JPH05197998A - Separation type optical pickup - Google Patents

Abstract

PURPOSE:To enable servo control with good accuracy and to reduce costs by surely preventing the reflected light from a transparent plate from being condensed to a photodetector. CONSTITUTION:An exit window 9 and an incident window 10 are respectively opened and formed in the part where a stationary optical system A and a moving optical system B face each other. These windows are covered with the transparent plates 4, 4'. The part of the stationary optical system A where the exit window 9 thereof is opened is a slope inclined by an angle theta with the plane perpendicular to the optical axis of a light beam. The transparent plate 4 is, therefore, mounted by inclining the plate by the theta with the plane perpendicular to the optical axis of the light beam. The angle theta of inclination is more specifically set at the angle at which the reflected light from the transparent plate 4 is prevented from being condensed onto the photodetector 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup used in an optical recording / reproducing apparatus, and more particularly to a split type optical pickup which is divided into a movable optical system and a fixed optical system.

[0002]

2. Description of the Related Art An optical disc is a recording medium (information recording medium) having a large capacity, capable of contactless information writing / reading and medium exchange, and is an optical disc file or an external storage of a computer. It is attracting attention as a medium.

In recent years, a separate optical pickup has been developed for the purpose of achieving high-speed access to such an optical disc. This separation type optical pickup forms a fixed optical system with various optical system parts including a semiconductor laser, and mounts a relatively lightweight objective lens and its actuator section on a movable optical system separated from the fixed optical system. Only the actuator portion is movable in the disc radial direction of the optical disc.

Laser light (hereinafter referred to as a light beam) of the fixed optical system and the movable optical system of such a separation type optical pickup.
An exit window and an entrance window are formed in the portions facing each other on the optical path. That is, the light beam emitted from the semiconductor laser is guided to the objective lens in the movable optical system through the emission window formed in the fixed optical system and the incident window formed in the movable optical system, and the light beam focused by the objective lens is guided. The spot is adapted to irradiate the disc surface of the optical disc.

By the way, the exit window and the entrance window are covered with a transparent plate in order to prevent foreign matter such as dust from entering the optical pickup from the outside and causing trouble. As a conventional example of a separation type optical pickup provided with this kind of transparent plate, there is one proposed in Japanese Patent Application No. 2-203434.

[0006]

By the way, in the above-mentioned conventional example, no reasonable consideration is given to the mounting mode of the transparent plate, and the mounting is so made as to be perpendicular to the optical axis of the light beam. Is. Therefore, a part of the light beam incident on the transparent plate is reflected by the surface of the transparent plate, and the reflected light is detected by the photodetector composed of the multi-division light receiving element,
There is a drawback that an offset (DC offset) occurs in the servo signal and accurate servo control cannot be performed.

The reason will be specifically described below with reference to FIGS. 4 and 5. FIG. 4 shows a general structure of a separation type optical pickup, which is divided into a fixed optical system A and a movable optical system B. The fixed optical system A has a rectangular box shape and is fixed on the base of the disk drive device. Inside the fixed optical system A, the semiconductor laser 1, the diffraction elements 2 and 45 are provided.
Other optical components such as a mirror 3, a photodetector 6 and a collimating lens are arranged.

In addition, an exit window 9 for guiding the light beam emitted from the semiconductor laser 1 to the movable optical system B is formed in the portion of the fixed optical system A facing the movable optical system B. The exit window 9 is covered with a transparent plate 4 in order to prevent foreign matter such as dust from entering the interior thereof and causing trouble.

On the other hand, an entrance window 10 is formed in the portion of the movable optical system B facing the exit window 9 and is similarly covered with a transparent plate 4 '.

In the above structure, the light beam emitted from the semiconductor laser 1 is diffracted by the diffractive element 2, then converted into parallel rays by an optical component (not shown) such as a collimator lens, and then reflected by the 45 ° mirror 3.
It is guided into the movable optical system B through the exit window 9 and the entrance window 10. The light beam guided inside the movable optical system B is focused and irradiated onto the disk surface of the optical disk D by the objective lens 5.

The light beam reflected from the disk surface of the optical disk D returns to the position of the diffractive element 2 following the path opposite to the above, and is subsequently diffracted by the diffractive element 2 toward the photodetector 6. The light is focused on the photodetector 6. The condensed beam condensed on the photodetector 6 is photoelectrically converted into an electric signal by the photodetector 6, and thereby a control signal for focusing and a control signal for tracking are generated.

By the way, in the above-mentioned conventional example, since the transparent plate 4 is mounted perpendicularly to the optical axis of the incident light beam, a part of the light beam incident on the transparent plate 4 is reflected by the surface of the transparent plate 4 and is reflected. The phenomenon of returning to the detector 6 occurs.
This becomes a serious problem particularly in an optical pickup having a structure in which light is guided to the photodetector 6 by the diffraction element 2 as shown in FIG.

The reason will be described below with reference to FIGS. 5 (a) and 5 (b). In the figure, the light beam emitted from the semiconductor laser 1 is split by the diffraction element 2 into 0th-order diffracted light and ± 1st-order diffracted light. The 0th-order diffracted light in this is shown in FIG.
As shown in (a), the light is incident on the transparent plate 4 after being converted into parallel light on the way. At this time, most of the light passes through the transparent plate 4 and goes to the movable optical system B, but a part of the light is reflected by the surface of the transparent plate 4 and returns to the diffraction element 2 again. This return light is diffracted again by the diffraction element 2, goes toward the photodetector 6, and is condensed on the photodetector 6. Since the output of the return light on the photodetector 6 does not have the information of focusing and tracking of the optical disc D, an offset occurs, which becomes an obstacle in the actual servo control.

Further, as shown in FIG. 5B, a part of the + 1st order diffracted light is reflected on the surface of the transparent plate 4 and returns to the diffraction element 2. Of this reflected light, the 0th-order diffracted (transmitted) light is focused on the photodetector 6 in the direction of the photodetector 6. This focused light also hinders the servo control.

For the above reason, in the optical pickup having the diffraction element 2, in addition to the reflected light of the main beam toward the optical disc D, the + 1st order diffracted light is also condensed on the photodetector 6 as unnecessary reflected light. Therefore, in such an optical pickup, in performing accurate servo control,
A particularly big problem arises.

In order to solve the above drawbacks, in the conventional example, a method of forming an antireflection film on the surface of the transparent plate 4 is often used, but an antireflection film capable of removing reflection almost completely is attached. However, there is a new drawback that the number of parts increases and the cost increases.

Therefore, the conventional separation type optical pickup has a limit in improving the accuracy of servo control.

The present invention solves the drawbacks of the prior art as described above, and it is possible to reliably prevent the reflected light from the transparent plate from being condensed on the photodetector, and to enable the accurate servo control. Another object of the present invention is to provide a separate optical pickup.

Another object of the present invention is to realize a separation type optical pickup which can reduce the cost.

[0020]

A separation type optical pickup of the present invention is provided with a movable optical system having an objective lens arranged to face an information recording medium and an optical component such as a light source. A split type optical pickup that is divided into a fixed optical system that guides a light beam from the optical system to the movable optical system through an emission window, and has a transparent plate that covers the emission window,
The transparent plate has a predetermined angle θ with respect to a plane perpendicular to the optical axis of the light beam.
It is only tilted, which achieves the above purpose.

Preferably, the fixed optical system includes a diffractive element that diffracts the return light from the information recording medium and a photodetector that detects the diffracted light, and the inclination angle θ of the transparent plate.
Is diffracted by the diffractive element when the transparent plate is not tilted, and the optical axis that connects the reflected optical axis intersection point on the diffractive element of the transparent plate and the end point of the photodetector. The angle α formed by the diffracted light axis of the diffracted light is set to have a relationship of θ ≧ 2 / α.

Further, preferably, the fixed optical system includes a lens for condensing return light from the information recording medium on a photodetector, and an inclination angle θ of the transparent plate is condensed by the lens. The angle β between the optical axis of the light and the optical axis connecting the center of the lens and the end point of the photodetector is set to have a relationship of θ ≧ 2 / β.

[0023]

When the inclination angle θ of the transparent plate is set to have the above relationship, the reflected light of the incident light beam reflected from the surface of the transparent plate is condensed on the photodetector for the reason described later. Never. Therefore, a good focusing servo signal and tracking servo signal without offset can be obtained.

[0024]

EXAMPLES Examples of the present invention will be described below.

FIG. 1 shows an embodiment of the separation type optical pickup of the present invention. This separated optical pickup is divided into a fixed optical system A and a movable optical system B. Fixed optical system A
Has a rectangular box shape and is fixed on the base of the disk drive device. Inside the fixed optical system A, a semiconductor laser 1, a diffraction element 2, a 45 ° mirror 3, a photodetector 6, a collimating lens, and other optical components are arranged.

In addition, an exit window 9 for guiding the light beam emitted from the semiconductor laser 1 to the movable optical system B is formed in the portion of the fixed optical system A facing the movable optical system B. The exit window 9 is covered with a transparent plate 4 in order to prevent foreign matter such as dust from entering the interior thereof and causing trouble.

On the other hand, an entrance window 10 is formed in the portion of the movable optical system B facing the exit window 9 and is similarly covered with a transparent plate 4 '.

In the above structure, the light beam emitted from the semiconductor laser 1 is diffracted by the diffractive element 2 and then collimated by the collimator lens 7 (see FIG. 2 (a)), and then by the 45 ° mirror 3. It is reflected and guided into the movable optical system B through the exit window 9 and the entrance window 10. The light beam guided inside the movable optical system B is focused and irradiated onto the disk surface of the optical disk D by the objective lens 5. Reference numeral 20 in the figure denotes a pair of guide shafts for guiding the movable optical system B to move the optical disc D in the disc radial direction.

The light beam reflected from the disk surface of the optical disk D returns to the position of the diffractive element 2 following the path opposite to the above, and is subsequently diffracted by the diffractive element 2 toward the photodetector 6. The light is focused on the photodetector 6. The condensed beam condensed on the photodetector 6 is photoelectrically converted into an electric signal by the photodetector 6, and thereby a control signal for focusing and a control signal for tracking are generated.

In this embodiment, the transparent plate 4 is the transparent plate 4.
It is mounted at an angle of θ with respect to the plane perpendicular to the optical axis of the light beam incident on. That is, the portion of the fixed optical system A facing the movable optical system B is formed as an inclined surface inclined by an angle θ, and the transparent plate 4 is attached to the exit window 9 formed in the inclined surface. Specifically, this tilt angle θ is such that the reflected light from the transparent plate 4 does not fall on the photodetector 6.
That is, the angle is set to be equal to or larger than the angle at which light is not focused on the photodetector 6. The details will be described below.

As described above, when the transparent plate 4 is tilted by an angle θ with respect to the plane perpendicular to the optical axis of the light beam, the optical axis of the reflected light reflected by the transparent plate 4 is shown in FIG. 2 (a). like,
The optical axis of the forward path, that is, the optical axis of the incident light beam, is inclined by 2θ. Therefore, the focal point P of the reflected light on the surface of the photodetector 6 is also closer to the semiconductor laser 1 in the figure by an amount according to the inclination of the optical axis.

Here, in order to prevent the condensing point P of the reflected light from falling on the photodetector 6, the original condition when the transparent plate 4 is not tilted as shown in FIG. Assuming that the optical axis of the reflected light is GP ₂ shown by the broken line in the figure, the diffracted optical axis of the return light diffracted again by the diffractive element 2 is outside the optical axis GP ₁ shown by the dashed line in the figure. Alternatively, it can be achieved by setting the inclination angle θ of the transparent plate 4 so as to be located outside the optical axis GP ₃ shown by the chain double-dashed line in the figure. That is,
Since the optical axes GP ₁ and GP ₃ are specifically the optical axes connecting the reflection optical axis intersection point G on the diffraction element 2 and the end points P ₁ and P _{3 of the} photodetector 6, these optical axes are If the tilt angle θ of the transparent plate 4 is set so as to be located outside of, the reflected light will not be condensed on the photodetector 6.

Here, the optical axis GP ₂ and the optical axis GP _{1 form} an angle α.
And the optical axis GP ₂ and the optical axis GP ₃ form an angle α ′.
Now, assuming that P ₂ is the midpoint between P ₁ and P ₃ , α> α ′ due to the geometrical relationship, so in the present embodiment the optical axis is set to the angle α.
If tilted above, the reflected light will not be collected on the photodetector 6.

The above α is from the semiconductor laser 1 to the photodetector 6
It is determined by the distances L ₁ , L ₂ , L ₃ to the above points P 1, P ₂ , P ₃ and the height t from the diffractive element 2 to the photodetector 6, and is represented by the following formula.

Α = tan ⁻¹ (L ₂ / t) −tan ⁻¹ (L ₁ / t) ... The above formula is derived by the following procedure. In FIG. 2B, if the intersection of the perpendicular from the reflection optical axis intersection G and the extension of the line segment P ₃ P ₁ is O and ∠P ₂ GO = b and ∠P ₁ GO = a, α = Since b−a, and a = tan ⁻¹ (L ₁ / t) b = tan ⁻¹ (L ₂ / t), α = b−a = tan ⁻¹ (L ₂ / t) −tan ⁻¹ (L ₁ / t) ...

This α is equal to the inclination amount 2θ of the reflected light axis in FIG. 2A, and the relation 2θ = α is established. Therefore,
In order that the reflected light is not finally collected on the photodetector 6,
θ ≧ α / 2 = 1/2 {tan ⁻¹ (L ₂ / t) −tan ⁻¹
(L ₁ / t)} would be good.

The above is an example of the case where the reflected light of the main beam is first-order diffracted by the diffractive element 2 and is about to enter the photodetector 6, but ± of the diffractive element 2 on the outward path to the optical disc D. The reflected light of the first-order diffracted light on the transparent plate 4 is shown in FIG.
As shown in (c), the same applies to the case where the diffraction element 2 is diffracted in the 0th order and enters the photodetector 6. Also at this time, if θ ≧ α / 2, it is possible to prevent the reflected light from being condensed on the photodetector 6 in the same manner as described above.

FIG. 3 shows another embodiment of the present invention. This embodiment shows an example in which the present invention is applied to a separation type optical pickup which does not use a diffraction element in a signal detecting method. The optical pickup of this embodiment has a lens 8 as means for condensing a light beam on the photodetector 6.

Now, the end points of the photodetector 6 are P ₁ and P ₃ , the original condensing point of the light beam when the transparent plate 4 is not tilted is P ₂ , the focal length of the lens 8 is f, P ₂ -P ₃ , and P ₂ -P ₁ are L _4, and if β ≧ tan ⁻¹ (L ₄ / f) is set as in the above embodiment, the light beam does not illuminate the photodetector. Therefore, θ ≧ β / 2 = 1/2 tan ⁻¹ (L ₄ / f)
When the transparent plate 4 is arranged at an inclination angle of 1, the reflected light from the transparent plate 4 does not return to the photodetector 6.

Here, β is an angle formed by the optical axis of the light focused on the photodetector 6 by the lens 8 and the optical axis connecting the center of the lens 8 and the end point of the photodetector 6. ..

In each of the above embodiments, the direction in which the transparent plate 4 is tilted needs to be set in consideration of each optical system. However, as described above, of the optical axes incident on the end points of the photodetector 6, the transparent plate is adjusted in accordance with the case where the angle α with the original optical axis incident on the center of the photodetector 6 is the largest. If the tilt angle θ of 4 is set, it is not necessary to pay particular attention to the tilting direction.

In each of the above embodiments, the case where the transparent plate 4 covering the exit window 9 of the fixed optical system A is tilted has been described, but if the entrance window 10 of the movable optical system B is also tilted, the transparent plate 4 is tilted. Since the reflected light from 4'can be prevented from being condensed on the photodetector 6, there is an advantage that more accurate servo control can be performed.

[0043]

According to the above-mentioned separation type optical pickup of the present invention, the transparent plate is formed at an angle at which the reflected light of the incident light beam reflected from the surface of the transparent plate is not focused on the photodetector. Since it is attached to the fixed optical system by inclining to, it is possible to obtain good focusing servo signal and tracking servo signal without offset. Therefore, it is possible to realize a separation type optical pickup capable of highly accurate servo control. Furthermore, since it is not necessary to provide an antireflection film or the like, the number of parts can be reduced. Therefore, it can greatly contribute to cost reduction.

In particular, according to the separate type optical pickup of the second or third aspect, there is an advantage that the servo control can be performed with higher accuracy.

[Brief description of drawings]

FIG. 1 is a plan sectional view showing an embodiment of an optical pickup of the present invention.

FIG. 2 is a schematic diagram for explaining a tilt angle with respect to an optical axis of a light beam of a transparent plate that can prevent reflected light from entering a photodetector.

FIG. 3 is a schematic view for explaining a tilt angle of a transparent plate according to another embodiment of the present invention with respect to an optical axis of a light beam.

FIG. 4 is a plan sectional view showing a conventional example of an optical pickup.

FIG. 5 is a schematic diagram for explaining a problem of the optical pickup shown in FIG.

[Explanation of symbols]

1 Semiconductor laser 2 Diffraction element 3 45 ° mirror 4 Transparent plate 5 Objective lens 6 Photodetector 8 Lens for condensing a light beam on the photodetector A Fixed optical system B Movable optical system P Focus point θ Incident of transparent plate Angle of inclination of the light beam with respect to the plane perpendicular to the optical axis α The optical axis that connects the intersection of the reflected optical axis on the diffraction element of the light beam reflected from the transparent plate and the end point of the photodetector, and the diffraction element when the transparent substrate is not tilted Angle formed by the diffractive optical axis of the diffracted light diffracted by the β lens and the optical axis of the light condensed on the photodetector,
Angle formed by the optical axis connecting the center of the lens and the end point of the photodetector

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukio Kurata 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation

Claims (3)

[Claims] 1. A movable optical system having an objective lens mounted opposite to an information recording medium and optical components such as a light source are mounted, and a light beam from the light source is passed through an emission window to the movable optical system. A split type optical pickup divided into a fixed optical system for guiding has a transparent plate that covers the exit window, and the transparent plate is inclined by a predetermined angle θ with respect to a plane perpendicular to the optical axis of the light beam. Separate optical pickup. 2. The fixed optical system comprises a diffractive element for diffracting the return light from the information recording medium and a photodetector for detecting the diffracted light, and the inclination angle θ of the transparent plate is the transparent plate. Of the diffracted light diffracted by the diffractive element when the transparent plate is not tilted, and the optical axis connecting the cross point of the reflected optical axis of the light beam reflected from the diffractive element and the end point of the photodetector. 2. The separation type optical pickup according to claim 1, which has a relationship of θ ≧ 2 / α with respect to an angle α formed by the diffracted optical axis. 3. The fixed optical system is provided with a lens for condensing return light from the information recording medium on a photodetector, and an inclination angle θ of the transparent plate is set to a value of the light condensed by the lens. 2. The separation type optical pickup according to claim 1, which has a relationship of θ ≧ 2 / β with respect to an angle β formed by the optical axis and the optical axis connecting the center of the lens and the end point of the photodetector.