A power-interrogated sensor which allows for simultaneous measurement of temperature and torsion is proposed and experimentally demonstrated, which is based on utilization of paired helical long-period fiber gratings (HLPG) with opposite helicities. Unlike most of the previous fiber grating-based sensing system, here the paired HLPGs are simultaneously used as both the sensing and the interrogating elements and thus the bulk and high-cost wavelength-interrogating device can be eliminated. Moreover not only the torsion but also the torsion direction can be determined simultaneously. The temperature sensitivity obtained is estimated to be ~41 pm/°C within a range of 20-150 °C, and the torsion responsivities obtained are ~-1.414 nm/rad/m and ~1.276 nm/rad/m, respectively within a rotation angle of −360°~360°.
© 2014 Optical Society of America
In the past few decades, long-period fiber grating (LPG) based sensors have been comprehensively studied and have found wide applications in the fields of civil engineering, industry, biomedicine, chemistry, etc [1–9]. There are two common interrogation methods that are used for LPG-based fiber sensors: a wavelength-interrogation approach and a power/intensity interrogation approach. For the first method, to precisely measure a wavelength shift, either an optical spectrum analyzer (OSA) with a high wavelength-resolution or an extremely narrow line-width tunable laser with both a wide tuning region and a high wavelength-scanning speed are desired. However, all of the devices mentioned above are extremely expensive, bulky, and thus not available for most practical applications. For the second approach, an additional linearly wavelength-dependent optical filter (or the so-called optical edge filter) is generally desired, which could linearly transform the wavelength shift into a change in optical power. Thus, only an optical power meter is needed for the measurement instead of an OSA, which allows for a more rapid, more compact, and far more cost-effective system than the former method. However, an additional wideband optical edge filter is needed, which is rather difficult to obtain in optics. To date variety of optical torsion sensors have been reported with utilizing different fiber devices. Such as titled fiber Bragg grating (TFBG) , distributed Bragg reflector (DBR) fiber laser , coreless square-fiber  and LPFGs [13–19] based torsion sensors. Most recently, various photonic crystal fibers (PCFs) based torsion sensors have been proposed and demonstrated [20–24]. However, all of the torsion sensors reported above have been developed either by using the cost-inefficient wavelength-interrogation method or by adding an additional power-interrogating optical filter as well as the expensive PCFs. On the other hand, it is commonly known that for LPGs based sensor systems [13–19], the resonance wavelength of a LPG is very sensitive to both temperature and torsion, thus, there has always existed a need to develop a simple and new technique enabling to discriminate these two effects simultaneously. The simultaneous measurement of temperature and torsion is an important topic and strongly desired for practical application of the fiber sensor.
In this paper, a novel HLPGs-based sensor that allows for simultaneous measurement of the temperature and torsion angle within ranges of 20 °C-150 °C, and −360°-360° on a 0.277 m twisted fiber, respectively, is proposed. The proposed sensor is based on the use of paired HLPGs but with opposite helicities, i.e., one is clockwise-helical LPG (cHLPG), the other is counterclockwise–helical LPG (ccHLPG). Unlike most of the previous reports on torsion sensors [1–24], a power-interrogation technique was only employed in this study, without the need of the generally-used wavelength measurement. Moreover, the proposed paired HLPGs were simultaneously utilized as both the sensing and the interrogating elements.
2. Principle and experimental setup
Figure shows experimental setup for fabrication of the paired-HLPGs, where the sapphire tube is heated by using a high-frequency CO2 laser (SYNRAD-20, emission central wavelength of 10.6 μm) which works at a high repetition rate (5 kHz) and with an output power of 18 W. In the experiments conducted in this study, the paired HLPGs were fabricated by homogenously twisting two sections of a single-mode fiber (SMF) (FutureGuide®-SR15E provided by Fujikura Inc.) through a rotation motor (ThorLabs:PRM1/MZ8) while it passes through a sapphire tube (to be used as a miniature oven). Noted that here the sapphire tube is fixed but the fiber is moved passing through the tube at constant of 40 μm/s by using a motored translation stage (ThorLabs:MTS50/M). At beginning, the first section of the SMF was continuously twisted to the clockwise direction with the rotation speed of 19°/s until that a cHLPG was successfully fabricated, which has a length of 25 mm and grating period of 758 μm. Sooner after the cHLPG was generated, the second section of the SMF was then twisted to the counterclockwise direction at rotation speed of −20 o/s, and a ccHLPG with a length of 30 mm and a grating period of about 720 μm was fabricated. Separation between the paired HLPGs was about 3 mm in our case. Inset given in Fig. 1 schematically shows the structure of the paired HLPGs. Transmission spectrum of the two cascaded HLPGs was measured by using an optical spectrum analyzer (OSA) at room temperature, which is shown in Fig. 2. Itcan be seen that there exist two attenuation peaks located at wavelengths near 1540 nm and 1570 nm, respectively (corresponding to the HLPGs with period of 720 μm and 758 μm), which can be attributed to the strong coupling between the forward core-mode and one of cladding-modes. Moreover, it can be seen that the transmission spectrum of the paired HLPGs nearly contains two linear regions, i.e., P1 (1530-1540 nm) and P2 (1572-1582 nm) with a wavelength span of ~10 nm, slopes of which are a little different and with opposite signs.
In order to investigate the thermal and torsional performance of the paired-HLPGs, we first measured the transmission spectra at three different temperatures (i. e., 30 °C, 90 °C, and 150 °C) respectively by employing a wide-band optical source (ASE) and OSA while no torsional stress was applied. It is found that as the temperature is increased, the spectrum is linearly shifted to the right direction and the thermal responsivity obtained is ~41 pm/°C, which is nearly the same level with that reported in . We then measured the transmission spectra of the paired HLPGs under three different torsional angles (i.e., -360°, 0°, and 360°) at room temperature, which are shown in Fig. 3. It is very interesting to find that when the paired HLPGs are twisted in the clockwise direction (for example 360o), attention peaks (resonant wavelengths) of the cHLPG and ccHLPG are shifted conversely, i.e., they are linearly shifted to a shorter and longer wavelength, respectively. In a counterclockwise (for example −360o) twist, the opposite results to the above can be obtained. Torsional responsivities for the obtained cHLPG and ccHLPG are ~-1.414 nm/rad/m and ~1.276 nm/rad/m, respectively, which are the same level as those reported in . Moreover, it can be easily found that as a torsional stress is applied to the fiber in which the paired HLPGs are included, the linear regions P1 and P2 shown in the initial spectrum shift linearly to the opposite directions which in return means that both the torsion angle and the torsional direction can be simultaneously determined from the spectra shifts of the paired HLPGs. Therefore, we believe that one can find a suitable application of the paired HLPGs to power-interrogation measurements, which are strongly desired for the practical application of all-fiber-based sensors . As a typical application example, Fig. 4 shows an experimental setup for simultaneous measurement of temperature and torsion, where the HLPGs represents the paired HLPGs with the same transmission spectrum as shown in Fig. 2. The paired HLPGs were laid in a temperature chamber and mounted on rotation motor at near end and a fiber holder at far end. Length of the twisted fiber L (between the fiber holder and the rotation motor) is 0.277 m. Moreover, the HLPGs were placed in a temperature chamber, allowing the temperature to be discretely varied from 20 to 150 °C with a resolution of ± 1°C. LD1 and LD2 represent two distributed-feedback semiconductor lasers (DFB) that have an optical output power of approximately 2 mW and operate at wavelengths of 1538 nm and 1581 nm, respectively. An optical switch was used to select the individual light from the two DFBs at each measurement instant. PD1 is a power meter which was utilized to monitor the input power through a 90:10 coupler. PD2 is another power meter that was used to measure the output powers of the transmitted light (from the paired HLPG) at the two wavelengths, respectively. Both of the power meters have a power resolution of about 0.01 dB. Since the spectral shifts shown in Fig. 3 are very sensitive to both the temperature and the torsion, and changes linearly with these two parameters; therefore, the power variations measured at PD2 (as shown in Fig. 4) can be expressed as:Eq. (3), the condition of must be satisfied. Moreover, for high-resolution measurement, a large absolute value of is strongly desirable . Fortunately, the above two conditions are inherently satisfied in our case, because from Fig. 2 and Fig. 3, one can easily deduce that the parameters A1 and A2 have opposite signs, meanwhile the parameters B1 and B2 have the same signs owing to the two HLPGs which have opposite helicities. On the other hand, it must also be noted that for a practical application, the utilized sensor is generally mounted or embedded into a composite object rather than the stages as shown in Fig. 4, which will inevitably make the grating‘s spectrum changes a little and results in wavelength shifts for the two critical linear regions P1 and P2 shown in Fig. 2, therefore for practical application, any changes for the spectrum of the HLPGs must be verified again after the sensor is embedded into the testing object and then the two critical linear regions P1 and P2 in the spectrum can be simply determined by using an OSA.
3. Experimental results and analysis
To allow the proposed sensing system to operate in the linear region, working wavelengths of the two lasers were fixed to 1538 nm and 1581 nm, respectively. In order to determine the four coefficients in Eq. (1) and Eq. (2), we then experimentally investigated the dependence of the output power on temperature and the applied torsion, respectively by using the setup shown in Fig. 4. Figure 5 shows the measurement results for dependence of the relative output powers (i.e.,(PD2-PD1 + 30) dBm) on temperature at wavelengths 1538 nm (LD1) and 1581 nm (LD2), respectively, where no torsional stress was applied to the paired HLPGs. A good linear relationship (R21538nm = 0.9901 and R21581nm = 0.9887) between the output powers and the ambient temperature was observed. Slopes of the two fitted lines are 0.0373 dB/°C and −0.0208 dB/°C, respectively. Compared with the fitted data, accuracy of the temperature measured was estimated to be ± 2 °C within the temperature range of 30 °C −150 °C. Figure 6 shows the measurement results for the dependence of the relative output power on the applied torsional stresses at room temperature. A good linear relationship (R21538nm = 0.9961 and R21581nm = 0.9891) can also be observed for both wavelengths and slopes of the fitted lines are 0.0073 dB/o and 0.0029 dB/o, respectively. Compared with the fitted data, accuracy of the torsional angle measured was estimated to be ± 3° within the torsional angle range of −360°~360°. To verify the accuracy and the repeatability of the proposed sensing scheme for simultaneous measurement of temperature and the applied torsional stress, we further investigated the dependence of the relative output power on the temperature under the condition of three different torsional stresses (−360°, 0° and −360°) and the dependence of the relative output power on the applied torsional stress under the condition of three different temperatures (30 °C, 90 °C, and 120 °C), respectively. The measuring results are shown in Fig. 7, and Fig. 8, respectively. From both the Figs. 7 and 8, it can be seen that for all the six cases given above, there nearly exists an ideal linear relationship between the relative laser powers and the ambient temperature (shown in Fig. 7); and the applied torsional stress (shown in Fig. 8). Moreover, the corresponding slopes of the fitted lines shown in Figs. 7 and 8 are almost the same as those obtained in Fig. 5 and Fig. 6, respectively (with fractional deviation of 2% for all the three cases), which in return means that the cross-sensitivity between torsional stress and temperature and high order of coefficients of B1 and B2 in terms of the temperature  can be neglected in our case. Average slopes for A1 and A2 in Eq. (3) are 0.0371dB/°C and −0.0204 dB/°C, respectively; for B1 and B2 are 0.0074 dB/o and 0.0028 dB/o, respectively.
Equation (3) then can be rewritten as:10–18]. In view of the above results (shown in Figs. 7 and 8), it can be concluded that the four coefficients A1, A2, B1, and B2 obtained here are temperature- and torsion-insensitive, which in return means that the proposed system works well and has a small cross-correction in term of the temperature and the applied torsion.
In conclusion, a novel sensor which allows for the simultaneous measurement of temperature and twisted angle within ranges of 20 °C-150 °C and −360° to 360° on a fiber with length of 0.277 m (−22.67 rad/m to 22.67 rad/m), respectively, is proposed, which is based on the use of paired HLPGs with opposite helicities. Temperature and torsion resolutions for the proposed system are estimated to be about ± 0.4 °C and ± 2.3°, respectively. Unlike most of the previous reported temperature and torsion sensors, the paired HLPGs were simultaneously utilized as both the sensing and the interrogating elements and thus only a power-interrogation technique was employed in this study without the need of the generally-used wavelength-shift measurement. Moreover, both the torsion and torsional direction can be simultaneously determined in the proposed sensing system, which potentially provide a higher speed, more compact, and more cost-effective sensing system than ever before.
This work was partly supported by the Grant-in-Aid for JSPS in Japan.
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