JP6088350B2 - Optical transmission system - Google Patents

Description

  The present invention relates to an optical transmission system, and more particularly to an RF signal optical transmission system that compensates for the influence of chromatic dispersion that occurs when an RF signal is optically transmitted.

  In order to respond to the recent increase in communication traffic, high-speed and large-capacity optical transmission systems have become widespread. As a technique for realizing such a system, for example, there is a wavelength division multiplexing (WDM) transmission, a multi-level modulation technique for adding information to a plurality of states of light intensity and optical phase, and the like. Proposed. Also in the optical access network, a PON (Passive Optical Network) system that enables high-speed data communication of 1 Gbit / s per subscriber is widespread.

  In the high-speed and large-capacity optical transmission system as described above, it is desirable to provide another redundant optical line in case the optical line is physically disconnected due to a disaster or the like. However, the redundancy is lost when a line break occurs at the same time in the redundant optical line, or when a line break occurs at a plurality of locations at the same time.

  2. Description of the Related Art When an optical line failure occurs, a technique is known that bypasses an optical line failure location with a wireless line such as FWA (Fixed Wireless Access). Since wireless communication is possible in free space, it has the advantage of high disaster resistance and high portability. However, since a wireless line generally has a narrower communication band than optical fiber communication, it is difficult to realize high-speed and large-capacity communication as much as an optical transmission system.

  On the other hand, in recent years, high-speed wireless communication technology using a millimeter-wave band RF (radio frequency) carrier such as 60 GHz, such as WirelessHD and WiGig, is known. In wireless communication using such a millimeter-wave band RF carrier, a wide communication band on the order of GHz can be secured, so that high-speed wireless transmission of several Gbit / s class can be realized. Therefore, it is expected that high-speed and large-capacity backup communication can be realized by using a wireless line realized by such a wireless communication technique as a backup line when a failure occurs in an optical line.

  Further, as one of optical and wireless fusion technologies, a radio over fiber (RoF) technology is known in which an optical signal is intensity-modulated with an RF signal and transmitted over an optical fiber. In order to transmit an optical signal modulated by an RF signal in an optical transmission system using RoF technology, it is generally necessary to remove the influence of chromatic dispersion that occurs when the optical signal propagates through an optical fiber. In particular, when using double sideband (DSB) modulation for transmitting both an RF upper sideband and an RF lower sideband obtained by intensity modulation of an optical signal with an RF signal, an optical fiber transmission line is used. Due to the chromatic dispersion that occurs, RF phase fading occurs between the signals in the double sidebands. For this reason, when an RF signal is transmitted through an optical fiber transmission line using DSB modulation, it is necessary to compensate for the influence of chromatic dispersion with an appropriate amount of dispersion compensation.

  Non-Patent Document 1 describes a technique for compensating for chromatic dispersion in a system in which an optical fiber is shared between an optical wave fusion master station and a PON. Specifically, by disposing a dispersion adder in the optical wave fusion master station and giving appropriate chromatic dispersion in advance, radio signals that interfere with the subscriber side terminal of the PON system are removed, and the optical wave fusion slave station In addition, by disposing a dispersion adder for compensating for chromatic dispersion, a radio signal having an appropriate RF signal strength is extracted.

  Non-Patent Document 2 discloses an RF phase generated during long-distance transmission by combining a light intensity modulator and an optical phase modulator and appropriately adjusting the bias voltage, RF signal intensity, and phase applied to each modulator. A technique for suppressing fading is described.

Tsutsumi, Ikeda, "Experimental study of interference suppression effect on optical baseband signal by chromatic dispersion control of optical radio fusion signal," IEICE Technical Report, Vol. 112, No. 398, MWP2012-63, January 2013 . Y. Cui et al., "Overcoming chromatic-dispersion induced power fading in ROF links using parallel modulators," IEEE Photon. Technol. Lett., Vol. 24, No. 14, pp. 1173-1175 (2012).

  As described above, in preparation for the occurrence of a line break in an optical fiber optical line, in an optical transmission system provided with a redundant wireless line as a backup, the connection line at the location where the line break has occurred is switched from the optical line to the wireless line. . As a result, a part of the optical line between the optical transmission / reception devices is replaced with a wireless line, thereby changing the distance that the optical signal propagates through the optical fiber between the optical transmission / reception devices. Further, the distance that the optical signal propagates through the optical fiber varies depending on the location and number of occurrences of line breaks in the optical line between the optical transceivers. For this reason, in such an optical transmission system, when transmitting an optical signal modulated with an RF signal, the amount of dispersion compensation to be compensated changes depending on the location and number of occurrences of line disconnection. Become. Therefore, the techniques using the dispersion compensation amount as a fixed value as in Non-Patent Documents 1 and 2 described above cannot be applied to dispersion compensation in such an optical transmission system.

  The present invention has been made in view of the above-described problems. The present invention relates to a technique for performing dispersion compensation with an appropriate amount of dispersion compensation in accordance with the state of occurrence of line breakage in an optical transmission system provided with a redundant radio line as a backup in preparation for occurrence of line breakage in an optical line. The purpose is to provide.

  The present invention can be realized as an optical transmission system, for example. An optical transmission system according to an aspect of the present invention includes an optical transmission device that transmits an optical signal modulated with an RF signal, an optical reception device that receives the optical signal, and from the optical transmission device to the optical reception device. A node pair provided in the middle of an optical transmission line and made up of an upstream node and a downstream node respectively positioned upstream and downstream in the transmission direction of the optical signal, the upstream node and the downstream node The connection line between the upstream node and the downstream node is switched from the optical line of the optical transmission line to the wireless line of the wireless transmission line when a line break occurs in the optical transmission line between And an optical transmission system for transmitting the optical signal from the optical transmission device to the optical reception device via the node pair on the optical transmission path, wherein the optical transmission device transmits the optical signal Change with RF signal And optical modulation means for transmitting the modulated optical signal via the optical transmission line, and test light received from the downstream side via the optical transmission line, and the downstream via the optical transmission line. First upstream reflection means for reflecting so as to propagate to the side, and the upstream node receives the optical signal received from the upstream side via the optical transmission line when the connection line is switched to the wireless line. When the connection line is switched to the wireless line, the optical signal compensated by the first compensating means is converted into a wireless signal and transmitted via the wireless line. Transmitting means for transmitting, and when the connection line is switched to the wireless line, the downstream node receives the wireless signal from the upstream node via the wireless line and transmits the wireless signal to the optical line. Convert to signal Receiving means for transmitting to the downstream side via the optical transmission line; and when the connection line is switched to the wireless line, the test light received from the downstream side via the optical transmission line is transmitted to the optical line. Second reflecting means for reflecting the light so as to propagate to the downstream side via a transmission line, and the optical receiver transmits the test light to the upstream side via the optical transmission line, First measuring means for measuring a round-trip propagation delay time (RTT) of the test light by receiving the test light reflected by the first reflecting means or the second reflecting means and propagating through the optical transmission line; Receiving the optical signal transmitted from the optical transmission device, second compensation means for performing chromatic dispersion compensation on the optical signal with a compensation amount according to the RTT measured by the first measurement means; It is characterized by providing.

  According to the present invention, in preparation for occurrence of a line break in an optical line, in an optical transmission system provided with a redundant wireless line as a backup, dispersion compensation is performed with an appropriate dispersion compensation amount according to the state of occurrence of the line break. Technology can be provided.

1 is a block diagram showing a schematic configuration example of an optical transmission system that optically transmits an RF signal according to an embodiment of the present invention. The figure which shows the 1st example of a structure of DCF44-k. The figure which shows the 2nd example of a structure of DCF44-k. 1 is a block diagram showing a schematic configuration example of an optical transmission system that optically transmits an RF signal, which is a premise of an embodiment of the present invention. The figure which shows an example of the relationship between an optical fiber transmission distance and RF throughput.

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

  First, with reference to FIG. 4, an optical transmission system which is a premise for describing an embodiment of the present invention will be described. In the optical transmission system illustrated in FIG. 4, the optical transmission device 400 transmits an optical signal modulated by the RF signal to the optical reception device 402, thereby optically transmitting the RF signal.

  4A shows a signal transmission path during normal operation where no optical line failure (line disconnection) has occurred, and FIG. 4B shows a signal transmission path when an optical line failure occurs. As shown in FIGS. 4A and 4B, the optical transmitter 400 and the optical receiver 402 are connected via an optical fiber, and optical / wireless node pairs 404-1 to 404-n are provided in the middle. Yes. Each of the optical / wireless node pairs 404-1 to 404-n includes an upstream node and a downstream node respectively located on the upstream side and the downstream side in the optical signal transmission direction. Optical fibers (optical lines) 406-1 to 406-n are respectively connected between the upstream side node and the downstream side node in each of the optical / wireless node pairs 404-1 to 404-n. Further, between the upstream side node and the downstream side node in each of the optical / wireless node pairs 404-1 to 404-n, an optical communication antenna is provided for backup (detour) of the optical lines 406-1 to 40-n. Radio lines 408-1 to 408-n of the radio transmission path are provided.

  As shown in FIG. 4A, during normal operation in which an optical line failure has not occurred, the optical signals transmitted from the optical transmission device 400 are the optical / radio node pairs 404-1 to 404-n and the optical line 406-1. ˜n are propagated and received by the optical receiver 402. On the other hand, when an optical line failure occurs in the optical lines 406-1 to 40-n corresponding to the optical / wireless node pairs 404-1 to 404-n, respectively, and the line breaks, each optical / wireless node pair uses the connection line as an optical line. Switch from to wireless. That is, the upstream side node and the downstream side node of the optical / wireless node pair corresponding to the optical line in which the line break has occurred switches the connection line from the optical line 406-1 to n to the wireless line 408-1 to n- Perform backup communication.

  FIG. 4B shows, as an example, a case where a line break has occurred in the optical lines 406-1 and 406-n. The optical / wireless node pair 404-1, 404-n is respectively connected to the upstream node and the downstream node. Connection lines with the side nodes are switched from optical lines 406-1 and 406-n to wireless lines 408-1 and 408-n. As described above, when a line disconnection occurs at a plurality of locations on the optical line between the optical transmission device 400 and the optical reception device 402, the connection line at each location is switched to a backup wireless line, thereby providing an optical line. Communication between transmitting and receiving devices can be continued.

Here, in an optical transmission system that optically transmits an RF signal as shown in FIGS. 4A and 4B, the influence of chromatic dispersion generated when an optical signal modulated by the RF signal propagates through an optical fiber is affected. It needs to be removed. For example, if the carrier frequency of the optical signal is f ₀ and the carrier frequency of the RF signal is f _RF , the optical signal intensity-modulated with the RF signal is expressed by the following equation (1), where m is the optical modulation degree. The
cos (2πf ₀ t) · {1 + m · cos (2πf _RF t)}
= Cos (2πf ₀ t)
+ (M / 2) cos {2π (f ₀ + f _RF ) t} + (m / 2) cos {2π (f ₀ −f _RF ) t} (1)
On the right side of Equation (1), the first term represents the optical signal carrier, and the second and third terms represent the RF signal components in the upper sideband and the lower sideband, respectively.

  As shown in equation (1), when using double sideband (DSB) modulation that transmits both upper sideband and lower sideband RF signal components, the RF signal intensity after propagation through the optical fiber is And the sum of the intensities of the RF signal components in the lower sideband. This sum changes depending on the phase relationship between these two RF signal components. For this reason, the throughput of the RF signal depends on the phase relationship between the RF signal components of the upper sideband and the lower sideband. In particular, when the phase difference is π, the RF signals of the upper sideband and the lower sideband are mutually connected. Canceling each other, the throughput becomes zero. The change in the phase relationship between the RF signal components in the upper sideband and the lower sideband is caused by the influence of chromatic dispersion. Such a change in RF signal intensity due to chromatic dispersion is generally referred to as RF phase fading. (Hereinafter, such a change in RF signal intensity caused by RF phase fading is referred to as a “dispersion penalty”.)

For example, the RF signal throughput P _RF after propagating through a length L optical fiber is expressed by the following equation (2).
P _RF ∝10 · log ₁₀ {cos ² (πLDλ ₀ ² f _RF ² / c)} (2)
Here, D is the dispersion value of the optical fiber, λ ₀ is the wavelength of the optical signal carrier, and c is the speed of light in the optical fiber. In formula (2), for simplification, the propagation loss of the optical fiber is not taken into consideration, but only the RF phase fading due to chromatic dispersion is taken into consideration.

FIG. 5 is a diagram showing a dispersion penalty with respect to the optical fiber transmission distance, which is calculated by using Expression (2), using the carrier frequency f _RF of the RF signal as a parameter. In the figure, D = 17 [ps / nm / km], λ ₀ = 1550 [nm], and the carrier frequency f _RF of the RF signal is calculated as three patterns of 2 GHz, 30 GHz, and 60 GHz. As shown in FIG. 5, at 2 GHz, which is widely used in mobile communication and the like, a dispersion penalty hardly occurs even after an optical signal propagates 10 km through an optical fiber. On the other hand, at 30 GHz belonging to the millimeter wave band, a dispersion penalty of several dB occurs in optical transmission of several kilometers. Further, it can be seen that at 60 GHz, a significant dispersion penalty occurs in 1 km optical transmission, and even 1 km optical transmission cannot be realized.

Thus, with an increase in the carrier frequency f _RF, due to the RF phase fading due to wavelength dispersion, the change of the RF signal strength at the optical receiving apparatus increases, the change is remarkable, especially the millimeter wave band. Therefore, in an optical transmission system that optically transmits an RF signal, it is necessary to compensate for the influence of chromatic dispersion that occurs when an optical signal modulated by the RF signal propagates through an optical fiber transmission line with an appropriate amount of dispersion compensation.

  Furthermore, as shown in FIGS. 4A and 4B, in the case of an optical transmission system provided with a redundant wireless line in preparation for the occurrence of a line break in the optical line, as described above, the light between the optical transceivers The dispersion compensation amount to be compensated changes depending on the occurrence location and the number of occurrences of the line break in the line. This is because the distance at which the optical signal propagates through the optical fiber changes between the optical transceivers when the connection line at the location where the line break has occurred is switched to a wireless line. For this reason, in an optical transmission system such as that shown in FIGS. 4A and 4B, it is necessary to perform dispersion compensation with an appropriate dispersion compensation amount according to the situation of occurrence of a line break.

<Configuration of optical transmission system>
Next, a configuration example of an optical transmission system that optically transmits an RF signal according to an embodiment of the present invention will be described with reference to FIG. The optical transmission system illustrated in FIG. 1 includes an optical transmission device 100 that transmits an optical signal modulated with an RF signal, an optical reception device 102 that receives the optical signal, and light from the optical transmission device 100 to the optical reception device 102. And optical / wireless node pairs 104-1 to 104-n provided in the middle of the transmission path. In the present embodiment, the optical / wireless node pair 104-1 is an example of a first node pair, and the optical / wireless node pairs 104-2 to n are examples of a second node pair. The optical transmission system may comprise any number of optical / radio node pairs 104-2 -n.

  As shown in FIG. 1, the optical transmitter 100 and the optical / radio node pair 104-1 are connected by an optical fiber 20-1, and the optical / radio node pair 104-n and the optical receiver 102 are optical fibers. 20- (n + 1). Of the optical / wireless node pairs 104-1 to 104-n, two adjacent optical wireless node pairs are connected by optical fibers 20-2 to 20-n, respectively.

  The optical / radio node pair 104-1 includes an upstream node 104-1a and a downstream node 104-1b located on the upstream side and the downstream side in the optical signal transmission direction, respectively. The same applies to n. The upstream node and the downstream node in each of the optical / wireless node pairs 104-1 to 104-n are connected by optical fibers 106-1 to 106-n, respectively. In the optical / wireless node pair 104-1, an optical fiber (optical line) 106-1 is provided between the transmitting antenna 28-1 of the upstream node 104-1a and the receiving antenna 30-1 of the downstream node 104-1b. A backup (detour) wireless line is provided, and the optical / wireless node pairs 104-2 to n are the same. As will be described later, the optical / wireless node pairs 104-1 to 104-n are respectively connected to the upstream side when an optical transmission line (optical fibers 106-1 to 106-n) is disconnected between the upstream node and the downstream node. The connection line between the node and the downstream node is switched from the optical line of the optical transmission line to the wireless line of the wireless transmission line.

  The optical transmission system shown in FIG. 1 is normally operated (that is, when no line disconnection occurs), the optical transmission path from the optical transmitter 100 to the optical receiver 102 and the optical / radio node pairs 104-1 to 104-1. The optical signal modulated by the RF signal is transmitted from the optical transmitter 100 to the optical receiver 102 via n. That is, an RF signal is transmitted from the optical transmission device 100 to the optical reception device 102 as an optical signal. In this case, the optical transmission path between the optical transmission device 100 and the optical reception device 102 includes optical fibers 20-1 to (n + 1) and optical fibers 106-1 to 106-1 as shown in FIG. The

  In the present embodiment, the optical fibers (optical lines) 106-1 to 106-n are provided in places where there is a higher possibility of line disconnection in the event of a disaster than the optical fibers 20-1 to (n + 1). For example, it is assumed that it is wired under an aerial or under a bridge. It is desirable that the redundant wireless line is provided corresponding to an optical line that is relatively likely to cause a line disconnection.

  In the optical transmission system shown in FIG. 1, the lengths of optical fibers (not shown in FIG. 1) other than the optical fibers 106-1 to 10-n and the optical fibers 20-1 to (n + 1) are ignored by the influence of chromatic dispersion. It should be as short as possible. In FIG. 1, for ease of explanation, an optical amplifier, an RF amplifier, or the like that is inserted at an arbitrary position on the optical transmission line is omitted. Moreover, although the example of the one-way communication from the optical transmitter 100 to the optical receiver 102 is shown, two-way communication may be used.

<Signal transmission during normal operation>
Next, signal transmission during normal operation in the optical transmission system shown in FIG. 1 will be described. In the optical transmission device 100, a light source (LD: Laser Diode) 10 emits continuous light with a wavelength of λ ₁ and constant light intensity, and the continuous light enters the optical modulator 12. The optical modulator 12 intensity-modulates the continuous light incident from the LD 10 with an RF signal 14 to be optically transmitted. The intensity of the RF signal 14 supplied to the optical modulator 12 may be amplified by appropriately inserting an RF amplifier or the like. The description of the bias voltage applied to the optical modulator 12 is omitted.

In the optical transmission apparatus 100, an optical signal having a wavelength λ ₁ (hereinafter also referred to as a “data optical signal”) intensity-modulated by the RF signal 14 is a wavelength division multiplexing optical multiplexer / demultiplexer (WDM OC: Wavelength Division Multiplexing Optical). Is output via the coupler 16 and enters the optical fiber 20-1. The WDM OC 16 is a passive optical component that multiplexes and demultiplexes a data optical signal having a wavelength λ _{1 and} an optical signal having a wavelength λ ₂ described later. The optical signal propagated through the optical fiber 20-1 is incident on the optical switch 22-1 in the upstream node 104-1a of the optical / wireless node pair 104-1.

  In the optical / wireless node pair 104-1, the optical switch 22-1 is an active optical device that switches an optical signal transmission path by switching between an A → B connection and an A → C connection. The connection can be switched according to control from the outside. During normal operation, the optical switch 22-1 is controlled to a connection state of A → B. Thereby, the optical signal propagated through the optical fiber 20-1 is supplied to the optical line 106-1. The optical signal propagated through the optical line 106-1 between the upstream node 104-1a and the downstream node 104-1b is incident on the optical switch 23-1 of the downstream node 104-1b. The optical switch 23-1 has a configuration similar to that of the optical switch 22-1, and is controlled to a connection state of B → A during normal operation. The optical signal that has passed through the optical switch 23-1 is output from the downstream node 104-1b of the optical / wireless node pair 104-1, and enters the optical fiber 20-2. The optical signal propagates through the optical fiber 20-2 and enters the optical switch 22-2 in the upstream node 104-2a of the optical / wireless node pair 104-2.

  As will be described later, when a line disconnection occurs in the optical line 106-1, the optical switch 22-1 switches to the connection of A → C according to the control from the outside, and the optical switch 23-1 The connection is switched from C to A according to the control from the outside.

  In the optical / wireless node pair 104-2, the optical switch 22-2 of the upstream node 104-2a and the optical switch 23-2 of the downstream node 104-2b have the same configuration, The connection state of A → B and B → A is controlled. As a result, the optical signal propagates through the optical line 106-2 between the upstream node 104-2a and the downstream node 104-2b. The optical signal that has passed through the optical switch 22-2, the optical line 106-2, and the optical switch 23-2 propagates through the optical fiber 20-3, and the upstream node 104-3a of the optical / radio node pair 104-3 (the upstream node 104-3a). Is incident on the optical switch 22-3).

  The optical / wireless node pairs 104-3 to n have the same configuration as the optical / wireless node pair 104-2. Therefore, during normal operation, as described above, the optical signal propagates through the optical transmission line composed of the optical fibers 20-4 to (n + 1) and the optical lines 106-3 to n, and the WDM in the optical receiver 102. Incident on the OC60.

In the optical receiving apparatus 102, the WDM OC 60 is a passive optical component that combines and demultiplexes a data optical signal having a wavelength λ _{1 and} an optical signal having a wavelength λ _{2 to be} described later, like the WDM OC 16. The WDM OC 60 supplies a data optical signal having a wavelength λ ₁ to a dispersion compensator (DCF) 62 and an optical signal having a wavelength λ ₂ to an optical transceiver (TRX) 68.

  In the optical receiving apparatus 102, the DCF 62 compensates for the influence of the chromatic dispersion that the data optical signal receives while propagating through the optical transmission line composed of the optical fibers 20-1 to (n + 1) and the optical lines 106-1 to 106-n. To do. That is, the effects of RF phase fading and waveform deterioration caused by chromatic dispersion are compensated. The data optical signal that has passed through the DCF 62 enters an optical / electrical converter (O / E) 64, is converted from an optical signal to an electrical signal (wireless signal), and is output as an RF signal 70. The

<Signal transmission when optical line failure occurs>
Next, signal transmission when an optical line failure (line disconnection) occurs in the optical transmission system shown in FIG. 1 will be described. Specifically, in the optical / wireless node pair corresponding to the optical line where the line break has occurred among the optical / wireless node pairs 104-1 to 104-n, the connection line is switched from the optical line to the redundant wireless line. Signal transmission will be described.

(When a line break occurs in the optical line 106-1)
When a line break occurs in the optical line 106-1 corresponding to the optical / wireless node pair 104-1, the optical / wireless node pair 104-1 detects this, and the upstream node 104-1a and the downstream node 104 are detected. -1b is switched from the optical line 106-1 to the redundant wireless line. Specifically, the optical / wireless node pair 104-1 switches the optical switch 22-1 to the connection A → C and the optical switch 23-1 to the connection C → A. As a result, the data optical signal propagated through the optical fiber 20-1 enters the DCF 24 via the optical switch 22-1 at the upstream node 104-1a. The DCF 24 includes a dispersion compensating fiber, a fiber Bragg grating (FBG), and the like, and performs dispersion compensation with a negative chromatic dispersion value for a data optical signal.

  When the connection line in the optical / wireless node pair 104-1 is switched to a wireless line, the DCF 24 may compensate for the influence of RF phase fading due to chromatic dispersion received by the optical fiber 20-1. For this reason, the DCF 24 performs dispersion compensation on the data signal with a compensation amount corresponding to the chromatic dispersion amount received by the optical fiber 20-1. Note that the dispersion compensation amount of the DCF 24 can be set to a fixed value. This is because the distance that the data optical signal propagates through the optical fiber between the optical transmission device 100 and the upstream node 104-1a of the optical / wireless node pair 104-1 is the same as that during normal operation and when line disconnection occurs. Is also constant. Therefore, the DCF 24 may perform chromatic dispersion compensation with a compensation amount corresponding to the transmission path length of the optical transmission path between the optical transmission device 100 and the optical / wireless node pair 104-1. In the present embodiment, the DCF 24 is an example of a first compensation unit for the first node pair.

  The data optical signal that has been subjected to dispersion compensation in the DCF 24 is converted into an electrical signal (wireless signal) by an optical / electrical converter (O / E: Optical-to-Electrical converter) 26. The electric signal is radiated from the upstream node 104-1a to the free space with a predetermined beam directivity and intensity through the transmitting antenna 28-1. In this way, the dispersion-compensated optical signal is converted into a radio signal and transmitted via a radio line. The radiated electric signal is received by the downstream node 104-1b via the receiving antenna 30-1, and is input to an electric / optical converter (E / O) 32-1. Note that an amplifier for amplifying an electrical signal may be inserted immediately before the transmitting antenna 28-1 in the upstream node 104-1a and immediately after the receiving antenna 30-1 in the downstream node 104-1b.

In the downstream node 104-1b of the optical / wireless node pair 104-1, the E / O 32-1 generates an optical signal (data optical signal) intensity-modulated with the input electrical signal (RF signal), and the WDM OC 34 To -1. Similar to the WDM OCs 16 and 60, the WDM OC 34-1 is a passive optical component that multiplexes and demultiplexes a data optical signal having a wavelength λ _{1 and} an optical signal having a wavelength λ _{2 to be} described later. The WDM OC 34-1 is an optical switch. 23-1. Thereafter, the optical signal supplied to the optical switch 23-1 reaches the upstream node 104-2a of the optical / wireless node pair 104-2 via the optical switch 23-1 and the optical fiber 20-2. In this way, even when a line disconnection occurs in the optical line 106-1 corresponding to the optical / wireless node pair 104-1, communication (signal transmission) via the wireless line is used as a backup of the optical line 106-1. Is possible.

(When a line break occurs in the optical line 106-2)
When the line disconnection occurs in the optical line 106-2 corresponding to the optical / wireless node pair 104-2, the optical / wireless node pair 104-2 detects this, and the upstream node 104-2a and the downstream side are detected. The connection line with the node 104-2b is switched from the optical line 106-2 to the redundant radio line. Specifically, the optical / wireless node pair 104-2 switches the optical switch 22-2 from A to C connection and the optical switch 23-2 from C to A connection. As a result, the optical signal propagated through the optical fiber 20-2 enters the WDM OC 42-2 via the optical switch 22-2 in the upstream node 104-2a. Similar to the WDM OCs 16 and 60, the WDM OC 42-2 multiplexes and demultiplexes the data optical signal with the wavelength λ _{1 and} the optical signal with the wavelength λ ₂ and sends the demultiplexed data optical signal with the wavelength λ ₁ to the DCF 44-2. Supply.

  In the DCF 24 of the optical / wireless node pair 104-1 as described above, the dispersion compensation amount only needs to be compensated with a compensation amount corresponding to only the chromatic dispersion amount received by the optical fiber 20-1, so the dispersion compensation amount is set to a fixed value. Is possible. On the other hand, in the DCF 44-2 of the optical / wireless node pair 104-2, it is necessary to determine the compensation amount in dispersion compensation in consideration of the connection state of the optical line 106-1. Specifically, when the optical line 106-1 is in a normal state, the DCF 44-2 corresponds to the total amount of chromatic dispersion received by the optical fibers 20-1, 20-2 and the optical fiber (optical line) 106-1. Dispersion compensation is performed with the compensation amount to be applied. Further, when a line break occurs in the optical line 106-1, the DCF 44-2 performs dispersion compensation with a compensation amount corresponding to the sum of the chromatic dispersion amounts received by the optical fibers 20-1 and 20-2. As described above, there are two patterns in the chromatic dispersion amount to be compensated by the DCF 44-2 depending on the connection state of the optical line 106-1.

  The data optical signal after dispersion compensation by the DCF 44-2 is converted into an electrical signal in the same manner as the processing in the upstream node 104-1a when the line disconnection occurs in the optical line 106-1, and is transmitted from the upstream node 104-2a. It is transmitted to the downstream node 104-2b via the wireless line. That is, the data optical signal output from the DCF 44-2 is incident on the O / E 46-2, converted into an electrical signal, and transmitted through the transmission antenna 28-2. Since the downstream node 104-2b has the same configuration as the downstream node 104-1b, the downstream node 104-2b performs the same processing as the downstream node 104-1b. As a result, the optical signal output from the WDM OC 34-2 is transmitted to the optical / wireless node pair 104-3 via the optical switch 23-2 and the optical fiber 20-3.

(When a line break occurs in the optical line 106-k)
When a line break occurs in the optical line 106-k corresponding to the optical / wireless node pair 104-k (k = 3, 4,..., N), the line break occurs in the optical line 106-2 as described above. Dispersion compensation by DCF 44-k is performed in the same manner as when it occurred.

  For example, in the DCF 44-3 of the optical / wireless node pair 104-3, it is necessary to determine the compensation amount in the dispersion compensation in consideration of the connection state of the optical lines 106-1 and 106-2. For this reason, there are three patterns of chromatic dispersion amounts to be compensated by the DCF 44-3 depending on the connection state of the optical lines 106-1 and 106-2. Further, when generalized to the optical / radio node pair 104-k (k = 2, 3,..., N), the chromatic dispersion amount to be compensated by the DCF 44-k of the optical / radio node pair 104-k is: There are k patterns depending on the connection state of the optical lines 106-1 to (k-1). Since the optical receiving apparatus 102 corresponds to the optical / radio node pair 104- (n + 1), there are (n + 1) patterns in the chromatic dispersion amount to be compensated by the DCF 62.

  It should be noted that the data optical signal by the optical / wireless node pair 104-k after the dispersion compensation by the DCF 44-k when the line disconnection occurs in the optical line 106-k (k = 3, 4,..., N). The processing for is the same as the processing by the optical / wireless node pair 104-2 described above.

  Thus, in the optical transmission system according to the present embodiment, when a line break occurs in the optical line 106-k (k = 1, 2,..., N), the connection line at the place where the line break occurs is used for backup. Switch to the wireless line. As a result, even when a line break occurs on the optical transmission line, it is possible to continue optical transmission of the RF signal between the optical transmission device 100 and the optical reception device 102.

<Configuration of DCF44-k>
The DCF 44-k (k = 2, 3,..., N) is composed of a dispersion compensating fiber, FBG, and the like, similarly to the DCFs 24 and 62. However, the DCF 44-k needs to have a configuration in which dispersion compensation can be performed with a dispersion compensation amount of k patterns according to the connection state of the optical lines 106-1 to (k-1). In the following, an example (FIG. 2) in which the DCF 44-k is configured with a dispersion compensating fiber will be described as a first example, and an example (FIG. 3) in which the DCF 44-k is configured with FBG will be described as a second example. . Note that the configuration when k = n + 1 corresponds to the configuration of the DCF 62 of the optical receiving apparatus 102 (k = n + 1). In the present embodiment, the DCFs 44-2 to n are examples of the first compensation means of the second node pair, and the DCF 62 is an example of the second compensation means.

FIG. 2 is a diagram illustrating a schematic configuration example of the DCF 44-k including a plurality of dispersion compensating fibers. The data optical signal having the wavelength λ ₁ incident from the WDM OC 42-k enters the k dispersion compensating fibers 82-1 to 82-k via the optical switch 80. The optical switch 80 has a 1 × k configuration and is controlled by a controller (CTL) 52-k in the optical / wireless node pair 104-k (upstream node 104-ka). In the present embodiment, the CTL 52-k controls the dispersion compensation amount for the data optical signal by switching the optical connection of the optical switch 80 in accordance with the dispersion compensation amount determined by a method described later. The data optical signal that has propagated through any of the dispersion compensating fibers 82-1 to 82-k enters the O / E 46-k via an optical coupler (OC) 84.

  Each of the dispersion compensating fibers 82-1 to 82-k has a different length, and each length corresponds to a different compensation amount of chromatic dispersion. That is, by selecting which of the dispersion compensating fibers 82-1 to 82-k is used, it is possible to perform dispersion compensation for the data optical signal with a compensation amount corresponding to the length. The dispersion compensating fiber 82 is obtained by grasping in advance the length of each of the optical fibers 20-1 to k and the optical fibers (optical lines) 106-1 to (k−1) or the amount of chromatic dispersion received by each optical fiber. Each length of −1 to k can be determined. That is, k chromatic dispersions to be compensated depending on the connection state of each optical fiber arranged between the optical transmitter 100 and the optical / wireless node pair 104-k (upstream node 104-ka). The amount may be obtained in advance, and the length of each dispersion compensating fiber may be determined based on the obtained amount.

Next, FIG. 3 is a diagram illustrating a schematic configuration example of the DCF 44-k including a plurality of FBGs. The data optical signal having the wavelength λ ₁ incident from the WDM OC 42-k enters one of k optical circulators 86-1 to 86-k via the optical switch 80. The optical circulators 86-1 to 86 -k are passive optical components that guide optical signals only in a specific direction, and supply the data optical signals incident from the optical switch 80 to the FBGs 88-1 to k, respectively.

  The FBGs 88-1 to k reflect the incident data optical signal at the grating portion, and supplies the reflected optical signal to the optical circulators 86-1 to 86-k again. By changing the grating period of the FBGs 88-1 to k in the longitudinal direction of the optical fiber, light of different wavelengths is reflected by different grating portions. Thereby, a group delay time difference depending on the wavelength can be generated. Therefore, by designing the grating portion so as to have the inverse characteristics of chromatic dispersion received by the optical fibers 20-1 to k and the optical fibers (optical lines) 106-1 to (k-1), the FBGs 88-1 to k are It will operate as a chromatic dispersion compensator.

  The optical signal reflected by any one of the FBGs 88-1 to k passes through the corresponding optical circulators 86-1 to 86-k and enters the OC 84. The optical signal incident on the OC 84 is incident on the O / E 46 -k via the OC 84.

<Determination Method of Dispersion Compensation Amount in DCF44-k>
In order to determine the dispersion compensation amount in the DCF 44-k (k = 2, 3,..., N) and the DCF 62, the chromatic dispersion amount to be compensated, that is, the optical / radio node pair 104-k or the optical receiver. It is necessary to grasp the length of the optical fiber through which the optical signal has propagated before reaching 102. Hereinafter, the dispersion compensation amount determination method will be described using the optical / wireless node pair 104-2 as an example.

  The optical line 106-2 corresponding to the optical / wireless node pair 104-2 is disconnected, and the connection line between the upstream node 104-2a and the downstream node 104-2b is switched to the wireless line. Assuming that In this case, the CTL 52-2 of the upstream node 104-2a needs to determine a compensation amount of chromatic dispersion in the DCF 44-2.

First, the CTL 52-2 activates an optical transceiver (TRX: Optical Transceiver) 54. The TRX 54-2 transmits / receives an optical signal having a wavelength λ ₂ different from the wavelength λ ₁ on a trial basis in order to measure the length of the optical fiber through which the data optical signal having the wavelength λ ₁ has propagated. The optical signal of wavelength λ ₂ transmitted by TRX 54-2 may be a single pulse or a control signal, and any optical signal can be used.

The TRX 54-2 refers to the clock (CLK) from the CTL 52-2, and upstream of the optical signal having the wavelength λ ₂ (hereinafter also referred to as “test light”) through the optical transmission line at a constant period. To the side. The dispersion compensation amount of the DCF 44-2 can be determined by transmitting the test light only once. However, when a disaster occurs, the connection state of the optical line 106-1 can change depending on the situation. Therefore, it is recommended to measure regularly.

The test light output from the TRX 54-2 is incident on the optical switch 22-2 via the WDM OC 42-2. The optical switch 22-2 has already been switched to the connection C → A due to the occurrence of a line break in the optical line 106-2. For this reason, the optical switch 22-2 supplies the incident test light to the optical fiber 20-2. As a result, the test light is output from the upstream node 104-2a of the optical / wireless node pair 104-2 to the upstream optical fiber 20-2. The optical signal (test light) having the wavelength λ ₂ propagated through the optical fiber 20-2 reaches the downstream node 104-1b of the optical / wireless node pair 104-1, and enters the optical switch 23-1.

(When optical line 106-1 is normal)
Here, when the optical line 106-1 corresponding to the optical / wireless node pair 104-1 is normal, the connection states of the optical switches 23-1, 22-1 are A → B and B → A, respectively. Therefore, the test light reaching the optical switch 23-1 propagates through the optical line 106-1 from the downstream node 104-1b to the upstream node 104-1a, and further propagates through the optical fiber 20-1. It reaches the optical transmitter 100 and enters the WDM OC 16. The test light having the wavelength λ ₂ demultiplexed by the WDM OC 16 is reflected by the optical reflector 18 and is incident on the WDM OC 16 again. The light reflector 18 is preferably configured to substantially totally reflect incident light, and can be configured by a mirror or FBG. In this way, the light reflector 18 reflects the test light received from the downstream side via the optical transmission path so as to propagate to the downstream side via the optical transmission path.

The test light reflected by the optical reflector 18 is combined with the data optical signal having the wavelength λ ₁ in the WDM OC 16 and output from the optical transmitter 100 to the optical fiber 20-1. The optical signal including the test light output from the optical transmission device 100 propagates through the optical fiber 20-1, the optical line 106-1 and the optical fiber 20-2, and again upstream of the optical / radio node pair 104-2. It reaches the side node 104-2a. In the upstream node 104-2a, the test light having the wavelength λ ₂ is received by the TRX 54-2 through the WDM OC 42-2. In this way, the TRX 54-2 transmits the test light to the upstream side via the optical transmission path, and is reflected by the light reflector 18 or the light reflector 36-1 (here, the light reflector 18) to reflect the light. The test light propagated through the transmission line is received.

  The CTL 52-2 knows the transmission time of the test light instructed to the TRX 54-2. Therefore, the CTL 52-2 measures the difference between the time when the TRX 54-2 transmits the test light and the time when the test light is received, that is, the round trip time (RTT), thereby measuring the test light. Can be calculated. Specifically, if the speed of light in the optical fiber is multiplied by half the value of RTT, the length of the optical fiber through which the test light has propagated can be obtained.

  As described above, assuming that the optical fiber lengths other than the optical fibers 20-1 and 20-2 and the optical line 106-1 are negligibly short, the data optical signal propagates to the optical fiber length through which the test light propagates. Equal to the optical fiber length Therefore, the CTL 52-2 can determine the optical fiber length through which the data optical signal propagates, that is, the chromatic dispersion amount to be compensated, by obtaining the optical fiber length through which the test light propagates. Therefore, the CTL 52-2 controls switching of the connection of the optical switch 80 in the DCF 44-2 so that the compensation amount of chromatic dispersion in the DCF 44-2 becomes a value corresponding to the length of the optical fiber through which the data optical signal propagates. Just do it. Thus, the DCF 44-2 performs chromatic dispersion compensation with a compensation amount that compensates for chromatic dispersion received during propagation through the optical fiber length (transmission path length) through which the optical signal propagates.

(When a line disconnection occurs in the optical line 106-1)
On the other hand, when the line disconnection has occurred in the optical line 106-1 corresponding to the optical / wireless node pair 104-1, the connection states of the optical switches 23-1, 22-1 are A → C and C → A, respectively. It is. Therefore, the test light having the wavelength λ ₂ that has reached the optical switch 23-1 is incident on the light reflector 36-1 through the WDM OC 34-1. Similar to the light reflector 18, the light reflector 36-1 reflects the incident test light and makes it incident on the WDM OC 34-1 again. As with the light reflector 18, the light reflector 36-1 is preferably configured to substantially totally reflect incident light, and can be configured with a mirror or FBG. In this way, the light reflector 36-1 reflects the test light received from the downstream side via the optical transmission path so as to propagate to the downstream side via the optical transmission path.

The test light reflected by the optical reflector 36-1 is combined with the data optical signal having the wavelength λ ₁ in the WDM OC 34-1 and the combined optical signal enters the optical switch 23-1. The optical signal passes through the optical switch 23-1, and is output from the downstream node 104-1b of the optical / radio node pair 104-1 to the optical fiber 20-2. The optical signal including the test light output from the downstream node 104-1b propagates through the optical fiber 20-2 and reaches the upstream node 104-2a of the optical / radio node pair 104-2 again. In the upstream node 104-2a, the test light having the wavelength λ ₂ is received by the TRX 54-2 through the WDM OC 42-2. In this way, the TRX 54-2 transmits the test light to the upstream side via the optical transmission path, and is reflected by the light reflector 18 or the light reflector 36-1 (here, the light reflector 36-1). The test light propagated through the optical transmission line is received.

  Thereafter, the CTL 52-2 calculates the length of the optical fiber through which the test light has propagated by measuring the RTT in the same manner as described above. Further, the CTL 52-2 controls switching of the connection of the optical switch 80 in the DCF 44-2 so that the compensation amount of chromatic dispersion in the DCF 44-2 becomes a value corresponding to the length of the optical fiber through which the data optical signal propagates. Just do it.

  In order to perform the switching control of the connection of the optical switch 80, the CTL 52-2 may hold a table in which the RTT and the connection of the optical switch 80 are associated in advance. The CTL 52-2 specifies the connection of the optical switch 80 associated with the measured RTT by referring to the table, and performs control to switch the optical switch 80 to the specified connection.

  In addition, the test light transmitted from the upstream node of any of the optical / radio node pairs 104-3 to n located downstream of the optical / radio node pair 104-2 propagates through the optical fiber 20-3. When the downstream node 104-2b is reached, the test light propagates as follows. When the optical line 106-2 is normal, the test light propagates through the optical fiber 20-2 to the downstream node 104-1b of the optical / wireless node pair 104-1, and then from the downstream node 104-1b. The light is reflected by the light reflector 36-1 or the light reflector 18 of the optical transmitter 100. The reflected test light returns on the optical transmission path that has passed before reflection, and is received by TRXs 54-3 to n that transmit the test light.

On the other hand, when the line disconnection has occurred in the optical line 106-2, the optical signal enters the WDM OC 42-2 via the optical switch 23-2. The test light having the wavelength λ ₂ demultiplexed by the WDM OC 42-2 is reflected by the light reflector 36-2. The reflected test light is output again to the optical fiber 20-3 via the WDM OC 42-2 and the optical switch 23-2, and then returns to the optical transmission path that has passed before reflection, and the test light is transmitted. Received by TRX 54-3 to n.

  When the TRXs 54-3 to n receive the reflected test light, the corresponding CTLs 52-3 to n can measure the RTT and determine the dispersion compensation amount by the DCFs 44-3 to n as described above. That is, as shown in FIG. 1, since the optical / wireless node pair 104-3 to n has the same configuration as the optical / wireless node pair 104-2, the same applies to the optical / wireless node pair 104-3 to n. With this procedure, the compensation amount of chromatic dispersion in the DCFs 44-3 to n can be controlled.

  In the above-described embodiment, the chromatic dispersion can be compensated by electrical dispersion compensation (EDC), which has been described with respect to the example in which the chromatic dispersion is compensated by the DCFs 24, 44-1 to n, 62. In this case, a signal processing unit having an EDC function may be inserted before the transmitting antennas 28-1 to 28-n and after the O / E 64. The RTT can be measured by the same method as described above. The CTLs 52-2 to n and 66 may control the EDC of the signal processing unit based on the RTT measurement result.

  As described above, the optical transmission system according to the present embodiment has an optical / radio node pair consisting of an upstream node and a downstream node in the middle of an optical transmission path between the optical transmitter 100 and the optical receiver 102. 104-1 to n. Each optical / wireless node pair switches the connection line between both nodes from the optical line to the wireless line when a line break occurs in the optical transmission line (optical fibers 106-1 to 106-n) between the both nodes. The optical receiver 102 transmits the test light to the upstream side via the optical transmission line, and receives the test light reflected by the downstream node of the optical transmitter 100 or any one of the optical / wireless node pairs. Measure the RTT of the light. Furthermore, the optical receiving apparatus 102 performs dispersion compensation on the optical signal received from the optical transmitting apparatus 100 by the DCF 62 with a compensation amount corresponding to the measured RTT.

  According to the present embodiment, even when a line break occurs in an optical line corresponding to an optical / wireless node pair capable of switching between an optical line and a wireless line, between the optical transmission apparatus 100 and the optical reception apparatus 102, It is possible to obtain the optical fiber length (transmission path length) through which the optical signal modulated with the RF signal propagates. Furthermore, by performing dispersion compensation based on the obtained transmission path length, it is possible to perform dispersion compensation on an optical signal modulated with an RF signal with an appropriate dispersion compensation amount according to the state of occurrence of a line break. .

  In the present embodiment, the upstream nodes of the optical / wireless node pairs 104-2 to 10-n also measure the RTT of the test light using the test light in the same manner as the optical receiver 102, and according to the measured RTT. With the compensation amount, dispersion compensation is performed on the optical signal received from the optical transmitter 100 by the DCFs 44-2 to n. This makes it possible to perform dispersion compensation on an optical signal modulated with an RF signal with an appropriate amount of dispersion compensation, regardless of the number of lines in which the line break has occurred among the optical lines 106-1 to 106-n.

<Example using SSB modulation>
In the above-described embodiment, it is assumed that the optical signal is subjected to DSB modulation with the RF signal and optical transmission is performed. However, when the optical signal is subjected to SSB modulation with the RF signal, as shown in FIG. The optical transmission system shown may be changed. In general, when an optical signal is SSB modulated with an RF signal, phase fading does not occur as in the case of using DSB modulation. Therefore, as described above, it is not necessary to measure the RTT using the test light and control the dispersion compensation amounts of the DCFs 44-1 to n, 62 based on the result. That is, the light reflectors 18, 36-1 to n, WDM OCs 16, 34-1 to n, 60, CTLs 52-1 to n, 66, and TRXs 54-1 to n, 68 in FIG.

  The DCFs 44-1 to 44-n have the chromatic dispersion amount received by the data optical signal reaching the optical receiver 102 when the line disconnection occurs in the optical lines 106-1 to 106-n. Dispersion compensation for the data optical signal may be performed with a positive dispersion compensation value so that the same amount as that in the case where is normal. Thus, the dispersion compensation amount of the DCF 62 can be a fixed value regardless of the connection state of the optical lines 106-1 to 106-n.

Claims (14)

An optical transmitter for transmitting an optical signal modulated with an RF signal;
An optical receiver for receiving the optical signal;
A node pair that is provided in the middle of an optical transmission path from the optical transmitter to the optical receiver, and is composed of an upstream node and a downstream node that are located on the upstream side and the downstream side in the optical signal transmission direction, respectively. When a line break occurs in the optical transmission line between the upstream node and the downstream node, a connection line between the upstream node and the downstream node is connected from the optical line of the optical transmission line. The node pair that switches to a wireless line of a wireless transmission path, and
An optical transmission system for transmitting the optical signal from the optical transmission device to the optical reception device via the node pair in the optical transmission path,
The optical transmitter is
Optical modulation means for modulating an optical signal with an RF signal, and transmitting the modulated optical signal via the optical transmission path;
First reflecting means for reflecting the test light received from the downstream side via the optical transmission path so as to propagate to the downstream side via the optical transmission path;
The upstream node is
When the connection line is switched to the wireless line, first compensation means for performing chromatic dispersion compensation on the optical signal received from the upstream side via the optical transmission line;
A transmission means for converting the optical signal after compensation by the first compensation means into a radio signal and transmitting the radio signal via the radio line when the connection line is switched to the radio line;
The downstream node is:
When the connection line is switched to the wireless line, the wireless signal is received from the upstream node via the wireless line, and the wireless signal is converted into an optical signal, and the downstream side via the optical transmission line Receiving means for transmitting to the side,
When the connection line is switched to the wireless line, a second reflection that reflects the test light received from the downstream side via the optical transmission line so as to propagate to the downstream side via the optical transmission line. Means, and
The optical receiver is
By transmitting the test light to the upstream side through the optical transmission line and receiving the test light reflected by the first reflecting means or the second reflecting means and propagating through the optical transmission line. First measuring means for measuring a round-trip propagation delay time (RTT) of the test light;
Receiving the optical signal transmitted from the optical transmitter, a second compensating unit for performing chromatic dispersion compensation on the optical signal with a compensation amount corresponding to the RTT measured by the first measuring unit; An optical transmission system comprising: The optical transmission system includes a plurality of the node pairs including a first node pair and a second node pair provided between the first node pair and the optical receiving device,
The upstream node of the second node pair is
By transmitting the test light to the upstream side through the optical transmission line and receiving the test light reflected by the first reflecting means or the second reflecting means and propagating through the optical transmission line. , Further comprising second measuring means for measuring the RTT of the test light,
The first compensation means of the second node pair is:
2. The optical transmission system according to claim 1, wherein chromatic dispersion compensation is performed on the optical signal with a compensation amount corresponding to RTT measured by the second measuring unit. The first compensation means and the second compensation means of the second node pair are:
Based on the RTT and the speed of light in the optical transmission path, the transmission path length through which the test light propagates is calculated, and the optical signal propagates through the optical transmission path of the transmission path length from the calculated transmission path length. 3. The optical transmission system according to claim 2, wherein chromatic dispersion compensation is performed with a compensation amount that compensates for chromatic dispersion received therebetween.   4. The optical transmission system according to claim 2, wherein a plurality of the second node pairs are provided at different positions in the optical transmission path between the first node pair and the optical receiver. 5. . The first compensation means of the second node pair is:
5. The optical transmission according to claim 2, comprising a plurality of dispersion compensating fibers each having a different compensation amount, and performing chromatic dispersion compensation with a dispersion compensating fiber having a compensation amount corresponding to the RTT. system. The first compensation means of the second node pair is:
5. The chromatic dispersion compensation according to claim 2, further comprising: a plurality of fiber Bragg gratings (FBGs) each having a different compensation amount, and performing chromatic dispersion compensation using a compensation amount of FBG corresponding to the RTT. Optical transmission system. The first compensation means of the first node pair is:
7. The chromatic dispersion compensation is performed on the optical signal with a compensation amount corresponding to a transmission path length of the optical transmission path between the optical transmission device and the first node pair. The optical transmission system according to any one of the above. The second compensation means includes
8. The optical transmission according to claim 1, comprising a plurality of dispersion compensating fibers each having a different compensation amount, and performing chromatic dispersion compensation by a dispersion compensating fiber having a compensation amount corresponding to the RTT. system. The second compensation means includes
8. The optical transmission system according to claim 1, comprising a plurality of FBGs each having a different compensation amount, and performing chromatic dispersion compensation by a compensation amount of FBG corresponding to the RTT. The wavelength of the optical signal is a first wavelength;
The optical transmission system according to claim 1, wherein a wavelength of the test light is a second wavelength different from the first wavelength. The optical transmitter is
The test light that is connected to the optical transmission path and is incident from the downstream side via the optical transmission path is guided to the first reflecting means, and the optical signal that is incident from the light modulating means, and the first reflecting The optical transmission system according to claim 10, further comprising a multiplexer / demultiplexer that multiplexes the test light reflected by the means and guides the test light to the optical transmission path. The optical receiver is
The test light that is connected to the optical transmission line and guides the test light incident from the first measurement means to the optical transmission line, and demultiplexes the test light from a signal received from the upstream side, thereby the first measurement means. The optical transmission system according to claim 10, further comprising a multiplexer / demultiplexer that demultiplexes the optical signal and demultiplexes the optical signal to the second compensation unit. The downstream node is:
The test light that is connected to the optical transmission path at the downstream side and is incident from the downstream side via the optical transmission path is guided to the second reflecting means, and the optical signal that is incident from the receiving means, and 13. The optical transmission according to claim 10, further comprising a multiplexing / demultiplexing unit that multiplexes the test light reflected by the second reflecting unit and guides the test light to the optical transmission path. system.   The optical transmission system according to claim 1, wherein the optical modulation unit performs double sideband (DSB) modulation on the optical signal with the RF signal.

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