JP5773521B2 - Mode multiplexer / demultiplexer, optical transceiver, and optical communication system - Google Patents

Description

  The present invention relates to a technique for generating or demultiplexing an optical signal having a plurality of modes in an optical communication system using a multimode optical fiber.

  Currently, traffic in optical fiber networks is increasing, and transmission capacity has been expanded using various methods such as transmission speed increase, multi-level modulation technology, and wavelength division multiplexing (WDM). However, expansion of transmission capacity using existing transmission lines and conventional transmission systems is expected to become difficult in the future, so expansion of the wavelength region, new transmission fibers and transmission systems are being studied.

  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, the usable wavelength band is considered to be limited. Furthermore, since it is difficult to realize an optical amplifier that can amplify over a wide wavelength range, there are many problems in realizing WDM in a wide wavelength range.

  Regarding a new transmission fiber, Non-Patent Document 1 proposes a fiber structure capable of expanding the effective area (Aeff) in order to suppress waveform distortion due to fiber nonlinearity. Fiber nonlinear suppression leads to an increase in the input power that can be input to the fiber, and can provide advantages such as higher transmission speed and further multi-value. However, the expansion of Aeff is based on a single mode operation, and there is a problem that it is difficult to greatly expand Aeff because the bending loss and the single mode operation are in a trade-off relationship.

  Regarding the new transmission method, Non-Patent Document 2 discloses OFDM (Orthogonal Frequency Division Multiplexing) that uses orthogonal frequency components used to improve frequency utilization efficiency in a wireless transmission method. Then, MIMO (Multiple Input Multiple Output) using a multimode optical fiber is being studied. However, since the transmitter / receiver requires complicated signal processing, there are problems such as speeding up of arithmetic processing.

JP-A-8-288911

T. T. et al. Matsui, T .; Sakamoto, K .; Tsujikawa and S.T. Tomota, “Single-mode photonic crystal fiber with low bending loss and Aeff of> 200 μm2 for ultra high-speed WDM transmission,” OFC20, PD20. Benn C.I. Thomsen, “MIMO enabled 40 Gb / s transmission using mode division multiplexing in multimode fiber,” OFC2010, OThM6 (2010). C. P. Tsekrekos and A.M. M.M. J. et al. Koonen, “Mode-selective spatial filtering for increased robustness in a mode group diversity multiplexing link,” Opt. Lett. 32, 1041-1043 (2007).

 In Patent Document 1, a multiplexing method using a propagation mode of an optical fiber is proposed. However, a method for exciting a desired higher-order mode has not been proposed, and since it is assumed that a single wavelength is used, there is a problem that it is difficult to realize a large capacity.

  Non-Patent Document 3 proposes a mode multiplexer / demultiplexer for using a propagation mode of an optical fiber. Here, mode coupling is performed by varying the excitation position in the optical fiber for each mode, and mode separation is performed by varying the reception position in the optical fiber for each mode. In other words, no precise mode multiplexing / demultiplexing method and multiple mode multiplexing / demultiplexing methods have been proposed, so there are losses between multiple signals and crosstalk, resulting in a complicated configuration and long distance and large capacity. There is a problem that it is difficult to realize transmission.

  In Patent Document 1 and Non-Patent Document 3, in a mode multiplex transmission using an optical fiber having a plurality of propagation modes and using each of the propagation modes of the optical fiber as a carrier, the existing fiber device operates in the basic mode. Since it is a premise, it is difficult to realize mode multiplex transmission using an existing fiber device as it is.

  Therefore, in order to solve the above problems, the present invention improves the accuracy of mode multiplexing and demultiplexing, reduces loss during multiplexing and crosstalk during demultiplexing, and uses existing fiber devices. The purpose is to realize mode multiplex transmission.

In order to achieve the above object, in the mode multiplexer / demultiplexer, the first waveguide includes an optical signal having an n _1st order mode and an (n ₁ + m) order mode (n ₁ and m are positive integers). The second waveguide can propagate an optical signal having the n _second- order mode (n ₂ is a positive integer), and the (n ₁ + m) -th order of the _first waveguide. The mode and the n _second- order mode of the second waveguide can be coupled.

Mode when multiplexing, the first waveguide is input an optical signal having a first n ₁ order mode, the second waveguide is input an optical signal having a second n ₂ order mode, first waveguide first n ₁ primary The optical signal having the mode and the (n ₁ + m) -th order mode is output.

During mode demultiplexing, the first waveguide is input an optical signal having a first n ₁ order mode and the (n 1 _{+ m)} order mode, and outputs an optical signal by the second waveguide having a second n ₂ order mode, the first waveguide is arranged to output an optical signal having a first n ₁ order mode.

Specifically, the present invention is a mode multiplexer including a first waveguide and a second waveguide , each having a rectangular cross section and having the same height, wherein the first waveguide is in height, the n _1-order mode (n ₁ is a positive integer) optical signal and the (n 1 _{+ m)} order mode (m is a positive integer) width and the relative refractive possible propagate optical signals having having set the rate difference, the second waveguide, the said height, the (n 1 _{+ m)} the n _2-order mode can be combined with a next mode (n ₂ of the first waveguide and n _{1 +} m A width and relative refractive index difference capable of propagating optical signals having different positive integers), and the first waveguide and the second waveguide are close to each other with a predetermined waveguide interval and interaction length in further comprising a mode multiplexing section that is parallel, the (n 1 _{+ m)} of the first waveguide follows The effective refractive index of the over-de is substantially coincident with the effective refractive index of the n _2-order mode of the second waveguide, and unlike the effective refractive index of the n _1-order mode of the first waveguide, said mode coupling The wave unit converts an optical signal having the n _second- order mode propagating in the second waveguide into an optical signal having the (n ₁ + m) -th mode of the first waveguide, and the first waveguide Is a mode multiplexer that multiplexes an optical signal having the (n ₁ + m) -order mode of the first waveguide after conversion into an optical signal having the n- _first order mode propagating by .

  According to this configuration, it is possible to provide a mode multiplexer that improves the accuracy of mode multiplexing and reduces loss during multiplexing.

In addition, the present invention is a mode multiplexer characterized by n ₁ = n ₂ .

Further, the present invention is a mode multiplexer in which n ₁ = n ₂ = 1, and the n _1st order mode and the n _2nd order mode are fundamental modes.

  According to this configuration, since the first waveguide and the second waveguide commonly input an optical signal having the same mode, for example, the fundamental mode, an existing transmitter that outputs an optical signal having the same mode, for example, the fundamental mode, is used. Mode multiplex transmission can be realized.

Further, the present invention is a mode duplexer including a first waveguide and a second waveguide each having a rectangular cross section and having the same height, wherein the first waveguide is at the height. , the n _1-order mode (n ₁ is a positive integer) optical signal and the (n 1 _{+ m)} order mode (m is a positive integer) in width and relative refractive index difference optical signal can be propagated with having is set, the second waveguide, the said height, said first waveguide first (n 1 _{+ m)} the n _2-order mode can be combined with a next mode (n ₂ are positive different from n _{1 +} m The width and relative refractive index difference are set such that an optical signal having an integer) can be propagated, and the first waveguide and the second waveguide are arranged in parallel with each other with a predetermined waveguide interval and interaction length. mode demultiplexer further comprising a first (n 1 _{+ m)} the following modes of the first waveguide The effective refractive index is substantially coincident with the effective refractive index of the n _2-order mode of the second waveguide, and unlike the effective refractive index of the n _1-order mode of the first waveguide, said mode demultiplexing unit The optical signal having the (n ₁ + m) order mode propagating in the first waveguide is converted into the optical signal having the n _second order mode of the _second waveguide, and the first waveguide propagates. The optical signal having the n _second order mode of the _second waveguide after the conversion is demultiplexed from the optical signal having the n _first order mode and the optical signal having the (n ₁ + m) order mode. It is a mode splitter.

  According to this configuration, it is possible to provide a mode demultiplexer that improves the accuracy of mode demultiplexing and reduces crosstalk during demultiplexing.

The present invention is also a mode duplexer characterized in that n ₁ = n ₂ .

In addition, the present invention provides a mode duplexer in which n ₁ = n ₂ = 1, and the n _1st order mode and the n _2nd order mode are fundamental modes.

  According to this configuration, the first waveguide and the second waveguide commonly output an optical signal having the same mode, for example, a fundamental mode. Mode multiplex transmission can be realized.

The present invention also light having a mode multiplexer, a first transmitter for outputting an optical signal having a first n ₁ order mode in the first waveguide, the second n ₂ order mode to the second waveguide And a second transmitter that outputs a signal.

  According to this configuration, mode multiplexing accuracy is improved, loss during multiplexing is reduced, and mode multiplexing transmission is realized using an existing transmitter that outputs an optical signal having the same mode, for example, a basic mode An optical transmission device can be provided.

In addition, the present invention includes a mode duplexer, a first receiver that receives an optical signal having the nth _first- order mode from the _first waveguide, and an nth-order mode from the _second waveguide. And a second receiver to which an optical signal is input.

  According to this configuration, mode demultiplexing accuracy is improved, crosstalk during demultiplexing is reduced, and mode multiplexing transmission is realized using an existing receiver that inputs an optical signal having the same mode, for example, a basic mode. An optical receiving device can be provided.

According to another aspect of the present invention, there is provided an optical signal having the n _first- order mode of the first waveguide and the first waveguide connected to the optical transmitter, the optical receiver, the optical transmitter and the optical receiver. And a multimode optical fiber that transmits an optical signal having the (n ₁ + m) th order mode.

  According to this configuration, the accuracy of mode multiplexing and demultiplexing is improved, loss at the time of multiplexing and crosstalk at the time of demultiplexing are reduced, and existing signals that input and output optical signals having the same mode, for example, the basic mode It is possible to provide an optical communication system that realizes mode multiplexing transmission using a transceiver.

  The present invention improves the accuracy of mode multiplexing and demultiplexing, reduces the loss at the time of multiplexing and the crosstalk at the time of demultiplexing, and can realize mode multiplexing transmission using an existing fiber device.

FIG. 3 is a diagram illustrating a configuration of a mode multiplexer / demultiplexer according to the first embodiment. 6 is a diagram illustrating a configuration of a mode multiplexer / demultiplexer according to Embodiment 2. FIG. 6 is a diagram illustrating a relationship between a relative refractive index difference and a waveguide width in Embodiment 2. FIG. It is a figure which shows the relationship between the relative refractive index difference and interaction length in Embodiment 2. It is a figure which shows the relationship between the use wavelength in Embodiment 2, and the normalized transmittance | permeability. It is a figure which shows the design method of the mode multiplexer / demultiplexer of Embodiment 2. FIG. It is a figure which shows the structure of the mode multiplexer / demultiplexer of Embodiment 3. FIG. It is a figure which shows the structure of the optical communication system of Embodiment 4.

  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.

(Embodiment 1)
The configuration of the mode multiplexer / demultiplexer of the first embodiment is shown in FIG. The mode multiplexer / demultiplexer 1 includes a first waveguide 11 and a second waveguide 12. The first waveguide 11, the n _1-order mode (n ₁ is a positive integer) (in m positive integer) optical signal and the (n 1 _{+ m)} order mode having a can propagate an optical signal having a. The second waveguide 12 can propagate an optical signal having an n _second- order mode (n ₂ is a positive integer). The (n 1 _{+ m)} the n _2-order mode of the next mode and the second waveguide 12 of the first waveguide 11 can be coupled, This will be described.

In the lower side of FIG. 1, in the first waveguide 11 and the second waveguide 12, the actual refractive index is indicated by a solid line, and the effective refractive index of each mode is indicated by a broken line. The actual refractive index increases at the core portions of the first waveguide 11 and the second waveguide 12 and decreases at the cladding portions of the first waveguide 11 and the second waveguide 12. The (n 1 _{+ m)} the effective refractive index of the following modes of the first waveguide 11 is substantially coincident with the effective refractive index of the n _2-order mode of the second waveguide 12, the n _1-order first waveguide 11 Less than the effective refractive index of the mode. Therefore, the (n 1 _{+ m)} the n _2-order mode of the next mode and the second waveguide 12 of the first waveguide 11 can be attached is attained phase matching.

A case where the mode multiplexer / demultiplexer 1 functions as a mode multiplexer will be described. The left side of FIG. 1 is the input side, and the right side of FIG. 1 is the output side. The first waveguide 11 is input an optical signal having a first n ₁ order mode. The second waveguide 12 receives an optical signal having an n _second order mode. In the mode multiplexing unit in which the first waveguide 11 and the second waveguide 12 are parallel, the optical signal having the n _second- order mode propagating through the _second waveguide 12 is transmitted to the (n ₁ + m) th of the _first waveguide 11. ) is converted into an optical signal having the following mode, the optical signal having a first n ₁ order mode in which the first waveguide 11 propagates light having a first (n 1 _{+ m)} the following modes of the first waveguide 11 after conversion Combine the signals. Thus, an optical signal having a first n ₁ order mode and the (n 1 _{+ m)} The following modes are modes combined.

  In the present invention, unlike Non-Patent Document 3, mode coupling is not performed by changing the excitation position in the optical fiber for each mode, and different modes are used between the first waveguide 11 and the second waveguide 12. Mode coupling is performed by coupling between the two. Therefore, it is possible to provide a mode multiplexer that improves the accuracy of mode multiplexing and reduces loss during multiplexing.

During mode _{multiplexing,} it may be n 1 = _{n 2,} an n 1 _{= n} 2 = 1, the _{n 1-order} mode and the _{n 2-order} mode can be a fundamental mode. At this time, since the first waveguide 11 and the second waveguide 12 commonly input an optical signal having the same mode, for example, the fundamental mode, an existing transmitter that outputs an optical signal having the same mode, for example, the fundamental mode, is used. Mode multiplex transmission can be realized. In Embodiment _2-4, a n 1 _{= n} 2 = 1, as the _{n 1-order} mode and the _{n 2-order} mode is the fundamental mode will be described.

A case where the mode multiplexer / demultiplexer 1 functions as a mode duplexer will be described. The right side of FIG. 1 is the input side, and the left side of FIG. 1 is the output side. The first waveguide 11 is input an optical signal having a first _{n 1} order mode and the _(n 1 + m) order mode. In the mode demultiplexing unit in which the first waveguide 11 and the second waveguide 12 are parallel, the optical signal having the (n ₁ + m) -th mode propagating through the first waveguide 11 is transmitted to the nth of the second waveguide 12. _{The second} waveguide after conversion from the optical signal having the n-th _first- order mode and the optical signal having the (n ₁ + m) -th order mode, which is converted into the optical signal having the _second- order mode and propagates through the first waveguide 11 An optical signal having 12 n _second- order modes is demultiplexed. Thus, an optical signal having a first n ₁ order mode and the (n 1 _{+ m)} The following modes are modes demultiplexing.

  In the present invention, unlike Non-Patent Document 3, the receiving position in the optical fiber is not changed for each mode, and mode separation is not performed, and different modes are used between the first waveguide 11 and the second waveguide 12. Mode demultiplexing is performed by coupling between the two. Therefore, it is possible to provide a mode demultiplexer that improves the accuracy of mode demultiplexing and reduces crosstalk during demultiplexing.

During mode demultiplexing _may be an n 1 = _{n 2,} an n 1 _{= n} 2 = 1, the _{n 1-order} mode and the _{n 2-order} mode can be a fundamental mode. At this time, since the first waveguide 11 and the second waveguide 12 commonly output an optical signal having the same mode, for example, the fundamental mode, an existing receiver that inputs the optical signal having the same mode, for example, the fundamental mode, is used. Mode multiplex transmission can be realized. In Embodiment _2-4, a n 1 _{= n} 2 = 1, as the _{n 1-order} mode and the _{n 2-order} mode is the fundamental mode will be described.

(Embodiment 2)
The configuration of the mode multiplexer / demultiplexer of the second embodiment is shown in FIG. The mode multiplexer / demultiplexer 1 includes a first waveguide 11 and a second waveguide 12. The first higher-order mode of the first waveguide 11 and the fundamental mode of the second waveguide 12 can be coupled. At the time of mode multiplexing, as in the first embodiment, an optical signal having a fundamental mode is input to the first waveguide 11 and the second waveguide 12, and the first waveguide 11 has the fundamental mode and the first higher-order mode. An optical signal is output. At the time of mode demultiplexing, as in the first embodiment, an optical signal having a fundamental mode and a first higher-order mode is input to the first waveguide 11 and has a fundamental mode from the first waveguide 11 and the second waveguide 12. An optical signal is output.

The lower side of FIG. 2 shows the actual refractive index distribution in the first waveguide 11 and the second waveguide 12. Refractive index of the core portion of the first waveguide 11 and the second waveguide 12 is n _1, the refractive index of the cladding portion of the first waveguide 11 and the second waveguide 12 is n _0, the relative refractive index difference Is Δ = (n ₁ ² −n ₀ ² ) / (2n ₁ ² ). Within the plane perpendicular to the propagation direction, the cross-sectional shape of the first waveguide 11 is a rectangular shape having a width w1 and a height h (= w2), and the cross-sectional shape of the second waveguide 12 is It is a rectangular shape having a width w2 and a height h (= w2). In a plane perpendicular to the propagation direction, the waveguide interval between the first waveguide 11 and the second waveguide 12 is g, and in the propagation direction, the first waveguide 11 and the second waveguide 12 are The parallel interaction length is L.

  W2, h, and Δ for the second waveguide 12 are set so that only the fundamental mode exists in the used wavelength band. W1, h, and Δ for the first waveguide 11 have a fundamental mode and a first higher-order mode in the used wavelength band, and the first higher-order mode of the first waveguide 11 and the fundamental mode of the second waveguide 12 Are set to combine. g is set to be narrow so that the coupling efficiency is high, and L is set to be long so that the coupling efficiency is high.

  FIG. 3 shows the relationship between the relative refractive index difference and the waveguide width in the second embodiment. The used wavelength band is assumed to be 1450 nm or more. It can be seen that the width of the first waveguide 11 may be approximately 2.6 times the width of the second waveguide 12 within the range of the relative refractive index difference shown in FIG.

  FIG. 4 shows the relationship between the relative refractive index difference and the interaction length in the second embodiment. The interaction length was calculated so that the coupling efficiency would be the desired efficiency when the waveguide spacing was 2 μm and 3 μm. The interaction length was calculated when an optical signal having x polarization and y polarization was incident. Although the cross-sectional shapes of the first waveguide 11 and the second waveguide 12 are rectangular, the polarization dependence of the interaction length is hardly recognized, and the mode multiplexer / demultiplexer 1 is used in combination with polarization multiplexing. I understand that I can do it.

  FIG. 5 shows the relationship between the wavelength used and the normalized transmittance in the second embodiment. The used wavelength band is assumed to be from 1450 nm to 1650 nm. The normalized transmittance for the fundamental mode is the ratio of the intensity of the optical signal having the fundamental mode output from the first waveguide 11 to the intensity of the optical signal having the fundamental mode input to the first waveguide 11. . The normalized transmittance for the first higher-order mode is the optical signal having the first higher-order mode output from the first waveguide 11 with respect to the intensity of the optical signal having the fundamental mode input to the second waveguide 12. Is the ratio of strength. In the upper stage, middle stage, and lower stage of FIG. 5, Δ = 0.35%, 0.45%, and 0.60%. g is 2 μm, and L is calculated based on FIG. 4 for Δ and g described above.

  In any case of FIG. 5, the normalized transmittance for the fundamental mode is close to 100% in all used wavelength bands, and the normalized transmittance for the first higher-order mode is 90% in all used wavelength bands. Over. In any case of FIG. 5, the normalized transmittance for the fundamental mode and the first higher-order mode has almost no polarization dependence in the entire used wavelength band.

  A design method of the mode multiplexer / demultiplexer of the second embodiment is shown in FIG. Δ and w2 are determined for the second waveguide 12 (step S1). In the second waveguide 12, it is confirmed whether only a single mode exists in the used wavelength band (step S2). If YES in step S2, the process proceeds to step S3. If “NO” in the step S2, the process returns to the step S1, and Δ and w2 are determined again for the second waveguide 12. In steps S1 and S2, waveguide analysis such as a finite element method can be applied.

  For the first waveguide 1, w1 is determined (step S3). It is confirmed whether the first higher-order mode of the first waveguide 11 and the fundamental mode of the second waveguide 12 can be coupled (step S4). If YES in step S4, the process proceeds to step S5. If NO in step S4, the process returns to step S3, and w1 is determined again for the first waveguide 1. In steps S3 and S4, a waveguide analysis such as a finite element method can be applied.

  g and L are determined (step S5). It is confirmed whether or not a desired coupling efficiency can be obtained (step S6). If YES in step S6, the design is terminated. If “NO” in the step S6, the process returns to the step S5, and g and L are determined again. In steps S5 and S6, propagation analysis such as a beam propagation method can be applied.

  The refractive index distribution and the cross-sectional shape of the second waveguide 12 are not limited to the refractive index distribution and the cross-sectional shape shown on the lower side of FIG. 2 if only the fundamental mode exists at the wavelength used. The refractive index distribution and the cross-sectional shape of the first waveguide 11 have a fundamental mode and a first higher-order mode at the wavelength used, and the first higher-order mode of the first waveguide 11 and the fundamental mode of the second waveguide 12 are If coupled, the refractive index profile and the cross-sectional shape shown in the lower side of FIG. 2 are not limited.

(Embodiment 3)
The configuration of the mode multiplexer / demultiplexer of the third embodiment is shown in FIG. The mode multiplexer / demultiplexer 1 arranges the mode multiplexer / demultiplexer 1-1 and the mode multiplexer / demultiplexer 1-2 in a column. The mode multiplexer / demultiplexer 1-1 includes a first waveguide 11-1 and a second waveguide 12-1. The mode multiplexer / demultiplexer 1-2 includes a first waveguide 11-2 and a second waveguide 12-2. The first waveguide 11-1 and the first waveguide 11-2 are connected to each other. The first higher-order mode of the first waveguide 11-1 and the fundamental mode of the second waveguide 12-1 can be coupled. The second higher-order mode of the first waveguide 11-2 and the fundamental mode of the second waveguide 12-2 can be coupled.

  The mode multiplexing will be described. First, in the mode multiplexer / demultiplexer 1-1, an optical signal having a fundamental mode is input to the first waveguide 11-1 and the second waveguide 12-1, and the fundamental mode and the first waveguide are transmitted from the first waveguide 11-1. An optical signal having a higher order mode is output. Next, in the mode multiplexer / demultiplexer 1-2, an optical signal having a fundamental mode and a first higher-order mode is input to the first waveguide 11-2, and an optical signal having the fundamental mode is input to the second waveguide 12-2. Are input, and an optical signal having a fundamental mode, a first higher-order mode, and a second higher-order mode is output from the first waveguide 11-2.

  The mode demultiplexing will be described. First, in the mode multiplexer / demultiplexer 1-2, an optical signal having a fundamental mode, a first higher-order mode, and a second higher-order mode is input to the first waveguide 11-2, and the fundamental is transmitted from the second waveguide 12-2. An optical signal having a mode is output, and an optical signal having a fundamental mode and a first higher-order mode is output from the first waveguide 11-2. Next, in the mode multiplexer / demultiplexer 1-1, an optical signal having a fundamental mode and a first higher-order mode is input to the first waveguide 11-1, and an optical signal having the fundamental mode is input from the second waveguide 12-1. Is output, and an optical signal having a fundamental mode is output from the first waveguide 11-1.

The lower left of FIG. 7 shows the actual refractive index distribution in the first waveguide 11-1 and the second waveguide 12-1. Refractive index of the core portion of the first waveguide 11-1 and the second waveguide 12-1 is _{n 1,} the refractive index of the cladding portion of the first waveguide 11-1 and the second waveguide 12-1 n ₀ . Within the plane perpendicular to the propagation direction, the cross-sectional shape of the first waveguide 11-1 is a rectangular shape having a width w1 and a height h (= w2). The cross-sectional shape is a rectangular shape having a width w2 and a height h (= w2). In the plane perpendicular to the propagation direction, the waveguide interval between the first waveguide 11-1 and the second waveguide 12-1 is g1, and in the propagation direction, the first waveguide 11-1 and The interaction length with which the second waveguide 12-1 is parallel is L1.

The lower right of FIG. 7 shows the actual refractive index distribution in the first waveguide 11-2 and the second waveguide 12-2. Refractive index of the core portion of the first waveguide 11-2 and the second waveguide 12-2 is _{n 1,} the refractive index of the cladding portion of the first waveguide 11-2 and the second waveguide 12-2 n ₀ . Within the plane perpendicular to the propagation direction, the cross-sectional shape of the first waveguide 11-2 is a rectangular shape having a width w1 and a height h (= w2). The cross-sectional shape is a rectangular shape having a width of w3 and a height of h (= w2). In the plane perpendicular to the propagation direction, the waveguide interval between the first waveguide 11-2 and the second waveguide 12-2 is g2, and in the propagation direction, the first waveguide 11-2 and The interaction length with which the second waveguide 12-2 is parallel is L2.

  The refractive index distribution and the cross-sectional shape of the first waveguide 11-1, the second waveguide 12-1, the first waveguide 11-2, and the second waveguide 12-2 in the third embodiment are the same as those in the first embodiment. What is necessary is just to set like the refractive index distribution and cross-sectional shape of the waveguide 11 and the 2nd waveguide 12. FIG.

(Embodiment 4)
FIG. 8 shows the configuration of the optical communication system according to the fourth embodiment. The optical communication system includes an optical transmitter T, a multimode optical fiber F, and an optical receiver R. The optical transmission device T includes a mode multiplexer 1M and a first transmitter and a second transmitter described below. The optical receiving device R includes a mode demultiplexer 1D and a first receiver and a second receiver described below.

  The mode multiplexer 1 of the second embodiment is applied as the mode multiplexer 1M. The first transmitter outputs an optical signal having a fundamental mode to the first waveguide 11. The second transmitter outputs an optical signal having a fundamental mode to the second waveguide 12. When three transmitters are arranged, the mode multiplexer / demultiplexer 1 of the third embodiment may be applied as the mode multiplexer 1M. When N transmitters are arranged, the mode multiplexer 1M is used as the mode multiplexer 1M. What is necessary is just to apply what arrange | positioned the mode multiplexer / demultiplexer 1 of Embodiment 2 in the column of (N-1).

  The mode multiplexer / demultiplexer 1 of the second embodiment is applied as the mode duplexer 1D. The first receiver inputs an optical signal having a fundamental mode from the first waveguide 11. The second receiver receives an optical signal having a fundamental mode from the second waveguide 12. When three receivers are arranged, the mode multiplexer / demultiplexer 1 of the third embodiment may be applied as the mode demultiplexer 1D. When N receivers are arranged, the mode demultiplexer 1D is What is necessary is just to apply what arrange | positioned the mode multiplexer / demultiplexer 1 of Embodiment 2 in the column of (N-1).

The first transmitter and the second transmitter use both the wavelength multiplexing method using the used wavelengths λ _{1 to} λ _N and the polarization multiplexing method using x and y polarization. However, the first transmitter and the second transmitter do not need to apply the multiplexing method, and may apply any multiplexing method other than the mode multiplexing method.

The first (2) transmitter is used by using the light source 21-1, ..., 21-N (22-1, ..., 22-N) and the wavelength multiplexer 71 (72). A wavelength multiplexing method using wavelengths λ _{1 to} λ _N is applied. The first (2) transmitter includes optical demultiplexers 31-1,..., 31-N (32-1,..., 32-N), polarization controllers 51X-1, 51Y-1,. ..., 51X-N, 51Y-N (52X-1, 52Y-1, ..., 52X-N, 52Y-N), and optical multiplexers 61-1, ..., 61-N (62- 1,..., 62-N), and a polarization multiplexing scheme using x, y polarization is applied. The first (2) transmitter includes optical modulators 41X-1, 41Y-1, ..., 41X-N, 41Y-N (42X-1, 42Y-1, ..., 42X-N, 42Y- N) is used to perform modulation processing of an optical signal having each wavelength and each polarization.

The first receiver and the second receiver use both the wavelength multiplexing method using the used wavelengths λ _{1 to} λ _N and the polarization multiplexing method using the x and y polarized waves. However, the first receiver and the second receiver do not need to apply the multiplexing method, and may apply any multiplexing method other than the mode multiplexing method.

The first (2) receiver uses the wavelength demultiplexer 81 (82) and applies the wavelength multiplexing method using the used wavelengths λ _{1 to} λ _N. The first (2) receiver uses the polarization splitters 91-1,..., 91-N (92-1,..., 92-N) and uses the x and y polarizations. Apply the polarization multiplexing method. The first (2) receiver includes light receiving circuits 101X-1, 101Y-1, ..., 101X-N, 101Y-N (102X-1, 102Y-1, ..., 102X-N, 102Y-N). ) To receive and demodulate an optical signal having each wavelength and each polarization.

  The first transmitter and the second transmitter need only process the optical signal having the fundamental mode, and may not process the optical signal having the higher order mode. The first receiver and the second receiver need only process the optical signal having the fundamental mode, and may not process the optical signal having the higher order mode. Only the mode multiplexer 1M, the multimode optical fiber F, and the mode duplexer 1D process optical signals having a fundamental mode and a higher-order mode. Therefore, mode multiplexing transmission can be realized using an existing transceiver that inputs and outputs an optical signal having a basic mode.

  When performing optical amplification in the transmission path of the multimode optical fiber F, the mode demultiplexer of the present invention is used to convert the higher order mode into the fundamental mode, and an optical signal having the fundamental mode is amplified. The basic mode is converted into a higher-order mode using the mode multiplexer of the present invention. Therefore, mode multiplex transmission can be realized using an existing optical amplifier that amplifies an optical signal having a fundamental mode, and long-distance transmission can be realized.

  The mode multiplexer / demultiplexer, optical transmission / reception apparatus, and optical communication system according to the present invention can be applied not only to the mode multiplexing method but also to the mode multiplexing method and other multiplexing methods (such as WDM and polarization multiplexing). It can be applied to the method.

1, 1-1, 1-2: mode multiplexer / demultiplexer 1M: mode multiplexer 1D: mode duplexers 11, 11-1, 11-2: first waveguides 12, 12-1, 12-2 : Second waveguide T: optical transmitter R: optical receiver F: multimode optical fiber 21, 22: light source 31, 32: optical demultiplexer 41, 42: optical modulator 51, 52: polarization controller 61, 62: optical multiplexer 71, 72: wavelength multiplexer 81, 82: wavelength demultiplexer 91, 92: polarization demultiplexer 101, 102: light receiving circuit

Claims (9)

A mode multiplexer having a first waveguide and a second waveguide, each having a rectangular cross section and the same height,
Wherein the first waveguide in the height (in m positive integer) optical signal and the (n 1 _{+ m)} The following mode the n _1-order mode (n ₁ is a positive integer) having an optical signal having a Set to the width and relative refractive index difference that can propagate,
The second waveguide has an n _second- order mode (n ₂ is a positive integer different from n ₁ + m) that can be coupled with the (n ₁ + m) -th mode of the _first waveguide at the height. Set to a width and relative refractive index difference capable of propagating an optical signal having,
A mode multiplexing unit in which the first waveguide and the second waveguide are arranged in parallel in a state of being close to each other with a predetermined waveguide interval and interaction length;
The effective refractive index of the (n ₁ + m) th order mode of the first waveguide substantially matches the effective refractive index of the n _second order mode of the _second waveguide, and the n _1st order of the _first waveguide. Unlike the effective refractive index of the next mode,
The mode multiplexing unit converts an optical signal having an n _second order mode propagating in the second waveguide into an optical signal having an (n ₁ + m) order mode of the first waveguide, and the optical signal having a first n ₁ order mode 1 waveguide propagates, the (n 1 _{+ m)} of the first waveguide after conversion mode, characterized in that for multiplexing the optical signal having the mode multiplexer vessel. The mode multiplexer according to claim 1, wherein n ₁ = n ₂ . n 1 ₌ a _n 2 = _1, wherein the first n ₁ order mode and the n _2-order mode is the basic mode, the mode multiplexer according to claim 1 or 2. A mode duplexer having a first waveguide and a second waveguide, each having a rectangular cross section and the same height,
Wherein the first waveguide in the height (in m positive integer) optical signal and the (n 1 _{+ m)} The following mode the n _1-order mode (n ₁ is a positive integer) having an optical signal having a Set to the width and relative refractive index difference that can propagate,
The second waveguide has an n _second- order mode (n ₂ is a positive integer different from n ₁ + m) that can be coupled with the (n ₁ + m) -th mode of the _first waveguide at the height. Set to a width and relative refractive index difference capable of propagating an optical signal having,
A mode demultiplexing unit in which the first waveguide and the second waveguide are arranged in parallel with each other with a predetermined waveguide interval and interaction length;
The effective refractive index of the (n ₁ + m) th order mode of the first waveguide substantially matches the effective refractive index of the n _second order mode of the _second waveguide, and the n _1st order of the _first waveguide. Unlike the effective refractive index of the next mode,
The mode demultiplexing unit converts an optical signal having the (n ₁ + m) th order mode propagating in the first waveguide into an optical signal having the n _second order mode of the _second waveguide, and The optical signal having the n _second- order mode of the _second waveguide after conversion is separated from the optical signal having the n-th _first- order mode propagating in one waveguide and the optical signal having the (n ₁ + m) -th order mode. A mode duplexer characterized by wave. The mode duplexer according to claim 4, wherein n ₁ = n ₂ . 6. The mode duplexer according to claim 4, wherein n ₁ = n ₂ = 1, and the n _1st order mode and the n _2nd order mode are fundamental modes. A mode multiplexer according to any one of claims 1 to 3,
A first transmitter for outputting an optical signal having an n _1st- order mode to the first waveguide;
A second transmitter that outputs an optical signal having an n _second- order mode to the second waveguide;
An optical transmission device comprising: A mode duplexer according to any one of claims 4 to 6,
A first receiver for inputting an optical signal having a first n ₁ order mode from said first waveguide,
A second receiver that receives an optical signal having an n _second- order mode from the second waveguide;
An optical receiving device comprising: An optical transmitter according to claim 7;
An optical receiver according to claim 8,
The optical transmitter and the optical receiver are connected to transmit an optical signal having the n _first- order mode of the first waveguide and an optical signal having the (n ₁ + m) -th mode of the first waveguide. A multimode optical fiber;
An optical communication system comprising:

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