Backward-pumped Raman Amplifiers Research Articles

Distributed Backward Pumped Raman Amplifiers Gain without Attenuation: A New Approach

Optical amplifiers have made a great achievement and a huge revolution in the world of optical communications, but some of the shortcomings must be occurs, for example, backward pump Raman amplifier suffers from decrease in gain. The main objective of this paper is to solve the problem of decrease that occurs in gain of backward pumped Raman amplifier. To solve this problem, we design a model of two of backward pumped Raman amplifiers are used in a cascaded form to overcome the attenuation in gain. We present the results in the case of using one backward pumped Raman amplifier compared to the model that we designed to show the difference between the two cases. Three types of fibers with different gain parameters are used. We also simulate and analyze the parameters that affect the Raman gain of fiber Raman amplifier for three different types of fiber. The gain depends heavily on the length of the fiber and the pumping power, according.

Approaching Terabits Per Carrier Metro-Regional Transmission Using Beyond-100GBd Coherent Optics With Probabilistically Shaped DP-64QAM Modulation

This research work proposed and experimentally demonstrated beyond-100GBd probabilistically shaped coherent optics that support the transmission of a ~1.2 Tbps coded line rate and a ~0.9 Tbps net bit rate per optical carrier over metro-regional distances. Pursuing next-generation transmission systems having both high capacity approaching terabits per wavelength and high spectral efficiency require engineering efforts from different aspects, which may include the enhancement of analog-to-digital or digital-to-analog interfaces, the improvement of channel characteristics, and also the realization of better receiver sensitivities, preferably through DSP techniques such as probabilistic shaping (PS). By doing so, this work achieved a single-channel and a 112.5-GHz WDM transmission of 900G wavelengths over 800 and 400 km, respectively, which employed a PS DP-64QAM modulation format at 105 GBd, yielding a net spectral efficiency of 8.05 bits/s/Hz. Such a high symbol rate was demonstrated using a commercially available DAC unit without band-interleaving. The enhanced optical channel consists of ITU-T G.654 compliant Terawave SLA+ fiber spans with the backward-pumped Raman amplifiers. In addition, with the same PS DP-64QAM modulation format and the same enhanced channel, this research work also demonstrated that 400G wavelengths can be delivered over 2800 km supporting long-haul applications.

Transmission Performance of 256-Gb/s PDM-16QAM With Different Amplification Schemes and Channel Spacings

We experimentally investigate the performance of a 256-Gb/s polarization-division-multiplexed 16-ary quadrature-amplitude-modulation (PDM-16QAM) transmission system in dispersion uncompensated TeraWave SLA+ fiber spans using erbium-doped-fiber amplifiers (EDFAs), hybrid EDFA/Raman amplifiers, and all backward-pumped Raman amplifiers. The effects of 37.5- and 50-GHz channel spacings and cascaded optical add/drop multiplexers (ROADMs) on the system performance are studied. We find that when there are no ROADMs, hybrid EDFA/Raman amplifiers and all-backward-pumped Raman amplifiers can increase the reach by about 70% and 100%, respectively, compared to EDFA-only amplifiers, from 2000 to about 3500 and 4200 km, respectively, assuming 20% soft-decision forward-error-correction (SD-FEC) for the system with both 37.5- and 50-GHz channel spacings. We show that cascaded ROADMs have a small impact on the system with 50-GHz channel spacing, but can severely degrade the performance of the system with 37.5-GHz channel spacing and virtually eliminate the reach gain from Raman amplifiers in such systems due to large penalties and higher error floors induced by ROADM filtering effects. We also show that for the same number of channels, the system with 37.5- and 50-GHz channel spacings has similar performance when there are no ROADMs.

Simple design method for gain-flattened three-pump Raman amplifiers

We propose a simple and efficient approach to the design of 3-wavelength backward-pumped Raman amplifiers with increased gain flatness over a wide spectral band. Three different methods dealing with the optimization of one or two different simple parameters are studied. Various examples are provided for illustration, and the dependence of both amplifier gain and gain flatness with pump power is shown. Gain flatness in the spectral range of 1,520–1,595 nm of about 1 dB for a 7.5 dB gain and of 1.8 dB for a 13.5 dB gain is demonstrated using only three pumps with wavelengths within the 1,420–1,480 nm interval.

A Novel Algorithm for Backward-Pumped Raman Amplifier

In this article, a novel semi-analytical algorithm for calculating backward-pumped Raman amplifier is proposed. The algorithm does not need iteration and can greatly save computational time. Simulation results for Raman amplifiers with 25 km, 50 km, and 100 km fibers show that the accuracy of the method is quite satisfying in comparison with the shooting algorithm and the deviation is within 0.02 dB.

Modeling backward-pumped Raman amplifiers

We describe a robust shooting algorithm to model backward-pumped Raman amplifiers. This algorithm uses a continuation method and a Jacobi weight in conjunction with the shooting algorithm. We compare this algorithm to the commonly used relaxation algorithm. We find that the shooting algorithm is more flexible, in that it can be applied to amplifiers in which one fixes the gain, in contrast to the standard relaxation algorithm, which can be applied only to two-point boundary-value problems. However, it is less efficient when applied to two-point boundary-value problems, in that it requires more computer time.

Gain saturation spectrum of backward-pumped broad-band Raman amplifiers

The gain saturation of optical amplifiers due to changes in wavelength-division-multiplexed channel loading is characterized using a technique that determines the saturation effect on each channel. This channel gain saturation (CGS) measurement is applied to a broad-band backward-pumped Raman amplifier and used to study how the CGS spectral tilt is controlled by the relative contributions of the pump-to-pump and channel-to-channel interactions. The CGS is also used to measure the gain dependence on the channel add-delete pattern.

Effect of a nonlinear photonic Crystal fiber on the noise characterization of a distributed Raman amplifier

The first experimental study of the effect of a nonlinear photonic crystal fiber (PCF) on the noise characteristics of a distributed backward-pumped Raman amplifier is reported. The PCF has a highly nonlinear Raman efficient, and a high Rayleigh scattering parameter. When an optical signal first passes through a 100-m nonlinear PCF followed by a 25-km single-mode fiber, the optical signal-to-noise ratios (OSNRs) of the amplified spontaneous emission and the double Rayleigh scattering (DRS) are improved because the high Raman gain efficiency of the PCF makes the Raman gain of the signal at the beginning of the link increase, and the signal power over the length of the transmission becomes near a constant. However, the improvement of the OSNR of DRS compared with the OSNR of amplifier spontaneous emission is limited by the large Rayleigh scattering in the PCF.

Analytical characterization of optical power and noise figure of forward pumped Raman amplifiers

We show that it is possible to find analytic expressions for characterizing the evolution of signal and noise photon numbers along the active fiber of a forward-pumped Raman amplifier with unequal signal and pump loss coefficients. We confirm the validity of the result by comparing the analytical solutions with numerical calculations and by analytically deriving the well-known 3 dB noise figure limit for high Raman gain. Apart from aiding the analysis and design of forward pumped Raman amplifiers, these results also enable one to find approximate analytical solutions for bidirectional Raman amplifiers and backward pumped Raman amplifiers with Rayleigh backscattering and Brillouin scattering.

Transient responses to slowly varying input waveforms in backward pumped Raman amplifiers

In this article, a novel numerical algorithm to simulate power transient in backward pumped Raman amplifiers is proposed. Based on this algorithm, transient response to a slowly varying input power waveform is investigated for the first time. We find that a properly designed, slowly varied input leading edge can reduce the output up-shoot drastically in a single-channel condition, while in a multi-channel condition, a gradually changed input waveform can eliminate the abrupt power up-shoot due to forward signal-to-signal Raman interactions and obtain a better gain clamping result by using pump adjustment. These conclusions will play a role in Raman amplified optical burst mode switching (or packet switching) systems and DWDM networks.

Cross modulation of pump-signals in distributed Raman amplifiers, theory and experiment

Cross modulation of the pump and signal in a backward pumped distributed Raman amplifier (DRA) is investigated theoretically and experimentally. An analytical theory describing the frequency response of cross modulation in a DRA with pump perturbation is developed for the first time. The corner frequency of this response is simply determined by the walkoff parameter of the two counterpropagation lights and the loss coefficient of the Raman fiber. This theory was confirmed by the agreement of theoretical results and experimental measurements.