Optical Signal-to-noise Ratio Research Articles

Direct detection transmission of a PAM signal with power fading mitigation based on Alamouti coding and dual-drive MZM.

Power-fading impairment induced by fiber dispersion and photodiode detection imposes a fundamental limitation on the intensity-modulation direct-detection (IM-DD) transmission systems. In this work, we propose a cost-effective pulse-amplitude modulation (PAM) signal transmission scheme with power-fading mitigation enabled by Alamouti coding and dual-drive Mach-Zehnder modulator (DDMZM). By interleaving the symbol blocks in the time domain for upper- and lower-arm of the DDMZM, flat end-to-end frequency response can be obtained without spectral nulls after combining the photocurrents at odd and even time slots. For single channel demonstration, we experimentally transmit up to 160Gb/s PAM-4, 140Gb/s PAM-6, and 108Gb/s PAM-8 signals over 80 km standard single-mode fiber (SSMF) with bit-error rates (BERs) below the 20% soft-decision forward error correction (SD-FEC) threshold of 2.0×10-2. For wavelength division multiplexing (WDM) transmission, 8λ×150Gb/s PAM-4 signals spacing at 100 GHz can also achieve 80 km reach. Moreover, we compare the optical signal-to-noise ratio (OSNR) sensitivity with single sideband (SSB) scheme and evaluate the tolerance of bias deviation in numerical simulation. The proposed Alamouti coding-based scheme provides a hardware-efficient and dispersion-tolerant candidate for high-speed inter-data center interconnect (DCI) applications.

Design of Optical Fiber Communication System Under Carrier Photodiode/Compact PCI Bus and Signal Data Modulation

This study was developed to design a high-speed and high-power uni-traveling-carrier photodiode (UTC-PD). Gaussian doping is adopted in the absorption layer, and a thin-doped cliff layer is introduced above the collection layer. The effects on the high-speed performance of UTC-PD are analyzed, and the thickness and concentration of absorption layer, cliff layer, and collection layer are optimized. Then, the carrier optical detector is used in optical fiber communication system, and a small pluggable SFP fiber module is used to realize the conversion of photoelectric signal. Based on the test requirements, the test platform of optical fiber communication system is built, and the CPCI bus based on PCI9054 is designed to communicate with the industrial computer. Meanwhile, a multi-polarization and high-order modulation scheme is designed for optical signal. The effects of transmission power, transmission distance, and optical signal-to-noise ratio (OSNR) on the transmission performance of orthogonal amplitude modulation (DP-16QAM, DP-32QAM) signals are analyzed by simulation. Simulation analysis shows that the optimized UTC-PD has improved its high speed and saturation, whose 3 dB bandwidth is up to 56 GHz, and responsiveness up to 0.28 A/W. In high-speed serial differential data transmission, the optical fiber communication system based on carrier optical detector/CPCI bus receives and sends reliably, and the error code value is always 0. Based on the signal modulation scheme in this study, the increase of OSNR can reduce the bit error rate (BER), and the bit error rate of 2A8P-16QAM and 4A8P-32QAM will decrease, that is, the signal transmission needs a higher optical signal-to-noise ratio.

Real-time demonstration of 103.125-Gbps fiber-THz-fiber 2 × 2 MIMO transparent transmission at 360-430 GHz based on photonics.

In this Letter, we experimentally demonstrate the first real-time transparent fiber-THz-fiber 2 × 2 multiple-input multiple-output (MIMO) transmission system with a record line rate of 125.516 Gbps at 360-430 GHz based on photonic remote heterodyning, hybrid optoelectronic down-conversion, and commercial digital coherent modules. The 103.125-Gbps net data rate using dual-polarization quadrature phase-shift keying (DP-QPSK) modulation is successfully transmitted over two spans of 20-km standard single-mode fiber (SSMF) and 60-cm wireless distance under 15% soft-decision forward error correction (SD-FEC) for a pre-FEC bit error ratio (BER) threshold of 1.56 × 10-2 (post-FEC BER < 10-15). The optical signal to noise ratio (OSNR) margin and the stability of the transmission system are extensively investigated. To the best of our knowledge, this is the first time to realize >100-Gbps real-time transparent fiber-THz-fiber link transmission at beyond the 350-GHz band, making it a promising scheme to pave the way towards a practical seamless integration of a fiber-THz-fiber link to the future 6G mobile communication system.

Joint multi-parameter optical performance monitoring scheme based on trajectory information for a Stokes vector direct detection system.

In this study, we propose and verify a joint multi-parameter optical performance monitoring (OPM) scheme based on trajectory information for the Stokes vector direct detection (SVDD) system, for the first time, to the best of our knowledge. Here, the proposed scheme first performs quantification of the trajectory to construct trajectory information, which not only presents diversity of the received symbols in spatial dimension, but also records the jump pattern among symbols in time dimension. Subsequently, eigenanalysis is introduced to extract critical features hidden in trajectory information and simultaneously achieve the purpose of dimensionality reduction. The effectiveness of the scheme is verified through 14/28 GBaud SVDD binary phase shift keying/quadrature phase shift keying/-8 quadrature amplitude modulation (QAM)/-16QAM/-32QAM/-64QAM simulation systems. Under the scenario of joint modulation format (MF) identification and optical signal to noise ratio (OSNR) monitoring, the identification rates of all six kinds of MFs achieve 100% within their corresponding reasonable OSNR ranges. Besides that, the average mean absolute error (MAE) of the monitored OSNRs are obtained as 0.03 dB, 0.22 dB, 0.36 dB, 0.41 dB, 0.46 dB, and 0.49 dB for those six kinds of MFs, respectively. Under the scenario of multi-parameter OPM, SVDD-8QAM/-16QAM/-32QAM signals are 100% successfully identified when residual chromatic dispersion (RCD) is located in the ranges of 0-200 ps/nm, 0-190 ps/nm, and 0-160 ps/nm, respectively. The average MAE of OSNR monitoring and RCD estimation for these three commonly used MFs are 1.08 dB and 3.23 ps/nm, respectively. Moreover, the study also demonstrates the robustness for baud rates and a relatively simpler calculation complexity about the proposed OPM scheme.

Optical performance monitoring using SOI-based spectral analysis.

A novel optical performance monitoring (OPM) method based on Fourier transform spectrum analysis (FTSA) is designed for optical signal-to-noise ratio (OSNR) monitoring, modulation format and baud rate recognition in the presence of fiber nonlinearities. The interference intensities, which reflect spectral features of signals, are obtained by exploiting the FTSA consisting of two-stage Mach-Zehnder interferometer (MZI) arrays. Then, the mapping between the OPM parameters and modulated interference intensity (MII) is characterized using neural networks without prior knowledge of the configuration of the communication network. Results show that optical performance parameters are monitored simultaneously. Meanwhile, the accuracy of modulation format and baud rate recognition is 94.8% and most (over 86%) OSNR monitoring errors are less than ±1 dB under complex transmission conditions in presence of frequency offset and delay jitter. Besides, the FTSA can be fabricated on a silicon on insulator (SOI) platform with a large fabrication tolerance, and it has broad working bandwidth to support the full optical communication band. Therefore, the proposed OPM method is capable of integration and miniaturization, which can be ubiquitously applied in network intermediate nodes to support the construction of smart optical networks.

Effect of Phase Noise on the Optical Millimeter-Wave Signal in the DWDM-RoF System

In this study, we examined the effect of phase noise on the optical millimeter-wave (mm-wave) signal in a dense wavelength division multiplexing radio-over-fiber (DWDM-RoF) system. A single modulator was used to generate the optical mm-wave signal in the DWDM-RoF system. This paper addresses the impact of phase noise, which results from phase imbalance, on the optical mm-wave signal. To lower the effect of phase noise on the optical mm-wave signal, the phase imbalance should be controlled. The phase imbalance can be controlled and decreased by adjusting the phase at the phase shift (PS). The system performance was analyzed using various parameters such as bit error rate (BER), signal-to-noise ratio (SNR), optical signal to noise ratio (OSNR), and error vector magnitude (EVM). From the results, we found the phase imbalance affected the optical mm-wave signal due to the imbalanced splitting of the signal intensity at the MZM. The phase imbalance impacts the phase noise, which impacts the optical mm-wave signal. The phase noise could be decreased by controlling the phase imbalance at the phase of 5π/12. The best results at the phase of 5π/12 were collected for phase noise at 0.02 degrees.

4×112 Gb/s/λ MCF Transmission Using Field PDM-PAM4 and Coherent Detection for Datacenter Applications

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

Fan-beam projection for modulation classification in optical wireless communication systems.

The focus of most research nowadays in the field of communication technology is on increasing bandwidth in wireless connectivity. Adaptive modulation can be used to enhance efficiency of communication systems. Adaptive modulation requires modulation format identification (MFI) at the receiver side to avoid the overhead required to determine the modulation type at the receiver. We present an MFI algorithm based on fan-beam projection to generate patterns from the constellation diagrams that are more discriminative. The constellation diagrams are obtained as images for eight different modulation formats (2/4/8/16 - PSK and 8/16/32/64 - QAM). Different classifiers such as AlexNet, VGG16, and VGG19 are studied and compared for the task of MFI. Evaluation of this proposed algorithm is performed by estimating the classification accuracy at different optical signal-to-noise ratios (OSNRs) ranging from 5 to 30 dB. The simulation results reveal that the proposed algorithm succeeds in identifying the wireless optical modulation format blindly with a classification accuracy up to 100% even at low OSNR values less than 8 dB compared with the related work.

A Multi-Wavelength Thulium-Doped Fiber Laser Using a Photonic Crystal Fiber-Based Sagnac Loop

A multi-wavelength thulium-doped fiber laser incorporating a photonic crystal fiber (PCF) Sagnac loop was presented and experimentally demonstrated. In the laser cavity, the Sagnac loop mirror used a 0.5 m PCF as a comb filter. A nonlinear polarization rotation (NPR)-based 600 m single-mode fiber (SMF) was used to suppress the mode competition. By adjusting the polarization controllers (PCs), up to nine stable wavelength outputs near the 2 μm spectrum were obtained at room temperature with an optical signal-to-noise ratio (OSNR) of up to 30 dB. The channel spacing of this fiber laser is 4.88 nm, and the 10 dB bandwidth encompasses 1950.39 to 1989.43 nm.

IMPROVEMENT OF MATHEMATICAL MODELS OF FIBER-OPTIC LINEAR PATH

The article describes the main problem of making a decision about the possibility and effectiveness of the use of fiber-optic communication facilities on communication lines with random parameters. As an example, fiber-optic communication lines of a trusted operator or an adversary are considered. The main reason is indicated, which is the lack of methods that allow predicting the possibility and expediency of using specific fiber-optic communication facilities in given environmental conditions. The necessity of developing and applying a probabilistic mathematical model of a fiber-optic linear path is explained as the basis of the methodology for evaluating the effectiveness of the use of fiber-optic communication means. An analysis of existing mathematical models is given and their shortcomings and imperfections are indicated. A probabilistic mathematical model of the fiber-optic linear path has been developed, which allows calculating the numerical characteristics of the quality indicators of the fiber-optic linear path: Optical Signal-to-Noise-Ratio (OSNR) and Bit Error Ratio (BER), based on the known technical data of the communication means used. The mathematical expectation of a random variable is used as a numerical characteristic, as an idealized average of OSNR and BER. Based on the mathematical expectation, other numerical characteristics are calculated, which are also used when making a decision as a risk measure, and allow us to judge its magnitude and significance. The developed probabilistic mathematical model is proposed as the basis of the efficiency assessment methodology for evaluating the effectiveness of work on communications.

Wavelength-switchable single-longitudinal-mode thulium-doped fiber laser at 2.05 µm using a superimposed fiber Bragg grating

Herein, we present a wavelength-switchable single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL), enabled by a three-channel superimposed fiber Bragg grating (SI-FBG), a compound-subring-cavity (CSC), and an unpumped TDF-based saturable absorber (SA). By adjusting a drop-in polarization controller (DI-PC), three single-wavelength operations can be obtained and switched flexibly. In each operation, the fiber laser exhibited a maximum optical signal-to-noise ratio (OSNR) of 62 dB as well as the wavelength and power fluctuations of ≤0.01 nm and ≤0.641 dB, respectively, in a 50-min time span. Furthermore, the SLM lasing characteristics of the proposed TDFL with the CSC and SA have been investigated in detail.

Joint monitoring of OSNR and frequency offset for coherent optical fiber communication systems based on QPSK training sequence

We propose a joint monitoring scheme of optical signal-to-noise ratio (OSNR) and frequency offset (FO) based on the analysis of a quadrature phase-shift keying training sequence (QPSK-TS) spectrum for coherent optical fiber communication systems. In the proposed scheme, the FO estimation (FOE) consists of a coarse FOE stage and a fine FOE stage. Firstly, the coarse FOE stage is operated by searching for the peak of QPSK-TS spectrum. Then, after conducting coarse FO compensation (FOC) and down-sampling process to the QPSK-TS signal, the fine FOE stage can be realized by operating fast Fourier transform (FFT) to the 4th power of the signal. Compared with the traditional FOE scheme based on FFT with 4th-power operation, the FOE range of the proposed scheme can be extended to 3 times without losing the FOE resolution. After the FOC procedure with the proposed FOE scheme, OSNR can be monitored accurately by obtaining the whole in-band amplified spontaneous emission (ASE) noise from QPSK-TS spectrum. It is noteworthy that both the proposed FOE scheme and the OSNR monitoring scheme are insensitive to chromatic dispersion (CD) and intra-channel nonlinearity (NL). Simulation results for a 28 GBaud system with 2400 km transmission length show that when the OSNR range is 10∼28 dB and FO is 10.25 GHz, the mean OSNR estimated error is < 0.9 dB and the standard deviation is < 0.3 dB. Furthermore, the back-to-back (B2B) experiment results with 20 GBaud show that when the OSNR range is 10∼26 dB and FO is 7 GHz, the mean OSNR estimated error is < 0.8 dB and the standard deviation is also < 0.3 dB.

Simultaneous monitoring of residual chromatic dispersion and OSNR of NRZ-OOK signal based on asynchronous delay-tap sampling and image processing

We demonstrated a simultaneous residual chromatic dispersion (CD) and optical signal-to-noise ratio (OSNR) monitoring method for non-return-to-zero on-off keying signals by employing the delay-tap sampling and image processing techniques. Delay-tap sampling scatter plots reflect the signal pulse shape change and image processing can extract the contour of the scatter plots. Numerical simulation shows that the OSNR monitoring range is from 12 to 32 dB with <0.5 dB error and the CD monitoring range is from 0 to 1360 ps/nm. In the experiment, the OSNR monitoring range is from 14 to 30 dB and the CD monitoring range can go up to 1275 ps/nm.

Machine Learning Based Low-Cost Optical Performance Monitoring in Mode Division Multiplexed Optical Networks

Real-time optical performance monitoring (OPM) is of the utmost importance in adaptive optical networks to enable awareness of channel conditions and to achieve high quality of service. In single-mode fiber (SMF)-based networks, optical signal-to-noise ratio (OSNR) and chromatic dispersion (CD) monitoring have been extensively studied in the literature. In this work, we consider OPM in few-mode fiber (FMF) networks employing non-coherent detection. OPM in such networks is a challenging task, as FMF has an additional performance-limiting impairment over SMF, namely mode coupling (MC). Here, we propose an OPM scheme to estimate three FMF channel parameters: OSNR within the range of 8 to 20 dB, CD within the range of 160 to 1120 ps/nm, and different levels of MC. The proposed scheme uses a stacked auto-encoder (AE) to extract features with reduced dimensionality compared to the original data. These features are used to train an artificial neural network (ANN) regressor. Simulation results show that the proposed OPM scheme can accurately estimate the OSNR, CD, and MC with root mean square error (RMSE) values of 0.0015 dB, 0.28 ps/nm, and 7.88 × 10−6, respectively. The performance of proposed OPM scheme is also evaluated against different types of features commonly used in literature.

Performance degradation and recovery of irradiated spaceborne fiber amplifier.

The output performance of the spaceborne fiber amplifier will degrade when exposed to radiation in aerospace environment. In this work, we have proposed a two-stage erbium-ytterbium-co-doped fiber amplifier (EYDFA) and studied the degradation performance under irradiation and recovery through the annealing process without employing any additional device. An irradiation experiment is conducted to test the performances of the amplifier under the irradiation environment, and experimental results indicate that the output power as well as optical signal-to-noise ratio (OSNR) exhibit similar irradiation responses. The amplifier was annealed through photobleaching and natural annealing processes. The output performance almost recovers to its initial level, and it is verified that the annealing efficiency for high intensity photobleaching is about 240 times as much as that for the case of natural annealing. This work confirms that the spaceborne fiber amplifier could achieve performance recovery after being irradiated, which is beneficial to the enhancement of the radiation hardness for future spaceborne fiber optical devices.

Efficient utilization of Hough transform and orthogonal-triangular decomposition for optical wireless modulation format recognition.

Two schemes for optical wireless modulation format recognition (MFR), based on the orthogonal-triangular decomposition (OTD) and Hough transform (HT) of the constellation diagrams, are proposed in this paper. Constellation diagrams are obtained at optical signal-to-noise ratios (OSNRs) ranging from 5 to 30 dB for seven different modulation formats (2/4/8/16-PSK and 8/16/32-QAM) as images. The first scheme depends on applying the HT of the obtained images; the second scheme is based on utilization of the decomposition of each of the obtained image matrices into an orthogonal matrix (Q) and an upper triangular matrix (R) followed by the HT. Different classifiers, including AlexNet, VGG16, and VGG19, are used for the MFR task. Model setups and results are provided to study the scheme efficiency at different levels of OSNR. The proposed schemes provide unique signatures for constellation diagrams. Moreover, it reveals that the main pattern corresponding to each constellation diagram is more distinguishable for both proposed schemes at different levels of OSNR. The obtained results achieve high accuracy at low OSNR values.

Multi-wavelength thulium-doped fiber laser at 2.05 μm incorporating a superimposed polarization-maintaining fiber Bragg grating

A switchable multi-wavelength thulium-doped fiber laser with a high optical signal-to-noise ratio (OSNR) and stability is proposed and experimentally demonstrated at 2.05 μm. For the first time, a superimposed polarization-maintaining fiber Bragg grating (SI-PM-FBG) was utilized as a multi-channel narrow bandwidth filter to select the lasing wavelengths in the laser cavity in the 2-μm band. It also introduced polarization hole burning (PHB) to inhibit the strong wavelength competition induced by homogeneous gain broadening in the thulium-doped fiber. By adjusting the two polarization controllers (PCs), four stable switchable single-wavelength operations were obtained. The OSNR reached 72 dB at a pump power of 2.89 W, and the maximum wavelength and power fluctuations were 0.01 nm and 0.162 dB, respectively. Four combinations of dual-wavelength operations were obtained by adjusting the PCs. The polarization states between each dual-wavelength lasing were orthogonal. The stability of the dual-wavelength operation was investigated in detail. The proposed laser also has the capability for three- or four-wavelength operation. The excellent performance enables this fiber laser to be used in atmospheric sensing and free-space optical communication.

Use of symmetric sagnac dual-ring scheme for tunable single-mode erbium fiber laser

In this paper, an erbium-doped fiber (EDF) laser with symmetric Sagnac dual-ring scheme is experimentally studied to achieve tunable and stable continuous-wave (CW) single-longitudinal-mode (SLM) presentation. In the measurement, the obtained wavelength-tuning bandwidth can be reached from 1518.0 to 1578.0 nm based on a C-band EDF gain-medium. Moreover, the corresponding output power, optical signal to noise ratio (OSNR) and wavelength linewidth of the designed EDF laser are studied and performed simultaneously.

1.5-kW all-fiberized Yb-doped MOPA laser at 1105 nm with near-diffraction-limited beam quality and narrow spectral width

We demonstrate an all-fiberized ytterbium-doped fiber laser (YDFL) at 1104.9 nm with a high optical signal-to-noise ratio (OSNR) by using ytterbium gain purely. The system is built based on FBG-based master oscillator power amplifier (MOPA) configuration, producing 1.5-kW power with a near-diffraction-limited beam quality (M2∼1.4) and a narrow spectral linewidth (FWHM∼70 GHz). The OSNR is ∼33 dB. A theoretical model is developed to calculate the amplified spontaneous emission (ASE) in the final amplifier. Both the calculated and experimental results suggest that the ASE or even parasitic lasing is the primary limitation for further power scaling. To the best of our knowledge, in >1100 nm wavelength range, this is the highest output power narrow spectral YDFL (with <100 GHz 3-dB spectral width) by using FBG-based MOPA configuration.

Joint Modulation Format Identification and Optical Signal-to-Noise Ratio Monitoring Based on Ternary Neural Networks

Modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring are essential for elastic optical networks. A method for joint modulation format identification and OSNR monitoring based on ternary neural networks was proposed. Further, a ternary neural network was established, and the constellation images after constant modulus algorithm (CMA) equalization were used as input features. Four commonly used modulation formats were distinguished, including dual-polarization (DP)-QPSK, 8QAM, 16QAM, and 64QAM. A 32G baud simulation system was constructed, and the results were analyzed. First, we investigated the influence of resolution and sample length on the identification accuracy. The values 32 and 7000 were selected for the two parameters, respectively, based on the balance between resource consumption and accuracy. Then, we compared the performance of the binary neural network (BNN), ternary neural network (TNN), and full-precision neural network (FNN). The results indicate that the memory consumption and extraction time of the TNN are similar to those of the BNN, and the accuracy is further improved. Moreover, the robustness of the system was analyzed. The results validate that the proposed method can tolerate fiber nonlinearity (from 500 km to 1500 km). Finally, an experiment was conducted to prove the practicality of the proposed method.