Comparison of different methods for optical gain spectroscopy

The internal losses of green and blue laser diodes are challenging to determine because of the narrow longitudinal mode spacing. Furthermore, the internal losses of state-of-the-art blue and green laser diodes are in the range of only a few inverse centimeter. Therefore the dynamical range given by the maxima and minima of the longitudinal mode spectrum is very large, even for moderate optical gain. Under these conditions, the usually employed, so-called Hakki–Paoli method to determine the optical gain becomes inaccurate. Now, we compare this with two other methods, the Cassidy method and an evaluation based on a Fourier transformation for a green laser diode. An error estimation as well as a correction of the systematic error caused by the spectral resolution of the setup were established. The overall highest gain was measured with the Cassidy method in the range of the lasing wavelength, as this method is least affected by the spectral resolution. In comparison of all methods, the highest gain for the wavelengths above the lasing wavelength is observed for one variation of the Fourier method, because background noise has the least influence on this method. For wavelengths below lasing wavelength we see similiar optical gain for all methods.