Intersubband spectroscopy of quantum cascade lasers under operating conditions

A technique is described that enables the mid infrared spectroscopic study of the intersubband electron distribution in quantum cascade lasers under operating conditions. By measuring the transmission of a broadband thermal source through the waveguide of a λ∼7.4μm InGaAs∕AlInAs∕InP quantum cascade laser, the redistribution of the electrons in the active region confined levels has been investigated for a wide range of operating currents and temperatures. This technique also allows direct determination of key parameters such as the temperature of the core region under operating conditions, the modal gain coefficient, and the waveguide loss.