Coherent fading noise (also known as speckle noise) affects the SNR and sensitivity of Distributed Acoustic Sensing (DAS) systems and makes them random processes of position and time. As in speckle noise, the statistical distribution of DAS SNR is particularly wide and its standard deviation (STD) roughly equals its mean (σSNR/〈SNR〉 ≈ 0.89). Trading resolution for SNR may improve the mean SNR but not necessarily narrow its distribution. Here a new approach to achieve both SNR improvement (by sacrificing resolution) and narrowing of the distribution is introduced. The method is based on acquiring high resolution complex backscatter profiles of the sensing fiber, using them to compute complex power profiles of the fiber which retain phase variation information and filtering of the power profiles. The approach is tested via a computer simulation and demonstrates distribution narrowing up to σSNR/〈SNR〉 < 0.2.
In Rayleigh-scattering-based Distributed Acoustic Sensing (DAS) an optical fiber is transformed into an array of thousands of 'virtual microphones'. This approach has gained tremendous popularity in recent years and is one of the most successful examples of a fiber-optic sensing method which made its way from the academia to the market. Despite the great amount of work done in this field, sensitivity, which is ones of the most critical parameters of any sensing technique, was rarely investigated in this context. In particular, little attention was given to its random characteristics. Without careful consideration of the random aspects of DAS, any attempt to specify its sensitivity or to compare between different DAS modalities is of limited value. Recently we introduced a new statistical parameter which defines DAS sensitivity and enables comparison between the performances of different DAS systems. In this paper we generalize the previous parameter and give a broader, simple and intuitive definition to DAS sensitivity. An important attribute of these parameters is that they can be easily extracted from the static backscatter profile of the sensing fiber. In the paper we derive the relation between DAS sensitivity and the static backscatter profile and present an experimental verification of this relation.
In this paper we study the SNR associated with acoustic detection in Rayleigh-based Distributed Acoustic Sensing
(DAS) systems. The study is focused on phase sensitive DAS due to its superiority in terms of linearity and sensitivity.
Since DAS is based on coherent interference of backscattered light from multiple scatterers it is prone to signal fading.
When left unresolved, the issue of signal fading renders the associated SNR randomly dependent on position and time.
Hence, its proper measurement and characterization requires statistical tools. Here such tools are introduced and a
methodology for finding the mean SNR and its distribution is implemented in both experiment and simulation. It is
shown that the distribution of the DAS-SNR can be obtained from the distribution of backscattered power in OTDR and
the mean DAS-SNR is proportional to the energy of the interrogation pulse.
The field of Optical Fiber Sensors (OFS) is gaining tremendous popularity in recent years. OFS natural immunity to electromagnetic disturbances, inherent biocompatibility and compactness making them highly attractive for ultrasound sensing. Moreover, their compatibility with photoacoustics can make them useful in situations where traditional piezoelectric probes are inadequate. However, the issue of multiplexing individual OFS into an array remains a challenging and costly task. In this work, we demonstrate a straightforward approach for multiplexing multiple broadband OFS for ultrasound sensing by exploiting most of the photoreceiver's bandwidth. The design is based on a recently developed system in which all sensing elements are connected to a single interrogator and to a single digitizing circuit. To mitigate aliasing, the system employs I/Q coherent detection. Synchronization of the sensor interrogation with the excitation enables very high repetition rates (kHz) making it ideal for applications where imaging of dynamic processes is desired.
Cumulative acoustically-induced phase modulation along the sensing fiber significantly degrades the performance of Optical Frequency Domain Reflectometry (OFDR) systems. Here we present a new method to mitigate this phenomenon using hybrid time-frequency interrogation and analysis. The method, which we term Gated-OFDR (G-OFDR), achieves remarkable results: ultra-sensitive dynamic sensing at z≈101km with 1.4m spatial resolution and acoustical sampling rate of 600Hz. As an example, the system detected and recorded, with high SNR, falls of two ~1g paperclips from height of ~20cm, on two fiber sections, 10m apart, at the end of the 101km fiber, without any crosstalk artifacts.
Fiber ultrasound (US) sensing is gaining popularity in recent years. Unique characteristics such as immunity to electromagnetic interference and embedding compatibility makes them advantageous in many applications. Multiplexing of US fiber sensors, however, remains a challenge. Here, a new multiplexing approach is introduced. Based on Swept Frequency Interferometry (SFI), it enables practical multiplexing of tens of US sensors. For demonstration, a 3-sensors setup was excited by ultrasound tone-bursts. While using low driving voltage (2.5-10V vs. ~100-400V in similar studies) and not implementing acoustic-impedance optimization or optical-resonance sensitivity enhancement, the sensors detected the excitation with high SNR (~25dB).
In traditional OFDR systems, the backscattered profile of a sensing fiber is inefficiently duplicated to the negative band of spectrum. In this work, we present a new OFDR design and algorithm that remove this redundancy and make use of negative beat frequencies. In contrary to conventional OFDR designs, it facilitates efficient use of the available system bandwidth and enables distributed sensing with the maximum allowable interrogation update-rate for a given fiber length. To enable the reconstruction of negative beat frequencies an I/Q type receiver is used. In this receiver, both the in-phase (I) and quadrature (Q) components of the backscatter field are detected. Following detection, both components are digitally combined to produce a complex backscatter signal. Accordingly, due to its asymmetric nature, the produced spectrum will not be corrupted by the appearance of negative beat-frequencies. Here, via a comprehensive computer simulation, we show that in contrast to conventional OFDR systems, I/Q OFDR can be operated at maximum interrogation update-rate for a given fiber length. In addition, we experimentally demonstrate, for the first time, the ability of I/Q OFDR to utilize negative beat-frequencies for long-range distributed sensing.
We introduce a new approach for multiplexing fiber-based ultrasound sensors using Optical Frequency Domain Reflectometry (OFDR). In the present demonstration of the method, each sensor was a short section of Polyimide-coated single-mode fiber. One end of the sensing fiber was pigtailed to a mirror and the other end was connected, via a fiber optic delay line, to a 1X4 fiber coupler. The multiplexing was enabled by using a different delay to each sensor. Ultrasonic excitation was performed by a 1MHz transducer which transmitted 4μs tone-bursts above the sensor array. The ultrasound waves generated optical phase variations in the fibers which were detected using the OFDR method. The ultrasound field at the sensors was successfully reconstructed without any noticeable cross-talk.
A highly sensitive OFDR system capable of detecting and tracking fast acoustic wave propagation is described. The system was tested by dropping a screw (50gr) and a paperclip (<5gr) at one end of an 18m PVC pipe. The sensing fiber detected the wave propagation (v ≈ 1750m/s) along the entire pipe. Fast phase variations due to the impact of the screw led to a transient shift in the frequency of the interrogating light which corrupted the observed response. By tracking the beat frequencies of predefined reflectors and extracting their amplitudes significant improvement in the system's output was obtained.
We introduce a phase sensitive, dynamic and long range fiber-optic sensing system with fully distributed audio recording capabilities. The proposed system implements a recently developed OFDR design, which is based on double interrogation of a sensing fiber with equally-spaced discrete reflectors. In this paper, the ability of each sensing segment to operate as an independent, purely optical audio recorder with little cross-talk artifacts is demonstrated.
Interferometric phase microscopy (IPM) enables to obtain quantitative optical thickness profiles of transparent samples, including live cells in-vitro, and track them in time with sub-nanometer accuracy without any external labeling, contact or force application on the sample. The optical thickness measured by IPM is a multiplication between the cell integral refractive index differences and its physical thickness. Based on the time-dependent optical thickness profile, one can generate the optical thickness fluctuation map. For biological cells that are adhered to the surface, the variance of the physical thickness fluctuations in time is inversely proportional to the spring factor indicating on cell stiffness, where softer cells are expected fluctuating more than more rigid cells. For homogenous refractive index cells, such as red blood cells, we can calculate a map indicating on the cell stiffness per each spatial point on the cell. Therefore, it is possible to obtain novel diagnosis and monitoring tools for diseases changing the morphology and the mechanical properties of these cells such as malaria, certain types of anaemia and thalassemia. For cells with a complex refractive-index structure, such as cancer cells, decoupling refractive index and physical thickness is not possible in single-exposure mode. In these cases, we measure a closely related parameter, under the assumption that the refractive index does not change much within less than a second of measurement. Using these techniques, we lately found that cancer cells fluctuate significantly more than healthy cells, and that metastatic cancer cells fluctuate significantly more than primary cancer cells.
We propose an off-axis interferometric imaging system as a simple and unique modality for continuous, non-contact and
non-invasive wide-field imaging and characterization of drug release from its polymeric device used in biomedicine. In
contrast to the current gold-standard methods in this field, usually based on chromatographic and spectroscopic
techniques, our method requires no user intervention during the experiment, and only one test-tube is prepared.
We experimentally demonstrate imaging and characterization of drug release from soy-based protein matrix, used as skin
equivalent for wound dressing with controlled anesthetic, Bupivacaine drug release. Our preliminary results demonstrate
the high potential of our method as a simple and low-cost modality for wide-field imaging and characterization of drug
release from drug delivery devices.