The measurement accuracy of dynamic fiber-optic sensing interrogators, which use frequency scanning to determine the value of the measured, err as either the event bandwidth approaches half the instrument sampling frequency or when the event dynamic range comes close to the instrument designed value. One main source of error is the common practice of assigning sampling at a non-uniform grid to a uniform one. Harmonics higher than -20 dB are observed for signal frequencies exceeding 25% of the sampling rate and/or for signal amplitudes higher than 15% of the instrument dynamic range. These findings have applications to fiber-Bragg-grating and Brillouin interrogators.
The ability of Brillouin-based fiber-optic sensing to detect damage in a moving cantilever beam is demonstrated. A fully
computerized, distributed and high spatial resolution (10cm) Fast-BOTDA interrogator (50 full-beam Brillouin-gain-spectra
per second) successfully directly detected an abnormally stiffened (i.e., ‘damaged’) 20cm long segment in a 6m
Aluminum beam, while the beam was in motion. Damage detection was based on monitoring deviations of the measured
strain distribution along the beam from that expected in the undamaged case.
Embedded fiber-optic strain sensing networks for airworthy assessment of operational Unmanned Aerial Vehicles (UAVs) are presented. Sensing is based on in-flight fiber Bragg grating technology, as well as on on-ground Rayleigh backscattering distributed strain sensing. While the in-flight instrumentation monitors loads, looking for excessive values, the Rayleigh-based technique is used for high spatial resolution strain distribution along the UAV wings, under prescribed loading. Consistency of measurements over time indicates structural integrity. Simultaneous strain measurements using both distributed Rayleigh and fiber Bragg gratings, on the same fiber, promises to combine high spatial resolution, though practically static measurements with dynamic, though discrete ones.
The effect of the gain dependency of the Brillouin linewidth on double-slope-assisted Brillouin optical domain, dynamic sensing techniques is studied. These double-slope methods are immune to pump-power related peak Brillouin gain variations, however, they are not immune to gain-dependent variations of the shape of the Brillouin gain spectrum. For a 15ns pump pulse, this gain-dependent shape of the normalized Brillouin gain is shown to lead to a 5.5 [%/dB] strain error when the double-slope sum-difference approach is used, compared with a 7 [%/dB] error when the double-slope ratio approach is used. Ways to compensate for these issues are discussed.
Using a tailored-frequency probe, this paper presents a distributed slope-assisted BOTDA (SA-BOTDA) measurement of an optical fiber, bonded to a bent cantilever, where the fiber's static Brillouin Frequency Shift continuously varies along its length by an amount of the order of or larger than the Brillouin bandwidth. While standard SA-BOTDA methods, employing a fixed probe frequency, may suffer from distortions in measuring vibrations around this Brillouininhomogeneous static state, a tailored probe preserves the full dynamic range offered by the slope of Brillouin gain spectrum. The computerized measurement system is capable of real-time continuous monitoring of the strain, including averaging.
We report a Brillouin-based fully distributed and dynamic monitoring of a strain wave, propagating at a speed of
~4km/sec in a 6m long fiber. Using an audio speaker, a mechanical impulse was introduced to one end of a 6m long
optical fiber, initiating a strain wave propagating towards the other fixed end of the fiber. Employing a simplified
version of the Slope-Assisted Brillouin Optical Time Domain Analysis (SA-BOTDA) technique, the whole length of the
fiber was interrogated every 1μs (before averaging). A dynamic spatially and temporally continuous map of the strain was obtained, from which the wave velocity could be deduced. With a trade-off among sampling rate, range and signal to noise ratio, kHz sampling rates and hundreds of meters of range can be covered with spatial resolutions down to a few centimeters.
We report a 10cm spatial resolution in a Brillouin-based distributed sensing system using two simultaneously launched
gain and loss pump pulses, having slightly different durations. Post-recording subtraction of the probe signal, excited by
the shorter pulse, from the corresponding one, obtained from the longer pump pulse, is no longer required, since it is
done automatically by the overlapping parts of the gain and loss pump pulses. Using a 30ns gain pump pulse and a 29ns
loss pump pulse we were able to improve upon previously published results, achieving a distributed strain measurement
along a standard single mode optical fiber with a spatial resolution of ~10cm. This technique does not broaden the
involved Brillouin gain spectra so that the strain/temperature sensitivity is not compromised.
We present two recently developed new methods for fast and distributed strain/temperature sensing in optical fibers,
based on Brillouin optical time domain analysis (BOTDA). Both methods make use of very fast (<1ns) and controlled
tuning of the optical frequency of a light wave. In the Fast BOTDA (F-BOTDA) method the complete Brillouin gain
spectrum is scanned, while in the Slope-Assisted technique (SA-BOTDA) only a single frequency point on the slope of
the Brillouin gain spectrum is probed. A sensing speed of a few hundred Hz is experimentally demonstrated, limited only
by the fiber length and the need for averaging.
The ramifications of optical fiber vibrations on the calibration phase of slope-assisted fast distributed Brillouin optical
time-domain analysis, is studied. It is theoretically and experimentally found that for not too severe vibrations the
Brillouin gain spectrum, as determined by classical BOTDA, is only negligibly broadened, still enabling the correct
estimation of its -3dB frequency points along the fiber.
Employing stimulated Brillouin scattering (SBS), we present a novel method for the quasi-simultaneous distributed
measurement of dynamic strain along an entire Brillouin-inhomogeneous optical fiber. Following classical mapping of
the temporally slowly varying Brillouin gain spectrum (BGS) along the fiber, we use a specially synthesized and
adaptable probe wave to always work on the slope of the local BGS, allowing a single pump pulse to sample fast strain
variations along the entire fiber. Strain vibrations of tens of Hertz and up to 2KHz are demonstrated, simultaneously (i.e.,
using the same pump pulse) measured on two different segments of the fiber, having different static Brillouin shifts.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.