Tomas Horvath

Joint accurate time and stable frequency distribution infrastructure sharing fiber footprint with research network

Abstract. 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.

Φ-OTDR signal amplification

Phase-sensitive optical time-domain reectometry (Φ-OTDR) seems to be the most appropriate solution for acoustic vibration along standard optical fiber detection. In general the sensing system measures phase changes of the received Rayleigh back-scattered signal in the fiber. Since the back-scattered signal intensity is decreased about tens of decibels in comparison to the forward propagating pulse power level, the received signal power level is very low. That is why the main limiting parameter of the system is the power level of the back-scattered signal, which limits maximum achievable distance. For long reach sensing it is necessary to create high power optical pulses with short time-duration. Direct pulse amplification by erbium doped fiber amplifier (EDFA) is an issue because of the pulses low repetition rate. We have designed and verified a simple method using a holding beam for amplifying of pulses with low repetition rate by standard telecommunication EDFA booster instead of deployment of an expensive optical shutter. A second CW laser with a different wavelength for EDFA stabilization is used in our setup. Because a pulse losses its energy during propagation in the fiber and with longer distances by 1st order Raman amplifier (RA). In telecommunications this amplifier is used to compensate for fiber losses. The second setup uses remote amplification by remotely pumped erbium doped fiber (EDF) placed after a few tens of kilometers of sensing fiber. A pump laser is placed in the transmitter part of the system from where EDF is pumped. In this paper, we present an overview of few techniques for Φ-ODTR signals amplification and their verification by measurement.

Simultaneous transmission of distributed sensors and data signals

Nowadays, fiber-optic sensors are widely used in many areas of an industry. To confirm increasing interest in these sensors we performed a short survey of market analysis not only for recent years but also market prediction for a near future. We performed also a short survey of current trends in fiber-optic sensors based on latest published results. For special applications we designed two possible setups of simultaneous transmission of data and sensor signals in the same fibers. By measurement we have verified suitability of interferometric methods for sensor applications if fibers with active data transmissions are used. For back-scattered based sensors a high-power optical signal is necessary hence a measurement of dependence of pulse duration on intensity of the Brillouin peaks was performed.

Simultaneous transmission of the high-power phase sensitive OTDR, 100Gbps dual polarisation QPSK, accurate time/frequency, and their mutual interferences

Currently, fibre networks are only way how to satisfy the ever growing needs for more bandwidth. Thanks to that the optical fibre can be found almost anywhere and new applications and services can be transmitted through the networks. Accurate time transfer, ultra-stable frequency transfer and fibre-optic sensors networks have been rather common. High speed data transmission, time and frequency transmission, and fibre-optic sensors must share the common fibre-optic infrastructure because it would not be economically feasible to build separate fibre networks for long distances. Each system has individual transmission requirements and is prone to another type of interference. Data transmission systems based on DP-QPSK or DP-xQAM use digital signal processing for signal recovering but it cannot fully compensate signal degradation due to polarization dependent loss and nonlinear effects which are the most dominant sources of signal degradation. Accurate time signals are slow and often OOK modulated, therefore may experience the degrading effect of chromatic dispersion. Ultra-stable frequency signals are not modulated at all information transmitted is the frequency of photons and such signals are continuous wave, but they suffer from phase noise also environmentally introduced, e.g. by vibrations. For phase sensitive OTDR sensor systems the high power pulses are necessary to use which may cause interference with other signals. For this reason, parallel and simultaneous transmission in DWDM spectral grids of standard data, time, frequency, and sensing signals is rather new and unexplored area of research.

Simultaneous transmission of standard data, precise time, stable frequency and sensing signals and their possible interaction

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.

Interference of Data Transmission in Access and Backbone Networks by High-Power Sensor System

ABSTRACT Currently, individual optical fibers are mostly used for each non-data application, which is very inefficient and uneconomical. Sharing a single fiber for multiple applications is a promising solution. However, in the case of a non-data application, the situation is much more complicated compared to data because of special application´s requirements. In laboratory setup, we performed a measurement with a standard G.652D optical fiber for analyzing possible interaction of stable frequency/accurate time transmission, 1.25/10Gbps data transmission (typical bitrates for access point-to-point networks), and high-power sensor signal for different channel spacing and different pulse duration of sensor signal.

Transmission convergence layer in XG-PON

The article deals with the Transmission Convergence Layer in 10 Gigabit passive optical networks. There is a description of conditions, under which the Optical Network Units (ONU) pass through, when is activated. Additionally, this article focuses on the processes of connecting ONUs to the Passive Optical Networks (PONs). The simulations demonstrate that as the number of simultaneous connections to ONU increase, longer wait time for each additional connection becomes. This also solves the collision conditions, that might arise in communication and the Equalization Delay (EqD). Finally, the article deals with the influence of a refractive index on timing relationships. The speed up of the frame transfer between Optical Line Terminal (OLT) and ONUs has been proven.

Towards secure Gigabit Passive Optical Networks: Signal propagation based key establishment

Nowadays, the Passive Optical Networks (PONs) technology is widely deployed in broadband access networks. This paper deals with the security issues of Gigabit PON (GPON) standardized by the International Telecommunications Union (ITU), namely, standard ITU-T G.984 that is widely implemented in Europe these days. We describe and analyze the security of this standard and show its security risks. In spite of that transmitted data are encrypted to provide their confidentiality on a multipoint fibre connection, session secret keys during their establishment can be observed by adversaries. To address this security flaw, we propose a key establishment protocol that securely sets the session secret keys between two communication parties in GPON. Furthermore, we provide the security analysis of the proposed protocol.

On security in gigabit passive optical networks

The Gigabit Passive Optical Networks (GPONs) are widely deployed in Europe these days. For example in the Czech Republic, telecommunication access networks will deploy GPONs in order to fulfill a minimal transmission speed 30 Mbps which is planned in 2020. Generally, GPONs are defined by the family of recommendations ITU-T G.984. Nevertheless, the recommendations ITU-T G.984 do not define the security in the default configuration. The key exchange mechanism is only optional and keys are sent in plain texts. This article deals with the security issues of GPONs. We analyze security risks and threats in GPONs and propose our robust solution to enhance security and privacy in GPONs.

Joint stable optical frequency and precise time transfer over 406 km of shared fiber lines — Study

We present a preliminary study of bidirectional ultra-stable optical frequency and precise time transmission over 406 km long path in the telecom-grade fiber optic infrastructure of dense wavelength division multiplexing. The main challenge in this application is the need of bidirectional amplification to compensate for signal loss. Directional non-reciprocities of the time transfer are evaluated together with their uncertainties. Solutions are proposed to significantly limit the final uncertainty.