WO2012099275A1 - Optical coupler and method of branch control - Google Patents

Optical coupler and method of branch control Download PDF

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Publication number
WO2012099275A1
WO2012099275A1 PCT/JP2012/051727 JP2012051727W WO2012099275A1 WO 2012099275 A1 WO2012099275 A1 WO 2012099275A1 JP 2012051727 W JP2012051727 W JP 2012051727W WO 2012099275 A1 WO2012099275 A1 WO 2012099275A1
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Prior art keywords
waveguide
refractive index
output
light
multimode
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PCT/JP2012/051727
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French (fr)
Japanese (ja)
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松本　崇
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日本電気株式会社
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Publication of WO2012099275A1 publication Critical patent/WO2012099275A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/122—Basic optical elements, e.g. light-guiding paths
    • G02B6/125—Bends, branchings or intersections
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/29—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the position or the direction of light beams, i.e. deflection
    • G02F1/31—Digital deflection, i.e. optical switching
    • G02F1/313—Digital deflection, i.e. optical switching in an optical waveguide structure
    • G02F1/3132—Digital deflection, i.e. optical switching in an optical waveguide structure of directional coupler type
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12133—Functions
    • G02B2006/12147—Coupler
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2808—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs
    • G02B6/2813—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs based on multimode interference effect, i.e. self-imaging
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0147—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on thermo-optic effects
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G02F1/217—Multimode interference type

Definitions

  • the present invention relates to an optical coupler for branching a light wave transmitted through an optical waveguide and a branch control method thereof.
  • the 2 ⁇ 2 optical coupler divides light from an input port (waveguide) into two, and outputs each from two output ports (waveguides).
  • Examples of the 2 ⁇ 2 optical coupler include a directional coupler type (DC) shown in FIG. 12A and a Mach-Zehnder interferometer type (MZI: Mach-Zehnder Interferometer) shown in FIG.
  • the 2 ⁇ 2 coupler is required to be designed and manufactured so as to obtain a different branching ratio depending on the application.
  • coherent detection it is necessary to design and manufacture so that even if an optical signal is input from which input port, the optical signal is equally branched. This is because the accuracy of coherent detection and further the communication error rate are determined.
  • Optical couplers are required to have no complicated structure, be small, have low loss, have low wavelength dependency, and be resistant to manufacturing variations. More recently, the importance of optical couplers capable of performing optical branch control with high accuracy has been increasing. However, each of the above optical couplers has the following problems. In a DC type optical coupler, it is difficult to control the distance between two adjacent waveguides, and it is difficult to suppress manufacturing variations.
  • the MZI type optical coupler can control (trimming) the branching ratio by changing the optical path length between both arms.
  • an MZI type optical coupler needs to control the optical path length of about 10 nm and perform branching ratio control.
  • Patent Document 1 describes a multi-mode interferometer type optical coupler.
  • the control of the width of the MMI for obtaining a stable branching ratio is about 100 nm. Therefore, compared with the MZI type, the controllable region is about 10 times wider and the manufacturing tolerance is wider.
  • the wavelength characteristics are uniform and can be used over the entire optical communication wavelength band.
  • the MMI coupler is less wavelength dependent than the directional coupler, and a coupling rate of about 50% can be obtained in a wide wavelength range.
  • the optical coupler of the multimode interferometer described in Patent Document 1 needs to change the structure itself when the branching ratio is controlled. In other words, to arbitrarily control the branching ratio, the structure must be changed each time. Therefore, the optical coupler described in Patent Document 1 has a problem that the branching ratio cannot be easily controlled depending on the situation.
  • An object of the present invention is to provide an optical coupler that solves the above-described problems.
  • An optical coupler includes a multimode waveguide, at least two input waveguides that are connected to the multimode waveguide and input light, and at least two output waveguides that are connected to the multimode waveguide and output light. And a refractive index control unit that adjusts the refractive index of the multimode waveguide, and the refractive index control unit is arranged substantially parallel to the traveling direction of light.
  • the optical coupler in the present invention can arbitrarily change the branching ratio.
  • FIG. 1 is a top view of the optical coupler according to the first embodiment.
  • FIG. 2 shows a simulation result in which the refractive index is controlled in the first embodiment.
  • FIG. 3 is a top view of the optical coupler according to the second embodiment.
  • FIG. 4 is a top view when the refractive index at both ends of the multimode waveguide is increased in the second embodiment.
  • FIG. 5 shows a simulation result when the refractive indexes of both end portions of the multimode waveguide are increased in the second embodiment.
  • FIG. 6 is a top view when the refractive index of the central portion of the multimode waveguide is increased in the second embodiment.
  • FIG. 7 shows a simulation result when the refractive index of the central portion of the multimode waveguide is increased in the second embodiment.
  • FIG. 8 is a cross-sectional view of a multimode waveguide according to the second embodiment.
  • FIG. 9 is a diagram illustrating a state of input light and output light of the optical coupler.
  • FIG. 10 is a diagram illustrating a state of input light and output light of the optical coupler.
  • FIG. 11 is a diagram illustrating a coherent mixer including an optical coupler.
  • FIG. 12A is a schematic diagram showing a directional coupler type.
  • FIG. 12B is a schematic diagram showing a Mach-Zehnder interferometer type.
  • FIG. 1 is a top view of an optical coupler 1 in the present embodiment.
  • the optical coupler 1 in this embodiment includes a first input waveguide 2, a second input waveguide 3, a multimode waveguide 4, and a first output waveguide 5. And a second output waveguide 6 and a refractive index control unit 7.
  • the first input waveguide 2, the second input waveguide 3, the first output waveguide 5, and the second output waveguide 6 are waveguides that transmit light in a single mode.
  • the multimode waveguide 4 is a waveguide that transmits light in multimode.
  • the first input waveguide 2 and the second input waveguide 3 are connected to the multimode waveguide 4 from the same direction.
  • the first input waveguide 2 and the second input waveguide 3 input the propagated light to the multimode waveguide 4.
  • the multimode waveguide 4 is rectangular, and the first input waveguide 2 and the second input waveguide 3 are connected to the same side.
  • the light that has interfered in the multimode waveguide 4 forms a self-image and is output to the first output waveguide 5 and the second output waveguide 6, respectively.
  • first output waveguide 5 and the second output waveguide 6 are multimode waveguides on the side opposite to the side where the first input waveguide 2 and the second input waveguide 3 are connected to the multimode waveguide 4. It is connected to the waveguide 4.
  • the first input waveguide 2 and the first output waveguide 5 are provided at positions facing each other across the multimode waveguide 4.
  • the second input waveguide 3 and the second output waveguide 6 are provided at positions facing each other with the multimode waveguide 4 interposed therebetween.
  • the rate at which the light input from the first input waveguide 2 is output from the first output waveguide 5 is referred to herein as a bar port output.
  • the ratio at which the light input from the first input waveguide 2 is output from the second output waveguide 6 is referred to as a cross-port output.
  • the multimode waveguide 4 forms a refractive index distribution so as to be line symmetric with respect to the traveling direction of light.
  • the light traveling direction refers to the direction in which light entering from the first input waveguide 2 and the second input waveguide 3 travels as a whole. Specifically, the direction is from the first input waveguide 2 to the first output waveguide 5 or from the second input waveguide 3 to the second output waveguide 6.
  • the refractive index is uniformly formed with respect to the traveling direction of light.
  • the refractive index control unit 7 extends in the light traveling direction at the central portion on the multimode waveguide 4 or at both end portions on the side along the light traveling direction or in the vicinity thereof.
  • the refractive index control unit 7 is arranged at a position that is substantially parallel or line symmetric with respect to the light traveling direction, and controls the refractive index of the multimode waveguide 4.
  • the operation and action in this embodiment will be described.
  • the branching ratio of the light output from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 depends on the width and length of the multimode waveguide 4 and the first input waveguide 2.
  • the second input waveguide 3, the first output waveguide 5, and the second output waveguide 6 are determined by the incident / exit position where the multimode waveguide 4 is connected.
  • the multimode waveguide 4 has a uniform refractive index, and under ideal conditions such that the multimode waveguide 4 is not bent downward or the like, the input light is converted into the first output waveguide 5. And can equally branch to the second output waveguide 6.
  • the width and length of the multimode waveguide 4 and the positional relationship between the input / output waveguides and the multimode waveguide 4 cannot be easily changed after they are determined in the manufacturing process. .
  • the optical coupler 1 in the present embodiment controls the refractive index distribution of the multimode waveguide 4 by disposing the refractive index control unit 7 at a position that is substantially parallel or line symmetric with respect to the light traveling direction.
  • FIG. 2 shows a simulation result in which the refractive index of the multimode waveguide 4 is changed by the refractive index control unit 7 provided in the central portion of the multimode waveguide 4 so as to extend in the light traveling direction. . As shown in FIG.
  • the bar port is higher than the cross port.
  • the ratio of the light input from the first input waveguide 2 to be output from the first output waveguide 5 disposed at the facing position is from the second output waveguide 6 disposed in the oblique direction. It becomes larger compared to the output ratio.
  • the horizontal axis in FIG. 2 indicates the difference in refractive index between the central portion and both end portions. Specifically, “0” indicates that the refractive index is the same at the center and both ends, the + side has a higher refractive index at the center than the both ends, and the ⁇ side has an opposite refractive index at the center.
  • the optical coupler 1 in the present embodiment controls the refractive index of the multimode waveguide 4 by the refractive index control unit 7 disposed substantially parallel to or in line symmetry with the light traveling direction. With the above structure, the branching ratio of light output from the multimode waveguide 4 can be easily controlled.
  • the branching ratio of the signals output from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 depends on manufacturing variations in the manufacturing process and external factors. Even when the refractive index distribution is distorted or cannot be equally branched, the branching ratio can be easily controlled by controlling the refractive index of the multimode waveguide 4 with the refractive index control unit 7. Further, the branching ratio control of the refractive index control unit 7 in the present embodiment is not limited to the case where the refractive index distribution of the multimode waveguide 4 is distorted.
  • FIG. 3 is a top view of the optical coupler 1 in the present embodiment.
  • the optical coupler 1 in this embodiment uses a thin film heater 8 as the refractive index control unit 7.
  • Other structures and connection relationships are the same as those in the first embodiment, and the first input waveguide 2, the second input waveguide 3, the multimode waveguide 4, the first output waveguide 5, And a second output waveguide 6.
  • the thin film heater 8 is provided to extend in the light traveling direction at the central portion on the multimode waveguide 4, on both end portions on the side along the light traveling direction, or at least one of the vicinity thereof. . That is, the thin film heater 8 is disposed at a position that is substantially parallel or line symmetric with respect to the light traveling direction.
  • Each thin film heater 8 is preferably longer than the multimode waveguide 4.
  • Each thin film heater 8 includes electrodes 9 at both ends provided to extend, and generates heat when energized through the electrodes 9. Note that all three thin film heaters 8 are not necessarily required, but are illustrated in FIG. 3 for the sake of clarity.
  • FIG. 4 As shown in FIG. 4, at the time of manufacturing the optical coupler 1, the light input from the first input waveguide 2 is compared with the second output waveguide 6 positioned in the oblique direction.
  • the ratio of output from the first output waveguide 5 located in the opposing direction is large. That is, in the multimode waveguide 4, the ratio of output from the bar port is considered to be larger than the ratio of output from the cross port. (In FIG.
  • the refractive index at both ends of the multi-mode waveguide 4 can be increased by increasing the amount of current flowing through the thin film heaters 8 at both ends and raising the temperature at both ends.
  • the ratio output to the second output waveguide 6 can be increased, the ratio output to the first output waveguide 5 can be decreased, and the branching ratio can be made equal.
  • the horizontal axis in FIG. 5 indicates the amount of increase in the refractive index at both ends, and the vertical axis indicates the branching ratio. Before energization, that is, when the horizontal axis is 0, the branching ratio is 7: 3, but when the refractive index is increased to 10 ⁇ 10 ⁇ 4 , the branching ratio is 1: 1.
  • the branching ratio can be easily controlled by heating the multimode waveguide 4.
  • the refractive index temperature coefficient of a silica-type material is about 10 ⁇ -5 > / T, what is necessary is just to heat about 100 degreeC from normal temperature.
  • the temperature immediately below the thin film heater 8 becomes the highest temperature, and the temperature decreases as the distance from the thin film heater 8 increases. Therefore, a temperature gradient is formed in the multimode waveguide 4 in the direction perpendicular to the traveling direction.
  • the refractive index distribution can be corrected by this temperature gradient to obtain a desired distribution.
  • the light input from the first input waveguide 2 is obliquely compared with the first output waveguide 5 positioned in the opposite direction.
  • the ratio of the output from the second output waveguide 6 positioned at is large.
  • the multimode waveguide 4 is considered to have a higher rate of output from the cross port than a rate of output from the bar port. (In FIG. 7, it is 6: 4.)
  • distortion occurs in the refractive index distribution of the multimode waveguide due to manufacturing variations in the manufacturing process and external factors.
  • the refractive index of the multimode waveguide 4 is greater at both ends than at the center. It is thought that it became high. Therefore, as shown in FIG.
  • the multimode waveguide 4 has a flat refractive index distribution as shown by a broken line by energizing a thin film heater 8 provided at the center to heat the center of the multimode waveguide 4.
  • the thin film heaters 8 at both ends are not shown in FIG. 6 because they are unnecessary.
  • FIG. 7 by increasing the refractive index at the center of the multimode waveguide 4, the ratio of output to the first output waveguide 5 is increased and the ratio of output to the second output waveguide 6 is increased. , And the branching ratio can be made equal. That is, the branching ratio can be easily controlled by heating the multimode waveguide 4.
  • FIG. 7 shows that is, the branching ratio can be easily controlled by heating the multimode waveguide 4.
  • the horizontal axis represents the amount of increase in the refractive index at the center
  • the vertical axis represents the branching ratio.
  • the branching ratio control by the thin film heater 8 in the present embodiment is not limited to the case where the refractive index distribution of the multimode waveguide 4 is distorted.
  • the branching ratio deviates from equal branching due to factors other than refractive index distortion. Even in this case, the branching ratio can be easily controlled by controlling the refractive index of the multimode waveguide 4 with the thin film heater 8.
  • the branching ratio is controlled by changing the refractive index of the multimode waveguide 4 from the outside of the optical coupler 1 by heating using the thin film heater 8.
  • the refractive index of the multimode waveguide 4 it is not limited to the thin film heater 8, and a carrier plasma effect, an electro-optic effect, a magneto-optic effect, or the like can be used.
  • FIG. 8 is a cross-sectional view of the multimode waveguide 4.
  • the refractive index control unit 7 is the cladding thickness adjusting unit 10.
  • the multimode waveguide 4 is formed by laminating at least a lower cladding 13, a core 12, and an upper cladding 11 in order from the bottom.
  • the clad thickness adjusting unit 10 is provided on the multimode waveguide 4 and adjusts the thickness of the upper clad 11. When the thickness of the upper clad 11 is reduced, the refractive index of the multimode waveguide 4 is lowered. On the other hand, when the upper cladding 11 is thickened, the refractive index of the multimode waveguide 4 increases.
  • the clad thickness adjusting portion 10 is provided to extend in the light traveling direction at the central portion on the multimode waveguide 4 or at least one of both end portions in the direction perpendicular to the light traveling direction. That is, the cladding thickness adjusting unit 10 is disposed at a position that is substantially parallel or line-symmetric with respect to the light traveling direction.
  • an FIB (Focused Ion Beam) apparatus or the like can be considered. By irradiating the upper clad 11 with a focused ion beam such as gallium ions, the FIB apparatus can scrape off atoms on the surface. That is, the thickness of the upper clad 11 can be reduced.
  • vapor deposition can be performed by irradiating the upper cladding 11 with a metal such as Cu, Al, or Au as a low energy ion beam. That is, the thickness of the upper clad 11 can be increased.
  • the clad thickness adjusting unit 10 is not limited to this as long as the thickness of the upper clad 11 can be adjusted. [Description of Functions and Effects]
  • the clad thickness adjusting unit 10 is distorted in the refractive index distribution of the multi-mode waveguide 4 due to manufacturing variations in the manufacturing process and external factors in view of the branching ratio. Is estimated to be high, the thickness of the upper clad 11 provided at the center is reduced.
  • the branching ratio can be easily controlled by reducing the refractive index at the center.
  • the branching ratio can be easily controlled to be equal by increasing the refractive index at both ends of the multimode waveguide 4.
  • the clad thickness adjusting unit 10 is presumed that the refractive index distribution of the multimode waveguide 4 is distorted due to manufacturing variations in the manufacturing process and external factors in view of the branching ratio, and as a result, the central refractive index is low. In this case, the thickness of the upper clad 11 provided at both ends is reduced. Since the multimode waveguide 4 can reduce the refractive index when the thickness of the upper clad 11 is reduced, the refractive index distribution can be flattened by reducing the refractive indexes at both the upper and lower ends. In the above case, not only the thickness of the upper clad 11 provided at both ends of the multimode waveguide 4 may be reduced, but the center portion of the multimode waveguide 4 may be increased.
  • the branching ratio can be easily controlled to be equal by increasing the refractive index at the center of the multimode waveguide 4.
  • the irradiation ion beam amount is adjusted by the FIB apparatus, and the amount of cutting the upper clad 11 may be increased or decreased in the direction from the center portion to the end portion of the multimode optical waveguide 4.
  • the refractive index is changed, it is only necessary to perform additional machining once on the multimode waveguide 4, so that continuous power consumption can be reduced as compared with the thin film heater 8 described in the second embodiment.
  • the branching ratio can be controlled without the need.
  • the cladding thickness adjusting unit 10 is not limited to the cladding thickness adjusting unit 10 as long as the refractive index of the multimode waveguide 4 can be changed without requiring continuous power consumption, and is a method of controlling with an impurity. And a method of controlling by UV irradiation.
  • the branching ratio control by the cladding thickness adjusting unit 10 in the present embodiment is not limited to the case where distortion occurs in the refractive index distribution of the multimode mode waveguide 4. When the coupling from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 is different between the two output waveguides, the branching ratio deviates from equal branching due to factors other than refractive index distortion.
  • FIGS. 9 and 10 are top views showing the states of input light and output light of the optical coupler 1.
  • the multimode waveguide 4 receives strong light from the first input waveguide 2 and weak light from the second input waveguide 3, strong light from the first output waveguide 5 Suppose that weak light is output from 6.
  • the ratio of barports is high.
  • the refractive index control unit 7 increases the refractive index at both ends of the multimode waveguide 4 or decreases the refractive index at the center by performing the above-described additional processing. That is, the rate at which the light input from the first input waveguide 2 is output to the first output waveguide 5 is decreased and the rate at which the light is output to the second output waveguide 6 is increased. As a result, the light output from the first output waveguide 5 becomes weak, and the light output from the second output waveguide 6 becomes strong, thereby realizing equal branching of the light output from the multimode waveguide 4. be able to.
  • the multimode waveguide 4 receives strong light from the first input waveguide 2 and weak light from the second input waveguide 3, weak light from the first output waveguide 5 Suppose that strong light is output from 6.
  • the refractive index control unit 7 lowers the refractive index at both ends of the multimode waveguide 4 or increases the refractive index at the center by performing the above-described additional processing.
  • FIG. 11 is a diagram illustrating a coherent mixer including the optical coupler 1.
  • the present embodiment includes an optical coupler 1, a splitter 14, a photodiode 15, and an amplifier 16.
  • the optical coupler 1 has the same structure and connection relationship as those of the first to third embodiments.
  • the optical coupler 1 has a first input waveguide 2, a second input waveguide 3, a multimode waveguide 4, and a first output waveguide.
  • a waveguide 5, a second output waveguide 6, and a refractive index control unit 7 are provided.
  • at least two splitters 14 are provided.
  • the signal light is incident on one splitter 14, and the local light that interferes with the signal light is incident on the other splitter 14 in order to demodulate the phase information.
  • the splitter 14 branches the incident signal light and local light.
  • at least two optical couplers 1 are provided.
  • One optical coupler 1 causes the signal light branched by the splitter 14 and the locally transmitted light to interfere with each other, and performs output in equal branches.
  • the other optical coupler causes the signal light branched by the splitter 14 and the locally transmitted light to interfere with each other and output in equal branches.
  • the two couplers have a difference of ⁇ / 4 in the optical path length of the waveguide that guides light from the splitter 14.
  • At least four photodiodes 15 are provided.
  • the photodiode 15 converts the four output lights output from the two optical couplers 1 into current signals.
  • the amplifier restores the current signal output from the photodiode 15 to a voltage signal.
  • the coherent mixer according to the present embodiment is a 90-degree optical hybrid interference that extracts phase information of a polarization-separated signal from a TE (Transverse Electric) optical signal and a TM (Transverse Magnetic) optical signal from the above configuration. It can be used for counting.
  • An important characteristic that affects the performance of a DP-QPSK receiver using a 90-degree optical hybrid interferometer is a characteristic called CMRR (Common-Mode Rejection Ratio). This is a measure of how much an input signal component common to two inputs can be removed in a differential amplifier circuit used for voltage conversion of a signal.
  • CMRR Common-Mode Rejection Ratio
  • the ideal branching characteristic of this optical coupler is that the branching ratio is 1: 1.
  • the optical coupler 1 in this embodiment can control the branching ratio of the output light to 1: 1 by providing the refractive index control unit 7. That is, the coherent mixer in the present embodiment can realize an amplifier having a high CMRR. As a result, since a high voltage offset can be applied, the influence of shot noise can be greatly reduced and high reception sensitivity can be realized.
  • the present invention has been described with reference to the above-described embodiment and examples, the present invention is not limited only to the configuration of the above-described embodiment and examples, and within the scope of the present invention. It goes without saying that various modifications and corrections that can be made by those skilled in the art are included. This application claims priority based on Japanese Patent Application No. 2011-011168 filed on Jan. 21, 2011, the entire disclosure of which is incorporated herein.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Nonlinear Science (AREA)
  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

An optical coupler according to the present invention is provided with a multimode waveguide, at least two input waveguides which are connected to the multimode waveguide to input light thereinto, at least two output waveguides which are connected to the multimode waveguide to output light therefrom and a refractive index control unit for adjusting the refractive index of the multimode waveguide, wherein the refractive index control unit is arranged substantially in parallel with a light traveling direction.

Description

光カプラと分岐制御方法Optical coupler and branching control method
　本発明は、光導波路を伝わる光波の分岐を行う光カプラと、その分岐制御方法に関する。 The present invention relates to an optical coupler for branching a light wave transmitted through an optical waveguide and a branch control method thereof.
　光通信におけるコヒーレント検波の広がりにより、干渉計に用いられる光カプラの分岐比特性の重要性が増している。なおコヒーレント検波は、信号光と局発光を光カプラで干渉させ、その差動動作により多値信号の復号を行う。
　近年、平板光回路（ＰＬＣ：Ｐｌａｎａｒ　Ｌｉｇｈｔｗａｖｅ　Ｃｉｒｃｕｉｔ）を用いた２×２光カプラが多く用いられている。２×２光カプラは、入力ポート（導波路）からの光を２つにわけ，それぞれを２つの出力ポート（導波路）からそれぞれ出力を行う。
　上記の２×２光カプラとしては、例えば図１２Ａに示す方向性結合器型（ＤＣ：Ｄｉｒｅｃｔｉｏｎａｌ　Ｃｏｕｐｌｅｒ）や、図１２Ｂに示すマッハツェンダ干渉計型（ＭＺＩ：Ｍａｃｈ−Ｚｅｈｎｄｅｒ　Ｉｎｔｅｒｆｅｒｏｍｅｔｅｒ）、などがあげられる。
　上記の２×２カプラは、その用途により、異なる必要となる分岐比が得られるように設計、製作することが求められている。コヒーレント検波の場合には、どちらの入力ポートから光信号が入力されたとしても、等分岐となるように設計、製作することが必要である。これはコヒーレント検波の精度、さらには通信のエラーレートが決定するためである。また光カプラは、複雑な構造でないことや、小型であること、低損失であること、波長依存性が低いこと、製作ばらつきに強いことなどが求められている。さらに最近では、高精度に光の分岐制御を行うことができる光カプラの重要性が高まってきている。
　しかしながら上記の光カプラは、それぞれ以下の問題点が存在する。ＤＣ型の光カプラは、近接させた２本の導波路の間隔を制御することが難しく、製作バラつきを抑えることが難しい。そのため分岐比を安定して得ることが難しい。
　一方、ＭＺＩ型の光カプラは、両アーム間の光路長を変化させることで、分岐比の制御（トリミング）が可能である。しかしＭＺＩ型の光カプラは所望の分岐比を得るためには、１０ｎｍ程度の光路長を制御し、分岐比制御を行う必要となる。一方で、屈折率変化でトリミングを行う場合には，熱光学効果を用いた場合でも、光路長を変化させるためには長いアーム長が必要となり、小型化という点で問題があった。
　そこで特許文献１には、マルチモード干渉計型（Ｍｕｌｔｉ　Ｍｏｄｅ　Ｉｎｔｅｒｆｅｒｏｍｅｔｅｒ）の光カプラに関する記載がされている。ＭＭＩ型は、安定した分岐比を得るためのＭＭＩの幅の制御が１００ｎｍ程度である。そのためＭＺＩ型と比較して、制御を行える領域が１０倍程度広く、製作トレランスが広い。また、波長特性も均一であり、光通信波長帯全域にわたって、使用可能である。また、ＭＭＩカプラは方向性結合器よりも波長依存性が小さく，広い波長範囲で結合率を約５０％得ることができる。 With the spread of coherent detection in optical communications, the importance of the branching ratio characteristics of optical couplers used in interferometers is increasing. In the coherent detection, signal light and local light are interfered by an optical coupler, and a multilevel signal is decoded by a differential operation.
In recent years, a 2 × 2 optical coupler using a planar optical circuit (PLC) is often used. The 2 × 2 optical coupler divides light from an input port (waveguide) into two, and outputs each from two output ports (waveguides).
Examples of the 2 × 2 optical coupler include a directional coupler type (DC) shown in FIG. 12A and a Mach-Zehnder interferometer type (MZI: Mach-Zehnder Interferometer) shown in FIG. 12B.
The 2 × 2 coupler is required to be designed and manufactured so as to obtain a different branching ratio depending on the application. In the case of coherent detection, it is necessary to design and manufacture so that even if an optical signal is input from which input port, the optical signal is equally branched. This is because the accuracy of coherent detection and further the communication error rate are determined. Optical couplers are required to have no complicated structure, be small, have low loss, have low wavelength dependency, and be resistant to manufacturing variations. More recently, the importance of optical couplers capable of performing optical branch control with high accuracy has been increasing.
However, each of the above optical couplers has the following problems. In a DC type optical coupler, it is difficult to control the distance between two adjacent waveguides, and it is difficult to suppress manufacturing variations. Therefore, it is difficult to stably obtain the branching ratio.
On the other hand, the MZI type optical coupler can control (trimming) the branching ratio by changing the optical path length between both arms. However, in order to obtain a desired branching ratio, an MZI type optical coupler needs to control the optical path length of about 10 nm and perform branching ratio control. On the other hand, when trimming by changing the refractive index, even when the thermo-optic effect is used, a long arm length is required to change the optical path length, and there is a problem in terms of miniaturization.
Therefore, Patent Document 1 describes a multi-mode interferometer type optical coupler. In the MMI type, the control of the width of the MMI for obtaining a stable branching ratio is about 100 nm. Therefore, compared with the MZI type, the controllable region is about 10 times wider and the manufacturing tolerance is wider. In addition, the wavelength characteristics are uniform and can be used over the entire optical communication wavelength band. Further, the MMI coupler is less wavelength dependent than the directional coupler, and a coupling rate of about 50% can be obtained in a wide wavelength range.
特開２００１−２１５４５２JP 2001-215458 A
　しかしながら、特許文献１に記載のマルチモード干渉計の光カプラは、分岐比の制御を行う場合、構造自体を変化させる必要があった。つまり任意で分岐比を制御するには、その度に構造を変更しなければいけない。そのため、特許文献１に記載の光カプラは、状況に応じて、容易に分岐比の制御を行えないという問題があった。
　本発明の目的は、上述した課題を解決する光カプラを提供することである。 However, the optical coupler of the multimode interferometer described in Patent Document 1 needs to change the structure itself when the branching ratio is controlled. In other words, to arbitrarily control the branching ratio, the structure must be changed each time. Therefore, the optical coupler described in Patent Document 1 has a problem that the branching ratio cannot be easily controlled depending on the situation.
An object of the present invention is to provide an optical coupler that solves the above-described problems.
　本発明における光カプラは、マルチモード導波路と、マルチモード導波路に接続し光を入力する少なくとも２つの入力導波路と、マルチモード導波路に接続し光を出力する少なくとも２つの出力導波路と、マルチモード導波路の屈折率を調整する屈折率制御部とを備え、屈折率制御部は、光の進行方向に対して略平行に配置されていることを特徴とする。 An optical coupler according to the present invention includes a multimode waveguide, at least two input waveguides that are connected to the multimode waveguide and input light, and at least two output waveguides that are connected to the multimode waveguide and output light. And a refractive index control unit that adjusts the refractive index of the multimode waveguide, and the refractive index control unit is arranged substantially parallel to the traveling direction of light.
　本発明における光カプラは、分岐比を任意に変化させることができる。 The optical coupler in the present invention can arbitrarily change the branching ratio.
　図１は、第１の実施形態における光カプラの上面図である。
　図２は、第１の実施形態において屈折率の制御を行ったシミュレーション結果である。
　図３は、第２の実施形態における光カプラの上面図である。
　図４は、第２の実施形態においてマルチモード導波路の両端部の屈折率を上げたときの上面図である。
　図５は、第２の実施形態においてマルチモード導波路の両端部の屈折率を上げたときのシミュレーション結果である。
　図６は、第２の実施形態においてマルチモード導波路の中央部の屈折率を上げたときの上面図である。
　図７は、第２の実施形態においてマルチモード導波路の中央部の屈折率を上げたときのシミュレーション結果である。
　図８は、第２の実施形態におけるマルチモード導波路の断面図である。
　図９は、光カプラの入力光と出力光の状況を示す図である。
　図１０は、光カプラの入力光と出力光の状況を示す図である。
　図１１は、光カプラを備えたコヒーレントミキサを示す図である。
　図１２Ａは、方向性結合器型を示す模式図である。
　図１２Ｂは、マッハツェンダ干渉計型を示す模式図である。 FIG. 1 is a top view of the optical coupler according to the first embodiment.
FIG. 2 shows a simulation result in which the refractive index is controlled in the first embodiment.
FIG. 3 is a top view of the optical coupler according to the second embodiment.
FIG. 4 is a top view when the refractive index at both ends of the multimode waveguide is increased in the second embodiment.
FIG. 5 shows a simulation result when the refractive indexes of both end portions of the multimode waveguide are increased in the second embodiment.
FIG. 6 is a top view when the refractive index of the central portion of the multimode waveguide is increased in the second embodiment.
FIG. 7 shows a simulation result when the refractive index of the central portion of the multimode waveguide is increased in the second embodiment.
FIG. 8 is a cross-sectional view of a multimode waveguide according to the second embodiment.
FIG. 9 is a diagram illustrating a state of input light and output light of the optical coupler.
FIG. 10 is a diagram illustrating a state of input light and output light of the optical coupler.
FIG. 11 is a diagram illustrating a coherent mixer including an optical coupler.
FIG. 12A is a schematic diagram showing a directional coupler type.
FIG. 12B is a schematic diagram showing a Mach-Zehnder interferometer type.
　以下に、本発明を実施するための好ましい形態について図面を用いて説明する。但し、以下に述べる実施形態には、本発明を実施するために技術的に好ましい限定がされているが、発明の範囲を以下に限定するものではない。
　〔第１の実施形態〕次に、本実施形態について図面を参照して詳細に説明する。図１は、本実施形態における光カプラ１の上面図である。
　〔構造の説明〕図１に示すように、本実施形態における光カプラ１は、第１入力導波路２と、第２入力導波路３と、マルチモード導波路４と、第１出力導波路５と、第２出力導波路６と、屈折率制御部７とを備えている。
　第１入力導波路２と、第２入力導波路３と、第１出力導波路５と、第２出力導波路６は、シングルモードで光を伝送する導波路である。また、マルチモード導波路４は、マルチモードで光を伝送する導波路である。
　第１入力導波路２と第２入力導波路３は、同じ方向からマルチモード導波路４に接続している。そして第１入力導波路２と第２入力導波路３は、伝搬してきた光をマルチモード導波路４に入力する。なお図１では、マルチモード導波路４は矩形であり、第１入力導波路２と第２入力導波路３とが同じ辺に接続している。
　マルチモード導波路４内で干渉した光は、自己結像を形成し、それぞれ第１出力導波路５と第２出力導波路６とに出力される。なお第１出力導波路５と第２出力導波路６は、第１入力導波路２と第２入力導波路３とがマルチモード導波路４と接続した辺と反対側の辺において、マルチモード導波路４と接続している。
　第１入力導波路２と第１出力導波路５は、マルチモード導波路４を挟んで対向した位置に設けられている。また第２入力導波路３と第２出力導波路６は、マルチモード導波路４を挟んで対向した位置に設けられている。第１入力導波路２から入力された光が、第１出力導波路５から出力される割合を、ここではバーポート出力と呼ぶことにする．また第１入力導波路２から入力された光が、第２出力導波路６から出力される割合を、クロスポート出力と呼ぶこととする．
　マルチモード導波路４は、光の進行方向に対して線対称となるように屈折率分布を形成している。本実施形態では、光の進行方向とは第１入力導波路２、第２入力導波路３から入った光が、全体として進行する方向を言う。具体的には、第１入力導波路２から第１出力導波路５へ向かう方向、あるいは第２入力導波路３から第２出力導波路６へ向かう方向である。
　なお屈折率は、光の進行方向に対しては一様に形成されている。
　屈折率制御部７は、マルチモード導波路４上の中央部、あるいは光の進行方向と沿った側の両端部、またはその近傍に、光の進行方向に延在して設けられている。（図示せず。）つまり屈折率制御部７は、光の進行方向に対して略平行、または線対称となる位置に配置されており、マルチモード導波路４の屈折率の制御を行う。
　〔作用の説明〕次に、本実施形態における動作と作用について説明する。
　第１入力導波路２と第２入力導波路３とを伝搬してきた光は、マルチモード導波路４に入ると、いくつかのモードに展開され、それぞれが互いに干渉し合う。
　一般的に、マルチモード導波路４が第１出力導波路５、第２出力導波路６に出力する光の分岐比は、マルチモード導波路４の幅や長さや、また第１入力導波路２、第２入力導波路３、第１出力導波路５、第２出力導波路６がマルチモード導波路４と接続する入出射位置などで決まる。
　ここでマルチモード導波路４は、屈折率が一様であり、またマルチモード導波路４が下方向などにたわんでいないなど理想的な条件の場合、入力された光を第１出力導波路５と、第２出力導波路６に等しく分岐することができる。
　しかしマルチモード導波路４の幅や長さ、また各入出力導波路と、マルチモード導波路４とが接続する位置関係は、製造工程において決定された後には、容易に変更することが出来ない。つまり製造工程における製作ばらつきや、外的な状況などにより、マルチモード導波路４が第１出力導波路５と第２出力導波路６とに出力する分岐比が変動したとしても、製造後には分岐比を容易に制御することはできなかった。
　そこで本実施形態における光カプラ１は、光の進行方向に対して略平行、または線対称の位置に屈折率制御部７を配置して、マルチモード導波路４の屈折率分布の制御を行う。またマルチモード導波路４の中央部に光の進行方向に延在して設けられた屈折率制御部７により、マルチモード導波路４の屈折率を変化させたシミュレーレーション結果を、図２に示す。
　図２に示すように、マルチモード導波路４の中央部の屈折率を、両端部の屈折率より高くした場合、クロスポート（ｃｒｏｓｓ　ｐｏｒｔ）よりバーポート（ｂａｒ　ｐｏｒｔ）が高くなる。つまり第１入力導波路２から入力された光が、対向している位置に配置されている第１出力導波路５から出力する割合は、斜め方向に配置されている第２出力導波路６から出力される割合に比べて大きくなる。
　なお、図２の横軸は中央部と両端部の屈折率の差を示す。具体的には、「０」は中央部と両端部で屈折率が同じである場合、＋側は中央部の屈折率が両端部より高い場合、−側は逆に中央部の屈折率が両端部より低い場合を示す。
　上記のシミュレーションから、マルチモード導波路４における中央部の屈折率を、両端部より高くした場合、第１入力導波路２から入力された光は、中心部を通過しにくくなり、対向する位置に配置された第１出力導波路５から出力される割合が多くなることが分かる。
　一方、マルチモード導波路４の中央部の屈折率を両端部の屈折率より低くした場合、バーポートよりクロスポートが高くなる。つまり、第１入力導波路２から入力された光が、斜め方向に配置された第２出力導波路６から出力する割合は、対向する位置に配置された第１出力導波路５から出力される割合に比べて大きくなる。
　上記のシュミレーショーンから、マルチモード導波路４における中央部の屈折率を、両端部より低くした場合、第１入力導波路２から入力された光は、中心部を通過して、斜め方向に設けられた第２出力導波路６から多く出力される割合が多くなることが分かる。
　〔効果の説明〕次に本実施形態における効果について説明する。
　本実施形態における光カプラ１は、光の進行方向に対して略平行、または線対称に配置された屈折率制御部７により、マルチモード導波路４の屈折率を制御する。上記構造によりマルチモード導波路４が出力する光の分岐比を容易に制御することができる。
　つまり本実施形態における光カプラ１は、マルチモード導波路４が第１出力導波路５と第２出力導波路６とに出力する信号の分岐比が、製造工程における製作ばらつきや外的要因などで屈折率の分布に歪みが生じたりして等しく分岐できない場合においても、屈折率制御部７でマルチモード導波路４の屈折率を制御することで、分岐比を容易に制御することができる。
　また本実施形態における屈折率制御部７の分岐比制御は、マルチモード導波路４の屈折率分布に歪みが生じた場合に限らない。マルチモード導波路４から第１出力導波路５、第２出力導波路６への結合が２つの出力導波路間で異なる場合など、屈折率分布の歪み以外の要因で分岐比が等分岐からずれた場合においても、屈折率制御部７でマルチモード導波路４の屈折率を制御することで、分岐比を容易に制御することができる。
　〔第２の実施形態〕次に、第２の実施形態について説明を行う。
　本実施形態では、製造工程に起因する応力などの理由で、マルチモード導波路４の分岐比が設計値の１：１からずれた場合に、１：１に修正することを示す。図３は、本実施形態における光カプラ１の上面図である。光カプラ１には、比屈折率差１．５％のシリカ系光導波路を用いる。
　〔構造の説明〕本実施形態における光カプラ１は、屈折率制御部７として薄膜ヒータ８を用いる。それ以外の構造・接続関係は、第１の実施形態と同様であり、第１入力導波路２と、第２入力導波路３と、マルチモード導波路４と、第１出力導波路５と、第２出力導波路６とを備えている。
　薄膜ヒータ８は、マルチモード導波路４上の中央部、あるいは光の進行方向に沿った側の両端部上、またはその近傍の少なくとも一方に、光の進行方向に延在して設けられている。つまり薄膜ヒータ８は、光の進行方向に対して略平行、または線対称となる位置に配置されている。各薄膜ヒータ８は、マルチモード導波路４より長いことが望ましい。なお各薄膜ヒータ８は、延在して設けられている両端部に電極９を備えており、電極９を通して通電することで発熱する。なお薄膜ヒータ８は、３本全部が必要とは限らないが、分かりやすくするために、図３では全部記載している。
　〔作用・効果の説明〕図４に示すように、光カプラ１の製造時点において、第１入力導波路２から入力された光が、斜め方向に位置する第２出力導波路６に比べて、対向する方向に位置する第１出力導波路５から出力する割合が大きい場合を考える。つまりマルチモード導波路４は、バーポートから出力する割合が、クロスポートから出力する割合より大きいと考えられる。（図４では、７：３である。）この分岐比を、以下のようにして１：１に修正する。
　分岐比からみて、製造工程における製作ばらつきや、外的に要因によりマルチモード導波路の屈折率分布に歪みが生じ、その結果、マルチモード導波路４の屈折率は、両端部より中央部のほうが高くなってしまったと考えられる。そのため、図４に示すようにマルチモード導波路４の両端部に設けられた薄膜ヒータ８をそれぞれ通電しマルチモード導波路４の加熱を行うことで、両端部の屈折率を上げて破線で示すように屈折率分布をフラットにする。なお、中央部の薄膜ヒータ８は、不要のため、図４では記載しない。
　両端部の薄膜ヒータ８に通電する電流量を増加し、両端部の温度を上昇させることで、マルチモード導波路４の両端部の屈折率を上げることができる。その結果、第２出力導波路６に出力する割合を増加させ、第１出力導波路５に出力する割合を減少させ分岐比を等しくすることができる。
　図５の横軸は、両端部の屈折率の上昇量を示し、縦軸は分岐比を示す。通電前つまり横軸が０のときの分岐比は７：３であるが、屈折率を１０×１０−４に増加させると分岐比は１：１となる。このように、マルチモード導波路４に加熱を行うことで容易に分岐比を制御することができる。なおシリカ系材料の屈折率温度係数が１０−５／Ｔ程度であるため、常温から１００℃程度加熱すればよい。
　なお、薄膜ヒータ８の直下が最も高温になり、薄膜ヒータ８から離れるに従って温度が低下していく。そのため、マルチモード導波路４内に、その進行方向と垂直方向に温度勾配が形成される。この温度勾配によって屈折率分布を修正して所望の分布にすることができる。
　一方、図７に示すように、上記で述べたケースとは逆に、第１入力導波路２から入力された光が、対向する方向に位置する第１出力導波路５に比べて、斜め方向に位置する第２出力導波路６から出力する割合が大きい場合を考える。つまりマルチモード導波路４は、クロスポートから出力する割合が、バーポートから出力する割合より大きいと考えられる。（図７では、６：４である。）
　分岐比からみて、製造工程における製作ばらつきや、外的に要因によりマルチモード導波路の屈折率分布に歪みが生じ、その結果、マルチモード導波路４の屈折率は、中央部より両端部のほうが高くなってしまったと考えられる。そのため、図６に示すようにマルチモード導波路４は中央部に設けられた薄膜ヒータ８を通電しマルチモード導波路４の中央部を加熱することで、破線で示すように屈折率分布をフラットにする。なお、両端部の薄膜ヒータ８は、不要であるため図６では記載していない。
　その結果、図７に示すように、マルチモード導波路４における中央部の屈折率を上げることで、第１出力導波路５に出力する割合を増加させ、第２出力導波路６に出力する割合を減少させ、分岐比を等しくすることができる。つまり、マルチモード導波路４に加熱を行うことで容易に分岐比を制御することができる。図７は、横軸は中央部の屈折率の上昇量で、縦軸は分岐比を示す。
　また本実施形態における薄膜ヒータ８による分岐比制御は、マルチモード導波路４の屈折率分布に歪みが生じた場合に限らない。マルチモード導波路４から第１出力導波路５、第２出力導波路６への結合が２つの出力導波路間で異なる場合など、屈折率の歪み以外の要因で分岐比が等分岐からずれた場合においても、薄膜ヒータ８でマルチモード導波路４の屈折率を制御することで、分岐比を容易に制御することができる。
　本実施形態では、薄膜ヒータ８を用いて加熱することで、光カプラ１の外部からマルチモード導波路４の屈折率を変化させて分岐比を制御した。しかしマルチモード導波路４の屈折率を外部から変化させることができるのであれば薄膜ヒータ８に限定されず、キャリアプラズマ効果、電気光学効果、磁気光学効果などを用いることができる。
　〔第３の実施形態〕次に、第３の実施形態について説明を行う。図８は、マルチモード導波路４の断面図である。
　〔構造の説明〕本実施形態における光カプラ１は、屈折率制御部７がクラッド厚調整部１０である。それ以外の構造・接続関係は、第１の実施形態と同様であり、第１入力導波路２と、第２入力導波路３と、マルチモード導波路４と、第１出力導波路５と、第２出力導波路６とを備えている。
　図８に示すように、マルチモード導波路４は、少なくとも下クラッド１３とコア１２と上クラッド１１とが下から順に積層されて形成されている。そしてクラッド厚調整部１０は、マルチモード導波路４上に設けられており、上クラッド１１の厚さの調整を行う。
　上クラッド１１の厚さを薄くした場合、マルチモード導波路４の屈折率は下がる。一方、上クラッド１１を厚くした場合、マルチモード導波路４の屈折率は上がる。
　クラッド厚調整部１０は、マルチモード導波路４上の中央部、あるいは光の進行方向とは垂直方向における両端部の少なくとも一方に、光の進行方向に延在して設けられている。つまりクラッド厚調整部１０は、光の進行方向に対して略平行、または線対称となる位置に配置されている。
　クラッド厚調整部１０の具体的な例としては、ＦＩＢ（Ｆｏｃｕｓｅｄ　Ｉｏｎ　Ｂｅａｍ）装置などが考えられる。ＦＩＢ装置は、ガリウムイオンなどの集束イオンビームを上クラッド１１に照射することで、表面の原子を弾き飛ばして削ることができる。つまり、上クラッド１１の厚さを薄くすることができる。
　一方、Ｃｕ、Ａｌ、Ａｕ、などの金属を低エネルギーのイオンビームとして上クラッド１１に照射することで蒸着を行うことができる。つまり上クラッド１１の厚さを厚くすることができる。なおクラッド厚調整部１０は、上クラッド１１の厚さを調整できるものであればこれに限定されない。
　〔作用・効果の説明〕クラッド厚調整部１０は、分岐比からみて製造工程における製作ばらつきや、外的要因によりマルチモード導波路４の屈折率分布に歪みが生じ、その結果、中央の屈折率が高いと推測される場合は、中央部に設けられた上クラッド１１の厚さを薄くする。マルチモード導波路４は、上クラッド１１の厚さを薄くすると屈折率を下げることができるため、中央部の屈折率を下げることで、容易に分岐比を制御することができる。
　なお上記の場合、マルチモード導波路４の中央部に設けられた上クラッド１１の厚さを薄くするだけでなく、マルチモード導波路４の両端部を厚くしてもよい。つまり、マルチモード導波路４の両端部の屈折率を上げることでも、容易に分岐比が等しくなるように制御することができる。
　一方、クラッド厚調整部１０は、分岐比からみて製造工程における製作ばらつきや、外的要因によりマルチモード導波路４の屈折率分布に歪みが生じ、その結果、中央の屈折率が低いと推測される場合は、両端部に設けられた上クラッド１１の厚さを薄くする。マルチモード導波路４は、上クラッド１１の厚さを薄くすると屈折率を下げることができるため、上下両端部の屈折率を下げることで、屈折率分布をフラットにすることができる。
　なお上記の場合、マルチモード導波路４の両端部に設けられた上クラッド１１の厚さを薄くするだけでなく、マルチモード導波路４の中央部を厚くしてもよい。つまり、マルチモード導波路４の中央部の屈折率を上げることでも、容易に分岐比が等しくなるように制御することができる。
　なお、上クラッド１１を光の進行方向と垂直方向に徐々に薄くする、あるいは厚くすると、第２の実施形態で述べた温度勾配と同様な効果を得ることができる。例えば、ＦＩＢ装置で照射イオンビーム量を調整して、上クラッド１１を削る量をマルチモード光導波路４の中央部から端部に向かう方向に多く、あるいは少なくすればよい。
　本実施形態では屈折率を変更する場合、マルチモード導波路４に対して１度追加工を行えばよいので、第２の実施形態に記載の薄膜ヒータ８と比べて、継続的な消費電力を必要とせずに、分岐比の制御を行うことができる。
　つまりクラッド厚調整部１０は、マルチモード導波路４の屈折率を継続的な消費電力を必要とせずに変化させることができるのであればクラッド厚調整部１０に限定されず、不純物によって制御する方法やＵＶ照射によって制御する方法などが考えられる。
　また本実施形態におけるクラッド厚調整部１０による分岐比制御は、マルチもモード導波路４の屈折率分布に歪みが生じた場合に限らない。マルチモード導波路４から第１出力導波路５、第２出力導波路６への結合が２つの出力導波路間で異なる場合など、屈折率の歪み以外の要因で分岐比が等分岐からずれた場合においても、クラッド厚調整部１０でマルチモード導波路４の屈折率を制御することで、分岐比を容易に制御することができる。
　〔分岐比制御方法〕次に、マルチモード導波路４における分岐比の制御方法について説明する。図９、図１０は、光カプラ１の入力光と、出力光の状況を示す上面図である。
　まず図９の場合について説明する。マルチモード導波路４には、第１入力導波路２から強い光が、第２入力導波路３から弱い光が入力された時に、第１出力導波路５から強い光が、第２出力導波路６から弱い光が出力されたとする。
　上記の場合、第１入力導波路２から入力された光の大半が、第１出力導波路５から出力されているため、バーポートの割合が高いと考えられる。つまり図９に示すような状態の場合、マルチモード導波路４は、中央部の屈折率が、両端部に比べて高いことが分かる。
　そこで屈折率制御部７は、上述の追加工を行うことで、マルチモード導波路４の両端部の屈折率を上げる、もしくは中央部の屈折率を下げる。つまり第１入力導波路２から入力された光が、第１出力導波路５に出力する割合を下げて、第２出力導波路６に出力する割合を上げる。その結果、第１出力導波路５から出力される光が弱くなり、第２出力導波路６から出力される光が強くなることで、マルチモード導波路４が出力する光の等分岐を実現することができる。
　次に、図１０の場合について説明する。マルチモード導波路４には、第１入力導波路２から強い光が、第２入力導波路３から弱い光が入力された時に、第１出力導波路５から弱い光が、第２出力導波路６から強い光が出力されたとする。
　上記の場合、第１入力導波路２から入力された光の大半が、第２出力導波路６から出力されているため、クロスポートの割合が高いと考えられる。つまり図１０に示すような状態の場合、マルチモード導波路４は、中央部の屈折率が、両端部に比べて低いことが分かる。
　そこで屈折率制御部７は、上述の追加工を行うことで、マルチモード導波路４の両端部の屈折率を下げる、もしくは中央部の屈折率を上げる。つまり第１入力導波路２から入力された光が第２出力導波路６に出力する割合を下げて、第１出力導波路５に出力する割合を上げる。その結果、第１出力導波路５から出力される光が強くなり、第２出力導波路６から出力される光が弱くすることで、マルチモード導波路４が出力する光の等分岐を実現することができる。
　〔第４の実施形態〕次に、第４の実施形態について説明を行う。図１１は、光カプラ１を備えたコヒーレントミキサを示す図である。
　本実施形態は、光カプラ１とスプリッタ１４とフォトダイオード１５と増幅器１６とを備える。なお光カプラ１は、構造・接続関係、第１~３の実施形態と同様であり、第１入力導波路２と、第２入力導波路３と、マルチモード導波路４と、第１出力導波路５と、第２出力導波路６と、屈折率制御部７とを備えている。
　図１１に示すように、スプリッタ１４は少なくとも２つ設けられている。一方のスプリッタ１４には信号光が入射され、他方のスプリッタ１４には位相情報を復調するために信号光と干渉させる局発光が入射される。そしてスプリッタ１４は、それぞれ入射された信号光と局発光の分岐を行う。
　光カプラ１も同様に、少なくとも２つ設けられている。一方の光カプラ１は、スプリッタ１４により分岐された信号光と局部発信光を干渉させ、等分岐で出力を行う。また他方の光カプラも同様に、スプリッタ１４により分岐された信号光と局部発信光を干渉させ、等分岐で出力する。但し２つのカプラは、スプリッタ１４から光を導く導波路の光路長にλ／４の差がある。
　フォトダイオード１５は、少なくとも４つ設けられている。フォトダイオード１５は、２つの光カプラ１から出力された４つの出力光を電流信号に変換する。
　増幅器は、フォトダイオード１５から出力された電流信号を電圧信号に復元する。
　〔作用・効果の説明〕本実施形態におけるコヒーレントミキサは、上記構成からＴＥ（Ｔｒａｎｓｖｅｒｓｅ　Ｅｌｅｃｔｒｉｃ）光信号とＴＭ（Ｔｒａｎｓｖｅｒｓｅ　Ｍａｇｎｅｔｉｃ）光信号とに偏波分離した信号の位相情報を取り出す９０度光ハイブリッド干渉計に用いることができる。
　９０度光ハイブリッド干渉計を用いたＤＰ−ＱＰＳＫレシーバの性能を左右する重要な特性に、ＣＭＲＲ（Ｃｏｍｍｏｎ−Ｍｏｄｅ　Ｒｅｊｅｃｔｉｏｎ　Ｒａｔｉｏ：同相信号除去比）と呼ばれる特性がある。これは信号を電圧変換する際に使用する差動増幅回路などにおいて、２つの入力に共通する入力信号成分をどれだけ除去できるかの尺度である。
　ＣＭＲＲ性能に影響する特性の一つが、光カプラ２の分岐光強度比である。この光カプラの理想的な分岐特性は、分岐比が１：１となることである。本実施形態における光カプラ１は、屈折率制御部７を設けることで、出力する光の分岐比を１：１に制御することができる。
　つまり本実施形態におけるコヒーレントミキサは、ＣＭＲＲが高い増幅器を実現できる。その結果、高い電圧オフセットを加えることができるため、ショット雑音の影響を大幅に軽減し、高い受信感度を実現することができる。
　以上、本発明を上記実施の形態及び実施例に即して説明したが、本発明は、上記実施の形態、及び実施例の構成のみに限定されるものでなく、本発明の範囲内で当業者であればなし得るであろう各種変形、修正を含むことはもちろんである。
　なお、この出願は、２０１１年１月２１日に出願された日本出願特願２０１１−０１１１６８を基礎とする優先権を主張し、その開示の全てをここに取り込む。 Hereinafter, preferred embodiments for carrying out the present invention will be described with reference to the drawings. However, the preferred embodiments described below are technically preferable for carrying out the present invention, but the scope of the invention is not limited to the following.
[First Embodiment] Next, this embodiment will be described in detail with reference to the drawings. FIG. 1 is a top view of an optical coupler 1 in the present embodiment.
[Description of Structure] As shown in FIG. 1, the optical coupler 1 in this embodiment includes a first input waveguide 2, a second input waveguide 3, a multimode waveguide 4, and a first output waveguide 5. And a second output waveguide 6 and a refractive index control unit 7.
The first input waveguide 2, the second input waveguide 3, the first output waveguide 5, and the second output waveguide 6 are waveguides that transmit light in a single mode. The multimode waveguide 4 is a waveguide that transmits light in multimode.
The first input waveguide 2 and the second input waveguide 3 are connected to the multimode waveguide 4 from the same direction. The first input waveguide 2 and the second input waveguide 3 input the propagated light to the multimode waveguide 4. In FIG. 1, the multimode waveguide 4 is rectangular, and the first input waveguide 2 and the second input waveguide 3 are connected to the same side.
The light that has interfered in the multimode waveguide 4 forms a self-image and is output to the first output waveguide 5 and the second output waveguide 6, respectively. Note that the first output waveguide 5 and the second output waveguide 6 are multimode waveguides on the side opposite to the side where the first input waveguide 2 and the second input waveguide 3 are connected to the multimode waveguide 4. It is connected to the waveguide 4.
The first input waveguide 2 and the first output waveguide 5 are provided at positions facing each other across the multimode waveguide 4. The second input waveguide 3 and the second output waveguide 6 are provided at positions facing each other with the multimode waveguide 4 interposed therebetween. The rate at which the light input from the first input waveguide 2 is output from the first output waveguide 5 is referred to herein as a bar port output. Further, the ratio at which the light input from the first input waveguide 2 is output from the second output waveguide 6 is referred to as a cross-port output.
The multimode waveguide 4 forms a refractive index distribution so as to be line symmetric with respect to the traveling direction of light. In the present embodiment, the light traveling direction refers to the direction in which light entering from the first input waveguide 2 and the second input waveguide 3 travels as a whole. Specifically, the direction is from the first input waveguide 2 to the first output waveguide 5 or from the second input waveguide 3 to the second output waveguide 6.
The refractive index is uniformly formed with respect to the traveling direction of light.
The refractive index control unit 7 extends in the light traveling direction at the central portion on the multimode waveguide 4 or at both end portions on the side along the light traveling direction or in the vicinity thereof. That is, the refractive index control unit 7 is arranged at a position that is substantially parallel or line symmetric with respect to the light traveling direction, and controls the refractive index of the multimode waveguide 4.
[Explanation of Action] Next, the operation and action in this embodiment will be described.
When the light propagating through the first input waveguide 2 and the second input waveguide 3 enters the multimode waveguide 4, it is developed into several modes, and each interferes with each other.
In general, the branching ratio of the light output from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 depends on the width and length of the multimode waveguide 4 and the first input waveguide 2. The second input waveguide 3, the first output waveguide 5, and the second output waveguide 6 are determined by the incident / exit position where the multimode waveguide 4 is connected.
Here, the multimode waveguide 4 has a uniform refractive index, and under ideal conditions such that the multimode waveguide 4 is not bent downward or the like, the input light is converted into the first output waveguide 5. And can equally branch to the second output waveguide 6.
However, the width and length of the multimode waveguide 4 and the positional relationship between the input / output waveguides and the multimode waveguide 4 cannot be easily changed after they are determined in the manufacturing process. . That is, even if the branching ratio output from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 fluctuates due to manufacturing variations in the manufacturing process, external conditions, and the like, branching occurs after manufacturing. The ratio could not be easily controlled.
Therefore, the optical coupler 1 in the present embodiment controls the refractive index distribution of the multimode waveguide 4 by disposing the refractive index control unit 7 at a position that is substantially parallel or line symmetric with respect to the light traveling direction. FIG. 2 shows a simulation result in which the refractive index of the multimode waveguide 4 is changed by the refractive index control unit 7 provided in the central portion of the multimode waveguide 4 so as to extend in the light traveling direction. .
As shown in FIG. 2, when the refractive index at the center of the multimode waveguide 4 is higher than the refractive indexes at both ends, the bar port is higher than the cross port. In other words, the ratio of the light input from the first input waveguide 2 to be output from the first output waveguide 5 disposed at the facing position is from the second output waveguide 6 disposed in the oblique direction. It becomes larger compared to the output ratio.
The horizontal axis in FIG. 2 indicates the difference in refractive index between the central portion and both end portions. Specifically, “0” indicates that the refractive index is the same at the center and both ends, the + side has a higher refractive index at the center than the both ends, and the − side has an opposite refractive index at the center. The case where it is lower than the part is shown.
From the above simulation, when the refractive index of the central portion of the multimode waveguide 4 is made higher than both ends, the light input from the first input waveguide 2 becomes difficult to pass through the central portion, and is positioned at the opposite position. It can be seen that the ratio of the output from the arranged first output waveguide 5 increases.
On the other hand, when the refractive index at the center of the multimode waveguide 4 is lower than the refractive indexes at both ends, the cross port is higher than the bar port. That is, the proportion of the light input from the first input waveguide 2 that is output from the second output waveguide 6 disposed in the oblique direction is output from the first output waveguide 5 that is disposed at the opposite position. Larger than the proportion.
When the refractive index of the central portion of the multimode waveguide 4 is made lower than both ends from the above-mentioned Schirley Shaun, the light input from the first input waveguide 2 passes through the central portion and is provided in an oblique direction. It can be seen that the ratio of a large amount of output from the second output waveguide 6 is increased.
[Explanation of Effects] Next, effects of the present embodiment will be described.
The optical coupler 1 in the present embodiment controls the refractive index of the multimode waveguide 4 by the refractive index control unit 7 disposed substantially parallel to or in line symmetry with the light traveling direction. With the above structure, the branching ratio of light output from the multimode waveguide 4 can be easily controlled.
In other words, in the optical coupler 1 according to the present embodiment, the branching ratio of the signals output from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 depends on manufacturing variations in the manufacturing process and external factors. Even when the refractive index distribution is distorted or cannot be equally branched, the branching ratio can be easily controlled by controlling the refractive index of the multimode waveguide 4 with the refractive index control unit 7.
Further, the branching ratio control of the refractive index control unit 7 in the present embodiment is not limited to the case where the refractive index distribution of the multimode waveguide 4 is distorted. When the coupling from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 is different between the two output waveguides, the branching ratio deviates from equal branching due to factors other than distortion of the refractive index distribution. Even in this case, the branching ratio can be easily controlled by controlling the refractive index of the multimode waveguide 4 with the refractive index control unit 7.
[Second Embodiment] Next, a second embodiment will be described.
In the present embodiment, when the branching ratio of the multi-mode waveguide 4 is deviated from 1: 1 as a design value due to a stress caused by the manufacturing process, it is corrected to 1: 1. FIG. 3 is a top view of the optical coupler 1 in the present embodiment. As the optical coupler 1, a silica-based optical waveguide having a relative refractive index difference of 1.5% is used.
[Description of Structure] The optical coupler 1 in this embodiment uses a thin film heater 8 as the refractive index control unit 7. Other structures and connection relationships are the same as those in the first embodiment, and the first input waveguide 2, the second input waveguide 3, the multimode waveguide 4, the first output waveguide 5, And a second output waveguide 6.
The thin film heater 8 is provided to extend in the light traveling direction at the central portion on the multimode waveguide 4, on both end portions on the side along the light traveling direction, or at least one of the vicinity thereof. . That is, the thin film heater 8 is disposed at a position that is substantially parallel or line symmetric with respect to the light traveling direction. Each thin film heater 8 is preferably longer than the multimode waveguide 4. Each thin film heater 8 includes electrodes 9 at both ends provided to extend, and generates heat when energized through the electrodes 9. Note that all three thin film heaters 8 are not necessarily required, but are illustrated in FIG. 3 for the sake of clarity.
[Description of Functions and Effects] As shown in FIG. 4, at the time of manufacturing the optical coupler 1, the light input from the first input waveguide 2 is compared with the second output waveguide 6 positioned in the oblique direction. Consider a case in which the ratio of output from the first output waveguide 5 located in the opposing direction is large. That is, in the multimode waveguide 4, the ratio of output from the bar port is considered to be larger than the ratio of output from the cross port. (In FIG. 4, it is 7: 3.) This branching ratio is corrected to 1: 1 as follows.
In view of the branching ratio, distortion occurs in the refractive index distribution of the multimode waveguide due to manufacturing variations in the manufacturing process and external factors. As a result, the refractive index of the multimode waveguide 4 is greater at the center than at both ends. It is thought that it became high. Therefore, as shown in FIG. 4, the thin film heaters 8 provided at both ends of the multimode waveguide 4 are energized to heat the multimode waveguide 4, thereby increasing the refractive index at both ends and indicated by broken lines. Thus, the refractive index distribution is made flat. The thin film heater 8 at the center is not shown in FIG.
The refractive index at both ends of the multi-mode waveguide 4 can be increased by increasing the amount of current flowing through the thin film heaters 8 at both ends and raising the temperature at both ends. As a result, the ratio output to the second output waveguide 6 can be increased, the ratio output to the first output waveguide 5 can be decreased, and the branching ratio can be made equal.
The horizontal axis in FIG. 5 indicates the amount of increase in the refractive index at both ends, and the vertical axis indicates the branching ratio. Before energization, that is, when the horizontal axis is 0, the branching ratio is 7: 3, but when the refractive index is increased to 10 × 10 −4 , the branching ratio is 1: 1. In this way, the branching ratio can be easily controlled by heating the multimode waveguide 4. In addition, since the refractive index temperature coefficient of a silica-type material is about 10 <-5 > / T, what is necessary is just to heat about 100 degreeC from normal temperature.
The temperature immediately below the thin film heater 8 becomes the highest temperature, and the temperature decreases as the distance from the thin film heater 8 increases. Therefore, a temperature gradient is formed in the multimode waveguide 4 in the direction perpendicular to the traveling direction. The refractive index distribution can be corrected by this temperature gradient to obtain a desired distribution.
On the other hand, as shown in FIG. 7, contrary to the case described above, the light input from the first input waveguide 2 is obliquely compared with the first output waveguide 5 positioned in the opposite direction. Let us consider a case where the ratio of the output from the second output waveguide 6 positioned at is large. In other words, the multimode waveguide 4 is considered to have a higher rate of output from the cross port than a rate of output from the bar port. (In FIG. 7, it is 6: 4.)
From the viewpoint of the branching ratio, distortion occurs in the refractive index distribution of the multimode waveguide due to manufacturing variations in the manufacturing process and external factors. As a result, the refractive index of the multimode waveguide 4 is greater at both ends than at the center. It is thought that it became high. Therefore, as shown in FIG. 6, the multimode waveguide 4 has a flat refractive index distribution as shown by a broken line by energizing a thin film heater 8 provided at the center to heat the center of the multimode waveguide 4. To. Note that the thin film heaters 8 at both ends are not shown in FIG. 6 because they are unnecessary.
As a result, as shown in FIG. 7, by increasing the refractive index at the center of the multimode waveguide 4, the ratio of output to the first output waveguide 5 is increased and the ratio of output to the second output waveguide 6 is increased. , And the branching ratio can be made equal. That is, the branching ratio can be easily controlled by heating the multimode waveguide 4. In FIG. 7, the horizontal axis represents the amount of increase in the refractive index at the center, and the vertical axis represents the branching ratio.
Further, the branching ratio control by the thin film heater 8 in the present embodiment is not limited to the case where the refractive index distribution of the multimode waveguide 4 is distorted. When the coupling from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 is different between the two output waveguides, the branching ratio deviates from equal branching due to factors other than refractive index distortion. Even in this case, the branching ratio can be easily controlled by controlling the refractive index of the multimode waveguide 4 with the thin film heater 8.
In the present embodiment, the branching ratio is controlled by changing the refractive index of the multimode waveguide 4 from the outside of the optical coupler 1 by heating using the thin film heater 8. However, as long as the refractive index of the multimode waveguide 4 can be changed from the outside, it is not limited to the thin film heater 8, and a carrier plasma effect, an electro-optic effect, a magneto-optic effect, or the like can be used.
[Third Embodiment] Next, a third embodiment will be described. FIG. 8 is a cross-sectional view of the multimode waveguide 4.
[Description of Structure] In the optical coupler 1 in the present embodiment, the refractive index control unit 7 is the cladding thickness adjusting unit 10. Other structures and connection relationships are the same as those in the first embodiment, and the first input waveguide 2, the second input waveguide 3, the multimode waveguide 4, the first output waveguide 5, And a second output waveguide 6.
As shown in FIG. 8, the multimode waveguide 4 is formed by laminating at least a lower cladding 13, a core 12, and an upper cladding 11 in order from the bottom. The clad thickness adjusting unit 10 is provided on the multimode waveguide 4 and adjusts the thickness of the upper clad 11.
When the thickness of the upper clad 11 is reduced, the refractive index of the multimode waveguide 4 is lowered. On the other hand, when the upper cladding 11 is thickened, the refractive index of the multimode waveguide 4 increases.
The clad thickness adjusting portion 10 is provided to extend in the light traveling direction at the central portion on the multimode waveguide 4 or at least one of both end portions in the direction perpendicular to the light traveling direction. That is, the cladding thickness adjusting unit 10 is disposed at a position that is substantially parallel or line-symmetric with respect to the light traveling direction.
As a specific example of the clad thickness adjusting unit 10, an FIB (Focused Ion Beam) apparatus or the like can be considered. By irradiating the upper clad 11 with a focused ion beam such as gallium ions, the FIB apparatus can scrape off atoms on the surface. That is, the thickness of the upper clad 11 can be reduced.
On the other hand, vapor deposition can be performed by irradiating the upper cladding 11 with a metal such as Cu, Al, or Au as a low energy ion beam. That is, the thickness of the upper clad 11 can be increased. The clad thickness adjusting unit 10 is not limited to this as long as the thickness of the upper clad 11 can be adjusted.
[Description of Functions and Effects] The clad thickness adjusting unit 10 is distorted in the refractive index distribution of the multi-mode waveguide 4 due to manufacturing variations in the manufacturing process and external factors in view of the branching ratio. Is estimated to be high, the thickness of the upper clad 11 provided at the center is reduced. Since the multimode waveguide 4 can reduce the refractive index when the thickness of the upper clad 11 is reduced, the branching ratio can be easily controlled by reducing the refractive index at the center.
In the above case, not only the thickness of the upper clad 11 provided in the central portion of the multimode waveguide 4 may be decreased, but both end portions of the multimode waveguide 4 may be increased. That is, the branching ratio can be easily controlled to be equal by increasing the refractive index at both ends of the multimode waveguide 4.
On the other hand, the clad thickness adjusting unit 10 is presumed that the refractive index distribution of the multimode waveguide 4 is distorted due to manufacturing variations in the manufacturing process and external factors in view of the branching ratio, and as a result, the central refractive index is low. In this case, the thickness of the upper clad 11 provided at both ends is reduced. Since the multimode waveguide 4 can reduce the refractive index when the thickness of the upper clad 11 is reduced, the refractive index distribution can be flattened by reducing the refractive indexes at both the upper and lower ends.
In the above case, not only the thickness of the upper clad 11 provided at both ends of the multimode waveguide 4 may be reduced, but the center portion of the multimode waveguide 4 may be increased. That is, the branching ratio can be easily controlled to be equal by increasing the refractive index at the center of the multimode waveguide 4.
When the upper clad 11 is gradually made thinner or thicker in the direction perpendicular to the light traveling direction, the same effect as the temperature gradient described in the second embodiment can be obtained. For example, the irradiation ion beam amount is adjusted by the FIB apparatus, and the amount of cutting the upper clad 11 may be increased or decreased in the direction from the center portion to the end portion of the multimode optical waveguide 4.
In the present embodiment, when the refractive index is changed, it is only necessary to perform additional machining once on the multimode waveguide 4, so that continuous power consumption can be reduced as compared with the thin film heater 8 described in the second embodiment. The branching ratio can be controlled without the need.
In other words, the cladding thickness adjusting unit 10 is not limited to the cladding thickness adjusting unit 10 as long as the refractive index of the multimode waveguide 4 can be changed without requiring continuous power consumption, and is a method of controlling with an impurity. And a method of controlling by UV irradiation.
Further, the branching ratio control by the cladding thickness adjusting unit 10 in the present embodiment is not limited to the case where distortion occurs in the refractive index distribution of the multimode mode waveguide 4. When the coupling from the multimode waveguide 4 to the first output waveguide 5 and the second output waveguide 6 is different between the two output waveguides, the branching ratio deviates from equal branching due to factors other than refractive index distortion. Even in this case, the branching ratio can be easily controlled by controlling the refractive index of the multimode waveguide 4 by the cladding thickness adjusting unit 10.
[Branch Ratio Control Method] Next, a branch ratio control method in the multimode waveguide 4 will be described. FIGS. 9 and 10 are top views showing the states of input light and output light of the optical coupler 1.
First, the case of FIG. 9 will be described. When the multimode waveguide 4 receives strong light from the first input waveguide 2 and weak light from the second input waveguide 3, strong light from the first output waveguide 5 Suppose that weak light is output from 6.
In the above case, since most of the light input from the first input waveguide 2 is output from the first output waveguide 5, it is considered that the ratio of barports is high. That is, in the state shown in FIG. 9, it can be seen that the multi-mode waveguide 4 has a higher refractive index at the center than at both ends.
Therefore, the refractive index control unit 7 increases the refractive index at both ends of the multimode waveguide 4 or decreases the refractive index at the center by performing the above-described additional processing. That is, the rate at which the light input from the first input waveguide 2 is output to the first output waveguide 5 is decreased and the rate at which the light is output to the second output waveguide 6 is increased. As a result, the light output from the first output waveguide 5 becomes weak, and the light output from the second output waveguide 6 becomes strong, thereby realizing equal branching of the light output from the multimode waveguide 4. be able to.
Next, the case of FIG. 10 will be described. When the multimode waveguide 4 receives strong light from the first input waveguide 2 and weak light from the second input waveguide 3, weak light from the first output waveguide 5 Suppose that strong light is output from 6.
In the above case, since most of the light input from the first input waveguide 2 is output from the second output waveguide 6, it is considered that the ratio of cross ports is high. That is, in the state as shown in FIG. 10, it can be seen that the multimode waveguide 4 has a lower refractive index at the center than at both ends.
Therefore, the refractive index control unit 7 lowers the refractive index at both ends of the multimode waveguide 4 or increases the refractive index at the center by performing the above-described additional processing. That is, the rate at which the light input from the first input waveguide 2 is output to the second output waveguide 6 is decreased, and the rate at which the light is output to the first output waveguide 5 is increased. As a result, the light output from the first output waveguide 5 becomes stronger and the light output from the second output waveguide 6 becomes weaker, thereby realizing equal branching of the light output from the multimode waveguide 4. be able to.
[Fourth Embodiment] Next, a fourth embodiment will be described. FIG. 11 is a diagram illustrating a coherent mixer including the optical coupler 1.
The present embodiment includes an optical coupler 1, a splitter 14, a photodiode 15, and an amplifier 16. The optical coupler 1 has the same structure and connection relationship as those of the first to third embodiments. The optical coupler 1 has a first input waveguide 2, a second input waveguide 3, a multimode waveguide 4, and a first output waveguide. A waveguide 5, a second output waveguide 6, and a refractive index control unit 7 are provided.
As shown in FIG. 11, at least two splitters 14 are provided. The signal light is incident on one splitter 14, and the local light that interferes with the signal light is incident on the other splitter 14 in order to demodulate the phase information. The splitter 14 branches the incident signal light and local light.
Similarly, at least two optical couplers 1 are provided. One optical coupler 1 causes the signal light branched by the splitter 14 and the locally transmitted light to interfere with each other, and performs output in equal branches. Similarly, the other optical coupler causes the signal light branched by the splitter 14 and the locally transmitted light to interfere with each other and output in equal branches. However, the two couplers have a difference of λ / 4 in the optical path length of the waveguide that guides light from the splitter 14.
At least four photodiodes 15 are provided. The photodiode 15 converts the four output lights output from the two optical couplers 1 into current signals.
The amplifier restores the current signal output from the photodiode 15 to a voltage signal.
[Description of Actions and Effects] The coherent mixer according to the present embodiment is a 90-degree optical hybrid interference that extracts phase information of a polarization-separated signal from a TE (Transverse Electric) optical signal and a TM (Transverse Magnetic) optical signal from the above configuration. It can be used for counting.
An important characteristic that affects the performance of a DP-QPSK receiver using a 90-degree optical hybrid interferometer is a characteristic called CMRR (Common-Mode Rejection Ratio). This is a measure of how much an input signal component common to two inputs can be removed in a differential amplifier circuit used for voltage conversion of a signal.
One of the characteristics affecting the CMRR performance is the branch light intensity ratio of the optical coupler 2. The ideal branching characteristic of this optical coupler is that the branching ratio is 1: 1. The optical coupler 1 in this embodiment can control the branching ratio of the output light to 1: 1 by providing the refractive index control unit 7.
That is, the coherent mixer in the present embodiment can realize an amplifier having a high CMRR. As a result, since a high voltage offset can be applied, the influence of shot noise can be greatly reduced and high reception sensitivity can be realized.
Although the present invention has been described with reference to the above-described embodiment and examples, the present invention is not limited only to the configuration of the above-described embodiment and examples, and within the scope of the present invention. It goes without saying that various modifications and corrections that can be made by those skilled in the art are included.
This application claims priority based on Japanese Patent Application No. 2011-011168 filed on Jan. 21, 2011, the entire disclosure of which is incorporated herein.
　１　　光カプラ
　２　　第１入力導波路
　３　　第２入力導波路
　４　　マルチモード導波路
　５　　第１出力導波路
　６　　第２出力導波路
　７　　屈折率制御部
　８　　薄膜ヒータ
　９　　電極
　１０　　クラッド厚調整部
　１１　　上クラッド
　１２　　コア
　１３　　下クラッド
　１４　　スプリッタ
　１５　　フォトダイオード
　１６　　増幅器 DESCRIPTION OF SYMBOLS 1 Optical coupler 2 1st input waveguide 3 2nd input waveguide 4 Multimode waveguide 5 1st output waveguide 6 2nd output waveguide 7 Refractive index control part 8 Thin film heater 9 Electrode 10 Clad thickness adjustment part 11 Upper clad 12 core 13 lower clad 14 splitter 15 photodiode 16 amplifier

Claims (10)

  1. 　マルチモード導波路と
    　前記マルチモード導波路に接続し光を入力する少なくとも２つの入力導波路と、
    　前記マルチモード導波路に接続し光を出力する少なくとも２つの出力導波路と、
    　前記マルチモード導波路の屈折率を調整する屈折率制御部とを備え、
    　前記屈折率制御部は、光の進行方向に対して略平行に配置されていることを特徴とする光カプラ。     A multimode waveguide and at least two input waveguides connected to the multimode waveguide for inputting light;
    At least two output waveguides connected to the multimode waveguide and outputting light;
    A refractive index control unit for adjusting the refractive index of the multimode waveguide;
    The optical coupler according to claim 1, wherein the refractive index control unit is disposed substantially parallel to a traveling direction of light.
  2. 　前記屈折率制御部は、前記マルチモード導波路上の中央部、あるいは、前記マルチモード導波路の端部のうち光の進行方向に沿った側の両端部またはその近傍、の少なくとも一方に、光の進行方向に延在して設けられていることを特徴とする請求項１に記載の光カプラ。 The refractive index control unit is configured to transmit light to at least one of a central portion on the multimode waveguide, or both ends of the multimode waveguide along the light traveling direction or the vicinity thereof. The optical coupler according to claim 1, wherein the optical coupler is provided so as to extend in a traveling direction.
  3. 　前記屈折率制御部は、前記マルチモード導波路の屈折率を外部から調整することを特徴とすることを特徴とする請求項２に記載の光カプラ。 3. The optical coupler according to claim 2, wherein the refractive index control unit adjusts the refractive index of the multimode waveguide from the outside.
  4. 　前記屈折率制御部は、前記マルチモードを加熱する薄膜ヒータであることを特徴とする請求項３に記載の光カプラ。 4. The optical coupler according to claim 3, wherein the refractive index control unit is a thin film heater that heats the multimode.
  5. 　前記薄膜ヒータは、前記マルチモード導波路より長いことを特徴とする請求項４に記載の光カプラ。 The optical coupler according to claim 4, wherein the thin film heater is longer than the multimode waveguide.
  6. 　前記屈折率制御部は、前記マルチモード導波路に追加工を行うことで屈折率を調整することを特徴とすることを特徴とする請求項２に記載の光カプラ。 3. The optical coupler according to claim 2, wherein the refractive index control unit adjusts the refractive index by performing additional processing on the multimode waveguide.
  7. 　前記屈折率制御部は、前記マルチモード導波路のクラッドの厚さを調整するクラッド厚調整部であることを特徴とする請求項５に記載の光カプラ。 6. The optical coupler according to claim 5, wherein the refractive index control unit is a cladding thickness adjusting unit that adjusts a thickness of a cladding of the multimode waveguide.
  8. 　前記マルチモード導波路は、光の進行方向に対して屈折率が一様であることを特徴とする請求項１乃至７に記載の光カプラ。 8. The optical coupler according to claim 1, wherein the multi-mode waveguide has a uniform refractive index with respect to a traveling direction of light.
  9. 　請求項１~８のいずれかに記載の前記光カプラを備えたことを特徴とするコヒーレントミキサ。 A coherent mixer comprising the optical coupler according to any one of claims 1 to 8.
  10. 　入力導波路からマルチモード導波路に入力された光が、
    前記入力導波路と斜め方向の位置に設けられた出力導波路より、
    前記入力導波路と対向した位置に設けられた出力導波路から出力する割合が大きい場合は、前記マルチモード導波路の両端部の屈折率を増加させ、
    　前記入力導波路からマルチモード導波路に入力された光が、
    前記入力導波路と対向した位置に設けられた出力導波路より、
    前記入力導波路と斜め方向の位置に設けられた出力導波路から出力する割合が大きい場合は、前記マルチモード導波路の中央部の屈折率を増加させることを特徴とする分岐比制御方法。     The light input from the input waveguide to the multimode waveguide is
    From the output waveguide provided at a position oblique to the input waveguide,
    If the ratio of output from the output waveguide provided at a position facing the input waveguide is large, increase the refractive index at both ends of the multimode waveguide,
    The light input from the input waveguide to the multimode waveguide is
    From an output waveguide provided at a position facing the input waveguide,
    A branching ratio control method, comprising: increasing a refractive index at a central portion of the multimode waveguide when a ratio of output from an output waveguide provided obliquely to the input waveguide is large.
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