A white light interferometric detection device and method based on a 3*3 fiber coupler

Abstract

The application belongs to the field of optical detection, and provides a white light interference detection device and method based on a 3*3 optical fiber coupler. A 3*3 optical fiber coupler, a sample arm and a reference arm are arranged, a circulator and a detector are arranged, and the angle of a vibrating mirror is changed to change the optical path of reference light. Finally, the method realizes obtaining three interference signals in the case of the same optical path of reference light, and is not affected by the interference spectrum envelope, so that the phase calculation accuracy is improved. The low-precision solution demodulation uses intensity demodulation to find the coherent peak position of the envelope curve, avoids phase wrapping, and increases the measurement range; in the case of the same optical path of reference light, one collection is collected at the same time, three interference signals with phase shift are generated, a larger sampling step can be selected during measurement, and the measurement speed is faster.

Description

Technical Field

[0001] This invention relates to the field of optical detection, and in particular to a white light interferometry detection device and method based on a 3*3 fiber coupler. Background Technology

[0002] White light interferometry (WLI) is a non-contact optical technique that uses a white light source for detection. Due to the shorter coherence length of white light, the interference fringes exhibit a distinct zero optical path difference position, significantly improving resolution and measurement range. During measurement, the optical path difference is altered by moving the sample arm or reference arm to acquire data. The acquired data is processed, and a demodulation algorithm determines the coherence peak position (equi-optical path point) of the interference signal, thereby determining the measurement height. The advent of WLI has brought tremendous progress to ultra-precision inspection technology, making quality control of products in ultra-precision machining processes much more convenient.

[0003] Currently, WLI demodulation algorithms can be divided into intensity demodulation and phase demodulation, which are based on the demodulation of amplitude or phase terms. Commonly used demodulation algorithms can be divided into extreme value method, phase shift method, Fourier transform method, etc. The phase shift method has high accuracy, but the current phase shift algorithm has the following defects: (1) Assuming the center wavelength of the light source is λ0, considering the round trip of the reference light and the sample light, the maximum measurement range is [-λ0 / 2, λ0 / 2]. Due to the periodicity of the trigonometric function, phase winding will occur if this range is exceeded. For common white light sources, this range is generally at the micrometer level. When imaging a large area, phase dewinding is required, which will reduce the measurement efficiency; (2) The phase shift method requires continuous acquisition of at least three images to calculate the position of a certain point. During the calculation, the amplitude term is approximated. When the sampling step size is large, the amplitude terms of these points are no longer approximately equal. Generally, the sampling step size is selected as λ0 / 8, corresponding to a phase shift of π / 2. This results in the phase shift method selecting a very small sampling step size, generally at the level of hundreds of nanometers, which will limit the measurement speed. (3) During scanning measurement, PZT or precision translation stage are generally used to change the optical path to generate phase shift. This method is prone to causing vibration of the interference system, reducing system stability and affecting measurement accuracy.

[0004] The existing technology still has the following problems:

[0005] 1. Current phase-shifting algorithms have a very small measurement range. When scanning samples with a large height range, phase entanglement will occur. The extra operation of phase de-entanglement will reduce the measurement efficiency. In addition, the accuracy of the measurement results will also be affected by the phase de-entanglement algorithm.

[0006] 2. The phase-shifting method has a scanning step size on the order of hundreds of nanometers. When used for samples with a large height range, the number of scanning steps is large, the scanning speed is slow, the amount of interference image data is large, and the measurement efficiency is low. However, increasing the scanning step size will distort the interference signal and cause errors in the demodulation process.

[0007] 3. Optical path scanning is performed by driving the reference mirror or system through PZT or a precision translation stage. This optical path scanning method involves mechanical movement, which can easily cause vibration in the interference system, reduce system stability, introduce errors, and affect measurement accuracy. Summary of the Invention

[0008] To address the problems existing in the prior art, this invention proposes a white light interferometry detection device and method based on a 3*3 fiber coupler.

[0009] The technical solution of the present invention is as follows: A white light interferometric detection device based on a 3*3 fiber coupler includes a light source 1, a circulator 3, a 3*3 coupler 4, a sample arm, a reference arm, and a detector; the light emitted from the light source 1 enters the a port of the circulator 3 through an optical fiber, exits through the b port of the circulator 3 to the 3*3 coupler 4, and is split into three beams: the first beam is used as a reference beam, the second beam is used as a sample beam, and the third beam is suspended and not connected to the output terminal;

[0010] The sample arm includes a collimating lens b9, a focusing lens b10, and a sample 11. After the sample light is collimated by the collimating lens b9, it is focused on the sample 11 by the focusing lens b10. The sample light reflected by each reflecting surface of the sample 11 returns to the 3*3 coupler 4 along the original path.

[0011] The reference light entering the reference arm is reflected by the reference arm and returns to the 3*3 coupler 4;

[0012] The reference light and sample light entering the 3*3 coupler 4 are split into three interference light signals by the 3*3 coupler 4. One interference light signal enters the b port of the circulator 3 and exits from the c port of the circulator 3, and is transmitted to the data acquisition card 12 via detector a14. The other two interference light signals are transmitted to the data acquisition unit 12 via detector b15 and detector c16 respectively. The data acquisition unit 12 transmits the signals to the processing unit 2 for signal demodulation.

[0013] The reference arm includes a collimating lens a5, a galvanometer 6, a focusing lens a7, a wedge prism 8, and a reference mirror 13. After being collimated by the collimating lens a5, the reference light is reflected by the galvanometer 6 and then focused by the focusing lens a7 and the wedge prism 8 onto the reference mirror 13. The reference light reflected by the reference mirror 13 returns to the 3*3 coupler 4 along the original path. When the galvanometer 6 is rotated at different angles, the corresponding reference light passes through different thicknesses of the wedge prism 8, resulting in different optical paths of the reference light. While changing the optical path of the reference light, detectors a14, b15, and c16 simultaneously acquire signals.

[0014] A white light interferometry detection method based on a 3*3 fiber coupler is implemented using the aforementioned white light interferometry detection device based on a 3*3 fiber coupler; the specific steps are as follows:

[0015] Step 1: At the i-th optical path position, the interference light signals collected by detectors a14, b15, and c16 are Ia(z) and Ia(z) respectively. i ), Ib(z) i ) and Ic(z i );

[0016] Ia(z i )=g(z i -h)*cos(4π(z i -h) / λ)+D0 1)

[0017] Ib(z i )=k1*g(z i -h)*cos(4π(z i -h) / λ+θ1)+D1 2)

[0018] Ic(z i )=k2*g(z i -h)*cos(4π(z i -h) / λ+θ2)+D2 3)

[0019] In the formula, λ is the center wavelength of light source 1, and z i Let D0, D1, and D2 be the DC components, and θ1 be the reference light's shift position; θ1 be the Ib(z) value. i ) and Ia(z i The inherent phase difference of ), θ2 is Ic(z) i ) and Ia(z i The inherent phase difference of ); k1, k2 are the splitting ratio coefficients of port 4b of the 3*3 coupler relative to port a, and port c relative to port a, respectively; h is the position of the equipathic point to be solved, which is the position of the reflecting surface of sample 11; g(z i -h) represents the amplitude envelope of the interference signal;

[0020] Step 2: Filter out the three interference signals Ia(z) i ), Ib(z) i ) and Ic(z i The DC components D0, D1, and D2 in the equation are used to obtain I0(z). i ), I1(z i ) and I2(z i ):

[0021] I0(z i)=g(z i -h)*cos(4π(z i -h) / λ) 4)

[0022] I1(z i )=k1*g(z i -h)*cos(4π(z i -h) / λ+θ1) 5)

[0023] I2(z i )=k2*g(z i -h)*cos(4π(z i -h) / λ+θ2) 6)

[0024] Step 3: Calculate the amplitude envelope of the interference signal g(z) i -h)

[0025]

[0026] Calculate the location of the maximum point of the envelope line using the centroid formula:

[0027]

[0028] In all z i In the process, the optical path position of the reference light closest to h′ is determined and set as z0. The signals collected at position z0 are I0(z0), I1(z0) and I2(z0). The distance between h′ and the point with equal optical path is set as Δh.

[0029] I0(z0)=cos(4πΔh / λ) 9)

[0030] I1(z0)=k1cos(4πΔh / λ+θ1) 10)

[0031] I2(z0)=k2cos(4πΔh / λ+θ2) 11)

[0032] get

[0033]

[0034] The position of the sample reflective surface is

[0035] h=h′+Δh 13)

[0036] Step 4: Calculate the position of each layer in the multilayer sample according to equations 8), 12), and 13). There are a total of m reflective surfaces, and the position of each reflective surface is h. i Then the distance between the reflective surface of the j-th sample layer and the reflective surface of the i-th sample layer is:

[0037] Δ ij =hi -h j 14).

[0038] The beneficial effects of this invention are:

[0039] 1. Current white-light interferometric phase-shifting methods require acquiring three (or more) interference signals at different times, and when the reference light has three (or more) different phase shifts. The sampling step size is very small, resulting in low efficiency. This invention acquires the reference light once at the same time along the same optical path, generating three interference signals with phase shifts. During measurement, a larger sampling step size can be selected, resulting in faster measurement speed.

[0040] 2. The present invention uses intensity demodulation for low-precision demodulation, which finds the position of the coherent peak of the envelope curve, avoids phase entanglement, and increases the measurement range;

[0041] 3. This invention uses a galvanometer and a wedge prism to replace the current PZT or electric displacement stage for optical path scanning. There is no mechanical movement, and the scanning part and the sample end are connected by optical fiber. Therefore, no additional mechanical vibration is generated at the sample end, which improves the stability of the system.

[0042] 4. Existing phase-shifting methods acquire interference signals at different optical path positions of the reference light, so the amplitude of each interference signal is affected by the interference spectrum envelope, which reduces the accuracy of phase calculation. This method obtains three interference signals under the condition of the same optical path of the reference light, and is not affected by the interference spectrum envelope, thus improving the accuracy of phase calculation. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of a white light interferometry detection device based on a 3*3 fiber coupler;

[0044] Figure 2 For signal Ia(z) i ), Ib(z) i ), Ic(z i ), g(z) i -h) diagram.

[0045] In the figure: 1-Light source; 2-Processing unit; 3-Circulator; 4-3*3 coupler; 5-Collimating lens a; 6-Galvanometer; 7-Focusing lens a; 8-Wedge prism; 9-Collimating lens b; 10-Focusing lens b; 11-Sample; 12-Data acquisition unit; 13-Reference mirror; 14-Detector a; 15-Detector b; 16-Detector c. Detailed Implementation

[0046] White light interferometry detection device based on 3*3 fiber coupler, such as Figure 1As shown, the light emitted from the light source 1 enters port a of the circulator 3 through the optical fiber, exits through port b of the circulator 3 to the 3*3 coupler 4, and is split into three beams: the first beam is used as the reference beam, the second beam is used as the sample beam, and the third beam is not processed.

[0047] The sample arm includes a collimating lens b9, a focusing lens b10, and a sample 11. After the sample light is collimated by the collimating lens b9, it is focused on the sample 11 by the focusing lens b10. The sample light reflected by each reflecting surface of the sample 11 returns to the 3*3 coupler 4 along the original path.

[0048] The reference light entering the reference arm is reflected by the reference arm and returns to the 3*3 coupler 4;

[0049] The reference light and sample light entering the 3*3 coupler 4 are split into three paths by the coupler 4. The reference light and sample light generate three interference light signals at the three outputs of the 3*3 coupler 4. One interference light signal enters port b of the circulator 3 and exits from port c of the circulator 3, then is transmitted to the data acquisition card 12 via detector a14. The other two interference light signals are transmitted to the data acquisition unit 12 via detectors b15 and c16, respectively. The data acquisition unit 12 transmits the signals to the processing unit 2 for demodulation. During measurement, the galvanometer 6 is rotated at different angles, and the corresponding reference light passes through different thicknesses of the wedge prism 8, thus changing the optical path of the reference light. Simultaneously with this change in optical path, the signals are acquired by detectors a14, b15, and c16.

[0050] The specific signal demodulation process is as follows:

[0051] (1) At the i-th optical path position, the interference signals collected by detectors a14, b15, and c16 are Ia(z) and Ia(z) respectively. i ), Ib(z) i ) and Ic(z i ):

[0052] Ia(z i )=g(z i -h)*cos(4π(z i -h) / λ)+D0 1)

[0053] Ib(z i )=k1*g(z i -h)*cos(4π(z i -h) / λ+θ1)+D1 2)

[0054] Ic(z i )=k2*g(z i -h)*cos(4π(z i -h) / λ+θ2)+D2 3)

[0055] In the formula, λ is the center wavelength of light source 1, and z i Let D0, D1, and D2 be the DC components, and θ1 be the reference light's shift position; θ1 be the Ib(z) value. i ) and Ia(z i The inherent phase difference of ), θ2 is Ic(z) i ) and Ia(z i The inherent phase difference of ); k1, k2 are the splitting ratio coefficients of port b relative to port a of the 3*3 coupler (4), and the splitting ratio coefficient of port c relative to port a, respectively; h is the position of the equipathic point to be solved, and is the position of the reflecting surface of sample (11); g(z i -h) represents the amplitude envelope of the interference signal;

[0056] (2) Filter out the three interference signals Ia(z) i ), Ib(z) i ) and Ic(z i The DC components D0, D1, and D2 in the equation are used to obtain I0(z). i ), I1(z i ) and I2(z i ):

[0057] I0(z i )=g(z i -h)*cos(4π(z i -h) / λ) 4)

[0058] I1(z i )=k1*g(z i -h)*cos(4π(z i -h) / λ+θ1) 5)

[0059] I2(z i )=k2*g(z i -h)*cos(4π(z i -h) / λ+θ2) 6)

[0060] Interference signal I0(z) i ), I1(z i ) and I2(z i ) and the amplitude envelope of the interference signal g(z) i -h) as Figure 2 As shown, the dashed line represents I0(z) i The solid line represents I1(z). i The dotted line represents I2(z). i The dashed line represents g(z). i -h).

[0061] (3) Calculate the amplitude envelope of the interference signal g(z) i -h)

[0062]

[0063] The location of the maximum point, i.e., the coherence peak, of the envelope curve is calculated using the centroid formula:

[0064]

[0065] All z i In the process, the optical path position of the reference light closest to h′ is determined and set as z0. The signals collected at position z0 are IO(z0), I1(z0) and I2(z0). The distance between h′ and the point with equal optical path is set as Δh.

[0066] I0(z0)=cos(4πΔh / λ) 9)

[0067] I1(z0)=k1cos(4πΔh / λ+θ1) 10)

[0068] I2(z0)=k2cos(4πΔh / λ+θ2) 11)

[0069] get

[0070]

[0071] The position of the sample reflective surface is

[0072] h=h′+Δh 13)

[0073] (4) Calculate the position of each layer in the multilayer sample according to Equations 8), 12), and 13). There are a total of m reflective surfaces, and the position of each reflective surface is h. i Then the distance between the reflective surface of the j-th sample layer and the reflective surface of the i-th sample layer is:

[0074] Δ ij =h i -h j 14).

Claims

1. A white light interferometry detection method based on a 3*3 fiber coupler, characterized in that, This is based on a white light interferometric detection device based on a 3*3 fiber optic coupler. The white light interferometric detection device based on a 3*3 fiber optic coupler includes a light source (1), a circulator (3), a 3*3 coupler (4), a sample arm, a reference arm, and a detector. The light emitted from the light source (1) enters the a port of the circulator (3) through the fiber optic cable, and is emitted to the 3*3 coupler (4) through the b port of the circulator (3). The light is split into three paths: the first path is used as the reference light, the second path is used as the sample light, and the third path is suspended and not connected to the output end. The sample arm includes a collimating lens b (9), a focusing lens b (10), and a sample (11). After the sample light is collimated by the collimating lens b (9), it is focused on the sample (11) by the focusing lens b (10). The sample light reflected by each reflective surface of the sample (11) returns to the 3*3 coupler (4) along the original path. The reference light entering the reference arm is reflected by the reference arm and returns to the 3*3 coupler (4). The reference light and sample light entering the 3*3 coupler (4) are split into three interference light signals by the 3*3 coupler (4). One interference light signal enters the b port of the circulator (3) and exits from the c port of the circulator (3), and is transmitted to the data acquisition unit (12) via detector a (14). The other two interference light signals are transmitted to the data acquisition unit (12) via detector b (15) and detector c (16) respectively. The data acquisition unit (12) transmits the signal to the processing unit (2) for signal demodulation. The specific steps of the white light interferometry detection method are as follows: Step 1: At the i-th optical path position, the interference light signals collected by detectors a (14), b (15), and c (16) are respectively , ; 1） 2） 3） In the formula, It is the center wavelength of the light source (1). The reference light's position is defined by D0, D1, and D2, which are the DC components. for and Inherent phase difference for and The inherent phase difference; These are the splitting ratio coefficients of port b relative to port a and port c relative to port a of the 3*3 coupler (4). The position of the equioptic path point to be solved is the position of the reflecting surface of the sample (11); The amplitude envelope of the interference signal; Step 2: Filter out the three interference signals , The DC components D0, D1, and D2 in the data are obtained. , and : 4） 5） 6） Step 3: Calculate the amplitude envelope of the interference signal 7） Calculate the location of the maximum point of the envelope line using the centroid formula: 8） In all In the middle, determine the distance The nearest reference optical path position is set to ,exist The location-collected signal is , and ,set up The distance between the points with equal optical path lengths is ; 9） 10） 11） get 12） The position of the sample reflective surface is 13） Step 4: Calculate the position of each layer in the multilayer sample according to equations 8), 12), and 13). There are a total of m reflective surfaces, and the position of each reflective surface is... Then the distance between the reflective surface of the j-th sample layer and the reflective surface of the i-th sample layer is: 14）。 2. The white light interferometry detection method based on a 3*3 fiber coupler according to claim 1, characterized in that, The reference arm includes a collimating lens a (5), a galvanometer (6), a focusing lens a (7), a wedge prism (8), and a reference mirror (13). After the reference light is collimated by the collimating lens a (5), it is reflected by the galvanometer (6) and then focused by the focusing lens a (7) and the wedge prism (8) in sequence onto the reference mirror (13). The reference light reflected by the reference mirror (13) returns to the 3*3 coupler (4) along the original path. When the galvanometer (6) is rotated at different angles, the corresponding reference light passes through the wedge prism (8) at different thicknesses, resulting in different optical paths of the reference light. While changing the optical path of the reference light, detectors a (14), b (15), and c (16) simultaneously collect signals.

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