Random fiber laser

Conventional approaches enhance random distributed feedback (R-DFB) strength by upgrading scattering elements. Nevertheless, these methods have reached a performance plateau and neglect the physical mechanism underlying photonic localization. To overcome this limitation, a hetero-frequency symmetric cladding fiber Bragg gratings (CLFBGs) structure was proposed and fabricated via a femtosecond point-by-point inscription technique to generate multipath interference feedback. This configuration facilitates the coupling of cladding modes at distinct frequencies with the core mode individually, thereby increasing the number of interfering photons within the cavity and effectively enhancing the modal degrees of freedom. Moreover, the symmetric structure significantly suppresses the radial energy dissipation in cladding modes, resulting in stronger core-cladding mode coupling. Therefore, it produces R-DFB enhancement and enables the transition from extended modes to localized modes (LM) in weakly disordered media. Experimental results demonstrated that, compared to single-path interference feedback, the new structure increased the spectral oscillation amplitude by 486% and reduced output power fluctuations from 0.54 mW to 0.33 mW. Moreover, the mean cavity quality factor (Qmean) increased from 2821 to 5239, indicating operation of the random laser (RL) in the LM regime. Further integration with a strongly scattering reflector minimized power fluctuations to 0.02 mW, and improved Qmean to 8422. These findings confirm that the designed random fiber laser (RFL) achieves exceptional temporal stability and strong photonic localization. As a result, the proposed laser could be applied in secure communications, optical sensing, and imaging.