Optical Signal-to-noise Ratio Research Articles

Modulation format recognition using CNN-based transfer learning models

Transfer learning (TL) appears to be a potential method for transferring information from general to specialized activities. Unfortunately, experimenting using various TL models does not yield good results. In this paper, we propose a model built from scratch with the Hough transform (HT) of constellation diagrams to improve modulation format recognition. The HT is utilized to project points on the constellation diagrams on the Hough space. The HT translates constellation diagram points into lines. Features can then be extracted from the HT domain. Constellation diagrams for eight different modulation formats (2/4/8/16—PSK and 8/16/32/64—QAM) are obtained at optical signal-to-noise ratios (OSNRs) ranging from 5 to 30 dB. The proposed system is based on classification and TL. The obtained results indicate that even at low OSNR values, the proposed system can blindly recognize the wireless optical modulation format with a classification accuracy of up to 99%.

Tunable and switchable dual-wavelength SLM narrow-linewidth fiber laser with cascaded passive double-ring cavity narrowband filters

In this article, we have proposed and demonstrated a tunable, switchable dual-wavelength linear cavity fiber laser, which has a passive dual ring cavity structures (DRCs) and polarization-maintaining fiber Bragg grating (PM-FBG), and can be used for the operation of single-longitudinal mode(SLM), narrow-linewidth, and orthogonal single polarization. The passive DRCs, consisted of four 2 × 2 optical couplers (OCs), work as the narrow-band filter and mirror. The tunable and switchable wavelength of the fiber laser is realized by stretching PM-FBG and adjusting the polarization controller (PC). The optical signal-to-noise ratio (OSNR) at the dual-wavelength operating state can be more than 45 dB in the tunable range. While the OSNR of fiber laser at the each-wavelength operating state can reach 50 dB with the maximum wavelength and power fluctuation of 0.01 nm and 1.2 dB, during the monitoring time of 40 min. In addition, the measured laser linewidths are 375 Hz and 420 Hz, respectively. Meanwhile, the states of polarization are orthogonal single polarization, and the degree of polarization (DOP) is close to 100 %.

Quadrant Multiplexing-based Geometrically Shaped Four-Dimensional Modulation Format for Polarization Multiplexed Coherent Optical Communication System

We present in detail the recently proposed novel geometric shaping four-dimensional (GS-4D) modulation format based on quadrant multiplexing and evaluate the signal from various aspects. GS-4D achieves a generalized mutual information (GMI) gain of 1.6 bits/4D-sym compared to polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16QAM) at low signal-to-noise ratio (SNR) regions. Set partitioning (SP) at the expense of one bit of spectral efficiency is proposed to exponentially reduce the amount of constellation points and increase the Euclidean distance. For fiber channels influenced by additive white Gaussian noise (AWGN) and phase noise, a further GS scheme with binary labeling is designed. The scheme takes bit error ratio (BER) as fitness, preserves the feature of this format while using mirror symmetry to reduce the complexity, and is implemented by an improved particle swarm optimization (PSO) algorithm. Results demonstrate that the proposed scheme has high optical SNR tolerance and low BER performance. Compared with the PM-16QAM, the GS-4D, GS-4D with SP and GS-4D with SP-PSO can provide 3.6 dB, 5.6 dB and 6.6 dB optical SNR gain, respectively. With the equal distance of transmission, the BER of GS-4D is decreased by around one order of magnitude compared with PM-16QAM, the BER of GS-4D with SP is lowered by around two orders of magnitude, and the BER of GS-4D with SP-PSO is reduced by around three orders of magnitude.

Determination of the optimal input channel powers using particle swarm optimization algorithm in a WDM system with In-Line optical amplifiers

In this paper, a new application of the particle swarm optimization (PSO) algorithm has been carried out in the field of the optical fiber (OF) communication system. In this theoretical work, the most suitable input channel powers to reduce the disruptive effects of four-wave mixing (FWM) occurring in OF and amplified spontaneous emission (ASE) noise from optical amplifiers are determined with the help of two different particle swarm optimization (PSO) in terms of inertia weight. The figure of merit is the optical signal-to-noise ratio (OSNR) of each channel at the output end of OF. Each of the input channel powers is optimized to maximize the OSNR of the channel with the lowest OSNR. The values obtained by both optimization methods are well good compared to those obtained with equal powers allocation (EPA). The best improvement in OSNR with PSO is obtained as 1.105846 dB (2.762778 %). Moreover, the performances of both algorithms and EPA are also comparatively evaluated in the study.

Empirical analysis of polarization division multiplexing-dense wavelength division multiplexing hybrid multiplexing techniques for channel capacity enhancement

<span>This paper exemplifies dense wavelength division multiplexing combined with polarization division multiplexing with C-band frequency range-based single-mode fiber. In the proposed link, 32 independent channels with 16 individual wavelengths are multiplexed with two different angles of polarization. Each carrying 130 Gbps dual-polarization data with 200 GHz channel spacing claiming a net transmission rate of 4.16 Tbits/s with spectral efficiency of 69% with 20% side-mode-suppression-ratio (SMSR) and optical signal to noise ratio (OSNR) 40.7. The performance of the proposed techniques has been analyzed using optimized system parameters securing a minimum bit error rate (BER) 10-9 at a transmission distance up to 50 km.</span>

Demonstration of Uncoupled 4-Core Multicore Fiber in Submarine Cable Prototype With Integrated Multicore EDFA

We developed a 15.2-km submarine cable prototype which can accommodate up to 16 fiber pairs (FPs) of 125 μm standard cladding diameter 4-core uncoupled multicore fiber (UC-MCF). Fiber properties such as loss, chromatic dispersion, and inter-core crosstalk were measured and compared before and after cabling. We found that the cabling process, such as fiber rewinding, copper tubing and polyethylene coating completion, did not significantly change the fiber properties such as loss. To verify the influence of cabling process on long-haul transmission performance, we fabricated a 60.8-km length 4-core UC-MCF transmission span by serially splicing four FPs from the 15.2-km length cabled 4-core UC-MCF. An experimental loop transmission setup was established using the cabled transmission span, and Q-value was measured in real-time. The 120 wavelength multiplexed channels were transmitted. The five channels were modulated by 100 Gbps 34.7 GBd polarization multiplexed quadrature phase shift keying (PM-QPSK) signal and the other channels were loading channels shaped from amplified spontaneous emission (ASE). We changed the transmission direction one core at a time in the 4-core UC-MCF based transmission line to examine the effect of inter-core crosstalk on the transmission line. The −25-dB inter-core crosstalk caused negligible signal degradation in the 4-core UC-MCF and fan-in fan-out (FIFO) a total of 88 spans. Furthermore, we found that the 5,350-km transmission using cabled 4-core UC-MCF was limited by optical signal to noise ratio (OSNR). We integrated the transmission 4-core UC-MCF with the 4-core UC-MC-erbium doped fiber amplifier (EDFA) without using FIFO and clarified experimentally a Q-value improvement of 0.6 dB by removing half number of the FIFOs.

A 500 km Long Transmission with Coherent Optical OFDM Using Dual Polarization Quadrature Phase Shift Keying (QPSK) and Double Sideband Suppressed‐Enhanced Carrier (DSS‐EC)

AbstractCoherent optical orthogonal frequency division multiplexing (CO‐OFDM) systems are chosen for this task because of their high spectral efficiency, low inter‐symbol interference (ISI), low intercarrier interference (ICI), robustness against chromatic dispersion (CD), and high nonlinearity interference tolerance (HNIT). The DSS‐EC technique is utilized in this study to investigate a high‐speed 100 Gbps CO‐OFDM with a QPSK encoder over a 500 km uncompensated connection distance. The proposed system is compared to CO‐OFDM‐QPSK and CO‐OFDM‐16QAM systems in terms of log BER at various input optical signal to noise ratios (OSNRs). Results reveal that due to enhanced available bandwidth use of communication channels, the proposed CO‐OFDM‐DSS‐EC‐QPSK system provides minimum log BER at lower input OSNRs while the rest two systems show identical log BER performance at high input OSNRs. Therefore, proposed technique is a prominent solution for future generations high data demands and for a cost‐effective solution due to an uncompensated transmission channel.

Impact of the input OSNR on data-driven optical fiber channel modeling

The data-driven approach is promising for computationally efficient modeling of optical fiber channels. Here, for the first time to our knowledge, we investigate the impact of the input optical signal-to-noise ratio (OSNR) on the accuracy of data-driven modeling for optical fiber channels, based on a conditional generative adversarial network (cGAN). Initially, considering a single span of 80 km of standard single mode fiber (SSMF), we vary the launched optical power for the ease of emulating both linear and nonlinear transmission scenarios. When the input OSNR of a root raised cosine (RRC)-shaped 16 quadrature amplitude modulation signal with a roll-off factor of 0.1 varies, we identify that there occurs an OSNR threshold for accurately modeling a single span of 80 km of SSMF in order to reach the normalized mean square error of less than 0.02 in relevance to the traditional split-step Fourier method. The OSNR thresholds are 19 dB and 21 dB, respectively, under scenarios of linear and nonlinear transmissions. Moreover, we verify that the OSNR threshold of accurate modeling is insensitive to both the modulation format and the RRC shaping of the input signal. Furthermore, we find that the cascaded use of cGAN well-trained for the single SSMF span is capable of modeling the multiple-span SSMF transmission in the case that the input OSNR threshold of 22.6 dB is satisfied. We believe that the identification of the OSNR threshold for data-driven optical fiber channel modeling is useful for the accurate and efficient emulation of long-haul fiber optical transmission.

Transmission of 4096-QAM OFDM at D-band.

We experimentally demonstrate the transmission of multi-order quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) millimeter wave signals at D-band. Meanwhile, the system nonlinearity analysis is also given, which is originated from the unsatisfactory optoelectronic devices, multi-order QAM and the high peak-to-average power ratio (PAPR) of OFDM signal. To alleviate the system nonlinearity, Volterra nonlinearity compensation (VNC) is adopted. Probabilistic shaping (PS) has been regarded as an effective approach to ensure the system robustness. By using the technologies of probabilistic shaping and Volterra nonlinearity compensation, 57.21-Gbit/s 4096-QAM OFDM signal at 117 GHz can be delivered over 13.42-m wireless distance in our experiment, achieving the normalized general mutual information (NGMI) threshold of 0.83 with 25% soft-decision forward-error correction (SD-FEC) overhead. In addition, we simulated the D-band millimeter wave simulation system in VPI software. The NGMI performance between conventional and discrete Fourier transform-spread (DFT-S) 16-QAM OFDM has been compared in simulation at the same optical signal-to-noise ratio (OSNR). The case of conventional 16-QAM OFDM has better performance. To the best of our knowledge, this is the first demonstration of transmission of multi-order QAM OFDM millimeter wave signal at D-band using PS and VNC methods.

Stable and Tunable PT-Symmetric Single-Longitudinal-Mode Fiber Laser Using a Nonreciprocal Sagnac Loop

A stable and tunable parity-time (PT) symmetric single-longitudinal-mode fiber laser using a nonreciprocal Sagnac loop is proposed, which is experimentally and numerically demonstrated. The nonreciprocal Sagnac loop only including a 3-dB optical coupler (OC) and a polarization controller (PC) is incorporated into a fiber ring cavity, in which nonreciprocal light transmission between the frequency-degenerate clockwise (CW) and counterclockwise (CCW) resonator modes is induced to suppress multiple-longitudinal-mode oscillation of the laser. The two light paths along the CW and CCW directions in the Sagnac loop are respectively defined as the gain and loss loop of the PT-symmetric laser, due to a controllable birefringence induced by the PC. By controlling the polarization states of the two light waves, the PT symmetry is broken when the gain coefficient is larger than the coupling coefficient, single-mode lasing is thus achieved. Experimental results show that the optical signal to noise ratio (OSNR) of the single-mode lasing at 1550 nm is 43.0 dB, proving the great superiority of PT symmetry for lasing mode selection. By tuning an optical tunable band-pass filter incorporated into the fiber cavity, the wavelength of output single-mode lasing varies from 1530 to 1560 nm, and their 3-dB Lorentzian linewidth varies within a range from 529 to 687 Hz. During a 30-min observation period, the variations of wavelength drift and OSNR of the single-mode lasing are less than 6 pm and 2.42 dB, respectively. The advantages of the proposed scheme are obvious, which include simple structure, low cost, simple operation, and good stability.

A Field Trial of Multi-Homodyne Coherent Detection over Multi-Core Fiber for Encryption and Steganography

The concept of multi-homodyne coherent detection is used for stealth and secured optical communications, demonstrating a transmission rate of 20 Gbps under negative (in dB) optical signal-to-noise ratio (OSNR), in a deployed multi-core fiber (MCF) network. A 10 GBd QPSK signal is assigned with a bandwidth of multi-hundreds GHz, optically encoded with spectral phase mask, and transmitted below ASE noise in the MCF channel. To enable such transmission scheme, a semiconductor based gain-switched laser (GSL) frequency comb, used as both the signal carrier and local oscillator (LO), are then mixed in intradyne coherent receiver. The ultra-short time-domain overlapping between the pulsed signal and the pulsed LO allows the authorized user to collect the information, while discriminating most of the noise. Consequently, the electrical SNR is dramatically improved compared to the optical SNR, resulting in optical processing gain. Nevertheless, ultra-short pulses mixing is rather susceptible to a chromatic dispersion (CD), which imposes a major challenge in this spread-spectrum transmission technique. In this contribution, the effect of CD in multi-homodyne coherent detection is analyzed in an analytic model and in experiments. The model suggests a modified CD transfer function, comprised of the optical sampling of a conventional CD transfer function. In addition, the SNR penalty associated with the CD effect is quantified, as well as its dependency in propagation length and overall GSL's bandwidth. To circumvent the devastating effect of the CD, multi-core fibers are proposed. Two different weakly-coupled cores of a homogeneous MCF allow the signal and the LO to co-propagate through a very similar dispersive channel. A field trial in a deployed 6.29 km MCF network has validated an encrypted and stealthy 20 Gbps coherent transmission, at -2 dB OSNR.

Wavelength-switchable L-band fiber laser assisted by random reflectors

A wavelength-switchable L-band erbium-doped fiber laser (EDFL) assisted by an artificially controlled backscattering (ACB) fiber reflector is here presented. This random reflector was inscribed by femtosecond (fs) laser direct writing on the axial axis of a multimode fiber with 50 μm core and 125 μm cladding with a length of 17 mm. This microstructure was placed inside a surgical syringe to be positioned in the center of a high-precision rotation mount to accurately control its angle of rotation. Only by rotating this mount, three different output spectra were obtained: a single wavelength lasing centered at 1574.75 nm, a dual wavelength lasing centered at 1574.75 nm and 1575.75 nm, and a single wavelength lasing centered at 1575.5 nm. All of them showed an optical signal-to-noise ratio (OSNR) of around 60 dB when pumped at 300 mW.

Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system under desert conditions

To meet the needs of future wireless optical networks, this paper introduces a high-speed, hybrid multiplexed, coherent free-space optical (FSO) communication system that integrates an orbital angular momentum (OAM) multiplexed signal with an orthogonal frequency division multiplexing (OFDM) technique. Two independent QAM polarized beams, each carrying in-phase and quadrature (I/Q) phase 16-QAM-OFDM modulated data, are combined using mode division multiplexing (MDM) to increase the capacity of the proposed system. The reason of choosing OFDM is its capability to support higher data rate, and mitigating intersymbol interference (ISI). The signal is detected using a coherent detection-based digital signal processing (DSP) algorithm at the receiver end. The proposed hybrid FSO system is evaluated in low and heavy dust environments using bit error rate (BER), link distance, optical signal-to-noise ratio (OSNR), and received optical power performance matrices. The simulation results demonstrate the successful transmission of a 120 Gb/s single carrier over the longest link ranges of 1.5 and 0.40 km, respectively, under low and heavy dust weather environments below the signal degradation threshold value (forward error correction (FEC) limit) of BER 2.2 × 10–3 in strong turbulent conditions.

Optical Field Recovery in Jones Space

Optical full-field recovery allows for fiber impairments compensation such as chromatic dispersion and polarization mode dispersion (PMD) in the digital signal processing. For cost-sensitive short-reach optical networks, some advanced single-polarization (SP) optical field recovery schemes are recently proposed to avoid chromatic dispersion-induced power fading effect and improve the spectral efficiency for larger potential capacity. Polarization division multiplexing (PDM) can further double both the spectral efficiency and the system capacity of these SP carrier-assisted direct detection (DD) schemes. However, the so-called polarization fading phenomenon induced by random polarization rotation is a fundamental obstacle that prevents SP carrier-assisted DD systems from polarization diversity. In this paper, we propose a receiver of Jones-space field recovery (JSFR) to realize polarization diversity with SP carrier-assisted DD schemes. Different receiver structures and simplified recovery procedures for JSFR are explored theoretically. The proposed JSFR pushes SP DD schemes towards PDM without an extra optical signal-to-noise ratio (OSNR) penalty. In addition, the JSFR shows good tolerance to PMD since the optical field recovery is conducted before polarization recovery. In the concept-of-proof experiment, we demonstrate 448-Gb/s reception over 80-km single-mode fiber using the proposed JSFR based on 2 × 2 couplers. Furthermore, we qualitatively compare the optical field recovery in Jones space and Stokes space in terms of the modulation dimension and hardware complexity.

Cascaded modulation format identification and optical signal-to-noise ratio estimation method for optical communication system

Modulation format identification (MFI) and optical performance monitoring are important for elastic optical networks. We propose a cascaded module-based MFI and optical signal-to-noise ratio (OSNR) estimation method. Polar coordinates were used as features to distinguish the modulation formats (MFs) and estimate the OSNR. Dilated convolution was adopted to increase the receptive field and reduce computational complexity. Four commonly used MFs were investigated: dual-polarization (DP) quadrature phase shift keying, 8QAM, 16QAM, and 32QAM. A 32G baud DP transmission system is established, and the results are discussed. The results show that MFI accuracy can reach 100% when the OSNR is less than the 7% forward error correction limit, and the accuracy of OSNR monitoring is close to 100%. Two factors, sample length and nonlinearity, were further investigated. The results show that the MFI and OSNR estimators can achieve perfect performance when the sample length is 3000. Simultaneously, the accuracy of the MFI remained at 100%, and the accuracy of the OSNR estimator decreased by 1% to 2% when the launch power changed from 0 to 5 dBm. Furthermore, two modules with the same structure, DC-CNN and CNN, were compared. The results show that the two models can achieve similar accuracies and that the DC-CNN has the least number of parameters. Finally, experimental verification was conducted to ensure the practical applicability of the proposed method.

Simultaneous modulation format identification and OSNR monitoring based on optoelectronic reservoir computing.

An approach for simultaneous modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring in digital coherent optical communications is proposed based on optoelectronic reservoir computing (RC) and the signal's amplitude histograms (AHs) obtained after the adaptive post-equalization. The optoelectronic RC is implemented using a Mach-Zehnder modulator and optoelectronic delay feedback loop. We investigate the performance of the proposed model with the number of symbols, bins of AHs and the hyperparameters of optoelectronic RC. The results show that 100% MFI accuracy can be achieved simultaneously with accurate OSNR estimation for different modulation formats under study. The lowest achievable OSNR estimation mean absolute errors for the dual-polarization (DP)-quadrature phase-shift keying signal, the DP-16-ary quadrature amplitude modulation (16QAM) signal, and the DP-64QAM signal are 0.2 dB, 0.32 dB and 0.53 dB, respectively. The robustness of the proposed scheme is also evaluated when the optoelectronic RC is in presence of additive white Gaussian noises. Then, a proof of concept experiment is demonstrated to further verify our proposed method. The proposed approach offers a potential solution for next-generation intelligent optical performance monitoring in the physical layer.

High-capacity free-space optical communications using wavelength- and mode-division-multiplexing in the mid-infrared region.

Due to its absorption properties in atmosphere, the mid-infrared (mid-IR) region has gained interest for its potential to provide high data capacity in free-space optical (FSO) communications. Here, we experimentally demonstrate wavelength-division-multiplexing (WDM) and mode-division-multiplexing (MDM) in a ~0.5 m mid-IR FSO link. We multiplex three ~3.4 μm wavelengths (3.396 μm, 3.397 μm, and 3.398 μm) on a single polarization, with each wavelength carrying two orbital-angular-momentum (OAM) beams. As each beam carries 50-Gbit/s quadrature-phase-shift-keying data, a total capacity of 300 Gbit/s is achieved. The WDM channels are generated and detected in the near-IR (C-band). They are converted to mid-IR and converted back to C-band through the difference frequency generation nonlinear processes. We estimate that the system penalties at a bit error rate near the forward error correction threshold include the following: (i) the wavelength conversions induce ~2 dB optical signal-to-noise ratio (OSNR) penalty, (ii) WDM induces ~1 dB OSNR penalty, and (iii) MDM induces ~0.5 dB OSNR penalty. These results show the potential of using multiplexing to achieve a ~30X increase in data capacity for a mid-IR FSO link.

Performance analysis of free space optic transmission link for broadband access networks

Abstract Since the last two decades free space optical (FSO) communication transcends the RF communication and microwave systems because of its capability of carrying high bandwidth, license-free, long-range, small size, high bandwidth, low cost and ease of deployment. FSO communication has few limitations as well like beam dispersion, scintillation, weather effects etc. This paper demonstrates the implementation of FSO link and analysis of transmission performance of FSO channel for broadband access networks. The proposed FSO link is simulated in opti-system to analyze the physical characteristics of transmitter and receiver. The transmission performances of FSO link are validated using on/off keying (OOK) and differential phase shift keying (DPSK) at various atmospheric conditions with different data rates and transmission ranges. The transmission performance of the FSO link is analyzed under the influence of key parameters like launch power, beam divergence, atmospheric attenuation, receiver sensitivity and receiver aperture. The proposed model of the FSO link is investigated using key parameters like optical signal to noise ratio (OSNR), noise figure and bit error rate (BER).

Modulation Format Identification Based on Signal Constellation Diagrams and Support Vector Machine

In coherent optical communication systems, where multiple modulation formats are mixed and variable, the correct identification of signal modulation formats provides the foundation for subsequent performance improvement using digital algorithms. A modulation format identification (MFI) scheme based on signal constellation diagrams and support vector machine (SVM) is proposed. Firstly, the signal constellation diagrams are divided by the fractal dimension of the weighted linear least squares (WLS-FD) algorithm, and the fractal dimension (FD) in each region is calculated, which is regarded as one of the image features. Then, the feature values of the image in different directions are extracted by the gray-level co-occurrence matrix (GLCM), and their mean and variance are calculated, which is regarded as another feature. Finally, the two features are input into the modulation format classifier constructed by the SVM to achieve MFI in coherent optical communication systems. To verify the feasibility and superiority of the scheme, we compare it with the MFI scheme based on higher-order statistical (HOS) features, GLCM features, and FD features, respectively. Further, we built a 30 GBaud coherent optical communication system with fiber lengths of 80 km and 120 km, where the optical signal-to-noise ratio (OSNR) ranges from 0 dB to 30 dB. The proposed MFI scheme identifies seven modulation formats: QPSK, 8QAM, 16QAM, 32QAM, 64QAM, 128QAM, and 256QAM. The results show that compared with the other three schemes, our proposed scheme has a better identification accuracy at low OSNR. In addition, the identification accuracy of this scheme can reach 100% when the OSNR ≥ 10 dB.

All-Optical Multi-Channel Aggregator for QPSK to 16QAM Based on Time-Lens and Phase Reloader in Elastic Optical Networks

An all-optical aggregator consisting of time-lens and a phase reloader for two four-channel multiplexed quadrature phase shift keying (QPSK) signals to a wavelength division multiplexed (WDM) 16-ary quadrature amplitude modulation (16QAM) signal is proposed. The feasibility of the scheme is confirmed by the waveforms of the intermediate process and the constellations of input-output signals. The performance of bit error rate (BER) and error vector magnitude (EVM) values under different optical signal-to-noise ratios (OSNR) are analyzed to evaluate the system, and the OSNR penalties of the aggregated signal at the forward error correction (FEC) threshold are between 2.25 dB and 2.45 dB. This scheme potentially enables reconfigurable service and improves compatibility in elastic optical networks (EON).