JP2011182256A - Optical communication switching system using duplex lines and method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical communication switching system using duplex lines which reduce optical interference noise when duplexing communication light. <P>SOLUTION: The optical communication switching system using the duplex lines has a configuration where the duplex lines are formed by connecting a first optical communication line 3 (actually used line) and second optical communication line 4 (bypass line) between an internal transmission device 1 (transmitter) and external terminal device 6 (receiver) and there is provided an optical frequency converter 5 (or wavelength converter) that changes an optical frequency of the communication light in the second optical communication line 4 (bypass line). The optical communication switching system suppresses the deterioration of communication signals by transferring the frequency component of beat interference generated when the communication light travelling on each of the duplex lines is multiplexed to a high frequency side which is outside of a receiving band of the external terminal device 6 (receiver) by using the optical frequency converter 5 (or wavelength converter). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical communication switching system and method including a duplex line by a first optical communication line (working line) and a second optical communication line (detour line) as an optical communication line, and communication caused by duplication of communication light. The present invention relates to a technique for reducing degradation of signal quality and transferring a signal on an active line to a detour line without interrupting a communication service while continuing a transmission logical link.

  In order to construct an economical optical communication system, a PON (Passive Optical Network) system in which a plurality of external transmission terminal devices are aggregated by a single in-house transmission device has been proposed (for example, see Patent Document 1). As a result, the cost of communication equipment has been greatly reduced, and the trend toward opticalization is in full swing. In addition, due to the expansion of transmission capacity such as high-density wavelength multiplexing and high-density time division multiplexing, services that require real-time properties and high value-added services such as video and optical telephones have become widespread.

  In the past, there have been cases in which communication routes have been forced to change due to road widening work, bridge replacement work, or other equipment work (new construction or repair of electricity, water, etc.) for off-site optical line equipment. It often occurs (hereinafter referred to as troubled relocation work). In such a situation, when trouble relocation work occurs in the communication equipment that supports the service as described above, it is a work to stop a lot of traffic at a time, so the influence on many users is immeasurable. In order to reduce the impact, facilities have been operated with inefficiencies, such as by dividing the construction period or performing switching work in a time zone with a small traffic volume, for example, from midnight to early morning.

  Under such circumstances, a tool for shortening the optical line switching connection time as much as possible and shortening the communication recovery time has been commercialized (for example, see Patent Document 2). However, even with this system, transmission data loss and transmission logical link mismatches are avoided due to temporary disconnection of the optical line and differences in the line length between the working line and the detour line (difference in arrival time of the transmission signal). I can't.

  Even if the transmission route is doubled at the time of construction of the optical line equipment, the working signal is provided in advance to the two routes from the transmission device, and switching with the switching connection time almost zero is performed, There is a problem that a mismatch in the transmission logical link is unavoidable (see, for example, Patent Documents 3 and 4).

  In any case, in consideration of such technical limitations of medium switching, it is recommended that the construction period be distributed so that the influence on the user (service degradation) is kept to a minimum and the midnight work should be further prolonged. There is no change in the situation that cannot be done.

JP-A-8-102710 Japanese Patent No. 3573606 JP-A-5-252548 JP-A-10-117168 JP 2009-253484 A

  In the conventional cable switching work, since the service is temporarily stopped, the construction date and time has been adjusted in consideration of the intention of the service user. However, as represented by the PON system, it is very difficult to adjust the construction date and time in a communication system in which many users are stopped at once due to troubled transfer work. As a result, troubled relocation work is lacking in planning and forced to extend the construction period. It should be noted that even if the user is a single user such as an SS network (single star network) system, the situation is the same as that of the PON system in a dedicated line that is extremely difficult to stop service.

  In view of the above circumstances, duplicated lines having the same line length (optical path length) are created and the phase of the transmission signal is matched to avoid data loss and transmission logical link mismatch (see, for example, Patent Document 5). .) This makes it possible to carry out obstacle relocation work without stopping the service. However, this method has a problem that the optical interference noise generated when the communication light is duplicated deteriorates the communication quality.

  It is an object of the present invention to provide an optical communication switching system and method using a duplex line that reduces optical interference noise generated when the communication light is duplexed.

  To achieve the above object, the present invention relates to a first optical communication line having the same signal transmission time between a first optical transmission device (transmitter) and a second optical transmission device (receiver). Another second optical communication line is connected to form a duplex line, and either the first optical communication line or the second optical communication line is selected while temporarily superimposing communication light. An optical communication switching system comprising an optical frequency converter or a wavelength converter for changing an optical frequency of the communication light in the second optical communication line, and using the optical frequency converter or the wavelength converter The frequency component of beat interference generated when the communication light propagating through each of the duplex lines is combined is shifted to the high frequency side outside the reception band of the second optical transmission device, thereby degrading the communication signal. It is characterized by suppressing.

  The present invention converts the optical frequency of communication light propagating through one of the duplex lines into a component different from the optical frequency of communication light propagating through the other, and beats the optical frequency difference component generated when they are combined By shifting the interference noise to a high frequency range where the optical transmission device (receiver) cannot respond, deterioration of the code error of the communication signal is suppressed. As a result, a highly reliable optical communication line switching work can be expected.

It is composition explanatory drawing which shows the optical communication switching system by the duplex line which concerns on embodiment of this invention. It is an image figure which shows the duplex communication optical waveform with beat interference which concerns on embodiment of this invention. It is an image figure which shows the level fluctuation | variation of the duplex communication light accompanying the beat interference which concerns on embodiment of this invention. It is an image figure which shows level fluctuation reduction of the duplex communication light which concerns on embodiment of this invention. It is composition explanatory drawing which shows the verification experiment type | system | group of the optical communication switching system by the duplex line which concerns on embodiment of this invention. It is an eye diagram (intermediate image displayed on a display) of communication light which is not duplexed according to an embodiment of the present invention. It is an eye diagram which shows the change in the communication light before the countermeasure by embodiment of this invention. It is an eye diagram which shows the duplexed communication light after the countermeasure which concerns on embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail.
FIG. 1 is an explanatory diagram showing a configuration of an optical communication switching system using a duplex line according to an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an in-house transmission apparatus which is a first optical transmission apparatus such as a transmitter, 2, 2 ′ is an optical coupler, 3 is a first optical communication line (working line), and 4 is a second light. A communication line (detour line), 5 is an optical frequency converter (or wavelength converter), and 6 is an external termination device that is a second optical transmission device such as a receiver.

  In FIG. 1, an output end of a local transmission device (transmitter) 1 is connected to an input end of an optical coupler 2, and one output end of the optical coupler 2 is optically transmitted through a first optical communication line (working line) 3. Connected to one input end of the coupler 2 '. The other output end of the optical coupler 2 is connected to the other input end of the optical coupler 2 ′ via the second optical communication line (detour line) 4 and the optical frequency converter (or wavelength converter) 5, and the optical coupler The output end of 2 'is connected to the input end of the external termination device (receiver) 6.

  That is, a second optical communication line different from the first optical communication line (working line) 3 having the same signal transmission time between the in-house transmission device (transmitter) 1 and the external termination device (receiver) 6. (Detour path) 4 is connected to form a duplex line, and either of the first optical communication line (working line) 3 and the second optical communication line (detour path) 4 while temporarily superimposing communication light. In the optical communication switching system for selecting the optical frequency, an optical frequency converter (or wavelength converter) 5 for changing the optical frequency of the communication light is provided in the second optical communication line (detour line) 4. A frequency component of beat interference generated when the communication light propagating through each of the duplex lines is combined using the optical frequency converter (or wavelength converter) 5 Move to the high-frequency side outside the receiving band of the terminating device (receiver) 6 Characterized in that to suppress deterioration of the communication signal Te.

  The communication light transmitted from the in-house transmission device (transmitter) 1 is branched into a first optical communication line (working line) 3 and a second optical communication line (detour line) 4 by a duplex line, and then multiplexed again. Then, since the communication light propagating through each of the duplex lines is input to the external termination device (receiver) 6 with a slight difference in arrival time, the optical frequency difference of the communication light becomes beat interference noise. appear.

  Thus, the optical frequency converter (or wavelength converter) 5 installed in the second optical communication line (detour path) 4 transmits communication light propagating through the second optical communication line (detour path) 4. The optical frequency is converted into a component different from the optical frequency of the communication light propagating through the first optical communication line (working line) 3, and beat interference noise of the optical frequency difference component generated when they are combined is excluded The termination device (receiver) 6 is moved to a high frequency range where it cannot respond, and the deterioration of the communication signal is suppressed.

  Next, the concept of beat interference noise reduction in an optical communication switching system using a duplex line will be described with reference to FIGS. 1 and 2.

  As shown in FIG. 1, the communication light transmitted from the in-house transmission device (transmitter) 1 is composed of a first optical communication line (working line) 3 and a second optical communication line (detour line) 4. When passing through a Mach-Zehnder type duplex line, light is received with beat interference noise corresponding to the optical path length difference ΔL (= Δt · c / n). This is because the communication light from the in-house transmission device (transmitter) 1 is a bit pulse whose intensity is directly modulated, and the optical frequency of the communication light is chirped. That is, when the chirped communication light passes through the duplex line and is multiplexed, the communication lights having different optical frequency components overlap due to the optical path length difference ΔL, so that the beat interference of the difference frequency component Δω. Will occur.

  FIG. 2 illustrates the state at that time, that is, the power and optical frequency of each communication optical bit pulse (1 bit) that has propagated through the duplex line, and the beat interference waveform. In FIG. 2, 7 is a bit pulse of working-side communication light, 8 is a bit pulse of bypass-side communication light, 9 is a frequency chirp curve of working-side communication light, 10 is a frequency chirp curve of bypass-side communication light, and 11 is beat interference. The waveform, 12 is the optical frequency difference (Δω).

Now, communication light φ ₁ branched by the optical coupler 2 and propagating through the first optical communication line (working line) 3 and communication light φ ₂ propagating through the second optical communication line (detour line) 4 are respectively transmitted. When approximated by plane waves, they are expressed by equations (1) and (2).
φ ₁ {L ₁ , ω (L ₁ )} = A · exp [−i {k ₀ · n · L ₁ −ω (L ₁ ) · t + φ ₀ }]... Equation (1)
φ ₂ {L ₂ , ω (L ₂ )} = B · exp [−i {k ₀ · n · L ₂ −ω (L ₂ ) · t + φ ₀ }] (2)

L ₁ and L ₂ are optical path lengths of the first optical communication line (working line) 3 and the second optical communication line (detour line) 4, and ω (L ₁ ) and ω (L ₂ ) are optical path lengths L ₁ and L ₁ . The optical frequency at L ₂ , A and B are amplitudes, k ₀ is the wave number in vacuum, n is the refractive index of the core, and φ ₀ is the initial phase.

Here, since the current value I measured by the off-site termination device (receiver) 6 is proportional to the square of the interference wave obtained by superimposing the communication lights φ ₁ and φ ₂ , the current value I is expressed by Equation (3). Is done. However, it is assumed that the optical-electric conversion efficiency is 1, and the polarization coupling efficiency at the time of multiplexing is 1 assuming that the beat interference becomes the largest.
I = | φ ₁ + φ ₂ ^* | ² …………… Formula (3)

* Represents a complex conjugate. Substituting Equation (1) and Equation (2) into Equation (3) yields the following Equation (4).
I = | A | ² + | B | ² + 2 · | A | · | B | · cos (k ₀ · n · ΔL−Δω · t) Equation (4)
However, ΔL = L ₁ −L ₂ and Δω = ω (L ₁ ) −ω (L ₂ ).

  Here, as can be seen from FIG. 2, when there is no optical path length difference (ΔL = 0), the frequencies of the communication lights to be combined also coincide, and the optical frequency difference Δω = 0. The cos (cosine) part of the third term becomes “1”, and the alternating current component disappears from the current value I. That is, the current value I is expected to be a constant value.

  However, considering that the length of the communication line expands and contracts depending on the ambient temperature environment, and the frequency of the communication light source fluctuates even if the optical path length difference completely matches (ΔL = 0), the equation (4 It is practically impossible for the AC component (beat interference term) to disappear from). Therefore, in the embodiment of the present invention, on the premise that beat interference occurs due to an optical path length difference ΔL or the like, countermeasures for reducing the interference noise are taken.

  As can be seen from equation (4), beat interference noise is obtained by making the frequency of the third term (cos part) representing beat interference larger than the reception band of the external termination device (receiver) 6, that is, by shifting to a high frequency state. The degree can be reduced.

As shown in FIG. 1, an optical frequency converter (or wavelength converter) 5 is installed in the middle of the second optical communication line (detour line) 4 to shift the optical frequency of the communication light φ ₂ by Ω. . When this time of the communication beam and phi _3, the formula (5).
φ ₃ {L ₂ , ω (L ₂ )} = B · exp [−i [k ₀ ′ · n · L ₂ − {ω (L ₂ ) + Ω} · t + φ ₀ ]]]. )

k ₀ ′ is the wave number after the optical frequency is shifted by Ω, and is expressed by the following equation (6).
k ₀ ′ = 2πc / (cλ + λ ² Ω)... Equation (6)

λ is the wavelength of communication light φ ₃ , and c is the speed of light. When Expression (1), Expression (5), and Expression (6) are substituted into Expression (3), the current value I generated by the superposition of the communication lights φ ₁ and φ ₃ is given by the following Expression (7). .
I = | A | ² + | B | ² + 2 · | A | · | B | · sin {(k ₀ · L ₁ −k ₀ '· L ₂ ) · n− (Δω + Ω) · t} ... Formula (7)

Using Expression (7), combining the optical frequency of the communication light φ ₂ propagating through the second optical communication line (detour line) 4 by Ω (φ ₃ ) and not (φ ₂ ) The current value I of communication light is calculated. In this calculation, the amplitudes of the communication lights φ ₁ and φ ₃ propagating through the first optical communication line (working line) 3 and the second optical communication line (detour line) 4 in FIG. ₁ are the same (| A | = | B | = 1), it was assumed that the optical path lengths were almost the same (ΔL = 2 mm).

  Further, since the chirp amount of the optical frequency of communication light (bit pulse) transmitted from the in-house transmission apparatus (transmitter) 1 of 1.25 Gbps is generally about several GHz per transmission speed, 2 GHz / 0.00. 8 ns (= 2 GHz / 160 mm), and 0.025 GHz was used for the optical frequency difference Δω generated when the optical path length difference ΔL = 2 mm.

The result is shown in FIG. 3A shows a case where the optical frequency of the communication light φ ₂ is not shifted by the optical frequency converter (or wavelength converter) 5 (Ω = 0 GHz), and FIG. 3B shows a case where it is shifted by 4.25 GHz. The state of the level fluctuation of the duplex communication light in this case is shown. As can be seen from FIGS. 3A and 3B, beat interference occurs regardless of the presence or absence of optical frequency shift, and the minimum level is zero. However, the frequency of the beat interference waveform is a long-period level change due to Δω (= 0.025 GHz) in FIG. 3A, whereas in FIG. 3B, the optical frequency shift Ω (= 4. 25 GHz) is the dominant factor, and the level changes in a short period.

  Next, FIG. 4 shows a level fluctuation image of the current value I when the external termination device (receiver) 6 receives the above-described beat interference waveform in a band of about 2 GHz. 13 is a beat interference waveform before the optical frequency shift (Ω = 0 GHz), and 14 is a beat interference waveform after the optical frequency shift (Ω = 4.25 GHz).

  Since the beat interference waveform 13 that does not shift the optical frequency (Δω = 0.025 GHz) is within the reception band (2 GHz) of the off-site termination device (receiver) 6, the beat interference waveform of FIG. Reproduced, the minimum level of communication light remains zero.

On the other hand, the beat interference waveform 14 obtained by shifting the optical frequency by Ω (Δω + Ω = 4.28 GHz) is out of the reception band (2 GHz) of the external termination device (receiver) 6, that is, the external termination device. Since the response speed of the (receiver) 6 is slower than the frequency of beat interference, it is measured as an averaged waveform. Therefore, the lowest level of the communication light does not become zero, and approaches 2 which is the sum of the amplitudes (| A | = | B | = 1) of the communication lights φ ₁ and φ ₃ .

  As described above, by providing an optical frequency converter or wavelength converter on one side of the duplex line and designing the frequency of beat interference of the combined communication light to exceed the reception band of the external termination device (receiver) 6 , Communication light level fluctuation (beat interference noise) can be reduced. As is clear from the beat interference waveform 14 in FIG. 4, the level fluctuation (beat interference noise) of the communication light is further increased by further increasing the optical frequency shift amount of the optical frequency converter (or wavelength converter) 5. Needless to say, it gets smaller.

  FIG. 5 is a verification experiment system of an optical communication switching system using a duplex line in the present invention. In FIG. 5, the same parts as those in FIG. In FIG. 5, 17 is a pulse pattern generator, 18 is an optical level adjuster, 19 is a polarization controller, 20 is an O / E (optical / electrical) converter, and 21 is an oscilloscope.

  That is, the communication light transmitted from the on-site transmission device (transmitter) 1 is divided by the optical coupler 2 into the first optical communication line (working line) 3 and the second optical communication line (detour line) 4, and the optical coupler. The signal is multiplexed again at 2 ', received by the O / E converter 20, and measured as a voltage value by the oscilloscope 21.

At this time, the communication light (φ ₂ [L ₂ , ω (t)]) propagating through the second optical communication line (detour line) 4 is an optical frequency converter (or wavelength) on the line 4. The optical frequency is shifted by Ω by the converter 5 (φ ₃ [L ₂ , ω (t) + Ω]), and propagates through the communication light φ ₃ and the first optical communication line (working line) 3. It is combined with communication light (φ ₁ [L ₁ , ω (t)]).

Here, the optical level adjuster 18 adjusts the level of communication light propagating through the first optical communication line (working line) 3 and the second optical communication line (detour line) 4. The polarization controller 19 also transmits communication light (φ ₁ [L ₁ , ω (t)]) of the first optical communication line (working line) 3 and communication light of the second optical communication line (detour line) 4. The polarization plane of (φ ₃ [L ₂ , ω (t) + Ω]) is made to coincide. These are used for maximizing the amplitude of beat interference (level fluctuation) of duplexed communication light and reducing the communication quality to the minimum.

  Now, the optical path length difference ΔL between the first optical communication line (working line) 3 and the second optical communication line (detour line) 4 is matched within 2 mm, and the level of communication light of each line is adjusted by the optical level adjuster 18. While adjusting the difference within 1 dB, the polarization controller 19 matched the polarization plane of the communication light from each line, and verified the effect of the concept of beat interference noise reduction in the optical communication switching system using the above-described duplex line.

First, FIG. 6 is an eye diagram of communication light (φ ₁ [L ₁ , ω (t)]) that is not duplexed and propagates only through the first optical communication line (working line) 3. This eye diagram is clearly and stably opened, and is received as a digital signal with good communication quality by the external termination device (receiver) 6.

Next, FIGS. 7A to 7D show changes in the eye diagram of the duplex communication light before the countermeasure (optical frequency conversion) of the embodiment of the present invention. As can be seen from this figure, the state of the eye diagram representing the quality of the digital signal causes a level fluctuation. In FIG. 7D, the eye diagram is completely closed, that is, until the digital signal disappears. Has reached. This is because a large level fluctuation (beat noise) occurs after the communication lights φ ₁ and φ ₂ constituting the digital signal are combined, and the level fluctuations shown in FIGS. It is repeating. Thus, the digital signal including the level fluctuation (beat noise) causes an error with respect to the threshold value for determining the codes “1” and “0” when the digital signal is received by the external termination device (receiver) 6. As a result, the communication quality deteriorates.

  On the other hand, FIG. 8 is an eye diagram of duplex communication light after the countermeasure (optical frequency conversion) of the embodiment of the present invention. In this case, the level fluctuation of the duplex communication light confirmed before the countermeasure of the embodiment of the present invention disappeared completely, and the eye diagram was not blocked. Note that some beat interference detected in the reception band (about 2 GHz) of the O / E converter 20 appears above the duplex communication light, but the shape of the eye diagram (FIG. 6) of the communication light not duplexed is maintained. You can see that Thereby, when the digital signal is received by the off-site termination device (receiver) 6, there is no error in the determination of the codes “1” and “0”, and as a result, good communication quality can be maintained.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

  DESCRIPTION OF SYMBOLS 1 ... Local transmission apparatus, 2, 2 '... Optical coupler, 3 ... 1st optical communication line (working line), 4 ... 2nd optical communication line (detour line), 5 ... Optical frequency converter (or wavelength conversion) ), 6 ... an external termination device, 17 ... a pulse pattern generator, 18 ... an optical level adjuster, 19 ... a polarization controller, 20 ... an O / E (optical / electrical) converter, 21 ... an oscilloscope.

Claims (2)

The first optical communication line and the second optical communication line are connected between the first optical transmission device and the second optical transmission device to form a duplex line, and the communication light is temporarily superimposed while the communication light is superimposed In the optical communication switching system that selects either the first optical communication line or the second optical communication line,
An optical frequency converter or a wavelength converter for changing an optical frequency of the communication light in the second optical communication line;
The frequency component of beat interference generated when the communication light propagating through each of the duplex lines using the optical frequency converter or wavelength converter is combined is out of the reception band of the second optical transmission device. An optical communication switching system using a duplex line characterized by shifting to the high frequency side. A duplexed line is formed by connecting a first optical communication line and a second optical communication line having the same signal transmission time between the first optical transmission apparatus and the second optical transmission apparatus, and temporarily transmits communication light. In the optical communication switching method of selecting either the first optical communication line or the second optical communication line while superimposing
The communication light propagating through each of the duplex lines is multiplexed using an optical frequency converter or a wavelength converter that changes the optical frequency of the communication light provided in the second optical communication line. An optical communication switching method using a duplex line, wherein a frequency component of beat interference that occurs sometimes is shifted to a high frequency side outside the reception band of the second optical transmission device to suppress deterioration of a communication signal.

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