This PDF file contains the front matter associated with SPIE Proceedings Volume 8237, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

State-of-the-art diffraction-limited fiber lasers are presently capable of producing kilowatts of power. Power levels produced by single elements are gradually increasing but beam combining techniques are attractive for rapidly scaling fiber laser systems to much higher power levels. We discuss both coherent and spectral beam combining techniques for scaling fiber laser systems to high brightness and high power. Recent results demonstrating beam combination of 500-W commercial fiber laser amplifiers will be presented.

The recent development of kW fiber laser sources makes the concept of laser systems operating at power levels from tens of kilowatts up to 100-kilowatt levels a reality. The use of volume Bragg gratings for spectral beam combining is one approach to achieve that goal. To make such systems compact, lower the complexity and minimize the induced thermal distortions we propose and demonstrate the use of special volume Bragg elements which have several Bragg gratings written inside as combining optical components. The multiplexed volume Bragg gratings (MVBGs) were recorded in photo-thermo refractive glass and three beams with total power of 420 W were successfully combined using one MVBG. The combining efficiency was 97% and there was no significant beam quality degradation. The results demonstrated that the approach of using multiplexed volume Bragg gratings for spectral beam combining is an excellent extension to the current state of the art combining techniques. Especially valuable is the capability to reduce the number of optical elements in the system and while being able to manage the expected thermal load when kilowatt level sources are used for beam combining.

The coherent combining of ultrashort pulses is a concept for scaling the pulse energy and average power of laser systems emitting ultrashort pulses. In this contribution the experimental results of a coherently combined femtosecond fiber CPA laser system consisting of 2 amplifiers is presented. Combining efficiencies as high as 89% and compressed pulse energies of up to 3mJ were achieved. Additionally, the system showed excellent long-term stability.

We propose and demonstrate an architecture that achieves passive coherent combination of two fiber femtosecond chirped-pulse amplifiers. The setup consists in the use of a well-balanced amplifying Sagnac interferometer. The experiment shows that the temporal, spectral, and spatial qualities of each beam are retained, with the generation of 250 fs pulses at 35 MHz repetition rate, an uncompressed average power of 10 W, and a combining efficiency of 96%. The behavior of this architecture in the presence of high accumulated nonlinear phase is investigated.

We present a physical model that may describe the observed phenomenon of modal instability in high power fiber amplifiers. In the power range of several hundred watts, large-mode-area, cladding-pumped, Yb³⁺-doped fiber amplifiers (both step-index and photonic crystal fibers), exhibit a sudden transition in the output beam profile from the fundamental mode to a higher order mode. We show how this behavior can be caused by a thermally induced mode coupling that leads to exponential gain of the higher order mode, and we implement a numerical model that quantitatively predicts the instability threshold for any large-mode-area step index fiber amplifier.

Detailed simulations on the temperature profile inside of a large mode area fiber in high power operation with mode interference are presented. These simulations show that the pump power is not homogeneously absorbed along the fiber, which in turn gives rise to an oscillating temperature profile along the fiber. This longitudinal temperature profile creates an index grating with the right period to transfer energy between the interfering modes. Two cases are analyzed: mode beating between the fundamental mode and a radially anti-symmetric mode and mode beating between the fundamental mode and a radially symmetric mode.

The quality of Yb-doped fused bulk silica produced by sintering of Yb-doped fused silica granulates has improved greatly in the past five years [1 - 4]. In particular, the refractive index and doping level homogeneity of such materials are excellent and we achieved excellent background fiber attenuation of the active core material down to about 20 dB/km at 1200 nm. The improvement of the Yb-doped fused bulk silica has enabled the development of multi-kW fiber laser systems based on a single extra large multimode laser fiber (XLMA fiber). When a single active fiber is used in combination with the XLMA multimode fiber of 1200 μm diameter simple and robust high power fiber laser setups without complex fiber coupling and fiber combiner systems become possible. In this papper, we will discuss in detail the development of the core material based on Yb-doped bulk silica and the characterization of Yb-doped fibers with different core compositions. We will also report on the excellent performance of a 4 kW fiber laser based on a single XLMA-fiber and show the first experimental welding results of steel sheets achieved with such a laser.

We have demonstrated a monolithic (fully fused), 1.2-kW, Yb-doped fiber laser with near-single-mode beam quality. This laser employs a new generation of high-brightness, fiber-coupled pump sources based on spatially multiplexed single emitters, with each pump providing 100 W at 915 nm within 0.15 NA from a standard 105/125 μm fiber. The fiber laser is end pumped through the high-reflector FBG using a 19:1 fused-fiber pump combiner, eliminating the need for pump/signal combiners. The output wavelength is 1080 nm, with a linewidth of < 0.5 nm FWHM. A peak power of 1.5 kW was reached in modulated operation (1-ms pulse duration) with M² < 1.2.

We report on the generation of femtosecond pulses from an all-normal-dispersion fiber laser featuring a large-mode-area ytterbium-doped photonic-crystal fiber. Nonlinear polarization evolution assisted by passive spectral filtering in combination with the large-pitch fiber design enables a significant peak power enhancement with the generation of multi-megawatt pulses. 65 W of average power at 76.5 MHz repetition rate, corresponding to 850 nJ pulses are generated in a compact oscillator setup. The output pulses are extra-cavity dechirped down to 111fs with 6 MW of peak power. To the best of our knowledge, these are the highest average and peak powers ever reached by a mode-locked fiber laser.

We present the nearly diffraction-limited propagation of 800 W of cw laser power through 100 m of delivery fiber having a core diameter of 30 μm. The laser source was a single-mode MOPA consisting of a fiber oscillator and two amplifier stages and was matched to the delivery fiber through a 500 mm long taper. At a maximum power of 800 W, a M² value of 1.35 was measured after 100 m of passive fiber. A minor Stokes-shifted spectral content was observed above 780 W.

We demonstrate a cascaded Raman fiber laser with a record output power of 104 W at 1480 nm. We achieve this with an 1117 nm Yb-doped fiber laser pumping a cascaded Raman conversion to 1480nm. Enhanced efficiency is achieved in the Raman cavity using a fiber with a long wavelength cut-off which prevents further Raman conversion of 1480 nm light and thus allowing for longer cavity lengths. The output is single mode making it a bright source for core or cladding pumping of erbium-doped fiber lasers.

We present a high power Er-doped fiber laser operating at 1555 nm, core pumped by a Raman fiber laser operating at 1480 nm. We achieve 55 W total output power for 67 W of pump power with a slope efficiency of 81%. To the best of our knowledge, this is the highest output power achieved from a core-pumped Er-doped (Yb free) fiber laser to date.

We developed a high power picosecond laser system at 80 MHz for third harmonic generation. We obtained 291 W at 1030 nm under 580 W of pump power at 976 nm from an all fiber master oscillator power amplifier based on rod-type fibers. By frequency tripling, we obtained 63 W at 343 nm with excellent beam quality (M²<1.2).

We report on the development and performance of a key components that enable the construction of multi-kW fiber amplifiers for government and industrial applications that are both reliable and highly affordable. The usefulness of these components span the range from single frequency near diffraction limited kW class fiber lasers to multimode wide linewidth fiber lasers for welding and cutting applications. The key components for these amplifiers are a novel multi fiber-coupled laser diode stack and a monolithic 6+1x1 large fiber pump/signal multiplexer. The precisely aligned 2-D laser diode emitter array found in laser diode stacks is utilized by way of a simple in-line imaging process with no mirror reflections to process a 2-D array of 380-450 elements into 3 400/440μm 0.22NA pump delivery fibers. The fiber combiner is an etched air taper design that transforms low numerical aperture (NA), large diameter pump radiation into a high NA, small diameter format for pump injection into an air-clad large mode area PCF, while maintaining a constant core size through the taper for efficient signal coupling and throughput. The fiber combiner has 6 400/440/0.22 core/clad/NA pump delivery fibers and a 20/440 PM step-index signal delivery fiber on the input side and a 40/525 PM undoped PCF on the output side. The etched air taper transforms the six 400/440 μm 0.22 NA pump fibers to the 500 μm 0.55 NA core of the PCF fiber with a measured pump combining efficiency of 92% with zero brightness drop. The combiner also operates as a stepwise mode converter via a 30 μm intermediate core region in the combiner between the 20 μm core of the input fiber and the 40 μm fiber core of the PCF with a measured signal efficiency of 90% while maintaining polarization with a measured PER of 20 dB. We report the signal coupling efficiency and power handling capability as well.

A high-spectral-flatness mid-infrared supercontinuum fiber source is presented. It generates highly stable single-mode broadband beam with a FWHM bandwidth of ~600nm and a 20dB-down spectra spanning from 1.8μm up to ~ 2.7μm. The supercontinuum pulses are emitted from a single-mode fiber with ~15kW peak power and >20GW/cm² laser peak intensity. This compact fiber source, in combination with a commercial mid-infrared optical spectrum analyzer (Yokogawa), offers a powerful tool for mid-infrared component characterization with high spectral resolution.

Ultraviolet absorption and laser excitation spectroscopy (LES) measurements are presented for rare-earth-doped optical fibers produced from both glassy and crystalline preforms. Absorption spectra are obtained via broad-spectrum UV LEDs emitting in the 250nm region. LES measurements are obtained utilizing a tunable UV laser source. The tunable laser employed is a frequency-doubled titanium:sapphire laser-pumped optical parametric amplifier (OPA) operating down to a minimum wavelength of about 225nm. Our results indicate a roughly linear relationship between the concentration of oxygen deficiency centers (ODC) and rare-earth content, regardless of the preform type, and the slope of the line is found to vary significantly with the rare earth. Additionally, LES measurements are used to elucidate the energy transfer mechanism from pumping in the UV to emission by the rare-earth. In all cases the fibers are Al codoped and those produced from glassy preforms are manufactured via standard methods. Fibers produced from crystalline preforms start with a pure silica-sleeved rare-earth doped YAG crystal rod that becomes glassy (amorphous) post-draw.

We present a comparative theoretical and experimental study of different schemes of fiber lasers with tapered doubleclad fiber (T-DCF) as an active medium. We have developed a theoretical model for obtaining the distribution of power and power density inside the fiber for the considered laser schemes with different longitudinal shapes of T-DCFs. The slope efficiencies, spectral characteristics, and beam quality deterioration associated with the mode conversion in the TDCF were also studied experimentally for different laser schemes. In addition, a spontaneous transition to a self-pulsing regime under certain conditions, and the associated problems have been investigated. Characteristics of T-DCF lasers constructed in co- and counter-propagating schemes are discussed and compared to those of regular fiber lasers, equivalent in active volume or in brightness.

Polymer optical fibres (POF) have historically focused on applications in data transmission over short distances, using highly multimode step-index or graded-index fibre designs. This paper will focus on a qualitatively different type of polymer fibres - microstructured polymer optical fibres (mPOF) - which allow a wider variety of fibre designs and optical properties to be achieved. Fibres with similar properties to conventional step- and graded-index POF can be made for data transmission applications, as well as single-mode fibres which can be used for grating inscription and gratingbased sensing. The use of microstructures can also be extended to longer wavelengths for the transmission of THz radiation, and both solid-core and hollow-core mPOF-based THz waveguides have been demonstrated. Finally, the development and extension of mPOF to form metal-dielectric structures for the manufacture of metamaterials using fibre-drawing methods will be discussed. Such drawn-metamaterials with electric and magnetic responses at THz frequencies have been demonstrated.

We report on a flexible large mode area (LMA) photonic crystal fiber (PCF) for high power laser and amplifier applications. We designed a fiber cross-section that includes two regions with different air-filling factors on opposite sides of the fiber core. This design allows suppressing higher order modes (HOMs) when the fiber is bent around a 10 cm radius at a wavelength of 1064 nm. The fiber then exhibits single-mode (SM) operation within a very large core with a diameter larger than 60 μm. The loss of the fundamental mode (FM) is lower than 0.01 dB/m. We can therefore consider building compact high power fiber lasers and amplifiers with a nearly diffraction limited beam output using our LMA PCF.

We present the correlation filter method for measuring the modal power spectrum of beams emerging from multi-mode optical fibers. An optical filter acts as central element of the measurement process performing the integral relation of correlation. This filter is realized by a computer-generated hologram with a specifically designed transmission function based on the spatial distribution of guided modes in the investigated waveguide. The correlation filter method provides modally resolved information about amplitudes, phase differences and polarization states. Hence, the full information about the optical field becomes available. Beside a detailed presentation of the measurement process, the setup and the design of the correlation filters, the major advantage of the method, the ability to perform real-time measurements, is introduced.

We report on an ytterbium doped single mode distributed mode filtering rod fiber in an amplifier configuration delivering high average output power, up to 292 watts, using a mode-locked 30ps source at 1032nm with good power conversion efficiency. We study the modal stability of the output beam at high average output power levels and demonstrate a 44% power improvement before the threshold-like onset of mode instabilities by operating the rod fiber in a leaky waveguide regime. We investigate the guiding dynamics of the rod fiber and explain the improved performance by thermally induced refractive index profile change.

Today multimode optical fibers are used to transport and generate high brilliant beams of considerable power. External perturbation of the fiber, e.g., induced by bending or strain, will influence the guided light, i.e., change modal content and beam properties. We present a detailed experimental investigation of fiber bending and strain induced changes of the modal content, based on the Correlation Filter Method that performs a modal decomposition with computer-generated holograms. Using this technique the modal amplitudes and phases can be monitored in real-time, i.e., currently with up to 30 Hz, such that variations in the modal composition can be observed instantaneously. The fast measurement rate can be used for adjustment purposes, e.g., to evaluate quantitatively the change of beam quality with varying bending diameter. We have applied our method to different kinds of multimode fibers such as step-index, photonic crystal and multicore fibers, whereas the results for a step-index and a multicore large mode area fiber are exemplarily presented, including the impact of bending on the beam quality.

We report on a new approach for experimentally analyzing transversal mode coupling in few mode fiber Bragg gratings (FBGs). Utilizing a spatial light modulator (SLM) enables real-time modulation of the incident spatial light distribution coupled into the fiber. Thus, individual transversal fiber modes can be excited, allowing for effort-less mode switching and mode rotation. Simultaneously, the transmitted and reflected modes are analyzed. We use this setup to demonstrate LP mode discrimination by asymmetric FBGs which favor reflection of a designated LP₁₁ mode orientation for few mode large mode area fibers.

We present current and near-term uses of high-power fiber lasers and amplifiers for NASA science and spacecraft applications. Fiber lasers and amplifiers offer numerous advantages for the deployment of instruments on exploration and science remote sensing satellites. Ground-based and airborne systems provide an evolutionary path to space and a means for calibration and verification of space-borne systems. NASA fiber-laser-based instruments include laser sounders and lidars for measuring atmospheric carbon dioxide, oxygen, water vapor and methane and a pulsed or pseudo-noise (PN) code laser ranging system in the near infrared (NIR) wavelength band. The associated fiber transmitters include high-power erbium, ytterbium, and neodymium systems and a fiber laser pumped optical parametric oscillator. We discuss recent experimental progress on these systems and instrument prototypes for ongoing development efforts.

We introduce the most recent progress in the optimization of ultrafast fiber lasers for building ultralow timing jitter signal sources. Using a sub-20-attosecond-resolution timing jitter measurement technique, we optimize the timing jitter of optical pulse trains from mode-locked Er-fiber and Yb-fiber lasers to 70 attoseconds and 175 attoseconds, respectively, when integrated from 10 kHz to 40 MHz offset frequency. To our knowledge, these results correspond to the lowest rms timing jitter demonstrated from fiber lasers so far, the equivalent phase noise of which is comparable to that of the best microwave sources available, with much reduced cost and engineering complexity.

Medium and high-power fiber lasers operating in the 1um region have proven their capabilities for cutting and welding in industrial manufacturing applications. This paper reviews the process performance capabilities of medium power (up to 500W) fiber lasers in a range of precision cutting, fusion-welding and additive manufacturing applications.

Diffraction-limited high power lasers represent a new generation of lasers for materials processing, characteristic traits of which are: smaller, cost-effective and processing "on the fly". Of utmost importance is the high beam quality of fiber lasers which enables us to reduce the size of the focusing head incl. scanning mirrors. The excellent beam quality of the fiber laser offers a lot of new applications. In the field of remote cutting and welding the beam quality is the key parameter. By reducing the size of the focusing head including the scanning mirrors we can reach scanning frequencies up to 1.5 kHz and in special configurations up to 4 kHz. By using these frequencies very thin and deep welding seams can be generated experienced so far with electron beam welding only. The excellent beam quality of the fiber laser offers a high potential for developing new applications from deep penetration welding to high speed cutting. Highly dynamic cutting systems with maximum speeds up to 300 m/min and accelerations up to 4 g reduce the cutting time for cutting complex 2D parts. However, due to the inertia of such systems the effective cutting speed is reduced in real applications. This is especially true if complex shapes or contours are cut. With the introduction of scanner-based remote cutting systems in the kilowatt range, the effective cutting speed on the contour can be dramatically increased. The presentation explains remote cutting of metal foils and sheets using high brightness single mode fiber lasers. The presentation will also show the effect of optical feedback during cutting and welding with the fiber laser, how those feedbacks could be reduced and how they have to be used to optimize the cutting or welding process.

Numerous achievements have been made recently by researchers in the areas of fiber laser beam combining and power scaling. Industry has demonstrated multi-kW power from a single fiber amplifier, and a US national laboratory has coherently combined eight fiber amplifiers totaling 4 kW. This paper will survey the recent literature and then focus on fiber laser results from the Laser Division, Directed Energy Directorate of the Air Force Research Laboratory (AFRL). Progress has been made in the power scaling of narrow-linewidth fiber amplifiers, and we are transitioning lessons learned from PCF power scaling into monolithic architectures. SBS suppression has been achieved using a variety of techniques to lower the Brillioun gain, including acoustically tailored fiber, laser gain competition resulting from multitone seeding and inducing a longitudinal thermal gradient. We recently demonstrated a 32-channel coherent beam combination result using AFRL's phaselocking technique and are focused on exploring the limitations of this technique including linewidth broadening, kW-induced phase nonlinearities and auto-tuning methods for large channel counts. Additionally, we have recently refurbished our High Energy Laser Joint Technology Office-sponsored 16-amplifier fiber testbed to meet strict PER, spatial drift, power stability and beam quality requirements.

The stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) threshold powers in passive small mode area optical fibers (SMAFs) and large mode area optical fibers (LMAFs) are determined from a distributed light scattering model. The model employs the pump and Stokes rate equations together with the equivalent thermal Brillouin and Raman noise powers to provide a numerical solution for the evolution of the pump and Stokes powers along the fiber length without approximation and is applicable in the non-depleted and depleted pump regimes. A simple transcendental equation for the threshold powers is obtained in the non-depleted pump, low fiber loss regime that is of similar form to the standard threshold relations first derived by Smith: P_th=κA_eff/gL_eff where Pth is the threshold power, g is the non-linear gain coefficient and Leff is the fiber effective interaction length. The numerical coefficient κ=ln(P_S/P_N) is found to be a weak function of the Stokes power P_S and the thermal noise power PN and differs for the two types of fibers due to the differing order-of-magnitude Stokes powers and noise powers. It is found that the SRS threshold power of the LMAF exceeds that first derived by Smith by 1.7 dB and that the SBS threshold power in the SMAF is 1.2 dB less than that derived by Smith. The SRS threshold in SMAF and the SBS threshold in the LMAF are in agreement to within <1.0 dB with the Smith relations. The transcendental approximation is useful for estimating the threshold powers in all fibers at any pump power and pump-to-Stokes conversion efficiency η<0.1. It is found that the Smith relations do not apply to the pump-depletion regime although they are defined for conversion efficiencies η of 0.5 and 1.0.

Gain dynamics and corresponding refractive index changes in fiber amplifiers are important for low noise fiber amplifiers and when using doped fibers as phase actuators. Furthermore self-induced refractive index changes are a possible explanation for mode fluctuations in high power fiber amplifiers, which currently present a major challenge for high power fiber lasers. Since these processes are all time dependent, a dynamic model of the refractive index change is very important. We will present measurements of phase shift dynamics in a low power ytterbium fiber amplifier. Using a simplified analytic model, which was originally developed to predict power modulation in telecom amplifiers, we were able to model the part attributed to the Kramers-Kronig-Relations and to obtain a much better understanding of the involved timescales. This will help modeling of phase noise in low noise amplifiers and possibly mode fluctuations in fiber amplifiers.

The effect of broad-range self-sweeping in Yb-doped fiber laser has been demonstrated experimentally for the first time. The self-sweeping effect is observed in an all-fiber laser configuration with a double-clad Yb-doped fiber and a cavity formed by a broad-band fiber loop mirror and Fresnel reflection from one cleaved end. The sweep range is limited by the width of the broad-band reflector and reaches up to 16nm. It is found that the self-sweeping effect is related to selfpulsations. So the sweep rate is increased with an increase in pump power and is decreased with increasing cavity length. RF and optical spectra (linewidth is measured to be not more than 100 MHz) show that during the evolution of a single pulse a small number of longitudinal modes take a part in lasing. Based on these results we propose a model describing dynamics of the laser frequency. The model is based on the spatial hole burning effect and the gain saturation in Yb laser transition, and takes into account self-pulsations of the laser. Theoretical estimation for pulse to pulse change of lasing frequency is in good agreement with experimental data.

We present experimental studies of PM Yb-doped photonic crystal fibers possessing acoustic and Yb-ion concentration tailoring. In the initial design, the concentration of dopants in two regions of the core were selected such that the corresponding Brillouin shifts were sufficiently separated to allow for further stimulated Brillouin scattering suppression through thermal effects. The Yb-ion concentration was maintained uniformly throughout the entire core. When this fiber was utilized in a counter-pumped amplifier configuration, ~500 W of single-frequency (kHz linewidth) output was obtained in a 10 m long fiber. Further power scaling with good beam quality beyond 500 W was limited by modal instabilities. A second fiber design was developed in which the Yb-ion concentration was modified to have preferential overlap with the fundamental mode as well as reduced pump absorption. The onset of the modal instabilities was sufficiently suppressed to allow for an output of 990 W with a nominal linewidth of 300 MHz and good beam quality.

We present experimental and modeling results investigating rare earth-doped (Er and Yb) YAG-derived silica fibers (RYDF) as candidates for use in high-power narrow-linewidth fiber lasers. Fabrication of the RYDFs via a rod-in-tube method, starting from crystalline YAG, and its transformation to the amorphous state is described. Stimulated Brillouin scattering (SBS) suppression results from material properties that cooperate to yield a low Brillouin gain coefficient (BGC), namely reduced photoelastic constant and increased acoustic velocity, mass density, and Brillouin spectral width relative to silica. We find typical BGC values for large-mode-area RYDFs to be around 0.5×10^-11 m/W. Utilizing a materials model, these results are extrapolated to compositionally-design further reductions to the BGC, including introducing other co-dopants to the mixture. Finally, we focus on the Yb-doped RYDF (YYDF) and show that it is a good candidate for use in kW-class narrow-linewidth fiber lasers via spectroscopy and amplifier experiments. The RYDFs are found to be very similar to conventional aluminosilicate fibers. We also find that YYDFs offer improved compatibility with phase modulating the laser for further increases in the SBS threshold. Using novel fiber fabrication methods with less-common and low-silica materials can lead to promising glass recipes with ultra-low intrinsic BGCs.

In this paper, we present a complete overview of the optical and noise performances of a high power single-frequency and single-mode fiber laser system operating at 780 nm. It provides up to 1.8W of linearly polarized output power at 780 nm, with 150 kHz linewidth and a perfect diffraction limited Gaussian beam. This newly developed fiber laser source is dedicated to trapping and cooling of Rb atoms for Bose-Einstein condensation or gravimetric applications. A great attention was paid to the laser intensity and frequency noises, linewidth and beam characterization of this all-fiber system.

We report on a single-frequency large mode area Er-doped photonic crystal fiber amplifier system. The active fiber had a core with 40 μm diameter and a numerical aperture of <0.04. This resulted in a mode field diameter of about 31 μm around 1550 nm wavelength. In a master-oscillator power-amplifier scheme, 70.8 W of output power at 1556 nm have been obtained. The maximum output power was limited by the amount of available pump power. Very high fractional TEM₀₀ content - and thus near diffraction limited output beam quality - has been verified by measurements with a non-confocal scanning ring cavity.

1178 nm single-frequency amplification by Yb-doped photonic bandgap fiber has been demonstrated. 24.6 W output was obtained without stimulated Brillouin scattering. 1.8 dB suppression of Brillouin gain by an acoustic antiguiding effect has been found in the low-index core antiresonant reflecting optical waveguide.

Raman fiber lasers (RFL) with a narrow spectral bandwidth are important light sources, e.g. for applications that require second-harmonic generation. Non-linear optical processes within the fiber like four-wave mixing (FWM) lead to spectral broadening, due to the generation of frequency components outside of the initial laser bandwidth. This mainly results from FWM of the longitudinal modes generated in the laser cavity. Consequently a reduced number of modes with increased frequency spacing, which is obtained by using a short laser cavity, is expected to reduce the spectral broadening. We investigate a Raman master oscillator power amplifier (MOPA) configuration, consisting of a RFL with a very short cavity length of 17 cm and a subsequent Raman amplifier of 55 m length. Pumping with 8 W power of an Ytterbium-doped fiber laser at 1100 nm, the Stokes wave at 1151 nm is generated, yielding 0.7 W Stokes power with a narrow full-width at half-maximum bandwidth of 60 pm at the MO output. From the measured frequency spacing of 607 MHz between the longitudinal modes a maximum number of 24 modes can be calculated to oscillate within the bandwidth of the MO output. The residual pump power is used to amplify the Stokes wave in a subsequent Raman amplifier. At the MOPA output the amplified Stokes power is 4.3 W with a strongly broadened bandwidth of 328 pm. Thus our results show that even for a Stokes wave with only few longitudinal modes at the MO output, strong FWM induced spectral broadening occurs within the amplifier.

The resonant pumping of holmium fibre lasers provides a fibre laser platform capable of addressing a wide range of applications that benefit from operation at wavelengths beyond 2.1 μm. This architecture can provide laser devices that promise high efficiency sources in the 2.1 μm spectral region with excellent beam quality and power scalability. This paper presents our recent results of resonantly pumped pulsed and CW holmium fibre lasers. We have demonstrated the first efficient resonantly cladding-pumped holmium fibre laser source. We have investigated both single mode and large mode area fibres and achieved CW output powers of ~140 W with excellent beam quality. In addition to these CW lasers we have investigated core-pumped, pulsed holmium fibre lasers and demonstrated output energies of up to 16 μJ at a repetition rate of up to 600 kHz. These developments provide an opportunity to build a high average power, pulsed 2.1 μm laser for the purpose of power scaling future mid-infrared sources.

We report a large aperture PPMgLN based OPO generating 21W of average output power at a slope efficiency of 45%, pumped by the output from a polarization maintaining Ytterbium doped fiber MOPA operating at 1060nm producing 58W of average output power and 20ns pulses at a repetition rate of 100kHz. A maximum of 5.5W of optical power was recorded at the idler wavelength of 3.82μm without thermal roll-off. We have experimentally verified that the pulse rise/fall time plays a significant role in the OPO conversion efficiency and that further enhancement in the OPO conversion efficiency will be possible using sub-nanosecond rise and fall times.

We report on a Tm:fiber master oscillator power amplifier system producing 100 W output power, with >10 dB polarization extinction ratio and diffraction-limited beam quality. To our knowledge, this is the highest polarized output power from an integrated Tm:fiber laser. The oscillator uses polarization maintaining (PM) single mode fiber with 10/130 μm core/cladding diameters, and the amplifier uses large mode area PM fiber with 25/400 μm core/cladding diameters. The oscillator and amplifier are pumped using 793 nm diodes spliced with pump combiners, and the oscillator is spliced to the amplifier via a mode field adaptor.

Mode-locked fiber lasers based on Tm-doped and Tm-Ho-codoped silicate fibers have been studied. The first modelocked fiber laser oscillator beyond 2 μm was demonstrated. The first mode-locked 2μm fiber laser with ~GHz repetition rate was demonstrated. The broadest spectrum (50nm FWHM) was obtained from dispersion managed 2μm mode-locked fiber laser. Various operation regimes of mode-locked thulium fiber lasers are presented, as well as all-fiber 2μm pulse amplifiers.

Numerical and analytical considerations of nonlinear compression in noble-gas-filled hollow core fiber pulse compressors are shown. Experiments are presented that enhance the peak power of existing fiber CPA architectures from 1.8 GW to 7.4 GW in a single stage. The pulses are shortened to 40 fs, which corresponds to a pulse shortening of more than one order of magnitude. Sending these pulses into a second hollow fiber further shortens the pulses to 25 fs at a pulse energy of 430 μJ at 30 kHz equaling an average power of 13 W. The peak power after the second stage is higher than 10 GW. In addition, we present very recent experiments on average power scaling of hollow fiber pulse compressors to the 100 W level. We compressed 200 W, 200 μJ, 500 fs pulses from a fiber CPA to 93 W, 93 μJ, 81 fs. These unique pulse parameters should be very well suited for high harmonic generation.

In the paper, a 2 μm high energy fs fiber laser is presented based on Tm doped fiber at a low repetition rate. The seed laser was designed to generate pulse train at 2 μm at a pulse repetition rate of 2.5 MHz. The low repetition rate seed oscillator eliminated extra devices such as AO pulse picker. Two-stage fiber amplifier was used to boost pulse energy to 0.65 μJ with chirped pulse amplification.

Chirped pulse amplification is a durable and widely used scheme for producing short pulses with duration less than 1ps and pulse energies from μJ to even mJ levels. The compressor unit needs to be able to handle high peak powers and is therefore traditionally made out of free space diffraction gratings. The stretcher unit, on the other hand, only has to handle low peak power and can therefore be realized with a dispersion managed fiber. We review our work on stretcher fiber design and dispersion managed modules in the Ytterbium gain band and Erbium gain band and present a modified stretcher fiber for managing numerically higher third and fourth order dispersion. The modified design allows for a lower incident angle on the compressor gratings and thereby reducing the grating separation and compressor size.

In this work we report on what we believe is the highest pulse energy obtained from an effectively single-mode Qswitched fiber laser system to date. This result has been obtained using rare-earth-doped double-clad large-pitch fibers which are able to delocalize the higher-order transverse modes from the active region of the core. We have built an actively Q-switched oscillator consisting of a 1.3m long large-pitch fiber with a mode field diameter of 50μm. This oscillator delivered pulses with 500μJ energy at a repetition rate of 5kHz. These seed pulses have been amplified in two fiber amplifiers which were pumped at 976nm. The pre-amplifier employs a 1.2m long large-pitch fiber with a mode field diameter of 80μm and delivered 3.5mJ pulse energy. Then, a 135μm core diameter large-pitch fiber was used as booster amplifier. Under high power operation the mode field diameter of this fiber is ~100μm. The setup can deliver average powers of up to 130W, resulting in 26mJ pulses with pulse durations of ~50ns. Additionally, since this is an effective single-mode laser system, the beam quality is close to the diffraction-limit.

Novel tapered Yb-doped polarization-maintaining (PM) large-mode-area (LMA) fibers were fabricated, with 25/250μm and 40/400μm core/clad at each end, and used in the last stage of a multi-stage Yb-fiber MOPA. The fiber-MOPA is controlled by high-speed FPGA for real-time control of the seed laser diode modulation, electro-optic modulator and acousto-optic modulator. Stable 0.3nsec pulses at 10kHz rate with 0.3mJ pulse energy (1MW peak power) is demonstrated. At 1nsec pulsewidth, scaling to >0.5mJ pulse energy is demonstrated. Polarization extinction ratio of 17dB is obtained with diffraction limited beam quality and excellent (<3%) power stability. Such compact and robust pulsed fiber amplifiers enable next generation of airborne and space lidar transmitters.

We demonstrate an inband, core-pumped master oscillator power amplifier (MOPA) with a maximum pulse energy of 1.56 mJ at a repetition rate of 1.25 kHz, seeded by an actively Q-switched Erbium/Ytterbium-codoped fiber (EYDF) ring laser, producing 150-ns pulses at 1562.5 nm. To maximize energy extraction whilst minimizing signal saturation effects, a 40μm Er³⁺-doped larger mode area (LMA) fiber was used as the gain medium. A 1535 nm single mode fiber laser was used for in-band pumping of the LMA fiber. The output beam quality (M²) was measured to be ~1.6. This is to the best of our knowledge is the highest reported pulse energy for a pulse fiber laser at 1.5 μm with M²~1.6.

30ns, 3μJ Q-switched pulse laser with the repetition rate of 10 kHz at 1950 nm was amplified with a 50cm long piece of thulium(Tm)-doped silicate fiber, and 300μJ pulses with the pulse width of 20 ns were obtained. The heavily Tm-doped silicate fiber has the cladding-pump absorption of 22dB/m at 793nm and the fiber maintains solid single-mode operation at 1950nm. To further increase output pulse energy, the Q-switched seed was pre-amplified to 20 μJ with core-pumping at 1567 nm. The power amplifier features of a 55cm long piece of Tm-doped silicate fiber with 21μm core. ~600μJ pulses with peak power up to 28 kW have been achieved. The ASE was suppressed 40dB below the laser pulses due to the short highly Tm-doped silicate fiber. Mode-locked pulses with the pulse energy of 0.3 nJ and the spectral width of 14 nm were amplified with a 30cm long Tm-fiber with 10 μm core, pulses with 16kW peak power as well as 20nJ pulses were achieved. With another Tm-doped silicate fiber (22μm core), 36nJ pulses with 1.1ps pulse width were obtained in all-fiber configuration. The high peak power pulses at 2 μm reported here should be useful for mid-IR generation.

Ultrafast UV and deep UV (DUV) sources are useful for many industrial applications. We present our research on developing a compact and high power picosecond Yb-fiber source for UV light generation (down to ~ 257 nm wavelength) via single pass second and third harmonics frequency conversion processes. Currently, the laser system delivers 260 mW average power at 343 nm (using THG) and 310 mW at 257 nm (using SHG from 515 nm).

The in-line chemical sensing device with novel C-type fiber and photonic crystal fiber was successfully fabricated. We further improved the sensitivity or light coupling efficiency of our in-line chemical sensing device with optimized the Ctype fiber length. The results show that sensitivity and response time of device are significantly enhanced with optimization of cleaving and splicing process. The gas sensing experiments with the optimized conditions are demonstrated for detecting partial pressure of acetylene. We also numerically analyzed the sensitivity of ring-core defect photonic crystal fiber which was used in this experiment, through full-vectorial finite element method.

We demonstrate a low noise fiber laser (LN-FL) based on gain feedback controlled high efficiency fiber amplifier chain (FAC) which connect with other optical devices in a circle manner. The FAC contains two cascaded fiber amplifiers with core pumped and double-clad pumped Erbium and Erbium-Ytterbium co-doped fibers. Gain saturation effect and amplified spontaneous emission noise in the FAC is analyzed and suppressed through gain control method. Lasing mode in the LN-FL is stabilized with fiber pigtails setting in special orbits and ensuring fiber device splicing loss low without any filters. For continuous wave operation, mode-hopping free laser spectrum with output power of 2W and SNR of 50dB is achieved. The narrowest bandwidths are about 0.2nm and 0.01nm for lasing cavity without and with filters, respectively. Lasing wavelength can be tuned in a wavelength span of 7nm by adjusting of a fiber pigtailed polarization controller. Pulsed operation of the laser under different pump seeds injection is experimental investigated and analyzed.

We experimentally investigated the role of the saturable absorber (SA) in a mode-locked fiber laser. It is shown that the modulation depth of the SA significantly affects the pulse duration at the laser output for a given cavity design (mainly cavity dispersion and nonlinearity).

We show that the cavity quality for generated wavelengths in dual-wavelength laser must be substantially different for stable dual wavelength operation. In experiments the linear cavity included two FBGs and a fiber optical loop mirror (FOLM) with a high-birefringence fiber in the loop. The reflection spectrum of the FOLM shifts through temperature control of the hi-bi fiber loop that allows an adjustment of the ratio between FOLM reflections at generated wavelengths. The dual wavelength operation required an appropriate adjustment of this ratio. We measured the ratio necessary for dual wavelength operation at different wavelength separation. Wavelength separation is tuned by axial tension applied to one of FBG in the range 0.6 - 3 nm. Shorter wavelength always required higher reflection, and ratio between short wavelength reflection and long wavelength reflection increased strongly with wavelength separation. The tolerance of dual wavelength generation to reflection adjustment was measured to be about 1%.

Holmium (Ho³⁺)-doped ZBLAN glasses have been investigated for the purpose of achieving efficient fiber lasers at 1.2 μm. Because of the long lifetime of the upper laser level and the small phonon energy in Ho³⁺-doped ZBLAN glasses, strong fluorescence at 1.2 μm that usually cannot be observed in Ho-doped silica glass has been measured. Fluorescence of 1 mol%, 3 mol%, and 6 mol% Ho³⁺-doped ZBLAN glasses are reported. The effect of cerium and terbium ions on the emission of Ho³⁺-doped ZBLAN glass has also been studied. Obstacles to achieving an efficient Ho³⁺-doped ZBLAN laser are analyzed and discussed. In studies of a commercial Ho³⁺-doped ZBLAN fiber laser, it was found that the 3 μm four-energy-level laser can easily overwhelm the 1.2 μm laser, which is a three-energy-level system having the same upper laser level with the 3 μm laser. In order to effectively suppress the competiting 3 μm transition, advanced Ho³⁺-doped ZBLAN fiber has been designed and fabricated for 1.2 μm fiber lasers. Fiber lasers at 1.2 μm using the new Ho³⁺-doped ZBLAN fiber have been developed. Our experiments demonstrate that the new Ho³⁺-doped ZBLAN fiber is an efficient gain medium for lasers at 1.2 μm.

Over 500W output power Yb doped single-mode fiber laser demonstrates excellent beam quality of less than 1.1 M2 value. Use of single mode fiber as delivery cable enables small spot-size; however delivery length is limited by SRS in fiber. To mitigate this characteristic, use of multi-mode fiber for delivery is one of approach. We present our design of 500W single mode fiber laser and beam quality measurement of multi-mode delivery fiber. How they vary, how we can homogenize beam depend on different condition of connection style, etc. We also present material process results in different beam performance.

Atomic layer deposition (ALD) was used to fabricate an ytterbium (Yb)-doped silica fiber in combination with the conventional modified chemical vapor deposition (MCVD) method. An MCVD soot-preform with a porous layer of SiO₂ doped with GeO₂ was coated with layers of Yb₂O₃ and Al₂O₃ prior to sintering, using the ALD method. ALD is a surface controlled CVD-type process enabling thin film deposition over large substrates with good thickness control, uniformity and high conformality. A materials analysis study showed that the dopants successfully penetrated the full thickness of 320 μm of the soot layer. Preliminary preform and fiber experiments on refractive index profiles, background losses, lifetime and the characteristic gain-loss curve were performed demonstrating the potential of this method for fabricating Yb-doped fibers with high concentration of dopants.

A novel arc calibration method has been developed for fusion splicing optical fibers with a large variety of glass diameters. This method heats the fiber with multiple short arcs and measures the amount of meltback at the corner of the fiber-ends. The fiber corner melting speed is found to be proportional to the fiber temperature. By varying the arc power of the multiple arcs, we can determine the desired arc power and appropriate melting speed for the tested fiber. This method has tested to be consistent and accurate using a newly released splicer with a controllable plasma zone. The splicer can automatically select the correct arc power for various fiber glass diameters. It enables the optimized splice parameters to be easily transferred to multiple splicers in production lines, resulting in consistent, high quality splice results.

We report on the ultrashort pulse inscription of chirped fiber Bragg gratings (FBG) using a constant phase mask and a deformed wave front. To analyze the influence of wave front deformations on the grating period, a numerical model was developed. It is based on a ray optical solution of the diffraction of an arbitrary wave front at a phase mask with a constant period, where the wave front is described by Zernike polynomials. Generally, a plane wave front generates an interference pattern with half the phase mask period. A defocus changes the grating period, while higher order aberrations like coma or spherical aberration yield a chirp along the inscribed grating. Additionally, the grating period depends on the distance of phase mask and fiber. To experimentally deform the wave front of the inscription beam, a cylindrical tuning lens was introduced to the setup. Due to decentering and tilting of the tuning lens, higher order aberrations were generated. Furthermore the fiber was tilted with respect to the phase mask. A chirped FBG with a FWHM bandwidth of 6.2 nm was realized.

We present a new approach in which the nonlinear Schrodinger equation (NLSE) coupled with beam propagation method (BPM) is used to quantitatively simulate pulse propagation in multimode cladding pumped fiber amplifiers. The complexity of fiber structures including fiber shapes and multiple cores which strongly affects the pump propagation and pump absorption and therefore also affects the fiber gain can be described by BPM modeling. The signal gain obtained from BPM is then used for modeling pulse amplification using the standard Step Split Fourier Method (SSFM). The simulation results are in quantitative agreement with our experiments.

We report on an all-fiber single-mode master oscillator power amplifier system delivering high peak power picosecond solitons at 1960 nm. The Bragg stabilized oscillator delivers 4.5 ps transform-limited pulses at 11.2 MHz. The pulse energy is equal to 62 pJ. Solitons are amplified up to 32 nJ in a short highly Thuliumdoped single-mode fiber amplifier pumped into the core at 1560 nm. The average output power achives 360 mW and the pulse peak power reaches 9.1 kW with little self-phase-modulation distortion. The amplified pulse duration is about 3.5 ps and the slope efficiency is equal to 13 %.

In this work we present a new design of Er/Yb co-doped fiber amplifier with controlled amplified spontaneous emission (ASE) at 1 μm. The Yb-ASE is controlled by stimulating a laser emission at 1064 nm in the amplifier, by providing a positive 1 μm signal feedback loop. The results were discussed and compared to a conventional amplifier setup without 1 μm ASE control. We have shown, that applying a 1064 nm signal loop in an Er/Yb amplifier can increase the output power at 1550 nm and provide stable operation without parasitic lasing at 1 μm.

In this work we present a high-power laser source developed in Master Oscillator Power Amplifier (MOPA) configuration. Presented MOPA source consists of three amplifying stages: pre-amplifier based on erbium-doped fiber, medium-power stage based on double-clad Er/Yb co-doped fiber and the power stage, based on Er/Yb co-doped doubleclad LMA fiber. Whole system is set up in All-In-Fiber technique, without any bulk, free-space optical elements. It provides over 20W of output power in the eye-safe 1550 nm band.

New generation systems are expected to include more intelligent amplifiers able to adapt to many conditions including different gains, channel load, temperature, aging and transient events.¹ To face the challenge and meet these new requirements, having an accurate control on the Er environment within the fiber core matrix has never appeared to be so necessary and predominant as it is the case now. Unlike conventional solution doping techniques where Erbium ions are randomly incorporated in the fiber core, our process makes use of a soft chemical synthesis to initially produce Erbium-doped nanoparticles (NPs). Erbium ions are therefore incorporated in the fiber core together with their local environment. So far, our investigations2 first showed that, from the material point of view, quenching levels are intimately linked to the design of the NPs through their chemical composition. Then, from the system perspective, we evidenced the higher power conversion efficiencies exhibited by NP fibers when compared to their conventional counterparts in high power amplifier configurations. In this paper, we address our most recent work focusing on the NP optimisation towards quenching-free Erbiumdoped fibers with a particular focus on core-shell alumino-silicate NPs. Completing our first amplifier results obtained in high power configurations, we also explore new NP fiber profiles that extend the range of their applications. Gain and noise characteristics of typical WDM operating points serve as key indicators on the benefits our NP doping process could provide.

We present the numerical study of the statistical properties of the partially coherent quasi-CW high-Q cavity Raman fiber laser. The statistical properties are different for the radiation generated at the spectrum center or spectral wings. It is found that rare extreme events are generated at the far spectral wings at one pass only. The mechanism of the extreme events generation is a turbulent-like four-wave mixing of numerous longitudinal generation modes. The similar mechanism of extreme waves appearance during the laser generation could be important in other types of fiber lasers.

We describe lasing in polarizing thulium doped PCF fiber for the first time. The ~4 m long fiber had 50/250 μm core/cladding diameters and hole-diameter to pitch ratio of 0.18. CW lasing was achieved by end pumping with a 793 nm diode in an oscillator configuration. Slope efficiencies of ~35 % have been obtained with single mode beam quality (M² <1.15). The polarization extinction ratio (PER) was >13 dB without any intra-cavity polarizing elements. This fiber, with MFD of ~36 μm and ~5.8 dB/m cladding pump absorption, is an attractive option for high energy pulsed amplifiers in the 2 μm wavelength regime.

Magnesium alloys constitute an attractive solution for cardiovascular stent applications due to their intrinsic properties of biocompatibility and relatively low corrosion resistance in human-body fluids, which results in as a less intrusive treatment. Laser micromachining is the conventional process used to cut the stent mesh, which plays the key role for the accurate reproduction of the mesh design and the surface quality of the produced stent that are important factors in ensuring the mechanical and corrosion resistance properties of such a kind of devices. Traditionally continuous or pulsed laser systems working in microsecond pulse regime are employed for stent manufacturing. Pulsed fiber lasers on the other hand, are a relatively new solution which could balance productivity and quality aspects with shorter ns pulse durations and pulse energies in the order of mJ. This work reports the study of laser micromachining and of AZ31 magnesium alloy for the manufacturing of cardiovascular stents with a novel mesh design. A pulsed active fiber laser system operating in nanosecond pulse regime was employed for the micromachining. Laser parameters were studied for tubular cutting on a common stent material, AISI 316L tubes with 2 mm in diameter and 0.2 mm in thickness and on AZ31 tubes with 2.5 mm in diameter and 0.2 in thickness. In both cases process parameters conditions were examined for reactive and inert gas cutting solutions and the final stent quality is compared.

An increase in energy of pulses generated in a similariton mode-locked femtosecond fiber laser is shown experimentally by increasing the length while reducing the doping of the ytterbium-doped fiber gain medium. Mode-locking is achieved by nonlinear polarization rotation evolution in a cavity using a combination of fiber and bulk optical components. The level of doping of the gain medium is varied by using lengths of differently doped ytterbium fiber. Experimental results verify that an increase in length of gain medium with a lower doping results in an increase in the output pulse energy.

The modal cut-off properties of Yb-doped Double-Cladding Photonic Crystal Fibers (DC-PCFs) with 19-cell core have been analyzed considering an innovative approach, where the avoided crossing between the guided modes and the highest-index cladding mode of the real fiber is taken as the cut-off condition. A finite element-based full-vector modal solver has been applied to calculate the modes of DC-PCFs with different normalized air-hole size and core refractive index. Simulations have shown the possibility to find generalized results for the modal cut-off wavelengths of 19-cell DC-PCFs.

We present for the first time a cascaded Raman fiber laser where the Yb-doped fiber laser and Raman fiber are combined into a single fiber. We achieve 42.6 % slope efficiency at 1236 nm with respect to launched pump power.

We report our developments of a 3-visible-wavelengths (Green, Yellow, Red) CW fiber laser system providing more than 3W of output power per colour on a single output path, in a simple and robust design. The system is based on three infrared narrow linewidth linearly polarized all-fiber lasers, combined into the same output PM fiber by a fused-fiber PM multiplexer. This component has been specially developed for the wavelengths of interest and to sustain high powers in CW operation. The common polarized infrared output is sent into a non linear optical stage to generate visible wavelengths, using periodically-poled non linear crystals and single-pass frequency doubling configuration. The system provides either 4W at 532 nm, 3W at 577 nm or 3.5W at 633 nm with a perfect beam quality and a free-space output.

We report on the generation of a supercontinuum in a chalcogenide microstructured tapered fiber. The suspended core diameter of the fiber is reduced from 5.5 μm to 0.8 μm in the waist of the tapered region. The zero dispersion wavelength is below 2 μm in the tapered region. To pump the fiber, we use a modelocked laser of 4 ps, with a central wavelength of 1960 nm. With only 150 W peak power in the fiber a supercontinuum is generated from 1300 to 2600 nm taking the supercontinuum wavelength edge at -30 dB from the continuum.

This report presents a time-frequency-domain white-light interferometer for modal resolved group-velocity dispersion measurements in photonic crystal fibers via dual-channel detection in the spectral range from 0.4 μ;m up to 1.7 μm. Supercontinuum emerging from a photonic crystal fiber illuminates a Mach-Zehnder-interferometer. Temporally resolved spectrograms recorded at distinct delay positions enable the detection of interference fringes for the equalization wavelength. By applying a Sellmeier polynomial to the wavelength dependent differential group delays the dispersion can be derived. The dispersion for several fiber modes is investigated over a broad spectrum of 1,300 nm.

By confining the Yb doping within a smaller radius in the center of the core of the few-mode large-mode-area fiber, the fundamental mode, which overlaps better with the Yb ions, sees higher gain than the higher order modes, and dominates the output. Hence, improved beam quality can be achieved. A confined-doped fiber with 41/395μm core/cladding diameters is fabricated by Direct Nanoparticle Deposition (DND) process. The fiber is characterized in an amplified spontaneous emission (ASE) source setup. Near-diffraction-limited beam quality (M²~1.3) is experimentally demonstrated.

We report on a high-power ultra-short fiber laser for thin film solar cells micromachining. The laser is based on Chirped Pulse Amplification (CPA) scheme. The pulses are stretched to hundreds of picoseconds prior to amplification and can be compressed down to picosecond at high energy. The repetition rate is adjustable from 100 kHz to 1 MHz and the optical average output power is close to 13 W (before compression). The whole setup is fully fibred, except the compressor achieved with bulk gratings, resulting on a compact and reliable solution for cold ablation.

In this paper we report and summarize an extensive investigation of photodarkening in Yb-doped silica fibers. We made a set of similar fibers in order to cover a large Yb concentration range. The results show that once uniform inversion is reached all along the test fiber it is possible to show self-similarity of the time evolutions. The stretching parameter has a little influence. We can therefore provide two unique numbers to define photodarkening performance: the saturation loss and the time scale parameter. We also propose a possible figure of merit to compare different fibers. This may allow to evaluate the impact of photodarkening on laser/amplifier devices.

As shown recently, a long telecommunication fibre may be treated as a natural one-dimensional random system, where lasing is possible due to a combination of random distributed feedback via Rayleigh scattering by natural refractive index inhomogeneities and distributed amplification through the Raman effect. Here we present a new type of a random fibre laser with a narrow (~1 nm) spectrum tunable over a broad wavelength range (1535-1570 nm) with a uniquely flat (~0.1 dB) and high (>2 W) output power and prominent (>40 %) differential efficiency, which outperforms traditional fibre lasers of the same category, e.g. a conventional Raman laser with a linear cavity formed in the same fibre by adding point reflectors. Analytical model is proposed that explains quantitatively the higher efficiency and the flatter tuning curve of the random fiber laser compared to conventional one. The other important features of the random fibre laser like "modeless" spectrum of specific shape and corresponding intensity fluctuations as well as the techniques of controlling its output characteristics are discussed. Outstanding characteristics defined by new underlying physics and the simplicity of the scheme implemented in standard telecom fibre make the demonstrated tunable random fibre laser a very attractive light source both for fundamental science and practical applications such as optical communication, sensing and secure transmission.

Q-switched operation in a thulium doped photonic crystal fiber (PCF) is shown for the first time. The slope efficiencies of 25.9, 31.9 and 33 % were observed for 10, 20 and 50 kHz repetition rates. The high peak powers of >5 kW in the PCF oscillator was used to introduce modulation instability initiated >350 nm broadening in ~50 m SMF-28 fiber.

Here, we demonstrate all-fiber direct amplification of 11 picosecond pulses from a gain-switched laser diode at 1063 nm. The diode was driven at a repetition rate of 40 MHz and delivered 13 μW of fiber-coupled average output power. For the low output pulse energy of 0.33 pJ we have designed a multi-stage core pumped preamplifier based on single clad Yb-doped fibers in order to keep the contribution of undesired amplified spontaneous emission as low as possible and to minimize temporal and spectral broadening. After the preamplifier we reduced the 40 MHz repetition rate to 1 MHz using a fiber coupled pulse-picker. The final amplification was done with a cladding pumped Yb-doped large mode area fiber and a subsequent Yb-doped rod-type fiber. With our setup we achieved amplification of 72 dBs to an output pulse energy of 5.7 μJ, pulse duration of 11 ps and peak power of >0.6 MW.

We present a power scalable, all-fibre, monolithic pulsed source based on a thulium doped fibre master-oscillator power amplifier design. This source produced 200 μJ pulses with 20 - 40 ns duration at a repetition rate of up to 75 kHz resulting in up to 12 W of linearly polarized light at 2.044 μm. This was used to pump a walk-off compensated ZnGeP2 optical parametric oscillator which generated 3 W in the mid-infrared, with a conversion efficiency of 25%.

A novel design of Yb-doped double-cladding hybrid photonic crystal fibers, with three rows of high-index inclusions on each side of the core, has been numerically analyzed with a full-vector modal solver based on the finite element method. Simulation results have demonstrated that a stronger filtering effect and a higher bending tolerance can be obtained with an increased number of high-index resonators. Moreover, the unique bending properties of hybrid fibers with anti-symmetric design have been investigated and explained through the comparison with the ones of hybrid symmetric fibers, with high-index inclusions of the same dimension on both sides of the core.

We propose a sub-100 μm double-clad single-crystalline Tm:YAG square core adhesive-free bond (AFB) waveguide structure as a prototype gain medium for an LMA ~2-μm fiber laser with ~790-nm diode laser pumping and present its mode simulation. The structure uses an undoped YAG and ceramic spinel for the inner and outer claddings, respectively. The mode simulation indicates that such double-clad structure can have over 1000-μm² mode area in a 2% Tm:YAG core which will maintain a single transverse mode laser operation. The structure also has a large numerical aperture (~ 0.22) for the un-doped YAG inner cladding available for pumping.

A large-mode-area Ytterbium-doped photonic crystal fiber amplifier with efficient suppression of amplified spontaneous emission is presented. The fiber cladding consists of a hexagonal lattice of air holes, where three rows are replaced with circular high-index inclusions. Seven missing air holes define the large-mode-area core. Light confinement is achieved by combined index and bandgap guiding, which allows for single-mode operation and distributed spectral filtering of amplified spontaneous. The fiber properties give control of the gain shape and are ideal for amplification in the long wavelength regime of the Ytterbium gain spectrum above 1100 nm.

High-power fiber amplifiers for pulsed applications require large mode area (LMA) fibers having high pump absorption and near diffraction limited output. This improves the limiting factor of nonlinear effects, while maintaining good beam quality. Photonic crystal fibers allow realization of short LMA fiber amplifiers having high pump absorption through a pump cladding that is decoupled from the outer fiber. However, achieving ultra low NA for single-mode (SM) guidance is challenging, and thus different design strategies must be applied to filter out higher order modes (HOMs). The novel distributed modal filtering (DMF) design presented here enables SM guidance, and previous results have shown a SM mode field diameter of 60 μm operating in a 20 nm SM bandwidth. The DMF rod fiber has high index ring-shaped inclusions acting as resonators enabling SM guidance through modal filtering of HOMs. Large preform tolerances are compensated during the fiber draw resulting in ultra low NA fibers with very large cores. In this paper, design optimization of the SM bandwidth of the DMF rod fiber is presented. Analysis of band gap properties results in a fourfold increase of the SM bandwidth compared to previous results, achieved by utilizing the first band of cladding modes. This covers of a large fraction of the Yb emission band, where wavelengths of 1030 nm and 1064 nm can be included.

We report on a novel approach for the generation of radially and azimuthally polarized light employing a fiber mode filter. The mode filter consists of a Fiber Bragg Grating written in a strongly guiding fiber with lifted modal degeneracy. These kinds of fibers guide radially and azimuthally polarized modes with non-degenerated, i.e. distinct, effective refractive indexes. The Fiber Bragg Grating reflects light only if the Bragg condition is fulfilled. In case of strongly guiding fibers the radially and azimuthally polarized modes are guided with different effective refractive indices and, consequently, the Bragg condition is fulfilled at different wavelengths. If the reflection bandwidth of the Fiber Bragg Grating is narrow enough, the radially and azimuthally polarized modes are spectrally separated. Thus, with such a mode filter it is possible to filter the radially or azimuthally polarized mode. This filter is suitable for its integration in a resonator for stable, compact and high polarization purity azimuthally and radially polarized all-fiber oscillators. In a first experiment an azimuthally polarized mode filter consisting of a commercially available step index fiber and a femtosecond written Fiber Bragg Grating was fabricated. The experimental results are presented and discussed.

Coherent combining is a novel approach to scale the performance of laser amplifiers. The use of ultrashort pulses in a coherent combining setup results in new challenges compared to continuous wave operation or to pulses on the nanosecond timescale, because temporal and spectral effects such as self-phase modulation, dispersion and the optical path length difference between the pulses have to be considered. In this paper the impact of these effects on the combining process has been investigated and simple analytical equations for the evaluation of this impact have been obtained. These formulas provide design guidelines for laser systems using coherent combining.

We present an ultrabroadband supercontinuum (SC) frequency comb generation covering the optical spectrum more than 1.2 octave from 570 nm to 1350 nm with a near flat-top power spectrum by using a mode-locked Yb fiber laser at 1030 nm as a master oscillator (MO). The repetition rate of the MO that has a spectral-width of 28 nm is 152 MHz and is phase-locked to a reference frequency synthesizer. A power amplifier is used to boost up the output power up to 1.3 W and the positively chirped amplified pulses are compressed by using a transmission grating pair. We used a commercially-available 10-cm long photonic crystal fiber that has two zero-dispersion wavelengths at 800 nm and 1093 nm, respectively, to generate an ultrabroadband SC frequency comb. Optical spectrum depending on the frequency chirp and pulse-width of the amplified pulses are analyzed to find an optimum coupling power as well as an optimum chirp parameter. We found that a dimensionless chirp parameter of 8.6 and a pulse-width of 517 fs for a chirped Gaussian pulse resulted in the optimum spectrum width of 1.2 octave.

We report a core pumped thulium-doped fiber amplifier that generates 1.4 μJ pulses at 1980 nm with a repetition rate of 3.6 MHz preserving the original spectral bandwidth of the oscillator. The amplifier chain is seeded by a passively modelocked fiber laser with 5 mW output power and the pulses are stretched to 800 picoseconds. The amplifier is core pumped by a single mode erbium fiber laser. The slope efficiency is 35%. To the best of our knowledge, this is the first demonstration of sub nanosecond pulses with energies higher than 1 μJ coming out of a thulium-doped fiber amplifier.

We present a hybrid laser system based on all-fiber amplification of a gain-switched laser diode. The diode emits low energy pulses with several tens of picoseconds pulse duration at a wavelength of 1550 nm and a repetition rate of 1 MHz. The three-stage fiber amplifier reaches an overall gain of 55 dB boosting the pulse energy to 0.48 μJ. Much care is taken to preserve an almost bandwidth limited pulse with a spectral width of 0.1 nm and negligible spectral broadening due to nonlinearities. The laser system has been designed such that two or more can be electronically synchronized with the aim to combine them for exploring frequency mixing scenarios. Here, we report first cross-correlation measurements of two synchronized laser systems and present a method to characterize the relative timing-jitter.

We have now demonstrated and characterized gas-filled hollow-core fiber lasers based on population inversion from acetylene (¹²C₂H₂) and HCN gas contained within the core of a kagome-structured hollow-core photonic crystal fiber. The gases are optically pumped via first order rotational-vibrational overtones near 1.5 μm using 1-ns pulses from an optical parametric amplifier. Transitions from the pumped overtone modes to fundamental C-H stretching modes in both molecules create narrow-band laser emissions near 3 μm. High gain resulting from tight confinement of the pump and laser light together with the active gas permits us to operate these lasers in a single pass configuration, without the use of any external resonator structure. A delay between the emitted laser pulse and the incident pump pulse has been observed and is shown to vary with pump pulse energy and gas pressure. Furthermore, we have demonstrated lasing beyond 4 μm from CO and CO₂ using silver-coated glass capillaries, since fused silica based fibers do not transmit in this spectral region and chalcogenide fibers are not yet readily available. Studies of the laser pulse energy as functions of the pump pulse energy and gas pressure were performed. Efficiencies reaching ~ 20% are observed for both acetylene and CO₂.

We report on investigations of direct bonding of mono-crystalline Terbium Gallium Garnet (TGG) to sapphire for use as Faraday isolator in high power fiber laser applications. The technology of direct bonding allows joining of inorganic crystalline and non-crystalline materials without any auxiliary materials at high bonding strength. Lack of additional joining material and related absorption is very promising for high power laser applications. The ability to join different materials allows inserting sapphire as a heat spreader into the beam path for TGG cooling to avoid thermal lensing. All samples were subjected to extensive chemical cleaning and subsequent low temperature plasma activation before bonding. Bonding was performed under compressive forces in a high vacuum environment at moderate temperature. Optically transparent bonds with a very low fraction of defects were obtained. The size of the bonded area depends significantly on the samples' flatness.

Fiber lasers emitting in the 2 μm wavelength range doped with thulium ions can be used as highly efficient pump sources for nonlinear converters to generate mid-infrared radiation. For spectroscopic purposes, illumination and countermeasures, a broad mid-infrared emission spectrum is advantageous. This can be reached by supercontinuum generation in fibers, e.g. fluoride fibers, which up to now has, however, only been presented with either low average power, complex Raman-shifted 1.55 μm pump sources or multi-stage amplifier pump schemes. Here we present recent results of a new actively-mode-locked single-oscillator scheme that can provide the high-repetition rate sub-ns pump pulses needed for pumping supercontinuum generators. A thulium-doped silica fiber laser is presented that provides > 11 W of average power CW-mode-locked pulses at 38 MHz repetition rate at ~ 38 ps pulse width. Upgrading the setup to allow Q-switched mode-locked operation yields mode-locked 40 MHz pulses arranged in 60 kHz bunched Q-switch envelopes and thus increases further the available peak power. In this Q-switched mode-locked regime over 5 W of average power has been achieved.