Optical Fiber 1 Jacket 300 m Cladding 125

Optical Fiber -1 - Jacket ( = 300 m ) Cladding ( = 125 m ) Core ( = 5~10 m ) Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

Fiber Optic Sensors I -2 - • Advantages of fiber optic sensors – nonelectrical and remotable – allow access into normally inaccessible areas with electrical sensors – no EMI (dielectric material) – easy to embed in composites – flexibility of the sensor size (mm ~ km) o o – wide temperature range (-200 C ~ 1500 C) – capability of multiplexing – non-corrosion in the various environments Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

Fiber Optic Sensors II -3 - • Sensor systems with optical fiber – intensity based fiber optic sensor systems – fiber optic sensor systems using interferometer – fiber optic sensor systems using polarization – FBG(fiber Bragg grating) sensor systems • Advantages of FBG sensor systems – fiber optic advantage – absolute measurement – small size – multiplexing Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

-4 - • Bragg Grating Sensor • Fabry-Perot Sensor Merits absolute measurements excellent strain resolution minimal strength degradation fast response time multiplexed Demerits Strain resolution may be limited with difficulty in signal processing difficulty in multiplexing broadband light source Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

Literature Survey I - Fabrication History of FBG -5 - • K. O. Hill et al. (Appl. Phys. Lett. , 1993, Vol. 62) – phase mask method – a simpler and mass production technique Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

-6 - Grating fabrication method using a phase mask Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

Fiber Bragg Grating Reflected Signal -7 - Index Grating Fiber Core Signal In Signal Out L Index of Refraction of Fiber Core Optical Fiber 1 mm to 20 mm L ne Dn 10 -5 to 10 -3 z 1 z 2 z Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

FBG sensor wavelength-encoding operation -8 - Broadband source Input Signal Wavelength Detection I Input spectrum l I Transmitted Signal L Reflected Siganl Transmitted Signal l. B l I Reflected signal l. B l Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

Theory -9 - • Bragg wavelength by Bragg condition : Effective refractive index : grating period • Center wavelength shift by external interference • The relationship between strain and center wavelength shift only when DT=0 Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

FBG Sensor system by WSFL - 10 - • Wavelength-division-based multiplexed FBG sensor system WSFL I scanning waveform e 2 e 1 I Input spectrum l e 3 I l 1 l e 4 I l 2 l Wavelength encoded returns I l 3 l l 4 l VFP Filter Controller swept range Dl 1 Dl 2 Dl. N source profile I output l 1 l 2 l. N Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

Wavelength Swept Fiber Laser - Broadband Source - 11 - Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

- 12 - Peak-hold optical spectrum by OSA Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

EFPI 센서의 측정원리 - 13 - Io I 1 I I 2 s Gauge Length Inor = A ( 1 + B cos 2 ks ) Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory

EFPI signal - 14 - Department of Aerospace Engineering 11/29/2020 Smart Structures and Composites Laboratory