Spectral broadening and power leakage in CW Yb-doped fibre laser

Spectral broadening of the fibre laser emission can degrade system efficiency, complicate frequency doubling and is a non-desirable effect in many applications. On the other hand, effect of the spectral broadening lies in a very background of the operation of supercontinuum laser sources. Therefore, study of this nonlinear physical phenomenon is both an interesting fundamental and practical engineering problem relevant to laser performance. Spectral broadening of powerful laser radiation might lead to large changes of the emission characteristics during one round trip, breaking assumption that is typically used in the classical laser models (see e.g. [1]). In this work we study experimentally and theoretically spectral broadening of radiation in CW Yb-doped fibre laser. Averaged models dealing with an average optical power and neglecting the phase of optical field and time dynamics of the radiation are commonly used for analysis of CW fibre lasers (see e.g. [2] and references therein). However, time evolution of the optical radiation might play an important role in performance of high power fibre laser even for so-called CW lasers. In this work we undertake experimental and numerical study of spectral broadening of radiation in fibre laser operating at 1084nm. Experimental setup is shown in Fig. 1(left): 35 meter Yb-doped fibre (GTWave) with 7µm core was pumped with 910 nm laser diode at up to 23W output power. Laser operates in CW regime with maximal output power of 12W. The FBG (HR2) at the left end of the cavity has maximum reflectivity at 1084nm and spectral width of ∼1nm. At the other end, cavity was formed by the 4% back reflection from the fibre edge. A fraction of power leaves the cavity through leakage around HR2 FBG, Fig. 1 (right). This effect can be quantitatively treated as an effective “decrease of the FBGs reflectivities”.