JP2563379B2 - Optical pickup - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical pickup for an optical disc, a CD, a CD-ROM, a WORM or the like.

2. Description of the Related Art Generally, an optical pickup of this type is constructed as shown in FIG. 10, for example. First, the laser light emitted from the semiconductor laser 1 is focused and irradiated onto the recording surface of the optical information recording medium (optical disc) 4 via the polarization beam splitter 2 and the condenser lens 3. The reflected light from the optical disc 4 passes through the condenser lens 3 again, and then the polarized beam splitter 2
Is separated from the incident light by. On the one hand, the reflected light thus separated passes through the beam splitter 5 and then, for example, by a cylindrical lens 6 for generating astigmatism,
The light is focused on the photodetector 7 having the divided light receiving element structure. As a result, the photodetector 7 detects a focus error signal based on the well-known astigmatism method, and the focus error signal is used for focus servo control of the optical pickup. On the other hand, the lights split in different directions by the beam splitter 5 are focused on a photodetector 8 for detecting a tracking error signal. This photodetector 8 has 2
It has a divided light-receiving element structure, and a tracking error signal is detected by processing the detection signal by the known push-pull method, and is used for tracking servo control of the optical pick-up. The information signal of the optical disk 4 is detected by processing the detection signals of these photodetectors 7 and 8.

However, since such conventional optical pickups are composed of bulk optical components, they are large and heavy as a whole, and the access time tends to be long accordingly. Further, since there are many parts to be assembled and adjusted, the mechanical stability is poor, and problems such as deterioration with time tend to occur.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to obtain an optical pickup that is small and lightweight, easy to assemble, and requires less adjustment.

In order to achieve the above object, the present invention comprises an optical waveguide, a dielectric layer on which reflected light from the optical information recording medium is incident with a higher refractive index than the optical waveguide, and the optical waveguide and the dielectric layer. A material layer disposed between and having an opening partly removed, the real part of the permittivity of which is negative; and a photodetector which is integrated with the optical waveguide and receives light guided in the optical waveguide. It is characterized by consisting of.

The first embodiment of the present invention will be described below with reference to FIGS. 1 and 2. First, a semiconductor laser 12 is arranged so as to face an optical disc 11 as an optical information recording medium.
Between the semiconductor laser 12 and the optical disk 11, a collimator lens 13, a polarization beam splitter 14, a quarter-wave plate 15 and an objective lens 16 are arranged on the optical axis in order from the semiconductor laser 12 side. It is arranged. As a result, the light emitted from the semiconductor laser 12 is focused on the optical disk 11 by the objective lens 16. A part of the light is reflected by the optical disc 11, passes through the objective lens 16 and the quarter-wave plate 15 again, and is separated from the incident light by the polarization beam splitter 14.
The reflected light from the optical disk 11 separated by the polarized beam splitter 14 enters the detection optical system 20 which is a characteristic of this embodiment.

Now, the detection optical system 20 will be described. In this detection optical system 20, first, an optical waveguide 23 is laminated on a Si substrate 21 via a buffer layer 22 made of SiO ₂ . On the optical waveguide 23, a metal thin film layer 24 which is a predetermined material layer is laminated and formed. Here, the metal thin film layer 24 is provided with an opening 24a formed by removing a part of the light so that the light from the polarized beam splitter 14 can be transmitted and incident on the optical waveguide 23. . Further, a triangular prism 25, which is a dielectric having a refractive index higher than that of the optical waveguide 23, is mounted and fixed on the metal thin film layer 24.

As a result, the reflected light from the optical disk 11 is reflected by the polarization beam splitter 14, and then the prism 25 of the detection optical system 20.
Incident on. Here, this light is usually a metal thin film layer
It will be totally reflected at the opening 24a of 24. However,
If the guided mode of the optical waveguide 23 and the incident light are phase-matched, the incident light is coupled to the optical waveguide 23 through the opening 24a,
It becomes guided light. At this time, the metal thin film layer 24 behaves as a dielectric substance having a negative dielectric constant and plays a role as a cladding layer of the optical waveguide 23.

This point will be described in more detail. First, the metal thin film layer 24 has a property that the real part of the dielectric constant is negative. In general, in the optical wavelength region of the metal has a complex refractive index towards the imaginary part is larger than the real part (for example, in the case of silver Ag, aluminum _{Al, A g = 0.065 + i4.0} , A l = 1.2 +
i7.0. The wavelength λ of light is 6328Å. In addition, since the wave equation and its solution directly include the complex dielectric constant ² instead of the complex refractive index, in the case of the optical waveguide 23 including the medium having the complex refractive index, the real number of the complex dielectric constant ² is used. The part determines the optical property and the imaginary part determines the propagation loss. Therefore, the metal generally behaves as a dielectric having a negative dielectric constant and a loss. For example, with respect to light having a wavelength of 6328Å and dielectric constant and Al permittivity of _Ag, the _{^{A g 2 = -15.996 + i0.520 A}} l 2 = -47.56 + i16.8.

Then, as described above, when light is incident on the prism 25, the light is reflected at the interface of the portion corresponding to the metal thin film layer 24 on the bottom surface of the prism 25.
The light is absorbed by 24 and the light does not enter the optical waveguide 23.
However, if the opening 24a formed in the metal thin film layer 24 on the bottom surface of the prism 25 is a dielectric material having a refractive index lower than that of the optical waveguide 23 (in this embodiment, an air layer just opened). The incident light seeps out into the opening 24a as an evanescent wave. At this time, the light is reflected by the optical waveguide 23.
The light enters the optical waveguide 23 if it is phase-matched with the waveguide mode of. Further, the light once entering the optical waveguide 23 as described above propagates in a state of being completely confined in the optical waveguide 23 because the metal thin film layer 24 has a negative dielectric constant. That is, there is no decoupling that exudes again as an evanescent wave and is emitted from the prism 25.

On the optical waveguide 23, as shown in FIG. 2, waveguide lenses 26a and 26b divided into two are formed or loaded so as to be positioned in the waveguide direction of such guided light. Therefore, the light guided into the optical waveguide 23 from the opening 24a of the metal thin film layer 24 is divided into two by these waveguide lenses 26a and 26b and is condensed at each position. The photodetector 27 is formed or loaded integrally with the optical waveguide 23 so as to be positioned on the side of the two-divided light collecting position. Here, the photodetector 27 is composed of four photodetectors 27a to 27d, the light from the waveguide lens 26a is condensed at the central position between the photodetectors 27a and 27b, and the photodetectors 27c and 27d are separated from each other. It is positioned so that the light from the waveguide lens 26b is condensed at the central position of. Reference numerals 28a to 28d are electrodes for extracting signals a to d corresponding to the photodetectors 27a to 27d to the outside.

With such a detection system configuration, first, the information signal of the optical disk 11 may be detected as a + b + c + d. or,
The focus error signal is detected as (a + d)-(b + c), and the tracking error signal is (a + b)-(c + d).
Should be detected as

By the way, a specific method of manufacturing the photodetector 27 and the like will be described. First, SiO ₂ is thermally oxidized on the Si substrate 21.
Are laminated to form a buffer layer 22. Then, the SiO ₂ layer in the photodetector portion is removed by a known photolithography technique, and a photodiode having a pin structure is formed in the Si substrate 21 by impurity diffusion in the removed portion, an ion implantation method, etc. Secure the vessels 27a to 27d. Then, for example, Corning # 7059 glass is laminated on the buffer layer 22 by sputtering to form the optical waveguide 23.
After that, the waveguide lenses 26a and 26b are formed. The waveguide lenses 26a and 26b have a refractive index T higher than that of the optical waveguide 23.
A material such as iO ₂ or SiN is deposited on the optical waveguide 23 to form it, or the refractive index of only the lens portion in the optical waveguide 23 is increased by impurity diffusion in a lens pattern, ion exchange, or proton exchange. You may form it.

Then, for example, silver Ag is vapor-deposited or sputtered on the optical waveguide 23 to form the metal thin film layer 24. As the material of the metal thin film layer 24, silver, which is a metal having a small imaginary part of the complex dielectric constant and a small loss, is effective, but other than this, for example, gold may be used. At this time, a mask is used to prevent Ag from adhering to the opening 24a. However, the entire metal thin film layer 24
After the formation of the openings, etching was performed using a nitric acid-based etchant based on the photolithographic method to form the openings.
24a may be formed. Further, a prism 25 having a right-angled triangular shape made of a medium having a refractive index higher than that of the optical waveguide 23, for example, heavy flint glass or rutile, is mounted and fixed on the metal thin film layer 24.

By the way, when the refractive index of the prism 25 is 2.08, the angle of the slope (vertical angle) is 45 °, the film thickness of the optical waveguide 23 is 4200Å, the refractive index is 1.54, and the wavelength of the laser light used is 7800Å, The incident angle of the thin film layer 24 on the opening 24a is 45 °.

As described above, according to the present embodiment, the polarization beam splitter 5, the cylindrical lens 6, and the detection optical system 20 corresponding to the photodetectors 7 and 8 which are conventional bulk optical components can be integrally configured. As a result, it can be made compact and lightweight as an optical pickup. Also, the assembly can be handled by 20 units of the detection optical system, and it is not necessary to individually consider the photodetector and the like,
It will be easy. This is the same in terms of adjustment,
It may require little adjustment. Particularly, according to the present embodiment, the opening 24 in the detection optical system 20 is
The metal thin film layer 24 on which a is formed is used as a prism 25 and an optical waveguide 23.
Since the light from the prism 25 side is coupled and guided in the optical waveguide 23 by being provided between them, the waveguide efficiency is good, a good detection operation can be performed, and the incident condition is advantageous. Can be one.

FIG. 3 shows a modified example of the opening of the metal thin film layer 24.
24a is formed by being divided into two in a direction parallel to the track groove of the optical disk 11. By dividing the opening 24a into two in this manner, it is possible to effectively take in light in a region having a large intensity distribution in the reflected light from the optical disk 11 side into the optical waveguide 23.

2 and 3, the opening 24a has a rectangular shape, but the shape is not limited to such a shape, and the opening 24a may have a circular shape, an elliptical shape, or the like. In addition, the size of the opening 24a is preferably set to be approximately the same as or slightly smaller than the incident beam diameter in order to enhance the coupling efficiency to the optical waveguide 23. Further, the inside of the opening 24a is in a low refractive index state due to an air layer (a space of about 1000Å), but a low refractive index material such as a photopolymer having a refractive index of 1.49 may be poured into the opening 24a and filled therein. .
In particular, an adhesive or the like is used for fixing between the metal thin film layer 24 and the prism 25, but if the refractive index of the transparent adhesive is lower than that of the optical waveguide 23, the adhesive may be filled in the opening 24a. .

Next, a second embodiment of the present invention will be described with reference to FIG. The same parts as those shown in the above-mentioned embodiment are designated by the same reference numerals. The basic structure of the constituent members is the same as that of the above-mentioned embodiment, except that the polarization beam splitter 14 is omitted, the arrangement of the semiconductor laser 12, the detection optical system 20, etc. is changed, and the optical waveguide 23 is only in the TE ₀ mode. Will be propagated. Specifically, the optical waveguide 23 has a refractive index of 1.54 and a film thickness of
The buffer layer 22 is made of Si having a refractive index of 1.46.
It is formed of O ₂ .

In such a configuration, the laser light having a wavelength of 780 nm emitted from the semiconductor laser 12 is collimated by the collimator lens 1
The light is collimated by 3 and enters a prism 25 having a refractive index of 2.075. At this time, the polarization direction of the semiconductor laser light is p-polarized light which is oriented parallel to the paper surface in FIG. As a result, the light incident on the prism 25 reaches the interface between the prism 25, the metal thin film layer 24, and the opening 24a.
Here, most of the light is reflected by the metal thin film layer 24, and only part of it is absorbed in the metal thin film layer 24. However, the light reaching the opening 24a is p-polarized light, and the waveguide mode TE _{0 of the} optical waveguide 23
Since and cannot be phase-matched with each other, they are totally reflected. As a result, the light from the semiconductor laser 12 is deflected to the optical disk 11 side. The light reflected by such an interface passes through the quarter-wave plate 15 to become right-handed circularly polarized light, and is condensed and irradiated onto the optical disk 11 by the objective lens 16. The light reflected by the optical disk 11 becomes left-handed circularly polarized light, and after passing through the objective lens 16, passes through the quarter-wave plate 15 again to become an s-polarized state, and then enters the prism 25 again.

Also in this case, the prism 25, the metal thin film layer 24, and the opening 24
It reaches the interface with a, but this time the guided mode T of the optical waveguide 23
E ₀ and s-polarized light (polarized light component perpendicular to the paper surface) can be phase-matched. In the case of the present embodiment, the incident angle of light to the opening 24a is 45
If it is set to °, the phase matching condition with the optical waveguide 23 is satisfied when s-polarized. Therefore, the light reaching the opening 24a is transmitted to the optical waveguide 23
It becomes a guided light by coupling inside. After that, the light is focused on the photodetectors 27a to 27d side through the waveguide lenses 26a and 26b as in the above-described embodiment, and the information is read by signal processing.

In the present embodiment, the example in which the optical waveguide 23 is treated as a single mode waveguide has been described, but a multimode waveguide configuration may be used. Even in this case, the light incident on the opening 24a and one waveguide mode having the optical waveguide 23 may be configured to satisfy the phase matching condition. The phase matching condition may not be satisfied when the laser light travels to the optical disk 11, and the phase matching condition may be satisfied when the reflected light from the optical disk 11 is incident.

Next, a third embodiment of the present invention will be described with reference to FIG. The present embodiment is based on the above-mentioned embodiment, and the lens system is also integrated with the detection optical system 20. First, on the incident side of the prism 25 from the semiconductor laser 12, a Fresnel lens 29 is mounted instead of the normal collimator lens 13. Also, a 1/4 plate 15 is provided on the surface of the prism 25 facing the optical disk 11.
And the Fresnel lens 30 is mounted on the 1/4 plate 15 in place of the normal objective lens 16. As described above, according to the present embodiment, all the optical pick-up optical system constituent members except the semiconductor laser 12 can be integrated by the prism 25, and the size and weight can be further reduced.

A fourth embodiment of the present invention will be described with reference to FIG.
In the present embodiment, a gradient index lens 31 is used as a collimating lens instead of the Fresnel lens 29 of the above embodiment. As a result, according to the present embodiment, by mounting the semiconductor laser 12 also on the end surface of the gradient index lens 31, all of them can be integrated.

Instead of the Fresnel lens or the distributed refractive rod lens in the third and fourth embodiments, another lens such as a distributed refractive index flat plate microlens may be used as appropriate.

Further, a fifth embodiment of the present invention will be described with reference to FIG. In the present embodiment, a predetermined quadrangular prism 32 is used in place of the triangular prism 25, and the elliptical beam emitted from the semiconductor laser 12 is caused by making the laser light from the semiconductor laser 12 incident on the inclined surface 32a. It has a beam shaping function. That is, the beam diameter is increased in the direction parallel to the plane of the drawing. Here, the metal thin film layer 24
The angle of incidence of the laser light on the opening 24a is larger than 45 °.

Next, a sixth embodiment of the present invention will be described with reference to FIG. In this embodiment, the Si substrate 21 is not used, the prism 25 itself is also used as a substrate, and the waveguide structure is simplified.
Only the detection optics 20 is shown and shown. First, in the present embodiment, a metal material such as silver is vapor-deposited on the bottom surface of the prism 25,
A film is formed by a sputtering method or the like to form the metal thin film layer 24. Then, a part of the metal thin film layer 24 is removed by etching using a well-known photolithography technique to form an opening.
Forming 24a. Then, in the opening 24a, the optical waveguide 23
A transparent medium 33 having a lower refractive index is filled. Furthermore, the optical waveguide
23 is formed into a film, and a photodetector is formed on the end face of the optical waveguide 23 in the waveguide direction.
While mounting 27, the waveguide lens 26 is formed at a predetermined position in the same manner as in the above-described embodiment.

Furthermore, a seventh embodiment of the present invention will be described with reference to FIG. In this embodiment, the optical waveguide 23 is formed on the substrate 34 having a lower refractive index than the optical waveguide 23, and the photodetector 27 is attached to the end face of the optical waveguide 23. Next, the metal thin film layer 24 having the opening 24a is laminated on the optical waveguide 23, and is adhered and fixed to the bottom surface of the prism 25.

Incidentally, the photodetector 27 in the sixth and seventh embodiments
As a method, an individual photo detector may be attached, or an amorphous silicon detector may be laminated and manufactured.

Advantageous Effects of the Invention The present invention includes the optical waveguide, the dielectric layer on which the reflected light from the optical information recording medium is incident with a higher refractive index than the optical waveguide, and the optical waveguide and the dielectric layer. A material layer having a removed opening and a negative real part of the dielectric constant;
Since the detection optical system is composed of a photodetector that is integrated with the optical waveguide and receives guided light, most of it has an integrated structure,
The size and weight of the optical pickup can be reduced,
Moreover, the integrated structure of the detection system facilitates the assembly of the optical system and reduces the need for adjustment.

[Brief description of drawings]

FIG. 1 is a partially sectional front view showing a first embodiment of the present invention, FIG. 2 is a plan view of an optical waveguide surface, FIG. 3 is a plan view showing a modified example, and FIG. FIG. 5 shows a partially sectional front view showing a second embodiment of the present invention, FIG. 5 shows a partially sectional front view showing a third embodiment of the present invention, and FIG. 6 shows a fourth embodiment of the present invention. FIG. 7 is a partially sectional front view showing a fifth embodiment of the present invention, and FIG. 8 is a partially sectional front view showing a sixth embodiment of the present invention. FIG. 9 is a partially sectional front view showing a seventh embodiment of the present invention, and FIG. 10 is a front view showing a conventional example. 11 ... Optical disc (optical information recording medium), 23 ... Optical waveguide, 24 ... Metal thin film layer (material layer), 24a ... Opening, 25
…… Prism (dielectric layer), 27 …… Photodetector, 32 …… Prism (dielectric layer)

Claims (1)

(57) [Claims] 1. An optical waveguide, and a dielectric layer into which reflected light from an optical information recording medium is incident with a higher refractive index than the optical waveguide,
A material layer, which is provided between the optical waveguide and the dielectric layer and has an opening partly removed, and whose real part of the dielectric constant is negative, and a material layer which is integrated with the optical waveguide and guides in the optical waveguide. An optical pickup comprising a photodetector for receiving light that emits light.