Transcript
Laser Beam Analysis
Laser Line Generators for Light- sectioning in Rail Inspection 3D-Measurement and process control for research and industrial environments Modern measuring techniques often involve using lasers specifically designed for the demanding requirements of industrial tasks. An increasing number of laser diodes have become available in the green and blue region of the visible spectrum so that laser-diode based modules now cover the full range from UV to IR. A variety of beam-shaping techniques can be used to produce the particular beam profile necessary for a specific measuring task. One application is the continuous monitoring of rail profiles during rail production or of their deterioration in use by mounting the test system on the train itself. The geometry of the rail can be determined by using laser lines and the laser light-sectioning method. The in situ measurement of wear ensures that the repair of the affected rail sections is performed in time. Laser light-sectioning is one of the main inspection techniques used for example during the rail production and when the tracks are in use. A laser line is directed at the rail at a particular angle and a camera records an image that visualizes the upper rail head geometry (Fig. 1). Plastic deformations, for example, of the rail head (i.a. squats) that arise from stress during operation (e.g. from high axle load of freight trains or in curves) are detected and provide continuous feedback about rail wear. A signal with high signal-to-noise-ratio and high precision is i.a. achieved by using high power laser lines with well-defined line geometry, providing spatial resolutions in the micrometer range. In general, not only the rail head but the complete rail profile is examined and this information is then stored with the exact position information, so that rail maintenance can be performed systematically.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
the Authors Anja Krischke Anja Krischke studied Physics at the University of Würzburg with a focus on the description of ultrashort laser pulses and quantum control. She joined Schäfter+Kirchhoff in 2011 and now works in optics development.
Peter Gips Peter Gips studied Information Engineering at the Technical University Dresden. He joined Schäfter+Kirchhoff in 1991 after research in the field of digital signal processing and is now responsible for image processing and software development.
Anja Krischke Schäfter+Kirchhoff GmbH 22525 Hamburg, Germany Tel: +49 40 853 997 - 0 E-mail:
[email protected]
Peter Gips Schäfter+Kirchhoff GmbH 22525 Hamburg Tel: +49 40 853 997 - 0 E-mail:
[email protected]
Christian Knothe Christian Knothe first studied Physics at the University of Freiburg i. Br. with a focus on laser spectroscopy before completing his doctoral thesis in fiber optics at the Technical University of Hamburg-Harburg. Since he joined Schäfter+Kirchhoff in 2005, he has been responsible for the advanced fiber optic applications.
Ulrich Oechsner Dr. Ulrich Oechsner studied Physics before completing his doctoral thesis at the University of Hamburg. After research in the fields of electrophysiology and physiological optics, he joined Schäfter+Kirchhoff in 2000 where he is responsible for optical design and system development.
Dr. Christian Knothe Schäfter+Kirchhoff GmbH 22525 Hamburg, Germany Tel: +49 40 853 997 - 0 E-mail:
[email protected]
Laser modules for industry Industrial applications, such as the inspection of rails during production and operation place specific demands on the laser source. The laser module 40TE-PO (Fig. 1) is encased in a rugged full-metal, potential free ESDprotected housing and has an integrated microcontroller and serial RS232 interface. The controller regulates i.a. laser diode cur-
Dr. Ulrich Oechsner Schäfter+Kirchhoff GmbH 22525 Hamburg, Germany Tel: +49 40 853 997 - 0 E-mail:
[email protected]
rent and temperature. If the inserted laser diode is without its own TE-cooler, the casing is additionally equipped with a temperature sensor and Peltier element to adjust the diode temperature. Vital data about the laser module and diode are stored and can be read out, such as the laser diode‘s hours of operation or the diode current consumption, which provides information about diode degradation. Re-
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Laser Beam Analysis
Figure 1: High power laser line generator 13LRM+40TE-PO. The module is specially designed to meet the requirement for industrial measurement tasks, e.g. the measurement of rail deterioration using laser light-sectioning.
ducing the diode temperature slows diode degradation and increases its life time. Furthermore, the electronics include reverse polarity and overvoltage protection, a power and current limitation, as well as digital (up to 250 kHz) and simultaneous analog modulation (up to 10 Hz). Both singlemode and multimode diodes in a variety of different diode casings are used as light sources in the 40TE -PO, with the singlemode diodes producing thinner lines. The complete system is built in a modular fashion with casing, diode, appropriate collimation lens and carefully selected beam-shaping optics. The optics can be optionally protected by a window made from scratch-resistant sapphire glass. Focus settings are adjusted easily, with both line length and line width increasing in direct proportion with greater working distance.
Laser light-sectioning with laser lines Laser light-sectioning is a 3D-measurement technique that uses laser triangulation to determine the height profile of an incident section (Fig. 2). A camera measures the lateral displacement and distortion of the incident laser line. The image contains the complete height information of the section defined by the incident laser beam. The 3D profile is acquired by continuous scanning, as the object passes through the detection system. A complete rail profile is, for example, obtained by positioning several laser line generators and cameras radially around the rail (Fig. 3). By evaluating the height variations detected by the cameras the complete geometry of the profile is extracted. If different laser lines
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overlap, e.g. laser lines with different wavelengths and adequate filters for the detecting cameras are used to separate the signals. The reflective characteristics of the object are critical for the success of the measurement. If reflection from the object is specular (mirror-like), no signal reaches the camera. At least partly diffusive reflection is needed. Most technical surfaces exhibit a reflective behavior consisting of both diffuse and specular reflection, with the fraction of diffuse reflection (itself isotropic) decreasing for higher incidence angles α. However, the homogeneity of the laser line directed on a diffusively reflecting object is disrupted by laser speckle, a multi-interference effect on optically rough surfaces. The intensity centers of the laser line are displaced laterally from the beam axis in a stochastic manner, disturbing the measurement. Laser speckle can be prevented by using a low coherence light source, e.g. a superluminescent diode. Choosing the right camera lens aperture setting contributes to minimizing
FIGURE 3: Light-Sectioning setup using two laser lines. Cameras and lasers are in Scheimpflug configuration. An region of interest of 500 × 100 pixels is set for the Photon Focus cameras that allows 1447 fps. The f-stop does not need to be increased, which means that an inspection train can travel at least twice as fast without losing light intensity.
laser speckle as well. The aperture acts as a spatial frequency filter, with a larger aperture producing a high frequency speckle pattern (Fig. 4A) that is less disturbing when compared with the low frequency pattern with larger gaps caused by a smaller aperture setting (Fig. 4B). The height-measuring range and the resolution are determined by the triangulation angle α between the planes of the dissecting laser line and the optical axis of the camera lens (Fig. 2). Larger angles α cause larger line displacements and, thus, a higher resolution for the same height change in comparison with smaller incident angles. The height-measuring range decreases with α. In order to ensure constant signal amplitude at the sensor, the depths of focus of both the dissecting laser line and the detecting camera system must be large enough to cover the required height-measurement range. However, since the signal to be acquired is non-parallel with the camera sensor the depth of focus of the camera is often not large enough to produce a sharp image of the whole dissecting plane. This problem can be overcome by using the Scheimpflug configuration for the camera/laser setup (see Figure 3 and Box 1, Fig. 6).
Choosing the right laser line: micro and macro laser lines FIGURE 2: Laser triangulation principle. The angle α between the laser beam and the camera determines the resolution and the height measurement range.
Every laser line is focussed at a predefined working distance from the beam source. At distances outside of this point, the laser line becomes wider and its power density decreases. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Laser Beam Analysis
FIGURE 4: Laser speckle behavior imaged with a small f-number k=2.8 (A) and with a large f-number k=22 (B).
Laser micro line generators produce narrow laser lines with a Gaussian intensity profile across the laser line. By convention, the depth of focus is defined as the range in which the laser line does not increase by more than a factor of 1.41. For a laser line with line width B (at the 13.5 % level) and wavelength λ, the depth of focus is defined as the Rayleigh range 2zR where:
2 zR =
π B2 . 2λ
In contrast, laser macro line generators produce laser lines with an extended depth of focus 2zM:
2 zM = 1.75
πB 2 2λ
with the intensity profile across the laser line approximately Gaussian with lobes on the side.
For a particular line width B, the depth of focus of a macro line is almost twice that of a micro line. At the same working distance A, macro lines are 2 to 5-times wider and have a depth of focus 7 to 35-times larger than the equivalent micro line. Compared with a micro line (Fig. 5A) the power density of a macro line is reduced (Fig. 5B), but does not change significantly over a greater range. Micro lines have a higher power density close to their focus but line width increases and power density falls drastically outside the focus . Laser line power density is often crucial for good signal quality. For measurements from a fast moving train (>100 km/h, low integration times, high test frequency), a good signal quality can only be achieved with high power laser lines. A high laser power also enhances signal contrast in bright environment lighting and ensures a sufficiently high power density when using longer laser lines at greater working distances. Since the height information is obtained from the displacement of the laser line, the line width does not generally restrict the resolution of the height measurement (often the displacement can even be measured more precisely). However, defects in the scanning direction will remain undetected if the line width is larger than the defect itself. The width of the laser line B (conventionally defined at the 1/e²-level) cannot be infinitesimally small but is limited for ideal optics to
Figure 5: Micro lines (A) exhibit a high power density at the focus, but line width increases and power density decreases considerably outside this point. Macro lines (B) have a lower power density but an extended depth of focus (7-35-times larger).
B=
4λ A , ØS π
for a particular wavelength λ, working distance A and incident beam diameter ØS at the line optics perpendicular to the laser line. In order to produce very thin lines, the beam diameter at the line optics needs to be large. A compromise must be found for each application between the benefits of a larger depth of focus accompanied by a compara-
infobox Scheimpflug configuration for image acquisition When acquiring an image, commonly the object, lens and image planes are parallel with each other. For laser light-sectioning, the object defined by the angle of incidence is non-parallel with the camera lens and sensor. Often the depth of focus needs to be increased to acquire a fully sharp image. The depth of focus 2zK depends on the fnumber k, the interpixel distance ∆x and the magnification β (= sensor size/image area):
2 z K = 2 ∆x k
1+ β . β2
The depth of focus can be increased by simply reducing the aperture size. However, the signal amplitude decreases by a factor of 2 for each additional f-stop, and the optical resolution also decreases. The Scheimpflug configuration allows a fully
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
focussed image of an oblique image if the object and the image plane both meet the lens plane along one straight line (Figure 6):
tan γ =
a' tan α a
α and γ are the angle between object and lens plane, and between the lens and image plane, respectively. The parameters a and a‘ are the distances of the major planes of the object and lens, and the lens and the image. The magnification for the height difference h is not constant
h=
ax cos γ a 'sin α − x cos (α − γ )
and the relationship between height difference h and peak position x on the sensor (relative to the optical axis of the lens) is non-linear.
FIGURE 6: Scheimpflug configuration: a sharp image is produced if the object, lens and image planes all meet in one straight line.
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Laser Beam Analysis
multimode edge emitter, which produces a line width of 1.41 mm. The 3D and 2D profile (Fig. 7D/7E) show a homogeneous intensity distribution along the laser line, the intensity distribution across the line is non-Gaussian. The difference in line width results from the way the emitter is aligned to the grid lens. For a singlemode diode, the minor emitter axis is imaged and the major axis is stretched. This does not work for the multimode diode, as the multimode intensity distribution produces inhomogeneity in the laser line. As a consequence the major axis of the emitter is imaged and the minor axis is stretched, producing a homogeneous but wider high power laser line.
Conclusion
FIGURE 7: Beam analysis setup (A) with line scan camera (1) and a high power laser line generator 13LRM+40TE-PO (2) mounted on a translation stage (3). The 3D beam profile (B, D) shows an almost homogeneous intensity distribution for a 220 mW singlemode diode (B) and good linearity (C, please note the non-quadratic grid). The line produced with the 450 mW multimode emitter is very homogeneous (D) but line width is increased (E).
tively larger line width and a lower power density, and smaller lines with a high power density and a smaller depth of focus. A detailed laser beam analysis reveals further characteristics of micro and macro laser lines.
Laser Beam Analysis Most laser diodes have a divergent radiation cone with an elliptical cross-section. A standard cylinder lens transforms it into a laser line by stretching the major semi-axis in relation to the minor semi-axis. The elliptical characteristics as well as the Gaussian intensity distribution along the line remain. A closer look at the laser line with the help of a camera reveals the elliptical character, characterized by a greater intensity and width at the center of the line. The useful range for light-sectioning is restricted. Aspherical cylinder lenses and grid lenses are used to produce laser lines with constant line width and homogeneous intensity profile along the line, which improves the performance capabilities of light-sectioning considerably. Quality control of laser lines is performed using a setup consisting of a Schäfter+Kirchhoff line scan camera (SK8160GKO-LB, monochrome, 5 µm pixels and 60 MHz pixel frequency), software for image acquisition and evaluation, and the
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laser line generator (40TE -PO) mounted in a xyz-adjustable console on a translation stage (Fig. 7A). The line scan camera sensor and the laser line axis are aligned perpendicular to one another, so that the camera acquires the cross-section of the laser line point for point. By moving the laser line generator on the translation stage, a scanned beam profile over the whole laser line is acquired. Line length, line width and the linearity of the laser line are evaluated. An algorithm determines the position of the intensity centers of the line for each measurement point. The maximum difference between the intensity centers then serves as a measure for the linearity of the laser line. Due to inaccuracies the laser line can be bent („banana-shaped“). Figure 7B-E shows the beam profile of a 40TE -PO with a grid lens (13LR M25-M125-1.5, A=110 mm macro line) as beam-shaping optics and two different laser diodes. The first laser diode is a 829 nm 220 mW singlemode edge emitter. After defining the usable laser line range the software depicts the 3D image (Fig. 7B) that reveals an almost homogeneous intensity distribution along the laser line. The intensity distribution across the laser line is Gaussian, with a line width of 0.115 mm. Over a line length of 334 mm, the intensity centers show a maximum deviation of 0.1 mm (Fig. 7C). The line width is narrow compared with the same laser module using a 640 nm 450 mW
High power laser line generators with carefully selected laser line beam-shaping optics (available for the wavelength range 4051550 nm) are e.g. used for laser light-sectioning of rail profiles. Depending on the prerequisites for depth of focus of the laser line, either micro lines with a high power density and smaller depth of focus or macro lines with larger line width and increased depth of focus are appropriate. Integrated electronics allows the monitoring of safety and life time data of the laser diode via serial interface, making maintenance easier. Schäfter+Kirchhoff also supplies micro and macro focus as well as laser pattern generators.
the company Schäfter+Kirchhoff GmbH Hamburg, Germany Schäfter+Kirchhoff has accumulated extensive experience in the development of opto-mechanical and opto-electronic systems for use in research, aviation and in space, as well as for demanding medical and industrial applications. Schäfter+Kirchhoff designs and manufactures their own CCD line scan camera systems, laser sources, beam-shaping optics and fiber-optic components for customers worldwide. www.SuKHamburg.com
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
