Wavelength-tunable narrow-linewidth semiconductor fiber-ring laser

A passive sub-ring was inserted into a semiconductor fiber-ring laser as the Vernier configuration to enable stable lasing spectrum. Wavelength tuning over 20 nm and fine tuning of less than 0.0001 nm were obtained by rotating the polarization controller and adjusting the voltage on the phase modulator, respectively. A narrow linewidth of less than 10 kHz and a sidemode suppression ratio of higher than 35 dB were demonstrated with single longitudinal-mode operation.