JP2006106372A - Optical branching apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical branching apparatus that lowers optical loss. <P>SOLUTION: The optical branching apparatus is composed of an input waveguide 1, a mode conversion tapered waveguide 2 connected to the input waveguide 1, and two output waveguides 3, 4 connected to the mode conversion tapered waveguide 2. In the propagation of a light wave from the input waveguide 1 to the mode conversion tapered waveguide 2, the ground mode and the high-order mode of the mode conversion tapered waveguide 2 are excited, with no ground mode dissipated in the structure. In addition, interference fringes by the ground mode and the high-order mode localize its strength in the connecting position with the output waveguides 3, 4 in the mode conversion tapered waveguide 2 in the structure. Thus, the input waveguide 1 and the output waveguides 3, 4 are efficiently coupled with each other, thereby lowering the light wave loss. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical branching device in optical fiber communication.

  Due to the rapid development of optical communication technology, various types of optical components have been researched and developed, and among these, waveguide type optical components based on optical waveguides on a flat substrate occupy the most important position. This is because the waveguide-type optical component has a feature that it can be mass-produced with high reproducibility with accuracy below the optical wavelength by photolithography technology and microfabrication technology. In particular, an optical branching device composed of a silica-based optical waveguide can be easily arrayed, and is excellent in terms of stability and controllability over time, and researched vigorously with application to FTTH in mind. It is being advanced.

  Non-Patent Document 1 below discloses an optical branching device as shown in FIG. In this optical branching device 10, a light wave incident from an input waveguide 11 having a width W1 passes through a mode filter waveguide 12 having a width W2 (W1> W2) and has a length L, a start width W2, and a termination width We. The light enters the tapered waveguide 13 that linearly widens its width. When an optical signal is input to the tapered waveguide 13, the light wave excites the fundamental mode of the tapered waveguide 13, and the width of the electric field distribution is gradually increased according to the propagation. Two output waveguides 14 and 15 having a width W1 are connected to the end face 13a which is the output end face of the tapered waveguide 13, and the optical electric field at the output end face of the tapered waveguide 13 is directed to the output waveguides 14 and 15. Join each one. However, in such an optical branching device 10, the connection position where the tapered waveguide 13 and the two output waveguides 14 and 15 are connected to each other is limited due to manufacturing limitations, that is, photolithography resolution and etching resolution limitations. In the meantime, a gap G inevitable in manufacturing is formed. Since a part of the light wave propagating through the tapered waveguide 13 leaks from the gap G, the loss in the entire optical branching device 10 increases.

Non-Patent Document 2 describes a technique related to a 1 × 2 multimode interference coupler. This multimode interference coupler has an optical branching function similar to that of the optical branching device 10. As shown in FIG. 4, in the 1 × 2 multimode interference coupler 20, a light wave incident from an input waveguide 21 that is a single-mode waveguide having a width W is a multimode waveguide having a width Ws (W <Ws) and a length L. 22 is incident. In the multimode waveguide 22, higher order modes are excited in addition to the fundamental mode, and the optical power forms a mountain at a specific position due to interference of each mode. Furthermore, the mountain changes its position according to the propagation of the light wave. At the end face 22a of the multimode waveguide 22, the interference fringes is localized in two peaks leaving the optical power by a distance S _0. Therefore, the optical power is efficiently coupled to the output waveguides 23 and 24 having a width W connected to the localized position of the optical power on the end face 22a of the multimode waveguide 22 by a distance S ₀ , respectively. Operates as a branching device. The output waveguides 23 and 24 are single mode waveguides.

  As described above, since the multi-mode interference coupler 20 needs to set a peak of interference fringes at the connection position of the output waveguides 23 and 24 using the interference of the second and higher order modes, the multi-mode waveguide It is necessary to set the width Ws of 22 wide. However, since the chromatic dispersion of each mode is different in the multimode waveguide 22, wavelength dependence also occurs in the state of the interference fringes. Furthermore, as a result of sufficiently widening the width Ws of the multimode waveguide 22, the fundamental mode of the multimode waveguide 22 is dissipated from the side surface 22 b of the multimode waveguide 22, thereby increasing the loss. In addition, in the multimode waveguide 22, since the equivalent refractive index that is generally felt depends on the polarization plane of light, there is also polarization dependency of loss.

A. Klekamp, P. Kersten, and W. Rehm, “An Improved Single-Mode Y-Branch Design for Cascaded 1: 2 Splitters”, IEEE J. Lightwave Technol., Vol. 14, No. 12, 2684-2686, 1996 M. Bouda, JWM van Uffelen, C. van Dam, and BH Verbeek, “Compact 1x16 power splitter based on symmetric 1x2 MMI splitters,” Electron. Lett., Vol. 30, No. 21, pp. 1756-1757, 1994 . W.K.Burns, A.F.Milton, A.B Lee, “Optical waveguide parabolic coupling hours,” Appl.Phys.Lett., Vol.30, pp.28-30,1997

As described above, the conventional optical branching device has a problem of increasing the loss due to the limitation in manufacturing. The inventor verified each parameter of the optical branching device 10 with a quartz optical waveguide having W1 = 7 μm, W2 = 5 μm, L = 400 μm, We = 16 μm and a relative refractive index difference of 0.5%. According to this verification, the optical branching device 10 has an excess loss of about 0.6 dB to 0.7 dB in addition to the loss of 3 dB that occurs in principle. Such excess loss gives a total increase of (loss excess per stage) × Log ₂ N in a 1 × N splitter configured by cascading optical branching devices in multiple stages in a tree shape. For example, it has been found that an excess loss of about 1.8 dB to 2.1 dB is caused in a 1 × 16 optical splitter in which four stages of optical branching devices are connected in cascade.

  In addition, the inventor verified each parameter of the multimode interference coupler 20 with a silica-based optical waveguide with W = 6 μm, Ws = 28 μm, L = 200 μm and a relative refractive index difference of 0.5%. At this time, the position where the interference fringes form a mountain was So = 20 μm. In the multimode interference coupler 20 having such parameters, the excess loss is 0.9 dB for 1.2 μm non-polarized light and 0.2 dB for 1.6 μm non-polarized light, and exhibits a large wavelength dependence. At 1.55 μm, it was 0.3 dB for TE polarized light and 0.6 dB for TM polarized light, indicating a large polarization dependence.

  Therefore, the present invention has been proposed in view of the above-described problems, and an object of the present invention is to provide an optical branching device that reduces wavelength dependency and polarization dependency and reduces loss.

  An optical branching device according to a first aspect of the present invention that solves the above-described problem is an optical branching device using an optical waveguide formed on a substrate, and includes an input waveguide and a mode conversion taper guide connected to the input waveguide. A waveguide and two output waveguides connected to the mode conversion taper waveguide, and in the propagation of light waves from the input waveguide to the mode conversion taper waveguide, the fundamental mode of the mode conversion taper waveguide And a high-order mode is excited and the base mode is not dissipated, and interference fringes due to the base mode and the high-order mode are connected to the output waveguide in the mode conversion tapered waveguide. It has a structure that localizes strength.

  An optical branching device according to a second invention that solves the above-described problem is the optical branching device according to the first invention, wherein the fundamental mode in the propagation from the incident end to the output end of the mode conversion tapered waveguide The length of the mode conversion taper waveguide is set so that the difference between the phase change of π and the phase change of the higher-order mode becomes π at the input optical signal wavelength.

  An optical branching device according to a third invention for solving the above-described problem is the optical branching device according to the first invention or the second invention, wherein the square of the width of the mode conversion tapered waveguide is the input waveguide. It is characterized by a linear function having a variable z as the distance z in the propagation direction of the light wave from the connection position between the waveguide and the mode conversion tapered waveguide.

  An optical branching device according to a fourth invention that solves the above-described problem is the optical branching device according to any one of the first to third inventions, wherein the mode conversion tapered waveguide has an incident light wavelength. In the present invention is characterized by having a structure of width, height, and relative refractive index difference that allows existence of a fundamental mode and higher-order modes up to the second order.

  According to the optical branching device according to the first aspect of the present invention, there is provided an optical branching device using an optical waveguide configured on a substrate, the input waveguide, a mode conversion tapered waveguide connected to the input waveguide, A two-wavelength output waveguide connected to the mode conversion taper waveguide, and in the propagation of the light wave from the input waveguide to the mode conversion taper waveguide, the fundamental mode and the higher order mode of the mode conversion taper waveguide Is excited and the fundamental mode does not dissipate, and the interference fringes due to the fundamental mode and the higher-order mode localize the intensity at the connection position of the mode conversion tapered waveguide with the output waveguide. By having a structure that allows the mode conversion taper waveguide to be connected between modes regardless of the gap formed between the two output waveguides at the end face of the mode conversion taper waveguide. By using interference fringes, the light wave and the two output waveguides of the mode conversion tapered waveguide can be efficiently coupled respectively. In addition, since the configuration avoids the dissipation of the fundamental mode, there is no increase in loss that has occurred in the conventional multimode interference coupler. As a result, loss can be reduced.

  According to a second aspect of the present invention, there is provided the optical branching device according to the first aspect, wherein the phase change of the fundamental mode in the propagation from the incident end to the output end of the mode conversion tapered waveguide is By setting the length of the mode conversion taper waveguide so that the difference in phase change of the higher-order mode is π at the input optical signal wavelength, the two output waveguides are connected to the mode conversion taper waveguide. A crest of interference fringes is present at the connection position where the two are connected, and the optical power at the end face of the mode conversion taper waveguide can be efficiently coupled to the two output waveguides.

  According to the optical branching device of the third invention, the optical branching device according to the first invention or the second invention, wherein the square of the width of the mode conversion tapered waveguide is the input waveguide and the mode The propagation angle of the fundamental mode is changed from the change angle of the width of the mode conversion taper waveguide by changing the linear function with the distance z in the propagation direction of the light wave from the connection position with the conversion taper waveguide as a variable. Since it becomes smaller, the fundamental mode is not dissipated.

  According to an optical branching device according to a fourth aspect of the present invention, there is provided the optical branching device according to any one of the first to third aspects, wherein the mode conversion tapered waveguide has a fundamental mode and an incident light wavelength. By having a width, height, and relative refractive index difference structure that allows the presence of higher-order modes up to the second order, the influence of chromatic dispersion and polarization mode dispersion up to the fundamental mode and the second-order mode is minimized. Can be suppressed.

  Hereinafter, the best mode for carrying out the optical branching apparatus according to the present invention will be specifically described based on examples.

  FIG. 1 is a schematic diagram of an optical branching device according to a first embodiment of the present invention, and FIG. 2 is output from two output waveguides in the optical branching device according to the first embodiment of the present invention. It is the graph which showed the amount of attenuation of the sum of optical power.

As shown in FIG. 1, the optical branching device according to the first embodiment of the present invention includes an input waveguide 1 having a width Ws, a mode conversion tapered waveguide 2 connected to the input waveguide 1, and a mode conversion. It consists of two output waveguides 3 and 4 connected to the tapered waveguide 2. The light wave incident on the input waveguide 1 propagates to the two output waveguides 3 and 4 through the mode conversion taper waveguide 2. The width W (z) of the mode conversion taper introduction path 2 is such that the square of the width W (z) of the mode conversion taper waveguide 2 is equal to the input waveguide 1 and the mode conversion taper waveguide 2 as shown in the following equation (1). Is changed by a linear function with the distance z in the propagation direction of the light wave from the connection position as a variable, and gradually becomes thicker in the propagation direction of the light wave.

Where λ is the wavelength of the optical signal, n is the effective refractive index of the mode conversion taper waveguide 2, α is a constant, the x axis is the width direction of the mode conversion taper waveguide 2, and the xz coordinate system is shown in FIG. Define as follows. That is, the mode conversion taper waveguide 2 has a predetermined length L, and its width is We at the end face 2a.

Further, the start end faces 3 a and 4 a of the output waveguides 3 and 4 at the connection position with the mode conversion taper waveguide 2 have a width W ₀ . A gap G that is unavoidable in manufacturing is determined between the connection positions of the two output waveguides 3 and 4 on the end face 2a of the mode conversion taper waveguide 2 and determined by the resolution of photolithography. Starting surface 3a of the output waveguides 3 and 4, the width W ₀ at 4a, a gap G, between the width We of the end face 2a of the mode conversion tapered waveguide 2, a relationship shown by the following equation (2) Have.
2 × W ₀ + G = We (2)

  The output waveguides 3 and 4 extend in parallel from the start end faces 3a and 4a toward the end faces 3b and 4b, and subsequently have a radius of curvature R determined by the relative refractive index difference Δ of the waveguides 3 and 4. Are bent so as to be convex toward the opposite sides, and then bent so as to be convex toward the outside with a radius of curvature R. Further, the output waveguides 3 and 4 are arranged so as to extend in parallel to each other at the termination surfaces 3b and 4b up to the interval S, and the widths of the termination surfaces 3b and 4b are the same as the width Ws of the input waveguide 1. It has a structure having a taper while being bent.

よって、モード変換テーパ導波路２の幅の変化角度θ_hは、以下の（４）式となる。

上記（４）式を式変形して、下記（５）式となる。

一方、基底モードの伝搬角θ_pは、下記（６）式で表される。

よって、θ_hとθ_pとの間には、下記（７）式の関係が成り立つ。
θ_h＝αθ_p （７）
ここで、αは１より小さい（α＜１）ので、下記（８）式が成り立つ。
θ_h＜θ_p （８）
したがって、モード変換テーパ導波路２の幅の変化角度θ_hは基底モードの伝搬角θ_pより小さくなるので、基底モードは散逸しないようになる。 Here, it is shown below that the fundamental mode is not dissipated in the optical branching device 6.
In the case of the above equation (1), the rate of change of the width W (z) of the mode conversion taper waveguide 2 is obtained by differentiating the above equation (1) by the variable z and the following equation (3).

Therefore, the change angle θ _h of the width of the mode conversion tapered waveguide 2 is expressed by the following equation (4).

The above equation (4) is transformed into the following equation (5).

On the other hand, the propagation angle θ _p of the fundamental mode is expressed by the following equation (6).

Thus, between the theta _h and theta _p, it holds the relationship of the following equation (7).
θ _h = αθ _p (7)
Here, since α is smaller than 1 (α <1), the following equation (8) is established.
θ _h <θ _p (8)
Therefore, since the change angle θ _h of the width of the mode conversion taper waveguide 2 is smaller than the propagation angle θ _{p of} the base mode, the base mode is not dissipated.

The above-described propagation angle θ _p of the fundamental mode is a reflection angle with respect to the fundamental mode described in the light ray theory, and the angle beyond this is the limit angle at which light leaks to the cladding at the core-cladding boundary. That is, when the change angle θ _h of the width of the mode conversion taper waveguide 2 becomes larger than θ _p , the fundamental mode leaks into the clad. That is, under the condition of equation (8), since the change angle θ _h is smaller than the propagation angle θ _p of the fundamental mode, it is possible to increase the width of the mode conversion tapered waveguide 2 while repeating total reflection. Become. Here, θ _h and θ _p are the same definitions as θ _h and θ _p shown in Non-Patent Document 3 above.

  In Example 1, an optical branching device was manufactured using Ws = 7.5 μm, L = 290 μm, α = 0.25, R = 15 mm, Δ = 0.45%, and S = 250 μm as the parameters described above. . The height in the direction perpendicular to the surface formed by the height of the mode conversion taper waveguide 2 waveguide, that is, the length L of the mode conversion taper waveguide 2 and its width W (z) was set to 7 μm. In particular, the gap G, which is unavoidable in production, is set to a typical value of about 3 μm. Therefore, the width Wo at the start end faces 3a and 4a of the output waveguides 3 and 4 is 13.5 μm, respectively. According to the calculation using the above-described parameters, the termination surface 2a of the mode conversion tapered waveguide 2 is allowed to have higher-order modes (primary mode and secondary mode) up to the fundamental mode and the secondary mode, respectively. Are equivalent to n0 = 1.4540, n1 = 1.4566, and n2 = 1.4571. As described above, since the structure allows only the higher order modes up to the second order, the influence of the chromatic dispersion and the polarization mode dispersion up to the fundamental mode and the second order mode can be minimized.

  Further, according to the optical branching device 6, the light wave incident from the input waveguide 1 to the mode conversion taper waveguide 2 excites only the even mode of the mode conversion taper waveguide 2. This is because the incident position on the mode conversion taper waveguide 2 is line symmetric with respect to the propagation of light. In addition, the calculation shows that the excitation ratio between the fundamental mode and the secondary mode is about 3: 1. Therefore, the interference fringes in the fundamental mode and the secondary mode have a definition of 0.75.

  Further, when the length L of the mode conversion taper waveguide 2 is 290 μm, the interference fringes form peaks at the positions where the output waveguides 3 and 4 exist. That is, in the optical branching device 6 having the above-described parameters, the phase change of the fundamental mode in the propagation of the light wave from the input end face of the mode conversion tapered waveguide 2 to which the input waveguide 1 is connected to the termination face 2a that is the output end face. And the difference in phase change between the second-order modes has a condition that becomes π at the input optical signal wavelength. In other words, the optical branching device of the present invention can be realized by giving the lengths L and α where π is the difference in phase change between the fundamental mode and the secondary mode at the wavelength.

  In production, a quartz glass is deposited on a silicon substrate using a flame deposition method (FHD), and an optical waveguide structure is formed by photolithography and reactive ion etching. Realized by applying the method. At this time, the gap G determined by photolithography and reactive ion etching was about 2 μm. As is apparent from FIG. 2, the attenuation is suppressed to 0.15 dB or less from the wavelength range of 1.2 μm to 1.6 μm in both TE and TM polarizations, and it is understood that there is no wavelength dependence. Since the difference between the polarization loss and the TM polarization loss is 0.02 dB at the wavelength of 1.3 μm at which the difference is greatest, it can be seen that there is no polarization dependency.

  Therefore, according to the optical branching device 6 according to the first embodiment of the present invention, the mode is independent of the gap G formed between the output waveguides 3 and 4 in the termination surface 2a of the mode conversion tapered waveguide 2. By utilizing the interference fringes between them, the light wave of the mode conversion taper waveguide 2 and the output waveguides 3 and 4 can be efficiently coupled. In addition, since the configuration avoids the dissipation of the fundamental mode, there is no increase in loss that has occurred in the conventional multimode interference coupler.

  In addition, although the conventional multimode interference coupler uses the same interference effect as that of the optical branching device 6, the loss is increased due to the dissipation of the fundamental mode in the multimode waveguide. On the other hand, since the optical branching device 6 is configured to avoid the dissipation of the base mode, there is no increase in loss that has occurred in the multimode interference coupler. As a result, the optical branching device can reduce loss.

1 is a schematic diagram of an optical branching apparatus according to a first embodiment of the present invention. It is the graph which showed the attenuation amount of the sum of the optical power output from two output waveguides in the optical branching apparatus which concerns on 1st Example of this invention. It is the schematic of the conventional optical branching device. It is the schematic of the conventional multimode interference coupler.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Input waveguide 2 Mode conversion taper waveguide 3, 4 Output waveguide 6 Optical branching device

Claims (4)

An optical branching device using an optical waveguide configured on a substrate,
An input waveguide, a mode conversion taper waveguide connected to the input waveguide, and two output waveguides connected to the mode conversion taper waveguide,
In the propagation of the light wave from the input waveguide to the mode conversion taper waveguide, the fundamental mode and the higher order mode of the mode conversion taper waveguide are excited and the base mode is not dissipated.
An optical branching device having a structure in which interference fringes due to the fundamental mode and the higher-order mode localize the intensity at a connection position of the mode conversion tapered waveguide with the output waveguide. The optical branching device according to claim 1,
The mode conversion taper guide is such that the difference between the phase change of the fundamental mode and the phase change of the higher order mode in the propagation from the entrance end to the exit end of the mode conversion taper waveguide is π at the input optical signal wavelength. An optical branching device characterized by setting a length of a waveguide. The optical branching device according to claim 1 or 2,
The square of the width of the mode conversion taper waveguide is changed by a linear function using a distance z in the propagation direction of a light wave from a connection position between the input waveguide and the mode conversion taper waveguide as a variable. Optical branching device. An optical branching device according to any one of claims 1 to 3,
The optical branching device, wherein the mode conversion tapered waveguide has a structure of width, height, and relative refractive index difference that allows existence of a fundamental mode and higher-order modes up to the second order at an incident light wavelength.

Images