FBG Reflectors for Raman Fiber Lasers

Introduction

The past decade has witnessed remarkable progress in high-power fiber lasers, with single cavity output exceeding 10 kW, driven by both industrial and defense applications. Ytterbium-doped fiber lasers have been instrumental in this advancement, converting low-brightness light from semiconductor laser diode modules into high-brightness light using double-clad optical fibers.
Despite prolific power scaling, the reliance on ytterbium as the primary gain medium confines fiber laser emission to a narrow region (1040–1120 nm, typically). Other rare-earth dopants like thulium and erbium are also available outside this range but offer more limited power and efficiency due to various technical challenges. These limitations underscore the need for high-power lasers spanning a broader wavelength spectrum to address applications outside of the rare-earth doped emission bands.

Raman Fiber Lasers

Raman fiber lasers (RFL) can emit light across a wide range of wavelengths by utilizing a suitable pump wavelength. This characteristic gives RFL a significantly broader emission spectrum compared to lasers relying uniquely on rare earth-doped fibers. Consequently, RFL technology is increasingly being used for generating high-power lasers at specific wavelengths that are quite difficult or even impossible to achieve using rare earth-doped fibers only.

Raman fiber lasers operate from the stimulated Raman scattering (SRS) produced by the molecular vibrations of the fiber material when excited at high powers. The spectrum of the propagating pump signal is shifted to a lower frequency Stokes component as illustrated below. The Raman shift is typically 13.2THz (440 cm-1) for silica fiber and up to 40THz (1330 cm-1) for phosphosilicate fibers.

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