Spectroscopy and microscopy analysis of semiconductor lasers in student laboratories

Light emitting diodes and semiconductor lasers are key elements of photonic systems, and every student enrolled in applied optics or photonics programs at universities learns about their fundamental physics and applications. We present part of a laboratory course for students of a photonics Master’s program which complements theoretical lectures on these fundamental photonic components. During this course, students investigate diode lasers using four different techniques with emphasis on the transition from spontaneous emission (LED mode) to lasing action. The first two parts—recording diode characteristics of current versus voltage and output power versus current, as well as determining the spatial characteristics of the commercial diode lasers—are common in most similar laboratory courses. Here we focus on the remaining two investigations. On the one hand, diode laser dimensions are quantitatively analyzed using visible microscopy and also electron microscopy. This includes images of the laser chip during operation in LED or laser mode which allows us to gain an understanding of light propagation in and around the active layer waveguide. On the other hand, changes of the emission spectra while increasing the injection current from LED to laser operation are measured with high accuracy using a Fourier transform spectrometer. This allows determination of the longitudinal mode spacing, which is in very good agreement with intuitive, theoretical predictions, though parts of the analysis are surprising for many students.