Immune to electromagnetic interference and able to provide multiplexed data along very long lengths of a railway or pipeline for the in situ single-headend measurement of vibration, strain, temperature, and fault locations or other problems, optical-fiber-based distributed sensors can be a better choice for structural health monitoring than discrete piezoelectric-based systems.
Among fiber-optic sensing architectures, fiber Bragg grating (FBG)-based accelerometer systems are more than adequate for perimeter security and other applications where events span frequencies <700 Hz and acceleration shock values of <30 G with sensitivities of about 16 pm/G. However, fiber sensors are desired with higher acceleration peak value and broader frequency ranges for railway monitoring applications.
In a first-of-its-kind demonstration, researchers at The Hong Kong Polytechnic University (Kowloon, Hong Kong) have developed a fiber-optic accelerometer using a Sagnac interferometer (SI) architecture with polarization-maintaining photonic-crystal fiber (PM-PCF) that achieves adequate >40 G acceleration values, but with 8 pm/G sensitivity for frequency values up to 1 kHz and is operable up to 2.5 kHz.1 The vibration sensor has been applied successfully to monitor track integrity in a railway application.
Exploiting photonic-crystal fiber sensitivity
Depending on the external parameter to be measured, PCF air-hole structures in the fiber cross-section can be modified to optimize the stress-induced measurement effect. Here, for the vibration-sensitive accelerometer, a commercially available length (about 0.35 m) of PM-PCF from NKT Photonics (Birkerød, Denmark) was coiled (in a 15 mm diameter) between two arms of a 50/50 splitter (3 dB coupler) in a SI configuration and sandwiched between a 56 g stainless-steel mass (25 mm diameter, 15 mm height) that applies lateral force on the looped fiber (see figure).
The slow axis was marked along the entire length of the PM fiber and it was carefully coiled such that the axis orientation was uniformly maintained in one plane as the fiber was epoxied to the substrate, ensuring that the vibration force was applied to only one polarization axis of the PM-PCF.
By launching broadband light into the coupler and monitoring the interferometric fringe pattern, lateral forces applied to the PM-PCF from one polarized axis produce birefringence changes that directly correlate with force parameters, such as vibration. By placing the accelerometer on a moving train, the reflection spectrum can be monitored in real time to pinpoint problems with the track of a railway—for example, indicating cracks, corrugations, or points of weakness in the track.
Experimental measurements for the device subjected to vibration frequencies of 100, 400, 700, and 1000 Hz show a signal-to-noise ratio (SNR) of >25 dB free of harmonic frequencies and with increasing acceleration values. The operational frequency of the PM-PCF accelerometer can reach 1 kHz without any significant fluctuation in the approximate 8 pm/G sensitivity value—much better than FBG-based accelerometers.
In field tests on an in-service railway train comparing the accelerometer to both FBG-based and piezoelectric types, the PM-PCF accelerometer performed comparable to the PZT device, but better than the FBG device in detecting a known track break.
“We have fabricated a novel microstructured optical fiber that increased measurement sensitivity of available systems up to 5X and we plan to use this new fiber to build additional accelerometers for field tests later this year,” says Zhengyong Liu, senior research fellow in the Photonics Research Center at The Hong Kong Polytechnic University. Professor Hwa-yaw Tam, team leader of the project, adds, “The novel accelerometer is an important component of an all-optical fiber sensing network that provides a huge amount of data to support the implementation of AI for predictive maintenance of railways.”
REFERENCE
1. Z. Liu et al., Opt. Express, 27, 15, 21597–21607 (Jul. 22, 2019).
