A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Temperature Sensor <s> A reflectively monitored optical-fiber Fabry–Perot interferometer was embedded in a graphite–epoxy composite material. Its performance as a temperature sensor was demonstrated from 20 to 200°C. The change in relative phase shift with temperature, Δϕ/ϕΔT, was measured to be 8.0 × 10−6/°C for this embedded sensor. This value is 4% lower than for one employing a similar fiber in an air ambient. A thermal expansion coefficient for the composite material in the direction of the fiber axis is estimated from these data to be 2.1 × 10−7/°C. <s> BIB001 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Temperature Sensor <s> An interferometric fiber-optic sensor using a light-emitting diode (LED) as the optical source is analyzed and demonstrated. The sensor arrangement employs two Fabry-Perot interferometers (FPIs) in series, one for sensing and one which serves as a reference. The optical output from the LED is spectrally modulated by reflection from the sensing FPI. Then, reflection or transmission by the reference FPI produces an interferometric beat response similar to that observed when a laser is used with the sensing interferometer alone. Best fringe visibility is obtained when the optical path lengths of the two interferometers are matched, and the fringes disappear when the path length difference becomes substantially greater than the coherence length of the LED. > <s> BIB002 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Temperature Sensor <s> A modified Sagnac interferometer-based fiber temperature sensor is proposed. Polarization independent operation and high temperature sensitivity of this class of sensors make them cost effective instruments for temperature measurements. A comparison of the proposed sensor with Bragg grating and long-period grating fiber sensors is derived. A temperature-induced spectral displacement of 0.99 nm/K is demonstrated for an internal stress birefringent fiber-based Sagnac interferometer. <s> BIB003 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Temperature Sensor <s> We demonstrate a fiber Bragg grating (FBG) sensor with controllable sensitivity by connecting two metal strips that have different temperature-expansion coefficients. By changing the lengths of the metal strips we successfully controlled and improved the temperature sensitivity to 3.3 times of that of bare FBG. <s> BIB004 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Temperature Sensor <s> Photoinduced long-period gratings are shown as versatile sensors for temperature, axial strain and index of refraction measurements. The principle of operation of such devices is discussed and the application to simultaneous temperature and strain is demonstrated. <s> BIB005 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Temperature Sensor <s> In this paper we demonstrate a simple technique for forming a fibre optic long period grating (LPG) with enhanced temperature sensitivity by surrounding the LPG with a material with a high thermo-optic coefficient, thus exploiting both the temperature and external refractive index sensitivity of the LPG. The operating temperature and measurement ranges are determined by the attenuation band that is monitored, and the refractive index and the thermo-optic coefficient of the surrounding material. <s> BIB006 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Temperature Sensor <s> We present a comprehensive study of the strain and temperature-sensing characteristics of single-mode-multimode-single-mode (SMS) structures based on the modal interference of guided modes of graded index multimode fiber (MMF) section spliced in between two single-mode fibers. A detailed theoretical study of the structures in terms of the refractive index distribution, effect of dopant and their concentrations, and the variation of core diameter has been carried out. Our study shows that for the SMS structure with a GeO2-doped MMF there exists a critical wavelength on either side of which the spectrum shows opposite spectral shift with a change in temperature/strain, whereas for structures with a P2O5-doped MMF it shows monotonic red shift with increasing temperature/strain. It has been found that the critical wavelength shifts toward higher wavelengths with decreasing ldquoqrdquo value/doping concentration. Using different MMFs, both the red and blue spectral shifts have been observed experimentally. It has also been found that the SMS structure has higher sensitivity toward this critical wavelength. The study should find application in designing strain-insensitive high-sensitive temperature sensors or vice versa. <s> BIB007
By comparison to traditional electronic temperature sensors, an optical fiber based temperature sensor has the merits of immunity to electromagnetic field interference and a capability to work in harsh environments and for these reasons optical fiber based temperature sensors have attracted significant research interest. A series of optical fiber based temperature sensors have been reported, most of which are based on FBGs   BIB004 , LPGs BIB005 BIB006 , Fabry-Perot configurations BIB001 BIB002 , a Sagnac interferometer BIB003 , and an SMS fiber structure BIB007 . In this sub-section we review various temperature sensors which utilise TOF technology, such as sensors based on an MMF tip, an MFC and MFC tip. Since the experimental setup utilized and the process for temperature sensing are similar for all of these fiber structures, only an MFC tip based temperature sensor is described here. Firstly, an MFC tip is fabricated by dividing a complete MFC structure into two equal parts, as previously illustrated in Figure 18 . Two MFC tip samples, denoted as sample 1 and sample 2, were fabricated in the experiments with diameters of 12.56 µm and 4.8 µm, respectively. Considering the mechanical stability and temperature measurement range of this temperature sensor, a large diameter is preferred. This large diameter supports multiple modes resulting in multimodal interference that is strongly temperature sensitive. Figure 24 presents the experimental setup for temperature sensing measurement based on MFC tips. A BBS (Fianium Ltd., Southampton, UK) launched broadband light with wavelength range of 450-1800 nm to port P1, and the reflected light out of port P2 was received and recorded by an OSA. The microheater used (NTT-AT, Tokyo, Japan) can provide a wide experimental temperature range up to 1500 • C. Experimental results show that the measured intensity for both sample 1 and sample 2 monotonically decrease for temperature increases. The difference between two samples is that sample 1 had a lower sensitivity of 1.514 × 10 −3 dB/ • C but a wider dynamic measurement than sample 2. Considering that an achievable temperature range has a more significant meaning for temperature sensor, a high temperature sensor based on MFC tip with waist diameter of 12.56 µm was proposed. It is noteworthy that this proposed temperature sensor can work well within a wide temperature range from room temperature to 1511 • C, which is the highest temperature reported for a silica optical fiber sensor. Moreover, its response time and resolution were also measured as 16.6 ms and 0.66 • C. The sensing performance of other temperature sensors based on TOF structure (MMF tip and MFC) is included in Table 2 for comparison. 