9:23 AM - 9:38 AM
[STT42-02] Long Distance DAS Measurement using Bidirectional Optical Amplifier and Repeater: Verification in the Tokaido Shinkansen Line Optical Fiber
Keywords:DAS, bi-directional optical amplification repeater,, long-distance measurement, seismic observation
In DAS measurements, noise due to light attenuation occurs as the measurement distance increases. We report a DAS measurement result with an application of bi-directional optical amplifiers to the optical communication cables laid along the Tokaido Shinkansen tracks.
We have been conducted DAS measurement with optical cables owned by JR Central along the Tokaido Shinkansen line (Yoshimi et al. JpGU2021, SSJ2022). The measured distances in these observations were about 70 km in maximum, a significant S/N decrease was observed beyond 50 to 60 km, which was caused by light attenuation.
As DAS equipment is currently expensive, devise ways to extend the measurement distance of a single device is desired. Optical cables installed by the Shinkansen line are routed into optical junction boxes every 30 to 45 km. We inserted optical amplification repeaters there to test the long-distance measurement capability of DAS. Two routes were set up for the verification: an about 180 km section from Kyoto to Shin-Osaka with a turnaround at Maibara, and an about 112 km section from Kyoto to Gifu-Hashima Station.
The DAS equipment used was an AP Sensing N5226B R100, which was modified to process data up to 200 km with the cooperation of the manufacturer. The optical amplification repeater was a "bidirectional amplifier" consisting of two EDFAs facing away from each other and coupled via circulators to amplify both the light from the interrogator and the Rayleigh backscattered light (cooperated with OCC Corp., NK Systems Ltd.). To stabilize the operating point of the EDFAs, gain clamp light (continuous light) was wavelength-multiplexed with the DAS pulse light and sent out. The EDFA on the pulsed light side was equipped with a bandpass filter passing only the DAS pulse light and clamp light, and the EDFA on the returned light side was equipped with a bandpass filter that passes only the Rayleigh backscattered light, to suppress the effect of the broadband noise light (ASE light) emitted by the EDFAs.
Figure 1 shows waterfall diagrams comparing the effect of the optical amplification repeaters over approximately 181 km. The vibration of the train can be recognized. Without the amplification repeater, the noise increases beyond about 100 km, and the tracks gradually become difficult to distinguish. While with optical repeaters, low noise measurements can be made up to 181 km.
Figure 2 shows the OTDR trace. Since the OTDR is measured using the DAS signal light, it is useful for understanding the behavior of the DAS signal light in the distance direction. Without amplifiers, the Rayleigh backscattered light becomes undetectable beyond 110 km, while with amplifiers, the backscattered light can be measured up to 181 km away. In this case, appropriate level adjustment of the amplification was necessary.
Figure 3 shows the waterfall diagram during the earthquake occurred at 13:49 and 13:50 on January 27, 2023, while a 112 km fiber route was observed using the amplifiers. The propagation of seismic waves over a distance of about 100 km could be recorded by a single DAS.
In general, there is a trade-off between spatial resolution and loss budget in DAS measurements. Coarse spatial resolution increases the loss budget and measurements up to about 100 km can be made without amplifiers (Figure 1, upper diagram): while with finer spatial resolution, the signal is limited only over a short distance of several 10 km because of less loss budget resulted by reduced power of the delivered pulsed light. It has been difficult to achieve both long-distance measurement and high spatial resolution in DAS measurement, but by using the optical amplifying repeaters in combination, measurement with higher spatial resolution becomes possible. Figure 4 shows an example of measurement of the same section as in Figure 3 with a gauge length of 3.75 m.
We have been conducted DAS measurement with optical cables owned by JR Central along the Tokaido Shinkansen line (Yoshimi et al. JpGU2021, SSJ2022). The measured distances in these observations were about 70 km in maximum, a significant S/N decrease was observed beyond 50 to 60 km, which was caused by light attenuation.
As DAS equipment is currently expensive, devise ways to extend the measurement distance of a single device is desired. Optical cables installed by the Shinkansen line are routed into optical junction boxes every 30 to 45 km. We inserted optical amplification repeaters there to test the long-distance measurement capability of DAS. Two routes were set up for the verification: an about 180 km section from Kyoto to Shin-Osaka with a turnaround at Maibara, and an about 112 km section from Kyoto to Gifu-Hashima Station.
The DAS equipment used was an AP Sensing N5226B R100, which was modified to process data up to 200 km with the cooperation of the manufacturer. The optical amplification repeater was a "bidirectional amplifier" consisting of two EDFAs facing away from each other and coupled via circulators to amplify both the light from the interrogator and the Rayleigh backscattered light (cooperated with OCC Corp., NK Systems Ltd.). To stabilize the operating point of the EDFAs, gain clamp light (continuous light) was wavelength-multiplexed with the DAS pulse light and sent out. The EDFA on the pulsed light side was equipped with a bandpass filter passing only the DAS pulse light and clamp light, and the EDFA on the returned light side was equipped with a bandpass filter that passes only the Rayleigh backscattered light, to suppress the effect of the broadband noise light (ASE light) emitted by the EDFAs.
Figure 1 shows waterfall diagrams comparing the effect of the optical amplification repeaters over approximately 181 km. The vibration of the train can be recognized. Without the amplification repeater, the noise increases beyond about 100 km, and the tracks gradually become difficult to distinguish. While with optical repeaters, low noise measurements can be made up to 181 km.
Figure 2 shows the OTDR trace. Since the OTDR is measured using the DAS signal light, it is useful for understanding the behavior of the DAS signal light in the distance direction. Without amplifiers, the Rayleigh backscattered light becomes undetectable beyond 110 km, while with amplifiers, the backscattered light can be measured up to 181 km away. In this case, appropriate level adjustment of the amplification was necessary.
Figure 3 shows the waterfall diagram during the earthquake occurred at 13:49 and 13:50 on January 27, 2023, while a 112 km fiber route was observed using the amplifiers. The propagation of seismic waves over a distance of about 100 km could be recorded by a single DAS.
In general, there is a trade-off between spatial resolution and loss budget in DAS measurements. Coarse spatial resolution increases the loss budget and measurements up to about 100 km can be made without amplifiers (Figure 1, upper diagram): while with finer spatial resolution, the signal is limited only over a short distance of several 10 km because of less loss budget resulted by reduced power of the delivered pulsed light. It has been difficult to achieve both long-distance measurement and high spatial resolution in DAS measurement, but by using the optical amplifying repeaters in combination, measurement with higher spatial resolution becomes possible. Figure 4 shows an example of measurement of the same section as in Figure 3 with a gauge length of 3.75 m.