Characterization of quantum-cascade lasers using single-pass transmission spectroscopy

The study of the electron distribution and intersubband transitions in quantum cascade (QC) lasers provides fundamental insights into the laser operation. Measurements on intersubband transitions are often performed on QC lasers under nonoperating conditions, i.e., at zero electric field. In this study, a single-pass transmission technique that allows for probing of QC lasers under operating conditions was used. The measurements and analysis were done on QC lasers with vertical and diagonal transition active region designs operating at λ ∼ 10 μm. The single-pass modal gain coefficients, extracted for the vertical transition design, were 24.04 cm/kA at 100 K, 21.36 cm/kA at 125 K, 14.25 cm/kA at 150 K, and 5.26 cm/kA at 175 K.