JPH04298837A - Polarizing beam splitter and magneto-optical head device - Google Patents

Abstract

PURPOSE:To simplify the structure of a magneto-optical head and to achieve high efficient light using ratio. CONSTITUTION:A magneto-optical head having one axis system is constituted of using a polarising beam splitter 14 which is composed by alternately combining hologram elements 11, 12 having polarizing properties to diffract only light in the specified polarization direction and Faraday rotation elements 11, 12 rotating the plane of polarization with incident light, a semiconductor laser 1, and an image formation lens 18. A beam 2 outgoing from the semiconductor laser 1 transmits the polarizing beam splitter while only rotating the plane of polarization and goes to a magneto-optical disk 8. Reflected light from the magneto-optical disk 8 is diffracted by the polarizing beam splitter 14 and made incident on optical detectors 45, 46, 47 and 48.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarizing beam splitter and a magneto-optical head device using the polarizing beam splitter for recording and reproducing optical disks.

0002

2. Description of the Related Art Conventional magneto-optical head devices have various configurations, and FIG. 3 shows an example of a magneto-optical head device using a hologram element, which is closely related to the present invention. This conventional optical head device uses a polarizing hologram element for detecting track and focus error signals and for detecting magneto-optical signals.

[0003] The emitted light 2 from the light source is passed through a collimating lens 3.
is converted into parallel light, enters the polarizing beam splitter 6 as p-polarized light, and after 50% of it passes through the converging lens 4.
The light is focused on the 8 surfaces of the magneto-optical disk. The return light reflected by the magneto-optical disk 8 passes through the opposite optical path and is reflected and separated by the polarizing beam splitter 6. Here, the reflectance of the p-polarized light component is 50%, and 100% of the s-polarized light component produced by the Kerr effect during reflection from the magneto-optical disk is reflected. This reflected light is converted into convergent light by the lens 7 and enters the polarizing hologram element 5.

This hologram element is composed of four hologram regions having different diffraction directions, as shown in FIG. 4, for error signal detection. That is, two first and second hologram areas 36 are arranged symmetrically with respect to a dividing line 44 perpendicular to the track direction on the magneto-optical disk 8.
, 37, and two third and fourth hologram areas 38 and 39 arranged symmetrically with respect to the optical axis on the dividing line. The focus error signal is from the +1st-order diffracted light from the first and second hologram areas 36 and 37, and the tracking error signal is from the third and fourth hologram areas 38,
It is detected from the +1st order diffracted light from 39. FIG. 5 is a diagram for explaining the state of the diffracted light converged on the eight-divided photodetector 9. As shown in FIG. The +1st-order diffracted lights from the first, second, third, and fourth hologram areas converge at points 40, 41, 42, and 43. FIG. 5(b) is a diagram showing the state in which the light beam is focused on the magneto-optical disk, and FIGS. 5(a) and (c) are diagrams showing the state in which the disk surface is displaced and moves away from the converging lens 4. This is the case when you get close to it. Therefore, the outputs of the four photodetecting elements 31, 32, 33, and 34 in the 8-divided detector 9 are set to V(31).
, V(32), V(33), and V(34), the focus error signal is (V(31)+V(34))−(V
(32)+V(33)).

On the other hand, the track error signal is detected by utilizing the fact that when the focus spot on the magneto-optical disk 8 deviates from the center of the track, the light intensity distribution of the returned light becomes unbalanced. That is, when a tracking error occurs, a difference occurs in the amount of light incident on the third hologram area 38 and the fourth hologram area 39. If the outputs of the two photodetecting elements 30 and 29 on which the diffracted lights converge and enter from the respective regions 38 and 39 are V(30) and V(29), the tracking error signal is V(29) - V(30) More can be obtained.

A differential detection method is used to detect the magneto-optical signal, but this requires an analyzer to separate the returned light into two 45 degree polarized components. This hologram element has polarization properties in its diffraction efficiency for detecting magneto-optical signals. This polarizing hologram element 5 has a birefringent diffraction grating type structure shown in FIG. If the X plate or Y plate of lithium niobate 52, which is a birefringent crystal, is subjected to proton exchange with benzoic acid, the optical axis of the crystal will change, for example, for light with a wavelength of 0.83 μm, which is commonly used in optical disk devices. The refractive index for extraordinary light, which is light polarized parallel to
．． The refractive index for ordinary light, which is light polarized perpendicular to the optical axis, decreases by about 0.04. Therefore, if a grating is formed in which exchange regions 53 subjected to proton exchange and non-exchange regions 54 not subjected to proton exchange are arranged periodically, it acts as a diffraction grating. When a phase compensation film 55 of an appropriate thickness is formed on the exchange area in order to cancel the phase difference between the ordinary light passing through the exchange area and the ordinary light passing through the non-exchange area, this grating becomes It does not function as a diffraction grating and can be transmitted without diffraction. In other words, this grid looks like a simple transparent substrate. By changing the depth of the exchange region 53 while satisfying the phase difference cancellation condition for the ordinary light, the extraordinary light is completely diffracted when the phase difference with respect to the extraordinary light is π. Such birefringent diffraction gratings are being considered for application as polarizers, and are reported in Technical Digest of the Second Optoelectronics Conference, pages 167 to 1.
“BIREFRINGENT” by Kaino and others published on page 69
GRATING POLARIZER”.

From the above, the crystal optical axis of this polarizing hologram element serves as the axis of the analyzer. In the optical head device, the optical axis of the crystal is tilted at about 45 degrees with respect to the polarization plane of the return light from the magneto-optical disk 8 for use as an analyzer for differential detection of magneto-optical signals. The two 45-degree polarized components of this returned light are separated as ordinary light and extraordinary light, corresponding to the diffracted lights of the 0th and 1st order lights, so the magneto-optical signal is based on the amount of light of the 0th and 1st order lights. Can be detected from the difference. That is, the photodetection element 27,
If the outputs of 28 are V(27) and V(28), the magneto-optical signal is V(27)-(V(28)+V(29)+V(30)
)+V(31)+V(32)+V(33)+V(34)
) can be obtained from

[0008]

[Problems to be Solved by the Invention] In the above-mentioned conventional magneto-optical head device, the return light from the magneto-optical disk is directed off the optical axis using a prism-type polarizing beam splitter, making it difficult to miniaturize the device. . Furthermore, since 50% of the light from the light source to the magneto-optical disk is not used, the light utilization rate is also low.

SUMMARY OF THE INVENTION An object of the present invention is to provide a polarizing beam splitter that can constitute a magneto-optical head device that is small and lightweight and has a high light utilization efficiency, and a magneto-optical head device using the same.

0010

[Means for Solving the Problems] A polarizing beam splitter of the present invention includes first and second hologram elements which are birefringent diffraction grating type elements made of birefringent crystal and which diffract only light in a specific polarization direction; Consisting of a first Faraday rotation element that rotates the polarization plane of incident light by 45 degrees, and a second Faraday rotation element that rotates the polarization plane of incident light by a predetermined angle,
The hologram element and the Faraday rotation element are arranged such that the polarization directions of the light diffracted by the first and second hologram elements intersect by 45 degrees, and the first Faraday rotation element is between the two hologram elements. They are characterized by being arranged alternately.

Further, the magneto-optical head device of the present invention includes a light source, an imaging lens system that focuses the light emitted from the light source onto a recording medium, and a lens system that is provided between the light source and the imaging lens system. The polarizing beam splitter described above is arranged in the order of a hologram element and a Faraday rotation element from the light source side, and a photodetector that receives the light separated by the polarizing beam splitter. .

0012

[Operation] The operation and principle of the present invention are as follows. In the present invention, in order to miniaturize the magneto-optical head device and increase the efficiency of light utilization, the head is configured as a uniaxial system using a polarizing beam splitter composed of a polarizing hologram element and a Faraday rotation element.

FIGS. 7A and 7B are diagrams for explaining the principle of the polarizing beam splitter of the present invention, and show the case where it is used in a magneto-optical head. The first and second hologram elements 10 and 12 transmit ordinary light and diffract extraordinary light. Each crystal optic axis 20, 21 forms an angle of 45 degrees. Further, the first Faraday rotation element 11 has a polarization plane of 4
The second Faraday rotation element 13 rotates the plane of polarization by 22.5 degrees.

FIG. 7(a) shows the case where the emitted light 2 from the light source is directed toward the magneto-optical disk 8. This polarizing beam splitter only rotates the polarization plane of the light, and the loss due to the separation of the light is eliminated. Let it pass through. The emitted light 2 from the light source enters the first hologram element 10 as ordinary light and is transmitted without being diffracted, and then rotated by +45 degrees by the first Faraday rotation element 11. The light also enters the second hologram element 12 as ordinary light and is transmitted without being diffracted. Finally the second
It passes through the Faraday rotation element 13 and advances to the magneto-optical disk.

On the other hand, FIG. 7(b) shows a case in which light is reflected by a magneto-optical disk and returns, and this polarizing beam splitter uses this returned light 22 as diffracted light to divide it into two parts at +45 degrees and -45 degrees. It is separated into two orthogonal polarized components and is not returned to the light source. The returned light 22 is transmitted through the second Faraday rotation element 1
3, it enters the second hologram element 12 with an inclination of 45 degrees with respect to the optical axis direction 21 of the crystal. The polarized light component is transmitted as ordinary light. Next, this -45 degree polarized light component is rotated by 45 degrees by the first Faraday rotation element 11, enters the first hologram element 10 as extraordinary light, and is diffracted. On the other hand, the +45 degree component diffracted by the second hologram element 12 enters the first hologram element 10 as ordinary light, and therefore passes through without being diffracted.

That is, this polarizing beam splitter functions simply as a transparent substrate for the light traveling from the light source to the magneto-optical disk, and functions as an analyzer for differential detection of the magneto-optical signal for the returning light. Generally, by changing the rotation angle of the polarization plane of the second Faraday rotation element, it is possible to separate the light into two orthogonal polarization components at arbitrary angles.

[0017]

Embodiments Next, embodiments of the present invention will be described with reference to the drawings. Components that are the same as those in the prior art are indicated by the same symbols. FIG. 1 is a sectional view showing a first embodiment of a polarizing beam splitter of the present invention. Polarizing beam splitter 1 of the present invention
4, the grating pattern of one of the first or second hologram elements 10, 12 for detecting an error signal of the optical head is constituted by a four-divided hologram area as described in the prior art, and the other is a simple grating. . Further, the Faraday rotation element is made of a magnetic material, and the rotation angle of the polarization plane of the incident light can be set by the length of the magnetic material. For example, YIG(Y3 Fe5
O12) is a typical material for Faraday rotation elements. In this embodiment, each hologram element and the Faraday rotation element are formed in close contact with each other, but they may be spatially separated.

FIG. 2 shows a first embodiment of a magneto-optical head device of the present invention constructed using this polarizing beam splitter 14. As described in the operation section, when light is incident on the first hologram element 10 as ordinary light, the polarizing beam splitter 14 transmits the incident light only by rotating the polarization plane of the incident light. Conversely, when light with the same polarization direction as this emitted light is incident from the second Faraday rotation element 13, it is separated into two polarization components of +45 degrees and -45 degrees as diffracted light, and is not allowed to pass through. In the magneto-optical head of the present invention, the emitted light 2 from the semiconductor laser 1 enters the first hologram element 10 of the beam splitter 14 as ordinary light, and after passing through the polarizing beam splitter 14, it is directed onto the magneto-optical disk 8 by the imaging lens 18. The light is focused. The light reflected from the magneto-optical disk 8 passes through the opposite optical path and enters the polarizing beam splitter 14 again. The +45 degree polarized component of the return light from the magneto-optical disk 8 is diffracted by the second hologram element 12 and sent to the photodetector 45,
46. In addition, the remaining -45 degree polarized light component is the first polarized light component.
The light is diffracted by the hologram element 10 and enters the photodetectors 47 and 48. The magneto-optical signal recorded on the magneto-optical disk 8 is
It can be detected from the difference between the sum of the outputs of the photodetectors 45 and 46 and the sum of the outputs of the photodetectors 47 and 48 using a differential detection method. Figure 8
are photodetectors 45, 46, 47, when the first hologram element 10 is composed of four divided hologram areas using the conventional technology.
48 is a diagram showing a convergence state of diffracted light on 48. FIG. Track and focus error signals are detected from the output of the photodetector 48.

FIG. 9 shows the second embodiment of the magneto-optical head device of the present invention.
It is a figure showing the condensation state of the diffracted light on the photodetector in Example. By changing the inclination of the grating of the hologram element in the polarizing beam splitter, the focal point of the diffracted light and the position of the photodetector are rotated around the semiconductor laser 1.

FIG. 10 is a diagram showing the focusing state of the diffracted light on the photodetector in the third embodiment of the magneto-optical head device of the present invention. In the figure, the third-order light and higher are omitted. Error signal detection with this head is the same as in the first embodiment, and is detected by the photodetector 48. The magneto-optical signal is detected from the difference between the sum of outputs of a photodetector that receives diffracted light of first-order light or higher from the first hologram element and the sum of outputs of a photodetector that receives diffracted light of first-order or higher order of the second hologram element. , increasing the utilization rate of light.

FIG. 11 shows a pattern of a hologram element used in a fourth embodiment of the polarizing beam splitter of the present invention. The first hologram element 49 is a grating pattern that generates diffracted light with astigmatism for detecting focus error signals, and the second hologram element 50 is composed of two hologram areas with different grating pitches to detect tracking error signals. In order to do this, they are partitioned by dividing lines 51 in a direction parallel to the track direction 60.

FIG. 12 is a diagram showing the focusing state of the diffracted light on the photodetector in the fourth embodiment of the magneto-optical head device of the present invention constructed using this polarizing beam splitter.
This shows the state when the magneto-optical disk is defocused. The focus error signal is detected from the output of the 4-split photodetector 57 using the astigmatism method. The track error signal is detected from the output of the two-split photodetector 59 using a push-pull method. The magneto-optical signal is detected from the difference between the sum of outputs of photodetectors 56 and 57 and the sum of outputs of photodetectors 58 and 59.

[0023]

[Effects of the Invention] The polarizing beam splitter of the present invention is constructed using a plate-shaped hologram element and a Faraday rotation element, so it can be made thinner, and since this hologram element can be manufactured by a photolithography process, mass production is possible. is possible.

Furthermore, since the magneto-optical head device of the present invention is configured using a polarizing beam splitter using the above-mentioned polarizing hologram element, a uniaxial configuration can be adopted, and the device can be made smaller and lighter. This is possible, and furthermore, it is possible to improve the efficiency of light utilization.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a first embodiment of a polarizing beam splitter of the present invention.

FIG. 2 is a diagram showing a first embodiment of the magneto-optical head device of the present invention.

FIG. 3 is a basic configuration diagram of a conventional magneto-optical head device.

FIG. 4 is a diagram showing a lattice pattern of a hologram element used in a conventional magneto-optical head device.

FIG. 5 is a diagram for explaining the state of light incident on an eight-divided photodetector in a conventional magneto-optical head device.

FIG. 6 is a diagram for explaining the structure of a polarizing hologram element.

FIG. 7 is a diagram for explaining the principle of the polarizing beam splitter of the present invention.

FIG. 8 is a diagram for explaining the state of diffracted light focused on the photodetector of the first embodiment of the magneto-optical head device of the present invention.

FIG. 9 is a diagram for explaining the state of diffracted light focused on a photodetector of a second embodiment of the magneto-optical head device of the present invention.

FIG. 10 is a diagram for explaining the diffraction state of light condensed on a photodetector in a third embodiment of the magneto-optical head device of the present invention.

FIG. 11 is a diagram showing a grating pattern of a hologram element in a polarizing beam splitter used in a fourth embodiment of the magneto-optical head device of the present invention.

FIG. 12 is a diagram for explaining the state of diffracted light focused on a photodetector of a fourth embodiment of the magneto-optical head device of the present invention.

[Explanation of symbols]

1 Semiconductor laser 2 Emitted light 3 Collimating lens 4 Converging lenses 6, 14 Polarizing beam splitter 7 Lens 8 Magneto-optical disk 9 8-divided photodetector 5, 10, 12, 49, 50 Hologram element 11,
13 Faraday rotation element 18 Imaging lenses 20, 21 Crystal optical axis 22 Return light from the magneto-optical disk 27, 28, 29, 30, 31, 32, 33, 34
Photodetection elements 36, 37, 38, 39 Hologram areas 40, 41
, 42, 43 Convergence points 44, 51 Parting lines 45, 46, 47, 48, 56, 58, 61, 62, 6
3, 64 Photodetector 52 Lithium niobate 53 Exchange area 54 Non-exchange area 55 Phase compensation film 57 4-split photodetector 59 2-split photodetector 60 Track direction

Claims (2)

[Claims] [Claim 1] Diffracting only light in a specific polarization direction;
First and second hologram elements which are birefringent diffraction grating type elements made of birefringent crystal, a first Faraday rotator that rotates the polarization plane of incident light by 45 degrees, and a first Faraday rotation element that rotates the polarization plane of incident light by a predetermined angle. The polarization direction of the light diffracted by the first and second hologram elements intersects by 45 degrees, and the first Faraday rotation element is configured to rotate the second Faraday rotation element. A polarizing beam splitter, characterized in that the hologram element and the Faraday rotation element are alternately arranged in between. 2. A light source, an imaging lens system that focuses the light emitted from the light source onto a recording medium, and a hologram element, a Faraday element, and a hologram element, which are provided between the light source and the imaging lens system from the light source side. Claim 1 in which the rotating elements are arranged in this order.
A magneto-optical head device comprising the polarizing beam splitter described above and a photodetector that receives light separated by the polarizing beam splitter.