Sub-nanometric precision displacement sensing using low mass and rough surfaces as movable targets

A novel speckle interferometric method has been developed to measure lengths and displacements over a distance of 100 μm with a resolution less than 1 nm using rough surfaces as movable targets instead of using conventional cube corners or mirrors. The speckle method is able to use Michelson and new configurations to combine the two scattered beams or fields from rough movable and stationary reference targets. The targets can be attached and can be produced directly onto movable actuator type of mechanisms. The phase information is extracted from the randomly modulated signals by employing an apertured disc system or an alternative optical fibre arrangement. This method exploits the concept of using narrow angle scatterers for economical use of laser light, enhanced reliability and modulation criteria. A prototype low-mass interferometric displacement sensor has been constructed to provide a compact and a flexible system. Apart from the measurement of changes in displacements in force and pressure measuring devices, it can also be used for the assessment of small range scanning probes, stylus profilers and in medical field the measurement of elastic properties of ear drums, etc. The sub-nanometre resolution and displacement values are repeatable to within ±1 nm can be demonstrated over distances of up to 2 mm by employing appropriate actuators and translation devices. The performance has been confirmed by comparison work against a conventional interferometric transducer.