Fiber Launch Power Research Articles

Deep intelligent spectral labelling and receiver signal distribution for optical links.

A unique automatic receiver signal distribution strategy is proposed for private optical networks based on the concept of non-orthogonality. A non-orthogonal signal waveform can compress the spectral bandwidth, which not only fits a signal in a bandwidth limited scenario, but also enables the compression ratio information for labelling. Depending on a unique value of spectral compression, an end user destination can be correlated. A network edge node will rely on deep learning to intelligently identify each raw signal and forward it to corresponding end users with no sophisticated digital signal pre-processing. In this case, signal identification and distribution are faster while computationally intensive signal compensation and detection will be shifted to each end user since the receiver is highly dynamic and user-defined in private optical networks. An intelligent signal classifier will be trained considering various fiber transmission factors such as transmission distance, training dataset size and launch power. At the end, a universal classifier is obtained, which can be used to identify signals in a system for any fiber transmission distance and launch power.

Design and investigation of N1-class next-generation passive optical network-2 (NG-PON2) coexistence architecture in the presence of Kerr effect and four-wave mixing (FWM) for fiber to the home (FTTX) access networks

Abstract In this paper, we investigated N1-class Time and Wavelength Division Multiplexing (TWDM) Next-Generation Passive Optical Network-2 (NG-PON2) supporting symmetric and coexistence configuration in a worst-case scenario. The network is optimized in the presence of linear and nonlinear impairments such as chromatic dispersion (CD), self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). On-off key (OOK) modulation technique incorporating hybrid optical distribution network (ODN) i.e., wavelength select (WS) and wavelength-routed (WR) ODN is used to connect downstream (D/S) and upstream (U/S) channels supporting symmetric 80 Gbps data rate operating at 2.5/2.5 and 10/10 Gbps, respectively. G.652 single-mode (SM) fiber channel is modeled for fiber launch power PTODN = 3 dBm and 4 dBm for D/S and U/S channels to implement N1-class network. The D/S and U/S receiver sensitivity (Rxs) demonstrated are −44.23/−37.46 dBm and −28.46/−35.32 dBm and inter-channel crosstalk (Cc) is −31.97/−31.39 dB accommodating 640 splitter configuration at 50 km in the presence of impairments at 10E-7 bit error rate (BER) and Quality (Q)-factor of 5 for 10 Gbps channel.

Dither-Frequency Tuning Technique for RSOA-Based Coherent WDM PON

Increasing the transmission distance is an attractive technological option for lowering the cost of loopback-configured coherent wavelength-division-multiplexed passive optical networks implemented by using reflective semiconductor optical amplifiers (RSOAs). In this regard, it is necessary to increase the fiber launch power of the seed light provided from the central office. However, the stimulated Brillouin scattering (SBS) limits this power to less than 6 dBm. It is well-known that the frequency-dithering technique increases the SBS threshold effectively, allowing us to launch a higher-power continuous-wave light. However, the linewidth of the seed laser (caused by the frequency dither) makes the carrier phase estimation (CPE) very difficult at the coherent receiver. To solve this problem, we propose a dither-frequency tuning technique in this letter. We set the dither frequency to be the integer times the inverse of the round-trip propagation delay. Then, we can make the frequency dither of the upstream signal unseen at the receiver since it becomes in phase with the frequency dither of the local oscillator. Thus, the CPE performance would not be limited by the frequency dither, but by the natural linewidth of the seed laser. The efficacy of the proposed technique is experimentally demonstrated over a 60-km loopback link using a 28-Gb/s quadrature phase-shift keying signal generated from RSOA.

Optical QPSK Signal Quality Degradation due to Phase Error of Pump Light in Optical Parametric Phase-Sensitive Amplifier Repeaters

The influence of pump phase error on phase-sensitive optical amplifier (PSA) repeaters and the waveform degradation due to chromatic dispersion and fiber nonlinearities in the optical multi-relay transmission of quadrature phase-shift keying phase-conjugated twin waves are considered theoretically. First, the influence of noise from the pump phase error, optical local oscillator, receiver, and the amplified spontaneous-emission (ASE) in PSA repeaters is investigated with the assumption that transmission fibers are linear lossy channels. The bit-error rate (BER) is estimated as a function of the signal-to-noise ratio, and the relationship between the number of transmission relays and the fiber launch power is clarified. Waveform degradation due to chromatic dispersion and the optical fiber nonlinearities in transmission fibers are investigated with the noiseless condition, and the maximum repeatable number as a function of the fiber launch power is calculated. Finally, we show the relationship among the maximum repeatable number, standard deviation of pump phase error in PSA repeaters, and the fiber launch power to clarify the optimum transmission condition with consideration of the noise and the waveform degradation.

Nonlinear Signal-to-Noise Ratio Estimation in Coherent Optical Fiber Transmission Systems Using Artificial Neural Networks

For high symbol rate fiber optic networks, the estimation and monitoring of time varying link performance parameters are critical for delivering optimal network performance. In this paper, a method is presented for estimating the nonlinear signal-to-noise ratio ${\text{(SNR}}_{{\text{nl}}})$ using an artificial neural network (ANN). The ANN is trained with the fiber nonlinearity induced amplitude noise covariance and phase noise correlation extracted from received symbols. The data used for training are simulation results for a 34.5 Gbaud dual polarization 16-ary quadrature amplitude modulation signal transmitted over a wide range of link configurations with varying fiber types and number of wavelength division multiplexed channels. Using 734 input-output sets, high accuracy is demonstrated for training, testing, and validation for simulation data with a maximum normalized root-mean-square error of 0.37% for ${\text{SNR}}_{{\text{nl}}}$ . Validation using experimental data exhibits less than 0.25 dB deviation from the true ${\text{SNR}}_{{\text{nl}}}$ for estimates obtained with varying fiber length and launch power.

Constant envelope OFDM scheme for 6PolSK-QPSK

A constant envelope OFDM scheme with phase modulator (PM-CE-OFDM) for 6PolSK-QPSK modulation was demonstrated. Performance under large fiber launch power is measured to check its advantages in counteracting fiber nonlinear impairments. In our simulation, PM-CE-OFDM, RF-assisted constant envelope OFDM (RF-CE-OFDM) and conventional OFDM (Con-OFDM) are transmitted through 80 km standard single mode fiber (SSMF) single channel and WDM system. Simulation results confirm that PM-CE-OFDM has best performance in resisting fiber nonlinearity. In addition, benefiting from the simple system structure, the complexity and cost of PM-CE-OFDM system could be reduced effectively.

A PAPR reduction scheme based on a new spreading code in optical direct detection OFDM system

In this paper, we propose and experimentally demonstrate a peak-to-average power ratio (PAPR) reduction scheme based on a new spreading code in direct detection optical orthogonal frequency division multiplexing (OFDM) system. The new spreading code with low cross correlation and high auto-correlation can support $$2N+1$$2N+1 users. Thus, $$2N+1$$2N+1 users or data symbols can be transmitted over only N subcarriers. The experimental results show that, after transmission over 70 km single-mode fiber, at the bit error rate of $$10^{-3}$$10-3, with fiber launch power of 2.75 dBm, the receiver sensitivity can be improved 2.1 dB by using the proposed scheme based on new spreading code. The PAPR can be reduced about 4.6 dB, compared with the original OFDM signal at a complementary cumulative distribution function of $$10^{-4}$$10-4.

PAPR reduction technique by inserting a power-concentrated subcarrier for CO-OFDM

Optical orthogonal frequency division multiplexing (OFDM) signals suffer from the nonlinear effects of an electric-to-optical modulator and fiber due to high peak to average power ratio (PAPR) characteristics. We propose a power-concentrated subcarrier method for use in OFDM systems to reduce PAPR of the OFDM signal without increasing system complexity or side information. The PAPR can be reduced by simply inserting a power-concentrated subcarrier (PCS) to replace the zero in the end of the signal spectrum at the zero padding stage. The simulation is performed with a conventional CO-OFDM transmission system with a PCS under dispersion and a nonlinear effective fiber to estimate PAPR using complementary cumulative distribution function (CCDF), error vector magnitude (EVM), and bit-error-rate (BER) characteristics. After a 2000-km transmission, the conventional CO-OFDM had an optimal fiber launch power of −6dBm from the BER characteristics. In contrast, the proposed CO-OFDM with a PCS improves transmission performance in terms of log(BER) as much as −0.67 by an increased fiber launch power of −1dBm with 60% concentrated-power of the PCS.

All-Optical Simultaneous Multilevel Amplitude and Phase Regeneration

Simultaneous amplitude and phase noise reduction of multiple signal states using a nonlinear amplifying loop mirror with integrated directional phase-sensitive amplifier are presented for the star-eight quadrature amplitude modulation transmission format as an example. The performance of this combined regenerator scheme is compared with that of a cascade of separate phase and amplitude regenerators. It could be shown that an improvement in the error vector magnitude of 4 dB for the high-power states with simultaneous improvement of 5 dB for the low-power states is possible in both cases. Transmission improvement by regeneration is considered for two noise types: 1) amplified spontaneous emission and 2) nonlinear phase noise. Both schemes can either improve the bit error rate by an order of magnitude or enable an increase of the fiber launch power by 3 dB.

Constant Envelop Minimum-Shift Keying OFDM Coherent Optical Communication System

In this paper, we propose a constant envelop minimum-shift keying orthogonal frequency division multiplexing (CE-MSK-OFDM) modulation scheme. This scheme can mitigate the inherent drawbacks of CE quadrature phase shift keying OFDM (CE-QPSK-OFDM) and MSK-OFDM. Advantages of the proposed CE-MSK-OFDM modulation scheme are theoretically investigated. A CE-MSK-OFDM coherent optical experimental system is presented. In our experiment, 2.5 Gb/s CE-MSK-OFDM signal and 2.5 Gb/s CE-QPSK-OFDM signal are transmitted over a 100 km standard single mode fiber (SSMF) without phase compensation to check its advantages in mitigating intercarrier interference (ICI). In addition, performance under large fiber launch power is also measured to check its advantages in counteracting fiber nonlinear impairments. The experimental results show that the proposed system performs well in decreasing ICI and peak-to-average power ratio (PAPR) as the theoretical analysis reveals.

DFT-Precoded Coherent Optical OFDM With Hermitian Symmetry for Fiber Nonlinearity Mitigation

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems have inferior nonlinear performance due to their high peak to average power ratio (PAPR) characteristics. In order to mitigate fiber nonlinearities for the CO-OFDM systems, we propose a novel discrete Fourier transform (DFT)-precoded coherent optical OFDM combined with a Hermitian symmetry (DFT-precoded HS CO-OFDM) system, which is a combination of the DFT-spread OFDM and the Hermitian symmetry of the DFT. Furthermore, the proposed system can be easily implemented by using a fast Fourier transform (FFT) algorithm. Because the proposed system has a lower PAPR than the conventional CO-OFDM system, the DFT-precoded HS CO-OFDM system provides superior nonlinear tolerance. Moreover, the proposed system is superior to the conventional DFT-spread coherent optical OFDM (DFT-spread CO-OFDM) system in terms of maintaining low PAPR in optical fiber transmission. The low PAPR in the proposed scheme is suitable for long-haul optical transmission systems, compared with the conventional DFT-spread CO-OFDM. For the DFT-precoded HS OFDM system, we have developed a theoretical framework illustrating the principles of the proposed system. The numerical results show that the DFT-precoded HS CO-OFDM system outperforms the conventional CO-OFDM by 5.4% EVM performance after 1200-km transmission and the conventional DFT-spread CO-OFDM by 4.5% EVM performance after 2800-km transmission at a fiber launch power of ${-}2$ dBm.

Compensation of Nonlinear Phase Noise Using the Effective Negative Nonlinearity of a Nonlinear Amplifying Loop Mirror

The nonlinear amplifying loop mirror (NALM) has been explored for use as a nonlinear phase-shift compensator (NPSC). Operation conditions for a tunable effective negative nonlinearity are considered and the NALM parameter optimization is discussed for direct bit-error-ratio (BER) improvement by postcompensation after a nonlinear transmission line. In this configuration, the fundamental limits for NPSC are estimated for differential quadrature phase-shift keying (DQPSK) using a simplified model. Numerical simulations of a 20 Gb/s RZ-DQPSK transmission system confirmed the applicability of this model and showed a significant BER improvement in a realistic transmission line. Alternatively, the fiber launch power per span could be increased by 2 dB for the same BER.

Experimental research of triangular optical pulse generation in normal dispersive fiber

Generation of triangular optical pulses in conventional normal dispersive (ND) fiber is experimentally realized using a passive nonlinear pulse shaping method based on a combination of pulse pre-chirping, group-velocity dispersion and self-phase modulation in a section of ND fiber. It is found that by optimizing the parameters of launching pulse power level and ND fiber length, high-quality triangular optical pulses with nearly constant gradients of leading and trailing edges and linear frequency chirp profiles can be obtained for a wide range of different pulse pre-chirping values. For different pulse pre-chirpings, high launch power is required for triangular optical pulse generation. The tolerance of triangular pulse formation to ND fiber length and launch power level is improved with the increase of pulse pre-chirping, which means that it is easy to obtain triangular pulses for higher pulse pre-chirping.

Multilevel Phase-Preserving Amplitude Regeneration Using a Single Nonlinear Amplifying Loop Mirror

A possibility of multilevel phase-preserving amplitude regeneration using a nonlinear amplifying loop mirror (NALM) is presented for the optical star-8 quadrature amplitude modulation (QAM) transmission format as an example. Two significantly different state power ratios for the QAM signal, 1:3 and 1:7, were investigated. After the optimization of the coupler splitting ratio and the directional phase bias in the NALM, amplitude noise can be efficiently suppressed at both signal power levels simultaneously. Bit-error-ratio (BER) simulations have shown that in a system limited by nonlinear phase noise, the deployment of the NALM allows an increase of the fiber launch power by 1.9 and 2.2 dB at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for a state power ratio of 1:3 and 1:7, respectively. The regeneration limits due to imperfections of the power transfer characteristic are also discussed.

Optical Coherent Receiver With a Switchable Electrical Dispersion Compensator for 10 Gb/s DPSK Transmission up to 300 km of SSMF in Metro Optical Networks

The purpose of this paper is to demonstrate the potential of an electrical signal processing technique to mitigate the effect of intersymbol interference in metro optical networks, using optical coherent detection. Intersymbol interference in fiber-optic systems can severely degrade the system performance and subsequently limit both the maximum transmission distance and data rate. The main sources of intersymbol interference include chromatic dispersion (CD) and polarization mode dispersion (PMD). The typical reach for 10 Gb/s metro optical systems is on the order of 300 km. To bridge this distance either with or without repeaters, we demonstrate the potential use of 10 Gb/s differential phase-shift keying (DPSK) modulation format in conjunction with an optical coherent receiver with a built-in switchable electrical dispersion compensator. The overall performance of the optical coherent receiver is compared with that of the corresponding direct detection (DD) receiver based on Mach-Zehnder delay interferometer. The optical signal-to-noise (OSNR) required to operate the optical coherent receiver at the enhanced forward error correction (EFEC) threshold of 2.1 X 10-3 is 6 dB/0.1 nm resolution bandwidth. The optical coherent receiver outperforms in absolute required OSNR by 0.5 dB at the EFEC threshold when compared to a DD receiver. A four position switchable CD compensator compensates up to ?5100 ps/nm of CD, using the method of heterodyne detection. The optical coherent receiver has a first-order PMD tolerance of 36.2 ps for 1 dB OSNR penalty. The utility of our optical coherent receiver is demonstrated by doing transmission experiments over 304 km of standard single-mode fiber (SSMF) in both repeatered (multiple spans) and repeaterless (single span) to measure the self-phase modulation (SPM) tolerance of 10 Gb/s nonreturn-to-zero (NRZ)-DPSK. The measured SPM tolerance of NRZ-DPSK over 304 km of SSMF in three spans of ~100 km each is +8.8 dBm fiber launch power for 1.5 dB OSNR penalty. In addition, the best Q factor of ~12 dB is obtained for the fiber launch power of +18 dBm over single repeaterless span of 304 km of SSMF, which gives ~3 dB of system margin at the second generation EFEC threshold of 9.1 dB.

Nonlinear Phase Noise Reduction in a DPSK Transmission System Using Cascaded Nonlinear Amplifying Loop Mirrors

The performance of a nonlinear amplifying loop mirror as a phase-preserving amplitude 2R regenerator in a differential phase-shift-keying transmission system with nonlinear phase noise as dominant limiting effect has been investigated in a recirculating fiber-loop setup. The experimental results show that cascaded regenerators can efficiently prevent the accumulation of nonlinear phase noise in such systems. It was possible to significantly increase the transmission quality; alternatively, a considerable increase of fiber launch power could be achieved for the same bit-error ratio. As a limiting effect, the amplified Rayleigh backscattering in the highly nonlinear fiber is identified when the regenerator is passed multiple times.

Phase-Preserving Amplitude Regeneration in DPSK Transmission Systems Using a Nonlinear Amplifying Loop Mirror

A phase-preserving 2R regenerator based on a nonlinear amplifying loop mirror was implemented in a RZ-DPSK transmission system. Its performance has been investigated in numerical simulations and experimentally. The results show that amplitude regeneration using a NALM can efficiently prevent accumulation of nonlinear phase noise in a 10 Gb/s DPSK transmission system. In the experiments, significant improvements of eye opening and of BER as well as a 3 dB increase in fiber launch power have been demonstrated. Simulations at 10 Gb/s and 100 Gb/s indicated that the enhancement of the transmission quality is smaller at 100 Gb/s. The reason is that at 100 Gb/s nonlinear intra-channel effects rather than pure nonlinear phase noise are the main limiting factor and the NALM can only reduce the accumulation of amplitude noise in that case.

Coherent Optical OFDM Transmission Up to 1 Tb/s per Channel

Coherent optical frequency-division multiplexing (CO-OFDM) is one of the promising pathways toward future ultrahigh capacity transparent optical networks. In this paper, numerical simulation is carried out to investigate the feasibility of 1 Tb/s per channel CO-OFDM transmission. We find that, for 1 Tb/s CO-OFDM signal, the performance difference between single channel and wavelength division multiplexing (WDM) transmission is small. The maximum Q is 13.8 and 13.2 dB respectively for single channel and WDM transmission. We also investigate the CO-OFDM performance on the upgrade of 10-Gb/s to 100-Gb/s based DWDM systems with 50-GHz channel spacing to 100-Gb/s systems. It is shown that due to the high spectral efficiency and resilience to dispersion, for 100-Gb/s CO-OFDM signals, only 1.3 dB Q penalty is observed for 10 GHz laser frequency detuning. A comparison of CO-OFDM system performance under different data rate of 10.7 Gb/s, 42.8 Gb/s, 107 Gb/s and 1.07 Tb/s with and without the impact of dispersion compensation fiber is also presented. We find that the optimum fiber launch power increases almost linearly with the increase of data rate. 7 dB optimum launch power difference is observed between 107 Gb/s and 1.07 Tb/s CO-OFDM systems.

Externally Modulated Optical Minimum Shift Keying Format

This paper presents the generation and the performance evaluation of optical minimum shift keying (MSK) for high-speed and high-spectral-efficiency wavelength-division multiplexing systems. The detailed principle on optical MSK generation and detection is derived and analyzed. Optical MSK data (10.7 Gb/s) are successfully generated by an experiment using the proposed scheme. The results show that the optical MSK signal exhibits a very compact optical spectrum. The performance of the optical MSK modulated system is evaluated and compared with those of 50% duty-cycle return-to-zero differential phase shift keying (RZ-DPSK) and 50% duty-cycle return-to-zero on-off keying (RZ-OOK) modulated systems, via both simulation and experiment, in terms of dispersion tolerance, linear crosstalk, self-phase modulation (SPM) tolerance, stimulated Brillouin scattering (SBS) threshold, optimal receiver filter bandwidth, and optimal link launch power into single-mode fibers and dispersion compensation fibers. Experimental and simulation results show that the MSK signal exhibits the largest tolerance against fiber dispersion and SPM effect as compared with RZ-DPSK and RZ-OOK. Experimental results also confirm the negative penalty against the SPM effect. The SBS threshold of both the MSK and RZ-DPSK systems is more than 10 dB higher than that of the RZ-OOK system. The MSK that is generated with conventional carrier-suppressed return-to-zero (CSRZ) pulses exhibits a narrower optimal optical filter bandwidth but a wider electrical filter bandwidth compared with those of RZ-OOK and RZ-DPSK. The MSK signal that is generated with pure dual-mode pulses has a similar optimum range to RZ-OOK and RZ-DPSK at a high optical signal-to-noise ratio (OSNR) but has a narrower optimal optical filter bandwidth at a low OSNR. The optimum fiber launch power study shows that the MSK signal that is generated using the conventional CSRZ pulses exhibits higher optimum launch power, which agrees with the SPM tolerance study.

High-power supercontinuum generation in highly nonlinear, dispersion-shifted fibers by use of a continuous-wave Raman fiber laser.

High-power supercontinua are demonstrated in highly nonlinear, dispersion-shifted fibers with a continuous-wave Raman fiber laser. Supercontinuum growth is experimentally studied under different combinations of fiber length and launch power to show output powers as high as 3.2 W and bandwidths greater than 544 nm. Modulation instability (MI) is observed to seed spectral broadening at low launch powers, and the interplay between MI and stimulated Raman scattering plays an important role in the growth of the continuum at high launch powers. The effect on continuum generation of parametric four-wave mixing coupled with the higher-order dispersion properties of the fiber is investigated.