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January, 2004 EEEL Demonstrates Fast and Accurate Index Profile Measurements for Fiber Bragg Gratings |
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Shellee Dyer, Joey Espejo, and Megan Schendel of EEEL’s Optoelectronics Division recently demonstrated two separate systems for measuring the index profile of a fiber Bragg grating (FBG). FBGs are a periodic modulation of refractive index written directly into the core of an optical fiber. This periodicity causes a narrow spectral bandwidth of the light traveling in the fiber to be selectively reflected from the FBG; therefore FBGs are frequently used as filters and dispersion compensators. The center wavelength of the reflected light is very sensitive to strain and temperature changes, making FBGs attractive as small, lightweight, networkable sensors.
One measurement system determines the index profile from a measurement of the power diffracted from the FBG when it is illuminated with a helium-neon laser at the Bragg angle. The other measurement determines the index profile from a low-coherence interferometric measurement of the grating’s complex reflection spectrum through a calculation process known as inverse scattering. An intercomparison of the measurement results on a nominally 1.4 mm long uniform profile FBG shows that the measured widths of the FBG’s index profile agree to better than 100 micrometers, and an uncertainty analysis indicates that both measurement systems have uncertainty less than 100 micrometers. The measurements of FBG profile length also agree well with a simulation based on a measurement of the FBG reflection spectrum.
The low-coherence interferometric measurement is particularly exciting since it is applicable to the simultaneous measurement of multiple gratings in a single optical fiber. Potential applications include high-spatial resolution strain and/or temperature sensing, where spatial resolutions better than 100 micrometers could be achieved. The measurement may also have exciting applications in the field of in-situ monitoring of the FBG writing process to provide active control of the FBG’s parameters. This may help improve NIST’s next-generation wavelength references that are based on FBGs.
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