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• The explosive growth of information has put forward higher requirements for the transmission performance of communication systems, but the accumulation of nonlinear impairments has become the primary factor limiting the performance of the system. • Digital backpropagation (DBP) offers a new approach to enhancing system capacity without affecting system performance. • However, due to the repeated Fourier transform and inverse Fourier transform of the split-step Fourier method (SSFM), the complexity of the algorithm is greatly increased. • In this paper, a correlated backpropagation algorithm with separation of walk-off effect is introduced, which improves the computational step size and significantly reduces the complexity of the algorithm. • Simulation results demonstrate that in a PDM-16QAM system with 6 channels, the proposed algorithm reduces the computational complexity by 73.96 % compared to the traditional DBP algorithm for an 800 km transmission. Digital backpropagation (DBP) is a commonly used method to compensate for the nonlinear impairments of optical fiber channels in the electrical domain, offering a new approach to enhancing the data capacity without affecting the system performance. However, due to the repeated Fourier transform and inverse transform of the split-step Fourier method (SSFM), the complexity of DBP algorithm has increased significantly. In this paper, a correlated backpropagation algorithm with the separation of the walk-off effect is developed, which improves the computational step and significantly reduces the complexity of the algorithm. Simulation results demonstrate that in a 6-channel PDM-16QAM system, the proposed hybrid algorithm reduces the computational complexity by 73.96 % compared to the traditional DBP algorithm for an 800 km transmission. [ABSTRACT FROM AUTHOR]

As a form of multi-carrier modulation, universal filter multicarrier (UFMC) combined with optical communication systems provides significant advantages for long-distance transmission, including large bandwidth, low attenuation, and flexible spectrum resource allocation. While polarization division multiplexing (PDM) enhances system capacity, it concurrently introduces various interference leading to signal distortion. Notably, phase noise has a profound impact on the performance of multi-carrier modulation systems. Phase noise results in stochastic variations to the signal, making the recovery of receiver signals challenging. In this paper, we propose a phase noise suppression method based on Gaussian Basis Expansion (GBE), specifically designed for PDM coherent optical universal filter multicarrier (CO-UFMC) system. We analyze the fundamental principles of phase noise suppression using GBE and validate its effectiveness in optical UFMC transmission systems. In comparison to traditional phase noise suppression methods, our approach exhibits superior phase noise robustness, while maintaining comparable computational complexity. The GBE method significantly enhances the system's tolerance to laser phase noise and improves its robustness against phase noise. The theoretical analysis is corroborated through numerical simulations of a 16-quadrature amplitude modulation (QAM) optical UFMC system. • The combination of universal filtered multi-carrier (UFMC) with optical communication systems offers significant advantages for long-distance transmission. • However, in multi-carrier modulation systems, phase noise can lead to random signal variations, posing challenges for signal recovery at the receiver. • In this study, we propose a phase noise suppression method based on Gaussian Basis Expansion (GBE), specifically designed for the PDM coherent optical universal filtered multi-carrier (CO-UFMC) system. • We analyze the fundamental principles of using GBE to suppress phase noise and validate its effectiveness in optical UFMC transmission system. • The GBE method greatly enhances the system's tolerance to laser phase noise, thereby improving its robustness against phase noise. [ABSTRACT FROM AUTHOR]

• Multi-class classification targeting infrastructure types in the field was performed. • Feature extraction designed for a sensing technique with conventional optical fiber. • Spectral envelope shape offers the prospect of improved accuracy. • Classification using both frequency and time-domain features show good performance. • Reclassification by considering the similarity of neighboring classes. We present a feature extraction method for a feedforward-type neural network (FNN) designed to realize a distributed acoustic sensing (DAS) technique that suits conventional optical fiber. This is, to the best of our knowledge, the first trial in which an FNN is used to interpret the field communication infrastructure type surrounding optical cables. Three classes are taken to represent the field environment: the cable tunnel (class 1), circular duct (class 2), and overhead area (class 3). We investigate and compare frequency- and time-domain feature extraction. We also show that the frequency-domain features yielded by spectral envelope shape (SES) processing have better performance than simple fast Fourier transform features. Two types of time-domain features are verified: one is the short-time maximum magnitude (STMM), which shows the largest value in the time frame, and the other is the short-time average magnitude (STAM), which indicates the average value in a time frame. Note that all features are optimized for multi-class classification. In this paper, we present the suitable number of both features and the number of training iterations. An accuracy rate of 79.0% is achieved using FNN analysis with the features studied here. Furthermore, by considering the similarity of neighboring classes, classes are refined into higher probability classes. As a result, accuracy is improved to 87.2%. [ABSTRACT FROM AUTHOR]

• Reveal the design principles of trench-assisted bridge fibers (TABFs), offering an effective methodology for fabricating TABFs for FIFO devices. • Demonstrate the feasibility of utilizing TABFs for fabricating low-loss FIFO devices. • The results indicated that the fabricated FI/FO device exhibited a remarkably low loss (<0.50 dB), and relatively low crosstalk (<-55.2 dB) in the C-band. Multi-core fibers (MCF) technology is the feasible path to extend the capacity of optical fiber communication systems. However, there is a lack of low-loss and low-crosstalk multi-core fan-in and fan-out (FIFO) devices for MCF communication in the C-band. In this paper, we proposed all-fiber FIFO devices with seven trench-assisted bridge fibers (TABFs). The special structure of the TABFs could facilitate the matching of single-core fiber with MCF during the tapering progress. By using the TABFs, the average taper loss was less than 0.06 dB. The novel method can reduce the insertion loss of FIFO devices. The experiments indicated that the fabricated FIFO devices exhibited lower than 0.50 dB insertion loss and less than −55.2 dB crosstalk in the C-band. This work provides novel TABFs and an efficient way to fabricate low-loss low-crosstalk FIFO devices for MCF communication systems. [ABSTRACT FROM AUTHOR]

In this work we introduce new numerical compact finite-difference algorithms for modeling nonlinear signal propagation in transmission systems based on multimode optical fibers, in the presence of nonlinearity and random linear mode coupling. We compare the computational efficiency of these methods with respect to the standard split-step Fourier method, for different regimes of random mode coupling, such as the weak, the intermediate, and the strong coupling regime. We reveal that, in the intermediate random coupling regime, a non-iterative version of the compact scheme can be twice faster than the standard Fourier method. • Numerical algorithms for modeling signal propagation in multimode transmission links. • Compact finite-difference algorithm of higher order of accuracy. • Different propagation regimes depending on the strength of random linear coupling. • Comparison of the proposed algorithms with the split-step Fourier method. [ABSTRACT FROM AUTHOR]

• Novel bidirectional full-duplex transmitting WDM-PON over single SSMF is proposed. • SRS + FWM impact on the proposed system performance is evaluated. • SXR-input power, SXR-channel spacing, SXR-channel length variations are simulated. • Results show a reliable communication with the system under SRS + FWM impact. • Proposed system and analyses are important for 5G and beyond fronthaul networks. Wavelength division multiplexing (WDM) is an important method in modern optical communication systems for both long-haul and access networks. Particularly, after being standardized by Telecommunication Standardization Sector of International Telecommunication Union (ITU-T), WDM passive optical network (WDM-PON) becomes a considerable choice for fronthaul implementations of 5G networks. Stimulated Raman scattering (SRS) and four-wave mixing (FWM) are crucial nonlinear impacts that significantly limit the performance of WDM-based optical fiber communication systems. In this paper, an ultra-dense/dense WDM-PON (UDWDM/DWDM-PON) architecture has been proposed and combined dual impact of SRS and FWM on performances of upstream channels (USCs) and downstream channels (DSCs) of the proposed UDWDM/DWDM-PON has been numerically analyzed. USCs have been assumed to operate in 1310 nm region while operating wavelength region of DSCs has been taken as 1550 nm. Simulations have been performed on both UDWDM-PON architectures with 3.125 GHz and 6.25 GHz channel spacings and DWDM-PON architectures with channel spacings between 12.5 GHz and 100 GHz. 2x15-channel and 2x63-channel system structures have been considered in simulations. Signal-to-crosstalk ratio (SXR) has been chosen as performance parameter and its variation with channel input powers, channel spacings and channel lengths have been simulated for both USCs and DSCs of 2x15- and 2x63-channel UDWDM/DWDM-PONs. Results show that the proposed UDWDM/DWDM-PONs using single G.652 fiber can exhibit a reliable communication with minimum 23 dB SXR performance under combined dual impact of SRS and FWM unless some highlights about input powers and channel lengths mentioned in the paper are neglected. [ABSTRACT FROM AUTHOR]

• This paper presents a systematic discussion of the frequency-domain channel transmission model for PDM CO-OFDM/OQAM systems. • Through a thorough analysis of ASE noise correlation, we introduce a novel approach, Combined Intra Symbol Frequency-Domain Averaging (CISFDA) channel estimation method for PDM OFDM/OQAM systems. • In comparison to the Full-Loaded Frequency-Domain (FL-FD) method, CISFDA enhances system robustness against ASE noise and fiber nonlinear effects significantly. Intrinsic imaginary interference (IMI) induced by chromatic dispersion (CD) and polarization mode dispersion (PMD) seriously degrades the performance of optical orthogonal frequency division multiplexing offset-quadrature amplitude modulation (O-OFDM/OQAM) systems. Therefore, effective channel estimations are required for such systems to maintain good transmission performance. In the previous publications, both the full loaded (FL) and the half loaded (HL) frequency-domain channel estimation methods for polarization-division-multiplexed (PDM) coherent optical OFDM/OQAM (CO-OFDM/OQAM) systems have been proposed. However, for these frequency-domain methods, the channel frequency-domain responses caused by the correlation of the amplified spontaneous emission (ASE) noise are not considered. In this paper, we systematically discuss the frequency-domain channel transmission model for PDM CO-OFDM/OQAM systems. With the analysis of the correlation of the ASE noise, we propose a combined intra symbol frequency-domain averaging (CISFDA) channel estimation method for PDM OFDM/OQAM systems. Compared with the full-loaded frequency-domain channel estimation (FL-FD) method, the CISFDA method promotes the system robustness against the ASE noise and fiber nonlinear effect significantly. The proposed method is validated by numerical Monte Carlo simulations of the PDM CO-OFDM/OQAM system. Our new algorithm outperforms the FL-FD method in the 36 Gb/s PDM OFDM CO-OFDM/OQAM system, and the transmission distance is improved by 500 km with the bit-error-rate (BER) target at 3.8 × 10 - 3 . [ABSTRACT FROM AUTHOR]

• Switching the internal state machine from FPGA is proposed to adjust the operating bias-point control of Mach-Zehnder electro-optic modulator, which indicates the proposed method can realize automatic bias-point control in real time without circuit, feedback or algorithm to compensate the drift. • A pulsed laser source of Mach-Zehnder electro-optic modulator based on the FPGA is established experimentally to demonstrate the modulation depth and the extinction ratio of pulse are improved by 40.50% and 32.85 dB, separately. • The sensing performances of ring-down cavity are improved including the initial curve rise of ring-down signals and the distortion of ring-down waveform. The good linearity, repeatability, stability and accuracy of temperature sensing system are investigated experimentally at the continuous temperature variation from −30℃ to 30℃. Mach-Zehnder electro-optic modulator as the essential component of high-speed modulation in the optical fiber communication and sensing system, its stability control of operating bias-point plays a key role in determining the transmission signal quality. Based on the output characteristic principle of Mach-Zehnder electro-optic modulator, researches on its bias-point drifting for causing the reduction mechanism of modulation depth and extinction ratio are analyzed. Switching its internal state machine from FPGA is proposed to automatically adjust the bias voltage in real time realizing the operating bias-point control of Mach-Zehnder electro-optic modulator, which is studied through the theoretical investigation. A pulsed laser with the temperature sensing system of fiber Bragg grating ring-down cavity based on FPGA is established experimentally to demonstrate its effectiveness. The experimental results show the modulation depth and the extinction ratio of pulsed laser are improved by 40.45% and 32.85 dB, respectively. Output characteristic curve of the sensing system is optimized, which includes the phenomenon of initial curve rise of ring-down signals and the distortion of ring-down waveform are eliminated. The good linearity, repeatability, stability and accuracy of sensing system are investigated at the continuous variation temperature from −30℃ to 30℃. [ABSTRACT FROM AUTHOR]

At present days, metropolitan and access communication networks are markedly developing utilizing a variety of technologies. Their bearer communication infrastructures will be mostly exploiting the optical transmission medium where wavelength division multiplexing techniques will play a key role. Wavelength-Division Multiplexed Passive Optical Networks (WDM-PON) attract considerable attention for the next generation optical metropolitan and access networked systems. Simultaneously, the utilization of common optical network resources in wavelength domains will be more important. The main purpose of this paper is presented by the evaluation of resource efficiencies for the non-symmetric dynamic wavelength allocation method applied in the WDM-PON network design with a conventional topology. This contribution introduces a novel non-symmetric Dynamic Wavelength Allocation (DWA) method for a selected network design with the utilization only passive optical splitters in remote nodes. This novel wavelength allocation method is oriented on minimizing a number of working wavelengths that will be used for compliance of customers' requests for data transmitting in the common point-to-multipoint WDM–PON network design. For evaluation of wavelength scheduling, a simulation model related to the conventional WDM-PON network design is realized in the MATLAB programming environment is presented with results. Finally, different traffic loading scenarios are determined and, subsequently, resource efficiencies of the presented non-symmetric DWA method are evaluated from a viewpoint of application in future WDM-PON network designs. [ABSTRACT FROM AUTHOR]

Optical Fiber Communication is known for its extremely high data-carrying capacity among all the existing transmission technologies. However, its capacity to the fullest is restrained by the effects of nonlinearity, thereby urging nonlinear compensation. In this paper, we have attempted to enhance the performance of Nonlinear Frequency Division multiplexing (NFDM) based nonlinearity compensation, a recent attraction due to its high throughput nature. Here, we design optimal transmit signals, emphasizing lowering the Peak to Average Ratio (PAPR). With appropriate investigation, optimal eigenvalue and Quadrature Amplitude Modulation (QAM) constellations for encoding have been procured that aid in constructing low PAPR NFDM signals. To upgrade the optimal QAM design, two novel Spiral QAM models, Model I and Model II are proposed that diversify signal envelopes in addition to lowering the PAPR. This improves the overall fault tolerance of the NFDM system. For a 2-eigenvalue NFDM system, using the proposed constellations, the PAPR is reduced by 0.24 dB (Model I) and 1.42 dB (Model II), compared to the conventional scheme. Thus, we have designed optimum NFDM transmit pulses and subsequently accomplished a high performing 10 Gbps NFDM system using the proposed constellation models. • Realized an enhanced 10 Gbps NFDM transmission system. • Designed optimal NFDM transmit signals with low Peak to Average Ratio. • Proposed novel Spiral QAM constellation models to diversify signal envelopes. • Improvement in the overall fault tolerance of the NFDM system. • Achieved better transmission reach. [ABSTRACT FROM AUTHOR]

As the capacity and the transmission distance of medium-reach optical fiber communication systems continue to increase, it is a major trend in demanding lower optical link budgets while maintaining the quality of signal transmission. We experimentally compared the transmission performance of the Kramers–Kronig (KK) and the coherent receivers in a 20-Gbaud quadrature phase shift keying (QPSK) system over a ∼ thousand-kilometer standard single-mode fiber (SMF) link. Experimental results show that the KK receiver requires lower cost and simpler structure, and behaves similarly with the coherent receiver in the transmission under 1260 km, despite higher transmitted power and computational complexity. [ABSTRACT FROM AUTHOR]

• Taper in etch SMF (TiESMF) sensing platform is not explored for gas sensing applications yet. • TiESMF sensors exhibited good sensitivity and selectivity towards hydrogen. • The developed system can be integrated with optical fiber communication networks such as FTTH. A hydrogen (H 2) sensing application is proposed and experimentally demonstrated using a multimode fiber (MMF)–tapered-in-etch single-mode fiber (SMF)–MMF (MMF–TiESMF–MMF)-based Mach–Zehnder interferometer (MZI). The MZI is made of an SMF segment placed between two MMF segments. The MMF segments function as mode couplers, splitting and combining light due to a core diameter discrepancy between the MMF and the SMF. The SMF segment undergoes cladding modification by chemically etching to achieve a 50 µm diameter over a 2 cm length, followed by the tapering to obtain a 15 µm waist diameter with a 1 cm length. A layer of palladium–graphite nanofibers is applied to the modified area of the TiESMF to produce H 2 adsorption. The sensor is investigated against H 2 over the 0.25–1% concentration range at room temperature. When H 2 molecules were desorbed on the sensing layer, its index of refraction was changed so that the output spectrum resulting from the MZI is blue-shifted due to H 2 molecules desorption. The MMF-TiESMF-MMF MZI exhibits a sensitivity of 0.332 nm/vol% and a linear fit coefficient of 99.72% for H 2 concentrations in the range selected. MZI based sensors showed a linear, stable, and repetitive response. [ABSTRACT FROM AUTHOR]

• Based on EFPI sensor, PB-AOF and wavelength division multiplexing technology, the system realizes one-way four-channel passive audio parallel transmission. • Achieve 20 km long distance optical fiber audio communication. • No difference between the EFPI sensors used by each channel. • Be used in scenarios with special requirements, such as security monitoring and mines. To meet sound transmission requirements in complex work environments, this paper constructs a passive audio pickup sensor based on an extrinsic Fabry-Perot interferometer (EFPI) and develops a one-way four-channel passive audio parallel transmission system based on a passive bidirectional audio over fiber(PB-AOF) transmission system and wavelength division multiplexing technology. The system realizes optical fiber audio transmission over a distance of 20 km. The experimental results show that the signal-to-noise ratio (SNR) is greater than 64 dB, and the highest sensitivity of the four channels in the proposed system is 1.06 V/Pa. Thus, the proposed long-distance terminal passive audio communication network is ideally suited for applications in certain fields. [ABSTRACT FROM AUTHOR]

• PM-FM elliptical hollow-core optical fibers with simple triple-layer structure. • Parameters of air hole and ring core can be adjusted for PM performance. • 10 effectively separated polarization modes with Δn eff > 1.93 × 10−4at 1550 nm. • Expandable number of guided PM modes with modified structural parameters. • Those fibers could be promising candidates toward high-capacity SDM systems. We propose polarization-maintaining few-mode elliptical hollow-core optical fibers with simple triple-layer structure toward space-division-multiplexing based high-capacity optical communication systems. The proposed fibers include a central elliptical air hole, a concentric elliptical ring core and a circular cladding. The structure-dependent polarization-maintaining performance is comprehensively investigated via numerical simulations. With appropriate structural parameters, 10 distinct polarization-maintaining modes can be guided in this fiber. The effective refractive index differences (Δn eff) between all adjacent modes are beyond 1.93 × 10−4 at 1550 nm, and the values are higher than 1.8 × 10−4 over 1510–1630 nm. The modal chromatic dispersions are analyzed subsequently. Furthermore, the number of guided polarization-maintaining modes can be expanded to 14 with modified structural parameters meanwhile keeping all Δn eff values no less than 1.33 × 10−4 in range of 1510–1630 nm. [ABSTRACT FROM AUTHOR]

Highlights • Wavelength reuse based bidirectional coherent OFDMA access network. • Enhancement of spectral efficiency by using the proposed reflective coherent PON. • Verification of the proposed technique in future WDM-PON. Abstract This paper demonstrates wavelength reused bidirectional transmission for a reflective coherent passive optical network (R-Co-PON). By reusing the downstream optical signal as the upstream optical carrier, the proposed R-Co-PON enhances the optical spectral efficiency. In an experimental demonstration, the proposed R-Co-PON achieved a symmetrical bidirectional transmission rate of 12 Gb/s using a 73-km-long fiber loopback architecture. The experiments confirmed the potential applicability of the proposed system in future optical access networks, particularly those using wavelength-division multiplexing. [ABSTRACT FROM AUTHOR]

Optical studies of ex vivo brain slices where blood is absent show that neural activity is accompanied by significant intrinsic optical signals (IOS) related to activity-dependent scattering changes in neural tissue. However, the neural scattering signals have been largely ignored in vivo in widely-used IOS methods where absorption contrast from hemoglobin was employed. Changes in scattering were observed on a time scale of seconds in previous brain slice IOS studies, similar to the time scale for the hemodynamic response. Therefore, potential crosstalk between the scattering and absorption changes may not be ignored if they have comparable contributions to IOS . In vivo , the IOS changes linked to neural scattering have been elusive. To isolate neural scattering signals in vivo , we employed 2 implantable optodes for small-separation (2 mm) transmission measurements of local brain tissue in anesthetized rats. This unique geometry enables us to separate neuronal activity-related changes in neural tissue scattering from changes in blood absorption based upon the direction of the signal change. The changes in IOS scattering and absorption in response to up-states of spontaneous neuronal activity in cortical or subcortical structures have strong correlation to local field potentials, but significantly different response latencies. We conclude that activity-dependent neural tissue scattering in vivo may be an additional source of contrast for functional brain studies that provides complementary information to other optical or MR-based systems that are sensitive to hemodynamic contrast. [ABSTRACT FROM AUTHOR]

• A five wavelength-switchable single-longitudinal-mode erbium-doped fiber laser was proposed. • A cascaded PS-FBG was designed and analyzed. To the best of our knowledge, this is the first time that the PS-FBG is used as a multi-channel filter to realize stable multiple SLM single-wavelength outputs. • Five single-wavelength switchable SLM outputs were generated in stable condition over 60 min. • The minimum linewidth at an integration time of 0.001 s was 1.3102 kHz. Wavelength-switchable fiber lasers play an important role in optical fiber communication systems. Although phase-shifted fiber Bragg grating (PS-FBG) has a narrow 3 dB bandwidth, it has rarely been reported as a multi-channel optical filter. Therefore, a five wavelength-switchable single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) was implemented and analyzed, where the cascaded PS-FBG with a 3 dB bandwidth of each channel narrower than 0.024 nm was used as a five-channel filter. The design, fabrication, and characterization of cascaded PS-FBG were implemented and analyzed in detail. By tuning the micro-displacement platform, five-single wavelength switchable SLM outputs with a high optical signal-to-noise ratio (OSNR) were generated and the outputs were monitored over 60 min. The measured maximum wavelength drift and the peak power fluctuation were 0.02 nm and 1.73 dB. The relaxation oscillation peaks were lower than −100 dB/Hz and the minimum linewidth at an integration time of 0.001 s was 1.3102 kHz. To the best of our knowledge, this is the first time that the PS-FBG is used as a multi-channel filter to realize multiple SLM single-wavelength outputs. [ABSTRACT FROM AUTHOR]

• Distributed optical fiber Brillouin optical time-domain reflection sensing system is used as passive railway monitoring system, which has advantages of anti-electromagnetic interference, high sensitivity, strong stability and low maintenance cost. • Monitoring Brillouin scattering signal of whole optical cable can be completed by using a redundant cable of railway communication optical fiber, which greatly saves construction cost and capital cost. • The railway communication optical fiber Brillouin scattering signal is used to complete position determination of track area/slopes/tunnels, and complex manual calibration work is completed by technological ways, which reduces time cost and capital cost of calibration and fault resolution. • Accumulated Brillouin scattering signal data of railway communication optical fiber for a long time has potential to assist in the analysis and judgment of problems such as abnormal structural state of railway track area/slopes/tunnels. Wide coverage and complex environment along the railway make a huge demand for calibration in critical areas such as tunnels or slopes. In this paper, an on-line identification method of railway tunnels and slopes based on Brillouin scattering signal is proposed. By monitoring Brillouin scattering signal of a redundant communication optical fiber, tunnels and slopes can be recognized by feature analysis. Experimental results indicate that the on-line identification method of railway tunnels and slopes based on Brillouin scattering signal is effective, invulnerable and accurate. [ABSTRACT FROM AUTHOR]

In this paper we focused on the experimental investigations for fabricating a surface plasmon resonance (SPR) based fiber optic sensor for the detection of the extent of adulteration in petrochemicals: petrol and diesel by kerosene. Primarily it is observed that the refractive index of the petrol and diesel changes if we mix kerosene in it. The variation in refractive index is linear in nature. Utilizing the phenomenon of surface plasmon resonance in Krestchmann configuration on optical fiber, the percentage of adulteration in petrol and diesel is detected. The detection level of adulteration is quantified systematically for both the petrol and diesel. The study carried out here explores the possibility of utilizing SPR technique for the detection of the level of adulteration in petrochemicals. The suitability of the optical fiber for remote sensing and its immunity towards electromagnetic interaction makes this probe very useful for such endeavor. High sensitivity, easy construction and its portability, makes this study important in the development of SPR based optical fiber sensors for petrochemical industries. Apart from this various aspects of environment polluting hazardous/toxic gases as an emission product of automobile fuels has also been discussed. [ABSTRACT FROM AUTHOR]

A sensitive curvature sensor based on MMF-SCF-MMF (MMF: multimode fiber; SCF: seven core fiber) structure is proposed. The multimode fiber (MMF) are used to improve the light coupling efficiency between the input singlemode fiber (SMF) and the seven-core fiber (SCF), and the seven-core fiber is used as the main element for curvature measurement. Experimental results show that the best curvature sensitivity reaches 41.46453 nm/m −1 in the range of 0.094 m −1 –0.567 m −1 . The temperature sensitivity is up to 59.02 pm/°C in the range of 20 °C–55 °C. The optical curvature sensors are widely used for buildings structure health monitoring and mechanical engineering due to the advantages of compact structure, anti-electromagnetic interference, and low cost. [ABSTRACT FROM AUTHOR]

We numerically and experimentally compared the BER performance of 128-level set-partitioning quadrature amplitude modulation (128-SP-QAM) and polarization multiplexed 16-ary quadrature amplitude modulation (PM-16QAM). From our experimental results and numerical simulation, it can be concluded that 128-SP-QAM has significant BER improvement compared to PM-16QAM when optical signal to noise ratio (OSNR) is high. However, when it comes to low OSNR scenario such as transmission over a very long distance, 128-SP-QAM and PM-16QAM have similar performance. [ABSTRACT FROM AUTHOR]

A new approach for realizing a wideband optical frequency comb (OFC) generator based on driving cascaded modulators by a Gaussian-shaped waveform, is proposed and numerically demonstrated. The setup includes N-cascaded MZMs, a single Gaussian-shaped waveform generator, and N-1 electrical time delayer. The first MZM is driven directly by a Gaussian-shaped waveform, while delayed replicas of the Gaussian-shaped waveform drive the other MZMs. An analytical model that describes the proposed OFC generator is provided to study the effect of number and chirp factor of cascaded MZM as well as pulse width on output spectrum. Optical frequency combs at frequency spacing of 1 GHz are generated by applying Gaussian-shaped waveform at pulse widths ranging from 200 to 400 ps. Our results reveal that, the number of comb lines is inversely proportional to the pulse width and directly proportional to both number and chirp factor of cascaded MZMs. At pulse width of 200 ps and chirp factor of 4, 67 frequency lines can be measured at output spectrum of two-cascaded MZMs setup. Whereas, increasing the number of cascaded stages to 3, 4, and 5, the optical spectra counts 89, 109 and 123 frequency lines; respectively. When the delay time is optimized, 61 comb lines can be achieved with power fluctuations of less than 1 dB for five-cascaded MZMs setup. [ABSTRACT FROM AUTHOR]

In this paper, we propose and experimentally demonstrate a tunable erbium-doped fiber laser (EDFL) with Sagnac interference loop with 45° angle shift spliced polarization maintaining fibers (PMFs). In the Sagnac loop, two PMFs with similar lengths. The Sagnac loop outputs a relatively complex interference spectrum since two beams transmitted in clockwise and counterclockwise encounter at the 3 dB coupler, interfere, and form two interference combs when the light transmitted in the Sagnac loop. The laser will excite and be stable when two interference lines in these two interference combs overlap together. Then by adjusting the polarization controller, the wide wavelength tuning is realized. Experimental results show that stable single wavelength laser can be realized in the wavelength range of 1585 nm–1604 nm under the pump power 157.1 mW. The side-mode suppression ratio is not less than 53.9 dB. The peak power fluctuation is less than 0.29 dB within 30 min monitor time and the side-mode suppression ratio is great than 57.49 dB when the pump power is to 222.7 mW. [ABSTRACT FROM AUTHOR]

In this paper, an optimized intra-symbol frequency-domain averaging (ISFA) scheme is proposed and experimentally demonstrated in intensity-modulation and direct-detection (IMDD) multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband over fiber (UWBoF) system. According to the channel responses of three MB-OFDM UWB sub-bands, the optimal ISFA window size for each sub-band is investigated. After 60-km standard single mode fiber (SSMF) transmission, the experimental results show that, at the bit error rate (BER) of 3.8 × 10 −3 , the receiver sensitivity of 128-quadrature amplitude modulation (QAM) can be improved by 1.9 dB using the proposed enhanced ISFA combined with training sequence (TS)-based channel estimation scheme, compared with the conventional TS-based channel estimation. Moreover, the spectral efficiency (SE) is up to 5.39 bit/s/Hz. [ABSTRACT FROM AUTHOR]

In this paper, unique design of an optimal broadband optical dispersion flattened fiber (DFF) is proposed, which is capable of supporting the data rate of the order of Tb/s. The analysis of the single mode fiber for the design of the proposed DFF has been carried out by employing the quadratic Finite Element Method (FEM) with generalized refractive index (R. I.) profile. The minimization of the dispersion with respect to various profile parameters within the specified wavelength band is the essential optimization criteria. Computations show that a DFF can be designed where the overall dispersion can be restricted within ± 1 ps/km-nm over the entire spectral span ranging from 1290 to 1540 nm (250 nm) exhibiting a very small maximum value of dispersion slope ( ± 0.02  ps / ( nm 2 -km ) ) in particular. The detailed performance analysis of the proposed DFF with different modulation techniques has been carried out in order to critically evaluate the performance of the DFF with respect to various significant parameters. The results suggest an excellent design of broadband optical waveguide capable of supporting high-speed data rate (40 Tb/s) through the single DFF, ideally suitable for the long haul dense wavelength division multiplexing (DWDM) optical transmission systems. [ABSTRACT FROM AUTHOR]

This paper investigates the design details of cladding rods-assisted (CRA) depressed-core (DC) few-mode fibers (FMFs) that feature more equally spaced linearly polarized (LP) modal effective indices, suitable for high-spatial-density weakly-coupled mode-division multiplexing systems. The influences of the index profile of cladding rods on LP mode-resolved effective index, bending sensitivity, and effective area A eff , are numerically described. Based on the design considerations of LP modal A eff -dependent spatial efficiency and LP modal bending loss-dependent robustness, the small LP 21 -LP 02 and LP 22 -LP 03 modal spacing limitations, encountered in state-of-the-art weakly-coupled step-index FMFs, have been substantially improved by at least 25%. In addition, the proposed CRA DC FMFs also show sufficiently large effective areas (in excess of 110 μm 2 ) for all guided LP modes, which are expected to exhibit good nonlinear performance. [ABSTRACT FROM AUTHOR]

This paper presents a centralized and fault localization technique for Ethernet Passive Optical Access Network. This technique employs L-band Amplified Spontaneous Emission (ASE) as the monitoring source and various fiber Bragg Gratings (FBGs) as the fiber's identifier. An FBG with a unique combination of Bragg wavelength, reflectivity and bandwidth is inserted at each distribution fiber. The FBG reflection spectrum will be analyzed using an optical spectrum analyzer (OSA) to monitor the condition of the distribution fiber. Various FBGs reflection spectra is employed to optimize the limited bandwidth of monitoring source, thus allows more fibers to be monitored. Basically, one Bragg wavelength is shared by two distinct FBGs with different reflectivity and bandwidth. The experimental result shows that the system is capable to monitor up to 32 customers with OSNR value of ∼1.2 dB and monitoring power received of −24 dBm. This centralized and simple monitoring technique demonstrates a low power, cost efficient and low bandwidth requirement system. [ABSTRACT FROM AUTHOR]

By employing sophisticated routing and spectrum assignment (RSA) algorithms together with a finer spectrum granularity (namely frequency slot) in resource allocation procedures, flex-grid optical networks can accommodate diverse kinds of services with high spectrum-allocation flexibility and resource-utilization efficiency. However, the continuity and the contiguity constraints in spectrum allocation procedures may always induce some isolated, small-sized, and unoccupied spectral blocks (known as spectrum fragments) in flex-grid optical networks. Although these spectrum fragments are left unoccupied, they can hardly be utilized by the subsequent service requests directly because of their spectral characteristics and the constraints in spectrum allocation. In this way, the existence of spectrum fragments may exhaust the available spectrum resources for a coming service request and thus worsens the networking performance. Therefore, many reactive defragmentation algorithms have been proposed to handle the fragmented spectrum resources via re-optimizing the routing paths and the spectrum resources for the existing services. But the routing-path and the spectrum-resource re-optimization in reactive defragmentation algorithms may possibly disrupt the traffic of the existing services and require extra components. By comparison, some proactive defragmentation algorithms (e.g. fragmentation-aware algorithms) were proposed to suppress spectrum fragments from their generation instead of handling the fragmented spectrum resources. Although these proactive defragmentation algorithms induced no traffic disruption and required no extra components, they always left the generated spectrum fragments unhandled, which greatly affected their efficiency in spectrum defragmentation. In this paper, by comprehensively considering the characteristics of both the reactive and the proactive defragmentation algorithms, we proposed a time-aware service-classified (TASC) spectrum defragmentation algorithm, which simultaneously employed proactive and reactive mechanisms in suppressing spectrum fragments with the awareness of services’ types and their duration times. By dividing the spectrum resources into several flexible groups according to services’ types and limiting both the spectrum allocation and the spectrum re-tuning for a certain service inside one specific spectrum group according to its type, the proposed TASC defragmentation algorithm cannot only suppress spectrum fragments from generation inside each spectrum group, but also handle the fragments generated between two adjacent groups. In this way, the proposed TASC algorithm gains higher efficiency in suppressing spectrum fragments than both the reactive and the proactive defragmentation algorithms. Additionally, as the generation of spectrum fragments is retrained between spectrum groups and the defragmentation procedure is limited inside each spectrum group, the induced traffic disruption for the existing services can be possibly reduced. Besides, the proposed TASC defragmentation algorithm always re-tunes the spectrum resources of the service with the maximum duration time first in spectrum defragmentation procedure, which can further reduce spectrum fragments because of the fact that the services with longer duration times always have higher possibility in inducing spectrum fragments than the services with shorter duration times. The simulation results show that the proposed TASC defragmentation algorithm can significantly reduce the number of the generated spectrum fragments while improving the service blocking performance. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate 40-Gb/s four-level pulse amplitude modulation (PAM4) transmission with 10 GHz devices and low-complexity equalizers for next-generation passive optical network (PON) systems. Simple feed-forward equalizer (FFE) and decision feedback equalizer (DFE) enable 20 km fiber transmission while high-complexity Volterra algorithm in combination with FFE and DFE can extend the transmission distance to 40 km. A simplified Volterra algorithm is proposed for reducing computational complexity. Simulation results show that the simplified Volterra algorithm reduces up to ∼75% computational complexity at a relatively low cost of only 0.4 dB power budget. At a forward error correction (FEC) threshold of 10 - 3 , we achieve 31.2 dB and 30.8 dB power budget over 40 km fiber transmission using traditional FFE-DFE-Volterra and our simplified FFE-DFE-Volterra, respectively. [ABSTRACT FROM AUTHOR]

In the fabricating process of a 3 × 3 fused tapered coupler, the three fibers are usually twisted to be close-contact. The effect of twist on 3 × 3 fused tapered couplers is investigated in this paper. It is found that though a linear 3 × 3 coupler may realize equal power splitting ratio theoretically by twisting a special angle, it is hard to be fabricated actually because the twist angle and the coupler’s length must be determined in advance. While an equilateral 3 × 3 coupler can not only realize approximate equal power splitting ratio theoretically but can also be fabricated just by controlling the elongation length. The effect of twist on the equilateral 3 × 3 coupler lies in the relationship between the equal ratio error and the twist angle. The more the twist angle is, the larger the equal ratio error may be. The twist angle usually should be no larger than 90° on one coupling period length in order to keep the equal ratio error small enough. The simulation results agree well with the experimental data. [ABSTRACT FROM AUTHOR]

A method is described for the high-resolution measurement of the differential mode delay (DMD) of few-mode fiber based on the frequency-modulated continuous wave (FMCW) technique. With the conventional approach, a wider optical bandwidth is required for higher resolution, however, the effect of chromatic dispersion becomes critical and limits the actual resolution. This paper proposes and demonstrates a phase reference technique for FMCW signals that achieves high-resolution measurements without increasing the optical bandwidth. We can also further improve the resolution of our proposed method by reducing the phase fluctuation with signal averaging. The feasibility of the proposed method is confirmed experimentally with several kinds of FMFs including an offset-spliced fiber where mode couplings occur. [ABSTRACT FROM AUTHOR]

In this paper, the cross-section images, of two different types of polarization maintaining (PM) optical fibers, are employed to estimate the optical phase variation due to transverse optical rays passing through these optical fibers. An adaptive algorithm is proposed to recognize the different areas constituting the PM optical fibers cross-sections. These areas are scanned by a transverse beam to calculate the optical paths for given values of refractive indices. Consequently, the optical phases across the PM optical fibers could be recovered. PM optical fiber is immersed in a matching fluid and set in the object arm of Mach-Zehnder interferometer. The produced interferograms are analyzed to extract the optical phases caused by the PM optical fibers. The estimated optical phases could be optimized to be in good coincidence with experimentally extracted ones. This has been achieved through changing of the PM optical fibers refractive indices to retrieve the correct values. The correct refractive indices values are confirmed by getting the best fit between the estimated and the extracted optical phases. The presented approach is a promising one because it provides a quite direct and accurate information about refractive index, birefringence and beat length of PM optical fibers comparing with different techniques handle the same task. [ABSTRACT FROM AUTHOR]

Signal-to-signal beating interference (SSBI) is one of the main drawbacks in direct-detection based optical transmission systems. Volterra filter is a common equalization method to mitigate the nonlinear distortion. However, the computational complexity may be unacceptable as the transmission capacity increases. In this paper, we propose a sparse Volterra model combining the feed forward equalization (FFE) and higher order terms of a modified Volterra filter with Schmidt orthogonal searching to mitigate the linear and nonlinear interference and reduce the complexity significantly in an optical single-side band (SSB) Nyquist pulse-shaped four-level pulse amplitude (NPAM-4) system. The experimental results show that the sparse Volterra filter and full Volterra filter have comparable performance, but the former only needs half kernels of the latter. [ABSTRACT FROM AUTHOR]

In order to minimize energy consumption in Time Division Multiplexing-Passive Optical Network (TDM-PON), IEEE and ITU-T have mandated sleep mode mechanism for Optical Network Units (ONUs) in the latest TDM-PON standards (e.g. IEEE P1904.1 SIEPON, ITU-T G.sup45). The sleep mode mechanism is a promising mean for maximizing energy saving in an ONU. An ONU in sleep mode flips between sleep and active state depending on the presence or absent of upstream and downstream frames. To ensure Quality of Service (QoS) of upstream frames, the recent TDM-PON standards introduced an early wake-up mechanism, in which an ONU is forced to leave the sleep state on upstream frame arrival. When the Optical Line Terminal (OLT) of a TDM-PON allows early wake-up of its connected ONUs, it allocates gratuitous grants for the sleeping ONUs along with allocating upstream grants for the ONUs in active state . Note that, the gratuitous grants control message sent periodically by the OLT on Inter-Gratuitous grant Interval (IGI) time. After leaving sleep state due to the arrival of upstream frame, the ONU uses its allocated gratuitous grant to send a control message mentioning the amount of upstream bandwidth (upstream grant) required in order to forward the remaining frames in its buffer. However, the existing early wake-up process of ONU can lead to increase the energy consumption of an ONU. It is because of the ONU wakes-up immediately from the sleep state on arrival of the upstream frame, but even so, it needs to wait for forwarding the frame until its allocated gratuitous grant period, resulting in spending energy unnecessarily. In addition, current energy saving solution for TDM-PONs do not provide a clear solution on how to manage different types of grants (e.g. listening grant, upstream transmission grant) within a Dynamic Bandwidth Allocation (DBA) polling cycle. To address this problem, we propose a state-of-art Grant Management Procedure (GMP) in order to maximize energy saving in a TDM-PON with sleep mode enabled ONUs. GMP contributes in defining the location of the different types of grants during a DBA polling cycle. Furthermore, GMP devises a mechanism so as to allow an ONU to predict its assigned gratuitous grant control message arrival time, thereby allowing an ONU to remain its transceiver unit powered off until the arrival period of the next gratuitous grant control message, increasing the energy saving of the ONU. Results show that, with the increment of IGI, the energy saving performance of an ONU with GMP increases noticeably in compare to a conventional ONU (an ONU that does not use GMP) without imposing any additional upstream frame delay. [ABSTRACT FROM AUTHOR]

Optical fiber is the most used medium for current telecommunication networks. Besides data transmissions, special advanced applications like accurate time or stable frequency transmissions are more common, especially in research and education networks. On the other hand, new applications like distributed sensing are in ISP’s interest because e.g. such sensing allows new service: protection of fiber infrastructure. Transmission of all applications in a single fiber can be very cost efficient but it is necessary to evaluate possible interaction before real application and deploying the service, especially if standard 100 GHz grid is considered. We performed laboratory measurement of simultaneous transmission of 100 G data based on DP-QPSK modulation format, accurate time, stable frequency and sensing system based on phase sensitive OTDR through two types of optical fibers, G.655 and G.653. These fibers are less common than G.652 fiber but thanks to their slightly higher nonlinear character, there are suitable for simulation of the worst case which can arise in a real network. [ABSTRACT FROM AUTHOR]

We present experimental work on a gigabit-capable and long-reach hybrid coherent UWDM-PON plus FSO system for supporting different applications over the same fiber infrastructure in the mobile backhaul (MBH) networks. Also, for the first time, we demonstrate a reconfigurable real-time DSP transmission/reception of DP-QPSK signals over standard single-mode fiber (SSMF) and FSO links. The receiver presented is based on a commercial field-programmable gate array (FPGA). The considered communication links are based on 20 UDWDM channels with 625 Mbaud and 2.5 GHz channel spacing. We are able to demonstrate the lowest sampling rate required for digital coherent PON by employing four 1.25 Gsa/s ADCs using an electrical front-end receiver that offers only 1 GHz analog bandwidth. We achieved this by implementing a phase and polarization diversity coherent receiver combined with the DP-QPSK modulation formats. The system performance is estimated in terms of receiver sensitivity. The results show the viability of coherent PON and flexible dual-polarization supported by software-defined transceivers for the MBH. [ABSTRACT FROM AUTHOR]

A hybrid fiber-optic sensor consisting of a micro extrinsic Fabry-Perot Interferometer (MEFPI) and an etched fiber Bragg grating (FBG) is proposed, which can measure strain and magnetic field simultaneously. The etched FBG is sealed in a capillary with ferrofluids to detect the surrounding magnetic field. FBG with small diameter will be more sensitive to magnetic field is confirmed by simulation results. The MEFPI sensor that is prepared through welding a short section of hollow-core fiber (HCF) with single-mode fiber (SMF) is effective for strain detection. The experiment shows that strain and magnetic field can be successfully simultaneously detected based on hybrid MEFPI/FBG sensor. The sensitivities of the strain and magnetic field intensity are measured to be up to 1.41 pm/με and 5.11 pm/mT respectively. There is a negligible effect on each other, hence simultaneously measuring strain and magnetic field is feasible. It is anticipated that such easy preparation, compact and low-cost fiber-optic sensors for simultaneous measurement of strain and magnetic field could find important applications in practice. [ABSTRACT FROM AUTHOR]

We propose a fiber nonlinearity-tolerant optical signal-to-noise ratio (OSNR) estimation method for high speed long haul coherent optical fiber transmission system. The correlation function of the amplitude noise and a calibration factor ξ are utilized to correct nonlinearity-induced distortions for the first time when the statistical moments-based method (SMB method) is applied. ξ depends on not only transmission distances but also launch powers. Besides, we put forward a fitting formula for optimal ξ and OSNR estimation results still keep valid regardless of transmission distances and launch powers. Compared with current existing OSNR estimation methods, the proposed method functions well in highly nonlinear systems and is demonstrated to be feasible and more accurate. OSNR estimation errors are below 0.94 dB after calibration over a wide OSNR range from 10 dB to 28 dB in a 112-Gb/s polarization-multiplexed quadrature phase shift keying (PM-QPSK) system when the launch power is up to 8 dBm and the transmission distance is as long as 2000 km. [ABSTRACT FROM AUTHOR]

The fabrication and characterization of IR chalcogenide polarization-maintaining (PM) step-index optical fibers with elliptical-core and 1-in-line-core have been reported for the first time. An improved isolated co-extrusion method was used to fabricate these core-shaped PM fibers. The elliptical core had a horizontal radius of a = 3.66 μm, vertical radius of b = 1.83 μm and the 1-in-line core of a = 4.83 μm, b = 1.42 μm, respectively. Single-mode PM beam spots were observed for the elliptical-core and 1-in-line-core fibers in the near-field energy distributions. The highest values of birefringence of the elliptical-core and 1-in-line-core fibers are 2.09 × 10 −4 at 2.7 μm and 3.272 × 10 −4 at 2.8 μm, respectively. The extinction ratios of −3.7 dB and −2 dB were achieved in fibers of 0.5 m long with elliptical-core and 1-in-line-core, respectively. [ABSTRACT FROM AUTHOR]

A study of the orthogonal polarization modes crosstalk changes in the point of different mechanical actions (pressure force) in the polarization-maintaining fiber with straining elliptical cladding is presented. It was found that by increasing of the pressure force the polarization extinction ratio increases nonlinearly. Also revealed the dependence of the extinction coefficient and the angle between vector of the mechanical action and polarization axes of the test fiber, which leads to change the extinction coefficient variable from −57 dB to −25 dB under the pressure force of 0.7 N. Also it was found that the cross angle of the fiber axes doesn’t influence on the extinction ratio value of the mechanical induced polarization crosstalk. [ABSTRACT FROM AUTHOR]

In this paper, we proposed and experimentally demonstrated simultaneous transmission of wired and wireless signals based on double sideband optical carrier suppression. By properly adjusting the bias point of the dual-output mach-zehnder modulator (MZM), a central carrier in one output port and a pair of first-order sidebands in another output port are generated. The pair of first-order sidebands are fed into a second MZM to generate second-order order sidebands. A wired signal is embedded on the central carrier while a wireless signal is embedded on the second-order sidebands. Unlike other schemes, we did not use optical filter to separate the carrier from the optical sidebands. The measured bit error rate (BER) and eye-diagrams after a 25 km single-mode-fiber (SMF) transmission proved that the proposed scheme is successful for both wired and wireless signals transmission. Moreover, the power penalty at the BER of 10 −9 is 0.3 and 0.7 dB for wired and wireless signals, respectively. [ABSTRACT FROM AUTHOR]

This paper proposes a multiband imaging method for optical fiber intrusion signals to realize the multi-dimensional analysis and the characteristic spectrum extraction. Firstly, the time-frequency analysis technique is performed on the initial collected intrusion signals to obtain the time-frequency distribution at each spatial intrusion position, on which energy band-wise integration is carried out along the frequency direction to generate spatial-temporal two-dimensional images in each frequency band. Then, these images are further stacked up as layers resulting in a spatial-temporal-frequency three-dimensional (3-D) cubic data. Based on this data, we extract the characteristic spectrums as the features of the intrusion signals. The real data experiments verify the effectiveness of this method. [ABSTRACT FROM AUTHOR]

Generally, orbital angular momentum (OAM) can be generated by combining two vector modes (VMs) or combining two linearly polarized modes (LPMs). The vortices generated by the first method are circularly polarized and carry spin angular momentum (SAM). While, the vortices generated by the second method are linearly polarized with no SAM. Although these two methods are closely related, they are essentially different. The difference and relation between the two methods are theoretically clarified in this paper. Two different kinds of OAM modes are experimentally demonstrated and validated in a two-mode fiber. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate a novel method based on electrical spectral interferometry with optical frequency comb for characterizing the modal dispersion between the higher-order modes and fundamental mode in a 1- km four-mode few-mode fiber (FMF). In this method, an optical frequency comb (OFC) is used to generate many subwavelengths with the same frequency interval to improve the measurement accuracy and speed. Based on the electrical spectral interferometry, the differential mode group delay (DMGD) and modal dispersion of the tested FMF are obtained by applying the simple FFT processing. The measured DMGD and modal dispersion within the whole C-band are about (3.2032–3.3033) ps/m , (2.5573–4.2085) ps/nm.km and (5.5055–5.7057) ps/m , (−11.2112–−4.1486) ps/nm.km and (11.1111–11.3113) ps/m , (−10.2454–−14.1332) ps/nm.km for LP11, LP21 and LP02 modes respectively. Meanwhile, we achieved a precision of ±0.002 ps/m for DMGD measurement. Compared with previous methods, our method has the advantage of high measurement accuracy, simple configuration and fast speed. The experimental results show that the demonstrated method could be a good solution to the characterization of a long FMF used in large capacity mode-division-multiplexing transmission systems. [ABSTRACT FROM AUTHOR]

We have demonstrated up to 56-Gb/s cost-efficient transmission for intra-data center interconnection over 10-km SMF-28 with negative power penalty. This 56-Gb/s transmitter comprises a low-cost 10-G-class directly-modulated distributed-feedback (DFB) laser without requiring expensive digital to analog converter (DAC), analog-to-digital converter (ADC) and power-consumptive digital signal processing (DSP). [ABSTRACT FROM AUTHOR]

To realize ultra-high capacity long-haul transmission, space-division multiplexing (SDM) has emerged as a promising solution, in which each data channel is modulated into an individual spatial/polarization modes in few-mode fibers (FMF) to increase the overall number of parallel channels. In this paper, we review the latest advances in SDM technology on the FMF, component, digital signal processing (DSP), as well as transmission demonstrations. First, we introduce the FMF characteristics, fabrication and manufacturing issues including modal dispersion, mode coupling, and nonlinearities. We next discuss in detail several key SDM components such as spatial multiplexers/demultiplexers (MUX/DeMUX), optical amplifiers, mode converters and SDM reconfigurable optical add-drop multiplexer (ROADM). Accordingly, we explore the DSP algorithms for SDM systems, covering least mean squares (LMS), recursive least squares (RLS), hardware complexity analysis, and mode dependent effects. Besides, a number of recent experimental validations are evaluated enabling higher transmission capacity for short, medium and long distances. [ABSTRACT FROM AUTHOR]

We propose a promising fiber nonlinearity-tolerant optical signal-to-noise ratio (OSNR) monitoring technique for high speed long haul polarization multiplexed-quadrature phase shift keying (PM-QPSK) Nyquist wavelength division multiplexing (WDM) coherent optical fiber transmission system, in which the correlation function between the symbols in all states of polarization as well as neighboring channels are exploited instead of the traditional GN model, thus it does not need the prior knowledge of the fiber transmission link nor the complicated integrals. The method is demonstrated to have the OSNR measurement error below 1.28 dB over a wide OSNR range from 10 dB to 28 dB in 100 Gb/s per channel PM-QPSK-Nyquist-WDM system when the launch power is up to 4 dBm and transmission distance is as long as 1600 km. [ABSTRACT FROM AUTHOR]

All-fiber distributed vibration sensing is attracting attention in relation to structural health monitoring because it is cost effective, offers high coverage of the monitored area and can detect various structural problems. And in particular the demand for high-speed vibration sensing operating at more than 10 kHz has increased because high frequency vibration indicates high energy and severe trouble in the monitored object. Optical fiber vibration sensing with phase-sensitive optical time domain reflectometry (phase-OTDR) has long been studied because it can be used for distributed vibration sensing in optical fiber. However, pulse reflectometry such as OTDR cannot measure high-frequency vibration whose cycle is shorter than the repetition time of the OTDR. That is, the maximum detectable frequency depends on fiber length. In this paper, we describe a vibration sensing technique with frequency-multiplexed OTDR that can detect the entire distribution of a high-frequency vibration thus allowing us to locate a high-speed vibration point. We can measure the position, frequency and dynamic change of a high-frequency vibration whose cycle is shorter than the repetition time. Both frequency and position are visualized simultaneously for a 5-km fiber with an 80-kHz frequency response and a 20-m spatial resolution. [ABSTRACT FROM AUTHOR]

In this work, we exploit the space/time duality in optics to implement a temporal lenticular lens allowing to simultaneously sample and magnify an arbitrary-shaped optical signal. More specifically, by applying a sinusoidal phase-modulation, the signal under test is propagated through a discrete dispersive element that samples and magnifies its initial waveform. Thanks to this temporal lenticular lens, optical sampling associated to an intensity magnification factor of 3.6 is experimentally demonstrated at a repetition rate of 10 GHz. [ABSTRACT FROM AUTHOR]

The paper presents the results of experimental investigations of the fiber optic hydrophone array consisting of six sensors, placed in one thin sensitive cable. Sensors were formed by pairs of Bragg gratings spaced 1.5 m apart and recorded in a birefringent optical fiber with the elliptical stressed coating. To form an extended sensor array the optical fiber was additionally covered with a silicone material RTV655 and protective coatings. Experimental investigations of the array showed that fiber-optic sensors pressure sensitivity increases as the acoustic frequency decreases at average value from −169.4 dB re rad/uPa at 495 Hz to −143.7 dB re rad/uPa at 40 Hz. The minimum detectable pressure was at average value from 53 mPa/√Hz at 495 Hz to 8.3 mPa/√Hz at 40 Hz. The obtained results might be used for developing and producing long thin hydroacoustic arrays for geophysical investigations and other hydroacoustic applications. [ABSTRACT FROM AUTHOR]