Thermal effects of critical passive and active kilowatt-splices in all fiber master-oscillator power amplifier (MOPA) are investigated. Proper designs for cooling apparatus are proposed and demonstrated experimentally, for the purpose of minimizing splice heating which is critical for the reliability of high power operation. By using these optimized methods, we have demonstrated numerically and experimentally for standard 125 µm, 200 µm and 400 µm Yb fibers temperature rise of critical active splice < 0,1 K/W.
We investigate hollow-core fibers for fiber delivery of high power ultrashort laser pulses. We have benchmarked 3 types of hollow-core fiber, respectively photonic bandgap, nested, and kagome fibers. nested and Kagome reach the 100W average power level, and are characterized experimentally for the delivery of 300 fs- 10 ps pulse range . Systematic experimental study of limitations in terms of beam quality, depointing, non linear effects, polarization or fiber coiling is reported.
Multi-kW combiner is a key component to achieve very high-power fiber laser. In this study, of interest for various industrial applications, two different optical designs were investigated for high brightness 6+1 to 1 KW-class combiners using 30/200 LMA output fiber.
The first one is a co-pumped combiner, which consists in making a bundle with 105/125 (0.22NA) multimode fibers with a 10/125 (0.1NA) signal fiber. This latter is tapered down to the output fiber diameter. An excellent multimode transmission up to 97% and a low single insertion loss below 0.5dB have been demonstrated.
The second one is a counter-pumped combiner, based on the development of a special technology to overcome the tapering of the 30/200 input signal fiber. We demonstrate the same specification concerning multimode transmission and signal insertion loss below 0.2dB.
The main issue consists of the reduction of the thermal heating, particularly for the counter pumped combiner, greatly dependent upon seeding and wavelengths operating conditions. To define power scaling limitations of the component, we have investigated the thermal effects origins and evolutions. These effects mainly depend on the type of loss along the optical structure, polymer absorption or contamination.
We will discuss on a process optimization, of packaging and CO2-laser-based processing machine to reach KW-class level fiber combiners. To the best of our knowledge, our component presents record performances, in term of high brightness conservation for 200µm LMA fibers, power handling, insertion loss and thermal optimization leading to a rise of 10°C/kW pumping at 976m.
ALPhANOV has developed expertise around the interfacing and integration of specialty optical fibers. We propose to present new developments concerning 100W-class fiber laser based on Yb-doped Large Mode Area fiber amplifier and a new plug-and-play connector for ultrashort pulse beam delivery by using Hollow Core Photonic Crystal Fibers.
In terms of high power fiber amplifier, we recently developed a high performance, fully monolithic PCF amplifier module. The module is based on the DC-200/40-PZ-Yb of NKT Photonics and on a homemade fiber fused component allowing us to couple up to 6 pumps of 50 W at 976 nm together with 5 W of signal, leading to an achieved power of 210 W at 1064 nm, which is to the best of our knowledge 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 100 days at > 100W average power with an excellent power stability < 1%.
Concerning fiber beam delivery solution, hollow core photonic crystal fiber has shown great potential for an industrial solution. To penetrate the industry, high power femtosecond lasers need low-loss stable and plug-and-play connectors to couple light into hollow-core single-mode fibers and carry it to the target. We recently developed such connector and tested it on different industrial femtosecond laser, 50W and 100W class. The new design of the connector compatible with vacuum or noble gas and 300W class femtosecond laser will also be presented.
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 [1]. Large mode area (LMA) fibers, transverse acoustically tailored fibers [2], coherent beam combining and also tapered fiber [3] 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.
KEYWORDS: Picosecond phenomena, Thermal effects, Fiber lasers, High power fiber amplifiers, Laser development, High power fiber lasers, Cladding, Photonic crystal fibers, Color centers
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.
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