A Heterodyne Mach-Zehnder Interferometer employing static and dynamic phase demodulation techniques for live-cell imaging

This paper describes a temporal carrier based Heterodyne Interferometer and associated phase demodulation techniques which are suitable for phase imaging of live cells. A Mach-Zehnder Interferometer is integrated to the microscope and two acousto-optic modulators are employed, to generate a temporal carrier that allows heterodyne approach to phase demodulation. Two demodulation schemes are presented: (a) Digital heterodyne phase extraction technique to extract the static phase information of the carrier signal, and (b) dynamic phase extraction technique for extracting phase variation in the carrier signal. The Heterodyne interferometer enables fast phase imaging and coupled with digital heterodyne phase extraction process, the system provides excellent temporal phase stability (standard deviation < 2 nm for 16 second measurement). This technique is employed for quantitative phase imaging of 3T3 fibroblast cells immersed in cell media. When there is phase variation, the temporal carrier signal is modulated and its instantaneous frequency is directly related to the variation. The dynamic phase extraction technique first determines the instantaneous frequency, which is then integrated with respect to time to obtain timevarying phase. The algorithm is able to extract a time varying phase, caused by a stimulated vibration at 30 Hz and 40 nm amplitude.