We report high‐power frequency conversion of a Yb‐doped fiber laser using a double‐pass pumped external‐cavity diamond Raman oscillator. Pumping with circular polarization is shown to be efficient while facilitating high‐power optical isolation between the pump and Raman laser. We achieved continuous‐wave average power of 154 W with a conversion efficiency of 50.5% limited by backward‐amplified light in the fiber laser. In order to prove further scalability, we achieved a maximum steady‐state Raman‐shifted output of 381 W with 61% conversion efficiency and excellent beam quality using 10 ms pump pulses, approximately a thousand times longer than the transient thermal time‐constant. No power saturation or degradation in beam quality is observed. The results challenge the present understanding of heat deposition in Raman crystals and foreshadow prospects for reduced thermal effects in diamond than originally anticipated. We also report the first experimental evidence for stimulated Brillouin scattering in diamond.
