JPH10307219A - Light guide type filter and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a small-sized light guide which has small variation in characteristics against temperature variation and it small-sized and good in productivity by making optical wave guides, which differ in optical path length from each other, different in sectional geometric structure and making selected wavelength not vary with temperature. SOLUTION: This fiber is equipped with optical wave guides IIIA and IIIC which differ in optical path length from each other and the light guides IIIA and IIIC are made different in geometric structure such as core thickness or width. Here, the core thicknesses and widths of the light guides IIIA and IIIC are related preferably so that at least one of the core thickness and width of the light guide IIIC having long optical path length is less than that of the light guide IIIA having short optical path length. Consequently, the light guides are made different in the length of an area wherein a coefficient of confinement of guide light to the core and clad is different between the optical wave guides. Consequently, the optical wave guide type filter can be obtained which has small passing band characteristics against variation in environmental temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide filter, and more particularly, to a small optical waveguide filter having a small change in characteristics with respect to a temperature change and having good manufacturability, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Generally, when the environmental temperature changes, the refractive index of a material such as a semiconductor or quartz constituting an optical waveguide filter changes, and as a result, the pass band of the filter changes. Therefore, it is desired to realize a filter having a small change in the pass band characteristic with respect to a change in the environmental temperature.
Here, as an example of an optical waveguide type filter having a small change in filter characteristics with respect to a change in environmental temperature, a conventional example proposed in the past for a Mach-Zehnder type optical filter will be described, and its structure and operation principle will be described.

FIG. 19 shows a conventional example of a buried 1.55 μm-band Mach-Zehnder optical filter (Tanobe et al., Integrated Photonics Research, pp.
90-93, 1996). In FIG.
Is an input optical waveguide, II is a demultiplexing part, III is a phase difference waveguide, IV is a multiplexing part, and V is an output optical waveguide. In this example, the optical path length of the optical waveguide IIIB is longer than the optical path length L of the optical waveguide IIIA by ΔL. 20 and 21 are sectional views taken along lines AA 'and BB' of FIG. 19, that is, the optical waveguide III of the phase difference waveguide, respectively.
FIG. 22 is a sectional view including A and IIIB, and FIG. 22 is a sectional view taken along the line CC ′ of FIG. 20 to 2
In 2, 1 is an InP upper cladding, 2 is a first core made of InGaAsP (hereinafter referred to as 1.1Q) having a width and a thickness of 1.5 μm, 0.23 μm and a band gap wavelength of 1.1 μm, respectively, and 3 is InGaAsP having a thickness of 0.3 μm and a band gap wavelength of 1.3 μm (hereinafter, referred to as InGaAsP)
1.3Q), 4 is an InP lower cladding, 5 is an InP substrate, and 6 is a 0.3 μm thick 1.C.
The second core of 1Q composition, 1 'is an InP spacer. The signal light is supplied to the second core 3 by the upper and lower InP claddings in the vertical direction and by the first cladding in the horizontal direction.
It is confined to (6). FIG. 19 shows the waveguide pattern formed in this manner, which is substantially equal to the pattern of the first core 2.

As apparent from the above description, in this conventional example, the cores of the optical waveguide IIIA and the optical waveguide IIIB are made of two different materials, that is, a second material having a 1.3Q composition.
A core 3 and a second core 6 having a 1.1Q composition are used. Further, the lengths of the second core 3 having the 1.3Q composition and the second core 6 having the 1.1Q composition are different as L and L + ΔL, respectively. In FIG. 22, IIIJ is a butt joint surface (butt joint surface) of the second core 3 having the 1.3Q composition and the second core 6 having the 1.1Q composition.

The operation principle of this conventional example will be briefly described below with reference to the above-mentioned literature. The equation describing the operation of the Mach-Zehnder filter is as follows.

[0006]

(Equation 1)

Here, n _1.3Q and n _1.1Q are the refractive indices of the second core 3 having the 1.3Q composition and the second core 6 having the 1.1Q composition, λ is the wavelength, and m is the interference in the Mach-Zehnder filter. And L is the second core 3 having a 1.3Q composition.
, L + ΔL is the length of the second core 6 having the 1.1Q composition.

When the environmental temperature changes, the refractive index of the second core 3 having the 1.3Q composition and the refractive index of the second core 6 having the 1.1Q composition change with temperature. At this time, when both sides of equation (1) are differentiated with respect to temperature, approximately

[0009]

(Equation 2)

Is obtained. Now, the temperature dependence of n _1.1Q and n _1.3Q is different, and in fact,

[0011]

(Equation 3)

Therefore, if ΔL is determined so that the right side of equation (2) becomes 0, that is, dλ / dT = 0, it is possible to realize an optical filter in which the light pass band does not change with respect to the environmental temperature. become. For example, if L is 2 mm,
ΔL is about 800 μm.

[0013]

In the optical waveguide filter having the conventional structure, two types of second filters having different materials are used.
Core, that is, a second core 3 having a 1.3Q composition and 1.1Q
Since the second core 6 having the composition is used, a butt joint of a core made of a different material must be performed for actual production. The procedure of this butt joint will be described below.

(1) As shown in FIG.
After a lower InP cladding 4, a second core 3 having a 1.3Q composition and an InP spacer 1 'are sequentially grown on the
An O ₂ film 7 is deposited, and a photoresist 8 is spin-coated thereon.

(2) Next, as shown in FIG. 24, the photoresist 8 is partially removed at the butt joint surface.

(3) Next, as shown in FIG. 25, using the remaining photoresist 8 as a mask, the SiO ₂ film 7, the InP spacer 1 ', and the second core 3 having the 1.3Q composition are wet-etched. And partially remove it.

(4) Next, as shown in FIG. 26, after removing the remaining photoresist 8, using the SiO ₂ film 7 as a mask, the second core 6 of 1.1Q composition and the InP spacer 1 '. The second core 3 having the 1.3Q composition and the second core having the 1.1Q composition are recrystallized on the lower InP clad 4 exposed by removing the second core 3 having the 1.3Q composition.
The core 6 is butt-jointed.

(5) Finally, after removing the SiO ₂ film 7, as shown in FIG. 27, a second core 6 having a 1.1Q composition and an upper InP clad 1 are formed by crystal regrowth.

Thus, the butt joint between the second core 3 having the 1.3Q composition and the second core 6 having the 1.1Q composition is completed. As described above, the crystal-grown second core 3 having the 1.3Q composition and the InP spacer are partially removed by etching, and then the crystal is re-grown twice, so that the process is complicated. As schematically shown in FIG. 26, the butt joint portion has the second core 6 of 1.1Q composition and the InP spacer 1 'has 6a,
A bulge indicated by 1'a is generated, and the bulge is reflected on the first core 2 as shown in FIG. 27 to generate a bulge 2a. Second cores 3 of 1.3Q composition and 1.1
Due to the swelling of the butt joint portion of the second core 6 having the Q composition, radiation loss of light occurs at the joint portion.

An object of the present invention is to solve such a problem and to realize an optical waveguide filter which is small in size and has good manufacturability with little change in characteristics with respect to temperature change, little light radiation loss. And

[0021]

In order to achieve the above-mentioned object, an optical waveguide filter according to the present invention comprises a plurality of optical waveguides having different optical path lengths, and branches input light into the plurality of optical waveguides. Then, after propagating through each optical waveguide and multiplexing and interfering, in the optical waveguide filter that selects the input light based on the wavelength, the geometrical structure of the cross section of the optical waveguide is changed, and as a result, By making the optical path lengths of the regions having different confinement factors of the guided light into the cores of the respective optical waveguides different from each other among the plurality of optical waveguides, the wavelength to be selected by the optical waveguide filter is not changed by the temperature. It is characterized by.

Here, preferably, the optical path lengths of the regions in which at least one of the thickness and the width of the core of the optical waveguide is different between the plurality of optical waveguides, or the core of each of the plurality of optical waveguides has a different length. At least one of the thickness and the width of the core of the optical waveguide having a long optical path is smaller than the thickness and the width of the core of the optical waveguide having a short optical path.

Preferably, the optical waveguide type filter is a Mach-Zehnder type filter, and at least one of the thickness and the width of the core of the optical waveguide having a longer optical path length is an optical waveguide having a shorter optical path length. Smaller than the thickness and width of the waveguide core.

Preferably, the thickness of the core of the optical waveguide having a long optical path is smaller than the thickness of the core of the optical waveguide having a short optical path, and the optical waveguide having a short optical path has a stepped portion. The thickness of the core of the optical waveguide having a long optical path is smaller than the thickness of the core of the optical waveguide having a short optical path, and the thickness of the core is continuously increased at both ends.

Preferably, the optical waveguide filter is an array grating filter, and the width of each core of the plurality of optical waveguides is smaller as the optical waveguide has a longer optical path length, or the optical waveguide filter is an array grating filter. In the filter, the length of the region where at least one of the thickness and the width of the core of the optical waveguide is small is longer as the optical waveguide has a longer optical path length.

A method of manufacturing an optical waveguide filter according to the present invention comprises providing two optical waveguides having different optical path lengths, splitting the input light into the two optical waveguides, and propagating each of the optical waveguides. In the method of manufacturing a Mach-Zehnder type optical waveguide filter for multiplexing and interfering and selecting the input light based on the wavelength, after forming the cores of the two optical waveguides, the core of the optical waveguide having a long optical path length is formed. Is etched to reduce at least one of the thickness and the width, and a step of forming an upper clad on the cores of the two optical waveguides.

Preferably, prior to the step of forming the upper cladding, a step of forming a step is also formed in the core of the optical waveguide having a short optical path length, or the core of the optical waveguide having the long optical path length is etched. A step of continuously increasing the thickness of both ends in the length direction of the core of the optical waveguide having a long optical path length, following the step of reducing the thickness.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A core and a clad constituting a waveguide have different refractive index temperature coefficients. In the present invention, by changing the geometric structure of the optical waveguide such as the thickness or width of the core, the length of the region where the confinement coefficient of the guided light to the core and the clad is different between the respective optical waveguides is set. Different between waveguides. as a result,
An optical waveguide filter having a small change in the pass band characteristic with respect to a change in the environmental temperature can be realized.

[0029]

Embodiment 1 FIG. 1 shows a top view of a first embodiment of the optical waveguide filter according to the present invention. This embodiment is an example of a Mach-Zehnder optical filter operating in the 1.55 μm band. FIG.
3 is a cross-sectional view taken along line BB 'in FIG. 1, and FIG. 4 is a cross-sectional view taken along line CC' in FIG. 1 to 4, the same parts as those of the conventional example shown in FIGS. 18 to 21 are denoted by the same reference numerals, and description thereof will be omitted.
In FIG. 1, the core of the optical waveguide IIIC is the optical waveguide II.
Thinner than IA core. 2 to 4, 9 is InP.
Upper claddings 10 and 12 are 1.3Q cores having different thicknesses from each other, and 11 is a lower cladding. Here, since the optical waveguide has a high mesa structure, 13 is air. 1 and 4, reference numeral 12a denotes a step.

The basic principle of the present invention is as follows. In the two optical waveguides, a region is provided in which the confinement coefficient (Γ factor) of the guided light to the core and the clad is different. Since the core and the clad are made of different materials, the amount of change in the refractive index with respect to the temperature change is largely different.
The average temperature coefficient of the refractive index at 5 ° C. to 75 ° C. is InP.
InGaAs of 2.01 × 10 ^{-4 /} ° C and 1.1Q composition
2.66 × 10 ^{-4 /} ° C with P, 1.3Q InGaAs
3.68 × 10 ^-4 / ° C in sP). As a result, if an optical path length difference ΔL is provided between the two optical waveguides, the pass band characteristic of the filter is not dependent on a change in environmental temperature by utilizing the difference in the temperature coefficient of the refractive index between the core and the clad. Can be designed. Hereinafter, the principle will be described using equations.

In the present invention, equations corresponding to equations (1) and (2) are as follows.

[0032]

(Equation 4)

[0033] Here, gamma ₁₀ optical path length is L 1.3Q
Gamma factor of the guided light into the core 10 of composition, gamma ₁₂ is a gamma factor of the guided light into the core 12 of 1.3Q composition optical path length is L + [Delta] L.

FIG. 9 shows the results obtained by semi-vector analysis of the Γ factor of the 1.3Q composition of the guided light to the core when the core width is fixed to 2 μm and the thickness is used as a variable. The Γ factor for the clad may be obtained by subtracting the value in the figure from 1. As can be seen from the figure, by changing the thickness of the core, the Γ factor can be greatly changed.

[0035] Therefore, the value of gamma ₁₀ and gamma ₁₂ and L and ΔL satisfies Equation (4), be determined simultaneously right side of the equation (5) to zero, pass to changes in environmental temperature characteristics Can be realized.
For example, when the thickness of the 1.3Q composition core is changed from 0.4 μm to 0.2 μm, the Γ factor becomes 77% to 3%.
4%, less than half. Incidentally, if L = 2 mm, ΔL is about 500 μm.

FIGS. 5 to 8 show portions of the manufacturing process of this embodiment corresponding to FIGS. 22 to 25 of the conventional example.

(1) As shown in FIG. 5, a lower InP clad 11, a 1.3Q core 10 (or 12) is crystal-grown on an InP substrate 5, and a photoresist 8 is formed thereon.
Is spin-coated.

(2) As shown in FIG. 6, the 1.3Q core 1
Photoresist 8 bordering the surface from which 0 (or 12) is removed
Is partially removed.

(3) As shown in FIG. 7, the 1.3Q core 10 (or 12) is partially removed by wet etching using the remaining photoresist 8 as a mask.
At this time, the InP is inserted into the 1.3Q core 10 (or 12).
If an etch stopper layer is provided, it is convenient for wet etching and has good crystallinity for subsequent regrowth.

(4) The remaining photoresist is removed, and as shown in FIG. 8, the process is completed by recrystallizing the InP upper clad 9.

As described above, in the manufacturing process of this embodiment, it is not necessary to perform a butt joint, so that the manufacturing process is simplified and the number of times of crystal regrowth is one. Therefore, there is an advantage that the manufacturing process is extremely simplified as compared with the conventional example.

Embodiment 2 FIGS. 10 and 11 show a second embodiment of the present invention. In the first embodiment, steps 12a are formed at both ends because the thickness of the core on the waveguide side having a long optical path length is reduced. Radiation loss occurs at this step, and as a result, the extinction ratio as a filter cannot be sufficient due to the difference in power between the light propagating on the long waveguide side and the light propagating on the short waveguide side. In this case, the same number of steps may be provided on the waveguide side having the shorter optical path length as on the waveguide side having the longer optical path length. FIG. 10 is a top view of an optical waveguide filter in which the same number (two each) of steps are formed in two waveguides, and FIG. 11 is DD ′ in FIG.
FIG. In both figures, reference numeral 10a denotes a groove provided in the core 10 of the shorter waveguide IIIa, and a step is formed at both ends thereof. Since the two waveguides have the same number of steps, a high extinction ratio can be obtained.

Embodiment 3 FIG. 12 is a top view of a third embodiment of the present invention, and FIG. 13 is a sectional view taken along line CC 'of FIG. This embodiment is a second embodiment.
2 is an example of a structure in which radiation loss due to a step in FIG. In FIG. 13, reference numeral 12b denotes a portion where the thickness of the core changes smoothly. Such a smooth change of the thickness can be realized by using a known region selective growth technique or the like. Since the thickness of the end of the thin core 12 changes smoothly and there is no step, radiation loss can be reduced, and a large extinction ratio can be realized without processing the shorter core 10.

Embodiment 4 FIG. 14 is a top view of a fourth embodiment of the present invention, and FIG. 15 is a sectional view taken along AA 'and BB' in FIG.
As shown in FIG. In this embodiment, unlike the first to third embodiments,
By changing the width of the buried core in the two waveguides IIIA and IIIB, the Γ factor of the guided light to the core and the clad is changed. 15 and 16, 1
4 is a cladding, 15 is a buried core of a waveguide having a short optical path length, and 16 is a buried core of a waveguide having a long optical path length.
In this embodiment as well, the effect becomes more remarkable if the thicknesses of the buried cores 15 and 16 are made different.

Embodiment 5 FIG. 17 shows a fifth embodiment of the present invention. The present embodiment is an example in which the present invention is applied to an array waveguide section forming an array grating filter. In FIG. 17, VI is an input waveguide, VII is an input slab waveguide, VIII is an array waveguide, IX is an output slab waveguide, and X is an output waveguide. In the present embodiment, each waveguide is drawn by the same line in the drawing for the sake of simplicity, but the width of each waveguide in the arrayed waveguide section is changed from a waveguide having a short optical path length to a waveguide having a long optical path length. It gradually narrows toward. As a result, adjacent waveguides have a relationship similar to the equations (4) and (5), and the Γ factor of the core and the cladding is changed. As a result, it is possible to realize an array grating filter having a small change in the pass characteristic with respect to a change in the environmental temperature.

Embodiment 6 FIG. 18 shows a sixth embodiment of the present invention. The present embodiment is an example in which the present invention is applied to an array waveguide section forming an array grating filter. In FIG. 18, VI is an input waveguide, VII is an input side slab waveguide, VIII is an arrayed waveguide section, IX is an output side slab waveguide, and X is an output waveguide. In the present embodiment, each waveguide is drawn by the same line on the drawing to avoid complication, but the region VIII
In the region other than A, at least one of the thickness and the width of the core is smaller than the region VIIIA. That is, in the present embodiment, each optical waveguide constituting the arrayed waveguide has a region having a different confinement coefficient of the guided light into the core, and the length of the region is different.

In the above description, the material constituting the core is the same between the optical waveguides. However, for example, the refractive index of the core of the optical waveguide having a short optical path length is made higher than that of the core of the optical waveguide having a long optical path length. It is even more effective.

In the above description, InGaAsP having a 1.3Q composition was used as the core.
In the present invention, it is sufficient that the temperature coefficients of the refractive indexes of the core and the clad are different, and the temperature coefficient does not depend on the material. Further, the present invention can be applied not only to semiconductors but also to optical waveguide filters made of other materials such as quartz or LiNbO ₃ .

[0049]

As described above, according to the present invention,
By making the geometrical structure of the waveguide such as the thickness or width of the core different between each waveguide, without performing the complicated butt joint of the manufacturing process, with a simple manufacturing process,
The Γ factor of the light guided to the core and the clad having different temperature coefficients of the refractive index can be made different between the respective waveguides.
As a result, it is possible to realize a waveguide filter having a small change in the pass band characteristic with respect to a change in the environmental temperature.

[Brief description of the drawings]

FIG. 1 is a top view of a first embodiment of the present invention.

FIG. 2 is a sectional view taken along the line AA 'of FIG.

FIG. 3 is a sectional view taken along line BB 'of FIG.

FIG. 4 is a sectional view taken along the line CC ′ of FIG. 1;

FIG. 5 is a cross-sectional view illustrating a manufacturing process of the first embodiment.

FIG. 6 is a cross-sectional view illustrating a manufacturing process of the first embodiment.

FIG. 7 is a cross-sectional view illustrating a manufacturing process of the first embodiment.

FIG. 8 is a cross-sectional view illustrating a manufacturing process of the first embodiment.

FIG. 9 is a diagram showing a calculation result of a confinement coefficient (Γ factor) of guided light.

FIG. 10 is a top view of the second embodiment of the present invention.

FIG. 11 is a sectional view taken along line DD ′ of FIG. 10;

FIG. 12 is a top view of the third embodiment of the present invention.

FIG. 13 is a sectional view taken along the line CC ′ of FIG. 12;

FIG. 14 is a top view of the fourth embodiment of the present invention.

FIG. 15 is a sectional view taken along line AA ′ of FIG. 14;

FIG. 16 is a sectional view taken along the line BB ′ of FIG. 14;

FIG. 17 is a top view of the fifth embodiment of the present invention.

FIG. 18 is a top view of the sixth embodiment of the present invention.

FIG. 19 is a top view of a conventional optical waveguide filter.

FIG. 20 is a sectional view taken along line AA ′ of FIG. 19;

FIG. 21 is a sectional view taken along the line BB ′ in FIG. 19;

FIG. 22 is a sectional view taken along the line CC ′ of FIG. 19;

FIG. 23 is a diagram illustrating a manufacturing process of a conventional example.

FIG. 24 is a diagram illustrating a manufacturing process of a conventional example.

FIG. 25 is a diagram illustrating a manufacturing process of a conventional example.

FIG. 26 is a diagram illustrating a manufacturing process of a conventional example.

FIG. 27 is a diagram illustrating a manufacturing process of a conventional example.

[Explanation of symbols]

1 InP upper cladding 1 'second core 7 SiO ₂ film of the second core 4 InP lower cladding 5 InP substrate 6 1.1Q composition of the first core 3 1.3Q composition of InP spacer 2 1.1Q composition 8 photoresist 9 InP upper cladding 10 1.3Q composition core 11 lower cladding 12 1.3Q composition core 13 air 14 cladding 15, 16 embedded core

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masaki Shintoku Nippon Telegraph and Telephone Corporation 3-19-2, Nishishinjuku, Shinjuku-ku, Tokyo

Claims (11)

[Claims] A plurality of optical waveguides having different optical path lengths are provided. The input light is split into the plurality of optical waveguides, propagated through the respective optical waveguides, multiplexed and interfered, and the input light has a wavelength. In the optical waveguide filter selected based on, the geometric structure of the cross section of the optical waveguide is different, as a result, the optical path length of the region where the confinement coefficient of the guided light to the core of each optical waveguide is different, the plurality of An optical waveguide filter, wherein wavelengths to be selected by the optical waveguide filter are not changed by temperature by making the optical waveguides different from each other. 2. The optical waveguide filter according to claim 1, wherein an optical path length of a region in which at least one of a thickness and a width of a core of the optical waveguide is different is made different among the plurality of optical waveguides. Optical waveguide filter. 3. The optical waveguide filter according to claim 1, wherein at least one of the thickness and the width of the core of the plurality of optical waveguides is at least one of the thickness and the width of the core of the optical waveguide having a long optical path length. An optical waveguide filter characterized by being smaller in thickness and width of a core of an optical waveguide having a short optical path length. 4. The optical waveguide filter according to claim 1, wherein the optical waveguide filter is a Mach-Zehnder filter, and a thickness and a width of a core of the optical waveguide having a longer optical path length among the two optical waveguides. Wherein at least one of the two is smaller than the thickness and width of the core of the optical waveguide having a short optical path length. 5. The optical waveguide filter according to claim 4, wherein the thickness of the core of the optical waveguide having the long optical path is smaller than the thickness of the core of the optical waveguide having the short optical path, and the optical waveguide having the short optical path is formed by a step. An optical waveguide filter having a portion. 6. The optical waveguide filter according to claim 4, wherein the thickness of the core of the optical waveguide having the long optical path is smaller than the thickness of the core of the optical waveguide having the short optical path, and the core is provided at both ends. An optical waveguide filter having a continuously increasing thickness. 7. The optical waveguide filter according to claim 1, wherein the optical waveguide filter is an array grating filter, and the core of each of the plurality of optical waveguides has a core having a longer optical path length. An optical waveguide filter characterized by being small. 8. The optical waveguide filter according to claim 1, wherein the optical waveguide filter is an array grating filter, and a length of at least one of a thickness and a width of a core of the optical waveguide is small. An optical waveguide type filter characterized in that an optical waveguide having a longer optical path length is longer. 9. An optical system comprising: two optical waveguides having optical path lengths different from each other, splitting input light into the two optical waveguides, propagating the respective optical waveguides, and multiplexing and interfering with each other. In the method for manufacturing a Mach-Zehnder type optical waveguide filter for selecting based on wavelength, after forming the cores of the two optical waveguides, etching the core of the optical waveguide having a long optical path length and etching at least one of the thickness and the width And a step of forming an upper clad on the cores of the two optical waveguides. 10. The method according to claim 9, wherein
A method of manufacturing an optical waveguide filter, comprising a step of forming a step also in a core of an optical waveguide having a short optical path length before the upper cladding forming step. 11. The manufacturing method according to claim 9, wherein
Subsequent to the step of etching the core of the optical waveguide having a long optical path length to reduce the thickness, a step of continuously increasing the thickness of both ends in the length direction of the core of the optical waveguide having the long optical path length is provided. A method for manufacturing an optical waveguide filter, comprising the steps of: