Radio-frequency spectra of long-cavity diode lasers under CW pumping

High-frequency spectra of free-running 5-mm-long triple-quantum-well graded-index separate-confinement heterostructure broad-area diode lasers emitting at ~1 μm are investigated in the range of 1-20 GHz using RF spectrum analyzer. The spectra reveal stable beat lines at ~8 and ~16 GHz, corresponding to single and double mode spacings between adjacent longitudinal modes. A current-dependent peak, varying from 0.6 to 2 GHz, is associated with the relaxation resonance. Measurements of mode beating spectra provide additional characterization of diode laser emission for coherent light applications.