A device and a method for measuring the wall of a glass tube based on time-domain spectral interferometry

Abstract

The application discloses a kind of measuring device and measuring method of glass tube wall based on time-domain spectrum interference, the measuring device includes for emitting coherent light source femtosecond laser, with the optical fiber circulator of femtosecond laser connection, with the optical fiber collimator of optical fiber circulator connection, with the pulse signal of optical fiber amplifier for amplifying femtosecond pulse back light, with the dispersion compensation optical fiber of optical fiber amplifier connection, with the photoelectric detection and data acquisition device of dispersion compensation optical fiber connection.The application utilizes the two back light pulses of glass wall upper and lower surface reflection spectrum interference effect, uses interference fringe period to solve pulse interval and further solve wall thickness.The application can realize measuring function by simple laser transceiver and data processing device, scheme has the advantages of simple and reliable, fast and high precision.

Description

Technical Field

[0001] This invention relates to a measuring device and method for glass tube walls, and more particularly to a measuring device and method for glass tube walls based on time-domain spectral interferometry, belonging to the field of laser interferometry. Background Technology

[0002] Glass tubes have a wide range of applications in production and daily life. Some applications, such as pharmaceutical glass bottles and measuring cylinders, have very high requirements for the outer diameter, inner diameter, and wall thickness of glass tubes.

[0003] To ensure the consistency and accuracy of glass tubes, it is necessary to measure and control the thickness of the glass tubes online during production. Due to the annular structure of glass tubes, traditional laser thickness gauges used for measuring the thickness of planar plates are no longer suitable. Therefore, mechanical measurement and manual reading are currently employed, such as using calipers or micrometers to measure the wall thickness of the tube ends, with the results read manually. However, this method has several significant problems: First, it lacks real-time performance, lagging behind the manufacturing process and unable to perform online manufacturing control, resulting in a large number of defective tubes and waste. Second, manual visual inspection is inefficient, inaccurate, and inconsistent, making it difficult to meet the needs of mass production and quality control. Finally, this method can only measure the tube ends, representing a localized measurement and lacking the ability to measure the overall thickness distribution of the glass tube. It suffers from measurement lag, randomness, and localization issues.

[0004] The light transmission properties of glass make it relatively easy to measure thickness using optical methods. For example, Luoyang Longhao Glass Co., Ltd., a subsidiary of Luoyang Glass Group, proposed a glass plate thickness monitoring device (authorization announcement number CN 206056517U). This device uses a swept-frequency continuous laser incident on the glass and detects the intensity of the reflected light signal. Utilizing the Fabry-Perot (FP) effect of the glass plate, the signal is enhanced when the laser wavelength reaches the transmission peak of the glass plate. The glass thickness is calculated using the wavelength difference between the two enhanced signals. However, this method relies on the FP effect of the glass plate and cannot be used in curved glass tubes. Furthermore, light intensity detection disturbances and the wavelength scanning accuracy of the laser severely affect the measurement function and accuracy. The optical path debugging and calibration are also complex, making it unsuitable for industrial applications. Summary of the Invention

[0005] Objective of this invention: The objective of this invention is to provide a simple, reliable, fast, and high-precision measurement device for glass tube walls based on time-domain spectral interferometry. This device measures pulse thickness using a femtosecond laser spectral interferometry method, achieving spectral-time domain mapping conversion of the femtosecond pulses through dispersion-compensating optical fibers. This allows for the acquisition of spectral interferometry period information using simple and universal data acquisition equipment, and the rapid analysis of glass tube thickness information, providing real-time measurement data for production. Another objective of this invention is to provide a measurement method for glass tube walls based on time-domain spectral interferometry.

[0006] Technical Solution: The present invention provides a measurement device for the glass tube wall based on time-domain spectral interferometry. The measurement device includes a femtosecond laser for emitting a coherent light source, an optical fiber circulator connected to the femtosecond laser, an optical fiber collimator connected to the optical fiber circulator, an optical fiber amplifier connected to the optical fiber circulator for amplifying the pulse signal of the femtosecond pulse return light, a dispersion compensation fiber connected to the optical fiber amplifier, and a photoelectric detection and data acquisition device connected to the dispersion compensation fiber. The centerlines of the femtosecond laser, the optical fiber circulator, and the optical fiber collimator are at the same horizontal position. The coherent light source emitted by the femtosecond laser passes sequentially through the optical fiber circulator and the optical fiber collimator to illuminate the glass tube under test. The upper and lower surfaces of the tube wall reflect the light to form two femtosecond pulse return lights with a certain time interval. The femtosecond pulse return lights pass sequentially through the optical fiber collimator, the optical fiber circulator, the optical fiber amplifier, and the dispersion compensation fiber before being acquired by the photoelectric detection and data acquisition device.

[0007] As a further improvement to the above scheme, the femtosecond laser is connected to the fiber optic circulator via a pigtail connector at the input end of the fiber optic circulator; the pigtail at the straight-through output end of the fiber optic circulator is fused to the pigtail of the fiber optic collimator, and the pigtail at the other end of the fiber optic circulator is fused to the fiber optic input end of the fiber optic amplifier.

[0008] As a further improvement to the above scheme, the output fiber of the fiber amplifier is fused to the input fiber of the dispersion compensation fiber, and the other end of the dispersion compensation fiber is connected to the signal light input connector of the photoelectric detection and data acquisition device.

[0009] As a further improvement to the above solution, the measuring device also includes a computer connected to the photoelectric detection and data acquisition device, the computer being connected to the photoelectric detection and data acquisition device via a data cable.

[0010] Preferably, the femtosecond laser meets the following conditions: center wavelength 1560nm±20nm, full width at half maximum (FWHM) >0.5nm, repetition frequency 10kHz~50MHz, and single pulse energy >1nJ.

[0011] Preferably, the wall thickness of the glass tube to be measured by the measuring device is 0.1 to 50 mm.

[0012] Preferably, the total dispersion of the dispersion-compensating fiber is >500 ps / nm.

[0013] On the other hand, the present invention provides a method for measuring the glass tube wall based on time-domain spectral interferometry, the method comprising the following steps:

[0014] S1. The femtosecond laser is incident on the upper and lower surfaces of the glass tube under test through the fiber optic circulator and fiber optic collimator. The tube wall reflects twice, forming two femtosecond pulses with a certain time difference. The femtosecond pulses return to the fiber optic collimator to form a return pulse signal.

[0015] S2. The return pulse signal is amplified by the fiber amplifier after passing through the fiber optic circulator, and then enters the dispersion compensation fiber for time stretching. The spectral information of the return pulse is mapped to the time domain, and then passes through the photoelectric detection and data acquisition device to obtain the time domain signal.

[0016] S3. Calculate the thickness of the glass tube wall using the acquired time-domain signal.

[0017] As a further improvement to the above scheme, in step S3, the thickness of the glass tube wall is calculated by a computer connected to the photoelectric detection and data acquisition device.

[0018] As a further improvement to the above scheme, the calculation method for the thickness of the glass tube wall in step S3 is as follows:

[0019] The photoelectric detection and data acquisition device uses a time-domain signal (t, U), where t represents time and U represents voltage. Continuously acquired backlight pulses are framed (T, U) according to the pulse period length T. Then, based on the characteristic intervals of the pulse spectrum, the mapping coefficient K of the time-domain spectrum is determined. K characterizes how long a 1nm spectral width can be stretched to. The time-domain coordinate T can then be converted into a spectrum using coefficient K, and further converted into frequency coordinates. Where c is the speed of light, λ0 is the center wavelength of the light source, and an inverse fast Fourier transform (IFFT) is performed on each frame to obtain the autocorrelation trace curve. The frequency domain interference period obtained by finding the peak of the curve is the time interval ΔT. The glass wall thickness is... Where n is the glass refractive index.

[0020] This invention proposes a simple, reliable, fast, and high-precision glass tube wall thickness measurement technique based on femtosecond laser time-domain spectral interferometry. Utilizing the spectral interference effect between two reflected pulses from the upper and lower surfaces of the glass wall, the pulse interval is calculated using the interference fringe period, and then the wall thickness is determined. A dispersive stretched Fourier transform method is employed to map the spectrum into the time domain, and a data acquisition card is used to measure the interference spectrum. Since the spectral refresh rate equals the pulse repetition frequency, the measurement is very fast, solving the problem of measurement lag. Compared to mechanical measurement, the laser can be emitted to any area of ​​the glass tube, allowing for motion scanning with a production machine tool, facilitating the determination of the global wall thickness and providing support for the production process. Compared to continuous laser wavelength scanning interferometry, the reflected light detection method has lower requirements for the glass surface shape and optical path collimation, and the spectral interference fringes are insensitive to fluctuations in light intensity, resulting in high repeatability accuracy of glass wall thickness measurement, typically exceeding 1%. The measurement function can be achieved through a simple laser transceiver and data processing device, offering advantages of simplicity, reliability, speed, and high precision.

[0021] Unlike traditional spatial interferometry, temporal interferometry, or triangulation thickness measurement techniques that use continuous light sources or lasers, this invention utilizes a femtosecond laser with a broad spectrum (typically above 10 nm @ 1550 nm), making it possible to measure the spectral fringe period. Furthermore, the femtosecond laser exhibits coherence, allowing the measurement of its spectrum to be converted into a time-domain intensity measurement using dispersion-compensated fiber. Spectral interference fringe information can be acquired using a general-purpose data acquisition card, eliminating the need for complex spectrometers. Due to the very short optical frequency oscillation period, the corresponding time period of the spectral interference fringes is also very short, resulting in high resolution for thickness measurement. For example, a 1 nm interference fringe period at 1560 nm corresponds to a time difference of 1 / 125 GHz = 8 ps between the return pulses from the upper and lower surfaces, corresponding to a thickness of approximately 1.6 mm in the glass. If the spectral detection resolution reaches 0.01 nm, a high resolution of 0.016 mm can be achieved.

[0022] Beneficial Effects: Compared with existing technologies, this invention has the following significant advantages: Since the technical principle of this solution is based on spectral interferometry, it has low requirements for the control of spatial beam quality, diameter, and angle, and is not sensitive to light intensity fluctuations. It is suitable for cylindrical curved surface thickness measurement and industrial production environments. Its measurement range and high resolution are very suitable for millimeter-level thickness measurement of glass tubes, and it has the advantages of simplicity, reliability, real-time high precision. It solves the problems of measurement lag, randomness, and localization in existing technologies. Attached Figure Description

[0023] Figure 1This is a schematic diagram of the structure of the present invention; reference numerals: 1. Femtosecond laser; 2. Fiber circulator; 3. Fiber collimator; 4. Glass tube under test; 5. Fiber amplifier; 6. Dispersion compensation fiber; 7. Photoelectric detection and data acquisition device; 8. Computer;

[0024] Figure 2 This is a data processing flowchart of the present invention;

[0025] Figure 3 The image shows the output spectrum of a femtosecond laser, where the two peaks correspond to a spectral width of 14.7 nm.

[0026] Figure 4 The spectrum of the dispersion-compensating fiber measured by a detector and oscilloscope, with the two peaks corresponding to 7.38 ns;

[0027] Figure 5 This is a graph of the acquired time-domain signal;

[0028] Figure 6 This is a time-domain signal diagram after framing.

[0029] Figure 7 The autocorrelation trace is the result of the inverse fast Fourier transform.

[0030] Figure 8 This is a graph showing the glass thickness values ​​output after the calculation. Detailed Implementation

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0032] like Figure 1 As shown, Embodiment 1 of the present invention provides a measurement device for glass tube walls based on time-domain spectral interferometry. The measurement device includes a femtosecond laser 1, an optical fiber circulator 2, an optical fiber collimator 3, a glass tube under test 4, an optical fiber amplifier 5, a dispersion-compensating optical fiber 6, a photoelectric detection and data acquisition device 7, and a computer 8. Specifically, the femtosecond laser 1 is connected to the input end of the optical fiber circulator 2 via a pigtail connector; the straight-through output end of the optical fiber circulator 2 is fused to the pigtail of the optical fiber collimator 3; the other end of the optical fiber circulator 2 is fused to the input end of the optical fiber of the optical fiber amplifier 5; the output end of the optical fiber amplifier 5 is fused to the input end of the dispersion-compensating optical fiber 6; the other end of the dispersion-compensating optical fiber 6 is connected to the signal light input connector of the photoelectric detection and data acquisition device 7; and the photoelectric detection and data acquisition device 7 is connected to the computer 8 via a data cable.

[0033] The main principle of this invention is as follows: Unlike narrow-spectrum continuous lasers, femtosecond lasers are broad-spectrum coherent light sources. Reflection from the upper and lower surfaces of the glass tube wall forms two femtosecond pulses with a certain time interval. These pulses exhibit spectral interference characteristics, and the period of the interference fringes is related to the thickness of the glass tube. By introducing large-dispersion-stretching femtosecond pulses using dispersion-compensating fibers, the pulse spectrum is mapped to the time domain. This eliminates the need for expensive high-resolution spectral measurement equipment, enabling rapid and high-precision wall thickness measurement within the time domain.

[0034] like Figure 2 As shown, this embodiment of the invention provides a method for measuring the glass tube wall using the above-mentioned device. The specific process is as follows: A femtosecond laser (center wavelength λ0, pulse period T) is incident on the upper and lower surfaces of the glass tube wall through an optical fiber circulator and collimator, resulting in two reflections and forming two pulses with a certain time difference. These pulses return to the collimator, forming a backlight signal. The backlight pulse signal is amplified by an optical fiber amplifier after passing through an optical fiber circulator, and then enters a dispersion-compensating fiber for time stretching. The spectral information of the backlight pulse is mapped to the time domain, and then the time domain signal (t, U) is obtained through a photoelectric detection and data acquisition device, where t represents time and U represents voltage. The continuously acquired backlight pulses are framed according to the pulse period length T (T, U). Then, the mapping coefficient K of the time domain spectrum is determined according to the characteristic interval of the pulse spectrum. K characterizes how long the spectral width of 1 nm can be stretched to. The time domain coordinate T can then be converted into a spectrum through the coefficient K, and further converted into frequency coordinates. Where c is the speed of light and λ0 is the center wavelength of the light source, an inverse fast Fourier transform (ifft) is performed on each frame to obtain the autocorrelation trace curve. The frequency domain interference period obtained by finding the peak of the curve is the time interval ΔT. Glass wall thickness

[0035] Example 1

[0036] according to Figure 1 Set up the measuring equipment. Turn on each device to measure the wall thickness of the glass tube.

[0037] The femtosecond fiber laser has an output spectrum as shown in... Figure 3 As shown, the pulse exhibits a cat-ear spectral structure with a center wavelength of 1560 nm and a characteristic spectral width of 14.7 nm between the two spectral peaks. Using a dispersive fiber with a total dispersion of 600 ps / nm at 1560 nm, the pulse propagation is broadened, mapping the spectrum to the time domain. This can be measured using a photodetector and oscilloscope, as shown in the figure. Figure 4 As shown, the time-domain pulse has a structure similar to the spectrum, with a 6.8 ns interval between the two wings, and the time-domain spectral mapping coefficients... Comparable to the dispersion stretching, the pulse repetition period T = 26.8 ns.

[0038] Place the glass tube to be measured, approximately 1 mm thick, at the measurement position, and use a data acquisition card with a data acquisition speed of 5 GSa / s to acquire the signal. Figure 5 The acquired time-domain signal shows that spectral interference occurs between the reflected light from the upper and lower surfaces of the glass tube, resulting in interference fringes. The period of the interference fringes is the flight time ΔT between two reflected light pulses.

[0039] The continuous time-domain signal is divided into frames, each frame having a length equal to the pulse repetition period of 26.8 ns, such as... Figure 6 As shown. According to Transform the time-domain coordinates into frequency-domain coordinates. After the coordinate transformation, perform a Fourier transform on each frame of the time-domain signal, and the autocorrelation trace is as follows: Figure 7 As shown, ΔT is obtained by finding the peak of the autocorrelation trace, according to the formula. The glass thickness can then be calculated, where the glass refractive index n = 1.42. One measurement can be obtained for each frame. Figure 8 The results are from 10 consecutive frames of measurement, with an average wall thickness of 9.75 mm and a repeatability accuracy of 0.08 mm.

[0040] This invention proposes a simple, reliable, fast, and high-precision glass tube wall thickness measurement technique based on femtosecond laser time-domain spectral interferometry. Utilizing the spectral interference effect between two reflected pulses from the upper and lower surfaces of the glass wall, the pulse interval is calculated using the interference fringe period, and then the wall thickness is determined. A dispersive stretched Fourier transform method is employed to map the spectrum into the time domain, and a data acquisition card is used to measure the interference spectrum. Since the spectral refresh rate equals the pulse repetition frequency, the measurement is very fast, solving the problem of measurement lag. Compared to mechanical measurement, the laser can be emitted to any area of ​​the glass tube, allowing for motion scanning with a production machine tool, facilitating the determination of the global wall thickness and providing support for the production process. Compared to continuous laser wavelength scanning interferometry, the reflected light detection method has lower requirements for the glass surface shape and optical path collimation, and the spectral interference fringes are insensitive to fluctuations in light intensity, resulting in high repeatability accuracy of glass wall thickness measurement, typically exceeding 1%. The measurement function can be achieved through a simple laser transceiver and data processing device, offering advantages of simplicity, reliability, speed, and high precision.

Claims

1. A method for measuring the wall of a glass tube based on time-domain spectral interferometry, characterized in that, The measurement method is implemented using a measuring device, which includes a femtosecond laser (1) for emitting a coherent light source, an optical fiber circulator (2) connected to the femtosecond laser (1), an optical fiber collimator (3) connected to the optical fiber circulator (2), an optical fiber amplifier (5) connected to the optical fiber circulator (2) for amplifying the pulse signal of the femtosecond pulse return, a dispersion compensation fiber (6) connected to the optical fiber amplifier (5), and a photoelectric detection and data acquisition device (7) connected to the dispersion compensation fiber (6); wherein, the femtosecond laser (1) The centerlines of the fiber circulator (2) and the fiber collimator (3) are at the same horizontal position. The coherent light source emitted by the femtosecond laser (1) passes through the fiber circulator (2) and the fiber collimator (3) in sequence and illuminates the glass tube under test. The upper and lower surfaces of the tube wall under test reflect to form two femtosecond pulse backlights with a certain time interval. The femtosecond pulse backlights pass through the fiber collimator (3), the fiber circulator (2), the fiber amplifier (5) and the dispersion compensation fiber (6) in sequence and are then collected by the photoelectric detection and data acquisition device (7). The method includes the following steps: S1. The femtosecond laser is incident on the upper and lower surfaces of the glass tube under test through the fiber optic circulator and fiber optic collimator. The tube wall reflects twice, forming two femtosecond pulses with a certain time difference. The femtosecond pulses return to the fiber optic collimator to form a return pulse signal. S2. The return pulse signal is amplified by the fiber amplifier after passing through the fiber optic circulator, and then enters the dispersion compensation fiber for time stretching. The spectral information of the return pulse is mapped to the time domain, and then passes through the photoelectric detection and data acquisition device to obtain the time domain signal. S3. The thickness of the glass tube wall is calculated using the acquired time-domain signal. The calculation method for the thickness of the glass tube wall is as follows: The photoelectric detection and data acquisition device uses a time-domain signal (t, U), where t represents time and U represents voltage. Continuously acquired backlight pulses are framed according to their pulse period length (T, U). Then, based on the characteristic intervals of the pulse spectrum, the mapping coefficient K of the time-domain spectrum is determined. K characterizes how long a 1nm spectral width can be stretched to. The time-domain coordinate T can then be converted into a spectrum using coefficient K, and further converted into frequency coordinates. ,in It's the speed of light. The center wavelength of the light source is used as the basis for inverse fast Fourier transform. The autocorrelation trace curve is obtained by performing an inverse fast Fourier transform on each frame. The frequency domain interference period obtained by finding the peak of the curve is the time interval. glass wall thickness , where n is the glass refractive index.

2. The method for measuring the glass tube wall based on time-domain spectral interferometry according to claim 1, characterized in that, The femtosecond laser (1) is connected to the fiber optic circulator (2) through the pigtail connector at the input end of the fiber optic circulator (2); the pigtail at the straight-through output end of the fiber optic circulator (2) is fused to the pigtail of the fiber optic collimator (3), and the pigtail at the other end of the fiber optic circulator (2) is fused to the fiber optic input end of the fiber optic amplifier (5).

3. The method for measuring the glass tube wall based on time-domain spectral interferometry according to claim 1, characterized in that, The output fiber of the fiber amplifier (5) is fused to the input fiber of the dispersion compensation fiber (6), and the other end of the dispersion compensation fiber (6) is connected to the signal light input connector of the photoelectric detection and data acquisition device (7).

4. The method for measuring the glass tube wall based on time-domain spectral interferometry according to claim 1, characterized in that, The measuring device also includes a computer (8) connected to the photoelectric detection and data acquisition device (7), and the computer (8) is connected to the photoelectric detection and data acquisition device (7) via a data cable.

5. The method for measuring the glass tube wall based on time-domain spectral interferometry according to claim 1, characterized in that, The femtosecond laser (1) meets the following conditions: center wavelength 1560nm±20nm, full width at half maximum (FWHM) of the spectrum > 0.5 nm, repetition frequency 10 kHz~50 MHz, and single pulse energy > 1 nJ.

6. The method for measuring the glass tube wall based on time-domain spectral interferometry according to claim 1, characterized in that, The measuring device is applicable to glass tubes with a wall thickness of 0.1~50mm.

7. The method for measuring the glass tube wall based on time-domain spectral interferometry according to claim 1, characterized in that, The total dispersion of the dispersion-compensating fiber (6) is >500ps / nm.

8. The method for measuring the glass tube wall based on time-domain spectral interferometry according to claim 1, characterized in that, In step S3, the thickness of the glass tube wall is calculated by a computer (8) connected to the photoelectric detection and data acquisition device.

Images