High Dispersion Tolerance Research Articles

Hardware Efficient and Chromatic Dispersion Tolerant Symbol-Rate Equalization Scheme for Short-Reach Coherent Transmission System

Chromatic dispersion enhanced spectrum aliasing, a nonlinear effect in the frequency domain, is theoretically analyzed under symbol-rate sampling conditions, and the relationship between its nonlinear strength and chromatic dispersion is investigated in detail. To address chromatic dispersion enhanced spectrum aliasing efficiently, a hardware efficient and chromatic dispersion tolerant equalization scheme is proposed to deal with inter-symbol-interference, chromatic dispersion, chromatic dispersion enhanced spectrum aliasing and phase noise simultaneously. Compared with the conventional T/2-spaced adaptive equalizer (AEQ) and symbol-rate AEQ, the proposed scheme can deal with large accumulated chromatic dispersion in a cost-efficient manner, which is extremely attractive for ultra-high baud rate and short-reach coherent optical interconnects. The experimental results also indicate that the proposed scheme can provide more than 3 dB received optical power sensitivity improvement over symbol-rate Sim-AEQ after 20 km fiber transmission links. With 7% FEC threshold, the proposed scheme with the same tap number can provide around 250 ps/nm and 90 ps/nm chromatic dispersion tolerance improvement over symbol-rate Sim-AEQ and conventional T/2-spaced equalization schemes, respectively. Additionally, compared with T/2-spaced 2 × 2 MIMO, the proposed scheme can also save around 50% real value multiplier because of the simplified equalization structure. Consequently, the proposed scheme with high chromatic dispersion tolerance and low complexity can be considered a promising DSP solution for ultra-capacity short-reach coherent optical interconnects such as datacenter interconnects and access applications.

Comparison of RZ and NRZ Modulation Techniques by Varying Duty Cycle and Mach Zehnder Modulator

Abstract: The aim of the research being undertaken is to examine the performance of advanced modulation formats such as RD and NRZ modulation schemes and their long-haul repeater-free fibre-traffic (SMF). Modular transmission of signals by SMF has been highly attracted by several benefits, such as high chromatic dispersion tolerance (CD), simplicity of deployment and higher spectrum efficiencies. These are the essential performance indicators of an optimal modulation technique for the creation of multiplexed (DWDM) transmission networks of the future generation of dense wavelengths. Based on a comparison of the two modulation approaches I give my findings. To mimic both modulation schemes, OptiSystem software is employed. Eye diagram with measurement of the Q factor and BER is generated. Keywords: Return to Zero, Non-Return to Zero, Eye Diagram, Bandwidth, BER, Q factor.

Physical Layer Encryption in DMT Based on Digital Multi-Scroll Chaotic System

Physical layer encryption based on digital chaos emerged as a promising methodology for securing data transmission. Discrete multi-tone (DMT) is a widely investigated advanced modulation format in intensity modulation and direct detection (IM/DD) systems, because of its high spectral efficiency, low cost and high dispersion tolerance. In this letter, we successfully performed a physical layer chaotic encryption method on an IM/DD system and experimentally demonstrated the transmission of 100 Gb/s encrypted 32QAM-DMT and 125 Gb/s pre-equalized encrypted 32QAM-DMT signals over 2-km standard single mode fiber (SSMF) with bit error rate (BER) under the hard decision forward error correction (HD-FEC) threshold of 3.8×10 -3 . The proposed encryption algorithm utilizes three independent pseudo-random sequences generated by the multi-scroll chaotic system for secure data transmission, and has no substantial impact on the transmission performance.

All-optical virtual private network utilizing bit allocation in OFDM-PON system

We propose a new all-optical virtual private network (AOVPN) using bit allocation technique in an orthogonal frequency-division multiplexing-based passive optical network (OFDM-PON) for the first time. The proposed system makes full use of the fundamental advantages of OFDM such as high spectral efficiency, dispersion tolerance, and flexibility in bandwidth allocation. It enables the simultaneous transmission of upstream (U/S) data and VPN data by multiple users. Since the U/S signal and the VPN signal are separated into bits within a high-order symbol, flexible resource management is possible according to real-time requirements. The feasibility of the proposed system is demonstrated in a 20-km single-fiber loopback link as a proof of concept. The experiment is focused on the optical transmission of VPN and U/S data. The results include performance variations of VPN transmission according to the bit allocation in a full-band uplink without multiple access and verification of dynamic bandwidth allocation (DBA) considering the multiple access by multi-users

Adaptive Blind Stokes-Space Based Equalizer for RSOP in SV-DD Systems With High Chromatic Dispersion Tolerance

Polarization de-multiplexing becomes challenging in Stokes vector-direct detection (SV-DD) systems, especially when the rotation of state of polarization (RSOP) and fiber chromatic dispersion (CD) are induced. To address this issue, an adaptive blind RSOP equalizer based on Stokes space is proposed. After theoretical analysis, it is found that the distribution of transmitted signals in Stokes space keeps a paraboloid. Based on this analysis, an adaptive scheme is proposed to track the symmetry axis of paraboloid for the first time. It is worth noticing that not only CD but also high-order modulation formats could not change the paraboloid distribution. Therefore, our algorithm has strong robustness to CD and good suitability for SV-DD systems with different modulation formats. In our simulation, the conventional Stokes space-based method and the training sequences (TS) method are both evaluated for comparison. The results show that our scheme can track RSOP of 20 Mrad/s and 8 Mrad/s at hard-decision forward error correction threshold in 28 Gbaud 4-level quadrature amplitude modulation (4-QAM) and 16-QAM SV-DD systems after 100 km fiber transmission, respectively. No additional computational complexity is required compared to the conventional Stokes space-based method. Moreover, compared to the TS-based method, there is no need to know the actual channel information, contributing to good flexibility.

Low-Complexity Soft-Decision Viterbi Algorithm for IM/DD 56-Gb/s PAM-4 System

In this letter, a soft-decision Viterbi algorithm (SOVA) combined with low-density parity check is investigated to achieve high chromatic dispersion tolerance and alleviate the high optical signal-to-noise ratio (OSNR) requirement in pulse amplitude modulation 4-level (PAM-4) systems. To reduce the complexity increase over a maximum likelihood sequence equalizer, we propose a non-binary SOVA algorithm combining the conventional SOVA and the hard decision VA. To achieve low complexity, only one competing path in a state and time index is used to update reliability. After the updating process, reliabilities that remain un-updated are set to a fixed value according to the hard decided VA output. Also, the proposed algorithm generates reliability scalars instead of a reliability vector. The proposed algorithm was verified experimentally in a 56-Gb/s PAM-4 system with the transmission distance of 20 km. The results demonstrate that the proposed algorithm has an OSNR gain of 0.3 dB in comparison to the conventional SOVA with lower complexity at the BER of $10^{\mathbf {-6}}$ .

Tributary Mapping Multiplexing an Efficient Technique for High Speed Fiber Optic Communication

<p>A novel technique of multiplexing called Tributary Mapping Multiplexing (TMM) is<br />applied to a single channel wavelength division multiplexing system and performance is monitored on the basis of simulation results. To elaborate the performance of TMM in this paper, a 4-User TMM system over single wavelength channel is demonstrated. TMM showed significant tolerance against narrow optical filtering as compared to that of conventional TDM at the rate of 40 Gbit/s. The above calculations are made by optical filter bandwidth and dispersion tolerance that was allowed at minimum. The spectral efficiency achieved by this TMM was 1 b/s/Hz and it was executed by using transmitters and receivers of 10 Gbit/s without polarized multiplexing. The high spectral efficiency, high dispersion tolerance and tolerance against strong optical filtering makes TMM an efficient technique for High<br />Speed Fiber Optic Communication.</p>

Four-amplitude shift keying-single sideband millimeter-wave signal generation with frequency sextupling based on optical phase modulation

We have proposed and demonstrated a scheme to generate a frequency-sextupling amplitude shift keying (ASK)-single sideband optical millimeter (mm)-wave signal with high dispersion tolerance based on an optical phase modulator (PM) by ably using the−4th-order and +2nd-order sidebands of the optical modulation. The ASK radio frequency signal, superposed by a local oscillator with the same frequency, modulates the lightwave via an optical PM with proper voltage amplitudes, the +2nd-order sideband carries the ASK signal with a constant slope while the −4th-order sideband maintains constant amplitude. These two sidebands can be abstracted by a wavelength selective switch to form a dual-tone optical mm-wave with only one tone carrying the ASK signal. As only one tone bears the ASK signal while the other tone is unmodulated, the generated dual-tone optical mm-wave signal has high dispersion tolerance.

Simulation and Analysis of Radio over Fiber (RoF) Systems using Frequency Up-Conversion Technique

Radio-over-Fiber (RoF) technology is an integration of microwave and optical networks [1]. RoF technology uses optical fiber as a backend technology due to the enormous advantages offered by it. SCM-WDM RoF systems (Subcarrier based Wavelength division multiplexing RoF system) improves spectral efficiency with high dispersion tolerance [7]. The data to be transmitted is electrically modulated with an IF (Intermediate frequency)or a RF (Radio frequency) carrier and electrically multiplexed using a multiplexer these signals are further optically modulated using an optical modulator and multiplexed using WDM to exploit the high bandwidth offered by the optical fiber. A comparison between IF over fiber and RF over has been analyzed in terms of quality factor and BER (Bit error rate) in a RoF system. In which the IF over fiber shows better performance than RF over fiber in terms of BER and Quality factor. IF signals are up-converted using HBT (Hetero-junction bipolar transistor) electronic mixer in the remote antenna units to the RF frequencies with the help of LO (local oscillator) and transmitted wirelessly [27]. General Terms Wireless communication systems, Optical fiber communication, Subcarrier multiplexing, Wavelength division multiplexing, Radio-over fiber systems.

All optical up-converted signal generation with high dispersion tolerance using frequency quadrupling technique for radio over fiber system

A novel all optical up-converted signal generation scheme with optical single-sideband (OSSB) technique for radio over fiber (RoF) application is presented and experimentally demonstrated using low-bandwidth devices. The OSSB signal is generated by one low-bandwidth intensity LiNbO3 Mach-Zehnder modulator (LN-MZM) under frequency quadrupling modulation scheme and one low-bandwidth LN-MZM under double sideband carrier suppressed modulation (DSB-CS) scheme. The proposed all OSSB generation scheme is capable of high tolerance of fiber chromatic dispersion induced power fading (DIPF) effect. Benefiting from this novel OSSB generation scheme, a 26GHz radio frequency (RF) signal up-conversion is realized successfully when one sideband of the optical LO signal is reused as the optical carrier for intermediate frequency (IF) signal modulation. The received vector signal transmission over long distance single-mode fiber (SMF) shows negligible DIPF effect with the error vector magnitude (EVM) of 15.7%rms. In addition, a spurious free dynamic range (SFDR) of the OSSB up-converting system is measured up to 81dBHz2/3. The experiment results indicate that the proposed system may find potential applications in future wireless communication networks, especially in microcellular personal communication system (MPCS).

Analysis of intensity modulation performance in WDM-PON network

This paper mainly does research and analysis on anti-dispersive performance and anti-nonlinear performance for intensity modulation formats such as NRZ, RZ, CS-RZ, DRZ, MD-RZ in 40Gbps high-speed single-channel optical transmission system. It compares the five pattern modulation formats in dispersion tolerance, nonlinear tolerance, and advantages and disadvantages in transmission distance. Anti-nonlinear effect ability of CSRZ and RZ are stronger than that of NRZ code; in the single-channel transmission system, DRZ and improved modulation format MDRZ have better resistance for anti-nonlinear performance, so they are suitable for long-distance transmission. RZ has the minimum dispersion tolerance. NRZ has higher spectrum efficiency than RZ, and thus have a better dispersion tolerance. CSRZ code has higher spectral efficiency, higher dispersion tolerance and nonlinear tolerance than RZ code, making it more suitable for long distance transmission.

Research on the Modulation Performance in GPON System

This paper mainly does research and analysis on anti-dispersive performance and anti-nonlinear performance for intensity modulation formats such as NRZ, RZ, CS-RZ, DRZ, MD-RZ in 40Gbps high-speed single-channel optical transmission system. It compares the five pattern modulation formats in dispersion tolerance, nonlinear tolerance, and advantages and disadvantages in transmission distance. Anti-nonlinear effect ability of CSRZ, RZ are stronger than that of NRZ code; in the single-channel transmission system, DRZ and improved modulation format MDRZ have better resistance foranti-nonlinear performance, so they are suitable for long-distance transmission. RZ has the minimum dispersion tolerance. NRZ than RZ has higher spectrum efficiency than RZ, and thus have a better dispersion tolerance. CSRZ code has higher spectral efficiency, higher dispersion tolerance and nonlinear tolerance than RZ code, making it more suitable for long distance transmission.

Ultra high capacity 1.28 Tbps DWDM system design and simulation using optimized modulation format

In this paper, an ultra high capacity optical system with transmission capacity of 1.28Tbps using optimized modulation format is presented. The system design is achieved by using a 64-channel 20Gbps dense wavelength division multiplexing (DWDM) technique. The proposed optimized modulation format has narrower bandwidth than conventional intensity modulation (IM) schemes. It has also high dispersion tolerance. Thus, makes it possible to attain an ultra high capacity long distance transmission. The performance characteristics of the system for different modulation formats are compared through simulation at 20Gbps. The system is also analyzed for pre, post and symmetrical dispersion compensation techniques. We have also investigated the filtering performance of the system, using narrow-band optical filters at each input and output channel, to improve the coverage range of the proposed system. These filters have been used for analysing the DWDM system performance, for channel spacings of 100GHz and 50GHz. The system is designed and simulated for a coverage distance of 200km and more. The proposed ultra high capacity system is expected to be more technically viable due to the use of optimized modulation format. Results and analysis of the proposed system are also presented. This system will be highly useful for high speed data processing, in the present communication and computer networks.

Heterodyne mixing and polarization diversity techniques in radio over fiber system with high sensitivity and dispersion tolerance

We propose and experimentally demonstrate the heterodyne mixing and polarization diversity techniques in radio over fiber (ROF) systems with the advantages of high sensitivity and dispersion endurance. The generation of 2.5 Gb/s optical DPSK and PSK signals, fiber propagation and wireless transmission over 15 GHz subcarrier are illustrated with very low sensitivity of −26 and −32 dBm, respectively. Moreover, based on the wireless polarization diversity techniques, we successfully performed the transmission of 1.3 Gb/s uncompressed high definition television (HDTV) signals with high quality.

Dispersion Tolerance for Optical Duobinary Transmitters based on a Mach-Zehnder Modulator and an Optical Filter

We theoretically investigated dispersion tolerance of an optical duobinary transmitter employed a Mach-Zehnder modulator and an optical filter. Compared to the optical transmitter based on a Mach-Zehnder modulator and an optical delay interferometer by optimizing the applied voltage for improving the dispersion tolerance, the demonstrated duobinary transmitter provides improved receiver sensitivity and higher dispersion tolerance while maintaining narrow spectral bandwidth.

Pulsed-incoherent-light-injected Fabry-Perot laser diode for WDM passive optical networks

We propose and demonstrate a pulsed-incoherent-light-injected Fabry-Perot laser diode (FP-LD) which generates incoherent return-to-zero (RZ) signals for wavelength-division-multiplexing passive optical networks. For the generation of the RZ signals, we first convert the continuous-wave (CW) amplified spontaneous emission (ASE) into an ASE pulse train with a pulse carver, spectrum-slice it into multiple channels with a waveguide grating router, and then inject them into FP-LDs for data modulation. Thanks to a wide slicing bandwidth of the injected incoherent light, the spectral linewidth of the generated RZ signals is determined by the slicing bandwidth, without being affected by the use of the RZ format. Thus, compared to incoherent non-return-to-zero (NRZ) signals generated with CW-ASE-injected FP-LDs, the RZ signals have a similar spectral linewidth but a wide timing margin between adjacent bits. Thus, the proposed transmitter can offer better dispersion tolerance than the NRZ signals. For example, our experimental demonstration performed at 1.25 Gb/s shows approximately 50% higher dispersion tolerance than the NRZ signals generated with CW ASE-injected FP-LDs. Despite the large slicing bandwidth of 0.67 nm for the injected ASE, we were able to transmit 1.25-Gb/s signals over 45-km standard single-mode fiber without dispersion compensation. The receiver sensitivity is also improved by 1.5 dB by using the RZ format.

A Novel Bidirectional 60-GHz Radio-Over-Fiber Scheme With Multiband Signal Generation Using a Single Intensity Modulator

We experimentally demonstrated a novel radio-over-fiber system to simultaneously generate dispersion-tolerant multiband downstream signals, including millimeter-wave, microwave, and baseband signals, based on multicarrier modulation in an intensity modulator and a subsequent optical filter. The uplink connection is realized by remodulation of downlink optical carrier and by baseband detection in the central office. The high-dispersion tolerance comes from the subcarrier cross-selection with only one data-bearing tone before signal beating in the receiver. The power penalty of 1.4 dB for 60-GHz carrier and negligible degradation for baseband and upstream are achieved for 2.5-Gb/s signal after 50-km single-mode fiber (SMF-28) and 4-m air link transmission. The theoretical analysis is also provided to obtain the optimal operation point.

Improved dispersion tolerance of an optical duobinary transmitter based on a Mach–Zehnder modulator and an optical delay interferometer

We present a method for improving the dispersion tolerance of an optical duobinary transmitter based on a dual-arm Mach–Zehnder (MZ) modulator and an optical delay interferometer without electrical low-pass filters. Since such a duobinary modulator using a standard 1-bit delay cannot provide high dispersion tolerance, we investigated the impact of the modulator driving voltages and the bit delay in the interferometer to improve the transmitter’s dispersion tolerance.

Performance of alternate-block-inversion modulation format in optical communication systems with no dispersion compensation

Phase-shaped binary transmission (PSBT) is known to be an attractive modulation format in system with no inline dispersion compensation because of its resilience to chromatic dispersion. We show that the phased amplitude-shift signaling alternate-block-inversion (PASS-ABI) modulation format has a more compact spectrum and higher dispersion tolerance than PSBT while requiring similar transmitter complexity.

Improvement of dispersion tolerance using phase-Modulated NRZ signals in 40-gb/s transmission systems

Compared with the conventional nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed return-to-zero, and chirped RZ signals by numerical simulation, a higher dispersion tolerance in the presence of fiber nonlinearities and amplifier noise has been obtained in this letter by using the phase-modulated NRZ signals. This scheme aims to improve resistance to residual dispersion by an adapted phase modulation at the transmitter.