Picosecond Yb-doped single-trench fiber amplifier with diffraction limited output

Abstract

We propose a novel fiber design single-trench fiber (STF) for large mode area fiber laser and amplifier. Fiber offers cylindrical symmetry and also offer higher refractive index of core compared to cladding. This avoids the need of stack and draw process and refractive index compensation of core doped with index raising rare earth and co-dopants ions, which are an indispensable condition in most of other fiber designs. That is why, this fiber design can be fabricated with conventional modified chemical vapour deposition process in conjunction with solution doping process, which can dramatically reduce the fabrication cost, hence suitable for mass production. Fiber offers very high loss (>10dB/m) and low power fraction in core (<50%) to the higher order modes for low loss of fundamental mode (<0.1dB/m) and high power fraction in core (>80%) thanks to the resonant coupling between core and ring modes. We fabricated a preform for 30μm core STF using MCVD process in conjunction with solution doping process in a single step, without using any micro-structuration and pixilation of core. Experiments ensure the robust single mode behaviour irrespective of launching condition of input beam. Furthermore, this fiber used in three stages MOPA provides 80kW peak pulses with repetition rate of 500 kHz, average power up to 10W, with M² ~ 1.14. Moreover, all solid structure ensures easy cleaving and splicing. In nutshell, an ultra-low cost, monolithic, compact, and an effective single mode fiber amplifier device can be achieved using single-trench fiber.