This PDF file contains the front matter associated with SPIE Proceedings Volume 6952, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Sandia National Laboratories' program in high-power fiber lasers has emphasized development of enabling technologies for power scaling and gaining a quantitative understanding of fundamental limits, particularly for high-peak-power, pulsed fiber sources. This paper provides an overview of the program, which includes: (1) power scaling of diffraction-limited fiber amplifiers by bend-loss-induced mode filtering to produce >1 MW peak power and >1 mJ pulse energy with a practical system architecture; (2) demonstration of a widely tunable repetition rate (7.1-27 kHz) while maintaining constant pulse duration and pulse energy, linear output polarization, diffraction-limited beam quality, and <1% pulse-energy fluctuations; (3) development of microlaser seed sources optimized for efficient energy extraction; (4) high-fidelity, three-dimensional, time-dependent modeling of fiber amplifiers, including nonlinear processes; (5) quantitative assessment of the limiting effects of four-wave mixing and self-focusing on fiber-amplifier performance; (6) nonlinear frequency conversion to efficiently generate mid-infrared through deep-ultraviolet radiation; (7) direct diode-bar pumping of a fiber laser using embedded-mirror side pumping, which provides 2.0x higher efficiency and much more compact packaging than traditional approaches employing formatted, fiber-coupled diode bars; and (8) fundamental studies of materials properties, including optical damage, photodarkening, and gamma-radiation-induced darkening.

We report on an eye safe fiber laser generating >5 Watts of average power at 50 kHz packaged in a cylinder measuring 6" in diameter and 3.75" in length to show compatibility with advanced seeker concepts. To our knowledge, this represents the highest average power per unit volume from an eye safe pulsed fiber laser generating multi-Watts of average power.

We present the results of the experimental study and comparison of Yb-free, Er-doped, all-fiber, alignment free, single frequency (SF) fiber amplifiers operating under 980-, 1470- and 1530-nm pumping for the core- and clad-pumping architectures. In the single-mode core-pumped configuration Er-doped fiber amplifiers demonstrated 52% and 60% pump to output efficiencies for 980 and 1480 nm pump wavelength, respectively, producing over 140 mW of SF output power at seed wavelength ~1560 nm and over 180 mW at seed wavelength 1605 nm for 300 mW of pump power. At the same time, when clad pumped, Er-doped 20/125 DC LMA gain fiber demonstrates laser efficiencies of 22.4% pumped at 980 nm - up to 20 W of fiber-coupled diode laser pumping. The same LMA fiber demonstrates 33% optical-to-optical efficiency (46% slope efficiency versus absorbed power) when cladding-pumped with 1520-1530-nm fiber-coupled laser diode modules. Detailed analysis of these experiments is presented.

An analysis of the parametric interaction and the initial fiber geometry to achieve wavelength conversion from common laser sources operating in the 1030-1064nm spectral band into the 900-950nm wavelength range has been performed. The preliminary analysis shows that new fiber designs involving fibers with cores engineered with crystal-like shapes and also pulsed fiber sources operating at wavelengths in the 1030-1064nm will be required to achieve efficient emission within the desired wavelength range. Both the fiber required for phase-matching the parametric nonlinear process and the pulsed fiber laser pump source are within reach of current technology. They both require engineering efforts to produce a packaged, rugged and compact source.

Spectral Beam Combination (SBC) of multiple fiber laser outputs has been shown to be an effective way to scale the power of fiber laser systems while maintaining near-diffraction-limited beam quality. The fiber SBC system maintains many of the key advantages of individual fiber lasers, such as high efficiency, excellent beam quality independent of output power and relaxed thermal management requirements. Several approaches to spectral beam combination have been demonstrated including single grating in linear oscillator, single grating in master oscillator power amplifier (MOPA), dual grating MOPA and dual grating ring oscillator configurations. Each of these variations has certain advantages in terms of the system design and fiber laser requirements. In this paper we analyze the different approaches and compare them in terms of combined beam quality, line-width requirements of the individual fiber laser channels, power scalability and system complexity. The results obtained using the different SBC approaches at Aculight are summarized in the context of this analysis.

We present recent advances in high power semiconductor laser bars and arrays at near infrared and eye-safe wavelengths. We report on increased spectral brightness with internal gratings to narrow and stabilize the spectrum and increased spatial brightness in multimode and single mode devices. These devices have the potential to dramatically improve diode pumped systems and enable new direct diode applications.

One of the recent advances in solid-state laser (SSL) defense technology is the 100W level Er-doped "eye-safe" laser with low quantum defect pumping at 1.53μm. Major technical challenges in achieving high-wattage devices include increasing the system power conversion efficiency and arranging the removal of heat from both the crystal and the pumps. It is known that performance of the crystal can be improved dramatically by cryogenic cooling. Hence, it is desirable to have cryo-cooled pumps to realize ergonomic and efficient diode-pumped SSL with unified cryogenic cooling. In this paper we report on the development of LN2-cooled InP-based λ~1.5-1.6 μm diode pumps. The broad area lasers demonstrated 11W in continuous-wave (CW) regime at an operating current of 20A. Despite the highest CW power measured to date from an InP-based emitter, we did not observe catastrophic optical mirror damage. The spectral width of the radiation from a cooled device decreased 1.5-2 from its room-temperature value, which will significantly improve pumping efficiency. We show that laser diode design has to be optimized for performance at cryogenic temperatures. Reviewing the data on LN2 cooled lasers emitting in the wavelength range of 1.13 - 1.8 μm, we discuss the route to increase the power conversion of the LN2 cooled InP-based pumps to greater than 60% and further narrow and stabilize the laser emission spectrum.

Recent progress in rare-earth doped fibers has allowed Yb-doped fiber lasers to be power-scaled to several kW's. Remarkably, the continued rise of the fiber laser output power into multi-kW range is being limited by the pump diodes rather than the fibers themselves. In this article we discuss our recent progress in the development of high-brightness fiber-coupled laser diode modules for pumping Yb and Er doped lasers. Pumps based on laser diode arrays as well as on multiple single emitter platforms will be described. The prospects of power scaling as well as expected limitations to different designs will be discussed. We demonstrate 976 nm pump module with 55W ex-fiber output power from 105 μm core diameter fiber. The coupling efficiency was 58%. Similar approach was used for realization of 1450 nm diodes and as a result over 15 W CW power was achieved from the fiber with the same aperture.

Using wavelength beam combination, we report greater than 100 W out of a 100 μm core, 0.2 numerical aperture fiber. We emphasize that this is reliable CW power from an optical system that does not suffer from distortion due to heating in gold-coated, polymer-based diffraction gratings. We show that using high-brightness bars with single-mode emitters, the wavelength beam combination technique is capable of achieving high power out of a 50 μm core, 0.2 numerical aperture fiber with good coupling efficiency.

We report on recent progress in the control of optical modes toward the improvement of commercial high-performance diode laser modules. Control of the transverse mode has allowed scaling of the optical mode volume, increasing the peak output power of diode laser emitters by a factor of two. Commercially-available single emitter diodes operating at 885 nm now exhibit >25 W peak (12 W rated) at >60% conversion efficiency. In microchannel-cooled bar format, these lasers operate >120 W at 62% conversion efficiency. Designs of similar performance operating at 976 nm have shown >37,000 equivalent device hours with no failures. Advances in the control of lateral modes have enabled unprecedented brightness scaling in a fiber-coupled package format. Leveraging scalable arrays of single emitters, the conductively-cooled nLIGHT Pearl^TM package now delivers >80 W peak (50 W rated) at >53% conversion efficiency measured from a 200-μm core fiber output and >45 W peak (35 W rated) at >52% conversion efficiency measured from a 100-μm fiber output. nLIGHT has also expanded its product portfolio to include wavelength locking by means of external volume Bragg gratings. By controlling the longitudinal modes of the laser, this technique is demonstrated to produce a narrow, temperature-stabilized spectrum, with minimal performance degradation relative to similar free-running lasers.

Diode pumped alkali-vapor (cesium, rubidium and potassium) lasers (DPALs) are attractive sources for high-power applications due to their high quantum efficiency, excellent optical beam quality and reduced thermal load. DPALs require optical pump sources that can reliably emit energy within the narrow (about 10 GHz) absorption bands of the alkali vapor. Single laser diodes (LD) and laser bars (LB) integrated into wavelength selective external cavities with volume diffraction gratings can simultaneously achieve narrow linewidths and high output power. A diode laser bar with a volume Bragg grating output coupler emitting at 780 nm has demonstrated a CW output power up to 30 W with a slope efficiency of 0.8 W/A, a spectral width (FWHM) below 10GHz, and a tunability over 400 pm. The output power of a diode bar in an external cavity exceeded 90% of the output power of the free-running bar. More than 90% of the laser emission was absorbed by Rb cell.

Characterization of thermal conductivity with temperature dependence is much important for designing of the high power lasers. A simple model of thermal conductivity for various optical materials, such as Y₃Al₅O₁₂, YVO₄, GdVO₄, stoichiometric and congruent LiTaO₃, and synthetic quartz, has been established: one parameter for specific heat and two parameters for thermal diffusivity. Authors also discussed the dependence of laser active ion doping concentration such as Yb³⁺ or Nd³⁺. This thermal conductivity model was verified from room temperature to 200°C in the various optical materials.

This paper presents the results of experimental studies on Q-switched flash-lamp pumped Nd:YAG laser with resonator formed by volume Bragg gratings. This novel design results in single-frequency mode operation with millijoules pulse energy. The mode selection is performed only by volume Bragg gratings that dramatically simplifies laser design.

Quantum cascade lasers (QCL) are a new class of solid-state lasers capable of delivering mid-infrared (mid-IR) radiation wavelengths from 3.5 μm to 25 μm. QCLs are finding extensive use in chemical sensing applications due to the abundance of absorption features in the molecular fingerprint region spanned by these sources. They are also being exploited in the field of electro-optical infrared countermeasures. QCL devices exhibit an elliptical emission profile that is highly divergent in the fast axis of the laser waveguide. The far-field profile of the QCL emission, 62° and 32° ± 2° for the fast and slow axes, respectively, places stringent demands on the design of efficient collimation lenses. Because of the current lack of commercially available mid-IR optical components, QCL users must design and fabricate custom micro-optics to efficiently collect, collimate, and focus the QCL emission. In this paper, we report the design, fabrication, and characterization of germanium aspheric collimating and focusing optics designed for mid-IR Fabry-Perot QCLs with an emission wavelength of 8.77 μm. Custom aspheric collimating and focusing lenses with a numerical aperture (NA) of 0.85 and 0.20, respectively, were designed and fabricated using single-point diamond turning. Measurements of the transmitted wavefront error at mid-IR wavelengths showed diffraction-limited performance with Strehl ratios >0.94 and >0.99 for the collimation and focusing lenses, respectively. A beam propagation figure of merit (M²) of 1.8 and 1.2 was measured for the fast and slow axes, respectively, of a Fabry-Perot QCL using a confocal optical system comprised of these lenses.

Active modelocking of multiple polariton lasers mediated by real time sensing offers novel capabilities for optically based sensing. We outline a strategy based in part on short range polariton-polariton interactions and in part on an actively managed external optical field coherent with each of the individual polariton lasers. This actively managed coherent optical field is required to establish long range coherence between multiple spatially distinct polariton lasers. Polariton lasers offer nonlinear behavior at excitation levels of a few quanta of the optical field, time constants of picoseconds or less, and optical wavelength dimensions of individual lasers. Achievement of useful long range, hundreds of meters, polariton based optical sensing appears useful, but to require active cohering of arrays of polariton lasers. Continuous metrology and active control of the system coherence offer unique opportunities for sensing approaching quantum limited operation. We consider strategies and capabilities of sensing systems based on such arrays of spatially distinct, but collectively coherent, polariton lasers. Significant advances in a number of technical areas over decades appear needed to achieve such systems.

We discuss progress towards a kilowatt class CW Yb:YAG cryogenic laser. Cryogenically-cooled crystalline solid-state lasers, and Yb:YAG lasers in particular, are attractive sources of scalable CW output power with very high wallplug efficiency and excellent beam-quality that is independent of the output power. Results are presented for a high power Yb:YAG oscillator that has produced over 550 W of output power with good slope and optical-optical efficiencies while maintaining single transverse mode output. We also describe a new oscillator-amplifier cryogenic Yb:YAG system nearing completion, that will build on the work presented here and result in CW power output of > 1 kW while maintaining near-diffraction-limited beam quality. The oscillator described here consists of a distributed array of seven highly-doped thin Yb:YAG-sapphire disks in a folded multiple-Z resonator. Individual disks are pumped from opposite sides using 100 W fiber-coupled 940 nm pump diodes. The laser system produces a near-diffraction-limited TEM₀₀ output beam with the aid of an active conduction-cooling design. In addition, the device can be scaled to very high average power in an oscillator-amplifier configuration, by increasing the number and diameter of the thin disks, and by increasing the power of the pump diodes with only minor modifications to the current design. We will present experimental results including output power, threshold power, and slope and optical-optical efficiencies.

We present our recent developments in high-power, high-efficiency cryogenic Yb:YAG laser systems. Specifically, we will discuss our 2.3-kW master oscillator power amplifier (MOPA) which has shown optical wall-plug efficiencies above 30-% (diode-driver input to optical output). This laser system has been operated for long run times with continuous wave and pulsed output formats. The beam quality factor, M², of the MOPA has been measured to be less than 2 and it is currently limited by the master oscillator. We are working to improve the device's beam quality and output power. In addition, we have demonstrated an all-cryogenic Yb:YAG laser that produced 29 W of optical power. Use of cryogenic diode laser pumps represents our next step towards achieving greater than 50% efficient high-power laser systems.

Adhesive-Free Bonded (AFB^(R)) composite components for solid state lasers essentially are only held together by Van der Waals attractive forces. Composites that have been evaluated include single crystal YAG, optical ceramic YAG, sapphire, crystal YAG/ sapphire, ceramic YAG/sapphire, single crystal spinel, ceramic spinel, single crystal spinel/ single crystal YAG. These composites are of interest for high average power AFB^(R) slabs, waveguides and disks. Since AFB^(R) composites of laser media do not fail in tensile fracture at the interface but at random surface flaws; an interferometric technique has been developed to measure the E-modulus which in turn is a measure of the cohesive strength of the bulk material. The E-modulus of composites has been determined to be on average only about 5-10% lower than that of the bulk material, confirming the excellent bond strength of Van der Waals composites.

Remote monitoring of carbon dioxide (CO₂) is becoming increasingly important for homeland security needs as well as for studying the CO₂ distribution in the atmosphere as it pertains to global warming problems. So, efficient solid-state lasers emitting in the 1.55 - 1.65 μm spectral range, where CO₂ absorption lines are, (i), plentiful and, (ii), carry significant relevant information, are in great demand. Reported here is the first laser performance of resonantly pumped Er³⁺-doped scandia (Sc₂O₃) ceramic. The laser was operated in the cryogenically-cooled regime with the quantum defect (QD) of only 4.5%, which, along with superior thermal conductivity of scandia, offers significant eye-safe power scaling potential with nearly diffraction limited beam quality. Slope efficiency of 77% and Q-CW output power of 2.35 W were obtained at 1605.5 nm which has significant utility for counter-IED applications.

The laser performance of resonantly pumped Er:YAG as the gain medium for an eye-safe high-power laser was investigated theoretically using a new thermo-optical model. The presented model takes into account the full spatially resolved temperature dependence of the most important parameters in the gain medium. Among those are the thermo-mechanical parameters (e.g. heat conduction), spectroscopic and multiphonon-relaxation lifetimes of the first four manifolds and the full spectral information of emission and absorption (⁴I_15/2 ↔ ⁴I_13/2) as well as excited-state absorption and re-emission (⁴I_13/2 ↔ ⁴I_9/2). All spectral lines are modeled as temperature dependent by calculating their line positions and line widths assuming two-phonon Raman interactions with the host. From these spectra the temperature dependent upconversion loss parameters can also be derived. The gain medium - cavity interaction is modeled by the rate equations for the first four manifolds and spectrally resolved radiation transport for pump and laser fields. Simultaneous solving this together with the heat generation and heat transport in the gain medium gives a realistic view into the Er:YAG laser performance. It predicts high optical-to-optical efficiences of > 60% at output powers of multiple kW from a single gain medium. The model is compared with experimental data of diode and fiber laser pumped Er:YAG lasers with good agreement.

A comprehensive model for determining the phase matching conditions for biaxial nonlinear crystals of general orientations of given Sellmeir equations for OPO applications has been developed. The model calculates the phase matching angle for a given pump wavelength and ranges of generated wavelengths, and the walk-off angle and refractive indexes and the polarization states of the fast and slow rays. Walk-off is proposed to be compensated by an Adhesive-Free Bond (AFB^(R)) twist 180° twin pair configuration the length of which is a function of the crystal type, crystal orientation, pump wavelength, converted wavelength, tuning curve, and beam diameter. The walk-off corrected design overcomes an obstacle that has prevented using biaxial crystals oriented in other than principal planes in terms of greater d_eff values. Experimental results on biaxial and uniaxial nonlinear single crystals have validated the model, allowing efficient evaluation of new nonlinear crystals and optimization of existing ones.

In this paper we review the current status of and progress towards higher power and more wavelength diverse diode-pumped solid-state miniature lasers. Snake Creek Lasers now offers unprecedented continuous wave (CW) output power from 9.0 mm and 5.6 mm TO type packages, including the smallest green laser in the world, the MicroGreen^TM laser, and the highest density green laser in the world, the MiniGreen^TM laser. In addition we offer an infrared laser, the MiniIR^TM, operating at 1064 nm, and have just introduced a blue Mini laser operating at 473 nm in a 9.0 mm package. Recently we demonstrated over 1 W of output power at 1064 nm from a 12 mm TO type package, and green output power from 300-500 mW from the same 12 mm package. In addition, the company is developing a number of other innovative new miniature CW solid-state lasers operating at 750 nm, 820 nm, 458 nm, and an eye-safe Q-switched laser operating at 1550 nm. We also review recently demonstrated combining volume Bragg grating (VBG) technology has been combined with automatic power control (APC) to produce high power MiniGreen^TM lasers whose output is constant to ± 10 % over a wide temperature range, without the use of a thermoelectric cooler (TEC). This technology is expected to find widespread application in military and commercial applications where wide temperature operation is particularly important. It has immediate applications in laser pointers, illuminators, and laser flashlights, and displays.

Fiber lasers are advancing rapidly due to their ability to generate stable, efficient, and diffraction-limited beams with significant peak and average powers. This is of particular interest as fibers provide an ideal pump source for driving parametric processes. Most nonlinear optical crystals which provide phase-matching to the mid-IR at commercially available fiber pump wavelengths suffer from high absorption above 4μm, resulting in low conversion efficiencies in the 4-5μm spectral region. The nonlinear optical crystals which combine low absorption in this same spectral region with high nonlinear gain require pumping at longer wavelengths (typically >1.9μm). In this paper, we report a novel mid-IR OPO pumped by a pulsed thulium-doped fiber laser operating at 2-microns. The eyesafe thulium-fiber pump laser generates >3W of average power at >30kHz repetition rate with 15-30ns pulses in a near diffraction-limited beam. The ZnGeP₂ (ZGP) OPO produces tunable mid-IR output power in the 3.4-3.99μm (signal) and the 4.0-4.7μm (idler) spectral regions in both singly resonant (SRO) and doubly resonant (DRO) formats. The highest mid-IR output power achieved from this system was 800mW with 20% conversion efficiency at 40kHz. In a separate experiment, the 3W of 2-micron light was further amplified to the 20W level. This amplified output was also used to pump a ZGP OPO, resulting in 2W of output power in the mid-IR. To our knowledge, these are the first demonstrations of a fiber-pumped ZGP OPO.

An alternative to Quasi Phase Matching (QPM), called Semi Quasi Phase Matching (SQPM) is proposed, wherein the segments of crystals with inverted axis are substituted by segments of small polycrystal grains of the same material. These segments produce negligible nonlinear interaction, while still providing the π phase change necessary. A SQPM device of length L should produce the same nonlinear output as a QPM device of length L/2. Crystals with higher nonlinear coefficients and larger transparency ranges like III-V and II-VI compounds, not normally amenable to inversion of axes, can be used. This is especially important for doubling of CO₂ laser lines.

We report on a passively Q-switched end pumped Nd:YLF laser including a noncritically phase-matched KTP singly resonant intracavity optical parametric oscillator (IOPO-KTP). For the Q-switching operation we have used Cr:YAG saturable absorber. The optimized passively Q-switched Nd:YLF laser without IOPO generated linearly polarized pulses of 11.5 ns and 1.07 mJ at 1047 nm. The conversion efficiency of the optimized Q-switched pulse energy at 1047 nm to 1547 nm of signal approached about 47%. For optimizing both Nd:YLF laser and IOPO we have numerically solved theoretical model. We have achieved 1.6-ns duration pulses at 1547 nm with energy of 0.5 mJ and peak power above of 300 kW. The beam quality was excellent (M²≈1).

Er:YAG solid state lasers offer an "eye-safe" alternative to traditional Nd:YAG lasers for use in military and industrial applications such as range-finding, illumination, flash/scanning LADAR, and materials processing. These laser systems are largely based on diode pumped solid state lasers that are subsequently (and inefficiently) frequency-converted using optical parametric oscillators. Direct diode pumping of Er:YAG around 1.5 μm offers the potential for greatly increased system efficiency, reduced system complexity/cost, and further power scalability. Such applications have been driving the development of high-power diode lasers around these wavelengths. For end-pumped rod and fiber applications requiring high brightness, nLIGHT has developed a flexible package format, based on scalable arrays of single-emitter diode lasers and efficiently coupled into a 400 μm core fiber. In this format, a rated power of 25 W is reported for modules operating at 1.47 μm, with a peak electrical to optical conversion efficiency of 38%. In centimeter-bar on copper micro-channel cooler format, maximum continuous wave power in excess of 100 W at room temperature and conversion efficiency of 50% at 6C are reported. Copper heat sink conductively-cooled bars show a peak electrical-to-optical efficiency of 43% with 40 W of maximum continuous wave output power. Also reviewed are recent reliability results at 1907-nm.