Method for measuring tilt of diffraction grating and method for adjusting the same

Abstract

A diffraction grating (102) installed in front of an imaging unit (103) is irradiated with light (106) by a light projection unit (104), the irradiated light is converted by the diffraction grating (102) into diffracted light, the imaging unit (103) receives the diffracted light as image data, and an arithmetic unit (105) calculates a numerical index corresponding to the tilt amount of the diffraction grating from the image data acquired from the imaging unit (103) and adjusts the tilt of the diffraction grating with respect to the imaging unit so that the numerical index becomes a predetermined value.

Description

Method for measuring and adjusting tilt of diffraction grating

[0001] The present disclosure relates to a method for measuring and adjusting the tilt of a diffraction grating, which measures and adjusts the tilt between a diffraction grating and an imaging unit of a wavefront analyzer configured with the diffraction grating and an imaging unit.

[0002] In measuring instruments such as shearing interferometers or wavefront sensors, a diffraction grating is used to split the light to be measured through the diffraction phenomenon, generating multiple diffracted lights. The interference fringes that occur when the diffracted lights interfere with each other are captured as image data by an image sensor, and the state of the light wavefront can be measured by analyzing the image data.

[0003] For example, an invention has been disclosed relating to an apparatus and method for analyzing the wavefront of light with high resolution by combining a diffraction grating and an imaging element (see, for example, Patent Document 1). This wavefront analyzer is composed of a diffraction element and a sensor, converts light incident on the diffraction element into diffracted light, and receives a signal of the diffracted light at the sensor. Furthermore, it is possible to measure the wavefront with high precision by acquiring multiple signals corresponding to multiple relative positions between the diffraction element and the sensor and calculating the phase from these signals.

[0004] Special table 2019-533174 publication

[0005] A method for measuring the tilt of a diffraction grating according to one aspect of the present disclosure is a method for measuring the tilt of the diffraction grating relative to an imaging unit of a wavefront analyzer including a diffraction grating that divides light by a diffraction phenomenon and an imaging unit that images the light that has passed through the diffraction grating, the method comprising: irradiating light onto the diffraction grating from a light projection unit for irradiating light; imaging the diffracted light that has passed through the diffraction grating as image data in the imaging unit; Fourier transforming the image data from the imaging unit to generate a Fourier transform image in a calculation unit; and calculating a numerical index corresponding to the amount of tilt of the diffraction grating relative to the imaging unit from the position of a diffraction peak in the Fourier transform image in the calculation unit.

[0006] 1 is a diagram illustrating the configuration of a wavefront analyzer for a diffraction grating tilt adjustment device according to an embodiment of the present disclosure, and the measurement and adjustment of the diffraction grating tilt; 2 is a flowchart illustrating an example of a method for measuring and adjusting the diffraction grating tilt according to an embodiment of the present disclosure; 3 is a flowchart illustrating an example of an analysis process in a calculation unit according to an embodiment of the present disclosure; 4 is a flowchart illustrating an example of additional analysis process in a calculation unit when measuring the tilt of parallel light; 5 is a schematic diagram illustrating an example of an image acquired by an imaging unit according to an embodiment of the present disclosure; and 6 is a schematic diagram illustrating an example of analysis process in a calculation unit according to an embodiment of the present disclosure.

[0007] The wavefront analyzer described in Patent Document 1 has a problem in that if an error occurs in the tilt between the diffraction grating and the imaging unit, the error is also added to the wavefront that is the analysis result. Therefore, it is necessary to perform assembly while suppressing this tilt error.

[0008] An object of the present disclosure is to provide a method for measuring and adjusting the tilt of a diffraction grating, which measures and adjusts with high precision the tilt between a diffraction grating and an imaging unit that constitutes a wavefront analyzer.

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0010] 1A is a diagram illustrating the configuration of a diffraction grating tilt adjustment device 100 capable of implementing a method for measuring and adjusting the tilt of a diffraction grating according to an embodiment of the present disclosure. FIG. 1B is an explanatory diagram illustrating the configuration of a wavefront analyzer 101 of the diffraction grating tilt adjustment device 100 according to an embodiment of the present disclosure, and the measurement and adjustment of the diffraction grating tilt.

[0011] 1A and 1B, a diffraction grating tilt adjustment device 100 includes a wavefront analyzer 101, a light projecting unit 104, and a calculation unit 105. The wavefront analyzer 101 is composed of a diffraction grating 102 and an imaging unit 103, such as an image sensor, and analyzes distortions in an interference image that occur behind the diffraction grating when light is irradiated, thereby restoring the wavefront. The light projecting unit 104 and the calculation unit 105 are also used to measure and adjust the tilt of the diffraction grating 102.

[0012] The light projecting unit 104 irradiates the diffraction grating 102 with light 106 .

[0013] The diffraction grating 102 splits light by means of a diffraction phenomenon, and transmits light 106 to convert it into diffracted light, which is then projected onto the image capturing unit 103 .

[0014] The image capturing unit 103 captures and acquires the diffracted light from the diffraction grating 102 as image data.

[0015] The calculation unit 105 performs frequency analysis on the image data from the image capturing unit 103 to calculate a numerical index corresponding to the amount of tilt of the diffraction grating 102 .

[0016] <Light Projection Unit 104> A pseudo-point light source formed by combining a pinhole with a coherent light source such as a laser or an incoherent light source such as an LED lamp can be used as the light projection unit 104. When light 106 is irradiated onto the diffraction grating 102 from the light projection unit 104, interference fringes are generated due to interference between the diffracted lights. Use of such a light source is expected to improve the contrast of the interference fringes, which will be advantageous for quantifying the tilt of the diffraction grating 102.

[0017] As an example, parallel light, i.e., collimated light, can be used as the light 106 irradiated from the light projecting unit 104. The position of a diffraction peak, which will be described later, depends on the wavefront aberration of the light 106, the angle of incidence, the number of vertical and horizontal pixels of the image data acquired by the imaging unit 103, the in-plane rotation angle of the grating pattern of the diffraction grating 102, the pitch of the grating pattern of the diffraction grating 102, the pixel pitch of the imaging unit 103, or the tilt of the diffraction grating 102 with respect to the imaging unit 103, but using collimated light can eliminate the influence of the wavefront aberration of the light 106, making it easier to detect the diffraction peak.

[0018] Furthermore, by irradiating light 106 from light projector 104 at an angle of incidence θ of 3 degrees or more and 64 degrees or less from a direction perpendicular to the surface of diffraction grating 102, the tilt of diffraction grating 102 relative to imaging unit 103 can be measured and adjusted with an accuracy of 0±0.05 degrees to 0±0.001 degrees. Within this range of incident angle θ, the larger the incident angle θ, the higher the tilt measurement sensitivity and adjustment accuracy. When incident angle θ is less than 3 degrees, the tilt of diffraction grating 102 relative to imaging unit 103 cannot be measured with an accuracy of 0±0.05 degrees or better due to errors in the numerical index caused by fluctuations in the brightness of interference fringes. When incident angle θ is greater than 64 degrees, the tilt of diffraction grating 102 relative to imaging unit 103 cannot be adjusted with an accuracy of 0±0.001 degrees or better due to dimensional errors in the fixing members of diffraction grating 102 or imaging unit 103, errors in the tilt amount caused by thermal expansion, etc.

[0019] <Diffraction Grating 102 > The diffraction grating 102 converts the light 106 from the light projecting unit 104 into diffracted light, and projects the diffracted light onto the imaging unit 103 .

[0020] The diffraction grating 102 may be a one-dimensional or two-dimensional amplitude grating, a phase grating or a combination of amplitude and phase gratings, or a Hartmann mask or modified Hartmann mask (MHM) as described in patent application FR 2 795 175 A1 under reference GR.

[0021] When light 106 is irradiated onto the diffraction grating 102 from the light projecting unit 104, diffracted light of multiple orders is generated. These diffracted light beams interfere with each other, so an interference image can be obtained using the imaging unit 103.

[0022] <Image Capturing Unit 103> The image capturing unit 103 is composed of an image sensor having a plurality of pixels, and receives diffracted light from the diffraction grating 102 and detects it as image data. By analyzing this image data with the calculation unit 105, the amount of tilt of the diffraction grating 102 can be quantified.

[0023] The imaging unit 103 may be a CMOS camera or a CCD camera.

[0024] 1B , the imaging unit 103 is fixed to one surface of, for example, a rectangular parallelepiped imaging unit holding base 123, and a rectangular plate-shaped diffraction grating 102 is arranged on top of the imaging unit 103 so as to overlap with a gap between them, and the four corners of the diffraction grating 102 are adjustably attached to the imaging unit holding base 123 with screws 122. Therefore, the tilt amount of the diffraction grating 102 with respect to the imaging unit 103 can be adjusted by turning each screw 122 forward or backward to tighten or loosen it.

[0025] <Calculation unit 105> The calculation unit 105 receives image data from the imaging unit 103 and analyzes the received image data to calculate a numerical index corresponding to the amount of tilt of the diffraction grating 102. That is, the calculation unit 105 acquires image data from the imaging unit 103 and performs frequency analysis on the luminance distribution of the acquired image data to calculate a numerical index corresponding to the amount of tilt of the diffraction grating 102.

[0026] <Method of Measuring and Adjusting Tilt of Diffraction Grating> FIG. 2A is a flowchart illustrating an example of a method of measuring and adjusting tilt of a diffraction grating according to an embodiment of the present disclosure.

[0027] An example of a method for measuring and adjusting the diffraction grating tilt according to the embodiment is as follows.

[0028] First, in step S1, the screw 122 is turned to adjust the tilt of the diffraction grating 102 relative to the imaging unit 103. When starting assembly, this step may be omitted and the process may start from step S2.

[0029] Next, in step S2, light 106 is irradiated onto the diffraction grating 102 from the light projecting unit 104, and the diffracted light transmitted through the diffraction grating 102 is captured as image data by the imaging unit 103, thereby obtaining an interference image.

[0030] Next, in step S3, the image data acquired by the image capturing unit 103 is subjected to a calculation process in the calculation unit 105, and the calculation unit 105 calculates a numerical index corresponding to the tilt amount of the diffraction grating 102 relative to the image capturing unit 103.

[0031] Next, in step S4, the calculation unit 105 compares whether the calculated numerical index matches a target value, which is the theoretical value of the numerical index when the diffraction grating 102 and the image capture unit 103 are parallel. If they match, the measurement method and adjustment method are terminated. If they do not match, the process returns to step S1, and the tilt of the diffraction grating 102 with respect to the image capture unit 103 is adjusted so that the numerical index matches the target value. Here, steps S2 to S3 are the measurement method, and steps S1 to S4 are the adjustment method.

[0032] <Calculation Processes in the Calculation Unit 105 in Steps S3 and S4> The calculation processes in the calculation unit 105 for image data in steps S3 and S4 will be described below.

[0033] Fig. 2B is a flowchart showing an example of analysis processing in the calculation unit 105 during tilt measurement and adjustment. Fig. 2C is a flowchart showing an example of analysis processing in the calculation unit 105 during tilt measurement of parallel light.

[0034] FIG. 3 is a schematic diagram showing an example of an interference image obtained by the image capturing unit 103 when an MHM is used as the diffraction grating 102.

[0035] FIG. 4 is a schematic diagram showing a Fourier transform image obtained by two-dimensionally Fourier transforming the interference image of FIG.

[0036] First, in a window processing step of step S31, window processing is performed on the image data to reduce the influence of noise on peak extraction in step S33. As a function for window processing, for example, a Hanning window or the like is used.

[0037] Next, in step S32, the Fourier transform of the image data that has been windowed in step S31 is calculated to obtain a Fourier transform image.

[0038] Next, in step S33, the positions of the diffraction peaks are detected in the Fourier transform image calculated in step S32. The positions of these peaks depend on the wavefront aberration of the light 106, the angle of incidence, the number of vertical and horizontal pixels of the image data acquired by the image capturing unit 103, the in-plane rotation angle of the grating pattern of the diffraction grating 102, the pitch of the grating pattern of the diffraction grating 102, the pixel pitch of the image capturing unit 103, or the tilt between the diffraction grating 102 and the image capturing unit 103. As an example, when an MHM is used as the diffraction grating 102, the positions of the four first-order diffraction peaks 107 in FIG. 4 are extracted.

[0039] Next, in step S34, a numerical index corresponding to the tilt amount of the diffraction grating 102 with respect to the image capturing unit 103 is calculated from the position of the diffraction peak 107 extracted in step S33. As an example of the numerical index, a numerical index calculated from the sum or difference of the distances of the diffraction peak positions can be used. The distances between points A and B, between points B and C, between points AC, and between points BD of the first-order diffraction peak 107 in FIG. 4 are respectively calculated as d ＡＢ , d ＢＣ , d ＡＣ , and d ＢＤ Then, the numerical indices I and J can be calculated by taking the sum or difference of these as follows:

[0040] I = (D ｓ +D ｄ ) / 2 J = (D ｓ -D ｄ ) / 2 where D ｓ = d ＢＣ -(d ＡＢ +d ＢＣ ) / 2D ｄ = d ＢＤ -(d ＡＣ +d ＢＤ ) / 2.

[0041] Furthermore, if the light emitted from the light-projecting unit 104 is parallel light and a two-dimensional amplitude grating, a phase grating, a combination of an amplitude grating and a phase grating, a Hartmann mask, or an MHM is used as the diffraction grating 102, then in step S35 following step S34, the irradiation angle of the light-projecting unit 104 relative to the diffraction grating 102 is changed, thereby changing the incident angle θ of the light 106 from the light-projecting unit 104 relative to the diffraction grating 102.

[0042] Next, in step S36, the calculation unit 105 calculates the difference between the numerical indexes before and after the change in the incident angle.

[0043] Next, in step S37, the value of this difference is regarded as a numerical index that quantifies the tilt of the diffraction grating 102, thereby reducing the influence of errors in the pitch of the grating pattern of the diffraction grating 102 and the pixel pitch of the imaging unit 103 on the numerical index, and making it possible to measure the tilt of the diffraction grating 102 relative to the imaging unit 103.

[0044] In step S4 following step S34 or step S37, the calculation unit 105 compares whether the numerical index calculated in step S34 or step S37 matches the target value. If the calculation unit 105 determines that the calculated numerical index matches the target value, the measurement method and adjustment method are terminated. If the calculation unit 105 determines that the calculated numerical index does not match the target value, the process returns to step S1, and the screw 122 is turned to adjust the tilt of the diffraction grating 102 relative to the imaging unit 103 so that the numerical index matches the target value. Specifically, for example, if the relationship between the numerical index and the tilt angle is obtained through experiment or simulation, and the amount of change in tilt angle per rotation of the screw 122 is calculated from the pitch of the screw 122 and the spacing between the screws, it can be determined from the numerical index how much the screw 122 should be rotated.

[0045] Here, as the target value, before performing step 4, the calculation unit 105 calculates the theoretical value of the numerical index when the diffraction grating 102 and the imaging unit 103 are parallel, and stores this value as the target value in the calculation unit 105.

[0046] When the light 106 is not parallel, i.e., when it has wavefront aberration, the theoretical values ​​of the numerical indices can be calculated based on optical simulations. On the other hand, when the light 106 is parallel, i.e., when it does not have wavefront aberration, the theoretical values ​​of the numerical indices can be calculated from the number of vertical and horizontal pixels in the image data acquired by the image capture unit 103, the in-plane rotation angle of the grating pattern of the diffraction grating 102, the pitch of the grating pattern of the diffraction grating 102, and the pixel pitch of the image capture unit 103. As an example, when an MHM having a square grating pattern with a uniform pitch is used as the diffraction grating 102, the pixel pitch of the image capture unit 103 is uniform, and the light 106 is parallel, the target values ​​of the numerical indices I and J are 0, so the tilt of the diffraction grating 102 with respect to the image capture unit 103 is adjusted so that the values ​​of I and J are 0.

[0047] In this embodiment, with this configuration, light 106 is irradiated onto diffraction grating 102 from light projecting unit 104, the diffracted light is captured by imaging unit 103 as image data, and calculation unit 105 acquires the image data from imaging unit 103. Based on this image data, calculation unit 105 calculates a numerical index corresponding to the amount of tilt of diffraction grating 102.

[0048] With this configuration, the tilt amount of the diffraction grating 102 relative to the imaging unit 103 can be measured and adjusted.

[0049] As a result, the tilt adjustment amount of the diffraction grating 102 can be measured and adjusted easily and with high accuracy, which has the great effect of improving the performance of the wavefront analyzer 101.

[0050] It should be noted that any of the various embodiments or modifications described above can be appropriately combined to achieve the effects of each. In addition, combinations of embodiments, combinations of examples, or combinations of embodiments and examples are possible, and combinations of features from different embodiments or examples are also possible.

[0051] <Additional Notes> The above description of the embodiments discloses the following techniques.

[0052] (Technology 1) A method for measuring the tilt of a diffraction grating relative to an imaging unit of a wavefront analyzer that includes a diffraction grating that divides light by a diffraction phenomenon and an imaging unit that images light that has passed through the diffraction grating, the method comprising: irradiating light onto the diffraction grating from a light projection unit for irradiating light; imaging the diffracted light that has passed through the diffraction grating as image data in the imaging unit; Fourier transforming the image data from the imaging unit to generate a Fourier transform image in a calculation unit; and calculating a numerical index corresponding to the amount of tilt of the diffraction grating relative to the imaging unit from the position of a diffraction peak in the Fourier transform image in the calculation unit.

[0053] (Technology 2) The method for measuring the tilt of a diffraction grating according to Technology 1, wherein the light irradiated from the light projecting unit is parallel light.

[0054] (Technology 3) The method for measuring the tilt of a diffraction grating according to Technology 1 or 2, wherein the light is irradiated from the light projecting unit at an incident angle of 3 degrees to 64 degrees from a direction perpendicular to the surface of the diffraction grating.

[0055] (Technology 4) The method for measuring the tilt of a diffraction grating according to any one of Techniques 1 to 3, wherein the calculation unit performs window function processing on the image data obtained from the imaging unit before the Fourier transform of the image data.

[0056] (Technology 5) A method for measuring the tilt of a diffraction grating according to any one of technologies 1 to 4, wherein, when the light irradiated from the light-projecting unit is parallel light, the calculation unit calculates the numerical index corresponding to the tilt amount, then changes the angle of incidence of the light from the light-projecting unit with respect to the diffraction grating, and the calculation unit calculates the difference between the numerical indexes before and after changing the angle of incidence, and the calculation unit uses the difference between the numerical indexes as a numerical index corresponding to the tilt of the diffraction grating with respect to the imaging unit.

[0057] (Technology 6) A method for adjusting the tilt of a diffraction grating, comprising: calculating the numerical index by the calculation unit in the method for measuring the tilt of a diffraction grating described in any one of Technologies 1 to 5; and then adjusting the tilt of the diffraction grating relative to the imaging unit so that the numerical index matches a target value, which is a theoretical value of the numerical index when the diffraction grating and the imaging unit are parallel.

[0058] According to each of the configurations of Techniques 1 to 5, the diffraction grating is irradiated with light from a light projecting unit, the diffracted light from the diffraction grating is acquired as image data by an imaging unit, and a numerical index corresponding to the tilt amount of the diffraction grating is calculated from the image data, thereby making it possible to measure the tilt of the diffraction grating constituting the wavefront analyzer simply and with high accuracy.

[0059] Furthermore, according to the configuration of Technology 6, by adjusting the numerical index so that it coincides with a target value, which is the theoretical value of the numerical index when the diffraction grating and the imaging unit are parallel, the tilt of the diffraction grating relative to the imaging unit can be adjusted easily and with high precision.

[0060] According to the above aspect of the present disclosure, the tilt of the diffraction grating constituting the wavefront analyzer can be measured easily and with high accuracy by irradiating light onto the diffraction grating from the light projecting unit, acquiring the diffracted light from the diffraction grating as image data with the imaging unit, and calculating a numerical index corresponding to the tilt amount of the diffraction grating from the image data.

[0061] Furthermore, according to the above aspect of the present disclosure, by adjusting the numerical index so that it matches a target value, which is the theoretical value of the numerical index when the diffraction grating and the imaging unit are parallel, the tilt of the diffraction grating relative to the imaging unit can be adjusted easily and with high precision.

[0062] The method for measuring and adjusting the tilt of a diffraction grating according to the above aspect of the present disclosure can measure and adjust the tilt of a diffraction grating with high accuracy by irradiating the diffraction grating with light from a light projector, converting the light into diffracted light by the diffraction grating, acquiring the diffracted light as image data by an imaging unit, and calculating a numerical index corresponding to the tilt of the diffraction grating based on the luminance distribution of the pixels that make up the image data. Therefore, by using this measurement method and adjustment method in the assembly of a wavefront analyzer, it is possible to suppress wavefront measurement errors due to the tilt of the diffraction grating and improve the accuracy of wavefront measurement and adjustment.

[0063] 101 Wavefront analyzer 102 Diffraction grating 103 Imaging unit 104 Light projecting unit 105 Calculation unit 106 Light 107 First-order diffraction peak

Claims

1. A method for measuring the tilt of a diffraction grating relative to an imaging unit of a wavefront analyzer comprising: a diffraction grating that divides light by diffraction; and an imaging unit that images the light that has passed through the diffraction grating, comprising: irradiating light onto the diffraction grating from a light projection unit for irradiating light; imaging the diffracted light that has passed through the diffraction grating as image data in the imaging unit; performing a Fourier transform on the image data from the imaging unit to generate a Fourier transform image in a calculation unit; and calculating a numerical index corresponding to the amount of tilt of the diffraction grating relative to the imaging unit from the position of a diffraction peak in the Fourier transform image in the calculation unit.

2. The method for measuring the tilt of a diffraction grating according to claim 1, wherein the light irradiated from the light projecting unit is parallel light.

3. A method for measuring the tilt of a diffraction grating according to claim 1 or 2, wherein the light is irradiated from the light projection unit at an incident angle of 3 degrees to 64 degrees from a direction perpendicular to the surface of the diffraction grating.

4. The method for measuring the tilt of a diffraction grating according to claim 1 or 2, wherein the calculation unit performs window function processing on the image data obtained from the imaging unit before the Fourier transform of the image data.

5. A method for measuring the tilt of a diffraction grating as described in claim 1, wherein the light irradiated from the light-projecting unit is parallel light, and after the calculation unit calculates the numerical index corresponding to the amount of tilt, the angle of incidence of the light from the light-projecting unit with respect to the diffraction grating is changed, and the calculation unit calculates the difference between the numerical indexes before and after the change in the angle of incidence, and the calculation unit determines the difference between the numerical indexes as a numerical index corresponding to the tilt of the diffraction grating with respect to the imaging unit.

6. A method for adjusting the tilt of a diffraction grating, comprising the steps of: calculating the numerical index by the calculation unit in the method for measuring the tilt of a diffraction grating as described in claim 1; and then adjusting the tilt of the diffraction grating relative to the imaging unit so that the numerical index coincides with a target value, which is the theoretical value of the numerical index when the diffraction grating and the imaging unit are parallel.