JPH05326906A - Optical waveguide - Google Patents

Abstract

PURPOSE:To obtain an optical waveguide, which can readily perform the position alignment of light emitting elements, the optical waveguide and photodetectors in high accuracy in a signal transmitting system using light without increasing the cost of the optical waveguide caused by the requirement for high position aligning accuracy. CONSTITUTION:An optical waveguide 1 optically by connects a plurality of light-emitting elements 32 and a plurality of photodetectors 34 corresponding to the light-emitting elements. The optical waveguide 1 comprises a plurality of columnar core parts 12 and a clad part 14. A plurality of the columnar core parts are arranged in parallel to each other. The surrounding part of the core part is filled with the clad part having the refractive index smaller than the refractive index of the core part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide for optically connecting a plurality of light emitting elements and a plurality of light receiving elements corresponding to the light emitting elements.

[0002]

2. Description of the Related Art Various advantages are brought about by using light for signal transmission to and from various systems, semiconductor chips, or boards equipped with semiconductor chips (hereinafter, also simply referred to as systems). These advantages include, for example, high-speed (high bit rate) signal transmission that does not depend on the CR constant, reduction of mutual interference between systems, reduction of electromagnetic noise, and the like. A light emitting element and a light receiving element are usually used for signal transmission by light. Then, in order to optically connect them, there are roughly divided cases where an optical waveguide including an optical fiber or the like is used and a case where a space is used as it is without using such an optical waveguide. In particular, it is preferable to use an optical waveguide for short-distance signal transmission such as signal transmission inside a system or the like, or for signal transmission in a two-dimensionally arranged system or the like. In addition, it is preferable to use the space as it is for signal transmission in a three-dimensionally arranged system or the like.

[0003]

When signal transmission is performed by an optical waveguide, as is often seen in research examples of optical ICs,
A major issue is how to connect the light emitting element and the light receiving element optically by a highly accurate, simple and inexpensive method.

For example, FIG. 8 shows an example in which an optical fiber is used as an optical waveguide to optically connect a light emitting element and a light receiving element. The light emitted from the light emitting element 100 is converged by the lens 102, propagated through the core portion of the optical fiber 104 including the core portion and the clad portion, and the light emitted from the optical fiber 104 is converged by the lens 106 and enters the light receiving element 108. .. Lenses 102, 106
By arranging, the loss of light at the connection end of the optical fiber 104 can be minimized. As the lens, a spherical lens, a converging rod lens or the like is usually used, and these are the light emitting element 100 or the light receiving element 108.
It is integrated with and modularized. However, such integration work requires high accuracy, which is a major obstacle to cost reduction of the system and the like.

It is important to make efficient optical connection between the light emitting element and the optical fiber or between the optical fiber and the light receiving element. When performing optical signal transmission with high efficiency in the transmission band, it is necessary to use a single mode fiber without mode dispersion as an optical waveguide. The diameter of the core portion of this single mode fiber is several μm to ten and several μm. Therefore, it is extremely important that light is efficiently incident on the core portion of such a single mode fiber. When using an optical connector for optical connection between an optical fiber and a light emitting element or a light receiving element,
From the viewpoint of alignment between the optical fiber and the light emitting element or the light receiving element, the optical connector is required to have high accuracy, which leads to an increase in the cost of the optical connector and, in turn, an increase in the cost of the entire system and the like.

On the other hand, there is also a method of improving the coupling efficiency by processing an optical fiber into a front lens fiber and a tapered fiber. However, such processing of an optical fiber requires high technology and complicated work, and it is not possible to satisfy the demand for cost reduction of the entire optical fiber and system.

For example, an optical connection between semiconductor chips by an optical fiber is disclosed in Japanese Patent Laid-Open No. 1-130112. According to this publication, the optical connection between the light emitting element array and the light receiving element array is performed by polishing the substrate on which the V-shaped groove is formed and the end face disposed in the V-shaped groove and obliquely with respect to the optical axis. The optical fiber is used. Also in this case, high precision is required for the alignment between the light emitting element and the optical fiber, and between the optical fiber and the light receiving element, and a substrate having a very high precision V-shaped groove is required. ..

As described above, the light emitting element in the light signal transmission system including the light emitting element, the optical waveguide, and the light receiving element,
The increase in the cost of various optical transmission optical components such as optical fibers and optical connectors, which is caused by the requirement for the alignment accuracy of the optical waveguide and the light receiving element, and the high alignment accuracy, is especially due to the large number of optical transmission lines inside the system. This is the biggest problem when using a waveguide, that is, when using a large number of optical waveguides arranged in an array. Therefore, unless problems such as high-accuracy alignment and high-precision processing of various optical transmission optical components can be solved, it is difficult to reduce the cost of various optical transmission optical components. Widespread use of optical waveguides as a large number of signal transmission paths is extremely difficult in terms of cost.

Therefore, it is an object of the present invention to perform alignment of a light emitting element, an optical waveguide, and a light receiving element in a signal transmission system by light with high accuracy and easily, and an optical waveguide generated from the requirement of high alignment accuracy. Another object of the present invention is to provide an optical waveguide which does not increase the cost.

[0010]

The above-mentioned object is an optical waveguide for optically connecting a plurality of light emitting elements and a plurality of light receiving elements corresponding to the light emitting elements, wherein It is composed of a columnar core part and a clad part, and a plurality of columnar core parts are arranged in parallel with each other, and the periphery of the core part is filled with a clad part having a refractive index smaller than that of the core part. It can be achieved by the featured optical waveguide of the present invention.

The cross-sectional shape of the core portion is circular, oval, square,
It can be rectangular, polygonal, or any other shape. The combination of the materials of the core part and the clad part is such that the refractive index of the core part is larger than that of the clad part, that is, n.
Any material may be used as long as ₁ > n ₂ and can satisfy various characteristics conventionally required for the core portion and the clad portion. From the viewpoint of formation, SiO ₂ (refractive index n ₁ = 1.5) / polyimide resin (refractive index n ₂ = 1.
4 to 1.5), SiO _X N _Y (refractive index n ₁ = 1.5 to 2.
0) / polyimide resin, Al ₂ O ₃ (refractive index n ₁ = 1.
7) / SOG (Spin On Glass) (refractive index n ₂ = 1.4 to
1.5) is preferred.

In a preferred embodiment of the optical waveguide of the present invention, light is guided in a single mode.

In another preferable aspect of the optical waveguide of the present invention, an optical connection between one light emitting element and one light receiving element corresponding to the light emitting element is provided through at least two adjacent core portions. Done.

In still another preferred aspect of the optical waveguide of the present invention, the clad portion is made of a material having an absorbing effect on the light emitted by the light emitting element. As such a material, a polyimide resin in which carbon black or the like is dispersed or SOG can be exemplified. By using such a material, it is possible to prevent mutual interference of the core parts due to light exceeding the critical acceptance angle, so-called crosstalk.

In a further preferred aspect of the optical waveguide of the present invention, at least one of the light input / output portions of the optical waveguide is constituted by a surface other than perpendicular to the axial direction of the optical waveguide,
Input and output of light to and from the light input / output unit having such a surface is performed at an angle other than the axial direction of the optical waveguide. As a result, the degree of freedom in arranging the light emitting element and the light receiving element can be increased.

[0016]

The principle of the present invention will be described below. In the optical waveguide as shown in FIG. 6, the propagation mode is such that light is introduced into the core portion of the optical waveguide at an angle θ (complementary incident angle) with respect to the axis of the core portion and total reflection occurs at a critical angle θ _C or less. It is formed under the condition that the sum of the phase changes in the X direction becomes 2πm (where m = 1, 2, ...) In the process of repeating and propagating. The total amount of phase change when light having a propagation constant K = 2π / λ makes one round trip in the X direction as shown in FIG. 6 is given by (2a · K · sin θ + ψ _g ) × 2. Here, a is the radius (core diameter) of the core portion of the optical waveguide, and ψ _g is the Goose-Henschen shift, which is given by the following equation. It should be noted that ψ _g is 0 when θ ≧ θ _C and −π when θ = 0. For the TE _{wave: tan (ψ g / 2)} = {√ (n 1 2 sin 2 φ-n 2 2)} /
(N ₁ cos φ) For TM wave: tan (ψ _g / 2) = {n ₁ √ (n ₁ ² sin ² φ−
n ₂ ² )} / (n ₂ ² cosφ) where φ = (π / 2) −θ, n ₁ is the refractive index of the core portion, and n ₂ is the refractive index of the cladding portion.

FIG. 7 shows the relationship of the selection rule with respect to the phase change of each mode (the relationship between the mode order and the incident complementary angle) at this time. From FIG. 7, it is understood that the condition for the optical waveguide to be a single mode is θ ₁ > θ _{C, that} is, sin ⁻¹ {(π−ψ _g ) / 2aK}> θ _C. Here, θ _C = sin ⁻¹ {(n ₁ ² −n ₂ ² ) / n ₁ ² } ^1/2 ≈sin ⁻¹ (√ (2Δ)). Here, Δ is a relative refractive index difference and is represented by Δ = (n ₁ −n ₂ ) / n ₁ . Therefore, (π−ψ _g ) / 2aK)> √ (2Δ) (1).

Now, when the wavelength λ of light introduced into the optical waveguide is constant, that is, when the propagation constant K = 2π / λ is constant,
From the formula (1), it is possible to realize a single mode optical waveguide by reducing the core diameter a (see FIG. 7). When the value of the wavelength λ becomes smaller, the first-order mode light is propagated, and the wavelength λ at this time is called the cut-off wavelength of the single mode optical waveguide.

On the other hand, as the wavelength λ of light is longer, a single mode optical waveguide can be realized with a larger core diameter. For example,
The core diameter for single mode fiber for optical communication is
When the wavelength is 0.85μm, it is about 7μm
Is about 10 μm, and the wavelength is 1.55 μm, about 12 μm
m. As is clear from FIG. 7, in order to realize a single-mode optical waveguide with a large core diameter, θ _C can be reduced by reducing the relative refractive index difference Δ between the core and the clad. .. Further, when it is desired to increase the critical acceptance angle, the relative refractive index difference Δ between the core portion and the clad portion may be increased.

Based on the above principle, a single mode optical waveguide having a fine core diameter and a high transmission band at a certain wavelength can be realized. For example, the wavelength of light is about 1μ
m, relative refractive index difference between core and cladding Δ = (n ₁ −n ₂ ).
When / n ₁ = 1%, from the formula (1), the radius a of the core portion for becoming the single mode is 5 to 10 times the wavelength, that is,
It becomes about 5 to 10 μm. In this case, the cross-sectional shape of the core portion is circular, but single-mode light propagation can be achieved with a similar size even when the cross-sectional shape of the core portion is rectangular. An optical waveguide having a core portion of this size can be manufactured by using a normal photolithographic technique.

By thus reducing the core diameter of the optical waveguide, it becomes possible to propagate light of a considerably long wavelength in a single mode. However, since the light receiving efficiency of the light emitted from the light emitting element in the optical waveguide deteriorates, there is usually a limit to the miniaturization of the core diameter.

The optical waveguide of the present invention comprises a plurality of columnar core portions and clad portions, the plurality of columnar core portions are arranged in parallel with each other, and the periphery of the core portion has a refractive index smaller than that of the core portion. Is filled with a clad portion having. Therefore, if the arrangement of the core portions is made sufficiently redundant as compared with the light spot size (for example, several tens of μm) of the light emitting element, that is, for example, the light emitted from the light emitting element is emitted to at least two adjacent core portions. If the core portions are arranged so as to be incident, even if the core diameter is made fine, the light receiving efficiency of the entire optical waveguide does not deteriorate. When light is incident on the optical waveguide of the present invention, the incident angle of light is the critical acceptance angle θ _MAX = sin.
If it is smaller than (NA / n ₁ ), the light propagates in the core part in the single mode. Here, NA is the aperture ratio, that is, n
₁ sin θ _C = n ₁ √ (2Δ).

If the core portions are arranged as described above,
It is only necessary to accurately align the light emitting element and the light receiving element, and it is not necessary to highly accurately align the optical waveguide of the present invention with the light emitting element or the light receiving element. This is because the core diameter of the core portion is smaller than the spot size (for example, several tens of μm) of the light emitted from the light emitting element, and the light from the light emitting element is incident on at least two adjacent core portions. Further, as a light receiving element, a PI having a comb-shaped electrode and meander electrode structure having an area of about 50 μm square to 200 μm square.
If you use N photo detector or MSM photo detector,
Since it is not necessary to use a condenser lens, the alignment accuracy can be further roughened.

[0024]

EXAMPLE FIG. 1 shows a schematic sectional view of an optical waveguide of the present invention. The optical waveguide 1 shown in FIG. 1A includes a core portion 10 made of SiO ₂ and a clad portion 12 made of a clad material formed on a substrate 10 made of silicon. The clad material is a polyimide resin. The cross-sectional shape of the core part is rectangular. The width of the core is 2-5
The gap L _H in the horizontal direction between the core portions is 0.5 to 1.0 μm, and the gap L _{V in the} vertical direction is 0.3 to 0.5 μm. The refractive index n ₁ of the core part is 1.5, and the refractive index n ₂ of the clad part is 1.4.
~ 1.5. In addition, in FIG. 1A, the beam of the signal light is shown by a dashed circle.

The optical waveguide shown in FIG. 1A can be manufactured by the following steps. [Step-10] As shown in the schematic partial cross-sectional view of FIG. 2A, first, a thickness of 1.0 is formed on the substrate 10 by the CVD method.
A μ ₂ SiO ₂ film 20 is formed. After forming a resist 22 on this SiO ₂ film, the resist is patterned by a normal photolithographic method. After that, reactive ion etching or ion milling (Ar gas sputter etching) is performed using a gas such as CF ₄ to pattern the SiO ₂ film 20 (FIG. 2).
(See (B)). The horizontal gap L _H of the SiO ₂ film 20 was set to 0.5 μm. Thus, one core layer in which a plurality of columnar core portions 12 made of SiO ₂ are arranged in parallel is formed.

[Step-20] Next, the resist 22 is removed, a polyimide resin is applied to the entire surface to form a polyimide resin layer 24, and then primary heating is performed to evaporate unnecessary organic components by heating (FIG. 2). (See (C)). Next, the polyimide resin layer 24 is etched back to remove the polyimide resin layer 24.
Flatten. Polyimide resin layer 24 on core 12
Thickness L _V was 0.3 μm. Thus, the core portion 12
The periphery of is filled with the clad portion 14 made of polyimide resin (see FIG. 2D). Secondary heating is performed as necessary to cure the polyimide resin layer 24. Secondary heating may be performed after forming a plurality of core layers.

Then, on the clad portion 14, [Step-1
0] and [Step-20] are repeated a required number of times to form a required number of core layers. Through the above steps, the optical waveguide 1 shown in FIG. 1A is completed.

FIG. 3 is a perspective view showing an example of signal transmission by light between semiconductor chips using the optical waveguide 1 of the present invention. In FIG. 3, 30A and 30B are semiconductor chips, 32 is a light emitting element, and 34 is a light receiving element. Semiconductor chip 30A,
A plurality of 30B, the light emitting elements 32, and the light receiving elements 34 are arranged in an array. 1 for the light emitting element 32 and the light receiving element 34
They may be mixed in one array. In this example, the light input / output portion of the optical waveguide is constituted by a surface 16 perpendicular to the axial direction of the optical waveguide, and the input / output of light to / from the optical input / output portion is parallel to the axial direction of the optical waveguide.

As is apparent from FIG. 3, the light emitting element 32
It is not necessary to align the optical waveguide 1 with the optical waveguide 1 and the optical receiver 1 with the optical waveguide 1 with high accuracy. This is because one light emitting element and one light receiving element corresponding to this light emitting element are optically connected via at least two adjacent core portions. Semiconductor chips 30A, 30B
If the substrate 46 on which the optical waveguide 1 is mounted is sufficiently flat and the light emitting elements 32 and the light receiving elements 34 are arranged in an array at a predetermined interval, at least one set of a plurality of light emitting elements and light receiving elements is provided. All the light emitting elements and the light receiving elements can be aligned by only accurately aligning.

An example of signal transmission by light between an optoelectronic integrated circuit (OEIC) using the optical waveguide 1 of the present invention and an OEIC is shown in a side view and a perspective view in FIG. 40 in FIG.
A and 40B are OEICs. The optical waveguide 1 is formed on a predetermined region of the substrate 10 by the method described above. That is, the optical waveguide 1 is integrally formed on the substrate on which the OEIC is mounted. OEIC 40A, 40
At the end of B, the light emitting element module 42 and the light receiving element module 44 are formed at equal intervals and at the same height.
The OEICs 40A and 40B are flip-chip mounted on a predetermined region of the substrate 10 so that the input and output of light to and from the end face 16 of the optical waveguide 1 are substantially vertical. The light emitting element emits light substantially parallel to the substrate 10.

At this time, it is not necessary to align all the light emitting elements and the light receiving elements of the OEICs 40A and 40B, and any one or two sets of the light emitting elements and the light receiving elements may be monitored, for example, the actual optical signal. do it,
Just align them. The larger the light emission angle of the light emitting element and the larger the spread of the signal beam, the coarser the alignment accuracy can be. In this case, if the light emission angle of the light emitting element is too large, light interference with the core portion corresponding to the adjacent light emitting element, so-called crosstalk occurs. In order to prevent this, the clad portion may be made of a material having an absorbing effect on the light emitted by the light emitting element, for example, a polyimide resin or SOG in which carbon black is dispersed.

At least one of the light input / output portions of the optical waveguide is constituted by a surface other than perpendicular to the axial direction of the optical waveguide,
FIG. 5 is a side view showing an example in which light is input / output to / from the light input / output unit having such a surface at an angle other than the axial direction of the optical waveguide. In the example shown in FIG. 5A, the OEICs 40A and 40B are mounted on the substrate 46 to which the circuit is mounted, and the light emitting element module 42 and the light receiving element module 44 formed in the OEICs 40A and 40B are
The optical waveguide 1 is attached.

The optoelectronic integrated circuit 40 shown in FIG.
A and 40B input and output light perpendicularly to the substrate 46 to which these circuits are attached. That is, the light emitting element module 42 emits light in the vertical direction with respect to the substrate 46, and the light receiving element module 44 receives the light in the vertical direction. In this case, after cutting the substrate 10 on which the core portion and the clad portion are formed into a predetermined size, the end face 16 is polished to 45 degrees.
By doing so, the light input / output portion of the optical waveguide is constituted by the surface 16 other than perpendicular to the axial direction of the optical waveguide (in this embodiment, the surface inclined by 45 degrees with respect to the axial direction). Then, the input and output of light to and from the light input / output unit having the surface 16 is performed at an angle (90 degrees with respect to the axial direction in this embodiment) other than the axial direction of the optical waveguide.

If the substrate 46 can transmit the light used, the optical waveguide 1 may be formed on the back surface of the substrate 46 and the OEICs 40A and 40B may be mounted on the front surface thereof, as shown in FIG. 5B. It is possible. In addition, in FIG. 5, the optical path is shown by a broken line.

The present invention has been described above based on the preferred embodiments, but the present invention is not limited to these embodiments. As shown in FIG. 1B, the core portions 12 of the optical waveguide 1 can be arranged in a staggered pattern. In addition, the cross-sectional shape of the core portion 12 of the optical waveguide 1 can be circular as shown in FIG. The arrangement state of the light emitting element, the light receiving element, and the optical waveguide is not limited to that in the embodiment, and any arrangement can be adopted. The dimensions of the core portion of the optical waveguide, the materials of the core portion and the cladding portion, etc. are merely examples, and can be changed as appropriate.

[0036]

In the optical waveguide of the present invention, the light emitting element and the light receiving element may be accurately aligned, and the light emitting element and the optical waveguide, or the light receiving element and the optical waveguide may be aligned with low accuracy. Therefore, the cost of the optical waveguide and the cost of the light emitting element, the light receiving element, etc., which are caused by the demand for high alignment accuracy, are not increased. Further, it is possible to easily reduce the cost for aligning and assembling the optical components for optical transmission.

Further, even when each of the light emitting element and the light receiving element is formed into an array, the optical waveguide can be manufactured independently of the interval of the array, and at least one set of the light emitting element and the light receiving element is aligned. This eliminates the need to align all the arrays. Therefore, a large number of optical waveguides can be easily formed with high alignment accuracy, and the cost of optical wiring can be reduced.

By using the optical waveguide of the present invention, it is possible to arrange the OEIC and the like with a high degree of freedom. Furthermore, since the structure of the optical waveguide of the present invention is basically a single mode, signal transmission at a high bit rate is possible.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an optical waveguide of the present invention.

FIG. 2 is a schematic cross-sectional view of an optical waveguide for explaining each step of the method for producing an optical waveguide of the present invention.

FIG. 3 is a diagram showing an example of signal transmission by light between semiconductor chips using the optical waveguide of the present invention.

FIG. 4 is a diagram showing an example of signal transmission by light between optoelectronic integrated circuits using the optical waveguide of the present invention.

5 is a diagram showing another example of signal transmission by light between optoelectronic integrated circuits using the optical waveguide of the present invention, which is different from FIG.

FIG. 6 is a diagram for explaining a propagation state of light in an optical waveguide.

FIG. 7 is a diagram showing a relationship between a mode order and an incident complementary angle.

FIG. 8 is a diagram showing a conventional example in which an optical fiber is used as an optical waveguide to optically connect a light emitting element and a light receiving element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Optical waveguide 10 Substrate 12 Core part 14 Clad part 16 End face of substrate 20 SiO ₂ film 22 Resist 24 Polyimide resin layer 30A, 30B Semiconductor chip 32 Light emitting element 34 Light receiving element 40A, 40B Optoelectronic integrated circuit (OEIC) 42 Light emitting element module 44 Light receiving element module 46 Substrate

Claims (5)

[Claims] 1. An optical waveguide for optically connecting a plurality of light emitting elements and a plurality of light receiving elements corresponding to the light emitting elements, wherein the optical waveguide comprises a plurality of columnar core portions and clad portions. A plurality of columnar core portions are arranged in parallel with each other, and the periphery of the core portions is filled with a clad portion having a refractive index smaller than that of the core portions. 2. The optical waveguide according to claim 1, which guides light in a single mode. 3. One light emitting element and one corresponding to the light emitting element
The optical waveguide according to claim 1, wherein the optical connection between the two light receiving elements is performed via at least two adjacent core portions. 4. The optical waveguide according to claim 1, wherein the clad portion is made of a material having an absorbing effect on the light emitted by the light emitting element. 5. At least one of the light input / output portions of the optical waveguide is constituted by a surface other than perpendicular to the axial direction of the optical waveguide,
The optical waveguide according to claim 1, wherein the input and output of light to and from the light input / output unit having such a surface is performed at an angle other than the axial direction of the optical waveguide.