Extrinsic Fabry-Perot Sensors Research Articles

Direct Non-Invasive Measuring Techniques of Nanometric Liquid Level Variations Using Extrinsic Fiber Fabry–Perot Interferometers

This article investigates two different non-contact non-invasive solutions for measuring nanometric-order liquid-surface displacements with Extrinsic Fiber Fabry-Perot interferometers. They are investigated for developing hydrostatic leveling sensor (HLS) systems targeting the detection of very slowly-evolving movements in geophysics and geotechnics. In the first technique, the sensing beam from the interferometer traverses a liquid of known refractive index and is reflected by a mirror submerged at the bottom of the HLS vessel. The liquid-level variation is thus sensed as a variation of the optical path length of the interrogating beam. The second solution, on the other hand, directly exploits the reflection of the sensing beam at the air-liquid interface in the absence of a reflective surface in the vessel. The subsequent variation of liquid level is then measured directly as the beam's optical path variation in air. The common denominator of these two techniques is an Extrinsic Fabry-Perot sensor with nanometric precision operating at a wavelength of ~1310 nm. The interrogating beam suffers from high IR absorption in water, hence the latter solution is more advantageous in terms of dynamic range. In applications where liquids other than water can be employed, the use of low optical absorption liquids such as Polydimethylsiloxanic fluids is recommended at this operating wavelength. Being more viscous and less volatile than water, these fluids can significantly improve the noise floor of HLS systems, hence contributing to a larger dynamic range, lower instrumental drift and higher signal-to-noise ratio.

Highly sensitive force sensor based on optical microfiber asymmetrical Fabry-Perot interferometer

An asymmetrical Fabry-Perot interferometric (AFPI) force sensor is fabricated based on a narrowband reflection of low-reflectivity fiber Bragg grating (LR-FBG) and a broadband Fresnel reflection of the cleaved fiber end. The AFPI sensor includes a section of microfiber made by tapering and it achieves a force sensitivity of 0.221 pm/μN with a tapered microfiber of 40 mm length and 6.1 μm waist diameter. Compared with similar AFPI structure in 125 μm-diameter single mode fiber, the force sensitivity of the microfiber AFPI structure is greatly enhanced due to its smaller diameter and can be optimized for different force scales by controlling the diameter. The fabrication process of the AFPI sensor is simple and cost-effective. The AFPI sensor has better multiplexing capacity than conventional extrinsic fiber-optic Fabry-Perot sensors, while it also release the requirement on the wavelength matching of the FBG-pair-based FPI.

Extrinsic and intrinsic fiber optic interferometric sensors for acoustic detection in high-voltage environments

Incipient fault diagnosis is closely related to insulation condition assessment. A great number of methods are available for condition monitoring and diagnosis of power transformer insulation systems, but only few of them can take direct measurements inside the transformer. Fiber optic sensors can be applied to incipient fault diagnosis. In particular, acoustic sensors have been developed for detection and location of partial discharges in oil-filled power transformers. We report the study of extrinsic and intrinsic fiber Fabry-Perot sensors that can be used to detect the acoustic waves that are generated by a partial discharge inside a power transformer. A comparative analysis is done to determine the best sensing head configuration and some methods to improve the parameter readout sensitivity are proposed. The sensing head behaviour when immersed in different fluids (air, water, and oil) is also investigated.

Vibration: history and measurement with an extrinsic Fabry–Perot sensor with solid-state laser interferometry

We have studied the history of vibration and demonstrate a laser-based noncontact interferometric vibration sensor. The sensor promises the measurement of microdisplacement by using a Fabry-Perot cavity formed between a partially coated gradient-index lens and a movable reflector. Displacement is determined by the detection of interference fringes caused by phase modulation within the cavity. The sensor was tested in conjunction with both multimode and single-mode fiber transmission. Calibration with multimode fiber produced a fringe-contrast function that decreased monotonically with displacement. This calibration allowed at least 30 fringes to be discriminated, giving a displacement resolution of 0.034 microm across a range of 10.2 microm. Dynamic tests demonstrated a working range of at least 3.74 microm at frequencies as high as 2 kHz. Similar tests in which single-mode fiber was used indicated a dynamic working range of at least 4.29 microm.

A multiplexed optical fibre-based extrinsic Fabry-Perot sensor system for in-situ strain monitoring in composites

The detection of impact damage in fibre reinforced composites is of significant concern because such damage can reduce the load-bearing ability of the composite. A number of factors can influence the nature and extent of impact damage development in composites including: (a) the type of reinforcing fibre and resin system; (b) the magnitude of the residual (fabrication) stresses; (c) the lay-up sequence; and (d) other factors such as the nature of the impactor, impact velocity, impact energy, temperature, moisture content in the composites, etc. From a structural health monitoring point of view, it is necessary to investigate the distribution of damage through the thickness of the composite. This paper reports on a simple, partially multiplexed optical fibre strain sensor system for in-situ strain and residual strain measurements in a carbon fibre reinforced epoxy composite. An extrinsic Fabry-Perot interferometric (EFPI) sensor design was used along with single-mode fibres. The multiplexing scheme was based on wavelength division via the use of two super luminescent diodes (SLDs) at different wavelengths. A low-cost fibre optic CCD spectrometer was used as the detector. The multiplexing scheme was demonstrated using two EFPI sensors. In principle, a number of EFPI sensors can be multiplexed using the proposed scheme provided that each sensor is illuminated at a specified and different wavelength. The feasibility of detecting the residual strain in the composite was demonstrated successfully at two specified positions within a 16-ply carbon fibre reinforced composite panel. Preliminary results indicated that the sensor system was also capable of detecting the effects of a 3.2 J impact. Excellent correlation was obtained between the EFPI sensor output and that obtained using surface mounted strain gauges.

In-line fiber etalon (ILFE) fiber-optic strain sensors

This paper describes an optical fiber interferometer that uses a short segment of silica hollow-core fiber spliced between two sections of single-mode fiber to form a mechanically robust in-line optical cavity. The hollow-core fiber is specifically manufactured to have an outer diameter that is equal to the outer diameter of the single mode lead fibers, thereby combining the best qualities of existing intrinsic and extrinsic Fabry-Perot sensors. Uniaxial tension and pure bending strength tests are used to show that the new configuration does not diminish the axial strength of bare fiber and reduces the bending strength by 17% at most. Similar tests confirm that the fiber sensor has 1.96% strain to failure. Axisymmetric finite element analysis is used to investigate the reliability of the in-line etalon when it is embedded in a typical thermoset composite, and parametric studies are performed to determine the mechanically optimal cavity length. The fiber optic sensor is tested using low coherence interferometry with pseudo-heterodyne demodulation under strain and temperature fields. The strain response compares well with resistance strain gages, and the temperature tests confirm the low thermal apparent strain of this sensor. >

In-line fiber étalon for strain measurement

A description and demonstration of a fiber interferometer that uses a short segment of silica hollow-core fiber spliced between two sections of single-mode fiber to form a mechanically robust in-line cavity are presented. The hollow-core fiber is specifically manufactured to have an outer diameter that is equal to the outer diameter of the single-mode lead fibers, thereby combining the best qualities of existing intrinsic and extrinsic Fabry-Perot sensors. A dynamic strain resolution of ~22 nepsilon/ radicalHz at frequencies of >5 Hz with a sensor gauge length of 137 microm is demonstrated.

Extrinsic Fabry-Perot sensor for strain and crack opening displacement measurements from -200 to 900 degrees C

The authors present the design of a fiber optic extrinsic Fabry-Perot interferometric sensor element and its operation in several applications. The sensor has been demonstrated in a coal-fired combustor for the measurement of both thermally induced strains and crack opening displacements in ceramic materials. As a strain sensor, the device is demonstrated at temperatures ranging from -200 to 900 degrees C. It is shown that a Fabry-Perot strain gage with a one centimetre gage length can be operated in differential mode with 0.01 microstrain resolution in real-time, and in an absolute mode with 0.5 microstrain resolution with a three-second scan time.