JP6653886B2 - Mode multiplexer / demultiplexer and mode multiplex transmission system - Google Patents

Description

  The present disclosure relates to a mode multiplexer / demultiplexer and a mode multiplex transmission system including the same.

  In an optical fiber communication system, a non-linear effect or a fiber fuse generated in an optical fiber becomes a problem, and an increase in transmission capacity is limited. In order to alleviate these limitations, it is necessary to reduce the density of light guided to the optical fiber, and a large core fiber is being studied as described in Non-Patent Document 1.

  However, there is a trade-off between the reduction of the bending loss, the expansion of the single mode operation region, and the expansion of the effective area, and there is a problem that the amount of expansion of the effective area under predetermined conditions is limited. Therefore, a mode multiplexing transmission system that uses a multimode fiber as a transmission fiber and performs parallel transmission using a plurality of propagating modes has been studied as a technology for achieving a dramatic increase in capacity (for example, see Non-Patent Documents). 2).

  In a mode multiplex transmission system, a mode multiplexer / demultiplexer has been proposed to propagate a plurality of signals emitted from a transmitter as separate modes in an optical fiber. (For example, see Non-Patent Documents 3 and 4)

T. Matsui, et al. , "Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Side-Wide-Band Transmission Overseas Comm. Lightwave Technology. 27, 5410-5416, 2009. N. Hanzawa et al. , “Demonstration of mode-division multiplexing transmission over 10 km two-mode fiber with mode coupler,” OFC2011, paper OWA4 (2011) N. Hanzawa et al. , "Asymmetric Parallel Waveguide with Mode Conversion for Mode and Wavelength Division Multiplexing Transmission", OFC2012, OTul. 4. N. Hanzawa et al, "Mode multi / demultiplexing with parallel waveguide for mode division multiple transmission," Opt. Express vol. 22, pp. 29321-29330 (2014) T. Uematsu et al. , "Low-loss and broadband PLC-type mode (de) multiplexer for mode-division multiplexing transmission," OFC / NFOEC2013, paper OTh1B. 5 (2013) P. J. Winzer et al. , "Penalties from In-Band Crosstalk for Advanced Optical Modulation Formats," ECOC 2011, paper Tu. 5. B. 7 (2011). N. Hanzawa et al. , "Four-mode PLC-based mode multi / demultiplexer with LP11 mode rotator on one chip for MDM transmission," ECOC 2014, We. 1.1.1 N. Hanzawa et al. , "Demonstration of PLC-based six-mode multiplexer for mode division multiplexing transmission," ECOC2015, P.E. 2.5

  However, as described in Non-Patent Documents 3 and 4, in a mode multiplexing system using coupling between modes in a parallel waveguide, a coupling characteristic depends on a wavelength, and a mode multiplexer / demultiplexer has a desired low loss. The problem is that the operating wavelength that can be coupled to the mode is limited.

  In view of the fact that current optical communication systems use a wavelength division multiplexing method, it is necessary to have a performance capable of multiplexing and demultiplexing modes in a wide wavelength band with low loss. In particular, C is a commonly used communication wavelength band. It is desired to obtain a characteristic capable of combining and demultiplexing modes with low loss in the band and the L band.

  Therefore, an object of the present disclosure is to multiplex / demultiplex a desired mode with low loss at each wavelength used in a wavelength multiplexing type optical communication system in a wide wavelength range from C band to L band.

  According to the present disclosure, a Mach-Zehnder interferometer is configured by a multi-mode waveguide, a desired mode is coupled at each coupling portion with a branch ratio of approximately 1: 1, and a period of a wavelength variation of a transmission characteristic is used in optical communication. The wavelength interval of the wavelength multiplexed signal to be obtained.

Specifically, the mode multiplexer / demultiplexer of the present disclosure includes:
A mode multiplexer / demultiplexer for multiplexing / demultiplexing wavelength-multiplexed signal light,
where n is an integer of 2 or more, the first to (n−1) th waveguides;
An n-th waveguide through which the n-th mode propagates from the first mode different from each other;
Assuming that k is an integer of 1 or more and (n-1) or less, the k-th waveguide and the n-th waveguide are arranged close to each other, and the wavelength multiplexed signal light propagating through the k-th waveguide is A k-th first coupling unit and a k-th second coupling unit coupled to the (k + 1) -th mode of the n-th waveguide;
A k-th delay unit that is arranged between the k-th first coupling unit and the k-th second coupling unit and causes a delay difference in the optical path length between the k-th waveguide and the n-th waveguide When,
Equipped with a,
The delay difference generated by the k-th delay unit is a phase delay difference that makes a peak wavelength interval of transmission characteristics of the mode multiplexer / demultiplexer coincide with a wavelength interval of wavelength-multiplexed signal light propagating through the k-th waveguide. It is .

Further, in the mode demultiplexer of the present disclosure, the second coupling portion of the first coupling portion and the first k of the k-th, respectively, the propagation of the k-th waveguide coupled to the waveguide of the first n 0.4-0.6 times the optical power of the mode may couple from the k- th waveguide to the n-th waveguide.

Further, in the mode demultiplexer of the present disclosure, the central wavelength of the wavelength-multiplexed signal light lambda, the wavelength interval [Delta] [lambda], when the Δτ phase delay difference, the phase delay difference between the wavelength interval Δλ = 2πλ / Δτ may be satisfied.

Further, in the mode multiplexer / demultiplexer according to the present disclosure, at least one of the k-th first coupling portion and the k-th second coupling portion has the k- th waveguide or the n-th waveguide. The waveguide width may change with respect to the longitudinal direction of the waveguide.

Further, in the mode multiplexer / demultiplexer of the present disclosure, in at least one of the k-th first coupling portion and the k-th second coupling portion, the k-th waveguide is provided from a plurality of locations of the k-th waveguide. wavelength-multiplexed signal light propagating in the waveguide may be coupled to the waveguide of the first n.

Specifically, the mode multiplex transmission system of the present disclosure
n is an integer of 2 or more, from the first transmitter to the n-th transmitter;
It consists mode demultiplexer according to any one of claims 1 to 5, k as one or more (n-1) an integer, guiding of the k-th wavelength-multiplexed signal light from the transmitter of the k The wavelength-multiplexed signal light from the n-th transmitter is input to one end of the n-th waveguide, and the mode-multiplexed signal light having a different mode for each transmitter is transmitted to the other of the n-th waveguide. A mode combiner that outputs from the end,
A multimode fiber for propagating the mode multiplexed signal light from the mode multiplexer,
Consists mode demultiplexer according to any one of claims 1 to 5, wherein the mode-multiplexed signal light is inputted to one end of the waveguide of the first n, waveguides and the said first k depending on the mode the a mode demultiplexer that outputs a wavelength multiplexed signal light from one of the other ends of the n waveguides,
A first receiver to an n-th receiver that respectively receive the wavelength multiplexed signal light output from the k-th waveguide and the n-th waveguide of the mode multiplexer;
Is provided.

  The above disclosures can be combined as much as possible.

  According to the present disclosure, a desired mode can be multiplexed / demultiplexed with low loss at each wavelength used in a wavelength multiplexing optical communication system in a wide wavelength range from the C band to the L band.

It is a structural example of a two-mode multiplexer / demultiplexer. 1 is a configuration example of a mode multiplexer / demultiplexer according to Embodiment 1. 5 is a transmission characteristic of the mode multiplexer / demultiplexer according to the first embodiment. 4 is a calculation result of crosstalk of the mode multiplexer / demultiplexer according to the first embodiment. 6 is a calculation result of an insertion loss of the mode multiplexer / demultiplexer according to the first embodiment. 9 is a calculation result of a branching ratio, crosstalk, and insertion loss of the mode multiplexer / demultiplexer according to the first embodiment. 2 is an example of a mode multiplexer / demultiplexer according to Embodiment 1. 9 is an example of a mode multiplexer / demultiplexer according to Embodiment 2. 9 is a first structural example of a coupling section of the mode multiplexer / demultiplexer according to the second embodiment. 9 is a second structural example of the coupling section of the mode multiplexer / demultiplexer according to the second embodiment. 9 is an example of a structural parameter of a coupling unit according to the second embodiment. 9 is a calculation result of transmission characteristics of a coupling unit according to the second embodiment. 14 is another example of the structure example of the coupling unit according to the second embodiment. 14 is another example of the structure example of the coupling unit according to the second embodiment. 14 is another example of the structure example of the coupling unit according to the second embodiment. 14 is another example of the structure example of the coupling unit according to the second embodiment. 9 is an example of a structural parameter of a coupling unit according to the second embodiment. 9 is an example of a structural parameter for each structure of a joint according to a second embodiment. 9 is a calculation result of transmission characteristics of a coupling unit according to the second embodiment. 9 is a calculation result of a transmission characteristic of the mode multiplexer / demultiplexer according to the second embodiment. 9 is an example of a structure parameter of the mode multiplexer / demultiplexer according to Embodiment 2. It is an example of the structure parameter of the mode multiplexer / demultiplexer which concerns on related technology. 9 is a calculation result of a transmission characteristic of the mode multiplexer / demultiplexer according to the second embodiment. 9 is another embodiment of the mode multiplexer / demultiplexer according to the second embodiment. FIG. 14 is a schematic diagram of an optical transmission system according to a third embodiment.

  Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments described below. These embodiments are merely examples, and the present disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In the specification and the drawings, components having the same reference numerals indicate the same components.

(Embodiment 1)
A first embodiment of the present disclosure will be described. FIG. 1 shows a configuration of a two-mode multiplexer / demultiplexer according to the related art. Here, a planar lightwave circuit type mode multiplexer / demultiplexer using a rectangular embedded waveguide made of a Si-based material is assumed.

The waveguide 42 is a waveguide in which two modes propagate, and the waveguide 41 is a waveguide in which one mode propagates. Here, the two modes indicate the basic mode LP01 mode and the first higher-order mode LP11 mode. The two waveguides have a region (coupling portion) that is partly close to each other. In that portion, the effective refractive index (n _eff ) of the LP11 mode propagating through the waveguide 42 and the LP01 mode propagating through the waveguide 41 are determined. The waveguide width and the relative refractive index difference are adjusted so that the effective refractive indices that propagate are matched. Thus, the propagating light of the LP01 mode propagating in the waveguide 41 is coupled to the LP11 mode of the waveguide.

  When the effective refractive indices match, mode conversion occurs in a region where the waveguides 41 and 42 are close to each other, and the power of the light propagating through the waveguide 41 is periodically transferred to the waveguide 42. Here, when the length L of the coupling portion is adjusted so that the power is completely transferred to the waveguide 42, the signal light incident from the port P1 is output to the port P4 as the LP11 mode of the waveguide 42. Generally, in order to match the effective refractive index between different LP modes, an asymmetric parallel waveguide structure in which the waveguide structures are not the same is required.

  On the other hand, the light incident from the port P2 propagates through the waveguide 42 as a fundamental mode, and the effective refractive index of the LP01 mode of the waveguide 42 does not match the LP01 mode of the waveguide 41. As a result, no mode conversion occurs, and the mode is output to the port P4 as the LP01 mode. As a result, by injecting signals into the ports P1 and P2, a signal in which two modes are multiplexed can be obtained at the port P4, and a function as a mode multiplexer can be realized. A detailed design method is disclosed in Non-Patent Document 3.

  Further, by inputting the mode-multiplexed light from the port P4, it is possible to separate the signals into the port P1 and the port P2. Thereby, the same structure disclosed in FIG. 1 can be used as a mode splitter.

  In this structure, the light input to the input side waveguide of the mode multiplexer may be in the fundamental mode for all the waveguides. In this case, since the light propagates as the fundamental mode at the output end, the matching and connectivity with the transceiver based on the single mode fiber are good, and good loss characteristics can be obtained.

  The coupling efficiency in the coupling portion depends on the wavelength. As described in Non-Patent Document 3, for example, when the used wavelength is 1.45 to 1.65 μm, the transmission loss characteristic is deteriorated in the wavelength region at both ends. You can see that it is doing. Therefore, a method of extending the operating wavelength by providing two coupling sections and providing an appropriate delay difference between the waveguides in a delay section in which coupling between the coupling sections does not occur has been studied (for example, Non-Patent Document 1). Reference 5). However, even if this method is used, there is wavelength dependence, and depending on the wavelength, loss and deterioration of the mode extinction ratio (the amount of coupling to a desired mode divided by the amount of coupling to other modes) are avoided. It is not possible.

  FIG. 2 shows a configuration example of the mode multiplexer / demultiplexer according to the present embodiment. The mode multiplexer / demultiplexer according to the present embodiment includes waveguides 11 and 12 functioning as a first waveguide and a second waveguide, and a coupling part 21 functioning as a first coupling part and a second coupling part. , 22 and a delay unit 23. The mode multiplexer / demultiplexer according to the present embodiment has a plurality of coupling portions 21 and 22 as compared with the configuration of the related art, and causes a propagation delay difference between the parallel waveguides 11 and 12 between the coupling portions. It has a delay unit 23.

  The difference from the configuration in Non-Patent Document 5 is the mode branching ratio in the coupling sections 21 and 22. In Non-Patent Document 5, the waveguide length (hereinafter referred to as a coupling length) and the waveguide interval of the coupling section are set so that the power of the mode propagating in each coupling section shifts to one waveguide. On the other hand, in the coupling sections 21 and 22 in the present disclosure, the coupling length and the waveguide interval are set so that 3 dB, which is half the power of the propagating mode, moves to the other waveguide.

  FIG. 3 shows transmission characteristics of the mode multiplexer / demultiplexer of the present disclosure. By adopting the configuration as shown in FIG. 2, the transmission characteristic is obtained from the delay difference of the delay unit 23 as in the case of the Mach-Zehnder interferometer used in the single mode waveguide, as shown in FIG. At predetermined intervals. In the Mach-Zehnder interferometer of the related art, all waveguides are constituted by single-mode waveguides, and are used as wavelength filters having periodic transmission characteristics. The present disclosure differs from the related art Mach-Zehnder interferometer in that the present disclosure includes a waveguide through which a plurality of modes propagate.

Here, assuming that adjacent peak wavelengths of the transmission characteristics are λ1 and λ2, Δλ (λ2−λ1) can be arbitrarily changed by adjusting the waveguide length ΔL of the delay unit 23. Generally, when a light wave split into two in the first coupling unit 21 interferes in the second coupling unit 22, the phase delay difference Δτ generated in the delay unit 23 has the following relationship.
(Equation 1)
Δλ = 2πλ / Δτ (1)
Here, the phase delay difference Δτ is obtained by the following equation.
(Equation 2)
Δτ = (ωt−β ₂₁ L ₂₁ ) − (ωt−β ₂₂ L ₂₂ ) (2)
Here, ω is the angular frequency, t is time, β ₂₁ and L ₂₁ are the propagation constant and the propagation distance in the waveguide 12 of one coupling part 21, and β ₂₂ and L ₂₂ are the waveguide 12 of the other coupling part 22, respectively. Are the propagation constant and the propagation distance.

  The propagation constant may be different depending on the waveguide width, height and relative refractive index difference of each waveguide. The propagation distance difference can be obtained from the waveguide difference of the non-coupling portion. It is preferable to design Δλ also in consideration of the phase delay difference generated in the steps 22 and 22. In other words, by adjusting the delay unit 23, the wavelength interval of the wavelength-division multiplexed signal to be used can be matched with the peak wavelength interval of the transmission characteristic, and the mode can be multiplexed over a wide wavelength range without loss or deterioration of the mode extinction ratio. it can.

Regarding the branching ratio at the coupling sections 21 and 22, the characteristics are best when the branching ratio is 1: 1 (here, this is defined as 0.5), but it is not necessary to strictly split the optical power to 1: 1. Absent. 4 and 5 show the calculation results. The structural parameters used in the calculation are w ₁ = 0.4 μm, w ₂ = 0.837 μm, the waveguide gaps g ₂₁ and g ₂₂ in the coupling parts 21 and ₂₂ are 0.2 μm, and the waveguide height h is 0.21 μm. The coupling ratio was changed by adjusting the coupling lengths L ₂₁ and L ₂₂ at 9.09 μm, the distance between the waveguides at 29.58 μm, and the coupling lengths L ₂₁ and L ₂₂ at the coupling portions 21 and 22, respectively. As shown in FIG. 5, the insertion loss is suppressed to 0.25 dB or less even when the branching ratio becomes 0.4 to 0.6. Note that the sign is inverted since it is represented as the transmittance T [dB] in the figure.

  According to FIG. 4, if the branching ratio is in the range of 0.4 to 0.6, the crosstalk between modes can be reduced to about −15 dB. In QPSK (Quadrature Phase Shift Keying), which is a signal format generally used in optical communication, the crosstalk must be -15 dB or less in order to suppress the penalty due to the crosstalk to 1 dB or less (for details, see Non-Patent See Reference 6.). Therefore, the branching ratio is desirably 0.4 to 0.6.

FIG. 6 shows the relationship between the branch ratio, crosstalk (XT), and insertion loss. In the figure, G _X represents a value of crosstalk, and _GL represents a value of insertion loss. When the crosstalk is -20 dB or less, the amount of signal degradation due to the crosstalk is so small as to be negligible, and higher quality signal transmission / reception can be performed. In order for the crosstalk to be -20 dB or less, the branching ratio needs to be 0.42 to 0.58. In order to sufficiently suppress signal degradation due to crosstalk, the branching ratio is in the range of 0.42 to 0.58. It is desirable that

As described in Non-Patent Documents 7 and 8, in order to realize an n-mode multiplexer / demultiplexer, n−1 waveguides 11 are provided, and a region where the waveguides 11 are close to each other is defined as (2n−2). ), The number of modes for multiplexing / demultiplexing can be extended. For example, a mode multiplexer / demultiplexer that multiplexes / demultiplexes three modes LP01, LP11, and LP21 includes two waveguides 11-1 and 11-2 as shown in FIG. The combining units 21 and 22 and the delay unit 23 are installed for each 11-2 . Its mode LP11 and LP21 of respectively may be installed waveguides 11-1 and 11-2 multiplexes.

(Embodiment 2)
FIG. 8 shows a configuration example of the mode multiplexer / demultiplexer according to the present embodiment. In the mode multiplexer according to the present embodiment, the arrangement of the coupling units 21 and 22 and the delay unit 23 is the same as that of the first embodiment, but the width of the waveguide in the coupling units 21 and 22 is linear with respect to the propagation direction. It changes to the configuration. As described in the first embodiment, it is required that half of the mode propagating in the waveguide 11 is coupled to the other waveguide 12 in the coupling portions 21 and 22, but the coupling characteristics in the coupling portions 21 and 22 are wavelength-dependent. It has nature.

For example, FIG. 9 shows a first structural example of the coupling unit 21, and FIG. 10 shows a second structural example of the coupling unit 21. The parameters of the waveguides 11 and 12 in FIGS. 9 and 10 are as described in FIG. The first structure is the waveguide width w ₁ and w ₂ in the coupling section 21 is constant, the second structure is an example in which the width w ₁ of the waveguide 11 is increased linearly with propagation direction I have.

  In the first structure, the branching ratio can be set to 0.5 by matching the effective refractive index of each waveguide mode at a specific wavelength and adjusting the waveguide interval and the coupling length. If they are different, the wavelength dependence of the effective refractive index in each waveguide is different, so that the waveguide spacing and the coupling length required to obtain the same branching ratio are different. For this reason, it is difficult for the first structure to obtain a constant branching ratio over a wide wavelength range. For example, if the C-L band is used, there are wavelengths where the branching ratio does not become 0.4-0.6, mode crosstalk is -15 dB or more at that wavelength, and insertion of a mode multiplexer / demultiplexer is performed. The loss may be 0.25 dB or more.

On the other hand, as in the second structure, the waveguide structure (here, the distance g between the waveguide width w ₁ and the waveguide) to change the relative direction of propagation, the structure effective refractive index match at any wavelength By providing a structure having the same in the coupling portion, a constant branching ratio can be obtained in a wide wavelength band. For example, the branching ratio can be set to 0.4 to 0.6 in the C to L bands.

The figure shows the calculation of the coupling amount from the port P4 on the output side of the lower waveguide 12 to the LP01 mode when the LP01 mode is incident from the port P1 of the upper waveguide 11 using the waveguide parameters described in FIG. Shown as 12. In FIG, G ₁ represents a first structure, G ₂ denotes a second structure. Note that h in FIG. 11 is the height of the waveguide.

When the parameters shown in FIG. 11 are applied to the first structure, as shown in FIG. 12, the coupling amount changes to about 0.3 to 0.7 for the wavelength of 1.5 to 1.6 μm, In some cases, the branching ratio cannot be maintained at 1: 1. On the other hand, in the second structure, the branching ratio can be maintained at 1: 1 over a wide wavelength range. Branching ratio of 1: loss characteristics as a mode demultiplexer if not 1 is deteriorated, it is possible to obtain a low loss characteristic in a wide wavelength range by changing the waveguide width w ₁ of the coupling portion 21 and 22 it can.

13 to 16 show third to fifth structural examples of the coupling portion 21. FIG. Structure of the third to fifth examples of the change in the waveguide width w ₁ at the coupling portion 21. In addition to the linear waveguide width w ₁ is increased in the configuration of FIG. 13 with respect to the propagation direction shown in Figure 10, a third example structure waveguide width w ₁ is increased to V-shape (valley) after reduction to configure the fourth structural example configuration, the fifth structure example of varying the waveguide width w ₁ as a W-type shows a configuration of waveguide width w ₁ is changed in a zigzag.

FIGS. 17 and 18 show examples of the structural parameters of the coupling portion in the respective structural examples. FIG. 19 shows the calculated characteristics of the coupling sections 21 and 22 using the parameters shown in FIGS. In the figure, G _{1 to} G ₅ indicate first to fifth structures, respectively. The input / output ports are the same as in FIG. Although the value of ΔW is negative in the V-type which is the third structural example, this means that it has a valley-shaped shape instead of the mountain-shaped shape shown in FIG. . It can be seen that each of the structures has a low wavelength dependency, and the second to fifth structures have a branching ratio within the range of 0.4 to 0.6 in the C to L bands, and It can be seen that the structure has a branching ratio of 0.4 to 0.6 over a wide wavelength range. Therefore, for the waveguide width w ₁ of the coupling portion 21 and 22, waveguide width w ₁ is changed in the propagation direction, as the change in the waveguide width w _1, may not be a linear, there a curve You may.

FIG. 20 shows the transmission characteristics of the mode multiplexer / demultiplexer when the wavelength multiplexed signal is designed on the assumption that the wavelength interval is 20 nm. G _3M and G _4M represent the second structure, G _3A and G _4A represent the structure shown in FIG. 1, and G _3T and G _4T represent the structure in which the coupling portion in FIG. 1 is tapered. The structural parameters of the second structure shown in _G3M and _G4M are as described in FIGS. The structural parameters shown in G _3T , G _4T , G _3T and G _4T are as described in FIG. Note that the refractive index _{n clad} of a refractive index _{n core} and cladding of the waveguide section is the same as FIG. 10. The characteristics from the port P1 to the port P3 are indicated by solid lines, and the characteristics from the port P1 to the port P4 are indicated by broken lines. That is, the solid line can be considered as a crosstalk amount assuming a duplexer, and a lower one is desirable.

In G _3A and G _4A , the crosstalk amount is low at the coupling wavelength designed as shown by the solid line G _3A , and the mode can be multiplexed / demultiplexed with a low loss and a high extinction ratio. It can be seen that has deteriorated. On the other hand, in G _3M and G _4M , which are examples of the mode multiplexer / demultiplexer of the present disclosure, the crosstalk amount greatly fluctuates depending on the wavelength, but the characteristics are good at intervals of 20 nm, and the wavelength multiplexed signal It can be seen that when the wavelengths match the wavelength intervals at which good characteristics can be obtained, the modes can be multiplexed / demultiplexed with low loss and high mode extinction ratio in a wide wavelength range.

Although an example (G _3T , G _4T ) in which the coupling portion in FIG. 1 is a taper is also described, the wavelength dependence is lower than that of G _3A. It is not possible to realize the characteristic that a high mode extinction ratio can be obtained in a wide wavelength range enough to cover. For example, the crosstalk value is desirably -15 dB or less (or -20 dB or less) as described above, and it can be seen that the wavelength used in G _3T and G _4T is limited to the C band. In _G3M and _G4M to which is applied, since the transmission characteristics periodically show peaks, it can be seen that a very wide range (for example, S band to L band) can be covered.

FIG. 23 shows transmission characteristics when the vertical axis in FIG. 20 is 0 to -2 dB. The insertion loss of the mode multiplexer / demultiplexer corresponds to the broken line in the figure. However, like the crosstalk amount, the loss characteristics of G _3A and G _4A are deteriorated at wavelengths other than the designed coupling wavelength. Understand. On the other hand, _G3M and _{G4M, which} are examples of the present disclosure, show that low loss characteristics are obtained at the designed wavelength interval, and that a wavelength multiplexed signal can be mode-multiplexed / demultiplexed with low loss. Although the case where the width w _{1 of the} waveguide 11 is changed has been described, the width w _{2 of the} waveguide 12 may be changed in the propagation direction instead of the width w ₁ of the waveguide 11. .

  As described in Non-Patent Document 5, a technique of reducing the wavelength dependence of the coupling characteristics by installing two or more coupling portions can be applied to the present disclosure. FIG. 24 shows another mode of the mode multiplexer / demultiplexer according to the present embodiment. The coupling portions 31 and 32 according to the present embodiment each include a plurality of adjacent regions. In the coupling portion 31 functioning as a first coupling portion, light propagated through the waveguide 11 is coupled to the waveguide 12 from a plurality of locations of the waveguide 11. Further, also at the coupling portion 32 functioning as the second coupling portion, the propagation light of the waveguide 11 is coupled to the waveguide 12 from a plurality of locations of the waveguide 11. As described above, by configuring the coupling portions 21 and 22 in the present embodiment with a plurality of coupling portions, a mode having a branching ratio of 0.4 to 0.6 in a wide band without using the tapered structure described above. A branch can be realized.

(Embodiment 3)
FIG. 25 illustrates a configuration example of an optical transmission system according to the present disclosure. In this system, transmitters 81-1 to 81-n that emit n wavelength-multiplexed signal lights, receivers 84-1 to 84-n that separate and receive n wavelength-multiplexed signals, The mode multiplexer / demultiplexer (mode multiplexer 82, mode duplexer 83) according to the first or second embodiment for multiplexing / demultiplexing the signals and the number of propagation modes connecting the mode multiplexer / demultiplexer is n And the wavelength interval of the wavelength multiplexed signal is Δλ. In the mode multiplexer 82, the signal light from each of the transmitters 81-1 to 81-n is input to a different waveguide 11 for each of the transmitters 81-1 to 81-n, and the transmitters 81-1 to 81-n A mode multiplexed signal light having a different mode is output from the waveguide 12 every time. The multimode fiber 91 propagates the mode multiplexed signal light from the mode multiplexer 82. The mode splitter 83 receives the mode multiplexed signal light in the waveguide 12 and outputs the signal light from the different waveguides 11 for each mode.

  As described in the first embodiment or the second embodiment, the wavelength division multiplexed signal is adjusted by adjusting the delay difference ΔL in the delay unit 23 of the mode multiplexer / demultiplexer and by matching the wavelength interval of the wavelength multiplexed signal transmitted and received with Δλ. Mode multiplexing / demultiplexing can be performed with low loss.

  In this specification, an embodiment relating to a planar lightwave circuit using a Si-based material has been described, but the material may naturally be another material. For example, even with a planar lightwave circuit using a semiconductor such as glass or InGaAsP, or an organic material such as a polymer, the same effects as those of the embodiment described in this specification can be obtained.

  The wavelength band to be used is set to about 1.5 to 1.6 μm in the embodiment described in the present specification, but may be a mid-infrared region (2 μm or more) having a longer wavelength or a visible light band. Absent.

(Appendix 1)
a waveguide 1 capable of propagating n or more modes (n is an integer of 2 or more);
Waveguides 2 to m (m is a positive integer of 2 or more and n or less) parallel to the waveguide 1;
Each of the waveguides 1 and the waveguides 2 to m arranged in parallel has two or more coupling portions close to each other,
Having a delay waveguide portion, which is a region not coupled between the waveguides, between the coupling portions,
The phase delay difference Δτ of the light wave generated in the delay part of the waveguide 1 and the waveguides 2 to m is adjusted by the propagation distance difference ΔL of the delay part,
In a portion where the waveguide 1 and the waveguides 2 to m (m is a positive integer of 2 or more and n or less) are close to each other,
The waveguide width and the waveguide width are set so that the effective refractive index of the mode propagating through the waveguides 2 to m exclusively matches the effective refractive index of the (m−1) modes of the waveguide 1 for the desired light wavelength range. Height and relative refractive index differences are adjusted,
The length of the coupling portion and the interval between the waveguides at the coupling portion are determined so that 0.4 to 0.6 times the optical power guided through the waveguides 2 to m by mode conversion propagates through the waveguide 1. Mode multiplexer / demultiplexer.

(Appendix 2)
2. The mode multiplexer / demultiplexer according to claim 1, wherein a waveguide width of the waveguide 1 or the waveguides 2 to m in the coupling portion changes in a propagation direction.

(Appendix 3)
3. The mode multiplexer / demultiplexer according to claim 1 or 2, wherein the period Δλ of the variation of the transmission characteristic at the frequency of the mode multiplexer / demultiplexer coincides with the wavelength interval of the wavelength multiplexed signal used in the communication. And a propagation delay difference [Delta] L between delay sections of the waveguides 2 to m is adjusted.

(Appendix 4)
The mode multiplexer / demultiplexer according to any one of supplementary notes 1 to 3, wherein a phase delay difference Δτ (rad.) Of a light wave generated in the delay section between the waveguide 1 and the waveguides 2 to m is a wavelength used in communication. A mode multiplexer / demultiplexer, wherein the following relationship is satisfied with respect to a center wavelength λ (m) of a multiplex signal and a wavelength interval Δλ (m).
(Number) Δλ = 2πλ / Δτ (λ: center wavelength in use wavelength band)

(Appendix 5)
a transmitter for emitting n wavelength multiplexed signal lights,
a receiver that wavelength-separates and receives n wavelength multiplexed signals;
A mode multiplexer / demultiplexer according to any one of supplementary notes 1 to 4, which multiplexes / demultiplexes the wavelength multiplexed signal;
A multimode fiber having n propagation modes for connecting the mode multiplexer / demultiplexer,
A wavelength division multiplexing / mode multiplexing transmission system, wherein a wavelength interval of the wavelength division multiplexing signal is Δλ.

(Appendix 6)
a waveguide 1 capable of propagating n or more modes (n is an integer of 2 or more);
Waveguides 2 to m (m is a positive integer of 2 or more and n or less) parallel to the waveguide 1;
Each of the waveguide 1 and the waveguides 2 to m arranged in parallel has a close coupling portion,
In a portion where the waveguide 1 and the waveguides 2 to m (m is a positive integer of 2 or more and n or less) are close to each other,
The waveguide width of the waveguide 1 or the waveguides 2 to m at the coupling portion changes in the propagation direction, and the waveguide 1 and the waveguides 2 to m at an arbitrary wavelength in a communication wavelength band (for example, 1530 to 1625 nm). The coupling section has a waveguide width that matches the effective refractive index of the desired mode, and the length of the coupling section and the interval between the waveguides at the coupling section are equal to 0 of the optical power guided through the waveguides 2 to m. A mode multiplexer / demultiplexer characterized in that 4 to 0.6 times is determined to propagate through the waveguide 1.

  The present disclosure is to realize a mode multiplexer / demultiplexer that can perform mode multiplexing / demultiplexing with a low loss in a wide wavelength range for a wavelength multiplexed signal, and realizes a large capacity by using wavelengths and modes in an optical fiber. -Long distance communication can be realized.

11, 12: waveguides 21, 22: coupling section 23: delay sections 31, 32: mode branching section 41: waveguide 2 of related art
42: Waveguide 1 of Related Technology
81: Transmitter 82: Mode multiplexer 83: Mode splitter 84: Receiver 91: Multimode fiber

Claims (6)

A mode multiplexer / demultiplexer for multiplexing / demultiplexing wavelength-multiplexed signal light,
where n is an integer of 2 or more, the first to (n−1) th waveguides;
An n-th waveguide through which the n-th mode propagates from the first mode different from each other;
Assuming that k is an integer of 1 or more and (n-1) or less, the k-th waveguide and the n-th waveguide are arranged close to each other, and the wavelength multiplexed signal light propagating through the k-th waveguide is A k-th first coupling unit and a k-th second coupling unit coupled to the (k + 1) -th mode of the n-th waveguide;
A k-th delay unit that is arranged between the k-th first coupling unit and the k-th second coupling unit and causes a delay difference in the optical path length between the k-th waveguide and the n-th waveguide When,
Equipped with a,
The delay difference generated by the k-th delay unit is a phase delay difference that makes a peak wavelength interval of transmission characteristics of the mode multiplexer / demultiplexer coincide with a wavelength interval of wavelength-multiplexed signal light propagating through the k-th waveguide. mode demultiplexer is. In the k-th first coupling portion and the k-th second coupling portion, the optical power of the propagation mode of the k-th waveguide coupled to the n-th waveguide is 0.4 to 0.6, respectively. A double couples from the k-th waveguide to the n-th waveguide,
The mode multiplexer / demultiplexer according to claim 1. When the center wavelength of the wavelength multiplexed signal light is λ, the wavelength interval is Δλ, and the phase delay difference is Δτ, the wavelength interval and the phase delay difference satisfy a relationship of Δλ = 2πλ / Δτ,
The mode multiplexer / demultiplexer according to claim 1 . In at least one of the k-th first coupling portion and the k-th second coupling portion, the waveguide width of the k-th waveguide or the n-th waveguide is set to be longer than the longitudinal direction of the waveguide. Changing,
Mode demultiplexer according to any one of claims 1 to 3. In at least one of the k-th first coupling portion and the k-th second coupling portion, the wavelength division multiplexed signal light propagating through the k-th waveguide from a plurality of locations of the k-th waveguide is transmitted to the n-th waveguide. Coupled to the waveguide of
Mode demultiplexer according to any one of claims 1 to 3. n is an integer of 2 or more, from the first transmitter to the n-th transmitter;
It consists mode demultiplexer according to any one of claims 1 to 5, k as one or more (n-1) an integer, guiding of the k-th wavelength-multiplexed signal light from the transmitter of the k The wavelength-multiplexed signal light from the n-th transmitter is input to one end of the n-th waveguide, and the mode-multiplexed signal light having a different mode for each transmitter is transmitted to the other of the n-th waveguide. A mode combiner that outputs from the end,
A multimode fiber for propagating the mode multiplexed signal light from the mode multiplexer,
Consists mode demultiplexer according to any one of claims 1 to 5, wherein the mode-multiplexed signal light is inputted to one end of the waveguide of the first n, waveguides and the said first k depending on the mode the a mode demultiplexer that outputs a wavelength multiplexed signal light from one of the other ends of the n waveguides,
A first receiver to an n-th receiver that respectively receive the wavelength multiplexed signal light output from the k-th waveguide and the n-th waveguide of the mode multiplexer;
Mode multiplex transmission system comprising:

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