Waveguide Type Optical Coupler

Abstract

Provided is a waveguide-type optical coupler (20) where a branching ratio is kept constant without wavelength dependence or polarization dependence in a wide wavelength band. In the waveguide-type optical coupler (20) constituted by a Mach-Zehnder interferometer having two arm waveguides (23, 24) between two directional couplers (21, 22), the widths of the two waveguides at coupling parts of the directional couplers (21, 22) differ from each other.

Description

[Technical field]

[0001] The present invention relates to an optical coupler used in an optical waveguide device. [Background technology]

[0002] Optical couplers are important circuit elements in constructing optical functional devices. A known example of a waveguide-type optical coupler is a directional coupler that has two waveguides extending close to each other and transfers optical power to one waveguide by adiabatic coupling of the optical field propagating through the other waveguide. However, the amount of light leaking out of the waveguide varies depending on the wavelength of the optical field, so the coupling rate (branching ratio) is wavelength dependent.

[0003] Figure 1 shows the wavelength characteristics of a typical conventional waveguide-type optical coupler. This waveguide-type optical coupler consisting of a directional coupler is designed to have a branching ratio of 50% at a wavelength of approximately 1.53 μm, i.e., a transmission loss of 3 dB. However, in the wavelength band used in optical communications, that is, from 1.3 μm to 1.65 μm, the transmission loss varies from a transmittance of -7 dB (branching ratio of 20%) to -1.6 dB. When optical couplers are applied to wavelength division multiplexing communications, such wavelength dependency is observed as a difference in optical power for each wavelength channel. This optical power difference needs to be compensated for in the optical communications system, and this is a challenge in building such a system.

[0004] In order to solve such problems, a wavelength-independent coupler (WINC) that reduces wavelength dependency using a Mach-Zehnder interferometer has been proposed (see, for example, Non-Patent Document 1). The WINC provides an optical path length difference between two waveguides that constitute the arms of the Mach-Zehnder interferometer, and furthermore, appropriately sets the coupling ratio of two directional couplers that constitute the Mach-Zehnder interferometer, thereby obtaining flat coupling characteristics in the target wavelength band.

[0005] FIG. 2 shows the configuration of a conventional WINC. A WINC 10 has two arm waveguides 13 and 14 between two directional couplers 11 and 12. An optical path length difference ΔL is provided between the arm waveguide 13 (long arm) and the arm waveguide 14 (short arm). The two waveguides constituting the directional couplers 11 and 12 have the same waveguide width. The coupling ratio κ of the directional coupler 11 is 1 and the coupling ratio κ of the directional coupler 12 2 By appropriately setting and , a transmission loss of 3 dB (branching ratio of 50%) is achieved in the wavelength band (1.3 μm-1.65 μm) used in optical communications. The coupling ratio of a directional coupler is determined by the length of the coupling section where the two waveguides are brought into close proximity (coupling length), the distance between the waveguides, and the waveguide width.

[0006] Figure 3 shows the wavelength characteristics of a conventional WINC. This is the result of calculating the wavelength characteristics of the WINC based on the above parameters. The transmitted light has a flat characteristic with a transmittance of -3 dB, but both the TE and TM polarizations show polarization dependent loss (PDL) over the above wavelength band. The reason for this is that the This is because the coupling ratio of the directional coupler has polarization dependence. The PDT (Polarization Dependent Transmittance), which is the difference between TE and TM polarization, has a polarization dependence of approximately 0.1 dB over this wavelength band.

[0007] For example, in a WINC using a silica-based planar lightwave circuit fabricated through high-temperature heat treatment such as flame deposition, the polarization dependence of the coupling rate of the directional coupler is thought to be due to the following reasons. That is, due to stress inside the optical waveguide during heat treatment, a difference in internal stress occurs between the substrate direction and the direction perpendicular to the substrate. This difference in internal stress causes birefringence inside the directional coupler, resulting in the appearance of polarization dependence. On the other hand, LiNbO 3In optical waveguides made of ferroelectric crystals such as InP, birefringence occurs based on the crystal orientation, and similar polarization dependence occurs. In optical semiconductor waveguides such as InP, the same polarization dependence occurs because they are crystalline waveguides. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] K. Jinguji; N. Takato; A. Sugita; M. Kawachi, "Mach-Zehnder interferometer type optical waveguide coupler with wavelength-flattened coupling ratio", Volume 26, Issue 17, 16 August 1990, p. 1326-1327 Summary of the Invention

[0009] An object of the present invention is to provide a waveguide type optical coupler which has no wavelength dependency or polarization dependency over a wide wavelength range and which maintains a constant branching ratio.

[0010] In order to achieve the above object, the present invention provides a waveguide type optical coupler including a Mach-Zehnder interferometer having two arm waveguides between two directional couplers, the Two The two waveguides at the coupling part of the directional coupler have different widths. An optical path length difference is provided between the two arm waveguides, and the waveguide width of a portion of the arm waveguide having a longer optical path out of the two arm waveguides is wider than the waveguide width of the arm waveguide having a shorter optical path, and the narrower one of the two waveguide widths at the coupling portion of the two directional couplers is connected to the arm waveguide having the longer optical path out of the two arm waveguides. It is characterized by the fact that [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 shows the wavelength characteristics of a conventional waveguide-type optical coupler. [Diagram 2] FIG. 2 shows the configuration of a conventional WINC. [Diagram 3] FIG. 3 shows the wavelength characteristics of a conventional WINC. [Figure 4]FIG. 4 is a diagram showing a configuration of a WINC according to a first embodiment; [Diagram 5] FIG. 5 is a diagram showing a configuration of a WINC according to a second embodiment; [Figure 6] FIG. 6 is a diagram showing wavelength characteristics of the WINC according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In this embodiment, an example using a silica-based optical waveguide is shown, but the material of the waveguide is not specified. This embodiment can be applied not only to a silica-based optical waveguide, but also to a case where a waveguide of other material systems such as a silicon (Si) waveguide, an indium phosphide (InP) waveguide, or a polymer-based waveguide is used. In addition, as a specific example of a waveguide design, a waveguide with a relative refractive index difference Δ of 2% will be taken up and described. This embodiment is not limited to these basic parameters of the waveguide, and the same concept can be applied to other parameters.

[0013] The WINC has two arm waveguides between two directional couplers, with an optical path length difference ΔL between the arm waveguides. As mentioned above, polarization dependence exists due to the wavelength dependence of the coupling rate of the directional coupler. Therefore, in order to eliminate the polarization dependence of the WINC, the phase difference between the arm waveguides of the Mach-Zehnder interferometer is made polarization dependent. The transfer matrix of the WINC is shown below. The transfer matrix of the first directional coupler that constitutes the Mach-Zehnder interferometer is C 1 , the transfer matrix of the arm waveguide section is A, and the transfer matrix of the second directional coupler is C. 2 Then, the transfer matrix M of the entire WINC is

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[0014] Here, κ is the coupling ratio of the directional coupler, β is the propagation constant of the arm waveguide, ΔL is the path length difference between the two arm waveguides, and z 1 , z 2 is the coupling length of the coupling part in the directional coupler.

[0015] When an optical signal is introduced into one of the input waveguides of the first directional coupler of the WINC, the input vector is [1,0] t Therefore, using the above formula, the branching strength (connection rate) I of WINC is

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[0016] In order to eliminate the polarization dependency of the WINC, there are two methods: using an asymmetric directional coupler, and providing a difference in the width of the waveguides in the two arms. These methods will be explained in order below.

[0017] [First embodiment] FIG. 4 shows the configuration of the WINC according to the first embodiment. FIG. 4(a) shows the overall configuration, and FIG. 4(b) shows an enlarged view of the directional coupler. The WINC 20 is composed of a Mach-Zehnder interferometer having two arm waveguides 23, 24 between two directional couplers 21, 22. An optical path length difference ΔL is provided between the arm waveguide 23 (long arm) and the arm waveguide 24 (short arm). The directional couplers 21, 22 are asymmetric directional couplers, and the two waveguides have different widths at the coupling portion. As shown in FIG. 4(b), the waveguide width on the side of the arm waveguide 23 constituting the long arm is W 1 , the waveguide width on the side of the arm waveguide 24 constituting the short arm is W 2 It states that:

[0018] In a symmetrical directional coupler, in which the two waveguides constituting the directional coupler have the same waveguide width, the phase relationship between the optical signals output from the two output waveguides is always 90°. On the other hand, in an asymmetrical directional coupler, the output phase has a phase difference that is determined by the transfer matrix in the following equation. That is, the transfer matrix C is expressed as follows:

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[0019] In a commonly used symmetric directional coupler, β 1 =β 2 , that is, δ=0, so the transfer matrix C is

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[0020] However, as shown in FIG. 4, when the widths of the two optical waveguides that constitute the coupling portion of the directional coupler are asymmetric, the propagation constant β 1 , β 2 Therefore, the phase relationship φ, as obtained from equation (2), is, for example,

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[0021] Width of the optical waveguide at the coupling part W 1 ,W 2 Asymmetrical propagation constant β 1 , β 2 By making the phase relationship φ different, the phase relationship of the light coming out of the output waveguide is changed from π / 2. As described above, the coupling rate differs depending on the polarization, so the generated phase difference can also have polarization dependency. By adjusting this phase relationship φ, the polarization dependency of the coupling part of the directional coupler in the first to third terms on the right side of equation (1) is compensated for, and the polarization dependency of the WINC is eliminated.

[0022] In the first embodiment, in the asymmetric directional coupler, the waveguide width on the long arm side is narrower and the waveguide width on the short arm side is wider. However, depending on the optical path length difference between the arm waveguides and the coupling ratio of the directional coupler, the opposite may be true as long as the two waveguide widths are different from each other.

[0023] [Second embodiment] FIG. 5 shows the configuration of a WINC according to the second embodiment. FIG. 5(a) shows the overall configuration, and FIG. 5(b) shows an enlarged view of a directional coupler. The WINC 30 is composed of a Mach-Zehnder interferometer having two arm waveguides 33 and 34 between two directional couplers 31 and 32. An optical path length difference ΔL is provided between the arm waveguide 33 (long arm) and the arm waveguide 34 (short arm). Furthermore, a portion of the arm waveguide 33 has a waveguide width W that is larger than the width W of the waveguides of the two arms. B The directional couplers 31 and 32 are asymmetric directional couplers, similar to the first embodiment, and each has a waveguide width on the long arm side of W 1 , the width of the waveguide on the short arm side is W 2 It states that:

[0024] In formula (1), in order to eliminate the polarization dependency as a WINC, a method of compensating by imparting polarization dependency to the phase term due to the arm section, i.e., cosβΔL in the third term on the right side, is also effective. By imparting polarization dependency to the phase term of cosβΔL, and combining it with the polarization dependency of the coupling section of the directional coupler in the first to third terms on the right side, the polarization dependency as a WINC is eliminated. In the second embodiment, an asymmetric directional coupler is used, and a difference is provided between the widths of the waveguides of the two arms to impart polarization dependency to the propagation constant β.

[0025] In the second embodiment, the waveguide width on the long arm side is wider than the normal waveguide width, but the waveguide width on the short arm side may be narrower than the normal waveguide width. Depending on the optical path length difference between the arm waveguides and the coupling ratio of the directional coupler, the relationship between the wide and narrow may be reversed, and it is sufficient that the two waveguide widths are different from each other to eliminate the polarization dependency.

[0026] FIG. 6 shows the wavelength characteristics of the WINC according to the second embodiment. 1 ,W 2 Asymmetric, and the waveguide width on the long arm side is W 1 The figure shows the result of calculating the wavelength characteristics when the phase term caused by the arm is made polarization dependent. As can be seen by comparing with Figure 3, the PDL and PDT of TE polarization and TM polarization, respectively, are significantly improved in the wavelength band used in optical communications.

[0027] According to the first and second embodiments, it is possible to provide a waveguide-type optical coupler that has wavelength dependence over a wide wavelength range, has suppressed polarization dependence, and maintains a constant branching ratio.

Claims

[Claim 1] In a waveguide-type optical coupler constituted by a Mach-Zehnder interferometer having two arm waveguides between two directional couplers, the two waveguide widths at the coupling portions of the two directional couplers are different from each other; an optical path length difference is provided between the two arm waveguides; a waveguide width of a part of the arm waveguide having a longer optical path out of the two arm waveguides is wider than a waveguide width of the arm waveguide having a shorter optical path; a waveguide-type optical coupler, characterized in that the narrower of the two waveguide widths at the coupling portion of the two directional couplers is connected to the arm waveguide having the longer optical path of the two arm waveguides.

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