JP2923371B2 - Optical head - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in an optical head.

[0002]

2. Description of the Related Art FIG. 16 shows a conventional waveguide type optical pickup (prior art 1: "optical integrated disk pickup head", Optronics (1989) No. 2, p150). In the figure, 61 is a Si substrate, 62 is a buffer layer formed between the Si substrate 61 and the waveguide 63, 64 is an optical disk, 65 is a focusing grating coupler (concentration diffraction grating; FGC),
66 is a grating beam splitter, 67 is a photo diode, and 68 is a laser diode. In the optical pickup having such a configuration, laser light emitted from the laser diode 68 propagates through the waveguide 63, passes through the grating beam splitter 66, is diffracted by the FGC 65, and is collected on the external optical disk 64. Be lighted. Next, the laser beam reflected by the optical disk 64 is incident on the FGC 65, reaches the grating beam splitter 66, and changes its optical path to reach the photodiode 67.

FIG. 17 shows a schematic view of an optical head device (prior art 2; Japanese Patent Application No. 1-508307). 1 in the figure is Si
It is a substrate. A transparent layer 2 having a low refractive index is formed on the substrate 1, and a transparent layer 3 (3a, 3b) having an anti-refractive index is formed thereon. On the surfaces of the transparent layers 3a and 3b, concentric or spiral grating couplers 4a and 4b are formed with respect to the central axis L. A transparent layer 3c having a high refractive index is formed on the surface of the transparent layer 3a with a transparent layer 5a of a low refractive index tank interposed therebetween, and the transparent layer 3c is formed on the inner peripheral side of the transparent layer 3b and its grating coupler 4b. Touch in the area where it was done. On the surface of the transparent layer 3c, a concentric or spiral grating coupler 4c is formed on a circular region concentric with the central axis L. A transparent tank 5b having a low refractive index is formed on the surface of the transparent layer 3b so as to cover the area of the grating coupler 4b, and the refractive index of the transparent layer 5b is equal to that of the transparent layer 5a. The transparent layer 3a,
3b, the photodetectors 6a, 6b
Are formed, and a reflective film 7 is formed in the transparent layer 5 so as to cover the photodetectors. In the figure, 8 is a semiconductor laser, 9 is a condenser lens, 10a and 10b are polarizers, 11, 13, and
14, 17a and 17b are light, 12 is guided light, 16 is a reflection surface, 18a and 18
b indicates guided light.

[0004]

However, the prior art 1 has the following problems. (1) Since the laser diode (LD) 68 and the waveguide layer 63 are directly end-face-coupled, the light use efficiency is extremely low. That is, most of the light escapes by scattering without being guided.

(2) Since the intensity distribution of light emitted from the opening of the FGC 65 is strongest in the front (on the side closer to the LD 68) and then decreases exponentially, the intensity of the condensed spot The intensity distribution does not become circular or side lobes appear on one side.

(3) The waveguide layer itself is thin, but when light is emitted from the FGC 65, it is inclined by about 15 degrees because emitting vertically from the waveguide layer substrate causes a large decrease in efficiency.
For this reason, the advantage of thinness is not actually used.

Therefore, the above prior art 2 has the above problem.
The object of the present invention is to improve (1) to (3) and to improve poor light-collecting properties due to the different polarization directions. However, according to the prior art 2, it is difficult to create a liquid crystal or a wave plate for aligning the polarization direction in the circumferential direction in one direction with the current technology. Moreover, even if it can be realized, it becomes a considerably expensive component and the price of the optical head itself becomes high. Thus, at present, there is no inexpensive polarizing element that aligns polarized light along the circumferential tangential direction or radial direction in one direction. Further, according to the prior art 2, it is difficult to produce a transparent substrate having such a multilayered waveguide structure and having a PD formed in the waveguide, and the production process is complicated and diversified, so mass production cannot be expected. . The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an optical head that is small and can be easily manufactured.

[0008]

SUMMARY OF THE INVENTION The present invention is directed to a laser light source, a transparent substrate having a waveguide formed on one principal surface, and a laser light from the laser light source formed on the transparent substrate. the guides, a first grating of concentric or spiral, the first grade of the transparent substrate
Formed on the outer periphery of the coating and guided to the waveguide
Directing the laser light in the direction of the central axis of the first grating;
Only by radiating from two directions at least orthogonal, and the second grating of concentric <br/> or spiral, the transparent
Between the radiation side of the laser light on the substrate and the information recording medium.
Provided between the second grating and the recording surface.
Make sure that the direction of deflection of the emitted laser light is uniform.
The laser beam is emitted from one of two orthogonal laser beams.
Of rotating the deflection direction by 90 degrees with respect to the laser beam
Wave plate, the laser light on the recording surface of the information recording medium
After being reflected, the second grating, the waveguide, the first
And an optical detecting means for receiving return light having passed through the grating .

Further, the laser beam passing through the half-wave plate
This half wave is used so that the polarization direction of the light is rotated by 90 degrees.
Optical axis of long plate and tangent to second grating
The optical head is characterized in that the angle formed by the optical head is 45 ° . Also, from the second grating
Radiated in the direction of the central axis of the first grating
Laser light is coupled across the central axis of the first grating.
Two facing regions consist of two pairs of regions that are orthogonal to each other
An optical head is characterized in that light is divided and emitted from four regions . 1 and 2 show the principle of an optical head according to the present invention. FIG. 1 is a front view, and FIG. 2 is a perspective view.

As shown in FIGS. 1 and 2, light emitted from a laser diode (LD) 20 is converted into circularly polarized light through a quarter wavelength plate 21. A first grating 22 having a circular tip.
As a result, the light is guided into the waveguide 24 on the transparent substrate 23, and is radiated and focused by the outer circular second grating. In the present invention, the second condensing grating is divided into a plurality of condensing gratings (FGC) 25a, 25b, 25c,
25d. The 1 / 2λ plate 26 is set so that the optical axis is aligned with the central polarization direction of the polarized light radiated from some of the zones (for example, FGCs 25a and 25c).

FIGS. 3A and 3B are views of the FGC as viewed from directly above, and the polarization direction of light emitted by each FGC is the direction of the FGC, that is, the tangential direction of the circumference. FIG.
The smaller the angle θ in (A), the smaller the variation in the polarization direction in that zone. However, if the distance is too small, the space between the zones becomes large, and the light use efficiency decreases. Therefore, the angle θ is determined by the balance between the two.
Is determined.

FIG. 3B shows the state of polarization of two of these zones after passing through a 1 / 2.lambda. Plate 26. FIG. The optical axis of each wave plate is FG in the middle of the zone.
The angle of the tangent to C is 45 degrees, and the polarized light emitted from the FGC rotates 90 degrees when passing through the wave plate, and becomes substantially uniform in all the zones as shown in the figure.

FIGS. 4 and 6 are characteristic diagrams showing the light intensity distribution at the focal position of the light condensed from the respective donut-shaped openings. FIG. 4 shows the distribution corresponding to the linearly polarized light in FIG.
Indicates a distribution corresponding to the polarized light in FIG. The intensity distribution changes greatly depending on the state of polarization at that time. Linearly polarized light (Fig. 5
(See FIG. 7) and circularly polarized light having the same phase (see FIG. 8), the light intensity is 1 / e ^2. Becomes 3 μm or more, and cannot be used for high-density optical recording. In FIGS. 5, 7 and 8, hatched portions indicate white portions where light is cast. Where F
It is preferable to divide the GC as much as possible in consideration of light use efficiency. However, since the structure is complicated, the number of divisions is limited.

In the present invention, considering mass productivity and cost,
There is no step other than the grating and the two-layer structure of the transparent substrate and the waveguide or the three-layer structure including the clad layer. Also, for the same reason, the photodetector (PD) is not formed on the waveguide. The reflected light from the disk returns and is radiated from the central circular grating as shown in an embodiment described later. Diffracting the light by a hologram element to obtain a servo focus, a tracking error signal and an RF signal is one method for realizing the present invention. The hologram element converts the first-order diffracted light into two
By giving each one astigmatism,
Obtain a casellar signal. The tracking error signal uses a push-pull method.

[0015]

In the present invention, taking FIG. 9 as an example, the light emitted from the LD passes through the hologram element (light separating means) and becomes 1/4.
It becomes circularly polarized light by the wave plate. The light is incident on the FGC formed on the transparent substrate, becomes a guided light, and propagates so as to spread outward from the center of the substrate. The guided light reaches four FGCs separated by 90 degrees and is emitted upward therefrom. A state in which the deflection direction is rotated by 90 degrees and the deflection surfaces are aligned by, for example, setting the half-wave plate with the optical axis aligned at 45 degrees with respect to the deflection direction on the facing FGC among the four FGCs, for example. And focus. This light is reflected on the recording surface of the information recording disk, returns to the opposite optical path, reaches the hologram element, where it is diffracted, and the zero-order light and 1
Divided into the next light. At this time, the difference between the right and left primary lights is pu
It becomes a sh-pull signal. The right and left primary lights are received by the PD divided into three, respectively. However, since the astigmatism of the primary light is given in opposite phases at the right and left, the received light amount as shown in FIG. By taking the difference, a focus error signal is obtained. In addition, the sum of all becomes an RF signal.

[0016]

【Example】

 (Example 1)

FIG. 9 is an explanatory diagram of the optical head according to the first embodiment of the present invention. In the figure, reference numeral 31 denotes a transparent substrate made of, for example, quartz and having a waveguide 32 formed thereon. The waveguide layer 32
For example, 7059 glass manufactured by Corning Incorporated was formed to a thickness of about 0.9 μm by RF sputtering. On the waveguide 32, there are formed four converging grating couplers (FGCs) 25a, 25b, 25c, and 25d, as shown in FIG. Here, the FGC is formed by forming a silicon nitride film to a thickness of 25 nm by the same RF sputtering method, applying a resist thereon, drawing a grating pattern by electron beam, and etching. Is formed. A 波長 wavelength plate 33 is arranged below the transparent substrate 31. Among the FGCs, for example, FG
Half-wave plates 34 and 35 are provided directly above the C25a and 25c so that the optical axes are aligned with the central polarization direction of the polarized light emitted from the FGCs 25a and 25c. A hologram element (light separating means) 36 for separating light is disposed immediately below the 波長 wavelength plate 33. Further, near the hologram element, photodetectors (photodiodes; PDs) 37 and 38 for receiving light from the hologram element 36 are arranged. In the drawing, reference numeral 39 denotes an information recording disk, and reference numeral 40 denotes a laser diode (LD).

The operation of the optical head having such a configuration is as described below. The light emitted from the LD 39 passes through the hologram element 36 and is converted into circularly polarized light by the 波長 wavelength plate 33. The light is incident on the FGC formed on the transparent substrate 31, becomes a guided light, and propagates so as to spread outward from the center of the substrate. The guided light is divided into four FGCs 25a and 25 separated by 90 degrees.
b, 25c, 25d, from which it is emitted upwards.
Of the four FGCs, for example, facing FGCs 25a and 25c
Align the optical axis at 45 degrees to the deflection direction
By installing the wave plates 34 and 35, the light is condensed by rotating the deflection direction by 90 degrees and aligning the deflection surfaces. This light is reflected on the recording surface of the information recording disk 39, returns to the opposite optical path, reaches the hologram element 36, where it is diffracted and split into zero-order light and primary light. At this time, the difference between the right and left primary lights becomes a push-pull signal. The right and left primary lights are received by the PD 37 divided into three parts, respectively. However, since the astigmatism of the primary light is given in opposite phases to the right and left, the received light amount as shown in FIG. By taking the difference, a focus error signal is obtained. In addition, the sum of all becomes an RF signal.

FIG. 11 is a diagram showing how a focus error signal is generated. For tracking, general push
As with the -pull signal, it is obtained by diffraction through the guide groove of the disk. 12 (A) to 12 (F) respectively show P in FIG.
The beam shape corresponding to D37, D37 is shown, (A),
(B) shows the case where the focal point is far, (C) and (D) show the case where the focal point matches, and (E) and (F) show the case where the focal point is close.

According to the optical head according to the above embodiment, the transparent substrate 31 having the waveguide 32 formed on the upper surface, the hologram element 36 for separating the laser light from the LD 40, and the transparent substrate 31
First circular grating 22 formed on 31 and guiding laser light passing through hologram element 36 to waveguide 32
A plurality of divided circular FGCs 25a to 25d formed on the transparent substrate 31 and guiding the laser light from the LD 40 through the hologram element 36 to the waveguide layer 32; A half-wave plate in which an optical axis is set at an appropriate angle with respect to a part of the emitted light so that the polarized light of the divided emitted light is uniformly aligned in polarization direction.
35, after reflecting on the recording surface of the information recording disc 39 ,
FGC, waveguide 32, first grating 22, waveguide
And PDs 37 and 38 for receiving the return light passing through 32. Therefore, a smaller optical head can be realized by shortening the optical path length from the LD 40 to the surface of the disk 39. Further, an optical head can be easily manufactured as compared with the related art. FIGS. 13 (A) and 13 (B) are diagrams showing the effects of the above embodiment. Even when the size of the optical head is reduced by using the hologram element, as shown in FIGS.
It is clear that a smaller optical head can be realized by shortening the optical path length from the LD 40 to the surface of the disk 39.

FIG. 14 is a plan view of a main part of the optical head according to the second embodiment. The second grating (FG) of the first embodiment
C) was divided into four, but the light between the gratings was discarded without any use. Therefore, in the second embodiment, even when the image is divided into the same four, FIG.
As shown in the figure, a grating lens 51 having a light condensing function is disposed in the waveguide, and the light guided from the center in all directions is divided into four without being emitted, and emitted from the FGCs 25a to 25d, respectively. It has the advantage of being able to. (Example 3)

FIG. 15 is an explanatory diagram of a main part of the optical head according to the third embodiment. In the third embodiment, the radiation FGC is not divided, but is formed into a circular FGC 52, and the waveguide is divided with a mask 53 having an opening 53a at a predetermined position. As described above, even if the FGC 52 for radiation is not divided and the extra portion is shielded after radiation, the first embodiment can be used.
The same effect can be obtained.

In the first and second embodiments, the FGC is divided into four parts. However, as described above, the number is not limited. For example, the number of divisions depends on whether priority is given to mass productivity or light use efficiency. Is selected.

[0024]

As described in detail above , according to the present invention,
Improved light collection, compact size, and easy fabrication
An optical head can be provided .

[Brief description of the drawings]

FIG. 1 is a schematic front view for explaining the principle of an optical head according to an embodiment of the present invention.

FIG. 2 is a perspective view of FIG. 2;

FIG. 3 is a principle diagram of aligning condensed light using a half-wave plate.

FIG. 4 is a light intensity distribution diagram at a focal position of light condensed from a donut-shaped opening.

FIG. 5 is an explanatory diagram of linearly polarized light.

FIG. 6 is a light intensity distribution diagram at a focal position of light condensed from a donut-shaped opening.

FIG. 7 is an explanatory diagram of circularly polarized light.

FIG. 8 is an explanatory diagram of a polarization tangential direction.

FIG. 9 is an explanatory diagram of the optical head according to the first embodiment of the invention.

FIG. 10 is a view for explaining the principle in the case where a focus error signal is obtained based on a difference in the amount of light received from the photogram element.

FIG. 11 is an explanatory diagram showing how a focus error signal is generated.

FIG. 12 is a pattern diagram of a photodiode when the focal point coincides with the disk surface, when the focal point is far, and when the focal point is close.

FIG. 13 is an explanatory diagram when the optical head is reduced in size according to the present invention.

FIG. 14 is an explanatory diagram of a main part of an optical head according to a second embodiment of the invention.

FIG. 15 is an explanatory diagram of a main part of an optical head according to a third embodiment of the invention.

FIG. 16 is an explanatory diagram of Conventional Technique 1.

FIG. 17 is an explanatory diagram of Conventional Technique 2.

[Explanation of symbols]

20, 40 ... Laser diode, 21, 34, 35 ... 1
/ 2λ plate, 22: first grating, 23, 31: transparent substrate, 24, 32: waveguide, 25a to 25d, 52: second grating (FGC), 36: hologram element,
37, 38: Photo diode, 39: Disk, 51
... Grating lens, 53 ... Mask.

Claims (3)

(57) [Claims] A laser light source; a transparent substrate having a waveguide formed on one principal surface thereof; and a laser light formed on the transparent substrate and guiding laser light from the laser light source to the waveguide. A concentric or spiral first grating, and an outer periphery of the first grating on the transparent substrate
Formed in the portion, the laser light guided to the waveguide,
At least straightforward in the direction of the central axis of the first grating
Interlinks 2 radiated from a direction, concentric or a spiral second grating, radiation side and information recording of the laser light in the transparent substrate
A second recording medium provided between the recording medium and the recording surface of the recording medium;
The deflection direction of the laser light emitted from the ring is uniform
Thus, from one of the two orthogonal laser beams,
Rotate the deflection direction by 90 degrees with respect to the emitted laser light
A half-wave plate, and the laser light was reflected by the recording surface of the information recording medium.
Then, the second grating, the waveguide, and the first grating
The optical head is characterized by comprising an optical detection means for receiving the return light having passed through the Ingu. 2. The laser light passing through the half-wave plate
The half-wave plate is rotated so that the polarization direction is rotated by 90 degrees.
Between the optical axis and a tangent to the second grating
2. The optical head according to claim 1, wherein the angle is 45 [deg .]. 3. The first grating from the second grating .
Laser light emitted toward the center axis of the rating
Are the two opposing each other across the central axis of the first grating
Regions consisting of two pairs of regions where the regions are orthogonal to each other
2. The optical head according to claim 1, wherein the optical head is divided and radiated .