JP2011250037A - Polarization multiplexing optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization multiplexing optical transmission system, capable of reducing an influence of PDL (polarization-dependent loss).SOLUTION: In the polarization multiplexing optical transmission system performing transmission between a first node and a second node by multiplexing two optical signals having an identical wavelength using mutually orthogonal polarized waves, the first node includes: a receiving unit for obtaining status information of the two optical signals transmitted from the second node; and a transmitting unit for transmitting the status information to the second node. Also, the second node includes: a receiving unit for receiving the status information transmitted from the first node; and based on the status information, a control processing unit for controlling the incident power level difference between the two optical signals input to a transmission path to be transmitted to the first node.

Description

  The present invention relates to a polarization multiplexed optical transmission system. In particular, the present invention relates to a polarization multiplexed optical transmission system that reduces the influence of polarization dependent loss (PDL) existing in an optical transmission line or an optical node on polarization multiplexed optical transmission.

  In conventional all-optical networks, optical signals are transmitted with a single polarization. In recent years, the introduction of a polarization multiplexing scheme in addition to wavelength multiplexing has been studied in order to increase the transmission speed and the efficiency of bandwidth utilization. This polarization multiplexing method utilizes the fact that two polarization states that are orthogonal to each other exist at the same wavelength, and transmits and receives two independent signals corresponding to these two polarization states. Two signals are obtained by performing polarization separation on the side.

  By the way, in an actual optical transmission line, there is a polarization dependent loss (PDL), and it is known that the signal light is excessively attenuated depending on the polarization state of the signal light. However, in a single-polarization optical transmission system, this attenuation due to PDL can be compensated by the optical node.

  However, in the case of the polarization multiplexing system, one of the two optical signals is greatly reduced in level at the receiving end, and there is a fear that good transmission characteristics cannot be obtained (see Non-Patent Document 1). This will be described in detail below.

  The polarization multiplexing method is a method in which an optical signal having the same wavelength is transmitted by polarization multiplexing at the transmission source in a state where the polarization is orthogonal. FIG. 1 is a diagram for explaining the influence of PDL on a polarization multiplexed optical signal. As shown in FIG. 1A, at the transmission source, two optical signals are polarized and multiplexed into two polarizations (X polarization and Y polarization) orthogonal to each other. If, for example, the direction of X polarization coincides with the loss axis (horizontal axis in FIG. 1) of the PDL of the optical transmission line, the X polarization is attenuated after propagation as shown in FIG. The greater the PDL, the greater the attenuation of X polarization.

T. Duthel et al., "Impact of polarization dependent loss on coherent POLMUX-NRZ-DQPSK", OThU5, OFC / NFOEC2008, 2008. Kazuaki Kikuchi, “Basics of Polarization Demultiplexing and Polarization Dispersion Compensation Technology Using a Digital Coherent Optical Receiver”, OCS2009-T01, OCS Second Class Study Group 1 “New Optical Transmission Technology Using Digital Signal Processing”, July 2009. T. Matsuda et al., "Field trial of 43-Gbit / s RZ-DQPSK transmission in aerial fiber with rapidly changing SOP", NWD1, OFC / NFOEC2009, 2009.

  With reference to FIG. 2 and FIG. 3, a change in power level when an optical signal is transmitted through an optical transmission line will be described. FIG. 2 is a diagram for explaining a change in power level in the case of single polarization optical transmission. Consider the case where an optical signal is transmitted from node A to node C in FIG. In the case of non-polarization multiplexing in the single polarization method as shown in FIG. 2, the channel is not defined by the polarization, and is in a single polarization state. Even if it is excessively attenuated by the PDL of the optical transmission line (between BC in FIG. 2B), it can be restored to a predetermined level by the optical amplifier of the optical node (position D in FIG. 2B). Then it is sent to the next node. However, at the receiving end (position F in FIG. 2B), there is a possibility of being affected by the PDL in the optical transmission path between node B and node C (between DE in FIG. 2B).

  FIG. 3 is a diagram for explaining a change in power level in the case of polarization multiplexed optical transmission. In the case of the polarization multiplexing system as shown in FIG. 3, the current optical node is controlled so that the sum of the powers of optical signals in two different polarization states is constant. For this reason, even if the power of either channel is excessively attenuated and reduced by PDL, the power is not recovered sufficiently (position D in FIG. 3B) and is sent to the next node. (In FIG. 3B, the X polarization is not recovered). Furthermore, if the signal light (X-polarized wave in FIG. 3B) again suffers excessive attenuation (between DE in FIG. 3B), the optical signal level becomes extremely small at the receiving end, which is necessary. An optical signal noise ratio (OSNR) may not be obtained.

  In addition, when the level deviation between two signals is large in this way, it is expected that polarization separation at a coherent receiver will be difficult in a digital coherent system that has been developed recently (see Non-Patent Document 2). ). As described above, when the polarization multiplexing method is introduced into the all-optical network, the transmission characteristics may be deteriorated due to the PDL of the optical transmission line if only the current technology is applied. In addition, since there are some PDLs in the parts constituting the optical node itself, it is considered that the influence cannot be ignored when the optical signal passes through many optical nodes.

  As the above solution, for example, polarization separation is performed by some means for each channel (wavelength) at each optical node, and control is performed so that the levels of X polarization and Y polarization are the same, and then polarization multiplexing is performed again. However, it is conceivable to send it to the optical transmission line. However, in this case, since the function is implemented for every several tens of channels (wavelengths), it is considered that each node becomes complicated and the cost is significantly increased. Therefore, it is desirable to reduce the influence of PDL by simple means.

  An object of the present invention is to provide a polarization multiplexed optical transmission system capable of reducing the influence of PDL with a simple configuration.

The polarization multiplexed optical transmission system of the present invention,
A polarization multiplexing optical transmission system that multiplexes and transmits two optical signals having the same wavelength with mutually orthogonal polarizations between a first node and a second node,
The first node is
A receiving unit for acquiring state information of two optical signals transmitted from the second node;
A transmitter for transmitting the state information to the second node;
Have
The second node is
A receiving unit for receiving the state information transmitted from the first node;
A control processing unit for controlling a difference in level of incident power to a transmission path of two optical signals to be transmitted to the first node based on the state information;
It is characterized by having.

  According to the present invention, it is possible to provide a polarization multiplexed optical transmission system capable of reducing the influence of PDL with a simple configuration.

Diagram for explaining the effect of PDL on polarization multiplexed optical signals Diagram for explaining the change in power level in the case of single polarization optical transmission The figure for explaining the change of the power level in the case of the polarization multiplexing optical transmission Diagram showing an example of an all-optical network in a bidirectional communication state The figure which shows the example of the all-optical network of the two-way communication state by the polarization multiplexing transmission system Diagram showing an example configuration of an optical node with an optical add / drop function The figure which shows the concept of the polarization multiplexing optical transmission system which concerns on embodiment of this invention The figure which shows the example of the polarization multiplexing transponder function part in an optical node The figure which shows the example of the polarization multiplexing transponder function part in the optical node which concerns on embodiment of this invention The block diagram which shows the example of the control process part in the optical node which concerns on embodiment of this invention

  Hereinafter, embodiments of the present invention will be described in detail.

  In the embodiment of the present invention, in polarization multiplexed optical transmission in which two optical signals having the same wavelength are multiplexed with polarizations orthogonal to each other and transmitted through the transmission line, in order to reduce or alleviate the influence of the PDL of the transmission line, The receiving side acquires the status information (power level, bit error rate, optical signal-to-noise ratio, Q value, etc.) of the two optical signals, and based on the acquired status information to the transmission path of the two optical signals of the transmission source Controls the level difference of the incident power. For example, the reception levels of two signals are compared, the level of the lower signal is increased at the transmission source, and control is performed with the goal of matching the power levels at the time of reception.

  In order to control the level difference at the transmission source, it is necessary to send the status information of the two signals on the reception side to the transmission source. Here, in general, the polarization state of the signal light propagating through the optical fiber fluctuates with time (see Non-Patent Document 3), and therefore, the influence of the PDL of the transmission path on the optical signal also changes with time. The level varies with time. Therefore, it is necessary for the receiving side to sequentially transmit state information, and it is necessary to prepare an optical path for transmitting control information in addition to a normal optical path. In the case of long-distance to ground transmission where the optical transmission line is fixed, it is easy to prepare a separate optical path for control information, but an optical add / drop system that frequently adds and deletes optical paths such as metro networks In this case, it is practically difficult to prepare a separate optical path for control information.

  In general, in an all-optical network, when an optical path is generally set from one node to another node, the optical path is set in both directions and is in a bidirectional communication state. FIG. 4 shows a schematic diagram of bidirectional communication in a general all-optical network. The upper part of FIG. 4 shows an optical path from the node A to the node C, and an optical signal is transmitted from the transmission unit Tx of the node A to the reception unit Rx of the node C. Similarly, the lower part of FIG. 4 shows an optical path from the node C to the node A, and an optical signal is transmitted from the transmission unit Tx of the node C to the reception unit Rx of the node A.

  The configuration of the polarization multiplexed optical transmission system is also as shown in FIG. The upper part of FIG. 5 shows an optical path from the node A to the node C, and the deviation of the node C from the transmission unit Tx (X) for X polarization and the transmission unit Tx (Y) for Y polarization of the node A is shown. An optical signal is transmitted to the receiver Rx (X / Y) with a wave separation function. Similarly, the lower part of FIG. 5 shows an optical path from the node C to the node A, and the node C transmits the X polarization transmission unit Tx (X) and the Y polarization transmission unit Tx (Y) to the node. The optical signal is transmitted to the receiving unit Rx (X / Y) with the polarization separation function of A.

  As described above, the transmission unit Tx and the reception unit Rx are provided in one node, and the transmission unit Tx and the reception unit Rx are usually 1 as a transponder function unit in an optical node having an optical add / drop function as shown in FIG. Contained in one package. Note that FIG. 6 shows the case of single polarization optical transmission, but similarly in the case of polarization multiplexed optical transmission, the transmission unit and the reception unit are accommodated in one package. Therefore, it is possible to transfer the state information obtained by the receiving unit to the transmitting unit.

<Configuration of Polarization Multiplexed Optical Transmission System According to Embodiment of the Present Invention>
FIG. 7 is a diagram showing a concept of the polarization multiplexed optical transmission system according to the embodiment of the present invention. As described with reference to FIG. 5, also in the polarization multiplexed optical transmission system, one node (node A, node C) has a transmission unit Tx (X) (11A, 21C) for X polarization and a Y polarization. There are a wave transmitting unit Tx (Y) (12A, 22C) and a polarization multiplexing unit POLMUX (13A, 23C), and a receiving unit Rx (X / Y) (10A, 20C) with a polarization separation function. . In the embodiment of the present invention, when an optical path is set between the node A and the node C, the optical path (A → C) from the node A to the node C as shown in FIG. The reception unit Rx (X / Y) (20C) detects the state of the two signals, and immediately sends the state information to the transmission unit Tx (X), Tx (Y) (21C, 22C) of the same node C. Are transmitted toward the receiving unit Rx (X / Y) (10A) of the node A. When the receiving unit Rx (X / Y) (10A) of the node A receives the state information together with the normal transmission signal, it immediately sends it to the transmitting units Tx (X), Tx (Y) (11A, 12A) of the same node A, Transmitters Tx (X) and Tx (Y) (11A, 12A) control the output of transmission light. In this way, the influence of PDL can be reduced or alleviated corresponding to the polarization fluctuation of the signal light. The optical path from node C to node A (C → A) is processed in the same way, and both paths can reduce the influence of PDL.

  In this way, in the case of bidirectional communication, since the state information of the signal light can be sent together with the transmission signal using the opposite optical path, the state information can be fed back at a high speed, resulting in high-speed polarization fluctuations. Yes. Note that the state information of the signal light does not necessarily have to be transmitted together with a normal transmission signal, and may be transmitted by a route other than the opposing optical path, for example, a control information path.

  The difference from a typical polarization multiplexed optical transmission system is the configuration and function of a transponder function unit (integration unit). FIG. 8 shows an example of a transponder function unit in a typical optical node. The part that performs the transmission function of the transponder includes a transmission unit Tx (X) (11A, 21A) for X polarization and a transmission unit Tx (Y) (12A, 22A) for Y polarization. Multiplexer POLMUX (13A, 23A) is implemented. Unlike the single-polarization transponder receiving unit, the receiving unit Rx (X / Y) (10A, 20A) has a polarization separation function. Normally, the transmission units Tx (X), Tx (Y) and the reception unit Rx (X / Y) of the transponder do not exchange information with each other.

  FIG. 9 shows an example of the transponder function unit in the optical node according to the embodiment of the present invention. Here, the transponder function unit of the node A is illustrated. The receiving unit Rx (X / Y) (20A) of the transponder function unit acquires the state information of the two signals transmitted from the ground node. A power level may be used as the state information. In addition to the power level, an optical signal-to-noise ratio (OSNR), an error rate, a Q value, and the like correlate with the power level of the signal and can be used as status information for controlling the transmission output. Any one of such state information may be used, and any combination may be used.

  This state information is used for level control of two signals transmitted from the ground node. For this reason, the control processing unit (24A) of the transponder function unit transmits the acquired state information to the transmission units Tx (X), Tx (Y) (21A, 22A) in the same package. The control processing unit (24A) converts this state information into a signal and transmits it along with transmission signals transmitted from the transmitting units Tx (X) and Tx (Y) (21A, 22A) to the ground. In FIG. 9, the state information is transmitted from the receiving unit Rx (X / Y) (20A) to the transmitting units Tx (X), Tx (Y) (21A, 22A) via the control processing unit (24A). However, the state information may be directly transmitted from the reception unit Rx (X / Y) (20A) to the transmission units Tx (X) and Tx (Y) (21A, 22A).

  On the other hand, the state information of the signal of the node transmitted to the ground is sent together with the normal transmission signal from the ground node, and is received by the receiving unit Rx (X / Y) (10A). The control processing unit (14A) extracts the state information and sends it to the transmission units Tx (X), Tx (Y) (11A, 12A). The transmission units Tx (X), Tx (Y) (11A, 12A) control the output of the X polarization and the Y polarization of their transmission light based on this information, and transmit toward the ground. At that time, for example, a variable optical attenuator (VOA) (15A, 16A) as shown in FIG. 9 may be used to adjust the incident power to the transmission path of the two optical signals. Specifically, instead of the transmitters Tx (X), Tx (Y) (11A, 12A) directly controlling the output of the X polarization and the Y polarization, the control processing unit (14A) is controlled by the variable attenuator (15A). 16A), the output of the X polarization and the Y polarization may be controlled.

  Although FIG. 9 shows the polarization multiplexing transponder function unit in one optical node (node A), the ground optical node (node C) is similarly configured. That is, the node C includes the same functional units 10C to 16C and 20C to 26C as the node A.

  FIG. 10 shows an example of a control processing unit in the optical node according to the embodiment of the present invention. The upper control processing unit (14A) in FIG. 10 corresponds to the transmission source control processing unit, and the lower control processing unit (24C) in FIG. 10 corresponds to the reception side control processing unit. Due to the polarization fluctuation as disturbance, the reception power Px of the X polarization and the reception power Py of the Y polarization change in the reception unit Rx (X / Y) (20C). However, since the sum of the powers of both polarizations is normally controlled at each optical node, increase / decrease in Px and increase / decrease in Py are reversed. That is, for example, when Px increases, Py decreases accordingly. That is, the level ratio of Px and Py varies. The current ΔP is acquired by the signal processing circuit (27C) from the reception powers Px and Py at the reception unit Rx (X / Y) (20C). The control circuit (18A) sets the target set value as ΔP = Px−Py = 0 and determines the transmission output. The determined transmission output is used for output control of the VOA (15A) via the VOA driver (19A). Both X polarization and Y polarization are affected by PDL, but usually the total sum of X polarization and Y polarization is kept constant at each optical node on the optical path as described above. Otherwise, output control only for X polarization may be performed. Also, if there is no PDL at all, if the power of X polarization and Y polarization is equal at the time of transmission, the power of X polarization and Y polarization is essentially the same at the receiver, but that power is the target setting value, The transmission output may be determined by the control circuit (18A) using the difference from the value of Px acquired from the signal processing unit (27C). By controlling the level difference or level ratio between Px and Py in this way, the influence of polarization fluctuation can be mitigated.

  In the case of the digital coherent system, polarization separation is performed in the transponder reception unit (coherent receiver) by using optical polarization separation means such as a polarization beam splitter in combination with digital signal processing. However, since there is a large level difference between the two optical signals when the optical path is set, there is a possibility that the polarization separation operation by digital signal processing cannot be performed at first. In that case, first, only the optical polarization separation means may be used to obtain the state information of two rough optical signals, and the state information may be transmitted to the transmission source. The transmission source controls the levels of the two optical signals at the transmission source up to an area where digital signal processing is possible based on the state information. When the level difference between the two optical signals becomes equal to or less than a predetermined threshold value, the actual operation may be started by switching to the combined use of optical polarization separation means and digital signal processing at the receiving unit.

<Effect of embodiment>
As described above, according to the embodiments of the present invention, a polarization multiplexed optical transmission system that can reduce or mitigate the influence of the polarization dependent loss (PDL) of the optical transmission line on the transmission characteristics can be simplified. And can be useful in future all-optical networks.

  In order to reduce or mitigate the effects of PDL, polarization separation is performed for each relay node, control is performed so that the levels of X polarization and Y polarization are the same, and then polarization multiplexing is performed again to obtain an optical transmission line. A solution of sending to the network is also possible. However, when such a PDL compensation function is implemented in each relay node, it is necessary to implement a PDL compensation function for each wavelength. This is because the influence of PDL varies because each wavelength passes through a different optical path. In the embodiment of the present invention, the PDL compensation function is not implemented in each relay node, but the polarization output is controlled between the transmission source and the transmission destination. It can be mitigated and the cost of the entire system can be reduced.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications and applications are possible within the scope of the claims.

  The present invention is applied to an all-optical network, and is particularly useful for a wavelength division multiplexing optical network having an optical add / drop function.

10A, 20A, 20C Polarization separation function receiving unit 11A, 21A, 21C X polarization transmission unit 12A, 22A, 22C Y polarization transmission unit 13A, 23A, 23C Polarization multiplexing unit 14A, 24A, 24C Control processing unit 15A , 25A Variable optical attenuator 16A, 26A Variable optical attenuator 27C Signal processing circuit 18A Control circuit 19A VOA driver

Claims (5)

A polarization multiplexing optical transmission system that multiplexes and transmits two optical signals having the same wavelength with mutually orthogonal polarizations between a first node and a second node,
The first node is
A receiving unit for acquiring state information of two optical signals transmitted from the second node;
A transmitter for transmitting the state information to the second node;
Have
The second node is
A receiving unit for receiving the state information transmitted from the first node;
A control processing unit for controlling a difference in level of incident power to a transmission path of two optical signals to be transmitted to the first node based on the state information;
A polarization multiplexed optical transmission system.   2. The polarization multiplexing according to claim 1, wherein the transmission unit of the first node transmits the state information to the second node using an opposite optical path when an optical path is set in both directions. Optical transmission system.   The polarization multiplexing optical transmission system according to claim 1, wherein the control processing unit of the second node controls the level difference by controlling an attenuation amount of an optical variable attenuator.   4. The polarization multiplexed light according to claim 1, wherein the state information is at least one of a power level, a bit error rate, an optical signal-to-noise ratio, and a Q value. 5. Transmission system.   The receiving unit of the first node has a polarization separation unit by optical processing and a polarization separation unit by digital signal processing, and a level difference between two optical signals transmitted from the second node is predetermined. The polarization multiplexed optical transmission system according to any one of claims 1 to 4, wherein state information of the two optical signals is acquired by the polarization separation unit using the optical processing until the threshold value is equal to or less than a threshold value. .

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