High power high energy lasers have recently emerged as potential solution for several applications such as secondary rays generation, nuclear fusion and medical therapy. One major limitation of such systems for high energy extraction is the laser-induced damage threshold (LIDT) of laser components.
In this context, many studies have been devoted to the determination of the LIDT of laser materials under certain operational conditions and the identification of the limiting factors at the nanoseconds and femto/sub-picoseconds regime 1–3.However, these measurements do not consider that in most high intensity laser chains such as chirped pulsed amplification (CPA), pulses are stretched to larger duration such as hundreds of picoseconds. Thus, measuring the LIDT of laser materials under stretched pulses irradiation becomes critical.
In this work, we report a study of the influence of the coating treatment and fatigue in the LIDT of Yb:YAG crystals under stretched pulses by means of 1-on-1, Rasterscan and S-on-1 tests. We use a 1mJ, 1kHz laser (S-Pulse model from Amplitude Systèmes) modified to support 150 ps-pulse duration. We show a lower LIDT of the AR-coating compared to that of the HR-coating (7J/cm2) and preliminary outcomes point out a non-deterministic effect of the fatigue at this regime of pulse duration. These results show the importance of testing critical components at hundred-picoseconds regime for high power and high energy lasers.
1. Sozet, M. et al. Laser damage density measurement of optical components in the sub-picosecond regime. 40, 2–5 (2015).
2. Smith, A. V & Do, B. T. Bulk and surface laser damage of silica by picosecond and nanosecond pulses at 1064 nm. (2008).
3. Jensen, L. O. et al. Investigations on SiO 2 / HfO 2 mixtures for nanosecond and femtosecond pulses a Laser. 7842, 1–10 (2017).
A high intensity Gamma source is required for Nuclear Spectroscopy, it will be delivered by the interaction between accelerated electron and intense laser beams. Those two interactions lasers are based on a multi-stage amplification scheme that ended with a second harmonics generation to deliver 200 mJ, 5 ps pulses at 515 nm and 100 Hz. <p> </p>A t-Pulse oscillator with slow and fast feedback loop implemented inside the oscillator cavity allows the possibility of synchronization to an optical reference. A temporal jitter of 120 fs rms is achieved, integrated from 10 Hz to 10 MHz. <p> </p>Then a regenerative amplifier, based on Yb:YAG technology, pumped by fiber-coupled QCW laser diodes, delivers pulses up to 30 mJ. The 1 nm bandwidth was compressed to 1.5 ps with a good spatial quality: M2 of 1.1. This amplifier is integrated in a compact sealed housing (750 x 500 x 150 mm), which allows a pulse-pulse stability of 0.1 % rms, and a long-term stability of 1,9 % over 100 hours (with +/-1°C environment). <p> </p>The main amplification stage uses a cryocooled Yb:YAG crystal in an active mirror configuration. The crystal is cooled at 130 K via a compact and low-vibration cryocooler, avoiding any additional phase noise contribution, 340 mJ in a six pass scheme was achieved, with 0.9 of Strehl ratio. The trade off to the gain of a cryogenic amplifier is the bandwidth reduction, however the 1030 nm pulse was compressed to 4.4 ps. As for the regenerative amplifier a long-term stability of 1.9 % over 30 hours was achieved in an environment with +/-1°C temperature fluctuations <p> </p>The compression and Second Harmonics Generation Stages have allowed the conversion of 150 mJ of uncompressed infrared beam into 60 mJ at 515 nm.
A high intensity Gamma source is required for Nuclear Spectroscopy, it will be delivered by the interaction between accelerated electron and intense laser beams. Those two interactions lasers are based on a multi-stage amplification scheme that ended with a second harmonics generation to deliver 200 mJ, 3.5 ps pulses at 515 nm and 100 Hz. <p> </p>A t-Pulse oscillator with slow and fast feedback loop implemented inside the oscillator cavity allows the possibility of synchronization to an optical reference. A temporal jitter of 120 fs rms is achieved, integrated from 10 Hz to 10 MHz. <p> </p>Then a regenerative amplifier, based on Yb:YAG technology, pumped by fiber-coupled QCW laser diodes, delivers pulses up to 30 mJ. The 1 nm bandwidth was compressed to 1.5 ps with a good spatial quality: M<sup>2</sup> of 1.1. This amplifier is integrated in a compact sealed housing (750x500x150 cm), which allows a pulse-pulse stability of 0.1% rms, and a long-term stability of 1,9% over 100 hours (with +/-1°C environment). <p> </p>The main amplification stage uses a cryocooled Yb:YAG crystal in an active mirror configuration. The crystal is cooled at 130 K via a compact and low-vibration cryocooler, avoiding any additional phase noise contribution, 340 mJ in a six pass scheme was achieved, with 0.9 of Strehl ratio. The trade off to the gain of a cryogenic amplifier is the bandwidth reduction, however the 1030 nm pulse was compressed to 3.5 ps.
A gain switched pulsed laser based on ytterbium doped rod PCF type fiber is presented. The high performance pump
system was based on 976 nm laser diodes incorporating high speed and high current laser diode drivers with active
feedback loop based control that enable high pulse to pulse stability. Furthermore the temperature control ensure the
adequate output spectrum of the pump laser diodes in order to match maximum of the absorption peak of Yb doped
active medium. The pulses duration in range between 48 ns to 75 ns were achieved with peak powers up to 3.6 kW.
Further, the change of the laser output spectrum in regard to the pump pulse power is observed.
We demonstrate the amplification of a 1064nm pulse-programmable fiber laser with Large Pitch Rod-Type Fibers of various Mode field diameters from 50 to 70 μm. We have developed a high power fiber amplifier at 1064nm delivering up to 100W/1mJ at 15ns pulses and 30W/300μJ at 2ns with linearly polarized and diffraction limited output beam (M²<1.2). The specific seeder from ESI – Pyrophotonics Lasers used in the experiment allowed us to obtain tailored-pulse programmable on demand at the output from 2ns to 600ns for various repetition rates from 10 to 500 kHz. We could demonstrate square pulses or any other shapes (also multi-pulses) whatever the repetition rate or the pulse duration. We also performed frequency conversion with LBO crystals leading to 50W at 532nm and 25W at 355nm with a diffraction limited output. Similar experiments performed at 1032nm are also reported.
Many industrial applications such as glass cutting, ceramic micro-machining or photovoltaic processes require high average and high peak power Picosecond pulses. The main limitation for the expansion of the picosecond market is the cost of high power picosecond laser sources, which is due to the complexity of the architecture used for picosecond pulse amplification, and the difficulty to keep an excellent beam quality at high average power. Amplification with fibers is a good technology to achieve high power in picosecond regime but, because of its tight confinement over long distances, light undergoes dramatic non linearities while propagating in fibers. One way to avoid strong non linearities is to increase fiber’s mode area. Nineteen missing holes fibers offering core diameter larger than 80μm have been used over the past few years [1-3] but it has been shown that mode instabilities occur at approximately 100W average output power in these fibers . Recently a new fiber design has been introduced, in which HOMs are delocalized from the core to the clad, preventing from HOMs amplification . In these so-called Large Pitch Fibers, threshold for mode instabilities is increased to 294W offering robust single-mode operation below this power level . We have demonstrated a high power-high efficiency industrial picosecond source using single-mode Large Pitch rod-type fibers doped with Ytterbium. Large Pitch Rod type fibers can offer a unique combination of single-mode output with a very large mode area from 40 μm up to 100μm and very high gain. This enables to directly amplify a low power-low energy Mode Locked Fiber laser with a simple amplification architecture, achieving very high power together with singlemode output independent of power level or repetition rate.
We demonstrated growth of YAG, LuAG and CALGO single crystal fibers with doping Nd, Yb, Er, and Ce by the
micro-pulling-down technique. Those fibers have applications in high power lasers and scintillating detectors. For laser
operation, average power of 65 W energy of 4 mJ and peak power above 7 MW have been demonstrated in various
configurations. Those results push the limits of end-pumped bulk crystals in terms of average power and exceed the
limits of pulsed fibers lasers in terms of energy. For scintillating applications, high density/high light yield detectors are
developed for nuclear science and medical applications.
We developed a high power picosecond laser system at 80 MHz for third harmonic generation. We obtained 291 W at
1030 nm under 580 W of pump power at 976 nm from an all fiber master oscillator power amplifier based on rod-type
fibers. By frequency tripling, we obtained 63 W at 343 nm with excellent beam quality (M<sup>2</sup><1.2).
We present the realization of an actively mode-locked laser based on a 30 μm core diameter single-mode double clad
photonic crystal fiber. For 19 W of pump power at 976 nm, it yields an average power of 10 W at 40 MHz. The delivered
pulses, centered at 1030 nm, have a duration of 15 ps. This corresponds to an energy of 250 nJ per pulse and a peak
power of 17 kW.
Recent developments of the micro-pulling down technique lead to efficient laser demonstration with Nd:YAG single
crystal fibers. Indeed these media which benefit from the spectroscopic and thermal properties of bulk crystals and from
the thin and long shape of glass fibers are ideal candidates for high average and high peak power laser systems. In this
work, we investigate the potential of Yb:YAG single crystal fibers. After a careful design taking into account the quasithree
level structure of the Yb<sup>3+</sup> ions, we grew single crystal fibers by the micro-pulling down technique. With a 1 at.%
doped and 40 mm long single crystal fiber of 1 mm in diameter, we obtained a power of 50 W in CW operation under
200 W of incident pump power. In the Q-switched regime, we achieved pulses with an energy of 1.8 mJ at 5 kHz and a
duration of 13 ns for 120 W of pump power. We measured a M² value below 2.5. We also investigated the thermal
management of our system by the use of thermal cartography and Finite Element Analysis, showing a maximum
temperature smaller than 120°C reached on the pumped end face for 200 W of pump power. These results are a very
promising to design high average power and high peak power laser sources.
We designed single-crystal fibers to combine excellent spectroscopic and thermo-mechanical properties of bulk crystals
and ability of pump guiding and good heat repartition of doped glass fibers. Such single-crystal fibers of excellent optical
quality were grown by the micro-pulling-down technique. A remarkable advantage of this technique is that pump
guiding is achieved in the directly grown fiber without additional polishing on the cylinder. We designed 0.2%-Nd doped
YAG crystal fibers sample of 50 mm and 1 mm diameter and AR coated on both end faces. It was longitudinally pumped
by a fiber-coupled laser diode with a maximum output power of 120 W at 808 nm. Laser emission at 1064 nm was
achieved inside a two concave mirrors cavity. We obtained 20 W of laser emission with a M<sup>2</sup> quality factor of 6, for an
incident pump power of 120 W and a slope efficiency of 18% without any thermal management problems. Besides, a
power of 16 W with linearly polarized laser emission has been obtained under the same pump power by introducing a
thin plate polarizer in the cavity. An acousto-optical modulator was inserted inside the cavity and 360 kW of peak power
with 12 ns pulses at 1 kHz repetition rate were achieved under 60 W of pump power. This work shows real
improvements of laser performances in directly grown single crystal fibers. A complete thermal study confirms a good
heat management and demonstrates scalability to high average power laser sources.