Time-resolved orientation detection system with quantum cascade lasers

A system with the combination of quantum cascade lasers (QCLs) and a photoelastic modulator (PEM) has been designed and constructed, which can achieve orientation detection with a time resolution of nearly 20 µs based on the principle of infrared dichroism, several orders of magnitude higher than that of the general mid-infrared spectrum. PEM with an inherent frequency of 50 kHz is employed to modulate the polarization direction of infrared light rapidly, the controller of which is used to provide the external trigger signal. A double frequency and delay circuit is fabricated to match the frequency of QCLs and PEM as well as overcome the delay during transfer of the trigger signal to a QCL controller, which can realize a minimum delay resolution of 5 ns. Also, a data acquisition program is compiled to reduce the data size, making continuous collection possible and lowering difficulty in data processing. The system is combined with the home-made biaxial stretching equipment to conduct the sequential biaxial stretching of β-polypropylene (PP) films. It shows that the orientation factor of polymer chains increases from 0.04 to 0.36 during machine stretching, which decreases to 0 during transverse stretching, fitting well with the orientation factors estimated with FTIR. The result robustly proves the feasibility of the system for rapid orientation detection.