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High-power NIR fiber lasers

par LAROCHE Mathieu - publié le , mis à jour le

This research theme mainly concerns the development of Neodymium-doped fiber laser systems for a direct emission in the spectral domain near 900 nm and subsequent frequency doubling for the generation of high power blue radiation. A first objective is to optimize the Nd-doped fibers in terms of laser efficiency and output spatial beam quality. Therefore we are exploring new fiber geometries and designs in close collaboration with iXFiber. In parallel we are developing laser sources such as amplifiers or oscillators in CW regime or pulsed regimes (ns, ps, fs) to achieve new IR or visible emission wavelengths.

 
I. High-power Nd-doped fiber laser sources near 900nm

Absorption and emission spectra of a Nd-dopded fiber

The main objective is the power scaling of Nd-doped fiber lasers in continuous and pulsed regime. The major problem to achieve high laser efficiency on the 4F3/2-4I9/2 transition of Nd3+ is related to the strong parasitic laser or amplified stimulated emission (ASE) at 1060 nm that normally prevents laser emission near 910 nm. However, we have shown that efficient laser operation near 910 nm may be achieved by increasing the core/clad surface ratio. Indeed, the high pump fluence results in a strong increase of the population inversion and gain difference between the two transitions can be small enough to allow laser emission at 910 nm. The large overlap of pump modes with the doped large mode area (LMA) core also permits to reduce the Nd3+ concentration and to avoid ions clustering effect which may strongly decrease the laser efficiency. A numerical modeling of the competition between the two laser transitions was developed and it was anticipated that cladding-to-core area ratio lower than 16 was necessary to obtain efficient laser operation at 910 nm. As a matter of fact, 20 W of output power was achieved at 910 nm using a Nd-doped fiber with a 20 um core diameter and a 80 um clad diameter [1].

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New fibers with an inner cladding reduced to 60 um were recently developed to optimize the laser performance below 900nm and improve the spatial quality of the output beam. We developed a fiber laser source which was tunable in wavelength by means of a volume Bragg grating (VBG). A high-power laser tunable over the spectral range 872 – 936 nm was demonstrated for the first time in a silica optical fiber. The laser source delivered up to 22 W of output power with a spectral width of 0.035 nm and a laser conversion efficiency close to 47% [2].

 
II. Nd-doped fiber laser in femtosecond regime at 930 nm

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This work was conducted as part of SOLAIR project in collaboration with CORIA – Rouen. A passively mode-locked neodymium-doped oscillator operating at 930 nm was studied and developed thanks to the expertise of the team “Sources laser et diagnostics en milieu denses” (CORIA) in this domain. The laser cavity comprised a dispersion managed line and a saturable absorber semiconductor (SESAM), generating parabolic pulses with 10 ps duration. The measured output power is 34 mW at a repetition rate of 15 MHz, which corresponds to more than 2.2 nJ pulse energy. This is the highest pulse energy that was reported so far for a mode-locked fiber oscillator at this wavelength. These pulses were then compressed to less than 126 fs. An amplification stage was developed at CIMAP and has extended the performance of the oscillator to an energy of 20 nJ energy (average power of 300 mW) in picosecond regime [3].

 
III. Non-linear frequency conversion in the blue spectral domain

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The main objective of the VERTICAL project was to generate laser pulses in the blue spectral domain and to evaluate the performances for LIDAR systems in underwater environment. For that purpose, a MOPA laser system operated in pulsed regime was developed around the new Nd-doped LMA fibers. Pulses with duration of 10 ns were amplified to an output power of 6 W at a repetition rate of 1 MHz. A peak power of 600 W was achieved without spectral or temporal distortions. The frequency doubling experiments in a MgO doped PPLN crystal have generated a power of near 1 W at 457 nm. [4]

 
References

  1. 20 W continuous-wave cladding-pumped Nd-doped fiber laser at 910 nm
    M. Laroche, B. Cadier, H. Gilles, S. Girard, L. Lablonde and T. Robin
    Optics Letters 38 (2013) 3065-7
    doi : 10.1364/ol.38.003065
  2. Extended tunability of Nd-doped fiber lasers operating at 872–936 nm
    B. Leconte, B. Cadier, H. Gilles, S. Girard, T. Robin, and M. Laroche
    Opt. Lett. 40 (2015) 4098-4101
    doi : 10.1364/OL.40.004098
  3. Mode-locked Nd-doped fiber laser at 930 nm
    K. Qian, H.J. Wang, M. Laroche & A. Hideur
    Optics Letters 39 (2014) 267-270
    doi:10.1364/ol.39.000267
  4. Generation of 520 mW pulsed blue light by frequency doubling of an all-fiberized 978 nm Yb-doped fiber laser source
    M. Laroche, C. Bartolacci, B. Cadier, H. Gilles, S. Girard, L. Lablonde and T. Robin
    Optics Letters 36 (2011) 3909-11
    doi : 10.1364/OL.36.003909