White light interferometric imaging apparatus and method based on quadrature demodulation

Abstract

The application belongs to the technical field of optical detection, and proposes a white light interference imaging device and method based on orthogonal demodulation. A galvanometer is used to replace the current PZT or potential displacement table to perform optical path scanning, without mechanical movement, and the scanning part and the sample end are connected by optical fibers, so that additional mechanical vibration is not generated at the sample end, and the system stability is improved. The sampling adopts a scanning mode of trigger signals and voltage signals, combined with the scanning of the galvanometer, to collect the interference spectrum in two time intervals. The optical path difference of the reference light of the two trigger signals in each group is π / 2, that is, the orthogonal signal, and the values of the interference spectrum envelope are calculated by using the two trigger signals. There is a large interval between groups, which is not limited by λ0 / 4, and a larger interval can be adopted, so that the number of collected interference spectrum images can be reduced, the acquisition speed and calculation speed are improved, and the measurement efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of optical detection technology, and in particular to a white light interferometric imaging device and method based on orthogonal demodulation. Background Technology

[0002] Vertical scanning white light interferometry (VSI) is an optical technique for measuring surface topography, capable of acquiring high-precision three-dimensional topographic information of an object's surface. To obtain the topographic information of the entire surface, VSI acquires a series of interference patterns by scanning the optical path in either the sample arm or reference arm. By analyzing and processing these interference patterns, the equipathic points of the interference signals are determined, thereby identifying the heights of different reflecting surfaces. VSI has wide applications in many fields, including materials science, manufacturing, optical components, and microelectronics.

[0003] The current VSI technology has two drawbacks: (1) The scanning step size of the optical path is limited. Assuming the center wavelength of the light source is λ0, according to the sampling law, the maximum optical path scanning step size is λ0 / 4 (considering the round trip between the reference light and the sample light). When the scanning step size exceeds λ0 / 4, the interference signal is distorted and depth demodulation cannot be performed. Therefore, the maximum scanning step size is generally on the order of hundreds of nanometers. When imaging at a large depth range, a large number of optical path scans are required, resulting in slow scanning speed, large amount of interference image data, and low measurement efficiency; (2) Currently, most VSI uses PZT or precision translation stage to drive the reference mirror or interference objective lens for optical path scanning. This optical path scanning method is prone to vibration of the interference system due to mechanical movement, which reduces system stability and affects measurement accuracy.

[0004] For example, in the invention patent "A rapid measurement method for microscopic morphology applicable to white light scanning interferometry (application number 202110223278.5)," piezoelectric ceramics are required to drive the interferometer microscope objective at equal intervals within a specified total stroke for vertical scanning. This involves mechanical movement, affecting system stability. The scanning step size of the piezoelectric ceramics is set to λ / 8, where λ is the center wavelength of white light. Within a 2.13 μm range, 32 interferometric images need to be acquired for calculation. The scanning interval is small, and when used for measurements over a larger height range, the number of interferometric images required is large, limiting the imaging speed. In the invention patent "A 3D morphology restoration method combining phase-shifting interferometry and vertical scanning interferometry" (application number 202110223278.5),... The same problem exists in application number 201611015451.8. Controlling the vertical scanning of piezoelectric ceramics via a host computer program with micro-steps affects system stability. The use of a five-frame phase-shift algorithm to calculate the phase requires acquiring five interferometric images within a single wavelength range, limiting the imaging speed. The patent "Narrowband Non-Monochromatic Light n+1 Amplitude Phase-Shift Test Algorithm for Smooth Surface Topography Measurement" (application number 201911302926.5) uses a phase shifter for a z-axis temporal vertical scan with a phase interval of π / 2 and a scanning step size of λ0 / 8, where λ0 is the center wavelength of the narrowband non-monochromatic light. It requires acquiring eight interferometric images within a single wavelength range, further limiting the imaging speed.

[0005] Therefore, the existing technology has the following problems:

[0006] 1. Currently, the optical path scanning step size of VSI is in the hundreds or tens 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.

[0007] 2. Optical path scanning is performed by driving the reference mirror or system through a PZT or precision translation stage. This optical path scanning method involves mechanical movement, which can easily cause vibration in the interference system, reduce system stability, and affect measurement accuracy. Summary of the Invention

[0008] To address the shortcomings of existing technologies, a white light interferometric imaging device and method based on orthogonal demodulation is designed.

[0009] The technical solution of the present invention is as follows: A white light interferometric imaging device based on orthogonal demodulation includes a white light source 1, a camera 2, a coupler 3, a circulator 4, a collimating lens, a galvanometer 6, a wedge glass 8, a reference mirror 9, a beam splitter 12, a dispersive lens 13, and a lens; the light source 1 is connected to one end of the coupler 3 via an optical fiber; the other end of the coupler 3 is divided into two branches; the first branch leads to the first port 17 of the circulator 4, and the collimating lens a5, galvanometer 6, lens a7, wedge glass 8, and reference mirror 9 are arranged sequentially outside the second port 15 of the circulator 4; the collimating lens c11 is arranged between the third port 18 of the circulator 4 and one side of the beam splitter 12; the second branch leads to one side of the collimating lens b10, and the other side of the collimating lens b10 is located on one side of the beam splitter 12; the other side of the beam splitter 12 is divided into two directions, one direction in which the dispersive lens 13 and sample 14 are arranged sequentially; the other direction in which the lens b16 and camera 2 are arranged sequentially; the camera 2 is connected to a computer.

[0010] Light emitted from light source 1 enters coupler 3 via optical fiber, where coupler 3 splits the light into sample light and reference light. The reference light enters from port 17 of circulator 4 and exits from port 15 of circulator 4. After being collimated by collimating lens a5, the reference light is reflected by galvanometer 6 and then passes through lens a7 and wedge glass 8 to be focused onto reference mirror 9. The reference light reflected by reference mirror 9 returns along the original path and exits through port 15 and port 18 of circulator 4. After being collimated by collimating lens c11, it is transmitted through beam splitter prism 12. The sample light is collimated by collimating lens b10 and then transmitted through beam splitter prism 12. It is focused onto sample 14 by dispersive lens 13. The sample light reflected by sample 14 returns along the original path to beam splitter prism 12. The sample light reflected by beam splitter prism 12 and the reference light transmitted by beam splitter prism 12 are imaged onto camera 2 by lens b16. Camera 2 acquires the interference image and sends it to computer for processing.

[0011] A white light interferometric imaging method based on orthogonal demodulation is implemented using the aforementioned white light interferometric imaging device based on orthogonal demodulation. The control voltage of the galvanometer 6 is changed, thereby altering the angle of the galvanometer 6 and causing the reference light to pass through different positions on the wedge-shaped glass 8, thus changing the optical path length of the reference light. Simultaneously, the computer transmits the galvanometer control voltage to the galvanometer 6 and a trigger signal to the camera 2. Each trigger signal triggers the camera 2 to acquire one frame of interferometric image, meaning the galvanometer control voltage signal and the trigger signal are synchronized. Two adjacent trigger signals form a group, and the galvanometer control voltage signal corresponding to this group is different, resulting in the galvanometer 6 being at two different angles. This causes the optical path difference of the reference light at these two angles to be π / 2, and the optical path interval of the reference light between two adjacent groups of trigger signals is equal.

[0012] While changing the optical path of the reference beam, interference images I1(x,y;n) and I2(x,y;n) are acquired, where (x,y) represents the position of the sample, n = 1…N, and n represents the nth set of images acquired; I1 and I2 represent the first and second frame interference images of each set; the height of the sample at each point is calculated based on the N sets of acquired interference images.

[0013] 1) Calculate the background signal.

[0014]

[0015] 2) Calculate the envelope of the interference spectrum.

[0016]

[0017] 3) Calculate the x-coordinate h(x,y) of the maximum point of the envelope S(x,y;n).

[0018]

[0019] h(x,y) represents the position of the point with equal optical path in the interference spectrum. Calculate the relative height H(x,y) of point (x,y).

[0020]

[0021] 'a' represents the optical path interval of the reference light corresponding to the two sets of trigger signals.

[0022] The galvanometer 6 is connected to a computer. Changing the control voltage of the galvanometer 6 changes its angle.

[0023] Compared with existing white light interferometry techniques, the present invention has the following advantages:

[0024] 1. This invention uses a galvanometer instead of 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.

[0025] 2. This invention employs a scanning method combining trigger signals and voltage signals, along with galvanometer scanning. Interference spectra are acquired at two time intervals. The optical path difference between the reference beams of the two trigger signals within each group is π / 2, which constitutes orthogonal signals. The values ​​of the interference spectrum envelope are calculated using these two trigger signals. Furthermore, there can be a larger interval between groups, not limited by λ0 / 4, thus reducing the number of interference spectrum images acquired, increasing acquisition and calculation speed, and improving measurement efficiency. Attached Figure Description

[0026] Figure 1This is a schematic diagram of a white light interferometric imaging device based on orthogonal demodulation;

[0027] Figure 2 This is a schematic diagram of the control voltage and trigger signal.

[0028] In the diagram: 1-Light source, 2-Camera, 3-Coupler, 4-Circulator, 5-Collimating lens a, 6-Galvanometer, 7-Lens a, 8-Wedge glass, 9-Reference mirror, 10-Collimating lens b, 11-Collimating lens c, 12-Beam splitter prism, 13-Dispersion lens, 14-Sample, 15-Second port, 16-Lens b, 17-First port, 18-Third port. Detailed Implementation

[0029] White light interferometric imaging devices based on orthogonal demodulation, such as Figure 1 As shown, light source 1 is a white light source. Light emitted from light source 1 enters coupler 3 via optical fiber. Coupler 3 splits the light into sample light and reference light. The reference light enters from the first port 17 of circulator 4 and exits from the second port 15. The reference light beam is collimated by collimating lens a5, reflected by galvanometer 6, and finally focused by lens a7 and wedge glass 8 onto reference mirror 9. The reference light is reflected by reference mirror 9 and returns along the original path to the second port 15 of circulator 4, exits from the third port 18 of circulator 4, collimated by collimating lens c11, and transmitted through beam splitter prism 12. The sample light is collimated by collimating lens b10, and then focused by beam splitter prism 12 and dispersive lens 13 onto sample 14. The light reflected by sample 14 returns along the original path to beam splitter prism 12. The sample light reflected by beam splitter prism 12 and the reference light transmitted by beam splitter prism 12 are imaged onto camera 2 by lens b16. Camera 2 acquires the interference image and transmits it to computer for processing.

[0030] The galvanometer 6 changes the position of the reference light as it passes through the wedge-shaped glass 8, thereby achieving optical path scanning of the reference light. The lens b16 can focus sample light of different wavelengths at different height positions, achieving a greater depth of field.

[0031] By changing the control voltage of galvanometer 6, the angle of galvanometer 6 is changed, allowing the reference light to pass through different positions on wedge glass 8, thus achieving optical path scanning of the reference light. Simultaneously, camera 2 acquires interference images while the voltage of galvanometer 6 is changed. Figure 2 The '+' symbol represents the control voltage signal from the computer to the galvanometer 6, causing the reference light to move along the surface of the wedge-shaped glass 8, thus changing the optical path of the reference light sequentially. The dotted line represents the trigger signal from the computer to the camera 2. The trigger signal and the galvanometer control voltage signal are synchronized. Each trigger signal triggers the camera to acquire one frame of interference image. Two adjacent trigger signals form a group, and the optical path difference between the reference lights corresponding to the two trigger signals in a group is π / 2, which means they are orthogonal signals. The optical path interval of the reference light between two adjacent groups of trigger signals is equal.

[0032] Assume that N sets of interference images are acquired. Each set of interference images includes two frames, I1(x,y;n) and I2(x,y;n), where (x,y) represents the lateral coordinates on the sample, n represents the nth set of images acquired, and the subscripts 1 and 2 represent the first and second frames of each set of interference images.

[0033] The process of calculating the height of a point (x, y) is as follows.

[0034] (1) Calculate the background signal.

[0035]

[0036] (2) Calculate the envelope of the interference spectrum

[0037]

[0038] (3) The centroid sampling method is used to calculate the x-coordinate g(x,y) of the maximum point of the envelope S(x,y;n), for example...

[0039]

[0040] h(x,y) represents the position of the point with equal optical path in the interference spectrum. Calculate the relative height H(x,y) of point (x,y).

[0041]

[0042] 'a' represents the optical path interval corresponding to the distance between the two sets of trigger signals.

Claims

1. A white-light interferometric imaging method based on orthogonal demodulation, characterized in that, This is achieved using a white light interferometric imaging device based on orthogonal demodulation. The white light interferometric imaging device based on orthogonal demodulation includes a white light source (1), a camera (2), a coupler (3), a circulator (4), a collimating lens, a galvanometer (6), a wedge-shaped glass (8), a reference mirror (9), a beam splitter (12), a dispersive lens (13), and a lens. The light source (1) is connected to one end of the coupler (3) via an optical fiber. The other end of the coupler (3) is divided into two branches. The first branch leads to the first port (17) of the circulator (4), and the second port (15) of the circulator (4) is arranged sequentially outside the first port (15). Collimating lens a (5), galvanometer (6), lens a (7), wedge glass (8), reference mirror (9); collimating lens c (11) is arranged between the third port (18) of the circulator (4) and one side of the beam splitter (12); the second branch is to one side of collimating lens b (10), and the other side of collimating lens b (10) is located on one side of the beam splitter (12); the other side of the beam splitter (12) is divided into two directions, one direction is arranged with dispersive lens (13) and sample (14) in sequence; the other direction is arranged with lens b (16) and camera (2) in sequence; camera (2) is connected to computer; The white light interferometric imaging method described above changes the control voltage of the galvanometer (6), thereby changing the angle of the galvanometer (6) and causing the reference light to pass through different positions of the wedge glass (8), thus changing the optical path of the reference light. Simultaneously, while the computer transmits the galvanometer control voltage to the galvanometer (6), the computer also transmits a trigger signal to the camera (2). Each trigger signal triggers the camera (2) to acquire one frame of interference image. The galvanometer control voltage signal is synchronized with the trigger signal. Two adjacent trigger signals form a group. The two galvanometer control voltage signals synchronized with this group are different, causing the galvanometer (6) to be at two different angles, resulting in an optical path difference of the reference light at the two angles. The reference optical path interval between two adjacent sets of trigger signals is equal. While changing the optical path of the reference beam, an interference image is acquired. and , Indicates the location of the sample. , where n represents the nth set of images acquired; and This represents the first and second interferometric images of each group; the height of the sample at each point is calculated based on the N groups of interferometric images collected. 1) Calculate the background signal. (1) 2) Calculate the envelope of the interference spectrum. (2) 3) Calculate the envelope x-coordinate of the maximum point , (3) Indicate the position of the isopathic point in the interference spectrum, and calculate relative height of the point , (4) This indicates the optical path interval of the reference light corresponding to the two sets of trigger signals.

2. The white light interferometric imaging method based on orthogonal demodulation according to claim 1, characterized in that, The light emitted from the light source (1) enters the coupler (3) through the optical fiber. The coupler (3) splits the light into sample light and reference light. The reference light enters from the first port (17) of the circulator (4) and exits from the second port (15) of the circulator (4). After being collimated by the collimating lens a (5), the reference light is reflected by the galvanometer (6) and then passes through the lens a (7) and the wedge glass (8) to be focused onto the reference mirror (9). The reference light reflected by the reference mirror (9) returns along the original path and passes through the second port (15) and the third port of the circulator (4) in sequence. The light emitted from the aperture (18) is collimated by the collimating lens c (11) and then transmitted through the beam splitter (12). The sample light is collimated by the collimating lens b (10) and then transmitted through the beam splitter (12). It is then focused onto the sample (14) by the dispersive lens (13). The sample light reflected by the sample (14) returns to the beam splitter (12) along the original path. The sample light reflected by the beam splitter (12) and the reference light transmitted by the beam splitter (12) are imaged onto the camera (2) by the lens b (16). The camera (2) acquires the interference image and sends it to the computer for processing.

Images