JP2682052B2 - Optical head - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical head for reproducing information recorded on a recording surface of an optical disc.

2. Description of the Related Art For example, an optical head used when reproducing information from an optical disc in which information is recorded by forming a large number of pits along a track is usually
Optical elements such as a laser light source, a condenser lens, a polarization beam splitter that separates incident light and reflected light, and a λ / 4 wavelength plate are fixed in their mutual positional relationship with their optical axes aligned. Therefore, it has a drawback that it is large and the optical axis adjustment at the time of assembly is complicated. Further, when the optical head becomes large, the inertial weight becomes large, the tracking operation becomes slow, and the access time to the optical disk becomes long.

For example, in the optical head shown in FIG. 7 of JP-A-63-191327, a λ / 4 wave plate for making light returning from a recording medium into a TE (TM) mode is fixed on a substrate. According to this, not only the optical head becomes large due to the attachment of the λ / 4 wavelength plate, but also the optical axis adjustment is complicated because it is necessary to position the crystal direction with high accuracy.

The present invention has been made in view of the above circumstances, and its purpose is to eliminate the need for optical axis adjustment,
Moreover, it is to provide an optical head which is simple and small in size.

Means for Solving the Problem In order to achieve such an object, the gist of the present invention is to irradiate a recording surface of an optical disk with irradiation light and detect reflected light from the recording surface to thereby provide the recording surface with the irradiation light. An optical head for reproducing recorded information, comprising: (a) having a first port at one end and covered with a metal film.
In addition to the first branch waveguide provided with a mode selection unit for attenuating the propagation light of TM mode, the second branch waveguide having the second port at one end, the first branch waveguide and the second branch waveguide, One end is connected to the other end, the other end has a common port, and a magnetic field of a predetermined angle is formed, so that the propagation mode of light is TE only in the direction of propagation from the common port to the first port or second port side. Equipped with a Y-shaped three-dimensional optical waveguide consisting of a common waveguide provided with a unidirectional mode converter that converts from mode to TM mode, the light supplied to the first port is exclusively propagated to the common port, A solid-state optical circulator for exclusively propagating the light supplied to the port to the second port; (b) a light source element for making the irradiation light incident on the first port; and (c) a reflected light output from the second port. With an optical sensor to detect (D) It is connected to the common port, guides the irradiation light made incident on the first port from the light source element, radiates it toward the recording surface of the optical disc, and receives reflected light from the recording surface. And an optical fiber leading to the common port.

With this configuration, the light source element and the optical sensor are connected to the first port and the second port of the solid-state optical circulator, and the optical head is connected to the common port of the solid-state optical circulator by connecting the optical fiber. A first branching waveguide having a mode selecting unit configured to attenuate the propagating light of TM mode by having a first port at one end and being covered with a metal film. A second branch waveguide having a port at one end, one end connected to the other ends of the first branch waveguide and the second branch waveguide, a common port at the other end, and a magnetic field of a predetermined angle formed. As a result, the unidirectional mode change that converts the optical propagation mode from TE mode to TM mode only in the direction in which the common port propagates from the first port or the second port to the side. Since it is configured by including a Y-shaped three-dimensional optical waveguide including a common waveguide provided with a conversion unit, a laser light source, a condenser lens, a polarization beam splitter for separating incident light and reflected light, and a λ / 4 wavelength Compared to a conventional optical head in which optical elements such as plates are fixed in their mutual positional relationship while their optical axes are aligned with each other, the number of parts is reduced and the structure is simpler. Therefore, it is not necessary to adjust the optical axes of the optical elements. Further, since the structure is simplified and the size and weight are reduced, the access speed to the optical disc is increased.

Embodiment Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

In FIG. 1, the solid-state optical circulator 10 has a function of exclusively propagating the irradiation light incident on the first port 12 to the common port 14 and causing the reflected light incident on the common port 14 to exclusively enter the second port 16. There is. The solid-state optical circulator 10 is made of a transparent material such as GGG (gadolinium gallium garnet, Gd ₃ Ga ₅ O ₁₂ ), SGG (samarium gallium garnet), and NGG (neodymium gallium garnet). Substrate 18, YIG (yttrium, iron, garnet, Y ₃ Fe ₅ O ₁₂ ), GdIG (gadolinium,
Iron / Garnet, Gd ₃ Fe ₅ O ₁₂ ), Bi _X Y _3-X Fe ₅ O ₁₂ , Bi _X Gd _3-X with some of them replaced by bismuth
Magnetic thin films 19 and 20 having a magneto-optical effect such as Fe ₅ O ₁₂ (Bi: YIG, Bi: GdIG, etc.) are formed on a substrate by well-known thin film forming means such as liquid phase epitaxy (LPE), vapor deposition, and sputtering.
Sticked on 18. The magnetic thin film 19 is the magnetic thin film 20.
And a substrate 18 such as GGG, and has a refractive index slightly smaller than that of the magnetic thin film 20. And a magnetic thin film
The 20 is provided with a three-dimensional optical waveguide 21 formed by removing an unnecessary portion by applying an etching technique such as chemical etching using phosphoric acid or the like. This 3
As shown in detail in FIG.
Of the common waveguide 26 connected to the common port 14 and the common waveguide 26.
The first branch waveguide 28 is branched from the common waveguide 26 and is connected to the first port 12, and the second branch waveguide 30 is branched from the common waveguide 26 and connected to the second port 16. As a result, the light traveling from the first port 12 to the common port 14 is propagated exclusively to the common port 14 without being demultiplexed, but the light traveling from the common port 14 to the first port 12 is split at the branch point. It is propagated toward the second port 16 as well as the first port 12.

A metal film 32 made of a material such as aluminum, nickel or gold is fixed on the first branch waveguide 28 by a well-known thin film forming means. The portion of the three-dimensional optical waveguide 21 covered with the metal film 32 constitutes a mode selection unit 34 that allows only the TE mode propagating light to pass by exclusively attenuating the TM mode propagating light.

The common waveguide 26 is provided with the unidirectional mode converter 36 shown in FIG. That is, a magnetic field is applied by the magnetic field forming device 38 so that the magnetization M of the magnetic film is inclined by θ from the film surface in a plane perpendicular to the light propagation direction. The magnetic field forming device 38 is composed of a permanent magnet 40 and a pair of yokes 42 that are connected to both ends of the permanent magnet 40 and form magnetic poles S and N at their tips. The direction of the magnetic field from the magnetic poles N to S is the direction M.
It is arranged so that Since the yoke 42 forming the magnetic pole N is located below the substrate 10,
Not shown in the figure.

Returning to FIG. 1, a semiconductor laser element 44 functioning as a light source element is provided at the first port 12 of the solid-state optical circulator 10 configured as described above, and an optical sensor 46 such as a photodiode is provided at the second port 16. Each one is provided. Further, the common port 14 of the solid-state optical circulator 10 guides the irradiation laser light to the optical disc 48 and
A relatively short polarization-maintaining optical fiber 50 for guiding the reflected light from 48 to the common port 14 is connected. The polarization-maintaining optical fiber 50 has a function of holding the polarization plane of incident light as it is and outputting it. Further, the polarization-maintaining optical fiber 50 has a rounded tip,
A condenser lens 52 is formed. Here, the polarization-maintaining optical fiber 50 is fixed to the substrate 18 with an appropriate fixing tool after the end face 24 of the substrate 18 is polished, or adhered with an ultraviolet curable resin or an epoxy resin. At this time,
A hole for fixing may be formed in the common port 14 of the substrate 18, and the end of the polarization-maintaining optical fiber 50 may be fitted into the hole.

The solid-state optical circulator 10 and the polarization-maintaining optical fiber 50 are fixed to a machine frame (not shown), and the machine frame is held by a tracking and focusing drive mechanism (not shown) to perform tracking control and focusing control. The tip of the polarization-maintaining optical fiber 50 is positioned at a height where a suitable focused spot can be obtained on a desired track.

 Hereinafter, the operation and effect of the present embodiment will be described.

The laser light emitted from the semiconductor laser element 44 is
After being incident on the port 12, it is guided by the first branch waveguide 28. The laser light incident on the first port 12 becomes only the TE mode in the mode selection unit 34. That is, assuming that the light propagation direction is the z-axis, the film thickness direction of the magnetic thin film forming the three-dimensional optical waveguide 21 is the x-axis, and the direction orthogonal to them is the y-axis, the field components (H _x , E _y , H _z ) And field components (E _x , H _y ,
The TM mode whose polarization plane is perpendicular to the TE mode with E _z ) propagates in the first branching waveguide 28.
At, since the attenuation constant for the TM mode is extremely large, it is exclusively attenuated, so only the TE mode is allowed to pass. The length dimension of the metal film 32 is determined so that the TM mode is sufficiently attenuated.

Here, the dielectric constant tensor of the magnetic film Is Indicated by

At this time, ε _1xy = 2f ₄₄ M ² sinθ _cosθ (2) ε _1xy = jf ₁ ^e Msinθ (3) ε _1zx = jf ₁ ^e Mcosθ (4) However, f ₂ ^e : First-order magneto-optical factor f ₄₄ : Second-order magneto-optical factor.

That, E of the TE mode by _ε 1xy _y and TM modes of E _x
Mode conversion by coupling with and E of TE mode by ε _1yz
Mode conversion by combining _y and TM mode E _z occurs simultaneously. If general, the mode conversion by epsilon _1yz very small compared with the case of epsilon _1Xy, wherein, close to the refractive index of the magnetic thin film 20 the refractive index of the magnetic thin film 19 as an intermediate layer is a waveguide layer, Since the phase matching point is near the cutoff,
E _z component becomes large. Further, by appropriately selecting θ, the magnitude of mode conversion by ε _1xy and ε _1yz can be made equal. The mode conversion by ε _1xy and ε _1yz cancels each other in the forward direction and adds in the opposite direction, so that mode conversion does not occur in the forward direction and the TE mode is emitted while the TE mode is emitted in the reverse direction. Is completely converted to TM mode, and unidirectional mode conversion occurs. That is, the TE-mode laser light that has passed through the mode selection unit 34 is not subjected to mode conversion when passing through the unidirectional mode conversion unit 36 provided in the common waveguide 26. And
The TE mode wave is output from the common port 14. On the contrary, all the reflected light (TE wave) from the common port 14 is converted to the TM mode. That is, the light (polarization of the TE mode wave) output from the common port 14 to the outside is guided by the polarization-maintaining optical fiber 50, and the condenser lens 52 formed at the tip thereof.
The light is condensed by the laser beam and is condensed on a predetermined track of the optical disc 48. The reflected light reflected by the optical disk 48 and received by the condenser lens 52 is the polarization-maintaining optical fiber 50.
It is incident on the common port 14 by being guided by. This reflected light is held in the TE mode wave by the polarization maintaining function of the polarization maintaining optical fiber 50, and is completely converted from the TE mode wave to the TM mode wave in the unidirectional mode converter 36.

In this way, the reflected light converted into the TM mode wave in the unidirectional mode conversion unit 36 is demultiplexed at the branch point of the three-dimensional optical waveguide 21 to the first branch waveguide 28 and the second branch waveguide 30. Although propagated, the reflected light propagated to the first branch waveguide 28 is attenuated in the mode selection unit 34. Therefore, since the reflected light does not reach the semiconductor laser element 44, the oscillation operation of the semiconductor laser element 44 becomes stable. Then, the reflected light is exclusively guided to the second port 16 through the second branch waveguide 30 and detected by the optical sensor 46 there. Since information is recorded in the form of pits on the tracks, the reflected light detected by the optical sensor 46 is disturbed corresponding to the existence of the pits, and its intensity changes. Therefore, the information recorded on the optical disk 48 is reproduced based on the change in the intensity of the reflected light.

As described above, according to the optical head of this embodiment, the first port 12 and the second port 16 of the solid-state optical circulator 10 are provided.
Since the optical head is configured by simply connecting the semiconductor laser element 44 and the optical sensor 46 to the common port 14 of the solid-state optical circulator 10 and connecting the polarization-maintaining optical fiber 50 to the optical port, Compared to a conventional optical head of a type in which optical elements such as a polarization beam splitter for separating light and reflected light, λ / 4 wavelength plates and the like have their optical axes aligned with each other and fixing the mutual positional relationship,
The number of parts is reduced, the structure is simplified, and mutual optical axis adjustment between optical elements is not required.

Further, according to the present embodiment, the optical head is small and lightweight, so that there is an advantage that the access speed to the optical disk 48 is increased.

Next, another embodiment of the present invention will be described. In the following embodiments, the same reference numerals are given to portions common to the above description, and the description will be omitted.

In the above-described embodiment, the semiconductor laser device 44 and the optical sensor 46 are directly attached to the substrate 18, but as shown in FIG. 3, the condenser lenses 56 and 58 are provided in the semiconductor laser device 44.
Between the first port 12 and the optical sensor 46 and the second port
It may be provided between 16 and.

In addition, if necessary, other functional elements may be added to the three-dimensional optical waveguide 21.
Can be provided. FIG. 4 shows an example thereof, and the modulator 60 is provided in the first branch waveguide 28.

Further, as shown in FIG. 5, a polarization-maintaining optical fiber 62 that fixes the optical circulator 10 and is relatively long and bent into an L-shape is used.
By fixing the tip portion of 62 to the tracking and focusing drive mechanism, only the tip portion may be positioned in association with the tracking control and the focusing control.

In addition, as shown in FIG.
Instead of the condensing lens 52 formed at the tip of, the condensing lens 64 may be provided independently.

Further, three sets of optical heads configured as shown in FIG. 5 may be installed side by side as shown in FIG. At this time, if the polarization-maintaining optical fibers 62 are fixed to each other so that the positions of the spots 66, 68, 70 of the irradiation light from the respective optical heads have the relationship shown in FIG. A tracking signal representing the deviation from the track 72 can be obtained by the reflected light from.

Also, a plurality of optical circulators 10 may be provided on a common substrate. FIG. 9 shows a board 74 in which three sets of optical heads are common.
The example shown in FIG.

As mentioned above, although one Example of this invention was described based on drawing, this invention is applied also in another aspect.

For example, a dielectric three-dimensional optical waveguide may be formed on a substrate made of a material having a magneto-optical effect.

In addition, the unidirectional mode converter 36 provided in the common waveguide 26 is a substrate in a plane that is orthogonal to the nonreciprocal portion to which a magnetic field parallel to the traveling direction (forward direction) of light is applied and the traveling direction.
The reciprocal portion may be provided with a magnetic field in a direction inclined by a predetermined angle from a line perpendicular to the surface of 18.

Further, the three-dimensional optical waveguide 21 does not have to be symmetrical as shown in FIGS. 1 and 2, and may be asymmetric. In particular, since the light propagating from the common port 14 to the second port 16 is demultiplexed at the branch point, it is desirable that the light is largely demultiplexed to the second branch waveguide 30 side. Further, it is possible to use a three-dimensional optical waveguide branched into three instead of the above three-dimensional optical waveguide 21 branched into two.

Also, instead of the magnetic field forming device 38 of FIG. 2, a type in which an electromagnet is provided in the yoke, or a type in which an electric wire that forms a magnetic field by passing an electric current is arranged near the optical waveguide is used. May be.

Further, a protective layer made of a dielectric material such as SiO ₂ may be further laminated on the three-dimensional optical waveguide 21.

An anisotropic crystal layer may be provided on the upper layer of the three-dimensional optical waveguide 21.

Further, a buffer layer for improving the light propagation efficiency may be provided between the metal film 32 of the mode selection unit 34 and the magnetic thin film 20, and the metal film 32 or the magnetic thin film 20 of the mode selection unit 34 may be provided. Alternatively, a birefringent crystal may be used. This birefringent crystal shows the refractive index of the magnetic thin film that constitutes the optical waveguide.
_Assuming that n _f , the refractive index satisfies the condition of n ₀ > n _f > n _e . Refractive index of such birefringent crystal for extraordinary light
If n _e coincides with the polarization direction of the TE mode wave, the TM mode wave leaks into the birefringent crystal, so that only the TE mode wave is guided.

In addition, a mode selection unit that attenuates the TM mode wave exclusively like the mode selection unit 34 may be provided between the unidirectional mode conversion unit 36 of the common waveguide 26 and the common port 14.
With this configuration, even if the TM mode wave is guided to the common port 14 from the outside, the TM mode wave is appropriately attenuated.

Further, in the solid-state optical circulator 10 of the above-described embodiment, the mode selection unit 34 and the unidirectional mode conversion unit are provided on the three-dimensional optical waveguide 21 formed on the single substrate 18.
Although the 36 is integrally provided, the mode selection unit 34 and the unidirectional mode conversion unit 36 are respectively provided on the two substrates, and the optical waveguides of those substrates are coupled by a polarization-maintaining optical fiber. You may do it.

Further, in the above-described embodiment, a Faraday rotation glass layer may be used instead of the magnetic thin films 19 and 20 fixed on the substrate 18. In this case, substrates made of different materials can be used.

Further, although the polarization-maintaining optical fiber is used as the optical fiber, if the optical fiber is short, it is not necessary to use the polarization-maintaining optical fiber, and a normal optical fiber may be used.

The above is merely an embodiment of the present invention, and the present invention can be variously modified without departing from the spirit thereof.

[Brief description of the drawings]

FIG. 1 is a front view showing the configuration of an embodiment of the present invention. FIG. 2 is a perspective view showing the solid-state optical circulator of FIG. 1 in detail. FIG. 3 is a diagram showing a main part of another embodiment of the present invention. FIG. 4 is a view corresponding to FIG. 1 showing another embodiment of the present invention. FIG. 5 is a diagram for explaining a usage state of another embodiment of the present invention. FIG. 6 is a diagram showing a main part of another embodiment of the present invention. FIG. 7 is a view showing a usage state of another embodiment of the present invention, and FIG. 8 is a view showing a relative positional relationship of spots formed in the embodiment of FIG. FIG. 9 is a front view showing another embodiment of the present invention. 10: Solid-state optical circulator 12: 1st port 14: Common port 16: 2nd port 44: Semiconductor laser device (light source device) 46: Optical sensor 48: Optical disk 50, 62: Polarization-maintaining optical fiber (optical fiber)

Claims (1)

(57) [Claims] 1. An optical head for reproducing information recorded on a recording surface of an optical disc by irradiating the recording surface with irradiation light and detecting reflected light from the recording surface, the first port having one end And a second branch waveguide having a second port at one end, the first branch waveguide being provided with a mode selection unit for attenuating TM mode propagation light by being covered with a metal film. One end is connected to the other ends of the branching waveguide and the second branching waveguide and has a common port at the other end, and a magnetic field of a predetermined angle is formed, so that the common port causes the first port or the second port. The Y-shaped three-dimensional optical waveguide including a common waveguide provided with a unidirectional mode conversion unit for converting the propagation mode of light from the TE mode to the TM mode only in the direction of propagation to the side, and the first port The light supplied to the common port A solid-state optical circulator that exclusively propagates the light supplied to the common port to the second port, a light source element that causes the irradiation light to enter the first port, and an output from the second port. An optical sensor that detects reflected light; and a first light source element that is connected to the common port.
An optical fiber that guides the irradiation light incident on the port and radiates it toward the recording surface of the optical disc, and receives the reflected light from the recording surface and guides it to the common port. head.