Ceramic substrate with negative thermal expansion for athermalization of Fiber Bragg Gratings

Several packaging methods for athermalization of Fiber Bragg Grating(FBG), which is equipped with negative expansion substrates, have been proposed. H owever, those methods have some deficiency resulted from the substrates suc h as complex structure or poor thermal expansion characteristics. In order to provide a suitable substrate for the athermalization of FBG, the authors have developed a Negative Expansion Ceramic Substrate (NECS) which has sim ple structure and suitable thermal expansion characteristics. NECS consists of polycrystalline beta-quartz solid solution (Li2O-Al2O3-nSiO(2), n > 2), and has thermal expansion coefficient of about -65 to -85 X 10(-7)/degrees C, which is sufficient large enough for total compensation of the Bragg wa velength shift. No difference in the thermal expansion was observed between the specimen as prepared and the one on which an epoxy adhesive was applie d. NECS is produced by means of a sintering method, which enables flexible design of the chemical composition. It was found that the hysteresis in the rmal expansion of the NECS depends upon the chemical composition and crysta lline structure. We decreased thermal expansion hysteresis by controlling t he SiO2 ratio in the composition and the crystal grain size. We confirmed t hat the temperature dependence of the FBG mounted on the NECS with an epoxy adhesive was decreased to -2.3 X 10(-3) nm/degrees C from 10.0 X 10(-3) nm /degrees C, in good agreement with the calculated value of -2.6 X 10(-3) nm /degrees C. The hysteresis in Bragg wavelength shift was less than 0.03 nm, that is sufficiently small enough for practical use. It was confirmed that NECS has suitable thermal expansion characteristics for the athermalizatio n of FBG.