The second approach proposed is a 10 Gb/s bidirectional fiber single-wavelength WDM-PON reuse with DPSK to DS signal and Mach-zender modulator (MZM) remodulation with US OFDM signal. 57 4.3 BER performance of 10 Gb/s downstream DPSK-modulated signal for both fiber transmission and over 25 km.
Introduction
It is most commonly known as Fiber in home technology in fiber optic communication. Technical advances were brought about by the quest to achieve higher data speeds, longer distances, lower cost and more functionality.
Literature Review
Among these, research on wavelength reuse scheme with bidirectional approach and colorless optical network unit (ONU) design to reach high data rates (~10 Gb/s) is of particular importance to understand the motivation for this work. In this thesis work, 10 Gbps wavelength reused bidirectional WDM-PON is designed based on RSOA as a colorless modulator in ONU with OFDM modulation to overcome the limited bandwidth response of RSOA to reach 10 Gbps.
Motivation
The proposed architecture is severely limited by the residual modulation of the DS in the US, so it is necessary to maintain the optimal DS extinction ratio (ER) for acceptable performance in bidirectional transmission. In addition, the immunity to fundamental noises such as re-modulation and Rayleigh backscattering can be effectively improved with the proposed system [7].
Thesis Objectives
Thesis Outline
Ciaramella, “10 Gbit/s symmetric WDM-PON based on cross-wavelength reuse to avoid Rayleigh backscatter and maximize bandwidth utilization,” Electron. 34;Centralized Lightwave WDM-PON System with Symmetric Rate of 10 Gbit/s Using Cost-Effective Directly Modulated Laser." ECOC 2009.
Access Networks: An Overview
Access network as opposed to the core network (e.g. the Network Switching Subsystem in GSM) that connects local providers. We can identify three ellipses that represent the core network, the edge network and the access network.
FTTx Architectures
Another architecture in Figure 2.3 (b) uses a curb switch to reduce the deployed fiber, but the curb switch is an active component that needs electrical power in addition to reserve power. The network topology of the last two architectures Figures 2.3 (c), (d) are usually called a passive optical network where only passive optical devices are used, namely fibers and splitters/couplers or combiners or wavelength multiplexers/demultiplexers.
Passive Optical Network (PON)
TDM-PON
TDM-PON is currently the most popular FTTP approach and is starting to be widely adopted in various regions of the world [6]. To make TDM-PON cost-effective and practical for widespread use, standardization of algorithms and protocols must be established.
WDM-PON
AWG gates are periodic in nature where multiple spectral rows of input wavelength channels are routed to the same output gate. Within each wavelength window, the wavelengths are separated by a wavelength spacing defined by the AWG gate.
WDM Wavelength Allocation
If the full wavelength band of 1270-1610 nm is used, it can contain 18 individual wavelength channels, as shown in Figure 2.8. Because DWDM can allocate many wavelength channels within a narrow range, it is considered the ultimate solution for WDM-PON.
Bidirectional Transmission in PONs
However, the use of conventional single-mode optical fiber for these systems limits the number of available channels due to the presence of water peak attenuation in the 1370–1410 nm range. The latter strategy represents a clear advantage for WDM networks, as wavelengths are now available for both directions, especially for CWDM networks where the number of wavelengths is limited.
WDM PON Architectures
Tunable Laser WDM PON
Alternatively, identical tunable lasers can be used in all ONUs with each laser tuned to the pre-assigned transmission wavelength [16,17]. Potential candidate technologies include tunable distributed feedback (DFB) lasers [17] and tunable vertical cavity surface-emitting lasers (VCSELs) [18]. The use of tunable lasers avoids the need for centralized light sources compared to other solutions, and subsequently the Rayleigh backscatter penalty of using these CW sources. An additional limitation on tunable lasers for use in a dynamic WDM PON is the tuning speed [19, 20].
Wavelength Reuse WDM PON
However, a true colorless property requires prior knowledge of what wavelength each laser should be tuned to. Some form of wavelength control must also be implemented to minimize crosstalk between wavelength channels during operation and to maintain wavelength alignment between the AWG and the lasers. Advantages of the wavelength reuse scheme include re-modulating the DS wavelength channel, eliminating the need for seeding sources, being cheaper than using tunable lasers, and directly modulating the RSOA.
Coherent injection and seeding WDM PON
As shown in Figure 2.13, injection-locked or wavelength-seeded light can be provided with CW light from a centralized light source located at the CO. The wavelength seeding scheme is the same as the injection gate scheme except for the use of an RSOA which amplifies and modulates the input CW light. Since the transmit wavelength of a colorless ONU is determined externally by the wavelength of the input light, all ONUs can be implemented with the same FP-LD or RSOA.
RSOA in WDM PON
In a two-way single-fiber system, CW light is transmitted from the CO, propagates through the fiber, is modulated and reflected by the ONU, and finally sent back to the CO. However, since the DS CW signal and US modulated data share the same fiber, system performance is inherently limited by RB noise.
Impairments
Rayleigh Backscattering (RB)
In this section, the statistical properties of the backscattered signal are presented and also the effect on the transmission of a counter-propagating signal is studied. For deriving the statistical properties of the RB signal, we use a two-dimensional model for the fiber represented in Figure 2.16. The backscattered intensity increases with the fiber length and converges to about 20km, depending on the fiber parameters.
Remodulation Noise
Available Modulation Formats
- Non Return To Zero (NRZ)
- Differential Phase Shift Key (DPSK)
- Orthogonal Frequency Division Multiplexing
- Introduction to OFDM
- Orthogonality
- OFDM Subcarriers
- OFDM Spectrum
- Spectrum efficiency of OFDM
- Inter symbol interference (ISI)
- Inter carrier interference (ICI)
- Cyclic prefix (CP)/ Guard interval
- Discrete Multi Tone (Modulation)
Each subcarrier is like a Fourier series component of the composite signal, an OFDM symbol. The rectangular windowing of the transmitted OFDM symbol results in a sinc function at each subcarrier frequency in the frequency response. The spectrum of each subcarrier is overlaid to illustrate the orthogonality of the subcarriers.
Introduction
Transmitter
Mach-Zehnder Modulator (MZM)
For strength modulation applications, the MZM should be biased at the quadrature bias point, while for linear field modulation applications, the MZM should be biased at the zero bias point [1]. The schematic symbol of MZM in VPI is given in Figure 3.4 (a), and the default parameter of MZM is shown in Figure 3.4 (b). The schematic symbol of the laser driver in the VPI is given in Figure 3.5 (a), and the taken drive amplitude and bias to drive the OFDM signal are shown in Figure 3.5 (b).
Pseudo Random Bit Sequence (PRBS)
Receiver
Photo Detector (PIN)
Low Pass Filter (LPF)
Bit Error Rate Tester (BERT)
Channel
Single Mode Fiber (SMF)
Downstream Modulation
Upstream Modulation
Different modules for transmitter, channel and receiver for signal generation and reception are explained with parameters.
Introduction
Two-Fiber Network: High Tolerance against Re-modulation Noise
Proposed Network
The CO consists of a 10 Gb/s DPSK signal for each WDM wavelength, which is multiplexed before being sent over SMF's optical distribution network. The other part is used to generate the US-modulated signal, where the DPSK-modulated optical DS signal is used as the starting wavelength for RSOA remodulation. Because the RSOA has a very limited bandwidth (~1 GHz), a spectrally efficient modulation format such as OFDM is used to achieve 10 Gb/s US data transmission.
Simulation setup
Another part of the DPSK-modulated DS optical signal is seeded into the RSOA for re-modulation with a 10 Gb/s OFDM signal. The bandwidth of the OFDM signal is set to 2.5 GHz, where the subcarriers are modulated with the 16-QAM format and thus ensure a data transfer rate of 10 Gb/s. A Dispersion Compensating Fiber (DCF) (compensating 400 ps/nm) is used to overcome the effect of chromatic dispersion on US performance.
Results and analysis
The received OFDM signal is also shown in Figure 4.2, which indicates the limited modulation response in the high frequency region due to the low bandwidth of RSOA. The inset of Figure 4.5 shows the constellation for the 16-QAM data format for both BB and 25 km fiber transmission with/without DCF. Finally, Figure 4.6 shows the BER for various US received power in both back-to-back (BB) and 25 km fiber optic transmission with dispersion compensation technique.
Single Fiber Network: Rayleigh Backscattering (RB) Noise analysis
Simulation setup
The remaining 16 subcarriers are the complex conjugate of the above 16 subcarriers to maintain Hermitian symmetry at the input of IFFT processing blocks. The RB noise was extracted from the optical circulator (OC-1) when the source light, including the modulated signal, passed through a 30 km fiber span. Because the optical power at the fiber input was fixed at 0 dBm, the RB noise label was found to have a value of −33 dBm.
Results and analysis
Since the RB tolerance depends on the interferometric beat noise falling within the Rx bandwidth, the RB tolerance of the wavelength-shifted signal can be improved by reducing spectral overlap with the carrier wavelength. The CW only keeps a line width of 10MHz while NRZ and DPSK have wider spectrum (see fig 4.13). Since CW generates less crosstalk noise as it has very narrow bandwidth compared to NRZ or DPSK.
Single Fiber Network: Both RB and Re-modulation Noise
Simulation Setup
The DS signal is then transmitted to the ONU over 20 km standard SMF (chromatic dispersion D=16 ps/nm/km, 4 dB propagation loss). As a DPSK demodulator, a narrow optical filter [Gaussian profile, full width half maximum (FWHM) is 60% of data rate] is used to convert phase information to intensity. Finally, the DS signal is received by an optical receiver with direct detection. of variable optical attenuator (VOA), PIN photodiode and electrical low-pass Bessel filter (bandwidth is 75% of data rate). Another part of the DPSK modulated optical DS signal is seen to the MZM for remodulation with 10 Gb/s OFDM signal.
Results and Analysis
Chow, Wavelength remodulation using DPSK down-and-upstream with high extinction ratio for 10 Gb/s DWDM passive optical networks, IEEE Photonics Technol. Chi, “Signal remodulation of OFDM-QAM for carrier-distributed long-range passive optical networks,” IEEE Photon. Prat, “Reduction of Rayleigh scattering using optical frequency dithering in passive optical networks with remotely located ONUs,” IEEE Photon.
Conclusions
Future Work
Appendix