Small displacement measurement based on surface plasmon resonance and heterodyne interferometry

A new method for small displacement measurement based on surface plasmon resonance and heterodyne interferometry is presented. A heterodyne light is focused on a mirror and reflected from it, and then it is incident on a prism which was coated with a thin gold film. When the mirror or the objective lens has a small displacement, the light will be converging or diverging into the prism, and the phase variation between two parts of the test beam under the condition of surface plasmon resonance (SPR) can be measured by using a two-segment photodiode and a lock-in amplifier. This phase difference between two parts of the test beam is proportional to the departure of the mirror from the focal plane, so the displacement can be obtained in real-time. It has some merits, such as, simple, stable, very high sensitivity and resolution. And its resolution is better than 1 nm.