This PDF file contains the front matter associated with SPIE Proceedings Volume 6873, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

High-power fiber lasers and amplifiers have gained tremendous momentum in the last five years, and many of the traditional manufactures of gas and solid-state lasers are pursuing the attractive fiber-based systems, which are now displacing the old technology in many areas. High-power fiber laser systems require specially designed fibers with large cores and good power handling capabilities - requirements that are all met by the airclad fiber technology. In the present paper we go through many of the building blocks needed to build high-power systems and we show an example of a complete airclad laser system. We present the latest advancements within airclad fiber technology including a new 70 μm single-mode polarization-maintaining rod-type fiber capable of amplifying to MW power levels. Furthermore we describe the novel airclad based pump combiners and their use in a completely monolithic 350 W CW fiber laser system with an M² of less than 1.1. Finally, we briefly touch upon the subject of photo darkening and its origin.

We present the first direct measurements of enhanced nonlinearities in large-mode-area fibers due to bend induced reductions in effective area. Both Raman scattering and self-phase modulation are observed to increase in tightly coiled fibers. The measured increase in nonlinearity compares well with predictions from simulations of the modal effective area.

Most of the current large mode area (LMA) fibers are few-moded designs using a large, low numerical aperture (N.A.) core, which promotes mode coupling between core modes and increases bending losses (coupling with claddingmodes), which is undesirable both in terms ofmode area and beamquality. Furthermore, short LMA fiber lengths and small cladding diameters are needed to minimize nonlinear effects and maximize pump absorption respectively in high-power pulsed laser systems. Although gain fiber coiling is a widely used technique to filter-out unwanted modes in LMA fibers, coupling between modes can still occur in component leads and relay fibers. In relay fiber, light coupled into higher-order modes can subsequently be lost in the coiling or continue as higher-order modes, which has the overall effect of reducing the effective transmission of the LP₀₁ mode and degrading the beam quality. However, maximum transmission of the LP₀₁ mode is often required in order to have the best possible beam quality (minimal M²). Launching in an LMA fiber with a mode field adapter (MFA)¹ provides an excellent way of ensuring maximum LP₀₁ coupling, but preservation of this mode requires highmodal stability in the output fiber. Small cladding, low N.A. LMA fibers have the disadvantage of being extremely sensitive to external forces in real-life applications, which is unwanted for systems where highly sensitive mode coupling can occur. In this paper, we present a detailed experimental and theoretical analysis of mode coupling sensitivity in LMA fibers as a function of fiber parameters such as N.A., core diameter and cladding diameter. Furthermore, we present the impact of higher N.A. as a solution to increase mode stability in terms of its effect on peak power, effective mode area and coupling efficiency.

We report our recent progress in the design and fabrication of a completely monolithic linearly-polarized pulsed Yb-doped fiber laser, with >10kW peak power, tunable 2ns-0.2μs pulse duration, tunable 50kHz-50MHz repetition rate and 50W average power in a diffraction-limited, linearly polarized and stabilized 0.8nm line-width output beam operating at 1064nm. The innovative all-fiber design of the laser is desirable for deployment in industrial applications. A wide range of independently-tunable pulse durations and repetition rates make this laser capable to address a large variety of laser applications, including high-power nonlinear wavelength conversion processes, LIDAR, etc.

We report on the pulse shape of an actively Q-switched fiber laser. This master oscillator power amplifier architecture generates pulses with multiple peaks due to its intrinsic dynamics. Modeling and experimental results provide us a detailed understanding of the relative importance of the different time constants on the dynamics of the laser, which allows us to define optimized design parameters that lead to smooth and controlled pulse shapes. This solution is simple and robust; operation over a broad range of repetition rate and output power is achieved without any adjustment of the laser settings, and the corresponding variation of the optical performances is minimal.

Space weather, the study of the Earth's upper atmosphere and forecasting its response due to solar events, depends on knowledge of the state parameters of the neutral and ionized upper atmosphere. In this work, we present a ground-based diode-seeded, high-power, narrow-linewidth Yb-fiber amplifier-based lidar operating at 1083 nm for measuring temperature and density of the neutral atmosphere from 300-1000 km. The current state of the lidar system will be addressed, as well as ongoing work to increase 1) signal to noise ratio through power scaling and 2) spatial resolution and wind measurement capability via pulsed operation.

A four-stage all-fiber single-frequency single-mode continuous-wave (cw) master-oscillator power-amplifier (MOPA) at 1083 nm is presented. Small mode area (SMA) and large mode area (LMA) amplifier stages are mode matched with a fiber mode converter (MC) and the signal and pumps are combined with tapered fiber bundles (TFBs). The final power stage uses a LMA Yb doped SBS-suppressing fiber. A single-frequency output power of 194 W is demonstrated with optical net and slope efficiencies of 73% and 80%, respectively. Numerical simulations for the signal output power and the SBS-induced Stokes backscattered power in the 4th stage amplifier agree with the experimental results. Pulse amplifier measurements showed a 400 W peak power output that was limited by the forward output ASE. The SBS reflectivity at 400 W output was only 2.75 x 10^-4.

11.2 dB suppression of stimulated Brillouin scattering (SBS) in an Yb-doped, Al/Ge co-doped large mode area (LMA) gain fiber is demonstrated with a ramp-like acoustic index profile exhibiting an acoustic index contrast of 0.09 and acoustic index slope of 0.01/μm.

Previous research has shown that temperature gradients along a fiber can broaden the Stimulated Brillouin Scattering (SBS) gain profile and thereby increase the SBS threshold. However, within practical temperature ranges this method has been limited to SBS thresholds of a few hundred Watts. It is also well known that strain gradients applied to a fiber can broaden the SBS resonance. To suppress the SBS threshold to kW levels in fiber amplifiers of length ~5 m requires broadening of the SBS resonance width to ~1 GHz, which can be achieved with a strain of 1 - 2%. Although tensile strain is generally limited by fiber failure to less than ~1%, compressive strain has been employed to the level of many percent in a number of applications in the tuning of fiber Bragg gratings. We demonstrate the effect of SBS gain broadening and suppression by strain gradients at high power (~ 190 W) for the first time to our knowledge, and explore scaling of this method to kW output levels.

This article will review several new modes of pulse formation and propagation in fiber lasers. These modes exist with large normal cavity dispersion. Self-similar evolution can stabilize high-energy pulses in fiber lasers, and this leads to order-of-magnitude increases in performance: fiber lasers that generate 10-nJ pulses of 100-fs duration are now possible. Pulse-shaping based on spectral filtering of a phase-modulated pulse yields similar performance, from lasers that have no intracavity dispersion control. These new modes feature highly-chirped pulses in the laser cavity. Instruments based on these new pulse-shaping mechanisms offer performance that is comparable to that of solid-state lasers but with the major practical advantages of fiber.

Femtosecond fiber lasers are currently of great interest due to their small size, stable operation, long lifetime and low cost compared to bulk lasers. However, for operation in the 1 µm wavelength range of Yb lasers, a major obstacle has been the lack of suitable fibers with anomalous dispersion that can compensate for the normal dispersion of the conventional active and passive fibers used. However, a new promising fiber device using a higher order mode (HOM) with anomalous dispersion in the 1 μm range has recently been demonstrated. The device comprises integrated all fiber mode converters based on long period gratings (LPG), and hence has the potential to be low loss and easy to splice, while offering a large effective area, and the possibility of third order dispersion compensation. In this paper, optimization of HOM fibers with anomalous dispersion in the 1 μm range has been investigated theoretically and experimentally. Fibers with dispersion coefficients ranging from +50 to +300 ps/(nm·km) at 1060 nm have been fabricated and devices including integrated LPG mode converters have been characterized. Modeled and measured properties of the modules, such as dispersion, grating bandwidth etc., are found to correlate well. It is shown that there is a tradeoff between a high dispersion coefficient and the bandwidth of LPG mode converters. The characteristics of such HOM devices have been studied in a linear, passively mode-locked laser-cavity using SESAM as saturable absorber.

We report on the generation of 265 nJ ultra-short pulses from a mode-locked Ytterbium-doped short-length large-mode-area fiber laser operating in the dispersion compensation free regime. The self-starting oscillator emits 2.7 W of average power at a pulse repetition rate of 10.18 MHz. The pulses have been compressed down to 400 fs, corresponding to 500 kW peak power. Numerical simulations confirm the stable solution and reveal the mechanisms for self-consistent intra-cavity pulse evolution. The pulse energy is one order of magnitude higher than so far reported for fiber oscillators in the 1 μm wavelength region. To our knowledge this is the first time that mode-locked fiber oscillators can compete in terms of pulse energy and peak power with most advanced bulk solid-state femtosecond lasers.

We report on a all-normal dispersion mode-locked fiber laser based on a large-mode-area ytterbium-doped microstructure fiber and using a high nonlinear modulation depth semiconductor saturable absorber mirror (SESAM). The laser delivers 3.3 W average output power with positively-chirped 5.5 ps pulses at a center wavelength of 1033 nm. The pulse repetition rate is 46.4 MHz, which results in an energy per pulse of 71 nJ. These pulses are extra-cavity de-chirped down to 516 fs using bulk gratings. The average power of the de-chirped pulses is 2.3 W, which corresponds to a peak power of more than 96 kW.

The possibility to build up an optical source of femtosecond pulses that are smoothly tuned in the telecommunication range using a dispersion-decreasing fiber is demonstrated. The smooth tuning is based on the Raman frequency conversion of ultrashort pulses, which can be effectively tuned due to the compression mechanism for maintaining of a relatively high pulse intensity in the medium with a monotonically decreasing anomalous dispersion. The generation of a 90-fs soliton pulse whose wavelength is smoothly tuned in the wavelength range 1.55 - 1.85 μm is experimentally demonstrated.

Our objective is to understand the mechanism that generates catastrophic optical damage in pulsed fiber amplifiers. We measured optical damage thresholds of bulk fused silica at 1064 nm for 8 ns and 14 ps pulses. The 8 ns pulse is single longitudinal mode from a Q-switched laser, and the 14 ps pulse is from a Q-switched mode-lock laser. The beams in both cases are TEM₀₀ mode, and they are focused to a 7.5 μm spot inside a fused silica window. The pulse-to-pulse energy variations are 1% for 8 ns pulses and 5% for 14 ps pulses. Under these conditions optical damage is always accompanied by plasma formation at the focal spot; we found the damage threshold fluences are 3854 ± 85 J/cm² for the 8 ns pulses and 25.4 ± 1.0 J/cm² for the 14 ps pulses. These fluences are corrected for self focusing. Both damage thresholds are deterministic, in contrast to the claim often made in the literature that optical damage is statistical in the nanosecond range. The measured damage threshold fluences for 8 ns and 14 ps pulses do not fit a square root of pulse duration scaling rule. We interpret the damage in terms of plasma formation initiated by multiphoton ionization and amplified by an electron avalanche. The damage threshold irradiance can be matched with a simple rate equation model that includes multiphoton ionization, electron avalanche, and electron-hole recombination. The damage morphologies are dramatically different in the nanosecond and picosecond cases because of the large difference in deposited energy. However, both morphologies are reproducible from pulse to pulse. We also measured surface damage thresholds for silica windows polished by different methods. We find that cerium oxide polished surfaces damage at approximately 40% of the bulk threshold, with a large statistical spread. Surfaces prepared using an Al₂O₃ polish damaged between 50% and 100% of the bulk damage limit, with a substantial fraction at 100%. Surfaces polished using first the Al₂O₃ polish and then an SiO₂ polish exhibit surface damage values equal to the bulk damage value at nearly every point. We also measured damage thresholds for different sized focal spots. Some earlier reports have claimed that damage thresholds depend strongly on the size of the focal spot, but we find the surface threshold is independent of the spot size.

Critical power, nonlinear guided stationary mode and transient dynamics in nonlinear waveguides are studied analytically. Under Gaussian mode approximation, critical power for nonlinear self-focus is derived analytically and is found to be independent of waveguide parameters. Nonlinear guided stationary mode is found to be the stable solution of nonlinear waveguide below critical power for nonlinear self-focus and has a reduced mode size dependent on optical power and V value of the waveguide. Equation governing transient dynamics of the nonlinear waveguide modes is also found. Transient dynamics scale with Rayleigh Raleigh range similar to that in bulk media. Mode is found to evolve adiabatically towards the local stationary mode in a fiber below self-focus limit. Mode will collapse to a singular point at a self-focus distance at and above critical power. Larger V value increases transient and self-focus distance. The self-focus distance becomes proportional to square root of optical power at higher power level and independent of V value. B integral are calculated for various amplifiers considering the impact of gradual collapse of beam size along the amplifier.

We report to the best of our knowledge for the first time on the fabrication and characterization of CO₂-laser written long-period gratings in a large-mode area photonic crystal fiber possessing a core diameter of 25 μm. The gratings have low insertion losses (<1 dB) and high attenuation (>10 dB) at the resonant wavelengths, making them particularly interesting for high power applications.

Recent advances in the field of phosphate glass fiber lasers are reviewed. Fabrication of microstructured fiber and writing of fiber Bragg gratings in passive and active phosphate glass fiber are demonstrated. Based on these novel components we fabricate cm-long, Watt-level fiber lasers that allow for tunable, single longitudinal mode operation.

We have developed a new technique to produce a Yb-doped fused silica bulk glass which is very well suited for fiber laser applications. The starting point is a liquid suspension of SiO₂ particles which is doped by a solution of rare earth ions. After dehydration, purification and vitrification we achieve a bubble-free homogeneous Yb-doped fused bulk silica, which is further processed by the plasma outside deposition (POD) technique into preforms for active laser fibers with a large active fiber core. The laser function of our Yb-doped silica was successfully proved in a side-pumped fiber laser setup. We present the results of the laser experiments.

This paper considers the development of high-power laser-DEW systems required to achieve significant effects on potential targets. The emphasis is on the progress with the technology and systems required to support the out-of-band approach needed to achieve a robust defeat mechanism; various laser-induced effects are discussed. The advantages and disadvantages of laser-based directedenergy weapon (DEW) systems will be reviewed. Evolution of laser-source technology is reviewed, with a particular emphasis on modern approaches such as the various novel-source investigations, including fibre-bundle devices. The paper discusses some of the research studies that are underway that may make this type of laser-weapon system viable. Some of the challenges currently faced by laser-based DEW systems are considered in this paper.

Spectral beam combining (SBC) has been extensively used for power scaling of laser systems. SBC is an incoherent technique of combining laser radiation from multiple sources with offset wavelengths into a single near-diffractionlimited beam with increased energy brightness. SBC by means of volume Bragg gratings (VBGs) recorded in photo-thermo- refractive (PTR) glass has been shown to be a simple and robust technique for combining high-power laser radiation. High-efficiency large-aperture VBGs were fabricated in PTR glass wafers. While being photosensitive in the UV, PTR glass offers high transmittance in the near-IR and visible parts of spectrum. Excellent mechanical properties and refractive index independent of temperature enable VBGs in PTR glass to withstand high-power laser radiation, making them ideal elements for high-power SBC. We report spectral combination of five randomly polarized fiber lasers with 0.5 nm spectral separation between channels around 1064 nm using reflecting VBGs in PTR glass. Maximum output power of the system is 773 W, corresponding to 91.7% combining efficiency. It is shown that VBGs introduce no significant beam distortions under high-power operation. Additionally, a common-cavity configuration for SBC with automatic wavelength control of sources by intra-cavity VBGs is suggested. Two fiber lasers are combined using this technique and automatic wavelength control is demonstrated. We show how simple power scaling allows obtaining multi-kW near-diffraction-limited laser radiation via SBC with volume Bragg gratings in PTR glass.

Theoretical and numerical analyses are presented of the passive coherent phasing of an array of fiber lasers that are combined in a single laser cavity by an Nx1 coupler. In an initial linear analysis it is found that the brightness gain of the passive coherent array grows linearly for a small number of fibers, however, for practical parameters, the coherent brightness gain saturates at ~ 8 - 12 for large arrays. An intensity dependent index (Kerr) nonlinearity is then introduced and it is shown that the expected maximum improvement is modest, with the coherent brightness gain saturating at 10 - 14, depending on the strength of the nonlinearity. These results are compared with recent experiments.

Coherent beam combining of fiber amplifier arrays is a promising way to increase power of fiber lasers, and overcome the physical limitations to fiber laser power scaling. We performed the coherent combining of fiber amplifier arrays using active control of the phase of each amplifier. The phase fluctuations in the fiber amplifiers have been measured and their effect on the beam combining process stability evaluated. We extended the coherent beam combining technique to perform wavefront shaping, in order to deliver a high brightness beam after turbulent atmospheric propagation. We present experimental results exhibiting the capability of the modulation multiplexing technique that we implemented to compensate phase fluctuations due to turbulent atmospheric propagation on the laser beam path. Moreover, and for the first time to our knowledge, we demonstrate automatic coherent combining of fiber amplifiers on a diffuse surface, after propagation through turbulent atmosphere, without any external turbulence measurement subsystem.

We show spectral combination of pulsed fiber laser systems for the first time to our knowledge. In this proof of principle experiment, two directly modulated wavelength-stabilized tunable external cavity diode lasers (ECDL) serve as independent seed sources. Each signal is amplified in a two stage ytterbium-doped fiber amplifier. The spatial overlap is created using a transmission grating with a combining efficiency as high as 92 %. No beam quality degradation has been observed for the combined beam compared to a single emission. An electronic delay is used to adjust the temporal overlap of the pulses from the spatially separated amplifier setups. The presented approach offers an enormous scaling potential of pulsed fiber laser systems, which are generally limited by nonlinear effects or fiber damage. We show that the huge gain bandwidth of Yb-doped fiber amplifiers and the high diffraction efficiency of dielectric reflection gratings in this wavelength range yield potential for a combination of up to 50 channels. For state-of-the-art ns-amplifier systems > 100 MW of peak power, > 100 mJ of pulse energy and average powers of > 10 kW seem feasible.

This paper describes the development of a measuring equipment capable of analysing the beam profile at high optical powers emitted by delivery fibers used in manufacturing processes. Together with the optical delivery system, the output beam quality from the delivery fiber and the shape of the focused spot can be determined. The analyser is based on the principle of a rotating wire being swept though the laser beam, while the reflected signal is recorded [1]. By changing the incident angle of the rotating rod from 0° to 360° in relation to the fiber, the full profile of the laser beam is obtained. Choosing a highly reflective rod material and a sufficiently high rotation speed, these measurements can be done with high laser powers, without any additional optical elements between the fiber and analyzer. The performance of the analyzer was evaluated by coupling laser light into different fibers, and measuring the output beam profiles. Fibers with different core diameters and different surface qualities were tested.

An analytical approach for the thermal design of high-power-fiber laser components is presented. The modular structure of the model allows adaption to different fiber designs and gives insight into the governing parameters of the heat transport. Furthermore the analysis and analytic optimization of interacting effects of groups of layers is possible with this method and is presented in this work. A previously suggested cooling scheme for a heat load of 120 W/m is analyzed. Applying the analysis to air-clad-fibers is leading to results differing up 40 % from previous works. The FEM-analysis of the cooling of splices shows that the cooling scheme suggested for the active fiber is not sufficient for splices for a fiber resonator in the kW-range. Using a one-dimensional model it can be shown that if a small percentage of loss in the splice is absorbed inside the recoat, it is necessary to reduce the recoat thickness.

We report a 29 W CW supercontinuum spanning from 1.07 μm to 1.67 μm with a spectral power density of 50 mW/nm up to 1.4 μm. The continuum is produced in a short length of photonic crystal fiber (PCF) with two zero dispersion wavelengths. Ultimately the second zero limits the long wavelength edge of the continuum. We also find that despite using much shorter lengths of PCF the affects of OH^- absorption are still visible in the supercontinuum produced.

Supercontinuum generation in highly nonlinear fibers (HNLF) pumped with femtosecond pulses is an area of large interest for applications such as broad band light sources, tunable femtosecond sources, frequency metrology, and fluorescence microscopy. In the last few years, a lot of focus has been put on optimizing photonics crystal fibers for supercontinuum application. In this paper, we will focus on conventional silica based HNLF, which e.g. have the advantage of precise dispersion control, and easy splicing to standard single mode fibers. We have performed a systematic experimental study of the effect of dispersion, of the HNLF as well as the input power to the HNLF. To pump the fiber we have build an femtosecond fiber laser consisting of a passive mode locked figure eight oscillator followed by an amplifier. The dispersion before coupling into the HNLF was optimized for broadest supercontinuum generation. Supercontinuum generation in both standard and polarization maintaining HNLF are studied.

A model description of photo darkening based on 30 characterised fibres in an un-seeded amplifier setup is presented. Photo darkening of ytterbium / aluminium and/or phosphorous co-doped silica fibres is found to saturate following prolonged exposure to pump radiation. The photo darkening is associated with non-binding oxygen at surfaces of ytterbium / aluminium clusters. The dominant colour centre at near infrared wavelengths in MCVD material is a combination of 1.9 eV (FWHM of 0.62 eV) and 2.4 eV (FWHM of 0.85 eV) absorption dependent on average phonon energy of the glass material.

We report the measurement of photodarkening in single-mode phosphate fibers with a Yb³⁺ concentrations of 7.1 × 10²⁶ m^-3 and 1.42 × 10²⁷ m^-3 (6 wt.% and 12 wt.% Yb₂O₃).We compare the photodarkening resistance of these phosphate fibers with that of three single-mode Yb³⁺-doped silica fibers. Our data shows that under strong pumping conditions, phosphate fibers allow Yb³⁺ concentrations that are least 6 times greater than the most photodarkening-resistant silica fibers to date without the onset of photodarkening at 660 nm.

A strong charge-transfer band at UV-wavelengths is found to play a major role for the observed induced optical losses (photodarkening) in ytterbium doped high-power fiber lasers. This is correlated to the valence stability of the ytterbium ion in the silicate glass matrix, which we believe is the origin of photodarkening. We have performed UV-irradiation experiments on ytterbium-doped preform samples and accelerated photodarkening experiments on Yb-doped fibers, by using 915 nm high power diodes. Our results show that photodarkening can be reduced, to low levels, either by preparing the preform glass in a reducing atmosphere or by hydrogen loading the fiber in a pressure chamber at room temperature.

Gamma-radiation-induced photodarkening has been observed and characterized in a suite of Yb-doped, Er-doped and Yb/Er co-doped optical fibers. Significant reduction in the optical transmission of the fibers under passive (not pumped) conditions was observed for wavelengths across the infrared spectrum. In general, it was found that the co-doped fiber tested showed the strongest radiation resistance whereas the Er-doped fibers tested exhibited the greatest radiation sensitivity. A dependence on dose rate was also observed in all fibers.

We investigated the influence of the atmosphere during preform collapsing (Cl₂/O₂, He, reducing gases as CO and H₂) on the photodarkening process in ytterbium doped silica fibers. The measurements were performed for a probe wavelength of 633nm in-situ during cladding pumping at 915 nm in dependence on the Yb inversion. The equilibrium values of the core excess loss were found to be remarkably lower in the fibers from preforms collapsed with the treatment of He or reducing gases, whereas the photodarkening rate constants are rather similar. Subsequent measurements of the fluorescence properties (pump wavelength 915nm or 976 nm) were carried out at the photodarkened fibers and compared to earlier results obtained at fibers as drawn. Laser experiments with the different fibers at low Yb inversion (no photodarkening) show a decrease of the laser slope efficiency in parallel to the degree of reduction of the doped core glasses. For the He treatment, an optimum of lowered photodarkening loss and reasonable laser efficiency can be obtained.

We demonstrate a chirped-pulse amplification system generating 25 μJ compressed pulses at a center wavelength of 1552.5 nm. The seed module and the amplifier chain are all in-fiber (with a few small fiber-pigtailed free-space components), followed by a free-space diffraction grating pulse compressor. The amplifier chain contains a pre-amplifier and a booster whose gain fibers are 45/125 μm core/cladding-diameter, core-pumped Er-doped fibers. The pump lasers for both amplifiers are single-mode 1480 nm Raman lasers capable of up to 8 W output. The seed module generates up to 2 ns chirped pulses that are amplified and subsequently compressed to <800 fs duration. At a repetition rate of 50 kHz, the 2 ns pulses from the seed module were amplified to 72 μJ, resulting in 25 μJ after pulse compression. The corresponding peak power levels after the amplifier chain and compressor were 36 kW and 31 MW, respectively.

We report on a high repetition rate noncollinear optical parametric amplifier system (NOPA) seeded by a cavity dumped Ti:Sapphire oscillator. The pump pulses for parametric amplification are generated via soliton generation in a highly nonlinear photonic crystal fiber with a subsequent fiber-based amplification stage and are therefore synchronized. The system is capable of producing high energy ultra-short pulses at repetition rates up to 2 MHz.

We report on experimental generation of wave-breaking-free pulses from an environmentally stable Yb-doped all-fiber laser. The compact linear cavity is constructed with saturable absorber mirror directly glued to the fibers end-facet as nonlinear mode-locking mechanism and chirped fiber Bragg grating (CFBG) for dispersion management, thus, without any free-space optics. Further, the laser was intrinsically environmentally stable, as only polarization maintaining (PM) fibers were used. In the wave-breaking-free regime, the fiber laser directly generates positively-chirped picosecond pulses at a repetition rate of 20.30 MHz. These pulses can be compressed to 218 fs in a HC-PBG providing a femtosecond all-fiber laser system. Adapting the intra cavity dispersion we have also generated chirped pulses with a parabolic spectral profile in the stretched pulse regime. We confirm numerically the wave-breaking-free pulse and stretched pulse evolution and discuss advantages and disadvantages of both regimes in terms of pulse quality.

We demonstrate here an all-fiber passively mode-locked laser using an integrated fiber-end mirror and photonic band-gap fiber-based dispersion compensator. The refined technology of thin-film coatings made with electron beam evaporation on a single-mode fiber facet results in a compact dichroic pump combiner/output coupler. The dichroic mirror made of ZrO₂ and SiO₂ provides a low reflectivity (0.4 %) for the 980 nm pump and over 40 % reflectivity for the 1040 nm signal wavelength, which enabled us to build a short-length mode-locked ytterbium fiber laser. The laser cavity consisted of 8 cm of highly doped ytterbium fiber, 10 cm of anomalous dispersion photonic bandgap fiber, a semiconductor saturable absorber mirror (SESAM) and the dichroic mirror. Pump and signal wavelengths were separated by a fiber coupler placed outside the cavity contrary to conventional geometry. A butt-coupled SESAM provided reliable self-starting at a pump power of 150 mW. The all-fiber design using dichroic fiber mirror combined with photonic bandgap fiber dispersion compensator is highly stable and requires virtually no alignment. The mode-locked laser produces 572-fs soliton pulses at 571.03 MHz fundamental repetition rate. To the best of our knowledge, this is the highest fundamental repetition rate fiber laser operating around 1 μm reported to date.

The output characteristics of the conventional one-stage Raman fiber laser are described theoretically by using the optical wave turbulence formalism. Simple analytical expressions describing RFL output power and its spectral shape are presented as well as square-root law for the output spectrum broadening law has been discovered. The indications of the turbulent-like spectral broadening in other types of CW fiber lasers and propagation phenomena in fibers are also discussed.

We present results on characterization of lasers with ultra-long cavity lengths up to 84km, the longest cavity ever reported. We have analyzed the mode structure, shape and width of the generated spectra, intensity fluctuations depending on length and intra-cavity power. The RF spectra exhibit an ultra-dense cavity mode structure (mode spacing is 1.2kHz for 84km), in which the width of the mode beating is proportional to the intra-cavity power while the optical spectra broaden with power according to the square-root law acquiring a specific shape with exponential wings. A model based on wave turbulence formalism has been developed to describe the observed effects.

We present the first experimental demonstration of strong coupling between the core modes in multi-core fibers (MCF) regardless of large spacing (~28μm) between them. The effect is very sensitive to bending of the fiber and is observed in the MCF laser as well as in the probe beam schemes. We explain the observed effect by a mechanism of the mode coupling based on their indirect interaction inside the fiber via intermediate cladding mode, analogues to the Bragg mode. 70% of power conversion from one core to another with beating length of tens of centimeters in 4-core MCF is measured.

A new technique for mitigating stimulated Brillouin scattering (SBS) effects in narrow-linewidth Yb-doped fiber amplifiers is demonstrated with a model that reduces to solving an 8×8 system of coupled nonlinear equations with the gain, SBS, and four-wave mixing (FMW) incorporated into the model. This technique uses two seed signals, or 'two-tones', with each tone reaching its SBS threshold almost independently and thus increasing the overall threshold for SBS in the fiber amplifier. The wavelength separation of these signals is also selected to avoid FWM, which in this case possesses the next lowest nonlinear effects threshold. This model predicts an output power increase of 86% (at SBS threshold with no signs of FWM) for a 'two-tone' amplifier with seed signals at 1064nm and 1068nm, compared to a conventional fiber amplifier with a single 1064nm seed. The model is also used to simulate an SBS-suppressing fiber amplifier to test the regime where FWM is the limiting factor. In this case, an optimum wavelength separation of 3nm to 10nm prevents FWM from reaching threshold. The optimum ratio of the input power for the two seed signals in 'two-tone' amplification is also tested. Future experimental verification of this 'two-tone' technique is discussed.

We propose a novel approach to increase the repetition rate of all-fiber Q-switched laser. This proposed ring cavity fiber laser consists of a fiber Bragg grating (FBG) functions as the wavelength discriminator and a chirped FBG Fabry-Perot (FP) etalon serve as the transmission filter inside the ring cavity. The Q-switching operation is achieved by periodically tuning of the FP etalon and hence modulating the loss of the cavity. A numerical model is developed to simulate this type fiber ring laser with consideration of FBGs' spectra. Our simulation shows that the repetition rate of the Q-switched pulse can be increased by multiple times that depend on the tuning range and the bandwidth of the chirped FBG FP etalon. Experimentally we achieved 14 kHz Q-switched pulses under 3.5 kHz PZT modulation frequency.

We present an all-fiber monolithically integrated fiber laser based on a custom tapered fused bundle pump combiner with 32 inputs ports connected to a double clad gain fiber. The pump combiner is designed to provide high isolation between signal and pumps fibers providing intrinsic pump protection. This configuration can generate more than 100W of continuous wave (CW) laser light using single-chip multimode pumps enabling long term reliability.

We propose and experimentally demonstrate an effective method to reduce far-field speckle noise in multimode fiber with a short cylindrical piezoelectric transducer (PZT) vibrating in radial direction. In this study, the fiber was coiled as tightly as possible around the mandrel of the PZT and periodic stretching effect was caused by the radial oscillations of the actuator. This technique can be adapted at a high modulation frequency, so the speckle patterns can be time-averaged. The output of the optical fiber was intensively observed by a CCD camera. By counting all the pixels corresponding to relative intensity graded 256 levels in selected area and by calculating the mean value and standard deviation of the intensity, we can measure the speckle contrast and vibration effect in quantitative measurands. It was clearly observed that the characteristics of the speckle pattern in vibration-ON-state were signinficantly reduced than that of vibration-OFF-state by comparing the proposed measurands as well as direct CCD images. We expect that the proposed speckle reduction technology would find viable applications in realization of fiber laser, laser marking, optical trapping and projection display systems.

We describe a pulsed green and ultraviolet laser source based on a frequency converted photonic crystal fiber (PCF) amplifier. The flexible-format all-fiber polarization maintaining (PM) seed source consists of cascaded ytterbium doped amplifiers seeded by a directly modulated 1064 nm laser diode. The seed generates pulses fixed at 3.5 ns duration from 20 kHz to 10 MHz. Up to 2.9 W of 532 nm (green) and 1 W of 355 nm (UV) output were obtained in tests from 20-100 kHz using LBO crystals for both the doubling and tripling conversion process. Scaling the pulse energies demonstrated at low repetition rates, to the high repetition rate operation possible with the flexible format seed source, shows the potential for high power, high repetition rate pulsed green and UV output for advanced applications.

An analytical model has been developed to minimize the effective mode area in a saturable absorber for an external-cavity passively Q-switched fiber laser. Under the condition of minimum effective mode size inside the saturable absorber, the analytical expressions for two key parameters of an optimum system have been derived. One parameter is the optimum focal position that is analytically derived to be a function of the thickness and initial transmission of the saturable absorber. The other parameter is the optimum magnification of the re-imaging optics that is analytically derived to be in terms of the numerical aperture and core radius of the laser fiber as well as the thickness and initial transmission of the saturable absorber. The present model provides a straightforward procedure to adopt a focusing system for the output performance. A diode-pumped ytterbium-doped fiber lasers with a Cr:YAG crystal as a saturable absorber is considered to illustrate the utility of the present model

Coherent Beam Combining (CBC) technology holds the promise of enabling laser systems with very high power and near-ideal beam quality. In this work we propose and demonstrate a novel CBC servo system using optical phase lock loops for phase control. This servo system is based on entirely electronic components and, consequently, can be considerably more compact and less expensive compared to servo systems made of optical phase/frequency shifters. In the proof-of-concept experiments we have combined two 100mW 1064nm commercial semiconductor lasers with the filled-aperture approach at an efficiency of 94% and also two 50mW 1538nm commercial semiconductor lasers using the tiled-aperture approach with a strehl ratio of 0.9. In addition, we also present a theoretical consideration of the influence of various sources of noise on the combining efficiency of a cascaded filled-aperture CBC system.

We report on reliable single-mode laser modules at 1060 nm used in pulsed operation for efficient seeding of fiber amplifiers. The modules incorporate InGaAlAs single quantum well diodes with a design inherited from telecom qualified devices. Pulse parameters can be widely varied with laser intrinsic modulation capability in GHz range. 2.5 W peak power is exhibited in a single-mode fiber at a current of 5 A with 200 ns pulses. Reliability is proven by lifetest in pulsed operation up to 3.5 A. Wavelength stabilization with fiber Bragg gratings is obtained over a wide range of operating conditions.

We report, to the first time to our knowledge, on a passively mode-locked single-polarization single-transverse-mode large-mode-area photonic crystal fiber laser operating in the dispersion compensation free regime. In the single-pulse regime, the laser generates 1.6 W of average power with 3.7 ps pulses at a repetition rate of 63 MHz, corresponding to a pulse energy of 25 nJ. Stable and self-starting operation is obtained by adapting the spot size at the saturable absorber mirror to the pulse evolution in the low-nonlinearity fiber. The pulses are compressible down to 750 fs. The presented approach demonstrates the scaling potential of fiber based short pulse oscillators towards high-power ultra-compact allfiber environmentally-stable configuration.

We have designed, built and tested an actively mode-locked fiber laser, operating at 1550 nm, for use as the sampling waveform in an opto-electronic analog-to-digital converter (ADC). Analysis shows that, in order to digitize a 10-GHz signal to 10 bits of resolution, the sampling pulsewidth must be less than 2.44 ps, the RMS timing jitter must be below 31.0 fs, and the RMS amplitude jitter must be below 0.195%. Fiber lasers have proven to have the capability to narrowly exceed these operating requirements. The fiber laser is a "sigma" laser consisting of Er-doped gain medium, dispersion-compensating fiber, nonlinear fiber, a Faraday rotation mirror, polarization-maintaining fiber and components, and diode pump lasers. The active mode-locking is achieved by a Mach-Zehnder interferometer modulator, driven by a frequency synthesizer operating at the desired sampling rate. A piezo-electric element is used in a feedback control loop to stabilize the output PRF against environmental changes. Measurements of the laser output revealed the maximum nominal PRF to be 16 GHz, the nominal pulsewidth to be 7.2 ps, and the nominal RNS timing jitter to be 386 fs. Incorporating this laser into a sampling ADC would allow us to sample a 805-MHz bandwidth signal to a resolution of 10 bits as limited by timing jitter. Techniques to reduce the timing-jitter bottleneck are discussed.

Development of photodarkening in two similar large-mode-area ytterbium doped fibers from different sources is compared. The excess loss induced by photodarkening is derived from transmission loss measurements of pristine and pumped or photodarkened samples. To accelerate the photodarkening process, cladding pumping is used so as to achieve high and uniform inversion through the sample. Further, intensity profiles are measured and compared in effort to detect possible radial variations in the induced losses.

The ability to strip cladding light from double clad fiber (DCF) fibers is required for many different reasons, one example is to strip unwanted cladding light in fiber lasers and amplifiers. When removing residual pump light for example, this light is characterized by a large numerical aperture distribution and can reach power levels into the hundreds of watts. By locally changing the numerical aperture (N.A.) of the light to be stripped, it is possible to achieve significant attenuation even for the low N.A. rays such as escaped core modes in the same device. In order to test the power-handling capability of this device, one hundred watts of pump and signal light is launched from a tapered fusedbundle (TFB) 6+1x1 combiner into a high power-cladding stripper. In this case, the fiber used in the cladding stripper and the output fiber of the TFB was a 20/400 0.06/0.46 N.A. double clad fiber. Attenuation of over 20dB in the cladding was measured without signal loss. By spreading out the heat load generated by the unwanted light that is stripped, the package remained safely below the maximum operating temperature internally and externally. This is achieved by uniformly stripping the energy along the length of the fiber within the stripper. Different adhesive and heat sinking techniques are used to achieve this uniform removal of the light. This suggests that these cladding strippers can be used to strip hundreds of watts of light in high power fiber lasers and amplifiers.