JP6422799B2 - Optical transmission system, optical transmitter and control method thereof - Google Patents

Description

  The present invention relates to an optical communication technique, and more particularly to a technique for measuring crosstalk between modes in an optical transmission system to which a mode multiplex transmission technique is applied.

  In recent years, in optical fiber communication, space division multiplexing (mode division multiplexing) transmission technology has attracted attention in addition to wavelength division multiplexing (WDM) and polarization multiplexing as a technique for increasing the transmission capacity. Mode division multiplexing (mode multiplexing) technology is a technology that increases the transmission capacity by the number of modes to be used by modulating and demodulating the light of each mode with an independent signal using a fiber having a plurality of spatial modes. It is.

  On the other hand, in an existing optical transmission system using a single mode fiber (SMF), for example, long distance transmission by WDM is performed. In such an optical transmission system, it is important to flatten (equalize) the wavelength dependence of gain in the optical repeater amplifier. However, gain deviation may remain between channels in the transmission band after gain equalization due to manufacturing deviation of optical components that can be used for gain equalization and changes in the surrounding environment. Such gain deviation may cause variations in reception characteristics (optical signal-to-noise ratio (OSNR)) among a plurality of channels of the WDM signal received by the optical receiver. In order to correct such variations in reception characteristics, generally, a technique (pre-emphasis) that makes a difference in signal light power level between channels of a WDM signal is used in an optical transmission apparatus (for example, Patent Document 1). ).

  In contrast, in future optical transmission systems to which mode multiplexing transmission technology is applied, not only equalize the wavelength dependence of the signal light received by the optical receiver, but also equalize the received power level between modes. There is a need to. This is because the mode multiplexing and demultiplexing unit consisting of a mode converter and a mode multiplexer / demultiplexer to realize mode multiplexing transmission, mode multiplexing transmission path, and mode multiplexing amplifier have different losses and gains for each mode. This is because a difference occurs in reception characteristics (OSNR) between modes. In mode multiplex transmission, it is known that crosstalk between modes occurs in a mode demultiplexer, a transmission line, and an amplifier. In a mode where the intensity of crosstalk from other modes is large, the transmission performance of signal light is significantly degraded. Such crosstalk intensity (crosstalk amount) generally depends on the characteristics of the mode demultiplexer, transmission line, and amplifier, and also varies depending on the type and combination of modes. For this reason, it is difficult to sufficiently suppress the influence of such crosstalk between modes by individually optimizing each device (component) constituting the optical transmission system.

  To deal with the above-mentioned problems in mode multiplex transmission, although at the laboratory level, as in pre-emphasis between wavelengths in WDM, the transmission power is set for each mode so that the reception characteristics are equal between modes. Techniques for adjustment have been proposed. For example, in Non-Patent Document 1, the transmission power is adjusted for each mode so that the bit error rate (BER) (Q value) of each mode after reception is equal. In Non-Patent Document 2, the crosstalk characteristics between modes of the mode demultiplexer and the transmission path are measured in advance, and the transmission power is set for each mode so that the amount of crosstalk between modes of each mode on the receiving side is equal. It is adjusting.

Japanese Patent Laid-Open No. 2005-138074

S. Massimiliano, et al., "Mode division multiplexed transmission with a weakly-coupled few-mode fiber", The Optical Fiber Communication Conference and Exposition (OFC 2012), March 2012, OTu2C.5 P. Genevaux, et al., "3 Modes transmission using hybrid separation with high mode selectivity and low losses spatial mode multiplexer", European Conference on Optical Communications (ECOC 2014), September 2014, We.1.1.5 T. Mizuno, et al., "Modal crosstalk measurement based on intensity tone for few-mode fiber transmission systems", Optical Fiber Communication Conference and Exposition (OFC 2014), March 2014, W3D.5

  However, in the method of Non-Patent Document 1, only the BER on the receiving side is used for adjusting the transmission power for each mode. However, when the BER deterioration factor is a factor other than the crosstalk between modes (for example, non-linear effect), it is difficult to equalize the reception characteristics between the modes accurately by adjusting the transmission power.

  In the method of Non-Patent Document 2, it is necessary to measure in advance the crosstalk characteristics of the mode demultiplexer, the transmission line, and the relay amplifier. However, the crosstalk characteristic is usually measured by inputting the test light for measurement from the transmission side for each mode to be multiplexed and measuring the power of each mode light output from the mode separator on the reception side. It is calculated based on the measurement result. For this reason, as the number of modes to be multiplexed increases, the measurement of necessary crosstalk characteristics becomes more complicated. Therefore, it is desirable that the crosstalk characteristic can be measured more easily in an actually operated system.

  In general, it is difficult to measure the crosstalk characteristic during actual communication (signal transmission) in the optical transmission system. However, in a system that is actually operated, the crosstalk characteristics may fluctuate with time due to changes in the external environment (bending of the optical fiber, temperature changes of each device, etc.) in the mode demultiplexer, transmission line, and relay amplifier. For this reason, it is necessary to measure crosstalk characteristics that vary with time and to equalize reception characteristics between modes in response to such time fluctuations.

  The present invention has been made in view of the above-described problems. The present invention more easily measures the amount of crosstalk between modes generated in mode multiplexed light transmitted in an optical transmission system that performs mode multiplexing transmission, and more accurately determines the reception characteristics of each mode included in the mode multiplexed light. The purpose is to provide a technique that enables equalization.

The present invention can be realized as, for example, an optical transmission system, an optical transmitter, and an optical receiver. An optical transmission system according to an aspect of the present invention includes: an optical transmission device that transmits mode multiplexed light in which a plurality of modes of light are multiplexed; and an optical reception device that receives the mode multiplexed light from the optical transmission device. The optical transmission system includes a plurality of test lights for measuring the amount of crosstalk between modes in the optical receiver, each corresponding to a different mode, and each having a different wavelength. Generating a plurality of test lights, and mode multiplexing to be transmitted to the optical receiver by multiplexing the plurality of test lights generated by the generation means as light of corresponding modes, respectively. Multiplexing means for generating light, wherein the optical receiver corresponds to each of the plurality of modes from the mode multiplexed light received from the optical transmitter A demultiplexing means for separating the light to be measured, and a measurement for measuring the amount of crosstalk between the modes by measuring the wavelength spectrum of each mode based on the light corresponding to each mode separated by the demultiplexing means. And the optical transmitter further includes a plurality of optical amplifiers for amplifying light of different modes for adjusting the transmission power of the light of each mode to be transmitted to the optical receiver, Based on the crosstalk amount measured for each of the plurality of modes by the measuring unit, the gains of the plurality of optical amplifiers are adjusted so that the crosstalk amount is equal between the plurality of modes. that, characterized in that.

An optical transmission system according to another aspect of the present invention includes an optical transmitter that transmits mode multiplexed light in which a plurality of modes of light are multiplexed, and an optical receiver that receives the mode multiplexed light from the optical transmitter. , Wherein the optical transmitter is a wavelength multiplexed signal in which light of a plurality of wavelengths excluding unused wavelengths is multiplexed, and the unused wavelengths differ for each mode. By generating a signal as signal light corresponding to each mode, and multiplexing a plurality of signal lights corresponding to the plurality of modes generated by the generation means as light of corresponding modes, Multiplexing means for generating mode multiplexed light to be transmitted to the optical receiver, wherein the optical receiver is configured to receive each of the plurality of modes from the mode multiplexed light received from the optical transmitter. The amount of crosstalk between modes is measured by measuring the wavelength spectrum of each mode based on the demultiplexing means for separating the corresponding light and the light corresponding to each mode separated by the demultiplexing means. Measuring means, and for each of the plurality of modes, the measuring means appears as a power of a component of a wavelength other than the unused wavelength and a component of the unused wavelength in the measured wavelength spectrum. The difference from the power of the crosstalk component from other modes is measured as the crosstalk amount .

  An optical transmission system according to another aspect of the present invention includes an optical transmitter that transmits mode multiplexed light in which a plurality of modes of light are multiplexed, and an optical receiver that receives the mode multiplexed light from the optical transmitter. The optical transmission device generates a plurality of signal lights corresponding to the plurality of modes, and signals corresponding to the generated modes of microwaves having different frequencies for each mode. A generating means for superimposing on the light, and superimposing on the signal light corresponding to each mode so that the frequency difference between the two corresponding microwaves differs for each combination of the two possible modes among the plurality of modes. The generating means for generating the microwave to be generated, and the plurality of signal lights generated by the generating means and corresponding to the plurality of modes, as light of the corresponding modes, respectively. And multiplexing means for generating mode multiplexed light to be transmitted to the optical receiver, the optical receiver from the mode multiplexed light received from the optical transmitter. Demultiplexing means for separating the light corresponding to each of the above, and measuring the frequency spectrum of the microwaves that are separated by the demultiplexing means and superimposed on the light corresponding to each mode, among the plurality of modes Measuring means for measuring a crosstalk amount between two possible modes based on a frequency component corresponding to a frequency difference between two microwaves corresponding to the two modes in the frequency spectrum. And

An optical transmission apparatus according to an aspect of the present invention is an optical transmission apparatus that transmits mode multiplexed light, in which light of a plurality of modes is multiplexed, to an optical reception apparatus via an optical transmission line, and in the optical reception apparatus A plurality of test lights for measuring the amount of crosstalk between modes, the generation means for generating the plurality of test lights corresponding to different modes and having different wavelengths, and generated by the generation means And multiplexing means for generating mode multiplexed light to be transmitted to the optical receiver by multiplexing the plurality of test lights as corresponding modes of light, and in the optical receiver, the generated mode The generated signal is measured to measure the amount of crosstalk between modes by measuring the wavelength spectrum of each mode based on the light corresponding to each mode separated from the multiplexed light. Mode includes a transmission unit for multiplexing optical transmitting to the optical receiver, wherein the optical transmission apparatus, for adjusting each mode transmission power of the light to be transmitted to the optical receiver, different modes of each A plurality of optical amplifiers for amplifying light, and based on the crosstalk amount measured for each of the plurality of modes, the plurality of crosstalk amounts are equalized between the plurality of modes. Each gain of the optical amplifier is adjusted .

An optical transmission system according to another aspect of the present invention includes an optical transmitter that transmits mode multiplexed light in which a plurality of modes of light are multiplexed, and an optical receiver that receives the mode multiplexed light from the optical transmitter. , Wherein the optical transmitter is a wavelength multiplexed signal in which light of a plurality of wavelengths excluding unused wavelengths is multiplexed, and the unused wavelengths differ for each mode. By generating a signal as signal light corresponding to each mode, and multiplexing a plurality of signal lights corresponding to the plurality of modes generated by the generation means as light of corresponding modes, Multiplexing means for generating mode multiplexed light to be transmitted to the optical receiver, wherein the optical receiver is configured to receive each of the plurality of modes from the mode multiplexed light received from the optical transmitter. The amount of crosstalk between modes is measured by measuring the wavelength spectrum of each mode based on the demultiplexing means for separating the corresponding light and the light corresponding to each mode separated by the demultiplexing means. Measuring means, and the optical transmission device further includes a plurality of optical amplifiers for amplifying light of different modes for adjusting the transmission power of the light of each mode to be transmitted to the optical reception device. The gains of the plurality of optical amplifiers are adjusted based on the crosstalk amounts measured for each of the plurality of modes by the measuring unit so that the crosstalk amounts are equal between the plurality of modes. It is characterized by that.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to measure more easily the amount of crosstalk between modes produced in the mode multiplexing light transmitted in the optical transmission system which performs mode multiplexing transmission. Also, based on the measurement result, it is possible to equalize the reception characteristics of each mode included in the transmitted mode multiplexed light with higher accuracy.

1 is a block diagram illustrating a configuration example of an optical transmission system according to Embodiments 1 and 2. FIG. The block diagram which shows the structural example of the optical signal generation part to which WDM is applied. The block diagram which shows the structural example of the optical transmission system by which the 1 or more relay amplifier is arrange | positioned in the middle of the optical transmission line. FIG. 6 is a block diagram illustrating a configuration example of an optical transmission system according to Embodiments 3 and 4. FIG. 10 is a block diagram illustrating a configuration example of an optical transmission system according to a fourth embodiment. FIG. 10 is a block diagram illustrating a configuration example of an optical transmission system according to a fifth embodiment. FIG. 10 is a block diagram illustrating a configuration example of an optical transmission system according to a fifth embodiment.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In the following drawings, components that are not necessary for the description of the embodiments are omitted from the drawings.

[Example 1]
FIG. 1 is a block diagram illustrating a configuration example of an optical transmission system to which mode multiplexing transmission is applied according to the first embodiment. As shown in FIG. 1, the optical transmission system includes an optical transmission device 10 that transmits mode multiplexed light in which light of a plurality of modes is multiplexed, and a mode transmitted from the optical transmission device 10 via an optical transmission line 30. It is comprised with the optical receiver 20 which receives multiplexed light. First, basic operations of the optical transmitter 10 and the optical receiver 20 in the optical transmission system will be described.

  The optical transmission device 10 includes N optical signal generators 11-n, N optical amplifiers 12-n, N mode converters 13-n, and mode multiplexers 14 (n = 1, 2). , ..., N). In the optical transmitter 10, the signal light generated by the N optical signal generators 11-n is amplified by the optical amplifier 12-n, and converted into light of different desired modes by the mode converter 13-n. Mode conversion is performed. The optical amplifier 12-n amplifies light of different modes. The mode multiplexer 14 generates a mode multiplexed light (mode multiplexed signal light) to be transmitted to the optical receiver 20 by multiplexing a plurality of (N) mode signal lights obtained by the mode conversion. Then, the generated mode multiplexed light is output to the optical transmission line 30. The optical amplifier 12-n adjusts the transmission power of the light in each mode (to be transmitted to the optical receiver 20) when it is output to the optical transmission line 30 as well as for amplification of the signal light. Used for. The mode multiplexed light output from the optical transmitter 10 reaches the optical receiver 20 via the optical transmission path 30.

  The optical receiver 20 includes a mode demultiplexer 21, N mode converters 22-n, and N optical signal receivers 23-n (n = 1, 2,..., N). In the optical receiver 20, the mode multiplexed light received from the optical transmitter 10 is demultiplexed into a plurality of (N) modes of light by the mode demultiplexer 21. The mode converter 22-n converts each of a plurality of (N) modes of light output from the mode splitter 21 into light corresponding to each mode before mode conversion in the optical transmission device 10 ( That is, inverse conversion is performed, and the signals are output to the corresponding optical signal receivers 23-n. As a result, the optical signal receiving unit 23-n receives the signal light corresponding to each mode multiplexed on the mode multiplexed light. The optical receiver 20 further includes a measurement unit 24-n that measures the amount of crosstalk between modes. Each measurement unit 24-n is composed of an optical spectrum analyzer in this embodiment.

  Here, the mode converter 13-n and the mode multiplexer 14, the optical transmission line 30, and the mode duplexer 21 and the mode converter 22-n of the optical receiver 20 in each mode of the optical transmitter 10 Loss is different. For this reason, the signal light corresponding to each mode received by the optical signal receiving unit 23-n has a difference in reception power between the modes. Further, signals are transmitted between modes in the mode converter 13-n and the mode multiplexer 14, the optical transmission line 30, and the mode duplexer 21 and the mode converter 22-n of the optical receiver 20 of the optical transmitter 10. Crosstalk (inter-mode crosstalk) occurs. As described above, the strength (magnitude) of the crosstalk between modes is the characteristic of the mode converter 13-n, the mode multiplexer 14, the optical transmission line 30, the mode duplexer 21, and the mode converter 22-n. As well as depending on the type and combination of modes to be multiplexed.

  In the present embodiment, in order to more easily measure the intensity of crosstalk (crosstalk amount) between modes generated in mode multiplexed light transmitted in the optical transmission system, the optical transmitter 10 and the optical receiver 20 are as follows. To work. In the measurement of the present embodiment, not only the characteristics of crosstalk between modes itself but also characteristics including loss and gain deviation between modes in the entire optical transmission system are measured as crosstalk characteristics.

  Specifically, in the optical transmitter 10, first, the optical signal generator 11-n generates a plurality of test lights (for the number of modes to be multiplexed) for measuring crosstalk between modes in the optical receiver 20. . A plurality of (N) test lights corresponding to different modes generated by the optical signal generator 11-n have different wavelengths in order to enable measurement of inter-mode crosstalk in the optical receiver 20. have. Further, the mode converter 13-n and the mode multiplexer 14 multiplex the generated plurality of test lights as corresponding mode lights, thereby generating mode multiplexed light and transmitting it to the optical receiver 20. To do.

  On the other hand, in the optical receiver 20, the mode demultiplexer 21 and the mode converter 22-n separate light corresponding to each mode from the mode multiplexed light received from the optical transmitter 10 (that is, demultiplexing is performed). Do). Furthermore, the measurement unit 24-n measures the inter-mode crosstalk amount by measuring the wavelength spectrum of each mode based on the light of each mode separated from the mode multiplexed light.

  FIG. 1 shows the wavelength (WL) spectrum of test light transmitted and received in each mode on the transmission side and the reception side. As shown in the figure, the optical transmitter 10 of the present embodiment transmits test light having a different wavelength for each mode to the optical receiver 20. Thereby, in the wavelength spectrum of each mode observed (measured) by the optical receiving device 20, the crosstalk (XT) component from other modes other than the mode has a wavelength other than the test light wavelength corresponding to the mode. It appears as a component of the wavelength. Therefore, by measuring the wavelength spectrum of each mode by the measurement unit 24-n of the optical receiving device 20, it is possible to specify which mode component and how much leaks to other modes as crosstalk. it can.

  The measurement unit 24-n (n = 1, 2,..., N) can calculate an index indicating the amount of crosstalk between modes from the measured wavelength spectrum. For example, the measurement unit 24-n calculates the difference between the power of the wavelength component of the corresponding test light and the power of the crosstalk component of the wavelength other than the wavelength of the test light in the measured wavelength spectrum. It may be measured as a quantity. More specifically, the measurement unit 24-n calculates the sum of the peak value of the corresponding wavelength component of the test light and the peak value of the crosstalk component of the wavelength other than the wavelength of the test light in the measured wavelength spectrum. May be measured as the amount of crosstalk between modes.

  In the present embodiment, it is assumed that the above-described measurement is performed before actual communication is started in the optical transmission system. For this reason, both signal light including a signal to be transmitted to the optical receiver 20 and CW light (continuous light) not including a signal to be transmitted to the optical receiver 20 can be used as test light. Further, the signal light used as the test light may be a wavelength division multiplexed (wavelength multiplexed) signal (WDM signal) in which light of a plurality of wavelengths is multiplexed. In this case, the optical signal generator 11-n (n = 1, 2,..., N) includes a plurality of optical signal generators that generate signal lights having different wavelengths, as shown in FIG. And a multiplexer that multiplexes the signal lights having a plurality of wavelengths to generate a WDM signal. It is also possible to measure the wavelength dependence of the inter-mode loss / gain deviation and inter-mode crosstalk as described above by sweeping the test light in the wavelength band used for the WDM signal.

  As described above, according to this embodiment, mode multiplexed light in which a plurality of test lights having different wavelengths are multiplexed is transmitted from the transmission side to the reception side, and the wavelength spectrum of each mode is measured on the reception side. Thus, the crosstalk amount between modes for each mode can be measured. That is, it is possible to measure the inter-mode crosstalk amounts for a plurality of modes at once, instead of measuring the inter-mode crosstalk amounts individually for each mode. Therefore, it is possible to more easily measure the amount of crosstalk between modes generated in mode multiplexed light transmitted in the optical transmission system.

  In the present embodiment, as shown in FIG. 3, one or more relay amplifiers 31 are disposed in the middle of the optical transmission line 30 between the optical transmitter 10 and the optical receiver 20. , As well as applicable. Further, the present embodiment can be similarly applied even when the test light (signal light) transmitted through the optical transmission line 30 as shown in FIG. 3 is a WDM signal.

[Example 2]
In the second embodiment, the gain of the optical amplifier 12-n (n = 1, 2,..., N) is adjusted in the optical transmitter 10 based on the measurement result of the inter-mode crosstalk amount in the first embodiment. The transmission power in each mode is changed, and the reception characteristics (OSNR) in each mode in the optical receiver 20 are equalized. Hereinafter, for simplification of description, description of portions common to the first embodiment is omitted.

  In this embodiment, as in the first embodiment, it is assumed that the amount of crosstalk between modes is measured in advance and the gain of the optical amplifier 12-n is adjusted in advance before actual communication is started in the optical transmission system. doing. Specifically, based on the inter-mode crosstalk amount measured for each of the plurality of modes by the measurement unit 24-n, the crosstalk amount is made equal between the plurality of modes (that is, in the optical receiver 20). Each gain of the optical amplifier 12-n is adjusted (n = 1, 2,..., N) so that the reception characteristics (OSNR) are equalized between the modes).

  According to the present embodiment, it is possible to equalize the reception characteristics of each mode included in the transmitted mode multiplexed light more accurately based on the measurement result of the inter-mode crosstalk amount. In the present embodiment, as shown in FIG. 3, one or more relay amplifiers 31 are disposed in the middle of the optical transmission line 30 between the optical transmitter 10 and the optical receiver 20. , As well as applicable. Further, in the present embodiment, even when a WDM signal is used as test light (signal light) and when such a WDM signal is transmitted through an optical transmission line 30 as shown in FIG. The same applies.

  In this embodiment, the gain of the optical amplifier 12-n is adjusted, but a combination of a fixed gain optical amplifier and a variable attenuator is used instead of the variable gain optical amplifier 12-n. Is also possible. In that case, the variable attenuator corresponding to each mode is set so that the crosstalk amount is equal between the plurality of modes based on the crosstalk amount between the modes measured for each of the plurality of modes by the measurement unit 24-n. What is necessary is just to adjust attenuation amount.

[Example 3]
In the third embodiment, as a modification of the second embodiment, during the actual communication (signal transmission) in the optical transmission system, based on the measurement of the crosstalk amount between modes and the measurement result, An example is shown in which the gain of the optical amplifier 12-n (n = 1, 2,..., N) is adjusted. In the following, for the sake of simplification of description, description of portions common to the first and second embodiments is omitted.

  As described above, in an optical transmission system in which communication is actually performed (operated), the crosstalk characteristic varies with time due to changes in the external environment in the mode multiplexer / demultiplexer, transmission path, and relay amplifier. (OSNR) can also vary over time. For this reason, it is necessary to measure crosstalk characteristics that vary with time, and to equalize reception characteristics between modes in response to such time fluctuations. Therefore, in this embodiment, in order to deal with such time fluctuations of the crosstalk characteristic and the reception characteristic (OSNR), the amount of crosstalk in the optical receiver 20 is monitored, and the monitoring result is fed back to the optical transmitter 10. To do. As a result, the gain of the optical amplifier 12-n can be dynamically adjusted in the optical transmission apparatus 10.

  FIG. 4 is a block diagram illustrating a configuration example of an optical transmission system to which mode multiplexing transmission is applied according to the third embodiment. The optical transmission system shown in FIG. 4 is different from the first and second embodiments (FIG. 1) in that the optical transmission device 10 and the optical reception device 20 include control units 15 and 25, respectively. In this embodiment, since communication is actually performed (signal light is transmitted) in the optical transmission system, test light having a wavelength that is not used as the wavelength of signal light for communication is set to the mode. Used to measure the amount of crosstalk.

  In the optical receiving device 20, the control unit 25 feeds back the measurement result of the crosstalk amount between modes for each mode by the measuring unit 24-n to the optical transmitting device 10. As described above, the measurement unit 24-n (n = 1, 2,..., N), for each mode, the peak value of the wavelength component of the corresponding test light in the wavelength spectrum, and the test light You may acquire the difference with the sum total of the peak value of the crosstalk component of wavelengths other than a wavelength as a measurement result of a crosstalk amount. In this case, the larger the measured difference, the smaller the crosstalk amount, and the smaller the measured difference, the larger the crosstalk amount. Such feedback can be performed using an arbitrary line, and may be performed via an optical transmission line, or may be performed via a dedicated line for transmitting a control signal.

  In the optical transmission device 10, the control unit 15 makes the crosstalk amount equal among a plurality of modes based on the crosstalk amount between modes for each mode fed back from the optical reception device 20 (that is, optical The gain of the optical amplifier 12-n (n = 1, 2,..., N) is adjusted so that the reception characteristic (OSNR) in the receiver 20 is equalized between the modes. For each mode, the control unit 15 decreases the gain of the optical amplifier 12-n when the measured amount of crosstalk is small, and increases the gain when the amount of crosstalk is large. The gain is adjusted so that the crosstalk amount becomes equal.

  As described above, according to the present embodiment, in an optical transmission system that is actually operated, the amount of crosstalk between modes that fluctuates over time is measured. It is possible to equalize the reception characteristics.

  In the present embodiment, as shown in FIG. 3, one or more relay amplifiers 31 are disposed in the middle of the optical transmission line 30 between the optical transmitter 10 and the optical receiver 20. , As well as applicable. Further, in this embodiment, even when a WDM signal is used as signal light, and such signal light (WDM signal) is transmitted through the optical transmission line 30 as shown in FIG. The same applies. In this case, test light having a wavelength that is not used as the wavelength of the WDM signal used as signal light may be generated and used for measurement.

[Example 4]
In the third embodiment, test light having a wavelength that is not used as the wavelength of signal light for communication is prepared for the number of modes (that is, test light for the number of modes is prepared separately from the signal light). ing). However, there may be a case where the wavelength for the test light cannot be secured due to the limitation of the usable wavelength band. Therefore, in the fourth embodiment, when a WDM signal is used as signal light, the amount of crosstalk between modes is measured without using a wavelength other than the wavelength band of the WDM signal. In the following, for the sake of simplification of description, description of portions common to the first and second embodiments is omitted.

  FIG. 5 is a block diagram illustrating a configuration example of an optical transmission system according to the fourth embodiment. The configuration of the optical transmission system shown in FIG. 5 is the same as that of the first embodiment (FIG. 1). However, in FIG. 5, the wavelength spectra shown on the transmission side and the reception side are different from those in FIG. As shown in FIG. 5, in this embodiment, the optical signal generator 11-n (n = 1, 2,..., N) is a WDM in which light of a plurality of wavelengths excluding unused wavelengths is multiplexed. A signal is generated as signal light corresponding to each mode. At that time, the optical signal generator 11-n generates WDM signals having different unused wavelengths for each mode. The generated WDM signal (signal light) corresponding to each mode is multiplexed as light of the corresponding mode, and transmitted to the optical receiver 20 as mode multiplexed light.

  In the optical receiver 20, as in the first to third embodiments, the measurement unit 24-n (n = 1, 2,..., N) is based on the light of each mode separated from the mode multiplexed light. The crosstalk amount between modes is measured by measuring the wavelength spectrum of each mode. Here, in the wavelength spectrum of each mode, as shown in FIG. 5, crosstalk (XT) components from other modes other than the mode are added as components of unused wavelengths corresponding to the mode. Will appear.

  For this reason, the measurement unit 24-n calculates the power of the wavelength component other than the unused wavelength corresponding to each mode in the measured wavelength spectrum and the power of the crosstalk component expressed as the unused wavelength component. The difference can be measured as the amount of crosstalk between modes. More specifically, the measurement unit 24-n includes a peak value of a component of a wavelength other than an unused wavelength corresponding to each mode in the measured wavelength spectrum, and a crosstalk component that appears as a component of the unused wavelength. The difference from the peak value can be measured as the amount of crosstalk between modes.

  As described above, according to the present embodiment, when a WDM signal is used as signal light, the amount of crosstalk between modes can be reduced without using a wavelength other than the wavelength band of the signal light (WDM signal). Measurements can be made.

  Note that the adjustment of the gain of the optical amplifier 12-n based on the measurement result of the inter-mode crosstalk amount in the second and third embodiments can be applied to the present embodiment. Thereby, similarly to the second and third embodiments, it is possible to equalize the reception characteristics (OSNR) between the modes. In the present embodiment, as shown in FIG. 3, one or more relay amplifiers 31 are arranged in the middle of the optical transmission line 30 between the optical transmitter 10 and the optical receiver 20. , As well as applicable.

[Example 5]
In the fifth embodiment, as a modification of the first to fourth embodiments, the amount of crosstalk between modes is increased by using light generated by the optical signal generator 11-n (n = 1, 2,..., N). An example in which such a measurement is performed using a microwave instead of the measurement will be described.

  Here, in Non-Patent Document 5, in the transmitter, a microwave (low frequency component) having a different frequency for each mode is superimposed on the signal light, and the frequency spectrum of each mode using a spectrum analyzer in the electrical stage of the receiver. Is observed. Furthermore, the amount of crosstalk between modes is derived from the observation results. In this example, measurement using microwaves is performed as in Non-Patent Document 5, but it is different from the method of Non-Patent Document 5 in that the amount of crosstalk between modes can be directly measured.

  FIG. 6 is a block diagram illustrating a configuration example of an optical transmission system according to the fifth embodiment. In the optical transmission system shown in FIG. 6, the optical transmitter 10 includes a microwave generator 16-n (n = 1, 2,..., N), and each optical signal generator 11-n This embodiment is different from the first to fourth embodiments in that a microwave modulator is provided and a microwave detector is provided in each optical signal receiving unit 23-n. In this embodiment, since microwaves are used for measuring the amount of crosstalk between modes, measurement can be performed regardless of whether communication is actually performed in the optical transmission system.

  Specifically, in the optical transmitter 10, the microwave generator 16-n (n = 1, 2,..., N) generates microwaves having different frequencies for each corresponding mode, and the corresponding light. The signal is input to the signal generator 11-n. In the present embodiment, the microwave generator 16-n not only has a different frequency for each corresponding mode, but also has a corresponding 2 for each combination of two modes that can be taken from a plurality of (N) modes. A microwave corresponding to each mode is generated so that the frequency difference between the two microwaves is different. The optical signal generator 11-n generates signal light corresponding to each mode and modulates the generated signal light with the input microwave, thereby superimposing the microwave on the generated signal light. Thereby, microwaves with different frequency intervals for each combination of two possible modes among a plurality of (N) modes are superimposed on the signal light of each mode. The mode converter 13-n and the mode multiplexer 14 respectively multiplex the plurality of signal lights corresponding to the plurality of modes on which the microwaves are superimposed as the light of the corresponding mode, thereby converting the mode multiplexed light. It is generated and transmitted to the optical receiver 20.

  On the other hand, in the optical receiver 20, the mode demultiplexer 21 and the mode converter 22-n separate light corresponding to each mode from the mode multiplexed light received from the optical transmitter 10 (that is, demultiplexing is performed). Do). Further, the optical signal receiving unit 23-n takes out the microwave superimposed on the light corresponding to each mode by the detector and outputs the microwave to the measuring unit 24-n. The measurement unit 24-n measures the frequency spectrum of the input microwave, and based on the frequency (beat) component corresponding to the frequency difference of the microwave corresponding to the specific mode and the other mode in the frequency spectrum. To measure the amount of crosstalk between modes.

  Here, the mode multiplexed light transmitted from the optical transmission device 10 not only has a different frequency for each corresponding mode, but also has different frequency intervals for each combination of two possible modes among the N modes. Microwaves are superimposed on the light corresponding to each mode. For this reason, in the microwave frequency spectrum measured by the measurement unit 24-n, a component corresponding to crosstalk between the corresponding mode and another mode includes two microwaves corresponding to the two modes. Will appear at a frequency equal to the frequency difference.

  Therefore, the measurement unit 24-n corresponds to the amount of crosstalk between two modes that can be taken among a plurality of modes, in the measured frequency spectrum, as a frequency difference between two microwaves corresponding to the two modes. Measure based on frequency components. For example, the measurement unit 24-n directly uses a peak value of a frequency component corresponding to a frequency difference between two microwaves corresponding to two modes in the measured frequency spectrum as a crosstalk amount between the two modes. Can be measured automatically.

  As described above, according to the present embodiment, the amount of crosstalk between modes can be directly measured by measurement using microwaves without using the light generated by the optical signal generator 11-n. Is possible. Note that the adjustment of the gain of the optical amplifier 12-n based on the measurement result of the inter-mode crosstalk amount in the second and third embodiments can be applied to the present embodiment. For example, when the measurement result of the inter-mode crosstalk amount is fed back from the optical receiver 20 to the optical transmitter 10 as in the third embodiment, an optical transmission system may be configured as shown in FIG. As a result, similarly to the second and third embodiments, it is possible to equalize reception characteristics (OSNR) between modes. In the present embodiment, as shown in FIG. 3, one or more relay amplifiers 31 are arranged in the middle of the optical transmission line 30 between the optical transmitter 10 and the optical receiver 20. , As well as applicable. Furthermore, the present embodiment can be similarly applied when a WDM signal is used as signal light.

Claims (20)

An optical transmission system comprising: an optical transmitter that transmits mode multiplexed light in which a plurality of modes of light are multiplexed; and an optical receiver that receives the mode multiplexed light from the optical transmitter,
The optical transmitter is
A plurality of test lights for measuring the amount of crosstalk between modes in the optical receiver, and generating means for generating the plurality of test lights corresponding to different modes and having different wavelengths;
Multiplexing means for generating mode multiplexed light to be transmitted to the optical receiver by multiplexing the plurality of test lights generated by the generating means as corresponding mode lights, respectively,
The optical receiver is
Demultiplexing means for demultiplexing light corresponding to each of the plurality of modes from the mode multiplexed light received from the optical transmitter;
Measurement means for measuring the amount of crosstalk between modes by measuring the wavelength spectrum of each mode based on the light corresponding to each mode separated by the demultiplexing means ,
The optical transmission device further includes a plurality of optical amplifiers for amplifying light of different modes for adjusting the transmission power of the light of each mode to be transmitted to the optical reception device,
Based on the crosstalk amount measured for each of the plurality of modes by the measuring unit, the gains of the plurality of optical amplifiers are adjusted so that the crosstalk amounts are equal between the plurality of modes. that the optical transmission system, characterized in that. The measurement means, for each of the plurality of modes, in the measured wavelength spectrum, the power of the component of the wavelength of the corresponding test light and the component from the other mode that appears as a component of a wavelength other than the wavelength of the test light The difference from the power of the crosstalk component is measured as the crosstalk amount.
The optical transmission system according to claim 1. For each of the plurality of modes, the measuring means calculates the difference between the peak value of the wavelength component of the corresponding test light and the sum of the peak values of the crosstalk component in the measured wavelength spectrum. Measure as quantity,
The optical transmission system according to claim 2. The test light is signal light including a signal to be transmitted to the optical receiver.
The optical transmission system according to any one of claims 1 to 3, characterized in that. The test light is continuous light that does not include a signal to be transmitted to the optical receiver.
The optical transmission system according to any one of claims 1 to 3, characterized in that. The measurement means further feeds back the crosstalk amount measured for each of the plurality of modes to the optical transmission device,
The optical transmission device is configured to make the crosstalk amounts equal among the plurality of modes based on the crosstalk amounts for the plurality of modes fed back by the measuring unit. Adjusting means for adjusting the respective gains of the amplifiers;
The optical transmission system according to claim 1 , wherein the optical transmission system is an optical transmission system. The generating means generates the plurality of test lights so as to have a wavelength that is not used as a wavelength of signal light including a signal to be transmitted to the optical receiver.
The optical transmission system according to claim 6 . The signal light is a wavelength multiplexed signal obtained by multiplexing light of a plurality of wavelengths.
The optical transmission system according to claim 4 or 7 , The multiplexing means includes
Conversion means for performing mode conversion for converting each of the plurality of test lights generated by the generation means into light of a corresponding mode;
Combining the plurality of test lights after the mode conversion to generate the mode multiplexed light, and
The demultiplexing means includes
Demultiplexing means for demultiplexing the mode multiplexed light received from the optical transmission device into the light of the plurality of modes;
Each of the output said plurality of modes light from said dividing means, before the mode conversion, and inverse conversion means for converting the light corresponding to each mode, from claim 1, characterized in that it comprises a 8 The optical transmission system according to any one of the above. An optical transmission system comprising: an optical transmitter that transmits mode multiplexed light in which a plurality of modes of light are multiplexed; and an optical receiver that receives the mode multiplexed light from the optical transmitter,
The optical transmitter is
A wavelength multiplexing signal in which light of a plurality of wavelengths excluding an unused wavelength is multiplexed, and the wavelength multiplexing signal having a different unused wavelength for each mode is generated as signal light corresponding to each mode; ,
Multiplexing means for generating mode multiplexed light to be transmitted to the optical receiving device by multiplexing the plurality of signal lights corresponding to the plurality of modes generated by the generating means as light of the corresponding modes; With
The optical receiver is
Demultiplexing means for demultiplexing light corresponding to each of the plurality of modes from the mode multiplexed light received from the optical transmitter;
Measurement means for measuring the amount of crosstalk between modes by measuring the wavelength spectrum of each mode based on the light corresponding to each mode separated by the demultiplexing means ,
For each of the plurality of modes, the measurement means includes a power of a component of a wavelength other than the unused wavelength in the measured wavelength spectrum and a cross from another mode that appears as a component of the unused wavelength. An optical transmission system characterized in that a difference from the power of a talk component is measured as the crosstalk amount . For each of the plurality of modes, the measurement means calculates a difference between a peak value of a component of a wavelength other than the unused wavelength and a peak value of the crosstalk component in the measured wavelength spectrum. Measure as quantity,
The optical transmission system according to claim 10 . The optical transmission device further includes a plurality of optical amplifiers for amplifying light of different modes for adjusting the transmission power of the light of each mode to be transmitted to the optical reception device,
Based on the crosstalk amount measured for each of the plurality of modes by the measuring unit, the gains of the plurality of optical amplifiers are adjusted so that the crosstalk amounts are equal between the plurality of modes. The
The optical transmission system according to claim 10 , wherein the optical transmission system is an optical transmission system.   An optical transmission system comprising: an optical transmitter that transmits mode multiplexed light in which a plurality of modes of light are multiplexed; and an optical receiver that receives the mode multiplexed light from the optical transmitter,
  The optical transmitter is
    A wavelength multiplexing signal in which light of a plurality of wavelengths excluding an unused wavelength is multiplexed, and the wavelength multiplexing signal having a different unused wavelength for each mode is generated as signal light corresponding to each mode; ,
    Multiplexing means for generating mode multiplexed light to be transmitted to the optical receiving device by multiplexing the plurality of signal lights corresponding to the plurality of modes generated by the generating means as light of the corresponding modes; With
  The optical receiver is
    Demultiplexing means for demultiplexing light corresponding to each of the plurality of modes from the mode multiplexed light received from the optical transmitter;
    Measurement means for measuring the amount of crosstalk between modes by measuring the wavelength spectrum of each mode based on the light corresponding to each mode separated by the demultiplexing means,
  The optical transmission device further includes a plurality of optical amplifiers for amplifying light of different modes for adjusting the transmission power of the light of each mode to be transmitted to the optical reception device,
  Based on the crosstalk amount measured for each of the plurality of modes by the measuring unit, the gains of the plurality of optical amplifiers are adjusted so that the crosstalk amounts are equal between the plurality of modes. An optical transmission system characterized by that. The measurement means further feeds back the crosstalk amount measured for each of the plurality of modes to the optical transmission device,
The optical transmission device is configured to make the crosstalk amounts equal among the plurality of modes based on the crosstalk amounts for the plurality of modes fed back by the measuring unit. Adjusting means for adjusting the respective gains of the amplifiers;
The optical transmission system according to claim 12 or 13 , An optical transmission system comprising: an optical transmitter that transmits mode multiplexed light in which a plurality of modes of light are multiplexed; and an optical receiver that receives the mode multiplexed light from the optical transmitter,
The optical transmitter is
Generating means for generating a plurality of signal lights corresponding to the plurality of modes, and superimposing a microwave having a different frequency for each mode on the signal light corresponding to each generated mode; Generating the microwave to be superimposed on the signal light corresponding to each mode so that the frequency difference between the two corresponding microwaves is different for each combination of the two modes that can be taken in
Multiplexing means for generating mode multiplexed light to be transmitted to the optical receiving device by multiplexing the plurality of signal lights corresponding to the plurality of modes generated by the generating means as light of the corresponding modes; With
The optical receiver is
Demultiplexing means for demultiplexing light corresponding to each of the plurality of modes from the mode multiplexed light received from the optical transmitter;
The frequency spectrum of the microwave superimposed on the light corresponding to each mode separated by the demultiplexing means is measured, and the amount of crosstalk between two modes that can be taken out of the plurality of modes is determined as the frequency. Measuring means for measuring based on a frequency component corresponding to a frequency difference between two microwaves corresponding to the two modes in the spectrum;
An optical transmission system comprising: The measurement means measures a peak value of a frequency component corresponding to a frequency difference between two microwaves corresponding to the two modes in the frequency spectrum as the crosstalk amount between the two modes.
The optical transmission system according to claim 15 . The measurement means further feeds back the crosstalk amount measured for each of the plurality of modes to the optical transmission device,
The optical transmitter is
A plurality of optical amplifiers for amplifying light of different modes for adjusting the transmission power of light of each mode to be transmitted to the optical receiver;
Based on the crosstalk amount for each of the plurality of modes fed back by the measuring means, the gains of the plurality of optical amplifiers are set so that the crosstalk amounts are equal between the plurality of modes. Adjusting means for adjusting,
The optical transmission system according to claim 15 or 16 , A relay amplifier that performs relay amplification of the mode multiplexed light transmitted through the transmission path in the middle of the transmission path between the optical transmission device and the optical reception device;
The optical transmission system according to any one of claims 1 to 17, characterized in that. An optical transmitter that transmits mode multiplexed light in which light of a plurality of modes is multiplexed to an optical receiver via an optical transmission line,
A plurality of test lights for measuring the amount of crosstalk between modes in the optical receiver, and generating means for generating the plurality of test lights corresponding to different modes and having different wavelengths;
Multiplexing means for generating mode multiplexed light to be transmitted to the optical receiver by multiplexing the plurality of test lights generated by the generating means as light of corresponding modes,
The optical receiver is configured to measure the crosstalk amount between modes by measuring the wavelength spectrum of each mode based on the light corresponding to each mode separated from the generated mode multiplexed light. Transmitting means for transmitting the mode multiplexed light to the optical receiver ,
The optical transmission device further includes a plurality of optical amplifiers for amplifying light of different modes for adjusting the transmission power of the light of each mode to be transmitted to the optical reception device,
Based on the crosstalk amount measured for each of the plurality of modes, the gain of each of the plurality of optical amplifiers is adjusted so that the crosstalk amount is equal between the plurality of modes. An optical transmission device. A method of controlling an optical transmission device that transmits mode multiplexed light in which light of a plurality of modes is multiplexed to an optical reception device via an optical transmission line,
A plurality of test lights for measuring the amount of crosstalk between modes in the optical receiver, and generating the plurality of test lights corresponding to different modes and having different wavelengths;
A multiplexing step for generating mode multiplexed light to be transmitted to the optical receiver by multiplexing the generated plurality of test lights as corresponding mode lights,
The optical receiver is configured to measure the crosstalk amount between modes by measuring the wavelength spectrum of each mode based on the light corresponding to each mode separated from the generated mode multiplexed light. Transmitting the mode multiplexed light to the optical receiver , and
The optical transmission device further includes a plurality of optical amplifiers for amplifying light of different modes for adjusting the transmission power of the light of each mode to be transmitted to the optical reception device,
Based on the crosstalk amount measured for each of the plurality of modes, the gain of each of the plurality of optical amplifiers is adjusted so that the crosstalk amount is equal between the plurality of modes. A method for controlling an optical transmission device.

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