JPH1068833A - Optical waveguide, method of manufacturing the same, and optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical waveguide which is not fluctuated in operating wavelength with respect to a change in an environment temp. by forming an upper clad layer consisting of a photopolymer material on the upper part of a part including a core. SOLUTION: A lower clad quartz glass layer 12 is formed by a flame deposition method on a silicon substrate 11. Next, a groove 13 for forming the core is formed by a photograph stage and reactive ion etching. Core glass soot is then deposited by the flame deposition method and is held in an electric furnace, by which the soot is sintered and a transparent core glass layer 14 is formed. Next, the core glass layer 14 is removed by reactive ion etching to form the core 15, by which an air clad optical waveguide is formed. Finally, the upper clad layer 16 consisting of the photopolymer material, such as polymethyl methacrylate(PMMA) is formed. Then, the so-called thermal waveguide having the refractive index consistent with the temp. change is obtd. by controlling the refractive index of the photopolymer material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide used in the fields of optical communication, optical signal processing, and optical measurement and a method of manufacturing the same, and further includes a 1.3 μm band, a 1.55 μm band, and the like having the optical waveguide. For example, an optical multiplexing / demultiplexing circuit for signal light at
The present invention relates to an optical circuit such as a wavelength selection circuit, a laser light source circuit, a directional coupler, a Mach-Zehnder interferometer, and a relief Bragg grating.

[0002]

2. Description of the Related Art A silica-based waveguide-type optical component is attracting attention as a means for realizing a practical waveguide-type optical component because it can be connected to a silica-based optical fiber with low loss. This silica-based optical waveguide is basically formed by a technique of forming a silica-based glass film consisting of a core or a clad having a thickness of several μm to several tens μm, and forming the formed core glass film to a thickness of several μm using photolithography. It is manufactured by combining with a technique of processing into a width pattern shape, and various optical circuits have been realized so far.

In particular, optical circuits such as a directional coupler having an optical multiplexing / demultiplexing function, a Mach-Zehnder interferometer, and a relief type Bragg grating are characterized by having a high degree of freedom in characteristics in circuit design and low insertion loss. It is expected as an important optical component for use in optical communication.

However, the operating wavelength of a conventional optical multiplexing / demultiplexing circuit or wavelength selection circuit composed of a buried silica-based waveguide in which a core and a clad are made of a silica-based glass film fluctuates with changes in its environmental temperature. There is a problem. For example, it has been reported that in an Er-doped waveguide DBR laser with an integrated Bragg grating, the oscillation wavelength shifts to the longer wavelength side as the temperature increases (Kitakawa et al., 19).
Proceedings of the 1995 IEICE General Conference, C-26
7).

[0005] This is interpreted as follows. If the pitch of the grating is Λ, the Bragg wavelength λ _B is 2nΛ
It is represented by At a wavelength of 1500 nm, for a silica-based optical waveguide, Λ is about 500 nm, and since the refractive index temperature coefficient dn / dT of the silica-based glass is about 10 × 10 ⁻⁶ , dλ _B / dT is 0.01 nm. / ° C. This value matches the temperature coefficient of the DBR laser oscillation wavelength.

In order to suppress a change in the operating wavelength due to such a temperature change, a so-called athermal waveguide has been conventionally proposed (Y. Kokubun et al.).
al, Electron. Lett. , Vol. 30,
pp / 1223-1224).

FIG. 4 is a conceptual sectional view of a so-called athermal waveguide conventionally proposed. In FIG.
Represents a Si substrate, 02 represents a lower cladding layer, 03 represents a core glass layer, and 04 represents a PMMA loading layer. here,
The so-called athermal waveguide refers to a waveguide having the property of an athermal state in which a change in the refractive index with respect to a temperature change is zero. In the waveguide of FIG. 4, PMMA (polymethyl methacrylate) is loaded on the core glass layer 03 to form the PMMA loading layer 04, and the waveguide structure is a rib type.

When such a waveguide structure is used,
The dn / dT of PMMA in the PMMA loading layer 04 is -105.
Since it is × 10 ⁻⁶ / ° C., by adjusting the signal light intensity in the PMMA loaded layer 04, the temperature coefficient of the equivalent refractive index of the waveguide becomes zero (0) in principle, that is, a so-called athermal state. The state becomes possible.

The signal light intensity adjustment in the PMMA loading layer 04 is controlled by the width and height of the PMMA loading layer. It has been shown that when such an athermal waveguide is used, the temperature coefficient of the effective refractive index can be reduced to −0.24 × 10 ⁻⁶ / ° C., and can be suppressed to 2% as compared with the buried quartz optical waveguide. .

[0010]

However, since the athermal waveguide whose conceptual cross-sectional view is shown in FIG. 4 has a rib-type waveguide structure, the refractive index difference between the core glass layer 03 and the lower cladding layer 02 is different. At most 10 ⁻³ , there is a problem that the loss in the bent waveguide becomes remarkable. That is,
When the effective refractive index difference is 10 ^-3 , the allowable bending radius is several c
m, which is not suitable for constructing a practical optical circuit including branching and bending at a length of several cm or less.

On the other hand, in a silica-based optical waveguide,
A three-dimensional optical waveguide in which the top and side surfaces of the core are in contact with a photopolymer material (Hibino, 1991 Autumn Meeting of Applied Physics, 10
p-ZN-15) and a three-dimensional optical waveguide in which the side surface of the core is in contact with quartz glass and the upper surface is in contact with a photopolymer material have been proposed (M. Ishii et al., M.
OC 1993).

However, these waveguides are manufactured by embedding a core glass layer after processing the core glass layer by a photolithography process and etching.
Roughness due to etching of the core side surface causes an increase in propagation loss, and the latter essentially has a problem that the core upper surface is lowered after being buried in the upper cladding layer.

In view of the circumstances described above, an object of the present invention is to provide an optical waveguide whose operating wavelength does not fluctuate in response to changes in environmental temperature and an optical circuit including the waveguide as a component.

[0014]

According to the present invention, there is provided a method of manufacturing an optical waveguide, comprising: a core for transmitting light formed on a planar substrate; and a cladding having a lower refractive index than the core. In the method for manufacturing an optical waveguide, after forming a lower cladding layer on the planar substrate, a groove structure serving as a core is formed in the lower cladding layer, and the core is formed on the lower cladding layer on which the groove structure is formed. After the formation, the portion other than the groove portion of the quartz-based core layer is removed, and an upper clad layer made of a photopolymer material is formed above the portion including the core.

An optical waveguide according to the present invention is an optical waveguide comprising a core formed on a flat substrate and formed by processing a groove for transmitting light, and a cladding layer having a lower refractive index than the core. A cladding layer made of a photopolymer material is formed on the upper surface of the core.

An optical circuit according to the present invention includes the optical waveguide.

The optical circuit is a directional coupler, a Mach-Zehnder interferometer, a relief type Bragg grating, an optical multiplexing / demultiplexing circuit for signal light, a wavelength selection circuit, or a laser light source circuit.

According to the present invention, as the optical waveguide having the core disposed in the groove constituting the optical circuit, the clad provided on the upper surface of the core is a three-dimensional optical waveguide made of a photopolymer material. The so-called athermal waveguide, in which the refractive index is constant with respect to a change in temperature, can be realized by controlling the refractive index.

Further, in the three-dimensional optical waveguide of the present invention, since the light confinement effect in the lateral direction is exerted by the cladding layer, the light confinement effect is large and the bending loss is small as compared with the conventional rib waveguide structure. For example, when the refractive index difference between the cladding layer and the core is 0.75%, the allowable bending radius is 5 mm, and when the refractive index difference is 2.0%, the allowable bending radius is 2 mm.
While the allowable bending radius of the conventional rib waveguide is several cm, it is 1/10 or less.

Further, the present invention employs a concave process in which a core structure is formed in a lower clad glass layer by a photo process and an etching process, and after the core glass layer is formed, portions other than the groove portions of the core glass layer are removed. Therefore, the number of manufacturing steps can be reduced as compared with a conventional three-dimensional optical waveguide structure athermal waveguide manufactured by a combination of a convex process and etching, and as a result, the yield can be improved.

Furthermore, in the concave process, the etching roughness on the side surface of the groove formed in the lower clad is reduced in the high-temperature sintering step at the time of forming the core, so that the scattering loss due to the etching roughness on the core side surface can be suppressed.

As described above, according to the present invention, an optical waveguide in a so-called athermal state having a small bending loss and scattering loss can be manufactured with a high yield. Also,
The operating wavelength of the obtained optical waveguide and the optical circuit including the optical waveguide as constituent elements do not fluctuate with changes in environmental temperature.

[0023]

Embodiments of the present invention will be described below.

[First Embodiment] FIG. 1 is a diagram showing a manufacturing process of a concave type optical circuit according to an embodiment of the present invention. As shown in FIG. 1, a silicon substrate 11 has a thickness of about 30 mm by a flame deposition method.
A lower clad silica glass layer 12 having a thickness of μm is formed (see FIG. 1A). Next, the width 6 for forming the core
A groove 13 having a thickness of 6 μm and a depth of 6 μm is formed by a photo process and reactive ion etching (FIG. 1B). Next, a core glass soot is deposited by a flame deposition method, and the soot is sintered at 1300 ° C. for 2 hours in an electric furnace to form a transparent core glass layer 14 (see FIG. 1C).
Next, the core glass layer 14 is formed by reactive ion etching.
Is removed to form a core 15, and an air-clad optical waveguide is manufactured (FIG. 1D). Finally, an upper cladding layer 16 made of a photopolymer material such as PMMA is formed (FIG. 1).
(E)).

[Embodiment 2] An arrayed waveguide grating having a multiplexing / demultiplexing function was manufactured using the optical circuit manufacturing method of Embodiment 1 described above. FIG. 2 shows an outline of the circuit configuration. 2, reference numeral 21 denotes a silicon substrate, 22 denotes a lower clad silica glass layer, 23 denotes an input waveguide, 24 denotes an input side slab waveguide, 25 denotes an array waveguide diffraction grating, 26 denotes an output side slab waveguide, and 27 denotes an output side slab waveguide. Is the output waveguide, 28 is PMMA
Are respectively shown in FIG.

Based on this embodiment, an air-clad array waveguide diffraction grating is manufactured by a concave process, and a PMM
The A film was formed as the upper cladding 28.

The waveguide manufactured in this embodiment has a core size of 6 μm × 6 μm and a refractive index difference between the core and the cladding layer.
0.75%. In designing the optical multiplexer / demultiplexer, the wavelength interval in the 1.55 μm band used in optical communication is
In order to obtain 0.4 nm (optical frequency interval: 50 GHz), the optical path length difference between the channel waveguides constituting the array waveguide was set to 63.0 μm.

To operate this optical multiplexer / demultiplexer, first, a single-mode optical fiber on the transmission side is connected to the input waveguide 23,
A frequency multiplexed signal light is incident. Input side slab waveguide 24
In, the signal light spread by the diffraction effect propagates to a plurality of channel waveguides constituting the arrayed waveguide diffraction grating, reaches the output side slab waveguide 26, and is further focused on the output waveguide 27.

In this case, by changing the lengths of the individual channel waveguides constituting the arrayed waveguide diffraction grating 25 to provide an optical path length difference, the phase of the signal light after the propagation of the channel waveguides is shifted. The wavefront of the convergent light in the output side slab waveguide 26 is tilted according to the phase shift amount. The light condensing position is determined by the inclination angle.

The amount of phase shift depends on the signal light frequency, and the focus position of the frequency multiplexed light is determined for each frequency. By arranging an output waveguide at the focus position, the signal light can be extracted for each optical frequency. it can.

By using the optical circuit manufacturing method of the present embodiment, the insertion loss of the athermal optical multiplexing / demultiplexing circuit is essentially reduced without changing the design parameters such as the allowable bending radius of the ordinary embedded quartz optical waveguide. The value is equivalent to that of the conventional optical multiplexing / demultiplexing circuit.

The transmission spectrum of this optical multiplexing / demultiplexing circuit at a wavelength of 1.55 μm was measured.
A good multiplexing / demultiplexing characteristic was obtained in which the frequency, the temperature coefficient of the peak wavelength was 14 MHz / ° C., the crosstalk between channels was −27 dB or less, and the insertion loss at the peak wavelength was 3.0 dB.

On the other hand, the spectrum shift of the conventional arrayed waveguide type diffraction grating composed of only silica-based glass is 1.4.
GHz / ° C.

Accordingly, it has been found that the temperature coefficient of the peak wavelength of the optical multiplexing / demultiplexing circuit constituted by the athermal waveguide of the present embodiment can be reduced to 1% as compared with the conventional one.

Third Embodiment FIG. 3 shows a configuration of a wavelength selection circuit manufactured by using the optical circuit manufacturing method according to the third embodiment. In FIG. 3, reference numeral 31 denotes a silicon substrate, 32 denotes a lower clad quartz glass layer, 33 denotes a quartz glass core,
Numeral 4 denotes a Bragg grating in which refractive index modulation is written by using a photo-induced refractive index change, and numeral 35 denotes an upper cladding layer using an ultraviolet curable resin.

In order to manufacture the present wavelength selection circuit, an air-clad optical waveguide is manufactured by a concave process similar to that of the first embodiment. Next, ArF is passed through a phase mask.
The core portion is irradiated with excimer laser light to form the Bragg grating 34 on the quartz glass core. Finally, an ultraviolet-curable resin was applied to the upper surface of the substrate and cured with ultraviolet light to form the upper cladding layer 35, thereby producing a three-dimensional optical waveguide. When the reflection characteristics of the Bragg grating were measured using an LED light source having a wavelength band of 1.5 μm, the center wavelength was 1.55 μm, and the temperature coefficient dλ _{B of the} reflection wavelength.
/ DT 1.0 × 10 ⁻⁴ nm / ° C., a reflectance of 99%, a reflection band of 0.1 nm, and an excess loss of graying of 0.2 dB were obtained. Therefore, it has been found that the reflection wavelength temperature dependency of the wavelength selection circuit of the present embodiment can be improved to 1% as compared with the case where the conventional quartz-based ℃ is used.

As described above, in this embodiment, PMMA and an ultraviolet curable resin are used as the photopolymer material.
In addition, it goes without saying that a photopolymer material having a generally negative temperature coefficient of refractive index, such as a silicone resin or a polyimide resin, and a refractive index matching agent can be used as the upper cladding.

In the present embodiment, the upper clad made of a photopolymer material is formed on the entire circuit. However, a part of the optical circuit constituted by the quartz optical waveguide is made of a photopolymer material by using photolithography technology. It is also possible.

In the present embodiment, the flame deposition method is used for producing a glass film, because this method is suitable for depositing a relatively thick and high quality glass film. In some cases, another method of synthesizing a glass film, for example, a CVD method or a sputtering method may be partially or entirely used. Further, the core layer and the cladding layer are not limited to quartz glass.

In this embodiment, an array waveguide diffraction grating has been described as an optical multiplexing / demultiplexing circuit, and a photo-induced Bragg grating has been described as a wavelength selection circuit. However, an athermal waveguide is widely used and an optical circuit utilizing optical wave interference is generally used. Since the temperature dependence of the operating wavelength can be improved, a directional coupler,
It is useful to apply the present invention to a Mach-Zehnder interferometer, a relief Bragg grating, and an optical circuit combining these circuits. Further, as an optical waveguide, a wavelength of 1.55
The present invention can be applied to a rare earth-doped optical waveguide such as an Er-doped optical waveguide having an optical amplification function in the μm band. For example, a single longitudinal mode D integrating a Bragg grating
If a BR laser is manufactured by the manufacturing method of the present invention, an athermal laser light source can be realized.

[0041]

As described above, according to the present invention, according to the present invention, a quartz glass having a positive refractive index temperature coefficient and a photo-resist having a negative refractive index temperature coefficient in a concave process. By combining with a polymer material, it is possible to realize a so-called athermal three-dimensional optical waveguide in which the temperature coefficient of the effective refractive index of the waveguide is essentially small. Therefore, it is possible to realize an optical circuit in which the fluctuation of the operating wavelength of the optical multiplexing / demultiplexing circuit and the laser light source is significantly reduced with respect to the change of the environmental temperature. Also,
A waveguide structure having a large light confinement effect and a small bending loss can be realized. Furthermore, since the concave process is used, the optical circuit has an advantage that the loss is low and the number of manufacturing steps is minimized, so that the yield is high.

[Brief description of the drawings]

FIG. 1 is a waveguide sectional view showing an optical circuit process according to an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram of an arrayed waveguide type circuit grating according to an embodiment of the present invention.

FIG. 3 is a circuit structure diagram of the waveguide type Bragg grating according to the embodiment of the present invention.

FIG. 4 is a conceptual sectional view showing a conventional athermal waveguide structure.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Silicon substrate 12 Lower clad silica glass layer 13 Groove 14 Transparent core glass layer 15 Core 16 Upper clad layer 21 Silicon substrate 22 Lower clad silica glass layer 23 Input waveguide 24 Input slab waveguide 25 Array waveguide diffraction grating 26 Output side slab waveguide 27 Output waveguide 28 Upper cladding layer 31 Silicon substrate 32 Lower cladding silica glass layer 33 Quartz glass core 34 Bragg grating 35 Upper cladding layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akira Himeno 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Inventor Yoji Omori 3-192-1, Nishi-shinjuku, Shinjuku-ku, Tokyo No. Japan Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. A method for manufacturing an optical waveguide comprising a core formed on a planar substrate for transmitting light and a clad having a lower refractive index than the core, wherein a lower cladding layer is formed on the planar substrate. Thereafter, a groove structure serving as a core is formed in the lower clad layer. After the core is formed on the lower clad layer having the groove structure formed thereon, portions other than the groove portion of the quartz core layer are removed. A method for manufacturing an optical waveguide, comprising forming an upper cladding layer of a photopolymer material on an upper portion of a portion including the optical waveguide. 2. An optical waveguide comprising a core formed by a groove processing for propagating light formed on a planar substrate and a cladding layer having a lower refractive index than the core, wherein an upper surface of the core is provided. An optical waveguide characterized in that a cladding layer made of a photopolymer material is formed in a portion. 3. An optical circuit comprising the optical waveguide according to claim 2. 4. The optical circuit according to claim 3, wherein the optical circuit is a directional coupler, a Mach-Zehnder interferometer, a relief Bragg grating, an optical multiplexing / demultiplexing circuit for signal light, a wavelength selection circuit, or a laser light source circuit. Optical circuit.