Signal Launch Power Research Articles

All-optical phase-preserving amplitude-regeneration technology based on bidirectional orthogonal-pumped semiconductor optical amplifier configuration

The phase-preserving amplitude regeneration scheme based on the bidirectional orthogonal-pumped semiconductor optical amplifier (SOA) is proposed in this work. Experimental investigation into the multiple four-wave mixing (FWM) process from the pump, the signal and their corresponding reflective fields is carried out in detail. The regeneration performance obtained from the product between co-propagating fields is also discussed, including its dependence on the signal launch power and the signal quality, to quantify the amplitude regeneration and the phase preserving behaviors. The amplitude distortion is suppressed by 2.2 dB experimentally, confirming the regeneration capability of the proposed scheme. Moreover, the regeneration performance is further investigated for multiple phase shift keying (MPSK) signals through the simulation. According to the numerical results, the operational parameters of the regenerator are the same for advanced modulation formats, proving the robust operation of the proposed bidirectional orthogonal-pumped SOA configuration.

Pilot Tone Power Limits of Brillouin Amplified Carrier Recovery for Optical Communications

The benefit of Brillouin amplification for optical carrier recovery from pilot tones is evaluated in application to coherent signal detection without a conventional local oscillator (LO) laser. The focus being on the narrow gain bandwidth suppressing broadband spectral noise power relative to the pilot tone. Calculations indicate its use at the receiver with realistic 30 dB gain and 30 MHz gain bandwidth permit lowering the minimum pilot tone to signal power ratio (PSR) for low distortion signal demodulation by up to ≈24 dB in the case of a 10 GHz wide signal channel, and even larger for broader channels. The ultimate limit being its ratio to the narrowness of the Brillouin gain bandwidth. Low noise Brillouin gain is also accounted for from characterization by a coherent receiver and digital signal processing approach for an optical fiber used as the gain medium in experiments. Its use in optical carrier recovery from a 3 channel WDM 48 Gb/s 64-QAM signal with polarization multiplexed pilot tones demonstrates improvement consistent with predictions in reducing the impact of pilot tone noise to near that for a conventional LO. The estimated low PSR limit for a bit error rate below the hard decision FEC limit was extrapolated as ≈-38 dB. This rose to ≈-30 dB with inclusion of 80 km link transmission due to the restricted signal launch power for avoiding nonlinear distortion. The insight highlights the wide-ranging capabilities for enhancing performance in advanced coherent communications.

Combating Fiber Nonlinearity Using Dual-Order Raman Amplification and OPC

We experimentally demonstrate, for the first time, a significant 34-dB nonlinear product power compensation in a mid-link optical phase conjugation (OPC) system using an optimized dual-order distributed Raman amplification (DRA) technique. The dual-order backward (BW)-pumping scheme shows a record signal power symmetry of 97% over 50.4-km single-mode fiber (SMF) spans. We also demonstrate that the required accuracy for span-to-span power alignment is within ±1 dB in order to maintain 20-dB nonlinear product compensation. For a 256-Gb/s dual-polarization-16 QAM transmission over 100.8 km ( $2\times50.4$ km) SMF with mid-link OPC, the proposed Raman amplification scheme enables the OPC to improve the nonlinear threshold by ~7 dB and the optimum signal launch power by ~5 dB compared with the system without OPC.

Hollow-core conjoined-tube negative-curvature fibre with ultralow loss

Countering the optical network ‘capacity crunch’ calls for a radical development in optical fibres that could simultaneously minimize nonlinearity penalties, chromatic dispersion and maximize signal launch power. Hollow-core fibres (HCF) can break the nonlinear Shannon limit of solid-core fibre and fulfil all above requirements, but its optical performance need to be significantly upgraded before they can be considered for high-capacity telecommunication systems. Here, we report a new HCF with conjoined-tubes in the cladding and a negative-curvature core shape. It exhibits a minimum transmission loss of 2 dB km−1 at 1512 nm and a <16 dB km−1 bandwidth spanning across the O, E, S, C, L telecom bands (1302–1637 nm). The debut of this conjoined-tube HCF, with combined merits of ultralow loss, broad bandwidth, low bending loss, high mode quality and simple structure heralds a new opportunity to fully unleash the potential of HCF in telecommunication applications.

Network Equipment and Their Procurement Strategy for High Capacity Elastic Optical Networks

In elastic optical networks, the success of providing high network capacity depends on the optical signal-to-noise ratio (OSNR) values of network lightpaths. As each lightpath's OSNR value defines the modulation format and capacity it can support, having high OSNR light-paths is always beneficial. Hence, with a given set of modulation formats, service providers need to optimize their optical infrastructure, including in-line amplifiers and reconfigurable optical add-drop multiplexers (ROADMs), given the size and topology of their core networks. This also will have a direct impact on vendors who need strong insight into the requirements of service providers and their networks in terms of equipment and new technology. Therefore, in this paper a comprehensive model based on the local optimization which leads to a global network optimization (LOGON) strategy of the Gaussian noise (GN) model has been proposed, which helps in estimating the lightpath OSNR and clearly quantifies the noise contributions from in-line amplifiers and post-amplification at the ROADMs. The model introduces closed-form expressions to calculate nonlinear impairment (NLI) contributions for various span lengths while using either erbium-doped fiber amplifiers (EDFAs) or H-Raman amplifiers, which helps in optimizing the signal launch power to achieve maximum link OSNR In addition to this, an offline strategy has been proposed that can help service providers to optimize their procurement of network equipment upfront and give insight into how much of the capacity bottleneck is alleviated in their networks if they do this. To demonstrate all of the above, the UK, Pan-European, and US Core networks have been considered, which illustrate differences in link lengths and reduced node density. It is seen that improving the OSNR conditions at the ROADM increases the network capacity when noise from in-line amplifiers is significantly reduced. Among the three networks, we found that the UK network responded the most to improved OSNR conditions at the ROADM nodes due to small link lengths and less line noise. Among the amplifiers, we found that improving ROADMs while having H-Raman in the links resulted in a maximum capacity increase. For the UK network at FG = 12.5 GHz, the capacity increases by 6650 Gbps, while for the larger Pan-European and US networks, the capacity increase reduces to 4550 and 1600 Gbps due to increased link lengths and line noise. Further, following the offline strategy, we are able to accommodate 1737, 1481, and 615 100G demands using H-Raman for the UK, Pan-EU, and US networks at FG = 12.5 GHz until 10% blocking is reached. Thereby, H-Raman provides 7.5%, 35.8%, and 94.9% extra capacity, respectively, for the UK, Pan-EU, and US. Finally, using H-Raman, all lightpaths in the UK network operate at PM-64QAM with maximum capacity at the end of the procedure.

Impact of Intercore Crosstalk on the Transmission Distance of QAM Formats in Multicore Fibers

We investigate the impact of intercore crosstalk on the achievable transmission distance of three square quadrature amplitude modulation (QAM) formats. We show that increasing intercore crosstalk across an 18 dB range starting from −43.4 dB/100 km reduces the achievable transmission distance for all formats, with a greater impact on higher order modulation formats. For a crosstalk level arising from equal signal launch power in each core of a homogeneous 7-core fiber, we measure a reduced transmission distance at $\text{BER}=1.5\times 10^{-2}$ of 24%, 38% and 54%, for polarization-division-multiplexed-quadrature phase-shift keying (PDM-QPSK), PDM-16QAM, and PDM-64QAM, respectively. Finally, we investigate the potential impact that dynamic crosstalk variation could have in transmission systems based on multicore fiber and estimate the achievable reach for an outage probability of $1\times 10^{-5}$ in the presence of dynamically varying intercore crosstalk.

Application of a digital non-linear compensation algorithm for evaluating the performance of root-raised-cosine pulses in 112 Gbit s−1 DP-QPSK transmission

In this paper, we numerically investigate the non-linear tolerance of root-raised-cosine (RRC) pulse shaping by interpolating finite impulse response (FIR) filters in conjunction with digital backward propagation (DBP) in coherent 112 Gbit s−1 dual-polarization quadrature phase shift keying (DP-QPSK) transmission. The results depict that RRC pulses are more tolerant to intra-channel non-linearities, i.e. self-phase modulation (SPM), as compared to standard RZ-33 and NRZ pulses. The non-linear threshold point is improved by using RRC pulses by a factor of 2 dB signal input power as compared to RZ pulses and by 4 dB signal launch power as compared to NRZ pulses. The behavior of RRC pulses is also investigated with standard single mode fiber (SMF), non-zero dispersion shifted fiber (NZDSF) and next-generation large Aeff pure silica core fiber (LA-PSCF). Most importantly multi-span DBP is implemented and in the case of RRC pulses the computational efforts of the conventional DBP algorithm are reduced by 80% with a diminutive Q-penalty of 0.74 dB. The duty cycle of the RRC pulses is further optimized for efficient system performance. We have also compared the performance of single-channel transmission with the multi-channel transmission, where the performance is limited due to inter-channel non-linear effects. Furthermore, the non-linear tolerance of RRC pulses is investigated with; (a) different amplifier spacing and (b) variation in transmission link design information for the DBP algorithm.

Transmission Performance of a 485-Gb/s CO-OFDM Superchannel With PDM-16QAM Subcarriers Over ULAF and SSMF-Based Links

We experimentally compare the transmission performance of a 485-Gb/s coherent optical orthogonal frequency-division-multiplexed (CO-OFDM) superchannel with PDM-16QAM subcarrier modulation over two types of fiber links, one based on ultra-large-area fiber (ULAF) spans, and the other on standard single-mode-fiber (SSMF) spans. Two amplification schemes, hybrid Raman/erbium-doped fiber amplifier (EDFA) and EDFA-only, are studied. We find that the optimum signal launch power is about 2 dB higher in ULAF than in SSMF for transmitting over 20 80-km spans under both amplification schemes. With EDFA-only amplification, the ULAF offers ~2-dB higher optimum Q2 factor than the SSMF after 1600-km transmission. With hybrid Raman/EDFA amplification, a transmission distance of 1600 km is achieved at Q2 = 9.2 dB by using the ULAF spans, which is 67% longer than that obtained with the SSMF spans.

Optimized digital backward propagation for phase modulated signals in mixed-optical fiber transmission link

The parametric optimization of Digital Backward Propagation (DBP) algorithm for mitigating fiber transmission impairments is proposed and numerically demonstrated for phase modulated signals in mixed-optical fiber transmission link. The optimization of parameters i.e. dispersion (D) and non-linear coefficient (γ) offer improved eye-opening (EO). We investigate the optimization of iterative and non-iterative symmetric split-step Fourier method (S-SSFM) for solving the inverse non-linear Schrödinger equation (NLSE). Optimized DBP algorithm, with step-size equal to fiber module length i.e. one calculation step per fiber span for obtaining higher computational efficiency, is implemented at the receiver as a digital signal processing (DSP) module. The system performance is evaluated by EO-improvement for diverse in-line compensation schemes. Using computationally efficient non-iterative symmetric split-step Fourier method (NIS-SSFM) upto 3.6 dB referenced EO-improvement can be obtained at 6 dBm signal launch power by optimizing and modifying DBP algorithm parameters, based on the characterization of the individual fiber types, in mixed-optical fiber transmission link.

Enhanced stimulated Brillouin scattering threshold through phase control of multitone phase modulation

We show that controlling the phase difference among the various tones of a multitone phase modulator can substantially increase the stimulated Brillouin scattering (SBS) threshold during optical fiber transmission. We present detailed modeling and experimental validation of the effect. This result should be especially useful for optical networks requiring high signal launch power.

Gain transients in copumped and counterpumped Raman amplifiers

Gain transients in both copumped and counterpumped distributed Raman amplifiers were investigated as a function of signal launch power, fiber type, and surviving signal wavelength for the first time. We found that gain transients are much more pronounced in a copumping scenario than in a counterpumping scenario for the same operating conditions. We also found that the dependency of gain transients on signal launch power, fiber type, and surviving channel wavelength follows the pump-depletion theory. A simple transient suppression technique with better than 0.2-dB ripple capability was also demonstrated.