Simple design for singlemode high power CW fiber laser using multimode high NA fiber

A large number of high power CW fiber lasers described in the literature use large mode area (LMA) double cladding fibers. These fibers have large core and low core numerical aperture (NA) to limit the number of supported modes and are typically operated under coiling to eliminate higher order modes. We describe here multimode (MM) high NA ytterbium doped fibers used in single mode output high power laser/amplifier configuration. Efficient single mode amplification is realized in the multimode doped fiber by matching the fundamental mode of the doped fiber to the LP01 mode of the fiber Bragg grating (FBG) and by selecting the upper V-number value that limits the overlap of the LP01 to the higher order modes. We show that negligible mode coupling is realized in the doped fiber, which ensures a stable power output over external perturbation without the use of tapers. Fundamental mode operation is maintained at all time without coiling through the use of FBG written in a single mode fiber. We show that such fiber is inherently more photosensitive and easier to splice than LMA fiber. We demonstrate an efficient 75W singlemode CW fiber laser using this configuration and predict that the power scaling to the kW level can be achieved, the design being more practical and resistant to photodarkening compared to conventional low NA LMA fiber.