JPH10133096A - Focus detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a focus detector having a wide focus detection range, as for a focus detector using a double mode waveguide. SOLUTION: Light emitted from a light source 1 is condensed by light condensing optical systems 3 and 4 so as to irradiate an object to be inspected 5. The light reflected by the object to be inspected 5 is made incident from the incident end faces of double mode waveguides 21 to 25 and transmitted. The inclination of electric field distribution of the light transmitted through the double mode waveguides 21 to 25 is detected by detecting means 51 to 55. In this case, a splitting means 10 is arranged so as to split light reflected by the object to be inspected into luminous fluxes equal to the number of the double mode waveguides and to guide respective fluxes to the incident end faces of plural double mode waveguides. Plural double mode waveguides are arranged so that each center of the incident end face may be shifted from the optical axis of the luminous flux guided by the splitting means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a focus detecting device or a displacement 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 use of the optical waveguide has the advantages that the size and weight of the optical system can be reduced and the adjustment of the optical axis is not required.

[0003] As one of measuring instruments using an optical waveguide,
For example, as described in JP-A-6-331841,
A focus detection device has been proposed. JP-A-6-33184
The focus detection device of Japanese Patent Publication No. 1 uses a double mode waveguide as an optical waveguide as shown in FIG.

When a laser beam is incident on a double mode waveguide, only the 0th mode or the 0th and 1st modes are excited in the waveguide according to the characteristics of the laser beam at the incident end. When the zero-order mode and the first-order mode are excited, the two modes interfere with each other in the waveguide, and the interference light propagates meandering in the double-mode waveguide. Therefore, asymmetry occurs in the light intensity distribution in the width direction of the optical waveguide.

Here, the characteristics of the incident light that excites the 0th-order mode light and the 1st-order mode light in the double mode waveguide are as follows.
There are two elements. One is when the intensity distribution of the incident laser light has asymmetry in the width direction of the waveguide, and the other is when the phase of the incident laser light has asymmetry in the width direction of the waveguide. is there. By appropriately selecting the length of the double-mode waveguide and detecting the asymmetry of the transmitted light, any element of the incident light can be detected (H. Ooki and j. Iwasaki, Optics Comm.
unications 85 (1991) 177).

The focus detection device disclosed in Japanese Patent Application Laid-Open No. Hei 6-331841 can detect a displacement (focal position shift) of incident laser light in the optical axis direction by using mode interference in the double mode waveguide. In FIG. 5, the light source 10
The light emitted from 1 is reflected by the half mirror 102, becomes parallel light by the collimator lens 103, and is then focused on the test object 105 by the objective lens 104. The light reflected on the test object 105 passes through the objective lens 104 and the collimator lens 103 again and passes through the half mirror 102
And is condensed on the incident end 106 of the double mode waveguide 111 formed on the substrate 110. Incident end face 106
Is located at a position slightly deviated from the optical axis of the reflected light from the test object 105.

When the test object 105 is shifted from the focal position of the objective lens 104, an inclination occurs in the equiphase plane of the laser spot light at the incident end face 106, so that the phase distribution in the width direction of the double mode waveguide 111 is generated. There is asymmetry. The 0th-order mode light and the 1st-order mode light are excited in the double mode waveguide 111, and the interference light of both modes propagates in the double mode waveguide 111 while meandering. By using this interference to detect the asymmetry of the phase distribution of the light incident on the double mode waveguide, it is possible to detect the focal position shift.

In order to detect the asymmetry of the phase distribution of the incident light from the asymmetry of the light propagating through the double mode waveguide 111, the apparatus shown in FIG. Branch into the waveguides 112 and 113. Then, the intensities of the light emitted from the waveguides 112 and 113 are detected by the photodetectors 114 and 115, and the difference between these outputs is obtained by the operational amplifier 116, whereby the asymmetry of the light propagating through the double mode waveguide 111 is obtained. Is detected, whereby the focal position shift of the test object 105 can be known.

[0009]

However, the focus detection device described in Japanese Patent Application Laid-Open No. 6-331841 has a high resolution, but has a small detectable range, so that the focus detection signal pull-in range is small. There was a problem that was small.

An object of the present invention is to provide a focus detection device using a double mode waveguide, which has a wide focus detectable range.

[0011]

According to the present invention, there is provided the following focus detecting apparatus. That is, a light source, a condensing optical system for condensing light from the light source and irradiating the object to be inspected, and a plurality of double modes for propagating reflected light from the object to be incident on an incident end face and propagating the same. A waveguide, having a detecting unit for detecting the deviation of the electric field distribution of light that has propagated through the double mode waveguide, between the test object and the plurality of double mode waveguides, Dividing means for dividing the reflected light from the test object into light beams of the number of the double mode waveguides, and guiding each of the light fluxes to the incident end faces of the plurality of double mode waveguides is arranged, and the plurality of double mode waveguides are arranged. Are focus detection devices in which the centers of the incident end faces are respectively displaced from the optical axis of the light beam guided by the splitting means.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A focus detecting device according to an embodiment of the present invention will be described.

First, the configuration of a focus detection device according to a first embodiment of the present invention will be described with reference to FIG.

On the substrate 20, five double mode waveguides 21, 22, 23, 24 and 25 are formed in parallel at predetermined intervals. Double mode waveguide 21, 2
2, 23, 24, and 25 are arranged on the side surface of the substrate 20 with one end as an incident end surface. Further, the other ends are connected to branching units 201, 202, 203, 204, and 205, respectively, for branching the light propagating therethrough in two directions. Two waveguides 26 and 27 that propagate the split light are connected to the split unit 201. Similarly, the waveguides 28 and 29 are connected to the branch 202, the waveguides 30 and 31 are connected to the branch 203, the waveguides 32 and 33 are connected to the branch 204,
The waveguides 34 and 35 are connected to the branch portion 205. The ends of the waveguides 26 to 35 are connected to the substrate 20 as emission ends.
And photodetectors 41 to 50 are respectively attached.

The photodetectors 41 and 42 are connected to a differential amplifier 51 for outputting the difference between the two outputs. Similarly, a differential amplifier 52 is connected to the photodetectors 43 and 44, a differential amplifier 53 is connected to the photodetectors 45 and 46, and a differential amplifier 54 is connected to the photodetectors 47 and 48. Are connected, and the differential amplifier 55 is connected to the photodetectors 49 and 50.

A half mirror array 10 is mounted on the side surface of the substrate 20 on which the incident end faces of the double mode waveguides 21 to 25 are arranged. Half mirror array 10
Has a configuration in which five half mirrors 11 to 15 are provided in parallel at the same interval as the interval between the double mode waveguides 21 to 25.

In the regions of the double mode waveguides 21 to 25 and the branch portions 201 to 205, the length of the region in which light can propagate in the double mode is L, and the complete coupling length (the phase difference between the 0th mode and the 1st mode is L = Lc (2m + 1) / 2 (m = 0, 1, 2,...)
·) The lengths of the double mode waveguides 21 to 25 and the shapes of the branch portions 201 to 205 are determined so that Under this condition, the asymmetry of the phase distribution of the light incident on the incident end faces of the double mode waveguides 21 to 25 is due to the asymmetry of the branch portion 20.
It is selectively detected as the asymmetry of the electric field intensity distribution of the propagating light in 1 to 205.

A half mirror 2, a collimator lens 3,
Objective lenses 4 are sequentially arranged on the optical axis. The light source 1 is disposed on the optical axis deflected by the half mirror 2.

The half mirror array 10 is positioned at a position where the reflected return light from the test object 5 is sequentially reflected by the half mirrors 11 to 15 to be incident on the incident end faces of the double mode waveguides 21 to 25, respectively. Be placed.
At this time, the double-mode waveguides 21 to 25 are arranged such that the centers of the incident end faces are slightly displaced from the optical axis of the light reflected by the half mirrors 11 to 15 in the width direction of the waveguide.

When the test object 5 is at the focal position of the objective lens 4, the reflected light is reflected by the five double mode waveguides 21.
The distance between the substrate 20, the collimator lens 3 and the objective lens 4 is set so as to focus on the incident end face of the double mode waveguide 23 at the center of the ダ ブ ル 25. That is, the light source 1
And the double mode waveguide 23 are arranged such that the end faces thereof are conjugate to the half mirror 2.

In the present embodiment, a silicon substrate is used as the substrate 20, and a SiO.sub.2
_Two layers are formed. An SiO ₂ layer to which germanium is added is sequentially formed thereon, and this layer is formed into double mode waveguides 21 to 25, branch portions 201 to 205, and waveguides 26 to 205.
After patterning by photolithography 35 shape, by covering the SiO ₂ layer as an upper clad, double mode waveguide 21 to 25, the branch portion 201 to
205, and waveguides 26 to 35 are formed.

In the half mirror array 10, as shown in FIG. 7, multilayer films 271 to 275 functioning as half mirrors 11 to 15 are formed on thin glass plates 261 to 266, respectively. 266 are bonded together with an adhesive having the same refractive index to form a laminate. This is cut at 45 ° with respect to the main plane as shown by dotted lines 267 and 268 in FIG. Thereby, the multilayer film 271 is formed.
To 275 manufacture the half mirror array 10 acting as the half mirrors 11 to 15. Note that the intervals between the half mirrors 11 to 15 are determined so as to give a necessary optical path length difference to the light flux split by the half mirrors 11 to 15. The thickness of the glass thin plates 261 to 266 is the half mirror 11 to
It is determined to be 1 / √2 of the interval of 15. The distance between the double mode waveguides 21 to 25 is equal to the half mirror 11.
It is determined so as to match the interval of ~ 15.

As the light source 1, for example, a semiconductor laser having a wavelength of emitted light of 670 nm is used.

In the focus detection device having such a configuration,
The linearly polarized light emitted from the light source 1 is
After being reflected by the collimator lens 3 and becoming parallel light, the light is condensed on the object 5 by the objective lens 4. The light reflected on the test object 5 passes through the objective lens 4 and the collimator lens 3 again, and passes through the half mirror 2.

The light transmitted through the half mirror 2 passes through the half mirror array 10 including the half mirrors 11 to 15 and passes through the double mode waveguides 21 to 21 formed on the substrate 20.
25.

The optical path lengths of the reflected light from the test object 5 before entering the double mode waveguides 21 to 25 differ from one another by the optical path lengths at the intervals of the half mirrors 11 to 15, respectively.

When the test object 5 is located at the focal position of the objective lens 4, the reflected light from the test object 5 is arranged so as to be focused on the incident end face of the double mode waveguide 23, so that the double mode In the waveguides 24 and 25, the reflected light from the test object 5 is focused on the test object side 5 from the incident end face. In the double mode waveguides 21 and 22, the reflected light is focused behind the incident end face.

When the object 5 deviates from the focal position of the objective lens 4 by a certain amount on the rear side (farther side than the objective lens 4), the reflected light from the object 5 is transmitted through the double mode waveguide 22. Focus on the end face (Figure 2). When the test object 5 further shifts to the rear side, the reflected light focuses on the end face of the double mode waveguide 21.

On the other hand, when the object 5 is displaced from the focal position of the objective lens 4 by a certain amount on the front side, the reflected light from the object 5 is focused on the end face of the double mode waveguide 24 (FIG. 3). Further, when the test object 5 is shifted to the near side, the reflected light is focused on the end face of the double mode waveguide 25.

That is, the end face of the double mode waveguide where the reflected light focuses differs depending on the position of the test object 5.

When the reflected light from the test object 5 enters the double mode waveguide and the reflected light is focused at the position of the incident end face, the wavefront of the reflected light from the test object 5 becomes
Light that is parallel to the incident end face at the incident end face and enters the double mode waveguide with a phase symmetrical in the width direction of the double mode waveguide. On the other hand, when the focal point of the reflected light is shifted from the incident end face position, the wavefront of the reflected light from the test object 5 becomes a curved surface at the incident end face. At this time, since the center of the double mode waveguide has a positional relationship slightly shifted from the optical axis of the reflected light from the test object 5, light having an asymmetric phase is incident on the double mode waveguide.

The length of the double mode waveguides 21 to 25 is
Since the length L is set as described above, when light having a phase symmetrical incidence on the double mode waveguides 21 to 25, the electric field intensity distribution of the propagation light reaching the branch portions 201 to 205 is:
It becomes symmetrical in the width direction of the waveguide, and the intensities of the light branched in two directions at the branch portions 201 to 205 become equal. When light having an asymmetric phase is incident on the double mode waveguides 21 to 25, the electric field intensity distribution of the propagation light reaching the branch portions 201 to 205 has an asymmetry corresponding to the asymmetry of the incident light. Then, a difference occurs in the intensity of light branched in two directions by the branch portions 201 to 205. The intensity difference corresponds to the asymmetry of the incident light.

Therefore, the light propagating through the respective double mode waveguides 21 to 25 at the branching portions 201 to 205 is branched in two directions, propagated by the waveguides 26 to 35, and the light intensity is detected by the photodetectors 41 to 41. By detecting the difference with the differential amplifier 50 and obtaining the difference with the differential amplifiers 51 to 55, it is possible to obtain an output corresponding to the displacement of the focal point of the reflected light from the incident end face.

Specifically, the outputs of the differential amplifiers 51 to 55 are different from the shift amounts of the reflected light from the focal points at the respective incident end faces of the double mode waveguides 21 to 25 in FIG.
Draw an S-shaped curve like.

At this time, the double mode waveguides 21 to 25
There is an optical path length difference between the half mirrors 11 to 15 in the reflected light reaching the respective incident ends of
The amount of deviation of the reflected light from the focal point at each incident end also varies. Therefore, the differential amplifiers 51 to 5
Outputs 61 to 65 output from the object 5 correspond to deviation amounts of the object 5 from the focal point of the objective lens 4 as shown in FIG.
The shape becomes slightly shifted.

Therefore, as shown in FIG. 4, the reflection until reaching the incident end face of each of the double-mode waveguides 21 to 25 so that the linear portion of the S-shaped curve slightly overlaps with the linear portion of the adjacent S-shaped curve. By setting the optical path length difference of light, even when the displacement amount of the test object 5 is large, it can be detected in the linear region of the S-shaped curve of any one of the focus error signals of the differential amplifiers 51 to 55. Therefore, the focus detection device of the present embodiment can detect the displacement of the test object in a wider range than the focus detection range using one double mode waveguide.

Note that S output from the differential amplifiers 51 to 55
In order to make the straight line portions of the S-shaped curve overlap little by little, as shown in FIG. 4, when the range of the straight-line region of one S-shaped curve is S and the interval between adjacent S-shaped curves is Δd,
d <S must be satisfied. Here, Δd is the difference between the optical path length Δl of the reflected light until reaching the incident ends of the double mode waveguides 21 to 25, and the objective lens 4 and the collimator lens 3
Divided by the square of the magnification α of the optical system consisting of Further, S is determined in advance by experiments or the like because it is determined by the shapes of the double mode waveguides 21 to 25 and the shift amount of the optical axis. Thus, the optical path length difference Δl of the reflected light can be determined in consideration of the magnification of the objective lens 4 and the collimator lens 3 so as to satisfy Δd <S. The half mirror array 1 is arranged so as to satisfy the optical path length difference Δl.
The distance between the half mirrors 11 to 15 is 0
It is determined in consideration of the refractive index of 61 to 266. The distance between the incident end faces of the double mode waveguides 21 to 25 is determined to be equal to the distance between the half mirrors 11 to 15. Thereby, it is possible to obtain a focus error signal in which the linear portions of the S-curve output from the differential amplifiers 51 to 55 are overlapped little by little (FIG. 4).

Therefore, in the focus detection device of the present embodiment, if any of the detection ranges of the five double mode waveguides 21 to 25 includes the defocus amount of the test object 5,
The amount of defocus can be detected from the output of the differential amplifier connected to the double mode waveguide. Thus, a focus detection device having a wide detection range can be provided.

In order to select the output of the differential amplifier of the double mode waveguide in which the amount of defocus of the test object 5 is within the detection range, the following circuit can be provided. That is, as shown in FIG. 1, an adder 301 for obtaining the sum of the outputs of the photodetectors 41 and 42, an adder 302 for obtaining the sum of the outputs of the photodetectors 43 and 44, and the sum of the outputs of the photodetectors 45 and 46. , An adder 304 for calculating the sum of the outputs of the photodetectors 47 and 48, and an adder 305 for calculating the sum of the outputs of the photodetectors 49 and 50. Of these,
The double mode waveguide to which the adder having the largest output is connected is the double mode waveguide in which the defocus amount of the test object at that time is within the detection range. Therefore, the selection circuit 306 connecting the adders 301 to 305 selects the largest adder, so that the user can selectively select the output of the differential amplifier of the double mode waveguide connected to the selected adder. Can be used.
As a result, the test object selects the double-mode waveguide differential amplifier in the detection range without any hesitation, and the test object 5
Can be detected.

The amount of reflected light that reaches the incident end faces of the double mode waveguides 21 to 25 decreases every time it passes through the half mirrors 11 to 15. For this reason, since the outputs of the photodetectors 49 to 50 are lower than the outputs of the photodetectors 41 and 42, as shown in FIG.
In order to obtain two S-shaped curves, the amplification factors of the differential amplifiers 51 to 55 are adjusted and preset so that the peak output levels become the same.

As described above, in the focus detection device of the present embodiment, the reflected light when the test object 5 deviates from the focal position of the objective lens 4 is detected in a plurality of double modes.
Since a focus error signal can be obtained in any of the double mode waveguides according to the amount of defocus, it is possible to detect defocus over a wide range. Further, the inclination of the focus error signal obtained by each double mode waveguide is the same as that of the focus detection device using one double mode waveguide, so that the detection range is expanded while maintaining the detection accuracy. be able to.

Although the present embodiment has been described with an array having five double mode waveguides, it is possible to detect a larger defocus amount by further increasing the number of double mode waveguides. Needless to say. In this case, a half mirror array having half mirror surfaces corresponding to the number of double mode waveguides is required. In the present embodiment, a half mirror array is manufactured by cutting a laminate of glass and a multilayer film. Therefore, a half mirror array having many half mirror surfaces can be easily manufactured by increasing the number of layers. Usually, in order to overlap the linear portions of the adjacent S-shaped curves little by little as shown in FIG. 4, it is necessary to set the interval between the half mirrors 11 to 15 to a minute interval of about several tens μm. In the embodiment, since the half mirror array is manufactured by cutting the laminated body of the glass and the multilayer film, the half mirrors 11 to 15 are manufactured.
Can be set by the thickness of the thin glass plate.
Therefore, the half mirror 11 with a minute interval of about several tens μm
To 15 can be easily manufactured.

Since the above-described focus detection device can detect the amount of deviation of the object 5 from the focal position of the objective lens 4, it is necessary to check whether the object 5 is at the focal position of the objective lens 4. In addition to the focus detection device that detects whether
It can be used as displacement detection means for detecting the displacement amount of the test object 5.

[0044]

As described above, according to the present invention, it is possible to provide a focus detection device using a double mode waveguide and having a wide focus detection range.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing an optical path when a test object 5 is displaced in the focus detection device of FIG.

FIG. 3 is a block diagram showing an optical path when a test object 5 is displaced in the focus detection device of FIG. 1;

FIG. 4 is a graph showing a focus error signal of the focus detection device of FIG.

FIG. 5 is a block diagram showing a configuration of a focus detection device according to a conventional example.

FIG. 6 is a graph showing the shape of the output from one differential amplifier in the focus detection device of FIG. 1;

FIG. 7 is an explanatory view showing a manufacturing process of the half mirror array of the focus detection device in FIG. 1;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Half mirror, 3 ... Collimating lens, 4 ... Objective lens, 5 ... Object to be examined,
10: half mirror array, 11, 12, 13, 1
4, 15: half mirror, 20: substrate, 21,
22, 23, 24, 25 ... double mode waveguide, 2
6, 27, 28, 29, 30, 31, 32, 33, 3
4, 35 ... waveguide, 41, 42, 43, 44, 4
5, 46, 47, 48, 49, 50 ... photodetector, 5
1, 52, 53, 54, 55 ... differential amplifier, 20
1, 202, 203, 204, 205 ... branch part, 3
01, 302, 303, 304, 305...
306 ... Selection circuit.

Claims (4)

[Claims] 1. A light source, a condensing optical system for condensing light from the light source and irradiating the object with a light, and a plurality of light sources for reflecting light from the object to be incident on an incident end face and propagating the light. A double mode waveguide, and a detecting means for detecting the bias of the electric field distribution of the light propagated through the double mode waveguide, between the test object and the plurality of double mode waveguides A dividing unit that divides the reflected light from the test object into luminous fluxes corresponding to the number of the double mode waveguides and guides each of the luminous fluxes to the incident end face of the plurality of double mode waveguides; A focus detection device, wherein each of the mode waveguides is arranged such that the center of the incident end face is shifted from the optical axis of the light beam guided by the splitting means. 2. The double mode waveguide according to claim 1, wherein the plurality of double mode waveguides are formed on the same substrate, and the incident end faces of the double mode waveguide are arranged side by side on the substrate. A focus detection device, comprising: a half mirror array having a plurality of half mirror surfaces, wherein the half mirror array is disposed on the side surface of the substrate. 3. The apparatus according to claim 1, wherein said detecting means comprises: a branch portion for branching the light propagating through said plurality of double mode waveguides in two directions, respectively; A focus detection device comprising: a photodetector for detection; and subtraction means for calculating a difference between the intensities of the two lights detected by the photodetector. 4. A light source, a condensing optical system for condensing light from the light source and irradiating the object with light, and a light condensing optical system for causing reflected light from the object to enter and propagate from an incident end face A plurality of focus detection devices each including: a double mode waveguide; and a detection unit configured to detect a bias of an electric field distribution of light propagating through the double mode waveguide. The waveguide is formed on a common substrate, and the light source and the condensing optical system are disposed only in a single set in common to the plurality of focus detection devices, and between the test object and the substrate, Dividing means for dividing the reflected light from the test object into light fluxes of the number of the double mode waveguides, and guiding each of the light fluxes to the incident end faces of the plurality of double mode waveguides, is provided. The difference between the optical path lengths of the light beams A focus detection device, wherein a predetermined optical path length difference is set so that a detection range of a detection unit of the focus detection device partially overlaps.