In the field of fiber optic sensing, the quest for longer measurement ranges, higher spatial resolution, and increased multiplexing capacity has consistently been a catalyst for innovation. The new study published in Optics Letters by Dr. Bing Han from Northeastern University and Dr. Han Wu from Sichuan University represents an important study in this endeavor. Their research focuses on the utilization of sixth-order random fiber lasing amplification (RFLA) in chaotic Fiber Bragg Grating (FBG) sensing systems, achieving the longest correlation optical time-domain reflectometry (COTDR) up to now.
FBGs have long been recognized for their remarkable sensitivity to environmental factors such as strain and temperature changes. Their unique properties, including high sensitivity, immunity to electromagnetic interference, and tolerance to harsh environments, have made them indispensable in high-precision measurement applications. To enable quasi-distributed sensing using FBGs, multiplexing techniques like wavelength division multiplexing, time division multiplexing (TDM), space division multiplexing, and combinations of these methods have been developed. TDM-based systems rely on optical time-domain reflectometry (OTDR) to interrogate FBGs along a single fiber link. But the spatial resolution of conventional OTDR is typically limited to several meters due to pulse width constraints.
The demand for higher spatial resolution and longer measurement ranges in fiber systems has driven the development of novel sensing techniques. COTDR emerged as a promising solution by using continuous wave (CW) chaotic lasing as the probe light. COTDR analyzes the cross-correlation function of the echo signal and chaotic reference light to achieve high spatial resolution, independent of measurement range, extending over tens of kilometers.
Wavelength-scanning COTDR, employing a wavelength-tunable chaotic light, was introduced for FBG sensing and location with high spatial resolution. However, the demonstrated chaotic FBG sensing systems had limited sensing distances, restricting their practical applications.
To extend the working distance of COTDR, researchers turned to distributed Raman amplification (DRA) pumped by low-noise semiconductor lasers. This approach led to the development of a 100-km-long COTDR capable of reflection event location with an impressive spatial resolution of 8.2 cm. Higher-order DRA promised further extensions in signal transmission distance, but cavity-based fiber lasers are mainly employed as Raman pump sources.
While cavity-based fiber lasers have been used effectively in DRA, they present a challenge due to their autocorrelation function (ACF) showing a time-delay signature (TDS) attributed to longitudinal modes. This TDS could be transferred to chaotic light in COTDR systems through relative intensity noise transfer during DRA. Several techniques have been employed to mitigate TDS, but amplifying chaotic light along the fiber without introducing additional TDS remained a challenge.
In their study, Dr. Han and Dr. Wu introduced high-order RFLA pumped by random fiber laser as a solution to the TDS problem of the long-distance COTDR. Via employing a 1090 nm ytterbium-doped random fiber laser as the pump source, cascaded random Raman fiber lasing, generated over a long fiber span, provides up to sixth-order distributed amplification for chaotic probe light and its echo signal at 1.55 μm. This innovation significantly extends the sensing distance of COTDR. As a proof-of-concept, the researchers successfully located three FBGs with 6-cm spatial resolution in the end of the 152 km-long fiber span and achieved temperature sensing of a specific FBG with a temperature sensitivity of 0.25 dB/°C and negligible cross talk. The achieved sensing distance sets a new record for COTDR.
The introduction of sixth-order RFLA in chaotic FBG sensing systems opens up exciting possibilities for a wide range of applications. The ultra-long sensing distance, high spatial resolution, and multiplexing capabilities make it an attractive choice for structural health monitoring, powerline monitoring, and beyond.
In conclusion, Dr. Bing Han and Dr. Han Wu’s research work in sixth-order RFLA for ultra-long chaotic FBG sensing represents an important advancement in the field of fiber optic sensing. Their creative approach has the potential to benefit various industries by enabling ultra-long-distance, high-precision sensing capabilities that were once considered unattainable.
Han B, Wu H. Ultra-long chaotic FBG sensing with high-order random fiber lasing amplification. Opt Lett. 2023 ;48(5):1280-1283. doi: 10.1364/OL.483973.