Investigation of the fiber Bragg grating inscribed in multimode fiber by femtosecond laser

Multimode fiber Bragg grating (MFBG) is inscribed in graded-index multimode fiber using infrared femtosecond laser and phase mask. Multiple dips are observed in the transmission spectrum of MFBG. There is not only self-coupling effect between the forward and backward principal modes, but also the mutual coupling effect between the neighbor modes. The induced refractive index modulation of 6.4 × 10 ⁻⁴ is achieved for a peak power density of 4.1 × 10¹³ W/cm² without any fiber sensitization. Thermal experiment results show that the MFBG belongs to type I grating. The transmission spectrum of MFBG shifts linearly with the strain and pressure at sensitivities of 1.27 ± 0.002 pm/με and −3.3 ± 0.06 pm/MPa, respectively. The MFBGs are expected to be used for multichannel filters, multiwavelength fiber lasers, and multiparameter sensing.