JPH07253516A - Optical waveguide expanded in mode field diameter and its production - Google Patents

Abstract

PURPOSE:To provide an optical waveguide expanded in a mode field diameter, good in optical coupling characteristic and low in loss and the same production. CONSTITUTION:A glass film for cores is formed on a quartz glass substrate 1. The film is patterned by photolithography and dry etching to the cores 2 of a prescribed size (stage 1). Next, the middle parts of the cores 2 on the substrate 1 are locally heated and melted by a CO2 laser beam 5 and are thereby deformed flat (stage 2). In the succession, the soot which constitutes a clad layer 6 is deposited on the quartz glass substrate 1 and is then vitrified by sintering (stage 3). The clad layer 6 is formed on the surface of the cores after the cores 2 are deformed by heating and, therefore, the mode field diameter of the optical waveguides is expanded without thermally deforming the clad layer 6. A slit 8 for cutting the cores 2 of the parts 4 irradiated with the laser is formed at the optical waveguide 7 with the clad (stage 4) and an optical element is inserted into the slit 8, by which the low-loss flush type optical parts are obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide formed on a glass substrate or a Si substrate, and more particularly to an optical waveguide in which a mode field diameter is locally enlarged in order to improve optical coupling characteristics and a manufacturing method thereof. It is a thing.

[0002]

2. Description of the Related Art In order to improve the sophistication of optical fiber communication systems and expand the range of application, in addition to light emitting / receiving elements and optical fibers, optical directional couplers, optical star couplers, optical multiplexers / demultiplexers, optical switches, etc. Optical passive components with optical signal processing and control functions are needed.

The form of the optical passive component can be classified into a bulk type, an optical fiber type and an optical waveguide type. Among them, the optical waveguide type component is the component which is most expected to be miniaturized, economically constructed, and has a composite / integrated function, and is under active development.

As one of the optical waveguide type components, studies are being made on an embedded optical component which is formed by processing a slit in the optical waveguide and inserting an optical element into the slit. With such an embedded optical component, not only can the performance of the optical waveguide component be improved, but it is also possible to add a function (for example, an isolator) which was difficult with the optical waveguide alone.

[0005]

In order to reduce the size and increase the number of functions of the optical waveguide type component, it is important to increase the relative refractive index difference Δ of the optical waveguide so that the bending loss characteristic of the optical waveguide is good. Is. However, if Δ is increased, the mode field diameter becomes smaller, so the connection loss increases due to the mismatch of the mode field diameters of the optical waveguide and the optical fiber, and there is a problem that the characteristics of the optical waveguide cannot be sufficiently drawn to the outside. . An example is shown below.

It is known that when the mode field diameter of the optical waveguide is 2ω ₁ and the mode field diameter of the optical fiber is 2ω ₂ , the optical coupling loss η caused by the mismatch of the mode field diameters is expressed by the following equation. There is.

Η = −20log (2ω ₁ ω ₂ / (ω ₁ ² + ω ₂ ² )) (1) As an example, the calculation result of the coupling loss between the optical waveguide and the optical fiber when 2ω ₁ is 7 μm and 10 μm is shown. As shown in FIG. From the figure, the mode field diameter 2ω ₂ of the optical fiber is 10μ.
When 2ω ₁ is 7 μm, the optical coupling loss is about 0.5
It becomes dB.

On the other hand, by forming a slit in the optical waveguide,
As a result of studying an embedded optical component in which an optical element is inserted and fixed in the slit, it was found that the practical slit width is at most about 30 μm because the diffraction loss increases as the slit width increases. With such a narrow slit, there is a problem that the types of optical elements that can be inserted are very limited.

As a method for solving these problems, a local mode field conversion technique for an optical waveguide by a thermal diffusion method has been reported (Yanagisawa et al., 1993 IEICE Spring Conference C).
-240). If the mode field diameter at any location of the optical waveguide can be increased, the coupling loss with the optical fiber can be reduced and the diffraction loss caused by forming the slit can be reduced.

However, in this method, even if the mode field diameter 2ω is increased from 5 μm to 10 μm, it is necessary to heat the optical waveguide at a temperature of 1300 ° C. for 10 hours, which is a problem in working efficiency.

Further, in order to reduce the diffraction loss, it is necessary to increase the mode field diameter to 20 μm or more. Therefore, when an attempt was made to further increase 2ω by this method, the cladding glass layer was deformed and the optical waveguide There was a problem that the loss of the company would increase. Further, in this method, since the position where the optical waveguide can be enlarged is limited to both ends of the optical waveguide, there is a problem that the mode field diameter of the optical waveguide at the place where the slit is formed cannot be enlarged.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to provide a mode field diameter expanding optical waveguide having good optical coupling characteristics and low loss, and a method for manufacturing the same.

[0013]

In order to achieve the above object, in the mode field diameter enlarged optical waveguide of the present invention, the middle of the core formed on the substrate is deformed flat due to local heating, After shaping, a clad layer is formed on the surface of the core. The ratio of the major axis to the minor axis of the core cross section of the flatly shaped portion is preferably less than 0.12.

Further, according to the method of manufacturing an optical waveguide with expanded mode field diameter of the present invention, a core is formed on a substrate, the middle of the core is locally heated to be deformed into a flat shape, and then a clad layer is formed on the surface of the core. It is characterized in that it is formed.

[0015]

[Function] The mode field diameter is expanded without thermally deforming the clad layer by locally heating the middle part of the core formed on the substrate to deform it into a flat shape and then forming the clad layer on the core surface. It becomes possible. If a CO ₂ laser, a micro-heater or the like is used as a heating source for locally heating the middle of the core, any place on the substrate can be heated and the heating range can be controlled arbitrarily.

It is known that the diffraction loss generated when a slit having a width d without a wave guiding action is formed in the middle of a waveguide is obtained by the following equation.

Η = 1 / (1 + z ² ) (2) where z = λd / (2πnω ₁ ² ) (3) where λ is the wavelength, n is the refractive index of the optical element inserted in the slit, and 2ω ₁ is It is the mode field diameter of the optical waveguide.

FIG. 2 shows the calculated value of the diffraction loss with respect to the slit width when 2ω is a parameter (λ = 1.55 μ
m, n = 1.0). From the figure, it can be seen that the larger 2ω is, the smaller the diffraction loss is.

FIG. 3 shows the calculation result of the mode field shape when an optical waveguide having a core size of 8 μm × 8 μm has a core cross-sectional area unchanged (64 μm ² ) and is deformed into a flat cross section. Marcatili's theory was applied to the calculation. The relative refractive index difference of the core was 0.3%. This figure is a diagram showing one quadrant of the mode field divided into four, where ε is the core long-short axis ratio (ε = short axis diameter / long axis diameter), a is the radius on the long axis side of the mode field, and b is The radius on the short axis side is shown. It can be seen from the figure that the diameter of the mode field on the major axis side can be increased by decreasing the major axis ratio ε of the core.

FIG. 4 shows in detail the relationship between the long-short axis ratio ε of the core and the radius b of the mode field on the short axis side. From the figure, it can be seen that when ε <0.12, b also becomes larger than before the deformation.

[0021]

Next, an embodiment of the present invention will be described with reference to FIG. The figure shows the outline of the manufacturing process of the mode-field-diameter expanding optical waveguide of one embodiment of the present invention.

First, a core glass film (composition: TiO ₂ -SiO _{2) having} a relative refractive index difference Δ = 0.3 is formed on a quartz glass substrate 1.
%), And the film is patterned by photolithography and dry etching into a core 2 having dimensions of 8 μm × 8 μm (step 1). The core pattern of this embodiment has a wavelength of 1.31 μm from the first port P1 to the core 2
Directional coupler type designed to emit 1.55 μm light from the second port P2 and 1.31 μm light from the third port P3 when 1.55 μm light is incident. This pattern has the optical demultiplexer 3 in the center of the substrate 1. Next, the middle of the core 2 in the vicinity of the second port P2 (portion where the slit is formed) on the substrate 1 was locally heated by a CO ₂ laser (carbon dioxide gas laser) 5 for 5 minutes (step 2). As a result, the core 2 of the laser irradiation part (heating part) 4 was melted and expanded in the horizontal direction and contracted in the vertical direction to be deformed into a flat cross section. As another heating source, for example, a micro heater, an oxyhydrogen flame heater, or the like can be used. By using these heating sources, it is possible to heat any place on the quartz glass substrate 1. Also, the heating range can be controlled arbitrarily. Subsequently, a soot to be the cladding layer 6 was deposited on the quartz glass substrate 1 by the flame volume method, and then sintered and vitrified (step 3). In this optical waveguide with a clad 7,
Width to cut the core 2 of the laser irradiation part 4 almost vertically 100μ
m slits 8 were formed (step 4). This slit 8
For the processing, for example, a mechanical cutting method using a diamond saw or a chemical method such as RIE (reactive ion etching) can be used. It is also possible to change the reflection characteristics by changing the angle between the slit 8 and the optical axis of the optical waveguide. Finally, the port P1 of the optical waveguide 7
Optical fibers 10, 11 and 12 are fusion-spliced to P3 to obtain a slitted mode field diameter expanding optical waveguide 9. For comparison, a slitted optical waveguide having the same core size was also manufactured by the conventional method without the above heat treatment.

After the above step 3, the loss of the optical waveguide of this example was measured and compared with the conventional optical waveguide without heat treatment. It was confirmed that the loss difference between the two was as small as 0.1 dB or less, and there was almost no increase in loss due to heat treatment. Also,
When the optical waveguide was cut and the mode field diameter of that part was measured by the far field pattern method, the mode field diameter of the part that was heat-treated by laser irradiation was longer on the major axis side than the part that was not heat-treated. Three times,
The minor axis side was magnified 1.4 times.

In this embodiment, quartz glass is used for the optical waveguide substrate 1, but the material of the substrate 1 is, for example, S.
The present invention can be applied to the case of i or multi-component glass.

Comparing the insertion loss of the optical waveguide of the present example and the conventional optical waveguide with the slit after forming the slits, the optical waveguide of the present example has a loss of 2.5 dB lower than that of the conventional optical waveguide with the slit. It was This can be said to be the effect of increasing the mode field diameter.

Further, a 1.31 μm transmission / 1.55 μm blocking optical filter (thickness 90 μm) was inserted into the slit 8 of the slitted optical waveguide 9 produced in this example, and fixed with an adhesive.
A filter-embedded duplexer was manufactured. The transmission loss in the 1.31 band was 1 dB or less and the isolation was 50 dB or more. Compared with the conventional optical waveguide type demultiplexer, isolation was improved by 20 dB or more.

In addition to this embodiment, an optical attenuator, an optical branching device, an optical multiplexer / demultiplexer, an optical isolator, an optical circulator, etc. are produced by changing the optical element to be inserted in the slit. It has been confirmed that these are also effective.

Further, the end portions (ports P1, P2, P3) of the core 2 were heat-treated to produce an optical waveguide having an expanded mode field diameter, and the coupling characteristics with the optical fibers 10, 11, 12 were evaluated. However, the result is that the connection loss is reduced and the loss due to the optical axis shift between the optical waveguide and the optical fiber is also reduced.

[0029]

In summary, according to the present invention, the following effects can be obtained.

(1) In the mode-field-diameter-increasing optical waveguide according to claim 1, the mode-field diameter is expanded by deforming the middle of the core into a flat shape before forming the cladding on the core. As described above, the cladding layer is not thermally deformed and the transmission loss is extremely small.

(2) In the mode-field-diameter-enlarged optical waveguide of claim 2, the major-minor axis ratio of the core cross section of the flatly deformed portion is set to less than 0.12. Since the diameters on both sides increase, the diffraction loss caused by forming the slit in this portion can be suppressed as much as possible, and the loss of the embedded optical component configured by inserting the optical element into this slit can be reduced.

(3) According to the method of manufacturing a mode field diameter enlarged optical waveguide of claim 3, a core is formed on a substrate, the middle of the core is locally heated to be deformed to a flat shape, and then the core is deformed. Since the clad layer is formed on the surface, the mode field diameter can be expanded at any position of the core without thermally deforming the clad layer.

[Brief description of drawings]

FIG. 1 is a graph showing an example of a calculation result of connection loss due to mode field diameter mismatch.

FIG. 2 is a graph showing an example of a result obtained by investigating a relationship between a slit width and a diffraction loss with a mode field diameter as a parameter.

FIG. 3 is a graph showing an example of a result of calculation of change in mode field shape on the minor axis side when the major axis and minor axis ratios of the core are changed.

FIG. 4 is a graph showing an example of the result of calculation of changes in mode field diameter on the minor axis side when the major axis and minor axis ratios of the core are changed.

FIG. 5 is a series of process charts showing a method for manufacturing a mode field diameter enlarged optical waveguide of the present invention.

[Explanation of symbols]

1 Substrate 2 Core 4 Laser Irradiation Part (Heating Part) 5 CO ₂ Laser Light (Heating Means) 6 Clad Layer 7 Optical Waveguide 8 Slit

Claims (3)

[Claims] 1. An optical waveguide in which a core and a clad covering the core are formed on a substrate, wherein the middle of the core is deformed to be flat by local heating. . 2. The expanded mode field diameter optical waveguide according to claim 1, wherein the major-minor axis ratio of the core cross section of the flatly deformed portion is less than 0.12. 3. A mode field diameter expansion, characterized in that a core is formed on a substrate, the middle of the core is locally heated to be deformed into a flat shape, and then a clad layer is formed on the surface of the core. Manufacturing method of optical waveguide.