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

Integration of electronic and optical techniques in the design and fabrication of pressure sensors

Since the introduction of micro-electro-mechanical systems fabrication methods, piezoresistive pressure sensors have become the more popular pressure transducers. They dominate pressure sensor commercialization due to their high performance, stability and repeatability. However, increasing demand for harsh environment sensing devices has made sensors based on Fabry-Perot interferometry the more promising optical pressure sensors due to their high degree of sensitivity, small size, high temperature performance, versatility, and improved immunity to environmental noise and interference. The work presented in this dissertation comprises the design, fabrication, and testing of sensors that fuse these two pressure sensing technologies into one integrated unit. A key innovation is introduction of a silicon diaphragm with a center rigid body (or boss), denoted as an embossed diaphragm, that acts as the sensing element for both the electronic and optical parts of the sensor. Physical principles of piezoresistivity and Fabry-Perot interferometry were applied in designing an integrated sensor and in determining analytic models for the respective electronic and optical outputs. Several test pressure sensors were produced and their performance was evaluated by collecting response and noise data. Diaphragm deflection under applied pressure was detected electronically using the principle of piezoresistivity and optically using Fabry-Perot interferometry. The electronic part of the sensor contained four p-type silicon piezoresistors that were set into the diaphragm. They were connected in a Wheatstone bridge configuration for detecting strain-dependent changes in resistance induced by diaphragm deflection. In the optical part of the sensor, an optical cavity was formed between the embossed surface of the diaphragm and the end face of a single mode optical fiber. An infrared laser operating at 1.55 was used for optical excitation. Deflection of the diaphragm, which causes the length of the optical cavity to change, was detected by Fabry-Perot interference in the reflected light. Data collected on several sensors fabricated for this dissertation were shown to validate the theoretical models. In particular, the principle of operation of a Fabry-Perot interferometer as a mechanism for pressure sensing was demonstrated. The physical characteristics and behavior of the embossed diaphragm facilitated the integration of the electronic and optical approaches because the embossed diaphragm remained flat under diaphragm deflection. Consequently, it made the electronic sensor respond more linearly to applied pressure. Further, it eliminated a fundamental deficiency of previous applications of Fabry-Perot methods, which suffered from non-parallelism between the two cavity surfaces (diaphragm and fiber), owing to diaphragm curvature after pressure was applied. It also permitted the sensor to be less sensitive to lateral misalignment during the fabrication process and considerably reduced back pressure, which otherwise reduced the sensitivity of the sensor. As an integrated sensor, it offered two independent outputs in one sensor and therefore the capability for measurements of: (a) static and dynamic pressures simultaneously, and (b) two different physical quantities such as temperature and pressure

Fibre interferometry for differential measurements

This thesis investigates the use of interferometry as an interrogation technology for the measurement of differential length at two widely separate locations. Differential length measurements are essential and can have many applications in industrial processes, therefore accurate measurements can be a critical. Such differential length measurements can be applied to aspects of differential pressure. Using an all optical fibre approach, the research utilises the effects of light interference for both low coherent and high coherent light sources for the determination of a differential length between individual sensing cavities separated by up to 10’s of meters. The construction of the differential length interrogation system makes use of two Fabry-Perot cavities arranged in a tandem configuration, as a means of determining the differential length between them. Such an arrangement provides a common path through which an optical broadband light source at a central wavelength of 1550 nm can propagate. As a consequence of this configuration, differential lengths are made simply using one single measurement, removing the need to determine each individual length. An additional benefit of this common optical path prevents environmental factors such as temperature and air pressures from affecting the measurement length in question. Using a scanning reference Michelson interferometer to induce an optical path change, low coherence interference effects are present when the optical path length of the differential Fabry-Perot cavities is equal to the optical path length difference in the Michelson interferometer. Using a separate DFB laser light source to illuminate the reference interferometer high coherence interference fringes, present when the optical path length of one interferometer arm is changing due to a piezo fibre stretcher, can be analysed to provide an accurate length determination. Taking into consideration the noise within the system the interrogation technique has a length measurement resolution of 27.43 nm. Demonstrations show that a differential length of 82.539 μm could be measured with an uncertainty of 41.00 nm. Through the characterisation of a deformable silicon diaphragm, it would be possible to construct a sensing system capable of measuring a differential pressure of 1 Pa in 100 kPa. This however would require a 9.13 mm thick diaphragm, with a radius of 0.35 m. Such a diaphragm would be out of the question and so further investigation into reducing the length measurement resolution would need to be carried out

Fiber inline pressure and acoustic sensor fabricated with femtosecond laser

Pressure and acoustic measurements are required in many industrial applications such as down-hole oil well monitoring, structural heath monitoring, engine monitoring, study of aerodynamics, etc. Conventional sensors are difficult to apply due to the high temperature, electromagnetic-interference noise and limited space in such environments. Fiber optic sensors have been developed since the last century and have proved themselves good candidates in such harsh environment. This dissertation aims to design, develop and demonstrate miniaturized fiber pressure/acoustic sensors for harsh environment applications through femtosecond laser fabrication. Working towards this objective, the dissertation explored two types of fiber inline microsensors fabricated by femtosecond laser: an extrinsic Fabry-Perot interferometric (EFPI) sensor with silica diaphragm for pressure/acoustic sensing, and an intrinisic Fabry-Perot interferometer (IFPI) for temperature sensing. The scope of the dissertation work consists of device design, device modeling/simulation, laser fabrication system setups, signal processing method development and sensor performance evaluation and demonstration. This research work provides theoretical and experimental evidences that the femtosecond laser fabrication technique is a valid tool to fabricate miniaturized fiber optic pressure and temperature sensors which possess advantages over currently developed sensors --Abstract, page iii

Investigation of shock waves in explosive blasts using fibre optic pressure sensors

The published version of this article may be accessed at the link below. Copyright @ IOP Publishing, 2006.We describe miniature all-optical pressure sensors, fabricated by wafer etching techniques, less than 1 mm(2) in overall cross-section with rise times in the mu s regime and pressure ranges typically 900 kPa (9 bar). Their performance is suitable for experimental studies of the pressure-time history for test models exposed to shocks initiated by an explosive charge. The small size and fast response of the sensors promises higher quality data than has been previously available from conventional electrical sensors, with potential improvements to numerical models of blast effects. Results from blast tests are presented in which up to six sensors were multiplexed, embedded within test models in a range of orientations relative to the shock front.Support from the UK Engineering&Physical Sciences Research Council and Dstl Fort Halstead through the MoD Joint Grants Scheme are acknowledged. WN MacPherson is supported by an EPSRC Advanced Research Fellowship

Femtosecond Laser Micromachining of Advanced Fiber Optic Sensors and Devices

Research and development in photonic micro/nano structures functioned as sensors and devices have experienced significant growth in recent years, fueled by their broad applications in the fields of physical, chemical and biological quantities. Compared with conventional sensors with bulky assemblies, recent process in femtosecond (fs) laser three-dimensional (3D) micro- and even nano-scale micromachining technique has been proven an effective and flexible way for one-step fabrication of assembly-free micro devices and structures in various transparent materials, such as fused silica and single crystal sapphire materials. When used for fabrication, fs laser has many unique characteristics, such as negligible cracks, minimal heat-affected-zone, low recast, high precision, and the capability of embedded 3D fabrication, compared with conventional long pulse lasers. The merits of this advanced manufacturing technique enable the unique opportunity to fabricate integrated sensors with improved robustness, enriched functionality, enhanced intelligence, and unprecedented performance. Recently, fiber optic sensors have been widely used for energy, defense, environmental, biomedical and industry sensing applications. In addition to the well-known advantages of miniaturized in size, high sensitivity, simple to fabricate, immunity to electromagnetic interference (EMI) and resistance to corrosion, all-optical fiber sensors are becoming more and more desirable when designed with characteristics of assembly free and operation in the reflection configuration. In particular, all-optical fiber sensor is a good candidate to address the monitoring needs within extreme environment conditions, such as high temperature, high pressure, toxic/corrosive/erosive atmosphere, and large strain/stress. In addition, assembly-free, advanced fiber optic sensors and devices are also needed in optofluidic systems for chemical/biomedical sensing applications and polarization manipulation in optical systems. Different fs laser micromachining techniques were investigated for different purposes, such as fs laser direct ablating, fs laser irradiation with chemical etching (FLICE) and laser induced stresses. A series of high performance assembly-free, all-optical fiber sensor probes operated in a reflection configuration were proposed and fabricated. Meanwhile, several significant sensing measurements (e.g., high temperature, high pressure, refractive index variation, and molecule identification) of the proposed sensors were demonstrated in this dissertation as well. In addition to the probe based fiber optic sensors, stress induced birefringence was also created in the commercial optical fibers using fs laser induced stresses technique, resulting in several advanced polarization dependent devices, including a fiber inline quarter waveplate and a fiber inline polarizer based on the long period fiber grating (LPFG) structure

Precision-microfabricated fiber-optic probe for intravascular pressure and temperature sensing

Small form-factor sensors are widely used in minimally invasive intravascular diagnostic procedures. Manufacturing complexities associated with miniaturizing current fiber-optic probes, particularly for multi-parameter sensing, severely constrain their adoption outside of niche fields. It is especially challenging to rapidly prototype and iterate upon sensor designs to optimize performance for medical devices. In this work, a novel technique to construct a microscale extrinsic fiber-optic sensor with a confined air cavity and sub-micron geometric resolution is presented. The confined air cavity is enclosed between a 3 μm thick pressure-sensitive distal diaphragm and a proximal temperature-sensitive plano-convex microlens segment unresponsive to changes in external pressure. Simultaneous pressure and temperature measurements are possible through optical interrogation via phase-resolved low-coherence interferometry(LCI). Upon characterization in a simulated intravascular environment, we find these sensors capable of detecting pressure changes down to 0.11 mmHg (in the range of 760 to 1060 mmHg) and temperature changes of 0.036°C (in the range 34 to 50°C). By virtue of these sensitivity values suited to intravascular physiological monitoring, and the scope of design flexibility enabled by the precision-fabricated photoresist microstructure, it is envisaged that this technique will enable construction of a wide range of fiber-optic sensors for guiding minimally invasive medical procedures

Optical Fiber High Temperature Sensor Instrumentation for Energy Intensive Industries

This report summarizes technical progress during the program “Optical Fiber High Temperature Sensor Instrumentation for Energy Intensive Industries”, performed by the Center for Photonics Technology of the Bradley Department of Electrical and Computer Engineering at Virginia Tech. The objective of this program was to use technology recently invented at Virginia Tech to develop and demonstrate the application of self-calibrating optical fiber temperature and pressure sensors to several key energy-intensive industries where conventional, commercially available sensors exhibit greatly abbreviated lifetimes due primarily to environmental degradation. A number of significant technologies were developed under this program, including • a laser bonded silica high temperature fiber sensor with a high temperature capability up to 700°C and a frequency response up to 150 kHz, • the world’s smallest fiber Fabry-Perot high temperature pressure sensor (125 x 20 μm) with 700°C capability, • UV-induced intrinsic Fabry-Perot interferometric sensors for distributed measurement, • a single crystal sapphire fiber-based sensor with a temperature capability up to 1600°C. These technologies have been well demonstrated and laboratory tested. Our work plan included conducting major field tests of these technologies at EPRI, Corning, Pratt & Whitney, and Global Energy; field validation of the technology is critical to ensuring its usefulness to U.S. industries. Unfortunately, due to budget cuts, DOE was unable to follow through with its funding commitment to support Energy Efficiency Science Initiative projects and this final phase was eliminated