100G光通信调制技术
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Keywords: 100 Gb/s, coherent optical receiver, CPM, MSK, QPSK
1. INTRODUCTION
DWDM systems are now the foundation for the ever expanding global communication network. Trends toward higher capacity have been in place for decades, creating the current need of 100 Gb/s per wavelength and beyond in the near future. The existing infrastructure for metro and regional networks comprises multiple reconfigurable optical add/drop multiplexers (ROADMs) to allow for flexible network configuration. ROADM channel filters are commonly implemented at 50 GHz and 100 GHz channel spacing. Transition to the 50 GHz grid is anticipated for 100 Gb/s operation to enable higher utilization of the optical transmission spectrum and maintaining high flexibility. To achieve adequate performance at a nominal 100 Gb/s payload rate, line rates of 112Gb/s are needed to accommodate overhead including forward error correction (FEC) [1]. FEC has dramatically altered the design and capabilities of optical links; requiring a pre-FEC BER of approximately 10-3. The 3dB pass-band of 50 GHz channels through cascaded ROADMs is less than 40 GHz, though an exact measure is dependent upon the deployed equipment. Therefore, spectral efficiency (SE) of at least 112/40 = 2.8 b/s/Hz is required for 100 Gb/s transmission over a single wavelength.
Proc. of SPIE Vol. 7621 76210J-1
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/03/2015 Terms of Use: http://spiedl.org/terms
be expected from them.
In this paper, we examine QPSK, offset QPSK (OQPSK), continuous phase frequency shift keying (CPFSK), and multilevel minimum shift keying (MLMSK). A discussion of the features of each format is provided in Sec. 2. For each format, a Mach-Zehnder modulator (MZM) structure is provided that attains the necessary transmitted signal behavior. Additionally, the temporal and frequency domain characteristics are presented for each format to give insight to the structure and to highlight differences between them. In Sec. 3, receiver architectures are shown for the formats under consideration, both for differential and coherent receivers. Section 4 provides a description of the simulation environment used for analyzing the formats, as well as a description of the testbed at the Georgia Tech 100G Consortium. Section 5 discusses the simulation results, and quantifies the differences between each format.
Quaternary modulation formats for 100-Gbps optical links
Thomas F. Detwiler*a, Steven M. Searcya, Robert Lingle, Jr.b, E. Bert Baschc, Stephen E. Ralpha aGeorgia Institute of Technology, 85 Fifth St. NW, Atlanta, GA, USA 30308 bOFS, 2000 Northeast Expressway, Norcross, GA, USA 30071 cVerizon, 40 Sylvan Rd., Waltham, MA, USA 02451
ABSTRACT
The demand for 100 Gb/s optical links is rapidly spreading across all levels of the optical networking infrastructure. Many of the first deployments will be in the local area network (LAN) and metro-core and regional network environments. To address needs in LAN, the upcoming IEEE standard (IEEE P802.3ba) seeks 100 Gb/s over distances up to 40km. Furthermore metro-core/regional dense wavelength division multiplexing (DWDM) architectures require reach of several hundred km and the ability to pass through ten or more ROADMs. However, a number of fundamental challenges remain including the selection of appropriate modulation formats that are robust to a variety of nonlinearities, are sufficiently spectrally efficient, and able to withstand the strong optical filtering of cascaded ROADMs. Here we compare a variety of single-carrier quaternary modulation formats, each providing 2 bits/symbol/polarization and each likely to provide some advantages at 100Gb/s. Each format is presented with an appropriate MZM-based transmitter, and constrained by practical signal fidelity limitations that also enable comparison to experimental results from our 100G testbed. We primarily examine direct detection for cost-sensitive metro networks; however we also quantify the performance of coherent receivers, where applicable. Simulation results demonstrate the relative OSNR penalty (at a pre-FEC BER of 10-3) for a range of launch powers and adjacent channel formats.
There are several modulation formats that are capable of SE ≥ 2.8 b/s/Hz, however careful consideration must be given to achieve optimal performance with a low-complexity implementation. The leading candidate for 100 Gb/s DWDM deployment is quadrature phase shift keying (QPSK) transmitted over a 100 GHz channel or independently over a polarization multiplexed (POLMUX) 50 GHz channel [2-3]. Simulations and experiments have achieved 100 Gb/s using return-to-zero differential QPSK (RZ-DQPSK) with direct detection in such configurations [4-5]. The use of coherent optical reception on POLMUX RZ-DQPSK can be used to improve reach and incorporate compensation for linear channel effects, primarily chromatic dispersion (CD) and polarization mode dispersion (PMD) [6]. In each of these experiments, the baud rate selected was half the post-FEC bit rate per polarization (~56 Gbaud for single polarization, ~28 Gbaud for dual polarization). Interestingly, the transmitted pulse shape selected in each trial is RZ, Although RZ initially occupies a bandwidth significantly higher than the ROADM filters, the pulse shape at the decision point is more robust to chromatic dispersion and other ISI impairments. Alternatives to QPSK formats have been examined [e.g. 7], but consideration to other quaternary formats is of high interest to determine what benefits, if any, can
*tdetwile@gatech.edu; phone (919)Байду номын сангаас72-8596; 100g.gatech.edu
Optical Metro Networks and Short-Haul Systems II, edited by Werner Weiershausen, Benjamin Dingel, Achyut Kumar Dutta, Atul K. Srivastava, Proc. of SPIE Vol. 7621, 76210J · © 2010 SPIE CCC code: 0277-786X/10/$18 · doi: 10.1117/12.842454
1. INTRODUCTION
DWDM systems are now the foundation for the ever expanding global communication network. Trends toward higher capacity have been in place for decades, creating the current need of 100 Gb/s per wavelength and beyond in the near future. The existing infrastructure for metro and regional networks comprises multiple reconfigurable optical add/drop multiplexers (ROADMs) to allow for flexible network configuration. ROADM channel filters are commonly implemented at 50 GHz and 100 GHz channel spacing. Transition to the 50 GHz grid is anticipated for 100 Gb/s operation to enable higher utilization of the optical transmission spectrum and maintaining high flexibility. To achieve adequate performance at a nominal 100 Gb/s payload rate, line rates of 112Gb/s are needed to accommodate overhead including forward error correction (FEC) [1]. FEC has dramatically altered the design and capabilities of optical links; requiring a pre-FEC BER of approximately 10-3. The 3dB pass-band of 50 GHz channels through cascaded ROADMs is less than 40 GHz, though an exact measure is dependent upon the deployed equipment. Therefore, spectral efficiency (SE) of at least 112/40 = 2.8 b/s/Hz is required for 100 Gb/s transmission over a single wavelength.
Proc. of SPIE Vol. 7621 76210J-1
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/03/2015 Terms of Use: http://spiedl.org/terms
be expected from them.
In this paper, we examine QPSK, offset QPSK (OQPSK), continuous phase frequency shift keying (CPFSK), and multilevel minimum shift keying (MLMSK). A discussion of the features of each format is provided in Sec. 2. For each format, a Mach-Zehnder modulator (MZM) structure is provided that attains the necessary transmitted signal behavior. Additionally, the temporal and frequency domain characteristics are presented for each format to give insight to the structure and to highlight differences between them. In Sec. 3, receiver architectures are shown for the formats under consideration, both for differential and coherent receivers. Section 4 provides a description of the simulation environment used for analyzing the formats, as well as a description of the testbed at the Georgia Tech 100G Consortium. Section 5 discusses the simulation results, and quantifies the differences between each format.
Quaternary modulation formats for 100-Gbps optical links
Thomas F. Detwiler*a, Steven M. Searcya, Robert Lingle, Jr.b, E. Bert Baschc, Stephen E. Ralpha aGeorgia Institute of Technology, 85 Fifth St. NW, Atlanta, GA, USA 30308 bOFS, 2000 Northeast Expressway, Norcross, GA, USA 30071 cVerizon, 40 Sylvan Rd., Waltham, MA, USA 02451
ABSTRACT
The demand for 100 Gb/s optical links is rapidly spreading across all levels of the optical networking infrastructure. Many of the first deployments will be in the local area network (LAN) and metro-core and regional network environments. To address needs in LAN, the upcoming IEEE standard (IEEE P802.3ba) seeks 100 Gb/s over distances up to 40km. Furthermore metro-core/regional dense wavelength division multiplexing (DWDM) architectures require reach of several hundred km and the ability to pass through ten or more ROADMs. However, a number of fundamental challenges remain including the selection of appropriate modulation formats that are robust to a variety of nonlinearities, are sufficiently spectrally efficient, and able to withstand the strong optical filtering of cascaded ROADMs. Here we compare a variety of single-carrier quaternary modulation formats, each providing 2 bits/symbol/polarization and each likely to provide some advantages at 100Gb/s. Each format is presented with an appropriate MZM-based transmitter, and constrained by practical signal fidelity limitations that also enable comparison to experimental results from our 100G testbed. We primarily examine direct detection for cost-sensitive metro networks; however we also quantify the performance of coherent receivers, where applicable. Simulation results demonstrate the relative OSNR penalty (at a pre-FEC BER of 10-3) for a range of launch powers and adjacent channel formats.
There are several modulation formats that are capable of SE ≥ 2.8 b/s/Hz, however careful consideration must be given to achieve optimal performance with a low-complexity implementation. The leading candidate for 100 Gb/s DWDM deployment is quadrature phase shift keying (QPSK) transmitted over a 100 GHz channel or independently over a polarization multiplexed (POLMUX) 50 GHz channel [2-3]. Simulations and experiments have achieved 100 Gb/s using return-to-zero differential QPSK (RZ-DQPSK) with direct detection in such configurations [4-5]. The use of coherent optical reception on POLMUX RZ-DQPSK can be used to improve reach and incorporate compensation for linear channel effects, primarily chromatic dispersion (CD) and polarization mode dispersion (PMD) [6]. In each of these experiments, the baud rate selected was half the post-FEC bit rate per polarization (~56 Gbaud for single polarization, ~28 Gbaud for dual polarization). Interestingly, the transmitted pulse shape selected in each trial is RZ, Although RZ initially occupies a bandwidth significantly higher than the ROADM filters, the pulse shape at the decision point is more robust to chromatic dispersion and other ISI impairments. Alternatives to QPSK formats have been examined [e.g. 7], but consideration to other quaternary formats is of high interest to determine what benefits, if any, can
*tdetwile@gatech.edu; phone (919)Байду номын сангаас72-8596; 100g.gatech.edu
Optical Metro Networks and Short-Haul Systems II, edited by Werner Weiershausen, Benjamin Dingel, Achyut Kumar Dutta, Atul K. Srivastava, Proc. of SPIE Vol. 7621, 76210J · © 2010 SPIE CCC code: 0277-786X/10/$18 · doi: 10.1117/12.842454