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Abstract: In this paper, a novel color-coding method for absolute phase measurement is proposed. A three steps phase-shift algorithm was adopted to calculate the relative phase (wrapped phase) because of the least need of images. Color strips were used to mark each 2π phase-change period and three neighboring colors as a group were used as codewords to identify the fringe orders. With the two parts, relative phase and fringe orders, the absolute phase can be obtained directly. We selected five colors (black, red, green, blue, and white) to test our algorithm, and nearly 60 different codewords were embedded in one image. The fringe orders and the relative phase are calculated simultaneously, and this algorithm can circumvent the phase unwrapping process. Moreover, errors will not diffuse, because the relative phase and fringe orders are uncorrelated. Besides, no need of unwrapping processing will lead to high speed in 3D profile measurement. In addition, large object can be measured with high precision using this method owe to enormous codewords can be embedded in one color image. One experiment with a small object and a big complex object was designed to test the algorithm for separate objects, and experimental results showed the validity of our algorithm for absolute phase measurement. [Copyright &y& Elsevier]

We have designed and configured a novel wavelength-tunable add/drop multiplexer (ADM) device that employs long-period gratings (LPGs) and piezoelectric ceramic fiber stretchers. The modeling for this ADM predicts that 50 International Telecommunication Union (ITU) dense wavelength division multiplexing (DWDM) channel signals could be selected in the wavelength range from 1526.25 to 1563.75 nm with 0.75-nm channel spacing, which covers the whole C-band gain spectrum of erbium-doped fiber amplifiers. The cross-talk between channels is less then −39 dB, and the total insertion loss of the device is about 0.24 dB which includes the splicing losses, the mode mismatching loss, and the expected losses in the two side-by-side coupled fibers. The tuning of add/drop channels within a limited region can be provided by changing the voltage, which is applied to the piezoelectric ceramic fiber stretchers. This technique is suitable for the in-line implementation of optical communication networks. [Copyright &y& Elsevier]

Abstract: A continuous-wave pumped, all-fiber optical parametric oscillator (OPO) around 1523nm based on the mixing interaction of parametric process and stimulated Raman scattering (SRS) effects are investigated in this paper. We study and give the detailed analysis of the generation and output characteristics of all-fiber OPO with the influence of the SRS effects. Experimental results show that there exists the saturation output effect for this kind of OPO due to the coexistence of first order SRS effect, second order SRS and the optical parametric oscillation. The maximal output power of OPO is about 100mW as the pump power reaches 1.4W. Furthermore, a depolarized L-band super-continuum light source from 1570nm to 1640nm based on the combined interaction of parametric process and SRS effects can also be obtained. [Copyright &y& Elsevier]

Mode-division multiplexing (MDM) technology, an effective approach to boost the transmission capacity, has seen momentous progress in the past decade. Wideband, low-noise, and power-efficient optical amplifiers for the MDM systems are highly desired. We propose and theoretically demonstrate a broadband few-mode optical parametric amplifier (OPA) using a multimode thin-film periodically poled lithium niobate (PPLN) waveguide with multiple quasi-phase matching (QPM). The few-mode OPA relies on intramodal and intermodal difference-frequency generation (DFG) processes, obtaining strong parametric amplification of MDM signals using a single-frequency fundamental-mode pump. Differential modal gain (DMG) is optimized by adjusting the effective length of parametric interaction for each mode. In addition, parasitic DFG processes to cause nonlinear modal crosstalk is suppressed due to the large phase mismatch ensured by the multiple QPM design. The results show that amplification of signals carrying three modes with >11.2-dB gain per mode over 41 nm bandwidth covering extended C band can be achieved with a 600-mW pump, while the DMG is less than 1.1 dB and nonlinear modal crosstalk is negligible. [ABSTRACT FROM AUTHOR]

The nonlinear distortion resulted from the Kerr effect in optical fibers limits the transmission capacity in long-haul optical fiber communications. In wavelength-division multiplexing (WDM) systems, the inter-channel nonlinear distortion is generally stronger than the intra-channel one, and also more difficult to be compensated, despite that there exist many reports on the nonlinearity compensation methods. Recently, recurrent neural networks have emerged as a potential solution without relying on detailed transmission link information. However, these networks often suffer from high computational complexity and lack interpretability due to their black-box nature. In this paper, we propose a novel overlapped-dense long short-term memory network (OD-LSTM). It can track the correlation of cross-phase-modulation (XPM) and consequently is capable of compensating the inter-channel nonlinear distortion. For a lower complexity, the overlapped-dense structure reduces the hidden units at time steps of different regions, since the correlation of XPM is reduced for the more distant symbols. To evaluate the effectiveness of our proposed scheme, we conduct a numerical simulation on a nine-channel WDM coherent optical transmission over a 1600-km standard single-mode fiber (SSMF) with DP-16QAM at 40 Gbaud. At the optimal optical power of −3 dBm, we observe a 0.52 dB Q-factor gain with the OD-LSTM equalizer, outperforming the digital back-propagation (DBP) with three steps per span and the Co-LSTM regressor. In terms of complexity, our proposed approach achieves a 13.5 % reduction compared to the Co-LSTM classifier, and its complexity is slightly lower than that of the ultralow complexity Co-LSTM regressor. We also discuss the visual understanding of the OD-LSTM, which provides insights into the internal mechanisms of the network. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we propose a novel photonic crystal fiber design to achieve high birefringence, high nonlinearity with a flattened dispersion profile. The air holes which are embedded in silica are arranged in triangular lattice configuration. In the core region, an air hole is introduced as the central defect to flatten the dispersion curve. The shape of the defect air hole is deliberately designed elliptical so that high birefringence in the PCF is obtained for polarization-maintaining purpose. The structural parameters are carefully selected to optimize the optical properties. [Copyright &y& Elsevier]

Abstract: We propose a novel tunable photonic crystal fiber (PCF) coupler scheme, that the light is coupled between a total internal reflection (TIR) guiding core and a photonic bandgap (PBG) guiding core. By introducing the thermo-optic responsive nematic liquid crystals (NLC) into the air holes around the PBG core, the hybrid PCF based coupler exhibits enhanced thermal tunability. The thermo-optical characteristics of the proposed coupler are numerically studied using finite element method (FEM) and beam propagation method (BPM). The proposed design can find promising applications in filtering and sensing devices. [Copyright &y& Elsevier]

Abstract: We propose a three-dimensional (3D) finite-difference time-domain (FDTD) method to analyze the pulse propagation characteristics in microstructured optical fibers (MOFs). The computation domain size is greatly reduced by adopting the technique of moving problem space. The propagating pulse is virtually held in the buffer cell of the problem space as simulation continues. This method is capable to investigate the temporal evolution of the propagating pulse. Spectral information can be obtained by Fourier analysis. As an example, the influence of the kerr nonlinearity on the optical pulse propagation in a Lorentz dispersive MOF is demonstrated. The model is also used to simulate the nonlinear interactions between the pump spectral broadening and third harmonic generations in a highly nonlinear fused silica nanowire with good agreement with the generalized nonlinear envelop equation (GNEE) model. [Copyright &y& Elsevier]

Abstract: The design of fiber Bragg grating to achieve different chirp amount in the selective transmission channel is presented. The method depends on the combination effect of introduced equivalent chirp with the linearly inherent chirp provided by the phase mask. Since these two chirps can be incorporated, or be counteracted, the different chirp amount can be easily obtained. [Copyright &y& Elsevier]

Due to transmission impairments, optical regeneration is needed to extend the length of a lightpath beyond its transparent length. Spare transmitter (Tx) and receiver (Rx) modules in an optical network node can be used for optical regeneration. A Max-spare algorithm for locating the regeneration nodes for a lightpath is proposed and compared with a Greedy algorithm in conjunction with two routing algorithms, namely, wavelength weighted (WW) and length weighted (LW) routing algorithms. Through simulation we find that significant number of lightpaths that cannot be established due to the violation of transparent length can be established if the regeneration algorithm is used to select transit nodes that have spare Tx and Rx for 3R regeneration. The translucent optical network with limited number of transceivers can still perform close to a transparent optical network with full set of transceivers. We find that for the same routing algorithm, the Max-spare algorithm yields better results than the Greedy algorithm. In addition; for the same regeneration nodes selection algorithm, the WW algorithm has better performance than the LW algorithm. [Copyright &y& Elsevier]

In this paper, a joint baud-rate and modulation format identification (BR-MFI) is proposed based on asynchronous delay tap picture (ADTP) analyzer by using convolutional neural network (CNN). Considering 8 types of signals under different channel conditions of OSNR, CD and DGD, the proposed BR-MFI can achieve 100% accuracy after 6 training epochs, and just 2 epochs for MFI. Here, two test number of samples are about 15% of total samples. This paper also investigates the influence of CNN structure on the identification accuracy. The results show that CNN has better performance for image processing than back-propagation Artificial Neural Network (BP-ANN). [ABSTRACT FROM AUTHOR]

A comprehensive comparison among different post-equalizers with relatively low complexity is studied for C-band 100-km dispersion-uncompensated standard single-mode fiber (SSMF) intensity modulation and direct detection (IM/DD) transmission systems with data rate beyond 100 Gb/s/ λ. These representative post-equalizers include feed-forward equalizer (FFE), polynomial-based nonlinear FFE (P-NFFE), absolute-term based nonlinear FFE (AT-NFFE), FFE combined with decision-feedback equalizer (FFE-DFE), polynomial based nonlinear FFE-DFE (P-NLE), absolute-term based nonlinear FFE-DFE (AT-NLE), and their lower-complexity versions of FFE-DFE-WS, P-NLE-WS, and AT-NLE-WS which use weight sharing (WS) to eliminate weight redundancy. We demonstrate 100-Gb/s/ λ and 112-Gb/s/ λ on-off keying (OOK) transmissions over a 100-km SSMF. For 112-Gb/s transmission, BERs of FFE-DFE(-WS), P-NLE(-WS), and AT-NLE(-WS) can reach 7% hard-decision forward error correction (HD-FEC) limit at 3.8 × 10 − 3. Under 7% HD-FEC limit, AT-NLE-WS with 32 real-valued multiplications shows similar BER performance as P-NLE and AT-NLE with a small power penalty of < 0.4 dB, but outperforms P-NLE-WS, AT-NLE, and P-NLE in huge complexity reduction of approximately 83.2%, 92.4%, and 92.7%, respectively. Therefore, taking both the BER performance and the operation complexity into account, AT-NLE-WS is the optimal equalizer and is a strong candidate to be applied in low-cost C-band IM/DD interconnections with data rates beyond 100 Gb/s/ λ and transmission distance up to 100 km. • The demonstrations of ≥ 50-Gb/s and ≥ 100-km IM/DD transmissions in terms of DSP, data rate, distance, and FEC threshold are summarized • A comprehensive comparison among different receiver-side equalizers is conducted for C-band 100-km SSMF based IM/DD systems. • The AT-NLEWS is a strong candidate to be applied in low-cost C-band IM/DD interconnections with transmission distance up to 100 km. [ABSTRACT FROM AUTHOR]

We discuss the mode properties for orbital angular momentum (OAM) modes supporting multi-mode ring-core fiber (MM-RCF). For strongly-guiding MM-RCF, by introducing one more layer to the ring-core, a two-layer ring-core fiber (TL-RCF) is proposed. It is proven that this TL-RCF can increase the separation degree between the vector eigenmodes and meanwhile weaken the spin–orbit coupling. For mode-group supporting MM-RCF, the maximum mode number is limited by the thickness of ring-core and the cut-off of radial higher-order mode. We also present the tradeoff relationship between the mode group effective index difference and inter-eigenmode differential mode delay in one mode group. The discussion in this paper can give guidance for the design of MM-RCF. • Describe why OAM mode need a ring-type core in detail. • Introduce two categories of OAM mode fibers, which are strongly-guiding fiber and mode-group supporting fiber. • Propose a two-layer ring-core fiber to separate the eigenmodes and weaken the spin-orbit coupling. • Reveal the tradeoff relationship between the mode group Δ n eff and inter-eigenmode DMD. [ABSTRACT FROM AUTHOR]

Abstract: A simple broadband and linear photonic RF phase shifter is proposed and experimentally demonstrated based on distributed Bragg reflector (DBR) fiber lasers and polarization sensitive effect of optical phase modulator. Photonic RF signal sources with frequency range from 1.6186-GHz to 3.7016-GHz are investigated. The full 360° phase shift of the broadband photonic RF signals can be achieved and tuned stably, continuously and linearly with low electrical control voltage. [Copyright &y& Elsevier]

Abstract: We have analyzed the thermal influence on the bandgap properties of liquid-crystal photonic crystal fibers. The bandgap parameters which affect the transmission conditions have been investigated. It is observed that the photonic bandgap can be thermally tuned, i.e. the red or blue shift of the bandgap results from the temperature dependence of the refractive index of the liquid crystal. For the planar alignment of liquid-crystal filled cladding, the ordinary refractive index plays a major role in determining the bandgap properties; the extraordinary refractive index comes into influence while the ordinary refractive index is relatively constant of temperature. The analyses agree well with the experiments results. [Copyright &y& Elsevier]

Abstract: This communication reports a simple configuration for the implementation of a compact and all-fiber based width-tunable pulse generator using four-wave mixing (FWM) effect in only 1.9-m of bismuth oxide based highly nonlinear fiber (Bi-HNLF). The width- tunable pulses are generated at repetition rates of 5 and 10GHz. In the performed experiment, the pulsewidth of a 5GHz repetition rate pulse train is tuned from 83 to 23ps whereas the pulsewidth of a 10GHz repetition rate pulse train is continuously tuned from 32 to 18ps. An analytical model is presented for width-tunable pulse generation and the results agree well with the experimental findings. The effects of pump source timing jitter and intensity fluctuation on pulse generation are discussed. [Copyright &y& Elsevier]

Abstract: A study on fiber Bragg grating (FBG) strain sensor, based on erbium-doped fiber (EDF) laser, is presented. A strain-sensing element, FBG, also acts as the lasing wavelength selecting component. When strain is applied on the FBG, the laser cavity loss changes, leading to a modification of the laser transient. Strain measurements are obtained in the time domain by simply measuring the EDF laser build-up time. Relative variation in the build-up time of up to 190%, for a strain range from 0με to 2350με, is achieved with a resolution corresponding to a strain of better than 2.35με. This study demonstrates a novel fiber sensor concept and the technical feasibility to develop fiber strain measurement. [Copyright &y& Elsevier]

Abstract: This paper presents the detailed principle on the generation and detection of optical minimum shift keying (MSK) format for high-speed and high spectral efficiency WDM systems. 10.7Gb/s optical MSK data is successfully generated by experiment using the proposed scheme. The results show that optical MSK signal exhibits very compact optical spectrum, which is promising for high spectral efficiency WDM applications. Simulation results on dispersion and nonlinear tolerance comparison confirm that MSK signal has better dispersion tolerance and nonlinear tolerance compared with 50% duty cycle return-to-zero differential phase shift keying (RZ-DPSK) and 50% duty cycle RZ on-off-keying (RZ-OOK) formats. [Copyright &y& Elsevier]

Abstract: In this paper, we first derived the BER expression of an optical channel with chromatic dispersion impairment. The derived BER expression takes into account the influence of pulse spreading, transmitter rise/fall times, receiver Q-factor and noises. It is simpler and hence easier to use than many existing channel BER models. Then we proposed a wavelength-interleaved FEC scheme to mitigate the chromatic dispersion impairment in optical transmission system and analyzed its decoding performance. The minimum wavelength separation required for such a wavelength-interleaving FEC system to obtain the highest coding gain is also determined. Based on this design, the chromatic dispersion tolerance of the proposed system with 2-wavelength and 4-wavelength-interleaving are shown to improve by about 13% and 22%, respectively, over a non-interleaved system. In essence, wavelength-interleaving averages out the performances of a set of channels experiencing higher and lower dispersion, hence the interleaved channels have better performance than the non-interleaved channels experiencing higher dispersion, and worse performance than the non-interleaved channels experiencing lower dispersion. However, since WDM systems are typically designed based on the worst-case channel, the overall system performance is improved by wavelength-interleaving. [Copyright &y& Elsevier]

Abstract: Based on dual-order stimulated Raman scattering (SRS) of a single 1395nm Raman fiber laser in 75km single mode fiber and its corresponding dispersion compensation module, a hybrid Raman/Erbium doped fiber amplifier (EDFA) for long wavelength band (L-band) amplification is realized by inserting a segment of EDF within the span. By comparing the performance of gain and noise in four hybrid amplifiers with different span configurations, we find that the distribution of the secondary L-band amplification obtained from the EDF along the link has a great influence on the performance of the hybrid amplifier. Both gain and noise performance of hybrid amplifier can be improved significantly by optimizing the location of the EDF. Moreover, we can extend the flat gain bandwidth from L-band to central wavelength band (C-band) plus L-band by recycling the residual first-order SRS to pump a segment of EDF with proper length. [Copyright &y& Elsevier]

Abstract: We consider and investigate an improved chromatic dispersion monitoring method using two RF tones with an inserted dispersion offset. This improved technique can be used to monitor both the positive and negative accumulated dispersion caused by optical fibers as well as other optical components in optical networks. We experimentally demonstrate that the monitoring range of the improved technique can be greater than 1150ps/nm and the monitoring sensitivity better than 0.064dB/ps/nm by selecting appropriate RF frequencies and dispersion offsets. Our investigations reveal that the RF modulation index should be greater than 10% but less than 20% so as to acquire a large monitoring range with a small power penalty. We also examine the CD monitoring errors caused by polarization mode dispersion (PMD) and self-phase modulation, and show that the use of a dispersion offset can effectively reduce the PMD-induced monitoring errors. [Copyright &y& Elsevier]

Abstract: We propose a novel remote Raman pumps monitoring technique based on the pump-to-signal four-wave-mixing (FWM) in a 50-m long highly nonlinear dispersion-shifted photonic crystal fiber. The health information of the pumps can be reproduced to the new FWM signal accompanying the transmission signal through the pump-to-signal FWM. As a result, the modulation frequency limitation for cross gain modulation can be avoided effectively, and no degradation of the transmission system is induced by this technique. [Copyright &y& Elsevier]

Abstract: We propose and demonstrate a temperature insensitive fiber Bragg grating (FBG) sensor interrogation system that uses a highly birefringent photonic crystal fiber (HiBi-PCF) Sagnac loop filter. Because of the high birefringence and the low temperature sensitivity of the HiBi-PCF, the loop fiber length is greatly reduced and the temperature stability of the system is dramatically improved. Experiment has been carried out by interrogating a FBG with applied strain. Good performances have been obtained. [Copyright &y& Elsevier]

We propose and characterize a dispersion slope compensator using a Fabry–Perot Etalon formed by double FBGs. Enhanced delay and dispersion by the multiple round trips of the light in the Fabry–Perot cavity are obtained using two uniform gratings, which have small delay and dispersion. The different shape of the dispersion and dispersion slope envelope can be gotten by the appropriate design of Fabry–Perot Etalon. [Copyright &y& Elsevier]

We report what is believed to the first example of graded index multimode fiber slanted grating. The gratings are realized by the same technique as the single mode fiber gratings. The periodic perturbation causes fundamental core mode to higher order core modes, cladding modes and radiation modes coupling. There is a unique feature in the spectra of certain angle-slanted gratings (2.5°–4°). Hope this phenomenon can cause a new type of fiber components that can be widely used in the LAN and sensing systems. [Copyright &y& Elsevier]

We report the development and demonstration of a long-period grating refractometer. The principle of operation is based on the using of a long-period grating that is structurally induced by a CO2 laser, and where the resonance wavelengths are shifted as the refractive index of medium surrounding the cladding of the long-period grating. The different concentrations for three types of solutions (ethylene glycol, salt, and sugar) were experimentally measured, and results show that, as a refractometer, this fiber-based device not only can differentiate chemicals based on their refractive index, but it can also become a concentration indicator of a particular chemical solution, and applied in the oil and petroleum industry. [Copyright &y& Elsevier]

A novel method was suggested about pedestal reduction and chirp elimination of high-order compressed solitons by using SOA and tunable filter. Results of numerical simulation demonstrate that, under certain conditions, the pedestals and chirp can be successfully mitigated without pulse broadening. [Copyright &y& Elsevier]

In this paper, an asynchronous single channel sampling (ASCS) based joint modulation format and baud rate identification (MF–BRI) method is proposed for M-QAM optical systems using Convolutional Neural Network (CNN), which significantly reduces the cost and complexity of monitoring system setup. Based on the three influencing factors of OSNR, CD and DGD, three sampling rates are analyzed to generate ASCS images to build the database. The simulation and experiment accuracy of MFI and MF–BRI is above 99.8%. We also discuss the differences between two-channel sampling (TCS) and single-channel sampling (SCS). [ABSTRACT FROM AUTHOR]