Analysis of sub-picosecond mid-infrared pulse propagation in a quantum cascade laser below and above threshold

Spectroscopy done on the light emitted by quantum cascade lasers (QCLs) provides only limited information on the optical , thermal, and temporal properties in such a laser. We study such lasers by time domain electric field spectroscopy of sub-picosecond pulse trains propagating through the QCL cavity. This is done by generating broadband mid-infrared pulses by phase matched difference frequency mixing in a 30 mum thick GaSe crystal.