JP3092355B2 - Light 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 optical head for recording or reproducing information on an optical disk.

[0002]

2. Description of the Related Art In recent years, in the field of optical disk systems, there has been a demand for techniques for reducing the size and weight of optical heads and manufacturing them in large quantities at low cost. The development of a waveguide type optical head in which the configuration of an optical head using a bulk type optical element such as the above is replaced with a thin film structure using a waveguide is being activated.

Hereinafter, a conventional waveguide type optical head will be described with reference to the drawings. FIG. 4 is a diagram showing a configuration of a conventional waveguide type optical head. In FIG. 4, the Si substrate 1
A transparent layer 2 having a low refractive index is formed thereon, and a transparent layer 3A having a high refractive index is formed on the transparent layer 2 in a ring shape having L as a central axis. Further, a transparent layer 5 having a low refractive index is formed in a circular shape with L as a central axis so as to cover a part of the inner peripheral side of the transparent layer 2 and the transparent layer 3A, and further covers these transparent layers as a whole. The transparent layer 3B is formed. On the surface of the transparent layer 3B, a concentric grating 4B having L as a central axis is formed in a circular region having L as a central axis, and an L is formed in an annular zone having L as a central axis.
Is formed as a concentric grating 4 </ b> A having the center axis as the center axis. On the Si substrate 1, a photodetector 6 is formed at a position corresponding to the inner peripheral end of the transparent layer 3A. The portion near the central axis L of the Si substrate 1 is made hollow by etching. 7 is a semiconductor laser light source, 8 is a base on which the laser light source is mounted, 9 is a collimating lens, 10A,
10B is an element for converting the polarization state of light.

The operation of the thin-film optical head constructed as described above will be described below. The linearly polarized light emitted from the semiconductor laser 8 is condensed by a condenser lens 9 and is condensed by a polarizing element 10A. The light 11 having an electric field vector in a concentric tangential direction (or radial direction) with L as a central axis.
Is converted to This light 11 is a concentric grating 4
By being incident on B, TE-mode (or TM-mode) guided light 12B is excited in the waveguide layer 3B. Guided light 1
2B emits light to the outside of the waveguide layer while propagating in the layer toward the outer periphery and propagating in the formation region of the grating 4A. If the guided light 12B is in the TE mode, the radiated light 13 is light in a polarization state in which the electric field vector is directed in a concentric tangential direction (hereinafter, referred to as concentric polarized light). It becomes light in a state (hereinafter, referred to as radiation polarization), and is converted into linearly polarized light via the polarizing element 10B.
Focus on

When the light reflected by the reflecting surface 15 is incident on the grating 4A, the TE-mode (or TM-mode) guided light 16 that propagates in the waveguide layer 3B again in the inner circumferential direction is excited. Is done. The guided light 16 is the waveguide layer 3
The light becomes the guided light 17 propagating through A, is emitted at the innermost end of the waveguide layer 3A, and is received by the photodetector 6.

[0006]

The optical head having the above configuration has the following problems. Hereinafter, this will be described with reference to FIGS.

FIG. 5A is a diagram showing the concept of the radiation loss coefficient of a grating coupler formed by a combination of an optical waveguide and a grating. In general, when the guided light 42 propagates through the formation region of the grating 40, the phase matching condition shown in the equation (1) is given by: sin θ = qλ / Λ−N (1) (Λ: period of the grating coupler, λ: wavelength of light,
If there is a solution that satisfies N: equivalent refractive index of the waveguide with respect to the guided light, θ: light emission angle, q: coupling order), light is emitted outside the waveguide layer 41 and the light quantity of the guided light 42 is gradually attenuated. I will do it. A coefficient indicating the degree of this attenuation is called a radiation loss coefficient,
The intensity I (z) of the emitted light (z; the distance in the propagation direction of the guided light) can be generally expressed by equation (2) using the radiation loss coefficient α.

I (z) = I ₀ exp (−2αz) (I ₀ ; intensity of emitted light at Z = 0) (2) FIG. 5B shows the distance z in the propagation direction on the horizontal axis and the radiation loss on the vertical axis. A plot of equation (2) as the coefficient α is shown. The dashed line in the figure indicates the case where the radiation loss coefficient is larger than that indicated by the solid line, and the one-dot chain line indicates the case where the radiation loss coefficient is smaller than that indicated by the solid line.
FIG. 5B shows that light is emitted in a smaller region when the radiation loss coefficient is larger, and conversely, in a larger region when the radiation loss coefficient is smaller. This means
Conversely, the same applies to the case where light is input from the outside of the waveguide layer to the inside of the waveguide layer. The larger the radiation loss coefficient of the coupler, the more light can be input in the small grating region, and conversely, the smaller the radiation loss coefficient A larger grating area is required to input more light.

FIG. 6B is a graph for explaining a change in a radiation loss coefficient with respect to each parameter of the grating coupler shown in FIG. 6A. In the configuration as shown in FIG. 6A, the radiation loss coefficient α shows a value as shown in FIG. 6B with respect to the grating depth h. Here, T ₁ represented by a solid line and T ₂ represented by a broken line are respectively
The case where the parameters of the waveguide layer are set as shown in (Table 1) so that the value of the equivalent refractive index N in the formation part of the grating coupler becomes substantially the same value (N = 1.59) is shown.

[0010]

[Table 1]

As shown in the figure, when gratings having the same depth are formed, the radiation loss coefficient is smaller when the optical waveguide layer is formed of a lower refractive index and a thicker film, and the refractive index and the film thickness are constant. For example, it can be seen that the smaller the grating depth, the smaller the radiation loss coefficient.

In the configuration of the conventional optical head shown in FIG. 4, a concentric grating 4B for inputting light and a concentric grating for radiating light to the outside of the waveguide layer and condensing the light at a point outside the waveguide layer. 4A was formed on the same waveguide layer 3B. Further, since the gratings 4A and 4B were formed by simultaneously etching with the same etching means, the grating depths were also the same.

That is, the radiation loss coefficient was the same in the formation region of the grating 4B and the formation region of the grating 4A. For this reason, if an optical waveguide layer having a high refractive index and a small film thickness is used, or if the grating depth is increased, the value of the radiation loss coefficient increases, so that the light input efficiency in the formation region of the grating 4B is reduced. Will be higher,
In the orbicular zone where the grating 4A is formed, most of the light is radiated in the minute portion on the inner peripheral side (that is, the aperture area is small), so that the light collecting property at the light collecting point is deteriorated. Conversely, if an optical waveguide layer having a low refractive index and a large film thickness is used or the grating depth is reduced, the value of the radiation loss coefficient decreases, and the effective area of the grating 4A increases (that is, the opening area). Becomes larger), and a good light-collecting property can be obtained at the light-collecting point, but there is a problem that the light input efficiency in the area where the grating 4B is formed becomes low.

[0014]

In order to solve the above-mentioned problems, an optical head according to the present invention comprises a laser light source, an optical waveguide layer formed in a plane perpendicular to the optical axis thereof, and a light source of the optical waveguide layer. and optical coupling means a obtained by forming a concentric periodic structure C whose central axis coincides with the optical axis in a circular region centered axis an axis, said optical imaging
The annular region whose central axis coincides with an optical axis of the optical waveguide layer in the outer peripheral side of the engagement means A, and an optical coupling means B obtained by forming a concentric periodic structure D whose central axis coincides with the optical axis, and Optical coupling means A
Radiation loss coefficient of light due to radiation loss coefficient alpha _A and the optical coupling means B of light by alpha _B are those satisfying the relationship α _A> α _B. Further, the radiation loss due to optical coupling means A coefficient alpha _A, more satisfies optical coupling means the optical radiation loss coefficient alpha _B are each by B, alpha _A> 10 a _{(1 / mm) α B <} 2 (1 / mm) High practicality.

The optical waveguide layer comprises a first circular portion having the optical axis of the laser beam as a central axis, and a second annular portion having the optical axis as the central axis. The optical coupling means A is formed on the second portion, and the optical coupling means B is formed on the second portion, and the outermost peripheral portion of the first portion and the innermost peripheral portion of the second portion overlap each other. When the refractive index and the film thickness of the portion are n _A and t _A , respectively, and the refractive index and the film thickness of the second portion are n _B and t _B , respectively, n _A > n _B t _A <t _B The radiation loss coefficient α of light by the optical coupling means A
_A and radiation loss coefficient of light by the optical coupling means _B α B, α _A>
to the relationship of α _B.

Alternatively, when the periodic structure C and the periodic structure D are formed of an uneven structure, and the depth of the periodic structure C is h _C and the depth of the periodic structure D is h _D , h _C > h _D. And the radiation loss coefficient α of light by the optical coupling means A
_A and radiation loss coefficient of light by the optical coupling means _B α B, α _A>
to the relationship of α _B.

The optical waveguide layer comprises a first circular portion having the optical axis of the laser beam as the central axis and a second annular portion having the optical axis as the central axis. Periodic structure C
However, the periodic structure D is formed in the second portion, the outermost peripheral portion of the first portion overlaps the innermost peripheral portion of the second portion, and the periodic structures C and D Has a concavo-convex structure formed by periodically etching the optical waveguide layers of the first portion and the second portion by the same etching means, respectively, and is formed by the etching means of the optical waveguide layer of the first portion. The first portion and the second portion of the optical waveguide layer may be composed of a film having an etching rate higher than the etching rate of the second portion of the optical waveguide layer by the etching means. Alternatively, a configuration in which an annular transparent layer centered on the optical axis of the laser beam is further provided on the periodic structure D may be employed.

[0018]

According to the present invention, there is provided an optical coupling means for inputting a laser beam from a laser light source into an optical waveguide layer, and radiating the laser beam outside the waveguide layer and condensing the laser beam at a point outside the waveguide layer. Since the radiation loss coefficients of the optical coupling means B can be optimized independently of each other, an optical head having a high light transmission efficiency and a high light collecting effect can be realized.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A thin film optical head according to an embodiment of the present invention will be described below with reference to the drawings. Note that the same components as those in the conventional example are denoted by the same reference numerals, and detailed description is omitted.

FIG. 1 is a sectional view showing a part of the structure of an optical head according to a first embodiment of the present invention. In FIG. 1, a buffer layer 2 having a low refractive index is formed on a substrate 1,
An optical waveguide layer 20 is formed on the buffer layer 2 in an annular zone centered on L. Further, on the buffer layer 2, L
The buffer layer 21 having a low refractive index is formed in an annular zone region having a central axis, and the outer peripheral portion thereof overlaps the inner peripheral portion of the optical waveguide layer 20 and forms a gentle taper toward the outer periphery. I have. Furthermore, an optical waveguide layer 22 covering the inner peripheral portion of the optical waveguide layer 20 and a part of the transparent layer 21 and the transparent layer 2
Are formed in a circular shape with L as the central axis, and the outer peripheral portion also has a gentle tapered shape toward the outer periphery. On the optical waveguide layer 20, L
Is formed on the optical waveguide layer 22, and a concentric grating 24 having L as the central axis is formed on the optical waveguide layer 22 in a circular region having L as the central axis.

In the optical head configured as described above, the laser beam 11 from the laser light source is incident on the region where the grating 24 is formed, and propagates in the optical waveguide layer 22 in the radial direction around the L axis. Guided light 2
5 is excited. The guided light 25 travels along the taper of the outer peripheral portion of the optical waveguide layer 22, enters the optical waveguide layer 20, and becomes the guided light 26 that propagates inside the optical waveguide layer 20 toward the outer peripheral side. When the guided light 26 propagates through the region where the concentric grating 23 is formed, light is emitted to the outside of the optical waveguide layer 20, and the emitted light 27 is collected at a point on the L axis.

According to such a configuration, the input grating coupler (grating 21 and grating 2) for inputting the laser light from the laser light source into the optical waveguide layer.
Structure consisting of the optical waveguide in the region where 1 is formed)
And a condensing grating coupler (a structure including the grating 24 and an optical waveguide in a region where the grating 24 is formed) for emitting light to the outside of the optical waveguide layer and condensing the light at a point outside the waveguide layer, respectively. Since the optical waveguide is constituted by the different optical waveguide layers (22 and 20), the radiation loss coefficient can be different in each region.

[0023] Here, the radiation loss coefficient of the formation region of the grating 24 alpha _A, when the radiation loss coefficient for light in the formation area of the grating 23 was set to alpha _B, if so as to satisfy the relationship of α _A> α _B In the area where the grating 24 is formed, a large amount of light is input with a small area, and at the same time, the area where the light is radiated becomes large in the area where the grating 23 is formed. That is, it is possible to realize an optical head having a high light input efficiency and a good aperture with a large aperture.

Here, as a practical configuration, for example, if the focal length of the optical head is 1 mm or more and the aperture of the condensing grating coupler is an annular zone aperture of about 0.45 to 0.7,
The width of the annular zone of the grating 24 is 0.5 mm or more. Generally, since the effective diameter of a laser beam involved in focusing is defined at a position corresponding to 1 / e ² of the intensity of light at the center of the beam, the intensity of light emitted at the outermost periphery of the orb In order to have an intensity of 1 / e ² or more of the light radiated at the innermost periphery of the band, from the formula (2), if exp (−2 × α _B × 0.5)> e ⁻² is satisfied. Good. Therefore the value of alpha _B is alpha _B if <2 (1 / mm), the effective opening area of the condensing grating coupler can be secured.

When laser light from a laser light source is input to an optical waveguide layer, generally, a concentric grating 2
By reducing the area of No. 4, the difference in light amount due to the propagation azimuth of the guided light 22 with respect to the L axis can be reduced. Therefore, the radius of the circular area where the grating 24 is formed is generally 0.1 mm or less. Here, the light amount distribution I ₁ (z ₁ ) of the incident laser light in the radial direction of the circular region.
(z ₁ ; radial distance from the center of the circle in the radial direction) is approximately expressed by using the radiation loss coefficient α _A of the input grating coupler. Equation (3): I ₁ (z ₁ ) = I ₀₁ exp (−2α _A z ₁ ) (I ₀₁ ; intensity of incident light at z ₁ = 0) (3). Therefore, the intensity of the incident light at the outermost peripheral portion of the grating forming region is 1 / e ^{2 of the} central intensity.
If it is made to correspond to the following, from Expression (3), α _A may be determined so as to satisfy exp (−2 × α _A × 0.1) <e ⁻² . Therefore, if the value of α _A is set to α _A > 10 (1 / mm), the light input efficiency into the waveguide layer 22 by the input grating coupler can be sufficiently ensured.

Next, a description will be given of an embodiment in which the radiation loss coefficient is optimized in the formation region of the grating 23 and the formation region of the grating 24 in the configuration of FIG. In FIG. 1, an optical waveguide layer 20 and an optical waveguide layer 22 are
Although the equivalent refractive indices are substantially equal to each other, the refractive index and the thickness of the optical waveguide layer A and the optical waveguide layer B are different from each other, and the refractive index and the thickness of the optical waveguide layer 22 are n _A and t _A , respectively. , the refractive index of the optical waveguide layer 20, the film thickness, respectively, n _B, when the t _B, a structure satisfying the relationship _{n a> n B t a <} t B simultaneously. As described earlier with reference to FIG. 6, for the same grating depth and equivalent refractive index, a grating coupler constituted by a thinner optical waveguide layer with a higher refractive index has a higher radiation loss coefficient. Therefore, according to the configuration satisfying the above relationship, the grating 2
Even if the depths of 3 and 24 are the same, the formation area of the grating 24, that is, the radiation loss coefficient of the input grating coupler, is relatively larger than the formation area of the grating 23, that is, the radiation loss coefficient of the focusing grating coupler. can do.

Next, another embodiment for optimizing the radiation loss coefficient of the grating coupler in each forming region will be described with reference to FIG. This embodiment is different from the previous embodiment in that the refractive index and the film thickness of the optical waveguide layer 20 and the optical waveguide layer 22 may be the same, and instead, the depth of the grating may be changed in each formation region. It is formed differently. That is, in the present embodiment, when the depth of the grating 24 is h _C and the depth of the grating 23 is h _D in FIG. 1, the relationship of h _C > h _D is satisfied.

As described above with reference to FIG. 1, the radiation loss coefficient of the grating coupler increases as the grating depth increases. Therefore, with a configuration that satisfies the above relationship, the radiation loss coefficient of the area where the grating 24 is formed can be made relatively larger than the radiation loss coefficient of the area where the grating 23 is formed. Further, the refractive index and the film thickness of the optical waveguide layer 20 and the optical waveguide layer 22 were made different as shown in the previous embodiment, and the depths of the gratings 23 and 24 were further increased as in this embodiment. If they are different, the difference between the radiation loss coefficient values can be further increased.

Here, a description will be given of an embodiment in which the depths of the grating 24 and the grating 23 are made different in each forming region. For example, when a SiON film formed by the plasma CVD method is used as the optical waveguide layer 20 and an SiN film similarly formed by the plasma CVD method is used as the optical waveguide layer 22, buffer hydrofluoric acid (hydrofluoric acid: aqueous solution of ammonium fluoride) is used. The etching rate of each of the mixed solutions having a volume ratio of 1:20 is 1: 1.
The SiN film is larger at a ratio of about 5.

Therefore, the SiON film can be made thin with a slightly lower refractive index.
(E.g., a refractive index of 1.71 and a film thickness of 0.5 μm).
The iN film has a high refractive index and is slightly thicker (for example, a refractive index of 1.82).
3. After forming a film having a thickness of 0.24 μm), pattern exposure and development of the grating are performed, and the optical waveguide layers 20 and 22 are etched with buffered hydrofluoric acid. It is possible to make the loss coefficient relatively larger and more optimal than the radiation forming area of the grating 24, that is, the radiation loss coefficient of the condensing grating coupler.

That is, if each of the optical waveguide layers 22 is formed of a film in which the etching speed of the optical waveguide layer 20 is higher than that of the optical waveguide layer 20 by the same etching means, the grating 2 can be formed in one etching step.
4 and the grating 23 can be formed with different depths, so that the radiation loss coefficient of the grating coupler can be different in each forming region, and the productivity is good.

Next, another embodiment for optimizing the radiation loss coefficient of the grating forming portion in each forming region will be described with reference to FIG. The configuration shown in FIG. 2 is substantially the same as the configuration shown in FIG. 1, but is different from the embodiment described with reference to FIG.
3 is that the transparent layer 25 is formed in the formation region. FIG. 3B shows a change in the radiation loss coefficient α with respect to the refractive index n _C of the cladding layer adjacent to the optical waveguide layer on which the grating is formed in the configuration of the grating coupler shown in FIG. FIG. 3B shows that the smaller the refractive index n _C of the cladding layer is smaller than the refractive index of the optical waveguide layer, the smaller the radiation loss coefficient. That is, in the configuration shown in FIG. 2, the refractive index of the cladding layer in the region where the grating 23 is formed is larger than that of the cladding layer (air layer; refractive index 1.0) in the region where the grating 24 is formed. The radiation loss coefficient of the region can be made relatively larger than the radiation loss coefficient of the region where the grating 24 is formed.

[0033]

As described above, the present invention provides an input grating coupler for inputting laser light from a laser light source into a waveguide layer, and a collector for radiating the guided light and condensing it at a point outside the waveguide layer. Since it is possible to independently optimize the radiation loss coefficient with the optical grating coupler,
A thin-film optical head having high light use efficiency and high light condensing ability can be provided, and the effect is great.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a part of a configuration of an optical head according to an embodiment of the present invention.

FIG. 2 is a sectional view showing a part of a configuration of an optical head according to another embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the refractive index of the cladding layer adjacent to the optical waveguide layer on which the grating is formed and the radiation loss coefficient of the grating coupler.

FIG. 4 is a configuration diagram of a conventional waveguide type optical head.

FIG. 5 is a principle diagram of a grating coupler.

FIG. 6 is a diagram showing a relationship between each parameter of the grating coupler and a radiation loss coefficient.

[Explanation of symbols]

 20, 22 optical waveguide layer 23, 24 concentric grating 25, 26 guided light 27 emitted light

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kiyoko Oshima 1006 Oaza Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-4-315831 (JP, A) JP-A-3-3 179320 (JP, A) JP-A-4-117638 (JP, A) WO 89/6424 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) G11B 7/12-7 / 22 G02B 6/122 G02B 6/34

Claims (6)

(57) [Claims] A laser light source; an optical waveguide layer formed in a plane perpendicular to an optical axis of the laser light; and a circular region centered on the optical axis of the optical waveguide layer. a concentric periodic structure C optical coupling means by forming a a to the axis, the annular region of the optical axis of the outer circumferential side of the optical waveguide layer and the central axis from said light coupling means a, the optical axis Coupling means B formed by forming a concentric periodic structure D having a central axis
Laser light from the laser light source is input to the optical waveguide layer by the optical coupling means A to generate guided light propagating in a radial direction from the optical axis, and the optical coupling means B generates the guided light. radiating, an optical head for condensing a point outside the optical waveguide layer, the radiation loss coefficient alpha _B of light by said light coupling means B and the optical coupling means a by light radiation loss coefficient alpha _a, An optical head satisfying a relationship of α _A > α _B. Wherein said optical coupling means the optical radiation loss coefficient alpha _A by A, the light coupling means optical radiation loss coefficient alpha _B are each by _{B, α A> 10 (1} / mm) α B <2 (1 The optical head according to claim 1, wherein the optical head satisfies the following formula: 3. The optical waveguide layer according to claim 1, wherein the optical waveguide layer comprises a circular first portion having the optical axis of the laser beam as a central axis, and an annular second portion having the optical axis as the central axis. most of the <br/> optical coupling means a in part, of the said optical coupling means B to the second part are formed respectively, and the second portion and the outermost peripheral portion of the first portion the optical head of the inner peripheral portion and overlap each other to have claim 1 or claim 2, wherein the refractive index of the first portion, the film thickness, respectively, n _a, t _a, of the second portion 3. The optical head according to claim 1, wherein n _A > n _B t _A <t _B , where n _B and t _B are the refractive index and the film thickness, respectively. Wherein said periodic structure C and the periodic structure D consists uneven structure, and the depth of the periodic structure C h _C,
Wherein when the depth of the periodic structure D was h _D, according to claim 1 or 2 optical head, wherein the a h _C> h _D. 5. The optical waveguide layer according to claim 1, wherein the optical waveguide layer comprises a circular first portion having the optical axis as the central axis and a second annular portion having the optical axis as the central axis. the <br/> periodic structure C in the portion of, the innermost circumference of the said periodic structure D in the second section are formed respectively, and the second portion and the outermost peripheral portion of the first portion the optical head and the overlapping has claim 1 or claim 2, wherein parts, optical waveguide of the periodic structure C and the periodic structure D is the second portion and the optical waveguide layer of each of the first portion A layer having a concavo-convex structure formed by periodically etching the layer by the same etching means, wherein the etching rate of the first portion of the optical waveguide layer by the etching means is equal to the etching rate of the second portion of the optical waveguide layer; Film material greater than the rate of etching by means The first part of claim 1 or 2 optical head wherein the optical waveguide layer of the optical waveguide layer and said second portions configured respectively. 6. The optical head according to claim 1, further comprising a ring-shaped transparent layer centered on the optical axis of the laser beam on the periodic structure D.