JPH02287408A - Waveguide type optical branching element - Google Patents

Abstract

PURPOSE:To relieve the existence of dependency of a power coupling rate on wavelength by forming two pieces of optical waveguides in the coupling region in a directional coupler in such a manner that the widths thereof vary from each other and the depths are the same as each other. CONSTITUTION:Two pieces of the optical waveguides 1a, 1b in the coupling region in the directional coupler 2 are so formed that the widths thereof vary from each other and the depths are the same as each other. The structure in which the widths of the waveguides 1a, 1b in the coupling region of the directional coupler vary is adopted for the directional coupler in such a manner. The waveguide type optical branching element of a low loss with which the existence of dependency of the coupling rate on wavelength is relieved to, for example, 50%+ or -10% in a desired wavelength region, for example, a wavelength region including 1.3 to 1.55mum with the practicable structure is thereby obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a waveguide type optical branching element used in the field of optical communications, etc., and more specifically relates to a waveguide type optical branching element used in the field of optical communication. This invention relates to a wave type optical branching element.

[Prior Art] In order to popularize optical fiber communication, various optical components are required in addition to optical fibers and receiving/emitting elements. Among these, the optical branching element is the most basic optical component, and a branching element having a branching ratio (coupling ratio) such as 50% branching, 20% branching, several % branching, etc. is required. In particular, there is a great demand for optical branching elements that have little wavelength dependence over a wide wavelength range.

The optical branching element is also called an optical cutter, and depending on its form,
They can be broadly classified into (1) bulk type, (2) fiber type, and (3) waveguide type.

The bulk type is composed of a combination of microlenses, prisms, interference film filters, etc., and can provide a branching element with little wavelength dependence, and although it is at a practical level, it takes a long time to assemble and adjust. Problems remain in terms of long-term reliability, price, and size.

The fiber type is constructed using the optical fiber itself through polishing, fusing, and stretching processes, and it is possible to create a type with reduced wavelength dependence, but the manufacturing process requires craftsmanship. However, it has the disadvantage of poor reproducibility and cannot be mass-produced.

On the other hand, among the waveguide types, it is difficult to accurately control waveguide parameters such as structure when producing optical branching elements using ion diffusion methods, ion exchange methods, etc., and the reproducibility is poor.

In contrast, waveguide type optical branching elements using the dry etching method, which has a proven track record as an LSI (Large Scale Integrated Circuit) processing technology, have the advantage of being mass-produced on a flat substrate using a photolithography process. It is attracting attention as a future type optical branching device due to its flexibility and the possibility of compact integration.

[Problems to be Solved by the Invention] There is a directional coupler type optical branching element as a conventional waveguide type optical branching element, and an example of its configuration is shown in FIG.

The same figure (^) is a plan view, the same figure CB) is a sectional view taken along cutting line c-c', and the same figure (C) is a sectional view taken along cutting line DD'. In the figure, two optical waveguides 1a and lb having equal widths are arranged on a flat substrate l, some of which are close to each other to form a directional coupler 2. Directional coupler 2
outputs the signal light incident from the input port 3a)
- It is designed to branch output to 3b and 4b. At this time, it is possible to set the power coupling ratio of the directional coupler 2 to a desired value at a certain desired wavelength, but when using an optical branching element in a wide wavelength range, the power coupling ratio The problem was the wavelength dependence of

Figure 1θ shows an example of the wavelength dependence of the coupling efficiency of the conventional directional coupler type optical branching element shown in Figure 9. In this case,
Since the propagation constants of the two-tree optical waveguides 1a- and lb are the same, their output power (coupling ratio) changes approximately sinusoidally with respect to the wavelength, and the maximum coupling ratio is 1 (
100J). In this example, 50
% coupling rate, at a wavelength of 1.55 μm, 1o0
The coupling ratio was close to 96, and it was not possible to operate as an optical branching element at the wavelength of 1.3 μm and at the wavelength of 1.55 μm at the same time.

On the other hand, in the above-mentioned directional coupler type optical branching element, if the propagation constants of the two optical waveguides in the coupling region are different, the maximum coupling ratio will be 1 (100H is not reached) due to the mismatch of the propagation constants. , the coupling rate is relaxed in a certain wavelength range.This can be expressed as the following equation (1
) is shown as the formula (P1
15 ■Asakura Shoten, September 20, 1980, first edition (1st printing)
.

C: Coupling rate: Coupling coefficients βa, βb: Propagation constant L when there is no coupling between waveguides a and b
: Length of the coupling part Furthermore, the object of the present invention is to connect optical fibers, which have hitherto been in great demand in the field of optical fiber communication.
No one has achieved the desired coupling rate with little wavelength dependence over a wide wavelength range from μm to 1.8 μm.

In addition, the so-called Y-branch type is known as another configuration method of the waveguide type optical branching element, but in this case, the coupling rate (
Although the wavelength dependence of the branching ratio is small, it is 1 dB at the branching point.
There was a wood-related drawback in that a certain degree of light loss was unavoidable.

In view of the above-mentioned problems, an object of the present invention is to fabricate a film using a waveguide I-simulation deposition method (e.g., flame deposition method, sputtering method, etc.) and microfabrication brainer technology such as photolithography or reactive ion etching. It provides a practical structure for a directional coupler type optical branching element, and in a desired wavelength range, for example, a wavelength range including 1.3 μm to 1.55 μm,
The object of the present invention is to provide a low-loss waveguide type optical branching element in which the wavelength dependence of the coupling rate is relaxed to, for example, 50%±10%.

An object of the present invention is to provide a waveguide type optical branching element having a structure that facilitates the following.

[Means for Solving the Problems] In order to achieve the above object, a first embodiment of the present invention includes a substrate, two optical waveguides disposed on the substrate, and a portion of the two optical waveguides that are connected to each other. In a waveguide type optical branching element consisting of one directional coupler placed close to each other, one end of the optical waveguide is an input port, and the other end of the optical waveguide is a main output port and a sub-output port, respectively. The two optical waveguides in the coupling region in the optical coupler have different widths and the same depth.

In the second embodiment of the present invention, the width of one optical waveguide in the coupling region in the directional coupler is approximately 8 μm, the width of the other optical waveguide is approximately 4 μm to less than 8 μm, and the depth of the image optical waveguide is approximately 8 μm.
It is characterized in that it is set so that the wavelength dependence of the coupling rate of power from the input port to the sub-output port is relaxed over a range of about 1.2 μl to 1.8 μm by making it about μm.

In the third embodiment of the present invention, the width of the waveguide in the vicinity of the input port and the output port is set to about 8 μm, which is comparable to the size of the optical fiber core to be connected to the port, and the directionality is It is characterized in that it is connected to the coupling region of the coupler.

[Function] The directional coupler of the present invention is characterized by a waveguide type optical branching element having a structure in which the waveguide widths in the coupling region are different. In principle, by changing the waveguide parameters such as the height of each waveguide and the refractive index difference Δ, the propagation constant of each waveguide can be effectively changed, and if the above-mentioned waveguide widths are different. However, in the present invention, the design and design method using microfabrication brainer technology such as photolithography and reactive ion etching can be realized.
A major feature of the present invention is that only the waveguide width is made asymmetric in consideration of ease of manufacturing.As a result, the structure of the present invention is extremely simple, and it is different from conventional directional coupler type optical fibers. Since it is almost the same size as the branch element, integration...
It also has a great advantage in terms of miniaturization.

Therefore, in the present invention, waveguide film deposition methods (e.g., flame deposition methods,
A practical structure of a directional coupler type optical branching element manufactured by a combination of well-known techniques such as sputtering method, etc.) and microfabrication brainer technology such as photolithography and reactive ion etching is used to obtain the desired wavelength. area, for example 1.3
In the wavelength range including μm to 1.55 μm, it is possible to provide a low-loss waveguide type optical branching element in which the wavelength dependence of the coupling rate is relaxed to, for example, 50%±lθ%. Furthermore, the present invention can provide a structure in which the width of the waveguide near the input/output port is tapered to facilitate connection with an optical fiber.

Therefore, such a waveguide type optical branching element according to the present invention,
It is expected to have a wide range of uses, including for distributing optical signals over a wide wavelength range, for monitoring, and for tapping.

Furthermore, by connecting the optical branching elements of the present invention in multiple stages on a flat substrate, it is easy to expand to a four-branching element or an eight-branching element. It is also possible to form optical branching elements in an array on the same substrate and connect them to, for example, an optical fiber array with a pitch of 250 μm.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

In the following example, a silica-based single-mode waveguide formed on a silicon substrate is used as an optical waveguide. This is because an excellent and practical waveguide type optical branching element can be provided, and the present invention is not limited to such a silica-based optical waveguide.

(First Example) First, with reference to FIG. 1, the structure of a waveguide type optical branching element according to a first example of the present invention will be described.

In Figure 1, (^) is a plan view, and (B) is a cutting line ^-
A sectional view taken along A°, and (C) a sectional view taken along cutting line B-8'. In Fig. 1, ■ is a silicon substrate, la,
lb is a quartz-based optical waveguide formed on a silicon substrate 1 using a silica-based glass material. The optical waveguides la and 1b have different widths and are close to each other at one location to form a directional coupler 2. The optical waveguides la and lb have a film thickness of 50
It consists of a 5i02-TiOz glass core portion with a length and width of about 8 μI embedded in a 5iCh glass layer 5 with a size of about μH, and is composed of a straight line pattern and an arc pattern with a radius of curvature of 50 n+m.

Such silica-based optical waveguides 1a and lb can be formed by a combination of known techniques with a glass film deposition technique using flame hydrolysis reaction of silicon tetrachloride or titanium tetrachloride, and a microfabrication technique such as reactive ion etching. . The coupling part of the directional coupler 2 has both optical waveguides 1a and lb spaced apart by s=3.
Keep it at about μm to 6μm, length L = 1.5++a
It is constructed by arranging them in parallel across a distance of about 2.5 mm. Further, in this embodiment, the distance between the input port 3a and the secondary port 4a, and the distance between the output port 3b and the secondary output port 4b are both 0.25h.
It is designed to m.

Next, referring to FIG. 2, as a first embodiment of the waveguide type optical branching element of the present invention, a branching element having a coupling rate of 50%±1O% in the wavelength range of 1.25μI to 1.8μI is described. Explain the coupling rate and wavelength dependence of

In Figure 2, the straight line ■ and the curve ■ are the envelope and coupling characteristics of a conventional symmetrical directional coupler (each waveguide width ^ B - 8 μm, waveguide height H - 8 μm), respectively. ,
Curves ■ and Curves ■ are the envelope and coupling characteristics of the asymmetric directional coupler (A-8 μm, Toro μvr, I (・8 μm)) of the embodiment of the present invention, respectively. However, the refractive index difference Δ-0
, 25, and in the curve ■, the waveguide spacing s-3,0μ
re, the joint length L-1, adjusted to 4 mm.

In the curve ■, the coupling rate increases monotonically as the wavelength increases in the wavelength range from 1.0 μI to 1.65 μm, but in the curve ■, it shows a gentle change with a peak near 1.4 μm, and the coupling rate increases from 1.0 μI to 1.65 μm. The coupling rate is maintained at 50%±lθ% over a wide wavelength range from 1.80 μm to 1.80 μm.

This is because, in equation (1) above, the monotonically increasing characteristic (curve ■) of the symmetric directional coupler is suppressed by the effect of the difference in propagation constant (βa - βb) due to asymmetry. be. Furthermore, due to the asymmetry, the maximum amplitude of the coupling characteristic can be expanded from 1.0! By suppressing the wavelength to 3 lines and adjusting the phase term of 5IN (sine), the wavelength band in which the suppressed maximum coupling rate can be obtained is set to be located approximately in the middle of the desired wavelength range. become.

The structural parameters in the embodiments of the present invention are waveguide width A,
B, the waveguide spacing S, and the like, and their setting requires submicron or less precision, which is easy to achieve with today's photolithography technology, as described above.

Furthermore, even if there is a small error in those parameters, the envelope in equation 11 mentioned above is
As shown in FIGS. 8 to 8, there is no extremely large change, and it can be sufficiently suppressed by adjusting the phase term. Furthermore, the phase term itself can be controlled freely and precisely by appropriately setting the coupling length to 0-ω.

Here, the dimensions of the optical branching element of this embodiment are almost the same as the size of a single symmetrical directional coupler. It is possible to fabricate about 800 pieces at once on a Si (silicon) wafer substrate.

Further, the loss value of this optical branching element was extremely small at about 0.2 dB. The loss of the optical branching element, including the connection loss with the single-mode optical fiber connected to the output port, is also 0.5d.
It was confirmed that it was small, below grade B, and was at a level sufficient for practical use. This is in contrast to the conventional Y-branch type optical branching element, in which the excess loss including fiber connection loss was about 1.5 dB or more. This is because a singular point such as a Y-branch is not included, and the optical element is composed of only a smooth pattern.

(Second actual tJζ example) The third (frame diagram) is 20% ± 5% in the wavelength range 1.25 μm to 1.70 μm configured as the second practical hζ example of the present invention.
FIG. 3 is a plan view showing the structure of a light branching element having a coupling ratio of . It is roughly the same as the first embodiment, but the input port 3a
, 4a differs from the arrangement in FIG. 1 in that the output ports 3b and llb are vertically symmetrically arranged.

FIG. 3(B) is a characteristic diagram illustrating the coupling rate and wavelength dependence of the optical branching element of the second practical hf example of the present invention. The straight line and the curve ■ are respectively symmetrical directional couplers (A-B-8μm, l
1-8 μm), and the curve ■
and curve ■ are respectively for an asymmetric directional coupler (^-8 μm,
These are the envelope and coupling characteristics for B-6μm, H-8μm). However, the refractive index difference Δ=0.25! The wavelength range is 1.25 μm to 1.70 μm. In um, 2
The waveguide spacing s = 4 so that the coupling rate is 0% ± 5%.
μm.

The length of the joint part was adjusted to L = 2.0 mm. In the curve ■, the coupling rate increases monotonically as the wavelength increases, but in the curve ■, it shows a gentle change with a peak near 1.45 μm, and the coupling rate increases over a wide wavelength range from 1.25 μl to 1.70 μm. The binding rate is maintained at %±5%.

(Third Embodiment) Figure 4(A) shows 5% ± 1 in the wavelength range l, 25μI11 to 1.75μm configured as the third embodiment of the present invention.
FIG. 3 is a plan view showing the structure of a light branching element having a coupling rate of The waveguide width in the vicinity of the sub-input port 4a and the sub-output port 4b is made the same as the width of the other waveguides 3a and 3b, and the directional coupler 2 is roughly the same as the first embodiment. The difference is that the coupling region is connected by a sufficiently smooth tapered waveguide 5 so that no radiation mode occurs.

FIG. 4(B) is a characteristic diagram illustrating the coupling rate/wavelength dependence of the optical branching element of the third embodiment of the present invention. The straight line ■ and the curve ■ are symmetrical directional couplers (＾-B・8μva, H
-8 μIn), and the curve ■
and the curve ■ are respectively the asymmetric directional coupler (^
-8 μm, B-5 μm, If-8 μm). However, refractive index difference Δ=0.25
N, and in the curve ■ the wavelength range is 1.25 μm -1,75
In order to obtain a coupling rate of 5%±1% in μm, the waveguide spacing s was adjusted to 3.0 μm; the coupling length L−1.4 mm. In curve ■, the coupling rate increases monotonically as the wavelength increases, but in curve ■, it shows a gentle change with a peak near 1.4 μI, and from 1.25 μI to 1
．． The coupling rate is maintained at 5%±1% over a wide wavelength range of 6μI.

The coupling characteristics of each embodiment described above are summarized as shown in FIG. 5, and it is possible to design a desired coupling rate in a desired wavelength range. Note that in the present invention, the cutoff wavelength shifts to the shorter wavelength side as the value of each waveguide parameter, especially the waveguide width B, decreases, so this may also have to be taken into consideration during design. .

The structural parameters of the coupling part of the directional coupler have been described in each of the examples above, but since the directional coupler is an extremely structure-sensitive optical circuit element, manufacturers should take into consideration the peculiarities of each manufacturing process. You can then change its parameters.

FIGS. 6 to 8 show the wavelength dependence of the envelope on waveguide parameters, which is most important when designing a waveguide type optical branching element according to the invention. The key is to determine the waveguide width B and waveguide spacing S that can achieve the center wavelength of the desired wavelength range and its coupling rate.
, the value of the refractive index difference Δ may be estimated from the envelope characteristics of FIGS. 6 to 8, and then the coupling length may be changed to 0−ω to find an appropriate value.

In addition, in each of the above embodiments, the case where the waveguide widths at the coupling portion of one directional coupler are made different from each other is dealt with, but this can be extended to connect N directional couplers. It is also possible to achieve an overall flatter wavelength-dependent characteristic.

Further, in each of the above embodiments, the optical branching element was constructed by the silica-based (SiO, -TiO,) optical waveguides 1a and 1b on the silicon substrate 1, but the substrate is not limited to silicon, and the substrate may be made of quartz glass. It is also possible to change to a substrate or the like. Furthermore, a 5j02-Ge02 optical waveguide using Gem2 as the main dopant in the core can also be used. Furthermore, as described above, the present invention is not limited to these silica-based optical waveguides, but can also be applied to other waveguide material systems, such as multi-component glass waveguide systems and lithium niobate waveguide systems.

Additionally, in each of the above embodiments, the difference in propagation constant was set by the difference in waveguide width, but in some cases, the width of the optical waveguides may be the same and there may be a slight difference in the refractive index values of the two optical waveguides. The effective propagation constant difference may be set by a method of giving , or by compositing the core region with different types of core materials. For example, a desired optical branching element can be realized by installing a thin film heater above the optical waveguide between the directional couplers and adjusting the refractive index value of the optical waveguide on one side using the thermo-optic effect. In addition to the preset propagation constant difference, a preliminary thin film heater is loaded on one of the waveguides, and coupling rate characteristics can be switched (wavelength dependent) by turning on and off the thin film heater. It is also possible to have a small wavelength dependence (high wavelength dependence).

[Effects of the Invention] As explained above, according to the present invention, the waveguide film deposition method (
We provide a practical structure for a directional coupler type optical branching element manufactured by a combination of known techniques such as flame deposition method, sputtering method, etc.) and microfabrication brainer technology such as photolithography and reactive ion etching. the desired wavelength range,
For example, in a wavelength range including 1.3 μm to 1.55 μm, it is possible to provide a low-loss waveguide type optical branching element in which the wavelength dependence of the coupling rate is relaxed to, for example, 50%±1O%. Furthermore, the present invention can provide a structure in which the width of the waveguide near the input/output port is tapered to facilitate connection with an optical fiber.

Therefore, such a waveguide type optical branching element according to the present invention,
It is expected to have a wide range of uses, including for distributing optical signals over a wide wavelength range, for monitoring, and for tapping.

Furthermore, by connecting the optical branching elements of the present invention in multiple stages on a flat substrate, it is easy to expand to a four-branching element or an eight-branching element. It is also possible to form optical branching elements in an array on the same substrate and connect them to, for example, a 250 μm pitch optical fiber array.

Furthermore, since it can be easily manufactured in large quantities on a flat substrate, it is expected that the cost will be reduced, and the optical branching element of the present invention and its application elements are expected to greatly contribute to the spread of optical communication systems.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of a waveguide type optical branching element according to a first embodiment of the present invention.
) is a sectional view taken along line 8B', FIG. 2 is a sectional view taken along line 8B', and FIG.
%) is a characteristic diagram showing the wavelength dependence of
This is a diagram showing an example (design coupling rate 20% ± 5%).
8) is a plan view showing the structure of the element, FIG. 4 (B) is a characteristic diagram showing wavelength dependence, and FIG. , Figure (8) is a plan view showing the structure of the element, Figure (B) is a characteristic diagram showing wavelength dependence,
Figure 5 is a characteristic diagram summarizing the wavelength dependence in the wavelength range of 1.2 μm to 1.8 μm when designed with a coupling rate of 10096.50% and 2H, 5t. Figures 6 to 8 are respectively A characteristic diagram showing the wavelength dependence of the maximum coupling rate (envelope) for each waveguide parameter (waveguide width B, waveguide spacing S, refractive index difference Δ). It is a diagram showing the structure of a branching element, in which (A) is a plan view and (CB) is a plan view.
(C) is a cross-sectional view taken along line DD', and Figure 1θ shows the coupling rate and wavelength of the conventional directional coupler type optical branching element shown in Figure 9. It is a characteristic diagram of an example which shows dependence. l... Substrate (silicon), Ia, H+... Optical waveguide, 2... Directional coupler, 2a, 2b... Coupling part waveguide of directional coupler 2, 3a
...Input port, 3b...Output port, 4a...Sub-input port, 4b...Sub-output port, 5...5i02-based glass membrane. Patent applicant Nippon Telegraph and Telephone Corporation

Claims (1)

[Scope of Claims] 1) Consisting of a substrate, two optical waveguides arranged on the substrate, and one directional coupler that is a part of the two optical waveguides and placed close to each other, In a waveguide type optical branching element in which one end of an optical waveguide is an input port and the other end of the optical waveguide is a main output port and a sub-output port, respectively, two optical guides in a coupling region in the directional coupler are provided. A waveguide type optical branching element characterized in that wave paths have different widths and the same depth. 2) By setting the width of one optical waveguide in the coupling region in the directional coupler to about 8 μm, the width of the other optical waveguide from about 4 μm to less than 8 μm, and the depth of both optical waveguides to about 8 μm, The wavelength dependence of the coupling rate of power from the input port to the sub-output port is set from 1.2 μm to 1.2 μm.
2. The waveguide type optical branching element according to claim 1, wherein the waveguide type optical branching element is set to be relaxed over about 8 μm. 3) The width of the waveguide near the input port and the output port is about 8 μm, which is comparable to the core size of the optical fiber to be connected to the port, and the directional coupling is performed through the tapered transient region. 3. The waveguide type optical branching element according to claim 1, wherein the waveguide type optical branching element is connected to a coupling region of a device.