In this work, we present a new compact diffractive laser encoder system, which serves as a positional detection apparatus in precision machine applications. The encoder records displacement information in terms of grating period using the Doppler effect. Using the Littrow configuration, the novel encoder provides high alignment tolerances. The design is special such that a change in the gap between the grating and the optical head does not affect the measurements. Therefore, a Michelson interferometer can be added to the system to measure the out-of-plane displacement. This system will be developed as a threedimensional displacement sensor in the future. Within a measurement distance of 100 mm, even in the laboratory environment, the maximum error is 53 nm and the repeatability is within AE20 nm.
This investigation presents a planar diffractive laser encoder system (PDLENS), which serves as a two-dimensional (2D) position detection apparatus for precision machine applications. Traditional 2D position detection utilizes a pair of linear encoders in crossed construction and so maintaining the perpendicularity between this pair of encoders is difficult. Besides, the rigorous alignment requirements among various components of the encoder system impose a serious user adaptation bottleneck. Of all alignment tolerances, the head-to-scale alignment tolerance is the most important problem for applications. In this work, a 2D grating is employed as the scale and the PDLENS is based on the retro-reflection configuration. Therefore the new encoder can provide good perpendicularity and tolerate larger alignment errors than the conventional encoder does. The grating pitch is 1.6μm and the period of the output signal is 0.4μm due to the double diffraction. Electronic interpolation with a factor of 400 leads to a readout resolution of 1 nm. The new encoder and a capacitive sensor were employed to simultaneously measure a circular motion with a radius of 1μm generated by a piezo stage. Comparing the measured positions, the deviation is less than 30 nm and the repeatability is better than 8 nm.
Many mechanical and optical components contain step features whose surface
height changes far exceed the optical wavelength. Therefore, this work presents an
interferometer based on variable synthetic wavelength interferometry (VSWI) and
differential heterodyne configuration to measure large step heights directly and
unambiguously. This largely common-path configuration can substantially reduce
the influence of environmental disturbances, which are the main sources of error in
the VSWI. Only one external cavity diode laser (ECDL) is employed to synthesize
a series of synthetic wavelengths in descending order. The wavelengths
are combinations of the varied wavelengths and the initial wavelength
of the ECDL. In contrast to wavelength scanning interferometry, this
method does not require the laser wavelength to be continuously tuned.
The step height is sequentially measured at these synthetic wavelengths
and a lock-in amplifier resolves the corresponding synthetic fractional
fringes. The step height is determined following a succession of optical
path difference calculations, in terms of the synthetic wavelengths and
measured synthetic fractional fringes. Three known step heights, verified
by a gauge block interferometer, were used to confirm the performance
of the proposed system. The results reveal that the uncertainty in the
measurement is approximately 80 nm when the measured height is up to 25 mm.
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