JPH05288943A - Curved optical waveguide and its production - Google Patents

Abstract

PURPOSE:To suppress the propagation loss of a guided wave due to unevenness of an etching surface and the radiation loss due to the curvature by decreasing the thickness-directional equivalent refractive index of at least an outside clad nearby the curved part of a core with the increase of distance from the core. CONSTITUTION:The outside clad 5 is gradually reduced in thickness and an inside clad 6 is gradually increased in thickness so as to correct the refractive index distribution. Therefore, when this curved optical waveguide is mapped on a straight optical waveguide at an equal angle, the outside and inside refractive index distributions of the curved part in the section become constant. Further, the inclinations of the thicknesses of the clads 5 and 6 are so determined that the refractive index distribution becomes flat at the time of conversion into the straight optical waveguide by the equalangle mapping. In this case, the outside nearby part of the curved part is etched a little deeper than the inside nearby part so as to equivalently correct the inclination of the refractive index distribution generated at the mesa part of the curved part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a curved optical waveguide for changing the traveling direction of light, and more particularly to a curved optical waveguide having an extremely low loss.

[0002]

Bent optical waveguides are used to change the optical path of guided light. However, conventional bent waveguides cause radiation loss at the bent portions. First, taking a bent optical waveguide without a loss suppression measure as an example, the cause of the bending loss is considered, and the conventionally used countermeasures and their drawbacks are described.

FIG. 15 shows an example of a bent optical waveguide without a loss suppression measure (a ridge type optical waveguide is taken as an example of the waveguide). Here, in the figure, 1 is a core (as a material,
For example, InGaAs / InAlAs MQW and InGa
It only needs to have a refractive index higher than that of the cladding such as AsP), 2
Is an upper InP clad, 3 is a lower InP clad, 4 is an InP substrate, 5 is an outer side clad, 6 is an inner side clad, and 7 is air. Here, the curved outer side clad 5 and the curved inner side clad 6 have the same thickness. The guided light propagates in the core 1 along the upper clad 2. With respect to the optical waveguide of FIG. 15, FIG. 16 shows an equivalent refractive index distribution obtained by converting a curved optical waveguide into a linear optical waveguide by using the conformal mapping method and a field distribution of guided light propagating in the curved portion.
Shown in. The polygonal line A is the equivalent refractive index distribution, and the curve B is the guided light field distribution.

That is, the stepwise refractive index distribution of the curved optical waveguide having the radius of curvature R is converted into a refractive index distribution by this conformal mapping.

[0005]

## _EQU1 ## n _curv (x, s) = n (x, s) (1 + x / R) (1) is converted into a linear optical waveguide. Here, x is the distance in the width direction with the center in the thickness direction of the core 1 and the center in the width direction of the clad 2 as the origin in the cross section shown in FIG.
s is the length along the light propagation direction, and n (x, s) is the actual refractive index distribution in the waveguide cross section. FIG.
As shown in the figure, since the field distribution of the guided light is biased to the outside of the bent portion, the power of the light is radiated to the outside of the bent portion, resulting in radiation loss in the bent portion.

FIG. 17 shows a conventional example for suppressing the radiation loss at this bent portion. As can be seen from the figure, in this conventional example, the outer side cladding 5 of the bent portion is deeply etched as compared with the inner side cladding 6, so that the equivalent refractive index in the thickness direction on the outer side is changed to the equivalent refractive index in the thickness direction on the inner side. To lower the radiation loss (ie, bending loss). FIG. 18 shows an equivalent refractive index distribution (A) and a guided light field distribution (B) when the curved optical waveguide is converted into a linear optical waveguide.

[0007]

However, in this conventional example, since the clad near the core of the optical waveguide is deeply etched, a propagation loss occurs due to the roughness of the etching surface. Further, if deep etching is performed to suppress the radiation loss due to bending as much as possible, there is a drawback that the single mode propagation condition cannot be maintained and multimode propagation occurs.

Therefore, an object of the present invention is to solve these problems and to provide a bent optical waveguide and a manufacturing method thereof which are excellent in terms of propagation loss due to roughness of an etching surface and radiation loss due to bending. is there.

[0009]

In order to achieve such an object, a curved optical waveguide according to the present invention is an optical waveguide having at least a core and a clad, and at least an outer clad near the curved part of the core. The equivalent refractive index in the thickness direction decreases as the distance from the core increases.

It is desirable that the equivalent refractive index in the thickness direction inside the bent portion increases as the distance from the core increases.

Further, it is preferable that at least one of the core and the clad has an inclined thickness or a plurality of steps.

A method of manufacturing a bent optical waveguide according to the present invention is as follows.
The method is characterized by including a step of etching at least one of the core and the clad using a mask having a thickness that changes stepwise or gradually.

A step of changing the thickness of the core or the clad in a stepwise manner by using at least one of wet etching and dry etching using an etch stop layer, and adjusting the thickness of at least one of the core and the clad. It is more preferable to include the step of removing the mask and re-etching to change the shape of the thickness gently after changing the thickness by etching.

[0014]

According to the present invention, an extremely low loss (lossless in an ideal shape) curved optical waveguide can be realized by giving a refractive index distribution so as to correct the field distribution of guided light in the curved portion.

[0015]

Embodiments of the present invention will be described in detail below with reference to the drawings.

Embodiment 1 FIG. 1 is a schematic partial perspective view for explaining a first embodiment of a curved optical waveguide according to the present invention.

In the first embodiment of the present invention, as shown in FIG. 1, the outer clad 5 is gradually thinned and the inner clad 6 is gradually thinned so as to correct the refractive index distribution given by the equation (1). It is thick. Therefore, when this curved optical waveguide is conformally mapped to a linear optical waveguide, the refractive index distribution in the cross section becomes constant as shown by A in FIG. 2 on the outside and inside of the curved portion. Therefore, the deviation of the field distribution B of the guided light is remarkably improved as compared with the conventional example, and the radiation loss due to the bending can be suppressed. Clad 5 and 6
The slope of the thickness makes the refractive index distribution flat when converted into a linear optical waveguide by conformal mapping. That is, in the case of a two-dimensional problem, the inclination is 1 / R with respect to the radius of curvature R. In the example of FIG. 2, in order to equivalently correct the gradient of the refractive index distribution generated in the mesa portion of the curved portion in the figure, the outermost portion of the curved portion is etched a little deeper than the innermost portion thereof. ..

Embodiment 2 FIG. 3 is a second embodiment in which the present invention is applied to a double core embedded type optical waveguide. In the figure, 1 is a first core and 8 is a second core. In addition, as the second core, an InGaAsP layer or MQW may be used. Second core 8
Is effective for confining the guided light in the width direction. The thickness of the second core 8 is thick inside the bent portion and thin outside.
Its upper surface is preferably parallel to the upper surfaces of the claddings 5 and 6. The thickness of the second core 8 is arbitrary. Further, the second core 8 may be arranged below the first core 1. The transformed refractive index profile obtained as a result of the conformal mapping in this case is shown in FIG. As can be seen from the figure, in this case, a completely flat refractive index distribution can be obtained, and as a result, a lossless curved optical waveguide can be realized.

Embodiment 3 FIG. 5 is a third embodiment of the present invention in which a region where a field distribution of guided light exists inside and outside a curved portion is etched stepwise. The height and width of each step are appropriately selected, but the average slope formed by the steps should be equal to the slope described in the first embodiment. FIG. 6 shows a converted refractive index distribution (A) and a guided light field distribution (B) obtained by conformally mapping the curved waveguide of FIG. 5 into a linear optical waveguide. As can be seen from the figure, although the refractive index distribution is wavy, the average refractive index distribution is constant, and the radiation loss due to bending can be greatly reduced. Moreover, since this step-like shape is along the wave propagation direction, the propagation loss is not increased.

Embodiment 4 FIG. 7 is a fourth embodiment in which the present invention is applied to a double core embedded type optical waveguide, and the regions where the field distribution of the guided light exists inside and outside the curved portion are made stepwise. Etch. The thickness of the second core 8 is uniform. FIG. 8 shows the converted refractive index distribution (A) and the guided light field distribution (B) obtained as a result of the conformal mapping in this case. Also in this case, as in the case of Example 3 shown in FIG. 6, although the refractive index distribution becomes wavy, the average refractive index distribution becomes constant, and the radiation loss due to bending can be significantly reduced. Become.

Example 5 and Example 6 FIGS. 8 and 9 are examples in which the first core 1 has a thickness distribution. That is, in the fifth embodiment shown in FIG. 8, the thickness of the first core 1 is uniformly thinned from the inside to the outside of the bent portion, and the second core 8 having a uniform thickness is formed thereon.
In the sixth embodiment shown in FIG. 9, the first
The core 1 and the second core 8 are both stepwise etched to reduce the bending loss. In this case, even in the completely filled state, the same effect as the case of changing the thickness of the clad described so far can be obtained.

Example of Manufacturing Method The manufacturing method of the present invention in which the thickness shown in FIG. 1 is inclined will be described.

An InP layer, for example, having a thickness of 1 μm is deposited as a lower clad on the InP substrate by a known method such as MOVPE method. Then, as a core, for example, InGa
An As / InAlAs MQW layer is deposited to a total thickness of 0.4 μm, and InP is used as an upper clad to a thickness of 1.
Deposit 5 μm. Next, a method for inclining the upper clad (outer and inner clads) will be described with reference to FIG. Reference numerals 9 and 10 respectively denote a shadow mask and a spacer formed of SiO _2, etc., and 11 denotes, for example, a vapor-deposited alumina thin film mask. Reference numeral 12 is a semiconductor growth film to be etched, and in this case, the outer and inner claddings. As can be seen from the figure, the alumina thin film 11 vapor-deposited through the shadow mask 9 has an inclination in the thickness direction. After that, the shadow mask 9 and the spacer 10 are removed, and then the whole is dry-etched. At this time, since the thin portion of the alumina thin film mask 11 disappears early, the etching amount increases after a certain period of time, and the outer and inner clads having the inclined thickness can be realized. In the case of the present invention, etching may be performed using the mask 11 formed by applying the shadow mask 9 having the shape of the bent portion of the optical waveguide. Further, it is needless to say that, for example, the alumina thin film is uniformly formed on the entire surface, and then the alumina thin film is dry-etched stepwise, and then the same method as above can be used to realize a shape with a changed thickness. After that, the upper clad 2 shown in FIG. 1 is formed. The second core 8 having the inclined thickness shown in FIG. 3 and the core 1 having the inclined thickness shown in FIG. 8 can be formed in the same direction.

In order to change the thickness shown in FIGS. 5, 7 and 9 in a stepwise manner, FIGS.
As shown in FIG. 4, the etch stop layer 1 is formed on each layer in the thickness direction.
3 is put in advance, and wet etching may be performed using the photoresist as a mask. As an etch stop layer,
For the InP clad, an InGaAsP layer and InG
An InP layer is used for the aAs / InAlAs MQW layer. In the case of dry etching, the etch stop layer is unnecessary.

In the final stage, if the whole is dry-etched, the edge portion of the staircase is quickly etched, so that the staircase has a gentle shape.

In the above description, the thickness distribution is provided both on the outside and inside of the bend. However, since the refractive index distribution inside the bend does not significantly affect the radiation loss, it is only on the outside of the bend. Even if the present invention is applied, there is a remarkable effect in reducing the bending loss.

[0027]

As described above, according to the present invention, unlike the conventional example, since the equivalent refractive index in the thickness direction is gradually changed in the radial direction, the guided light caused by the roughness of the etching surface is not generated. Propagation loss can be suppressed. Further, since the vicinity of the waveguide is not deeply etched, the single mode propagation condition is not broken. Therefore, it is possible to realize a curved waveguide with a low loss and a small radius of curvature.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram illustrating the principle of the first embodiment of the present invention.

FIG. 3 is a schematic diagram showing a second embodiment of the present invention.

FIG. 4 is a diagram illustrating the principle of the second embodiment of the present invention.

FIG. 5 is a schematic view showing a third embodiment of the present invention.

FIG. 6 is a diagram for explaining the principle of the third embodiment of the present invention.

FIG. 7 is a schematic view showing a fourth embodiment of the present invention.

FIG. 8 is a diagram for explaining the principle of the fourth embodiment of the present invention.

FIG. 9 is a schematic view showing a fifth embodiment of the present invention.

FIG. 10 is a schematic view showing a sixth embodiment of the present invention.

FIG. 11 is a schematic diagram illustrating an example relating to the production method of the present invention.

FIG. 12 is a schematic view illustrating another example of the production method of the present invention.

FIG. 13 is a schematic view illustrating another example of the production method of the present invention.

FIG. 14 is a schematic view illustrating another example of the production method of the present invention.

FIG. 15 is a schematic view showing an example of a conventional bent optical waveguide.

16 is a diagram illustrating the principle of the conventional example shown in FIG.

FIG. 17 is a schematic view showing an example of another conventional curved optical waveguide.

FIG. 18 is a diagram illustrating the principle of the conventional example shown in FIG.

[Explanation of symbols]

 1 core 2 upper InP clad 3 lower InP clad 4 InP substrate 5 outer side clad 6 inner side clad 7 air 8 second core 9 shadow mask 10 spacer 11 alumina mask 12 semiconductor growth film 13 etch stop layer

Claims (6)

[Claims] 1. In a curved optical waveguide having at least a core and a clad, the equivalent refractive index in the thickness direction of at least the outer clad near the curved part of the core becomes lower as the distance from the core increases. A curved optical waveguide characterized in that 2. The curved optical waveguide according to claim 1, wherein the equivalent refractive index in the thickness direction of at least the inner cladding near the curved portion increases as the distance from the core increases. A bent optical waveguide. 3. The curved optical waveguide according to claim 1, wherein the thickness of at least one of the core and the clad has an inclination or changes in a plurality of steps. Curved optical waveguide. 4. The method for producing a curved optical waveguide according to claim 1, wherein the core or the clad is formed by using a mask whose thickness is changed stepwise or inclined. A method for manufacturing a bent optical waveguide, comprising the step of etching at least one of the above. 5. The method for manufacturing a bent optical waveguide according to claim 1, wherein the core or the clad is formed by using at least one of wet etching and dry etching using an etch stop layer. A method of manufacturing a bent optical waveguide, comprising the step of changing the thickness in a stepwise manner. 6. The method for manufacturing a curved optical waveguide according to claim 1, wherein at least one of the core and the clad is changed in thickness by etching, and then the mask is formed. A method of manufacturing a bent optical waveguide, comprising a step of grading a change in thickness by removing and etching again.