A heterogeneous step-index fiber Bragg grating fabricated by femtosecond laser point-by-point technique for laser generation

Step-index fiber Bragg gratings (SI-FBGs) are increasingly recognized for their potential applications in optical communications, sensing, and laser systems. This study introduces three novel types of heterogeneous step-index fiber Bragg gratings (Hetero-SI-FBGs), fabricated using the femtosecond laser point-by-point technique. To the best of our knowledge, this work represents the first demonstration of fabricating 3 mm femtosecond gratings in heterogeneous step-index (Hetero-SI) fibers. When compared with homogeneous step-index fiber Bragg gratings (Homo-SI-FBGs) produced via the traditional ultraviolet-laser phase-mask technique, the developed Hetero-SIFBGs offer an increased number of wavelength output channels, enhanced optical signal-to-noise ratio (OSNR), and expanded multifunctional optical properties. The OSNR for two distinct wavelength channels in the reflection spectrum measured 22.19 dB and 20.67 dB, respectively. Furthermore, the femtosecond laser technique employed here exhibits greater flexibility, reduced fiber damage, and enhanced potential for multifunctional integration than the ultraviolet-laser phase-mask technology. To address current research limitations in Hetero-SI fibers, the Hetero-SI-FBGs were implemented as wavelength reflection filters in random fiber lasers (RFLs), facilitating stable random laser with reduced frequency noise. This advancement heralds a new era for SIFBGs in multi-channel laser systems and underscores their promising capabilities in fiber sensing and wavelength-division multiplexing.