JPH0652165B2 - Interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention splits incident light into two by a beam splitter, gives an optical path difference between the two split lights, and causes them to interfere again on the beam splitter. Two speed of light interferometers.
This type of interferometer is used in a wide range of applications such as inspection equipment for optical parts, rangefinders, and interferometers.

[Prior art]

A two-beam interferometer is shown in FIG. 7 as a device to which this type of interferometer is applied. In FIG. 7, the light emitted from the light source 1 is narrowed down by the diaphragm 2 and then made into parallel rays by the collimating reflecting mirror 3, and the two rays 6, 7 are made by the semitransparent film 5 formed on the substrate 4 by vapor deposition or the like. It is divided into The substrate 4 is usually a plane parallel plate and is made of a material that allows light to pass therethrough. Glass or fused quartz is used for a visible spectroscope, and single crystal materials of KBr, CsI, and KRS-5 are used for an infrared region. In the far infrared, the substrate 4 and the correction plate 8 are not necessary, and the semipermeable membrane 5 uses a polymer membrane such as polyethylene terephthalate.

The correction plate 8 is made of a material having the same thickness as the substrate 4, and is arranged so as to sandwich the semipermeable membrane 5. As an example, the light beam divided into two at the point A is reflected by the movable plane mirror 9 and the fixed plane mirror 10 and causes interference at the point B. At this time, if the correction plate 8 is not provided, the light beam 7 is divided into two and then passes through the substrate 4 twice,
The light ray 6 does not pass even once, and the refractive index of the substrate 4 varies depending on the wavelength of light, so that the optical distance between the fixed reflecting mirror 10 and the semitransparent film 5 varies depending on the wavelength. When it is desired to cause interference with a constant optical path difference irrespective of wavelength, by inserting a correction plate 8 of the same thickness and material into the light beam 6, the same wavelength-dependent optical distance as that of the light beam 7 side can be obtained. I also need to make it.

In the two-beam interference spectrometer of FIG. 7, it is necessary that all lights having wavelengths in the spectral region interfere with each other with the same optical path difference, and the correction plate 8 is always inserted.

The propulsion device 11 is for moving the movable plane mirror 9, and its moving distance, that is, the optical path difference between the light beam 6 and the light beam 7 is detected by the laser interferometer length measuring device 12 in units of laser wavelength,
Input to the computer 13 in real time. Since the movable plane mirror 9 and the fixed plane mirror 10 have a significant adverse effect on the measurement due to an error in the light reflection direction and an error in the optical path due to a slight inclination of the mirrors, the bearings in the propulsion device 11 are generally provided with air (not shown). Bearings are used and fixed plane mirror 10
An elevation adjusting device is attached to the device to adjust the elevation angle of the fixed plane mirror 10 periodically.

The light that interferes with a predetermined optical path difference is reflected by the irradiation mirror 14 to the sample 15
The light of various wavelengths is absorbed according to the spectral characteristics of the sample 15, and is collected by the condenser mirror 16. Further, the light is focused on the detector 18 by the detection mirror 17 and input to the computer 13 as an electric output. The computer 13 stores the detector output as data as a function of the optical path difference measured by the laser interferometer and stores the predetermined optical path difference data, and then obtains the spectral characteristic by Fourier transform.

[Problems to be solved by the invention]

Substrate 4, semi-permeable membrane 5 and compensator 8 constituting the beam splitter
The structure of the substrate 4 and the semipermeable membrane 5 is shown in FIG. Usually, it is a disk, and the thickness of the substrate 4 is required to be about 10% of the outer diameter for polishing which will be described later. The substrate 4 and the correction plate 8 are made of a light transmissive material. For example, a KBr single crystal material which is generally expensive in the mid-infrared region is used. Further, it is necessary to optically polish the surface, but since the KBr crystal is relatively soft to be polished and has deliquescent property, the polishing cost is considerably high. Further, the substrate 4 and the correction plate 8 need to have the same thickness, and a high-resolution spectrometer is required to have an accuracy of about ± 10 μm. The cost will rise further. In the visible region, fused silica is used as the material, so the material cost is relatively low, but the flatness required by polishing must be higher than that for infrared because the measurement wavelength is short. It is necessary that the difference in thickness between the substrate 4 and the correction plate 8 is also small, and the cost is about the same. In this way the beam splitter 19
The cost is large, and the cost is 20 to 40% of the direct material cost of the spectroscope excluding the computer 13.

Since the interferometer has dramatically improved sensitivity compared to the grating type spectrometer, it has been remarkably popularized in recent years and is becoming an indispensable device for the high-tech industry. However, the price is still higher than that of the grating type. In the main body of the spectroscope, the above-mentioned beam splitter 19 and the computer 13 required for Fourier transform are the main causes of the high price. The computer 13 is being developed at a low cost and with high performance due to the progress of electronic technology.
The beam splitter 19 is basically the same as that of several decades ago, and it is expected that the beam splitter 19 will be inexpensive and have good performance.

The present invention aims to provide an inexpensive and high performance interferometer with a new beam splitter.

[Means for solving problems]

According to the present invention, an interferometer is composed of a beam splitter formed of parallel plane plates having light-semi-transparent surfaces on both sides and two retro-reflecting mirrors arranged at a predetermined distance from both sides of the splitter. It is composed of

[Action]

With this configuration, the incident light is split on one surface of the beam splitter, the split incident light is reflected by each of the retroreflectors, and the split incident light is split by another surface. Since the generated incident light is interfered with, it is possible to remove the conventionally required correction plate from the interferometer. Further, since it is not necessary to use a material such as KBr that has deliquescent properties, is difficult to polish, and is hard to deposit, it is possible to provide an interferometer having excellent environmental resistance and stable operation.

〔Example〕

First, the structure of a retroreflector will be described before showing an embodiment of the present invention. Corner cube reflector as retroreflector
20 is shown in FIG. 10 (a). FIG. 10 (a) is an external view of the corner cube reflector 20. It consists of three plane mirrors that are perpendicular to each other, and the incident ray 21 is reflected three times on three surfaces and reflected as a reflected ray 22, but the reflected ray 22 is in the opposite direction and parallel to the joining ray 21. . FIG. 10 (b) shows the reflection property of the two-dimensional corner cube reflector. The incident light ray 21 emitted from the point A is reflected at the points B and D and becomes the reflected light 22 and reaches the point E. Corner cube reflector 20
If is inclined about the apex P as shown by the chain double-dashed line in Fig. 10 (b), the semi-radiation points become B ', D'and the optical path becomes like a dotted line, but the position and direction of the reflected light 22 are No change, ▲
In the two-dimensional case of ▼ = ▲ ▼ = 2 × ▲ ▼, which is clear from FIG. 14, the same can be proved in the geometrically three-dimensional case. FIG. 10 (c) is viewed from a direction forming an equal angle with respect to each reflecting surface of the corner cube reflector 20, and the incident light 21 is incident perpendicularly on the paper surface,
It is reflected as a thick solid line and becomes reflected light 21. At this time point A and point E, or point B and point D appear to be point symmetrical with respect to the vertex P. This is because it is viewed from the same direction as the light ray 21 (equal to the reflection direction of the light ray 22). At this time, even if the reflector is tilted about the apex P, the length of {circle around ()} is kept constant and the length is equal to 2 × OP.

FIG. 2 shows an interferometer according to an embodiment of the present invention. A fixed corner cube reflecting mirror 23 and a movable corner cube reflecting mirror 24 are arranged with respect to a plane parallel plate 25 made of germanium (Ge) as shown in the figure. The light beam 21 incident on the interferometer is a plane parallel plate.
A part of the light which is reflected by the point A ₀ on the front surface 26 of the 25 and is transmitted is further reflected by a point B ₀ on the back surface 27 of the plane-parallel plate 25 and partially transmitted. The point where the axis parallel to the reflected light 28 at the point A ₀ passes through the vertex P ₁ of the fixed corner cube reflecting mirror 23 and the front surface 26 is defined as O _1, and passes through the vertex P ₂ of the movable corner cube reflector 24. The point where the axis parallel to the transmitted light 29 at B ₀ collides with the back surface 27 is O ₂ . The fixed corner cube reflecting mirror 23 is moved to the movable corner cube reflecting mirror so that O ₁ O ₂ is perpendicular to the front surface 26 (and thus the back surface 27 is also vertical).
24 is adjusted by the xy stage attached to the fixed corner cube reflecting mirror 23. The moving direction of the movable corner cube reflecting mirror 24 is adjusted by a tilting device (not shown) so as to be parallel to the transmitted light 29 at the point B ₀ . By arranging the corner cube reflecting mirrors 23 and 24 and the plane parallel plate 25 in this way, the light rays 30 reflected by the fixed corner cube reflecting mirror 23 can be obtained.
A point D ₀ reaching the back surface 27 coincides with a point where the light ray 31 reflected by the movable corner cube reflector 24 reaches the back surface 27. We will prove this below.

That is, according to the laws of reflection and refraction, the reflected ray 28 and the refracted ray A
The three straight lines of ₀ B ₀ and the perpendicular 32 of the front surface 26 passing through the point A ₀ are on the same plane (this surface is referred to as S ₁ ), while the ray 30 and the refracted light C ₀ D ₀ Perpendicular 33 of front surface 26 passing through point C ₀
The three straight lines are also on the same plane. Since rays 28 and 30 are parallel and perpendicular 33 is also parallel, S ₁ and S ₂ are parallel to each other.
It is a plane. Therefore, if the intersection of the perpendicular line 32 and the back surface 27 is A ′ ₀ and the intersection of the perpendicular line 33 and the back surface 27 is C ′ ₀ , A ′ ₀ B ₀ is the intersection line of S ₁ and the back surface 27, and C ′ ₀ D ₀ is Since S ₂ and the back surface 27 are intersecting lines, A ′ ₀ B ₀ is parallel to C ′ ₀ D ₀ and both refractions have the same refraction angle, and therefore have the same length. On the other hand, due to the nature of the fixed corner cube reflector 23, A ₀ and C ₀ are symmetrical with respect to the point O, and O ₁ O ₂ is adjusted perpendicularly to the front surface 26 (and hence the back surface 27). a _'0 and C' ₀ becomes symmetrical with respect to the point O ', therefore the point B ₀ and the point D ₀ is symmetric with respect to the point O _2. The point at which the reflected light 31 reaches the back surface is a point symmetrical to the point B ₀ with respect to the point O ₂ , and eventually coincides with D ₀ ,
31 and 30 interfere with each other.

The rays that are not transmitted at the point B ₀ and are reflected are the points A ₁ , B ₁ , A ₂ ,
B ₂ ... is also divided into transmitted light and reflected light, and these lights interfere at points C ₁ D ₁ C ₂ D ₂ . Light rays that interfered with C ₁ C ₂ on the front surface 26 are returned to the incident side of the light, but D on the back surface 27
_The light beams that interfere with ₀ D ₁ D ₂ ... Are emitted as the output of the interferometer. It is assumed that the incident light amount is 1 and the amplitude intensity of outgoing light interfered with by D ₀ D ₁ D ₂ ... And E ₀ E ₁ E ₂ ... For example, in E ₂ ,
Ray A ₀ C ₀ D ₀ C ₁ D ₁ C ₂ D ₂ and ray A ₀ B ₀ A ₁ C ₁ D ₁ C ₂
D ₂ and light rays A ₀ B ₀ A ₁ B ₁ D ₁ C ₂ D ₂ include those that have passed through various paths, and these all interfere with each other. The amplitude reflectance of the plane-parallel plate 25 and the air is r _{1 and the} amplitude transmittance is t. For light of wavelength λ Let l (λ) be the optical length (refractive index × length) of, for example, the phase difference between the amplitude of the light ray A ₀ C ₀ D ₀ C ₁ D ₁ C ₂ D ₂ and the point A ₀ is Is. The negative sign of -r in the equation (1) is the parallel plane plate 2 from the air.
This is because the phase changes by π when the light incident on 5 is reflected at the point A ₀ . This amplitude and phase difference uses a complex number Can be written.

When the amplitude and phase difference of E ₀ E ₁ E ₂ ... E _n are examined, the results are as follows.

Becomes When the energy J = | E | ² is calculated with the energy reflectance R = r ² and the energy transmittance T = t ^2. Next J ₀ , J ₁ …… J _n are all optical path differences It can be seen that the light interferes with and does not depend on l (λ).

This is because all the light rays that converge at the point Dn pass through the parallel plane plate 25 the same number of times (2n + 1), and therefore, as in the conventional example of FIG. , Even if the correction plate 8 of the same material and the same thickness is not purposely arranged,
As shown in the embodiment of the figure, if an interferometer is configured with a parallel plane plate 25 and retroreflectors such as corner cube reflecting mirrors 23 and 24, the phase is corrected, and an interferometer that interferes with a constant optical path difference is provided. Obtainable. Here, FIG. 9 shows what happens when an interferometer is configured by the plane mirrors 9 and 10 and the plane parallel plate 25 without using the retroreflector. In FIG. 9, for example, the output E ₀ has a ray A ₀ C ₀ A ₀ B ₀ , a ray A ₀ B ₁ D ₁ B ₁ A ₀ B ₀ , and a ray A _0.
B ₁ A ₁ C ₁ A ₁ B ₁ A ₀ B _{0 and the} like are included. But these lights are all out of phase. For example, when the output E ₀ is calculated, the refraction angle of the refracted light A ₀ B ₀ is defined as Therefore, those having different phase differences cause multi-beam interference. Of the light reflected by each of the fixed reflecting mirror 10 and the movable reflecting mirror 9, the strongest one is left as an approximation. Becomes If you calculate the energy And the optical path difference Although 1 (λ) remains in the optical path difference, the phase is not corrected even if the approximation as shown in equation (12) is performed. Similarly, it can be proved that the phases of E ₁ E ₂ ... are not corrected. Therefore, an interferometer cannot be constructed with the plane mirrors 9 and 10 and the parallel plane plate 25, and retroreflectors 23 and 24 are required instead of the plane mirrors 10 and 9 as shown in FIG. 2 of the embodiment of the present invention.

Although the incident light is shown by a single line 21 in FIG.
In the two-beam interference spectrometer shown in FIG. 7 of the conventional example, a collimated parallel beam is incident by the collimating reflecting mirror 3. Therefore, when the light ray 21 is considered, E ₀ E ₁ E ₂ ... Are independent, and mutual interference does not pose a problem, but becomes problematic when a parallel light beam is incident. FIG. 3 shows the case where a parallel light beam is incident in the present invention. In FIG. 3, one wave front W of the incident light 21 is a plane parallel plate 25, a fixed corner cube reflector.
23. Due to the action of the movable corner cube reflector, split interference occurs to form a wavefront W ₀ W ₁ W ₂ ... And emitted light from the interferometer. W ₀ W ₁ ...... If the mutual interference is not taken into consideration, the intensity of each light beam is set to the incident energy of 1 and the third formula and the seventh formula are used.
It becomes E ₀ E ₁ E ₂ ... represented by the formula. Wave front W ₀ W ₁ W ₂
... are emitted with a shift perpendicular to the traveling direction of light. When the thickness of the plate is d, the incident angle of the incident light ray 21 is θ, and the refraction angle is the refraction angle, this interval a is a = 2d tan cos θ (14). When the wavefront is deviated in a direction perpendicular to the traveling direction, how much coherence can occur depends largely on the type of light and the surface state of the collimating reflecting mirror 3.

When the incident light is laser light, the wavefront W ₀ is coherent over almost the entire surface, and if there is a portion where W ₀ and W ₁ overlap, interference occurs, but in the case of an ordinary light source, coherence in the lateral direction is generated. When collimated with a small, polished concave mirror, coherence is sufficiently lost with a deviation of about ± 500 M with respect to mid-infrared light. Less for visible light. Recently, an aspherical mirror having a surface roughness of about 30M can be made by cutting an aluminum surface, but in this case, a deviation of about ± 100M will not sufficiently interfere. On the contrary, by using the collimator reflecting mirror 3 having an appropriate surface roughness, it is possible to limit the coherence in the lateral direction, and by increasing the thickness of the plane-parallel plate 25, the incidence of the light ray 21 is also increased. It can also be limited by increasing the angle θ.

From the above, E ₀ E ₁ E ₂ can be treated as independent light, so that the interferometer according to the present invention has an optical path difference with respect to a parallel light flux from a normal light source other than a laser. Can be interfered with.

Consider the case where germanium (Ge) is used for the plane-parallel plate 25 for the amplitude of the interference signal expressed by the equations (8) and (9). Ge is about T = 0.3 for reddish light having a wavelength of 20μ. If R = 0.7 without considering absorption Becomes When formula (18) is calculated for R = 0.7 and T = 0.3, the interference strength = 0.30, which is 71% of the maximum efficiency (2RT) 0.42 when R = 0.7 and T = 0.3. Wavefront W ₁ W ₂ ...
.. is affected by the deviation a, but the interference strength = 0.13 even with only the first term (wavefront W ₀ ) of the equation (18), and at least 60% of the maximum efficiency is guaranteed. Ge is almost ideal at about T = 0.5 for infrared light having a wavelength of 2 μm to 10 μm. R = 0.5
Then, 2RT ² = 0.25, which is equal to the maximum efficiency and there is no problem. The amplitude of the interference signal is actually different from the above equation. Ie optical path difference If is increased, the coherent light in normal light decreases,
Output decreases. However, this is exactly the same as in the prior art, and it can be said that there is no problem with the magnitude of the interference signal as described above.

An embodiment in which the interferometer according to the present invention is applied to the two-beam interference spectrometer shown in FIG. 7 is shown in FIG. The difference is clear when compared with FIG.

That is, in this embodiment, the correction plate required in the conventional example of FIG. 7 is not necessary. As a result, the cost of materials and the cost of polishing the beam splitter 19 can be reduced. Further, conventionally, an accuracy of about ± 10 μm was required for a high-resolution spectrometer with respect to a thickness error between the substrate 4 and the correction plate 8, but it is no longer necessary to make the thickness uniform as a set of two sheets. . Since this work of adjusting the thickness is repeated polishing and measurement, and a considerable amount of time is spent, the effect of this embodiment is even greater. Further, in the conventional case, it is necessary to perform an assembling work in which a spacer is provided between the substrate 4 and the correction plate 8 to be incorporated in one holder. At this time, since an air layer is formed between the substrate 4 and the correction plate 8, multi-beam interference occurs between them and a specific wavelength is often absorbed. Substrate 4 in this case
And the correction plate 8 must be assembled with a slight inclination. In this embodiment, the above assembling work is not necessary at all, the holder is simply fitted, and there is no fear of multi-beam interference. In this respect, the cost will be further reduced.

Next, in this embodiment, it is not necessary to deposit the semipermeable membrane 5. The vapor deposition work is usually performed while heating the substrate 5. For example KBr
When a semi-permeable film of Ge is vapor-deposited on the substrate, the film quality and film strength increase as the temperature of the substrate 5 rises, but if the temperature is raised too much, the surface accuracy of the KBr that has been polished will deteriorate, and its control is considerably controlled. difficult. The cost of the beam splitter 19 can be further reduced because no such labor of vapor deposition is required. The fact that a semi-permeable membrane is not necessary is advantageous not only in cost but also in performance. If it goes down, as an example, a conventional beam splitter that deposits a semi-transmissive film of Ge on KBr utilizes interference of a Ge thin film, but the wavelength range of light that can be split is limited by the thickness of the thin film.
For example KBr is 400 cm ^-1 through light well from (infrared light) about 30000Cm ^-1 (visible), but make only those to around at most 5000 cm ^-1 When made beam splitter from 400 cm ^-1 in Ge film Absent. But according to the invention K
It is possible to utilize the entire Br light transmission region. However, the refractive index of KBr is about 1.5 to 1.7 in this region, and the reflectance R in the case of vertical incidence is several percent, so it is necessary to increase the reflectance by increasing the incident angle, but 400 cm ^-1
It is possible to make a beam splitter effective from ¹ to 30000 cm ^-1 . Since the material does not need to be particular about KBr, the incident angle can be reduced by using thallium bromochloride (KRS-6) of thallium bromoiodide (KRS-5) having a high refractive index.

Although germanium or thallium bromoiodide has been described as a material for the plane-parallel plate in the above-described embodiments, the present invention is not limited to this. For example, a semiconductor material such as silicon or gallium and an oxide, zinc selen (ZnSe), or the like. Dielectric materials can be used.

If the incident angle cannot be increased by any means, vapor deposition films 34 and 35 may be formed on both surfaces of the plane-parallel plate 25 as in the other embodiment shown in FIG. In this case, the merit caused by not requiring the above-mentioned vapor deposition is lost, but the merit caused by not requiring the correction plate is still effective. Compared with the conventional one in terms of cost, the substrate 4 of FIG.
Evaporation is also required on one side, but if you devise a jig and evaporate on both sides without breaking the vacuum of the evaporation system at the time of evaporation,
It does not increase the cost significantly.

Another embodiment of the present invention shown in FIG. 17 is the same as that shown in FIG. 4, in which semipermeable membranes are vapor-deposited on both sides, but a transparent portion is formed. That is, in FIG. 17, the semi-permeable film 225 is vapor-deposited on the upper half of the front surface 224 of the substrate 204 made of a plane-parallel plate made of a light-transmissive material, and the semi-permeable film 227 is vapor-deposited on the lower half of the back surface 226. Has been done. FIG. 18 is an optical path diagram of an interferometer in which the beam splitter 219 shown in FIG. 17 and the retroreflectors 10 and 9 are combined. FIG. 18 (a) is a semi-transparent film in which the incident light beam AB is deposited on the upper half of the front surface 224 of the beam splitter 219.
FIG. 18 (b) shows the case where the incident light beam AB first enters the 225, and the incident light beam AB first enters the semitransparent film 227 deposited on the lower half of the back surface 226 of the beam splitter 219. In FIG. 18 (a), the incident light beam AB is the semipermeable membrane 225 of the beam splitter 219.
And two rays ▲ ▼ and ▲ ▼
The points F and F'are coincident with each other and interfere in the same manner as in the embodiment shown in FIG. Both optical paths pass through the substrate 204 only once until they interfere with each other after being divided at the point B, and their phases are corrected. The same applies to the case of FIG. 18 (b). Since the beam splitter 219 does not divide the light many times as shown in FIG. 2, it is possible to use not only normal light but also light with high coherence such as laser light. It is the same as the embodiment shown in FIG. 7 that vapor deposition is necessary but other advantages are not lost.

FIG. 11 shows another embodiment of the present invention in which two kinds of semi-transparent films 128 for infrared light and semi-permeable film for visible light 129 are provided on the same beam splitter 119. In this example, the semitransparent film 129 for visible light serves as a beam splitter for the laser light emitted from the laser interference side distance device 12 shown in FIG. 1 or 12. According to the conventional method, the visible semi-permeable film could only be formed in the area A of FIG. 11, but according to the present invention, the deposition locations can be alternated as shown in FIG. Can be clearly distinguished from each other, and thus various problems occurring at the boundary between two types of semipermeable membranes, which have occurred in the conventional method, can be solved. Therefore, if two kinds of semipermeable membranes are provided on the back surface of the substrate in a staggered manner with the front surface by the same method,
It is possible to make a beam splitter 119 capable of simultaneously measuring from the infrared region to the visible region.

FIG. 12 (a) is a still another embodiment of the present invention, which is different from that shown in FIG. 4 in that the semipermeable membrane 125 on the front surface 124 and the back surface 126 are different.
That is, a slight gap d is formed at the boundary with the semipermeable membrane 127 above. FIG. 12 (b) is a modification of FIG. 4,
As described above, since the diaphragm 2 of FIG. 1 or 7 has a finite size, the light beam incident on the interferometer is not a perfect parallel light beam, but contains a slightly tilted component. This inclination is about 3 degrees at most. This tilted light is shown in Fig. 12 (b).
As described above, after the B is divided into two, the C is divided into two again. Therefore, in the case of an ideal semipermeable membrane, the light ray CD is the light ray A.
1/4 of B, 1/4 of C'D 'and 1/4 of BC' are also halved, and the light amount of the light beam CD which should originally interfere decreases, and the light beam CD 'and the light beam CD interfere with each other. . However, since it is a small amount when viewed from the total light amount, it is not a problem normally, but when the light amount near the center is particularly strong, it appears as stray light. The embodiment shown in FIG. 12 (a) solves this problem. For example, the substrate 4 has a refractive index of 1.5 and a thickness of 6
mm, and the tilt angle of the incident light from the parallel light is 3 degrees at maximum, from the law of refraction, Becomes

FIG. 13 shows still another embodiment of the present invention, in which the beam splitter is vertically divided. FIG. 14 is an example in which an interferometer is configured using the beam splitter 119 shown in FIG.
A gap is provided between the semi-permeable membrane 125 on the front surface and the semi-permeable membrane 127 on the back surface so that the semi-permeable membrane 125 on the front surface and the semi-permeable membrane 127 on the rear surface do not divide into two at the point B and further overlap at the point C. In FIG. 14, the light split into two by B interferes with F, and the light split with F interferes with B, so that the outgoing light has a vertical gap corresponding to ▲ ▼.
On the other hand, in FIG. 7, the irradiation mirror 14 and the condensing mirror 16 measure the sample 15 that transmits light. Generally, these mirrors can be replaced as an attachment, for example, a reflection type sample or light. There are optics that measure scattered powder. However, it is necessary to adjust the optical axis when exchanging these optics. First
In this embodiment, the laser 130 for adjusting the optical axis is emitted from the reflecting mirror 131 using the gap CG in FIG.

In addition, the beam splitter configuration is shown in Fig. 15 (a) (b).
Such an embodiment is also possible. Figure 15 (a) shows an example in which the beam splitter is divided into several parts, and (b) shows an example in which the optical axis is shifted.

Further, as the retroreflector 23 or 24, the corner cube reflecting mirror is used as shown in FIG. 2 in the embodiment, but it goes without saying that other retroreflectors may be used. For example, a corner cube prism shown in Fig. 10 (a), a retroreflector called a cat's eye reflector in Fig. 10 (b), and a mirror like two plane mirrors attached at right angles as shown in Fig. 10 (c). Alternatively, a triangular prism having two surfaces at right angles can be used.
The corner cube prism is similar to a corner cube reflector in the form of a prism, and has been widely used in recent years in a side finder for surveying using a laser.

The interferometer according to the present invention can be applied not only to the interferometry device but also to other applications. FIG. 6 shows an example applied to a distance measuring device. In FIG. 6, the XY stage 36 is moved in the direction of the arrow by a drive motor (not shown). X-coordinate retroreflector
The 37 and Y coordinate retroreflector 38 is a retroreflector of the type shown in FIG. 10 (c) and is attached to the XY stage. A laser beam 40 emitted from a laser light source 39 is split into two by a beam splitter 41, one of which is used for measuring the X coordinate and the other of which is used for measuring the Y coordinate. Since the method of measuring the X coordinate and the Y coordinate is the same, only the X direction will be described. The light separated by the beam splitter 42 at the point A is reflected by the X-coordinate retroreflector, and the other light is reflected by the reference retroreflector 43, causing interference at the point B of the beam splitter 42. The light reflected after the interference is detected by the length measurement detector 44, and the light transmitted after the interference is detected by the direction detector 45. Laser light
Reference numeral 40 denotes linearly polarized light, which has a P component and an S component with respect to the semipermeable membrane surface on the beam splitter 42, but the 1/4 wavelength plate 46 has a P component.
The component is delayed by 1/4 wavelength, and the direction detector 45 detects the P component by the polarizing plate 47. The S component interferes without phase shift, and is detected by the detector 44 by the action of the polarizing plate 48. In this way, the outputs of the detector 44 and the detector 45 are out of phase with each other by about λ / 4, and the directions can be detected by the same principle as the rotary encoder. Now, the important point is that the beam splitter
42 and the reference retroreflector 43 do not move. If these positions move during measurement, the detector 44 naturally outputs an output as if the XY stage 36 moved by that amount, resulting in erroneous measurement. Since the laser light is monochromatic, chromatic dispersion due to the material does not matter. However, if a conventional beam splitter with the correction plate 8 removed is used instead of the beam splitter 42, the optical path difference on one side changes due to the thermal expansion of the material, so an expensive material with a small expansion coefficient is required. . However, when the correction plate is inserted, the above-mentioned problems of the correction plate, such as an error in the thickness and cost, occur. Therefore, if the beam splitter 42 according to the present invention is used so that it can be expanded toward the B side with respect to the point A side, the effect of thermal expansion can be completely corrected. Moreover, since the structure is simple, the advantages described above can be used as they are.

〔The invention's effect〕

As is apparent from the above description, according to the present invention, a beam splitter composed of parallel plane plates having light semi-transmissive surfaces on both sides and a pair of retroreflectors are used, and the light split on one surface is Since the interferometer was configured to interfere in the plane of
Before the split light beams interfered with each other, they passed through the same distance in the substrate, and it was possible to obtain the same correction effect as the function of the correction plate that was necessary in the past. As a result, the correction plate, which was required in the past, is no longer required, and the cost as well as the performance can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an interferometry device to which the interferometer according to the present invention is applied, FIGS. 2, 3, and 4 are block diagrams showing an embodiment of the interferometer according to the present invention, and FIG. FIG. 6 is a schematic diagram showing an example of a retroreflector, FIG. 6 is a block diagram showing another embodiment of an apparatus to which the interferometer according to the present invention is applied, and FIG. 7 is a conventional interferometer using the interferometer. FIG. 8 is a block diagram, FIG. 8 is a schematic diagram showing a conventional beam splitter, FIG. 9 is a block diagram showing the action when the plane mirror of the beam splitter of the present invention is combined, and FIG. 10 is the action of a retroreflector. Schematic diagram for explaining, FIGS. 14 and 17 are configuration diagrams showing an embodiment of an interferometer according to the present invention, respectively.
11 to 13 and 15 to 16 are schematic views showing examples of the beam splitter according to the present invention. 4: substrate, 5: semipermeable membrane, 8: correction plate, 19: beam splitter, 23: fixed retroreflector, 24: movable retroreflector, 25, 12
5: Semipermeable membrane, 27, 127: Semipermeable membrane.

Claims (10)

[Claims] 1. An interference, comprising: a beam splitter made of parallel plane plates having light-semi-transparent surfaces on both sides; and two retroreflectors arranged at a predetermined distance from both sides of the splitter. Total. 2. The interferometer according to claim 1, wherein the beam splitter is a plane-parallel plate made of a semiconductor material. 3. The beam splitter according to claim 1, wherein the beam splitter is formed by forming semi-transmissive films at different positions on both surfaces of a plane-parallel plate made of a light-transmissive material. Interferometer. 4. The interferometer according to claim 2, wherein the semiconductor material is germanium. 5. An interferometer according to claim 2, wherein the semiconductor material is silicon. 6. The interferometer according to claim 2, wherein the semiconductor material is potassium arsenide. 7. An interferometer according to claim 1, wherein the material of the plane-parallel plate is a dielectric material. 8. An interferometer according to claim 7, wherein the dielectric material is zinc selenide. 9. An interferometer according to claim 7, wherein the dielectric material is thallium bromoiodide. 10. An interferometer according to claim 7, wherein the dielectric material is thallium bromochloride.