Optical Signal-to-noise Ratio Research Articles

A selectable single-frequency erbium fiber laser with septuple-ring apparatus for narrow linewidth and uniform power output

A single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) with septuple-ring is demonstrated to acquire tunable and stable wavelength output. The demonstrated septuple-ring design not only can result in mode-filter operation to diminish the dense longitudinal-mode oscillations, but also can reach sub-kHz linewidth. The presented fiber laser also can extend the tuning bandwidth from 1512.0 to 1572.0 nm based on C-band gain-medium and lead to a uniform power output range within 0.36 dB power fluctuation. Furthermore, the obtained output linewidth, output power (Pout), output stability and optical signal to noise ratio (OSNR) of the EDFL are also executed and discussed.

Intelligent joint multi-parameter optical performance monitoring scheme based on HT images and MT-ResNet for elastic optical network

In elastic optical network (EON), joint multi-parameter optical performance monitoring (OPM) can effectively manage, diagnose, and reduce operational costs for transmission optical links. In this paper, we propose a novel joint monitoring scheme utilizing Hough transform (HT) images combined with multi-task residual neural network (MT-ResNet) for EON. The scheme can realize baud rate identification (BRI), modulation format identification (MFI), residual chromatic dispersion identification (CDI), optical signal-to-noise ratio (OSNR) and residual differential group delay (DGD) estimation at the same time. The HT image is obtained by preprocessing original constellation diagram, which is a key feature of parameter monitoring for EON signals and presents obvious differentiation in parameter space. By optimizing the skip connections in MT-ResNet, we effectively resolve the issue of details information loss or incompleteness caused by the transmission of impaired optical signals in the neural network. The simulation results demonstrate that the identification success rate can reach 100 % for two common BRs, five mainstream MFs, and seven residual CD values with different impairment degrees. The mean absolute errors (MAEs) of OSNR and residual DGD estimates are 0.42 dB and 0.014 times symbol period respectively. The scheme has excellent tolerance for fiber nonlinear effects. In experimental verification, the accuracies of BRI and MFI are 100 %, and the MAEs of corresponding OSNR estimation for PDM-QPSK/16QAM/32QAM are 0.25 dB, 0.36 dB, and 0.40 dB, respectively. Compared with the existing typical schemes, our scheme significantly improves performance and reduces complexity while simultaneously monitoring a large number of parameters.

Single-frequency fiber laser at 1627 nm based on a self-designed Er-doped hybridized glass fiber.

Aiming at applications like expanding usable wave band of optical telecommunication and preparing Sr optical lattice clocks, a 1627 nm single-frequency fiber laser (SFFL) is demonstrated based on a 7-m-long self-designed Er-doped hybridized glass fiber (EDHF) and a linear cavity configuration with a loop mirror filter (LMF). By inserting a 10-m-long unpumped commercial Er-doped fiber as a dynamic Bragg grating into the LMF, a stable single-longitudinal-mode (SLM) laser with an output power of about 10 mW is obtained. The optical signal-to-noise ratio (OSNR) of SFFL is over 50 dB, and the linewidth is about 3.7 kHz. The measured relative intensity noise (RIN) is less than -140 dB/Hz at frequencies of over 0.5 MHz, and a power variation in 1 h is less than ±0.26%. To our best knowledge, it is the first demonstration of a SFFL operating at the U-band. This 1627 nm SFFL can provide advanced light source technology support for many cutting-edge applications.

A 75 GHz Brillouin Stokes Channels For High Data Rate DWDM Communication Systems

The experimental results of a 75 GHz Brillouin Stokes channels are demonstrated in this paper. Three dispersion compensation fiber (DCF) spools and one single mode fiber (SMF) spool are used as a Brillouin gain medium. An erbium doped fiber amplifier (EDFA) sections are used to amplify the generated Stokes channels and to reduce the Brillouin Stokes threshold inside the cavities. Laser pump of 1480 nm with total power of 470 mW are used for the EDFA sections as a pump power unit. In our proposed setup, four channels with 75 GHz spacing, high Stokes power of about 10 mW, and large optical signal to noise ratio (OSNR) of about 52 dB are achieved at Brillouin pump power of 25 mW. The achieved channels are tuned over 15 nm from 1565 to 1580 nm. within the entire tuning range up to 25 Stokes channels with 75 GHz spacing can be utilized. High data rate of about 800 Gb/s can be obtained per single channel at this wide channel spacing. Therefore, the total data rate that could be achieved within the obtained tuning range is about 20 Tb/s.

O-band tunable multiwavelength Brillouin-Raman fiber laser based on a wavelength-agile Raman pump.

Multiwavelength Brillouin-Raman fiber laser (MBRFL) features broadband multiwavelength generation with flat-amplitude and high optical signal to noise ratio (OSNR), which has great potential in optical fiber communication applications. Till now, the spectral regions of MBRFLs are mostly concentrated at conventional C- and L-band and the tunability of MBRFL is limited by using the Raman pump with fixed wavelength. Here, by utilizing wavelength-agile random fiber laser which can emit tunable lasing at 1.2 µm band as the Raman pump, we experimentally demonstrate the tunable MBRFL in the O-band for the first time, to the best of our knowledge. At Raman and Brillouin pump powers of 920 mW and -3 dBm, respectively, up to 90 Stokes lines with 0.13 nm wavelength spacing and >13 dB OSNR can be obtained when the Raman and Brillouin pump wavelength are set at 1231 nm and 1300 nm, respectively. Moreover, by tuning the wavelength of Brillouin pump from 1295 nm to 1330 nm, tunable MBRFL can be achieved with similar multiwavelength generation bandwidth by simultaneously tuning the Raman pump wavelength, and the number of Stokes lines are beyond 85 across the tuning range. The bandwidth of the demonstrated O-band MBRFL is also the widest wavelength span ever reported for multiwavelength Brillouin fiber lasers at 1.3 µm band. Our work indicates that the use of wavelength-agile random fiber laser as Raman pump in MBRFL can provide an effective way to extend the spectral regions of MBRFL and also improve the tunability performance of MBRFL.

Design and performance investigation of a coherent optical system based multi-band OFDM with various indexes

Abstract In this work, a comprehensive study of a multi-band optical orthogonal frequency division multiplexing (MB-OOFDM) system is investigated. Different OFDM indexes (128,256,512, and 1024) are utilized with the coherent detection CD technique. High-order modulation 16-quadrature amplitude modulation (16-QAM) is used to obtain high spectral efficiency. Transmission studies indicated that 128 subcarriers with 2-bands exhibit a higher optical signal-to-noise ratio (OSNR) of 23 dB with lower bit error rate (BER) of 1 × 10−4, while 1024 subcarriers with 8-bands shows a lower OSNR of 22 dB with a higher BER of 3 × 10−3. Even though the higher subcarrier type has the lowest BER, it provides a superior solution when combined with wavelength-division multiplexing (WDM) in the presence of forward error correction (FEC). The performance of the system with 40 Gbps along 800 km of uncompensated fiber has designed and simulated using Optisystem software.

Experimental evaluation on OSNR and DMG performance of cascade FM-EDFA systems by equivalent input spectrum method

An all-fiber few-mode erbium-doped fiber amplifier (FM-EDFA) is constructed by the integration of few-mode isolator wavelength division multiplexers (FM-IWDMs), which act as the multiplexing or demultiplexing function between signal and pump light. The gain performance of the FM-EDFAs under forward, bidirectional and backward pumping cases is investigated by experiment. For LP01, LP11e and LP11o-mode signals, the minimum differential mode gain (DMG) of 0.14 dB can be achieved by forward LP11e and LP11o-mode pumping. We propose the equivalent input spectrum method to experimentally simulate the cascade FM-EDFA systems. For the three-mode amplification, the optical signal-to-noise ratio (OSNR) and overall DMG after 8-stage transmission are respectively 13.89 dB and 1.59 dB, which may theoretically support 1000 km-long transmission of 100 Gb/s DP-QPSK signal. A cascade amplification system composed of six-mode EDFA with the DMG of 1.35 dB is also considered to verify the feasibility of the equivalent input spectrum method.

On the design of low phase noise and flat spectrum optical parametric frequency comb

Optical frequency combs (OFCs) have become increasingly pervasive in recent years, with their advantageous frequency coherence properties enabling significant developments in numerous fields, such as optical communications, spectroscopy, and microwave signal processing. Recent interest in OFC development emphasizes minimizing and mitigating phase noise of individual comb lines for high-quality signal generation, processing, and detection. Cavity-less electro-optic combs and parametric combs are attractive sources for these applications in that they permit flat spectra, tunable tone spacing, and robustness to temperature variations. Although previous research has demonstrated broadband parametric OFC generation, the scaling of the phase noise has not been systematically investigated. Here, we demonstrate a 25 GHz-spacing cavity-less parametric OFC generator and investigate the interaction between electronic and optical noise sources that affect its phase noise and linewidth. In addition, we study the optimal design of a nonlinear amplified loop mirror based pulse shaper with a focus on the impact of pump power on the signal-to-pedestal power ratio, which ultimately influences the spectral flatness and the optical signal-to-noise ratio (OSNR) after the parametric expansion. Notably, we design the OFC using all polarization-maintaining (PM) components, demonstrating the performance of PM highly nonlinear fibers in parametric comb generation. This results in a PM cavity-less comb with <9 dB power variation over 110 nm, >0 dBm power per tone, <10 kHz linewidth, and >23 dB OSNR. These characteristics make it highly desirable for application in communication and signal processing.

ASE noise mitigation with digital frequency offset loading for discrete spectrum nonlinear frequency division multiplexing systems.

We propose an amplified spontaneous emission (ASE) noise mitigation scheme utilizing digital frequency offset loading (DFO-loading) for discrete spectrum nonlinear frequency division multiplexing (DS-NFDM) systems. Firstly, based on the one-to-one mapping relationship between frequency offsets and eigenvalue positions, the transmitter side encodes 4-bit information onto 16 kinds of different digital frequency offsets. Then, a sliding window-assisted eigenvalue position (SWA-EP) decoding technology is further proposed to substitute the classical channel equalization and carrier phase recovery processes, with the purpose of recovering the original information. The numerical and experimental results demonstrate that, compared with b-coefficient 16 quadrature amplitude modulation (QAM) scheme, Q-factor gains are 2.1 dB under 15 dB optical signal-to-noise ratio (OSNR) and 1.8 dB after 800 km fiber transmission, respectively. Moreover, its complexity is only 0.6% of the b-coefficient scheme. The DFO-loading scheme offers an effective and low-complexity way to mitigate ASE noise of DS-NFDM system.

Compensating Chromatic Dispersion and Phase Noise using Parallel AFB-MBPS For FBMC-OQAM Optical Communication System

Filter Bank Multi-Carrier Offset-QAM (FBMC-OQAM) is one of the hottest topics in research for 5G multi-carrier methods because of its high efficiency in the spectrum, minimal leakage in the side lobes, zero cyclic prefix (CP), and multiphase filter design. Large-scale subcarrier configurations in optical fiber networks need the use of FBMC-OQAM. Chromatic dispersion is critical in optical fiber transmission because it causes different spectral waves (color beams) to travel at different rates. Laser phase noise, which arises when the phase of the laser output drifts with time, is a major barrier that lowers throughput in fiber-optic communication systems. This deterioration may be closely related among channels that share lasers in multichannel fiber-optic systems using methods like wavelength-division multiplexing with frequency combs or space-division multiplexing. In this research, we use parallel Analysis Filter Bank (AFB) equalizers in the receiver part of the FBMC OQAM Optical Communication system to compensate for chromatic dispersion (CD) and phase noise (PN). Following the equalization of CD compensation, the phase of the carriers in the received signal is tracked and compensated using Modified Blind Phase Search (MBPS). The CD and PN compensation techniques are simulated and analyzed numerically and graphically to determine their efficacy. To evaluate the FBMC's efficiency across various equalizers, 16-OQAM is taken into account. Bit Error Rate (BER), Optical Signal-to-Noise Ratio (OSNR), Q-Factor, and Mean Square Error (MSE) were the primary metrics we utilized to evaluate performance. Single-tap equalizer, multi-tap equalizer (N=3), ISDF equalizer with suggested Parallel Analysis Filter Banks (AFBs) (K=3), and MBPS were all set aside for comparison. When compared to other forms of Nonlinear compensation (NLC), the CD and PN tolerance attained by Parallel AFB equalization with MBPS is the greatest.

Single-longitudinal-mode narrow linewidth broadband tunable Brillouin random laser including dispersion compensation fibers

We propose and demonstrate a single-longitudinal mode (SLM) narrow linewidth broadband tunable Brillouin random laser, including two segment dispersion compensated fibers (DCFs), in which enough narrowband Brillouin gain provided by a 2-km-length DCF to overcome the cavity loss, and Rayleigh backscattering (RBS) provided by a 1-km-length DCF form a mirrorless open cavity to continuously compress the laser linewidth. Compared to ordinary single mode fibers, DCF has an enhanced stimulated Brillouin scattering gain coefficient and RBS coefficient, which is attributed to its small core diameter structure, making the laser more compact and stable. Stronger random feedback effectively lowers the laser threshold, and enough Brillouin gain for multiple amplifications in random mode makes SLM operations more accessible. The proposed laser is with a threshold of ∼11 mW and an optical signal-to-noise ratio (OSNR) of ∼55.75 dB, the 3-dB linewidth and linewidth compression ratio of the output laser reaches ∼550 Hz and ∼236, respectively. By changing the wavelength of the pump laser, the laser wavelength can be continuously tuned in a broad wavelength range, which is only limited by tunable range of Brillouin pump laser. The laser wavelength fluctuation of 0.02 nm and the OSNR fluctuation of about 3 dB in a 30-minute measure period, indicating that the laser is with a good stability.

Reconfigurable all-optical pattern-matching system for phase modulation formats based on phase-sensitive amplification in highly nonlinear fiber

All-optical pattern-matching system is to compare the input data with the target sequence to implement the data filtering directly in the optical layer. However, for current all-optical pattern-matching systems, one pattern-matching system can only match one modulation format, which limits its application. In this paper, a reconfigurable all-optical pattern-matching system for phase modulation formats is proposed. Taking advantage of phase-sensitive amplification (PSA) in highly nonlinear fiber (HNLF), the system can match various phase modulation formats through optical switches configured by a controller. We first evaluate the feasibility of the proposed system for binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) modulation formats with the transmission rates of 100 Gbaud and 200 Gbaud. The results show that our proposed system can correctly recognize and locate the position of the target sequence. Next, we analyze the extinction ratio (ER), contrast ratio (CR), Q factor, and bit error rate (BER) by changing the optical signal-to-noise ratio (OSNR) values of BPSK and QPSK signals to evaluate the noise performance. The proposed reconfigurable all-optical pattern-matching system provides an effective method to recognize signals with different phase modulation formats, which is suitable for future dynamically reconfigurable optical networks.

High-speed spatial light modulation enabling 25-Gbit/s twisted light encoding/decoding and 260-m security free-space data transmission.

Spatial domain of light beam is an important degree of freedom to be extensively explored. As a set of spatial domains, twisted lights have some natural properties such as orthogonality and security, providing great potentials in optical communications especially for data encoding/decoding. However, the speed of traditional spatial light modulators has always been criticized. Here we present a hundred-meter security free-space data transmission based on high-speed spatial light modulation by exploiting temporal-to-spatial domain mapping. We demonstrate 25-Gbit/s twisted light encoding/decoding and 260-m security free-space data transmission in the experiment. The encoding/decoding link will lead to 3-dB improvement in bit error rate (BER) performance compared with a single channel in theory and ∼1-dB optical signal-to-noise ratio (OSNR) penalty at the forward error correction (FEC) threshold of 3.8e-3 in practice. The experiment results also show favorable security performance of the proposed encoding/decoding link system.

Watt-level broadband PM Tm-doped fiber amplifier for the 1750–1910 nm window

We report broadband experimental and simulated performance of a single-clad polarization-maintaining (PM) thulium-doped fiber amplifier (TDFA) optimized for operation in the 1750–1910 nm wavelength band. With the two-stage amplifier architecture, a maximum output power of 2.4 W was obtained at 1850 nm for 7.0 W of launched pump power at 1567 nm, with a corresponding optical-to-optical efficiency of 31.4 %. The measured polarization extinction ratio (PER) was larger than 22.6 dB, and the long-term power stability exhibited a peak-to-peak variation of 2.4 %. By employing a tunable seed laser covering the 1750–1910 nm wavelength range, we measured a maximum output power level of 2.38 W with an optical signal-to-noise ratio (OSNR) greater than 53 dB/0.1 nm. Our simulations are in good agreement with the experimental data over the whole 160 nm operating spectral bandwidth.

Weighted Decision Enabled Phase Retrieval Receiver With Adaptive Intensity Transformation

Phase-retrieval (PR) receivers can reconstruct complex-valued signals from two de-correlated intensity measurements by the use of iterative PR algorithm, without the assistance of any optical carriers. However, both the computational complexity with hundreds of iterations and the limited PR accuracy prevent it from being applied to high-speed photonic interconnections. In current submission, we propose a weighted decision enhanced PR receiver with adaptive intensity transformation (WD_AIT_PR). The weighted decision based on a compressed sigmoid nonlinear function is helpful to avoid the hard-decision (HD) errors, leading to the performance enhancement of the PR receiver, in terms of both convergence speed and PR accuracy. When the 56 GBaud 16QAM signal after transmission over 80 km standard single mode fiber (SSMF) is reconstructed, the WD_AIT_PR receiver is numerically verified to possess the faster convergency speed with only 70 iterations and the best steady BER performance by reducing the required optical signal-to-noise ratio (OSNR) by 0.8 dB, 1.8 dB and 1.8 dB in order to reach 7% HD forward error correction (HD-FEC), in comparison with the HD_AIT_PR receiver by replacing WD with HD, the PR receiver with the phase reset (PR_PR) and the PR receiver with adaptive intensity transformation (AIT_PR), respectively. Meanwhile, the WD_AIT_PR receiver is robust to transmission impairments, including transmitter-side I/Q imbalance, receiver-side skew, the noise imbalance between two intensity measurements, laser phase noise, and laser wavelength drift. Finally, the enhanced performance of the WD_AIT_PR receiver is experimentally verified through recovering 25 GBaud 16QAM signals under the condition of 40 km SSMF transmission.

Power equalization-based disaggregated optical network using switched gain equalization controlled amplifiers.

Optical network disaggregation is an attractive agreement to avoid vendor lock-in in choosing desirable partners based on lead time of delivery and facilitates strong negotiations. Disaggregation of conventional coherent transceivers (XCVRs) with 400G ZR+ technologies (of similar bandwidth density and form factor QSFP-DD) allows operators to enjoy significant cost reduction and substantial power savings in high-capacity IP over a WDM transmission. However, interoperability between low power and high back-to-back required optical signal-to-noise ratio (OSNR) of ZR+ and high power with relatively lower back-to-back required OSNR of conventional coherent XCVRs limits the performance of the latter, which makes disaggregation more complex. This can be avoided by equalizing the power of ZR+ and conventional XCVRs, by wavelength-selective switches (WSSs) in re-configurable optical add/drop multiplexers. However, such power equalization can lead to lower link OSNR, originating from insertion loss and penalty caused by cascaded WSSs. In this Letter, we use a dynamic gain equalizer of a switched gain equalization controlled (SGEC) amplifier at each in-line amplifier site to equalize the powers, without introducing insertion loss or filter penalty. We report a field trial of a low power 400G-ZR+ QSFP-DD-DCO as an alien channel over ten high-power coherent host channels, interoperating with open forward error correction, between two different vendors. We obtain a similar Q-margin in the absence and presence of 400-ZR+ channel, suggesting the equalization by SGEC amplifiers. We further simulate the power and OSNR evolution of channels along the field trial link to verify the equalization. Simulation results are consistent with that of the field trial.

A novel PAPR reduction scheme based on nonlinear real-valued support vector regression for CO-OFDM systems

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems suffer from the high peak-to-average ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals, which leads to the back-off of laser power and fiber nonlinear effects, thereby deteriorating the bit error rate (BER) performance and limiting the transmission distance. Iterative clipping and filtering (ICF) scheme, as a direct, simple and effective PAPR reduction scheme that only processes signals at the transmitter, but requires multiple fast Fourier transform (FFT)/inverse fast Fourier transform (IFFT), resulting in high computational complexity. We propose a novel scheme using the machine learning (ML) technique, which is based on nonlinear real-valued support vector regression (NRSVR) and trained on ICF scheme. The simulation results show that compared with ICF scheme, the proposed scheme has a similar PAPR0 for 10−4 complementary cumulative distribution function (CCDF), an additional 15 km of single mode fiber (SMF) transmission for 16 dB optical signal-to-noise ratio (OSNR) and 10−3BER, and a significant time complexity reduction. Compared with other ML-based schemes, the proposed scheme also achieves better performance and lower computational complexity.

Selectable and stable single-frequency fiber laser with flattened power range and sub-kHz linewidth output

To achieve stable and tunable single-longitudinal-mode (SLM) wavelength output, an erbium-doped fiber (EDF) laser with two fiber ring architecture and unpumped EDF based saturable absorber (SA) is demonstrated experimentally. The acquired optical signal to noise ratio (OSNR), wavelength linewidth, output stability and power of the fiber laser over an available tuning bandwidth of 1523.0 to 1569.0 nm are also implemented. Furthermore, this EDF laser configuration also can distribute the output power uniformly in the span of 1527.0 to 1569.0 nm within 0.53 dB power difference.

Thulium-doped fiber laser with bidirectional single-longitudinal-mode output using a cascaded triple-ring cavity filter

Single-longitudinal-mode (SLM) fiber lasers play important roles in optical fiber communication. However, there has been little reported research on bidirectional SLM output, especially in the 2-μm band. In the work reported here, a bidirectional SLM output thulium-doped fiber laser (TDFL) using a cascaded triple-ring cavity (CTRC) filter was proposed and investigated. To realize bidirectional output, the proposed TDFL had a simple structure, using only two 2 × 2 optical couplers. To realize SLM output, the transmission spectrum, as well as the reversibility of the CTRC filter were discussed and investigated in detail. The direction-switchable SLM output was realized by polarization-dependent loss. By tuning the polarization controller carefully, each direction could generate SLM output with an optical signal-to-noise ratio (OSNR) > 72 dB. The maximum wavelength and peak power fluctuation over 60 min were, respectively, 0.01 nm and 0.48 dB. The variation of output power with pump power, linewidth, relaxation oscillation, and relative intensity noise were also investigated. To the best of our knowledge, this is the first time that a sub-ring cavity filter has been used to realize bidirectional SLM output.

All-optical wavelength and format conversion of DQPSK-to-PAM4 using DLI and pump assisted NOLM

An all-optical wavelength and format conversion scheme of 20 Gbps differential quadrature phase shift keying (DQPSK)-to-4-ary pulse amplitude modulation (PAM4) signals based on the delay line interferometer (DLI) and the pump assisted nonlinear optical loop mirror (NOLM) is proposed and numerically simulated. The input DQPSK signal is converted into two star-16-ary quadrature amplitude modulation (star-16QAM) signals (namely, star-16QAM-I and star-16QAM-Q) using DLI with the time delay of 1-symbol and different linear phase shifts of π/4 and −π/4, respectively. The star-16QAM signals are converted into the on-off keying (OOK) signals (namely, OOK-I and OOK-Q) using the pump assisted NOLM based on the self-phase modulation (SPM) and the cross-phase modulation (XPM) in the highly nonlinear fiber (HNLF). The two OOK signals can be aggregated into different types of PAM4 signals by adjusting their power ratio (PR) and relative phase shift (RPS) flexibly. Error vector magnitude (EVM) and bit error rate (BER) for the input and output optical signals are calculated to verify the scheme performance. Compared to the DQPSK signal with an input optical signal-to-noise ratio (OSNR) of 30 dB, there are about 2.5 dB penalty of receiver OSNR for the converted OOK-I and OOK-Q signals. For the corresponding aggregated equidistant PAM4 and equiamplitude PAM4 signals, they have about 6.3 dB and 12.1 dB penalty of receiver OSNR. The proposed scheme of DQPSK-to-PAM4 can be deployed at the all-optical gateway to bridge the long-hual and short-reach optical networks.