The present disclosure relates more particularly to active optical fibers, amplified spontaneous emission (ASE) sources using such active optical fibers, and imaging and detection systems and methods using such ASE sources. In one aspect, the disclosure provides an active optical fiber that includes a rare earth-doped gain core configured to emit radiation at at least a peak wavelength emitted wavelength when pumped with pump radiation having a pump wavelength; a pump core surrounding the gain core; and a cladding surrounding the pump core, wherein the value M=16R2(NA)2/λ2 in which R is the gain core radius, NA is the active optical fiber numerical aperture, and λ is the peak emitted wavelength, is at least 50, or at least 100. The present disclosure also provides an optical source that includes the optical fiber coupled to a pump source.