A rare-earth-doped-fiber light source with wavelength stability under radiation from the environment having a ratio of mean wavelength change to a mean wavelength of less than a few hundred to a few thousands ppm, the rare-earth-doped-fiber light source comprising: a rare-earth doped fiber used as an active medium for the broadband light source, the rare-earth doped fiber having a host material and a cross-sectional structure, the cross-sectional structure of the rare-earth doped fiber including a fiber core doped with rare-earth ions; an undoped fiber having the same host material and the same cross-sectional structure as the rare-earth doped fiber, the cross-sectional structure of the undoped doped fiber including a rare-earth-dopant-free fiber core , wherein the undoped fiber and the rare-earth doped fiber are placed in proximity to each other by being attached to each other using attachment fixtures, wherein the length (L) of the undoped fiber is one of the same as that of the doped fiber or optimized to match a radiation sensitivity of the doped fiber so as to exhibit the same host-glass loss and loss recovery dynamics as the rare-earth doped fiber; a coupler configured to direct a first portion of pump power from a pump laser to the undoped fiber, wherein the first portion of pump power was twice passed through the coupler; and a wavelength division multiplexer configured to input a second portion of pump power from the pump laser which is output from the coupler to the rare-earth doped fiber; a first photodetector configured to provide a signal indicative of the power of the first portion of the pump power ; a second photodetector configured to provide a signal indicative of the output power of the undoped fiber ; and a controller configured to control the mean wavelength of the rare-earth-doped-fiber source through pump power adjustment based on the signals of the first photodetector and the second photodetector. a rare-earth doped fiber used as an active medium for the broadband light source, the rare-earth doped fiber having a host material and a cross-sectional structure, the cross-sectional structure of the rare-earth doped fiber including a fiber core doped with rare-earth ions; an undoped fiber having the same host material and the same cross-sectional structure as the rare-earth doped fiber, the cross-sectional structure of the undoped doped fiber including a rare-earth-dopant-free fiber core , wherein the undoped fiber and the rare-earth doped fiber are placed in proximity to each other by being attached to each other using attachment fixtures, wherein the length (L) of the undoped fiber is one of the same as that of the doped fiber or optimized to match a radiation sensitivity of the doped fiber so as to exhibit the same host-glass loss and loss recovery dynamics as the rare-earth doped fiber; a coupler configured to direct a first portion of pump power from a pump laser to the undoped fiber, wherein the first portion of pump power was twice passed through the coupler; and a wavelength division multiplexer configured to input a second portion of pump power from the pump laser which is output from the coupler to the rare-earth doped fiber; a first photodetector configured to provide a signal indicative of the power of the first portion of the pump power ; a second photodetector configured to provide a signal indicative of the output power of the undoped fiber ; and a controller configured to control the mean wavelength of the rare-earth-doped-fiber source through pump power adjustment based on the signals of the first photodetector and the second photodetector. <EOS>