JPH11194011A - Interference apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an interference apparatus by which the face shape of an object to be measured can be measured with high accuracy, by a method wherein a background component which is detected in advance is removed from interference light waves. SOLUTION: A shielding plate 15a is changed over to the outside of an optical path, and a shielding plate 15b is changed over to the inside of the optical path. Only measuring light waves which contain a background component (interference fringes or the like generated due to dust particles stuck to an object to be measured and to an optical element, due to an irregularity in a coating or the like or due to the optical element other than the object to be measured) are detected by a CCD camera 11 so as to be stored in a frame memory 13b for background. Then, the shielding plate 15a is changed over to the inside of the optical path, and the shielding plate 15b is changed over to the outside of the optical path. Only reference light waves which contain the background component are detected by the camera 11 so as to be stored in the memory 13b. Then, both shielding plates 15a, 15b are changed over to the outside of the optical path, and interference light waves are photographed by the camera 11 so as to be stored in a frame memory 13a for interference light waves. Then, the interference light waves and the detected background component are computed between frames. Information on the light-wave face of the object to be measured from which the background component is removed by a subtracter 16, is adjusted by a level regulator 17 so as to be output to a monitor 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer for measuring the shape and refractive index distribution of an optical component using an interferometer.

[0002]

2. Description of the Related Art An interferometer detects a wavefront shape of light as a pattern of interference fringes by utilizing a light interference phenomenon.
Precise measurement of optical component shapes such as lenses and mirrors,
As a method for precisely measuring the refractive index distribution of glass, it is widely used for industrial purposes. A fringe scan interferometer is known as an interferometer capable of measuring the shape of a lens with relatively high accuracy using an interferometer.

FIG. 3 is a schematic view of a main part of an optical system of a conventional fringe scan interference device.

In FIG. 1, reference numeral 1 denotes a laser light source that emits a coherent light beam, 2 denotes a beam expander that expands the diameter of the incident light beam and emits the light beam, and 3 denotes a reflection according to the polarization state of the incident light beam. Alternatively, the polarizing beam splitters 4a and 4b that transmit light are λ / 4 plates that convert between linearly polarized light and circularly polarized light.
Is a collimator lens to be incident on the object, and 6 is an object to be measured having a concave surface. Here, the surface shape 6a is measured. Reference numeral 7 denotes a reference plane mirror for generating a reference light wave, 8 denotes a piezoelectric element serving as means for moving the reference plane mirror 7 in the optical axis direction, and 9 denotes a specific polarization component among light waves having two orthogonal linear polarization components. Is a polarizing plate having a 45 ° azimuth for extracting and interfering light, 10 is a condensing lens that plays a role of capturing the interference light wave into the CCD camera 11, and 12 is an image captured by the CCD camera 11 (an interference image). ) Monitor to display. 13 is a frame memory for storing light wavefront information of the interference lightwave, and 14 is a computer.

A light beam emitted from a laser 1 shown in FIG. 3 is expanded by a beam expander 2 and then divided by a polarization beam splitter 3 into reflected light and transmitted light. The reflected light changes from linearly polarized light to circularly polarized light by passing through the λ / 4 plate 4a, enters the DUT 6 via the collimator lens 5 that creates a reference spherical wave, and is reflected as a measurement light wave. This measurement light wave has surface shape information 6a of the DUT 6. The reflected measurement light wave returns to the original optical path,
The light again passes through the λ / 4 plate 4a, becomes linearly polarized light rotated by 90 ° as compared with the outgoing light, and is transmitted by the polarization beam splitter 3 this time. The other transmitted light changes from linearly polarized light to circularly polarized light through the λ / 4 plate 4b, enters the reference plane mirror 7, and is reflected as a reference light wave. The reflected reference light wave passes through the λ / 4 plate 4b again, becomes linearly polarized light rotated by 90 ° as compared with the normal direction, and is reflected by the polarization beam splitter 3 this time. The measuring light wave and the reference light wave overlap again by the polarizing beam splitter 3 and pass through the polarizing plate 9 in the 45 ° direction to become an interference light wave. This interference light wave is imaged by the CCD camera 11 via the condenser lens 10, and the state of the interference fringes can be observed on the monitor 12. The object 6 is moved and positioned so as to be in a null state while observing the interference fringes projected on the monitor 12. In this state, C
The light wavefront information of the interference lightwave captured from the CD camera 11 is stored in the frame memory 13. Next, using a piezoelectric element,
The reference plane mirror 7 is moved in the optical axis direction by λ / 4, and is similarly recorded in the frame memory 13. Thereafter, the same operation is performed once again, and the light wavefront information for a total of three times is stored in the frame memory 1.
Record it in 3.

The phases of the three pieces of wavefront information can be calculated by using a well-known bucket method.

V1 = V0 (1 + cosφ) Expression 1 V2 = V0 (1−sinφ) Expression 2 V3 = V0 (1−cosφ) Expression 3 When these expressions are expanded, φ = tan ^{− 1} ((V3−V2) / (V1−V2))...

Therefore, if the phase is calculated from Equation 4 for each measurement point of the interference light wave and is restored as a surface, the surface shape information 6a of the measured object can be measured.

As a method of identifying a background area, Japanese Patent Application Laid-Open Nos.
No. 05.

Here, a brief description will be given by taking JP-A-2-12001 as an example.

FIG. 4 is a flowchart for identifying a background area. The interference fringes are generated by a fringe scan interference device. First, a plurality of light wavefront information is detected by the method described above and stored in the frame memory.
(Three pieces of light wavefront information are detected in the above-described example, but it is better to detect as many light wavefront information as possible.) Next, the maximum value and the minimum value of the lightwave front information are detected for each pixel, and S = maximum. Value−minimum value H = ((maximum value + minimum value) / 2) / X (X = coefficient)

The magnitudes of S and H are compared for each pixel, and S <
If H is true, the pixel is identified as a signal area; if false, the pixel is identified as a background area.

According to such a method, a numerical value based on the offset level is compared with the amount of change in the sinusoidal signal to discriminate the signal region from the background region. There are features that can be identified without errors.

[0014]

SUMMARY OF THE INVENTION The above-described conventional interference apparatus has an object to distinguish a signal area and a background area in a measurement area. However, interference light waves are usually detected by a CCD camera and observed directly on a monitor.
The signal region and the background region could not be clearly distinguished, which could adversely affect the alignment of the interference light waves. Also, components that do not depend on the optical wavefront information of the device under test even within the signal area, such as the device under test and a reference plane mirror,
Dust and coating irregularities on the optical element surface, irregularities in laser power, interference fringes caused by optical elements other than the measured object, etc. are also detected as light wavefront information of the measured object, and phase components are measured. Was affecting.

According to the present invention, when measuring the phase distribution of light wavefront information of an object to be measured, a background component which has an adverse effect is removed from the lightwave front information, whereby the surface shape of the object to be measured is measured with high accuracy. The purpose of the present invention is to provide an interference device capable of performing the above-described operations.

[0016]

An interference device according to the present invention comprises:
(1-1) In an interferometer that detects an interference light wave between a measurement light wave including light wavefront information of an object to be measured and a reference lightwave and measures a phase distribution of the light wavefront information of the object to be measured, a background that has been detected in advance. It is characterized in that components are removed from the interference light wave.

In particular, (1-1-1) the above-mentioned background component is removed by detecting a measurement lightwave and a reference lightwave independently and storing them in a frame memory and a frame storing an interference lightwave detected in the measurement. Computing between frames using a memory and a subtractor.

(1-1-2) The interferometer has shielding means for shielding in the non-common optical path, and the shielding means blocks either the measuring light wave or the reference light wave. .

(1-1-3) The interference device detects a background component by greatly tilting one of an object to be measured and a reference plane mirror forming a reference light source, and greatly shifting the optical path of the measurement light wave or the reference light wave. That you are doing.

(1-1-4) The wavefront information from which the background component has been removed is characterized in that the level is adjusted and output to a monitor.

As a result, only the signal component of the light wavefront information of the object to be measured can be observed on the monitor, the alignment of the interference fringes can be easily performed, and the background component causing an error can be removed, so that the measurement can be performed with high accuracy. Is possible.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic view of a main part of a first embodiment of the present invention. This embodiment is significantly different from the conventional example shown in FIG. 3 in that a shielding function is provided in each optical path in order to detect the reference light wave and the measurement light wave independently.
A background memory and a frame memory for an interference light wave are provided, a background component of the interference light wave is removed using a subtractor, the level is adjusted, and then output to a monitor and a computer.

In FIG. 1, reference numeral 1 denotes a laser light source that emits a coherent light beam, 2 denotes a beam expander that expands the diameter of the incident light beam and emits the light beam, and 3 denotes a reflection according to the polarization state of the incident light beam. Alternatively, a polarizing beam splitter for transmitting light, 15a and 15b are shielding plates having a role of blocking light waves, 4a and 4b are λ / 4 plates for converting linearly polarized light and circularly polarized light, and 5 is for condensing an incident light beam to be described later. DUT 6
The collimator lens 6 is a concave object to be measured. Here, the surface shape is measured. Reference numeral 7 denotes a reference plane mirror for generating a reference light wave, 8 denotes a piezoelectric element serving as a means for moving the reference plane mirror 7 in the optical axis direction, and 9 denotes a specific polarization component among light waves having two orthogonal linear polarization components. Is a 45 ° azimuth polarizer for extracting and interfering light, 10 is a condensing lens which plays a role of causing the interference light wave to be captured by the CCD camera 11, and 12 is a monitor for displaying an image captured by the CCD camera 11. It is. 13a and 13b are frame memories for storing light wavefront information of the interference lightwave and the background component;
Is a frame memory for storing a plurality of interference light waves taken out of phase. Here, the background component has already been removed. Reference numeral 16 denotes a subtractor that removes a background component from the interference light wave, and reference numeral 17 denotes a level adjuster that adjusts the level of the background-removed light wavefront information output from the subtractor 16 and outputs the information to the monitor 12. Reference numeral 14 denotes a computer that calculates a phase distribution of the DUT from a plurality of pieces of light wavefront information from which background has been removed.

The light beam emitted from the laser light source 1 shown in FIG. 1 is expanded by a beam expander 2 and then divided by a polarization beam splitter 3 into reflected light and transmitted light. The reflected light changes from linearly polarized light to circularly polarized light by passing through the λ / 4 plate 4a, enters the DUT 6 via the collimator lens 5 that creates a reference spherical wave, and is reflected as a measurement light wave. (At this time, the shielding plate 15a is out of the optical path.)
This measurement light wave has surface shape information 6a of the DUT 6. The reflected measurement light wave returns to the original optical path, passes through the λ / 4 plate 4a again, becomes linearly polarized light rotated by 90 ° as compared with the outgoing light, and is transmitted through the polarization beam splitter 3 this time. The other transmitted light is a λ / 4 plate 4
The light passes through b, changes from linearly polarized light to circularly polarized light, enters the reference plane mirror 7, and is reflected as a reference light wave. (The shielding plate 15b at this time is in a state outside the optical path.) The reflected reference light wave passes through the λ / 4 plate 4b again, and is 90 times larger than the normal light wave.
と な り The light becomes the rotated linearly polarized light, and is reflected by the polarization beam splitter 3 this time. The measuring light wave and the reference light wave overlap again by the polarizing beam splitter 3 and pass through the polarizing plate 9 in the 45 ° direction to become an interference light wave. This interference light wave is imaged by the CCD camera 11 via the condenser lens 10.

However, since the interference light wave thus captured is detected by a CCD camera and directly observed by a monitor, the signal region and the background region cannot be clearly distinguished, which may adversely affect the alignment of interference fringes. Was. Even in the signal area, components that do not depend on the light wavefront information of the device under test, such as the device under test, the reference plane mirror, dust and coating on the surface of the optical element, unevenness in the laser power, and other than the device under test. The interference fringes generated by the optical element are detected together as the light wavefront information of the object to be measured, which affects the measurement of the phase distribution. Therefore, when measuring the phase distribution of the light wavefront information of the device under test, high-accuracy measurement can be realized by removing background components that adversely affect the wavefront information.

First, a method of detecting a background component will be described. The shielding plate 15b shown in FIG. 1 is switched into the optical path, and the shielding plate 15a is switched out of the optical path. CCD in this state
The light wave taken in from the camera 11 is only the measurement light wave. Accordingly, the light wavefront information of the measurement lightwave at this time includes dust and unevenness of the coating on the measured object and the optical element, unevenness of the laser power, interference fringes generated by optical elements other than the measured object, and the like. The measurement lightwave detected by the CCD camera 11 is stored in the background frame memory 13b. Next, the shielding plate 15a is moved into the optical path,
The shielding plate 15b is switched out of the optical path. CCD in this state
The light wave taken in from the camera 11 is only the reference light wave. Therefore, the light wavefront information of the reference lightwave at this time includes dust and unevenness of the coating on the reference plane mirror and the optical element, unevenness of the laser power, interference fringes generated by the optical element other than the measured object, and the like. CCD camera 11
The measurement lightwave detected in step 3 is also stored in the background frame memory 13b. The light wavefront information of the measurement lightwave and the reference lightwave detected in this way are background components, respectively, and when the lightwavefront information of the device under test is detected later, the background component detected here is removed from the interference light wave. become.

After detecting the background component, light wavefront information of the device under test is detected. That is, the shielding plate 1 shown in FIG.
5a and 15b are switched out of the optical path, the interference light wave is photographed by the CCD camera 11, and the interference light wave frame memory 13a
To be stored. Thereafter, the subtractor 16 compares the interference light wave captured earlier with the background frame memory 13.
The detected background component stored in b is calculated between frames. At this time, if parallel image processing is performed, arithmetic processing can be performed in real time. Thereafter, the light wavefront information of the device under test excluding the background component is adjusted to the input level of the monitor 12 by the level adjuster 17,
Output to monitor. Therefore, the light wavefront information of the device under test from which the background component has been removed can be observed from the monitor 12 in real time. The object 6 is moved and positioned so as to be in a null state while observing the interference fringes projected on the monitor 12. In this state, the light wavefront information of the DUT from which the background component output from the subtractor 13 has been removed is stored in the frame memory 13. Next, the reference plane mirror 7 is moved in the optical axis direction by λ / 4 using the piezoelectric element 8, and is similarly recorded in the frame memory 13. Thereafter, the same operation is performed once again, and the light wavefront information for a total of three times is recorded in the frame memory 13.

The phases of these three pieces of wavefront information can be calculated by using a well-known bucket method.

V1 = V0 (1 + cosφ) V2 = V0 (1−sinφ) V3 = V0 (1−cosφ) When these equations are expanded, φ = tan ⁻¹ ((V3−V2) / (V1−V2)). Become.

Accordingly, the phase shape information 6a of the object to be measured can be measured by calculating the phase for each measurement point of the interference light wave from Equation 4 and restoring it as a plane.

As described above, in this embodiment, since only the signal component of the light wavefront information of the device under test can be observed on the monitor, the alignment of the interference lightwaves is facilitated, and the background component which is an error component is removed. Therefore, highly accurate measurement is possible.

Next, a second embodiment of the present invention will be described. In the present embodiment, the detection of the background component may be performed as follows.

In the first embodiment described above, when detecting the measurement lightwave or the reference lightwave independently, the shielding plate is inserted into the optical path so as to block the reference lightwave or the measurement lightwave. However, the detection of the measurement lightwave or the reference lightwave
This can also be realized by greatly shifting the optical path of one of the light waves, which can be achieved by inclining the reference plane mirror or the object under test. This method has an advantage that it is not necessary to provide a shielding function for blocking light waves.

[0034]

According to the present invention, there is provided an interferometer for interfering a measurement light wave including light wavefront information of an object to be measured with a reference lightwave, detecting the generated interference lightwave, and measuring the phase distribution of the lightwavefront information of the object to be measured. In, the background component is grasped by previously detecting the measurement lightwave and the reference lightwave independently, and the background component is removed from the interference lightwave detected at the time of measurement. As a result, only the signal components of the light wavefront information of the device under test can be observed on the monitor, making it easier to align the interference fringes. Also, it is possible to remove background components that cause errors, thus enabling highly accurate measurement. Become.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a main part of a first embodiment of the present invention.

FIG. 2 is a flowchart of a method for removing background noise according to the present invention;

FIG. 3 is a schematic view of a main part of a general fringe scan interference device.

FIG. 4 is a flowchart of a conventional background area identification method.

[Explanation of symbols]

 Reference Signs List 1 laser 2 beam expander 3 polarizing beam splitter 4a, 4bλ / 4 plate 5 collimator lens 6 object to be measured (concave lens) 6a surface shape of object to be measured 7 reference plane mirror 8 piezoelectric element 9 polarizing plate 10 condensing lens 11 CCD Camera 12 Monitor 13, 13a, 13b Frame memory 14 Computer 15a, 15b shielding plate 16 Subtractor 17 Level adjuster

Claims (5)

[Claims] 1. An interferometer for detecting an interference light wave of a measurement light wave including light wavefront information of an object to be measured and a reference light wave and measuring a phase distribution of the light wavefront information of the object to be measured, the background having been detected in advance. An interference device for removing a component from an interference light wave. 2. The method according to claim 1, wherein the background component is removed by using a frame memory that independently detects and stores the measurement lightwave and the reference lightwave, a frame memory that stores the interference lightwave detected during the measurement, and a subtractor. 2. The interference apparatus according to claim 1, wherein the calculation is performed between frames. 3. The interferometer according to claim 1, further comprising shielding means for shielding in the non-common optical path, wherein the shielding means blocks one of the measuring lightwave and the reference lightwave. Item 2. The interference device according to Item 2. 4. The interferometer detects a background component by greatly tilting one of an object to be measured and a reference plane mirror forming a reference light source, and largely shifting an optical path of a measurement light wave or a reference light wave. The interference device according to claim 2, wherein: 5. The interference apparatus according to claim 1, wherein the light wavefront information from which the background component has been removed is adjusted in level and output to a monitor.