Novel homodyne frequency-shifting interference pattern locking system

We present a novel homodyne frequency-shifting interference pattern locking system to enhance the exposure contrast of interference lithography and scanning beam interference lithography(SBIL). The novel interference pattern locking system employs a special homodyne redundant phase measurement interferometer(HRPMI) as the sensor and an acousto-opto modulator(AOM) as the actuator. The HRPMI offers the highly accurate value as well as the direction recognition of the interference pattern drift from four quadrature interference signals. The AOM provides a very fine resolution with a high speed for phase modulation. A compact and concise system with a short optical path can be achieved with this new scheme and a small power laser head in tens of microwatts is sufficient for exposure and phase locking, which results in a relatively low-cost system compared with the heterodyne system. More importantly, the accuracy of the system is at a high level as well as having robustness to environmental fluctuation. The experiment results show that the short-time(4 s) accuracy of the system is 0.0481 rad e3σT at present. Moreover, the phase of the interference pattern can also be set arbitrarily to any value with a high accuracy in a relatively large range, which indicates that the system can also be extended to the SBIL application.