Abstract
Raman lasing in a graded-index fiber (GIF) attracts now great deal of attention due to the opportunity to convert high-power multimode laser diode radiation into the Stokes wave with beam quality improvement based on the Raman clean-up effect. Here we report on the cascaded Raman generation of the 2nd Stokes order in the 1.1-km long GIF with 100-μm core directly pumped by 915-nm diodes. In the studied all-fiber scheme, the 1st Stokes order is generated at 950–954 nm in a linear cavity formed at GIF ends by two fiber Bragg gratings (FBGs) securing beam quality improvement from M2 ≈ 30 to M2 ≈ 2.3 due to special transverse structure of FBGs. The 2nd Stokes wave is generated either in linear (two FBGs) or half-open (one FBG) cavity with random distributed feedback via Rayleigh backscattering. Their comparison shows that the random lasing provides better beam quality and higher slope efficiency. Nearly diffraction limited beam (M2 ≈ 1.6) with power up to 27 W at maximum gain (996 nm), and 17 W at the detuned wavelength of 978 nm has been obtained, thus demonstrating that the 2nd-order random lasing in diode-pumped GIF with FBGs provides high-efficiency high-quality beam generation in a broad wavelength range within the Raman gain spectral profile.
Highlights
Outstanding performance of high-power fiber lasers in both CW and pulsed regimes[1,2,3] enables their wide implementation for material processing in automotive, aerospace and oil & gas industries, and for micromachining in microelectronics, photovoltaics, LED, medical devices etc
Coupling of several laser diodes (LDs) to the graded-index fiber (GIF) is provided by a special multimode fiber pump combiner[24,25], a linear cavity formed by special fiber Bragg gratings (FBGs) secures the 1st Stokes wave (~954 nm) generation with relatively high beam quality (M2 ≈ 2.3 at 35 W) at low quality of the LD pump beam (M2 ≈ 30), see[27] for details
It should be noted that the HR FBG is inscribed by CW UV interference pattern formed in the core area, while the LR FBG is inscribed point-by-point by femtosecond pulses tightly focused in the center of the GIF core providing the selection of fundamental transverse mode[23]
Summary
Zoomed view of the 2nd Stokes spectrum shows that in the case of 486 cm−1 Raman shift (Fig. 4c) the spectrum contains additional blue-shifted lines corresponding to the resonant coupling of higher-order transverse modes of the GIF with HR FBG since they have higher Raman gain than the fundamental one This result is similar to that one obtained for the conventional RFL pumped by multimode LDs when the shift corresponds to the long-wavelength slope of the Raman gain profile[27]. Comparison of the 2nd Stokes power for three different RRFL configurations (Fig. 5a) shows that the closer Raman shift to the gain maximum (corresponding positions on the measured Raman gain profile[27] are shown in Fig. 5b), the lower the threshold and the higher slope efficiency and maximum output power of the laser at available pumping. Nearly diffraction limited beam is generated in the configuration with Rayleigh scattering based distributed feedback
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