Ceramic bestows temperature independence on gratings
Formed within the core of an optical fiber, a fiber Bragg grating (FBG) serves the same purpose for fiberoptics that a wavelength-selective dielectric coating does for bulk optics. Fiber Bragg gratings and dielectric coatings share many properties, including a shift of peak reflectance with temperature—an effect that is primarily due to the temperature-dependent refractive index of optical materials. Because the reflectance peak of an FBG is so narrow (far more so than for most narrowband reflective optical coatings), even a tiny change in temperature can negatively affect its function.
In one previous attempt to force an FBG into being temperature-insensitive, a pretensioned optical fiber containing an FBG was mounted at two points to a hybrid substrate composed of two materials with differing thermal expansion coefficients; thermal changes in the substrate caused the fiber to change length in a way that offset temperature-dependent refractive index (see figure).1 However, the mechanical arrangement was complex.
A negative-expansion ceramic substrate (NECS) developed by researchers at Nippon Electric Glass (Ostu-shi and Shiga-ken, Japan) promises to overcome the shortcomings of earlier substrates. Based on a polycrystalline ß-quartz solid solution, the NECS was produced by a sintering method free from the precipitation of coarse crystals occurring during the formation of glass in the competing glass-crystallization method. Raw materials were mixed to their prescribed compositions, formed by pressing or casting, then fired (typically at 1350°C for 15 h).
A variety of substrates were created, each with slightly different chemical composition and firing conditions. Choosing one specimen for its desirable thermal-expansion coefficient, the researchers determined that it had a highly linear thermal expansion. As do most ceramic materials with grain-boundary gaps, the NECS showed hysteresis By testing the various compositions, the researchers showed that a higher concentration of silicon dioxide produced a lower hysteresis. Increasing both grain size and proportion of silicon dioxide resulted in a NECS with acceptably low hysteresis.
A pretensioned FBG was mounted at two points (using epoxy glue) to a groove in a 3 x 3 x 40-mm NECS. The substrate had a thermal-expansion coefficient of -82 x 10-7/°C. The resulting experimental wavelength shift of the FBG versus temperature was -2.3 x 10-3 nm/°C, contrasting with a value of 10.0 x 10-3 nm/°C for an unmounted FBG. The Bragg wavelength shift recorded under repeated heat cycles between -40 and 85°C showed a hysteresis of less than 0.03 nm, a value sufficiently small for practical use.
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- G. W. Yoffe, P. A. Krug, F. Ouellete, and D. Thorncraft, OFC '95 Tech. Digest, 134 (1995).