Broadband optical interferometric roughness measurement device and method based on common-path transmission

Abstract

The present application relates to a kind of broadband light interference type roughness measuring device based on common path transmission, including super radiation emitting diode, 1×2 optical fiber coupler, optical fiber collimator, cube spectroscope, reference arm, probe arm, spectrometer and host computer;Collimator, spectroscope, reference arm and probe arm constitute interference device and install on adjustable mounting frame, and measured object is set on displacement table;The two interfaces of optical fiber coupler same side connect super radiation emitting diode and spectrometer, and the single output port of other side connects interference device;Super radiation emitting diode radiates broadband light, which is transmitted through optical fiber coupler, then is divided into reference light and probe light through collimator and spectroscope, respectively enters reference arm and probe arm, respectively after being reflected by reference mirror and measured object surface, converges and interferes, enters spectrometer through optical fiber coupler, then enters linear array CMOS camera and is imaged and transmitted to host computer.The device and method are beneficial to improve the accuracy of non-contact roughness measurement.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of non-contact optical measurement, in particular to a broadband optical interference type roughness measurement device and method based on common path transmission. BACKGROUND

[0002] Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important influence on the service life and reliability of mechanical products. Using the principle of light interference to measure roughness is a common non-contact measurement method. However, the traditional interference type roughness meter is mostly a free space type system, which has poor flexibility and poor usability. The optical fiber type interference system is greatly affected by external interference such as fiber disturbance, vibration and bending, and the measurement result is prone to disturbance noise, which affects the roughness measurement result. SUMMARY

[0003] The purpose of the present application is to provide a broadband optical interference type roughness measurement device and method based on common path transmission, which is beneficial to improve the accuracy of non-contact roughness measurement.

[0004] In order to achieve the above purpose, the technical scheme adopted by the present application is: a broadband optical interference type roughness measurement device based on common path transmission, comprising a super radiation light emitting diode, a 1x2 optical fiber coupler, an optical fiber collimator, a cubic beam splitter, a reference arm, a detection arm, an adjustable mounting frame, a displacement table, a spectrometer and an upper computer; the optical fiber collimator, the cubic beam splitter, the reference arm and the detection arm constitute an interference device, the interference device is installed on the adjustable mounting frame, and a measured object is arranged on the displacement table; the super radiation light emitting diode radiates broadband light with a certain bandwidth as a detection source; the optical fiber coupler has three connecting ports, two interfaces on the same side are connected with the super radiation light emitting diode and the spectrometer respectively, and a single output port on the other side is connected with the interference device; the detection source is transmitted through the optical fiber coupler, output from the single output port and collimated into parallel light by the optical fiber collimator, and then the parallel light is divided into reference light and detection light by the cubic beam splitter; the reference light enters the reference arm and is focused by the microscopic focusing lens in the reference arm, and is focused on a point on the surface of the reference mirror; the detection light enters the detection arm and is focused by the microscopic focusing objective lens in the detection arm, and is converged on a point on the surface of the measured object; the reference light and the detection light are reflected by the reference mirror and the surface of the measured object respectively, and then converge at the cubic beam splitter and interfere, the interference light enters the spectrometer after being transmitted through the optical fiber coupler, the interference signal is collimated into parallel light by the large beam optical fiber collimator in the spectrometer, and then irradiated to the transmission grating, and then focused by the focusing mirror and entered into the linear array CMOS camera after being expanded by the transmission grating according to the wavelength; the imaging of the linear array CMOS camera is transmitted to the upper computer.

[0005] Further, the reference arm comprises an adjustable sleeve, a microscopic focusing lens, a reference mirror and an optical adjusting frame, the adjustable sleeve is arranged in front of the microscopic focusing lens for adjusting the optical path of the reference light, and the reference mirror is arranged on the optical adjusting frame for adjusting the posture of the reflected reference light, so that the interference device can achieve better interference effect.

[0006] Further, the detection arm comprises a fixed sleeve and a microscopic focusing objective lens, and the detection arm moves up and down under the driving of the adjustable mounting frame to adjust the relative distance between the detection arm and the measured object.

[0007] Further, the displacement table is an XY displacement table, which drives the measured object to move in the X direction and the Y direction.

[0008] Further, the spectrometer comprises a large-beam collimator, a transmission grating, a focusing mirror and a linear array CMOS camera.

[0009] Further, the reference arm and the detection arm are connected to the same optical fiber port of the optical fiber coupler, and the reference light and the detection light are transmitted in the same optical fiber to avoid system noise caused by external interference.

[0010] The application also provides a common-path transmission-based wideband optical interference type roughness measurement method based on the above device, comprising the following steps:

[0011] Step S1: placing the mirror on the displacement table, adjusting the detection arm up and down through the adjustable mounting frame, and focusing the detection light on the surface of the mirror;

[0012] Step S2: adjusting the optical adjusting frame of the reference arm to make the coincidence degree of the reference light and the detection light optimal, improving the signal-to-noise ratio of the system interference signal; at the same time, adjusting the length of the adjustable sleeve to change the optical path difference between the detection light and the reference light, so that the number of interference signal periods reaches a set value, to distinguish from the zero optical path position, and avoid interference when using the Hann window energy center method for signal demodulation;

[0013] Step S3: replacing the mirror with the metal sample to be measured, adjusting the detection arm up and down through the adjustable mounting frame, focusing the detection light on the surface of the metal sample, and making the number of interference signal periods the same as the set value in step S2; controlling the displacement table to move and traverse all detection points on the metal sample to collect the interference signals of all detection points;

[0014] Step S4: performing Fourier transform on the collected interference signals, and then using the Hann window energy center method to calculate the surface height of each detection point;

[0015] Step S5: substituting the surface height of each detection point into the roughness calculation formula to calculate the roughness of the metal sample.

[0016] Further, the interference signal collected by the spectrometer is:

[0017]

[0018] wherein the first term is a direct current term, and the second term is an interference signal term, wherein carries the position information of the metal sample surface relative to the reference mirror, when the height of the metal sample surface changes, the optical path difference between the reference light and the probe light changes, and the interference signal density changes; S r (λ) is the spectral power distribution function of the reference light; S p (λ) is the spectral power distribution function of the probe light reflected by the metal sample surface; λ is the wavelength of the light; 2d represents the optical path difference between the probe light reflected by the metal sample surface and the reference light; Re represents the real part of the complex domain interference signal;

[0019] The formula (1) is converted to the wave number domain, specifically:

[0020]

[0021] wherein k is the wave number, and the relationship between the wave number and the wavelength is

[0022] The surface height information of the metal sample is obtained by performing a fast Fourier transform on the interference signal of formula (2), specifically:

[0023]

[0024] wherein δ is the Euler-K function.

[0025] Further, the metal sample surface height of each detection point is accurately calculated using the Hanning window energy barycenter method, specifically:

[0026]

[0027] wherein h represents the depth position of the detection point, z res is the axial resolution of the system, G k+i is the energy at the k+i point, λ1 is the starting wavelength of the broadband light source, and λ2 is the terminal wavelength of the broadband light source.

[0028] Further, the metal sample surface height calculated by formula (4) is substituted into the roughness calculation formula to calculate the roughness of the metal sample, specifically:

[0029]

[0030] wherein h(x) is the height of the xth detection point, and n is the number of sampling points.

[0031] Compared with the prior art, the present application has the following beneficial effects: the present application provides a broadband light interference type roughness measurement device and method based on common path transmission, which combines the advantages of free space type system and optical fiber type system, carries reference light and probe light in a common path transmission mode, effectively isolates the interference of external factors such as disturbance, bending and temperature on the system, so that the device has the advantages of strong anti-noise stability of the free space type system and high flexibility of the optical fiber type system, not only improves the accuracy of roughness measurement, but also is easy to operate and use. In addition, the present application uses the Hann window energy barycenter method to realize nanoscale precision signal demodulation, improves the axial resolution of the system, and further realizes the accurate measurement of the surface height of the metal sample and calculates the surface roughness of the metal sample. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a device structure schematic diagram of an embodiment of the present application;

[0033] Figure 2 is a metal sample physical object diagram in an embodiment of the present application;

[0034] Figure 3 is a metal sample surface height calculation principle diagram in an embodiment of the present application;

[0035] Figure 4 is a metal sample surface height measurement result diagram in an embodiment of the present application.

[0036] In the figure: 1 - super radiation light emitting diode, 2 - optical fiber coupler, 3 - reference arm, 4 - optical adjustment frame, 5 - reference mirror, 6 - microscopic focusing lens, 7 - adjustable sleeve, 8 - optical fiber collimator, 9 - cubic beam splitter, 10 - fixed sleeve, 11 - microscopic focusing objective lens, 12 - adjustable mounting frame, 13 - probe arm, 14 - metal sample, 15 - XY displacement table, 16 - upper computer, 17 - spectrometer, 18 - linear array CMOS camera, 19 - focusing mirror, 20 - transmission grating, 21 - large beam collimator. DETAILED DESCRIPTION

[0037] The present application will be further described below in combination with the drawings and embodiments.

[0038] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as generally understood by those skilled in the art to which the present application belongs.

[0039] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0040] like Figure 1 As shown, this embodiment provides a broadband optical interferometric roughness measurement device based on common-path transmission, including a superluminescent diode 1, a 1×2 fiber coupler 2, a fiber collimator 8, a cubic beam splitter 9, a reference arm 3, a probe arm 13, an adjustable mounting bracket 12, a displacement stage 15, a spectrometer 17, and a host computer 16. The fiber collimator 8, the cubic beam splitter 9, the reference arm 3, and the probe arm 13 form an interferometric device, which is mounted on the adjustable mounting bracket 12, and the object to be measured 14 is placed on the displacement stage 15. The superluminescent diode 1 emits broadband light with a certain bandwidth as a detection source; the fiber optic coupler 2 has three connection ports, with two ports on the same side connecting to the superluminescent diode 1 and the spectrometer 17 respectively, and a single output port on the other side connecting to an interferometer; the detection source is transmitted through the fiber optic coupler 2, output from its single output port and collimated into parallel light by the fiber optic collimator 8, and then the parallel light is split into reference light and detection light by the cubic beam splitter 9; the reference light enters the reference arm 3, is focused by the micro-focusing lens 6 in the reference arm 3, and is focused at a point on the surface of the reference mirror 5; the detection light enters the detection arm 13, is focused by the micro-focusing objective lens 11 in the detection arm 13, and converges at a point on the surface of the object under test 14; the reference light and the detection light are reflected by the reference mirror 5 and the surface of the object under test 14 respectively, and then converge and interfere at the cubic beam splitter 9, and the interference light is transmitted through the fiber optic coupler 2 and enters the spectrometer 17. The spectrometer 17 includes a large-beam collimator 21, a transmission grating 20, a focusing lens 19, and a linear CMOS camera 18. The interference signal is collimated into parallel light by the large-beam fiber collimator 21 in the spectrometer 17 and then illuminates the transmission grating 20. After being expanded by the transmission grating 20 according to wavelength, it is focused by the focusing lens 19 into the linear CMOS camera 18; the image from the linear CMOS camera 18 is transmitted to the host computer 16.

[0041] In the embodiment, the reference arm 3 comprises an adjustable sleeve 7 arranged in front of the microscopic focusing lens 6 for adjusting the optical path of the reference light, a reference mirror 5 arranged on the optical adjusting frame 4 for adjusting the posture of the reflected reference light so as to make the interference device achieve better interference effect. The detection arm 13 comprises a fixed sleeve 10 and a microscopic focusing objective 11, and is driven by the adjustable mounting frame 12 to move up and down so as to adjust the relative distance between the detection arm and the measured object.

[0042] The reference arm 3 and the detection arm 13 are connected to the same optical fiber port of the optical fiber coupler, and the reference light and the detection light are transmitted in the same optical fiber, so as to effectively avoid the system noise caused by external interference such as fiber disturbance, vibration and bending, and improve the noise resistance performance of the system.

[0043] In the embodiment, the displacement table 15 is an XY displacement table, which drives the measured object to move in the X direction and the Y direction.

[0044] Based on the above device, the embodiment further provides a corresponding wideband light interference type roughness measurement method based on common path transmission, comprising the following steps:

[0045] Step S1: placing the mirror on the XY displacement table, and adjusting the detection arm up and down by the adjustable mounting frame so as to focus the detection light on the surface of the mirror.

[0046] Step S2: adjusting the optical adjusting frame of the reference arm so as to make the coincidence degree of the reference light and the detection light optimal, and improving the signal noise ratio of the system interference signal; at the same time, adjusting the length of the adjustable sleeve, changing the optical path difference between the detection light and the reference light, and making the interference signal period number about 100 so as to be distinguished from the zero optical path position, and avoiding the interference when the Hann window energy center method is used for signal demodulation.

[0047] Step S3: replacing the mirror with the metal sample to be measured, adjusting the detection arm up and down by the adjustable mounting frame so as to focus the detection light on the surface of the metal sample, and making the interference signal period number the same as that in step S2, i.e. about 100; controlling the displacement table to move and traverse each detection point on the whole metal sample, and collecting the interference signals of each detection point.

[0048] Step S4: performing Fourier transform on the collected interference signals, then using the Hann window energy center method (HnWECM) to accurately estimate the interference signal period number, and calculating the surface height of each detection point.

[0049] Step S5: substituting the surface height of each detection point into the roughness calculation formula to calculate the roughness of the metal sample.

[0050] Figure 2is the real object diagram of the metal sample in the embodiment. Figure 3 is the calculation principle diagram of the surface height of the metal sample in the embodiment.

[0051] In the above method, the interference signal collected by the spectrometer is:

[0052]

[0053] wherein the first term is a direct current term, and the second term is an interference signal term, wherein carries the position information of the surface of the metal sample relative to the reference mirror, and when the surface height of the metal sample changes, the optical path difference between the reference light and the probe light changes, and the interference signal density changes; S r (λ) is the spectral power distribution function of the reference light; S p (λ) is the spectral power distribution function of the probe light reflected by the surface of the metal sample; λ is the wavelength of the light; 2d represents the optical path difference between the probe light reflected by the surface of the metal sample and the reference light; Re represents the real part of the complex domain interference signal.

[0054] Convert formula (1) to the wave number domain, specifically:

[0055]

[0056] wherein k is the wave number, and the relationship between the wave number and the wavelength is

[0057] Perform fast Fourier transform on the interference signal of formula (2) to obtain the surface height information of the metal sample, specifically:

[0058]

[0059] wherein δ is the Euler-K function.

[0060] Use the Hann window energy barycenter method to accurately calculate the surface height of the metal sample at each detection point, specifically:

[0061]

[0062] wherein h represents the depth position of the detection point, z res is the axial resolution of the system, G k+i is the energy at the k+i point, λ1 is the starting wavelength of the broadband light source, and λ2 is the terminal wavelength of the broadband light source.

[0063] Figure 4 is the measurement result diagram of the surface height of the metal sample in the embodiment.

[0064] The surface height of the metal sample calculated by formula (4) is substituted into a roughness calculation formula to calculate the roughness of the metal sample, specifically as follows:

[0065]

[0066] Wherein, h(x) is the height of the xth detection point, and n is the number of sampling points.

[0067] The wideband light interference type roughness measuring device and method based on common path transmission provided by the application utilize the same optical fiber to transmit reference light and detection light, so that the reference light and the detection light are modulated by the same interference signal, the interference signal can be cancelled when interference occurs, the system has the advantages of strong anti-noise stability of the free space type system and high flexibility of the optical fiber type system. At the same time, the Hanning window energy barycenter method is used as a signal demodulation method to improve the axial resolution of the system, realize the measurement of the nanoscale metal surface height, and further realize the accurate measurement of the surface roughness of the metal sample.

[0068] The above is only a preferred embodiment of the application, and is not intended to limit the application in other forms. Any skilled person in the art can modify or change the above disclosed technical content to obtain equivalent embodiments. However, any simple modification, equivalent change and modification made on the basis of the technical essence of the application to the above embodiments without departing from the technical solution of the application still falls within the protection scope of the application.

Claims

1. A broadband optical interferometric roughness measurement device based on common-path transmission, characterized in that, The application relates to a super-radiation light-emitting diode, a 1*2 optical fiber coupler, an optical fiber collimator, a cubic beam splitter, a reference arm, a detection arm, an adjustable mounting frame, a displacement table, a spectrometer and a host computer; the optical fiber collimator, the cubic beam splitter, the reference arm and the detection arm constitute an interference device, the interference device is installed on the adjustable mounting frame, and a measured object is arranged on the displacement table; the super-radiation light-emitting diode radiates broadband light with a certain bandwidth as a detection source; the optical fiber coupler has three connecting ports, two interfaces on the same side are connected with the super-radiation light-emitting diode and the spectrometer respectively, and a single output port on the other side is connected with the interference device; the detection source is transmitted through the optical fiber coupler, is output from the single output port and is collimated into parallel light by the optical fiber collimator, and then the parallel light is divided into reference light and detection light by the cubic beam splitter; the reference light enters the reference arm, is focused by a microscopic focusing lens in the reference arm and is focused on a point on the surface of a reference mirror; the detection light enters the detection arm, is focused by a microscopic focusing objective lens in the detection arm and is converged on a point on the surface of the measured object; the reference light and the detection light are reflected by the reference mirror and the surface of the measured object respectively, are converged at the cubic beam splitter and interfere, the interference light is transmitted through the optical fiber coupler and enters the spectrometer, the interference signal is collimated into parallel light by a large-beam optical fiber collimator in the spectrometer, is irradiated into a transmission grating and is focused into a linear array CMOS camera by a focusing mirror after being expanded according to wavelengths by the transmission grating; imaging of the linear array CMOS camera is transmitted to the host computer; the reference arm comprises an adjustable sleeve, a microscopic focusing lens, a reference mirror and an optical adjusting frame, the adjustable sleeve is arranged on the front side of the microscopic focusing lens and is used for adjusting the optical path of the reference light, and the reference mirror is arranged on the optical adjusting frame and is used for adjusting the posture of the reflected reference light; the detection arm comprises a fixed sleeve and a microscopic focusing objective lens, and the detection arm moves up and down under the driving of the adjustable mounting frame to adjust the relative distance between the detection arm and the measured object.

2. The common-path transmission based broadband optical interferometric roughness measurement device of claim 1, wherein, The displacement table is an XY displacement table and drives the measured object to move in the X direction and the Y direction.

3. The common-path transmission based broadband optical interferometric roughness measurement device of claim 1, wherein, The spectrometer comprises a large-beam collimator, a transmission grating, a focusing mirror and a linear array CMOS camera.

4. The common-path transmission based broadband optical interferometric roughness measurement device of claim 1, wherein, The reference arm and the detection arm are connected to the same optical fiber port of the optical fiber coupler, and the reference light and the detection light are transmitted in the same optical fiber to avoid system noise caused by external interference.

5. A common-path transmission based broadband optical interferometric roughness measurement method based on the apparatus of any one of claims 1-4, characterized in that, The application further relates to a signal demodulation method, and the method comprises the following steps: S1: placing a reflecting mirror on the displacement table, adjusting the detection arm up and down through the adjustable mounting frame to make the detection light focus on the surface of the reflecting mirror; S2: adjusting the optical adjusting frame of the reference arm to make the coincidence degree of the reference light and the detection light reach the best, improving the signal noise ratio of the system interference signal; meanwhile, the length of the adjustable sleeve is adjusted to change the optical path difference between the detection light and the reference light, so that the number of interference signal periods reaches a set value to distinguish from the zero optical path position, avoiding the interference when the Hann window energy center method is used for signal demodulation. Step S3: replace the mirror with the metal sample to be tested, adjust the detection arm up and down through the adjustable mounting bracket, focus the detection light on the surface of the metal sample, and make the number of interference signals the same as the set value in step S2; control the displacement table to move and traverse all detection points on the metal sample, and collect the interference signals of each detection point; Step S4: Fourier transform the collected interference signals, and then calculate the surface height of each detection point by using the Hanning window energy center method; Step S5: substitute the surface height of each detection point into the roughness calculation formula to calculate the roughness of the metal sample.

6. The common-path transmission based broadband optical interferometric roughness measurement method of claim 5, wherein, The interference signal collected by the spectrometer is: wherein the first term is a direct current term, the second term is an interference signal term, wherein carries information about the position of the surface of the metallic sample relative to the reference mirror, and wherein the density of the interference signal changes when the surface height of the metallic sample changes; S r (λ) is a spectral power distribution function of the reference light; S p (λ) is a spectral power distribution function of the probe light reflected from the surface of the metallic sample; λ is the wavelength of the light; 2d represents the optical path difference between the probe light reflected from the surface of the metallic sample and the reference light; Re represents the real part of the complex domain interference signal; Convert formula (1) to the wave number domain, specifically: where k is the wave number, which has the following relationship with the wavelength The surface height information of the metal sample is obtained by performing fast Fourier transform on the interference signal of formula (2), specifically: Wherein, δ is the Euler function.

7. The common-path transmission based broadband optical interferometric roughness measurement method of claim 5, wherein, The surface height of each detection point of the metal sample is accurately calculated by using the Hanning window energy center method, specifically: where h represents the depth position of the probe point, z res is the axial resolution of the system, G k+i is the energy at the k+1 point, λ1 is the starting wavelength of the broadband light source, and λ2 is the terminal wavelength of the broadband light source.

8. The common-path transmission based broadband optical interferometric roughness measurement method of claim 7, wherein, Substitute the surface height of the metal sample calculated by formula (4) into the roughness calculation formula to calculate the roughness of the metal sample, specifically: Wherein, h(x) is the height of the xth detection point, and n is the number of sampling points.

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