Distributed Fiber Raman Amplifier Research Articles

Dense wavelength division multiplexing scheme based on effective distributed inline light fiber Raman amplifier configuration

Abstract This paper demonstrated the dense wavelength division multiplexing scheme based on effective distributed inline light fiber Raman amplifier configuration. Various forward/backward and bidirectional pumping power configurations are studied versus fiber reach. Output light signal power is demonstrated against fiber reach without Raman amplification technique. Output light signal power in the forward Raman amplification scheme is clarified with pumping power of both 500 mW and 700 mW in various fiber channel configurations. As well as output light signal power in the backward Raman amplification scheme with pumping power of both 500 mW and 700 mW in various fiber channel configurations. Amplification Raman gain parameter coefficient is demonstrated with various values of pumping power pump based on various fiber channel configurations. Backward amplification net parameter gain is studied for different single mode/true wave/freelight fibers channel configuration at different both pumping power values and fiber reach. As well as the forward amplification net parameter gain is clarified for different single mode/true wave/freelight fibers channel configuration at different both pumping power values and fiber reach.

Chaos synchronization of semiconductor lasers over 1040-km fiber relay transmission with hybrid amplification

Optical chaos communication and key distribution have been extensively demonstrated with high-speed advantage but only within the metropolitan-area network range of which the transmission distance is restricted to around 300 km. For secure-transmission requirement of the backbone fiber link, the critical threshold is to realize long-reach chaos synchronization. Here, we propose and demonstrate a scheme of long-reach chaos synchronization using fiber relay transmission with hybrid amplification of an erbium-doped fiber amplifier (EDFA) and a distributed fiber Raman amplifier (DFRA). Experiments and simulations show that the hybrid amplification extends the chaos-fidelity transmission distance thanks to that the low-noise DFRA suppresses the amplified spontaneous emission noise and self-phase modulation. Optimizations of the hybrid-relay conditions are studied, including launching power, gain ratio of DFRA to EDFA, single-span fiber length, and number of fiber span. A 1040-km chaos synchronization with a synchronization coefficient beyond 0.90 is experimentally achieved, which underlies the backbone network-oriented optical chaos communication and key distribution.

Solution of fiber Raman amplifier model using binary search equation-based adaptive artificial bee colony algorithm

Due to the need for high bandwidth, there is growing interest in distributed fiber Raman amplifiers (FRA). In addition to having wide bandwidth, FRAs have the advantages of a low noise factor and simplicity of use. However, the optimization of the wavelength and power values of the pump lasers to be used in FRAs for a broadband and flat gain region is an important problem to be solved. In this study, a distributed FRA system was set using 100 signals with a 50-km propagation distance and 8 pump signals in the opposite direction, which were gain-flattened. The attenuation coefficient of SMF-28 type optical fiber was used in the system. First, optimum pump wavelengths and power levels were found by the binary search equation based adaptive artificial bee colony algorithm. Then, the FRA boundary value problem (BVP) was solved with the MATLAB BVP solver. Finally, the analytical Jacobian matrix required for the solution of the equation was included in the system, and faster and more effective results were obtained. The results show that the gain ripple of 100 optical signals was 0.5 dB at the maximum.

Automatic Gain Control Method and Implementation of Distributed Raman Fiber Amplifier

Automatic Gain Control Method and Implementation of Distributed Raman Fiber Amplifier

On the Evolution of Noise in Multiple-Span Transmission With Forward Pumped Raman Amplifiers

The evolution of noise in transmission systems with multiple spans featuring forward pumped distributed Raman fiber amplifiers has been investigated experimentally and by numerical simulations. We found evidence that the transfer of relative intensity noise from the pump to the signal reveals characteristics of a nonlinear process, which cannot fully be captured by a Gaussian random variable added to the signal power. Power fluctuations of the pump radiation result in an intensity modulation of the amplified signal by stimulated Raman scattering. We have observed that a concatenation of multiple forward pumped amplifiers can lead to an increase of the bandwidth of the noise added to the signal compared to a single amplifier. A process acting like modulation instability of the pump radiation during propagation through the fiber was identified as a potential source of the bandwidth increase from span to span.

Gain-forming effect on optical signal-to-noise ratio in Raman fiber amplifier

Abstract We have introduced the comprehensive optical-to-noise ratio (OSNR) to investigate simultaneously the effects of all three major linear noise sources in distributed Raman fiber amplifier (DRFA). The total noise power contains double Rayleigh backscattering (DRB) noise, amplified spontaneous emission (ASE) noise, and relative intensity noise (RIN). Also, we have introduced an approximated gain profile approach, which helps in evaluating the OSNR for higher-order pumping (HOP) configuration for backward, forward, and bidirectional DRFA schemes. Finally, by applying the dynamic and static full-numerical computations on a system of eight wavelength division multiplexing (WDM) channels (for amplifier lengths of 70, 140, and 210 km), we demonstrated that for RIN frequencies greater than 100 kHz, the higher-order bidirectional pumping scheme exceeds the backward schemes by 12 dB.

A Channel Model to Deal with Distributed Noises and Nonlinear Effects in a Fiber System with Distributed Raman Amplifiers

Nowadays, the distributed fiber Raman amplifier (FRA) has become more and more popular in long-haul fiber systems, owing to its lower noise figures and weaker nonlinear effects in the link. The critical issue in distributed FRAs is the presence of various kinds of noises and their interactions with the signal. However, the existing Raman channel models and their numerical solving methods can only partially describe how the randomly distributed noises interact with the signal. This causes the difficulties in analyzing the distributed FRA precisely and the inconveniences for the applications and the maintenance of FRA systems. In this paper, we propose a modified Raman channel model to describe more comprehensively the interactions between the distributed noises and the signal under the influence of loss, distributed gain, dispersion, and nonlinear effects in the distributed FRA systems. With the comparisons of the error–vector magnitude (EVM) curves, our model can get lower errors in the experimental results regarding bidirectional pumped FRA single-span fiber systems and multi-span systems with backward-pumped FRAs.

A Comparative Analysis of Dual-Order Bidirectional Pumping Schemes in Optical Fiber Raman Amplification

Abstract In this paper, dual-order bidirectional pumping schemes of distributed fiber Raman amplifier are compared with standard first-order pumping in wavelength division multiplexed optical transmission systems. The novel comparison analysis is carried out in terms of Optical signal-to-noise ratio and Q-factor, on-off gain and noise figure by varying optical input power and fiber lengths. The results indicate that dual-order schemes present 0.02 dB higher OSNR and 5 dB higher Q-factor in comparison to first-order pumping when input optical power is varied from −4 to 5 dBm. Similarly, there is 4 dB higher on-off gain with dual order comparatively to first order when fiber length varied from 10 to 100 km. However, there is degradation in noise figure and Q-factor due to DRBS noise with dual-order pumping when fiber length from 10 to 100 km. Further, the signal power evolutions along fiber length show that there is 5 dBm improvement for 100 km fiber. The novelty of the work is that comparative analysis exhibits improvement in OSNR, on-off gain and Q-factor using dual-order bidirectional pumping.

Performance Analysis of Distributed Raman Amplifier Using Different Pumping Methods in DWDM Communication Systems

Abstract The objective of this work is to investigate and compare the impact of forward, backward and bidirectional pumping methods in repeater less distributed Raman fiber amplifier system and to analyze its benefits in long haul optical communication links. The effect of different pump wavelengths and pump powers are also analyzed. By adjusting the forward pump power as well as backward pump powers carefully, the transmission performance can be improved. The pump power ratio is adjusted to 20% so that the degradation of signal due to non-linear effect is reduced. The distributed gain and pump penetration depth for different pump power ratios are also examined. A comparison of different pumping methods reveals that bidirectional pumping gives the optimum performance in terms of higher gain and lower noise figure when compared to single pumps and unidirectional pumping. Low pump power ratios lead to better transmission performance than higher pump ratios due to lesser non-linear effects. The simulation results indicate that signal power evolution decreases with the increase in length of the fiber. It is also found that the pump power is more relevant during the first 50 km of the distributed fiber line and it decreases as it goes beyond 100 km. By choosing the optimum pumping methods and number of pumps, Raman Amplifiers can be used in almost all places where optical amplifiers are required.

Performance analysis of distributed fiber Raman amplifiers employing higher order pumping schemes in optical transmission systems

Performance of fiber Raman amplifier is explored with special emphasis on higher order pumping configurations for wavelength division multiplexed optical transmission systems. The amplification analysis is done in terms of equivalent noise figure (ENF), optical signal-to-noise ratio (OSNR), and double Rayleigh backscattering (DRBS) noise powers. The investigations reveal that at 36 dB on--off gain, ENF improvement of 1 dB (using second order pumping) and of 2 dB (using third order pumping configuration) can be achieved. On similar lines, by varying input optical power and fiber length, OSNR and DRBS are reported for the higher order of pumping configurations. Overall, the work presents improvements of about 1 dB in ENF and 0.03 dB in OSNR. These investigations are produced at lower input signal powers.

Analysis of Spontaneous Raman and Rayleigh Scatterings in Distributed Fiber Raman Amplification Systems Based on a Random Distribution Model

The properties of light wave propagation with stochastic scattering in a fiber are particularly attractive because of their influence in various applications. Although stochastic scattering in fibers is of great significance, until now, no accurate theoretical model has described the randomness of the distributions and variations in scattering since the scattering sources are regarded as centralized or homogeneously distributed. In this paper, we proposed a model to analyze the random variation in scattering with time and location. Then, we employed the model to simulate stochastic scattering in distributed fiber Raman amplification (FRA) systems, including Rayleigh backscattering and spontaneous Raman scattering, which are crucial limitations of the distributed FRA systems. The simulations agreed well with our experimental measurements, proving that our model efficiently described stochastic scattering spectra and distributions in FRAs. Our theory accurately analyzed the distribution and evolution of scattering along the fiber and is a promising tool for optimizing the performance of distributed fiber systems, especially systems with distributed amplification such as fiber communication systems, random feedback fiber lasers, and fiber sensors.

Efficient evaluation of impairment induced by distributed fiber Raman amplifier using error vector magnitude techniques in unrepeated coherent communication system

We investigate the impairment induced by relative intensity noise (RIN) of Raman pump in an ultra-long unrepeated multi-level modulated coherent optical communication system. By adopting error vector magnitude (EVM) techniques, we proposed a simple and high efficient numerical method to calculate and analyze the impact of Raman pump RIN on the coherent receiver system. Both intensity and phase noise are taken into consideration in our numerical simulations when choosing Raman pump lasers with different RIN and using different signals. Our simulation result shows that higher-order phase-modulated signal is more sensitive to RIN of the Raman pump. Comparing to the phase noise, intensity noise induced by RIN of the Raman pump can generally be ignored. Apart from the well-known walk-off parameter, nonlinear parameters and Raman-gain coefficient also play important roles in the complex noise transfer process. Our calculation makes it possible to quickly and accurately evaluate the hybrid distributed fiber Raman amplification (DFRA) along with remotely-pumped erbium-doped fiber amplification (EDFA) in ultra-long unrepeated transmission systems.

Effect of stimulated Brillouin scattering on the gain saturation of distributed fiber Raman amplifier and its suppression by phase modulation**Project supported by the National Natural Science Foundation of China (Grant No. 61177073) and the Major Application Basic Research Project of National University of Defense Technology, China (Grant No. ZDYYJCYJ20140701).

For distributed fiber Raman amplifiers (DFRAs), stimulated Brillouin scattering (SBS) can deplete the pump once occurring and consequently generate gain saturation. On the basis of such a theory, theoretical gain saturation powers in DFRAs with various pump schemes are obtained by calculating SBS thresholds in them, and the experimental results show that they are in excellent agreement with the calculation results. The saturation power of the DFRA with a 300 mW forward pump is as low as 0 dBm, which needs to be enhanced by phase modulation, and the effect is quantitatively studied. A simple model taking both modulation frequency and index into consideration is presented by introducing a correction factor to evaluate the effect of phase modulation on the enhancement of saturation power. Experimentally, it is shown that such a correction factor decreases as the modulation frequency increases and approaches zero when the modulation frequency becomes high enough. In particular, a phase modulation with a modulation frequency of 100 MHz and a modulation index of 1.380 can enhance the saturation power by 4.44 dB, and the correction factor is 0.25 dB, in which the modulation frequency is high enough. Additionally, the factor is 1.767 dB for the modulation frequency of 25 MHz. On this basis, phase modulations with various indexes and a fixed frequency of 25 MHz are adopted to verify the modified model, and the results are positive. To obtain the highest gain saturation power, the model is referable. The research results provide a guide for the design of practical DFRAs.

Performance of megahertz amplified optical time-stretch optical coherence tomography (AOT-OCT).

Enabled by the ultrahigh-speed all-optical wavelength-swept mechanism and broadband optical amplification, amplified optical time-stretch optical coherence tomography (AOT-OCT) has recently been demonstrated as a practical alternative to achieve ultrafast A-scan rate of multi-MHz in OCT. With the aim of identifying the optimal scenarios for MHz operation in AOT-OCT, we here present a theoretical framework to evaluate its performance metric. In particular, the analysis discusses the unique features of AOT-OCT, such as its superior coherence length, and the relationship between the optical gain and the A-scan rate. More importantly, we evaluate the sensitivity of AOT-OCT in the MHz regime under the influence of the amplifier noise. Notably, the model shows that AOT-OCT is particularly promising when operated at the A-scan rate well beyond multi-MHz--not trivially achievable by any existing swept-source OCT platform. A sensitivity beyond 90 dB, close to the shot-noise limit, can be maintained in the range of 2 - 10 MHz with an optical net gain of ~10 dB. Experimental measurement also shows excellent agreement with the theoretical prediction. While distributed fiber Raman amplification is mainly considered in this paper, the theoretical model is generally applicable to any type of amplification schemes. As a result, our analysis serves as a useful tool for further optimization of AOT-OCT system--as a practical alternative to enable MHz OCT operation.

Relative phase noise induced impairment in CO-OFDM optical communication system with distributed fiber Raman amplifier.

In this Letter, we demonstrate that the interplay between Raman pump relative intensity noise and cross-phase modulation leads to a relative phase noise (RPN) that brings non-negligible performance degradation to coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission systems with co-pumped Raman amplification. By theoretical analysis and numerical simulation, we proved that RPN brings more system impairment in terms of Q-factor penalty than the single carrier system, and relatively larger walk-off between pump and signal helps to suppress the RPN induced impairment. A higher-order modulated signal is less tolerant to RPN than a lower-order signal. With the same spectral efficiency, the quadrature-amplitude modulation format shows better tolerance to RPN than phase-shift keying. The reported findings will be useful for the design and optimization of Raman amplified CO-OFDM multi-carrier transmission systems.

New trend for optical signal-to-noise ratio of disturbed Raman fiber amplifier

In a distributed Raman fiber amplifier (DRFA), Raman amplification allows a lower signal launch powers to transverse the span above the noise floor while still increasing the optical signal-to-noise ratio (OSNR). It improves the noise figure and reduces the nonlinear penalty of fiber systems. In this paper, we demonstrate a new trend of OSNR at different pump configurations: forward, backward and bidirectional pumping for DRFAs as a function of fiber length. We also present the variation of OSNR with both input pump power and input signal power. It is found that forward pumping provides the highest OSNR, reaching its maximum value of 37dB. However, backward pumping provides the smallest OSNR that has its maximum of 22dB and the bidirectional pumping provides the moderate OSNR between the others having its peak of 26dB.

Flexible single pump hybrid fiber amplifier for the S+C bands

This work proposes optical amplification in the S+C bands using a hybrid optical fiber amplifier (HFA) that uses a single wavelength pump. Its configuration is based on an erbium doped amplifier (EDFA) to provide gain over the C band and a distributed Raman fiber amplifier (RFA) to attain gain over the S band. Moreover, the proposed amplifier uses standard commercially available fibers. The HFA was numerically characterized and its optimal configuration was calculated using optimization procedures based on genetic algorithms. A flexible HFA configuration was achieved allowing a span length up to 100km.

Signal-to-noise analysis in a counter-pumped fiber Raman amplifier

The light gain efficiency due to Raman scattering in a distributed fiber Raman amplifier is a subject of our investigation. The detailed analysis of noise properties at one-wave optical signal amplification in a single-mode fiber in the scheme of counter-pumped fiber Raman amplifier is realized in the paper. Experimental results of the amplified spontaneous emission with backward pumping as a base of optical noise evaluation are presented. Raman gain spectrum for a coherent Stokes low-powered signal is calculated and it is compared with the observed spectrum of non-coherent amplified spontaneous emission noise. It is shown that an output optical signal has higher gain than the amplified spontaneous emission and it results in the appreciable growth in the output optical signal-to-noise ratio.

Optimization on bandwidth in the third-order pumped distributed fiber Raman amplifier

The effect of higher-order amplified spontaneous scattering in Raman amplification is investigated. With the effect, a structure of third-order distributed fiber Raman amplifier (DFRA) is introduced and its bandwidth is studied and optimized. A method of multiorder pumps with multiwavelengths is adopted, which is different from that of multiwavelength pumps for the conventional DFRA. Simulation results indicate that pumps in different orders have different functions: the first-order pump can improve the gain flatness and extend the bandwidth, the second-order pump can increase the gain value, whereas the third-order pump can provide powers for pumps in other orders and improve the noise figure (NF) together with them; a flat spectrum with the bandwidth of 80 nm, gain of 24 dB, ripple of less than 1 dB, and a low NF over a span of 120 km is achieved. All these results are beneficial to design a broadband higher-order Raman pumping DFRA in future experimental work. A modified genetic algorithm is also proposed in optimizing the bandwidth.

Relative phase noise induced impairment in M-ary phase-shift-keying coherent optical communication system using distributed fiber Raman amplifier

We show for the first time, to the best of our knowledge, that, in a coherent communication system that employs a phase-shift-keying signal and Raman amplification, besides the pump relative intensity noise (RIN) transfer to the amplitude, the signal's phase will also be affected by pump RIN through the pump-signal cross-phase modulation. Although the average pump power induced linear phase change can be compensated for by the phase-correction algorithm, a relative phase noise (RPN) parameter has been found to characterize pump RIN induced stochastic phase noise. This extra phase noise brings non-negligible system impairments in terms of the Q-factor penalty. The calculation shows that copumping leads to much more stringent requirements to pump RIN, and relatively larger fiber dispersion helps to suppress the RPN induced impairment. A higher-order phase-shift keying (PSK) signal is less tolerant to noise than a lower-order PSK.