Table 1.

Chronological developments of different optical fiber-based humidity sensors and their important results using different principles and materials.

Year	Type	Material	Response Time	Range (% RH)	Ref.
1985	Evanescent wave	CoCl2 doped gelatin film	<1 min	50–80	[31]
1988	Evanescent wave	Porous SiO2 optical fibre cladding	-	25–95	[32]
1988	Direct spectroscopic	Etched borosilicate opticalfibre segment doped with CoCl2	<5 min	20–50	[33]
1989	Interferometric	SiO2–TiO2–SiO2 cavity	1 min	0–80	[34]
1991	Interferometric	Nafion	<1 min	-	[35]
1995	Evanescent wave	CoCl2 doped gelatin film	1 s	20–80	[36]
1995	Direct spectroscopic	Aluminium/morin metalion–organic complex doped PVP membrane	-	0–80	[37]
1996	Evanescent wave	a surface plasmon waveguide with a thin layer of Nafionfluoropolyme	-	20–50	[38]
1997	Direct spectroscopic	Crystal violet doped Nafion film	-	40–82	[39]
1998	Direct spectroscopic	Rhodamine B doped HPC film	∼2 min	0–95	[40]
1999	Interferometric	SiO2–[Au:PDDA + /PSS-]-air cavity using ISAM technique	1.5 s	11–100	[41]
2000	Evanescent wave	Agarose gel	<1 min	30–80	[42]
2001	Interferometric	SiO2–[PDDA + /PS-119]-air cavity using ISAM technique	3 s	0–97
2002	In-fiber grating	Polyimide	-	10–90	[43]
2002	Evanescent wave	CoCl2 doped PVA film	-	S: >78 U: 3–90	[44]
2003	Evanescent wave	HEC/PVDF film	<5 s	20–80	[45]
2004	Direct spectroscopic	Porous sol-gel fibre segment doped with CoCl2	-	2–10	[46]
2004	Evanescent wave	Porous sol-gel cladding	<1 min	3–90	[47]
2005	In-fibre grating	CoCl2 doped PEO film	<1 s	I: 70–80 W: 40–80	[48]
2006	Direct spectroscopic	Ruthenium-based complex doped PTFE membrane	∼2 min	4–100	[49]
2006	Evanescent wave	PDDA/Poly R-478 nanostructured sensing overlay using ISAM technique	-	75–100	[50]
2008	Interferometric	PVA	-	33–97	[51]
2008	In-fiber grating (FBG)	PI	∼25 min	22–97	[52]
2008	Evanescent wave	Gelatin	<0.5 s	9–94	[53]
2008	In-fiber grating (LPG)	PVA	<1 min	33–97	[54]
2008	Absorption	SiO2nano-particles	<1 s	75–100	[55]
2009	In-fiber grating (FBG)	PVA	<2 s	20–98	[56]
2009	In-fiber grating (LPG)	Poly(ethylene oxide)/CoCl2	<10 s	50–95	[57]
2009	Evanescent wave	ZnO	30 s	5–90	[58]
2009	Grating + interferometric	Hydrogel	-	60–100	[59]
2009	Absorption	ITO	-	20–80	[60]
2010	In-fiber grating (FBG)	PI	-	30–80	[61]
2010	Evanescent wave	Ag-Polyaniline	30 s	5–95	[62]
2010	Absorption	Xerogel	10 s–2 min	10–70	[29]
2011	In-fiber grating (FBG)	PI	-	0–75	[63]
2011	In-fiber grating (LPG)	SiO2nano-sphere film	<1 s	20–80	[64]
2011	Evanescent wave	PVA	-	50–89	[65]
2011	Absorption	PVA/SiO2/CoCl2	<2 min	25–65	[66]
2011	Interferometric	Tin dioxide	-	2–40	[67]
2012	Absorption	Au-NP/boehmite	<20 s	-	[67]
2012	Interferometric	PVA	<6 s	20–80	[68]
2012	Evanescent wave	TiO2	<0.5 s	24–95	[69]
2013	Evanescent wave	PAA electrospunnanowires	<0.5 s	30–95	[70]
2013	Grating + interferometric	PI	-	20–80	[71]