  We realize a fiber Bragg grating InGaN based laser diode emitting at 400 nm
and demonstrate its high coherency. Thanks to the fabrication of a narrow band
fiber Bragg grating in the near-UV, we can reach single-mode and
single-frequency regimes for the self-injection locked diode. The device
exhibits 44 dB side-mode-suppression-ratio and mW output power. Detailed
frequency noise analysis reveals sub-MHz integrated linewidth and 16 kHz
intrinsic linewidth. Such a narrow linewidth laser diode in the near-UV domain
with a compact and low-cost design could find applications whenever coherency
and interferometric resolutions are needed.
