We present a comprehensive experimental study of the technique of Longitudinal Mode Filling (LMF) applied to the
reduction of Stimulated Brillouin Scattering (SBS), in Ytterbium Doped Fibre Amplifiers (YDFA) at the wavelength of
1064 nm. Pulse durations and Mode Field Diameters (MFD) lie in the ranges of 10 - 100 ns and 10 - 35 μm,
respectively. Input pulse-shaping is implemented by means of direct current modulation in multimode Laser-Diode
seeds. This evidences a number of interests in the development of robust and low cost Master Oscillator Power
Amplifiers (MOPA). Highly energetic, but properly shaped, nanosecond pulses may be produced this way without any
need of additional electro-optical means for in-line phase and amplitude modulation. Seeds consist of Distributed Feed-
Back (DFB) and Fibre Bragg Gratings (FBG) with different fibre lengths. We demonstrate the benefit of LMF with
properly controlled mode spacing, in combination with chirp effects due to fast current transients in the semiconductors,
in order to deal with SBS thresholds in the range of a few to some hundred μJ. The variations of the SBS threshold are
discussed versus the number of longitudinal modes, the operating conditions of the selected seed and pulse-shaping
conditions.
We demonstrate an Ytterbium-doped fiber laser system generating high energy pulses at the non-conventional
wavelength of 977 nm. An actively Q-switched master fiber oscillator delivers 1.2 W of average power in 12 ns pulses at
82 kHz of repetition rate. This pulsed fiber source is then amplified in an ultra-large core photonic crystal fiber amplifier
up to 71 W. Deducing the fraction of power contained in interpulse ASE, we obtained 0.7 mJ pulses at 977 nm, resulting
in a pulse peak-power of >55 kW. To the best to our knowledge, this system delivers the highest performances ever
demonstrated in this spectral window.
High peak power femtosecond oscillators exhibit great potential for many applications such as micro- and nanomachining and structuring, waveguide writing in glass, nonlinear frequency conversion or seeding of ultrafast fiber and bulk amplifiers. Ultrashort pulse durations below 50 fs are routinely produced by Ti:sapphire lasers. However, due to the need for a green pump laser, Ti:Sapphire lasers suffer from a greater complexity. Diode-pumped Ytterbium femtosecond lasers on the other hand are compact and reliable lasers, but, because of the limited amplification bandwidth, typically exhibit pulse duration greater than 60 fs. We present a directly diode-pumped 40-fs laser source with pulse energies higher than 120 nJ, more than 2 MW peak power, and a pulse repetition rate of 9 MHz. The laser setup is compact and fits in a 60 x 40 cm footprint. The laser source consists of a passively mode-locked femtosecond oscillator and fiber-based post-compression module. The oscillator operates at 9 MHz pulse repetition rate and produces pulse energies up to 300 nJ at 370 fs pulse duration. The oscillator is then focused into a standard single mode fiber in order to broaden the pulse spectrum to about 60 nm bandwidth. Owing to the high initial pulse energy the used fiber is
as short as 15 mm. After collimation it was sufficient to reflect the beam 8 times on 2 parallel chirped mirrors having 250 fs2 each. The overall transmission of this pulse compression module was about 80% resulting in 120 nJ transmitted pulse energy in 40-fs pulses.
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