Factors to be considered in DWDM networking : In this article we will discuss what are the factors that are considered in DWDM networking. There are some factors that can consider in DWDM networking. Dispersion limited distance, power , OSNR and some other factors are to be considered in DWDM networking. In this article I will explain about these points.
Factors, Dispersion limited distance, power , OSNR and some other factors play very important role in DWDM networking. You might be having some difficulty in understanding now, but when you read the entire article, the whole thing will be understood. If you work on a DWDM network, then it is mandatory for you to know about all the factors.
The length of this fiber is only on the order of several tens or hundreds. There is little difference in the qualitative dispersion of rare-earth-doped fibers with fibers defined in ITU-T recommendations G.652, G.653 and G.655. For systems of tens or hundreds of kilometers, the effect of this spread is negligible.
In single mode optical fiber, the major dispersion include material dispersion and wave-guide dispersion. There are different time delays for different frequency components when they arrive at the optical receiver after being transmitted through the optical fiber.
In the time zone, this broadens optical pulses, causes crosstalk and eye pattern erosion between them, and eventually leads to a degradation in system error performance.
Different frequency components in the signal are originated from the laser source optical spectrum characteristics, including wavelength, spectrum width, laser chirp, etc. At present, the -20dB optical spectrum width of SLM lasers at 1550nm region can be up to 0.05nm. In this case, the laser chirp is the determinant limiting the regenerative length.
In some subsystems of optical amplifier, a type of passive dispersion compensation device can be assembled with the optical amplifier to form an amplifier subsystem. This subsystem will add limited chromatic dispersion to the system. And the dispersion coefficient, inverse to the optical fibers of the system, will reduce the system chromatic dispersion. This device can be installed together with an EDFA to compensate the loss related to the passive dispersion compensation function.
Additionally, adopting G.655 optical fiber and G.653 optical fiber is favorable for chromatic dispersion reduction. If nonlinear faults are well considered, the G655 optical fiber has optimal over-all properties in long-haul transmission.
When we calculate dispersion during DWDM network design, the typical dispersion coefficient at 1550nm window is 17ps/nm.km because optical fibers employed in China are primarily G.652 fiber. But in engineering design, 20ps / nm.km is adopted for the budget.
During practical calculation, power budget is only conducted for two pieces of adjacent equipment in the transmission network instead of conducting unified power budget for the whole network. The distance (attenuation) between two pieces of adjacent equipment in the transmission network is called regeneration distance (attenuation).
Optical amplifier creates light around the signal wavelength, i.e. amplified spontaneous emission (ASE). In a transmission system with several cascading EDFAs, ASE noise of optical amplifiers will repeat a periodic attenuation and amplification process. Because the ASE noise input in each optical amplifier is amplified and charged to the ASE generated by that optical amplifier.. Hence the total ASE noise power will increase with the number of amplifiers in approximate proportion and the signal power will decrease. The noise power may exceed the signal power.
The ASE noise frequency spectrum extends along the length of the distribution system. When ASE noise from the first optical amplifier is sent to the second, the gain distribution of the second optical amplifier will change due to ASE noise due to the gain saturation effect. In this case similarly, the effective gain distribution of the third optical amplifier will also change. This effect will be transmitted downward to the next optical amplifier. Even though narrow-band filters are used in each optical amplifier, ASE noise will also accumulate. This is because noise is present in the signal frequency band.
Self-filtering method is suitable for systems with tens of or more optical amplifiers. This method adjusts the signal wavelength to the self-filtering wavelength in order to reduce the ASE noise received by the detector, similar to using a narrow band filter. This is most effective when the approach of reducing optical amplifier interval and employing low gain optical amplifier is used to reduce the initial ASE noise.
If all-optical DWDM closed ring network is adopted, the self-filtering method isn't suitable. In fact, the peak formed in the whole gain frequency spectrum of the optical amplifier may severely affect system performance. In this case, utilizing ASE filtering method can utmost reduce ASE noise accumulation. This is achieved via the approach of filtering the DWDM channels which are not sent to the network node before being switched out the node.
For systems with some optical amplifiers, the self-filtering method is not as effective as the ASE filtering method. The ASE filtering method can flexibly select the signal wavelength and has other advantages. The characteristics of the filter must be chosen carefully because the cascading filter has a near-band compared to the signal filter (unless it has a rectangular frequency band). For different network applications, OSNR requirements are almost the same, with slight differences as shown in below table-
OSNR is one of the most important factors that affect the DWDM system error performance. For a DWDM system with multiple cascading optical line amplifiers, the noise power is dominated by amplified spontaneous emission (ASE) noise.
In practical DWDM systems, EDFA gain inequality may cause difference in channel output power and EDFA noise coefficient. So during design, the OSNR of the worst channel should meet the requirements and have sufficient redundancy.
SBS effect has a minimum threshold power. However, research indicates that different types of optical fibers and even different optical fibers of the same type have different SBS threshold power. For external modulation systems adopting narrow spectrum line lasers, the typical SBS threshold power is on the order of 20~30mw. Since the effective core area of G.653 fiber is relatively small, the SBS threshold power of systems adopting G.653 fiber is lightly lower than that of systems adopting G.652 fiber. This is true for all the nonlinear effects. SBS threshold power is sensitive to the spectrum line breadth of the laser and the power level.
Transmission limitations : SBS greatly limits the optical power transmittable in the fiber. Below image describes this effect for narrow band lasers, where all the signal power falls into the Brillouin bandwidth. Then the forward transmission power gradually saturates and the backward scattering power rapidly increases.
Methods of reducing the effect : In a system whose laser line breadth is apparently larger than the Brillouin bandwidth or whose signal power smaller than the threshold power, SBS impairment won't occur.
Transmission limitations : SRS can occur in single wavelength systems and multi-wavelength systems. In systems with single wavelengths and no line amplifiers, the signal may be impaired by its effect when its power exceeds 1W. However, in multi-wavelength systems of relatively wider channel spacing, the channels of shorter wavelength will lose a portion of power to the higher-wavelength channels due to the effect of SRS, leading to a degradation of the signal-to-noise ratio performance.
This may limit the total capacity of systems with fixed total number of channels, channel spacing, mean launched optical power and total system length. The SRS dispersion threshold of systems adopting G.653 optical fiber is slightly lower than that of systems employing G.652 fiber because G.653 fiber has smaller equivalent core area. SRS will not cause practical degradation for single wavelength systems. However, it may limit the capacity of DWDM systems.
Methods of reducing the effect : In single wavelength systems, optical filters can be used to filter the unwanted frequency components. However, up to now there are no practical technologies for multi-wavelength systems to eliminate the effect of SRS. The effect of SRS effects can also be released by reducing the signal power. Nevertheless, no apparent SRS limitation has appeared in the carefully-designed DWDM systems implemented at the present time.
Transmission limitations : Typically, SPM is relatively obvious only in systems with high accumulated dispersion or ultra long lengths. Dispersion limiting systems may be unable to tolerate SPM effects. In multi-wavelength systems of narrow channel spacing, spectral widening due to SPM can cause interference between adjacent channels.
In G.652 optical fiber, SPM of the low chirp intensity modulated signal leads to compression of the pulse. For G.655 optical fiber of abnormal dispersion characteristic, the SPM effect of the signal is a function of the transmitter power. Pulse compression can suppress the chromatic dispersion and provide certain dispersion compensation. However, the maximal dispersion limitation and the corresponding transmission distance limitation still exist.
As per given above image illustrates compression of the transmission pulse caused by SPM of the low chirp intensity modulated signal in G.652 optical fiber, also it can be regarded as spectral broadening.
Methods of reducing the effect : To adopt G.653 optical fiber and configure the signal channels near the zero-dispersion zone will benefit the reduction of SPM effects. For systems employing G.652 optical fiber and less than 100km in length, the effects of SPM can be controlled by using dispersion compensation in appropriate intervals. The SPM effects can also be weakened by reducing the input optical power or configuring the system operating wavelengths over the zero dispersion wavelength of G.655 fiber.
Methods of reducing the effect : In this factor XPM can be controlled by selecting the appropriate channel spacing. Study shows that the signal distortion caused by XPM in multi-wavelength systems only occurs between adjacent channels. In a 3-channel system, the signal-to-noise ratio (SNR) of the central channel is nearly equal to that of single channel systems.
This is because the channel spacing has increased. Hence, the effect of XPM is negligible if signal channels have appropriate spacing. In simulation experiments for systems with a channel power consumption of 5mw, it is approved that a channel spacing of 100GHz is enough for reducing the effects of XPM. The dispersion penalty caused by XPM can be controlled by adopting dispersion compensation in proper intervals along the system.
When channel spacing is equal, these products will right enter the adjacent signal channels. If the phase matched condition is reached between the sideband and the initial signal, these two light-waves propagating along the optical fiber will generate highly efficient FWM.
Transmission limitations : Occurrence of FWM side-bands may cause remarkable reduction to the signal power. Even more critically, residual interference occurs when mixture products directly enter the signal channels. This kind of interference is determined by the interaction between the phases of the signals and the side-bands and indicated by the increase and decrease of the signal pulse amplitude.
Residual loss leads to closure of the eye pattern of the receiver and causes bit-error rate (BER) performance degradation. The effect of FWM can be reduced by the breakdown function of frequency spacing and chromatic dispersion and the phase matching among light-waves. G.652 fiber-optic systems suffer less loss of FWM compared to those adopting G.653. opposite of this, if a signal is right located at or near the zero-dispersion point, FWM may surge in a relatively short fiber length (i.e. tens of kilometer). Moreover, FWM is sensitive to the channel spacing.
Four-wave mixing may cause severe damage to multi-wavelength systems adopting ITU-T G.652 optical fiber because signals can merely tolerate a very small chromatic dispersion. In single channel systems, FWM interaction may occur between signals and ASE noises, as well as the main modes and sidemodes of the transmitters. The ASE phase noises accumulated by optical Herr effect are superimposed to the signal carriers and cause the broadening of the rear part of the signal spectrum.
Methods of reducing the effects : As mentioned above, FWM band can be suppressed by utilizing the fiber dispersion such as G.655. Here we can say that FWM damage can be released by arranging uneven channel spacing. To lower the power level of G.653 fiber-optic systems can permit multi-wavelength operation, but this will weaken the advantages of the optical amplifiers.
To properly suppress the generation of mixing products, a scheme has been proposed (an existing recommendation or new recommendation for future study) to adopt the optical fiber with a minimum permissible dispersion (non-zero dispersion) in the amplification bandwidth of EDFA. It is also a possible scheme to use the non-zero dispersion optical fiber of inverse dispersion characteristic as replacement section.
However, this replacement may encounter difficulties during installation, operation and maintenance because of the introduction of another kind of fiber. Some similar methods are discovered to adopt long fiber sections of limited dispersion and short fiber sections of inverse but relatively greater dispersion (for compensation).
A scheme has been proposed to adopt uneven and relatively large channel spacing to reduce the nonlinear effects and allow to arrange DWDM systems in G.653 fiber to reduce the effect of FWM. To use uneven channel spacing can guarantee that the mixing products caused by three or more channels won't fall into the wavelengths of other channels.
However, the power transfer from the signals to the mixing products (i.e. power loss of the signals) keep fixed due to the configuration of uneven channel spacing, and will still lead to remarkable closure phenomenon of the eye pattern. Increase of the channel spacing can also reduce the effect of FWM. This kind of remission technology may be restricted since the gain spectrum will be narrowed due to the cascading of optical amplifiers and the amplification spectrum will be narrowed due to the access of optical amplifiers.
For birefringent optical fibers, the first term generates a group delay time called polarization dispersion. This kind of polarization dispersion leads to a group delay difference between the orthogonal polarization states, as shown in below given image.
Although PMD effect randomly changes the polarization state of pulses propagating in optical fiber, a pair of orthogonal states or primary states can be determined, i.e. the signal incident to the fiber at the input end keeps its polarization state at the output end.
For the first term, these states are independent to the wavelength. However, in some cases the occurrence of the primary states may be related to the wavelength. This, together with the chromatic dispersion of the optical fiber, will lead to further degradation.
Birefringence of optical fiber is randomly introduced due to factors such as stress, bending, twisting and temperature. Random birefringence mechanism redetermines the local birefringence axis along the optical fiber and leads to the coupling between polarization modes. The fiber length between this change is called coupling length. The coupling length of an optical fiber refers to the sum of the average value of total local coupling length.
Transmission limitations : In digital transmission systems, PMD leads to inter-symbol interference. When the total dispersion is equal to 0.4T (T is the bit period), an optical power penalty of about 1dB is introduced. Current study shows that, optical fibers or optical fiber cables are apt to be standardized according to mean PMD, as well as digital transmission systems. It is predicted via computer simulation that the probability for the optical power penalty of a system to exceed 1dB is less than 10-9 if the mean PMD isn't greater than 0.1T.
In a long distance amplified systems employing polarization scrambler (a component which deliberately modulates the polarization state of the laser and makes it work in an unpolarized state), PMD leads to the increase of signal polarization. The interaction between polarization dependent loss and polarization hole-burning causes the degradation of system performances. When additional polarization dependent loss occurs in the system, greater secondary loss will be aroused.
The secondary effect may generate coupling between PMD and chromatic dispersion and increase the statistical component of the dispersion. This field is under study.
Methods of reducing the effects : Since the problem is caused by birefringence, all the efforts for reducing the effects of PMD are related to reducing the birefringence introduced during optical fiber cable manufacturing, such as optimizing optical fiber manufacturing, guaranteeing the concentration of optical fiber, reducing the residual of fiber core and employing accurate cable structure. Typical mean PMD of optical fiber cables is in the following range:
Another method is to add polarization controllers at the input end and the output end. A polarization splitter is connected after the output polarization controller and used to generate an error signal. The output polarization controller searches this error signal and readjusts the polarization controller to minimize the error signal. At the zero-error signal point, the input polarization state is the primary state of the system. This technology has been used to compensate a 5Gbit/s system. Coherent frequency division multiplexing systems also adopt a similar technique.
Transmission limitations : In amplified systems, the amplifiers operate in the power conservative mode. The signal and the noise are affected by PDL. However, the signal and the noise suffer different effects because the noise is unpolarized. The noise can be divided into a component parallel to the signal and another one orthogonal to the signal.
Methods of reducing the effects : For PMD, it is important to reduce the polarization mode dependent loss of the components. To be pointed out, the effect of polarization mode dependent loss increases with the number of the amplifiers. For example, this requirement is extremely strict in long-range submarine systems. In short distance systems with only several amplifiers, the effect of polarization mode dependent loss is for further study.
Although erbium ions are randomly distributed in the glass fiber rod material, dipoles related to the erbium ions are anisotropic in the micro level. When the linear polarization saturated signal is equidirectional to the primary axis of the dipoles, the polarization hole-burning has the greatest effect. However, when the polarization state of the saturated signal is elliptical or circular, its effect decreases.
Because the total differential gain is the vector sum of these two effects, both the signal laser and the pumping laser will affect the total effect. The degree of hole-burning is in direct proportion to the polarization. Unpolarized saturated signals have no hole-burning problem. This case, as a whole, is similar to the case of circular polarization signal.
Factors, Dispersion limited distance, power , OSNR and some other factors play very important role in DWDM networking. You might be having some difficulty in understanding now, but when you read the entire article, the whole thing will be understood. If you work on a DWDM network, then it is mandatory for you to know about all the factors.
Dispersion Limited Distance
What is the importance of dispersion in the DWDM network we all know. What is dispersion effect and why this factor to be considered in DWDM network, that we will discuss here.Dispersion effect description
Chromatic dispersion, due to the transmitter optical spectrum characteristic and optical fiber chromatic dispersion, is a major factor that limits transmission efficiency. Generally, adding an optical amplifier to a system will not significantly change the total chromatic dispersion. As active gain media in EDFA, rare-earth-doped optical fibers will cause slightly chromatic dispersion.The length of this fiber is only on the order of several tens or hundreds. There is little difference in the qualitative dispersion of rare-earth-doped fibers with fibers defined in ITU-T recommendations G.652, G.653 and G.655. For systems of tens or hundreds of kilometers, the effect of this spread is negligible.
Transmission limitations
As transmission rates in optical fiber communication systems increase continuously and as the optical amplifier increases the no-electrical-repeater optical transmission distance, the total dispersion and the associated dispersion penalty of the entire transmission link can be very large and must be dealt with seriously. The spread limit has currently become a determinant of regenerative segment length.In single mode optical fiber, the major dispersion include material dispersion and wave-guide dispersion. There are different time delays for different frequency components when they arrive at the optical receiver after being transmitted through the optical fiber.
In the time zone, this broadens optical pulses, causes crosstalk and eye pattern erosion between them, and eventually leads to a degradation in system error performance.
Different frequency components in the signal are originated from the laser source optical spectrum characteristics, including wavelength, spectrum width, laser chirp, etc. At present, the -20dB optical spectrum width of SLM lasers at 1550nm region can be up to 0.05nm. In this case, the laser chirp is the determinant limiting the regenerative length.
Methods of reducing the effect
Since the presence of optical amplifiers doesn't affect the chromatic dispersion effects in the system, it is not required to regulate specific methods of reducing these effects to minimum. However, EDFA, which makes possible long distance no-regenerative-repeater system, will aggravate the impairment caused by the chromatic dispersion in the system.In some subsystems of optical amplifier, a type of passive dispersion compensation device can be assembled with the optical amplifier to form an amplifier subsystem. This subsystem will add limited chromatic dispersion to the system. And the dispersion coefficient, inverse to the optical fibers of the system, will reduce the system chromatic dispersion. This device can be installed together with an EDFA to compensate the loss related to the passive dispersion compensation function.
Additionally, adopting G.655 optical fiber and G.653 optical fiber is favorable for chromatic dispersion reduction. If nonlinear faults are well considered, the G655 optical fiber has optimal over-all properties in long-haul transmission.
Considerations in network design
In DWDM network design, firstly the whole network is divided into several regenerator sections, letting the length of each section less than the dispersion limited distance of the laser. Hence, the performance of the whole network can tolerate the effect of dispersion.When we calculate dispersion during DWDM network design, the typical dispersion coefficient at 1550nm window is 17ps/nm.km because optical fibers employed in China are primarily G.652 fiber. But in engineering design, 20ps / nm.km is adopted for the budget.
Power
Long distance transmission of optical signal requires that the signal power is enough to compensate the attenuation of the optical fiber. Generally, the attenuation coefficient of G.652 optical fiber at 1550nm window is about 0.25dB/km. When factors such as optical connectors and optical fiber redundancy are taken into consideration, the combined optical fiber attenuation coefficient is generally less than 0.275dB/km. |
| Regeneration Attenuation |
As shown in the picture above, S is the transmit point of station A, R is the reference point of station B and L is the transmission distance between point S and point R. Then: Regeneration distance = Pout - Pin) /a.
Pout: As above image Channel output power of point S (in dBm). The optical power of point S is related to the configuration of point A.
Pin: The minimum input power (in dBm) of the permissible channel point R.
A: Optical fiber cable attenuation per kilometer (dB / km) (using 0.275dB / km according to IT-T recommendations. This has the effect of various factors, including connectors and redundancy).
Optical Signal-to-Noise Ratio ( OSNR )
Generation principles of noise
The ASE noise frequency spectrum extends along the length of the distribution system. When ASE noise from the first optical amplifier is sent to the second, the gain distribution of the second optical amplifier will change due to ASE noise due to the gain saturation effect. In this case similarly, the effective gain distribution of the third optical amplifier will also change. This effect will be transmitted downward to the next optical amplifier. Even though narrow-band filters are used in each optical amplifier, ASE noise will also accumulate. This is because noise is present in the signal frequency band.
In simple way we can define Optical signal-to-noise ratio (OSNR) as :
OSNR = channel optical signal power / channel noise optical power.
Transmission limitations
ASE noise accumulation affects the system SNR because SNR degradation of the received signal is mainly caused by ASE related beat noise. This type of beat noise linearly increases with the number of optical amplifiers. Hence, error rate degrades as the number of optical amplifiers increases. Besides, the noise is accumulated in exponential form to the gain amplitude of the amplifier.
As the result of optical amplifier gain, the ASE noise frequency spectrum will have a wavelength peak after accumulating in many optical amplifiers. To specially point out, when adopting closed all-optical ring, the ASE noise will infinitely accumulate if cascading infinite number of amplifiers. Although in systems with filters ASE accumulation is remarkably decreased due to the filters, intraband ASE will still increase as the number of optical amplifiers increases. Hence, SNR will degrade with the increase of amplifiers.
As the result of optical amplifier gain, the ASE noise frequency spectrum will have a wavelength peak after accumulating in many optical amplifiers. To specially point out, when adopting closed all-optical ring, the ASE noise will infinitely accumulate if cascading infinite number of amplifiers. Although in systems with filters ASE accumulation is remarkably decreased due to the filters, intraband ASE will still increase as the number of optical amplifiers increases. Hence, SNR will degrade with the increase of amplifiers.
Methods of ASE reduction
ASE noise accumulation may decrease as the interval of optical amplifiers reduces (when the total gain is equal to the total transmission channel attenuation) because ASE accumulates in exponential form with the increase of the gain amplitude of the amplifier. One of the following filter techniques can further reduce unexpected ASE noise, ie adopting ASE noise filter or using self-filtering effect (self-filtering method).Self-filtering method is suitable for systems with tens of or more optical amplifiers. This method adjusts the signal wavelength to the self-filtering wavelength in order to reduce the ASE noise received by the detector, similar to using a narrow band filter. This is most effective when the approach of reducing optical amplifier interval and employing low gain optical amplifier is used to reduce the initial ASE noise.
If all-optical DWDM closed ring network is adopted, the self-filtering method isn't suitable. In fact, the peak formed in the whole gain frequency spectrum of the optical amplifier may severely affect system performance. In this case, utilizing ASE filtering method can utmost reduce ASE noise accumulation. This is achieved via the approach of filtering the DWDM channels which are not sent to the network node before being switched out the node.
For systems with some optical amplifiers, the self-filtering method is not as effective as the ASE filtering method. The ASE filtering method can flexibly select the signal wavelength and has other advantages. The characteristics of the filter must be chosen carefully because the cascading filter has a near-band compared to the signal filter (unless it has a rectangular frequency band). For different network applications, OSNR requirements are almost the same, with slight differences as shown in below table-
 |
| OSNR requirements |
In practical DWDM systems, EDFA gain inequality may cause difference in channel output power and EDFA noise coefficient. So during design, the OSNR of the worst channel should meet the requirements and have sufficient redundancy.
Other Factors
Stimulated Brillouin Scattering (SBS)
Principles of Stimulated Brillouin Scattering (SBS) : In the intensity-modulated system employing narrow spectrum line breadth laser, the strong forward transmission signal will convert to backward transmission once the signal optical power exceeds in the system the stimulated Brillouin scattering (SBS) threshold. In SBS, the forward transmission light is scattered in the form of photons. Only the backward scattered light is in single mode optical fiber. The scattered light is shifted from 1550nm by about 11GHz.SBS effect has a minimum threshold power. However, research indicates that different types of optical fibers and even different optical fibers of the same type have different SBS threshold power. For external modulation systems adopting narrow spectrum line lasers, the typical SBS threshold power is on the order of 20~30mw. Since the effective core area of G.653 fiber is relatively small, the SBS threshold power of systems adopting G.653 fiber is lightly lower than that of systems adopting G.652 fiber. This is true for all the nonlinear effects. SBS threshold power is sensitive to the spectrum line breadth of the laser and the power level.
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SBS threshold of
the narrow band laser
|
Stimulated Raman Scattering (SRS)
Principles : SRS is a broadband effect related to the interaction of light with silicon atom vibration modes. SRS makes the signal wavelength works as a Raman pump of the channels of longer wavelength or the Raman-shifted light of spontaneous scattering. In any circumstance, the signals of shorter wavelength will always be weakened by this process. At the same time, the signals of longer wavelength will be enhanced.This may limit the total capacity of systems with fixed total number of channels, channel spacing, mean launched optical power and total system length. The SRS dispersion threshold of systems adopting G.653 optical fiber is slightly lower than that of systems employing G.652 fiber because G.653 fiber has smaller equivalent core area. SRS will not cause practical degradation for single wavelength systems. However, it may limit the capacity of DWDM systems.
Methods of reducing the effect : In single wavelength systems, optical filters can be used to filter the unwanted frequency components. However, up to now there are no practical technologies for multi-wavelength systems to eliminate the effect of SRS. The effect of SRS effects can also be released by reducing the signal power. Nevertheless, no apparent SRS limitation has appeared in the carefully-designed DWDM systems implemented at the present time.
Self-phase Modulation (SPM)
Principles : Because of the Herr effect, instantaneous variations in the power of an optical signal result in self modulation. This effect is called self-phase modulation. In single wavelength systems, SPM effect will broaden the signal's spectrum when changes in the signal's intensity result in variations in its phase, as shown in given below image. In the normal dispersion zone of optical fiber, signals propagating along the fiber will experience a longer instantaneous widen once the frequency spectral broadening is caused by SPM due to the chromatic dispersion. In the abnormal dispersion zone, the chromatic dispersion of optical fiber and SPM may compensate with each other. Thus the signal broadening will be smaller. |
| Compression and spectrum broadening of the transmission pulse caused by self-phase modulation |
Transmission limitations : Typically, SPM is relatively obvious only in systems with high accumulated dispersion or ultra long lengths. Dispersion limiting systems may be unable to tolerate SPM effects. In multi-wavelength systems of narrow channel spacing, spectral widening due to SPM can cause interference between adjacent channels.
In G.652 optical fiber, SPM of the low chirp intensity modulated signal leads to compression of the pulse. For G.655 optical fiber of abnormal dispersion characteristic, the SPM effect of the signal is a function of the transmitter power. Pulse compression can suppress the chromatic dispersion and provide certain dispersion compensation. However, the maximal dispersion limitation and the corresponding transmission distance limitation still exist.
As per given above image illustrates compression of the transmission pulse caused by SPM of the low chirp intensity modulated signal in G.652 optical fiber, also it can be regarded as spectral broadening.
Cross-phase Modulation (XPM)
Principles : In multi-wavelength systems, when variations in light intensity lead to a phase shift, cross-phase modulation will generally broaden the signal spectrum due to the interaction between adjacent channels. The spectral broadening caused by XPM is related to the channel spacing.
This is because the dispersion caused by difference in group velocities may lead to the interaction among the pulses which should separately propagate along the optical fiber. In case that XPM results in spectral broadening, the signals will suffer a relatively large instantaneous spectral broadening due to the chromatic dispersion effect when propagating along the optical fiber.
Transmission limitations : Impairment caused by XPM in G.652 fiber-optic systems is more obvious than that in G.653 and G.655 fiber-optic systems. The broadening, caused by XPM, leads to interference between adjacent channels in multi-wavelength systems.
Transmission limitations : Impairment caused by XPM in G.652 fiber-optic systems is more obvious than that in G.653 and G.655 fiber-optic systems. The broadening, caused by XPM, leads to interference between adjacent channels in multi-wavelength systems.
This is because the channel spacing has increased. Hence, the effect of XPM is negligible if signal channels have appropriate spacing. In simulation experiments for systems with a channel power consumption of 5mw, it is approved that a channel spacing of 100GHz is enough for reducing the effects of XPM. The dispersion penalty caused by XPM can be controlled by adopting dispersion compensation in proper intervals along the system.
Four-wave Mixing (FWM)
Principles : Four-wave mixing (FWM), also called four-phonon mixing, occurs in the case that two or three light-waves with different wavelength interact and cause new lightwaves at other wavelengths. These additional wavelengths are called mixture products or side-bands. This interaction can occur between signal and EDFA ASE noise in multi-wavelength systems, and between main mode and side mode. In case of 3 signals, the mixing products are shown below in given image. |
| The mixing products caused by 3-wave interaction |
Transmission limitations : Occurrence of FWM side-bands may cause remarkable reduction to the signal power. Even more critically, residual interference occurs when mixture products directly enter the signal channels. This kind of interference is determined by the interaction between the phases of the signals and the side-bands and indicated by the increase and decrease of the signal pulse amplitude.
Residual loss leads to closure of the eye pattern of the receiver and causes bit-error rate (BER) performance degradation. The effect of FWM can be reduced by the breakdown function of frequency spacing and chromatic dispersion and the phase matching among light-waves. G.652 fiber-optic systems suffer less loss of FWM compared to those adopting G.653. opposite of this, if a signal is right located at or near the zero-dispersion point, FWM may surge in a relatively short fiber length (i.e. tens of kilometer). Moreover, FWM is sensitive to the channel spacing.
Four-wave mixing may cause severe damage to multi-wavelength systems adopting ITU-T G.652 optical fiber because signals can merely tolerate a very small chromatic dispersion. In single channel systems, FWM interaction may occur between signals and ASE noises, as well as the main modes and sidemodes of the transmitters. The ASE phase noises accumulated by optical Herr effect are superimposed to the signal carriers and cause the broadening of the rear part of the signal spectrum.
To properly suppress the generation of mixing products, a scheme has been proposed (an existing recommendation or new recommendation for future study) to adopt the optical fiber with a minimum permissible dispersion (non-zero dispersion) in the amplification bandwidth of EDFA. It is also a possible scheme to use the non-zero dispersion optical fiber of inverse dispersion characteristic as replacement section.
However, this replacement may encounter difficulties during installation, operation and maintenance because of the introduction of another kind of fiber. Some similar methods are discovered to adopt long fiber sections of limited dispersion and short fiber sections of inverse but relatively greater dispersion (for compensation).
A scheme has been proposed to adopt uneven and relatively large channel spacing to reduce the nonlinear effects and allow to arrange DWDM systems in G.653 fiber to reduce the effect of FWM. To use uneven channel spacing can guarantee that the mixing products caused by three or more channels won't fall into the wavelengths of other channels.
However, the power transfer from the signals to the mixing products (i.e. power loss of the signals) keep fixed due to the configuration of uneven channel spacing, and will still lead to remarkable closure phenomenon of the eye pattern. Increase of the channel spacing can also reduce the effect of FWM. This kind of remission technology may be restricted since the gain spectrum will be narrowed due to the cascading of optical amplifiers and the amplification spectrum will be narrowed due to the access of optical amplifiers.
Polarization Mode Dispersion (PMD)
Principles : As we know, the fundamental mode in a circular symmetric dielectric waveguide is dual-degenerate. In a physical optical fiber, this degeneration is separated by birefringence. For polarization-maintaining fibers, birefringence is deliberately introduced. However, for general communication optical fibers, birefringence is an unexpected product which is randomly introduced due to the stress perturbation the fiber suffers.For birefringent optical fibers, the first term generates a group delay time called polarization dispersion. This kind of polarization dispersion leads to a group delay difference between the orthogonal polarization states, as shown in below given image.
 |
| Occurrence of group delay between the orthogonal polarization states |
For the first term, these states are independent to the wavelength. However, in some cases the occurrence of the primary states may be related to the wavelength. This, together with the chromatic dispersion of the optical fiber, will lead to further degradation.
Birefringence of optical fiber is randomly introduced due to factors such as stress, bending, twisting and temperature. Random birefringence mechanism redetermines the local birefringence axis along the optical fiber and leads to the coupling between polarization modes. The fiber length between this change is called coupling length. The coupling length of an optical fiber refers to the sum of the average value of total local coupling length.
In a long distance amplified systems employing polarization scrambler (a component which deliberately modulates the polarization state of the laser and makes it work in an unpolarized state), PMD leads to the increase of signal polarization. The interaction between polarization dependent loss and polarization hole-burning causes the degradation of system performances. When additional polarization dependent loss occurs in the system, greater secondary loss will be aroused.
The secondary effect may generate coupling between PMD and chromatic dispersion and increase the statistical component of the dispersion. This field is under study.
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Polarization Dependent Loss (PDL)
Principles : Polarization dependent loss is caused by dichromatism of optical passive components such as isolator and coupler. When a signal passes through a dichromatic component, its electric field part parallel to the loss axis will suffer certain attenuation. Like PMD, the axis direction which determines the PDL is randomly changed.
Optical amplification may increase the component orthogonal to the signal. Additionally, variations of the signal polarization lead to mode dispersion. Thus, magnitude of the orthogonal component of the noise is time-varied. This will reduce the signal-to-noise ratio at the receiver end and cause impairment to the system.
Polarization Hole-burning (PHB)
Principles : Polarization hole-burning (PHB) is the result of the anisotropic saturation caused by the polarization saturated signal light incident in the erbium-doped optical fiber. This will reduce the options of stimulated states utilizing the polarization field to locate. Hence, the available gain in the orthogonal direction is relatively large.Although erbium ions are randomly distributed in the glass fiber rod material, dipoles related to the erbium ions are anisotropic in the micro level. When the linear polarization saturated signal is equidirectional to the primary axis of the dipoles, the polarization hole-burning has the greatest effect. However, when the polarization state of the saturated signal is elliptical or circular, its effect decreases.
Because the total differential gain is the vector sum of these two effects, both the signal laser and the pumping laser will affect the total effect. The degree of hole-burning is in direct proportion to the polarization. Unpolarized saturated signals have no hole-burning problem. This case, as a whole, is similar to the case of circular polarization signal.
Transmission limitations : Because it makes the noise formed along the link larger than the noise budget calculated according to the simple linear theory, PHB will affect the performances of the system. The effects are that the signal-to-noise ratio decreases due to the PHB and that the ultimately measured Q value fluctuates under PMD and PDL.
Since there are two factors affecting PHB, there are two ways to affect the system performances. The total effect is in direct proportion to the gain saturation and increases with the saturation.
Firstly, we consider the effect of the polarized pumping laser. To reach the purpose of the discussion, the pumping polarization can be assumed as fixed. Pumping causes differential gain in the orthogonal polarization axis direction. The noise orthogonal to the pump is greater than the noise equidirectional to the pump. However, the polarization axes of the pumping lasers of the amplifiers along the link are incoherent to each other.
The accumulation effect shall be similar to a random walk. The pump which results in PHB can be regarded as a related factor to the PDL of an amplifier. Hence, the noise obtained by averaging the number of the amplifiers should be linear, the same as the budget calculated by the simple linear theory.
Signal lasers which cause PHB are slightly different. The lasers are used to propagate signals, so the polarization noise equidirectional to a signal laser obtains the same gain as the signal. However, the noise orthogonal to the signal laser is always orthogonal to the polarization axis of the signal. Hence, the signal increases in a nonlinear mode along the amplified link.
The total differential gain caused by PHB will change with the variations of the signal polarization state along the amplified link (caused by PMD). It changes because the hole-burning effect of the signal is related to the pump effect.
When staying in their corresponding polarization states, the signal laser and the pumping laser will change the amplitude of the differential gain variation. Hence, although this makes the total noise increase in a nonlinear form, the noise may be time-varied. As mentioned above, the signal-to-noise ratio will decrease and be time-varied.
Methods of reducing the effect : There are several methods for reducing the effect of PHB. It is a feasible method to amplify in the small-signal area, but it is not always possible. In many cases, it can meet the demands.
Actually, the simplest method is to adopt unpolarized signals which can be generated via many approaches. The most common approach is to adopt polarization scramble to generate signal. If a phase modulator is used, the polarization state will change between the two orthogonal states at all time. Thus, the signal seems to have no polarization.
This indicates that it's better to arrange the polarization modulation according to a double bit rate because the PDL in the amplifier will be converted from polarization modulation to amplitude modulation.
By adopting double bit rate polarization modulation, the amplitude fluctuation stays at the rate above the bandwidth of the detector and is not sensible to the receiver.
If this technology is used, the performances of very long distance systems will be improved and reach the expected purpose of high reliability. Polarization modulation has become the standard implementation method for overseas large systems.
Since there are two factors affecting PHB, there are two ways to affect the system performances. The total effect is in direct proportion to the gain saturation and increases with the saturation.
Firstly, we consider the effect of the polarized pumping laser. To reach the purpose of the discussion, the pumping polarization can be assumed as fixed. Pumping causes differential gain in the orthogonal polarization axis direction. The noise orthogonal to the pump is greater than the noise equidirectional to the pump. However, the polarization axes of the pumping lasers of the amplifiers along the link are incoherent to each other.
The accumulation effect shall be similar to a random walk. The pump which results in PHB can be regarded as a related factor to the PDL of an amplifier. Hence, the noise obtained by averaging the number of the amplifiers should be linear, the same as the budget calculated by the simple linear theory.
Signal lasers which cause PHB are slightly different. The lasers are used to propagate signals, so the polarization noise equidirectional to a signal laser obtains the same gain as the signal. However, the noise orthogonal to the signal laser is always orthogonal to the polarization axis of the signal. Hence, the signal increases in a nonlinear mode along the amplified link.
The total differential gain caused by PHB will change with the variations of the signal polarization state along the amplified link (caused by PMD). It changes because the hole-burning effect of the signal is related to the pump effect.
When staying in their corresponding polarization states, the signal laser and the pumping laser will change the amplitude of the differential gain variation. Hence, although this makes the total noise increase in a nonlinear form, the noise may be time-varied. As mentioned above, the signal-to-noise ratio will decrease and be time-varied.
Methods of reducing the effect : There are several methods for reducing the effect of PHB. It is a feasible method to amplify in the small-signal area, but it is not always possible. In many cases, it can meet the demands.
Actually, the simplest method is to adopt unpolarized signals which can be generated via many approaches. The most common approach is to adopt polarization scramble to generate signal. If a phase modulator is used, the polarization state will change between the two orthogonal states at all time. Thus, the signal seems to have no polarization.
This indicates that it's better to arrange the polarization modulation according to a double bit rate because the PDL in the amplifier will be converted from polarization modulation to amplitude modulation.
By adopting double bit rate polarization modulation, the amplitude fluctuation stays at the rate above the bandwidth of the detector and is not sensible to the receiver.
If this technology is used, the performances of very long distance systems will be improved and reach the expected purpose of high reliability. Polarization modulation has become the standard implementation method for overseas large systems.
However, in long distance amplified systems, PMD will result in secondary polarization and cause PHB which leads to the performance degradation of the systems. This effect improves the complex properties of the interaction of polarization effects in amplified links.
Last Word
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