JPH09210619A - Focus detection device using optical waveguide - Google Patents

Abstract

A focus detection device having a small size and a simple structure is provided. SOLUTION: An emission end face 203 of a light source 21 and a position 27 slightly deviated from the center of an end face 202 of a double mode waveguide 28 are arranged at conjugate positions with a polarization micro beam splitter 23 sandwiched therebetween. The linearly polarized illumination light emitted from the end face 203 is deflected by the beam splitter 23,
It is condensed by the condenser lens 25. The reflected light from the object 26 to be inspected is condensed by the condenser lens 25, passes through the beam splitter 23, and enters the double mode waveguide 28. By setting the length of the double-mode waveguide so that the asymmetry of the phase distribution of the light incident on the double-mode waveguide can be detected, it does not correspond to the case where the object 26 to be measured is located at the focal point of the condenser lens. The case and can be detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus detecting device or a step measuring device using an optical waveguide.

[0002]

2. Description of the Related Art In recent years, optical waveguides have attracted attention in the fields of optical communication and optical measurement. The reason is that the size and weight of the optical system can be reduced by using the optical waveguide.
This is because there is an advantage that adjustment of the optical axis becomes unnecessary.

As one of measuring instruments utilizing an optical waveguide,
For example, Japanese Patent Application Laid-Open No. 6-331841 proposes a device for detecting a focus.

In Japanese Unexamined Patent Publication No. 6-331841, a double mode waveguide is used as an optical waveguide, and light condensed from the end face of the double mode waveguide is made incident. At this time, the optical axis of the incident light is slightly deviated from the center of the double mode waveguide. The modes excited in the double-mode waveguide are different depending on whether the focus of the condensed light matches the end face of the double-mode waveguide or not. Therefore, the symmetry of the light propagating through the double mode waveguide differs depending on the focal position of the incident light. Therefore, the focus position of the incident light is detected by detecting the symmetry of the light propagating through the double mode waveguide.

In Japanese Patent Laid-Open No. 6-331841, a polarization-selective diffraction grating is used for converging illumination light on an object and separating the reflected light from the illumination light on the end face of the double mode waveguide. And an objective lens are used.

[0006]

Problems to be Solved by the Invention Japanese Patent Application Laid-Open No. 6-33184
The configuration of the device described in Japanese Patent Publication No. 1 is a configuration in which a polarization selective diffraction grating must be used. However, there is a problem that it is difficult to manufacture a polarization-selective diffraction grating with good performance.

An object of the present invention is to provide a compact focus detection device having a simple structure.

[0008]

In order to achieve the above object, according to the present invention, an illumination optical system for emitting illumination light,
Of the illumination light and the reflected light, in order to collect the illumination light and irradiate the object to be inspected, and to separate the illumination light and the reflected light from each other, ,
A deflection optical system that transmits one and reflects the other, a double-mode waveguide for allowing the reflected light separated by the deflection optical system to enter from an end face and propagate, and a double-mode waveguide that propagates through the double-mode waveguide Symmetry detecting means for detecting the symmetry of the light intensity distribution, the deflection optical system is arranged between the illumination optical system and the condensing optical system, A focus detection device is provided, wherein a position deviated from the center of the end face and an emission center of the illumination light of the illumination optical system are arranged at a conjugate position with the deflection optical system interposed therebetween.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described.

First, the structure of the focus detecting apparatus according to the first embodiment of the present invention will be described with reference to FIG.

On the lithium niobate substrate 20, a double mode waveguide 28 and a branching portion 201 for branching the double mode waveguide 28 into two.
Two waveguides 29 and 30 connected to the branch portion 201 are formed. These are formed by diffusing Ti into the substrate 20. Double mode waveguide 28
Is exposed at one side surface of the substrate 20,
It is 2. In addition, the ends of the waveguides 29 and 30 are also exposed on one side surface of the substrate 20, and the photodetectors 32 and 3 are respectively exposed.
3 is attached. The outputs of the photodetectors 32 and 33 are input to the differential amplifier 34. A pair of electrodes 31 is arranged on the double mode waveguide 28 with a buffer layer (not shown) interposed therebetween.

The substrate 20 is arranged so that the end face 202 of the double mode waveguide 28 faces the object 26 to be measured. Between the end face 202 and the object 26 to be inspected, a micro beam splitter 23 of a minute size that reflects and transmits light depending on the polarization direction, a quarter wave plate 24, and an objective lens 25 are sequentially arranged. Further, the end surface 203 of the polarization plane preserving fiber 22 is arranged on the optical axis separated from the optical axis connecting the end surface 202 and the object 26 to be measured by the microbeam splitter 23. A light source 21 that emits linearly polarized illumination light is attached to the other end surface of the fiber 22.

The linearly polarized illumination light emitted from the light source 21 propagates through the polarization-maintaining fiber 22 with the polarization direction being preserved, and is emitted from the end face 203. The micro beam splitter 23 is arranged so as to reflect the linearly polarized illumination light emitted from the end face 203 and transmit the light in the polarization direction orthogonal to this. The quarter-wave plate is arranged so as to convert the linearly polarized illumination light reflected by the microbeam splitter 23 into circularly polarized light. On the other hand, the inspection object 26
The reflected light of the illumination light due to is converted into a polarization direction orthogonal to the illumination light by passing through the quarter-wave plate 24, and thus passes through the microbeam splitter 23.

The objective lens 25 collects and forms an image of the illumination light emitted from the end face 203 of the optical fiber 22 and, when the object 26 to be inspected is located on the image forming surface, the reflected light of the objective lens 25 is sloppy. The waveguide 28 is arranged so as to form an image at a position 27 slightly deviated from the center of the end face of the waveguide 28 in the width direction. That is, the emission end face 203 of the optical fiber 22
And the position 27 slightly deviated from the center of the incident end face 202 of the double-mode waveguide 28 are arranged so as to have a conjugate positional relationship with the micro beam splitter 23 interposed therebetween.

Next, the operation of the focus detection device shown in FIG. 2 will be described.

The linearly polarized illumination light emitted from the light source 21 propagates through the polarization-maintaining fiber 22 and is guided to the microbeam splitter 23. Micro beam splitter 2
Since 3 is a configuration that reflects the linearly polarized illumination light emitted from the light source 21, the illumination light is reflected by the microbeam splitter 23 and converted into circularly polarized light by the quarter-wave plate 24. Then, it is condensed by the objective lens 25 and irradiated onto the object 26 to be inspected. The light reflected by the object 26 to be inspected is condensed again by the objective lens 25, converted into linearly polarized light orthogonal to the linearly polarized light of the illumination light by the ¼ wavelength plate 24, transmitted through the microbeam splitter 23, The light is focused at a position 27 slightly deviated from the center of the end face 202 of the double mode optical waveguide.

At this time, when the object 26 to be inspected is located at a position corresponding to the image plane of the illumination light, the reflected light is reflected by the objective lens 25 to the end face 202 of the double mode waveguide 28.
An image is formed at the position 27 as shown in FIG. On the other hand, when the object 26 to be inspected is not located at the position coincident with the image plane of the illumination light, the reflected light is located at the position 27 on the end face 202 of the double mode waveguide 28 as shown in FIG. 5B. Does not form an image.

When the reflected light is imaged at the position 27 of the end face 202 of the double mode waveguide 28, as can be seen from FIG. 5A, the wave front of the light at the time when the reflected light reaches the end face 202. 51 becomes parallel to the end face 202. For this reason, FIG.
In the case of (a), it is equivalent to the case where light having a symmetrical phase distribution in the width direction of the double mode waveguide 28 is incident on the double mode waveguide 28.
Is excited with 0th-order mode light and 1st-order mode light.

Further, when the reflected light does not form an image at the position 27 of the end face 202 of the double mode waveguide 28, FIG.
As is clear from (b), the wavefront 51 of the light when the reflected light reaches the end face 202 is not parallel to the end face 202. Therefore, in the case of FIG. 5B, it is equivalent to the case where light having an asymmetrical phase distribution in the width direction of the double mode waveguide 28 is incident on the double mode waveguide 28, and Causes 0th-order mode light and 1st-order mode light to be excited.

Since the 0th-order mode light propagating in the double-mode waveguide is an even mode, the light intensity distribution is symmetrical with respect to the width direction of the waveguide. On the other hand, when the 0th-order mode light and the 1st-order mode light are excited, both modes interfere with each other in the waveguide, and the interference light propagates while meandering in the waveguide. It is known that an asymmetry occurs in the light intensity distribution of.

Utilizing this, the double mode waveguide 28
By setting the length L of the beam to an appropriate length and detecting the symmetry of the intensity distribution of the light propagating in the double mode waveguide,
It is also known that the asymmetry of the phase distribution incident on the double mode waveguide can be detected. (H. Oki and
J. Iwasaki, Optics Communi
Cations 85 (1991) 177). The appropriate length L of the double-mode waveguide 28 is a complete coupling length (length at which the phase difference between the 0th-order mode and the 1st-order mode is π) L
For c, L = Lc (2m + 1) / 2 (m = 0, 1, 2, ...
・) The length is such that

In practice, however, light propagates in the double mode in the portion where the waveguide spacing is narrow even in the branching portion 201, so the double mode waveguide 28 and the branching portion 201.
It is necessary to set L to a length that allows light to propagate substantially in the double mode including the above.

In the present embodiment, by applying a voltage from the electrode 31 to the double mode waveguide 28, the complete coupling length Lc is adjusted by the electro-optic effect, and the length L of the double mode waveguide 28 is calculated by the above formula. Trying to meet.

Therefore, when the object 26 to be inspected is located on the image forming plane of the illumination light by the objective lens, as described above, the light whose wavefront is parallel to the end face 202 is incident on the double mode waveguide 28. The 0th-order mode light and the 1st-order mode light are excited in the double-mode waveguide 28. The 0th-order mode light and the 1st-order mode light excited in the double-mode waveguide 28 propagate while interfering with each other. The length L of the double mode waveguide is
Since the asymmetry of the phase distribution is detected by the above equation, the waveguide 29,
Evenly divided into 30. Therefore, the photodetector 32,
An equal amount of light is detected at 33.

On the other hand, when the object to be inspected is not located on the image plane of the illumination light from the objective lens, the light whose wavefront is not parallel to the end face 202 is incident on the double mode waveguide 28,
Zero-order mode light and first-order mode light are excited in the double-mode waveguide 28. The 0th-order mode light and the 1st-order mode light interfere with each other, and the interference light meanders and propagates to reach the branching unit 201.
At this time, since the length L of the double mode waveguide 28 satisfies the above expression, the interference light is biased to either the left or right in the width direction of the double mode waveguide 28, and the branch portion 20.
At 1, there is a difference in the amount of light split into the waveguides 29 and 30. Therefore, the light amounts detected by the photodetectors 32 and 33 are not equal.

Therefore, by detecting the difference between the amounts of light detected by the photodetectors 32 and 33 with the differential amplifier 34, whether the object 26 to be measured is positioned on the image plane of the illumination light by the objective lens 25 or not. That is, it is possible to detect whether or not it is located at the focal point of the objective lens 25.

As described above, in this embodiment,
The position 27 slightly deviated from the center of the end face 202 of the double mode waveguide and the emission center of the end face 203 of the optical fiber 22 for emitting the illumination light are set to the micro beam splitter 2
By arranging 3 in a conjugate position with sandwiching 3, it was possible to configure the focus detection device with a simple configuration. The micro beam splitter 23 is an optical component of a small size of several mm square. Furthermore, in the present embodiment, the microbeam splitter 23 can be attached to the end face of the double mode waveguide 28, so that the optical path length can be shortened and alignment and attachment work can be facilitated. Therefore, a compact focus detection device with a simple configuration,
It can be manufactured efficiently.

Although the lithium niobate substrate 20 having the electro-optical effect is used in the above-mentioned embodiment, other materials having the electro-optical effect such as GaAs or Si having no electro-optical effect are used. Can be used.

When a material having no electro-optical effect is used, the length L of the double mode waveguide 28 may be geometrically adjusted to manufacture. If the electro-optic effect is not used, the electrode 31 is unnecessary.

Further, in this embodiment, the double mode waveguide 28, the branching portion 201, the waveguides 29 and 30 are connected to the substrate 20.
Although formed above, it is also possible to configure these with optical fibers.

Further, as a second embodiment of the present invention,
A configuration as shown in FIG. 3 can be used. The configuration of FIG. 3 is almost the same as the configuration of FIG. 2, but instead of using the objective lens 25, a condenser grating lens 35 is integrated in the microbeam splitter 23.

The condensing grating lens 35 is composed of a concentric circular grating as shown in FIG. 4, and has a condensing function like an objective lens.

In the configuration of FIG. 3, the illumination light is condensed on the object 26 to be inspected by the condensing grating lens 35, and the reflected light from the object 26 to be inspected is condensed by the condensing grating lens 35 into the double mode waveguide. The light is focused on the end face 28.

With such a structure, the optical path length becomes shorter and the number of optical parts becomes smaller than that in the case of using the objective lens 25 of the first embodiment, and the optical system becomes simple. The other configurations and operations are the same as those in the first embodiment, and therefore their explanations are omitted.

Next, a focus detecting device according to a third embodiment of the present invention will be described with reference to FIG.

The lithium niobate substrate 2 has the double mode waveguide 28 and the branching portion 20 as in the first embodiment.
1, the single mode waveguide 3 is formed in addition to the waveguides 29 and 30.

One end face 4 of the single mode waveguide 3
Is arranged on the side surface of the substrate 2 where the end face 202 of the double mode waveguide 28 is located. On the other end face of the single mode waveguide 3, a light source 1 that emits a linear illumination light
Is attached.

A collimator lens 5, a birefringent prism 6, a quarter-wave plate 7 and an objective lens 8 are sequentially arranged between the substrate 2 and the object 9 to be inspected. The birefringent prism 6 is
A Rochon prism, which is a commonly used Rochon prism, is a prism that makes an ordinary ray go straight and refracts an extraordinary ray in a direction different from the incident axis. The linearly polarized illumination light emitted from the single mode waveguide 3 becomes an ordinary ray to the birefringent prism 6, and the linearly polarized light whose polarization direction is orthogonal to the illumination light becomes an extraordinary ray of the birefringent prism 6. The birefringent prism 6 is arranged. Further, the light reflected by the object 9 to be inspected is refracted by the birefringence prism, and this refraction prism is collimated by the double mode waveguide 2 by the collimator lens 5.
The end face 4 of the single mode waveguide 3 and the end face 4 of the single mode waveguide 3 so as to be condensed at a position 27 slightly deviated from the center of the end face 202.
The distance from the end face 202 of the double mode waveguide 28 is predetermined and manufactured.

The linearly polarized light emitted from the light source 1 propagates through the single mode waveguide 3 formed on the substrate 2 and exits from the end face 4. The emitted linearly polarized illumination light is collimated by the collimator lens 5 and then enters the birefringent prism 6.

Since the illumination light is an ordinary ray of the birefringent prism 6, it travels straight through the birefringent prism 6. And 1 /
It is converted into circularly polarized light by the four-wave plate 7 and is condensed on the object 9 to be inspected by the objective lens 8.

The light reflected by the object 9 to be inspected again passes through the objective lens 8 and the quarter-wave plate 7 to be converted into linearly polarized light whose polarization directions are orthogonal to each other. Since this plane of polarization becomes an extraordinary ray with respect to the birefringent prism 6, the light refracted by the birefringent prism 6 and the optical path changed is
The light is condensed through the collimator lens 5 at a position 27 slightly deviated from the center of the end face 202 of the double mode waveguide 28.

Similar to the first embodiment, when the object 9 to be measured is located at the focal point of the objective lens 8, light having parallel wavefronts is incident on the double mode waveguide 28 and is displaced from the focal point. When the object 9 to be inspected is located, light whose wavefront is inclined is incident. By setting the length of the double-mode waveguide to a length that detects the asymmetry of the phase distribution of the light incident on the double-mode waveguide, the difference in the amount of light detected by the photodetectors 32 and 33 is differentiated. By using the amplifier 34, it is possible to know the focus position shift of the object 9 to be inspected.

In the configuration of the third embodiment, the focus detection device can be constructed by combining the birefringent prism, the collimator lens, and the objective lens, which are easy to produce, so that the polarization which is difficult to produce as in the conventional case. Since it is not necessary to use a selective diffraction grating, manufacturing is easier than ever before.

In each of the above-described embodiments, the focus detection device has been described, but the step difference of the object to be inspected can be measured with the same configuration.

[0045]

According to the present invention, it is possible to provide a focus detection device having a small size and a simple structure.

[Brief description of drawings]

FIG. 1 is a block diagram illustrating a configuration of a focus detection device according to a third embodiment of the present invention.

FIG. 2 is a block diagram illustrating a configuration of a focus detection device according to the first embodiment of the present invention.

FIG. 3 is a block diagram illustrating a configuration of a focus detection device according to a second embodiment of the present invention.

FIG. 4 is an explanatory diagram illustrating a configuration of a condenser grating lens used in a second embodiment of the present invention.

5A and 5B are explanatory diagrams showing a relationship between an end face 202 of the double mode waveguide 28 of the focus detection device of FIG. 1 and an image plane of reflected light.

[Explanation of symbols]

1, 21 ... Light source, 2, 20 ... Substrate, 3 ... Single-mode waveguide, 22 ... Optical fiber, 23 ...
Microbeam splitter, 24 ... Quarter wave plate, 25 ... Objective lens, 26 ... Object to be inspected, 28
... Double mode waveguide, 29,30 ... Waveguide,
31 ... Electrodes, 32, 33 ... Photodetectors, 34 ...
-Differential amplifier.

Claims (6)

[Claims] 1. An illumination optical system for emitting illumination light; a focusing optical system for focusing the illumination light to irradiate an object to be inspected and focusing reflected light thereof; and the illumination light and the reflected light. In order to separate the illumination light and the reflected light, a deflection optical system that transmits one of the illumination light and the reflected light and reflects the other of the illumination light and the reflected light that is separated by the deflection optical system is incident from an end face and propagates. And a symmetry detecting means for detecting the symmetry of the intensity distribution of the light propagating through the double mode waveguide, wherein the deflection optical system includes the illumination optical system and the collecting optical system. A position displaced from the center of the end face of the double mode waveguide and an emission center of the illumination light of the illumination optical system, which are disposed between the optical system and the double mode waveguide, are arranged at positions conjugate with each other with the deflection optical system interposed therebetween. A focus detection device characterized by being provided. 2. The illumination optical system according to claim 1, wherein the illumination optical system emits linearly polarized light, the deflection optical system is a polarization beam splitter, and there is 1 between the deflection optical system and the object to be inspected. A focus detection device in which a quarter-wave plate is arranged. 3. The illumination optical system according to claim 1, further comprising a light source that emits linearly polarized illumination light and an optical fiber that propagates the illumination light and emits the light from an end face. The focus detection device, wherein the emission center is an end face of the optical fiber. 4. The condensing optical system according to claim 2, wherein the condensing optical system is a grating lens, the grating lens is attached to the polarization beam splitter, and the ¼ wavelength plate includes a grating lens and an object to be inspected. A focus detection device, which is arranged between the two. 5. The symmetry detection means according to claim 1, wherein the divergence detecting unit detects a difference in intensity between light branched into two and light branched in the double mode waveguide. A focus detection device having a detector. 6. An illumination optical system that emits linearly polarized illumination light, a condensing optical system that condenses the illumination light and irradiates an object to be inspected, and condenses the reflected light, and the reflected light. A polarization direction conversion means for converting the polarization direction of the illumination light into a direction different from the polarization direction of the illumination light, and in order to separate the illumination light and the reflected light, one of the illumination light and the reflected light is transmitted, A deflection optical system for refracting the other, a double mode waveguide for allowing the reflected light separated by the deflection optical system to enter from an end surface and propagate, and an intensity distribution of the light propagated through the double mode waveguide And a symmetry detection unit for detecting symmetry, wherein the double-mode waveguide is such that the reflected light condensed by the condensing optical system is condensed at a position deviated from the center of the end face. And the deflection optics are Is a prism using folding, the polarization direction conversion means is disposed between the deflection optical system and the object to be inspected, the illumination optical system, one of the ordinary ray and the extraordinary ray of the deflection optical system. The linearly polarized light that coincides with the polarization direction of, and the polarization direction conversion means converts the reflected light into linearly polarized light that coincides with the other polarization direction of the ordinary ray and the extraordinary ray of the deflection optical system. Characteristic focus detection device.