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Fiber
Bragg Grating Longitudinal Index Profile Measurements
We assembled two systems for measuring longitudinal refractive index
profile in Fiber Bragg Gratings (FBG). One technique determines the
index profile from 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 using a low-coherence interferometric measurement
of the grating's complex reflection spectrum through a calculation process
known as inverse scattering. A comparison of the measurement results
on a nominally 1.4 mm long uniform profile FBG shows that the widths
measured with the two different techniques agree to better than 100
mm.
 High-Accuracy
Fiber Bragg Grating Sensor Calibrator
We designed, built, tested, and recently delivered a calibration instrument
for optical fiber Bragg grating (FBG) strain and temperature sensors.
This instrument provides four stabilized FBGs at different wavelengths.
The center wavelength of each FBG is well-characterized so that the
device can be used to calibrate strain sensor readout units. The FBG
center wavelengths were accurately measured using both a tunable laser
measurement system and low-coherence interferometry. Uncertainty analysis
on both techniques yielded a 2 pm expanded uncertainty on the center
wavelengths of the FBGs in the calibration instrument.
High-Spectral-Resolution
PMD Measurement System Completed
We have completed characterization of a 2.46 GHz Modulation Phase Shift
(MPS) system for the measurement of PMD in narrow spectral bandwidths.
We achieve a single-measurement uncertainty of 40 fs in an approximately
40 pm bandwidth. Averaging (along with reorientation of the fiber leads)
brings this value down to 9 fs. MPS is a useful technique for PMD measurement
because it is capable of measuring PMD in a narrow bandwidth with a
high spectral efficiency.
Chromatic
Dispersion Error Source Identified as ASE
We have demonstrated that modulation phase shift measurements of
chromatic dispersion can be negatively effected by the presence of Amplified
Spontaneous Emission (ASE) from the source laser. We found errors as
large as 3 % in measurements of chromatic dispersion in fibers where
the zero-dispersion wavelength significantly overlapped the ASE spectrum.
We have quantified this effect and demonstrated that it is mitigated
by the use of a ASE-blocking filter for the laser.
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