A scanning heterodyne interferometer with immunity from microphonics

The advantage of optical profilometers based on a common path type interferometer is their insensitivity to microphonics. This means that such systems are capable of very accurate phase measurements. In this paper we will present a system with this characteristic. Two beams are directed onto the sample along the same path. One beam is focused onto the sample surface and the second remains collimated, acting as a large area reference. This is achieved using a specially constructed lens which allows the relative size of the illuminated areas on the sample to be chosen arbitrarily, giving a large reference and a high signal to noise ratio. In most existing systems this ratio is not independent and there is a trade-off between the effective ratio of the spots and interference efficiency. In our system no such trade-off exists. Each reflected beam is interfered with a third frequency shifted beam. They are then detected separately, resulting in two AC signals. The phase of these signals are then compared to provide the object surface phase structure. Path length fluctuations due to microphonics are cancelled by this comparison. Results obtained from a prototype version of the system will be presented.