1：Experimental Design and Method Selection:

The study involved fabricating fiber Bragg gratings (FBGs) in polymer optical fibers (POFs) using a phase mask technique with a multiple order phase mask. The experimental setup allowed for precise control of the tilt angle φ between the phase mask and the fiber. Reflection spectra were monitored during UV irradiation to observe peak splitting and evolution.

2：Sample Selection and Data Sources:

A step-index POF made of PMMA cladding and PMMA/PS copolymer core was used, fabricated at Maria Curie-Sklodowska University. The fiber was annealed at 85°C for 10 hours before inscription to improve spectrum quality.

3：List of Experimental Equipment and Materials:

Equipment included a CW 30 mW He-Cd laser (λUV = 325 nm), a phase mask from Ibsen Photonics (Λ = 1052 nm), plano-convex lenses (focal lengths 75 mm and 25 mm), a silica fiber coupler, a supercontinuum source (SuperK Versa, NKT Photonics), an optical spectrum analyzer (AQ6370C, Yokogawa Electric Corporation), index matching gel, goniometer stage (OptoSigma), rotational stage (OptoSigma), and camera with DLTCamViewer software for angle measurement.

4：Experimental Procedures and Operational Workflow:

The UV beam was focused on the fiber core using lenses. The phase mask was tilted at various angles φ, and reflection spectra were recorded during irradiation. Angles were adjusted and measured with high precision. The inscription process involved monitoring peak positions and separations over time.

5：Data Analysis Methods:

Data were analyzed by comparing experimental peak positions and separations with theoretical predictions based on a model considering interference of diffraction orders. Linear fits were applied to peak separation data, and statistical analysis included fitting errors.