JP5553270B2 - Optical communication system, optical transmitter, optical receiver, and optical communication method - Google Patents

Description

  The present invention relates to a multiplexed optical transmission technique with improved frequency utilization efficiency.

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

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

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

  Regarding the new transmission method, OFDM (Orthogonal Frequency Division Multiplexing) using orthogonal frequency components used to improve frequency utilization efficiency in the wireless transmission method shown in Non-Patent Document 2 or non-patent Application of MIMO (Multiple Input Multiple Output) to a multimode optical fiber has been studied as shown in Reference 3, but since a complicated signal processing is required in the transmitter / receiver, the calculation processing speed is increased. There are issues such as.

  As for a new transmission method, a multiplexing method using a propagation mode of an optical fiber has been proposed in Patent Document 1, but a method for exciting a desired higher-order mode has not been proposed. Since it is assumed to be used, there is a problem that it is difficult to realize a large capacity.

JP-A-8-288911

Matsui et al., “Single-mode photonic fiber with low bending loss and Aeff of> 200 μm 2 for ultra high-speed WDM transmission”, OFC 2010, PA. S. L. Jansen et al., “10 × 121.9-Gb / s PDM-OFDM Transmission with 2-b / s / Hz Spectral Efficiency over 1,000 km of SSMF” Benn C.I. Thomsen, “MIMO enabled 40 Gb / s transmission using mode division multiplexing in multimode fiber”, OFC2010, OThM6. S. Savin et al., “Tunable mechanically induced long-period fiber gratings” POTICCS LETTERS / Vol. 25, no. 10 / May 15,2000

  Accordingly, in order to solve the above-described problems, the present invention provides an optical communication system, an optical transmitter, and an optical receiver capable of improving the frequency utilization efficiency and expanding the transmission capacity by using an existing transmission line and a conventional transmission method. And an optical communication method. The target transmission line is not limited to an existing transmission line, and an optical communication system, an optical transmitter, an optical receiver, and an optical communication method capable of expanding the transmission capacity even in a transmission line that will be newly established in the future. It is also intended to provide.

  To achieve the above object, the optical communication system, the optical transmitter, the optical receiver, and the optical communication method according to the present invention employ a multiplexing method that uses each propagation mode as a carrier in a multimode optical fiber cable. It was.

Specifically, an optical communication system according to the present invention includes an optical transmitter that transmits an optical communication signal, an optical receiver that receives the optical communication signal, and between the optical transmitter and the optical receiver. A multimode transmission line that connects and propagates light of a predetermined wavelength in a plurality of modes,
The optical transmitter generates a light of one wavelength out of the predetermined wavelengths that can be propagated in the multimode transmission line in a fundamental mode in the multimode transmission line, and branches the light from the light source into a plurality of light sources An optical demultiplexer, an optical modulator that modulates each of the lights branched by the optical demultiplexer, and outputs the optical signal as an optical signal, and the optical signal output by the optical modulator has different modes. A mode converter for mode conversion; and a mode multiplexer for multiplexing the optical signals output from the mode converter or the optical modulator as a mode multiplexing unit, and the mode multiplexer The optical signal is the optical communication signal,
The optical receiver mode-divides a mode demultiplexer that demultiplexes the received optical communication signal into the optical signal for each mode, and a light receiving circuit that receives the optical signal output by the mode demultiplexer. It has as a unit.

In the optical communication method according to the present invention, the optical transmitter generates light of one wavelength of the predetermined wavelengths that can be propagated in the multimode transmission line in a fundamental mode in the multimode transmission line. A generation procedure, an optical demultiplexing procedure for branching the light generated in the light generation procedure into a plurality, an optical modulation procedure for modulating each of the light branched in the optical demultiplexing procedure and outputting as an optical signal, A mode conversion procedure for mode-converting the optical signals output in the optical modulation procedure so that the modes are different from each other, and a mode multiplexing procedure for multiplexing the optical signals output in the mode conversion procedure or the optical modulation procedure And as the mode multiplexing step, the optical signal combined in the mode multiplexing procedure is the optical communication signal,
A mode demultiplexing procedure for demultiplexing the received optical communication signal into the optical signal for each mode and a light receiving procedure for receiving the optical signal output in the mode demultiplexing procedure in the optical receiver. It is characterized by being executed as a division step.

  The optical communication system and the optical communication method according to the present invention use each of a plurality of propagation modes existing in a multimode transmission line as a carrier. For this reason, even if it is one wavelength, a some carrier can be formed. Therefore, it is possible to provide an optical communication system and an optical communication method capable of improving the frequency utilization efficiency and expanding the transmission capacity with the existing transmission line and the conventional transmission method.

The optical transmitter of the optical communication system according to the present invention includes a plurality of mode multiplexing units having different wavelengths of the light source, and combines the optical communication signals having different wavelengths output from the mode multiplexing units. Further having a propagation mode holding wavelength multiplexer that outputs
The optical receiver includes a plurality of the mode division units, demultiplexes the received optical communication signal for each wavelength, and couples them to the mode demultiplexer of the mode division unit. It further has a corrugator.

In the optical communication method according to the present invention, the optical transmitter performs a plurality of the mode multiplexing steps having different wavelengths of the light source, and the optical communication signals having different wavelengths output in the respective mode multiplexing steps. Proceed further with the propagation mode holding wavelength multiplexing procedure to combine and output
The optical receiver performs a propagation mode holding wavelength demultiplexing procedure for demultiplexing the received optical communication signal for each wavelength, and each of the optical communication signals demultiplexed by the propagation mode holding wavelength demultiplexing procedure Processing is performed in the mode dividing step.

The mode multiplexing unit of the optical transmitter of the optical communication system according to the present invention has two mode multiplexers, adjusts the optical signals to orthogonal polarizations, and performs the mode multiplexing for each polarization. Have a polarization controller coupled to the
The optical transmitter further includes a propagation mode maintaining polarization multiplexer that combines and outputs the optical communication signals having different polarizations output from the mode multiplexing unit while maintaining the polarization.
The mode division unit of the optical receiver has two of the mode duplexers,
The optical receiver further includes a propagation mode maintaining polarization demultiplexer that separates the received optical communication signal for each polarization and couples the signals to the mode demultiplexer of the mode division unit. And

In the optical communication method according to the present invention, the optical transmitter performs a polarization control procedure for adjusting the optical signal to orthogonal polarization before the mode multiplexing procedure in the mode multiplexing step, and the mode In the multiplexing procedure, the optical signal is combined for each polarization to be the optical communication signal,
After the mode multiplexing step, further performs a propagation mode holding polarization multiplexing procedure for multiplexing and outputting the optical communication signals having different polarizations output in the mode multiplexing step while maintaining the polarization,
The optical receiver performs a propagation mode maintaining polarization demultiplexing procedure for separating the received optical communication signal for each polarization, and the optical communication signal separated by the propagation mode maintaining polarization demultiplexing procedure is converted to the mode. It is characterized by processing in a division step.

The mode multiplexing unit of the optical transmitter of the optical communication system according to the present invention has two mode multiplexers, adjusts the optical signals to orthogonal polarizations, and performs the mode multiplexing for each polarization. Have a polarization controller coupled to the
The optical transmitter has a plurality of the mode multiplexing units having different wavelengths of the light source, and maintains the polarization of the optical communication signals having different polarizations from the wavelengths output by the mode multiplexing units. It further has a propagation mode holding wavelength multiplexer that multiplexes and outputs,
The mode division unit of the optical receiver has two of the mode duplexers,
The optical receiver includes a plurality of the mode division units, and is demultiplexed by a propagation mode holding wavelength demultiplexer that demultiplexes the received optical communication signal for each wavelength and the propagation mode holding wavelength demultiplexer. And a propagation mode holding polarization demultiplexer for separating the optical communication signal for each polarization and coupling each of the signals to the mode demultiplexer of the mode division unit.

In the optical communication method according to the present invention, the optical transmitter performs a plurality of the mode multiplexing steps in which the wavelengths of the light sources are different from each other,
In the mode multiplexing step, a polarization control procedure for adjusting the optical signal to orthogonal polarization is performed before the mode multiplexing procedure, and the optical signal is multiplexed for each polarization in the mode multiplexing procedure. Propagation mode holding for outputting the optical communication signal, after the mode multiplexing step, combining and outputting the optical communication signal having a different wavelength and polarization output in the mode multiplexing step while maintaining the polarization. Perform further wavelength multiplexing procedure,
The optical receiver performs a propagation mode maintaining wavelength demultiplexing procedure for demultiplexing the received optical communication signal for each wavelength, and each of the optical communication signals demultiplexed by the propagation mode maintaining wavelength demultiplexing procedure is A propagation mode maintaining polarization demultiplexing procedure is performed for each wave, and the optical communication signal separated by the propagation mode maintaining polarization demultiplexing procedure is processed in the mode dividing step.

  An optical communication system and an optical communication method according to the present invention include a conventional WDM technique that uses a plurality of signal lights having different wavelengths, and polarization division multiplexing (PDM) that superimposes signals on different polarizations. ) Can be combined with technology. The present invention can further improve the frequency utilization efficiency by combining these conventional techniques, and an optical communication system and an optical communication capable of further expanding the transmission capacity with an existing transmission line and a conventional transmission system. A method can be provided.

  The optical transmitter according to the present invention is an optical transmitter provided in the optical communication system.

  An optical receiver according to the present invention is an optical receiver included in the optical communication system.

  The present invention provides an optical communication system, an optical transmitter, an optical receiver, and an optical communication method capable of improving the frequency utilization efficiency and expanding the transmission capacity with an existing transmission line and a conventional transmission method. it can.

It is a figure explaining the optical communication system which concerns on this invention. It is a figure which shows the result of having calculated the space | interval of the grating for converting a fundamental mode into a 1st higher order mode in a fiber Bragg grating. It is a figure which shows the principle of a mode multiplexer and a mode splitter. It is a figure which shows the transmittance | permeability of the fundamental mode and 1st higher order mode in a parallel waveguide. It is a figure explaining the optical communication system which concerns on this invention. It is a figure explaining the optical communication system which concerns on this invention. It is a figure explaining the optical communication system which concerns on this invention.

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

(Embodiment 1)
FIG. 1 is a diagram illustrating an optical communication system 100 according to the present embodiment. The optical communication system 100 includes an optical transmitter 111 that transmits an optical communication signal, an optical receiver 118 that receives the optical communication signal, and an optical transmitter 111 and the optical receiver 118 that have a predetermined wavelength. A multimode transmission line 117 that propagates light in a plurality of modes. The optical transmitter 111 generates light of one wavelength out of a predetermined wavelength that can be propagated through the multimode transmission path 117 in the basic mode in the multimode transmission path 117, and branches the light from the light source 112 into a plurality of light sources. The optical demultiplexer 113, the optical modulator 114 that modulates each of the lights branched by the optical demultiplexer 113, and outputs the optical signal as an optical signal, and the optical signal output from the optical modulator 114 has different modes. A mode converter 115 that performs mode conversion, and a mode multiplexer 116 that combines optical signals output from the mode converter 115 or the optical modulator 114, as a mode multiplexing unit 151. The combined optical signal is used as an optical communication signal. The optical receiver 118 includes a mode division unit that includes a mode demultiplexer 119 that demultiplexes the received optical communication signal into an optical signal for each mode, and a light receiving circuit 120 that receives the optical signal output from the mode demultiplexer 119. 161.

  The optical transmitter 111 has a mode multiplexing unit 151. The mode multiplexing unit 151 includes a light source 112, an optical demultiplexer 113, an optical modulator 114, a mode converter 115, and a mode multiplexer 116. The optical receiver 118 has a mode division unit 161. The mode division unit 161 has a mode duplexer 119 and a light receiving circuit 120.

  As shown in FIG. 1, an optical transmitter 111 in an optical communication system 100 includes a light source 112 having a wavelength λ, an optical demultiplexer 113 that equally splits light having a wavelength λ into N, and an optical modulation that modulates light having a wavelength λ. And a mode converter 115 that converts the fundamental propagation mode of the wavelength λ into a desired higher-order mode and a mode multiplexer 116 that multiplexes the generated N propagation modes. The optical receiver 118 also receives a mode demultiplexer 119 that separates the multiplexed N propagation modes into respective modes, and a light receiving circuit 120 that receives the demultiplexed N signal lights and converts them into electrical signals. It is comprised by.

  The mode multiplexer 116 of the optical transmitter 111 and the mode duplexer 119 of the optical receiver 118 are connected by a multimode transmission line 117. The light output from the light source 112 of the optical transmitter 111 is equally branched by the optical demultiplexer 113, and the equally branched light is modulated by the optical modulator 114 at each port. The modulated fundamental propagation mode is converted into a different higher-order propagation mode by the mode converter 115 at each port. The N propagation modes including the fundamental mode are multiplexed by the mode multiplexer 116, and the combined signal light is propagated by the multimode transmission line 117 and is transmitted by the mode demultiplexer 119 of the optical receiver 118. The light is separated into a propagation mode, received by the light receiving circuit 120, and converted into an electric signal. Since the optical transmitter 111 propagates in the fundamental mode before the mode converter 115, the apparatus can be configured using a conventional optical device.

The mode converter 115 is constituted by a long-period fiber Bragg grating. As shown in Non-Patent Document 4, the grating interval Λ for converting the fundamental mode to the higher-order mode at the wavelength λ is given by Equation (1). _{N eff} and _{n cl} ^m is the effective refractive index of the fundamental mode at the respective wavelength lambda _m of formula (1) shows the effective refractive index of the m-th order mode. The fiber grating spacing is determined by the structural parameters of the fiber used, the wavelength, and the mode order to be converted. After determining the wavelength to be used, numerical analysis is performed from the structural parameters of the fiber to calculate the effective refractive index of the fundamental mode and the desired higher-order mode. The required grating period is determined using these calculation results and equation (1).

  Assuming a step-type refractive index profile fiber with a relative refractive index difference Δ of 0.35% and a core radius of 4.5 μm, the fundamental mode is changed to the first higher-order mode using equation (1). FIG. 2 shows the result of calculating the grating interval for conversion. In FIG. 2, the horizontal axis indicates the wavelength λ, and the vertical axis indicates the grating interval Λ for converting the fundamental mode to the first higher-order mode. From this result, the fundamental mode with a wavelength of 1000 nm can be converted to the first higher-order mode by setting the grating interval to about 480 μm.

  Next, the mode multiplexer 116 and the mode duplexer 119 can be realized by symmetric parallel waveguides. FIG. 3 shows the principle diagram of the mode multiplexer and mode splitter. By changing the distance between the two waveguides, the separation of each mode into a parallel port and a diagonal port is realized using the difference in coupling length between the modes. In FIG. 4, the horizontal axis indicates the distance between the cores of the parallel waveguide, and the vertical axis indicates the transmittance of the fundamental mode and the first higher-order mode of the parallel port. The waveguide parameter of the parallel waveguide is assumed to be a fiber having a refractive index profile used in the calculation of FIG. It can be seen that when the inter-core distance is changed, the transmittance of the fundamental mode and the first higher-order mode is changed, and the transmittance between the modes is different, so that the parallel port and the diagonal port can be separated. In the calculation of FIG. 4, the length in which two waveguides are parallel and light interaction occurs between the waveguides is defined as the interaction length, and the interaction length is 20 cm. Since the transmittance of the fundamental mode is 1 and the transmittance of the first higher order mode is 0, the fundamental mode is output to the parallel port, and the first higher order mode is output to the diagonal port. It can be seen that it can be separated into each mode. This method can also be applied to higher-order modes. A parallel waveguide capable of separating desired higher-order modes by designing the inter-core distance and the interaction length appropriately is designed, and the number of multiplexed modes N Accordingly, it is possible to separate a plurality of modes by connecting parallel waveguides in series.

  The optical communication system 100 generates an optical signal to be superimposed for each propagation mode in the fundamental mode, and converts the optical signal in the fundamental mode into a higher-order mode using a long-period fiber Bragg grating. Then, a signal is superimposed on each of the N modes including the basic mode, and they are multiplexed and multiplexed. For this reason, the optical communication system 100 can increase the transmission capacity by N + 1 times compared to the optical communication system only in the basic mode.

(Embodiment 2)
FIG. 5 is a diagram illustrating the optical communication system 200 according to the present embodiment. The optical communication system 200 is a combination of the optical communication system 100 of FIG. 1 and WDM. In the optical communication system 200, the optical transmitter 211 has a plurality of mode multiplexing units 151 having different wavelengths of the light source 112, and combines and outputs optical communication signals having different wavelengths output from the respective mode multiplexing units 151. Further having a propagation mode maintaining wavelength multiplexer 217, and an optical receiver 219 having a plurality of mode division units 161, demultiplexing the received optical communication signal for each wavelength, and each mode. The difference from the optical communication system 100 is that it further includes a propagation mode maintaining wavelength demultiplexer 220 coupled to the mode demultiplexer 119 of the division unit 161.

  The optical transmitter 211 has N mode multiplexing units 151 arranged in parallel for each wavelength. The optical receiver 118 has N mode division units 161 arranged in parallel for each wavelength.

The optical transmitter 211 includes N light sources 112 from λ ₁ to λ _N , an optical demultiplexer 113 that equally divides light of each of N wavelengths, and an optical modulator 114 that modulates light of each of N wavelengths. A mode converter 115 that converts a fundamental propagation mode of each wavelength into a desired higher-order mode, a mode combiner 116 that combines N propagation modes generated at each wavelength, and a propagation mode of each wavelength. A propagation mode holding wavelength multiplexer 217 that multiplexes N wavelengths in a held state.

  In addition, the optical receiver 219 separates the combined N wavelengths at the wavelength demultiplexed by the propagation mode holding wavelength demultiplexer 220 and the propagation mode wavelength demultiplexer which perform wavelength separation while maintaining the propagation mode. A mode demultiplexer 119 that separates N propagation modes and a light receiving circuit 120 that receives N signal beams demultiplexed by the mode demultiplexer 119 and converts them into electrical signals.

  The propagation mode holding wavelength multiplexer 217 of the optical transmitter 211 and the propagation mode holding wavelength demultiplexer 220 of the optical receiver 219 are connected by a multimode transmission line 117. The N lights output from the light source 112 of the optical transmitter 211 are equally branched by the optical demultiplexer 113, and the equally branched light is modulated by the optical modulator 114 at each port. The modulated fundamental propagation mode is converted into a different higher-order propagation mode by the mode converter 115 at each port. The N propagation modes including the fundamental mode are combined by the mode combiner 116, and the combined signal light is further combined by the propagation mode holding wavelength combiner 217 and propagated by the multimode transmission path 117. The light is separated into N wavelengths by the propagation mode holding wavelength demultiplexer 220 of the optical receiver 219, separated into N propagation modes by the mode demultiplexer 119, received by the light receiving circuit 120 and converted into an electrical signal. Is done.

  The optical communication system 200 is configured by using a simple fiber device for two multiplexing methods of mode division multiplexing and wavelength division multiplexing, and dramatically increases the transmission capacity as compared with the optical communication system of only the basic mode. be able to.

(Embodiment 3)
FIG. 6 is a diagram illustrating the optical communication system 300 according to the present embodiment. The optical communication system 300 is a combination of the optical communication system 100 of FIG. 1 and a PDM. In the optical communication system 300, the mode multiplexing unit 152 includes two mode multiplexers 116, and adjusts the optical signal to orthogonal polarizations and couples the polarization controller 317 to the mode multiplexer 116 for each polarization. In addition, the optical transmitter further includes a propagation mode maintaining polarization multiplexer 319 that combines and outputs optical communication signals of different polarization output from the mode multiplexing unit 152 while maintaining the polarization. The mode division unit 162 has two mode demultiplexers 119, and the optical receiver demultiplexes the received optical communication signal for each polarization, and separates each of them into the mode demultiplexer 119 of the mode division unit 162. The optical communication system 100 is different from the optical communication system 100 in that it further includes a propagation mode maintaining polarization splitter 322 coupled to the optical communication system 100.

  The optical transmitter 311 has a mode multiplexing unit 152. The mode multiplexing unit 152 includes a light source 112, an optical demultiplexer 313, an optical demultiplexer 113, an optical modulator 114, a mode converter 115, and a mode multiplexer 116. The mode multiplexing unit 152 has two systems for each polarization between the optical demultiplexer 113 and the mode multiplexer 116. The optical receiver 321 includes a mode division unit 162. The mode division unit 162 has a mode demultiplexer 119 and a light receiving circuit 120, and has two systems for each polarization.

  The optical transmitter 311 includes a light source 112 having a wavelength λ, an optical demultiplexer 313 that divides the light having the wavelength λ into two, and an optical demultiplexer 113 that divides the light having the wavelength λ that is branched by the optical demultiplexer 313 into N pieces. And an optical modulator 114 that modulates the fundamental mode of wavelength λ, a mode converter 115 that converts the fundamental propagation mode of wavelength λ to a desired higher-order mode, and the polarization of each of the generated N propagation modes is adjusted. A polarization controller 317 that combines the N propagation modes, a mode multiplexer 116 that combines the N propagation modes, and a propagation mode holding polarization multiplexer 319 that combines the two polarizations while maintaining the respective propagation modes. Has been.

  Further, the optical receiver 321 includes a propagation mode holding polarization demultiplexer 322 that separates polarization while holding the combined N propagation modes, and a mode demultiplexer 119 that separates each of the N propagation modes. The light receiving circuit 120 receives N signal lights demultiplexed by the mode demultiplexer 119 and converts them into electric signals.

  The propagation mode maintaining polarization multiplexer 319 of the optical transmitter 311 and the propagation mode maintaining polarization splitter 322 of the optical receiver 321 are connected by a multimode transmission line 117. The light of wavelength λ output from the light source 112 of the optical transmitter 311 is bifurcated by the optical demultiplexer 313, and the bifurcated light is further equally divided into N by the optical demultiplexer 113. The light equally branched into N is modulated by the optical modulator 114 at each port. The modulated fundamental propagation mode is converted into a different higher-order propagation mode by the mode converter 115 at each port. The polarization controller 317 connected to the subsequent stage of the mode converter 115 is adjusted so that one of the two beams of the wavelength λ is X-polarized and the other is Y-polarized. The N propagation modes are multiplexed for each of the X polarization and the Y polarization, and are multiplexed by the propagation mode maintaining polarization multiplexer 319. The combined signal light is propagated through the multimode transmission line 117, separated into X polarization and Y polarization by the propagation mode maintaining polarization demultiplexer 322 of the optical receiver 321, and then the mode demultiplexer 119. Are separated into N propagation modes, received by the light receiving circuit 120 and converted into electrical signals.

  The optical communication system 300 is configured by using simple fiber devices for the two multiplexing methods of mode division multiplexing and polarization division multiplexing, and the transmission capacity is dramatically increased compared to the optical communication system of only the basic mode. Can be made.

(Embodiment 4)
FIG. 7 is a diagram illustrating the optical communication system 400 of the present embodiment. The optical communication system 400 is a combination of the optical communication system 100 of FIG. 1, WDM, and PDM. The optical communication system 400 includes a polarization controller 317 in which the mode multiplexing unit 152 has two mode multiplexers 116, adjusts optical signals to orthogonal polarizations, and couples them to the mode multiplexers 116 for each polarization. In addition, the optical transmitter 411 has a plurality of mode multiplexing units 152 with different wavelengths of the light source 112, and maintains polarization of optical communication signals having different polarizations from the wavelengths output by the mode multiplexing units 152. A propagation mode holding wavelength multiplexer 419 that multiplexes and outputs in this state, the mode division unit 161 has two mode demultiplexers 119, and the optical receiver 421 has a plurality of mode divisions. A propagation mode holding wavelength demultiplexer 422 that demultiplexes the received optical communication signal for each wavelength, and a propagation mode holding wavelength demultiplexer 422; The optical communication system 100 is different from the optical communication system 100 in that it further includes a propagation mode holding polarization demultiplexer 322 that separates the optical communication signal for each polarization and couples them to the mode demultiplexer 119 of the mode division unit 161. It is.

  The optical transmitter 411 parallels N mode multiplexing units 152 for each wavelength. The optical receiver 421 has N mode division units 161 arranged in parallel for each wavelength.

The optical transmitter 411 includes N light sources 112 from λ ₁ to λ _N , an optical demultiplexer 313 that divides the light of each of the N wavelengths into two, and further divides the bifurcated light into N equal parts. Corresponding to the optical demultiplexer 113, the optical modulator 114 that modulates each of the fundamental modes of N wavelengths, the mode converter 115 that converts the fundamental propagation mode of each wavelength to a desired higher order mode, and each of the N wavelengths A polarization controller 317 that adjusts the polarization of each of the N higher-order modes generated at each wavelength, and a mode multiplexer 116 that combines the N propagation modes generated at each wavelength, respectively. And a propagation mode holding wavelength multiplexer 419 that multiplexes N wavelengths in a state where the propagation mode of the wavelength is held.

  In addition, the optical receiver 421 demultiplexes the multiplexed N wavelengths by the propagation mode holding wavelength demultiplexer 422 and the propagation mode wavelength demultiplexer 422 that perform wavelength separation while holding the propagation mode. A propagation mode holding polarization demultiplexer 322 for separating polarized waves while maintaining N propagation modes at a wavelength, a mode demultiplexer 119 and a mode demultiplexer 119 for separating N propagation modes. It comprises a light receiving circuit 120 that receives the demultiplexed N signal lights and converts them into electrical signals.

  The propagation mode maintaining wavelength multiplexer 419 of the optical transmitter 411 and the propagation mode maintaining wavelength demultiplexer 422 of the optical receiver 421 are connected by a multimode transmission line 117. The N light beams output from the light source 112 of the optical transmitter 411 are branched into two by an optical demultiplexer 313, and the bifurcated light is further branched equally into N by an optical demultiplexer 113. The light equally branched into N is modulated by the optical modulator 114 at each port. The modulated fundamental propagation mode is converted into a different higher-order propagation mode by the mode converter 115 at each port. The polarization controller 317 connected to the subsequent stage of the mode converter 115 is adjusted so that one of the two lights of the N wavelengths λ is divided into two and the other is a Y polarization, N propagation modes including the fundamental mode are multiplexed in the number of N modes for each of the X polarization and the Y polarization, and are multiplexed by the mode multiplexer 116. The N propagation modes adjusted to the X polarization and the Y polarization combined at the respective wavelengths are further divided into N by a propagation mode holding wavelength multiplexer 419 that combines the wavelengths while holding the propagation modes. Multiple wavelengths are combined. The signal light combined with all of the polarization, wavelength, and propagation mode is propagated through the multi-mode transmission line 117, separated into N wavelengths by the propagation mode holding wavelength demultiplexer 422 of the optical receiver 421, and N Each of the wavelengths is further separated into an X-polarized wave and a Y-polarized wave by the propagation mode-maintaining polarization demultiplexer 322. After polarization separation, the mode demultiplexer 119 separates each of the N propagation modes, and the light receiving circuit 120 receives the light and converts it into an electrical signal.

  The optical communication system 400 is configured by using simple fiber devices for the three multiplexing methods of mode division multiplexing, polarization division multiplexing, and wavelength division multiplexing, and has a capacity that can be transmitted as compared with an optical communication system only in the basic mode. It can be increased dramatically.

  The optical communication system of the present invention is not limited to expanding the capacity of an existing transmission line. The optical communication system of the present invention can be applied even to a transmission line newly constructed or a transmission line constituted by a newly developed optical fiber.

100, 200, 300, 400: Optical communication system 111, 211, 311, 411: Optical transmitter 118, 219, 321, 421: Optical receiver 112: Light source 113: Optical demultiplexer 114: Optical modulator 115: Mode Converter 116: Mode multiplexer 117: Multimode transmission line 119: Mode duplexer 120: Light receiving circuit 151, 152: Mode multiplexing unit 161, 162: Mode division unit 217: Propagation mode holding wavelength multiplexer 220: Propagation Mode holding wavelength demultiplexer 313: Optical demultiplexer 317: Polarization controller 319: Propagation mode preserving polarization multiplexer 322: Propagation mode preserving polarization demultiplexer 419: Propagation mode preserving polarization multiplexer 422: Propagation Mode-maintaining wavelength demultiplexer

Claims (10)

An optical transmitter for transmitting an optical communication signal;
An optical receiver for receiving the optical communication signal;
A multimode transmission line that connects between the optical transmitter and the optical receiver and propagates light of a predetermined wavelength in a plurality of modes;
An optical communication system comprising:
The optical transmitter is
A light source that generates light of one wavelength among the predetermined wavelengths that can propagate in the multi-mode transmission line in a fundamental mode in the multi-mode transmission line;
Where N is an integer of 2 or more, an optical demultiplexer for equal splitting the light from the light source into N,
And N optical modulator for outputting an optical signal by modulating each of the equal branched N optical by the optical demultiplexer,
N-1 mode converters for mode-converting N-1 optical signals respectively output from N-1 of the optical modulators from a fundamental mode to a higher-order mode so that the modes are different from each other;
A mode multiplexer for multiplexing the N-1 optical signals output by the mode converter and the optical signal output by the remaining one of the optical modulators;
As a mode multiplexing unit, the optical signal combined by the mode multiplexer is the optical communication signal,
The optical receiver is:
A mode demultiplexer for demultiplexing the received optical communication signal into N optical signals for each mode;
A light receiving circuit for receiving the optical signal output from the mode duplexer;
Have a as a mode division unit,
The mode converter is a long-period fiber Bragg grating in which a grating interval is set according to a higher-order mode to be converted,
The optical communication system, wherein the mode multiplexer and the mode splitter are fiber devices in which a plurality of symmetric parallel waveguides having different inter-core distances are connected in series . The optical transmitter is
A plurality of the mode multiplexing units having different wavelengths of the light source;
A propagation mode holding wavelength multiplexer that multiplexes and outputs the optical communication signals having different wavelengths output by the respective mode multiplexing units;
The optical receiver is:
A plurality of the mode division units;
2. The propagation mode holding wavelength demultiplexer according to claim 1, further comprising a propagation mode maintaining wavelength demultiplexer that demultiplexes the received optical communication signal for each wavelength and couples the demultiplexed signals to the mode demultiplexer of the mode division unit. Optical communication system. The mode multiplexing unit of the optical transmitter is:
Have two mode multiplexers,
The optical signal is adjusted to orthogonal polarization, and further has a polarization controller coupled to the mode multiplexer for each polarization,
The optical transmitter is
A propagation mode maintaining polarization multiplexer that combines and outputs the optical communication signals with different polarizations output by the mode multiplexing unit while maintaining the polarization;
The mode division unit of the optical receiver is:
Have two mode splitters,
The optical receiver is:
2. The propagation mode maintaining polarization demultiplexer further comprising a propagation mode maintaining polarization demultiplexer for separating the received optical communication signal for each polarization and coupling each of the received signals to the mode demultiplexer of the mode division unit. The optical communication system described. The mode multiplexing unit of the optical transmitter is:
Have two mode multiplexers,
The optical signal is adjusted to orthogonal polarization, and further has a polarization controller coupled to the mode multiplexer for each polarization,
The optical transmitter is
A plurality of the mode multiplexing units having different wavelengths of the light source;
Propagation mode holding wavelength multiplexers that combine and output the optical communication signals having different polarizations and wavelengths output from each mode multiplexing unit while maintaining polarization,
The mode division unit of the optical receiver is:
Have two mode splitters,
The optical receiver is:
A plurality of the mode division units;
A propagation mode holding wavelength demultiplexer for demultiplexing the received optical communication signal for each wavelength;
A propagation mode maintaining polarization demultiplexer for separating the optical communication signal demultiplexed by the propagation mode maintaining wavelength demultiplexer for each polarization and coupling each of the signals to the mode demultiplexer of the mode dividing unit; ,
The optical communication system according to claim 1, further comprising:   An optical transmitter provided in the optical communication system according to claim 1.   An optical receiver included in the optical communication system according to claim 1. An optical transmitter for transmitting an optical communication signal;
An optical receiver for receiving the optical communication signal;
A multi-mode transmission line that connects between the optical transmitter and the optical receiver and propagates light of a predetermined wavelength in a plurality of modes;
An optical communication method for an optical communication system comprising:
In the optical transmitter,
A light generation procedure for generating light of one wavelength among the predetermined wavelengths capable of propagating in the multimode transmission path in a fundamental mode in the multimode transmission path;
Where N is an integer of 2 or more, the optical demultiplexing procedure for equal splitting the light generated by said light generating procedure into N,
A light modulation procedure is output as N optical signal by modulating each of the equal branched N optical in the optical demultiplexing procedure,
A mode conversion procedure for mode-converting N-1 of the optical signals output in the optical modulation procedure from a basic mode to a higher-order mode so that the modes are different from each other;
A mode combining procedure for combining the N-1 optical signals output in the mode conversion procedure and the remaining one optical signal output in the optical modulation procedure;
Is performed as a mode multiplexing step, and the optical signal combined in the mode multiplexing procedure is used as the optical communication signal,
In the optical receiver,
A mode demultiplexing procedure for demultiplexing the received optical communication signal into N optical signals for each mode;
A light receiving procedure for receiving the optical signal output in the mode demultiplexing procedure;
As a mode split step ,
In the mode conversion procedure, mode conversion is performed with a long-period fiber Bragg grating in which a grating interval is set according to a higher-order mode to be converted.
In the mode multiplexing procedure and the mode demultiplexing procedure , an optical communication method is characterized by multiplexing and demultiplexing with a fiber device in which a plurality of symmetric parallel waveguides having different inter-core distances are connected in series .
In the optical transmitter,
Performing a plurality of the mode multiplexing steps in which the wavelengths of the light sources are different from each other;
Further performing a propagation mode holding wavelength combining procedure for combining and outputting the optical communication signals having different wavelengths output in each of the mode multiplexing steps,
In the optical receiver,
Performing a propagation mode holding wavelength demultiplexing procedure for demultiplexing the received optical communication signal for each wavelength,
8. The optical communication method according to claim 7, wherein each of the optical communication signals demultiplexed by the propagation mode maintaining wavelength demultiplexing procedure is processed in a plurality of the mode division steps. In the optical transmitter,
In the mode multiplexing step,
Performing a polarization control procedure to adjust the optical signal to orthogonal polarization before the mode multiplexing procedure,
In the mode multiplexing procedure, the optical signal is combined for each polarization to be the optical communication signal,
After the mode multiplexing step,
Further performing a propagation mode holding polarization multiplexing procedure for combining and outputting the optical communication signals having different polarizations output in the mode multiplexing step while maintaining the polarization,
In the optical receiver,
Propagating mode maintaining polarization demultiplexing procedure for separating the received optical communication signal for each polarization,
8. The optical communication method according to claim 7, wherein the optical communication signal separated in the propagation mode maintaining polarization demultiplexing procedure is processed in the mode division step. In the optical transmitter,
Performing a plurality of the mode multiplexing steps in which the wavelengths of the light sources are different from each other;
In the mode multiplexing step,
Performing a polarization control procedure to adjust the optical signal to orthogonal polarization before the mode multiplexing procedure,
In the mode multiplexing procedure, the optical signal is combined for each polarization to be the optical communication signal,
After the mode multiplexing step,
Further performing a propagation mode holding wavelength multiplexing procedure for multiplexing and outputting the optical communication signal having different polarization and wavelength output in each mode multiplexing step while maintaining polarization,
In the optical receiver,
Performing a propagation mode holding wavelength demultiplexing procedure for demultiplexing the received optical communication signal for each wavelength,
Performing a propagation mode holding polarization demultiplexing procedure for separating the optical communication signals demultiplexed in the propagation mode holding wavelength demultiplexing procedure for each polarization,
8. The optical communication method according to claim 7, wherein the optical communication signal separated in the propagation mode maintaining polarization demultiplexing procedure is processed in the mode division step.

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