Polymer optical fiber for large strain measurement based on multimode interference

Optical Sensing Using Fiber-Optic Multimode Interference Devices: A Review of Nonconventional Sensing Schemes

We review fiber-based multimode interference (MMI) devices with a particular focus on optical fiber-based sensing applications. The present review complements a recently published, extensive review where the sensing of conventional physical variables such as refractive index, temperature, displacement, and strain was covered. This review focuses on MMI fiber sensors for nonconventional physical variables, including mechanical, electromagnetic, chemical, and optical, covering around fifteen years of work in the field. Finally, by the end of this paper, we also review some new trends of MMI-based schemes based on polymer fibers, for wavelength-locking applications, for retrieving the thermo-optic coefficient of liquid samples, and for measuring the dynamics of complex fluids.

A Refractive Index Sensitive Liquid Level Monitoring Sensor Based on Multimode Interference

According to the beam propagation method, a fiber refractive index-sensitive multimode interference (MMI) structure fabricated by splicing a self-made silica glass rod between two single mode fibers (SMF–NCF (no core fiber)–SMF structure) is proposed for liquid level monitoring. Theoretical and experimental investigation was carried out meticulously using a 4.5 cm and a 9.5 cm long silica glass rod. It is proved that the simple and economical sensor with the shorter length has high sensitivity, satisfactory repeatability, and favorable stability. The sensitivity climbs with the increase in refractive index of the measured liquid, which is 204 pm/mm for pure water, 265.8 pm/mm for 10% glycerin solution, and 352.5 pm/mm for 25% glycerin solution. The proposed sensor can be standardized in certain application circumstances to achieve accurate liquid level monitoring.

Wide-range strain sensor based on Brillouin frequency and linewidth in an As2Se3-PMMA hybrid microfiber

We propose a wide-range strain sensor based on Brillouin frequency and linewidth in a 50 cm-long As₂Se₃-polymethyl methacrylate (As₂Se₃-PMMA) hybrid microfiber with a core diameter of 2.5 µm. The distributed information over the hybrid microfiber is measured by a Brillouin optical time-domain analysis (BOTDA) system. The wide dynamic range strain from 0 to 15000 µɛ is enabled by measuring the Brillouin frequency and linewidth due to the low Young's modulus of As₂Se₃ core and the high mechanical strength of PMMA cladding. The deformation of the As₂Se₃-PMMA hybrid microfiber is observed when the strain is greater than 1500 µɛ by measuring the distributed Brillouin frequency and Brillouin linewidth over the 50 cm-long hybrid microfiber. The measured errors based on the Brillouin frequency in the range of 0-1500 µɛ and 1500-15000 µɛ are 42 µɛ and 105 µɛ, respectively. The measured error based on the Brillouin linewidth is 65 µɛ at 0-1500 µɛ and the maximum error is 353 µɛ when the tensile strain is 15000 µɛ. No strain memory effect is observed compared with the polymer optical fiber due to Young's modulus in As₂Se₃ is larger than that in polymer. Numerical simulations are developed to accurately predict the strain dependence of Brillouin frequency in the As₂Se₃-PMMA hybrid microfiber.

Multimode interferometer-based torsion sensor employing perfluorinated polymer optical fiber

This paper reports a torsion sensor based on the multimode interference theory. The sensor is fabricated by sandwiching a section of perfluorinated polymer optical fiber (POF) between two silica single mode fibers to construct a single-mode-multimode-single-mode (SMS) structure. The perfluorinated POF is easily connected to the optical fiber via the precise alignment of ceramic ferrules and ceramic mating sleeve. With the considerable flexibility and deformability of the perfluorinated POF, the proposed sensor is especially suitable for torsion measurement. Experimental results show that a wavelength sensitivity of 106.762 pm/(rad/m) and an intensity sensitivity of 0.165 dBm/(rad/m) are obtained within a large torsion rate of -100∼100 rad/m.

Highly stretchable hybrid silica/polymer optical fiber sensors for large-strain and high-temperature application

The large-range and high-sensitivity strain measurement in high-temperature ambiance is a great challenge in engineering applications. Because of the frangibility of the glass material, the traditional optical fiber strain sensors cannot endure a limit strain of 1%. To break through the limit, we propose a hybrid silica/polymer optical fiber sensor. It can endure extraordinarily large strain. The maximum strain of 35% is confirmed by experiments. To achieve high sensitivity and detect a small change in strain, a phase tracking method is used. The sensitivity of the sensor is 28 pm/με which is 28 times larger than that of the traditional FBG sensors. In addition, because of the excellent high-temperature endurance of polyimide (PI) and adhesive, the sensor can survive in the high temperature up to 220 °C. The proposed hybrid silica/polymer optical fiber sensor has potentials to monitor deformation in plastic products, structure health in composite materials, and even strain in biomaterials.

Temperature-insensitive refractometer based on an RI-modulated singlemode-multimode-singlemode fibre structure

A temperature-insensitive refractometer based on a refractive index (RI)-modulated singlemode-multimode-singlemode (RMSMS) fibre structure is proposed and experimentally demonstrated. In this investigation, a combination of no-core fibre (NCF) and multimode fibre (MMF) regions provides an RI modulation region due to the difference in RI between the NCF and the MMF. In effect, by periodically embedding the NCF within the MMF section of a singlemode-multimode-singlemode (SMS) fibre structure, a long-period grating (LPG) can be effectively introduced in the MMF section, and the excited cladding modes are therefore able to sense surrounding RI variation. The modulation parameters are determined from the numerical simulations, and the experimental results show the maximum RI sensitivity of the fabricated sample is as high as 206.96 nm/RIU. In addition, the proposed RMSMS fibre structure is proven to be unaffected by external temperature variation (in the wavelength domain), which is a very attractive feature in practical sensing applications.

Highly sensitive strain sensor based on composite interference established within S-tapered multimode fiber structure

In this paper, a novel strain sensor based on composite interference established within an S-tapered multimode (STM) fiber structure is proposed and experimentally demonstrated. The STM fibre structure is simply realized by non-axially tapering a traditional single-mode-multimode-single-mode (SMS) fiber into S-shape using a fusion splicer. This fabricated S-tapered structure provides an extra Mach-Zehnder interferometer (MZI) that is introduced within the multimode fibre (MMF) section; therefore, composite interference based on the inherent multimode interference (MMI) of an SMS and the introduced MZI is successfully established. This resultant composite interference greatly enhances the performance of traditional SMS fibre structures for strain sensing, with a maximum strain measurement sensitivity as high as -103.8 pm/με achieved with a detectable strain resolution of 0.2 με. Benefiting from the experimentally determined high sensitivity and good repeatability, this low-cost strain sensor can be realistically applied in many areas where high accuracy strain measurement is required.

Displacement and Strain Measurement up to 1000 °C Using a Hollow Coaxial Cable Fabry-Perot Resonator

We present a hollow coaxial cable Fabry-Perot resonator for displacement and strain measurement up to 1000 °C. By employing a novel homemade hollow coaxial cable made of stainless steel as a sensing platform, the high-temperature tolerance of the sensor is dramatically improved. A Fabry-Perot resonator is implemented on this hollow coaxial cable by introducing two highly-reflective reflectors along the cable. Based on a nested structure design, the external displacement and strain can be directly correlated to the cavity length of the resonator. By tracking the shift of the amplitude reflection spectrum of the microwave resonator, the applied displacement and strain can be determined. The displacement measurement experiment showed that the sensor could function properly up to 1000 °C. The sensor was also employed to measure the thermal strain of a steel plate during the heating process. The stability of the novel sensor was also investigated. The developed sensing platform and sensing configurations are robust, cost-effective, easy to manufacture, and can be flexibly designed for many other measurement applications in harsh high-temperature environments.

An embeddable optical strain gauge based on a buckled beam

We report, for the first time, a low cost, compact, and novel mechanically designed extrinsic Fabry-Perot interferometer (EFPI)-based optical fiber sensor with a strain amplification mechanism for strain measurement. The fundamental design principle includes a buckled beam with a coated gold layer, mounted on two grips. A Fabry-Perot cavity is produced between the buckled beam and the endface of a single mode fiber (SMF). A ceramic ferrule is applied for supporting and orienting the SMF. The principal sensor elements are packaged and protected by two designed metal shells. The midpoint of the buckled beam will experience a deflection vertically when the beam is subjected to a horizontally/axially compressive displacement. It has been found that the vertical deflection of the beam at midpoint can be 6-17 times larger than the horizontal/axial displacement, which forms the basis of a strain amplification mechanism. The user-configurable buckling beam geometry-based strain amplification mechanism enables the strain sensor to achieve a wide range of strain measurement sensitivities. The designed EFPI was used to monitor shrinkage of a square brick of mortar. The strain was measured during the drying/curing stage. We envision that it could be a good strain sensor to be embedded in civil materials/structures under a harsh environment for a prolonged period of time.

Fiber-Optic Point-Based Sensor Using Specklegram Measurement

Here, we report a fiber-optic point-based sensor to measure temperature and weight based on correlated specklegrams induced by spatial multimode interference. The device is realized simply by splicing a multimode fiber (MMF) to a single-mode fiber (SMF) with a core offset. A series of experiments demonstrates the approximately linear relation between the correlation coefficient and variation. Furthermore, we show the potential applications of the refractive index sensing of our device by disconnecting the splicing point of MMF and SMF. A modification of the algorithm in order to improve the sensitivity of the sensor is also discussed at the end of the paper.

Extended-bandwidth frequency sweeps of a distributed feedback laser using combined injection current and temperature modulation

This article details the generation of an extended-bandwidth frequency sweep using a single, communication grade distributed feedback (DFB) laser. The frequency sweep is generated using a two-step technique. In the first step, injection current modulation is employed as a means of varying the output frequency of a DFB laser over a bandwidth of 99.26 GHz. A digital optical phase lock loop is used to lock the frequency sweep speed during current modulation, resulting in a linear frequency chirp. In the second step, the temperature of the DFB laser is modulated, resulting in a shifted starting laser output frequency. A laser frequency chirp is again generated beginning at this shifted starting frequency, resulting in a frequency-shifted spectrum relative to the first recorded data. This process is then repeated across a range of starting temperatures, resulting in a series of partially overlapping, frequency-shifted spectra. These spectra are then aligned using cross-correlation and combined using averaging to form a single, broadband spectrum with a total bandwidth of 510.9 GHz. In order to investigate the utility of this technique, experimental testing was performed in which the approach was used as the swept-frequency source of a coherent optical frequency domain reflectometry system. This system was used to interrogate an optical fiber containing a 20 point, 1-mm pitch length fiber Bragg grating, corresponding to a period of 100 GHz. Using this technique, both the periodicity of the grating in the frequency domain and the individual reflector elements of the structure in the time domain were resolved, demonstrating the technique's potential as a method of extending the sweeping bandwidth of semiconductor lasers for frequency-based sensing applications.

Tuning whispering gallery lasing modes from polymer fibers under tensile strain

Wavelength tuning of whispering gallery lasing modes has been observed from Rhodamine-B-doped polymer fibers under tensile strain. Good quality whispering gallery lasing modes are produced from both solid and hollow fibers by transverse optical pumping. The lasing modes are shifted linearly toward the shorter wavelength side when the fiber is elongated in the axial direction. Compared with solid fiber, the lasing modes of hollow fiber can be tuned over the entire gain spectrum with a tuning range of ∼5  nm. It is found that the tuning of the lasing modes of hollow fiber is reversible.

Microwave interrogated large core fused silica fiber Michelson interferometer for strain sensing

A Michelson-type large core optical fiber sensor has been developed, which is designed based on the optical carrier-based microwave interferometry technique, and fabricated by using two pieces of 200-μm diameter fused silica core fiber as two arms of the Michelson interferometer. The interference fringe pattern caused by the optical path difference of the two arms is interrogated in the microwave domain, where the fringe visibility of 40 dB has easily been obtained. The strain sensing at both room temperature and high temperatures has been demonstrated by using such a sensor. Experimental results show that this sensor has a linear response to the applied strain, and also has relatively low temperature-strain cross talk. The dopant-free quality of the fused silica fiber provides high possibility for the sensor to have promising strain sensing performance in a high temperature environment.

Multiplexed displacement fiber sensor using thin core fiber exciter

This letter reports a multiplexed optical displacement sensor using a thin core fiber (TCF) exciter. The TCF exciter is followed by a stripped single mode optical fiber. A small section of buffer is used as the movable component along the single mode fiber. Ultra-weak cladding mode reflection (< - 75 dB) was employed to probe the refractive index discontinuity between the air and buffer coating boundary. The position change of the movable buffer segment results in a delay change of the cladding mode reflection. Thus, it is a measure of the displacement of the buffer segment with respect to the glass fiber. The insertion loss of one sensor was measured to be less than 3 dB. A linear relationship was evaluated between the measurement position and absolute position of the moving actuator. Multiplexed capability was demonstrated and no cross talk was found between the sensors.

Highly sensitive liquid level monitoring system utilizing polymer fiber Bragg gratings

A novel and highly sensitive liquid level sensor based on a polymer optical fiber Bragg grating (POFBG) is experimentally demonstrated. Two different configurations are studied and both configurations show the potential to interrogate liquid level by measuring the strain induced in a POFBG embedded in a silicone rubber diaphragm, which deforms due to hydrostatic pressure variations. The sensor exhibits a highly linear response over the sensing range and a good repeatability. For comparison, a similar sensor using a FBG inscribed in silica fiber is fabricated, which displays a sensitivity that is a factor of 5 smaller than the POFBG. The temperature sensitivity is studied and a novel multi-sensor arrangement proposed which has the potential to provide level readings independent of temperature and the liquid density.

Design and experiment of FBG-based icing monitoring on overhead transmission lines with an improvement trial for windy weather

A scheme for monitoring icing on overhead transmission lines with fiber Bragg grating (FBG) strain sensors is designed and evaluated both theoretically and experimentally. The influences of temperature and wind are considered. The results of field experiments using simulated ice loading on windless days indicate that the scheme is capable of monitoring the icing thickness within 0–30 mm with an accuracy of ±1 mm, a load cell error of 0.0308v, a repeatability error of 0.3328v and a hysteresis error is 0.026%. To improve the measurement during windy weather, a correction factor is added to the effective gravity acceleration, and the absolute FBG strain is replaced by its statistical average.

A coaxial cable Fabry-Perot interferometer for sensing applications

This paper reports a novel coaxial cable Fabry-Perot interferometer for sensing applications. The sensor is fabricated by drilling two holes half-way into a coaxial cable. The device physics was described. The temperature and strain responses of the sensor were tested. The measurement error was calculated and analyzed.