Multimode Fiber Links Research Articles

Power-over-fiber-based optical wireless communication systems towards 6G

This paper reports two implementations of power-over-fiber (PoF) solutions applied to radio-over-fiber (RoF) and optical wireless communication (OWC) systems, in the context of an industrial environment. We employ a conventional 62.5-µm multimode fiber (MMF) to deliver optical power to different communication links based on RoF, free-space optics (FSO), and visible light communication (VLC) technologies aiming beyond 5G (B5G) and 6G applications. First, a 3.5-GHz 5G New Radio (5G NR) signal is transmitted throughout a 20-km single-mode optical fiber (SMF) link using RoF technology. Regarding the PoF system, a 5-W optical power is transmitted through a 100-m MMF link. A photovoltaic power converter (PPC) and a DC/DC converter are employed to convert the power from the optical to the electrical domain and adjust the voltage level, respectively, with the purpose of energizing a remote RoF module. The attainable optical and electrical power transmission efficiencies (OPTE and PTE) are 80% and 19%, respectively. Posterior, a second PoF system is implemented to power a hybrid RoF/FSO/VLC B5G system, comprising a 200-m MMF and an additional DC/DC converter. Over 10.5 W of optical power is transmitted to feed an electrical amplifier (EA) and a white LED from the VLC link. In this configuration, we achieve 78% and 18.5% of OPTE and PTE, respectively. Furthermore, a performance investigation based on the root mean square error vector magnitude (EVMRMS) metric is conducted to evaluate the signal using the implemented PoF systems and a conventional electrical power supply. In the first implementation, a throughput of 600 Mbps is achieved with 100-MHz bandwidth without performance degradation, when compared to the conventional-powered RoF system, whereas, in the second implementation, 60-Mbps throughput is achieved when employing the FSO and VLC technologies simultaneously, demonstrating the applicability and potential of the PoF technique for B5G and 6G industrial communications.

Joint-Transceiver Equalization Technique over a 1.4 km Multi-Mode Fiber Using Optical MIMO Technique in IM/DD Systems

In optical fiber communication, recent advances in multiple-input and multiple-output (MIMO) systems using space-division multiplexing have helped achieve higher spectral efficiency and data rates. Propagating higher-order modulation formats over MIMO systems further strengthens the capacity of the transmission link. In the optical-MIMO system, the dispersion impairments originating from a 1.4 km long multi-mode fiber (MMF) are mitigated using the proposed joint-transceiver equalization technique. A numerical convex optimization algorithm is used to compute and optimize the pre- and post-equalization (PPE) coefficients jointly restricted by cost and power budgets. The potential of the proposed joint-PPE technique is tested on an MMF link, which is severely degraded by dispersion compared to a single-mode fiber channel. From the experimental results, the average optical received power gain necessary to reach 10−4 bit-error rate is improved by nearly 2.5 dB using the joint-PPE compared to the post-equalization only based on the minimum mean-squared error principle. When the efficiency of the conventional zero-forcing (ZF) principle-based PPE and the joint-PPE is compared, the joint-PPE scheme outperforms the ZF-PPE by approximately 1.5 dB. The enhancement in the transmission quality is observed with experimentally measured eye diagrams using the joint-PPE scheme. Under the analyzed scenarios, computer simulation also confirms the hypothesis, which establishes the effectiveness of the proposed joint-transceiver equalization over the conventional ZF-PPE scheme. Moreover, the simulated performance benefits of the joint-PPE are evaluated using the singular value decomposition (SVD) technique. Improvement of ≈3.86 dB in the average optical received power gain required to reach 10−4 bit-error rate is witnessed with the PAM-4 format. Overall, the joint-transceiver equalization technique is proven to be beneficial in optical MIMO systems.

Wheel-Based MDM-PON System Incorporating OCDMA for Secure Network Resiliency

Wheel-based network resilience passive optical network (PON) based on mode division multiplexing (MDM) can be integrated with optical code division multiple access (OCDMA) schemes efficiently for the fixed and backhaul traffic under normal and break/failure fiber operating conditions. In this work, a bidirectional 10/2.5 Gbit/s hybrid MDM-OCDMA-PON system using multi-weight zero cross-correlation (MWZCC) code is proposed. Donut modes 0 and 1 are incorporated by the MDM technique in the proposed system. The benefit of this work is to offer an inexpensive, high-bandwidth and advanced long-haul network with satisfactory resource utilization ability for fiber links with protection against faults and to improve the reliability along with survivability of the network. The simulation results show the successful realization of the multimode fiber (MMF) link at 1.6 km in the uplink and 1.2 km in the downlink directions under an acceptable bit error rate (BER). The minimum accepted received power of −31 dBm in uplink and −27 dBm in downlink over 1 km link at 10/2.5 Gbit/s rate is obtained. Moreover, the minimum received power of −20 dBm in uplink and −30 dBm downlink is achieved by using MWZCC code compared to other codes handling 58 simultaneous end users. Further, the influence of fiber impairments and connected devices on the proposed approach is numerically evaluated. Moreover, it is shown that the wheel based proposed approach performs well than other topologies for the bidirectional network resilience transmission.

High-Speed Spiral-Phase Donut-Modes-Based Hybrid FSO-MMF Communication System by Incorporating OCDMA Scheme

Hybrid free-space optics (FSO) and optical fiber have been viewed as vital transmission techniques to satisfy high bandwidth and extended transmission range requirements under adverse environment conditions in the future last-mile obstruction problem. In this investigation, 80 Gbps data is transmitted on a hybrid FSO and multimode fiber (MMF)-based network using mode division multiplexing of two donut modes, Donut mode 0 and 1, and optical code-division multiplexing (OCDMA) schemes. For the OCDMA schemes, modified new zero-cross-correlation (MNZCC) codes are used, whereas, to add the phases into donut modes, a spiral phase diffuser is used. The purpose of the investigation is to provide an economical, high-speed and advanced last-mile network with adequate resource utilization for hybrid wired/wireless-based systems. The results obtained show achievement of an acceptable BER up to a fixed 100 m FSO link, with the combination of a 385 m MMF link under clear weather conditions. In another case, when the MMF link was fixed at 100 m, an acceptable bit error rate (BER) is achieved at 2.07 km FSO link. Furthermore, the results were obtained in the presence of strong and weak turbulences. A comparison of log-normal and gamma-gamma modeling for scintillations is presented.

Securing Data in Multimode Fibers by Exploiting Mode-Dependent Light Propagation Effects.

Multimode fibers hold great promise to advance data rates in optical communications but come with the challenge to compensate for modal crosstalk and mode-dependent losses, resulting in strong distortions. The holographic measurement of the transmission matrix enables not only correcting distortions but also harnessing these effects for creating a confidential data connection between legitimate communication parties, Alice and Bob. The feasibility of this physical-layer-security-based approach is demonstrated experimentally for the first time on a multimode fiber link to which the eavesdropper Eve is physically coupled. Once the proper structured light field is launched at Alice's side, the message can be delivered to Bob, and, simultaneously, the decipherment for an illegitimate wiretapper Eve is destroyed. Within a real communication scenario, we implement wiretap codes and demonstrate confidentiality by quantifying the level of secrecy. Compared to an uncoded data transmission, the amount of securely exchanged data is enhanced by a factor of 538. The complex light transportation phenomena that have long been considered limiting and have restricted the widespread use of multimode fiber are exploited for opening new perspectives on information security in spatial multiplexing communication systems.

Joint Pre- and Post-Equalization with Higher-Order Modulation Formats in SDM-Based Optical MIMO Systems

The multiple-input and multiple-output (MIMO) technology is a promising area of research to cope up with the demands of higher data rates and capacity. In the optical communication domain, the combination of space-division multiplexing (SDM) with higher-order modulation (HOM) formats over an optical MIMO system actively addresses these challenges. By allowing multi-level signaling with limited increment in the transmitter’s complexity, a jointly designed pre- and post-equalization (PPE) for an optical MIMO system with a multi-mode fiber (MMF) link is proposed. Cost-effectiveness of the system is incorporated by utilizing intensity modulation/direct detection (IM/DD) with HOM formats such as pulse-amplitude modulation (PAM) schemes. With the aid of a numerical optimization algorithm, the proposed joint-PPE filter coefficients are optimized with respect to the MMF channel and the transmit power constraint. In contrast to existing research on the single-mode fiber (SMF) based optical systems, the effectiveness of the proposed joint-PPE filter is analyzed on an MMF link, which is considerably degraded by the modal dispersion. In the analyzed experimental scenario, the proposed joint-PPE scheme confirms to be beneficial as compared to the post-equalization only (PE-only) in terms of bit-error rate (BER) performance. Furthermore, the required average received optical power to reach a BER 10−4 by the joint-PPE scheme is improved by 2 dB with comparison to the minimum mean-squared error (MMSE) PE-only.

Coherent Communication Over Multi Mode Fibers for Intra-Datacenter Ultra-High Speed Links

Multimode fibers (MMFs) links using vertical cavity surface-emitting lasers (VCSELs) are still the solution of choice for today’s data center shorter distances thanks to their low cost and robustness. Currently, the market is moving towards 400G-capable systems using multiple lanes at 50 Gbps per lane, based on intensity modulation (IM) and direct detection (DD). A technology switch will probably be required to overcome the 100 Gbps per lane limit that will likely be the bottleneck of future IM-DD MMF-based links. In this manuscript we propose an intra-data center system based on coherent detection and MMF, analyzing experimentally and analytically the performance of polarization multiplexed coherent communication. We show that the advantages enabled by the coherent technology can be fully exploited in an MMF-based link when central launch is ensured. In particular, we study in detail the effect of lateral offsets introduced by the connectors at the interface between two MMF facets, in terms of net optical loss and signal-to-noise ratio (SNR) degradation. A statistical analysis is performed analytically exploiting a large database of OM3 and OM4 fibers modal delays and results are presented for various combinations of fiber length, type and number of MMF-to-MMF connections along the link. We show quantitative results on the maximum acceptable lateral offsets in the MMF connectors, highlighting that in practical conditions average offsets of up to 3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m can be tolerable.

Implementation of a Full Optically-Powered 5G NR Fiber-Wireless System

We report the implementation of a full optically-powered 5G new radio (5G NR) fiber-wireless (FiWi) system based on power-over-fiber (PoF) and radio-over-fiber (RoF) technologies. Our approach enables the simultaneous transmission of a 5G NR signal at 3.5 GHz with bandwidth up to 100 MHz and a 2.2-W optical power signal employing dedicated fiber-optics links. The optical-wireless data link consists of a 12.5-km single-mode fiber (SMF) optical fronthaul followed by a 10-m wireless propagation environment, which is the longest wireless reach reported in literature up to now, regarding optically-powered FiWi systems. The proposed PoF system is able to deliver stable electrical power up to 475 mW, by means of using a 100-m multimode fiber (MMF) link, with the purpose of optically powering a 5G NR remote antenna unit (RAU). An overall power transmission efficiency (PTE) of 23.5% is experimentally demonstrated in a real 5G NR system. Furthermore, the FiWi system performance is investigated in accordance with the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3^{\mathrm rd}$</tex-math></inline-formula> generation partnership project (3GPP) Release 15 requirements, in terms of root mean square error vector magnitude ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mathrm EVM}_{\mathrm RMS}$</tex-math></inline-formula> ). The proposed optically-powered 5G NR FiWi system provides 500 Mbit/s throughput with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mathrm EVM}_{\mathrm RMS}$</tex-math></inline-formula> as low as 3.9%, employing 64-quadrature amplitude modulation (QAM) without using optical amplification.

Performance analysis of wavelength division multiplexing MDM-PON system using different advanced modulations

Abstract Mode division multiplexing (MDM) is very competent next generation multiplexing technique and is becoming popular among researchers these days. In this research article, an integrated passive optical network (PON) using MDM and wavelength division multiplexing (WDM) is proposed at 25 Gbps over 3 km multimode fiber (MMF) link distance. For MDM, diverse Laguerre–Gaussian (LG) such as LG12, LG15, LG18, LG111 and LG114 are incorporated and also for cost reduction, vertical cavity surface emitting laser (VCSEL) is located in optical line terminal (OLT). Performance of diverse advanced modulations such as compressed spectrum return to zero (CSRZ), duo-binary return to zero (DRZ) and modified duo-binary return to zero (MDRZ) is evaluated and compared with non-return to zero (NRZ) in terms of Bit error rate (BER) at varied MMF link lengths. Results revealed that CSRZ performance stand out and NRZ provide worst performance.

Mitigation of Mode Partition Noise in VCSEL-MMF Links by Optimizing Launch Conditions

The multimode characteristics of vertical cavity surface emitting laser (VCSEL) sources lead to mode partition noise (MPN) impairing high-speed multimode fiber (MMF) links. In this letter, we propose a low-cost and straightforward MPN mitigation scheme by optimizing the launch condition between the VCSEL and the MMF. The dependence of the MPN on launch conditions is theoretically assessed. Simulation shows that the mode power redistribution induced by rearranging the launch condition has great potential to reduce the MPN. Experiments are carried out to demonstrate the MPN reduction. In our case, using a 15- $\mu \text{m}$ radial offset between the VCSEL and the MMF, we observe efficient noise suppression in both the frequency domain and the time domain.

Low-Noise Graded-Index Plastic Optical Fiber Achieved by Specific Copolymerization Process

Ultra-high-definition (UHD) technologies have recently received attention on account of their practical applications in consumer electronics. UHD devices require uncompressed video transmission at a data rate exceeding 100 Gb/s; thus, an optical fiber connection is essential. In consumer applications, optical fibers are very short. Moreover, their connections must permit variations in the fiber alignment, which accommodates rough handling by consumers. Under this condition, the transmitted signal quality is significantly degraded owing to noise and instabilities that strongly depend on the fiber alignment conditions in optical modules and connectors. Therefore, graded-index plastic optical fibers (GI POFs) are promising optical cables for consumer applications because of their flexibility, safety, and high bandwidth. Recently, the authors experimentally demonstrated that GI POFs can reduce interferometric noise, such as modal noise and multipath interference noise, in a multimode fiber link based on a vertical-cavity surface-emitting laser (VCSEL). This noise reduction effect results from strong mode coupling of GI POFs, which is closely related to microscopic heterogeneous structures of a core polymer matrix. In this paper, a control method of mode coupling using the copolymerization process for fiber core materials is proposed. The formation of composition fluctuations by copolymerization increases the mode coupling in the GI POF core. It thereby enables highly stable and robust data transmission with a large fiber misalignment tolerance in a VCSEL-based multimode fiber link. The proposed method for mode coupling control is expected to contribute to household optical communication systems in the upcoming UHD era.

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.

Spectral-dependent electronic-photonic modeling of high-speed VCSEL-MMF links for optimized launch conditions.

We present spectral-dependent electronic-photonic modeling of vertical-cavity surface-emitting laser (VCSEL)-multimode fiber (MMF) links for next-generation high-speed interconnects. The beam coupling processes, between the VCSEL and the MMF and between the MMF and the photodetector (PD), are discussed, with spectral-dependent three-dimensional launch conditions analyzed. The model accounts for fiber effects on the transmission performance, specifically modal attenuation, dispersion, mode mixing, and mode partition noise. An advanced split-step small-segment (4-S) method simulates the signal evolution over the MMF with high accuracy and high efficiency. Experimental validation at 25 Gbps confirms the high accuracy of the VCSEL-MMF link model. The model reveals that larger radial offsets can further excite lower-order mode groups reducing the power distributed to higher-order groups when a tilted beam couples to the input fiber facet. With an optimized misalignment launch, the modal bandwidth is greatly improved by 3.8-fold compared to the conventional center launch. The model helps determine the optimum launch condition to improve link performance metrics such as transmission reach.

Multi-Gigabit CO-OFDM System over SMF and MMF Links for 5G URLLC Backhaul Network

The 5G cellular network aims at providing three major services: Massive machine-type communication (mMTC), ultra-reliable low-latency communications (URLLC), and enhanced-mobile-broadband (eMBB). Among these services, the URLLC and eMBB require strict end-to-end latency of 1 ms while maintaining 99.999% reliability, and availability of extremely high data rates for the users, respectively. One of the critical challenges in meeting these requirements is to upgrade the existing optical fiber backhaul network interconnecting the base stations with a multigigabit capacity, low latency and very high reliability system. To address this issue, we have numerically analyzed 100 Gbit/s coherent optical orthogonal frequency division multiplexing (CO-OFDM) transmission performance over 400 km single-mode fiber (SMF) and 100 km of multi-mode fiber (MMF) links. The system is simulated over optically repeated and non-repeated SMF and MMF links. Coherent transmission is used, and the system is analyzed in a linear and non-linear regime. The system performance is quantified by bit error ratio (BER). Spectrally efficient and optimal transmission performance is achieved for 400 km SMF and 100 km MMF link. The results designate that MMF links can be employed beyond short reach applications by using them in the existing SMF infrastructure for long haul transmission. In particular, the proposed CO-OFDM system can be efficiently employed in 5G backhaul network. The multi-gigabit capacity and lower BER of the proposed system makes it a suitable candidate especially for the eMBB and URLLC requirements for 5G backhaul network.

112-Gb/s PAM4 Transmission Over 1 km of MMF With Mode-Field Matched Center- Launching in 850-nm Band

We evaluate possibility of utilizing the mode-field matched center-launching (MCL) technique in the 850-nm band. For this purpose, we realize the MCL technique by fusion-splicing the HI-780 pigtail fiber of the transmitter to the OM2-type transmission multi-mode fiber (MMF). The results show that we can achieve the high coupling efficiency of >92% by optimizing the splicing conditions such as the arc-discharge duration and fiber-push distance. We confirm that, in the case of utilizing the MCL technique, the frequency-response characteristics of the MMF link becomes almost identical to that of the single-mode fiber (SMF) link. As a result, we could demonstrate the successful transmission of 112-Gb/s four-level pulse amplitude modulation (PAM4) signal operating at 852 nm over 1 km of the OM2-type MMF by utilizing the MCL technique. The performance of this link is measured to be very stable even in the presence of the mechanical perturbations such as fiber bending and fiber shaking.

Multiport swept-wavelength interferometer with laser phase noise mitigation employing a broadband ultra-weak FBG array

Optical vector network analyzers (OVNAs) based on swept-wavelength interferometry are applied widely in optical metrology and sensing to measure the complex transfer functions of optical components, devices, and fibers. Phase noise from laser sweep nonlinearities degrades the measurement quality as the distance increases and limits the usage of the OVNA in characterizing systems with long impulse responses as required in space-division multiplexing links with a high mode count or in the presence of large modal differential group delay (DGD). In this Letter, we use a densely distributed broadband ultra-weak fiber Bragg grating array to directly measure the distortion due to phase noise at a 5-m increment up to 400 m and use this measured data to directly eliminate the distortion. We experimentally extend the measurement range of the swept-wavelength OVNA over 400 m and successfully characterize a 2-km six-mode multimode fiber link with an accumulated impulse response as wide as 20 ns.

RFID over low cost VCSEL‐based MMF links: experimental demonstration and distortion study

Radio-over-fibre (RoF) distributed antenna system (DAS) technology has been investigated for the distribution of ultra-high frequency (UHF) radio frequency identification (RFID) signals. RoF DAS allows for reduced number of readers and centralised placement of readers which facilitates easy system maintenance, but it is important to find a low-cost solution that can achieve comparable performance to a conventional RFID system. In this work, a low-cost vertical-cavity surface-emitting laser (VCSEL)-based multimode fibre (MMF) link has been developed and demonstrated for passive UHF RFID applications. The reported results show almost the same performance when compared with a conventional RFID system. In addition, simple spatial antenna diversity schemes are tested, with improved performance reported in comparison with an RFID-RoF system without diversity. Also, an investigation of RFID over fibre with RoF non-linearity is carried out showing that phase-reversal amplitude shift keying (PR-ASK) RFID modulation allows for higher levels of RF carrier and modulated signal power than the ASK RFID, for low levels of non-linearity.

A full‐duplex ultra‐wideband over multimode fiber link for internet of things based smart home applications

AbstractWe propose a bidirectional ultra‐wideband (UWB) over multi‐mode fiber link for internet of things based smart home applications that is compatible with globally emerging green radio trend. Employing direct modulation and coupled with carrier‐reuse makes the proposed architecture cost efficient. UWB signals are all‐optically generated by converting phase modulation to intensity modulation by using a shaping filter that has a Gaussian profile. Bidirectional data at the rate of 2 Gbps is transmitted while achieving low receiver sensitivities.

Si3N4 photonic integration platform at 1 µm for optical interconnects.

Vertical-cavity surface-emitting lasers (VCSELs) are the predominant technology for high-speed short-range interconnects in data centers. Most short-range interconnects rely on GaAs-based multi-mode VCSELs and multi-mode fiber links operating at 850 nm. Recently, GaAs-based high-speed single-mode VCSELs at wavelengths > 1 µm have been demonstrated, which increases the interconnect reach using a single-mode fiber while maintaining low energy dissipation. If a suitable platform for passive wavelength- and space-multiplexing were developed in this wavelength range, this single-mode technology could deliver the multi-Tb/s interconnect capacity that will be required in future data centers. In this work, we show the first passive Si3N4 platform in the 1-µm band (1030-1075 nm) with an equivalent loss < 0.3 dB/cm, which is compatible with the system requirements of high-capacity interconnects. The waveguide structure is optimized to achieve simultaneously single-mode operation and low bending radius, and we demonstrate a wide range of high-performance building blocks, including arrayed waveguide gratings, Mach-Zehnder interferometers, splitters and low-loss fiber interfaces. This technology could be instrumental in scaling up the capacity and reducing the footprint of VCSEL-based optical interconnects and, thanks to the broad transparency in the near-infrared and compatibility with the Yb fiber amplifier window, enabling new applications in other domains as optical microscopy and nonlinear optics.

A fiber modal adapter for upgrading 850 nm multimode fiber links to 1310 nm single-mode transmission

Abstract We propose a simple and compact adapter using specially designed modal conditioning single-mode fiber for fundamental mode transmission through multimode fiber. The modal conditioning single-mode fiber has a mode field diameter at 1310 nm roughly matching that of the fundamental mode of the 50-μm core multimode fibers. The fiber is designed for integration in a compact adapter due to its low fiber cutoff wavelength. The adapters are attached to both the input and output ends of the multimode fiber to form a link suitable for single-mode transmission. The detailed characteristics of the link such as insertion loss and multi-path interference are measured. Using the adapters, we demonstrate error-free transmission over 1-km multimode fiber using a 100G CWDM4 transceiver, which carries 4 × 25 Gb/s transmission at four wavelengths around 1310 nm. The proposed SMF adapter not only enables newly deployed multimode fiber to be future proof for higher data rate SM transmission, but also offers an upgrade solution for legacy OM2 installations which otherwise cannot support higher data rate and in some cases are very costly to replace.