The energy structure and the value of the electrooptic effect in heteroepitaxial structures (HES) with multiple InGaAlAs / InAlAs quantum wells have been studied. The interferometric method has been developed to determine small changes in the refractive index under transverse transmission of light through a layered structure. The length of the Mach-Zehnder interferometer for the modulator at HES with multiple InGaAlAs / InAlAs quantum wells has been chosen.
We report the theoretical and experimental study of calibration-free heterodyne phase-sensitive dispersion spectroscopy (HPSDS) in the mid-infrared using a direct current modulated mid-infrared quantum cascade laser (QCL). The modulation of QCL current at several hundred MHz or higher generates the synchronous frequency and intensity modulation of the QCL emission. An analytical model of the phase of the beat note signal in HPSDS is derived by considering the absorption and dispersion processes and incorporating the QCL modulation parameters. In the experiment, a 4.5 μm QCL modulated at 350 MHz was used to measure N2O at 200 Torr in a 10 cm gas cell. The N2O concentrations inferred from the analytical model were compared with the nominal values to show good agreement over the concentration range of 189−805 ppm with a standard deviation <3%. When the QCL wavelength was locked at the line-center of the molecular transition, it was of interest to find that the theoretical model was simplified to that used for near-infrared HPSDS with an electro-optical modulator for laser modulation.
Heterostructure InGaAlAs/InAlAs on the InP substrate for the electro-optical modulator based on the quantum confined Stark effect