This PDF file contains the front matter associated with SPIE Proceedings Volume 8733, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The latest developments in AlGaInN laser diode technology are reviewed. The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v. to the visible, i.e., 380-530nm, by tuning the indium content of the laser GaInN quantum well. Of specific interest for defence applications is blue-green laser diode technology for underwater telecommunications and sensing applications. Ridge waveguide laser diode structures are fabricated to achieve single mode operation with optical powers of <100mW in the 400-420nm wavelength range with high reliability. Low defectivity and highly uniform GaN-substrates allow arrays and bars of nitride lasers to be fabricated. In addition, high power operation of AlGaInN laser diodes is demonstrated with the operation of a single chip, ‘mini-array’ consisting of a 3 stripe common p-contact at powers up to 2.5W cw in the 408-412 nm wavelength range and a 16 stripe common p-contact laser array at powers over 4W cw.

We report on our research in power scaling VECSEL around 1 μm to exceed 100W per chip. Recently, we have utilized these optimized VECSEL chips to achieve a new record for a mode-locked VECSEL. The output power of the laser was 3.4W. This corresponds to a pulse energy of 7.5nJ and a pulse peak power of 13.3kW. Both are record values for a semiconductor laser in the femtosecond regime. These optimized structures have also been used to demonstrate high power operation with a highly coherent TEM₀₀ mode and to demonstrate a record single frequency output power of 15W.

At MBDA Germany a concept for a high-energy laser weapon system is investigated, which is based on existing industrial laser sources. Due to the enormous progress in the field of high-power fiber lasers, commercial industrial fiber lasers are now available delivering a nearly-diffraction limited beam quality with power levels of up to 10 kW. By using a geometric beam coupling scheme, a number of individual high-power fiber laser beams are combined together using one common beam director telescope. A total laser beam power of more than 100 kW can be achieved, which is sufficient for an operational laser weapon system. The individual beams from the different lasers are steered by servo-loops using fast tip-tilt mirrors. This principle enables the concentration of the total laser beam power at one common focal point on a distant target, also allowing fine tracking of target movements and first-order compensation of turbulence effects on laser beam propagation. The proposed beam combination concept was demonstrated by using different experimental set-ups. A number of experiments were performed successfully to investigate laser beam target interaction and target fine tracking, also at large distances and at moving targets. Content and results of these investigations are reported, which demonstrate the complete engagement sequence for a C-RAM scenario. This includes subsequent steps of target acquisition by radar and IR optics, followed by large angle coarse tracking, active fine tracking and destruction of the target by the laser system. This successful implementation of geometric beam combining is an important step for the realization of a laser weapon system in the near future.

The realization of a high-energy laser weapon system by coupling a large number of industrial high-power fiber lasers is investigated. To perform the combination of the individual beams of the different fiber lasers within the optical path of the laser weapon, a special optical set-up is used. Each optical component is realized either as reflective component oras refractive optics. Both possibilities were investigated by simulations and experiments. From the results, the general aspects for the layout of the beam-guidance optics for a high-power fiber laser system are derived.

We report on recent progress made in the development of highly compact, single mode, distributed feedback laser (DFB) seed laser modules with integrated drive electronics for lidar and spectroscopy applications from space based platforms. One of the intended application of this technology is in the NASA’s Active Sensing of CO₂ Emissions over Nights, Days, and Seasons (ASCENDS) mission. NASA Langley Research Center (LaRC) is working on a prototype laser based spectroscopy system for simultaneous measurement of CO₂ and O₂ for planned Active Sensing of CO₂ Emissions over Nights, Days, and Seasons (ASCENDS) mission application. For this purpose, 1571 nm spectral band for CO₂ sensing and 1262 nm spectral band for oxygen sensing have been selected. In this paper, we discuss recent progress made in the development of single mode, compact and stable, seed laser technologies for CO₂ and O₂ transmitters with focus on linewidth and noise measurements. The 1571 nm and 1262 nm DFB laser modules with integrated drive electronics have advanced current and temperature drivers built into them. A combination of temperature and current tuning allows coarse and fine adjustment of the diode wavelengths. The current tuning was demonstrated at a rate of ~0.7 pm/mV over a working range of ~1 V for a total of 0.7 nm. Also, temperature tuning at a rate of ~2 pm/mV over a working range of ~1 V for a total wavelength range of ~2 nm was demonstrated. The current tuning was performed at a rate of up to 200 kHz allowing rapid adjustment and dithering of the laser frequency. Furthermore, the best performance of laser linewidth observed was ~11 kHz with frequency stability <10 MHz over 1 hour period. The microcooler arrangement embedded inside these modules has provided significant reduction in power consumption. The electronics has been designed, prototyped and tested using space-qualified components within a hermetically sealed package of volume less than 2" x 2" x 0.5".

We report on a high-power diode laser pump source for diode-pumped alkali lasers (DPAL), specifically rubidium alkali vapor lasers at 780nm, delivering up to 100W/bar with FWHM spectral line width of 0.06nm (~30GHz). This pump is based on a micro-channel water-cooled stack with collimation in both-axes. Wavelength-locking of the output spectrum allows absorption in one of the very narrow resonance lines of the atomic rubidium alkali vapor. To achieve these results, research was conducted to deliver the highest performance on all key components of the product from the diode laser bar which produces the optical power at 780nm to the external Bragg gratings which narrow the spectrum line width. We highlight the advancements in the epitaxy, device design, beam collimation, grating selection, alignment, tunability and thermal control that enable realization of this novel pump-source for DPALs. Design trade-offs will be presented.

Contemporary operating environment requires a wide range of tools to respond to a myriad of regular and irregular threats. Accordingly, conventional weapons do not suffice in some cases. As technology improves exponentially, the dominance of conventional weapons is slowly fading away by the advances in laser technology. This study first outlines the characteristics of laser weapons, then provides the military applications of them in land, maritime, air and space domains and finally exhibits implications for battlefield functions. This study concludes that any country that is seeking primacy in military terms must allocate extra time and resources to obtain this emerging technology. Since it seems that there are not adequate studies about the military applications and operational concepts of the laser weapons, this study tries to increase awareness about their potential advantages.

A compact, robust and efficient nanosecond pulsed optical parametric oscillator (OPO) generating radiation in the mid- IR spectral range is reported. The OPO is based on periodically poled material for the efficient non-linear processes of up-converting 1064 nm radiation to 1538 and 3450 nm respectively. Pulsed emission exceeding 130 mW average power at the idler (3450 nm) with a total conversion efficiency of 30%, including both signal and idler, has been reached. The maximum pulse energy of the idler is 11 μJ, pulse duration around 4 ns and peak power close to 3 kW. The results are achieved for an optical pump power of 1.4 W at the entrance of the OPO and an electrical pump power of 14 W. The total size of the OPO device is only 125x70x45 mm³ (LxWxH) including the pump laser at 1064 nm. The idler output radiation is narrowed by spectral filtering to < 1.5nm and temperature tuneable over > 50 nm. The OPO has a robust design and withstands shocks up to 60g at 8 ms and the storage temperature is -20 °C to + 60 °C. The compact size and low power consumption make this OPO device suitable for many kinds of molecular spectroscopy applications in the areas of environmental monitoring and pollution control as well as in combustion physics and process control. Integration of the OPO source into compact equipment for Photo Acoustic Spectroscopy (PAS) allowing fast and highly sensitive detection of methane and ethanol at ppb-levels is also described.

We discuss approaches to increasing the cw output power of the interband cascade lasers (ICLs) for the midwave infrared spectral region. While most of the attention to date has been focused on reducing the operating power of the ICL, the optimization for maximum output power proceeds in a different direction. We find that increasing the number of stage is beneficial, in that it boosts the slope efficiency with only a modest penalty due to higher threshold power density and extra heating. The critical figure of merit for realizing high-power ICLs is the internal loss, which can be estimated from the external differential quantum efficiency (EDQE) per stage. The internal loss can be controlled by varying the thickness of the low-doped GaSb separate-confinement layers (SCLs). We demonstrate room-temperature EDQEs approaching 45% for broad-area 7-stage ICLs with 800-nm-thick SCLs.

We have studied the broadband mid-IR supercontinuum generation in a lead-silicate tapered microstructured fiber for femto-second input pulses at 1550 nm. The supercontinuum generated extends from ~1000 to ~ 5000 nm for a suitably designed tapered fiber. The coherence properties of the supercontinuum depends on the input pulse parameters. It is possible to generate perfectly coherent supercontinuum with a flat broadened spectrum extending to ~5000 nm in this fiber taper.

We present the burn-in behavior and power stability of multiple quantum cascade lasers (QCLs) that were measured to investigate their long-term performance. For these experiments, the current to the QCL was cycled every ten minutes, and the output power was monitored over time for durations as long as two months. A small increase in power for a given injection current is observed for almost all of the QCLs tested during the burn-in period. Although the amount and duration of the burn-in varied among the devices tested, we observed that QCLs that operated with a lower threshold current exhibited a smaller burn-in change from initial conditions for the first ten hours of operation. This correlation, however, disappeared at longer operation times. The effect of packaging the QCLs is also investigated to determine its impact on performance and reliability. The power stability is measured for the packaged QCLs along with changes in the operational conditions. Although the temperature of the QCL is kept constant with a thermistor and thermoelectric cooler (TEC) inside the package, the case temperature is varied to monitor its correlation with any changes in power or frequency. Power changes and small frequency shifts are observed under these conditions. One possible explanation for these changes is the influence of optical feedback from the anti-reflection (AR) coated window in the package. The data from all of these experiments is presented.

The spectroscopic properties of Ho³⁺-doped YVO4 were studied at cryogenic and room temperatures in the 2 μm spectral region to clarify recent observations of efficient dual-wavelength laser operation in this material. Polarized absorption cross sections were measured, and stimulated emission cross sections were determined using the reciprocity method coupled with Füchtbauer-Ladenburg calculations. The observed laser emission wavelengths were at 2041.7 nm, 2054.2 nm, and 2068.5 nm; the first two corresponding to pi transitions and the third to a sigma transition. Gain cross section calculations were used to predict which of the three wavelengths would lase for a given output coupler reflectivity. In depth analysis of the gain cross section in the region between 80 K and 100 K showed that the laser output wavelength is very susceptible to minor changes in temperature.

Due to the favorable thermal properties of sesquioxides as hosts for rare earth laser ions, we have recently studied the spectroscopy of Er:Lu₂O₃ in the 1400-1700 nm wavelength range, and here report its comparison with our earlier results on Er:Y₂O₃ and Er:Sc₂O₃. These studies include absorption and fluorescence spectra, fluorescence lifetimes, and inference of absorption and stimulated emission cross sections, all as a function of temperature. At room temperature, optical absorption limits practical laser operation to wavelengths longer than about 1620 nm. In that spectral range, the strongest stimulated emission peak is that at 1665 nm in Er:Sc₂O₃, with an effective cross section considerably larger than those of Er:Y₂O₃ and Er:Lu₂O₃. At 77K, the absorption is weak enough for efficient laser operation at considerably shorter wavelengths, where there are peaks with much larger stimulated emission cross sections. The three hosts all have peaks near 1575-1580 nm with comparably strong cross sections. As we have reported earlier, it is possible to lase even shorter wavelengths efficiently at this temperature, in particular the line at 1558 nm in Er:Sc₂O₃. Our new spectroscopic studies of Er:Lu₂O₃ indicate that its corresponding peak, like that of Er:Sc₂O₃, has a less favorable ratio of stimulated emission to absorption cross sections. Reasons for the differences will be discussed. We conclude that for most operating scenarios, Er:Sc₂O₃ is the most promising of the Er-doped sesquioxides studied for laser operation around 1.5-1.6 microns.

The transient theory of stimulated Brillouin Scattering (SBS) in optical fibers is used to investigate the effects of weak feedback of an incident broadband laser field. When the linewidth of the laser approaches the Brillouin frequency shift, a weak reflection of the laser field overlaps with the SBS gain and can be amplified in the fiber resulting in an apparent reduction in SBS threshold. We show that a reflection of 0.01% (-40 dB) of a laser field with a linewidth equal to the Brillouin shift can reduce the SBS threshold by a factor of 2. These results have implications on the design of high power fiber laser systems that utilize spectral broadening to suppress SBS.

High power (<0.5kW) experiments using low NA (~0.07), very large mode area (VLMA) step index fibers (SI) (with core/clad diameters: 45/375, 60/500um) and gain tailored step index (GT-SI) fibers (with doped-core/core/clad diameters: 38/60/400, 50/80/533um) are presented. In fiber amplifier experiments with multi-moded beam (M² 1.5- 3) outputs, Stimulated Thermal Rayleigh scattering (STRS) threshold is determined by comparing gain dependence of output mode quality between high power (<200W) and low power (<100W) experiments for a given fiber layout. Beam quality degradation with signal power is characterized well above the instability threshold where a saturation of the phenomena is observed. For SI fibers degree of beam quality degradation is found to be significantly worse for tighter fiber coil diameters. GT-SI fibers exhibit significantly less modal degradation compared to SI fibers. STRS instability threshold is further verified with signal power dependent multi-path interference spectrum (MPI) measurements which exhibited exponential broadening above the threshold. Strength of STRS nonlinear coupling coefficients are estimated from experimental data using a comprehensive 3-dimensional transverse spatial hole burning (TSHB) fiber MOPA numerical model, phenomenologicaly extended to include STRS.

We demonstrated nearly a quantum defect limited CW operation of a 41-mm-long Er:YAG large area crystalline waveguide laser with a diffraction limited output, which was resonantly pumped by a fiber laser at 1532 nm. Using a Er(0.25%):YAG, 62 μm x 62 μm waveguide, surrounded by a 3 x 5 mm rectangular cladding of undoped YAG, an output power of 9.1 W with slope efficiency of 92.8% has been achieved. The output laser beam had a Gaussian profile with a ~ 2.8 x 10^-2 rad divergence, which is in good agreement with the divergence expected from a waveguide with a low NA value. The waveguide laser operated simultaneously at two wavelengths, 1617 nm and 1645 nm, when the transmission of the laser cavity output coupler was less than 20-25%, and operated only at 1617 nm when the laser output mirror had a higher transmission.

Numerous methods to increase the stimulated Brillouin scattering (SBS) threshold have been previously implemented. Some are passive, based on acousto-optic fiber designs that incorporate longitudinally- or radially-tailored optical and/or acoustic index profiles, leading to broadened Brillouin gain spectra (BGS) with reduced peak gain. Some are active, relying on an applied temperature or strain distribution, also resulting in broadened BGS. Broadening the laser spectrum still represents the most effective method to-date to obtain large-scale (> 20 dB) decreases in the gain, but the suitability of this method depends largely on the application and system requirements on the laser spectrum. Despite these technologies, some introduced only in the last decade, the vast majority of high-energy, narrow-linewidth fiber laser systems are still limited by SBS rather than the availability of pump power. We present an alternative approach; rather than focusing on ‘suppressing’ SBS in waveguide or other designs, we propose implementing materials with intrinsically low Brillouin gain. We focus on high-density, high-soundvelocity, large acoustic-damping-coefficient, and low-photoelastic-constant materials wherein the correct balancing of physical characteristics gives rise to extremely low Brillouin gain. In general, the approach requires the use of compositions that would be considered to be highly unconventional and unachievable utilizing standard fiber fabrication methods. For example, we describe recent results on sapphire-derived fibers (among other compositions) wherein a Brillouin gain nearly 20 dB lower than those of more conventional fibers has been realized. Other compositions will also be presented, including new results on a novel baria doped fiber, including others predicted to have zero-valued photoelastic constants, and therefore zero Brillouin gain.

The advent of high energy, high power fiber lasers hinges on the availability of new fibers and suitable cladding materials. New fiber materials with the needed thermal and optical properties are urgently needed for high powered fiber lasers; a viable cladding process using suitable cladding materials is equally critical. These two fundamental technologies are being developed. Most recently, the technical feasibility of applying optically transparent undoped YAG as the fiber cladding was explored via a single source electron beam deposition process. The technical feasibility of depositing such fiber claddings was successfully demonstrated on various model fibers. Amorphous Y_xAl_1-xO₃ coatings with various chemistries and high optical transmittances were deposited and shown to be stable even after high temperature annealing, while retaining the optical quality.

Single crystal fibers are an intermediate between laser crystals and doped glass fibers. They can combine the advantages of both by guiding laser light and matching the efficiencies found in bulk crystals, making them ideal candidates for high-power laser and fiber laser applications. In particular, a very interesting feature of single crystal fiber is that they can generate high power in the eye-safe range (Er:YAG) with a high efficiency, opening new possibilities for portable directed energy weapons. This work focuses on the growth of a flexible fiber with a core of dopant (Er, Nd, Yb, etc…) that will exhibit good waveguiding properties. Direct growth or a combination of growth and cladding experiments are described. We have, to date, demonstrated the growth of a flexible foot long 45 microns doped YAG fiber. Scattering loss measurements at visible wavelengths along with dopant profile characterization are also presented. Laser characterization for these fibers is in progress.

In this paper, we present our recent progress in the development of rare-earth (Yb³⁺ or Ho³⁺) doped Lu₂O₃ and Y₂O₃ sesquioxides for high power solid state lasers. We have fabricated high quality transparent ceramics using nano-powders synthesized by a co-precipitation method. This was accomplished by developments in high purity powder synthesis and low temperature scalable sintering technology developed at NRL. The optical, spectral and morphological properties as well as the lasing performance from our highly transparent ceramics are presented. In the second part of the paper, we discuss our recent research effort in developing cladded-single crystal fibers for high power single frequency fiber lasers has the potential to significantly exceed the capabilities of existing silica fiber based lasers. Single crystal fiber cores with diameters as small as 35μm have been drawn using high purity rare earth doped ceramic or single crystal feed rods by the Laser Heated Pedestal Growth (LHPG) process. Our recent results on the development of suitable claddings on the crystal fiber core are discussed.

We present progress on 3 laser systems operating near 3 μm: a continuous-wave (cw) widely tunable narrow linewidth system, a high peak power Q-switched system, and a passively mode-locked system. The cw system emitted average powers of < 1 W over a wavelength range of 130 nm, with a spectral width of < 1 nm. The Q-switched system produced pulses with 33 ns and peak power of 576 W. The mode-locked system produced ~20 ps pulses at a repetition rate of 27 MHz.

Application of the Transverse Chirped Bragg Grating (TCBG - a reflecting volume Bragg grating with continuously variable resonant wavelength across the aperture) for the narrowband tunable ring-cavity fiber laser is presented. The main advantage of the use of TCBG is that its linear translation allows continuous tuning of emission wavelength within 5-10 nm band. Yb doped fiber laser operating in the wavelength range of 1050-1055 nm of narrowband emission up to 2.3 W is demonstrated.

We report a master-oscillator/power-amplifier laser system featuring a polarizing and coilable 40-micron-core Yb-doped photonic crystal fiber as the final-stage amplifier. The laser source generates 3.4 ns pulses at a repetition rate 19 kHz, with maximum pulse energy 1.2 mJ, maximum average power 22.8 W, near diffraction-limited (M² < 1.1) beam quality, and 20% electrical to optical efficiency in a compact package. This pulsed-fiber laser flight system provides high pulse energy, average power, peak power, diffraction limited beam quality, and high efficiency all in a thermally and mechanically stable compact package.

Non-planar ring resonators are widely used for high precision ring laser gyroscopes including Zero-Lock Laser Gyroscopes. The analysis of optical-axis perturbation in nonplanar ring resonators is important for resonator design. Ray matrix approach based on appropriate coordinate systems has been employed to analyze the optical-axis perturbation in nonplanar ring resonators. The sensitivities of optical-axis decentration (SD) and optical-axis tilt (ST) in nonplanar resonators with 90° and 270° image rotation are discussed in detail in the region of 0< K <8, where K is the ratio of the total cavity length to the radius of the curvature mirrors. There are both four singular points in the whole region of 0<K<8. On the left of the first singularity, it is found that the longer the mirror radius, the less the optical-axis decentration sensitivity. This is opposite the behavior of planar ring resonators, but the behaviors of optical-axis tilt sensitivity in planar and nonplanar ring resonators are similar. In planar resonators, it also demonstrates that in the region of 0<K<2, larger the mirror radius is, higher sensitivity of optical-axis decentration will be, but lower sensitivity of optical-axis tilt will be. These results are confirmed by related experiments. It is worth to note that SD and ST in the nonplanar resonator with certain parameters have the similar singularities. The analysis in this paper is important for the resonator design, improvement and beam position control nonplanar ring resonators.

Beam propagation from a laser phased array system through the turbulent atmosphere is simulated and the ability of such a system to compensate for the atmosphere via piston-only phase control of the sub-apertures is evaluated. Directed energy (DE) applications demand more power than most lasers can produce, consequently many schemes for high power involve combining the beams from many smaller lasers into one. When many smaller lasers are combined into a phased array, phase control of the individual sub-apertures will be necessary to create a high-quality beam. Phase control of these sub-apertures could then be used to do more, such as focus, steer, and compensate for atmospheric turbulence. Atmospheric turbulence is well known to degrade the performance of both imaging systems and laser systems. Adaptive optics can be used to mitigate this degradation. Adaptive optics ordinarily involves a deformable mirror, but with phase control on each sub-aperture the need for a deformable mirror is eliminated. The simulation conducted here evaluates performance gain for a 127 element phased array in a hexagonal pattern with piston-only phase control on each element over an uncompensated array for varying levels of atmospheric turbulence. While most simulations were carried out against a 10 km tactical scenario, the turbulence profile was adjusted so performance could be evaluated as a function of the Fried Parameter (r0) and the log-amplitude variance somewhat independently. This approach is demonstrated to be generally effective with the largest percentage improvement occurring when r0 is close to the sub-aperture diameter.

Experimental evaluation of a flash lamp pumped Er: glass laser over temperature extremes is presented. In this study, first behavior of flash lamp pumped Er: glass laser system has been experimentally investigated over temperature extremes. Next, the generated data during experimentation has been analyzed and system parameters are optimized such that fairly constant laser output is maintained over temperature extremes. This work also enables the development of a compact, simple, low cost, light weight, low repetition rate and high peak power laser source working in eye safe region for defense application.

Mode-locked fiber lasers with low timing jitter and high repetition rate is desired for various applications including photonics analog-to-digital conversion, microwave synthesis and high-precision clock distribution. We demonstrate a compact all fiber laser mode locked by semiconductor saturable absorber mirror with linear cavity. The laser is operating near 1.55 μm. It has a repetition rate of 250 MHz and the root-mean-square timing jitter is 8 fs integrated from 100 Hz to 1 MHz measured at the third harmonic frequency of 750 MHz.