A beat-frequency tunable dual-mode fiber-Bragg-grating external-cavity laser

A dual-mode external cavity laser employing two spatially separated fiber Bragg gratings has been investigated. We experimentally demonstrated that, without utilizing polarization separation techniques, a closely spaced (∼0.3 nm) dual-mode emission can be stabilized if a specific in-phase relation between the external feedbacks and the gain chip facet residual feedback is satisfied at both lasing wavelengths. The radio-frequency (RF) beat signal generated by the dual-mode optical signal had a typical extinction ratio of 25 dB above the noise floor and a 3-dB linewidth of approximately 2 MHz. The RF beat signal had a frequency tuning range of 200 MHz, where the dual-single-mode spectrum can be maintained.