Performance limit of a multi-frequency probe based coherent optical time domain reflectometry caused by nonlinear effects

In order to meet the length calibration requirements of coherent optical time domain reflectometer(COTDR) in state monitoring of long-distance communication, a new coherent optical time domain reflectometer length calibration method was proposed. The method utilized the principle that light circulated continuously in the fiber ring. And based on the principle, the COTDR length calibration device was developed. The device used the pulse frequency and pulse width emitted by the function generator to control the breaking time of the optical switch. Thereby it could realize the control of the propagation time of the light in the fiber loop. If a fiber amplifier was added to the fiber loop, simulation of fiber links of different lengths could be realized. Finally, the length calibration of the coherent optical time domain reflectometer was achieved. The experimental results showed that the length of the fiber used in the device is 150km, and the optical pulse could be rotated up to 7 times in the fiber loop. Therefore, the maximum range of the coherent optical time domain reflectometer length calibration was 1050km. Its extended uncertainty was 7.0m, and the containing factor was 2. This method could not only promote the development of coherent optical time domain reflectometry technology, but also provided a good idea for COTDR length calibration.

The inter-channel crosstalk (XT) in frequency division multiplexing coherent optical time domain reflectometry (FDM-OTDR) is described theoretically. The inter-channel XT degrades the spatial resolution of the OTDR trace at the position, where the reflectivity changes precipitously, and is dependent on the frequency channel spacing, pulse waveform, and weighting function of the signal processing. We show that the deterioration of the OTDR trace can be fully simulated by using impulse response analysis, and that apodization of the probe pulse shape and weighting function effectively suppress the inter-channel XT. Moreover, we propose a novel nonlinear self-phase modulation (SPM) suppression scheme to adapt apodized probe pulse in our experiment. The experimental results for the impulse response, OTDR trace, and SPM suppression are discussed.

A bidirectional full duplex radio over fiber (RoF) system is proposed for multi-band signal generation and transmission. A multi-band signal is generated at the central station (CS), which includes 10 GHz baseband, 40, and 70 GHz millimeter-wave (mm-wave) by optical frequency multiplication technique of radio frequency (RF) local oscillator (LO) with frequency of 15 GHz. The simultaneous transmission of the 16-QAM OFDM-based multi-band downlink signal and 4-QAM uplink signal over a single fiber is demonstrated. The carrier for uplink transmission is extracted from the unmodulated downlink signal with tunable FBG which is modulated by OFDM-based 4-QAM uplink data obtained from the user end via an antenna. The frequency reuse from the downlink signal makes the base station (BS) free from the laser source. At the user end, the multi-band signals for multi-serve application are obtained by heterodyne beating of multiple bands. An increase in launch power induces fiber dispersion and non-linearities like self-phase modulation (SPM) and cross phase modulation (XPM), which depends on the nonlinear refractive index of the fiber. Different industrial standard fibers with common specifications and modulation format along with co-shared components are deployed, and system performance is analyzed. The stimulated Brillouin scattering (SBS) can be mitigated by optimizing the launch power within the SBS threshold. The nonlinear effects are minimized at the system level without the requirement of additional digital signal processor (DSP) . The theoretical and simulation analysis shows that the designed bidirectional RoF link exhibits an improved system performance for bidirectional transmission with negligible power penalty.

Recent progress in optical-fiber amplifiers has proportionately increased the optical power traveling through the optical fibers, and high-power transmission has induced nonlinear problems that result in the signal degradation. Optical-fiber amplifiers have also provided technology for realizing multigigabit, multithousand-kilometer transmission, but a problem of accumulated nonlinear effects, which has not been observed in past transmission systems, has appeared. The nonlinear effects observed in practical optical fibers are stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and phenomena induced by the optical-power-dependent refractive index, such as self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). This paper reviews measurement techniques, especially for SBS and nonlinear refractive index, and describes countermeasures against these fiber nonlinearities from the standpoint of the transmission fibers.

Signal fading severely affects the performance of optical fiber distributed acoustic sensing (DAS) systems based on coherent optical time domain reflectometry (COTDR), especially in terms of differentiated responses between channels. To eliminate fading, a polarization diversity COTDR system is established, which generates two pulse feedbacks with different fading components for P and S polarization. The rayleigh backscattered (RBS) signals from the two separate probing pulses exhibit different fading positions under the influence of external interference but have the same phase change rate. Through an improved rotational vector summation (IRVS) algorithm, the complex RBS signals of the two polarization states are effectively synthesized, simultaneously suppressing fading phenomena caused by orthogonal polarization. Experimental results demonstrate that the demodulated signal-to-noise ratio (SNR) can reach 65 dB@200 Hz. Moreover, by using 20% of the mean amplitude as a threshold, the probability of fade events decreases from 4.9% to 0.12%. The proposed fade mitigation distributed sensing system exhibits excellent performance and plays a crucial role in practical DAS systems.

We propose and experimentally demonstrate a novel method based on coherent optical time domain reflectometry for locating excess mode-dependent loss in coupled multi-core fibers for the first time.

Fiber optic distributed acoustic sensing (DAS) is a potential method to achieve partial discharge (PD) detection and fast localization in gas insulated metal enclosed transmission lines (GIL). In order to detect and locate the weak partial discharge signals in the GIL as early as possible, this paper proposes an integrated distributed fiber optic sensor installation method for the GIL, as well as a distributed partial discharge acoustic detection and localization system. Based on the fully insulating nature of the optical fiber, the optical fiber is installed inside the GIL to improve the sensitivity to partial discharge acoustic signals, and the length of the sensor unit is increased by repeated winding to further improve the detection performance of the acoustic signals. A true-to-life GIL partial discharge test platform was built to investigate the multi-point distributed sensing performance of the proposed coherent optical time domain reflectometer system for metal tip discharge. The test results show that the sensing distance of the sensing system can reach 120m, which can achieve the multipoint detection and localization of the partial discharge acoustic signal of 29.7pC. The proposed sensing method and system provide a new solution for GIL weak partial discharge optical fiber distributed acoustic signal detection.

This paper describes a coherent optical time domain reflectometer (OTDR) with a high dynamic range for locating faults in optical cable systems incorporating repeaters based on erbium-doped fiber amplifiers (EDFA). We also present experimental results achieved with transmission lines containing several fiber spans and EDFAs. >

In the recent years, there has been an unpredicted increase in data transmission. To cope with this increase, a reliable technology is required. The next-generation passive optical network 2 (NG-PON2) was used in integration with future technology that works on radio over fiber (RoF). However, the optical communication system is subject to the following nonlinear effects: Self-phased modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), stimulated Brillouin scattering (SBS), and stimulated Raman scattering (SRS). These effects are dominant, and they risk the performance of the optical fiber system. Therefore, digital signal processing (DSP) technique is used in this study to minimize and offset the nonlinear effects. In this study, three digital filters working on (DSP) are compared for the same data transmission rate at different distances from the transmitter without using compensation techniques. The results were compared by bit error rate (BER) and the error vector magnitude (EVM). Thus, the error vector magnitude (EVM) was noticed to be ~19.58% without using DSP. Nonlinear effects were mitigated by filters of Gaussian, Bessel and DSP-64QAM raised cosine to ~9.09%, ~14.22% and ~17.54%, respectively. Hence, the bit error rate (BER) of the best technique after comparison was minimized to DSP-64QAM raised cosine. It occurred after minimizing the nonlinear effects as 1.11E−14 at a distance of 40 km and 100 km. The BER value was 3.43E−10, which is within the acceptable values of performance for the proposed optical communication system with nonlinear effects. The system was simulated using the optical simulator optisystem 15.

In this paper we report on the performance of a modular single mode pulsed fiber laser system operating in the C-band. With off-the-shelf telecom components and specialty-designed electronics, 3 kW peak power can be generated in a short (1 ns) pulse at 10 kHz at 1545 nm; however, the onset of nonlinear optical effects (SRS, FWM, and SPM) is observed at a 1kW peak power level. Using highly doped erbium fibers, peak powers up to 13kW and pulse energies of up to 20μJ have been generated with a pulse duration range of 0.6-5 nsec, repetition rates between 3kHz to 1 MHz, and at a wavelength of 1545.3nm and 1567.5 nm before the onset of nonlinear effects became noticeable. Therefore, with the use of highly doped erbium fiber, the onset of nonlinear effects can be increased by an order of magnitude. For narrowband amplification, stimulated Brillouin scattering (SBS) is the limiting nonlinear process. In this regime we recorded the onset of SBS at 8μJ/pulse with a duration of 2.5 nsec. Depending on the pulse shape and pulse duration, self phase modulation (SPM) was also observed, which spectrally broadens the output centered at the signal wavelength; however, the spectral broadening due to SPM is only minor compared to SRS and FWM. It was also demonstrated that pulse steepening is minimized with an appropriate seed waveform. A 3 ns, shaped, input pulse nearly maintained its pulse duration after amplification. Without pulse shaping, the pulse shortened to 1.1 ns.

We presented a concept of converged coexistence (CC) access network and generate, and test model is demonstrated for optimization of channel launch power PTODN in the distribution network which subsequently confirm power budget class of NG-PON2 in this paper. Multiplexing of wavelength and time (TWDM) technology is incorporated for multichannel NG-PON2 configurations for 5G, internet-of-things (IoT) applications, and fiber-to-the-x (FTTX) services. CC NG-PON2 symmetric configuration supporting 2.5 and 10 Gbps channels are analyzed and compared in the sight of channel nonlinear impairments includes Kerr-effect (γ) spanning from self phase modulation (SPM) and cross-phase modulation (CPM), chromatic dispersion (CD), and four wave mixing (FWM) in worst-case scenario. Intensity modulation (IM) technique incorporated before multiplexing the D/S channels by coexisting element (CEx) and coupled to wavelength routed (WR) optical distribution network (ODN). U/S channels are routed using wavelength select (WS) ODN. ODN is implemented using G.652 non zero dispersion shift (NZ-DSF) single-mode (SM) fiber. Multi-channel spectrum envelope propagating in either direction is optimized to PTODN = 3/5/7/9 dBm and 4 dBm, respectively. The theoretical modeling and simulative results confirms the proposed network configuration supports incremental receiver sensitivity (Rxs) as −42.19/−42/−39.75/−37.97 dBm and −35.45/−35.38/−33.45/0.38/−42 dBm for λ1 2.5 Gbps and λ8 10 Gbps TWDM and PtP WDM channels, respectively, for all-class networks supporting splitter configuration ratio of 640 at 50 km mitigating effect of channel nonlinearities like CD, SPM, cross-phase modulation (XPM), and FWM for λ8 10 Gbps channel.

Optical fiber distributed acoustic sensing (DAS) is a potential approach to realize on-line monitoring and rapid localization of power cable partial discharges (PDs). Distributed detection of discharges requires a DAS system with a wide frequency band and low noise level. For this purpose, an improved coherent optical time-domain reflectometry (COTDR) method based on dual-frequency probe pulses was developed. The method doubled the length-bandwidth product (LBP) of traditional DAS systems without significantly increasing the complexity of signal demodulation. A discharge signal extraction algorithm was also proposed based on the detection results of the dual-frequency pulses to exclude the effects of fading noises and vibrations. The improved COTDR system demonstrated the frequency band extension by detecting and recovering the 50 kHz sinusoidal vibration. The sensing distance was 1.1 km and the spatial resolution was 7.15 m. Afterward, the multi-point weak spark discharge acoustic signals were correctly measured and localized, and the frequency components up to 71.36 kHz were detected. The proposed COTDR method provides a prospective solution for distributed PD detection for power cables.

Chapter 11 - Highly Nonlinear Fibers

The third-order nonlinearity in optical fibers is the cause of the related phenomena of self-phase modulation (SPM), four-wave mixing (FWM), and cross-phase modulation (XPM). The FWM generation efficiency in a single fiber segment depends on the propagation constant mismatch Δβ between the participating waves.1 A common technique to reduce FWM in WDM networks is to use high-dispersion fibers, but a drawback is that high dispersion in turn leads to pulse shape distortion. Hence the need arises to compensate for this high dispersion. Means have been introduced to do so, including: localized dispersion compensation (DC) by placing a dispersion compensator after each segment;2 alternating dispersion (AD), i.e. reversing the sign of the dispersion coefficient in consecutive fiber segments;3 phase conjugation (PC) between two consecutive segments.4,5 Here we show that by broadening the approach to FWM reduction, to encompass all the tools listed above, one can find a combination which can in principle completely cancel all third-order nonlinear effects in a two-segment link. We first present the proof for FWM, and then discuss its extension to XPM and SPM.

It is well known that the information capacity of a lightwave communication system is ultimately limited by the nonlinear interactions between the information signals and the fiber medium. These optical non-linear interactions can lead to interference, distortion, and attenuation of the signals, resulting in system degradation. There are four types of non-linear interactions in optical fibers. (1) stimulated Raman scattering (SRS); (2) stimulated Brillouin scattering (SBS); (3) self-and cross phase modulation (SPM and CPM), and (4) four-photon mixing (FPM). These nonlinear effects degrade both the single channel and multichannel systems in different ways. The SBS and FPM effects depend on the signal modulation format and detection schemes (direct or coherent) respectively and whereas the SPM and CPM effects are of concern in angle modulated systems. The effects of SRS, CPM, and FPM induce interchannel crosstalk in wavelength division multiplexing (WDM) systems and affects the system capacity. In this paper, the effects of nonlinearities in lightwave communication systems are reviewed with the emphasis on recent developments. Additionally, techniques to minimize the effects of nonlinearities are discussed.