JP2003124912A - Wdm optical transmission system using polarized surface preserved optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a high-speed WDM optical transmission system having dense wavelength arrangement as compared with conventional one by using polarized surface preserved optical fibers in an optical transmission line. SOLUTION: There are means (3-1, 1-1, 1-2) that divide an optical transmission signal that is subjected to wavelength multiplexing into even and odd channels, polarizers 4-1, 4-2 that convert each divided optical signal to linear polarization that is vertical mutually, a polarization synthesizer 5-1 that allows each optical signal from the polarization element to be subjected to polarization synthesis, an element that inputs the optical signal from the polarization synthesizer 5-1 to the polarization axes that orthogonally cross each other of a polarized surface preserved optical fiber 11, a polarization separator 5-2 that separates the optical signal from the polarization surface storage optical fiber 11 to the linear polarization that is vertical mutually, and a WDM demultiplexer 2 that allows the output signal of the polarization separator to be subjected to wavelength separation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexing optical transmission system, and particularly to a wavelength arrangement of 50 GHz (0.4 mm).
The present invention relates to a densely arranged, high capacity wavelength division multiplexing optical transmission system as described below.

[0002]

2. Description of the Related Art With the increase in the amount of information in recent years, in the conventional wavelength division multiplexing optical transmission system (WDM system), the sideband thereof is limited to the expansion of the transmission capacity and the extension of the transmission distance due to the expansion of the modulation frequency band. Is giving. Further, the transmission power increases due to wavelength multiplexing, and there is a problem of deterioration of transmission quality due to second-order and third-order nonlinear effects (self-phase modulation, four-wave mixing, etc.).

For example, as the nonlinearity of an optical fiber that limits the transmission capacity, there is a phenomenon (optical Kerr effect) in which the refractive index of the optical fiber changes with the intensity of an optical signal. The change in index of refraction modulates the phase of the optical signal propagating in the optical fiber, resulting in frequency chirping that modifies the signal spectrum. This phenomenon is called self-phase modulat.
ion: SPM). The spectrum is expanded by SPM, and the waveform distortion due to wavelength dispersion is further increased.

In order to avoid these problems, the following measures have been proposed.

(1) Limitation of increase in transmission capacity by sideband In Japanese Patent Laid-Open No. 10-145336, an optical filter or an optical multiplexer / demultiplexer is used to perform residual sideband modulation to suppress the spread of the optical signal band. An "optical communication device" that realizes a high-density wavelength division multiplexing optical transmission system has been proposed.

(2) Suppression of non-linear effect US Pat. No. 6,175,435, "Optical communi"
cationsystem using Optical Phase conjugation to su
ppress waveform distortion caused by chromatic dis
"Persion and Kerr effect" Watanabe Shigeki, inserts a phase conjugate element in the middle of a transmission single-mode optical fiber (SMF) or polarization-maintaining optical fiber (PMF) and gives reverse distortion by self-phase modulation (SPM) in advance. In addition, a method has been proposed in which distortion due to SPM in an actual transmission optical fiber is canceled.

Further, US Pat. No. 6011892 "Optical fiber for Wavelength Division multiplexi"
ng ”Chvaphyvy Anbrew R.el.al has a dispersion value of 1.5 to 8 PS / nm · km given to the transmission SMF, and the dispersion slope is also set to less than 0.095 PS / nm ² · km. It is proposed to suppress the deterioration of transmission quality due to four-wave mixing.

[0008]

However, these methods are only those in which multiple wavelengths are divided and multiplexed in one optical transmission line, and single sideband modulation is not proposed for sidebands. However, coupling between sidebands is inevitable when the wavelength arrangement becomes dense.

From the above, in order to realize WDM optical transmission of higher speed and larger capacity than the currently considered WDM optical transmission system, there are the following problems.

(1) The wavelength arrangement is dense and the influence of sidebands is eliminated.

(2) In the dense wavelength arrangement state, the transmission rate per channel is set to 10 GbPS or more to suppress polarization coupling.

(3) Under the conditions (1) and (2) above, nonlinear distortion and distortion due to dispersion are suppressed.

In order to solve the above-mentioned problems associated with the ultrahigh-speed and super-dense wavelength arrangement of the prior art, the present invention uses a polarization-maintaining optical fiber in an optical transmission line to realize a WDM with a denser wavelength arrangement at a higher speed than conventional. The purpose is to realize an optical transmission system.

By introducing the polarization-maintaining optical fiber according to the present invention, amplification of an optical signal and a change of the add / drop multiplexer become problems, and a solution to this problem is also provided.

[0015]

In order to achieve the above object, the present invention is configured as follows.

(1) In a WDM optical transmission system using a polarization-maintaining optical fiber according to the invention of claim 1, means (1, 3-1) for separating the wavelength-multiplexed optical transmission signal into an even channel and an odd channel. 1-1, 1-2), a polarization element (4-1, 4-2) that converts the separated optical signals into mutually perpendicular linearly polarized light, and a polarization of each optical signal from the polarization element. A polarization combining element (5-1) to be combined, and a polarization maintaining optical fiber (1) for transmitting an optical signal from the polarization combining element.
1) elements that are input to mutually orthogonal polarization axes, a polarization separation element (5-2) that separates an optical signal from the transmission polarization-maintaining optical fiber output into mutually perpendicular linear polarizations, and the polarization And a demultiplexer (2) that wavelength-separates the output signal of the wave separation element.

As means for separating the wavelength-division-multiplexed optical transmission signal into even-numbered channels and odd-numbered channels, as shown in FIG. 1, the wavelength-division-multiplexed optical transmission signal is even-numbered by the wavelength splitter (3-1). Channels and odd channels are separated and two groups of wavelength division multiplexed optical signals λ1, λ3 ... λ2n + 1, λ2, λ4, ... λ2n by two multiplexers (1-1, 1-2) as shown in FIG. There are two forms of creating.

According to a second aspect of the present invention, in the WDM optical transmission system using the polarization-maintaining optical fiber according to the first aspect, the polarization separating element (5-2) and the polarization combining element (5-1). A wavelength-selective add / drop module (optical signal inserting / branching device) (6) capable of inserting / branching an optical signal in units of wavelength is inserted between and (see FIG. 3).

According to a third aspect of the present invention, in the WDM optical transmission system using the polarization-maintaining single-mode optical fiber according to the first aspect, the polarization-maintaining single-mode fiber outputs are provided in the middle of the transmission polarization-maintaining single-mode fiber. A polarization splitter (PS) (5-3) for separating two orthogonal linearly polarized lights, and an optical amplification element (7-1, 7-2) for optically amplifying each polarization signal.
And a polarization combining element (PC) (5-4) that combines the outputs of the optical amplifying elements with polarization.

According to a fourth aspect of the present invention, in the WDM optical transmission system using the polarization-maintaining optical fiber according to any one of the first to third aspects, as the polarization-maintaining optical fiber for transmission, its dispersion is about 2 PS / nm · km. It is characterized by using a polarization-maintaining optical fiber whose dispersion slope is suppressed to a finite value and whose dispersion slope is flat with less than 0.1 PS / nm ² · km.

According to a fifth aspect of the present invention, in the WDM optical transmission system using the polarization-maintaining optical fiber according to the first to third aspects, the wavelength band division of the even channels and the odd channels is 1. WDM wavelength bands. 53 to 1.57 μm
C band defined by the range of 1.57 to 1.61
The L band is defined in the range of μm.

<Main point of the invention> In order to solve the above-mentioned problems of the prior art, the present invention uses two wavelength division multiplexed signals.
One group (eg even channel, odd channel)
And the respective wavelength bands are input to the polarization axes of the polarization-maintaining optical fiber which are orthogonal to each other and propagated.

Although the respective propagated wavelengths are different, the same wavelength may exist between the respective polarization axes. However,
When long-distance transmission is performed, coupling is likely to occur at the same wavelength or between modulation sidebands depending on the extinction ratio of the polarization-maintaining optical fiber. Therefore, in order to suppress the case where the wavelengths are the same, it is preferable that the signal wavelengths be different.

As described above, in order to propagate WDM optical signals of different wavelength bands in two axes that are orthogonal to each other in the polarization-maintaining optical fiber, an element for inputting the optical signal into the transmission optical fiber, and a polarization plane. An element is required to recover the original signal from the storage optical fiber output. Therefore, the wavelength-division-multiplexed signal is divided into two groups, the linearly polarized light orthogonal to each is assigned, and the element for polarization-combining the polarized light is constituted by, for example, a polarizer or a polarization splitting / combining device (birefringence element).

The outline of the operation will be described with reference to FIG.

The wavelength-division-multiplexed signal wavelength as shown in FIG. 6A is separated into an even number channel and an odd number channel by a wavelength splitter, and each has a polarization state orthogonal to each other by passing through a polarizer. There are a polarization wavelength band (FIG. 6B) and an S polarization wavelength band (FIG. 6C). The WDM signals having these two polarization states are polarization-combined by a polarization combiner (a birefringent element or a polarization beam splitter), matched with the polarization axes orthogonal to each other of the polarization-maintaining optical fiber, and input.

In this way, since two groups of signals, which are wavelength-distributed so as not to be influenced by sidebands, are propagated with different polarization axes, it is possible to propagate a WDM optical signal having a virtually denser wavelength disposition. it can.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below based on the illustrated embodiments.

FIG. 1 shows a first preferred embodiment of the present invention. A WDM signal (FIG. 6A) obtained by wavelength-multiplexing optical signals of respective channels having different wavelengths λ1, λ2, and λn by a multiplexer (modulator) 1 is converted into a wavelength splitter 3-
1 separates into an even channel group and an odd channel group. Here, the divided wavelength bands of the even channel and the odd channel are 1.53 to 1.5 of the WDM wavelength band, respectively.
The C band is defined in the range of 7 μm and the L band is defined in the range of 1.57 to 1.61 μm.

By passing each of these two wavelength bands through a polarizer (P-polarized light) 4-1 and a polarizer (S-polarized light) 4-2, which are polarization elements, linearly polarized light beams orthogonal to each other, that is, FIG. It is assumed that there are two groups of WDM optical signals of P-polarized light (two-way arrow in FIG. 1) shown in FIG. 6B and S-polarized light (arrow tip in FIG. 1) shown in FIG. Polarization combining of these two optical signal groups is performed by the polarization combiner 5-1 which is a polarization combining element,
The polarization plane-maintaining optical fiber 11 for transmission is made to enter the polarization axes orthogonal to each other.

The WDM signal propagated through the transmission polarization-maintaining optical fiber 11 is a polarization separator (polarization separator) 5 which is a polarization separating element arranged at the output end of the transmission polarization-maintaining optical fiber 11. The polarized light is separated by 2 and the wavelength is combined by the waveband combiner 3-2.
Are separated into channels of wavelengths λ1, λ2, ... λn.

Another embodiment is shown in FIG. This embodiment is particularly effective when it is desired to implement wavelength arrangement in a more virtual dense manner.

That is, when performing wavelength division multiplexing, the wavelength λ is set by the two multiplexers (modulators) 1-1 and 1-2.
1, λ3 ... λ2n + 1 odd-numbered channels and λ2, λ4 ... λ2n even-numbered channels are divided into two groups and multiplexed, and each obtained wavelength division multiplexed optical signal (WDM optical signal) is used as a polarization element. (P-polarized light) 4-1 and a polarizer (S-polarized light) 4-2 are allowed to pass therethrough, so that orthogonal polarization states (P-polarized light and S-polarized light) are obtained.
Form two WDM optical signal groups with
The two WDM optical signal groups are combined by the polarization combiner 5-1 which is a polarization combining element, and are made incident on the polarization axis of the transmission polarization-maintaining optical fiber 11 in alignment.

The WDM optical signal propagated through the transmission polarization-maintaining single-mode fiber 11 is a polarization splitter 5- which is a polarization splitting element arranged at the output end of the transmission polarization-maintaining single-mode fiber 11.
Polarization is separated by 2 and wavelengths λ of 2 groups are respectively separated by WDM demultiplexers (demodulators) 2-1 and 2-2.
Channels are separated into optical signals of 1, λ3 ... λ2n + 1, λ2, λ4 ... λ2n.

As described above, the polarization-maintaining fiber (PM
By using F) in the optical transmission line, although the coupling depending on the extinction ratio occurs between the wavelengths propagating in the orthogonal polarization axes, the coupling length becomes extremely short, and the self-phase modulation due to the nonlinear effect is also controlled. It becomes possible. Incidentally, the effect of chirping due to self-phase modulation is generally proportional to the coupling length, as represented by the following equation (1), so the effect of chirping can also be suppressed.

Δω (t) = (− 2πn2 / λ) · (∂ | Et | ² / ∂t) · Δz ... (1) Et: electric field intensity n2 of light: second-order nonlinear refractive index λ: carrier wavelength Δω (t): Optical frequency change due to chirping Δz: Coupling length (interaction length) Furthermore, wavelength allocation that is not affected by the modulation sideband is given to two groups, and different polarization states are assigned to the respective wavelength groups. Therefore, even if there is a portion that overlaps with the sideband wavelength range, the ratio of coupling during propagation is approximately the extinction ratio.

A finite dispersion value (for example, 2.0 PS / nm · km or less) is given to each polarization axis of the polarization-maintaining optical fiber to flatten the dispersion slope (less than 0.1 PS / nm · km). As a result, four-wave mixing (third-order nonlinear effect) can be suppressed, and flattening of the dispersion slope is effective in suppressing waveform distortion and controlling waveform deformation due to chirping.

This is because the waveform deformation due to the chirping causes a red shift (or blue shift) and a blue shift (or red shift) of the wavelength at the pulse rising portion and the pulse falling portion due to the second-order nonlinear effect.
This is because the red-shifted portion decreases (or increases) the group velocity and the blue-shifted portion increases (or decreases) the group velocity due to the presence of the dispersion slope, and thus the waveform is deformed. That is, since the group velocity of the sideband is different due to chirping, waveform deformation occurs.

The above numerical values of dispersion and dispersion slope are obtained from US Pat. No. 6011892. US Pat. No. 6011892 describes a general single-mode optical fiber (SMF) and not a polarization-maintaining optical fiber (PMF), but basically causes the same dispersion. And is feasible.

Further, the WDM optical signal wavelength is set to an even channel,
Instead of dividing into odd-numbered channels, the divided channels are divided into, for example, 1.53 to 1.57 μm in the WDM wavelength band.
If a short wavelength band or a conventional wavelength band (C band) defined in the range 1 and a long wavelength band (L band) defined in the range of 1.57 to 1.61 μm are given, WDM propagating in each polarization axis The optical signals are in a state of less coupling, and the propagation quality can be improved.

Next, modified examples will be described with reference to FIGS.

FIG. 3 shows the polarization combiner 5-1 and the polarization separator 5 in the middle of the optical transmission line (FIG. 1 or 2) of the present invention.
-2, a wavelength selective add / drop module 6 capable of inserting / branching an optical signal in units of wavelength, that is, optical A
A DM (Add Drop Multiplexer) is added.

In FIG. 3, 11 is a polarization-maintaining optical fiber for transmission, 5-3 is a polarization splitter, 5-4 is a polarization combiner, and 6
Is a wavelength selective ADM (hereinafter referred to as WS-ADM) capable of inserting / branching an optical signal in wavelength units.

The optical signal propagating through the polarization-maintaining optical fiber 11 for transmission is polarization-split by the polarization splitter 5-3, enters the WS-ADM 6, and the drop signal is split by the wavelength selection function of the WS-ADM 6. The inserted signal is inserted, the polarization states are again assigned to the even-numbered channel and the odd-numbered channel, combined by the polarization combiner 5-4, and aligned with the polarization axis of the polarization-maintaining optical fiber 11 for transmission. Is transmitted.

FIG. 4 shows another modification. This is, in the middle of the polarization-maintaining optical fiber 11 for transmission shown in FIG. 1 or FIG. 2, a polarization splitter 5-3 for separating the output of the polarization-maintaining optical fiber into two linearly polarized lights orthogonal to each other, and Optical amplifiers 7-1 and 7-2 which are elements for optically amplifying polarized signals
And a polarizer 4 that assigns a polarization state orthogonal to its output
WDM using a polarization-maintaining optical fiber including -1, 4-2 and, more precisely, a polarization combiner 5-4 which is a polarization combining element for polarization combining the output of the polarizer. It is an optical transmission system.

FIG. 5 shows another modification. This is to insert an extinction ratio improver in the middle of the polarization maintaining fiber 11 for transmission shown in FIG. 1 or 2 for the purpose of improving the extinction ratio when the polarization states are coupled by long-distance transmission. It is a configuration example.

That is, the polarization-maintaining optical fiber 1 for transmission is used.
The optical signal propagating through 1 is separated into two groups of WDM signals in polarization states orthogonal to each other by the polarization splitter 5-3, and the demultiplexers 2-1 and 2-1 each belong to each wavelength group once. Demultiplexed in S / N
After increasing the ratio, the multiplexers 1-1 and 1-
The polarization states are orthogonalized by the polarizers 4-1 and 4-2, are combined by the polarization optical combiner 5-1 and are incident on the polarization-maintaining optical fiber 11 for transmission. As a result, the S / N is improved, and as a result, the extinction ratio is improved.

By inserting the extinction ratio improver having the above structure at any desired position in the PMF optical transmission line, high-speed and large-capacity transmission with good quality of the optical transmission line becomes possible.

<Use Method and Application System> As described above, the present invention uses the polarization-maintaining optical fiber.
Therefore, it can be used for multi-data transmission to a measurement system or a distributed system (using the add / drop function) that needs to transmit using a polarization plane.

Further, the present invention was devised in order to prevent coupling between the signals between the two polarization axes, and therefore, signals of two different wavelength bands are used for rising and falling. It is also possible to transmit data to the destination.

In carrying out the present invention, the results will be maximized if the following points are taken into consideration. That is, since the system of the present invention uses the polarization-maintaining optical fiber and expects that there is no interaction between the signals propagating in the two orthogonal polarization axes, strain or temperature change is added to the optical fiber. Then, coupling occurs between the orthogonal polarized waves. Therefore, it is necessary to provide a laying method (for example, in an underground tunnel) so that stress is not applied to the optical fiber as much as possible and there is no influence of temperature change.

By constructing the WDM optical transmission system of the present invention under such consideration, it is possible to avoid the influence of the waveform distortion due to the polarization dispersion, which is said to be a problem at 10 GbPS / ch or more. A high quality WDM optical transmission system can be constructed.

[0053]

As described above, according to the present invention, a wavelength-division-multiplexed signal is divided into two groups, an even channel and an odd channel, and the respective wavelength bands are divided into polarization-maintaining optical fibers. Since the light is input to the polarization axes orthogonal to each other and propagates, it is possible to realize a WDM optical transmission system having a denser wavelength arrangement at a higher speed than conventional. Therefore, according to the present invention, it is possible to realize an ultra-high-speed, ultra-large-capacity optical transmission system of several 100 wave multiplexing, which is expected in the future.

Further, external stress does not act on the optical fiber,
In addition, by adopting a laying method that is not affected by temperature changes,
The influence of waveform distortion due to polarization dispersion, which is said to be a problem at 10 GbPS / ch or more, can be avoided, and a higher quality WDM optical transmission system can be constructed.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of a WDM optical transmission system of the present invention.

FIG. 2 is a diagram showing a second embodiment of the WDM optical transmission system of the present invention.

FIG. 3 shows a modified example of the WDM optical transmission system of the present invention, and is a diagram showing a configuration example in which an add / drop module is inserted in the optical transmission line.

FIG. 4 is a diagram showing a modification of the WDM optical transmission system of the present invention, and is a diagram showing a configuration example in which an optical amplifier is inserted in an optical transmission line.

FIG. 5 is a diagram showing a modified example of the WDM optical transmission system of the present invention, and is a diagram showing a configuration example in which an extinction ratio improver is inserted in an optical transmission line.

FIG. 6 is a diagram showing operation waveforms of main parts of the WDM optical transmission system of the present invention.

[Explanation of symbols]

1, 1-1, 1-2 Multiplexer 2, 2-1 Demultiplexer 3-1 Wavelength splitter 3-2 Waveband combiner 4-1, 4-2 Polarizer (polarizing element) 5-1, 5-2 Polarization Wave combiner (polarization combiner) 5-3 Polarization splitter 5-4 Polarization combiner (polarization combiner) 6 Add / drop module (optical signal inserting / branching device) 7-1, 7-2 Optical amplifier (Optical amplification element) 11 Polarization preserving optical fiber (PMF) for transmission

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Claims (5)

[Claims] 1. A means for separating a wavelength-multiplexed optical transmission signal into even-numbered channels and odd-numbered channels, a polarization element for converting each of the separated optical signals into mutually perpendicular linearly polarized light, and respective lights from the polarization elements. A polarization combining element for polarization combining signals, an element for inputting an optical signal from the polarization combining element into mutually orthogonal polarization axes of a transmission polarization maintaining optical fiber, and a transmission polarization maintaining optical fiber A polarization-maintaining optical fiber comprising: a polarization splitting element for splitting an optical signal from an output into mutually perpendicular linearly polarized light; and a demultiplexer for wavelength-splitting the output signal of the polarization splitting element. WDM optical transmission system. 2. A WDM optical transmission system using a polarization-maintaining optical fiber according to claim 1, wherein an optical signal is inserted / dropped in wavelength units between the polarization separation element and the polarization combining element. A WDM optical transmission system using a polarization-maintaining optical fiber, in which a wavelength-selective add / drop module that can be used is inserted. 3. A WDM optical transmission system using a polarization-maintaining optical fiber according to claim 1, wherein the output of the polarization-maintaining optical fiber is orthogonal to each other in the middle of the polarization-maintaining optical fiber for transmission. A polarization-maintaining light, which is provided with a polarization splitter for splitting into polarized light, an optical amplification element for optically amplifying each polarization signal, and a polarization combining element for polarization combining the output of the optical amplification element. W using fiber
DM optical transmission system. 4. A WDM optical transmission system using a polarization-maintaining optical fiber according to claim 1, wherein said dispersion-maintaining optical fiber for transmission has a dispersion of 2
Suppressed to a finite value of about PS / nm · km, use a polarization-maintaining optical fiber characterized by using the polarization-maintaining optical fiber is assumed flat of 0.1 ps / nm less than ² · miles dispersion slope WDM optical transmission system. 5. A WDM optical transmission system using a polarization-maintaining optical fiber according to claim 1, wherein the even-numbered channel and the odd-numbered channel are divided into wavelength bands of 1.53 to 1.57 μm, respectively. 2. A WDM optical transmission system using a polarization-maintaining optical fiber, wherein the C band is defined in the range of 1 and the L band is defined in the range of 1.57 to 1.61 μm.