CW Yb-fibre laser with wavelength-variable efficient intracavity frequency doubling in partially coupled enhancement cavity

Abstract

ABSTRACT This paper presents the results on second harmonic generation in a tuneable Yb-fibre laser with an enhancement cavity partially coupled to the laser resonator. The maximal second harmonic output power was 880 mW at 536 nm when pumped with 6.2 W at 976 nm. The output radiation line width of the second harmonics of the Yb-fibre laser did not exceed 0.5 nm with a tuning range of 521–545 nm and the output power at the ends of this range 220 and 450 mW respectively. Further presented is an analysis of different frequency doubling configurations both with and without an enhancement cavity in a broad range of outp ut powers of the fundamental radiation. Keywords : fiber laser, nonlinear optics, second harmonic generation, enhancement cavity 1. INTRODUCTION Frequency doubling optical configurations relying on enhancement cavities have demonstrated their high efficiency in a variety of different lasers [1–7]. Application of such cavities to fibre lasers made it possible not only to efficiently convert non-single-frequency radiation [8, 9], but also to remove the need of automatic alignment of the enhancement cavity length to the wavelength of the incident radiation. Due to relatively long resonators of fibre lasers, spectral density of longitudinal modes in these resonators far exceeds that in the enhancement cav ities. Therefore, certain frequencies of the fibre laser resonator coincide or nearly coincide with transmission peaks of the enhancement cavity. For these frequencies, the enhancement cavity is resonant and provides efficient power enhancement inside the cavity. Practicality of such configurations was demonstrated earlier for cases when the enhancement cavity is external to the fibre laser resonator and when the enhancement cavity is contained within the fibre laser resonator (see Fig. 1(a) and 1(b)). The present work proposes a new approach to second harmonic generation in fibre lasers: in our case, the enhancement cavity is partially coupled to the fibre laser resona tor owing to a small shared optical path, at the same time belonging both to the fibre laser resonator and to the enhanc ement cavity (Fig. 2). This beca me possible because the fibre laser output coupler was placed inside the enhancement cavity.