JPH0829124A - Displacement measuring device and optical pickup - Google Patents

Abstract

PURPOSE:To detect a focus error signal highly accurately by splitting the reflected light from a substance to be measured into two luminous fluxes, generating an interference light by overlapping optical paths, reading the change in interference fringes caused by the movement of the substance to be measured, and detecting the displacement into the direction of the optical axis. CONSTITUTION:A reflecting and transmitting means 10 comprises a first partial reflecting-part transmitting mirror 11, which transmits a part of the light from a beam splitter 3 and reflects another part, a reflecting mirror 12, which reflects the transmitted light TR from the mirror 11, a reflecting mirror 13, which reflects the reflected light RF from the mirror 11, and a second partial-reflecting-part transmitting mirror 14, into which the respective light beams from the mirrors 12 and 13 enter in the overlapped state. The mirrors 11, 12 and 13 are set so that the interference fringes are generated in one of two luminous fluxes BM1 and BM2. When the luminous flux BM1 or BM2 from the mirror 14 enters and the interference fringes are generated, light receiving means 7 and 8 read the phases and pitches the interference fringes, which are changed by the movement of a substance to be measured 5 that is caused by the movement into the direction of the optical axis, and detects the displacement.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement measuring device and an optical pickup for measuring the displacement of an object to be measured such as a record carrier such as an optical disk.

[0002]

2. Description of the Related Art Conventionally, there is known an optical pickup that reproduces information recorded on an optical record carrier such as an optical disk by using its reflected light or records information.

A focus servo system is used in this kind of optical pickup, and as the focus servo system, there are an astigmatism method, a critical angle method, an if edge method and the like. Among them, the astigmatism method (optical disc technology by Radio Morio Co., Ltd. under the supervision of Morio Onoe, p99) is widely used not only for optical heads for magnetic discs but also for general optical discs including compact discs and laser discs.

The principle is that if the light receiving means (photodetector) is a four-division light receiving surface (each output is A, B, C, D), the incident light is focused on the disk. At that time, the image of the reflected light becomes circular on the 4-division light receiving surface of the photodetector,
At this time, the differential output (A + C)-(B + D) between the diagonal light receiving surfaces, which is the focus error output, becomes zero. On the other hand, if the disc becomes far or near from the objective lens,
The image of the four-division light receiving surface of the photodetector changes from a circular shape to an elliptical shape. Therefore, the focus error output becomes positive (far) or negative (close), so the position of the objective lens is adjusted so that this output becomes zero.

[0005]

By the way, generally, in this type of apparatus, in order to shorten the access time, it is important to reduce the size and weight of the optical pickup. However, in the focus detection method such as the conventional astigmatism method, a change in the shape of the beam is detected. Sensitivity cannot be obtained, and therefore there is a limit to miniaturization. In addition, the spot on the light receiving element is quite small, from several microns to several tens of microns, adjustment is difficult, and an offset occurs depending on the environment, which is unstable.

For this reason, the applicant of the present invention has devised a displacement measuring device (optical pickup) as shown in FIG. 1, for example.
Referring to FIG. 1, this displacement measuring apparatus condenses and irradiates light from a light source 1 onto an object to be measured (for example, an optical disk) 5 through an objective lens 4 via a collimator lens 2 and a beam splitter 3, and the object to be measured 5 The reflected light from the objective lens 4,
Two diffraction gratings 6a and 6b through the beam splitter 3
When the parallel light is made incident on the double diffraction grating 6 made of, and the parallel light is made incident on the double diffraction grating 6, the phase and pitch of the interference fringes generated by the interference between the diffracted lights are detected by a photodiode or the like. Light is received by the light receiving means (light receiving element) 7, and the displacement of the DUT 5, for example, the optical axis direction of the DUT 5 (the optical axis direction of the light incident on the DUT; the optical axis direction of the objective lens 4) x The displacement (movement amount) of is detected.

The principle of interference fringe generation using the double diffraction grating 6 consisting of the first diffraction grating 6a and the second diffraction grating 6b will be described with reference to FIGS. Now
It is assumed that the first diffraction grating 6a and the second diffraction grating 6b of the double diffraction grating 6 have pitches Λ ₁ and Λ ₂ , respectively, and light of the wavelength λ is vertically incident on the first diffraction grating 6a. . Note that the generality of the present invention is not lost even if the incidence is not normal.

In the double diffraction grating 6, the first diffraction grating 6a generates ± n-order light (n is positive), and the second diffraction grating 6a generates the second light.
In the diffraction grating 6b, the + m-order light of the + n-order light (m is positive) and the + m-order light of the -n-order light (m is positive) are generated. Then, the ± generated by the second diffraction grating 6a
The m-th order lights are caused to interfere with each other to generate interference fringes. In addition, +
Indicates that the incident light is diffracted to the left in the traveling direction, and-represents the opposite case.

Here, the diffraction condition of the + nth order light in the first diffraction grating 6a is expressed by the following equation. In the case of -nth order, n may be replaced with -n, and therefore the description thereof is omitted below.

[0010]

## EQU1 ## sin θ ₁ = nλ / Λ ₁

The diffraction condition at the second diffraction grating 6b is expressed by the following equation.

[0012]

2 −sin θ ₂ + sin θ ₁ = mλ / Λ ₂

From the equations 1 and ₂ , the following equation is derived for θ ₂ .

[0014]

(3) sin θ ₂ = λ (n / Λ ₁ −m / Λ ₂ )

As shown in FIG. 4 (a), when the incident angle of θ ₂ is 2
Pitch Λ _{0 of} interference fringes by two lights (plane waves) BM ₁ and BM ₂
Is expressed by Expression 4, and the phase of the interference fringes due to the relative phase is expressed by Expression 5.

[0016]

[Formula 4] Λ ₀ = λ / (2sin θ ₂ )

[0017]

[Formula 5] β ₀ = β ₁

Here, β ₀ is the phase of the interference fringe, and β ₁ is the phase between the plane waves BM ₁ and BM ₂ . Therefore, the relationship between the pitch of the interference fringes and the pitch of the double diffraction grating is expressed by Equation 6 using Equations 3 and 4.

[0019]

## EQU6 ## 1 / (2Λ ₀ ) = n / Λ ₁ −m / Λ ₂

Regarding the phase relationship in the case of the double diffraction grating 6 (see FIG. 4B), if the problem is only interference of diffracted light of positive and negative orders, the phase relationship immediately after the diffraction grating is reversed. Therefore, the phase of the interference fringe is expressed by the equation 7.

[0021]

(7) β ₀ = 2β ₂

From this, the pitch of the interference fringes is the pitch of the double diffraction grating 6 (that is, the pitch Λ _{1 of} the first diffraction grating 6a).
And the pitch Λ ₂ ) of the second diffraction grating 6b, it is completely independent of the wavelength λ of the incident light. When the light diameter is W ₀ and the right side and the left side of Expression 6 are multiplied, Equation 8 is obtained.

[0023]

[Equation 8] _{(W 0 / Λ 0) /} 2 = nW 0 / Λ 1 -mW 0 / Λ 2

Here, W ₀ / Λ ₀ is the number of interference fringes generated within the light diameter, and nW ₀ / Λ ₁ and mW ₀ / Λ ₂ are in the first diffraction grating 6a and the second diffraction grating 6b. It is obtained by multiplying the number of diffraction gratings within the light diameter by each order. That is,
It becomes the following formula.

[0025]

## EQU9 ## << Number of interference fringes >> / 2 = Order * << Number of first diffraction grating >>-Order * << Number of second diffraction grating >>

As described above, the relationship between the number of interference fringes, the number of the first and second diffraction gratings 6a and 6b, and the respective orders thereof has been clarified. Although interference fringes are generated regardless of which order is used, ± 1st order light is superior to high order diffracted light because it has high diffraction efficiency. That is, the + 1st-order light generated by the first diffraction grating 6a and the -1st-order light of the second diffraction grating 6b.
(E in FIG. 3) and the −1st order light generated by the first diffraction grating 6a and the + 1st order light (F in FIG. 3) of the second diffraction grating 6b are most effective.

As an example of the number of interference fringes, when only ± 1st order light is used, when a diffraction grating having a very high density of Λ ₁ = 0.948 μm is used in order to achieve high resolution, Λ ₀ = 1 mm is set to a large value. Therefore, Λ ₂ = 0.94768 μm.

The difference between Λ ₁ and Λ ₂ is about 0.03%, which is very small, but can be created. If the diameter of the collimated light is set to about 2 mm, one or two interference fringes will be observed.

The above is the principle of interference fringe generation using the double diffraction grating 6.

A method of measuring the displacement of the object to be measured using this double diffraction grating will be described below with reference to FIG.

Referring to FIG. 5, a lens 101 is set on the surface of the object to be measured 5 so as to be substantially in focus, and a double diffraction grating 6 is set on the optical axis of the lens 101.

In FIG. 5, the focal length of the lens 101 is f, the distance between the lens and the surface of the object to be measured is b ₁ , the focusing position on the double diffraction grating side is b ₂ , and the lens aperture is A. There is. Further, the distance (defocus amount) between the lens focus position and the surface of the object to be measured is set to d, and the double diffraction grating 6 (6a, 6a,
The angle of incidence on 6b) is θ (the upper angle of the optical axis is θ ₁ and the lower angle is θ ₂ ). In this case, when the distance between the first and second diffraction gratings 6a and 6b of the double diffraction grating 6 is T and d << f, the following formula is established.

[0033]

## EQU10 ## 1 / f = 1 / b ₁ + 1 / b ₂ θ = A / b ₂ b ₁ = f + d

From Equation 10, b ₂ is expressed by the following equation.

[0035]

B ₂ = fb ₁ / (b ₁ −f)

From equations 10 and 11 θ is expressed by

[0037]

[Equation 12] θ = A (b ₁ −f) / fb ₁ = Ad / f (f + d)
≒ Ad / f ²

Here, it is assumed that the defocus amount d is minute, that is, d << f. In this case, the lens 10
The light emitted from the lens 1 is close to the collimated state,
And the double diffraction grating 6 are close to each other, the x-axis (x = 0 on the optical axis) along the first diffraction grating 6a as shown in FIG.
, And when taking the X axis (X = 0 on the optical axis) along the second diffraction grating 6b, A = x can be obtained, so
Is given by

[0039]

[Equation 13] θ = xd / f ²

As described above, the incident angle of the light beam differs depending on the position (x). The light which has entered the double diffraction grating 6 from both sides with respect to the optical axis and which has been diffracted twice by the double diffraction grating 6 is, as shown in FIG. 5, an emission surface (interference fringe generation surface). Meet at. Since these two lights BM ₃ and BM ₄ have different emission angles,
Interference occurs between these and interference fringes occur.

Next, the pitch of the interference fringes at each position is obtained.
The angle θ ₃ at which light above the optical axis is incident on x at y = 0 and exits at the exit surface is represented by the following equation (see FIG. 7).

[0042]

(14) sin θ ₁ −sin θ ₃ = λ (1 / Λ ₂ −1 / Λ ₁ )

Since θ _{1 to} 0 and θ _{3 to} 0, θ ₃ is given by the following equation.

[0044]

[Equation 15] θ ₃ = θ ₁ + λ (1 / Λ ₁ −1 / Λ ₂ )

Substituting equation 13 into equation 15, the following equation is obtained.

[0046]

[Equation 16] θ ₃ = xd / f ² + λ (1 / Λ ₁ −1 / Λ ₂ )

It is desired to define the position X of the light on the emission surface (y = T) of the second diffraction grating 6b of the double diffraction grating 6, but for the sake of simplicity, the diffraction angle of the first diffraction light is 45 °. Then, the following equation is obtained.

[0048]

X = x−T

Substituting equation 17 into equation 16, the following equation is obtained.

[0050]

[Equation 18] θ ₃ = d (X + T) / f ² + λ (1 / Λ ₁ −1 / Λ ₂ )

Similarly, the angle θ ₄ at which light below the optical axis is incident on x at y = 0 and exits at the exit surface is expressed by the following equation.

[0052]

Θ ₄ = d (X−T) / f ² + λ (1 / Λ ₁ −1 / Λ ₂ )

The pitch Λ ₀ of the interference fringes when the incident angles of the two light beams are θ ₃ and θ ₄ and θ ₃ ˜0 and θ _{4 = 0} are expressed by the following equation.

[0054]

[Formula 20] Λ ₀ = λ / (| sin θ ₃ + sin θ ₄ |) = λ /
(| Θ ₃ + θ ₄ |)

Substituting the equations 18 and 19 into the equation 20, the following equation is obtained.

[0056]

Λ ₀ (d) = λ / [| 2dT / f ² + 2λ (1 /
Λ ₁ -1 / Λ ₂₎ |]

Here, as described above, λ is the wavelength and T is 2
The distance between the two diffraction gratings 6a and 6b, f is the focal length of the objective lens 4, Λ ₁ is the pitch of the first diffraction grating 6a, and Λ ₂ is the pitch of the second diffraction grating 6b. From this equation, it can be seen that the interference fringes generated by the double diffraction grating 6 are fringes of equal pitch Λ ₀ (d) that depend on the defocus amount d regardless of the position X. When the diffraction grating 6a and the diffraction grating 6b have the same pitch (Λ ₁ = Λ ₂ ), the pitch Λ ₀ (d) of the interference fringes is expressed by the following equation.

[0058]

[Formula 22] Λ ₀ (d) = f ² / [| (d / λ) | 2T]

When there is no defocus (when d = 0)
From Equation 22, Λ ₀ → ∞, but interference fringes occur when defocus occurs. Therefore, it is possible to obtain the displacement (more accurately, a minute displacement) d of the object to be measured or the focus error signal Fo by reading the change due to the defocus of the pitch or phase of the interference fringes.

For example, two diffraction gratings 6 having the same pitch
Parallel light is incident on the double diffraction grating 6 composed of a and 6b, and the + 1st order light at the first diffraction grating 6a and the −1st order light at the second diffraction grating 6b (referred to as E light) ) And the −1st-order light at the first diffraction grating 6a and the + 1st-order light (referred to as F light) at the second diffraction grating 6 are caused to interfere with each other, and the pitch Λ
An interference fringe of ₀ (d) can be generated.

Here, the phases of the two diffraction gratings 6a and 6b are
Intentionally shift (the distance between the peaks of the diffraction grating). 8 to 10 show a state where the phases of the two diffraction gratings 6a and 6b are shifted. That is, in FIG. 8 to FIG.
The pitch of the first diffraction grating 6a and the second diffraction grating 6b is Λ
When (= Λ ₁ = Λ ₂ ), the phase difference between the peaks of the first diffraction grating 6a and the valleys of the second diffraction grating 6b is set to Λ / 8, and the ± 1st order light is diffracted light. The case where (the above-mentioned E light and F light) is used is shown, and in this case, the phases of the two diffracted lights (E light and F light) from the second diffraction grating 6b are 90 °.
(1/4 pitch = λ / 4) More specifically, when not in defocus, the E light and the F light have wavefronts parallel to each other, and their equiphase surfaces enter into each other like a comb. At this time, since the E-phase and F-light equiphase surfaces do not intersect, no interference fringes occur.

As described above, FIG. 8 shows the case where there is no defocus as described above. However, when defocus d occurs, the wavefronts of E light and F light are microscopically as shown in FIGS. Each is curved as shown in FIG. Due to this curvature, the equal phase planes intersect, and interference fringes of the pitch Λ ₀ (d) expressed by Formula 22 are generated. The interference fringe is formed by the intersection of the E and F light wavefronts, and the intersection moves depending on whether the defocus is positive or negative, as indicated by CLS in the figure. This means that the left and right phases are reversed. The light amount distribution of the interference fringes qualitatively changes due to defocusing as shown in FIG. 11, and the left side LT and the right side RT in the interference plane are inverted at the boundary of d = 0. Therefore, the defocus d can be known by reading this with the light receiving means.

Specifically, the left side LT and the right side R in the interference plane
A light receiving element (eg, photodiode) is installed at each T, and the amount of light detected by the light receiving element on the left side (output)
The difference DI between A'and the detected light amount (output) B'of the light receiving element on the right side
When F (= A'-B ') is detected, a so-called S-shaped curve as shown in FIG. 12 is obtained.

FIG. 13 is a diagram showing a configuration example of an optical pickup to which the above principle is applied. Here, a semiconductor laser (LD) is generally used as the light source 1. This optical pickup is used in an optical recording / reproducing apparatus that records and reproduces information by condensing and irradiating light from a light source onto a record carrier. In the configuration of FIG. 13, the light from the light source 1 is collimated by a collimating lens. The light is collimated by 2 and is incident on the objective lens 4 through the beam splitter 3, and is condensed by the objective lens 4 to irradiate the record carrier as the DUT 5. The reflected light from the record carrier 5 again enters the double diffraction grating 6 via the objective lens 4 and the beam splitter 3. In the double diffraction grating 6, the reflected light incident on the double diffraction grating 6 causes interference fringes according to the above-described principle, and the generated interference fringes are received by the light receiving means (for example, FIG. 14).
Light is received by the two-divided light receiving element (7) as shown in (a), and the defocus amount d of the record carrier 5 is detected based on the output difference DIF (= A'-B ') of the two-divided light receiving element. The focus error signal Fo is obtained based on the defocus amount d and the focus servo is performed.

In order to detect not only the focus error signal Fo but also the track error signal Tr, the focus detection method is used and the light receiving means 7 is divided into four light receiving elements (each output is as shown in FIG. 14B). A, B, C, D) may be used. Then, the focus error signal Fo is given by
3 and the track error signal Tr is calculated by using the push-pull method.

[0066]

(23) Fo = (A + B)-(C + D)

[0067]

[Equation 24] Tr = (A + D)-(B + C)

When it is not necessary to detect a track, a two-divided light receiving element (outputs A ', B') as shown in FIG. 14A is sufficient instead of a four-divided light receiving element. In this case, the focus error can be detected by the output difference A′−B ′ of the two light receiving elements. Similarly, when detecting a track by the wobbling method, a focus error is detected by the output difference A′−B ′ of the two light receiving elements, and a track error is detected by the sum A ′ + B ′ of the outputs of the two light receiving elements. be able to.

In the above example, the two diffraction gratings 6
The pitches Λ ₁ and Λ _{2 of} a and 6b are the same, and the interference fringe is expressed by
2. In this case, the interference fringes do not occur in the absence of defocus, and the interference fringes occur in the case of defocus. Quantity detected, but two diffraction gratings 6
The pitches Λ ₁ and Λ ₂ of a and 6b may be different. In addition,
In this case, the interference fringes are generated according to Equation 21. That is, the interference fringes are generated even when there is no defocus, and therefore when the defocus is present, the pitch of the interference fringes is changed to detect the displacement of the object 5 and the defocus amount. can do.

As described above, by using the interference fringes formed by the double diffraction grating 6, it is possible to provide a displacement measuring device and an optical pickup suitable for miniaturization.

However, when a diffraction grating is used, a diffraction grating for this displacement measuring device (optical pickup) must be prepared, and when a diffraction grating is used, the diffraction angle due to the wavelength change of the light source 1 must be adjusted. There are problems such as having to pay attention to changes when designing.

It is an object of the present invention to provide a displacement measuring device and an optical pickup which can be designed with a high degree of freedom by a simple optical element without using a diffraction grating.

[0073]

In order to achieve the above-mentioned object, the invention according to claims 1 and 2 uses reflection / transmission means without using a mechanism called diffraction. As a result, it is possible to use conventional general-purpose simple optical elements, which is extremely easy to fabricate. Also, when using a diffraction grating, pay attention to changes in the diffraction angle due to changes in the wavelength of the light source. It is possible to design with a high degree of freedom.

In the third aspect of the invention, the reflecting / transmitting means is composed of one prism. As a result, stable measurement can be performed without moving each optical element due to the influence of vibration or the like.

In the invention according to claim 4, the reflecting / transmitting means has one surface having a function as a partially reflecting and partially transmitting mirror, and the other surface having a function as a mirror for reflecting at least a part of light. The optical element has a reflected light from the object to be measured is incident on one surface of the optical element to generate first reflected light and transmitted light, and transmitted light is incident on the other surface. Second reflected light is generated, and interference fringes are generated by the interference between the first reflected light and the second reflected light. Thereby, the interference fringes can be generated with a simple configuration having only two surfaces of one optical element.

In the fifth aspect of the present invention, the reflection / transmission means has a function of selectively transmitting P-polarized light (S-polarized light) on one surface and selectively reflecting S-polarized light (P-polarized light). Then, the other surface has an optical element having a function of reflecting at least a part of the P-polarized light (S-polarized light) transmitted through the one surface,
Further, the S-polarized light (P-polarized light) reflected on one surface and the P-polarized light (S-polarized light) transmitted on one surface, reflected on the other surface, and further transmitted on the other surface are overlapped so that the optical paths of one of them overlap. The distance between the surface and the other surface and the angle of incidence of the light on one surface are set, and a means is set so that the S-polarized light (P-polarized light) and the P-polarized light (S-polarized light) interfere with each other in the part where the optical paths overlap. There is. Further, since the means for selecting by polarized light is provided, multiple reflection does not occur, interference fringes are not blurred, and detection can be performed accurately.

Further, in the invention described in claim 6, means for separating the light into two polarized lights which are parallel to each other and are orthogonal to each other is used. This makes it possible to easily achieve separation and superposition of light beams with a simple structure.

Further, the invention according to claim 7 is an optical pickup device to which the displacement measuring device according to any one of claims 1 to 6 is applied. This allows
It is possible to realize an optical pickup having an extremely simple structure.

According to the present invention, the recording signal and the track error signal are also detected by utilizing a part of the light from the reflection / transmission means. As a result, all signals detected by the optical pickup can be detected with a simple structure.

[0080]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 15 is a configuration diagram of an embodiment of a displacement measuring device (for example, an optical pickup device) according to the present invention. With reference to FIG. 15, this displacement measuring apparatus includes a light source (for example, a semiconductor laser) 1, a collimating lens 2 for collimating the light from the light source 1, a beam splitter 3, and a collimating lens 2. Light to be measured (e.g. record carrier)
5 is generated by the objective lens 4 for converging and irradiating the light 5 and the reflected light from the DUT 5 through the objective lens 4 and the beam splitter 3 to generate interference fringes. The interference fringes are projected, the interference fringes are received, and the information on the displacement of the DUT 5 based on the interference fringes (the defocus amount in the optical axis direction and / or the direction orthogonal to the optical axis direction) is tracked. Light receiving means (light receiving elements such as photodiodes) 7 and 8 for detecting (displacement in the radial direction of the record carrier). Here, as the reflection / transmission means 10, the reflected light from the DUT 5 is used.
What has a function of generating interference fringes by overlapping the optical paths while separating the two light fluxes is used.

In the example of FIG. 15, the reflection / transmission means 10 transmits a part of the light from the beam splitter 3 and reflects a part thereof.
And the transmitted light T from the first partially reflecting partially transmitting mirror 11.
A reflection mirror 12 that reflects R and a reflection mirror 1 that reflects the reflected light RF from the first partially reflective partially transmissive mirror 11.
3 and a second partially reflective partially transmissive mirror (half mirror) 14 on which the light from the reflective mirror 12 and the light from the reflective mirror 13 are incident in an overlapping state.

At this time, the second partially reflecting partially transmitting mirror 1
Two light fluxes BM ₁ and BM ₂ are emitted from 4. At this time, the light beam BM ₁ is in a state in which the two light beams BM ₁₁ and BM ₁₂ are overlapped with each other, and the light beam BM ₂ is two light beams BM ₂₁ and
The BM ₂₂ is emitted in an overlapping state. In this case, 2
The mirrors 11, 12, and the mirror 14 are arranged so that interference fringes are generated in at least one of the two light beams BM ₁ and BM ₂ .
13 is set.

Further, the light beam BM ₁ from the second partially reflecting and partially transmitting mirror 14 is incident on the light receiving means 7, and the light beam BM ₁
When an interference fringe is generated in the light receiving means 7, the light receiving means 7 causes the DUT 5 to be in the optical axis direction (the optical axis direction of the incident light on the DUT 5;
When moving in the optical axis direction x) of the objective lens 4, the phase and pitch of the interference fringes of the light beam BM ₁ that changes with this movement are read, and based on this, the optical axis direction x of the DUT 5 is read. The amount of movement to, that is, the displacement is detected.

Similarly, the light beam BM ₂ from the second partially reflecting and partially transmitting mirror 14 is incident on the light receiving means 8 and the light beam B
When the interference fringes are generated in M ₂ , the light receiving means 8
When the DUT 5 moves in the optical axis direction (the optical axis direction of the incident light on the DUT 5; the optical axis direction of the objective lens 4) x, the interference of the light beam BM ₂ which changes with the movement. The phase and pitch of the stripes are read, and the amount of movement of the DUT 5 in the optical axis direction x, that is, the displacement is detected based on this.

Next, the operation of the displacement measuring device having such a configuration will be described. The light from the light source 1 is collimated by the collimator lens 2, condensed by the objective lens 4, and irradiated onto the object 5 to be measured. The reflected light from the DUT 5 is separated by the beam splitter 3 via the objective lens 4 and enters the first partially reflective partially transmissive mirror 11. Here, transmitted light TR and reflected light RF are generated, and transmitted light TR
The reflected light RF and the reflected light RF are reflected by the mirrors 12 and 13, respectively, and are incident on the second partially reflective partially transmissive mirror 14 such that their optical paths overlap each other.

At this time, the respective mirrors 11, 12, 13 can be set so that the two lights TR, RF are parallel or orthogonal to each other. Or two optical TR, RF
The angle formed by can also be set to an angle other than 0 ° or 90 ° (that is, a slight angle can be added).

When the two lights TR and RF are set to be parallel or orthogonal to each other, the equiphase surfaces of the light BM ₁₁ and the light BM ₁₂ are positioned with respect to each other and the equiphase surface of the light BM _21. Since the mutual positions of the equiphase surfaces of the light BM ₂₂ can be controlled, the two diffraction gratings 6a and 6b in the double diffraction grating 6 described above are controlled.
Same state as when the pitch is the same, namely, in the absence of defocus, in a similar condition (e.g. light BM ₁ and shown in FIG. 8 equiphase surfaces of the two light is staggered, equiphase surface of the BM ₁₁ 9 and the equiphase surface of BM ₁₂ are not parallel to each other and interference fringes do not occur, and when defocus occurs, depending on whether the defocus is negative or positive, FIG. in the same state (for example, light BM ₁ and shown in FIG. 10, the equiphase surface of the BM ₁₁ and B
It is possible to obtain a state in which interference fringes are generated by intersecting the M ₁₂ equiphase surface. Therefore, when each of the light receiving means 7 and 8 and the two-divided light receiving element as shown in FIG. 14A (the outputs are A ′ and B ′, respectively) are used, the output difference (A′−
B ′) makes it possible to detect the displacement of the DUT 5 in the optical axis direction x, that is, the defocus amount.

Further, the two lights TR and RF are not parallel or orthogonal, and the angle between them is 0 ° or 90 °.
When it is not °, as in the case where the pitches of the two diffraction gratings 6a and 6b in the double diffraction grating 6 are different,
It is possible to obtain a state in which interference fringes are generated even when there is no defocus, and by using the fact that the pitch of the interference fringes changes when defocus occurs (for example, as shown in FIG. The displacement of the DUT 5 in the optical axis direction x, that is, the defocus amount can be detected by detecting the change in the output difference (A′−B ′) with the divided light receiving element).

Although two light receiving means 7 and 8 are arranged in the example of FIG. 15, only one light receiving means 7 or 8 may be arranged.

As described above, in the displacement measuring device of the above embodiment, the diffraction grating is not used, and only the simple optical element is used.
The displacement of the DUT 5 in the optical axis direction x can be measured easily and accurately.

In the example of FIG. 15, as the reflection / transmission means 10, the first partial reflection partial transmission mirror 11 and the reflection mirror 1 are used.
The optical elements 2, 13 and the second partial reflection partial transmission mirror 14 are used, but in order to prevent the mirrors from moving due to the influence of vibration or the like, these optical elements may be integrated. it can.

FIG. 16 is a diagram showing an example in which a prism in which the above optical elements are integrated is used as the reflection / transmission means 10. In this case, partial transmission and partial reflection can be realized by one prism 20. Further, in the figure, since the angle φ is about 45 °, whether the material of the prism 20 is glass or plastic, the refractive index is 1.3.
As described above, since total reflection occurs, it is not necessary to form a reflecting means such as a dielectric multilayer film or a metal film. Therefore, by using only this one prism 20, the first partially reflecting partially transmitting mirror 11, the reflecting mirrors 12, 13 shown in FIG.
Each function of the optical element of the second partially reflecting and partially transmitting mirror 14 can be realized.

Instead of the optical elements shown in FIG. 15, one surface 22 as shown in FIG. 17 is replaced by an optical element 24 which functions as a partially reflecting and partially transmitting mirror and the other surface 23 functions as a mirror. It can also be used. FIG.
In the example of FIG. 7, the optical element 24 is configured as a parallel plate, and the surface 22 that functions as a partially reflective partially transmissive mirror.
And the surface 23 functioning as a mirror are parallel to each other.
When the optical element 24 configured as shown in FIG. 17 is used as the reflection / transmission means 10, the reflected light from the DUT 5 is
The light is incident on the partially reflective partially transmissive mirror of the one surface 22 via the beam splitter 3, and transmitted light TR and reflected light RF are generated. The transmitted light TR is reflected by the mirror on the other surface 23 and is incident on the one surface 22 again so that the optical path of the transmitted light TR and the optical path of the transmitted light TR overlap. As a result, the two light beams BM ₁₁ and BM ₁₂ are emitted from the one surface 22 with their optical paths overlapping each other, and these two BM ₁₁ and BM ₁₂ are made incident on the light receiving means (for example, two-divided light receiving element) 27. The displacement of the DUT 5 in the optical axis direction x can be measured in the same manner as described above.

In FIG. 17, the parallel plate is used as the optical element 24, but instead of this, a wedge substrate as shown in FIG. 18 may be used. In this case, even when the DUT 5 has no defocus, the two light beams BM ₁₁ and BM ₁₂ are not parallel and have a predetermined angle, and therefore interference fringes are generated. In addition, the displacement of the DUT 5 in the optical axis direction x can be measured by utilizing the fact that the pitch of the interference fringes changes when defocus occurs.

By the way, when two reflecting surfaces of one optical element 24 are used as shown in FIGS. 17 and 18, FIG.
As shown in FIG. 3, multiple reflection occurs and two or more light beams are generated, and as a result, the interference fringes may be slightly blurred. In order to solve this, as shown in FIG. 20, it is possible to select transmission and reflection on one surface 22 by polarization. That is, a polarization selection film that functions to reflect S-polarized light and transmit P-polarized light may be provided on one surface 22. In this case, when the reflected light from the DUT 5 enters the one surface 22 of the optical element 24 through the beam splitter 3, first, the S-polarized light is reflected and the P-polarized light is transmitted. The P-polarized light transmitted through the one surface 22 is reflected by the other surface 23 of the optical element 24 and is incident on the one surface 22 again, but since the P-polarized light is transmitted through the surface 22, multiple reflection does not occur. It is possible to prevent blurring of interference fringes. In the above example, as the polarization selection film, a film that reflects S polarized light and transmits P polarized light is used, but a film that transmits S polarized light and reflects P polarized light may be used. That is, which one is used can be appropriately selected according to the design of the optical pickup.

As described above, in each of the above-described examples, various reflection / transmission means having the function of separating the light beams into two and superimposing them have been shown. It is also possible to separate and superimpose the light flux into two by using an optical element such as a cubic crystal or a prism using the same. FIG. 21 shows examples of various optical elements having a function capable of separating and superposing light beams into two. 21 (a) is a beam displaying prism, FIG. 21 (b) is a Thomson prism, and FIG.
(c) is a Wollaston prism. These are commercially available and can be easily obtained (for example, refer to the Melles Griot company catalog). When the beam displaying prism 31 of FIG. 21 (a) is used, it is used as it is to separate the light flux into two. Can be stacked. Further, the other two prisms 32 and 33 shown in FIGS.
When using, two can be arranged in series as shown in FIGS. 22 (a) and 22 (b).

When the prisms shown in FIGS. 21 (a), 21 (b) and 21 (c) are used, the two light beams emitted from these prisms are two polarized lights which are orthogonal to each other, so that they do not interfere with each other. Therefore, in order to generate the interference fringes in such a case, for example, as shown in FIG.
It is possible to set the polarizing plate 35 that allows only the polarization direction forming 5 ° to pass. That is, by setting the polarizing plate 35, the polarization directions of the two light beams are aligned in the same direction and become the same polarization, so that interference fringes can be generated. In addition, in the example of FIG. 23, the case where the beam displaying prism 31 is used is shown. However, when the prism configuration of FIGS. 22A and 22B is used instead of the prism 31, the same applies. By providing the polarizing plate 35, interference fringes can be generated.

As described above, in the displacement measuring device of the present invention,
The displacement of the DUT 5 in the optical axis direction x can be measured using only simple optical elements such as mirrors and prisms that do not require a diffraction mechanism, without using a diffraction grating. This eliminates the need to consider the change in the diffraction angle due to the change in the wavelength of the light source, for example, so that the design is significantly facilitated and the displacement can be measured with high accuracy and reliability. Therefore, when this displacement measuring device is applied to an optical pickup, the optical pickup can be easily designed and the focus error signal of the record carrier (for example, an optical disc) which is the DUT 5 can be detected with high accuracy and reliability. it can.

Further, for example, in the example of FIG. 15, FIG. 17, FIG. 18, or FIG. 20 described above, the reflecting mirror 12 or 23 is used as a part of the optical element, but as shown in FIG. It is also possible to obtain a recording signal and a track signal of the DUT 5 (record carrier) 5 by using this part as a partially reflecting partially transmitting mirror 37 instead of 12, 13 and using partially transmitted light from this mirror 37. it can. For example, by partially condensing the partially transmitted light from the mirror 37 by the condensing lens 38 and making it enter the light receiving means 39, the light receiving means 39 can detect the recording signal or the track signal. At this time, a small photodiode (PD) can be used as the light receiving means 39 by collecting light with the collecting lens 38. As a result, it is possible to eliminate the situation that the light receiving element has a large diameter due to a large spot in the sharing interferometry (a large light receiving element area results in high cost and lack of high-speed response).

More specifically, this light receiving means (light receiving element)
39 can be used for detecting a recording signal, or can be used for track detection by the push-pull method by configuring the light receiving means 39 as a two-divided light receiving element. It is also possible to apply the push-pull method to only the two-divided light receiving element and detect the sum of the respective outputs of the two-divided light receiving element as a recording signal.

Further, in each of the above-mentioned examples, if the light receiving means 7 or 8 is a four-division light receiving element as shown in FIG. 14B, not only the focus error signal but also the light receiving means 7 or 8 is used. It is possible to detect the track error signal and further the recording signal with high accuracy and reliability.

In the above embodiment, the beam splitter 3 transmits the light from the light source 1 and the collimator lens 2 and reflects the reflected light from the DUT 5. On the contrary, the light from the light source 1 and the collimator lens 2 may be reflected and incident on the DUT 5, and the light reflected from the DUT 5 may be transmitted.

[0103]

As described above, according to the first and second aspects of the present invention, since the reflection / transmission means is used without using the mechanism of diffraction, the conventional general-purpose simple optical element is used. Can be used and is extremely easy to fabricate, and a highly flexible design can be performed without paying attention to the change in the diffraction angle due to the change in the wavelength of the light source, which should be noted when using the diffraction grating. .

According to the third aspect of the present invention, since the reflection / transmission means is composed of one prism, stable measurement can be performed without moving each optical element due to the influence of vibration or the like. be able to.

According to the fourth aspect of the present invention, the reflection / transmission means has one surface having a function as a partial reflection / partial transmission mirror and the other surface as a mirror for reflecting at least a part of the light. It is composed of an optical element having a function, makes the reflected light from the object to be measured incident on one surface of the optical element to generate the first reflected light and the transmitted light, and the transmitted light to the other surface. Since the second reflected light is made incident and the interference fringes are generated by the interference between the first reflected light and the second reflected light, only two surfaces of one optical element are simple. Interference fringes can be generated with various configurations.

According to the fifth aspect of the invention, the reflection / transmission means has a function of selectively transmitting P-polarized light (S-polarized light) on one surface and selectively reflecting S-polarized light (P-polarized light). Have
The other surface has an optical element having a function of reflecting at least a part of the P-polarized light (S-polarized light) transmitted through the one surface, and the S-polarized light (P-polarized light) reflected by the one surface. , So that the optical paths of P-polarized light (S-polarized light) transmitted through one surface, reflected by the other surface, and further transmitted through one surface overlap, and the distance between one surface and the other surface A means for setting an incident angle and causing interference between S-polarized light (P-polarized light) and P-polarized light (S-polarized light) is set at a portion where optical paths overlap. Also, since a means for selecting by polarized light is provided,
Multiple reflection does not occur, interference fringes do not blur, and detection can be performed accurately.

According to the sixth aspect of the invention, since the means for separating the light into the two polarized lights which are parallel to each other and orthogonal to each other is used, the light beams can be easily separated and overlapped with each other with a simple structure. Achievement can be achieved.

Further, according to the invention described in claim 7, the optical pickup device is constructed by applying the displacement measuring device according to any one of claims 1 to 6, so that the configuration is extremely simple. The optical pickup of can be realized.

According to the invention described in claim 8, since the recording signal and the track error signal are also detected by utilizing a part of the light from the reflection / transmission means, it is possible to detect all the signals made in the optical pickup. The detection can be performed with a simple structure.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of a displacement measuring device.

FIG. 2 is a diagram for explaining generation of interference fringes.

FIG. 3 is a diagram for explaining generation of interference fringes.

FIG. 4 is a diagram for explaining generation of interference fringes.

FIG. 5 is a diagram for explaining a method of measuring a minute displacement.

FIG. 6 is a diagram for explaining a method of measuring a minute displacement.

FIG. 7 is a diagram for explaining a method of measuring a minute displacement.

8A and 8B are views for explaining detection of a defocus amount of an object to be measured by the displacement measuring device of FIG.

9A and 9B are views for explaining detection of a defocus amount of an object to be measured by the displacement measuring device of FIG.

10 is a diagram for explaining detection of a defocus amount of the object to be measured by the displacement measuring device of FIG.

FIG. 11 is a diagram showing a light amount distribution of interference light depending on a defocus amount.

FIG. 12 is a diagram showing a change in output difference between two light receiving elements according to a change in defocus amount.

FIG. 13 is a diagram showing a configuration example of an optical pickup.

FIG. 14 is a diagram showing a configuration example of a light receiving unit.

FIG. 15 is a configuration diagram of an embodiment of a displacement measuring device according to the present invention.

FIG. 16 is a diagram showing an example in which a prism is used as a reflection / transmission unit.

FIG. 17 is a diagram showing another configuration example of the reflection / transmission means.

FIG. 18 is a diagram showing another configuration example of the reflection / transmission means.

FIG. 19 is a diagram showing how multiple reflections occur.

FIG. 20 is a diagram showing another configuration example of the reflection / transmission means.

FIG. 21 is a diagram showing another configuration example of the reflection / transmission means.

FIG. 22 is a diagram showing another configuration example of the reflection / transmission means.

FIG. 23 is a diagram showing another configuration example of the reflection / transmission means.

FIG. 24 is a diagram showing another configuration example of the reflection / transmission means.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light source 2 collimator lens 3 beam splitter 4 objective lens 5 object to be measured 10 reflection / transmission means 7, 8 light receiving means 11, 12 partially reflective partial transmission mirror 12, 13 mirror 20 prism 24 optical element 22, 23 optical element surface 31 beam Displaying prism 32 Thomson prism 33 Wollaston prism 35 Polarizing plate 37 Mirror 38 Condensing lens 39 Light receiving means

Claims (8)

[Claims] 1. An object to be measured is focused and irradiated with light from a light source,
In a displacement measuring device that measures the displacement of an object to be measured based on the reflected light from the object to be measured, the reflected light from the object to be measured is separated into two light fluxes, and optical paths are overlapped to generate an interference fringe. Reflection / transmission means and the displacement of the measured object in the optical axis direction are read by reading the change in the interference fringes accompanying the movement of the measured object in the optical axis direction of the light incident on the measured object. A displacement measuring device having a light receiving means for detecting. 2. The displacement measuring device according to claim 1,
The reflection / transmission means has a first partially-reflective / partial-transmission mirror that generates transmitted light and reflected light when the reflected light from the object to be measured is incident thereon, and the transmitted light and the reflected light are respectively predetermined. And a second partially reflective partially transmissive mirror for generating interference fringes when the transmitted light and the reflected light from the reflecting means are incident and are superposed on each other. Displacement measuring device characterized by the above. 3. The displacement measuring device according to claim 1,
The displacement measuring device, wherein the reflection / transmission means is composed of one prism. 4. The displacement measuring device according to claim 1, wherein
The reflection / transmission means has an optical element having one surface having a function as a partially reflecting and partially transmitting mirror and the other surface having a function as a mirror for reflecting at least a part of the light. Reflected light from an object is incident on one surface of the optical element to generate first reflected light and transmitted light, and transmitted light is incident on the other surface to generate second reflected light. The displacement measuring device is characterized in that interference fringes are generated by the interference between the first reflected light and the second reflected light. 5. The displacement measuring device according to claim 1, wherein
The reflection / transmission means has a function that one surface selectively transmits P-polarized light (S-polarized light) and selectively reflects S-polarized light (P-polarized light), and the other surface transmits one surface. P polarization (S
Polarized light) having an optical element having a function of reflecting at least a part of the light, and further, the S-polarized light reflected by the one surface.
(P-polarized light) and the one surface and the other surface so that the optical paths of the P-polarized light (S-polarized light) transmitted through the one surface, reflected by the other surface, and further transmitted through the one surface overlap. And the angle of incidence of the light on the one surface are set, and the S-polarized light (P-polarized light) and the P-polarized light (S-polarized light) are provided in a portion where the optical paths overlap.
A displacement measuring device, characterized in that means for causing interference with the displacement measuring device is set. 6. The displacement measuring device according to claim 1,
The reflection / transmission means separates the reflected light from the object to be measured into two polarizations that are parallel to each other and orthogonal to each other, and
A displacement measuring device, comprising: a polarization direction adjusting unit that aligns the directions of two polarizations that are orthogonal to each other to generate interference fringes. 7. An optical pickup using the displacement measuring device according to claim 1, wherein a focus error signal is detected by reading a change in the interference fringes due to defocusing. Characteristic optical pickup. 8. The optical pickup according to claim 7, wherein a recording signal and a track error signal are also detected by utilizing a part of the light from the reflection / transmission means.