This document reviews the polarization-nulling technique for monitoring optical-signal-to-noise ratio (OSNR) in dynamic wavelength-division multiplexing (WDM) networks. It describes how the technique utilizes different polarization properties of optical signals and noise to measure OSNR. However, its performance can be affected by factors like polarization-mode dispersion, nonlinear birefringence, and polarization-dependent loss in the transmission link. The document analyzes these effects and introduces techniques to overcome problems and accurately measure OSNR even with large differential group delays or nonlinear birefringence. It also evaluates the technique's performance in different fiber link experiments.
IRJET- Transmission Distortion Compensation using Optical Phase Conjugation i...IRJET Journal
This document discusses using optical phase conjugation (OPC) in a chalcogenide waveguide to compensate for transmission distortions in a dense wavelength division multiplexing (DWDM) system. OPC can reverse the effects of fiber dispersion and nonlinearities by inverting the signal spectrum in the middle of the transmission link. Simulation results show that OPC placed in the middle of a 4x56 Gb/s DQPSK DWDM system over 10 spans of standard single mode fiber can simultaneously compensate for both fiber dispersion and nonlinearities. The effectiveness of compensation is investigated using the symmetric power-accumulated dispersion diagram. Chalcogenide waveguides are proposed for realizing broadband and flexible OPC due to their ultra-high
Received Power performance in downlink architecture of Radio-over-Fiber Trans...IOSR Journals
Abstract : The In this paper, we studied the RoF system and analyzed the received power performance in downlink architecture of RoF system. The RoF system employs a Mach–Zehnder modulator (MZM) and a phase shifter to externally generate an optical single sideband (OSSB) signal since the OSSB signal is tolerable for power degradation due to a chromatic fiber-dispersion effect. The received power performance is analyzed by calculating a factor called Power Penalty. It is shown that Power penalty is increased exponentially as the differential delay increased with the distance due to chromatic dispersion with the change in laser linewidth (𝛾𝑅𝐹) from 10MHz to 1000MHz. The results are calculated for various transmission distances (𝐿𝐹𝐼𝐵𝐸𝑅) 1km to 40km for optical distances. The frequency of laser taken is 30-GHz RF carrier (𝑓𝑅𝐹) and wavelength 1550-nm laser (λ) with zero line width, fiber dispersion parameter (D) 17 ps/nm·km.
Keywords: Chromatic dispersion, DEMZM, Laser line width, Power penalty and Received power.
The effects of optical nonlinearities in optical fibers are ubiquitous. These effects can be deleterious, but
also have many useful applications, especially in the field of implementing all-optical functionalities in optical
networks. The paper is devoted to modeling the evolution of signal at 400 Gbps during non linear propagation for
DP 16-QAM modulation format in order to show the impact of optical launch power and fiber span transmission
on the propagation. This study is focused on the non linear propagation of fiber to obtain the relationship between
Optical Launch Power (OLP) and Error Vector Magnitude (EVM). It is found that the for the range of launch
power (-8dBm to 4dBm) , EVM is below 16%.
Keywords: DP, EVM, OLP, QAM.
Attosecond pulses produced by using HHG in gases, it is possible to make a few simple statements: attosecond pulses are unique tools for the investigation of ultrafast electronic processes in atoms, molecules, nano structures and solids. Impressive progress has been demonstrated from the technological point of view, with the possibility to routinely generate attosecond pulses in perfectly reproducible ways.
Investigating the single and doubly periodic mapping in fully Dispersion mana...SachidanandChikkpeti
Implementation and comparison of classical and the
doubly periodic dispersion mapping for the 40gb/s single channel
long haul optical transmission system for on-off keying(OOK)
modulation, the system has been analyzed using in-line and
post-compensation scheme setting pre-compensation to zero with
maintaining system’s net residual dispersion to zero in the
nonlinear regime.
This document compares techniques for reducing out-of-band interference in OFDM systems, including carrier nulling, windowing, and active interference cancellation (AIC). It implements these techniques in MATLAB and analyzes their effectiveness in lowering interference levels. Specifically, it shows that AIC provides the most interference suppression but results in power peaks as the number of AIC tones increases. To address this, the document proposes incorporating an optimization technique to restore power levels when varying the number of AIC tones.
The document discusses advance optical modulation formats for high speed optical networks. It explains that using proper modulation formats can help achieve high speed networks by reducing linear and nonlinear impairments. It compares various intensity and phase modulation formats and their effectiveness in mitigating distortions at 10Gbps, 40Gbps and 100Gbps network speeds. The document proposes further research on modulation formats that can combat fiber nonlinear impairments like correlatve coding and partial response formats to design high capacity optically routed networks.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
IRJET- Transmission Distortion Compensation using Optical Phase Conjugation i...IRJET Journal
This document discusses using optical phase conjugation (OPC) in a chalcogenide waveguide to compensate for transmission distortions in a dense wavelength division multiplexing (DWDM) system. OPC can reverse the effects of fiber dispersion and nonlinearities by inverting the signal spectrum in the middle of the transmission link. Simulation results show that OPC placed in the middle of a 4x56 Gb/s DQPSK DWDM system over 10 spans of standard single mode fiber can simultaneously compensate for both fiber dispersion and nonlinearities. The effectiveness of compensation is investigated using the symmetric power-accumulated dispersion diagram. Chalcogenide waveguides are proposed for realizing broadband and flexible OPC due to their ultra-high
Received Power performance in downlink architecture of Radio-over-Fiber Trans...IOSR Journals
Abstract : The In this paper, we studied the RoF system and analyzed the received power performance in downlink architecture of RoF system. The RoF system employs a Mach–Zehnder modulator (MZM) and a phase shifter to externally generate an optical single sideband (OSSB) signal since the OSSB signal is tolerable for power degradation due to a chromatic fiber-dispersion effect. The received power performance is analyzed by calculating a factor called Power Penalty. It is shown that Power penalty is increased exponentially as the differential delay increased with the distance due to chromatic dispersion with the change in laser linewidth (𝛾𝑅𝐹) from 10MHz to 1000MHz. The results are calculated for various transmission distances (𝐿𝐹𝐼𝐵𝐸𝑅) 1km to 40km for optical distances. The frequency of laser taken is 30-GHz RF carrier (𝑓𝑅𝐹) and wavelength 1550-nm laser (λ) with zero line width, fiber dispersion parameter (D) 17 ps/nm·km.
Keywords: Chromatic dispersion, DEMZM, Laser line width, Power penalty and Received power.
The effects of optical nonlinearities in optical fibers are ubiquitous. These effects can be deleterious, but
also have many useful applications, especially in the field of implementing all-optical functionalities in optical
networks. The paper is devoted to modeling the evolution of signal at 400 Gbps during non linear propagation for
DP 16-QAM modulation format in order to show the impact of optical launch power and fiber span transmission
on the propagation. This study is focused on the non linear propagation of fiber to obtain the relationship between
Optical Launch Power (OLP) and Error Vector Magnitude (EVM). It is found that the for the range of launch
power (-8dBm to 4dBm) , EVM is below 16%.
Keywords: DP, EVM, OLP, QAM.
Attosecond pulses produced by using HHG in gases, it is possible to make a few simple statements: attosecond pulses are unique tools for the investigation of ultrafast electronic processes in atoms, molecules, nano structures and solids. Impressive progress has been demonstrated from the technological point of view, with the possibility to routinely generate attosecond pulses in perfectly reproducible ways.
Investigating the single and doubly periodic mapping in fully Dispersion mana...SachidanandChikkpeti
Implementation and comparison of classical and the
doubly periodic dispersion mapping for the 40gb/s single channel
long haul optical transmission system for on-off keying(OOK)
modulation, the system has been analyzed using in-line and
post-compensation scheme setting pre-compensation to zero with
maintaining system’s net residual dispersion to zero in the
nonlinear regime.
This document compares techniques for reducing out-of-band interference in OFDM systems, including carrier nulling, windowing, and active interference cancellation (AIC). It implements these techniques in MATLAB and analyzes their effectiveness in lowering interference levels. Specifically, it shows that AIC provides the most interference suppression but results in power peaks as the number of AIC tones increases. To address this, the document proposes incorporating an optimization technique to restore power levels when varying the number of AIC tones.
The document discusses advance optical modulation formats for high speed optical networks. It explains that using proper modulation formats can help achieve high speed networks by reducing linear and nonlinear impairments. It compares various intensity and phase modulation formats and their effectiveness in mitigating distortions at 10Gbps, 40Gbps and 100Gbps network speeds. The document proposes further research on modulation formats that can combat fiber nonlinear impairments like correlatve coding and partial response formats to design high capacity optically routed networks.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
IRJET- Regeneration Analysis using Erbium Doped Fiber AmplifierIRJET Journal
This document summarizes a research paper that analyzes regeneration using erbium-doped fiber amplifiers. It begins with an abstract that outlines the goal of realizing an all-optical 3R regenerator for high-speed optical networks. It then discusses various components used in the system, including erbium-doped fiber amplifiers, semiconductor optical amplifiers, and fiber Bragg gratings. Simulation results are presented to evaluate the performance of the regenerator design for different parameters. The conclusions indicate that compensation techniques are needed to achieve high quality factors for transmission links operating at data rates of 40Gb/s and above.
This document summarizes research on the effects of third-order dispersion (TOD) in long-haul optical fiber communication systems. The research used simulation software to model propagation of high-speed pulses through standard single-mode fiber (SSMF) and non-zero dispersion-shifted fiber (NZDSF) systems with dispersion compensation fiber (DCF). Results showed increased pulse broadening and oscillatory tails with higher bit rates due to TOD. NZDSF systems demonstrated lower distortion compared to SSMF. TOD effects were also dependent on transmission distance, duty cycle, and fiber types used. The outcomes provide insights useful for designing very high-speed, long-distance optical networks.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
This document describes a simulation of transmitting 20 channels of either 10 Gbps or 20 Gbps data over 80 km spans of conventional single mode fiber using wavelength division multiplexing in the L-band spectrum region. Both pre-compensation and post-compensation techniques for dispersion were considered. Post-compensation provided better performance with transmission distances of up to 720 km for a total throughput of 200 Gbps and 320 km for 400 Gbps, and lower bit error rates compared to pre-compensation. Non-linear effects and amplified spontaneous emission noise were also accounted for in the simulation.
Mode-Locked Erbium Doped Pulse Fiber Laser Using the Kerr EffectKyle McSwain
This document describes an experiment to build a pulsed fiber laser using erbium-doped fiber and the Kerr effect. The researchers were able to generate ≤400ps pulses within a 12 meter ring cavity at a peak spacing of about 60 ns. They believe this places the pulses in either the soliton or stretched pulse regime. The laser utilizes an artificial saturable absorber composed of polarizing elements to generate pulses in the 1550 nm spectrum through passive mode-locking via the Kerr effect and self-phase and self-amplitude modulation.
Reduction of Four-Wave Mixing in DWDM System using Electro-Optic Phase Modula...IJECEIAES
In this paper, electro-optic phase modulator (EOPM) is used to reduce the effect of four-wave mixing (FWM), which is placed after 64 DWDMchannels multiplexer. It was found that the FWM is very sensitive to the phase deviation of the EOPM, and it can be reduced by introducing a phase shift between pulses. The simulation results confirmed the ability of the EOPM in improving the system performanceas indicated by the bit error rates. In term of comparison, the system of 64 channels based intensity modulated/ direct detection (IM/DD) transmission achieved bit error rate of 10 over 30 km and 70km without and with EOPM, respectively. -26
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
RF testing has remained hype for most of us. But seriously it is not so. It can be very interesting and one can develop a lot of interest in this if given an opportunity.
In this paper, authors have started with the some basic concepts of radio engineering which we studied in engineering and built upon these concepts to use in practical applications.
We have also described the basic principles of Signal Analyzer and Signal Generator which are the most common test tools used for any radio testing.
This document summarizes a research article that experimentally demonstrates a multi-wavelength fiber laser based on a Lyot filter. Six laser lines with over 5dB extinction ratio were generated using a combination of four-wave mixing in a highly nonlinear fiber and a Lyot filter mechanism in the laser cavity. The wavelength spacing of the multi-wavelength output was 0.15nm, which matches the characteristics of the Lyot filter used consisting of a polarization maintaining fiber. The laser provides potential applications in optical communication systems and instrumentation.
An optical add-drop multiplexer was developed by integrating silicon waveguide optical circulators and a Bragg reflector. A silicon waveguide Bragg reflector was fabricated that achieved 30 dB reflection for TM mode signals of a particular wavelength. An optical add-drop multiplexer device was demonstrated by combining two optical circulators with the Bragg reflector. It was able to add and drop signal light of a specific wavelength while passing other wavelengths through.
This document provides an overview of microwave link fundamentals, including:
1. Microwaves are electromagnetic radiations between 1-30 GHz used in telecommunications. Higher frequencies allow for higher bandwidth but require more advanced processing capabilities.
2. Microwave links are used in telecom industry applications like BTS connectivity and point of interconnect connectivity. Frequency allocation depends on distance, with shorter distances using higher frequencies.
3. Key factors that affect microwave links include reflection, refraction, diffraction, scattering, and absorption in the atmosphere. Diversity techniques like frequency and space diversity can help overcome some of these factors.
Spm limited long haul optical transmission in rz dpsk for varying input launc...Alexander Decker
This document summarizes a study on the performance of return-to-zero differential phase shift keying (RZ-DPSK) modulation for optical fiber transmission systems. The study uses numerical simulations to analyze transmission over 4000 km for varying input launch powers and data rates of 10Gbps and 20Gbps. The results show that RZ-DPSK modulation provides better performance than other formats, with maximum Q values achieved at around 7-8dB of input power. Performance degradation is seen with increasing data rate and transmission distance due to linear and nonlinear impairments.
This document discusses band pass filters that are constructed using a combination of low pass and high pass filters integrated with operational amplifiers. It provides equations to calculate key filter parameters like resonance frequency, bandwidth, and cutoff frequencies based on resistor and capacitor values. Simulation results show that as the near infrared operating wavelength increases, the filter capacitance increases, resistance decreases, resonance frequency and cutoff frequencies decrease, and gain increases. The document examines these filter characteristics over a wide range of parameters.
This paper summarizes research on using fiber Bragg gratings (FBG) for dispersion compensation in a 10 Gbps optical communication system using return-to-zero (RZ) modulation format. Simulation results showed that using an FBG to compensate for dispersion in a 3200 km long single mode fiber transmission link improved the bit error rate and Q-factor. Eye diagrams taken at 100 km, 1600 km, and 3200 km after dispersion compensation by the FBG demonstrated reduced pulse spreading and improved eye openings compared to the uncompensated case. The FBG provided effective dispersion compensation to improve transmission performance over long distances.
In this paper, an adaptive control system is employed in a novel implementation technique of feedforward linearization system for optical analog communication systems’ laser transmitter. The adaptive control system applies the Newton trust-region dogleg algorithm, which is a numerical optimization algorithm, to automatically tune the adjustment parameters in the feedforward loops to optimize the feedforward system performance and adapt to process variations. At the end of the paper, significant reductions of over 20 dBm in the third-order intermodulation distortion products have been achieved for operating frequencies from 5.0 to 5.8 GHz.
This document summarizes key concepts about optical amplifiers and networks. It discusses how optical amplifiers work by stimulating emission to amplify light, and describes common amplifier types like semiconductor optical amplifiers, doped fiber amplifiers, and Raman amplifiers. It also provides details on erbium-doped fiber amplifiers, including their operation, architecture, and use in telecommunication networks using SONET/SDH protocols. SONET/SDH networks are often configured in ring topologies to provide protection against link and node failures.
A high efficient narrow linewidth fiber laser based on fiber Bragg grating Fabry-Perot (FBG F-P) cavity was demonstrted. The spatial hole burning effect was restrained by fiber Faraday rotator(FR). Two short FBG F-P cavities as narrow bandwidth filters discriminated and selected the laser longitudinal modes efficiently. Stable single frequency 1550nm laser was acquired. Pumped by two 976nm LD, the fiber laer exhibited a 11mW threshold. The 73mW output power was obtained upon the maximum 145mW pump power. The optical-optical efficciency was 50 and the slope efficiency was 55. The 3dB linewidth of laser was less than 10kHz, measured by the delayed self-heterodyne method with 10km mono-mode fiber. The high power narrow linewidth fiber lasr can be used in high resolution fiber sensor system .
Investigation of optimal duty cycle for gvd undercompensatedIAEME Publication
This document summarizes an investigation into optimizing the duty cycle of input pulses in an undercompensated optical fiber link. The link consists of 5 loops, each with 50km of standard single mode fiber followed by dispersion compensating fiber. Simulation results show that higher duty cycle pulses (66-75%) permit greater undercompensation (up to 1.3% reduction in dispersion compensating fiber length) while maintaining reliable communication compared to lower duty cycle pulses (25-33%) which only tolerate up to 1% undercompensation. The optimal duty cycle balances fiber nonlinearities and accumulated dispersion to maximize the quality factor over the greatest range of input powers for a given level of undercompensation.
This document discusses coherent optical orthogonal frequency division multiplexing (CO-OFDM) for visible light communications. It begins with an introduction describing increasing bandwidth demand and the benefits of CO-OFDM. It then explains the basic principles of coherent optical transmission and CO-OFDM. The document proceeds to describe simulations conducted using OptiSystem software to model a 4-QAM CO-OFDM system over varying transmission distances. The results show degradation of the signal constellation and optical signal-to-noise ratio as distance increases due to attenuation and chromatic dispersion. Finally, the document concludes with a review of the CO-OFDM fundamentals and simulation results.
6 ijaems jan-2016-15-comparative analysis of free space optics and single mod...INFOGAIN PUBLICATION
In this paper we have done comparative analysis of free space optics and single mode fibre optical channels system. Performance study is done for variable ranges 1km, 50km and 100km for the FSO and SMF channels. Single Mode Fiber and Free Space Optical Channel is observed that maximum Q factor, minimum bit error rate and signal power. FSO requires no licensing or frequency synchronization. It also provides the transmission of data with unlimited bandwidth. Results expose the ability of improving the Q factor by using these channels and the optimum effect concluded at (1km) length of optical rang by using the FSO channel i.e. the quality factor and signal power were increased to reach (1943 ) and (22.46dBm) respectively and MIN. BER is zero.
Analysis of propagation of modulated optical signal in an integrated optic enviIAEME Publication
This document analyzes the propagation of modulated optical signals in integrated optic environments. It begins by discussing how modulation distributes an optical signal's power across multiple frequency components, requiring analysis beyond the monochromatic assumption. It then models a slab waveguide numerically to show how propagation constants and mode profiles vary with frequency. Finally, it simulates the phase variations experienced by different frequency components of a modulated signal propagating through integrated optic structures like straight waveguides and bends, demonstrating their ability to support wide bandwidths.
IRJET- Regeneration Analysis using Erbium Doped Fiber AmplifierIRJET Journal
This document summarizes a research paper that analyzes regeneration using erbium-doped fiber amplifiers. It begins with an abstract that outlines the goal of realizing an all-optical 3R regenerator for high-speed optical networks. It then discusses various components used in the system, including erbium-doped fiber amplifiers, semiconductor optical amplifiers, and fiber Bragg gratings. Simulation results are presented to evaluate the performance of the regenerator design for different parameters. The conclusions indicate that compensation techniques are needed to achieve high quality factors for transmission links operating at data rates of 40Gb/s and above.
This document summarizes research on the effects of third-order dispersion (TOD) in long-haul optical fiber communication systems. The research used simulation software to model propagation of high-speed pulses through standard single-mode fiber (SSMF) and non-zero dispersion-shifted fiber (NZDSF) systems with dispersion compensation fiber (DCF). Results showed increased pulse broadening and oscillatory tails with higher bit rates due to TOD. NZDSF systems demonstrated lower distortion compared to SSMF. TOD effects were also dependent on transmission distance, duty cycle, and fiber types used. The outcomes provide insights useful for designing very high-speed, long-distance optical networks.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
This document describes a simulation of transmitting 20 channels of either 10 Gbps or 20 Gbps data over 80 km spans of conventional single mode fiber using wavelength division multiplexing in the L-band spectrum region. Both pre-compensation and post-compensation techniques for dispersion were considered. Post-compensation provided better performance with transmission distances of up to 720 km for a total throughput of 200 Gbps and 320 km for 400 Gbps, and lower bit error rates compared to pre-compensation. Non-linear effects and amplified spontaneous emission noise were also accounted for in the simulation.
Mode-Locked Erbium Doped Pulse Fiber Laser Using the Kerr EffectKyle McSwain
This document describes an experiment to build a pulsed fiber laser using erbium-doped fiber and the Kerr effect. The researchers were able to generate ≤400ps pulses within a 12 meter ring cavity at a peak spacing of about 60 ns. They believe this places the pulses in either the soliton or stretched pulse regime. The laser utilizes an artificial saturable absorber composed of polarizing elements to generate pulses in the 1550 nm spectrum through passive mode-locking via the Kerr effect and self-phase and self-amplitude modulation.
Reduction of Four-Wave Mixing in DWDM System using Electro-Optic Phase Modula...IJECEIAES
In this paper, electro-optic phase modulator (EOPM) is used to reduce the effect of four-wave mixing (FWM), which is placed after 64 DWDMchannels multiplexer. It was found that the FWM is very sensitive to the phase deviation of the EOPM, and it can be reduced by introducing a phase shift between pulses. The simulation results confirmed the ability of the EOPM in improving the system performanceas indicated by the bit error rates. In term of comparison, the system of 64 channels based intensity modulated/ direct detection (IM/DD) transmission achieved bit error rate of 10 over 30 km and 70km without and with EOPM, respectively. -26
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
RF testing has remained hype for most of us. But seriously it is not so. It can be very interesting and one can develop a lot of interest in this if given an opportunity.
In this paper, authors have started with the some basic concepts of radio engineering which we studied in engineering and built upon these concepts to use in practical applications.
We have also described the basic principles of Signal Analyzer and Signal Generator which are the most common test tools used for any radio testing.
This document summarizes a research article that experimentally demonstrates a multi-wavelength fiber laser based on a Lyot filter. Six laser lines with over 5dB extinction ratio were generated using a combination of four-wave mixing in a highly nonlinear fiber and a Lyot filter mechanism in the laser cavity. The wavelength spacing of the multi-wavelength output was 0.15nm, which matches the characteristics of the Lyot filter used consisting of a polarization maintaining fiber. The laser provides potential applications in optical communication systems and instrumentation.
An optical add-drop multiplexer was developed by integrating silicon waveguide optical circulators and a Bragg reflector. A silicon waveguide Bragg reflector was fabricated that achieved 30 dB reflection for TM mode signals of a particular wavelength. An optical add-drop multiplexer device was demonstrated by combining two optical circulators with the Bragg reflector. It was able to add and drop signal light of a specific wavelength while passing other wavelengths through.
This document provides an overview of microwave link fundamentals, including:
1. Microwaves are electromagnetic radiations between 1-30 GHz used in telecommunications. Higher frequencies allow for higher bandwidth but require more advanced processing capabilities.
2. Microwave links are used in telecom industry applications like BTS connectivity and point of interconnect connectivity. Frequency allocation depends on distance, with shorter distances using higher frequencies.
3. Key factors that affect microwave links include reflection, refraction, diffraction, scattering, and absorption in the atmosphere. Diversity techniques like frequency and space diversity can help overcome some of these factors.
Spm limited long haul optical transmission in rz dpsk for varying input launc...Alexander Decker
This document summarizes a study on the performance of return-to-zero differential phase shift keying (RZ-DPSK) modulation for optical fiber transmission systems. The study uses numerical simulations to analyze transmission over 4000 km for varying input launch powers and data rates of 10Gbps and 20Gbps. The results show that RZ-DPSK modulation provides better performance than other formats, with maximum Q values achieved at around 7-8dB of input power. Performance degradation is seen with increasing data rate and transmission distance due to linear and nonlinear impairments.
This document discusses band pass filters that are constructed using a combination of low pass and high pass filters integrated with operational amplifiers. It provides equations to calculate key filter parameters like resonance frequency, bandwidth, and cutoff frequencies based on resistor and capacitor values. Simulation results show that as the near infrared operating wavelength increases, the filter capacitance increases, resistance decreases, resonance frequency and cutoff frequencies decrease, and gain increases. The document examines these filter characteristics over a wide range of parameters.
This paper summarizes research on using fiber Bragg gratings (FBG) for dispersion compensation in a 10 Gbps optical communication system using return-to-zero (RZ) modulation format. Simulation results showed that using an FBG to compensate for dispersion in a 3200 km long single mode fiber transmission link improved the bit error rate and Q-factor. Eye diagrams taken at 100 km, 1600 km, and 3200 km after dispersion compensation by the FBG demonstrated reduced pulse spreading and improved eye openings compared to the uncompensated case. The FBG provided effective dispersion compensation to improve transmission performance over long distances.
In this paper, an adaptive control system is employed in a novel implementation technique of feedforward linearization system for optical analog communication systems’ laser transmitter. The adaptive control system applies the Newton trust-region dogleg algorithm, which is a numerical optimization algorithm, to automatically tune the adjustment parameters in the feedforward loops to optimize the feedforward system performance and adapt to process variations. At the end of the paper, significant reductions of over 20 dBm in the third-order intermodulation distortion products have been achieved for operating frequencies from 5.0 to 5.8 GHz.
This document summarizes key concepts about optical amplifiers and networks. It discusses how optical amplifiers work by stimulating emission to amplify light, and describes common amplifier types like semiconductor optical amplifiers, doped fiber amplifiers, and Raman amplifiers. It also provides details on erbium-doped fiber amplifiers, including their operation, architecture, and use in telecommunication networks using SONET/SDH protocols. SONET/SDH networks are often configured in ring topologies to provide protection against link and node failures.
A high efficient narrow linewidth fiber laser based on fiber Bragg grating Fabry-Perot (FBG F-P) cavity was demonstrted. The spatial hole burning effect was restrained by fiber Faraday rotator(FR). Two short FBG F-P cavities as narrow bandwidth filters discriminated and selected the laser longitudinal modes efficiently. Stable single frequency 1550nm laser was acquired. Pumped by two 976nm LD, the fiber laer exhibited a 11mW threshold. The 73mW output power was obtained upon the maximum 145mW pump power. The optical-optical efficciency was 50 and the slope efficiency was 55. The 3dB linewidth of laser was less than 10kHz, measured by the delayed self-heterodyne method with 10km mono-mode fiber. The high power narrow linewidth fiber lasr can be used in high resolution fiber sensor system .
Investigation of optimal duty cycle for gvd undercompensatedIAEME Publication
This document summarizes an investigation into optimizing the duty cycle of input pulses in an undercompensated optical fiber link. The link consists of 5 loops, each with 50km of standard single mode fiber followed by dispersion compensating fiber. Simulation results show that higher duty cycle pulses (66-75%) permit greater undercompensation (up to 1.3% reduction in dispersion compensating fiber length) while maintaining reliable communication compared to lower duty cycle pulses (25-33%) which only tolerate up to 1% undercompensation. The optimal duty cycle balances fiber nonlinearities and accumulated dispersion to maximize the quality factor over the greatest range of input powers for a given level of undercompensation.
This document discusses coherent optical orthogonal frequency division multiplexing (CO-OFDM) for visible light communications. It begins with an introduction describing increasing bandwidth demand and the benefits of CO-OFDM. It then explains the basic principles of coherent optical transmission and CO-OFDM. The document proceeds to describe simulations conducted using OptiSystem software to model a 4-QAM CO-OFDM system over varying transmission distances. The results show degradation of the signal constellation and optical signal-to-noise ratio as distance increases due to attenuation and chromatic dispersion. Finally, the document concludes with a review of the CO-OFDM fundamentals and simulation results.
6 ijaems jan-2016-15-comparative analysis of free space optics and single mod...INFOGAIN PUBLICATION
In this paper we have done comparative analysis of free space optics and single mode fibre optical channels system. Performance study is done for variable ranges 1km, 50km and 100km for the FSO and SMF channels. Single Mode Fiber and Free Space Optical Channel is observed that maximum Q factor, minimum bit error rate and signal power. FSO requires no licensing or frequency synchronization. It also provides the transmission of data with unlimited bandwidth. Results expose the ability of improving the Q factor by using these channels and the optimum effect concluded at (1km) length of optical rang by using the FSO channel i.e. the quality factor and signal power were increased to reach (1943 ) and (22.46dBm) respectively and MIN. BER is zero.
Analysis of propagation of modulated optical signal in an integrated optic enviIAEME Publication
This document analyzes the propagation of modulated optical signals in integrated optic environments. It begins by discussing how modulation distributes an optical signal's power across multiple frequency components, requiring analysis beyond the monochromatic assumption. It then models a slab waveguide numerically to show how propagation constants and mode profiles vary with frequency. Finally, it simulates the phase variations experienced by different frequency components of a modulated signal propagating through integrated optic structures like straight waveguides and bends, demonstrating their ability to support wide bandwidths.
The document discusses Coherent Optical OFDM (CO-OFDM) and analyzes its performance at different data rates such as 1Gbps, 20Gbps, and 100Gbps. 4QAM-OFDM signals are generated and transmitted through 250km of fiber. The results show that distortions in the OFDM spectrum and QAM constellations increase as the data rate increases, due to nonlinearities in the fiber. CO-OFDM has advantages over other modulation formats as it provides both coherent detection and OFDM modulation, but its performance is affected by fiber nonlinearities at high data rates due to its high peak-to-average power ratio.
Analysis of Nonlinear Signal Interference to Optimize the Coherent Optical Tr...IOSRJECE
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2. LEE et al.: REVIEW OF THE POLARIZATION-NULLING TECHNIQUE FOR MONITORING OSNR IN WDM NETWORKS 4163
Fig. 1. Operating principle of the polarization-nulling technique.
use in the transmission link consisting of aerial fibers, this
technique should be able to track the fast fluctuation of the
state-of-polarization (SOP) of optical signal caused by wind
and electric currents in the neighboring power line [21]–[23].
Several techniques have been developed to overcome some
of these problems [10]–[13]. These techniques either calibrate
out the small amount of signal power leaked into the noise
in the orthogonal polarization state (due to PMD or nonlinear
birefringence) by using an additional optical filter or measure
the noise power at the slope of the signal’s spectrum to mitigate
the effect of PMD or nonlinear birefringence.
In this paper, we review the polarization-nulling tech-
nique for monitoring the OSNR in dynamic WDM networks.
In Section II, we describe the operating principle of the
polarization-nulling technique and its performances measured
in various types of fiber links. The results show that this
technique is susceptible to various polarization effects that oc-
curred in the transmission link. Thus, to identify the dominant
source of errors, we investigate the deleterious effects of PMD,
nonlinear birefringence, PDL, and fast fluctuation of SOP in
Section III. We then introduce the techniques developed to
mitigate these problems in Section IV. Finally, this paper is
summarized in Section V.
II. POLARIZATION-NULLING TECHNIQUE
A. Operating Principle
Fig. 1 shows the operating principle of the polarization-
nulling technique. This technique utilizes different polarization
properties of optical signals and ASE noises. The SOP of the
optical-signal incident on the OSNR monitor can be linear,
circular, or elliptical due to the random polarization rotation
along the transmission fiber. However, this arbitrarily polarized
signal can be changed to a linearly polarized signal simply by
using a polarization controller. The linearly polarized signal and
unpolarized ASE noise can then be split into two orthogonal
polarization components (in which one polarization component
consists of the signal and polarized ASE noise, while the other
has polarized ASE noise only) by using two linear polarizers.
Thus, the signal power, together with the polarized ASE noise,
can be measured with the first linear polarizer, which is aligned
Fig. 2. Experimental setup to evaluate the performance of the polarization-
nulling technique in various types of fiber links. (a) Back-to-back, (b) 640 km
of SMF (average PMD = 1.32 ps), (c) 640 km of NZDSF (average
PMD = 1.52 ps), and (d) 640 km of SMF+DCF (average PMD = 3.22 ps).
with the signal’s polarization. The polarized ASE noise (i.e.,
half of the total ASE noises) can be measured by using the
second linear polarizer, which is aligned to be orthogonal from
the signal’s polarization. Thus, the optical powers measured
after the first and second linear polarizers can be expressed as
Pp = Ps + 0.5Pn (1)
Po = 0.5Pn (2)
where Ps and Pn represent the optical powers of the signal
and ASE noise, respectively. Using these measured powers
(i.e., Pp and Po), OSNR can be obtained as
OSNR =
Ps
Pn
Bn
Br
=
Pp − Po
2Po
Bn
Br
(3)
where Bn is the noise equivalent bandwidth, and Br is the
resolution bandwidth. Bn is determined by the passband of
the demultiplexing filter. Thus, we can estimate the OSNR at
the resolution of Br simply by measuring Pp and Po. It should
be noted that both Pp and Po are measured right at the signal’s
wavelength. Thus, it would be possible to monitor the “true”
value of OSNR by using the polarization-nulling technique.
B. Measured Performances in Various WDM Links
To evaluate the performance of the polarization-nulling tech-
nique under the realistic networking environment, we measured
the OSNR in various types of 640-km-long fiber links. Fig. 2
shows the experimental setup. We multiplexed the outputs of
six DFB lasers operating in the range of 192.5–193.5 THz
and modulated with either 2.5- or 10-Gb/s nonreturn-to-zero
(NRZ) signal (pattern length: 231
− 1) by using a LiNbO3
modulator. The channel spacing was 200 GHz. The extinction
ratio of the modulated signal was about 13 dB. The multiplexed
WDM signals were first sent to a 13-km-long single-mode fiber
(SMF) for decorrelation and traversed through eight Erbium-
doped fiber amplifier (EDFA) modules followed by eight
80-km-long SMFs [Fig. 2(b) and (d)] or nonzero dispersion
3. 4164 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 11, NOVEMBER 2006
Fig. 3. Measured OSNR in various types of fiber links. (a) 2.5 Gb/s.
(b) 10 Gb/s.
shifted fibers (NZDSFs) [Fig. 2(c)]. All the EDFA modules
in Fig. 2(d) consisted of a two-stage EDFA and a disper-
sion compensating fiber (DCF). The average PMD for these
640-km-long SMF, NZDSF, and SMF+DCF links were mea-
sured to be 1.32, 1.52, and 3.22 ps, respectively. The input chan-
nel power was set to 0 dBm for both SMF and NZDSF, while it
was reduced to −3 dBm for DCF. We used an additional EDFA
module as a preamplifier in front of the demultiplexer. After
640-km transmission, the signal was demultiplexed by using
an arrayed waveguide grating (AWG) and sent to an optical
attenuator to set the optical-power incident to the monitoring
module to about −20 dBm. The 3-dB channel bandwidth of the
AWG was 0.944 nm.
Fig. 3 shows the measured OSNR by using the polarization-
nulling technique. For each link in Fig. 2, we measured the
OSNR for more than 10 h. The OSNR was set to about 25 dB
in all cases. In the back-to-back experiment shown in Fig. 2(a),
it was necessary to use an additional noise source to set the
OSNR to 25 dB. In this case, the monitoring error was measured
to be less than 0.4 dB for every channel, regardless of the bit
rate. This monitoring error was still maintained to be less than
0.6 dB, even when we transmitted 2.5-Gb/s signals over the
640-km-long SMF link [in Fig. 2(b)], as shown in Fig. 3(a).
However, when we replaced the SMF link with either NZDSF
link in Fig. 2(c) or SMF+DCF link in Fig. 2(d), the maximum
errors were increased up to about 4 and 6 dB, respectively.
On the other hand, when we increased the bit rate to 10 Gb/s,
the maximum errors were somewhat decreased to 1.2 and
3.7 dB for the 640-km-long NZDSF link and SMF+DCF link,
respectively, as shown in Fig. 3(b). We attributed these errors
to PMD and nonlinear birefringence [9]. For example, if a
modulated optical signal is transmitted through the fiber link
with nonnegligible PMD, its spectral components could have
different polarization states (i.e., the signal could be depolar-
ized) [24], [25]. Thus, it would be impossible to make all these
spectral components linearly polarized at the same time, which,
in turn, causes an error in the measured OSNR. The optical
signal could also be depolarized by nonlinear birefringence
[19], [26], [27]. When multiple optical signals are transmit-
ted through optical fiber, the polarization state of one signal
could be modulated by the other intensity-modulated signals.
If this nonlinear polarization modulation is faster than the time
required for the polarization-nulling process, the polarization-
nulling technique could overestimate the ASE noise power due
to the depolarized signal component and become erroneous.
III. PERFORMANCE-LIMITING FACTORS
The polarization-nulling technique utilizes different polariza-
tion properties of the optical signal and ASE noise. As a result,
the performance of this technique is bound to be affected by
various polarization effects that occurred in the transmission
link. For example, as described in the previous section, this
technique becomes erroneous if the signal is depolarized due to
PMD or nonlinear birefringence. The accuracy of this technique
could also be deteriorated if the ASE noise becomes polarized
due to PDL. In addition, the failure in tracking the fast fluc-
tuation of the SOP of optical signal could cause a large error.
We investigated these potential problems to identify the major
limiting factors on the performance of the polarization-nulling
technique.
A. Polarization-Mode Dispersion (PMD)
When the optical signal (broadened by modulation) propa-
gates through the fiber link having nonnegligible PMD, each
spectral component could have different polarization state at
the end of the fiber due to PMD [24]. Thus, it is impossible
to nullify all the spectral components of the signal (by using
the polarizer aligned to the orthogonal state from the signal’s
polarization) at the same time, which, in turn, would result in
the monitoring errors. To include this effect of PMD, we can
rewrite (1) and (2) as
Pp = Ps(1 − εPMD) + 0.5Pn (4)
Po = PsεPMD + 0.5Pn (5)
where εPMD represents the fraction of the optical signal leaked
into the orthogonal polarization state from the signal’s polariza-
tion due to PMD. Assuming that there exists only the first-order
PMD, this term can be estimated as [25]
εPMD =
1
2
−
1
2
cos2 θ+sin2
θ
cos(τ∆ω)P(∆ω)d∆ω
2
(6)
where θ represents the polarization angle between the input
SOP of the optical signal and the PMD vector of the transmis-
sion link, τ is the magnitude of the PMD vector, ∆ω is the
optical angular-frequency offset from the center frequency, and
P (∆ω) is the power spectral density function of the modulated
optical signal defined as
P(∆ω)d∆ω = 1. Thus, by using
(3)–(5), the OSNR error caused by PMD can be described as
Error (dB) = 10 log
OSNRr
OSNRm
= 10 log
1 + 2εPMDOSNRr(Br/Bn)
1 − 2εPMD
(7)
4. LEE et al.: REVIEW OF THE POLARIZATION-NULLING TECHNIQUE FOR MONITORING OSNR IN WDM NETWORKS 4165
Fig. 4. OSNR errors due to PMD measured in the 640-km-long SMF+DCF
link [9]. The average PMD of this link was 3.22 ps. In this paper, we used
only one channel to measure the OSNR errors without the effect of nonlinear
birefringence.
where OSNRr is the real OSNR, and OSNRm is the measured
OSNR. As expected, the OSNR error caused by PMD increases
with εPMD. Since εPMD is dependent on the magnitude of
the PMD vector and the spectral width of the optical signal,
the OSNR error increases with PMD and transmission speed.
This equation shows that the OSNR error is also dependent on
OSNRr. Thus, if OSNR is high, even a small εPMD could cause
a large monitoring error.
To evaluate the effect of PMD (without the effect of nonlin-
ear birefringence), we transmitted only one channel over the
SMF+DCF link in Fig. 2(d), which had the largest PMD due to
DCF (3.22 ps). The bit rate was changed from 1 to 10 Gb/s, and
the OSNR was set to 25 dB after the transmission. The moni-
toring error fluctuated significantly due to the random nature of
PMD. Thus, we measured the OSNR at each bit rate for more
than 10 h. Fig. 4 shows the average monitoring errors (solid
circles) in comparison with the theoretically calculated curve
(which was obtained by approximating (6) and (7) for small
PMD, and then averaging the results for τ and θ). In this figure,
the error bars represent the fluctuations of the measured OSNR
errors. This figure shows that the monitoring error caused by
PMD was increased with the bit rate. For example, when the
bit rate was 2.5 Gb/s, the maximum error was measured to be
only about 0.6 dB. However, the maximum error was increased
up to about 2 dB at 10 Gb/s. This was because the spectral
bandwidth of the optical signal was broadened as we increased
the bit rate. Thus, the effect of PMD on the performance of
the polarization-nulling technique could not be neglected if the
system is operating at high speed (≥ 10 Gb/s) over the fiber link
with large PMD.
B. Nonlinear Birefringence
When multiple optical signals are transmitted through optical
fiber, the polarization state of one signal (probe) can be affected
by the other signals (pumps) due to nonlinear birefringence
[26], [27]. Thus, the polarization state of the probe signal
could be modulated by the intensity-modulated pump signal.
However, this modulation frequency is typically faster than
the polarization-adjusting time of the polarization-nulling tech-
nique. Thus, it would be difficult to make this polarization-
modulated signal to be completely linearly polarized during
the OSNR measurement. This effect would result in the over-
estimation of the noise power and cause monitoring errors. We
analyzed the effects of nonlinear birefringence in an M-channel
WDM system. Because of the nonlinear birefringence, a small
portion of the signal power could be included in the noise power
measured by using the linear polarizer set in the polarization
state orthogonal to the signal. Neglecting the pulse distortion
caused by chromatic dispersion, this portion of the signal power
for the ith
channel in an M-channel WDM system could be
estimated as (for the worst-case analysis) (8), shown at the
bottom of the page, where ŝi and t̂ are the normalized Stokes
vectors of the ith
channel and polarizer, respectively, · denotes
the time average, ω is the angular frequency, J0 is the Bessel
function of the first kind of order 0, N is the number of spans,
ηij is the link enhancement factor (ηij = N for the dispersion-
compensated link, and ηij = | sin(NωdijL/2)/ sin(ωdijL/2)|
for the link without dispersion compensation) [28], L is the
span length, dij is the group velocity mismatch between chan-
nels i and j, Φij is the ac portion of the nonlinear phase shift of
the ith channel caused by the jth channel, and ϕij is the phase
retardation factor [9], [26]. The monitoring error caused by
nonlinear birefringence could be obtained by replacing εPMD
with εNL in (7) as
Error (dB) = 10 log
1 + 2εNLOSNRr(Br/Bn)
1 − 2εNL
. (9)
To evaluate the effect of nonlinear birefringence, we mea-
sured the monitoring errors in a simple two-channel experi-
ment. We multiplexed a continuous wave (CW) signal (probe)
and a modulated signal (pump) and transmitted the multiplexed
signals over 40-km-long SMF. The total PMD of this link
was only about 0.3 ps. The average optical powers of the
probe and pump signals were measured to 0 and 7 dBm at the
input of SMF, respectively. The channel spacing was 200 GHz,
and the OSNR was set to 25 dB. In this paper, the effect of
PMD could be neglected since we measured the OSNR of
εNL,i =
1
2
1 − ŝi · t̂
P(ω)dω
=
1
2
1 − J0
2
3
M
j=1,j=i
ηijΦij(ω) cos {ϕij(ω)}
2
+
M
j=1,j=i
ηijΦij(ω) sin {ϕij(ω)}
2
1
2
P(ω)dω (8)
5. 4166 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 11, NOVEMBER 2006
Fig. 5. (a) OSNR errors due to nonlinear birefringence measured in a two-
channel experiment. (b) OSNR errors measured in the various fiber links shown
in Fig. 2 [9]. The lines represent the theoretically calculated maximum errors
by using (8). The symbols are the measured data including the effect of PMD
(maximum values).
the CW signal. Fig. 5(a) shows the maximum errors caused
by the nonlinear birefringence in this system. The measured
data agree well with the calculated curve by using (8) and (9).
When the modulation frequency was smaller than 100 MHz, the
monitoring error was measured to be as large as 1 dB. However,
the error was substantially reduced as the modulation frequency
was increased. This was because the polarization modulation
caused by the nonlinear birefringence was averaged out more
quickly due to the group velocity mismatch between channels
caused by chromatic dispersion as the modulation frequency of
the pump signal was increased. We also evaluated the effect of
nonlinear birefringence using the various fiber links described
in Fig. 2. Fig. 5(b) shows the measured maximum OSNR errors
in Fig. 3 in comparison with the calculated curves by using (8)
and (9). In general, the measured data agreed well with the
calculated values (although the effect of PMD was included
in the measured data). Thus, we attributed the OSNR errors
observed in Fig. 3 mostly to the effect of nonlinear birefrin-
gence. The largest OSNR error was observed in the SMF+DCF
link. This was because the DCF had a small effective area,
and the effect of the nonlinear birefringence generated in each
span could linearly accumulate in the dispersion-compensated
fiber link. In addition, the OSNR error in the NZDSF link was
measured to be larger than that in the SMF link due to its
smaller dispersion and effective area. Fig. 5(b) also shows that
when we transmitted the 10-Gb/s signals over the SMF+DCF
link (which had the largest PMD of 3.22 ps), the measured error
was much larger than the calculated value due to the effect of
PMD. Thus, we concluded that the dominant error source of
the polarization-nulling technique was nonlinear birefringence,
except when PMD was quite large.
C. Polarization-Dependent Loss (PDL)
The polarization-nulling technique estimates the power of
the ASE noise within the signal’s bandwidth by measuring
only the noise power in the polarization state orthogonal to the
signal since the ASE noise is assumed to be fully unpolarized.
However, if the ASE noise is partially polarized due to PDL, the
noise power in the orthogonal polarization state may no longer
be identical to the noise power in the state parallel to the signal.
Fig. 6. Experimental setup to measure the effect of PDL on the polarization-
nulling technique (sw: Optical switch, Att: Optical attenuator, PC: Polarization
controller, and BPF: Bandpass filter).
Thus, it could cause an error in the measured OSNR by using
the polarization-nulling technique. In the case where OSNR is
much higher than 10 dB, this monitoring error (caused by the
partially polarized ASE noise) can be estimated by
Error (dB) = 10 log [1 − DOPASE(ŝ · n̂)] (10)
where DOPASE is the degree of polarization (DOP) of the
ASE noise, ŝ and n̂ represent the normalized Stokes vectors of
the signal and the partially polarized ASE noise, respectively,
and ŝ · n̂ represents their inner product [20], [29]. It has been
reported that DOPASE should have Maxwellian distribution,
since the polarized portion of the ASE noise arisen from each
amplifier due to PDL should be added randomly [30]. Thus,
the average DOP of the ASE noise (after the transmission of N
amplifier spans) can be described as
DOPN 2
=
1 +
2
3
Γ2
1 −
1
N
2
DOPN−12
−
16
9π
1 −
1
N
Γ2
DOPN−1 +
8
3π
Γ2
(11)
for N ≥ 2 and DOP1 = Γ, where the parameter Γ represents
the magnitude of the PDL vector of each span defined as
(1 − 10−PDL_dB/10
)/(1 + 10−PDL_dB/10
), and PDL_dB is
the PDL in decibels [30]. Using this result, the probability that
the error in the measured OSNR using the polarization-nulling
technique becomes larger than x dB can be obtained as
Probability {|error| ≥ x(dB)}
≈
1
2
erfc
2(10x/10
− 1)
√
πDOPN
+ erfc
2(1 − 10−x/10
)
√
πDOPN
(12)
for DOPN
6. 1 and x 3 [29].
Fig. 6 shows the experimental setup used to investigate the
effect of PDL. We transmitted 20 WDM channels (channel
spacing: 50 GHz) through the link consisted of 15 EDFA spans.
The first 10 channels (channel number: 1–10) operated in the
range of 1547.3–1550.9 nm, while the other channels (channel
number: 11–20) operated in the range of 1556.2–1559.8 nm.
An additional laser (reference channel) operating at 1553.8 nm
was used with an optical switch to measure the Stokes para-
meters of the signal and the ASE noise at this wavelength.
Each span consisted of an EDFA, an optical attenuator, a
polarization controller, and a PDL element. The PDL of each
7. LEE et al.: REVIEW OF THE POLARIZATION-NULLING TECHNIQUE FOR MONITORING OSNR IN WDM NETWORKS 4167
Fig. 7. (a) Cumulative probability of the errors in the measured OSNR’s by
using the polarization-nulling technique (due to the partially polarized ASE
noise) in a transmission link consisted of 15 spans (average PDL/span =
0.57 dB) [29]. The solid curve represents the theoretically calculated values
by using (12). (b) Probability that the error in the measured OSNR by using the
polarization-nulling technique becomes larger than 1 dB (due to the partially
polarized ASE noise caused by PDL) [29].
span was set to about 0.57 dB by using PDL elements. We
first measured the Stokes parameters of the reference signal by
using a polarization analyzer. We then turned off the optical
switch to measure the Stokes parameters of the ASE noise at
the same wavelength. Using these measured Stokes parameters,
we evaluated the accuracy of the polarization-nulling technique
by comparing the total noise power with the noise power in the
polarization state orthogonal to the signal.
Fig. 7(a) shows the cumulative probability of the measured
OSNR errors in comparison with the theoretical curve obtained
by using (12). The measured data agreed well with the the-
oretically calculated curve. For the transmission link used in
this paper (where the average PDL/span was 0.57 dB), the
probability that the errors in the measured OSNR (caused by
the partially polarized ASE noise due to PDL) became larger
than 1 dB was about 10−2
. Fig. 7(b) shows this probability (i.e.,
the probability that the error in the measured OSNR becomes
larger than 1 dB) as a function of PDL/span and the number
of spans. The result shows that if the PDL/span is smaller than
0.2 dB (which is a typical value for current systems [31]), the
effect of PDL on the measured OSNR using the polarization-
nulling technique is very small even for an ultralong-distance
transmission system. For example, the probability that the error
in the measured OSNR becomes larger than 1 dB is less than
10−4
in a 50-span transmission system, as long as the PDL/span
is smaller than 0.2 dB. However, the PDL/span could be raised
above 0.2 dB in the network utilizing reconfigurable optical
add/drop multiplexers (ROADMs). In such cases, the increased
PDL/span would certainly increase the error probability. For
example, if the PDL/span is increased to 0.3 dB, this probability
could be increased to 2 × 10−3
. Thus, in this case, the number
of amplifier spans should be reduced to 23 to maintain the error
probability within 10−4
.
D. Fast Polarization Fluctuation
When the optical signal is transmitted through an installed
fiber (especially aerial fiber), its SOP could fluctuate rapidly in
response to the environmental conditions [21]–[23]. To evaluate
the effect of this polarization fluctuation, we first measured the
Fig. 8. (a) Fourier components of the Stokes parameters of the optical signal
measured in a 120-km aerial fiber link [21], [22]. (b) Measured OSNR by using
the polarization-nulling technique in a 120-km aerial fiber link [21], [22].
SOP fluctuation of an optical signal in a 120-km-long aerial
fiber link installed in the field. Fig. 8(a) shows that the SOP
fluctuation measured in this aerial fiber link had two strong
frequency components at 60 and ∼0.3 Hz. We attributed the
60-Hz peak to the Faraday rotation caused by the current in
the electrical power transmission line, since it was identical
to the current frequency used in the electrical power line. The
other dominant peak at ∼0.3 Hz was caused by the wind and
the pendulum motion of the optical ground wire [21]–[23].
Fig. 8(b) shows the measured OSNR in this aerial link us-
ing the polarization-nulling technique. We intentionally
changed the OSNR from 19 to 28 dB to evaluate the perfor-
mance of the polarization-nulling technique at various OSNR
values. The optical-power incident to the OSNR monitor was
maintained to −22 dBm by using a variable optical attenuator.
The OSNR was measured for 10 min at each OSNR value (19,
23, 26, and 28 dB). The result shows that the polarization-
nulling technique could monitor the OSNR with accuracy better
than ±0.3 dB in most cases. This was because our OSNR
monitoring setup could track the polarization fluctuation up to
∼400 Hz. The accuracy of the measured OSNR was slightly
degraded to ±1 dB when the OSNR was extremely high
(28 dB). However, no significant change was observed in the
measurement accuracy during our long-term experiment, which
lasted one week.
IV. SOLUTIONS FOR IMPROVED PERFORMANCE
The performance of the polarization-nulling technique could
be deteriorated if the signal is depolarized (due to PMD or
nonlinear birefringence), or the ASE noise is polarized (due
to PDL). However, it has been shown in the previous sec-
tion that the effect of PDL could be neglected unless the
PDL/span was very large, and the transmission distance was
transoceanic. Thus, for practical use, it is necessary to improve
the polarization-nulling technique so that it could endure the
effects of PMD and nonlinear birefringence. Several techniques
have been proposed for this purpose [10]–[13]. These tech-
niques either calibrated out the small amount of signal power
leaked into the noise in the orthogonal polarization state (due
to PMD or nonlinear birefringence) by using an additional
optical filter or measured the noise power at the side of the
signal’s spectrum to mitigate the effects of PMD or nonlinear
8. 4168 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 11, NOVEMBER 2006
Fig. 9. Schematic diagram of the polarization-nulling technique improved
by using an additional optical filter (PBS: Polarization beam splitter, BPF:
Bandpass filter, and PD: Photodetector).
birefringence. In this section, we describe these improved ver-
sions of the polarization-nulling techniques in details.
Fig. 9 shows the schematic diagram of the improved
polarization-nulling technique implemented by using an addi-
tional optical filter [10]. The optical signal, which is demulti-
plexed by using the first optical bandpass filter (BPF), is sent to
a polarization beam splitter (PBS) via a polarization controller.
This controller is used to maximize the signal power P1 in one
arm. Thus, the signal and ASE noise are split into two polar-
ization components after the PBS. If the effects of PMD and
nonlinear birefringence could be neglected, one polarization
component should have the polarized ASE noise only, while the
other has the signal and polarized ASE noise. However, if the
signal is depolarized by PMD and/or nonlinear birefringence,
a small amount of signal power could be leaked into the arm
where we intend to measure only the noise power. To calibrate
this out, we split the polarized ASE noise component into two
parts (P2 and P3) by using a 3-dB coupler and then filter one
part by using the second optical BPF. This filter is used to
reduce the bandwidth of the ASE noise. Neglecting the loss of
each path (which can be calibrated, if necessary), the optical
powers detected at photodiodes can be described as
P1 = Ps (1 − ε) + 0.5Pn (13)
P2 = 0.5Psε + 0.25Pn (14)
P3 = 0.5Psε + 0.25αPn (15)
where ε is the ratio of the signal power excluded from P1 (thus,
included in P2 and P3) due to PMD, nonlinear birefringence,
and/or incomplete polarization control, and α is the bandwidth
reduction factor determined by the transmission characteristics
of the first and second BPFs. Using these equations, the OSNR
can be obtained as
OSNR =
Ps
Pn
Bn
Br
=
(P1 + 2P2)
4(P2 − P3)/(1 − α)
− 1
Bn
Br
.
(16)
To evaluate the performance of this improved technique, we
measured the OSNR in the 640-km-long fiber links described
in Fig. 2. The passbands of the first and the second BPFs
were 0.944 and 0.668 nm, respectively. The parameter α was
measured to 0.577. Fig. 10 shows the OSNR measured by this
technique for 10 h. The results show that this improved version
of the polarization-nulling technique could measure the OSNR
Fig. 10. Measured OSNR by using the improved technique illustrated in
Fig. 9 [10]. The OSNR was measured for 10 h in the various fiber links shown
in Fig. 2.
Fig. 11. Schematic diagram of the polarization-nulling technique improved
by using a narrow tunable filter and a PMDC.
with accuracy better than ±1 dB, regardless of the bit rates
and/or types of the fiber links.
Although the technique illustrated in Fig. 9 could effectively
negate the deleterious effects of PMD and nonlinear birefrin-
gence, it requires the precise alignment of two tunable filters.
This may not be an easy task since these filters should be
tuned together rapidly for every WDM channels. To avoid
this complexity, another version of the improved polarization-
nulling technique has been proposed by using only one tunable
filter [11]–[13]. This technique measures the noise power both
at the center and slope of the signal’s spectrum to mitigate the
effect of nonlinear birefringence. Fig. 11 shows the schematic
diagram of the proposed technique. The optical signal is first
sent to a PMD compensator (PMDC) to eliminate the effect of
PMD on the OSNR measurement. After passing through the
PMDC, the signal is sent to a tunable BPF (having bandwidth
much narrower than the signal). This filter is first adjusted to
the center of a specific WDM channel. Then, the power of the
filtered signal (Pλ1) is measured before the linear polarizer. At
the same time, the ASE noise power polarized orthogonal to the
signal (Pλ1,null) is measured by using the polarization-nulling
technique. The tunable BPF is then adjusted again to measure
the powers of signal (Pλ2) and polarized ASE noise (Pλ2,null)
on the slope of the signal’s spectrum (i.e., at the frequency a few
gighertz apart from the center frequency). If the optical signal
is slightly depolarized after the transmission due to nonlinear
birefringence, there can be a small portion of optical signal
9. LEE et al.: REVIEW OF THE POLARIZATION-NULLING TECHNIQUE FOR MONITORING OSNR IN WDM NETWORKS 4169
Fig. 12. Effects of PMD on the performance of the improved technique
illustrated in Fig. 11 [22]. In this experiment, we changed the DGD value from
0 to 60 ps by using a PMD emulator. OSNROSA represents the OSNR values
measured by using an optical spectrum analyzer.
in addition to the polarized ASE noise in Pλ1,null or Pλ2,null.
Thus, Pλ1,null and Pλ2,null can be expressed as
Pλ1,null = (Pλ1 − Pn)εNL +
1
2
Pn (17)
Pλ2,null = (Pλ2 − Pn)εNL +
1
2
Pn. (18)
Since Pλ2 is smaller than Pλ1, the portion of the signal power
transferred into Pλ2,null (due to nonlinear birefringence) is also
smaller than the portion transferred into Pλ1,null. As a result,
in case the signal is depolarized due to nonlinear birefringence
(i.e., εNL = 0), the OSNR derived from Pλ2,null becomes more
accurate than the OSNR obtained by using Pλ1,null [11], [12].
However, if εNL were large, this method could still suffer from
large errors since the portion of Pλ2 transferred into Pλ2,null
cannot be neglected. This problem can be solved by eliminating
εNL in (17) and (18) [13]. Thus, we can now estimate the
power of ASE noise and OSNR accurately without the effect of
εNL as
Pn =
2(Pλ1Pλ2,null − Pλ1,nullPλ2)
Pλ1 − Pλ2 − 2Pλ1,null + 2Pλ2,null
(19)
OSNR =
Pt − PnBt/Bf
PnBr/Bf
(20)
where Pt is the total power of the optical signal and ASE noise
within the bandwidth Bt (measured by scanning the tunable
filter over the signal’s whole spectrum), and Bf is the bandwidth
of the tunable BPF.
We implemented an OSNR monitor based on this technique
and evaluated the effect of PMD and nonlinear birefringence.
The PMDC was implemented by using a piece of high-
birefringent fiber and a polarization controller. The bandwidth
of the tunable BPF was about 3 GHz. Because of this extremely
narrow bandwidth, this OSNR monitor could be relatively
insensitive to the effect of PMD, even without the PMDC.
However, we used the PMDC to extend the PMD limit to
beyond several tens of picoseconds. We first investigated the
effect of PMD for a 10-Gb/s NRZ signal by using a first-
order PMD emulator. Fig. 12 shows that the OSNR could
Fig. 13. Effect of nonlinear birefringence on the improved technique illus-
trated in Fig. 11. (a) Channel powers incident on the SMF and DCF were set
to −5 dBm. (b) Channel powers incident on the SMF and DCF were increased
to 0 dBm.
be measured with accuracy of better than ±1 dB, even when
the differential group delay (DGD) was as large as 60 ps
(at OSNR = 27 dB). It has been reported that 30-ps DGD
could incur a power penalty of ∼1.4 dB for the 10-Gb/s NRZ
signal (in an optically preamplified system) [32]. Thus, this
technique could measure the OSNR accurately as long as the
PMD-induced power penalty is not extremely large. The effect
of the higher order PMD is not a concern here since the
bandwidth of the BPF is merely 3 GHz. To evaluate the effect
of nonlinear birefringence, we also measured the OSNR for
10 h in the 640-km-long SMF+DCF link described in Fig. 2(d).
In this experiment, we transmitted eight channels operating
at 10 Gb/s and reduced the channel spacing to 100 GHz to
induce large nonlinear birefringence. The channel power was
set to either −5 or 0 dBm at the input of SMF and DCF.
The OSNR was set to 20 dB after transmission. Fig. 13 shows
the performance of the proposed technique in comparison with
that of the conventional polarization-nulling technique. For fair
comparison, a PMDC was also used in front of the conventional
polarization-nulling technique so that we could investigate the
effect of nonlinear birefringence without the influence of PMD
in both cases. Fig. 13(a) shows that the performance of the
conventional technique was seriously affected by nonlinear
birefringence, even when the channel power was set to as low as
−5 dBm at the input of SMF and DCF. The OSNR error caused
by nonlinear birefringence in the conventional technique was
1.7 ± 1.3 dB. However, this version of the improved technique
could measure the OSNR with accuracy of better than ±0.3 dB
under the same condition. There was no significant difference
in the accuracy of the measured OSNR when we changed
the transmission speed to 2.5 Gb/s. Fig. 13(b) shows that this
technique could also measure the OSNR accurately, even when
the channel power was increased up to 0 dBm, while the
OSNR error in the conventional technique was as high as 8 dB.
We also evaluated the performance of this technique in an
ultralong-distance transmission link by using a recirculating-
loop experiment (made of a 640-km-long SMF+DCF link). We
launched eight 10-Gb/s channels in this recirculating loop and
measured OSNR as a function of the transmission distance. The
result in Fig. 14 shows that this technique could measure the
OSNR accurately, even with a transmission distance as long
as 3200 km.
10. 4170 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 11, NOVEMBER 2006
Fig. 14. Measured OSNR in the recirculating loop made of 640-km-long
SMF+DCF link.
V. SUMMARY
For proper operation and maintenance of a dynamic WDM
network, it would be necessary to monitor the OSNR of
each channel. However, the conventional linear interpolation
technique cannot measure the true value of OSNR in such
networks. To overcome this problem, it has been proposed to
utilize the different polarization properties of optical signal and
ASE noise. For example, it has been demonstrated that the
polarization-nulling technique could measure the true value of
OSNR in a dynamic networking environment. In this paper,
we reviewed this polarization-nulling technique and discussed
its performance-limiting factors and possible solutions to over-
come these limitations.
The operating principle of the polarization-nulling technique
is based on the assumptions that the signal is highly polarized
and that the ASE noise is completely unpolarized. However,
in practice, these assumptions are easily violated by various
polarization effects in the transmission link. For example, the
signal could be depolarized by PMD and nonlinear birefrin-
gence, and the ASE noise could be partially polarized due to
PDL. In addition, for use in aerial fiber links, this technique
should be able to track the fast fluctuation of polarization
caused by winds and electric currents in the neighboring power
lines. Several techniques have been developed to overcome
some of these problems. These improved techniques either
calibrated out the small amount of signal power leaked into
the noise in the orthogonal polarization state (due to PMD
or nonlinear birefringence) by using an additional optical fil-
ter or measured the noise power at the slope of the signal’s
spectrum.
To evaluate the possibility of using these improved
polarization-nulling techniques in the real systems, we thor-
oughly investigated the effects of PMD, nonlinear birefrin-
gence, PDL, and fast polarization fluctuation. The results
showed that these techniques could monitor the OSNR with
accuracy of better than ±1 dB, even when the first-order PMD
was as large as 60 ps (at OSNR ≤ 27 dB). The effect of
the signal depolarization caused by nonlinear birefringence
was measured to be negligible, even in a highly nonlinear
transmission link. We also investigated the effect of the partially
polarized ASE noise caused by PDL. The results showed that
as long as the PDL/span was smaller than 0.2 dB (as in most
current systems [31]), the OSNR could be monitored accurately
by using the polarization-nulling technique, even in a long-
distance system. For example, when the PDL/span was 0.2 dB,
the probability that the error in the measured OSNR became
larger than 1 dB was merely 10−4
in the transmission link
made of 50 amplifier spans. To verify the practicality of the
polarization-nulling technique, we measured the OSNR of the
optical signals transmitted through a 120-km-long aerial fiber
link for one week. In this aerial fiber link, the SOP of the
optical signal was measured to be fluctuated at ∼0.3 and 60 Hz
due to winds and electric currents in the neighboring power
lines, respectively. Despite these slow and fast fluctuations,
the polarization-nulling technique could monitor OSNR with
accuracy better than ±1 dB (at OSNR = 19 ∼ 28 dB). No
significant degradation in the monitoring accuracy was ob-
served during this long-term measurement. We also evaluated
the performance of the polarization-nulling technique in an
ultralong-distance transmission link by using a 640-km-long
recirculating loop. The results showed that this technique could
measure the OSNR accurately, even in the transmission link
longer than 3200 km.
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amplified spontaneous emission noise on Q-factor estimation using op-
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nication Conf., San Diego, CA, 1999, Paper WE3.
J. H. Lee was born in Yangju, Korea, in 1977. He received the B.S., M.S., and
Ph.D. degrees in electrical engineering from the Korea Advanced Institute of
Science and Technology (KAIST), Daejeon, Korea, in 1999, 2001, and 2005,
respectively.
His current research interests include polarization mode dispersion,
polarization-dependent loss, and optical performance monitoring (OPM) in
wavelength-division-multiplexed (WDM) networks.
H. Y. Choi received the B.S. degree in electronics from University of Seoul,
Seoul, Korea, and the M.S. degree in electrical engineering from the Korea
Advanced Institute of Science and Technology (KAIST), Daejeon, Korea,
in 2003 and 2005, respectively. She has been working toward the Ph.D. degree
in optical communication systems at KAIST since 2005.
Her current research interests include advanced modulation format and OPM
in WDM networks.
S. K. Shin was born in Kapung, Korea, in 1970. He received the B.S., M.S.,
and Ph.D. degrees in electrical engineering from the Korea Advanced Institute
of Science and Technology (KAIST), Daejeon, Korea, 1992, 1994, and 2001,
respectively.
He is currently with Teralink Communications, Inc., Daejeon. His current
research interests include OPM in WDM networks and optical wireless com-
munications for the dedicated short-range communication (DSRC).
Y. C. Chung (S’81–M’83–SM’03–F’05) was with Los Alamos National Labo-
ratory, Los Alamos, NM, under the AWU-DOE Graduate Fellowship Program,
from 1985 to 1987. From 1987 to 1994, he was with the Lightwave Systems
Research Department at ATT Bell Laboratories. In 1994, he joined the Korea
Advanced Institute of Science and Technology (KAIST), Daejon, Korea, as
a Professor of electrical engineering. His current research activities include
high-capacity WDM transmission systems, all-optical WDM networks, WDM
monitoring techniques, WDM PON, and fiber-optic networks for wireless
communications, etc. He has published over 300 journal and conference papers
in these areas and is the holder of over 60 patents.
Dr. Chung is currently serving as an Associate Editor for the IEEE/Optical
Society of America (OSA) JOURNAL OF LIGHTWAVE TECHNOLOGY. He has
also served as Conference Chair and Committee Member for numerous interna-
tional conferences including Optical Fiber Communication Conference (OFC),
International Conference on Integrated Optics and Optical Fibre Communi-
cation (IOOC), Opto-Electronics and Communications Conference (OECC),
Asia-Pacific Optical Communications Conference (APOC), etc. He is a Fellow
of OSA.