Inscription method for chirped fiber Bragg gratings with high spectral flatness based on a nonlinear phase mask

Degradation of the spectral flatness of chirped fiber Bragg gratings (CFBGs) in femtosecond fiber lasers ultimately leads to reduced laser performance. Degraded spectral flatness stems from spectral tilt due to short-wavelength loss and the Fabry-Pérot (F-P) effect, which produces high-frequency ripples in the reflection spectrum. To overcome these issues, this study proposes an inscription method for high spectral flatness CFBGs based on a nonlinear phase mask (PM) in which the diffraction efficiency and period exhibit nonlinear distributions. Linearly decreasing the diffraction efficiency of the PM from short to long wavelengths compensates for short-wavelength loss, while superimposing a super Gaussian apodization function with q = 4 creates a nonlinear diffraction efficiency distribution that suppresses the F-P effect. The proposed method achieves precise control of the fringe contrast of the interference field formed by the ± first-order diffracted light from the PM and modulates the axial refractive index profile of the CFBG. Simultaneously, a quadratic term is introduced into the period function of the PM, which enables the CFBG to match the multi-order dispersion parameters of the pulse compressor, achieving high-quality pulsed laser output. The method is validated using an ultraviolet laser lithography system, with the CFBG inscribed on a PM980-XP fiber achieving a spectral flatness of 97.41%, corresponding to a reflectivity ripple amplitude of 1.90%. Integration of the fabricated CFBG into a femtosecond laser system yielded a laser output with an average power of 5 W, a pulse width of 391 fs, a Gaussian-shaped pulse profile, and no substantial pulse side lobes or pedestal.