COMPARING SENSITIVITY OF STATE OF POLARIZATION MONITORING AND DISTRIBUTED ACOUSTIC SENSING IN THE SVALBARD ARCTIC SUBMARINE COMMUNICATION CABLE

By Kristina Shizuka Yamase

Abstract

The existing optical fiber telecommunication infrastructure is an attractive alternative for creating a global geophysical activity sensing network. This paper compares State of Polarization (SOP) sensing and Distributed Acoustic Sensing (DAS) in a field measurement on a telecommunication cable in Svalbard, Norway. SOP sensing is used on an alien wavelength in a live single-span passive submarine cable communication system. Unlike DAS, direct physical fiber perturbation is measurable without saturating, while distributed signals from earthquakes are still measurable. The combined environmental and background noise limits the minimum detectable SOP variations to corresponding to signals from ML 2-3 earthquakes in our infrastructure.

1. INTRODUCTION

Sub-sea fibre telecommunication infrastructures are spanning the world. Additional utilization of this telecommunication network for monitoring environmental and geophysical parameters is attractive as it enables high granularity worldwide monitoring. Using Distributed Acoustic Sensing (DAS), detecting phase changes in a back-reflected signal is a well-established technique. Various studies have demonstrated submarine sensing of both earthquakes, ship passing, and whale soundings [1]. However, DAS systems currently have a range constraint of approximately 150 km [2]. The SOP of an optical signal in a fiber cable is susceptible to mechanical disturbance brought on by geophysical vibrations. In [3], where a long active subsea cable of 10,000 km was used, SOP-based sensing was used to detect earthquakes as well as ocean swells. The work demonstrated that

SOP information may be extracted from telecommunication systems in operation by accessing telemetry data from the coherent receivers. This simplifies detection compared to the more advanced DAS method, using a dedicated instrument containing a laser with high demands to phase stability. Therefore, SOP signals have recently been studied as an attractive alternative for global geophysical activity sensing network systems showing susceptibility to mechanical disturbances and reaching longer distances compared to DAS [3, 4]. SOP detection is simpler than DAS, but there are limitations in location information and sensitivity. Limits in detection, and utilization of data from SOP variations, have not been as thoroughly explored as the well-established DAS technique. In particular, simultaneous DAS and SOP monitoring for comparing sensitivity and data content has not been explored.

In this contribution, we perform simultaneous monitoring using DAS and SOP and report results from a field test conducted in August 2022 in Isfjorden, Svalbard, Norway. The field study was implemented between Longyearbyen and New Aalesund on a 250 km passive fiber. DAS interrogators were connected to each side of a dedicated dark fiber within the same cable as the communication system, allowing the entire 250 km length of the fiber to be covered by DAS sensing, investigating how and when the acoustic wave affected the cable. Simultaneously, SOP fluctuations were observed using a polarimeter connected to an alien wavelength in a live communication system using EDFA and Raman amplifiers. In this type of system, downloading SOP data from coherent receivers were not supported. Earthquakes are observed on both SOP and DAS data, and the comparison of sensitivity and interpretation of the field test results are discussed.

2. FIELD TEST SETUP

Figure 1 displays a schematic representation of the SOP sensing system as well as a map of the optical fiber cable. The DWDM system and an EDFA amplifier at Ny-Aalesund were utilized to transmit modulated light with a wavelength of 1542.94 nm into an “alien wavelength” using a wavelength-specific 1 Gb/s Ethernet (GBE) SFP module. The polarimeter (PM1000, Novoptel) is placed in Longyearbyen, receiving the light at the alien wavelength after travelling through the 250 km fiber. The signal was wavelength-multiplexed with the live communication traffic wavelengths. The signals are amplified by a Raman amplifier and an EDFA and before demultiplexing. The degree of polarization (DOP) and optical power at the polarimeter were 0.85 and -8.6 dBm, respectively. The polarimeter sampling frequency was set to 3100

Hz. From both sides of a dedicated dark fiber (type G.652D), two DAS interrogators (OptoDAS, ASN) were connected within the same cable as the fiber used for SOP measurements. The DAS interrogator operated at 1550 nm wavelength to sample 30,000 channels with 4.08 m spacing from 0 to 120 km from both sides, with a sampling frequency of 645.16Hz. Both SOP and DAS monitoring data were transmitted to southern Norway through the network of Sikt.

Analysis And Results

3.1 ANALYSIS METHOD

The polarimeter outputs the three normalized Stokes parameters. To study the SOP variation, we write , where is the initial Stokes vector. For small polarization variations, is in a plane transverse to . We therefore characterize the SOP variations using the vector . The SOP data were smoothed using a Gaussian-weighted moving average with a window size of 150 samples, followed by downsampling to 30Hz. For the time-frequency analysis, the power spectral density (PSD) of the length of was estimated using 2 seconds windows, sliding one second at a time. In addition, the time-frequency results were smoothed by averaging over the nearest neighbor samples in both frequency and time domain.

3.2 IMPACT OF HUMAN ACTIVITIES

To access the impact of human activities close to the endpoints we recorded both DAS and SOP signals during different local human activities. Figure 2 shows the time-frequency analysis of the SOP. Strong local perturbations, such as touching the fiber directly or hitting the wall in the server room, results in large SOP variations. However, local vibrations due to human activity on the ground above buried land cables is below the system noise floor. The average noise floor, without any local human activity varied from day to day. A noise floor of -65 dB/Hz in the 0.5 to 3 Hz band as seen in Figure 2 corresponds to an angular deviation of over the Poincare sphere [4]. Signals from marine and geophysical activity will to a large extent be limited to frequencies below 3 Hz. Directly touching the fiber resulted in signals varying up to -25 dB/Hz, while for vibration on the ground over the cable, the variations were up to -52 dB/Hz. For DAS, directly touching the fiber causes signal saturation, while vibrations on the ground above the cable were within the dynamic range.

3.2 EARTHQUAKES

For earthquake analysis, the arrival times of the P- and S-waves and the duration of the signal from the earthquake were verified by analysing DAS data. Figure 3 shows the time-frequency analysis of the SOP signal during two low-magnitude earthquakes. According to seismic data from NORSAR, Earthquake A, an ML3.47 earthquake, occurred on the 19th of August, while Earthquake B, an ML2.70 earthquake, was on the 21st of August. The time the P- and S-waves first hits the cable is indicated. However, part of the SOP signal is significantly delayed since waves first reaches other part of the cable much later. Figure 4 shows the corresponding frequency analysis of the DAS data during the P- and S-waves. Note that the apparent strong signal around 120 km is noise. The earthquakes are seen as strain in the 1 to 3 Hz range along the whole cable. The peak strain is on the order of 10-11/Hz for earthquake A. The magnitude of an earthquake cannot directly be converted to strain magnitude on the fiber. For the two earthquakes considered here, earthquake A caused factor of 10 more strain on the fiber than Earthquake B, in spite of the fact that the excursion, based on the reported magnitude, should be a factor 6 or larger. However, due to the high background noise during Earthquake A, especially the P-wave was challenging to identify in the SOP data. The average signal level below 3 Hz prior to the earthquakes were -64 dB/Hz and -70 dB/Hz for earthquake A and B respectively. Thus, earthquake B caused less strain on the fiber, but due to the lower noise level both the P- and S-waves could be identified.

The sensitivity of the SOP signal, here defined as the minimum detectable SOP variation, is strongly affected by background environmental noise. Both the SOP and the DAS data shows that the environmental noise varies from day to day. Factors contributing to the background noise floor of the SOP measurement are noise from the optical amplifiers, receiver noise from the polarimeter, and possibly effects from other data traveling in the same fiber. Environmental noise varying from day to day could be due to temperature, weather, and ocean activities. Future work would be to establish a methodology to characterize and differentiate both noise and periodic signal trends for SOP sensing.

Conclusion

Integrating SOP sensing for monitoring low-frequency geophysical parameters in live telecommunication networks using an alien wavelength is demonstrated. Using a polarimeter, we are able to detect an ML2.7 earthquake. Local signals on land, such as human activity over a buried cable, is found to create weak SOP variation below the noise floor. The level of the environmental noise is shown to vary on different time scales, including day-to-day variations. In future work, effort will be put into enhancements in the processing of the monitored signal and limiting noise for enhancing sensitivity and detection capabilities. STF

Aknowledgement

The authors acknowledge the Norwegian Research Council and the sponsors of the Centre for Geophysical Forecasting at NTNU for financial support (Grant No. 309960), ASN for the support with configuring the DAS interrogator, and Sikt for letting us use the DWDM-system.

References

[1] M. Landrø et al., “Sensing whales, storms, ships and earthquakes using an Arctic fibreoptic cable,” Earth and Space Science Open Archive, pp. 39, 2021

[2] O. Waagaard et al., “Real-time low noise distributed acoustic sensing in 171 km low loss fiber,” OSA Continuum, vol. 4, no. 2, pp. 688-701, 2021

[3] Z. Zhan et al., “Optical polarization-based seismic and water wave sensing on transoceanic cables,” Science., vol. 371, no. 6532, pp. 931–936, 2021

[4] A. Mecozzi et al. “Polarization sensing using submarine optical cables”, Optica, vol. 8, no. 6, pp. 788-795, 2021.

KRISTINA SHIZUKA YAMASE SKARVANG is a PhD student at the SFI Centre for Geophysical Forecasting (CGF), Department of Electronic Systems, Norwegian University of Science and Technology (NTNU). She is working on embedding optical polarization-based geophysical activity sensors in optical data communication systems. The project aims to develop a sensor system that can be integrated into an optical fibre infrastructure to differentiate geophysical signals from other activities.