Abstract
We review our recent experimental results on the cascaded Raman conversion of highly multimode laser diode (LD) pump radiation into the first- and higher-order Stokes radiation in multimode graded-index fibers. A linear cavity composed of fiber Bragg gratings (FBGs) inscribed in the fiber core is formed to provide feedback for the first Stokes order, whereas, for the second order, both a linear cavity consisting of two FBGs and a half-open cavity with one FBG and random distributed feedback (RDFB) via Rayleigh backscattering along the fiber are explored. LDs with different wavelengths (915 and 940 nm) are used for pumping enabling Raman lasing at different wavelengths of the first (950, 954 and 976 nm), second (976, 996 and 1019 nm) and third (1065 nm) Stokes orders. Output power and efficiency, spectral line shapes and widths, beam quality and shapes are compared for different configurations. It is shown that the RDFB cavity provides higher slope efficiency of the second Stokes generation (up to 70% as that for the first Stokes wave) with output power up to ~30 W, limited by the third Stokes generation. The best beam quality parameter of the second Stokes beam is close to the diffraction limit (M2~1.3) in both linear and half-open cavities, whereas the line is narrower (<0.2 nm) and more stable in the case of the linear cavity with two FBGs. However, an optimization of the FBG reflection spectrum used in the half-open cavity allows this linewidth value to be approached. The measured beam profiles show the dip formation in the output pump beam profile, whereas the first and second Stokes beams are Gaussian-shaped and almost unchanged with increasing power. A qualitative explanation of such behavior in connection with the power evolution for the transmitted pump and generated first, second and third Stokes beams is given. The potential for wavelength tuning of the cascaded Raman lasers based on LD-pumped multimode fibers is discussed.
Highlights
Raman fiber lasers (RFL) are known as wavelength-agile laser sources utilizing almost all the advantages of fiber lasers, such as high efficiency, beam quality and stability, in a reliable, adjustment-free, all-fiber cavity; see [1,2] for a review
Highly multimode (M2~30) 940 nm laser diode (LD) pump radiation up to ~200 W power coupled through a fiber pump combiner to a 100 um core graded-index fiber is serially converted into the first, second and third Stokes beam
A linear cavity composed of fiber Bragg gratings (FBGs) inscribed in the fiber core is formed to provide feedback for the first Stokes order, whereas for the second order, both the linear cavity consisting of two FBGs and the half-open cavity with one FBG and random distributed feedback (RDFB) via Rayleigh backscattering along the fiber are explored
Summary
Raman fiber lasers (RFL) are known as wavelength-agile laser sources utilizing almost all the advantages of fiber lasers, such as high efficiency, beam quality and stability, in a reliable, adjustment-free, all-fiber cavity; see [1,2] for a review. A broad SRS bandwidth extended up to several tens of THz from the pump wavelength and higher-order SRS at high-power pumping enable Raman generation at almost any wavelength in a transparency window of silicabased fibers ranging from ~1 to ~2 μm when pumped by a high-power Yb-doped fiber laser (YDFL) operating at ~1 μm. The cavity of such lasers may be ring or linear with broadband or fixed-wavelength reflection, e.g., by a pair fiber Bragg gratings (FBG) resonant to a chosen Stokes wavelength. A sequential beam quality improvement from M2~34 up to a diffraction-limited beam (M2 < 1.3) for the highest order with high conversion efficiency is demonstrated
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