Standard optical fibers are successfully embedded within a model wing of an unmanned aerial vehicle, constructed of carbon fiber and epoxy, during its production. Time-gated Brillouin optical correlation domain analysis along the embedded optical fibers is performed with a spatial resolution of 4 cm. Tests were carried out using a portable measurement setup prototype. The results represent an important step towards applications of high-resolution Brillouin analysis outside the research laboratory.
The scanning-free, precise reconstruction of the local value of the Brillouin frequency shift in a section of optical fiber is
proposed and demonstrated experimentally. The Brillouin shift is identified based on transient analysis of the step
response of the amplified signal wave. The measurement protocol requires a single trace, taken at an arbitrary frequency
offset between the central optical frequencies of the pump and signal waves. The frequency offset may be chosen
anywhere within ±100 MHz of the Brillouin frequency shift, corresponding to a measurement range of ±100 °C or
±2,000 μepsilon. The experimental error between the estimated Brillouin shift and its true value is less than±1 MHz. The
protocol is suitable to high-resolution Brillouin optical correlation domain analysis.
Forward stimulated Brillouin scattering (FSBS) between two co-propagating optical waves is observed in 220 meterslong sections of standard single-mode fibers. The interaction is mediated by high-order, radial acoustic modes that are supported by the fiber. The acoustic resonance frequency and the gain spectrum of an individual mode are characterized using a single-frequency stimulation procedure, with no need for frequency scanning. The measurement protocol is employed in the characterization of the temperature dependence of the resonance frequency. Good agreement with previous literature is achieved. The temperature sensitivity of the measurements is ±0.65 °C.
High-resolution Brillouin optical correlation domain analysis of fibers embedded within beams of composite materials is performed with 4 cm resolution and 0.5 MHz sensitivity. Two new contributions are presented. First, analysis was carried out continuously over 30 hours following the production of a beam, observing heating during exothermal curing and buildup of residual strains. Second, the bending stiffness and Young's modulus of the composite beam were extracted based on distributed strain measurements, taken during a static three-point bending experiment. The calculated parameters were used to forecast the beam deflections. The latter were favorably compared against external displacement measurements.
Brillouin analysis of a 8.8 km-long fiber with 2 cm resolution is reported. Pump and signal are jointly modulated by a binary phase Golomb code. Over 2,000 correlation peaks are simultaneously introduced and interrogated in a single trace. All 440,000 resolution points are covered in just 211 scans of peaks positions. The pump wave is amplitude modulated by a second, 10,000 bits long code. Post-processing of the output signal provides a measurement signal-tonoise ratio that is equivalent to that of 5,000 single-pulse acquisitions. A 7cm-long hot spot is identified. The uncertainty in Brillouin frequency measurements is ±3.5 MHz.
A combined time-domain and correlation-domain Brillouin sensing scheme is demonstrated. Pump and signal waves are phase modulated by a high-rate, perfect Golomb code, which confines their interaction to discrete, 2 cm-long correlation peaks. The properties of the code substantially reduce off-peaks interactions. The pump wave is also amplitude modulated by a single pulse, so that amplifications at different correlation peaks can be separated in the time domain. The Brillouin gain spectra at all 80,000 resolution points of a 1,600 m-long fiber are reconstructed experimentally, using only 127 scans per frequency. A 3 cm-long hot spot is recognized.
A new technique for Brillouin optical correlation domain analysis is proposed and demonstrated, in which the pump and signal waves are drawn from the filtered amplified spontaneous emission of an erbium-doped fiber amplifier. An estimated spatial resolution of 3.3 mm is obtained using a 33 GHz-wide source. The reconstruction of the Brillouin gain line and the recognition of a localized hot spot are demonstrated in a proof-of-concept experiment. Unlike phase-coded correlation domain analysis methods, the proposed scheme is not restricted by the bandwidth of available electro-optic modulators or pattern generators. Resolution is scalable to less than one millimeter.
Dynamic Brillouin gratings (DBGs), inscribed by co-modulating two writing pump waves with a bi-phase code, are analyzed theoretically, simulated numerically, demonstrated and characterized experimentally. A comparison is made between modulation by pseudo-random bit sequences (PRBS) and perfect Golomb codes. Numerical analysis shows that Golomb codes provide lower off-peak reflectivity, due to the unique properties of their cyclic auto-correlation function. Golomb coded DBGs can therefor allow for the long variable delay of one-time probe waveforms with higher signal-tonoise ratios, and without averaging. A figure of merit is proposed, in terms of the optical signal-to-noise ratio of reflected waveforms and the delay-bandwidth product of the setup. As an example, the variable delay of return-to-zero, on-off keyed data at 1 Gbit/s rate, by as much as 10 ns, is successfully demonstrated. The eye diagram of the reflected waveform remains open, whereas PRBS modulation of the pump waves results in a closed eye.
Distributed temperature measurements with 1.2 cm resolution based on stimulated Brillouin scattering (SBS) in standard
fibers are reported. High resolution is achieved by phase-coding both pump and probe waves with a high-rate, pseudo-random
binary phase code. The SBS interaction is effectively confined to narrow correlation peaks. The separation
between adjacent peaks, signifying the unambiguous measurement range, scales with the length of the modulation code
and can therefore be made arbitrarily long. Measurements were performed over 40 meters of fiber, or 3300 resolution
points. The technique is applicable to distributed measurements of birefringence and Brillouin frequency shift over
polarization maintaining fibers.
In this work, we apply a recent technique for the generation of stimulated Brillouin scattering (SBS) dynamic gratings
that are both localized and stationary to realize high-resolution distributed temperature sensing. The gratings generation
method relies on the phase modulation of two pump waves by a common pseudo-random bit sequence (PRBS), with a
symbol duration that is much shorter than the acoustic lifetime. This way the acoustic wave can efficiently build up in
the medium at discrete locations only, where the phase difference between the two waves does not temporarily vary. The
separation between neighboring correlation peaks can be made arbitrarily long. Using the proposed method, we
experimentally demonstrate distributed temperature sensing with 5 cm resolution, based on modifications to both the
local birefringence and the local Brillouin frequency shift in polarization maintaining fibers. The localization method
does not require wideband detection and can generate the grating at any random position along the fiber, with complete
flexibility. The phase-coding method is equally applicable to high-resolution SBS distributed sensing over standard
fibers.
Reflections from movable, dynamic acoustic gratings in polarization maintaining (PM) fibers are employed in the long
variable delay of periodic, isolated pulses. The gratings are introduced by stimulated Brillouin scattering (SBS)
interaction between two counter-propagating pump waves, which are spectrally detuned by the Brillouin frequency shift
of the PM fiber and are both polarized along one of its principal axes. The gratings are interrogated by the reflections of
read-out signals that are polarized along the orthogonal principal axis. High-rate phase modulation of both pump waves
by a pseudo-random binary sequence introduces dynamic gratings that are both localized and stationary, at specific
locations in which the modulated pumps are correlated. The separation between adjacent correlation peaks can be made
arbitrarily long. Long variable delays are readily obtained by scanning the grating along the fiber, via changing either the
length or the rate of the modulation sequence. At the same time, the short length of the gratings, on the order of a cm,
accommodates the delay of broadband pulses. The technique is therefore free of the delay-times-bandwidth product
limitation that undermines the performance of SBS-based 'slow light' delay: we report the delay 1-ns long pulses by as
much as 770 ns. In addition, the combined reflections from two dynamic gratings with a variable separation are used to
implement radio-frequency photonic filters of tunable free spectral range. At the current stage, the technique is restricted
by noise from residual scattering that takes place outside of the correlation peaks. Hence, it is thus far limited to the
processing of repetitive signals, for which the noise may be effectively averaged out.
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