JP3225819B2 - Waveguide type optical branching device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a waveguide type optical branching device.

[0002]

2. Description of the Related Art In order to construct an optical communication system such as an optical subscriber network, it is essential to commercialize a small and high-performance optical passive component. This type of optical passive component includes a Y-branch type circuit and M
There is a Z-type WINC and the like.

For example, a Y-branch type circuit includes an asymmetric Y-branch element having one input and two outputs and different branch ratios (Shirata et al .:
“Quartz-based waveguide type 1 × N splitter for optical communication system”, IEICE Electronics Society Conference, SC-1-15, p. 337 (199
5)). FIG. 11 is a diagram showing an optical circuit using an asymmetric Y-branch element as a conventional example of a waveguide type optical branch element. FIG.
2 is a partially enlarged view of FIG. FIG. 13 is a diagram showing an increase rate of the waveguide width of the branch waveguide shown in FIG.
FIG. 4 is a diagram showing a state of mode coupling of the branch waveguide shown in FIG.

The asymmetric Y-branch shown in FIG. 11 has a width W1 where the input waveguide 1 widens in a tapered shape from the width W and the width becomes "W1 + WB + W1" as shown in FIG.
Branching waveguides 2 and 3. The branch waveguides 2 and 3 gradually widen the waveguide width and eventually reach the width W. At this time, the increasing rate of the waveguide width is different between the branch waveguide 2 and the branch waveguide 3, and FIG. As shown in the figure, the increase rate of the waveguide width of the branch waveguide 3 is L1 / L2 times the increase rate of the branch waveguide 2. By changing L1 / L2, the branching ratio can be changed. The principle will be described with reference to FIGS.

When light is incident on the center of the input waveguide 1,
Only the even mode is excited (cross-sectional view along line AA). Although the light is slightly coupled to the radiation mode in accordance with the gradual spread of the waveguide, it may be considered that only the even mode exists at the position indicated by the line BB. When the input waveguide 1 branches, B-
The even mode having the B-line as the reference structure is coupled to the even mode having the CC cross-sectional view as the reference structure, but no coupling to the odd mode occurs. The coupling to the radiation mode at this time becomes a loss, and it is estimated that the loss is about 0.1 to 0.2 dB with the optimum structure at this stage. Thereafter, the even mode is gradually coupled to the odd mode because the waveguide widths of the branch waveguides 2 and 3 change asymmetrically. At this time, as the distance between the branch waveguide 2 and the branch waveguide 3 increases, the difference between the propagation constants of the even mode and the odd mode becomes smaller. Deterioration of the wavelength independence of the branching ratio due to interference with the mode (ripple of the branching ratio with respect to wavelength) can be ignored. When the branch waveguides 2 and 3 are sufficiently separated from each other and the waveguide widths become equal,
At the position indicated by the E line, the propagation constant of the even mode and the propagation constant of the odd mode are equal, and the even mode and the odd mode are L1 / L2
, L1 / L2 independent of wavelength.
Is obtained.

As a waveguide type optical branching element which realizes a different branching ratio with two inputs and two outputs in a wavelength-independent manner, an MZ-type W
INC (K. JINGUJI et al, "MACH-ZEHNDER INTERFEROMETE
RTYPE OPTICAL WAVEGUIDE COUPLER WITH WAVELENGTH-FL
ATTENED COUPLING RATIO ", Electron.Lett., 1990 Vol.26
No.17 p.1326-1327, JP-A-3-213829)
Is typical. As shown in FIG. 15, the MZ type WINC is composed of two directional couplers 10 and 11 and two waveguides 12 and 13 whose lengths are different by ΔL, and ΔL is a value at a short wavelength end of a desired operating wavelength range. By setting it to be slightly shorter than that, a branching ratio with small wavelength dependence is realized. FIG. 15 is an explanatory diagram of an MZ type WINC using an optical demultiplexer as a conventional example of a waveguide type optical branching element.

[0007]

However, the Y-branch type circuit shown in FIG. 11 cannot realize a waveguide type optical branching element having two inputs and two outputs. The one-input two-output type branch element 20 is, for example, a single optical fiber 21 as shown in FIG.
In an optical communication system that performs one-way communication, a monitor light can be used as a branching element for looping back to the transmission side. However, in a system that performs two-way communication with one optical fiber 30 as shown in FIG. It cannot be used as a branch element 31 for looping monitor light back to the transmitting side. In addition, the Y-branch type circuit transmits the reflected return light to the transmitter side by using the optical fiber 3 in the system shown in FIG.
It cannot be used as a branch element required in a system (FIG. 18) to which an OTDR function for checking a disconnection of 0 or the like is added. FIGS. 16 to 18 are optical circuit diagrams using a conventional waveguide type optical branching element.

Further, the MZ type WINC shown in FIG. 15 is an interference type circuit utilizing the phase difference given by the optical path length difference between the two waveguides 12 and 13, and the material (quartz) forming the waveguides 12 and 13 is used. Glass, etc.), and is susceptible to birefringence, etc.
PDL (Polarization Dependent Loss, loss change due to polarization) is also large. In particular, when applied to a long-distance communication system requiring a small temperature characteristic and a low PDL, improvement is required.

Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide a waveguide type optical branching element having a small branching ratio with low temperature dependence, low PDL, wavelength independence, and different branching ratios. It is in.

[0010]

In order to achieve the above object, the present invention has two input ports and two output ports, and connects each input port and each output port with a waveguide, respectively. In a waveguide-type optical branching element in which a coupling part is formed by gradually approaching a waveguide, from the input port to the emission end of the coupling part, when light is input from one input port, only substantially even mode is excited. However, when light is input from the other input port, it has a structure that excites substantially the odd mode only, and changes the waveguide width asymmetrically from the coupling portion to the output port to change the branching ratio. .

In addition to the above configuration, the present invention provides that the two waveguides make the waveguide width asymmetric from the input port to the coupling portion, and that the difference between the two waveguides is gradually reduced at the coupling portion while the two waveguides remain close to each other. The widths of the two waveguides may be made equal at the output end of the coupling portion, and the two waveguide widths may be asymmetrically changed from the coupling portion to the output port while being changed asymmetrically.

In addition to the above configuration, the present invention increases the rate of change of the waveguide width of the coupling portion on the incident side, and the smaller the difference between the two waveguide widths, the smaller the rate of change of the waveguide width. The waveguide may be formed such that the change rate of the waveguide width becomes substantially zero on the emission side where the width is equal.

According to the present invention, in addition to the above configuration, one of the waveguides of the coupling portion has a constant waveguide width, and the other waveguide has a waveguide width of the entrance end larger than the exit end of the coupling portion, and Of the boundary lines between the core and the cladding, the boundary line closer to the one waveguide is parallel to the one waveguide, and the inclination of the boundary line farther from the one waveguide with respect to the other waveguide is It may be formed so that it is larger on the incident side of the coupling portion, becomes smaller as the difference between the two waveguide widths becomes smaller, and becomes substantially zero on the emission side of the coupling portion having the same waveguide width.

In addition to the above-described structure, the present invention provides a method for forming a boundary line between the core and the clad of the other waveguide, which is farthest from one of the waveguides, on the boundary line at the incident end of the coupling unit. It may be formed so as to have a cosine function curve with a point as a starting point and a line extending in parallel with one of the waveguides as a reference line.

In addition to the above-described structure, the present invention provides a method of forming a boundary line between the core and the clad of the other waveguide, which is farthest from one of the waveguides, on the boundary line of the incident end of the coupling unit. It may be formed so as to be a sine function curve with a line connecting a point and a point on the boundary between the emission end of the coupling portion as a reference line.

According to the above configuration, when light is input from one input port, only a substantially even mode is excited in the waveguide on the input port side, and when light is input from the other input port, the waveguide on the input port side is excited. Then, only the almost odd mode is excited. Although only the even mode or the odd mode exists on the input side of the coupling section, the coupling between the even and odd modes hardly occurs at the coupling section since the core width changes gradually.

In particular, by providing a smooth structure in which the boundary between the core and the clad changes like a sine function or a cosine function, coupling between even and odd modes can be suppressed, and almost no unnecessary mode is generated. This is because the coupling amount between the even and odd modes is proportional to the product of the electric field amplitude of the even mode, the electric field amplitude of the odd mode, and the increase rate of the dielectric constant (square of the refractive index) in the propagation constant direction, and thus the waveguide width is increased. This is because the coupling is unlikely to occur even when the change in the waveguide width is increased at the asymmetrical position.

While the two waveguides move away from each other, the widths of the waveguides increase at different increasing rates. However, since the widths of the two waveguides increase gradually, the mode gradually changes from the even mode to the odd mode or from the odd mode to the even mode. Binding occurs. The branching ratio is related to the amount of coupling between the even and odd modes, and the amount of coupling is related to the ratio of the increase rate of the two waveguide widths. Therefore, the branching ratio can be controlled by the ratio of the increasing rate of the width of the two waveguides.

[0019]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. The description will be made with reference to numerical values, but is not limited.

FIG. 1 is a plan view showing an embodiment of the waveguide type optical branching device of the present invention.

In FIG. 2, two input ports 40 and 41 and two output ports 42 and 43 arranged on a substrate (not shown) are connected to two conductors made of a core covered with a clad 44. The coupling portions 4 are connected by the waveguides 45 and 46, respectively, and the two waveguides 45 and 46 are gradually approached.
7 are formed.

In the input ports 40 and 41, both waveguides 45,
46 and DSF (Dispersi) with a spot radius of about 4.2 μm
The waveguide width is about 6 μm to reduce the connection loss with the on-shifted fiber (dispersion-shifted fiber) (F
-Position indicated by F line). The width of the waveguide 46 on the one input port 41 side is tapered so as not to cause radiation loss in order to suppress generation of an unnecessary mode, and is about 3.5 μm at a position indicated by a GG line. . The waveguide 45 on the other input port 40 side is gradually approached to the waveguide 46 on the one input port 41 side, and the two waveguides 45 and 46 are made parallel at the position indicated by the line HH, and the distance between them is set to be equal. It was about 3.5 μm. The distance between the waveguides 45 and 46 is kept constant from the position indicated by the line HH to the position indicated by the line I-I, and the width of the waveguide 45 is gradually narrowed to the position indicated by the line I-I. In the example, the widths of the waveguides 45 and 46 are equal (about 3.5).
μm).

At this time, the rate of change of the waveguide width in which the waveguide width decreases toward the output port is large where the asymmetry of the waveguide width is large as shown in FIG. 2, and the waveguide widths are equal. It is smaller at the value. FIG. 2 is a diagram showing a waveguide pattern of the waveguide type optical branching element shown in FIG. 1, and FIG. 3 is an enlarged view of a coupling portion of the waveguide type optical branching element.

As shown in FIG. 3, the boundary line La between the core and the clad has the x-axis (reference line) inclined with respect to the longitudinal direction of the waveguides 45 and 46 (inclined to the average inclination of the entire coupling portion).
(Y = Asin (cx), where cx is 0 to -π) (the boundary line La is defined by the direction of the average inclination of the entire joint portion). Sine function curve). As a result, the occurrence of unnecessary modes can be suppressed to 1/20 or less as compared with the case where the waveguide width is changed linearly.

Next, in FIG. 1, from the position indicated by the line II to the position indicated by the line KK, the two waveguides 45 and 46 move away from each other and the width of the waveguide is from about 3.5 μm to about 6 μm.
It is spread to μm. At this time, the width of the waveguide 45 is widened to be about 6 μm at the position indicated by the JJ line, whereas the width of the waveguide 46 is about 4.2 μm at the position indicated by the JJ line. The increase rate of the waveguide width is about 1: 0.7 between the waveguide 45 and the waveguide 46. This branching ratio is calculated from the simulation value of FIG.
It can be seen that this corresponds to a branching ratio of 7 dB. FIG. 4 is a diagram showing the relationship between the ratio of the increase rate of the waveguide width and the branch ratio. The horizontal axis is the axis of the increase rate of the waveguide width, and the vertical axis is the branch ratio axis. The rate of increase (L2 / L) on the horizontal axis shown in FIG.
1) is a waveguide 4 on the output port 42, 43 side shown in FIG.
This is the ratio between the taper lengths L1 and L2 when the widths 5 and 46 increase from W1 to W0, respectively. FIG. 5 is a diagram showing the relationship between the taper length and the core width of the waveguide shown in FIG. 4. The horizontal axis is the taper length axis, and the vertical axis is the core width axis.

FIG. 6 is a graph showing the relationship between the length of the coupling portion shown in FIG. 2 and the power ratio of the odd mode. The horizontal axis represents the length, and the vertical axis represents the power ratio of the odd mode (odd mode / ( (Odd mode + even mode)).

In the figure, the solid line (on the x-axis) shows the characteristics when the width of the core of the waveguide 45 of the coupling portion 47 in this embodiment changes in a sine function, and the broken line shows the conventional coupling portion. Shows the characteristics when the width of the core of the waveguide changes linearly. From this figure, it can be seen that the odd mode power ratio of the coupling section 47 in the present embodiment is approximately 0, and that no odd mode has occurred.

The constituent material of the present waveguide type optical branching element is that the substrate is SiO ₂ and the cladding 44 is SiO ₂ -P ₂ O ₅ -B _2.
O ₃ , and the core is SiO ₂ —TiO ₂ . The refractive index of the substrate and the cladding is 1.458, and the refractive index of the core is 1.4657.

The manufacturing process was performed by techniques such as EB (electron beam) evaporation, photolithography, and flame deposition.

Next, the operation of the present waveguide type optical branching element will be described with reference to FIG. FIG. 7 is a diagram showing the relationship between the two waveguides shown in FIG. 1 and the mode.

When light is input to one input port 40,
Only the even mode is excited at the position indicated by the line GG. When light is input from the other input port 41, only the odd mode is excited at the position indicated by the GG line. Two waveguides 45, from the position indicated by the line GG to the position indicated by the line HH,
Since 46 gradually approaches, only the even mode or only the odd mode exists at the position indicated by the line HH.

At the position indicated by the line HH, two waveguides 4
The widths of the waveguides 45 and 46 are the same at the position indicated by the line II. Since the core width gradually changes from the position indicated by the line HH to the position indicated by the line II, little coupling occurs between the even and odd modes. In particular, in the case of a structure in which the core width changes like a sine function, the coupling between the even and odd modes can be suppressed, and there is almost no occurrence of an unnecessary mode.
Since it is proportional to the product of the even-mode electric field amplitude, the odd-mode electric field amplitude, and the rate of change of the dielectric constant (square of the refractive index) in the direction of propagation constant, the change in the waveguide width is large where the waveguide width is asymmetric. Bonding is unlikely to occur).

As the two waveguides 45 and 46 move away from each other, the widths of the waveguides 45 and 46 increase at different increasing rates. The width of the waveguide width is gradual, and the coupling from the even mode to the odd mode or from the odd mode to the even mode occurs gradually. The branching ratio is related to the amount of coupling between even and odd modes occurring here, and the amount of coupling is determined by the two waveguides 45 and 46.
The width of the rate of increase is related to the ratio. Therefore, the amount of coupling can be controlled by controlling the ratio of the increase rates of the two waveguide widths.

Here, if light is input from one input port 40, the ratio of the light output from one output port 42 to the light output from the other output port 43 is 7:
If light is input from one of the output ports 42, even mode and odd mode occur at a ratio of 7: 3 on the output ports 42 and 43 side of the coupling unit 47, and both input ports 40 , 41 is 7: 3. When light is input from the other output port 43, the ratio is 3: 7.

That is, the two waveguides 45 and 46 having different waveguide widths are gradually approached (when light is input from the thicker waveguide 45, only the even mode is excited and the narrower waveguide 46 is excited). (A structure that excites only the odd mode when light is input from) The width of the waveguide is changed while keeping the two waveguides 45 and 46 close to each other until the width of the two waveguides becomes equal. Is gradually reduced (a structure in which light is distributed to the two waveguides 45 and 46 without causing coupling between the even mode and the odd mode).
The width of the waveguide is gradually widened while keeping the 46 away from each other, and the rate of increase in the width of the two waveguides 45 and 46 is made different (coupling occurs gradually between even and odd modes, and the propagation constant of even and odd modes is gradually reduced. (Degenerate structure) structure . That
Therefore, in a position indicated by line I-I, to excite only the even mode in the case of inputting light at one of the input ports 40,
When light is input to the other input port 41, only the odd mode is excited so that no unnecessary mode is generated . Then , the waveguide width is gradually asymmetrically increased from the position indicated by the line II to the position indicated by the line KK, and
By separating one waveguide 45 and the other waveguide 46 from each other, the even-odd mode propagation constants are made closer to each other, so that the even-odd mode coupling is gradually generated.

The conversion from the even mode to the odd mode occurs from the position indicated by the line II to the position indicated by the line KK in accordance with the ratio of the increase rate of the waveguide width. Is input to the output port 42, the input light is about 1.7.
Light whose dB has been attenuated is output, and light whose input light has been attenuated by about 4.8 dB is output to the output port 43.

FIG. 8 is a diagram showing the loss wavelength characteristics at both output ports 42 and 43 when light is input to one input port 40 of the waveguide type optical branching element shown in FIG.
In the figure, the horizontal axis is the wavelength axis, and the vertical axis is the loss axis. Pt represents the loss of the output light of one output port 42, and Pc represents the loss of the output light of the other output port 43.

The loss at the output port 43 is wavelength λ = 1.31.
A wavelength-independent branching ratio of 4.9 dB at μm and 4.77 dB at λ = 1.55 μm was obtained. In addition, the temperature characteristic is 0.1.
001 dB / ° C. or less, and PDL was 0.01 dB or less. On the other hand, the MZ type WINC has design parameters,
If the manufacturing conditions are different, the value is different, so that it cannot be said unconditionally.

As described above, in the present embodiment, the temperature characteristics of the material are in principle less affected by the birefringence, so that the temperature characteristics are small (in the embodiment, 0.001 dB / ° C. or less), and the low PDL (the embodiment) 0.001dB or less)
Thus, it is possible to realize a waveguide type optical branching element having a wavelength-independent branching ratio of two inputs and two outputs.

In the case of an asymmetric Y-branch, since the structure may change greatly, the radiation loss is 0.1 to 0.5.
Although it is about 2 dB, in the present embodiment, the structure changes smoothly, so that there is no loss in theory.

In the above-described embodiment, the waveguide 4 of the boundary between the core and the clad of the waveguide 45 of the coupling portion 47 is formed.
Although the boundary line La farthest from the line 6 is formed so as to be a sine function curve in which the reference line is inclined with respect to the longitudinal direction of the waveguide 46, the present invention is not limited to this. As shown in FIG. 9, the waveguide 45 of the coupling portion 47a
The boundary line Lb, which is farthest from the waveguide 46a, of the boundary line between the core and the clad of a is set to a cosine function curve with respect to the longitudinal direction of the waveguide 46a, that is, on the boundary line of the incident end of the coupling portion 47a. A cosine-function-like curve (y = Acos (cx), where cx is π to 2π) having a point as a starting point and a line extending in parallel with the waveguide 46a as an x-axis (reference line) may be formed. Good.

FIG. 9 is an explanatory view showing another embodiment of the present invention. FIG. 10 is a diagram showing the relationship between the waveguide and the mode of the waveguide type optical branching element having the core pattern shown in FIG. As shown in the figure, a part of the cosine function curve and the boundary line Lb between the core and the cladding of the waveguide 45a are shown.
Are equal to each other. Also in the waveguide type optical branching device having such a core pattern, the temperature dependence of the branching ratio is small, low PDL, wavelength independence, and different branching like the waveguide type optical branching device shown in FIG. Having a ratio.

In the above-described embodiment, SiO ₂ -GeO ₂ is used as the material of the core. However, the present invention is not limited to this, and other dielectrics, semiconductors, organic substances, and the like may be used. Further, a dielectric, a semiconductor, an organic material, or the like may be used for the substrate and the clad.

The waveguide type optical branching device according to the present embodiment is applicable to an optical communication system. In particular, it is suitable as an optical branching element used in a PDL or a long-distance optical communication system requiring low temperature characteristics.

[0045]

In summary, according to the present invention, the following excellent effects are exhibited.

The temperature dependence of the branching ratio is small, low PDL,
It is possible to realize a waveguide type optical branching element having wavelength independence and having different branching ratios.

[Brief description of the drawings]

FIG. 1 is a plan view showing an embodiment of a waveguide type optical branching device of the present invention.

FIG. 2 is a diagram showing a waveguide pattern of a waveguide type optical branching element.

FIG. 3 is a view showing a pattern of a coupling portion of the waveguide type optical branching element shown in FIG. 1;

FIG. 4 is a diagram illustrating a relationship between a ratio of an increase rate of a waveguide width and a branching ratio.

5 is a diagram showing a relationship between a taper length and a core width of the waveguide shown in FIG.

FIG. 6 is a diagram illustrating a relationship between a length of a coupling unit illustrated in FIG. 2 and a power ratio of an odd mode.

FIG. 7 is a diagram showing a relationship between two waveguides shown in FIG. 1 and a mode.

8 is a diagram illustrating loss wavelength characteristics at both output ports when light is input to one input port of the waveguide type optical branching element illustrated in FIG. 1;

FIG. 9 is an explanatory diagram showing another embodiment of the present invention.

10 is a diagram showing a relationship between a waveguide and a mode of the waveguide type optical branching element having the core pattern shown in FIG.

FIG. 11 is a diagram showing an optical circuit using an asymmetric Y-branch element as a conventional example of a waveguide type optical branch element.

FIG. 12 is a partially enlarged view of FIG. 11;

13 is a diagram showing an increase rate of the waveguide width of the branch waveguide shown in FIG.

FIG. 14 is a diagram showing a state of mode coupling of the branch waveguide shown in FIG. 11;

FIG. 15 is an explanatory diagram of an MZ type WINC using an optical demultiplexer as a conventional example of a waveguide type optical branching element.

FIG. 16 is an optical circuit diagram using a conventional waveguide type optical branching element.

FIG. 17 is an optical circuit diagram using a conventional waveguide type optical branching element.

FIG. 18 is an optical circuit diagram using a conventional waveguide type optical branching element.

[Explanation of symbols]

 40, 41 input port 42, 43 output port 45, 46 waveguide 47 coupling part La boundary line

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tomoyuki Shirata 5-1-1, Hidaka-cho, Hitachi City, Ibaraki Prefecture Inside of Opto-Systems Research Laboratory, Hitachi Cable Co., Ltd. (56) References JP-A-59-19829 JP, A) JP-A-2-287408 (JP, A) JP-A-4-212108 (JP, A) JP-A 4-212109 (JP, A) JP-A-5-313028 (JP, A) JP 6-308338 (JP, A) JP-A-7-230014 (JP, A) JP-A-9-113743 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 6 / 12-6/14

Claims (6)

(57) [Claims] A waveguide having two input ports and two output ports, wherein each input port and each output port are connected by a waveguide, and the two waveguides are gradually approached to form a coupling portion. In the type optical branching element, from the input port to the emission end of the coupling portion, when light is input from one of the input ports, only substantially even mode is excited,
When light is input from the other input port, it has a structure that excites only the almost odd mode, and changes the branch width asymmetrically by changing the waveguide width asymmetrically from the coupling portion to the output port. Waveguide type optical branching element. 2. The two waveguides make the width of the waveguide asymmetrical from the input port to the coupling portion, and the coupling portion gradually reduces the difference between the two waveguides while keeping both waveguides close to each other. 2. The waveguide type according to claim 1, wherein both waveguide widths are made equal at an emission end of the portion, and both waveguide widths are asymmetrically changed from the coupling portion to an output port while being changed asymmetrically, and both waveguide widths are made equal at an output port. Optical branching element. 3. The output in which the change rate of the waveguide width of the coupling portion is large on the incident side, and the change rate of the waveguide width is small as the difference between the two waveguide widths becomes small, and the two waveguide widths are equal. 3. The waveguide type optical branching device according to claim 2, wherein the waveguide side is formed such that the change rate of the waveguide width is substantially zero on the side. 4. The waveguide of one of the coupling portions has a constant waveguide width, the other waveguide has a waveguide width of an incidence end larger than an emission end of the coupling portion, and has a core and a cladding. Among the boundary lines, the boundary line closer to the one waveguide is parallel to the one waveguide, and the inclination of the boundary line farther from the one waveguide with respect to the one waveguide is 4. The waveguide according to claim 3, wherein the waveguide is formed so as to be larger on the incident side of the coupling portion and smaller as the difference between the two waveguide widths becomes smaller, and to be substantially zero on the emission side of the coupling portion having the same waveguide width. Type light splitting element. 5. A boundary line, which is farther from one waveguide among the boundary lines between the core and the clad of the other waveguide of the coupling portion, is set to a point on the boundary line of the incident end of the coupling portion as a starting point. 5. The waveguide type optical branching device according to claim 4, wherein the waveguide type optical branching device is formed so as to have a cosine function curve with a line extending parallel to one of the waveguides as a reference line. 6. A coupling line between the core and the cladding of the other waveguide, which is farther from one of the waveguides, is connected to a point on the boundary at the incident end of the coupling portion. 5. The waveguide type optical branching device according to claim 4, wherein the waveguide type optical branching device is formed to have a sine function curve with a line connecting a point on the boundary line of the emission end of the portion as a reference line.