Search

The performance of high-speed intensity modulation direct detection (IM-DD) transmissions is severely degraded by the linear inter-symbol interference (ISI), due to both bandwidth-constrained optoelectrical components and the chromatic dispersion (CD). Meanwhile, nonlinear ISI due to intensity modulation and square-law detection can further deteriorate the IM-DD system performance significantly. Here, we first propose three metrics to quantify those impairments in IM-DD transmissions. Then, we compared both the BER performance and computational complexity of different DSP schemes, using a C-band PAM-4 experimental test-bed with an end-to-end 10-dB bandwidth of 17 GHz. Those DSP schemes include receiver-side feedforward equalization (FFE), receiver-side Volterra filter equalization (VFE), transmitter-side Tomlinson-Harashima precoding (THP) together with receiver-side FFE, transmitter-side THP together with receiver-side VFE, channel-shortening filter (CSF) together with maximum likelihood sequence estimation implemented with linear branch metrics (MLSE-LI-BM), and CSF together with MLSE implemented with nonlinear branch metrics (MLSE-NL-BM). Our experimental results indicate that, only the combination of CSF and MLSE-NL-BM enables a 112 Gbit/s transmission over 5 km fiber at the KP4-FEC threshold, but with an increased computational complexity.

Optical filter based on liquid crystal on silicon (LCOS) is widely used in wavelength selective switch (WSS) to allocate the optical spectrum flexibly. Constrained by the achievable spectral resolution of single grating used in the WSS, the roll-off bandwidth of optical filter is challenging to improve for the purpose of precise and efficient spectrum allocation. In this article, we demonstrate a novel double gratings configuration to realize higher spectral resolution by repeatedly enlarging and accumulating the dispersion, under the condition of the maintenance of the final light spot size. After examining the filtering performance with respect to the diffraction number, we are able to achieve a spectral resolution of 1.63 GHz, indicating of almost tenfold enhancement in comparison with traditional single grating configuration. In particular, an LCOS-based optical filter with a spectral roll-off bandwidth of 1.26 GHz and addressable resolution of 187 MHz is demonstrated. Meanwhile, the insertion loss and the range of operation wavelength are investigated to determine the optimal number of diffractions for various applications. The proposed LCOS-based filter enables spectral manipulation of GHz roll-off bandwidth, fulfilling the performance requirements of future flexible optical networking.

Linear optical sampling (LOS) is a powerful technique to circumvent the electronic bottleneck arising in optical modulation analyzer (OMA). To extend the range of operation wavelength for the LOS-based OMA, the fiber optical frequency comb (FOFC) is proposed to replace the narrowband mode-locked fiber laser (NMFL) as an optical sampling source. However, this procedure reduces the signal-to-noise ratio (SNR), due to the constraints of the unideal response of balanced photodetector (BPD) and the limited power per FOFC tone. Here, we investigate the biased balance detection (BBD) scheme to overcome such issue. Our simulation results show that, the characterization performance of the FOFC-based LOS with the help of the BBD scheme is almost the same as that of the NMFL-based LOS, when the PDM-QPSK/16QAM/64QAM signals are set as the signal under test (SUT). Meanwhile, we find that the dynamic range of SUT input power is determined by the BPD imbalance and the FOFC power. By increasing either the average power of the FOFC from 4 to 10 mW or the imbalance of the BPD from 1.03 to 1.04, we can enhance the dynamic range of SUT input power by 8.8 mW and 6.6 mW, respectively. Finally, we develop the equipment and experimentally verify the BBD scheme by a precise characterization of 32 GBaud PDM-QPSK signal.

In this article, trellis shaping technique is numerically and experimentally investigated to improve the performance of optical orthogonal frequency division multiplexing (OFDM) signal. Different from other constellation shaping techniques, trellis shaping can be designed to reduce the correlation value of OFDM data sequence and average power simultaneously in one OFDM symbol. It means that optical OFDM signals generated by trellis shaping can improve the performance under both linear and nonlinear channels. For comparison with trellis shaping technique, we also consider optimal Maxwell–Boltzmann (MB) shaping and uniform signaling techniques at spectral efficiency (SE) of 10 and 14 bits/4D-sym. Numerical simulations show that trellis shaping can provide certain shaping gain over uniform signaling under linear additive white Gaussian noise (AWGN) channel. The fiber transmission results indicate that OFDM signals with trellis shaping have higher effective signal-to-noise ratio (SNR) than those with MB shaping and uniform signaling. It means that trellis shaping technique has the ability to mitigate the fiber nonlinear noise. Considering both linear and nonlinear noise in the fiber link, trellis shaping can extend the transmission distance by $\sim$ 110 km and $\sim$ 200 km over MB shaping and uniform signaling at SE of 10 bits/4D-sym. At SE of 14 bits/4D-sym, trellis shaping provides similar performance as MB shaping but $\sim$ 120-km extension reach in comparison with uniform signaling. The performances of OFDM signals with trellis shaping, MB shaping and uniform signaling are also experimentally investigated in a four-channel wavelength division multiplexing (WDM) fiber transmission systems. The experimental results show that trellis shaping can also mitigate the nonlinear distortions induced by the clipping effects at the transmitter side. Compared with MB shaping, trellis shaping extends the transmission reach by $\sim$ 240 km and $\sim$ 90 km at SE of 10 and 14 bits/4D-sym, respectively.

We experimentally demonstrate digital signal processing (DSP)-free 8 × 10 Gb/s 4-level pulse-amplitude modulation (PAM-4) transmission for cost-effective coherent wavelength division multiplexing passive optical network (WDM-PON) with 20 GHz channel spacing over the C-band. The polarization-independent coherent receiver at optical network unit (ONU) including a 3 × 3 fiber coupler and a polarization beam splitter (PBS) followed by three single-ended photodiodes (PDs), is simple and cost-effective, because neither DSP nor phase/frequency locking is necessary for the coherent detection of PAM-4 signal. We numerically compare the performance between the transmitted signal diversity and the local oscillator (LO) signal diversity schemes, and optimize the parameters of heterodyne detection, in order to enhance the power budget. Based on the LO signal diversity scheme, we can achieve an average receiver sensitivity of −26 dBm for eight wavelength channels at a bit error ratio (BER) below the hard-decision forward error correction (HD-FEC) threshold (i.e., 3.8 × 10−3), leading to a power budget of 29 dB after 25 km standard single-mode fiber (SSMF) transmission. Moreover, we verify an unequally spaced PAM-4 signaling, leading to contribution of 1 dB receiver sensitivity improvement for 10 Gb/s/λ PON.

An elegant method of nonlinear Fourier transform (NFT) has attracted worldwide research interests and such NFT methodology provides a new viewpoint on the physics of laser dynamics. Recently, the use of the NFT has been proposed for the investigation of laser radiation, indicating the capability to characterize the ultrashort pulse in the nonlinear frequency domain. Here, pure solitons are numerically separated from the resonant continuous wave (CW) background in a fiber laser by utilizing NFT. It is identified that the soliton and the resonant CW background have different eigenvalue distributions in the nonlinear frequency domain. Similar to water distillation, we propose the approach of soliton distillation, by making NFT on a steady pulse generated from a fiber laser, then filtering out the eigenvalues of the resonant CW background in the nonlinear frequency domain, and finally recovering the soliton by inverse NFT (INFT). Simulation results verify that the soliton can be distinguished from the resonant CW background in the nonlinear frequency domain and pure solitons can be obtained by INFT.

Self-accelerating beams have attracted worldwide research attentions due to their intriguingly curved trajectory with localized peak intensity and the characteristic of self-healing after obstacles. We first put forward a generation vector to analytically describe general trajectory manipulation for 2-D Airy beam. Based on the successful trajectory manipulation of 2-D Airy beam, a 100 Gbps NRZ-OOK real-time free-space photonic interconnection is experimentally demonstrated. Our results indicate that the bending trajectory can break the constraints of traditional free-space photonic interconnection where the receiver has to be set under the condition of line-of-sight. Meanwhile, by taking the power correction factor into account of the beam width variation, we propose a metric of normalized power density (NPD) in order to characterize the power density variation of beam main lobe along the free-space propagation. After calculating the 3 dB NPD range of both Airy beam and Gaussian beam, we identify that the power density of Airy beam main lobe can be maintained for a longer free-space reach than a Gaussian beam with the same minimum width.

For future-oriented space-division multiplexing (SDM) networks based on multicore fibers (MCFs), intentional inter-core signal coupling and reconfigurable switching are desired to transfer signals between cores and establish various network topologies. In this paper, we inscribed long-period gratings (LPGs) into the MCF using the programmable electrical arc discharge (EAD) method. When the MCF is held straight, the LPGs initiate resonations between core modes and cladding modes for each individual core, with the signals at certain wavelengths transferred among cores through shared cladding modes. When the MCF-LPG is directionally bent, the inter-core power coupling is blocked due to the bending loss. We realized efficient power coupling among three ambient cores in a seven-core fiber at the C-band. The minimum end-to-end insertion loss is 11.3 dB, including that of the fan-in/fan-out multiplexers, and the maximum extinction ratio between the switching ON/OFF states is 39 dB. A six-wavelength SDM coherent transmission system is established to evaluate the performance of signal switching, with each wavelength channel modulated with a 224 Gb/s 16QAM-OFDM signal. The transmission results validate that all spatial output ports have good performances in both switching states, which indicates that reconfigurable inter-core signal switching is feasible for enabling flexible network functions within MCFs.

In this paper, we proposed a simple uniform entropy loading (UEL) algorithm to adaptively load the same entropy for all the subcarriers according to the channel state information. Thanks to the signal-to-noise ratio (SNR) equalization effect of precoding, only one distribution matcher is used for UEL, maximally reducing the complexity of entropy loading. The proposed algorithm is based on the relationship between the optimal information bits per symbol (IBPS) and SNR for probabilistically shaped quadrature amplitude modulation (PS QAM) under the constraint of normalized generalized mutual information (NGMI) considering the implementation of constant composition distribution matching (CCDM) and off-the-shelf forward error correction (FEC) code. Different from generalized mutual information (GMI), IBPS intuitively represents practical performance. Besides, the relationship between IBPS and SNR for conventional QAM and time domain hybrid QAM (TDHQ) are also calculated to perform the well-known bit power loading (BPL) algorithm, Levin-Campello (LC) algorithm, and TDHQ assisted adaptive loading (AL) algorithm, respectively. Based on theoretical analysis, to our best knowledge, we first found that precoding procedure adopted by TDHQ assisted AL and UEL scheme degrades Shannon capacity compared to water-filling algorithm, the upper limit of BPL algorithms. However, benefiting from shaping gain of PS QAM, the proposed UEL algorithm achieves higher net data rate (NDR) and lower peak-to-average power ratio (PAPR) than the LC algorithm, which is demonstrated by the simulations and experiments in single sideband discrete multi-tone (SSB-DMT) systems. According to the experimental results, up to 3.2% NDR improvement and 0.82 dB PAPR reduction are realized by the UEL algorithm compared to the LC algorithm. The UEL algorithm also outperforms TDHQ assisted AL algorithm by the average NDR gain of up to 8.0%. The simple and effective advantages make the proposed UEL scheme suitable for the short-to-medium reach optical fiber communication systems.

Efficient synchronization is of great importance for fiber optical coherent communication system. Here, we propose an accurate and low-complexity timing/frequency synchronization scheme based on sparse fast Fourier transform (S-FFT). The proposed scheme consists of coarse timing/frequency synchronization, fine timing synchronization, and fine frequency synchronization. Inspired by the idea of S-FFT, we take full advantages of the sparse nature of training symbols (TSs) and realize the synchronization in the frequency domain. The proposed scheme enables accurate timing synchronization at low signal-to-noise ratio (SNR) region. A complete frequency offset estimation (FOE) range of [−symbol rate/2, + symbol rate/2] can be realized with high resolution. Finally, the proposed scheme is experimentally verified through 10 Gbaud dual-polarization (DP) 16/32-quadrature amplitude modulation (QAM) transmission, without the BER performance penalty. In particular, the total computation complexity is reduced by nearly 200 times in comparison with that using conventional sliding window correlation scheme.

Optical signal-to-noise ratio (OSNR) monitoring is essential to both the operation of reliable and reconfigurable network and the supply of high quality-of-service. Recently, deep learning technique has been proposed for the implementation of OSNR monitoring. However, the potential of deep learning technique for OSNR monitoring has not been fully exploited in terms of the monitoring accuracy and robust operation. Here, we propose an OSNR monitoring scheme with high accuracy and short response time using the long short-term memory neural network (LSTM-NN). The use of LSTM-NN is helpful to identify the relationship between the time-varied data and corresponding OSNR without manual feature extraction. We investigate the optimal number of time steps for the LSTM-NN-based OSNR monitoring and the monitoring accuracy with respect to various modulation formats with variable baud-rates. Finally, we implement an experimental verification for 34.94-GBd polarization division multiplexing-16 quadrature amplitude modulation signal, achieving a mean absolute error of 0.05 dB over the OSNR range from 15 to 25 dB. The advantages of LSTM-NN-based OSNR monitoring include high accuracy and smart response, which is ideal for future agile optical network.

In recent years, many conventional image denoising techniques have been intensively studied to enhance the signal to noise ratio (SNR) of Brillouin optical time domain analyzer (BOTDA), due to their superior denoising performance to one-dimensional methods. However, in the case of low sampling rate, the details of the signal are smoothed out due to less useful information, resulting in a degradation of the spatial resolution. Moreover, these conventional denoising algorithms are quite time-consuming compared with the BOTDA measuring time. To overcome these drawbacks, we employ a feed-forward convolutional neural networks (CNN) based image denoising for BOTDA. A conventional BOTDA system with 15 ns pulse width is implemented to demonstrate the effectiveness of the exploited CNN-based denoising method. The actual electrical noise signals of the BOTDA at different sampling rates are collected to synthesize training samples. The CNN model is trained with the noise and simulated BOTDA signals. Experimental results show that SNR improvement of 13.43 dB, 13.57 dB, and 12.9 dB is achieved at a sampling rate of 500 MSa/s, 250 MSa/s, and 125 MSa/s, respectively, via the trained CNN denoiser. No spatial resolution distortion can be observed in the denoised BOTDA signals. Besides, the CNN denoiser only takes 0.045 s to process a 151 × 50000 image benefiting from GPU computing. This processing time is negligible compared with the acquisition time of BOTDA, which makes real-time denoising possible.

We experimentally demonstrate the generation of 107-Gbit/s net-rate optical discrete multitone signal using a 1.5- μ m single-mode vertical cavity surface emitting laser (VCSEL) with modulation bandwidth of 22 GHz. Utilizing a nonlinearity-tolerant channel equalization algorithm for digital signal processing, total net-rates of 726.6-Gbit/s over 2.5-km dispersion-uncompensated 7-core fiber and 533.1-Gbit/s over 10-km dispersion-compensated 7-core fiber below 7% overhead hard-decision forward error correction limit have been experimentally achieved with a 1.5- μ m VCSEL-based intensity-modulation direct-detection system. The features of the 1.5- μ m single-mode VCSEL, 2.5-km/10-km multi-core fibers, and fan-in/fan-out modules are presented. Besides, the Volterra series-based nonlinearity-tolerant channel equalization algorithm, which improves the signal-to-noise ratio with more than 5 dB, is mathematically described and experimentally validated. The results have demonstrated that 1.5- μ m single-mode VCSEL and multi-core-fiber-based transmission can be a promising candidate to solve the capacity challenges in short-reach optical interconnects.

We have proposed and demonstrated a hybrid optical-fiber sensor that enables simultaneous distributed acoustic sensing (DAS) and distributed temperature sensing (DTS). The hybrid fiber sensor is realized through space-division multiplexed (SDM) reflectometers in a multicore fiber (MCF), where Raman optical time-domain reflectometry (ROTDR) for DTS is implemented simultaneously with phase-sensitive optical time-domain reflectometry (Φ-OTDR) for DAS through space-division multiplexing. The SDM reflectometers share an identical pulse source, but use separate interrogation fiber cores, allowing simultaneous measurement of ROTDR and Φ-OTDR. The proposed hybrid sensor based on MCF does not suffer from the incompatible pump power levels issue existing in its counterpart based on single mode fiber thanks to the SDM implementation. Thus it effectively eliminates the restriction imposed by fiber nonlinear effects (e.g., modulation instability). Wavelet transform denoising method is employed to reduce the noise of temporal ROTDR traces; as a result, the worst temperature uncertainty is reduced from 4.1 to 0.5 °C over 5.76 km sensing range. The proposed SDM hybrid fiber sensor can realize simultaneous distributed intrusion detection and temperature monitoring. It offers great potential in long-term real-time pipeline monitoring for oil and gas industry.

We report the design, fabrication, and characterization of a panda type few-mode fiber (panda-FMF) capable of both mode profile and polarization maintenance, due to the use of two applied stress rods into the conventional FMF. The panda-FMF can support six spatial modes including LP01, LP11a, LP11b, LP21a , LP21b, and LP02, and each possesses two orthogonal linear polarization modes. According to the characterization results of panda-FMF, we theoretically identify that the effective refractive index difference between either two spatial or orthogonal linear polarization modes is more than 1 × 10−4 over the C-band. Moreover, the experimental results verify that both mode profile and polarization of high-order radially asymmetric modes can be perfectly maintained even under severe twist rate of 1.57 rad/m.

We experimentally demonstrate a stable and compact dual-loop optoelectronic oscillator (OEO) using multicore fiber and self-polarization-stabilization technique. Two fiber cavities using two standard single mode fibers with different length in traditional dual-loop OEOs are replaced by only one multicore fiber where several cores are linked together to form the short and the long cavity simultaneously. Thanks to this fiber-integrated medium, the OEO relative stability is enhanced and the required cavity length is decreased. To avoid the influence from unstable polarization states of optical signals in these two fiber cavities, a self-polarization-stabilization technique is proposed. Therefore, no additional polarization controller and polarization maintaining fiber are required, contributing to further shorter fiber length in cavities and more compact structure. Moreover, the frequency and power stability of the generated RF signal in OEO is improved a lot compared with the traditional dual-loop OEO using the polarization division multiplexing technique. The experimental results show that the frequency and power drift of the generated RF signal with frequency of 7.8 GHz are within 0.59 ppm and 0.44 dB during 1 h, respectively. Furthermore, by introducing a tunable microwave photonics filter, a frequency tunable OEO is achieved. The experimental results show that the frequency of the generated RF signal could be tuned from 3.5 to 17.1 GHz, the phase noise performance at 10 kHz frequency offset is around −100 dBc/Hz in whole frequency range, and the side mode suppression ratio is 61 dB.

We propose a scheme for security improvement in the orthogonal frequency division multiplexing (OFDM) passive optical network based on a fixed-point digital chaos algorithm with a low computational precision. The dynamical degradation problem is improved by introducing the transmitted data as a perturbation source. In our demonstration, aperiodic key streams are generated from a fixed-point chaos with its trajectory disturbed by the upstream data, and then used to encrypt the downstream data. A 7.64-Gbps encrypted OFDM signal is successfully transmitted over a 25-km standard single mode fiber. The experimental results indicate that the proposed scheme suggests an effective and promising solution to meet the demands for low implementation complexity and high security level.

We propose and experimentally demonstrate two types of infinite impulse response (IIR) microwave photonic filters (MPFs) with different configurations, named cascaded configuration and parallel configuration. Both configurations are based on the homogeneous multicore fiber (MCF) in which the intercore power coupling is enabled by the tapering technique. The quality factors (Q factors) of cascaded and parallel IIR-MPFs are 143 and 136, respectively, which is much higher than that of one single IIR filter. The geometric structure of the tapered MCF and the number of spatial cores open new dimensions to manipulate the performance of MPFs. Theoretically, higher Q factor and rejection ratio can be obtained by using more cores of MCF to build more cascaded or parallel filters. These two types of IIR-MPFs show good reconfigurability and tunability by adjusting optical power and time delay through simulations. We anticipate that microwave photonics systems will benefit from these two types of IIR-MPFs in terms of compactness, reconfigurability, tunability, and performance stability.

Polarization mode coupling (PMC) is a detrimental effect that limits the polarization maintaining performance of polarization maintaining fibers (PMF). In this paper, we propose and demonstrate a novel method based on a principle similar to that of a microwave photonic filter (MPF) for the PMC distribution measurement. By regarding the PMF with PMC taking place as an MPF and measuring its transfer function in frequency domain, we are able to identify the intensities and locations of PMC events along PMFs. After detailed theoretical analysis and numerical simulations, validation experiments have been conducted on four PMF segments with rotational splicing between each other to manually generate PMCs. The measurement results of PMC positions and strengths are in accordance with the preset values. With a frequency scanning step of 16 MHz and a range of 14.4 GHz, such method can achieve a spatial resolution of 30 m and a measurement range of 27 km. The proposed method is suitable for the measurement of PMCs along PMFs up to tens of kilometers.

A novel automatic bias control (ABC) method which could lock an optical IQ modulator at arbitrary bias points is proposed and experimentally demonstrated for the first time. In the proposed method, the sequences and parameters of the used dither signals and the switch of the applied bias voltages are specially designed to minimize the influence of the power coupling between three MZMs in optical IQ modulator and the nonideal extinction ratios. By this way, precise electro-optical characteristic parameters of these three MZMs can be obtained by calculating the relevancy of the produced dither signals and the optical output signal. Based on these obtained parameters, any-bias-locking operation could be achieved via a feedback control loop. The accuracy and stability of the proposed ABC scheme are experimentally verified by using 1 GHz sine signals and 6 Gbaud single-carrier QPSK signals, and the accuracy of the proposed ABC algorithm is about 0.4° due to the limit of DAC resolution. Moreover, thanks to this ABC scheme, a simple and stable 29.2 GHz millimeter wave generation with SNR of 30 dB by optical frequency quadrupling technique could be realized without any optical or electrical filter.

The set-partitioning quadrature amplitude modulation (SP-QAM) formats are highly preferred for the next generation fiber optical transmission system, due to the Euclidean distance increment of constellation and its power efficiency. However, the existing carrier phase recovery (CPR) scheme of SP-QAM is independently implemented between two polarization signals, and the laser linewidth tolerance of SP-QAM is restricted the same as that of conventional QAM signals. Here, we propose a novel four-dimensional (4-D) CPR scheme for the SP-QAM formats. After the phase offset estimation and precompensation between two polarizations, we implement blind phase search (BPS) algorithm in the 4-D space. Meanwhile, traditional decision circuit is replaced by 4-D detection of SP-QAM and 4-D distance is calculated as the cost function for identifying the desired test phase angle. We carry out numerical simulations under conditions of 28 Gbaud SP-128-QAM, SP-512-QAM, and SP-2048-QAM coherent transmissions. The CPR performance of our proposed 4-D BPS scheme is much better than that of conventional BPS. Given the 1 dB required optical signal-to-noise ratio penalty at ${\rm{BER}} = 1 \times {10^{-3}}$ , the linewidth times symbol duration products of $5.2 \times {10^{-4}}$ , $2 \times {10^{-4}}$ , and $7 \times {10^{-5}}$ are tolerable for SP-128-QAM, SP-512-QAM, and SP-2048-QAM, respectively. We further experimentally verify the performance of the proposed 4-D BPS under the scenarios of 28 Gbaud SP-128-QAM back-to-back and standard single-mode fiber transmission from 750 to 1500 km, indicating of the robustness to the amplified spontaneous emission noise and transmission impairments.

A novel low-complexity digital signal processing solution is presented to compensate the impairments of limited bandwidth and nonlinearity. It is based on the equalizer, adaptive noise-whitening postfilter, and maximum likelihood sequence detection (MLSD). The system performance loss is induced by higher noise power at the signal band edges after the equalizer can be mitigated by the postfilter and MLSD. Given the above structure, it is possible to enrich the equalizer by providing nonlinear compensation capability. A memory polynomial equalizer (MPE), instead of Volterra equalizer (VE), is applied as the equalizer to compensate the nonlinearity impairments in order to make a tradeoff between complexity and performance. For the adaption of MPE, the blindly adaptive multistep-size decision-directed least-mean-square algorithm is selected. By using a dual-drive Mach–Zehnder and direct detection, 64-Gb/s single-sideband 4-ary pulse amplitude modulation (SSB-PAM4) transmission over 120-km dispersion-uncompensated standard single-mode fiber (SSMF) with 10-dB bandwidth roughly 13.5 GHz is experimentally demonstrated. Given a 20% overhead soft-decision forward-error correction with bit error rate threshold of $2\times 10^{{-2}}$ , the dispersion-uncompensated SSMF transmission distance can be significantly increased from 40 to 120 km with the proposed receiver side solution. The optical signal noise ratio performances for different fiber reach and receiver structure are investigated. Furthermore, we compare the computational complexity of MPE with VE and conclude that MPE can substantially reduce computation complexity with negligible performance loss.

We present a detailed investigation of the singe-channel 100G polarization-multiplexed quadrature-phase-shift-keying-based unrepeatered coherent transmissions using various order distributed Raman amplifiers (DRAs). The impact of main factors on the system performance is discussed including intrachannel nonlinearity, amplified spontaneous emission, double Rayleigh backscattering, and pump relative intensity noise (RIN)-induced impairments. After optimizing the pump power configuration, it is found that compared to a first- and a third-order DRA, a second-order DRA can provide the best performance owing to relative high optical signal-to-noise ratio (OSNR) and low pump RIN-induced penalty. By using the Giles model, the optimization of a remote optically pumped amplifier is performed in terms of the Erbium-doped fiber length and its farthest position, with given input signal's power and OSNR.

We proposed and demonstrated an in-band optical signal-to-noise ratio (OSNR) monitoring method for a polarization-division-multiplexed coherent optical orthogonal frequency-division-multiplexing (PDM-CO-OFDM) system. OSNR of the PDM signals on both polarization tributaries can be simultaneously monitored by calculating the electrical SNR with low complexity by taking advantage of the uncorrelation property between the noise and training symbol, which is used for conventional frame synchronization. This OSNR monitoring technique is verified in 103.57-Gb/s 16 quadrature amplitude modulation-based PDM CO-OFDM transmission system with 240—1440-km standard single-mode fiber (SSMF) links. Experimental results show that after calibration, the OSNR estimation error is less than 0.6 dB in a dynamic range of 12—27 dB, the estimation remains accurate even after 1440-km SSMF transmission, regardless of the chromatic dispersion and polarization mode dispersion in the fiber. To further investigate the robustness of the proposed OSNR monitoring method, additional simulation is implemented on VPI and MATLAB platform. It turned out to be insensitive to nonlinear effect within reasonable range as our monitoring technique shows good performance with launching power below −2 dBm after 500-km SSMF transmission. The proposed technique could still accurately monitor the individual OSNR of each polarization tributary of the PDM signal even in the presence of 5-dB polarization-dependent loss.

A fiber-based all-optical switch accomplished by pump depletion in a single-pump phase sensitive (PS) amplifier is theoretically investigated using an inhomogeneous fiber model including the zero-dispersion wavelength (ZDW) fluctuations. We find that the off -state (maximum pump depletion) of the PS all-optical switch is very sensitive to the ZDW fluctuations, whereas the on -state (minimum pump depletion) is more robust. Meanwhile, the nonreciprocal transmission characteristic of the PS all-optical switch induced by the ZDW fluctuations is investigated with the help of the proposed inhomogeneous fiber model. Our theoretical results are useful to optimize the experimental implementation of PS all-optical switch.

We propose an intercarrier interference (ICI) mitigation scheme for dual-carrier superchannel transmission. Using the modified decision directed least mean square (DD-LMS) based multi-input multi-output (MIMO) processing, we can simultaneously realize both polarization division demultiplexing and ICI mitigation for PDM-m-PSK and PDM-m-QAM formats. Thanks to the innovative modification of frequency-shift operation, the MIMO equalization enables synchronous output of dual-carrier signals with robust performance against frequency offset, laser phase noise, as well as polarization mode dispersion (PMD). The experimental back-to-back transmission of PDM-16QAM signal has 8.3-dB OSNR improvement using the proposed MIMO processing. Three dual-carrier 160-Gb/s PDM-16QAM superchannels within 25-GHz grid are experimentally transmitted over 640-km standard single-mode fiber (SSMF), leading to net spectral efficiency of 5.7b/s/Hz.

We apply the signal- and channel-independent constant amplitude zero autocorrelation sequence (CAZAC) precoding technique to a bandwidth-limited longer-reach DDO-OFDM system, and propose a simplified adaptive modulation scheme according to the equalized SNR per subcarrier after CAZAC precoding. Simulation results show that with the optimized modulation index, conventional 8.23-Gb/s double sideband (DSB) modulated QPSK-OFDM signal cannot realize 100 km standard single-mode fiber (SSMF) transmission at the given received optical power, while CAZAC precoding can enable 100 km SSMF transmission with dispersion-induced power penalty as low as 2.7 dB. The advantage becomes more obvious when CAZAC precoding is employed to 16.46-Gb/s 16QAM-OFDM systems. Thanks to the frequency diversity, OFDM system with CAZAC is proved to be more dispersion tolerant with improved receiver sensitivity. We also analyze the signal-to-noise ratio (SNR) of each individual subcarrier and its flatness is confirmed to be much improved independent of modulation formats and transmission distances when precoding is used. Based on the priori knowledge of flat SNR curve, we further propose a simplified adaptive modulation scheme using only three steps where bit loading can be applied with power loading omitted. Simulation and experimental results demonstrate that the BER performance can be further improved with negligible data rate reduction and system complexity aggravation using our proposed simplified adaptive modulation method.

In this paper, we propose and demonstrate a blind chromatic dispersion (CD) estimation method based on fractional Fourier transformation (FrFT). Through numerical simulations, the proposed CD estimation method is shown to be robust against the amplified spontaneous emission noise and nonlinear interference. Only 2048 samples are required for reliable CD estimation for single-carrier 28 GBd DP-QPSK or 32 GBd DP-16QAM signals, and the standard deviation can be as low as 98.9 and 103.6 ps/nm, respectively. The feasibility of the proposed CD estimation method has been experimentally verified using 28 GBd DP-QPSK and 14 GBd DP-16QAM signals over various transmission distances with CD ranges of 46 332–77 220 ps/nm and 5148–51 480 ps/nm, respectively. Compared with some other CD estimation methods, the method based on FrFT has advantages in the aspects of less computation complexity and robustness to transmission impairments.

We propose a coherent optical OFDM (CO-OFDM) system based on 3-dimensional (3-D) OFDM technique in this paper. By using the 3-D signal mapper and 2-D IFFT module, the minimum Euclidean distance (MED) in constellations is increased by 8.7%, and the peak-to-average power ratio of the generated OFDM signal is reduced by 4 dB. The experimental results show that the 3-D OFDM method can improve nonlinearity tolerance by 2 dB for 100-km standard single mode fiber in a 16.27-Gb/s 2-D coherent optical OFDM system without polarization modulation. Theoretical analysis and simulation results show that the OSNR penalty could be attributed to the increased MED, the extended decision boundaries and the coding gain. To further analyze the performance in long haul transmission system, the simulation setup of 1000-km SSMF CO-OFDM system is built up. The results verify that the proposed method could effectively enhance nonlinearity tolerance and transmission performance of the CO-OFDM system.

Low-complexity carrier phase recovery (CPR) for M-ary offset quadrature amplitude modulation (OQAM) formats is proposed and numerically verified, based on the technique of constellation classification. Although the OQAM signal has very poor tolerance to the laser linewidth due to the phase noise-induced crosstalk, we find that both even and odd samples of OQAM signal only have conventional constellation rotation with respect to phase noise. Through linear fitting of the classified constellation, carrier phase estimation can be derived from the slope of linear fitting function. Compared with our recently proposed modified blind phase search scheme, the computational complexity (CC) of proposed CPR does not increase much with the OQAM modulation level of M. Taking the CPR of 16-OQAM into account, we show that the CC can be significantly reduced by a factor of 8.47 (or 7.34) in the form of multipliers (or adders) with performance comparable to MBPS (B = 24) under the condition of Δf × T S = 5 × 10 -6 . In particular, a tolerance of linewidth and symbol duration product of 8 x 10-5, under the condition of CD = 300 ps/nm and DGD = 10 ps, can be secured for 16-OQAM, given 0.7-dB required-OSNR penalty.

Frequency offset correction (FOC) is essential for a coherent optical orthogonal frequency-division multiplexing (CO-OFDM) system. In this paper, we proposed a robust and efficient FOC algorithm by designing training symbols in frequency domain. This algorithm has an advantage in overcoming imperfect frequency response induced by transmitter devices and physical channel implementations. To further improve the stability of the FOC algorithm against the optical signal-to-noise ratio degradation, a two-step iterative operation is introduced. The proposed algorithm has been utilized in a practical 100-km 16QAM-CO-OFDM transmission system to compensate the frequency offset dynamically. The efficiency and robustness of the algorithm have been validated by outstanding and stable experimental results.

We investigate five-channel polarization division multiplexed Nyquist WDM superchannel using offset-16QAM and 16QAM modulation format, under the condition of transmitter-side digital spectral shaping technique, respectively. The offset-16QAM scheme can greatly relax the implementation complexity, because the tap number of 35 is sufficient for the required finite impulse response (FIR) filter, compared with 81-tap FIR filter of the 16QAM scheme. About 1.3-dB back-to-back sensitivity improvement is obtained for the offset-16QAM scheme. In particular, the offset-16QAM scheme has better tolerance of DACs’ constraints, including the untracked jitter, sampling rate, and resolution. Finally, the practical implementation of offset-16QAM-based Nyquist WDM superchannel with TS-DSS is evaluated, by taking the phase difference deviation among wavelength channels and phase noise into account. No more than 1.3-dB required-OSNR penalty can be obtained, in case the phase difference is varied from 0° to 360°. When the laser with a linewidth of 100 kHz is used, there still exists about 1-dB overall performance improvement, compared with the 16QAM scheme.

We report a fiber-optic magnetic field sensor with ultra-high sensitivity based on a precisely configured extrinsic fiber-optic Fabry–Perot interferometer (EFFPI) and Terfenol-D slab. The EFFPI was simply formed by placing two well-cleaved single-mode fibers with carefully designed spacing and it was bonded to the surface of a Terfenol-D slab by epoxy resin. The experiments demonstrate good linear relationship between the applied magnetic field strength and the wavelength shift up to 560 Oe and the measurement range is only limited by the available bandwidth of the light source. The maximal sensitivity of the magnetic field measured by the proposed sensor is 854.73 pm/Oe through monitoring the shift of wavelength dip of the spectrum reflected from the EFFPI, which is significantly larger than most of the reported results. We also evaluated the repeatability of the proposed sensor and the performance of the proposed sensor working at a direct-current (dc)-biased magnetic field. Results indicated that the proposed fiber-optic magnetic field sensor exhibits good restorable measurement performance up to 140 Oe and a preapplied dc magnetic field can be used to extend the linear measurement dynamic range.

We present theoretical investigation of all-optical differential phase-shift keying (DPSK) regenerative wavelength multicasting using dual-pump degenerate phase sensitive amplifier (PSA), based on seven-wave model rather than traditional three-wave model. We first test the accuracy of the seven-wave model in the context of 40 Gb/s DPSK all-optical regenerative multicasting. We find that the seven-wave model is more accurate than the three-wave model. Meanwhile, we successfully optimize optical signal power, frequency spacing between two pumps, and regeneration capability of all multicasting channels. All-optical DPSK regenerative one-to-nine wavelength multicasting can be successfully realized. Both eye diagrams and the calculated bit error rate curve demonstrate that the optical signal to noise ratio penalties of all multicasting channels are improved by around 3 dB. Finally, to the best of our knowledge, we investigate, for the first time, the effects of the high-order four-wave mixing (FWM) products on the evolution of pump-to-signal intensity modulation transfer (IMT) and IMT from the pump to all wavelength-multicasting channels in a dual-pump degenerate PSA. We find that high-order FWM products contribute to the suppression of pump-to-signal IMT after proper optimization of dual-pump's frequency spacing and signal optical power. Both regenerative one-to-nine wavelength multicasting and pump-to-signal IMT smaller than 1 can be simultaneously achieved under the gain saturation region.

We propose a novel method to simultaneously improve the physical layer security and the transmission performance of the orthogonal frequency division multiplexing (OFDM) passive optical network system by using chaos and fractional Fourier transform (FrFT) techniques. The designed 3-D chaotic sequences are used to form the training sequence for time synchronization, to perform the OFDM subcarriers masking, and to control the fractional order of the FrFT operation. The analyses show that the whole key space size of the proposed scheme could be beyond 1050, and the peak-to-average-power-ratio of the transmitted OFDM signal can be decreased by about 0.5 dB. Furthermore, we successfully demonstrate an 8.18 Gbps 16-quadrature-amplitude- modulation (QAM)-OFDM data transmission experiment with chaotic and FrFT operations over 25 km single mode fiber. The results show that the proposed scheme could effectively enhance the system security and the transmission performance without additional bandwidth requirement.

Although the pump relative intensity noise transfer phenomenon in traditional Raman amplified amplitude-shift keying fiber communication systems has been investigated intensively, the stochastic intensity fluctuation of Raman pump laser will interplay with fiber nonlinearity and leads to additional phase noise of signal in the multilevel modulated coherent optical communication system. Such phase noise is defined as relative phase noise (RPN). In this paper, we present comprehensive analysis regarding the characteristics and impairments of RPN through both theoretical derivation and Monte Carlo simulation. We derive the analytical expressions of phase error variance for M-ary PSK and 16QAM modulation formats in consideration of RPN using decision-aided maximum-likelihood phase estimation algorithm. The analysis can be used to predict RPN induced impairment theoretically. Bit error rate (BER) performance is evaluated and compared for QPSK, 8PSK, 16PSK, and 16QAM modulation formats, and the result shows that in the context of RPN 16QAM signal outperforms 16PSK which has the same spectral efficiency. Finally, we extend the analytical result obtained in pump nondepletion regime to depletion regime via numerical analysis. It is observed by comparison that Q-factor penalty will increase in depletion regime.

With development of high power GaN laser diode (LD) and Bragg gratings writing in fluoride glass and fibers, it's necessary and important to develop a numerical model for visible down conversion Pr 3+-doped fiber (PDF) lasers based on GaN LD and fiber Bragg gratings (FBGs) to optimize its performance for practical use. We propose a numerical model to investigate both steady state and Q-switching dynamics of a visible down conversion PDF lasers. FBGs are placed at both ends of PDF to form a linear cavity. First, the effect of fiber length, boundary reflectivity and the doping concentration on the output power at steady state was obtained and analyzed. The simulation result at steady state qualitatively agree well with previous experimental demonstrations and the FBG based cavity exhibits advantages over conventional PDF system in terms of low threshold and high slope efficiency. After that, by modulating the reflecting spectrum of FBG by using a piezoelectric transducer, Q-switched pulse behavior has been established to illustrate the impact of FBG feedback, pulse repetition frequency (PRF), pumping power, boundary reflectivity and the doping concentration on pulse shape and pulse characteristics. The larger gain factor is found to be the main reason to explain obvious multipeaks, spurious pulses, and considerable energy leakage in red color Q-switching process. Methods to mitigate above mentioned phenomenon have been proposed and demonstrated in this paper.

A novel approach to implementing instantaneous frequency measurement (IFM) based on an amplified fiber-optic recirculating delay loop and a broadband incoherent light source (ILS) is proposed, analyzed, and experimentally demonstrated. Since the semiconductor optical amplifier-based fiber-optic delay loop has an infinite impulse response that varies from a large positive value to negative infinity on a log scale, a unique relationship between the output power, and the frequency of the input continuous-wave (CW) microwave signal is established. Meanwhile, it is experimentally shown that the use of the ILS can greatly improve the stability of the proposed IFM system. When the input power of CW microwave signal is within the range of -7 dBm to -16 dBm, the measured errors remain within ±400 KHz over a frequency range of 6.94-6.958 GHz. The measurement error, the complexity and cost of the proposed IFM system can be considerably reduced by only using one ILS, one modulator, and one photodetector. Since the proposed IFM system has a capability of optical integration, it is theoretically estimated that the measurement range can be extended to 20 GHz with a measurement resolution of 1.36 dB/GHz.

A 40 Gb/s multifunction optical format conversion module based on nondegenerate four-wave mixing (ND-FWM) arising in a single semiconductor optical amplifier (SOA) is proposed and demonstrated. It has the ability to achieve NRZ-OOK/DPSK/DQPSK to CSRZ-OOK/DPSK/DQPSK, RZ-OOK/DPSK/DQPSK to CSRZ-OOK/DPSK/DQPSK, and CSRZ-OOK/DPSK/DQPSK to RZ-OOK/DPSK/DQPSK format conversions. A comprehensive dynamic model to investigate twelve FWM-related waves in SOA is presented. Using this model, the proposed module with multiple format conversions is numerically investigated. We find that the SOA based scheme can be used for high-speed all-optical multifunction format conversion, because one of the FWM participators also serves as an assist light to accelerate SOA gain recovery. Meanwhile three identical converted signals with different wavelengths can be obtained simultaneously for all-optical wavelength multicast application. Finally, the bit-error-rate (BER) measurements of some 10 Gb/s operations are demonstrated in order to further verify the proposed multifunction format conversion module. The power penalty is measured to be less than 1 dB at the BER level of 10-9.

Based on single-sideband (SSB) modulation, a novel scheme to simultaneously implement all-optical microwave bandpass filtering and up-conversion is proposed, which is favorable for radio-over-fiber (ROF) applications. SSB modulation with different sidebands will theoretically bring pi phase difference on a microwave signal by utilizing dual-electrode Mach-Zehnder modulators. This effect is the key to the realization of bandpass filtering, which was experimentally verified. With the help of several other photonic devices, a five-tap bandpass filter integrated with heterodyne-detection-based microwave frequency up-conversion was demonstrated theoretically. Besides the up-conversion function, the simulation certification of our proposed scheme also indicated some well filtering characteristics, such as more than 2-dB secondary sidelobe ratio improvement, the reconfigurability by applying different window functions, the filtering tunability with the center intermediate frequency (IF) from 2.16 to 4.08 GHz, and the moderate tolerance to a large dynamic IF input signal. The proposed scheme features the feasibility of the integration of some cost-effective frequency multiplication methods and the flexible bandpass-filtering characteristics for millimeter-wave ROF systems.

We analyze an active mode-locked laser under the influence of asynchronous spontaneous emission (ASE) noise by using eight mathematical series Sn, Qn, Rn, an, bn, cn, rn, and Pn to trace the evolution of the noise. The series are easily calculated from the laser parameters and are used to determine the steady-state pulse of the active mode-locked laser operating in both exactly tuned and detuned conditions. Series analysis results of ideal noiseless laser models are consistent with that of classical self-consistence methods. The advantage of our series approach is that it can be used for studying laser model even in the presence of the inline optically amplified noise. Analysis of the laser model with ASE noise reveals that the large noise figure amplifier and high cavity loss degrade the signal-to-noise ratio (SNR). Large decrease of SNR caused by detuning limits the locking range of the laser. The series method can be used not only to determine the characteristics of the steady state pulse but also to study the transient process of the noise inside the laser and to determine the locking range of the laser when the ASE noise is considered. Our analytical results are visualized by simulation results.

To extend the buffer depth of a fiber loop optical buffer, we have experimentally demonstrated an enhanced semiconductor optical amplifier (SOA)-based dual-loop optical buffer (DLOB) for storing variable-length optical packets. We have theoretically derived constraints governing the buffer depth of the DLOB, in which the SOA not only provides a nonlinear phase shift in the loop to implement the buffer function but also compensates for the fiber loop attenuation during long-time storage. It is found that the maximum allowable length of a stored packet to avoid the counter-propagation packet collision inside the SOA depends on the SOA bias position as well as the length of the fiber loop. To demonstrate the effectiveness of the proposed enhanced configuration, we have successfully demonstrated the storage of 2.5-Gbps variable-length packets even when the length of the input packet exceeds the corresponding length of the fiber loop. Another unique advantage of the proposed enhanced DLOB configuration is that it can also overcome the problem of power leakage of the stored packet due to a directional gain difference of single SOA and gain saturation. Index