INFLUENCE OF LIGHTNING STRIKES ON THE OPERATION OF COHERENT FOCL

For the first time, the influence of lightning strikes on the operation of fiber-optic communication lines (FOCL) overhead laying using optical cables in a ground wire (OPGW) was investigated in the works of M. Kurono and colleagues [1, 2]. In the articles, the dynamics of polarization at the moment of a lightning strike into a ground wire with an optical cable (OC) is investigated theoretically, experimentally and using direct field measurements. It is noted that lightning strikes cause very rapid changes in polarization that can disrupt the operation of coherent communication systems.

Analysis of the polarimeter signal shape showed that the polarization changes at a maximum rate within a short time of several tens of microseconds, which corresponds to the rise time of the lightning current. The largest recorded polarization shift was 165⁰. Almost simultaneously with the work [1], in 1996, the article by S.A. Sokolov [3] was published, in which the manifestation of the Kerr and Faraday effects in the optical cable was considered.

However, the first generation of coherent communication systems, which was actively studied in the late 1990s, did not find practical application, and lightning discharges do not significantly affect the operation of fiber-optic communication lines with systems based on amplitude modulation and direct detection. Therefore, works [1–3] were hardly noticed by the scientific and engineering communities.

A new generation of coherent communication systems using digital signal processing (DSP) has proven to be extremely effective, quickly become widely used in practice and has provided a significant increase in the capacity of fiber-optic lines and communication networks.

Through the use of polarization multiplexing and multi-level quadrature modulation formats, the channel rate has been increased from 10 Gbps, first to 100 Gbps, and then to 400 Gbps, and continues to increase. It is important to note that for the operation of coherent communication systems, the use of dispersion compensators at the physical level is not required due to its electronic compensation carried out in the DSP unit.

However, during operation, coherent communication systems using the infrastructure of overhead power lines (air cable, FOCL over a high-voltage line) have one drawback. Errors occur in the channel of a coherent transmission system in those periods of time when the air FOCL is in the zone of a thunderstorm front.

Today, the nature of this phenomenon is generally understood. During a thunderstorm, a lightning strike forms a strong longitudinal magnetic field in the fiber, a change in which, in turn, leads to rotation of the state of polarization of the optical signal (SOP) and the appearance of errors on the receiving side [1–5]. The error is caused by the insufficient speed of compensation in the DSP for strong SOP rotations caused by the magnetic field during lightning strikes.

In works [4, 5], the dynamics of the Stokes parameters in the laid FOCL was measured. In [4], the maximum measured angular rate of change in the state of polarization was 5.1 Mrad/s. A close maximum value (5.3 Mrad/s) for one observation season was recorded in [5]. In addition to the maximum rate of change of lightning current, other factors, such as cable design and grounding methods, also affect the SOP rate of change.

To eliminate interruptions in the operation of coherent communication systems due to lightning strikes, two approaches are possible. The first one is the development of faster digital information processing systems for coherent receivers (DSP). The second one is the design of new types of cable, the induced magnetic field in which is much lower than in the structures used today. As will be shown below, the most effective way to finally solve the problem of communication interruption due to lightning strikes is to combine these two approaches.

With typical parameters of OPGW and lightning (length of lay is 0.2 m, distance between supports 150 m, lightning current 20 kA), the polarization rotation of the plane of the light wave exceeds 10 rad! At the leading edge of the lightning current pulse (with a duration of 1–20 μs, see Fig. 1), rapid changes in the current dI/dt lead to the rotation of the polarization state at a rate of 0.5 to 10 Mrad/s [5]. In [6], similar values are given: the average values of lightning currents (at their peak) are about 30 kA with a steepness of the current front up to 24 kA/μs.

The influence of a rapid change in SOP on the operation of coherent communication systems was experimentally investigated in [5]. In this work, the dependence of the required optical signal-to-noise ratio (OSNR) on the angular velocity of the SOP rotation was measured in two modes of operation: with the SOP rotation compensation algorithm turned off and on.

Without the use of the SOP rotation suppression algorithm, already at a polarization rotation speed of 2 Mrad/s, the penalty exceeds 3 dB, which is greater than the operational margin. As a result, the frequency of communication interruptions due to lightning strikes becomes unacceptably high, even in regions with an average level of thunderstorm activity.

With the use of the SOP fast rotation compensation algorithm, only a small fraction of lightning causes polarization rotation at a speed of more than 6 Mrad/s, creating an OSNR penalty of more than 1.5 dB. However, the number of such events, which can reach one in three months, remains unacceptably large. In such a situation, in order to avoid interruption of communication, it is necessary to include an additional OSNR margin (about 2 dB) during design, which ultimately leads to an increase in the cost of fiber-optic communication lines.

According to various estimates, an increase in the margin by 2 dB can lead to an increase in the cost of fiber-optic cable infrastructure by 10–20%, depending on the direction of the efforts being made to improve. For example, a 2 dB reduction in attenuation in a cable in a typical amplification span of 100 km will require a switch from standard optical fiber (OF) to a reduced attenuation OF, which will increase the cost of line-cable structures by about 10-15%.

Eliminating the requirement to use an additional OSNR margin for protection against communication interruptions during lightning strikes makes it possible to use of an optical cable in a ground wire of a special design in FOCL, which reduces the magnetic field in the optical fiber by about an order of magnitude [8].

Currents of the order of 100 kA, created by lightning strikes, not only lead to heating of the conductive elements of the cable, but also form a longitudinal pulsed magnetic field inside it. This effect can be understood by imagining the cable as a solenoid of a collection of helically twisted wires around the SST with little transient conductance.

We will proceed from the fact that the amplitude of the magnetic induction in the central part of the OPGW depends on the design of the cable, in which the outer layers of wires behave like a series of solenoids. From simple physical considerations, it follows that a two-layer design with the opposite direction of twisting can drastically reduce the magnitude of the magnetic field in the center of the cable created by a lightning strike, and thereby reduce the rate of SOP change.

To solve the problem of lightning resistance of high-speed communication systems, in [8], a mechanism was proposed to compensate for the effect of lightning by using an additional layer of wires in the OPGW design, which has an opposite direction with respect to the first. In this case, there is a mutual compensation of oppositely directed magnetic fields, and hence the degree of influence on the optical fiber. The maximum field compensation occurs with the optimal selection of the OPGW design parameters: steps of winding layers, wire materials and their diameters.

These assumptions were verified experimentally on a setup, the diagram of which is shown in Fig. 4. On the test bench, on insulated suspensions, two OPGW samples, each 98 m long, are placed in a taut horizontal state. The impulse current is formed by a capacitor current source (U = 24 V; C = 0.1 F) with TRIAC control with current regulation using a resistance box (0.1–1.0 Ohm), the shape of the discharge current is recorded using a USB oscilloscope. A test continuous signal at a wavelength of 1550 nm is fed from a laser optical source EXFOFLS600 to the input of the first fiber of the OPGW with a length of about 100 m. All 12 fibers of the G.652 type in each of the cables under test are spliced into a circular loop, the total length of the optical fiber exposed to the discharge current, is 1175 m. The polarization state at the output of the 12-fiber stub is measured by the SOP Keysight N7781N analyzer.