We report frequency doubling of an extended-cavity diode laser in an a-cut KTP crystal. Continuous-wave, single-mode green light at 540 nm with a power of 22.8 mW was generated from an input of 44.2 mW by type II non-critical phase matching. Stable operation with intensity fluctuations smaller than 1 % peak-to-peak for more than two hours was achieved by employing resonant optical feedback. The optical feedback also reduced the fundamental-wave linewidth to the upper limit of 41 kHz.
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