Standard Single-mode Fiber Research Articles

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.

Physical layer secure key generation and distribution based on noise variances in optical fiber communications systems

Considering the key demand in the secure optical fiber communication, a key generation and distribution scheme based on random noise variances is proposed. The noise of optical communication systems is inevitable due to the influence of various factors. The additive noise that cannot be eliminated is random after digital signal processing, which ensures the randomness of the key. However, the range of system noise variances is small and the noise of links is exposed. A random Gaussian white noise is added at receivers, which increases the noise fluctuation and enhances security. Legitimate parties obtain secure keys by calculating the data inconsistency rate, where the consistency comes from the correlation of noise variances in adjacent time slots over a period of time. Experimental results demonstrate that the error-free key is obtained with the key generation rate of 1.6 Mbit/s over 105 km standard single-mode fiber. Moreover, the security analysis shows that the proposed key generation and distribution scheme can resist fiber-tapping attacks.

Proof-of-concept demonstration of double-slope-assisted Brillouin optical correlation-domain reflectometry

We develop a new configuration of distributed strain and temperature sensing technology called double-slope-assisted Brillouin optical correlation-domain reflectometry. Its loss-independent operation is demonstrated through simplified simulation and proof-of-concept experiments using a standard silica single-mode fiber.

Fiber-optic temperature sensor based on glass Fabry–Perot interferometer

The paper presents a theoretical and experimental study of a fiber-optic all-glass microthermometer based on a Fabry–Perot interferometer. The interferometer is fabricated by applying borosilicate glass to the cleaved end of a standard telecom single-mode optical fiber using a microforge. According to the results of experimental studies, the spectral sensitivity of the fabricated interferometer was about 40 pm/°C. A mathematical model of the Fabry–Perot interferometer based on the method of scattering and transfer matrices is presented and verified, which makes it possible to calculate the spectral response of an interferometer with arbitrary structure parameters.

Multivariable Analysis of Nonlinear Optical Loop Mirror Operating Parameters Using Jones Matrices and Three-Dimensional Renderings

Nonlinear optical loop mirrors (NOLMs) are used in modern fiber optic devices and optical communications. In this study, we present numerical analyses of the multiple variables involved in the operation of an NOLM in low- and high-power transmissions. The Jones matrix formalism was used to model linear and circular polarization inputs. We used three-dimensional (3D) plots to identify the characteristics required in the experimental operation of the NOLM. These characteristics, including the critical power, low- and high-power transmission, and dynamic range, depend on parameters such as the fiber loop length, input power, angle of retarder plate, and input polarization. A standard single-mode fiber (SMF-28) with high twist loop lengths of 100, 300, and 500 m and input powers of 0–100 W was simulated. Three-dimensional surface graphics provided a comprehensive view of the NOLM transmission and considerably enhanced the optimal transmission by manipulating adjustable device components including the power and polarization control plates. Optimal transmission facilitates its use in integrating ultrafast pulse generation, optical signal processing, optical communication systems, and photonic integrated circuit applications.

100Gb/s coherent optical secure communication over 1000 km based on analog-digital hybrid chaos.

In recent years, the transmission capacity of chaotic secure communications has been greatly expanded by combining coherent detection and multi-dimensional multiplexing. However, demonstrations over 1000 km fiber are yet to be further explored. In this paper, we propose a coherent optical secure transmission system based on analog-digital hybrid chaos. By introducing an analog-digital converter (ADC) and a bit extraction into the feedback loop of entropy source, the broadband analog chaos is converted into a binary digital signal. This binary digital signal is then mapped to a 65536-level pulse amplitude modulation (PAM) signal and injected into the semiconductor laser (SL) to regenerate the analog chaos, forming a closed loop. The binary digital signal from the chaos source and the encrypted signal are transmitted via wavelength division multiplexing (WDM). By using conventional digital signal processing (DSP) algorithms and neural networks for post-compensation, long-haul high-quality chaotic synchronization and high-performance secure communication are achieved. In addition, the probability density distribution of the analog chaotic signal is effectively improved by adopting the additional higher-order mapping operation in the digital part of the chaos source. The proof-of-concept experimental results show that our proposed scheme can support the secure transmission of 100 Gb/s quadrature phase shift keying (QPSK) signals over 1000 km of standard single-mode fiber (SSMF). The decrypted bit error rate (BER) reaches 9.88 × 10-4, which is well below the 7% forward error correction (FEC) threshold (BER = 3.8 × 10-3). This research provides a potential solution for high-capacity long-haul chaotic optical communications and fills the gap in secure communications based on analog-digital hybrid chaos.

Fast polarization state tracking for dual-polarization direct detection with Jones-space field recovery.

Direct detection system is expected to possess the phase and polarization diversity in order to achieve high spectral efficiency and fiber impairment compensation such as chromatic dispersion and polarization rotation. In this Letter, we theoretically extend the concept of the proposed Jones-space field recovery (JSFR) to include a dynamic polarization rotation matrix and experimentally demonstrate the rapid polarization state tracking ability of the JSFR receiver based on a 3 × 3 optical coupler. Under a rotation of the state of polarization at a rate of 1 Mrad/s, we successfully transmit 59-GBd dual-polarization 16-ary quadrature-amplitude-modulation signals over an 80-km standard single-mode fiber based on a decision-directed least mean square (DD-LMS) or a recursive least square (DD-RLS), with a bit-error rate below the 14% hard-decision forward error correction threshold of 1 × 10-2. The experimental results indicate that the legacy polarization tracking algorithms designed for coherent optical communication are also applicable for this direct detection scheme. To our best knowledge, this work demonstrates the first polarization rotation-tolerant direct detection system with phase and polarization diversity, providing a low-cost and high-speed solution for short-reach communications.

All-fiber hollow-core fiber gas cell

We present a hollow-core fiber (HCF) gas cell with light launched and collected via standard solid-glass core single-mode fibers (SMFs). Gas delivery is realized via a 100μm gap between the SMF and HCF with light low-loss coupling optimized for such a gap. Coupling into higher-order modes as well as the Fresnel reflections at the SMF-HCF interface are strongly suppressed to minimize unwanted multi-path interference. The assembled HCF gas cell including the gas inlet is encapsulated in a conventional metallic T-piece for easy gas filling, venting, and sealing. Thanks to its numerous features, such as alignment-free design (once fabricated), above-described promising optical performance, and compactness, we believe it will be of interest for applications in gas sensing and as gas references.

MMW Signal Gain in DML-Based Microwave Photonic Links Under Large Signal Regime

This paper presents a theoretical and experimental analysis of the millimeter wave (mmW) signal gain observed in photonically generated signals by using carrier suppressed (CS) external modulation in a centralized network architecture while the data is transmitted over a directly modulated laser (DML) operating under different signal regimes. Measurements of the frequency response under small and large signal regimes are presented for the sake of comparison. Moreover, the mmW signal gain has been measured for different standard single mode fiber (SSMF) lengths in order to identify the conditions and impact of large signal regime operation. The results are also validated by the experimental transmission of a 25 MHz QPSK signal transmitted over 40 GHz since the error-vector-magnitude (EVM) is measured for different input electrical power values as well as several received optical power (RoP) levels. Moreover, the spurious-free dynamic range (SFDR) ratio is measured to compare the third-order intermodulation distortion (IMD3) between small and large signal regime. The quantitative results demonstrate that proper system conditions allow the exploitation of the mmW signal gain towards the future deployment of high energy efficient networks.

Low-Complexity Conditional Generative Adversarial Network (c-GAN) Based Nonlinear Equalizer for Coherent Data-Center Interconnections

Nonlinear impairments emerging from both fiber optical links and optoelectrical devices are the major bottleneck for enhancing the throughput of photonic data-center interconnection (DCI). Here, we propose a data-driven nonlinear equalizer based on a conditional generative adversarial network (c-GAN) for high-speed and large-capacity photonic DCI. Its performance is experimentally evaluated by transmitting C-band 40-channel 60-GBaud dual polarization-16 quadrature amplitude modulation (DP-16QAM) dense wavelength division multiplexing (DWDM) signals over 20-km standard single-mode fiber (SSMF). Our experimental results verify that, the proposed c-GAN equalizer outperforms both traditional nonlinear equalizers and other data-driven counterparts, in terms of both the bit-error ratio (BER) and computational complexity. Only the proposed c-GAN equalizer can guarantee the BER performance of all 40 channels to reach the threshold of 20% soft decision forward error correction (SD-FEC) at BER=0.027, leading to a net transmission capacity of 16 Tb/s. Meanwhile, the computational complexity of the proposed c-GAN equalizer can be significantly reduced by 31.8%, 80.7%, and 98.8%, respectively, in comparison with Volterra filter equalizer (VFE), multilayer perceptron (MLP), and long short-term memory neural network (LSTM-NN) schemes.

Direct detection system for independent triplet-sideband signals based on a single photodiode.

This paper proposes a novel, to the best of our knowledge, independent triplet-single-sideband (triplet-SSB) transmission system scheme aimed at increasing channel capacity and improving spectrum efficiency. The conventional independent multiband transmission systems are limited by their complexity and high computational requirements, which hinder the improvement of spectrum efficiency and channel capacity. To address these challenges, this scheme uses three independent signals, modulated by an in-phase/quadrature (I/Q) modulator, and transmits them over a 5-km standard single-mode fiber (SSMF). At the receiver end, a single photodiode (PD) is used for signal reception, and the signals are separated using digital signal processing (DSP) algorithms. Through simulation and verification, the feasibility and reliability of the system are demonstrated, with the bit error rates (BERs) of all three signals below the hard decision forward error correction (HD-FEC) threshold value of 3.8 × 10-3. This independent triplet-SSB transmission system scheme effectively improves spectrum efficiency and channel capacity, providing a valuable solution for meeting the growing demands of data transmission.

Partial response THP for high-speed PAM-4 transmission

Intensity-modulation direct-detection (IM/DD) pulse-amplitude modulation (PAM-4) signals are promising for high-speed data center networks. However, high-speed PAM-4 signals always are subject to critical impairments due to the bandwidth constraint. In the conventional PAM-4 system, a direct detection-faster than Nyquist (DD-FTN) with high complexity is usually appended to the feed-forward equalizer (FFE) for colored noise suppression to maximize transmission performance. In order to yield a simple PAM-4 scheme with similar performance as that of the DD-FTN, we proposed a nonlinear differential coding-generalized Tomlinson-Harashima Precoding (NLDC-GTHP) PAM-4 system. The proposed system consists of a NLDC-GTHP at the transmitter and a simplified two-dimensional constellation distortion (S2DCD) module at the receiver. We experimentally demonstrated the proposed NLDC-GTHP PAM-4 system for 62.5-Gb/s PAM-4 signal transmission over 2-km standard single mode fiber (SSMF) with the 10-dB bandwidth of about 12 GHz. By introducing the NLDC-GTHP and S2DCD, we achieved the similar receiver sensitivity as that of the DD-FTN at the hard-decision forward-error-correction (HD-FEC) bit error rate (BER) threshold, with the decrease of 86% computational complexity.

Optical Fiber Sensor with Novel Structure and Its Applications in Oil and Gas Exploration

Compared to electronic sensors, Optical Fiber Sensors (OFSs) have received increasing attention due to their advantages, such as small size, light weight, anti-electromagnetic interference, easy reuse, and remote sensing. This paper proposes a novel all-fiber mode interferometer. This interferometer excites higher-order cladding modes through mode mismatch between standard single-mode fibers and thin cores. There is interference between higher-order cladding mode and core layer mode, resulting in interference fringes used as sensing signals. Sensitivity testing is conducted on the designed OFS to demonstrate whether it meets the application requirements for petroleum exploration. Transmission-type Thin-Core Fiber Modal Interferometers (TCFMIs) with different lengths of Thin-Core Fibers (TCFs) (20 mm, 40 mm, and 60 mm) are produced in the experiment. Among them, TCFMIs at 60 mm TCF length can obtain ideal sensing signals. The TCFMI (with a length of 20 mm TCF) is encapsulated in a self-made aluminum groove in rosin to test its Refractive Index (RI). The results show that as the RI increases, the central wavelength shifts towards the long wavelength direction. Its sensitivity reaches 146 nm/R.I.U. When the temperature is increased, the central wavelength shifts towards the long wavelength direction, resulting in lower temperature sensitivity. In the sensitivity test, the phase change obtained by designing OFSs is proportional to the vibration acceleration. It is fixed on the vibration table to keep the acceleration of the vibration table constant and adjust the vibration frequency of the vibration table. The results show that the vibration spectral line of the sensor is relatively flat within 100 Hz, and resonance occurs within the range of 200 Hz to 350 Hz. Through phase demodulation, sensors loaded with different oscillator masses increase linearly in the low-frequency range. When the vibration frequency approaches the resonance frequency of the sensor, the phase sensitivity of the sensor increases nonlinearly.

Observation of strong photorefractive-like effect in silica micro-fibers

We observe the strong photorefractive (PR)-like effect in silica micro-fibers, which is different from the previously reported nonlinear behaviors in optical fiber devices. The micro-fiber is fabricated by tapering a standard silica single-mode fiber using the hydrogen flame. Two fiber Bragg gratings are fused at each end of the micro-fiber to form a Fabry–Pérot cavity, enabling us to observe the nonlinear dynamics. By situating the silica micro-fiber in a gas medium (i.e., N2, Ar, He, and air), we observe the remarkable PR-like effect, especially under the low-pressure condition below 80 Torr. In fact, the nonlinear effect increases with the reduced pressure as a power function. Such a nonlinear effect is also affected by the laser wavelength detuning rate, input laser power, and micro-fiber diameter. Finally, a nonlinear cavity dynamics model is adopted to well describe the observed nonlinear effect.

Pilot-scale testing of natural gas pipeline monitoring based on phase-OTDR and enhanced scatter optical fiber cable

In this paper, we present the results of lab and pilot-scale testing of a continuously enhanced backscattering, or Rayleigh enhanced fiber cable that can improve distributed acoustic sensing performance. In addition, the Rayleigh-enhanced fiber is embedded within a tight buffered cable configuration to withstand and be compatible for field applications. The sensing fiber cable exhibits a Rayleigh enhancement of 13 dB compared to standard silica single-mode fiber while maintaining low attenuation of ≤ 0.4 dB/km. We built a phase-sensitive optical time domain reflectometry system to interrogate the enhanced backscattering fiber cable both in lab and pilot-scale tests. In the laboratory experiment, we analyzed the vibration performance of the enhanced backscattering fiber cable and compared it with the standard single-mode telecom fiber. Afterward, we field validated for natural gas pipeline vibration monitoring using a 4-inch diameter steel pipeline operating at a fixed pressure level of 1000 psi, and a flow rate of 5, 10, 15, and 20 ft/s. The feasibility of gas pipeline monitoring with the proposed enhanced backscattering fiber cable shows a substantial increase in vibration sensing performance. The pilot-scale testing results demonstrated in this paper enable pipeline operators to perform accurate flow monitoring, leak detection, third-party intrusion detection, and continuous pipeline ground movement.

FPGA implementation for 44.2368-Gbit/s PAM8 signal transmission with pruned pre-equalization.

In this experiment, we demonstrated an intensity modulation and direct-detection (IM/DD) system based on a field-programmable gate array (FPGA). The PAM8 signals are successfully delivered at 44.2368 Gbit/s over a 45-km standard single-mode fiber (SSMF), satisfying the soft-decision forward error correction (SD-FEC) criterion of 2.4 × 10-2, and the net bit rate may reach 36.864 Gbit/s without the need of optical amplifiers. At the transmitter, we used a pruned pre-equalization algorithm to process the PAM8 signals, and the high-speed parallel PAM8 signals were processed at the receiver using 64 parallel constant modulus algorithm (CMA) and decision-directed least mean square (DD-LMS) equalizers. Additionally, we analyzed the bit error rate (BER) performance of the DD-LMS equalizer in FPGA simulation with various data lengths, both before and after equalization.

Convergence of compressed sensing encryption and deep network recovery in RoF system

In this paper, a security scheme based on the convergence of compressed sensing (CS) encryption and deep network recovery is proposed in radio-over-fiber (RoF) system. In this scheme, CS encryption and deep network recovery are performed at the transmitter and receiver respectively. At the transmitter, the original data are selected based on the compression ratio (CR) and then protected by a three-stage encryption process after CS compression, and the dynamic key (DK) is generated from the original data. In the encryption process, the compressed information is scrambled by 3D rotation, then singular value decomposition (SVD) and DK embedding is implemented, followed by constellation symbol masking. At the receiver, a deep network is used to recover the original data and improve recovery performance. The proposed scheme is experimentally demonstrated and the encrypted OFDM signals are successfully transmitted over 50 km standard single-mode fiber (SSMF) and 5 m wireless channel. The experimental results show that the scheme has a key space of 1.34×10233, which can resist brute intrusion by eavesdroppers, and the bit error rate (BER) is below 10−3 when the received optical power (ROP) is greater than 4 dBm. Additionally, the mean structural similarity (MSSIM) is higher than 0.97, which provides a gain of 0.2 compared to traditional methods. So, the proposed scheme has good security performance and recovery performance, which has a significant potential for application in future optical networks.

Mode transition phenomena into an in-fiber Mach-Zehnder interferometer

This work reports on a novel in-fiber Mach-Zehnder interferometer (MZI) device which is able to exploit the mode transition phenomena to tailor the sensitivity to external refractive index (RI). The proposed MZI structure is extremely simple: it consists of a small section of single mode double cladding fiber (DCF) in between two sections of standard single mode fiber using the core-offset splicing approach. The DCF presents a W-shaped RI profile with the outer cladding having a greater RI than the inner cladding. Therefore, the mode transition of cladding modes from outer to inner cladding of the DCF can be induced by tailoring the cladding thickness, in turn enhancing the sensitivity to surrounding refractive index. Specifically, wet chemical etching employing hydrofluoric acid is used to reduce the outer cladding diameter. Here, we report a comprehensive study of the phenomena while fiber diameter is decreased by 30 µm. Moreover, a specific study case with DCF diameter of 112 µm demonstrates a sensitivity gain of 2.5 over unetched fiber in the refractive index range of 1.33–1.39. Temperature and mechanical stress cross sensitivities are also evaluated. The suggested sensing device's advantages of considerable compactness, low manufacturing cost, simple fabrication technique, and higher sensitivity will be useful for future applications in the bio-chemical sensing and others.

Performance of Kramers–Kronig and coherent receivers in Medium-Reach optical communication

As the capacity and the transmission distance of medium-reach optical fiber communication systems continue to increase, it is a major trend in demanding lower optical link budgets while maintaining the quality of signal transmission. We experimentally compared the transmission performance of the Kramers–Kronig (KK) and the coherent receivers in a 20-Gbaud quadrature phase shift keying (QPSK) system over a ∼ thousand-kilometer standard single-mode fiber (SMF) link. Experimental results show that the KK receiver requires lower cost and simpler structure, and behaves similarly with the coherent receiver in the transmission under 1260 km, despite higher transmitted power and computational complexity.

Q-Band MMW Transmission Enabled by Joint Probabilistic Shaping and Precoding With MZM-Based OCS Modulation

In this letter, we propose a novel and straightforward scheme for Q-band (33-50GHz) millimeter-wave signal generation enabled by a Mach-Zehnder modulator (MZM)-based joint algorithm, which combines probabilistic shaping (PS) and precoding technology, to enhance the anti-interference ability of the transmission system. Based on the proposed scheme, we experimentally demonstrate the generation and transmission of 1.4-Gbaud 46-GHz PS-16-quadrature-amplitude-modulation (PS-16QAM) vector signals, showcasing improved bit-error-ratio (BER) in both standard single-mode fiber (SSMF) and wireless transmission cases. The proposed scheme exhibits a clear advantage in terms of nonlinear interference suppression and satisfies the hard-decision forward-error-correction (HD-FEC) threshold with a BER of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> in radio-over-fiber (RoF) systems.