JPH02234108A - Optical branching and multiplexing circuit - Google Patents

Abstract

PURPOSE:To obtain an optical branching and demultiplexing circuit which is reduced in branching loss by forming >=2 branching optical waveguides on a substrate in a shape based upon a square-law cosine function. CONSTITUTION:The optical branching and multiplexing circuit is constituted of the substrate 1, a main waveguide 2 which is formed by a Ti diffusing method, the branching waveguides 3a and 4a which are formed by the Ti diffusing method as well to equal width, and a branching point 5a, and those main waveguide 2 and branching waveguides 3a and 4a are single-mode optical waveguides. Then the optical branching and multiplexing circuit is formed so that the shape of the part of the branching waveguides 3a and 4a shown by length L, i.e. what is called a Y branching part is based upon the square-law cosine function. Consequently, the inclination of the optical waveguides 3a and 4a in the vicinity of the branching (multiplexing) point is decreased gradually to zero and the inside refractive index can be made smaller than the outside refractive index in the vicinity of the branching point, so the inclination of the wave front of light can be controlled properly and the optical branching and demultiplexing circuit which is reduced in branching loss is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an optical branching/multiplexing circuit that is composed of two or more branching optical waveguides formed on a substrate and performs branching and multiplexing of light. It is.

(Prior Art) In optical control circuits such as optical modulators and optical switches, optical branching circuits and optical multiplexing circuits are essential as basic components. Conventionally, such optical branching/multiplexing circuits have two
Y-shaped branch optical waveguides having more than one branch leading waveguide are known, and reducing the loss thereof is extremely important for improving the performance of optical control circuits.

FIG. 2(a) is a configuration diagram showing an optical branching/multiplexing circuit consisting of a conventional Y-shaped branch leading waveguide. In the figure, 1 is the substrate,
2 is a main waveguide, 3 and 4 are branch waveguides, 5 is a branch point, and 6 is a taper portion. The optical branching/combining circuit in FIG. 2(a) has a branching angle of 2θ, a width W2 of the main waveguide 2 and a width W3 and W4 of the branching waveguides 3 and 4, and
The width of the tip of the so-called wedge-shaped part near branch point 5 (
It has an ideal shape in which the tip width (hereinafter referred to as tip width) is zero.

The production of this type of optical branching/multiplexing circuit involves, for example, attaching the main waveguide 2, the branching waveguides 3 and 4, and the tapered part 6 to a substrate 1 made of L i N b O 3, which is an electro-optic material. By diffusing impurities such as Ti into the desired Y-shaped region, or by performing ion conversion 4, the refractive index (ng) of the Y-shaped region is made larger than the refractive index (ns) of the substrate 1, and a waveguide is formed. This is done by forming a

To explain the operation of such a configuration, for example, as an optical branching circuit, part of the light propagating from the main waveguide 2 side is radiated by mode conversion etc. in the taper part 6, and the rest reaches the branching point 5. Furthermore, the light that has passed through the branch point 5 is split into branch waveguides 3 and 4 and propagated therein. At this time, ideally each branched light beam should be tilted by an angle of (+) θ and (−) θ1 in the branch waveguides 3 and 4, respectively.

(Problem to be Solved by the Invention) However, when an optical branching/multiplexing circuit consisting of a Y-shaped branch leading waveguide is actually manufactured, the taper portion 6 is determined by the T1 film thickness, TI pattern width, diffusion conditions, etc. In the refractive index distribution, the inclination of the wavefront cannot be properly controlled, resulting in radiation loss.

FIG. 3 is a graph showing the relationship between the branching angle 2θ1 of the optical branching/combining circuit shown in FIG. represents loss. In addition, this graph assumes that the wavelength λ of light is 1.52
The calculation results are shown when the refractive index OS of μm1 substrate 1 is 2.146, the TI pattern width is 8 μm%, the Ti pattern spacing is 15 μm, and the Ti film thickness DTI is used as a parameter (in this case, the so-called The length of the Y-branch is about 2 steps). As can be seen from Figure 3, in order to suppress branch loss,
Even when the branching angle 2θ1 is set to 1 degree or less, the branching loss is about 1 dB, which cannot be suppressed to a satisfactory value. As a result, in Ti evaporation, variations in Tf film thickness occur, resulting in the problem that it is not possible to stably manufacture an optical branching/multiplexing circuit consisting of a low-loss Y-shaped branch leading waveguide.

Furthermore, in processes such as exposure and diffusion, the width of the tip of the wedge-shaped portion near the branching point 5 is
As shown in a), if the width is not zero and is formed with a predetermined width W5, a large branching loss will occur.

Figure 4 is a graph showing the relationship between the tip width W5 and the branching loss of the optical branching/combining circuit shown in Figure 2(b), where the horizontal axis represents the tip width W, and the vertical axis represents the branching loss. , shows the calculation results when the branching angle 2θ1 is 0.6 degrees.

As can be seen from FIG. 4, for example, the Ti film thickness DTl is 8
0mm, and when the tip width W5 is 3μm, the branch loss is 1.6
dB is also increasing.

The object of the present invention has been made in view of the above circumstances, and the object is to provide an optical branching/multiplexing circuit 9 in which branching loss is reduced.

(Means for Solving the Problems) In order to achieve the above object, in the present invention, in an optical branching/multiplexing circuit consisting of two or more branching leading waveguides formed on a substrate, the branching optical waveguides are squared. The shape was based on a cosine function.

(Operation) According to the present invention, a curved branch leading wavepath given by a raised cosine function is formed.

(Embodiment) FIG. 1 is a block diagram showing a first embodiment of an optical branching/multiplexing circuit according to the present invention, and the same components as those in FIG. 2 showing a conventional example have the same reference numerals. Expressed as

That is, 1 is a substrate made of LiNbOs, 2 is a substrate made of T
A main waveguide formed by the i-diffusion method, 3a and 4a are branch waveguides formed with equal widths by the TI diffusion method, 5a is a branch point, and the main waveguide 2 and the branch waveguide 3 are
a. , 4a are single mode optical waveguides.

The optical branch/white wave circuit shown in FIG. 1 includes branch waveguides 3a and 4.
In Fig. 1 of A, the shape of the portion indicated by the length, the so-called Y-branch, is expressed by the squared cosine (
It is formed into a shape based on a function (ra1sed eoslne).

Fo(Z)=1/4G [1-eos(πZ/L)]
...(1) F1(Z)-1/2W+1/4 (W+G)[1-c
os(πZ/L)] −(2) Note that equation (1) is
Equation (2) shows the shape of the inner part of the branch, and equation (2) shows the shape of the outer part of the Y branch.

However, G is the gap between the branch waveguide 3 and the branch waveguide 4a (
(hereinafter referred to as Ti pattern gap), and w represents the waveguide width (hereinafter referred to as Tt pattern width).

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

First, a Y-shaped optical waveguide pattern of Ti is formed on a LiNb03 substrate 1 by a normal lift-off method. That is, for example, a photoresist is exposed on the L t N b O a substrate 1 and then developed to form a groove having a width of several μm and having the same shape as the optical waveguide pattern.

Next, after several tens of OA of Ti is deposited on the entire surface, the resist is removed with a remover to form a Ti optical waveguide pattern. Then, over a period of several hours, the temperature was lowered to about 10
The temperature is raised to about 00°C, and Ti is diffused into the L I N b 0 3 substrate 1. As a result, as shown in Figure 1,
T1 diffusion L IN b O a optical waveguide 2.3a, 4a
, is formed, and the fabrication of the optical branching/multiplexing circuit is completed.

Figure 5 is a graph showing the relationship between the length L of the Y branch and the branch loss in the optical branching/combining circuit shown in Figure 1, where the horizontal axis is Y
The length L1 of the branch portion represents the branch loss. In addition,
The graph shows the wavelength λ of light as l. The refractive index ns of the 52llms substrate 1 is 2.141i, and the T L pattern width W is 8.
μm, Ti when the Ti pattern gap G is 15 μm
Calculation results are shown for film thicknesses DTI of 60 nm, 70 nw, and 80 nIl.

As can be seen from FIG. 5, in the optical branching/combining circuit of the first embodiment, the branching loss uniformly decreases with respect to the length L of the Y-branching part, and compared to the conventional optical branching/multiplexing circuit shown in FIG. Y of optical branching/combining circuit
The branch loss is approximately constant at 0.2 dB or less for a length of 2 ml1, which is about the same as the length of the branch, and compared to the conventional circuit, the branch {H loss is significantly reduced. This is an optical branching (combining) circuit with lower loss.

Moreover, FIG. 6 is a graph showing the relationship between the tip width W5 shown in FIG. 2 (b) of the optical branching/combining circuit in FIG. 1 and the branching loss, where the horizontal axis is the tip width W1 and the vertical axis is The axis shows branch loss in Table 17. Note that this graph, similar to FIG. 5, shows that the Ti film thickness D
TI is 60 rv, 70 nm, 80 rv
The results of each calculation are shown.

As can be seen from FIG. 6, for example, the Ti film thickness DTI is 8
In the case of 0nrA and tip width W5 of 3 μm, the increase in branch loss is about 0.5 dB.

That is, according to the first embodiment, since the slope θ of the waveguide in the vicinity of the branch point 5 asymptotically approaches zero, it is possible to reduce the increase in loss due to the slope of the wavefront. Furthermore, near the branch point 5, the refractive index of the inner part is smaller than the refractive index of the outer part, and the phase velocity of the light propagating inside becomes faster than the phase velocity of the outer part.
This tilts the wavefront, so it is possible to reduce loss.

FIG. 7 is a configuration diagram showing a second embodiment of the optical branching/multiplexing circuit according to the present invention. The difference between this second embodiment and the first embodiment is that the shapes of the inner and outer parts of the Y-branch, indicated by the length in the figure, are expressed by the following squared cosine function:
3) and (4), and the tapered portion 6 is provided as in FIG. 2 which shows the conventional example.

F o '(Z) −−1/2W+1/4 (W+
G) [1 −cos<yr Z/L) ] −(3)
F1 '(Z) - 1/2W + 1/4 (W 10G) [
1-cos<yr Z/L)]-(4
) These (3). Even in the optical branch/& wave circuit formed based on equation (4), the same effects as in the first embodiment can be obtained.

FIG. 8 is a configuration diagram showing a third embodiment of the optical branching/multiplexing circuit according to the present invention. The difference between the third embodiment and the first embodiment is that the width W3a of the branch waveguide 3a and the width W3a of the branch waveguide 4a (in the third embodiment, W3a>W4) are different. (However, the width W2 of main waveguide 2
"4a).

In this case, by appropriately setting the parameters of the raised cosine function, it is possible to achieve low loss and to make the ratio of the output optical powers of the branch waveguides 3a and 4a 1:1.

In the above first to third embodiments, the characteristics of a so-called optical branching circuit have been explained as an example, but the same effect can be obtained with an optical multiplexing circuit with exactly the same configuration.

Furthermore, even if the number of branch waveguides is three or more, the same effects as in the first to third embodiments can be obtained by appropriately setting the parameters of the raised cosine function.

Furthermore, in the first to third embodiments, T1 diffusion L
Although the iN b O a waveguide has been described, it goes without saying that the present invention can be applied to other waveguides such as proton exchange waveguides. Furthermore, it goes without saying that the present invention is also applicable to cases where the refractive index distribution is step-shaped, such as in glass waveguides, semiconductor waveguides, and the like.

Further, in the first to third embodiments, the explanation has been given by taking a single mode guiding waveguide as an example, but it goes without saying that the present invention can be applied to a multimode optical waveguide as well.

(Effects of the Invention) As explained above, according to the present invention, since the branch leading waveguide has a shape based on a raised cosine function, for example, the slope of the optical waveguide near the branching (combining) point can be asymptotically approached to zero, In addition, since the inner refractive index can be made smaller than the outer refractive index near the branch point, the inclination of the light wavefront can be appropriately controlled, providing a low-loss optical branching/combining circuit that reduces branching loss. There are advantages that can be achieved.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a block diagram showing a first embodiment of an optical branching/multiplexing circuit according to the present invention, FIG. 2 is a block diagram of a conventional optical branching/multiplexing circuit, and FIG. The figure is a graph showing the relationship between branching angle and branching loss in a conventional optical branching/multiplexing circuit. Figure 4 is a graph showing the relationship between tip width and branching loss in a conventional optical branching/multiplexing circuit. The figure is a graph showing the relationship between the length of the Y branch of the optical branching/multiplexing circuit in Fig. 1 and the branching loss, and Fig. 6 is a graph showing the relationship between the tip width of the optical branching/multiplexing circuit and the branching loss. ,
FIG. 7 is a block diagram showing a second embodiment of the optical branching/multiplexing circuit according to the present invention, and FIG. 8 is a block diagram showing a third embodiment of the optical branching/multiplexing circuit according to the present invention. be. In the figure, 1... substrate, 2... main waveguide, 3a, 4a,
- Branch waveguide, 5...branch point, 6...Taber part. Patent Applicant Nippon Telegraph and Telephone Corporation Representative Patent Attorney, Seitaka Yoshida 2: Main waveguide 1 board (a) Figure 1 (b) Configuration diagram of conventional optical branching/multiplexing circuit Figure 2 Branch angle 2θ1 ( (degrees) Graph showing the relationship between minute angle change and branch discontinuity in the conventional example (Length of the Y branch part L<mm) Sekiho of the length of the Y branch part and the branch loss of the optical branching/multiplexing circuit in Fig. 1 Graph showing the relationship between the tip width and branching loss in the conventional example Graph showing the relationship between the tip width and branching loss of the optical branching/combining circuit shown in Fig. 1 Diagram

Claims (1)

[Claims] An optical branching/combining circuit comprising two or more branching optical waveguides formed on a substrate, characterized in that the branching optical waveguide has a shape based on a raised cosine function. Multiplexing circuit.