JP2864171B2 - Observation optical system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for forming a clear interference image by forming interference fringes.

[0002]

2. Description of the Related Art The interferometer considered here has, as necessary components, an optical surface for separating a light beam from a light source into a reference light beam and a measurement light beam, and an optical surface for matching the reference light beam and the length measuring light beam. What is necessary is just to have an optical system for projecting and observing the interference fringes with a projection lens and an imaging lens.
Regardless of the type of interferometer such as Fizeau type, Mach-Zehnder type, Twyman Green type, etc.
A description will be given using a Fizeau interferometer as an example.

FIG. 10 shows an optical system of a general Fizeau interferometer. The incident light flux enters the Fizeau reference lens 4 through the condenser lens 1, the half mirror 2, and the collimator lens 3 as shown by a solid line in the figure, and a part of the light flux enters the Fizeau reference surface 5 of the Fizeau reference lens 4. The light is reflected, and a part of the light is incident on the test surface 6, where it is reflected. The return light from the Fizeau reference surface 5 and the return light from the test surface 6 interfere with each other, pass through the Fizeau reference lens 4 and the collimator 3 again, are converted into a parallel light beam by the projection lens 7, and the rough surface of the opal glass 8 Projected above. The above is the optical path of the zero-order light that is not diffracted from the return light from the test surface 6.

[0004] However, the return light from the test surface 6 generates diffraction to be blocked by dust and dirt on the test surface, the outer edge of the optical surface, etc., and becomes a light beam having a spread as shown by a dotted line in the figure. . If this light flux is not imaged on the opal glass 8 as the projection surface, including the 0th-order light, high-frequency components are lost from the projected interference fringes, resulting in a blurred interference image. Interferes with the zero-order light and bends interference fringes. Therefore, as shown in FIG. 10, when the opal glass 8 is moved in the optical axis direction and the point on the test surface 6 and the projected image point on the opal glass 8 are placed in an object-image relationship,
Since the diffracted light on the test surface is projected on the same point as the zero-order light by the Fizeau reference lens 4, collimator 3, and projection lens 7, clear interference fringes can be obtained. Test surface 6
Since the position on the object side with respect to the Fizeau reference lens 4, the collimator 3, and the projection lens 7 always changes depending on the measurement shape, it is necessary to adjust the position of the opal glass 8 each time. This is the focus of the interferometer.

[0005] In the prior art, an opal glass 8 on a disc is used.
Is rotated to temporally reduce the coherency of the projected image, and at the same time, also serves as a spatial low-pass filter, reducing the background noise of the interference image due to dust and dirt to make the interference fringe image appear clear . The interference fringe image diffused by the opal glass is formed on a CCD-TV 10 as an optical image detector by an imaging lens 9 and can be monitored. The feature of this conventional method is that the interference fringes are once projected on the opal glass,
The point is that a real image is formed by the image forming lens and the image is formed on the CCD-TV. Focusing is performed by moving the imaging lens and the opal glass together.

[0006]

The reason for using an opal glass in the interferometer having the above configuration is as follows.
CCD-T
This is to facilitate the processing of a still image of interference fringes obtained from V and to prevent the occurrence of moire fringes due to the pixel pitch of the CCD and background noise. However, in the phase interferometry, which has been generalized in recent years as a method of processing interference signals with higher precision, DC noise such as background noise has no effect on the processing accuracy, and the number of pixels of the CCD also jumps. Since the band of the moiré fringes to be generated has a high frequency, the background noise that is out of focus has almost no high-frequency component, so that the moire fringe problem does not occur. On the contrary, the opal glass impairs the spatial high-frequency component of the interference fringe image, and also causes a loss in light quantity. Opal glass is a parallel flat plate, so that the reflection on the front and back surfaces overlaps the observation light beam, and the rough surface is a diffusion surface that has no effect even if an antireflection coating is applied, so that the reflection on the front and back surfaces reaches several percent. Are superimposed on the observation light beam and reduce the contrast of interference fringes. Furthermore, the opal glass is rotated in order to reduce coherency in order to reduce noise on the rough surface of the opal glass. Occurs. This flare is averaged by the rotation of the opal glass. However, unless the error of the perpendicularity is considerably suppressed, the flare of the projected interference fringe is not averaged at about 1000 rpm. This means that when the interference fringes are moved and the phase is specified with high accuracy using the phase interference method, the measurement accuracy is reduced by affecting the phase as an error. Rather, as the precision of the optical element under test becomes ultra-precise, interferometers are required to have a measured value closer to a true value and an accurate lateral resolution. In other words, there is no longer any reason to use opal glass to endure the above drawbacks.

Further, the observation magnification is changed by using a zoom lens as an image forming lens and changing the zoom magnification. However, the zoom lens generally has large distortion and the magnification is set to be close to 6 times. And distortion is several percent
As a result, the interference fringe image for observation and breaking is distorted, causing an error in the broken value of the interference fringes.

In order to strictly keep the observation magnification constant during focusing, an opal glass,
It is necessary to move the imaging lens and the CCD-TV integrally in the optical axis direction. If they are not integrated, the opal glass and the CCD-TV surface deviate from the image relationship with the imaging lens due to focusing, and out-of-focus and fluctuations in observation magnification occur in this part, thus reducing the amount of observation information. This is because an error occurs in the analysis result of interference fringes. However, in the conventional method, only the opal glass and the imaging lens are integrated and moved in the optical axis direction, and the CCD-TV is fixed. For this reason, the distance between the CCD-TV and the imaging lens is increased (at least 50 cm), and the depth of focus of the imaging lens on the CCD-TV side is increased. The problem is solved by setting the level to a level that does not cause a problem, but the observation optical system becomes extremely large, and the size of the entire interferometer is increased. The present invention relates to a focusing optics and a variable power optics of an interferometer that overcomes all of these disadvantages of the conventional method.

[0009]

SUMMARY OF THE INVENTION An observation optical system having a focusing optical system of an interferometer according to the present invention is an optical system for focusing an interference fringe in the observation system on a position of a test surface, and shares a focal position with a collimator. A projection lens for collimating the zero-order light from the test surface of the interference fringes, and an imaging lens for projecting the light beam onto the optical image detector, and the imaging lens and the optical image detector are integrated into the optical axis direction. The above-mentioned problem has been solved by forming diffracted light of the first and subsequent orders at the zero-order light position by changing the distance from the projection lens.

In this focusing optical system, the distance between the optical image detector and the imaging lens is fixed to twice the rear focal length f of the imaging lens, and the distance between the projection lens and the optical image detector is fixed. The right-angle prism is extracted and the optical axis is turned back, and the right-angle prism is moved forward and backward with respect to the projection lens in the optical axis direction to perform focusing. Further, the right-angle prism described above is interposed between the optical image detector and the imaging lens. It is desirable to provide a right-angle prism having a folded surface orthogonal to the folded surface.

Further, it is preferable to dispose a beam expander or a beam compressor on the rear side of the projection lens to perform zooming. At this time, the beam expander and the beam compressor having different magnifications are connected to a modularized optical system. It is preferable that a plurality of beam expanders or beam compressors having different magnifications are circumferentially fixed to the rotating plate, and that the observation magnification can be changed by rotating the rotating plate. Also, the sine condition error of each optical element is kept at 0.1% or less.

[0012]

The major feature of this method is that no opal glass or the like is used, and the interference fringes are not realized.
Thereby, observation and breaking can be performed without dropping the high-frequency component of the interference fringe by the opal glass described above. In addition, there is no loss of light amount, and there is no superposition of unnecessary light due to reflection on the front and back surfaces, and it is possible to observe interference fringes with extremely high sharpness.

[0013]

[Embodiments] Each of the above-mentioned structures will be described in detail with reference to embodiments. FIG. 1 shows this embodiment in a Fizeau interferometer. The light flux from the light source is guided to the collimator 3 through the focusing lens 1 and the half prism 2, and interference fringes are generated by the return light from the reference surface 5 and the test surface 6 of the Fizeau reference lens 4. This 0-order return light beam passes through the half prism 2 and is converted into a parallel light beam by the projection lens 7. This optical path is similar to that of the conventional interferometer shown in FIG. In the present invention, the CCD-
A defocused image of zero-order light is projected on the TV 10. Since the interference fringes are spatially present as an intensity distribution in a plane perpendicular to the optical axis direction, the CCD-TV plane may be brought to any position other than the focal point of the imaging lens for the zero-order light. Here, when the distance between the CCD-TV surface and the imaging lens is fixed and integrated, and this is moved back and forth in the optical axis direction, there is a position where the diffracted light forms an image at the same position as the 0th-order light. Focusing can be achieved by adjusting the focus. That is, the aerial image of the diffracted light by the projection lens 7 is converted by the imaging lens 9 into the CCD-TV 1
Image on 0.

When the distance between the imaging surface of the CCD-TV and the imaging lens is fixed to a position twice as long as the rear focal length of the imaging lens, the diffraction image by the projection lens and the CCD-focus at the in-focus position
The size of the projected image on the image plane of the TV becomes equal, and an interference fringe image having the same size as the exit pupil diameter of the projection lens is obtained. At this time, since the front and rear groups of the imaging lens can be made exactly the same element as shown in FIG. 2, distortion and sine condition error can be suppressed low, so that the cost of the imaging lens can be greatly reduced. it can.

FIG. 3 shows an embodiment in which a right-angle prism 11 is inserted between the projection lens 7 and the imaging lens 9. For convenience, a plane parallel to the plane of FIG. 3 is referred to as a folded surface of the right-angle prism in FIG. The optical system up to the projection lens is omitted because it is the same. With the right-angle prism 11, the apparent object-image distance of the imaging lens can be reduced to about ／ to 上 apparently. In addition, by moving the right-angle prism 11 in the optical axis direction of the projection lens, it is possible to focus while the imaging lens 9 and the CCD-TV 10 are fixed. Moreover, since the right-angle prism is a single component, focusing can be performed with a smaller driving force and a smaller driving mechanism than when the imaging lens and the CCD-TV are integrally moved. Further, since the optical path is turned back, the actual focusing optical path length changes by twice the moving distance of the right-angle prism, and a focusing effect can be obtained quickly and with a small movement. As in the case where is not used, the observation magnification does not change due to focusing. Since the right angle prism does not use the vicinity of the apex at all, the right angle portion may be cut off as shown in FIG. 4 to provide a more compact focusing optical system.

FIG. 5 shows an embodiment in which the right-angle prism 12 is inserted between the imaging lens and the CCD-TV, and the optical path is further turned back. Thus, the object-image distance of the imaging lens is
Apparently, it can be reduced to almost half. Further, when the right-angle prism 12 is made exactly the same as the right-angle prism 11 inserted between the projection lens and the imaging lens, distortion and aberration can be reduced even if the front and rear groups including the right-angle prism of the imaging lens are the same element. Since the sine condition error can be kept low, the cost can be greatly reduced.

Moreover, when the planes containing the folded light beams of the two right-angle bristles are arranged as shown in FIG. 6 so as to be at right angles, the right-angle prism is shifted in the direction on the folded surface perpendicular to the respective incident light beams. And CCD-T
The interference images on V can be moved independently of each other, for example, vertically and horizontally. Thereby, the final optical axis alignment of the CCD-TV and the imaging optical system is
It will be very easy to do.

In the embodiment shown in FIG. 7, a beam expander 13 is inserted behind the projection lens to increase the observation magnification. The optical system thereafter may or may not include the above-described right-angle prism and focus on it. The beam expander receives a parallel light beam, expands its diameter, and emits the light beam as a parallel light beam. Therefore, the eccentricity due to the shift of the optical system does not affect the image quality at all, and is a very strong optical system against tilt. Moreover, since it is designed as a single-magnification lens, distortion and sine condition errors when combined with a projection lens are reduced compared to conventional zoom lenses.
It can be easily designed small with a small number of lenses,
Unnecessary light due to surface reflection of the lens is smaller than that of the zoom lens, and high sharpness and low distortion can be easily maintained.

By providing a plurality of such beam expanders and providing a mechanism capable of exchanging them, zooming can be performed while maintaining a higher-quality interference image than in the case of using a conventional zoom lens. An example is shown in FIG. The magnification of the observation magnification is performed by rotating the beam expanders 13, 13 ', 13 "and the like arranged on the circumference of the rotating plate 14 by the rotating shaft 15. The rotating accuracy and the stopping angle accuracy of the rotating plate 14 are slightly increased. At worst, it has little effect on the observed interference image for the reasons described above.

Not only in the beam expander but also in the beam compressor, the incoming and outgoing light beams are parallel light,
Since the influence of the shift and tilt on the image quality is insensitive, it is possible to change the observation magnification using the beam compressor without deteriorating the image quality.

When a large number of beam expanders and beam compressors having different magnifications are used as in the embodiment shown in FIG. 8, there is a disadvantage that this variable power system becomes expensive. Therefore,
If a high magnification is performed with a combination of low magnifications, the magnification can be changed over a wide range without increasing the number of types of lens systems, so that the cost can be reduced due to mass production effects. In the example of FIG. 9, using a double beam expander 13, this is connected to a series as a module, and attached to a rotating plate, and x1 (no beam expander), x2 (13,
An example in which four observation magnifications of x4 (two 13 'beam expanders), x8 (three 13 "beam expanders) and x8 (three 13" beam expanders) are obtained. Since it can be realized only by making six expanders at a time, the price is lower than making three types of beam expanders one by one.In this way, cascade of modularized beam expanders or beam compressors It can be used because the optical system of the beam expander and beam compressor is extremely insensitive to tilt and shift, and because it has a single magnification, each aberration can be designed to be low.

[0022]

As described above, in the present invention, since the interference fringes are not realized as in the conventional art without using an opal glass or the like, the high-frequency components of the interference fringes due to the opal glass can be observed without any loss. And can be broken.
In addition, there is no loss of light amount, and there is no superposition of unnecessary light due to reflection on the front and back surfaces, and it is possible to observe interference fringes with extremely high sharpness. Although the observation optical system, which was conventionally easy to increase in size, becomes compact, it was possible to further reduce the size by inserting a prism in the optical path.

[Brief description of the drawings]

FIG. 1 is an optical path diagram of an embodiment when the present invention is applied to a Fizeau interferometer.

FIG. 2 is an optical arrangement diagram showing an example of a configuration of an imaging lens.

FIG. 3 is an optical arrangement diagram of an observation optical system in which a right-angle prism is inserted between a projection lens and an imaging lens.

FIG. 4 is a plan view showing an example of the shape of a right-angle prism.

FIG. 5 is an optical arrangement diagram of an embodiment in which a right-angle prism is also inserted in the imaging optical system.

FIG. 6 is an optical arrangement diagram of another embodiment in which a right-angle prism is also inserted in the imaging optical system.

FIG. 7 is an optical arrangement diagram of an embodiment in which an observation magnification is increased by a beam expander.

FIG. 8 is an optical arrangement diagram of an embodiment in which magnification is changed by a plurality of beam expanders.

FIG. 9 is a conceptual diagram showing an embodiment in which a beam expander is modularized.

FIG. 10 is an optical path diagram of a conventional example of a Fizeau interferometer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Focal lens 2 Half prism 3 Collimator 4 Fizeau reference lens 5 Fizeau reference surface 6 Test surface 7 Projection lens 8 Opal glass 9 Imaging lens 10 CCD-TV 11, 12 Right angle prism 13 Beam expander 14 Rotating plate 15 Rotation axis

Claims (8)

(57) [Claims] In an optical system for focusing an interference fringe in an observation system on a position of a test surface, a projection lens that shares a focus position with a collimator and collimates zero-order light of the interference fringe from the test surface, and a light beam thereof Is formed on the optical image detector, and the image forming lens and the optical image detector are integrally moved back and forth in the optical axis direction, and the first and subsequent diffracted lights are changed to the 0th order by changing the distance from the projection lens. An observation optical system having a focusing optical system for forming an image at a light position. 2. An observation optical system having a focusing optical system according to claim 1, wherein the distance between the optical image detector and the imaging lens is fixed to twice the rear focal length f of the imaging lens. 3. A projection lens according to claim 1, wherein a right-angle prism is inserted between the projection lens and the optical image detector to turn the optical axis back, and the right-angle prism is moved back and forth with respect to the projection lens in the optical axis direction. Optical system having a focusing optical system characterized in that focusing is performed by 4. The interference according to claim 1, wherein a right-angle prism having a folded surface orthogonal to the folded surface of the right-angle prism is provided between the optical image detector and the imaging lens. Observation optical system with focusing optical system 5. The observation apparatus according to claim 1, wherein a variable power optical system of the interferometer is provided by arranging a beam expander or a beam compressor behind the projection lens. Optical system 6. The observation optical system according to claim 5, wherein the beam expander and the beam compressor having different magnifications are configured by a combination of modularized optical systems. 7. The method according to claim 5, wherein a plurality of beam expanders or beam compressors having different magnifications are fixed to a rotating plate in a circumferential shape, and the observation magnification can be changed by rotating the rotating plate. Observation optical system with variable magnification optical system of interferometer 8. An observation optical system of an interferometer according to claim 1, wherein a sine condition error of each optical element is set to 0.1% or less.