Optical transmission system and wavelength dispersion compensation method

Abstract

An optical transmission system comprising multi-stage-configuration relay mechanisms in a relay interval between an optical transmitter and an optical receiver, wherein at least one or more relay mechanisms among the multi-stage-configuration relay mechanisms comprises: a first optical fiber that transmits an optical signal; a second optical fiber that has opposite dispersion characteristics from the first optical fiber and that compensates for wavelength dispersion occurring in the transmission of the first optical fiber due to a nonlinear optical effect; and a third optical fiber that is an optical fiber comprised at a subsequent stage to the second optical fiber and having the same dispersion characteristics as the first optical fiber, said third optical fiber having opposite dispersion characteristics to the second optical fiber and compensating for wavelength dispersion occurring in the transmission of the second optical fiber due to a nonlinear optical effect.　

Description

Optical transmission system and wavelength dispersion compensation method

[0001] The present invention relates to an optical transmission system and a wavelength dispersion compensation method.

[0002] Conventionally, an Analog-Radio-over-Fiber (A-RoF) transmission technology is known, in which an optical signal is intensity-modulated with an analog waveform, or an analog waveform is modulated using another modulation method such as FM (Frequency Modulation), and then the optical signal is intensity-modulated with the modulated signal, and the optical signal in the form of an analog waveform is transmitted through an optical fiber. This analog waveform can be, for example, an audio signal, a video signal, or a radio wave signal, but is not limited to these and can be any analog waveform. Alternatively, a digital waveform can be treated as an analog waveform and transmitted using A-RoF. Systems that use this A-RoF method to transmit wideband analog waveform signals on the order of GHz over long distances have been put into practical use (see, for example, Non-Patent Documents 1 and 2). Figure 6 is a diagram showing an example configuration of an optical transmission system using a conventional A-RoF method. In the optical transmission system shown in Figure 6, the optical transmitter 10 generates a modulated signal by intensity-modulating an optical signal using a wideband signal output from a signal source 11, and transmits this modulated signal to the optical receiver 12 via an optical fiber. The connection between the optical transmitter 10 and the optical receiver 12 is divided into a relay section and an access section.

[0003] The relay section is equipped with N (where N is an integer greater than or equal to 2) optical amplifiers 13, (N-1) single-mode fibers (SMFs) 14, and (N-1) dispersion-compensating fibers (DCFs) 15. For example, in the relay section, the optical amplifiers 13, single-mode fibers 14, and dispersion-compensating fibers 15 are arranged in the order of optical amplifier 13-1, single-mode fiber 14-1, dispersion-compensating fiber 15-1, ..., optical amplifier 13-(N-1), single-mode fiber 14-(N-1), dispersion-compensating fiber 15-(N-1), and optical amplifier 13-N. In addition, the access section is equipped with one single-mode fiber 16.

[0004] The optical amplifier 13 amplifies the power of an optical signal that attenuates in optical fiber transmission. The optical amplifier 13 is, for example, an EDFA (Erbium Doped Fiber Amplifier). In an optical transmission system, as shown in FIG. 6, in order to suppress a decrease in signal quality due to wavelength dispersion occurring in the single-mode fiber 14 in the relay section, a dispersion compensation fiber 15 is used at the output destination of the single-mode fiber 14 to compensate for wavelength dispersion. Note that in FIG. 6, only one single-mode fiber 16 and one optical receiver 12 in the access section are shown, and the configuration is such that there is one optical transmitter and one optical receiver (a one-to-one configuration of optical transceivers). However, a configuration may also be used in which the fiber branches from the subsequent stage of each optical amplifier 13 and is connected to an access section or an optical receiver not shown (a one-to-many configuration of optical transceivers).

[0005] "Transmission equipment for transferring multi-channel television signals over optical access networks by frequency modulation conversion", Recommendation ITU-T J.185, International Telecommunication Union, June 2012. Shimoba, et al., "Optical video distribution technology using the FM batch conversion method", IEICE CS Research Group, 2019

[0006] In the optical transmission system shown in Figure 6, Self-Phase Modulation (SPM), a type of nonlinear optical effect, occurs in each single-mode fiber 14 and dispersion compensation fiber 15 in the relay section. Therefore, experimental results have shown that the more relay stages there are in fiber transmission (i.e., the longer the transmission distance), the greater the level of the sidewaves in the optical spectrum and the broader the bandwidth (see legend in Figure 6). Here, "number of relay stages" is a number that represents how many times the signal is amplified by the optical amplifier 13 in the relay section. Since chromatic dispersion occurs during transmission even for sidewaves whose levels have increased in this way, dispersion compensation is necessary. The area near the center of the optical spectrum is a spectrum that has existed since the first stage of long-distance transmission (e.g., stage 0), so a large amount of chromatic dispersion is accumulated during the transmission process. The outer part of the optical spectrum, outside the center, is a sidewave newly generated during transmission, so the accumulation of chromatic dispersion is small, resulting in a bias in the amount of chromatic dispersion.

[0007] As described above, sidewaves whose levels increase in the single-mode fiber 14 section can be compensated for by the dispersion compensation fiber 15 section provided immediately afterward. The single-mode fiber 14 section is the section from one end of the single-mode fiber 14 to the other end. However, in conventional optical transmission systems, dispersion compensation is not possible for sidewaves whose levels increase in the dispersion compensation fiber 15 section. The dispersion compensation fiber 15 section is the section from one end of the dispersion compensation fiber 15 to the other end. As a result, this leads to a decrease in the optical modulation degree immediately after the dispersion compensation fiber 15 in each stage, causing a deterioration in the system's carrier-to-noise ratio (CNR). Therefore, in order to solve this problem, in addition to compensating for general wavelength dispersion that can be compensated for by dispersion compensation fibers, it is necessary to compensate for special wavelength dispersion that occurs in the transmission path of the dispersion compensation fiber due to nonlinear optical effects.

[0008] It should be noted that this phenomenon is not a major problem in other common optical networks using configurations such as PON (Passive Optical Network) (for example, the FLET'S Hikari (registered trademark) network), but is a problem that requires particular attention in optical networks using the RoF method. One reason for this is that while other common optical networks using configurations such as PON have short transmission distances on the order of tens of kilometers, optical networks using the RoF method have ultra-long-distance transmission on the order of hundreds of kilometers, making it easier for chromatic dispersion to accumulate. The second reason is that optical networks using the RoF method have higher optical power input to the fiber compared to other common optical networks using configurations such as PON, resulting in stronger nonlinear optical effects. It should be noted that although the above problem requires particular attention in optical networks using the RoF method, it may also affect other common optical networks, including PON and SS (Single Star) configurations, so it is desirable to address this problem as well.

[0009] In view of the above circumstances, the present invention aims to provide a technology that can improve the carrier-to-noise ratio in optical networks.

[0010] One aspect of the present invention is an optical transmission system comprising a multi-stage relay mechanism in a relay section between an optical transmitter and an optical receiver, wherein the multi-stage relay mechanism comprises a first optical fiber for transmitting an optical signal, and a second optical fiber having dispersion characteristics opposite to those of the first optical fiber and compensating for wavelength dispersion caused by nonlinear optical effects during transmission of the first optical fiber, and further comprising a third optical fiber located downstream of the second optical fiber, having the same dispersion characteristics as the first optical fiber, having dispersion characteristics opposite to those of the second optical fiber, and compensating for wavelength dispersion caused by nonlinear optical effects during transmission of the second optical fiber, or an optical transmission system comprising a first optical fiber of a length that satisfies the condition that the measurement result of an index in the section comprising the first optical fiber is equal to or greater than a threshold.

[0011] One aspect of the present invention is a wavelength dispersion compensation method for an optical transmission system that includes a multi-stage relay mechanism in a relay section between an optical transmitter and an optical receiver, wherein the multi-stage relay mechanism includes a first optical fiber and a second optical fiber, the first optical fiber transmits an optical signal, the second optical fiber has dispersion characteristics opposite to those of the first optical fiber and compensates for wavelength dispersion caused by nonlinear optical effects during transmission of the first optical fiber, and at least one of the multi-stage relay mechanisms includes a third optical fiber, or the first optical fiber having a length that satisfies the condition that the measurement result of an index in the section containing the first optical fiber is equal to or greater than a threshold, and if at least one of the relay mechanisms includes the third optical fiber, the third optical fiber is provided downstream of the second optical fiber, has the same dispersion characteristics as the first optical fiber, has dispersion characteristics opposite to those of the second optical fiber and compensates for wavelength dispersion caused by nonlinear optical effects during transmission of the second optical fiber.

[0012] This invention makes it possible to improve the carrier-to-noise ratio in optical networks.

[0013] This figure shows an example configuration of the optical transmission system in the first embodiment. This figure shows experimental results regarding the evaluation of the effectiveness of the invention. This figure shows an example configuration of the optical transmission system in a modified version of the first embodiment. This figure shows an example configuration of the optical transmission system in the second embodiment. This figure shows an example configuration of the optical transmission system in the third embodiment. This figure shows an example configuration of the optical transmission system using the conventional A-RoF method.

[0014] One embodiment of the present invention will be described below with reference to the drawings.

[0015] (First Embodiment) Figure 1 shows an example of the configuration of an optical transmission system 100 in the first embodiment. The optical transmission system 100 includes an optical transmitter 10, a signal source 11, an optical receiver 12, an optical amplifier 13, a single-mode fiber 14, a dispersion compensation fiber 15, a single-mode fiber 16, and a single-mode fiber 17. The optical transmitter 10 and the optical receiver 12 are connected by an optical fiber, which is divided into a relay section and an access section.

[0016] In the first embodiment, the relay section is equipped with N optical amplifiers 13, (N-1) single-mode fibers 14, (N-1) dispersion compensation fibers 15, and (N-1) single-mode fibers 17. Thus, the relay section has a multi-stage relay mechanism for relay transmission. One relay mechanism is composed of a combination of one optical amplifier 13, one single-mode fiber 14, one dispersion compensation fiber 15, and one single-mode fiber 17. That is, the relay mechanism in the first embodiment is equipped with one optical amplifier 13, one single-mode fiber 14, one dispersion compensation fiber 15, and one single-mode fiber 17.

[0017] The multiple relay mechanisms provided in the relay section in the first embodiment have the same configuration. The number of relay mechanisms increases as the transmission distance between the optical transmitter 10 and the optical receiver 12 increases.

[0018] In the relay section, optical amplifiers 13, single-mode fibers 14, dispersion compensation fibers 15, and single-mode fibers 17 are provided in the order of optical amplifier 13-1, single-mode fiber 14-1, dispersion compensation fiber 15-1, single-mode fiber 17-1, ..., optical amplifier 13-(N-1), single-mode fiber 14-(N-1), dispersion compensation fiber 15-(N-1), single-mode fiber 17-(N-1), and optical amplifier 13-N.

[0019] The combination of optical amplifier 13-1, single-mode fiber 14-1, dispersion compensation fiber 15-1, and single-mode fiber 17-1 constitutes the first-stage relay mechanism, while the combination of optical amplifier 13-(N-1), single-mode fiber 14-(N-1), dispersion compensation fiber 15-(N-1), and single-mode fiber 17-(N-1) constitutes the (N-1)-stage relay mechanism. Thus, a single-mode fiber 17 is provided at the end of each relay mechanism. In addition, one single-mode fiber 16 is provided in the access section. Note that in Figure 1, only one single-mode fiber 16 and one optical receiver 12 are shown in the access section, resulting in a configuration of one optical transmitter and one optical receiver (one-to-one optical transceiver configuration). However, it is also possible to have a configuration where fibers branch off from the downstream stage of each optical amplifier 13 and connect to an access section and optical receivers not shown (one-to-many optical transceiver configuration). The same applies to the following embodiments.

[0020] In the configuration shown in Figure 1, a special wavelength dispersion occurs in the dispersion compensation fiber 15 section due to nonlinear optical effects. Therefore, it is assumed that this special wavelength dispersion due to nonlinear optical effects can be compensated for by a fiber having inverse dispersion characteristics with the dispersion compensation fiber 15 (for example, a single-mode fiber). Accordingly, in the optical transmission system 100 of the first embodiment, a new single-mode fiber 17 is provided immediately after each dispersion compensation fiber 15 section in order to compensate for the special wavelength dispersion due to nonlinear optical effects.

[0021] The signal source 11 generates a broadband signal using an FM batch conversion method. The optical transmitter 10 uses the broadband signal generated by the signal source 11 to intensity modulate the optical signal and generate a modulated signal (optical signal) in the form of an analog waveform signal (broadband signal). The optical transmitter 10 sends the generated modulated signal (optical signal) through an optical fiber.

[0022] The optical amplifier 13 amplifies the power of the input optical signal. The optical amplifier 13 is, for example, an EDFA. For example, the optical amplifier 13-1 amplifies the power of the modulated signal (optical signal) transmitted from the optical transmitter 10. The single-mode fiber 14 is provided downstream of the optical amplifier 13 and transmits the optical signal amplified by the optical amplifier 13. Due to nonlinear optical effects, wavelength dispersion occurs in the transmission of the optical signal through the single-mode fiber 14.

[0023] The dispersion compensation fiber 15 is provided downstream of the single-mode fiber 14 and compensates for chromatic dispersion that occurs during transmission in the single-mode fiber 14 section due to nonlinear optical effects. In other words, the dispersion compensation fiber 15 compensates for dispersion of side waves in the optical spectrum of the optical signal whose level has increased in the single-mode fiber 14 section. Furthermore, chromatic dispersion occurs during the transmission of the optical signal by the dispersion compensation fiber 15 due to nonlinear optical effects.

[0024] The single-mode fiber 17 is located downstream of the dispersion compensation fiber 15 and compensates for the chromatic dispersion that occurs during transmission in the dispersion compensation fiber 15 section due to nonlinear optical effects. In other words, the single-mode fiber 17 compensates for the dispersion of the side waves in the optical spectrum of the optical signal whose level has increased in the dispersion compensation fiber 15 section. In this way, the single-mode fiber 17 compensates for the special chromatic dispersion that occurs in the transmission path of the dispersion compensation fiber 15 due to nonlinear optical effects.

[0025] Here, the length of the single-mode fiber 17 is determined as follows. For example, it is desirable that the administrator measures the transmission quality (e.g., CNR and SNR (Signal Noise Ratio)) in the insertion section of the single-mode fiber 17 in advance using an optical receiver or the like, and set the length to one that improves the transmission quality based on the measurement results.

[0026] One method for determining the length of the single-mode fiber 17 in the first embodiment is to gradually increase the length of the single-mode fiber 17 by a predetermined length (for example, several meters), and determine the length of the additional single-mode fiber 17 to be the length at which the measured transmission quality obtained when the length of the single-mode fiber 17 is set to each length (for example, 10m, 20m, 30m, etc.) is maximized.

[0027] A second method for determining the length of the single-mode fiber 17 in the first embodiment involves gradually increasing the length of the single-mode fiber 17 by a predetermined length (for example, several meters), and determining the length of the additional single-mode fiber 17 to be added when the measured transmission quality obtained when the length of the single-mode fiber 17 is set to each length (for example, 10m, 20m, 30m, etc.) exceeds a preset threshold. Figure 1 shows the case where a single-mode fiber 17 of the length obtained when the preset threshold is exceeded is added (see the callout in Figure 1).

[0028] Furthermore, the insertion point of the single-mode fiber 17 may be downstream of the dispersion compensation fiber 15, or it may be downstream of the dispersion compensation fiber 15 and at a point where the transmission quality is less than or equal to threshold 1. Threshold 1 is a value smaller than the threshold.

[0029] The single-mode fiber 16 transmits the optical signal transmitted through the relay section to the optical receiver 12. The optical receiver 12 receives the optical signal transmitted by the single-mode fiber 16 and demodulates the broadband signal.

[0030] (Operation) Next, the processing flow of the optical transmission system 100 in the first embodiment will be described. The optical transmitter 10 generates a modulated signal (optical signal) based on the broadband signal generated by the signal source 11. The optical transmitter 10 sends the generated modulated signal (optical signal) to the optical fiber. The modulated signal (optical signal) sent from the optical transmitter 10 is input to the optical amplifier 13-1. The optical amplifier 13-1 amplifies the power of the input modulated signal (optical signal). The optical amplifier 13-1 sends the amplified modulated signal (optical signal) to the optical fiber. The amplified modulated signal (optical signal) sent from the optical amplifier 13-1 is input to the single-mode fiber 14-1.

[0031] The single-mode fiber 14-1 transmits the amplified modulated signal (optical signal) that is input to it. The amplified modulated signal (optical signal) transmitted through the single-mode fiber 14-1 section undergoes wavelength dispersion due to nonlinear optical effects. The amplified modulated signal (optical signal) transmitted by the single-mode fiber 14-1 is input to the dispersion compensation fiber 15-1.

[0032] The dispersion compensation fiber 15-1 compensates for the chromatic dispersion that occurred in the single-mode fiber 14-1 section by transmitting the amplified modulated signal (optical signal) that is input to it. In other words, the dispersion compensation fiber 15-1 has inverse dispersion characteristics with respect to the single-mode fiber 14-1. The amplified modulated signal (optical signal) transmitted in the dispersion compensation fiber 15-1 section experiences chromatic dispersion due to nonlinear optical effects. The amplified modulated signal (optical signal), whose chromatic dispersion has been compensated by the dispersion compensation fiber 15-1, is input to the single-mode fiber 17-1.

[0033] The single-mode fiber 17-1 compensates for the chromatic dispersion that occurred in the dispersion compensation fiber 15-1 section by transmitting the amplified modulated signal (optical signal) that is input to it. The amplified modulated signal (optical signal), which has had its chromatic dispersion compensated by the single-mode fiber 17-1, is input to the optical amplifier 13-2. The above process is then repeated. For example, if there are five single-mode fibers 14, five dispersion compensation fibers 15, and five single-mode fibers 17 in the relay section, the above process will be repeated four times.

[0034] Subsequently, the optical amplifier 13-N amplifies the power of the amplified modulated signal (optical signal), which has been wavelength dispersion compensated, through the single-mode fiber 17-(N-1). The optical amplifier 13-N sends the amplified modulated signal (optical signal) to the optical fiber. The amplified modulated signal (optical signal) sent from the optical amplifier 13-N is input to the single-mode fiber 16. The single-mode fiber 16 transmits the input amplified modulated signal (optical signal) to the optical receiver 12. The optical receiver 12 receives the amplified modulated signal (optical signal) transmitted through the single-mode fiber 16.

[0035] (Experimental Results) Next, we will explain the experimental results conducted in an actual transmission environment using Figure 2. The left figure of Figure 2 shows the CNR measurement results in the optical transmission system (conventional system) shown in Figure 6. As shown in the left figure of Figure 2, it can be seen that the CNR decreases as the transmission distance of the relay section increases. In contrast, the right figure of Figure 2 shows the result of adding a single-mode fiber 17 to the optical transmission system (conventional system) shown in Figure 6. Note that this experiment shows the result of adding a single-mode fiber 17 to the final stage of the relay section in the optical transmission system (conventional system). As shown in the right figure of Figure 2, it can be seen that the CNR is greatly improved by inserting an additional 10 km of single-mode fiber 17.

[0036] The optical transmission system 100 configured as described above includes a multi-stage relay mechanism in the relay section between the optical transmitter 10 and the optical receiver 12. The multi-stage relay mechanism includes a single-mode fiber 14, a dispersion compensation fiber 15 having dispersion characteristics opposite to those of the single-mode fiber 14 and compensating for wavelength dispersion caused by nonlinear optical effects during transmission of the single-mode fiber 14, and a single-mode fiber 17 provided downstream of the dispersion compensation fiber 15, which is an optical fiber having the same dispersion characteristics as the single-mode fiber 14, but with dispersion characteristics opposite to those of the dispersion compensation fiber 15 and compensating for wavelength dispersion caused by nonlinear optical effects during transmission of the dispersion compensation fiber 15.

[0037] Thus, the optical transmission system 100 is newly equipped with a fiber (single-mode fiber 17) having inverse dispersion characteristics with respect to the dispersion compensation fiber 15, downstream of the dispersion compensation fiber 15. This makes it possible to compensate for the special wavelength dispersion that occurs in the dispersion compensation fiber 15 due to nonlinear optical effects. As a result, it becomes possible to improve the carrier-to-noise ratio (CNR) in optical networks using the RoF method.

[0038] Furthermore, in the optical transmission system 100, the length of the additional single-mode fiber 17 is set to a length that improves the transmission quality in the section where the single-mode fiber 17 is inserted. Therefore, in each section where the single-mode fiber 17 is inserted, the special wavelength dispersion that occurs in the dispersion compensation fiber 15 due to nonlinear optical effects can be compensated more reliably. As a result, it becomes possible to improve the carrier-to-noise ratio (CNR) in an optical network using the RoF method.

[0039] (Modification 1 in the First Embodiment) In the first embodiment described above, the order of fibers through which the optical signal transmitted from the optical transmitter 10 passes was single-mode fiber 14 ⇒ dispersion compensation fiber 15. In contrast, the order of fibers through which the optical signal transmitted from the optical transmitter 10 passes may be dispersion compensation fiber 15 ⇒ single-mode fiber 14. In this configuration, the wavelength dispersion that occurs in the side waves of the optical spectrum of the optical signal whose level has increased in the dispersion compensation fiber 15 section is dispersed and compensated in the immediately following single-mode fiber 14 section, but the wavelength dispersion that occurs in the side waves of the optical spectrum of the optical signal whose level has increased in the single-mode fiber 14 section is not dispersed and compensated. Therefore, in this case, the fiber inserted additionally after the single-mode fiber 14 is a dispersion compensation fiber. This will be explained in detail below.

[0040] Figure 3 shows an example of the configuration of an optical transmission system 100a in a modified version of the first embodiment. The optical transmission system 100a includes an optical transmitter 10, a signal source 11, an optical receiver 12, an optical amplifier 13, a single-mode fiber 14, a dispersion compensation fiber 15, a single-mode fiber 16, and a dispersion compensation fiber 18.

[0041] As shown in Figure 3, the relay section is equipped with N optical amplifiers 13, (N-1) dispersion compensation fibers 15, (N-1) single-mode fibers 14, and (N-1) dispersion compensation fibers 18. For example, the optical amplifiers 13, dispersion compensation fibers 15, single-mode fibers 14, and dispersion compensation fibers 18 are provided in the relay section in the order of optical amplifier 13-1, dispersion compensation fiber 15-1, single-mode fiber 14-1, dispersion compensation fiber 18-1, ..., optical amplifier 13-(N-1), dispersion compensation fiber 15-(N-1), single-mode fiber 14-(N-1), dispersion compensation fiber 18-(N-1), and optical amplifier 13-N.

[0042] In the optical transmission system 100a shown in Figure 3, the order in which the single-mode fiber 14 and dispersion compensation fiber 15 are installed is reversed compared to the optical transmission system 100. Therefore, in the configuration shown in Figure 3, a special wavelength dispersion due to nonlinear optical effects occurs in the single-mode fiber 14 section. It is assumed that this special wavelength dispersion due to nonlinear optical effects can be compensated for by a fiber with inverse dispersion characteristics (e.g., a dispersion compensation fiber) to the single-mode fiber 14. Accordingly, in the optical transmission system 100a, a new dispersion compensation fiber 18 is provided immediately after each single-mode fiber 14 section to compensate for the special wavelength dispersion due to nonlinear optical effects.

[0043] In the case of such a configuration, the dispersion compensation fiber 15 is provided at the subsequent stage of the optical amplifier 13 and transmits the optical signal amplified by the optical amplifier 13. Due to the transmission of the optical signal by the dispersion compensation fiber 15, wavelength dispersion caused by the non-linear optical effect occurs. The single-mode fiber 14 is provided at the subsequent stage of the dispersion compensation fiber 15 and compensates for the wavelength dispersion generated in the dispersion compensation fiber 15. That is, the single-mode fiber 14 compensates for the side waves of the optical spectrum of the optical signal whose level has increased in the dispersion compensation fiber 15 section. Thus, the single-mode fiber 14 compensates for the special wavelength dispersion that occurs in the transmission path of the dispersion compensation fiber 15 due to the non-linear optical effect. Also, due to the transmission of the optical signal by the single-mode fiber 14, wavelength dispersion caused by the non-linear optical effect occurs.

[0044] The dispersion compensation fiber 18 is provided at the subsequent stage of the single-mode fiber 14 and compensates for the wavelength dispersion generated by the transmission in the single-mode fiber 14 section. That is, the dispersion compensation fiber 18 compensates for the side waves of the optical spectrum of the optical signal whose level has increased in the single-mode fiber 14 section. Here, the length of the dispersion compensation fiber 18 is determined by the same method as the method (the first method or the second method) for determining the length of the single-mode fiber 17 in the first embodiment described above. Also, the insertion point of the dispersion compensation fiber 18 may be at the subsequent stage of the single-mode fiber 14, or may be at the subsequent stage of the single-mode fiber 14 and at a point where the transmission quality becomes threshold 1 or less.

[0045] (Modification 2 in the First Embodiment) In the first embodiment described above, a configuration was described in which a single-mode fiber 17 is newly inserted immediately after the dispersion compensation fiber 15 in each relay mechanism. In Modification 1 described above, a configuration was described in which a dispersion compensation fiber 18 is newly inserted immediately after the single-mode fiber 14 in each relay mechanism. In contrast, the configuration may be made to shorten the length of the existing optical fiber without adding a new optical fiber. In the configuration shown in Figure 1, for example, instead of newly inserting a single-mode fiber 17 immediately after the dispersion compensation fiber 15 in each relay mechanism, the configuration may be made to shorten the length of the single-mode fiber 14 in each relay mechanism. In the configuration shown in Figure 3, for example, instead of newly inserting a dispersion compensation fiber 18 immediately after the single-mode fiber 14 in each relay mechanism, the configuration may be made to shorten the length of the dispersion compensation fiber 15 in each relay mechanism. The length to which the single-mode fiber 14 or dispersion compensation fiber 15 is shortened is determined based on the method for determining the length of the single-mode fiber 17 in the first embodiment (the first method or the second method). However, the method for determining the length of the single-mode fiber 17 in the first embodiment (the first method or the second method) is to gradually increase the length of the fiber by a predetermined length, whereas here it differs in that the length of the fiber is gradually shortened by a predetermined length.

[0046] (Modification Example 3 in the First Embodiment) In the above-described embodiment, for each section of the relay mechanism, it may be determined whether to add a new fiber or shorten the length of an existing fiber, and the addition of a new fiber or the shortening of the length of an existing fiber may be performed for each section of the relay mechanism. The addition of a new fiber as referred to here is the addition of the single-mode fiber 17 described in FIG. 1 or the dispersion compensation fiber 18 described in FIG. 3. The shortening of the length of an existing fiber is the shortening of the length of the single-mode fiber 14 or the dispersion compensation fiber 15 described in Modification Example 2. Whether to perform the addition of a new fiber or the shortening of the length of an existing fiber may be determined by measuring the change in transmission quality in each section of the relay mechanism, and the method that improves the transmission quality more may be selected for each section of the relay mechanism.

[0047] (Second Embodiment) In the first embodiment, the configuration in which a single-mode fiber (or a dispersion compensation fiber) is newly inserted immediately after the dispersion compensation fiber (or the single-mode fiber) in each relay mechanism has been described. On the other hand, a configuration in which the wavelength dispersion generated in a plurality of relay mechanisms is compensated collectively is also assumed. Therefore, in the second embodiment, the configuration in which the wavelength dispersion generated in a plurality of relay mechanisms is compensated collectively will be described.

[0048] FIG. 4 is a diagram showing a configuration example of the optical transmission system 100b in the second embodiment. The optical transmission system 100b includes an optical transmitter 10, a signal source 11, an optical receiver 12, an optical amplifier 13, a single-mode fiber 14, a dispersion compensation fiber 15, a single-mode fiber 16, and a single-mode fiber 17.

[0049] In the second embodiment, the relay section is equipped with N optical amplifiers 13, (N-1) single-mode fibers 14, (N-1) dispersion compensation fibers 15, and (N-2) single-mode fibers 17. In Figure 4, an example is shown where there are (N-2) single-mode fibers 17, but the number of single-mode fibers 17 is not limited to this. The number of single-mode fibers 17 changes depending on the number of dispersion compensation fiber sections that are compensated together. In the optical transmission system 100b, the number of single-mode fibers 17 should be less than the number of dispersion compensation fibers 15.

[0050] In the second embodiment, the relay section also has a multi-stage relay mechanism, similar to the first embodiment. However, unlike the first embodiment, the relay mechanism in the second embodiment has two configurations: a first relay mechanism and a second relay mechanism. The first relay mechanism consists of a combination of one optical amplifier 13, one single-mode fiber 14, and one dispersion compensation fiber 15. That is, the first relay mechanism comprises one optical amplifier 13, one single-mode fiber 14, and one dispersion compensation fiber 15. The second relay mechanism consists of a combination of one optical amplifier 13, one single-mode fiber 14, one dispersion compensation fiber 15, and one single-mode fiber 17. That is, the second relay mechanism comprises one optical amplifier 13, one single-mode fiber 14, one dispersion compensation fiber 15, and one single-mode fiber 17.

[0051] As shown in Figure 4, the relay section is provided, for example, with optical amplifiers 13, single-mode fibers 14, dispersion compensation fibers 15, and single-mode fibers 17 in the order of optical amplifier 13-1, single-mode fiber 14-1, dispersion compensation fiber 15-1, single-mode fiber 17-1, optical amplifier 13-2, single-mode fiber 14-2, dispersion compensation fiber 15-2, single-mode fiber 17-1, ..., optical amplifier 13-N.

[0052] The single-mode fiber 17-1 compensates for the special wavelength dispersion generated during optical signal transmission by the dispersion compensation fiber 15-1 provided in the first relay mechanism, and the special wavelength dispersion generated during optical signal transmission by the dispersion compensation fiber 15-2 provided in the second relay mechanism.

[0053] As described above, the single-mode fiber 17 in the second embodiment is provided after the multiple stages of dispersion compensation fibers 15, and collectively compensates for the chromatic dispersion generated during transmission in each of the dispersion compensation fiber 15 sections provided in the first relay mechanism (e.g., dispersion compensation fiber 15-1) and the dispersion compensation fiber 15 sections provided in the second relay mechanism (e.g., dispersion compensation fiber 15-2). In other words, the single-mode fiber 17 provides dispersion compensation for the side waves of the optical spectrum of the optical signal whose level has increased in the multiple dispersion compensation fiber 15 sections. The method for determining the length of the additional single-mode fiber 17 is the same as in the first embodiment.

[0054] With the optical transmission system 100b configured as described above, it is not necessary to provide an optical fiber downstream of each dispersion compensation fiber 15 to compensate for the special wavelength dispersion that occurs in the transmission path of the dispersion compensation fiber 15 due to nonlinear optical effects, as in the first embodiment. Therefore, the optical transmission system 100b can reduce the number of single-mode fibers 17 compared to the first embodiment. As a result, it is possible to obtain the same effects as the first embodiment while keeping the overall system cost down.

[0055] (Modification 1 in the second embodiment) In the optical transmission system 100b, similar to the first embodiment, the order in which the optical signal transmitted from the optical transmitter 10 passes through the fibers may be, instead of single-mode fiber 14 ⇒ dispersion compensation fiber 15, dispersion compensation fiber 15 ⇒ single-mode fiber 14. In this configuration, the optical transmission system 100b includes dispersion compensation fibers 18 to compensate for the chromatic dispersion that occurs in the side waves of the optical spectrum of the optical signal whose level has increased in each of the multiple single-mode fiber 14 sections. The dispersion compensation fibers 18 are provided after the multiple stages of single-mode fibers 14 and compensate for the chromatic dispersion that occurs in each of the multiple single-mode fibers 14 collectively. The method for determining the length of the additional dispersion compensation fibers 18 is the same as in the first embodiment.

[0056] (Modification 2 in the second embodiment) The optical transmission system 100b may be configured to shorten the length of an existing optical fiber instead of adding a new optical fiber, as in Modification 2 in the first embodiment.

[0057] (Modification 3 in the second embodiment) The optical transmission system 100b may decide whether to add a new fiber or shorten the length of an existing fiber in a section of a relay mechanism, similar to Modification 3 in the first embodiment, and may add a new fiber or shorten the length of an existing fiber in any section of a relay mechanism. In the optical transmission system 100b, a new fiber is not added in each section of a relay mechanism, as in the first embodiment. Therefore, the optical transmission system 100b may decide whether to add a new fiber or shorten the length of an existing fiber in each section containing multiple relay mechanisms, and may add a new fiber or shorten the length of an existing fiber in each section containing multiple relay mechanisms.

[0058] (Third Embodiment) In the first embodiment, transmission quality was used as an example of a measurement target for determining the length of a new optical fiber (single-mode fiber or dispersion-compensated fiber) to be added to the relay section. In the third embodiment, a configuration in which optical modulation is used as a measurement target for determining the length of a new optical fiber (single-mode fiber or dispersion-compensated fiber) to be added to the relay section will be described.

[0059] Figure 5 shows an example configuration of the optical transmission system 100c in the third embodiment. The optical transmission system 100c comprises an optical transmitter 10, a signal source 11, an optical receiver 12, an optical amplifier 13, a single-mode fiber 14, a dispersion compensation fiber 15, a single-mode fiber 16, and a single-mode fiber 17. The configuration of the optical transmission system 100c is the same as in the first embodiment.

[0060] In the optical transmission system 100c of the third embodiment, the method for determining the length of the additional single-mode fiber 17 differs from that of the first embodiment. For example, in the optical transmission system 100c of the third embodiment, the optical modulation index is used as an indicator for determining the length of the additional single-mode fiber 17. In this case, it is desirable for the administrator to measure the optical modulation index in the insertion section of the single-mode fiber 17 in advance using an optical receiver or the like, and to set the length to one that is considered to improve the optical modulation index based on the measurement results.

[0061] One method for determining the length of the single-mode fiber 17 in the third embodiment is to gradually increase the length of the single-mode fiber 17 by a predetermined length (for example, several meters), and determine the length of the additional single-mode fiber 17 to be the one at which the measured value of the optical modulation degree obtained when the length of the single-mode fiber 17 is set to each length (for example, 10m, 20m, 30m, ..., etc.) is maximized.

[0062] A second method for determining the length of the single-mode fiber 17 in the third embodiment is to gradually increase the length of the single-mode fiber 17 by a predetermined length (for example, several meters), and determine the length of the additional single-mode fiber 17 when the measured optical modulation depth obtained when the length of the single-mode fiber 17 is set to each length (for example, 10m, 20m, 30m, etc.) exceeds a preset threshold. Figure 5 shows the case where a single-mode fiber 17 of the length obtained when the preset threshold is exceeded is added (see the callout in Figure 5).

[0063] Furthermore, the insertion point of the single-mode fiber 17 may be downstream of the dispersion compensation fiber 15, or it may be downstream of the dispersion compensation fiber 15 and at a point where the optical modulation degree is less than or equal to threshold 2. Threshold 2 is a value smaller than the threshold.

[0064] With the optical transmission system 100c configured as described above, the same effects as in the first embodiment can be obtained.

[0065] (Modification 1 in the Third Embodiment) Similar to the first embodiment, the optical transmission system 100c may have a fiber sequence through which the optical signal transmitted from the optical transmitter 10 passes, not single-mode fiber 14 ⇒ dispersion compensation fiber 15 ⇒ single-mode fiber 14. In this configuration, the optical transmission system 100c includes a dispersion compensation fiber 18 for compensating for the chromatic dispersion that occurs in the sidewaves of the optical spectrum of the optical signal whose level has increased in the single-mode fiber 14 section. The dispersion compensation fiber 18 is provided downstream of each single-mode fiber 14 and compensates for the chromatic dispersion that occurs in the single-mode fiber 14. Here, the length of the dispersion compensation fiber 18 is determined by the same method as the method for determining the length of the single-mode fiber 17 in the third embodiment described above (the first or second method). The insertion point of the dispersion compensation fiber 18 may be downstream of the single-mode fiber 14, or it may be downstream of the single-mode fiber 14 and at a point where the optical modulation degree is less than or equal to a threshold of 2.

[0066] (Modification 2 in the Third Embodiment) The optical transmission system 100c may be configured to shorten the length of existing optical fibers instead of adding new optical fibers, as in Modification 2 in the First Embodiment. In the configuration shown in Figure 5, for example, instead of inserting a new single-mode fiber 17 immediately after the dispersion compensation fiber 15 in each relay mechanism, the configuration may be made to shorten the length of the single-mode fiber 14 in each relay mechanism. In the configuration shown in Modification 1 in the Third Embodiment, for example, instead of inserting a new dispersion compensation fiber 18 immediately after the single-mode fiber 14 in each relay mechanism, the configuration may be made to shorten the length of the dispersion compensation fiber 15 in each relay mechanism. The length to which the single-mode fiber 14 or dispersion compensation fiber 15 is shortened is determined based on the method for determining the length of the single-mode fiber 17 in the Third Embodiment (the first method or the second method). However, the method for determining the length of the single-mode fiber 17 in the Third Embodiment (the first method or the second method) is a method of gradually increasing the length of the fiber by a predetermined length, but here it differs in that the length of the fiber is gradually shortened by a predetermined length.

[0067] (Modification 3 in the third embodiment) The optical transmission system 100c may decide whether to add a new fiber or shorten the length of an existing fiber in a section of the relay mechanism, similar to Modification 3 in the first embodiment, and may add a new fiber or shorten the length of an existing fiber in any section of the relay mechanism. Whether to add a new fiber or shorten the length of an existing fiber can be determined by measuring the change in optical modulation in each section of the relay mechanism and selecting the option that better improves the optical modulation for each section of the relay mechanism.

[0068] (Modifications common to the first to fourth embodiments) In the embodiments described above, the application to optical networks using the RoF method, which requires particular attention in conventional configurations, was explained as an example. However, the application is not limited to optical networks using the RoF method, and may also be to other general optical networks including PON configurations and SS configurations.

[0069] While embodiments of this invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and includes designs and the like that do not depart from the spirit of this invention.

[0070] This invention can be applied to optical transmission systems that utilize the RoF transmission method.

[0071] 10…Optical transmitter, 11…Signal source, 12…Optical receiver, 13, 13-1 to 13-N…Optical amplifier, 14, 14-1 to 14-(N-1), 16, 17, 17-1 to 17-(N-1)…Single-mode fiber, 15, 15-1 to 15-(N-1), 18, 18-1 to 18-(N-1)…Dispersion-compensated fiber, 100, 100a, 100b, 100c…Optical transmission system

Claims

1. An optical transmission system comprising a multi-stage relay mechanism in a relay section between an optical transmitter and an optical receiver, wherein the multi-stage relay mechanism comprises: a first optical fiber for transmitting an optical signal; and a second optical fiber having dispersion characteristics opposite to those of the first optical fiber and compensating for wavelength dispersion caused by nonlinear optical effects during transmission of the first optical fiber, and wherein at least one of the multi-stage relay mechanisms further comprises: a third optical fiber provided downstream of the second optical fiber, having the same dispersion characteristics as the first optical fiber, having dispersion characteristics opposite to those of the second optical fiber, and compensating for wavelength dispersion caused by nonlinear optical effects during transmission of the second optical fiber, or the first optical fiber having a length that satisfies the condition that the measurement result of an index in the section comprising the first optical fiber is equal to or greater than a threshold.

2. In the case where at least one of the multi-stage relay mechanisms includes the third optical fiber, the multi-stage relay mechanism is composed of a first relay mechanism including the first optical fiber and the second optical fiber, and a second relay mechanism including the first optical fiber, the second optical fiber and the third optical fiber, wherein the third optical fiber provided in the second relay mechanism compensates for wavelength dispersion caused in the transmission of the second optical fiber due to nonlinear optical effects in the first and second relay mechanisms, respectively, the optical transmission system according to claim 1.

3. In the case where at least one of the multi-stage relay mechanisms is equipped with the third optical fiber, the length of the third optical fiber provided in the one or more relay mechanisms is such that the measurement result of the index in the insertion section of the third optical fiber is maximized, or the measurement result is equal to or greater than a predetermined threshold, the optical transmission system according to claim 1 or 2.

4. A wavelength dispersion compensation method for an optical transmission system that includes a multi-stage relay mechanism in a relay section between an optical transmitter and an optical receiver, wherein the multi-stage relay mechanism comprises a first optical fiber and a second optical fiber, the first optical fiber transmits an optical signal, the second optical fiber has dispersion characteristics opposite to those of the first optical fiber and compensates for wavelength dispersion caused by nonlinear optical effects during transmission of the first optical fiber, and at least one of the multi-stage relay mechanisms includes a third optical fiber, or the first optical fiber having a length that satisfies the condition that the measurement result of an index in the section containing the first optical fiber is equal to or greater than a threshold, and if at least one of the relay mechanisms includes the third optical fiber, the third optical fiber is provided downstream of the second optical fiber, has the same dispersion characteristics as the first optical fiber, has dispersion characteristics opposite to those of the second optical fiber, and compensates for wavelength dispersion caused by nonlinear optical effects during transmission of the second optical fiber.

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