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A detailed theoretical study is conducted on the nonlinear interference in the same-wavelength bidirectional coherent optical fiber communication systems. The Gaussian noise (GN) model used to evaluate nonlinear interference (NLI) in unidirectional systems is applied and extended to bidirectional transmission scenarios. The extended NLI model shows that in a bidirectional transmission communication system, the backward signal almost does not introduce additional nonlinear crosstalk to the forward signal due to the strong walk-off effect between forward and backward transmitted signals. Specifically, the ratio of the nonlinear crosstalk introduced by the forward and backward signals is about 21 dB, which means that the traditional GN model is also applicable in the bidirectional scenario. This conclusion is validated on the platform of a same-wavelength bidirectional coherent optical communication system based on Optisystem software.

In this paper, an interference fading mitigation method based on subband phase-shift transform in coherent phase-sensitive optical time-domain reflectometer ($\Phi$-OTDR) is proposed. The operation principle of the subband phase-shift transform with complementary amplitude characteristic is presented. The phase shift transform is performed in the digital domain without requiring complex frequency modulation. After subband phase-shift transform, the interference fading can be effectively mitigated through optimal tracking and rotated vector sum (RVS) methods. The experimental results show that the fluctuation of phase intensity is reduced from $\sim$70 dB to $\sim$20 dB and the average standard deviation of the recovered phase is also reduced from 0.223 rad to 0.046 rad. In addition, different disturbance signals are successfully recovered, verifying the effectiveness of the proposed method.

Downscaling Chlorophyll-a (Chl-a) concentration derived from satellite image is crucial for refined applications such as water quality monitoring. However, the precision of downscaling is usually constrained by various environmental factors. In this paper, we develop a downscaling method for Chl-a concentration to improve precision, especially for inland lakes with different surrounding environment. The method downscales the Sentinel-3 Chl-a concentration from 300 m to 30 m, based on the multivariate analysis (MVA) and the gradient boosting decision tree (GBDT) model. Firstly, we analyzed 21 Chl-a concentration related indices to identify optimal factors for Chl-a concentration variability. Secondly, a GBDT model is constructed to convey the non-linear relationship between the optimal factors and Chl-a concentration at coarse resolution. Finally, fine-resolution Chl-a concentrations were produced by employing the model to refine cofactors for 12 distinct lakes. The results indicated that the proposed MVA-GBDT method effectively inferred the variability of Chl-a concentration with a mean RMSE of 4.505 mg/m3, an improvement of 5%–39% over other methods. Furthermore, for lakes with large water quality heterogeneity, the method led to a cross validation RMSE and a difference in accuracy of 5.371 mg/m3 and 0.866 mg/m3, respectively. In addition, this study examined the significance of the auxiliary factors and found that the NDCI and WST were the two most important factors for MVA-GBDT to detect Chl-a concentration distributions, particularly for NDCI in lakes with high nutrient contrasts. These findings contribute to the generation of fine-scale Chl-a concentrations in lakes and support related applications.

The inter-data center links are meeting the challenge of increasing transmission capacity continuously. Here, the tradeoff between transmission capacity and cost and power consumption especially needs to be considered. The field modulation polarization-division-multiplexed (PDM) four-level pulse amplitude modulation (PAM4) with coherent detection is a potential candidate for 400 Gb/s and beyond inter-data center transmissions, which has several advantages including low-cost, low optical signal to noise ratio (OSNR) requirement and high receiver sensitivity. In this paper, the feasibility and effectiveness of the field PDM-PAM4 coherent system are investigated over multi core fiber (MCF). In order to adapt to the field PAM signals, the signal-phase aid least-mean square (SP-LMS) and training multi-modulus algorithm (TMMA) are modified and analyzed for equalization and polarization demultiplexing. Finally, it is demonstrated that 4×112 Gb/s field PDM-PAM4 is successful transmitted over 16 km MCF and 384 km SSMF in C-band with a BER below 3.8e-3.

In this paper, a nonlinear model is established for the Kaplan turbine generator set (KTGS) with dual input governor and generator excitation to solve unsatisfactory regulation performance caused by the deviation from the combination curve during the transient process. According to the model characteristics, the constraint effect of the objective function in multi-objective feedback nonlinear control design is utilized to replace the associative relationship between the wicket gate and the blade. Then, a three-variable integrated nonlinear control strategy (TVINCS) is applied to the wicket gate, blade governor, and generator excitation. Simulation results demonstrate the precise tracking performance of hydraulic and electrical parameters controlled by TVINCS. The controller is designed to guarantee the turbine’s output efficiency, and effectively coordinate the wicket gate and blade actions in the dynamic regulation process, providing a superior dynamic adjustment performance for the system under various disturbances.

This paper presents the balanced-to-balanced Gysel filtering power divider with arbitrary power division for first time. Based on the multicoupled series-resonator bandpass prototype network, a detailed theory analysis is presented and the corresponding design equations are concluded. Two filtering power dividers with the division ratio of 2 and 3 are realized using the half-wavelength resonator and the short-stub-loaded resonator. Finally, two example circuits are fabricated and measured for verification. The measured results show good agreement with the simulated and confirm the theory.

Regional electricity consumption presents important network structure features. This article applies the social network approach to construct the regional electricity consumption network in China during the period from 2000 to 2018. It analyses global features, local features and the block division of the networks. Then, the dynamic effect of the global and local features on regional electricity intensity is empirically verified through a quantile regression model. The results first show that the spatial correlation of regional electricity consumption has formed a network structure, exhibiting the characteristics of increasing network density, from 0.2 to 0.24, and decreasing network hierarchy and network efficiency, with network hierarchy decreasing from 0.73 to 0.24 and network efficiency decreasing from 0.64 to 0.56; second, the regional electricity consumption network can be divided into four blocks that play different roles and interact to maintain the sustainable evolution of the network; and third, changes in network structure can have a significant effect on regional electricity intensity, and overall, the effect of density is approximately -0.96, and the effects of hierarchy and efficiency are 0.15 and 0.46, respectively. Finally, some policy suggestions beneficial for reducing regional electricity intensity are provided.

We propose a direct detection scheme for pulse amplitude modulation (PAM) signals over long-reach fiber transmission with phase noise mitigation. In this scheme, an optical carrier is transmitted as pilot together with the optical signal, whose frequency is located at the edge of the optical signal. The electrical field of the optical signal can be retrieved via the beating between the pilot and optical signal in one photodetector at the receiver side, which facilitates the digital compensation of chromatic dispersion effect. To enable real value signal processing, digital carrier regeneration is proposed to mitigate the phase noise fluctuation in the proposed scheme. To further improve the performance, the digital signal processing including Kramers-Kronig field reconstruction and Volterra equalizer are applied to reduce the signal-to-signal beat noise and nonlinear distortions after fiber transmission. The experimental results for both 56- and 64-Gb/s PAM-4 signal transmission cases show that the performances can be significantly improvement based on the proposed scheme and DSP methods. Both 56- and 64-Gb/s PAM-4 signal transmissions over 720- and 1440-km standard single mode fiber have been successfully achieved under 7% and 20% forward error correction codes, respectively.

We propose a signal-to-signal beating interference (SSBI) cancellation scheme in two-mode fiber (TMF) transmission system based on balanced detection without requirement of a guard band. The SSBI can be eliminated based on the mode selectivity characteristic of mode coupler. The mode selectivity in mode coupler relies on the coupling behavior between the fundamental mode LP01 and the first-order mode LP11, which is highly dependent on the spatial orientation of the fiber modes. We design and fabricate a fused-type fiber mode coupler (FFMC) based on circular-core TMF. Based on the fabricated FFMC, we experimentally demonstrate a multi-band 100-Gb/s optical orthogonal frequency division multiplexing signal transmission over 10-m TMF with SSBI cancellation. Significant performance improvement is observed when compared with conventional direct detection scheme under the condition of insufficient bandwidth of a guard band.

A low-complexity mobile fronthaul architecture via digital code-division multiplexing (CDM) is proposed to enable channel aggregation of 4G-LTE signals. In comparison with traditional frequency division multiplexing based aggregation scheme, the fast Fourier transformation/inverse fast Fourier transformation operations are replaced by simple sign selection and addition, leading to the significant reduction of computational complexity. Moreover, synchronous transmission of both the I/Q waveforms of wireless signals and the control words (CWs) used for the purpose of control and management using the CDM approach is also presented to be compliant with the common public radio interface (CPRI). In a proof-of-concept experiment, we demonstrate the transmission of 48 × 20 MHz LTE signals with CPRI equivalent data rate of 59 Gb/s, achieving an average error vector magnitude (EVM) of ~3.6% and ~4.3% after 5 and 20 km transmission over standard single-mode fiber (SSMF), respectively. Furthermore, we successfully demonstrate the transmission of 32 × 20 MHz LTE signals together with CPRI-compliant CWs, corresponding to CPRI-equivalent data rate of 39.32 Gb/s, only using single optical wavelength channel with analog bandwidth of ~1.96 GHz. After transmission over 5 km SSMF, CWs can be error-free recovered while the LTE signals are recovered with an EVM of ~3.6%.

Collision detection and avoidance between solid bodies are one of the most important problems in path planning for robotics machining or multi-axis machining. While planning toolpath for multi-axis milling, accurate collision detection is usually time-consuming among complex solid bodies in the computer environment. Furthermore, how to avoid the collision automatically within limited space in the path planning stage still needs lots of experience. To this end, this paper presents a general collision detection and tool posture automatic adjustment approach for the multi-axis milling process. First, by analyzing the contact state of the tool-workpiece, the calculation model of the interference quantity is determined. A unified tool constraint mathematical model based on the interference quantity and the interference type is established. Second, three types of tool adjustment strategies are constructed, the sequential quadratic programming method is used to solve the model and the graphics processing unit-based high-performance computing technology is employed to accelerate the solution process. Finally, the developed method is validated for automatic collision and tool posture adjustment in the five-axis milling of a blisk. The presented method can be integrated into commercial CAD/CAM software for rapid tool collision detection and tool orientation modification.

Machining efficiency could be increased and chatter vibrations could be suppressed by employing irregular pitch angles onto milling cutters. Applying variable pitch cutters could improve machining stability by disrupting the regenerative chatter mechanism and reducing cutting forces due to decreased nonlinear dynamic chip thicknesses. Previous studies on variable pitch cutters have been mainly investigating vibration suppression and dynamic modeling. There is a lack of efficient and practical variable pitch cutter design methods for practical industrial applications. An analytical method for designing milling cutters with alternating variable pitches has been proposed in this paper. In this method, a new variable, the generic sum of phase difference between the inner and outer waves, has been defined to model the machining dynamics and the stability of the variable pitch cutter. The phase difference value has then been investigated to obtain its generic optimal solution to the stability limit considering different cutting conditions. Based on this, the analytical variable pitch cutter design method is derived. The experimental results have manifested that, compared with uniform pitch cutter, the variable pitch cutter, designed using the proposed method, was able to achieve: 1) a critical stable axial depth of cut improvement of 126% under the desired spindle speed; 2) a cutting force reduction of 53%; 3) a decrease of 75% and 52% in Sq (the root mean square length of the scale limited surface) and Sz (the maximum height of the scale limited surface) of the machined surfaces, respectively. These significant improvements approved the superiority and robustness of the proposed design method for chatter suppression and may promote the application of variable pitch cutter technologies.

In the milling process, large tool diameter can get higher rigid and machining efficiency. To improve machining efficiency for 2 1/2-axis part, an image skeleton-based method is proposed in this paper. First, a rasterization-based method for converting the part section graphics into an image for image analysis with high accuracy is developed. Second, the Euclidean distance field is calculated for the image to get the initial skeleton; then to get the accurate skeleton of the image, a partial differential equation-based method is employed to refine the initial skeleton. After that, an objective function based on the residual area of the material is established, which is solved by the genetic algorithm to obtain the optimal tool combination for a given number of tools. Finally, the developed approach is validated in the milling of a port part for tool selection. Results show that the selected toolset can remove most of the material effectively. Furthermore, analysis results show that the increase of tool types will increase the available cutting area ratio, but it will decrease the growth.

In this paper, a delta-sigma modulation is proposed for the orthogonal frequency division multiplexed-passive optical network (OFDM-PON) to enable transmission of OFDM signals via binary intensity modulation and direct detection (IM-DD) channels. The delta-sigma modulation is an independent digital signal processing block that converts the analog OFDM signal into binary bit sequence. The OFDM signal can be simply retrieved from the bit sequence, using a low-pass filter at the receiver. Neither the analog-to-digital converter (ADC)/digital-to-analog converter (DAC) nor the linear modulator/detector are required, which means the proposed system can be fulfilled by employing cost-effective 10- or 40-Gb/s binary optics and electronics. Besides, the delta-sigma-modulated OFDM-PON is able to offer excellent robustness against noises and nonlinear distortions. In the experimental demonstration, the performance of proposed delta-sigma-modulated OFDM-PON system has been investigated. It is found that around a 4-dB improvement in receiver sensitivity is achieved, compared to a traditional analog OFDM-PON scheme.

We experimentally demonstrate on-chip dual-channel all-optical AND logic operations based on four-wave mixing (FWM) for on-off keying (OOK) signals in a multimode silicon waveguide. A two-mode (de)multiplexing circuit consisting of tapered directional coupler-based (de)multiplexers and a multimode waveguide is designed and fabricated for this application. Experimental results show open eye-diagrams and confirm the logic operations with moderate power penalties for both modes.

Single-channel 112-Gb/s PAM-4 transmission based on low-cost intensity modulation and direct detection (IM/DD) optics is experimentally demonstrated over 1-km standard single-mode fiber. By employing a digital precompensation, duobinary encode/decoding with PAM-4 signal and 7-level training-sequence-aided least mean square (TS-LMS) algorithm, we successfully achieve a receiver sensitivity of about -2 dBm with 7% overhead HD-FEC. Chirp management is applied at the transmitter side to extend the reach to 1 km for transmission at 1500 nm. This is the first demonstration, to the best of our knowledge, that 112-Gb/s PAM-4 modulation is achieved using only a 18-GHz (3-dB-bandwidth) commercial directly modulated laser. The method proposed in this paper is both bandwidth and computationally efficient, which is thought to be feasible in the low-cost short-reach optical applications.

Digital coherent superposition (DCS) for optical orthogonal frequency-division multiplexing (OFDM) subcarrier pairs with Hermitian symmetry can simultaneously reduce the inter-carrier interference (ICI) from laser phase noise and cancel the fiber nonlinearity to the first order. In this paper, we study an interesting scenario, where we upgrade the conventional OFDM with 4-quadrature amplitude modulation (QAM) to DCS-OFDM with 16-QAM, which have equal spectrum efficiency and require no change on hardware. We conduct a simulation to compare the tolerance to the laser phase noise and fiber nonlinearity, which are the main concerns for such an upgrading. First, the 16-QAM DCS-OFDM relaxes the laser linewidth requirement and performs better with a smaller fast Fourier transform size. Second, the nonlinear tolerance of 16-QAM DCS-OFDM is better when the nonlinear effects appear. The nonlinear tolerance of DCS-OFDM depends on the dispersion compensation scheme. The maximum SNR improvement is 3.4 dB with full post-dispersion compensation, and it can increase to 6.6 dB with symmetrical dispersion compensation at the launch power of 4 dBm. Finally, we verify that the maximum SNR improvement of 16-QAM DCS-OFDM with full post-dispersion compensation is 3.7 dB at the launch power of 6.2 dBm in the experiment.