A Simple All-Optical Water Level Monitoring System Based on Wavelength Division Multiplexing with an Arrayed Waveguide Grating

Abstract

We propose and demonstrate a simple water level monitoring system based on the wavelength division multiplexing (WDM) for the spent fuel pool (SFP) at a nuclear power plant. The basic principle is based on the measurement of the optical power spectra by the Fresnel reflection according to the change of the refractive index at the end facet of the optical fiber tip (OFT). An arrayed waveguide grating (AWG) is employed to achieve multi-channel sensing capability with a C-band broadband light source (BLS) based on amplified spontaneous emission (ASE). The feasibility of the proposed scheme is investigated with a simulation and experimentation. We also investigate the limiting factor for remote transmission. The system performance is degraded by the Rayleigh backscattering of the BLS light, but it can be operated over long distances within 10 km with 5 dB of difference peak power margin.

Long-Reach DWDM-Passive Optical Fiber Sensor Network for Water Level Monitoring of Spent Fuel Pool in Nuclear Power Plant

“…The SUs are installed in the SFP where each unit represents a specific water level. Note that two channels of AWG (e.g., the first channel 1 and the last channel N) are used as reference channels that are supposed to be in the air all the time for comparison to other channels [ 15 ].…”

“…The spectrum-sliced ASE lights are delivered to the SUs of SFP. At the end of the SU’s fiber-optic connector, a small portion of the light is back-reflected towards the EOF site, where the reflectance depends on whether the fiber-optic connector is in the air or in the water [ 15 ]. The reflected light is multiplexed by the AWG and sent back to the monitoring station via the SMF.…”

“…In the simulation, we assumed that the refractive index of fiber ( ) is 1.4492, air ( ) is 1.0002739, and water ( ) is 1.3152, respectively [ 16 ]. Note that we considered the refractive indices to be constant within the C-band as those are nearly independent of the wavelength and temperature [ 15 ]. Based on Equation (1), the power coefficients for the Fresnel reflection are approximately −26.3 dB (0.23% power reflection) in the water ( ) and −14.7 dB (3.36% power reflection) in the air ( ), respectively.…”

“…For this, we utilize the ASE light (i.e., BLS in Figure 1 ) and a 32-channel flat-top passband AWG with channel spacing of 100 GHz and 3 dB bandwidth of 80 GHz, subsequently enhancing the measurement resolution. The output light of the BLS has a large bandwidth (>30 nm) where its spectral flatness is less than 2.5 dB [ 15 ]. The AWG has relatively low (e.g., <5 dB) insertion loss, which does not visibly vary for wavelengths (i.e., channels).…”

“…Recently, we proposed and demonstrated a simple all-optical water level monitoring method based on the use of spectrum-sliced amplified spontaneous emission (ASE) light, for its application in nuclear power plant safety [ 15 , 16 ]. This method provides a simple way of multichannel sensing, at a high spatial resolution for low insertion loss, having great robustness to external temperature changes.…”