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We report on experimental results on a high power, all-fibred, linearly polarized, mode-locked laser at 1.03 µm. The laser generates pulses of 40 ps wide at a repetition rate of 52 MHz, exhibiting 12 kW peak power. Dispersion in optical fibres is controlled to obtain both high power and narrow spectral linewidth. The average output power reached is 25 W with a spectral linewidth of 380 pm and a near diffraction limit beam (M² < 1.2). This laser is an ideal candidate for applications like IR spectroscopy, where high peak power and narrow linewidth are required for subsequent wavelength conversion. Keywords: Ytterbium, passive mode locking, Self Phase Modulation 1. INTRODUCTION The inherent advantages of all fibred lasers are compactness, reliability, and low cost. Short pulse lasers are useful for many applications like life scienc es, and material processing [1]. A number of mode-locking mechanisms have been reported in various operation regimes like stretched-pulse regime, and wave-breaking free regime [2]. The output pulse characteristics of a mode-locked fibre laser are related to the regime of operation that is determined by the cavity Group Velocity Dispersion (GVD). Unfortunately, amplification of picoseconds pulses in a fibre system is still challenging. Non linearity results in severe distortion on the pulses. Solutions consist in an increase of the effective mode area of fibres and a scaling of pulse energy by broadening the optical pulse. Recently, significant results in fibred amplifiers employing flexible Large Mode Area (LMA) Photonic Crystal Fibres (PCF) have been demonstrated. These fibres are really a good opportunity to scale up the output power of fibred lasers. However, the use of microstructured fibres needs specific tools to make end caps or splices. Moreover, the cladding diameter is not conventional, and specific Mode Field Adapta tor (MFA) is needed. Besides, doped rod type amplifiers are of strong interest to scale up power of laser systems. The core of the rod can reach a diameter of 80 µm, keeping a very good spatial beam quality and increasing the threshold of non linear effects. Because this kind of amplifier is not flexible by nature, it is rather not compact, and it needs to be carefully aligned in a free space configuration. In parallel, large core step index optical fibres have been improved thanks to recent progress in the management of dopants distribution [3, 4]. Indeed, highly doped fibres with polarization preservation and good beam quality are now on the market and allow laser manufacturers to in crease the extractable energy from amplifiers. The use of standard 125 µm cladding diameter is another advantage, allowing the use of standard equipment and all fibred components [5]. Large core pump combiners are another key component for fibred systems. The improvement of the expertise in this domain leads to the development of new combiners adapted to the previously mentioned optical fibres. In this paper, we present a high power short pulse genera tion from a passively mode-locked Ytterbium (Yb) doped fibre oscillator operating at 1.03 µm, based on a linearly polarized single mode Fabry-Perot (FP) cavity. The obtained source produces 5 ps pulses at a repetition rate of 52 MHz. The oscill ator pulse is then stretched to 40 ps with a Chirped Fibre Bragg Grating (CFBG). These pulses are then amplified by two amplifiers in a Master Oscillator Power Amplifier (MOPA) configuration. The first amplifier constituted by Polarization Maintaining (PM) single mode fibres with 5 µm core diameter allows reaching 100 mW average power. A second amplification stage is used to reach 25 Watts, limited by available pump power. The last booster stage is based on a commercially available step index LMA fibre with a core diameter of 20 µm, a cladding diameter of 125 µm, and a numerical aperture of 0.08. With an optimum management of Self Phase Modulation (SPM) during amplification, the spectral linewidth can be adjusted. The efficiency of the high power amplification stage is 50%, with no significant temporal distortion. Such laser can be useful as a seeder for frequency conversion in the green and UV. We will present here the experimental setup and results of the laser system. Then, we will present the simulated results do ne with the commercial software Fiberdesk. |
