A device and method for detecting an orthogonal light beam

Abstract

The application relates to the technical field of atomic magnetometer and discloses a detection device and method of orthogonal light beams, the detection device of the orthogonal light beams comprising a substrate; a polarization beam combination device used for combining pump light and detection light received in an atomic magnetometer, rotating the pump light or the detection light by 90 degrees, and making the pump light and the detection light exit after being combined; a detector used for collecting a light spot image of the combined pump light and detection light, the light spot image comprising a first light spot generated based on the pump light and a second light spot generated based on the detection light; and a processing unit used for acquiring the light spot image, extracting the center coordinates of the first light spot and the second light spot in the light spot image, and determining the orthogonal state of the pump light and the detection light according to the center distance between the center coordinates of the two light spots. The application can realize quantitative evaluation of the orthogonal state of the pump light and the detection light, thereby providing intuitive and accurate feedback for optical alignment, reducing assembly errors, and improving the overall performance of the atomic magnetometer.

Description

Technical Field

[0001] This invention relates to the field of atomic magnetometer technology, and specifically to a detection device and method for orthogonal beams. Background Technology

[0002] The pump-detection orthogonal configuration dual-beam spin-exchange relaxation-free (SERF) atomic magnetometer is an all-optical atomic magnetometer with advantages such as high sensitivity and easy integration. It has important application prospects in fields such as biomagnetic field measurement and inertial navigation, and has therefore attracted widespread attention.

[0003] In the working principle of this type of magnetometer, the spatial orthogonality of the pump beam and the detection beam is the core prerequisite for ensuring its excellent performance. Currently, determining whether the pump beam and the detection beam are orthogonal mainly relies on observing whether the angle between the pump beam and the detection beam is 90 degrees, and adjusting the angle between the pump beam and the detection beam accordingly. However, due to limitations in production level and assembly process, directly measuring the angle between the pump beam and the detection beam in miniaturized magnetometer systems is not accurate enough, often making it difficult to achieve strict spatial orthogonality. This leads to a reduction in the overlap area of ​​the two beams, introducing additional cross-coupling noise, reducing the effective signal amplitude and signal-to-noise ratio, and thus affecting the measurement sensitivity and long-term stability of the magnetometer. Summary of the Invention

[0004] This invention provides a detection device and method for orthogonal beams to solve the technical problem that the results of determining the spatial orthogonality of pump light and detection light in atomic magnetometers are not accurate enough.

[0005] In a first aspect, the present invention provides a detection device for orthogonal beams, applied to an atomic magnetometer, comprising: A polarization beam combiner is used to rotate the pump light and the detection light in the receiving atomic magnetometer by 90 degrees so that the pump light and the detection light are combined and emitted. The pump light is circularly polarized and the detection light is linearly polarized. The detector is used to acquire spot images of the pump light and the detection light after beam combining. The spot images include a first spot generated based on the pump light and a second spot generated based on the detection light. The processing unit, connected to the detector, is used to acquire a light spot image, extract the center coordinates of the first light spot and the center coordinates of the second light spot in the light spot image, calculate the center-to-center distance between the center coordinates of the first light spot and the center coordinates of the second light spot, and determine the orthogonal state of the pump light and the detection light based on the center-to-center distance.

[0006] The orthogonal beam detection device of the present invention rotates the pump beam or the detection beam by 90 degrees through a polarization beam combiner, projects the two beams onto the same detection plane, and acquires the beam spot image after beam combining through a detector. The processing unit extracts the center coordinates of the first beam and the center coordinates of the second beam in the beam spot image. The center-to-center distance between the center coordinates of the first beam and the second beam can be used to quantitatively evaluate the orthogonal state of the pump beam and the detection beam, thereby providing intuitive and accurate feedback for optical alignment, reducing assembly errors, and improving the overall performance of the atomic magnetometer.

[0007] Optionally, the polarization beam combining device includes a polarization beam splitter, wherein pump light is incident on the polarization beam splitter from a plane perpendicular to a first direction, and detection light is incident on the polarization beam splitter from a plane perpendicular to a second direction, wherein the first direction is perpendicular to the second direction.

[0008] In this method, taking into account the characteristics of circularly polarized and linearly polarized light, a polarizing beam splitter can be used to efficiently combine the two beams, ensuring that the detector can acquire a clear light spot image and improving the accuracy of subsequent light spot center extraction and orthogonality determination.

[0009] Optionally, the polarization beam combiner also includes a reflector, the reflective surface of which forms a preset angle with the combined beam emitted from the polarization beam splitter, so as to reflect the combined pump light and detection light along a third direction.

[0010] In this method, the output direction of the combined beam is changed by a reflector, which allows for flexible adjustment of the detector's installation position and saves space required for device layout.

[0011] Optionally, the preset angle is 45 degrees, and the third direction is perpendicular to the plane containing the first and second directions.

[0012] In this method, by setting the reflecting surface of the mirror and the polarizing beam splitter at a 45-degree angle, the output direction of the combined beam is perpendicular to the incident direction of the original pump and detection beams. This maximizes the utilization of the three-dimensional space of the magnetometer, optimizes the overall layout of the device, makes the structure compact, and improves the integration.

[0013] Optionally, the reflector is a triangular reflector.

[0014] Optionally, the reflector is a plane reflector.

[0015] In a second aspect, the present invention provides a method for detecting orthogonal beams, applied to an orthogonal beam detection apparatus as described in any of the first aspects of the present invention, comprising: Acquire the light spot image collected by the detector; Extract the center coordinates of the first spot and the center coordinates of the second spot from the spot image; Calculate the center coordinates of the first light spot and the center distance between the two light spots; The orthogonal state of the pump light and the detection light is determined based on the center-to-center distance.

[0016] The orthogonal beam detection method of the present invention acquires a beam-combined image by a detector, extracts the center coordinates of the first beam and the center coordinates of the second beam in the beam image, and uses the center-to-center distance between the center coordinates of the first beam and the second beam to achieve a quantitative assessment of the orthogonal state of the pump beam and the detection beam, thereby providing intuitive and accurate feedback for optical alignment, reducing assembly errors, and improving the overall performance of the atomic magnetometer.

[0017] Optionally, the center coordinates of the first spot and the center coordinates of the second spot in the spot image are extracted, including: Contour edge extraction is performed on the light spot image to obtain the contours of the first light spot and the second light spot; Based on the coordinates and grayscale values ​​of the pixels within the outline of the target spot, the center coordinates of the target spot are calculated using a preset formula, where the target spot is either the first spot or the second spot.

[0018] In this method, the center coordinates are calculated by combining the coordinates of pixels within the light spot outline and the grayscale values. This fully utilizes the imaging characteristics of the light spot, avoids errors caused by relying solely on edge coordinates, and improves the accuracy of center coordinate extraction. It is especially suitable for small-sized light spots and uneven grayscale distribution in miniaturized scenarios.

[0019] Optionally, the default formula is:

[0020]

[0021] In the formula, The center coordinates of the target light spot The coordinates of the pixel. For pixels The grayscale value is R, where R is the outline range of the target spot.

[0022] This method can make full use of the grayscale information of each pixel of the light spot, effectively suppress the influence of factors such as light noise and blurred light spot edges, and calculate the center coordinates with high accuracy.

[0023] Optionally, the orthogonality of the pump light and the detection light is determined based on the center-to-center distance, including: The angle between the pump light and the detection light is calculated based on the center-to-center distance and the distance from the detector to the exit surface of the polarization beam combiner. The orthogonality of the pump and detection beams is determined by the angle between them. The smaller the angle, the better the orthogonality of the pump and detection beams.

[0024] In this method, the center distance is converted into the angle between the two beams, realizing the quantitative judgment of the orthogonality state. This can accurately reflect the degree of orthogonality between the two beams, providing more specific and accurate data support for optical path calibration, and facilitating targeted adjustment of the optical path angle. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the structure of the orthogonal beam detection device according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the polarization beam combining device according to an embodiment of the present invention; Figure 3 This is a schematic flowchart of the orthogonal beam detection method according to an embodiment of the present invention; Figure 4 This is a schematic diagram illustrating the principle of calculating the angle between the pump light and the detection light after beam combining in an embodiment of the present invention. Figure 5 This is a schematic diagram of the spot distribution of the pump light and the detection light in an embodiment of the present invention; Explanation of reference numerals in the attached figures: 1. Pump light; 2. Detection light; 3. Polarization beam combiner; 31. Polarization beam splitter; 32. Mirror; 4. Detector; 5. Processing unit. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] It should be noted that the terms "installation," "connection," and "linkage" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection of two components. The terms "parallel," "perpendicular," and "equal" include the described situation and situations that are similar to the described situation, where the range of similarity is within an acceptable deviation range, which is determined by a person skilled in the art taking into account the measurement under discussion and the error associated with the measurement of a particular quantity (i.e., the limitations of the measurement system). For example, "parallel" includes absolute parallelism and approximate parallelism, where the acceptable deviation range for approximate parallelism can be, for example, within 5°; "perpendicular" includes absolute perpendicularity and approximate perpendicularity, where the acceptable deviation range for approximate perpendicularity can also be, for example, within 5°; "equal" includes absolute equality and approximate equality, where the acceptable deviation range for approximate equality can be, for example, the difference between the two equals being less than or equal to 5% of either one. For a person skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0029] According to an embodiment of the present invention, a detection device for orthogonal beams is provided, applicable to an atomic magnetometer. For example... Figure 1 As shown, the detection device for the orthogonal beam includes: The polarization beam combiner 3 is used to rotate the pump light 1 and the detection light 2 in the receiving atomic magnetometer by 90 degrees so that the pump light 1 and the detection light 2 are combined and emitted. The pump light 1 is circularly polarized light and the detection light 2 is linearly polarized light. Detector 4 is used to acquire the spot image of the pump light 1 and the detection light 2 after beam combining. The spot image includes a first spot generated based on the pump light 1 and a second spot generated based on the detection light 2. The processing unit 5 is connected to the detector 4 and is used to acquire a light spot image, extract the center coordinates of the first light spot and the center coordinates of the second light spot in the light spot image, calculate the center distance between the center coordinates of the first light spot and the center coordinates of the second light spot, and determine the orthogonal state of the pump light 1 and the detection light 2 based on the center distance.

[0030] Specifically, in an atomic magnetometer, pump light 1 is circularly polarized light and detection light 2 is linearly polarized light. In order to improve the sensitivity and signal stability of the magnetometer, it is necessary to strictly ensure that pump light 1 and detection light 2 are spatially orthogonal, that is, the incident directions of pump light 1 and detection light 2 need to be perpendicular.

[0031] Pump light 1 and detection light 2 are incident from two mutually perpendicular incident surfaces of polarization beam combiner 3. Through polarization beam combiner 3, pump light 1 or detection light 2 is rotated by 90 degrees, so that pump light 1 and detection light 2 are combined and emitted.

[0032] It should be understood that when the incident directions of pump light 1 and detection light 2 are perpendicular, pump light 1 and detection light 2 can be completely combined. When the angle between the incident directions of pump light 1 and detection light 2 is not 90 degrees, the transmission directions of pump light 1 and detection light 2 after being combined will have a certain deviation, but they will still be transmitted in roughly the same direction, so that the detector 4 can simultaneously collect the first spot and the second spot corresponding to the two beams.

[0033] Detector 4 is a photoelectric detection device that can convert light signals into electrical signals or digital images, such as CCD, CMOS or position-sensitive detector 4, to obtain light spot position information.

[0034] Processing unit 5 is an electronic device with functions such as image acquisition, data processing, and algorithm calculation, such as FPGA, MCU, DSP, or computer, used to automatically calculate the center of the light spot and determine the orthogonality of the light beam.

[0035] After acquiring the light spot image, the processing unit 5 can obtain the contour of the first light spot generated based on the pump light 1 and the contour of the second light spot generated based on the detection light 2 according to the image processing algorithm. It further extracts the center coordinates of the first light spot and the center coordinates of the second light spot. Based on the distance calculation formula between the two points, it calculates the center distance between the center coordinates of the first light spot and the center coordinates of the second light spot. When the center distance is zero, it means that the orthogonality of the pump light 1 and the detection light 2 is completely orthogonal. Otherwise, it means that the pump light 1 and the detection light 2 have not reached complete orthogonality. Generally, the larger the center distance, the worse its orthogonality is, and the incident angle of the pump light 1 or the detection light 2 needs to be further adjusted.

[0036] The orthogonal beam detection device of this invention rotates the pump beam 1 or the detection beam 2 by 90 degrees through the polarization beam combiner 3, projecting the two beams onto the same detection plane. The detector 4 collects the beam-combined spot image, and the processing unit 5 extracts the center coordinates of the first spot and the second spot in the spot image. The center-to-center distance between the center coordinates of the first spot and the second spot can be used to quantitatively evaluate the orthogonal state of the pump beam 1 and the detection beam 2, thereby providing intuitive and accurate feedback for optical alignment, reducing assembly errors, and improving the overall performance of the atomic magnetometer.

[0037] In some embodiments, such as Figure 2As shown, the polarization beam combining device 3 includes a polarization beam splitter 31. Pump light 1 is incident on the polarization beam splitter 31 from a plane perpendicular to the first direction, and detection light 2 is incident on the polarization beam splitter 31 from a plane perpendicular to the second direction. The first direction is perpendicular to the second direction.

[0038] Specifically, the first direction is the Y direction, and the second direction is the X direction.

[0039] The polarizing beam splitter 31 is a cubic structure formed by depositing a multilayer film structure on the inclined surface of a right-angle prism and then bonding them together, with the adhesive layer tilted at 45° relative to the X direction.

[0040] The detection light 2 is linearly polarized light, and its polarization angle is used as the S-ray incident light of the polarization beam splitter 31. Therefore, the polarization beam splitter 31 can reflect all of the detection light 2 to the positive Y direction.

[0041] Pump light 1 is circularly polarized light, possessing both P-components and S-components. After passing through polarizing beam splitter 31, part of the light is transmitted to the positive Y-direction, and part is reflected to the negative X-direction. The pump light 1 transmitted to the positive Y-direction and the detection light 2 reflected to the positive Y-direction are combined and emitted. At this time, detector 4 can be placed in the output optical path of the combined beam to acquire the spot image of the combined pump light 1 and detection light 2.

[0042] Furthermore, if the polarization angle of the detection light 2 is used as the P-light incident on the polarization beam splitter 31, then the polarization beam splitter 31 can transmit all of the detection light 2 to the negative X-axis direction. At this time, the pump light 1 reflected to the negative X-axis and the detection light 2 transmitted to the negative X-axis direction are combined and emitted. At this time, beam combining can also be achieved. Correspondingly, the position of the detector 4 can be adjusted according to the emission direction of the combined beam so that it can collect the light spot of the combined beam.

[0043] In this embodiment, taking into account the characteristics of circularly polarized light and linearly polarized light, the polarizing beam splitter 31 can efficiently combine the two beams, ensuring that the detector 4 can acquire a clear light spot image and improve the accuracy of subsequent light spot center extraction and orthogonality determination.

[0044] In some embodiments, the polarization beam combiner 3 further includes a reflector 32, the reflective surface of which forms a preset angle with the beam combined by the polarization beam splitter 31, so as to reflect the combined pump light 1 and the detection light 2 along a third direction.

[0045] Specifically, the reflector 32 is located on the output optical path of the combined beam emitted by the polarizing beam splitter 31. The reflecting surface of the reflector 32 forms a preset angle with the combined beam (including the detection light 2 and part of the pump light 1), thereby deflecting the transmission direction of the combined beam to a third direction. At this time, the detector 4 is correspondingly set on the output optical path of the combined beam after being reflected by the reflector 32, and its photosensitive surface is perpendicular to the third direction.

[0046] The reflector 32 and the polarizing beam splitter 31 can be an integrated structure or a separate structure.

[0047] The reflector 32 can be a triangular reflector 32 or a plane reflector 32. In a preferred embodiment, the reflector 32 is a triangular reflector 32, and it and the polarizing beam splitter 31 can be an integral structure, which can reduce the size of the device.

[0048] By changing the output direction of the combined light by the reflector 32, the installation position of the detector 4 can be flexibly adjusted, saving space required for device layout.

[0049] Furthermore, the preset angle is 45 degrees, and the third direction is perpendicular to the plane containing the first and second directions.

[0050] By setting the reflective surface of the reflector 32 and the polarizing beam splitter 31 at a 45-degree angle, the output direction of the combined beam is perpendicular to the incident direction of the original pump beam 1 and the detection beam 2. This maximizes the utilization of the three-dimensional space of the magnetometer, optimizes the overall layout of the device, makes the structure compact, and improves the integration.

[0051] This invention also provides a method for detecting orthogonal beams, applied to the orthogonal beam detection device described in the above embodiments of this invention, and executed by processing unit 5. Figure 3 As shown, the method includes: Step S301: Obtain the light spot image collected by detector 4; Step S302: Extract the center coordinates of the first spot and the center coordinates of the second spot from the spot image; Step S303: Calculate the center-to-center distance between the center coordinates of the first light spot and the center coordinates of the second light spot; Step S304: Determine the orthogonal state of pump light 1 and detection light 2 based on the center distance.

[0052] The method for detecting orthogonal beams in this embodiment of the invention acquires a beam-spot image after beam combining using detector 4, extracts the center coordinates of the first beam and the center coordinates of the second beam in the beam-spot image, and uses the center-to-center distance between the center coordinates of the first beam and the second beam to achieve a quantitative assessment of the orthogonal state of the pump beam 1 and the detection beam 2, thereby providing intuitive and accurate feedback for optical alignment, reducing assembly errors, and improving the overall performance of the atomic magnetometer.

[0053] In some embodiments, step S302, extracting the center coordinates of the first light spot and the center coordinates of the second light spot in the light spot image, includes: Step S3021: Extract the contour edges of the light spot image to obtain the contours of the first light spot and the second light spot. Step S3022: Calculate the center coordinates of the target light spot based on the coordinates and grayscale values ​​of the pixels within the contour range of the target light spot, combined with a preset formula, wherein the target light spot is either the first light spot or the second light spot.

[0054] Specifically, an image processing algorithm is used to extract the contour edges of the acquired light spot image to obtain the contours of the first light spot and the second light spot.

[0055] Using the first and second light spots as target light spots in sequence, the center coordinates of the two light spots are calculated sequentially based on the following preset formula:

[0056]

[0057] In the formula, The center coordinates of the target light spot The coordinates of the pixel. For pixels The grayscale value is R, where R is the outline range of the target spot.

[0058] By combining the coordinates of pixels within the light spot outline and their grayscale values ​​to calculate the center coordinates, the imaging characteristics of the light spot are fully utilized, avoiding errors caused by relying solely on edge coordinates, thus improving the accuracy of center coordinate extraction. This is especially suitable for small-sized light spots with uneven grayscale distribution in miniaturized scenarios.

[0059] Furthermore, the preset formula used can make full use of the grayscale information of each pixel of the light spot, effectively suppress the influence of factors such as light noise and blurred edge of the light spot, and the calculated center coordinates are highly accurate.

[0060] In some embodiments, step S304, determining the orthogonal state of pump light 1 and detection light 2 based on the center-to-center distance, includes: Step S3041: Calculate the angle between pump light 1 and detection light 2 based on the center spacing and the distance from detector 4 to the exit surface of polarization beam combiner 3. Step S3042: Determine the orthogonal state of pump light 1 and detection light 2 based on the angle between pump light 1 and detection light 2. The smaller the angle between pump light 1 and detection light 2, the better the orthogonality between pump light 1 and detection light 2.

[0061] Specifically, such as Figure 4As shown, the distance from detector 4 to the exit surface of polarization beam combiner 3 is L. Then, the angle between pump light 1 and detection light 2 is... for:

[0062] In the formula, The center-to-center distance.

[0063] When the center-to-center distance d between the two light spots is 0, the included angle It is also 0, at which point pump light 1 and detection light 2 are completely orthogonal.

[0064] In some embodiments, L=40mm, and the center-to-center distance between the two light spots is 1.2mm. In this case, the light spot morphology of the two light spots displayed by the processing unit 5 is as follows: Figure 5 As shown. If the angle θ between the two beams is less than 1.7°, it means they are not completely orthogonal. The larger the angle θ, the worse their orthogonality.

[0065] In this embodiment, the center distance is converted into the angle between the two beams to achieve a quantitative judgment of the orthogonality state. This can more accurately reflect the degree of orthogonality between the two beams, providing more specific and accurate data support for optical path calibration, and facilitating targeted adjustment of the optical path angle.

[0066] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A detection device for orthogonal beams, applied to an atomic magnetometer, characterized in that, include: A polarization beam combiner is used to rotate either the pump light or the detection light in a receiving atomic magnetometer by 90 degrees, so that the pump light and the detection light are combined and emitted, wherein the pump light is circularly polarized light and the detection light is linearly polarized light. A detector is used to acquire a spot image of the pump light and the detection light after they are combined, the spot image including a first spot generated based on the pump light and a second spot generated based on the detection light; The processing unit, connected to the detector, is used to acquire the light spot image, extract the center coordinates of the first light spot and the center coordinates of the second light spot in the light spot image, calculate the center distance between the center coordinates of the first light spot and the center coordinates of the second light spot, and determine the orthogonality of the pump light and the detection light based on the center distance.

2. The detection device for orthogonal beams according to claim 1, characterized in that, The polarization beam combining device includes a polarization beam splitter prism. The pump light is incident on the polarization beam splitter prism from a plane perpendicular to a first direction, and the detection light is incident on the polarization beam splitter prism from a plane perpendicular to a second direction, wherein the first direction is perpendicular to the second direction.

3. The detection device for orthogonal beams according to claim 2, characterized in that, The polarization beam combiner also includes a reflector, the reflective surface of which forms a preset angle with the beam combiner emitted from the polarization beam splitter, so as to reflect the combined pump light and the detection light along a third direction.

4. The detection device for orthogonal beams according to claim 3, characterized in that, The preset angle is 45 degrees, and the third direction is perpendicular to the plane containing the first and second directions.

5. The detection device for orthogonal beams according to claim 3, characterized in that, The reflector is a triangular reflector.

6. The detection device for orthogonal beams according to claim 3, characterized in that, The reflector is a plane reflector.

7. A method for detecting orthogonal beams, characterized in that, The detection device for orthogonal beams as described in any one of claims 1 to 6 comprises: Acquire the light spot image collected by the detector; Extract the center coordinates of the first spot and the center coordinates of the second spot from the spot image; Calculate the center-to-center distance between the center coordinates of the first light spot and the center coordinates of the second light spot; The orthogonality of the pump light and the detection light is determined based on the center-to-center distance.

8. The method for detecting orthogonal beams according to claim 7, characterized in that, Extracting the center coordinates of the first spot and the center coordinates of the second spot in the spot image includes: The contour edges of the light spot image are extracted to obtain the contours of the first light spot and the second light spot; The center coordinates of the target light spot are calculated based on the coordinates and gray values ​​of the pixels within the outline range of the target light spot, combined with a preset formula, wherein the target light spot is either the first light spot or the second light spot.

9. The method for detecting orthogonal beams according to claim 8, characterized in that, The preset formula is: In the formula, The center coordinates of the target light spot are: The coordinates of the pixel. For pixels The grayscale value is R, and R is the contour range of the target light spot.

10. The method for detecting orthogonal beams according to claim 7, characterized in that, Determining the orthogonality of the pump light and the detection light based on the center-to-center distance includes: The angle between the pump light and the detection light is calculated based on the center spacing and the distance from the detector to the exit surface of the polarization beam combiner. The orthogonality of the pump light and the detection light is determined based on the angle between them. The smaller the angle between the pump light and the detection light, the better their orthogonality.

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