Displacement Sensing
Precision measurement of sub-nanometer movements utilizes the phase difference between a reference light beam and a beam reflected from a vibrating surface. Implementing an optical interferometric readout allows for the non-contact detection of mechanical displacement in high-frequency resonators. This technique provides exceptional sensitivity, making it a primary tool for characterizing micro-machined sensors and calibrating reference transducers.
Optical Configuration
A laser source, a beam splitter and a photodetector form the core of the measurement system. In a typical optical interferometric readout, the light is split into two paths, one of which interacts with the moving sensor element. The reflected light recombines with the reference beam, creating interference fringes that the photodetector converts into an electrical signal.
This optical method avoids the parasitic capacitance issues associated with electrical sensing.
Calibration Procedure
System calibration requires aligning the laser beam to the point of maximum displacement on the vibrating structure. Metrologists use a piezo-actuated stage to calibrate the voltage-to-displacement conversion factor, ensuring that the sensor response is mapped accurately to the wavelength of the laser. Since temperature changes alter the refractive index of air and cause thermal expansion in the optical mounts, the entire setup is housed in a temperature-controlled chamber to prevent drift.
Limit of Detection
Shot noise in the photodetector establishes the fundamental limit of the measurement resolution. In practice, the system can resolve displacements down to the picometer scale when the laser intensity is stable.
