In the ever-advancing realm of modern technology, the demand for unparalleled precision and stability in timekeeping and frequency control has surged to unprecedented heights. As our interconnected world rellies more than ever on intricate synchronization and seamless communication, the development of cutting-edge optical infrastructure has emerged as a cornerstone in meeting these exacting demands. There has been obvious increased continuous focus on precise time and frequency transmission dissemination at a national and international level recently. We would like to present the situation in the Czech Republic, our strategy, approach, and our experience with a non-commercial, costeffective solution that utilizes optical networks shared with other traffic. The presented solution provides accurate time and stable frequency at a lower operational cost, using the shared spectrum of the CESNET3 network infrastructure.
There has been an increased focus on precise time and frequency transmission dissemination at a national and international level recently. We would like to present the situation in the Czech Republic, our strategy, approach, and our experience with a non-commercial, cost-effective solution that utilizes shared optical networks. The presented solution provides accurate time and stable frequency at a lower operational cost, utilizing the shared spectrum of the CESNET3 network infrastructure. We are committed to future developments and upgrades that will include the next wavelength bands and geographic extensions. Additionally, we have implemented bidirectional dark channels on various wavebands, which utilize shared leased fibers and offer bidirectional compensation for fiber losses. However, operating precise time and frequency requires a single path with bidirectional amplification performed by optical amplifiers, which are sensitive to feedback from the fiber line induced by back-scattering, and reflections, and which can cause unwanted oscillations. We have addressed this issue by carefully solving the interference with parallel data transmissions. In summary, we have implemented a cost-effective solution for precise time and frequency dissemination in the Czech Republic, which utilizes shared optical networks. We are committed to future developments, and we are also part of a consortium that plans to realize a Pan-European network to offer time and frequency services to a broad range of users.
Precise Time and Frequency dissemination and its essential infrastructure on national or even international level are being focused recently. We present here the situation in Czechia, our strategy and approach to contribute with experience of non–commercial solution, open activity utilizing optical networks shared with other data traffic. The cost efficient solution of accurate time and very stable frequency is realized, and operated within shared spectrum of CESNET network infrastructure; the costs are under control then. We are focused on future developments, plans, upgrades concerning wavelength bands and geographic solutions and extensions. Bidirectional dark channels on various wavebands, we do realize them to utilize shared leased fibers, bidirectional compensation of the fiber losses is the benefit or must for these solutions. When operating precise Time and Frequency, single path with bidirectional amplification performed by optical amplifiers is preferred, however it is sensitive to feedbacks by fiber line, caused mainly by back scattering, reflections, and potential to cause unwanted oscillations. The interference with parallel data transmissions is the issue being carefully solved. Within this paper, we also briefly mention CLONETS Design Study project where we share the experience, and the consortium is about to plan and realize the coherent Pan-European Time and Frequency dissemination network to interconnect national research network, and to provide various Time and Frequency services for a wide range of users, research, non-commercial and commercial as well.
This contribution focuses on experimental verification of the QKD system deployment in a multi-domain network environment managed by Czech and Polish National Research and Educational Network (NREN) operators. We demonstrate full functionality of such a solution for transmission of secret keys in boundary conditions, and with this we open up new possibilities for further use of extremely secure communication between two neighboring network entities, and the services built upon it. Moreover, we have shared the cross-border link among strong QKD service channels, accurate time, and classical data channels together with weak quantum channel to reduce the total number of optical fibers needed for transmission. To our knowledge, this is the first shared cross-border QKD transmission in the region of Central and Eastern Europe (CEE).
National time and frequency dissemination networks are being developed in many countries; also international connections are being established. In the contribution we present Czech Infrastructure for Time and Frequency as a non-commercial, open activity focused on the transfer of accurate time and very stable frequency using optical networks. The national optical infrastructure for time and frequency transfer is operated on top of the CESNET network infrastructure, to have operational cost under control. We also address actually running and planned upgrades and future development plans regarding wavelength bands and considered geographic extensions. We will also focus on creation of bidirectional dark channels on different wavebands within shared fibers together with bidirectional compensation of fiber losses. Single path bidirectional amplification utilizing lumped optical amplifiers is sensitive to feedback from fiber line like back scattering and reflections and in case of increased feedback can produce unwanted oscillations, which potentially interfere with parallel data transmissions. We will also briefly mention the CLONETS-DS project working on design study for coherent Pan-European time and frequency dissemination network, which would connect national networks and provide different services based on time and frequency for a wide range of users.
KEYWORDS: Optical amplifiers, L band, Fiber amplifiers, Tunable lasers, Erbium, Dense wavelength division multiplexing, Signal to noise ratio, Signal attenuation, Optical networks, Internet
The advent of 5G mobile communications has amplified the demand for a scalable and sustainable telco networking model. Traditionally, the L-band or long band use to expand the capacity of terrestrial Dense Wavelength Division Multiplexing (DWDM) optical networks. With the acceleration of internet usage and connectivity worldwide, submarine cables carry upwards of 99 percent of global Internet traffic between continental landmasses. On-demand basis, submarine cables reaching close to Shannon’s limit which made the role for the L-band to double the maximum amount of data traffic. This paper deliberates regarding the performance of CzechLight TM (CLA) bi-directional (BiDi) Erbium Doped Fibre Amplifier (EDFA) in L-band specifically in 1595-1610 nm of 100 GHz ITU Grid with a single mode fibre transmission system. The tunable laser in this experiment finds an efficient technique to have an ultra-stable frequency gain over conventional techniques in the optical networks.
Precise time and ultra-stable optical frequency transfers over fiber networks are deployed relatively often these days. When size of such infrastructure for precise time and frequency bidirectional transmission is becoming significant, aspects associated with infrastructure operational cost and time needed for deployment of time and frequency transmission must be considered. First can be decreased via fiber sharing with telecommunication traffic, however spectral allocation must be considered carefully to avoid mutual disturbance of time and frequency transmission versus data and allow future accommodation of growing demands. In text, we show and discuss alternative spectral bands to be used for time and frequency transmission. Time to deployment can be quite excessive especially when transmission must be established via multiple networks or network domains, also there is a chance of blocking. In case of precise time and optical radio frequency transmission it is possible to use conversion from optical to electrical and back to optical domain with wavelength change. This possibility removes danger of blocking and improves time to deployment for such services. We also address possibility to change wavelength or just extend reach by using simple re-amplify and reshape approach.
Article summarizes past and continuous development, and especially current state of Czech national research infrastructure for Clock Network Services and future development plans. The focus is on used transmission means and stabilization techniques, available and planned wavelength bands and also plans for geographic extensions.
Fibre based transmission of ultra-precise time and ultra-stable optical frequency is quickly becoming common reality, not only in the fibres used for research but also in other operational fibre networks. At the moment, a fibre represents the best precision of such transmissions. In order to achieve highest possible transmission stability up to 10-18for 1000 s averaging, the bidirectional transmission within single fibre is required including exclusive optical amplification. We present here the use and results of Optical Time Domain Reflectometry (OTDR) technique for detection of disturbances as connector losses, reflections, bending etc. on live fibres with present Amplified Spontaneous Emission (ASE) from bidirectional Erbium Doped Fibre Amplifiers (EDFAs).
Multiple studies have been carried out to analyze the mutual interaction of a phase-sensitive optical time-domain reflectometer (Φ-OTDR) and parallel digital data traffic. However, interactions with analog transmission, e.g., community antenna television (CATV), have not been addressed. Our study examines and presents the influence of a developed sensing system Φ-OTDR when operated simultaneously on parallel fibers with the transfer of data from an analog CATV system. Three scenarios are suggested and discussed for the measurements and optimization of the data network in the analog CATV data transfer. These scenarios are suggested to enable the verification of the mutual interactions between data networks with the analog data transfer and suggested sensing system and to determine how the data network with the analog data transfer may or may not be influenced by the sensing system. The optimization and measurements prove that the analog CATV data transfer was negatively influenced by the sensing system Φ-OTDR, and the channel bit error rate increased by nearly half. The implementation of the sensing system Φ-OTDR was realized for a data network, followed by the testing and optimization, which proved that the safe spectral distance between the CATV and the sensing system is 150 GHz and higher.
The paper focuses on showing past, current and ready to start time and frequency (metrology) projects taking place in Europe aimed to long haul time and frequency transfers over optical fibers. It addresses both EURAMET and Horizon2020 projects’ scope. Focus will be given to Coordination and Support Action project CLOck NETwork Services (CLONETS) - Strategy and innovation for clock services over optical-fibre networks and proposed subsequent design study CLONETS-DS. Attention will be given also to effort of Research and Education Networks (NRENs) as pioneer network operators into this field.
Optical fibers are becoming commonly used beside data transmissions for dissemination of ultra-precise and stable quantities or alternatively as distributed sensors of for example acoustic and mechanic vibrations, seismic waves, temperature etc. There have been developed methods for these transfers and their stabilization, allowing thus to achieve excellent performances. Such performance is bound with utilization of single physical medium for both ways of propagation. These methods are attractive both for very high-performance applications and as a secure alternative complementary to radio and satellite-based transfer methods. From economical point of view, sharing fibers with regular data traffic is an advantage, especially for longer distances and large infrastructures. Unfortunately, the most often used wavelengths are located almost in the middle of telecommunication band. Due to continuous data traffic growth and utilization of flexible spectral allocation, the collision in wavelength plan will occur more and more often. In this paper we overview alternative wavelengths suitable for these transfers, we also propose suitable methods for all-optical reach extension, by all-optical amplification. Shared line design allowing transfer of ultra-stable quantities in three different spectral bands is proposed and such design is evaluated.
The reach of any all-optical transmission is limited by attenuation of transmission path and other factors as signal to noise ratio, and it can be extended by all-optical amplification. Bidirectional single fibre transmission introduces an issue of bidirectional symmetrical amplifiers in order not to lose advantage of path symmetry. In case of time transfer, quasibidirectional amplification might be acceptable when supported by specific arrangements, e.g. as much as possible equal arrangement for disjoint segments of the path. Time transfer with best available accuracy or optical frequency transfers require single path optical amplifiers that are further considered. In this constitution, unfortunately, reflections together with Rayleigh back-scattering will create feedback. In case feedback is strong enough and discrete amplifier operates in high gain regime (about 20dB), the whole system will start to oscillate. It saturates the gain of amplifiers and also can generate errors, when lasing in a transmission band. In the article, we review possible all optical amplification methods including those allowing to use untraditional transmission bands (outside C band).
Long distance precise frequency and accurate time transfer methods based on optical fiber links have evolved rapidly in recent years, demonstrating excellent performance. They are attractive both for very high-performance applications and as a secure alternative complement to radio- and satellite-based methods. In this paper, we present development of infrastructure for such transmission containing 700+km of transmission lines, with planned cross border optical frequency connectivity. According to our knowledge, this will be the third such line globally. The infrastructure also shares fibers with existing data transmissions, both amplitude and phase modulated, which poses high demands on mutual isolation and insensitivity to cross talks.
In this paper, we propose and present verification of all-optical methods for stabilization of the end-to-end delay of an optical fiber link. These methods are verified for deployment within infrastructure for accurate time and stable frequency distribution, based on sharing of fibers with research and educational network carrying live data traffic. Methods range from path length control, through temperature conditioning method to transmit wavelength control. Attention is given to achieve continuous control for relatively broad range of delays. We summarize design rules for delay stabilization based on the character and the total delay jitter.
Since optical fibre is a standard medium for all current and new networks, these optical networks offer possibility for connecting new applications over long distances almost to anywhere. However with increasing number of applications, the large number of dedicated fibres will be necessary. This constitution is quite unpractical in terms of costs, however since wavelength division multiplexing enables transmission of multiple different signals over one fibre it is more than suitable to use this technology for cost reduction and network efficiency increase. Wavelength division multiplexing technology is common in data networks where parameters of all signals may be optimized (especially maximum optical power launched into the fibre) for simultaneous transmission. In case of non-data applications the situation is more difficult because each application is connected by different type of signal and with its own requirements for transmission parameters. Hence it is necessary to evaluate possible interactions before field deployment. In this paper we deal with possible interaction of a coherent 100 Gb/s dual polarisation QPSK data signal with new applications like accurate time and stable frequency transmission and high-power pulse signal used for distributed sensing. In laboratory setup we performed a measurement with a standard G.652D single mode optical fibre and also with G.655 fibre which can also be found in some networks and may be source of more nonlinear interactions. All signals were transmitted in a grid with 100GHz spacing according to ITU standard. Results confirmed our assumptions that 100GHz spacing is not large enough and also that G.655 optical fibre is prone to more non-linear interactions.
The infrastructure essentialities for accurate time and stable frequency distribution are presented. Our solution is based on sharing fibers for a research and educational network carrying live data traffic with time and frequency transfer in parallel. Accurate time and stable frequency transmission uses mainly dark channels amplified by dedicated bidirectional amplifiers with the same propagation path for both directions of transmission. This paper targets challenges related to bidirectional transmission, particularly, directional nonreciprocities.
In this paper, we present infrastructure for accurate time and stable frequency distribution. It is based on sharing of fibers of research and educational network carrying data traffic. Accurate time and stable frequency transmission uses mainly created dark channels amplified by special bidirectional amplifiers with the same propagation path for both directions. Paper also targets challenges joined with bidirectional transmission, which represents directional non-reciprocities and interaction with parallel data transmissions.
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