Lih-Horng Shyu

The Comparison of Environmental Effects on Michelson and Fabry-Perot Interferometers Utilized for the Displacement Measurement

The optical structure of general commercial interferometers, e.g., the Michelson interferometers, is based on a non-common optical path. Such interferometers suffer from environmental effects because of the different phase changes induced in different optical paths and consequently the measurement precision will be significantly influenced by tiny variations of the environmental conditions. Fabry-Perot interferometers, which feature common optical paths, are insensitive to environmental disturbances. That would be advantageous for precision displacement measurements under ordinary environmental conditions. To verify and analyze this influence, displacement measurements with the two types of interferometers, i.e., a self-fabricated Fabry-Perot interferometer and a commercial Michelson interferometer, have been performed and compared under various environmental disturbance scenarios. Under several test conditions, the self-fabricated Fabry-Perot interferometer was obviously less sensitive to environmental disturbances than a commercial Michelson interferometer. Experimental results have shown that induced errors from environmental disturbances in a Fabry-Perot interferometer are one fifth of those in a Michelson interferometer. This has proved that an interferometer with the common optical path structure will be much more independent of environmental disturbances than those with a non-common optical path structure. It would be beneficial for the solution of interferometers utilized for precision displacement measurements in ordinary measurement environments.

Influence of intensity loss in the cavity of a folded Fabry-Perot interferometer on interferometric signals

Fabry-Perot interferometer is often used for the micro-displacement, because of its common optical path structure being insensitive to the environmental disturbances. Recently, the folded Fabry-Perot interferometer has been investigated for displacement measurements in large ranges. The advantages of a folded Fabry-Perot interferometer are insensitive to the tilt angle and higher optical resolution. But the design of the optical cavity has become more and more complicated. For this reason, the intensity loss in the cavity will be an important parameter for the distribution of the interferometric intensity. To obtain a more accurate result of such interferometer utilized for displacement measurements, the intensity loss of the cavity in the fabricated folded Fabry-Perot interferometer and the modified equation of the folded Fabry-Perot interferometer will be described. According to the theoretical and experimental results, the presented model is available for the analysis of displacement measurements by a folded Fabry-Perot interferometer.

Fabry–Pérot interferometer utilized for displacement measurement in a large measuring range

The optical configuration of a Fabry-Pérot interferometer is uncomplicated. This has already been applied in different measurement systems. For the displacement measurement with the Fabry-Pérot interferometer, the result is significantly influenced by the tilt angles of the measurement mirror in the interferometer. Hence, only for the rather small measuring range, the Fabry-Pérot interferometer is available. The goal of this investigation is to enhance the measuring range of Fabry-Pérot interferometer by compensating the tilt angles. To verify the measuring characteristic of the self-developed Fabry-Pérot interferometer, some comparison measurements with a reference standard have been performed. The maximum deviation of comparison experiments is less than 0.3 μm in the traveling range of 30 mm. The experimental results show that the Fabry-Pérot interferometer is highly stable, insensitive to environment effects, and can meet the measuring requirement of the submicrometer order.

Developed of a Multi-Degree of Freedoms Measuring System and an Error Compensation Technique for Machine Tools

In this paper, a new measuring system, Multi-Degree of Freedoms Measuring System(MDFMS), is presented. It is integrated a miniature laser interferometer with a DVD pickup and straightness measuring optical system. The five-degrees of freedoms motion errors to simultaneously measure machine tools. And, it reduces the overall time for measuring the geometric errors. The resolution and accuracy of straightness measurement are about 0.3μm and the resolution and accuracy of the angular error measurement are about 0.6arcsecs within the measuring range of 150mm. MDFMS is characteristic by simple set up, low cost, fast measuring and high accuracy for machine tools.

Multi-interferometric displacement measurement system with variable measurement mirrors

Laser interferometers have been widely implemented for the displacement sensing and positioning calibration of the precision mechanical industry, due to their excellent measuring features and direct traceability to the dimensional definition. Currently some kinds of modified Fabry-Perot interferometers with a planar mirror or a corner cube prism as the measurement mirror have been proposed. Each optical structure of both models has the individual particularity and performance for measuring applications. In this investigation, a multi-interferometric displacement system has been proposed whose measurement mirror can be quickly and conveniently altered with a planar mirror or a corner cube reflector depending on the measuring demand. Some experimental results and analyses about the interpolation error and displacement measurements with both reflectors have been demonstrated. According to the results, suggestions about the choice of a measuring reflector and interpolation model have been presented. With the measuring verifications, the developed system with a maximum standard deviation less than 0.2081 μm in measuring range of 300 mm would be a compact and robust tool for sensing or calibrating the linear displacement of mechanical equipment.

Development of A four-Degrees-of-Freedom Diffraction Sensor

The development of a diffraction-type four-degrees-of-freedom sensor based on the three-dimensional diffraction method and collimation method is described in this paper. This sensor is designed to be integrated with a linear laser encoder to allow five-degrees-of-freedom measurement. It is composed of a miniature collimation-type sensor, a reflective diffraction grating and a quadrant detector to simultaneously measure a straightness error and three rotational angles. Based on the diffraction method, the reflective diffraction grating reflects the incident laser from the collimation-type sensor to several diffractive laser rays and only the 0 and +1-order diffractive laser rays are detected by the collimation-type sensor and a quadrant detector respectively. According to the changed spot positions of the diffraction laser lights on the collimation-type sensor and the quadrant detector, a straightness error and the three rotational errors can be solved simultaneously.

Small displacement measurement based on surface plasmon resonance and heterodyne interferometry

A new method for small displacement measurement based on surface plasmon resonance and heterodyne interferometry is presented. A heterodyne light is focused on a mirror and reflected from it, and then it is incident on a prism which was coated with a thin gold film. When the mirror or the objective lens has a small displacement, the light will be converging or diverging into the prism, and the phase variation between two parts of the test beam under the condition of surface plasmon resonance (SPR) can be measured by using a two-segment photodiode and a lock-in amplifier. This phase difference between two parts of the test beam is proportional to the departure of the mirror from the focal plane, so the displacement can be obtained in real-time. It has some merits, such as, simple, stable, very high sensitivity and resolution. And its resolution is better than 1nm.

Modified Fabry-Perot interferometer for displacement measurement in ultra large measuring range

Laser interferometers have demonstrated outstanding measuring performances for high precision positioning or dimensional measurements in the precision industry, especially in the length measurement. Due to the non-common-optical-path structure, appreciable measurement errors can be easily induced under ordinary measurement conditions. That will lead to the limitation and inconvenience for in situ industrial applications. To minimize the environmental and mechanical effects, a new interferometric displacement measuring system with the common-optical-path structure and the resistance to tilt-angle is proposed. With the integration of optomechatronic modules in the novel interferometric system, the resolution up to picometer order, high precision, and ultra large measuring range have been realized. For the signal stabilization of displacement measurement, an automatic gain control module has been proposed. A self-developed interpolation model has been employed for enhancing the resolution. The novel interferometer can hold the advantage of high resolution and large measuring range simultaneously. By the experimental verifications, it has been proven that the actual resolution of 2.5 nm can be achieved in the measuring range of 500 mm. According to the comparison experiments, the maximal standard deviation of the difference between the self-developed Fabry-Perot interferometer and the reference commercial Michelson interferometer is 0.146 μm in the traveling range of 500 mm. With the prominent measuring characteristics, this should be the largest dynamic measurement range of a Fabry-Perot interferometer up till now.

Flatness and defect measurements for a plane glass using a heterodyne SPR sensor

An application of surface plasmon resonance (SPR) angular sensor in the surface defect measurement is proposed. The method based on geometrical optics, SPR effects and heterodyne interferometry technique could transform a phase shift into a surface height. As a beam normally is incident into a plate that is with a very small apex angle, the angle is a function of the flatness or defect directly. Thus, to scan the specimen, the surface defect or its flatness is detected. It has some merits, such as, simple, sensitive, and real-time measurements.