JP2017142301A - Strict natural mode multi/demultiplexer and strict natural mode multiplex transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a strict natural mode multi/demultiplexer and a strict natural mode multiplex transmission system with which it is possible to selectively multiplex or demultiplex a strict natural mode of multi-mode fibers.SOLUTION: A strict natural mode multi/demultiplexer 1 comprises: an LP mode multi/demultiplexer 3 for multiplexing/demultiplexing an LP mode in order to selectively excite a strict natural mode and separate a transmission channel from a received strict natural mode on the basis of a relationship of linear combination between the LP mode and the strict natural mode; a polarization rotator 4 for rotating the polarization direction of propagation light in one optical path of a plurality of incidence/emission ends of the LP mode multi/demultiplexer; and an inphase/antiphase generation separator 2 for generating/separating an inphase component and an antiphase component based on the relationship of linear combination between the LP mode component and strict natural mode component of the LP mode multi/demultiplexer and polarization rotator.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a strict eigenmode multiplexer / demultiplexer and a strict eigenmode multiplexing transmission system.

  Experimental examples of mode multiplexing transmission using higher-order modes of several mode fibers are known. Conventionally, an LP (Linearly Polarized) mode is used as a mode used for mode multiplexing transmission.

The strictly eigenmodes of the cylindrical optical fiber are strictly the HE _lm and EH _lm (TE _0m and TM _0m when 1 = 0) modes (Non-patent Document 1). However, since the propagation constants of the HE _{l + 1, m} and EH _{l-1, m} modes are in a pseudo-degenerate state, weak waveguide approximation and LP mode have been proposed without using strict eigenmodes (Non-Patent Document 2). ).

  Further, since the laser light incident on the fiber incident end is also linearly polarized light, the LP mode is used as a transmission channel in recent ultra-high capacity mode multiplex transmission (Non-patent Document 3).

  On the other hand, it has been pointed out that even if the LP mode (linearly polarized light) is excited at the incident end, it is no longer the LP mode (linearly polarized light) after transmission (Non-Patent Documents 4 to 7). Although the transmission characteristics according to the LP mode are unknown, since the transmission channels can be separated by the MIMO process, the transmission mode has not been strictly examined.

E. Snitzer, “Cylindrical dielectric waveguide modes,” J. Opt. Soc. Am., Vol.51, no.5, pp.491-498, 1961. D. Gloge, “Weakly guiding fibers,” Appl. Opt., Vol.10, no.10, pp.2252-2258, 1971. D. Soma, et al., “2.05 Peta-bit / s Super-Nyquist-WDM SDM Transmission Using 9.8-km 6-mode 19-core Fiber in Full C band,” 29th European Conference on Optical Communication (ECOC2015), Valencia , PDP.3.2, Oct. 1, 2015. H. Kogelnik and P. J. Winzer, “Modal Birefringence in Weakly Guiding Fibers,” J. Lightwave Technol., Vol.30, no.14, pp.2240-2245, 2015. J. von Hoyningen-Huene, et al., “LCoS-based mode shaper for few-mode fiber,” Optics Express, vol.21, no.15, pp.18097-18110, 2013. E-L. Lim, et al., “Vector Mode effects in Few Moded Erbium Doped Fiber Amplifiers,” OFC / NFOEC2013, OTu3G.2, Anaheim, 2013. Richerdson, “Fiber Amplifiers for SDM Systems,” OFC / NFOEC2013, Anaheim, OTu3G.1, 2013. Jun Liu, et al., “Demonstration of Few Mode Fiber Transmission Link Seeded by a Silicon Photonic Integrated Optical Vortex Emitter,” 29th European Conference on Optical Communication (ECOC2015), Valencia, P.2.18, Sept. 29, 2015. N. Hanzawa, et al., “Mode multi / demultiplexing with parallel waveguide for mode division multiplexed transmission,” Opt. Exp. Vol. 22, no. 24, pp. 29321-29330, Dec.1, 2014. E. Ip, et al., “88 × 3 × 112-Gb / s WDM Transmission over 50 km of Three-Mode Fiber with Inline Few-Mode Fiber Amplifier,” ECOC2011, Geneva, Th.13.C.2, 2011 .

The LP mode is an approximate mode having an electromagnetic field distribution of linearly polarized light, and is a strictly eigenmode such as an HE mode or an EH mode (in the case of the LP _1m mode, the HE _2m ^even mode and the TM _0m mode, and the HE _2m ^odd mode and the TE _The linearly polarized light can be formed by superimposing ( _{0 m} mode).

  Conventionally, mode multiplex transmission is performed using the LP mode without using a strict eigenmode as a transmission channel due to the following technical background.

  (A) The laser beam of the light source incident from the incident end of the number mode fiber is generally linearly polarized light. Since LP mode is always excited when linearly polarized light is incident on a few mode fiber using laser light as a light source, it is difficult to selectively excite only the strict eigenmode.

The only TE mode and TM mode exciters with a diffraction grating on the side wall of a microring resonator have been reported (Non-patent Document 8). It operates only at the resonant wavelength, and the HE ₂₁ mode cannot be multiplexed / demultiplexed.

  (B) The strict eigenmode of the constituent elements constituting the LP mode by superposition has a substantially degenerated propagation constant. Therefore, on the premise that when the LP mode configured as a superposition of these strict eigenmodes at the entrance end is excited, transmission of the mode multiplexed transmission using the LP mode is performed on the premise that the electromagnetic field distribution is maintained as it is. Is going. A mode in which the electromagnetic field distribution is different but the propagation constant is the same is called a degenerate mode.

  The following points can be cited as problems of the above-described LP mode excitation and mode multiplex transmission caused by LP mode excitation.

  (C) Due to the group delay difference between the strict eigenmodes constituting the LP mode, a pulse width broadening due to mode dispersion occurs in mode multiplexing transmission.

Although the propagation constants of the strict eigenmodes constituting the LP mode are substantially equal, strictly speaking, there is a very small difference of about 10 ⁻⁵ to 10 ^{−7 with} respect to the value of the propagation constant. Due to this minute difference, the phase relationship of the superposition of the strict eigenmodes changes, so that the electromagnetic field distribution changes with a period of several tens of centimeters to several meters, resulting in an electromagnetic field distribution that can no longer be called the LP mode. Due to this change in the electromagnetic field distribution, a signal mixed with strictly eigenmodes, which is no longer linearly polarized light, is output from the output end of the mode multiplexed optical fiber transmission. The phenomenon that the electromagnetic field distribution is not linearly polarized occurs even if the number mode fiber is ideally circular in cross section and the optical axis is linear.

For example, in a three-mode fiber, it has been conventionally considered that four modes degenerated in the LP ₁₁ mode are transmitted in addition to the orthogonal polarization mode in the LP ₀₁ mode. However, the LP ₁₁ mode is actually represented by a linear combination of four exact eigenmodes: HE ₂₁ ^even , TM ₀₁ , HE ₂₁ ^odd , and TE ₀₁ . For example, the LP _11-x ^even mode having even symmetry with respect to the x-axis coordinate of the LP ₁₁ mode and linearly polarized in the x-axis direction is a linear combination of the HE ₂₁ ^even mode and the TM ₀₁ mode, which are strictly eigenmodes. Since there is a propagation constant difference between the HE ₂₁ ^even mode and the TM ₀₁ mode, ^{even if the} LP _11-x ^even mode is excited at the incident end, the electromagnetic field distribution changes with a period of several m to several tens of m (non-patent document). Literature 4, non-patent literature 5).

  (D) It is difficult to accurately separate the signal in which the electromagnetic field distribution is not maintained and the strict eigenmode is mixed by the demultiplexing of the mode demultiplexer that separates the LP mode.

  Therefore, in the conventional mode multiplexing optical fiber transmission experiment using several mode fibers, the LP mode component at the incident end is restored by digital signal processing such as MIMO processing of the mode mixed signal at the output end of the mode demultiplexer. . MIMO processing cannot be realized by real-time processing because the processing load increases as the number of modes increases.

The inventors of the present application have also found that there is a problem in the analysis of the coupling between modes.
(E) In mode multiplex transmission using LP mode excitation, not only the intensity distribution changes, but also the polarization state of the electric field changes locally to become elliptically polarized light depending on the location in the cross section. For this reason, when the LP mode is used, an erroneous result is brought about in the analysis result of connection or inter-mode coupling.

  As shown in the above (c) to (e), in order to eliminate the problem of mode multiplexing transmission caused by LP mode excitation, selective excitation of exact eigenmode for mode multiplexing transmission using exact eigenmode Is required.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned conventional problems and selectively multiplex or demultiplex strictly eigenmodes of several mode fibers.

  Another object of the present invention is to construct a strict eigenmode multiplexer / demultiplexer that enables selective multiplexing or demultiplexing of strict eigenmodes of several mode fibers, and mode multiplex transmission using the strict eigenmode.

  The present invention uses a strictly eigenmode as a base mode used for mode multiplexing in mode multiplexing transmission in which a plurality of modes are propagated through one optical fiber core and each propagation mode is used as a transmission channel. A strict eigenmode multiplexer / demultiplexer for multiplexing and demultiplexing modes, and a strict eigenmode multiplex transmission system including a strict eigenmode multiplexer / demultiplexer.

  The strict eigenmode multiplexer / demultiplexer of the present invention excites a strict eigenmode among the higher-order modes of a few mode fiber at the incident end and transmits it to the number mode fiber, and demultiplexes the strict eigenmode at the output end. . By using the strict eigenmode, it is possible to prevent the change of the electromagnetic field distribution during the propagation through the several-mode fiber and the mixing of the LP mode components at the output end caused by the change of the electromagnetic field distribution.

  The strict eigenmode multiplexer / demultiplexer of the present invention is based on the linear coupling relationship between the LP mode and the strict eigenmode, based on the selective excitation of the strict eigenmode and the transmission channel from the received strict eigenmode. In order to perform separation, an LP mode multiplexer / demultiplexer that multiplexes / demultiplexes the LP mode, a polarization rotator that rotates the polarization direction of propagating light, an LP mode multiplexer / demultiplexer, and the polarization rotator And an in-phase / anti-phase generator / separator that is arranged in the preceding stage and performs linear combination conversion between the LP mode component and the strict eigenmode component.

  The LP mode multiplexer / demultiplexer has a function of demultiplexing incident signal light into a plurality of LP mode components, and a function of multiplexing a plurality of LP mode components to excite a plurality of LP modes in one outgoing light. It is a component. The demultiplexing of the LP mode can be performed by the propagation constant difference between the LP modes included in the incident light, or by the difference in the electromagnetic field distribution in the case of the degenerated LP mode. In the case of the degenerated LP mode, the difference in the electromagnetic field distribution.

The polarization rotator is a component having a function of rotating the electromagnetic field direction of the LP mode. The LP mode rotates the polarization direction between x-polarized light and y-polarized light by passing through the polarization rotator. For example, polarization rotation is performed between the LP _11-x ^even component and the LP _11-y ^even component.

  The in-phase / anti-phase generator / separator is a component that performs conversion based on a linear coupling relationship between an LP mode component that enters and exits the LP mode multiplexer / demultiplexer and a strict eigenmode component. Since the LP mode is represented by a linear combination of strictly eigenmodes, by combining a plurality of LP mode components with a fixed phase relationship based on the relationship of the linear combination, the LP mode component can be combined according to the combination of LP mode components to be combined. While it is possible to obtain a strict eigenmode, it is possible to separate and obtain single mode waveguide output light of a plurality of LP mode components corresponding to the strict eigenmode to be demultiplexed by demultiplexing the strict eigenmode. it can.

  The strict eigenmode multiplexer / demultiplexer of the present application uses the LP mode multiplexer / demultiplexer as the LP mode multiplexer when the strict eigenmode is selectively excited. Strict eigenmode multiplexers split incident light into two output ports in phase or in opposite phase by an in-phase / anti-phase generator / separator corresponding to the incident light, and polarize one of the two output ports. After the polarization direction is rotated by the rotator, the two outgoing lights are multiplexed by the LP mode multiplexer to excite the strict eigenmode, and the strict eigenmode is excited in the several mode optical fiber.

  On the other hand, the strict eigenmode multiplexer / demultiplexer of the present application uses the LP mode multiplexer / demultiplexer as an LP mode demultiplexer when a transmission channel is separated from a plurality of strict eigenmodes transmitted through several mode fibers. The exact eigenmode multiplexer / demultiplexer demultiplexes the LP mode formed by the linear combination of the exact eigenmode by the LP mode demultiplexer and separates it into a plurality of LP mode components. After rotating the polarization direction with a polarization rotator, the two LP mode components are separated with an in-phase / anti-phase generation separator to obtain an output corresponding to a strictly eigenmode.

[Configuration of in-phase / negative-phase generator / separator]
The in-phase / negative-phase generation separator of the present application can be configured in a plurality of forms. Each form of the in-phase / anti-phase generator / separator is a bidirectional 2-input 2-output in-phase / anti-phase multiplexing / demultiplexing circuit that generates or separates the in-phase component and the anti-phase component of incident light.

(First form of in-phase / negative-phase generation separator)
In the first form of the in-phase / anti-phase generator / separator, the first set of two single-mode waveguides having different propagation constants and the second set of two single-mode waveguides having the same propagation constant are combined. It is composed of an asymmetric X junction branch circuit that intersects at an intersection.

  In the asymmetric X junction branch circuit, in one traveling direction, the phase relationship of the light waves separated into the two single mode waveguides in the second set of two single mode waveguides is in phase or out of phase. Two output lights are output according to the waveguide in which light is incident in the two waveguides of the first set. Further, in the other traveling direction, the two single mode waveguides of the first set simultaneously enter the in-phase component or the combined light of the opposite phase components into the two single mode waveguides of the second set. The light is selectively output from one waveguide according to the in-phase component or the anti-phase component of the two input lights.

(Second form of in-phase / negative-phase generation separator)
The second form of the in-phase / anti-phase generator / separator has a 2-input 1-output or 1-input 2-output port, and one single-mode waveguide having two inputs is changed to a two-mode waveguide by a tapered structure. An asymmetric directional coupler that couples the primary mode of the two-mode waveguide and the fundamental mode of the other waveguide, and is connected to the output side of the two-mode waveguide of the asymmetric directional coupler. It is comprised with an optical circuit provided with a branch junction circuit.

  This optical circuit excites a fundamental mode or a primary mode in a two-mode waveguide according to the incident port in the traveling direction from the 2-port side to the 1-port side, and branches the emitted light by an equal branching junction circuit. Then, the outgoing light is branched to the two output ports in phase or in opposite phase. In addition, the optical circuit converts the in-phase component or the anti-phase component of the two input lights joined by the equi-branching junction circuit in the traveling direction from the 1 port side to the 2 port side of the asymmetric directional coupler according to the phase component. A fundamental mode corresponding to the in-phase component or a primary mode corresponding to the anti-phase component is excited in the two-mode waveguide, and is selectively emitted from the two output ports by the asymmetric directional coupler.

(Third embodiment of in-phase / negative-phase generation separator)
In the third embodiment of the in-phase / anti-phase generation separator, an incident optical path incident from the high refractive index side and an incident incident from the low refractive index side with respect to the same point on the boundary surface between two dielectrics having different refractive indexes. The optical device includes an optical element including an optical path and an outgoing optical path that exits from the high refractive index side and an outgoing optical path that exits from the low refractive index side. A prism can be used as the optical element.

  In the configuration using this optical element, an incident beam incident from one of the two incident optical paths has a phase relationship between two light waves propagating through the two outgoing optical paths according to the incident incident optical path. Two light waves having the same phase or opposite phases are emitted from the two outgoing optical paths. In addition, the configuration using this optical element combines the in-phase component or the anti-phase component of two light waves incident from the incident light paths of the two incident light paths, and 2 2 in accordance with the phase components of the two incident light waves. The light is selectively emitted from one of the two outgoing light paths.

[Port configuration of common-phase / negative-phase generator / separator]
The in-phase / anti-phase generator / separator can have a plurality of forms of port configurations for entering and exiting a plurality of incident light or outgoing light.

(First form of port configuration)
The first form of the port configuration is a configuration in which a polarization multiplexer / demultiplexer is connected to the in-phase / negative-phase generator / separator as a component of a port for entering / exiting a plurality of incident light or outgoing light.

  The polarization multiplexer / demultiplexer selectively inputs or outputs x-polarized light and y-polarized light with respect to the in-phase / negative-phase generator / separator. The polarization multiplexer / demultiplexer has a function of a polarization demultiplexer (or polarization separator) and a function of a polarization multiplexer. For example, in a 2-input 2-output polarization multiplexer / demultiplexer, if light is incident from one port with 2 input ports, the light is x-polarized component and y-polarized component (or TE-polarized component and TM-polarized component). , The function of a polarization demultiplexer (or polarization separator) that separates and outputs the x-polarized component at one of the two output ports and the y-polarized component at the other port, and two input ports If the x-polarized light is incident from one side and the y-polarized light is incident from the other side, a polarization multiplexer function of combining and outputting both polarization components to one output port is provided.

  In a configuration using a bidirectional 2-input 2-output in-phase / anti-phase multiplexer / demultiplexer as an in-phase / anti-phase generator / separator, by connecting a polarization multiplexer / demultiplexer to an input port to which light is incident, x-polarized light or y-polarized light can be selectively input to the input port of the in-phase / anti-phase generator / separator, and the in-phase / in-phase generator is generated by connecting a polarization multiplexer / demultiplexer to the output port for emitting light. The x-polarized light or the y-polarized light output from the output port of the separator can be selectively emitted. The configuration including this polarization multiplexer / demultiplexer can be applied to the first form and the second form of the in-phase / negative-phase generation / separation unit.

(Second form of port configuration)
The second form of the port configuration is such that two in-phase / anti-phase multiplexing / demultiplexing circuits are provided, and each in-phase / anti-phase multiplexing / demultiplexing circuit is provided with a port component for entering / exiting a plurality of incident light or outgoing light. is there.

  The second form of the port configuration is a configuration comprising two in-phase / anti-phase multiplexing / demultiplexing circuits, connected to one port of two in-phase / anti-phase multiplexing / demultiplexing circuits, and polarized between x-polarized light and y-polarized light Polarization rotator that converts 90 degrees (polarization 90 degree converter) and two in-phase / anti-phase multiplexing / demultiplexing circuits, connected to the polarization converter within the ports of one in-phase / anti-phase multiplexing / demultiplexing circuit A polarization multiplexer that polarizes and combines the output light of the port that is not connected and the output light of the polarization converter connected to the port of the other in-phase / anti-phase multiplexing / demultiplexing circuit, It is the structure connected with LP mode multiplexer / demultiplexer. The configuration including the second configuration of the port configuration can be applied to the first to third configurations of the in-phase / negative-phase generation separator.

(Strict eigenmode multiplex transmission system)
The strict eigenmode multiplexing transmission system of the present invention includes the strict eigenmode multiplexer / demultiplexer of the present invention and a number mode fiber, and the strict eigenmode multiplexer / demultiplexer is connected to the transmission end and the reception end of the number mode fiber. Is done.

  The strict eigenmode multiplexer / demultiplexer at the transmitting end functions as a mode multiplexer, and excites the strict eigenmode in the several mode fiber. The number mode fiber performs mode multiplexing transmission in a strict eigenmode. The strict eigenmode multiplexer / demultiplexer at the receiving end functions as a mode demultiplexer, separates the strict eigenmode received from the several-mode fiber, converts it into an optical signal, and outputs it.

  According to the strict eigenmode multiplex transmission system of the present invention, since the mode multiplex transmission of the several mode fiber in the strict eigenmode, there is no change in the electromagnetic field distribution during the propagation, and the number mode fiber of the transmission line is ideal. If the crosstalk is low, no crosstalk occurs in principle at the output end.

  According to the strict eigenmode multiplex transmission system of the present invention, by performing mode multiplex transmission using the strict eigenmode, the fundamental crosstalk at the output end is prevented as in the mode multiplex transmission using the conventional LP mode. be able to.

  In addition, since the group speeds of the strict eigenmodes are different from each other, the mode dispersion due to the synthesis of the strict eigenmodes occurs and the compensation becomes complicated when the LP mode is used, but according to the strict eigenmode multiplex transmission system of the present invention, Dispersion compensation can be easily performed by separating the transmission into strictly eigenmodes.

  As described above, according to the strict eigenmode multiplexer / demultiplexer of the present invention, the strict eigenmodes of several mode fibers can be selectively multiplexed or demultiplexed.

  Also, mode multiplexing transmission can be configured using a strict eigenmode multiplexer / demultiplexer that enables selective multiplexing or demultiplexing of strict eigenmodes of several mode fibers and a strict eigenmode.

It is a figure for demonstrating the outline of the exact eigenmode multiplexer / demultiplexer of this invention, and an exact eigenmode multiplexing transmission system. It is a figure for demonstrating the exact eigenmode multiplexer / demultiplexer using 1st Embodiment of the common-phase / inverse phase production | generation separator of this invention. It is a figure for demonstrating 1st Embodiment of the in-phase reverse phase production | generation separator of this invention. It is a figure for demonstrating operation | movement of the exact eigenmode multiplexer / demultiplexer using 1st Embodiment of the in-phase / inverse phase production | generation separator of this invention. It is a figure for demonstrating the exact eigenmode multiplexer / demultiplexer using 2nd Embodiment of the in-phase reverse phase production | generation separator of this invention. It is a figure for demonstrating operation | movement of the exact eigenmode multiplexer / demultiplexer using 2nd Embodiment of the in-phase / inverse phase production | generation separator of this invention. It is a figure for demonstrating the exact eigenmode multiplexer / demultiplexer using 3rd Embodiment of the in-phase / inverse phase production | generation separator of this invention. It is a figure for demonstrating 3rd Embodiment of the in-phase reverse phase production | generation separator of this invention. It is a figure for demonstrating operation | movement of the exact eigenmode multiplexer / demultiplexer using 3rd Embodiment of the in-phase production | generation reverse_phase | separation separator of this invention. It is a figure for demonstrating operation | movement of the exact eigenmode multiplexer / demultiplexer using 3rd Embodiment of the in-phase production | generation reverse_phase | separation separator of this invention. It is a figure for demonstrating the 2nd form of the port structure of the exact eigenmode multiplexer / demultiplexer of this invention. It is a figure for demonstrating the structural example of a LP mode multiplexer / demultiplexer. It is a figure for demonstrating the structural example of a LP mode multiplexer / demultiplexer.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Hereinafter, an outline of the strict eigenmode multiplexer / demultiplexer and the strict eigenmode multiplexing transmission system of the present invention will be described with reference to FIG. 1, and the first to third embodiments of the strict eigenmode multiplexer / demultiplexer of the present invention will be described. 2 to 4, 5 to 6, and 7 to 10, and the second mode of the port configuration will be described with reference to FIG. 11, and the LP mode coupling used in the strict eigenmode multiplexer / demultiplexer will be described. A configuration example of the duplexer will be described with reference to FIGS.

  A schematic configuration of the strict eigenmode multiplexer / demultiplexer and the strict eigenmode multiplexing transmission system will be described with reference to FIG. FIG. 1A shows a schematic configuration of a strict eigenmode multiplexer / demultiplexer, and FIGS. 1B and 1C show a schematic configuration of a strict eigenmode multiplex transmission system.

(Schematic configuration of strictly eigenmode multiplexer / demultiplexer)
In FIG. 1A, an exact eigenmode multiplexer / demultiplexer 1 includes an in-phase / anti-phase generator / separator 2 that outputs two output lights in phase or in phase to two output ports corresponding to an incident port, and LP A polarization rotator 4 that rotates the polarization direction of the mode component, and an LP mode multiplexer / demultiplexer 3 that excites different LP modes depending on the incident port.

  The in-phase / anti-phase generator / separator 2 outputs two outgoing lights so that the light incident on one of the two incident ports is in phase or opposite phase to the two outgoing ports according to the incident port, Alternatively, when light is incident in opposite directions, the in-phase component or the opposite-phase component of the two incident lights is selectively coupled to the two ports.

The polarization rotator 4 is a component that rotates the polarization direction of propagating light. The polarization rotator 4 rotates the polarization direction of the propagating light and changes the polarization state of x-polarized light and y-polarized light. For example, by passing through the polarization rotator 4, the polarization state is bi-directionally converted between the LP _11-x ^even component and the LP _11-y ^even component.

  The LP mode multiplexer / demultiplexer 3 is a component that performs conversion based on a linear coupling relationship between the LP mode component and the strict eigenmode component. Since the LP mode and the strict eigenmode are in a linear combination relationship, the strict eigenmode can be obtained by combining a plurality of LP modes with a fixed phase relationship. In this LP mode multiplexing, by changing the combination of LP modes to be combined, a strict eigenmode corresponding to the combination can be obtained. In the combination of the LP modes, the polarization state of the LP mode component to be combined is adjusted by changing the polarization direction with the polarization rotator 4. On the other hand, by demultiplexing the strict eigenmode, it is first demultiplexed into a plurality of LP mode components constituting the strict eigenmode, and then separated into output light corresponding to the strict eigenmode by the in-phase and antiphase generator / separator. it can.

(Schematic configuration of strictly eigenmode multiplex transmission system)
FIGS. 1B and 1C show a schematic configuration of a strict eigenmode multiplex transmission system. In the strict eigenmode multiplex transmission system 10, the strict eigenmode multiplexer / demultiplexer 1a on the transmission side and the strict eigenmode multiplexer / demultiplexer 1b on the reception side are connected to both ends of the number mode fiber 11, and the strict eigenmode multiplex transmission system 1b receives the transmission signal. The eigenmode multiplexer / demultiplexer 1a excites the number mode fiber 11 in the strict eigenmode, the number mode fiber performs mode multiplexing transmission in the strict eigenmode, and the strict eigenmode multiplexer / demultiplexer 1b that receives the strict eigenmode is strict. The eigenmode is separated, converted into a received optical signal, and output. The strict eigenmode multiplexer / demultiplexer 1a at the transmitting end functions as a mode multiplexer, and the strict eigenmode multiplexer / demultiplexer 1b at the receiving end functions as a mode demultiplexer.

  When the exact eigenmode is selectively excited, the exact eigenmode multiplexer / demultiplexer 1 uses the in-phase / anti-phase generator / separator 2 as the in-phase / in-phase generator 2a, and outputs two outputs corresponding to the incident ports. Two output lights whose phase relation of incident light is in phase or opposite phase are output, and one polarization direction of the two output lights is rotated by the polarization rotator 4a, and then the LP mode multiplexer / demultiplexer 3a. It combines and excites a strict eigenmode, and excites a strict eigenmode in the number mode fiber 11.

  On the other hand, when the transmission channel is separated from the strict eigenmode, the strict eigenmode multiplexer / demultiplexer 1 uses the in-phase / anti-phase generator / separator 2 as the in-phase / anti-phase separator 2b. The exact eigenmode multiplexer / demultiplexer 1b demultiplexes the exact eigenmode by the LP mode multiplexer / demultiplexer 3b and separates it into two LP mode components. One of the separated two LP mode components is polarized wave rotator 4b. Then, the two LP mode components are combined by the in-phase / anti-phase separator 2b to obtain an output corresponding to the strict eigenmode.

[Strict eigenmode multiplexer / demultiplexer configuration]
The exact eigenmode multiplexer / demultiplexer of the present application can be configured in a plurality of forms according to different configurations of the in-phase / anti-phase generation separator. The in-phase and anti-phase generator / separator can be composed of a bidirectional 2-input 2-output in-phase / anti-phase multiplexing / demultiplexing circuit that multiplexes / demultiplexes the in-phase component and anti-phase component of two LP mode components. Depending on the configuration of the anti-phase multiplexing / demultiplexing circuit, a plurality of forms can be provided.

  Hereinafter, the structural example is demonstrated about the multiple form of an in-phase / negative phase production | generation separator. The first and second embodiments show a waveguide type in-phase and out-of-phase generator / separator, and the third embodiment shows a space beam type in-phase and out-of-phase generator / separator.

(First form of in-phase / negative-phase generation separator)
A first embodiment of the in-phase / negative-phase generation separator will be described with reference to FIGS.
The first strict eigenmode multiplexer / demultiplexer 1A includes an in-phase / antiphase generator / separator 2A, an LP mode multiplexer / demultiplexer 3A, and a polarization rotator (Pol. Rotator) 4A. The in-phase / anti-phase multiplexing / demultiplexing circuit constituting the in-phase / anti-phase generator / separator 2A has the same propagation constant as the first waveguide set including two single mode waveguides 2Aa and 2Ab having different propagation constants. It is composed of an asymmetric X merge branch circuit in which a pair of second waveguides including two single mode waveguides 2Ac and 2Ad intersect at one intersection.

Incident light or outgoing light is led out to the waveguide 2Aa through the polarization multiplexer 5Ao to the HE ₂₁ ^odd port and TM ₀₁ port, and to the waveguide 2Ab, the polarization multiplexer 5Ae. Incident light or outgoing light is led out to and from the HE ₂₁ ^even port and the TE ₀₁ port.

  FIG. 2 shows a configuration in which the propagation constants are made different by making the width of the waveguide 2Aa wider than the width of the waveguide 2Ab in the first set of waveguides. The propagation constant may be varied depending on the thickness or refractive index of the waveguide in addition to the width of the waveguide.

  FIG. 3 is a diagram for explaining generation and separation of the in-phase component and the anti-phase component by the asymmetric X junction branch circuit. FIG. 3 shows a case where the difference in propagation constant is configured by the difference in waveguide width.

  FIGS. 3A and 3B show a case where the light is incident on one waveguide of the first set of waveguides having different waveguide widths. As shown in FIG. 3A, incident light introduced from the waveguide 2Aa having a wide waveguide width is branched into two outputs in phase with the waveguides 2Ac and 2Ad. On the other hand, as shown in FIG. 3B, the incident light introduced from the waveguide 2Ab having a narrow waveguide width is branched into two outputs having opposite phases with respect to the waveguides 2Ac and 2Ad.

  FIGS. 3C and 3D show the case where the traveling direction of the above-described example is reversed, and the incident light enters two waveguides of the second waveguide set having the same waveguide width. ing. As shown in FIG. 3C, when in-phase light is incident from the waveguide 2Ac and the waveguide 2Ad, light in which the in-phase component is coupled to the waveguide 2Aa having a wide waveguide width is emitted. On the other hand, as shown in FIG. 3D, when light of opposite phase is incident from the waveguide 2Ac and the waveguide 2Ad, light having the opposite phase component coupled to only the waveguide 2Ab having a narrow waveguide width is emitted. The

  Due to the generation and separation of the in-phase component and the anti-phase component described above, the asymmetric X-merging / branching circuit is configured so that the two waveguides 2Ac and 2Ad in the second set are in phase or in-phase 2 in one traveling direction. One output light is selectively output as a phase component in accordance with a waveguide into which light is incident in the two waveguides 2Aa and 2Ab of the first set.

  Further, in the other traveling direction, the two waveguides 2Aa and 2Ab in the first set are changed in accordance with the phase relationship between the two incident lights input from the two waveguides 2Ac and 2Ad in the second set. The two waveguides 2Aa and 2Ab are selectively output from one waveguide according to the phase component of the in-phase component or the anti-phase component.

There is a linear coupling relationship between the LP mode and the strict eigenmode, which is expressed by the following equations (1) and (2).

  Equation (1) shows the conversion from the strict eigenmode to the LP mode, and Equation (2) shows the conversion from the LP mode to the strict eigenmode.

  FIG. 4 is a schematic diagram of one of the two output lights generated by the in-phase / negative-phase generator / separator 2A, passed through the polarization rotator 4A, and then incident on the LP mode multiplexer / demultiplexer 3A. An example is shown in which an eigenmode is selectively excited and a few mode fiber is excited in a strict eigenmode.

(TM ₀₁ mode)
From the equation (2), the TM ₀₁ mode of the strict eigenmode is expressed by the following equation (3) by LP _11-x ^even and LP _11-y ^odd of the LP mode.
TM ₀₁ = [LP _11-x ^even + LP _11-y ^odd ] / √2 (3)

FIG. 4A shows a state of multiplexing in the TM ₀₁ mode performed based on the relationship of linear combination represented by Expression (3).

The in-phase / negative-phase generator / separator 2A splits x-polarized incident light introduced from the waveguide 2Aa having a wide waveguide width into two to generate two output lights having the same phase relationship, and one of the output lights is It is output from the waveguide 2Ac of the second set of waveguides as an LP _11-x ^even component. The propagating light in the other waveguide 2Ad is rotated in the polarization direction by the polarization rotator 4A and becomes an LP _11-y ^odd component.

LP mode demultiplexer 3A is a port for LP ₁₁ ^{the even} modes introduced LP _11-x ^{the even} components from the waveguide 2Ac, in the output light polarization rotator from waveguide 2Ad port for LP ₁₁ ^odd mode By introducing the polarization-rotated LP _11-y ^odd component in phase, the exact eigenmode of the TM ₀₁ mode is excited.

(HE ₂₁ ^even mode)
From the equation (2), the HE ₂₁ ^even mode of the strict eigenmode is expressed by the following equation (4) by LP _11-x ^even and LP _11-y ^odd of the LP mode.
HE ₂₁ ^even = − [LP _11−x ^even −LP _11−y ^odd ] / √2 (4)

FIG. 4B shows a combined state of HE ₂₁ ^even that is performed based on the relationship of the linear combination represented by Expression (4).

The in-phase / anti-phase generator / separator 2A splits x-polarized incident light introduced from the waveguide 2Ab having a narrow waveguide width into two to generate two output lights having opposite phases, and LP _11-x ^{The even} component and the −LP _11-x ^odd component are output from the waveguides 2Ac and 2Ad of the second set of waveguides. The -LP11 _-x ^odd component of the waveguide 2Ad is rotated in the polarization direction by the polarization rotator 4A to become a -LP11 _-y ^odd component.

The LP mode multiplexer / demultiplexer 3A introduces the LP _11-x ^even component into the LP ₁₁ ^even mode port, and introduces the -LP _11-y ^odd component into the LP ₁₁ ^odd mode port in the reverse phase, thereby reducing the HE. ^Excites the exact eigenmode of ₂₁ ^even mode.

(TE ₀₁ mode)
From the equation (2), the strict eigenmode TE ₀₁ mode is expressed by the following equation (5) by the LP mode LP _11-y ^even and LP _11-x ^odd .
TE ₀₁ = [LP _11-y ^even −LP _11-x ^odd ] / √2 (5)

FIG. 4C shows a multiplexing state of TE ₀₁ that is performed based on the relationship of linear combination represented by Expression (5).

The in-phase / anti-phase generator / separator 2A splits the y-polarized incident light introduced from the waveguide 2Ab having a narrow waveguide width into two to generate two output lights having opposite phases and LP _11-y ^{The even} component and the −LP _11-y ^odd component are output from the waveguides 2Ac and 2Ad of the second set of waveguides. The -LP11 _-y ^odd component of the waveguide 2Ad is rotated in the polarization direction by the polarization rotator 4A to become a -LP11 _-x ^odd component.

The LP mode multiplexer / demultiplexer 3A introduces the LP _11-y ^even component into the LP ₁₁ ^even mode port, and introduces the -LP _11-x ^odd component into the LP ₁₁ ^odd mode port in the reverse phase, thereby obtaining the TE. _{Excites the 01} mode strictly eigenmode.

(HE ₂₁ ^odd mode)
From equation (2), the HE ₂₁ ^odd mode of the strict eigenmode is expressed by the following equation (6) by LP _11-y ^even and LP _11-x ^odd of the LP mode.
HE ₂₁ ^odd = [LP _11-y ^even + LP _11-x ^odd ] / √2 (6)

FIG. 4D shows a HE ₂₁ ^odd mode multiplexing state performed based on the relationship of the linear combination represented by Expression (6).

The in-phase / negative-phase generator / separator 2A branches y-polarized incident light introduced from the waveguide 2Aa having a wide waveguide width into two to generate two output lights having the same phase relationship, and LP _11-y ^even The component is output from the waveguide 2Ac of the second set of waveguides. The polarization direction of the LP _11-y ^odd component of the other waveguide 2Ad is rotated by the polarization rotator 4A to become an LP _11-x ^odd component.

The LP mode multiplexer / demultiplexer 3A introduces the LP _11-y ^even component into the LP ₁₁ ^even mode port, and introduces the LP _11-x ^odd component into the LP ₁₁ ^odd mode port in the same phase, thereby ^causing HE ₂₁ ^odd Excites the exact eigenmode of the mode.

The polarization multiplexer (PBS) 5A shown in FIG. 2 is connected to the two input ports of the in-phase / antiphase generator / separator 2A of the strict eigenmode multiplexer / demultiplexer 1A in order to multiplex four strict eigenmodes. In this configuration, four incident lights are introduced. The polarization multiplexer 5Ao has two ports, a TM ₀₁ input port that introduces incident light that multiplexes TM ₀₁ components, and a HE ₂₁ ^odd input port that introduces incident light that multiplexes HE ₂₁ ^odd components. It is connected to one input port of the in-phase / negative-phase generator / separator 2A. The polarization multiplexer 5Ae has two ports, a TE ₀₁ input port for introducing incident light for multiplexing the TE ₀₁ component and a HE ₂₁ ^even input port for introducing incident light for multiplexing the HE ₂₁ ^even component. Is connected to the other input port of the in-phase / negative-phase generation separator 2A.

(Second form of in-phase / negative-phase generation separator)
A second embodiment of the in-phase / negative-phase generator / separator will be described with reference to FIGS.
In FIG. 5, the first strict eigenmode multiplexer / demultiplexer 1B includes an in-phase / anti-phase generator / separator 2B, an LP mode multiplexer / demultiplexer 3B, and a polarization rotator 4B.

The second form of in-phase reverse-phase generator separator 2B is two inputs and one output, or 1 asymmetrical directional coupler 2B ₁ having an input 2 output port of a two-mode waveguides asymmetric directional coupler 2B ₁ The optical circuit includes an equal branching junction circuit 2B ₂ connected to a certain one port side. The asymmetric directional coupler 2B ₁ has two waveguides, ie, a main waveguide and a sub-waveguide, having different waveguide widths and thicknesses. The asymmetric directional coupler 2B ₁ has a coupling portion in parallel. It is an optical circuit that shifts.

The optical circuit, in the traveling direction from the second port side of the asymmetric directional coupler 2B ₁ toward the first port side, selectively fundamental mode or primary mode main waveguide is a bimodal waveguide according to the incident port excited to be branched at equal branching and joining circuit 2B _2. The equally-branching / merging circuit 2B ₂ has a phase relationship between the output lights emitted to the two output ports in the same phase when the mode excited in the main waveguide is the fundamental mode and in the opposite phase when the mode is the primary mode. To output. Further, the optical circuit is configured to change the in-phase component or the anti-phase component of the two input lights merged by the equal branching / merging circuit 2B ₂ in the traveling direction from the 1 port side to the 2 port side of the asymmetric coupler according to the phase component. Then, the light is selectively emitted from the two ports.

Polarization multiplexer shown in FIG. 5 (PBS) 5B, in order to multiplex the four strict eigenmodes, the phase reverse-phase generator separator 2B relative to the two input ports of the asymmetric directional coupler 2B ₁ In this configuration, four incident lights are introduced. The polarization multiplexer 5Bo has two ports, a TM ₀₁ input port that introduces incident light that multiplexes TM ₀₁ components, and a HE ₂₁ ^odd input port that introduces incident light that multiplexes HE ₂₁ ^odd components. connecting one input port of an asymmetric directional coupler 2B _1. The polarization multiplexer 5Be has two ports, a TE ₀₁ input port for introducing incident light for multiplexing the TE ₀₁ component and a HE ₂₁ ^even input port for introducing incident light for multiplexing the HE ₂₁ ^even component. and it is connected to the other input port of the asymmetric directional coupler 2B _1.

FIG. 6 shows two output lights branched by an in-phase / antiphase generator / separator 2B composed of an asymmetric directional coupler 2B ₁ and an equi-branch merging circuit 2B _2, and one of the output lights is passed through the polarization rotator 4B. After that, an example is shown in which two output lights are incident on the LP mode multiplexer / demultiplexer 3B to selectively excite the strict eigenmode and excite the several mode fiber in the strict eigenmode.

(TM ₀₁ mode)
FIG. 6A shows a state of multiplexing in the TM ₀₁ mode performed based on the relationship of linear combination represented by Expression (3).

The asymmetric directional coupler 2B ₁ introduces x-polarized incident light from the main waveguide, and excites the fundamental mode in a portion that becomes a two-mode waveguide by the tapered structure of the main waveguide. The equal branching / merging circuit 2B ₂ splits the incident light into two to generate two in-phase output lights. One of the two output lights is an LP _11-x ^even component, and the other is an LP _11-x ^odd component. The polarization direction of the LP _11-x ^odd component is rotated by the polarization rotator 4B to become the LP _11-y ^odd component.

The LP mode multiplexer / demultiplexer 3B introduces an LP _11-x ^even component into the LP ₁₁ ^even mode port, and introduces an LP _11-y ^odd component into the LP ₁₁ ^odd mode port in the same phase, thereby obtaining a TM ₀₁ mode. Excites the exact eigenmode of.

(HE ₂₁ ^even mode)
FIG. 6B shows a combined state of the HE ₂₁ ^even mode performed based on the relationship of the linear combination represented by the equation (4).

The asymmetric directional coupler 2B ₁ introduces x-polarized incident light from the sub-waveguide and couples it to the primary mode of the portion that has become a two-mode waveguide by the main waveguide taper structure. The equal branching / merging circuit 2B ₂ splits the incident light into two to generate two output lights having opposite phases. One of the two output lights is an LP _11-x ^even component, and the other is a -LP _11-x ^even component. The -LP11 _-x ^odd component is rotated in the polarization direction by the polarization rotator 4B to be in the -LP11 _-y ^odd mode.

The LP mode multiplexer / demultiplexer 3B introduces the LP _11-x ^even component into the LP ₁₁ ^even mode port, and introduces the -LP _11-y ^odd component into the LP ₁₁ ^odd mode port in the reverse phase, thereby reducing the HE. ^Excites the exact eigenmode of ₂₁ ^even mode.

(TE ₀₁ mode)
FIG. 6C shows a TE ₀₁ mode multiplexing state performed based on the relationship of linear combination represented by Expression (5).

The asymmetric directional coupler 2B ₁ introduces y-polarized incident light from the sub-waveguide, and couples it to the primary mode of the portion that has become the two-mode waveguide by the tapered structure of the main waveguide. The equal branching / merging circuit 2B ₂ splits the incident light into two to generate the LP _11-y ^even component and the -LP _11-y ^odd component having opposite phases. One -LP11 _-y ^odd component is rotated in the polarization direction by the polarization rotator 4B to become a -LP11 _-x ^odd component.

The LP mode multiplexer / demultiplexer 3A introduces the LP _11-y ^even component into the LP ₁₁ ^even mode port, and introduces the -LP _11-x ^odd component into the LP ₁₁ ^odd mode port in the reverse phase, thereby obtaining the TE. _Combines strictly eigenmodes of ₀₁ mode.

(HE ₂₁ ^odd mode)
FIG. 6D shows a combined state of the HE ₂₁ ^odd mode performed based on the relationship of the linear combination represented by Expression (6).

The asymmetric directional coupler 2B ₁ introduces y-polarized incident light from the main waveguide, and excites the fundamental mode in the portion that becomes a two-mode waveguide by the tapered structure of the main waveguide. The equal branching / merging circuit 2B ₂ splits the incident light into two to generate an LP _11-y ^even component and an LP _11-y ^odd component having the same phase. One LP _11-y ^odd component is rotated in the polarization direction by the polarization rotator 4B to become an LP _11-x ^odd component.

The LP mode multiplexer / demultiplexer 3A introduces the LP _11-y ^even component into the LP ₁₁ ^even mode port, and introduces the LP _11-x ^odd component into the LP ₁₁ ^odd mode port in the same phase, thereby ^causing HE ₂₁ ^odd Excites the exact eigenmode of the mode.

(Third embodiment of in-phase / negative-phase generation separator)
A third embodiment of the in-phase / negative-phase generation separator will be described with reference to FIGS.
The third strict eigenmode multiplexer / demultiplexer 1C includes an in-phase / antiphase generator / separator (optical elements 2C ₁ , 2C ₂ ), an LP mode multiplexer / demultiplexer 3C, and a polarization rotator 4C. This form is a configuration example for multiplexing and demultiplexing light beams.

In FIG. 7, the third form of the in-phase / negative-phase generation separator 2 </ b> C has an incident optical path and a low refractive index incident from the high refractive index side with respect to the same point on the boundary surface between two dielectrics having different refractive indexes. In this configuration, _two optical elements 2C ₁ and 2C ₂ having an incident optical path incident from the side, an outgoing optical path exiting from the high refractive index side, and an outgoing optical path exiting from the low refractive index side are provided. A prism can be used as the optical elements 2C ₁ and 2C ₂ .

In the configuration using the optical elements 2C ₁ and 2C ₂ , output light that is incident on one of the two incident optical paths is output to the two outgoing optical paths in accordance with the incident optical path. Are output from the two outgoing optical paths so that the phase relationship of Further, the configuration of the optical elements 2C ₁ and 2C ₂ is such that the in-phase component or the anti-phase component of the two input lights incident simultaneously from the incident optical paths of the two incident optical paths is changed according to the phase components of the two incident input lights. The light is selectively emitted from one of the two outgoing light paths.

7, the optical element 2C _1, the incident beam from the input port of the TM ₀₁ mode is incident on the high refractive index side of the optical element 2C ₁ via the mirror, from the input port of the HE ₂₁ ^{the even} modes the incident beam is incident on the low refractive index side of the optical element 2C ₁ via the mirror. The beam emitted from the high refractive index side of the optical element 2C _1, the variable phase controller and via the mirror is incident on the polarization multiplexer 5C _2, the beam emitted from the low refractive index side of the optical element 2C _1, The light enters the polarization multiplexer 5C ₁ via the mirror and the polarization rotator (polarization 90-degree converter) 4C ₁ . The polarization rotators 4C ₁ and 4C ₂ are elements that convert the polarization by 90 degrees, and can be composed of λ / 2 plates.

On the other hand, the optical element 2C _2, the incident beam from the input port of the HE ₂₁ ^odd mode, and enters the high refractive index side of the optical element 2C ₂ via the mirror, the incident beam from the input port of the TE ₀₁ and it enters the low refractive index side of the optical element 2C ₂ through the mirror. The beam emitted from the high refractive index side of the optical element 2C _2, the polarization rotator (polarization 90 degrees converter) 4C ₂ incident on the polarization multiplexer 5C ₂ via a low-refractive optical element 2C ₂ the beam emitted from the rate side, and enters the polarization multiplexer 5C ₁ via the mirror and phaser.

The polarization multiplexer 5C ₁ receives the light emitted from the low refractive index side of the optical element 2C ₁ and the optical element 2C ₂ and emits the light to the LP ₁₁ ^odd mode port of the LP mode multiplexer / demultiplexer 3C. On the other hand, the polarization multiplexer 5C ₂ receives the light emitted from the high refractive index side of the optical element 2C ₁ and the optical element 2C ₂ and emits the light to the LP ₁₁ ^even mode port of the LP mode multiplexer / demultiplexer 3C. .

  An example of generation of in-phase components and anti-phase components with respect to multiplexing / demultiplexing by the optical element will be described with reference to FIG. FIG. 8A shows a state where incident light is branched into two outputs in the same phase, and FIG. 8B shows a state where incident light is branched into two outputs in the opposite phase.

(Branch of common-mode output)
In FIG. 8 (a), from the first input port (Input port # 1) via the mirror 2C _b, toward the high refractive index side to the low refractive index side of the boundary surface 2C _c of the optical element 2C _a Incident beam is incident. The incident beam is divided into a reflected beam and a transmitted beam at the boundary surface 2C _c of the optical element 2C _a . In entering the boundary surface 2C _c, transmitted and reflected beams when incident from the high refractive index side are in phase. The reflected beam is incident to the variable phase controller 2C _e via mirrors 2C _d, is from the first output port after the phase adjustment (Output port # 1) emitted as a first output beam.

On the other hand, the transmitted beam is incident on the fixed phase shifter 2C _j via the mirrors 2C _h and 2C _i, and is emitted as a second output beam from the second output port (Output port # 2) after the phase change that has been fixed. The The phase of the first outgoing beam and the second outgoing beam can be adjusted by the variable phase shifter 2C _e . Thereby, the first outgoing beam and the second outgoing beam are branched into in-phase components by making the incident beam incident on the boundary surface 2C _c of the optical element 2C _a from the high refractive index side to the low refractive index side.

(Reverse phase output branch)
8 (b), the second input port (Input port # 2) mirror from _2C f, through 2C _g, the high refractive index side from the low refractive index side of the boundary surface 2C _c of the optical element 2C _a An incident beam is incident on the. The incident beam is divided into a reflected beam and a transmitted beam at the boundary surface 2C _c of the optical element 2C _a . In entering the boundary surface 2C _c, transmitted and reflected beams when incident from the low refractive index side opposite phases. Transmitted beam is incident to the variable phase controller 2C _e via mirrors 2C _d, is from the first output port after the phase adjustment (Output port # 1) emitted as a first output beam. On the other hand, the reflected beam enters the fixed phase shifter 2C _j via the mirrors 2C _h and 2C _i, and is output as a second outgoing beam from the second output port (Output port # 2) after the fixed phase change. The

The phase of the first outgoing beam and the second outgoing beam can be adjusted by the variable phase controller 2C _e . As a result, the first output beam and the second output beam are converted into two outputs of opposite phases by making the incident beam incident on the boundary surface 2C _c of the optical element 2C _a from the low refractive index side to the high refractive index side. Branch off.

When the optical element 2Ca is configured by _a prism, when the power reflectance and the power transmittance are designed to be 50%, when the refractive index of the prism is n ₁ = 2.00, in the case of TE polarized light. The prism apex angle θ _p is θ _p = 2θ ₁ so that the incident angle θ ₁ of the high refractive index side incident beam is 61.53 degrees and the incident angle θ ₂ of the low refractive index side incident beam is 17.56 degrees. = Set to 123.06 degrees.

  FIG. 9 shows two outgoing lights branched by the in-phase / anti-phase generator / separator 2C using in-phase / anti-phase branching by reflection and transmission in the optical element to the polarization rotator 4C and the LP mode multiplexer / demultiplexer 3C. An example is shown in which a strict eigenmode is selectively excited by being incident, and a few mode fiber is excited in the strict eigenmode.

(TM ₀₁ mode)
FIG. 9A shows a state of multiplexing in the TM ₀₁ mode performed based on the relationship of linear combination represented by Expression (3).

The optical element 2C ₁ introduces an x-polarized incident beam from the TM ₀₁ mode input port, and branches the incident beam into an in-phase LP _11-x ^even component and LP _11-x ^odd component. One LP _11-x ^odd component is rotated by 90 degrees in the polarization direction by the polarization rotator 4C ₁ and becomes an LP _11-y ^odd component.

The LP mode multiplexer / demultiplexer 3C introduces the LP _11-y ^odd component whose polarization direction is rotated 90 degrees by the polarization rotator 4C ₁ to the LP ₁₁ ^odd mode port via the polarization multiplexer 5C _1. and, the LP _11-x ^even components through the polarization multiplexer 5C ₂ by introducing in phase to the LP ₁₁ ^{the even} mode port, to excite the exact eigenmodes TM ₀₁ mode.

(HE ₂₁ ^even mode)
FIG. 9B shows a combined state of the HE ₂₁ ^even mode performed based on the relationship of the linear combination represented by the equation (4).

The optical element 2C ₁ introduces an x-polarized incident beam from the HE ₂₁ ^even mode input port, and branches the incident beam into an LP _11-x ^even component and an −LP _11-x ^odd component having opposite phases. One -LP _11-x ^odd component, polarization direction is rotated 90 degrees -LP _11-y ^odd components by the polarization rotator 4C _1.

The LP mode multiplexer / demultiplexer 3C transmits the −LP _11-y ^odd component whose polarization direction is rotated by 90 degrees by the polarization rotator 4C ₁ to the LP ₁₁ ^odd mode port via the polarization multiplexer 5C _1. introduced, by introducing in phase to the port for LP ₁₁ ^{the even} modes LP _11-x ^even components through the polarization multiplexer 5C _2, to excite the exact eigenmodes HE ₂₁ ^{the even} mode.

(TE ₀₁ mode)
FIG. 10A shows a TE ₀₁ mode combining state performed based on the relationship of linear combination represented by Expression (5).

The optical element 2C ₂ introduces an x-polarized incident beam from the TE ₀₁ mode input port, and branches the incident beam into an anti-phase LP _11-x ^even component and -LP _11-x ^odd component. One LP _11-x ^even component is rotated by 90 degrees in the polarization direction by the polarization rotator 4C ₂ and becomes an LP _11-y ^even component.

The LP mode multiplexer / demultiplexer 3C introduces the LP _11-y ^even component whose polarization direction is rotated by 90 degrees by the polarization rotator 4C ₂ to the LP ₁₁ ^even mode port via the polarization multiplexer 5C _2. Then, the strict eigenmode of the TE ₀₁ mode is excited by introducing the -LP _{11 -x} ^odd component into the LP ₁₁ ^odd mode port through the polarization multiplexer 5C ₁ in a reverse phase.

(HE ₂₁ ^odd mode)
FIG. 10B shows a combined state of the HE ₂₁ ^odd mode that is performed based on the relationship of the linear combination represented by Expression (6).

The optical element 2C ₂ introduces an x-polarized incident beam from the HE ₂₁ ^odd input port, and branches the incident beam into an in-phase LP _11-x ^even component and LP _11-x ^odd component. One LP _11-x ^even component is rotated by 90 degrees in the polarization direction by the polarization rotator 4C ₂ and becomes an LP _11-y ^even component.

The LP mode multiplexer / demultiplexer 3C introduces the LP _11-y ^even component whose polarization direction is rotated by 90 degrees by the polarization rotator 4C ₂ to the LP ₁₁ ^even mode port via the polarization multiplexer 5C _2. and, the LP _11-x ^odd component via the polarization multiplexer 5C ₁ by introducing in phase to the LP ₁₁ ^odd mode port, to excite the exact eigenmodes HE ₂₁ ^odd mode.

(Port configuration of in-phase / negative-phase generator / separator)
Strict eigenmode multiplexer / demultiplexer equipped with 2-input 2-output in-phase / antiphase generator / separator Port configuration for multiplexing / demultiplexing four modes of TM ₀₁ mode, HE ₂₁ ^even mode, HE ₂₁ ^odd mode, and TE ₀₁ mode As described above, in the first and second embodiments of the in-phase and out-of-phase generator / separator, a polarization multiplexer is connected to the in-phase / in-phase generator / separator as a component of a port for entering / exiting a plurality of incident light or outgoing light The third form of the in-phase / inverse-phase generator / separator having the first port configuration is provided with two in-phase / anti-phase multiplexing / demultiplexing circuits as a port configuration for entering / exiting a plurality of incident light or outgoing light. The port configuration is provided.

  In the first port configuration, the polarization multiplexer / demultiplexer selectively inputs or outputs x-polarized light and y-polarized light with respect to the in-phase / negative-phase generator / separator. In a configuration using a bi-directional 2-input 2-output in-phase / anti-phase multiplexing / demultiplexing circuit as an in-phase / anti-phase generation / separator, by connecting a polarization multiplexer to an input port to which light is incident, x Polarized light or y-polarized light is selectively input to the input port of the in-phase / anti-phase generator / separator, and the output of the in-phase / anti-phase generator / separator is connected to the output port for emitting light. X-polarized light or y-polarized light output from the port is selectively emitted.

  The third form of the in-phase / anti-phase generator / separator is the prism design in which the incident angle with respect to the TE polarized light (s-polarized light or x-polarized light), the apex angle of the prism, and the incident light with respect to the TM polarized light (p-polarized light or y-polarized light). Since the angle and the apex angle of the prism are different, it is not possible to apply a configuration in which a polarization multiplexer is added in front of the in-phase / anti-phase generation separator as in the first port configuration.

  The second port configuration is a configuration applied to the third form of the in-phase / anti-phase generation / separation device, and is a port configuration including two in-phase / anti-phase multiplexing / demultiplexing circuits. The second port configuration will be described with reference to FIG.

The strict eigenmode multiplexer / demultiplexer 1D is a polarization 90 that converts 90 degrees of polarization between x-polarized light and y-polarized light with respect to one port of the two in-phase / anti-phase multiplexer / demultiplexers 2D ₁ , 2D _2. Polarization rotators 4D ₁ and 4D ₂ are connected as degree converters, and in two in-phase / anti-phase multiplexing / demultiplexing circuits 2D ₁ and 2D ₂ , polarization rotation is performed within the ports of one in-phase / anti-phase multiplexing / demultiplexing circuit. Polarization multiplexers 5D ₁ and 5D for polarization multiplexing / demultiplexing the input / output light of the port not connected to the multiplexer and the input / output light of the polarization rotator connected to the port of the other in-phase / anti-phase multiplexing / demultiplexing circuit ₂ and connected to the polarization multiplexers 5D ₁ and 5D ₂ and the LP mode multiplexer / demultiplexer 3D. Although FIG. 11 shows the configuration of polarization multiplexing, it can also be applied to the configuration of polarization demultiplexing.

  The second port configuration is not limited to the third form of the in-phase / negative-phase generator / separator, but can be applied to the first and second forms of the in-phase / negative-phase generator / separator.

(Configuration example of LP mode multiplexer / demultiplexer)
Next, a configuration example of the LP mode multiplexer / demultiplexer will be described with reference to FIGS.
A configuration shown in FIG. 12 is known as a waveguide type LP ₁₁ mode multiplexer / demultiplexer (Non-patent Document 9). The LP mode multiplexer / demultiplexer 3 includes asymmetric directional couplers 3 ₁ and 3 ₃ and a mode rotator 3 ₂ .

In the asymmetric directional couplers 3 ₁ and 3 ₃ of FIG. 12, the propagation light of the fundamental mode incident from the ports 1 and 3 is coupled only to the LP ₁₁ ^even mode (the TE ₁₀ mode in the corresponding mode in the waveguide). and, the mode rotator _{3 2} (in the corresponding mode in the waveguide _{TE 01} mode) LP ₁₁ ^{the even} modes LP ₁₁ ^odd mode with the polarization direction (corresponding _{TE 10} mode in the mode in the waveguide) into.

Fundamental mode incident on Port2 bind to LP ₀₁ mode is the fundamental mode of a few-mode fiber as it propagates to the exit end, LP ₁₁ in the fundamental mode which is incident on Port3 asymmetric directional coupler 3 ₁ ^{the even} mode (guiding The corresponding mode in the waveguide is coupled to the TE ₁₀ mode). This LP ₁₁ ^{the even} modes (the corresponding mode in the waveguide TE ₀₁ mode) LP ₁₁ ^odd mode by the mode rotator 3 ₂ is converted to, no fundamental mode undergo a change. Further, the LP ₁₁ ^even mode is multiplexed by entering the fundamental mode from Port ₁ , and as a result, the three modes of the LP ₀₁ mode, the LP ₁₁ ^even mode, and the LP ₁₁ ^odd mode are multiplexed. Thus, _{LP 01} mode port Port2 causes the incident _{LP 01} components, Port3 is LP ₁₁ ^odd mode port for entering the LP ₁₁ ^odd components, Port1 operates as LP ₁₁ ^{the even} modes port for entering the LP ₁₁ ^{the even} component .

A configuration shown in FIG. 13 is known as a spatial beam type LP ₁₁ mode multiplexer / demultiplexer (Non-patent Document 10).

13 shows a configuration in which the LP mode multiplexer 3 ₄ and LP mode demultiplexer ₃₅ across the transmission path constituted by the beam splitter and the phase plate. The phase plate has a phase of 0 or π by partially changing the thickness. LP mode multiplexer 3 ₄ is a mode multiplexer for multiplexing the LP mode, the LP mode demultiplexer ₃₅ is de-mode multiplexer for demultiplexing the LP mode.

In each of the above embodiments, the combination of the HE ₂₁ ^even mode and the TM ₀₁ mode, and the combination of the HE ₂₁ ^odd mode and the TE ₀₁ mode are exemplified as a set of strictly eigenmodes that form the LP ₁₁ mode. Generally, the LP _lm mode is used. Is expressed by a linear combination of HE _{l + 1, m} mode and EH _{l-1, m} mode, which are strictly eigenmodes, and the present invention is not limited to the above embodiments. Various modifications can be made based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

  The strict eigenmode multiplexer / demultiplexer and the strict eigenmode multiplex transmission system of the present invention can be applied to long-distance large-capacity optical fiber transmission using several mode fibers, short-distance data transmission in a data center, and the like.

1, 1A to 1D, 1a, 1b Strict eigenmode multiplexer / demultiplexer 2 In-phase anti-phase generator / separator 2A In-phase / anti-phase generator / separator 2Aa-2Ad Waveguide 2B In-phase anti-phase generator / separator 2B ₁ Asymmetric directional coupler 2B ₂ equal branching junction circuit 2C in-phase / negative-phase generator / separator 2C _a optical element 2C _b mirror 2C _c interface 2C _e variable phase controller 2C ₁ , 2C ₂ optical element 2C _d , 2C _f , 2C _g , 2C _h , 2C _i mirror 2C _j fixed phase shifters 2D _1, 2D _{2-phase-reverse} congruent branching circuit 2a, 2b phase reverse-phase generator separator 3,3A～3D LP mode demultiplexer 3a, 3b LP mode demultiplexer 3 _1, 3 ₃ Asymmetric directional coupler 3 ₂ mode rotator 4, 4A, 4B, 4C, 4C ₁ , 4C ₂ polarization rotator 4D ₁ , 4D ₂ polarization rotator 4a, 4b polarization rotator 5Ao, 5Ae polarization Multiplexer 5Bo, 5 Be polarization combiner 5C ₁ , 5C ₂ polarization combiner 5D ₁ , 5D ₂ polarization combiner 10 Strict eigenmode multiplex transmission system 11 Number mode fiber θ ₁ incident angle θ ₂ incident angle θ _p apex angle

Claims (7)

LP mode multiplexer / demultiplexer that multiplexes / demultiplexes LP mode of several mode fiber;
A polarization rotator that rotates the polarization direction of the LP mode component at the input / output end of the LP mode multiplexer / demultiplexer;
The LP mode multiplexer / demultiplexer and the polarization rotator comprise an in-phase / anti-phase generator / separator that performs linear combination conversion between LP mode components and strict eigen components having different mode labels, Strict eigenmode multiplexer / demultiplexer. The in-phase / anti-phase generation / separation circuit is a bidirectional 2-input 2-output in-phase / anti-phase multiplexing / demultiplexing circuit that multiplexes / demultiplexes the in-phase component and the anti-phase component into two output ports.
The in-phase / anti-phase multiplexing / demultiplexing circuit is
An asymmetric X merge branch circuit in which a first set of two single-mode waveguides having different propagation constants and a second set of two single-mode waveguides having the same propagation constant intersect at one intersection;
In one traveling direction, the two waveguides in the second set transmit branched light whose phase relationship between the two output lights is in phase or opposite phase, and light in the first set of two waveguides. Selectively output the phase component according to the incident waveguide,
In the other traveling direction, the two waveguides of the first set transmit the combined light of the in-phase component or the anti-phase component of the two input lights simultaneously incident on the two input ports to the second set of waveguides. 2. The exact eigenmode multiplexer / demultiplexer according to claim 1, wherein the output is selectively output from one waveguide in accordance with an in-phase component or an anti-phase component of two input lights of the waveguide. The in-phase / anti-phase generation / separation circuit is a two-input two-output bidirectional in-phase / anti-phase multiplexing / demultiplexing circuit that multiplexes / demultiplexes the in-phase component and the anti-phase component into two output ports,
The in-phase / anti-phase multiplexing / demultiplexing circuit is
An asymmetric directional coupler having a port of two inputs and one output or one input and two outputs, and an equi-branching merging circuit connected to the one-port side formed into a two-mode waveguide by the tapered structure of the asymmetric directional coupler; An optical circuit comprising:
In the traveling direction from the 2-port side to the 1-port side of the asymmetric directional coupler, it selectively couples to the fundamental mode or the primary mode in the 2-mode waveguide according to the incident port, The component coupled to the fundamental mode after branching is emitted to the two output ports in the same phase, and the component coupled to the primary mode is emitted to the two output ports in the opposite phase,
In the traveling direction from the 1 port side to the 2 port side of the asymmetric directional coupler, the in-phase component or the anti-phase component of the two input lights merged by the equi-branch merging circuit is selectively selected from the 2 ports according to the phase component. The strict eigenmode multiplexer / demultiplexer according to claim 1, wherein The in-phase / anti-phase generation / separation circuit is a two-input two-output bidirectional in-phase / anti-phase multiplexing / demultiplexing circuit that multiplexes / demultiplexes the in-phase component and the anti-phase component into two output ports,
The in-phase / anti-phase multiplexing / demultiplexing circuit is
An incident optical path incident from the high refractive index side, an incident optical path incident from the low refractive index side, and an outgoing optical path exiting from the high refractive index side with respect to the same point on the boundary surface of the dielectric having different refractive indexes. An optical element including an exit optical path that exits from the low refractive index side,
An incident beam incident from one of the two incident optical paths is branched with the phase relationship of the two outgoing lights in phase or opposite in phase according to the incident incident optical path. Emanating from
The in-phase component or the reverse-phase component of the two incident lights incident simultaneously from the respective incident optical paths of the two incident optical paths are combined, and one of the two outgoing optical paths is selected according to the phase component of the two incident incident lights. 2. The exact eigenmode multiplexer / demultiplexer according to claim 1, wherein the strict eigenmode multiplexer / demultiplexer selectively emits from one of the emission optical paths. A polarization multiplexer / demultiplexer connected to the in-phase / anti-phase generator / separator;
The strict eigenmode coupling according to claim 2 or 3, wherein the polarization multiplexer / demultiplexer selectively inputs or outputs x-polarized light and y-polarized light to or from the in-phase / anti-phase generator / separator. Duplexer. Two in-phase / negative-phase multiplexing / demultiplexing circuits are provided,
A polarization rotator (polarization 90 degree converter) that is connected to one port of two in-phase / anti-phase multiplexing / demultiplexing circuits and converts the polarization between x-polarized light and y-polarized light by 90 degrees;
In the two in-phase / anti-phase multiplexing / demultiplexing circuits, the input / output light of the port not connected to the polarization rotator in the port of one in-phase / anti-phase multiplexing / demultiplexing circuit and the port of the other in-phase / anti-phase multiplexing / demultiplexing circuit A polarization multiplexer / demultiplexer for polarization multiplexing / demultiplexing the input / output light of the polarization rotator connected to
The strict eigenmode multiplexer / demultiplexer according to any one of claims 2 to 4, wherein the polarization multiplexer / demultiplexer is connected to the LP mode multiplexer / demultiplexer. A strict eigenmode multiplexer / demultiplexer according to any one of claims 1 to 6, and a number mode fiber,
The exact eigenmode multiplexer / demultiplexer is connected to the transmitting end and the receiving end of the number mode fiber,
The strict eigenmode multiplexer / demultiplexer at the transmitting end excites a strict eigenmode in the number mode fiber,
The number mode fiber performs multiplex transmission in strictly eigenmode,
The exact eigenmode multiplexer / demultiplexer at the receiving end converts the exact eigenmode received from the several mode fiber into an optical signal and outputs the optical signal.