JP2967428B2 - Wavelength independent star coupler - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a star coupler which is an essential optical component for optical signal distribution in a large-capacity LAN (Local Aera Network) system. The branching ratio is improved so that it can be used in a wide wavelength range of 1.3 μm to 1.6 μm, with the branching ratio being substantially constant without depending on the light wavelength.

<Prior Art> An N × N star coupler that uniformly branches optical power incident on an arbitrary one of N input optical waveguides into N output optical waveguides has a structure shown in FIG. Are 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 these input optical waveguides
The optical waveguide 11 is coupled to the output optical waveguide 12 via a 3 dB directional coupler 13 in three stages.

Therefore, the optical power incident on an arbitrary one of the eight input optical waveguides 11 is stepwise reduced by the 3 dB directional coupler 13 to 1/1 /.
The light is equally branched into 2, 1/4, and 1/8, and 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 above-described star coupler having the conventional structure, the optical power incident on an arbitrary input optical waveguide 11 is converted into a plurality of output optical waveguides.
Although it can be equally branched into 12, there is a problem that a large number of 3 dB directional couplers 13 are required as shown in the following equation.

M = (N / 2) log ₂ N (1) where M is the number of required 3 dB directional couplers 13.

For example, in the case of a large-capacity LAN system with N = 128, M
= 448, a huge number. Accordingly, the size of the star coupler is extremely large, and the production yield is low and the cost is high.

In addition, since the coupling ratio of a 3 dB directional coupler usually changes depending on the wavelength of light, a conventional star coupler can be used only at a specific wavelength.

The present invention has been made in view of the above prior art, and provides a star coupler suitable for a large scale and applicable to wavelength multiplexing by branching optical power constant regardless of wavelength. The purpose is to do so.

<Means for Solving the Problems> In a star coupler for uniformly branching the optical power incident on any one of one or more input optical waveguides to two or more output optical waveguides to achieve the object, An input optical waveguide array in which the input optical waveguides are arranged in a fan shape,
A slab optical waveguide having an output optical waveguide array in which the output optical waveguides are arranged in a sector shape and facing each other, and the input optical waveguide array and the output optical waveguide array having no lateral light confinement structure. And the center of curvature of the input optical waveguide array is located on the output optical waveguide array side from the arc end of the slab optical waveguide.

<Operation> The light incident on an arbitrary input optical waveguide is not confined in the slab optical waveguide in the lateral direction with respect to the traveling direction of the light, and thus spreads in the lateral direction. The spread light
Since the input optical waveguide array and the output optical waveguide array have the waveguides arranged in a fan shape, the input optical waveguide array and the output optical waveguide array are uniformly branched into the output optical waveguide. It is preferable that the center of curvature of the sector of the optical waveguide array is located on the optical waveguide array side from the arc end of the slab optical waveguide.

<Example> Hereinafter, the present invention will be described in detail with reference to examples shown in the drawings.

FIG. 1 shows an embodiment of the present invention. In this embodiment, N × N
It is related to a star coupler. That is, as shown in the figure, the star coupler of the present embodiment has the input optical waveguide 1,
Dummy waveguides 2 and 3, slab optical waveguide 4, output optical waveguide 5
And dummy waveguides 6 and 7. Input optical waveguide 1
Is constructed over the core of refractive index n ₁ in the cladding of refractive index n _0, a width 2a and thickness 2t of the core are normally equally designed. The output optical waveguide 5 and the dummy waveguides 2, 3, 6, and 7 have the same configuration as the input optical waveguide 1.

The input optical waveguide 1 and the 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 the 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 and are connected via the slab optical waveguide 4. As shown in an enlarged view of FIG. 2, the slab optical waveguide 4 is located at an intermediate portion between the input waveguide array and the output waveguide array which are arranged in a fan shape facing each other, and is transverse to the light traveling direction. It does not have a light confinement structure in the direction. Here, in FIG. 2, N _array is the dummy waveguides 2, 3 (or 6, 5) in the input optical waveguide array 8 (or the output optical waveguide array 9).
The total number of the waveguide, including 7), O is the center of curvature of the input optical waveguide array 8 arranged in a fan, O 'is the center of curvature of the output optical waveguide array 9 disposed in a fan, L _c is the curvature center OO' The distance between them, R _s is the radius of curvature when N _array optical waveguides are arranged without gaps, and (R _s + q _f ) is the radius of curvature of the actual fan-shaped optical waveguide array. Here is a q _f ≧ 0. The angle ψ at which N 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 _s is a slab optical waveguide central portion length is given by the following equation.

_{Y s = 2 (R s +} q f) -L c ... (3) Thus, for Y _s is given Knowing _{R s, q f, L c} , R s, q f, L c and ψ is This is an important parameter in the design of the wavelength-independent star coupler. As a simple case, q _f = 0,
In the case of L _c = R _s becomes Y _s = R _s. Therefore, given the core width 2a of the optical waveguide and the order N of the star coupler, if the angle 込 む for which the waveguide array is to be specified is specified, R _s is also determined, so that と is the most important parameter. I understand.
Note that the number of waveguides differs between FIG. 1 and FIG. 2 for easy viewing.

The wavelength-independent star coupler was designed using the beam propagation method (MDFeit et al., “Light propagation in g
raded-index optical fiber, Appl.Opt., Vol.17, no.24,
pp3990-3998 (1978)). FIG. 3 shows a simulation result of light propagation for a star coupler of order N = 16. The star coupler parameter is N _array
= 24 (four dummy waveguides on each side), L _c = 2650μ
m, is _{R s = 1850μm, q f =} 400μm, Y s = 1850μm, the wavelength of light lambda = 1.55 .mu.m. FIG. 3 (a) shows the case where light is incident on the center input optical waveguide 1 (12th from the left), and FIG. 3 (b) shows the leftmost input optical waveguide 1 (from the left).
(Fifth in counting) is a light propagation intensity distribution when light is incident. As shown in the figure, light input from an arbitrary input optical waveguide 1 spreads in the lateral direction in the slab optical waveguide 4 and is uniformly dispersed, and is transmitted to the 16 output optical waveguides 5 irrespective of the incident light position. It can be seen that the optical power is branched uniformly.

FIG. 4 shows the simulation result of FIG. 3 as a light propagation waveform. FIG. 9A shows the case where light is incident on the center input optical waveguide (12th from the left), and FIG. 9B shows the leftmost input optical waveguide (5th from the left). 7 shows a light propagation waveform when light is incident on the light source. Looking at the optical waveform at the boundary between the incident optical waveguide array 8 and the slab optical waveguide 4, the light is coupled to several adjacent waveguides, and the side lobes of the light are coupled to the slab optical waveguide 4 and the output optical waveguide array 9. Is very important in realizing a uniform light distribution at the boundary of. As shown in FIG. 4 (b), for the input optical waveguide 1 at the left end (or right end), a dummy waveguide 2 (3) is provided on the left (or right) side thereof.
It can be seen that it is necessary to provide similar side lobes at other incident positions.

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

FIGS. 5 (a) and 5 (b) show the optical waveguide under the following conditions.
For obtaining a wavelength-independent star coupler (order N = 16)
It shows the result of calculation of L _c (●), R _s (○), and q _f (、).

However, the relative refractive index difference Δ is (n ₁ −n ₂ ) / n ₁ .

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. It is considered that a star coupler using a wave path is practical.
FIG. 6 shows the results of calculating L _c , R _s , and q _f for an optical waveguide having a relative refractive index difference Δ of 0.3% while changing the order N of the star coupler. Further, FIG. 7 shows the estimated angle ψ of the waveguide array.
Is the result of calculating. In the range of practical refractive index,
= 0.06 to 0.10. The straight lines in FIGS. 5 to 7 are fitting straight lines for the respective design parameters, and are represented by the following equations.

L _c = (218−175) · N (μm)… (4) R _s = (150−115) · N (μm)… (5) _qr = (34−30) · N (μm)… (6) Ψ = 0.0496Δ + 0.054 (radian)… (7) However, the relative refractive index difference Δ is% Based on the above computer simulation of the star coupler, a wavelength-independent star coupler is manufactured using a quartz optical waveguide. did.

First, an SiO ₂ lower cladding layer is deposited on a Si substrate by a flame deposition method, and then a SiO ₂ glass core layer doped with TiO ₂ or GeO ₂ as a dopant is deposited. did. Next, the core layer was etched using a mask pattern produced based on computer simulation to form a predetermined input / output waveguide array and slab optical waveguide region, and finally, a SiO ₂ upper clad layer was deposited.

FIG. 8 (a) shows the optical branching characteristics of the star coupler for a wavelength λ = 1.3 μm, and FIG. 8 (b) shows a wavelength λ = 1.425 μm.
FIG. 3C shows the results of measurement for the wavelength λ = 1.55 μm. Fig. 8 (a)-
In (c), the horizontal axis is the light incident position (m) on the input optical waveguide array, and m = 5 to 20 because there are four dummy waveguides on the left and right, and the vertical axis is the input power _Pin. When the output power of the n-th output waveguide (n = 5 to 20) is P _out.n , it is a normalized branch output P _n defined as the following equation.

Therefore, when P _n = 1 (for all n), a uniform light distribution with zero insertion loss is obtained. The star coupler used for the measurement in FIG. 8 has an order N = 16, a relative refractive index difference Δ = 0.3%, a core width 2a = 8 μm, L _c = 2650 μm, R _s
= 1850μm, a _{q f = 400μm, N array =} 24 ( including four increments the dummy waveguide right and left).

FIG. 9 shows the average value of the optical branching ratio shown in FIG. 8 for each wavelength. In FIG. 9, ◎ indicates that the wavelength λ = 1.
3 μm, Δ: wavelength λ = 1.425 μm, ○: wavelength λ = 1.55 μm
It is shown about. As is clear from this figure,
It can be confirmed that a wavelength-independent star coupler having a substantially constant light distribution over a wide wavelength range of 1.3 μm to 1.55 μm can be realized.

FIG. 10 shows that the order N = 16, the relative refractive index difference Δ = 0.5%, the core width 2a = 7 μm, L _c = 2050 μm, R _s = 1450 μm, and q _f = 300.
[mu] m, the light branching ratio of the star coupler of N _array = 24, the wavelength lambda = 1.3 .mu.m, determined using 1.425Myuemu, light of 1.55 .mu.m, shows the average of the branching ratio in each wavelength.

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

<Effects of the Invention> As described above in detail based on the embodiments, the star coupler of the present invention uniformly branches into any single waveguide, and the branching ratio of the optical power does not depend on the wavelength. Since it is substantially constant, it has a great advantage in signal distribution in a large-scale LAN system, a wavelength multiplexing system, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration 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, FIG.
4 (a) and 4 (b) are explanatory diagrams showing simulation results of light propagation at different incident positions for N = 16 star couplers, and FIGS. 4 (a) and 4 (b) are respectively N = 16 star couplers. FIG. 5 (a) is a graph showing the relationship between the relative refractive index difference and the parameters L _c and R _s , while FIG.
FIG. 5 (b) is a graph showing the relationship between the relative refractive index difference and the parameter q _f , and FIG. 6 is a graph showing the parameter L _c ,
7 is a graph showing the relationship between R _s and q _f , FIG. 7 is a graph showing the relationship between the relative refractive index difference Δ and the angle 、, and FIGS. 8 (a) and (b).
(C) is a graph showing the result of measuring the optical branching characteristic of the star coupler, FIG. 9 is a graph showing the average value of the optical branching ratio shown in FIG. 8 for each wavelength, and FIG. 10 is each wavelength. Graph showing the average value of the optical branching ratio measured in
FIG. 11 is a graph showing the average value of the optical branching ratio measured at each wavelength, and FIG. 12 is an explanatory diagram showing the structure of a conventional N × N star coupler. In the drawing, 1,11 is an input optical waveguide, 2,3,6,7 are dummy waveguides, 4 is a slab optical waveguide, 5 and 12 are output optical waveguides, 8 is an input optical waveguide array, and 9 is an output optical waveguide array. , 13 are 3 dB directional couplers.

Claims (1)

(57) [Claims] 1. A star coupler for uniformly dispersing optical power incident on an arbitrary one of one or two or more input optical waveguides to two or more output optical waveguides, wherein the input optical waveguides are arranged in a fan shape. An input optical waveguide array and an output optical waveguide array in which the output optical waveguides are arranged in a sector shape are arranged to face each other, and the input optical waveguide array and the output optical waveguide array Coupled by a slab optical waveguide having no confinement structure, wherein the center of curvature of the input optical waveguide array is located closer to the output optical waveguide array than the output optical waveguide array-side arc end of the slab optical waveguide, and the output optical waveguide A wavelength-independent star, wherein a center of curvature of an array is located on the input optical waveguide array side from an arc end on the input optical waveguide array side of the slab optical waveguide. Ppura.