JP5235591B2 - Method for measuring transmitted wavefront of birefringent optical element - Google Patents

Description

  The present invention relates to a method for measuring a transmitted wavefront of an optical element such as a lens or a filter mounted on various optical devices, and is particularly suitable when the transmitted wavefront measurement of an optical element having birefringence is performed using linearly polarized light as measurement light. The present invention relates to a transmitted wavefront measuring method for a birefringent optical element.

  Conventionally, a technique of measuring aberrations of an optical element such as a lens by measuring a transmitted wavefront using an interferometer has been widely used for performance inspection of the optical element (see Patent Documents 1 and 2 below). In general, many interferometers use linearly polarized light such as laser light as measurement light, and interferometers for transmission wavefront measurement generally use linearly polarized light as measurement light.

  On the other hand, in recent years, many optical elements are formed of various resin materials typified by polycarbonate. Some of these resin optical elements have birefringence (optical anisotropy). In particular, in injection molded optical elements, the pressure distribution during resin injection is not uniform. Due to the internal stress and the like of the molded product produced more, there is known one that exhibits a considerably strong birefringence.

JP 2007-78593 A Japanese Patent No. 2951366

  When the transmitted wavefront measurement of the optical element having birefringence as described above (hereinafter referred to as “birefringent optical element”) is performed using an interferometer using linearly polarized light as measurement light, the imaged interference fringes In some areas, the wavefront shape information of the transmitted wavefront may not be correctly carried due to the influence of the birefringence of the optical element to be tested.

  As described above, when a region in which wavefront shape information is not properly carried (hereinafter sometimes referred to as an “inappropriate region”) is generated in the interference fringes, the conventional measurement method cannot properly perform the transmitted wavefront measurement. was there.

  The present invention has been made in view of such circumstances, and in transmitted wavefront measurement using linearly polarized light as measurement light, information on the wavefront shape of the transmitted wavefront is correctly carried by the influence of the birefringence of the optical element under test. It is an object of the present invention to provide a transmitted wavefront measuring method for a birefringent optical element capable of appropriately performing transmitted wavefront measurement even when an interference fringe having a region that has not been captured.

The method for measuring a transmitted wavefront of a birefringent optical element according to the present invention irradiates a test light element having birefringence with linearly polarized measurement light, and interferes the transmitted wavefront transmitted through the test optical element with a reference wavefront. A method for measuring aberration of the transmitted wavefront based on image data of interference fringes obtained by:
An interference fringe imaging step of imaging the interference fringe for each of a plurality of imaging arrangement positions set so that the relative positional relationship between the direction of the polarization plane of the measurement light and the optical element to be measured is different from each other;
In each of the interference fringes picked up at each of the plurality of pick-up positions, an improper region in which information on the wavefront shape of the transmitted wavefront is not properly carried due to the influence of the birefringence of the optical element to be tested is specified. An appropriate region identification step;
The wavefronts of the entire transmitted wavefront are connected by combining the wavefront shape information corresponding to the other areas excluding the inappropriate area of each interference fringe imaged at each of the plurality of imaging arrangement positions by the aperture synthesis method. A wavefront shape obtaining step for obtaining shape information;
The wavefront aberration measuring step of measuring the aberration of the transmitted wavefront based on the wavefront shape information of the entire transmitted wavefront is performed in this order.

  In the present invention, the test optical element may be an injection molded plastic lens.

  The “polarization plane of the measurement light” means a vibration plane of the magnetic vector of the measurement light (a plane including the traveling direction of the measurement light and the vibration direction of the magnetic field).

  According to the transmitted wavefront measuring method for a birefringent optical element according to the present invention, the following effects can be obtained by providing the above-described configuration.

  That is, in the present invention, attention is paid to the fact that the position of the improper region formed in the captured interference fringe changes as the relative positional relationship between the direction of the polarization plane of the measurement light and the optical element to be measured changes. The interference fringes are imaged for each of a plurality of imaging arrangement positions set so that the relative positional relationship is different from each other. Then, wavefront shape information of the entire transmitted wavefront is obtained by connecting the wavefront shape information corresponding to the other regions except the improper region of each interference fringe imaged at each of the plurality of imaging arrangement positions by the aperture synthesis method. Information is obtained, and transmitted wavefront aberration is measured based on this information.

  Therefore, even when an improper region is formed in each interference fringe, it is possible to obtain appropriate wavefront shape information of the entire transmitted wavefront, and thus it is possible to appropriately measure the aberration of the transmitted wavefront over the entire region. .

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of a light wave interference measuring apparatus used in a transmitted wavefront measuring method (hereinafter sometimes simply referred to as “transmitted wavefront measuring method”) of a birefringent optical element according to an embodiment of the present invention. FIG. 2 is a schematic diagram ((A) is a front view, and (B) is a plan view) showing the shape of a test lens (birefringent optical element in the present embodiment).

  The optical interference measuring apparatus shown in FIG. 1 constitutes a Fizeau interferometer, and includes an interferometer main body 20 and a subject positioning unit 30. The interferometer body 20 outputs linearly polarized light having a long coherence distance as measurement light, and includes a light source 21 such as a laser light source, a beam diameter expanding lens 22, a beam splitter 23, a collimator lens 24, and an imaging lens 25. And imaging means 26 having a light detection surface. The interferometer body 20 also includes an analysis device 27 including a computer that performs image processing of interference fringes picked up by the image pickup means 26, various arithmetic processes, and drive control of various adjustment units, and interference fringe images. A monitor device 28 for displaying and an input device 29 for making various inputs to the analyzing device 27 are provided. The reference plate 4 shown in FIG. 1 is usually included in the interferometer main body 20, but will be described in the subject positioning unit 30 for convenience of explanation in this specification.

  On the other hand, the subject positioning unit 30 supports the reference plate 4, the test lens 1, and the null mirror 6 in this order in the traveling direction of the measurement light from the interferometer body unit 20 (upward in FIG. 1). And it is comprised so that alignment adjustment may be carried out.

  That is, the reference plate 4 is supported by the manual biaxial tilt stage 11 and rotated around the X axis (axis extending in the left-right direction in FIG. 1) and the Y axis (axis extending perpendicular to the paper surface in FIG. 1). The angle (tilt) is adjusted in the preliminary adjustment stage. Further, the test lens 1 is supported by the θ stage 13 and the electric biaxial tilt stage 14 via the lens mounting jig 5 and has a rotation angle around the measurement optical axis L (shown by a one-dot chain line in the figure), The rotation angle (inclination) about the X axis and the Y axis is automatically adjusted when each lens 1 is measured. The null mirror 6 is supported by a manual biaxial tilt stage 12 and is further supported in turn by an electric X-axis stage 15, an electric Y-axis stage 16, and an electric Z-axis stage 17. A laser marker 18 is installed on the electric two-axis tilt stage 14. Although not shown, the reference plate 4 is provided with a fringe scan adapter for finely moving the reference plate 4 in the optical axis direction when performing the fringe scan measurement.

  In this embodiment, the test lens 1 is a plastic lens that is injection-molded as a lens for an optical sensor or the like designed to convert an input plane wave into a predetermined aspherical wave and output it. As shown, it consists of a lens part 2 and an overhang part 3. The lens portion 2 has a first lens surface 2a and a second lens surface 2b both aspherical, the overhang portion 3 is formed in a bowl shape, and both the upper surface 3a and the lower surface 3b are light from the lens portion 2. Designed to be perpendicular to the axis.

  The shape of the test lens 1 and its use are not limited to those of the above-described embodiment. For example, an optical recording medium such as CD, DVD (including AOD), BD (Blu-ray disc) is recorded / reproduced. It is possible to use an optical pickup lens of the apparatus or to attach a diffractive optical surface. In the present embodiment, it is assumed that the test lens 1 converts an incident plane wave into an aspherical wave and outputs it, but when a spherical wave is output from the test lens 1, Instead of the null mirror 6, a reference spherical reflecting mirror having a spherical reflecting surface is arranged as a null optical element.

  As shown in FIG. 1, the lens mounting jig 5 includes a central window 5a for measuring a transmitted wavefront with respect to the lens portion 2 of the lens 1 to be tested, and a protruding portion window 5b positioned outside the central window 5a. Further, a null mirror reflecting plane portion window 5c located outside the protruding portion window 5b is provided to support the protruding portion 3 of the lens 1 to be tested from the lower side in the figure.

  The null mirror 6 constitutes a reflection-type null optical element, and is a reflective aspherical part that retroreflects an aspherical wavefront that once converges and diverges after passing through the lens part 2 of the lens 1 to be examined. 6a and a reflective flat surface portion 6b arranged perpendicular to the central axis of the reflective aspherical surface portion 6a (the optical axis of the null mirror 6), and the X axis of the null mirror 6 by the manual biaxial tilt stage 12 And the inclination about the Y axis are adjusted in the preliminary adjustment stage, and the X axis, Y axis, Z axis (see FIG. 1) are adjusted by the electric X axis stage 15, the electric Y axis stage 16, and the electric Z axis stage 17. It is possible to adjust the movement in parallel with each direction of the vertical axis), whereby the alignment is automatically adjusted when the transmitted wavefront of the lens 1 is measured.

  Further, although not shown, the optical interference measuring apparatus includes a sample stage moving mechanism for automatically performing loading / unloading operations of the lens 1 to be examined. The sample stage moving mechanism is the same as that described in the above-mentioned Patent Document 1 and Japanese Patent Application Laid-Open No. 2008-46051 (hereinafter referred to as “Patent Document 3”), and detailed description thereof is omitted.

  Hereinafter, the execution procedure of the transmitted wavefront measuring method according to an embodiment of the present invention will be described. The transmitted wavefront measuring method of the present embodiment is applied when measuring the transmitted wavefront of the lens 1 to be measured using the above-described optical interference measuring apparatus.

  Note that when the transmitted wavefront measurement is performed, alignment adjustment of the reference plate 4, the null mirror 6, and the test lens 1 is performed. For these alignment adjustments, for example, the method disclosed in Patent Document 3 can be applied, but detailed description thereof is omitted here.

  <1> First, a plurality of (two in the present embodiment) imaging arrangement positions are set such that the relative positional relationship between the direction of the polarization plane of the measurement light and the lens 1 to be measured is different from each other. Specifically, two positions where the rotation angles of the lens 1 to be measured about the measurement optical axis L with respect to the null mirror 6 are different from each other by a predetermined angle (45 degrees in the present embodiment) are set as two imaging arrangement positions. In the present embodiment, the test lens 1 is rotated 2 around the measurement optical axis L by using the θ stage 13 while the direction of the polarization plane of the measurement light is fixed with respect to the space. The two imaging arrangement positions (hereinafter, one of the two imaging arrangement positions may be referred to as a first imaging arrangement position and the other may be referred to as a second imaging arrangement position). Thereby, the rotation angle around the measurement optical axis L of the test lens 1 with respect to the direction of the polarization plane of the measurement light is 45 around the measurement optical axis L at the first imaging arrangement position and the second imaging arrangement position. Will only change.

  <2> Next, an interference fringe carrying transmission wavefront information of the lens unit 2 of the lens 1 to be tested (hereinafter referred to as “interference fringe of the lens unit”) for each of the two set imaging arrangement positions is set as follows: Imaging is performed according to the procedure (interference fringe imaging step).

  (A) The test lens 1 is installed at the first imaging arrangement position, and the measurement light is irradiated to the lens portion 2 of the test lens 1 through the reference plate 4 and the null mirror central window 5a of the lens mounting jig 5. At this time, the light transmitted through the lens portion 2 of the lens 1 to be examined, retroreflected by the null mirror 6, and transmitted again through the lens portion 2 and the reference light (reflected light from the reference surface 4 a of the reference plate 4). An interference fringe (hereinafter referred to as “first interference fringe”) of the lens portion formed by the interference is imaged.

  (B) The test lens 1 is installed at the second imaging arrangement position, and the interference fringes (hereinafter referred to as “second”) of the lens portion formed in the same manner as the first interference fringes at the second imaging arrangement position. (Referred to as “interference fringes”). In addition, when imaging the interference fringes (first and second interference fringes) of the lens unit, a masking process is appropriately performed in order to shield an image related to a region other than the lens unit 2 (Patent Document 3 above). reference).

  <3> Subsequently, in the first and second interference fringes imaged at each of the two imaging arrangement positions, information on the wavefront shape of the transmitted wavefront is correctly carried by the influence of the birefringence of the lens 1 to be tested. The improper area that did not exist is identified (unsuitable area identifying step).

  FIG. 3 is a diagram showing an example of the first interference fringes imaged at the first imaging arrangement position, and FIG. 4 is a bird's-eye view of the transmitted wavefront created based on the first interference fringes shown in FIG. . FIG. 5 is a contour map of the transmitted wavefront created based on the first interference fringes shown in FIG.

  On the other hand, FIG. 6 is a diagram showing an example of the second interference fringe imaged at the second imaging arrangement position, and FIG. 7 is a bird's-eye view of the transmitted wavefront created based on the second interference fringe shown in FIG. It is. FIG. 8 is a contour map of the transmitted wavefront created based on the second interference fringes shown in FIG.

The coordinate axes X ₁ , Y ₁ , and Z ₁ shown in FIGS. 4 and 7 show the relative positional relationship between the transmitted wavefront obtained by the first interference fringe and the transmitted wavefront obtained by the second interference fringe. It is a common thing set as follows. Further, the coordinate axes X ₂ and Y ₂ shown in FIG. 5 and FIG. 8 are such that the relative positional relationship between the transmitted wavefront obtained by the first interference fringe and the transmitted wavefront obtained by the second interference fringe can be understood. The coordinate axes X ₁ , Y ₁ and the coordinate axes X ₂ , Y ₂ are different from each other in the origin setting position, but their directions are set to coincide with each other (become parallel). ing. 5 and 8 show a straight line extending obliquely different from the coordinate axes X ₁ and Y ₁ , this is for determining the cross-sectional position when obtaining the cross-sectional shape of the transmitted wavefront. In the description of the present embodiment, there is no particular relationship. 5 and 8 show the positions of the highest point P and the lowest point V on each contour line.

Further, in FIGS. 4 and 7, the direction of the polarization plane of the measuring beam has a direction parallel to the axis X ₁ in FIG. Further, FIGS. 4 and 7 show a gate position G when the test lens 1 is injection-molded. The gate position G shown in FIG. 4 is a position rotated by 45 degrees (around the measurement optical axis L) with respect to the direction of the polarization plane of the measurement light, and the gate position G shown in FIG. The position is rotated by 90 degrees with respect to the direction of the polarization plane (around the measurement optical axis L).

In FIGS. 3 to 5, the improper areas A ₁ and A ₂ are mainly incapable of correctly carrying information on the wavefront shape of the transmitted wavefront due to the influence of the birefringence of the lens 1 to be tested (information is missing). Has been identified as an important area.

On the other hand, in FIGS. 6 to 8, the improper areas B ₁ and B ₂ were not properly carrying the wavefront shape information of the transmitted wavefront due to the birefringence of the lens 1 (the information was missing). ) Has been identified as a major area.

<4> Next, the wavefront shape information corresponding to the other areas excluding the inappropriate areas A ₁ and A ₂ in the _first interference fringes, and the inappropriate areas B ₁ and B ₂ in the second interference fringes. Wavefront shape information corresponding to the other areas other than the above is connected to each other by the aperture synthesis method to obtain wavefront shape information of the entire transmitted wavefront of the lens unit 2 (wavefront shape acquisition step).

  As the aperture synthesis method, for example, a conventionally known method disclosed in JP 2002-162214 A, JP 8-219737 A, JP 10-332350 A, or the like can be used.

  <5> Then, aberration analysis of the transmitted wavefront is performed based on the obtained wavefront shape information of the entire transmitted wavefront of the lens unit 2, and the obtained wavefront aberration is developed by a Zernike polynomial so that the coma aberration of the transmitted wavefront is developed. Is obtained (wavefront aberration measurement step).

  Although one embodiment of the present invention has been described above, the present invention is not limited to such an embodiment, and various embodiments can be used.

  For example, in the above embodiment, in order to change the relative positional relationship between the direction of the polarization plane of the measurement light and the test lens 1, the direction of the polarization plane of the measurement light remains fixed with respect to the space. Is rotated around the measurement optical axis L, but the direction of the polarization plane of the measurement light is rotated around the measurement optical axis L while the lens 1 to be tested is fixed with respect to the space. It is also possible to rotate both the direction of the polarization plane of light and the lens 1 to be tested around the measurement optical axis L.

  In the above embodiment, an example of an injection-molded plastic lens is described as an example of a birefringent optical element. However, the present invention is not limited to various optical elements, for example, on the optical path in order to change the characteristics of light. It is possible to apply to a transmitted wavefront measurement of a filter plate arranged, a correction plate arranged on the optical path in order to compensate for the optical path length, and the like having birefringence. When performing transmission wavefront measurement of such a flat birefringent optical element, a reflective reference plate having a flat reflective reference surface for retroreflecting parallel light is arranged instead of the null mirror 6 described above. Is done.

  Further, in the above-described embodiment, two positions where the directions of the polarization planes of the measurement light irradiated on the test lens 1 are different from each other by 45 degrees around the measurement optical axis L are set as a plurality of imaging arrangement positions. However, two positions where the directions of the polarization planes of the measurement light are different from each other by an angle other than 45 degrees around the measurement optical axis L may be set as a plurality of imaging arrangement positions. It is also possible to set three or more arrangement positions.

  The present invention can also be applied to transmitted wavefront measurement using a transmission-type null optical element (null lens) as disclosed in Japanese Patent Application Laid-Open No. 2008-89356.

  In the above embodiment, the optical interference measuring apparatus is a Fizeau interferometer. However, the present invention is also applicable to transmitted wavefront measurement using another type of interferometer such as a Michelson type. It is possible.

1 is a schematic diagram of an optical interference measuring apparatus used in an aberration measurement error correction method according to an embodiment. Schematic diagram showing the shape of the test lens ((A) is a front view, (B) is a plan view) The figure which shows an example of the 1st interference fringe imaged in the 1st imaging arrangement position Bird's eye view of transmitted wavefront created based on first interference fringes Contour map of transmitted wavefront created based on first interference fringes The figure which shows an example of the 2nd interference fringe imaged in the 2nd imaging arrangement position Bird's eye view of transmitted wavefront created based on second interference fringes Contour map of transmitted wavefront created based on second interference fringes

Explanation of symbols

1 Test lens (birefringent optical element)
2 Lens portion 2a First lens surface 2b (of test lens) Second lens surface 3 (of test lens) 3 Overhang portion 3a Upper surface 3b (of overhang portion) Bottom surface 4b (of overhang portion) 4 Reference plate 4a Reference Surface 5 Lens mounting jig 5a Central window 5b Overhang window 5c Reflective plane window 6 Null mirror (reflective null optical element)
11 Manual 2-axis tilt stage (for reference plate adjustment)
12 Manual 2-axis tilt stage (for null mirror adjustment)
13 θ stage 14 Electric 2-axis tilt stage 15 Electric X-axis stage 16 Electric Y-axis stage 17 Electric Z-axis stage 18 Laser marker 20 Interferometer body 21 Light source 22 Beam diameter expanding lens 23 Beam splitter 24 Collimator lens 25 Imaging lens 26 Imaging means 27 Analyzing device 28 Monitor device 29 Input device L Measuring optical axis A ₁ , A ₂ , B ₁ , B ₂ inappropriate area X ₁ , Y ₁ , Z ₁ , X ₂ , Y ₂ coordinate axis G Gate position P Maximum point V lowest point

Claims (2)

Based on image data of interference fringes obtained by irradiating the test optical element having birefringence with linearly polarized measurement light and causing the transmitted wavefront transmitted through the test optical element to interfere with the reference wavefront, A method for measuring a transmitted wavefront of a birefringent optical element that measures aberration,
An interference fringe imaging step of imaging the interference fringe for each of a plurality of imaging arrangement positions set so that the relative positional relationship between the direction of the polarization plane of the measurement light and the optical element to be measured is different from each other;
In each of the interference fringes picked up at each of the plurality of pick-up positions, an improper region in which information on the wavefront shape of the transmitted wavefront is not properly carried due to the influence of the birefringence of the optical element to be tested is specified. An appropriate region identification step;
The wavefronts of the entire transmitted wavefront are connected by combining the wavefront shape information corresponding to the other areas excluding the inappropriate area of each interference fringe imaged at each of the plurality of imaging arrangement positions by the aperture synthesis method. A wavefront shape obtaining step for obtaining shape information;
A wavefront aberration measuring step of measuring aberration of the transmitted wavefront based on wavefront shape information of the entire transmitted wavefront, and performing in this order, a transmitted wavefront measuring method for a birefringent optical element. 2. The transmitted wavefront measuring method for a birefringent optical element according to claim 1, wherein the test optical element is an injection molded plastic lens.

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