Spectral characterization technique of self-organized distributed feedback in a self-sweeping fiber laser.

We report on spectral characterization technique of self-organized dynamical distributed feedback (DDFB) in a self-sweeping Yb-doped fiber laser. The DDFB is originated from gain and refraction index gratings formed (recorded by standing waves) in the laser's active medium and dynamically changes during frequency self-sweeping operation. Dynamic nature of the feedback requires fast characterization (reading) of corresponding reflection spectrum. The reading process can be separated from the recording one in time due to sufficiently long characteristic time of the dynamical gratings. The DDFB spectra are measured during off-state of the self-sweeping laser with a tunable probe radiation near the dynamical grating reflection maximum. The spectra reconstructed in a range of ~1 GHz from a sequence of shorter probe wave scans have narrow sharp peak with width of ~50 MHz and reflectivity of about 0.1%. A good agreement between theory and experimental results is demonstrated.