JPH03141044A - Optical pickup - Google Patents

Abstract

PURPOSE:To miniaturize the optical pickup and to allow it to have a high performance by reducing a position fluctuation of a spot on a photodetector caused by an oscillation wavelength shift of a laser light source and an offset of each signal, in a miniature pickup loaded with a semiconductor laser light source and a holographic diffraction grating. CONSTITUTION:A laser light beam 10 emitted from a semiconductor laser light source 1 passes through holographic differection gratings 102, 101 in an optical path converting element 100, condensed onto an optical disk 3 by an objective lens 2, and forms an optical spot 11. A laser light beam 12 reflected by this disk 3 passes through the lens 2 again and enters a first holographic diffraction grating 101 in the optical path converting element 100, diffracted by the grating 101, and a + primary diffracted light beam 13 is separated. Also, the light beam is diffracted by this + primary diffraction grating 102 again and a -primary diffracted light beam 14 is separated. This - primary diffracted light beam 14 is made incident on a photodector and optical intensity is detected, and an information signal and a position control signal of the optical spot 11 are reproduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical pickup used in an optical information recording/reproducing apparatus for optical discs, digital audio discs, video discs, and the like.

[Conventional technology]

Conventionally, an optical pickup is constructed by arranging independent optical elements such as a semiconductor laser light source, an objective lens, a beam splitter, a detection lens, and a photodetector in a pickup case.

For this reason, it is difficult to achieve a size reduction in size and weight compared to magnetic pickups, and this is a major obstacle to reducing the size and weight of the entire recording/reproducing device.

Recently, as a solution to such problems.

Optical pickups using holographic diffraction gratings with irregularly spaced and curved grating patterns have been proposed. In other words, in this optical pickup, a transmitted or reflected light beam from an optical information recording medium (hereinafter referred to as an optical disk) is diffracted by a holographic diffraction grating, and then incident on a photodetector. By omitting the lens, the optical pickup has no wrinkles and is lighter in weight.

Regarding optical pickups using holographic diffraction gratings, for example, Japanese Patent Application Laid-Open No. 1229-1983
No. 38, JP-A No. 62-97141.

JP-A-62-92242, JP-A-62-1577
56, JP-A No. 62-124656, and the like.

[Problem to be solved by the invention]

By the way, the oscillation wavelength of the semiconductor laser light source currently mounted on the optical pickup may deviate from the design value by at least ±30 y&wm due to temperature fluctuations during the initial adjustment. In the conventionally proposed compact optical pickup using a holographic diffraction grating, the separation angle of the laser beam that is diffracted and separated by one diffraction grating changes greatly due to the influence of this wavelength shift, and as a result, the photodetector The upper spot irradiation position varies significantly from the design value. If there is such a fluctuation in the irradiation position.

This causes a large offset in the focus control signal, tracking control signal, etc., resulting in a problem that the reliability of the optical pickup is significantly reduced.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pickup suitable for suppressing variations in the optical spot due to such wavelength shifts of laser light and reducing offsets of each control signal.

Means for Solving caIm] In order to achieve the above object, in the present invention, at least two holographic diffraction gratings with different grating patterns are arranged at a predetermined interval in an optical pickup, and the transmission or reflection from the optical disk is The light beam is sequentially diffracted by each of the holographic diffraction gratings, and is then incident on a photodetector.

[For production]

Among the light beams diffracted by the holographic diffraction grating, what are the diffraction directions of the +1st order diffraction light and the 11st order diffraction light?

It is approximately symmetrical with respect to the incident light beam.

Therefore, for example, by setting the grating patterns of the two holographic diffraction gratings to predetermined patterns, the change in the diffraction direction of the +1st-order diffraction source in the first holographic diffraction grating due to the wavelength fluctuation of the laser beam, and the change in the diffraction direction of the +1st-order diffraction source in the second
The change in the diffraction direction of the 1st-order diffracted light in the holographic diffraction grating can be canceled out.

Therefore, using an optical path conversion element provided with such a plurality of holographic diffraction gratings, the transmitted or reflected light beam from the optical disk is first diffracted by the first holographic diffraction grating.

The +1st-order diffracted light is further diffracted by a second holographic diffraction grating, and the -1st-order diffracted light is made incident on the photodetector. By this, the positional fluctuation of the spot on the photodetector due to the wavelength shift of the laser beam can be suppressed. can be reduced.

Note that the details of the grating pattern design method for each holographic diffraction grating will be described later.

〔Example〕

A first embodiment of the present invention will be described below with reference to FIG.

FIG. 1 is a configuration diagram showing the configuration of an optical pickup as a first embodiment of the present invention. 1 is a semiconductor laser light source, 2 is an objective lens, 5 is an optical disk, and 4 is a photodetector. Further, 100 is an optical path conversion element, 101 and 102
are each holographic gratings.

A laser beam 10 emitted from a semiconductor laser light source 1 passes through a holographic diffraction grating 102 in an optical path changing element 100.
．． 101, the light is focused onto the optical disk 3 by the objective lens 2, forming a light spot 11.

The laser beam 12 reflected from the optical fiber 3 is again
The light enters the first holographic diffraction grating 101 in the optical path conversion element 100 through the objective lens 2, is diffracted by the holographic diffraction grating 101, and is separated into a +1st-order diffracted light beam 13. Furthermore, this +1st order diffraction light beam 15 enters the second holographic 1st diffraction grating 102, is diffracted again by this holographic diffraction grating 102, and the -1st order diffraction light beam 14 is separated. This-
The first-order diffracted light beam 14 enters the photodetector 4, the light intensity is detected, and an information signal and position control Kl of the light spot 11 are generated.
Signals (focus, tracking control signals) are reproduced.

FIG. 2 is an explanatory diagram showing how the two holographic diffraction gratings in FIG. 1 reduce the deviation in the irradiation position of the laser beam due to wavelength deviation.

Figure 2 (6) shows the laser beam 1 with the design wavelength λ (cotton).
This shows the case where the light enters the holographic diffraction grating 101 of wJl in the two-way optical path conversion element 100, and the grating 10
The +1st-order diffracted light beam 15 separated by 1 travels at an angle α tilted in the + direction (1 clockwise direction in the example shown) with respect to the optical axis of the incident laser light beam 12, and passes through the second hologram. The light is incident on the diffraction grating 102. Lattice 102
Then, the -1st-order diffracted light beam 14 undergoes diffraction again and travels at an angle β in one direction (counterclockwise in the example shown) with respect to the optical axis of the incident laser light beam 15, and reaches the photodetector. 4 (in the example shown, point Q).

On the other hand, FIG. 2(b) shows a case where the wavelength of the laser light beam 120 is shifted from the design wavelength λ by lλ to the longer wavelength side. In such a case, the +1st order diffracted light beam 1 separated by the first holographic diffraction grating 101
The separation angle of 3 changes from α to α−Δα. However, the second holographic diffraction grating 102 separates the 11th-order diffracted light beam 14 from the +1st-order diffracted light beam 13.
The separation angle also changes from β to β − Δβ, and the angle change Δ
β is in the opposite direction to Δα.

However, using the design method described later, the grating pattern of each holographic diffraction grating 10t, 102 was designed so that the amount of change in the diffracted light separation angle of each holographic diffraction grating 101102 with respect to the wavelength shift Δλ becomes a predetermined value. By defining each holographic grating 101
．． By canceling the change in the diffracted light separation angle occurring at 102, the incident position Q' of the 1st-order diffracted light beam 140 incident on the photodetector 4 can be made to coincide with Q.

Next, a method of designing the grating patterns of each of the holographic diffraction gratings 101 and 102 provided in the optical path conversion element 100 will be described. The design method is 2 as shown below.
Divided into stages.

First, in the first stage, each holographic diffraction grating 101
．． The relationship between the average grating pitch of 102 and the amount of displacement of the irradiation position of the laser beam 14 due to wavelength shift is calculated. Third
The figure is an explanatory diagram showing each parameter during calculation. As shown in Figure 0, a holographic diffraction grating 101
, 102, the average grating pitch of Pl, P2 . grid 10
Distance d between 1 and 102, refractive index %d, grating 102~
Assuming that the distance between the photodetectors 4 is t, the refractive index is nt, and the wavelength of the incident laser beam is λ, the distance from the origin O in the figure to the point Q on the photodetector 4 where the 1st-order diffracted light beam 14 is incident is The distance is expressed as follows.

L=t-TANCΦ)-t-rAgCv] ...
・・・・・・・・・・・・・・・・・・・・・(1) Here, Φ and V are each holographic diffraction grating 101,
The average diffraction light separation angle is 102, and is expressed by the following formula.

Φ= 5rs-' [λ/(ru d-P1)] ・-・
・・・・・・・・・・・・・・・・・・・・・・・・(
21F = SIN"' (A - (J'2 - J'1
)/(nt-Pi-P2)]・(31 In the first stage of design, we use equations (1)2 to (31) to find an average lattice whose distance variation is less than a predetermined value within a predetermined wavelength range. Find pitch combinations.

FIG. 4 is a graph showing an example of the calculation result of the above formula.

In this calculation example, P1 = 1.0 μ horse 1j = 31 + nt
L = 1.511, t = 1.0mm, n
t = 1.0, and the relationship between the wavelength λ (700 sm to 850 sm) of the laser beam and the distance is plotted for five types of P2. For comparison, similar calculations were performed and plotted on the same graph for a small optical pickup configured to change the optical path using a single holographic diffraction grating as proposed in the past.

In the example of one wire as shown in the figure? 1=1.0μII, P2=
=0.524s f) When combined, 750s+s ~
810 nt f) wavelength range H (taking into consideration a wavelength shift of ±301 with respect to the center wavelength of 780 nt f), the variation in the irradiation position on the photodetector 4 can be suppressed to below α01−. This is a variation less than one-sixth of that in the case of a conventional single holographic diffraction grating.

Next, in the second stage of design, a detailed grating pattern of each holographic diffraction grating 101, 102 is determined by the following procedure. FIG. 5 is an explanatory diagram showing the design procedure.

First, as shown in FIG. 5(g), a predetermined position on the optical axis of the +1st-order diffracted light beam 15 that separates and advances from the laser light beam 12 at the diffracted light separation angle Φ determined using the above-mentioned formulas ( In the example, a point light source is assumed at the intersection R) of the optical axis and the plane containing the photodetector 4. Then, this point light source is emitted, and the grid 1
The diverging laser light beam 20 enters the grating plane of the grating 101 through the grating 101 and emits the semiconductor laser light source 1,
[Similarly, by arranging the position K of interference fringes generated when the laser beams 10 incident on the grating surface of the grating 101 overlap on the grating surface of the grating 101, and the convex portions or concave portions of the grating grooves, , determine the grid pattern of the grid 101.

That is, a point light source R is emitted, which passes through the grating 102 to the grating 10.
Let the optical path length of a ray that reaches an arbitrary point A (coordinates (J, y)) on 1 be 51 (go y), and similarly the optical path length of a ray that is emitted from the semiconductor laser light source 1 and reaches point A via the grating 102. When is SO(s * y), the linear formula % formula %
) (4) If the holographic diffraction grating 101 designed in this way is used, the protrusions or recesses of the grating grooves are arranged at the positions (g, y) where the following is true, and the grating pattern is sequentially obtained. The +1st-order diffracted light beam 13 separated by 101 is
The light beam travels along the same optical path as the laser light beam 20 described above but reaches the grating 102 in the opposite direction.

Next, the grating pattern of the grating 102 consists of a laser beam 20 emitting a single point light source R and a diverging laser emitting a point light source placed at the designed irradiation position Q on the photodetector 4, as shown in FIG. 5(A). The interference fringes generated when the light beam 30 and the light beam 30 overlap each other on the grating surface of the grating 102 are determined by arranging the convex or concave portions of the grating grooves.

That is, a point light source R is emitted, and an arbitrary point B (
The optical path length of the ray that reaches the coordinates (X, 1')) is 52(x,
Similarly, the optical path length of the light beam emitted from a point light source placed at the designed irradiation position Q on the photodetector 4 and reaching one point B is 53(
In this way, the convex or concave portions of the grating grooves are placed at the positions (x, y) where the linear formula % formula %) (5) holds true, and the grating pattern is sequentially obtained. When using the holographic diffraction grating 102 designed as shown in FIG. The light is focused at the irradiation position QK. Therefore, holographic diffraction gratings 101 and 102 whose patterns are designed using one or more design procedures are arranged at intervals of d, and the light beam 12 reflected or transmitted through the optical disk 3 is sequentially diffracted by each holographic diffraction grating 101 and 102. By doing so, the light can be made to be incident on the designed irradiation position on the photodetector 4.

The above is the design method for each grating pattern when two holographic diffraction gratings are used. As design examples, FIG. 6 (1) shows an example of the lattice pattern of the lattice 101, and FIG.

As can be seen from Figure 8, which depicts the locus of every 20 lattice grooves in an area of 1×1-, each lattice pattern has a curved shape with irregular intervals. There is.

In the embodiment shown in FIG. 1, the optical path conversion element 100 is composed of two holographic diffraction gratings. Three or more holographic gratings (in the example shown, 101,
102, 103) may be used.

FIG. 8 is a configuration diagram showing a third embodiment of the present invention.
Among the parts in the figures, parts similar to those in the embodiment of FIG. 1 are designated by the same numbers.

In this embodiment, as the optical path conversion element 100, holographic diffraction gratings are placed on both the front and back surfaces of a substrate 50 that is optically transparent like a glass substrate or has a predetermined transmittance.
This is an example in which 01 and 102 are provided.

By providing the holographic diffraction gratings 101 and 102 on both surfaces 5 of a single substrate as in this embodiment, the optical path conversion element 100 can be made smaller.

°Also, as in the fourth embodiment of the present invention shown in FIG.
Semiconductor laser light source 1. Photodetector 4 in a single package 6
It can also be used as a sealing glass for the back cage 6 by placing the optical path converting element 100 thereon. By adopting such a configuration, the semiconductor laser 1
, photodetector4. It is also possible to combine the optical path conversion element 100 into a single optical element and further downsize the optical pickup.

Furthermore, in the embodiment shown in FIG. 6 described above, the holographic diffraction grating 101,
However, as in the fifth embodiment of the present invention shown in FIG. The grid 101 avoids the passage area 50 of
The +1st-order diffracted light beam 130 is separated by the grating 102 and enters the grating 102 only in the passing region 51 and its neighboring region.
It is also good to set up the holographic grating 1, like this
By partially providing 02, unnecessary diffraction is prevented,
Stray light can be reduced and light utilization efficiency can be improved.

Incidentally, the grating pattern of each holographic diffraction grating 101, 102 used in the embodiments shown in FIGS. By setting the refractive index to [, the design can be performed in exactly the same manner as the embodiment shown in FIG.

FIG. 11 is a configuration diagram showing a sixth embodiment of the present invention. Components similar to those in each of the previous embodiments are designated by the same reference numerals.

Similar to the embodiment shown in FIG. 10, the feature of this embodiment is that in the optical path conversion element 100 in which holographic diffraction gratings 101 and 102 are provided on both sides of a single substrate 50, the laser beam located within the plane 5a of the semiconductor laser light source 1 is Passage area 5 of light beam 10
0 and its vicinity, a simple lattice 104 having a predetermined lattice pitch and a straight line shape is provided. This simple grid 1
04 is a laser light beam 1 emitted from a semiconductor laser light source 1
0 to the main beam 10g and two sub beams 10, 10el
C layer and this sub beam 10h.

10c is provided for detecting a tracking control signal using a so-called 3-beam system. As for the principle of detecting the tracking control signal by this three-beam system, it is already a known technique, so a description thereof will be omitted.

As in this embodiment, a plurality of holographic diffraction gratings 101 and 102 used for optical path conversion and a simple grating 104 used for detecting a tracking control signal of a 3-beam system are installed on both the front and back surfaces of a single substrate 50. By providing the optical path conversion element 1000, the functions of the optical path conversion element 1000 can be further compounded, and the optical pickup can be made smaller in size.

〔Effect of the invention〕

According to the present invention, in a small optical pickup equipped with a semiconductor laser light source and a holographic diffraction grating, positional fluctuations on the photodetector due to deviations in the oscillation wavelength of the semiconductor laser light source and associated offsets of each detection signal are reduced. Therefore, it is possible to provide a small-sized optical pickup with improved reliability.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention. Figure 2 is an explanatory diagram showing how the holographic diffraction grating in Figure 1 reduces the irradiation position shift of the laser beam due to wavelength shift. Figure 3 shows the grating pattern of the holographic diffraction grating in Figure 1. An explanatory diagram showing each parameter when designing, Fig. 4 is a graph showing the relationship between the distance and the wavelength λ of the laser beam in Fig. 5, and Fig. 5 is a grating pattern of the holographic diffraction grating in Fig. 1. FIG. 6 is a plan view showing an example of the grating pattern of the holographic diffraction grating in FIG. 1, and FIG. 7 is a diagram showing the 20th embodiment of the invention. Fig. 8 is a block diagram showing the fifth embodiment of the present invention, Fig. 9 is a partially cutaway perspective view of the fourth embodiment of the present invention, and Fig. 10 is a block diagram showing the fifth embodiment of the present invention. FIG. 5 is a configuration diagram showing a fifth embodiment of the invention. FIG. 11 is a block diagram showing a sixth embodiment of the present invention. 1... Semiconductor laser light source 2...
・・・・・・・・・Objective lens 3・・・・・・・・・
..... Optical disk 4 ..... Photodetector 5 ..... Substrate 100 ....
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Claims (1)

[Scope of Claims] 1. A semiconductor laser light source, an objective lens for condensing a laser beam emitted from the semiconductor laser light source onto an optical information recording medium, and a laser beam that transmits through the optical information recording medium or An optical pickup comprising an optical path converting element that converts the optical path of a laser beam reflected by an information recording medium and guides it to a photodetector, and the photodetector that detects the light intensity of the guided laser beam. , the optical path conversion element has two or more holographic diffraction gratings each having grating grooves arranged at unequal intervals and forming a curved trajectory or grating grooves arranged at equal intervals and forming a linear trajectory. , an optical pickup characterized in that each of the holographic diffraction gratings is arranged at a predetermined interval from each other. 2. The optical pickup according to claim 1, wherein the optical path conversion element is formed by forming the holographic diffraction grating on the front and back surfaces of the same substrate having a predetermined light transmittance, respectively. optical pickup. 3. The optical pickup according to claim 1, wherein the optical path changing element converts at least one grating of the holographic diffraction element into a main beam and at least two laser beams emitted from the semiconductor laser light source. 1. An optical pickup characterized in that the optical pickup is formed in the same plane of the same substrate together with a simple grating for separating the sub-beams. 4. The optical pickup according to claim 1, wherein the semiconductor laser light source, the optical conversion element, and the photodetector are arranged in the same package.