Time-resolved orientation detection system with quantum cascade lasers

[Ke Ye, Review of Scientific Instruments]   

Post doctor Meng and master Ye et al. develop a method which have a high time resolution in orientation detection based on quantum cascade lasers. The related work is published inReview of Scientific Instruments, 2018, 89(7): 073101.

A system with the combination of quantum cascade lasers (QCLs) and photoelastic modulator (PEM) has been designed and constructed, which can achieve orientation detection with time resolution of 20 µs based on the principle of infrared dichroism, several orders of magnitude higher than that of general mid-infrared spectrum. PEM with 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 the trigger signal to QCLs 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 of data processing. The system is combined with a home-made biaxial stretching equipment to conduct the sequential biaxial stretching of β-polypropylene (PP) film. 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. The result which fits well with the orientation factors estimated with FTIR robustly proves the feasibility of the system for rapid orientation detection.

Fig High time resolution orientation detection system based on quantum cascade lasers