Advancements in fiber-optic communication: Addressing nonlinear challenges

As is well known, chromatic dispersion (CD) and nonlinear effect such as self-phase modulation (SPM), cross-phase-modulation (XPM), and four-wave-mixing (FWM), as well as their impairments interaction with each other are recognized as a limiting impairment for a high bit rates optical systems. With the advent of the 40 Gb/s, it is necessary to study transmission performance, which clearly depends on the modulation format and the system design. A numerical comparison of non-return-to-zero (NRZ), return-to-zero (RZ) and differential phase shift keying (DPSK) formats is made at a bit rate of 40 Gbit/s for single-channel and WDM systems with different compensation method in attempt to find the optimum modulation format. The transmitter under consideration used a 1.5 um DFB-laser externally modulated by a MZM modulator with modulation format (NRZ, RZ, DPSK), 64 PRBS data. At the receiver end an optical filtering using gaussian, fabry-perot and rectangular filter is used. Between the compensation method the symmetrical design leaves the best results in comparison to pre- and post-compensation. The impact of SPM can be reduced considerably by the symmetrical design. NRZ shows a best toleranz again chromatic dispersion and DPSK a best toleranz to Nonlinearity.

Semianalytic models are developed to deterministically calculate the variances of degenerate and nondegenerate four-wave mixing (FWM) noises for dispersion-managed dense wavelength division multiplexing (DWDM) systems with 8-Ary modulations [i.e., 8-level amplitude- and differential phase-shift keying (8APSK) and constant-amplitude optical differential 8-level phase-shift keying (D8PSK)]. The semianalytic models include various important propagation effects for exact numerical results. A 5.28-Tb/s (40-Gs/s/ch) 100-GHz-spaced 33-channel DWDM system with a dispersion map is then numerically analyzed by using the newly derived semianalytic models. It is numerically validated that FWM impacts coming from 8APSK pump channels are more severe than those coming from D8PSK ones, where pump channels denote the channels whose energies are transferred to a probe channel through the FWM process. The numerical results show that although FWM tolerance of a central channel with 8APSK is worse than that with D8PSK, a central channel with 8APSK is still superior to that with D8PSK when some linear noises and FWM noise are simultaneously taken into account for our given system conditions, which is mainly attributed to a relatively larger minimum Euclidean distance for the 8APSK constellation than the D8PSK one.

Choice of modulation format plays a critical role in the design and performance of fiber optic communication systems. We discuss the basic physics of electro-optic phase and amplitude modulation and derive model transfer functions for ideal and non-ideal Mach-Zehnder modulators. We describe the generation and characteristics of the standard nonreturn-to-zero (NRZ) modulation format, as well as advanced formats such as return-to-zero (RZ), carrier-suppressed RZ (CSRZ), duobinary, modified duobinary, differential phase-shift keyed (DPSK), and return-to-zero DPSK (RZ-DPSK). Finally, we discuss the relative merits of these formats with respect to a variety of system impairments.KeywordsFiber opticsModulation formatsOptical communication systemModeling

Optical fiber communication technology is beneficial to satisfy today's increasing demands for high data rates. This need for higher data rates and bandwidth has driven the fast development of fiber optic communication technologies. But at the same time, some undesirable nonlinear effects in the optical fiber medium become noticeable at high power levels. These fiber nonlinearities affect the optical fiber communication systems. The light signal in Wavelength Division Multiplexing (WDM) system can be degraded due to nonlinear impairments in the fiber. So it has become necessary to study the nonlinear effects. In this paper, nonlinear effects called Self-Phase Modulation (SPM) and Four-Wave Mixing (FWM) are observed. Factors responsible for variations in SPM and FWM effects are analyzed using OptiSystem simulation tool.

Optical fiber communication has characteristics that really need attention. The characteristics of fiber optic transmission are very important in designing optical fiber networks, one of which is widening pulses called dispersions. Dispersion is the widening of light pulses in optical fibers which reduces data speed, signal to noise ratio and system quality. In addition to dispersion compensation, which is the most important feature needed in fiber-optic communication systems; transmission power needs to be considered to produce optimal signal output. In this paper, we will simulate the optimization effect of power and fiber dispersion compensation on Non Return to Zero (NRZ) and Return to Zero (RZ) modulation on optical transmission systems, with 5 dBm power parameters, 10 dBm 15 dBm and 0 dBm and DCF 5 km, 10 km, 15 km, 20 km and 25 km. Simulation results show that dispersion compensation is very influential on the transmission system. From these two modulations, there are differences in the results between NRZ and RZ. The results of dispersion compensation also affect the input power and DCF length.

Simultaneous demodulation and dispersion compensation of WDM DPSK channels using optical ring resonator

In this article optical minimum-shift keying (MSK) modulation format in 9×40 Gbit/s wavelength-division multiplexing (WDM) spacing of 100 GHz transmission systems is investigated and compared with return-to-zero differential phase-shift keying (RZ-DPSK) and return-to-zero (RZ). The performance of optical MSK in presence of amplification noise, chromatic dispersion and nonlinear effects is analytically determined. Operations over a wide range of parameters, such as span input power, precompensation value, and chromatic dispersion parameter are performed; experimental results show that optical MSK demonstrates special performance in transmission distance, nonlinear effects, and dispersion tolerance.

We propose a novel four-wave mixing (FWM) impairments compensation method based on cancellation by means of estimated FWM components. Digital coherent detection can compensate for fiber transmission degradation, including fiber nonlinearity impairments. However, in the backward propagation (BP) method, the out-of-band FWM components must be detected. With our proposed method, only the transmitted signal channels need to be detected. The compensation results indicate that the input optical power tolerance is improved by nearly 4 dB at 1-dB eye penalty. The compensation possibility is demonstrated experimentally in a three-channel wavelength-division-multiplexing (WDM) system with differential phase-shift keying (DPSK) modulation and digital coherent detection. The eye penalty of each signal is improved by more than 3 dB with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> =8 dBm. This compensation method is also effective in multichannel systems.

Crosstalk power penalties were calculated for a bidirectional fiber-optic wavelength division multiplexed (WDM) system (622 Mb/s at 1300 nm in one direction and 155 Mb/s at 1550 nm in the other) using the system simulation program Simulation of Fiber-Optic Communication Systems (SIFOCS). Nonreturn-to-zero (NRZ) and return-to-zero (RZ) modulation formats were investigated with the result that, in most cases, the power penalty was shown to depend only weakly on individual system properties and is close to the values found in the literature, but there are exceptions where it may be greater than usual (e.g. at 622 Mb/s RZ). These can be detected and investigated with the methods reported. The crosstalk power penalty becomes excessive only for signal-to-crosstalk power ratios well below 10 dB and is negligible for values greater than 15 dB. The calculation methods used can be easily adapted to other systems, e.g. to systems with more closely spaced wavelengths. >

We propose and demonstrate an in-band optical signal-to-noise ratio (OSNR) monitoring technique using the output power ratio (PR) of the nonlinear optical loop mirror (NOLM) theoretically and experimentally. Operation point (OP) is investigated to be the input average power which corresponds to the first maximum of the power ratio curve. The monitoring performances using PR are proved to be better than those just using one output port of NOLM for on-off keying (OOK) and differential phase-shift-keying (DPSK) signals with different duty cycles: non-return-to-zero (NRZ), carrier-suppressed return-to zero (CSRZ), return-to-zero (RZ) with 50% and 33% duty cycle (RZ 50 and RZ 33) at 40 Gb/s. For 80 Gb/s RZ-differential quadrature phase-shift keying (DQPSK) signal, we obtained 12.16 dB maximum power variation and 13-39.8 dB monitoring range experimentally. The power ratio scheme can improve the contrast ratio by 7.05 dB and the monitoring range by 13.8 dB compared with that just using the transmission port. The tolerance to residual chromatic dispersion of the monitoring performance is investigated to be less than 1.7 ps/nm. The experimental validations are consistent with the theoretical analyses. The proposed scheme can be used in ultra-speed transmission systems.

Direct-detection of differential phase shifted keying (DPSK) optical signals is implemented either by ad hoc optical filtering or one-bit delayed optical interferometers. We show a coherent receiver where the filtering is performed in the electrical domain after down-converting the incoming signal to the baseband or to an intermediate frequency. This can be particularly advantageous whenever optical filters cannot be used or when extremely narrow filtering (sub-GHz) would be required [for example ultra-dense wavelength division multiplexing (WDM) passive optical networks]. Electrical filters can be realized more accurately than their optical counterpart. In addition, in a coherent receiver, this operation is colorless and avoids digital signal processing. We show experimentally this reduced complexity receiver and compare it with the classical one based on the delay and multiply block.

A compact device with a two-level transfer function (TF) implemented with two semiconductor optical amplifier (SOA)-based stages is proposed and characterized. Each stage exploits nonlinear polarization rotation and self-phase modulation. The obtained improved TF with very flat top and bottom levels makes the scheme suitable for working as a reshaper in all-optical regeneration. The effectiveness of the device is verified in regenerating both nonreturn-to-zero (NRZ) and return-to-zero (RZ) data signals up to 40 Gb/s. Bit error rate measurements demonstrate increased threshold margin and extinction ratio improvement.

In the modern world, bandwidth and data rates are two most important parameters under consideration in any communication system. An optical fiber communication system is capable of transmitting the data at high bit rates and has a large bandwidth capacity. This high data rate and bandwidth capacity in an optical fiber communication system is further enhanced by using wavelength division multiplexing (WDM) in conjunction with the information transmission system, in which information from multiple sources are transferred using the same fiber, at the same time, but at different wavelengths. But the high capacity and data rates in optical WDM system is limited by nonlinear effects that occur in optical WDM system, which not only pose a limitation on the channel capacity and data rates but also degrades the performance of the data transmission system. In this paper, analysis has been done to investigate the efficiency of Four Wave Mixing (FWM) effect under the influence of different system parameters such as chromatic dispersion coefficient, channel spacing, transmission power level, effective area of fiber and fiber length in the form of input signals and output signals frequency spectrums.

Crosstalk due to four-wave mixing (FWM) is increasing sharply when lightwave channel is upgrading in wavelength division multiplexing (WDM) fiber-optical communication systems. A technique, designed the channel wavelength allocation to reduce the crosstalks due to FWM, is submitted, when multi-channel is extended easily from 3-channel to 22-channel in WDM system. The result of simulation is presented. It is shown that suitable unequally channel separations can be found for which no FWM product term is superimposed on any of the transmitted channels, and the bandwidth expansion factor is not always increasing, and the span between the FWM crosstalks and channel wavelengths are not always decreasing, when increasing WDM system channels.

We investigate the phase-preserving amplitude regenerative characteristics of the return-to-zero (RZ) differential-phase-shift-keying (DPSK) wavelength conversion based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). The Q-factor and the optical signal-to-noise ratio (OSNR) before and after conversion are experimentally obtained and analysed in different input noise power levels. In both the continuous-wave and synchronous clock pumping cases, we find that there is amplitude clamping in the FWM conversion due to the gain saturation of SOA, which can suppress the amplitude fluctuation of the converted DPSK signal before and after demodulation. We have achieved 2-dB Q penalty improvement in our experiment demonstration of 10Gbit/s RZ-DPSK signal with OSNR lower than 19dB.