This PDF file contains the front matter associated with SPIE Proceedings Volume 9081, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Polycrystalline YAG fiber has recently attracted considerable attention for the role it could play as a fiber-laser gain media. This primarily due to its large surface-to-volume ratio, high stimulated Brillouin scattering threshold, and its high thermal conductivity; all of which are superior to that of silica-glass fibers. As a consequence, techniques which enable the fabrication of poly- and single-crystalline YAG fibers have recently been the focus of a number of efforts. In this work we have endeavored to reduce the scattering loss of polycrystalline-YAG-core fibers while simultaneously demonstrating optical gain by enhancing our processing techniques using feedback from mechanical testing and through the development of a technique to encase doped YAG-core fibers with un-doped YAG claddings. To this end we have recently fabricated fibers with both core and claddings made up of polycrystalline YAG and subsequently confirmed that they indeed guide light. In this paper, the processes leading to the fabrication of these fibers will be discussed along with their characterization.

Trivalent holmium has 14 laser channels from 0.55 to 3.9 μm. The laser emission of most interest is the transition ⁵I₇→⁵I₈ near 2 μm because of its potential for use in eye-safe systems and medical applications. In this paper, we present our recent results in the development of Ho³⁺ doped laser materials for eye-safe solid state lasers. We report a calorimetric study of non-radiative losses in two micron pumped holmium doped laser host materials such as silica glass, yttrium aluminum garnet (YAG) crystal and Lu₂O₃ ceramics. Optical, spectral and morphological properties as well as the lasing performance from highly transparent ceramics are presented.

A new series Er:Lu_xGd_1-xVO₄ (x=0.1，0.24，0.48，0.57，0.79 and 0.9)mixed laser crystals have been successfully grown by the Czochralski method with 1% Er³⁺ concentration.The thermal properties of Er:Lu_xGd_1-xVO₄crystals series crystals were investigated systematically, including the thermal expansion, specific heat, thermal diffusion coefficients, and thermal conductivities. The anisotropy and variation of the thermal properties with the component x were also achieved and discussed based on their structure. All the results showed that this mixed crystals should have promising applications in the moderate-power lasers.

Fiber lasers are susceptible to a number of nonlinear and parasitic effects that can significantly inhibit performance. From a system perspective, requirements of a fiber laser may cover average or peak power, amplitude stability, and spectral content of the source. Nonlinear and parasitic phenomena, including Brillouin scattering, Raman scattering, mode stability, self-phase modulation, four-wave mixing in multi-wavelength lasers, etc. can each influence one or all of these performance characteristics. Conventionally, these system-limiting undesirables have been addressed with complex waveguide designs. However, each of these light-medium interactions can be described by material coefficients that lead to estimates of ‘threshold’ values where these phenomena become significant. For example, this may be the Brillouin gain coefficient for Brillouin scattering, the nonlinear refractive index, n₂, for self-phase modulation, or the thermo-optic coefficient, dn/dT, for mode stability. Furthermore, these coefficients tend to be strong functions of the material, and surprisingly wide ranges of values exist between known material systems. For example, both the Pockels’ photoelastic constant, p₁₂, and thermo-optic coefficient may be either positive or negative for a material. It follows logically that mixtures of materials with coefficients of opposing signs would then give rise to compositions where these coefficients may be zero. Example of such effect negation may include the barium aluminosilicate system for p₁₂ and the phosphosilicate or titanosilicate system for dn/dT. Compositional tailoring of the optical fiber is therefore suggested as an alternative means to suppressing these parasitics, and methods to do so will be discussed at the conference.

Optical systems as well as the human eye are susceptible to saturation or damage caused by high power lasers. We present a non-linear, solid-state passive wideband optical protection filter. As opposed to fixed spectral filters, which permanently block only specific wavelengths, the wideband filter is clear at all wavelengths until it is hit by damaging light. At input powers below threshold, the filter has high transmission over the whole spectral band. However, when the input power exceeds the threshold power, transmission is decreased dramatically. We propose a novel technology for protection of any imaging system, sensors and the human eye against laser threats from the visible and up to the IR.

Experimental measurements were performed to completely characterize the linear and nonlinear optical properties in butyl salicylate solution of a novel bipyridyl platinum(II) complex bearing benzothiazolylethynylfluorene ligands. This paper describes the analysis of the resulting experimental data and reports the values of the ground- and excited-state absorption cross sections and of the other photophysical parameters characterizing the chromophore/solvent system.

There is a variety of national and international standards for laser safety analysis requesting for risk reduction of hazards due to laser radiation. A lot of risks are already addressed by the standards. Thus appropriate evaluation methods for calculating dangerous radiation levels and measures for risk reduction are available for certain types of risks. However, when high energy lasers are applied in outdoor environments a variety of additional risks may arise. Due to high laser energy and varying outdoor environmental conditions, appearance and options of risks increase. Assuming that radiation can be held below certain Maximum Permissible Exposure (MPE) levels anywhere in the unmonitored surrounding is impracticable when high energy lasers (HEL) are applied in outdoor environments. Therefore, an adjusted approach of methodology and appropriate materials, i.e. data and algorithms for risk assessment, is necessary. In the study reported here, we conduct a probabilistic risk assessment (PRA) focused on an application of HEL radiation in outdoor field tests. Thus we identify hazards within a particular field test environment and estimate the risks associated to these hazards using a probabilistic approach. The risks are assessed for being acceptable by probability of occurrence and severity. General risk management processes then demand that risks that exceed an acceptable level need to be reduced through appropriate risk reduction measure that either reduce the probability of occurrence or the consequences of an accident associated to that hazard. After having verified that all our recommended measures are implemented to a sufficient extend, field tests may be conducted.

A non-contact method for analyzing of explosives traces from surfaces was developed. The method is based on the laser desorption of analyzed molecules from the surveyed surfaces followed by the laser ionization of air sample combined with the field asymmetric ion mobility spectrometry (FAIMS). The pulsed radiation of the fourth harmonic of a portable GSGG: Cr3+ :Nd3+ laser (λ = 266 nm) is used. The laser desorption FAIMS analyzer have been developed. The detection limit of the analyzer equals 40 pg for TNT. The results of detection of trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX) are presented. It is shown that laser desorption of nitro-compounds from metals is accompanied by their surface decomposition. A method for detecting and analyzing of small concentrations of explosives in air based on the laser ionization and the FAIMS was developed. The method includes a highly efficient multipass optical scheme of the intracavity fourthharmonic generation of pulsed laser radiation (λ = 266 nm) and the field asymmetric ion mobility (FAIM) spectrometer disposed within a resonator. The ions formation and detection proceed inside a resonant cavity. The laser ion source based on the multi-passage of radiation at λ = 266 nm through the ionization region was elaborated. On the basis of the method the laser FAIMS analyzer has been created. The analyzer provides efficient detection of low concentrations of nitro-compounds in air and shows a detection limit of 10-14 - 10–15 g/cm3 both for RDX and TNT.

Power scaling of fiber lasers is highly desirable in many applications but is mainly limited by nonlinear effects. Large-mode-area fibers have been used to mitigate this limit, such as the leakage channel fiber (LCF). The mode intensity profile in these fibers typically exhibits Gaussian-like structure with much reduced effective mode-area compared to the physical fiber core area. Thus, a flat-top mode with a uniform intensity distribution is more suitable for larger effective mode-area without having to increase core size. In this work, we demonstrate the first flat-top mode generated in a 50 μm-core Yb-doped LCF fiber. The mode flattening from Gaussian beam to a flat-top one is achieved by using a 30 μm uniform Yb-doped area in the core center with a refractive index very slightly below that of the background silica glass by 2×10^-4. The resulting flat-top mode has a significantly increased effective mode area of ~1880 um², which is ~50% larger than that of a conventional uniform core and ~6 times the effective mode area of the flat-top mode record demonstrated previously. A 6m-long fiber is also tested in a laser configuration with a slope efficiency of ~84% at 1026 nm with respect to the absorbed pump power at 976 nm.

Fibertek has developed a space qualifiable, highly efficient, high power (<5W), fiber based 1.5um laser optical module (LOM). The transmitter achieves 6W average and <1kW peak power out of a 2m long single mode delivery fiber with 8nsec pulses and <6Ghz linewidth. Stimulated Brillouin Scattering (SBS) is managed by precise linewidth control and by use of LMA gain fiber in the power stage while maintaining the required diffraction limited, and highly polarized (PER<20dB) output. Size and weight of the built LOM are 8”x10”x2.375” and 3 kg, respectively. With improvements in the modulation scheme and component specification, achieved LOM electrical to optical efficiency is over 17.0%. Highly efficient operation is sustained for a wide range of pulse-position modulation (16 to 128-ary PPM) formats with pulse widths varying from 8nsec to 0.5nsec and operation temperature 10-50C. Pressure stress analysis, random vibration analysis and thermal analysis of the designed LOM predicts compliance with NASA GEVS levels for vibration and thermal cycling in a vacuum environment. System will undergo both thermal vacuum and vibration testing to validate the design.

Within the past 10 years, thulium (Tm)-doped fiber lasers have emerged as a flexible platform offering high average power as well as high peak power. Many of the benefits and limitations of Tm:fiber lasers are similar to those for ytterbium (Yb)-doped fiber lasers, however the ~2 µm emission wavelength posses unique challenges in terms of laser development as well as several benefits for applications. In this presentation, we will review the progress of laser development in CW, nanosecond, picosecond, and femtosecond regimes. As a review of our efforts in the development of power amplifiers, we will compare large mode area (LMA) stepindex and photonic crystal fiber (PCF) architectures. In our research, we have found Tm-doped step index LMA fibers to offer relatively high efficiency and average powers at the expense of fundamental mode quality. By comparison, Tm-doped PCFs provide the largest mode area and quasi diffraction-limited beam quality however they are approximately half as efficient as step-index fibers. In terms of defense related applications, the most prominent use of Tm:fiber lasers is to pump nonlinear conversion to the mid-IR such as supercontinuum generation and optical parametric oscillators/amplifiers (OPO/A). We have recently demonstrated Tm:fiber pumped OPOs which generate ~28 kW peak power in the mid-IR. In addition, we will show that Tm:fiber lasers also offer interesting capabilities in the processing of semiconductors.

We have demonstrated efficient lasing of a Tm-doped fiber when pumped with another Tm-doped fiber. In these experiments, we use a 1908 nm Tm-doped fiber laser as a pump source for another Tm-doped fiber laser, operating at a slightly longer wavelength (~2000 nm). Pumping in the 1900 nm region allows for very high optical efficiencies, low heat generation, and significant power scaling potential due to the use of fiber laser pumping. The trade-off is that the ground-state pump absorption at 1908 nm is ~37 times lower than at 795nm. However, the absorption cross-section is still sufficiently high enough to achieve effective pump absorption without exceedingly long fiber lengths. This may also be advantageous for distributing the thermal load in higher power applications.

Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here for the first time realization of BH-QCLs with a single step regrowth of highly resistive (<1x10⁸ ohm•cm) semiinsulating InP:Fe in less than 45 minutes in a flexible hydride vapour phase epitaxy process for burying ridges etched down to 10-15 μm deep both with and without mask overhang. The fabricated BH-QCLs emitting at ~4.7 μm and ~5.5 μm were characterized. 2 mm long 5.5 μm lasers with ridge width 17-22 μm, regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for CW operation. 5 mm long 4.7 μm BH-QCLs of ridge widths varying from 6-14 μm regrown without mask overhang, besides being spatially monomode, TM₀₀, exhibited WPE of ~8-9% with an output power of 1.5 – 2.5 W at room temperature and under CW operation. Thus, we demonstrate a simple, flexible, quick, stable and single-step regrowth process with extremely good planarization for realizing buried QCLs leading to monomode, high power and high WPE.

Mid-infrared spectral region (2-4 μm) is gaining significant attention recently due to the presence of numerous enabling applications in the field of gas sensing, medical, environmental and defense applications. Major requirement for these applications is the availability of laser sources in the atmospheric transmission window free of water vapor absorption, such as the 2-2.3 μm spectral window. Type-I GaSb-based laser diodes are ideal candidates for these applications being compact, electrically pumped, power efficient and able to operate at room temperature in continuous-wave. Moreover, due to the nature of type-I transition these devices have characteristic low operation voltage, typically below 1 V, resulting in low power consumption, and high-temperature of operation. In this work, we present recent progress of 2.1 μm wavelength single-spatial mode GaSb type-I laser diode development at Brolis Semiconductors. Experimental device structures were grown by solid-source multi-wafer MBE, consisting of an active region with 2 compressively strained (~1.3 %) GaInAsSb quantum wells. Epi-wafers were processed into a ridge-waveguide devices and mounted on Cu or CuW heatsink. Presented devices feature state-of-the art performance in CW mode with < 1.2 W and 30 % WPE for single emitter device as well as 9 W and 28 % WPE for a laser diode bar.

We present a new method to lock an unlimited number of lasers using a simple, inexpensive hologram and photodiodes. Multiple fiber laser inputs are sent through a fiber shifters and then onto two holographic optical elements that split each beam in two and mix them in pairs. By taking the ratio of intensities measured from photodiodes, an error signal is generated which can be used to control the phase shifters to ensure continuous phase locking of pairs of lasers. We have constructed an autonomous system that locks 3 lasers and occupies a footprint no larger than a laptop. Locking is robust and can be configured to work with lasers of any power and wavelength. Our current system operates at bandwidths of up to 10kHz but has the potential of 100MHz or faster using a two-stage, woofer-tweeter approach.

For high-power diode lasers, asymmetric reflectivities of facets are employed in order to improve slope efficiency. In recent years, the cavity lengths of these laser diodes have been increased to better distribute heat in order to improve output power. However, these two methods result in an inhomogeneous longitudinal profile of photon density, which leads to a non-uniform gain profile and is typically referred to as longitudinal spatial hole burning (LSHB). LSHB is believed to one of the limiting factors in scaling the output power of diode lasers. In this work, the LSHB effect is confirmed experimentally. The longitudinal photon density distribution, carrier density distribution, and gain distribution were calculated using a finite difference method to solve the spatially-varying rate equations in an 808 nm high-power diode laser. The experimental work was carried out by direct observation of spontaneous emission from a window patterned into the top contact of a 1.5 mm cavity length 808 nm diode laser. Because the spontaneous emission rate is proportional to the square of carrier density, the carrier density distribution could be measured for the device. The non-uniformity observed in this device agrees with the calculated carrier density profile, strongly supporting the existence of the LSHB effect in the device.

Optical transient responses to a variety of pump-current pulses, modified in length and amplitude, are experimentally investigated under room temperature conditions for a Fabry-Perot semiconductor bulk laser with an 80 nm thick active layer (AL) comprised in a strongly asymmetric broadened waveguide structure. At best, gain-switched trailing oscillations free single optical pulses with ~2.48 nJ pulse energy, E_opt, and ~130 ps full width at half maximum (FWHM), equivalent to ~13 W peak power were achieved. The corresponding pump-current pulse was of ~6.9 A amplitude and ~1.26 ns FWHM generated with a compact driver circuit of ~1 cm² in size utilizing a commercial silicon avalanche transistor as an electrical switch. We found that the laser pulse temporal position correlates with a critical charge, Q^crit, arising from the current pulse, which scales with the laser diode oxide stripe width (90 μm) and cavity length (1.5 mm).

Power scaling of high power laser resonators is limited due to several nonlinear effects. Scaling to larger mode areas can offset these effects at the cost of decreased beam quality, limiting the brightness that can be achieved from the multi-mode system. In order to improve the brightness from such multi-mode systems, we present a method of transverse mode selection utilizing volume Bragg gratings (VBGs) as an angular filter, allowing for high beam quality from large mode area laser resonators. An overview of transverse mode selection using VBGs is given, with theoretical models showing the effect of the angular selectivity of transmitting VBGs on the resonator modes. Applications of this ideology to the design of laser resonators, with cavity designs and experimental results presented for three types of multimode solid state lasers: a Nd:YVO4 laser with 1 cm cavity length and 0.8 mm diameter beam with an M² of 1.1, a multimode diode with diffraction limited far field divergence in the slow axis, and a ribbon fiber laser with 13 cores showing M² improved from 11.3 to 1.5.

We have demonstrated a new form of Nd:YVO₄ amplifier operating at 1064 nm based on a 800 µm thick Nd:YVO₄ gain layer bonded to a 4H-SiC prism. The amplifier was tested in the ‘master oscillator - power amplifier’ (MOPA) configuration, where both the seed source and the single pass amplifier were operated in a quasi-continuous wave (Q-CW) regime: pulse duration 500 µs, pulse repetition frequency (PRF) - 100 Hz. The Nd:YVO₄gain element was pumped by a 808 nm laser diode bar stack to amplify seed inputs in the power range of 1 to 55 W with a gains of 4 to 2.6, respectively, with 25% optical-to-optical extraction efficiency. The temperature distribution of the gain medium was measured under operational conditions using thermography.

A 1um fiber laser outputting high energy (<1mJ) pulse-bursts with high peak powers (<15kW) and narrow linewidth (<300MHz) is an attractive pump source for tunable periodically poled crystal (PPx) based OPA’s which are used in gas sensing, imaging and communication applications. Here a turn-key 1064nm PM Yb-doped fiber amplifier capable of generating high pulse burst energies with transform limited linewidth is presented. The ~20W average power capable laser is optimized for high energy (0.5-2mJ) and high peak power (<10kW) operation at low duty cycles (<0.1%). The laser is capable of operating at <10x the saturation energy level of the final stage gain fiber and achieves a high level of pulse-to-pulse peak power uniformity within pulse-burst. Stimulated Brillion Scattering (SBS) limited micro pulse energy up to 40uJ is achieved and SBS dependence on micro pulse width and separation are characterized. High wall plug efficiency (<20%) for the FPGA controlled system is maintained by temporal and spectral ASE suppression and by spreading the necessary pulse pre-shaping losses (~12dB) to three different amplitude modulation points in the amplifier chain.