Online detection system and method for collimation wavefront quality of Mach-Zehnder interferometer

Abstract

The invention discloses an online detection system and method for collimation wavefront quality of a Mach-Zehnder interferometer. The system comprises a forward reflector and a turn-back reflector which are movably arranged on the Mach-Zehnder interferometer; the forward reflector and the turn-back reflector are plane mirrors; the method comprises the following steps: acquiring near-end and far-end interferograms, and measuring related parameters; judging and detecting unqualified optical elements according to the corresponding parameters and replacing the unqualified optical elements; re-measuring related parameters, determining whether the corresponding parameters in the two interferograms exceed threshold values or not, if the corresponding parameters do not exceed the threshold values, determining that the two interferograms are qualified, otherwise, determining that the two interferograms are unqualified; the device is simple in structure and low in cost, and can accurately and quantitatively detect the collimation wavefront quality of visible light or invisible light by directly utilizing an existing light path and interferogram acquisition and analysis system.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical measurement, and relates to collimated wavefront quality detection, in particular to a Mach-Zehnder interferometer collimated wavefront quality online detection system and method. BACKGROUND

[0002] The Mach-Zehnder interferometer (MZI) is an important optical instrument widely used in the fields of optical measurement, laser interference, precision sensing, etc., which detects the optical path difference or wavefront change through beam splitting, beam combining and interference. However, the traditional MZI is mainly used for measuring the optical path change or phase modulation, and has limited ability to detect the quality of its own collimated wavefront. In theory, the interference optical path of the MZI belongs to equal-optical-path double-beam interference, and the collimation characteristics of the light source do not affect the measurement accuracy. However, in actual application, due to the misalignment of the two beams, the unequal optical path caused by the measured object in the optical path, and the possible beam flipping, the collimation of the collimated wavefront (i.e. the parallelism of the wavefront) is not low to ensure the measurement accuracy. If the collimated wavefront has aberration, it will cause the distortion of the interference fringes, affecting the accuracy of the measurement results.

[0003] The existing collimated wavefront detection technology generally faces the problems of poor compatibility, high cost or inability to quantitatively analyze. For example, the shearing interferometry method needs to rely on complex optical hardware and is limited by the imaging aperture, and the five-prism scanning method needs to be equipped with a high-precision guide rail and a parallel light tube system, which significantly increases the equipment cost and the complexity of adjustment. Although the Shack-Hartmann wavefront sensor can quantitatively detect, it is expensive and needs an additional beam-reducing system to match the aperture of the interferometer, resulting in redundant optical paths and error accumulation. In addition, although the corner cube prism method simplifies the structure, it cannot return to the MZI imaging device only by reflecting the light path with the corner cube prism, so it cannot be directly used, and this method relies on subjective interpretation of the change of the interference fringes, which cannot quantitatively evaluate the wavefront quality and is difficult to expand to non-visible light waveband applications such as infrared light at low cost. The above-mentioned methods all need to introduce external equipment or independent detection systems, which not only destroys the original integrity of the MZI optical path, but also greatly increases the system complexity and implementation cost, making it difficult to realize online detection of the MZI.

[0004] In summary, there is no system or method in the prior art that can simultaneously meet the requirements of simple structure, low cost, direct application in the MZI, and quantitative and accurate detection of the collimated wavefront quality of visible light and non-visible light using the existing optical path. SUMMARY

[0005] The purpose of the present application is to provide a Mach-Zehnder interferometer collimated wavefront quality online detection system and method which has a simple structure, low cost and can directly and quantitatively detect the collimated wavefront quality of visible light and non-visible light using the existing optical path.

[0006] Technical solution: The Mach-Zehnder interferometer collimation wave front quality online detection system provided by the application comprises a removable forward mirror arranged in the transmission or reflection light path of a first beam splitter of a Mach-Zehnder interferometer, and a movable turnaround mirror arranged on the light path away from the light source after the light turned back by the forward mirror is split by the first beam splitter, and the forward mirror and the turnaround mirror are both plane mirrors and are perpendicular to the light path.

[0007] Based on the above system, by introducing the forward mirror and the turnaround mirror into the Mach-Zehnder interferometer, that is, the MZI, any one of the two original split beams split by the first beam splitter of the MZI is reflected back to the first beam splitter twice and interferes with the other original split beam along the MZI light path to the second beam splitter, and finally enters the collection system of the MZI and is received, and the phase shift interference sampling can be realized by moving the turnaround mirror along the light path, and whether the collimation wave front quality of the MZI is qualified can be determined by moving the turnaround mirror to generate interference fringes at different positions, the method does not need to damage the original structure of the MZI, only the movable forward mirror and the turnaround mirror are added, and the collimation wave front quality is detected by using the image acquisition and analysis software of the interferometer itself, without the need of additional detectors, low cost, simple structure, and fast detection can be realized.

[0008] The Mach-Zehnder interferometer collimation wave front quality online detection method provided by the application comprises the following steps:

[0009] S1, the turnaround mirror is moved to the set proximal position and distal position along the light path to obtain a proximal interference graph and a distal interference graph, and the peak-to-valley value, defocus value, astigmatism value, coma and spherical aberration in the two interference graphs are measured;

[0010] S2, whether the coma and spherical aberration in the two interference graphs are less than or equal to a threshold A is judged, if yes, step S3 is entered, otherwise, whether the optical elements in the beam collimation system meet the design requirements is rechecked, if not, debugging or replacement is performed until the design requirements are met and step S1 is returned;

[0011] S3, whether the peak-to-valley value in the proximal interference graph is less than or equal to a threshold B is judged, if yes, step S4 is entered, otherwise, whether the flatness of the first beam splitter, the forward mirror and the turnaround mirror meets the design requirements is rechecked, if not, debugging or replacement is performed, and after all meet the design requirements, step S1 is performed again to measure all parameters and step S3 is repeated;

[0012] S4, judging whether the difference between the defocus amounts and the difference between the amounts of astigmatism in the two interference patterns exceed a threshold C, if neither exceeds, determining that the collimated wavefront quality is qualified, otherwise, the position of the point light source in the collimating system on the optical axis of the collimating objective or on the focal plane needs to be adjusted to reduce the difference between the defocus amounts and the difference between the amounts of astigmatism respectively until the difference between the defocus amounts and the difference between the amounts of astigmatism are both less than or equal to the threshold C.

[0013] By step S2, it can be judged whether the quality of the optical element in the beam collimating system is qualified, if not, it is replaced until its quality is qualified, thus avoiding the influence of the unqualified optical element on the final detection result and improving the accuracy of the final detection; step S3 can judge whether the flatness quality of the first beam splitter, the forward mirror and the turnaround mirror matches the collimated wavefront precision requirement, if not, it also needs to be replaced until it meets the requirement, thus also avoiding the influence of the unqualified optical element on the final detection result; steps S2 and S3 improve the accuracy of the final detection; and step S4 quantitatively judges whether the optical axis adjustment of the collimated wavefront is in place through specific numerical values (only when the adjustment is in place can the quality of the collimated wavefront be determined to be qualified), thus realizing quantitative detection, which is more accurate than the artificial observation of the change of the fringe to judge whether the quality of the collimated wavefront is qualified. Since the detection is based on the related measurement data of the interference pattern instead of direct human eye observation, in addition to visible light, the quality of the collimated wavefront of non-visible light can also be detected without the need to increase additional and expensive non-visible light detectors. The entire detection method only relies on the simple and low-cost system described in the present application, thus the implementation of the present method is also low in cost and simple in operation and high in accuracy.

[0014] Advantages: Compared with the prior art, the significant effects of the present application are that the online detection system only includes the movable forward mirror and the turnaround mirror, and can be directly applied in the MZI to perform detection using the existing optical path therein, and even the phase shifter of the turnaround mirror can directly borrow the phase shifter of the MZI, only introducing a small amount of additional optical path and corresponding elements, which is low in cost and simple in structure; the online detection method measures the related data in the interference patterns at the near-end position and the far-end position to ensure that all optical elements are qualified, thus avoiding their influence on the detection result and improving the accuracy of the final detection result, and the final detection also relies on whether the measured data exceeds the corresponding threshold to judge whether the quality is qualified, thus realizing quantitative and accurate detection; at the same time, according to the detection data instead of direct observation, the quality of the collimated wavefront of non-visible light can also be detected in addition to visible light. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a schematic diagram of the overall structure of the existing MZI;

[0016] Figure 2The whole structure schematic diagram of the system when the front reflection mirror is located in the turning light path. DETAILED DESCRIPTION

[0017] The Mach-Zehnder interferometer collimating wave front quality online detection system comprises a front reflection mirror 2.1 which is arranged in the transmission or reflection light path of a first beam splitter 1.3 of the Mach-Zehnder interferometer and is perpendicular to the light path, forms a turning-back arm of the turning-back type Michelson interference, and is removable; and a turning-back reflection mirror 2.2 which is arranged on the light path away from the light source after the light turned back by the front reflection mirror 2.1 is split by the first beam splitter 1.3, is perpendicular to the light path, and is movable along the light path; the front reflection mirror 2.1 and the turning-back reflection mirror 2.2 are both plane mirrors, and the surfaces of the two mirrors are both coated with a broadband reflection film.

[0018] The Mach-Zehnder interferometer further comprises a light source 1.1, a first fixed reflection mirror 1.4, a second fixed reflection mirror 1.5, a second beam splitter 1.6, and an imaging and collecting system 1.7, and the first fixed reflection mirror 1.4 is further provided with an MZI phase shifter 1.9; an independent test phase shifter 2.3 can also be arranged on the turning-back reflection mirror 2.2 in the system, the imaging and collecting system 1.7, the MZI phase shifter 1.9, and the test phase shifter 2.3 are all connected with a control and analysis system 1.8; a collimating system is arranged between the light source 1.1 and the first beam splitter 1.3, and a beam expanding system can also be arranged between the collimating system and the first beam splitter 1.3, and the beam expanding system can be selected according to requirements.

[0019] The working process of the original Mach-Zehnder interferometer is as follows: the laser generated by the light source 1.1 is collimated and expanded by the collimating system and the beam expanding system, and then is shot to the first beam splitter 1.3 to generate two original split beams of turning and transmission, the two original split beams are shot to the first fixed reflection mirror 1.4 and the second fixed reflection mirror 1.5 respectively, and then are turned back to the second beam splitter 1.6 to interfere and are collected by the imaging and collecting system 1.7, and the imaging and collecting system 1.7 can adopt a CCD camera; the online detection system adopts the imaging and collecting system 1.7 of the MZI to collect the interference pattern, and does not need additional light path and equipment.

[0020] The system adds the front reflection mirror 2.1 in the turning or transmission light path of the first beam splitter 1.3, and arranges the turning-back reflection mirror 2.2 on the light path of the reflected light of the front reflection mirror 2.1, and the front reflection mirror 2.1 and the turning-back reflection mirror 2.2 are located on the two sides of the first beam splitter 1.3, and the specific conditions of the front reflection mirror 2.1 arranged at different positions are described as follows:

[0021] I. The forward mirror 2.1 is arranged in the folded light path, and the original split light of the folded light path is reflected by the forward mirror 2.1 back to the first beam splitter 1.3 to generate two beams of secondary split light, wherein the folded secondary split light is shot to the laser 1, and the transmitted secondary split light is shot upward, and the turnaround mirror 2.2 is arranged on the light path, and the folded secondary split light is reflected by the turnaround mirror 2.2 to generate two beams of tertiary split light, wherein the transmitted tertiary split light is shot to the fixed mirror 7 at the upper right, and the folded tertiary split light is reflected by the fixed mirror 7 to the second beam splitter 1.6 to interfere with the folded original split light, and the imaging acquisition system 1.7 acquires the interference image to determine whether the collimation wavefront quality is qualified.

[0022] II. The forward mirror 2.1 is arranged in the transmitted light path, and the original split light of the transmitted light path is reflected by the forward mirror 2.1 back to the first beam splitter 1.3 to generate two beams of secondary split light, wherein the transmitted secondary split light is shot to the laser 1, and the folded secondary split light is shot upward, and the turnaround mirror 2.2 is arranged on the light path, and the folded secondary split light is reflected by the turnaround mirror 2.2 to generate two beams of tertiary split light, wherein the transmitted tertiary split light is shot to the fixed mirror 7 at the upper right, and the folded tertiary split light is reflected by the fixed mirror 7 to the second beam splitter 1.6 to interfere with the folded original split light, and the imaging acquisition system 1.7 acquires the interference image to determine whether the collimation wavefront quality is qualified.

[0023] The online detection system described in the application, the forward mirror 2.1 is pluggable, realizes the quick switching of the MZI (Mach-Zehnder interferometer) and the composite interference mode, does not affect the original light path structure of the MZI, and can be flexibly selected to insert the forward mirror 2.1 in the transmission arm or the reflection arm of the first beam splitter 1.3 according to the arrangement characteristics of the MZI light path.

[0024] Before describing the Mach-Zehnder interferometer collimation wavefront quality online detection method described in the application, the determination basis of each threshold used in the method steps is described first:

[0025] Firstly, the target accuracy w0 of the MZI collimation wavefront quality is determined, and the value range of w0 is determined as 0.1λ, 0.5λ, λ is the wavelength of incident light, and the optical design requirement is 0.25λ; then each threshold is determined according to w0, and the specific is:

[0026] According to the analysis of the aberration source of the MZI collimation wavefront error, the coma (coma) and SA (spherical aberration) are mainly attributed to the collimation objective lens, and the error control threshold A should be in the range of , and the typical value is: threshold A = w0 / 5;

[0027] According to the analysis of the aberration source in the collimated wavefront error of the MZI, the irregular error is mainly attributed to the plane elements (the first beam splitter 1.3, the forward mirror 2.1 and the turnaround mirror 2.2). According to the error synthesis theory, if the flatness errors of the three plane optical elements involved are independent of each other, the control threshold of the irregular error is where PV0 is the flatness of the first beam splitter, PV1 is the flatness of the forward mirror, and PV2 is the flatness of the turnaround mirror.

[0028] According to the analysis of the aberration source in the collimated wavefront error of the MZI, the astigmatism (AST) and the defocus (Power) are attributed to the optical axis and the defocus adjustment error in the collimation optical path, and the error control threshold C should be in the range of , and the typical value is: threshold C = w0 / 5.

[0029] After determining the respective thresholds, the steps of the method are described in detail as follows:

[0030] S1, moving the turnaround mirror 2.2 along the optical path to a set proximal position and a set distal position to obtain a proximal interference pattern and a distal interference pattern, and measuring the peak-to-valley value, defocus, astigmatism, coma and spherical aberration in the two interference patterns;

[0031] The proximal position is located outside the Mach-Zehnder interferometer shell and closest to the first beam splitter 1.3, and the distal position is spaced apart from the proximal position by more than 1 meter.

[0032] S2, judging whether the coma and spherical aberration in the two interference patterns are less than or equal to the threshold A, if yes, entering step S3, otherwise, rechecking whether the optical elements in the beam collimation system meet the design requirements, if not, adjusting or replacing, and returning to step S1 after meeting the design requirements; here, the out-of-tolerance refers to not meeting the design requirements, and the same meaning is contained in the following, which will not be repeated.

[0033] S3, judging whether the peak-to-valley value in the proximal interference pattern is less than or equal to the threshold B, if yes, entering step S4, otherwise, rechecking whether the flatness of the first beam splitter 1.3, the forward mirror 2.1 and the turnaround mirror 2.2 meets the design requirements, if not, adjusting or replacing, and repeating step S3 after all meet the design requirements.

[0034] S4, judging whether the difference between the defocus and the astigmatism in the two interference patterns exceeds the threshold C, if not, recognizing that the collimated wavefront quality is qualified, otherwise, adjusting the position of the point light source in the collimation system on the optical axis of the collimation objective or the focal plane to respectively reduce the difference between the defocus and the astigmatism, until the difference between the defocus and the astigmatism is less than or equal to the threshold C.

[0035] In order to better illustrate the method, a specific example is further described as follows:

[0036] Take a mid-wave infrared laser crystal tester as an example, the wavelength is 3390nm, which is non-visible light. The main light path adopts a Mach-Zehnder interferometer structure, and the target accuracy requirement of the collimated wave front is w0=λ / 4. As shown in the figure, Figure 1 The fiber light source 1.1 is used as a point light source and collimated with the collimating lens 1.2 to form a collimated light path, two beams of light separated by the first beam splitter 1.3 pass through the first fixed mirror 1.4 and the second fixed mirror 1.5 respectively to combine at the second light splitter 1.6, enter the imaging acquisition system 1.7, and the control and analysis system 1.8 drives the MZI phase shifter 1.9 to perform phase shifting and collect multiple phase shift interference images, and the measurement result is obtained by analysis and calculation.

[0037] The MZI collimated wave front quality online detection system provided by the application, as shown in the figure, Figure 2 The forward mirror 2.1 is vertically arranged in the light path of the first beam splitter 1.3, the return mirror 2.2 is arranged in the outer light path, and the test phase shifter 2.3 drives the return mirror 2.2 to perform phase shifting. The forward mirror 2.1 and the return mirror 2.2 are both plane mirrors. When the forward mirror 2.1 is inserted into the light path, the reflected light of the first beam splitter 1.3 returns to the original path and enters the outer light path, and after being reflected by the return mirror 2.2 and the first light splitter 1.3 twice, the collimated light directly transmitted by the first beam splitter 1.3 is reflected by the two fixed mirrors and then combined at the second light splitter 1.6 to interfere and enter the MZI imaging acquisition system.

[0038] The threshold values determined by the application are that the threshold value A control interval of the collimating lens error COMA and SA is w0 / 5=0.05λ; the accuracy requirement of the plane optical element is 0.05λ, so the threshold value B control requirement is B≤ The adjustment error threshold value C control control interval of the collimating system is

[0039] The sampling required for debugging the collimated wave front is completely adopted by the mid-wave infrared CCD of the original MZI. After debugging is completed, the forward mirror 2.1 exits the MZI light path, the connection between the control and analysis system 1.8 and the test phase shifter 2.3 is cut off, the connection with the MZI phase shifter 1.9 is restored, and the return mirror 2.2 is removed, so that the Mach-Zehnder interference light path is restored.

Claims

1. A system for online detection of collimated wavefront quality of a Mach-Zehnder interferometer, characterized in that: The removable forward mirror (2.1) is arranged in the transmission or reflection path of the first beam splitter (1.3) of the Mach-Zehnder interferometer, and the removable turnaround mirror (2.2) is arranged on the light path away from the light source after the light turned back by the forward mirror (2.1) is split by the first beam splitter (1.3), both the forward mirror (2.1) and the turnaround mirror (2.2) are plane mirrors and are perpendicular to the light path.

2. The system of claim 1, wherein: The turnaround mirror (2.2) is mounted on a test phase shifter (2.3) which can be controlled by a measurement control system (1.8) of the MZI to realize phase shift sampling.

3. The system of claim 1, wherein: The surfaces of the forward mirror (2.1) and the turnaround mirror (2.2) are coated with a broadband reflective film.

4. A method for on-line testing of collimated wavefront quality using the Mach-Zehnder interferometer system of claim 1, characterized in that, The method comprises the following steps: S1, moving the turnaround mirror (2.2) to a set proximal position and a set distal position along the light path to obtain a proximal interferogram and a distal interferogram, and measuring the peak-to-valley value, defocus value, astigmatism value, coma and spherical aberration in the two interferograms; S2, judging whether the coma and spherical aberration in the two interferograms are less than or equal to a threshold A, if yes, entering step S3, otherwise, rechecking whether the optical elements in the beam collimation system meet the design requirements, if not, adjusting or replacing, and returning to step S1 after meeting the design requirements; S3, judging whether the peak-to-valley value in the proximal interferogram is less than or equal to a threshold B, if yes, entering step S4, otherwise, rechecking whether the flatness of the first beam splitter (1.3), the forward mirror (2.1) and the turnaround mirror (2.2) meets the design requirements, if not, adjusting or replacing, and repeating step S3 after all parameters are measured again according to step S1 and meeting the design requirements; S4, judging whether the difference between the defocus values and the difference between the astigmatism values in the two interferograms exceed a threshold C, if not, determining that the collimated wavefront quality is qualified, otherwise, adjusting the position of the point light source on the optical axis of the collimator objective or on the focal plane to respectively reduce the difference between the defocus values and the difference between the astigmatism values, until the difference between the defocus values and the difference between the astigmatism values are less than or equal to the threshold C.

5. The method of claim 3, wherein: The proximal position in step S1 is located outside the Mach-Zehnder interferometer housing and closest to the first beam splitter (1.3), and the distal position is spaced apart from the proximal position by more than 1 meter.

6. The method of claim 3, wherein: The collimated wavefront has a target accuracy w0, and the value range of w0 is [0.1λ, 0.5λ], where λ is the wavelength of the incident light.

7. The method of claim 5, wherein: The threshold value A and the threshold value C each have a value range of 8. The method of claim 3, wherein: The calculation formula of the threshold B in step S3 is wherein PV0 is the flatness of the first beam splitter, PV1 is the flatness of the forward mirror, and PV2 is the flatness of the turnaround mirror.

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