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.
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.
In this communication, the authors report on the first high peak-power emission obtained using a solid non-filamented core fully-aperiodic large pitch fiber manufactured by the REPUSIL method which is based on the sintering and vitrification of micrometric doped silica powders. Using a simple amplifier stage based on a 75 cm long piece of a fullyaperiodic large pitch fiber with a fiber core of 50 μm, an average output power of 95 W was achieved with an available pump power of 175 W, corresponding to an optical-to-optical efficiency of 54 %. The peak power reaches about 35 kW for pulse duration of 200 ps at a repetition rate of 13.5 MHz. A recent evolution of our set-up using a seeder delivering an average power of 4 W at 1 MHz with a pulse duration of 50 ps led to the emission of 71.4W in average power corresponding to a peak power of 1.42 MW. These results present the first demonstration of high average and high peak power in pulsed regime for these fibers.
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