Improving a commercially available heterodyne laser interferometer to sub-nm uncertainty

Abstract

Laser interferometer systems are known for their high resolution, and especially for their high range/resolution ratio. In dimensional metrology laboratories, laser interferometers are popular workhorses for the calibration of displacements. The uncertainty is usually limited to about 10 nm due to polarization-and frequency mixing. For demanding applications however nanometer uncertainty is desired. We adapted a commercially available heterodyne laser interferometer by feeding the measurement signal into a fast lock-in amplifier and use the laser interferometer reference signal as a reference. By measuring both the in-phase and quadrature component an uncorrected phase can be directly measured. By recording both components while the phase changes between 0 and 2π a typical ellipse is recorded from which the first and second harmonics of periodic deviations can be derived. These can be corrected independent of their origin. Measurements using a Babinet-Soleil compensator show that this method can reduce significant non-linearities (16 nm top-bottom) to a standard deviation down to about 0.01 nm. This is confirmed in the evaluation of some standard optical-set-ups. With this method, also optical set-ups can be analysed to predict the non-linearities when a non-compensated standard interferometer is used.

Recent advances of the metrological AFM at INRIM

“…A technique based on fast lock-in detection of phase-quadrature signals derived from the reference and measuring heterodyne beats, has been taken to determine the periodical error by the offset and elliptical path of the phase from the quadrature signals [11,12]. In principle, phase errors are sampled before and after AFM scans or images are taken, then used for an afterward correction of the interferometer readings.…”

Balanced plane-mirror heterodyne interferometer with subnanometer periodic nonlinearity

“…These combinations with imperfect polarizing optics, polarization alignment between the source and optics, and ghost reflections create additional periodic nonlinearity errors [4,6,7]. Periodic nonlinearity could be reduced using either compensation techniques [1,[8][9][10] or two spatially separated beams [10,11]. For example, Joo et al recently proposed three novel heterodyne interferometer configurations without periodic nonlinearity [12,13].…”

Influence of tilt on collinear calibration of a laser interferometer

“…Interferometer-based measurement methods and applications have become a hot issue for researchers and scholars [15][16][17][18][19][20][21][22]. The fundamental function is length measurement, which is the basis of other functions of laser interferometers.…”