A $\lambda/4$-Shifted Distributed-Feedback Laser Diode With a Fiber Ring Cavity Configuration Having an OSNR of 85 dB and a Linewidth of 7 kHz

We newly propose a lambda/4-shifted distributed-feedback laser diode (DFB-LD) that employs a fiber ring cavity configuration and which incorporates an ultranarrow band fiber Bragg grating (FBG) in the cavity. By controlling the FBG wavelength to match the oscillation wavelength of the DFB-LD, we achieved stable single-mode oscillation where the sidemodes of the DFB-LD and the Fabry-Perot modes induced by the ring configuration were sufficiently suppressed. Large performance improvements with an optical signal-to-noise ratio of 85 dB and a linewidth of 7 kHz were obtained.