Summary of the technology This technology relates to an L-band (Long wavelength-band) optical fiber optical source to raise radiating efficiency amplifying power of an L-band natural light. This technology, related to an L-band optical fiber optical source for raise the efficiency of the emitting light, is comprised of an erbium-added optical fiber amplifier pumped with a pumping light, an isolator that prevents from moving to opposite direction installed in the latter part of the afore said erbium-added optical fiber amplifier therein, a wavelength division multiplexer that receives a pumping source installed in the previous part of the afore said erbium-added optical fiber amplifier, a wavelength selective mixer that divides a amplified spontaneous emission into an L-band (Long wavelength-band) and a C-band (Conventional-band) in an erbium-added optical fiber amplifier, a reflector that reflects the L-band light installed in the previous part of the afore said wavelength selective mixer, and an FBG (Fiber Bragg Grating) that selectively reflects the C-band light.