Optical Signal-to-noise Ratio Research Articles

Optimization and Performance Evaluation of Single-Mode SOI Waveguides for Ultra-Broadband C-to-S Wavelength Conversion

In order to keep up with expected growth in data traffic, using multiple bands beyond the C band is an appealing solution as it can partly reuse deployed optical fibers. One way of handling multi-band data generation and reception is by using all-optical wavelength converters (AOWCs) to translate to or from the C band, where commercial equipment is easily available. In this work, we therefore designed and realized an AOWC targeting C-to-S band conversion. To respect the requirements of commercial applications, we aimed for a low-complexity high-bandwidth implementation with reasonably low power consumption, tolerance for typical process variations and potential for further integration. Thus we implemented the AOWC as a strip waveguide in a Silicon-on-insulator (SOI) technology. An optimization of the conversion bandwidth was conducted during design and initial characterizations confirmed a conversion efficiency of -32.8 dB and a high bandwidth of about 35 nm with grating couplers and likely beyond 50 nm for edge couplers. Using the fabricated converters, we were able to demonstrate a marginal 0.3 dB optical signal-to-noise ratio (OSNR) penalty for a back-to-back (b2b) transmission of a single-channel single-polarization 32 GBd quadrature-phase-shift keying (QPSK) signal converted from C to S band. Additionally, this converted signal was successfully transmitted over 100 km of standard single-mode fiber (SSMF). Finally, crosstalk issues arising in multi-channel conversion were investigated.

Simultaneous OSNR and chromatic dispersion monitoring based on optical tones power ratio

A novel technique is proposed for simultaneous monitoring of optical signal-to-noise ratio (OSNR) and chromatic dispersion (CD). The scheme is based on the cross-phase modulation effect in a highly nonlinear fiber, which induces a change in the optical tones according to the impairments. The power ratio of the optical tones at two separate frequencies is utilized for monitoring. Improvements in the maximum measurable OSNR, CD and dynamic range can be obtained using only the clock tone for the non-return-to-zero, return-to-zero with a 50% duty cycle differential quadrature phase-shift keying, differential phase-shift keying and on-off keying signals at 80 Gb/s. For the return to zero differential quadrature phase shift keying (RZ-DQPSK) system, the OSNR of 4–40 dB can be monitored with a 36.73 dB dynamic range, and a chromatic dispersion of 0–117 ps/nm can be monitored with a 48.68 dB dynamic range. A correlation is built to describe the relationship of the power ratio with the OSNR and chromatic dispersion which has a root-mean-squared error of 1.41. The delay group dispersion insensitivity, the impact of the input signal power, and the filter selection are investigated. The design enables accurate monitoring, simple operation and may be used in a wide range.

Multi-Tone Optical Source Generation for Applications in Next-Generation Passive Optical Networks using Photonic Structures

This study presents the design and simulation of an integrated multi-carrier optical source with a 227 GHz bandwidth for passive optical network (PON) applications. The optical comb generation attained using a photonic structure known as a micro-ring resonator fabricated in silicon nitride (Si3N4) facilitates cost reduction when produced on a large scale. Additionally, the generated optical comb accomplishes non-uniform tones in terms of the optical signal-to-noise ratio (OSNR), which allows for the dynamic assignment of carriers to retainable customers as a function of the data rate and transmission distance requirements. The design and simulation demonstrate the generation of frequency combs with optical carriers in a range of 5-40 tones, an OSNR range of 20-80 dB, and a free spectral range (FSR) of 50-3 610 GHz. To achieve these features, a geometric design of the device is proposed, and its response to variations of input laser parameters is described. In summary, the device uses two optical micro-resonators with radii of 100 and 450 µm and controls the power and the tuning of laser parameters. The proposed method allows generating a deterministic and reliable path to the frequency combs. Finally, the characteristics of the obtained combs are tested to determine their potential use in PON transmissions.

Five-wavelength-switchable single-longitudinal-mode erbium-doped fiber laser using CDRC filter and cascaded PS-FBG

Wavelength-switchable fiber lasers play an important role in optical fiber communication systems. Although phase-shifted fiber Bragg grating (PS-FBG) has a narrow 3 dB bandwidth, it has rarely been reported as a multi-channel optical filter. Therefore, a five wavelength-switchable single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) was implemented and analyzed, where the cascaded PS-FBG with a 3 dB bandwidth of each channel narrower than 0.024 nm was used as a five-channel filter. The design, fabrication, and characterization of cascaded PS-FBG were implemented and analyzed in detail. By tuning the micro-displacement platform, five-single wavelength switchable SLM outputs with a high optical signal-to-noise ratio (OSNR) were generated and the outputs were monitored over 60 min. The measured maximum wavelength drift and the peak power fluctuation were 0.02 nm and 1.73 dB. The relaxation oscillation peaks were lower than −100 dB/Hz and the minimum linewidth at an integration time of 0.001 s was 1.3102 kHz. To the best of our knowledge, this is the first time that the PS-FBG is used as a multi-channel filter to realize multiple SLM single-wavelength outputs.

Monolithic Quantum Dot Discrete Mode Laser on SOI

Single-longitudinal-mode light sources on a silicon platform exhibit tremendous potential in data transmission, integrated on-chip optical interconnects, and optical ranging. A unique InAs/GaAs quantum dot discrete mode (DM) laser directly grown on a silicon-on-insulator (SOI) substrate with embedded silicon gratings has been demonstrated here, which is regrowth-free and compatible with integrated silicon photonics. Attributed to refractive index perturbation and optical feedback from embedded silicon gratings prepatterned on the SOI substrate, this DM laser can provide stable single-mode operation with a threshold current of 45 mA and an output power of 9 mW. The maximum optical signal-to-noise ratio (OSNR) achieved is 43.6 dB with an optical linewidth of 157 kHz. The 3 dB direct modulation bandwidth is measured as 3.6 GHz. The DM laser can be externally modulated at 70 Gbit/s with non-return-to-zero (NRZ) modulation format and 80 Gbit/s under 4-level pulsed amplitude (PAM4) modulation format. Such SOI-based DM laser paves the way toward large-scale and high-power on-chip integrated lasers.

Strain FBG-Based Sensor for Detecting Fence Intruders Using Machine Learning and Adaptive Thresholding.

This paper demonstrates an intruder detection system using a strain-based optical fiber Bragg grating (FBG), machine learning (ML), and adaptive thresholding to classify the intruder as no intruder, intruder, or wind at low levels of signal-to-noise ratio. We demonstrate the intruder detection system using a portion of a real fence manufactured and installed around one of the engineering college's gardens at King Saud University. The experimental results show that adaptive thresholding can help improve the performance of machine learning classifiers, such as linear discriminant analysis (LDA) or logistic regression algorithms in identifying an intruder's existence at low optical signal-to-noise ratio (OSNR) scenarios. The proposed method can achieve an average accuracy of 99.17% when the OSNR level is <0.5 dB.

Multi-parameter estimation resistant to the random polarization effect for coherent optical receivers.

Optical parameter estimation based on the data obtained by coherent optical receivers is critical for optical performance monitoring (OPM) and the stable operation of the receiver digital signal processing (DSP). A robust multi-parameter estimation is intricate due to the interference of various system effects. By resorting to the cyclostationary theory, we are able to formulate a chromatic dispersion (CD), frequency offset (FO), and optical signal-to-noise ratio (OSNR) joint estimation strategy that is resistant to the random polarization effect, including polarization mode dispersion (PMD) and polarization rotation. The method uses data directly after the DSP resampling and matched filtering. Both numerical simulation and field optical cable experiment validate our method.

Investigation of MDRZ and DPSK modulation schemes employed in DWDM systems for smart city 5G access networks

Wavelength division multiplexing (WDM) is one of the emerging technologies for data transmission in 5G access network assembly between the antenna unit (AU) and the distribution unit (DU). Here, in this work, we evaluate and analyze a dense wavelength division multiplexing (DWDM) — passive optical networks (PONs) employing erbium–ytterbium-doped fiber amplifier (EYDFA) [Formula: see text] Raman hybrid optical amplifier (HOA) using two modulation schemes — modified duobinary return to zero (MDRZ) and differential phase shift keying (DPSK) for alternate channels, where the channel count has been varied from 8 to 40. The system has been analyzed over the wavelength span of 1550–1558.8[Formula: see text]nm with a channel spacing, input power and data rate of 25[Formula: see text]GHz, 0[Formula: see text]dB and 40[Formula: see text]Gb/s, respectively. An average gain, gain ripple and optical signal-to-noise ratio (OSNR) of 20.87, 0.95 and 30[Formula: see text]dB, respectively have been observed in the case of HOA along with the achievement of Q-factor of up to 9 at the receiving end of the system. The DPSK-modulated channels result in higher values of gain and OSNR along with lower gain ripple as compared to MDRZ channels making the proposed system a good option for 5G access network applications.

Combined image Hough transform based simultaneous multi-parameter optical performance monitoring for intelligent optical networks

The fast-emerging flexible optical networks require intelligent monitoring techniques for supervising the signal quality performance to achieve the desired efficiency. In this paper, we propose a multi-task convolutional neural network (MT-CNN) for multi-parameter monitoring using a unique combined image Hough Transform (CIHT) technique. The proposed system supports simultaneous modulation format identification (MFI), transmission distance detection (TDD), roll-off factor identification (ROFI), Laser linewidth detection (LLWD), centre wavelength detection (CWLD) along with the optical signal-to-noise ratio (OSNR) estimation. The proposed scheme realizes the multi-parameter optical performance monitoring using the advantage of Hough transform (HT) for efficient feature extraction. Rigorous simulations to prove the advantage of using Hough transform, and data augmentation has been performed to conclude the results achieved. The simulation results demonstrate 100% accuracy after 112 epochs for MFI, TDD, ROFI, LLWD, and CWLD, and the mean absolute error (MAE) for OSNR estimation has reduced to 0.354 dB after 180 epochs. Also, rigorous simulations with various conditions are performed to demonstrate the merits of the proposed method.

Tunable and stable single-frequency fiber laser by applying Rayleigh feedback light and saturable absorber filter

In this study, a stable and tunable erbium-doped fiber (EDF) ring laser architecture is demonstrated. To filter the dense longitudinal-mode noises and achieve the single-longitudinal-mode (SLM) performance, the Rayleigh backscattering (RB) feedback injection and unpumped EDF saturable absorber (SA) induced filter is applied inside the ring cavity. Here, an effective wavelength-selection scale of the EDF laser is achieved between 1528.0 and 1576.0 nm, when C-band EDF based gain-medium is utilized. Additionally, the wavelength linewidth, optical signal to noise ratio (OSNR), output power and output stability of the fiber laser are also discussed over the available wavelength range in the measurement.

Low-Complexity Modulation Format Identification Based on Amplitude Histogram Distributions for Digital Coherent Receivers

In this work, a prior-training-free and low-complexity modulation format identification (MFI) scheme, based on amplitude histogram distributions, was proposed and demonstrated, both numerically and experimentally, for autonomous digital coherent receivers. In the proposed scheme, after having performed power normalization, incoming polarization division multiplexed (PDM) signals were classified into QPSK, 8QAM, 16QAM, 32QAM and 64QAM signals, according to their ratios. Ratios were defined according to specific features of their amplitude histograms. The proposed MFI scheme used only amplitude information. As such, it was insensitive to carrier phase noise. Furthermore, the proposed scheme did not require any prior information, such as optical signal-to-noise ratio (OSNR). The performance of the proposed MFI scheme was numerically verified using 28GBaud PDM-QPSK/-8QAM/-16QAM/-32QAM/-64QAM signals. The numerical simulation results showed that the proposed scheme was able achieve a 100% correct identification rate for all five modulation formats when their OSNR values were higher than the thresholds corresponding to the 20% FEC correcting bit error rate (BER) of 2.4 × 10−2. To further explore the effectiveness of the proposed MFI scheme, proof-of-concept experiments in 28GBaud PDM-QPSK/-8QAM/-16QAM, and 21.5GBaud PDM-32QAM transmission systems were also undertaken, which showed that the proposed scheme as robust against fiber nonlinearities. To explore the scheme’s feasibility for use in practical transmission systems, the computational complexity analysis of the proposed scheme was conducted. It showed that, compared with relevant MFI schemes, the proposed MFI scheme was able to significantly reduce the computational complexity.

Modulation Format Identification and OSNR Monitoring Based on Multi-Feature Fusion Network

In this paper, we propose a multi-feature fusion network (MFF-Net) for a modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring scheme. The constellation map data used in this work comes from five modulation formats, namely 56 Gbit/s 4/8 phase shift keying (PSK) and 16/32/64 quadrature amplitude modulation (QAM). The constellation maps are input to one branch network of the MFF-Net, and then the constellation maps are processed by horizontal projection and used as input to another branch network as a way to fuse the two image features. The results show that the scheme achieves 100% MFI accuracy and 98.82% OSNR monitoring accuracy for the five modulation formats. In addition, the performance of MFF-Net and binarized convolutional neural network (B-CNN), visual geometry group network (VGG-Net), and traditional weighted multi-task learning (EW-MTL) are compared to present the superiority of the method. The effect of model structure on MFF-Net is also discussed. The robustness of the model is also evaluated for different transmission distances and bit rates.

Simulation and design of C + L broadband communication single mode fiber

A novelty CL fiber which can expand L-band communication is proposed to meet the urgent demand of optical fiber transmission bandwidth for large capacity communication system. The influence of L-band attenuation on the OSNR (optical signal-to-noise ratio) of the system is analyzed and calculated; through the simulation of DWDM (dense wavelength division multiplexing) system, the requirements for FAWD (fiber attenuation warping degree) of CL fiber in 100Gbit/s rate DWDM system are obtained; and the influence of waveguide structure design on FAWD is tested. The results show that, the OSNR of L-band for will be 3 dB–3.5 dB worse than traditional C-band on 80 km span system; the FAWD@1625 nm (△α1625) and the FAWDD (fiber attenuation warping degree difference) of CL fiber are recommended to be controlled below 0.015 dB/km and 0.005 dB/km respectively to meet the link requirements in relevant standards. Additionally, it is feasible to improve the attenuation flatness of L-band by optimizing the waveguide structure design which is presented in the paper, that will provide a reference for the large-scale acquisition of novelty CL fibers in the future.

Purely nonlinear amplified ultralow-noise transmission using hybrid second-order DRA and PPLN-based PSA.

We report ultralow-noise transmission over a 102-km single-mode fiber using a purely nonlinear amplification scheme consisting of a second-order distributed Raman amplifier (DRA) and a phase-sensitive amplifier (PSA) based on periodically poled LiNbO3 waveguides. The hybrid DRA/PSA features a broadband gain over the C and L bands and an ultralow-noise advantage, with a less than -6.3 dB effective noise figure in the DRA stage and a 1.6 dB optical signal-to-noise ratio (OSNR) improvement in the PSA stage. Compared with the unamplified link, the OSNR is improved by 10.2 dB for a 20-Gbaud 16QAM signal in the C band, resulting in error-free detection (a bit-error rate of less than 3.8 × 10-3) for the signal with a low link input power of -25 dBm. Mitigation of nonlinear distortion is also achieved by the proposed nonlinear amplified system due to the subsequent PSA.

Capacity Enhancement Analysis of an OAM-OFDM-SMM Multiplexed Free Space Communication System in Atmospheric Turbulence

To overcome atmospheric turbulence (AT) distortion during signal propagation through the optical link, orbital angular momentum (OAM) mode states employing multiple inputs and multiple outputs (MIMO) techniques have recently gained prominence in free space optical communication (FSO). As the various OAM modes propagate through the free space optical link, signal attenuation and crosstalk may occur, reducing system capacity and increasing the likelihood of bit errors. In this work, our objective is to propose a spectrally efficient, high-speed and channel capacity efficient crosstalk FSO communication system by combining the features of orthogonal frequency division multiplexing (OFDM), spatial mode multiplexing (SMM), and a mode diversity scheme into an existing OAM-FSO communication system. The incorporation of the OFDM-MIMO concept and spatial mode diversity into the existing OAM-MIMO-FSO system is extremely beneficial in enhancing the transmission capacity, mitigating multipath fading and atmospheric turbulence distortions. The Gamma–Gamma (GG) model is used to assess the performance of the proposed system under various atmospheric turbulence conditions in terms of the performance metrics such as BER vs. number of OAM states for different refractive index structure and Rytov constants, link distance, and an optical signal to noise ratio (OSNR). A FEC limit of 3.8 × 10−3 and a maximum link distance of 2 km are set to evaluate these performance parameters. Finally, the transmission capacity of the proposed system is compared to that of the existing MIMO and OAM-SMM-MIMO systems for different OSNR values under atmospheric turbulence conditions for the OAM state of l = +1, yielding an overall improvement of 3.3 bits/s/Hz compared to conventional MIMO systems and 1.6 bits/s/Hz for the OAM-SMM-MIMO system.

Frequency Comb Generation Based on Brillouin Random Lasing Oscillation and Four-Wave Mixing Assisted with Nonlinear Optical Loop Mirror

A frequency comb generator (FCG) based on dual-cavity Brillouin random fiber lasing oscillation in the 1.5 μm telecon spectral window is established and experimentally demonstrated. In the half-open main cavity of the dual cavity, the stimulated Brillouin scattering in highly nonlinear fiber (HNLF) and Rayleigh scattering in single-mode fiber are employed to provide sufficient Brillouin gain and the randomly distributed feedback, respectively, for random mode resonance. The sub-cavity includes an Er-doped fiber amplifier to couple back and boost lower-order Stokes and anti-Stokes light for the cascade of stimulated Brillouin scattering to generate multiple higher-order Stokes and anti-Stokes light. Meanwhile, efficient four-wave mixing is stimulated in the HNLF-based main cavity, further enhancing the number and intensity of the resonant Stokes and anti-Stokes light. By taking advantages of the unique transmission characteristics of nonlinear optical loop mirrors, the power deviation between Stokes and anti-Stokes lines is further optimized with 17 orders of stable Stokes lines and 15 orders of stable anti-Stokes lines achieved within the 10 dB power deviation, with maximum optical signal-to-noise ratio (OSNR) of ~22 dB and ~17 dB and minimum OSNR of ~10 dB and ~7.5 dB for Stokes and anti-Stokes lines, respectively. In addition, the dynamic characteristics of the proposed FCG have been experimentally investigated. Such an FCG with fixed frequency spacing will find promising applications in fields of optical communication, microwave, optical sensing, etc.

Frequency offset modeling in presence of ASE noise and corresponding low-complexity solution for discrete spectrum modulated nonlinear frequency division multiplexing system

For discrete spectrum modulated nonlinear frequency division multiplexing (DS-NFDM) transmission, the combined effect of laser frequency offset (FO) and amplifier spontaneous emission (ASE) noise would cause a perturbative shift of eigenvalues and corresponding spectral distortion. Moreover, the classical 4th-power feedforward frequency offset estimation (FOE) scheme is not suitable for DS-NFDM system with low baud rate. In this paper, we firstly deduce an FO impairment model with ASE noise in nonlinear Fourier domain (NFD). Meanwhile, we have proposed a training sequence-assisted FOE (TS-FOE) scheme, which effectively reduces the impact of ASE noise. The 2 GBaud DS-NFDM 16QAM simulation results illustrate that this scheme achieves an FOE error within 1 MHz using a training sequence (TS) with length of 64. Besides, its FOE range achieves [−Rs/2, +Rs/2], where Rs denotes the baud rate of signal. Furthermore, the effectiveness of TS-FOE scheme has been verified by 2 GBaud NFDM QPSK experimental system. The results demonstrate that when optical signal-to-noise ratio (OSNR) is set 13 dB, the TS-FOE scheme presents stable FO compensation performance for different FOs within 1 GHz, and the corresponding bit error rates (BERs) are all below the 7% hard decision forward error correction (HD-FEC) threshold. Meanwhile, its complexity is on the order of OL and related with the length L of TS, which is as one thousandth as that of angle search FOE scheme.

Auto Bias Control Technique for Silicon IQ Modulators Using the Dual-Arms Power Dithers

In high-capacity coherent transmission applications such as 400 Gb/s and beyond, silicon inphase and quadrature modulators (SIQMs) are widely used as a coherent transmitter assembly. The bias points of the SIQM deviate the optimum points because of environmental change. Consequently, auto bias control (ABC) is an essential technique to stabilize the bias points of the SIQM. In this paper, we described a novel method to achieve the bias point high-precision control for SIQMs. First, a pair of the differential power dithers are applied to the dual-arms of the channels I and Q, which can compose a push-pull bias locking mode. By this way, the bias error of the channel P induced by the channels I and Q can be eliminated compared with the conventional single-arm ABC method. Then an orthogonal integration operation is used to extract the dither frequency component of the output signal of the SIQM, and the intersections of the orthogonal-integral curves corresponding to the bias points of the channels I, Q, and P are the optimum bias thermal-power. The simulation and experimental results show that our method exhibits a good performance for locking the optimum thermal-power of the SIQM, and the optical signal-to-noise ratio (OSNR) penalty caused by the proposed method for a 512-Gb/s optical signal transmission with a dual-polarization 16-quadrature amplitude modulation format can be neglected in our experiment.

Penalty mitigation for OSC-induced XPM in long-haul WDM coherent systems by digital up-conversion coding.

Due to the cross phase modulation (XPM) effect, in long-haul high-speed dense wavelength division multiplexing (DWDM) coherent systems, using a low-speed on-off-keying (OOK) format optical supervisory channel (OSC) will introduce extra nonlinear phase noise, which restricts the transmission distance. In this paper, we propose a simple OSC coding method to mitigate the OSC-induced nonlinear phase noise. According to the split-step solution of the Manakov equation, we up-convert the baseband of the OSC signal out of the pass-band of the walk-off term to reduce the spectrum density of XPM phase noise. Experimental results show that the optical signal to noise ratio (OSNR) budget on the 400 G channel of 1280-km transmission is improved by 0.96 dB, which achieves almost the same performance with the no OSC case.

Stable wavelength-swept single-frequency erbium fiber laser in L-band tuning bandwidth

In the study, we demonstrate an erbium-doped fiber (EDF) laser system by integrating the Mach–Zehnder interferometer (MZI) and triple-fiber-ring structures to generate the single-longitudinal-mode (SLM) output in L-band bandwidth. Here, a sub-ring with a 5.5 m unpumped EDF based saturable absorber (SA) is also applied to induce the narrowband auto-tracking filter behavior. Based on the proposed fiber cavity induced mode-filter effect, the EDF laser can achieve the stable, narrow and wavelength-tunable output characteristics over an effective tuning range of 1568.0 to 1613.0 nm. Furthermore, the output stability, optical signal to noise ratio (OSNR), output power and laser linewidth of the laser are also performed and discussed.