Compensation for polarization mode dispersion and nonlinear birefringence in a multichannel optical fibre system
- Authors: Waswa, David Wafula
- Date: 2009
- Subjects: Fiber optics , Nonlinear optics , Polarization (Light)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10375 , http://hdl.handle.net/10948/885 , Fiber optics , Nonlinear optics , Polarization (Light)
- Description: Polarization mode dispersion (PMD) is stochastic in nature and continues evolving in an unpredictable manner according to the changing environment. Nonlinear birefringence in multichannel systems alters the polarization states of the bits, so that they vary from one bit to the next in a way that is difficult to predict. These are the two major signal-impairment effects that are inherent in optical fibre transmission links which can seriously degrade network performance. It is therefore extremely challenging to compensate for both linear and nonlinear birefringence in multichannel systems. The purpose of this thesis is to investigate the interaction between PMD and nonlinear induced birefringence in a fibre with consideration of mode coupling. A sound knowledge of this interaction is necessary in designing a linear and nonlinear polarization mode dispersion compensator for WDM systems, as was successfully carried out in this study. The investigation shows that the effect of nonlinear birefringence alone depolarizes the signal, while in high PMD links where polarization mode coupling is high, the nonlinear birefringence effect couples with second-order PMD such that it may reduce the penalty and improve the signal DOP. Further investigation shows that when nonlinear birefringence becomes significant, asymmetry arises between the two principal axes of the fibre, such that it is only one axis which experiences the effect of nonlinear birefringence. It is found out that along this vii axis, there exists a critical point in pump power where the nonlinear birefringence cancels PMD in the link and improves the signal. An adaptive compensator to cancel PMD and nonlinear birefringence was designed based on feedforward DOP-monitoring signal. The compensator was tested both at laboratory level and on the Telkom buried fibre link and found to be functioning as intended. It was able to adaptively track and compensate PMD in the link in less than a second. The compensator was able to cancel PMD in the link up to a maximum of 30 ps. The compensator improved the DOP of the worst signal by more than 100 percent.
- Full Text:
- Date Issued: 2009
- Authors: Waswa, David Wafula
- Date: 2009
- Subjects: Fiber optics , Nonlinear optics , Polarization (Light)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10375 , http://hdl.handle.net/10948/885 , Fiber optics , Nonlinear optics , Polarization (Light)
- Description: Polarization mode dispersion (PMD) is stochastic in nature and continues evolving in an unpredictable manner according to the changing environment. Nonlinear birefringence in multichannel systems alters the polarization states of the bits, so that they vary from one bit to the next in a way that is difficult to predict. These are the two major signal-impairment effects that are inherent in optical fibre transmission links which can seriously degrade network performance. It is therefore extremely challenging to compensate for both linear and nonlinear birefringence in multichannel systems. The purpose of this thesis is to investigate the interaction between PMD and nonlinear induced birefringence in a fibre with consideration of mode coupling. A sound knowledge of this interaction is necessary in designing a linear and nonlinear polarization mode dispersion compensator for WDM systems, as was successfully carried out in this study. The investigation shows that the effect of nonlinear birefringence alone depolarizes the signal, while in high PMD links where polarization mode coupling is high, the nonlinear birefringence effect couples with second-order PMD such that it may reduce the penalty and improve the signal DOP. Further investigation shows that when nonlinear birefringence becomes significant, asymmetry arises between the two principal axes of the fibre, such that it is only one axis which experiences the effect of nonlinear birefringence. It is found out that along this vii axis, there exists a critical point in pump power where the nonlinear birefringence cancels PMD in the link and improves the signal. An adaptive compensator to cancel PMD and nonlinear birefringence was designed based on feedforward DOP-monitoring signal. The compensator was tested both at laboratory level and on the Telkom buried fibre link and found to be functioning as intended. It was able to adaptively track and compensate PMD in the link in less than a second. The compensator was able to cancel PMD in the link up to a maximum of 30 ps. The compensator improved the DOP of the worst signal by more than 100 percent.
- Full Text:
- Date Issued: 2009
Polarization mode dispersion emulation and the impact of high first-order PMD segments in optical telecommunication systems
- Authors: Musara, Vitalis
- Date: 2009
- Subjects: Optical communications , Fiber optics , Polarization (Light)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10519 , http://hdl.handle.net/10948/1138 , Optical communications , Fiber optics , Polarization (Light)
- Description: In this study, focus is centred on the measurement and emulation of first-order (FO-) and second-order (SO-) polarization mode dispersion (PMD). PMD has deleterious effects on the performance of high speed optical transmission network systems from 10 Gb/s and above. The first step was characterising deployed fibres for PMD and monitoring the state of polarization (SOP) light experiences as it propagates through the fibre. The PMD and SOP changes in deployed fibres were stochastic due to varying intrinsic and extrinsic perturbation changes. To fully understand the PMD phenomenon in terms of measurement accuracy, its complex behaviour, its implications, mitigation and compensation, PMD emulation is crucial. This thesis presents emulator designs which fall into different emulator categories. The key to these designs were the PMD equations and background on the PMD phenomenon. The cross product from the concatenation equation was applied in order to determine the coupling angle β (between 0o and 180o) that results in the SO-PMD of the emulator designs to be either adjustable or fixed. The digital delay line (DDL) or single polarization maintaining fibre (PMF) section was used to give a certain amount of FO-PMD but negligible SO-PMD. PMF sections (birefringent sections) were concatenated together to ensure FO- and SO-PMD coexist, emulating deployed fibres. FO- and SO-PMD can be controlled by altering mode coupling (coupling angles) and birefringence distribution. Emulators with PMD statistics approaching the theoretical distributions had high random coupling and several numbers of randomly distributed PMF sections. In addition, the lengths of their PMF sections lie within 20% standard deviation of the mean emulator length. Those emulators with PMD statistics that did not approach the theoretical distributions had limited numbers of randomly distributed PMF sections and mode coupling. Results also show that even when an emulator has high random mode coupling and several numbers of randomly distributed PMFs, its PMD statistics deviates away from expected theoretical distributions in the presence of polarization dependent loss (PDL). The emulators showed that the background autocorrelation function (BACF) approaches zero with increasing number of randomly mode coupled fibre sections. A zero BACF signifies that an emulator has large numbers of randomly distributed PMF sections and its presence means the opposite. The availability of SO-PMD in the emulators made the autocorrelation function (ACF) x asymmetric. In the absence of SO-PMD the ACF for a PMD emulator is symmetric. SO-PMD has no effect on the BACF. Polarization-optical time domain reflectometry (P-OTDR) measurements have shown that certain fibre sections along fibre link lengths have higher FO-PMD (HiFO-PMD) than other sections. This study investigates the impact of a HiFO-PMD section on the overall FO- and SO-PMD, the output state of polarization (SOP) and system performance on deployed fibres (through emulation). Results show that when the wavelength-independent FO-PMD vector of the HiFO-PMD section is greater than the FO-PMD contributions from the rest of the fibre link, the mean FO-PMD of the entire link is biased towards that of the HiFO-PMD section and the SO-PMD increases (β ≠ 0o or 180o) or remains fixed (β = 0o or 180o) depending on the coupling angle β between the HiFO-PMD section and the rest of the fibre link. In addition, the FO-PMD statistics deviates away from the theoretical Maxwellian distribution. However, experimental results show that the HiFO-PMD section has negligible influence on the SOPMD statistical distribution. An increase in the amount of FO-PMD on a HiFO-PMD section reduces the output SOP spread to a given minimum, in this study the minimum was reached when the HiFO-PMD ≥ 35 ps. However, the outcome of the output SOP spread depends on the location of the HiFO-PMD section along the fibre link length. It was found that when the HiFO-PMD section introduces SO-PMD, the bit error rate (BER) is much higher compared to when it does not introduce SO-PMD.
- Full Text:
- Date Issued: 2009
- Authors: Musara, Vitalis
- Date: 2009
- Subjects: Optical communications , Fiber optics , Polarization (Light)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10519 , http://hdl.handle.net/10948/1138 , Optical communications , Fiber optics , Polarization (Light)
- Description: In this study, focus is centred on the measurement and emulation of first-order (FO-) and second-order (SO-) polarization mode dispersion (PMD). PMD has deleterious effects on the performance of high speed optical transmission network systems from 10 Gb/s and above. The first step was characterising deployed fibres for PMD and monitoring the state of polarization (SOP) light experiences as it propagates through the fibre. The PMD and SOP changes in deployed fibres were stochastic due to varying intrinsic and extrinsic perturbation changes. To fully understand the PMD phenomenon in terms of measurement accuracy, its complex behaviour, its implications, mitigation and compensation, PMD emulation is crucial. This thesis presents emulator designs which fall into different emulator categories. The key to these designs were the PMD equations and background on the PMD phenomenon. The cross product from the concatenation equation was applied in order to determine the coupling angle β (between 0o and 180o) that results in the SO-PMD of the emulator designs to be either adjustable or fixed. The digital delay line (DDL) or single polarization maintaining fibre (PMF) section was used to give a certain amount of FO-PMD but negligible SO-PMD. PMF sections (birefringent sections) were concatenated together to ensure FO- and SO-PMD coexist, emulating deployed fibres. FO- and SO-PMD can be controlled by altering mode coupling (coupling angles) and birefringence distribution. Emulators with PMD statistics approaching the theoretical distributions had high random coupling and several numbers of randomly distributed PMF sections. In addition, the lengths of their PMF sections lie within 20% standard deviation of the mean emulator length. Those emulators with PMD statistics that did not approach the theoretical distributions had limited numbers of randomly distributed PMF sections and mode coupling. Results also show that even when an emulator has high random mode coupling and several numbers of randomly distributed PMFs, its PMD statistics deviates away from expected theoretical distributions in the presence of polarization dependent loss (PDL). The emulators showed that the background autocorrelation function (BACF) approaches zero with increasing number of randomly mode coupled fibre sections. A zero BACF signifies that an emulator has large numbers of randomly distributed PMF sections and its presence means the opposite. The availability of SO-PMD in the emulators made the autocorrelation function (ACF) x asymmetric. In the absence of SO-PMD the ACF for a PMD emulator is symmetric. SO-PMD has no effect on the BACF. Polarization-optical time domain reflectometry (P-OTDR) measurements have shown that certain fibre sections along fibre link lengths have higher FO-PMD (HiFO-PMD) than other sections. This study investigates the impact of a HiFO-PMD section on the overall FO- and SO-PMD, the output state of polarization (SOP) and system performance on deployed fibres (through emulation). Results show that when the wavelength-independent FO-PMD vector of the HiFO-PMD section is greater than the FO-PMD contributions from the rest of the fibre link, the mean FO-PMD of the entire link is biased towards that of the HiFO-PMD section and the SO-PMD increases (β ≠ 0o or 180o) or remains fixed (β = 0o or 180o) depending on the coupling angle β between the HiFO-PMD section and the rest of the fibre link. In addition, the FO-PMD statistics deviates away from the theoretical Maxwellian distribution. However, experimental results show that the HiFO-PMD section has negligible influence on the SOPMD statistical distribution. An increase in the amount of FO-PMD on a HiFO-PMD section reduces the output SOP spread to a given minimum, in this study the minimum was reached when the HiFO-PMD ≥ 35 ps. However, the outcome of the output SOP spread depends on the location of the HiFO-PMD section along the fibre link length. It was found that when the HiFO-PMD section introduces SO-PMD, the bit error rate (BER) is much higher compared to when it does not introduce SO-PMD.
- Full Text:
- Date Issued: 2009
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