Mutual injection-locking of two individual double-clad fibre lasers

Abstract

Phase-locking and coherent beam combination of two individual double-clad fibre lasers by a novel extra-cavity mutual injectionlocking method are proposed and experimentally demonstrated. Steady interference stripes with high contrast ratio (about 0.5) are observed. The output power of the phase-locked array exceeds 11 W and the power-combining efficiency is about 80%. No power-restriction optical components are utilised in the phase-locking experiment and the output power can be further up-scaled.Introduction: Recent advances in high-power fibre lasers have shown that fibre lasers are one of the most efficient lasers and the output power of a single-fibre laser has exceeded the kilowatt level [1]. To scale-up the output power of a single-fibre laser to higher power level, one has to face significant challenges including nonlinear effects, thermal effects, and fibre damage, etc. Therefore, the coherent combination array of multiple fibre lasers, each with moderate power, is considered as an effective way to obtain high output power. In recent years, several approaches have been demonstrated in the combining of multiple fibre lasers including the master oscillator power amplifier (MOPA) system [2, 3], the fibre coupler combining method [4], the selfimaging confocal resonator [5,6] and the Michelson cavity [7] etc. Mutual injection phase-locking is a scaled-up technique for coherent combine multiple fibre lasers in the recent two years, which was first demonstrated by two erbium-doped singlemode fibre lasers using two fibre splitters for energy injection [8]. The mutual injection phaselocking method does not need to inspect and control either the fibre laser's phase or the fibre-length and the intensity of the individual lasers, and it can lead to a new architecture for high-power laser systems. In their experiment, though steady interference stripes were observed, the output power of their array was limited at milliwatt level because the fibre splitters cannot bear high power, hence their architecture can not be scaled up to high power output.In this Letter, we propose and demonstrate a novel extra-cavity mutual injection-locking method to realise phase-locking of two double-clad fibre lasers. The mutual injection structure consists of a beam splitter and a corner cube, with no other power-restriction optical components being utilised. This mutual injection structure can break through the power restriction of fibre splitters, which were utilised in [8]. In our experiment, steady interference stripes with high contrast ratio (about 0.5) were observed and more than 11 W of coherent output power was achieved with combining efficiency of 80%. To the best of our knowledge, this is the first report of mutual injection-locking of two double-clad fibre lasers. Successful attempts at phase-locking with the high output power of our experiment show that this kind of mutual injection-locking technique is a promising technique for fibre laser coherent combination.

Experimental study on phase-locking of two high-power all-fiber lasers

“…In the experiment steady interference strips with high visibility of 46% are observed. The maximum coherent output power of 407 W with approximate 98% combining efficiency is obtained when the pump power is 657 W. Compared with the previous work [18,[21][22][23], this new injection-locking configuration is very simple and easy to construct. It demonstrates great potential in the scaling-up of output power if there is enough pump power.…”