Integrated dual grating method for extended range interferometric displacement detection in probe microscopy

The authors introduce a method that uses quadrature phase-shifted dual gratings to increase the range of optical interferometric displacement detection in phase-sensitive grating based microsensors. The concept is experimentally demonstrated on a surface micromachined probe microscopy structure suspended over a quartz substrate and two reflective gratings. Quadrature phase shift is obtained via a micromachined step in the quartz substrate below one of the gratings. Measurement of the intensity of the reflected diffraction orders from each grating while the mechanical structure is moved using an integrated actuator shows that interferometric sensitivity is maintained well beyond the quarter wavelength limit of similar sensors.