrated by Cr:ZnS KLM laser pumped degenerate (subharmonic) parametric oscillators (OPOs) based on based on quasi-phase matching in orientation-patterned gallium arsenide, and random phase matching in polycrystalline ZnSe.

Optical amplification of a mid-IR laser using a Co²⁺ :CdTe amplifier was attempted by a double-pass technique. A 2.825 μm pump source was used to amplify a 3.780 μm seed source through the gain medium. Of transition-metal-doped chalcogenide lasers, iron (Fe²⁺) and chromium (Cr²⁺) have emerged as practical sources of tunable mid-infrared radiation, while cobalt (Co²⁺) still remains comparatively unexplored. The initial observations of directly pumping into the ⁴T₂ energy level are reported and discussed to better understand the potential of Co:CdTe as a mid-infrared gain medium.

The development of new solid-state laser materials for mid-infrared (mid-IR) laser sources continues to be interest for potential applications in remote sensing of bio-chemical agents, IR countermeasures, and IR spectroscopy. Fluorescent materials based on Ho3+ doped crystals and glasses with narrow phonon spectra cover a wide wavelength range between ~1-4 µm. In this work, spectroscopic characterization on infrared emission properties of trivalent holmium (Ho3+) doped potassium lanthanum chloride (K2LaCl5) were explored. K2LaCl5 is slightly hygroscopic but possesses a maximum phonon energy of 235 cm-1. The low maximum phonon energy of K2LaCl5 leads to low non-radiative decay rates and efficient IR fluorescence. The studied Ho3+ doped K2LaCl5 material was grown by Bridgman technique. Using ~900 nm excitation, IR emissions centered at ~1.66, ~1.995, and ~3.90 µm were observed from Ho:K2LaCl5 corresponding to the 5I5-->5I7, 5I7-->5I8, and 5I5-->5I6 transitions of Ho3+ ions. Spectroscopic results and data modeling including the Stark level energies, Judd-Ofelt analysis, transitions cross-sections, and fluorescence dynamics will be presented at the conference.

In the pursuit of efficient mid-Infrared laser host materials, we developed a dual-phase nanocomposite composed of a majority species (MgO) which provides high thermal conductivity and a rare earth doped minority species (Er:Y₂O₃) featuring a low maximum-phonon energy. This material was prepared using a co-precipitation method where both components are synthesized together for intimate mixing on the smallest scale. Preparation parameters were tuned to achieve a small crystallite size in order to limit scattering at the grain boundaries between the two different species. Optical characterization of the prepared materials included percent transmission (%T) as well as Raman measurements and Er fluorescence spectroscopy. Once suitable transmissivity was achieved, %T results were compared to Mie scattering calculations to gauge the average grain size in the material; and we determined the smallest average Y₂O₃ grain sizes achieved in our materials so far was 80 nm in diameter.

We examined the performance of potassium diode pumped alkali laser (K DPAL) using He, Ar, Methane (CH4), Ethane (C2H6) and a mixture of He and CH4 as a buffer gas to provide spin-orbit mixing of the 4P3/2 and 4P1/2 states of Potassium atoms. We found that pure helium as an efficient buffer gas for K DPAL with a static gain medium can be used only for pulsed operation with up to 50 µs pulse durations. The performance degradation of K DPAL with pure helium for longer pulses can be explained by ionization, which causes an effective reduction in neutral alkali atoms number density. Using a flowing system for the K DPAL allows improving its operation in continuous wave (CW) mode, but for efficient lasing with pure He buffer gas, a considerable flow speed of about 100 m/s is required. In contrast, using a small amount of methane or ethane (10-20 Torr) mixed with helium at total pressure of about 1 atm, an efficient continuous wave lasing can be achieved with very moderate flow speeds of about 1 m/s. Argon buffer gas was also tested in this experiments, but it did not support lasing neither in pulsed nor in CW mode of K DPAL operation.

A high brightness, high energy passively Q-switched (PQS) Nd:YAG laser is presented using volume transmitting Bragg gratings (TBGs) as angular filters. A planar cavity with a length of 1 cm, an 800 μm diameter pump beam, a 20% transmission Cr:YAG saturable absorber, and a 40% output coupling resulted in output pulse energies greater than 1 mJ and durations near 600 ps. In order to increase brightness without altering cavity length, a TBG with narrow angular selectivity placed in the cavity to suppress the higher order transverse modes allowing for near diffraction limited output.

Highly efficient, water cooled, resonantly pumped Ho3+ fluoride lasers have been demonstrated at 2 μm. The laser materials, 0.5 at.% Ho:YLF, 0.5 at.% Ho:BYF, and 0.5 at.% Ho:BaYLuF, were resonantly pumped with an IPG Tm:fiber laser operating at 1940 nm. Laser action occurred on the ⁵I₇ to the ⁵I₈ manifold with center wavelengths of 2062.6 nm, 2074.0 nm, and 2075.2 nm respectively. Continuous wave (CW) operation of 0.5% Ho:BYF achieved a maximum efficiency of approximately 68% with a 50% reflective output coupler. Pulsed operation of this laser system produced 4 mJ pulses at 500 Hz with a slope efficiency of 38%.

Chemical threat detection has long been of interest to military, law enforcement, environmental agencies, and forensic investigators. Recently, as the propensity for both foreign and homegrown terrorism, illegal drug manufacture, and concern for environmental regulation continues to grow, the demand for rapid, portable chemical threat detection capabilities has increased dramatically. In particular, the ability to identify chemical threats (explosives, narcotics, toxic industrial chemicals, etc.) at a distance (standoff) is of special interest as it increases the safety of the end user during interrogation. Traditional analytical laboratory techniques such as high-performance liquid chromatography or gas chromatography coupled with mass spectrometry offer excellent sensitivity for detection and identification of trace amounts of threatening material. However, these techniques often lack the portability necessary for remote on-site interrogation as samples must be physically collected and brought to a laboratory for analysis. Vibrational spectroscopic techniques offer both the chemical identification and miniaturization capabilities required for portable, on-site chemical threat detection. Most importantly, spectroscopic techniques are inherently and uniquely standoff, where emitted or scattered photons are collected at some distance from the sample. The challenge then becomes miniaturizing the instrumentation while maximizing the distance at which accurate chemical detection can be made. Here we report on portable chemical threat detection instrumentation developed by Alakai Defense Systems, which employs deep ultra-violet Raman spectroscopy. We discuss the general system aspects such as basic optical design and ambient light rejection techniques. We also present data on the performance capabilities using several substances including actual narcotics and other compounds commonly used as cutting agents. Lastly, we discuss possible future directions including the ability for rapid spectroscopy while maintaining high photon detection sensitivity by employing an intensified scientific CMOS (sCMOS) and the propensity for NIR standoff Raman detection using deep-depletion CCD technology.

An important consideration for the laser beam propagation through a long atmospheric path is the geometry of the optical path and random variations in the refractive index due to atmospheric turbulence. Here, we consider a plane wave propagation through a 10km medium to investigate the deep turbulence effects on the beam propagation using phase screen approach. The turbulence effects are modeled by non-Kolmogorov descriptions of energy cascade theory, known as beta-model. The beta-model incorporates space-filling concepts for the turbulent eddies in the inertial ranges using fractal descriptions for the eddies. Metrics based on intensity and phase variances and number of zero intensity values are analyzed for various levels of turbulence intensity (measured by Cn2) by choosing the value of power law exponent from a range of eligible values. It is observed that metrics based on intensity variance saturate but metrics based on phase variance show potential for characterizing stronger turbulence effects.