JP2589367B2 - Optical branching / combining circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical branching / combining circuit comprising a Y-branch optical waveguide formed on a substrate and performing light branching and multiplexing. .

(Prior Art) In a light control circuit such as an optical modulator or an optical switch, an optical branching circuit and an optical multiplexing circuit are indispensable as basic components. As such an optical branching / combining circuit, a Y-branch optical waveguide having a main waveguide and two branch waveguides obtained by branching the main waveguide has been conventionally known. It is extremely important for performance improvement.

FIG. 2A is a configuration diagram showing a conventional optical branching / combining circuit including a Y-branch optical waveguide. In the figure, 1 is a substrate, 2 is a main waveguide, 3 and 4 are branch waveguides, 5 is a branch point, 6
Is a tapered portion. Optical branching-combining circuit in (a) of FIG. 2 is to the branching angle between 2 [Theta] _1, the main waveguide 2 having a width W ₂
And the widths W ₃ and W _{4 of the} branch waveguides 3 and 4 are equalized, and the width of the tip of the so-called wedge-shaped portion near the branch point 5 (hereinafter referred to as the tip width) becomes an ideal shape of zero. ing.

This type of optical branching / combining circuit is manufactured by, for example, forming a Y-shape to be the main waveguide 2, the branch waveguides 3 and 4, and the tapered portion 6 on a substrate 1 made of an electro-optical material such as LiNbO _3. The impurity such as Ti is diffused in the region of the above, or ion conversion is performed, and the refractive index (ng) of the Y-shaped region is set to the substrate 1.
This is performed by forming a waveguide with a refractive index (ns) larger than the above.

For example, an operation as an optical branch circuit in such a configuration will be described. Light propagating from the main waveguide 2 side is partially radiated by mode conversion or the like in the tapered portion 6,
The rest reaches junction 5. Further, the light passing through the branch point 5 is branched into the branch waveguides 3 and 4 and propagates.
At this time, ideally, each branch light needs to be inclined by (+) θ ₁ and (−) θ _{1 in} the branch waveguides 3 and 4, respectively.

(Problems to be solved by the invention) However, when an optical branching / combining circuit composed of a Y-shaped branching optical waveguide is actually manufactured, the Ti film thickness, Ti pattern width,
In the refractive index distribution of the tapered portion 6 determined by the diffusion conditions and the like, the slope of the wavefront cannot be appropriately controlled, resulting in radiation loss.

Figure 3 is a graph showing the relationship between the branch loss caused by branching angle 2 [Theta] ₁ and the branch point 5 of the optical branching-combining circuit of FIG. 2 (a), the horizontal axis represents branching angle 2 [Theta] _1, the vertical axis Represents a branch loss. In this graph, the wavelength of light λ is 1.52μ.
m, the refractive index ns of the substrate 1 is 2.146, and the Ti pattern width is 8 μm.
The calculation results are shown for the case where m and the Ti pattern interval are 15 μm and the Ti film thickness DTI is a parameter (in this case, the length of the so-called Y branch portion is about 2 mm).

As can be seen from Figure 3, in order to suppress branching loss, even when setting the branching angle 2 [Theta] ₁ to less than 1 degree, branching loss becomes about 1 dB, can not be suppressed to a satisfactory value. As a result, in Ti vapor deposition, a variation in the thickness of the Ti film occurs, so that there has been a problem that it is impossible to stably manufacture a light-branching / combining circuit comprising a low-loss Y-branch optical waveguide.

Furthermore, in the process of exposure and diffusion or the like, the tip width of the wedge-shaped portion in the vicinity of the branch point 5 is, not zero, as shown in of FIG. 2 (b), with a predetermined width W ₅ forms In this case, a large branch loss will occur.

Figure 4 is a graph showing the relationship between the tip width W ₅ of the optical branching-combining circuit and the branch loss of FIG. 2 (b), the horizontal axis tip width W _5, the vertical axis represents branching loss And the branch angle 2θ ₁
The calculation result when 0.6 degrees is set is shown.

As can be seen from FIG. 4, for example, the Ti film thickness DTI is 80 mm,
If the tip width W ₅ is 3 [mu] m, the branch loss has increased 1.6 dB.

The present invention has been made in view of such circumstances,
An object of the present invention is to provide an optical branching / combining circuit with reduced branch loss.

(Means for Solving the Problems) In order to achieve the above object, according to the present invention, a light comprising a Y-shaped branched optical waveguide formed by forming a main waveguide and two branched waveguides obtained by branching the main waveguide on a substrate. In the branching / combining circuit, the branch point of the Y branch portion of the two branch waveguides is set as the origin,
When the Z coordinate is taken from the main waveguide toward the branch waveguide, the length of the Y branch portion is L, the width of the waveguide is W, and the distance between the branch waveguides is G. The shape based on the raised cosine function of F ₀ (Z) = G [1−cos (πZ / L)] L / 4, and the shape outside the Y branch portion is F ₁ (Z) = W / 2 + (W + G ) · [1-cos (πZ / L)] / 4 The shape was based on the raised cosine function.

(Operation) According to the present invention, the inclination of the waveguide near the branch point can be asymptotically reduced to zero by making the shape of the Y branch portion a shape based on the raised cosine function. The inner refractive index can be made smaller than the refractive index of

(Embodiment) FIG. 1 is a block diagram showing a first embodiment of an optical branching / combining circuit according to the present invention, and the same components as those in FIG. That is, 1 is a substrate made of LiNbO ₃ , 2 is a main waveguide formed by a Ti diffusion method, 3
Similarly, a and 4a are branch waveguides formed to have the same width by the Ti diffusion method, 5a is a branch point, and the main waveguide 2 and the branch waveguides 3a and 4a are single mode optical waveguides.

The optical branching / combining circuit of FIG. 1 includes branching waveguides 3a, 4a
In FIG. 1, the shape of the portion indicated by the length L, the so-called Y-branch, is defined by taking the Z coordinate from the main waveguide 2 toward the branch waveguides 3a and 4a with the origin at the branch point 5a of the Y-branch. , The raised cosine shown in the following equations (1) and (2)
e) It is formed into a shape based on a function.

F ₀ (Z) = G [1-cos (πZ / L)] / 4 (1) F ₁ (Z) = W / 2 + (W + G) · [1-cos (πZ / L)] / 4. ... (2) Expression (1) shows the shape inside the Y-branch, and Expression (2) shows the shape outside the Y-branch. Here, G is the distance between the branch waveguides 3a and 4a (hereinafter, referred to as Ti pattern gap), and W is the waveguide width (hereinafter, Ti pattern gap).
Pattern width).

The optical branching / combining circuit having such a configuration is manufactured by the following procedure.

First, on a LiNbO ₃ substrate 1 by a normal lift-off method
A Ti Y-shaped optical waveguide pattern is formed. That is, for example, Li
After exposing a photoresist on the NbO ₃ substrate 1, a groove having the same width as the optical waveguide pattern and having a width of several μm is formed by developing. Next, after depositing several hundreds of Ti on the entire surface, the resist is removed with a remover to form an optical waveguide pattern of Ti. Then, for several hours, bring the temperature to about 10
The temperature is raised to about 00 ° C. to diffuse Ti into the LiNbO ₃ substrate 1. As a result, as shown in FIG. 1, the Ti-diffused LiNbO ₃ optical waveguide 2,
3a and 4a are formed, and the fabrication of the optical branching / combining circuit is completed.

FIG. 5 is a graph showing the relationship between the length L of the Y branch portion and the branch loss in the optical branching / combining circuit of FIG. 1. The horizontal axis is the length L of the Y branch portion, and the vertical axis is the branch loss. Is represented. The graph shows that the Ti film thickness DTI is 60 nm, 70 nm, 80 n when the light wavelength λ is 1.52 μm, the refractive index ns of the substrate 1 is 2.146, the Ti pattern width W is 8 μm, and the Ti pattern gap G is 15 μm.
Each calculation result at m is shown.

As can be seen from FIG. 5, the optical branching device of the first embodiment
In the multiplexing circuit, the branch loss is reduced uniformly with respect to the length L of the Y-branch, and the length L of the multiplexing circuit is substantially equal to the length of the Y-branch of the conventional optical branching / multiplexing circuit shown in FIG. = 2 mm, the branch loss is almost constant at 0.2 dB or less, which is a lower loss optical branching (multiplexing) circuit with significantly reduced branch loss compared to conventional circuits. It has become.

Also, FIG. 6 is a graph showing the relationship between the tip width W ₅ and branch losses shown in the second diagram of an optical branching-combining circuit of Fig. 1 (b), the horizontal axis tip width W ₅ The vertical axis represents the branch loss. The graph shows that the Ti film thickness DTI was 6 as in FIG.
Each calculation result at 0 nm, 70 nm, and 80 nm is shown.

As can be seen from FIG. 6, for example, the Ti film thickness DTI is 80 n
m, when the tip width W ₅ is 3 [mu] m, an increase in branch loss, slight
0.5dB.

That is, according to the first embodiment, since the inclination θ of the waveguide near the branch point 5a gradually approaches zero, an increase in loss due to the inclination of the wavefront can be reduced. Further, near the branch point 5a, the inner refractive index is smaller than the outer refractive index, and the phase velocity of light propagating inside becomes faster than the outer phase velocity, thereby tilting the wavefront, thereby reducing loss. Can be achieved.

FIG. 7 is a configuration diagram showing a second embodiment of the optical branching / combining circuit according to the present invention. Different points of the the present second embodiment the first embodiment (in this second embodiment, W _3a> W _4a) width W _3a and the branch waveguides 4a of the branch waveguides 3a and is different from the (However, the width W ₂ of the main waveguide ₂ = W _4a ).

In this case, by appropriately setting the parameters of the raised cosine function, the ratio of the output light power of the branch waveguides 3a and 4a can be reduced to 1: 1 with low loss.

In the first and second embodiments, the configuration as a so-called optical branch circuit has been described as an example. However, the same effect can be obtained with an optical multiplexing circuit with the completely same configuration.

In the first and second embodiments, the Ti-diffused LiNbO ₃
Although the waveguide has been described, it goes without saying that the present invention can be applied to other waveguides such as a proton exchange waveguide. Furthermore, it goes without saying that the present invention can be applied even when the refractive index distribution has a stepped shape, such as a glass waveguide or a semiconductor waveguide.

In the first and second embodiments, a single mode optical waveguide is described as an example. However, it goes without saying that the present invention can be applied to a multimode optical waveguide.

(Effects of the Invention) As described above, according to the present invention, the shape of the Y-branch is based on the raised cosine function, so that, for example, the slope of the waveguide near the branch (combination) point is asymptotic to zero. Can and
Since the inner refractive index can be made smaller than the outer refractive index in the vicinity of the branch point, the tilt of the light wavefront can be appropriately controlled, and a low-loss optical branching / combining circuit with reduced branch loss can be provided. There is.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of an optical branching / combining circuit according to the present invention, FIG. 2 is a block diagram of a conventional optical branching / combining circuit, and FIG. FIG. 4 is a graph showing a relationship between a branch angle and a branch loss in a multiplexing circuit, FIG. 4 is a graph showing a relationship between a tip width and a branch loss in a conventional optical branching / multiplexing circuit, and FIG. FIG. 6 is a graph showing the relationship between the length of the Y-branch portion of the branch / combination circuit and the branch loss; FIG. 6 is a graph showing the relationship between the tip width of the optical branch / combination circuit and the branch loss;
FIG. 7 is a configuration diagram showing a second embodiment of the optical branching / combining circuit according to the present invention. In the figure, 1 ... substrate, 2 ... main waveguide, 3a, 4a ... branch waveguide, 5a ... branch point.

Claims (1)

(57) [Claims] 1. An optical branching / combining circuit comprising a Y-branch optical waveguide in which a main waveguide and two branch waveguides obtained by branching the main waveguide are formed on a substrate. Taking the branch point of the branch as the origin, taking the Z coordinate from the main waveguide to the branch waveguide, the length of the Y branch is L, the width of the waveguide is W, and the distance between the branch waveguides is G.
, The shape inside the Y-branch is based on the raised cosine function of F ₀ (Z) = G [1−cos (πZ / L)] / 4, and the shape outside the Y-branch is An optical branching / combining circuit having a shape based on a raised cosine function of F ₁ (Z) = W / 2 + (W + G) · [1-cos (πZ / L)] / 4.