JP2014119556A - Mode multiplexer/demultiplexer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To more efficiently perform multiplexing/demultiplexing in any of two or more propagation modes.SOLUTION: A mode multiplexer/demultiplexer includes: a plurality of single mode waveguides 111 and 112 having different waveguide widths; and one multi-mode waveguide 113 propagating multimode light which has a mode number more than the number of the single mode waveguides and in which a coupling part with each single mode waveguide is arranged in a longitudinal direction. The single mode waveguides 111 and 112 has a waveguide width such that an effective refractive index in a basic mode is equal to one effective refractive index in a higher mode propagating the multi-mode waveguide 113. A coupling efficiency of the single mode waveguides 111 and 112 with the multi-mode waveguide 113 in a use wavelength band has interaction lengths Land Lto be equal to or more than a desired value.

Description

  The present invention relates to a mode multiplexer / demultiplexer necessary 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 two 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.

  As for a new transmission fiber, a fiber structure has been proposed in which the effective area (Aeff) 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 Aeff is based on a single mode operation, there is a trade-off relationship between bending loss and single mode operation, and it is difficult to significantly increase Aeff. There is.

  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”, OFC 2010, 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 multiplexing of three modes or more has been proposed.

  An object of the present invention is to perform multiplexing / demultiplexing of an arbitrary number of propagation modes of two or more with high efficiency.

The mode multiplexer / demultiplexer of the present invention is
A plurality of single mode waveguides with different waveguide widths;
A multi-mode waveguide that propagates multimode light having a number of modes greater than the number of single-mode waveguides, and in which coupling portions with the single-mode waveguides are arranged in the longitudinal direction,
Each single mode waveguide has a waveguide width such that the effective refractive index of the fundamental mode is equal to one effective refractive index of the higher order mode propagating through the multimode waveguide;
The coupling portion has an interaction length such that the coupling efficiency between each single mode waveguide and the multimode waveguide in a used wavelength band is equal to or higher than a desired value.

The method of designing the mode multiplexer / demultiplexer of the present invention is as follows.
A plurality of single mode waveguides with different waveguide widths;
A multimode waveguide that propagates multimode light having a larger number of modes than the number of single-mode waveguides, and has a multimode waveguide in which coupling portions with the single-mode waveguides are arranged in the longitudinal direction. A method of designing a multiplexer / demultiplexer,
Determining the waveguide width of the multimode waveguide so as to propagate a number of modes larger than the number of single mode waveguides in the wavelength band used;
Determining the waveguide width of each single mode waveguide such that the effective refractive index of the fundamental mode is equal to the effective refractive index of one of the higher order modes propagating through the multimode waveguide;
Determining the interaction length of the coupling part so that the coupling efficiency in the wavelength band to be used is not less than a desired value;
Have

  Depending on whether the effective refractive index of the fundamental mode in the single-mode waveguide is designed to be equal to which effective refractive index of the higher-order mode in the multi-mode waveguide, any higher-order mode multiplexing / demultiplexing can be performed. In addition, since the coupling length between each single-mode waveguide and multi-mode waveguide in the wavelength band to be used exceeds the desired value, it is possible to perform propagation mode multiplexing / demultiplexing with high efficiency. it can. Therefore, the present invention can perform multiplexing / demultiplexing of any number of propagation modes of 2 or more with high efficiency.

The transmission device of the present invention includes:
A plurality of wavelength division multiplexing signal generation units for generating wavelength division multiplexing signals;
The wavelength multiplexed signal generator is connected to each of the single mode waveguide and one end of the multimode waveguide, and the wavelength multiplexed signal input from one end of the single mode waveguide and the multimode waveguide is set to a mode. A mode multiplexer / demultiplexer of the present invention that multiplexes and outputs from the other end of the multimode waveguide;
Is provided.

The receiving apparatus of the present invention is
A wavelength-division multiplexed signal is input to one end of the multi-mode waveguide, and the wavelength-division multiplexed signal obtained by demultiplexing the input signal for each mode is added to each of the single-mode waveguide and the other multi-mode waveguide. A mode multiplexer / demultiplexer of the present invention that outputs from the end;
A wavelength multiplexed signal receiving unit that receives the wavelength multiplexed signal output to the single mode waveguide and the multimode waveguide 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 that of the multimode waveguide;
Is provided.

  According to the present invention, multiplexing / demultiplexing of an arbitrary number of propagation modes of 2 or more can be performed with high efficiency.

It is a figure explaining the mode multiplexer / demultiplexer of Embodiment 1. FIG. An example of the change in effective refractive index from the fundamental mode to the fourth higher-order mode is shown. An example of determining each waveguide width is shown. An example of the wavelength dependence of the coupling efficiency of the first higher-order mode is shown. An example of the wavelength dependence of the coupling efficiency of the second higher-order mode is shown. The flowchart which showed the procedure of the design method of a waveguide is shown. It is a figure explaining the mode multiplexer / demultiplexer of Embodiment 2. FIG. It is a figure explaining the mode multiplexing communication system of Embodiment 3. FIG.

  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)
In the present embodiment, three-mode multiplex transmission using a fundamental mode, a first higher-order mode, and a second higher-order mode that are propagation modes in an optical fiber will be described. A combination of modes to be multiplexed is arbitrary, and may be, for example, a combination of a basic mode, a first higher-order mode, and a third higher-order mode, or a combination of a basic mode, a second higher-order mode, and a third higher-order mode. It may be a combination. Arbitrary combinations are possible for the propagation modes in the waveguide. In the present embodiment, for convenience, the basic mode, the first higher-order mode, and the second higher-order mode will be described as the LP ₀₁ , LP ₁₁ , and LP ₂₁ modes, respectively.

A first embodiment of the present invention will be described. FIG. 1 is a diagram illustrating a mode multiplexer / demultiplexer according to this embodiment. The mode multiplexer / demultiplexer of this embodiment includes a waveguide 111, a waveguide 112, and a waveguide 113. The waveguide 111 and the waveguide 112 are single mode waveguides, and the waveguide 113 is a multimode waveguide. Mode demultiplexer of this embodiment, the coupling portion between the waveguide 111 and the waveguide 113 having an interaction length L _1, and a coupling portion between the waveguide 112 and the waveguide 113 having an interaction length L ₂ Have

Each coupling portion is arranged in cascade in the longitudinal direction of the waveguide 113. In the coupling portion between the waveguide 111 and the waveguide 113, the LP ₀₁ mode of the waveguide 111 and the LP ₁₁ mode of the waveguide 113 are phase-matched using an asymmetric parallel waveguide. In the coupling portion between the waveguide 112 and the waveguide 113, the LP ₀₁ mode of the waveguide 112 and the LP ₂₁ 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.

In this description, the description is limited to the three modes of the LP ₀₁ mode, the LP ₁₁ mode, and the LP ₂₁ mode, but the mode multiplexer / demultiplexer is not limited to this. For example, one multimode waveguide only has to propagate multimode light having a number of modes larger than the number of single mode waveguides, and mode multiplexing is similarly performed for higher-order modes of the LP ₂₁ mode or higher. And mode demultiplexing is possible. Further, the present invention can be applied to an arbitrary number of modes of 2 or more. For example, if there are N single mode waveguides, an (N + 1) mode mode multiplexer / demultiplexer can be configured.

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 n ₁ of the waveguide and the refractive index n ₀ of the medium around the waveguide.

The waveguide 111 and the waveguide 112 have waveguide widths w ₁ and w ₂ so as to be a single mode in the used wavelength region. 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 the present embodiment, as an example, the height h of the waveguide is set equal to the waveguide width.

The waveguide 113 is capable of propagating at least the propagation mode of LP ₂₁ mode or more, and the waveguide width w ₃ is designed so that the effective refractive index of the LP ₁₁ mode of the waveguide 113 is equal to that of the LP ₀₁ mode of the waveguide 111 and is guided. The effective refractive index of the LP ₂₁ mode of the waveguide 113 and the LP ₀₁ mode of the waveguide 112 are designed to be equal. A design method for the waveguide 111, the waveguide 112, and the waveguide 113 will be specifically described below.

  Although the waveguide 111 and the waveguide 112 can couple the fundamental mode and the higher-order mode described above even in a state where a plurality of modes can propagate, the present embodiment is designed in a single mode.

As an example, effective from LP ₀₁ mode of wavelength 1530 nm to LP ₀₂ mode (third higher-order mode in an optical fiber) with respect to the waveguide width w when the relative refractive index difference Δ of each waveguide is 0.4%. The change in refractive index is shown in FIG. FIG. 2 shows the effective refractive index of the propagation mode of the waveguide on the vertical axis with the waveguide width w (the height h of the waveguide is equal to w) on the horizontal axis. FIG. 2 shows that when the relative refractive index difference Δ is 0.4%, three-mode propagation can be realized by setting the waveguide width w to 12.0 μm or more.

Figure 3 shows a diagram for determining the waveguide width w ₁ and the waveguide width w ₂ and the waveguide width w _3. FIG. 3 shows the value of the effective refractive index of the propagation mode at a wavelength of 1550 nm as an example. Here, the waveguide width w ₃ is 13.7 μm capable of propagating in three modes or more. At this time, the waveguide width w ₁ and the waveguide width w ₂ may be determined in a region where a single mode is used, and therefore the waveguide width may be set to 8 μm or less. Since the waveguide width w ₁ may be set equal to the effective refractive index of the LP ₁₁ mode of the waveguide 113, w ₁ is 7.17 μm. Further, since the waveguide width w ₂ should be equal to the LP ₂₁ mode of the waveguide 113 and the effective refractive index, w ₂ is 4.35 μm.

When the waveguide widths w ₁ , w _2, and w ₃ shown in FIG. 3 are used, the interaction length L ₁ necessary for converting the LP ₀₁ mode of the waveguide 111 into the LP ₁₁ mode of the waveguide 113 The interaction length L ₂ necessary for converting the LP ₀₁ mode of the waveguide 112 to the LP ₂₁ mode of the waveguide 113 is calculated by, for example, a beam propagation method, and the LP ₀₁ mode of the waveguide 111 is changed to the LP of the waveguide 113. _An appropriate interaction length is designed so that the coupling efficiency to the ₁₁ mode and the LP ₀₁ mode of the waveguide 112 have the maximum coupling efficiency to the LP ₂₁ mode of the waveguide 113. Here, the waveguide spacing _{g 2} waveguide interval _{g 1} and the waveguide 112 and the waveguide 113 of the waveguide 111 and the waveguide 113 when equal to 4.0μm by the beam propagation method _{LP 11} mode, _{LP 21} mode respectively the _{L 1} the coupling efficiency determining the interaction length with the maximum of 1.6 mm, _{L 2} was 1.7 mm.

In this embodiment, the design is performed up to the LP ₂₁ mode. However, the present invention is not limited to this, and a multiplexer / demultiplexer of a desired propagation mode such as using the LP ₁₁ mode and the LP ₀₂ mode is used. It is possible to realize.

Waveguide spacing g ₁ , g ₂ is 4.0 μm, waveguide width w ₁ is 7.17 μm, waveguide width w ₂ is 4.35 μm, waveguide width w ₃ is 13.7 μm, relative refractive index of each waveguide First higher-order mode coupling obtained when the difference Δ is 0.4%, the bending radius R of each waveguide is 50 mm, the interaction length L ₁ is 1.6 mm, and the interaction length L ₂ is 1.7 mm. FIG. 4A shows the wavelength dependency of efficiency, and FIG. 4B shows the wavelength dependency of the coupling efficiency of the second higher-order mode. FIG. 4 (a) shows the coupling efficiency (solid line) output as the LP ₁₁ mode, which is the first higher order mode, to the waveguide 113 when light of the LP ₀₁ mode, which is the fundamental mode, is incident on the waveguide 111. The coupling efficiency (dashed line) output to the waveguide 111 as the LP ₀₁ mode, which is the fundamental mode, is shown. FIG. 4 (b) shows the coupling efficiency (solid line) output when the light of the LP ₀₁ mode, which is the fundamental mode, is incident on the waveguide 112, and is output to the waveguide 113 as the LP ₂₁ mode, which is the second higher-order mode. The coupling efficiency (broken line) output as the LP ₀₁ mode, which is the fundamental mode, to the waveguide 112 as it is.

4 (a) and 4 (b), it can be seen that an extremely efficient three-mode multiplexer / demultiplexer having a coupling efficiency of 95% or more in the wavelength band of 1530 nm to 1565 nm can be realized. Here, when a fiber having a core diameter of 18 μm and a core relative refractive index difference Δ of 0.4% is processed as an optical fiber capable of propagating three or more modes, the mode field diameter (MFD) of the LP ₀₁ mode of the optical fiber is The MFD of the waveguide 113 described above at 15.3 μm was 13.7 μm, and the MFD mismatch loss was estimated using the approximate expression (2) described below, and was about 0.15 dB. Therefore, the connection loss between the asymmetric parallel waveguide of the present invention and the transmission fiber is expected to be very small. When the MFDs of the transmission fiber and the waveguide are extremely different, the connection between the waveguide and the fiber is considered. It is also possible to reduce the connection loss by reducing MFD mismatch by, for example, processing one or both of them in a tapered shape. The MFD described above is calculated by approximating the field of LP ₀₁ mode for both fiber and waveguide by Gaussian approximation.

ここで、Ｗ_１，Ｗ_２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。

Here, W ₁ and W ₂ are the MFD of the optical fiber and the MFD of the waveguide, respectively, and η indicates the coupling loss.

Although the first embodiment is designed under the condition where Δ of the waveguide is equal and the waveguide interval is equal, the present invention is not limited to this, and Δ of each of the three waveguides may be different. Further, the waveguide intervals g ₁ and g ₂ may be different.

Next, a method for designing a mode multiplexer / demultiplexer according to the present invention will be described. FIG. 5 is a flowchart showing the procedure of the waveguide design method according to the present embodiment.
First, the wavelength band to be used is determined (S101).
Next, the relative refractive index difference Δ and the waveguide width w ₃ are determined so as to achieve a three-mode operation or more at the lower limit wavelength of the wavelength band to be used (S102, S103). For example, Δ and w ₃ are determined by obtaining a cutoff wavelength using waveguide analysis such as a finite element method. In the following steps, waveguide analysis is performed using the center wavelength of the used wavelength band, and waveguide parameters are determined.
Next, the waveguide width w ₁ is determined so that the effective refractive indexes of the LP ₀₁ mode of the waveguide 111 and the LP ₁₁ mode of the waveguide 113 are equal (S104, S105). Also in determining the w _1, to calculate the effective refractive index with a waveguide analysis such as the finite element method.
Next, the waveguide width w ₂ is determined so that the effective refractive index of the LP ₂₁ mode of the waveguide 113 is equal to the effective refractive index of the LP ₀₁ mode of the waveguide 112 (S106, S107).
Next, the waveguide intervals g ₁ and g ₂ are determined (S108), and the interaction lengths L ₁ and L ₂ are determined using the determined waveguide intervals g ₁ and g ₂ (S109). The interaction lengths L ₁ and L ₂ are calculated by performing propagation analysis such as a beam propagation method to calculate the interaction length.
After calculating the interaction lengths L ₁ and L ₂ , 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 g ₁ and g ₂ are reset (S108), and the interaction length L _1, and calculates the _{L 2} (S109). When a desired coupling efficiency is obtained (Yes in S110), a mode conversion characteristic or a demultiplexing characteristic is extracted (S111). If the desired mode conversion characteristic or demultiplexing characteristic is obtained, the parameters determined so far are determined.

  As described above, the mode multiplexer / demultiplexer according to the present embodiment can achieve 95% or more of the demultiplexing efficiency of the fundamental mode and the two higher-order modes in the wavelength band of C band (1530 nm to 1565 nm). Therefore, it is possible to increase the speed and distance of mode multiplexing transmission using any number of propagation modes of 2 or more.

  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 is used. Thus, it becomes possible to realize mode multiplex transmission with an arbitrary number of modes of 2 or more. For this reason, it is possible to realize mode multiplex transmission of an arbitrary number of modes of 2 or more in combination with existing multiplexing techniques such as wavelength multiplexing with a simple configuration.

(Embodiment 2)
A second embodiment of the present invention will be described. This embodiment relates to a usage example of (2N + 1) mode multiplexing of the mode multiplexer / demultiplexer shown in the first embodiment (N is a natural number). Since the mode multiplexer / demultiplexer shown in the first embodiment is a parallel waveguide having two coupling portions, (2N + 1) mode conversions are made by connecting the mode multiplexer / demultiplexer shown in the first embodiment in cascade. It can correspond to. FIG. 6 shows a configuration example for converting up to LP ₃₁ mode (fourth higher-order mode in an optical fiber) when N = 2.

In the mode multiplexer / demultiplexer of FIG. 6, a mode multiplexer / demultiplexer 600 and a mode multiplexer / demultiplexer 601 are connected in cascade. The mode multiplexer / demultiplexer 600 includes a waveguide 611, a waveguide 612, and a waveguide 613. The mode multiplexer / demultiplexer 601 includes a waveguide 614, a waveguide 615, and a waveguide 613. The waveguide widths of the waveguides 611 to 615 are W _{1 to} W ₅ , respectively, and each waveguide has the same width and height. Waveguide gap of the waveguide 611 and the waveguide 613 are _{g 1,} waveguide spacing of the waveguide 612 and the waveguide 613 is _{g 2,} waveguide spacing of the waveguide 614 and the waveguide 613 is in _{g 3} , waveguide spacing of the waveguide 615 and the waveguide 613 is _{g 4.} Interaction length of the waveguide 611 and the waveguide 613 is _{L 1,} the interaction length of the waveguide 612 and the waveguide 613 is _{L 2,} the interaction length of the waveguide 614 and the waveguide 613 is an _{L 3} , the interaction length of the waveguide 615 and the waveguide 613 is _{L 4.}

The mode multiplexer / demultiplexer 600 sets the waveguide width w ₁ , the waveguide interval g _1, and the interaction length L ₁ so that the LP ₀₁ mode input to the waveguide 611 is converted into the LP ₁₁ mode of the waveguide 613. The waveguide width w ₂ , the waveguide interval g _2, and the interaction length L ₂ are set so that the LP ₀₁ mode input to the waveguide 612 is converted into the LP ₂₁ mode of the waveguide 613. The LP ₀₁ mode input to the waveguide 613 propagates as it is in the LP ₀₁ mode, and the LP ₀₁ mode, the LP ₁₁ mode, and the LP ₂₁ mode are output from the output of the waveguide 613 of the mode multiplexer / demultiplexer 600, and the mode combining is performed. The signal is input to the waveguide 613 of the wave 601.

Here, it is assumed that the waveguide 613 has a waveguide design capable of propagating five or more modes. The mode multiplexer / demultiplexer 601 includes a waveguide width w ₄ , a waveguide interval g ₃ , and an interaction length so that the LP ₀₁ mode input to the waveguide 614 is converted into the LP ₀₂ mode of the waveguide 613. L ₃ is set, the waveguide width _{w 5} of the waveguide 615 as _{LP 01} mode input waveguide 615 is converted into _{LP 31} mode of the waveguide 613, the waveguide spacing _{g 4,} the interaction length _{L 4} Is set. In the mode multiplexer / demultiplexer 601, the waveguide width w ₃ of the waveguide 613 is set so that the LP ₀₁ mode, the LP ₁₁ mode, and the LP ₂₁ mode of the waveguide 613 are not coupled to the LP ₀₁ mode of the waveguide 614 and the waveguide 615. Set. Thereby, mode conversion up to the LP ₃₁ mode is possible. Moreover, although the function of the mode multiplexer was demonstrated, the mode multiplexer / demultiplexer of this embodiment can be utilized as a mode splitter by inverting input / output.

  Further, by connecting the mode multiplexer / demultiplexer shown in Embodiment 1 in cascade, it is possible to convert to a higher order mode and to perform demultiplexing. In this embodiment, an example is shown in which a mode multiplexer / demultiplexer capable of (2N + 1) mode multiplexing / demultiplexing is configured by cascading the mode multiplexer / demultiplexers described in the first embodiment. The number of single-mode waveguides (waveguides 611, 612, 614, and 615 in the example of FIG. 6) coupled to the multimode waveguide (waveguide 613 in the example of FIG. 6) is an odd number. If configured, it is also possible to configure a mode multiplexer / demultiplexer capable of 2N mode multiplexing / demultiplexing. Therefore, a mode multiplexer / demultiplexer capable of multiplexing / demultiplexing an arbitrary number of two or more modes can be configured by the invention according to the present embodiment.

In this embodiment, the relative refractive index difference Δ of the waveguide is constant. However, the relative refractive index difference Δ and the height of the waveguide are not limited to this, and the waveguides having different values are used. You may design with a waveguide parameter. In addition, the embodiment has been described in which the waveguide width w ₃ of the waveguide 613 is the same size in the mode multiplexer / demultiplexer 600 and the mode multiplexer / demultiplexer 601, but is guided between the mode multiplexer / demultiplexers 600 and 601. The size of the waveguide may be changed within a range in which a desired number of propagation modes can be maintained, for example, by making the waveguide 613 tapered. By changing the size of the waveguide between the mode multiplexers / demultiplexers, the design for suppressing coupling other than the desired mode at the coupling portion of the waveguide 613 between the mode multiplexers / demultiplexers is facilitated.

(Embodiment 3)
Embodiment 3 of the present invention will be described. The present invention relates to a usage example of the mode multiplexer / demultiplexer shown in the first 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. 7 shows a configuration example of a system for performing three-mode multiplexing on WDM and PDM. The use 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 shown in FIG. 7, the polarization multiplexing wavelength division multiplexing transmission system includes a transmission device 511 and a reception device 520. The transmission device 511 includes three wavelength multiplexed signal generation units and a mode multiplexer 518 that multiplexes (mode converts) the propagation modes described in the first embodiment. Each wavelength multiplexed signal generation unit includes N light sources 512 from λ ₁ to λ _N , an optical demultiplexer 513 that splits the light of each of the N wavelengths into two, and light that modulates each of the fundamental modes of the N wavelengths. A modulator 514, a polarization controller 515 for adjusting the polarization, an optical multiplexer 516 for multiplexing the fundamental propagation modes of the respective wavelengths, and a wavelength multiplexer 517 for multiplexing the N wavelengths. It is comprised by.

  The receiving apparatus 520 also receives the mode demultiplexer 521 according to the first embodiment that demultiplexes the combined three propagation modes, and the wavelength at which the N wavelength multiplexed signals demultiplexed for each mode are received. And a multiple signal receiver. The wavelength multiplexed signal receiving unit includes a wavelength demultiplexer 522 for demultiplexing the N wavelengths demultiplexed for each mode into the respective wavelengths, and a polarization at each wavelength demultiplexed by the wavelength demultiplexer 522. A polarization demultiplexer 523 for separating the wave and a light receiving circuit 524 that receives the demultiplexed signal light and converts it into an electrical signal.

  The mode multiplexer 518 of the transmission device 511 and the mode duplexer 521 of the reception device 520 are connected by a multimode transmission line 519. By using the mode multiplexer / demultiplexer according to the first embodiment, processing is performed in the basic propagation mode in the transmission device and the reception device, and the signal is transmitted by being converted into the higher-order mode only in the transmission path. 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.

  Although this embodiment has been described by limiting to three-mode multiplexing, according to the present invention, a mode multiplexer / demultiplexer capable of multiplexing / demultiplexing any number of two or more modes as described in the second embodiment. Therefore, the present invention is not limited to three-mode multiplexing, and mode multiplexing transmission with an arbitrary number of modes equal to or greater than two becomes possible.

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

111, 112, 113, 611, 612, 613, 614, 615: Waveguide 511: Transmission device 512: Light source 513: Optical demultiplexer 514: Optical modulator 515: Polarization controller 516: Optical multiplexer 517: Wavelength multiplexing Demultiplexer 519: Multimode transmission line 520: Receiver 521: Mode demultiplexer 522: Wavelength demultiplexer 523: Polarization demultiplexer 524: Light receiving circuit 600, 601: Mode multiplexer / demultiplexer

Claims (5)

A plurality of single mode waveguides with different waveguide widths;
A multi-mode waveguide that propagates multimode light having a number of modes greater than the number of single-mode waveguides, and in which coupling portions with the single-mode waveguides are arranged in the longitudinal direction,
Each single mode waveguide has a waveguide width such that the effective refractive index of the fundamental mode is equal to one effective refractive index of the higher order mode propagating through the multimode waveguide;
A mode multiplexer / demultiplexer having an interaction length of the coupling portion such that the coupling efficiency between each single mode waveguide and the multimode waveguide in a used wavelength band is equal to or higher than a desired value. A plurality of wavelength division multiplexing signal generation units for generating wavelength division multiplexing signals;
The wavelength multiplexed signal generator is connected to each of the single mode waveguide and one end of the multimode waveguide, and the wavelength multiplexed signal input from one end of the single mode waveguide and the multimode waveguide is set to a mode. The mode multiplexer / demultiplexer according to claim 1, wherein the mode multiplexer / demultiplexer is multiplexed and output from the other end of the multimode waveguide;
A transmission apparatus comprising: A wavelength-division multiplexed signal is input to one end of the multi-mode waveguide, and the wavelength-division multiplexed signal obtained by demultiplexing the input signal for each mode is added to each of the single-mode waveguide and the other multi-mode waveguide. The mode multiplexer / demultiplexer according to claim 1, which outputs from an end,
A wavelength multiplexed signal receiving unit that receives the wavelength multiplexed signal output to the single mode waveguide and the multimode waveguide for each wavelength;
A receiving device. The transmission apparatus according to claim 2, which transmits a mode-multiplexed wavelength multiplexed signal;
The receiving apparatus according to claim 3, 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 that of the multimode waveguide;
A mode multiplexing communication system. A plurality of single mode waveguides with different waveguide widths;
A multimode waveguide that propagates multimode light having a larger number of modes than the number of single-mode waveguides, and has a multimode waveguide in which coupling portions with the single-mode waveguides are arranged in the longitudinal direction. A method of designing a multiplexer / demultiplexer,
Determining the waveguide width of the multimode waveguide so as to propagate a number of modes larger than the number of single mode waveguides in the wavelength band used;
Determining the waveguide width of each single-mode waveguide such that the effective refractive index of the fundamental mode is equal to one effective refractive index of the higher-order mode propagating through the multi-mode waveguide;
Determining the interaction length of the coupling part so that the coupling efficiency in the wavelength band to be used is not less than a desired value;
A method of designing a mode multiplexer / demultiplexer.

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