High power single frequency lasers are very attractive for a wide range of applications such as nonlinear conversion, gravitational wave sensing or atom trapping. Power scaling in single frequency regime is a challenging domain of research. In fact, nonlinear effect as stimulated Brillouin scattering (SBS) is the primary power limitation in single frequency amplifiers. To mitigate SBS, different well-known techniques has been improved. These techniques allow generation of several hundred of watts . Large mode area (LMA) fibers, transverse acoustically tailored fibers , coherent beam combining and also tapered fiber  seem to be serious candidates to continue the power scaling. We have demonstrated the generation of stable 200W output power with nearly diffraction limited output, and narrow linewidth (Δν<30kHz) by using a tapered Yb-doped fiber which allow an adiabatic transition from a small purely single mode input to a large core output.
Thermally induced transverse modal instabilities (TMI) have attracted these five years an intense research efforts of the entire fiber laser development community, as it represents the current most limiting effect of further power scaling of high power fiber laser. Anyway, since 2014, a few publications point out a new limiting thermal effect: fiber modal degradation (FMD). It is characterized by a power rollover and simultaneous increase of the cladding light at an average power far from the TMI threshold together with a degraded beam which does not exhibit temporal fluctuations, which is one of the main characteristic of TMI.
We report here on the first systemic experimental study of FMD in a high power photonic crystal fiber. We put a particular emphasis on the dependence of its average power threshold on the regime of operation. We experimentally demonstrate that this dependence is intrinsically linked to regime-dependent PD-saturated losses, which are nearly three times higher in CW regime than in short pulse picosecond regime. We make the hypothesis that the existence of these different PD equilibrium states between CW regime and picosecond QCW pulsed regime is due to a partial photo-bleaching of color centers in picosecond regime thanks to a higher probability of multi-photon process induced photobleaching (PB) at high peak power. This hypothesis is corroborated by the demonstration of the reversibility of the FMD induced in CW regime by simply switching the seed CW 1064 nm light by a short pulse, picosecond oscillator.
We report a high performance, fully monolithic 40 μm core, Yb-doped photonic crystal fiber amplifier module. The developed fused combiner allows us to couple 6 pumps of 50 W at 976 nm and 5 W of signal at 1064 nm in the PCF amplifier. We then produced up to 210 W of average power at 1064 nm which is the highest power ever delivered by a fully monolithic PCF amplifier. The module is entirely thermally controlled in a rugged package, and has run more than 25 days at > 100W average power with an excellent peak to peak power stability < 1%.
Directing high laser power spatially and temporally is of major interest for various applications. We developed a
compact and efficient system based on a DMD and consisting of a homemade multimode high-brightness fiber splitter
and a 60-Watt laser diode. This design enables computer-controlled distribution of several Watts of laser power to each
or several optical fibers in the bundle consisting of 7 fibers in this paper (but it can be extended to 19, 37 or more fibers).
The coupling efficiency and extinction ratio were measured and optimized. An overall efficiency of about 9% was
demonstrated by considering all losses due to DMD efficiency, geometric fill factor and fiber coupling efficiency, with
extinction ratios between 20 and 45dB.
Pump-probe techniques are widely used to measure events on time scales much shorter than
the resolution of electronic detectors, and are applied in such diverse fields as ultrafast
spectroscopy, photo-acoustics, TeraHertz imaging, etc. In ultrafast photoacoustics
measurements for instance, a pump beam launches in the sample acoustic waves, which are
detected by a second, temporally shifted probe beam. Typical detection methods rely on very
small changes in the reflection coefficient of the sample surface, requiring an averaging of the
signal to improve the signal to noise ratio. Traditional pump-probe methods use a mechanical
delay line to shift the two pulses in the time domain, where each measurement point
corresponds to a single mechanical position of the delay line. Although very efficient for
small measurement ranges, extending this method in the hundreds of picoseconds or
nanosecond lead to a very long acquisition time, and unpractical length for the delay line.
We present a new, compact detection system, using a compact dual-oscillator ultrafast
laser system, specifically designed for pump-probe measurements over time scales as long 20
ns, with a sub-picosecond resolution. This system does not use any mechanical delay line, and
allows for extremely fast acquisition time.