Stimulated Brillouin scattering interactions in optical fibers involve the exchange of energy and linear momentum between the optical and acoustic fields. In most settings, the angular momenta of the two fields remain unchanged. In this presentation, I discuss the vector properties of forward stimulated Brillouin scattering processes in standard, single-mode fibers. The process make take place through guided acoustic modes of two-fold azimuthal symmetry, which appear in degenerate pairs. Analysis shows that a pair of optical pump waves, with circular and orthogonal polarizations, may stimulate a superposition the pair of acoustic modes. That superposition takes up the form of a rotating acoustic vortex beam. The stimulation process is associated with the transfer of angular momentum quanta between the optical and acoustic fields. This type of interaction adds a new dimension to stimulated Brillouin scattering processes in fibers.
In this work, we demonstrate optomechanical measurements of radiation induced alterations of the acoustic velocity in a fluoroacrylate polymer coating of a silica optical fiber. The optomechanical measurement is based on forward Brillouin scattering initiated in the fiber core which stimulates acoustic waves that reach the fiber coating. The measurement may serve as an additional metric to quantify the dose of ionizing radiation to which the fiber was exposed. We have demonstrated that the stiffness of the coating increases following gamma irradiation, as measured by the time of flight of radial acoustic waves through the coating. The measurement was performed on few meters long fiber, but can be extended to a spatially distributed analysis in longer fibers. The tests showed a linear dependence of the acoustic time-of flight on the overall dosage of gamma irradiation. The time of flight decreased by as much as 15% following exposure to 180 Mrad from a 60Co source. In a follow-up study, we found that the stiffness of the previously exposed fiber coatings continued to slowly increase over months, after extraction from the radiation field. These results reveal the vulnerability of the specific coating to ionizing radiation and the potential complexities involved with dosimetry.
A distributed clad mode optical fiber sensor is reported for the first time. Random-access coupling of light to a clad mode of a standard single-mode fiber is achieved using Brillouin dynamic gratings. Coupling is restricted to a single, few-centimeters-long section that is scanned along a fiber under test. No permanent gratings are required, and all optical fields are launched and detected in the core mode. The coupling spectrum is affected by the local refractive index of the substance outside the cladding. Distributed mapping of surrounding media is reported over 2 meters of fiber with 8 centimeters resolution. The sensitivity of outside index measurements is between 4e-4 and 4e-3 refractive index units.
Frequency domain analysis of time-resolved fluorescence measurements (TRFM) is an extremely rapid technique for medical diagnostics thanks to its unique sensitivity to a wide variety of physical and chemical features. Nevertheless, the determination of the underlying fluorescence lifetime (FLT) data of samples by their frequency response data (FRD), demands fitting algorithms. Therefore, the interpretation of the precise changes in the FLT of complex environments in term of biochemical processes is a challenge as it involves uncertainties associated with the chosen fitting algorithm. This research suggests a novel characterization procedure based on the squared distance (D2) between the FRD of the samples that avoid the inherent blurring caused by the transformation of the FRD into FLT data. The D2 approach was validated through simulated data of 6 classes with similar FLT characteristics, where the accuracy of D2 classification was about 96%. In addition, this approach was tested on experimental FRD from 43 individual samples that their preliminary physician diagnosis divided them into 4 groups: 5 healthy samples served as controls, 9 samples diagnosed with diverse types of bacteria, 16 samples diagnosed with diverse types of viruses and 13 samples were negatives to any bacterial or viral infection, although presenting related symptoms. Using the D2 analysis, the classification of 28/30 matched the physician diagnosis and the classification of 41/43 samples matched earlier report. In conclusion, this work demonstrated that the D2 model can aid in disease identification and increase the specificity and sensitivity of conventional medical procedures or TRFM-based diagnosis.
Guided acoustic waves Brillouin scattering (GAWBS) processes in standard optical fibers allow for sensing of liquids outside the cladding boundary, where light does not reach. Optical waves stimulate the oscillations of elastic modes of the fiber. The linewidths of these modes, in turn, depend on the mechanical impedance of surrounding media. These linewidths are monitored through photo-elastic scattering of optical probe waves. However, acoustic guided waves scatter light in the forward direction. The timing of forward-scattering event cannot be identified directly, hence distributed analysis based on GAWBS could not be performed, and measurements were restricted to point-sensing only. In this work we show a way around the problem. The exchange of optical power among two optical tones that stimulate the acoustic wave is monitored through careful analysis of Rayleigh back-scatter. Distributed GAWBS analysis is performed over 3 km of fiber with resolution between 100-200 meters. The measurements successfully distinguish between water and ethanol based on mechanical impedance.
Guided acoustic waves Brillouin scattering (GAWBS) was recently shown to facilitate chemical sensing outside the cladding of an unmodified fiber, even though the guided light wave never reaches the substance under test. In this work, the study of GAWBS is extended to commercially-available, multi-core fibers. We show that GAWBS leads to inter-core cross-phase modulation, between cores that are otherwise optically isolated from one another. The observed modulation spectrum is in excellent agreement with the predictions of an opto-mechanical analysis of the fiber. Use of multi-core fibers would improve the signal-to-noise ratio in GAWBS-based sensors, and may enable distributed analysis.
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