Abstract

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 μm thick GaSe crystal. The coherent detection works up to 45 THz, employing the electro optic effect in a 7 μm thick ZnTe crystal. This detection scheme is invariant to the strong co-emitted light fields of the QCL above threshold, and thus, allows us to measure the whole gain dynamics below and above threshold. Due to the recovery of the complete amplitude and phase information we can iteratively solve for the complex refractive index in a wide spectral range (18-33 THz). We determine the waveguide and the mirror losses at the emission frequency of 25.4 THz in good agreement with the values deduced from the threshold current densities of different resonator lengths [1].

© 2009 IEEE