JPH0422481B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to, for example, branching light waves into a plurality of optical fibers, or combining light branches from a plurality of light sources or optical fibers into one optical fiber. Regarding optical couplers.

[Technical background of the invention and its problems]

Recently, a holo coupler as shown in Figure 20 has been reported as an optical coupler for branching and multiplexing (IEEE J.QUANTUM ELECTR0N.QE−11
(1975) 794−796). As shown in the figure, light from one optical fiber 102 is split into two optical fibers 104 by a Hoha coupler 100.
Such a holo coupler 100 is manufactured by holographically recording an interference pattern of light 106 emanating from one point and light 108, 110 converging at a plurality of points on a recording substrate 112, as shown in FIG. Ru. In this case, if a recording material having a thick emulsion layer is used as the recording substrate 112, a holocoupler 100 with relatively high diffraction efficiency can be obtained. However, in the method of fabricating the holo coupler 100 by holographic recording, if the wavelength used is different from the wavelength used for holographic recording, aberrations due to this will become a problem. For example, there is no suitable hologram recording material in the 1 μm to 1.6 μm wavelength band used for optical communications, so visible light wavelengths are used for recording, but there is aberration due to the difference in wavelength, so diffraction occurs. The light will not be focused to a single point and therefore will not be coupled to an optical fiber with high efficiency. Further, this reduction in coupling efficiency is particularly noticeable when a single mode fiber is used. Furthermore, the holographic method requires precise optical system adjustment during recording, making it unsuitable for mass production. In addition, as a characteristic of an optical coupler, the branched light intensity is required to be constant or, in some cases, to be branched to a predetermined branching ratio with good reproducibility, but the method described above allows for accurate control of the branching ratio. It is difficult to do so.

[Purpose of the invention]

An object of the present invention is to provide an optical coupler suitable for mass production, which has little aberration even when using light in various wavelength bands, and allows control of the branching ratio.

[Summary of the invention]

The first invention has a grating lens that diffracts incident light, and the pattern of the grating lens is approximately equal to the first function representing the pattern of the grating lens that converges the diffracted light to approximately one point. An optical coupler characterized in that the optical coupler is formed by a third function offset by a periodically changing second function, and is drawn on a two-dimensional plane viewed from the focal point of the grating lens. It provides:

Further, in a second invention, the optical waveguide formed on the substrate has a grating lens that diffracts the guided light propagating through the optical waveguide, and the pattern of the grating lens substantially reflects the diffracted light. The focal point of the grating lens is formed by a third function offset by a second function that changes approximately periodically with respect to a first function representing a pattern of the grating lens to be converged to one point; The present invention provides an optical coupler characterized in that it is drawn on a two-dimensional plane viewed from.

〔Effect of the invention〕

According to the present invention, by giving an offset of a periodic function to the period of the grating lens, it is possible to easily realize an optical coupler having an arbitrary number of branches and branching ratio. Moreover, the optical coupler according to the present invention can be easily manufactured for use in photomasks, etc., and is suitable for mass production. Furthermore, by providing the grating lens of the present invention in the optical waveguide, the number of branches can be increased.
An optical waveguide type optical coupler with a controlled branching ratio can be obtained.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the invention. In this structure, light emitted from a semiconductor laser 2 is diffracted by an optical coupler 6 having a grating lens 4, and then transmitted through a plurality of optical fibers 8 arranged at equal intervals.
It is now being injected into the Hereinafter, the principle of the optical coupler 6 as shown will be explained with reference to FIGS. 2 and 3. Here, the first
Grating lens 4 of optical coupler 3 similar to the figure
The xy coordinate and the ξη coordinate are respectively set on the surface and the focal plane 10 of this lens. This optical coupler 6 has a concentric grating pattern, and the phase Ω(x,y) of this grating lens 4 is given by the following equation.

Ω (x, y) = Ω ₀ (x, y) + Ω ₁ (x) ... Here, Ω ₀ (x, y) is the difference between the light 14 emitted from the light source 12 and the light 18 converging on the focal point 16. It is a phase difference in the xy plane, and Ω ₁ (x) is a function giving a phase offset, and is a periodic function whose period in the x direction is Λ. As described above, the present invention is characterized in that the phase function of the grating lens of the optical coupler is a function that changes periodically and is given an offset. In such a configuration, the intensity I (ξ, η) of the diffracted light from the optical coupler 6 at a point Q (ξ, η) on the focal plane 10 is expressed by the following equation using paraxial approximation.

I(ξ, η)∝{｜a(ξ)｜ ² sinC ² (Mπξ/d)
/sinC ² (πξ/d)}sinC ² (πn ₀ B/λfη)... (sinCx≡sinx/x) Here, λ is the wavelength of light, f is the distance between the grating lens 2 and the focal plane 10, n ₀ is the refractive index of the medium between the grating lens 2 and the focal plane 10, A and B are the sizes (lengths) of the grating lens in the x and y directions, and M and d are the following formulas. It is the amount given.

M=A/Λ... d=λf/(n ₀ Λ)... Also, a(ξ) in the equation is a function determined by the phase function Ω ₁ (x) and is given by the following equation.

a(ξ)=1/Λ ^A _O exp{i(Ω ₁ +2πn ₀ /λfξx)
}dx... As shown in the formula, I (ξ, η) is (md,
0) is a function that has a peak at (m=0,
±1, ±2,…). In other words, on the ξη plane, the distance d
The diffracted light will converge on multiple points located at equal intervals. Therefore, by using a grating lens having such a structure, a structure 1 as shown in FIG. 1 can be obtained.
By reversing the N branching or arrangement, an optical coupler having an N:1 multiplexing function can be realized. in this case,
The distance d between the plurality of focal points and the period Λ of the function Ω ₁ are given by the following equation.

FIG. 3 shows the intensity distribution in the ξ direction in the focal plane. In this figure, the solid line is sinC ² (Mπξ/d)/sinC ²
(πξ/d), and the broken line represents the distribution of |a(ξ)| ² , and the actual intensity distribution is the result of both. ξ=
sinC ² in md (m=0, ±1, ±2,…)
(Mπξ/d)/sinC ² (πξ/d) is 1, so
In the end, the intensity ratio of the light that converges at each focal point is |a(md)| ²
It will be decided. Since a(ξ) is expressed by the formula using Ω ₁ , by changing the function Ω ₁ ,
Optical couplers with various branch strength ratios can be designed.

FIG. 4a shows an example of a pattern when Ω ₁ (x)=0, that is, there is no phase offset, and corresponds to the pattern of a grating lens that converges incident light to only one point. As an example of how to give the function Ω ₁ in this case, as shown in Figure 4b, N
There are equal parts. Consider the case where Ω ₁ =α _o (n=0 to N-1)... in each of these N equally divided sections. Ω ₁ in the pattern shown in FIG. 4b is composed of α ₀ , α ₁ , . . . , α ₄ as shown in FIG. 4c. In general, from the equations |a(md)| ² = 1/N ² sinC ² mπ/N | _N-1 〓 ⁿ⁼⁰ exp {i(2πnm/N+α _o )}| ² ... Here, branch For example, if the number N is 5 as shown in Figure 4, then using the formula |a(-2d)| ² =|a(-d)| ² =|a(o)| ² =
α _o is found by solving |a(d)| ² = |a(2d) | ² ....

Other examples of the phase offset function Ω ₁ are shown in FIGS. 5a and 5b. In this case, if Ω ₁ (x) is set as shown in FIG. 5b, the pattern of the grating lens becomes a diagonal line pattern as shown in FIG. 5a. At this time, the respective deviations δ(x ₁ ) and δ(x ₂ ) of the pattern of x ₁ and x ₂ points within the period Λ from when Ω ₁ (x) = 0 are such that the pitch of the grating lens is L(x ), it is given by δ(x ₁ )=L(x ₁ )・Ω ₁ (x ₁ )/2π δ(x ₂ )=L(x ₂ )・Ω ₁ (x ₂ )/2π.

In addition, an example of the function Ω ₁ may be as shown in FIG. 6.

Next, FIG. 7 shows a simulation result of the diffracted light intensity distribution by the optical coupler as shown in FIG. 4. At the same time, adjust α _o to increase the number of branches to 3.
The results are shown in Figure 8. Figures 7 and 8
As can be seen by comparing the figures, the degree of freedom increases as the number of divisions N of Λ increases with respect to the number of branches, so it is possible to reduce the power of the diffracted light to branches other than the required branches. Furthermore, it is also possible to design the strength ratios to each branch to be different if necessary.

Although FIGS. 7 and 8 show cases of five branches and three branches, the intensity of each branched light can be controlled as appropriate. This branching ratio can be controlled by giving an arbitrary ratio to the light intensity at each focal point using the formula to obtain α _o .

Further, the pattern of the grating lens may be an amplitude type binary pattern, or as shown in FIG. The pattern may be changed depending on the situation. Such a pattern can be produced by direct drawing using electron beam exposure, exposure of a resist using a photomask, and phenomenon, and is also suitable for mass production using replicas.

Figure 10 shows that in order to further improve the diffraction efficiency,
This is an example of a blazed relief grating. Replica copying is also possible in the case of such a shape.

Next, a second embodiment of the present invention is shown in FIG.
The optical coupler 22 shown in this figure couples the light emitted from the optical fiber 24 to the two-dimensionally arranged optical fibers 26. In this case, the function Ω ₁ that offsets the phase of the grating lens 28 is:
It is a function that changes periodically not only in the x direction but also in the y direction.

Furthermore, although the above example shows the function of optical branching, optical multiplexing can also be realized by reversing the input and output. FIG. 12 shows a third embodiment of the present invention. In this example, light emitted from a plurality of semiconductor lasers 30 is coupled to one optical fiber 34 by an optical coupler 32. As described above, when the optical coupler according to the present invention is used as a multiplexer, there is an advantage that the tolerance for the wavelength error of the light source can be relatively large. This is because the grating lens is an inline type. Therefore, not only the same wavelength, but also 20 to 50
It can also be used as a wavelength multiplexing multiplexer that couples light from an ordinary light source to an optical fiber at wavelength intervals as narrow as Å.

In addition to the above-mentioned use as a transmission type grating, it is also possible to design an optical coupler using a reflection type grating by metal vapor deposition or the like.

Note that the equation is based on paraxial approximation, but if the size of the grating lens becomes much larger than the focal length, no large aberration will occur even if this approximation is used. When the numerical aperture is large and when the distance between the focal points is large, a certain amount of aberration occurs at the outer periphery of the grating lens. In this case, for example, in the case where the number of branches is 2, the aberration can be suppressed as follows. That is, as shown in FIG. 13, a function Ω ₁ (x, y) that provides an offset for two focal points F ₁ and F ₂ is given by the following equation.

Ω ₁ (x,y)=2πn ₀ /λ(l ₁ −l ₂ )... Here, l ₁ and l ₂ are the distances between the point (x, y) and F ₁ and F ₂ . At this time, the equiphase line 36 where Ω ₁ (x, y)=mλ (m: integer) becomes a hyperbola as expressed by the following equation.

4 {(n ₀ d) ² − (mλ) ² }x ² −4 (mλ) ² y ² = (mλ
) ²
{4(n ₀ f) ² + (n ₀ d) ² − (mλ) ² } ... If f is large, d≪f, mλ≪f, so the above equation becomes x=mλf/n ₀ d... , the interval for one period is Λ=λf/(n ₀ d), that is, the relationship expressed by the equation.

Next, a third embodiment of the present invention will be described with reference to FIG. In this example, an optical coupler is configured by a two-dimensional waveguide layer 52 formed on a dielectric substrate 50 and a waveguide grating lens 54 formed on this waveguide. According to this, the light emitted from the semiconductor laser 56 is transmitted to the optical waveguide layer 52.
This guided light 58 is further diffracted by the waveguide grating lens 54 and converged on a plurality of focal points located on the waveguide end face 60, and each converged light is transmitted through a plurality of fibers. 62. Furthermore, in Figure 15b, the 14th
An example of the pattern of the grating lens 54 shown in the figure is shown. The pattern shown in Figure 15a is Ω ₁
(x)=0, that is, this is the pattern when there is no phase offset, and corresponds to the pattern of a grating lens that converges the incident waveguide light to only one point.
On the other hand, the one shown on the right is a periodic relationship with period Λ as Ω ₁ (x), which deviates from the original pattern depending on the location of the phase of the grating. This deviation δ(x) is given by δ(x)=L(x)·Ω ₁ (x)/(2π)... using the grating pitch L(x).

A pattern such as that shown in FIG. 15 can be formed by direct drawing using electron beam exposure or by exposing a resist using a photomask.

Another example of the shape of the grating lens formed in the optical waveguide is shown in FIG. In this example, phase is added by changing the lens thickness in the traveling direction of the guided light. In the example shown in FIG. 17, the light incident from the optical fiber 64 is transmitted to the grating lens 6
6, becomes a radiation mode that propagates through the substrate 50, and is coupled to a plurality of optical fibers 70 connected to the substrate end surface 68. In this case, the equiphase line of the function Ω ₁ (x, y) that provides an offset to the phase is shown by a solid line in FIG. The broken line in the figure represents the grating lens pattern when no offset is provided. In this case, the interval Λ between equiphase lines on the straight line 72 separated by f from the straight line along which the optical fibers 70 in FIG. However, in this case n ₀
represents the refractive index of the substrate 1. Since the distance f varies depending on the value of Z, the equiphase line becomes a curve as shown in FIG.

FIG. 19 shows a third embodiment of the present invention. In this example, the optical fibers 74 are arranged two-dimensionally, and Ω ₁ corresponds to a function that changes periodically in two directions.

The above example shows optical branching, but optical multiplexing can also be realized by reversing the input and output.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of an optical coupler according to the present invention, FIGS. 2 to 13 are diagrams showing an optical coupler formed on an optical substrate, and FIGS. 14 to 19 are diagrams showing an optical coupler formed on an optical waveguide. 20 and 21 are diagrams showing conventional examples. 2, 12, 30, 56... light source, 4, 54, 6
6...Grating lens, 6, 22, 32...
...Optical coupler, 8, 24, 26, 30, 34, 6
2,64,70,74...optical fiber, 10...
Focal plane, 16... Focal point, 52... Optical waveguide layer.

Claims (1)

[Scope of Claims] 1. A grating lens that diffracts incident light, and a pattern of the grating lens that corresponds to a first function representing a pattern of the grating lens that converges the diffracted light to approximately one point. Light characterized by being formed by a third function offset by a second function that changes approximately periodically, and being drawn on a two-dimensional plane viewed from the focal point of the grating lens. coupler. 2 The period Λ of the second function is: Λ=λf/(n ₀ d) where λ: Wavelength of the light used d: Distance between focal points of the grating lens formed by the third function f: Third function Distance n ₀ between the pattern surface of the grating lens formed by the third function and the focal point: It is characterized by being expressed by the refractive index of the medium between the pattern surface of the grating lens formed by the third function and the focal point. An optical coupler according to claim 1. 3. The optical coupler according to claim 2, wherein the period of the second function is different in two directions of the grating lens pattern formed by the third function. 4. An optical waveguide formed on a substrate has a grating lens that diffracts the guided light propagating through the optical waveguide, and the pattern of the grating lens converges the diffracted light to approximately one point. is formed by a third function that is offset by a second function that changes approximately periodically with respect to a first function that represents a pattern of An optical coupler characterized by what is depicted.