Long range Coherent-Optical Frequency Domain Reflectometry for large scale distributed sensing

We report a noble coherent optical frequency domain reflectometry (OFDR) system that simply enables measurement range enhancement up to full coherence length of a laser source. The proposed technique is based on bidirectional determination of Rayleigh backscattered signal along a fiber. To do this, complex OFDR signals are acquired with the help of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pi / 2$</tex-math></inline-formula> phase-shifting interferometry in a detection part and an additional delay fiber in a reference arm. Through a bidirectional determination process using the complex OFDR signals, mirrored signals appearing at both sides of the spatial domain were completely discriminated, so that ambiguity arising due to a positional origin can be removed. Bidirectional distributed strain and temperature sensing is successfully performed for the first time. Spectral shifts by applying strain to and heating of the test fiber were found to be independent on both sides of the reference position, indicating that twofold enhancement of the measurement range can be obtained compared to previous systems.

Recent progress on novel long-range coherent optical frequency domain reflectometry is reviewed along with its applications to diagnosing problems with optical fiber cables. The measurement range of a conventional C-OFDR is limited to the coherence length of the laser used as the light source, since the phase noise of the laser degrades the sharpness of the beat spectrum. We have developed phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) to overcome this limitation by introducing a novel phase-noise compensation technique, and we achieved a very high-resolution measurement over the fiber link length. We describe the principle of PNC-OFDR and recent related developments, and discuss its use in diagnosing issues with fiber networks.

Volcanic eruptions can cause loss of life and livelihoods, damage critical infrastructure and have long-term impacts, including displaced populations and long-lasting economic implications. Many factors contribute to disasters from natural hazards. One of these is the institutional capacity to enable hazard assessment for pre-emergency planning to protect populations and environments, provide early warning when volcanoes threaten to erupt, to provide forecasts and scientific advice during volcanic emergencies, and to support post-eruption recovery and remediation. Volcano observatories play a critical role in supporting communities to reduce the adverse effects of eruptions [Chapter 15]. Their capacity to monitor volcanoes is thus a central component of disaster risk reduction.

The city of Auckland (population 1.3 million) is built on and around a potentially active basaltic intraplate volcanic system, the Auckland volcanic field. This monogenetic field of around 50 small volcanoes covers an area of 360 km2 and may have been active for ca. 250 ka. Volcano monitoring can be difficult in small distributed volcanic fields such as the AVF as the next vent location is not known, and traditional techniques such as geochemical and ground deformation monitoring may therefore not be feasible as there is no obvious target for measurements. In such cases, seismic monitoring may be the only technique suitable. High levels of background (cultural) seismic noise and extremely low levels of historical seismicity make volcano monitoring particularly challenging in the AVF. Here, we report on recent attempts to improve monitoring capability in the AVF, which include the installation of downhole seismometers and a seismic refraction study to refine the velocity model used to locate earthquakes. We conclude that downhole instrumentation in a volcano-monitoring network allows better detection of local earthquakes, but note that several downhole sensors would be required for a network to realise the full benefits of downhole instrumentation. Our refinement of the general velocity model for Auckland has some effect on calculated earthquake locations and depths, and we suggest that further work on refining the velocity model and then its incorporation into the GeoNet earthquake location system could help us be better prepared for a future AVF eruption.

Volcanoes are hazardous to local and global populations, but only a fraction are continuously monitored by ground‐based sensors. For example, in Latin America, more than 60% of Holocene volcanoes are unmonitored, meaning long‐term multiparameter data sets of volcanic activity are rare and sparse. We use satellite observations of degassing, thermal anomalies, and surface deformation spanning 17 years at 47 of the most active volcanoes in Latin America and compare these data sets to ground‐based observations archived by the Global Volcanism Program. This first comparison of multisatellite time series on a regional scale provides information regarding volcanic behavior during, noneruptive, pre‐eruptive, syneruptive, and posteruptive periods. For example, at Copahue volcano, deviations from background activity in all three types of satellite measurements were manifested months to years in advance of renewed eruptive activity in 2012. By quantifying the amount of degassing, thermal output, and deformation measured at each of these volcanoes, we test the classification of these volcanoes as open or closed volcanic systems. We find that ~28% of the volcanoes do not fall into either classification, and the rest show elements of both, demonstrating a dynamic range of behavior that can change over time. Finally, we recommend how volcano monitoring could be improved through better coordination of available satellite‐based capabilities and new instruments.

SUMMARY Real-time monitoring of volcanic earthquakes forms a critical component of assessing volcanic hazard but requires a widely dispersed land-based instrument network and analysis infrastructure to be effective. Here we show how a sparse network of hydrophones can be used for volcano monitoring as well. Detection of acoustic events by the Mid-Atlantic Ridge hydrophone array includes many events in the vicinity of the Cape Verde Islands, upon which a volcano monitoring network operates. Correlating the hydroacoustic events with earthquake locations determined by the network, we find that earthquakes of magnitudes as low as ML 2.0 can be hydroacoustically located to ±25 km at ranges up to 2000 km. All correlatable events are volcano-tectonic in character. Acoustic propagation of small-magnitude earthquake signals is more efficient if their sources are perched in the island’s volcanic edifice in the ocean sound transmission channel, which contributes to volcanic event detectability. The location accuracy at this low magnitude range, which includes magnitudes typical of seismic crises precursory to eruptions, provides an additional volcanic hazard monitoring capability. Seismicity and hydroacoustic activity in the northern Cape Verdes suggest that this monitoring element may be of use in assessing future risk there.

Volcano monitoring in Peru is carried out by the Instituto Geofísico del Perú (IGP), through its Centro Vulcanológico Nacional (CENVUL). CENVUL monitors 12 out of 16 volcanoes considered as historically active and potentially active in southern Peru and issues periodic bulletins about the volcanic activity and, depending on the alert-level of each volcano, also issues alerts and warnings of volcanic unrest, ash dispersion, and the occurrence of lahars. The information generated by CENVUL is disseminated to the civil authorities and the public through different information media (newsletters, e-mail, website, social media, mobile app, etc.). The IGP volcanology team was formed after the eruption of Sabancaya volcano in 1988. Since then, geophysical and geological studies, volcanic hazards assessments, and multidisciplinary monitoring realized by the IGP, have provided a comprehensive understanding of volcanic activity in Peru and forecast future eruptive scenarios. Currently, 80% of the historically active and potentially active volcanoes in Peru are equipped with networks of multiparameter instruments, with the seismic monitoring being the most widely implemented. In this report, we present the situation of volcanic monitoring in Peru, the monitoring networks, the techniques employed, as well as efforts to educate and inform the public and officials responsible for disaster risk management.

Using low power consumption photonic modules containing a low timing jitter optical laser source and an optical gate, we demonstrated potential all-optical sampling of a radio signal carried at &gt;5GHz with a 6 bits resolution.

We experimentally characterize high-power (up to 2 W) and low-noise lasers, based on counter-pumped discrete Raman amplifiers, for efficient first- and higher-order distributed Raman co-pumping. We show that a proper optimization of the active fiber parameters effectively minimizes the output pump RIN.

Active open-vent volcanoes produce intense infrasound airwaves, and volcanoes with prominent craters can create strongly resonant signals, which are inaudible to humans, and often peak around 1 Hz. Study of volcano infrasound is used to model eruption dynamics, the structure of volcanic craters, and can be used as a component of volcano monitoring infrastructure. We have developed a portable on-site real-time sonification device that emits an audible sound in response to an infrasonic airwave. This device can be used near an active volcano both as a real-time educational aid and as an accessible tool for monitoring the state of volcano activity. This paper presents this device with its hardware and software implementation, its parameter mapping sonification algorithm, recommendations for its use in the field, and strategies for future improvements.

&amp;lt;p&amp;gt;Volcanoes are increasingly better monitored around the world. Nonetheless, the detection and monitoring of volcanic ash plumes remains difficult, especially in remote areas. Intense electrical activity and lightning in volcanic plumes suggests that electrical monitoring of active volcanoes can aid the detection of ash emissions in near real-time. Current very low frequency and wide-band thunderstorm networks have proven to be able to detect plumes of large magnitude. However, the time delay and the relatively high number of non-detected explosive episodes show that the applicability of these systems to the detection of smaller (and often more frequent) ash-rich explosive events is limited. Here we use a different type of thunderstorm detector to observe electrical discharges generated by the persistent Vulcanian activity of Minamidake crater at Sakurajima volcano in Japan. The sensors consist of two antennas that measure the induced current due to the change in electric field with time. In contrast to the current thunderstorm networks, these sensors measure within the extremely low frequency range (1-45 Hz) and can detect lightning up to 35 kilometres distance.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Two detectors were installed at a distance of 3 and 4 kilometres from Minamidake crater and recorded almost continuously since July 2018. Within this period, the ash plumes reached a maximum height of 5.5 kilometres above the crater rim. Using a volcanic lightning detection algorithm and the catalogue of volcanic explosions compiled by the Japan Meteorological Agency (JMA), the number of electrical discharges was determined for each individual explosive event. In addition, the start of electrical discharges was compared to the eruption onset estimated by the JMA.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Preliminary results show that the detector closest to the crater had the highest detection efficiency. It detected electrical discharges during 60% of the eruptions listed by the JMA. This is significantly higher than for the World Wide Lightning Location Network, which detected electrical discharges (in the very low frequency range) within 20 kilometres of Sakurajima for less than 0.005% of the eruptions. Furthermore, the results show that for 40% of the detected eruptions, electrical discharges were detected before the estimated JMA timing. Hence, electrical discharges can mark the inception of the explosion with a higher precision and are an indication of ash emission. This demonstrates the value of the cost-effective sensors used here as a monitoring tool at active volcanoes.&amp;lt;/p&amp;gt;

The measurement of gravity over active volcanoes is usually aimed either at the development of sub-surface structural models1–4 or the investigation of temporal changes in gravity associated with magma movements5–13. The positive gravity anomaly over Poas volcano, Costa Rica4 indicates the presence of a cylindrical pipe of denser material extending to several kilometres depth. Although the volcano is active, this feature has remained apparently stable for at least the past four years. However, our programme of high-precision microgravity studies indicates changes with amplitudes of 120 µGal at the crater rim and 50 µGal on the flanks with a periodicty of ∼30 days. These data are interpreted in terms of either relative movements of several metres between the magma body and the stable volcanic edifice or of cyclic density change related to the degree of vesiculation or crystallization of the magma: there is a possibility that these variations are tidally induced. Such studies may have wider relevance in analysing previously unrecognized components of microgravity change and in the monitoring of active volcanoes.

Fuego volcano (Guatemala) is one of the most active and hazardous volcanoes in the world. Its almost persistent activity generates lava flows, pyroclastic density currents (PDCs) and airfall tephra. After the emplacement, its deposits are often eroded and mobilized by heavy rains, forming lahars. These processes cause frequent morphological change and threaten the surrounding areas. Fuego was very active in 2018, producing three main eruptions in February, June and November. This study mainly focused on the first of these events, later compared to the June event. A violent-Strombolian eruption happened between January 31st and February 1st producing lava fountains, lava flows directed SW from the summit, and PDCs down the east flank. A significant portion of the material from this activity was deposited in three of the seven channels -known locally as barrancas - which surround the edifice. An important amount of ash was also ejected and blow by the wind to the west. The activity was monitored by a webcam, recording the complete succession of events. Inaccessible terrain at Fuego makes ground-based assessment of recent eruptive deposits very challenging. Earth-orbiting satellites can provide unique observations of volcanoes during eruptive activity - when ground-based techniques may be too hazardous - and also during inter-eruptive phases, but have typically been hindered by relatively low spatial and temporal resolution. This study tested the use of a new source of Earth observation data for volcano monitoring: high resolution (~ 3 m pixel size) images acquired from a constellation of over 150 CubeSats (‘Doves’) operated by Planet Labs Inc. The Planet Labs constellation provides high spatial resolution at high cadence (

Very low frequency noise laser sources are key elements of many applications, such as: atom or ion optical clocks, ultra stable microwave (MW) frequency generation, gravitational wave detection , ultra stable optical frequency transfer, and so on.The laser frequency locking technique developed by Pound, Drever and Hall (PDH) is widespread as the commonly used method of laser frequency stabilization on optical cavities, and it has been successfully demonstrated on lasers of various wavelengths. It led to a fractional frequency instability lower than 10 -15 for 1 s averaging times and subhertz line width. This approach has intrinsically two weaknesses. First, it requires fine alignment of free space optical components, tight polarization adjustment, and spatial mode matching. In addition, the cavity has to be housed in a high vacuum enclosure with thermal radiation shielding. This makes the system relatively expensive, bulky and fragile. The second weakness is that the PDH method does not allow tuning the laser frequency. An alternative method is to use a two arm (Michelson or Mach-Zehnder) interferometer to measure the frequency fluctuations during a fixed time delay. This method requires a relatively large arm imbalance to obtain sufficient frequency discriminator sensitivity. Indeed, with a Michelson interferometer, the quality factor is proportional to the fiber delay. For example, using a 5 km fiber delay line the quality factor of the interferometer is about 30 billions for a 1.55 μm wavelength laser, which is equivalent to the quality factor of a 10 c m Fabry Perot cavity with finesse about 230 thousands. This stabilization technique allows a more robust, simpler, cheaper, transportable and frequency tunable laser with low frequency noise. In the experiment, the author use the frequency shifted heterodyne Michelson interferometer to stabilize laser frequency. Since the laser frequency can be chirped by setting a frequency offset onto the demodulation signal, the laser was called an “agile laser”.

Received signal from weather satellite is overlaid with WDM RoF technique on 10.7Gb/s (OTU2) data stream. For extending the signal transmission distance between the antenna and the receiver, a low noise amplifier is designed by using an EP-HEMT to achieve lower noise and higher gain so that the received signal level is as low as −130dBm. The received signal is amplified by using a laser driver which is also designed by using the EP-HEMT. A low noise and linear 1550nm DFB laser is modulated by the laser driver. The converted optical signal is overlaid on a 10.7Gb/ (OTU2) data streams by using a 1310/1550nm WDM. A clear image from the weather satellite is received after 40km SMF. There was no degradation in the eye diagram and total jitter of the 10.7Gb/s data signal.