Improved thermal and strain performance of annealed polymer optical fiber Bragg gratings

Abstract

Abstract:We report on a detailed study of the inscription and characterization of fiber Bragg gratings (FBGs) in commercial step index polymer optical fibers (POFs). Through the growth dynamics of the gratings, we identify the effect of UV-induced heating during the grating inscription. We found that FBGs in annealed commercial POFs can offer more stable short-term performance at both higher temperature and larger strain. Furthermore, the FBGs' operational temperature and strain range without hysteresis was extended by the annealing process. We identified long-term stability problem of even the annealed POF FBGs. [7][8][9][10][11][12][13][14][15][16]. 325nm has been employed as a mainstream wavelength for writing grating in PMMA POFs [1][2][3][4][5][8][9][10][11][12][13][14][15]. Other wavelength such as 355nm obtained from a frequency-tripled Nd:YAG laser has been used to write grating in CYTOP fiber developed by Asahi Glass Co. and Keio University [15,16]. On the other hand, 800nm femtosecond pulses from Ti:Sapphire laser or its double frequency was mainly used for point by point direct writing [6] or grating writing with a phasemask [7]. However, the mechanism of index change does not appear to be fully understood [5,13,[18][19][20]. It is believed that more than one process is involved in the photo-induced refractive index changes and hence in the grating formation dynamics [18][19][20]. The widely accepted point is that the principle mechanism of index change is an increase due to the photo-induced polymerization of the unreacted monomers [5,[18][19][20], while laser-induced heating in the irradiated region during the inscription may also contribute to the index change [5]. Previous reports indicated that annealing of the POF before FBG inscription can relieve the frozen-in stress induced by the fiber drawing process [21] and increase the linear operation temperature range of FBGs [22]. However, the effect of annealing on the strain sensitivity performance was not yet considered. Polymer optical FBGs have shown great potential for sensor applications to sense for example temperature and strain with higher sensitivity and wider tunability than its silica counterpart [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Those advantages are due to the lower Young's modulus and higher thermo-optic coefficient of POFs [23,24]. In addition, polymers are clinically

Fabrication and characterization of polycarbonate microstructured polymer optical fibers for high-temperature-resistant fiber Bragg grating strain sensors

“…The fiber was then annealed in order to improve the stability of the strain sensing [34,47]. Specifically, the annealing was done in two phases.…”

“…This margin can be thought of as the minimum one to ensure long-term stability of a POFBG [14]. Secondly, the combination of two distinct annealing phases at 120°C and 130°C was likely to have significant impact in this regard, as early investigations showed the annealing process to considerably contribute to enhancing the thermal stability of polymeric FBGs and extending the linearity in their response to temperature [34,47]. The particular temperature of 125°C makes PC POFBGs applicable to certain areas of automation technology and automotive engineering.…”

Bandpass transmission filters based on phase shifted fiber Bragg gratings in microstructured polymer optical fibers

“…However, since the first polymer FBG reported in 1999 [22], several promising devices, mainly for the measurement of strain and temperature using step-index fiber and micro-structured fiber have been demonstrated [23][24][25]. Despite of this, for the inscription of the grating, the photosensitivity of the material is important; so far great advances have been done both in doped [26][27][28] and undoped fibers [29], but grating time stability is still an issue if the FBG is not properly annealed [24,30]. Previous literature reports that power density have an effect on time stability of the grating [31] and also a chirped Bragg grating inscribed using an KrF excimer laser at 248nm kept unchanged for 13 weeks [32].…”

Simultaneous measurement of temperature and humidity with microstructured polymer optical fiber Bragg gratings

“…Some polymers, such as PMMA, are humidity sensitive and strongly absorb water 9 , while other polymers, for instance TOPAS and Zeonex, are insensitive to humidity [10][11][12] . Exploiting such unique and intrinsic properties of POFs, researchers in this area have developed different single parameter POFBG sensors such as strain, temperature, humidity, pressure and soon [13][14][15][16][17] . Only little work has been done in the development of multi-parameter POFBG sensors.…”