Fiber Optic Transport Research Articles

Bidirectional White-Lighting WDM VLC–UWOC Converged Systems

A bidirectional white-lighting wavelength-division-multiplexing (WDM) visible light communication (VLC)-underwater wireless optical communication (UWOC) converged system with an office-level white light illumination of 514 lux and a symmetric downstream/upstream data rate of 150 Gb/s is demonstrated successfully. Red, green, and blue (RGB) laser diodes with two-stage light injection and optoelectronic feedback schemes are operated to provide bidirectional acceptable office-lighting and high-transmission-rate. White light is created/separated by combining/dispersing RGB lights utilizing two cascaded transmission gratings at the transmitting/receiving side. By employing PAM4 modulation and RGB triple-light WDM technique, the transmission rate is enhanced remarkably to 150 Gb/s for downlink/uplink transmission. Acceptable office-lighting (>500 lux) is attained through 3 m plastic optical fiber transport. Additionally, good bit error rate performance of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> and accepted PAM4 eye diagrams are obtained over 10 m free-space transmission with 5 m ocean underwater link. The bidirectional white-lighting WDM VLC-UWOC converged system demonstrated here is beneficial for the development of white-lighting with high-speed optical wireless communications.

Coherent Optics Ready for Prime Time in Short-Haul Networks

Optical fiber transport is the key technology to enable the delivery of fast and affordable bandwidth for all services, such as 5G/mobile, business and residential broadband, cloud computing, and video transport. Coherent optics, a game-changer optical fiber transport technology, has completely transformed optical transmission systems and enabled a widespread upgrade and new deployment of DWDM networks to speeds of 100 Gb/s, 200 Gb/s, and 400 Gb/s per wavelength. Over the past decade, coherent optics has moved beyond its long-haul application origins and is now firmly entrenched in metro networks. 2020 marked the 10th anniversary of digital coherent optical technology being officially deployed in service providers' networks. Further development in CMOS processing, design simplification, and cost reduction of opto-electronic components, and the need for more efficient optical transport technologies have been factors driving coherent solutions into new market applications, specifically short-haul applications in edge and access networks. This migration model has been demonstrated in the optical industry before: the DWDM system technology started in long haul and subsequently migrating to metro and edge access; forward error correction encoding and decoding followed the same pattern. Benefiting from initial long-haul technology development, coherent optics for access networks is poised to follow the same natural progression.

Based on FP LDs with split band technique in two-channel fiber optical CATV transport system

This study proposes a two-channel fiber optic cable television (CATV) transport system based on Fabry–Perot laser diodes (FP-LDs) with a split band technique. To reduce the interference between channels, we apply the split band technique to two channels with different frequencies by using two converters at each moment. In this two-channel transmission scheme, composite second-order (CSO) and composite triple-beat (CTB) distortions induced by the systems are possibly confined in unused channels and result in good transmission performances. Through a 40 km standard single-mode fiber transmission, excellent performances of carrier-to-noise ratio (≥50 dB), CSO (≥70 dB), and CTB (≥72 dB) are obtained using the proposed fiber optic CATV transport systems. The proposed systems employing FP-LDs with a split band technique are also simpler and more cost effective than conventional externally modulated systems.

Investigation of utilizing a VCSEL diode to work as a tunable optical bandpass filter

Tunable optical band-pass filter (TOBPF) composed of a vertical-cavity surface-emitting laser (VCSEL) is proposed for multi-wavelength optical fiber transport systems. Experimental results prove that through properly adjusting VCSEL driving current, one of multiple injected lightwaves can properly flow through the proposed optical filter and others will be attenuated roughly 12 dB. Furthermore, by changing the VCSEL driving current, the pass-band window of the VCSEL-based TOBPF can be shifted to align with different designated injected lightwave and to block the others. By employing the TOBPF in multi-wavelength optical fiber transport systems, proper eye diagrams are experimentally observed for each dedicated optical signal. The proposed scheme is shown to be a practical and flexible component for multi-wavelength optical fiber transport systems.

Simulation of a long-haul fiber optic link with a two-mode optical fiber

Modern telecommunication networks approach the capacity crunch, which is associated with the so-called nonlinear Shannon limit. So, the passage to fiber optic links with few-mode optical fibers is considered as an alternative solution of the described problem concerned with high nonlinearity of conventional commercial single-mode optical fibers. Presently, various designs of few-mode optical fibers have been known, with the recently published works presenting experimental results demonstrating their potentialities for long-haul fiber optic links. A lot of models of long-haul fiber optic links with few-mode optical fibers have been developed based on which features of a few-mode optical fiber transport network were numerically simulated. This work presents the results of simulation of a 6000-km long-haul fiber optic link with a two-mode optical fiber and 100-km-per-span Erbium doped fiber optic amplifiers system under 100 Gbps DP-DQPSK data transmission. We studied the use of particular linearly polarized modes and optical vortices for signal transmission. The computation results were compared with the simulation of the same fiber optic link with a single-mode optical fiber under the identical conditions.

FTTH and Two-Band RoF Transport Systems Based on an Optical Carrier and Colorless Wavelength Separators

A hybrid fiber-to-the-home (FTTH) and radio-over-fiber (RoF) transport system is proposed to transmit one wired signal and two wireless signals simultaneously through an optical lightwave. By properly utilizing the polarization characteristic of a phase-remodulated and polarization-remodulated optical lightwave, the first- and second-order sidebands of the lightwave can be colorlessly separated for optical double- and quadruple-frequency RoF transmissions. Experimental results prove that the independent FTTH and two-band RoF signals can be simultaneously generated and separated by relatively low-frequency radio-frequency devices, optical modulators, and polarization beam splitters. The transmission performances are proven by proper bit error rate (BER) performance and clear eye and constellation diagrams. The interference among the transmitted wired signal and double- and quadruple-frequency RoF signals is acceptable. In this case, the wavelength of the optical carrier can be flexibly assigned, and the frequency of the transmitted wireless signals can be easily adjusted without affecting the functionality of the developed optical fiber transport system. The presented architecture can be flexibly employed to support various types of FTTH and microwave and millimeter-wave over fiber transmissions.

Note: Fiber optic transport probe for Hall measurements under light and magnetic field at low temperatures: Case study of a two dimensional electron gas

A fiber optic based probe is designed and developed for electrical transport measurements in presence of quasi-monochromatic (360-800 nm) light, varying temperature (T = 1.8-300 K), and magnetic field (B = 0-7 T). The probe is tested for the resistivity and Hall measurements performed on a LaAlO3-SrTiO3 heterointerface system with a conducting two dimensional electron gas.

A phase-modulated direct-detection fiber optical CATV transport system

A fiber optical CATV transport system based on phase modulation (PM) and direct-detection schemes to reduce noise and distortion induced by systems is proposed and experimentally demonstrated. The architecture of the proposed systems is simplified by the fiber-dispersion-induced PM-to-intensity modulation (IM) conversion technique. As a result, the optical signal is detected directly by a photodiode (PD), without using the sophisticated and expensive delay interferometer (DI). Impressive performances of carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple beat (CTB) were obtained in our proposed systems over a 100-km single-mode fiber (SMF) transport.

Reconfigurable digital coherent receiver for metro-access networks supporting mixed modulation formats and bit-rates

A single, reconfigurable, digital coherent receiver is proposed and experimentally demonstrated for converged wireless and optical fiber transport. The capacity of reconstructing the full transmitted optical field allows for the demodulation of mixed modulation formats and bit-rates. We performed experimental validation of different modulation formats, including VCSEL based OOK, baseband QPSK, RoF OFDM and wireless IR-UWB over a 78km deployed fiber link.

Hybrid Wireline and Wireless Transport System Based on Polarization Modulator

A polarization modulation and amplitude re-modulation scheme is proposed to achieve a hybrid wireline and wireless fiber optical transport system. With the assistance of a polarization beam splitter, the odd-order and even-order sidebands of an optical polarization modulated wireless signal can be separated colorlessly to achieve optical double frequency application and amplitude re-modulation scheme. The overall operation principle is experimentally demonstrated by up-converting a 625 Mb/s/20 GHz signal up to a 625 Mb/s/40 GHz signal. The transmission performance of the proposal is evaluated by communicating a hybrid 625 Mb/s baseband signal and a 625 Mb/s/14 GHz wireless signal over a 20 km single-mode fiber. The obtained error free transmissions and clear eye diagrams are great evidence to prove the practicality of this proposal.

A hybrid CATV/OFDM long-reach passive optical network architecture

A hybrid CATV/OFDM long-reach PON architecture is proposed and experimentally demonstrated. By multiplexing the OFDM and CATV signals in different frequency bands, the overall network structure is simplified and the available bandwidth for the OFDM signals is increased. Furthermore, by carefully adjusting the driving voltage of the employed CATV transmitter, the overall transmission performance can be optimized by eliminating a large amount of unwanted distortions. Unlike the current fiber optical CATV transport system in which the CATV signals need to be amplified in every 30 ~40 km transmission span, the proposed architecture has successfully extended the transmission span to 60 km. This can practically remove the needs of electrical power provisioning and monitoring between the central office (CO) and each local exchange (LE). Good transmission performances of carrier-to-noise ratio (CNR), composite second-order (CSO), composite triple beat (CTB) and bit error rate (BER) were obtained for the transmitted CATV and OFDM signals. All of these experimental results prove the practice and efficiency of the proposed architecture in providing simplicity and extended reach services to customers.

Fiber optical CATV transport systems based on PM and light injection-locked DFB LD as a duplex transceiver

A bidirectional fiber optical CATV transport system employing phase modulation (PM) scheme and frequency up-conversion technique to deal with downstream CATV signals, and using light injection-locked distributed feedback laser diode (DFB LD) as a duplex transceiver at the receiving site is proposed and experimentally demonstrated. With optimum injection wavelength and power level, a DFB LD is efficiently employed for both the transmitter and receiver operations. Such DFB LD is used to replace the functions of delay interferometer (DI) and CATV receiver, and also to be as the upstream light source. To the best of our knowledge, it is the first time to successfully utilize a DFB LD to detect the phase-modulated CATV signals. Impressive experimental results prove that our proposed systems not only can employ the PM scheme and the frequency up-conversion technique to optimize the overall performances of systems, but also can use an injection-locked DFB LD to detect the downstream phase-modulated CATV signals as well as to transmit the upstream CATV ones simultaneously.

An Upconverted Phase-Modulated Fiber Optical CATV Transport System

A fiber optical CATV transport system that employed an upconverted technique and a phase modulation scheme to reduce noise and distortion induced by system is proposed and experimentally demonstrated. In contrast to a fiber optical CATV transport system that used a conventional intensity modulation scheme, excellent performances of carrier-to-noise ratio, composite second order, and composite triple beat are obtained in our proposed systems over a 100-km single-mode fiber transmission. It verifies the feasibility of the proposed systems and reveals an outstanding one with excellent transmission performances.

Hybrid CATV/16-QAM OFDM in-building networks over SMF and GI-POF transport

A hybrid CATV/16-QAM OFDM in-building network over a combination of single-mode fiber (SMF) and graded-index plastic optical fiber (GI-POF) transport is proposed and experimentally demonstrated with good qualities of service. In this system, a 1556 nm optical signal is directly transmitted along with two fiber spans (20-km SMF + 25-m GI-POF). Without using any wavelength conversion or bridge circuit between SMF and POF connection, error free transmissions with sufficient low bit error rate (BER) values are achieved for 2.5 Gbps/2.5 GHz and 5 Gbps/2.5 GHz OFDM signals; as well as good performances of carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple beat (CTB) are obtained for CATV one. This proposed network reveals an outstanding one with economy and convenience to be installed.

Introduction to the Issue on Enabling Technologies for Digital Optical Communication Systems

This issue aims at providing a diverse coverage of the current state of research in digital optical communication technologies, ranging all the way from deep-space laser communications to terrestrial fiber-optic transport, packet, and access networks to digital optical communications for computer interconnects and on-chip integration.

A Radio-Over-GI-POF Transport System

A novel radio-over-graded index-plastic optical fiber (GI-POF) transport system is proposed based on vertical-cavity surface-emitting lasers (VCSELs) and light injection-locking technique. Comparing with other light sources, VCSEL provide a good balance between its cost and quality for intrabuilding network applications. To further enhance its transmission performance, we are the first one to integrate VCSELs with light injection-locking technique in radio-over-GI POF intrabuilding network. Impressive upgrading of third-order intermodulation distortion to carrier ratio, spurious-free dynamic range, error vector magnitude, and bit error rate were experimentally obtained and discussed in the paper.

Cryogenic confocal microscopy with rotation in a magnetic field

Two approaches to conducting magneto-optical confocal spectroscopy are described, in each of which the confocal head is rotatable with respect to the magnetic field. A coudé arrangement has been shown to give adequate performance for scanned imaging, but lower optical throughput than a system based on single-mode optical fiber transport. The design criteria for a cryogenic fiber-coupling objective are described, and the tolerances demanded in lens alignment are shown to be relatively benign, allowing manufacture without special techniques. The practical use of the rotating confocal system with commercial stick-slip positioners has been shown to be rigid enough, and asymmetric weight distribution and diamagnetic forces are small enough to permit single quantum emitters to be studied over a range of angles and field strengths.

Fiber Capacity Limits With Optimized Ring Constellations

In this letter, we calculate theoretical limits to fiber-optic transport capacity under the constraint of requiring the use of concentric multiring modulation alphabets. Subject to these conditions, we vary ring amplitude ratios and occupation probabilities to determine the capacity-optimized ring constellations. The technique used for fiber transmission calculations accurately captures all fiber nonlinear effects, with the accuracy limited by the noise and signal statistical properties only.

Complexity in Spanish optical fiber and SDH transport networks

Complex networks are important instances of technology-related complex systems. In this work we apply tools from complexity science to characterise two Telefónica España transport network systems: the optical fiber network and the SDH transport network. We compare both cases and derive its most important properties. Remarkably, our results show that in both cases several features of heterogeneous, hierarchical complex networks arise.

CSO/CTB PERFORMANCE IMPROVEMENT BY USING FABRY-PEROT ETALON AT THE RECEIVING SITE

An externally modulated NTSC 77-channel erbium-doped fiber amplifier (EDFA)-repeated system employing Fabry-Perot (FP) etalon at the receiving site to improve system performance was proposed and demonstrated. In comparison with conventional externally modulated fiber optical CATV transport systems, good performance of carrier-to-noise ratio (CNR), composite second order (CSO), and composite triple beat (CTB) were achieved for the full channel band over a 100-km single-mode fiber (SMF) transmission. Our proposed systems are suitable for the long-haul fiber optical CATV transport systems.