Fabrication of tilted fiber Bragg gratings in all-cyclo olefin polymer fibers.

In this study, UV phase mask and femtosecond (fs)-laser line-by-line (LbL) techniques are employed to inscribe tilted fiber Bragg gratings (TFBGs) into in-house fabricated all-cyclo olefin polymer (COP) optical fibers. Analysis of cross-sectional refractive index profiles shows that the UV phase mask technique induces a higher refractive index change in the cladding (3.3 × 10-3) than in the core (3.6 × 10-4), as the laser beam is absorbed by the cladding before reaching the core, despite similar absorption properties of both regions at the inscription laser wavelength. This technique also causes undesired physical deterioration within the irradiated fiber cladding region, which hinders the formation of cladding mode resonance comb. In contrast, TFBGs are successfully inscribed using the fs-laser LbL technique at tilted angles of 2°, 4°, and 6°, where the tightly focused laser beam results in a significantly lower spectral density in the cladding compared to the core, thereby avoiding mechanical degradation of the cladding. The structural changes of the all-COP optical fiber core and cladding are smooth and uniform, with a decrease in refractive index of 6.6 × 10-3 and 3.2 × 10-3, respectively. The resonance strength and spectral width of the cladding mode comb are up to 8.2 dB and 126.9 nm, respectively. This study serves as a foundation for TFBG fabrication in all-COP optical fibers, which offer advantages of long-term thermal stability and negligible water uptake over other polymers.