Tunable narrow linewidth DFB laser diode with artificially enhanced Rayleigh scattering-based external distributed feedback

We experimentally demonstrate a tunable narrow-linewidth distributed feedback (DFB) laser diode based on the artificially enhanced Rayleigh backscattering in a piece of high numerical aperture single-mode fiber. Taking advantage of femtosecond laser pulses induced randomly refractive index modulation in the core of the high numerical aperture single-mode fiber with a length of 15 m and a numerical aperture of 0.35, Rayleigh backscattering is artificially enhanced by 60 dB. By importing the external distributed feedback into the lasing cavity of the DFB laser diode, the frequency noise is suppressed from 1.2 × 10 5 Hz 2 /Hz to 5.6 Hz 2 /Hz, corresponding to a frequency noise suppression ratio of 43.31 dB. Moreover, the Lorentzian linewidth of the DFB laser diode measured by the delayed self-heterodyne interferometry is narrowed from 654 kHz to 1.24 kHz. It is kept at the same level in its full wavelength tuning range of ∼2 nm. The proposed method employing artificially enhanced Rayleigh scattering as the external feedback provides a method for compact integrated narrow linewidth fiber or semiconductor lasers, especially tunable lasers benefitting from the wavelength adaptivity of Rayleigh backscattering-based distributed feedback.