JPH032806A - Optical waveguide type branching/joining device - Google Patents

Abstract

PURPOSE:To reduce the wavelength dependency and to improve the wavelength characteristic by constituting the device so that between split waveguides being adjacent to each other, a code of a propagation constant difference between two pieces of partial waveguides for constituting its split waveguides is inverted. CONSTITUTION:On a silicon substrate 11, the lower clad layer 12 of 20mum thick, a core layer 13 of 2mum thick, and the upper clad layer 14 of 1.5mum thick are formed successively by quartz, and also, waveguides 1, 2 are formed like a ridge at an interval of 7.2mum thereon. In this case, as for the waveguides 1, 2, in both of them, road width is varied in an intermediate position of the whole, its cross sectional structure is varied, two equally split conductive wave paths A, B are formed, and also, between the split waveguides being adjacent to each other, a code of a mutual propagation constant difference is inverted. In such a way, the wavelength characteristic becomes flat, and the wavelength dependency becomes small.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical waveguide type branching/merging device, and more specifically,
This invention relates to an optical waveguide type branching/merging device in which the wavelength characteristics of the guided light are flat and the wavelength dependence is small.

(Prior Art) Conventionally, an optical waveguide type branching/merging device, also called an optical directional coupler, has a structure as shown in a plan view in FIG. 9(a). That is, for example, on the substrate 3 made of quartz,
Waveguides 1.2 having the same cross-sectional structure are arranged close to each other,
Propagation constant β1 of good waveguide 1.2. By making β2 equal, evanescent coupling is caused between the good waveguides 1 and 2. That is, in this type of waveguide type branch-combiner, the difference in propagation constant between both waveguides (Δβ−β).

−β is zero.

In the case of an optical directional coupler with this structure, complete optical power transfer occurs between both waveguides. For example, when light with optical power 1 is input only to waveguide l at the input end, the element length For the optical power in waveguide 1.2! t and Ig (optical power of the waveguide 2) change as shown in the graph of FIG. 9(b).

Now, if the element length is set to the perfect coupling length Lo, the waveguide l
Between the waveguide 2 and the waveguide 2, optical branching of taX can be achieved.

However, this perfect coupling length Lo varies depending on the wavelength of the guided light. In other words, as the wavelength of the guided light becomes longer, the confinement of light within the waveguide becomes worse.
The amount of light seeping out of the waveguides increases, resulting in stronger interactions between the waveguides. That is, the complete bond length Lo becomes shorter and the branching ratio deviates from 1:1.

As described above, the conventional optical waveguide type branching/merging device shown in FIG. 9 has a problem of poor wavelength dependence.

For this reason, in the waveguide as well, the propagation constant β1 of the waveguide 1 and the propagation constant β of the waveguide 2 are made different, as in the case of the fiber type branching/merging device (fused taper type). , the propagation constant difference (Δβ) between the waveguides is Δβ
-β and -β2 to 0 are being studied to realize a wide wavelength band.

In this case, complete transfer of optical power between the waveguides does not proceed, but only the transfer of optical power as shown in FIG. 10 occurs. The characteristics of this change in light power are:
Curve 1. 1. This means that the amount of change in element length near the peak or bottom of II is smaller than in the case of FIG. 9, etc.).

Therefore, if the operating point is now set at the position of Ll, the coupling between both waveguides will be wavelength dependent and even if some fluctuations occur,
The amount of change near the peak (or bottom) of optical power 1□ becomes small. That is, in the case of this structure, Fig. 9 (a
) The wavelength dependence is smaller than that of the branch-combiner structure shown in ().

(Problems to be Solved by the Invention) The present invention aims to provide an optical waveguide type branch/combiner with improved layer and wavelength characteristics by applying the method applied to the above-described fiber type branch/combiner. .

(Means for Solving the Problem) In order to achieve the above-mentioned object, in the present invention, a substrate and two waveguides disposed close to each other so as to cause evanescent coupling to the substrate are provided. In the optical waveguide type branching/merging device, the two waveguides are divided in the longitudinal direction to form a plurality of sets of divided waveguides, and the difference in propagation constant between adjacent divided waveguides is An optical waveguide type branching/merging device characterized in that the sign is reversed is provided.

In the optical waveguide type branching/merging device of the present invention, the fact that two waveguides are arranged close to each other on the substrate is the same as in the conventional case.

In the present invention, two objects that are connected to each other and are related to each other are
A book waveguide is divided into 7M pieces in the longitudinal direction to form a plurality of sets of divided waveguides, which have a continuous structure.

Between the divided waveguides adjacent to each other, the propagation constant difference Δβ between the two partial waveguides constituting the divided waveguide is
It is characterized in that the sign of is reversed.

As shown in the plan view of FIG. 1, the waveguide 1. A case will be explained in which the waveguide 2 is divided into La and L- to form a divided waveguide A1 and a divided waveguide B. Here, the propagation constants of waveguide 1 and waveguide 2 in divided waveguide A are respectively βIA+β2A, and waveguides 1 and 2 in divided waveguide B are respectively βIA+β2A. The propagation constant of waveguide 2 is β111, respectively.
．． Each waveguide is formed so that β.

In the case of the present invention, within the range of element length 0≦L≦L, the propagation constant difference Δβ is set to Δβ−βlAβ,>O (or <0), and LA≦L≦LA+Ll
Each divided waveguide is formed so that Δβ=βl−β2@<0 (or 0>) in the range of .

In order to invert the signs of Δβ between the divided waveguides, the width of the waveguide may be changed in the longitudinal direction, or the cross-sectional structure of the waveguide may be changed in the longitudinal direction.

(Operation) The operation of the branching and merging device of the present invention will be explained using the structure shown in FIG. For convenience of explanation, LA=L, l=L/2.
β1. −β, ,β2A=β18.

In a range where the element length is O≦L≦LA, the propagation constant difference: Δβ of this divided waveguide A is Δβ=δβ.

Here, δβ=βIA-β2A= (β1.-β2.
), therefore, LA≦L≦L a + L m
In the range of , the propagation constant difference of split waveguide β: △β is Δβ
=-δβ.

At this time, the optical output of the waveguide l 1. The optical output ■□ from the waveguide 2 is expressed by the following equation based on mode coupling theory when the incident power is 1.

1, = 1-1° (however, β.-k"+(Δβ)"/4.: number of bonds) 1 when K≦2Δβ. ．． 1. The relationship between and L is shown in Figure 2 (a
), and the relationship when K≧2Δβ is shown in Figure 2 (b).
Shown below.

If K≦2Δβ, r,,! The rate of change with respect to L at the peak (or bottom) of 2 is even flatter than in the case shown in FIG. That is, the optical waveguide type branching/combining device of the present invention has flatter wavelength characteristics and smaller wavelength dependence than the above-mentioned fiber type branching/merging device.

If maf, -1K≧2Δβ, then I, 1. At the peak (or bottom) of , it exhibits bimodal characteristics, and its wavelength characteristics become flat.

(Example) The present invention will be described in detail below with reference to the drawings.

A branching/merging device of the present invention as shown in the plan view of FIG. 3(a) and the cross-sectional view of FIG. 3(a) was manufactured.

On the silicon substrate 11, a lower cladding layer with a thickness of 20 μm, a core layer with a thickness of 2 μm, and an upper cladding jW14 with a thickness of 1.5 μm are sequentially formed of quartz. The waveguide 1.2 was formed into a ridge shape.

In both waveguides 1.2, the path width changes at the intermediate position of the whole, and the cross-sectional structure changes, resulting in two equally divided conductive waveguides A,
It forms B. That is, the waveguide 1 has a path width of 8.0 μm in divided waveguide A, and a path width of 10 μm in divided waveguide B.
．． It's 0 pm. The waveguide 2 is symmetrical to the waveguide 1. Therefore, the propagation constant difference Δβ in the divided waveguide A and the propagation constant difference Δβ in the divided waveguide B.

The relationship Δβ1=−Δβ holds true between them. And Δβ is Δβ−2,5π/L.

In this branching/merging device, when the center wavelength used is λ-1.3 μm, the complete coupling length L0 is Le=6.1 mm.

The element length is L=10 mm. In other words, the operating point is
The relationship L=1.64xt, 0 is satisfied.

The wavelength dependence of this branch-combiner for 1:1 branching is expressed as
It is shown as curve A in the figure. In addition, Δβ=0,
The wavelength dependence of the conventional branch-combiner with an operating point of L-0, 5XL is taken as curve B, and further, Δβ-〇, 8π/L, and the conventional wavelength dependence with an operating point of L=0.95xt, 0. Curve C shows the wavelength dependence of the fiber type branching/merging device.

As is clear from the figure, in the branch-combiner of the present invention, the dependence of the element length on the complete coupling length L0 is slower than in other cases, and its wavelength characteristics are flat.

FIG. 5 is a diagram illustrating a case where the cross-sectional structure of a divided waveguide is changed. FIG. 5(a) shows a cross section of one waveguide in a certain divided waveguide, and FIG. A cross section of a waveguide in which another landing layer 15 is further provided on one of the waveguides in the adjacent divided waveguides is shown.

FIG. 6 is a plan view showing the case where the waveguide is divided into three parts. In this case, if Δβ of the divided waveguide in area A is Δβ>0, Δβ of the divided waveguide in area B is Δβ×0. , Furthermore, Δβ of the divided waveguide in region C is 8β>0.

Figure 7 shows the following steps to facilitate separation of the waveguides at the input end and output end, and to facilitate connection with optical fibers.
Forming bent portions 16.16 at appropriate locations on the good waveguide,
Furthermore, in order to make the path width of each waveguide constant at the input end and output end, tapered portions 17 and 17 are placed at appropriate locations on the waveguide.
It is a top view which illustrates the case where it forms. This structure is practical.

Figure 8 differs from the above in that the Δβ of the divided waveguides is changed discontinuously, whereas the path width between the divided waveguides changes continuously in a linear function. FIG. 4 is a plan view illustrating a 4° structure with a transition area of 1 day.

(Effects of the Invention) As is clear from the above description, the optical waveguide type branching/merging device of the present invention has a structure including a substrate and two parts arranged close to each other so as to cause evanescent coupling to the substrate.
In an optical waveguide type branching/merging device consisting of a main waveguide,
The two waveguides are divided in the longitudinal direction to form a plurality of sets of divided waveguides, and the signs of the propagation constant differences between adjacent divided waveguides are reversed. Therefore, its wavelength characteristics become flat, and its wavelength dependence becomes even smaller than in the past.

Therefore, in a tree coupler or a star coupler in which branching/merging devices are connected in cascade, the improvement in wavelength characteristics becomes more remarkable, and the industrial value is extremely large.

[Brief explanation of drawings]

FIG. 1 is a plan view showing the basic structure of the waveguide type branching/merging device of the present invention, and FIGS. Graphs showing the relationship, FIGS. 3(a) and 3(b) are a plan view and a cross-sectional view of the embodiment, FIG. 4 is a graph showing the wavelength dependence of the branch-merging device, and FIGS. FIG. 5(t)l is a sectional view showing a waveguide of another embodiment, FIG. 6 is a plan view of another embodiment, FIG. 7 is a plan view of yet another embodiment, and FIG. 8 is a further A plan view of another embodiment, FIG. 9(a) is a plan view illustrating a conventional waveguide type branching/merging device, and FIG. 9(0)) is a plan view of the waveguide type branching/merging device of FIG. 9(a). Graph showing the relationship between optical power and element length. FIG. 1O is a graph showing the relationship between optical power and element length in a fiber type branching/merging device. 1.2...Waveguide, 3.11...Substrate, I2...
Lower cladding layer, 13... Core layer, 14.15...
Upper cladding layer, 16... Bent part, 17... Tapered part, 18... Transition region.

Claims (1)

[Claims] In an optical waveguide type branching/merging device consisting of a substrate and two waveguides arranged close to each other so that evanescent coupling occurs on the substrate, the two waveguides are divided in the longitudinal direction to form a plurality of waveguides. 1. An optical waveguide type branching/merging device comprising a set of divided waveguides, and the signs of propagation constant differences between adjacent divided waveguides are reversed.