JP2000275005A - Method for measuring wavefront shape - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly accurately measure a wavefront aberration of light waves output from an optical member with the use of a compact member without carrying out a complicate optical axis adjustment by performing an integration operation process to interference fringe data obtained with the use of a corner cube. SOLUTION: A laser luminous flux 16 from a light source 12 which is output from a collimator lens 10 is irradiated to a reference plate 20. The passing luminous flux is irradiated to a corner cube 24 positioned on a plate 22 nearly parallel to the reference plate 20. Interference fringe data generated by an interference between a reflecting light from the corner cube 24 and a reflecting light from the reference plate 20 is obtained every time the corner cube 24 is moved on the plate 22. Based on the interference fringe data, an angle of incidence θ of the luminous flux entering the corner cube 24 is obtained for each movement position of the corner cube 24. The obtained angle of incidence θis integrated, whereby a wavefront shape of light waves output from the collimator lens 10 is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavefront shape measuring method for measuring wavefront aberration of light waves output from various lenses and various mirrors, and more particularly, to a method for measuring a parallel light beam output from a collimator lens used in an interferometer or the like. The present invention relates to a method for measuring a wavefront shape for an entire aperture.

[0002]

2. Description of the Related Art An interferometer device is known as a device for measuring a surface shape of an object with high accuracy.

When the interferometer is used to measure the flatness of an object, for example, it is necessary to irradiate the reference plate and the object with a parallel light beam. Therefore, the divergent light beam from the light source is collimated by a collimator lens. However, when the collimator lens has a shape error or a refractive index distribution, and the wavefront aberration of the output parallel light beam becomes large, the wavefront aberration component superimposed on the obtained interference fringe data is obtained. Noise cannot be ignored, and it becomes difficult to measure the shape of the subject with high accuracy.

Therefore, it is necessary to verify the wavefront aberration output from such a collimator lens.

As a conventional technique for measuring the wavefront aberration of the output light from such a collimator lens, a spherical laser interferometer 100 as shown in FIG. 8 is used to measure the transmitted wavefront from the collimator lens 102. Things are known.

That is, the laser interferometer for spherical surface 100
, The laser beam from the light source is
Then, the collimator lens 102, which is the subject, is irradiated with the divergent light beam, and the parallel light beam from the collimator lens 102 is irradiated on the reference plate 106. The light beam reflected from the reference plate 106 returns to the reference lens 104 via the collimator lens 102, and interferes with the light beam from the light source reflected by the surface 104A of the reference lens 104 on the collimator lens 102 side. Therefore, by measuring the interference fringe data obtained by the optical interference, the transmitted wavefront of the collimator lens can be measured.

[0007]

However, a collimator lens 102 used for an ordinary interferometer or the like has a diameter of, for example, φ3.
Some have a size of about 00. Therefore, if the size of the collimator lens 102 is large, the reference lens 10
Since the distance between the collimator lens 4 and the collimator lens 102 increases, accurate measurement is difficult due to the influence of air disturbance, and optical axis adjustment of the optical system is also difficult.

[0010] Of course, such a problem is not limited to the measurement of the collimator lens, but also occurs in the measurement of other various lenses and various mirrors.

SUMMARY OF THE INVENTION The present invention has been made in view of such a problem, and measures the wavefront aberration of a light wave output from an optical member with high accuracy using a compact member without complicated optical axis adjustment. It is an object of the present invention to provide a method for measuring a wavefront shape to be obtained.

[0010]

According to the wavefront shape measuring method of the present invention, a light beam from a light source output from a subject is irradiated on a reference plate, and the light beam output from the reference plate is substantially applied to the reference plate. Irradiate the corner cube located on the parallel surface or an equivalent surface, and each time the corner cube is moved on the surface, the reflected light from the corner cube and the reflected light from the reference plate Of the light beam incident on the corner cube at each moving position of the corner cube based on the interference fringe data, and integrating the obtained incident angle information with an integral operation. To obtain the wavefront shape of the light wave output from the subject.

In addition, the incident angle information for each moving position of the corner cube is fitted by a power series polynomial,
It is desirable to integrate the function provided for the fitting with a function of the position.

Further, when the cubic two-dimensional orthogonal coordinate system is provided on the surface, the corner cube is moved on a plurality of straight lines parallel to the x-axis first, and then the y-axis is moved.
On at least one straight line parallel to the axis,
It is preferable that the skirt height of each data on each straight line parallel to the x-axis is determined based on the data on the straight line parallel to the y-axis.

Prior to each movement of the corner cube on the surface, the corner cube is placed at the center of the opening to obtain interference fringe data, and the inclination of the reference plate is determined based on the obtained interference fringe data. It is desirable to adjust.

Further, the light beam from the light source is a laser beam, and the interference fringe data is obtained by the light interference between the reflected light beam from the corner cube and the reflected light beam from the reference plate having different optical path lengths. It is possible to do so.

Further, based on the information on the wavefront shape obtained as described above, it is also possible to obtain position setting information for setting the lens or the mirror and the light source at mutually optimum positions.

The method of the present invention is also useful when the subject is a collimator lens.

Further, the method according to the present invention is characterized in that the object outputs a wavefront having a very large radius of curvature so that interference fringes can be observed by returning light from the corner cube when obtaining the wavefront shape. It is also useful in the case of a mirror, and in such a case, the radius of curvature of the lens or the mirror can be obtained based on the obtained information on the wavefront shape.

Further, the amount of aberration caused by inserting the reference plate is obtained in advance using a technique such as ray tracing, and the amount of aberration obtained in advance is subtracted from the information on the obtained wavefront shape. It is possible to obtain an analytically corrected wavefront shape.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram for explaining a wavefront shape measuring method according to an embodiment of the present invention.

That is, the method of this embodiment uses a Fizeau-type interferometer to measure the wavefront shape of the plane light beam output from the collimator lens 10 and obtain its wavefront aberration.

More specifically, a laser beam 16 output from the laser light source 12 and diverged by the lens 14 is converted into a parallel beam 18 by the collimator lens 10, and the parallel beam 1 is
8 to the reference plate 20, and further transmits the transmitted light to the reference plate 2.
Irradiate the plate 22 installed in parallel with 0.

A corner cube 24 for reflecting the incident light beam in the incident direction and without inverting the phase is disposed on the plate 22. The size of the corner cube 24 is smaller than the diameter of the collimator lens 10. Since it is small, only a part of the parallel light beam 18 is incident on the corner cube 24.

The light beam reflected and emitted by the corner cube 24 travels backward in the optical path of the incident light beam and enters the reference plate 20 again.

A part of the parallel light beam 18 from the collimator lens 10 is reflected on the reference surface 20A of the reference plate 20, and the wavefront of the reflected light beam and the wavefront of the return light from the corner cube 24 are reflected. If there is a shift, light wave interference occurs.

Then, the interference light is irradiated on the imaging surface of the imaging means 30 via the lens 14, the half mirror 26, and the lens 28 to form an interference fringe image.

Thereafter, the interference fringe image is converted to interference fringe analysis means 3
2 to obtain the incident angle information of the incident light beam incident on the corner cube 24, that is, the incident angle information at each incident position of the light beam incident on the reference surface 20A.

The incident angle information at each incident position of the light beam on the reference plane 20A is obtained by the interference fringe analyzing means 32 every time the corner cube 24 is moved to another position 24A on the plate 22. Since this incident angle information is obtained by differentiating the wavefront aberration with the position variable,
The integrating means 34 integrates the plurality of pieces of incident angle information with the position variable to determine the wavefront shape of the entire parallel light flux 18.

The obtained wavefront shape can be regarded as representing the amount of aberration of the wavefront output from the collimator lens 10, and the amount of aberration of the wavefront is displayed on the display means 36 as an image.

As the corner cube 24, it is desirable to use a cube for an optical wave length measuring instrument (for example, a cube whose reflection surface is coated with silver and whose squareness is high precision).

Also, the corner cube is placed at the optical axis position (center of the opening) so that the reference plate surface is perpendicular to the optical axis of the wavefront, and the inclination of the reference plate 20 is adjusted so that interference fringes do not appear. It is important to keep it.

Further, a plurality of pieces of incident angle information obtained by moving the corner cube 24 are plotted in a system in which the horizontal axis represents distance and the vertical axis represents incident angle values, and each point is plotted on one curve. Fitting with a polynomial power series etc.
It is desirable to obtain a continuous incident angle distribution function because the integration process becomes easy and the analysis error is reduced. Hereinafter, the principle of the above method will be described with reference to FIGS.

As shown in FIG. 2, the incident angle θ of the incident light beam 130 incident on the reference plate 20 with respect to the reference surface 20A is 0.
Rather, if it has an angle, the incident light flux 1
The reflected light beam 132 on the reference plane 20A of the light source 30 and the return light beam 13 of the incident light beam 130 from the corner cube 24.
4 intersects at an angle 2θ, and therefore the wavefront 132A of the reflected light beam 132 and the wavefront 134A of the return light beam 134 also intersect at an angle 2θ as shown in FIG. When the relationship between these two wavefronts is represented as shown in FIG. 3, the distance d corresponding to one side of the triangle is a value representing the fringe pitch information of the interference fringes.

Here, since the aperture φA of the corner cube 24 is known, and the distance d can be known from the obtained interference fringe data, the incident angle is calculated from these two values by the following equation (1). θ can be obtained.

Sin θ = d / 2A (1) Also, since the incident angle θ is generally small and the distance d is sufficiently smaller than φA, the above equation (1) can be replaced by the following equation (2). .

Tan θ = sin θ = d / 2A (2) Using these equations (1) or (2), the reference plane 2
The incident angle θ at each incident position of the light beam of 0 A can be obtained.

Then, each of the obtained incident angles θ is:
Since it represents the differential value of the position variable of the wavefront shape W of the incident light beam 130, if the size of the corner cube 24 is selected to be sufficiently smaller than the aperture diameter of the collimator lens 10, the following equation (3) is obtained. To establish. Here, the position variable is x.

DW / dx = tan θ = d / 2A (3) Therefore, by integrating the above equation (3) with the position variable x, the wavefront shape of the incident light beam 130 can be obtained, thereby obtaining a collimator. The wavefront aberration of the output light from the lens 10 can be known.

In the above description, it is assumed that the inclination of the reference plate 20 with respect to the optical axis has been adjusted first.

[0040]

Next, an embodiment in which the corner cube 24 is moved at predetermined intervals along a straight line on the plate 22 will be described.

That is, as shown in FIG. 4, in a circle 41 corresponding to the effective diameter of the collimator lens 10, the corner cube 24 is moved at predetermined intervals along a large-diameter straight line of the circle 41, which coincides with the x-axis. In this case, the incident angle θ (dW / d) is obtained at each of these 12 positions using the above method.
x) was measured.

The effective diameter of the collimator lens at this time is φ
The size of the corner cube 24 is φ10.

The following equation (4) was fitted to each shape of the corner cube 24, and the distribution of the incident angle θ (dW / dx) was obtained from the coefficient a for x and φ10.

F (x, y) = ax + by + c (4) At this time, the horizontal axis indicates the position (pixel numbers are converted into positions), and the vertical axis indicates the incident angle θ (dW / dx). When plotted in the coordinate system having the values of (1) and (2), the values are represented as shown in FIG.

Next, a ninth-order power series polynomial was fitted to the distribution of each measurement point. The fitted curve has a curved shape as represented by a dotted line in FIG.

Next, the function representing the fitted curve of FIG. 5 was integrated with the position variable x. FIG. 6 shows the result of the integration.

Thus, the entire wavefront shape on the x-axis shown in FIG. 4 was obtained.

Since the vertical axis of the graph shown in FIG. 6 may be considered to represent the wavefront aberration, the value is about 0.65 μm, and the wavelength of the laser beam from the light source 12 is λ (0.6328 μm ).

Next, an embodiment will be described in which the wavefront shape of the entire effective diameter of the collimator lens 20 is obtained.

In the same manner as the wavefront shape on the x-axis, a wavefront shape W on a plurality of straight lines parallel to the wavefront shape as shown in FIG. 7A was obtained.

Thereafter, for each position on the y-axis as shown by the circle 43 in FIG. 7B, the wavefront shape W is obtained by the above method, and based on the wavefront shape W in the x-direction obtained above. A plurality of wavefront shapes W were aligned in the y direction, and the wavefront shape W over the entire effective diameter of the collimator lens 10 was determined.

Note that the wavefront shape measuring method of the present invention is not limited to the above-described embodiment, and various modifications can be made as described later.

However, in the method of the present invention, it should be noted that the expected measurement becomes difficult when interference fringes generated by the return light from the corner cube and the return light from the reference plane are not observed. . Specifically, cases where it is difficult to observe interference fringes are as follows.

If the reference plane and the corner cube arrangement plane are not substantially parallel, the reflected light from the reference plane jumps out of the observation area or the interference fringes become too dense to read.

If the size of the corner cube is too small and the number of pixels of the interference fringe image is too small, analysis becomes impossible.

When the radius of curvature is small even for a spherical wave,
Interference fringes generated by the reflected light from the corner cube and the reflected light from the reference surface become too dense, and reading becomes impossible.

Note that the operation of moving the corner cube 24 and the process of fitting a predetermined function to the distribution of dW / dx obtained thereby are not limited to those described above, and other various methods (for example, x, y tilt function).

For example, the movement of the corner cube only needs to specify its movement position, and does not necessarily have to move regularly. Further, the function to be fitted to the distribution of dW / dx can be a power series polynomial of another order or another appropriate function suitable for shape fitting.

In the above embodiment, the object to be measured is a collimator lens. However, another optical member that outputs a plane wave, for example, a plane mirror may be used.

Further, since the method of the present invention can measure a spherical wave having a large radius of curvature, it can be applied to a diverging lens, a converging lens, a reflecting mirror, and the like having an optical surface having a large radius of curvature.

However, as described above, a condition is to output a wavefront having a very large radius of curvature so that interference fringes can be observed.

Further, it is possible to fit a wavefront shape measured under such conditions using a spherical surface or a function that approximates a spherical surface, and obtain a radius of curvature based on the result.

In addition, the generation of aberration which does not cause much problem when a parallel flat plate (reference plate) is inserted into a parallel light beam also becomes a serious problem when a parallel flat plate (reference plate) is inserted into a spherical wave. . Such a problem is not negligible even for a spherical wave having an extremely large radius of curvature as described above. In particular, since the aberration is spherical aberration, the error in calculating the radius of curvature naturally becomes large. Therefore, in such a case, the amount of aberration generated according to the thickness and the refractive index of the reference plate is calculated in advance by using a method such as ray tracing, and the calculation result is obtained by the wavefront obtained by the above-described method. By subtracting the information from the shape information and performing analytical correction, accurate measurement of the wavefront shape becomes possible.

Further, in the above embodiment, the Fizeau-type interferometer is applied when performing the measurement.
Other interferometer devices such as a Michelson type and a Mach-Zehnder type can be applied.

Further, in the above embodiment, a predetermined function is fitted to the obtained distribution of dW / dx, and this function is integrated. However, an integral operation is directly performed on the numerical value of dW / dx. Is also good.

Further, for example, in a parallel light beam emitting optical system composed of a diverging lens and a collimator lens, the diverging lens is set in consideration of an optimal relative positional relationship between the diverging lens and the collimator lens with the least wavefront aberration of the emitted light. On the other hand, the amount of wavefront aberration when the collimator lens moves and the relative positional relationship shifts is calculated in advance using a method such as ray tracing, and this is compared with the amount of wavefront aberration measured by the method of the present invention. Based on the comparison result, the amount of positional deviation (correction amount) can be obtained from the relational expression obtained by using the above-described ray tracing method or the like.

[0067]

As described above, according to the wavefront shape measuring method of the present invention, the corner cube arranged at the subsequent stage of the reference plate is moved within a range corresponding to the opening, and the corner cube is moved at each moving position. Interference fringe data is obtained by light wave interference between the light returned from the cube and the reflected light from the reference plate. After that, the information on the optical path can be obtained simply by integrating the incident angle information of the incident light on the reference plate obtained from these data. It is possible to measure the wavefront shape output from the subject arranged in the inside.

Accordingly, if the wavefront aberration generated by other optical elements in the optical path is set to be negligible, the wavefront aberration of the output light from the subject can be measured with an extremely simple configuration, and a large-sized apparatus can be used. In short, the setting is troublesome, and the practical effect is much larger than that of the conventional method in which the influence of disturbance must be considered.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining a method of an embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the embodiment method of the present invention.

FIG. 3 is a diagram for explaining the operation of the method according to the embodiment of the present invention;

FIG. 4 is a diagram for explaining a corner cube moving operation in the embodiment of the present invention.

FIG. 5 is a graph showing a relationship between a measurement point and a fitting function obtained according to an example of the present invention.

FIG. 6 is a graph showing a result obtained by an example of the present invention.

FIG. 7 is a diagram for explaining a corner cube moving operation in the embodiment of the present invention.

FIG. 8 is a schematic diagram for explaining a conventional technique.

DESCRIPTION OF SYMBOLS 10, 102 Collimator lens 12 Laser light source 14, 28 Lens 18 Parallel light flux 20, 106 Reference plate 20A Reference surface 22 Plate 24 Corner cube 30 Imaging means 32 Interference fringe analyzing means 34 Integrating means 36 Display means 41 Effective diameter 100 Laser interferometer for spherical surface 130 Incident light beam 132 Reflected light beam 134 Return light beam 132A, 134A Wavefront

Claims (9)

[Claims] 1. A light beam from a light source output from a subject is irradiated on a reference plate, and a light beam output from the reference plate is placed on a surface substantially parallel to the reference plate or a surface equivalent thereto. Each time the corner cube is moved on the surface, interference fringe data generated by interference between light reflected from the corner cube and light reflected from the reference plate is obtained. On the basis of the data, the incident angle information of the light beam incident on the corner cube at each moving position of the corner cube is obtained, and the obtained incident angle information is subjected to an integral operation, and the wavefront of the light wave output from the subject is obtained. A wavefront shape measuring method characterized by determining a shape. 2. The wavefront shape according to claim 1, wherein incident angle information for each moving position of the corner cube is fitted by a power series polynomial, and a function provided for the fitting is integrated by a position variable. Measuring method. 3. When the corner cube is moved on an xy two-dimensional orthogonal coordinate system on the surface, the corner cube is first moved on a plurality of straight lines parallel to the x axis, and then moved on the y axis. Performing on at least one parallel straight line, the x
The skirt height of each data on each straight line parallel to the axis
3. The wavefront shape measuring method according to claim 1, wherein the determination is performed based on data on a straight line parallel to the axis. 4. Before each movement of the corner cube on the surface, the corner cube is placed at the center of the opening to obtain interference fringe data, and the inclination of the reference plate is calculated based on the obtained interference fringe data. Adjusting, characterized in that it is adjusted
4. The wavefront shape measuring method according to any one of items 3 to 5. 5. A light beam from the light source is a laser beam,
The interference fringe data is obtained by optical interference between a reflected light beam from the corner cube and a reflected light beam from the reference plate having different optical path lengths.
5. The method of measuring a wavefront shape according to claim 4, wherein 6. The apparatus according to claim 1, wherein position setting information for setting a lens or a mirror and a light source at mutually optimum positions is obtained based on the obtained information on the wavefront shape. 2. The method for measuring a wavefront shape according to claim 1. 7. The wavefront shape measuring method according to claim 1, wherein the subject is a collimator lens. 8. When the subject is a lens or a mirror that outputs a wavefront having a very large radius of curvature such that interference fringes can be observed by returning light from the corner cube when obtaining a wavefront shape. 7. The wavefront shape measuring method according to claim 1, wherein a radius of curvature of the lens or the mirror is obtained based on the obtained information on the wavefront shape. 9. A wavefront corrected analytically by previously obtaining an amount of aberration caused by inserting the reference plate and subtracting the previously obtained amount of aberration from the obtained information of the wavefront shape. 9. The wavefront shape measuring method according to claim 8, wherein the shape is obtained.