JP2850996B2 - Optical coupling device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical coupling device for converting a spot size of a light wave propagating through an optical waveguide with low loss.

[0002]

2. Description of the Related Art In the case of optical coupling between a semiconductor laser diode (LD) and a single mode optical fiber, when the end face of the LD element and the optical fiber are directly butt-coupled (butt joint), light waves in the optical waveguides of each other are converted. Since the spot sizes are different, the coupling loss at the direct butting portion becomes a problem. Normally, the light spot size (mode radius: W) of the LD is 1 μm and the spot size of the optical fiber is about 5 μm, so that the coupling loss is about 10 dB.
become. Therefore, a method of reducing the coupling loss by converting the spot size by a lens is generally adopted.

FIG. 5 shows a conventional configuration example in which a single lens is used to optically couple an optical function element having a plurality of laser diodes (LDs) and an array fiber with one lens. In FIG. 5, reference numeral 503 denotes a semiconductor substrate, and 501 denotes an LD.
509 is a lens, 507 is an optical fiber, and 508 is a V-groove array for fixing the optical fiber at regular intervals. In such a configuration, as the integration scale of the LDs increases, the coupling loss increases due to the influence of lens aberrations and the like, so that the number of LDs that can be integrated on one semiconductor substrate is limited.

As shown in FIG. 6, an optical coupling device for converting a light spot size by a tapered optical waveguide is shown in FIG.
There is a method of optically coupling between an LD and an optical fiber with low loss by using it as a substitute for a lens. FIG. 6 (A)
Is a top view of a conventional optical coupling device, (B) is a cross-sectional view,
(C) is a characteristic diagram for explaining the operation principle. That is, as can be seen from FIG.
01 relative refractive index difference Δn [= (n−n ₁ ) / n ₁ , n ₁ ,
n is the refractive index of the cladding layer 603 and the refractive index of the core layer 601], and when the thickness t or the width w of the core 601 is gradually increased from 0, guided light (basic mode light) is obtained. The spot size W of (1) gradually decreases from an infinite size, takes a minimum value, and then increases again. Here, if t and w become too large, the waveguide becomes a multi-mode optical waveguide, and the loss due to higher-order mode conversion becomes large. Therefore, usually, the size of this region is not used. By utilizing this relationship, the core 6 of the optical coupling device is used.
In the design of the sizes t and w of 01, the light incident end side (L
D), the spot size of LD light (about 1 μm)
Dimensions w _i to m) to give the same degree of spot size W _i,
At t _i (= several 100 nm to several μm), on the light emitting end side,
Dimensions t ₀ , w ₀ (= number 10 to number 100) giving a size W ₀ comparable to the spot size (about 5 μm) of the optical fiber
nm). The length L of the region where the size of the core 601 is tapered is set to a length of 〜100 μm to several mm or more in order to reduce radiation loss. However, when the dimensions t ₀ and w ₀ on the light emitting end side are reduced and W ₀ is increased, the guided light intensity distribution of the optical fiber is substantially Gaussian, whereas the light intensity distribution of the optical waveguide is not. , Since the normalized frequency v is smaller than 1, an exponential function is formed. For this reason, there is a coupling loss due to the shape mismatch. In addition, since an optical waveguide having extremely weak light confinement is used, there is a disadvantage that radiation loss easily occurs due to a slight structural defect or material fluctuation in the optical waveguide. Further, the spot size W sharply increases in a region where the core dimensions w and t are small. For this reason, fluctuations in the dimensions at the time of fabrication greatly change the magnitude of the coupling loss with the optical fiber. Therefore, severe requirements were imposed on the reproducibility of the production.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks and to achieve low-loss optical coupling between two different optical functional elements, particularly an optical functional element in which a plurality of optical devices are integrated. To provide an optical coupling device that is easy to manufacture.

[0006]

An optical coupling device according to the present invention is an optical coupling device for optically coupling optical functional devices having different structures with low loss, wherein at least a width or a thickness of the optical functional device in a light propagation direction. Gradually changed,
At least one optical waveguide core, a first cladding region configured to surround the optical waveguide core, and at least a second cladding region configured to surround the first cladding region, and The refractive index of the second cladding is smaller than the refractive index of the first cladding.

[0007]

According to the present invention, the core portion of the optical waveguide is surrounded by two layers of cladding, and the refractive index of the inner first cladding region (layer) directly surrounding the core is set to the outer second cladding region (layer).
The refractive index of the core is larger than the refractive index of the core, and at least the width or thickness of the tapered shape of the core is changed in the light propagation direction to reduce the coupling loss with the optical fiber, and Light confinement is enhanced and radiation losses are reduced. Therefore, an optical coupling device having low loss characteristics and improved manufacturability can be realized.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments and principles of the present invention will be described in detail with reference to the drawings, but it is needless to say that the present invention is not limited thereto.

FIG. 1 shows an example of an optical coupling device according to the present invention. FIG. 1 (A) is a perspective view, FIG.
(C) is a top view. 101 is a core layer of an optical waveguide, 1
02 is a first cladding region I, 103 is a second cladding region II, 104 is incident light, 105 is outgoing light, 106 is a light incident end, and 107 is a light exit end. The tapered core layer gradually changes the spot size of the LD light wave of the incident light 104 and converts it to an appropriate size that reduces the coupling loss with the optical functional device (eg, optical fiber) connected to the light emitting end 107. doing. Here, the width and thickness of the first cladding region are set to _wc1 and _tc1 , respectively.
The width w and the thickness t of the core layer 101 are set to w _i and t _i that give the same size as the spot shape of the semiconductor optical function element at the connection portion with the laser, that is, the light incident end portion 106. At the emission end 107, w ₀ and t ₀ are set so as to reduce the coupling loss with the optical fiber connected thereto. The refractive indices of the core layer, the first cladding region, and the second cladding region are n, n ₁ , and n, respectively.
₂ and n> n ₁ > n ₂ .

FIGS. 2A and 2B are diagrams for explaining the principle of the present invention, and show the guided light field intensity distribution at the light entrance end and light exit end in the embodiment of FIG. 1, respectively. Represents At the light incident end (semiconductor laser side), light waves are distributed to the core and the first cladding region I, and the core is in a state of strong confinement. On the other hand, at the light emitting end, the core is in a state of weak confinement. n ₁ >
Due to the relationship of n ₂ , the light wave is mainly distributed in the core and the first cladding region I, and the field intensity is weak in the second cladding region II. FIG. 2C shows the field intensity distribution at the light emitting end of the conventional embodiment (corresponding to the case of w _c1 , t _c1 → ∞ in the present invention). In the present invention, the size of the first cladding region I, w _c1 , t _c1 , is set to an appropriate size as compared with the conventional example (FIGS. 6 (A), 6 (B)). And the radiation loss is reduced because light is strongly confined in the core and the first cladding region I.

FIG. 3 is a view for explaining the effect of the present invention. FIG. 3A shows the width w ₀ of the core at the light emitting end 107 (the fiber connection side) and the width of the first clad. FIG. 3B shows the relationship between w _c1 and the coupling loss characteristic of the optical fiber, and FIG. 3B shows the relationship between the taper length L and the radiation loss characteristic when the fundamental mode is excited from the light incident end 106 in the tapered optical waveguide. Shows the results calculated by the wave model,
(C) is a top view which shows the taper shape of the core part of an optical coupling device. Here, (A) shows the refractive index of the core n = 3.
3, the refractive index n _{1 of} the first cladding region I = 3.17, the refractive index n ₂ of the second cladding region = 1.0, the wavelength λ = 1.
At 55 μm, the coupling loss characteristics are obtained when the width w _c1 of the first cladding region I is variously changed (5 μm, 10 μm, 15 μm, ∞), and (B) shows the taper shown in the top view in (C). Using an optical coupling device having a shaped core, n = 3.3, n ₁ = 3.166, n ₂ = 1.0,
Radiation with various widths (10 μm, 20 μm, 30 μm, で) when the width w _i of the core on the light incident end side is 0.25 μm and the width w _b of the core on the light exit end side is 0.05 μm and w _i is variously changed. It is a loss characteristic, and w _c1 = ∞ represents the characteristic of the conventional example. FIG.
According to (A), by setting w ₀ and w _c1 to appropriate values, the coupling loss of the optical fiber can be reduced, and the manufacturing tolerance for the _value of w ₀ can be reduced as compared with the conventional example. I understand. This is because the change in the coupling loss in the region where w ₀ is small is smaller than in the conventional case. Further, from FIG. 3B, the taper length L
It can be understood that when the radiation loss is made the same as in the conventional example, the radiation loss can be reduced, and when the radiation loss is made the same, the taper length L can be greatly reduced. In this structure, since the width w _c1 of the first cladding region is finite, the multi-mode optical waveguide also guides higher-order modes. However, loss due to higher-order mode conversion is small. I understand. Although the calculated values are approximate values, an accurate optical waveguide structure can be designed by, for example, a finite difference method or a finite element method.

The magnitudes of the refractive indices n, n ₁ and n ₂ can be arbitrarily set by appropriately setting the impurity doping concentration by using the composition of a dielectric material or a semiconductor material or the plasma effect. For example, when semiconductor InP is used, n = 3.1 for light in the wavelength λ = 1.55 μm band.
7 Further, the refractive index of InGaAsP can be set to an arbitrary size from about 3.2 to about 3.5 depending on its composition. The effective refractive index can be arbitrarily set by using a multiple quantum well layer and selecting the material and thickness of the well layer and the barrier layer. When a semiconductor material is used, the optical coupling device of the present example can be manufactured by, for example, an epitaxial growth technique or a photolithography technique.

In the above description, the case where the refractive indices n ₁ and n ₂ of the first and second cladding regions are uniform has been described. By doing so, the same effect as the present invention can be obtained. FIG. 4 is a cross-sectional view showing another example of the optical coupling device of the present invention.
In this case, the second cladding layer 402 is formed and then the second cladding layer 406 is embedded. In this case, the semiconductor substrate 403 is configured as a second cladding layer, and even if the refractive index n ₂₁ of the substrate 403 is different from the refractive index n _{22 of} the second cladding layer 406, the first cladding layer 406 is formed.
Against the refractive index n ₁ of the clad _{layer, n 1> n 21, n} 22
In this case, the same effect as in the above-described embodiment can be obtained. Therefore, the second cladding layer 406 may be made of air. Further, even when the first cladding layer 402 is formed of a plurality of different materials, the same effect can be obtained even when the structure is asymmetric.

In the above description, a case where the spot size is enlarged by making the dimensions w ₀ , t ₀ of the core on the light emitting end side sufficiently smaller than the dimensions w _i , t _i of the core on the light incident end side is described. However, for example, w _i is wider and t _i is extremely thin to form a core, or a plurality of core layers having different dimensions or refractive indices are combined, and at least some of the layers have a tapered shape. To convert the spot size.

Furthermore, although the case where the tapered shape of the core is a straight line is shown, it may be a curved line such as a parabola or an exponential function. If the traveling direction of the light is reversed, reverse spot size conversion is also possible.

In the above description, the case where optical fibers are connected has been described. In addition to the above, the connection of the optical waveguide to any other optical waveguide component such as a semiconductor optical waveguide component or a glass optical waveguide component. The material of the optical coupling device waveguide according to the present invention is adjusted to match the light intensity distribution,
It is obvious that the characteristics of low coupling loss can be realized by setting the dimensions.

Since the optical coupling device of the present invention can be composed of a semiconductor material, the optical coupling device of the present invention can be mounted on the same substrate at the light input / output end of an optical functional element such as a semiconductor laser, an LD amplifier or an optical switch. It is also possible to realize a monolithically integrated optical device. In this case, when forming the optical functional device optical waveguide on the semiconductor substrate, the optical coupling optical waveguide of the present invention is simultaneously formed, or after the optical functional device portion is formed, the optical waveguides are directly joined to each other. May be formed with a tapered optical waveguide for optical coupling.

[0018]

As described above, according to the present invention, by forming the core portion of the optical waveguide into a tapered shape and forming a double cladding layer around the core portion, an optical coupling with low loss and excellent manufacturability is obtained. Makes the device feasible.

[Brief description of the drawings]

FIG. 1 shows one configuration example of an optical coupling device according to the present invention. (A) is a perspective view, (B) is a sectional view, and (C) is a top view.

FIG. 2 is a diagram for explaining an operation principle of the present invention;
(A) and (B) are diagrams showing the optical field intensity distribution at the input / output end of the tapered optical waveguide according to the present invention, respectively, and (C) is the optical field intensity at the output end of the conventional tapered optical waveguide. FIG. 3 is a diagram showing distribution.

FIG. 3 is a diagram for explaining the operation principle of the present invention;
(A) and (B) show the structure and material of the optical waveguide, respectively.
FIG. 3 is a diagram showing a relationship between dimensions, an optical fiber coupling loss, and a radiation loss at a tapered portion, and FIG. 4C is a schematic top view showing a tapered shape of a core portion.

FIG. 4 is a sectional view showing another configuration example of the optical coupling device according to the present invention.

FIG. 5 is a schematic top view showing a conventional optical coupling method.

6A and 6B are diagrams showing the structure and operation principle of a conventional optical coupling device, wherein FIG. 6A is a top view, FIG. 6B is a cross-sectional view, and FIG.
Is a characteristic diagram.

[Explanation of symbols]

Reference Signs List 101 core layer of optical waveguide 102 first cladding region 103 second cladding region or semiconductor substrate 104 incident light 105 outgoing light 106 light incident end 107 light emitting end 401 optical waveguide core layer 402 first cladding region 403 2 cladding region or semiconductor substrate 406 second cladding layer 501 core layer of optical waveguide 503 second cladding region or semiconductor substrate 507 optical fiber 508 V-groove array 509 lens 601 core layer of optical waveguide 603 second cladding region Or semiconductor substrate 604 incident light 605 outgoing light L taper length n core refractive index n ₁ refractive index of first cladding region n ₂ refractive index of second cladding region t ₀ thickness of first cladding region w ₀ th width w _c1 of the first width w _i light entering end side of the core the core of the first cladding region

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B ⁶ /12-6/122 G02B 6/42

Claims (1)

(57) [Claims] An optical coupling device for optically coupling optical functional devices having different structures with low loss, wherein at least one optical waveguide has at least a width or a thickness gradually changed in a light propagation direction. A core, a first cladding region configured to surround the optical waveguide core, and a second cladding region configured to surround the first cladding region.
And a refractive index of the second cladding is smaller than a refractive index of the first cladding.