JPS6376119A - Optical pickup - Google Patents

Abstract

PURPOSE:To contrive to reduce the tracking offset by using a reflection type diffraction grating having grating slots of unequal interval curve formed with a recess and a projection of a specific grating slit. CONSTITUTION:The + or - 1st-order diffracted light beams 4b, 4c separated by the reflection type diffraction grating 3 are regarded respectively as divergent luminous flux radiating a light given in an imaginary way. Moreover, the grating slot is provided at a position where the nearly middle part of the recess or projection of the grating slot is placed at a position with either a right stripe or a dark stripe of the interference stripe generated when the light beam from the virtual light source of the diffracted light of either the + 1st-order light 4b or the - 1st-order order diffracted light 4c and the actual incident light beam 2 made incident in the reflection type diffraction grating 3 and radiated from the laser light source 1 are overlapped in a coherent way. Furthermore, the grating slot pattern is formed as unequal interval curves. Thus, in the detection of tracking error by a 3-beam system, the tracking offset caused due to the positional deviation of the light spot of the + or - 1st-order diffracted light in the radial direction of the optical disk is reduced remarkably.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical pickup that performs tracking error detection using a 3-beam system, and particularly relates to an optical system configuration suitable for simplifying the optical configuration thereof. [Prior Art] Conventionally, as a tracking error detection means using a 3-beam system, as shown in Japanese Patent Application Laid-open No. 57-205833, a method for detecting tracking error using a three-beam system has been proposed, for example, as shown in Japanese Unexamined Patent Publication No. 57-205833. It is known that a phase-type diffraction grating with a large linear pattern of phase grooves is arranged perpendicular to the optical axis. This method uses a simple optical system to detect tracking errors using a 3-beam system. On the other hand, it is possible to separate the light beam incident on an optical information recording medium (hereinafter referred to as an optical disk) from the light beam reflected from the optical disk!
There was a problem in that two special optical components were required: a beam splitter to separate the incident light and a diffraction grating to separate the incident light and generate three light beams.

In contrast, the present inventors have developed an optical pickup that reduces the number of optical components by using an optical element that integrates a diffraction grating and a beam splitter by providing a beam splitter film on the grating grooves of a reflective diffraction grating. We are recommending consideration.

[Problem that the invention seeks to solve]

By the way, when using such an optical element that integrates a beam splitter and a diffraction grating, the plane on which the phase groove is provided is arranged at an angle with respect to the optical axis. In a reflection type diffraction grating, such as a diffraction grating, which has a pattern 2 of equally spaced linear grating grooves, a significantly large wavefront aberration occurs in the ±1st-order diffracted light separated by the reflection type diffraction grating, and the irradiation of the light spot Since a positional shift occurs, it is not possible to correctly irradiate the light spot of the ±1st-order diffracted light onto the optical disk. Therefore, in order to put into practical use optical pink-up that integrates a beam splitter and a diffraction grating, it is necessary to create a pattern of grating grooves that can reduce the effects of wavefront aberration and deviation of the irradiation position of the optical spot to a level that does not cause any practical problems. A new reflection-type diffraction grating is required.

An object of the present invention is to provide a reflection type diffraction grating and an optical pickup in which wavefront aberration and positional deviation of a light spot are improved as described above.

[Means for solving problems]

The above purpose is to form the grating groove pattern 2 of the reflective diffraction grating into a non-uniformly spaced curved shape as described later, and at least the light beam traveling on the optical axis of the light beams incident on the reflective diffraction grating. The direction of the arrangement of the grating grooves in the part where the reflection type diffraction grating is arranged is approximately parallel to the plane that includes both the incident optical axis and the normal to the plane on which the reflection type diffraction grating is provided. This is achieved by making the direction of propagation of the ±1st-order diffracted light substantially parallel to the plane that includes both the optical axis and the modulus We.

[Effect]

When a reflective diffraction grating is placed at an angle to the optical axis,
By forming the lattice groove pattern into a non-uniformly spaced curved pattern as described later, wavefront aberration occurring in the ±1st-order diffracted light can be significantly reduced. In addition, the direction of propagation of the ±1st-order diffraction light is, as mentioned above, relative to the plane that includes both the optical axis of incidence on the reflective diffraction grating and the normal to the surface on which the grating grooves of the reflective diffraction grating are provided. By making the optical disc substantially parallel to the tangential direction of the optical disc, the direction of the deviation of the irradiation position of the optical spots of the ±1st-order diffracted light, which is irradiated in parallel with the tangential direction of the optical disc, is adjusted to the direction T of the arrangement of the irradiation positions of each party spot, that is, the optical disc. They can be made to substantially coincide in the tangential direction, thereby making it possible to significantly reduce the positional deviation of the light spot in the radial direction of the optical disc. Therefore, when performing tracking error detection using the 6-beam method, it is possible to significantly reduce tracking offset caused by positional deviation of the optical spot of the ±1st-order diffracted light in the radial direction of the optical disk.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to FIG. 1. FIG. 1 is a front view of an example of the optical pickup of the present invention. A diverging light beam 2 emitted from a semiconductor laser light source for one minute is incident on a reflection type diffraction grating 6. This reflection type diffraction grating 3 is arranged so as to be inclined with respect to the optical axis (two axes) of the incident light beam 2. In addition, in FIG. 1, this inclination angle is approximately 45°. Moreover, this reflection type diffraction grating 3 is
A concavo-convex grating groove 3b is provided inside the grating groove 3b, and a beam splitter film C is formed above the grating groove 3b.

When the diverging light beam 2 incident on the reflective diffraction grating 3 is reflected by the uneven beam splitter film 3C, it is separated into a 0th order light beam 4&%+1st order diffracted light beam 4b, . . . -1st order diffracted light beam 4C. Each separated light beam 4a, 4b
, 4c is a y-axis parallel to the optical axis (y-axis) in FIG.
They propagate in a direction substantially parallel to the 7-Z plane, both containing the axis, and are individually focused onto the recording tracks 6& of the original disk 6 through the objective lens 5, and are focused on the recording tracks 6a as shown in FIG. (optical spora) 7a, 7b,
7cE is formed. Furthermore, light spots 7a, 7b, 7
The reflected light from the optical disk 6 shown in FIG. The photodetector 9 has, for example, six areas 9a, 9b, 9c, 9d, 9 as shown in FIG.
e, divided into 9f.

The reflected light 10& from the optical disk 6 of the 0th-order light beam (optical spora) 7a enters the areas 9a, 9b, 9c, and 9d, and the sum of the output signals of the areas 9& and 9d and the areas 9b and 90
By the astigmatism method, from the difference signal between the sum of the output signals of
A focal position Wt error signal of the optical spoiler) 78 is detected.

In addition, the remaining two regions 9e and 9f each contain ±1st-order diffracted light 4.
a.

The reflected lights 10b and 10c from the optical disks 6 of the optical spoilers 7b and 7c enter separately, and the tracking error signal of the optical spoiler 7a is detected from the difference signal of the surface areas. Such a tracking error detection means is generally called a 3-beam system. Note that the information signal recorded on the optical disc 6 is transmitted to each area 9a, 9 of the photodetector 9, for example.
It is detected from the sum of the output signals of b, 9c, and 9d.

By the way, when the reflective diffraction grating 33 is arranged obliquely with respect to the optical axis as shown in the embodiment of FIG. In this case, a large wavelength aberration of about 1/2 wavelength (rms value) occurs in the ±1st-order diffracted light, making it impossible to narrow down the optical spora (7b, 7c) to about the diffraction limit.

As a measure to reduce such wavefront aberration, the present invention has devised a reflection type diffraction grating having a grating groove pattern of unevenly spaced curves as shown below.

An embodiment of the lattice groove pattern of the present invention will be described below. This grating groove pattern is regarded as a luminous flux emitted from a virtually set light source, respectively, and the ±1st-order diffracted light 4b and da f separated by the reflection type diffraction grating 3 are divided into the +1st-order diffracted light 4b and the -1st-order diffracted light 4C. This occurs when the light beam from the virtual light source of one of the diffracted lights and the actual incident light beam 2 emitted by the laser light source 1 and incident on the reflective diffraction grating 3 are coherently superimposed on the reflective diffraction photon 5. This can be obtained by providing the grating grooves so that the approximate center of the concave or convex portions of the grating grooves is located at the position where either the bright or dark interference fringes appear.

FIG. 3 is a front view and a plan view for explaining the principle of creating lattice grooves/<turns of this embodiment.

The lattice groove pattern of this example is as shown in Fig. 3.
The optical distance R between the point P on the surface where the grating grooves of the reflective diffraction grating 3 are provided and the laser light source 1, -1st order diffracted light 40?
If the optical distance between the emitting virtual light source 1C and the point 2 is Ro, Ro Rγ=m, λ □ (1) where λ is the wavelength of the laser beam, and 1 is an arbitrary integer.

Either a concave portion or a convex portion of the lattice groove is provided at the position of the point P that satisfies the relationship of equation (1). Figure 4 shows the above relational expression? An example of a lattice groove pattern in which lattice grooves are provided at filling positions is shown. In FIG. 4, a black line 11 indicates a locus along which a substantially central portion of a convex portion or a concave portion of the lattice groove passes. As is clear from FIG. 4, the lattice groove pattern obtained by the formula (2) has an unevenly spaced curved pattern.

(Hereinafter, to simplify the lattice number of such unevenly spaced curves,
It is written as a curved lattice. ) Note that, as shown in FIG. 5(a), the actual reflection type diffraction grating 3 has a recess with a substantially rectangular grating groove cross section at the position P of the coordinates (z, y) that satisfy the relationship 2 of equation (1). Along the obtained interference fringes, so that the approximately central portion 3b-1 of the lattice groove is located, or the approximately central portion 3b-2 of the convex portion of the grating groove cross section is located as shown in FIG. A lattice groove 3b is provided. Also, Figure 5 (C)
The cross-sectional shape of the grating grooves may be approximately sinusoidal as shown in FIG. 5(d), or approximately triangular as shown in FIG. 5(,).

Also, similar to the creation principle described in equation (1) above, the light beam from the virtual light source 1b of the +1st order diffracted light 4b and the laser light source 1
Even if grating grooves are provided along the interference fringes obtained by superimposing the light beams 2 from the curved i-grating, a curved grating having the same performance as the curved i-grating described above can be obtained.

Furthermore, in the above embodiment, the ±1st-order diffracted light 4b on the position 2 reflection type diffraction grating 3 of the virtual light sources 1b and 1c.

It is precisely aligned with the position of a light source that emits two diverging lights that have exactly the same phase distribution as the phase distribution of 4c, but it is not limited to this. Virtual light source 1b at the position.

A lattice groove pattern may be created assuming 1C.

By using the curved grating obtained as described above as the reflection type diffraction grating 3 of No. 1 [4], the ±1st-order diffracted light Wavefront aberrations occurring at 4b and 4c can be significantly reduced. FIG. 6 shows a curved grating having a grating groove pattern as shown in FIG.
When used as
X direction in the figure) and the wavefront aberration (rm8 value) generated in each of the ±1st-order diffracted lights 4b and 4c.
As is clear from FIG. 6, by using the curved grating as described above, the laser light source can be moved from the ideal position to within ±0.
Even if there is a deviation of about 5 m, the wavefront aberration of the ±1st-order diffracted beams 4b and 4c can be reduced to about 1,/'20 wavelength (rms value) or less, and there is no practical problem with the optical slots 7b and 7c on the optical disk 6. It becomes possible to narrow down the results to a certain degree.

On the other hand, when such a curved grating is used as the reflection type diffraction grating 3, the ±1st-order diffraction light 4b irradiated onto the optical disk,
A positional shift occurs in the irradiation position on the light spot 4c) 7b or 7C. FIG. 7 shows the amount of positional deviation of the laser light source 1 and the optical slots of the ±1st-order diffracted lights 4b and 4c when a curved grating with a grating groove pattern as shown in FIG. 4 is used as in FIG. 6. , 7c shows the relationship between the amount of positional deviation from the ideal position. The horizontal axis in FIG. 7 shows the amount of positional deviation of the laser light source 1 in the X-axis direction, and the vertical axis shows the amount of positional deviation of the laser light source 1 in the X-axis direction. The amount of positional deviation of the optical stubs 7a and 7b in the i-axis direction obtained when the images are projected onto the optical disk 6 is shown.

As shown in FIG. 7, when the laser light source 1 is misaligned, the optical switches 7a and 7b are moved in opposite directions.
The irradiation position moves by approximately the same distance.

However, on the other hand, even if the source 1 is not misaligned, a misalignment of about 0.6 .mu.m will occur in the optical stub 7b.

Incidentally, such a positional shift of the optical switch 7b is caused by the difference between the light beam emitted from the virtual light source 1C of the first-order diffracted light 4C and the laser light source 1, as shown in the example shown in FIG. 3 and equation (1).
This occurs when a curved grating is used that has grating grooves along the interference fringes that appear when light beams emitted from In the case of a grating in which grating grooves are provided along interference fringes with the light beam from the light source 1, on the contrary, a positional shift occurs in the light spot 7C.

In this way, the light beams of the ±1st-order diffracted light (7b, 7c)
If a positional deviation occurs in only one of the optical spots, or if the absolute amount of positional deviation of both spots is different (hereinafter referred to as an asymmetric positional deviation), the direction of the positional deviation will be determined. If it is perpendicular to the direction of the recording track 6& of the optical disc 6, that is, in the radial direction of the optical disc 6, as shown in FIG.
7c are not lined up in a straight line substantially parallel to the recording track 6a, resulting in a relative positional shift in the radial direction of the optical disk. For this reason, a significant offset occurs in the tracking error signal by the 6-beam system. Regardless of the grating groove pattern, the direction in which such an asymmetric positional shift of the light spot occurs is determined by the relationship between the optical axis (Z axis) of the light beam 2 incident on the reflection grating 3 and the grating grooves of the reflection grating. The intersection line between the surface including the normal to the surface on which the grating grooves are provided, that is, the y-axis in FIG. 1, is the objective lens 5.
is limited only to the direction of the i-axis optically projected onto the optical disk 6.

Therefore, in order to reduce the positional deviation in the radial direction of the optical disc 6 of the optical disk 7b or 7C as described above and the tracking offset associated with the positional deviation, the 0th order and ±1st order are separated by the reflection type diffraction grating 3. The direction of propagation of the diffracted beams 4a, 4b, and 4c is a plane that includes both the optical axis of the incident light beam 2 and the normal line to the reflective diffraction grating 3, as mentioned earlier, that is, the plane shown in FIG. - Orient the grating groove pattern so that it is approximately parallel to the Z plane.

Note that in order to make the propagation direction of the 0th-order and ±1st-order diffraction lights 4a, 4b, and 4c approximately parallel to the 7-Z plane, the diffraction gratings 3 must be incident on the reflection type diffraction grating 3 as shown in the example of FIG. The direction in which the grating grooves 3b are arranged in the portion of the light beam 2 that is incident at least near the optical axis is y in FIG.
The grating groove pattern may be oriented so as to substantially coincide with the direction parallel to the -Z plane, that is, the y-axis direction.

That is, when creating a grating groove pattern according to equation (1), the position of the virtual light source is set to 1. as in the embodiment shown in FIG. −
Equation (1) can be calculated assuming two planes.

The above explanation was limited to the i direction in which the asymmetric positional deviation of the light spot occurs, but of course the positional deviation of the light spot also occurs in directions other than the i-axis.However, in any direction other than the i-axis direction, For example, as shown in FIG. 10, if there is no positional shift in the laser light source 1, the light beam will be blocked) 7b,
There is almost no positional deviation of 7c, and if there is a positional deviation of the laser light source 1, the optical switches 7b and 7c move in opposite directions by approximately the same distance. Therefore, even if there is a positional shift in the laser light source 1, the linearity of the irradiation positions of the light spots 7a, 7b, and 7c is always maintained, as shown in FIG. Therefore, the positional deviation of the optical spot can be easily corrected by a technique such as rotating the entire optical big amplifier around the optical axis.

In this way, the objective lens 5 allows the optical disk 6 to be
The light beams 7a, 7b, and 7c irradiated above are aligned approximately in a straight line parallel to the i-axis in FIG. In addition, when detecting tracking error signals using the 3-beam method, optical
Since the irradiation positions of 7&, 7b, and 7c are arranged on a straight line approximately parallel to the recording track 6a of the optical disc 6, the light beams 7a,
By arranging 7b and 7o in a straight line substantially parallel to the i-axis, an asymmetrical positional shift of the optical slots 7b and 7a occurs as shown in Figure 9. It can be made to roughly match the tangential direction of B. Therefore, Hikari Suboc) 7
It is possible to significantly reduce the irradiation position deviation in the radial direction of b and 7c, and at the same time, the tracking offset can also be greatly reduced. Reflected light 10b from the optical disc 6.

10c is also affected, and the reflected light iob or 10c is irradiated with a relative shift to the regions 9θ and 9f of the photodetector 9. However, taking into account such a positional deviation amount, as shown in FIG.
, 9e so as to be incident only on each of 9f.

If the light receiving surface of 9f is made wide, the rJJ problem will not occur.

Next, FIG. 13 shows another embodiment of the curved grating of the present invention.
It is a front view and a plan view for explaining the principle of its creation. In this embodiment, ±1st-order diffracted light 4b is separated by a reflection type diffraction grating as shown in FIG.
, 4c, in order to improve the point that the wavefront aberrations are different from each other,
Interference fringes between the +1st order diffracted light 4b of the light beam from the virtual light source 1b and the light beam from the laser light source 1, and interference fringes of the 1st order diffracted light 4C of the light beam from the virtual light source 1c and the light beam from the laser light source 1 A concave or convex portion of the grating groove is provided at the average position of the respective interference fringes. That is, the optical distance between the point P on the surface where the grating of the reflective diffraction grating B is provided and the laser light source 1 is Rγ, the optical distance ERG between the virtual light source 1b of the +1st order diffracted light 4b and the point 2, and the −1st order Virtual light source 1C and point 2 of folded light 4C
If the optical distance between them is '2Ro = (R, - Go.')
/2 = m, λ □ (2) where λ is the wavelength of the laser beam, m is an arbitrary integer, and a concave or convex portion of the grating groove is provided at a point P that satisfies the relationship of equation (2).

Figure 14 shows the first curved grid obtained by calculating equation (2).
An example B of the relationship between the amount of positional deviation (in the X-axis direction) of the laser light source and the amount of wavefront aberration (rms value) generated in the ±1st-order diffraction lights 4b and 4c when used as the reflection type diffraction grating 3 shown in the figure is shown.

As is clear from Fig. 14, when using the curved grating created from the relational expression (2), wavefront aberration occurs, unlike when using the curved grating created from the relational expression (1). Both the +1st-order diffracted light 4b and the -1st-order diffracted light 4C have the same value, and their absolute amounts are also slightly lower.

Figure 15 is a diagram showing how the position of the light spot shifts when a curved grating created from the relational expression (2) is used, and the vertical and horizontal axes of the diagram are the same as in Figure 7. . As is clear from FIG. 15, when the curved grating created from equation (2) is used, even when there is no positional shift in the laser light source 1, +1
The optical spot 7c of the second-order diffracted light 4b and the optical spot 7c of the first-order diffracted light 4C are shifted in the same direction by the same distance (about 0.2 μm).

In this case as well, if the direction of the positional shift is aligned with the radial direction of the optical disc B, the tracking offset will increase as in the case of the first embodiment.

Therefore, just like the first embodiment, the 0th order and ±1
By making the direction of propagation of the second-order diffracted lights 4a, 4b, and 4c substantially parallel to the y-Z plane in Fig. 1, the direction of the positional deviation of the optical discs 7b and 7c can be adjusted to the position of the optical disc B, as shown in Fig. 16. It is possible to substantially match them in the tangential direction, and the tracking offset can be significantly reduced.

In the above-described embodiments, only the case where the angle of inclination of the reflection type diffraction grating 3 with respect to the optical axis is 45 degrees is described, but of course, this angle of inclination may be any angle from 0 to 90 degrees, and (1) The lattice groove pattern may be created by calculating the relationship of the equation or (2).

Finally, the specific structure of the reflection type diffraction grating of the present invention will be explained with reference to FIG. 17 and an example thereof.

The reflection type diffraction grating 6 of the present invention has a substrate 3a made of a parallel flat plate of optical glass or optical plastic.
The uneven grating grooves 5bf are carved according to the grating groove pattern shown in the present invention, and a half mirror a3c made of, for example, a dielectric film or a metal film is formed thereon, and a refractive index close to that of the substrate 3& is formed thereon. Optical plastic layer, for example, an acrylic thread surface fat layer 3d that can be cured by ultraviolet rays.
It is created by forming the lattice grooves cut into the substrate 3a in such a manner as to embed them in the lattice grooves.

〔Effect of the invention〕

As described above, according to the present invention, ±1
Since the wavefront aberration of each optical spot formed by the next diffracted light on the optical disk, the positional deviation in the optical disk radial direction, and the accompanying tracking offset of the 6-pin system can be significantly reduced, the beam splitter and diffraction This will have a great effect on the practical application of optical pickups that integrate the gratings and reduce the number of optical components.

[Brief explanation of the drawing]

Figure 1 is a front view of an embodiment of the present invention, Figure 2 is a plan view showing the irradiation position of the original spot, Figures 3 and 4.
5 shows a schematic diagram 1 and a perspective view, FIG. 6, FIG. 7, FIG. 8, and FIG. 9, which illustrate the principle of creating a lattice groove pattern according to the first embodiment of the present invention. Figures 10, 11, and 12 are a schematic diagram and a plan view showing the characteristics of the lattice groove pattern of the embodiment shown in Figure 1;
3 is a schematic diagram showing the principle of creating a lattice groove pattern according to a second embodiment of the present invention, and FIGS. 14 and 15. FIG. 16 is a schematic diagram and a plan view showing the characteristics of the grating groove pattern of the second embodiment, and FIG. 17 is a sectional view showing an example of the configuration of the reflection type diffraction grating of the present embodiment. DESCRIPTION OF SYMBOLS 1... Laser light source, 3... Reflection type diffraction grating, 5... Objective lens, 6... Optical disk,
9...Photodetector. degree\

Claims (1)

[Claims] 1. A laser light source and an objective lens that focuses a diverging light beam emitted from the laser light source and irradiates it onto an optical information recording medium are provided, and in an optical path between the laser light source and the objective lens. A reflection type diffraction grating provided with uneven grating grooves is arranged obliquely with respect to the optical axis, and the reflection type diffraction grating reflects the diverging light beam emitted from the laser light source and converts the divergent light beam to zero. Separate into second-order and ±first-order diffracted light beams,
In the optical pickup configured to guide each of the diffracted light beams to the objective lens, the reflective diffraction grating includes each of the light source and a second light source provided at a predetermined distance from the light source. When mutually coherent light beams emitted from the reflective diffraction grating are superimposed on the reflective diffraction grating, there is an abbreviation of a cross section of a concave or convex part of the grating groove at the position of either the bright or dark interference fringes that occur. What is claimed is: 1. An optical pickup characterized in that it is a reflection type diffraction grating having curved grating grooves at irregular intervals formed by providing concave and convex portions of the grating grooves such that the central portion thereof is located. 2. The reflection type diffraction grating is such that the direction in which the grating grooves are lined up in the portion where at least the light beam traveling on the optical axis of the light beam incident on the reflection type diffraction grating is incident is aligned with the optical axis of the incident beam. 2. The optical pickup according to claim 1, wherein the optical pickup is substantially parallel to a plane that includes a normal line to a surface on which grating grooves of the reflection type diffraction grating are carved.