We obtained 51 W of UV laser light at 343nm for 8 ns temporally square-shape pulse at 400 kHz repetition rate which corresponds to a peak power of 42.5kW and a conversion rate of 38% from a 133W linearly polarized signal at 1030nm. This high UV power is achieved by third harmonic generation of infrared beam which is generated thanks to a newly developed Ytterbium-doped rod-type high-power amplifier effectively singlemode fiber with a hybrid hexagonal and aperiodic cladding design. Two fibers with MFD at 47µm and 67µm were tested. The 47µm MFD fiber allow to reach up to 200W of singlemode signal before the TMI appearance. This fiber can deliver 150 W of 1030 nm signal with a 250 W pump light, for different nanosecond pulse durations and repetition rates with excellent beam quality (M²<1.1).
In this communication, we report for the first time on a homemade 55 µm core VLMA “Yb-free” Er-doped aluminosilicate double-clad fiber manufactured by the REPUSIL powder sintering technology and its implementation within two different laser configurations emitting around 1560 nm, both pumped at 976 nm. First, a free-running free-space CW oscillator delivers up to 40 W of average power with optical-to-optical efficiency of 30 % and near-diffraction-limited beam, despite the large core size. In a second experiment, the fiber is used as the main amplifier of a MOPA system delivering up to 10 nJ pulses at GHz repetition rate.
We developed two Ytterbium-doped rod-type effective singlemode fibers based on new hexagonal FA-LPF design, exhibiting MFD of 47μm and 67μm. Both fibers can deliver 150W of 1030nm signal for 250W of pump power, characterized in a MOPA set-up for different nanosecond pulse durations and repetition rate with excellent beam quality (M2 ⪅ 1.1). Using the 47μm MFD fiber, TMI threshold has been measured for signal power slightly higher than 200W. Using the 67μm MFD fiber, we performed, through a third harmonic generation, the creation of 51W signal power at 343nm for 8ns temporally square pulse at 400 kHz repetition rate.
Many applications such as nonlinear microscopy and strong field optoelectonics require high-energy (> 100 nJ) ultrashort (< 100 fs) pulses above 1.55 µm out of a singlemode fiber. Here, we report on high-energy amplification in tapered Er-doped fiber fabricated by the powder technique. The system based on direct amplification is free from stretcher and compressor units. We generate 90 fs MW-class pulses at 1600 nm by amplification and management of nonlinear effects in the tapered fiber. Despite the output 100 µm core diameter, the emitted beam is near-diffraction limited.
In this work we present investigation of TMI in Yb-doped FA-LPF using spatially and temporally resolved imaging. We present ST-measurements that are clearly describing the system noises and that can be used to optimize amplifier systems in order to mitigate TMI. Comparative TMI measurements between the same FA-LPF in amplifier and laser configuration are also presented. These measurements show similar results with respect to optical efficiencies but large differences with respect to TMI.
Fiber lasers are a great source for tunable lasers due to the wide and relatively flat gain spectra of rare earth transitions in a glassy host (as compared to crystals). Thulium (Tm)-doped fibers, in particular, offer an extremely wide tunability of up to 330 nm in the 2μm wavelength region in a dual gain module configuration1. More recently, new concepts have emerged, which allow the synchronized emission of two or even more wavelengths2. These sources are particularly useful for nonlinear frequency conversion via four-wave mixing (FWM) or difference frequency generation (DFG). We will present a very versatile fiber-integrated approach based on Fiber-Bragg-Grating (FBG) arrays implemented in a theta-shaped cavity. The Tm-doped fiber source emits typical average powers of 0.5W and is tunable from 1931nm to 2040nm. The emission linewidth follows the spectral characteristic of the FBG and is typically 30GHz in our case. This concept allows a constant wavelength-independent repetition rate as well as a synchronous emission of two or even three independently tunable wavelengths. The tuning is performed purely electronically by optical gating, and in addition the pulse duration can be tuned between 4ns and 25ns. The switching speed is very fast and was measured to be less than 10μs. These experiments will be contrasted with a different approach based on a VLMA fiber associated to a set of two volume Bragg gratings (VBG), one of them being angle-tunable. This concept allows pulsed (Q-switched) as well as CW operation and features a continuous and wider tunability of up to 144nm especially and also the dual wavelength mode. The output power was > 4.5W in CW mode and pulse peak power of 12kW have been obtained in the Q-switched mode with pulse durations of 25ns.
High power diffraction-limited 1064 nm fiber lasers operating in the nanosecond regime can be used for long-range LIDAR and micromachining applications. Peak power is limited by non-linearities, there is therefore an interest to develop fibers exhibiting a very large mode field effective area. New fibers are being developed in the frame of the 4F consortium ("French laser Fibers for Factories of the Future") to fulfill this need. We report on results obtained with a new 39 μm core diameter polarization maintaining ytterbium doped fiber that has been manufactured using the powder sintering technology. It features a large cladding absorption close to 20 dB/m at 976 nm (small signal) and a mode field diameter close to 32 μm. We built a pulsed MOPA. The preamplifier generates 2.5 ns pulses at 1064 nm with 8.5 W average power at 1 MHz pulse repetition frequency. The power amplifier is based on the 39 μm core fiber with 215/230μm hexagonal cladding counterpumped at 976 nm. It features 72 % slope efficiency delivering 72.2 W average power at a pulse repetition rate of 1 MHz. An end-cap was spliced to the fiber output to increase the damage threshold. At 100 kHz a peak power of 351 kW was measured for an average power of 59.9 W. The efficiency is then 70 %. We also studied the influence of the bending radius on the slope efficiency. We do not observe any slope efficiency reduction down to 25 cm bending diameter. It decreased to 68 % for the 20 cm bending diameter. The laser shows a quasisinglemode output beam with a good quality factor M2 of 1.2.
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.
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.
Yb-doped Photonic Crystal Fibers (PCFs) have triggered a significant power scaling into fiber-based lasers. However thermally-induced effects, like mode instability, can compromise the output beam quality. PCF design with improved Higher Order Mode (HOM) delocalization and effective thermal resilience can contain the problem. In particular, Fully- Aperiodic Large-Pitch Fibers (FA-LPFs) have shown interesting properties in terms of resilience to thermal effects. In this paper the performances of a Yb-doped FA-LPF amplifier are experimentally and numerically investigated. Modal properties and gain competition between Fundamental Mode (FM) and first HOM have been calculated, in presence of thermal effects. The main doped fiber characteristics have been derived by comparison between experimental and numerical results.
The power scaling of fiber lasers and amplifiers has triggered an extensive development of large-mode area fibers among which the most promising are the distributed mode filtering fibers and the large-pitch fibers. These structures enable for an effective higher-order modes delocalization and subsequently a singlemode emission. An interesting alternative consists in using the fully-aperiodic large-pitch fibers, into which the standard air-silica photonic crystal cladding is replaced by an aperiodic pattern made of solid low-index inclusions cladding. However, in such a structure, the core and the background cladding material surrounding it must have rigorously the same refractive index. Current synthesis processes and measurement techniques offer respectively a maximum resolution of 5×10-4 and 1×10-4 while the indexmatching must be as precise as 1×10-5 . Lately a gain material with a refractive index 1.5×10-4 higher than that of the background cladding material was fabricated, thus re-confining the first higher-order modes in the core. A numerical study is carried out on the benefit of bending such fully-aperiodic fiber to counteract this phenomenon. Optimized bending axis and radius have been determined. Experiments are done in a laser cavity operating at 1030 nm using an 88cm-long 51μm core diameter ytterbium-doped fiber. Results demonstrate an improvement of the M2 from 1.7 when the fiber is kept straight to 1.2 when it is bent with a 100 to 60 cm bend radius. These primary results are promising for future power scaling.
Over the last decade, significant work has been carried out in order to increase the energy/peak power provided by fiber lasers. Indeed, new microstructured fibers with large (or very large) mode area cores (LMA) such as Distributed Mode Filtering (DMF) fibers and Large-Pitch Fibers (LPF) have been developed to address this concern. These technologies have allowed diffraction-limited emission with core diameters higher than 80 μm, and have state-of-the-art performances in terms of pulse energy or peak power while keeping an excellent spatial beam quality. Although these fibers were designed to reach high power levels while maintaining a single transverse mode propagation, power scaling becomes quickly limited by the onset of transverse modal instabilities (TMI). This effect suddenly arises when a certain average power threshold is exceeded, drastically degrading the emitted beam quality. In this work, we investigate the influence of the core dimensions and the refractive index mismatch between the active core and the background cladding material, on the TMI power threshold in rod-type Fully-Aperiodic-LPF. This fiber structure was specifically designed to enhance the higher-order modes (HOMs) delocalization out of the gain region and thus push further the onset of modal instabilities. Using a 400W pump diode at 976 nm, the power scaling, as well as the spatial beam quality and its temporal behavior were investigated in laser configuration, which theoretically provides a lower TMI power threshold than the amplifier one due to the lack of selective excitation of the fundamental mode.
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