JP3214544B2 - Planar optical waveguide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a planar optical waveguide which enables a highly reliable connection with an optical fiber or another planar optical waveguide.

[0002]

2. Description of the Related Art Research and development for forming a planar optical circuit (PLC) by forming a silica-based glass waveguide on a flat substrate such as Si or quartz are progressing. The quartz-based glass waveguide includes a rectangular core having a high refractive index and a clad surrounding the core and having a slightly lower refractive index than the core, and is manufactured by a quartz-based glass film forming technique and a reactive etching technique. P
In order to actually use an LC in a system, it is necessary to connect an optical fiber to input and output an optical signal. And
PLC-optical fiber connection technology is required to have low loss and high stability.

Usually, a PLC has a large number of input / output ports, so that a multi-core optical fiber is connected. Therefore, a fiber array is manufactured by fixing a multi-core fiber to a glass block with a UV adhesive, and the fiber array is further UV-coated.
It is fixed to the PLC with an adhesive.

Until now, the most versatile PLC type splitter has been put to practical use, in which both ends of the PLC are polished and connected to input / output fiber parts whose end faces are also polished.

FIG. 1 shows a conventional PLC to which an optical fiber is connected.
1 shows the structure of a mold splitter. In the figure, 11 is a splitter circuit (PLC), 12 is a single-core fiber for input, 13 is an 8-core tape fiber for output, 14 and 15 are glass blocks for fixing fibers, and 16a, 16b, 17a and 17b are glass for connection reinforcement. It is a board. PLC body size is 4 × 30
mm, and the entire length is 80 mm.

More specifically, the input single-core fiber 12 is connected with the glass block 14 and the glass plate 16a, and the output 8-core tape fiber 13 is connected with the glass block 15 and the glass plate 17a.
The end face of the PLC 11, together with the glass plates 16b and 17b, was polished to 8 degrees and connected with a UV adhesive.

In the circuit by the connection method described above, (1) a low-loss fiber connection is possible by flattening the surface roughness by cutting the substrate by polishing, and (2) the angle of the polished surface can be set arbitrarily. For example, when the angle is set to 8 degrees, the return loss can be greatly reduced, (3) the bonding strength can be increased by flattening, and high reliability can be realized by PLC-optical fiber connection. There is.

[0008]

However, the above-mentioned circuit has a disadvantage that cracks occur in the clad (glass) layer on the polished surface when the end face of the PLC is polished.

FIG. 2 schematically shows a PLC end face after polishing. In the figure, 21 is a Si substrate, 22 is a lower cladding layer, 23 is an upper cladding layer, and 24 is a core. Usually, cracks occur in the lower cladding (glass) layer 22 near the Si substrate 21. The cracks cause non-uniformity in the adhesive layer, deteriorating long-term reliability and deteriorating optical characteristics such as return loss.

Hereinafter, the internal stress in the glass layer which is considered to be the cause of the crack will be briefly estimated.
The internal stress σ generated in the difference in thermal expansion coefficient, a relationship of σα (Tg-Tr) (α g -α Si).

Here, Tg is the vitrification temperature of the cladding (glass) layer, Tr is room temperature, and α _g and α _Si are the thermal expansion coefficients of the cladding (glass) layer and the Si substrate, respectively. Further, as shown in FIG. 2, when the directions parallel and perpendicular to the plane substrate in the PLC cross section are x and y directions, respectively,
Relationship stress and refractive index is given by _{Δn x = C 1 σ x +} C 2 σ y Δn y = C 1 σ y + C 2 σ x.

[0012] Here, σ _x, σ _y each direction of stress, Δn _x, Δn _y is the refractive index change due to each of the axial stresses, C ₁ and C ₂ are photoelastic coefficient, C ₁ = −7.7 × 10 ⁻¹³ (Pa ⁻¹ ) C ₂ = −4.1 × 10 ⁻¹² (Pa ⁻¹ ).

Further, σ _x and σ _y have a relationship of σ _y = −κσ _x (Poisson's relationship). Here, κ (= 0.
17) is a Poisson coefficient.

[0014] From the foregoing relationship _{Δn y -Δn x = σ x (} -κC 1 + C 1 -C 1 + κC 2) = σ x (1 + κ) (C 2 -C 1) ...... (1) is obtained.

On the other hand, in the case of a lower clad layer having a thickness of about 30 μm in a quartz optical waveguide on a Si substrate, a typical value of the birefringence B is as follows: B = n _TM −n _TE = n _x −n _y = 4 × 10 ⁻⁴ (2)

If the compressive stress is taken to be unity, the internal stress is given by σ _x = 4 × 10 ⁻⁴ /1.171×3.3×10 ⁻¹² = 1.0 × according to the above equations (1) and (2). 10 ⁸ Pa = 0.1 GPa.

According to the above equation, the cladding (glass) layer has a thickness of 0.1 mm.
It can be seen that a compressive stress of about 1 GPa is acting.
This value depends on the thickness and composition of the cladding layer. It has been clarified that cracks basically occur due to this internal stress, and that crack formation is particularly likely to occur in the periphery of a thick clad (glass) layer. This is presumably because stress concentration easily occurs in the peripheral portion.

An object of the present invention is to provide a planar optical waveguide capable of preventing a crack from occurring at a connecting portion and enabling highly reliable connection with another optical waveguide.

[0019]

In order to solve the above-mentioned problems, according to the present invention, the thickness of the cladding (glass) layer at the connection portion is made non-uniform. Alternatively, by forming a clad only around the core, the internal stress around the core is reduced while maintaining the waveguide structure.

FIG. 3 shows the basic concept of the planar optical waveguide of the present invention. In the figure, 31 is a Si substrate, 32 is a lower cladding layer, 33 is an upper cladding layer, and 34 is a core.

Here, the characteristic is that the lower cladding layer 32 is thickened only around the core 31. From the viewpoint of stress, if there is no cladding under the core, almost no stress will be applied, but since there is no waveguide structure, loss will increase. Therefore, the cladding around the core cannot be eliminated.

Therefore, in the present invention, the cross-sectional area of the core is PLC
Taking advantage of the small proportion of the entire cross-sectional area, the clad is left only around the core, and the lower clad layer is eliminated or thinned in other portions. As a result, the internal stress can be greatly reduced, so that the generation of cracks is eliminated, and the PL having excellent long-term stability is obtained.
While the C-fiber connection is possible, its optical properties do not change.

[0023]

FIG. 4 shows a first embodiment of the present invention. Here, an example of a silica-based PLC 1 × 8 splitter circuit is shown. In the figure, reference numeral 41 denotes a splitter circuit (PL
C) and 42 are waveguides, 43 is an input side connection, and 44 is an output side connection.

FIG. 5 is an enlarged end view of the output side connection portion 44 after polishing along the line aa 'in FIG. 4, and FIG.
It is an expanded sectional view which follows a b 'line. In the figure, 45 is a Si substrate, 46 is a lower cladding layer, 47 is an upper cladding layer,
8 is a core.

In order to manufacture the structure shown in FIGS. 4 to 6, first, a clad shape is patterned on a Si substrate 45 by photolithography, and a core 48 is formed by wet etching.
A portion for forming a lower cladding layer is formed below the surface of the substrate. Next, the lower cladding layer 46 made of quartz glass is
It is deposited by the method and vitrified. Thereafter, the surface shape becomes uneven, so that it is flattened by polishing. After depositing the core layer, patterning and glass etching of the core 48 were performed using a marker, and finally the upper clad layer 47 was deposited.

In the manufactured 1 × 8 splitter, the pitch of the eight ports on the output side was set to 0.25 mm, and
3 and the width of the deformed cladding region of the output side connection portion 44 is 50
μm (centered on the core), the length was 2 mm, and the thickness of the cladding layer immediately below the core was 15 μm. The produced wafer was cut, and a glass plate 49 was fixed on the polished surface with a UV adhesive for protection. Thereafter, the end face of the splitter shown in FIG. 4 was polished by an ordinary method. No crack was observed on the polished end face. From these results, the effectiveness of the present invention was confirmed.

In this embodiment, the optical characteristics of the splitter are
This is the same as a conventional splitter without a deformed cladding region, and the effectiveness of the present invention was also confirmed in this regard.

FIG. 7 shows a second embodiment of the present invention, in which a PL with a wavelength multiplexing function for an optical transceiver is used.
The example of C is shown. In the figure, 51 is a splitter circuit (PL
C), 52 are waveguides, 53 is an interference film filter for wavelength multiplexing, 54 is an input / output connection portion, and 55 is an optical transceiver L.
D, PD mounting part. FIG. 8 is an enlarged end view of the input / output connection portion 54 after polishing along the line cc ′ in FIG.
In the figure, 56 is a Si substrate, 57 is a lower cladding layer, 58 is an upper cladding layer, and 59 is a core.

In order to fabricate the structure shown in FIGS. 7 and 8, first, a clad shape is patterned on a Si substrate 56 by photolithography, and a core 59 is formed by wet etching.
A portion for forming a quartz-based glass lower cladding layer is formed underneath. In this example, the etching amount of the Si substrate 56 is 1
The thickness was reduced to 0 μm so that unevenness after the formation of the clad was reduced. Next, a silica-based glass lower cladding layer 57 is deposited to a thickness of 10 μm by the FHD method, and is turned into a transparent glass.

Thereafter, without planarization by polishing,
Unnecessary cladding layers other than the portions where the substrate 56 is etched are removed by patterning by photolithography and glass etching. Further, a core layer is deposited, and a core 5
9 is patterned. In patterning the core 59,
Because of the small irregularities, precise patterning was possible without flattening by polishing. Thereafter, glass etching was performed, and finally, an upper clad layer 58 was deposited.

In the manufactured PLC with the wavelength multiplexing function for the optical transceiver, the width of the deformed cladding region of the input / output connection portion 54 was set to 50 μm (centered on the core) and the length was set to 2 mm. The produced wafer was cut, and a glass plate (not shown) was fixed on the polished surface with a UV adhesive for protection. Thereafter, the end face of the PLC of FIG. 7 was polished by a usual method. No crack was observed on the end face after polishing. From these results, the effectiveness of the present invention was confirmed.

In this embodiment, the optical characteristics of the splitter are
Same as conventional splitter without deformed cladding region. In this respect, the effectiveness of the present invention was confirmed.

FIG. 9 shows a third embodiment of the present invention.
It is an end elevation of the output side connection part of an LC splitter. In the figure,
61 is a Si substrate, 62 is a lower cladding layer, 63 is an upper cladding layer, and 64 is a core.

In order to produce the structure shown in FIG. 9, first, a lower clad layer is deposited by the FHD method and is made vitrified.
Next, the clad shape is patterned by photolithography, and only the lower clad layer 62 corresponding to the vicinity of the core is left by dry etching. After the formation of the core layer, patterning and glass etching of the core 64 were performed using a marker to form a rectangular core, and finally the upper clad layer 63 was deposited.

It is needless to say that the lower clad layer is present on the entire surface of the substrate except for the connection portion.

The produced wafer was cut, and a glass plate (not shown) was fixed on the polished surface with a UV adhesive for protection.
Thereafter, the end face of the splitter shown in FIG. 9 was polished by an ordinary method. No crack was observed on the polished end face. According to this example, since the etching and polishing of the Si substrate are not required, the deformed cladding region can be easily formed.

In this embodiment, the optical characteristics of the splitter are
This is the same as a conventional splitter without a deformed cladding, and the effectiveness of the present invention was also confirmed in this regard.

It should be noted that in the above description, the substrate is Si
However, the present invention can be applied to a substrate such as sapphire. In addition, dry etching or mechanical processing other than wet etching can be applied to processing of various substrates. Further, there is no limitation on the length of the deformed cladding region in the direction along the core, and the lower cladding layer of all the cores may be the deformed cladding region. However, only the end portions are preferable from the viewpoint of ease of fabrication. Further, the width can be reduced to a point where the optical characteristics do not deteriorate.
It is on the order of 0 μm.

[0039]

As described above, according to the present invention,
Since cracks can be prevented from being generated at the connection part due to polishing and a polished surface without cracks can be formed, a highly reliable PLC
-A fiber connection can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing the structure of a conventional PLC splitter.

FIG. 2 is an end view showing a PLC end face after polishing.

FIG. 3 is an end view showing the basic concept of the planar optical waveguide of the present invention.

FIG. 4 is a plan view showing a first embodiment of the planar optical waveguide of the present invention.

5 is an enlarged end view of the output side connection portion after polishing along the line aa ′ in FIG. 4;

FIG. 6 is an enlarged sectional view taken along the line bb ′ of FIG. 4;

FIG. 7 is a plan view showing a planar optical waveguide according to a second embodiment of the present invention.

FIG. 8 is an enlarged end view of the input / output connection portion after polishing along the line cc ′ in FIG. 7;

FIG. 9 is an end view of an output-side connecting portion showing a third embodiment of the planar optical waveguide of the present invention.

[Explanation of symbols]

31, 45, 56, 61 ... Si substrate, 32, 46, 5
7, 62 ... lower cladding layer, 33, 47, 58, 63 ...
Upper cladding layer, 34, 48, 59, 64 ... core, 4
1, 51: splitter circuit, 42, 52: waveguide, 43
... input side connection part, 44 ... output side connection part, 49 ... glass plate, 53 ... interference film filter for wavelength multiplexing, 54 ... input / output connection part, 55 ... LD, PD mounting part for optical transceiver.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yasufumi Yamada 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (72) Inventor Motohaya Ishii 3-19, Nishishinjuku, Shinjuku-ku, Tokyo No. 2 Nippon Telegraph and Telephone Corporation (72) Inventor Masahiro Yanagisawa 3-19-2 Nishi Shinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-8-43654 (JP, A) JP-A-7-27933 (JP, A) JP-A-9-166724 (JP, A) JP-A-9-166723 (JP, A) JP-A-8-201649 (JP, A) -501570 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 6/12-6/14 G02B 6/26-6/27 G02B 6/30-6/35 G02B 6 / 42-6/43

Claims (2)

(57) [Claims] An optical waveguide comprising a core for guiding and propagating light and a clad surrounding the core and having a refractive index slightly lower than the core is formed on a substrate, and the substrate is provided at least at one end of the substrate. In a planar optical waveguide provided with a connection portion for connecting the upper optical waveguide to another optical waveguide, the entire vicinity of the connection portion of the substrate except the lower portion of the core is provided.
And the lower part of the core near the connection part of the substrate is formed thicker than other portions of the substrate.
Independently of each other, from other portions of the substrate near the connection.
A flat optical waveguide , wherein the cladding is formed to be thin and the cladding is deposited flat on the substrate . 2. The core according to claim 1 , wherein said core is located near said connection portion of said substrate.
A cross section parallel to an end face of the connection portion, the cross section being a lower shape
2. The planar optical waveguide according to claim 1 , wherein the shape is a trapezoid whose base is on the side close to the core .