Fiber amplifier

A fiber amplifier, comprising:

• optical isolators at an input end and at an output end of the fiber amplifier, wherein the optical isolator and the input end is coupled to receive few-mode signal light signals;
• a pump laser;
• a pump and signal combiner;
• a few-mode doped fiber;
• and
• wherein:
• the pump laser is configured to output pump light;
• the pump and signal combiner is configured to couple the input few-mode signal light signals and the pump light into the few-mode doped fiber;
• periodic gratings etched into the fiber wherein refractive indexes of a fiber core of the few-mode doped fiber are distributed to be gradient along a radial direction of a cross section and an effective refractive index difference Δn created by the periodic gratings between adjacent mode groups of the few-mode signal light signals is equal to a wavelength of the few-mode signal light;
• the fiber core is etched with periodic gratings along an axial direction;
• and
• periods of the gratings, namely a product of the effective refractive index difference between adjacent mode groups is equal to a wavelength of few-mode signal light, satisfy a phase matching condition for passing the few-mode signal light signals and filtering other light signals.