CN1937460B - Crosstalk restraining method and device using optical fiber grating dispersion compensator - Google Patents
Crosstalk restraining method and device using optical fiber grating dispersion compensator Download PDFInfo
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- CN1937460B CN1937460B CN2005100864989A CN200510086498A CN1937460B CN 1937460 B CN1937460 B CN 1937460B CN 2005100864989 A CN2005100864989 A CN 2005100864989A CN 200510086498 A CN200510086498 A CN 200510086498A CN 1937460 B CN1937460 B CN 1937460B
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Abstract
In WDM system, the chromatic dispersion compensation (CDC) based on multi-channel fiber raster (FR) includes wideband FR, sampling FR, super superpose FR and serial narrowband FR, etc. In all these forms, system interference includes both linear interference induced from limited extinction rate of FR and non-linear interference from cross phase modulation. This invention possesses restraint function over both linear and non-linear interference. The invention induces the cross phase modulation restrainer (CPMR) with random time-delay difference only between the neighbor channels. This chromatic dispersion compensator based on chirp fiber raster is practically an integrative scheme solving both CDC and CPMR. Its structure is simple without extra insertion loss. The system cost raises little.
Description
Affiliated technical field
The present invention relates to a kind of use fibre-optical grating compensator with dispersion cross talk restraining method and device, belong to the optical fiber telecommunications system technical field.
Background technology
In recent years, in areas of information technology, along with optical fiber communication and dispersion compensation technology rapid development, the transmission rate of high speed optical communication experimental system has reached more than tens Gb/s, transmission channel reaches tens channels, and multichannel dispersion compensation technology becomes a key technology that promotes the optical fiber communication development.Up to now, mostly international, domestic is to adopt dispersion compensating fiber (DCF) to carry out dispersion compensation, yet, the DCF loss is big, optical fiber non-linear strong, this is very disadvantageous for multi channel Optical Fiber Transmission, and is difficult to realize the compensation of large-scale chromatic dispersion gradient with DCF.Compensate with of the chromatic dispersion of Chirp fiber grating conventional fiber, have cost low, simple in structure, insert many-sided advantage such as the little and dependable performance of loss.Therefore, adopting the optical fiber grating dispersion compensation is a kind of dispersion compensation method with application prospect.
But, when carrying out dispersion compensation, be with outer chromatic dispersion influential during owing to the grating series connection to the time delay of other gratings with optical fiber grating pair multi-channel optical fiber communication system, influence is relevant with the number of the grating of cascade with the grating wavelength interval.Someone adopts multichannel optical fiber grating (as intensity sampled-grating and super stack fiber grating) to carry out multi channel dispersion compensation, yet, consider the saturation effect of photon-induced refractive index modulation, the phase place of fiber grating and amplitude all can produce distortion when the number of channel increases, and this has just limited the more making of multichannel optical fiber grating greatly.Therefore, people have invented the phase-samplomh fiber grating again.But no matter when adopting any method to make the Chirp fiber grating, because the various factors influence, it all can not be desirable making the reflectance spectrum of fiber grating and group delay curve, and can produce secondary lobe.In falling into the channel of other wavelength, secondary lobe will produce the coherent crosstalk in the channel.
Compare with dispersion compensating fiber, fiber grating has smaller non-linear, but in dwdm system, and when the number of channel is a lot, especially the interval of interchannel very little (0.4nm) time, the cross-phase modulating action is strong.The uniform fiber grating that the useful multiplexing demultiplexing device of people adds fibre delay line and series connection is done cross-phase modulation inhibitor.But because the unfavorable factors such as cost of its complex structure, the loss that increases system and increase system, its application is very limited.At present, generally be to adopt complicated dispersion management to suppress the cross-phase modulating action.But for the system based on the DCF dispersion compensation, the dispersion compensation rate scope that it is optimum and the tolerance limit of residual dispersion are all very little.This is that the dispersion compensation rate of system will be near 1 because CHROMATIC DISPERSION IN FIBER OPTICS is bigger, otherwise serious broadening will take place in pulse, and the interior intersymbol interference of channel will be compared seriously; And the dispersion compensation rate of system will be near 1 the time, shown in as mentioned, can cause the resonance of cross-phase modulation again.Therefore, concerning based on the system of DCF+G.652 optical fiber, it is conflicting suppressing its requirement from phase modulated and cross-phase modulation, therefore, the tolerance limit of its dispersion compensation rate scope and residual dispersion is all very little, in fact, is difficult to apply in the over distance dwdm system.
Because nonlinear crosstalk (cross-phase modulation and four-wave mixing effect) is mainly caused by adjacent channel, therefore, in fact it is poor not need that each interchannel is all introduced random delay, and it is just passable as long as between adjacent channel, introduce the random delay difference, so, the dispersion compensator that this patent proposes, it is poor only to introduce random delay between adjacent channel, just can suppress cross-phase modulation effect effectively.
Summary of the invention
In order to overcome existing technical deficiency, the invention provides a kind of use fibre-optical grating compensator with dispersion cross talk restraining method and device.The technical solution adopted for the present invention to solve the technical problems is:
A kind of use fibre-optical grating compensator with dispersion cross talk restraining method; Contain following steps;
Step 1: channel is divided into groups, use fiber grating to compensate multi channel chromatic dispersion;
Step 2: when making fiber grating, every group of channel carried out dispersion compensation with a fiber grating, take intensity sampling optical-fiber grating or phase-samplomh fiber grating or super stack fiber grating etc.;
Step 3: between adjacent channel, introduce the random phase difference and suppress nonlinear crosstalk, fiber grating is not on the same group used than long optical fiber L1, L2 couples together, fiber grating 1 on the same group, 2, the existing super stacks of 3 usefulness (superimpose) fiber grating, any manufacture methods such as intensity sampling optical-fiber grating or phase-samplomh fiber grating are made;
Step 4: adopt the method that increases the length between the fiber grating, reduce the linear crosstalk of fiber grating, when fiber grating is connected, surpass the coherence length of light source.
A kind of fibre-optical grating compensator with dispersion restraining device of crosstalking: be a kind of fiber grating multichannel dispersion compensator that in optical communication, uses.This patent has proposed to have the dispersion compensator that suppresses the function of crosstalking, and only introduces the random delay difference and suppress the cross-phase modulation between adjacent channel.Its structure utilizes the staggered multi-wavelength optical fiber grating of several wavelength to form as shown in Figure 3.
It has the function that inhibition is crosstalked.In existing wdm system,, comprise band optical fiber grating, sampling optical-fiber grating and super stack fiber grating and adopt several modes such as narrow band fiber grating of connecting based on the dispersion compensation of multichannel optical fiber grating.No matter adopt which kind of mode, the crosstalking of system all comprises two kinds of the nonlinear crosstalks that linear crosstalk that the limited side mode suppression ratio by fiber grating causes and cross-phase modulation cause.And this fibre-optical grating compensator with dispersion that the present invention proposes is a kind of dispersion compensation mode between narrow band fiber grating dispersion compensation and multi-wavelength optical fiber grating dispersion compensator, and linear crosstalk and nonlinear crosstalk are all had inhibit feature.
Because the fiber grating that writes can not be the desirable logical or band resistance of band, always having sidelobe exists, the reflection of sidelobe can influence the interior flatness of band of other cascaded fiber gratings, even more serious is because the light of sidelobe reflection can drop in the band of other cascaded fiber grating, with the flashlight of this channel be the light of same frequency, coherent phenomena can take place, this will make the time delay ripple of other cascaded fiber gratings become big, causes by compensating signal waveform deterioration, and the Power penalty of system is increased.No matter use which kind of fiber grating, this crosstalking all exists.Consider that the actual laser that uses all is that certain bandwidth is arranged in the communication, therefore, can not simply treat as it is monochromatic light, and should regard a kind of partially coherent light as.It is concerned with in the scope in relevant space, is exactly noncoherent and exceeded outside the relevant space.Be in outside the relevant space if work as the light signal of the fiber grating reflection of band external reflectance and adjacent channel, two-beam just can not interfere, and like this, crosstalking of the fiber grating of adjacent channel just deteriorates to incoherent crosstalking by coherent crosstalk.Therefore, when fiber grating is connected, can investigate the coherence length of light source, make distance between the fiber grating, make crosstalking of fiber grating deteriorate to incoherent crosstalking greater than the coherence length of laser.
Effect of the present invention is introduced the influence that random delay missionary society weakens the cross-phase modulation at interchannel, and uniform fiber grating that the useful multiplexing demultiplexing device of people adds fibre delay line and series connection is done cross-phase modulation inhibitor.But because the unfavorable factors such as cost of its complex structure, the loss that increases system and increase system, its application is very limited.Because nonlinear crosstalk (cross-phase modulation and four-wave mixing effect) is mainly caused by adjacent channel, therefore, in fact it is poor not need that each interchannel is all introduced random delay, and it is just passable as long as between adjacent channel, introduce the random delay difference, so, this patent has proposed to have the dispersion compensator that suppresses the function of crosstalking, and only introduces the random delay difference and suppress the cross-phase modulation between adjacent channel.Only adjacent not on the same group interchannel to have introduced random delay poor, there is not delay inequality at the corresponding interchannel of interchannel on the same group, measuring the group delay curve of cascaded fiber grating observes, the narrow band fiber grating of its group delay curve and series connection is all different with single multichannel optical fiber grating, have only adjacent channel to introduce group delay, and the delay inequality of interchannel on the same group is consistent.
This patent propose based on having of chirped fiber grating suppress the to crosstalk dispersion compensator of function, simple in structure, the system cost increase seldom and does not have extra Insertion Loss.
Description of drawings
The present invention is further described below in conjunction with drawings and Examples.
The reflectance spectrum and the time delay of the single fiber grating of Fig. 1 .a.;
The reflectance spectrum and the time delay of this fiber grating after Fig. 1 .b. series connection;
Fig. 2. cascaded fiber grating reflection spectrum and time delay when being distance L＞3m;
Fig. 3. have the schematic diagram of the sampling optical-fiber grating of the function that suppresses to crosstalk;
Fig. 4. (a). the group delay curve synoptic diagram of common sampling optical-fiber grating;
Fig. 4. (b). have the group delay curve synoptic diagram of the sampling optical-fiber grating of the function that suppresses to crosstalk.
Embodiment
Embodiment 1: because the grating that writes can not be the desirable logical or band resistance of band, always having sidelobe exists, the reflection of sidelobe can influence the interior flatness of band of other cascaded fiber gratings, even more serious is because the light of sidelobe reflection can drop in the band of other cascaded fiber grating, with the flashlight of this channel be the light of same frequency, coherent phenomena can take place, this will make the time delay ripple of other cascaded fiber gratings become big, causes by compensating signal waveform deterioration, and the Power penalty of system is increased.No matter use which kind of fiber grating, this crosstalking all exists.
Though crosstalking of fiber grating is very weak, because relevant effect, it still can produce very adverse influence to system.Consider in the communication that actual to use the signal of laser and transmission all be that certain bandwidth is arranged, therefore, can not be its simply as being monochromatic.This is a kind of partially coherent light.It is concerned with in the scope in relevant space.When the light signal of the fiber grating reflection of band external reflectance and adjacent channel is within the relevant space, two-beam can interfere so.The group delay ripple of fiber grating can worsen, and certain distortion can take place the waveform of light pulse, and this transmission to light signal is very disadvantageous.But, being in outside the relevant space if work as the light signal of the fiber grating reflection of band external reflectance and adjacent channel, two-beam just can not interfere, and like this, crosstalking of the fiber grating of adjacent channel just deteriorates to incoherent crosstalking by coherent crosstalk.
For partially coherent light, cross-correlation function is
And the time coherence of quasi-monochromatic light ripple is:
As seen, the mould value of time coherence is with presenting property of the interval time τ variation of two light waves.The time interval is big more, and degree of coherence is more little.And the time interval is exactly a light beam by the time between two adjacent fiber gratings, so, the interval that increases two adjacent fiber gratings, just can increase the time interval, the coherence of two-beam is weakened, and when time interval greater than coherence time the time, this two-beam can be thought incoherent.This length is called coherence length:
L＝cΔt＝c/Δv (3)
Therefore, just can be used as incoherent light the distance more than L＞3m has treated.For tester EG﹠amp; The CD400 of G, because the modulating frequency of laser is 700MHz, figure record when more than L＞3m apart from the time, the time delay ripple of cascaded fiber grating, with Fig. 1. (a) compare, as can be seen, the time delay ripple does not worsen, coherent crosstalk is degenerated and to be crosstalked for incoherent, only is the noise of being introduced by the sidelobe of fiber grating to the influence of system.
Concerning any one common fiber grating, no matter its secondary lobe is to be caused by the systematic error or the error of randomness, its secondary lobe generally speaking all is stronger from the nearer local reflectivity of the main lobe of fiber grating, and a little less than main lobe place reflection far away, especially concerning the wavelength interval is the dwdm system of 0.4nm, the lobe reflections of fiber grating in the so near position of wavelength always is bigger, and the index that can reach preferably can only reach-28dB at present; But concerning sampling optical-fiber grating, because the fiber grating of each wavelength is superimposed, this secondary lobe is to belong to coherent crosstalk, and the influence of system is not allowed to ignore.And, therefore also lost the effect that suppresses the cross-phase modulation owing to do not introduce delay inequality between each wavelength of sampling optical-fiber grating.If when the making wavelength interval is the sampling optical-fiber grating of 0.4nm, the fiber grating of adjacent channel is separately become N the sampling optical-fiber grating that the wavelength interval is bigger, they are together in series carry out dispersion compensation, General N does not need too big, and it is just enough to get 2-4.Be the dispersion compensator of a N=3 shown in Fig. 3, the wavelength interval of a sampling optical-fiber grating will become 3 * 0.4nm, and crosstalking of bigger secondary lobe can deteriorate to incoherent crosstalking; And the cross-phase modulation of 3 interchannels of wavelength vicinity also can be effectively suppressed, for nonlinear crosstalk, because the cross-phase modulation effect of adjacent channel is the principal element that influences systematic function, therefore, the performance of whole system can be greatly improved.
After a sampling optical-fiber grating was divided into n, its linear crosstalk reduced to some extent, but should be consistent with common sampling optical-fiber grating on spectrum, and still, owing to introduced delay inequality between adjacent channel, its sampling optical-fiber grating is different.Group delay curve by the dispersion compensator of making among Fig. 3 should be as Fig. 4. (b), with Fig. 4. and (a) compare, adjacent several interchannels have been introduced different delay inequality τ
1And τ
2, its numerical value is by distance L between the fiber grating 1 and L2 decision.And every two channels, the 3rd channel is consistent with the initial value of the group delay of that channel of beginning.Because the delay inequality of interchannel is very big, the difference of the initial value of the group delay between the actual adjacent channel that records is at random.
Fig. 3 is the concrete manufacture method of the fiber grating when getting n=3, when n gets other value, makes according to same rule.Fiber grating is not on the same group used than long optical fiber L1, and L2 couples together.Fiber grating 1,2,3 on the same group can be with existing super stack (superimpose) fiber grating, and any manufacture methods such as intensity sampling optical-fiber grating or phase-samplomh fiber grating are made.This dispersion compensator can obtain by test group delay curve with the main distinction of the dispersion compensator of using always, as shown in Figure 4.
Embodiment 2:
1. not being to use a fiber grating to carry out dispersion compensation, but channel is divided into groups, reduce the interval of channel on the same group as far as possible, is N as total number of channel, and channel wavelength and interval are respectively λ
1λ
2... λ
nλ
N+1λ
N+2... λ
N, grouping can be n, then the wavelength of every group of channel is; λ
1λ
N+1..., λ
2λ
N+2... ..., λ
n... λ
N, in fact N needn't be the multiple of n; N is more than 1.
2. when making fiber grating, can carry out dispersion compensation to every group of channel with a fiber grating, like this, N channel need use n fiber grating (1＜n＜N), according to studies show that, n does not need too big, generally gets n=2～4 and just can effectively reduce and crosstalk;
3. for the laser of certain wavelength, can obtain its coherence length according to formula L=c Δ t=c/ Δ v, when connecting fiber grating can so that the optical path difference between the fiber grating greater than the coherence length of laser; In fact, as long as the distance between the proper extension fiber grating just can reduce linear crosstalk;
4. be not delay inequality all to be arranged, and it is poor just to have introduced random delay at adjacent on the same group interchannel, does not have delay inequality on the same group the corresponding interchannel of interchannel not at all interchannels.
Claims (3)
1. one kind is used the fibre-optical grating compensator with dispersion cross talk restraining method, it is characterized in that: contain following steps:
Step 1: channel is divided into groups, use fiber grating to compensate multi channel chromatic dispersion;
Step 2: when making fiber grating, every group of channel carried out dispersion compensation with a fiber grating, take intensity sampling optical-fiber grating or phase-samplomh fiber grating or super stack fiber grating;
Step 3: between adjacent channel, introduce the random phase difference and suppress nonlinear crosstalk, fiber grating is not on the same group used than long optical fiber (L1, L2) couple together, fiber grating (1 on the same group, 2,3) with existing super stack fiber grating, any manufacture method in intensity sampling optical-fiber grating or the phase-samplomh fiber grating is made;
Step 4: adopt the method that increases the length between the fiber grating, reduce the linear crosstalk of fiber grating, when fiber grating is connected, surpass the coherence length of light source.
2. a kind of use fibre-optical grating compensator with dispersion cross talk restraining method according to claim 1 is characterized in that: in the step 1 channel is divided into groups, use fiber grating to be n fiber grating, n is 2～4.
3. one kind is used the fibre-optical grating compensator with dispersion restraining device of crosstalking, it is characterized in that: be a kind of fiber grating multichannel dispersion compensator that in optical communication, uses, utilize the staggered multi-wavelength optical fiber grating of wavelength to form, total number of channel is N, and channel wavelength is respectively λ
1λ
2λ
nλ
N+1λ
N+2λ
N, be grouped into n, the satisfied 1＜n＜N that concerns of n; Then the wavelength of every group of channel is: first group of λ
1λ
N+1, second group of λ
2λ
N+2..., n organizes λ
nλ
NEvery group of channel carries out dispersion compensation with a multi-wavelength optical fiber grating, and (L1's fiber grating on the same group L2) does not couple together with long optical fiber; Introduce the random delay difference at adjacent on the same group interchannel and suppress the cross-phase modulation.
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| CN1428960A (en) * | 2001-12-23 | 2003-07-09 | 华为技术有限公司 | On-line dispersion compensation device of wavelength division high-speed optical transmission system |
| CN1434583A (en) * | 2002-01-25 | 2003-08-06 | 清华大学 | Bragg grating with new sampling structure for compensating dispersion and polarization mode dispersion |
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| CN1428960A (en) * | 2001-12-23 | 2003-07-09 | 华为技术有限公司 | On-line dispersion compensation device of wavelength division high-speed optical transmission system |
| CN1434583A (en) * | 2002-01-25 | 2003-08-06 | 清华大学 | Bragg grating with new sampling structure for compensating dispersion and polarization mode dispersion |
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