Showing papers on "Fiber optic sensor published in 2017"

Overview of Fiber Optic Sensors

Abstract: Over the past 20 years two major product revolutions have taken place due to the growth of the optoelectronics and fiber optic communications industries. The optoelectronics industry has brought about such products as compact disc players, laser printers, bar code scanners, and laser pointers. The fiber optic communications industry has revolutionized the telecommunications industry by providing higher performance, more reliable telecommunication links with ever decreasing bandwidth cost. This revolution is bringing about the benefits of high-volume production to component users and a true information superhighway built of glass.

Simultaneous measurement of refractive index and temperature using a SPR-based fiber optic sensor

Abstract: A surface plasmon resonance-based fiber-optic sensor for simultaneous measurement of refractive index and temperature of liquid samples is proposed and experimentally demonstrated. The sensor consists of a gold-coated MM-SM-MM optical fiber structure, whose sensitive section was partially covered with polydimethylsiloxane (PDMS) to generate two independent SPR resonance dips in the fiber transmission spectrum. One of the dips is generated by the bare gold-coated fiber section whose wavelength resonance is tuned by the refractive index and temperature of the surrounding medium. The other dip that is exclusively used to monitor the temperature variations of the liquid sample, whose central wavelength at 900 nm corresponds to PDMS refractive index at 20 °C, is produced by the polymerized gold-coated fiber section. The high refractive index and temperature sensitivity achieved, 2323.4 nm/RIU and −2.850 nm/°C respectively, the small size, the ease fabrication process, and the bio-compatibility of the proposed device are appealing characteristics that makes it ideal for practical bio-sensing applications.

Review and Analysis of Peak Tracking Techniques for Fiber Bragg Grating Sensors.

Abstract: Fiber Bragg Grating (FBG) sensors are among the most popular elements for fiber optic sensor networks used for the direct measurement of temperature and strain. Modern FBG interrogation setups measure the FBG spectrum in real-time, and determine the shift of the Bragg wavelength of the FBG in order to estimate the physical parameters. The problem of determining the peak wavelength of the FBG from a spectral measurement limited in resolution and noise, is referred as the peak-tracking problem. In this work, the several peak-tracking approaches are reviewed and classified, outlining their algorithmic implementations: the methods based on direct estimation, interpolation, correlation, resampling, transforms, and optimization are discussed in all their proposed implementations. Then, a simulation based on coupled-mode theory compares the performance of the main peak-tracking methods, in terms of accuracy and signal to noise ratio resilience.

Phase-detection distributed fiber-optic vibration sensor without fading-noise based on time-gated digital OFDR

Abstract: For a distributed fiber-optic vibration sensor (DFVS), the vibration signal extracted from the phase of backscattering has a linear response to the applied vibration, and is more attractive than that from the intensity term. However, the large phase noise at a random weak-fading-point seriously limits the sensor's credibility. In this paper, a novel phase-detection DFVS is developed, which effectively eliminates the weak-fading-point. The relationship between phase noise and the intensity of backscattering is analyzed, and the inner-pulse frequency-division method and rotated-vector-sum method are introduced to effectively suppress phase noise. In experiments, two simultaneous vibrations along the 35-kilometer-long fiber are clearly detected by phase detection with the signal-to-noise ratio (SNR) over 26 dB. The spatial resolution approaches 5 m and the vibration response bandwidth is 1.25 kHz.

Surface plasmon resonance biosensor based on gold-coated side-polished hexagonal structure photonic crystal fiber.

Abstract: The refractive index sensing characteristics of the side-polished photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor are detailed investigated in this paper. We used the finite element method (FEM) to study the influences of the side-polished depth, air hole size, lattice constant, and the refractive index (RI) of the PCF material on sensing performance. The simulation results show that the side-polished depth, air hole size, lattice pitch have significant influence on the coupling strength between core mode and surface plasmon polaritons (SPPs), but have little influence on sensitivity; the coupling strength and sensitivity will significant increase with the decrease of RI of the PCF material. The sensitivity of the D-shaped PCF sensor is obtained to be as high as 21700 nm/RIU in the refractive index environment of 1.33-1.34, when the RI of the PCF material is controlled at 1.36. It revealed a new method of making ultra-high sensitivity SPR fiber sensor. Then we experimental demonstrated a SPR refractive sensor based on the side-polished single mode PCF and investigated the sensing performance. The experimental results of the plasmon resonance wavelength sensitivity agree well with the theoretical results. The presented gold-coated D-shaped PCF SPR sensor could be used as a simple, cost-effective, high sensitivity device in bio-chemical detection.

A Non-Invasive Multichannel Hybrid Fiber-Optic Sensor System for Vital Sign Monitoring

Abstract: In this article, we briefly describe the design, construction, and functional verification of a hybrid multichannel fiber-optic sensor system for basic vital sign monitoring. This sensor uses a novel non-invasive measurement probe based on the fiber Bragg grating (FBG). The probe is composed of two FBGs encapsulated inside a polydimethylsiloxane polymer (PDMS). The PDMS is non-reactive to human skin and resistant to electromagnetic waves, UV absorption, and radiation. We emphasize the construction of the probe to be specifically used for basic vital sign monitoring such as body temperature, respiratory rate and heart rate. The proposed sensor system can continuously process incoming signals from up to 128 individuals. We first present the overall design of this novel multichannel sensor and then elaborate on how it has the potential to simplify vital sign monitoring and consequently improve the comfort level of patients in long-term health care facilities, hospitals and clinics. The reference ECG signal was acquired with the use of standard gel electrodes fixed to the monitored person's chest using a real-time monitoring system for ECG signals with virtual instrumentation. The outcomes of these experiments have unambiguously proved the functionality of the sensor system and will be used to inform our future research in this fast developing and emerging field.

Recent progress of using Brillouin distributed fiber optic sensors for geotechnical health monitoring

Abstract: Distributed optical fiber sensors (DOFS) have been attracted significant attention from geotechnical engineering communities for a few decades. Innovative development of structural design, encapsulation and implementation methods of optical fiber sensors leads to many new applications in geotechnical monitoring field. This paper presents a comprehensive review of Brillouin based DOFS for health monitoring of various geotechnical structures include geotextiles, soil nails, anchors, pipelines, piles, retaining walls, tunnels, and landslides. Recent application status of using two successful commercialized technologies including Brillouin Optical Time Domain Reflectometry (BOTDR) and Brillouin Optical Time Domain Analysis (BOTDA) for geotechnical health monitoring was reviewed and discussed in details. Particular emphasis was given to sensor design, encapsulation, and installation methods of DOFS in various successful geotechnical applications. Comparison analysis regarding typical advantages and limitations of different technologies (DOFS, fiber Bragg grating sensors, and conventional sensors) for geotechnical health monitoring was also presented and discussed in this paper.

Recent advancements in optical fiber hydrogen sensors

Abstract: A review for optical fiber hydrogen sensors based on palladium (Pd) and tungsten oxide (WO3) thin films is presented, with specific focus on the measurement methods, probe structures, and sensing properties of different sensors. Firstly, the theoretical models behind the optical fiber hydrogen sensors, as well as their practical limitations, are addressed. Secondly, four mainstream measurement methods, including intensity, fiber Bragg grating (FBG), interferometer, surface plasmon resonance (SPR), which have been proposed to sense the physicochemical properties variations of sensitive thin films when exposed to hydrogen, are reviewed. Then, the advantages and disadvantages of all the above measurement methods are also discussed and compared. Finally, the existing problems and future prospects of optical fiber hydrogen sensors are pointed out.

Highly flexible and stretchable optical strain sensing for human motion detection

Abstract: Strain sensors with high flexibility and stretchability are essential for quantifying strains generated by human activities due to the soft and curvilinear surfaces of the body. Here, we report the design and fabrication of a highly flexible and stretchable optical strain sensor that is applicable for the detection of human motions. The sensor is based on dye-doped polydimethylsiloxane optical fiber, which enables the quantitative detection of tensile strains by absorption changes of the light passing through the dye-doped fiber. Elaborate mechanical and strain tests confirm durability, reliability, and long-term stability of the sensor device. The sensor exhibits linear and repeatable responses in a large dynamic range up to 100%, far exceeding the stretchability of conventional silica fibers (<1%), and the strain precision is below +/−1%. We show that the sensor can be used to monitor various human activities, such as joint motion, speaking, and deep breathing in real time.

Fiber Grating-Assisted Surface Plasmon Resonance for Biochemical and Electrochemical Sensing

Abstract: Surface plasmon resonance (SPR) optical fiber sensors can be used as a cost-effective and relatively simple-to-implement alternative to well established bulky prism configurations for in situ high sensitivity biochemical and electrochemical measurements. The miniaturized size and remote operation ability offer them a multitude of opportunities for single-point sensing in hard-to-reach spaces, even possibly in vivo . Grating-assisted and polarization control are two key properties of fiber-optic SPR sensors to achieve unprecedented sensitivities and limits of detection. The biosensor configuration presented here utilizes a nanoscale metal-coated tilted fiber Bragg grating (TFBG) imprinted in a commercial single-mode fiber core with no structural modifications. Such sensor provides an additional resonant mechanism of high-density narrow cladding mode spectral combs that overlap with the broader absorption of the surface plasmon for high accuracy interrogation. In this paper, we briefly review the principle, characterization and implementation of plasmonic TFBG sensors, followed by our recent developments of the “surface” and “localized” affinity studies of the biomolecules for real life problems and the electrochemical actives of electroactive biofilms for clean energy resources.

U-bent fiber optic SPR sensor based on graphene/AgNPs

Abstract: We present a U-bent fiber optic SPR sensor based on the graphene/AgNPs, combined the virtues of graphene, AgNPs and U-bent fiber. By changing the laser-induced time, we achieved the optimal time for the deposition of AgNPs. With the proposed graphene/AgNPs U-bent fiber optic sensor, we discussed and investigated the SPR behaviors. The shifts up to 32 and 16 nm are observed respectively for the detection of 90% aqueous ethanol and 20% aqueous glucose, corresponding refractive index (RI) 1.3657 and 1.3557. The sensitivity of this proposed is calculated as 1198 nm/RIU and the response and recovery time of this U-bent fiber optic sensor are respectively 3 s and 80 s. These experimental results indicate that our proposed graphene/AgNPs U-bent fiber optic sensor will be expected to open new avenue for the detection in the field of medicine, biotechnology and food safety.

Multi-event waveform-retrieved distributed optical fiber acoustic sensor using dual-pulse heterodyne phase-sensitive OTDR.

Abstract: We demonstrate a novel type of distributed optical fiber acoustic sensor, with the ability to detect and retrieve actual temporal waveforms of multiple vibration events that occur simultaneously at different positions along the fiber. The system is realized via a dual-pulse phase-sensitive optical time-domain reflectometry, and the actual waveform is retrieved by heterodyne phase demodulation. Experimental results show that the system has a background noise level as low as 8.91×10−4 rad/√Hz with a demodulation signal-to-noise ratio of 49.17 dB at 1 kHz, and can achieve a dynamic range of ∼60 dB at 1 kHz (0.1 to 104 rad) for phase demodulation, as well as a detection frequency range from 20 Hz to 25 kHz.

Surface-plasmon-resonance-based optical-fiber temperature sensor with high sensitivity and high figure of merit.

Abstract: We propose and demonstrate a novel and compact optical-fiber temperature sensor with a high sensitivity and high figure of merit (FOM) based on surface plasmon resonance (SPR). The sensor is fabricated by employing a single-mode twin-core fiber (TCF), which is polished as a circular truncated cone and coated with a layer of gold film and a layer of polydimethylsiloxane (PDMS). Owing to the high refractive index sensitivity of SPR sensors and high thermo-optic coefficient of PDMS, the sensor realizes a high temperature sensitivity of −4.13 nm/°C to −2.07 nm/°C in the range from 20°C to 70°C, transcending most other types of optical-fiber temperature sensors. Owing to the fundamental mode beam transmitting in the TCF, the sensor realizes a high FOM of up to 0.034/°C, more than twice that of SPR sensors based on multimode fiber. The proposed temperature sensor is meaningful and will have potential applications in many fields, such as biomedical and biomaterial.

Continuous Multicore Optical Fiber Grating Arrays for Distributed Sensing Applications

Abstract: We describe the fabrication and distributed sensing capabilities of very long continuous fiber grating sensor arrays in a twisted multicore fiber. The continuous gratings are fabricated in fibers with UV transparent coating using a flexible and scalable reel-to-reel processing system. Single-frequency continuous gratings are characterized using optical frequency-domain reflectometry and a shape reconstruction algorithm to measure fiber bend radius. Broadband reflection gratings are shown to act as enhanced quasi-Rayleigh scattering elements allowing for distributed temperature measurements in the presence of 10-dB transmission loss.

Highly sensitive temperature sensor using a Sagnac loop interferometer based on a side-hole photonic crystal fiber filled with metal.

Abstract: A highly sensitive temperature sensor based on an all-fiber Sagnac loop interferometer combined with metal-filled side-hole photonic crystal fiber (PCF) is proposed and demonstrated. PCFs containing two side holes filled with metal offer a structure that can be modified to create a change in the birefringence of the fiber by the expansion of the filler metal. Bismuth and indium were used to examine the effect of filler metal on the temperature sensitivity of the fiber-optic temperature sensor. It was found from measurements that a very high temperature sensitivity of −9.0 nm/°C could be achieved with the indium-filled side-hole PCF. The experimental results are compared to numerical simulations with good agreement. It is shown that the high temperature sensitivity of the sensor is attributed to the fiber microstructure, which has a significant influence on the modulation of the birefringence caused by the expansion of the metal-filled holes.

Sensitivity amplification of fiber-optic in-line Mach–Zehnder Interferometer sensors with modified Vernier-effect

Abstract: In this paper, a novel sensitivity amplification method for fiber-optic in-line Mach-Zehnder interferometer (MZI) sensors has been proposed and demonstrated. The sensitivity magnification is achieved through a modified Vernier-effect. Two cascaded in-line MZIs based on offset splicing of single mode fiber (SMF) have been used to verify the effect of sensitivity amplification. Vernier-effect is generated due to the small free spectral range (FSR) difference between the cascaded in-line MZIs. Frequency component corresponding to the envelope of the superimposed spectrum is extracted to take Inverse Fast Fourier Transform (IFFT). Thus we can obtain the envelope precisely from the messy superimposed spectrum. Experimental results show that a maximum sensitivity amplification factor of nearly 9 is realized. The proposed sensitivity amplification method is universal for the vast majority of in-line MZIs.

All-optical phase shifter and switch near 1550nm using tungsten disulfide (WS 2 ) deposited tapered fiber

Abstract: All-optical phase shifters and switches play an important role for various all-optical applications including all-optical signal processing, sensing and communication. In this paper, we demonstrate a fiber all-optical phase shifter using few-layer 2D material tungsten disulfide (WS2) deposited on a tapered fiber. WS2 absorbs injected 980 nm pump (control light) and generates heat, which changes the refractive index of both WS2 and tapered fiber due to thermo-optic effect and achieves a maximum phase shift of 6.1π near 1550 nm. The device has a loss of 3.7 dB. By constructing a Mach-Zehnder interferometer with WS2 based phase shifter in one arm, an all-optical switch is also obtained with an extinction ratio of 15 dB and a rise time of 7.3 ms. This all fiber low-cost and compact optical phase shifter and switch demonstrates the potential of 2D transition metal dichalcogenides for all-optical signal processing devices.

High sensitivity D -shaped hole fiber temperature sensor based on surface plasmon resonance with liquid filling

Abstract: A high sensitivity D-shaped hole double-cladding fiber temperature sensor based on surface plasmon resonance (SPR) is designed and investigated by a full-vector finite element method. Within the D-shaped hole double-cladding fiber, the hollow D-section is coated with gold film and then injected in a high thermo-optic coefficient liquid to realize the high temperature sensitivity for the fiber SPR temperature sensor. The numerical simulation results show that the peaking loss of the D-shaped hole double-cladding fiber SPR is hugely influenced by the distance between the D-shaped hole and fiber core and by the thickness of the gold film, but the temperature sensitivity is almost insensitive to the above parameters. When the thermo-optic coefficient is −2.8×10−4/°C, the thickness of the gold film is 47 nm, and the distance between the D-shaped hole and fiber core is 5 μm, the temperature sensitivity of the D-shaped hole fiber SPR sensor can reach to −3.635 nm/°C.

Recent Development in the Distributed Fiber Optic Acoustic and Ultrasonic Detection

Abstract: Distributed acoustic sensors provide a powerful instrumentation for ultrasound testing to identify the internal crack and deformation with the location for assessing structural conditions and predicting the potential structural failure. This paper provides a tutorial review of the milestones leading to the development of the distributed acoustic sensor based on Rayleigh back scattering. It offers discussion on basic principles, optical configurations and system design requirement, various detection schemes, performance limitations and applications for the maximum measurable frequency, minimum detectable strain and highest spatial resolution. A final comment on the prospects of the further developments is presented.

Fiber-Optic Sensors for Measurements of Torsion, Twist and Rotation: A Review

Abstract: Optical measurement of mechanical parameters is gaining significant commercial interest in different industry sectors. Torsion, twist and rotation are among the very frequently measured mechanical parameters. Recently, twist/torsion/rotation sensors have become a topic of intense fiber-optic sensor research. Various sensing concepts have been reported. Many of those have different properties and performances, and many of them still need to be proven in out-of-the laboratory use. This paper provides an overview of basic approaches and a review of current state-of-the-art in fiber optic sensors for measurements of torsion, twist and/or rotation.Invited Paper.

Distributed Fiber-Optic Acoustic Sensor With Enhanced Response Bandwidth and High Signal-to-Noise Ratio

Abstract: This paper proposes a novel distributed fiber-optic acoustic sensor, which can solve both the tradeoff between the maximum measurable distance and the spatial resolution, and that between the measurement distance and the vibration response bandwidth. The system is based on frequency-division-multiplexing time-gated digital optical frequency domain reflectometry, which consecutively injects linear-frequency-modulated probe pulses with different frequency ranges. Undersampling method is introduced to reduce the sampling rate of the analog-to-digital converter and the data size, which can reduce the cost of the system and facilitate real-time data processing. In experiments, two simultaneous vibrations with frequency up to 9 kHz are detected over the 24.7-km-long fiber, with a sign-to-noise ratio of 30 dB and spatial resolution of 10 m.

Nearly-octave wavelength tuning of a continuous wave fiber laser.

Abstract: The wavelength tunability of conventional fiber lasers are limited by the bandwidth of gain spectrum and the tunability of feedback mechanism. Here a fiber laser which is continuously tunable from 1 to 1.9 μm is reported. It is a random distributed feedback Raman fiber laser, pumped by a tunable Yb doped fiber laser. The ultra-wide wavelength tunability is enabled by the unique property of random distributed feedback Raman fiber laser that both stimulated Raman scattering gain and Rayleigh scattering feedback are available at any wavelength. The dispersion property of the gain fiber is used to control the spectral purity of the laser output.

High spatial resolution distributed optical fiber dynamic strain sensor with enhanced frequency and strain resolution.

Abstract: A distributed optical fiber dynamic strain sensor with high spatial and frequency resolution is demonstrated. The sensor, which uses the ϕ-OTDR interrogation technique, exhibited a higher sensitivity thanks to an improved optical arrangement and a new signal processing procedure. The proposed sensing system is capable of fully quantifying multiple dynamic perturbations along a 5 km long sensing fiber with a frequency and spatial resolution of 5 Hz and 50 cm, respectively. The strain resolution of the sensor was measured to be 40 ne.

3D printed hollow core terahertz Bragg waveguides with defect layers for surface sensing applications.

Abstract: We study a 3D-printed hollow core terahertz (THz) Bragg waveguide for resonant surface sensing applications. We demonstrate theoretically and confirm experimentally that by introducing a defect in the first layer of the Bragg reflector, thereby causing anticrossing between the dispersion relations of the core-guided mode and the defect mode, we can create a sharp transmission dip in the waveguide transmission spectrum. By tracking changes in the spectral position of the narrow transmission dip, one can build a sensor, which is highly sensitive to the optical properties of the defect layer. To calibrate our sensor, we use PMMA layers of various thicknesses deposited onto the waveguide core surface. The measured sensitivity to changes in the defect layer thickness is found to be 0.1 GHz/μm. Then, we explore THz resonant surface sensing using α-lactose monohydrate powder as an analyte. We employ a rotating THz Bragg fiber and a semi-automatic powder feeder to explore the limit of the analyte thickness detection using a surface modality. We demonstrate experimentally that powder layer thickness variations as small as 3μm can be reliably detected with our sensor. Finally, we present a comparative study of the time-domain spectroscopy versus continuous wave THz systems supplemented with THz imaging for resonant surface sensing applications.

Photonic crystal fiber-based plasmonic biosensor with external sensing approach

Abstract: We propose a simple photonic crystal fiber (PCF) biosensor based on the surface plasmon resonance effect. The sensing properties are characterized using the finite element method. Chemically stable gold material is deposited on the outer surface of the PCF to realize the practical sensing approach. The performance of the modeled biosensor is investigated in terms of wavelength sensitivity, amplitude sensitivity, sensor resolution, and linearity of the resonant wavelength with the variation of structural parameters. In the sensing range of 1.33 to 1.37, maximum sensitivities of 4000 nm/RIU and 478 RIU−1 are achieved with the high sensor resolutions of 2.5×10−5 and 2.1×10−5 RIU using wavelength and amplitude interrogation methods, respectively. The designed biosensor will reduce fabrication complexity due to its simple and realistic hexagonal lattice structure. It is anticipated that the proposed biosensor may find possible applications for unknown biological and biochemical analyte detections with a high degree of accuracy.

Extreme Environment Sensing Using Femtosecond Laser-Inscribed Fiber Bragg Gratings

Abstract: The femtosecond laser-induced fiber Bragg grating is an effective sensor technology that can be deployed in harsh environments. Depending on the optical fiber chosen and the inscription parameters that are used, devices suitable for high temperature, pressure, ionizing radiation and strain sensor applications are possible. Such devices are appropriate for aerospace or energy production applications where there is a need for components, instrumentation and controls that can function in harsh environments. This paper will present a review of some of the more recent developments in this field.

Label-free detection of bovine serum albumin based on an in-fiber Mach-Zehnder interferometric biosensor

Abstract: We propose and experimentally verify an innovative label-free optical fiber biosensor based on a Mach-Zehnder interferometer for bovine serum albumin (BSA) concentration detection. The proposed fiber biosensor utilized a micro-cavity within a single-mode fiber to induce Mach-Zehnder interference. A remarkable feature of this biosensor is that external media can directly interact with the fiber core signal through microfluidic channels connected to the micro-cavity and sensor surface. The device was fabricated by means of femtosecond laser micromachining and chemical etching. A fiber interferometer of this type exhibits an ultrahigh refractive index sensitivity of −10,055 nm/RIU and a detection limit of 3.5 × 10−5 RIU. Different concentrations of BSA with an infinitesimally small refractive index difference can be clearly differentiated in situ by the interferential spectra of the structure. Experiments demonstrated the biosensor exhibited a BSA solution concentration sensitivity of −38.9 nm/(mg/mL) and a detection limit of 2.57 × 10−4 mg/mL, respectively. Moreover, this biosensor is a sub-microliter dose and ultrasensitive at the low concentrations detected in BSA, which make it a promising for biochemical applications such as DNA hybridization, cancer screenings, medicine examination and environmental engineering, etc.

Integration of a microfluidic polymerase chain reaction device and surface plasmon resonance fiber sensor into an inline all-in-one platform for pathogenic bacteria detection

Abstract: We presented the integration of polymerase chain reaction (PCR) microdevice and surface plasmon resonance (SPR) optical fiber sensor into an inline all-in-one device for fluorophore-free detection of PCR amplification The proposed integrated device consisted of the microfluidic PCR reactor that used channel position-dependent temperature change and the optical fiber SPR sensor with bimetallic (Ag/Al) coating This all-in-one device performed not only the amplification of DNA of Salmonella spp (injected volume of 20 microlitter) within 30 min, but also the immediately subsequent measurement of the DNA amplicon by the SPR fiber sensor part Results showed that the SPR sensor signal increased with the cycle number due to the amplicon refractive index smaller than the sum of its constituent dNTP molecules and its minimum resolvable DNA amplicon was that generated at the 15th cycle The integrated device could serve as a DNA amplification-to-detection instrument Both the label-free nature that could allow reusability of the presented device and further miniaturization of the whole system can find use in a point-of-care testing with early diagnosis genetic analysis

Highly sensitive fiber random-grating-based random laser sensor for ultrasound detection.

Abstract: A novel highly sensitive ultrasound sensor based on a random fiber laser is proposed and demonstrated for the first time, to the best of our knowledge. The random fiber laser is constructed with the erbium-doped fiber gain. A fiber random grating provides random-distributed feedback for the random laser and acts as an ultrasound sensing head. The random laser can operate in a single-mode lasing scenario, which offers linear and pure temporal responses to the broadband ultrasonic acoustic emission from 20 kHz to 0.8 MHz. The multiple-interfering reflection spectrum of the random grating provides a large number of steep peak areas over a broad spectral range, which significantly enhances the sensitivity of the random laser sensor and makes it an ideal sensor in harsh environments with large temperature or strain variations. The proposed laser sensor offers signal-to-noise ratio improvement up to 20 dB and higher sensitivity compared with conventional piezoelectric acoustic sensors.

Polarization-Assisted Fiber Bragg Grating Sensors: Tutorial and Review

Abstract: Fiber Bragg gratings (FBGs) are inherently sensitive to temperature, axial strain, and pressure, which can be easily measured by a shift of the Bragg wavelength in their reflected/transmitted power spectrum. FBG sensors acquire many more additional sensing modalities and applications when the polarization of the interrogating light is controlled. For the polarization to have an effect, the cylindrical symmetry of the fiber must be broken, either by the structure of the fiber itself, by that of the FBG, or by the perturbation to be measured. Polarization control allows for sensing parameters that are spatially oriented, such as lateral force, bending or twist, and also for measurements of the properties of anisotropic media. Most importantly, polarization control enables high quality all-fiber surface plasmon resonance (SPR) FBG sensors and localized SPR-assisted sensing. This tutorial will cover the theory of polarized measurements in fiber gratings, their experimental implementation, and review a selection of the most important applications.