Measurement of the Linewidth Enhancement Factor of InGaAlAs and InGaAsP laser diodes using the Fourier Series Expansion of the Amplified Spontaneous Emission spectrum

The optical gain and linewidth enhancement factor (alpha) of a semiconductor laser are important parameters for device performance and design, especially for widely tunable lasers. The Hakki-Paoli (H-P) method is the most common method for measuring alpha. However this method of determining alpha is dependent on the resolution bandwidth of the analyser used. Here we introduce the Fourier series expansion (FSE) method to calculate alpha by first finding the round trip gain. By performing a Fourier series expansion on each longitudinal mode in the cavity and finding the ratio of two successive Fourier coefficients.