IMRA's ultrashort pulse fiber laser products continue to evolve to expand the application scope. The latest prototype FCPA produces pulses with less than 500-fs pulse duration at a 50-kHz repetition rate. At the fundamental wavelength of 1045 nm, the pulse energy is greater than 10 mJ. The increase in pulse energy over the standard FCPA μJewel permits greater flexibility in the focusing conditions applicable for micromachining, enabling a wider variety of laser-machined structures and profiles. This paper describes the latest micromachining application areas being studied with this new laser.

Fiber-integrated high power fiber lasers are becoming the technology of choice for a diverse range of micromaterials processing applications due to their efficiency, operational stability and reliability. It is now clear that a much wider range of laser parameters are available when fiber lasers are compared to conventional solid-state lasers. Add to this the lack of additional variables associated with thermal lensing and process development is greatly simplified. Of even more importance, this parameter flexibility enables these lasers to perform well beyond the state-of-the-art in certain established applications where performance expectations are now very high. Similarly, due to its low M², the laser is shown here to perform well in applications and on materials that might not be immediately considered suitable for this type of continuous-wave modulated laser.

Single transverse mode fiber lasers have recently achieved output powers at the kilowatt level. These breakthroughs can be attributed to the maturation of high power diode pump technology at 980 nm and the use of large mode area (LMA) fibers. In the continous wave (cw) operation regime, LMA fibers, through their reduction of device length and increase of the effective area, have been effective in the reduction of deleterious nonlinear optical effects such as stimulated Raman scattering (SRS). The use of LMA fiber has been less effective in the suppression of stimulated Brillouin scattering (SBS), for which the threshold can be several orders of magnitude lower than for SRS. In this work we use refractive index profiles and index modifying dopant distributions for the mitigation of SBS. Our theoretical and modeling efforts led to an experimentally demonstrated increase in SBS threshold of 2.5 dB for single mode fiber and 6 dB for LMA amplifier fiber. We discuss the use of this SBS-suppressive fiber in the demonstration of a high-power, narrow linewidth fiber amplifier.

In this paper we present advances made in the development and fabrication of highly efficient, large-mode area fibers for eye-safe wavelengths (1.55 μm, 2.0 μm). LMA Er/Yb co-doped and Tm doped fibers have been successfully fabricated, with 25 μm core and 250 to 300 μm clad diameters, that are suitable for nanosecond pulsed amplification in LIDAR applications as well as high power CW amplification. Manufacturing challenges for these novel fibers are discussed. Measured and modeled data, for both types of fibers, are presented. The development of non-PM and PM-LMA fibers for eye-safe applications is expected to spur rapid progress in power scaling at these wavelengths, similar to that witnessed by the industry at 1.06 μm.

All-fiber contained laser systems play a key role, in the development of rugged, compact, and highly efficient eye-safe laser sources that can generate high peak and average powers and short (<5 ns) pulses. Application of such laser systems include spectroscopy, LIDAR, free-space communications, materials processing and nonlinear optics. In this paper we present further improvement on a novel high power all-fiber-based master oscillator power amplifier (MOPA) laser system operating in the C-band with <5 ns pulses and a repetition rate range of 6kHz − 200kHz. The system was optimized for performance of repetition rates between 6kHz and 18kHz. With this system, pulse energies of 322 μJ with a peak power of 170kW and an average power of 1.9W were generated using a custom designed Er:Yb co-doped double-clad fiber. The spectral output of the amplified pulses shows no spectral broadening due to Four-Wave-Mixing or Stimulated Raman scattering for pulse energies with less than 260μJ. Additionally, a beam quality M²=2.1+/-0.1 was achieved. The physical performance parameters of the all-fiber laser system make it very suitable for a variety of applications. The performance of the MOPA system and the experimental data are presented and discussed. To our knowledge the combination of the presented pulses energies, peak power, average power are the highest ever recorded in an all fiber system.

This report presents a discussion of the engineering issues and results of high power 2μm Tm³⁺-doped fibre lasers pumped at 790nm. To date we have achieved up to 85W from such devices with 54% slope efficiency relative to launched pump. More recently, through using Tm³⁺ concentrations of approximately 4(wt.)% to enhance the cross-relaxation process (³H₄,³H₆->³F₄,³F₄) we have demonstrated slope efficiencies of up to 67% relative to launched power. This represented ~170% quantum slope efficiency for the 790nm pumped 2μm laser.

We have performed numerical simulations to investigate the optimization of compound glass microstructured optical fibers for mid IR supercontinuum generation beyond the low loss transmission window of silica, using pump wavelengths in the range 1.55-2.25 mm. Large mode area fibers for high powers, and small core fiber designs for low powers, are proposed for a variety of glasses. Modeling results showed that for Bismuth and lead oxide glasses, which have nonlinearities ~10 x that of silica, matching the dispersion profile to the pump wavelength is essential. For chalcogenide glasses, which have much higher nonlinearities, the dispersion profile is less important. The pump pulses have duration of <1 ps, and energy <30 nJ. The fiber lengths required for generating continuum were <40 mm, so the losses of the fibers were not a limiting factor. Compared to planar rib-waveguides or fiber-tapers, microstructured fiber technology has the advantages of greater flexibility for tailoring the dispersion profile over a broad wavelength span, and a much wider possible range of device lengths.

Fiber lasers deliver excellent beam-quality and high efficiency in a robust and largely maintenance-free format, and are now able to do so with output powers in the kilowatt regime. Consequently, fiber lasers have become an attractive alternative to solid-state and gas lasers for e.g. material processing like welding, cutting and marking. The all-glass air-clad photonic crystal fibers (PCFs) combine large mode-field diameters (currently up to 40 μm), high numerical aperture (typically in the 0.6-0.65 range), high pump absorption (30 dB/m demonstrated in ytterbium) and excellent high-power handling (kW CW and mJ pulses demonstrated). These properties have made this fiber type one of the most promising candidates for the future high-power fiber laser and amplifier systems that are expected to replace many of the traditional systems in use today. To utilize the high numerical aperture and large mode-field diameters of the air-clad PCFs, special care must be taken in the system integration. In this paper, we will show examples of how these fibers can be integrated in laser and amplifier sub-assemblies with standard fiber pump-interfaces for use with single-emitter diodes or diode-bar pump sources. Moreover, we report on the most recent advances in fiber design including rod-type fibers and broadband polarizing ytterbium-doped large-mode-area air-clad fibers. Finally, we will review the latest results on PCF-based amplifier and laser configurations with special focus on high-power CW systems and high-energy pulsed configurations.

Photonic crystal fibers consisting of a solid fused silica core surrounded by a regular array of sub-micron air-holes have been shown to operate with single mode core well above 30 μm in active laser geometries as well as passive beam delivery fibers. Novel designs based on the combination of stress applying elements that are index matched to the holey cladding have recently been emerged. In this report we summarize the properties of these polarization maintaining photonic crystal fibers. Beside the characterization of the polarizing window and birefringence, high power laser and amplifier configurations using these fibers are demonstrated and first experiments concerning the temperature sensitivity of the polarizing properties are presented.

In this paper, we describe an intrinsically single-mode, Yb-doped photonic-crystal rod featuring a core of ~70?m diameter. When used as the final amplifier in a pulsed master-oscillator/power-amplifier system, the rod produced ~1ns, spectrally narrow, diffraction-limited pulses of ~2MW peak power at ~10 kHz repetition rate.

Silica-based hollow-core photonic bandgap (HC-PBG) fibers are of interest for high-power laser applications, due to the possibility of guiding the majority of the optical power in air, thus suppressing nonlinearities and the limitations set by the breakdown threshold of silica. In this contribution, we study numerically the laser-induced damage threshold in HC-PBG fibers as function of core size and cladding air-filling fraction, and compare to a typical silica-core large-mode area (LMA) fiber. Remarkably, the HC-PBG fibers yield no significant improvement over the LMA reference, indicating that radically new design ideas will be needed for HC-PBG fibers to be competitive as active components in a high-power laser system.

Hollow core photonic crystal fiber (HCPCF) amplifiers, in which Er³⁺- or Yb³⁺- doped glass acts as the gain medium, are proposed as a means of achieving high power pulse amplification. Double-clad configurations are identified which capture multimode pump light up to an NA of around 0.33. The nonlinear and breakdown properties of a HC-PCF amplifier with a mode area of approximately 50μm² are predicted to be comparable to those of a solid core fiber amplifier with a mode area of 1000μm². Mode competition effects within the HC-PCF amplifier strongly degrade the output signal unless the net gains of the unwanted guided modes are below that of the signal mode. This can be achieved if the ratio of amplifier gain to scattering loss is larger for the signal mode than any of the undesired guided modes. Assuming loss is dominated by hole interface roughness scattering, and an even doping profile produces the gain, the ratios for the unwanted guided modes of a typical HCPCF geometry are calculated to be similar to that for the signal carrying mode. The mode competition also places a lower bound on the active fiber length, typically implying a longer length is required than in a solid core fiber amplifier. This adversely affects the device efficiency due to scattering loss of the pump field incurred at the air/glass interfaces. To achieve a clean mode output and acceptable efficiency, alternative designs for the HC-PCF will need to be developed.

We propose a depressed clad hollow optical fiber with fundamental (LP01) mode cut-off suitable for high power short-wavelength, especially three-level, fiber laser operation by introducing highly wavelength dependent losses at longer wavelengths. The cut-off characteristic of such fiber structure was investigated. A Yb-doped depressed clad hollow optical fiber laser generating 59.1W of output power at 1046nm with 86% of slope efficiency with respect to the absorbed pump power was realised by placing the LP₀₁ mode cut-off at ~1100nm.

Optical coherence tomography (OCT) is an emerging technology for micrometer-scale, cross-sectional imaging of biological tissue and materials. One of the key limitations to achieving ultrahigh-resolution OCT imaging outside the laboratory setting has been the lack of compact, high-performance broadband light sources with sufficient power and stability to allow practical real-time imaging. The broad-bandwidth supercontinuum (SC) sources were recently demonstrated with femtosecond lasers in combination with nonlinear fibers. Using SC, we can demonstrate ultrahigh resolution OCT. However, wideband SC generally has large excess noise and significant fine structure. Low noise and smooth spectral shape are desired in the ideal supercontinnum source. In this paper, we describe recent studies on practical SC generation for ultrahigh-resolution OCT. SC generation is first analyzed both numerically and experimentally in terms of OCT imaging requirements and optimized conditions for generation are discussed. Supercontinua generated by use of highly nonlinear fiber which have a zero-dispersion wavelength near the pump wavelength, generally result in severe spectral modulation and fluctuating fine structure in the spectra. This spectral modulation produces sidelobes and reduced contrast in the interferometric point-spread function. In contrast, normally dispersive, highly nonlinear fibers (ND-HNFs) can generate smooth and Gaussian shaped supercontinua by the combination of self-phase modulation and normal dispersion. Low noise and wideband SC generation is demonstrated using ND-HNFs. Two colored SC generation is also demonstrated using a photonic crystal fiber which has two close zero dispersion wavelengths. The numerical results are almost in agreement with the experimental ones. Finally, low noise SC generation is demonstrated in an all fiber system based on an ultrashort pulse fiber laser. Wideband, low noise, near Gaussian shaped, high power SC is generated in the 1.55 μm wavelength region. In vivo, high-speed OCT imaging of human skin with ~5.5 μm resolution and 99 dB sensitivity is demonstrated.

We report on pulsed fiber-based sources generating high peak and average powers in beams of excellent spectral/spatial quality. In the first setup, a ~10-kHz pulse repetition rate (PRR), 1ns-pulse, Q-switched microlaser seeded a dual-stage amplifier featuring a 40-μm-core Yb-doped photonic-crystal fiber (PCF) as the power amplifier. From this amplifier, we obtained diffraction-limited (M² = 1.05), ~1ns pulses of 1.1mJ energy, ~1.1MW peak power, ~10.2W average-power, spectral linewidth ~9GHz, negligible nonlinearities, and slope efficiency >73%. In the second setup, we replaced the seed source with a shorter-pulse (<500ps) microchip laser of PRR ~13.4 kHz and obtained diffraction-limited (M²=1.05), ~450ps pulses of energy >0.7mJ, peak power in excess of 1.5 MW, average power ~9.5W, spectral linewidth <35 GHz. To show further power scaling, these pulses were amplified in a 140-μmcore Yb-doped fiber, which yielded multimode (M² ~ 9), 2.2mJ-energy, 30-W average-power pulses of peak power in excess of 4.5MW, the highest ever obtained in a fiber source, to our knowledge. In the third setup, an Yb-doped, 70μmcore, intrinsically single-mode photonic-crystal rod was used to generate diffraction-limited (M² ~ 1.1), ~10kHz PRR, ~1ns pulses of 2.05mJ energy, >2 MW peak-power (the highest ever reported in a diffraction-limited fiber source), ~20W average-power, ~13 GHz spectral linewidth, and spectral signal-to-noise ratio >50 dB. Finally, a single polarization large-core Yb-doped PCF was used to demonstrate high-peak-power harmonic generation. We obtained ~1ns pulses of peak powers >410 kW in the green (531nm) and >190kW in the UV (265.5 nm).

We report results from Yb-doped fiber amplifiers seeded with two microchip lasers having 0.38-ns and 2.3-ns pulse durations. The shorter duration seed resulted in output pulses with a peak power of >1.2 MW and pulse energy of 0.67 mJ. Peak power was limited by nonlinear processes that caused breakup and broadening of the pulse envelope as the pump power increased. The 2.3-ns duration seed laser resulted in output pulses with a peak power of >300 kW and pulse energy of >1.1 mJ. Pulse energies were limited by the onset of stimulated Brillouin scattering and ultimately by internal optical damage (fluences in excess of 400 J/cm² were generated). In both experiments, nearly diffraction-limited beam profiles were obtained, with M² values of <1.2. Preliminary results of a pulse-amplification model are in excellent agreement with the experimental results of the amplifiers operating in the low-to-moderate gain-depletion regime.

We have tested a series of Ytterbium doped large core fibers operating near 10Kpps and producing pulses of approximately 1ns. We have achieved 0.85mJ/pulse resulting in peak powers in excess of 2MW with 0.4ns pulses and near diffraction limited beams. In another fiber, we have achieved over 1.5mJ/pulse with pulses of 900ps corresponding to 1.65MW of peak power and M² of 2.5. In the latter case, wall-plug efficiencies, excluding cooling of the pump diode lasers, in excess of 15% were also achieved. This fiber amplifier has operated for 2 months without any degradation or observed optical damage.

We have developed an all-fiber format 10-mJ energy and 200 W average power Yb-doped laser. Based on a seed that utilizes first relaxation peaks and 65-um multimode fiber amplifier, the laser produces 300 ns pulses at 1-50 kHz variable repetition frequencies. Wall plug efficiency of 25% and 10 m delivery output with isolated head are essential features for industrial applications.

Fibre pulsed lasers are increasingly being adopted as the laser of choice in a number of industrial applications, such as micromachining, drilling and marking. In peak-power-driven applications, such as marking, it is essential to retain high peak powers (in excess of 2.5 to 5 kW) at high repetition rates in order to achieve faster character marking and increased throughput.

The operation and performance characteristics of three different modulation techniques for the Q-switching of Tm3+ doped silica fibre lasers operating near 2 um are described. Various Q-switched regimes are observed for a range of modulation frequencies and fibre lengths when the fibre is end-pumped with a high power Nd: YAG laser operating at 1.319 um in a linear bidirectional cavity. A larger multimode fibre core of 17 um diameter is used to increase the laser gain volume and achieve higher pulse energy. Experimentally this laser produced pulses with a peak power of 18.5 W, at higher repetition rate of approximately 20 kHz, a single Q-switched pulse of duration 300 ns at full width at half-maximum (FWHM), and 5.5 uJ are observed using an optical (mechanical) chopper. A peak power of ~ 3.3 kW and pulse duration of 320 ns (FWHM) at low repetition rate of 50-70 Hz and highest pulse energy up to 2.3 mJ by using an electro-optic modulator (EOM) is obtained. This peak power is enhanced to be 4.1 kW and the shortest pulse duration at FWHM of 150 ns at low repetition rates of 100-500 Hz when using an acousto-optic modulator (AOM). The results presented show that Q-switching is currently limited by the performance of the intracavity modulator. In this paper, an evaluation is presented of the high performance of active Q-switched Tm³⁺ doped fibre lasers using various techniques, some reasons for the difference in performance are discussed and some potential routes to further improvements are described.

We report on progress toward power scaling Yb fiber lasers beyond kW levels by an efficient and versatile architecture that maintains near diffraction limited beam quality. For this work, power scaling is performed at two distinct levels. The first utilizes a diffraction grating to spectrally beam combine (SBC) the output from several master-oscillator, poweramplifier (MOPA) fiber lasers with a goal of producing high quality combined beams with > 1 kW of power. The second involves scaling individual MOPA outputs to > 200 W, thereby reducing the number of lasers required for SBC. As a first step toward reaching these goals, we have developed Yb fiber MOPAs producing up to 208 W of polarized, narrow band, and near diffraction limited output and have demonstrated two-channel fiber laser SBC with a power combining efficiency of 93%, a combined beam power of 258 W, and a dispersed axis M² of 1.06. These results represent a significant advance in high brightness, spectrally beam combined laser systems.

The different concepts of combining fiber lasers for power-scaling are discussed. We report on three combined fibers with an output power of 100 W. Several proposals are made for further power scaling and the capacitance of a grating is tested in a simulation-experiment.

A four-element fiber array has demonstrated 470 watts of coherently phased, linearly polarized light energy in a single far-field spot. Each element consists of a single-mode fiber-amplifier chain. Phase control of each element is achieved with a Lithium-Niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channel's signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channel's phase modulator. This effort represents the results of a multi-year effort to achieve high power from a single element fiber amplifier and to understand the important issues involved in coherently combining many individual elements to obtain sufficient optical power for directed energy weapons. Northrop Grumman Corporation and the High Energy Laser Joint Technology Office jointly sponsored this work.

We report a novel coherent beam combining technique. This is the first actively phase locked optical fiber array that eliminates the need for a separate reference beam. In addition, only a single photodetector is required. The far-field central spot of the array is imaged onto the photodetector to produce the phase control loop signals. Each leg of the fiber array is phase modulated with a separate RF frequency, thus tagging the optical phase shift for each leg by a separate RF frequency. The optical phase errors for the individual array legs are separated in the electronic domain. In contrast with the previous active phase locking techniques, in our system the reference beam is spatially overlapped with all the RF modulated fiber leg beams onto a single detector. The phase shift between the optical wave in the reference leg and in the RF modulated legs is measured separately in the electronic domain and the phase error signal is feedback to the LiNbO₃ phase modulator for that leg to minimize the phase error for that leg relative to the reference leg. The advantages of this technique are 1) the elimination of the reference beam and beam combination optics and 2) the electronic separation of the phase error signals without any degradation of the phase locking accuracy. We will present the first theoretical model for self-referenced LOCSET and describe experimental results for a 3 x 3 array.

In this paper, we show that it is possible to arrange for an 18-core photonic crystal fibre (PCF) laser to operate in the fundamental in-phase supermode. The mode divergence is as small as 12.5 mrad. The equivalent mode field diameter is about 52 μm. Mode filtering is provided by a pinhole in the far field. The laser is Q-switched using an Acousto-Optic Modulator (AOM). An output power up to 65 W at a repetition rate of 50 kHz (corresponding to 1.3 mJ per pulse), with 22 ns short pulses, has been obtained with a slope efficiency of 46%. Ongoing amplification experiments are briefly described. Limiting factors (end facet damage threshold and thermal dissipation) are discussed for further scaling of this laser concept.

The compensation of nonlinear phase shifts by dispersion in femtosecond fiber amplifiers is explained. Contrary to previous understanding, a chirped-pulse fiber amplifier with mismatched stretcher and compressor can out-perform a matched system when the pulse acquires a significant nonlinear phase shift.

High average power single-mode fiber lasers have attracted significant attention as alternatives to conventional solidstate lasers owing to their relative high brightness, compactness and robustness. Likewise the turn-key operation of industrially qualified ultrafast fiber oscillators is well established. In recent years the convergence of reliable ultrafast fiber oscillators, high brightness pump diodes and high power fiber amplifiers has enabled ultrafast fiber lasers to surpass ultrafast solid-state lasers in terms of average power. While fiber lasers have generally not been able to match the ultrashort pulse energies produced by solid-state lasers, careful management of nonlinearities can overcome the conventional B-integral limit of π thereby permitting stable operation of practical ultrafast fiber lasers with pulse energies approaching the milli-Joule level. Here we review modes of nonlinear propagation in fibers which have enabled increases in ultrashort pulse energies from nano-Joule to milli-Joule levels, namely: solitons, similaritons and cubicons. As an example of a practical high energy ultrafast fiber laser, we demonstrate a cubicon Yb fiber chirped pulse amplification system producing 550 fs pulses with 50 μJ at >15 W.

We report on the generation of 50 fs pulses with an average output power of more than 50W. This is done by combining a high average power fiber CPA system with a microstructured large-mode-area fiber for nonlinear compression. The fiber CPA system delivers 300 fs pulses with a repetition rate of 73MHz in a linearly polarized beam with diffraction-limited quality. The average output power can exceed 100W. Nonlinear compression of these pulses is done by launching the light into a very short piece of a microstructured fiber and then removing the phase with a pair of chirped mirrors.

We report on a high power, high-energy femtosecond fiber source based on direct amplification of parabolic pulses from an environmental stable passively mode-locked fiber oscillator in an Yb-doped single-polarization photonic crystal fiber. The system delivers a pulse energy of 1.2 μJ (21 W average power) at a repetition rate of 17 MHz and a pulse duration of 240 fs in a linearly polarized beam with diffraction-limited quality. The special pulse shape allows for the generation of high quality femtosecond pulses beyond nonlinearity limits, which is confirmed by numerical simulations.

The performance of high average power and high energy femtosecond fiber laser systems is discussed. Remarkable evolutions in fiber technology made it possible to overcome restrictions due to nonlinear pulse distortions in the amplification fiber and revealed the full potential of rare-earth-doped fibers as a power-scalable solid-state laser concept in the short pulse regime. State-of-the-art femtosecond fiber lasers in our labs deliver average powers well above 100 W and pulse energies of several 100 μJ in the 1 μm wavelength region. This performance, in particular the significantly higher repetition rate compared to conventional femtosecond lasers, allows for unique approaches in several application fields. Beside the fiber designs, the setup, performance and limitations of these systems we will discuss selected applications.

We are developing an all fiber laser system optimized for providing input pulses for short pulse (1-10ps), high energy (~1kJ) glass laser systems. Fiber lasers are ideal solutions for these systems as they are highly reliable and enable long term stable operation. The design requirements for this application are very different than those commonly seen in fiber lasers. High-energy lasers often have low repetition rates (as low as one pulse every few hours), and thus high average power and efficiency are of little practical value. What is of high value is pulse energy, high signal to noise ratio (expressed as pre-pulse contrast), good beam quality, consistent output parameters and timing. Our system focuses on optimizing these parameters. Our prototype system consists of a mode-locked fiber laser, a compressed pulse fiber amplifier, a "pulse cleaner", a chirped fiber Bragg grating, pulse selectors, a transport fiber system and a large mode area fiber amplifier. We will review the system and present theoretical and experimental studies of critical aspects, in particular the requirement for high pre-pulse contrast.

We report on the generation of self-similar highly-stable femtosecond pulses from a side-pumped ytterbium-doped double-clad fiber laser. Positively-chirped parabolic pulses with 6.4 ps duration and more than 3.2 nJ energy are obtained. These pulses are extra-cavity compressed to 140 fs. The noise measurements using radio-frequency analysis show that this regime of emission ensures low-noise operation with less than 0.05 % amplitude fluctuations.

Heavily doped active fibers based on the soft phosphate glass offer an attractive gain medium for compact and high-power laser oscillators. We report a passively modelocked fiber oscillator at 1.5μm based on such active fiber. The standing-wave laser cavity consists of a 20cm-long piece of the side-pumped active phosphate fiber which is heavily co-doped with Er and Yb ions, and a low-ratio fused coupler. The length of the all-fiber laser cavity is 65cm. The modelocked operation of the oscillator is started and sustained by a Semiconductor Saturable Absorber Mirror (SESAM), and no additional pulse narrowing mechanism is used. In order to avoid a premature over-saturation of the SESAM, the fiber end which is butt-coupled to the SESAM is adiabatically tapered which expands the propagating fiber mode and decreases the power density incident on the absorber substantially. The stable modelocked operation of the laser oscillator occurs in the range between 0.65W and 2.3W of the average output power, which is limited by the maximum available pump power at 975nm. The peak pulse power is limited by the saturated SESAM at ~450W, and the pulse width grows from 11psec to 35psec as the pump power is increased. At the pulse repetition rate of 160MHz, the pulse energy reaches 14.4nJ. Our laser oscillator combines the convenience of the all-fiber construction with the power performance previously achievable only with the modelocked bulk-optic laser oscillators or more complex systems involving fiber amplifiers.

We report on the observation of both single pulse and bound states of an environmentally stable all-polarization maintaining (PM) mode-locked laser based on a saturable absorber. The laser operates in the self-similar regime, and parabolic pulse spectra were obtained. The pulses could externally be compressed to 212 fs (single pulse) and 248 fs (bound states). Results of a numerical model are also presented. The model reveals important information about the criteria for obtaining pulses with parabolic temporal shape.

We demonstrate a wide and fast wavelength-tunable mode-locked fiber laser based on tuning the mode-locking frequency. The laser is in a sigma-laser configuration and a wideband semiconductor optical amplifier (SOA) at 1.3 μm wavelength region is used as a gain medium. Mode-locking is achieved by direct modulation of the injection current to the SOA, and a dispersion compensation fiber (DCF) is used to provide desired intracavity dispersion. By tuning the modulation frequency, a wide tuning range over 100 nm is achieved. Lasing wavelength is measured to be in linearly proportion to the RF frequency applied to the SOA. The sweep rate over the entire wavelength range (100 nm) can be raised to be as high as 100 kHz.

We report a simple, environmentally-stable, passively mode-locked Yb-based fiber oscillator operating at 1035 nm with pulse duration of ~5 ps. Mode-locking was achieved using a saturable absorber mirror. The output of the laser exhibited a polarization extinction ratio >20 dB with the implementation a polarization maintaining fiber cavity and a polarization sensitive fiber coupler. The laser outputted near transform-limited pulses at 25-100 MHz at a pump threshold of 20-30 mW. We have tested operation of the laser using two different saturable absorber mirror structures: multiple quantum wells and quantum-dots at 1035 nm. Pulse properties and laser performance were comparable using quantum-dots and multiple quantum wells as the saturable absorber.

Coherent combining of laser beams into a single giant coherent beam with diffraction limited beam quality is one of the most challenging problems in laser physics. The effort given to this problem since the early sixties has been and continues to be enormous since this is the only viable way forward to further power scaling of lasers. The general idea of this approach is to split a single diffraction limited coherent beam into many beams which are then amplified by a parallel array of similar power amplifiers, the combined output from which will be like a single powerful diffraction limited beam. While there have been countless test-of-principle demonstrations at low power it is fair to say that none have had success in real high power systems. The main reason for this is the unavoidable and uncontrollable variation in phase shift (both linear and nonlinear) in the different amplification channels, rendering the combining process essentially incoherent even in a case when the original beam is perfectly coherent. The effect of spectral self-phase conjugation in SBS, recently discovered by the authors, offers a solution to this. We will discuss the basic principles of the effect and its main features and characteristics in regard to its application for coherent beam combining the output from a fiber amplifier array.

Phasing of two-channel cw master oscillator/power amplifier beams using an SBS phase conjugate mirror has been demonstrated. The phasing was achieved for a two-channel master oscillator/power amplifier system, which used a single-frequency Nd:YAG master oscillator, two parallel fiber amplifiers, and a fiber phase conjugate mirror in a double-pass configuration. The successful phasing of two cw amplifier beams with a fiber phase conjugate mirror greatly enhances the prospects for phasing of multiple laser amplifiers without complex servo-loop control systems.

In this paper, we present the results of a frequency domain analysis of the effect of coiling induced macro-bending on the phasing of the modes of 6 and 7-core large mode area photonic crystal fibers. These fibers may enable fiber laser CW power and pulse energy scaling due to their large effective mode area (~4000 square microns), however; phase differences between the cores degrade the output beam quality. We study the effects of bend axis, bend radius, and core geometry on the phasing of the fiber modes. Coiling causes the signals in the individual cores to suffer de-phasing relative to each other in a predictable manner. This allows the possibility of recovering phased output, and thus near diffraction limited beam quality, from amplifiers employing these fibers. Preliminary experimental results are also presented.

We have developed innovative high power polarized 1064nm pulsed fiber laser for efficient nonlinear frequency conversion and by frequency doubling its output generated 30W of average power with 68% conversion efficiency at 532nm. This new 1064nm pulsed fiber laser operates at 1.8MHz repetition rate with 1.3ns pulse duration and close to bandwidth-limited spectral linewidth. The developed laser delivers 46W average power at 1064nm in the linearly polarized output beam with a polarization extinction ratio 20dB. This laser has a truly solid-state design required for deployment into the industrial environment and can be used for nonlinear frequency conversion to generate high power emission in the visible and UV parts of optical spectrum as well as for other applications. The overall 16% efficiency demonstrated in generating 532nm is believed to be the highest wall plug efficiency achieved by any solid-state or fiber lasers with visible output. We expect that over 20% total efficiency for the fiber laser with tens of watts output in the green spectral range will be available.

We have demonstrated 3.5W of 589nm light from a fiber laser using periodically poled stoichio-metric Lithium Tantalate (PPSLT) as the frequency conversion crystal. The system employs 938nm and 1583nm fiber lasers, which were sum-frequency mixed in PPSLT to generate 589nm light. The 938nm fiber laser consists of a single frequency diode laser master oscillator (200mW), which was amplified in two stages to >15W using cladding pumped Nd³⁺ fiber amplifiers. The fiber amplifiers operate at 938nm and minimize amplified spontaneous emission at 1088nm by employing a specialty fiber design, which maximizes the core size relative to the cladding diameter. This design allows the 3-level laser system to operate at high inversion, thus making it competitive with the 1088nm 4-level laser transition. At 15W, the 938nm laser has an M² of 1.1 and good polarization (correctable with a quarter and half wave plate to >15:1). The 1583nm fiber laser consists of a Koheras 1583nm fiber DFB laser that is pre-amplified to 100mW, phase modulated and then amplified to 14W in a commercial IPG fiber amplifier. As a part of our research efforts we are also investigating pulsed laser formats and power scaling of the 589nm system. We will discuss the fiber laser design and operation as well as our results in power scaling at 589nm.

Pulses at 1178 nm were produced by pulsed Raman pumping at 1060 nm in an ytterbium doped fiber. Single-pass frequency doubling of the Raman pulses generated 1 W of average power at 589 nm.

Two-pump parametric devices using highly nonlinear optical fiber are reviewed. Physics of multiple-pump interaction is outlined and performance implications are illustrated. Multiple-band operation of two-pump devices is used for packet- and bit-scale optical switching at 40Gb/s. Generic two- and three-pump architectures are described, with applications in spectral and polarization switching.

The single mode fibers with chromatic dispersion varying along the length are attracting a considerable attention due to their value for optical soliton processing and applications in stable sources of ultra short optical pulses. In particular, dispersion decreasing fibers (DDF) have been recognized to be useful for high-quality soliton pulse compression and stable against pump noise continuum generation. The fibers with varying along length dispersion can have a lot of application in optical signal processing. The method to produce fibers varying along the length from standard preform had been developed. It is possible to draw fibers with a necessary length dependence on the diameter with high accuracy. During the drawing process information about the current diameter is processed by digital control unit and compared with a calculated value. The dispersion deviation from the prearranged value is less than 0.1ps/nm/km. High-quality pulse compression has been obtained in DDF. The compression factor is determined by the ratio of input to output dispersion and typically limited to about 20. Using DDF with optimum dispersion profile it is possible to generate pedestal-free pulses of less than 50fs duration. In addition, a new DDF design allows to increase the SBS threshold by 7dB over the conventional nonlinear fibers.

A Q-switched all-fiber laser application based on a novel micro-optical waveguide (MOW) on micro-actuating platform (MAP) light modulator is presented. A fused biconical taper (FBT) coupler acts as MOW, mounted on an electromechanical system, MAP, where an axial stress over the waist of FBT coupler is precisely controlled. The axial stress induced refractive index changes caused the coupling efficiency to result in modulation of optical power. The light modulator was implemented in a laser cavity as a Q-switching element. Q-switching of Yb³⁺-doped fiber laser was successfully achieved with the peak power of 192mW at 4.1W pump power and 699mW at 5.2W at the repetition rate of 18.6kHz. Further optimization of switching speed and modulation depth could improve the pulse extraction efficiency and the proposed structure can be readily applied in all-fiber Q-switching laser systems for marking applications.

Fiber lasers are attractive because they can avoid many problems associated with free-space optics. However, the large mode area required in high power fiber lasers and amplifiers causes several new difficulties in fiber and component design and in fusion assembly methods. Mode transformation technology allows optimization of the design of the gain fibers with independent control of the characteristics of the input and output beams. The quality of the output beam is decoupled from the mode shape in the gain fiber. Even with the desired signal properties, the high optical powers and multimode pump operation cause new issues with reliability of the fused fibers and components. To achieve high reliability operation, care must be taken to minimize and dissipate stray light.

Light absorption in structural adhesives constitutes the main source of heat in tapered fused bundle (TFB) devices. Efficient heat dissipation solutions were developed by studying these thermal loads. The relative merits of transparent vs. opaque package designs were established experimentally. In the former, light escapes without being absorbed by the package walls, whereas in the latter, the spurious optical signal is directly absorbed and dissipated. The fact that heat is generated directly in the adhesive largely favors the opaque package, which offers more efficient heat extraction. By using a thermally conductive package, a temperature rise of 1.1°C per Watt of dissipated power was measured. These numbers demonstrate that passive heat sinking at 20°C is sufficient to allow reliable operation up to 45Watts of dissipated power (1kW with 0.2dB optical loss) without compromising long-term reliability.

In this paper we examine the various components that conceivably may be used in fiber lasers, together with a suggested Taxonomy for testing these classes of parts. These classes include passives, actives and active modules. Test protocol is suggested for qualification based on current methodologies employed in the fiber optic communications industry, but adapted to space conditions. These modifications include the additional environmental conditions imposed by space, namely thermal, vibration and radiation. Additionally, this test protocol is verified by executing a series of vibration, thermal (including vacuum) and radiation tests to examine its validity. A selected set of recently-developed commercial off-the-shelf fiber optic components (at 106x nm) are chosen for these tests. These include doped fibers, combiners, sources, pumps, isolators and fiber Bragg gratings. The scope for this work is limited to the environmental conditions of lower Earth orbit satellites, 100 to 1000km orbital altitude and up to 60 degrees inclination.

Fiber-integrated high power fiber lasers (HPFLs) have demonstrated remarkable levels of parametric performance, efficiency, operational stability and reliability, and are consequently becoming the technology of choice for a diverse range of materials processing applications in the "micro-machining" domain. The design and functional flexibility of such HPFLs enables a broad operational window from continuous wave in the 100W+ power range, to modulated CW (to 50kHz prf and above), and to quasi-pulsed operation (kW/μs/mJ regime) from a single design of laser system. A long-term qualification program has been successfully completed to demonstrate the robustness and longevity of this family of fiber lasers. In this paper we report for the first time on the power-scaling extension of SPI's proprietary side-coupled cladding-pumped GTWave^TM technology platform to output power levels in the multi-hundred watt domain. Fiber and system design aspects are discussed for increasing both average power and peak power for CW and quasi-pulsed operation respectively whilst maintaining near-diffraction limited beam quality and mitigating non-linear effects such as Stimulated Raman Scattering. Performance data are presented for the new family of laser products with >200W CW output power, M² ~ 1.1 and modulation performance to 50kHz: Furthermore, the modular, flexible approach provided by GTWave^TM side-pumped technology has been extended to demonstrate a two-stage MOPA operating at >400W.

A new scheme for suppression of Stimulated Brillouin Scattering (SBS) in high power fiber amplifiers is proposed where the fiber core diameter varies along the fiber length. The fiber has an ultra-large core diameter at most locations to suppress SBS, while at certain locations the core is relatively small to reduce bending sensitivity. A numerical model based on SBS rate equations is used to compare the SBS threshold of a uniform fiber to that of a fiber with variations in physical properties vs position. The model takes into account SBS gain dependence on temperature distributions as well as inhomogeneous spectral broadening due to NA variations. The modeling results show that the SBS threshold can be increased significantly when the seed power, fiber length, pump configuration, and fiber parameters are optimized. As a result, a single frequency fiber amplifier can generate up to 1 kW output power without the onset of SBS. Furthermore, the nonuniform fiber allows a high power amplifier to be packaged in a much more compact enclosure than an ultra-low NA fiber based amplifier.

The design and optimization of high-power fiber lasers and amplifiers requires a detailed understanding of several important physical processes, both linear and nonlinear. The influence of bending on the overlap of the propagating mode as well as its resistance to deleterious nonlinear effects such as self-focusing must be accurately predicted. To this end we have developed a number of models, both analytic and numerical, that allow us to treat these effects in detail.

Many high power fiber laser applications require doped fibers having large mode area but still working in the single mode regime. The most common techniques to keep a large mode area fiber in the single mode regime are to reduce the core numerical aperture, to strip the high order modes by coiling the fiber, to launch only a single transverse mode, or to use photonic crystal fibers. All these methods have limits and disadvantages. In this paper we demonstrate by simulation the effectiveness of another method to suppress the high order modes in large mode area active fibers by optimizing the rare earth dopant concentration across the core while keeping the step index structure of the core of the fiber. This method was not previously employed because the traditional doped fiber manufacturing technologies do not have the required capability to radially control the dopant concentration. However, Direct Nanoparticle Deposition (DND) can be used to manufacture large mode area fibers having any radial distribution of active element concentration and any refractive index profile. Thus, DND fibers can be designed to benefit from this high order mode suppression technique. The simulation results presented in this paper have been obtained using Liekki Application Designer v3.1, a software simulator for fiber lasers and amplifiers.

Along with rapid progress in characteristics of single-fiber lasers much attention is paid these days to the problem of phase locking of multi-core fiber lasers. It is known that only strong-coupled system has a chance to be phase locked despite non-identity of channels. In case of the strong inter-channel coupling the traditional theoretical approach based on expansion over modes of individual cores fails. We have developed 3D diffraction numerical code, and results of its implementation to simulations of 7-core hexagonal fiber structure experimentally studied recently are reported. Detailed analysis of light propagation in this system, where self-organization effect was observed, shows that the dominant mechanism of coherence formation in an array of co-propagating beams is mode spatial filtering produced by the gain assembly. Non-linear resonance refraction exerts a weak influence on appearance of self-organization. The ways to achieve better selection of a single array mode are discussed. For the first time, the tolerance limits on micro-core parameters for keeping phase-locking are numerically found.

We report the investigation of stimulated Raman Scattering (SRS) in a standard fiber using a directly modulated DFB laser with wavelength equal to 1549 nm, amplified by a two stage Erbium-doped fiber amplifier. The amplifier provides amplification of 2-mW peak power input pulses to 100-W peak power output pulses. This setup provides a simple source of high power pulses for nonlinear investigation. However, the directly modulation of DFB lasers causes transient oscillations at the beginning of pulses. In our case pulses consisted of a 2-ns transient part followed by a steady-state plateau. We used a monochromator to measure the spectrum at the fiber output. A fast photodetector was placed at the monochromator output and pulse shapes were measured for different wavelengths. This technique allows the separate measurement of the spectrum of different parts of output pulses. We used the SMF-28 fiber with the standard dispersion of 20 ps/nm-km for our wavelength. We made measurements of the output spectra for four fiber lengths: 210 m, 600 m, 4.5 km, and 10 km. For short fiber lengths (210 m and 600 m) the spectra of the transient part and the plateau part were similar and manifest modulation instability (MI) and four-wave mixing (FWM). For 4.5-km long and 10-km long fibers we found that the transient part generates a continuous broadband spectrum while the plateau causes the Stokes wave with the spectrum maximum centered at 1656 nm.

Optically harmonic mode-locking and femtosecond compression of a semiconductor optical amplifier fiber laser (SOAFL) induced by backward injecting a dark-optical-comb pulse-train at 10 GHz is demonstrated for the first time. The injected dark-optical-comb with 25-ps pulsewidth can be generated by seeding tunable laser into a Mach-Zehnder intensity modulator (MZM) at DC biased point of 0 V. The MZM is driven by a commercial electrical comb generator under an input microwave power of 28 dBm at repetition frequency of 10 GHz. Theoretical simulation indicates that the backward injection of dark-optical-comb results in a wide gain-depletion width (as well as a narrow gain window of ≤25 ps) within one modulation period, providing a cross-gain-modulation mode-locking of the SOAFL with a shortest pulsewidth of 5.4 ps at 10 GHz. The difficulty in mode-locking the SOAFL by an optical short pulse (bright-optical comb) injection is also demonstrated and elucidated by the insufficient gain-depletion time (as well as modulation depth). After propagating through a 75m-long dispersion compensating fiber (DCF), the negatively chirped SOAFL pulsewidth that is mode-locked by the backward injected dark-optical-comb can be linearly dispersion-compensated and slightly shortened to 3.9 ps. By increasing the peak power of the dispersion-compensated SOAFL pulse to 51 W and propagating it through a 76.7m-long single mode fiber (SMF), an eighth-order nonlinearly soliton compression is achieved with the pulsewidth, linewidth, and time-bandwidth product of 560 fs, 4.5 nm and 0.31, respectively.

We previously reported that FM mode-locked erbium fiber ring laser could be stabilized with high supermode noise suppression ratio when a semiconductor optical amplifier (SOA) is inserted in the cavity to act as a fast saturable gain medium even in the presence of normal dispersion. In this paper, we further investigate the noise characteristics including relaxation oscillation, timing and energy jitter in both dispersion region when FM mode-locked pulse oscillate in an erbium fiber ring laser with a SOA.

We report that simple wavelength switching technique based on actively mode-locked Fabry-Perot laser diode (FPLD) coupled with high dispersion external cavity by slightly detuning the modulation frequency. When dispersion compensating fiber (DCF) with dispersion, -100ps/km/nm at 1550 nm, is used for timing filter, peak wavelength of the optical pulse switched from the shorter FP longitudinal mode to the longer FP longitudinal mode. Wavelength switching from the longer FP longitudinal mode to the shorter FP longitudinal mode was achieved by adopting a length of single mode fiber with dispersion, 16ps/km/nm at 1550 nm.

We report a wideband and almost flat spectrum supercontinuum (SC) generation by using a passive mode-locked Er-doped fiber laser (PML-EDFL) and a highly nonlinear dispersion shifted fiber (HN-DSF). The fiber laser pulses are characterized with a conventional second harmonic generation frequency resolved optical gating (SHG-FROG) method. The dependence of the SC spectral width on the HN-DSF length, the spectral stability of the generated SC spectrum, and conditions for obtaining smooth spectra without a fine structure are investigated experimentally. In this paper, wide spectrum covering 1100-1750 nm wavelength range is generated with only 8.5 cm long HN-DSF when laser is single pulse operation regime. In addition, the flat spectrum with ±1 dB uniformity was obtained at the wavelength region of 1130-1510 nm. And spectral stability is about ±0.2 dB at the uniformity wavelength region. We believe that our proposed all-fiber based SC source has many important applications in recently developed frequency-domain measurement techniques such as optical coherence tomography (OCT), Optical frequency domain imaging (OFDI), optical frequency domain reflectrometry (OFDR), and their instrumentation.

A linearly-polarized, 977 nm pulsed laser capable of 1 kW, 15 μJ output has been demonstrated. The laser is based on Yb³⁺-doped fiber technology, is core pumped and has a monolithic, all-fiber design. A 13 dB polarization extinction ratio was observed at the maximum measured output power. The output performance of the laser is pump-limited and shows no sign of non-linear effects at the demonstrated output powers. The laser emission is inherently near-diffraction limited due to the single-mode nature of the fibers used.

The high gain offered by Erbium doped fiber amplifiers has, since its first demonstration, been explored in lasers and super fluorescence sources. Although such devices have been the topic for numerous scientific publications only a few configurations have resulted in commercial products. We have identified the principal reasons for this to be the difficulty in obtaining single longitudinal mode laser operation in the inherently long laser cavities and the tendency of superfluorescence fiber sources (SFS) to show spurious lasing in high power operation. In this manuscript we show some results of our effort to deal with these obstacles. A technique based on a saturable absorber (SA) grating filter is shown to assure stable single longitudinal mode lasing even when the laser cavity is subject to temperature variations. The saturable absorber filter has a narrow passband and dynamically tracks the lasing mode. An all- polarization maintaining (PM) fiber ring cavity in combination with a saturable absorber filter provides a solution for stable single mode, single polarization laser operation. Progress on amplified spontaneous emission (ASE) superfluorescence sources is fueled by improvements in available pump power ratings. However spurious lasing is limiting the spectral power density of the broadband emission. We present techniques based on tailored optical feedback using filtered ASE seeding or Faraday rotator (FR) mirror which increases lasing threshold and thus the achievable output power. These advances have allowed the manufacture of fiber optic sources which maintain their performance parameters over time even when subject to temperature and vibration perturbations found in real applications.

We frequency doubled, tripled, and quadrupled linearly-polarized, 1062nm-wavelength, 1ns-long, ~9.6kHz-repetition-rate pulses from an Yb-doped photonic crystal fiber amplifier. We obtained pulse peak/average powers of 410kW/4W at 531nm, 160kW/1.5W at 354nm, and 190kW/1.8W at 265.5nm.

We report passive harmonic mode locking of a high-power Yb-doped double-clad fiber laser operating in both the normal- and the anomalous-dispersion regimes with a fundamental repetition rate of 20.4 MHz. In the anomalous-dispersion regime (total cavity GVD of -0.1 ps2), 1-ps, 125-pJ pulses are emitted at a repetition rate of 408 MHz. When the total net dispersion is close to zero (about -0.004 ps2), 680 fs, 48 pJ pulses are emitted at a repetition rate higher than 2 GHz. The supermodes suppression is than about 25 dB. In the normal-dispersion regime (total cavity GVD of +0.047 ps2), 116-fs, 1.7-nJ pulses are emitted at a repetition rate of 102 MHz with a supermodes suppression of more than 60 dB. We also report a new regime of multiple pulsing emission observed with this fiber laser : the stable emission of two pairs of bound pulses exhibiting different time separations and uniformly separated in the same cavity round trip. Keywords: Harmonic mode locking, multiple pulsing, bound states.