The doped fiber external cavity semiconductor laser (DFECL) has been reported with a simple structure, high power,
narrow linewidth, and stable wavelength. The DFECL is mostly suitable to be an optical carrier generator for external
modulation or microwave optical generation. Because of mode locking, the DFECL, with saturable absorber in its
external cavity, has the possibility to be direct modulated at its multiples of cavity resonant frequency. The useful
modulation frequency of the laser can be increased significantly.
In this paper, we present experimental results about the transmission response of direct modulation of an ultra-long
DFECL, and the modulated microwave signal transmission at the frequency of the 22nd. multiple of the cavity resonant
frequency. Modulated narrow bandwidth microwave signals at 2.4GHz were transmitted by this DFECL. The received
RF spectrum has no obvious distortion for a 10MHz narrow band microwave signal and, all the resonant and harmonic
frequencies in the 0~2.5GHz region are 50 dB lower than the transmitted wave. The results show that narrowband
modulated microwave can be transmitted at high frequency by the long DFECL; even through the cavity round-trip
frequency is very low. We conclude that this ultra-long doped fiber external cavity semiconductor laser can be used for
narrowband wireless communication with direct modulation.