A compact and low cost fiber sensor based on single multimode microfiber with Fresnel reflection is proposed and demonstrated for simultaneous measurement of refractive index and temperature. The sensor is fabricated with two simple steps including fiber tapering and then fiber endface cleaving. The reflection spectrum is an intensity modulated interference spectrum, as the tapered fiber generates interference pattern and the cleaved endface provides intensity modulation. By demodulating the fringe power and free spectrum range (FSR) of the spectrum, RI sensitivities of −72.247dB/RIU and 68.122nm/RIU, as well as temperature sensitivities of 0.0283dB/°C and −17pm/°C are obtained. Further, the sensing scheme could also provide the feasibility to construct a more compact sensing probe for dual-paramters measurement, which has great potential in bio/chemical detection.
© 2015 Optical Society of America
Dynamic refractive index (RI) and temperature sensing are of the most importance for applications in biomedical and environmental science. Comparing to traditional mechanical and electrical approaches, optical fiber sensing method exhibits many unique merits, such as compact structure, electromagnetic interference immunity, high sensitivity, and potential low cost [1–6]. As we all know, temperature will strongly influence RI of solutions due to the large thermo-optic coefficients; therefore, simultaneous or mutually independent measurement of RI and temperature appears particularly important. Some fiber structures such as the in-line fiber Mach-Zehnder interferometer (MZI) , dual-cavity fiber Fabry-Perot interferometer (FPFI) , and two-mode fiber interferometric probe  are investigated for simultaneous measurement of RI and temperature, however, these sensors require special fibers which bring relatively high insertion loss and the achieved sensitivities are relatively low.
Micro/nanofiber (MNF) is widely investigated for building fiber sensors due to its unique and promising optical properties of low transmission loss, tight optical confinement, high nonlinear effect, and large evanescent field [10–13]. Specially, large evanescent field makes MNF to have great application prospect in high sensitivity sensing which is especially important in trace detection. However, microfiber including the tapered PCF are always only used to be a RI sensor due to two reasons [14–16]. One is their relatively low temperature sensitivity in air, and the other is that these schemes cannot detect the RI and temperature simultaneously. In recent years, many novel fiber structures based on microfiber including tapered fiber Bragg grating combined with microfiber cavity (TMFC) , Tapered fiber MZI , microfiber Fabry-Perot interferometer (MFPI) , and our previous work of multimode microfiber (MMMF) based dual MZI  are developed for simultaneous measurement of RI and temperature as well as microfiber Bragg gratings (mFBGs) [21, 22] are devoted to temperature independent RI sensing. However, TMFC, MFPI, and mFBGs require UV laser photolithograph FBG, which increases equipment cost and manufacturing difficulty. Tapered fiber MZI shows similar sensitivities of the two measuring parameters due to the same working principle, which may bring imprecision in demodulation. MMMF based dual MZI still exists deficiency as the sensing structure is relative complex and incompact.
In this paper, a simple fiber sensor is demonstrated for simultaneous measurement of RI and temperature. The sensor is a multimode microfiber (MMMF) cascaded with a flat fiber endface, which is easily fabricated by nonadiabatic fiber tapering  and then fiber cleaving. The former one provides an interference spectrum while the latter one works as the spectrum reflector and intensity modulator. Dual-parameters sensing can be realized by monitoring the shift of the free spectrum range (FSR) and the fringe power along with surrounding RI (SRI) and temperature variation through sine function fit of the reflection spectrum, which is a simple but effective way to detect the SRI and temperature at the same time. Meanwhile, the reflective configuration of the senor yields easy solution for remote sensing in practical applications. Moreover, theoretical calculations are carried out to compare with the experimental results to demonstrate the feasibility and good consistency.
2. Schematic diagram and properties of the sensor
The schematic diagram of the sensor is exhibited in Fig. 1, which is constructed with three sections named Lead-in SMF (LSMF), MMMF, and Reflection SMF (RSMF). The sensor is fabricated with two simple steps: firstly, a conventional single mode fiber (SMF) is tapered down to several micrometers diameter with hydrogen flame to form the MMMF, and the two SMF sections; secondly, the tapered fiber is cleaved at the end of one SMF section to generate a flat fiber-air endface, and then the two SMF sections are defined as the LSMF and the RSMF. When light transmits from the LSMF into MMMF, two main modes i.e. HE11 mode and HE12 mode are firstly excited [24, 25].The two modes are coupled into RSMF through the second taper region to generate one MZI, and then reflected back at the endface of RSMF due to the Fresnel reflection. Next, the reflected MZI pattern is injected into the MMMF again and thus the other MZI is produced at the first taper region. It should be noted that the two MZIs have the same interference pattern due to the identical transmission path. Finally, a reflection spectrum with higher extinction ratio is obtained in LSMF by the overlapped patterns of the two MZIs. One highlight of the sensor is that the endface acts as not only a spectrum reflector for remote sensing but also a sensor to surrounding RI (SRI) variation.
The resonant dip λp of the reflected interference spectrum generated by the two modes can be expressed as:
For a tapered SMF with diameter of several micrometers, the core diameter of the tapered fiber is under sub-micrometer to be neglected, and thus, the tapered SMF are considered to be a uniform medium with the same material of the SMF cladding. β1 and β2 in MMMF can be calculated from the waveguide field equations as follows :
Due that Δneff dominates the shape of the interference spectrum and is dependent on many factors, like the diameter of MMMF, RI of MMMF material and surrounding medium, investigation on the Δneff variation trend along with these parameters seems to be very important. Figure 2(a) presents the calculated Δneff with different diameters of MMMF as a function of RI based on Eq. (2). It is clear that the Δneff of MMMF is at 10−2 order of magnitude, and it will decrease along with SRI increasing as well as the fiber diameter decreasing.
When MMMF is immersed in solution, temperature change will influence both the RI of MMMF and solution due to the thermo-optic effect, which leads to Δneff variation. Simulated Δneff of MMMF with diameter of 7.835μm in isopropanol, glycerin solution and sucrose solution along with temperature increasing are illustrated in Fig. 2 (b), which demonstrates that Δneff variation in isopropanol is larger than that in glycerin solution and sucrose solution. This is due to its high thermo-optic coefficient of −4.5 × 10−4 RIU/°C, while the values of glycerin solution and sucrose solution are −3.5 × 10−4 RIU/°C and −1 × 10−4 RIU/°C, respectively. Here, it should be noted that although the thermo-optic coefficient of silica is almost 50 times smaller than that of the glycerin solution, the thermal effect of fiber can’t be neglected as the light field is mainly distributed in the MMMF .
The reflection ratio of the interference spectrum power at the endface i.e. R can be calculated by the following equation :
3. Experimental results and discussions
Figure 3 exhibits the schematic diagram of the experimental setup, which contains an interrogation system (Micron Optics, Inc., sm125-500) including a broadband light source ranging from 1510nm to 1590nm and an optical spectrum analysis with wavelength scanning interval of 5 pm and accuracy of 1pm, and a signal processing system used to analyze and demodulate the reflection spectrum. For the sensing element, the length of the first taper region and the second taper region are around 2.4mm and 3.0mm, respectively. The uniform waist of MMMF has a length of 8.6mm and a diameter of about 7.835μm to support multimode transmission. Actually, the working part of the RSMF is its endface, while the rest part is acting as light transmission medium and could be as short as possible.
Figure 4(a) illustrates the typical reflection spectrums in air and water with SRI of 1 and 1.3320, respectively. As we can see from the picture, two main differences exist in the two reflection spectrums i.e. the FSR and fringe power. When surrounding medium changes from air to water, the FSR increases from 8.121nm to 11.814nm while the fringe power decreases from −25.469dBm to −38.806dBm. Fast Fourier transformation (FFT) of the reflection spectrums in air and water are depicted in Fig. 4 (b). It can be seen that there is only one spatial frequency peak, which means two main modes interference decides the reflection spectrum. In addition, the spatial frequency of peak1 for air is larger than that of peak2 for water, indicating smaller FSR in air than in water. As only two modes participate the interference, a sine curve is utilized to fit the interference spectrum to get the average fringe power, and then the FSR can be easily obtained from the period of the curve.
The RI experiment is implemented by immersing the sensing element in glycerin solution with different concentrations. Representative reflection spectrums for RI sensing are shown in Fig. 5(a), with the fringe power of −38.806dBm, −39.513dBm, and −40.097dBm, as well as the FSR of 11.814nm, 12.294nm, and 13.021nm at RI = 1.3320, 1.3410, and 1.3500, respectively. Figure 5(b) depicts the variations of the fringe power and FSR when RI increases from 1.3320 to 1.3550, with the sensitivities of −72.247dB/RIU and 68.122nm/RIU, respectively. Based on the relationship as , the positive sensitivity of FSR also demonstrates that Δneff decreases along with SRI increasing, which is identical with the simulation result in Fig. 2(a). Meanwhile, the negative sensitivity of the fringe power is in agreement with Eq. (2). Assuming that the RI of the RSMF core is 1.455, the calculated power-RI sensitivity should be −74.6dB/RIU. The small deviation between the experimental sensitivity and the theoretical sensitivity may come from the inaccurate value of the core RI as well as that of the calibrated SRI measured by Abbe refractometer with accuracy of only 2 × 10−4. The detectable SRI range of the sensor is predicted from 1 to 1.4200 due that the MMMF with diameter of 7.835μm will not support two modes transmission when the SRI is beyond 1.4200 based on Eq. (2).
Temperature sensing is realized by heating glycerin solution from 35°C to 60 °C. Typical reflection spectrums for temperature sensing are displayed in Fig. 5(a), with the fringe power of −41.681dBm, −41.336dBm, and −41.107dBm, as well as the FSR of 14.434nm, 14.277nm, and 14.119nm at T = 35°C, 45°C and 55°C, respectively. As presented in Fig. 5(b), when temperature goes up, the FSR decreases while the fringe power increases with sensitivities of −17pm/°C and 0.028dB/°C, respectively. The negative sensitivity of FSR is the combined action of thermo-optic effect of fiber and glycerin solution, which has been verified in Fig. 2(b). According to Eq. (2), the reflection ratio of the interference spectrum will increase as the temperature goes up, giving rise to the positive sensitivity of fringe power.
From Fig. 6, by tracking the fringe power and FSR of the reflective interference spectrum along with the RI and temperature variation, simultaneous measurement of dual-parameters can be implemented by calculating the following equations:
Certainly, the small optical power fluctuation of the light source will bring about measurement error of the fringe power. However, it can be greatly eliminated in real time by introducing a reference light from the light source through a tap coupler. Moreover, further improvement could be implemented by cutting at the uniform region of MMMF with the help of a sapphire blade or FIB to form a Fresnel reflection endface [21, 24], which can greatly shrink the sensor size. In this way, a high sensitive probe shape sensor with length of about 7mm and diameter of only 7.835μm can be constructed for dual-parameters measurement, which has great potential in medicine and physiology.
We have proposed and demonstrated a multimode microfiber sensor assisted with Fresnel reflection to realize simultaneous measurement of RI and temperature. By tracking the fringe power and FSR of the reflective spectrum, RI sensitivities of −72.247dB/RIU and 68.122nm/RIU, as well as temperature sensitivities of 0.0283dB/°C and −17pm/°C are achieved. The sensor presents unique properties including high sensitivities, compact size, simple structure, easy fabrication and low cost. Furthermore, we introduce the feasibility of a probe shape sensor with the help of a sapphire blade or FIB, which has great potential in medicine and physiology.
This work is supported by the National Natural Science Foundation of China (No. 61275004), the sub-Project of the Major Program of the National Natural Science Foundation of China (No. 61290315), the European Commission's Marie Curie International Incoming fellowship (Grant No. 328263), and the Natural Science Foundation of Hubei Province for Distinguished Young Scholars (No. 2014CFA036).
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