A single-frequency narrow linewidth fiber laser with linear laser cavity is presented and demonstrated experimentally.The moving refractive index grating and gain grating,which are induced by interfering between the feedback light from the part of output light and the lasing light in intra-cavity,are used to select laser longitudinal modes efficiently.A section high gain erbium-doped fiber(EDF) is used as gain medium and is pumped by a single mode 975 nm laser diode(LD),and a stable single-frequency laser a...