This PDF file contains the front matter associated with SPIE Proceedings Volume 8961, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

When seeding a high power fiber amplifier with a frequency-chirped seed, the backward Brillouin scattering can be kept at the spontaneous level because the coherent laser/Stokes interaction is interrupted. Operating a conventional vertical cavity surface-emitting diode laser in an optoelectronic feedback loop can yield a linear frequency chirp of ~1016 Hz/s at a constant output power. The simple and deterministic variation of phase with time preserves temporal coherence, in the sense that it is straightforward to coherently combine multiple amplifiers despite a large length mismatch. The seed bandwidth as seen by the counter-propagating SBS is large, and also increases linearly with fiber length, resulting in a nearly-length-independent SBS threshold. Experimental results at the 600W level will be presented. The impact of a chirped seed on multimode instability is also addressed theoretically.

A novel acoustic and gain tailored Yb-doped photonic crystal fiber is used to demonstrate over 800 W single-frequency output power with excellent beam quality at 1064 nm. The large mode area fiber core is composed of 7 individually doped segments arranged to create three distinct acoustic regions and preferential gain overlap with the fundamental optical mode. This design leads to suppression of both stimulated Brillouin scattering and modal instability. To the best of our knowledge, the output power represents the highest power ever reported from a near diffractionlimited single-frequency fiber laser. Furthermore, we show that by using a broadband seed, 1.22 kW of output power is obtained without the onset of the modal instability.

An investigation of the use of hollow-core photonic bandgap (PBG) fiber to transport high-power narrow-linewidth light is performed. In conventional fiber the main limitation in this case is stimulated Brillouin scattering (SBS) but in PBG fiber the overlap between the optical intensity and the silica that hosts the acoustic phonons is reduced. In this paper we show this should increase the SBS threshold to the multi-kW level even when including the non-linear interaction with the air in the core. A full model and experimental measurement of the SBS spectra is presented, including back-scatter into other optical modes besides the fundamental, and some of the issues of coupling high power into hollow-core fibers are discussed.

We report on the use of a Semiconductor Disk Laser (SDL) as a seed laser for an Ytterbium-Doped Photonic Bandgap Fiber (Yb-PBGF) amplifier in a Master-Oscillator Power-Amplifier (MOPA) configuration. The SDL comprised a GaInAs/GaAs/GaAsP gain chip, a 1-mm-thick etalon for mode selection, and a 3-mm-thick birefringent filter for wavelength tuning. The fiber amplifier consisted of an Yb-doped core surrounded by a structure of periodically arranged germanium rods with a pitch of 10.2 μm, and to maintain the polarization, the fiber comprised two boron rods. The output of the MOPA-configuration was 31 W and the linewidth of the amplifier output was 149±31 kHz.

Novel laser applications such as laser-wake-field acceleration of particles require extreme parameters from ultra-short-pulse systems. We propose a concept capable to realize simultaneously multi-TW peak powers and multi-kW average powers by employing spatially and temporally separated amplification of chirped laser pulses delivered by fiber-amplifiers. As a combining element for the temporally 100 ns separated pulses (10 MHz repetition rate) we suggest a non-steady-state enhancement cavity using a fast switching-element to dump out the enhanced pulses at 15 kHz.

Coherent beam combining (CBC) is a promising solution for high power directed energy weapons. We investigate several particular issues for this application: First, we study the evolution of phase noise spectrum for increasing pump power in 100 W MOPFA. The main variations in the spectrum are located in the low frequency region corresponding to thermal transfer between the fiber core heated by the pump absorption and the fiber environment. The phase noise root mean square evolves linearly with the pump power. Noise spectrum is not shifted to higher frequencies. Second, we investigate the influence of fiber packaging and amplifier packaging on the phase noise and estimate the LOCSET controller bandwidth (BW) requirement in each case. Results show large variation of BW depending on the packaging, and not on the power. Then, we investigate the performances of CBC in harsh environment. For this purpose, we implement CBC of a 20-W fiber amplifier and a passive fiber using the LOCSET technique and simulate harsh environment by applying strong vibrations with a hammering drill on the optical table. The applied vibration spectrum ranges from 1 Hz to ~10 kHz with a standard deviation of 9 m/s². CBC of the amplifier output and the passive fiber output is performed on a second table, isolated from vibrations. Measurements of the phase difference between both outputs and of the applied vibrations are simultaneously performed. Residual phase error of λ/40 (i.e. > 99 % CBC efficiency) is achieved under strong vibrations at 20 W. The -3 dB bandwidth of the LOCSET controller has been measured to be ~4.5 kHz. Results are in agreement with simulations.

This paper demonstrates a next-generation high-energy, eye-safe light detection and ranging (LIDAR) transmitter for the Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission. The system design is based on an advanced eye-safe, polarization-maintaining (PM) master oscillator-power amplifier (MOPA) LIDAR transmitter platform currently under development at Fibertek. This platform consists of a narrow linewidth (400 Hz – 1MHz) and highly stable seed laser, a flexible and reconfigurable pulse generator, and multiple stages of PM Erbium (Er)-doped fiber amplifiers (EDFAs) with increasing mode-field area. Using this architecture, we have demonstrated more than 20W continuous wave (CW) at 1571nm, up to 475 μJ energy per pulse at 1572 nm, and up to 250 μJ energy per pulse at 1529 nm wavelength with 1.5 μs pulsewidth and 10 kHz repetition rate. The output beams at the highest energy levels are diffraction limited, and the polarization extinction ratio (PER) is ~17dB. The optical efficiency is about 36% at CW operation and the optical-to-optical conversion efficiency is ~17% with respect to total pump power when the laser is in pulse operation mode. We also demonstrate a comparable optical efficiency (30%) with CW operation using radiationharden Er-doped gain fiber.

The subject of this paper is the requirements, design, fabrication, and testing of a prototype laser rock drilling system capable of penetrating even the hardest rocks found deep in the earth. The Oil and Gas industry still uses many of the technologies that were in use at the turn of the 19^th century. The drilling industry started with a great innovation with the introduction of the tri-cone bit by Howard Hughes in 1908. Since then, the industry has modified and optimized drilling systems with incremental advancement in the ability to penetrate hard crystalline rock structures. Most oil producing reservoirs are located in or below relatively soft rock formations, however, with the growing need for energy, oil companies are now attempting to drill through very hard surface rock and deep ocean formations with limited success. This paper will discuss the types of laser suitable for this application, the requirements for putting lasers in the field, the technology needed to support this laser application and the test results of components developed specifically by Foro Energy for the drilling application.

The single-mode properties of LMA AS-PBGFs have been numerically analyzed. It was shown that the analysis of the losses in single-cladding fibers provides a significantly different picture of the mode confinement with respect to the overlap integral, which is commonly considered when designing active double-clad fibers. Innovative designs, based on a so-called heterostructured cladding, have been considered to enhance HOM suppression, providing a significant improvement with respect to standard AS-PBGFs.

A novel multifocus tomographic algorithm for reconstructing an optical fiber’s cross sectional refractive index distribution from transverse projections is described. This new algorithm is validated against measurements of both microstructured and multicore optical fibers, which were not previously measurable. Optical fiber tomographic measurements recently made by several research groups using different technologies have all suffered from the same limitation, namely that typical fiber diameters (several hundred microns) exceed the imaging depth-of-field (approximately one micron) by several orders of magnitude. The new algorithm combines data acquired from a multiplicity of focal planes to overcome this limitation, yielding measurements with extremely fine spatial resolution over large transverse dimensions, thereby providing the first-ever high quality measurements of microstructured and multicore fibers. This new measurement approach is broadly applicable to any tomographic problem in which the depthof- field is greatly exceeded by the transverse dimension of the specimen. Many types of transverse optical fiber measurement technologies, including interference microscopy, quantitative phase microscopy (QPM), residual stress measurement, differential interference contrast (DIC) microscopy, and spontaneous emission tomography will benefit from this new algorithm, which will greatly facilitate characterization of optical fibers for high-power applications.

In this paper we consider a new type of hollow core microstructured optical fibers (HC MOFs) so called negative curvature hollow core fibers (NCHCFs). NCHCFs are known as hollow core fibers which allow to transmit a light under extremely high material loss of the cladding material. Such unique property of NCHCFs is due to the fact that their guiding mechanism is different from the guiding mechanisms in hollow core photonic crystal fibers (HC PCFs) and hollow core Bragg fibers (HC BFs). The two main factors which determine the guiding properties of NCHCFs are the ‘negative curvature’ (in a more general case, an alternating curvature) of the core – cladding boundary and the density of electromagnetic states of the cladding. It will be shown that the ‘negative curvature’ of the core – cladding boundary determines the type of interference which can lead to strong light localization in the air core. The interference which leads to air core mode formation in HC PCFs or HC BFs can be considered in terms of a linear momentum transfer by the photonic crystal cladding to the air core modes. In the case of NCHCFs the air core mode formation can be considered in terms of an azimuthal momentum transfer by the core – cladding boundary with an alternating curvature to the air core modes. The fabrication process of NCHCFs and several potential applications of NCHCFs in medicine, sensing and high power delivery are discussed.

This paper presents the newest development of high energy, high power ultrafast fiber lasers in 1 μm, 2 μm and 3 μm regimes at PolarOnyx Inc. For the 1 μm mJ fiber laser, a Yb-doped PCF optical amplifier was built to boost pulse energy to 1.1 mJ at 100 kHz, with pulse duration of 710 fs. In the kW experiment, up to 1050 W average power was obtained with a repetition rate of 80 MHz and pulse duration of 850 fs. In the Tm-doped fiber laser experiment, an average power of up to 76 W was achieved at wavelength of 2012 nm with a repetition rate of 31 MHz and pulse duration of 870 fs. In the Er:ZBLAN fiber laser experiment, an average power of up to 142 mW was achieved at wavelength of 2784 nm with a repetition rate of 16.4 MHz and pulse duration of 5 ps. This work lays out a foundation for further energy and power scaling of ultrafast fiber lasers.

Passive spatial and temporal coherent combining schemes are implemented to scale the output energy of a nonlinear temporal compression setup. By generating 32 replicas of the incident femtosecond pulses, the output of a high energy fiber chirped-pulse amplifier can be compressed using self-phase modulation in a large mode area rod-type fiber at peak power levels well beyond the self-focusing threshold of 4 MW. We demonstrate the generation of 71 fs 7.5 μJ pulses at 100 kHz repetition rate, corresponding to a peak power of 86 MW.

Divided-pulse amplification employing passive coherent beam combining implementations causes a strong degradation in efficiency. In this contribution typical implementations are analyzed and a solution using an active stabilization system is presented. With this 380 fs pulses at 1.25 mJ corresponding to a peak power of 2.9 GW have been achieved demonstrating the potential of this approach.

We describe unprecedented performance level from a femtosecond fiber laser system optimized for precision industrial micro-machining. The monolithic fiber chirped pulse amplifier chain enables system output of 215 μJ pulse energy, ~510 fs pulse duration and 16 W average power. We reveal the critical enabling technology to reach this unprecedented pulse energy level, the salient operating principles for the full chirped pulse amplification system, and the key experimental performance data for the laser system.

Fiber designs are proposed that allow distributed wavelength filtering far more selective than conventional designs, and which is consistent with conventional fiber fabrication. By including a gradient that pre-compensates the bend perturbation in the cladding, the proposed designs overcome the usual tradeoff between mode area and wavelength selectivity. Simulations shows that the resulting fiber performance enables delivery of multi-kW signals over long distances with modest net Raman gain, using bend-resistant fibers of convenient core size.

Diffraction-limited high power lasers in the region of 10s of kW to greater than 100 kW are needed for defense, manufacturing and future science applications. A balance of thermal lensing and Stimulated Brillouin Scattering (SBS) for narrowband amplifiers and Stimulated Raman Scattering (SRS) for broadband amplifiers is likely to limit the average power of circular core fiber amplifiers to 2 kW (narrowband) or 36 kW (broadband). A ribbon fiber, which has a rectangular core, operating in a high order mode can overcome these obstacles by increasing mode area without becoming thermal lens limited and without the on-axis intensity peak associated with circular high order modes. High order ribbon fiber modes can also be converted to a fundamental Gaussian mode with high efficiency for applications in which this is necessary. We present an Yb-doped, air clad, optical fiber having an elongated, ribbon-like core having an effective mode area of area of 600 μm² and an aspect ratio of 13:1. As an amplifier, the fiber produced 50% slope efficiency and a seed-limited power of 10.5 W, a gain of 24 dB. As an oscillator, the fiber produced multimode power above 40 W with 71% slope efficiency and single mode power above 5 W with 44% slope efficiency. The multimode M2 beam quality factor of the fiber was 1.6 in the narrow dimension and 15 in the wide dimension.

We propose a novel cylindrically symmetric hybrid fiber design that allows combining properties of the both fibers guiding light due to total internal reflection (low optical losses) and photonic bandgap fibers (anomalous dispersion at 1 μm). Refractive index profile of these fibers consist of the only few layers: low-index core (n_core-n_silica>0) surrounded with one or more high-index ring layers (n_high-n_core>0), a depressed layer (n_depress-n_silica<0) and silica cladding. Operating mode is one of the high-order modes (depending on high-index ring layers number) with intensity maximum at fiber axis. Because the other modes (including the fundamental mode LP₀₁) are guided in the high-index ring layer(s) the hybrid mode can be easily excited by splicing hybrid fiber and standard single-mode (λ~1μm) step-index fiber with appropriate mode field diameter. Moreover method of achievement of asymptotically singlemode regime of light propagation (suppression of the high-index ring layer modes) has been proposed. The main idea of it is doping narrow strong absorbing layer where hybrid mode has intensity of electric field closed to zero. Furthermore we have considered possibility to increase anomalous dispersion of the hybrid fiber (up to 100 ps/nm km) by usage more complicated refractive index profile with two high-index ring layers. In this work we have fabricated the technologically simplest hybrid fiber with the only one high-index layer. The hybrid LP₀₂ core mode had dispersion of 13 ps/(nm km) and optical loss of about 6 dB/km. Propagation of chirped pulses through the fabricated hybrid fiber allowed us to compress them from 8ps to 330fs.

We present a 3kW single-mode fiber laser based on an Yb-doped LMA fiber operating at 1080nm. The laser which is pumped by 9xxnm diode bars stacks. It is believed to be the highest power direct diode pumped single-mode fiber laser oscillator to date.

In this contribution we demonstrate a single mode continuous wave laser amplifier with 146 W of power at a wavelength of 1009 nm. On one hand this experiments constitutes an extension of the wavelength range of high power fiber lasers, furthermore, emission wavelength well below 1030 nm find use for efficient high-brightness tandem pumping of high power fiber amplifiers. The wavelength and bandwidth of the seed oscillator are defined by a pair of fiber Bragg gratings. This seed is amplified in a two-stage Ytterbium-doped rod-type amplifier to 146 W with a high slope efficiency of 64 %, an excellent beam quality and an ASE-suppression as high as 63 dB.

Power scaling of Yb-free Er-doped fiber lasers is extremely challenging due to low Er ion absorption cross-section and growth of unbleachable loss at high Er concentrations because of clustering effects. Hence, usual double-clad Er-doped fibers suffer from low efficiency. We present an efficient high power all-fiber amplifier based on our newly developed Yb-free Er-doped fiber. Proper core composition and relatively low Er3+ concentration mitigates clustering effect. Furthermore, large single-mode core diameter of 34 um increases the pump absorption and decreases the fiber length. Our amplifier consists in the specialty Er fiber pumped through a commercially available pump combiner by means of 6 pigtailed multimode diodes (D=105 um, NA=0.15, input pump power of 275W). The signal source is a low power continuous wave fiber laser spliced to the amplifier. Therefore we built truly all-fiber laser without any free space coupling. We obtained 103 W of amplified signal limited only by the available pump power. Pump conversion efficiency is as high as 37 %. To the best of our knowledge this is the highest power ever demonstrated for Yb-free Er-doped lasers pumped at 976 nm. This power level is similar to that obtained in resonantly pumped Er-doped fiber lasers.

We demonstrate three-level laser operation at 976 nm of a large-core Yb-doped aluminosilicate fiber, which is fabricated by powder-sinter technology and shows a very homogeneous refractive index profile. The investigated fiber has a core diameter of 126 μm and a numerical aperture of 0.18, well-matched to standard fiber coupled pump diodes. The core composition has been optimized to reduce photodarkening effects. Multimode and single mode operation with multiple Watts output power is presented for this fiber making it useful for the realization of high brightness fiber coupled pump sources.

A significant enhancement in the rate of material removal is demonstrated using a nanosecond-pulsed UV fiber laser in multi-pulsing burst mode, as compared to the case without bursting. Percussion drilling and scribing of thin-film and bulk material tests show that, in general, laser bursts with increased pulse count and reduced pulse spacing show higher rates of material removal. A considerable improvement in removal rate is demonstrated, when bursting is applied to scribing of mono-crystalline silicon (m-Si) and up to 30% in percussion drilling speed. Likewise, improved material removal is demonstrated for scribing of thin film of indium tin oxide (ITO) on glass or metal film on sapphire. Examples of material processing are given with and without bursting at similar experimental conditions of average power, scan speed, and burst/pulse energies. Experimental results included are for m-Si, ITO thin films on glass, and metal films on sapphire.

Pulsed fiber lasers and continuous-wave (cw) fiber lasers have become the tool of choice in more and more laser based industrial applications like metal cutting and welding mainly because of their robustness, compactness, high brightness, high efficiency and reasonable costs. However, to further increase the productivity with those laser types there is a great demand for even higher laser power specifications. In this context we demonstrate a pulsed high peak- and averagepower fiber laser in a Master Oscillator Power Amplifier (MOPA) configuration with selectable pulse durations between 1 ns and several hundred nanoseconds. To overcome fiber nonlinearities such as stimulated Raman scattering (SRS) and self-phase-modulation (SPM) flexible Ytterbium doped extra-large mode area (XLMA) step index fibers, prepared by novel powder-sinter technology, have been used as gain fibers. As an example, for 12 ns pulses with a repetition rate of 10 kHz, a pump power limited average laser output power of more than 400 W in combination with peak powers of more than 3.5 MW (close to self-focusing-threshold) has been achieved in stable operation. The potentials of this laser system have been further explored towards longer pulse durations in order to achieve even higher pulse energies by means of pulse shaping techniques. In addition, investigations have been conducted with reduced pulse energies and repetition rates up to 500 kHz and average powers of more than 500 W at nearly diffraction limited beam quality.

We demonstrate flexible performance in a fiber MOPA system based on nLIGHT’s PFL seed laser platform and chirally coupled core (3C®) fiber. The 33μm core, 27μm MFD 3C fiber used in these demonstrations is fabricated in volume at nLIGHT’s Finland facility. A variety of pulse formats are amplified to nonlinearity-limited peak power <300kW, including single pulses in the 50ps to 1ns regime at a variety of repetition rates from 10’s of kHz to MHz. Beam quality in these 3C based MOPAs is exceptional with M2<1.15 and circularity <95% at all power levels. Beam pointing often evident in other LMA fiber technologies due to higher order mode content is minimal in these fiber MOPAs. Burst mode operation of the seed laser system using flexible burst packet repetition rates (10’s of kHz to several MHz) and adjustable pulse-to-pulse spacing within bursts (<10ns to 100ns) is demonstrated and amplified in the same 3C fibers. Bursts of up to ten 50ps pulses amplified to total energies exceeding 160μJ are demonstrated at 200kHz burst repetition rate and 32W average power at high efficiency (74% slope). Bursts of up to five 500ps pulses are also amplified to up to 360μJ total energy. In both cases, the varying degree of pulse saturation win a burst and mitigation paths are reviewed.

We report an all-fiber passively Q-switched laser using a large mode area (LMA) Yb³⁺ -doped fiber claddingpumped at 915 nm and an unpumped single-mode (SM) Yb3⁺-doped fiber as the saturable absorber (SA). The saturable absorber SM fiber and LMA gain fiber were coupled with a fiber taper designed to match the fundamental spatial mode of the LMA fiber and the expanded LP01 mode of the single mode fiber. The amplified spontaneous (ASE) intensity propagating in the single mode SA saturates the absorption before the onset of gain depletion in the pumped fiber, switching the fiber cavity to a high Q-state and producing a pulse. Using this scheme we demonstrate a Q-switched all-fiber oscillator with 32 μJ 93 ns pulses at 1030 nm. The associated peak power is nearly two orders of magnitude larger than that reported in previous experimental studies using a single Yb⁺³ saturable absorber fiber. The pulse energy was amplified to 0.230 mJ using an Yb³⁺-doped cladding pumped fiber amplifier fusion spliced to the fiber oscillator, increasing the energy by eight fold while preserving the all-fiber architecture.

We report on the development and performance of a fully monolithic PCF amplifier that has achieved over 400 W with near diffraction limited beam quality with an approximately 1GHz phase modulated input. The key components for these amplifiers are an advanced PCF fiber design that combines segmented acoustically tailored (SAT) fiber that is gain tailored, a novel multi fiber-coupled laser diode stack and a monolithic 6+1x1 large fiber pump/signal multiplexer. The precisely aligned 2-D laser diode emitter array found in laser diode stacks is utilized by way of a simple in-line imaging process with no mirror reflections to process a 2-D array of 380-450 elements into 3 400/440μm 0.22NA pump delivery fibers. The fiber combiner is an etched air taper design that transforms low numerical aperture (NA), large diameter pump radiation into a high NA, small diameter format for pump injection into an air-clad large mode area PCF, while maintaining a constant core size through the taper for efficient signal coupling and throughput. The fiber combiner has 6 400/440/0.22 core/clad/NA pump delivery fibers and a 25/440 PM step-index signal delivery fiber on the input side and a 40/525 PM undoped PCF on the output side. The etched air taper transforms the six 400/440 μm 0.22 NA pump fibers to the 525 μm 0.55 NA core of the PCF fiber with a measured pump combining efficiency of over 95% with a low brightness drop. The combiner also operates as a stepwise mode converter via a 30 μm intermediate core region in the combiner between the 20 μm core of the input fiber and the 40 μm fiber core of the PCF with a measured signal efficiency of 60% to 70% while maintaining polarization with a measured PER of 20 dB. These devices were integrated in to a monolithic fiber amplifier with high efficiency and near diffraction limited beam quality.

The plasma outside deposition (POD) process is the basic technology for production of large core multimode silica fibers with highly fluorine doped cladding. Due to the all silica fiber construction such fibers can transmit several 10 kW of light power even with a core of 100 μm or less. An overview of the current capabilities and trends in high power laser applications will be presented, including very large fibers, shaped core and cladding designs and fibers with multiple claddings or multiple cores. These concepts can be applied to transmission fibers as well as fiber lasers. Heraeus is supporting these new developments by offering a growing number of materials, preforms and services.

We have studied the photodarkening effect in fiber preforms with an ytterbium-doped aluminosilicate glass core. The room-temperature stable Yb²⁺ ions formation in the glass matrix under both UV- and NIR-pumping irradiation was revealed by the method of absorption spectra analysis and the fluorescence spectroscopy technique. Comparative studies of preforms and crystals samples luminescence spectra, obtained under UV-excitation, were performed. A general mechanism of Yb²⁺ ions and aluminium oxygen-hole centers (Al-OHC) formation as a result of photoinduced process of Yb³⁺ ions excitation to "charge-transfer state" (CTS) was found for both Yb:YAG crystal and aluminosilicate glass.

In this paper, we report fabrication and investigation of ytterbium-doped phosphorsilicate fiber (P co-doped YDF) with high Yb content, low numerical aperture, and low background loss. The P co-doped YDF is fabricated by MCVD using the vapor sources of Yb, SiCl₄, AlCl₃, and POCl₃, and by the gas-phase doping method. The optical properties of this P co-doped YDF are compared with Al co-doped and Al:P co-doped YDFs with low background losses. The minimum background loss of the P co-doped YDF in the spectral range from 1100 to 1380 nm is as low as ~3 dB/km. This is nearly independent of the Yb and P contents because soot deposition and collapsing conditions are properly optimized (i.e., the P co-doped YDF from a non-optimized process shows a few hundred dB/km). The excess loss induced by PD, for the P co-doped YDF, was dramatically reduced compared to both Al co-doped and Al:P co-doped YDFs. The optical slope efficiency of the P co-doped YDF is about 80%, depending on the pumping wavelength and fiber length. The fiber colors during pumping are blue for both the P co-doped and Al:P co-doped YDFs. Based on the results from a prolonged test, the output power of the P co-doped YDF is highly stable, with an initial degradation of 2-3%; which demonstrate improvement in PD resistivity with P incorporation into the glass, compared to the Al:P co-doped YDF with degradation above 6%.

Resonantly pumped holmium fibre lasers present a range of opportunities for the development of novel fibre laser and amplifier devices due to the availability of mature, efficient high power thulium fibre pump lasers. In this paper we describe the operation of a large mode area holmium-doped fibre amplifier. The master-oscillator is an all-fibre linearly polarised, core pumped single mode laser operating at 27 W at 2.11 μm. This laser was amplified in a large mode area fibre producing up to 265 W of output power. This system is the first demonstration of a resonantly pumped holmiumdoped fibre amplifier. It is also the highest power fibre amplifier that is capable of operating in an atmospheric transmission window <2.05 μm. This monolithic all-fibre system is able to address a wide range of remote sensing, scientific, medical and defence applications.

In this contribution we propose a new way to simultaneously increase (almost double) the amplification efficiency and reduce the thermal issues of Thulium-doped fibers. This technique is based on pumping the excited state of the Thulium fiber, in order to bring the ions from the pump level to the upper laser level via stimulated emission instead of via phononic relaxation processes. Our simulations show that using this technique the efficiency of the amplification process can be increased by almost a factor of 2.

We report supercontinuum generation in the 1.7-2.9 μm range with up to 3.08 W of output power and in the range of 1.93-3.18 μm with up to 3.8 W of output power from all-fiber MOPA pulsed systems with Tm-doped fiber mode-locked seed laser. Supercontinuum generation was demonstrated in nonlinear germanate fibers and fluoride (ZBLAN) fibers. The supercontinuum bandwidth reached 1250 nm at -10 dB level.

The coherent combining of ultrashort pulses is a concept for scaling the pulse energy and average power of laser systems emitting ultrashort pulses. This concept has already been demonstrated for two-channel systems. In this contribution we report on a four-channel system that delivers 2.1 mJ pulse energies at a repetition rate of 100 kHz and 210 W of average power after compression. A combination efficiency of 88% was achieved and this demonstrates the scalability of the combining approach to a larger number of channels.

Coherent combination applied to state-of-the-art femtosecond fiber CPA systems has recently opened the route towards high energy and average power laser systems. The ~2 mJ, 340 fs, 196 W (100 kHz) pulses of such a system are coupled to an argon-filled hollow-core fiber for spectral broadening via self-phase modulation. Subsequent compression in a chirped mirror compressor leads to ~1 mJ, 45 fs, 96 W pulses. Under different conditions 580 μJ, 26 fs, 11.6 GW, 135 W (250 kHz) pulses are achieved. This is an unprecedented combination of average power and pulse energy.

We implement, in the same femtosecond fiber amplifier setup, both chirped pulse amplification and divided pulse amplification. With the generation of temporally delayed replicas this scheme allows an equivalent stretched pulse duration of more than 1ns in a compact tabletop system. The generation of 45 W of compressed average power at 100 kHz, together with 320 fs and 450 μJ pulses, is demonstrated using a rod-type ytterbium-doped fiber.

We demonstrate spectral coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using active phase stabilization based on an electro-optic phase modulator, an average power of 10 W before compression and a high gain factor of 30 dB is obtained. At this gain value, 130 fs pulses with a spectral width of 19 nm can be generated, highlighting the strong potential of pulse synthesis for the reduction of the minimum duration of ultrashort pulses in fiber chirped-pulse amplifiers.

Large mode area rod-type fibers have enabled amplification of ultra-short pulses to mJ pulse energy and MW peak powers. For very large mode field areas, fibers have to be designed as rigid rods with typical fiber lengths of around 1 m for efficient operation. A shorter fiber length can be desirable to reduce the packaging size of commercial systems and to decrease the impact of parasitic nonlinear effects for peakpower scaling. The fiber design presented here is based on a modified large-pitch fiber with an effectively higher ytterbium concentration in the fiber core. To achieve index matching the cladding index needs to be changed. In this contribution we propose to co-dope the passive host material with germanium to match both indices and to obtain a higher Yb-concentration within the active core. Compared to standard LPF, where the core index is reduced by co-doping the core with Flourine, the ytterbium doping concentration of this novel germanium-pedestal LPF is doubled. A detailed numerical and experimental investigation shows that with short fiber lengths <40cm is feasible to achieve output powers beyond 100W with 10W seed. Significantly higher gains, of nearly 30 dB, can be achieved for fiber lengths in the order of 60cm. A similar gain can be expected in a conventional LPF with 1.20 m length. In conclusion, we demonstrate a fiber design for significantly enhanced energy storage per fiber length and improved pump absorption. This concept will notably reduce the footprint of ultra-short fiber laser systems.

The use of double-clad fibers for short pulses amplification requires high active ions concentration in order to keep the active fiber length short. In the case of Er-doped fibers an increase of concentration leads to a significant drop of efficiency due to Er ions clustering. We have demonstrated through numerical simulation that efficiency of amplifiers based on double-clad P₂O₅-Al₂O₃-SiO₂ (PAS) Er-doped fibers decreases slower with Er-concentration growth if compared with standard Al₂O₃-SiO₂ fibers. In this paper, we present single-mode large-mode-area heavily Er-doped double-clad fiber based on PAS glass matrix for short pulses amplification. The developed PAS fiber has a 36 μm singlemode core and a small signal cladding absorption of 3 dB/m at 980 nm leading to an optimal fiber length in range of 5-8 m depending on the central wavelength. At first, an all-fiber nanosecond MOPA at 1560 nm was built using our PAS fiber as the final amplifier. We obtained 28 W of average output power (efficiency of 25 % with respect to the launched pump power at 976) limited by amplified spontaneous emission. Pulse energy of 1.5 mJ was achieved at pump power level of ~120 W. We believe that it is the first demonstration of mJ-energy level single-mode nanosecond fiber system. Then, direct amplification of 100-fs source was performed using this fiber. We obtained 12 nJ pulse energy and 100 kW of peak power from the fiber which is close to the record value for Er-doped fiber amplifiers.

We present results for increased transmission of ~99.5% in the near-IR through the end faces of silica single mode fibers by creating a random antireflective microstructure etched into the end face of the fiber. We demonstrate high laser damage thresholds for these fibers with AR structured surfaces.

Large-pitch fibers (LPFs) have enabled the current records for average power, pulse energy and pulse peak power in ultra-fast fiber laser systems. In this paper the working principle of LPFs, which is based on higher-order mode delocalization, is numerically analyzed paying special attention to thermal effects and index mismatch. An enhanced design concept is proposed with a reduced symmetry to improve the delocalization of higher-order modes. This enhanced design has been obtained by transferring the most important characteristics of spiral geometries to a common hexagonal lattice.

We demonstrate a record of over 20W yellow light by frequency-doubling the output of a novel Yb-doped fiber source. A pulsed source in the wavelength range of 1060-1100nm is efficiently shifted by SRS to the wavelength of a CW seed in the 1100-1180nm range. While pulse width is determined by the pulsed source, the wavelength is set by the narrowband CW seed. The extended frequency is doubled to yellow in a single pass SHG with an overall efficiency of 915nm pump converted to yellow exceeding 25%. The scheme can be used to generate scalable, high-power, costeffective sources in the 560-590nm range.

We report on two types of modal instabilities observed in high power Yb amplifiers based on Large Mode Area Fibers. The first is observed to occur at a Threshold Power, which we refer to as Threshold Power Modal Instabilities (TPMI). The modal instability is observed as a decrease in beam quality or reduced core light output as higher order modes leak into the fiber cladding. In PM 25/400 fiber amplifiers, we observe the threshold for the modal instability to vary depending on pump wavelength detuning, with the onset occurring at approximately 15 W/m peak heat load. In PM 20/400 and 25/400 fiber amplifiers without stress rods or other polarization control, we can achieve 1 kW output, limited by available pump power, without modal instabilities. The second type of modal instability is observed for certain cases where the fiber initially operates without any sign of MI but then degrades over an extended operating time, leading to a similar behavior as the TPMI. We refer to the second class as Fiber Degradation Modal Instabilities (FDMI). For these degraded fibers, we observe that fiber performance is unchanged below the critical power for modal instabilities. Experiments on degraded fiber show a wavelength dependent permanent change in the degraded fiber with a memory of the original operating wavelength.

We use our numerical model of mode instability to analyze the influences of spontaneous thermal Rayleigh scattering (sTRS) and laser gain saturation on instability threshold powers. sTRS is stronger than the quantum noise used as the seed power for stimulated thermal Rayleigh scattering in previous studies, so the threshold is reduced by 15-25% with sTRS seeding. Gain saturation is strong in any efficient amplifier and we show how it can be exploited to raise instability thresholds be a factor of two or more while staying below the stimulated Brillouin threshold.

In this paper the threshold for Stimulated Raman scattering (SRS) is analyzed experimentally and theoretically for monolithic LMA cq kW fiber oscillators. Four oscillators with different spectral widths of the low reflecting (LR) Fiber Bragg Gratings (FBG) (0.04 nm, 0.5 nm, 1.5 nm (FWHM) and without LR grating) were characterized. Experimental it was found that threshold of SRS depends on the spectral width of the out coupling FBGs, which is not yet understood completely. Attempts to describe such lasers by simulations are based on nonlinear Schrödinger equation supporting spectral broadening of cw-fiber laser, rate equation gain as well as broadband Raman gain. The experimental results and the simulations were compared and discussed.

Numerical analysis of the onset of modal instability in fiber amplifiers is presented. Specifically calculations of the evolution of the intensity fluctuation spectrum along the fiber for a sampling point offset from the core center are presented for different instability onset conditions. These include seeding with LP01 only, seeding with LP01 and LP11 at the same frequency, seeding with LP01 and LP11 at offset frequencies, and seeding taking quantum shot noise into account. The position dependent spectra are shown to be very similar for each of these cases suggesting a common instability mechanism.

High-power, high-brightness, fiber-coupled pump modules enable high-performance industrial fiber lasers with simple system architectures, multi-kW output powers, excellent beam quality, unsurpassed reliability, and low initial and operating costs. We report commercially available (element™), single-emitter-based, 9xx nm pump sources with powers up to 130 W in a 105 μm fiber and 250 W in a 200 μm fiber. This combination of high power and high brightness translates into improved fiber laser performance, e.g., simultaneously achieving high nonlinear thresholds and excellent beam quality at kW power levels. Wavelength-stabilized, 976 nm versions of these pumps are available for applications requiring minimization of the gain-fiber length (e.g., generation of high-peak-power pulses). Recent prototypes have achieved output powers up to 300 W in a 200 μm fiber. Extensive environmental and life testing at both the chip and module level under accelerated and real-world operating conditions have demonstrated extremely high reliability, with innovative designs having eliminated package-induced-failure mechanisms. Finally, we report integrated Pump Modules that provide < 1.6 kW of fiber-coupled power conveniently formatted for fiber-laser pumping or direct-diode applications; these 19” rack-mountable, 2U units combine the outputs of up to 14 elements™ using fused-fiber combiners, and they include high-efficiency diode drivers and safety sensors.

White noise phase modulation is an effective technique capable of increasing the SBS threshold in high power fiber amplifiers. Theoretical models predict the enhancement factor as a function of linewidth and fiber length, but have yet to be experimentally verified over wide ranges of these variables. We present results on a cut-back experiment performed on a passive fiber with a white-noise broadened laser, measuring the SBS enhancement factor as a function of fiber length and bandwidth. In addition, the experimental results will be compared to phase modulation models of the SBS process in optical fibers.

Heavy doping of common silica gain fibers is not practical; therefore long fibers are required for efficient amplification (usually 5-10m). This is undesirable due to nonlinearities that grow with fiber length. In contrast, NP Photonics phosphate-glass based fibers can be heavily doped without any side-effects, and hence can provide very high gain in short lengths (less than 0.5m). This enables an ideal pulsed fiber amplifier for a MOPA system that maximizes the energy extraction and minimizes the nonlinearities. We demonstrate 1W average power, 200μJ energy, and >10kW peak power from a SBS-limited all-fiber MOPA system at 1550nm, and 32W average power, 90μJ energy, and 45kW peak power from a SRS and SPM limited all-fiber MOPA system at 1064 nm. These results were limited by the seed and pump sources.

Coherent beam combining (CBC) by active phase control has proven to be an efficient and scalable way to increase the output power delivered by fiber laser systems. This study investigates the potential of this technique when applied to another type of laser sources: nonlinear optical frequency converters. Given that the efficiency of nonlinear conversion processes, here Second Harmonic Generation (SHG), relies on a phase-matching condition between the fundamental and the harmonic waves, an indirect control of the phase of an SHG beam through the fundamental wave is theoretically possible. This paper experimentally demonstrates such indirect phase control and its application to CBC of frequency converters. Two continuous-wave 1.55-μm fiber amplifiers are frequency doubled in PPLN crystals to generate 775-nm beams. These SH beams are coherently combined using frequency-tagging active phase control. A standard fibered electro-optic modulator (EOM) is used to control the phase of one of the 1.55-μm fiber amplifiers. This EOM provides both phase modulation for frequency-tagging and proper phase shifts to compensate for the phase fluctuations of the other 1.55-μm amplifier. Efficient coherent combining of these two 775-nm beams is successfully achieved: a λ/19.5 residual phase error is measured.

Ho-doped fiber lasers are of interest for high energy laser applications because they operate in the eye safer wavelength range and in a window of high atmospheric transmission. Because they can be resonantly pumped for low quantum defect operation, thermal management issues are anticipated to be tractable. A key issue that must be addressed in order to achieve high efficiency and minimize thermal issues is parasitic absorption in the fiber itself. Hydroxyl contamination arising from the process for making the Ho-doped fiber core is the principal offender due to a combination band of Si-O and O-H vibrations that absorbs at 2.2 μm in the Ho³⁺ emission wavelength region. We report significant progress in lowering the OH content to 0.16 ppm, which we believe is a record level. Fiber experiments using a 1.94 μm thulium fiber laser to resonantly clad pump a triple clad Ho-doped core fiber have shown a slope efficiency of 62%, which we also believe is a record for a cladding-pumped laser. Although pump-power limited, the results of these studies demonstrate the feasibility of power scaling Ho-doped fiber lasers well above the currently-reported 400-W level.¹

In this work we discuss the impact of visible light radiation on photodarkening generation in 1070-nm Yb-doped fiber lasers. Simultaneous photodarkening and photobleaching effects induced by 976 nm and 405 nm or 550 nm radiations respectively were investigated. We observed a significant photobleaching effect due to 405 nm radiation but not a complete recovery. A strong absorption of the 405 nm radiation by the excited ions (Excited-State Absorption) was also observed and found as a main limiting factor for the bleaching performance together with observation of photodarkening losses induced by ground-state absorption. To proper define the optimum bleaching wavelength we report, for the first time to the best of our knowledge, the Excited-State Absorption cross section in the visible range. The reported experiments allow to individuate the main parameters defining the optimum bleaching wavelength. In a final experiment, using optimized 550-nm wavelength bleaching radiation, we were able to operate a laser at 93% of its pristine power level compensating a power drop of about 45% in absence of bleaching. The method we present is an effective yet simple way to run laser using standard Al-silicate fibers with doping level over 10²⁶ ions/m³ and high inversion.

In this paper we demonstrate a Raman fiber oscillator for the generation of radially and azimuthally polarized beams. The Raman fiber oscillator comprises a high NA fiber and two Fiber-Bragg Gratings (FBGs). Due to the high NA of the fiber, radially and azimuthally polarized modes are guided with their own effective refractive indexes, i.e. they are not degenerated. Therefore, the FBGs reflect these modes at different wavelengths. The mode that oscillates in the resonator can be selected by controlling the coupling lens and the polarization of the pump beam. Unfortunately, at the output of the fiber oscillator the output beams exhibit a non-circularly symmetric intensity profile as a result of a slightly elliptical fiber core. Consequently, the impact of elliptical cores on the polarization degeneracy has been analyzed in detail. In order to compensate for the elliptical core we applied a transverse force on the last few cm of the fiber. With this force the waveguide characteristic of the fiber is changed in such a way that a radially or azimuthally polarized doughnutshaped beam profile is observed. Thereby an output power of 480mW (400mW) was reached for the azimuthal (radial) polarization. The presented concept is wavelength agile and suitable for all-fiber microscopic setups, especially for STED-microscopy.

For the implementation of novel fiber laser concepts, such as extra-large mode area (X-LMA) fiber lasers or multi-core fiber lasers alternative manufacturing processes for highly-doped silica glasses and the laser fibers fabricated from it are required. For efficient laser operation a high absorption of pump power in the active fiber core is a necessary condition. To increase the pump light absorption the fiber development aimed at the preparation of laser-active and adapted passive single-large core fibers up to multi-core structures with 7 large cores showing broken circular fiber symmetry. The optimization of the optical fibers which will be shown in detail is based on the combination of several innovative manufacturing methods such as the powder sintering technology (REPUSIL), the preform preparation by stack-and-draw technique and the fiber drawing process. The described procedure is particularly suitable to produce multifilament glass preforms resp. laser fibers with large cores in which the radial and lateral indices of refraction can be adjusted homogeneously and reproducibly. Due to the realized increase of the laser-active core volume in these fibers the pump light absorption could be considerably increased and the resulting shorter fiber length allows the use of fibers with a moderate attenuation. The results concerning the characterization of materials science and the optical aspects e. g. the dopant concentration distributions and related refractive index profiles as well attenuation and pump absorption spectra will be presented.

We have demonstrated a linearly polarized cw all-in-fiber oscillator providing 1 kW of output power and a polarization extinction ratio (PER) of up to 21.7 dB. The design of the laser oscillator is simple and consists of an Ytterbium-doped polarization maintaining large mode area (PLMA) fiber and suitable fiber Bragg gratings (FBG) in matching PLMA fibers. The oscillator has nearly diffraction-limited beam quality (M² < 1.2). Pump power is delivered via a high power 6+1:1 pump coupler. The slope efficiency of the laser is 75 %. The electro/optical efficiency of the complete laser system is ~30 % and hence in the range of Rofin’s cw non-polarized fiber lasers. Choosing an adequate bending diameter for the Yb-doped PLMA fiber, one polarization mode as well as higher order modes are sufficiently supressed1. Resulting in a compact and robust linearly polarized high power single mode laser without external polarizing components. Linearly polarized lasers are well established for one dimensional cutting or welding applications. Using beam shaping optics radially polarized laser light can be generated to be independent from the angle of incident to the processing surface. Furthermore, high power linearly polarized laser light is fundamental for nonlinear frequency conversion of nonlinear materials.

High concentrations of the rare-earth elements erbium, holmium and thulium have been successfully doped into single crystal (SC) yttrium aluminum garnet (YAG, Y3Al5O12) fibers by use of the laser heated pedestal growth (LHPG) method. The spontaneous emission spectra and fluorescence were measured in the near-infrared (NIR). The results show progress towards forming a solid state laser able to produce a wavelength in the NIR, for high power applications.

The phenomenon of mode instabilities has quickly become the most limiting effect for a further scaling of the average power of fiber laser systems. Consequently it is of great importance to find solutions for this problem. In this work we propose two concrete possible passive mitigation strategies: the first one is based on the reduction of the heat load in the fiber, whereas the second one is based on the reduction of the pump absorption. In both cases a significant increase of the threshold is expected.

Theoretical aspects of microstructured fibers fabrication from preforms with sealed holes at the top end are discussed. Dependences of the holes blowing degree on their diameters, on a ratio of the preform top end temperature to temperature in the center of a furnace and on other parameters are estimated. Experimental results of different microstructured fibers drawing in such a regime are presented. We have drawn the simplest microstructured fiber with one hole from a tube with outside diameter 6.2 mm and inner diameter 4.4 mm (capillary drawing). Also we have drawn MSFs from preforms with 6 and 60 small holes. To check the results of the theoretical analysis we have prepared a preform with different size holes and then drew it into a fiber. In all cases experimental results are in good agreement with theoretical estimations. Thus this method of fabrication gives the possibility to manufacture long length microstructured fibers with stable internal structural parameters, high reproducibility, ease of controlling and changing the fibers parameters and opportunity to make structures with holes of different sizes. The observed changes of the holes blowing degree in our experimental samples can be easily compensated with the help of additional heating of the preform top end. Besides, such heating makes it possible to control and change the holes blowing degree, particularly during the fibers drawing process.

In this paper we present a dual-wavelength fiber mode-locked laser based on CVD-graphene saturable absorber (SA). The laser setup is based on two ring cavities connected by a common branch with a graphene SA. As a gain media erbium (Er) and thulium (Tm) doped active fiber were used. The laser generate optical pulses centered at 1558.5 nm and 1937 nm. The repetition rates and pulse durations were of 23.2 MHz, 16.42 MHz and 0.95 ps, 1.03 ps for the solitons generated in Er- and Tm-doped cavities, respectively.

A Yb-doped, dual-core, double-clad, polarization-maintaining fiber is used to demonstrate passive coherent beam combining. A homemade Dammann grating is employed as a passive beam-combining optical element. Self-phasing is observed in this laser system, where we attribute the self-phasing behavior to the Kramers-Kronig effect. We experimentally demonstrate the importance of polarization on coherent beam combining efficiency as well as on Kramers-Kronig induced self-phasing.

A random lasing based on Rayleigh scattering in a passive fiber directly pumped by a high-power laser diode (LD) has been demonstrated. Owing to the random distributed feedback (RDFB) the low-quality LD beam (938 nm) is converted into the high-quality laser output (980 nm). Because of the relatively low excess above the threshold with the available LD, the RDFB laser output is limited in power on 0.5 W level. In the used gradient-index (GRIN) fiber the output beam has 4.5 lower divergence as compared with the pump beam thus demonstrating a new approach for high-power fiber lasers with high-quality output.

We report on the development of a nanosecond green laser based on efficient frequency conversion of an Yb-doped fibre laser externally modulated by a semiconductor optical amplifier. The laser system uses commercially available fibers and operates with pulse durations in the range 3-40 ns and repetition rates in the range 50-800 kHz. A maximum conversion efficiency of 66 % and a maximum average output power of 16 W have been reached for a repetition rate of 250 kHz and a pulse duration of 8 ns while preserving a high beam quality.

High-power fiber lasers have reached kW power levels. The most important non-linear process limiting power scaling of industrial fiber lasers is stimulated Raman scattering. Long period gratings (LPGs) couple forward propagating core light to forward propagating cladding light and are well suited as a filter for the unwanted Raman scattering. In this paper we show for the first time of our knowledge the inscription of LPGs in large-mode-area (LMA) fibers with ultra-short laser pulses. We investigate the influence of different inscription parameters with a 3D, spatially resolved measurement of the induced index change. We present results from gratings with an attenuation of 8.5 dB at the desired wavelength with a small out-of-band loss of 1 dB.

The relative phase between two fiber lasers is controlled via a high performance optical phase-locked loop (OPLL). Two parameters are of particular importance for the design: the intrinsic phase noise of the laser (i.e. its linewidth) and a high-gain, low-noise electronic locking loop. In this work, one of the lowest phase noise fiber lasers commercially available was selected (i.e. NP Photonics Rock fiber laser module), with sub-kHz linewidth at 1550.12 nm. However, the fast tuning mechanism of such lasers is through stretching its cavity length with a piezoelectric transducer which has a few 10s kHz bandwidth. To further increase the locking loop bandwidth to several MHz, a second tuning mechanism is used by adding a Lithium Niobate phase modulator in the laser signal path. The OPLL is thus divided into two locking loops, a slow loop acting on the laser piezoelectric transducer and a fast loop acting on the phase modulator. The beat signal between the two phase-locked lasers yields a highly pure sine wave with an integrated phase error of 0.0012 rad. This is orders of magnitude lower than similar existing systems such as the Laser Synthesizer used for distribution of photonic local oscillator (LO) for the Atacama Large Millimeter Array radio telescope in Chile. Other applications for ultra-low noise OPLL include coherent power combining, Brillouin sensing, light detection and ranging (LIDAR), fiber optic gyroscopes, phased array antenna and beam steering, generation of LOs for next generation coherent communication systems, coherent analog optical links, terahertz generation and coherent spectroscopy.

We present results on the amplifier performance and characteristics of Yb-doped Single Mode fiber amplifiers spanning a broad range of wavelengths from 1028 nm to 1100 nm. Both PM and non-PM amplifiers are discussed, with emphasis on the use of polarization controllers in intrinsically non-PM amplifiers to obtain high Polarization Extinction Ratios (PER). In general, outside the 1064nm region, there has been relatively little discussion or work towards developing high power fiber amplifiers for operation at either 1030 nm or 1100 nm with narrow line-width and high brightness, primarily due to amplifier design and architecture issues related to strong re-absorption and amplified spontaneous emission. Here we address key fiber and amplifier design characteristics aimed at mitigating these issues while highlighting performance attributes and challenges for operation near either end of the above defined spectral range.

We propose the use of a narrowband amplified spontaneous emission (ASE) fiber source as seeder in a pulsed master oscillator power amplifier (MOPA) configuration for power and energy pulse extraction scaling. The narrowband ASE master oscillator is compared with the standard configuration using a Fabry-Perot laser diode. We show the performance limitations imposed by the use of the Fabry-Perot lasers, caused by the backward high peak power pulses triggered due to stimulated Brillouin scattering (SBS). Free SBS operation is shown when using the narrowband ASE seeder in the same system, allowing for significant increase on the extractable power and energy.

We propose and report a 193 nm narrow-linewidth light generation by a frequency mixing of Yb and Er-fiber lasers as the seed for an ArF excimer laser. The Yb-fiber laser includes a pulsed distributed feedback (DFB) or external cavity diode laser (ECDL), acousto-optic modulator(AOM), fiber amplifiers and an Yb:YAG single crystal fiber (SCF) power amplifier with more than 7 W output. The Er-fiber laser consists of a continuous-wave (CW) DFB laser, a semiconductor optical amplifier (SOA), and fiber amplifiers. The second harmonic generation (SHG) and the fourth harmonic generation (FHG) of Yb laser at 515 nm and 258 nm reach 5 W and 1.5 W, respectively. Two stages of sum-frequency generation (SFG) produce the power of 100 mW for 193 nm laser by use of CLBO crystals.

We have developed an all-glass, fusion spliceable polarization maintaining (6+1)× 1 pump/signal combiner for fiber lasers and amplifiers. We utilize an enhanced tapered fiber bundle technology for multimode pump channels and a vanishing core fiber for the single mode polarization maintaining large mode area (PLMA) signal channel. The signal channel of the combiner is optimized to match a double-clad PLMA fiber with 20 micron core and 400 micron glass cladding with 0.065 numerical aperture (NA). The multimode pump channels have 200 micron core and 240 micron cladding with NA of 0.22 designed to deliver high power 980 nm pump light. The same double-clad PLMA fiber is used as both the signal input channel and the combined output for the device. Polarization axes of the input and output PLMA fibers are aligned during the fusion splices to achieve polarization crosstalk below -20 dB. Utilizing this approach, we have achieved coupling loss of ~0.4 dB for the signal channel as measured from the input PLMA to the output PLMA at a wavelength of 1060 nm and coupling loss below 0.01 dB for all pump channels as determined from the measured temperature rise of the combiner package temperature as the optical pump power at 974 nm is increased up to 45 W. Low signal and pump losses result in high efficiency lasing or amplification at over a kW of pump power for high power applications where a single mode, high polarization extinction ratio output is required.

Transverse mode instability (TMI) in rare-earth doped fiber amplifiers operating above an average power threshold is caused by intermodal stimulated thermal Rayleigh scattering due to quantum defect heating. We investigate thermally induced longitudinal waveguide perturbations causing power transfer from the fundamental mode (FM) to the higher order mode (HOM) by a nonlinear gain, which depends on the FM-HOM frequency shift and position along the fiber. We take temperature and mode profile evolution along the fiber into consideration to engineer fiber designs with increased TMI threshold and operation stability at higher average powers.

Designs of Tm-doped photonic crystal fibers for laser operation must take in account the strong thermo-optical effects due to the Tm quantum defect and the consequent corruption of the single mode guiding properties. A new fiber design with a ∼ 80 μm core diameter, based on the cladding mirror symmetry reduction is proposed and analyzed using a full-vector FEM-based modal solver. The thermal effects are investigated using a computationally efficient model. A large pitch fiber with similar core diameter, which represents the actual state-of-art of Tm-doped laser technology, has been investigated in order to have a basis of comparison. Optimizing some key parameters of the new symmetry free fiber, the possibility to achieve a wide band single mode operation under an heavy heat load of over 300 W/m is demonstrated. In particular a very high modal discrimination value larger than 0.5 is obtained.

Intense nanosecond emission with spectral broadening from 980 to 1600 nm was generated with peak power up to 117 kW, close to the damage threshold of fiber fuse. Both laser amplification and nonlinear conversion were simultaneously employed in a fiber power amplifier giving power scaling free from significant depletion. In a diode-seeded all-PM-fiber master oscillation power amplifier system under all normal dispersion, a core-pumped preamplifier using double-pass scheme can significantly improve the energy extraction. This produced the pulse energy of 1.2 mJ and duration of 6 ns with a conversion efficiency of 66% at the moderate repetition of 20 kHz, which is consistent with the coupled laser rate equations including the stimulated Raman scattering. For the comparable nonlinear strength in each stage from single to few modes, the onset and interplay of four kinds of fiber nonlinearities can be addressed.

High power fiber laser amplifier cascade can be simplified using double-pass scheme due to improvement of overall efficiency, especially for amplifiers with small input seed or high stored energy. The yield of stimulated Raman scattering (SRS) in the double-pass scheme is comparable to the level in amplifiers using counter-directional-pumped single-pass scheme if the pumping configuration is appropriate, even though the interaction length becomes twice for double pass scheme. In the study, insertions of Raman strippers along the active fiber with double-pass scheme is proved to be another choice to effectively suppress SRS besides the utilization of photonic band-gap fibers.

A large number of power delivery applications for optical fibers require beams with very specific output intensity profiles; in particular applications that require a focused high intensity beam typically image the near field (NF) intensity distribution at the exit surface of an optical fiber. In this work we discuss optical fiber designs that shape the output beam profile to more closely correspond to what is required in many real world industrial applications. Specifically we present results demonstrating the ability to transform Gaussian beams to shapes required for industrial applications and how that relates to system parameters such as beam product parameter (BPP) values. We report on the how different waveguide structures perform in the NF and show results on how to achieve flat-top with circular outputs.

We present detailed analysis of the algorithm for adjustment of double resonance in short-length Brillouin ring fiber laser. Adapted laser cavity is simultaneously resonant for the pump and Stokes radiations. Demonstrated approach is equally useful for design of single mode fiber lasers with ultra-narrow optical spectra, Q-switched Brillouin fiber lasers as well as for the applications which required high power fiber resonators free from stimulated Brillouin scattering.

We present an experimental study on mode coupling characteristics of few-mode large-mode-area (LMA) fibers, which are widely used in high power fiber lasers. The modal power allocation is measured by modal decomposition of the nearfield intensity profile of the output beam. Cut-back measurements are carried out with commonly-used fibers with different fiber geometries. The evolution of the modal power content due to mode coupling is presented. The influence of the fiber geometry on mode coupling is discussed.

Frequency conversion through spontaneous degenerate four wave mixing (FWM) is investigated in large mode area hybrid photonic crystal fibers. Different FWM processes are observed, phasematching between fiber modes of orthogonal polarization, intermodal phasematching across bandgaps, and intramodal phasematching within the same transmission band as the one containing the pump laser. Furthermore first and second order Ra- man scattering is observed. The interplay between the different FWM processes and Raman scattering are investigated.

Ytterbium doped multicore fibers have been recently employed in the field of high power and Quasi-Gaussian beam lasers to design truly single-mode multicore fiber lasers. The special design of these fibers offers low bending loss even for compact high power lasers and amplifiers. Moreover, the Multi-core fiber amplifier possesses a large effective mode area which results in a significant decrease of the related nonlinear effects. In the paper, modal resolved group-velocity dispersion measurements in active multicore fibers are performed using time-frequency-domain white-light interferometry. A Mach-Zehnder-type interferometer with dual-channel detection in the spectral range from 0.4 μm up to 1.7 μm and a home-made supercontinuum source are used. Temporally resolved spectrograms recorded at distinct delay positions enable the detection of interference fringes for the equalizationwavelength. The group-velocity dispersion can be derived by applying a Sellmeier polynomial fit to the wavelength dependent differential group delay function. The dispersion parameters for several LMA fibers are investigated over a broad spectral range of about 1.3 μm.

In the present paper, optical absorption and emission spectra and luminescence decay lifetimes of different concentrations, 0.1, 0.3, 0.5, 0.7 and 1.0 mol% of Er³⁺ and 0.1Er³⁺/0.5Yb³⁺ co-doped tellurite glasses (TeO₂-Bi₂O₃-ZnONb₂O₅) were reported. Judd-Ofelt intensity parameters were determined and used to calculate spontaneous radiative transition probabilities (A_rad), radiative lifetimes (τ_R), branching ratios (β) and stimulated emission cross-sections (σ_P) for certain emission transitions. NIR emission at 1.5μm and up-conversion spectra of Er³⁺ and Er³⁺/Yb³⁺ co-doped tellurite glasses were measured under excitation wavelength of 980 nm. The absorption, emission and gain cross-sections for ⁴I_13/2→⁴I_15/2 transition of Er³⁺ are determined. The peak emission cross-section of this transition is found to be higher (9.95×10^-21 cm²) for 0.1 mol% of Er³⁺ and lower (6.81×10^-21 cm²) for 1.0 mol% of Er³⁺ doped tellurite glasses, which is comparable to other oxide glasses. The larger peak emission cross-section for lower concentration of Er³⁺ is due to the high refractive index of glass matrix (2.1547), relation established from Judd-Ofelt theory. The observed full-widths at half maxima (FWHM) for lower and higher concentrations of Er³⁺ are 64nm and 96 nm respectively. The larger values of FWHM and peak emission cross-sections are potentially useful for optical amplification processes in the design of Erbium doped fiber amplifiers (EDFs). Under 980 nm excitation three strong up-conversion bands were observed at 530nm, 546nm and 665nm. The pump power dependent intensities and mechanisms involved in the up-conversion process have been studied. The luminescence decay profiles for ⁴I_13/2 level were reported for all glass matrices.

We report on the performance of a prototype pump combiner for use with thulium-doped photonic crystal fiber (PCF). This platform is attractive for “all-fiber” high energy and high peak power laser sources at 2 μm. We will report on the performance of this integrated amplifier in comparison to free space amplification in Tm:PCF. In particular, we carefully look for spectral/temporal modulation resulting from multimode interference between fundamental and higher order transverse modes in the amplifier to evaluate this for ultrashort chirped pulse amplification. The slope efficiency for the all-fiber amplifier is 22.1 %, indicating the need for further improvement. However, an M² < 1.07 demonstrates excellent beam quality, as well as amplified polarization extinction ratios of ~25 dB.

The capability of measuring the spectral and temporal phase of an optical signal is of fundamental importance for the advanced characterization of photonic and optoelectronic components, biochemical sensors, structural monitoring sensors and distributed sensor networks. To address this problem, several techniques have been developed (frequency-resolved optical gating (FROG), spectral phase interferometry for direct electric-field reconstruction (SPIDER), stepped-heterodyne technique, laser Doppler vibrometry (LDV) and Doppler optical coherence tomography (OCT)). However, such techniques often lack of versatility for the mentioned applications. Swept-wavelength interferometric techniques and, among these, optical frequency-domain reflectometry (OFDR) are flexible and highly sensitive tools for complete characterization of amplitude and phase of target devices. In this work, we investigate the spectral and temporal phase measurement capabilities of OFDR. Precise characterization of spectral phase information is demonstrated by retrieving the phase response of a commercial optical filter, the Finisar Waveshaper 1000 S/X, programmable in attenuation and phase over C+L band (1530– 1625 nm). The presented results show accurate retrieval of group delay dispersion (GDD) and discrete phase shift as well as filter attenuation profile. Although some intrinsic accuracy limitations of OFDR phase measurements may be encountered (and herein specified), we show that information encoded in OFDR reflectogram data is very rich when adequately exploited. In addition to previously published results, we demonstrate the high sensitivity of the technique to Doppler effects. From practical point of view, such sensitivity can be beneficially exploited for the characterisation of dynamical aspects of samples under test. Unlike LDV, OFDR allows the simultaneous retrieval of the temporal position of several localised reflecting target along the beam path. All these aspects make OFDR a highly promising candidate for the study of both static and dynamic aspects of complex photonic components or to probe a parallel sensor network, as needed for future applications.

Fundamental performance of the swept-source optical coherence tomography (SS-OCT) system is defined by its wavelength-swept laser. Especially narrower instantaneous spectral linewidth of the laser has the advantage in deeprange tomography. We have demonstrated narrow-linewidth actively mode-locked ring lasers (AMLL), employing anomalous dispersion configuration. The linewidth of an AMLL is determined by anomalous dispersion and self-phase modulation (SPM) in the semiconductor optical amplifier (SOA). For such soliton-like phenomenon of AMLLs, numerical calculation predicts that both of large dispersion and small SPM make the linewidth narrower. Since the dispersion restricts wavelength sweeping range of AMLLs, too large dispersion cannot be used. To weaken the SPM effect, low linewidth enhancement factor α of SOA is desirable. Quantum-dot(QD)-based SOA offers low α-factor in comparison with quantum-well SOA (QWSOA). In this study, we employ a QDSOA as a gain medium in an AMLL and also use a QWSOA for comparison. The wavelength band of the QWSOA-AMLL is 1.5 μm and that of QDSOA-AMLL is 1.0 μm. Since we employed the 10 ps/nm of net dispersion in both configurations, the dispersion parameter β2 for the QDSOA-AMLL is approximately half of that for the QWSOA-AMLL. The measured full-width half-maximum (FWHM) linewidths in a static state were 0.08 nm for the QWSOA-AMLL and 0.04nm for the QDSOA-AMLL. In spite of the small β2 the QDSOA-AMLL achieves narrower spectral than the QWSOA-AMLL. We also confirmed that the interference signal was improved by adopting the QDSOA.

Nonlinear phenomena in microstructured fibers (MSFs) is defined by dispersive properties of a fiber. Zero dispersion wavelength (ZDW) and pump source wavelength play an important role in estimating the nonlinear effects and thus are subject of wide investigations. Multiple nonlinear processes like: four wave mixing (FWM), cross phase modulation (XPM), cannot be very efficient without phase matching which is achieved when a fiber is pumped in anomalous dispersion region. On the other hand, other nonlinear processes, such as self-phase modulation (SPM) and Raman scattering (RS), profit from pumping fiber in normal dispersion region. Thus the efficiency of supercontinuum (SC) generation in a fiber is dependent on its chromatic dispersion properties, which can be tailored by the proper fiber geometry design, and by the pump source wavelength. In our paper we present experimental analysis of SC generation obtained for a series of nonlinear MSFs. Our fibers have different ZDW and therefore when pumped by the same pump source, different nonlinear effects contribute to the SC generation. We analyze and explain the influence of ZDW on nonlinear effects. Comparisons of nonlinear interactions for fibers pumped in anomalous and normal dispersion regimes are provided. In our silica MSFs an ultra-short UV radiation was obtained by nonlinear processes estimation. We provide experimental analysis of MSFs geometrical parameters influence on UV conversion efficiency. Our studies present effective SC generation in near infrared, visible and UV ranges. Unique information about the influence of MSFs geometry on UV generation efficiency gives possibility to increase its application potential.

Fundamental properties of pure silica microstructured fibres (MSFs) can be determined by their geometrical crosssection design. Investigation of nonlinear effects was widely evaluated in diverse types of MSFs with exactly defined dispersive properties. Proper design of Zero Dispersion Wavelength (ZDW) strongly influences generation of nonlinear processes resulting especially in supercontinuum generation (SC). ZDW shift to short wavelengths together with high nonlinearity (small effective mode area) can be obtained by dramatic decrease of microstructured fibre pitch and increase of air-filling ratio. Fibre geometry must be properly scaled preserving technological tolerances to obtain precisely defined position of ZDW near visible range. Additionally, higher air-filling ratio results in multimode guiding regime. Therefore, in the paper we present studies of novel type of fibre geometry with ZDW near visible range together with endlessly single mode propagation regime. Chromatic dispersion measurements and ZDW analysis are performed with use of interferometric method. Presented MSFs series (the same structure type, but different ZDW position) is manufactured by stack and draw method. Proposed MSFs geometry enables fabrication of desired chromatic dispersion characteristic while respecting all technological tolerances, which is very difficult in case of manufacturing typical photonic crystal fibres for supercontinuum generated with 780 nm wavelength pulses from titanium-sapphire laser. Additionally, proposed endlessly single mode operation provides high quality white light output beam, simultaneously with stable and flat SC source. Paper also reports on the SC generation with pumping in the anomalous and normal side of chromatic dispersion with femtoseconds and picoseconds pulses.

Several papers were reported on spectroscopic properties of rare earth doped different host glasses. A complete knowledge of fluorescence properties of rare earth ions in laser materials is necessary to achieve efficient, compact and cheap sources of laser radiation for NIR and mid-IR region. Tellurite glasses are potentially useful for generation of NIR and mid-IR laser radiation due to its special features such as lowest phonon energy (750 cm^-1) among oxide glasses, reasonably wide transmission region (0.35 - 5μm), good glass stability, good rare earth ion solubility, high linear and non-linear refractive index. In the present work, authors prepared Ho³⁺ and Tm³⁺ singly doped and Ho³⁺/Tm³⁺ co-doped tellurite glasses using conventional melt-quenching method. Spectroscopic measurements and analysis of energy transfer process in Ho³⁺, Tm³⁺ and Ho³⁺ /Tm³⁺ co-doped glasses pumped with 785nm and 451 nm excitation wavelengths have been performed. There are some spectroscopic properties which are important in understanding and modeling of rare earth doped laser materials. Using Judd-Ofelt theory, radiative transition rates (A_rad), radiative lifetimes (τ_R) and branching ratios (β) were estimated for certain excited states of Ho³⁺ and Tm³⁺ doped tellurite glasses. The emission cross-sections and gain coefficients have been determined from the absorption spectra of Ho³⁺ and Tm³⁺ ions in tellurite glasses. The energy transfer process such as ion cross-relaxation, Tm³⁺-Ho³⁺ energy transfer and energy transfer upconversion were studied and identified to specific candidate for laser operation.

The dependence of the filtering bandwidth on the mode locking technique was studied in details in two types of mode locked fiber laser cavities; one utilizes SESAM, while the other employs NPE. The results show that for the two cases, below certain value of filtering bandwidth, no mode locking can be observed and the cavities will be in a lossy region. Moreover, NPE-based all fiber cavities can support narrower spectral bandwidth compared to SESAM-based cavities. Hence, NPE-based cavities produce shorter pulse width than SESAM-based ones. Experimental investigation was carried out to verify our simulation results and good agreement was achieved.

A novel Automated Image Magnification system has been developed for a CO2 laser glass processing system (LZM-100 LazerMaster) enabling precise and accurate alignment of optical fiber and fiber based components. This system measures the size of the fiber image and adjusts the imaging optical path based on the fiber size and process requirement. By automatically adjusting the magnification, we can process fibers and components requiring a large field of view (<2500um) and those with micron or submicron features without having to modify the machine configuration.

In recent years the use of supercontinuum light sources has encouraged the development of various optical measurement techniques, like microscopy and optical coherence-tomography. Some disadvantages of common supercontinuum solutions, in particular the rather poor stability and the absence of modulation abilities limit the application potential of this technique. We present a directly controllable all-fiber laser source with appropriate parameters in order to generate a broad supercontinuum spectrum with the aid of microstructured fibers. Through the application of a laser seed-diode, which is driven by a custom built controller to generate nanosecond pulses with repetition rates in the MHz range in a reproducible manner, a direct control of the laser system is enabled. The seedsignal is amplified to the appropriate power level in a 2-step amplification stage. Wide supercontinuum is finally generated by launching the amplified laser pulses into different microstructured fibers. The system has been optimized in terms of stability, power-output, spectral width and beam-quality by employing different laser pulse parameters and several different microstructured fibers. Finally, the system as a whole has been characterized in reference to common solid state-laser-based supercontinuum light sources

When scaling CW single-mode fiber amplifiers to high power, the first nonlinear limitation that appears for narrowlinewidth seed lasers is stimulated Brillouin scattering (SBS). We present a dynamical simulation of Brillouin scattering in a Yb-doped fiber amplifier that numerically solves the differential equations in z and t describing the laser, Stokes and pump waves, the inversion, and the density fluctuations that seed the scattering process. We compare the model to experimental data, and show that a linearly chirped seed laser is an efficient form of SBS suppression; especially for long delivery fibers. The frequency chirp decreases the interaction length by chirping through the Brillouin resonance in a time that is short compared to the fiber transit time. The seed has a highly linear chirp of 10¹⁴ – 10¹⁶ Hz/s at 1064 nm which preserves a well-defined phase relationship in time. This method of SBS suppression retains a long effective coherence length for purposes of coherent combining, while at high chirps appears to the SBS as a large linewidth, increasing the threshold. An increase in fiber length increases the laser bandwidth as seen by the SBS, leading to a fiberlength- independent SBS threshold.