In Plane Laser-interferometric Sensor

Laser interferometer deliver non-contact and zero mass loading measurements. They provide unique opportunities by measuring the movement of macroscopic and microscopic objects with an extraordinary high resolution and precision. Usually laser interferometer measure along the out-of-plane direction, the measurement object is moved along the measurement beam. But there is an increasing need to determine the in-plane motion or vibration characteristics of structures like MEMS or industrial surfaces. An in-plane laserinterferometric sensor for measuring the lateral displacement of a surface was developed. The measuring principle of the sensor is based on the heterodyne detection of a moving object. Two laser beams with different frequencies are focused to a common point at the surface and the scattered light is collected by a common lens. The Doppler shift due to a lateral movement of the surface is detected. The sensitivity of the detection is a function of the incident light angle. This principle provides high sensitive measurements of the lateral velocity of the object. The in-plane laserinterferometric sensor consists of a measuring head and a separate controller which includes the laser light sources and the demodulation electronics. The light sources are fiber coupled, so the sensor size is only defined by the size of the optical components used and the reasonable incident angle. Due to the highly integrated design, a small sensor size can be realized by using standard optics. It can be shown that the in-plane laserinterferometric sensor is able to track surfaces with very low reflectivity as industrially rough surfaces. A short term noise with a standard deviation of 4 nm at 10 kHz bandwidth can be achieved. High precision measurements of objects displacement, speed, acceleration or frequencies are possible.