W-type fiber suppresses Raman scattering

For power scaling of pulsed fiber-laser systems, stimulated Raman scattering (SRS) continues to be a primary obstacle. Using short, large-core, low-numerical-aperture fibers reduces SRS, but the resulting multimode output has poor beam quality. In addition, attempts to bend conventional single-mode fiber to reduce the SRS threshold result in high loss at the signal wavelength. To rectify these problems, researchers at the University of Southampton (Southampton, England) have developed a single-mode W-type (depressed-clad) fiber that completely suppresses SRS in a high-peak-power fiber master-oscillator power-amplifier (MOPA) source.

Mathematical modeling was used to design a W-type fiber consisting of a 7 µm ytterbium-doped core diameter and a 7-µm-thick suppressed cladding region with index (n) differences of 0.003 and -0.002, respectively, with respect to the silica outer cladding (n = 1.45). Using a 23-m-long section of this fiber in the final stage MOPA (which was pumped by a 975 nm diode stack with a 103 ps pulse signal at a 32 MHz repetition rate), an average output power of 53 W-corresponding to a peak power of 13 kW-was achieved at the 1060 nm signal wavelength. Contact Jaesun Kim at [email protected].