JPH051968A - Light heterodyne interference device - Google Patents

Abstract

PURPOSE:To measure the shape of a measured object with high precision by splitting the synthesized measurement light wave and reference light wave into two with a polarizing optical element, detecting the signals obtained by interference as the measurement signal and reference signal respectively, and detecting the phase change of the measurement light wave. CONSTITUTION:The first light wave and the second light wave emitted from a light source means 1 pass through a beam expander 3 and are fed to a beam splitter 4. The first light wave passes through a lambda/4-plate 5b and a lens 7 and is reflected by a measured object 8 and returned to the splitter 4 as the reference light wave to be synthesized with the measurement light wave. The synthesized light wave is vectorially decomposed with the vibration plane via a polarizing optical element 9 and sent out as the interference light wave, and the beam signal can be extracted. These signals are detected by detecting means 11, 13 respectively, and the phase difference is detected by a phase measuring circuit 14. The two-dimensional distribution is obtained by the above work, and the shape of the measured object 8 can be measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical heterodyne interferometer for measuring the shape of an object to be measured such as an optical lens or a mirror by detecting an optical phase change of an incident light wave associated with the shape. The present invention relates to an optical heterodyne interferometer.

[0002]

2. Description of the Related Art Conventionally, an optical heterodyne interferometer is known as an apparatus capable of relatively accurately measuring the shape of optical members such as lenses and mirrors by utilizing light wave interference. For example, U.S. Pat. No. 4,188,122 and Applied Op.
tics Vol.19.No.1 (1980) P.154-P.160.

FIG. 4 is a schematic view of a main part of an optical system of a conventional optical heterodyne interferometer.

A laser 10 such as a Kr ion laser is shown in FIG.
The light wave emitted from the beam No. 1 is reflected by the mirror M1 and is split into two by the beam splitter 102 into reflected light and transmitted light. Of these, the transmitted light (first light wave) passes through the black cell 105a, the mirror M2, and the λ / 2 plate 104, and the reflected light (second light wave) passes through the black cell 105b and the mirror M3, and then at the beam splitter 103. The waves are being combined.

At this time, the first light wave that has passed through the black cell 105a undergoes a frequency shift of frequency f1, and the λ / 2 plate 1
The polarization plane is rotated through 90 degrees through 04. The second light wave that has passed through the black cell 105b has undergone a frequency shift of frequency f2.

Then, these light waves are combined by a beam splitter 103, then expanded in beam diameter by a beam expander 106, and are incident on a polarization beam splitter 107. Of these, the first light wave of frequency f1 passes through the polarization beam splitter 107, becomes circularly polarized by the λ / 4 plate 108b, and enters the DUT 111 through the collimator lens 110. The test wavefront (measurement light wave) reflected by the DUT 111 returns to the original optical path. At this time λ / 4
Since the light wave when re-incident on the plate 108b is reflected by the DUT 111 as compared with the time of incidence and thus becomes circularly polarized light in the reverse direction, the polarization direction (polarization plane) when passing through the λ / 4 plate 108b. Is displaced by 90 degrees compared to the time of incidence. For this reason, this time it is reflected by the polarization beam splitter 107.

On the other hand, the second light wave having the frequency f2 is reflected by the polarization beam splitter 107 and becomes circularly polarized by the λ / 4 plate 108a, and the reference wavefront (reference light wave) reflected by the reference plane mirror 109 becomes reverse circularly polarized light. Return to the original optical path.
Then, after passing through the λ / 4 plate 108a, it becomes a linearly polarized light having a polarization direction (polarization plane) displaced by 90 degrees from that at the time of incidence, and this time it passes through the polarization beam splitter 107 and is combined (combined) with the previous wavefront to be detected. ing.

Here, the two light waves of the test wave surface and the reference wave surface, which are recombined, are caused to interfere with each other as a wave surface capable of interfering with each other through the polarizing plate 112 formed of a linear polarization element. The interference light wave at this time is detected as a heterodyne signal (beat signal) having a frequency shift difference f1-f2 by a photodetector (photodetector) 15 and an image detection camera (image dissector camera) 116, respectively.

The image detection camera 116 is the computer 1
The camera is capable of selecting an arbitrary one point of a two-dimensional image according to a command from 18 and detecting the intensity signal of light incident on that point in real time.

The heterodyne signals f1-f2 in FIG.
If the reference plane mirror 109 is assumed to be an ideal plane, the phase distribution of 1 directly represents the error in the shape of the DUT 111 from the spherical wave created by the collimator lens 110.

That is, the phase distribution φ (x, y) based on the shape of the object to be measured 11 at this time represents the shape error distribution ε (x, y) of the object to be measured 111, and the relationship is as follows. Represented.

Ε (x, y) = φ (x, y) · λ / 4π (1) where λ is the wavelength of the measurement light, where the heterodyne signal obtained by the photodetector 115 is the reference signal R, Each signal from the image detection camera 116 is used as a measurement signal S and the reference signal R and the measurement signal S are measured by the phase meter 117.
And the phase difference φ between Further, each measurement point of the image detection camera 116 is two-dimensionally scanned by a command from the computer 118, and a two-dimensional distribution of the phase difference φ (x, y) between the measurement signal S and the reference signal R at each measurement point is obtained. . Thereby, the phase distribution ε (x, y) based on the shape of the DUT 111 is detected. For example, in the equation (1), if the electrical phase measurement is performed with an accuracy of about 1 degree, the distribution of the shape error can be detected with an accuracy of nm or less.

[0013]

Generally, in an optical heterodyne interferometer, the interference fringes on the detection surface oscillate at a very high speed. For this reason, a photon storage type light receiving means such as a CCD camera cannot be used, a photon non-storage type light receiving means with a quick response is required, and two signals of a reference light wave and a measurement light wave must be extracted.

Generally, since the photon non-accumulation type light receiving means has low sensitivity, when the reflectance of the object to be measured is low, the S / N ratios of both the reference signal and the measurement signal are deteriorated and the measurement accuracy becomes low. However, there is a problem that

However, in the conventional optical heterodyne interferometer shown in FIG. 4, the light beam reciprocated by the polarization beam splitter 107 back and forth between the measurement system for obtaining the measurement light wave and the reference system for obtaining the reference light wave has its polarization axis in the direction of 45 degrees. A linear polarization element 112, which is a rotated polarizing plate, extracts only the same vector component and causes it to interfere with each other to extract a beat signal. The beat signal is further divided into two, one of which is received by the photo detector 115 and the other of which is received by the image dissector camera 116.

Generally, the linear polarizing element has a good transmittance of about 40%, and since it is further divided into two, in the conventional optical heterodyne interference device, the amount of light reaching the light receiving means is emitted from the polarizing beam splitter 7. Since only about 20% of the light can be used, high accuracy measurement is difficult when the reflectance of the object to be measured is low, and a light source having an unnecessarily large power must be used. There was a point.

According to the present invention, the shape of the object to be measured can be measured with high accuracy by effectively utilizing the interference light flux of the measurement light wave and the reference light wave synthesized by the polarization beam splitter without using a light source of strong output. An object is to provide an optical heterodyne interference device.

[0018]

The optical heterodyne interferometer of the present invention makes one of the first light wave and the second light wave having different frequencies incident on the object to be measured through the polarization beam splitter and outputs the measured light wave. After obtaining the reference light wave from the other light wave via the reference surface, the two light waves are combined by the polarization beam splitter, and the combined measurement light waves of linearly polarized light and the reference light wave orthogonal to each other are polarized in the incident light. Is divided into two via a polarization optical element that emits and separates at a different angle, and the same vector component is interfered to obtain a beat signal. One of these beat signals is used as a reference signal.
Detected by the detecting means, the other beat signal is detected as the measuring signal by the second detecting means, and the optical phase change of the measuring light wave is detected by utilizing the signals obtained by the first and second detecting means. It is characterized by doing so.

In particular, the present invention is characterized in that the polarization optical element is a polarization beam splitter or a Wollaston prism whose polarization axis is rotated by 45 degrees with respect to the polarization direction of the linearly polarized light from the polarization beam splitter.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic view of the essential portions of a first embodiment of the present invention, and FIG.
FIG. 2 is an enlarged perspective view of a part of FIG. 1. In FIG. 2, the arrow indicates linearly polarized light vibrating in the direction of the arrow.

In the present embodiment, compared with the conventional optical heterodyne interferometer of FIG. 4, the incident light is separated and emitted between the polarization beam splitter 4 and the detecting means (11, 13) at different angles depending on the polarization direction. The polarization optical element 9 is arranged differently, and the other configurations are substantially the same.

Next, although partially overlapping the description of FIG. 4, the features of the present invention will be described with reference to FIGS. 1 and 2.

Reference numeral 1 denotes a light source means, which is composed of, for example, a two-frequency laser and emits coherent first light waves (frequency f1) and second light waves (frequency f2) in which the frequencies of linearly polarized light orthogonal to each other are slightly different. There is. Reference numeral 2 is an ND filter, which adjusts the amount of passing light.

A beam expander 3 expands the diameter of the incident light beam and emits it. Reference numeral 4 denotes a polarization beam splitter, which reflects or transmits the incident light beam according to the polarization state of the incident light beam to change the traveling direction of the light beam. Reference numerals 5a and 5b denote λ / 4 plates, which rotate the polarization plane of the incident light beam by 45 degrees and emit it. The reference plane mirror 6 forms a reference wavefront.
A collimator lens 7 collects an incident light beam and makes it incident on an object 8 to be measured which will be described later. The object to be measured 8 is composed of a mirror, a lens, and the like, and detects the three-dimensional shape of them, the uniformity of the material, and the like.

Reference numeral 9 denotes a polarizing optical element, which has an optical function of emitting the incident light at different angles and separating the incident light according to the polarization direction. The polarization optical element 9 splits a light wave having a specific polarization component from a light wave having two linearly polarized light components orthogonal to each other and causes the light waves to interfere with each other. In this embodiment, the polarizing optical element 9 has a reflection surface of 45 with respect to the polarization direction of the incident linearly polarized light.
It is composed of polarization beam splitters that are tilted.

Reference numeral 10 denotes a pinhole plate, which has a pinhole 10a for passing a light beam incident on a specific position. Reference numeral 11 is a first detection means, which is composed of a photodetector, and obtains a reference signal from the beat signals based on the interference light wave from the polarization optical element 9. A condenser lens 12 condenses (images) the interference light wave from the polarization optical element 9 onto the second detection means 13. The second detection means 13 comprises an image dissector camera (image detection camera),
The optical signal at the position addressed by the computer 15 is read out in real time. Reference numeral 14 denotes a phase detecting means (phase measuring circuit), which includes the first detecting means 11 and the second detecting means 11.
The phase difference between the two signals input from the detection means 13 is detected. The computer 15 calculates and calculates the shape of the DUT 8 based on the signal from the phase measuring circuit 14.

In this embodiment, two light beams of a linearly polarized first light wave (frequency f1) and second light wave (frequency f2) emitted from the light source means 1 and having different polarization directions orthogonal to each other are passed through the ND filter 2. After adjusting the light amount by the light, the light is incident on the beam expander 3.

The beam expander 3 expands the light beam diameter and makes it enter the polarization beam splitter 4. Of these, the first light wave of frequency f1 passes through the polarization beam splitter 4, is circularly polarized by the λ / 4 plate 5b, and is incident on the DUT 8 through the collimator lens 7. Then, the wavefront to be detected reflected by the DUT 8 returns to the original optical path. At this time, the light wave when re-incident on the λ / 4 plate 5b is reflected by the object to be measured 8 as compared with the time of incidence, and thus is circularly polarized in the reverse direction. The direction (plane of polarization) is displaced by 90 degrees compared with the time of incidence. Therefore, it is reflected by the polarization beam splitter 4 this time.

On the other hand, the second light wave having the frequency f2 is reflected by the polarization beam splitter 4 and becomes circularly polarized by the λ / 4 plate 5a, and the reference wavefront reflected by the reference plane mirror 6 becomes circularly polarized light in the reverse direction and the original optical path is changed. Return. Then, it passes through the λ / 4 plate 5a and becomes linearly polarized light whose polarization direction (polarization plane) is displaced by 90 degrees from that at the time of incidence. This time, it passes through the polarization beam splitter 4 and is combined (combined) with the previous wavefront to be detected. ing.

Here, the two light waves of the test wave surface and the reference wave surface, which are recombined, are caused to interfere with each other through the polarization optical element 9 as wavefronts capable of interfering with each other.

As shown in FIG. 2, the polarization beam splitter as the polarization optical element 9 in this embodiment is arranged with the polarization axis (reflection surface) inclined by 45 degrees with respect to the incident linearly polarized light, whereby two light waves are the same. The vector component is split into the interference light wave of the selected transmitted light and the reflected light bent at 90 degrees. As a result, the loss of the light flux when entering and exiting the polarization optical element 9 is reduced as compared with the case where a polarizing plate is used, and the light flux is effectively used. Then, the two interference light waves at this time are the first detection means 11 and the second detection means as a heterodyne signal (beat signal) having a frequency shift difference f1-f2.
Each is detected by the detection means 13.

The heterodyne signal f1- in this embodiment.
The phase distribution of f2 directly represents an error in the shape of the DUT 8 from the spherical wave generated by the collimator lens 7 if the reference plane mirror 6 is an ideal plane.

Therefore, for example, the heterodyne signal obtained by the first detecting means 11 is used as the reference signal R, each signal from the second detecting means 13 is used as the measurement signal S, and the position of the reference signal R and the measurement signal S is calculated by the phase measuring circuit 14. The phase difference φa is detected.

Further, each measuring point of the second detecting means 13 is two-dimensionally scanned by a command from the computer 15 _{, and} a two-dimensional distribution of the phase difference φ _{x, y} between the measuring signal S and the reference signal R at each measuring point is obtained. Ask. Thereby, the shape error of the entire measured object 8 is measured.

FIG. 3 is an enlarged perspective view of a part of the second embodiment of the present invention. In this embodiment, a Wollaston prism 21 is used instead of the polarization beam splitter as a polarization optical element for emitting and separating the incident light at different angles depending on the polarization direction of the incident light, as compared with the first embodiment of FIGS. 1 and 2. They are different, and the other configurations are substantially the same as those in the first embodiment.

In FIG. 3, the reference light wave among the interference light waves emitted by decomposing the vibrating surface in a vector manner by the polarization optical element 21 passes through the pinhole plate 10 and the first detecting means 11
And the measurement light wave is detected by the second detection means 13 via the condenser lens 12.

In this embodiment, by using the Wollaston prism 21 as the polarization optical element, the loss of the incident light flux is reduced as compared with the case where the polarizing plate is used as in the first embodiment, and the light flux is effectively used. .

[0038]

According to the present invention, the frequency difference of 2 is slightly different.
When obtaining the optical path difference distribution or the three-dimensional shape of the object to be measured by the optical heterodyne interferometer using two lights, the linearly polarized measurement light wave and the reference light wave whose polarization directions are orthogonal to each other are combined by the polarization beam splitter and emitted. After that, using the polarization optical element with the above-mentioned configuration, the vibrating surface is decomposed in vector and emitted as an interference light wave so that a beat signal can be taken out, and one is used as a reference signal and the other is used as a measurement signal for phase detection. To achieve an optical heterodyne interferometer capable of minimizing a loss of light and measuring a shape error with a high S / N ratio even for an object to be measured having an extremely low reflectance. You can

[Brief description of drawings]

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

FIG. 2 is an enlarged perspective view of a part of FIG.

FIG. 3 is a schematic view of the essential portions of Embodiment 2 of the present invention.

FIG. 4 is a schematic view of a main part of a conventional optical heterodyne interferometer.

[Explanation of symbols]

1 light source means 2 ND filter 3 beam expander 4 Polarizing beam splitter 5a, 5b λ / 4 plate 6 reference plane mirror 7 Collimator lens 8 DUT 9 Polarizing optical element 10 pinhole plate 11 First detecting means 12 Condensing lens 13 Second detection means 14 Phase measurement circuit 15 Computer 21 Wollaston Prism

Claims (2)

[Claims] 1. A first light wave of a different frequency and a second light wave of which one light wave is incident on an object to be measured via a polarization beam splitter to obtain a measurement light wave, and the other light wave is a reference light wave via a reference surface. After obtaining the above, the polarization optics that combine both light waves with the polarization beam splitter, and emit the separated measurement light waves of linearly polarized light and reference light waves that are emitted at different angles depending on the polarization direction of the incident light and separate A beat signal is obtained by dividing the same into two via an element and interfering the same vector component, one of these beat signals is detected as a reference signal by the first detection means, and the other beat signal is measured as a measurement signal. An optical heterodyne interferometer characterized in that the optical phase change of the measurement light wave is detected by using the signals detected by the second detecting means and obtained by the first and second detecting means. 2. The polarization optical element has a polarization axis of 4 with respect to a polarization direction of linearly polarized light from the polarization beam splitter.
The optical heterodyne interferometer according to claim 1, which is a polarization beam splitter or a Wollaston prism rotated by 5 degrees.