Dynamic tracking down-conversion signal processing method based on reference signal for grating heterodyne interferometer

Abstract

Traditional displacement measurement systems by grating, which purely make use of fringe intensity to implement fringe count and subdivision, have rigid demands for signal quality and measurement condition, so they are not easy to realize measurement with nanometer precision. Displacement measurement with the dual-wavelength and single-grating design takes advantage of the single grating diffraction theory and the heterodyne interference theory, solving quite well the contradiction between large range and high precision in grating displacement measurement. To obtain nanometer resolution and nanometer precision, high-power subdivision of interference fringes must be realized accurately. A dynamic tracking down-conversion signal processing method based on the reference signal is proposed. Accordingly, a digital phase measurement module to realize high-power subdivision on field programmable gate array (FPGA) was designed, as well as a dynamic tracking downconversion module using phase-locked loop (PLL). Experiments validated that a carrier signal after down-conversion can constantly maintain close to 100 kHz, and the phase-measurement resolution and phase precision are more than 0.05 and 0.2 deg, respectively. The displacement resolution and the displacement precision, corresponding to the phase results, are 0.139 and 0.556 nm, respectively.

Modeling and Analysis of Phase Fluctuation in a High-Precision Roll Angle Measurement Based on a Heterodyne Interferometer

“…From the mathematical model, reduction of the fluctuation amplitude of the frequency difference is a way to reduce the phase fluctuation, but the frequency stability of one of the better commercial dual frequency lasers can only reach 1 kHz, which is still too large and would cause an obvious error in phase metering. Frequency reduction is verified to be an effective method to reduce the frequency fluctuation [21,22]. In this study, a stabilized He-Ne laser with two acousto-optic modulators (AOM) is adopted to generate dual frequency beams [9,23], wherein two AOM (Gooch and Housego AOMO 3080-125) with adjustable RF drivers (AODS Synth DDS 8 CH) are employed, as shown in Figure 7.…”

“…It cannot be directly interpreted by air turbulence or other environmental factors or the repeatability of the phase-detecting electronics, because its fluctuation period is the same as the frequency fluctuation, which is mainly resulted from frequency fluctuation. Frequency fluctuation is a common phenomenon and can reach tens or hundreds of kHz, which has been also observed by other scholars [21,22]. These authors presented a dynamic base signal with a phase-locked loop to track the changing frequency in their frequency mixing and frequency reduction, but they did not mention the phase fluctuation, nor did they investigate the reason or mechanism.…”

Optimized design and error analysis of optical system for heterodyne grating interferometry

“…We have presented a heterodyne grating interferometer for displacement measurement in previous work 7 . In this study, we modified the optical configuration as shown in figure 1.…”

Compact grating displacement measurement system with a 3 \times 3 coupler

“…Classical grating displacement measurement systems, such as the dual-grating measurement system, work on principles of Moiré fringe in a long range but with low resolution [5,6] . Recently, various types of grating interferometers have been developed to measure linear displacement or multi-degree-of-freedom (DOF) position with nanometer resolution, such as single-metric grating measurement system [7,8] , Littrow-type grating interferometers [9,10] , heterodyne grating interferometers [11,12] , and DOF linear encoders [13] . Although most of these systems have their own merits and specific applications, they are usually built by relatively complicated optical setup, leading to difficulty in alignment.…”