Strain gradients for upscaling the power of single frequency fibre amplifiers (Conference Presentation)

Abstract

Fibre lasers and amplifiers have proven their interest in high power laser systems thanks to their compactness and their stability against temperature and mechanical fluctuations. The increase of emitted optical power is currently limited by nonlinear effects due to the intrinsically high confinement of the optical field in fibres. Among all the existing nonlinear effects, the Stimulated Brillouin Scattering (SBS) is predominant in the case of single frequency or narrow spectrum optical fields. The SBS effect comes from the interaction between the incident optical field and the acoustic (density) waves in the optical fibre material, creating an effective Bragg grating for the incident wave. This interaction results in the creation of a back-reflected optical wave depleting the power of the incident one. Because of the nonlinear nature of SBS, the power transfer between the incident and the reflected optical waves depends on the incident power. It is usual to define a threshold power 𝑃𝑆𝐵𝑆 at which the reflected wave arises. In order to increase the power of single frequency fibre lasers, many developments have been done in the past decades to mitigate SBS. We can class the different SBS mitigation techniques in two categories depending on whether they act on the optical field properties or on the optical fibre itself. All the techniques belonging to the first category are based on an active control of the phase or the frequency of the incident optical wave, in order to spread the incident energy in the spectral domain: sinusoidal Phase Modulation (PM) [1], non-sinusoidal PM [2], Gaussian noise PM, Pseudo-Random Binary Sequence (PRBS) PM [3] or frequency chirp modulation [4]. In the second category, there are different ways to increase 𝑃𝑠𝑏𝑠. Among them, we can maximize the core radius of the fibre, minimize the fibre length [5], or apply temperature gradients [6], dopant concentrations gradients or strain gradients to broaden the SBS spectrum and thus lower its effective gain. We present here two patented techniques allowing longitudinal tensile or compression strain gradients on optical fibres. We performed successful realizations on a wide variety of optical fibres, ranging from single mode and Large Mode Area (LMA) passive fibres for power transport, to (single-mode and LMA) active fibres used in amplifiers architectures. Thanks to their passive nature and their insensitivity to wavelength, we believe these techniques to be highly suitable for high power narrow spectrum laser systems. Figure 1 : Experimental output power and pulse energy of two realizations of single frequency pulsed fibre amplifiers used in optical sensing systems. Left : Erbium doped amplifier at 1580 nm with tensile strain gradient on LMA active fibre. Right : ErYb co-doped single mode active fibre at 1545 nm with compresive strain gradient. Figure 1 presents the obtained output power and pulse energy of two single frequency amplifiers with high SBS threshold, thanks to our strain gradient techniques. Figure 1 (left) corresponds to a LMA Er doped fibre whose core diameter is 40 μm mounted on our tensile strain gradient device. It delivers 150 ns long pulses at 4 KHz repetition rate with up to 1800 W peak power, instead of the 600 W peak power limit in absence of SBS mitigation. Figure 1 (right) presents the achievements made with the compressive strain gradient technique. This single frequency single mode fibre amplifier emits up to 385 W peak power at 10 KHz repetition rate instead of 120 W peak power before applying the compressive strain gradient. The details of the two patented strain gradients techniques and some experimental implementations of these techniques in high SBS threshold fibre amplifiers will be presented. The advantages and limitations of such techniques for narrow spectrum high power laser sources will also be discussed.