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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.

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).

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.

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.

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.

This work presents a review on the effect of transverse mode instability in high-power fiber laser systems and the corresponding investigations led worldwide over the last decade. This manuscript includes the description of the experimental observations, the physical origin of this effect, as well as some of the proposed mitigation strategies., Comment: (c) 2020 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited

In this Letter, we report on the generation of 1060 W average power from an ultrafast thulium-doped fiber chirped pulse amplification system. After compression, the pulse energy of 13.2 μJ with a pulse duration of 265 fs at an 80 MHz pulse repetition rate results in a peak power of 50 MW spectrally centered at 1960 nm. Even though the average heat-load in the fiber core is as high as 98 W/m, we confirm the diffraction-limited beam quality of the compressed output. Furthermore, the evolution of the relative intensity noise with increasing average output power has been measured to verify the absence of transversal mode instabilities. This system represents a new average power record for thulium-doped fiber lasers (1150 W uncompressed) and ultrashort pulse fiber lasers with diffraction-limited beam quality, in general, even considering single-channel ytterbium-doped fiber amplifiers.

A phase shift between the modal interference pattern and the thermally-induced refractive index grating is most likely the ultimate trigger for the damaging effect of transverse mode instabilities (TMI) in high-power fiber laser systems. By using comprehensive simulations, the creation and evolution of a thermally-induced phase shift is explained and illustrated in detail. It is shown that such a phase shift can be induced by a variation of the pump power. The gained knowledge about the generation and evolution of the phase shift will allow for the development of new mitigation strategies for TMI.

We propose, to the best of our knowledge, the first mitigation strategy for TMI based on controlling the phase shift between the thermally-induced index grating and the modal intensity pattern. In particular, in this work we present a study of transverse mode instabilities in burst operation in a high-power fiber laser system. It is shown that, with a careful choice of the parameters, this operation regime can potentially lead to the mitigation of TMI by forcing an energy transfer from the higher-order-modes into the fundamental mode during the burst.

Thermally-induced refractive index gratings (RIG) in high-power fiber laser systems lead to transverse mode instabilities (TMI) above a certain average power threshold. The effect of TMI is currently the main limitation for the further average power scaling of fiber lasers and amplifiers with nearly diffraction-limited beam quality. In this work we experimentally investigate, for the first time, the growth of the RIG strength by introducing a phase-shift between the RIG and the modal interference pattern in a fiber amplifier. The experiments reveal that the RIG is strong enough to couple energy between different transverse modes even at powers significantly below the TMI threshold, provided that the introduced phase-shift is high enough. This indicates that, as the strength of the RIG further increases with increasing average output power, the RIG becomes more and more sensitive to even small noise-induced phase-shifts, which ultimately trigger TMI. Furthermore, it is shown that a beam cleaning also occurs when a positive phase-shift is introduced, even above the TMI threshold. This finding will pave the way for the development of a new class of mitigation strategies for TMI, which key feature is the control of the introduced phase-shift.

In this contribution we demonstrate the nonlinear pulse compression of an ultrafast thulium-doped fiber laser down to 14 fs FWHM duration (sub-3 optical cycles) at a record average power of 43 W and 34.5 μJ pulse energy. To the best of our knowledge, we present the highest average power few-cycle laser source at 2 μm wavelength. This performance level in combination with GW-class peak power makes our laser source extremely interesting for driving high-harmonic generation or for generating mid-infrared frequency combs via intra-pulse frequency down-conversion at an unprecedented average power. The experiments were enabled by an ultrafast thulium-doped fiber laser delivering 110 fs pulses at high repetition rates, and an argon gas-filled antiresonant hollow-core fiber (ARHCF) with excellent transmission and weak anomalous dispersion, leading to the self-compression of the pulses. We have shown that ARHCFs are well-suited for nonlinear pulse compression around 2 μm wavelength and that this concept features excellent power handling capabilities. Based on this result, we discuss the next steps for energy and average power scaling including upscaling the fiber dimensions in order to fully exploit the capabilities of our laser system, which can deliver several GW of peak power. This way, a 100 W-class laser source with mJ-level few-cycle pulses at 2 μm wavelength is feasible in the near future.

The state of the art of science and technology in fiber lasers and amplifiers is reviewed. Prospects for future power scaling using advanced fiber designs in combination with modern laser and amplifier architectures are discussed.

It has been recently shown that photodarkening can significantly reduce the mode instability threshold in high power Yb-doped fiber amplifiers, thus resulting in an even more severe limitation to the scaling of the output average power of these systems. Therefore, an efficient reduction of photodarkening in an Yb-doped active fiber will lead to very significant gains in the output average power delivered by such systems. In this context, it has been reported that photodarkening can be significantly mitigated when co-doping a fiber core with Al and P, which makes this approach potentially appealing to increase the TMI threshold. Unfortunately co-doping the fiber core with Al and P also alters the effective cross-sections of the fiber, which has repercussion in the amplification efficiency. Thus, a fiber with a higher P concentration will exhibit lower cross-sections, therefore requiring a higher Yb-ion concentration to reach a certain desired amplification efficiency. However, increasing the Yb-ion concentration leads to higher photodarkening losses, which might potentially counteract the benefits of using P co-doping. In this paper we present a comparative analysis of the expected performance of different fiber amplifiers for a given constant average heat-load and amplification efficiency as a function of the ratio of Al:P concentration in the fiber core. This study indicates which core compositions are more beneficial for increasing the mode instability threshold in Yb-doped high-power fiber amplifier systems.

Coherent beam combining refers to the process of generating a bright output beam by merging independent input beams with locked relative phases. We report the first quantum mechanical noise limit calculations for coherent beam combining and compare our results to quantum-limited amplification. We highlight a favourable noise scaling for both pure and noisy input states.

We present nonlinear self-compression of ultrashort pulses from a powerful thulium- doped fiber laser using a gas-filled antiresonant hollow-core fiber. 104 sub-2 cycle pulses have been generated around 1.85 µm at 98 kHz repetition rate.

We present a thulium-doped fiber chirped pulse amplifier delivering 790 W average power, 250 fs pulses with diffraction-limited beam quality and a relative intensity noise of 0.23% (RF-band 10 Hz to 1 MHz).

We demonstrate a first fiber-laser driven two-color air-plasma based THz source operating at 30 kHz repetition rate. The generated THz spectrum covers 0.1 - 5 THz and has an average power in the mW-range.

We present efficient, ultrabroadband intrapulse difference-frequency generation driven by a Tm-doped fiber laser resulting in an average mid-infrared output power of 450 mW spanning more than one octave (−10 dB width) at a central wavelength of 12 µm.

During the past decade coherent anti-Stokes Raman scattering (CARS) microscopy has evolved to one of the most powerful imaging techniques in the biomedical sciences, enabling the label-free visualization of the chemical composition of tissue in vivo in real time. While the acquisition of high-contrast images of single cells up to large tissue sections enables a wide range of medical applications from routine diagnostics to surgical guidance, to date CARS imaging is employed in fundamental research only, essentially because the synchronized multiple wavelength pulsed laser sources required for CARS microscopy are large, expensive and require regular maintenance. Laser sources based on optical fibers can overcome these limitations combining highest efficiency and peak powers with an excellent spatial beam profile and thermal stability. In this review we summarize the different fiber-based approaches for laser sources dedicated to coherent Raman imaging, in particular active fiber technology and passive fiber-based frequency conversion processes, i.e. supercontinuum generation, soliton self-frequency shift and four-wave mixing. We re-evaluate the ideal laser parameters for CARS imaging and discuss the suitability of different laser concepts for turn-key operation required for routine application in clinics.

High-average power laser sources delivering intense few-cycle pulses in wavelength regions beyond the near infrared are promising tools for driving the next generation of high-flux strong-field experiments. In this work, we report on nonlinear pulse compression to 34.4 μJ-, 2.1-cycle pulses with 1.4 GW peak power at a central wavelength of 1.82 μm and an average power of 43 W. This performance level was enabled by the combination of a high-repetition-rate ultrafast thulium-doped fiber laser system and a gas-filled antiresonant hollow-core fiber.

Among other modern imaging techniques, stimulated Raman scattering (SRS) requires an extremely quiet, widely wavelength tunable laser [1], which, up to now, is unheard of in fiber laser systems. Here we present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber [2]. We employ pulses from an all-fiber frequency and repetition rate tunable laser, which after amplification and subsequent conversion in the linear OPO cavity arrangement are converted to wavelength ranges of 764–960 nm and 1164–1552 nm at a repetition rate of 9.5 MHz. Thus, all biochemically relevant Raman shifts from 922 to 3322 cm−1 may be addressed in combination with an additional output, which is tunable from 1024 to 1052 nm. This ultra-low-noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging.

The noise characteristics of high-power laser systems are becoming more and more important for a host of high-end applications that include, for example, attosecond pulse generation [1]. For these experiments, the noise level of the driving laser is very important since the HHG-process that drives the attosecond-pulse generation depends strongly on the driving laser field. Up to now, only thin-disk lasers have been systematically investigated regarding their noise characteristics [2]. In a previous work [3], we began with the examination of the transfer function of the relative intensity noise (RIN) in a high-power fiber laser system, and found out that it can be significantly attenuated due to saturation effects. This is in accordance with previous theoretical works [4]. We also found out that the noise characteristics of the fiber laser system depends on fiber design.

In recent years the evolution of the average output power emitted by high-power fiber-lasers and amplifiers has reached a level at which the onset of thermal effects has been observed. The most detrimental of these effects is the phenomenon of transverse mode instabilities (TMI) [1]. These instabilities are characterized by a sudden onset of spatial and temporal fluctuations in a formerly stable beam once a certain average output power threshold has been reached [2]. TMI are currently the most limiting effect for the further average output power scaling of fiber-laser systems and amplifiers with nearly diffraction-limited beam quality. Therefore, the development of mitigation strategies for TMI is critical to further enhance the performance of this technology.

The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Even though a significant amount of knowledge on TMI in single-pass fiber amplifiers has been generated in the last years, relatively little is known about this effect in multi-pass amplifiers and oscillators. In this contribution TMI is experimentally investigated in a double-pass fiber amplifier, for the first time to the best of our knowledge. The TMI threshold was found to be significantly lower in the double-pass configuration than in the single-pass arrangement. Furthermore, the investigations unveiled a complex dynamic behavior of the instabilities in the double-pass fiber amplifier.

Transverse mode instabilities (TMI) have become a very serious problem for the further scaling of the average power of fiber laser systems. Recently the strong impact that photodarkening (PD) has on the TMI threshold of Yb-doped fiber laser systems has been revealed. This is a remarkable finding since it opens the door to a significant increase of the average power of fiber laser systems in the near future. The key to achieve this is to reduce the amount of PD losses in the fiber, which can be done with an optimization of the glass composition in the fiber. In this work we perform a theoretical study on the impact that co-dopants such as Al and P have on PD and on the TMI threshold. This analysis tries to find the optimum glass composition from the point of view of TMI. It is shown that in a short rod type fiber, changing the glass composition only leads to a modest increase of the TMI threshold due to the degradation of the cross-sections. This demonstrates that the optimization of the glass cannot be done attending only to the PD losses at the cost of the laser cross-sections. In spite of this, changing the glass composition can bring benefits in pulsed operation in terms of the stored energy. Additionally, other fiber geometries different from the rod-type can benefit in a greater degree by introducing co-dopants in the glass.

The noise characteristics of high-power fiber lasers, unlike those of other solid-state lasers such as thin-disks, have not been systematically studied up to now. However, novel applications for high-power fiber laser systems, such as attosecond pulse generation, put stringent limits to the maximum noise level of these sources. Therefore, in order to address these applications, a detailed knowledge and understanding of the characteristics of noise and its behavior in a fiber laser system is required. In this work we have carried out a systematic study of the propagation of the relative intensity noise (RIN) along the amplification chain of a state-of-the-art high-power fiber laser system. The most striking feature of these measurements is that the RIN level is progressively attenuated after each amplification stage. In order to understand this unexpected behavior, we have simulated the transfer function of the RIN in a fiber amplification stage (~80μm core) as a function of the seed power and the frequency. Our simulation model shows that this damping of the amplitude noise is related to saturation. Additionally, we show, for the first time to the best of our knowledge, that the fiber design (e.g. core size, glass composition, doping geometry) can be modified to optimize the noise characteristics of high-power fiber laser systems.

Experimental demonstrations of Tm-doped fiber amplifiers (typically in CW- or narrow-band pulsed operation) span a wavelength range going from about 1700 nm to well beyond 2000 nm. Thus, it should be possible to obtain a bandwidth of more than 100 nm, which would enable sub-100 fs pulse duration in an efficient, linear amplification scheme. In fact, this would allow the emission of pulses with less than 20 optical cycles directly from a Tm-doped fiber system, something that seems to be extremely challenging for other dopants in a fused silica fiber. In this contribution, we summarize the current development of our Thulium-doped fiber CPA system, demonstrate preliminary experiments for further scaling and discuss important design factors for the next steps. The current single-channel laser system presented herein delivers a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. Special care has been taken to reduce the detrimental impact of water vapor absorption by placing the whole system in a dry atmosphere housing ( 100 W average power can be expected in the near future.

The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Thus, it is of high interest to develop efficient mitigation strategies to further enhance the performance of fiber laser systems. By actively modulating the pump power of an Yb-doped rod-type fiber amplifier, it was possible to weaken the thermally-induced refractive index grating along the fiber and, thus, to mitigate TMI to a large extent. A significant advantage of this approach is that it can be easily integrated in any existing fiber-laser system since no further optical components are needed. A function generator connected to the pump diode driver was used to achieve the modulation. With this setup we were able to extract a fully stabilized beam at ~ 1.5 times above the TMI threshold. Furthermore, a stabilization of the beam was still feasible at an average output power of 628 W, which is more than three times higher than the free-running TMI threshold of that particular fiber under identical conditions (e.g. seed power). This is the highest average output power reported from a single-channel rod-type fiber amplifier with a high-quality stabilized beam, to the best of our knowledge.

Thulium-doped fiber lasers are an attractive concept for the generation of mid-infrared (mid-IR) ultrashort pulses around 2 μm wavelength with an unprecedented average power. To date, these systems deliver >150 W of average power and GW-class pulse peak powers with output pulse durations of a few hundreds of fs. As some applications can greatly benefit from even shorter pulse durations, the spectral broadening and subsequent temporal pulse compression can be a key enabling technology for high average power few-cycle laser sources around 2 μm wavelength. In this contribution we demonstrate the nonlinear compression of ultrashort pulses from a high repetition rate Tm-doped fiber laser using a nitrogen gas-filled hollow capillary. Pulses with 4 GW peak power, 46 fs FWHM duration at an average power of 15.4 W have been achieved. This is, to the best of our knowledge, the first 2 μm laser delivering intense, GW-pulses with sub 50-fs pulse duration and an average power of >10 W. Based on this result, we discuss the next steps towards a 100 W-level, GW-class few-cycle mid-IR laser.

Ultrashort-pulse laser systems are an enabling technology for numerous applications. The stability of such systems is especially crucial for frequency metrology and high precision spectroscopy. Thulium-based fiber lasers are an ideal starting point as a reliable and yet powerful source for the nonlinear conversion towards the mid-IR region. Recently, we have demonstrated that nonlinear self-compression in a fused silica solid-core fiber allows for few-cycle pulse duration with up to 24 MW peak power using a high-repetition rate thulium-based fiber laser system operating at around 2 μm wavelength [1]. This experiment operates near the self-focusing limit of about 24 MW for circular polarization, which increases the requirements for the system stability due to the risk of a fiber damage. Here, we present a self-protecting nonlinear compression regime allowing for long-term operation and high output-pulse stability with very similar output performance.

Among other modern imaging techniques, stimulated Raman Scattering (SRS) requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber laser systems. We present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber. We employ an all-fiber frequency and repetition rate tunable laser in order to enable wideband conversion in the linear OPO cavity arrangement, the signal and idler radiation can be tuned between 764 and 960 nm and 1164 and 1552 nm at 9.5 MHz. Thus, all biochemically relevant Raman shifts between 922 and 3322 cm-1 may be addressed in combination with a secondary output, which is tunable between 1024 and 1052 nm. This ultra-low noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging. We measure the relative intensity noise of this output beam at 9.5 MHz to be between -145 and -148 dBc, which is low enough to enable high-speed SRS imaging with a good signal-to-noise ratio. The laser system is computer controlled to access a certain energy differences within one second. Combining FWM based conversion, with all-fiber Yb-based fiber lasers enables the construction of the first automated, turn-key and widely tunable fiber laser. This laser concept could be the missing piece to establish CRS imaging as a reliable guiding tool for clinical diagnostics and surgical guidance.

The combination of high-repetition-rate ultrafast thulium-doped fiber laser systems and gas-based nonlinear pulse compression in waveguides offers promising opportunities for the development of high-performance few-cycle laser sources at 2 μm wavelength. In this Letter, we report on a nonlinear pulse compression stage delivering 252 μJ, sub-50 fs-pulses at 15.4 W of average power. This performance level was enabled by actively mitigating ultrashort pulse propagation effects induced by the presence of water vapor absorptions.

We present the nonlinear self-compression of pulses from a high repetition rate thulium-doped fiber laser system using a gas-filled antiresonant hollow-core fiber. Sub-3-cycle pulses with several GW peak power at 21.4 W of average power have been generated.

The term coherent Raman scattering (CRS) summarizes imaging methods that do not only poses inherent 3D sectioning capabilities but also enables chemical sensitive detection of living tissue. Instead of relying on external markers that are potentially changing its behavior, CRS measures the vibration frequencies of the molecular bonds by the means of inelastic scattering known as Raman scattering. Among these techniques, stimulated Raman Scattering (SRS) has the advantage to be background-free but requires an extremely quiet, widely tunable dual-wavelength laser source. Up to now, such features could only be provided by large and sensitive laser systems that limited the used of this imaging modality to specialized laser laboratories. We present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber. We employ an all-fiber frequency- and repetition-rate tunable laser in order to enable wideband conversion in the linear OPO cavity arrangement. The signal and idler radiation can be tuned between 764 and 960 nm and 1164 and 1552 nm at 9.5 MHz. Thus, all biochemically relevant Raman shifts between 922 and 3322 cm−1 may be addressed in combination with a secondary output, which is tunable between 1024 and 1052 nm. This ultra-low noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging. We measure the relative intensity noise of this output beam at 9.5 MHz over the tuning range to be between −145 and −148 dBc, which is low enough to enable direct high-speed SRS imaging with a good signal-to-noise ratio without the use of balanced detection. The laser system is computer controlled to access a certain energy differences within one second. Combining FWM based conversion, with all-fiber Yb-based fiber lasers enables the construction of the first automated, turn-key and widely tunable fiber laser. This laser concept could be the missing piece to establish CRS imaging as a reliable guiding tool for clinical diagnostics and surgical guidance.

Tm-doped fused silica fibers allow for the efficient, linear amplification of broad spectra in the 1.9 µm wavelength region. The single-channel fiber chirped pulse amplification system presented herein delivers a pulse-peak power of >5 GW and a nearly transform-limited pulse duration of 90 fs at 10 kHz repetition rate. An increase of the repetition rate to 100 kHz allowed for unprecedented 45 W average power and >4 GW peak power in this wavelength regime.

In this Letter, we present, to the best of our knowledge, the largest effective single-mode fiber reported to date. The employed waveguide is a passive large pitch fiber (LPF), which shows the core area scaling potential of such a fiber structure. In particular, we achieved stable single-transverse mode transmission at a wavelength of 1.03 μm through a straight passive LPF with a pitch of 140 μm, resulting in a measured mode-field diameter of 205 μm.

High-power fibre lasers are in demand for industrial, defence and scientific applications. This review provides an overview of the present state of the art in the field and discusses present challenges and the future outlook.

Spatially and spectrally resolved imaging (S2) is a very sensitive, robust and elegant method to measure the power of excited modes in a fiber. For the common reconstruction technique an approximation is necessary, which is based on dominant fundamental mode content. In this work we present several algorithms that significantly improve the accuracy of modal reconstruction for weak excited fundamental mode content. We show that in some cases general analytical solutions exist that can completely overcome the former limitation. In addition, we introduce an iterative procedure to improve the mode modal reconstruction independent of the specific used algorithm.

This work presents an overview on the latest advancements in the understanding of transverse mode instabilities (TMI) together with guidelines to optimize high power fiber laser systems from the thermal point of view.

The average output power of Yb-doped fiber amplifier systems is currently limited by the onset of transverse mode instabilities. Besides, it has been recently shown that the transverse mode instability threshold can be significantly reduced by the presence of photodarkening in the fiber. Therefore, reducing the photodarkening level of the core material composition is the most straightforward way to increase the output average power of fiber amplifier systems but, unfortunately, this is not always easy or possible. In this paper we present guidelines to optimize the output average power of fiber amplifiers affected by transverse mode instabilities and photodarkening. The guidelines derived from the simulations do not involve changes in the composition of the active material (except for its doping concentration), but can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration and the core conformation, among others, will be studied and discussed.

The large-pitch fiber design and its fundamental operating principle – the delocalization of higher-order modes – have enabled the current performance records of ultrashort-pulse fiber laser systems. However, further average power scaling is limited by transverse mode instabilities. This effect has been linked to the average thermal load inside the active fiber and, consequently, it is strongly enhanced for short fiber amplifiers that are typically used for ultrashort-pulse amplification. Recently, it has also been discovered that photo-darkening is an additional, very important heat source in an active fiber that can be as strong as the quantum defect. A reduction of the Yb-doping concentration in high-power active fibers is a promising way to mitigate this problem. In this contribution we propose a new large-pitch design which has an increased delocalization of higher-order modes and, at the same time, allows for a larger core to clad ratio that enables reducing the Yb-ion concentration while maintaining a similar amplification efficiency. A comprehensive laser simulation that includes modal properties, thermo-optical effects and photo-darkening is used to highlight the important benefits of this design. Additionally, this novel and compact fiber design can be a promising unit-cell for a multi-core fiber, which should ultimately enable the development of multi-kW average power and multi-mJ pulse energy ultrashort-pulse fiber lasers in the near future.

Stimulated Raman Scattering requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber lasers. We present a compact and maintenance-free optical parametric oscillator based on degenerate four-wave mixing in a photonic crystal fiber. By employing an all-fiber frequency and repetition rate tunable laser as a seed source, we are able to generate tunable light between 1015 and 1065 nm. After amplification and subsequent conversion in the fiber OPO, signal and idler radiation between 785 and 960 nm and 1177 and 1500 nm may be generated with a repetition rate of 9 MHz. Therefore, we are able to address Raman shifts between 910 and 3030 cm-1. An additional output provides the Stokes radiation at 18 MHz required for the SRS process, which is passively synchronized to the tunable radiation. We measure the relative intensity noise of the Stokes beam at 9 MHz to be -150 dBc enabling high speed SRS imaging with a good signal-to-noise ratio. The combination of FWM based conversion, coupled with all-fiber Yb-based fiber lasers allows for the first turn-key, widely tunable and extremely compact laser systems developed for applications of CRS microscopy in clinics. This source could very well be the missing key instrument that CRS imaging requires for its real world transition.

Complementing ultrafast thulium-doped fiber-laser systems with a subsequent nonlinear pulse compression stage can enable unique laser parameters at around 2 μm operation wavelength. Significant pulse shortening and peak power enhancement have been accomplished using a fused silica solid-core fiber. In this fiber a pulse peak power of 24 MW was achieved without catastrophic damage due to self-focusing. As compared to operation in the well-explored 1 μm wavelength region, increasing the emission wavelength to 2 μm has a twofold advantage for nonlinear compression in fused-silica solid-core fibers. This is because, on the one hand the self-focusing limit scales quadratically with the wavelength. On the other hand the dispersion properties of fused silica allow for self-compression of ultrashort pulses beyond 1.3 μm wavelength, which leads to strong spectral broadening from very compact setups without the need for external compression. Using this technique we have generated 1.1 μJpulses with 24 fs FWHM pulse duration (

The maximum average power that can be emitted from an ytterbium-doped fiber-laser system is estimated. The analysis takes into account all the effects known so far that may limit the average power including transverse mode instabilities and photo darkening. Hereby, the recent experimental observation that transverse mode instabilities depend on the average heat load in a fiber amplifier is exploited. The results of this analysis show that there are three main limiting effects: stimulated Raman scattering, the brightness of the pump laser and transverse mode instabilities. Moreover, the analysis suggests that, disregarding possible practical constrains, the average output power of a fiber laser system can be, in principle, increased up to 70kW.

In this work we present guidelines to increase the transverse mode instability threshold of high power fiber amplifiers and also, for the first time to the best of our knowledge, of fiber oscillators. These guidelines do not involve changes in the composition of the active material (except for its doping concentration), but they can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration, the core conformation, the pump configuration and the mirror reflectivities in a fiber oscillator will be studied and discussed.

Thulium-fiber CPA with 200 fs pulses and >1 GW peak power at 50kHz repetition-rate enables nonlinear-compression to sub-30 fs with 140 µJ pulse energy at 2 µm wavelength - a unique laser for high photon-energy HHG.

Thulium-doped fibers with very large mode-field areas offer new opportunities for the power scaling of mid-IR ultrashort-pulse laser sources. The current single-channel laser system presented herein delivers a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. A tenfold improvement in peak power has been achieved by optimizing the pulse shape and by mitigating atmospheric water absorption. Ongoing efforts include the full exploitation of the thulium-gain bandwidth and the coherent combination of multiple amplifier channels aiming for peak powers of 20 GW at MHz repetition rate in the near future.

Thulium-doped fibers with ultra large mode-field areas offer new opportunities for the power scaling of mid-IR ultrashort-pulse laser sources. Here, we present a laser system delivering a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. This performance level has been achieved by optimizing the pulse shape, reducing the overlap with atmospheric absorption lines, and incorporating a climate chamber to reduce the humidity of the atmospheric environment.

A new liquid-level sensor with a multipoint layout is presented, which is based on power loss arising in laterally polished bent sections prepared along a plastic optical fibre. The polishing is applied to the fibre surface on top of several U-shaped bends until part of the core is also removed. The resultant bare flat area on the core is an elliptic surface in direct contact with the medium surrounding the fibre. Any variation in the optical and geometric parameters characterising our multimode fibre is analysed, since it will cause changes in the propagation of light along the polished bends. Experimental results included in the paper correspond to the prototype for our sensor, which consists of eight sensing probes placed sequentially along the fibre, in combination with an optoelectronic unit working as a liquid-level transducer.