Narrow Optical Filtering Research Articles

Transmission of 160-Gb/s Modified Manchester modulation over WDM system

ABSTRACTThe paper examines the performance of Modified Manchester (MM) modulation scheme over wavelength division multiplexing (WDM) in high-speed optical communication links. The MM as a new modulation technique has a narrow spectral width compared to conventional Manchester coding, which allows its implementation in WDM systems beneficial. In this study, the performance characteristics of MM and conventional Manchester modulation formats are assessed in WDM system at 10 Gb/s bitrate for each channel, for the least allowable channel spacing as well as tolerance to chromatic dispersion (CD). It is revealed from the results of simulation that MM performs meaningfully well in comparison with conventional Manchester in terms of tolerance against narrow optical filtering, spectral efficiency, which is improved by 32% and CD tolerance, which is improved by +100 ps/nm. Sixteen wavelength channels (16 × 10 Gb/s) are modulated to provide 160 Gb/s data capacity, which was transmitted successfully over 224 km standard single mode fibre (SSMF) using MM while the conventional Manchester only covered about 157 km.

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>

Narrow optical filtering in phase modulated communication links

Two variants of differential phase-shift keying with π and π/2 shifts are compared theoretically. The expressions are derived for dispersion averaged over all the realisations of the bit sequence. It is shown that in the presence of strong optical filtering the π/2 scheme yields less dispersion of pulses and much better Q-factor in agreement with the experiment. The inequality is confirmed by numerical calculation.

Balanced Detection Schemes for Optical Duobinary Communication Systems

This paper investigates balanced detection schemes for optical duobinary communication systems, and compares duobinary with differential phase-shift keying (DPSK) modulation. The balanced detection schemes are evaluated in duobinary and DPSK systems limited by amplified spontaneous emission noise, narrow optical filtering, and chromatic dispersion. We also consider the optimization of duobinary generating low-pass filter. Experimental data at 10 Gb/s show that balanced detection yields a significant advantage in receiver sensitivity compared with direct detection for duobinary systems, allowing duobinary systems to approach within ~1.5 dB the performance of DPSK. We also employ Monte Carlo simulations to evaluate balanced detection for 40 Gb/s duobinary transmission systems with narrow optical filtering designed for 50 GHz channel spacing. The simulations confirm our experimental results, and provide a more general investigation of balanced detection schemes. In particular, we propose a novel balanced detection scheme for duobinary systems designed to improve the tolerance to narrow optical filtering.

Nonlinear Pulse Reshaping With Highly Birefringent Photonic Crystal Fiber for OCDMA Receivers

We combine an erbium-doped fiber amplifier with a short segment of highly birefringent photonic crystal fiber to generate a nonlinear transfer function suitable for optical transmission systems based on optical code-division multiple-access (OCDMA). Such a functional element based on spectral broadening followed by an off-center narrow optical filtering provides an effective suppression of spurious signals such as multiple-access interferences in OCDMA receivers and can reduce the pulses amplitude jitter.

Architecture to integrate multiple PONs with long reach DWDM backhaul

This paper demonstrates the feasibility of an architecture that consolidates a number of deployed Passive Optical Network (PON) infrastructures into a long-reach, high-split ratio system which further increases equipment sharing between users. The demonstrated system allows the use of uncooled lasers with possible wavelength drift across a CWDM band (20 nm) with optical amplification and narrow optical filtering with no performance degradation. A complete study of potential implementations was performed with experimental results showing that a target performance of 10-10 could be achieved over 120 km of standard fiber with transmitter wavelengths from 1542 to 1558 nm and DWDM backhaul wavelengths from 1520 to 1535 nm. This gives the potential to support up to 2560 users.

Precompensation for Narrow Optical Filtering of 10-Gb/s Intensity Modulated Signals

Signals in spectrally efficient optical networks can be distorted by the narrowing of the overall bandwidth that occurs when optical add-drop multiplexers are concatenated. In this letter, it is demonstrated that the performance implications of this bandwidth narrowing can be mitigated by precompensation using arbitrary optical waveform generation. The synthesis of the required voltage waveforms for a dual-drive Mach-Zehnder modulator reduces the penalty in receiver sensitivity due to narrow optical filtering from 6.5 to 1.9 dB. Simultaneous precompensation for fiber dispersion is also demonstrated.

Chromatic dispersion tolerance in optimized NRZ-, RZ- and CSRZ-DPSK demodulation

We present the results of a comprehensive analysis optimizing the performance of DPSK systems with increased FSR and narrow optical filtering, establishing improved chromatic dispersion tolerance of NRZ-DPSK by 20%, RZ-DPSK by 71% and CSRZ-DPSK by 74% approximately. Transmitting a 40Gb/s signals on a spectrally efficient 50GHz DWDM grid still exhibit improvements of 7% for NRZ-DPSK, 37% for RZ-DPSK and 22% for CSRZ-DPSK, relative to a typical DPSK receiver. The optimized delay and optical filtering scale with the amount of chromatic dispersion. We also demonstrate the impact of limited transmitter bandwidth on optimal optical filtering and bit delay parameters.

Modeling of Coherent and Incoherent Rayleigh Crosstalk in Conventional Optical Links

Performance of Rayleigh-dominated transmission is modeled using approximated and exact noise power spectral densities (PSDs) and results are compared to experimental measurements for nonreturn-to-zero, return-to-zero, and duobinary modulation formats. Both models accurately predict penalties associated with coherent Rayleigh crosstalk for all three formats with typical accuracy within 1 dB. For incoherent Rayleigh crosstalk, the numerical model using exact PSDs improves prediction accuracy by as much as 4 dB over the analytic model. The large, previously unexplained disagreement is attributed to the set of analytic approximations. Our results indicate that a rigorous description of the channel spectrum should be used when calculating impairments caused by incoherent Rayleigh crosstalk, particularly in very dense wavelength-division-multiplexed systems requiring a narrow optical filtering.

Optical Equalization of 42.7-Gbaud Bandlimited RZ-DQPSK Signals

We demonstrate the use of an optical equalizer to allow 42.7-Gbaud (85.4-Gb/s) return-to-zero differential quadrature phase-shift keyed signals to better tolerate the narrow optical filtering required in high-spectral-efficiency systems. The equalizer passbands are repetitive, enabling the equalization of multiple channels.

Impact of Optical Filtering on Duobinary Transmission

An experimental investigation into the effects of optical filtering on 10-Gb/s duobinary transmission is presented. It is found that duobinary transmission systems benefit substantially from narrow optical filtering at the receiver. A 2-3- dB Q improvement in amplified spontaneous emission (ASE) noise-limited performance is obtained by optimizing the optical filter bandwidth. The duobinary modulation is compared to nonreturn-to-zero modulation as a function of optical filter bandwidth in ASE noise-limited and dispersion-limited regimes.

Multiterabit/s DWDM Terrestrial Transmission With Bandwidth-Limiting Optical Filtering

Wavelength-division multiplexing (WDM) systems based on 40-Gb/s channel bit rate appear as likely successors of current widespread N/spl times/10 Gb/s systems. However, they will provide larger throughputs than N/spl times/10 Gb/s systems only if some specific techniques are implemented, so as to improve the utilization ratio of the optical bandwidth, namely, the information spectral density (ISD). Narrow optical filtering is one such technique. Based on a review of some of our multiterabit/second transmission experiments in terrestrial configurations, we highlight the benefits of optical filtering to reshape the channel spectra. First, we show that vestigial sideband (VSB) narrow filtering of nonreturn-to-zero (NRZ) data makes 0.64-bit/s/Hz ISD possible over long-haul and ultra-long distances, provided that a specific wavelength allocation scheme is implemented. Using this scheme, a record 10-Tb/s capacity is demonstrated over 300 km by applying different data along each of the two polarization axes. However, along a single polarization axis and when channels are packed closer with NRZ-VSB filtering, at 0.8-bit/s/Hz ISD, nonlinear interactions between channels affect system performance and reduce the maximum error-free distance. We show that these interactions can be contained, still using narrow optical filtering, but by resorting to an alternative modulation format, namely, phase-shaped binary transmission.

On the use of NRZ, RZ, and CSRZ modulation at 40 Gb/s with narrow DWDM channel spacing

The use of nonreturn-to-zero (NRZ), return-to-zero (RZ), and carrier-suppressed return-to-zero (CSRZ) modulation formats in an ultradense wavelength-division multiplexing (UDWDM) scenario at 40 Gb/s is investigated. The results of a simulative analysis on back-to-back setups are presented. Narrow optical filtering at the receiver and at the transmitter, as well as the orthogonal polarization launch of adjacent channels, is studied in order to improve spectral efficiency. We demonstrate that standard setups do not allow acceptable performances with 50-GHz channel spacing for all the considered modulation formats, while the use of a transmission optical filter may dramatically improve the performance of RZ and CSRZ modulation formats that become suitable for the use in UDWDM systems. We show that, due to the narrow transmission filtering, the RZ pulse becomes NRZ-like, and the CSRZ modulation is duobinary coded. Furthermore, we demonstrate that NRZ modulation does not benefit from the introduction of a transmission optical filter, while it takes advantage of the orthogonal polarization launch of adjacent channels, but its performance is still worse than the RZ and CSRZ performance in a UDWDM scenario.