In this paper, we propose, through different theoretical examples of composite fibre designs, some alternative ways to push up the limits of active optical fibres. Highly ytterbium-doped large cores are required for high power delivery and low non linear limitations. Such cores usually support several guided modes. We demonstrate in this paper that by surrounding the core with a tailored cladding, single mode transverse emission under amplification or in the lasing regime is possible. A 3D modelling of transverse mode competition in active fibres is first presented and applied to several cladding designs. This numerical model, which is wavelength sensitive, is of great interest for laser cavity design in continuous wave regime for example as both composite fibre and cavity designs are considered. Accurate modelling of modal competition requires to compute overlap between electric field and active region for each electromagnetic mode (Gamma mn ) as a function of the transverse plane coordinates Gamma mn (x, y). The fibre design, the laser cavity, the pump power have to be considered in order to obtain an accurate description of 3D spatial population inversion and its consequences on modal competition under active regime. The numerical model is first applied to a fibre exhibiting an all solid cladding made of 2D array of high refractive index rods and then to a multi-layered fibre. The latter is used to illustrate our purpose in figure 1 which shows the comparison between lasers made of fibres with or without tailored cladding. Both fibres exhibit an Yb-doped (5625 wt-ppm) core with a diameter of 30 mum and a refractive index higher than that of silica (5times10 -3 ). The pump power is 60 W. When the core is surrounded by pure silica, 17 modes can propagate and LP 01 carries less than 20% of the total output power which is distributed among almost all modes. The laser obtained is highly multimode and the spatial quality of the beam is expected to be very low. On the contrary, when the structured cladding surrounds the core, almost all the laser power is emitted on the LP 01 mode. With a 4% - 99%, 2 m long cavity, less than 8% of total power is emitted on the LP 41 , all other modes do not exist.
We report an all solid state laser device producing tunable dual wavelength emission in the near IR region (1060nm, and 1550 nm) by use of an Er/Yb co-doped fiber. Generation of continuous-wave radiation around 630 nm is then demonstrated by extra-cavity sum frequency mixing in a Periodically Poled Lithium Niobate (PPLN) crystal. Quasi phase matching conditions are obtained over 7 nm to generate tunable coherent light in the red spectral range.
We experimentally demonstrate that spatial beam self-cleaning can be highly efficient when obtained with a few-mode excitation in graded-index multimode optical fibers. By using 160 ps long, highly chirped (6 nm bandwidth at -3dB) optical pulses at 1562 nm, we demonstrate a one-decade reduction of the power threshold for spatial beam self-cleaning, with respect to previous experiments using pulses with laser wavelengths at 1030-1064 nm. Self-cleaned beams remain spatio-temporally stable for more than a decade of their peak power variation. The impact of input pulse temporal duration is also studied.
Although singlemode fiber lasers become a mature technology, enhancements, in terms of output power, spatial beam quality, bend insensitivity are still required. A major trend is to increase the active core area to increase the thresholds of nonlinear effects while ensuring a transverse singlemode behavior. Actually, increasing the active ions' concentration is also demanded since it allows a drastic reduction of the fiber length, everything being equal. Two non-exclusive strategies are laid out to overcome fiber laser limitations. On the one hand, it is demonstrated that surrounding a highly multimode active core by a properly designed microstructured cladding, exhibiting specific resonant features, allows the fiber laser to be operated in the singlemode regime. On the other hand, a large mode area photonic bandgap fibre is shown to lead to a transverse singlemode fiber laser with very good lasing efficiency.
We experimentally demonstrate Kerr beam self-cleaning in the anomalous dispersion regime of graded-index multimode optical fibers. By using 90 ps duration, highly chirped (2 nm bandwidth at -3dB) optical pulses at 1562 nm, we demonstrate a 2 decades reduction, with respect to previous experiments in the normal dispersion regime, of threshold peak power for beam self-cleaning into the fundamental mode of the fiber, accompanied by more than 65% nonlinear increase of intensity correlation into the fundamental mode. Highly efficient self-selection of the LP11 mode is also observed. Self-cleaned beams remain spatio-temporally stable for more than a decade of variation of the peak pulse power.
