JP4977409B2 - Multimode optical transmission apparatus and multimode optical transmission system - Google Patents

Description

  The present invention relates to a multimode optical transmission apparatus and a multimode optical transmission system that transmit an optical signal via a multimode optical transmission line, and more specifically, to reduce deterioration of noise characteristics and distortion characteristics due to intermode interference. The present invention relates to a multimode optical transmission apparatus and a multimode optical transmission system.

  For example, Non-Patent Document 1 discloses an optical transmission system using a conventional multimode optical fiber. FIG. 9 is a block diagram showing an example of the configuration of a conventional optical transmission system disclosed in Non-Patent Document 1. In FIG. 9, the conventional optical transmission system has a configuration in which an electro-optical conversion unit 501 and a photoelectric conversion unit 502 are connected by a multimode optical transmission line 503.

  An electric signal is input to the electro-optical conversion unit 501. The electro-optical conversion unit 501 converts the input electric signal into an optical signal and sends the optical signal to the multimode optical transmission line 503. A multimode optical fiber is used for the multimode optical transmission line 503. The optical signal propagated through the multimode optical transmission line 503 is input to the photoelectric conversion unit 502. The photoelectric conversion unit 502 converts the input optical signal into an electrical signal and outputs it. Here, a semiconductor laser such as a Fabry-Perot laser is usually used for the electro-optical conversion unit 501. For the photoelectric conversion unit 502, a PIN photodiode, an avalanche photodiode, or the like is used.

Such an optical transmission system using a multi-mode optical fiber has a large core diameter and is easy to couple with a light source. Compared with a system using a single-mode optical fiber. An inexpensive system can be constructed. Taking advantage of these characteristics, many optical transmission systems using multimode optical fibers are currently introduced into relatively short-distance systems such as LANs in enterprises.
DR Hjelme by AR Mickelson, “Microbending and modal noise” (USA), Vol. 22, Applied Optics, 1983 1 December, p. 3874-3879

  However, in an optical transmission system using a multimode optical fiber, transmission characteristics are limited because a plurality of propagation modes exist in the optical transmission line. That is, in an optical transmission system using a multimode optical fiber, different propagation modes interfere with each other to generate noise called modal noise. Here, the modal noise is noise caused by temporal fluctuation of a speckle pattern generated by inter-mode interference. In addition, if the optical fiber is bent or there is a gap between the connectors, the transmission characteristics are greatly deteriorated.

  As described above, when optical transmission is performed using a multimode optical fiber, transmission characteristics deteriorate due to inter-mode interference. In particular, when the number of modes in the optical transmission path is small, or when the line width of the light source is narrow, the coherence between the modes becomes high, so that the transmission characteristics are greatly degraded. As an example of this, there may be mentioned a case where a surface emitting laser is used as a light source and a 1.3 μm band single mode optical fiber is used as an optical transmission line. The wavelength of the surface emitting laser is generally in the short wavelength band of 850 nm, and when the optical transmission is performed using the single mode optical fiber in the optical transmission line at that wavelength, the number of modes is reduced to about 2-3.

  In consideration of coupling between a light source and a single mode optical fiber, it is assumed that a single mode surface emitting laser having a far field pattern having a Gaussian distribution is used as the light source. Under these conditions, the number of modes in the optical transmission path is small and the line width of the light source is narrow, and it is considered that the noise characteristics and distortion characteristics of the multimode optical transmission path are significantly degraded due to inter-mode interference.

  Therefore, an object of the present invention is to provide a multimode optical transmission apparatus and a multimode optical device capable of reducing deterioration of noise characteristics and distortion characteristics due to intermode interference even under the condition where the influence of intermode interference becomes significant. It is to provide a transmission system.

  The present invention is directed to a multimode optical transmission apparatus that converts an input electrical signal into an optical signal and transmits the converted optical signal via a multimode optical transmission line. In order to achieve the above object, the multimode optical transmission apparatus of the present invention controls a signal output unit that outputs a predetermined signal and a signal output unit based on the input electrical signal, A control unit that adjusts a frequency component of the signal, and an electro-optical conversion unit that converts a predetermined signal into an optical signal and transmits the optical signal to a multimode optical transmission line. The control unit adjusts the frequency component of the predetermined signal based on the group delay information of each mode propagating through the multimode optical transmission line.

  Preferably, the signal output unit includes a modulation unit that modulates the input electric signal and outputs the modulated electric signal as a high frequency modulation signal. The control unit adjusts the frequency of the high frequency modulation signal output from the modulation unit based on the group delay information of each mode propagating through the multimode optical transmission line.

  The signal output unit modulates the input electric signal and outputs it as a high frequency modulation signal, and generates a signal having a predetermined frequency, and generates a superimposed signal that outputs the generated signal as a superimposed signal. And a multiplexing unit that frequency-multiplexes the high-frequency modulation signal output from the modulation unit and the superimposed signal and outputs the resultant signal as a predetermined signal. The control unit adjusts the frequency of the superimposed signal output from the superimposed signal generation unit based on the group delay information of each mode propagating through the multimode optical transmission line.

  Preferably, the input electrical signal is a baseband signal. In this case, the signal output unit generates a signal having a predetermined frequency, outputs the generated signal as a superimposed signal, frequency-multiplexes the baseband signal and the superimposed signal, and outputs a predetermined signal. And a multiplexing unit that outputs as The control unit adjusts the frequency of the superimposed signal output from the superimposed signal generation unit based on the group delay information of each mode propagating through the multimode optical transmission line.

  Preferably, the control unit adjusts the frequency of the superimposed signal output from the superimposed signal generation unit so that the harmonic component of the superimposed signal does not overlap the band of the high-frequency modulation signal output from the modulation unit.

  Preferably, the multimode optical transmission apparatus further includes a mode group delay information estimation unit that estimates group delay information of each mode propagating through the multimode optical transmission line. The mode group delay information estimation unit includes a short pulse signal generation unit that generates a short pulse signal, a photoelectric conversion unit that converts an optical signal received via a multimode optical transmission path, and a short pulse signal generation unit. A comparison operation unit that compares the short pulse signal generated by the signal and the electric signal converted by the photoelectric conversion unit and calculates group delay information of each mode propagating through the multimode optical transmission line. The electro-optical conversion unit converts the short pulse signal generated by the short pulse signal generation unit into an optical signal, and transmits the optical signal through the multimode optical transmission line. The optical signal received via the multimode optical transmission line includes a short pulse signal generated by the short pulse signal generation unit.

  The optical signal received via the multimode optical transmission line may include a predetermined signal output from the signal output unit. In this case, preferably, the multimode optical transmission apparatus further includes a mode group delay information estimation unit that estimates group delay information of each mode propagating through the multimode optical transmission line. The mode group delay information estimation unit converts the optical signal received via the multimode optical transmission path into an electrical signal, and outputs the electrical signal as a predetermined signal; the predetermined signal output by the signal output unit; Comparing with a predetermined signal output from the photoelectric conversion unit, the amount of distortion or noise generated in the multimode optical transmission line is calculated, and based on the calculated amount of distortion or noise, the multimode optical transmission line is calculated. And a comparison operation unit that estimates group delay information of each mode that propagates.

  The control unit may calculate group delay information of each mode propagating through the multimode optical transmission line based on the length of the multimode optical transmission line.

  Preferably, the control unit adjusts the frequency component included in the predetermined signal based on the group delay difference Δτ between the dominant modes propagating through the multimode optical transmission line.

  Let Δτ be the group delay difference between the first mode and the second mode propagating through the multimode optical transmission line, and n be an arbitrary natural number. Preferably, the modulation unit sets the frequency of the high frequency modulation signal to a frequency equal to n × 1 / Δτ.

  Let Δτ be the group delay difference between the first mode and the second mode propagating through the multimode optical transmission line, and n be an arbitrary natural number. Preferably, the modulation unit sets the frequency of the high-frequency modulation signal in a range of frequencies larger than (n−1 / 4) × 1 / Δτ and smaller than (n + 1/4) × 1 / Δτ.

  Let Δτ be the group delay difference between the first mode and the second mode propagating through the multimode optical transmission line, and n be an arbitrary natural number. Preferably, the superimposed signal generator sets the frequency of the superimposed signal to a frequency equal to (n−½) × 1 / Δτ.

  Let Δτ be the group delay difference between the first mode and the second mode propagating through the multimode optical transmission line, and n be an arbitrary natural number. Preferably, the superimposed signal generator sets the frequency of the superimposed signal to a frequency different from n × 1 / Δτ.

  In addition, the present invention provides a multimode optical transmission system including an optical transmission device and an optical reception device that convert an input electrical signal into an optical signal and transmit / receive the converted optical signal via a multimode optical transmission line. Is also directed. And in order to achieve the said objective, the multimode optical transmission system of this invention is provided with the optical transmitter of the following structures, and an optical receiver. The optical transmission device controls a signal output unit that outputs a predetermined signal based on the input electrical signal, a control unit that controls the signal output unit to adjust a frequency component of the predetermined signal, and a predetermined signal. An electro-optical conversion unit that converts the light into an optical signal and transmits the signal to a multimode optical transmission line. The control unit adjusts the frequency component of the predetermined signal based on the group delay information of each mode propagating through the multimode optical transmission line. The optical receiver includes a photoelectric conversion unit that converts an optical signal received via the multimode optical transmission path into an electrical signal.

  In addition, each process performed by the signal output unit, the control unit, and the electro-optical conversion unit included in the above-described multi-mode optical transmission apparatus converts the input electric signal into an optical signal, and converts the converted optical signal into the multi-mode. It can also be understood as a multimode optical transmission method for transmitting via an optical transmission line.

  As described above, according to the present invention, inter-mode interference is achieved by appropriately setting the frequency of the high-frequency modulation signal that modulates the optical signal based on the group delay information of each mode that propagates through the multimode optical transmission line. It is possible to reduce the deterioration of noise characteristics and distortion characteristics. As a result, even when a multimode optical fiber is used as an optical transmission line, it is possible to prevent a decrease in optical transmission quality and to construct a high-quality optical transmission system at a low cost.

  In addition, by appropriately setting the frequency of the superimposed signal that modulates the optical signal based on the group delay information of each mode that propagates through the multimode optical transmission line, the noise characteristics and distortion characteristics due to inter-mode interference are reduced. Can be reduced. Further, since the line width of the light source can be widened by frequency multiplexing the superposed signal with the high frequency modulation signal, it is possible to further reduce the deterioration of noise characteristics and distortion characteristics due to inter-mode interference.

  In addition, by providing the mode group delay information estimation unit, it is possible to estimate the group delay information of each mode propagating through the multimode optical transmission line. As a result, even when the network configuration changes, it is possible to reduce deterioration of noise characteristics and distortion characteristics due to inter-mode interference. Further, the mode group delay information estimation unit can accurately estimate the group delay information of each mode propagating through the multimode optical transmission line by using the short pulse signal.

  These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing an example of the configuration of a multimode optical transmission system 1 according to the first embodiment of the present invention. 1, the multimode optical transmission system 1 has a configuration in which an optical transmission device 11 and an optical reception device 21 are connected by a multimode optical transmission line 310. The optical transmission device 11 includes an electro-optical conversion unit 110, a modulation unit 120, and a control unit 130. The optical receiver 21 includes an opto-electric conversion unit 210.

  In FIG. 1, data D1 is input to the optical transmitter 11 as data to be transmitted. Data D1 is modulated by modulation section 120 and output as a high frequency modulation signal. The high frequency modulation signal output from the modulation unit 120 is converted into an optical signal by the electro-optical conversion unit 110 and sent to the multimode optical transmission line 310. An optical signal is input from the multimode optical transmission line 310 to the optical receiver 21. The optical signal input to the optical receiver 21 is converted into an electrical signal by the photoelectric conversion unit 210 and output as a high frequency modulation signal.

  However, in the multimode optical transmission system 1, inter-mode interference in the multimode optical transmission line 310 becomes a problem as described above. That is, the interference between optical signals (modes) having different propagation paths causes the noise characteristics and distortion characteristics of the high-frequency modulation signal output from the photoelectric conversion unit 210 to deteriorate. However, by appropriately setting the frequency of the high-frequency modulation signal that modulates the optical signal, it is possible to reduce deterioration of noise characteristics and distortion characteristics due to inter-mode interference.

  Therefore, the control unit 130 sets the frequency of the high frequency modulation signal output from the modulation unit 120 based on the group delay information C1 of each mode propagating through the multimode optical transmission line 310. Specifically, the control unit 130 sets the frequency of the high-frequency modulation signal output by the modulation unit 120 based on the group delay difference Δτ between dominant modes propagating through the multimode optical transmission line 310. In the following description, the case where the dominant modes are the basic mode and the primary mode will be described.

  Note that the modulation unit 120 is configured to output a signal to the electro-optical conversion unit 110 and may be referred to as a signal output unit. In addition, a multimode optical fiber cable is typically used for the multimode optical transmission line 310, but a single mode optical fiber cable may be used depending on use conditions.

Hereinafter, the frequency condition of the high frequency modulation signal that can reduce the deterioration of the noise characteristic and the distortion characteristic will be described using mathematical expressions and diagrams. (Expression 1) is obtained by directly modulating an optical signal with two-wave modulation signals (angular frequencies ω ₁ , ω ₂ ) and transmitting the signal using the multimode optical transmission line 310 (two modes having different propagation paths ( This represents a third-order distortion component I _IM3 (angular frequency 2ω ₁ −ω ₂ ) generated by interference between the fundamental mode and the first-order mode.

In (Equation 1), the term in the square root changes according to the angular frequencies ω ₁ , ω ₂ and the group delay difference Δτ. Therefore, the third-order distortion amount depends on the frequency of the modulation signal and the group delay difference Δτ. Dependent. 2 and 3 are diagrams for explaining the dependence of the third-order distortion generated by inter-mode interference on the modulation frequency. 2 and 3, the vertical axis represents the result of calculating (Equation 1) (that is, the third-order distortion I _IM3 ), and the horizontal axis represents the product (ω ₁ Δτ) of the angular frequency ω ₁ and the group delay difference Δτ. Represents. Here, ω ₂ is an angular frequency separated from ω ₁ by Δω. 2 and 3 show calculation results when ΔωΔτ takes various values. From FIG. 2, when Δω satisfies (Expression 2), the third-order distortion I _IM3 decreases within a range where ω ₁ satisfies (Expression 3). Similarly, as shown in FIG. 3, when Δω satisfies (Expression 4), the third-order distortion I _IM3 decreases within a range where ω ₁ satisfies (Expression 5). Therefore, if the value of the group delay difference Δτ is known, the interval Δω and the angular frequency ω ₁ of the two modulation signals are expressed by (Expression 2) to (Expression 3) or (Expression 4) to (Expression 5). By setting the range, the third-order distortion can be minimized.

In (Expression 2) to (Expression 5), it is predicted that the improvement effect of the transmission performance is highest when n = 0. That is, as the frequency for modulating the optical signal is lower, the FM modulation indexes B ₁ and B ₂ in (Equation 1) become larger, and as a result, J ₀ (B ₁ ) and J ₀ (B ₂ ) in (Equation 1). This is because the size can be reduced. Furthermore, when a desired transmission band is determined, by setting n to an optimal value so as to be within the desired transmission band, an effect of improving high transmission performance in the desired frequency band can be obtained. it can.

  Here, a method of measuring the group delay difference Δτ will be described with reference to FIG. FIG. 4 is a block diagram showing an example of the configuration of the multimode optical transmission system 1a that measures the group delay difference Δτ. 4, in the multimode optical transmission system 1a, in order to measure the group delay difference Δτ, the optical transmission device 11a includes a transmission-side mode group delay estimation unit, and the optical reception device 21a includes a reception-side mode group delay estimation unit. Further prepare. The transmission-side mode group delay estimation unit and the reception-side mode group delay estimation unit are connected by a multimode optical transmission line 320. The transmission-side mode group delay estimation unit includes a short pulse signal generation unit 141, a comparison calculation unit 142, and a photoelectric conversion unit 151. The reception-side mode group delay estimation unit includes an electro-optical conversion unit 251.

  The multimode optical transmission system 1a may include a transmission-side mode group delay estimation unit outside the optical transmission device 11a and a reception-side mode group delay estimation unit outside the optical reception device 21a.

  In the transmission-side mode group delay estimation unit, the short pulse signal generation unit 141 generates a short pulse signal. The short pulse signal generated by the short pulse signal generation unit 141 is converted into an optical signal by the electro-optical conversion unit 110 and transmitted to the multimode optical transmission line 310. The optical signal propagated through the multimode optical transmission line 310 is received by the optical receiver 21a. The optical signal received by the optical receiver 21a is converted into a short pulse signal by the photoelectric converter 210. The short pulse signal is input to the reception-side mode group delay estimation unit. In the reception-side mode group delay estimation unit, the short pulse signal is converted back to an optical signal by the electro-optical conversion unit 251 and transmitted to the multi-mode optical transmission line 320.

  In the transmission-side mode group delay estimation unit, the optical signal propagated through the multimode optical transmission line 320 is converted into a short pulse signal by the photoelectric conversion unit 151. The comparison calculation unit 142 compares the short pulse signal converted by the photoelectric conversion unit 151 with the short pulse signal generated by the short pulse signal generation unit 141, and converts the short pulse converted by the photoelectric conversion unit 151. Measure the signal waveform spread. The comparison calculation unit 142 calculates a group delay difference Δτ between the fundamental mode and the primary mode generated in the multimode optical transmission line 310 and the multimode optical transmission line 320 from the measured waveform spread.

  In the multimode optical transmission system 1a, the multimode optical transmission line 310 and the multimode optical transmission line 320 may be the same multimode optical transmission line. Even in such a configuration, the short pulse signal transmitted from the optical transmitter 11a to the optical receiver 21a and the short pulse signal transmitted from the optical receiver 21a to the optical transmitter 11a overlap in time. Because they are not, they will not collide. Therefore, the transmission-side mode group delay estimation unit can calculate the group delay difference Δτ between the basic mode and the primary mode more accurately.

  As described above, according to the multimode optical transmission system according to the first embodiment of the present invention, the high-frequency modulation that modulates the optical signal based on the group delay information of each mode propagating through the multimode optical transmission line 310. By appropriately setting the frequency of the signal, it is possible to reduce deterioration of noise characteristics and distortion characteristics due to inter-mode interference. As a result, even when a multimode optical fiber is used as an optical transmission line, it is possible to prevent a decrease in optical transmission quality and to construct a high-quality optical transmission system at a low cost.

  In addition, by providing the mode group delay estimation unit, it is possible to estimate the group delay information of each mode propagating through the multimode optical transmission line 310. As a result, even when the network configuration changes, it is possible to reduce deterioration of noise characteristics and distortion characteristics due to inter-mode interference. The mode group delay estimation unit can accurately estimate the group delay information of each mode propagating through the multimode optical transmission line 310 by using the short pulse signal.

  In addition, the multimode optical transmission system simply does not use the transmission-side mode group delay estimation unit and the reception-side mode group delay estimation unit, and based on the length of the multimode optical transmission line 310, You may estimate the group delay information of each mode which propagates the transmission line 310. FIG. Thereby, the multimode optical transmission system can estimate the mode group delay information without using a complicated configuration.

  In the above description, the third-order distortion caused by interference between two modes having different propagation paths has been described. However, the multimode optical transmission system 1 according to the present embodiment has other types of distortion ( For example, it is also useful for secondary distortion and noise. That is, the multimode optical transmission system 1 sets the frequency of the high frequency modulation signal so as to satisfy the conditions of (Expression 2) to (Expression 3) or (Expression 4) to (Expression 5). It is possible to improve the deterioration of characteristics other than the above. In addition, the multimode optical transmission system 1 according to the present embodiment can obtain the same effect even when there are a plurality of modes.

Furthermore, since the FM modulation indexes B ₁ and B ₂ are increased by increasing the amplitude of the high-frequency modulation signal, the values of the Bessel functions J ₀ (B ₁ ) and J ₀ (B ₂ ) can be reduced. Therefore, the multimode optical transmission system 1 according to the present embodiment sets the frequency of the high frequency modulation signal so as to satisfy the conditions of (Expression 2) to (Expression 3) or (Expression 4) to (Expression 5). By setting the amplitude of the high frequency modulation signal large, it is possible to more effectively reduce the deterioration of noise characteristics and distortion characteristics due to inter-mode interference.

(Second Embodiment)
In the first embodiment, the method of setting the frequency of the high frequency modulation signal based on the group delay difference Δτ between the fundamental mode and the primary mode has been described. In the second embodiment, a method for reducing deterioration of noise characteristics and distortion characteristics due to inter-mode interference by frequency-multiplexing a superposition signal on a high-frequency modulation signal and optimally setting the frequency of the superposition signal will be described.

  FIG. 5 is a block diagram showing an example of the configuration of the multimode optical transmission system 2 according to the second embodiment of the present invention. In FIG. 5, the multimode optical transmission system 2 has a configuration in which an optical transmission device 12 and an optical reception device 22 are connected by a multimode optical transmission line 310. The optical transmission device 12 includes an electro-optical conversion unit 110, a modulation unit 120, a control unit 130, a multiplexing unit 160, and a superimposed signal generation unit 170. The optical receiver 22 includes a photoelectric conversion unit 210.

  In the optical transmission device 12, the control unit 130 determines the frequency of the superimposed signal generated by the superimposed signal generation unit 170 based on the group delay difference Δτ between the fundamental mode and the primary mode propagating through the multimode optical transmission line 310. Set. The superimposed signal generation unit 170 generates a superimposed signal having a specific frequency. Data D1 is modulated by modulation section 120 and output as a high frequency modulation signal. The multiplexing unit 160 frequency-multiplexes the superimposed signal generated by the superimposed signal generation unit 170 and the high frequency modulation signal output from the modulation unit 120, and outputs the result as a high frequency modulation signal. The high frequency modulation signal output from the multiplexing unit 160 is converted into an optical signal by the electro-optical conversion unit 110 and sent to the multimode optical transmission line 310. The optical signal input from the multimode optical transmission line 310 to the optical receiver 22 is converted into a high frequency modulation signal by the photoelectric conversion unit 210.

  Note that the modulation unit 120 may output a signal modulated with a baseband signal instead of the high-frequency modulation signal. The modulation unit 120, the multiplexing unit 160, and the superimposed signal generation unit 170 are configured to output a signal to the electro-optical conversion unit 110, and may be referred to as a signal output unit.

  Here, the multimode optical transmission system 2 can reduce noise characteristics and distortion characteristics due to inter-mode interference by setting the frequency of the superimposed signal to be frequency-multiplexed with the high-frequency modulation signal to an appropriate value. . Hereinafter, the frequency condition of the superimposed signal that can reduce the deterioration of the noise characteristic and the distortion characteristic will be described using mathematical expressions and drawings.

(Equation 6) directly modulates an optical signal with two-wave modulation signals (angular frequencies ω ₁ , ω ₂ ) and one-wave superimposed signal (angular frequency ω _LO ), and uses a multimode optical transmission line 310. 3rd order distortion component I _IM3 (angular frequency 2ω ₁ −ω ₂ ) generated by interference between two modes (fundamental mode and first order mode) having different propagation paths.

In (Equation 6), the second term depends on the frequency of the superimposed signal, and the term in the square root of the second term depends on the product of the angular frequency ω _LO of the superimposed signal and the group delay difference Δτ. Since it changes, the amount of third-order distortion changes according to the angular frequency ω _LO of the superimposed signal and the group delay difference Δτ.

FIG. 6 is a diagram for explaining the effect of the superimposed signal on the third-order distortion caused by inter-mode interference. In FIG. 6, the vertical axis represents the result of calculating (Equation 6) (ie, third-order distortion I _IM3 ), and the horizontal axis represents the product (ω ₁ Δτ) of the angular frequency ω ₁ of the modulation signal and the group delay difference Δτ. ). FIG. 6 shows a calculation result when ω _LO Δτ is 2nπ and 2 (n + 1) π, and n is an integer. From FIG. 6, the third-order distortion I _IM3 is smaller when ω _LO Δτ is 2 (n + 1) π than when ω _LO Δτ is 2nπ. Therefore, by setting the angular frequency ω _LO of the superimposed signal to a value that satisfies (Equation 7), the third-order distortion I _IM3 can be reduced regardless of the frequency of the modulation signal. Furthermore, as shown in FIG. 6, it is possible to further reduce the third-order distortion I _IM3 by setting the frequency of the modulation signal to a value that satisfies (Equation 8).

  Here, since the method for detecting the group delay difference Δτ between the basic mode and the primary mode is the same as that in the first embodiment, detailed description thereof is omitted. Similarly to the first embodiment, the multimode optical transmission system 2 according to the second embodiment is simply based on the length of the multimode optical transmission line 310, and the basic mode and the primary mode. The group delay difference Δτ may be detected.

In (Expression 7), when n = 0, it is expected that the effect of improving the transmission performance is the highest. This is because the FM modulation index B _LO in (Expression 6) increases as the frequency for modulating the optical signal decreases, and as a result, J ₀ (B _LO ) in (Expression 6) can be reduced. Furthermore, by setting n to an optimal value so that the harmonic component of the superimposed signal does not overlap the transmission signal, the influence of the harmonic component of the superimposed signal on the transmission signal is avoided, and interference between modes is prevented. The accompanying deterioration of noise characteristics and distortion characteristics can be reduced.

  As described above, according to the multimode optical transmission system 2 according to the second embodiment of the present invention, the superposition for modulating the optical signal based on the group delay information of each mode propagating through the multimode optical transmission line 310. By appropriately setting the frequency of the signal, it is possible to reduce deterioration of noise characteristics and distortion characteristics due to inter-mode interference. Further, since the line width of the light source can be widened by frequency multiplexing the superposed signal with the high frequency modulation signal, it is possible to further reduce the deterioration of noise characteristics and distortion characteristics due to inter-mode interference.

ただし、ｎは任意の自然数である。 Note that the modulation unit 120 may set the frequency f of the high-frequency modulation signal so as to satisfy the condition of (Equation 9) or (Equation 10). As a result, the multimode optical transmission system 2 can secure the maximum amplitude of the high-frequency modulation signal output from the photoelectric conversion unit 210, so that higher quality transmission can be performed.

However, n is an arbitrary natural number.

  In the above description, the third-order distortion caused by interference between two modes having different propagation paths has been described. However, the multimode optical transmission system 2 according to the present embodiment has other types of distortion ( For example, it is also useful for secondary distortion and noise. That is, the multimode optical transmission system 2 can improve characteristic deterioration other than the third-order distortion by setting the frequency of the superimposed signal to satisfy (Equation 7). Further, the multimode optical transmission system 2 according to the present embodiment can obtain the same effect even when there are a plurality of modes.

Furthermore, since the FM modulation index B _LO increases by increasing the amplitude of the superimposed signal, the value of the Bessel function J ₀ (B _LO ) decreases, and the value of the first term in (Equation 6) can be decreased. it can. On the other hand, since the value of the Bessel function J ₁ (B _LO ) increases as B _LO increases, the value of the second term increases. However, the value of the second term can be reduced by setting the frequency of the superimposed signal to the above-described value. Therefore, the multimode optical transmission system 2 according to the present embodiment sets the frequency of the superimposed signal so as to satisfy the condition of (Equation 7) and increases the amplitude of the superimposed signal, thereby causing intermode interference. Deterioration of noise characteristics and distortion characteristics can be reduced more effectively.

(Third embodiment)
In the first embodiment and the second embodiment, by appropriately setting the frequency of the high-frequency modulation signal or the frequency of the superimposed signal that is frequency-multiplexed with the high-frequency modulation signal, the noise characteristics and distortion characteristics associated with inter-mode interference can be reduced. A method for reducing degradation was described. In the third embodiment, using the fact that the amount of distortion or noise changes when the frequency of the high-frequency modulation signal or superimposed signal is changed, the fundamental mode and the primary mode that propagate through the multimode optical transmission line 310 are utilized. A method for measuring the group delay difference Δτ between them will be described.

  FIG. 7A is a block diagram showing an example of the configuration of a multimode optical transmission system 3a according to the third embodiment of the present invention. 7A, the multimode optical transmission system 3a has a configuration in which an optical transmitter 13a and an optical receiver 23a are connected by a multimode optical transmission line 310 and a multimode optical transmission line 320. The optical transmission device 13 a includes an electro-optical conversion unit 110, a modulation unit 120, a control unit 130, an opto-electric conversion unit 151, and a comparison calculation unit 181. The optical receiving device 23 a includes an opto-electric conversion unit 210 and an electro-optical conversion unit 251.

  Note that the modulation unit 120 is configured to output a signal to the electro-optical conversion unit 110 and may be referred to as a signal output unit. In addition, the photoelectric conversion unit 151 and the comparison calculation unit 181 are configured to estimate the group delay difference Δτ between the fundamental mode and the primary mode on the transmission side, and are therefore referred to as a transmission side mode group delay estimation unit. May be. In addition, since the electro-optical conversion unit 251 is configured to estimate the group delay difference Δτ between the basic mode and the primary mode on the reception side, it may be referred to as a reception-side mode group delay estimation unit.

  In the optical transmission device 13a, the modulation unit 120 modulates the input data D1 and outputs it as a high frequency modulation signal. The high frequency modulation signal output from the modulation unit 120 is converted into an optical signal by the electro-optical conversion unit 110 and transmitted to the multimode optical transmission line 310. The optical signal propagated through the multimode optical transmission line 310 is received by the optical receiver 23a. The optical signal received by the optical receiver 23a is converted into a high-frequency modulated signal by the photoelectric converter 210. The high frequency modulation signal is input to the reception-side mode group delay estimation unit. In the reception-side mode group delay estimation unit, the high-frequency modulation signal is converted again into an optical signal by the electro-optical conversion unit 251 and sent to the multimode optical transmission line 320.

  In the transmission-side mode group delay estimation unit, the optical signal propagated through the multimode optical transmission line 320 is converted into a high-frequency modulation signal by the photoelectric conversion unit 151. The comparison calculation unit 181 compares the high-frequency modulation signal output from the modulation unit 120 with the high-frequency modulation signal converted by the photoelectric conversion unit 151, and is included in the high-frequency modulation signal converted by the photoelectric conversion unit 151 Distortion amount (for example, second-order harmonic distortion, third-order intermodulation wave distortion, etc.) is calculated. The comparison calculation unit 181 calculates the group delay difference Δτ between the basic mode and the primary mode generated in the multimode optical transmission line 310 and the multimode optical transmission line 320 based on the calculated distortion amount.

More specifically, as described in the first embodiment, an optical signal is directly modulated with _two modulation signals (angular frequencies ω ₁ and ω ₂ ) and transmitted using a multimode optical transmission line 310. In some cases, third-order distortion caused by inter-mode interference depends on the frequency of the modulation signal. That is, since the third-order distortion amount changes according to the frequency change of the high-frequency modulation signal, the comparison operation unit 180 determines the group delay difference based on the change of the third-order distortion amount when the frequency of the high-frequency modulation signal is changed. Δτ can be calculated.

  FIG. 7B is a block diagram showing an example of the configuration of a multimode optical transmission system 3b according to the third embodiment of the present invention. 7B, the multimode optical transmission system 3b has a configuration in which an optical transmission device 13b and an optical reception device 23b are connected by a multimode optical transmission line 310 and a multimode optical transmission line 320. The optical transmission device 13b includes an electro-optic conversion unit 110, a modulation unit 120, a control unit 130, an opto-electric conversion unit 151, a multiplexing unit 160, a superimposed signal generation unit 170, and a comparison calculation unit 182. The optical receiver 23b includes an opto-electric conversion unit 210 and an electro-optical conversion unit 251.

  Note that the modulation unit 120, the multiplexing unit 160, and the superimposed signal generation unit 170 are configured to output a signal to the electro-optical conversion unit 110, and may be referred to as a signal output unit. In addition, the photoelectric conversion unit 151 and the comparison calculation unit 182 are configured to estimate the group delay difference Δτ between the basic mode and the primary mode on the transmission side, and are therefore referred to as a transmission side mode group delay estimation unit. May be. In addition, since the electro-optical conversion unit 251 is configured to estimate the group delay difference Δτ between the basic mode and the primary mode on the reception side, it may be referred to as a reception-side mode group delay estimation unit.

  In the optical transmission device 13b, the modulation unit 120 modulates the input data D1 and outputs it as a high frequency modulation signal. The superimposed signal generation unit 170 generates a superimposed signal having a specific frequency. The multiplexing unit 160 frequency-multiplexes the high-frequency modulation signal output from the modulation unit 120 and the superimposed signal generated by the superimposed signal generation unit 170 and outputs the result as a high-frequency modulation signal. The high frequency modulation signal output from the multiplexing unit 160 is converted into an optical signal by the electro-optical conversion unit 110 and transmitted to the multimode optical transmission line 310.

  The optical signal propagated through the multimode optical transmission line 310 is received by the optical receiver 23b. The optical signal received by the optical receiver 23b is converted into a high-frequency modulated signal by the photoelectric converter 210. The high frequency modulation signal is input to the reception-side mode group delay estimation unit. In the reception-side mode group delay estimation unit, the high-frequency modulation signal is converted again into an optical signal by the electro-optical conversion unit 251 and sent to the multimode optical transmission line 320.

  In the transmission-side mode group delay estimation unit, the optical signal propagated through the multimode optical transmission line 320 is converted into a high-frequency modulation signal by the photoelectric conversion unit 151. The comparison calculation unit 182 compares the high-frequency modulation signal output from the signal output unit with the superimposed signal included in the high-frequency modulation signal converted by the photoelectric conversion unit 151 and compares the distortion amount (for example, 2nd order harmonic distortion, 3rd order intermodulation wave distortion, etc.) are calculated. The comparison calculation unit 182 calculates the group delay difference Δτ between the basic mode and the primary mode generated in the multimode optical transmission line 310 and the multimode optical transmission line 320 based on the calculated distortion amount.

More specifically, as described in the second embodiment, an optical signal is directly generated by two modulated signals (angular frequencies ω ₁ , ω ₂ ) and one superimposed signal (angular frequency ω _LO ). When modulated and transmitted using the multimode optical transmission line 310, the third-order distortion generated by inter-mode interference depends on the frequency of the superimposed signal. FIG. 8 is a diagram for explaining the frequency dependence of the third-order distortion amount generated by inter-mode interference on the superimposed signal. From FIG. 8, when the frequency of the modulated signal of two waves is set to an appropriate value (here, ω ₁ Δτ = 2nπ, ω ₂ Δτ = ω ₁ Δτ + 0.1π, n is an arbitrary integer), the value of ω _LO Δτ Accordingly, the third-order distortion I _IM3 changes. That is, since the third-order distortion amount changes according to the frequency change of the superimposed signal, the comparison calculation unit 180 calculates the group delay difference Δτ based on the change of the third-order distortion amount when the frequency of the superimposed signal is changed. Can be calculated.

  In the above description, the group delay difference Δτ between the basic mode and the primary mode is calculated from the distortion amount. However, the group delay difference Δτ can also be calculated by monitoring the noise amount. Specifically, the optical transmission device 13b transmits an optical signal modulated by a frequency-multiplexed signal of an unmodulated carrier (hereinafter referred to as a monitor signal) and a superimposed signal via the multimode optical transmission line 310. . When the optical transmission device 13b demodulates the optical signal received via the multimode optical transmission line 320 by the photoelectric conversion unit 151, the noise near the monitor signal changes according to the frequency of the superimposed signal. Since the period of the change depends on the group delay information of the mode propagating through the multimode optical transmission line 310, the group delay difference Δτ can be calculated by obtaining the relationship between the frequency of the superimposed signal and the noise level in the vicinity of the monitor signal. Can be sought. Note that the group delay difference Δτ can be measured in the same manner even if the optical signal is modulated only by the monitor signal. That is, when the frequency of the monitor signal is changed, the noise in the vicinity of the monitor signal changes in accordance with the frequency of the monitor signal. Therefore, if the frequency of the monitor signal and the noise level in the vicinity of the monitor signal are measured, the group delay difference Δτ Can be calculated.

  In the multimode optical transmission systems 3a and 3b according to the present embodiment, the multimode optical transmission line 310 and the multimode optical transmission line 320 may be the same multimode optical transmission line. For example, the multimode optical transmission systems 3a and 3b can transmit wavelength-multiplexed optical signals through the same multimode optical transmission line by wavelength multiplexing optical signals in both directions. Thereby, the group delay difference Δτ between the basic mode and the primary mode can be calculated more accurately.

  As described above, according to the multimode optical transmission system according to the third embodiment of the present invention, it is possible to estimate the distortion component of the modulation signal by changing the frequency of the superimposed signal. As a result, the group delay difference Δτ in the multimode optical transmission line 310 can be estimated without using the short pulse signal generator 141 as shown in the first embodiment. In addition, the multimode optical transmission system 3 according to the present embodiment can estimate the group delay difference Δτ in the multimode optical transmission line 310 by using the same method even when there are a plurality of modes.

  Although the present invention has been described in detail above, the above description is merely illustrative of the present invention in all respects and is not intended to limit the scope thereof. It goes without saying that various improvements and modifications can be made without departing from the scope of the present invention.

  The multimode optical transmission system of the present invention is useful for reducing deterioration of noise characteristics and distortion characteristics due to intermode interference.

1 is a block diagram showing an example of the configuration of a multimode optical transmission system 1 according to a first embodiment of the present invention. The figure explaining the modulation frequency dependence of the third-order distortion generated by inter-mode interference The figure explaining the modulation frequency dependence of the third-order distortion generated by inter-mode interference Block diagram showing an example of the configuration of a multimode optical transmission system 1a for measuring the group delay difference Δτ The block diagram which shows an example of a structure of the multimode optical transmission system 2 which concerns on the 2nd Embodiment of this invention. The figure explaining the effect of the superimposition signal with respect to the third order distortion generated by inter-mode interference The block diagram which shows an example of a structure of the multimode optical transmission system 3a which concerns on the 3rd Embodiment of this invention. The block diagram which shows an example of a structure of the multimode optical transmission system 3b which concerns on the 3rd Embodiment of this invention. The figure explaining the frequency dependence with respect to the superimposition signal of the 3rd order distortion amount generated by inter-mode interference Block diagram showing an example of the configuration of a conventional optical transmission system using a multimode optical fiber

Explanation of symbols

1, 1a, 2, 3a, 3b Multi-mode optical transmission system 11-13 Optical transmitter 110, 251 Electro-optical converter 120 Modulator 130 Controller 141 Short pulse generator 142 Comparator 151, 210 Photoelectric converter 160 Multiplexer 170 Superimposition signal generator 180 Comparison operation unit 21, 21a, 22, 23a, 23b Optical receiver 310, 320 Multimode optical transmission line

Claims (17)

A multimode optical transmission device that transmits an optical signal generated based on transmission data via a multimode optical transmission line,
A modulation unit that generates a high-frequency modulated signal modulated based on the transmission data;
An electro-optical converter that converts the high-frequency signal into an optical signal and transmits the optical signal to the multi-mode optical transmission line;
And a control unit that adjusts the frequency of the high-frequency modulation signal so that a third-order distortion component generated by inter-mode interference is reduced based on group delay information of each mode propagating through the multimode optical transmission line. A multi-mode optical transmission device. A multimode optical transmission device that transmits an optical signal generated based on transmission data via a multimode optical transmission line,
A modulation unit that generates a high-frequency modulated signal modulated based on the transmission data;
A superimposed signal generator for generating a superimposed signal having a predetermined frequency;
A multiplexing unit that generates a multiplexed signal obtained by frequency-multiplexing the high-frequency modulation signal and the superimposed signal;
An electro-optical converter that converts the multiplexed signal into an optical signal and transmits the optical signal to the multi-mode optical transmission line;
And a control unit that adjusts the frequency of the superimposed signal so that a third-order distortion component generated by inter-mode interference is reduced based on group delay information of each mode propagating through the multimode optical transmission line. A multi-mode optical transmission device. A multimode optical transmission device that transmits an optical signal generated based on transmission data via a multimode optical transmission line,
A modulator for generating a baseband signal based on the transmission data;
A superimposed signal generator for generating a superimposed signal having a predetermined frequency;
A multiplexing unit that generates a multiplexed signal obtained by frequency-multiplexing the baseband signal and the superimposed signal;
An electro-optical converter that converts the multiplexed signal into an optical signal and transmits the optical signal to the multi-mode optical transmission line;
And a control unit that adjusts the frequency of the superimposed signal so that a third-order distortion component generated by inter-mode interference is reduced based on group delay information of each mode propagating through the multimode optical transmission line. A multi-mode optical transmission device.   When the control unit adjusts the frequency of the superimposed signal so that the third-order distortion component generated by inter-mode interference is reduced, the harmonic component of the superimposed signal is within the band of the high-frequency modulation signal output by the modulation unit. The multimode optical transmission apparatus according to claim 2, wherein the frequency of the superimposition signal is adjusted within a range that does not overlap. A mode group delay information estimation unit for estimating group delay information of each mode propagating through the multimode optical transmission line;
The mode group delay information estimation unit
A short pulse signal generator for generating a short pulse signal;
A photoelectric conversion unit that converts an optical signal received via the multimode optical transmission path into an electrical signal;
Comparison in which the short pulse signal generated by the short pulse signal generator and the electrical signal converted by the photoelectric converter are compared to calculate group delay information for each mode propagating in the multimode optical transmission line. With an arithmetic unit,
The electro-optical conversion unit converts the short pulse signal generated by the short pulse signal generation unit into an optical signal, and transmits the optical signal through the multimode optical transmission line.
The optical signal received through the multimode optical transmission line is converted into an optical signal by the receiving side device that has received the optical signal obtained by converting the short pulse signal generated by the short pulse signal generation unit. 4. The multimode optical transmission device according to claim 1, wherein the short pulse signal obtained in this way is electro-optically converted and transmitted to the multimode optical transmission line. 5. A mode group delay information estimation unit for estimating group delay information of each mode propagating through the multimode optical transmission line;
The mode group delay information estimation unit
An optical signal received through the multimode optical transmission line is converted into an electric signal, and an optical signal received through the multimode optical transmission line is received by receiving the optical signal transmitted from the electric optical conversion unit. A photoelectric conversion unit that is a device in which a side apparatus performs photoelectric conversion on an electrical signal obtained by photoelectric conversion of a received optical signal and transmits the converted signal to the multimode optical transmission line;
Comparing the signal input to the electro-optical conversion unit and the signal output from the photoelectric conversion unit to calculate the amount of distortion or noise generated in the multi-mode optical transmission line, the calculated amount of distortion or 4. A multi-operation device according to claim 1, further comprising: a comparison operation unit that estimates group delay information of each mode propagating through the multi-mode optical transmission line based on a noise amount. 5. Mode optical transmission device.   The said control part calculates the group delay information of each mode which propagates the said multimode optical transmission line based on the length of the said multimode optical transmission line, The one of the Claims 1 thru | or 3 characterized by the above-mentioned. The multimode optical transmission apparatus according to one. When the group delay difference between the first mode and the second mode propagating through the multimode optical transmission line is Δτ, and an arbitrary natural number is n,
The multimode optical transmission apparatus according to claim 2, wherein the modulation unit sets the frequency of the high-frequency modulation signal to a frequency equal to n × 1 / Δτ. When the group delay difference between the first mode and the second mode propagating through the multimode optical transmission line is Δτ, and an arbitrary natural number is n,
The modulation unit sets the frequency of the high-frequency modulation signal in a range of frequencies larger than (n−1 / 4) × 1 / Δτ and smaller than (n + 1/4) × 1 / Δτ. The multimode optical transmission apparatus according to claim 2. When the group delay difference between the first mode and the second mode propagating through the multimode optical transmission line is Δτ, and an arbitrary natural number is n,
The multimode optical transmission apparatus according to claim 2, wherein the superimposition signal generation unit sets the frequency of the superimposition signal to a frequency equal to (n−1 / 2) × 1 / Δτ. When the group delay difference between the first mode and the second mode propagating through the multimode optical transmission line is Δτ, and an arbitrary natural number is n,
The multimode optical transmission apparatus according to claim 2, wherein the superimposed signal generation unit sets the frequency of the superimposed signal to a frequency different from n × 1 / Δτ. A multimode optical transmission system comprising an optical transmitter and an optical receiver that transmit and receive an optical signal generated based on transmission data via a multimode optical transmission line,
The optical transmitter is
A modulation unit that generates a high-frequency modulated signal modulated based on the transmission data;
An electro-optical converter that converts the high-frequency signal into an optical signal and transmits the optical signal to the multi-mode optical transmission line;
A controller that adjusts the frequency of the high-frequency modulation signal so that a third-order distortion component generated by inter-mode interference is reduced based on group delay information of each mode propagating through the multimode optical transmission line;
The optical receiver is
A multi-mode optical transmission system comprising an opto-electric conversion unit that converts an optical signal received through the multi-mode optical transmission path into an electric signal. A multimode optical transmission system comprising an optical transmitter and an optical receiver that transmit and receive an optical signal generated based on transmission data via a multimode optical transmission line,
The optical transmitter is
A modulation unit that generates a high-frequency modulated signal modulated based on the transmission data;
A superimposed signal generator for generating a superimposed signal having a predetermined frequency;
A multiplexing unit that generates a multiplexed signal obtained by frequency-multiplexing the high-frequency modulation signal and the superimposed signal;
An electro-optical converter that converts the multiplexed signal into an optical signal and transmits the optical signal to the multi-mode optical transmission line;
A controller that adjusts the frequency of the superimposed signal so that a third-order distortion component generated by inter-mode interference is reduced based on group delay information of each mode propagating through the multimode optical transmission line;
The optical receiver is
A multi-mode optical transmission system comprising an opto-electric conversion unit that converts an optical signal received through the multi-mode optical transmission path into an electric signal. A multimode optical transmission system comprising an optical transmitter and an optical receiver that transmit and receive an optical signal generated based on transmission data via a multimode optical transmission line,
The optical transmitter is
A modulator for generating a baseband signal based on the transmission data;
A superimposed signal generator for generating a superimposed signal having a predetermined frequency;
A multiplexing unit that generates a multiplexed signal obtained by frequency-multiplexing the baseband signal and the superimposed signal;
An electro-optical converter that converts the multiplexed signal into an optical signal and transmits the optical signal to the multi-mode optical transmission line;
A controller that adjusts the frequency of the superimposed signal so that a third-order distortion component generated by inter-mode interference is reduced based on group delay information of each mode propagating through the multimode optical transmission line;
The optical receiver is
A multi-mode optical transmission system comprising an opto-electric conversion unit that converts an optical signal received through the multi-mode optical transmission path into an electric signal. A multi-mode optical transmission method for transmitting an optical signal generated based on transmission data via a multi-mode optical transmission line,
Generating a high-frequency modulated signal modulated based on the transmission data;
Converting the high-frequency signal into an optical signal and transmitting it to the multimode optical transmission line;
Adjusting the frequency of the high-frequency modulation signal based on group delay information of each mode propagating through the multimode optical transmission line so as to reduce a third-order distortion component generated by inter-mode interference. how to. A multi-mode optical transmission method for transmitting an optical signal generated based on transmission data via a multi-mode optical transmission line,
Generating a high-frequency modulated signal modulated based on the transmission data;
Generating a superimposed signal having a predetermined frequency;
Generating a multiplex signal obtained by frequency-multiplexing the high-frequency modulation signal and the superimposed signal;
Converting the multiplexed signal into an optical signal and transmitting it to the multimode optical transmission line;
Adjusting the frequency of the superimposed signal based on group delay information of each mode propagating through the multimode optical transmission line so as to reduce a third-order distortion component generated by inter-mode interference. ,Method. A multi-mode optical transmission method for transmitting an optical signal generated based on transmission data via a multi-mode optical transmission line,
Generating a baseband signal based on the transmission data;
Generating a superimposed signal having a predetermined frequency;
Generating a multiplexed signal by frequency multiplexing the baseband signal and the superimposed signal;
Converting the multiplexed signal into an optical signal and transmitting it to the multimode optical transmission line;
Adjusting the frequency of the superimposed signal based on group delay information of each mode propagating through the multimode optical transmission line so as to reduce a third-order distortion component generated by inter-mode interference. ,Method.

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