JP2014026005A - Optical multiplexer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem in that it is difficult for a conventional PLC multiplexer to separate a mode having an intensity distribution that is vertically odd symmetrical.SOLUTION: An optical multiplexer multiplexes mode having intensity distribution that is vertically odd symmetrical by introducing geometrical asymmetry into a clad 30 of a planar light wave circuit (PLC). The optical multiplexer according to the invention is formed with a PLC. The optical multiplexer is provided with a slit 51 in the clad 30, thereby multiplexing a mode having a distribution that is vertically odd symmetrical.

Description

  The present invention relates to an optical multiplexer / demultiplexer used for optical communication or measurement thereof. More specifically, when connecting an optical component / optical transmission line (such as an optical fiber) having a plurality of modes and an optical component / optical transmission line having a single mode, combining and demultiplexing (separating) while maintaining mode matching Is related to an optical multiplexer / demultiplexer.

  In recent years, optical communication technology that enables high-speed data communication has been widely adopted in the information communication field. In the conventional large-capacity optical communication technology, a single mode optical fiber has been used as a transmission medium. In order to further expand the communication capacity, research on a mode multiplex transmission system that uses two or ten optical fibers having a plurality of propagation modes (several mode fibers) and carries different information in each propagation mode is progressing. In the current mode multiplex transmission method, a single mode device is used for signal generation, a plurality of single mode signals are combined into a mode multiplex signal at the time of transmission, transmitted in a batch using a multimode transmission line, and similarly The receiving unit separates this into a plurality of single mode signals, and then decodes them using a single mode device. For this reason, a multiplexer / demultiplexer that multiplexes and demultiplexes a plurality of single mode signals stably and simply is used (for example, see Non-Patent Documents 1 to 3).

Sebastian Randel et al., “6 × 56-Gb / s mode-division multiplexed transmission over 33-km new-mode fiber enabled by 6 × 6 MIMO equalization, Opt. 19, no. 17,16697 (2011) N. Hanzawa et al., “Demonstration of mode-division multiplexing transmission over 10 km two-mode fiber with mode coupler”, OFC 2011 OWA4 (2011) Sergio G. Leon-Saval et al., “Photonic Lantern”, Optics Express, Vol. 18, no. 8, 8430 (2010). Hidehiko Takara, et al .; “Two-mode coupling / demultiplexing planar lightwave circuit and its coupling to several-mode fiber”, Laser Society 32nd Annual Conference, G301aIV06, 2012

A typical mode of spatial distribution (cross section) of modes in a multimode optical fiber that can be used for mode multiplexing communication is shown in FIG. The name of the mode is the name in the weak waveguide approximation. In mode multiplex communication, these multiple modes are transmitted simultaneously in a single optical fiber, and different information is transmitted in each mode, so it is necessary to synthesize and separate them in the transmission / reception unit. . In the LP11 mode, a plurality of modes exist in a degenerated state, and it is difficult to optically separate them. In the LP ₂₁ mode, a plurality of modes are similarly degenerated. FIG. 2 shows the modes of the rectangular waveguide and the electric field distribution in the vertical direction, and the names of the modes indicate the names in the Markatiri approximation. The E00, E10, E20 modes, etc. in FIG. 2 have a mirror-related mode distribution in the vertical direction of the core. Here, this is generically called an evenly symmetrical electric field distribution in the vertical direction. On the other hand, the E01, E11 modes, etc. are point-symmetrical distributions in the vertical direction and are collectively referred to as odd-symmetrical electric field distributions in the vertical direction.

  As seen in Non-Patent Document 1, a spatial optical system is used as a multiplexer / demultiplexer, but a translucent mirror is used for multiplexing / demultiplexing. Considering a demultiplexer, in the case of using n modes, it is necessary to divide the original light into n, so that the light loss is large in principle. Similarly, light loss occurs during synthesis. Further, when a space system is used, there is a limit to miniaturization, and it is difficult to improve stability.

  As in Non-Patent Document 2, an optical fiber coupler can also be used. In this case, it is difficult to combine and separate a plurality of modes at once, and it is practical to use only two modes.

  As in Non-Patent Document 3, it is also possible to receive modes collectively in a plane, but it is necessary to separate the modes by performing advanced signal processing after reception. It is not.

  As a method for overcoming these drawbacks, a mode multiplexer / demultiplexer using a quartz planar lightwave circuit (PLC) is also used. (Non-Patent Document 4) However, since the PLC is normally configured in a two-dimensional shape as the name suggests, it is suitable for combining and separating modes having an even-symmetric electric field distribution in the vertical direction, but the E01, E11 modes, etc. For a mode having an odd-symmetrical electric field distribution in the vertical direction, the coupling from the upper half and the lower half is canceled and the operation as an optical multiplexer / demultiplexer is not performed, and the modes cannot be separated. In order to perform longitudinal mode synthesis / separation using a PLC, it is necessary to use a laminated PLC, and it is difficult to manufacture with high accuracy.

  In order to achieve the above object, in the present invention, longitudinal mode synthesis / separation is realized by a two-dimensional PLC. Specifically, this is realized by introducing vertical asymmetry in the clad portion of the PLC. Since the light propagation in the PLC is reciprocal, it is obvious that the same effect can be obtained even if the relationship between the input and the output is reversed, and an optical multiplexer and an optical demultiplexer can be realized with the same structure.

  Specifically, an optical multiplexer / demultiplexer according to the present invention is configured by using a planar lightwave circuit in which a waveguide is formed in a clad, and combines or separates a plurality of single mode signals into one multimode signal. In the optical multiplexer / demultiplexer, the dielectric constant is made asymmetric in the thickness direction of the planar lightwave circuit between the single mode waveguide transmitting the single mode signal and the multimode waveguide transmitting the multimode signal. For this purpose, a dielectric adjustment unit is disposed.

  In the optical multiplexer / demultiplexer according to the present invention, the dielectric adjustment unit may be a slit provided on an upper surface of the planar lightwave circuit.

  In the optical multiplexer / demultiplexer according to the present invention, the slits of the single mode waveguide and the multimode waveguide are based on a position closest to the top surface of the single mode waveguide and the multimode waveguide. It may be deeper than the position of 5% of the average thickness and shallower than the position of 80% of the average thickness of the single mode waveguide and the multimode waveguide.

  In the optical multiplexer / demultiplexer according to the present invention, a medium having a dielectric constant different from that of the cladding may be disposed in a part or all of the slit.

  In the optical multiplexer / demultiplexer according to the present invention, the medium disposed in the slit has an electro-optic effect, a magneto-optic effect, or a thermo-optic effect, and applies electricity, magnetism, or heat to the medium disposed in the slit. Thus, the optical characteristics of the medium disposed in the slit may be adjusted.

  In the optical multiplexer / demultiplexer according to the present invention, the thickness of the clad disposed below the single mode waveguide and the multimode waveguide is an average thickness of the single mode waveguide and the multimode waveguide. And the thickness of the cladding disposed below the single mode waveguide and the multimode waveguide is equal to the average thickness of the single mode waveguide and the multimode waveguide. It may be 25% or less.

  In the optical multiplexer / demultiplexer according to the present invention, the thickness of the clad disposed below the single mode waveguide and the multimode waveguide is an average thickness of the single mode waveguide and the multimode waveguide. It may be 15% or less.

  The above inventions can be combined as much as possible.

  Since the optical multiplexer / demultiplexer of the present invention can be manufactured only by the PLC manufacturing process, it is possible to manufacture a mode multiplexer / demultiplexer with less mode interference (high extinction ratio). Further, since there is little mixing of unnecessary modes, it is possible to reduce the fundamental optical loss.

It is an example of a propagation mode of a circular optical waveguide (optical fiber). It is an example of the propagation mode of a rectangular optical waveguide. It is a top view which shows an example of the optical multiplexer / demultiplexer which concerns on this embodiment. An example of A-A 'cross section is shown. It is the perspective view which showed the 1st Embodiment of this invention. It is the cross-sectional view which showed the 1st Embodiment of this invention. It is the figure which calculated the change of the optimal length of the waveguide by the depth of a slit. It is the perspective view which showed the 2nd Embodiment of this invention. It is the cross-sectional view which showed the 2nd Embodiment of this invention. It is the figure which calculated the change of the optimal length of the waveguide length by the thickness of an upper clad. It is the cross-sectional view which showed the 3rd Embodiment of this invention. It is the cross-sectional view which showed the 4th Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  FIG. 3 is a top view showing an example of the optical multiplexer / demultiplexer according to the present embodiment. The optical multiplexer / demultiplexer according to the present embodiment is configured using a planar lightwave circuit, and includes a multimode waveguide 10 that transmits a multimode signal, a single mode waveguide 20 that transmits a single mode signal, and a planar lightwave. And a dielectric constant adjusting unit 50 that makes the dielectric constant asymmetric in the thickness direction of the circuit. The dielectric constant adjusting unit 50 is an arbitrary medium having a dielectric constant different from that of the cladding, and the dielectric constant thereof may be larger or smaller than that of the cladding.

  FIG. 4 shows an example of the A-A ′ cross section. The dielectric constant adjusting unit 50 is disposed at a position where coupling occurs between the single mode signal and the multimode signal. For example, the clad is disposed between the single mode waveguide 20 and the multimode waveguide 10. If the coupling occurs between the single mode signal and the multimode signal, the dielectric constant adjusting unit 50 is disposed at a position closer to the upper surface than the single mode waveguide 20 and the multimode waveguide 10. Alternatively, the single mode waveguide 20 and the multimode waveguide 10 may be disposed closer to the lower surface.

The dielectric constant adjusting unit 50 is disposed at a position where the dielectric constant becomes asymmetric in the thickness direction of the planar lightwave circuit. For example, with respect to the center line LC of the single mode waveguide 20 and the multimode waveguide 10, it may be arranged only at a position close to the upper surface as shown in FIG. May be. Further, with the center line LC as a boundary, the volume of the dielectric constant adjusting unit 50 may be different between a position closer to the upper surface than the center line LC and a position closer to the lower surface. In this case, the length in the thickness direction may be different, the width in the surface direction may be different, or the thickness direction and the surface direction may be different.
Hereinafter, a specific configuration will be described.

(First embodiment)
5 and 6 are a perspective view and a cross-sectional view, respectively, of a portion where optical coupling occurs in the optical multiplexer / demultiplexer according to the first embodiment of the present invention. In the present embodiment, a slit 51 is provided on the upper surface of the planar lightwave circuit as the dielectric constant adjusting unit 50. By making slits in the clad 30, the mode coupling becomes asymmetric in the vertical direction, and a mode coupler of a longitudinal higher order mode can be configured.

  A medium having a dielectric constant (refractive index) different from that of the clad 30 is disposed in the slit 51. It is easy to use air as a medium for the slit 51 by making a cut in the clad 30. Here, although the calculation result when the medium is air is shown, the medium is not limited to air.

  Introducing specific parameters in the present embodiment, the thickness TC of the clad 30 is 20 μm, the cross section of the multimode waveguide 10 is 10 μm thick, the width is 14 μm, Δ = 0.7%, and the single mode waveguide 20 The cross section is 10 μm thick, the width is approximately 4 μm, Δ = 0.7%, the distance between the multimode waveguide 10 and the single mode waveguide 20 is 4 μm, and the slit 51 is the multimode waveguide 10 and the single mode waveguide 20. The width of the waveguide is 1 μm. The width of the single mode waveguide 20 is designed so that the coupling power to the multimode waveguide 10 is the highest. In the calculation, the width of the single-mode waveguide 20 is adjusted so that 98% or more of the power is coupled when the width of the multimode waveguide 10 is constant and the waveguide length has the highest coupling power. The waveguide length here refers to the effective length of the waveguide in which coupling occurs between the multimode signal and the single mode signal in which the distance between the multimode waveguide 10 and the single mode waveguide 20 is 4 μm. In an actual PLC, the characteristics may be finely adjusted by heating the waveguide.

  FIG. 7 is a graph obtained by calculating the waveguide length at which the coupling rate is highest when the depth of the slit 51 is changed. The present embodiment works most effectively when the depth d of the slit 51 is 50% of the thickness of the multimode waveguide 10 and the single mode waveguide 20, but the depth d of the slit 51 is arbitrary. Even if it exists, the optical multiplexer / demultiplexer which performs isolation | separation and a synthesis | combination of 100% by adjusting waveguide length can be produced. In consideration of making the waveguide length within 50 mm from the viewpoint of manufacturing the optical multiplexer / demultiplexer, the depth d of the slit 51 is based on the upper surface HH of the multimode waveguide 10 and the single mode waveguide 20. The multimode waveguide 10 and the single mode are deeper than the position of 5% of the average thickness of the multimode waveguide 10 and the single mode waveguide 20 and below the top surface of the multimode waveguide 10 and the single mode waveguide 20. It is preferably shallower than the position where the average thickness of the waveguide 20 is 80%.

(Second Embodiment)
8 and 9 are a perspective view and a cross-sectional view, respectively, of an optical multiplexer / demultiplexer according to the second embodiment of the present invention. Instead of inserting the slit 51, the thickness of the clad 30 is made asymmetrical in the vertical direction, so that a vertical asymmetry is generated in the mode distribution to constitute a mode coupler of the longitudinal higher order mode. For example, the thickness DD of the lower clad disposed below the single mode waveguide 20 and the multimode waveguide 10 and the upper side disposed above the single mode waveguide 20 and the multimode waveguide 10. For the thickness DU of the cladding, DU <DD.

  Introducing specific parameters in the present embodiment, the thickness DD of the lower clad 30 is 20 μm, the cross section of the multimode waveguide 10 is 10 μm thick, 20 μm wide, Δ = 0.7%, single mode waveguide The cross section 20 has a thickness of 10 μm, a width of approximately 3.5 μm, Δ = 0.7%, and the interval between the multimode waveguide 10 and the single mode waveguide 20 is 4 μm.

  In principle, it is sufficient if there is a difference between the thickness DU of the upper clad and the thickness DD of the lower clad. However, since the electric field in the portion of the clad 30 becomes exponentially weaker as the distance from the core increases, Δ becomes 0. In a normal PLC of several percent or more, the thickness of the clad 30 is set to be equal to or greater than the thicknesses of the multimode waveguide 10 and the single mode waveguide 20, so that there is practically no difference from an infinite clad. It becomes a characteristic. Therefore, in this embodiment, the thickness DD of the lower cladding is calculated as 20 μm, which is twice the thickness of the multimode waveguide 10 and the single mode waveguide 20. Although the thickness of the clad 30 may be reversed upside down, it is preferable to make the lower clad 30 thick in order to use the substrate.

  FIG. 10 is a graph obtained by calculating the waveguide length at which the coupling rate is highest when the thickness DU of the upper cladding is changed. In consideration of making the waveguide length within 50 mm from the viewpoint of manufacturing, the thickness DU of the upper cladding is preferably thinner than 15% of the average thickness of the multimode waveguide 10 and the single mode waveguide 20. . Further, when the waveguide length condition is relaxed to within 100 mm, the upper cladding thickness DU should be less than 25% of the average thickness of the multimode waveguide 10 and the single mode waveguide 20.

(Third embodiment)
FIG. 11 is a cross-sectional view of an optical multiplexer / demultiplexer according to the third embodiment of the present invention. By adding the first embodiment to the second embodiment, the degree of freedom in manufacturing can be increased.

(Fourth embodiment)
In the optical multiplexer / demultiplexer of the present embodiment, a medium having a dielectric constant different from that of the clad 30 is disposed in part or all of the slit 51 described in the first embodiment. FIG. 12 is a cross-sectional view of an optical multiplexer / demultiplexer according to the fourth embodiment of the present invention. 12A, the entire slit 51 is filled with a medium 52 having a dielectric constant different from that of the cladding, and FIG. 12B is filled with a medium 52 having a dielectric constant different from that of the cladding at the bottom of the slit 51. In FIG. 12C, a medium 52 having a dielectric constant different from that of the clad is arranged in the middle part of the slit 51, and in FIG. As described above, by adjusting the dielectric constant and the amount of the medium 52 to be filled, it is possible to finely adjust the characteristics of the slit 51 after the production.

(Fifth embodiment)
The optical multiplexer / demultiplexer of the present embodiment finely adjusts the optical characteristics of the medium 52 by heating the medium 52 disposed in the slit 51 described in the fourth embodiment with a heater electrode or applying an electric field or magnetic field. In particular, if a medium with a large thermo-optic effect, electro-optic effect, or magneto-optic effect is selected as the filling medium, the optical characteristics can be changed greatly with a small change in temperature and change in electric field strength, and the adjustment range can be expanded. Is possible. It is also possible to adjust the optical characteristics of the medium 52 disposed in the slit 51 by filling the slit 51 with an anisotropic medium such as liquid crystal and changing the direction of the optical axis by an electric field / magnetic field. . A method of providing an electrode in a PLC or other waveguide or in the vicinity thereof and applying a heating or electric / magnetic field is a known technique.

  In the calculation examples in the above embodiment, the calculation is performed assuming a PLC using quartz glass, but the material of the planar lightwave circuit is not limited to this as long as it is a medium capable of propagating a single mode signal or a multimode signal.

  The present invention is an optical multiplexer / demultiplexer used for mode multiplexing optical communication or measurement thereof, and has an optical component / optical transmission line (optical fiber, etc.) having a plurality of modes and an optical component having a single mode. An optical multiplexer / demultiplexer that combines and demultiplexes (separates) while matching the modes when connecting optical transmission lines.

  Since the optical multiplexer / demultiplexer of the present invention can be manufactured by a technically established PLC manufacturing process, it is possible to manufacture a mode multiplexer / demultiplexer with less mode interference (high extinction ratio). Further, since there is little mixing of unnecessary modes, it is possible to reduce the fundamental optical loss.

10: Multimode waveguide 20: Single mode waveguide 30: Clad 50: Dielectric constant adjustment unit 51: Slit 52: Medium

Claims (7)

In an optical multiplexer / demultiplexer configured by using a planar lightwave circuit in which a waveguide is formed in a clad and combining or separating a plurality of single mode signals into one multimode signal,
A dielectric adjustment unit for making the dielectric constant asymmetric in the thickness direction of the planar lightwave circuit between the single mode waveguide that transmits the single mode signal and the multimode waveguide that transmits the multimode signal. Arranged optical multiplexer / demultiplexer.   2. The optical multiplexer / demultiplexer according to claim 1, wherein the dielectric adjustment unit is a slit provided on an upper surface of the planar lightwave circuit.   The slit is located at a position that is 5% of the average thickness of the single-mode waveguide and the multimode waveguide, with reference to the position closest to the top surface of the singlemode waveguide and the multimode waveguide. 3. The optical multiplexer / demultiplexer according to claim 2, wherein the optical multiplexer / demultiplexer is deep and shallower than a position of 80% of an average thickness of the single mode waveguide and the multimode waveguide.   4. The optical multiplexer / demultiplexer according to claim 2, wherein a medium having a dielectric constant different from that of the cladding is disposed in part or all of the slit. The medium disposed in the slit has an electro-optic effect, a magneto-optic effect or a thermo-optic effect,
5. The optical coupling according to claim 2, wherein the optical characteristic of the medium disposed in the slit is adjusted by applying electricity, magnetism, or heat to the medium disposed in the slit. Waver. The thickness of the clad disposed below the single mode waveguide and the multimode waveguide is at least one times the average thickness of the single mode waveguide and the multimode waveguide;
The thickness of the clad disposed below the single mode waveguide and the multimode waveguide is 25% or less of the average thickness of the single mode waveguide and the multimode waveguide. An optical multiplexer / demultiplexer according to any one of claims 1 to 5.   The thickness of the clad disposed below the single mode waveguide and the multimode waveguide is 15% or less of the average thickness of the single mode waveguide and the multimode waveguide. The optical multiplexer / demultiplexer according to claim 6.

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