Femtosecond laser plane-by-plane inscribed ultrahigh-order fiber Bragg grating and its application in multi-wavelength fiber lasers

We propose an ultrahigh-order fiber Bragg grating (UHO-FBG) containing dense resonants and its application as a novel filtering device in multi-wavelength lasers. The UHO-FBG is fabricated by femtosecond laser plane-by-plane direct inscription. Thanks to the plane-by-plane inscription, high-order Bragg resonances can be formed with multiple reflectance peaks of comparable reflectance in the range of the fiber operating bandwidth and without the transmission depression of long-period gratings in the transmission spectrum. We also experimentally demonstrate the use of UHO-FBG pairs in a distributed Bragg reflector laser, enabling the excitation of multi-wavelength lasers.