Conventional Multimode Fiber Research Articles

Optically powered FiWi system for B5G hybrid VLC/RF access networks.

This paper proposes the concept and reports the demonstrator of an optically powered fiber/wireless (FiWi) system based on a hybrid visible light communication (VLC) and RF access network for beyond 5G(B5G) indoor applications. The power-over-fiber (PoF) technology is properly applied to simultaneously energize two components from the remote site, namely: a radio-over-fiber (RoF) module, which contains a photodetector and an RF amplifier; a red laser from the VLC system. The proposed PoF system is composed of a 500 m conventional multimode optical fiber (MMF) with a 62.5 μm core diameter, which enables the transmission of up to 12 W optical power with a power transmission efficiency (PTE) of 14%. In the proof of concept, 8.1W optical power is generated to feed the 5G system components. Regarding the communication setup, a 5G new radio (5GNR) signal is initially transported through a RoF fronthaul, composed of 10 km of single-mode fiber (SMF), followed by the hybrid VLC/RF access network. The performance investigation of the B5G communication system is based on an analysis of the root mean square error vector magnitude (EVMRMS) parameter. Experimental results indicate 1.2Gbps throughput in the VLC link using a 200MHz bandwidth and 64-quadrature amplitude modulation (64-QAM), resulting in approximately 4.02% of EVMRMS. Moreover, the RF link at 3.5GHz provides 360 Mbps with an EVMRMS of 2.67%. Finally, a comparison between the PoF system and conventional power supply validates the proposed approach's applicability for B5G wireless communications.

In-situ background-free Raman probe using double-cladding anti-resonant hollow-core fibers.

This study presents the development of an in-situ background-free Raman fiber probe, employing two customized double-cladding anti-resonant hollow-core fibers (AR-HCFs). The Raman background noise measured in the AR-HCF probe is lower than that of a conventional multi-mode silica fiber by two orders of magnitude. A plug-in device for fiber coupling optics was designed that was compatible with a commercially available confocal Raman microscope, enabling in-situ Raman detection. The numerical aperture (NA) of both AR-HCF claddings exceeds 0.2 substantially enhancing the collection efficiency of Raman signals at the distal end of the fiber probe. The performance of our Raman fiber probe is demonstrated by characterizing samples of acrylonitrile-butadiene-styrene (ABS) plastics, alumina ceramics, and ethylene glycol solution.

Joint WDM and OAM Mode Group Multiplexed Transmission Over Conventional Multimode Fiber

To exploit the advantages of multimode fiber (MMF)-based transmission and to improve the overall capacity, transmission of OAM modes over a single wavelength needs to be replaced by multichannel transmission with the proper experimental demonstration in terms of achievable bit error rates (BER), cross-talk(x-talk), etc. over the entire transmission band. In this paper, we have experimentally shown the successful transmission of two OAM mode groups multiplexed signals jointly with three channels using wavelength division multiplexing (WDM) over a 1 km conventional MMF, namely OM3. In this experimental study, each OAM mode over each WDM channel uses 10 Gbps of on-off keying (OOK) modulation format. At the receiver end, the achieved bit error rate (BER) is found well below the required pre-FEC BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3.8\times 10^{-3}$</tex-math></inline-formula> without using any MIMO-DSP. To establish the efficacy of the signal transmission, the time evolutions of OAM mode x-talk and BER over wavelengths spanning the entire C-band have been experimentally measured. Under the influence of different x-talks, the BER performances of all three 50 GHz-spaced WDM channels for both OAM modes are measured. With proper adjustment of polarization at the receiver end, a BER value below the pre-FEC threshold is achieved over 900 seconds. The experimental results also revealed that the power penalty contribution of WDM x-talk is negligible compared to OAM mode x-talk.

Orbital Angular Momentum in Fibers

Orbital angular momentum (OAM) of light beams with helical phase front, as an alternative spatial dimension resource of photons, has attracted extensive attention over the past decades. Very recently, OAM in fibers has facilitated rapid development of various fiber-based applications, such as optical communications, optical manipulation, optical sensing, optical imaging, nonlinear optics, and quantum science. Comprehensive and in-depth understanding of OAM in fibers is of great interest to researchers in this area. Here, we give a tutorial overview of OAM in fibers. We first introduce the basic theories, including the eigenmode analysis method, mode coupling theory, and spin-orbit mapping of light in fibers. We then give a detailed classification of OAM fibers and discuss versatile spatial mode bases in fibers. After that, we review various OAM fiber designs, including conventional single-mode fibers, conventional multi-mode fibers, ring-core fibers, air-core fibers, photonic crystal fibers, negative-curvature hollow-core fibers, multi-core fibers, active fibers (e.g., erbium-doped fibers), etc. Besides, we describe the state-of-the-art commercial fiber manufacturing processes as well as testing and characterization methods for OAM fibers. We also review various active and passive OAM fiber devices. Moreover, we introduce several typical advanced applications based on OAM fibers, such as capacity scaling fiber-optic communications, remote vectorial Doppler velocimetry, super-resolution optical imaging, and high-dimensional quantum cryptography. Finally, we briefly discuss future challenges and prospects of OAM in fibers.

Investigation of Crosstalk between Data Signals and Feed Light in Power-over-Fiber

In power-over-fiber systems, simultaneous data signals and power transmission using a single optical fiber is an attractive step towards simplifying the systems. However, when simultaneously transmitting data signals and feed light in a single core, the transmission quality of the data signal degrades owing to the crosstalk based on the characteristics of the feed light source. In this study, we investigate and evaluate the crosstalk between the data signal and high-power feed light in a power-over-fiber system. To observe the effect of the crosstalk more clearly, we use a power-over-fiber link using a single-core, conventional multimode fiber, and a high-power Raman fiber laser as the feed light source. Consequently, it is found that the feed light induces relative intensity noise related to the longitudinal mode beating and mode partition noise determined by the characteristics of the laser. In addition, it depends on the input power and transmission distance, and has a significant impact on the quality of the data signal by the noise transfer. To evaluate these noises quantitatively, we also evaluate the error-vector-magnitude and bit-error-rate characteristics of analog and digital data signals, which show that similar characteristics are observed.

A 96-Channel Electrophysiology Catheter with Integrated Read-Out ASIC and Optical Link.

This paper describes a realization of an electrophysiology (EP) catheter with 96 electrodes which requires no electrical wiring to the outside by relying on an optical link for both power supply and data communication. The catheter tip is constructed from a liquid crystal polymer (LCP) material. It features 96 gold electrodes, which are uniformly arranged along an expandable basket. An integrated ASIC amplifies, filters and digitizes the EP signals and establishes communication to a data processing unit outside the patient's body. The optical interface consists of a conventional multi-mode fiber and a single blue LED inside the catheter. The external unit used to generate optical power, establish communication and perform data post-processing comprises a laser module, optics, and electrical components. The catheter is designed to capture EP signals in the range of 600 µVpp to 20 mVpp in a frequency range between 8 Hz and 120 Hz.

Spatial Beam Self-Cleaning in Bi-Tapered Multimode Fibers

We report the spatial beam self-cleaning in bi-tapered conventional multimode fibers (MMFs) with different tapered lengths. Through the introduction of the bi-tapered structure in MMFs, the input beam with poor beam quality from a high-power fiber laser can be converted to a centered, bell-shaped beam in a short length, due to the strengthened nonlinear modes coupling. It is found that the bi-tapered MMF with longer tapered length at the same waist diameter shows better beam self-cleaning effect and larger spectral broadening. The obtained results offer a new method to improve the beam quality of high-power laser at low cost. Furthermore, it may be interesting for manufacturing bi-tapered MMF-based devices to obtain the quasi-fundamental mode beam in spatiotemporal mode-locked fiber lasers.

Focal-ratio degradation (FRD) mitigation in a multimode fibre link using mode-selective photonic lanterns

ABSTRACT We present a new way to mitigate focal-ratio degradation (FRD) when using optical fibres to transport multimode light. Our approach exploits a custom multicore fibre (MCF) with six dissimilar cores that are single-mode at ∼1550 nm wavelength and minimally coupled over 7 m. We fabricated adiabatic mode-selective photonic lanterns (PLs) at each end of the MCF to create a fibre link with multimode ports, the PLs coupling each spatial mode of the multimode ports to a specific core of the MCF and vice versa. The PL-MCF-PL link exhibits superior FRD behaviour compared to a conventional multimode fibre that also supports six modes, because it inhibits the transfer of light from lower order modes to higher order modes. These results open up a potential powerful new approach to mitigate FRD in multimode fibre links, with particular applications in astronomical instruments.

CURRENT STATE OF METHODS AND MEANS OF REGISTRATION OF HIGH TEMPERATURES AND MECHANICAL STRESSES IN STRUCTURES

An analytical review of physically possible methods and available achievements in registering hydrostatic pressure or mechanical stresses using fiber optic fibers and sensors based on them based on published works that can be used in harsh environmental conditions is carried out. The results of the review show that fully distributed or quasidistributed fiber-optic systems for recording hydrostatic pressure or mechanical stress can be implemented on the following physical principles and apparatus with measures to compensate or suppress the influence of temperature: polarizing sensors on birefringent single- mode light guides and OTDR equipment; micro-flexible sensors with OTDR equipment on conventional multimode fibers; measuring systems on fiber Bragg gratings; on discrete sensors, in particular, on sealed fiber Fabry–Perot interferometers; Brillouin distributed sensors on single-mode fibers that are not sensitive to temperature changes. It is shown that single-mode birefringent fibers with hollow holes in the shell and fiber Bragg gratings written in the core have a good linear sensitivity to hydrostatic pressure and a weak dependence on temperature. Lattices in phosphorous-containing single-mode light guides have increased high-temperature properties up to ~500 C and higher. A number of discrete fiber sensors’ structures and pressure recorders are investigated. Various structures of sensitive elements of pressure sensors on sealed fiber Fabry–Perot interferometers and fiber gratings in spherical and cylindrical small-sized cases are investigated. Sensors based on Fabry–Perot fiber interferometers soldered into a glass capillary and protected from water by external high-temperature hermetic coatings showed good linearity in the pressure range of 0…540 ATM at temperatures up to ~200 C. The sensors are efficient at temperatures up to 600 °C and are promising for use in severe and special external conditions. The possibility of compensating the temperature sensitivity by selecting external coatings is shown. Pressure sensors were tested on local areas with microbends and it was shown that they can measure pressures up to ~24 МPа at temperatures up to ~450 C, but to compensate for the dependence of the readings on temperature, it must be measured by an independent sensor. The possibility of independent and simultaneous measurement of hydrostatic pressure and temperature along a single fiber using spontaneous Brillouin scattering is shown. Pressure is measured by the frequency shift of Brillouin scattering, and temperature by its intensity. The operation of the Brillouin recorder in the pressure range 0…34 MРа is demonstrated. The pressure resolution was ~0,2 МРа. New methods are proposed for detecting Brillouin scattering – a heterodyne signal with a high signal-to-noise ratio and based on frequency modulation of a semiconductor single-frequency distributed feedback laser. The measurement range has been increased by more than 10 km and the coordinate resolution has been increased. The Brillouin scattering method is promising for creating distributed systems for measuring hydrostatic pressure or mechanical stress for severe physical conditions, including temperatures of ≥3000 C.

Noise and Distortion Reduction in OFDM Radio-Over-Fiber Link by Graded-Index Plastic Optical Fiber

We demonstrate that a graded-index plastic optical fiber (GI POF) enables higher-quality radio-over-fiber (RoF) transmission of an orthogonal frequency-division multiplexing (OFDM) signal, compared with a conventional multimode fiber, for short-distance household applications (< 100 m). It is shown that the improvement in OFDM signal transmission, using the GI POF, can be attributed to the reduction in noise and intermodulation distortion, owing to the external optical feedback from the transmission fibers. The low noise and low distortion in the signal transmission are mainly due to the strong mode coupling in the GI POF, which is closely related to the microscopic heterogeneous structures in core polymer materials. The developed GI POF will be invaluable for RoF networks in households as a solution for indoor broadband wireless communications in the upcoming 5G era.

Low-noise radio over graded-index plastic optical fiber.

We developed a graded-index plastic optical fiber (GI POF) that enables lower-noise radio frequency (RF) transmission than conventional multimode fibers for short-distance household applications (<100m). It is shown that reflection noise degrades RF transmission, regardless of the carrier frequency, through the spurious generation that accompanies the RF modulation of a vertical-cavity surface-emitting laser. The GI POF with distinctive mode coupling, which is closely related to its microscopic polymer structure, suppresses noise and spurious generation to improve transmission quality. Our low-noise radio-over-GI-POF technology will offer significant advantages for optical wiring systems for broadcast and communication in small- and medium-scale buildings.

Temporal ghost imaging with random fiber lasers.

Ghost imaging in the time domain has opened up new possibilities to retrieve ultrafast waveforms. A pre-requisite to ghost imaging in the time domain is a light source with random temporal intensity fluctuations that are fully uncorrelated over the duration of the temporal waveform being imaged. Here, we show that random fiber lasers are excellent candidates for ghost imaging in the time domain. We study the temporal correlations of the intensity fluctuations of a random fiber laser in different operating regimes and compare its performance in temporal ghost imaging configurations with that of a conventional multi-mode cavity-based fiber laser. Our results demonstrate that random fiber lasers can achieve superior performance for ghost imaging as compared to cavity-based fiber lasers where strong correlations at the cavity round-trip time can yield artefacts for waveforms of long duration.

All-fiber second-order orbital angular momentum generator based on a single-helix helical fiber grating.

An all-fiber orbital angular momentum (OAM) generator enabling direct turning of the fundamental mode (${{\rm HE}_{11}}$HE11) to the second OAM modes (${ l} = {\pm 2}$l=±2) with an efficiency of $\sim90\% $∼90% has been proposed and experimentally demonstrated, which is realized based on utilization of a second-order helical fiber grating written in a few-mode fiber. This is the first time, to the best of our knowledge, that an all-fiber second-order OAM has been achieved with using only one component, i.e., the helical long-period fiber grating. The proposed method opens a new way to efficiently generate an all-fiber higher-order OAM using a conventional multimode fiber.

Weakly-coupled mode division multiplexing over conventional multi-mode fiber with intensity modulation and direct detection

Multi-mode fiber (MMF) links are expected to greatly enhance capacity to cope with rapidly increasing data traffic in optical short-reach systems and networks. Recently, mode division multiplexing (MDM) over MMF has been proposed, in which different modes in MMF are utilized as spatial channels for data transmission. Strongly-coupled MDM techniques utilizing coherent detection and multiplex-input-multiplex-output (MIMO) digital signal processing (DSP) are complex and expensive for short-reach transmission. So the weakly-coupled approach by significantly suppressing mode coupling in the fiber and optical components has been proposed. In this way, the signals in each mode can be independently transmitted and received using conventional intensity modulation and direct detection (IM-DD). In this paper, we elaborate the key technologies to realize weakly-coupled MDM transmission over conventional MMF, including mode characteristic in MMF and weakly-coupled mode multiplexer/demultiplexer (MUX/DEMUX). We also present the up-to-date experimental results for weakly-coupled MDM transmission over conventional OM3 MMF. We show that weakly-coupled MDM scheme is promising for high-speed optical interconnections and bandwidth upgrade of already-deployed MMF links.

Directly using 88-km conventional multi-mode fiber for 6-mode orbital angular momentum multiplexing transmission

Twisted light carrying orbital angular momentum (OAM), which featuring helical phase front, has shown its potential applications in diverse areas, especially in optical communications in free space and specially designed fibers, e.g. a vortex fiber. Instead of specially designed fibers extensively used in the reported OAM-based fiber transmission experiments, here we demonstrate the viability of a conventional graded-index multi-mode fiber (MMF) for OAM multiplexing transmission with less digital signal processing (DSP) complexity. We demonstrate a 120-Gbit/s quadrature phase-shift keying (QPSK) signal transmission in an 8.8-km OM4 MMF by using OAM mode multiplexing with all the modes in the first two mode-groups (OAM01L, OAM01R, OAM+11L, OAM+11R, OAM-11L, OAM-11R) with only 2×2 and 4×4 multiple-input-multiple-output (MIMO) equalization. Moreover, we demonstrate the data-carrying two OAM mode groups multiplexing transmission over the 8.8-km MMF without MIMO equalization. These demonstrations may open up new perspectives to enable the realistic use of OAM-based MMF solution in data centers and super-computers.

Optically powered radio-over-fiber system based on center- and offset-launching techniques using a conventional multimode fiber.

We demonstrate radio-over-fiber (RoF) transmission with a 10-W feed power-over-fiber (PWoF) using a conventional multimode fiber (MMF). In this scheme, the modal dispersion and feed light crosstalk in the MMF are effectively mitigated by the combination of center-launching (CL) and offset-launching (OL) techniques. The CL is used for propagating the feed light into lower-order modes in the MMF, while the OL is used not only for propagating the optical data signals into higher-order modes in the MMF, but also for mitigating the modal dispersion. We successfully achieved significant improvement in the RoF transmission performance with the 10-W feed PWoF and extended the link length up to 4km, owing to the two techniques.

Fundamental-mode MMF transmission enabled by mode conversion

Modal dispersion in conventional multi-mode fiber (MMF) will cause serious signal degradation and an effective solution is to restrict the signal transmission in the fundamental mode of MMF. In this paper, unlike previous methods by filtering out higher-order modes, we propose to adopt low-modal-crosstalk mode converters to realize fundamental-mode MMF transmission. We design and fabricate all-fiber mode-selective couplers (MSC), which perform mode conversion between the fundamental mode in single-mode fiber (SMF) and fundamental mode in MMF. The proposed scheme is experimentally compared with center launching method under different MMF links and then its wavelength division multiplexing (WDM) transmission performance is investigated. Experimental results indicate that the proposed mode conversion scheme could achieve better transmission performance and works well for the whole C-band.

Orbital angular momentum mode groups multiplexing transmission over 2.6-km conventional multi-mode fiber.

Twisted light carrying orbital angular momentum (OAM) is a special kind of structured light that has a helical phase front, a phase singularity, and a doughnut intensity profile. Beyond widespread developments in manipulation, microscopy, metrology, astronomy, nonlinear and quantum optics, OAM-carrying twisted light has seen emerging application of optical communications in free space and specially designed fibers. Instead of specialty fibers, here we show the direct use of a conventional graded-index multi-mode fiber (MMF) for OAM communications. By exploiting fiber-compatible mode exciting and filtering elements, we excite the first four OAM mode groups in an MMF. We demonstrate 2.6-km MMF transmission using four data-carrying OAM mode groups (OAM0,1, OAM+1,1/OAM-1,1, OAM+2,1, OAM+3,1). Moreover, we demonstrate two data-carrying OAM mode groups multiplexing transmission over the 2.6-km MMF with low-level crosstalk free of multiple-input multiple-output digital signal processing (MIMO-DSP). The demonstrations may open up new perspectives to fiber-based OAM communication/non-communication applications using already existing conventional fibers.

Experimental study on the statistic characteristics of a 3x3 RF MIMO channel over a single conventional multimode fiber.

Based on the observed random fluctuation phenomenon of speckle pattern across multimode fiber (MMF) facet and received optical power distribution across three output ports, we experimentally investigate the statistic characteristics of a 3×3 radio frequency multiple-input multiple-output (MIMO) channel enabled by mode division multiplexing in a conventional 50µm MMF using non-mode-selective three-dimensional waveguide photonic lanterns as mode multiplexer and demultiplexer. The impacts of mode coupling on the MIMO channel coefficients, channel matrix, and channel capacity have been analyzed over different fiber lengths. The results indicate that spatial multiplexing benefits from the greater fiber length with stronger mode coupling, despite a higher optical loss.

Power Flow in a Large-Core Multimode Fiber under External Perturbation and its Applications

Large core optical multimode fiber provides benefits such as a large light-coupling tolerance, easy handling, and delivery of higher light power without undesirable nonlinear effects. In this research, we exploit the effects of external perturbation on the power flow within the large core fiber and present two relevant applications, namely a perturbation sensor and a doughnut beam tuner. Since conventional multimode fiber power flow model does not take into consideration the perturbation effect, we modify the power flow model so that the influence of time varying perturbation can be theoretically analyzed. Based on our theory, we further conduct the numerical simulation and experiments on these two applications. For the fiber vibration sensor, the proposed numerical model shows that the sensor sensitivity depends on the intensity profile of the launched beam and also the higher-order harmonics that were not reported previously can become interferences to affect the signal. For the beam tuner application, we prove both theoretically and experimentally that the doughnut intensity profile at the fiber output can be tuned in real-time by applying external perturbations to the fiber. We expect that the results can be useful to further exploit the external perturbation on large core fiber in various applications.