Review of Fiber Optic Displacement Sensors

Displacement Measurements Are of Significant Importance in a Variety of Critical Scientific and Engineering Fields, Such as Gravitational Wave Detection, Geophysical Research, and Manufacturing Industries. Due to the Inherent Advantages Such as Compactness, High Sensitivity, and Immunity to Electromagnetic Interference, in Recent Years, Fiber Optic Sensors Have Been Widely Used in an Expansive Range of Sensing Applications, Ranging from Infrastructural Health Monitoring to Chemical and Biological Sensing. of Particular Interest Here, Fiber Optic Displacement Sensors Have Gained Wide Interest and Have Evolved from Basic Intensity Modulation-Based Configurations to More Advanced Structures, Such as Fiber Bragg Grating (FBG)-Based and Interferometric Configurations. This Article Reviews Specifically the Advanced Fiber Optic Displacement Sensing Techniques that Have Been Developed in the Past Two Decades. Details Regarding the Working Principle, Sensor Design, and Performance Measures of FBG-Based, Interferometers-Based (Including the Fabry-Perot Interferometer, the Michelson Interferometer, and the Multimode Interferometer), Microwave Photonics-Based, and Surface Plasmon Resonance-Based Fiber Optic Displacement Sensors Are Given. Challenges and Perspectives on Future Research in the Development of Practical and High-Temperature Tolerant Displacement Sensors Are Also Discussed

NOVEL OPTICAL MICRORESONATORS FOR SENSING APPLICATIONS

Optical microresonators have been proven as an effective means for sensing applications. The high quality (Q) optical whispering gallery modes (WGMs) circulating around the rotationally symmetric structures can interact with the local environment through the evanescent field. The high sensitivity in detection was achieved by the long photon lifetime of the high-Q resonator (thus the long light-environment interaction path). The environmental variation near the resonator surface leads to the effective refractive index change and thus a shift at the resonance wavelength. In this Dissertation, we present our recent research on the development of new optical microresonators for sensing applications. Different structures and materials are used to develop optical resonator for broad sensing applications. Specifically, a new coupling method is designed and demonstrated for efficient excitation of microsphere resonators. The new coupler is made by fusion splicing an optical fiber with a capillary tube and consequently etching the capillary wall to a thickness of a few microns. Light is coupled through the peripheral contact between inserted microsphere and the etched capillary wall. Operating in the reflection mode and providing a robust mechanical support to the microresonator, the integrated structure has been experimentally proven as a convenient probe for sensing applications. Microspheres made of different materials (e.g., PMMA, porous glass, hollow core porous, and glass solid borosilicate glass) were successfully demonstrated for different sensing purposes, including temperature, chemical vapor concentration, and glucose concentration in aqueous solutions. In addition, the alignment free, integrated microresonator structure may also find other applications such as optical filters and microcavity lasers

Advances in Fiber-Optic Extrinsic Fabry-Perot Interferometric Physical and Mechanical Sensors: A Review

Fabry-Perot Interferometers Have Found a Multitude of Scientific and Industrial Applications Ranging from Gravitational Wave Detection, High-Resolution Spectroscopy, and Optical Filters to Quantum Optomechanics. Integrated with Optical Fiber Waveguide Technology, the Fiber-Optic Fabry-Perot Interferometers Have Emerged as a Unique Candidate for High-Sensitivity Sensing and Have Undergone Tremendous Growth and Advancement in the Past Two Decades with their Successful Applications in an Expansive Range of Fields. the Extrinsic Cavity-Based Devices, I.e., the Fiber-Optic Extrinsic Fabry-Perot Interferometers (EFPIs), Enable Great Flexibility in the Design of the Sensitive Fabry-Perot Cavity Combined with State-Of-The-Art Micromachining and Conventional Mechanical Fabrication, Leading to the Development of a Diverse Array of EFPI Sensors Targeting at Different Physical Quantities. Here, We Summarize the Recent Progress of Fiber-Optic EFPI Sensors, Providing an overview of Different Physical and Mechanical Sensors based on the Fabry-Perot Interferometer Principle, with a Special Focus on Displacement-Related Quantities, Such as Strain, Force, Tilt, Vibration and Acceleration, Pressure, and Acoustic. the Working Principle and Signal Demodulation Methods Are Shown in Brief. Perspectives on Further Advancement of EFPI Sensing Technologies Are Also Discussed

High Sensitivity Optical Fiber Interferometric Sensors

Optical fiber interferometers have been widely employed and investigated for monitoring the changes in both physical and chemical parameters, with the advantages of compact size, light weight, immunity to electromagnetic interference, high sensitivity, capability to work in harsh environments and remote operation capabilities. Among the different kinds of fiber sensors based on interferometry, singlemode-multimode-singlemode (SMS) structures has attracted considerable interest due to their inherent advantages of high sensitivity, ease of fabrication and interconnection to other fiber systems and low cost. However, the challenge is that the sensitivity of the traditional SMS based fiber structure is not sufficient in some cases, for example in bio-chemical applications, where detection of a very small variation in a bio-chemicals’ concentration is required. There is thus a need to investigate how to modify or enhance an SMS structure to achieve ultrahigh sensitivity. This thesis presents research and its applications concerning approaches to improve the sensitivity and detection accuracy of a traditional SMS fiber structure based sensor. The key achievements of this thesis include: Traditional SMS fiber structure for breathing state monitoring A bend SMS structure is investigated as a breathing sensor by attaching it to a thin plastic film in an oxygen mask. Breath rate can be monitored using this sensor by detecting power variations due to the macro bending applied to the SMS section during each inhalation and exhalation cycles. Different types of breathing conditions including regular and irregular breath patterns can be distinguished. The proposed sensor is capable of working in a strong electromagnetic field and radioactive environment. Tapered small core singlemode fiber (SCSMF) for the detection of refractive index (RI), ammonia, and volatile organic compounds (VOCs) A modified SMS structure based on a tapered SCSMF is proposed and investigated with significantly improved RI sensitivity. It is found that the sample with a smaller waist diameter gives higher sensitivity. In the experiment, a maximum sensitivity of 19212.5 nm/RIU (RI unit) in the RI range from 1.4304 to 1.4320 has been demonstrated when the waist diameter of the SCSMF is tapered down to 12.5 μm. The best corresponding theoretical resolution of the proposed sensor is 5.025 × 10-7 RIU which is over 10 times higher than that of many previous reported optical fiber based RI sensors. The proposed structure is capable of monitoring relative humidity level change even without coating of the fiber sensor’s surface with a layer of hygroscopic material. A silica sol-gel based coating has been used as a sensitive material to ammonia for the first time, by applying it to the surface of the tapered SCSMF for the detection of ammonia in water. The proposed sensor shows an ultra-high sensitivity of 2.47 nm/ppm with short response and recovery time of less than 2 and 5 minutes respectively. The corresponding theoretical detection limit of ammonia in water is calculated to be 4 ppb, which is 3 orders of magnitude improvement compared to the previous reported interferometry based ammonia sensor. In addition, the sensor has good performance in terms of repeatability of measurement and selectivity for sensing ammonia compared to that of other common ions and organic molecules in water. VOCs sensors are also demonstrated by coating a mixture of sol-gel silica and Nile red on the surface of two different types of tapered fiber sensors (tapered SCSMF) and a microfiber coupler (MFC)). The MFC based sensor shows better sensitivities to ethanol and methanol than that based on a tapered SCSMF due to its smaller waist diameter. The detectable gas concentration changes of the MFC based sensor are calculated to be ~77 ppb and ~281 ppb for ethanol and methanol respectively which are over one order of magnitude improvement than many other reports. The sensors also show fast response times of less than 5 minutes and recovery times varied from 7 to 12 minutes. Simultaneous measurement of ethanol and methanol is achieved by utilizing two different coating recipes. Hollow core fiber (HCF) structure for high temperature and twist sensing. A modified SMS structure with much improved spectral quality factor (Q) is investigated both theoretically and experimentally. The modified structure is based on a HCF. It is found that periodic transmission dips with high spectral extinction ratio and high Q factor are excited because of the multiple beam interferences introduced by the cladding of the HCF. The HCF structure can be used as a high sensitivity (up to 33.4 pm/°C) temperature sensor in a wide working temperature range (from room temperature to 1000 °C). By coating a thin layer of silver (~ 6.7 nm) on one side of the HCF surface, a twist sensor with a maximum sensitivity of 0.717 dB/°has been achieved, which is the highest twist sensitivity reported for intensity modulation based fiber sensors, with excellent measurement repeatability. Further theoretical and experimental investigation attributes this high twist sensitivity to the polarization dependent reflection coefficient at the outer HCF surface associated with the partial silver coating

Singlemode-Multimode-Singlemode Fiber Structures for Sensing Applications – A Review

A singlemode-multimode-singlemode (SMS) fiber structure consists of a short section of multimode fiber fusion-spliced between two SMS fibers. The mechanism underpinning the operation of an SMS fiber structure is multimode interference and associated self-imaging. SMS structures can be used in a variety of optical fiber systems but are most commonly used as sensors for a variety of parameters, ranging from macro-world measurands such as temperature, strain, vibration, flow rate, RI and humidity to the micro-world with measurands such as proteins, pathogens, DNA, and specific molecules. While traditional SMS structures employ a short section of standard multimode fiber, a large number of structures have been investigated and demonstrated over the last decade involving the replacement of the multimode fiber section with alternatives such as a hollow core fiber or a tapered fiber. The objective of replacing the multimode fiber has most often been to allow sensing of different measurands or to improve sensitivity. In this paper, several different categories of SMS fiber structures, including traditional SMS, modified SMS and tapered SMS fiber structures are discussed with some theoretical underpinning and reviews of a wide variety of sensing examples and recent advances. The paper then summarizes and compares the performances of a variety of sensors which have been published under a number of headings. The paper concludes by considering the challenges faced by SMS based sensing schemes in terms of their deployment in real world applications and discusses possible future developments of SMS fiber sensors

Non-resonant Mie scattering: Emergent optical properties of core-shell polymer nanowires

Cataloged from PDF version of article.We provide the in-depth characterization of light-polymer nanowire interactions in the context of an effective Mie scattering regime associated with low refractive index materials. Properties of this regime sharply contrast with these of resonant Mie scattering, and involve the formation of strictly forward-scattered and coupling-free optical fields in the vicinity of core-shell polymer nanowires. Scattering from these optical fields is shown to be non-resonant in nature and independent from incident polarization. In order to demonstrate the potential utility of this scattering regime in one-dimensional (1D) polymeric nanostructures, we fabricate polycarbonate (PC) -polyvinylidene difluoride (PVDF) core-shell nanowires using a novel iterative thermal drawing process that yields uniform and indefinitely long core-shell nanostructures. These nanowires are successfully engineered for novel nanophotonics applications, including size-dependent structural coloration, efficient light capture on thin-film solar cells, optical nano-sensors with ultrahigh sensitivity and a mask-free photolithography method suitable for the straightforward production of 1D nanopatterns

Refractometer based on fiber Bragg grating Fabry-Pérot cavity embedded with a narrow microchannel

We report on inscription of microchannels of different widths in optical fiber using femtosecond (fs) laser inscription assisted chemical etching and the narrowest channel has been created with a width down to only 1.2µm. Microchannels with 5µm and 35µm widths were fabricated together with Fabry-Pérot (FP) cavities formed by UV laser written fiber Bragg gratings (FBGs), creating high function and linear response refractometers. The device with a 5µm microchannel has exhibited a refractive index (RI) detection range up to 1.7, significantly higher than all fiber grating RI sensors. In addition, the microchannel FBG FP structures have been theoretically simulated showing excellent agreement with experimental measured characteristics