Plastic Optical Fibre Sensors for Structural Health Monitoring: A Review of Recent Progress

Table 1

Sensor performance and other comparison criteria.

POF sensing technology	Type of POF used	Sensor type	Strain resolution	Dynamic range	Relative sensor cost	Relative equipment cost	Key features & remarks
Intensiometric
[3, 4, 7, 9–11, 14, 28, 34]	SI MMF	Intrinsic	∼50 𝜇𝜀	∼1.2%	Very low (<US$1/m)	Very low (<US$200)	Simple design; high cost-effectiveness; demonstrated for flexural and axial loading, crack detection in concrete; monitoring low-velocity impact in advanced composites; POF embedded in fibre composites for axial strain measurement; capable of monitoring quasistatic and dynamic loading (∼30 Hz or higher).
[12–15, 35, 36]	SI MMF	Extrinsic	∼5 𝜇𝜀 (liquid-filled type);	∼0.12%	Very low (<US$1/m)	Very low (<US$200)	Simple design; high cost-effectiveness; though strain limit demonstrated up to 40%, higher strains possible; demonstrated for crack detection, monitoring crack-width opening, impact damage, geotextile strain, capable of monitoring quasistatic and dynamic loading (∼1 kHz or higher).
			∼100 𝜇𝜀   (air-filled type)	∼40% or more
OTDR-based
[37–39]	SI MMF	Intrinsic	∼0.5%	∼50%–100%	Very low (<US$1/m)	Very high (>US$50,000)	Distributed measurement; High strain measurement, however, POF axial strain elastic limit ∼5%; general ability to discriminate loading types based on backscatter shape trace; quasistatic measurement only. Demonstrated to be capable of supporting 100% strain applied at 5 locations along a 100 m POF.
			Spatial resolution: ∼2 m for 50 m POF to 5 m for 100 m POF
[32, 33, 40, 41, 44]	SI & GI MMF	Intrinsic	∼0.1% (SI POF)	∼40%	Very low (<US$1/m)	Very high (>US$50,000)	Distributed measurement of 100 m (SI POF) and 500 m (GI POF); high strain measurement; demonstrated for monitoring geotextile strain, geotextile-retrofitted masonry structures; generally only quasistatic measurement only or very low frequency oscillation (∼0.25 Hz) if high-speed OTDR unit used.
			∼2% (GI POF)
			Spatial resolution: ∼0.1–1 m for 100 m (GI POF)
Interferometric
[30, 31]	SM POF	Intrinsic	0.05 𝜇𝜀 (for gauge length of 0.1 m)	15.8%	High (∼USD$50/m)	High (∼USD$6000–USD$7000)	Large strain measuring capability; High measurement precision; less intrusive due to smaller fibre diameter; Difficulty in cleaving and coupling the single mode fibres in the field; strain rate of ∼3% strain/min possible.
[43]	SI MMF	Intrinsic	10 𝜇𝜀	500 𝜇𝜀	Very low (<US$1/m)	Low (<US$1000)	Simple concept; demonstrated dynamic monitoring on a rotor blade up to 5 Hz.
Grating-based
[17, 45, 46]	PMMA; SM POF	Intrinsic	<1 𝜇𝜀	3.6%	Currently not commercially available. Estimated cost: High (∼>US$100)	High to very high ∼US$10,000–US$30,000 for OSA-based interrogators and broadband light source	Bragg gratings inscribed on in-house made dye-doped PMMA perform with UV-beam; strain measurement conducted by means of simple tension of POF. Scheme proposed allow temperature and strain to be measured simultaneously; depending on interrogator used, the sensor could monitor quasistatic and dynamic loading conditions.
[20–25]	mPOF	Intrinsic	∼1 𝜇𝜀	8%	Currently not commercially available. Estimated cost: High >US$100	High to very high ∼US$10,000–US$30,000 for OSA-based interrogators and broadband light source	Long period grating mechanically imprinted onto microstructured POF; linear response limited to 2% strain; creeping observed beyond 3%; viscoelastic effects noted above 2% but minimal below 2%; significant material relaxation under constant strain.
[44]	SI MMF; GI MMG;	Intrinsic	Not reported	Not reported	Currently not commercially available.	High to very high ∼US$10,000–US$50,000 for spectrometer, OTDR and broadband light source	Grating photo-induced onto tapered 0.2 mm SI-MMF and non-tapered GI POF with UV beam; OTDR used as signal interrogator instead of conventional OSA.
	mPOF	Intrinsic	Not reported estimated: ∼0.02%	∼1% for initial test	Estimated cost: High >US$100		Long period grating on mPOF integrated to geotextile; length of sensor extended using silica fibres.