JPH0789415B2 - Optical head - Google Patents

Description

The present invention relates to an optical head for optically recording / reproducing information, and more particularly to a magneto-optical recording / reproducing device for detecting magneto-optically recorded information. Regarding the optical head.

[Conventional technology]

Conventionally, an optical head for reading information recorded magneto-optically, an optical system for detecting a change in polarization state of reflected light or transmitted light caused by the recorded information,
It was constructed separately from the optical system for detecting the servo error signal or the change in the amount of reflected light from the reflected light or transmitted light, and had the drawback that the optical system became complicated and the optical head became large. . That is, the reflected light or transmitted light from the medium is separated into two beams by a beam splitter such as a half mirror, and then one beam is guided to a polarization state detection system such as a polarization beam splitter and an analyzer to record the recorded information. The other beam is used for detection, and the other beam is guided to an optical system for detecting a focus error signal, a track error signal, a reaction optical change, and the like which are irrelevant to the polarization state.

[Problems to be solved by the invention]

The above-described conventional magneto-optical recording / reproducing optical head has a complicated structure because the optical system for detecting the change in the polarization state and the optical system for detecting the servo signal are separately arranged. There is a drawback that it becomes large and it is difficult to reduce the size and cost of the optical head.

[Means for Solving the Problems] The optical head of the present invention is arranged in the optical path of reflected light or transmitted light from a recording medium, and its main axis is in a plane orthogonal to the optical axis of the reflected light or transmitted light. And a first prism that forms a predetermined angle with respect to the plane of polarization of the reflected light or transmitted light and its main axis is in a plane orthogonal to the optical axis of the reflected light or transmitted light and the first prism Against the main axis of
A second prism that forms an angle greater than 45 degrees and less than 135 degrees, and the intensity of the reflected light or the transmitted light changes depending on its polarization state, and the light intensity by the polarization. On the same plane as the polarization separation means for separating the third beam without change and the first, second and third beams that have passed through the polarization separation means to separate and receive them. It is characterized in that it is provided with a light detecting means arranged.

〔Example〕

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the optical head of the present invention. FIG. 2 is a perspective explanatory view showing a part of FIG. 1 in detail.

The light emitted from the semiconductor laser 1 becomes a parallel light flux by the collimator lens 2, passes through the beam splitter 3, and is converged by the objective lens 4 to become a converged light.
Is irradiated. The light beam reflected by the recording medium 5 travels backward through the objective lens 4 and enters the beam splitter 3 again. The beam splitter 3 has, for example, a transmittance of 70% and a reflectance of 30.
%, And the optical path of a part (30%) of the light beam reflected by the recording medium 5 is bent and directed toward the condenser lens 6. The light passing through the condenser lens 6 passes through the cylindrical lens 7 and enters the Wollaston prism 10.

As shown in FIG. 2, the Wollaston prism 10 is made of a uniaxial crystal such as a crystal having a principal axis orthogonal to the central axis of the light transmitted through the cylindrical lens and tilted at an angle of about 45 ° with respect to the polarization direction thereof. The prism 31 and the prism 32, which is the same member as the prism 31, is displaced from the main axis of the prism 31 by a certain angle and has a main axis in a direction orthogonal to the central axis of light. The light that has passed through the Wollaston prism 10 is separated into two beams 300 and 400 that change in intensity depending on the polarization state of the incident light and one beam 500 that does not change in intensity depending on the polarization state of the incident light. Of these, the beam 500, which has no intensity change due to the polarization state, is not affected by the magneto-optical information recorded on the recording medium 5, so that the change in the reflectance of the recording medium 5 is detected (or the information recorded in the reflectance changing type). Read out) and detection of servo control error signals such as focus error and tracking error. In this embodiment, astigmatism is generated in the focused light by the functions of the condenser lens 6 and the cylindrical lens 7 arranged in the optical path of the reflected light, and the change is caused by the four-division light arranged in the central portion of the photodetector 11. An astigmatism method focus error detection system that obtains a focus error signal by detection with the detection element 23 is configured.

Two beams 300 and 4 that change their intensity depending on the polarization state
00 has intensity changes of mutually opposite phases depending on the polarization state (polarization inclination) of the reflected light from the recording medium 5, so that the respective photodetection elements 21 and 22 receive the light and the difference between their outputs is taken. A change in the polarization state of the reflected light, that is, information recorded magneto-optically on the recording medium 5 can be detected. Further, the three beams emitted from the Wollaston prism 10 are condensed on almost the same plane although the traveling directions thereof are different, so that the photodetector element receiving each beam is on the same plane in one photodetector 11. Will be placed.

In the above-mentioned configuration, although the signals can be detected in principle regardless of the values of the intensity ratios of the three beams separated by the Wollaston prism 10, reflection by the magneto-optical recording is possible. The change in polarization state of light or transmitted light is extremely small, and in order to detect highly reliable information, the intensity of beams 300 and 400 for detecting change in polarization state is used for servo error signal detection, etc. It is preferable that the intensity of the beam 500 is larger than that of the beam 500. Therefore, in the present invention, by adjusting the inclinations of the principal axes of the two prisms 31 and 32 that form the Wollaston prism 10, two beams 300 and 400 that change in intensity depending on the polarization state are obtained.
The sum of the light intensities of the above is set to be larger than the light intensity of the beam 500 that has no intensity change due to polarization. An adjustment method for making the Wollaston prism 10 have such characteristics will be described below.

FIG. 3 is a vector diagram schematically showing how reflected light is separated from a medium having linearly polarized light by a Wollaston prism.

In the same figure (A), the linearly polarized reflected light 100 entering the Wollaston prism 10 is reflected by the prism 31 to be an ordinary light 101 and an extraordinary light 102
The state of being separated into. The prism 31 has a principal axis in the direction shown in FIG. 3G, and the light 101 having a polarization orthogonal to the principal axis becomes the extraordinary light 102 having the polarization parallel to the ordinary optical axis. At this time, the polarization of the incident light 100 to the prism and the ordinary light 1
If θ is the angle without polarization direction of 01, and the amplitude of the incident light is 1, the sealing width of the ordinary light 101 is cos θ, and the amplitude of the extraordinary light is sin.
θ.

In FIG. 3B, the ordinary light 101 in the prism 31 is the prism 32.
Shows that the light is further separated into ordinary light and extraordinary light. Since the principal axis of the prism 32 has a direction inclined by φ from the principal axis of the prism 31 as shown in FIG. 7A, both the ordinary ray 101 and the extraordinary ray 102 in the prism 31 become ordinary rays and extraordinary rays in the prism 32. To be separated. When the amplitude of the ordinary light 101 of the prism 31 is cos θ, the amplitude of the ordinary light 110 in the prism 32 separated from this is cos θ cos φ, and the amplitude of the extraordinary light 111 is cos θ sin φ. Also, the extraordinary light 1 at the prism 31
02 also shows the ordinary light 12 in the prism 32 as shown in FIG.
It is separated into 0 and extraordinary light 121, and the amplitude is sin θ sin
φ and sin θ cos φ.

Since the ordinary light and the extraordinary light have different refractive indexes in the prisms 31 and 32, the ordinary light in the prism 31 and the light that becomes the extraordinary light in the prism 32, and vice versa, the extraordinary light in the prism 31
The light that becomes ordinary at 32 has its optical path bent at the boundary of the prism. On the other hand, in the prisms 31 and 32, the light that continues in the ordinary light state or the abnormal light state proceeds straight without being bent at the prism boundary. The ordinary light in the prism 31 and the extraordinary light 111 in the prism 32 have amplitude and polarization as shown in FIG. 2D, and the Wollaston prism 10 bends the beam 300 upward as shown in FIG. Becomes The extraordinary light in the prism 31 and the ordinary light 120 in the prism 32 have the amplitude and polarization as shown in FIG. 2F, and become a beam 400 that bends downward as shown in FIG. Light 110 and prism 31 that become ordinary light at prism 31 and ordinary light at prism 32
The extraordinary light and the extraordinary light 121 in the prism 32 have the amplitude and polarization shown in FIG. 3 (E), respectively, and go straight through the Wollaston prism 10 to form the central beam 500 shown in FIG. Become.

Since the intensity of each beam is calculated by the squared value of the above amplitude, the light incident on the Wollaston prism 10
If the intensity of 0 is 1, the intensity of beam 111 is cos ² θ sin
² φ, the intensity of the beam 120 is sin ² θ sin ² φ. In addition, the intensity of the straight beam that is a combination of the two beams 121 and 110 is the sum of the intensities of the two beams cos ² θ cos ² φ + sin ² θcos ²
φ = cos ² φ. Therefore, the ratio of the intensities of the three beams separated by the Wollaston prism 10 is the angle θ between the ordinary light polarization direction of the prism 31 and the polarization direction of the light incident on the prism and the prism 31 of the Wollaston prism 10. by the difference of the inclination phi of the main axis of the spindle and the prism ^{32 cos 2 θ sin 2 φ:} cos 2 φ: expressed as sin ² θsin ² φ.

The fact that the polarization state of the light incident on the Wollaston prism 10 changes is the same as the change of θ in the above equation, so the light intensity whose value changes with θ is affected by the polarization state. The intensity of the central straight beam is cos ²
Since the frequency of φ and θ is not included, the intensity change due to polarization does not appear. On the other hand, the intensities of the beam 300 and the beam 400 change in proportion to cos ² θ sin ² θ, respectively, so that they change in opposite phases with respect to changes in polarization.

Here, the intensities of the two beams 300 and 400 whose intensities change depending on the polarization state are cos ² θ sin ² φ and sin ² θs, respectively.
Since it is in ² φ, the sum of the light intensities of the two beams becomes cos ² θ sin ² φ + sin ² θsin ² φ = sin ² φ, and the intensity of the previous two beams is To make it larger, sin ² φ> cos ² φ, that is, 45 ° <| φ | <135 °. However, since cos ² φ≈0 near | φ | = 90 °, the intensity of the beam 500 that does not change due to polarization becomes extremely small, and three beams cannot be substantially obtained. Also, two beams 300 and 4 whose intensity changes with polarization
In order to take a magneto-optical change due to the intensity difference of 00, it is desirable that the two beams have almost the same intensity in a balanced state. For this purpose, sin ² θ = cos ² θ, that is, | θ |
Is preferably set to around 45 °.

As described above, by appropriately adjusting the directions of the principal axes of the two prisms 31 and 32 forming the Wollaston prism 10, three beams having a desired light intensity ratio are emitted. You can

〔The invention's effect〕

As described above, according to the present invention, an optical head capable of detecting and reproducing magneto-optically recorded information with high sensitivity can be obtained with a simple structure. It should be noted that the beam splitter transmittance and the focus detection means by the astigmatism method and the like in the present embodiment are not limited to this configuration, and it goes without saying that any configuration having the same effect can be employed. . Also, the structure of the Wollaston prism is not limited to this example.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an optical head of the present invention,
FIG. 2 is a perspective view showing a part of FIG. 1 in detail, and FIG. 3 is a vector diagram schematically showing how reflected light from a medium having linearly polarized light is separated by a Wollaston prism. 1 ... Semiconductor laser, 2 ... Collimating lens, 3 ...
Beam splitter, 4 ... Objective lens, 5 ... Recording medium, 6 ... Condensing lens, 7 ... Cylindrical lens,
10 …… Wollaston prism, 11 …… Photodetector.

Claims (1)

[Claims] 1. A recording medium on which information is stored magneto-optically is irradiated with focused light, and the information recorded magneto-optically is detected by a change in polarization state of reflected light or transmitted light from the recording medium. In the optical head, which is arranged on the optical path of the reflected light or the transmitted light, the main axis is in a plane orthogonal to the optical axis of the reflected light or the transmitted light, and with respect to the polarization plane of the reflected light or the transmitted light. An angle that is greater than 45 degrees and less than 90 degrees with respect to the principal axis of the first prism and its principal axis that forms a predetermined angle in the plane orthogonal to the optical axis of the reflected light or transmitted light. Alternatively, a first prism and a second beam, which are composed of a second prism having an angle larger than 90 degrees and smaller than 135 degrees, and whose intensity changes depending on the polarization state of the reflected light or the transmitted light, and light by polarization. Change in intensity And a photodetector for separating the first, second and third beams, which have passed through the polarization separation means, respectively, and arranged them on the same plane. An optical head comprising: a light detecting unit.