Measurement of spontaneous emission spectra of diode laser structures

Abstract

The unamplified spontaneous emission spectrum emitted under high injection conditions provides a valuable insight into the behaviour of semiconductor laser structures. To determine this spectrum directly, amplification or absorption of the emission must be avoided, for example by observation through a top-contact window. While it is possible, under favourable circumstances, to transform the emission spectrum to a gain spectrum, it is preferable to measure optical gain by more direct methods, usually by analysis of the amplified spontaneous emission from the end of the device, for example by a single-pass stripe length method. A complete characterisation of the semiconductor gain medium therefore requires a combination of techniques for measurement of spontaneous emission and of optical gain as has been done by fabricating a segmented-contact structure for a single-pass gain measurement with a window in the top contact for observation of spontaneous emission. However this is a complex device and it is necessary to observe emission from the device in two different directions. Furthermore the gain and emission spectra are obtained from different regions of the sample. In this paper we show that the gain and true spontaneous emission spectrum can be obtained from amplified spontaneous emission spectra alone. We confirm the validity of the process by comparison with the true spontaneous emission spectrum observed through a top-contact window and show how the emission spectrum may be expressed in real units. This approach provides considerable simplification of the technique and enables both the TE and TM emission spectra to be measured, which is not possible with window methods.