Jiwen Cui

FBG Interrogation Method with High Resolution and Response Speed Based on a Reflective-Matched FBG Scheme

In this paper, a high resolution and response speed interrogation method based on a reflective-matched Fiber Bragg Grating (FBG) scheme is investigated in detail. The nonlinear problem of the reflective-matched FBG sensing interrogation scheme is solved by establishing and optimizing the mathematical model. A mechanical adjustment to optimize the interrogation method by tuning the central wavelength of the reference FBG to improve the stability and anti-temperature perturbation performance is investigated. To satisfy the measurement requirements of optical and electric signal processing, a well- designed acquisition circuit board is prepared, and experiments on the performance of the interrogation method are carried out. The experimental results indicate that the optical power resolution of the acquisition circuit border is better than 8 pW, and the stability of the interrogation method with the mechanical adjustment can reach 0.06%. Moreover, the nonlinearity of the interrogation method is 3.3% in the measurable range of 60 pm; the influence of temperature is significantly reduced to 9.5%; the wavelength resolution and response speed can achieve values of 0.3 pm and 500 kHz, respectively.

Optical fiber probe based on spherical coupling of light energy for inner-dimension measurement of microstructures with high aspect ratios.

An optical fiber probe, based on spherical coupling of light energy, is proposed to transform the lateral displacement of a spherical coupler into the deflection of a light energy center, while the light energy is transmitted in the reverse direction. Therefore, the shadowing effect of a microcavity can be eliminated. The probe has a high displacement sensitivity, allowing precision inner-dimension measurements of microstructures with high aspect ratios. Measurements of microholes and fuel injection nozzles indicate that, for a microstructure with an aspect ratio of up to 15:1, a probing force <1 μN, a resolution of up to 0.05 μm can be achieved using the proposed probe, which is easy to exchange and low cost.

A Twin Fiber Bragg Grating Probe for the Dimensional Measurement of Microholes

A fiber probe with a novel structure of twin fiber Bragg grating sensitive to both contacts along x-axis and z-axis is proposed for measurement of microholes with high aspect ratio. The main conundrums during the dimensional measurement of microholes with high aspect ratio, such as shadowing effect, limited probing space, and requirement of multidimensional tactile sense are well solved using the proposed probe. Sensing principle and decoupling algorithm has been verified through experiments. Experimental results indicate that the influence of manufacture defects and the misalignment between probe and measurement coordinates can be reduced through rotation and tilt calibration. The contact displacement sensitivity is 12.91 pm/μm and 49.75 pm/μm along x-axis and z-axis, respectively. A microhole with an aspect ratio of 10:1 can be measured using the proposed twin fiber Bragg grating probe.

Fiber deflection probing method based on micro focal-length collimation

In order to improve the low displacement sensitivity in sensing the 2D deflection of a fiber probe used for measurement of micro cavities with high aspect ratio, a fiber stem with a ball mounted on its end is used as a probe and a small segment of it is used as a cylindrical lens to collimate a point light source and image it to a camera. The deflection of the fiber stem can be inferred from the change in image acquired by the camera with ultrahigh displacement sensitivities of 10,000 and 20,000 in two dimensions using a fiber stem of 44 mum in diameter, and the corresponding resolutions are better than 1 nm and 3 nm respectively.

Further improvement of edge location accuracy of charge-coupled-device laser autocollimators using orthogonal Fourier-Mellin moments

In order to further improve edge location accuracies of charge-coupled-device laser autocollimators, we use rotation invariance and lower radial orders of orthogonal Fourier-Mellin moments to efficiently extract the characteristics of edges, set the edges in the vertical direction, and establish the expressions used for edge location. We accomplish this by analyzing the interrelationship of orthogonal Fourier-Mellin moments. We compensate errors caused by the differences between the masks selected (by correcting the locations in proportion to the dimensions of the masks selected) and by the differences between actual and ideal edge patterns (by correcting the bias errors in proportion to the distance from the center of the sampling area to the edge). Experimental results show that an edge location accuracy of 0.06 pixel can be achieved for straight lines, 0.08 pixel for curves, and ±0.12 pixel for an actual circular target acquired by a charge-coupled-device laser autocollimator. It is therefore concluded that the proposed method can be used as an efficient approach to achieve the higher edge location accuracies specified for such optoelectronic image measurements instruments as a charge-coupled-device laser autocollimator.

A Twin FBG Probe and Integration With a Microhole-Measuring Machine for the Measurement of Microholes of High Aspect Ratios

To enhance the precision manufacture of the microholes of high aspect ratios, a fiber Bragg grating (FBG) probe with a twin fiber structure based on capillary-driven self-assembly fabrication method is developed for a micro-hole-measuring machine to do the high-precision measurement. The sensing principle of the twin FBG probe is investigated through analyzing the strain distribution within the probe under an axial or radial contact displacement. Then, the design and fabrication method of the twin FBG probe is demonstrated. To improve the accuracy of the measurement, both data processing method and measuring method are proposed. A data processing method for acquiring the triggering position with the transformation of the signal domain and multiple fitting is implemented to raise the accuracy of the triggering position. And a method for measuring the diameter of a microhole with perpendicular bisector of the chords and dichotomization scanning is utilized to enhance the determination accuracy of the diameter. Experimental and simulation results indicate that the data processing method and measuring method are robust, and the accuracy of the measurement can be improved.

Improvement of vision measurement accuracy using Zernike moment based edge location error compensation model

This paper presents the Zernike moment based model developed to compensate edge location errors for further improvement of the vision measurement accuracy by compensating the slight changes resulting from sampling and establishing mathematic expressions for subpixel location of theoretical and actual edges which are either vertical to or at an angle with X-axis. Experimental results show that the proposed model can be used to achieve a vision measurement accuracy of up to 0.08 pixel while the measurement uncertainty is less than 0.36μm. It is therefore concluded that as a model which can be used to achieve a significant improvement of vision measurement accuracy, the proposed model is especially suitable for edge location of images with low contrast.

Enhancing laser beam directional stability by single-mode optical fiber and feedback control of drifts

A system, which uses a single-mode optical fiber to suppress the laser beam drifts, and uses feedback control to further reduce the drifts of a primary collimated beam, is proposed to enhance the directional stability of laser beams. The linear and angular drifts of laser beams are separated through light path arrangement and detected using quadrant position detectors, and are controlled using two-dimensional linear and angular drift feedback control systems according to their magnitudes detected. Theoretical analyses and experimental results indicate that the collimation accuracy of 0.7×10−7rad can be achieved at a collimation distance of 500 mm using single-mode optical fiber and separate control of linear and angular drifts.

Double fiber probe with a single fiber Bragg grating based on the capillary-driven self-assembly fabrication method for dimensional measurement of micro parts

Focusing on the ultra-precision dimensional measurement of parts with micro-scale dimensions and high aspect ratios, a two-dimensional double fiber probe with a single fiber Bragg grating (DS-FBG probe) is investigated in detail in this paper. The theoretical analysis of the sensing principle is verified by spectrum simulations of the DS-FBG probe with a modified transfer matrix method using the strain distribution within the DS-FBG probe. The fabrication process and physical principle of the capillary-driven self-assembly of double fibers in the UV adhesive with a low viscosity are demonstrated. Experimental results indicate that resolutions of 30 nm in radial direction and 15 nm in axial direction can be achieved, and the short-term displacement drifts within 90 seconds are 28.0 nm in radial direction and 7.9 nm in axial direction, and the long-term displacement drifts within 1 hour are 61.3 nm in radial direction and 17.3 nm in axial direction. The repeatability of the probing system can reach 60 nm and the measurement result of a standard nozzle is 300.49 μm with a standard deviation of 20 nm.

Three-dimensional fiber probe based on orthogonal micro focal-length collimation for the measurement of micro parts.

A 3-dimensional fiber probe based on orthogonal micro focal-length collimation (MFL-collimation) is proposed for the measurement of micro parts with high aspect ratios. The probe consists of a fiber stylus which acts as a micro focal-length cylindrical lens (MFLC-lens) of the two orthogonal MFL-collimation optical paths and a probe tip fixed on the free end of the fiber stylus for touching the workpiece. The fiber stylus will deflect (deflection mode) or buckle (buckling mode) under contacts, and the deflection or buckling of the fiber stylus will cause corresponding shifts of the fringe images of the two orthogonal MFL-collimation optical paths. Therefore, the 3-dimensional displacements of the probe tip are transformed into the centroid position shifts of the zero-order fringe images. Experimental results indicate that the fiber probe has a measuring capability in 3-dimensional tactility, and a radial and axial resolution of 5 nm and 3 nm can be obtained respectively. The probe is easily applied in the measurement of micro parts because of its high resolution, low cost, high measurable aspect ratio, low probing forces and capability in three-dimensional tactility.