JP6288772B2 - Mode multiplexer / demultiplexer and mode multiplexing communication system - Google Patents

Description

  The present invention relates to a mode multiplexer and a mode duplexer required for mode multiplexing transmission, which is a multiplexing method in which each propagation mode is used as a carrier in an optical fiber cable having three or more propagation modes.

  Currently, traffic in optical fiber networks is increasing, and transmission capacity can be increased by using various methods such as increasing transmission speed, increasing the number of wavelength division multiplexing using wavelength division multiplexing (WDM) technology, and multi-level modulation. I have been trying to expand. However, it is expected that it will be difficult to expand the transmission capacity using existing transmission lines and conventional transmission methods in the future, so the expansion of the wavelength range, new transmission fibers, and new transmission methods have been studied. Yes.

  As a method for expanding the wavelength region, studies have been made to increase the transmission capacity by realizing WDM in a wide wavelength region using a wavelength band that is not currently used. However, since the transmission loss varies depending on the wavelength band, it is considered that the usable wavelength band is limited, and furthermore, it is difficult to realize an optical amplifier that can amplify over a wide wavelength range, so that WDM in a wide wavelength range is practically used. There are many challenges to reach.

Regarding a new transmission fiber, a fiber structure has been proposed in which the effective area (A _eff ) can be increased in order to suppress waveform distortion due to fiber nonlinearity. Fiber nonlinear suppression leads to an increase in input power that can be input to the fiber. If the input power can be increased, advantages such as higher transmission speed and further multi-value can be obtained. However, as shown in Non-Patent Document 1, since the expansion of A _eff is based on single-mode operation, it is difficult to significantly increase Aeff because bending loss and single-mode operation are in a trade-off relationship. There are challenges.

  As for the new transmission scheme, OFDM (Orthogonal Frequency Division Multiplexing) using orthogonal frequency components used for improving the frequency utilization efficiency in the wireless transmission scheme shown in Non-Patent Document 2, As shown in Patent Document 3, application of MIMO (Multiple Input Multiple Output) to a multimode optical fiber has been studied. However, since complicated signal processing is required in a transceiver, high-speed processing is required. There are issues such as

  Furthermore, although a multiplexing method using a propagation mode of an optical fiber has been proposed in Patent Document 1, a method for exciting a desired higher-order mode has not been proposed, and it is assumed that it is used at a single wavelength. Therefore, there is a problem that it is difficult to realize a large capacity. As shown in Non-Patent Document 3, as a mode demultiplexer for using the propagation mode of an optical fiber, a method of demultiplexing a mode by changing a light receiving position has been proposed. In such a situation, crosstalk between modes becomes large, and the design of the optical receiver becomes complicated. Therefore, it is not preferable for use in mode multiplex transmission using a propagation mode as a carrier.

JP-A-8-288911

Matsui et al., “Single-mode photonic fiber with low bending loss and Aeff of> 200 μm 2 ultra high-speed WDM transmission”, OFC 2010, PA. Benn C.I. Thomsen, “MIMO enabled 40 Gb / s transmission using mode division multiplexing in multimode fiber”, OFC2010, OthM6. C. P. Tsekrekos et al., “Mode-selective spatial filtering for increased robustness in a mode group diversity multiplexing”, OPTIC LETTERS, Vol. 32, no. 9, 2007. R. Ryf et al., “Optical Coupling Components for Spatial Multiplexing in Multi-Mode Fibers”, ECOC2011, Th. 12 B. 1. N. Hanzawa, et al., "Asymmetric parallel waveguide with mode conversion for mode and wavelength division multiplexing transmission", OFC2012, OTU1. 4).

  In mode multiplex transmission using an optical fiber with multiple propagation modes and each propagation mode of the optical fiber as a carrier, the existing device is assumed to operate in the basic mode. It is difficult to realize mode multiplexing transmission by using. In addition, the mode multiplexer / splitters proposed so far have poor demultiplexing efficiency, making long-distance transmission and high-speed signal transmission difficult, and using a spatial system makes the configuration complicated. There are challenges. In the method using the spatial system, it is possible to achieve high demultiplexing efficiency for each mode as described in Non-Patent Document 4, but the insertion loss is as large as 8 dB or more, and it is difficult to reduce the size of the device. There are also other issues.

  In recent years, as shown in Non-Patent Document 5, a two-mode multiplexer / demultiplexer using an asymmetric parallel waveguide has been proposed. By using an asymmetric parallel waveguide, signals can be handled only in the fundamental mode at the transmitting and receiving ends. It is possible to use an existing device as it is. However, the multiplexer / demultiplexer described in Non-Patent Document 5 is limited to two modes, and no multiplexing / demultiplexing method for realizing three-mode multiplexing has been proposed. When realizing a three-mode multiplexer / demultiplexer with an optical waveguide, it may be necessary to produce a waveguide with a different waveguide height within the same chip, but the waveguide height is the same. There is a technical problem that it is difficult to change within the chip.

  The present invention is an optical waveguide that has a restriction that the height of the waveguide is constant, and a higher-order mode having a field distribution in the height direction of the waveguide. An object of the present invention is to realize a mode multiplexer / demultiplexer having three or more modes.

The mode multiplexer / demultiplexer of the present invention is
A first waveguide propagating LP01 mode;
A second waveguide propagating LP11b mode;
A third waveguide propagating in a predetermined first higher order mode and second higher order mode;
A fourth waveguide propagating LP01 mode;
A fifth waveguide propagating in the LP11a mode;
A first coupling unit coupling the LP01 mode light of the first waveguide and the first higher-order mode light of the third waveguide;
A second coupling unit coupling the LP11b mode light of the second waveguide and the second higher-order mode light of the third waveguide;
A third coupling unit coupling the LP01 mode light of the fourth waveguide and the LP11a mode light of the fifth waveguide;
The fifth waveguide and the second waveguide are connected, and the LP11a mode light of the fifth waveguide is rotated to LP11b mode light and input to the second waveguide, and the second waveguide A mode rotator that rotates the LP11b mode of the waveguide to the LP11a mode and inputs the mode to the fifth waveguide;
With
The first higher-order mode is at least one of an LP1Xa mode (where X is an integer of 1 or more) and an LP0Y mode (where Y is an integer of 2 or more);
The second higher order mode is, LPZ1 mode (where, Z is an integer of 2 or more.) Is.

In the mode multiplexer / demultiplexer of the present invention,
The first waveguide, the second waveguide, and the third waveguide are formed by a first parallel waveguide having the same height and formed in the same plane,
The fourth waveguide and the fifth waveguide are formed by a second parallel waveguide having the same height and formed in the same plane,
The mode rotator is formed by a sixth waveguide having the same height formed in the same plane,
The first parallel waveguide, the second parallel waveguide, and the sixth waveguide may be formed on the same plane.

  In the mode multiplexer / demultiplexer of the present invention, the sixth waveguide may have a trench layer in which a groove is formed in at least a part of a cross section.

  In the mode multiplexer / demultiplexer of the present invention, the sixth waveguide may have a waveguide having an asymmetry in the cross section.

In mode demultiplexer of the present invention, at least one of said LP1Xa mode and the LP0Y mode is LP11a mode, the LPZ1 mode Ru cormorants be LP21 mode.

The transmission device of the present invention is
A plurality of wavelength division multiplexing signal generation units for generating wavelength division multiplexing signals;
The wavelength multiplexed signal generator is connected to one end of each of the first waveguide, the third waveguide, and the fourth waveguide, and mode multiplexed the wavelength multiplexed signal input from the wavelength multiplexed signal generation And outputting from the other end of the third waveguide, the mode multiplexer / demultiplexer of the present invention,
Is provided.

The receiving device of the present invention is
A wavelength-division multiplexed signal that is mode-multiplexed is input to one end of the third waveguide, and the wavelength-division multiplexed signal obtained by demultiplexing the input signal for each mode is converted into the first waveguide, the third waveguide, and the A mode multiplexer / demultiplexer of the present invention that outputs from each other end of the fourth waveguide;
A wavelength multiplexed signal receiver that receives the wavelength multiplexed signal output from the mode multiplexer / demultiplexer for each wavelength;
Is provided.

The mode multiplexing communication system of the present invention is
A transmission apparatus of the present invention for transmitting a mode-multiplexed wavelength division multiplexed signal;
A receiver of the present invention for receiving a mode-multiplexed wavelength multiplexed signal from the transmitter;
A multimode optical fiber that connects the transmitting device and the receiving device and is capable of propagating a mode number equal to or greater than the number of modes that the third waveguide propagates;
Is provided.

The design method of the mode multiplexer / demultiplexer of the present invention is as follows:
A first waveguide propagating LP01 mode;
A second waveguide propagating LP11b mode;
A third waveguide propagating in a predetermined first higher order mode and second higher order mode;
A fourth waveguide propagating LP01 mode;
A fifth waveguide propagating in the LP11a mode;
The fifth waveguide and the second waveguide are connected, and the LP11a mode light of the fifth waveguide is rotated to LP11b mode light and input to the second waveguide, and the second waveguide A sixth waveguide that functions as a mode rotator that rotates the LP11b mode of the waveguide to the LP11a mode and inputs the mode to the fifth waveguide;
A first coupling unit coupling the LP01 mode light of the first waveguide and the first higher-order mode light of the third waveguide;
A second coupling unit coupling the LP11b mode light of the second waveguide and the second higher-order mode light of the third waveguide;
A third coupling unit coupling the LP01 mode light of the fourth waveguide and the LP11a mode light of the fifth waveguide;
A mode multiplexer / demultiplexer design method comprising:
Determining the waveguide width of the third waveguide, the first waveguide, and the second waveguide, and determining the interaction length of the first coupling portion and the second coupling portion; The parallel waveguide part design procedure of
A second parallel waveguide section design procedure for determining a waveguide width of the fifth waveguide and the fourth waveguide, and determining an interaction length of the third coupling section;
The waveguide width of the sixth waveguide is determined so as to propagate the LP11a mode and the LP11b mode in the used wavelength band, and the sixth waveguide is rotated so that the propagation mode rotates in the sixth waveguide. The mode rotor design procedure for determining the interaction length of the sixth waveguide so that the coupling efficiency from the LP11a mode to the LP11b mode in the wavelength band to be used is not less than a desired value. When,
In order,
The first higher-order mode is at least one of an LP1Xa mode (where X is an integer of 1 or more) and an LP0Y mode (where Y is an integer of 2 or more);
The second higher order mode is, LPZ1 mode (where, Z is an integer of 2 or more.) Is.

  The above inventions can be combined as much as possible.

  Since the mode multiplexer / demultiplexer of the present invention includes the first waveguide and the third waveguide, the first multimode light such as the LP01 mode light and the LP11a mode using the waveguide having a constant height. Can be combined. In addition, since the mode multiplexer / demultiplexer of the present invention includes the second waveguide and the third waveguide, a second multi-channel such as LP11b mode light and LP21 mode is used by using a waveguide having a constant height. The mode light can be multiplexed / demultiplexed. Therefore, the mode multiplexer / demultiplexer of the present invention can realize the multiplexer / demultiplexer capable of realizing the mode multiplexing transmission of three modes or more with an optical waveguide having a restriction that the height of the waveguide is constant.

  Further, since the mode multiplexer / demultiplexer of the present invention includes the fourth waveguide and the fifth waveguide, the multiplexing / demultiplexing of the LP01 mode light and the LP11a mode light is performed using a waveguide having a constant height. The LP11b mode light can be input to the second waveguide through the mode rotator. For this reason, the mode multiplexer / demultiplexer of the present invention enables mode multiplexing transmission of three or more modes using a propagation mode in an optical fiber as a carrier by using a conventional optical device, and the height of the waveguide is increased. Even if there is a limit of constant, it is possible to realize a mode multiplexer / demultiplexer having three or more modes with high efficiency.

An example of the 1st parallel waveguide part for carrying out multiplexing / demultiplexing to LP21 mode of 1st Embodiment is shown. An example of the 2nd parallel waveguide part concerning a 1st embodiment is shown. An example of the mode rotor which concerns on 1st Embodiment is shown. The 2nd example of the mode rotor concerning a 1st embodiment is shown. The 3rd example of the mode rotor which concerns on 1st Embodiment is shown. An example of a mode multiplexer / demultiplexer using a first parallel waveguide portion, a second parallel waveguide portion, and a mode rotator is shown. It is a figure which shows the effective refractive index of the propagation mode with respect to the width | variety of a waveguide in relative refractive index difference (DELTA) 0.4% of a waveguide. It is a figure for determining the waveguide width of three waveguides. It is a figure which shows the wavelength dependence of the coupling efficiency from LP11a mode to LP11b mode. It is a figure which shows the 2nd wavelength dependence of the coupling efficiency from LP11a mode to LP11b mode. It is a figure which shows the 3rd wavelength dependence of the coupling efficiency from LP11a mode to LP11b mode. It is a figure which shows the coupling efficiency of LP01 mode and LP11a mode of a parallel waveguide which made the waveguide space | interval 5 micrometers. It is a figure which shows the coupling efficiency of LP11b mode and LP21 mode of a parallel waveguide which made the waveguide space | interval 5 micrometers. It is a flowchart which shows an example of the design method of the 1st parallel waveguide. It is a flowchart which shows an example of the design method of a 2nd parallel waveguide. It is a flowchart which shows an example of the design method of a mode rotor. An example of the 1st parallel waveguide part concerning a 2nd embodiment is shown. An example of the coupling efficiency of the LP01 mode and the LP11a mode in the second embodiment is shown. An example of the coupling efficiency of LP11b mode and LP21 mode in 2nd Embodiment is shown. It is a figure which shows the structure of the 3rd Embodiment of this invention.

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

In the present invention, it is assumed to be used for three-mode multiplex transmission using a fundamental mode, a first higher-order mode, and a second higher-order mode (LP ₀₁ , LP ₁₁ , LP ₂₁ ) that are propagation modes in an optical fiber doing. In this specification, regarding the propagation modes in the waveguide, for convenience, the fundamental mode, the first higher-order mode, the second higher-order mode, and the third higher-order mode are referred to as LP01, LP11a, LP11b, and LP21 modes, respectively.

  The present invention provides a parallel waveguide section 101 composed of three waveguides having the same relative refractive index difference Δ and the same waveguide height, and two waveguides having the same waveguide height. By connecting three waveguides 120 and three mode rotors having the same waveguide height, the three-mode multiplexing / demultiplexing in the optical fiber is realized.

  The three waveguides of the first parallel waveguide section 110 are waveguides in which the central waveguide 113 can propagate the LP21 mode or higher, and the width of the waveguide for propagating the LP01 mode in the used wavelength range. A waveguide 111 in which w1 is set, a waveguide 112 in which a width w2 of a waveguide different from that of the waveguide 111 capable of propagating at least the LP11b mode in the used wavelength range, and a central waveguide in which the LP21 mode or higher can propagate 113, the effective refractive index of the LP01 mode of the waveguide 111 is equal to that of the first multimode of the waveguide 113, and the effective refractive index of the LP11b mode of the waveguide 112 is equal to that of the second multimode of the waveguide 113. It is assumed that the asymmetric parallel waveguide is constituted by the waveguide 113 in which the width w3 of the waveguide is set.

  Here, the first multi-mode is a mode having a field distribution having one or more peaks in the horizontal direction of the waveguide including the center of the waveguide and at most one peak in the height direction. It is at least one of the LP1Xa mode (where X is an integer of 1 or more) and the LP0Y mode (where Y is an integer of 2 or more). The second multimode is a mode having a field distribution having two or more peaks in the height direction of the waveguide. For example, at least one of the LPZ1 modes (where Z is an integer of 2 or more). It is. In the present embodiment, the case where the first multimode is the LP11a mode with X = 1 and the second multimode is the LP21 mode with Z = 2, the present invention is listed here. Any of the first multimode and the second multimode may be combined.

  For the two waveguides of the second parallel waveguide section 120, at least two or more propagation modes propagate with the fourth waveguide 114 in which the waveguide width w4 for propagating the LP01 mode is propagated in the used wavelength range. This is possible, and the asymmetric parallel structure is formed by the fifth waveguide 115 in which the waveguide width w5 is set so that the effective refractive index of the LP01 mode of the fourth waveguide 114 is equal to the effective refractive index of the LP11a mode. Let it be a waveguide.

  Using the two parallel waveguide sections 110 and 120 described above, the mode rotator of the waveguide 116 is connected in a subordinate manner between the waveguide 115 and the waveguide 112, and the output LP11a of the parallel waveguide section 112 is used. By passing the mode rotator and exciting the LP11b mode of the waveguide 112, mode conversion and mode demultiplexing are performed by phase matching of higher order modes including the fundamental mode and the LP21 mode. Here, the asymmetrical parallel waveguide includes not only the size of the waveguide but also the case where the relative refractive index difference of the waveguide is different. The waveguide 111 is a waveguide capable of propagating the first higher-order mode or higher. A waveguide may be configured.

  As a result, in the wavelength band of the C band (1530 nm to 1565 nm), it is possible to realize the demultiplexing efficiency of the fundamental mode and the two higher-order modes at 98% or more. High-speed and long-distance transmission of mode multiplex transmission is possible.

  In addition, since propagation is performed in the basic mode in the previous stage of mode multiplexing and the subsequent stage of mode demultiplexing, that is, the section other than the transmission line, the fiber device can be operated in the basic mode, so the existing fiber device can be used. Thus, three-mode multiplex transmission can be realized. For this reason, it is possible to realize three-mode multiplex transmission in combination with existing multiplexing techniques such as wavelength multiplexing with a simple configuration.

  In addition, by using the proposed method, the height of the parallel waveguides can be made constant and in the same plane, so it is technically difficult to use the method of changing the height of each waveguide. Therefore, it is possible to realize a three-mode multiplexer / demultiplexer.

(First embodiment)
A first embodiment of the present invention will be described. FIG. 1 is a diagram for explaining a first parallel waveguide section for performing multiplexing / demultiplexing up to the LP21 mode of the present embodiment. The first parallel waveguide unit 110 includes a waveguide 111, a waveguide 112, and a waveguide 113. The waveguide 111 propagates LP01 mode light, the waveguide 112 propagates LP11b mode light, and the waveguide 113 propagates LP21 or higher. The first parallel waveguide section 110 includes a coupling portion C1 between the waveguide 111 having the interaction length L1 and the waveguide 113, and a coupling portion C2 between the waveguide 112 having the interaction length L2 and the waveguide 113. Have.

  FIG. 2 is a diagram illustrating a second parallel waveguide section for generating LP11a mode light. The first parallel waveguide section 120 includes a waveguide 114 and a waveguide 115. The waveguide 114 propagates LP01 mode light, and the waveguide 115 propagates LP11a mode light. The second parallel waveguide section 120 includes a coupling section C3 between the waveguide 114 having the interaction length L3 and the waveguide 115.

  FIG. 3 is a diagram illustrating a waveguide that functions as a mode rotator for coupling LP11a mode light to LP11b mode light. The mode rotor 130 is a waveguide 116 having a waveguide width w8 and height h, and includes a trench layer in which a groove is provided in at least a part of a cross section. The trench layer includes grooves having an upper width w83, a lower width w84, and a depth d between the widths w81 and w82 of the waveguide at the width w8. The groove is provided asymmetrically with respect to the center on the width w8 in the cross section, and w81 and w82 are different. The groove may be provided on the upper surface of the waveguide as illustrated, but may be provided on the lower surface. The shape of the groove is arbitrary, and may be a rectangle as shown in FIG. 3 or a triangle.

  As shown in FIG. 3, the mode rotor 130 has asymmetry in the cross section. 4 and 5 show other embodiments of the mode rotor 130. FIG. As shown in FIGS. 4 and 5, the mode rotor 130 may have a trench layer at the end of the waveguide where the width w82 becomes 0 in the waveguide width w8. If an asymmetry occurs in the cross section, the shape of the trench layer is arbitrary. For example, as shown in FIGS. 4 and 5, a groove is provided only on one side with respect to the center in the left-right direction of the cross section of the waveguide. Further, a groove may be arranged at the center in the left-right direction of the cross section, and the width of the groove may be asymmetric with respect to the center of the cross section. The shape of the groove is also arbitrary. For example, the width w83 of the upper part of the groove and the width w84 of the lower part of the groove may be the same or different. For example, the shape of the groove is a square having the same width w83 at the upper portion of the trench and width w84 at the lower portion of the trench as shown in FIG. Further, the shape of the groove may be, for example, a triangle having a width w84 at the bottom of the trench of 0 as shown in FIG. Thus, the shape of the trench of the trench layer can be any polygon.

  FIG. 6 is a diagram for explaining the configuration of a mode multiplexer / demultiplexer that multiplexes / demultiplexes up to LP21 mode using the first parallel waveguide section 110, the second parallel waveguide section 120, and the mode rotator 130. . The waveguide 112 and the waveguide 115 are connected by a mode rotator 130. LP01 mode light is input to the waveguide 114 and LP11a mode light is output from the waveguide 115. The mode rotator 130 has a function of rotating the LP11a mode light output from the waveguide 115 by 90 degrees. Therefore, LP11b mode light is input to the waveguide 112. The mode rotator may be a waveguide type using a waveguide as in the present proposal, or may be an optical element.

  The coupling portions C <b> 1 and C <b> 2 are arranged in cascade in the longitudinal direction of the waveguide 113. In the coupling part C1, the LP01 mode of the waveguide 111 and the LP11a mode of the waveguide 113 are phase-matched using an asymmetric parallel waveguide. In the coupling part C2, the LP11b mode of the waveguide 112 and the LP21 mode of the waveguide 113 are phase-matched using an asymmetric parallel waveguide. Thereby, the mode multiplexer / demultiplexer of this embodiment functions as a mode multiplexer that performs mode conversion and also functions as a mode multiplexer that performs mode demultiplexing. The asymmetric parallel waveguide may have the same relative refractive index difference Δ if the size of the waveguide is different. Moreover, not only the size of the waveguide is different, but also the case where the relative refractive index difference of the waveguide is different is included.

The waveguide 111, the waveguide 112, and the waveguide 113 are parallel waveguides having the same relative refractive index difference Δ. Δ is expressed by the following equation using the refractive index n1 of the waveguide and the refractive index n0 of the medium around the waveguide.

  The waveguide 111 has at least the width w1 of the waveguide that propagates the LP01 mode in the used wavelength range, and the waveguide 112 has at least the width w2 of the waveguide that propagates the LP11b mode in the used wavelength range. Here, the cutoff wavelength is determined by determining the waveguide height h and the relative refractive index difference Δ, determining the waveguide width w and the wavelength band to be used, and performing a waveguide analysis using, for example, the finite element method. Find the wavelength. In this embodiment, the height h of the waveguide is set equal to the width of the central waveguide 113.

  The waveguide 113 is capable of propagating at least a propagation mode of LP21 mode or more, the waveguide width w3 of the waveguide 113 is designed so that the effective refractive index of the LP11a mode of the waveguide 113 and the LP01 mode of the waveguide 111 are equal, and The LP21 mode of the waveguide 113 and the LP11b mode of the waveguide 112 are designed to be equal in effective refractive index. A design method for the waveguide 111, the waveguide 112, and the waveguide 113 will be specifically described below.

  As an example, FIG. 7 shows changes in the effective refractive index from the LP01 mode to the LP21 mode with a wavelength of 1550 nm with respect to the waveguide width w when the relative refractive index difference Δ of each waveguide is 0.4%. FIG. 7 shows the effective refractive index of the propagation mode of the waveguide on the vertical axis with the width w of the waveguide (the height h of the waveguide is constant at 13.7 μm) on the horizontal axis. FIG. 7 shows that LP21 mode propagation can be realized by setting the waveguide width w to 11.0 μm or more when the relative refractive index difference Δ is 0.4%.

  FIG. 8 is a diagram for determining the widths of the waveguide 111, the waveguide 112, and the waveguide 113 of the first parallel waveguide section 110. FIG. Here, the width w3 of the waveguide 113 is 13.7 μm capable of propagating the LP21 mode or higher. At this time, the width w1 of the waveguide 111 may be set equal to the effective refractive index of the LP11a mode of the waveguide 113, so that w1 is 4.79 μm. Further, the width w2 of the waveguide 112 is only required to be equal to the effective refractive index of the LP21 mode of the waveguide 113 and the effective refractive index of the LP11b mode of the waveguide 112, so w2 is 4.62 μm.

  When the waveguide width w1, the waveguide width w2, and the waveguide width w3 shown in FIG. 8 are used, the interaction length L1 and the LP11b mode necessary for converting the LP01 mode to the LP11a mode are converted to the LP21 mode. The interaction length L2 required for the calculation is calculated by, for example, a beam propagation method, and the coupling efficiency of the LP01 mode of the waveguide 111 to the LP11a mode of the waveguide 113 and the LP11b mode of the waveguide 112 to the LP21 mode of the waveguide 113 are calculated. Design an appropriate interaction length to maximize the coupling efficiency of Here, when the waveguide interval g1 between the waveguide 111 and the waveguide 113 and the waveguide interval g2 between the waveguide 112 and the waveguide 113 are equal to 5.0 μm, the coupling efficiencies of the LP11a mode and the LP21 mode are obtained by the beam propagation method. When the maximum interaction length was determined, L1 was 1.5 mm and L2 was 1.0 mm.

  In the present embodiment, the LP11b of the waveguide 112 is used to excite the LP21 mode in the first parallel waveguide section 110. The mode rotor 130 shown in FIG. 3 is used for exciting the LP 11b. The second parallel waveguide section 120 includes a waveguide 114 capable of propagating the LP01 mode and a waveguide 115 capable of propagating the LP11a mode or higher. As the waveguide parameters of the waveguide 114 and the waveguide 115 for exciting the LP 11a, the parameters of the coupling portion of the waveguide 111 and the waveguide 113 of the first parallel waveguide section 110 may be used. That is, the waveguide width w4 is equal to the waveguide width w1, the waveguide width w5 is equal to the waveguide width w3, the waveguide interval g3 of the coupling portion C3 is equal to the waveguide interval g1, and the interaction length L3 is the interaction length. Equal to L1.

  In the mode rotor 130, the height h of the waveguide and the relative refractive index difference Δ are equal to those of the parallel waveguide portion 110 and the parallel waveguide portion 120, respectively, 13.7 μm and 0.4%. This time, the mode rotor 130 was configured with the following parameters. In the present embodiment, the width w8 of the parallel waveguide section 116 is 14.0 μm, the width w81 of the parallel waveguide section 116 is 3.0 μm, the widths w83 and w84 of the trench layers are 1.0 μm, the depth is 6.8 μm, The interaction length L4 was 2.57 mm. FIG. 9 shows the wavelength dependence of the coupling efficiency from the LP11a mode to the LP11b mode calculated with this parameter. FIG. 9 shows the efficiency (solid line) that the field distribution rotates and couples to the LP11b mode when the LP11a mode is input to the waveguide 116 and the efficiency (dashed line) that is output as the LP11a mode without being fully rotated. Yes. The parameter of the mode rotator 130 is not limited to this, and it is only necessary that the rotation of a desired mode can be realized by configuring according to the design of the parallel waveguide unit 116 described below.

  FIG. 10 shows an example of the wavelength dependence of the coupling efficiency when the mode rotor 130 shown in FIG. 3 is employed. The waveguide Δ is 0.45%, the height h is 11.0 μm, the waveguide width w8 is 11.3 μm, w82 is 2.0 μm, and the trench layer widths w83 and w84 are 1 in the mode rotor parameters. The depth d was set to 5.4 μm, and the interaction length L4 was set to 1.46 mm. FIG. 11 shows an example of the wavelength dependence of the coupling efficiency when the mode rotator 130 shown in FIG. 4 is employed. w82 was set to 0.0 μm, w83 and w84 were set to 1.5 μm, the depth d was set to 3.9 μm, and the interaction length L4 was set to 2.99 mm. Both FIG. 10 and FIG. 11 show high coupling efficiency from the LP11a mode to the LP11b mode, and it can be seen that even if a trench is arranged at an arbitrary position in the cross section, it functions as a mode rotor. The parameter of the mode rotator is not limited to this, and the parameter may be set so that a desired mode can be rotated.

  In the present invention, the first parallel waveguide section 110 and the second parallel waveguide section 120 are replaced with the waveguide 115 and the first parallel waveguide section of the second parallel waveguide section 120 as shown in FIG. By connecting a mode rotator 130 between the waveguide 112 and the waveguide 112, the LP11b mode of the waveguide 112 is excited to perform multiplexing / demultiplexing of the LP21 mode. Therefore, the waveguide 111, the waveguide 113, and the waveguide 114 are used as input / output ports, and the LP01, LP11, LP21 only in the LP01 mode, which is the fundamental mode, at the input part of the mode multiplexer and the output part of the mode duplexer. The three modes can be combined / demultiplexed.

  The waveguide spacing g1, g2 is 5.0 μm, the waveguide width w1 of the waveguide 111 is 4.79 μm, the waveguide width w2 of the waveguide 112 is 4.62 μm, the width w3 of the waveguide 113 is 13.7 μm, The height h of the waveguide is 13.7 μm, the relative refractive index difference Δ is 0.4%, the bending radius R of the waveguide is 50 mm, the interaction length L1 is 1.5 mm, and the interaction length L2 is 1.0 mm. FIG. 12 shows the wavelength dependence of the coupling efficiency of the LP11a mode obtained in this case, and FIG. 13 shows the wavelength dependence of the coupling efficiency of the LP21 mode.

  FIG. 12 shows the coupling efficiency (solid line) output as the LP11a mode to the waveguide 113 and the coupling efficiency (dashed line) output to the waveguide 111 as the LP01 mode when LP01 mode light is incident on the waveguide 111. ing. FIG. 13 shows the coupling efficiency (solid line) output as the LP21 mode to the waveguide 113 when LP11b mode light is incident on the waveguide 112 and the coupling efficiency (dashed line) output as the LP11b mode to the waveguide 112 as it is. Show.

ここで、Ｗ１、Ｗ２はそれぞれ光ファイバのＭＦＤ、導波路のＭＦＤとし、ηは結合損失を示す。 From FIG. 9 to FIG. 13, it can be seen that an extremely efficient three-mode multiplexer / demultiplexer having a coupling efficiency of 98% or more in the wavelength band of 1530 nm to 1565 nm can be realized. Assuming a fiber having a core diameter of 18 μm and a core relative refractive index difference Δ of 0.4% as an optical fiber capable of propagating three or more modes, the mode field diameter (MFD) of the fundamental mode of the optical fiber is 15. .3 μm, and the MFD of the waveguide 113 described above is 13.7 μm. When these values were used to estimate the MFD mismatch loss using the approximate expression (2), it was about 0.15 dB.

Here, W1 and W2 are the MFD of the optical fiber and the MFD of the waveguide, respectively, and η indicates the coupling loss.

  From this, the connection loss between the asymmetric parallel waveguide of the present invention and the transmission fiber is expected to be very small. If the transmission fiber and the MFD of the waveguide are extremely different, the connection loss is reduced by reducing the mismatch of the MFD by processing one or both of them at the connection portion between the waveguide and the fiber in a tapered shape. It is also possible to reduce this. The MFD described above is calculated by approximating Gaussian approximation of the fundamental mode field for both the fiber and the waveguide.

  Although the first embodiment is designed under the condition that the relative refractive index difference Δ of the waveguide is equal and the interval between the waveguides is equal, the present invention is not limited to this, and each of the three waveguides is designed. The relative refractive index difference Δ may be different, and the waveguide intervals g1 and g2 may be different. In addition, the second parallel waveguide section 120 may be designed with a waveguide parameter capable of multiplexing / demultiplexing the LP01 mode and the LP11a mode without using the parameter of the first parallel waveguide section 110.

Next, a method for designing a mode multiplexer / demultiplexer according to the present invention will be described. FIG. 14, FIG. 15 and FIG. 16 are flowcharts showing the procedure of the waveguide design method. The design procedure includes a first parallel waveguide section design procedure, a second parallel waveguide section design procedure, and a mode rotor design procedure.
In the first parallel waveguide section design procedure shown in FIG. 14, the first parallel waveguide section 110 is designed.
First, the wavelength band used in the mode multiplexer / demultiplexer is determined (S101).
Next, the relative refractive index difference Δ and the waveguide width w3 are determined so that the LP21 mode or higher can propagate in the wavelength band to be used (S102, S103). The relative refractive index difference Δ and the waveguide width w3 are determined by, for example, obtaining the cutoff wavelength using waveguide analysis such as a finite element method.
Next, the waveguide width w1 is determined so that the effective refractive indexes of the LP01 mode of the waveguide 111 and the LP11a mode of the waveguide 113 are equal (S104, S105). Also in the determination of the waveguide width w1, the effective refractive index is calculated using waveguide analysis such as a finite element method.
Next, a waveguide width w2 is determined such that the effective refractive index of the LP21 mode of the waveguide 113 (S108) is equal to the effective refractive index of the LP11b mode of the waveguide 112 (S106, S107).
Next, the waveguide intervals g1 and g2 are determined (S108), and the interaction lengths L1 and L2 are determined using the determined waveguide intervals g1 and g2 (S109). The interaction lengths L1 and L2 are calculated by performing propagation analysis such as a beam propagation method to calculate the interaction length.
After calculating the interaction lengths L1 and L2, it is determined whether or not the desired coupling efficiency is obtained (S110). If the desired coupling efficiency is not obtained, the process proceeds to step S108. For example, when a coupling efficiency of 80% or more is desired in the wavelength band to be used and a coupling efficiency less than that is obtained, the waveguide intervals g1 and g2 are reset (S108), and the interaction lengths L1 and L2 are set. Is calculated (S109), and when the desired characteristics are obtained (Yes in S110), the parameters determined so far are confirmed.

In the second parallel waveguide section design procedure shown in FIG. 15, the second parallel waveguide section 120 is designed.
In this procedure, the design proceeds with the used wavelength band used in the first parallel waveguide unit 110 (S201). The relative refractive index difference Δ and the waveguide width w5 are determined so that the LP11a mode or higher can be propagated in the wavelength band to be used (Yes in S203) (S202). The relative refractive index difference Δ and the waveguide width w5 are determined by obtaining a cut-off wavelength using waveguide analysis such as a finite element method similarly to the first parallel waveguide section 110.
Next, the waveguide width w4 is determined so that the effective refractive indexes of the LP01 mode of the waveguide 114 and the LP11a mode of the waveguide 115 are equal (S204, S205). Also in the determination of the waveguide width w4, the effective refractive index is calculated using waveguide analysis.
Next, the waveguide interval g3 is determined (S206), and the interaction length L3 is determined (S207). For the calculation of L3, propagation analysis is performed to calculate the interaction length.
After calculating the interaction length L3, it is determined whether or not the desired binding efficiency is obtained (S208). If the desired binding efficiency is not obtained, the process proceeds to step S206. For example, when a coupling efficiency of 80% or more is desired in the wavelength band to be used and a coupling efficiency less than that is obtained, the waveguide interval g3 is reset (S206), and the interaction length L3 is calculated. (S207) When a desired characteristic is obtained (Yes in S208), each parameter determined so far is confirmed.

In the mode rotor design procedure shown in FIG. 16, the mode rotor 130 is designed.
In this procedure, the design proceeds in the wavelength band used in the first parallel waveguide section 110 and the second parallel waveguide section 120 (S301). The waveguide height h, relative refractive index difference Δ, and waveguide width w8 are determined so that both degenerate modes of the LP11a mode and LP11b mode can propagate in the wavelength band to be used (Yes in S303) (S302). . For the determination of the waveguide height h, the relative refractive index difference Δ and the waveguide width w8, the waveguide analysis such as the finite element method is used similarly to the first parallel waveguide section 110 and the second parallel waveguide section 120. To determine the cutoff wavelength.
Next, a procedure for providing a trench layer in the waveguide is performed. In order to determine the position of the trench layer, the width w82 from the end of the waveguide is determined (S304). w82 is arbitrarily determined to some extent, and the following procedure is performed. After determining w82, the widths w83 and w84 and the depth d of the trench layer are determined (S305, S306), and the interaction length L4 is determined so that the LP11a mode rotates to the LP11b mode (S307). L4 is determined using waveguide analysis such as a beam propagation method. Fine adjustment of the parameters from S305 to S307 is performed, and when a desired coupling efficiency from the LP11a mode to the LP11b mode is obtained (Yes in step S308), the parameters determined so far are confirmed. If the desired characteristics cannot be obtained by adjusting the parameters in S305 to S307 (No in Step S308), it is determined whether or not the coupling efficiency is assumed when w82 is determined in Step S304 (S309). When it is the assumed coupling efficiency, the process proceeds to step S304, and when it is not the assumed coupling efficiency, the process proceeds to step S305. Then, the procedure from S305 to S307 is performed again, and the procedure from S304 to S307 is repeated until a desired characteristic is obtained, thereby determining each parameter.

(Second Embodiment)
A second embodiment of the present invention will be described. FIG. 17 shows an example of the first parallel waveguide section 110 according to the present embodiment. The present embodiment relates to an embodiment of the multiplexer / demultiplexer corresponding to the LP21 mode provided with a tapered portion that changes the thickness of the central waveguide at the coupling portion of the LP11a mode and the coupling portion of the LP21 mode.

  The first parallel waveguide unit 110 according to the present embodiment includes a waveguide 611, a waveguide 612, and a waveguide 613. The waveguide 611 propagates LP01 mode light, the waveguide 612 propagates LP11b mode light, and in the waveguide 613, the LP11a mode propagates in the width w3 portion, and the LP21 mode or more propagates in the waveguide width w6 portion. Make it possible. The first parallel waveguide portion 110 includes a coupling portion C1 between the waveguide 611 and the waveguide 613 having the interaction length L1, and a coupling portion C6 between the waveguide 612 and the waveguide 613 having the interaction length L6. Have.

  In this embodiment, the relative refractive index difference Δ is 0.45%, and the waveguide height h is 9 μm. In this case, the parameters of the waveguide are obtained using the design method shown in the first embodiment. In the present embodiment, it is assumed that the wavelength range used is 1530 nm to 1565 nm which is C-band (S101). The waveguide widths w3 and w6 of the waveguide 613 are set to 12.9 μm and 19.1 μm, respectively.

In steps S104 and S105, the waveguide width w1 of the waveguide 611 may be set so that the effective refractive index of the LP11a mode in the waveguide width w3 of the waveguide 613 matches the LP01 mode of the waveguide 611. w1 is determined to be 4.5 μm.
Next, in steps S106 and S107, the waveguide width w2 of the waveguide 612 may be set so that the effective refractive index of the LP21 mode in the waveguide width w6 of the waveguide 613 and the LP11b mode of the waveguide 612 are equal. The waveguide width w2 is determined to be 7.0 μm.
Next, in step S108, the waveguide interval between the waveguides 611 and 613 is g1, the interval between the waveguides 612 and 613 is g6, and g1 and g6 are 4 μm and 9 μm, respectively. Interaction in which the coupling efficiency from the LP01 mode of the waveguide 611 to the LP11a mode of the waveguide 613 and the coupling efficiency from the LP11b mode of the waveguide 612 to the LP21 mode of the waveguide 613 are both 90% or more at this waveguide interval. When the lengths L1 and L2 are obtained (Yes in S110), they are 0.65 mm and 8.9 mm, respectively.

  The parameter of the second parallel waveguide 120 connected to the waveguide 612 is the same as the parameter of the coupling portion C2 of the waveguides 611 and 613, and the mode rotator 130 is connected to the output end of the second parallel waveguide 120. To the waveguide 612.

  FIG. 18 and FIG. 19 show the results of obtaining the coupling efficiency from the waveguides 611 to 613 and from the waveguide 612 to the waveguide 613 using the waveguide parameters described above. It can be seen that the determined waveguide parameter has a coupling efficiency of 90% or more in the C-band. The present invention is not limited to the waveguide parameters shown in the first embodiment and the second embodiment, and the width, interval, and interaction length of the waveguide may be adjusted according to desired characteristics.

(Third embodiment)
A third embodiment of the present invention will be described. This embodiment relates to a usage example of the mode multiplexer / demultiplexer shown in the first embodiment and the second embodiment. The mode multiplexer / demultiplexer of the present invention can be used in combination of three multiplexing methods of a wavelength division multiplexing communication system (WDM) and a polarization multiplexing communication system (PDM) and a mode multiplexing communication system. It is conceivable that these three multiplexing methods are used individually or only two multiplexing methods are used, and any of these methods can be applied. FIG. 20 shows a configuration example of the mode multiplexing communication system according to this embodiment. The mode multiplexing communication system according to the present embodiment is a configuration example of a system for performing three-mode multiplexing for WDM and PDM. The usage example of the mode multiplexer / demultiplexer of the present invention is not limited to this in FIG. As for a light source having a certain wavelength λ, a single light source may be used with power splitting, or a plurality of light sources having the same wavelength may be prepared to generate a signal having a wavelength λ.

As illustrated in FIG. 20, the polarization multiplexing wavelength multiplexing mode multiplexing transmission system includes a transmission device 711 and a reception device 720. The transmission device 711 includes three wavelength multiplexed signal generation units and a mode multiplexer 718 that combines (mode-converts) the propagation modes described in the first embodiment. Each wavelength multiplexed signal generation unit modulates N light sources 712 from λ ₁ to λ _N , an optical demultiplexer 713 that splits each of the N wavelengths into two, and a fundamental mode of N wavelengths. An optical modulator 714, a polarization controller 715 for adjusting the polarization, and an optical multiplexer 716 for multiplexing the fundamental propagation modes of the respective wavelengths.

  In addition, the receiving device 720 includes the mode demultiplexer 721 described in the first and second embodiments for demultiplexing the combined three propagation modes, and the N demultiplexers for each mode. And a wavelength multiplexed signal receiving unit for receiving the wavelength multiplexed signal. The wavelength multiplexed signal receiving unit includes a wavelength demultiplexer 722 for demultiplexing the N wavelengths demultiplexed for each mode into the respective wavelengths, and a polarization at each wavelength demultiplexed by the wavelength demultiplexer 722. A polarization demultiplexer 723 for separating waves and a light receiver 724 that receives the demultiplexed signal light and converts it into an electrical signal.

  The mode multiplexer 718 of the transmission device 711 and the mode duplexer 721 of the reception device 720 are connected by a multimode transmission line 719. By using the mode multiplexer and demultiplexer of the first embodiment and the second embodiment, processing is performed in the basic propagation mode in the transmission device and the reception device, and is converted into a higher-order mode only in the transmission path. The signal is transmitted. Therefore, a conventional optical device can be used as it is by using the mode multiplexer / demultiplexer of the present invention. When signal amplification is required in the transmission line, the mode multiplexer / demultiplexer of the present invention is used to convert the higher-order mode to the fundamental mode and perform amplification using an optical amplifier, and then mode multiplexing / demultiplexing. A long distance transmission is also possible by converting to a higher-order mode using a device and transmitting.

  The present invention can be applied to the information communication industry.

111, 112, 113, 114, 115, 116, 611, 612, 613: waveguide 110: first parallel waveguide section 120: second parallel waveguide section 130: mode rotator 711: transmission device 712: light source 713: Optical demultiplexer 714: Optical modulator 715: Polarization controller 716: Optical multiplexer 717: Wavelength multiplexer 718: Mode multiplexer 719: Multimode transmission path 720: Receiver 721: Mode duplexer 722 : Wavelength demultiplexer 723: Polarization demultiplexer 724: Light receiver

Claims (9)

A first waveguide propagating LP01 mode;
A second waveguide propagating LP11b mode;
A third waveguide propagating in a predetermined first higher order mode and second higher order mode;
A fourth waveguide propagating LP01 mode;
A fifth waveguide propagating in the LP11a mode;
A first coupling unit coupling the LP01 mode light of the first waveguide and the first higher-order mode light of the third waveguide;
A second coupling unit coupling the LP11b mode light of the second waveguide and the second higher-order mode light of the third waveguide;
A third coupling unit coupling the LP01 mode light of the fourth waveguide and the LP11a mode light of the fifth waveguide;
The fifth waveguide and the second waveguide are connected, and the LP11a mode light of the fifth waveguide is rotated to LP11b mode light and input to the second waveguide, and the second waveguide A mode rotator that rotates the LP11b mode of the waveguide to the LP11a mode and inputs the mode to the fifth waveguide;
With
The first higher-order mode is at least one of an LP1Xa mode (where X is an integer of 1 or more) and an LP0Y mode (where Y is an integer of 2 or more);
The second higher order mode is, LPZ1 mode (where, Z is an integer of 2 or more.) Is the mode demultiplexer. The first waveguide, the second waveguide, and the third waveguide are formed by a first parallel waveguide having the same height and formed in the same plane,
The fourth waveguide and the fifth waveguide are formed by a second parallel waveguide having the same height and formed in the same plane,
The mode rotator is formed by a sixth waveguide having the same height formed in the same plane,
The first parallel waveguide, the second parallel waveguide, and the sixth waveguide are formed in the same plane;
The mode multiplexer / demultiplexer according to claim 1.   The mode multiplexer / demultiplexer according to claim 2, wherein the sixth waveguide has a trench layer in which a groove is formed in at least a part of a cross section.   The mode multiplexer / demultiplexer according to claim 2, wherein the sixth waveguide includes a waveguide having an asymmetry in a cross section. At least one of the LP1Xa mode and the LP0Y mode is an LP11a mode,
The LPZ1 mode is an LP21 mode.
The mode multiplexer / demultiplexer according to any one of claims 1 to 4. A plurality of wavelength division multiplexing signal generation units for generating wavelength division multiplexing signals;
The wavelength multiplexed signal generator is connected to one end of each of the first waveguide, the third waveguide, and the fourth waveguide, and mode multiplexed the wavelength multiplexed signal input from the wavelength multiplexed signal generation And outputting from the other end of the third waveguide, the mode multiplexer / demultiplexer according to any one of claims 1 to 5,
A transmission apparatus comprising: A wavelength-division multiplexed signal that is mode-multiplexed is input to one end of the third waveguide, and the wavelength-division multiplexed signal obtained by demultiplexing the input signal for each mode is converted into the first waveguide, the third waveguide, and the The mode multiplexer / demultiplexer according to any one of claims 1 to 5, which outputs from each other end of the fourth waveguide,
A wavelength multiplexed signal receiver that receives the wavelength multiplexed signal output from the mode multiplexer / demultiplexer for each wavelength;
A receiving device. The transmission device according to claim 6, which transmits a mode-multiplexed wavelength multiplexed signal;
The receiving apparatus according to claim 7, wherein the receiving apparatus receives a mode multiplexed wavelength multiplexed signal from the transmitting apparatus;
A multimode optical fiber that connects the transmitting device and the receiving device and is capable of propagating a mode number equal to or greater than the number of modes that the third waveguide propagates;
A mode multiplexing communication system. A first waveguide propagating LP01 mode;
A second waveguide propagating LP11b mode;
A third waveguide propagating in a predetermined first higher order mode and second higher order mode;
A fourth waveguide propagating LP01 mode;
A fifth waveguide propagating in the LP11a mode;
The fifth waveguide and the second waveguide are connected, and the LP11a mode light of the fifth waveguide is rotated to LP11b mode light and input to the second waveguide, and the second waveguide A sixth waveguide that functions as a mode rotator that rotates the LP11b mode of the waveguide to the LP11a mode and inputs the mode to the fifth waveguide;
A first coupling unit coupling the LP01 mode light of the first waveguide and the first higher-order mode light of the third waveguide;
A second coupling unit coupling the LP11b mode light of the second waveguide and the second higher-order mode light of the third waveguide;
A third coupling unit coupling the LP01 mode light of the fourth waveguide and the LP11a mode light of the fifth waveguide;
A mode multiplexer / demultiplexer design method comprising:
Determining the waveguide width of the third waveguide, the first waveguide, and the second waveguide, and determining the interaction length of the first coupling portion and the second coupling portion; The parallel waveguide part design procedure of
A second parallel waveguide section design procedure for determining a waveguide width of the fifth waveguide and the fourth waveguide, and determining an interaction length of the third coupling section;
The waveguide width of the sixth waveguide is determined so as to propagate the LP11a mode and the LP11b mode in the used wavelength band, and the sixth waveguide is rotated so that the propagation mode rotates in the sixth waveguide. The mode rotor design procedure for determining the interaction length of the sixth waveguide so that the coupling efficiency from the LP11a mode to the LP11b mode in the wavelength band to be used is not less than a desired value. When,
In order,
The first higher-order mode is at least one of an LP1Xa mode (where X is an integer of 1 or more) and an LP0Y mode (where Y is an integer of 2 or more);
The second higher order mode is, LPZ1 mode (where, Z is an integer of 2 or more.) In which the mode demultiplexer design method.

Images