JPH025242B2 - - Google Patents

Abstract

PURPOSE:To enable the measurement with high accuracy, by changing the state of polarization in at least one optical path of bisected beams of light so as to vary the state of polarization of both beams in an optical path for generation of an interference pattern and making the intensity of light of both beams after passage through an analyzer roughly constant. CONSTITUTION:The exit surface 31 of the 2nd collimator of an interference device is a plane, and is a beam splitter through which a part of beam passes and from which a part of the beam reflects. An object 51 to be measured of a transmission type is inserted after the passage through the beams splitter. The light transmitted through the same passes through a polarization plate 42 and is then reflected on a reference plane mirror 53. The reflected light is reflected together with the light reflected on the splitter 31 by a beam splitter 300 and the reflected light is transmitted through an analyzer 7 in an optical path for generation of an interference 13 so that both beams of the light interfere with each other. The intensities of both beams are thus made equal, the max. contrast is obtained, and the measurement of high accuracy is easily realized.

Description

[Detailed Description of the Invention] The present invention relates to an interference method and apparatus for measuring the flatness, sphericity, or lens performance of an object, particularly for measuring various objects with different reflectances and transmittances with high precision. The present invention relates to an interference method and an interference device.

Conventional interference devices have been used to measure flat or spherical surfaces, but when measuring objects with different transmittances and reflectances, several types of beam splitters with different separation ratios are used to split the coherent light into two. A different beam splitter was prepared each time the transmittance or reflectance of the object to be measured changed, so that the light intensity ratio of both lights in the interference pattern generation optical path was approximately equal. In addition, a filter was inserted into one of the two divided optical paths to attenuate the amount of light.
However, when attempting to perform high-precision measurement with this method, the following disadvantages occur.

When trying to achieve a measurement accuracy of λ/10 or higher, the surface accuracy of the optical components used in the measurement optical system, such as the beam splitter and the mirror in the reference optical path, is at the same level as the measurement accuracy. The surface accuracy measurement result is affected by this, and measurement accuracy of approximately λ/5 or higher cannot be obtained. Therefore, as shown in the patent filed by the present inventors (Japanese Patent Application No. 55-28066), the distortion of the measurement optical system is measured in advance and this information is stored. By correcting the measurement result of an arbitrary object to be measured using this stored information, high-precision measurement becomes possible for the first time. However, as mentioned above, when replacing beam splitters and filters for samples with different reflectances and transmittances, it is necessary to change the correction value each time.
If you turn it upside down, the correction value will naturally change.
Moreover, frequent replacement of such beam splitters and filters requires a great deal of time for operation and adjustment.

The present invention solves the above-mentioned conventional problems, and easily equalizes the intensity of both lights in the interference fringe pattern generation optical path with a simple configuration for objects to be measured with different reflectances and transmittances. By making equal, we get
without changing the distortion of the measurement optical system, that is, the correction value.
It is an object of the present invention to provide an interference method and apparatus that make it possible to always use the same correction value.

In order to achieve the above object, the present invention provides means for changing the polarization state on at least one of the optical paths that divide the coherent light into two. By doing this, the polarization states of both lights in the interference pattern generation optical path are different. Assuming that the reference light and the reflected light (or transmitted light) from the object are both linearly polarized light, and their polarization directions are orthogonal to each other, then
The intensity of the reference light Ir and the intensity of the reflected light from the object to be measured are
If Io, these two lights do not interfere, but by inserting an analyzer, such as a polarizing plate, into the interference pattern generation optical path, and tilting the polarization direction (direction that transmits 100% of linearly polarized light) by θ with respect to the reference light. Deploy. In this way, the intensities, I _dr and I _dp of the reference light and object light transmitted through this polarizing plate are given by the following equations.

I _dr = I _r cos ² θ (1) I _dp = I _p sin ² θ (2) Also, since each polarization direction naturally matches the polarization direction of the polarizing plate, both lights cause interference. At this time I _dr
When and I _dp are equal, the contrast of the interference fringes is maximized. θ that satisfies this condition is given by the following equation.

θ＝tan ^-1 √ _{r p} (3) From the above explanation, for samples with different reflectances of the measured object, that is, for different I _p , only the polarization direction of the polarizing plate can be changed to satisfy equation (3). By making adjustments, it is easy to always obtain the maximum interference fringe contrast.

According to the present invention, as described above, it is not necessary to replace the beam splitter with a mirror of a standard reference surface that has a different reflectance each time the reflectance of the object to be measured differs, or to insert a different filter. Therefore, the wavefronts of the reference light and object light are not changed at all. Therefore, the maximum interference fringe contrast can always be obtained without changing the shape of the interference pattern of both lights at all, so that a highly accurate interference device can be obtained. In addition, an analyzer is inserted into the optical path where the interference pattern is generated, and the distortion of the optical components present in the interference optical system is measured using, for example, a highly accurate plane mirror or spherical mirror, and the measured value is used as a correction value. By using this constant correction value, it is possible to always correct the measurement results of the object to be measured and realize highly accurate interference measurement.

Hereinafter, the present invention will be explained in detail with reference to embodiments shown in the drawings.

FIG. 1 shows an embodiment of the present invention. A linearly polarized laser beam is emitted from the laser beam 1, and the beam is converted into a laser beam 10 having a desired beam diameter by a collimator lens 2. This light is split into two by the beam splitter 3, and one light is sent to the object optical path 1.
1 and reflected by the object to be measured 50. The other beam split into two by the beam splitter 3 is guided to a reference optical path 12, in which a quarter-wave plate 4 is arranged to convert the incident linearly polarized light into circularly polarized light. When the circularly polarized light is reflected by the reference mirror 6 and transmitted through the quarter-wave plate again, it becomes linearly polarized light whose polarization direction is orthogonal to the linearly polarized light in the object optical path.
This light is reflected by the beam splitter 3, guided to the interference pattern generation optical path 13, and interferes with the light reflected by the object to be measured 50. However, in the current polarization state, the polarization directions of both lights are orthogonal. , no interference pattern occurs. Interference pattern generation optical path 13
An analyzer 7 made of a polarizing plate is arranged inside, and can be rotated by a rotation mechanism 71 with the direction of the optical axis as the rotation axis direction. Interference pattern generation optical path 1
3, the ratio of the intensity I _p of the reflected light from the object to be measured and the intensity I _r of the reference light changes when the object to be measured changes, so the polarization direction of the polarizing plate of the analyzer 7 is set to the reference light. tilted by θ with respect to the linear deflection direction. However, θ is
This is a value that satisfies equation (3). In this way, the light intensity I _d of both lights transmitted through the analyzer becomes equal (I _d = I _r
cos ² θ=I _p sin ² θ), and the light becomes the same linearly polarized light, and the contrast of the interference fringes is maximized. The interference fringes are projected onto the imaging surface 91 of the imaging device 9 by the lens 8 .

FIG. 2 shows another embodiment of the present invention. The same numbers as in FIG. 1 represent the same items. This embodiment is an interference device that measures a transmission type object 51 to be measured. The linearly polarized laser beam is divided into two parts and a reference optical path 12
A half-wave plate inside converts the incident linearly polarized light into linearly polarized light orthogonal to it. Beam splitter 31
As a result, both lights are guided to an interference fringe generation optical path 13, and by the analyzer 7 rotating around the optical axis, the intensity of both lights becomes equal, resulting in maximum contrast, and an image is captured by the imaging device 9.

FIG. 3 shows an embodiment of the invention. The laser light source 1' is a non-polarized laser. The light emitted from the laser light source becomes an expanded parallel beam by a collimator lens 21, passes through a beam splitter, and becomes an expanded parallel beam 11 by a second collimator lens 22. The exit surface 31 of this second collimator is a flat surface, and serves as a beam splitter that transmits some of the beams and reflects some of the beams. After passing through the beam splitter, a transmission type object to be measured 51 is inserted. After passing through the polarizing plate 42, the light is reflected by the reference plane mirror 53, and is reflected by the beam splitter 300 together with the light reflected by the beam splitter 31. 7), and both lights interfere. Here, if the transmittance of the object to be measured with the smallest transmittance is Tmin, then the beam splitter has a reflectance r and a transmittance (1-r) (assuming that there is no absorption).
Further, it is assumed that the reference plane mirror 53 reflects 100% and the polarizing plate does not absorb any absorption), and r is set so that the following formula holds true.

r=Tmin ² (1-r) ² In this way, when the transmittance is minimum, the polarization direction of the polarizer 7 can be changed to the polarization direction of the light from the object to be measured, that is, the polarization direction of the polarizing plate 42. When the directions match, the intensity of both lights will be equal and maximum contrast will be obtained. Further, when the transmittance of the object to be measured is generally T, the intensities of both lights become equal by rotating the polarizer by θ shown by the following equation from the polarization direction in the case of the above-mentioned minimum transmittance.

cos ² θ=Tmin ² /T ² FIG. 4 shows an embodiment of the present invention. 1 in Figure 4
The same numbers as those in the figures represent the same items. In this embodiment, a case is shown in which a spherical mirror 52 is measured as the object to be measured. Reference numeral 101 is a condenser lens, and the lens is arranged so that the light condensing position by this lens coincides with the center of curvature of the spherical mirror of the object to be measured. . In the reference optical path 12, a wedge glass 120 is arranged outside the λ/4 plate. This wedge glass 120 is slightly moved in the direction of arrow A by a drive device 121, and the optical path length of the reference light is modulated. For example, by changing the optical path length at a pitch of about λ/8 to λ/250, the image pickup device 9 captures the interference pattern and sends it to the control circuit 90. Measure the deviation from the spherical shape. The control circuit 90 has a built-in memory 91, which first measures a nearly perfect spherical reference spherical mirror as an object to be measured, and stores the value φ _p (xi,
yi) is stored in the memory. Next, as shown in FIG. 4, a general object to be measured 52 is measured. If the measured value is φ′ (xi, yi), φ _p (xi, yi) represents the distortion of the measurement optical system, so the true value of the measured object φ
(xi, yi) is given by the following formula.

φ (xi, yi) = φ′ (xi, yi) − φ _p (xi, yi) Such correction is not limited to the embodiment shown in Fig. 4, but also applies to the optical systems of the embodiments shown in Figs. 1 to 3. The same can be done for In the case of Fig. 1, the object to be measured 50
Instead, the correction value may be obtained by performing measurements using a highly accurate planar principle. Also, Figures 2 and 3
In the case of the embodiment shown in the figure, it is sufficient to remove the transmission type object to be measured, perform the measurement, and obtain the correction value.

According to the present invention, when measuring objects having various reflectances and transmittances, conventional methods require preparing several types of beam splitters with different reflectances or preparing several types of filters with different transmittances. Previously, the device had to be prepared and replaced every time the object to be measured changed, but this is no longer necessary, and with a simple operation, the intensity ratio of both lights can be equalized and interference fringes with maximum contrast can be obtained.
For example, when measuring a plane or spherical mirror that is being processed, the reflectance of the object being measured is a few percent, but the reflectance of a mirror coated with a reflective coating is close to 90%. Without intercalation, it was not possible to obtain high contrast reflectance for both uncoated and coated samples. Conventionally, this involved replacing the beam splitter or inserting a filter to increase the contrast. However, since the surface accuracy of beam splitters and filters is about a fraction of a wavelength, the measurement accuracy is only about this level. Also, it is not impossible to measure and correct the surface accuracy of beam splitters and filters in advance, but measuring such correction values for a large number of beam splitters and filters makes operations complicated and difficult. Not only is it necessary to prepare a large number of memories, but it is also necessary to replace and install beam splitters and filters with sufficient precision.

According to the present invention, an analyzer that adjusts the contrast to the maximum is inserted outside the reference optical path and the object optical path, so even if the analyzer is rotated and adjusted, it has no effect on the interference pattern itself other than changing the contrast. Therefore, the rotation adjustment has no effect on the interference results. Therefore, by always using the same correction value, highly accurate measurement can be easily achieved, and the effect is very large.

[Brief explanation of drawings]

Fig. 1 is an embodiment of the present invention, which uses a 1/4 wavelength plate as the polarization state changing means, and Fig. 2 is an embodiment of the present invention, in which a 1/4 wavelength plate is used as the polarization state change means. FIG. 3 is a diagram showing an embodiment of the present invention using a two-wavelength plate, and FIG. 4 is a diagram showing an embodiment of the present invention using a polarizing plate as a polarization state changing means. FIG. 3 is an example diagram illustrating a device equipped with means for storing the amount of distortion of the measuring optical system in a memory and correcting the measured value of the object to be measured. 1 is a laser light source, 2 is a collimator lens, 3 is a beam splitter, 4, 41, 42 are means for changing the polarization state, 50, 51, 52 are objects to be measured, 6 is a reference reflector, 7 is an analyzer, 8 is the lens,
9 is an imaging device, 90 is a control circuit, 91 is a memory,
120 and 121 are optical path length changing means (phase modulation means).

Claims (1)

[Scope of Claims] 1. Light emitted from a coherent light source is divided into two parts, one part is irradiated onto an object to be measured, and the light transmitted or reflected from the object to be measured is the same as the other divided reference light. In an interference method in which light is guided into an optical path, that is, an interference pattern generation optical path, and interference is caused, the polarization state of at least one of the two divided beams is changed to make the polarization states of both lights in the interference pattern generation optical path different; An interference method characterized by making the light intensity of both lights substantially constant after passing through an analyzer provided in an optical path for generating an interference pattern. 2. A high-precision measurement is performed by using the result of interference measurement using a high-precision plane mirror or spherical mirror as the object to be measured as a correction value, and correcting the value obtained by interferometrically measuring an arbitrary object to be measured with the correction value. An interference method according to claim 1. 3. A coherent light source, a means for dividing the light emitted from the coherent light source into two, and an object to be measured on one of the two divided optical paths, and the light reflected or transmitted through the object to be measured and the means for dividing the light into two. In an interference device that guides the other reference beam to the same interference pattern generation optical path to generate interference fringes, a means for changing the polarization state of at least one of the two divided beams is provided in the interference pattern generation optical path. An interference device characterized in that the polarization states of both lights are made to differ, a variable analyzer is provided in an interference pattern generating optical path, and the light intensity of both lights is made substantially constant after passing through the analyzer. 4. The interference device according to claim 3, further comprising a means for changing the polarization state of the two lights in the interference pattern generation optical path so that the polarization states of the two lights are mutually orthogonal linearly polarized light. 5. The interference device according to claim 4, characterized in that one of the two divided optical paths is provided with means for changing the polarization state consisting of a 1/4 wavelength plate or a 1/2 wavelength plate. 6 Equipped with a memory for storing the interference measurement results of the object to be measured, and storing the measurement results using a high-precision plane mirror or spherical mirror as the object to be measured in the memory as correction values, and interferometrically measuring any object to be measured. 4. The interference device according to claim 3, wherein the interference result is corrected using a correction value stored in the memory.