Probing the electronic and optical properties of quantum cascade lasers under operating conditions

We report the realisation of spectroscopic broadband transmission experiments on quantum cascade lasers (QCLs) under continuous wave operating conditions for drive currents up to laser threshold. This technique allows, for the first time, spectroscopic study of light transmission through the waveguide of QCLs in a very broad spectral range (λ~1.5-12 μm), limited only by the detector response and by interband absorption in the materials used in the QCL cladding regions. Waveguide transmittance spectra have been studied for both TE and TM polarization, for InGaAs/InAlAs/InP QCLs with different active region designs emitting at 7.4 and 10μm. The transmission measurements clearly show the depopulation of the lower laser levels as bias is increased, the onset and growth of optical amplification at the energy corresponding to the laser transitions as current is increased towards threshold, and the thermal filling of the second laser level and decrease of material gain at high temperatures. This technique also allows direct determination of key parameters such as the exact temperature of the laser core region under operating conditions, as well as the modal gain and waveguide loss coefficients.