OPTICAL FIBER SIGNAL MODE CONVERSION APPARATUS AND CONVERSION METHOD, AND OPTICAL FIBER TRANSMISSION SYSTEM

An example optical fiber signal mode conversion apparatus includes a non-single-mode optical fiber and a single-mode optical fiber. The single-mode optical fiber forms, with the non-single-mode optical fiber, a first coupling region and a second coupling region along a signal transmission direction in the non-single-mode optical fiber, where an effective refractive index of a fundamental mode signal of the single-mode optical fiber in the first coupling region is equal to an effective refractive index of a signal in a first mode, the signal in the first mode is coupled to a fundamental mode channel of the single-mode optical fiber, and an effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is equal to an effective refractive index of a signal in a second mode.

TECHNICAL FIELD

This application relates to the field of optical communication technologies, and in particular, to an optical fiber signal mode conversion apparatus, an optical fiber signal mode conversion method, and an optical fiber transmission system.

BACKGROUND

With explosive growth of information transmission requirements, a conventional single-mode optical fiber gradually approaches a transmission limit, and a new manner represented by few-mode multiplexing and multi-mode multiplexing is widely concerned. For example, in a few-mode multiplexing technology, an independent orthogonal mode in a few-mode optical fiber is used as a transmission channel, to multiply an optical transmission capacity.

One of core problems of few-mode optical fiber communication is mode conversion. The mode conversion mainly includes two types. One type is mode multiplexing and demultiplexing, where the mode multiplexing means that fundamental mode signals in a plurality of single-mode optical fibers are multiplexed to a few-mode optical fiber and are transmitted as fundamental mode signals and high-order mode signals (the fundamental mode signals and the higher-order mode signals in the few-mode optical fiber are synthesized into a few-mode multiplexed signal); and the mode demultiplexing means that the few-mode multiplexed signal is demultiplexed to fundamental mode signals in the plurality of single-mode optical fibers for transmission. The other type is that any mode in a few-mode multiplexed signal is converted into another mode for transmission, or a plurality of modes in the few-mode multiplexed signal are converted into a plurality of other modes. The second type of mode conversion has important applications in scenarios such as mode cyclic conversion and mode add/drop multiplexing.

SUMMARY

Embodiments of this application provide an optical fiber signal mode conversion apparatus, an optical fiber signal mode conversion method, and an optical fiber transmission system. A main objective is to provide an optical fiber signal mode conversion apparatus that can convert one mode into another mode.

To achieve the foregoing objective, the following technical solutions are used in embodiments of this application.

According to a first aspect, this application provides an optical fiber signal mode conversion apparatus, configured to convert a first mode into a second mode. The optical fiber signal mode conversion apparatus includes: a non-single-mode optical fiber, where the non-single-mode optical fiber includes a first mode channel and a second mode channel, the first mode channel is configured to transmit a signal in the first mode, and the second mode channel is configured to transmit a signal in the second mode; and a single-mode optical fiber, configured to form, with the non-single-mode optical fiber, a first coupling region and a second coupling region along a signal transmission direction in the non-single-mode optical fiber, where an effective refractive index of a fundamental mode signal of the single-mode optical fiber in the first coupling region is equal to an effective refractive index of the signal in the first mode, the signal in the first mode may be coupled to a fundamental mode channel of the single-mode optical fiber, an effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is equal to an effective refractive index of the signal in the second mode, and the fundamental mode signal of the single-mode optical fiber may be coupled to the second mode channel.

In the optical fiber signal mode conversion apparatus provided in this embodiment of this application, the single-mode optical fiber forms the first coupling region and the second coupling region with the non-single-mode optical fiber. Because the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the first coupling region is equal to the effective refractive index of the signal in the first mode, the signal, in the first mode, of the non-single-mode optical fiber can be coupled to the fundamental mode channel of the single-mode optical fiber; and because the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is equal to the effective refractive index of the signal in the second mode, the fundamental mode signal of the single-mode optical fiber can be coupled to the second mode channel of the non-single-mode optical fiber. Therefore, in this embodiment of this application, the effective refractive indexes of the fundamental mode signal of the single-mode optical fiber in the first coupling region and the second coupling region are set to be different, so that when the fundamental mode signal decoupled to the single-mode optical fiber is transmitted to the second coupling region, the effective refractive index of the fundamental mode signal changes, and finally, the fundamental mode signal coupled to the second mode is coupled to the second mode channel of the non-single-mode optical fiber, thereby converting the first mode into the second mode.

In a possible implementation of the first aspect, the non-single-mode optical fiber includes a first optical fiber, the first coupling region is formed between the single-mode optical fiber and the first optical fiber, and the second coupling region is formed between the single-mode optical fiber and the first optical fiber. In other words, the first mode may be converted into the second mode on a same optical fiber.

In a possible implementation of the first aspect, the non-single-mode optical fiber includes a first optical fiber and a second optical fiber, the first coupling region is formed between the single-mode optical fiber and the first optical fiber, and the second coupling region is formed between the single-mode optical fiber and the second optical fiber. In other words, the converted second mode may be converted to another optical fiber, to be used in a mode add/drop multiplexing scenario.

In a possible implementation of the first aspect, a length of the first coupling region is equal to a coupling length for the signal in the first mode to be coupled to the fundamental mode channel of the single-mode optical fiber. Because the length of the first coupling region is equal to the coupling length for the signal in the first mode to be coupled to the fundamental mode channel of the single-mode optical fiber, compared with a case in which the length of the first coupling region is equal to an integer multiple, greater than 1, of the coupling length for the signal in the first mode to be coupled to the fundamental mode channel of the single-mode optical fiber, a length of the non-single-mode optical fiber is significantly reduced, and for an all optical fiber transmission system, a length of the entire all optical fiber transmission system is also effectively reduced.

In a possible implementation of the first aspect, a length of the second coupling region is equal to a coupling length for the fundamental mode signal of the single-mode optical fiber to be coupled to the second mode channel. Because the length of the second coupling region is equal to the coupling length for the fundamental mode signal of the single-mode optical fiber to be coupled to the second mode channel, compared with a case in which the length of the second coupling region is equal to an integer multiple, greater than 1, of the coupling length for the fundamental mode signal of the single-mode optical fiber to be coupled to the second mode channel, the length of the non-single-mode optical fiber is significantly reduced, and for the all optical fiber transmission system, the length of the entire all optical fiber transmission system is also effectively reduced.

In a possible implementation of the first aspect, in the first coupling region, the non-single-mode optical fiber and the single-mode optical fiber are arranged in parallel, and a cladding of the non-single-mode optical fiber and a cladding of the single-mode optical fiber are spliced; in the second coupling region, the non-single-mode optical fiber and the single-mode optical fiber are arranged in parallel, and the cladding of the non-single-mode optical fiber and the cladding of the single-mode optical fiber are spliced; and a value range of a distance d between a center of a core of the non-single-mode optical fiber and a center of a core of the single-mode optical fiber is: d∈[Rf1+Rs1, Rf2+Rs2], where Rf1is a radius of the core of the non-single-mode optical fiber, Rs1is a radius of the core of the single-mode optical fiber, Rf2is a radius of the cladding of the non-single-mode optical fiber, and Rs2is a radius of the cladding of the single-mode optical fiber.

In a possible implementation of the first aspect, a refractive index of the core of the single-mode optical fiber in the first coupling region is a first refractive index, a refractive index of the core of the single-mode optical fiber in the second coupling region is a second refractive index, a refractive index of the core of the single-mode optical fiber in a non-coupling region between the first coupling region and the second coupling region is a third refractive index, and the third refractive index is between the first refractive index and the second refractive index.

Because the third refractive index is between the first refractive index and the second refractive index, when the single-mode optical fiber is processed and fabricated, compared with a case in which the third refractive index sometimes exceeds the first refractive index and sometimes exceeds the second refractive index, processing difficulty of the single-mode optical fiber is significantly reduced.

In a possible implementation of the first aspect, the first mode and the second mode are two modes in a degenerate mode; the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the first coupling region is equal to the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region; and the center of the core of the non-single-mode optical fiber and the center of the core of the single-mode optical fiber are on a first straight line along a cross section of the non-single-mode optical fiber in the first coupling region, the center of the core of the non-single-mode optical fiber and the center of the core of the single-mode optical fiber are on a second straight line along the cross section of the non-single-mode optical fiber in the second coupling region, and an included angle between the first straight line and the second straight line is equal to a phase difference between the first mode and the second mode.

When the first mode and the second mode are two modes in the degenerate mode, the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the first coupling region is equal to the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region, and the included angle between the first straight line and the second straight line is equal to the phase difference between the first mode and the second mode, so that conversion between the first mode and the second mode in the degenerate mode can be implemented. Therefore, in this embodiment of this application, mode conversion in the degenerate mode is implemented, and an application scenario of the optical fiber signal mode conversion apparatus is expanded.

In a possible implementation of the first aspect, the second mode is a degenerate mode, and the second mode includes a first submode and a second submode; the optical fiber signal mode conversion apparatus is configured to convert the first mode into the first submode, the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is n1, the optical fiber signal mode conversion apparatus is further configured to convert the first mode into the second submode, the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is n2, and n1=n2; and the optical fiber signal mode conversion apparatus is configured to convert the first mode into the first submode, the center of the core of the non-single-mode optical fiber and the center of the core of the single-mode optical fiber are on a third straight line along a cross section of the non-single-mode optical fiber in the second coupling region, the optical fiber signal mode conversion apparatus is further configured to convert the first mode into the second submode, the center of the core of the non-single-mode optical fiber and the center of the core of the single-mode optical fiber are on a fourth straight line along the cross section of the non-single-mode optical fiber in the second coupling region, and an included angle between the third straight line and the fourth straight line is equal to a phase difference between the first submode and the second submode.

When the second mode includes two submodes that are degenerate modes, in the second coupling region, n1=n2and the included angle between the third straight line and the fourth straight line is equal to the phase difference between the first submode and the second submode. In this way, the first mode can be converted into the first submode in the degenerate mode or the first mode can be converted into the second submode, to implement mode differentiation in the degenerate mode.

In a possible implementation of the first aspect, the optical fiber signal mode conversion apparatus is configured to convert N modes, and N is an integer greater than or equal to 2; the non-single-mode optical fiber includes N mode channels, and the N mode channels are in a one-to-one correspondence with the N modes; there are N single-mode optical fibers, and any single-mode optical fiber forms the first coupling region and the second coupling region with the non-single-mode optical fiber; a mode that is of the non-single-mode optical fiber and that is coupled to any single-mode optical fiber is one of the N modes; and a mode that is of any single-mode optical fiber and that is coupled to the non-single-mode optical fiber is one of the N modes, and the mode that is of the non-single-mode optical fiber and that is coupled to any single-mode optical fiber is different from the mode that is of the single-mode optical fiber and that is coupled to the non-single-mode optical fiber.

When the optical fiber signal mode conversion apparatus in the foregoing technical solution is used, the optical fiber signal mode conversion apparatus may form a cyclic mode conversion apparatus, to implement signal mode cyclic conversion when being used in an optical fiber transmission system. When the optical fiber signal mode conversion apparatus is applied to mode cyclic conversion, compensation for a group delay in a differential mode may be implemented, and a signal crosstalk may be further reduced.

In a possible implementation of the first aspect, a plurality of first coupling regions are successively arranged along an axial direction of the non-single-mode optical fiber, and a plurality of second coupling regions are successively arranged along the axial direction of the non-single-mode optical fiber.

According to a second aspect, this application further provides an optical fiber signal mode conversion method, applied to the optical fiber signal mode conversion apparatus. The method includes: when the signal in the first mode in the non-single-mode optical fiber is transmitted to the first coupling region, decoupling the signal in the first mode to the fundamental mode channel of the single-mode optical fiber, and transmitting a decoupled signal in the single-mode optical fiber as a fundamental mode signal; and when the fundamental mode signal in the single-mode optical fiber is transmitted to the second coupling region, coupling the fundamental mode signal of the single-mode optical fiber to the second mode channel of the non-single-mode optical fiber, and transmitting a coupled signal in the non-single-mode optical fiber in the second mode.

According to the optical fiber signal mode conversion method provided in this embodiment of this application, because the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the first coupling region is equal to the effective refractive index of the signal in the first mode, when the signal in the first mode that is transmitted in the non-single-mode optical fiber is transmitted to the first coupling region, the signal in the first mode is decoupled to the fundamental mode channel of the single-mode optical fiber according to a mode matching condition, and is transmitted in the single-mode optical fiber as a fundamental mode signal. In addition, because the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is equal to the effective refractive index of the signal in the second mode, when the fundamental mode signal in the single-mode optical fiber is transmitted to the second coupling region, the fundamental mode signal of the single-mode optical fiber is coupled to the second mode channel of the non-single-mode optical fiber according to a mode matching condition, thereby converting the first mode into the second mode.

According to a third aspect, this application further provides an optical fiber transmission system. The optical fiber transmission system includes: transmission optical fibers, where the transmission optical fibers include a first transmission optical fiber and a second transmission optical fiber, and the first transmission optical fiber and the second transmission optical fiber each include a first mode channel and a second mode channel. The foregoing optical fiber signal mode conversion apparatus is disposed at a node between the first transmission optical fiber and the second transmission optical fiber. An optical inlet of the non-single-mode optical fiber is opposite to an optical outlet of the first transmission optical fiber, and an optical outlet of the non-single-mode optical fiber is opposite to an optical inlet of the second transmission optical fiber.

The optical fiber transmission system provided in this embodiment of this application includes the optical fiber signal mode conversion apparatus in any of the foregoing technical solutions. In this way, a signal in a first mode on the first transmission optical fiber can be converted to a second mode channel of the second transmission optical fiber by using the optical fiber signal mode conversion apparatus, to be transmitted in the second transmission optical fiber in a second mode. In addition, the optical fiber transmission system provided in this embodiment of this application can resolve a same technical problem as the optical fiber signal mode conversion apparatus described in the foregoing technical solutions, and achieve a same expected effect.

In a possible implementation of the third aspect, the optical fiber transmission system is configured to transmit signals in N modes, where N is an integer greater than or equal to 2; there are N segments of transmission optical fibers, axial lengths of the N segments of transmission optical fibers are all equal, each segment of transmission optical fiber has N mode channels, and the N mode channels are in a one-to-one correspondence with the N modes; and there are N-1 optical fiber signal mode conversion apparatuses, and one optical fiber signal mode conversion apparatus is disposed at a node between two segments of transmission optical fibers. The optical fiber transmission system can compensate for a group delay in a differential mode.

DESCRIPTION OF EMBODIMENTS

Embodiments of this application relate to an optical fiber transmission system, an optical fiber signal mode conversion apparatus, and an optical fiber signal mode conversion method. The following describes in detail the optical fiber transmission system, the optical fiber signal mode conversion apparatus, and the optical fiber signal mode conversion method with reference to accompanying drawings.

In embodiments of this application, non-single-mode includes few-mode and multi-mode. In the following embodiments, a few-mode optical fiber and few-mode signal transmission are used as examples.

An embodiment of this application provides an optical fiber transmission system. Refer toFIG. 1. The optical fiber transmission system includes a first transmission optical fiber21and a second transmission optical fiber22. The first transmission optical fiber21and the second transmission optical fiber22each include a first mode channel and a second mode channel. The first mode channel is configured to transmit a signal in a first mode. The second mode channel is configured to transmit a signal in a second mode. An optical fiber signal mode conversion apparatus1is disposed at a node between the first transmission optical fiber21and the second transmission optical fiber22. The optical fiber signal mode conversion apparatus1is configured to convert the first mode into the second mode.

An embodiment of this application provides an optical fiber signal mode conversion apparatus. Refer toFIG. 2. The optical fiber signal mode conversion apparatus1includes a few-mode optical fiber11and a single-mode optical fiber12. The few-mode optical fiber11includes a first mode channel and a second mode channel. The first mode channel is configured to transmit a signal in a first mode. The second mode channel is configured to transmit a signal in a second mode. The single-mode optical fiber12forms, with the few-mode optical fiber11, a first coupling region13and a second coupling region14along a signal transmission direction in the few-mode optical fiber11. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12in the first coupling region13is equal to an effective refractive index of the signal in the first mode, and the signal in the first mode may be coupled to a fundamental mode channel of the single-mode optical fiber12. An effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the second coupling region14is equal to an effective refractive index of the signal in the second mode, and the fundamental mode signal of the single-mode optical fiber12may be coupled to the second mode channel.

Because the effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the first coupling region13is equal to the effective refractive index of the signal in the first mode, the signal in the first mode may be coupled to the fundamental mode channel of the single-mode optical fiber12. In this way, the first mode of the few-mode optical fiber11and a fundamental mode of the single-mode optical fiber12meet a mode matching condition in the first coupling region13, and the signal in the first mode in the few-mode optical fiber11can be decoupled to the single-mode optical fiber12and be transmitted in the single-mode optical fiber12in the fundamental mode. In addition, the effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the first coupling region13is different from the effective refractive index of the fundamental mode signal in the second coupling region14, and the effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the second coupling region14is equal to the effective refractive index of the signal in the second mode. In this way, the fundamental mode of the single-mode optical fiber12and the second mode of the few-mode optical fiber11meet a mode matching condition in the second coupling region14, so that the fundamental mode signal in the single-mode optical fiber12is coupled to the second mode channel of the few-mode optical fiber11and is transmitted in the few-mode optical fiber11in the second mode. Therefore, the first mode is converted into the second mode.

When the optical fiber signal mode conversion apparatus provided in this embodiment is used in an optical fiber transmission system, all optical fiber transmission (transmission optical fibers and the mode conversion apparatus are all optical fibers) may be implemented, and the optical fiber signal mode conversion apparatus can be well compatible with the optical fiber transmission system. One mode can be converted into another mode by using only one few-mode optical fiber11and one single-mode optical fiber12. Therefore, a structure is simple and costs of the transmission system are greatly reduced. In addition, one few-mode optical fiber11and one single-mode optical fiber12form only two coupling regions, so that an insertion loss is small.

As shown inFIG. 2, the optical fiber signal mode conversion apparatus is configured to convert an M1mode into an M2mode (an optical inlet of the few-mode optical fiber11is M1+M3+ . . . +Mn, and an optical outlet of the few-mode optical fiber11is M2+M3+ . . . +Mn). In this case, the few-mode optical fiber11includes an M1mode channel, an M2mode channel, an M3mode channel, . . . , and an Mnmode channel. The effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the first coupling region13is equal to an effective refractive index nM1of a signal in the M1mode, and the signal in the M1mode may be coupled to the fundamental mode channel of the single-mode optical fiber12. The effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the second coupling region14is equal to an effective refractive index nM2of a signal in the M2mode, and the fundamental mode signal of the single-mode optical fiber12may be coupled to the M2mode channel. In this way, the optical fiber signal mode conversion apparatus can convert the M1mode into the M2mode.

In some scenarios, a plurality of modes need to be converted into a plurality of modes. An embodiment of this application provides an optical fiber signal mode conversion apparatus, as shown inFIG. 3a. A service requirement is that M1, M2, and M3modes are converted into M4, M5, and M6modes (an optical inlet of a few-mode optical fiber11is M1+M2+M3+M7+ . . . +Mn, and an optical outlet of the few-mode optical fiber11is M4+M5+M6+M7+ . . . +Mn), and the few-mode optical fiber11includes an M1mode channel, an M2mode channel, an M3mode channel, . . . , and an Mnmode channel. Single-mode optical fibers include a single-mode optical fiber12a, a single-mode optical fiber12b, and a single-mode optical fiber12c. The single-mode optical fiber12a, the single-mode optical fiber12b, and the single-mode optical fiber12ceach form a first coupling region and a second coupling region with the few-mode optical fiber11. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12ain the first coupling region is equal to an effective refractive index nM1of a signal in the M1mode, and the signal in the M1mode may be coupled to a fundamental mode channel of the single-mode optical fiber12a. An effective refractive index of the fundamental mode signal of the single-mode optical fiber12ain the second coupling region is equal to an effective refractive index nM4of a signal in the M4mode, and the fundamental mode signal of the single-mode optical fiber12amay be coupled to the M4mode channel. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12bin the first coupling region is equal to an effective refractive index nM2of a signal in the M2mode, and the signal in the M2mode may be coupled to a fundamental mode channel of the single-mode optical fiber12b. An effective refractive index of the fundamental mode signal of the single-mode optical fiber12bin the second coupling region is equal to an effective refractive index nM5of a signal in the M5mode, and the fundamental mode signal of the single-mode optical fiber12bmay be coupled to the M5mode channel. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12cin the first coupling region is equal to an effective refractive index nM3of a signal in the M3mode, and the signal in the M3mode may be coupled to a fundamental mode channel of the single-mode optical fiber12c. An effective refractive index of the fundamental mode signal of the single-mode optical fiber12cin the second coupling region is equal to an effective refractive index nM6of a signal in the M6mode, and the fundamental mode signal of the single-mode optical fiber12cmay be coupled to the M6mode channel. In this way, the optical fiber signal mode conversion apparatus can convert the M1mode, the M2mode, and the M3mode into the M4mode, the M5mode, and the M6mode.

It should be noted that the foregoing conversion manner of the optical fiber signal mode conversion apparatus shown inFIG. 3ais not limited, and may alternatively be that the effective refractive indexes of the fundamental mode signals of the single-mode optical fibers12a,12b, and12cin the first coupling regions are respectively equal to the effective refractive indexes of the signals in the M1, M2, and M3modes, and the effective refractive indexes of the fundamental mode signals of the single-mode optical fibers12a.12b, and12cin the second coupling regions are respectively equal to the effective refractive indexes of the signals in the M5, M6, and M4modes.

In some scenarios, a plurality of modes need to be dynamically converted into a plurality of modes in real time or at a high frequency, and a conversion correspondence between the plurality of modes and the plurality of modes in previous dynamic conversion is different from a conversion correspondence between the plurality of modes and the plurality of modes in next dynamic conversion. An embodiment of this application provides an optical fiber signal mode conversion apparatus, as shown inFIG. 3b. A service requirement is that M1, M2, and M3modes are converted into M4, M5, and M6modes (an optical inlet of a few-mode optical fiber11is M1+M2+M3+M7+ . . . +Mn, and an optical outlet of the few-mode optical fiber11is M4+M5+M6+M7+ . . . +Mn), and the few-mode optical fiber11includes an M1mode channel, an M2mode channel, an M3mode channel, . . . , and an Mnmode channel. Single-mode optical fibers include a single-mode optical fiber12a, a single-mode optical fiber12b, a single-mode optical fiber12c, a single-mode optical fiber12d, a single-mode optical fiber12e, and a single-mode optical fiber12f. The single-mode optical fiber12a, the single-mode optical fiber12b, and the single-mode optical fiber12ceach form a first coupling region with the few-mode optical fiber11, and the single-mode optical fiber12d, the single-mode optical fiber12e, and the single-mode optical fiber12feach form a second coupling region with the few-mode optical fiber11. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12ain the first coupling region is equal to an effective refractive index nM1of a signal in the M1mode, and the signal in the M1mode may be coupled to a fundamental mode channel of the single-mode optical fiber12a. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12bin the first coupling region is equal to an effective refractive index nM2of a signal in the M2mode, and the signal in the M2mode may be coupled to a fundamental mode channel of the single-mode optical fiber12b. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12cin the first coupling region is equal to an effective refractive index nM3of a signal in the M3mode, and the signal in the M3mode may be coupled to a fundamental mode channel of the single-mode optical fiber12c. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12din the second coupling region is equal to an effective refractive index nM4of a signal in the M4mode, and the fundamental mode signal of the single-mode optical fiber12dmay be coupled to the M4mode channel. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12ein the second coupling region is equal to an effective refractive index nM5of a signal in the M5mode, and the fundamental mode signal of the single-mode optical fiber12emay be coupled to the M5mode channel. An effective refractive index of a fundamental mode signal of the single-mode optical fiber12fin the second coupling region is equal to an effective refractive index nM6of a signal in the M6mode, and the fundamental mode signal of the single-mode optical fiber12fmay be coupled to the M6mode channel. There is a dynamic optical switching apparatus31between the single-mode optical fibers12a.12b, and12cand the single-mode optical fibers12d,12e, and12f, and a function of the dynamic optical switching apparatus31is to respectively switch, to the single-mode optical fiber12d,12e, or12f, an optical signal transmitted in the single-mode optical fiber12a.12b, or12c, where a switching correspondence between the single-mode optical fiber12a,12b, or12cand the single-mode optical fiber12d,12e, or12fmay be dynamically changed in real time or at a high frequency. In this way, the optical fiber signal mode conversion apparatus can respectively convert the M1mode, the M2mode, or the M3mode into the M4mode, the M5mode, or the M6mode, and a mode conversion correspondence can be dynamically changed in real time or at a high frequency. For example, when the dynamic optical switching apparatus31is in an operating state1, the M1mode is converted into the M4mode, the M2mode is converted into the M5mode, and the M3mode is converted into the M6mode. After the dynamic optical switching apparatus31dynamically switches to an operating state2, the M1mode is converted into the M5mode, the M2mode is converted into the M6mode, and the M3mode is converted into the M4mode. It should be noted that the optical fiber signal mode conversion apparatus shown inFIG. 3bmay perform conversion between three pairs of modes at the same time, or may perform conversion between two pairs, four pairs, or more pairs of modes at the same time. This is not limited.

Usually, a refractive index of a core of the single-mode optical fiber12in a non-coupling region is between refractive indexes of the core in the two coupling regions. It is assumed that the refractive index of the core of the single-mode optical fiber12in the first coupling region is a first refractive index, the refractive index of the core of the single-mode optical fiber12in the second coupling region is a second refractive index, the refractive index of the core of the single-mode optical fiber12in the non-coupling region between the first coupling region and the second coupling region is a third refractive index, and the third refractive index is between the first refractive index and the second refractive index.

That the third refractive index is between the first refractive index and the second refractive index is implemented in a plurality of implementations. Refer toFIG. 4aandFIG. 4b. When the first refractive index of the core of the single-mode optical fiber in the first coupling region is greater than the second refractive index of the core of the single-mode optical fiber in the second coupling region, the third refractive index shows a gradient transition trend, for example, may show a linear gradient transition (a straight line1inFIG. 4a), a non-linear gradient transition (a curve2and a curve3inFIG. 4a), or a step transition shown inFIG. 4b.

To shorten a length size of the few-mode optical fiber11to reduce a volume of the conversion apparatus, refer toFIG. 5a, a length L of the first coupling region13is equal to a coupling length for the signal in the first mode to be coupled to the fundamental mode channel of the single-mode optical fiber. The coupling length refers to a shortest coupling length when optical signal energy is completely transferred from the few-mode optical fiber11to the single-mode optical fiber12for the first time. The length L of the first coupling region13is controlled to be equal to the coupling length for the signal in the first mode to be coupled to the fundamental mode channel of the single-mode optical fiber, and not to be an integer multiple, greater than 1, of the coupling length. In this way, the length size of the few-mode optical fiber can be shortened. Especially, when there are a plurality of first coupling regions, an effect of shortening the length size of the few-mode optical fiber is more obvious.

A length L of the second coupling region14should also be controlled to be equal to a coupling length for the fundamental mode signal of the single-mode optical fiber to be coupled to the second mode channel. The coupling length refers to a shortest coupling length when the optical signal energy is completely transferred from the single-mode optical fiber12to the few-mode optical fiber11for the first time.

In this embodiment of this application, in a radial direction of an optical fiber, the single-mode optical fiber12and the few-mode optical fiber11that are involved each include a core, a cladding, and a coating layer that are successively disposed from inside to outside. The core completes transmission of an optical signal. A refractive index of the cladding is different from that of the core. The optical signal is enclosed in the core for transmission, to protect the core. The coating layer serves as a protection structure of the core and the cladding.

Embodiments of this application further provide a manner of forming a coupling region. In the first coupling region, the few-mode optical fiber11and the single-mode optical fiber12are arranged in parallel, and the cladding of the few-mode optical fiber11and the cladding of the single-mode optical fiber12are spliced or are bonded through side polishing. In the second coupling region, the few-mode optical fiber11and the single-mode optical fiber12are also arranged in parallel, and the cladding of the few-mode optical fiber11and the cladding of the single-mode optical fiber12are spliced or are bonded through side polishing. Certainly, the few-mode optical fiber11and the single-mode optical fiber12may be alternatively connected through another structure.

FIG. 5bis a schematic diagram of a cross section of a coupling region. A distance between a center of the core11-2of the few-mode optical fiber11and a center of the core12-2of the single-mode optical fiber12is d, and a value range of the distance d is usually d∈[Rf1+Rs1, Rf2+Rs2], where Rf1is a radius of the core11-2of the few-mode optical fiber11, Rs1is a radius of the core12-2of the single-mode optical fiber12, Rf2is a radius of the cladding11-1of the few-mode optical fiber11, and Rs2is a radius of the cladding12-1of the single-mode optical fiber12.

Usually, a maximum coupling efficiency when decoupling is performed in the first coupling region is determined based on a first mode that needs to be converted, then the distance d between the center of the core of the few-mode optical fiber and the center of the core of the single-mode optical fiber in the first coupling region is determined based on the maximum coupling efficiency, and a coupling length of the first coupling region is determined based on a value of d. Similarly, a maximum coupling efficiency when decoupling is performed in the second coupling region is determined based on a second mode that needs to be converted, then the distance d between the center of the core of the few-mode optical fiber and the center of the core of the single-mode optical fiber in the second coupling region is determined based on the maximum coupling efficiency, and a coupling length of the second coupling region is determined based on a value of d. A specific parameter design is not described in detail again.

An embodiment of this application further provides a mode conversion manner. Refer toFIG. 6a. A few-mode optical fiber includes a first few-mode optical fiber111and a second few-mode optical fiber112, where a first coupling region13is formed between a single-mode optical fiber12and the first few-mode optical fiber111, and a second coupling region14is formed between the single-mode optical fiber12and the second few-mode optical fiber112. In other words, a converted second mode may be converted to another few-mode optical fiber, to be used in a mode add/drop multiplexing scenario.

Alternatively, more than two modes in one few-mode optical fiber may be dynamically converted into a plurality of modes in more than two other few-mode optical fibers. For example, refer toFIG. 6b. A few-mode optical fiber includes a first few-mode optical fiber111, a second few-mode optical fiber112, and a third few-mode optical fiber113. Single-mode optical fibers include a single-mode optical fiber121, a single-mode optical fiber122, a single-mode optical fiber123, and a single-mode optical fiber124. There is a dynamic optical switching apparatus31between the single-mode optical fibers121and122and the single-mode optical fibers123and124, and a function of the dynamic optical switching apparatus31is to respectively switch, to the single-mode optical fiber123or124, an optical signal transmitted in the single-mode optical fiber121or122, where a switching correspondence between the single-mode optical fiber121or122and the single-mode optical fiber123or124may be dynamically changed in real time or at a high frequency. In this way, the optical fiber signal mode conversion apparatus can respectively convert an M1mode or an M2mode in the first few-mode optical fiber111into an M3mode in the second few-mode optical fiber112or an M4mode in the third few-mode optical fiber113, and a mode conversion correspondence can be dynamically changed in real time or at a high frequency. In other words, a plurality of modes in one few-mode optical fiber may be dynamically converted into a plurality of modes in a plurality of other few-mode optical fibers, to be used in a mode add/drop multiplexing scenario in which a dynamic mode is adjustable.

When the first mode and the second mode are two modes in a group of degenerate modes, the optical fiber signal mode conversion apparatus provided in this embodiment of this application may still convert the first mode into the second mode. The effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the first coupling region13is equal to the effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the second coupling region14. In addition, the center of the core of the few-mode optical fiber11and the center of the core of the single-mode optical fiber12are on a first straight line along a cross section of the few-mode optical fiber in the first coupling region13; and the center of the core of the few-mode optical fiber11and the center of the core of the single-mode optical fiber12are on a second straight line along the cross section of the few-mode optical fiber11in the second coupling region14. An included angle between the first straight line and the second straight line is equal to a phase difference between the first mode and the second mode. Therefore, the optical fiber signal mode conversion apparatus provided in this embodiment of this application implements mode conversion in the degenerate modes.

It should be noted that, that an included angle between the first straight line and the second straight line is equal to a phase difference between the first mode and the second mode means that the included angle between the first straight line and the second straight line is exactly equal to the phase difference between the first mode and the second mode, and that the included angle between the first straight line and the second straight line is close to the phase difference between the first mode and the second mode also falls within the protection scope of this application.

Refer toFIG. 7a. The optical fiber signal mode conversion apparatus is configured to convert an M1amode into an M1bmode, where the M1amode and the M1bmode are two modes in a degenerate mode M1, and a phase difference φ between the M1amode and the M1bmode is 45°. The effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the first coupling region13and the effective refractive index of the fundamental mode signal of the single-mode optical fiber12in the second coupling region14are equal, and both are equal to an effective refractive index of a signal in the M1mode (nM1a=nM1b). Refer toFIG. 7b. The center of the core11-2of the few-mode optical fiber11and the center of the core12-2of the single-mode optical fiber12are on the first straight line L3along the cross section of the few-mode optical fiber in the first coupling region13. Refer toFIG. 7c. The center of the core11-2of the few-mode optical fiber11and the center of the core12-2of the single-mode optical fiber12are on the second straight line L4along the cross section of the few-mode optical fiber in the second coupling region14. The included angle α2 between the first straight line L3and the second straight line L4is equal to the phase difference φ between the M1amode and the M1bmode, that is, α2=45°.

If the second mode is a degenerate mode and the second mode includes a first submode and a second submode, when the optical fiber signal mode conversion apparatus is configured to convert the first mode into the first submode, the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is n1; when the optical fiber signal mode conversion apparatus is configured to convert the first mode into the second submode, the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is n2; and n1=n2. When the optical fiber signal mode conversion apparatus is configured to convert the first mode into the first submode, the center of the core of the few-mode optical fiber and the center of the core of the single-mode optical fiber are on a third straight line along the cross section of the few-mode optical fiber in the second coupling region; when the optical fiber signal mode conversion apparatus is configured to convert the first mode into the second submode, the center of the core of the few-mode optical fiber and the center of the core of the single-mode optical fiber are on a fourth straight line along the cross section of the few-mode optical fiber in the second coupling region; and an included angle between the third straight line and the fourth straight line is equal to a phase difference between the first submode and the second submode. Therefore, the optical fiber signal mode conversion apparatus provided in this embodiment of this application can implement mode differentiation in the degenerate mode.

It should be noted that, that an included angle between the third straight line and the fourth straight line is equal to a phase difference between the first submode and the second submode means that the included angle between the third straight line and the fourth straight line is exactly equal to the phase difference between the first submode and the second submode, and that the included angle between the third straight line and the fourth straight line is close to the phase difference between the first submode and the second submode also falls within the protection scope of this application.

Refer toFIG. 8a. The optical fiber signal mode conversion apparatus is configured to convert the M1mode into an M2amode (the M2amode and an M2bmode are two modes in a degenerate mode M2, and a phase difference φ between the M2amode and the M2bmode is 90°), and the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is equal to an effective refractive index nM2aof a signal in the M2amode. Refer toFIG. 9a. The optical fiber signal mode conversion apparatus is configured to convert the M1mode into the M2bmode, and the effective refractive index of the fundamental mode signal of the single-mode optical fiber in the second coupling region is nM2b, and nM2a=nM2b.

Refer toFIG. 8b. The center of the core11-2of the few-mode optical fiber and the center of the core12-2of the single-mode optical fiber are on the third straight line L1along the cross section of the few-mode optical fiber in the second coupling region shown inFIG. 8a. Refer toFIG. 9b. The center of the core11-2of the few-mode optical fiber and the center of the core12-2of the single-mode optical fiber are on the fourth straight line L2along the cross section of the few-mode optical fiber in the second coupling region shown inFIG. 9a. The included angle α1 between the third straight line L1and the fourth straight line L2is equal to the phase difference φ between the M2amode and the M2bmode, that is, α1=90°.

The optical fiber signal mode conversion apparatus provided in this embodiment of this application may be further applied to mode cyclic conversion, to be further used in a few-mode optical fiber transmission system and a multi-mode optical fiber transmission system. For example, a transmission system transmits signals in N modes, where N is greater than 1. To implement compensation for a delay in a differential mode, there are N segments of transmission optical fibers in the transmission system, and axial lengths of the N segments of transmission optical fibers are all equal. Each segment of transmission optical fiber has N mode channels, and the N mode channels are in a one-to-one correspondence with the N modes. There are N-1 optical fiber signal mode conversion apparatuses in the transmission system, and one optical fiber signal mode conversion apparatus is disposed at a node between two segments of transmission optical fibers. A few-mode optical fiber11of each optical fiber signal mode conversion apparatus includes N mode channels, and the N mode channels are in a one-to-one correspondence with N modes. There are N single-mode optical fibers12, and any single-mode optical fiber forms a first coupling region and a second coupling region with the few-mode optical fiber11. A mode that is of the few-mode optical fiber11and that is coupled to any single-mode optical fiber is one of the N modes, and modes that are of the few-mode optical fiber and that are coupled to the N single-mode optical fibers are different. A mode that is of any single-mode optical fiber and that is coupled to the few-mode optical fiber is one of the N modes, and modes that are of the N single-mode optical fibers and that are coupled to the few-mode optical fiber are different. In addition, the mode that is of the few-mode optical fiber11and that is coupled to any single-mode optical fiber is different from the mode that is of the single-mode optical fiber and that is coupled to the few-mode optical fiber. Therefore, cyclic conversion is formed.

The following describes an optical fiber transmission system with an optical fiber signal mode conversion apparatus by using examples.

As shown inFIG. 10, an optical fiber transmission system is configured to transmit signals in LP01, LP11, LP21, and LP02modes. Transmission optical fibers include a first transmission optical fiber211, a second transmission optical fiber212, a third transmission optical fiber213, and a fourth transmission optical fiber214. In addition, axial lengths of the first transmission optical fiber211, the second transmission optical fiber212, the third transmission optical fiber213, and the fourth transmission optical fiber214are all equal. There are three optical fiber signal mode conversion apparatuses1. One optical fiber signal mode conversion apparatus1is disposed at a node between the first transmission optical fiber211and the second transmission optical fiber212. One optical fiber signal mode conversion apparatus1is disposed at a node between the second transmission optical fiber212and the third transmission optical fiber213. One optical fiber signal mode conversion apparatus1is disposed at a node between the third transmission optical fiber213and the fourth transmission optical fiber214. In addition, the three optical fiber signal mode conversion apparatuses1are of a same structure.

FIG. 11shows an optical fiber signal mode conversion apparatus of a structure. Single-mode optical fibers include single-mode optical fibers121,122,123, and124. The single-mode optical fibers121,122,123, and124each form a first coupling region and a second coupling region with a few-mode optical fiber11. Effective refractive indexes of fundamental mode signals of the single-mode optical fibers121,122,123, and124in the first coupling regions are respectively equal to effective refractive indexes nLP01, nLP11, nLP21, and nLP02of the signals in the LP01, LP11, LP21, and LP02modes. Effective refractive indexes of the fundamental mode signals of the single-mode optical fibers121,122,123, and124in the second coupling regions are respectively equal to the effective refractive indexes nLP11, nLP21, nLP02, and nLP01of the signals in the LP11LP21, LP02, and LP01modes. In this way, the optical fiber signal mode conversion apparatus implements conversion from the LP01mode to the LP11mode, from the LP11mode to the LP21mode, from the LP21mode to the LP02mode, and from the LP02mode to the LP01mode. Therefore, the optical fiber transmission system using the optical fiber signal mode conversion apparatus shown inFIG. 11performs three times of conversion in a cyclic mode, so that a signal in each mode is transmitted in the LP01mode, the LP11mode, the LP21mode, and the LP02mode for an equal distance. Therefore, the optical fiber transmission system can implement compensation for a group delay in a differential mode.

As shown inFIG. 12, an optical fiber transmission system implements a mode cycle in a different sequence from that shown inFIG. 10. An optical fiber signal mode conversion apparatus shown inFIG. 13implements conversion from an LP01mode to an LP21mode, from the LP21mode to an LP02mode, from the LP02mode to an LP11mode, and from the LP11mode to the LP01mode. Similarly, the optical fiber transmission system using the optical fiber signal mode conversion apparatus shown inFIG. 12andFIG. 13performs three times of conversion in a cyclic mode, so that a signal in each mode is transmitted in the LP01mode, the LP11mode, the LP21mode, and the LP02mode for an equal distance. Therefore, the optical fiber transmission system can implement compensation for a group delay in a differential mode.

As shown inFIG. 14, an optical fiber transmission system is configured to transmit a signal in an LP01mode, a signal in an LP11amode, and a signal in an LP11bmode, where the LP11amode and the LP11bmode are two modes in a degenerate mode. Transmission optical fibers include a fifth transmission optical fiber215, a sixth transmission optical fiber216, and a seventh transmission optical fiber217. In addition, axial lengths of the fifth transmission optical fiber215, the sixth transmission optical fiber216, and the seventh transmission optical fiber217are all equal. There are two optical fiber signal mode conversion apparatuses1. One optical fiber signal mode conversion apparatus1is disposed at a node between the fifth transmission optical fiber215and the sixth transmission optical fiber216. One optical fiber signal mode conversion apparatus1is disposed at a node between the sixth transmission optical fiber216and the seventh transmission optical fiber217. In addition, the two optical fiber signal mode conversion apparatuses1are of a same structure.

As shown inFIG. 15a, an embodiment of this application provides an optical fiber signal mode conversion apparatus. Single-mode optical fibers include single-mode optical fibers125,126, and127. The single-mode optical fibers125,126, and127each form a first coupling region and a second coupling region with a few-mode optical fiber11. Effective refractive indexes of fundamental mode signals of the single-mode optical fibers125,126, and127in the first coupling regions are respectively equal to effective refractive indexes nLP01, nLP11a, and nLP11bof the signals in the LP01, LP11a, and LP11bmodes. Effective refractive indexes of the fundamental mode signals of the single-mode optical fibers125,126, and127in the second coupling regions are respectively equal to the effective refractive indexes nLP11a, nLP11b, and nLP01of the signals in the LP11a, LP11b, and LP01modes. The optical fiber signal mode conversion apparatus implements conversion from the LP01mode to the LP11amode, from the LP11amode to the LP11bmode, and from the LP11bmode to the LP01mode. Therefore, the optical fiber transmission system using the optical fiber signal mode conversion apparatus shown inFIG. 14andFIG. 15aperforms two times of conversion in a cyclic mode, so that a signal in each mode is transmitted in the LP01mode, the LP11amode, and the LP11bmode for an equal distance. Therefore, the optical fiber transmission system can implement compensation for a group delay in a differential mode.

Because the LP11amode and the LP11bmode are two modes of a degenerate mode, a phase difference between the LP11amode and the LP11bmode is φ. Refer toFIG. 15bandFIG. 15c. In the first coupling region of the single-mode optical fiber126, a center of a core11-2of the few-mode optical fiber11and a center of a core126-2of the single-mode optical fiber126are on a fifth straight line along a cross section of the few-mode optical fiber11. In the second coupling region, the center of the core11-2of the few-mode optical fiber11and the center of the core126-2of the single-mode optical fiber126are on a sixth straight line along the cross section of the few-mode optical fiber11. An included angle between the fifth straight line and the sixth straight line is equal to the phase difference between the LP11amode and the LP11bmode.

As shown inFIG. 16, an optical fiber transmission system implements a mode cycle in a different sequence from that shown inFIG. 14. An optical fiber signal mode conversion apparatus shown inFIG. 17implements conversion from an LP01mode to an LP11bmode, from the LP11bmode to an LP11amode, and from the LP11amode to the LP01mode. Similarly, the optical fiber transmission system using the optical fiber signal mode conversion apparatus shown inFIG. 16andFIG. 17performs two times of conversion in a cyclic mode, so that a signal in each mode is transmitted in the LP01mode, the LP11amode, and the LP11bmode for an equal distance. Therefore, the optical fiber transmission system can implement compensation for a group delay in a differential mode.

The optical fiber signal mode conversion apparatus provided in this embodiment of this application may be further used in a mode add/drop multiplexing scenario. Refer toFIG. 18. A signal in an LP01mode that is transmitted in a few-mode optical fiber111is decoupled to a single-mode optical fiber121, and a decoupled signal is finally converted into a signal in an LP11mode in a few-mode optical fiber112for transmission. A signal in the LP11mode that is transmitted in the few-mode optical fiber111is decoupled to a single-mode optical fiber122, and a decoupled signal is finally converted into a signal in an LP21mode in a few-mode optical fiber113for transmission. This implements switching of mode signals in different optical fibers, and implements add/drop multiplexing of a mode-multiplexed signal.

An embodiment of this application further provides an optical fiber signal mode conversion method. The optical fiber signal mode conversion method is applied to the foregoing optical fiber signal mode conversion apparatus, and includes the following steps:

S1: When the signal in the first mode in the few-mode optical fiber11is transmitted to the first coupling region13, decouple the signal in the first mode to the fundamental mode channel of the single-mode optical fiber12, and transmit a decoupled signal in the single-mode optical fiber12as a fundamental mode signal.

S2: When the fundamental mode signal in the single-mode optical fiber12is transmitted to the second coupling region14, couple the fundamental mode signal of the single-mode optical fiber12to the second mode channel of the few-mode optical fiber11, and transmit a coupled signal in the few-mode optical fiber11in the second mode. Therefore, the first mode is converted into the second mode.

In the descriptions of this specification, the described specific features, structures, materials, or characteristics may be combined in a proper manner in any one or more of the embodiments or examples.