This PDF file contains the front matter associated with SPIE Proceedings Volume 7195, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing

In this contribution, we report on an 80-μm core diameter Yb-doped rod type photonic crystal fiber laser emitting up to 94W in CW regime when operating at 977nm, which is to our knowledge the highest output power ever achieved from a single-mode solid-state laser operating at this wavelength. Key parameters of ytterbium doped 3-level laser, such as transparency pump intensity, pump absorption saturation, and gain competition between three and four level laser operation are then discussed in the particular context of high power fiber laser operating at 977nm. Possible applications of the demonstrated source are then discussed.

A large number of high power CW fiber lasers described in the literature use large mode area (LMA) double cladding fibers. These fibers have large core and low core numerical aperture (NA) to limit the number of supported modes and are typically operated under coiling to eliminate higher order modes. We describe here multimode (MM) high NA ytterbium doped fibers used in single mode output high power laser/amplifier configuration. Efficient single mode amplification is realized in the multimode doped fiber by matching the fundamental mode of the doped fiber to the LP₀₁ mode of the fiber Bragg grating (FBG) and by selecting the upper V-number value that limits the overlap of the LP₀₁ to the higher order modes. We show that negligible mode coupling is realized in the doped fiber, which ensures a stable power output over external perturbation without the use of tapers. Fundamental mode operation is maintained at all time without coiling through the use of FBG written in a single mode fiber. We show that such fiber is inherently more photosensitive and easier to splice than LMA fiber. We demonstrate an efficient 75W singlemode CW fiber laser using this configuration and predict that the power scaling to the kW level can be achieved, the design being more practical and resistant to photodarkening compared to conventional low NA LMA fiber.

Pump propagation and absorption in active tapered double-clad fiber has been analyzed based on a ray optics approach. Optimization of the longitudinal shape, absorption and angular distribution of the pump beam allowed for power scaling of a ytterbium fiber laser up to 600 W with high beam quality (M²≤1.08) and a slope efficiency of 63%. It is shown that the influence of vignetting in a tapered fiber can be avoided, resulting in high overall efficiency, in good agreement with the presented model.

Northrop Grumman Corporation has made significant progress in the development of compact, high power, continuous operation solid state lasers for military applications during the past six years. The Joint High Power Solid State Laser (JHPSSL) program is nearing completion of its third phase; its key objective is to demonstrate a 100kW solid state laser with excellent beam quality. Northrop's unique scalable architecture coherently combines modular 15kW lasers to produce power levels of 100kW and beyond with excellent beam quality and run times. This paper describes the JHPSSL program history, Northrop's high power solid state laser architecture and our demonstrated results.

High power fiber lasers deliver multiple kW laser power with a diffraction limited beam quality. A drawback for some applications is the arbitrary polarization. We report on experimental and theoretical results of kW class cw fiber lasers with linear polarization. A comparison of different concepts for generation of polarized high power output will be presented. The most feasible design for kW class power scaling will be selected. In order to find an empirical formula for calculating bend loss, additional measurements are carried out and are then compared to the theoretical results.

Fused fiber components are the key building blocks that enable reliable and efficient operation of high power fiber lasers. In this paper, we review fabrication techniques for the manufacture of such devices, including mode-field adaptors, fiber tapers, fused couplers, and fused combiners. We present the basic equations governing both the optical performance and fabrication requirements for these devices, and demonstrate how these apply to some common fiber laser applications. We then describe and discuss component fabrication techniques and available hardware.

We propose an in-fiber chiral optical isolator based on chiral fiber polarizer technology and calculate its performance by incorporating the magnetic field into the scattering matrix. The design will be implemented in a special preform, which is passed through a miniature heat zone as it is drawn and twisted. The birefringence of the fiber is controlled by adjusted the diameter of a dual-core optical fiber. By adjusting the twist, the fiber can convert linear to circular polarization and reject one component of circular polarization. In the novel central portion of the isolator, the fiber diameter is large. The effective birefringence of the circular central core with high Verdet constant embedded in an outer core of slightly smaller index of refraction is small. The central potion is a non-reciprocal polarization converter which passes forward traveling left circularly polarized (LCP) light as LCP, while converting backward propagating LCP to right circularly polarized (RCP) light. Both polarizations of light traveling backwards are scattered out of the isolator. Since it is an all-glass structure, we anticipate that the isolator will be able to handle several watts of power and will be environmentally robust.

The inscription of fiber Bragg gratings with femtosecond laser radiation gives access to a wide range of new materials beyond conventional UV-photosensitive and special hydrogen loaded materials. Typically, infrared wavelengths have been used until now for femtosecond Bragg grating inscription. The use of UV femtosecond laser radiation gives more direct access to two-photon absorption in materials with high band-gaps and could achieve gratings with high spatial resolution. The refractive index modulation process for UV femtosecond pulses is then obtained as a combination of classical color center absorption and high intensity structural modification. Besides the choice of the inscription laser wavelength, also the inscription method is of great practical importance for the grating properties. The most commonly used phase mask technique suffers from mask degradation effects in case of high intensity laser pulses and gives only limited flexibility in variation of the Bragg reflection wavelengths. We have therefore investigated the use of an interferometric technique in combination with UV femtosecond laser pulses. The relevant parameters of spatial and temporal coherence have been considered and the wavelength tuning and reflection properties have been tested for several different types of fibers.

We demonstrate 30 W of average UV power, at 353 nm, by harmonically converting the output of a seeded cascade of fiber amplifiers operating at 1060 nm. The UV output represents 46% harmonic conversion efficiency from the fundamental beam. The all-fiber-amplifier, MOPA architecture supports variable pulse repetition frequencies and pulse widths. We demonstrate pulse repetition frequencies up to 2 MHz and pulse widths as short as 2 ns. Two bulk LBO crystals, oriented for second and third harmonic conversion, are used to obtain stable UV output power. A turnkey system using this architecture is commercially available. The system is entirely air-cooled and operates from a standard wall plug electric service, facilitating integration into various material processing applications.

We present in this paper multi-watts CW operation of an Ytterbium-doped fiber laser directly emitting at wavelengths above 1150 nm for frequency-doubling in the yellow spectral range. A maximum output power of more than 15W has been obtained at 1154 nm in CW linearly polarized operation with a linewidth of less than 0.20 nm (FWHM) in an all-fiber configuration. Multi-watts second-harmonic generation (SHG) in the yellow spectral range with periodically-poled non linear crystals is presented.

Quasi-cw UV light sources are of interest for replacing frequency-doubled Ar-Ion lasers in several applications. Our target application in semiconductor inspection requires a narrow bandwidth cw or quasi-cw source at 258nm, which cannot be achieved by frequency converting the output of a (modelocked) Neodymium-based laser. We developed a fiber MOPA system which operates at a high repetition rate of 5MHz and generates 1ns long pulses. The system consists of a low power oscillator and four consecutive amplifier stages. which boost the average power to 40W at 1031nm. The IR output of the fiber system is frequency doubled and quadrupled using LBO and CLBO crystals for SHG and FHG, respectively. We achieved SHG conversion efficiencies of up to 82% and a UV power of up to 14W.

This paper reviews the progress in active fibers suitable for power scaling, highlighting the advances in fiber design that will enable the control of nonlinearities such as SRS and SBS in high power fiber lasers, as well as making feasible a practical high power three-level system.

We demonstrate suppression of amplified spontaneous emission at the conventional ytterbium gain wavelengths around 1030 nm in a cladding-pumped polarization-maintaining ytterbium-doped solid core photonic crystal fibre. The fibre works through combined index and bandgap guiding. Furthermore, we show that the peak of the amplified spontaneous emission can be shifted towards longer wavelengths by rescaling the fibre dimensions. Thereby one can obtain lasing or amplification at longer wavelengths (1100 nm - 1200 nm) as the amount of amplification in the fibre is shown to scale with the power of the amplified spontaneous emission.

Leakage channel fibers have demonstrated their ability to significantly extend the effective mode area of a fundamental mode while maintaining robust single mode operation. These fibers are designed to have strong built-in mode filtering which effectively suppresses the propagation of all higher order modes while keeping fundamental mode loss to a minimum, and, therefore, effectively extending the regime of single mode operation. Recently all-glass leakage channel fibers have been demonstrated as a significant improvement over designs with air holes. These all glass leakage channel fibers not only can be manufactured with much improved consistence and uniformity. They can also be handled and used as conventional fibers. More importantly, mode distortions from collapse of air holes in photonic crystal fibers during splicing and other end face treatments are largely eliminated. We will review some of the recent progress in this area.

We demonstrate the first operation and preliminary characterization of a dual core double-clad fiber laser where the laser operates in a coaxially doped thulium outer core region and in-turn resonantly pumps a laser with a holmium doped inner core. The fiber laser is 790nm pumped producing up to 1.5 W near 2.1μm.

We have developed a production process for rare earth doped bulk silica to fulfill the demand of such material for fiber laser applications. In contrast to the standard techniques such as a combination of MCVD (modified chemical vapor deposition) and solution doping, our novel technology is based on a granulate process that enables novel ultra large mode area fiber designs (XLMA) with active core diameters above 100 μm as well as larger batch sizes. Several Yb-doped fibers with two different fiber designs were manufactured and successfully tested in both side- and end-pumped fiber laser setups. Both fiber designs have been compared to similar MCVD fibers. The influence of the material composition on the photodarkening properties has been investigated.

Photodarkening is a detrimental phenomenon known to affect ytterbium doped fibers. Methods to study the spectral and temporal properties of the photodarkening induced loss were developed. The spectral shape of the photodarkening loss measured from multiple aluminosilicate samples indicate that visible wavelength(s) could be used in benchmarking fibers for their PD induced loss. Two principal methods, core and cladding pumping, were introduced to induce a known and repeatable inversion to fiber samples. The photodarkening rate could be parameterized using a single variable, inversion. More generally, the photodarkening rate was found to follow a simple power law and to be proportional to [Yb]^7±1 (the excited state Yb ion density). Two methods, stretched exponential and bi-exponential, were used to fit the rate measurements. Both fitting methods were found suitable, with the bi-exponential method having more potential in increasing the understanding of the mechanism(s) behind photodarkening. Coiling induced spatial changes in the inversion and subsequent photodarkening performance were demonstrated for a large-mode-area fiber laser.

In the last years, photodarkening in ytterbium doped silica based laser fibers turned out to be a critical factor for high power laser action. Several investigations have been carried out in order to characterize the time dependent increase of the fiber loss and to understand and model the complex optical phenomenon. Despite of progress in this field, there is still a lack of data concerning the detailed influence of fiber composition and preparation process parameters as well as concerning the role of atomic defects in the core glass. Here we report on investigations about the photodarkening in dependence on the glass composition of the fiber laser core. By MCVD, fibers with different codopants (additional to the active ytterbium doping) have been prepared in a well-defined manner, regarding process parameters and glass composition, and comprehensively characterized. In addition to the photodarkening measurements, further optical properties have been measured on the fibers and fiber performs, which are related to the photodarkening effect: intensity and spectral behaviour of the Yb³⁺ absorption and emission in the NIR, cooperative visible fluorescence, UV absorption and UV excited visible emission. The concentration of codopants which are commonly used for active and passive lightguide fibers (aluminium, germanium, phosphorus) was systematically varied and correlated with the optical properties.

In this paper we present how charge transfer processes influences the induced optical losses (photodarkening) in ytterbium doped fiber lasers. The location of the charge transfer absorption band is strongly composition dependent and is correlated to the valence stability of the ytterbium ion in the silicate glass matrix. An improved photodarkening performance can in general be observed for a charge-transfer band shifted to shorter wavelengths, although other routes are also possible to reduce photodarkening. Other parameters that affect the laser performance, such as absorption and emission cross section, must also be considered.

Spectroscopic studies of intrinsic defect centers in Yb doped silica fibers are presented. The relationship between the defect centers and photodarkening in Yb doped silica fibers is investigated. Photoluminescence from nonbridging oxygen hole center (NBOHC) and oxygen deficiency center (ODC) defects in several ytterbium-doped silica fibers are presented and analyzed. The photoluminescence spectra and temporal decays are given to determine if the Yb dopant and associated co-dopants which serve as network modifiers in the silica matrix could provide pathways whereby infrared photons could be upconverted into the UV, leading to damage in the fiber.

A model description of photo darkening of ytterbium cw fiber lasers based on long term tested fiber lasers is presented. Photo darkening of Yb/Al co-doped silica fibres is found to saturate following prolonged exposure to pump and signal radiation. The observed slope efficiency of Al co-doping is compared with modeled slope efficiency of P co-doping.

A Mid-InfraRed FIber Laser (MIRFIL) has been developed that generates super-continuum covering the spectral range from 0.8 to 4.5 microns with a time-averaged power as high as 10.5W. The MIRFIL is an all-fiber integrated laser with no moving parts and no mode-locked lasers that uses commercial off-the-shelf parts and leverages the mature telecom/fiber optics platform. The MIRFIL power can be easily scaled by changing the repetition rate and modifying the erbium-doped fiber amplifier. Some of the applications using the super-continuum laser will be described in defense, homeland security and healthcare. For example, the MIRFIL is being applied to a catheter-based medical diagnostic system to detect vulnerable plaque, which is responsible for most heart attacks resulting from hardening-of-the-arteries or atherosclerosis. More generally, the MIRFIL can be a platform for selective ablation of lipids without damaging normal protein or smooth muscle tissue.

Fiber lasers are an ideal pump source for nonlinear frequency conversion because they have the capability to generate short pulses with high peak-powers and excellent beam quality. Thulium-doped silica fibers allow for pulse generation and amplification in the 2-micron spectral band. This opens the door to a variety of nonlinear crystals, such as ZnGeP₂ (ZGP) and orientation patterned GaAs (OPGaAs), which cannot be pumped by Yb- or Er-doped fiber laser directly due to high losses in the near-IR band. These crystals combine low losses with high nonlinearities and transparency for efficient nonlinear mid-IR converters. Using such nonlinear crystals and a pulsed Tm-doped master oscillator fiber amplifier (MOFA), we have demonstrated efficient mid-IR generation with watts of output power in the 3-5μm region. The Tm-doped MOFA is capable of generating from 10 to 100W of average output power at a variety of repetition rates (10kHz - >500kHz) and pulse widths (10ns - >100ns). Total mid-IR power is only limited by thermal effects in the nonlinear materials. The use of Tm-doped fiber-pumped OPOs shows the path toward compact, efficient, and lightweight mid-IR laser systems.

A four-stage, Tm-doped fiber amplifier chain emitted 608 W of single-frequency (SF) output power with 53 dB gain, 54% slope efficiency, and M² = 1.05 beam quality. The output power was limited by available pump power. The final amplifier stage preserved the input <5-MHz linewidth and imposed negligible phase noise above 3 kHz. SBS limits at the 2040-nm operating wavelength were measured by splicing different lengths of passive fiber to the amplifier exit. Thermal limits of the fiber were explored analytically and are consistent with the measured power performance. Comparison of the SBS and thermal limits suggests a maximum SF power of ~750 W from this fiber configuration, with further potential to scale past 1 kW with different fiber parameters. To our knowledge, this is the highest power reported to date from any single-frequency, single-mode fiber laser.

We present an all-fiber high power tunable femtosecond soliton-based source incorporating a picosecond fiber laser and an 8 m long piece of hollow-core photonic bandgap fiber. Strongly chirped high energy 5.5 ps pulses produced by fiber amplification are compressed in the hollow core enabling formation of stable 520 fs-solitons with 77% conversion efficiency. Wavelength tunability was provided by exploiting Raman self-frequency shift of the solitons yielding 33nm tuning range. The transform limited output pulses were frequency doubled using a conventional nonlinear crystal with high conversion efficiency of 60%. Demonstration of a femtosecond green laser tunable from 534 nm to 548 nm with 180nJ pulse energy is also reported.

We report the generation of high temporal quality high energy ultra-short pulses from a moderately non-linear fiber chirped pulse amplifier. The system is based on a two amplification stages and delivers 270 fs pulses of 100 μJ energy at a repetition rate of 300 kHz. The excellent temporal quality of the recompressed pulses, down to 1.1 the Fourier limit, allows the production of ~ 340 MW of peak power. The fibre chirped pulse amplifier is design to compensate the nonlinear phase shift accumulated throughout the amplifier stages by third order of dispersion provided by mismatched temporal stretcher and compressor units.

We present a degenerated-parametric amplifier with gigawatt peak power operating at 1030 nm and 30 kHz repetition rate. Pulses of a fiber chirped pulse amplification (FCPA) system with 650 fs pulse duration and 1 mJ pulse energy are frequency doubled and used as pump source for a two stage optical parametric amplifier. Both the FCPA and the optical parametric amplifier (OPA) are seeded by the same YB:KGW oscillator. Spectral broadening of the OPA seed signal in a short-polarization-maintaining-step-index fiber creates enough bandwidth for sub 30 fs pulse generation, while temporal synchronization of pump and signal is realized by means of a multipass cell in the OPA signal beam path. Parametric amplification of the broadband signal takes place in two 1 mm BBO crystals. Pulse compression via chirped mirrors yields 81 μJ pulses as short as 29 fs. The corresponding pulse peak power is estimated to be as large as 2 GW. Together with the good beam quality (measured M²<1.8) this device enables high intensity experiments at high repetition rates.

We report on an ytterbium-doped fiber CPA system delivering 325 W of average power at 40 MHz repetition rate corresponding to 8.2 μJ pulse energy. The pulse duration is as short as 375 fs resulting in 22 MW of peak power.

We report on the generation of microjoule level picosecond pulses from a mode-locked Yb-doped LMA fiber laser operating in the purely normal dispersion regime. The self-starting oscillator stabilized with slow relaxation semiconductor saturable absorber (SAM) emits 11 W of average power at a pulse repetition rate of 10 MHz, corresponding to a pulse energy of 1.1 μJ. The laser produces a 0.4 nm narrow emission line with 310 ps output pulses. In the femtosecond operation, the oscillator stabilized with fast relaxation SAM emits 9 W of average power at a pulse repetition rate of 9.7 MHz, corresponding to a pulse energy of 927 nJ. The laser produces positively chirped output pulses of 8 ps which are compressed down to 711 fs, corresponding to megawatt peak power. To our knowledge this is the first time that mode-locked fiber oscillators can generate higher pulse energies of beyond microjoule-level at high average output power.

A femtosecond fiber system based on nonlinear chirped-pulse amplification is investigated. It is the first investigation on pulse properties from all-normal-dispersion fiber laser after amplification. Nonlinearities due to fibers are carefully managed. The system generates up to 5-μJ pulses, and delivers near diffraction-limit beam (M² < 1.1), polarization extinction ratio (40 dB) and polarization extinction ratio (36 dB).

We review what we have learned in the last few years from modeling nanosecond fiber amplifiers. We have developed a number of models that treat bent fiber mode profiles and bend loss, plus gain models and models of nonlinear processes such as self phase modulation, self focusing, SRS, SBS, and four wave mixing. The models have been validated by detailed comparisons with laboratory measurements.

High-speed, high-resolution materials processing strongly benefits from optical sources that deliver high peak power in short, high-repetition-rate pulses of excellent beam quality. These sources are also of interest for achieving high average power at nonlinearly-generated wavelengths. Until recently, high peak power, high-repetition-rate pulses have only been available from solid-state lasers. Fiber lasers and amplifiers offer significant advantages over solid-state lasers in terms of size and wall-plug efficiency. This paper presents a fiber-based master-oscillator/power-amplifier (MOPA) source at 1064nm featuring 84-μm-core, polarization-maintaining Yb-doped photonic crystal fiber that generates ~20ps - 100ps pulses at variable pulse repetition frequencies (PRFs), from 10kHz to 100MHz. The flexibility in pulse format allows the source to be tailored to the application. Where peak power is critical, the PRF is reduced to achieve maximum peak power. Where average power is needed, the PRF is increased to achieve high average power. Peak powers of ~4MW have been achieved at reduced PRF (100kHz), and average powers greater than 172W have been demonstrated at high PRF (100MHz) in a linearly-polarized output beam.

In this contribution, we report on spectral combination of four sub-5ns pulsed fiber amplifier systems with an average output power of 200W at 200kHz repetition rate resulting in 1mJ of pulse energy. A dielectric reflection grating is used to combine four individual beams to one output possessing a measured M² value of 1.3 and 1.8, respectively, independent of power level. Extraction of higher pulse energies and peak powers will be discussed.

Pulseshaping is important in high energy pulsed fiber MOPA system to mitigate non-linear effects and optimize the processing of different materials. However, pulseshaping is greatly limited by the spectral features of the semiconductor seed source commonly used as the master oscillator. Through the appropriate design of an external fiber Bragg grating (FBG) and adequate current modulation, the spectrum of the fiber-coupled seed laser was broadened to suppress stimulated Brillouin scattering occurring in the amplifier chain and the central emission wavelength and bandwidth were controlled. Pulseshaping is also quickly limited by the saturation energy and doping level of standard aluminosilicate ytterbium doped fibers used in the power amplifier even with large core diameter. Co-doping the fiber with phosphorus greatly increases the saturation energy of the system, which gives smoother pulseshape and significantly lower stimulated Raman scattering (SRS). It is shown that going from 1060 nm to longer emission wavelength at 1090 nm with this fiber increases further the pulseshaping capabilities and reduces SRS. The phosphorus codoping also allows higher ytterbium doping level without photo-degradation, which decreases nonlinear effects generation during the amplification while giving more flexible pump wavelength choice and efficiency.

We derive an expression describing pre-compensation of pulse-distortion due to saturation effects in short pulse laseramplifiers. The analytical solution determines the optimum input pulse required to obtain any arbitrary target pulse at the output of the saturated laser-amplifier. The relation is experimentally verified using an all-fiber amplifier chain that is seeded by a directly modulated laser-diode.

We report a new pulsed, narrow linewidth, single-mode, polarization maintaining (PM) all-fiber laser source in master oscillator and power amplifier (MOPA) configuration that can operate over the C-band. The single-frequency pulsed fiber laser seed was achieved by actively Q-switching a fiber laser using a piezo, with a wide pulse duration tuning range of 7.5 ns - 1.24 μs. We use single-mode PM large core highly Er/Yb co-doped phosphate glass fiber (LC-EYPhF) in the power amplifier stage of MOPA to achieve 54 μJ/pulse for 153-ns pulses at 1538 nm with repetition rate of 20 kHz and an estimated linewidth of ~ 5 MHz.

The power available from narrow-linewidth single-transverse-mode fiber amplifiers is primarily limited by the onset of stimulated Brillouin scattering. One approach for increasing the SBS threshold that has shown recent promise is to tailor the acoustic velocity within the fiber cross-section to suppress Brillouin gain. Relating the SBS threshold to an acousto-optic effective area has yielded a theory which contradicts experimental measurements that indicate the nonlinear optical effective area of the tested SBS suppressing and Higher Order Mode (HOM) fibers is of primary importance in the nonlinear process. In this work, we present a new formalism for determining the Brillouin gain in fibers with inhomogeneous acoustic velocity which may be implemented with a wide variety of computational methods. We find that the Brillouin gain amplitude and spectrum are independent of the acousto-optic effective area and that they reduce to the bulk result for conventional step-index fibers. Implementing a finite-element method, we find that an SBS-suppressing design employing a negative focal length acoustic lens exhibits a broadened gain spectrum and reduced gain amplitude relative to step-index fibers. The SBS threshold of this fiber is increased by 8.4 dB relative to a standard large mode area fiber, each with an identical 6 meter length. Designs that further flatten the Brillouin gain spectrum have the potential to further increase the SBS threshold leading to higher single-frequency output power from devices incorporating these fibers.

A single frequency fiber laser operating near 2 micron with over 50 mW output power has been demonstrated by using a short piece of newly developed single mode holmium-doped germanate glass fiber. Laser from 2004 nm to 2083 nm was demonstrated from a short Ho-doped fiber laser cavity. A heavily thulium-doped germanate fiber was used as an in-band pump source for the holmium-doped fiber laser. The single frequency fiber laser can be thermally tuned.

We report high power phase locked fiber amplifier array using the Self-Synchronous Locking of Optical Coherence by Single-detector Electronic-frequency Tagging technique. We report the first experimental results for a five element amplifier array with a total locked power of more than 725-W. We will report on experimental measurements of the phase fluctuations versus time when the control loop is closed. The rms phase error was measured to be λ/60. Recent results will be reported. To the best of the authors' knowledge this is the highest fiber laser power to be coherently combined.

Fiber lasers provide an attractive means of reaching high output laser power because of their advantages in terms of compactness, reliability, efficiency and beam quality. In order to obtain much higher output power than it is possible from a single fiber, beam-combining techniques have been investigated. In this communication, we present a new technique of coherent fiber combining, based on self adaptive digital holography that does not require any phase error measurement. A low power plane reference beam is first launched into the fiber amplifier array. The interference pattern between the beams with phase φ(x,y) issued from the fiber array and a plane reference beam is recorded on a digital camera and directly transferred to a Spatial Light Modulator (SLM) which acts as a programmable digital hologram. This hologram is read out simultaneously and a phase conjugate beam with phase -φ(x,y) is generated in order -1 of the diffraction pattern. This beam is then injected in the fiber amplifier array. At the output of the fiber amplifier array, the phase of each elementary beam are locked. Experimental demonstration of coherent beam combining by digital holography is demonstrated with polarization maintaining fibers operating at 1 μm. Digital holography is realized thanks to a CCD/CMOS camera and a liquid crystal SLM. Owing to the high resolution of existing SLMs and cameras, this technique could be applied to phase lock a large number of fiber amplifiers.

Volume holographic elements can potentially combine many spectrally distinct laser beams using multiplexed Bragg gratings. However, there are theoretical limits imposed by the interactions between these gratings. A method of numerical simulation has been developed to analyze the interactions between many hundreds of multiplexed gratings in a single volume holographic element. We apply this tool to the problem of spectral beam combining of incoherent laser sources. Various system parameters limit the number of beams that can be combined under maximum bandwidth and minimum efficiency constraints. System designs are shown that combine 180-350 lasers ranging over a 60nm bandwidth with theoretical efficiencies in excess of 99%.

Volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass are used in a wide range of high-power laser applications due to their unique spectral response and excellent optical and thermo-mechanical properties. Experimental results of applications of narrow-band reflecting VBGs to spectral beam combining (SBC) and wavelength control of fiber lasers are presented. Output power of 770 W from a system combining five fiber lasers with 91.7% efficiency is demonstrated with spectral separation between channels of 0.5 nm around 1064 nm and no distortions in diffracted beams. Similar system with 0.25 nm channel separation around 1550 nm is demonstrated with the same efficiency and M² of the spectrally-combined beam < 1.15. A novel compact monolithic multi-channel beam combiner based on stacked tilted VBGs is suggested. Absolute efficiency exceeding 90% is reported for a four-channel device with 0.7 nm spectral separation of channels. We show that a linear stack of monolithic combining elements enables compact spectrally-combined laser systems with output power of 10-100 kW. A common-cavity approach to multi-channel spectral beam combining of high-power lasers is demonstrated. In this configuration wavelengths of the sources are passively controlled by a combination of a common output coupler and intra-cavity VBGs, which also act as combining elements. Laser wavelengths are automatically selected to match resonant wavelengths of respective gratings and provide maximum combining efficiency. Stable operation of a passively-controlled system combining two amplifiers with 0.4 nm spectral separation is demonstrated. Wavelengths of amplifiers are shown to automatically follow Bragg condition of VBGs during heating of gratings.

Fiber lasers have been receiving considerable attention because of their advantages of high power, high beam quality and high efficiency, and are expected as one of the desirable heat sources for high-speed and deep-penetration welding. In our researches, therefore, the effects of laser powers and their densities on the weld penetration and the formation of sound welds were investigated in welding of Type 304 austenitic stainless steel, A5052 aluminum alloy or high strength steel plates with four laser beams of about 0.12 to 1 mm in focused spot diameter, and their welding phenomena were observed with high-speed video cameras and X-ray transmission real-time imaging system. It was found that the laser power density exerted a remarkable effect on the increase in weld penetration at higher welding speeds, but on the other hand at low welding speeds deeper-penetration welds could be produced at higher power. Laser-induced plume behavior and its effect on weld penetration, and the mechanisms of spattering, underfilling, porosity and humping were elucidated, sound welds without welding defects could be produced under the improved welding conditions. In addition, importance of the development of focusing optics and the removal of a plume during remote welding will be emphasized in terms of the stable production of constant deep-penetration welds and the reduction in welding defects in high power laser welding.

We report on recent advances in laser material processing using a novel pulsed fiber laser platform providing pulse shape agility at the nanosecond time scale and at high repetition rates. The pulse shapes can be programmed with a time resolution of 2.5 ns and with an amplitude resolution of 10 bits. Depending on the desired laser performances, the pulses are generated either by directly modulating the drive current of a seed laser diode or by modulating the output of a seed laser diode operated in CW with electro-optic modulators. The pulses are amplified in an amplifier chain in a MOPA configuration. Advanced polarization maintaining LMA fiber designs enable output energy per pulse up to 60 μJ at 1064 nm at a repetition rate of 200 kHz with excellent beam quality (M²< 1.1) and narrow line widths suitable for efficient frequency conversion. Micro-milling experiments were carried out with stainless steel, in which processing microstructures of a few tens of microns in size usually represents a challenge, and aluminum, whose thermal conductivity is about 20 times higher than stainless steel. The results obtained with two metals having very different thermal properties using different pulse shapes with durations varying between 3 ns and 80 ns demonstrate the benefits of using lasers offering flexible pulse durations and controllable pulse intensity profiles for rapidly optimizing a process in different applications while using the same laser with respect to conventional methods based on pulsed laser with fixed pulse shapes. Numerous applications are envisioned in a near future, like the micromachining of multi-layered structures, in particular when working with the harmonics of the laser.

Precise optical loss measurements are a prerequisite for the development of new optical materials and complex optical multilayers. A flat supercontinuum (400 nm - 1650 nm), generated by a photonic crystal fiber pumped with a train of kHz nanosecond Q-switched microchip laser pulses at 1064 nm is used for high sensitive cavity ring down (CRD) loss measurements. The supercontinuum based CRD-technique enables the precise determination of the reflectivity of highreflective coatings from R = 0.995 to R = 0.99995 or of the transmission loss of optical materials from τ = 0.005 to τ = 0.00005 with an accuracy better than 2 • 10^-6, and covers an extreme wide spectral range of more than 1000 nm.

The thermal degradation of double clad optical fiber coatings is known to be the prime limiting factor for the operation of high power CW fiber lasers. In this paper, we conduct a study of thermal effects in high power CW fiber lasers. A particular focus is put on heating at the splice points and in the doped fiber due to the quantum defect in 100-W class CW fiber lasers. A theoretical model and experimental measurements taken with a high resolution IR camera on 125 to 400 μm diameter fibers are presented. Thermal contact resistance between the fiber and its heat sink are considered in the conduction heat transfer model and measured for different geometries. Proper designs for cooling apparatus are proposed and optimization of the active fiber is discussed. Some predictions for power scaling and temperature management of fiber lasers to kW power level are also described.

Cr:forsterite (Cr:Mg₂SiO₄) single crystal fibers of diameter less than 100 μm were made for the first time to our knowledge. This novel fiber material will be used to make fiber light sources such as fiber lasers and broadband light sources for applications in biophotonics and optical communications. Cr:forsterite crystal has a broad emission spectrum ranging from 1.1 to 1.4 μm that traditional glass fibers or semiconductor light sources cannot offer. And fiber light sources are compact, efficient, maintenance-free and compatible with fiber-optic components potentially leading to new performance and functions. In this work, bulk Cr:forsterite crystal was melted, pulled and re-grown into a long fiber using laser heated pedestal growth (LHPG) technique. Single crystal rhombic structure was preserved and verified by Xray diffractometer. By using electron probe micro-analyzer, change in Cr dopant concentration and distribution profile for various fiber diameters and growth conditions was studied.

Manufacture fiber amplifiers and lasers in a versatile and cost effective way while controlling rare-earths chemical environment becomes a real technology differentiator. A MCVD compatible Nanoparticle Doping Process has been developed to master with a higher accuracy rare earth amplifier and ytterbium laser fibers. Improved doped erbium fibers with C-band gain shape were obtained with much less aluminum content and unprecedented low background attenuation losses. This process should better show its merits in high power regime paving the way to fiber amplifiers and lasers for low cost new performances.

We report recent advances in the domain of Highly Non-Linear Photonic Crystal Fibers (HNL-PCFs) especially designed as gain medium for Raman fiber lasers. Indeed, a fiber Raman coefficient as high as 42 W^-1.km^-1 at 1.12μm has been obtained, while keeping optical losses moderate, below 6 dB/km at this wavelength. We have calculated that only 2 meters of such a germanium doped HNL-PCF is required to obtain an output power in the order of 10W at 1.12 μm with an efficiency of 90%. Experimental output optical spectra of multi-cascades cavities are finally given.

Fibers used for high power delivery are designed to ensure single-mode operation (in order to guarantee good output beam quality), large effective areas (A_eff) and resistance to bend-induced distortions (in order to avoid non-linear effects). For simple step index fibers, the maximum Aeff of the fundamental mode that can practically be achieved at 1.06μm is ~350μm². All-solid-silica Bragg fibers with large cores were proposed as an alternative solution for high power delivery through their fundamental core mode. These fibers consist of a low-refractive index core surrounded by a multilayer cladding that acts as a Bragg mirror. The loss spectrum of such fibers consists of a concatenation of several transmission windows separated by high-loss peaks. Here, we simultaneously study, for the first time (at our knowledge), the bending impact on Bragg fibers for the three critical properties required for high power delivery: large A_eff, single-mode propagation and low bend losses for the fundamental mode. Thanks to their specific guiding mechanism, A_eff as large as ~500μm² at 1.06μm can be achieved in Bragg fibers, while maintaining single-mode operation and bend losses lower than 0.1dB/m. Our numerical results are validated by experimental measurements on a PCVD Bragg fiber with a 40μm diameter core.

As overall power increases in fiber lasers and amplifiers, the amount of optical power which must be dealt with in order to obtain high core to core and core to cladding isolation also increases. This unwanted light can represent hundreds of watts and must be managed adequately. By combining a proper termination (end cap) design and cladding stripping techniques it is possible to obtain a robust output beam delivery component. The cladding stripping techniques are inspired by previous work done on high power cladding strippers. All measurement presented here are done with a flat end cap. Both core to core and core to cladding isolation will be better with an angled end cap. A core-to-core isolation of over 25dB was measured, while core to cladding was over 30dB. Power handling was characterized by the capability of the device to handle optical power loss, rather than transmitted power. The component dissipated over 50 watts of optical power due to isolation. The above results show that understanding the mechanisms of optical loss for forward and backward propagating light in a end cap and the heat load that these losses generate is the key to deliver kilowatts of optical power and protect the integrity of the system.

A Q-switched fiber laser with all-fiber configuration is proposed in which an all-fiber Q-switching modulator is developed based on the long period fiber grating. The fiber laser cavity comprises a pair of fiber Bragg gratings which are used to confine oscillation wavelength of the laser, gain fiber and a long period fiber grating based modulator. The long period fiber grating modulator is inserted in the laser cavity as Q-switching device, where the long period fiber grating modulator contains a long period fiber grating and an actuator, the initial resonance loss of the long period fiber grating is matched to the fiber Bragg grating wavelength. While the actuator applies stress to a section of the long period fiber grating, transmission spectrum of the long period fiber grating can be changed accordingly; Q-factor of the fiber laser cavity can be alternately modulated thereby.

High-power combiner designs (such as kilowatt-class combiners and beyond) are increasingly aggressive on brightness conservation in order to reduce the brightness loss of the pumps as much as possible in both direct diode combining and pump and signal coupling, especially with the advent of next-generation high-power pumps. Since most of the pump loss is due to brightness loss across the combiner, tighter designs (close to the brightness limit) are considerably more sensitive to variations in the input power distribution as a function of numerical aperture; for instance, next-generation, high-power multi-emitter pumps are likely to have larger numerical apertures than conventional single-emitter diodes. As a consequence, pump insertion loss for a given combiner design sitting close to the brightness limit should be dependant on the input power distribution. Aside from presenting a manufacturing challenge, high brightness combiners also imply more sophisticated testing to allow a deeper understanding of the loss with respect to the far-field distribution of the pump inputs and thus enable the extrapolation of loss for an arbitrary, cylindrically symmetric radiant intensity distribution. In this paper, we present a novel test method to measure loss as a function of numerical aperture (NA) fill factor using a variable NA source with square-shaped far field distributions. Results are presented for a range of combiners, such as 7x1 and 19x1 pump combiners, with different brightness ratio and fiber inputs. Combiners violating the brightness conservation equation are also characterized in order to estimate the loss as a function of input power vs. NA distribution and fill factor.

We present an erbium doped fibre ring laser system to realize single frequency lasing by incorporating a reflector with ~2m of un-pumped polarization-maintaining erbium-doped fibre to act as a saturable absorber. Depending on the particular requirements, the fibre reflector may be a fibre Bragg grating (FBG), loop mirror (LM) or a reflective coating on the fibre end. In this way, a transient grating is formed in the saturable absorber which acts as a narrow-band optical filter, reducing the number of modes over which the laser can operate and hence suppressing mode hopping in the cavity. Polarization-maintaining (PM) components are used throughout the system, except for the EDFA, and a polarization controller is used for enhancing stability and to ensure that the state of polarization is properly aligned. With this system we have observed a long period of stable, narrow line-width and single mode operation, tuneable over 30nm. The intended application is for gas spectroscopy using wavelength scanning and pump modulation. A Sagnac loop filter (SLF) can be used to scan the centre wavelength over a gas absorption line while the pump modulation produces an amplitude modulated signal on the output, suitable for detection by a lock-in (phase-sensitive) amplifier. The method is useful for the recovery of absorption line-shapes in the near-IR where the overtone absorption lines are weak. Compared with the use of a traditional DFB laser source, the fibre laser offers the advantages of a much broader tuning range and recovery of distortion-free line-shapes since wavelength and amplitude modulation may be performed independently.

We experimentally studied the coherent beam combining characteristics of fiber laser arrays in all-fiber passive configurations using polarization maintaining fibers. In addition, we simulated the coherent performance by including fiber nonlinearity. The beam combining performance is affected by both optical feedback and the laser cavity length difference. In addition, Kramers-Kronig and n₂ induced nonlinearity plays an important role for the coherent phase locking. We describe the scalability of the coherent array to high power via scaling of laser power and fiber count. We show coherently combined output powers of 27.4 Watts and 12.2 Watts at 1083 nm in 2-laser and 4-laser arrays.

Coherent beam combining of fiber amplifier arrays is an efficient way to overcome the physical limitations to fiber laser power scaling. Moreover, coherent combining techniques involving active phase control of the laser emitters offer the largest versatility, as they can also be used for complex purposes such as beam steering, wavefront shaping or atmospheric turbulence compensation. We reported last year the first experimental demonstration of coherent combining of fiber amplifiers on a remote scattering surface, after propagation through turbulent atmosphere, using the backscattered signal. These results were achieved with a frequency-tagging technique, and appropriate spatial filtering to lower sensitivity to backward turbulence, and compensate only for onward turbulent propagation. We present now experimental measurements of turbulence strength and resulting residual phase error. With turbulence compensation using the backscattered signal for phase control, this error is λ/15 rms. We also present the theoretical analysis of this experiment, emphasizing how limiting the aperture and not the field of view of the phase difference measurement subsystem reduces sensitivity to backward turbulence, without decreasing the optical flux on the detector.

Experimental investigations concerning the operation characteristics of a coherent fiber laser MOPA array are presented. The experimental set up consists of a single frequency fiber coupled 35 mW DBR diode laser at 1063 nm as master oscillator and two polarization maintaining two stage 2 W Yb doped commercial fiber amplifiers. Phase control is accomplished by fiber coupled acousto-optic frequency shifters prior to the amplifiers and an opto-electric phase locked loop operating at 100 MHz. Phase measurement at the amplifier output is achieved by a combination of a free space and fiberoptic interferometer in combination with RF photodiodes. A heterodyne signal of the amplifier output signal is generated with respect to a reference signal derived from the master oscillator and works as input signal for the phase control. Phase coupling of the array is demonstrated and the degree of coherence is determined from the contrast of the far field diffraction pattern of the output beam as well as from analysis of the RF photodiode signals. The characteristics of the phase control and phase stability are investigated and residual phase disturbances resulting from thermal and acoustic effects as well as depolarization are identified. Achievable beam quality as a function of fill factor is compared to theoretical computations. Finally, perspectives concerning a coherent 4x15 W MOPA array with three stage amplifying systems are outlined.

In this paper we present for the first time to our knowledge an efficient and rugged light source in the visible, based on a gain switched Yb³⁺ doped fiber laser source, frequency doubled by a non-critically phase matched LiB₃O₅ crystal. The simple setup proves to be robust and durable against back reflections, which in turn remove the requirement for optical isolators along most of the system. Gain switched fiber lasers typically produce long pulses with low peak power, which are not optimal for frequency conversion applications. However, as opposed to MOPA laser configurations, based on a semiconductor laser diode as a seed source, the narrow spectral line width and chirp free operation of gain switched lasers render them suitable for efficient frequency doubling.

The time-dependence of photodarkening (PD) in Yb-doped fibers is commonly fitted with a stretched exponential function to determine typical parameters of the process. But, the experimental conditions to obtain consistent results from the comparison of PD for different pump powers, various concentrations of the dopants, or fibers from different manufacturers are not adequately regarded up to now. We discuss the requirements concerning the measurement method as well as the impact of the initial state of the fiber under test. Further on, the experimental results of PD characteristics are discussed in the framework of a kinetic model of the observed processes. The discussion of the measurable PD loss is expanded by introducing the concept of the "PD state" that defines the PD loss as a weighted mean and takes effect on the further evolution of the process. In this way, the basis of fiber characterization and the understanding of the PD kinetics will be improved.

Photodarkening is presently a major concern for the long term reliability and efficiency of high power Yb-doped fiber lasers and amplifiers. This phenomenon has been associated with the formation of color centers in the fiber core of single-clad and large mode area Yb-doped fibers. However, its origin is still not well understood and to date no comprehensive model that could predict the lifetime of Yb-doped fiber-based devices has been put forward. A semi-empirical approach seems at the moment the best way to gain a better understanding of the growth behavior of photo-induced losses in Yb-doped fibers in the presence of both photodarkening and photobleaching processes. A rate equation describing the activation and deactivation of color centers involving stretched exponential functions has been developed. For this approach to be effective and reliable, a minimum of parameters is used, four to describe photodarkening and three for photobleaching. A large mode area Yb-doped fiber fabricated at INO using the MCVD process has been characterized. By properly choosing the initial pumping conditions, each parameter of the stretched exponential functions has been measured separately from the others. The model has then been used to simulate the power decay from a 1 kW, 10 ns-pulse, 100 kHz Yd-doped LMA fiber power amplifier. We show that the photodarkening behavior predicted by the model is in good agreement with the experimental results over more than 6000 hours. Such a model is general in its application but the stretched exponential parameters are unique to the type of fiber tested. The model will be a useful characterization tool for developing photodarkening-resistant fibers and for evaluating the lifetime of Yb-doped fiber-based devices affected by photodegradation.

A combined photodarkening and thermal bleaching measurement of a large-mode-area (LMA) ytterbium-doped fiber (YDF) is presented. Photodarkened YDF sample is recovered to pre-photodarkened state by thermal annealing. As a result, this approach enables repeated measurements with the same sample and therefore eliminates uncertainties related to changing of the sample (such as sample length and splice losses). Additionally, our approach potentially improves the accuracy and repeatability of the photodarkening rate measurement, and also allows automation of the measurement procedure.

In this work we experimentally demonstrate the possibility to build up the L-band tunable GHz repetition rate fiber laser via a dispersion flattened dispersion decreasing fiber. High quality fully synchronized with clock source 0.9 ps pulses are generated.

We report on a novel simple configuration of an Yb-doped fiber laser cavity comprising only Yb-doped fiber and a saturable absorber element. Numerical results show that stable mode-locked operation does exist in such a laser cavity in limited range of parameters. The conditions to obtain stable pulses are investigated as a function of the Yb-fiber bandwidth, length, and gain coefficient. The temporal and spectral behavior of the femtosecond pulses are also studied for different input parameters. The instability dynamics of the mode-locked pulse is also elucidated.

Pulse-contrast degradation at the CPA-system's output is analyzed. If Kerr-nonlinearity is present, weak initial spectral phase and amplitude modulations are responsible for the decrease. The pulse is split into several sub-pulses. Bessel-functions describe the intensities of the side-pulses relative to the principal pulse. We provide the governing physical quantities.

We investigate theoretically multi-tone seeding of high power ytterbium-doped amplifiers using a numerical code that solves a two-point boundary problem. A large wavelength separation among the signals is used leading to efficient transfer of power through laser gain while increasing the stimulated Brillouin scattering threshold in the channel of interest. Two-tone and three-tone seeding configurations are considered. For two-tone seeding, it is shown that a combination of narrow linewidth and broadband signals employed in a co-propagating geometry can achieve the same level of SBS suppression as counter-pumping.

The paper presents a fiber optic tunable laser built with a band pass tunable optical filtering cascade made of two tiltedmirrors (Optune) interferometers as feedback element of an optical amplifier. A dynamic wavelength reference monitors the laser wavelength. The optical cascade has 200 nm tuning range, the band pass has 0.2 dB insertion loss flatness across the entire tuning range, 0.1 nm bandwidth at 3 dB, 45 dB rejection ratio and 160 dB/nm roll-off. According to the measurements, the line width of this tunable laser is below 41 MHz. The dynamic wavelength reference generates wavelength markers with 0.1 pm relative accuracy and with 1 pm absolute accuracy across 40°C temperature interval. These markers could be used to mitigate the non-linearity of the laser tuning. It was achieved 800 nm/ms tuning speed with the tuning element working below its resonant frequency. The experiments performed in open-loop operation in 1550 nm spectral region revealed 1 pm tuning accuracy and 0.1% tuning non-linearity versus the tuning voltage across 40 nm tuning interval. Laser line roll-off is steeper than 160 dB/nm. With appropriate reflective coatings, the filtering cascade can operate also in other spectral regions (visible, UV) with tuning accuracy limited by the wavelength reference and by the laser controller. A digital signal processor monitors the operation of the tunable laser to achieve optimum performance. This tunable laser source has applications in interrogators for fiber optic sensors and in optical coherence tomography.

We demonstrate a method of generating a multiwavelength Brillouin/Erbium fiber laser in a ring cavity, with stimulated Brillouin scattering as a mirror. All Four generated Stokes lines have peak powers above 0 dBm with equal spacing of 10 GHz (0.08 nm) at 130 mW pump power from a 1480 nm laser diode. Our technique suppresses other potential modes to circulate in the laser cavity, thus the self-lasing modes are eliminated. The tuning range over 39 nm from 1527 nm to 1566 nm was successfully demonstrated, which is only limited by the amplification bandwidth of the erbium gain.

With narrow linewidth fiber lasers steadily increasing in power, it becomes important to consider the passive fibers employed as fiber laser pigtails, component fibers, power delivery fibers, etc. Stimulated Brillouin scattering (SBS) can become the leading power impairment in systems incorporating 'reasonable' lengths of passive fiber. For example, this might be a 1-meter passive fiber pigtail on a long-pulse transform-limited fiber amplifier. We present some results towards the development of large-scale (>10 dB) SBS-suppressed passive fibers for these applications. The results of several developed fibers will be shown, including optical fibers that exhibit large-scale SBS suppression (>10dB), with one fiber having a Brillouin gain coefficient as low as 0.15 × 10^-11 m/W in the core. Some fibers are found to be limited by SBS in the cladding. As a result, we incorporate more complete SBS-suppression approaches that also take the cladding interaction into consideration. We analyze both the Brillouin spectra and power transmission properties of several developed fibers to verify SBS suppression. Brillouin spectra are obtained through a standard heterodyne technique, while power transmission tests are compared against standard Ge-doped telecommunications fiber as a control. Modeling results will be shown to be in excellent agreement with experimental data.

We present a novel scheme of reducing the spectral broadening effects of self phase modulation (SPM) in high-peak-power, narrow-band fiber lasers by preconditioning the input pulses of a master oscillator seeded high-peak-power fiber amplifier with a commercially available fiber-coupled phase modulator. We demonstrate an output emission linewidth reduction in a 1064 nm nanosecond pulse-width, 10 kW peak power fiber laser system. Modeling and measurements are presented for this system using a directly modulated diode seed laser and the resulting practical limitations imposed on SPM mitigation are discussed.

A four-wave mixing process of 4700 cm^-1 Stokes-shift is stimulated by pumping a 20-cm commercial large-mode-area photonic crystal fiber with amplified Ti:sapphire femtosecond pulses. The phase-matching condition is realized through an intermodal scheme which promotes the process in the largely normal dispersion regime of the fiber. The walk-off effect of the interacting pulses is minimized by introducing an initial chirp to the pump pulses. Conversion efficiency over 7% from near-infrared pump input to visible anti-Stokes signal can be achieved.

To realize a completely monolithic, pulsed, fiber laser without free space elements we describe a gain-switched fiber laser pumped with a pulsed diode laser at about 965 nm with more than 30 μJ in a 200 ns pulse. For best beam quality we use a single mode fiber with a 6 μm core diameter. We report lasing of an Yb-doped double-clad fiber at 1080 nm and a pulse energy of about 8 μJ with a variable repetition rate from 1 - 50 kHz. The experimental results are compared with the data of a time resolved simulation and basic analytically derived formulas.