In recent years fiber pulsed fiber lasers have began to challenge diode pumped solid state lasers in performance. In particular double-clad fiber lasers and amplifiers with mJ energies and near diffraction limited beam quality are gaining respect for applications such as materials processing, laser radar and remote sensing. Frequency conversion of single-polarization fiber lasers further increases the application space to such as underwater communications, underwater imaging, semiconductor processing and gas sensing. Yb fiber lasers have to date produced several mJ pulse energy and several MW peak power but, largely due to materials issues Er based fiber laser systems underperforms in comparison. Relevant technologies are reviewed.

We report on the recent progress in the design and development of completely monolithic linearly-polarized pulsed fiber amplifiers seeded by Q-switched fiber laser oscillators. We demonstrate near diffraction limited beam quality with ~ 20 kW peak power (1mJ pulse energy, ~ 45 nsec) pulses and an average power ~ 20 W at 20 kHz repetition rate with linearly polarized (> 17dB PER) output from a simple MOPA design. The laser produces spectrally narrow pulses with ~ 0.5 nm linewidth centered at 1064nm, suitable for various non-linear applications including generation of visible and UV light. The simple MOPA design consists of a monolithic fiber amplifier based on an optimized coil of polarization maintaining large mode area (PM-LMA) fiber with 30 &mgr;m core and low power Q-switched fiber oscillator. Excellent output beam quality is achieved through the mode selectivity of the coiled PM-LMA fiber in the amplifier stage. Such compact and robust fiber lasers are suitable for a variety of applications, such as nonlinear wavelength conversion processes using a variety of nonlinear materials, laser radars, etc.

Photonic crystal fiber designs for power scaling of single-polarization amplifiers are presented. These fibers incorporate a core with a refractive index slightly lower than that of pure silica and borosilicate stress rods embedded within the photonic crystal lattice. They are realizable as rod-type as well as conventional photonic crystal fibers. The core index change results in a fundamental mode profile that is flattened relative to that of standard photonic crystal fibers. A continuum of mode profiles ranging from approximately Gaussian to approximately top-hat can be achieved in this manner. The optimal parameters of these designs depend on the desired coiling radius, birefringence, and mode area. Preliminary experimental results are presented.

High-energy pulsed narrow-linewidth diffraction-limited ytterbium-doped power amplifiers in the 1030 to 1100 nm wavelength range and in the nanosecond regime require large mode area (LMA) fibers to mitigate stimulated Brillouin scattering (SBS). However, typical LMA fibers with mode-field diameters larger than 20 &mgr;m are inherently multimode. To achieve a diffraction-limited output, several techniques are available such as low core numerical aperture, fiber coiling and selective doping. The triple-clad fiber design takes advantage of the three techniques. The first clad located next to the core allows a reduction and a better control of the effective numerical aperture for high ytterbium doping that is difficult to achieve with the standard double-clad fiber design. Also, the thickness of the first clad gives an extra degree of freedom that allows either a nearly bending-insensitive output or mode filtering through bending losses that can be enhanced by a depressed-clad design. Incorporating to the triple-clad design an optimized selective rare-earth doping of the core favors the fundamental mode over higher-order modes by the gain differential. Using the right dopants, it can also favor SBS suppression by reducing the overlap between optical and acoustic field distributions. Ytterbium-doped LMA triple-clad fibers with a large depressed first clad and selective ytterbium doping are tested in a power amplifier configuration. Also, ytterbium-doped polarization-maintaining LMA triple-clad fibers with a thin first clad are tested for SBS.

We have numerically investigated the behavior of the LP₀₁ fundamental mode of a step-index, multimode (MM) fiber as the optical power approaches the self-focusing limit (P_crit). The analysis includes the effects of optical gain and fiber bending and are thus applicable to coiled fiber amplifiers. We find that at powers below P_crit, there exist stably propagating power-dependent modifications of the LP₀₁ mode, in contrast to some previous solutions that exhibited large-amplitude oscillations in beam waist along the fiber. For the first time, to our knowledge, we show that in a MM fiber amplifier seeded with the low-power LP₀₁ eigenmode, the transverse spatial profile will adiabatically evolve through power-dependent stationary solutions as the beam is amplified toward P_crit. In addition, for a given value of the nonlinear index, P_crit is found to be nearly the same in the bulk material and in a step-index fiber. These conclusions hold for both straight and bent fibers, although the quantitative details are somewhat different.

Various techniques are described for the efficient non linear conversion of high power fibre laser sources. These include the frequency doubling in periodically poled crystals of polarization preserving high power fibre Raman masteroscillator power fibre amplifier schemes, or the frequency doubling of a high power, narrow line bismuth-doped fibre laser, primarily for visible generation in the region of 589 nm. Above 2 microns where loss in silica based fibres prohibits efficient Raman generation, the use of heavily doped (~75%) GeO₂ fibres has been demonstrated as an efficient and effective all-fibre configuration to extend high power cw operation into the near infra red spectral region. For many applications, high peak power multiple wavelengths are required simultaneously. The integration of pulsed fibre lasers operating around 1 &mgr;m and photonic crystal fibre provides a simple mechanism to achieve this goal. Generally, the upper wavelength limit of supercontinuum generation is restricted by propagation loss and this has a consequential effect of inhibiting short wavelength generation through four wave mixing in the fibre. We have developed a technique employing long lengths of tapered PCFs that allow efficient phase matched four wave mixing to the short wavelength region and permits the generation of spectral power densities as high as 5mW/nmin the uv.

We review our recent work in the area of high power 2 &mgr;m and 3 &mgr;m fiber laser development. We present the recent results of our highly efficient directly diode-pumped Tm³⁺-doped silica and Ho³⁺-doped silica fiber lasers, passively switched 2&mgr;m fiber lasers, and discrete wavelength generation. We also present results from our recent Raman fiber laser experiments using chalcogenide fibers.

Broadly tunable fiber laser system has been demonstrated at room temperature. The IR fiber laser system consists of two high peak power individual pulse fiber lasers and their difference frequency generation (DFG) in Periodically Poled Lithium Niobate (PPLN) nonlinear optical crystal. Both lasers were operating at 20 kHz Pulse Repetition Rate and 200 ns pulse duration with diffraction limited beam quality. The first laser is a CW, < 16 GHz spectral bandwidth, tunable in 1050-1081 nm spectral range and Acousto- Optically extra cavity amplitude modulated Yb fiber laser ring oscillator - high power two stage amplifier which produced 280 mW of average and 70W of Peak power, respectively. The second laser is a DFB CW, single 1556 nm wavelength, < 1MHz spectral bandwidth and extra cavity amplitude modulated Er fiber laser oscillator - high power single stage amplifier with 160 mW of output average power and 40W of peak power. Synchronized pulses from two fiber lasers have been combined and fiber coupled into a single polarization maintained fiber using a fiber WDM combiner and then fibercoupled into the NovaWave Technologies, commercial DFG laser module which employed 50 mm PPLN crystal. The DFG stage of the system produced tunable radiation in 3236.4-3545.4 nm spectral range (309 nm). The difference frequency generation has a 9 mW average power, 20 kHz pulse repetition rate and 200 ns pulse duration which corresponds to 2.25 W of peak power. The demonstrated pulse DFG conversion efficiency is 0.2 W/W² (20%/W) which is ~ 100 times higher than that of CW operation. Further scaling of IR laser power was limited by optical damage of PPLN crystal and fiber lasers combining optics. Using a PPLN-MgO crystal and additional fiber laser amplifier stages based on Large Mode Area gain fibers is expected to allow us to achieve damage free difference frequency generation with up-to 100 mW of average power and peak power of up to 25 W.

We report a pulsed, Nd:YAG (1064 nm) microchip laser amplified by a mode-filtered, Yb-doped fiber amplifier. The system provided a widely tunable repetition rate (7.1-27 kHz) with constant pulse duration (1.0 ns), pulse energy up to 0.41 mJ, linear output polarization, diffraction-limited beam quality, and <1% pulse-energy fluctuations. Detailed spectral and temporal characterization of the output pulses revealed the effects of four-wave mixing and stimulated Raman scattering, and we investigated the effects of fiber length and Yb-doping level on system performance. The amplifier output was efficiently converted to a variety of wavelengths between 213 and 4400 nm by harmonic generation and optical parametric generation, with Watt-level output powers. The laser system employs a simple architecture and is therefore suitable for use in practical applications.

We report on the generation of pulse average power in excess of 1W (at pulse repetition rate ~100 kHz) in the 3.8-4micron wavelength range, obtained from a periodically-poled lithium niobate optical parametric oscillator pumped by a nanosecond-pulse, high-power 1545nm-wavelength pulsed fiber source.

Noise can limit the power in a fiber amplifier by competing with the signal for gain. Disributed filtering offers a way of overcoming current high-power limitations by selectively removing noise before it is amplified. Resonant mode suppression is a flexible strategy for wavelength filtering as well as suppression of unwanted transverse modes. An Yb-doped gain fiber with distributed filtering of stimulated Raman scattering (SRS) was demonstrated in a cladding-pumped amplifier. Substantial suppression of SRS was demonstrated in this fiber, with a core and resonant Ge-doped ring made using standard solid-fiber fabrication. Simulations explore generalizations to large-mode-area and filters with multiple band-stop features.

We report on an electronically tunable continuous-wave (CW) Ytterbium doped polarization maintaining fiber laser with more than 100 nm tuning range and more than 10 W linearly polarized single spatial mode laser output. The laser can be continuously tuned from 1027 nm to 1103 nm without any parasitic lasing peak with the 10dB linewidth of 0.1 nm. The maximum achievable tuning range of the fiber laser was from 1025 nm to 1124 nm with companying parasitic lasing peaks around 1040 nm. The usage of an intra-cavity acousto-optic tunable filter allows for the fast electronic tunability with less than 50 &mgr;s response time and random wavelength access capability.

A single cladding ytterbium doped fibre amplifier pumped at 980 nm that exhibits negligible amount of photodarkening over a long period of time is demonstrated. The output power as a function of time decreased by a very small factor compared to standard single mode ytterbium fibres. To achieve this photodarkening resistant amplifier, a special ytterbium doped fibre has been developed. Codoping with aluminium or other rare-earth such as erbium is shown to decrease the multi-excitation of ytterbium clusters and thus lower photodarkening. Photodarkening was characterized by comparing the amount of excess loss created by core pumping single cladding fibres at high intensity at 980 nm. Photodarkening was found to be directly proportional to the excitation of the ytterbium ions by comparing different pumping scheme and pump wavelength. Core pumping of a single cladding ytterbium doped fibre amplifier at 980 nm represents the worst case scenario for photodarkening. Engineering ytterbium fibres for low photodarkening is therefore critical in pulsed amplification where short length of fibre with high doping level is required as demonstrated with 6 &mgr;m core ytterbium fibre amplifier pumped in the core or in the cladding. Photodarkening was correlated to clustering from cooperative luminescence measurement at 500 nm produced by ytterbium clusters that would emit UV radiation under strong pumping.

In order to test power-handling at 1kW, a special splitter component had to be developed to make use of available sources. A tapered fused-bundle (TFB) 1X7 splitter using a 1.00mm core diameter 0.22NA input fiber coupled to seven 400 micron core 0.22 NA output fibers was tested up to 860W at 976nm. Surface temperature rise was measured to be less than 15°C with active heat sinking. The above results suggest that understanding the mechanisms of optical loss for forward and backward propagating light in a TFB and the heat load that these losses generate is the key to producing multi kW components, and demonstrates that reliable kW-level all fiber devices are possible.

We demonstrate direct diode-bar side pumping of a Yb-doped fiber laser using embedded-mirror side pumping (EMSP). In this method, the pump beam is launched by reflection from a micro-mirror embedded in a channel polished into the inner cladding of a double-clad fiber (DCF). The amplifier employed an unformatted, non-lensed, ten-emitter diode bar (20 W) and glass-clad, polarization-maintaining, large-mode-area fiber. Measurements with passive fiber showed that the coupling efficiency of the raw diode-bar output into the DCF (ten launch sites) was ~84%; for comparison, the net coupling efficiency using a conventional, formatted, fiber-coupled diode bar is typically 50-70%, i.e., EMSP results in a factor of 2-3 less wasted pump power. The slope efficiency of the side-pumped fiber laser was ~80% with respect to launched pump power and 24% with respect to electrical power consumption of the diode bar; at a fiber-laser output power of 7.5 W, the EMSP diode bar consumed 41 W of electrical power (18% electrical-to-optical efficiency). When end pumped using a formatted diode bar, the fiber laser consumed 96 W at 7.5 W output power, a factor of 2.3 less efficient, and the electrical-to-optical slope efficiency was lower by a factor of 2.0. Passive-fiber measurements showed that the EMSP alignment sensitivity is nearly identical for a single emitter as for the ten-emitter bar. EMSP is the only method capable of directly launching the unformatted output of a diode bar directly into DCF (including glass-clad DCF), enabling fabrication of low-cost, simple, and compact, diode-bar-pumped fiber lasers and amplifiers.

We report on an optical parametric amplification system which is pumped and seeded by fiber generated laser radiation. Due to its low broadening threshold, high spatial beam quality and high stability, the fiber based broad bandwidth signal generation is a promising alternative to white light generation in bulky glass or sapphire plates. As pump source we propose the use of a high repetition rate ytterbium-doped fiber chirped pulse amplification system.

The recent development of photonic bandgap fibers with solid cores enables the construction of dispersion compensated all-fiber ultrashort mode-locked fiber lasers. Solid-core photonic bandgap fibers (SC-PBG) can be spliced to standard fibers with relative low loss and negligible Fresnel reflection due to the matching indexes of the cores. The fibers can provide significant anomalous dispersion with low nonlinearity and are therefore ideal for dispersion compensation in ultrafast fiber lasers. We demonstrate the use of a SC-PBG fiber for intra cavity dispersion compensation in an ytterbium based mode-locked fiber laser. The limitations on pulse duration due to the relative high third order dispersion of the SC-PBG fiber are discussed.

The concept of spectral compression induced by self phase modulation is used to generate transform-limited 10ps pulses in a rare-earth-doped low nonlinearity fibre amplifier. The seed source of the amplifier stage is a high power, Yb³⁺:KGW bulk oscillator which delivers 500 fs transform-limited pulses at 10MHz repetition rate. After a reduction of the repetition rate down to 3MHz, the femtosecond pulses are negatively chirped by transmission gratings in a compressor arrangement. The resulting 10ps pulses are further seeded into the power amplifier and up to 32W output power is obtained while the spectral bandwidth is reduced to less than 0.5 nm by means of self phase modulation.

Passive combining of fiber oscillators may one day lead to a simple power scaling capability that multiplies the best results obtained by single devices. Along with HRL's unique approach and early experimental demonstrations, some extensive modeling is underway to sort through the current controversies in this area, to design for large numbers of fibers, and to create the most stable inphase beam output.

Fibre amplifiers exhibit rapid time dependent phase fluctuations due to the environment and to thermal and other effects associated with the pumping and lasing processes. We characterise these effects in a large mode area fibre amplifier having an output power of 260W limited only by pump power. The amplifier retains its coherence even at the highest available output power with negligible linewidth broadening. Phase fluctuations are characterised by a low-amplitude power-independent jitter superimposed on a power-dependent drift due to heating. We also measure the phase fluctuations in a COTS fibre preamplifier and find they are predominantly large amplitude periodic oscillations at 110Hz, probably induced by pump power fluctuations. The two amplifiers were combined in series to give a high gain amplifier chain and actively phase stabilised to high precision (~&lgr;/37 rms) using a piezo-ceramic fibre stretcher incorporated into a PC-based feedback loop.

With recent advances in high-power laser technology, Volume Bragg Gratings (VBG) have been recognized as important elements in different types of beam-combining applications, such as, design of optical correlators, coherent and incoherent power beam-combiners and in particular, spectral beam combiners (SBC) in which the output beams from several distinct laser sources are combined into a single-aperture, diffraction-limited beam. The obvious advantage of VBG's in these applications results from their narrow spectral and angular selectivity compared, for example, to any type of surface gratings. Almost a two order magnitude difference in spectral efficiency (number of channels per usable bandwidth) can potentially allow one to combine a much larger number of lasers into a single spot. The VBG recorded in a photo-thermo-refractive (PTR) glass exhibit long-term stability of all its parameters in high-power laser beams. With power density more than 1 MW/cm2 in the CW beam of total power on a kilowatt level the characteristics of these elements appear to be stable. In order to increase the spectral efficiency of such a "beam-combiner" the overall loss resulting from absorption and cross-talk between channels should be minimized. In this paper we consider architecturespecific beam-combining scheme and address cross-talk minimization problem based on optimal channel positioning. A mathematical model reveals the critical parameters for high efficiency spectral beam combining in which explicit equations are derived to relate the spectral density to the total system efficiency. Issue of system scalability for up to 200 channels is addressed. Coupled wave theory of thick hologram gratings is used in this analysis to characterize.

We describe a three-channel, spectrally beam combined (SBC), 1-&mgr;m fiber laser that features a SBC power combining efficiency of 93%, versatile master-oscillator, power-amplifier (MOPA) fiber channels with up to 260 W of narrowband, polarized, and near-diffraction limited output, and currently produces 522 W of average power with a dispersed (non-dispersed) beam quality at 522 W of 1.18x (1.22x) diffraction limited. To our knowledge, these results represent the best combination of output power and beam quality achieved by SBC to date.

Parametric devices based on four-wave mixing in fibers enable the amplification, frequency conversion and phase conjugation of optical signals. They can also be used to buffer and regenerate signals. In this paper some recent research on parametric devices is reviewed briefly.

We report on the development of a multi-channel all-fiber based laser system for LADAR applications. Two design pathways for the multi-channel laser system were investigated: (i) multiple externally triggered single channel MOPA (SCMOPA) systems and (ii) a single master-oscillator/multiple power-amplifier (SMOMPA) system. We designed and tested a single channel MOPA system that consists of a seed source, two single mode amplifiers and a power amplifier. With this system we were able to achieve up to 195&mgr;J/pulse at 6kHz with a pulse duration of 3ns. For evaluating the SMOMPA approach we built a bread board based on the single channel results with one master oscillator and 4 power amplifiers. With this breadboard we achieved pulse energies from each of the four power amplifiers of 120±4&mgr;J at 6kHz and 78-90&mgr;J at 18kHz. The temporal line-shapes emitted from each power amplifier were identical within the signal-to-noise level of the temporal traces and have a FWHM=2.2ns.

Nonlinear pulse compression techniques are of course very well-known, exploiting the initial spectral broadening and temporal compression phase of higher-order soliton evolution in the anomalous dispersion regime of an optical fiber. However, the presence of effects such as Raman scattering, higher-order dispersion or input pulse noise can induce pulse break-up and instabilities through soliton-fission processes. Recent studies of soliton fission in the context of supercontinuum generation have provided improved insight into the way these processes can be avoided, allowing significant improvement in achievable compressed pulse quality and duration. In this paper we focus on providing an overview of a series of our own experiments around 1550 nm where we have used parabolic pulse similariton amplifiers to generate low noise pulses that have been linearly and nonlinearly compressed to the sub-30 fs regime using controlled compression in highly nonlinear fiber. In addition, we also describe recent results where comparable pulse durations have been obtained using sub-10 cm lengths of highly nonlinear fiber directly spliced to the output pigtail of a commercial femtosecond source.

We have previously demonstrated ultrashort pulse amplification in fiber systems beyond the B-integral limit. Here we report on recent experiments to increase the average power of such systems, and their application to high-speed material processing. A compact fiber chirped pulse amplification system, producing sub-picosecond 50 &mgr;J pulses at a repetition rate of 1 MHz, is obtained by implementing a fiber stretcher and a 1780 l/mm dielectric diffraction grating compressor. Despite a substantial residual dispersion mismatch between stretcher and compressor, the cubicon fiber amplifier allows for the generation of sub-picosecond pulses with sufficient quality for high-speed micromachining applications. Moreover, the dielectric grating compressor allows power independent near-diffraction limited beam quality as required for precision micro-machining. We utilize this laser to mill aluminum, alumina, and glass targets with material removal rates >0.2 mm³/s in all three materials.

We report on an Yb-doped photonic crystal fiber based CPA system delivering 90.4 W average power of 500 fs pulses at a repetition rate of 0.9 MHz corresponding to a pulse energy of 100 &mgr;J.

We have described a frequency-stabilized, polarization-maintained erbium fiber ring laser. This laser has no frequency modulation at the output beam. A tunable single-mode laser has also been newly developed by simultaneously controlling a tunable FBG with a 1.5 GHz bandwidth and a PZT in the cavity. The frequency stability reached as high as 1.3 x 10^-11 for an integration time of 1 s and the linewidth was as narrow as 4 kHz. Using this coherent laser as a light source, we successfully transmitted a 20 Msymbol/s coherent quadrature amplitude modulation (QAM) signal over 525 km and achieved error free transmission.

Naval Research Laboratory (NRL) is developing chalcogenide glass fibers for applications in the mid and long wave IR wavelength regions from 2-12 &mgr;m. The chalcogen glasses (i.e., glasses based on the elements S, Se, and Te) are transparent in the IR, possess low phonon energies, are chemically durable and can be drawn into fiber. Both conventional solid core/clad and microstructured fibers have been developed. Chalcogenide glass compositions have been developed which allow rare earth doping to enable rare earth doped fiber lasers in the IR. Also, highly nonlinear compositions have also been developed with nonlinearities ~1000x silica which enables nonlinear wavelength conversion from the near IR to the mid and long wave IR. In this paper, we review rare earth doped chalcogenide fiber for mid and long wave IR lasers and highly nonlinear chalcogenide fiber and photonic crystal fiber for wavelength conversion in the mid and long wave IR.

Optical frequency comb systems have received much attention in recent years due to their enormous potential in optical frequency metrology and optical frequency synthesis applications. The expected transition to optical time standards, attosecond lasers and optical electron accelerators will all rely on the availability of rugged and stable frequency comb lasers, combining superior performance with utility. Fiber laser based frequency combs have a number of advantages over their Ti:sapphire cousins in that they are more compact, are capable of turnkey, long-term operation with power consumption, are compatible with existing fiber optics, and cover the telecommunication window. The first fiber-frequency comb systems showed excellent long-term performance but the short term performance was inferior to Ti:sapphire combs due to excess noise contribution. However, recently it was shown that the linewidth of the individual comb limes of a fiber frequency comb can reach the sub-Hertz level and fiber frequency combs are now approaching the performance of Ti:sapphire based systems.

High power fiber lasers have been recently demonstrated at the kilowatt level. The spectral linewidths of these lasers oscillators can exceed 20 nm. Whereas, such broad spectra are fine for many applications, such as materials processing where raw power is the primary requirement, other applications, including coherent beam combination, harmonic generation, or gravitational wave detection, require high powers beams with much narrower linewidths. Amplification of narrow linewidth signals in optical fibers is limited by stimulated Brillouin scattering (SBS). We discuss novel fiber designs that limit SBS allowing the amplification of narrow linewidth signals to kilowatt power levels.

Leakage channel fibers, where several air holes form a core, can be precisely engineered to create large leakage loss for higher order modes, while maintain negligible transmission loss for the fundamental mode. This unique property can be used for designing optical fibers with large effective mode area, which supports robust fundamental mode propagation. The large air holes in the design also enable the optical fibers to be bend-resistant. The principles of design and operation regime are outlined in this paper. Performance of an ytterbium-doped double clad leakage channel fiber and an ytterbium-doped polarization-maintaining (PM) double clad leakage channel fiber with ~50&mgr;m core diameter is also discussed.

The objective of this work is to understand catastrophic optical damage in nanosecond pulsed fiber amplifiers. We used a pulsed, single longitudinal mode, TEM₀₀ laser at 1.064 µm, with 7.5-nsec pulse duration, focused to a 7.45-&mgr;m-radius spot in bulk fused silica. Our bulk damage threshold irradiance is corrected to account for self focusing. The pulse to pulse variation in the damage irradiance in pure silica is less than 1%. Damage is nearly instantaneous, with an induction time much less than 1 ns. These observations are consistent with an electron avalanche rate equation model, using reasonable rate coefficients. The bulk optical breakdown threshold irradiance of pure fused silica is 5.0x1011 ±7% Watts/cm². We also measured the surface damage threshold irradiance of 1% Yb³⁺ doped fused silica preform of Liekki Yb1200 fiber, and found it is equal to that of pure silica within 2%. The optical damage morphology is reproducible from pulse to pulse. To facilitate the morphology study we developed a technique for locating the laser focus based on the third harmonic signal generated at the air-fused silica interface. This gives a very small uncertainty in focal position (~ 10 &mgr;m) which is important in interpreting the damage structure. The surface third harmonic method was also used to determine the laser focus spot size and verify beam quality. Earlier reports have claimed that the damage irradiance depends strongly on the size of the focal spot. We varied the focal volume to look for evidence of this effect, but found none.

We developed a gain-staged master-oscillator/power-amplifier source featuring an Yb-doped, 100&mgr;m-core rod-like photonic crystal fiber (PCF) as the final amplifier. From this PCF, we obtained 1ns pulses of energy in excess of 4.3 mJ, peak/average power ~ 4.5 MW / 42W, and spectral width ~20 GHz. The PCF emitted a near-Gaussian, single-transversemode beam of M²~1.3.

We present an adaptive and robust pulse-operated high power fiber laser, which has an average output power of 30 W. The fiber laser realizes flexible pulse duration without changing a repetition rate and an average power, which is desirable for laser processing, by employing a Q-switched (Q-SW) Master Oscillator Power Amplifier(MOPA). For instance, its pulse duration is controllable in the range of 50 ns to 100 ns at 50 kHz in repetition rate. In addition to the flexible pulse duration, the fiber laser provides high powers, a peak power of 27 kW at 30 kHz in repetition rate and an average power of 30 W at 50 kHz in repetition rate. Large mode area (LMA) fibers and end pumping structure in the laser also contribute the above mentioned features, pulse adaptiveness and high output power. In terms of the robustness of the laser, we employ newly-developed reflection-suppressing pump combiners, which protect pump laser diodes (LDs) from amplified reflected signal light from objectives. All those features make the fiber laser a more practical light source for laser processing.

Photonic bandgap fibers have already proved their huge potential for guiding light in air over kilometric lengths. Nowadays, solid-core bandgap fibers draw considerable attention due to their unusual properties. For instance, the bandgap effect may lead to very large mode area operation, management of the chromatic dispersion curve, spectral filtering or bend loss reduction, all features that could enhance fiber laser performances. Recent results about the design, fabrication and characterization of large mode area solid-core bandgap fibers are presented. Prospects of further development of bandgap fiber lasers are discussed.

The design and optimization of high-power fiber amplifiers requires a simulation tool capable of including a wide range of effects simultaneously, including mode distortion and loss due to bending, spatially-dependent saturable gain, guiding from arbitrary index of refraction profiles and self-focusing. In addition, the nonlinear effects are power dependent and thus will distort the pulse shape. We have constructed a numerical model to address these issues and serve as a platform for data analysis and system optimization.

We have numerically compared the performance of various designs for the core refractive-index (RI) and rare-earth-dopant distributions of large-mode-area fibers for use in bend-loss-filtered, high-power amplifiers. We first established quantitative targets for the key parameters that determine fiber-amplifier performance, including effective LP₀₁ modal area (A_eff, both straight and coiled), bend sensitivity (for handling and packaging), high-order mode discrimination, mode-field displacement upon coiling, and index contrast (manufacturability). We compared design families based on various power-law and hybrid profiles for the RI and evaluated confined rare-earth doping for hybrid profiles. Step-index fibers with straight-fiber A_eff values > 1000 &mgr;m² exhibit large decreases in A_eff and transverse mode-field displacements upon coiling, in agreement with recent calculations of Hadley et al. [Proc. of SPIE, Vol. 6102, 61021S (2006)] and Fini [Opt. Exp. 14, 69 (2006)]. Triangular-profile fibers substantially mitigate these effects, but suffer from excessive bend sensitivity at A_eff values of interest. Square-law (parabolic) profile fibers are free of modal distortion but are hampered by high bend sensitivity (although to a lesser degree than triangular profiles) and exhibit the largest mode displacements. We find that hybrid (combined power-law) profiles provide some decoupling of these tradeoffs and allow all design goals to be achieved simultaneously. We present optimized fiber designs based on this analysis.

We present numerical results suggesting that optical fibers with a strong central dip at the center of a strictly axisymmetric refractive index profile can guide stable fundamental modes at more than 10 times the bulk media critical power for self-focusing. The fibers are modeled using a numerical integration of the steadystate axisymmetric scalar paraxial non-linear Schrodinger (NLS) equation as well as by an axisymmetric scalar paraxial finite-difference beam-propagation-method (FD-BPM) code. Two different criteria demonstrated that the fundamental modes of these fiber designs are stable to radial disturbances above the bulk media critical power for self focusing. If these modes are also shown to be stable to azimuthal disturbances, they will significantly extend the performance of extremely high-peak-power (≫4MW) short-pulse (≪1 ns) optical fiber lasers and amplifiers.

Yb-doped fibers are widely used in applications requiring high average output powers and high power pulse amplification. Photodarkening is one limiting factor in these fibers. In this paper, characterization of photodarkening in large-mode-area (LMA) fibers is presented building upon our previous work, which indicated that meaningful comparison of photodarkening properties from different fibers can be made as long as care is taken to equalize the excited state Yb concentration between samples. We have developed a methodology that allows rapid and reproducible photodarkening measurements to be performed and that enables quantitative comparison of the photodarkening propensity among fibers with different compositions and under different operating conditions. We have shown that this measurement technique can be used effectively for LMA fibers by employing cladding pumping rather than the more standard core pumping. Finally, we observe a seventh-order dependence of the initial photodarkening rate on the excited-state Yb population for two different Yb-doped fibers; this result implies that photodarkening of a Yb-doped fiber source fabricated using a particular fiber will be strongly dependent on the device configuration.

Monolithic, all-fiber PM optical amplifiers have been investigated regarding components performance, amplifier design and narrow line-width amplification. PM-based components performed very well, the pump/signal combiner in particular handling > 300W pump power and still maintaining the PER of the input signal. A co-pumped amplifier configuration was chosen based on reliability and potential SBS mitigation (i.e. fiber length after the active fiber output). This system based on PLMA-YDF - 25/400 fiber appeared to give the best output power (155W-177W, depending on the length of passive delivery fiber), PER (~ 17 dB) and excellent beam quality (M²= 1.1). Altering the fiber temperature and length were necessary to provide the best results.

Over recent years, there has been a tremendous and rapid progress in power scaling Yb-doped fiber-based picosecond sources due to their high efficiency, excellent beam quality and immunity to thermo-optical effects. These remarkable properties are not only very attractive for many scientific and industrial applications but also for frequency doubling to generate green. Besides good beam quality, a high degree of polarization and a narrow linewidth, further increase in conversion efficiency requires high peak power and increased crystal length. High peak power can be obtained by employing a fiber master-oscillator power amplifier design (MOPA) where seed pulses with adequate duty cycle are amplified to high average powers. However in this arrangement minimizing nonlinear effects arising in the fiber amplifiers becomes a challenge. The amplification of picosecond pulses causes linewidth broadening and the spectral bandwidth of the crystal is reduced by a preferred longer length. This trade-off can result in lower frequency doubling efficiency. In this paper, as well as the benefits and limitations of fiber lasers applied to nonlinear frequency conversion, we will review the various design considerations for the development of a high average power picosecond green laser based on single-pass frequency doubling of a fiber MOPA system.

We report on the performance of seeded Raman fiber amplifiers based on multimode gradient index fiber. The "beam cleanup" improvement in beam quality of the Stokes beam over that of input pump beam (previously observed in unseeded fiber amplifier geometries using stimulated Raman scattering in multimode graded index fiber) is shown here to be limited by the beam quality of the input seed beam for seeded amplifier configurations. The amplifiers are characterized in terms of their capacity for beam-cleanup, their ability to amplify the seed and in terms of their output spectra. The advantages and disadvantages of a backward-pumped geometry versus a forward-pumped geometry are discussed. Depending on the geometry, amplified power can be readily distributed to a cascade of Stokes frequencies (unseeded forward-pumped geometry) or can be mostly contained in the seed frequency (seeded backward-pumped geometry).

A high-power, single-frequency, ytterbium doped photonic crystal fiber amplifier using a Nd:YAG non-planar ring oscillator seed source is reported. The system delivers up to 148 W of output power and a slope efficiency of 76% with respect to the launched pump power. At maximum output power the amplified spontaneous emission was suppressed by more than 40 dB and a polarization extinction ratio of 28 dB was obtained. Single transverse-mode operation with M² values better than 1.2 was measured by a standard method. To investigate the real overlap of the photonic crystal fiber transverse-mode with the Gaussian fundamental-mode, sensitive beam quality measurements with a Fabry-Perot ring-cavity premode cleaner will be presented.

An all-fiber approach is utilized to phase lock and select the in-phase supermode of compact multicore fiber lasers. Based on the principles of Talbot imaging and waveguide multimode interference, the fundamental supermode is selectively excited within a completely monolithic fiber device. The all-fiber device is constructed by simply fusion splicing passive non-core optical fibers of controlled lengths at both ends of a piece of multicore fiber. Experimental results upon in-house-made 19- and 37-core fibers are demonstrated, which generate output beams with high-brightness far-field intensity distributions. The whole fabricated multicore fiber laser device can in principle be a single fiber chain that is only ~10 cm in length, aligning-free in operation, and robust against environmental disturbance.

Incoherent spectral beam combining (SBC) by means of volume Bragg gratings (VBGs) has been shown to be a simple and robust technique for generating high-power laser radiation. Combination of laser radiation from multiple sources into a single near-diffraction-limited beam results in energy brightness increase, while spectral brightness is preserved. High-efficiency VBG recording in photo-thermo-refractive (PTR) glass has been recently developed. While being photosensitive in the UV, PTR glass offers high transmittance in the near-IR and visible parts of spectrum. Moreover, this glass has excellent mechanical properties and refractive index independent of temperature. These features enable VBGs in PTR glass to withstand high-power laser radiation, making them ideal elements for high-power SBC. We present experimental results of successful 5-channel SBC with reflecting VBGs in PTR glass with small channel spacing (~0.43 nm around 1064 nm). Absolute system efficiency of 93.5% is demonstrated. Combined beam is shown to be near-diffraction-limited with M²=1.11. Behavior of narrow-band reflecting VBGs in high-power beams is studied. VBGs are shown to withstand 570 W CW radiation around 1064 nm with diffraction efficiency in excess of 92%. Pathway to near-diffraction-limited high-power laser systems via SBC with VBGs is shown. High-efficiency SBC system with 0.2 nm channel spacing is designed.

Chalcogenide glass based optical waveguides offer many attractive properties in all-optical signal processing because of the large Kerr nonlinearity (up to 420 × silica glass), the associated intrinsic response time of less than 100 fs and low two-photon absorption. These properties together with the convenience of a fiber format allow us to achieve all-optical signal processing at low peak power and in a very compact form. In this talk, a number of non-linear processing tasks will be demonstrated including all-optical regeneration, wavelength conversion and femtosecond pedestal-free pulse compression. In all-optical regeneration, we generate a near step-like power transfer function using only 2.8 m of fiber. Wavelength conversion is demonstrated over a range of 10 nm using 1 m of fiber with 7 ps pulses, peak power of 2.1 W, and 1.4 dB additional penalty. Finally, we will show efficient compression of low-power 6 ps pulses to 420 fs around 1550 nm in a compact all-fiber scheme. These applications show chalcogenide glass fibers are very promising candidate materials for nonlinear all-optic signal processing.

We present an experimental and theoretical analysis of four-wave-mixing (FWM) in nanosecond pulsed fiber amplifiers FWM leads to a saturation of the in-band amplified pulse energy and to distortions of the spectral and temporal profiles, and it is the main limiting effect in the ~1 ns temporal regime. A simple model considering both Raman and FWM contributions provides a good description of the measured behaviours, allowing optimization and design tradeoffs to be explored for mitigating FWM.

We present an overview of nonlinear frequency conversion techniques which we developed and optimised for use with mode-locked Erbium fiber lasers. Starting with 70 fs, 3 nJ pulses at 1560 nm, we access the entire wavelength band from 500 to 2000 nm without gaps. Across this broad range of wavelengths, we adapt pulse parameters such as temporal duration and spectral width to the specific application requirements.

A simple Q-switch, consisting of a rotating planar-mirror where the reflectivity of portions of the surface are high and the reflectivity of the remaining portions are low, is reported. This novel Q-switch is demonstrated using a fiber laser. Using two meters of single-mode, 5 &mgr;m core, Yb-doped fiber, a 40% fiber Bragg-grating (FBG) at 1064 nm, and a single-mode diode-pump at 976 nm, a fiber laser with a threshold power and slope efficiency of approximately 25 mW and 33% was achieved, respectively, with the rotary mirror held stationary at the high-reflectivity region to form part of the laser cavity. With the mirror rotating at a fixed speed of 7200 rpm, pulses with a 140 ns full-width-half-maximum (FWHM) at a repetition rate of 480 Hz were observed. At an optical pump-power of 80 mW, the average power from the Q-switched fiber laser was 3.5 mW resulting in a calculated, peak pulse-power of 48.6 W.

Important progress in the development of rare earth doped high power fiber lasers was possible by large-mode-area (LMA) fibers with increased core diameters and reduced core apertures as low as 0.05. In this way, the excellent beam quality is maintained, but the power density can be reduced below critical values despite of very high output powers beyond 1 kW. Sophisticated concepts had to be developed in order to maintain the low NA in the case of high doping, e.g. the codoping by index-decreasing components as boron or fluorine. Here we report on the progress in the preparation of microstructured LMA laser fibers, the core area of which is composed of parts with high doping and parts with refractive index lower than the silica pump cladding. In contrast to the direct codoping, in this way the atomic environment of the active atoms can be tailored and optimized independently on the mean refractive index of the core. The preparation was carried out by stacking different rods in a multistep process, leading to cores with up to more than hundred single elements. Both for ytterbium and erbium/ytterbium doped fibers, good optical properties concerning basic attenuation and rare earth fluorescence could be reached by introducing additional purification steps. Different fiber structures were characterized concerning mode field distribution, pump power absorption and laser behavior.

A novel method to increase the pulse repetition rate by means of fission of second-order solitons in the fiber with periodically modulated dispersion is studied. The experiments confirm the results of numerical simulations. The efficient doubling of the pulse repetition rate takes place in dispersion oscillating fiber (DOF). Good agreement between theory and experiment was obtained.

Ytterbium-doped double-clad photonic crystal fibres (PCF) are ideal candidates for amplification of diffraction limited pulses to energies of several mJ. The combination of large mode field diameter (MFD), large pump area and high numerical aperture allows for low cost multimode diode pumping of short fiber lengths with high pump absorption and high thresholds for detrimental non-linear effects. We present results from ns-pulsed Q-switched laser and MOPA systems with pulse energies exceeding 1mJ and peak powers up to 17kW, by using a PCF with MFD of 24&mgr;m. The results are scalable with the PCF MFD to much higher pulse energies. Recent reports on truly single mode PCF with MFD up to 100&mgr;m indicates in this way, that pulse energies exceeding 10mJ could be reached.

Passively mode-locked fiber lasers are the best pulsed sources available today due to their simplicity and their ability to generate transform-limited pulses in the picosecond regimes. A drawback of the conventional passively mode-locked fiber lasers is that the pulse repetition rate is relatively low, at best a few tens of MHz, because of long cavity length. In order to raise repetition rate up to a few GHz, the cavity length has to be shortened below a few centimeters. Fiber lasers with such a short cavity require a high gain fiber and a small saturable absorber with low loss. Recently, the authors have proposed and demonstrated a small and low-loss saturable absorber device incorporating carbon nanotubes (CNT). Using CNT, we have realized very stable 2cm-long, 5GHz mode-locked Er:Yb-codoped silica-fiber laser, but the output power was limited to ~0.2mW due to insufficient gain in the Er:Yb-codoped silica-fiber. Here we used heavily Er:Yb-codoped phosphate fiber to form 1cm-long cavity with fiber mirrors, and succeeded in generating stable pulse trains with output power as high as 30mW and repetition rate as high as 10GHz at 1535nm.

Lasers operating at wavelengths that pass through the atmosphere are required for many applications, including lidar/ladar, spectroscopy, and pollution detection. One window of particular interest is between 2050 and 2300 nm. A Tm:silica fiber laser may be a candidate for operating in this window, but reported solutions using double clad fibers only achieved wavelengths up to 2090nm even though the emission spectrum of the lasing band extends beyond 2200nm. By carefully selecting the dopant concentration, mirror reflectivities, and fiber length the operating wavelength may be adjusted. A laser based on a double clad Tm:silica fiber coupled to a bulk grating for wavelength selection was constructed. By changing the output coupler reflectivity the maximum obtainable wavelength shifted from 2040nm to 2140nm, and another mirror resulted in 2188nm lasing operation.

We present a tunable high power ytterbium fiber laser system which is suitable for a variety of applications in nonlinear frequency conversion, spectroscopy or in medicine technique. The realized laser system consists of a master oscillator power amplifier arrangement with a fiber integrated pulse shaper. The continuous-wave core-pumped fiber oscillator with a grating in Littman configuration as tuning element defines the spectral properties of the system, like tuning range and spectral linewidth. A fiber integrated modulator is used for pulse generation and shaping. The losses due to this modulator are compensated in a core-pumped amplifier. The final amplifier stage applying a double clad fiber generates peak powers at a multi Watt level. At a repetition rate of 20 kHz and a pulse duration of 3 &mgr;s an average output power of 1.8 Watt has been achieved corresponding to a peak power of 88 W. The system is tunable from 1040 nm to 1070 nm with a linewidth of 10 MHz. Experimental results on tuning performance, pulse shaping and stability will be presented.

In this paper, we theoretically and experimentally investigated a two-stage erbium-doped fiber amplifier (EDFA) with a single-pump laser diode pumped at 980 nm in which a Sagnac interferometer filter is introduced to reduce amplified spontaneous emission (ASE) providing significant improvement on the amplifier performance. The erbium-doped fiber (EDF) parameters were measured in order to optimize parameters such as pump power, EDF length, ASE noise and signal gain. The Sagnac interferometer filter was designed to provide a periodic transmittance of 46 nm that allows by temperature to adjust the minimum transmission at 1530 nm (peak of the ASE noise) and maximum transmission at 1550 nm (signal wavelength). Experimental results show that with a simple configuration we obtained up to 53-dB amplification with only 75 mW of pump power, which can be enhanced easily by 3 dB providing total amplification up to more than 55 dB.

We report experimental results that demonstrate the mode-locking operation of a figure eight fiber laser based on a symmetrical nonlinear optical loop mirror (NOLM) with a twisted low-birefringence fiber in the loop. The mode-locking operation is achieved by nonlinear polarization rotation in the NOLM, where the counterpropagating beams accumulate a differential nonlinear phase shift when they have different polarizations. By adjusting a single quarter wave (QW) retarder in the NOLM it was possible to change the NOLM transmission behavior from a maximum to a minimum at low input power. Self-starting mode-locking operation was found for only specific settings of the QW retarder. The pulse repetition frequency was around 0.8 MHz. The mode-locked laser operated stably for hours and the adjustment procedure was straightforward. We achieved stable generation of picosecond pulses with milliwatts of average output power.

We designed a high output power double cladding erbium-ytterbium fibre amplifier that showed no amplified spontaneous emission (ASE) at 1.0 &mgr;m using a quasi singlemode fibre. The reduction of the amplified stimulated emission (ASE) at 1.0 &mgr;m was found to be the combination of fibre design and temperature effect in the core. A 10W output double cladding Er-Yb amplifier with a core/cladding fibre diameter of 10/125 &mgr;m was realized with a seed signal of 1.4 W at 1563 nm and with counter-propagating pump power of 35 W at 976 nm without any significant ASE generation at 1.0 &mgr;m. The fibre also exhibits singlemode behaviour with M² <1.1 and a high slope efficiency of 30%. The fibre was designed to minimize ASE at 1.0 &mgr;m by heavily doping the fibre and using the appropriate ratio between Yb³⁺ and Er³⁺ ions. By incorporating into our model the core temperature increase coming from the quantum defect of the Er-Yb system, we can also predict a raise in the absorption cross-section of the ytterbium ions around 1060 nm yielding to an increase of the 1 &mgr;m ASE threshold from 14 W to 35 W pump power, which allowed us to reach a 10 W output power at 1563 nm instead of 5 W normally predicted by the theory. These results show potential power scaling of the output power or double cladding erbium ytterbium amplifier using quasi singlemode core erbium ytterbium fibre avoiding the need of large core dimension that degrades the beam quality.

A high peak-power, high repetition rate Master Oscillator Power Amplifier (MOPA) system incorporating an Yb-doped fiber amplifier and its second harmonic generation were investigated in detail. The oscillator is actively Q-switch microchip laser at repetition rate of 50 kHz with a pulse width of 2.8ns. The amplifier employing Yb-doped polarization maintaining fiber, having a large mode area was exited by a laser diode with an optical power of 17W. As results, the amplified average output power of 10W and the optical conversion efficiency of 59% were achieved. In this MOPA system, second harmonic generation (SHG) experiments were performed using KTP and LBO crystals. The conversion efficiency of 21% and 40%, the maximum SHG power of 0.92 W and 3.3W were obtained for KTP crystal and LBO crystal respectively.

Various techniques have been suggested to build a fiber ring laser using a fiber Bragg gratings (FBG) as a lasing wavelength selection filter. For most of FBG filter, a circulator has been used as a key component to convert the reflection characteristic of FBG to the transmission spectrum, but it has been hard to tune the spectral reflectance or transmittance of FBG intrinsically. In this research, a multi-wavelength fiber ring laser is proposed based on a novel switchable bandpass filter to show tunable multiple lasing spectra with high extinction. The proposed switchable bandpass filter consists of multiple FBG's incorporating Sagnac loop interferometer configuration. Transmission spectra of bandpass filter can be greatly varied for more than 20 dB as changing the phase of polarization controller in the Sagnac loop. We experimentally demonstrate the lasing wavelength is easily selected by the switchable FBG's at multiple spectral positions.

Determination of the radiation response of doped-fiber laser materials, systems and components to relevant ionizing radiation fluxes is central to the prediction of long-term fiber-based laser performance/survivability in adverse and/or space-based environments. It is well known that optical elements that are placed into orbit around the Earth experience harsh radiation environments that originate from trapped-particle belts, cosmic rays, and solar events. Of particular interest to optical materials is the continuous flux of gamma photons that the materials encounter. Such radiation exposure commonly leads to the formation of color centers in a broad range of optical materials. Such color center formation gives rise to changes in optical transmission, loss and luminescent band structure, and, thus, impacts long-term optical device performance. In this paper we will present the results of our investigation of gamma-radiation-induced photodarkening on the passive optical transmittance of a number of ytterbium- (Yb-) doped optical fibers. We will discuss the evolution of the optical response of the fiber across the 1.0 to 1.6 micron wavelength window with increasing gamma exposure. Results indicate that these fibers exhibit reasonable radiation resistance to gamma exposures typical of a 5-year, low-earth-orbit environment. Maximum transmittance losses of less than 10% were observed for total gamma exposures of 2-5 krad (Si). In this paper we will present the results of our investigation of gamma-radiation-induced photodarkening on the optical transmittance of a number of ytterbium- (Yb-) doped optical fibers. We will discuss the evolution of the optical response of the fiber across the 1.0 to 1.6 micron wavelength window with increasing gamma exposure. Results indicate that these fibers exhibit reasonable radiation resistance to gamma exposures typical of a 5-year, low-earth orbit environment. Maximum transmittance losses of less than 10% were observed for total gamma exposures of 2-5 krad (Si).

We constructed a passively mode-locked Er-doped fiber laser (PML-EDFL). It generates ~ 1.3 ps pulses at a repetition rate of 12 MHz with an average output power of 0.7 mW. These pulses are amplified in a short Er-doped fiber amplifier (EDFA) which is composed of low nonlinearity EDF. The average power of amplified pulse is about 15 mW. And its pulse width is about 880 fs. An all-fiber supercontinuum (SC) is generated by putting the amplified fiber laser pulse at the wavelength of 1. 56 &mgr;m into the highly nonlinear dispersion shifted fiber (HN-DSF) whose zero dispersion wavelength is 1.537 &mgr;m and nonlinear coefficient is about 10.5/W/km at the input wavelength. The polarization state of the generated SC spectra is well defined such that it can be properly controlled by the polarization controller. By using a delayed pulsed method, we report an experimental study of the coherence of SC spectra generated through a HN-DSF. In this paper, the strong dependence of the spectral coherence on the HN-DSF length is observed experimentally. And optimal conditions for obtaining wide SC with high coherence are investigated in detail. We believed that our proposed all-fiber laser based SC source with high coherence has many important applications in recently developed frequencydomain measurement techniques such as optical coherence tomography (OCT), optical frequency domain imaging (OFDI), optical frequency domain reflectometry (OFDR) and their instrumentation.

We present record-breaking experimental data on high power transmission through novel 7X1 and 19X1 multimode combiners based on Photonic Crystal Fiber technology. Both combiners are monolithic, have losses of ~0.2 dB, show very high thermal robustness and can handle record high optical powers. We have transmitted 100 W through the 7X1 and 310 W through the 19X1 combiner without evidence of any degradation or critical heating. The powers were limited only by available pump power. The combiners are based on Air-clad technology, where a ring of air-holes running along the length of the device provide guiding for the light. This Air-clad offers three major advantages for the device: 1: It is well suited for high optical powers as no polymer coating gets into contact with the light; 2: it is much easier to package as mechanical contact can be made anywhere on the device without risk of optical performance penalty; 3: the Numerical Aperture of the light can be increased beyond the limits imposed by polymer coatings. The presented pump combiners are especially well suited for high power fiber lasers, since such combiners can be spliced directly onto the active fiber, thereby enabling a robust, stabile laser solution with excellent efficiency and beam quality.

This PDF file contains the front matter associated with SPIE Proceedings Volume 6453, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.