>Supercontinuum Laser Applications
>Continuous Spectrum Beam Measurement of Internal Surface Profile
This paper introduces the inner surface profile measurement with a smooth spatial distribution.
Fig. 1 Principle of inner surface profile measurement.
A schematic diagram of the internal surface profile measurement is shown in Figure 1. The light beam from the coherent light source (laser diode) is collimated and incident on the top of the cone mirror at a right angle. （ x，y，z ） 点的圆柱形样品的表面上。 The beam spreads out in a disc shape and shines on the surface of a cylindrical sample containing ( x, y, z ) points. （ξ，η） 位置图像平面上并由二维检测器捕获。 The optical slice contour is focused by the lens on the (ξ, η) position image plane and captured by a two-dimensional detector. Φ （ z，θ ） 分别是基线，透镜和像平面之间的长度，以及光学切片轮廓的图像角度。 θ is an angle of the point cloud; L, c and Φ ( z, θ ) are the baseline, the length between the lens and the image plane, and the image angle of the optical slice contour. r （ z，θ ） 表示锥镜和样品表面之间的长度，是位置z和角度θ的函数。 For the optical triangulation method, the radius r ( z, θ ) represents the length between the cone lens and the sample surface as a function of the position z and the angle θ.
However, when we use a coherent light source to illuminate the surface of the sample, laser speckle hinders the measurement of the inner surface profile. The mechanism of speckle formation in internal surface profile measurement is shown in Figure 2. For ease of understanding, the figure is a simplified sketch showing the sample surface, lens, and image plane. The sample has a rough surface longer than the wavelength of the light source, and the spatial intensity distribution of the interference caused by the rough surface is random, so speckle patterns are generated at the points indicated by AC on the image plane. A'-C' 所示的点扩展函数。 This means that the point spread functions shown by A'-C 'are integrated . For this reason, we need to discuss the speckle radius on the surface of the sample illuminated by the disc beam.
Fig. 2 Speckle formation of optical section outline.
After some theoretical processing, the speckle radius is
For this result, the speckle radius is limited by the wavelength λ, the distance c between the image plane and the lens, and the radius a of the lens. The distance c and the radius a are constant because they are parameters of the optical system. It means that the light intensity distribution is an integral point expansion function, which is smoothed by averaging the wavelength range of the supercontinuum beam. We investigated the effect of speckle reduction on the profile measurement of the inner surface through a supercontinuum beam experiment.
In order to evaluate the speckle reduction on the profile of an optical slice using a supercontinuum beam, we set up an experimental device for the profile measurement of the inner surface. The optical device consists of an oscillator, an amplifier, a cone lens, a reference ring gauge, and a complementary MOS (CMOS) camera with 4912 × 3684 pixels, as shown in Figure 3 (a). The supercontinuous laser source provides a wavelength-integrated average power of 2.4 W, a wavelength range of 1.0-2.1 μm, and a repetition frequency of 10 MHz. The outgoing beam from the supercontinuum is incident on the top of the cone mirror. The beam of light extends like a disk after reflecting from a conical mirror. The disc-shaped beam illuminates the surface of the reference ring gauge, as shown in Figure 3 (b). A CMOS camera captures light scattered on the surface of the reference ring gauge. The optical slice image, that is, the ring beam, is shown in Fig. 3 (c). Fig. 3 (d) shows the intensity distribution of the optical slice contour at an angle θ along the radius. 们将 We will r 0， Δr 和Δ r （ θ ） 定义为从光学切片图像获得的点云，平均半径，半径的标准偏差，以及点云与平均半径之间的半径差，以及 Δ r （ θ ） = r ( θ ) , r0 , Δr, and Δr ( θ ) are defined as the point cloud obtained from the optical slice image, the average radius, the standard deviation of the radius, and the radius difference between the point cloud and the average radius, and Δr ( θ ) = - r 0 。 r ( θ ) -r 0 . Δ r （ θ ） 的差异会略有变化，因为孔的点云具有不均匀的表面轮廓。 The difference in radius Δ r ( θ ) is expected to vary slightly because the point cloud of the hole has an uneven surface profile. In this experiment, we used a fiber collimator with a focal length f = 150mm. We also used He-Ne laser with a wavelength of 543.5 nm to compare the results with supercontinuum beams.
Figure 3 Schematic diagram of the inner surface profile measurement. (a) An optical device for measuring the inner surface profile with a supercontinuous laser light source; (b) The relationship between a reference ring gauge and a conical mirror; (c) an image taken by a CMOS camera; (d) an intensity distribution along a radius.
In practical applications of 3D contour measurement, it is important to reduce laser speckle. In this experiment, we used a reference ring gauge (Mitsutoyo Corporation) with a tolerance of 20 μm and a diameter of 100 mm as the test sample. Figures 4 (a) and 4 (b) show optical slice images captured on a reference ring gauge using a supercontinuum beam and a green He-Ne laser, respectively. The color of the optical slice image in FIG. 4 (a) is a false color, and the gap on the optical slice image is caused by a rod, which is used to hold a cone mirror, as shown in FIG. 3 (b). To make it easier to observe the contours of the optical slices, we enlarged the images, as shown in Figures 4 (c) and 4 (d). 光学切片轮廓，无法找到散斑效应，而由He-Ne激光器产生的光学切片轮廓中很容易发现散斑效应。 The speckle effect cannot be found for the optical slice profile generated by the supercontinuum beam , and the speckle effect is easily found in the optical slice profile generated by the He-Ne laser.
Figure 4. Comparison of experimental results between a supercontinuum beam and a green He-Ne laser. (a) and (b) Optical section images of a reference ring gauge with a diameter of 100 mm. (c) and (d) enlarged images. (e) and (f) The maximum distribution of the intensity distribution along the angle θ. (g) and (h) Two-dimensional distribution of point clouds. (i) and (j) Optical slice contours. Supercontinuum beams produce results in (a), (c), (e), (g), and (i); He-Ne lasers obtain (b), (d), (f), (h), and ( j).
示为 AA '， BB' ， CC' 和 DD '的典型位 置处所示的光学切片轮廓。 In addition, we evaluated the optical slice contours shown at typical locations shown as AA ', BB' , CC ', and DD ' in Figures 4 (a) and 4 (b) . Figures 5 (a) and 5 (b) show the optical cross-sectional profile along the radius using a supercontinuum beam and a green He-Ne laser. Note that the radius at the maximum intensity corresponds to r = 0. The beam profile of the supercontinuum spectrum forms a Gaussian-like shape, while the beam profile of the green He-Ne laser is a non-Gaussian beam. For optical slice images, in Figs. 4 (a) and 4 (b), we obtain the maximum intensity distribution along the angle θ in Figs. 4 (e) and 4 (f). A comparison of the maximum intensity distribution between a supercontinuum beam and a He-Ne laser shows that a supercontinuum beam achieves a more uniform intensity distribution than a He-Ne laser. The point cloud distribution is shown in Figures 4 (g) and 4 (h). A point cloud was calculated with 3600 points in the range of θ at 0.1 ° intervals. Comparing the two-dimensional distribution of point clouds, it is difficult to evaluate the difference between a supercontinuum beam and a He-Ne laser beam. Therefore, we extend the two-dimensional distribution of the point cloud to the non-uniformity of the radius as a function of the angle θ, as shown in Figures 4 (i) and 4 (j). Δ r （ θ ） = The difference in radius is Δ r ( θ ) = - r 0 ，其中 r ( θ ) -r 0 , where 和 r 0 分别是点云和平均半径[图4(i)和4(j)中的红线]。 r ( θ ) and r 0 are the point cloud and the average radius [red lines in Figures 4 (i) and 4 (j)]. Due to the rod holding the cone lens, the optical slice image has a blind angle in the range of 280 ° -288 °. According to this result, the speckle effect is reduced in a super-continuous beam compared to a He-Ne laser. These point cloud results illuminate the application of supercontinuum beams to 3D shape measurements.
Fig. 5 Comparison of beam profile along radius r between (a) supercontinuous beam and (b) green He-Ne laser. (b)中所示的AA'（实线），BB'（虚线），CC'（点划线）和DD'（虚 线）的典型位置处示出了强度分布。 Intensities are shown at typical positions of AA '(solid line), BB' (dotted line), CC '(dotted line), and DD' (dotted line ) shown in Figs. distributed. Note that the radius at the maximum intensity corresponds to r = 0. The beam profile of a super-continuous beam forms a Gaussian-like shape, while the beam profile of a green He-Ne laser is disconnected from the Gaussian distribution due to speckle noise.
A super-continuous beam is applied to a compact probe for internal surface profile measurement. Fig. 6 (a) shows a schematic diagram of a compact probe for measuring the inner surface profile using a super continuous beam. Compact probes for internal surface profile measurement include a ring beam device, rigid mirror, lens unit, CMOS camera, and supercontinuum. The tube frame covers the rigid frame and connects to the lens unit. The supercontinuum beam has a broadband spectrum, as shown by the solid line in Fig. 6 (b). In this experiment, we compared the measurement results of the inner surface profile of the supercontinuum beam and the laser diode, whose wavelength is 650nm, as shown by the dotted line in Fig. 6 (b). The ring beam device is composed of an optical fiber collimator and a conical lens aligned with a transverse beam, as shown in FIG. 6 (c). The blind angle of the ring beam device is 110 °, because the cross beam is used to connect the fiber collimator and the cone mirror.
Figure 6 Schematic diagram of an internal surface profile measuring instrument that uses a supercontinuous beam and a 650 nm wavelength beam from a laser diode. (a) Schematic diagram of a compact probe for internal surface profile measurement. (b) Spectra of supercontinuum beams and laser diodes. (c) Structure of the probe.
Figure 7 Experimental setup of an internal surface profile measuring instrument for measuring the wear depth of a hole with a diameter of 12.75 mm. (a) Photo of the experimental setup. (b) Optical slice image of a supercontinuum beam. (c) Supercontinuum beam point cloud. (d) Optical slice image of a laser diode with a wavelength of 650 nm. (e) Laser diode point cloud.
Fig. 7 (a) is a photograph of an experimental device. In this experiment, we measured a small hole. The valve body is a key device used in automotive transmissions. It is important to measure the wear depth of the small holes to understand the wear mechanism. Figures 7 (b) and 7 (c) show typical images and point clouds of the inner surface profile of the hole in the valve body at z = 0mm using a supercontinuum beam. When the output power of the laser diode with a wavelength of 650 nm is 30 mW, the exposure time of the captured image is 1 s, and the output power of the super-continuous beam is 2.4 w. To. A point cloud of 360 points is calculated at an interval of 1 ° within the range of θ. Due to the transverse beam and optical fiber, the measuring instrument has a blind angle at 50 ° -155 °. Figures 7 (d) and 7 (e) also show an image captured using a laser diode and a point cloud of the inner surface profile of the hole. In addition, we converted these into radius distributions to observe the details of the point clouds shown in Figures 7 (c) and 7 (e).
Figures 8 (a) and 8 (b) show the radius distribution of the supercontinuum beam and the laser diode along the angle, respectively. A comparison between a supercontinuum beam and a laser diode shows that the beam profile of the former is improved compared to the latter. In order to obtain the depth of wear in the hole, we measured the inner surface profile of the hole with two scans as the tube holder was rotated, because the measuring angle of the compact probe was limited to 250 °.
Figure 8 Radius distribution along the angles of (a) supercontinuum beam and (b) laser diode.
Using the obtained optical slice images of z = 0-16mm, we reconstruct the three-dimensional shape of the small holes in the valve body. In this experiment, we first capture images of 160 optical slice contours at 0.1 mm intervals. After the tube holder was rotated, images of 160 optical slice contours were further obtained. Figure 9 shows a three-dimensional map of the point cloud. Note that the sample well has two drainage holes. We studied the depth of wear from a three-dimensional profile. 示出了没有磨损的内表面轮廓，图10(b)中示出了有磨损的内表面轮廓。 In order to evaluate the inner surface profile difference between no wear (blue dotted line) and wear (solid red line) shown in FIG. 9, the inner surface profile without wear is shown in FIG. 10 (a), and FIG. 10 ( The inner surface profile with wear is shown in b). The blue and red graphs represent the first and second scans, respectively. 位置 （a0，b0） 。 Note that the center position (a0, b0) is determined by using a circle fitting algorithm for the point cloud . 用圆拟合算法的配准允许组合通过两次扫描获得的点云。 Registration using a circle fitting algorithm allows the point clouds obtained from two scans to be combined. For the circle fitting algorithm, we re-drawn the radius distributions without wear [Figure 10 (c)] and the wear [Figure 10 (d)] as a function of angle. As a result, the radius distribution in FIG. 10 (d) is larger than the radius distribution in FIG. 10 (c). As wear increases depth, the radius becomes larger. However, for sample point clouds without wear, the radius along the angle is uniform. Therefore, the abrasion-removed depth is 0.088 mm.
Figure 9 Point cloud of small holes in the valve body. The point cloud is composed of 52328 points and 320 images.
Figure 10 Point cloud without wear and tear. The point cloud obtained from the optical slice profile of the hole, (a) is not worn and (b) is worn. Blue diamonds and red circles show the first and second scans of the inner surface profile measurement. (c) and (d) wear-free and wear-resistant radius distributions, respectively. Note that the green line indicates a radius r = 6.375mm.
To visually assess the depth of wear in the hole, we applied a coordinate transformation from a Cartesian coordinate system to a polar coordinate system. Figures 11 (a) and 11 (b) show the derived radius and maximum intensity, respectively. The color gradation in Figure 11 (a) indicates the radius; wear and scratches are visualized. The changes in radius due to wear and scratches were determined to be 0.088mm and 0.050mm, respectively. The depth of the scratch is shown in Fig. 11 (a), and the maximum intensity distribution is shown in the gray level in Fig. 11 (b).
Figure 11 (a) View of the radius distribution and (b) the maximum intensity distribution.
Comparing the speckle effect of a supercontinuum beam with a conventional monochromatic laser, the standard deviation of a supercontinuum beam is doubled. Using a compact probe that measures the profile of the inner surface with a super-continuous beam, the depth eliminated by valve body wear is measured. The radial spatial resolution of the probe is 2 μm. Due to the suppression of speckle, it has the same order of magnitude as the wavelength of the supercontinuum beam. Compared with monochromatic lasers, supercontinuous beams can increase radial spatial resolution by a factor of five.
From <Three-dimensional measurement of an inner surface profile using a supercontinuum beam>