JP2731390B2 - Tapered waveguide - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a tapered waveguide which forms a radial focus only by a waveguide and is applicable to an optical head for recording / reproducing an optical disk signal.

(Prior Art) A tapered waveguide is known in which a waveguide layer formed on a substrate has a tapered end.
No. 06). In such a tapered waveguide, guided light propagating through a waveguide layer having a constant thickness is tapered,
It is known that the light is emitted to the substrate side, and the emitted light is divergent.

(Problems to be Solved by the Invention) If the light extracted from the tapered waveguide to the substrate side can be focused finely by appropriately designing the tapered shape from the divergent type, This can be used, for example, as an illumination spot on an optical disc,
Alternatively, it is expected to be used as an efficient light source.

As an optical head used for recording / reproducing an optical disk signal using a waveguide, there is known an optical head having a configuration in which a diffraction grating for optical coupling is formed on a flat optical waveguide (Japanese Patent Laid-Open Publication No. H11-163873). However, this type of head uses a diffraction grating to extract light to the outside and to introduce light from an optical disk into an optical waveguide.
For example, the focus position of the extracted light fluctuates due to the wavelength fluctuation of the semiconductor laser as the light source, and the light utilization efficiency for irradiating the optical disk is low because the light is extracted to both sides of the diffraction grating. However, there are problems such as difficulty in producing a diffraction grating for coupling.

On the other hand, in the tapered waveguide, the emission characteristic of the light extracted to the substrate side is stable with respect to the wavelength change of the light source,
Further, since substantially all of the light can be extracted to the substrate side as described later, the light utilization efficiency is high and the fabrication is easy, so that the optical head has high applicability.

However, even if it is possible to focus a minute focal point by appropriately designing the tapered shape as described above, the focal point is located very close to the interface between the substrate and the waveguide layer. Therefore, when the light is actually extracted out of the substrate, the light becomes a divergent light beam, and the focus of the extracted light cannot be directly used for illumination of an optical disk or the like.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a novel tapered waveguide that can focus light extracted to the substrate side at a position away from the boundary surface. It is in.

For the application of tapered waveguides to optical heads, see Takeda and Miyazaki: Pickup focusing characteristics of tapered waveguides for integrated optical disk heads (Preliminary meeting of the 1987 Tokai Section Alliance, 1987) and Takeda・ Miyazaki: Focusing characteristics of tapered waveguides for integrated optical disk heads (disclosed in IEICE Technical Report MW87-112 (1988)).

(Means for Solving the Problems) Hereinafter, the present invention will be described.

 The tapered waveguide of the present invention has a substrate and a waveguide layer.

The substrate has a flat surface for forming a waveguide layer, has a lower refractive index than the waveguide layer, and is transparent to the guided light (the guided light can be transmitted).

 The waveguide layer is formed on this substrate.

The above-mentioned waveguide layer is connected to the uniform-layer waveguide layer having a uniform layer thickness, and has the same layer thickness as the uniform-layer waveguide layer at the connection portion. The layer thickness gradually decreases as the distance increases, and the tapered waveguide layer emits guided light propagating through the waveguide layer having the uniform thickness to the substrate side.

The substrate is formed by connecting a propagation substrate and a radiation substrate that share a plane for forming the waveguide layer in the waveguide direction in the waveguide layer in this order, and the radiation substrate has a refractive index larger than that of the propagation substrate. Having. Of course, the refractive index of both the propagation substrate and the radiation substrate is smaller than the refractive index of the waveguide layer, and is transparent to the guided light.

The connecting portion between the propagation substrate and the radiation substrate is close to the radiation start position determined by the refractive index of the propagation substrate and the waveguide layer, and the shape of the tapered waveguide layer, and exceeds the radiation start position in the waveguide direction. Not set. Here, the radiation start position is actually a position uniquely determined by the refractive index of the propagation substrate and the waveguide layer and the shape of the tapered waveguide layer when the entire substrate is formed by the propagation substrate. In the case of using a substrate, leakage of light power from the radiation start position to the substrate side starts.

(Example) Hereinafter, a description will be given based on a specific example.

FIG. 1 is an explanatory diagram showing one embodiment of the present invention.

In the drawing, reference numeral 10 indicates a substrate, and reference numeral 20 indicates a waveguide layer.

The waveguide layer 20 includes a waveguide layer 21 having a uniform thickness and a tapered waveguide layer 22.

The substrate 10 is formed by connecting a propagation substrate 11 and a radiation substrate 12 to each other.The propagation substrate 11 and the radiation substrate 12 are both transparent to the guided light and share a plane for forming a waveguide layer. ,
Waveguide layer 20 consisting of uniform thickness waveguide layer 21 and tapered waveguide layer 22
Are formed on the plane.

For the following description, X and Z directions are determined as shown in the figure.
The Z direction corresponds to the plane of the substrate 10 and corresponds to the propagation direction of the guided light. In other words, the guided light is guided and propagated through the uniform thickness waveguide layer 21 to the right in FIG. The X-axis is orthogonal to the plane, and the position of the X-axis represents the connection between the waveguide layer 21 having a uniform thickness and the tapered waveguide layer 22. Therefore, the tapered waveguide layer 22
Is the right part of the X-axis in FIG.

The region TR where the tapered waveguide layer 22 is formed is called a tapered region.

The area above the waveguide layer 20 is the air area occupied by air. The relationship between the refractive indices is, for example,
The refractive index of 10 is 1.51 for the propagation substrate 11, 1.515 for the radiation substrate 12,
The refractive index of the waveguide layer 20 is 1.52, and the refractive index of air is 1. Therefore, the refractive index increases in the order of the air region, the propagation substrate 11, the radiation substrate 12, and the waveguide layer 20. The thickness of the uniform waveguide layer 21 is 2.63 μm, the length of the tapered region TR is 6000 μm,
The wavelength of the transmitted light is 0.6328μm (He-Ne laser light)
Is assumed.

Free surface of tapered waveguide layer 22 (boundary surface with air region)
As shown in the figure, the connecting portion with the uniform thickness waveguide layer 21 has the same layer thickness as the uniform layer thickness waveguide layer 21 as shown in the figure, and the taper is formed by decreasing the layer thickness as the distance from the connecting portion increases. doing.

However, as described above, since the thickness of the waveguide layer having a uniform thickness is extremely small as compared with the length of the tapered region, the inclination of the waveguide layer is extremely small even though it is tapered.

In the formation of the tapered region TR, various tapered shapes can be easily realized by controlling the moving speed of the mask by performing sputtering while moving the mask in the Z direction by the shadow mask sputtering method.

(Operation of the Invention) Hereinafter, the operation of the present invention will be described in relation to the embodiments.

The light wave propagating and propagating in the Z direction through the uniform thickness waveguide layer 21 has a wavelength
0.6328 μm, the guided mode is a TE ₀ mode which is a fundamental wave. Since the layer thickness of the waveguide layer 21 having a uniform thickness is 2.63 μm, the effective refractive index in this mode is N = 1.5107. In addition, we consider the propagation mode as a superposition of plane waves, and perform quasi-geometric optics consideration on the elementary waves as asymptotic fields. That is, a plane wave, which is an elementary wave, is handled as a set of light beams propagating in the normal direction of the wavefront. Using the effective refractive index, the incident TE ₀ mode is represented as a group of rays having a propagation angle θi of 86.42 degrees. In the following description, an angle is represented by an angle with the X axis unless otherwise specified.

The light beam propagating through the waveguide layer 21 having a uniform thickness propagates in a zigzag manner while repeating total reflection at the boundary surface with the substrate 10 and the boundary surface with the air region.

When such propagating light enters the tapered waveguide layer 22,
As shown in FIG. 2, the free surface of the tapered waveguide layer 22, that is, the tangent 2A at the interface with the air region is inclined with respect to the Z direction, so that it is totally reflected by the free surface and When re-entering the interface, the angle of incidence θ is
Is smaller than the propagation angle θi. As described above, in the tapered waveguide layer 22, the propagation angle becomes gradually smaller as the reflection in the tapered region TR is repeated.

On the other hand, in the embodiment, the critical angle of total reflection at the interface between the tapered waveguide layer 22 and the substrate 10 is 83.42 with respect to the propagation substrate 11.
The critical angle of total reflection at the interface between the tapered waveguide layer 22 and the air region is 41.14 degrees.

Here, for the sake of simplicity, assuming that the entire substrate is constituted by the propagation substrate 11, light incident on the tapered waveguide layer 22 propagates in a zigzag manner while repeating reflection,
10 and the angle of incidence on the boundary surface on the air region side, that is, the free surface, gradually decreases.
When it becomes smaller than 83.42 degrees, a part of the light power leaks to the substrate 10 side. The ratio of the leakage power at this time depends on the refractive index of the waveguide layer 20, the refractive index of the substrate 10, and the angle of incidence of the light beam on the reflection point, and may be calculated by a relational expression known as the power transmission coefficient of Fresnel. it can. As described above, the position at which power leakage starts is the radiation start position.

Further reflection is repeated, and when the angle of incidence on the air-area-side boundary surface becomes smaller than the critical angle 41.14 degrees, light leaks to the air region. Due to the large difference in the critical angle of reflection, substantially all of the light leaks to the substrate side after the light starts to leak to the substrate side and before the leak in the air region starts. Therefore, there is no need to consider leakage to the air region side.

The direction α of the light leaking to the substrate 10 can be known by Snell's law regarding refraction.

Now, assuming that the entire substrate is made of a propagation substrate as described above, the behavior of light leaking to the substrate side as described above, that is, radiation light, was examined as follows.

Referring again to FIG. 1, the light propagating through the uniform-thickness waveguide layer 21 uniformly propagates at a propagation angle θi of 86.42 degrees as described above. Therefore, when such light is incident on the tapered waveguide layer 22 at the connecting portion X axis, any incident light beam is X
It has an angle of θi with respect to the axis. Therefore, first, considering light that is incident on the tapered surface directly across the connecting portion obliquely rightward and upward, the total of this light is at the point 0 (connecting portion) in FIG. ) And light reflected at point P and incident on the tapered waveguide layer 22. The distance between point O and point P is 2.63 tan (86.42 degrees) = 42.04 μm using a layer thickness of 2.63. Becomes Similarly, the total of light rays that enter the connecting portion obliquely downward and to the right and that are reflected once at the boundary surface with the substrate 10 and then incident on the free surface of the tapered region are represented by points 0 and Q in FIG. It is the light that enters between. The distance between the zero point and the Q point is also 42.04 μm as described above.

Therefore, considering that the light rays incident on the tapered waveguide layer 22 through the connecting portion and reflected from the area IP in FIG. become.

Therefore, tracking was performed on a large number of rays totally reflected at an angle of θi from the area IP to the upper right.

First, in order to see the effect of the free surface shape of the tapered waveguide layer on the above-mentioned behavior, a third shape of the free surface was used.
Concave shape 4-1 as shown, linear shape 4-2, convex shape 4-3
Was assumed. These shapes have a = 2.63 μm, b = 1/6000
And letting the parameter be c, X = a ｛1− (b · Z) ^c ｝ (1) can be expressed as: concave shape 4-1, straight shape 4-2, convex shape
For 4-3, the parameter c is 0.5, 1, 2, respectively. As described in the embodiment, the thickness of the waveguide layer is uniform.
2.63 μm, and the length of the tapered region is 6000 μm.

In examining the behavior of the radiated light leaking to the substrate side, the degree of concentration of the radiated light in the substrate is examined. This degree of concentration is defined as the ray density as follows. That is, consider a plane Z _obs orthogonal to the Z axis as shown in FIG. 4, divide this plane Z _obs into sections of 2 μm in the X direction, count the number m of radiated rays passing through each section, and calculate the total radiated rays. The light density D is defined as the ratio (m / m _T ) to several m _T ≡D. The section having the maximum light density is called the maximum light density position, and the position of the maximum light density in the substrate is determined by setting the position of the plane Z _obs to 1 μm in the Z direction.
This is found by scanning in m steps. In examining the behavior of the emitted light, the waveguide layer accompanying the emission,
The power balance between the substrates is not taken into account. The reflected light in the waveguide layer and the radiated light in the substrate are traced with the power disregarded. The tracking is terminated when the angle of incidence of the reflected light on the substrate-side boundary surface becomes ≦ 0.

1 totally reflected at the position of X = Z = 0 in the area IP in FIG.
FIG. 5 shows the result of examining the propagation state in the substrate 10 of the light rays traced by the above-described method and a number of radiation rays leaking from the one light ray to the substrate side one after another. This figure is
The shape of the free surface of the waveguide layer on the taper is the convex surface 4-3 in FIG.
In the case of (c = 2), a group of radiation rays passing through the maximum ray density position (FIG. 5 (II)) and a group of radiation rays not passing through (FIG. 5 (I), (III)) are divided. It was found that the behavior was qualitatively the same regardless of whether the free surface shape of the tapered waveguide layer was convex, concave, or linear. That is, regardless of the shape of the free surface, in general, radiation that emits light rays that do not pass through the maximum ray density position is sandwiched by a radiation area (the area between AB in FIG. 5) that emits a ray group that passes through the maximum ray density position. There is an area.

However, when the position of incidence on the tapered waveguide layer 22 is moved within the region IP, how the above-mentioned maximum ray density position fluctuates is shown by three types of surface shapes shown in FIG. c = 0.5), a linear shape (c = 1), and a convex surface (c = 2) were examined. When the shape of the free surface was concave or linear, the starting position of the tracing ray was within the area IP. The maximum light density position fluctuated in both X and Z directions due to the fluctuation, but it did not fluctuate at all when the shape of the free surface was convex, and it was found that the convergence of the light was extremely stable.

FIG. 6 shows how the maximum ray density position varies with the variation of the starting position of the tracking ray. In FIGS. 1 (I) and 1 (II), the vertical axis Zi represents the Z coordinate of the starting point, and the horizontal axis represents the amount of change in the maximum ray density position in the Z and X directions, respectively. The broken line indicates the case where the free surface is concave (c = 0.5), and the solid line indicates the case where the free surface is linear (c = 1).

When the free surface shape is convex (c = 2), the maximum ray density position does not change at all with respect to the change of the starting point of the tracking ray. That is, the behavior of the emitted light beam is extremely stable with respect to the fluctuation of the starting position of the tracking light beam. Upon examination, it was found that this tendency is not limited to the case where c = 2, but also applies to cases where c = 3, 4,... And convex surfaces represented by trigonometric functions and the like.

The above description does not take into account the power budget in the tracing ray as described above. However, in actual radiation, of course, the power balance must be considered. Considering the power balance, there are the following problems.

That is, the radiation start position described above is located on the left side of the position indicated by the symbol A in FIG. This radiation start position is
10, the refractive index of the waveguide layer 20 is determined by the shape of the tapered region.If the entire substrate 10 is formed of the propagation substrate 11, light leaks from the radiation start position to the substrate side. Beginning, the amount of leakage is determined by the angle of incidence on the substrate and the reflectance. Therefore, when the power, that is, the light intensity, of each radiated light is calculated by the power transmission coefficient of Fresnel due to this leakage, the light propagates through the waveguide layer having a uniform thickness and radiates from the tapered waveguide layer to the substrate side. All of the power of the emitted light is emitted toward the substrate while the reflection and emission are repeated only about 10 times from the emission start position. As a result, in such a case, the power in the waveguide layer for each light beam for each light beam incident from the range of IP in FIG. 1 is radiated by at most about 10 radiated light beams. Focus on a relatively narrow space. FIG. 7 (I) schematically shows this state. However, as shown in the figure, the refraction angle of the radiated light from the vicinity of the radiation start position, that is, the radiated angle is close to 90 degrees, and therefore, the concentration of the power due to the convergence of the radiated light does not reach the waveguide layer 20 inside the substrate 10. Will occur at a location very close to the boundary surface. Therefore, in such a case, it is extremely difficult to set the concentration position of the power outside the substrate, and eventually, what can be taken out of the substrate is a divergent light beam.

In the present invention, the substrate 10 is composed of a propagation substrate 11 and a radiation substrate 12 having a larger refractive index than the propagation substrate 11, and the connection portion thereof is close to the radiation start position, and this radiation start position is in the above-mentioned waveguide direction. By setting so as not to exceed, the power concentration position of the radiated light can be set at a position distant from the boundary surface between the substrate and the waveguide layer.

Referring again to FIG. 1, the free surface shape of the tapered waveguide layer 22 in this embodiment is the same as the convex surface 4-3 in FIG. Radiation start position R on the substrate side when a laser beam having a wavelength of 0.6328 μm is guided to the waveguide layer 20 in the TE ₀ waveguide mode.
Is located 4058 μm from the position of Z = 0.

In the embodiment, the connecting portion between the propagation substrate 11 and the radiation substrate 12 is set at a position slightly shifted to the X-axis side from the radiation starting position R, but the setting position of the connecting portion is set at the radiation starting position as described above. Since it is a condition that it is close to and does not exceed this radiation start position in the above-mentioned waveguide direction, the position of the connecting portion can be set near the radiation start position R with the position of the radiation start position R as a limit.

Since the refractive index of the radiating substrate 12 is larger than the refractive index of the propagating substrate, when the propagating light is reflected by the tapered waveguide layer 22 and enters the radiating substrate 12, radiation is started into the radiating substrate 12.

If the whole board is composed of the propagation board,
As described above, the direction of the radiation beam emitted at the radiation start position is close to the interface between the substrate and the waveguide layer, and the power concentration occurs very near the interface (FIG. 7 (I)).
), The radiating substrate 12 has a higher refractive index than the propagating substrate, and therefore the difference in refractive index from the waveguide layer 20 is small. For this reason, the refraction angle of the radiation ray leaking into the radiation substrate 12, that is, the radiation angle, is smaller than the radiation ray radiated into the propagation substrate at the radiation start position, and as shown in FIG. The direction in which the power rises with respect to the surface and the power is concentrated due to the intersection of the radiated light rays is located at a position distant from the boundary surface.Therefore, the power of the radiated light rays may be actually concentrated outside the substrate. Will be possible. Since the radiation into the radiation substrate 12 starts near the radiation start position, in this state, the interval between the radiation positions of the adjacent radiation rays is relatively wide, and therefore, it is possible to focus radiation from a relatively wide area. it can.

The essence of the present invention is to change the propagation angle in the region I and start the radiation in the region II, as shown in FIG. The point is that a small radiating substrate is used in the region II, and the position of the power concentration is moved away from the substrate surface by extracting power with the radiated light beam rising to the substrate surface.

In the embodiment, the shape of the free surface of the tapered waveguide layer is a convex shape, but is not limited to this. But,
When a convex free surface is used, the stability of the power concentration position is improved.

(Effect of the Invention) As described above, according to the present invention, a novel tapered waveguide can be provided. In this tapered waveguide, the substrate is composed of a propagation substrate and a radiation substrate having a larger refractive index than that, and a radiation beam with a small radiation angle is extracted into the radiation substrate. It can be located at a position away from the boundary surface, and the stability of power concentration can be enhanced by making the free surface shape of the tapered waveguide layer convex as in the second aspect of the present invention. Therefore, applicability to an optical head for recording / reproducing optical disk information is improved.

FIG. 9 shows an example of an optical head using the tapered waveguide of the present invention. Reference numeral 50 indicates an optical disk. Since the light extracted to the substrate side of the tapered waveguide has no focusing property in the waveguide width direction (the direction orthogonal to the drawing in FIG. 1), the light extracted to the substrate side is spotted on the optical disc 50. In this example, the cylindrical lens 40 is used as an anamorphic optical system for focusing. As another example, a spot-shaped focal point can be realized by setting the shape of the tapered waveguide layer to a three-dimensional curved surface.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating one embodiment of the present invention, and FIG.
8 are diagrams illustrating the operation of the present invention, and FIG. 9 is a diagram illustrating an example of an optical head using the present invention. 10 ... board, 11 ... propagation board, 12 ... radiation board, 20 ...
Waveguide layer

Claims (2)

(57) [Claims] 1. A substrate having a plane for forming a waveguide layer and transparent to guided light, and a waveguide layer formed on the plane of the substrate and having a higher refractive index than the substrate. Wherein the waveguide layer has a uniform layer thickness waveguide layer having a uniform layer thickness, and is connected to the uniform layer thickness waveguide layer. The layer thickness gradually decreases as the connection part is separated, and the tapered waveguide layer emits guided light propagating through the uniform layer thickness waveguide layer to the substrate side, and the substrate forms the waveguide layer. A propagation substrate and a radiation substrate sharing a plane for use are connected in this order in the waveguide direction in the waveguide layer, and the radiation substrate has a larger refractive index than the propagation substrate. The connection between the radiation substrate and the radiation substrate is determined by the refractive index between the propagation substrate and the waveguide layer and the shape of the tapered waveguide layer. Close to the position, and a tapered waveguide of the radiation start position, characterized in that it is set so as not to exceed in the above-mentioned waveguide direction. 2. The method according to claim 1, wherein the free surface of the tapered waveguide layer protrudes from the substrate so as to form a stable maximum light density position of the light beam extracted toward the substrate. A tapered waveguide characterized by having a convex shape.