JPH05203408A - Phase difference detector - Google Patents

Abstract

PURPOSE:To reduce the space occupied by an optical element and photodetector in a phase difference detector so as to simplify the constitution and reduce the size of the detector. CONSTITUTION:The title detector is a phase difference detector which photoelectrically detects a plurality of signals having a prescribed phase difference between them from the output light of an interferometer and is provided with a quarter wave plate 2, polarizing means constituted by arranging a plurality of polarizers 3-6 having polarizing axes which are partially different from each other by prescribed angles so that the light receiving quantities of the polarizers 3-6 can become nearly equal to each other, and photodetectors 7a-7d which are made to correspond to the polarizers 3-6 one to one. The phase difference detector also has such an advantage that alignment can be easily made by utilizing its output signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase difference detector in which the space required for an optical element is reduced and the size is reduced.

[0002]

2. Description of the Related Art In a fringe counting type interferometer, in general, sin, c is used for discriminating a moving direction of an object and dividing a signal.
The configuration is such that an os two-phase signal is obtained.

FIG. 10 is a block diagram of a conventional example. 15 is a laser light source, 17, 24 and 26 are polarization beam splitters (PBS), 20 is a moving corner cube, 21
Is a fixed corner cube, 22 is a half-wave plate, and 23
Is a beam splitter (BS), 25 is a quarter-wave plate, and 27 to 30 are photodetectors (PD).

The luminous flux 16 emitted from the laser light source 15 is P
The light enters the BS 17 and is separated into a p-polarized component 18 which is a polarized component parallel to the paper surface and an s-polarized component 19 which is a polarized component perpendicular to the paper surface. The p-polarized component 18 is reflected by the moving corner cube 20 to become measurement light, which then enters the PBS 17 again. The s-polarized component 19 is reflected by the fixed corner cube 21 to become reference light, which is superposed on the measurement light by the PBS 17. The light flux from the PBS 17 passes through the half-wave plate 22 and the polarization directions of both the reference light and the measurement light are rotated by 45 ° and then split into two by the BS 23. The light flux transmitted through the BS 23 is split by the PBS 24, and the PD 27 detects the p-polarized component and the PD 29 detects the s-polarized component. The interference signals obtained at PD 27 and PD 29 are 180 ° out of phase with each other.

On the other hand, the light beam reflected by the BS 23 passes through the quarter-wave plate 25 and, after a phase delay occurs between the reference light and the measurement light, is split by the PBS 26 and the p-polarized component is PD.
At 30, the s-polarized component is detected at PD 28. PD28
And the interference signals obtained by the PD 30 are 180 degrees out of phase with each other.
It has a phase difference of 90 ° and a phase difference of 90 ° from the signals of PD27 and PD29. That is, four-phase signals whose phases differ by 90 ° are obtained. Further, when signals having phases different by 180 ° are subtracted from each other, the bias component is canceled and a two-phase signal with doubled amplitude is obtained.

[0006]

However, in such a structure, the optical element used occupies a large space.
Normally, four-phase signals are used to remove the bias component, but in order to obtain this, a polarization beam splitter and a space for arranging the photodetectors facing each other in the traveling directions of the four light beams are particularly necessary. .. When a semiconductor laser is used as a light source or an optical fiber is used to downsize the length measuring device, the size of the signal detection system becomes an important issue. One solution to this problem is disclosed in Japanese Patent Application Laid-Open No. 1-280803, but since a diffraction grating is used, it is inevitable that the cost will be increased if a device having excellent characteristics is manufactured.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a phase difference detector having a small size and a simple structure, which occupies a small space for optical elements and photodetectors.

[0008]

SUMMARY OF THE INVENTION The present invention is a phase difference detector for photoelectrically detecting a plurality of signals having a predetermined phase difference from each other by the output light of an interferometer. One wavelength plate, a polarizing means in which a plurality of polarizers in which at least a part of their respective polarization axes differ by a predetermined angle are arranged so that the amount of light received by each polarizer becomes substantially equal, and the polarizing means of the polarizing means. It is characterized in that it is provided with a photodetector corresponding to each polarizer in a one-to-one correspondence.

Further, in the above-described structure, the correction means for generating a signal for correcting the output of the photodetector is arranged, the optical branching means is provided in the optical path of the output light of the interferometer, and one of the light splitting means is used. The light flux of 1 is guided to the photodetector through the polarization means, and the other light flux is guided to the correction means.

[0010]

The luminous flux transmitted through the quarter-wave plate is transmitted through the polarizing means, so that the luminous flux is not divided into a plurality of luminous fluxes and is divided into a plurality of portions having different polarization directions. And then
The interference fringes generated by the partial light transmitted through each polarizer are projected on the corresponding photodetectors with a predetermined phase difference and photoelectrically detected. For this reason, the space occupied by the optical elements is small, and the photodetectors can be arranged close to each other, so that miniaturization is facilitated without using expensive elements.

[0011]

FIG. 1 shows a first embodiment of the present invention. Reference numeral 1 denotes a light beam which is the output light of the interferometer and includes p-polarized measurement light and s-polarized reference light. Reference numeral 2 is a quarter-wave plate, and reference numerals 3 to 6 are polarizing plates, so that the polarization axes come in the directions of the arrows (different by 45 °). Reference numeral 7 is a quadrant photodiode (PD) having elements 7a to 7d.

The light beam 1 is transmitted through the quarter-wave plate 2, the measurement light component and the reference light component are converted into circularly polarized lights having opposite rotations and transmitted through the polarizing plates 3 to 6. At this time, the polarizing plates 3 to 6
Since the polarization axes of are different by 45 °, the phase is 90 °.
It becomes a four-phase signal that is shifted by four, and the four-division photodiode 7
Is incident on each of the elements 7a to 7d. Similar to the conventional example, by subtracting signals having phases different by 180 ° from each other, a two-phase signal in which the bias component is canceled and the amplitude is doubled can be obtained.

FIG. 2 shows a second embodiment of the present invention. Reference numerals 1 to 7 are the same as those in FIG. 8 is a beam splitter (B) disposed between the quarter-wave plate 2 and the polarizing plates 3 to 6.
S), and 9 to 12 are another pair of polarizing plates, and the direction of the arrow (4
The polarization axis comes to 5 degrees. 1
Reference numeral 3 is another set of four-division photodiodes (PD) having elements 13a to 13d.

In the second embodiment, even if the light beam laterally shifts due to the inclination of the moving object, it is possible to prevent an extreme change in the amplitude of the interference signal and an increase in the bias component. Now, the interference signals of the respective elements 7a to 7d of the four-division photodiode 7 are represented by the following equations (1) to (4), respectively.

Asa = I _a [1 + cos φ] ... (1)

ASb = I _b [1 + cos (φ−π / 2)] ... (2)

ASc = I _c [1 + cos (φ−π)] ... (3)

ASd = I _d [1 + cos (φ−3π / 2)] ... (4)

Here, I _{a to} I _d are amounts corresponding to the incident amounts on the respective elements 7a to 7d, and are amounts that change with lateral deviation of the light flux. Neglecting the difference in reflection / transmission of the beam splitter and the absorption in the polarizing plate, the four-division photodiode 13
Each element 13a, the amount of light incident 13b, 13c, to 13d are respectively _{I b, I a, I d} , the I _c of. Therefore, each element 13a to 1 of the four-division photodiode 13 is
The signal intensities of 3d are expressed by the following equations (5) to (8).

BSa = I _b [1 + cos (φ−3π / 2)] ... (5)

BSb = I _a [1 + cos (φ−π)] ... (6)

BSc = I _d [1 + cos (φ−π / 2)] ... (7)

BSd = I _c [1 + cos φ] ... (8)

In order to obtain a two-phase signal, the following (9), (1
0) is calculated.

S1 = (ASa + BSd) − (ASc + BSb) = 2 (I _a + I _c ) cos φ ... ... (9)

S2 = (ASb + BSc) − (ASd + BSa) = 2 (I _b + I _d ) sin φ ...・ (10)

In other words, the four-division photodiode 7 and the four-division photodiode 13 are provided with polarizing plates so that the intensities of signals whose phases are different by 180 ° are equal to each other and correction is performed. Therefore, as shown in the equations (9) and (10), signals having the same phase are added to each other so that the signal and the phase are 18
The bias component can be removed by taking the difference between the signals that differ by 0 °. Further, when the values of I _{a to} I _d change due to the lateral deviation of the light flux, the changes of I _a and I _b change so as to be complemented by I _c and I _{d, and} therefore, the expressions (9) and (10) represent. Thus, changes in the amplitudes of the signals S1 and S2 are also suppressed.

FIG. 3 shows a third embodiment. This is a configuration in which the combination of polarizing plates in the second embodiment is changed. That is, two polarizing plates 3 and 5 whose polarization axes differ from each other by 90 ° are arranged in the adjacent quadrant in one optical path.
In the other optical path, the polarization axis direction is 4
Polarizing plates 4 and 6 different by 5 ° are arranged in the same manner. At this time, the interference signals at the respective elements 7a to 7d of the four-division photodiode 7 are expressed by the following equations (11) to (14).

ASA = I _a [1 + cos φ] (11)

ASb = I _b [1 + cos (φ−π)] ... (12)

ASc = I _c [1 + cos φ] (13)

ASd = I _d [1 + cos (φ−π)] ... (14)

Further, the signal intensities of the respective elements 13a to 13d of the four-division photodiode 13 are as follows (1)
Expressions 5) to (18) are obtained.

BSa = I _b [1 + cos (φ−π / 2)] ... (15)

BSb = I _a [1 + cos (φ−3π / 2)] ... (16)

BSc = I _d [1 + cos (φ−π / 2)] ... (17)

BSd = I _c [1 + cos (φ−3π / 2)] ... (18)

Here, in order to obtain a two-phase signal, the following (1
9) and (20) are calculated. As a result, it can be seen that a two-phase signal having the same amplitude can be obtained although the bias component remains.

S1 = (ASa + ASc) − (ASb + ASd) = (I _a + I _c ) − (I _b + I _d ) + (I _a + I _c + I _b + I _d ) cos φ ...・ ・ ・ ・ ・ ・ ・ ・ (19)

S2 = (BSa + BSc) − (BSb + BSd) = (I _d + I _b ) − (I _c + I _a ) + (I _d + I _b + I _c + I _a ) sin φ ...・ ・ ・ ・ ・ ・ ・ ・ (20)

FIG. 4 shows a fourth embodiment. The fourth embodiment is particularly effective when the intensity distribution of the light flux is not uniform and the difference in the amount of light entering the photodiode elements is large. Similar to the second embodiment, the quarter-wave plate 2 and the polarizing plates 3 to
A beam splitter (BS) 8 is provided between the beam splitter 6 and the beam splitter 6 to split the light beam. One of the divided luminous fluxes passes through the ND filter 14 and is divided into a four-division photodiode (PD) 13
Is incident on and detected. Then, the signal output of the element 7a of the four-divided photodiode (PD) 7 is divided by the signal output of the element 13b of the four-divided photodiode (PD) 13, so By processing the signal output, it is possible to obtain a signal output that is not affected by the lateral displacement of the light flux and the intensity distribution. The branching ratio of the beam splitter is more efficient when reflection is smaller than transmission, but is not limited. In addition, the ND filter 14 does not need to be used.

FIG. 5 shows a fifth embodiment. In the first to fourth embodiments, an image guide 40 is provided instead of placing the four-division photodiode immediately after the polarizing plate. As a result, an image of the intensity distribution of the interference fringes is projected on a photodiode located in a remote place, for example, in the controller.
In the fifth embodiment, the photodiode and the electric circuit can be separated from the detection system, so that the detector can be further downsized.

In the above embodiment, by monitoring the output of each element, the lateral deviation of the light beam, that is, the deviation of the alignment of the moving mirror or the like can be detected. For example, in the second embodiment, the element 7 on the right half of the quadrant photodiode 7 is
a and 7d and the outputs of the elements 13b and 13c on the left half of the four-division photodiode 13, the elements 7b and 7c on the left half of the four-division photodiode 7 and the elements 13a and 13d on the right half of the four-division photodiode 13 are calculated. By subtracting the sum of the outputs of the above, the horizontal alignment error signal S _dx that does not include the AC component (assumed to be the x direction) is obtained. That is

S _dx = (ASa + ASd + BSb + BSc)-(ASb + ASc + BSa + BSd) ... (21)

It becomes Then, alignment may be performed so that S _dx becomes 0. Similarly, the sum of the upper half of each element is subtracted from the sum of the lower half to obtain the alignment error signal _{Sdy in} the vertical direction (y direction).

S _dy = (ASa + ASb + BSa + BSb)-(ASc + ASd + BSc + BSd) ... (22)

In the third and fourth embodiments, for example, from the outputs of the four elements of the four-divided photodiode 13, the presence or absence of the deviation and its direction can be known by the following calculation.

S _dx = (BSa + BSb) − (BSc + BSd) (23)

S _dy = (BSa + BSd)-(BSb + BSc) (24)

FIG. 6 shows a sixth embodiment. In this embodiment, instead of placing the four-division photodiode immediately after the polarizing plate in the first to fourth embodiments, four optical fibers 4 are used.
1, 42, 43, 44 and four photodiodes 45, 46, 47, 48 arranged corresponding to these are used. The light transmitted through the polarizing plates 4 to 6 is incident on the optical fibers 41 to 44 which are arranged two by two vertically and horizontally, and after transmission, is emitted toward the light receiving surfaces of the four photodiodes 45 to 48, respectively. In addition to the advantages of the fifth embodiment, the present embodiment uses a photodiode and an optical fiber that are relatively easily available, and therefore has the advantage of being inexpensive.

FIG. 7 shows a seventh embodiment. In this embodiment, the polarizing plate and the photodiode are integrally formed. That is, four light receiving portions 50a, 50b, 50c, 50d are manufactured on the photodiode substrate 50, and polarizing plates having different polarization directions are attached to the respective light receiving portions. Signals from the respective light receiving portions 50a, 50b, 50c, 50d are transmitted to the circuit board 52 via the conductive wires in the flexible board 51, and the output from this circuit board is sent to a counter (not shown) via the cable 53. It is like this. In this embodiment, since the polarizing plate, the photodiode and the circuit board can be unitized, there is an advantage that the entire detector can be made extremely small.

FIG. 8 shows an eighth embodiment. In this embodiment, the light beam that has passed through the quarter-wave plate 2 is diffused and then applied to the polarizing plate. That is, FIG. 8 (a)
As shown in FIG. 5, the light beam that has passed through the quarter-wave plate 2 is diffused by the concave lens 54, passes through the polarizing plates 3 to 6, and is then received by the quadrant photodiode 7. in this case,
If the diameter of the light beam is sufficiently larger than that of the photodiode 7, even if the optical axis of the light beam deviates from the center of the photodiode, the difference in light intensity is small. Therefore, there is an advantage that the mounting tolerance of the photodiode 7 can be made gentle. FIG. 8B shows a modified example in which the concave plano lens 55 and the polarizing plates 3 to 6 are bonded together. By integrating the concave plano lens and the polarizing plate in this way, it becomes easy to attach each optical element.

FIG. 9 shows a ninth embodiment. In this embodiment, the light beam is divided into four and then made incident on each polarizing plate and the photodiode. That is, FIG. 9 (a)
As shown in FIG. 3, the light beam that has passed through the quarter-wave plate 2 is divided into four by a beam splitting element 56 made of optical glass formed into a quadrangular pyramid shape, and then passes through polarizing plates 3 to 6 to be divided into four. It is made incident on the divided photodiode 7. In the present embodiment, if the light beam is made incident on the center of the element 55, it is divided into four fan-shaped beams due to the difference in the incident angle, and each beam is prevented from entering the boundary line of the polarizing plate. Therefore, there is an advantage that it can be prevented from being affected by the bonding error of the four polarizing plates. Further, it is possible to use inexpensive optical parts as compared with the case where a diffraction grating is used as described in JP-A-1-250803.
Also, unlike the diffraction grating, it is possible to use light other than the primary light, which is also advantageous in that the light quantity is large. The beam splitting element 56 may be one in which the center of the element is retracted inward as shown in FIG. 9B, or one in which four lenses are stuck together as shown in FIG. 9C. Further, in this embodiment, a lens may be arranged in front of or behind the beam splitting element 56 to reduce the beam. In this case, since the diameter of the beam incident on the polarizing plate or the photodiode becomes small, there is an added advantage that the mounting tolerance of the polarizing plate or the photodiode can be made gentle.

[0054]

As described above, in the phase difference detector of the present invention, the space occupied by the optical element and the photodetector is reduced, and the miniaturization is facilitated with a simple structure. Further, the alignment can be easily performed by utilizing the output signal.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a second embodiment of the present invention.

FIG. 3 is a configuration diagram of a third embodiment of the present invention.

FIG. 4 is a configuration diagram of a fourth embodiment of the present invention.

FIG. 5 is a configuration diagram of a fifth embodiment of the present invention.

FIG. 6 is a configuration diagram of a sixth embodiment of the present invention.

FIG. 7 is a configuration diagram of a seventh embodiment of the present invention.

FIG. 8A is a configuration diagram of an eighth embodiment of the present invention.
(B) is a figure which shows the partial modification in 8th Example.

FIG. 9A is a configuration diagram of a ninth embodiment of the present invention.
(B) is a diagram showing a modification of the beam splitting element in the ninth embodiment. (C) is a figure which shows the other modification of the beam splitter in 9th Example.

FIG. 10 is a configuration diagram of a conventional example that does not use the present invention.

[Explanation of symbols]

 2 quarter-wave plate 3-6 polarizing plate 7 four-division photodiode 9-12 polarizing plate 13 four-division photodiode 22 quarter-wave plate 27-30 photodetector 40 image guide 41-44 optical fiber 45-48 photodiode 50 Photodiode Board 51 Flexible Board 52 Circuit Board 53 Cable 54 Concave Lens 55 Concave Plane Lens 56 Beam Splitting Element

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[Procedure amendment]

[Submission date] December 4, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0052

[Correction method] Change

[Correction content]

FIG. 8 shows an eighth embodiment. In this embodiment, the light beam that has passed through the quarter-wave plate 2 is diffused and then applied to the polarizing plate. That is, FIG. 8 (a)
As shown in FIG. 5, the light beam that has passed through the quarter-wave plate 2 is diffused by the concave lens 54, passes through the polarizing plates 3 to 6, and is then received by the quadrant photodiode 7. in this case,
If the diameter of the light beam is sufficiently larger than that of the photodiode 7, the difference in light intensity is small even if the optical axis of the light beam and the center of the photodiode are deviated. Therefore, the photodiode 7
There is an advantage that the mounting tolerance of can be loosened. FIG. 8B shows a modified example in which the concave plano lens 55 and the polarizing plates 3 to 6 are bonded together. By integrating the concave plano lens and the polarizing plate in this way, it becomes easy to attach each optical element.

 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Hiroshi Yukawa 2-43-2 Hatagaya, Shibuya-ku, Tokyo Olympus Optical Co., Ltd. (72) Inventor Yukio Eda 2-43-2 Hatagaya, Shibuya-ku, Tokyo Olympus Optical Co., Ltd.

Claims (2)

[Claims] 1. A phase difference detector for photoelectrically detecting a plurality of signals having a predetermined phase difference from the output light when the output light of the interferometer is incident on the quarter wave plate and each polarization. Polarizing means, in which a plurality of polarizers having at least a part of axes different from each other by a predetermined angle are arranged so that the amounts of light received by the respective polarizers are substantially equal, and one-to-one with each polarizer of the polarizing means. A phase difference detector comprising a corresponding photo detector. 2. A correction means for generating a signal for correcting the output of the photodetector, an optical branching means is provided in the optical path of the output light of the interferometer, and one light beam split by the optical branching means is a polarizing means. 2. The phase difference detector according to claim 1, wherein the photodetector is arranged so that the other light beam is guided to the correction means.