JPH0496007A - Wavelength-nondependent start coupler - Google Patents

Abstract

PURPOSE:To distribute a signal in a large-scale LAN system, a wavelength multiplex system, etc., by perform uniforming branching to one optional waveguide by the start coupler and making the branching ratio of light power nearly constant irrelevantly to the wavelength. CONSTITUTION:This is the start coupler which branches the light power, which is made incident on one optical waveguide, equally to >=2 output optical waveguides. An input optical waveguide array formed by arranging input optical waveguides in a sectorial shape and an output optical waveguide array formed by arranging output optical waveguides in a sectorial shape are installed opposite each other and coupled by a slab optical waveguide which has no light confinement structure laterally, and the angle mu from the center of curvature of the sector of the input optical waveguide array or from the center of curvature of the sector of the output optical waveguide array to the output optical waveguide array is set to 0.06 - 0.10 (radian).

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is applicable to large-capacity LAN (Local Aero Ne
This article relates to star couplers, which are essential optical components for optical signal distribution in (working) systems, and in particular, the splitting ratio of optical signal power is independent of the optical wavelength, is approximately constant, and has a wide wavelength range of 13 μm to 1.6 μm. It has been improved so that it can be used in

<Prior art> Conventionally, an NXN star coupler that uniformly branches the optical power incident on any one of N input optical waveguides to N output optical waveguides has a structure shown in FIG. 12. It has been known. The star coupler shown in the figure has N=8. That is, it has eight input optical waveguides 11 and eight output optical waveguides 12, and from these large-blade leading wave BII to the output optical waveguide 12 via a 3 dB directional coupler 13, the signal is transmitted in three stages. are combined.

Therefore, the optical power incident on any one of the eight input optical waveguides 11 is divided stepwise and equally into 1/2.1/4.1/8 by the 3 dB directional coupler 13. It is branched into all eight output optical waveguides 12. Therefore, in this example, twelve 3 dB directional couplers 13 are required.

<Problems to be Solved by the Invention> In the star coupler with the conventional structure described above, the optical power incident on any input optical waveguide 11 can be equally split into a plurality of output optical waveguides 12. As shown, there is a problem in that a large number of 3 dB directional couplers 13 are required.

M = (N/2) l o g zN...(
1) However, M is the number of necessary 3 dB directional couplers 13.

For example, in the case of a large-capacity LAN system with N=128, the number becomes an enormous number of M=448.

Therefore, not only does the size of the star coupler become very large, but also the production yield is poor and it becomes expensive.

Furthermore, since the coupling rate of a 3 dB directional coupler usually changes depending on the wavelength of light, conventional star couplers can only be used at a certain wavelength.

The present invention has been made in view of the above-mentioned prior art,
The object of the present invention is to provide a star coupler that is suitable for large scale operation and can also be applied to wavelength multiplexing by branching optical power at a constant rate regardless of wavelength.

<Means for Solving the Problems> The configuration of the present invention to achieve the above object uniformly branches the optical power incident on any one of one or more input optical waveguides to two or more output optical waveguides. In the star coupler, an input optical waveguide array in which the input optical waveguides are arranged in a fan shape and an output optical waveguide array in which the output optical waveguides are arranged in a fan shape are installed facing each other, and the input optical waveguide The array and the output optical waveguide array are laterally coupled by a slab optical waveguide having no optical confinement structure, and the angle at which the output optical waveguide array is viewed from the center of curvature of the sector of the input optical waveguide array or the output optical waveguide The angle ψ looking into the input optical waveguide array from the center of curvature of the fan-shaped array is 0.06 to 0.10 (radian)
It is characterized by

Effects> The light incident on any input optical waveguide is not confined in the transverse direction with respect to the traveling direction of the light in the slab optical waveguide, so it spreads in the transverse direction. Since the input optical waveguide array and the output optical waveguide array have waveguides arranged in a fan shape, the spread light is uniformly branched to the output optical waveguide.

In particular, setting the angle from the center of curvature of the fan-shaped waveguide array of one waveguide array to the other waveguide array to be 006 to 0.10 (radian) is suitable for the relative refractive index difference of optical waveguides used practically. .

<Examples> The present invention will be described in detail below with reference to examples shown in the drawings.

FIG. 1 shows an embodiment of the present invention. This embodiment relates to an NXN star coupler.

That is, as shown in the figure, the star coupler of this embodiment is
It has an input optical waveguide 1, dummy waveguides 2 and 3, a slab optical waveguide 4, an output optical waveguide 5, and a dummy waveguide 6.7. The input optical waveguide 1 has a core with a refractive index n1 and a core with a refractive index n0.
The width of the core is 2a and the thickness is 2t.
are usually designed to be equal. The output optical waveguide 5 and dummy waveguides 2.3 and 6.7 also have the same configuration as the input optical waveguide 1.

The input optical waveguides 1 and dummy waveguides 2 and 3 are arranged in a fan shape to form an input optical waveguide array 8, and the output optical waveguides 5 and dummy waveguides 6 and 7 are also arranged in a fan shape to form an output optical waveguide array. The input optical waveguide array 8 and the output optical waveguide array 9 are arranged to face each other,
They are coupled via a slab leading waveguide 4. As shown in an enlarged view of FIG. 2, the slab optical waveguide 4 is located at an intermediate portion between an input waveguide array and an output waveguide array that are arranged in a fan shape facing each other, and is transverse to the direction of light propagation. It does not have an optical confinement structure in the direction. Here, the second
In the figure, N a r r a is the total number of waveguides including dummy waveguides 2 and 3 (or 6.7) in the input optical waveguide array 8 (or output optical waveguide array 9), and O is the fan-shaped number. The center of curvature of the arranged input optical waveguide array 8, 0
' is the center of curvature of the output optical waveguide array 9 arranged in a fan shape, Le is the distance between the centers of curvature OO', and R3 is N a r
The radius of curvature when r a number of optical waveguides are arranged without gaps, (Rs+q+) is the radius of curvature of the actual fan-shaped optical waveguide array.

Here, q≧0. The angle ψ at which the eight output waveguide arrays or input waveguide arrays are viewed from the center of curvature O or O' is given by the following equation.

Further, Y is the length of the central portion of the slab optical waveguide, and is given by the following equation.

Y, = 2 (R, +Q+) -Lc ・-13) In this way, since Y can be given if R-, qt, and Lc are known, R-, qt, LC, and ψ are the wavelength-independent star coupler. This is an important parameter in the design. In a simple case,
In the case of q, = o, Lc = Rm, Y s '' R-. Therefore, when the core radiation 2a of the leading waveguide and the order N of the star coupler are given, the angle φ when looking at the waveguide array is If R8 is specified, R8 is also determined, so it can be seen that φ is the most important parameter.In order to make it easier to see, the number of waveguides is different between FIG. 1 and FIG.

The wavelength-independent star coupler was designed using the beam propagation method (M, D, Felt et al., rLightp
ropagation in graded-inde
x 0ptical fiberJ, Apl) 1.0
pt, , Vol, 17. no, 24. pl
) 3990-3998 (1978)). FIG. 3 shows simulation results of light propagation for a star coupler of order N=16. The parameters of the star coupler are: N 11 r T 11 a = 24 (4 dummy waveguides on the left and right), Lc” 2650 μm, Rs
=1850μm, qr=400μm, Ys=1
850 μm, and the wavelength of light λ=1.55 μm. Figure 3(a) shows the central large blade leading wave II (12 counting from the left).
(b) shows the optical propagation intensity distribution when the light is input to the input optical waveguide 1 at the left end (fifth from the left). As shown in the figure, light input from any input optical waveguide 1 spreads laterally and is evenly dispersed in the slab optical waveguide 4, and is distributed to 16 output optical waveguides B5 regardless of the position of the incident light. It can be seen that the optical power is evenly split.

FIG. 4 shows the simulation results of FIG. 3 as optical propagation waveforms. Figure (a) shows when light is incident on the central input optical waveguide (12th from the left), and Figure (b) shows the leftmost input optical waveguide (5th from the left). This is the propagation waveform of light when light is incident on . Looking at the optical waveform at the boundary between the input optical waveguide array 8 and the slab optical waveguide 4, the light is coupled to several adjacent waveguides, and this side lobe is coupled between the slab optical waveguide 4 and the output optical waveguide array 9. It can be seen that this is very important in achieving uniform light distribution at the boundary. As shown in FIG. 4('b), for the input optical waveguide I at the left end (or right end), a dummy waveguide 2 (or 3) is placed on the left (or right) side.
It turns out that it is necessary to create similar side lobes at other incident positions.

Figures 3 and 4 show the results of designing the structural parameters of a wavelength-independent star coupler in order to make the branching of light uniform regardless of the incident position of the light, and to make it constant regardless of the wavelength. It is shown in FIGS.

Figures 5(a) and 5(b) are for optical waveguides under the following conditions.
This figure shows the results of calculating Lc (*), R, (○), and qt (◎) to obtain a wavelength-independent star coupler (order N=16).

However, the relative refractive index difference Δ is (n+ nz)/n+.

Note that when the relative refractive index difference Δ is 1% or more, the loss of the optical waveguide increases, and the coupling loss with the optical fiber also increases, so the optical waveguide with a relative refractive index difference Δ of 196 or less A star coupler using

FIG. 6 shows the results of calculating Lc, R, , qt by changing the order N of the star coupler for an optical waveguide with a relative refractive index difference Δ of 0.3%. Furthermore, FIG. 7 shows the result of calculating the viewing angle φ of the waveguide array. In the practical refractive index range, φ=0.06 to 0. It can be seen that it is IO. The straight lines in FIGS. 5 to 7 are fitting straight lines for each design parameter, and are expressed by the following equations.

Lc= (228-175) -N (μrn')-1
4) R, = (150-115) ・N (μm)
-(5)q r= (34-30) -N (, czm
) (61φ = 0.0496Δ + 0.054 (radian) ... (7) However, the relative refractive index difference Δ is in % Based on the computer simulation of the star coupler above, using a quartz optical waveguide. A wavelength-independent star coupler was fabricated.

First, a lower cladding layer of Sin was deposited on a Si substrate by a flame deposition method, then a core layer of Sin and glass doped with TiOx or GeOt was deposited, and then transparent vitrification was performed in an electric furnace. Next, the core layer is etched using a mask pattern created based on computer simulation to form a predetermined input/output waveguide array and slab optical waveguide region, and finally, 5i
b The upper cladding layer was deposited.

Figure 8(a) shows the optical branching characteristics of the star coupler at wavelength λ
= 1.3 μm, the same figure (b) shows the wavelength λ = 1, 42
5 μm, and (C) of the same figure shows the results of measurements at a wavelength λ=1.55 μm.

In FIGS. 8(a) to (C), the horizontal axis is the light incident position a (m) on the input optical waveguide array, and since there are four dummy waveguides on each side, m = 5 to 20, The vertical axis represents the input power P1m, the nth output waveguide (n = 5 to 20
), the normalized branch output P is defined as the following equation.

P, P, Danyu, N...(8) P.I.

Therefore, when P, = 1 (for all n),
This shows that a uniform light distribution with zero insertion loss can be obtained.

The star coupler used for the measurements in FIG. 8 has an order N=16, a relative refractive index difference Δ=0.396, and a core width of 2 a
= 8 utn, Lc=2650μm, Rs=18
50μm.

q H”400 μm, N m+r−y=24 (including four dummy waveguides on each side).

FIG. 9 shows the average value of the optical branching ratio shown in FIG. 8 for each wavelength. In FIG. 9, ◎ indicates wavelength λ=1.
3 μm, intestine wavelength λ = 1.425 μm, ○ indicates wavelength λ =
This is shown for 1.55 μm.

As is clear from this figure, the wavelength is 1.3 μm-1,55
It can be confirmed that a wavelength-independent star coupler with substantially constant light distribution over a wide wavelength range of μm can be realized.

FIG. 10 shows order N-16, relative refractive index difference Δ=0.5%,
Core width 2 a = 7 μm, Lc = 2050 μm 5R
s=1450μm, qr=300μm, N,,,a
, =24, the optical branching ratio of the star coupler is given by the wavelength λ-1,
Measurements were made using light of 3 μm, 1425 μm, and 1.55 μm, and the average value of the branching ratio at each wavelength is shown.

From these Figures 9 to 12, the design of a wavelength-independent star coupler by computer simulation (Figures 5 to 7)
The validity of the figures and formulas (4) to (7)) was confirmed.

<Effects of the Invention> As specifically explained above based on the embodiments, the star coupler of the present invention can uniformly branch into any single waveguide, and the branching ratio of optical power is independent of the wavelength. Since it is substantially constant, it has a great advantage in signal distribution in large-scale LAN systems, wavelength multiplexing systems, and the like.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a wavelength-independent star coupler according to an embodiment of the present invention, FIG. 2 is an enlarged view of a slab optical waveguide, and FIG.
Figures (a) and (b) are explanatory diagrams showing the simulation results of light propagation with different incident positions for N-16 star couplers, and Figures 4 (a) and (b) are N-1 star couplers, respectively.
Figure 5 (a) is an explanatory diagram showing the simulation results of the optical propagation waveform when the incident position is changed for the star coupler No. 6.
5(b) is a graph showing the relationship of parameters qr to relative refractive index difference, and FIG. 6 is a graph showing the relationship of parameters Lc, Rs, qt to order N. Graph showing the relationship between
The figures are graphs showing the relationship between the angle ψ and the relative refractive index difference △. A graph showing the average value of the optical branching ratio shown in the figure for each wavelength, Figure 10 is a graph showing the average value of the optical branching ratio measured at each wavelength, and Figure 11 is a graph showing the optical branching ratio measured at each wavelength. 12 is an explanatory diagram showing the structure of a conventional NXN star coupler. In the drawing, I and 11 are large-blade leading waveguides, 2.3 and 6.7 are dummy waveguides, and 4 is a slab guiding waveguide, 5.12 is an output optical waveguide, 8 is an input optical waveguide array, 9 is an output optical waveguide array, and I3 is a 3 dB directional coupler.

Claims (1)

[Claims] In a star coupler that uniformly branches optical power incident on any one of one or more input optical waveguides into two or more output optical waveguides, the input optical waveguide array is formed by arranging the input optical waveguides in a fan shape. and an output optical waveguide array formed by arranging the output optical waveguides in a fan shape, are installed facing each other, and the input optical waveguide array and the output optical waveguide array are arranged in a slab having no optical confinement structure in the lateral direction. They are coupled by optical waveguides, and the angle at which the output optical waveguide array is viewed from the center of curvature of the sector of the input optical waveguide array, or the angle ψ at which the input optical waveguide array is viewed from the center of curvature of the sector of the output optical waveguide array is 0.06. ~0.1
A wavelength-independent star coupler characterized by being 0 (radian).