This PDF file contains the front matter associated with SPIE Proceedings Volume 10897, including the Title Page, Copyright information, Table of Contents, and Author and Conference Committee lists.

We analyze the average power scaling capabilities of ultrafast thulium-doped fiber lasers in the context of available pump brightness and transverse-mode instabilities. It has been theoretically predicted that thulium-doped fiber lasers emitting at around a wavelength of 2 μm can withstand much higher thermal loads before the onset of mode instability is observed. In this work we intend to verify this theoretical hypothesis experimentally. To do so, we use state of the art laser diodes to reach a combined pump power of more than 2 kW at 793 nm. Careful thermal management of the fiber amplifier and sufficient seed power are beneficial for exploiting cross-relaxation, which allows for high optical to optical efficiencies greater than 50%, which we demonstrate in a 50/250 thulium-doped photonic crystal fiber. An average power of 1060 W after pulse compression to 265 fs duration with stable diffraction-limited beam quality is demonstrated. We have achieved a pump-limited average output power of 1.15 kW prior to pulse compression without the onset of thermal mode instability. After a longer period of operation at this power level, a degradation of the amplifier performance occurred and thermal mode instability has been observed for average powers in excess of 847 W. This is the first time, to the best of our knowledge, that thermal mode instability has been observed in a thulium-doped fiber laser. According to simulations of the amplification process, the average heat load at the threshold of thermal mode instability is at least 95 W/m, which is about 3 times higher than the typical value for ytterbium-doped fiber laser.

In this work we have investigated the impact of pump-power noise on transverse mode instabilities (TMI) in high-power fiber laser systems. This is a crucial study since former works have shown that pump-power variations can induce a phase shift between the modal interference pattern and the thermally-induced refractive index grating and, thus, they are the most likely trigger for TMI. To experimentally investigate this behavior, we have generated white noise for different frequency bands with an arbitrary waveform generator and applied it to the pump diode. In a first experiment we have evaluated the frequency range of interest. It was found that only frequency components of the pump noise close to the main frequency of the TMI fluctuations influence the TMI threshold of the fiber laser system. In a second experiment we have measured the TMI threshold of the free-running system and compared it to the ones obtained when applying different pump-noise amplitudes. It was found that the TMI threshold can be decreased by almost a factor of three by increasing the noise of the pump source. This result is in good agreement with former theoretical and experimental studies and suggests that pump-power noise indeed acts as a main trigger for TMI. Furthermore, the findings indicate that the development of pump sources and drivers with a low noise level in a frequency range close to the main frequency of the TMI fluctuations could help to increase the TMI threshold of fiber laser systems.

In this work, we study the generation and evolution of phase-shifts between the modal interference pattern and the thermally-induced index grating due to pump-power changes. This study is not only important to understand new mitigation strategies based on controlling such phase-shifts, but also to comprehend how pump/signal noise can trigger TMI. Understanding how such a phase-shift can develop from a pump/signal change is not trivial, since the movement of both the MIP and the RIG are thermally driven and, therefore, should have similar time constants. Our simulations show unequivocally that a change of the pump power will lead to the generation of a phase-shift and the physical reason for this behavior is unveiled. The main reason is an increased sensitivity of the MIP to temperature variations because the local beat-length changes of the MIP are accumulated along the whole fiber length. Therefore, the further downstream the fiber a MIP maximum is (i.e. closer to the pump end in a counter-pumped configuration), the stronger and faster its position shift will be. This insight shows a way to obtain more TMI-resilient fiber designs and may help understanding the core area dependence of TMI.

Recently, significant work has been conducted to reach high energy or peak power in fiber lasers. Microstructured fibers with large mode areas were developed to address this concern [1,2] and have allowed to access to the best state-of-the-art performances in terms of pulse energy, average power and peak power [3,4]. Although these fibers were designed for power scaling while keeping a single transverse mode propagation, the onset of transverse modal instabilities (TMI) degrades significantly the beam quality owing to the re-confinement of one or more higher order modes (HOMs) in the gain area. That effect suddenly appears when a certain average power threshold is exceeded. To push further the TMI power threshold, an original aperiodic pattern made of solid low-index inclusions embedded into the optical cladding was proposed to enhance the HOMs delocalization out of the gain region and thus ensure an effective single-mode emission. Such fibers are called Fully-Aperiodic Large-Pitch Fibers (FA-LPF). In this work, we realize for the first time a burn-in experiment with a 84 µm core Yb-doped FA-LPF in amplification regime. Using a 400 W pump diode at 976 nm and two different seeders, the power scaling as well as the spatial beam quality and its temporal behavior [5] were investigated in amplifier configuration in two different temporal regimes (nanosecond and picosecond pulses). After 800 hours, the maximum extracted average signal power decreases from 139W to 128W in picosecond regime and no TMI have been observed. Explanations on the power decrease will be given during the conference.

Multi-core fiber capability to deliver several independent beams in a single structure has been deeply investigated to obtain spatial multiplexing in optical communication. Recently, the coherent beam multiplexing idea has been extended to high power fiber laser field, where multi-core fiber amplifiers, combining low power beams, promise to overcome thermal mode instability, which characterizes single-core fiber amplifiers. Although coherent output beam combination is advantaged in multi-core fiber, the understanding of core phase shifts is necessary to implement efficient beam combination. In presence of thermal load, induced by pump-to-signal conversion quantum defect, a refractive index gradient is formed on the multi-core fiber amplifier cross-section, thus changing core propagation properties and possibly creating unwanted core couplings. In this work a 9-core double-cladding fiber amplifier is numerically investigated by varying the core thermal load, from 2 to 15 W/m, in order to understand the structure propagation mismatch. The 9 cores are organized in a 3×3 regular grid, each core has a diameter of 19 μm and a spacing of 55 μm. Cores numerical aperture is 0.06. The outer cladding has a diameter of 340 μm. A comparison between a rod-type fiber amplifier configuration and a flexible fiber amplifier has been performed. Results show that the cores can be divided in three groups according to their propagation properties: central core, side cores, and corner ones. The phase shift between these groups, or equivalently the effective index difference, becomes higher with the increase of thermal load. These observations are fundamental to implement a model for beam propagation in presence of thermal effect, to investigate the amplification dynamics along z-direction.

We present a kW level pulsed laser based on a master oscillator power amplifier (MOPA) configuration. The directly modulated single frequency laser at 1952 nm was pulsed in the nanosecond regime with a repetition rate frequency from 10 kHz to 2 MHz. The MOPA topology was based on a two stage amplifier using single clad Thulium-doped fiber: it consisted of a pre-amplifier stage followed by a booster stage. We investigated the performance of this pulsed laser for two different TDFs with different saturating energies in the booster stage. The direct modulation allowed us to demonstrate more than 1 kW of output peak power over pulse repetition frequencies from 10 kHz to 500 kHz. For a pulse duration of 21 ns, we measured output energy of 13 μJ and 29 μJ for booster fiber saturating energies of 15 μJ and 30 μJ, respectively.

In this paper, a single frequency thulium doped silica fiber laser operating above 2 μm based on cascaded single-modemultimode- single-mode (SMS) fiber structures incorporating a Sagnac loop was experimentally demonstrated. Based on the theoretical analysis of the trans mission properties of the SMS fiber structure and mode selection characteristic of Sagnac loop, two cascaded SMS fiber devices with same multimode fiber lengths around 3.25 cm were used in our laser system to select the specific lasing wavelength above 2 μm as well as enhance the suppression of lasing around 1900 nm. And 6.5 m unpumped TDF was employed in Sagnac loop as saturable absorber to achieve single-frequency operation. Single-longitudinal-mode (SLM) operation of the fiber laser operating at 2004.05 nm was achieved with a signal to noise ratio (SNR) more than 60dB.

We present a non-destructive optical technique for a full characterization of rare-earth-doped optical preforms. Specifically, a combined profiling of refractive-index and active-dopant was carried out in the same region of optical preforms by performing ray deflection and photoluminescence projection measurements, respectively. In this way, the optical core, delineated by the refractive index profile, was used to define the actual active dopant distributions within the core. Using the same experimental set-up, it was also possible to uniquely provide the maximum value of active-ion concentration reached inside the core. The study was carried out on an optical fiber preform sample doped with Yb³⁺ ions in high concentration. Comparison with independent measurements, such as those performed by a commercial refractive index profiler and microstructural secondary-ion mass spectrometry analysis, confirms the accuracy of the proposed technique in the optical features evaluation.

We present and compare the performance of bidirectionally pumped Yb-doped monolithic amplifier and oscillator setups in 20/400 μm geometry tested up to signal powers of 3.5 kW and 5 kW without the occurrence of transverse mode instabilities and maintaining a single mode beam quality of M² ~ 1.3. The scaling was primarily limited by the nonlinear effect of Stimulated Raman Scattering. This contribution contains detailed analysis of the temporal and spectral behavior of both configurations. The results show the excellent feasibility of monolithic oscillators and FBG for high power operation, even outperforming the amplifier pendant in terms of output power.

We present highly efficient three-channel dual-grating spectral beam combining with a combined signal output power of 5.5 kW at an excellent beam quality of M² = 1.5. Three 2-kW all-fiber narrow-linewidth continuous-wave Ytterbium-doped fiber amplifiers at 1050 nm, 1070 nm and 1090 nm were combined using in-house fabricated polarizationindependent dielectric reflection gratings. The total combining efficiency was 94% at full power level, which is close to the expected value referred to the incorporated grating’s efficiency in a dual-grating setup.

In this paper, we present our current work towards a highly efficient XLMA (extra-large mode area) fiber-based laser, which is being performed in the EKOLAS consortium within the BMBF-funded EffiLAS (efficient high-performance laser beam sources) research alliance. To this end, the complete manufacturing process chain of the XLMA fiber was reviewed and optimized. The work started with the material composition of the active XLMA preform with the goal of improving the purity and thus the background loss. A successfully implemented fluorine co-doping process allows refractive index adjustment of the active core material which improves the beam quality of the laser fibers without changing the concentration of active ions in the glass composition. The preform is subjected to a screening in which possible scatter centers, e.g. bubbles, inclusions or contaminants, are mapped and categorized, in order to identify defects, which could lead to a failure in the drawn fiber, already at an early production stage. The subsequent fiber drawing is monitored for scattering using the emissions from the heated preform as well as for inhomogeneities of the dopants using a phase measurement technique. Finally, the fiber is tested for residual impurities and background losses using a multi-mode OTDR to ensure that the fibers are free of any defects.

VAD technology has been proposed for laser fiber fabrication to improve its productivity and reproducibility. Rare-earth ion, Yb³⁺ has been incorporated with Ce³⁺ and Al³⁺ by the conventional solution doping process. In order to obtain a transparent Yb-doped core rod, the pre-sintering and consolidation process for the large volume of silica soot deposited by VAD has been significantly investigated. In addition, 20/400μm Yb-doped fiber with octagonal cladding has been fabricated. The cladding absorption was 0.44dB/m at 915nm, which corresponds to ~ 0.2mol% of Yb concentration and the slope efficiency with respect to absorbed pump power at 976nm was 66.7%.

Accessing the molecular fingerprint region between 2 and 20 μm is a key aspect in modern metrology and spectroscopy. While the wavelength range from 2-5 μm can easily be addressed through nonlinear frequency conversion starting from well-matured 1 μm driving lasers, access to the deep mid-IR wavelength regime is difficult. This is, because of the limited transmission of non-oxide crystals (that offer high nonlinearity and good transmission for the aspired mid-IR idler) at the pump wavelength and/or multi-photon absorption. Shifting to a longer pump wavelength relieves these limitations. In this work we present an experiment based on intra pulse difference frequency generation (IPDFG) in GaSe driven by an ultrafast Tm-doped chirped pulse amplifier. This experiment led to an octave spanning mid-IR spectrum, covering the wavelength range between 7.2-16.5 μm (-10 dB width) with 450 mW of average power at 1.25 MHz repetition rate. This result outperforms comparable sources driven at 1 μm wavelength in average power and conversion efficiency, while providing much broader spectral coverage. To further facilitate the use of these promising sources in real-world spectroscopic applications, we have built a nonlinear amplifier, which, based on its compact and robust design is an ideal candidate in this respect. Optimizing the output ultimately led to high pulse quality 50 fs pulses with 250 nJ of pulse energy at 80 MHz of repetition rate and 20 W average output power, exceeding current designs in the anomalous dispersion regime by 1 order of magnitude. It is our ongoing effort to utilize this laser for parametric downconversion. Covering the wavelength regime beyond 5 μm wavelength would make it an enabling technology for next generation spectroscopy, fundamental and life sciences.

Frequency combs are an enabling technology for metrology and spectroscopic applications in fundamental and life sciences. While frequency combs in the 1 μm regime, produced from Yb-based systems have already exceeded the 100 W – level, high power coverage of the interesting mid-infrared wavelength range remains yet to be demonstrated. Tm- and Ho-doped laser systems have recently shown operation at high average power levels in the 2 μm wavelength regime. However, frequency combs in this wavelength range have not exceeded the 5 W-average power level. In this work, we present a high power frequency comb, delivered by a Tm-doped chirped-pulse amplifier with subsequent nonlinear pulse compression. With an integrated phase noise of <320 mrad, low relative intensity noise of <0.5% and an average power of 60 W at 100 MHz repetition rate (and <30 fs FWHM pulse duration), this system demonstrates high stability and broad spectral coverage at an unrivalled average power level in this wavelength regime. Therefore, this laser will enable metrology and spectroscopy with unprecedented sensitivity and acquisition time. It is our ongoing effort to extend the spectral coverage of this system through the utilization of parametric frequency conversion into the mid-IR, thus ultimately enabling high power fingerprint spectroscopy in the entire molecular fingerprint region (2 – 20 μm).

A bidirectional 793 nm diode-pumped actively Q-switched Tm³⁺, Ho³⁺-codoped silica polarization maintaining (PM) double-clad (DC) fiber laser is reported. An average output power of 55 W with 100 ns pulse width, 200 kHz repetition rate and 2.09 μm wavelength is obtained with this fiber laser. To the best of our knowledge, this is the highest ever demonstrated average output power emitted from a Q-switched single oscillator 2 μm fiber laser. Thanks to end-caps fusion-spliced on both fiber tips, this fiber laser source is presenting good beam quality factors (M² < 1.7) and no thermal-induced damaging is observed at 55 W average output power. Further power scaling of the laser is only pumppower- limited in the range of the total available pump power in this setup (180 W). Direct OPO ZnGeP₂ (ZGP) pumping with this fiber laser has been performed for optical nonlinear conversion in the mid-IR regime. More than 8 W of average output power has been generated in the 3-5 μm band.

Current developments in LIDAR and atmospheric sensing experiments highlight the need for multi-watt, large bandwidth, high dynamic range polarization-maintaining optical amplifiers in the eye safe 1.9—2.15 μm band. So far, as an illustration of the previous state of the art for high power devices, multi-watt Tm-doped fiber amplifiers (TDFAs) have been demonstrated by Goodno et al. with an output power of 608 W at a signal wavelength of 2040 nm. As for Ho-doped fiber amplifiers (HDFAs) Hemming et al. have reported output powers of 265 W at 2110 nm. For this HDFA, a double clad Ho-doped fiber pumped by high power fiber lasers made the configuration complex and yielded an optical slope efficiency of 41%. Both of these achievements were with standard (non-polarization-maintaining) fiber.

Thulium fiber lasers emit light of wavelengths spanning as low as 1650nm to 2200nm. This broad emission band is in the “eye-safety” wavelength regime and intersects with the IR atmospheric transmission window with its opacity subsiding past 1900nm wavelength. Consequently, a high power, single frequency, tunable thulium fiber laser with its tuning range from 1900nm to 2000nm has the unique capability of studying high power beam propagation through the atmosphere in regions of both weak and strong transmission. Moreover, such lasers can be made to tune across individual molecular absorption lines due to chemical species present in the atmosphere. This enables a detailed investigation on how individual molecular absorption lines affect the transmission of high power laser beams. In this paper, a 100kHz linewidth, near diffraction limited, 100W class, widely tunable CW thulium fiber laser system is described for atmospheric propagation studies. The fiber laser is of master oscillator power amplifier(MOPA) architecture with one pre-amp and a final power amplifier. The master oscillator is a 5mW class tunable external cavity diode and is tunable from 1900nm to 2000nm. The pre-amp amplifies the seed to 2-3W level, which is then further amplified to 100W by the final amplifier made from thulium doped 25um core 250um cladding 0.09NA fiber from Nufern. All fiber architecture allows efficient lasing at the lossy molecular absorption wavelengths.

High power thulium-doped fibers rely on 793 nm pumping and cross-relaxation. While this approach has been historically successful, low optical-to-optical efficiencies and high thermal loads impede multi-kW power scaling. Another option is to in-band pump the final amplifier. In-band pumped thulium-doped fibers enable >80% efficiencies and low thermal loads. Design concepts and simulations for scaling thulium-doped fibers >1 kW with in-band pumping are discussed. Developing the high power 1.9 µm pump units, the incoherent fiber combiner, and the specially designed final amplifier are detailed. Requirements on the seed source (wavelength, power, etc.) are also described.

Cascaded Raman fiber lasers is the only proven technology which enables high power continuous wave (CW) fiber laser sources outside rare-earth emission bandwidths. Among these systems, recently demonstrated cascaded Raman fiber lasers based on Random distributed feedback (RDFB) provide wavelength agility enabling high-power, ultra-broad band wavelength tunable fiber lasers. However, these systems are limited in terms of power scaling due to degrading spectral purity with increasing input pump power which in turn limits the applicability of these systems. This is due to conversion of desired wavelength into next higher order Raman stokes with increase in pump power. In this work, we demonstrate a high power, ultra-high spectral purity, broadly wavelength tunable cascaded Raman fiber laser. This was enabled by culmination of two significant advances over the last year. To terminate the Raman cascade at the required output wavelength, we utilized our recently proposed distributed filtered feedback mechanism. To achieve high spectral purity, we used a recently demonstrated technique of using high power fiber based Amplified Spontaneous emission (ASE) sources as the input pump. A maximum output power of ~33W at 1.5μm and ~27W at 1.4μm was achieved. High spectral purity of <97% at the final wavelength was achieved over a wide-range of output powers.

There are a number of spectral features around 1700 nm which are very attractive to the laser community. Strong C-H bond absorption and moderate water absorption lend this region to many applications such as polymer processing and laser surgery. Despite being a growing area of interest, development of high-power, laser sources in this region is quite challenging. Here we present preliminary results of in-band pumping a thulium-doped fiber laser (TDFL) with erbium-only doped fiber laser (EDFL) to generate 1700 nm Erbium-only was chosen to avoid the power scaling limitations of co-doping with ytterbium. Parasitic lasing on the ytterbium band at 1030 nm results in efficiency roll-off and self-pulsing. Because of this, single-mode Er/Yb laser systems are generally limited to moderate powers of ~20W and reliability remains an issue. An in-house double-clad large mode-area fiber, with a fundamental mode-field diameter of 20 um, was cladding pumped at 975 nm. Maximum power at 90 W launched was 31 W and the slope efficiency was 44.2% (35.4%) with respect to absorbed (launched) pump power. Suitability for high-brightness core pumping of a TDFL was confirmed with an M2 measurement of 1.1 ± 0.1. For 1700 nm generation, the 1580 nm pump light was free-space coupled into an in-house TDF (0.2 wt.% dopant concentration). For a maximum launched power of 23 W, 15 W of 1726 nm output was generated with a slope efficiency of 67.1% (64.7%) with respect to absorbed (launched). To the author’s best knowledge, this is the highest recorded power in this wavelength region from a TDFL.

High power and high efficiency random fiber laser working at 1.5μm with mixed Erbium-Raman gain was theoretically proposed. The numerical model based on rating equations was established to analysis the laser performance of Erbium-Raman gain random fiber laser by different pump schemes. The optical-optical conversion efficiency reached 90% when pump power is 100W by forward pump scheme in the 1.5μm regime. Our work supplied an effective design for high efficiency and high power random fiber laser at 1.5μm which can be used for optical fiber sensing, research and communication. In our simulations results we found that forward pump scheme is better than backward pump scheme and bidirectional pump scheme in high power 1.5μm random laser, besides forward pump scheme is better than the other two pump schemes because of its compact, high efficiency and low costs, we also found that long Er-doped fiber (EDF) fiber length had good effect on the laser efficiency, the fiber length of single mode fiber (SMF) from 200m to 1km had little effect on laser efficiency. This design of 1.5μm random fiber laser can be a useful theoretical guidance for experiment and diverse wavelength could be achieved by the seed of 1.5μm random fiber laser.

This paper describes a coherent Doppler lidar developed by NASA to address a need for a high-performance, compact, and cost-effective velocity and altitude sensor onboard its landing vehicles. Future robotic and manned missions to solar system bodies require precise ground-relative velocity vector and altitude data to execute complex descent maneuvers and soft landing at a pre-designated site. This lidar sensor, referred to as Navigation Doppler Lidar (NDL), transmits three laser beams at different pointing angles toward the ground and measures range and velocity along each beam using a frequency modulated continuous wave (FMCW) technique. The three line-of-sight measurements are then combined in order to determine the three components of the vehicle velocity vector and its altitude relative to the ground with about 2 cm/sec and 2 meters precision, respectively, dominated by the vehicle motion. The NDL can also benefit terrestrial aerial vehicles that cannot rely on GPS for position and velocity data. The NDL offers a viable option for enabling aircraft operation in areas where the GPS signal can be blocked or jammed by intentional or unintentional interference. A modified version of the NDL incorporating a beam steering device can produce 3-dimensional range and Doppler images that are critical for safe andefficient operation of autonomous ground vehicles. This paper describes the design of the NDL and its capabilities as demonstrated through extensive ground tests and flight tests onboard helicopters and autonomous rocket-powered vehicles. Then, the utilization of the NDL technologies for terrestrial vehicles will be discussed.

Mode-locked fiber lasers are currently limited to sub-3-μm wavelengths, despite application-driven demand for longer wavelength mid-IR pulse sources. Erbium- and holmium-doped fluoride fiber lasers are emerging for 2.7-2.9 μm emission, yet further extending this coverage is challenging. Here, we propose a new approach using dysprosium-doped fiber with frequency shifted feedback (FSF), achieving 33 ps pulses with up to 2.7 nJ energy, tunable from 2.97 to 3.30 μm. Notably, this is the longest wavelength mode-locked fiber laser and the most broadly tunable pulsed fiber source to date. Simulations are also performed, offering insights into the dynamics of FSF pulse generation.

All-fiber integrated super-continuum (SC) sources are described based on a platform architecture that can operate in the visible, near-infrared, short-wave infrared, mid-wave infrared and long-wave infrared, with demonstrated SC wavelengths ranging from 0.47 to 12 μm. Modulation instability initiated SC generation leads to a simple SC source with no moving parts and that uses o_-the-shelf components from the mature telecommunications and fiber optics industry. The resulting light sources are basically a cascade of fibers pumped by fiber-pigtailed laser diodes and some drive and control electronics; thus, the SC sources have the potential to be cost-effective, compact, robust and reliable. Starting from fused silica fibers, the SC spectrum can be extended to shorter or longer wavelengths by cascading fibers with appropriate dispersion and/or transparency. As one example, we demonstrate a long-wave infrared SC source that generates a continuous spectrum from approximately 1.57 to 12 μm using a fiber cascade comprising fused silica fiber followed by ZBLAN fluoride fiber followed by sulfide fiber and, finally, a high-numerical-aperture selenide fiber. The time-averaged output power is as high as 417 mW at 33% duty cycle, and we observe a near-diffraction-limit, single spatial-mode beam across the entire spectral range. A prototype is described that is based on a three-layer architecture with a form factor of 16.7 × 10 × 5.7 and that plugs into a standard wall plug. This SC prototype has been used in a number of field tests as the active illuminator for stand-off FTIR system over distances of 5 to 25 m, thus enabling identification of targets or samples based on their chemical signature. Further optimization of the SC source will also be described to increase the output power and to reduce the form factor.

Novel ultrafast sources based on fluoride fiber lasers and amplifiers have emerged as a new alternative for the efficient generation of coherent broadband signal in the mid-infrared (> 3 µm). It was shown recently that the amplification of a 2.8-µm ultrafast signal using Er3+: fluoride fiber leads to different output regimes in the mid-infrared (supercontinuum or tunable femtosecond pulses) depending on the input signal properties. Here, we study the underlying mechanisms that affect the amplification and spectral conversion dynamics of ultrashort pulses inside such amplifier through a complete numerical model based on accurate measurements of the amplifier properties. The numerical results, which are in very good agreement with the experiment, show an enhanced output stability due to the attractor properties of the amplifier and the possibility of generating a supercontinuum extending from 2.8 to 4.2 µm with more than 4 W of average power using off-the-shelf fluorozirconate fibers. By designing the appropriate amplifier, tunable and spectrally isolated femtosecond solitons with peak power up to 400 kW around 3.4 µm that can be shifted up to 4 µm could also be achieved. Future perspectives with novel mid-IR laser transitions and different fluoride glass compositions with extended transmission are also discussed.

We demonstrate a 70 mol % GeO2 doped fiber for mid-infrared supercontinuum generation. Experiments ensure a highest output power for a broadest spectrum from 1000nm to 3000nm from this fiber, while being pumped by a broadband 4 stage Erbium fiber based MOPA. Our investigations reveal the unexploited potential of Germania doped fiber for mid-infrared supercontinuum generation. To the best of our knowledge, this is the record power, for an ultrabroadband, all-fiberized, and compact device size supercontinuum light source based on Silica and Germania fiber, ever demonstrated to the date.

Diffraction-limited 976nm lasers can be used to core-pump ultrafast fiber lasers to overcome nonlinearities with significantly shortened fiber lengths or to pump ultrafast solid-state lasers for much improved beam quality. In order to make Yb fiber lasers operate efficiently as a three-level system at ~976nm, it is critical to use double-clad fibers with large core-to-cladding ratio and additional spectral filters, such as dichroic mirrors in free space or fiber Bragg gratings in all-fiber configurations, to suppress lasing at longer wavelengths. Diffraction-limited 94W at 976nm was achieved in 2008 with an efficiency of ~50% with respect to the launched pump powers at ~915nm using a rod-type PCF and multiple dichroic mirrors. However, the results from flexible fibers with the potential to be used for monolithic fiber lasers are far worse. In this work, the Yb-doped double-clad all-solid photonic bandgap fiber has a core diameter of ~25μm and a cladding diameter of ~125μm. The photonic bandgap was engineered to have its long wavelength band edge just beyond 976nm to suppress lasing at longer wavelengths. We demonstrate a record efficiency of ~54% with regard to the coupled pump power at ~915nm. Pump-limited 38W at ~977nm was achieved with a M2 of ~1.24. ASE at ~1026nm was suppressed by <30dB at all powers. This is possible due to the use of all-solid photonic bandgap fibers which provide both the necessary large core-to-cladding ratio and the additional suppression of the four-level system by strong out-of-band transmission loss.

We present theoretical and experimental investigations on effective single-transverse mode propagation in very large mode area (VLMA) fibers. Upscaling the mode area of fibers is the most effective approach to reduce the nonlinear interaction and, therefore, to allow for the confinement of high-power radiation without detrimental nonlinear effects. Even though the investigations are carried out in a passive large pitch fiber (LPF), they reveal an intrinsic scaling potential of this design which, if unlocked, will be beneficial for active VLMA fibers in the future. A commercial mode solver based on a full-vectorial finite-difference approach has been used to simulate the confinement losses of the fundamental and higher-order transverse modes. These simulations have revealed that the differential loss in one-missing-hole photonic crystal fibers can be tailored to be larger than 10 dB/m for fiber core sizes larger than 200 μm at 1 μm wavelength. In order to test the theoretical predictions experimental investigations have been performed. Therefore, a rod-type fiber has been fabricated and effective single-mode operation with unprecedented large mode-field diameters has been demonstrated. We were able to achieve single-mode propagation in a passive 1.3 m long LPF with a pitch of 140 μm possessing a mode-field diameter of 205 μm. Even a strong misalignment of the coupling condition did not lead to any significant appearance of higher order modes at the fiber exit, which proves the robustness of the singlemode operation. To the best of our knowledge these results represent the largest dimension of a fundamental transverse mode reported in a waveguide structure at 1 μm wavelength to date. Compared to previous results the mode area is scaled by a factor of about 4 (with respect to active fibers) and a factor of ~8 (with respect to passive fibers).

Phosphate glass is an attractive material for rare-earth-doped fiber manufacturing because high doping levels are possible without introducing negative effects such as up-conversion or increased non-radiative recombination. In this paper we present a novel PM heavily Yb-doped polarization maintaining large mode area phosphate fiber and a < 100 W power level amplifier based on this fiber. The fiber was fabricated by a rod-in-tube technique. An 18 cm long piece of the fiber was used to build a high-power all-fiber amplifier. 106 W of output power at 1030 nm was achieved with 55 % slope efficiency with respect to the launched pump power. To the best of our knowledge, this is the highest average power ever demonstrated for short phosphate fiber lasers.

Hollow-core fiber (HCF) has found plenty of interdisciplinary applications in areas ranging from ultra-intense pulse delivery, single-cycle pulse generation, low latency optical communication, UV light sources, mid-IR gas lasers to biochemical sensing, quantum optics and mid-IR to Terahertz waveguides. These applications calls for better performance HCF, especially in loss, bandwidth and mode quality. Here we present a new hollow-core fiber with conjoined-tubes in the cladding and a negative-curvature core shape. It exhibits a minimum transmission loss of 2 dB/km at 1512 nm and a <16 dB/km bandwidth covering the O, E, S, C, L telecom bands. The debut of this conjoined-tube hollow-core fiber, with combined merits of ultralow loss, broad bandwidth, low bending loss, high mode quality and simple structure heralds a new opportunity to fully unleash the potential of hollow-core fiber in laser and telecommunication related applications.

Yb-doped hybrid fiber with anomalous dispersion of 100-380 ps/(nm⋅km) near 1 μm was demonstrated. Single-mode propagation was achieved by selective mode amplification of the hybrid mode: only central part of the core was doped with Yb-ions, which allowed preferences for hybrid mode amplification (suppression of undesirable modes can exceed 20-30 dB, when splice conditions are optimized). Usage of the developed fiber as non-linear pulse compressor allowed us to demonstrate pulses with duration of 250 fs and peak power in excess of 3 kW.

We demonstrate a low-concentrated solar-pumped fiber laser with all-inorganic cesium lead halide perovskite quantum dots (QDs), which function as a sensitizer. The perovskite QDs exhibit substantial advantages for solarpumped laser applications because of their broad absorption and narrow emission spectra with high quantum yield. We successfully tuned the peak emission wavelength of the perovskite QDs by altering the I/Br ratio in order to achieve spectral overlap with Nd³⁺ ions, which have been widely used as a laser medium for solar-pumped lasers. The measurement results show that the laser output power is highly sensitive to the peak emission wavelength of the QDs. Although our synthesized QDs have a quantum yield of approximately 65%, which is less than that of conventional organic dyes, the laser performance was comparable because the fluorescence spectrum is tailored to the Nd³⁺ absorption band.

A comprehensive theoretical investigation of a dual wavelength, cascade cavity bismuth-doped fiber (BDF) laser operating in the wavelength range of 1.7μm is presented. The fiber laser model is based on parameters extracted from experimental characterization of the BDF. The BDF serves as an active medium with optical gain in the wavelength region from 1.65μm to 1.8μm. The laser cavity is defined by a 90% mirror on one end of the BDF and two fiber Bragg gratings (FBG1 and FBG2) separated with the BDF on the other end of the laser cavity. One of the gratings FBG1 with the peak reflectivity 95% is centered at 1.725μm. The second one FBG2 with peak reflectivity of 90% is centered at 1.729μm. Both FBGs have a 3-dB bandwidth of ~0.5nm. It is shown that the cascade laser can operate at two 1.725μm and 1.729μm wavelengths with different powers depending on the parameters of the structure.

A high power single-mode fiber laser has received a lot of attention in various materials processing fields. In order to carry out processing under proper conditions with single-mode fiber laser, the dynamic flows of fluids inside the materials should be precisely understood. We carried out the numerical analysis of materials processing using a 5-kW single-mode fiber laser. The dynamic flows of the copper fluid during the bead-on-plate tests are calculated and the results agree well with the experimental results. The processing with the different scan speed and spot size, and at the output power of 10 kW are numerically calculated as well. The proper processing conditions with less spatters and dross for each materials can be determined by the dynamic analysis.

Within the EKOLAS consortium, which is part of the BmBF-funded Effilas (Efficient high-performance laser beam sources) research alliance, we are developing Fiber Bragg Gratings (FBGs) written in extra-large mode area (XLMA) fibers. By using FBGs as fiber integrated output mirrors, which is state-of-the-art in single-mode fiber lasers, we aim to reduce complexity and increase robustness and reliability of multi-mode fiber resonators. To this end, we are investigating the use of FBGs as outcoupling mirrors with a reflectivity below 10 % in XLMA high-power multi-mode fiber lasers. As a first step, FBGs are written into a passive 105/125 μm multi-mode fiber. We present their application for frequency stabilization of a resonator based on XLMA fibers, and tested the FBGs up to an output power of 150 W at 1075 nm without failure. As the next step, transition from passive 105/125 μm fibers to active XLMA fibers is currently being investigated. For FBG inscription, we use a phase mask and an ultra-fast laser system (100 fs, 800 nm). The setup is adjustable in three translation and three rotation axes. Additionally it features a modular mechanical design for fast and flexible interchanging of fiber mounts, phase mask and process optics. The FBGs are pre-characterized in transmission with a white-light source and an optical spectrum analyzer.

Fiber lasers have experienced an explosive growth in output power over the last decade, enabling multi-kW output powers with nearly diffraction-limited beam quality. Transverse Mode Instability (TMI) is currently the ultimate limitation in average power scaling of CW fiber lasers. Therefore, new fiber designs and amplifier geometries will be required in the future to overcome TMI-induced limitations. To explore the parameter space, we have constructed several kW-class fiber amplifiers to examine the effects of different fiber designs and amplifier geometries using both commercially available fiber as well as custom, in-house drawn Yb-doped fiber. Investigations are ongoing which explore the effects of variations in the cladding diameter, core NA and fiber length. Results are presented which investigate the roles the cladding diameter and core NA play in system design with regard to thermal management and beam quality management, respectively. Finally, we will present our current progress towards long range, outdoor laser propagation at our TISTEF laser facility.

High-power fiber lasers have become one of the most important tools for material processing in the last decade. Laserline GmbH, which is primarily known for its multi-kilowatt direct-diode lasers, also introduced a fiber-laser platform as a brightness converter for its direct-diode lasers a few years ago. Currently, output powers up to 6 kW at beam qualities down to 4 mm mrad are commercially available. The platform is based on a single directly water-cooled ytterbium doped XLMA (extra-large mode area) fiber in an end-pumped configuration, which can easily be combined in series with a standard diode laser. In this paper, we present the latest progress in power scaling of our fiber-laser system. By improving the material properties of the active fiber, the thermal management and the thermo-mechanical stability of the resonator, up to 10 kW output power from a single, unidirectionally pumped fiber-laser oscillator with a beam parameter product in the range of 4 mm mrad to 8 mm mrad is demonstrated. Further power scaling up to 17.5 kW with 8 mm mrad was achieved by bidirectional pumping of the active fiber. In both cases, a rather high optical conversion efficiency of 75-77 % leads to a wall-plug efficiency of ~35 % for the whole laser system. Currently, we do not observe any physical limits, such as nonlinear effects for example.

We present a turn-key pulsed polarized Holmium doped fiber source at 2088 nm. In our experimental setup we are utilizing an acousto-optic modulator as a mode-locker or Q-switch. The linear cavity is comprised of a fiber Bragg grating (FBG) pair with central wavelength of 2088 nm in a polarization maintaining passive fiber. The active fiber is polarization maintaining, doped with Holmium, and pumped with an in-house made Thulium doped fiber laser with an emission wavelength of 1940 nm and a maximum output power of more than 10 W. In q-switch operation it was possible to generate stable nanosecond pulses between 5 and 250 ns at repetition frequencies between 10 and 250 kHz. Shaping of the AOM driving signal allows for some modification of output pulse shape and its length. The maximum output energy was measured to be 0.5 µJ. In mode-locked operation the laser emitted pulses at 27.35 MHz repetition rate which corresponds to the optical cavity roundtrip time. The calculated band-width limited pulse width was approx. 13 ps (sech2) and the pulse energy was 0.5 nJ. In both operation regimes the signal quality was very high with optical signal-to-noise ratio greater than 50 dB without amplified spontaneous emission (ASE). Similarly, the polarization extinction ratio (PER) was greater than 20 dB. The oscillator design is fully monolithic and polarization-maintaining. Its compactness, turn-key operation, and all-fiber design makes it a very promising platform for nonlinear frequency conversion.

We present a coherently-combined ultrafast fiber laser system consisting of four amplifier channels delivering 3.5 kW average power at 80 MHz repetition rate with a pulse duration of 430 fs FWHM and a close-to-diffraction-limited beam quality with an M² < 1.2. The system incorporates a fully automated self-adjustment of the beam combination, allowing for a quasi-turn-key operation of the system. At the date of publication, this system delivers the highest average power reported from an ultrafast laser.

We present a laser amplifier based on coherent combination of 16 channels from a single multicore fiber employing multi-channel components for beam splitting, combination and temporal phasing. Stretched femtosecond pulses (250 fs transform-limit) were combined with an efficiency of 80% at up to 205 W average power.

In this contribution, we present the newest results of the recently introduced pulse-energy-scaling technique electrooptically controlled divided-pulse amplification (EDPA) and its implementation in a high-power fiber laser system based on coherent combination. In this experiment, a burst of 8 stretched fs-pulses is amplified in two high-power fiber amplifier channels followed by coherent combination into a single pulse. Afterwards, the signal is compressed to a FWHM pulse duration of 255 fs with a pulse energy of 3 mJ and an average power of 105 W. The additional degrees of freedom provided by EDPA, such as direct access to the amplitudes and phases of all individual pulses in each burst, are exploited to compensate for gain saturation effects. Thus, a great temporal contrast of about 18.5 dB is reached and a very high combining efficiency of nearly 80%, including spatial as well as temporal combining, is reached. Furthermore, the system features three customized multi-pass cells as optical delay lines, minimizing the footprint of the combining stage to 0.5 m². For the time being, two amplifiers are employed in order to initially optimize the parameters of EDPA and the performance of temporal combining. However, the laser system comprises a total of 16 parallel main amplifier channels, potentially enabling spatio-temporal combination of 128 separately amplified pulses with the currently applied bursts of 8 pulses. This extension is part of upcoming experiments and will allow for significant further scaling of the pulse energy in the near future.

We show a new interferometer for coherently combining multiple pulses from multiple amplifiers for femtosecond, high energy pulses. A double pass provides passive interferometer stability, but we add a spatial offset to enable unidirectional amplification in fiber. Using intensity splitters and phase results in high spectral bandwidth and low dispersion. We demonstrate direct combination of eight 81 fs pulses made by nonlinear amplification, broadening, and compression within two separate fiber amplifiers with only one active stabilization for the relative fiber amplifier lengths. We verify coherence of the input and output pulses for frequency comb applications.

Coherent beam combining (CBC) is a major method for fiber laser power scaling. For many channels or feedback or when cameras as sensors, however, analog feedback loops become inconvenient and digital controllers offer advantages in flexibility. Here, we investigate the feasibility of using end to end reinforcement learning (RL) methods to achieve CBC. This should enable to achieve the goal of maximizing the power without manually setting any locking strategy or system model. In deep RL a neural network is assigned random weights in the beginning and trained to become a usable control policy. The output of our neural network is a 21-component vector which estimates the value for 21 hypothetical changes of the control voltage. For best control, we should pick the maximum and change the output accordingly. To train this we split up the process in 300 step episodes. Between episodes, we update the weights of the neural network. As our test setup, we used a simple two-channel system combining 1550 nm femtosecond pulses. We trained the RL agent and after half a day of training were able to achieve CBC. In the beginning of training, the agent, it tries many different seemingly random actuator movements but eventually, it converges to a suitable policy. After training for two hours this algorithm was capable of maximizing the output power reliably without us injecting any explicit information about optics or dither locking. The power noise of the trained controller was about 1% RMS.

We report on the generation of 102 μJ-, sub-2 cycle pulses with several GW of peak power centered at 1.73 μm wavelength. The intense few-cycle source features 8.8 fs pulse duration (full-width at half maximum, FWHM). It is operated at 196 kHz pulse repetition rate and provides a record average power of 20 W. This result is enabled by the combination of two average power scalable concepts: direct emission from an ultrafast Tm-doped fiber laser system and nonlinear pulse compression in a gas-filled antiresonant hollow-core fiber. Ultrafast Tm-doped fiber lasers are scalable to kW-level average power, which emphasizes the great potential of this approach for driving high cut-off HHG at high repetition rates.

The timeframe between the first demonstration of novel laser parameters and the commercial availability of systems delivering those parameters has been shrinking over the recent years. This development makes femtosecond laser systems with unparalleled parameters available to a wide range of applicants. We present the progress in setting up such a FCPA laser system delivering 5mJ pulse energy at 100kHz pulse repetition rate (i.e. 500W of average power) and with pulse durations of 6fs. The system, the so-called HR2 laser, will be implemented at the ELI-ALPS facility in Szeged, Hungary. Furthermore, the laser is optimized to generate output pulses with a stable carrier-envelope-phase (CEP). Since 6fs requires a bandwidth not supplied by Yb-based gain-media, and thus nonlinear compression is necessary, the chirped-pulse-amplification (CPA) system itself must deliver a higher energy to compensate for the losses in these nonlinear-compression-stages. Hence, the CPA is operated at 12mJ and 1.2kW delivering sub 300-fs pulses with an almost diffraction limited beam quality. To achieve such a parameter-set, the output of 16 main-amplifier channels is coherently combined using a polarization-based filled-aperture combining scheme. Maintaining the CEP stability of the oscillator throughout the entire system is another key challenge of the project. We finally discuss further power-scaling approaches and possibilities to reduce system size, complexity and, therewith, cost. By using these revolutionary laser systems, even demanding applications such as laser-based particle acceleration or XUV-generation via high-harmonic generation will be able to perform the important step from the laboratory to real-world applications allowing to access repetition rates orders of magnitude higher than before.

We present the most recent results of ultrafast fiber-laser driven generation of broadband THz radiation based on two-color gas-plasma. The experiment shows how energetic fiber-lasers can improve on an application today mainly dominated by Ti:sapphire lasers and power-scalability of this kind of THz sources is discussed. With a high-power driving laser THz radiation with more than 4 mW of average power is generated. This is the highest average power using this scheme so far.

Frequency doubled sub 50 fs Erbium-fiber lasers are ideal tool used to seed Ti:sapphire amplifier. Therefore, over last decade large number of all-fiber laser architecture has been reported for such application. Nevertheless, the emitted pulses are usually too long due to the gain bandwidth of Erbium or the laser architecture is not made with Polarization Maintaining (PM) fibers which will be a limitation for frequency doubling. We demonstrate a new design of an all-PM erbium doped fiber laser emitting sub 50 fs pulses with high pulse energy and we study its frequency doubling. Our architecture is based on a concatenation of three amplifiers having different group velocity dispersion. These amplifiers provide numerous degrees of freedom allowing to control the output pulse duration. Thanks to this new design, the laser produces 14 nJ pulse with a duration of 48 fs and an average power of 560 mW. This is to the best of our knowledge the shortest pulse duration with an energy higher than 10 nJ emitted by an all-fiber laser around 1.5-1.6 μm. The pulses are further converted by Second Harmonic Generation to 796 nm with an efficiency of 25 %. The average power of the doubled signal is 140 mW with 3.5 nJ pulse energy. The nonlinear crystal has been carefully chosen in order to cover all the spectral bandwidth of the pump and to ensure a sub 50 fs pulse at 796 nm.

Laser pulses in either laboratory or industry are typically complex objects. Unlike the classical electro-magnetic wave with uniform polarization distribution along the flat wave-front or a general vector beam under the paraxial approximation, the real light pulses, such as the pulses from the high power multimode fiber laser and the dechirped femtosecond pulses with structured wave-front, spectrum and polarization distributions, usually have non-vanishing component in the propagation direction. Therefore, the description of a general vectorial laser pulse should be implemented in multi-dimensional way, for the light is the combination of a three-dimensional(3D) vector field (electric field E) and a 3D pseudovector field (magnetic field B) in the 3D Euclidean space (R3). Here we report on a novel technique for the multi-dimensional characterization which includes the spatiotemporal amplitude and phase information as well as the vectorial features in 3D Euclidean space of the complex laser pulses, such as the intrinsically controlled femtosecond pulses with higher-order Poincaré sphere beams and vectoral spherical beams. A two-steps-based polarization-sensitive Mach–Zehnder interferometer temporal scan technique was used, at the first time, to capture the complete information of the pulses. The corresponding measurement device, placed on the collimated and attenuated beam at the laser output is consists of a special Mach–Zehnder interferometer, a polarizing beam-splitter and charge-coupled device (CCD) cameras. The reference beam with vertical and horizontal polarization is exported from the cavity and attenuated to a suitable intensity by using a neutral-density filter. After 3D phase unwrapping, removal of the reference curvature, correction of achromatic wave-front distortions, spectral phase and amplitude reconstructions, as well as the measurement for the intrinsic phase of the reference pulse, the complete information of the pulse, include the phase information of the three electronic components, will be obtained. This new measurement capability opens the way to in-depth characterizations and optimizations of the complex laser pulses and ultimately to the study of new phenomena of multimode fiber laser generated laser pulse as well as the interactions between materials and structured ultra-short laser beams.

In this work, an all-solid Ytterbium doped double-clad single-mode (SM) large-mode-area (LMA) photonic crystal fiber (PCF) is characterized in a monolithic amplifier setup. The characterized all-solid PCF combines the SM operation of LMA PCFs while maintaining simple splicing of all-solid fibers. The fiber has been tested in a pulsed monolithic forward pumped amplifier system achieving excellent modal properties. The fiber is polarization maintaining and has a mode field diameter of approximately 24 μm. At optimal fiber length the output signal power of the amplifier was measured to 43 W limited by pump power with an optical to optical efficiency of 61 %. Polarization extinction ratios (PER) above 18 dB was observed for all fiber lengths. Simple splicing with splice losses < 0.2 dB and maintained polarization are reported using a standard Fujikura fusion splicer.

Within the last ten years, ultrashort pulsed lasers have developed from complex scientific devices to reliable industrial tools for various applications. MOPA systems with fiber amplifiers offer an excellent combination of small size, reliable design and flexible parameters. However, their power and more specifically pulse energy levels could not reach the high level of other laser designs based on disk or rod amplifiers. In the latest development generation of TRUMPF’s TruMicro 2000 series, we present an all-fiber ultrashort pulse laser with a maximum pulse energy of 100 µJ based on industrial grade technologies. The high energy level allows for efficient frequency conversion to a wavelength of 515 nm which is beneficial for numerous applications. Furthermore, the linear amplifier concept combined with intelligent electronics and software allow for flexible adaption of parameters: The laser is equipped with a sophisticated pulse-on-demand function that allows variable pulse picking from the seed frequency of 50 MHz. This bears the challenge of fluctuating pulse ensures which is overcome by a combination of measures that ensure constant pulse energies even with variable pulse distances. As a result, the laser pulses can be synchronized to external trigger signals with a time jitter as low as 30 ns. Additionally, the TruMicro 2000 offers a flexible burst mode up to 8 pulses in one burst and the pulse duration can be changed continuously from femto- to picoseconds. With a newly patented technology, the full sweep from 300 fs to 20 ps is done in less than 500 ms without any influence on beam parameters.

We report the performance of a single frequency Raman DFB fiber laser operating at a wavelength of 1178-nm. The 20-cm long DFB laser operates in a robust single longitudinal mode with the slope efficiency of 6.9% and the output power of over 1Watt.

We report stimulated Brillouin scattering (SBS) measurements in orbital angular momentum (OAM) carrying modes of an optical fiber. The additional degree of freedom afforded by angular momentum selectivity leads to new possibilities to control SBS gain. We find that, depending on the OAM value in relation to the polarization state of the launched pump light, the SBS threshold can vary by as much as 2.6 dB, even when normalized for mode area. Combined with their inherently large mode areas and naturally polarization maintaining behavior, OAM fiber modes may offer an alternative means for scaling power in narrow-linewidth fiber lasers.

We demonstrate a prototype sensor capable of measuring specular and diffuse reflectance spectra from samples 3.6 m away. The sensor utilizes mid-wave to long-wave infrared supercontinuum light coupled into a rotational FTIR spectrometer to actively probe remote samples. We measure the diffuse reflectance of acetaminophen at 41.77 μg/cm² on a glass substrate and find that a modified Bobbert-Vlieger analysis can estimate the effects of particle size distribution on return spectra. We find that the measured return from stand-off particulate measurements depends not only on the chemical identity, but also the size and distribution of particles on the substrate.

Fiber-based single-frequency amplifiers, e.g. for gravitational wave detectors, are typically limited by the nonlinear effect of the stimulated Brillouin scattering (SBS). In addition to a decreasing output power level, SBS also initiates a broadband intensity noise in a frequency regime up to several MHz. Several published theories provide models to explain this excess intensity noise. Here, we report new theoretical and experimental studies, which suggest the conversion from phase to intensity noise by an asymmetric Brillouin gain spectrum. Due to the asymmetric Brillouin gain profile, upper and lower frequency sidebands experience a frequency-dependent gain-factor and a differential non-zero phase shift. Thus, the sidebands do not cancel each other out and the differential non-zero phase shifts provides an additional contribution to the intensity noise. Using a measured asymmetric Brillouin gain spectrum and the 1/f-frequency noise characteristics of the pump source, we are able to reconstruct the shape of the excess intensity noise fairly well. In addition, further evidence for an asymmetric SBS gain profile has been observed. Due to small back reflections at the fiber end faces corresponding resonances are evident in the intensity noise spectrum. Through the asymmetry of the SBS gain profile and the differential non-zero phase shift, the conditions for the resonator eigenfrequencies are influenced. Therefore, the beat frequency between the upper and lower eigenfrequencies with the carrier are not equal anymore, which becomes noticeable in a splitting of resonances in the intensity noise spectrum.

We demonstrate infrared spectroscopy systems that are capable of predicting acrylamide level in powders of potato fries based on all-fiber high power supercontinuum (SC) lasers in both the short-wave and mid-wave infrared spectral range. Two SC lasers used in this study cover wavelength range of 670nm to 2500nm and 1600nm to 11000nm, respectively. We use the spectroscopy system to measure 32 French fry samples with different acrylamide concentrations calibrated by gas chromatography-mass spectroscopy (GCMS). Our predicted acrylamide concentrations show a good linear correlation to the measured acrylamide concentration obtained through GCMS, and the partial least square regression analysis shows standard error down to 145ppb. Based on our results, our system could provide a non-destructive alternative method for determining the acrylamide in food samples at a stand-off distance, which could be important for near-line or in-line quality control purposes.

We describe the recent discovery of an ultrafast Raman scattering mechanism, soliton-self mode conversion (SSMC). It features power-scalable frequency conversion of ultrashort pulses with near-100% photon-conversion efficiency, and dominates over even concurrent conventional seeded processes in spite of being noise initiated. Realising spatially coherent, MW peak-power-level, transform-limited, sub-100-fs pulses at non-traditional wavelengths directly out of fibers facilitates applications that are currently accessible primarily by bulk optical parametric oscillators. In addition, this mechanism enables endoscopic or flexible delivery of ultrashort energetic pulses at user-defined colours. We show representative applications of SSMC in multiphoton microscopy at the biologically crucial 1300-nm spectral range.

In this work we report the results of a theoretical and experimental study that we have carried out on a Brillouin optical time domain analysis (BOTDA) sensing scheme using a novel low-noise actively-stabilized fiber Brillouin ring laser (BRL) as probe source. The BRL laser is based on a short-cavity (SC), < 4 meters long, layout achieving double-resonance (DR) operation for both pump and probe signals; an active wavelength-locking circuit is used to stabilize the signal and tune the signal frequency over a range of ̴ 200 MHz range. The wavelength-locked SC-DR BRL shows spectral linewidth of approximately 10 kHz and RIN values of ~-145 dB/Hz across the (0-600) MHz range; pump-probe frequency shift can be efficiently tuned across the entire Brillouin gain spectrum of the sensing fiber with sub-kHz precision (200 Hz) and high temporal stability for timescale of BOTDA measurements (more than 100 ms). A preliminary BOTDA measurement using a wavelength-locked long-cavity (LC) BRL yielded a Brillouin frequency shift (BFS) uncertainty of 1,5 MHz corresponding to temperature and strain sensitivity values of 1 K and 25 με, respectively, and a spatial resolution of 5 m for 50 ns-long pump pulses.

High power polarised narrow linewidth sources are of immense importance in coherent beam combining (increased path length accuracy), spectral beam combining (reduced angular spread), non-linear frequency conversion (increased parametric gain) and remote detection. Power scaling of narrow linewidth sources is primarily limited by Stimulated Brillouin Scattering which can be overcome by spectral broadening before amplification to higher powers. Various applications have different requirements of spectral purity and power which can be met if the source line width is tunable. We demonstrate a narrow line width polarisation maintaining (PM) laser with continuous linewidth tuning from ~2.88 GHz to ~9.88 GHz with over 20 W of output power with a polarisation extinction ratio (PER) greater than 20 dB. We achieve continuous linewidth tuning through pure phase modulation of a 1064 nm DBR laser with a white noise source whose bandwidth and power are tuned with a low pass filter bank and variable attenuation of the drive voltage to the modulator. This dual control mechanism enables gapless tuning of the linewidth of the source. A cascade of optimized PM Yb-doped fiber preamplifier and power amplifier is used to scale the output power to over 20W. Combined with the wavelength tuning of the DBR seed, the tunable spectral width makes it versatile for use in a wide range of applications.

Aiming at the problem of high-ring-cavity losses caused by the use of a high-insertion-loss filter components to tune the output wavelengths in a continuous-wave (CW) all-fiber optical parametric oscillator (FOPO), this paper proposes a low ring-cavity-loss tunable CW FOPO based on the cascaded single-mode-multimode-single-mode (SMS) fiber structures. By applying an axial pulling force to the SMS fiber device to adjust the transmission spectrum of the filter device, the cascaded SMS fiber devices were used to achieve a tunable CW output of the double sidebands in the range of 1494~1501 nm and 1638~1629 nm. Among them, the insertion losses of the cascaded SMS fiber devices at the selected wavelengths are less than 1 dB, and the total FOPO ring-cavity losses are no more than 5 dB.

Novel configuration of combined Er/Er-Yb fiber amplifier is proposed to join advantages of both approaches (high gain and high efficiency). Co-propagating pump at 976 nm and signal at 1556 nm were coupled into a short piece of Er-doped fiber, where signal was amplified to a few Watts level. Then signal together with unabsorbed pump passed to LMA Er- Yb fiber. In this configuration, 2 ns pulses were amplified to 20 W of average power and 37 kW of peak power (limited by four-wave mixing). Differential efficiency reaches 37%, which is a record for fiber lasers with this peak power level.

Passively mode-locked fiber lasers are capable of generating a variety of optical pulses, depending of the pump power and cavity adjustments, which makes them attractive for research in multidisciplinary fields. In this paper, we analyze the experimental results of a fiber laser operating in passively mode-locked regime. The figure-eight laser cavity configuration uses 4 m of a co-doped Er/Yb double clad fiber, as a gain medium, pumped by a 976 nm diode laser. The cavity is completed by an isolator for ensure unidirectional propagation, two quarter-wave retarders (QWR), and a nonlinear optical loop mirror (NOLM) with 120 m of twisted standard single-mode fiber in the loop acting as saturable absorber. We used a couple of quarter-wave retarders (QWR) QWR1 at the NOLM input and QWR2 into the NOLM loop. By carefully adjusting of the QWRs the laser generates noise-like square pulses at the 1.54 μm range. With the increase of the pump power, the laser generates harmonics of the mode-locking fundamental mode with repetition frequency of 1.38 MHz, from which we can obtain the second, third and fourth harmonic in a controlled mode.

We demonstrate an all fiber mode-locked ultrafast erbium laser oscillator by means of nonlinear polarization evolution (NPE) in polarization maintaining (PM) fibers. Here, the artificial saturable absorber (SA) is formed by a piece of PM active fiber and a Faraday mirror (FM). High repetition rate could be obtained thanks to the introduction of the PM active fiber. The transmission characteristics of the SA are studied through theoretical analyses as well as experimental demonstration. Channeled spectra caused by soliton-soliton interaction and birefringence-induced filter are also studied and discussed. A good agreement is achieved between the theoretical analyses and experimental results.

We report dual-wavelength mode-locked operation of a passively mode-locked Er-doped fiber laser (EDFL) including two sections of a fiber with equal length, one right hand twisted and another left hand twisted, and a double-pass amplifier with Faraday Mirror (FM). The configuration allows cancellation of the influence of both linear and circular birefringence and strict control of the polarization of pulses in the cavity. Depending on the polarization azimuth we observed synchronous dual-wavelength, 1532 nm and 1560 nm, noise-like pulses (NLP), and desynchronized soliton emission.

Single-frequency Yb³⁺ fiber amplifiers operating at 1064 nm are promising candidates to fulfill the challenging requirements for laser sources of the next generation of interferometric gravitational wave detectors. We present the current development progress of a fiber amplifier engineering prototype and compare the optical and thermal performance to the solid-state-laser source of advanced LIGO. The fiber amplifier system consists of two monolithic fiber amplifier stages which currently deliver more than 110 W (functional prototype demonstrated 215 W [9,11]) of output power. The fiber amplifier output beam has one to two orders of magnitude lower relative beam pointing and relative power noise in the lower frequency range of 1 Hz to 100 Hz compared to the solid-state-laser system. It also has a polarization extinction ratio above 21 dB and a TEM₀₀-mode content of more than 97.8 % ±0.6 % at 110 W output power. Besides the optical properties, repair and maintenance procedures are improved by a modular design of the system. Each of the modules can separately be maintained and repaired or easily be replaced by a preassembled module; it therefore minimizes laser downtimes. Another advantage is the lower heat load of approximately 500 W compared to the SSL, which produces more than 4500 W of heat, both at an optical output power of 200 W. The lower heat load simplifies cooling and reduces the complexity of the modules.

We report an all-fiber, linearly polarized, 134-W, 1064-nm fiber laser based on a three-stage Ytterbium fiber amplifier system seeded by a gain-switched diode laser with tunable pulse width from 21 to 200 ps at repetition rates of 0.5–1.5 GHz. Optical signal-to-noise ratio was more than 30 dB. Pulse bandwidth was within 1 nm at repetition rates of 0.5–1.5 GHz. The fiber laser offers excellent long term power stability.

Since the proof of concept of Photonic Crystal Fibers (PCF) by Knight et al., their development over the last two decades has led to progressive enlargement of core sizes while maintaining a transverse single-mode operation enabling power scaling in fiber lasers and amplifiers by pushing further the nonlinear effects and damage thresholds. Numerous fiber designs and laser/amplifier architectures have been investigated in order to make the most of the PCF technology and mainly to mitigate a new deleterious phenomenon responsible of beam quality degradation, the Transverse Mode Instability (TMI), which arose in parallel of the high average powers reached with those fibers. In this context, our research group has developed a PCF, so called Fully-Aperiodic Large-Pitch Fibers (FA-LPF) which proved its relevance with passive as well as active fibers, manufactured with the powder sintering technology known as REPUSIL. In this work, the refractive index of the FA-LPF core is slightly lower than that of the background cladding material (Δn ~ -5x10-5). This depressed-index core feature enables a thermal resilience ensuring an effective single-mode propagation above a certain average power for core size as high as 110µm. Experimental results in amplifier set-up with a 110 µm Yb-doped depressed core FA-LPF led to 110W of amplified signal for 300W of pump with a M² < 1.3. No TMI phenomenon was observed even at maximum pump power despite the average power and the very large mode area involved.

We demonstrated, for the first time to our best knowledge, an active tapered double clad fiber with circular birefringence and 35 μm core diameter. The output radiation had perfect beam quality (M²=1.18/1.1) and linearly polarized light with 15 dB of PER. The developed double clad active fiber was investigated for amplification of picosecond pulses in allfiber MOPA system. The MOPA system delivered 50 ps pulses with 55 W of the average power revealed 34.4 dB gain of the booster amplifier.

We pursue locking of optical frequency comb for absolute frequency measurement. For the purpose, the characteristics of frequency comb are measured. At first, the characteristics in the fs pulse laser are measured by an autocorrelation technique. Second, the frequency stability of the reputation rate of an optical frequency comb based on a mode-locked fiber laser is measured. The repetition frequency is locked to a frequency generated from a synthesizer, which is referred to a Rubidium frequency standard. The stability was 3 Hz at averaging time of 1 s under locking. The best stability was better than 0.3 mHz at averaging time of 100 s. Finally, the wavelength spectrum of the pulse laser is broadened using self-phase modulation in highly nonlinear fibers. The spectrum bandwidth was broadened to 824 nm.

It is possible to predict more accurately the laser fiber refractive index when we apply a stress induced index change model together with the measured preform index profile data. The improved fiber index difference estimates enable to fine tune the deposition process so that the index difference values of the drawn fibers will be close to the designed values.

Large-mode-area (LMA) photonic crystal fiber (PCF) amplifiers are attractive for high-power amplification of single-mode (SM) narrow-linewidth light. Traditionally, LMA PCF designs include air holes but splicing and interfacing of fibers with air holes is complicated. Recently, this was addressed with the development of a LMA PCF amplifier fiber without air holes but with SM operation maintained. This all-solid fiber can be spliced with standard splicers and thus enables integration of PCFs into all-fiber monolithic laser systems. The new fiber, named DC-250/30-PM-Yb-FUD, is demonstrated in a free-space configuration for high-power amplification of 1064 nm light with a narrow linewidth of less than 20 kHz. A seed power of 380 mW is amplified to 40 W, corresponding to a single-stage gain of 20 dB, with an optical to optical efficiency of 69%. No indication of stimulated Brillouin scattering is observed, and the output is stable during frequency modulation of the seed laser.

Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber is reported. Suppression of higher-order modes is enhanced compared with large mode area stepindex fibers owing to the depressed-index inner cladding and confined doping. Average power and pulse energy exceeding respectively 90 W and 50 μJ were achieved after multipart fiber amplifier stages, with final-stage amplifier gain larger than 40 dB. Pulse compression using a chirped volume Bragg grating later yields durations as short as 1 ps and peak powers exceeding 10 MW, with near diffraction-limited output. Pulse-on-demand and burst modes are straightforward, given the master oscillator/power amplifier scheme, with pulse generation first initiated from direct current modulation of a seed laser diode while subsequent external phase modulation and spectral selection yield pulse trains/bursts following digitized arbitrary waveforms. The proposed scheme is considered to be relevant for laser materials processing.

Simultaneous Q-switching and mode-locking (QML) pulse operation is demonstrated by using an in-fiber acousto-optic bandpass modulator (AOBM). The AOBM exhibits a high rejection efficiency of 28 dB for the non-resonant wavelengths, a broad modulation bandwidth of 0.9 nm, a modulation depth of 0.74 and easy implementation in an allfiber device. The effectiveness of the modulator is demonstrated in a rig cavity, mode locking is achieved by amplitude modulation of the transmitted light and matching the modulation period with the cavity round-trip time, simultaneously, the Q factor of the cavity is controlled by superimposing a rectangular wave modulation on the acoustic signal, with variable frequency and duty cycle, allowing a QML pulse operation. Experimental results demonstrate stable QML operation at 0.5 kHz repetition rate. A maximum energy of 1.4 μJ was achieved within a burst of 110 ps mode-locked sub-pulses contained in a 6.4 μs Q-switched envelope. These results demonstrate the potential of the AOBM to perform the double function of active mode locker and Q-switching device in an all-fiber laser.

Analyzing modal contents of arbitrary beams from optical fiber provides useful information for high-power fiber laser and mode-division-multiplexed optical communication. Numerical modal decomposition can be applied to diverse applications universally without complicated setup, but often leads to a false solution which is called as a local minimum. We propose several ways to solve this issue and compare the results to find the best way for global optimization based on stochastic parallel gradient descent algorithm.

We demonstrate a PM-NALM fiber laser utilizing a 3x3 coupler. While a 2x2 coupler causes no phase bias between counter-propagating lights in the NALM, the 3x3 coupler can generate a phase bias (2μ3) in the NALM without an additional nonreciprocal phase shifter. As a result, this 3x3 coupler-based NALM laser requires less than 2 times pump power for the mode-locking initiation compared to the 2x2 PM-NALM laser with a similar repetition-rate and configuration. The minimum rms relative-intensity-noise (RIN) is only 0.0048% integrated from 10 Hz to 1 MHz offset frequency, which is one of the lowest RINs demonstrated from free-running mode-locked lasers so far.

In this work, an Erbium doped ring fiber laser cavity based on in-line fiber optic interferometer (IFOI) for multiwavelength operation is proposed. To fabricate the modal filter small sections of thin-core fiber (TCF) and no-core fiber (NCF) were used. The TCF is a highly Erbium doped fiber that generated special characteristics such as: side mode suppression ratio of 43.10 dB, low threshold (75mA), minimal power instabilities (0.47dB) and lower wavelength drift (0.1nm). By controlling the bending of the fiber filter and adjusting the polarization stage is possible to obtain: single, dual and triple laser emission. The laser offers compactness and low-cost implementation.

We present a multi-wavelength ring fiber laser cavity based on modified loop fiber optic interferometer. Here, the interferometer is composed by three segments: two thin-core fiber (TCF) sections and one no-core fiber (NCF) section, these fibers are arranged as follows: TCF-NFC-TCF, then this structure is set into a fiber loop. Hence, by using a conventional ring laser cavity the proposed fiber optic loop is seed. Afterward, by controlling the modal fiber curvature and the ring cavity polarization state: single, dual and triple lasing modes can be achieved. The laser offers good power and wavelength stability, the maximal variations are 0.2 nm and 0.12 dB.

A linear mode-locked fiber laser based on the semiconductor saturable absorption mirror was demonstrated to realize mode locking in group velocity locked vector soliton regime and polarization rotation vector soliton regime. The highly fixed birefringence was introduced into the linear fiber laser cavity by using a piece of polarization maintaining Erdoped fiber. The vector nature of pulse of the mode-locked fiber laser was experimentally studied. It turned out that, this birefringence enhanced mode-locked fiber laser can generate both group velocity locked vector soliton and polarization rotation vector soliton though slightly changing the birefringence of the laser cavity. The experimental results reveal the fundamental physics of group-velocity locked vector soliton and polarization rotation vector soliton generation and demonstrate that the linear birefringence enhanced fiber laser is a good platform for investigating the vector soliton.

For studies of the European Space Agency ESA, Fraunhofer ILT develops and builds narrowband, power-stabilized fundamental mode fiber amplifiers especially for future space-based gravitational-wave detectors, e.g. LISA, and for Earth gravity field missions. In this paper, we present the status of our ongoing work, based on a highly stable fiber amplifier designed for a Next Generation Gravity field Mission (NGGM) pre-study, towards power scaling as well as enhancement of the Technology Readiness Level (TRL). Our amplifier has already demonstrated to meet the requirements for future gravity field missions. It features a design that is free of stimulated Brillouin scattering (SBS) and a feedback loop for power stabilization.

We are presenting an inversion estimator based controller for master oscillator power amplifier (MOPA) laser systems. The controller is here applied to a Ytterbium doped MOPA fiber laser. The control algorithm regulates the pump power to limit the amplifier gain, so operation is restricted to the boundaries set by damage threshold or nonlinearities. In particular, it allows low pulse repetition frequency operation down to single shot and arbitrary gating.

We demonstrate for the first time a fibre Raman laser delivering record-setting energy (60 nJ) of double-scale picofemtosecond pulses at wavelength of 1270 nm, which is of great interest in bio-medical applications. A Stokes wave was generated in single-stage Raman conversion of amplified radiation from a mode-locked F8 Yb-doped laser passing through a synchronously pumped phosphosilicate fibre ring cavity. The converted radiation at 1270 nm amounting to 47% of the total Raman laser output was maximised by tuning the repetition rate of the pulses generated in the Yb-doped master oscillator to the fundamental inter-mode spacing frequency of the phosphosilicate fibre cavity.