This PDF file contains the front matter associated with SPIE Proceedings Volume 7914, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The clinical solid-state Holmium:YAG laser lithotripter (λ=2120 nm) is capable of operating at high pulse energies, but its efficient operation is limited to low pulse rates during lithotripsy. The diode-pumped experimental Thulium Fiber Laser (λ=1908 nm) is limited to low pulse energies, but can operate at high pulse rates. This review compares stone ablation threshold, ablation rate, and retropulsion effects for Ho:YAG and TFL. Laser lithotripsy complications also include optical fiber bending failure resulting in endoscope damage and low irrigation rates leading to poor visibility. Both problems are related to fiber diameter and limited by Ho:YAG laser multimode spatial beam profile. This study exploits TFL spatial beam profile for higher power transmission through smaller fibers. A short taper is also studied for expanding TFL beam at the distal tip of a small-core fiber. Stone mass loss, stone crater depths, fiber transmission losses, fiber burn-back, irrigation rates, and deflection through a flexible ureteroscope were measured for tapered fiber and compared with conventional fibers. The stone ablation threshold for TFL was four times lower than for Ho:YAG. Stone retropulsion with Ho:YAG increased linearly with pulse energy. Retropulsion with TFL was minimal at pulse rates < 150 Hz, then rapidly increased at higher pulse rates. TFL beam profile provides higher laser power through smaller fibers than Ho:YAG laser, potentially reducing fiber failure and endoscope damage and allowing greater irrigation rates for improved visibility and safety. Use of a short tapered distal fiber tip also allows expansion of the laser beam, resulting in decreased fiber tip damage compared to conventional fibers, without compromising fiber bending, stone ablation efficiency, or irrigation rates.

In this paper, we demonstrate the benefits of using bursts of picosecond pulses for material micromachining and compare the results with those obtained when using a nanosecond source with similar pulse energy, pulse width and pulse shape. The picosecond laser source used for the experiments was delivering 60-ps pulses at a repetition rate of 1.8 GHz, grouped within arbitrarily-shaped bursts having a width that could be varied from 2.5 to 40 ns. The laser output central wavelength was at 1064 nm and the output beam M² value was below 1.15. Micro-milling experiments were performed on silicon for two levels of energy per burst and with different burst amplitude profiles. We show that the maximum material removal efficiency and the surface quality can be increased by more than 25% when using bursts of picosecond pulses with respect to nanosecond pulses with similar energy per pulse. Effect of shaping the burst envelope of the picosecond laser on the maximum material removal efficiency is also presented.

Pulse bursting is demonstrated using a pulsed MOPA fiber laser at 1064 nm for percussion drilling of stainless steel. Bursts are configured as fast pulse sequences at tens of MHz, with a temporal envelope in the range from 10 to 420 ns. Their use rapidly enhances the efficiency of material removal by enhancing pulse energy deposition, and quality improvement. Results are shown for various pulse conditions by changing number of pulses, spacing and peak powers. Each pulse burst is collectively triggered and amplified as a single pulse group, at repetition frequencies from singleshot to hundreds of kHz.

In this paper, we present results on a master-oscillator Yb-doped fiber amplifier with 1 kW cw output power (at 1064nm), and near-diffraction limited beam quality (M2<1.4), with internal quantum efficiency >83%. The final amplifier stage uses a very high Yb-doped 35-um core LMA fiber, using a new process recipe that virtually eliminates photo-darkening. As a result, high efficiency, SBS-free operation to 1 kW cw power level is obtained, with a phase modulation bandwidth of only 450MHz, well below other reported results. To enable single-frequency cw power scaling to kW levels, we investigate LMA fiber waveguide designs exploiting gain-discrimination, using partially Yb-doped LMA fiber cores, with various diameters up to 80-um. SBS-free, singlefrequency (few kHz) operation is demonstrated up to 0.9kW cw power. At the lower cw powers (<200W) neardiffraction limited beam-quality is obtained, but is observed to deteriorate at higher cw powers. We discuss potential causes, and present a detailed simulation model of kW large-core fiber-amplifiers, that includes all guided modes, fiber bend, transverse spatial hole burning, gain-tailoring, mode-scattering, SBS nonlinearity, and various thermal effects. This model shows good agreement with the observed single-frequency power scaling and beam-quality characteristics.

The output power of fiber optic laser systems has been exponentially increasing in the last years. However, non-linear effects, and in particular stimulated Raman scattering (SRS), are threatening to seriously limit the development pace in the near future. SRS can take place anywhere along the laser system, however it is actually the passive delivery fiber at the end of the system, the section where SRS is most likely to occur. The common way to combat this problem is to use the so-called Large Mode Area (LMA) fibers. However, these fibers are expensive and have a multimode nature that will either reduce the beam quality of the laser output or require a careful excitation of the fundamental mode. Furthermore, the larger the core area, the more complicated it will be to sustain single-mode operation. Therefore, it is becoming increasingly important to be able to determine which is the minimum core area required in the delivery fiber to avoid SRS. This calculation is usually carried out using the conventional formula for the Raman Threshold published by R.G. Smith in 1972: P_th =16A_eff g_RL_eff . In this work we demonstrate that this formula and the conclusions derived from it are inaccurate for short (several meters long) LMA fibers. For example, one widely spread belief (obtained from this expression) is that there is no dependence of the Raman intensity threshold (I_th=P_th/A_eff) on the mode area. However, our calculations show otherwise. Additionally, we have obtained an improved Raman threshold formula valid for short LMA fibers.

We describe a time-dependent model that describes the evolution of stimulated Brillouin scattering (SBS) in fibers under phase-modulated pump conditions. In order to accurately model fast modulations, the triply-coupled system of differential equations describing the interaction of SBS through optical and phonon fields is solved numerically. SBS is initiated from noise by using a Langevin term. We initially consider single-frequency sinusoidal modulations as a function of modulation amplitude and frequency. We then investigate the effects of SBS mitigation when a single-frequency seed is phase modulated with a broad-band white-noise source (WNS).

The efficiency of two tone fiber amplifiers can be changed rather significantly by altering the temperature of the external environment surrounding the gain fiber. It is shown experimentally that changes in the temperature of the core of the gain fiber has dramatic effects on the 1064 nm / 1040 nm power distribution in the output of narrow linewidth 1064 nm two tone amplifiers with a greater percentage of the output being 1064 nm at higher core temperatures. By increasing the environmental temperature of the gain fiber from 20 to 80°C, the efficiency of a 1064 nm two tone amplifier can be increased up to 40% with the greatest increases seen in amplifiers seeded hardest in 1040 nm, i.e., having the smallest 1064 nm / 1040 nm seed ratios. This has been attributed to temperature dependence of the absorption and emission cross-sections at the wavelengths of interest. Finally, the temperature of the gain fiber can be used as a design tool to enable a higher efficiency 1064 nm two tone amplifier.

We present experimental studies of a novel polarization-maintaining (PM) Yb-doped photonic crystal fiber (PCF) possessing a two-segment transverse acoustic profile and a mode field diameter (MFD) of approximately 30 μm. The concentrations of the dopants (fluorine, aluminum, germanium) in the two segments were selected such that the corresponding Brillouin shifts were sufficiently separated to allow for the introduction of a large thermal gradient for further SBS suppression. A pump-probe experiment was conducted in order to characterize the Brillouin gain spectrum (BGS) and to confirm the existence of two narrow Brillouin gain peaks. The separation of the two peaks was approximately 220 MHz and the bandwidth of each was estimated to be 50 MHz. The application of a step temperature profile resulted in the BGS displaying four distinct peaks; thus demonstrating further SBS suppression through a thermal gradient. By utilizing the thermal gradient obtained through quantum defect heating, we obtained 494 W of output power in a counter-pumped configuration without the onset of SBS. Measurements of the beam quality at various power levels and up to the highest reported power consistently indicated an M2 of less than 1.3.

The power handling capabilities of fiber lasers are limited by the technologies available to fabricate and assemble the key optical system components. Previous tools for the assembly, tapering, and fusion of fiber laser elements have had drawbacks with regard to temperature range, alignment capability, assembly flexibility and surface contamination. To provide expanded capabilities for fiber laser assembly, a wide-area electrical plasma heat source was used in conjunction with an optimized image analysis method and a flexible alignment system, integrated according to mechatronic principles. High-resolution imaging and vision-based measurement provided feedback to adjust assembly, fusion, and tapering process parameters. The system was used to perform assembly steps including dissimilar-fiber splicing, tapering, bundling, capillary bundling, and fusion of fibers to bulk optic devices up to several mm in diameter. A wide range of fiber types and diameters were tested, including extremely large diameters and photonic crystal fibers. The assemblies were evaluated for conformation to optical and mechanical design criteria, such as taper geometry and splice loss. The completed assemblies met the performance targets and exhibited reduced surface contamination compared to assemblies prepared on previously existing equipment. The imaging system and image analysis algorithms provided in situ fiber geometry measurement data that agreed well with external measurement. The ability to adjust operating parameters dynamically based on imaging was shown to provide substantial performance benefits, particularly in the tapering of fibers and bundles. The integrated design approach was shown to provide sufficient flexibility to perform all required operations with a minimum of reconfiguration.

A polarization-maintaining (PM), narrow-linewidth, continuous wave, thulium fiber laser is demonstrated. The laser cavity is formed from two femtosecond-laser-written fiber Bragg gratings (FBGs) and operates at 2054 nm. The laser output possesses both narrow spectral width (78 pm) and a high polarization extinction ratio of ~18 dB at 5.24 W of output power. This laser is a unique demonstration of a PM thulium fiber system based on a two FBG cavity that produces high PER without any free-space elements. Such a narrow linewidth source will be useful for applications such as spectral beam combining which often employ polarization dependent combining elements.

Recent progress on high efficiency Tm-doped silica fibers pumped at 790nm has enabled the demonstration of a 2μm CW fiber laser operating at the 1kW power level and with single mode beam quality [1]. In addition to this state-of-the-art high power research, Tm-doped fibers are now starting to find applications in lasers with nsec [2] and psec [3] pulsed operation, as well as lower power CW lasers (50-100W) in the 1940nm wavelength region for medical use [4]. As with Yb-doped fibers, the question of photodarkening needs to be addressed to ensure the long term fiber reliability is appropriate for the application. In a recent paper [5], we proposed that the up-conversion process in highly doped Tm-fibers was significantly quenched when compared with lower concentration fibers, under 790nm pumping. This trend was also observed in the photodarkening rate as measured in a CW 2μm fiber laser cavity operating around 20W output power. In this controlled experiment, the rate of photodarkening dropped from 15% per 1000 hours in low concentration fiber to less than 1% in a fiber doped with 4.6% Tm. In this paper we review the 20W results of our earlier work and then confirm the long term reliability of 1940nm CW fiber lasers operating at higher (40W) output power, presenting results for a laser operating for 1200 hours without significant loss of output power (around 12% total power variation). Over any given 24-hour period during this experiment, the laser operates open loop with around 8% total power variation, in an air cooled configuration. We believe these results confirm the appropriate level of long term reliability of Tm-doped fibers for CW fiber lasers at the ~50W power level, suitable for medical laser applications.

Photodarkening phenomenon in ytterbium (Yb)-doped silica glasses was experimentally investigated by measurements such as electron spin resonance (ESR), X-ray absorption fine-structure (XAFS), and optical transmittance. A predominant increase of Al-oxygen hole center (OHC) was observed for Al-Yb co-doped silica glasses both by the incidence of pump light and by gamma-ray irradiation. It was also recognized that the optical transmission loss similarly increased in both cases. These results indicate that the formation of Al-OHC is the prime cause of the excess loss induced by photodarkening. XAFS measurement indicated that the second-nearest-neighbor atoms around Yb may be related to the mechanism of photodarkening phenomenon.

Photodarkening-induced output power degradations and long-term stability in high power pulsed and CW fiber laser MOPA systems are discussed. The studied laser systems are based on aluminosilicate single-mode Yb-doped fibers and use the GTWave fiber technology for cladding pumping. The active fiber lengths are between 10-30m. We have tested Yb-doped fiber amplifiers operated under both pulsed and CW mode. Using OTDR background-loss measurements we show, for the first time, that the photodarkening-induced loss is non-uniformly distributed along the length of the active fiber. By calculating the average inversion along the fiber length, we show that the induced loss follows closely an Yb-inversion dependence to the power of 2. In addition we have studied the temperature dependence of the output power variation. It is shown that increasing (decreasing) operating temperature results in decrease (increase) of the laser output power, reaching the new equilibria over time scales of ~200hours. We also present data on the non-photodarkening SPI fiber which is used in all SPI products.

The Tm-doped silicate glass fibre laser that operates in the 2 micron region of the spectrum is fast becoming a mature technology with output powers already exceeding 1 kW. In this paper, I will review a number of current and future experiments that involve lasers pumped with the output from Tm-doped silicate glass fibre lasers including linear systems e.g., the optical excitation of rare earth ions and nonlinear systems e.g., Raman fibre lasers.

We report the simultaneous excitation of multiple Raman Stokes lines in a 250 m long fiber using multi-step pump pulses. The frequency doubled output of a single polarization all-fiber Yb-doped MOPA operating at 1060 nm was used as the pump source. By adjusting the pump power and the pulse profiles we achieved the simultaneous excitation of green (1st Stokes), yellow (4th Stokes) and red light (6th Stokes) using 3-step pulses or the combination of any two using 2-step pulses. Through the use of pulse shaping we generate sequences of colored pulses with the flexibility of providing dynamic, agile frequency tuning between well-defined wavelengths.

An all-fiber supercontinuum source extending from 1.5 to 5 μm has been demonstrated in single-mode step-index As2S3 fiber using a Raman shifted erbium doped mode-locked silica fiber laser pump source. 140 mW broadband power was demonstrated with a spectral intensity variation of 10 dB from 1.9 to 4.4 μm and 20 dB from 1.65 to 4.78 μm.

We have developed several high power picosecond sources using rod-type fiber systems. We will show that a very simple architecture can produce average power over 90 W @ 1030 nm, 57 W @ 515 nm and 20 W @ 343 nm, with pulse repetition rates ranging from 200 kHz to 80 MHz. Particular emphasis will be given on the control of non-linear effects in the fiber without relying on CPA.

We present second harmonic generation of a high average power, high energy femtosecond Yb doped fiber chirped pulse amplifier. This system is operated at various repetition rates at a central wavelength of 1040 nm. After two pre-amplification stages a main amplifier is used to achieve the required pulse energy for efficient second harmonic generation. It is comprised of a 1.2 m long photonic crystal fiber with a mode field diameter of 45 μm. A dielectric grating based compressor is used for compression to a pulse duration of about 406 fs. Second harmonic generation is then achieved in a 500 μm thick BBO crystal. The conversion efficiency of the second harmonic generation remained almost constant at >60 % for all repetition rates and average power levels. At 5.25 MHz the highest average power of 135 W at 520 nm was achieved. In addition this comes with an excellent beam quality which is validated by a measured M² < 1.2.

3C (Chirally-Coupled Core) optical fiber establishes a technological platform for high brightness, power scalable lasers with an engineerable fiber geometry that enables robustly single-mode performance of large core diameter fibers. Here we report the demonstration of robust polarization preserving performance of 35 μm core 3C fiber for short pulse systems. A polarization extinction ratio (PER) of ~ 20 dB is stably maintained with ambient temperatures varying over a 50°C range from a Yb-doped double clad 3C fiber amplifier. We also demonstrate that this high-PER polarization output is insensitive to temperature gradients and mechanical perturbations in the 3C fiber amplifier. The ability to deliver high peak power pulses at high average powers while maintaining exceptional beam quality and a stable polarization state in an easily integrated format makes 3C fiber laser systems extremely attractive for harmonic generation to visible and UV wavelengths.

It is relatively straightforward to completely measure both long (>10ns) and very short (<100ps) laser pulses in time. But intermediate pulse lengths-that of the most common laser pulses- remain nearly immeasurable and, not coincidentally, correspond to the least stable of all lasers. True, ultrahigh-bandwidth oscilloscopes and streak cameras can now resolve such pulses, but such exotic electronic devices are expensive and fragile and only yield the temporal intensity and not the temporal phase. Here we describe a simple, elegant, accurate, complete, compact, all-optical, entirely passive, and single-shot FROG device that solves the problem. It simultaneously achieves a very large delay range of ~10ns and very high spectral resolution of <1pm. It accomplishes both feats using high-efficiency, high-finesse etalons, the first to tilt the pulse by 8⪅9, .t9hat is, by several meters over a centimeter beam, and another to generate massive angular dispersion for a high-resolution spectrometer. We demonstrate this device for measuring pulses 100ps to several ns long from a fiber-amplified micro-disk laser.

We investigate the feasibility of pulsed fiber amplifier coherent combining. Therefore, we characterize phase fluctuations in low-peak-power pulsed fiber amplifiers using two different interferometric techniques. These measurements reveal that for low peak-powers, phase fluctuations remain moderate during the pulses. Noticeable phase fluctuations occurring between the pulses can be perfectly controlled using classical continuous-wave-efficient combining techniques. Results of such realization combining two low-peak-power pulsed fiber amplifiers, using classical frequency-tagging coherent combining techniques, are presented. Phase difference measurement is performed between pulses using a small signal leak from the common master oscillator. For the first time to our knowledge, successful coherent combining of two low-peak- power pulsed fiber amplifiers is thereby demonstrated.

Here we report a compact monolithic 2000nm pulsed laser with a single spatial mode output, ~1.5ns pulse duration, 8kW peak power and >200mW average power at 20 kHz repetition rate. The gain-switched laser, consisting of a pair of fiber Bragg gratings and 0.5m of thulium-doped single cladding fiber, was core pumped by a high peak power pulsed 1.5μm laser. When the input pulse energy of the 20 kHz pump pulses was sufficient enough to saturate the Thulium doped fiber, a stable 20 kHz pulse train was observed with measured linewidth of 0.05nm which corresponds to the limit of resolution for the Optical Spectrum Analyzer. This compact, pulsed 2000 nm laser, to the authors' limited knowledge, represents the first Tm-doped fiber laser with 8kW peak power and several ns pulse duration; which is less than the previously reported tens of ns pulsewidth previously reported from gain-switched Tm-doped fiber lasers.

We report here the successful realization of 25 millions wavelengths per second using an SOA based PL around 1565 nm at a 75 MHz repetition rate. The laser is simply composed of an SOA, a CFBG (10 ps/nm) with a 100 nm bandwidth, an optical circulator, an EOM (intensity modulator), and an output coupler (20%). Pulse duration is around 45 ps and OSNR of the pulse is around 35 dB at 1565 nm without sweeping. Tunable dispersion compensating module (TDCM) was used to compress the chirped pulse output and 10 ps pulse duration was obtained at 1548 nm. Finally 25 megawavelengths per second was realized with under 3 pulses per wavelength and 1024 discrete wavelengths. Linear k-space sweeping function was enabled in the swept-source OCT (SS-OCT) system through graphical user interface (GUI).

We present the realization of an actively mode-locked laser based on a 30 μm core diameter single-mode double clad photonic crystal fiber. For 19 W of pump power at 976 nm, it yields an average power of 10 W at 40 MHz. The delivered pulses, centered at 1030 nm, have a duration of 15 ps. This corresponds to an energy of 250 nJ per pulse and a peak power of 17 kW.

We report on dual-stage fibered Master Oscillator Power Amplifier system designed to meet stringent requirements of large-scale laser facilities front-end sources. The combination of high power continuous wave laser and electro-optical modulator allows the generation of multi-kHz nanosecond pulses with accurate tailored temporal pulse profiles. The seed pulses are amplified in a preamplifier to achieve 20 μJ pulse energy (33 dB gain) launched inside the power amplifier. We finally obtained a strictly single mode beam, narrow-bandwidth, multi-kHz nanosecond pulse amplification at mJ level, with more than 50 dB optical signal-to-noise ratio, 15 dB polarization extinction ratio and sub-nanosecond resolution temporal shaping. The choice of optimal parameters and the temporal pulse shaping have been performed thanks to numerical simulations (including forward and backward amplified spontaneous emission) in excellent agreement with the results.

Amplification of a gain-switched laser diode is demonstrated in an all-fiber based setup. The amplified spontaneous emission between two consecutive pulses was investigated quantitatively in the time domain. A maximum pulse energy of 13 μJ at a repetition rate of 1 MHz and a pulse duration of 40 ps was extracted, corresponding to a peak power of 270 KW. Temporal pulse deformation due to intrapulse Raman scattering was observed.

We report on nonlinear optical compression of passively Q-switched pulses accessing sub-10 ps domain, which is so far dominated by mode-locked systems. The concept implements the SPM-induced spectral-broadening of passively Q-switched microchip pulses in optical waveguides and a supplementary compression with bulk optics e.g. a pair of diffraction gratings or a chirped-Bragg-grating. Used seed-source is a fiber-amplified, passively Q-switched microchip laser operating on a single longitudinal mode and consists of a monolithically bonded combination of Nd:YVO₄-crystal and semiconductor saturable absorber mirror. The microchip laser provides pulses with durations of 100-150 ps, pulse energies of ~200 nJ at various repetition rates from hundreds of kilohertz to more than a megahertz, and line width of ~20 pm at wavelength of 1064nm. During the amplification process in the photonics crystal fiber, the pulses are spectrally broadened to up to ~0.7nm at energy of 17μJ. Using a diffraction grating compressor with 1740 l/mm, the pulses are compressed to duration as short as 6ps assuming a numerically calculated de-convolution factor of 0.735. To the best of our knowledge, this is the first reported realization of nonlinear compression of the Q-switched pulses and the shortest pulses from a passively Q-switched laser system.

The major challenge in the development of monolithic kW class CW fiber lasers is the efficient conversion of pump photons into a high brightness laser beam under the constraints of heat management, long term stability and nonlinearities. This article reviews the interaction of some fiber related aspects as e.g. fiber core composition, photodarkening and modality, as well as their influence on system complexity and power scalability. Recent work on active fibers, pump couplers, mode field adaptors and other fiber-optic components will be presented.

Progress in advanced specialty fibers is the foundation to further breakthroughs in fiber lasers. Recently, we have been working to advance several areas of developments in specialty fibers and would like to review these efforts here. The first topic is in the further development of all-glass large core leakage channel fibers (LCF) for robust and practical solutions for power scaling. The second area is the development of wide band air-core fibers with an innovative square lattice cladding and the demonstration of a factor of two improvements in bandgap over conventional hexagonal lattice. These air-core fibers are critical for fiber delivery solution of both CW and pulsed fiber lasers in the future. The last topic is a new development in design and simulation of SBS gains in optical fibers by incorporating leaky acoustic modes. These leaky acoustic modes have been mostly overlooked so far. It is essential that they are considered in SBS simulations in fibers, because they are normal solutions to the acoustic waveguide equations and have similar loss to guided acoustic modes where the acoustic mode loss is dominated by material loss. This leads to much improved resolution of SBS gain spectrum in fibers and to new design insights to the limit of SBS suppression based on anti-guide acoustic waveguide designs.

Ytterbium-doped Large Pitch Fibers with very large mode areas are investigated in a high power fiber amplifier configuration. An average output power of 294 W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62 μm. Compared to previous active large mode area designs the threshold of mode instabilities is increased by a factor of about 3.

We have developed a commercial 4-kW fiber laser consisting of seven, 600-W modules whose outputs are combined with a fused-fiber combiner. The system architecture has several practical advantages, including pumping with reliable single-emitter diodes, monolithic fused-fiber construction (no free-space beams), and end pumping using a 91:1 pump combiner (eliminating the need for complex pump/signal combiners). Typical results at 4-kW output power are a beamparameter product of 2.6 mm-mrad, 8-hr power stability of < 0.5% rms, central wavelength of 1080 nm, and linewidth of 1.2 nm FWHM. These lasers have been incorporated into Amada machines used for cutting metal sheet and plate and have been used to cut aluminum, mild steel, stainless steel, brass, titanium, and copper with a thickness up to 19 mm. A world-record cutting speed of 62 m/min has been demonstrated for 1-mm aluminum sheet metal.

We report on a novel concept to scale the performance of ultra-fast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. The splitting and combination process is based on the polarization combining technique using polarization cubes. A Hansch-Couillaud detector measures the polarization state of the combined beam. The error signal (deviation from linear polarization) is used to stabilize the optical path lengths in the different channels with a piezo mounted mirror. In a proof-of-principle experiment the combination of two femtosecond fiber-based amplifiers in a CPA systems is presented. A combining efficiency as high as 97% has been achieved. Additional measurements were carried out to investigate the stability of the system. The concept offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.

Lasers that produce 100 kW level diffraction limited power will require beam combining due to fundamental thermal and nonlinear limitations on the power of single aperture lasers. Towards this goal, we present high power, high spectral density beam combining by volume Bragg gratings of five 150 W beams with a spectral separation of 0.25 nm between beams, the narrowest to date for high power. Within 1 nm, 750 W of total power is combined with greater than 90 % efficiency. Combined beam quality is discussed including the effect of unequal individual beam divergences on the combined beam quality. The individual input beams may have unique divergences as they enter the system, and the heated volume Bragg gratings (VBGs) may introduce very slight changes in divergence to each beam. These small differences in beam divergence between the beams will not degrade the M² of the individual beams, but the composite M² after combination can be adversely affected if the beams do not have equivalent divergence at the output of the system. Tolerances on beam divergence variation are analyzed and discussed. High power beams transmitting through or diffracting from a VBG can experience different distortions resulting from thermal effects induced in the VBGs. Each beam also experiences a different aberration, as no two beams pass through the same number of identical VBGs. These effects are studied with experiment compared to modeling. Possible methods of beam quality improvement are discussed.

We perform sensitivity analyses on two different array configurations of coherently combined fiber amplifiers to study the impact of residual phase errors onto the combining efficiency. The arrays studied are: a square of 16 fibers and a hexagon of 19 fibers. For the hexagon, two different shaped wavefronts are studied. In this method a global analysisis performed: we modify simultaneously all the phase errors using numerical space filling designs. Then, the construction of a metamodel makes it possible to investigate more precisely the role of each fiber and specially the role of interactions between fibers onto the combination with less runs than classical approaches. Results exhibit different behaviors and specially interactions between fibers with respect to the array configurations and with respect to the two different shaped wavefronts. Finally, we demonstrate that we can study arrays of more than 100 fibers.

We demonstrate experimentally that few atomic layer graphene possesses ultrafast, super broadband saturable absorption, which can be exploited to passive mode lock fiber lasers of different operation wavelengths. Passive mode locking performance of an erbium-doped fiber laser operating at the 1.55μm and an ytterbium-doped fiber laser operating at the 1.06μm with a graphene saturable absorber mirror has been experimentally demonstrated, and wavelength tuneable solitons, vector solitons, as well as the bound state of solitons have been observed.

An Yb-based 78-MHz repetition rate fiber-amplified frequency comb is used to investigate the power scaling limitations of a standard-design bow tie high-finesse enhancement cavity for XUV generation. With a Xenon gas jet in the 22-μm-radius focus, the 200-fs intra-cavity circulating pulse reaches a maximum of 20 kW of time-averaged power. A novel cavity design is presented, conceived to overcome the observed enhancement limitations and offering the prospect of few-nm high-power high-harmonic generation. Several applications which come into reach for the first time are discussed.

In this contribution, we report the generation of high energy, high temporal quality, ultrashort pulses from a single stage double-pass short length Ytterbium-doped rod type fibre chirped pulse amplification setup. The fibre used as gain medium is a 95-cm long rod type microstructured fibre with a 80 μm core diameter and 200 μm pump cladding diameter seeded by 3 nJ pulses at 100 kHz of repetition rate. We demonstrate amplification and compression up to 200 μJ energy in 250 fs (assuming sech² pulse shape) with good temporal quality leading to ~ 750 MW peak power.

The mode-locking of dissipative soliton fiber lasers using large mode area fiber supporting multiple transverse modes is studied experimentally and theoretically. Experiments using large core step-index fiber, photonic crystal fiber, and chirally-coupled core fiber show that when the higher order mode content exceeds -27 dB, the maximum stable single-pulse energy is significantly reduced. The averaged mode-locking dynamics in a multi-mode fiber are studied using a distributed model. The co-propagation of multiple transverse modes is governed by a system of coupled Ginzburg-Landau equations (CGLEs). Simulations show that stable and robust mode-locked pulses can be produced. The maximum stable single pulse energy is found to increase with higher order mode filtering. This work demonstrates that mode-locking performance is very sensitive to the presence of multiple waveguide modes when compared to systems such as amplifiers and continuous-wave lasers, and gives a quantitative estimate of what constitutes effectively single-mode operation. Robust, distributed higher order mode filtering is necessary to maximize single-pulsing energy.

Amplified ultrashort pulses at 2 μm are of great interest for atmospheric sensing, medical, and materials processing applications. We describe the generation and amplification of femtosecond 2 μm pulses in thulium doped silica fiber. Mode-locked eye-safe laser pulses at ~2 μm were generated in a Tm:fiber oscillator using a single-walled carbon nanotube saturable absorber. Stable mode-locking was achieved at a repetition rate of 70 MHz with soliton pulses reaching energies of ~40 pJ with a spectral bandwidth of ~8 nm. Autocorrelation measurements indicated bandwidth limited pulses of ~500 fs duration. This oscillator was used to seed a Tm:fiber amplifier in both free space and fiber coupled configurations. Effects of dispersion compensation and pulse amplification are reported.

Non-adiabatic pulse compression of cascaded higher-order optical soliton is investigated. We demonstrate high degree compression of pulses with soliton order N=2, 3, 4 and 5 in two or three nonlinear fibers with different second-order dispersion coefficients. Each fiber length is shorter than half of its soliton period. This compression technique has significant advantages over the widely reported adiabatic and higher-order soliton compression.

For the first time, a non-destructive technique for spatially resolving the location and relative concentration of rare-earth dopants in an optical fiber is demonstrated. This novel technique is based on computerized tomographic detection of spontaneous emission and achieves micron-scale spatial resolution with the aid of oil-immersion imaging. In addition to elucidating interactions between the signal, pump, and dopant distributions, the measurement described here can reveal shortcomings in fiber manufacturing. Since the technique is non-destructive and can be scanned along the fiber length, it can map the full 3-dimensional distribution of complex rare-earth-doped fiber structures including gratings, physical tapers, fusion splices, and even couplers. Experimental data obtained from commercially available Yb-doped silica optical fibers is presented, contrasted, and compared to refractive index profile data. In principle the technique can also be applied to Er-, Bi-, or Tm-doped silica or non-silica optical fibers.

We report the energy scaling of mode-locked fiber lasers using a large-mode area chirally-coupled core fiber. This is a demonstration of the scaling of ultrafast fiber oscillators to large cores in an all-solid glass package that holds the lowest order fiber mode while maintaining compatibility with fiber fusion technology. An all-normal dispersion cavity design yields pulse energies above 40 nJ that dechirp to durations below 200 fs. Using lower net dispersion, pulses dechirping close to 100 fs are obtained with pump limited energies. Effectively single-mode operation is confirmed by beam quality as well as spectral interference measurements.

The properties of a large mode area Yb-doped double-cladding hybrid photonic crystal fiber, with antisymmetric high-index inclusions, have been analyzed. Simulations, carried out through a finite-element based modal solver, and experimental measurements have demonstrated the narrow spectral filtering capability of this fiber, with a passband of about 80 nm in the Yb gain region. Moreover, high-order mode suppression has been demonstrated with a proper air-hole size. Finally, the stress-induced birefringence due to the Ge-doped rods used as high-index insets has been investigated, accounting for the polarization-maintaining behaviour of the manufactured fiber.

We report on a fast measurement procedure for the widespread beam propagation ratio of light emerging from LMA fibers. The investigated beam is decomposed into its eigenmodes using a computer generated holographic filter. The modally resolved measurement of amplitudes and phases enables the reconstruction of the optical field. With the full field information, the propagation of the beam through free space is simulated and a virtual caustic measurement is realized. After comparing the presented method with ISO-standard measurements it is applied to conventional step-index fibers as well as to a multicore fiber.

Enabling Single-Mode (SM) operation in Large-Mode-Area (LMA) fiber amplifiers and lasers is critical, since a SM output ensures high beam quality and excellent pointing stability. In this paper, we demonstrate and test a new design approach for achieving ultra-low NA SM rod fibers by using a spatially Distributed Mode Filter (DMF). This approach achieves SM performance in a short and straight rod fiber and allows preform tolerances to be compensated during draw. A low-NA SM rod fiber amplifier having a mode field diameter of ~60μm at 1064nm and a pump absorption of 27dB/m at 976nm is demonstrated.

We investigate experimentally and theoretically the wavelength dependence of the pump absorption along Yb³⁺-doped fibers, for cladding-pumped single as well as coupled multimode (GTWave^TM) fibers. We show that significant spectral absorption distortions occur along the length with the 976nm absorption peak affected the most. We have developed a novel theoretical approach, based on coupled mode theory, to explain the observed effects. We have also investigated the mode mixing requirements in order to improve the absorption spectral distribution along the increase the overall absorption efficiency and discuss the implications on fiber laser performance.

Sintering of Yb-doped fused silica granulates is a well established technique developed by the IPHT and Heraeus Quarzglas and it produces very homogeneous rare earth doped bulk silica core rods for fiber laser applications. By using a newly developed laser induced deflection (LID) technique we are able to pre-characterize directly the material absorption properties of the bulk material prior to the laser fiber production. The bulk absorption results measured by LID are without scattering effects and they are typically in good agreement with the total attenuation measured in the fibers. We achieved a fiber background loss of 20 dB/km. Furthermore, we present detailed studies of the refractive index homogeneity of the Yb-doped bulk materials and laser fibers to show the unique features of the Yb-doped bulk silica. Multimode double cladding laser fibers with an extra large mode area XLMA fiber design (core diameter up to 100 μm) have been produced from the Yb-doped bulk silica rods by two different techniques. One is a classical jacketing method; the other employs the stacking of un-doped, Yb- and F-doped rods and F-doped tubes. Different fiber types have been tested in different fiber laser setups. The influence of the fiber end cap properties on the fiber laser focus shift is discussed in detail. We have achieved fiber laser output powers up to 1.925 kW, limited only by the pump power. We also investigated the long term laser stability at different power levels.

The single-mode regime of 19-cell Yb-doped double-cladding photonic crystal fibers, successfully exploited for high-power applications due to their large mode area, has been studied. The first higher-order mode cut-off wavelength has been evaluated taking into account the crossing between its dispersion curve, obtained with a full-vector modal solver based on the finite element method, and the one of the fundamental space-filling mode, calculated for an infinite cladding. Moreover, the overlap integral on the doped core of the higher-order mode at cut-off condition has been calculated, in order to investigate its effective suppression in the gain competition with the fundamental mode, by applying a spatial and spectral amplifier model. 19-cell double-cladding photonic crystal fibers with different core diameter and refractive index values have been considered. Simulation results have shown that the approach based on the fundamental space-filling mode effective index is not suitable for the cut-off analysis of large core fibers with a finite cladding dimension.

We demonstrate all-fiber passively mode-locked optical frequency comb source at 1.55 μm stabilized to the absorption line of the hydrogen cyanide. With reduced cavity length 169 MHz fundamental repetition frequency, 111 fs pulse duration and 44 nm wide optical comb were obtained. Frequency of the optical comb was tied to the frequency stabilized DFB laser diode Absolute stability of the single comb line was measured to be 9e-9. We expect that stability of 1e-10 within the whole comb bandwidth can be obtained with proposed configuration.

In this work, we have constructed an actively Q-switched T-DCF laser with an acousto-optic modulator (AOM). Actively Q-switched laser with T-DCF generated stable emission at 1065 nm over a wide repetition rate range of 0 Hz - 150 kHz, with the highest measured pulse energy of 1.6mJ limited by stimulated Brillouin scattering (SBS). We attribute the robust operation at low duty cycles to the intrinsically low amplified spontaneous emission (ASE), provided by the tapered fiber shape. In addition to the advantage of the large mode area at the output end of the fiber, the T-DCF provides several other attractive features. First, the vignetting of co-propagating ASE results in ASE power loss in wide-to-narrow end propagation. Second, the low-mode nature of counter-propagating spontaneous emission, arising from mode selection in the narrow part of the fiber, leads to weaker amplification of the spontaneous emission. These two effects contribute to the low ASE background. Finally, fiber diameter modulation is a known method for SBS suppression, and another inherent property of the T-DCF. These characteristics allowed for generation of 1.6mJ, 64 ns pulses at very low duty cycles, up to single shot operation, illustrating the potential of the T-DCF for high-energy pulse generation and amplification.

A new measurement technique for characterizing the magnitude of power coupling from the fundamental mode to higher order core- and cladding-bounded modes occurring in a fiber Bragg grating (FBG) inscribed in a large-mode-area (LMA) fiber is demonstrated and studied. The method is based on inducing mode selective fiber bending losses on the modes propagating in the core and monitoring the power guided by the cladding of the LMA fiber. Besides transmitted, also reflected distributions of modes can be resolved in terms of the relative powers carried by them and thus the fraction of higher order modes (HOMs) can be quantified. Additionally, the method can distinguish the mode content spectrally with high resolution. Sample FBGs having a chirped index profile are characterized using the method. It is shown that the method can yield information that is useful for better design and optimization of fiber optic devices utilizing FBGs in LMA fibers, such as fiber lasers.

Increasing the ablation efficiency of picosecond laser sources can be performed by bunching pulses in bursts¹ and benefit from heat accumulation effects^2-5 in the target. Pulsed fiber lasers are well suited for such a regime of operation, as the single pulse energy in a fiber is limited by the onset of nonlinear effects (SPM, SRS). Increasing the number of pulses to form a burst of pulses allows for average power scaling of picosecond fiber lasers. We are presenting in this paper a high-power fiber laser emitting arbitrarily-shaped bursts of picosecond pulses at 20 W of average output power. Burst duration can be varied from 2.5 ns to 80 ns. The burst repetition rate is externally triggered and can be varied from 100 kHz to 1 MHz. The single pulse duration is 60 ps and the repetition rate within a burst is 1.8 GHz. The output beam is linearly polarized (PER > 20 dB) and its M² value is smaller than 1.15. The laser source has a tunable central wavelength around 1064 nm and a spectral linewidth compatible with frequency conversion. Conversion efficiency higher than 60% has been obtained at 10 W of 1064-nm output power.

Optical fibers are used in various applications, e. g. optical communication, material processing, as a laser medium or to generate efficient supercontinua. For most of these applications the knowledge of the dispersion is an essential prerequisite. The dispersion and modal properties of photonic crystal fibers (PCF) strongly depend on the hole diameter and pitch. Since fabrication tolerances affect the structure of the photonic lattice, the dispersion behavior as well as the number of guided transverse modes can differ from numerical calculations. Dispersion measurement of singlemode photonic crystal fibers has been well described in recent papers. However, the determination of dispersion in the presence of higher-order modes is much more difficult. To measure the dispersion of optical fibers with high accuracy, a time-domain white-light interferometer based on a Mach-Zehnder interferometer is presented. The experimental setup allows to determine the wavelength-dependent differential group delay of light travelling through conventional fibers and PCFs within the wavelength range from VIS to NIR. Interferences appear due to superposition of two laser beams, one propagating through the tested fiber and the other travelling through air. Measuring the different group delays of a step-index fiber shows the sufficient accuracy of the interferometer. This paper demonstrates a simple yet effective way to suppress higher-order modes, making it possible to measure the chromatic dispersion of singlemode as well as multimode fibers.