JPH083906B2 - Optical head device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical system of what is generally called an optical disk such as a CD, an optical video disk and a recording / reproducing optical disk.

(Prior Art) FIG. 2 shows an optical system used in a conventional optical disc. The light beam emitted from the semiconductor laser 1 is made into substantially parallel rays by the collimator lens 2, and the wedge prism 8
Incident on. The wedge prism is used to eliminate the astigmatism of the emitted light beam and to correct the light distribution shape. FIG. 2 shows an example using two wedge prisms. The light beam that has passed through the wedge prism is
The light passes through the beam splitter 9 and enters the objective lens 4. The light beam converged by the objective lens passes through the substrate of the disk 5 and reaches the information recording / reproducing layer. The light beam reflected by the information recording / reproducing layer traces the outward path in the opposite direction, is reflected and branched by the beam splitter 9, passes through the lens group 6, and enters the detector 7. When the lens group 6 is a combination of a single-convex lens and a cylindrical lens, a general focusing servo signal called a so-called astigmatism method can be obtained. A tracking servo signal can be obtained by taking out the detector output using an appropriate circuit. The objective lens 4 can be two-dimensionally driven by using these servo signals, and servo can be performed so that the light beam incident on the disk 5 is converged on a certain track, but it is not directly related to the present invention. Since it is not present, detailed description is omitted.

FIG. 3 is another example of the conventional example. In this example, the Littrow beam splitter 10 in which the wedge prism and the beam splitter of the example shown in FIG. In the example of FIG. 3, the number of optical components is small, and it can be said that the assembling property of the optical system is much improved as compared with the example of FIG.

(Problems to be Solved by the Invention) In the conventional example as shown in FIG. 2, the number of optical components increases,
Considering optical axis adjustment and assembling, it is complicated and cost performance is poor. The conventional example shown in FIG. 3 compensates for this drawback. However, as is clear from this structure, since the same surface is used as a transmission and reflection surface, vapor deposition is used as a means for separating transmitted light and reflected light. There are technically difficult points such as the necessity of forming many films. Further, there is a drawback that retardation of the light beam occurs on the reflection surface of the Littrow beam splitter 10 and the incident light of linearly polarized light is generally elliptically polarized light. Where the retardation (Reta
rdation) means that an optical element such as a Littrow beam splitter is constructed by adhering prisms.
It means that photoelasticity and birefringence occur due to the cured state of the adhesive, stress strain, etc., and a phase difference occurs due to the polarization state of incident light. Therefore, when a phase difference occurs, the amount of light incident on the detector changes, or the amount of light returned to the semiconductor laser increases, which deteriorates the quality of the reproduced signal. The generation of the retardation causes a remarkable signal deterioration particularly in a technique called a magneto-optical recording / reproducing system that utilizes the Kerr effect of a recording film material.

The present invention provides a method that overcomes the drawbacks of the two methods described above and that is simple and that retardation does not easily occur.

(Means for Solving Problems) In order to solve the problems seen in the conventional example as described above, in the present invention, one wedge prism is used instead of the conventional combination of the wedge prism and the beam splitter. To do. When the light beam passes through this one wedge prism, it is possible to correct the beam shape and the astigmatism. Further, this wedge prism is used as a beam splitter to extract the surface reflected light on one surface thereof.

That is, the optical head device of the present invention includes a collimator lens that receives a light beam emitted from a radiation light source to form a substantially parallel beam, an objective lens that converges the light beam on a record carrier, and a space between the collimator lens and the objective lens. It has an optical element having two non-parallel surfaces arranged, and an optical path for causing the light beam reflected on the record carrier and transmitted again through the objective lens to be reflected by the surface of the optical element on the objective lens side and incident on the photodetector. When the incident angle on the collimating lens side of the optical element is α and the incident angle on the objective lens side is β, then 45 ° <β <α <75 °, and the output for the incident beam diameter in the optical element When the ratio of the beam diameters to be set is m, m = 1.5 to 2.0.

(Operation) According to the present invention, it is possible to reduce the number of optical components, facilitate the optical axis alignment and assembling of the optical head, and reduce the components to improve the reliability. Since the wedge prism used as a beam splitter uses surface reflection without an adhesive surface, birefringence and photoelasticity are less likely to occur, and therefore a phase difference (retardation) does not occur due to the polarization state. In particular, it is very advantageous in a system for recording / reproducing an information signal by utilizing a minute difference in polarization state.

(Example) An example of the present invention will be described with reference to FIG. The light beam emitted from the semiconductor laser 1 as a radiation source is
The collimator lens 2 makes a substantially parallel light beam. The light beam enters the wedge beam splitter 3 at an incident angle α. The value of α depends on the astigmatism distance of the semiconductor laser and the ellipticity of the semiconductor laser, which is the ratio of the radiation full angle at half maximum θ _‖ parallel to the joining direction of the semiconductor laser and the radiation half angle at full angle θ _⊥ perpendicular to the joining direction. . When the angle of emission from the wedge prism is β, the ellipticity of the light beam before and after the wedge prism is m times. At this time, the beam expansion rate m is Given in. Here, n is a refractive index. This relation m is always m> 1 even if α> β. In the case of a normal semiconductor laser, θ _‖ 10 ° θ _⊥ 25 °, but the elliptic correction factor of the optical system does not necessarily need to make the light beam isotropic, but rather considers the light beam transmission efficiency. , M =
It is preferable to set it to about 1.5 to 2.0.

The light beam emitted from the semiconductor laser has a Gaussian distribution as a fundamental mode. When the ellipse correction factor of the optical system is set to 2.5 times and the light beam is made isotropic, the light beam distribution becomes closer to the plane wave distribution than the Gaussian distribution. When light with a distribution close to a plane enters the objective lens,
At the focal plane of the objective lens, a light spot called Airy Disc is created. First-order diffracted light is generated around the Airy Disc, and the maximum intensity of the first-order diffracted light is about 4% of the 0-th order light. If this first-order diffracted light is on an adjacent track,
Crosstalk occurs. Track pitch 1.2 to 1.6 μm
Therefore, in an optical disk device having a wavelength of 770 to 800 nm and an NA of 0.45 to 0.6, the crosstalk becomes large because the first-order diffracted light comes exactly on the adjacent track. On the other hand, when entering the objective lens, the distribution of the light beam becomes close to Gaussian. For example, when the intensity around the objective lens becomes 1 / e ^{2 of} the center intensity, that is, about 13%, the full width at half maximum of the spot is half the full width at half maximum of Airy Disc. It expands by about 10% compared to the full width, but the 1st-order diffracted light intensity decreases to about 1/3. As a result, crosstalk from adjacent tracks is reduced. Therefore, in the present invention, if the correction of the ellipticity is not made to be a perfect circle for the purpose of suppressing the first-order diffracted light and the correction in one direction is 1.5 to 2.0 times, the transmittance is relatively large and an appropriate Gaussian is obtained. It can be a beam distribution. Since the incident angle is too large for α> 75 °,
Conditions for anti-reflection coating become difficult. On the other hand, β <45
If it is set to °, the angle of the wedge prism for correction becomes large, and the configuration of the optical system becomes complicated.

As a numerical example, if β = 45 ° is fixed, α = 64.8 ° when m = 1.5, and α = 71.9 ° and 45 ° when m = 2.0.
<Β <α <72 °.

Also, if we fix α = 75 °, β = 6 when m = 1.5
When 8.9 ° and m = 2.0, β = 59.6 °, 59 ° <β <α <
It becomes 75 °.

Therefore, in the present invention, 45 ° <β <α <75 °,
In addition, α and β may be set so as to satisfy m = 1.5 to 2.0.

The light beam that has passed through the wedge beam splitter 3 passes through the substrate of the disk 5 by the objective lens 4 and enters the information recording / reproducing layer. The light beam reflected by the information recording / reproducing layer again passes through the objective lens 4 and enters the wedge beam splitter 3 at an incident angle β. The light beam reflected by the wedge beam splitter 3 is a lens group 6
It passes through and enters the detector 7. Although the focusing and tracking signals and the information reproducing signal can be obtained from the output of the detector 7 as described in the section of the conventional example, other well-known servo signal detecting methods can be used in the present invention, and therefore detailed description will be given. Is omitted.

In the case of the present invention, by setting the beam expansion rate m of the wedge beam splitter 3 to 1.5 to 2.0, the high-order diffracted light of the light beam on the disk 5 is reduced, and it is used in the direction of reducing crosstalk from the adjacent track. Is a good idea.
That is, in FIG. 1, by setting the optical system so that the radial direction of the disk is in the vertical direction of the paper surface, it is possible to obtain a practically good reproduction signal characteristic.

(Effects of the Invention) As described above, according to the present invention, it is possible to reduce the number of optical components and still obtain equivalent or higher performance. In addition, it is possible to reduce the retardation in order to obtain the reproduction and servo signals by using the surface reflection of the non-parallel wedge-shaped beam splitter, and especially for the method of detecting the minute polarization state difference such as magneto-optical. Will be effective. The use of the present invention can improve the adjustment of the optical axis and the assembling property of the optical head as a ripple effect. This is, for example, the retro-
Even when compared with the case of using the beam splitter 10, the design based on the reflecting surface is easy, the optical path of the reflected light is geometrically simple, and the effect is greater than the conventional one. There is.

[Brief description of drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG.
FIG. 3 is a diagram showing a most basic configuration in which a wedge prism and a beam splitter are combined in the conventional example of FIG. 3, and FIG. 3 is a diagram showing a configuration using the Littrow beam splitter 10 in the second conventional example. 1 ... Semiconductor laser, 2 ... Collimate lens, 3 ...
Wedge beam splitter, 4 ... Objective lens, 5 ...
Disc, 6 ... Lens group, 7 ... Detector.

Claims (1)

[Claims] 1. A collimator lens for receiving a light beam emitted from a radiation light source to form a substantially parallel beam, an objective lens for converging the light beam on a record carrier, and a non-parallel lens disposed between the collimator lens and the objective lens. An optical element having two surfaces, and an optical path for reflecting the light beam reflected on the record carrier and transmitted again through the objective lens to be reflected by the surface of the optical element on the objective lens side and incident on a photodetector. Then
The incident angle on the collimating lens side of the optical element is α,
When the incident angle on the objective lens side is β, 45 ° <β <α <75 °, and when the ratio of the outgoing beam diameter to the incident beam diameter in the optical element is m, m = 1.5 to 2.0. An optical head device characterized by the above.