Brillouin Optical Correlation Domain Analysis in Composite Material Beams.

Structural health monitoring is a critical requirement in many composites. Numerous monitoring strategies rely on measurements of temperature or strain (or both), however these are often restricted to point-sensing or to the coverage of small areas. Spatially-continuous data can be obtained with optical fiber sensors. In this work, we report high-resolution distributed Brillouin sensing over standard fibers that are embedded in composite structures. A phase-coded, Brillouin optical correlation domain analysis (B-OCDA) protocol was employed, with spatial resolution of 2 cm and sensitivity of 1 °K or 20 micro-strain. A portable measurement setup was designed and assembled on the premises of a composite structures manufacturer. The setup was successfully utilized in several structural health monitoring scenarios: (a) monitoring the production and curing of a composite beam over 60 h; (b) estimating the stiffness and Young's modulus of a composite beam; and (c) distributed strain measurements across the surfaces of a model wing of an unmanned aerial vehicle. The measurements are supported by the predictions of structural analysis calculations. The results illustrate the potential added values of high-resolution, distributed Brillouin sensing in the structural health monitoring of composites.

Double-pulse pair Brillouin optical correlation-domain analysis.

Brillouin optical correlation-domain analysis (B-OCDA) allows for distributed measurements of strain and temperature with sub-cm resolution. Time-multiplexing techniques have previously extended B-OCDA to the monitoring of many km of fiber and two million resolution points. Thus far, however, the number of scans of correlation peaks positions, necessary to cover the fiber under test, was restricted to the order of 100 or more. In this work we report a B-OCDA protocol that is able to address an entire fiber using only 11 pairs of position scans per choice of frequency. The measurements protocol relies on a merger between B-OCDA principles and double-pulse-pair analysis, previously incorporated in time-domain Brillouin sensors. Phase coding of the pump and signal waves with a repeating, short and high-rate code stimulates Brillouin interactions in a large number of narrow correlation peaks, with substantial temporal overlap. Unambiguous measurements are achieved by repeating each experiment twice, using a pair of pump pulses of different durations, and subtracting the two output traces. The principle is demonstrated in the analysis of a 43 m-long fiber with 2.7 cm resolution. Several local hot-spots are properly identified in the measurements. The experimental uncertainty in the measurement of the local Brillouin frequency shift is estimated as ± 1.9 MHz. The proposed method requires broader detection bandwidth and a larger number of averages than those of previous time-gated B-OCDA setups. Hence the overall number of measurements is similar to that of previous setups.

High-resolution Brillouin optical correlation domain analysis with no spectral scanning.

Distributed Brillouin fiber sensors typically rely on the reconstruction of the steady-state Brillouin gain spectrum (BGS), through spectral scanning of the frequency offset between the pump and signal waves. In this work, we propose and demonstrate an alternative approach, in which the local Brillouin frequency shift (BFS) is extracted from temporal transient analysis of the step response of the amplified signal wave. Measurements are taken at only two arbitrary frequency offsets between pump and signal. No spectral scanning and no prior knowledge of a reference BGS are necessary. The principle is supported by analytic and numeric solutions of the differential equations of stimulated Brillouin scattering. The BFS of a 2 meters-long fiber under test was measured with 1 MHz accuracy and a dynamic range of 200 MHz. Transient measurements were also performed in a Brillouin optical correlation domain analysis (B-OCDA) experiment with 4 cm resolution, standard deviation of 2.4 MHz and 100 MHz dynamic range. A 4 cm-wide hot-spot was properly identified in the measurements. Multiple correlation peaks could be addressed in a single flight of a pump pulse. The results represent the first B-OCDA that is free of spectral scanning. This new measurement concept may be applicable to random-access distributed and dynamic monitoring of sound and vibration.

Time multiplexing super resolution using a Barker-based array.

We propose the use of a new encoding mask in order to improve the performance of the conventional time multiplexing super resolution method. The resolution improvement is obtained using a 2D Barker-based array that is placed upon the object and shifted laterally. The Barker-based array is a 2D generalization of the standard 1D Barker code. The Barker-based array has stable autocorrelation sidelobes, making it ideal for the encoding process. A sequence of low resolution images are captured at different positions of the array, and are decoded properly using the same array. After removing the low resolution image from the resulting reconstruction, a high resolution image is established. The proposed method is presented analytically, demonstrated via numerical simulation, and validated by laboratory experiment.

Super-resolution using Barker-based array projected via spatial light modulator.

The use of a two-dimensional Barker-based array in the conventional time multiplexing super-resolution (TMSR) technique was recently presented [Opt. Lett.40, 163-165 (2015)OPLEDP0146-959210.1364/OL.40.000163]. It enables achieving a two-dimensional SR image using only a one-dimensional scan, by exploiting its unique auto-correlation property. In this Letter, we refine the method using a mismatched array for the decoding process. The cross-correlation between the Barker-based array and the mismatched array has a perfect peak-to-sidelobes ratio, making it ideal for the SR process. Also, we propose the projection of this array onto the object using a phase-only spatial light modulator. Projecting the array eliminates the need for printing it, mechanically shifting it, and having a direct contact with the object, which is not feasible in many imaging applications. 13 phase masks, which generate shifted Barker-based arrays, were designed using a revised Gerchberg-Saxton algorithm. A sequence of 13 low resolution images were captured using these phase masks, and were decoded using the mismatched arrays, resulting in a high-resolution image. The proposed mismatched array and the design process of the phase masks are presented, and the method is validated by a laboratory experiment.

Monitoring the evaporation of fluids from fiber-optic micro-cell cavities.

Fiber-optic sensors provide remote access, are readily embedded within structures, and can operate in harsh environments. Nevertheless, fiber-optic sensing of liquids has been largely restricted to measurements of refractive index and absorption spectroscopy. The temporal dynamics of fluid evaporation have potential applications in monitoring the quality of water, identification of fuel dilutions, mobile point-of-care diagnostics, climatography and more. In this work, the fiber-optic monitoring of fluids evaporation is proposed and demonstrated. Sub-nano-liter volumes of a liquid are applied to inline fiber-optic micro-cavities. As the liquid evaporates, light is refracted out of the cavity at the receding index boundary between the fluid and the ambient surroundings. A sharp transient attenuation in the transmission of light through the cavity, by as much as 50 dB and on a sub-second time scale, is observed. Numerical models for the transmission dynamics in terms of ray-tracing and wavefront propagation are provided. Experiments show that the temporal transmission profile can distinguish between different liquids.

The squared distance approach to frequency domain time-resolved fluorescence analysis.

A frequency-domain (FD) analysis of fluorescence lifetime (FLT) is a unique and rapid method for cellular and intracellular classifications that can serve for medical diagnostics purposes. Nevertheless, its data analysis process demands nonlinear fitting algorithms that may distort the resolution of the FLT data and hence diminish the classification ability of the method. This research suggests a sample classification technique that is unaffected by the analysis process as it is based on the squared distance (D2 ) between the raw frequency response data (FRD). In addition, it presents the theory behind this technique and its validation in two simulated data sets of six groups with similar widely and closely spaced FLT data as well as in experimental data of 43 samples from bacterial and viral infected and non-infected patients. In the two simulated tests, the classification accuracy was above 95% for all six groups. In the experimental data, the classification of 41 out of 43 samples matched earlier report and 29 out of 31 agreed with preliminary physician diagnosis. The D2 approach has the potential to promote FD-time resolved fluorescence measurements as a medical diagnostic technique with high specifity and high sensitivity for many of today's conventional diagnostic procedures.

Optomechanical time-domain reflectometry.

Optical fibres constitute an exceptional sensing platform. However, standard fibres present an inherent sensing challenge: they confine light to an inner core. Consequently, distributed fibre sensors are restricted to the measurement of conditions that prevail within the core. This work presents distributed analysis of media outside unmodified, standard fibre. Measurements are based on stimulated scattering by guided acoustic modes, which allow us to listen where we cannot look. The protocol overcomes a major difficulty: guided acoustic waves induce forward scattering, which cannot be mapped using time-of-flight. The solution relies on mapping the Rayleigh backscatter contributions of two optical tones, which are coupled by the acoustic wave. Analysis is demonstrated over 3 km of fibre with 100 m resolution. Measurements distinguish between air, ethanol and water outside the cladding, and between air and water outside polyimide-coated fibres. The results establish a new sensor configuration: optomechanical time-domain reflectometry, with several potential applications.