JP4146788B2 - Optical waveguide connection module and method for fabricating the same - Google Patents

Description

  The present invention relates to an optical waveguide connection module having a core shape that matches an optical field distribution in order to reduce connection loss when connecting end faces of two or more optical waveguide substrates, and a method for manufacturing the waveguide.

  With the diversification of optical communication systems and the increase in communication capacity, optical signal processing modules are inevitably enhanced in function and capacity. For the development of such an optical signal processing module, it is a simple means to apply a planar optical waveguide having high uniformity and high degree of freedom in circuit layout by adapting photolithography technology. In particular, quartz-based planar lightwave circuits (PLCs) are well connected to communication optical fibers due to material consistency, and large-scale light mainly depends on the size of the silicon wafer that serves as the substrate. Circuit development is easily achieved.

  Furthermore, by adjusting the material added to form the core, the relative refractive index difference Δ between the core and the cladding can be easily changed, and by increasing Δ to reduce the core diameter It is possible to decrease the minimum bending radius of the optical circuit and increase the optical circuit integration density per unit area. There are 0.3%, 0.45%, 0.75%, and 1.5% as Δ that we currently employ. For example, Δ = 0.75% of optical waveguide and Δ = 1.5 The minimum bend radius of the% optical waveguide is 5 mm and 2 mm, respectively. Therefore, when the Δ = 1.5% optical waveguide is used, the optical circuit area can be greatly reduced to about ¼ that when the Δ = 0.75% optical waveguide is used.

On the other hand, the use of quartz PLC is also an effective means for enhancing the functionality of the optical signal processing module. Specifically, it is to apply an end face connection technique in which another optical waveguide substrate such as a crystal optical waveguide is connected to the end face of the PLC. The crystal optical waveguide has characteristics that cannot be obtained by PLC such as electro-optic effect, nonlinear optical effect, and acousto-optic effect, as typified by LiNbO ₃ (LN: lithium niobate). It becomes possible to merge. As an example, a wavelength conversion device using periodic polarization reversal LN is used as a crystal optical waveguide, and a multiplexer / demultiplexer and a 1-bit delay line are provided on the PLC side. There is an (OTDM) transmission module (see, for example, Non-Patent Document 1). The technology for connecting different types of optical waveguide substrates can be modularized regardless of the size of the optical waveguide substrate and the number of ports, and there is no effect when size fluctuations occur due to end face polishing. Compared to the case where the optical waveguide substrate is connected using a fiber, the module size is small, and there is an advantage that an optical path length error between ports does not occur.

  However, there is a problem in that coupling loss occurs in connection by end face butting between optical waveguide substrates. The factors include Fresnel reflection loss caused by the difference in refractive index due to the difference in materials, coupling efficiency decrease and scattering due to the difference in optical field distribution, distortion of the waveguide position due to manufacturing errors, and misalignment during mounting. Etc. Of these, Fresnel reflection can be sufficiently reduced by coating a non-reflective film on the end surface according to the material of the optical waveguide, and dimensional errors and positional deviations are aligned while monitoring the light intensity. It is solved by using an active alignment method. Therefore, matching the optical field distribution of the optical waveguide substrate and the mounting element is important in reducing the coupling loss.

  The optical field distribution of an optical waveguide that propagates a single mode largely depends on the manufacturing method of the core. In a general PLC, the cross-sectional shape of the core is a square, and the optical field distribution is a circle having a spread in the cladding layer. The magnitude of the optical field distribution is approximately 8 μm with a 0.75% Δ waveguide. On the other hand, in the LN waveguide, the core is manufactured by thermal diffusion of Ti and / or exchange of the protons, so that it depends on the diffusion multiplier, and its light field distribution is symmetrical in the horizontal direction and flat in the vertical direction from the substrate surface to the lower side. It becomes a shape. The minimum value in the vertical direction of LN devices currently in practical use is about 5 μm. When there is no optical axis shift or angle shift, the coupling efficiency is determined by the overlap integration of the optical field distribution.

  FIG. 6 shows how the PLC and the LN optical waveguide are connected by a conventional method. In this figure, 61 is an LN device, 62 is an LN optical waveguide substrate, 63 is an LN optical waveguide, 64 is a PLC, 65 is a PLC Si substrate, 66 is a cladding layer of PLC, and 67 is a PLC core. The end faces of the PLC 64 and the LN optical waveguide substrate 62 are connected to each other at a position where the 67 and the LN optical waveguide 63 are coupled. 69 is an LN optical field distribution, 70 and 71 are PLC optical field distributions, and 72 is an optical axis.

  The coupling loss between the general-purpose 0.75% Δ-PLC 64 and the IN device 61 is about 1 dB. When the end faces of the PLC 64 and the LN optical waveguide substrate 62 described as examples are connected to each other by matching, it is important to match the optical field distribution of both to reduce the coupling loss.

  The LN optical waveguide 63 exhibits a flat light field distribution 69 as shown in FIG. 6, and generally has a mode field diameter (MFD) smaller than that of an optical fiber. Focusing on this point, in order to connect the PLC and various LN devices with low loss, 1.5% Δ-PLC with strong optical confinement is adopted, and the connection portion with the LN is shown in FIG. An optical waveguide connection module to which such a spot-size converter (SSC) structure using a lateral taper structure is applied has been proposed (see Non-Patent Document 2). As a related technique, there has been proposed a technique in which an SSC structure using a vertical side taper structure is used to connect a single optical fiber and a PLC (see Non-Patent Document 3).

FIG. 7 shows a form of a butt connection between a PLC and a LN optical waveguide that employ a lateral taper structure in the connection portion. In general, in an LN optical waveguide, Ti deposited on the surface of the substrate along the circuit layout is diffused into the LN substrate by heat, or Li and the solution in the LN substrate 62 overheated by immersing in a solution of benzoic acid or the like. A core (LN optical waveguide) 63 is formed by a method of exchanging H ⁺ therein. Accordingly, the light field distribution 69 of the LN device 61 is symmetrical with respect to the optical axis in the horizontal direction, and has a flat shape determined by the diffusion constant and diffusion time of Ti and H ⁺ with respect to the LN substrate in the vertical direction. The axis is near the substrate surface. On the other hand, on the PLC 64 side, due to the effect of the lateral taper structure 68 of the PLC core 67, the optical field distribution 70 becomes a wide elliptical shape in the horizontal direction. By adjusting, it is possible to match the optical field distribution of the connected LN device.

  FIG. 8 shows the dependency of the optical field distribution of PLC on Δ and core size. According to FIG. 8, there is a large difference in the optical confinement effect between 0.75% -Δ and 1.5% -Δ, and 1.5%- It can be seen that the Δ PLC is suitable. It can also be seen that the MFD can be adjusted to an arbitrary size by increasing the core size of the PLC core 67.

Takashi Yamada et al., “160 Gbit / s OTDM-MUX Module Using PLC-PPLN End Connection”, 2002 IEICE Electronics Society Conference, C-3-22, p. 122 Takashi Yamada et al., “OTDM-MUX Module Using 1.5% ΔPLC-PPLN Connection”, 2003 Electronics Society Conference of the Institute of Electronics and Communication Engineers, C-3-98, p. 231 M. Itoh et., “Large reduction of single mode-fiber coupling loss in 1.5% Δ planar lightwave circuits using spot-size converters” ELECTRONICS LETTERS 17th January 2002 Vol.38 No.2 pp.72-73

  As described above, when two or more different optical waveguide substrates are used to connect the waveguide end faces to each other, the optical field distributions of the two are generally different, so that the coupling efficiency is reduced and a loss occurs. In particular, in the LN optical waveguide substrate, the optical field distribution of the LN optical waveguide substrate is largely flat in the substrate direction due to the core formation method, so that a large coupling loss occurs when matching PLCs having a circular optical field distribution. To do. This coupling loss has a great influence on the LN device using the nonlinear optical effect because its output characteristic depends on the incident light intensity.

  On the other hand, the optical waveguide connection module to which the spot size conversion structure based on the lateral taper structure shown in FIG. 7 proposed for the purpose of reducing the coupling loss is applied, the optical field distribution of the LN waveguide is flat and low. Since the direction is asymmetrical, it has been practically difficult to realize an optical field close to the LN waveguide on the PLC side.

  The present invention has been made in view of the above points, and in an optical signal processing module in which two or more different optical waveguide substrates are butt-connected, coupling caused by mismatching of the optical field distribution between the optical waveguide substrates. It is an object of the present invention to provide a core shape of a connecting portion on the PLC side and a manufacturing method thereof, which can further enhance the effect of reducing loss by a simple manufacturing process.

In order to achieve the above object, an optical waveguide connection module according to the present invention includes an optical waveguide connection module in which the end surfaces of two or more optical waveguide substrates having different optical field distribution shapes are connected to each other. In order to match the elliptical vertical asymmetric other light field distribution at the end of the core part of the optical waveguide substrate, a spot size conversion structure with a convex cross-sectional shape with different widths in the core thickness cross-section direction is formed And the one optical waveguide substrate constituting the optical waveguide connection module is a quartz-based planar lightwave circuit, and the optical waveguide substrate connected to the quartz-based planar lightwave circuit has a diffusion core structure. characterized in that it is an optical waveguide.

  Here, the spot size conversion structure may have a tapered shape in the planar direction and a stepped shape having a different width in the core thickness cross-sectional direction.

The crystal optical waveguide may be an optical waveguide using a LiNbO ₃ substrate.

In order to achieve the above object, the waveguide fabrication method of the present invention is such that when the core of the silica-based planar lightwave circuit constituting the optical waveguide connection module is formed, the cross-sectional shape of the core becomes convex. The core is subjected to two-stage etching with different etching widths.

In order to achieve the above object, another aspect of the waveguide fabrication method of the present invention is to etch the lower clad layer when forming the core of the silica-based planar lightwave circuit constituting the optical waveguide connection module. A core layer is deposited on the entire lower cladding layer where the convex portion has been dug, and then etching is performed on the core layer to form a convex core pattern on the lower side in the vertical direction. In this way, the core is manufactured.

  As described above, in the optical signal processing module in which the end faces of two or more different optical waveguide substrates (mainly PLC and LN optical waveguide device) are connected to each other, the present invention matches the optical field distributions of the optical waveguide substrates with each other. As described above, since the spot size conversion structure having a convex core structure is used on the PLC side, the coupling loss can be greatly reduced.

  In particular, even if the optical field distribution of the LN waveguide is flat, it is asymmetric in the vertical direction. Therefore, as in the present invention, by adjusting the length of each side, the LN waveguide can be adjusted by adjusting the length of each side. An optical field distribution close to can be realized, and the effect of reducing the coupling loss can be obtained more easily.

  The best mode for carrying out the present invention will be described below in detail with reference to the drawings. However, the present invention is not limited to these embodiments, and changes in shape and numerical values within the scope described in the claims, replacement of materials, design changes by addition of well-known techniques, etc. include.

(First embodiment)
Coupling loss between the PLC and the LN optical waveguide device when there is no optical axis deviation and radiation angle deviation can be expressed by the following equation by coupling of Gaussian beams.

Here, ωx and ωy are optical field distribution diameters in the horizontal direction and vertical direction of the PLC, and σx, σy ₁ and σy ₂ are optical field distribution diameters in the horizontal direction, vertical direction upper side and lower side of the LN optical waveguide. . Therefore, the coupling loss is minimized when the horizontal direction of the optical field distribution is the same in both cases and the flatness on the LN device side is small in the vertical direction. Actually, when the coupling loss between the 1.5% -ΔPLC employing the lateral taper having the structure shown in FIG. 7 and the LN optical waveguide device was measured, the value was about 0.3 dB. Therefore, it is reduced by about 0.7 dB compared to the coupling loss of the conventional structure of 0.75% -ΔPLC.

  FIG. 1 and FIG. 2 show a configuration of an optical waveguide connection module that employs a structure in which tapered structures having different waveguide widths are stacked in the vertical direction on the PLC side connection portion according to the present invention, that is, a convex core shape. Show. FIG. 1 shows a case where the projection is upward in the vertical direction, and FIG. 2 shows a case where the projection is downward.

  If the convex core structure 108 of the PLC core 107 is convex upward as shown in FIG. 1, the optical waveguide substrates 105 and 102 of either one of the PLC 104 or the LN device 101 are connected upside down. . On the other hand, when the convex core structure 108 is convex downward as shown in FIG. 2, both the optical waveguide substrates 105 and 102 are connected in the same direction to the PLC 104 and the LN device 101.

  The convex core structure 108 formed in the cladding layer 106 of the PLC 104 has an end face connected to the LN optical waveguide 103 and a cross section in the vicinity thereof having an inverted T shape or T shape, and this cross sectional shape is After extending by a predetermined length, the convex portion is expanded in a tapered shape until the convex portion reaches the core width of the PLC core 107. Due to this shape, the light field distribution 110 of the laterally tapered portion of the PLC 104 has the same shape as the light field distribution 109 of the LN device 101, and the light field distribution 111 of the developed portion of the PLC 104 is circular.

  FIG. 3A shows an example of a three-dimensional BPM (Beam Propagation Method) simulation result of the optical field distribution for the PLC 104 employing a convex core. FIG. 3B also shows the core shape of the emission end used in the analysis. This confirms that a flat light field distribution is obtained reflecting the convex shape. The coupling loss between the PLC 104 adopting this structure and the LN device 101 approaches zero as much as possible.

  On the LN device 101 side, producing a core shape that improves the flat light field distribution generally complicates the production process, such as the volume of different types of films and the diversification of diffusion materials. For example, if the LN device 101 is a modulator, the applied electric field may be affected. Therefore, it is easier to match the optical field distribution on the PLC 104 side with the LN device 101, and the manufacturing process is also suppressed to an additional etching process, as will be described later, as compared to a normal manufacturing process. Is possible.

  Also, even if the optical field distribution of the LN waveguide is flat, it is asymmetric in the vertical direction. Therefore, as in the present invention, by adjusting the length of each side, the LN waveguide can be adjusted by adjusting the length of each side. An optical field distribution close to can be realized.

  FIG. 4 shows an outline of a process for producing a convex core on the upper side in the vertical direction as shown in FIG. First, the lower clad layer 113 is deposited on the Si substrate 105 of the PLC 104, and then the core layer 114 is deposited (FIG. 4A). Next, a taper portion etching mask 401 is formed so as to leave the upper side of the convex core, and etching is performed using the taper portion etching mask 401, so that a part of the core and a portion other than the core have a desired height. The convex part 115 is obtained by removing (FIG. 4B). Further, using the waveguide pattern etching mask 402, the core 107 having the desired shape (convex shape upward in the vertical direction) is formed by etching the entire core along the circuit layout in the same manner as in a normal PLC manufacturing process. (FIG. 4C). Finally, the upper clad layer 106 is deposited until the core portion 107 is buried at a desired depth (FIG. 4D).

  FIG. 5 shows an outline of a process for producing a convex core on the lower side in the vertical direction as shown in FIG. First, the lower clad layer 113 is deposited on the Si substrate 105 of the PLC 104. At this time, in consideration of the protruding portion protruding from the respective clad layers that are usually required, the deposit is thickened by several μm (FIG. 5A). Next, by etching using the taper portion etching mask 501 composed of the upper shape of the convex core and the circuit layout, they are dug into the lower cladding layer 113 to a desired depth (FIG. 5B). ). Subsequently, the core layer 114 is volumed over the entire lower clad layer 113 in which the convex portion 116 is dug (FIG. 5C). Thereafter, etching is performed using a waveguide pattern etching mask 502 having a lower shape of the convex core and a circuit layout, thereby forming a core pattern 107 having a target shape (convex shape on the lower side in the vertical direction) (FIG. 5). (D)). Finally, the upper clad layer 106 is deposited until the core portion 107 is buried at a desired depth (FIG. 5E).

  As shown in FIGS. 4 and 5, a PLC having a convex core can be manufactured by a simple manufacturing process in which an etching process is added once compared to a normal manufacturing process.

It is a perspective view which shows a mode that PLC and LN optical waveguide which have convex shape in the perpendicular direction upper side in one Embodiment of this invention were couple | bonded. It is a perspective view which shows a mode that PLC and LN optical waveguide which have convex shape in the perpendicular direction lower side in one Embodiment of this invention were couple | bonded. (A) is a characteristic figure which shows an example of the three-dimensional BPM simulation result of light field distribution with respect to PLC which employ | adopted the convex core, (b) is a figure which shows the shape of the output end of the convex core. (A)-(d) is process drawing which shows the outline of the manufacturing process of PLC which has convex shape in the orthogonal | vertical direction upper side in one Embodiment of this invention. (A)-(e) It is process drawing which shows the outline of the manufacturing process of PLC which has convex shape in the perpendicular direction lower side in one Embodiment of this invention. It is a perspective view which shows a mode that the conventional PLC and the LN optical waveguide were couple | bonded. It is a perspective view which shows a mode that PLC which has a horizontal taper structure, and the LN optical waveguide were couple | bonded. It is a characteristic view which shows the core size and (DELTA) dependence of the optical field distribution diameter of PLC.

Explanation of symbols

101 LN device 102 LN substrate (optical waveguide substrate of LN device)
103 LN optical waveguide 104 PLC
105 Si substrate (PLC optical waveguide substrate)
106 Cladding layer 107 PLC core 108 Convex core structure 109 Optical field distribution of LN device 110 Optical field distribution of laterally tapered portion of PLC 111 Optical field distribution of developed portion of PLC 112 Optical axis 113 Lower cladding layer 114 Core layer 115 Convex type Part 116 convex part 401 taper part etching mask 402 waveguide pattern etching mask 501 taper part etching mask 502 waveguide pattern etching mask

Claims (5)

In the end face portion of the optical waveguide connection module the shape of the light field distribution formed by connecting the end faces of the two or more different optical waveguide substrate, an end portion of the core portion of one of the optical waveguide substrate, the elliptical Asymmetric Forming a spot-size conversion structure having a convex cross-sectional shape with a step shape having a different width in the core thickness cross-section in order to match the other light field distribution ; and
The one optical waveguide substrate constituting the optical waveguide connection module is a silica-based planar lightwave circuit, and the optical waveguide substrate connected to the silica-based planar lightwave circuit is a crystal optical waveguide having a diffusion core structure An optical waveguide connection module.   2. The optical waveguide connection module according to claim 1, wherein the spot size conversion structure has a tapered shape in a planar direction and a stepped shape having different widths in a core thickness cross-sectional direction. An optical waveguide connection module according to claim 1 or 2, wherein the crystal optical waveguide is characterized in that an optical waveguide using a LiNbO ₃ substrate. When forming the core of the silica-based planar lightwave circuit constituting the optical waveguide connection module according to claim 1 or 2 , the etching width of the core is such that the cross-sectional shape of the core is convex. A method for producing a waveguide, characterized in that different two-stage etching is performed. 3. A lower clad layer in which a convex portion is dug by etching the lower clad layer when forming a core of the silica-based planar lightwave circuit constituting the optical waveguide connection module according to claim 1 A core layer is deposited on the entire upper surface, and then the core is fabricated by performing etching on the core layer to form a convex core pattern on the lower side in the vertical direction. Waveguide manufacturing method.

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