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The mode-division multiplexing technique combined with a few-mode erbium-doped fiber amplifier (FM-EDFA) demonstrates significant potential for solving the capacity limitation of standard single-mode fiber (SSMF) transmission systems. However, the differential mode gain (DMG) arising in the FM-EDFA fundamentally limits its transmission capacity and length. Herein, an innovative DMG equalization strategy using femtosecond laser micromachining to adjust the refractive index (RI) is presented. Variable mode-dependent attenuations can be achieved according to the DMG profile of the FM-EDFA, enabling DMG equalization. To validate the proposed strategy, DMG equalization of the commonly used FM-EDFA configuration was investigated. Simulation results revealed that by optimizing both the length and RI modulation depth of the femtosecond laser-tailoring area, the maximum DMG (DMGmax) among the 3 linear-polarized (LP) mode-group was mitigated from 10 dB to 1.52 dB, whereas the average DMG (DMGave) over the C-band was reduced from 8.95 dB to 0.78 dB. Finally, a 2-LP mode-group DMG equalizer was experimentally demonstrated, resulting in a reduction of the DMGmax from 2.09 dB to 0.46 dB, and a reduction of DMGave over the C band from 1.64 dB to 0.26 dB, with only a 1.8 dB insertion loss. Moreover, a maximum range of variable DMG equalization was achieved with 5.4 dB, satisfying the requirements of the most commonly used 2-LP mode-group amplification scenarios.

The ever-increasing demand for the transmission capacity has stimulated intensive investigations, in terms of both new transmission window of standard single mode fiber (SSMF) and new fiber utilizing the spatial domain. Multi-band wavelength division multiplexing (WDM) transmission exploring the extended wavelength band, besides traditional C+L band, is an attractive solution for the rapid and cost-effective enhancement of transmission capacity. To this end, here we demonstrate the U-band WDM transmission over 125-km G.652.D fiber, when the C-band amplified spontaneous emission (ASE) source acts as the incoherent pump for Raman amplification (RA). Meanwhile, bidirectional long short-term memory neural network (BiLSTM-NN) is applied to inversely design the spectral shape of two C-band ASE sources. After the optimization through inverse design, the ROP variation at the U-band can be less than 1.2 dB under different net Raman gains. Consequently, we can experimentally achieve a net Raman gain of ∼15 dB over a 3 dB bandwidth of around 2.1 THz at the U-band, under the condition of distributed bidirectional Raman pump configuration. Finally, the performance of the U-band RA is experimentally verified by transmitting three-wavelength 20 GBaud DP-QPSK signals, when the threshold of 7% HD-FEC can be successfully reached.

Driven by the exponential increase of network traffic, the demand for the capacity enhancement of photonic data-center interconnection (DCI) becomes more and more urgent. The utilization of high-order modulation format is ideally preferred to enhance the transmission capacity, considering the bandwidth constraint of optoelectrical devices. Whereas, using high-order modulation format also imposes stringent requirement on signal-to-noise ratio (SNR). By introducing probabilistic shaping (PS) technique, we can realize a shaping gain as well as adaptive data rate for the photonic DCI. Nonetheless, when PS is employed for the intensity modulation direct detection (IMDD) system, the optimal probability shaping distribution is not compatible with the distribution of forward error correction (FEC) code. Here, we derive a new probability mass function (PMF) suitable for the IMDD system to realize the cooperation of PS and FEC, based on the establishment of Lagrange function. In comparison with the uniformly distributed eight-level pulse amplitude modulation (PAM-8) signal, a certain generalized mutual information (GMI) gain can be experimentally achieved utilizing Optimized pairwise Maxwell-Boltzmann PAM-8 (Optimized PMB-PAM-8) with derived PMF under the condition of same bit-rate. After 2-km standard single-mode fiber (SSMF) transmission, the net data rate can be increased from 197 Gbit/s of uniformly distributed PAM-8 to 212 Gbit/s by the use of Optimized PMB-PAM-8, which can satisfy the requirement of 200G per lane optical interconnections.

Realizing a spatial phase modulation (SPM) from 0 to 2π is vital for achieving precise light field manipulation, when the phase-only spatial light modulator (SLM) is driven by the voltage. However, insufficient bias voltage degrades the quality of the generated structured light. Here, we propose a superpixel method that consists of 2 × 2 adjacent pixels to equalize the imperfect SPM response, when the bias voltage is insufficient. Meanwhile, a look-up table (LUT) is established between the target light field and the superpixel equalized light field, for reducing the computational complexity of phase hologram generation. When the phase modulation depth is reduced from 2π to 1.67π, we can generate the vortex beams with better quality by the superpixel technique. Characterizing the spatial phase distribution of vortex beams through interferometry verifies the correct function of the superpixel enabled SPM equalization method. In addition, the peak signal-to-noise ratio (PSNR) is used as the metric to quantitatively evaluate the quality of the generated vortex beam. Compared with the phase modulation distortion caused by the insufficient bias voltage, the PSNR after the superpixel enabled SPM equalization is improved by 4.5 dB. Our results would facilitate various applications mediated by power-efficient phase modulation utilizing the SLM.

Generalized two-dimensional (2D) Airy beam is a kind of self-accelerating beam with variable initial angle between its two wings, manifested itself as an initial angle determined parabolic trajectory during the free-space propagation. However, the independent and flexible manipulation of both the transverse optical field and longitudinal trajectory of the generalized 2D Airy beam has not been achieved yet which limits its application in the various fields. Herein, we report on tailoring of the propagation properties of the generalized 2D Airy beam based on the catastrophe theory, where analytical expression of its propagation trajectory is derived. In order to clarify the relationship between the transverse optical field distribution and the longitudinal trajectory, we analytically put forward a generation vector, facilitating the tailoring of both longitudinal trajectory and transverse distribution of optical field simultaneously. Consequently, we can effectively generate the generalized 2D Airy beam and precisely manipulate the evolution of its peak intensity. Once the initial and terminal points of trajectory are determined in advance, we can flexibly tailor the trajectory of 2D Airy beam, with the help of corresponding generation vector. Meanwhile, when the longitudinal trajectory is fixed, we can flexibly rotate the transverse optical field distribution of the generalized 2D Airy beam and manipulate its initial angle. Experimental verifications of the manipulation capabilities for the longitudinal trajectory, initial angle, and the rotation of transverse optical field are provided to validate our theoretical results.

In order to deal with the chromatic dispersion-induced power fading issue for short-reach direct-detection optical fiber communication applications, such as the ever-increasing data-center interconnections (DCIs), optical filed recovery is intensively being under investigation. To date, various direct detection schemes capable of optical field recovery have been proposed, including the Kramers–Kronig (KK) receiver, asymmetric self-coherence detection (ASCD) receiver, carrier-assisted differential detection receiver (CADD), Stokes vector receiver (SVR), and carrier-free phase-retrieval (CF-PR) receiver. Among those, the CF-PR receiver attracts lots of research attention because it can circumvent the requirement of a strong continuous-wave (CW) optical carrier for the beating with the signal. Generally, the CF-PR receiver consists of only two single-ended photodiodes (PDs) and one dispersive element, for the field recovery of the quadrature amplitude modulation (QAM) signals. Based on the theoretical and experimental studies reported so far, this paper reviews the latest progress of CF-PR receivers designed for high-speed optical short-reach transmission links.

This work successfully demonstrates that thin-film lithium niobate platform can bring polarization management devices into a new phase of that higher speed, smaller size, and lower cost.

Internet of everything is being build up rapidly, which causes much higher requirements towards the flexibility of communication resources, especially the adaptivity of the fronthaul to various transmission distances and capacities. In this paper, we proposed a low-cost and spectrum efficient 5G fronthaul broadband connection solution that combines hierarchical modulation technology and delta-sigma modulation. The proposed scheme can double the number of accesses without occupying extra wavelengths. Moreover, such architecture can flexibly allocate the SNR of a group of remote radio units for different user requirement of transmission rate and reaches. Simulation results show that the proposed scheme can increase the access distance of the fronthaul network by 23% and the capacity by 16.7%. The feasibility of the solution is verified through a proof-of-concept experiment. After 20 km transmission, the OFDM-64QAM and -256QAM signal with a center frequency of 3.5 GHz and a bandwidth of 500 MHz can meet the EVM requirements of 8% and 3.5%, respectively. And when the ${R_{LM}}$ of the hierarchical PAM4 signal is 0.6, the ROP required by the most-significant bit branch can be reduced by ∼1.5 dB.

Self-accelerating beams have the unusual ability to remain diffraction-free while undergo the transverse shift during the free-space propagation. We theoretically identify that the transverse optical field distribution of 2D self-accelerating beam is determined by the selection of the transverse Cartesian coordinates, when the caustic method is utilized for its trajectory design. Based on the coordinate-rotation method, we experimentally demonstrate a scheme to flexibly manipulate the rotation of transverse optical field for 2D self-accelerating beams under the condition of a designated trajectory. With this scheme, the transverse optical field can be rotated within a range of 90 degrees, especially when the trajectory of 2D self-accelerating beams needs to be maintained for free-space photonic interconnection.

A broadband microwave photonic mixer with tunable phase shift and supporting dispersion-induced power-fading-free fiber transmission is proposed and demonstrated based on a dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM). In this scheme, the intermediate-frequency (IF)/radiofrequency (RF) signal and the local oscillation (LO) signal are applied to the four sub-MZMs biased at their minimum transmission points via a power splitter and a 90° hybrid coupler, respectively. Through mutual beating between the IF/RF and the LO modulation sidebands in a high-speed photodetector at the remote site, high-efficiency frequency conversion is achieved. The dispersion-induced power fading over long-distance fiber transmission is eliminated through setting the biased-induced phase difference between the parent-MZMs in the two sub-DPMZMs of the DP-DPMZM to be π/2. In addition, the phase shift of the frequency-converted signal can be continuously tuned over 360° through synchronously adjusting the bias voltages of the parent-MZMs in the two sub-DPMZMs. The proposed scheme is experimentally demonstrated, where a microwave photonic mixer with a 6-dB operation bandwidth of 40 GHz and supporting dispersion-induced power-fading-free transmission over 20 km SMF is realized. Meanwhile, a continuously tunable phase shift over 360° in the frequency range of 0.1 GHz to 29.9 GHz is achieved, where the power variation during phase tuning is smaller than 4 dB.

We present the theoretical study of polarization-maintaining multi-core few-mode fiber (PM MC-FMF) with a cladding diameter of 125 μm, in order to secure the maximum number of independent spatial channels arising in the single fiber. By taking both the inter-core crosstalk (XT) and the intra-core mode coupling into account, we identify that the PM five-core three-mode fiber is able to provide 19 parallel channels without the need of MIMO signal processing. After the numerical optimizations, the inter-core XT of MC-FMF is less than −30 dB/100km, due to the use of trench-assisted heterogeneous core distribution. Meanwhile, the effective refractive index (RI) differences among both spatial modes and polarization modes are more than 8 × 10−4 and 3.26 × 10−4 over the C+L band, respectively, due to the introduction of stress and geometric birefringence. Moreover, the cladding diameter of such MC-FMF is the same as that of the standard single-mode fiber (SSMF), leading to a convenient upgrading from the SSMF to the MC-FMF.

We propose a design of ring-core few-mode fiber (RC-FMF) with the erbium doping at the cladding region near the outer ring edge. After the parameter optimization of the RC-FM-EDF, the intensity overlapping difference between LP01 and LP11 modes can be minimized, leading to a DMG of 0.22 dB for the corresponding ring-core few-mode erbium-doped fiber amplifier (RC-FM-EDFA). Meanwhile, in comparison with uniform core doping design, the saturation input signal power of the RC-FM-EDFA can be enhanced from −17.7 dBm and −16 dBm to −8.5 dBm for LP01 and LP11 modes, respectively. Consequently, our proposed RC-FM-EDFA is characterized by both low differential modal gain (DMG) and the enhanced saturation input signal power. Those new characteristics of the RC-FM-EDFA is ideally desired for future agile networks.

Polarization-maintaining (PM) all-fiber cavity is always essential to realize stable ultrafast-pulse generation. With the help of five sections of PM fibers with independently specific splicing angles, we successfully obtain a femtosecond pulse output from a PM Ytterbium-doped fiber laser (YDFL) mode-locked by the nonlinear polarization evolution (NPE) technique. Using a hybrid component, the PM all-fiber cavity can be greatly simplified. Ultrafast-pulses with 24.5-MHz repetition rate, 20-nm spectral bandwidth, and 260-fs pulse duration after external compression can be steadily generated. In comparison with the non-PM fiber cavity, the proposed YDFL has excellent long-term environmental stability, making it ideal for various practical applications.

Polarization de-multiplexing becomes challenging in Stokes vector-direct detection (SV-DD) systems, especially when the rotation of state of polarization (RSOP) and fiber chromatic dispersion (CD) are induced. To address this issue, an adaptive blind RSOP equalizer based on Stokes space is proposed. After theoretical analysis, it is found that the distribution of transmitted signals in Stokes space keeps a paraboloid. Based on this analysis, an adaptive scheme is proposed to track the symmetry axis of paraboloid for the first time. It is worth noticing that not only CD but also high-order modulation formats could not change the paraboloid distribution. Therefore, our algorithm has strong robustness to CD and good suitability for SV-DD systems with different modulation formats. In our simulation, the conventional Stokes space-based method and the training sequences (TS) method are both evaluated for comparison. The results show that our scheme can track RSOP of 20 Mrad/s and 8 Mrad/s at hard-decision forward error correction threshold in 28 Gbaud 4-level quadrature amplitude modulation (4-QAM) and 16-QAM SV-DD systems after 100 km fiber transmission, respectively. No additional computational complexity is required compared to the conventional Stokes space-based method. Moreover, compared to the TS-based method, there is no need to know the actual channel information, contributing to good flexibility.

In this paper, we have explored the feasibility of 8-level pulse amplitude modulation (PAM8) format for realizing beyond 100 Gb/s transmission in a bandwidth-limited intensity modulation and direct-detection (IMDD) system. Generally, Volterra equalizer is a practical algorithm which can be used in bandwidth-limited system to combat the linear and nonlinear impairments. However, we observe that the performance of Volterra equalizer would be restricted by a phenomenon of level-dependent equalized effect, especially in low optical signal-to-noise ratio case. To cope with this problem, a tap coefficient decision directed Volterra equalizer (TDD-Volterra) with multiple sets of tap coefficients is proposed. The optimal set of tap coefficients is selected according to the decision results of input symbols before equalization. By this means, the TDD-Volterra is effective for overcoming the level-dependent equalized effect. By using the proposed TDD-Volterra, 180 Gb/s PAM8 signal is successfully transmitted over 2 km standard single mode fiber in an IMDD system with the 10-dB bandwidth of 17.5 GHz for the first time. The achieved bit error rate (BER) value is below the 7% overhead hard-decision forward error correction threshold of $3.8\times 10 ^{-3}$ . Moreover, the performance comparison among decision-directed least-mean square equalizer, Volterra Equalizer, and TDD-Volterra are analyzed in this paper. The experimental results show that compared to the conventional Volterra equalizer, the algorithm complexity of TDD-Volterra is decreased by over 50% for achieving a given BER value. Our research may provide a solution for beyond 100 Gb/s short-reach applications using bandwidth-limited electro-optical components.

Weakly coupled multicore fiber (WC-MCF) is promising for use in short-reach fiber telecommunication systems in the near future. Intercore crosstalk (XT) plays an important role in WC-MCF-based short-reach intensity-modulation/direct-detection (IM/DD) systems, because XT can significantly degrade system performance. For the first time, the combined effect of intercore XT and the frequency offset between optical carriers, which is defined as the carrier beating impairment (CBI), is measured and characterized by experiments and numerical simulations. The experimental results show that the CBI causes large fluctuations in the signal-to-noise ratio (SNR) with a normally tolerable XT level. Such SNR fluctuations can additionally reduce the system's XT tolerance (at worst 1-dB SNR degradation) by as much as 20 dB for 4-level pulse amplitude modulation (PAM-4) transmission. The simulation results show that the CBI is mainly affected by the specific properties of the laser source and the modulation depth of the transmitted signals. These results suggest that the XT should be designed to be much smaller for WC-MCF-based short-reach IM/DD systems when there is the CBI.

Quantum key distribution (QKD) is regarded as an alternative to traditional cryptography methods for securing data communication by quantum mechanics rather than computational complexity. Towards the massive deployment of QKD, embedding it with the telecommunication system is crucially important. Homogenous optical multi-core fibers (MCFs) compatible with spatial division multiplexing (SDM) are essential components for the next-generation optical communication infrastructure, which provides a big potential for co-existence of optical telecommunication systems and QKD. However, the QKD channel is extremely vulnerable due to the fact that the quantum states can be annihilated by noise during signal propagation. Thus, investigation of telecom compatibility for QKD co-existing with high-speed classical communication in SDM transmission media is needed. In this paper, we present analytical models of the noise sources in QKD links over heterogeneous MCFs. Spontaneous Raman scattering and inter-core crosstalk are experimentally characterized over spans of MCFs with different refractive index profiles, emulating shared telecom traffic conditions. Lower bounds for the secret key rates and quantum bit error rate (QBER) due to different core/wavelength allocation are obtained to validate intra- and inter-core co-existence of QKD and classical telecommunication.

An image-rejected multi-band frequency down-conversion scheme is proposed and experimentally demonstrated based on photonic sampling. The multi-band radio-frequency (RF) signals to be processed are copied into two replicas in quadrature, which are then sampled by an ultra-short optical pulse train via a polarization-multiplexed modulator. After polarization demultiplexing and detection using a pair of low-speed photodetectors, the multi-band RF signals are simultaneously down-converted to the intermediate frequency (IF) band. The image components can be suppressed by quadrature coupling the two generated IF signals via an electrical 90° hybrid coupler (HC). In the experiment, multi-band RF signals in the frequency range of 6 GHz to 39 GHz are down-converted to the IF band below 4 GHz using a local oscillator (LO) signal at 8 GHz to generate the ultra-short optical pulse train. Image rejection is achieved in the digital domain using digital signal processing to compensate for the amplitude and phase mismatch between the two IF signals and to implement quadrature coupling. In addition, through using an electrical phase shifter, an electrical attenuator, and an electrical 90° HC to achieve quadrature coupling of the two IF signals, image-rejected multi-band frequency down-conversion is also verified in the analog domain.

We propose and experimentally demonstrate a reconfigurable finite impulse response microwave photonic filter (FIR-MPF) based on long period gratings (LPGs) inscribed in multicore fibers. By exploiting two dimensions of optical wavelength and space, intentional inter-core signal switching is realized. For the diversity of optical wavelength, we tune the operation wavelength to change the inter-core power coupling according to the wavelength-dependent characteristic of LPGs. On the other hand, for the diversity of space, we use directional bending of LPGs to achieve switchable spatial channel allocation. Both configurations can implement three different two-tap FIR-MPFs individually. We show a promising solution for reconfigurable radio frequency signal processing in potential with low cost and high efficiency.

We design a panda type four-core fiber with a comparable optical property of the polarization maintenance single mode fiber (PM-SMF) and can support 8 non-polarization-degenerate eigenmodes with a birefringence of approximate 3.24 × 10-4 over the wavelength range from 1288 to 1630 nm. In particular, only four stress rods are efficiently used to achieve the PM characteristics for each core, in comparison with traditional design of panda type PM-SMF. Meanwhile, we identify that the optimal configuration of stress rods can not only enhance the birefringence, but also further suppress the crosstalk (XT) between two primary eigenstate of polarization (ESOP) arising in adjacent cores, although the corresponding XT at secondary ESOP remains unchanged. Moreover, our numerical results show that both chromatic dispersion (CD) together with mode field diameter (MFD) and the bending sensitivity are consistent among four cores. We believe that such specialty multi-core fiber is ideal for compact and cost-effective fiber-optic gyroscope (FOG) application, besides the potential in the MIMO-free spatial division multiplexing (SDM) interconnection.

A simple method based on a designed pseudo-pilot is proposed for both carrier frequency offset (CFO) and phase noise (PN) estimation for the coherent optical offset-quadratic-amplitude modulation (QAM) based filterbank multicarrier (CO-FBMC/OQAM) transmission. Numerical simulations are performed to evaluate the performance of the proposed scheme for a 20 GBd/s FBMC/OQAM system. The simulation results show that a robust and precise CFO and PN estimation can be achieved simultaneously in comparison with traditional methods. For the CFO estimation, our method can achieve a wide CFO estimation range of [-10, +10 GHz]. As for the PN compensation, the tolerable combined laser linewidths under 1 dB optical signal noise ratio (OSNR) penalty constraint for 4/8/16/32-QAM are around 1.6 MHz, 0.8 MHz, 500 kHz, and 250 kHz, respectively. The results indicate that the proposed scheme can be used for a wide range CFO and PN compensation with better transmission performance in high speed CO-FBMC/OQAM system.

In this paper, a novel image encryption scheme based on analog-digital hybrid electro-optic chaotic sources and compressive sensing is proposed. Distinguished from other existing chaos-based encryption schemes, analog-digital hybrid electro-optic chaotic sources are utilized, in which the synchronization of chaotic systems can be achieved by transmitting binary digital bits. The robustness of the encryption scheme can be significantly improved and the theoretical transmission distance will be much longer because of digital chaos synchronization. Moreover, owing to the compression of compressive sensing and the sensitivity to initial values of chaotic sources, the proposed scheme can greatly reduce the amount of data and strengthen safety. Exhaustive simulations and analysis demonstrate that the proposed scheme is of good peculiarities such as high efficiency, good robustness and security.

Nonlinear Fourier transform (NFT), based on the nonlinear Schrödinger equation, is implemented for the description of soliton propagation, and in particular focused on propagation of high-order solitons. In nonlinear frequency domain, a high-order soliton has multiple eigenvalues depending on the soliton amplitude and pulse-width. During the propagation along the standard single mode fiber (SSMF), their eigenvalues remain constant, while the corresponding discrete spectrum rotates along with the SSMF transmission. Consequently, we can distinguish the soliton order based on its eigenvalues. Meanwhile, the discrete spectrum rotation period is consistent with the temporal evolution period of the high-order solitons. The discrete spectrum contains nearly 99.99% energy of a soliton pulse. After inverse-NFT on discrete spectrum, soliton pulse can be reconstructed, illustrating that the eigenvalues can be used to characterize soliton pulse with good accuracy. This work shows that soliton characteristics can be well described in the nonlinear frequency domain. Moreover, as a significant supplement to the existing means of characterizing soliton pulses, NFT is expected to be another fundamental optical processing method besides an oscilloscope (measuring pulse time domain information) and a spectrometer (measuring pulse frequency domain information).

A secure key distribution scheme for orthogonal frequency division multiplexing (OFDM) passive optical network system is proposed and experimentally demonstrated. The training symbol (TS), which is used for time synchronization and channel estimation, is formed by adopting a noise-like chaotic sequence. The redundancy of the TS is utilized to embed secure keys. An experiment is performed, in which 7.64 Gb/s 16-quadrature-amplitude-modulation OFDM data and 28.4 Mb/s keys embedded in are successfully transmitted over 25 km standard single mode fiber. The results indicate a promising key distribution method for physical layer secure optical communication.

Due to the benefit of multiple parallel channels within single cladding and easy implementation of multi-input multioutput technique, we present the spatial division multiplexing (SDM)-based reflective intensity-modulated fiber optics displacement sensor (RIMFODS). Under the condition of single-input single-output, we experimentally compare the transfer function of RIMFODS by using a pair of standard single-mode fibers (SSMFs), single seven-core-single-mode fiber, and single seven-core-few-mode fiber. In comparison with the conventional SSMF-based RIMFODS, the use of seven-core-few-mode fiber can reduce the dead zone of measurement range from 200 to 100 μm. Meanwhile, the system sensitivity can be enhanced from 0.14 to 12.2 nW/μm. In particular, with the help of two-input-five-output technique, the dead zone can be further reduced to 70 μ m, with sensitivity reaching as high as 53.87 nW/μm. Finally, we theoretically investigate the effect of both the core size and the core spacing of multicore fiber on the transfer function of RIMFODS. Such an SDM-based RIMFODS with compact footprint has lots of potential to be exploited.

Orthogonal frequency-division-multiplexing (OFDM) technology is widely used in visible light communication (VLC) to achieve high data rate transmission. However, the traditional direct-current (DC)-biased optical OFDM (DCO-OFDM) VLC systems suffer from the high peak-to-average power ratio (PAPR) which causes signal clipping distortion, and, thus, performance degradation. Furthermore, severe high-frequency fading due to the limited system bandwidth results in poor bit error rate (BER) performance. Precoding matrix (PM) techniques have been proposed to enhance the performance of VLC OFDM transmission, but a little or no work has been carried out in investigating the theory of PM used in OFDM VLC systems. In this paper, we aim to reveal the theory of PM-DCO-OFDM for a VLC system. To figure out the intrinsic laws of a PM method, we investigate the principles of PAPR reduction, clipping distortion optimization, and signal-to-noise ratio (SNR) distribution equalization. Based on the analysis of PAPR, we theoretically proved the simplicity of PM as a method to reduce the possibility of high PAPR by improving the autocorrelation performance of input symbols. The clipping distortion could be improved due to the reduction of high PAPR. Moreover, the relatively uniform SNR distribution can be achieved by PM through equalizing the clipping and channel noise, which is beneficial to improve the BER performance in high-frequency constrained systems. However, the PM method used in a DCO-OFDM VLC system should consider the transmitting power, modulation format, and transmission distance as a whole to achieve the transmission performance improvement. The simulation results demonstrate the complementary cumulative distribution function of PAPR can be reduced ~3 dB, while the performance of clipping distortion power and clipping error probability are significantly improved. Furthermore, experiment is carried out with results showing that the PM method can improve the BER performance in the case that VLC OFDM transmission has enough transmitting power, but with the low transmitting power, the PM also can damage the BER performance. The simulation and experiment results are consistent with our theoretical analysis.

We have implemented a BOTDA that combines channel estimation and complementary coding technique for high performance distributed sensing. The use of OFDM-modulated probe reduces the measurement time sharply compared to the classical frequency sweeping technique. Meanwhile, the complementary coding provides a higher SNR of the sensing system, which further reduces the measurement time by mitigating averaging. The method of combining the channel estimation technique and complementary coding is theoretically analyzed. It has been demonstrated that the pulse coding scheme can benefit both the Brillouin gain spectrum (BGS) and Brillouin phase spectrum (BPS). Experiments are carried out using 512-bit codewords on a 10-km single-mode fiber with a spatial resolution of 25 m. The complex BGS distribution is measured within 2 ms, with a BFS uncertainty of 1.1 and 0.6 MHz for BGS and BPS, respectively.

We design a new kind of fractional Fourier transform superimposed training sequence (TS) to simultaneously estimate linear distortions including chromatic dispersion (CD), timing offset (TO), and frequency offset (FO), etc. Without sacrificing additional spectrum efficiency, the scheme can reduce the computational complexity sharply compared to a regular estimation method. The feasibility of the proposed scheme is verified by simulations of 28 GBaud dual polarization quadrature phase-shift keying (DP-QSPK) and 16-ary quadrature amplitude modulation (DP-16 QAM) system. Simulation results show that the superimposed TS has negligible negative effects on the transmission performance considering 7% forward error correction while the TO, FO, and rough CD estimation perform well after a long distance transmission. The max TO estimation (TOE) errors are, respectively, eight samples and 12 samples for QPSK and 16 QAM, and the max FO estimation errors are, respectively, 5.7 and 3.6 MHz for QPSK and 16 QAM. The rough CD estimation (CDE) error is below 946 ps/nm. The TOE and rough CDE separately decrease the complexity of frame synchronization and CDE. In total, the computation complexity of the estimations decreases by 91.8% compared to a regular blind estimation method.

In this paper, we propose and demonstrate a joint optical signal-to-noise ratio (OSNR) and a nonlinear distortions estimation method. The OSNR is calculated by considering either the link parameters or the correlation operations of the training symbols. The nonlinear distortions estimation in this calculation comes from different nonlinear sensitivities of these two methods. The indicator of NL penalty is defined as the ratio of nonlinear distortion power to the signal power. As the estimation method, based on link parameters, has low computational complexity, and the training symbol used for the OSNR estimation is inherently indispensable for frame synchronization, this nonlinear distortion estimation method introduces a few complexities and no extra overhead. Experimental results obtained by using a 200 Gb/s 16 QAM DWDM system establish the accuracy of the proposed OSNR estimations method, with errors less than 1 dB. The nonlinear distortions were also measured under different simulation conditions, using a coherent optical OFDM system, and the results are in accordance with the expectations.

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%.

Abstract We propose and experimentally demonstrate spatial rotation manipulation for radially asymmetric modes based on two kinds of polarization maintaining few-mode fibers (PM-FMFs). Theoretical finding shows that due to successful suppression of both polarization and spatial mode coupling, the spatial rotation of radially asymmetric modes has an excellent linear relationship with the twist angle of PM-FMF. Both elliptical core and panda type FMFs are fabricated, in order to realize manageable spatial rotation of LP11 mode within ±360° range. Finally, we characterize individual PM-FMF based spatial orientation rotator and present comprehensive performance comparison between two PM-FMFs in terms of insertion loss, temperature sensitivity, linear polarization maintenance, and mode scalability.

Abstract Physics phenomena of multi-soliton complexes have enriched the life of dissipative solitons in fiber lasers. By developing a birefringence-enhanced fiber laser, we report the first experimental observation of group-velocity-locked vector soliton (GVLVS) molecules. The birefringence-enhanced fiber laser facilitates the generation of GVLVSs, where the two orthogonally polarized components are coupled together to form a multi-soliton complex. Moreover, the interaction of repulsive and attractive forces between multiple pulses binds the particle-like GVLVSs together in time domain to further form compound multi-soliton complexes, namely GVLVS molecules. By adopting the polarization-resolved measurement, we show that the two orthogonally polarized components of the GVLVS molecules are both soliton molecules supported by the strongly modulated spectral fringes and the double-humped intensity profiles. Additionally, GVLVS molecules with various soliton separations are also observed by adjusting the pump power and the polarization controller.

We have experimentally demonstrated the signal-to-signal beating interference (SSBI) mitigation capability of Kramers-Kronig (KK) detection in a single sideband (SSB) four-ary pulse-amplitude-modulation (PAM4) direct detection (DD) system. A 112-Gbit/s (56-GBaud) SSB-PAM4 optical signal suitable for KK detection is generated by using a single commercial, low-cost dual-drive Mach-Zehnder modulator. The system bandwidth constraint is mitigated by Nyquist pulse shaping and bandwidth precompensation. The SSBI mitigation capability of KK algorithm is identified by comparing with two order Volterra filter. After 80-km dispersion-uncompensated standard single-mode fiber transmission, linear equalizer with KK algorithm and two order Volterra filter based nonlinearity equalizer show similar bit error rate (BER) performance, which are 3.2 × 10-3 and 2.8 × 10-3 , respectively. Both BERs are below 7% hard-decision forward error correction threshold of 3.8 × 10 -3. The experiment results present the excellent SSBI mitigation capability of KK detection in SSB-PAM4 DD system. Above all, our experiments reveal the potential of KK detection in DD system for data center interconnects.

The mode-division multiplexer/demultiplexer (MMUX/DeMMUX) is the crucial component for the implementation of mode-division multiplexing (MDM) transmission. We propose an on-chip two-mode MMUX/DeMMUX based on adiabatic couplers. Thanks to the principle of mode evolution, the proposed MMUX possesses advantages of broadband operation and high tolerance of fabrication error. The performance is experimentally evaluated by integrating the proposed MMUX and DeMMUX into an on-chip MDM link, with a low crosstalk of

Based on electrical arc discharges mechanisms, we inscribed spatially arrayed long period gratings (LPGs) into a heterogeneous seven-core fiber. The LPG fabrication platform was built upon the commercial fiber fusion splicer with a self-developed real-time control software. With the help of homemade fan-in/fan-out multiplexer, the high-quality grating spectra in seven cores are measured. The largest resonant dip of -42 dB and minimum insertion loss of 0.5 dB are achieved in the central core. To demonstrate potentials of the spatially diversified gratings, we measured the strain and temperature simultaneously with the spatial response matrix to eliminate the cross sensitivity. Accurate sensing results have been achieved with relative errors less than 4%.

We demonstrate an ultra-dense wavelength-division-multiplexing (UDWDM) passive optical network (PON) in a spatial-division-multiplexing system utilizing our home-made seven-core multicore fiber (MCF) with a self-homodyne coherent detection (SHCD) scheme. In the SHCD system, we transmit 12 channels of 40-Gb/s UDWDM-PON signals using polarization-division-multiplexing quadrature phase-shift keying (PDM-QPSK) with a 12-GHz grid through the six outer cores of the seven-core MCF with the central core used to transmit the pilot tone. An optical frequency comb generator is employed to supply the multi-carriers for cost saving. After 37-km low-crosstalk seven-core MCF long-reach transmission with compact fan-in/fan-out multiplexers, a large transmission capacity of 2.88 Tb/s and a power budget of 26 dB are obtained. At the optical network unit side, a narrow linewidth local oscillator laser is no longer necessary since SHCD is adopted in our system. The complexity of the digital signal processing procedure at the coherent receiver is also greatly reduced as the carrier phase recovery and carrier frequency offset estimation are not needed.

We propose a 3-D adaptive loading algorithm (ALA), which can individually allocate proper modulation formats, power levels, and the forward error correction codes to each orthogonal frequency division multiplexing (OFDM) subcarrier according to the estimated channel state information (CSI). The introduced look-up table operation can reduce the iterations and improve the convergence speed. To evaluate the performance of the proposed algorithm, we tailor the system parameters of direct detection optical OFDM (DDO-OFDM) to investigate the difference of effective data rate (EDR) between fixed schemes and proposed adaptive modulation and coding (AMC) schemes. When the fiber length is set to 90 km and received optical power (ROP) is -10 dBm in simulation, the AMC scheme achieves 7.86 Gb/s EDR, which is 1.93 times of the maximum EDR the fixed schemes can achieve under the bit error rate (BER) constraint. Compared with the Levin-Campello algorithm, the 3-D ALA improves about 3% and 6% data rates in poor and good CSI, respectively. In experiments, when the fiber length and ROP are 50 km and -18 dBm, respectively, the AMC scheme achieves the EDR of 4.96 Gb/s, which is 2.84 times of the maximum EDR of the fixed schemes under the BER constraint.

We propose two end-view-based digital image processing algorithms for high-precision rotational angle alignment of specialty optical fibers. The first technique is based on the cross-correlation calculation between images of two fiber ends. Accurate alignment can be achieved between identical specialty optical fibers with arbitrary end face structures. The other one is based on the self-correlation method. Two fibers with different refractive index profiles in the fusion splicer can be rotated to preset angles independently with high precision. Accurate angle alignment of less than 0.5° uncertainty and low average splicing loss for seven-core multicore fibers have been achieved in experiment. To speed up the alignment process, we further proposed two simplified algorithms for the above-mentioned techniques. Time consumption can be reduced by about 50% with acceptable alignment accuracy. We confirmed that all the methods are suitable for other kinds of specialty optical fibers, such as polarization maintaining fibers and photonic crystal fibers.

We develop a two-step infrared (IR) femtosecond fiber laser exposure technique to flexibly fabricate long period fiber gratings (LPFGs) with a high peak band-rejection efficiency of 35.4 dB, insertion loss of 4.36 dB, and 3 dB bandwidth of 13 nm. First, we etch periodic corrugations via IR femtosecond laser in the cladding of standard single mode fiber, leading to efficient laser beam-focusing on the cladding as well as stress-optic refractive index (RI) modulation. Then, we carry out femtosecond fiber laser irradiation under the corrugations, to further introduce RI modulation within fiber core. We experimentally investigate the impacts of fabrication parameters including laser focus location and individual exposure time with respect to the transmission spectra of LPFGs. The fabricated LPFG has a temperature coefficient of 114 pm/°C and a polarization dependent loss of 1.78 dB, which may find useful applications in sensors.

A high-linearity directly-modulated analog photonic link (APL) with simultaneous suppression of the even-order intermodulation distortion, third-order intermodulation distortion (IMD3), and cross-modulation distortion (XMD) is proposed and experimentally demonstrated by the corporation of push-pull structure and an adaptive compensation algorithm. In the proposed directly-modulated APL system, the push-pull structure is introduced to effectively eliminate all the even-order nonlinear distortions, including the second-order intermodulation distortion and the second-order harmonic distortions. To further simultaneously suppress the IMD3 and XMD, an adaptive compensation algorithm is designed and adopted at the receiver. The experimental results show that the second-order spurious-free dynamic range and the third-order spurious-free dynamic range is improved by 19.8 and 12.4 dB, respectively, corresponding to 7.15% error vector magnitude performance improvement for a 120 Mb/s 64QAM-OFDM RF signal transmission when the input RF power is 13 dBm.

We present an optically coupled chaotic system involving three-phase modulated electro-optic nonlinear loops and an optical coupler. The dynamical properties and the time delay signature (TDS) suppressing performance of the system is analyzed in detail. Numerical results show that the TDS can be suppressed not only under statistical analysis of a single output, but also under mutual statistical analysis of the multiple outputs. Compared with the intensity modulated electrically coupled scheme, the presented system has less interior noise due to the simpler construction.

In this paper, we have achieved a high repetition rate and tunable pulse generation near ∼2 μm through an all-optical actively mode-locked fiber laser. The performances of the fiber laser have been systematically investigated through numerical simulations. The thulium-doped fiber laser cavity is modulated by the optical injection at 1.55 μm via a nonlinear optical loop mirror in which a specially-designed group-velocity-matching highly nonlinear fiber is incorporated. According to the simulation results, up to 40-GHz pulse train with picosecond tunable pulse width and peak power can be produced. Moreover, the repetition rate can be further enlarged with the help of higher gain. The impacts of gain coefficient, cavity length, and repetition rate on the output are studied in details. The proposed mode-locked fiber laser can be potentially used for future 2- μm fiber communication.

An extension of the conventional dual Schottky diode-based predistortion circuit (PDC) is proposed to linearize directly modulated radio over fiber (RoF) system. The main advantages of the proposed PDC are simple configuration, excellent distortion suppression, and broad bandwidth from dc to 6 GHz. By using the proposed cascaded Schottky diodes-based PDC, the condition of third-order intermodulation nonlinearities suppression can be more easily satisfied and the optimal linearization effect can be achieved due to the fact that it has more circuit adjustment parameters. The experimental results show that by using the proposed PDC, the spurious-free dynamic range (SFDR) @2 GHz of the optical transceiver which is designed by ourselves can be improved from 93.8 dB·Hz2/3 to 112.1 dB·Hz4/5 in a 1-Hz bandwidth, corresponding to 7.21% error vector magnitude performance improvement for 20 MHz 64QAM-OFDM @2 GHz signal transmission over 10 km standard single mode fiber. The linearity improvement by using the proposed PDC has also been verified in a commercial optical transceiver based RoF system, and the achieved SFDR @ 2 GHz can be increased from 102.8 dB·Hz2/3 to 121.6 dB·Hz4/5 in a 1-Hz bandwidth.

Mode division multiplexing together with multiple-input multiple-output digital signal processing (MIMO-DSP) has the potential for fulfilling future ever-increasing transmission capacity demand. However, for the mitigation of severe mode coupling, the cost and computation complexity of MIMO-DSP become prohibitive with the growing numbers of guided modes arising in the traditional circular core few-mode fiber. We design, fabricate, and characterize panda type elliptical core few-mode fiber (PE-FMF) supporting six eigenmodes of LP01x, LP01y, LP11ax, LP11ay, LP11bx, and LP11by independent transmission. Such PE-FMF, with both low refractive index difference of 0.006 between the core and the cladding and low ellipticity of 1.449, can alleviate the trade-off between the eigenmode numbers and the desirable mode spacing occurring in traditional elliptical-core FMF design. The numerical results indicate that the minimal mode spacing among six eigenmodes is above 1.95 × 10−4 over the whole C + L band. The maintaining capability of both mode intensity profile and polarization is experimentally verified, when the PE-FMF is under a twisting rate range from 0 to 1.57 rad/m. During the fiber twisting, the mode intensity profile can be maintained and the polarization extinction ratio of individual eigenmode is above 20 dB over 50 m PE-FMF.

Theoretical investigation on the mechanism of temporal depolarization effect in a polarization optical time-domain reflectometry (POTDR) system is conducted. First, the relationship between the dynamic range of the incomplete POTDR system and the degree of polarization (DOP) of the backscattered optical signal is derived. Second, a temporal depolarization suppressed POTDR system is proposed by introducing quasi-distributed Fresnel reflections. Third, the DOP distribution of the proposed system is demonstrated by numerical simulations and experiments. It is shown that the temporal depolarization effect can be significantly suppressed to maintain sufficient dynamic range for fast and accurate polarimetric measurement. The signal-to-noise ratio (SNR) of the received optical signal is also improved with the help of Fresnel reflections. With high DOP and improved SNR, instantaneous intrusion sensing and accurate vibration frequency measurement along the fiber link have been achieved without performing any data averaging.

We propose and demonstrate a novel spatial-division multiplexed Mach-Zehnder interferometer (MZI) structure in heterogeneous multicore fiber (MCF) for simultaneous measurement of temperature and strain. The MZI structure is constituted of dual-tapered regions along heterogeneous MCF by using electrical arc discharge. Different spatial channels (cores) in heterogeneous MCF have dissimilar responses to the outer environment. With our in-house developed low-loss fan-in/fan-out devices, we experimentally achieved the temperature sensitivity of 47.37 pm/°C for the central core and 53.20 pm/°C for the outer core and strain sensitivity of 1.10 pm/με for the central core and 0.84 pm/με for the outer core. Using the temperature-strain joint matrix, the cross sensitivity is eliminated, and the relative measurement error is less than 5%. The spatial-division multiplexed interferometric sensors embedded in the telecommunication-grade MCF show great potential for multiparameter sensing.

We propose and experimentally demonstrate an all-fiber tunable LP11 mode rotator with 360° range, based on the bending and twisting of the two-mode fiber (TMF). A theoretical model of TMF perturbation and its effect on the spatial pattern of LP11 mode is put forward. We experimentally characterize the implementation of the LP11 mode rotator by an all-fiber polarization controller configuration. The results agree well with the theoretical investigations. We are able to demonstrate arbitrary LP11 mode rotation with 360° range. The insertion loss is less than 0.4 dB when the operation wavelength is varied from 1540 to 1560 nm.

A bidirectional transmission and massive multi-input multi-output (MIMO) enabled radio over a multicore fiber system with centralized optical carrier delivery is experimentally investigated in this paper. Optical carriers for upstream are delivered from the central office to the remote antenna unit via the inner core for the coreless implementation. In our experiment, as one of the fifth-generation (5G) waveform candidates, orthogonal frequency division multiplexing using offset quadrature amplitude modulation (OFDM/OQAM) is adopted in both uplink and downlink to increase the spectral efficiency and side lobes suppression ratio. An advanced 2 × 2 MIMO-OFDM/OQAM channel estimation algorithm is optimally designed to equalize the hybrid optical and wireless MIMO channels. The experimental results show that bidirectional transmission of 4.46 Gb/s 2 × 2 MIMO-OFDM/16-OQAM could be achieved over a 20-km seven-core fiber and a 0.4-m wireless link. The proposed scheme proves that the multicore fiber can realize the transparent transmission of bidirectional radio signals and effectively simplify the infrastructures in the future 5G cellular systems.

We report an all-fiber miniature exposed Fabry-Pérot (F-P) cavity microfluidic refractometer by splicing an ultrashort segment of thin fiber with standard single-mode fiber with large off-center alignment. Theoretical investigations of dynamic range and sensitivity issues for the proposed fiber sensors are carried out. The exposed F-P cavity can provide excellent interaction between embedded gas/liquid with guided light, with a high sensitivity of 1086 nm/RIU at ~1550 nm within the refractive index (RI) range from 1.332 to 1.3565. Meanwhile, the exposed F-P cavity structure is insensitive to temperature fluctuation. The simple and cheap fabrication method, together with ultracompact package, stable operation, and low-temperature cross-sensitivity, verify that it has great potential for microfluidic RI sensing for chemical, biological, and medical applications.

The dynamics of dissipative solitons (DSs) are explored in a high-repetition-rate normal-dispersion erbium-doped fiber laser for the first time. Despite the high fundamental repetition rate of 129 MHz and, thus, the low pulse energy, a DS train with a dechirped pulse width of 418 fs, period doubling of single and dual DSs, and 258-MHz second-order harmonic mode locking of DSs, can be observed in the fiber laser with increasing pump power and appropriate settings. A transmitted semiconductor saturable absorber and a wavelength division multiplexer/isolator/tap hybrid module are employed to simplify the laser configuration, thus not only increasing the repetition rate but enhancing the stability and robustness of the fiber laser due to the commercial availability of all the components as well.