Measurement of optical cavity properties in semiconductor lasers by Fourier analysis of the emission spectrum

We present several observations on a novel method for the evaluation of the internal loss properties in semiconductor lasers. The method we use involves Fourier analysis of the Fabry-Perot mode spectrum when operating the device below lasing threshold. The observation of various structural features in the Fourier transform domain allows us to extract important information on the laser cavity. As one example, the amount of cavity propagation loss/gain, or net gain, can be derived from the decay rate of harmonics of the Fourier spectrum. A comparison between experimental and calculated gain versus wavelength data for lasers fabricated in the AlGaAs, AlGaInP, and AlGaInN material systems is given. As a second example, this method also allows the identification of the density and strength of intracavity scattering centers. This is an important capability for the fabrication of blue diode lasers in the gallium-nitride material system.