In-line Amplifiers Research Articles

PMD Design for High-Speed WDM Backbone Network Systems Based on Field PMD Measurements

We propose a polarization mode dispersion (PMD) design for high-speed wavelength-division multiplexing (WDM) backbone network systems based on field PMD measurements on installed optical fibers for long-term commercial use. Implementing a high-speed network system on an installed fiber requires measuring PMD, because the PMD characteristics of most installed fibers are unknown. For enhanced practicality, we must be able to precisely evaluate PMD characteristics precisely with just one measurement. To understand the statistical properties of measured PMD values, we use the Jones Matrix Eigenanalysis (JME) method to conduct long-term (12 months) PMD measurements on installed fibers. We statistically analyze the measurement results and confirm that the measured values match the theory that considers the accuracy of the measurement instrument. This enables a PMD design of desired outage probability based on PMD measurements of installed fibers. We also carry out a 43-Gb/s return-to-zero differential quadrature phase shift keying (RZ-DQPSK) signal transmission with high PMD fibers in order to confirm the effectiveness of our PMD design. The PMD values of the in-line amplifier transmission line are settled so as to meet the worst value of the design. We confirm that 43-Gb/s RZ-DQPSK signals are stably transmitted at the design value.

On the correct modeling of semiconductor optical amplifier RIN and phase noise for optical phase shift keyed communication systems

Phase modulation schemes are attracting much interest for use in ultra-fast optical communication systems because they are much less affected by fiber nonlinearities than conventional modulation formats. Semiconductor optical amplifiers (SOAs) can be used to amplify and process phase modulated signals. However, existing SOA nonlinear phase noise (NLPN) models are simplistic and, sometimes, inaccurate. It is, therefore, important to correctly model their behavior since NLPN is the main drawback in these applications. In this paper we show that a more accurate model can be used leading to simple nonlinear noise expressions at the SOA output of differential phase shift keying systems. To demonstrate the utility of this model, we have used it to calculate the optical signal to noise ratio penalties introduced by a power booster SOA and the first inline amplifier of a 40 Gb/s NRZ-DQPSK single channel link. The model parameters have been estimated from measurements taken of a commercial SOA.

Noise performance of optical fiber transmission links that use non-degenerate cascaded phase-sensitive amplifiers

Based on semi-classical theory, the noise performance of a multi-span fiber optical transmission system employing a cascaded phase-insensitive amplifier (PIA) and phase-sensitive amplifiers (PSAs) is investigated. Compared with the pure-PIA and pure-PSA based in-line amplification schemes, the copier + PSA scheme is found to improve the system NF by up to 6 and 3 dB, respectively, in an optimized long-haul fiber link. In addition, this cascaded configuration will significantly relax the requirement for accurate phase- and wavelength-locking which is rigorously needed in the pure-PSA configuration. This scheme is also modulation-format independent. As a proof of concept, the NF of a fiber parametric amplifier based copier + PSA cascade with inter-stage attenuation representing the fiber link is measured, which shows a 1.8-dB total NF improvement over the conventional EDFA cascade.

Power Loss Resilience in Laser Diode-Based Optical Chaos Communications Systems

The robustness of laser diode-based optical chaos communications to system power losses has been experimentally studied. In-line optical amplification is shown to be effective in restoring the received optical power to enable successful message extraction at an authorized receiver. At the same time, eavesdropper alert can be achieved by measuring the cross-correlation coefficient.

Real-time transmission of 3Gb/s 16-QAM encoded optical OFDM signals over 75km SMFs with negative power penalties

Real-time optical orthogonal frequency division multiplexing (OOFDM) transceivers are experimentally demonstrated with advanced pilot subcarrier-assisted channel estimation being implemented. The channel estimation technique is, for the first time, proposed and experimentally verified rigorously, which offers a number of unique features including high accuracy, low complexity, small pilot bandwidth usage, excellent stability and buffer-free data flow. The fastest ever real-time end-to-end transmission of 3 Gb/s 16-QAM-encoded OOFDM signals over 75 km MetroCor single-mode fibres is achieved with negative power penalties of -2dB at BERs of 1.0 x 10(-4) in directly modulated DFB laser-based, intensity-modulation and direct-detection systems without in-line optical amplification and chromatic dispersion compensation.

Transmission Performance Analysis of Fiber Optical Parametric Amplifiers for WDM System

A numerical analysis is presented on the long-haul wavelength-division multiplexing (WDM) transmission system employing fiber-optic parametric amplifier (FOPA) cascades based on one-pump FOPA model with Raman Effect taken into account. The end-to-end equalization scheme is applied to optimize the system features in terms of proper output powers and signal-to-noise ratios (SNRs) in all the channels. The numerical results show that—through adjusting the fiber spans along with the number of FOPAs as well as the channel powers at the terminals in a prescribed way—the transmission distance and system performance can be optimized. By comparing the results generated by different lengths of fiber span, we come to the optimal span length to achieve the best transmission performance. Furthermore, we make a comparison among the long-haul WDM transmission systems employing different inline amplifiers, namely, FOPA, erbium-doped fiber amplifier (EDFA), and Fiber Raman Amplifier (FRA). FOPA demonstrates its advantage over the other two in terms of system features.

A 80 km reach fully passive WDM-PON based on reflective ONUs

We propose a novel line coding combination (Inverse RZ coding in downlink and RZ in uplink) that extends the reach of WDM Passive Optical Networks based on Reflective SOAs with no in-line amplification. We achieved full downstream remodulation even when feeding the reflective SOA with power levels as low as -35 dBm, thus increasing the system power budget. We experimentally assessed this scheme for a fully passive, full-duplex and symmetrical 1.25 Gb/s WDM-PON over a 80 km G.652 feeder.

Compensation of Dispersion and Nonlinear Impairments Using Digital Backpropagation

Optical fiber transmission is impacted by linear and nonlinear impairments. We study the use of digital backpropagation (BP) in conjunction with coherent detection to jointly mitigate dispersion and fiber nonlinearity. We propose a noniterative asymmetric split-step Fourier method (SSFM) for solving the inverse nonlinear Schrodinger equation (NLSE). Using simulation results for RZ-QPSK transmitted over terrestrial systems with inline amplification and dispersion compensation, we obtain heuristics for the step size and sampling rate requirements, as well as the optimal dispersion map.

107 Gb/s uncompensated transmission over a 470 km hybrid fiber link with in-line SOAs using MLSE and duobinary signals

We experimentally demonstrate uncompensated 8-channel wavelength division multiplexing (WDM) and single channel transmission at 10.7 Gb/s over a 470 km hybrid fiber link with in-line semiconductor optical amplifiers (SOAs). Two different forms of the duobinary modulation format are investigated and compared. Maximum Likelihood Sequence Estimation (MLSE) receiver technology is found to significantly mitigate nonlinear effects from the SOAs and to enable the long transmission, especially for optical duobinary signals derived from differential phase shift keying (DPSK) signals directly detected after narrowband optical filter demodulation. The MLSE also helps to compensate for a non-optimal Fabry-Perot optical filter demodulator.

Impact of optical modulation formats on SPM-limited fiber transmission in 10 and 40 Gb/s optimum dispersion-managed lightwave systems

In this paper, 10 and 40 Gb/s optical systems have been investigated for nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed return-to-zero (CSRZ) and RZ-differential phase-shift-keying (RZ-DPSK) data formats. For the range of the optical signal power from −5 to 15 dBm, a maximum self-phase modulation (SPM)-limited transmission distance L SPM is determined with eye-opening penalty (EOP) >1 dB .The observations are based on the modeling and numerical simulation of optimum dispersion-managed transmission link. Transmission over distances of the order of several hundreds of kilometers has been shown with and without amplified spontaneous emission (ASE) noise of the in-line erbium-doped fiber amplifiers (EDFAs).

Determining the maximum signal power in 10 Gbit/s WDM optical networks

Nowadays in optical networks there is a trend line to extend the maximum transmission distance. Several techniques are known; however, the simplest one is to increase the signal power in the optical fiber. The task of minimizing the number of inline amplifiers in order to increase the deployed system’s cost effectiveness leads to increasing the signal power. The only problem is the nonlinear behavior of the optical fiber which limits the signal power to be inserted into the optical fibers. In this paper, we present a model to calculate analytically the signal quality deterioration due to nonlinear effects at the receiver point. As a result, we give the exact value of the optimal signal power at the transmitter point or at the output of the inline amplifiers for 10 Gbit/s wavelength division multiplexed (WDM) systems.

Semiconductor Optical Amplifier for Detection Function in Radio Over Fiber Systems

This paper demonstrates that in broadband networks in-line semiconductor optical amplifiers (SOAs) beside providing system gain can also be used as simultaneous transceivers. The SOA branching stages are studied and presented for bus configuration. Using a steady-state model, we have made a small-signal approximation applying an equivalent circuit for detection based on the rate equations. The theory enables us to explain the detection functionality, the optimal working state of the multifunctional SOA, and to analyze noise properties. The measurements support the calculations. So the SOA-transceivers are potentially useful in subcarrier multiplexed access systems.

In-line phase-sensitive amplification of multi-channel CW signals based on frequency nondegenerate four-wave-mixing in fiber

We demonstrate phase-sensitive amplification of multiple wavelength-division-multiplexed continuous-wave (CW) signals by frequency nondegenerate four-wave-mixing process in optical fiber. By fine-tuning the optical wavelengths of the CW signals, simultaneous phase-sensitive in-line amplification of three signal channels is realized. This indicates the possibility of amplifying multiple data channels by an in-line phase-sensitive fiber parametric amplifier. We also discuss a potential system architecture employing such amplifiers.

Relative Intensity Noise in Optical Single Side Band Systems with Multiple In-Line Amplifiers: Analysis and Validation

This article analyzes and validates through simulation the influence of fiber nonlinearity, dispersion, and loss on the Relative Noise Intensity (RIN) spectrum at fiber output in externally modulated optical single side band (OSSB) systems. The analysis includes the RIN generated by the laser source, the effect of the OSSB modulator, and noise generated by the in-line optical amplifiers. The analysis takes into account conversion between phase noise and intensity noise introduced by fiber transmission. It is shown that, depending on the operating conditions, the system performance may be predominantly affected either by the laser RIN or by the noise generated by the in-line optical amplifiers. The impact of the OSSB modulator can be relevant when the RIN of the system is dominated by the laser noise and should therefore be considered.

Assessment of gain saturation in cascaded semiconductor optical amplifier for high-bit-rate wavelength-division-multiplexing signals over long transmission distance

In this paper, the investigation of 10 channels at 80-Gb/s wavelength-division-multiplexing (WDM) transmission over 910-km single-mode fiber and dispersion compensating fiber using cascaded in-line semiconductor optical amplifiers at a span of 70 km for soliton return zero differential phase-shift keying (RZ-DPSK) modulation format is achieved. With the narrow channel spacing (i.e., 200 GHz), the results of quality is more than 15 dB the received signals after covering 910-km transmission distance without any power drop. The semiconductor optical amplifier (SOA) structure parameters for in-line amplifier is optimized to obtain low cross talk in multichannel WDM systems at nil power penalty with sufficient gain. For the differential gain 200 atto cm 2 and length 750 mm of SOA has minimum SOA-induced cross talk is observed. The impact of optical power received and the Q factor at different differential gain, carrier lifetime, and length has been illustrated. It is found that for a transmission distance of 700 km, with decrease the channel spacing, there is an increase in cross talk among channels, hence the signal quality continues to decrease. A clear eye diagram and a good optical spectrum are observed at a transmission distance of 910 km in a soliton RZ-DPSK system.

Coherent transfer of an optical carrier over 251 km

We transfer an optical frequency over 251 km of optical fiber with a residual instability of 6x10(-19) at 100 s. This instability and the associated timing jitter are limited fundamentally by the noise on the optical fiber and the link length. We give a simple expression for calculating the achievable instability and jitter over a fiber link. Transfer of optical stability over this long distance requires a highly coherent optical source, provided here by a cw fiber laser locked to a high finesse optical cavity. A sufficient optical carrier signal is delivered to the remote fiber end by incorporating two-way, in-line erbium-doped fiber amplifiers to balance the 62 dB link loss.

Signal Design and Detection in Presence of Nonlinear Phase Noise

In optical fiber transmission systems using inline amplifiers, the interaction of a signal and amplifier noise through the Kerr effect leads to nonlinear phase noise that can impair the detection of phase-modulated signals. We present analytical expressions for the maximum-likelihood (ML) decision boundaries and symbol-error rate (SER) for phase-shift keying and differential phase-shift keying systems with coherent and differentially coherent detection, respectively. The ML decision boundaries are in the form thetas(r) = c2r2 + c1r + c0, where thetas and r are the phase and the amplitude of the received signal, respectively. Using the expressions for the SER, we show that the impact of phase error from carrier synchronization is small, particularly for transoceanic links. For modulation formats such as 16-quadrature amplitude modulation, we propose various transmitter and receiver phase rotation strategies such that the ML detection is well approximated by using straight-line decision boundaries. The problem of signal constellation design for optimal SER performance is also studied for a system with four signal points.

Power budget improvement for long-haul WDM transmission with optimum placement of SOAs

Sufficient power margin is investigated for ten-channels WDM transmission over 68,908 km by using cascaded in-line semiconductor optical amplifier for the differential phase-shift keying (DPSK) modulation format for the first time. For this, we used the structural optimization and placement scheme of semiconductor optical amplifiers (SOAs) for long-haul WDM transmission. The SOA model for in-line amplifier has low crosstalk, ASE noise power and low noise figure with sufficient gain. The impact of noise figure, amplification factor, ASE noise power, optical gain and crosstalk with signal input power for the SOA model has been illustrated, which shows that 400 mA is the optimum bias current. We observed that the optimized optical filter bandwidth for the 100 GHz channel spacing is 0.4 nm. We observe that as we decrease the channel spacing, the quality of signal is degraded. We show that the optimum span scheme-1 is used up to a transmission distance of 68,908 km with good quality for power margin more than 24 dB. This placement scheme of SOAs shows good power budget for long transmission distance. We show the optical spectrum and clear eye diagram at the transmission distance of 68,908 km for optimum span schemes. Finally, we investigate the maximum transmission distance with decrease in channel spacing, i.e., 20 and 50 GHz.

Interleaved Bidirectional Transmission of 16 $\times$ 10-Gb/s DWDM Signals Using DPSK Modulation Format and In-line Semiconductor Optical Amplifiers

We have performed both experimental and numerical analyses of interleaved bidirectional transmission of 16 times 10-Gb/s dense-wavelength-division-multiplexed (DWDM) signals using differential-phase-shift-keying (DPSK) modulation format and in-line semiconductor optical amplifiers (SOAs). The constant intensity of DPSK-modulated DWDM signals enabled us to mitigate the crosstalk effects in the bidirectional SOA. We have demonstrated error-free bidirectional transmission of 16 channels over an 80-km single-mode fiber (SMF) with matching dispersion-compensating fiber and in-line amplification provided by an SOA. SOAs can be used for bidirectional transmission if the facet reflections can be controlled. In order to understand further the effect of the SOA facet reflectivity on transmission results, we developed a simulation model for our system. Based on the good agreement between our simulation and experimental results, we have found out that the system reflections can be minimized by careful optimization of the bias current and reflectivity of the SOA

Comparative Analysis of BER Estimation Methods in Numerical Simulation of 40-Gb/s RZ-DPSK Transmission With In-Line SOAs

Applying direct error counting, we compare the accuracy and evaluate the validity of different available numerical approaches to the estimation of the bit-error rate (BER) in 40-Gb/s return-to-zero differential phase-shift-keying transmission. As a particular example, we consider a system with in-line semiconductor optical amplifiers. We demonstrate that none of the existing models has an absolute superiority over the others. We also reveal the impact of the duty cycle on the accuracy of the BER estimates through the differently introduced Q-factors