In this work we experimentally investigate the response time of humidity sensors based on polymer optical fiber Bragg gratings. By the use of etching with acetone we can control the poly (methyl methacrylate) based fiber in order to reduce the diffusion time of water into the polymer and hence speed up the relative wavelength change caused by humidity variations. A much improved response time of 12 minutes for humidity decrease and 7 minutes for humidity increase, has been achieved by using a polymer optical fiber Bragg grating with a reduced diameter of 135 microns.

The humidity response of poly(methyl methacrylate) (PMMA)-based optical fiber Bragg gratings (POFBGs) has been studied. The characteristic wavelength of the grating is modulated by water absorption-induced swelling and refractive index change in the fiber. This work indicates that anisotropic expansion may exist in PMMA optical fiber, reducing the humidity responsivity of the grating and introducing uncertainty in the responsivity from fiber to fiber. By pre-straining a grating, one can get rid of this uncertainty and simultaneously improve the POFBG response time. and affinity for water that leads to a swelling of the fiber and an increase of RI. The former feature offers a wellconditioned performance for overcoming the cross sensitivity issues existing in silica fiber [1], while the latter feature leads to an increase with humidity of the Bragg wavelength of a Bragg grating written in the fiber [2]. This is a potentially very useful property that has possible applications in the chemical processing, agricultural, food storage, paper manufacturing, semiconductor, and pharmaceutical industries, where humidity is monitored and controlled to ensure product quality. This property has also been exploited for the detection of saline concentration [3] and water in fuel [4]. There have been many reports on the use of PMMAbased fiber gratings for humidity sensing. However, the reported humidity responsivities show considerable variation (see, e.g., [1,2,5]). This variation has not been satisfactorily explained and causes great uncertainty concerning the POFBG's performance and applicability.The Bragg wavelength of a fiber grating depends on the effective core RI and the grating pitch, both of which depend on the temperature and the water content in the case of a PMMA-based POFBG. For constant temperature, the Bragg wavelength change of a POFBG against humidity can be expressed aswhere λ B is the initial Bragg wavelength, η is the normalized RI change with humidity %RH −1 , and β the swelling coefficient related to humidity induced volumetric change %RH −1 . The wavelength change in POFBG sensors varies linearly with humidity, if η and β are constant; however both η and β can be temperature or humidity dependent [6,7]. It should be pointed out that not every polymer shows this water affinity: Bragg gratings made in TOPAS polymer are insensitive to humidity [8] and are therefore suitable in applications where cross sensitivity to humidity needs to be avoided. Bragg gratings can be written into pure PMMA optical fibers by using UV illumination to induce polymerization of unreacted monomers that contribute to an increase in RI. Better performance has been achieved using dopedcore PMMA-based optical fibers [9]. Since additional dopants or copolymers are required to control the core index of step index fibers, the performance of PMMAbased fiber gratings may vary with the type of fiber used. The detailed composition of dopant and copolymer used in a specific PMMA-based optical fiber is often not known and neither are their dep...

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Abstract-The microwave photonic responses of superstructured fiber Bragg gratings in combination with dispersive fiber are investigated theoretically and experimentally. The superstructured gratings are optimized, taking account of the spectral response of the broad-band source, Erbium-doped fiber amplifier, and optical tunable filter to achieve a filter response with sidelobe suppression of more than 60 dB.Index Terms-Microwave photonic filter, superstructure fiber Bragg grating (SFBG).

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