Measurement of FM noise, AM noise, and field spectra of 1.3 µm InGaAsP DFB lasers and determination of the linewidth enhancement factor

The FM and AM noise spectra of 1.3 μm InGaAsP DFB lasers are measured in the frequency range from dc to 4 GHz. The relaxation resonances appearing in these spectra are compared to the semiclassical theory of laser noise. All the resonance parameters, i.e., the linewidth enhancement factor α, the resonance frequency f
R
, and the damping constant γ
e
, are determined from the FM noise spectra by successful curve fitting. The estimated value of α is 2.2. Field spectra for various bias currents are measured by using optical heterodyne detection. Theoretical lineshapes are obtained by using four noise-parameter values which have been determined from the FM noise and the linewidth measurements. The results are in excellent agreement with the measured spectra. This agreement verifies the estimation that
\alpha = 2.2.