Since the invention of the laser, electro-optical instrumentations and non-contact sensors made their appearance on worldwide markets as an alternative to traditional approaches. In fact, laser sources provide a light beam that has specific proprieties (spatial and temporal coherence, small divergence) that makes laser sensing techniques very reliable, accurate and suitable for non-contact measurements of vibrations, distance, velocity and displacement.
Traditionally, vibrational tests are carried out by using accelerometers. An accelerometer is an electromechanical device that is based on a mass-spring system and measures acceleration forces. The main problems of accelerometers are that they are contact sensor that must be glued onto the surface under test with the negative consequences that the mass of the sensor and of the electric cables influences the measurement and changes the resonance frequencies of the object under test, and that measurements in hardly accessible places, like cavity or holes, are not allowed.
Contrary to the accelerometers, optical sensing techniques allow to perform non-contact and non-invasive measurements on targets and objects that are difficult to access, or that are too small or too hot to attach a physical transducer. Furthermore, optical sensing techniques allow vibration measurement without mass-loading, which is especially important for the characterization of small objects (e.g., MEMS – Micro Electro-Mechanical Systems – …). Moreover, dynamical measurements can be performed with high sensitivity and resolution.
Main laser techniques for ranging are based on the following principles:
• Time of flight – The distance or position information is obtained by measuring the time T required for a light pulse to go to the target and back, with T=2L/c (where c is the speed of light). The distance L can be retrieved as L=c*T/2. This technique can be applied with both pulsed light or CW (Continuous Wave) sources.
• Triangulation – A combination of a laser source and a CCD sensor or PSD (Position Sensitive Detector) is used. The target is aimed by a laser beam, generally pulsed, and the back-scattered light is collected by a lens (that “looks” at the target from a different angle with respect to that of the incoming laser light) and imaged on the surface of a distributed optical sensor, (CCD or PSD). When the target moves, the position of the spot on the sensor changes, and the vibration or displacement can be measured.
• Interferometry – A coherent beam propagates to the target and back-travelling field is coherently detected by beating it with a reference field on a photodetector. Signals of the form cos(2ks) and sin(2ks) (where k=2π/λ and s the displacement) are obtained. Through an analysis of the signal cos(2ks) the displacement can be retrieved with a resolution of λ/2, or even better. This technique is used to measure vibration, displacement, and velocity.