Improvement of differential modal gain in a ring-core few-mode erbium-doped polymer optical waveguide amplifier.

A few-mode erbium-doped waveguide amplifier (FM-EDWA) with a confined Er3+ doped ring structure is proposed to equalize the differential modal gain (DMG). The FM-EDWA amplifying three spatial modes (LP01, LP11a and LP11b) is optimized by genetic algorithm and fabricated using precise lithography overlay alignment technology. We observe gain values of over 14 dB for all modes with DMG of 0.73 dB at 1529 nm pumped only with LP01 for the power of 200 mW. Furthermore, a flat gain of more than 10 dB is demonstrated across 1525-1565 nm, with a sufficiently low DMG of less than 1.3 dB.

Low-complexity LDPC decoding algorithms for ultra-high-order modulated signals.

Low-complexity decoding algorithms and ultra-high-order modulation formats are necessary to meet data rate requirements in excess of 1 Tbps. The information bottleneck algorithm has been effectively applied to the LDPC decoding algorithm in recent years, and its performance is comparable to that of the double-precision information propagation technique. However, the application of information bottleneck decoding algorithms in ultra-high order modulation forms has received little attention. Furthermore, the number of table lookups required for a single decoding loop is square to the node degree, which is undesirable for optical communications. We present a low-complexity LDPC decoding technique for ultra-high-order modulated signals in this study. First, the algorithm employs multivariate covariates to build an information bottleneck framework, which introduces the processing required for applying the information bottleneck algorithm to 1024-QAM signals and the requirement of combining higher-order modulation formats with LDPC codes. The technique makes use of a bidirectional recursive network and the symmetry of quantized information to reuse the same set of tables, considerably reducing the number of table lookup operations required in the decoding process. Constructing a coherent optical communication system with 1024-QAM signals proves that the proposed algorithm can operate effectively. The performance sacrifice of 0.2 ∼ 0.3 dB reduces the number of table lookup operations required for decoding from square to linear magnitude, which greatly reduces the decoding time required in optical communication.

Two-dimensional phased array antenna beamforming system based on mode diversity.

In this paper, a two-dimensional phased array antenna beamforming system based on mode diversity is demonstrated for the first time. The system uses few-mode long-period fiber gratings to excite different modes, and utilizes few-mode fiber Bragg gratings and 2 × 2 optical switches to control the propagation paths of optical signals, so as to realize the true time delay control of optical signals of different mode channels and complete the two-dimensional scanning of the beam. In order to prove the feasibility of the two-dimensional phased array antenna beamforming system based on mode diversity, we conduct experimental verification and performance testing of the system using optical switches to select the loop structures composed of the optical circulators. The far-field radiation patterns of 2 × 3 phased array antenna system of different frequencies are tested at different beam pointing angles. The experimental results are compared with the simulation results to demonstrate that the beam pointing angles have no squint. The beamforming system based on mode diversity takes modes as independent channels for the transmission of signals, and excites and processes signals of different modes in a single few-mode fiber core, which effectively reduces the volume and complexity of the optically controlled phased array radar system.

Measurement of differential mode group delay in few-mode fibers based on Fresnel reflection peaks.

Mode-division multiplexing communication based on few-mode fibers (FMFs) is the most competitive scheme to address the bandwidth exhaustion in single-mode fibers. The differential mode group delay (DMGD) among different modes in FMFs is an important aspect in characterizing FMFs, and the convenient, fast, and accurate measurement of DMGD in different modes of FMFs is a crucial task. This paper presents a single-ended measurement method for DMGD in FMFs based on Fresnel reflection peaks. Experimental measurements were conducted to determine the DMGD of a 3.689 km six-mode fiber and a 9.7 km three-mode fiber, and the results were compared with the traditional time-of-flight method. The findings demonstrate that our proposed method based on Fresnel reflection peaks exhibits good accuracy in measuring the DMGD of FMFs. Moreover, this method enables single-ended measurement and offers simplicity and practicality.

Mode add-drop technology based on few-mode fiber Bragg gratings.

By increasing the number of modes of transmission in optical fiber, the mode-division multiplexing (MDM) technology can effectively improve the transmission capacity. The mode add-drop technology is an important part of the MDM system and a key element for realizing flexible networking. In this paper, a mode add-drop technology based on few-mode fiber Bragg grating (FM-FBG) is reported for the first time. The technology utilizes the reflection characteristics of Bragg grating to realize the add-drop function in the MDM system. The grating is written in parallel according to the optical field distribution characteristics of different modes. By changing the writing grating spacing Δa to match the optical field energy distribution of the few-mode fiber, the few-mode fiber grating with high self-coupling reflectivity for the high-order mode is fabricated, and the performance of the add-drop technology is improved. The mode add-drop technology is verified in a 3 x 3 MDM system, which uses quadrature phase shift keying (QPSK) modulation and coherence detection. The experimental results show that the excellent transmission, add, and drop of 3 x 8 Gbit/s QPSK signals in 8 km few-mode fibers are achieved. The realization of this mode add-drop technology only requires Bragg gratings, few-mode fiber circulators, and optical couplers. It has the advantages of high performance, simple structure, low cost, and easy implementation, and it can be widely used in the MDM system.

Fiber Bragg grating-based linear-cavity fiber laser.

A fiber Bragg grating-based linear-cavity fiber laser is proposed and demonstrated experimentally. A single-mode fiber Bragg grating is used as a wavelength selector in the fundamental mode, and four few-mode fiber Bragg gratings are used to reflect LP01 mode, LP11 mode, LP21 mode and LP02 mode at the same wavelength. Different modes can be reflected at the same wavelength between a single-mode fiber Bragg grating and four few-mode fiber Bragg gratings to form four sub-linear-cavities. This fiber laser, which enables simultaneous lasing of LP01 mode, LP11 mode, LP21 mode and LP02 mode at the same wavelength, plays an important role in the field of mode-division multiplexing fiber-optic communications.

Measurement of differential mode group delay in few-mode fiber with correlation optical time-domain reflectometer.

A measurement system of differential mode group delay (DMGD) in few-mode fiber with correlation optical time-domain reflection was proposed. A photonic lantern used in the system can be utilized for mode separation and selection and can generate six LP modes: LP01,LP11a,LP11b,LP21a,LP21b, and LP02. The signal reflected by the end of the fiber is correlated with the data sequence sent, which realizes the single-ended measurement of 5 km few-mode fiber DMGD. This method is not destructive for detecting fiber transmission systems and is simple and easy to implement. It can be used for detecting fiber transmission characteristics in the mode-division multiplexing communication system.

Atmospheric turbulence resistant heterodyne coherent receiver of few-mode fiber.

We propose and implement a free-space optical (FSO) communication system based on few-mode heterodyne detection that can effectively suppress atmospheric turbulence effects. The experimental results show that the received power gain of the FSO communication system using six-mode fibres is about 6 dB over that using SMF under moderate to strong turbulence conditions.In addition, we have built a coherent detection system for space laser communications with few-mode heterodyne detection and reception, and verified the compensation of atmospheric turbulence effects by the few-mode heterodyne detection and reception technique. Experimental results show that the proposed scheme improves the power budget by 4∼5dB over the single-mode heterodyne coherent reception scheme at BER = 3.8×10-3 under moderate to strong turbulence conditions.

Gain Enhancement of the Optical Waveguide Amplifier Based on NaYF4/NaLuF4: Yb, Er NPs-PMMA Integrated with a Si3N4 Slot.

A Si3N4 slot waveguide has the ability to confine light tightly in the slot, shows weak absorption of 980 nm pump light, and has lower transmission loss compared to a Si slot. Hence, the optical waveguide amplifier based on Er3+ and Yb3+codoped was proposed to be integrated with a Si3N4 slot to increase the gain. The core-shell NaYF4/NaLuF4: 20%Yb3+, 2%Er3+ nanocrystals-polymeric methyl methacrylate covalent linking nanocomposites were synthesized and filled into the slot as gain medium. The concentrations of Er3+ and Yb3+ were increased compared with traditional physical doping methods. High-efficiency emission at 1.53 µm was achieved under 980 nm laser excitation. The slot waveguide was accurately designed using the semivector finite difference method in combination with the maximum confinement factors and the minimum effective mode area. The optimum width of the slot was 200 nm, and the optimum height and width of the silicon strip waveguide were 400 nm and 400 nm, respectively. The six-level spectroscopic model was presented, and the gain characteristics of the slot waveguide amplifier were numerically simulated. A net gain of 8.2 dB was achieved, which provided new ideas and directions for waveguide amplifiers.

Fault detection performance of a multi-mode transmission reflection analysis for a few-mode fiber link.

We propose a novel monitoring technique based on multi-mode transmission reflection analysis for a long-reach few-mode fiber (FMF) based mode division multiplexing system. By launching unmodulated continuous-wave optical light modes into the corresponding spatial modes of the FMF, the transmitted and reflected or backscattered optical powers can be measured and quantitatively analyzed to accurately characterize and locate the fault. The influences of the capture fraction, attenuation coefficient, and Rayleigh backscattering coefficient are discussed, and simulation results show that the proposed method can realize the fault location of the FMF link. Moreover, considering the influence of mode crosstalk on localization accuracy, it is clear that using the monitoring combination modes LP01 and LP21 gives a high precision of 3.58 m.

Few-mode polymer optical waveguide amplifier for mode-division multiplexed transmission.

We propose a few-mode erbium-ytterbium codoped polymer optical waveguide amplifier employing mode-selective photonic lanterns capable of multiplexing and demultiplexing. A reconfigurable pump configuration scheme is adopted to balance the modal gain per mode. A square few-mode waveguide amplifier supporting LP01, LP11a, and LP11b modes is designed and fabricated. The crosstalk effect and modal profiles are characterized. An average gain of 10.4 dB per mode is obtained in a 1.5 cm waveguide at 1555 nm through pumping of the LP01 mode at 320 mW and the LP21b mode at 120 mW at 976 nm. The ultralow differential modal gain is 0.4 dB. In addition, simultaneously amplified LP01, LP11a, and LP11b modes are also demonstrated across the whole C-band with low noise figures. To the best of our knowledge, this is the first report of an experimental realization of a few-mode optical waveguide amplifier.

Analysis of characteristics of few-mode fiber fusion splicing under dynamic spatial modecrosstalk.

In this paper, an analytical model of the fusion splicing loss of few-mode fiber (FMF) under the condition of dynamic crosstalk is proposed. Under different dynamic spatial modal crosstalk caused by fusion mismatches and rotation angles, we analyze the variation trend of the fusion splicing loss characteristics of FMF with LP01,LP11a, and LP11b, respectively. Simulation results show that under the condition of axial misalignment introducing modal crosstalk, the splice loss of the LP11a and LP11b modes is higher than that of LP01. On the other hand, the modal crosstalk caused by rotation angles makes the fusion splice loss of LP01 present symmetrical distribution, and the loss of fusion splice of LP01 is maximum at 45°. When rotating from 0° to 90°, for LP11a and LP11b modes, the fusion loss of LP11a decreases gradually, while the loss of LP11b increases gradually. Both are in a symmetrical distribution. Meanwhile, we build an FMF fusion splice loss measurement system based on photon lanterns and FMF circulators. The experimental results show that it is consistent with the simulation results, and the experimental and simulation results confirm the feasibility of the fusion splice fault loss analysis modal under dynamic crosstalk conditions.

Single- and dual-wavelength fiber laser with multi-transverse modes.

A single- and dual-wavelength fiber laser with multi-transverse modes is proposed. Mode interference is realized in the core of an optical fiber by writing a long period fiber grating on a few-mode fiber to obtain the LP01 mode and the LP11 mode simultaneously. A mode interferometer based on a few-mode long period fiber grating (FM-LPFG) is used as a comb filter in the ring-cavity fiber laser. Single- and dual-wavelength outputs can be achieved in the fiber laser by adjusting a polarization controller (PC). A mode-selective photonic lantern is used to realize mode conversion, and six LP modes, LP01, LP11a, LP11b, LP21a, LP21b, and LP02, can be generated. A single- and dual-wavelength fiber laser with multi-transverse modes can be achieved by combining a mode interferometer with a mode-selective photonic lantern. This work has potential applications in mode division multiplexing (MDM) systems to enlarge the capacity of optical communications.

Gain characteristics of the hybrid slot waveguide amplifiers integrated with NaYF4:Er3+ NPs-PMMA covalently linked nanocomposites.

Waveguide amplifiers based on slot waveguide have enormous capacity due to their ability to confine light strongly to a narrow slot waveguide. NaYF4:Er3+ nanoparticles-polymeric methyl methacrylate covalently linked nanocomposites were synthesized and filled into the slot. The stability and the Er3+ concentration doped in this novel material were improved. The slot waveguide was designed accurately. The rigorous numerical method, full-vector finite difference method, was used to analyze the modal characteristics and optimize the slot combined with the maximum power confinement in the slot and the minimum effective mode area of the slot. A four-level spectroscopic model pumped at 1480 nm was presented. The rate equations and propagation equations were solved and the gain characteristics of the slot waveguide amplifier were numerically simulated. The primary parameters were optimized. A net gain of 5.78 dB was achieved when the signal power was 0.001 mW at 1530 nm, pump power was 20 mW, Er3+ concentration was 1.3 × 1027 m-3, and the waveguide length was 1.5 cm.

Fault detection of few-mode fiber based on high-order mode with high fault detection sensitivity.

We report on the fusion splice loss characteristics of each mode in few-mode fiber (FMF). The fusion fault loss characteristics of all modes of the six-mode step-index and graded-index fibers are measured, respectively. The high-order modes with high fault detection sensitivity are presented. The fault events with different loss amplitudes can be accurately characterized by high-order modes. On this basis, a fault detection and location method for FMF based on backscattered light of high-order modes is proposed and experimentally demonstrated. Rayleigh backscattering light of high-order modes is utilized to detect the faults of 7.2 km six-mode step-index fiber with three fusion splice points with different fusion quality, the detection results of each mode are compared. The high-order modes LP21a, LP21b, and LP02 with high fault detection sensitivity are shown, three fault points located around 1.03, 3.1, and 3.2 km of the FMF are successfully detected. While the LP01, LP11a, and LP11b modes can detect only one fault point, which is located around 3.19 km of the FMF.

Threshold for stimulated Brillouin scattering in few-mode fibers.

In this paper, the stimulated Brillouin scattering (SBS) threshold of step-index few-mode fibers (FMFs) is theoretically analyzed. The scattering characteristics of FMFs are analyzed using the mathematical model of the Brillouin scattering spectrum. The gain spectrum of Brillouin scattering and the influence of fiber parameters on the SBS threshold of FMFs are discussed, and the SBS theoretical threshold is obtained according to the parameters of FMFs. A new experimental system for measurement of the SBS threshold of FMFs based on two photonic lanterns is built that can eliminate the Fresnel reflection. The measured SBS thresholds of the LP01 and LP11 modes over 9.8 km two-mode fiber are 10.93 dBm and 11.35 dBm, respectively, and the measured SBS thresholds of LP01, LP11, LP21, and LP02 over 11 km four-mode fiber are 10.18, 10.38, 11.71, and 12.23 dBm, respectively. The experimental results agree well with the theoretical analysis, and the deviation is less than 0.7 dB. The theoretical analysis has guiding significance for setting the incident power in FMF communication and sensing. The experimental system can accurately measure the SBS thresholds of FMFs.

Simultaneous measurement of mode dependent loss and mode coupling in few mode fibers by analyzing the Rayleigh backscattering amplitudes.

In this paper, we propose and experimentally demonstrate a nondestructive method for simultaneously measuring the mode-dependent loss (MDL) and mode coupling (MC) in few-mode fibers (FMFs). The method is based on analyzing the Rayleigh backscattering amplitudes obtained with an optical time domain reflectometer (OTDR). The experimental results show that, in about 4 km six-mode FMF, the MDL and MC values are 0.132 dB and -23.13 dB/km between LP01 and LP11a, 0.176 dB and -23.73 dB/km between LP01 and LP11b, 0.272 dB and -26.17 dB/km between LP01 and LP21a, 0.284 dB and -26.70 dB/km between LP01 and LP21b, and 0.380 dB and -20.21 dB/km between LP01 and LP02. And it also demonstrated that the proposed scheme can be scalable to measure MDL and MC between higher-order modes. The values of the MC and MDL obtained by the proposed method agree well with that by the conventional transmission method. However, the proposed method has the merits of simultaneous and single-end measurement of MDL and MC in the FMF and could be a good solution to the characterization of FMFs used in large-capacity mode-division-multiplexing transmission systems.