November 1, 2008 / Vol. 33, No. 21 / OPTICS LETTERS
2455
Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry–Perot interferometer Hae Young Choi,1 Kwan Seob Park,1 Seong Jun Park,1 Un-Chul Paek,1 Byeong Ha Lee,1,* and Eun Seo Choi2 1
Department of Information and Communications, Gwangju Institute of Science and Technology, 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea 2 Department of Physics, Chosun University, 375 Seosuk-dong, Dong-gu, Gwangju 501-759, South Korea *Corresponding author:
[email protected]
Received July 22, 2008; revised September 7, 2008; accepted September 9, 2008; posted September 23, 2008 (Doc. ID 99210); published October 21, 2008 A miniature Fabry–Perot (FP) interferometric fiber-optic sensor suitable for high-temperature sensing is proposed and demonstrated. The sensor head consists of two FP cavities formed by fusion splicing a short hollow-core fiber and a piece of single-mode fiber at a photonic crystal fiber in series. The reflection spectra of an implemented sensor are measured at several temperatures and analyzed in the spatial frequency domain. The experiment shows that the thermal-optic effect of the cavity material is much more appreciable than its thermal expansion. The temperature measurements up to 1000° C with a step of 50° C confirm that it could be applicable as a high-temperature sensor. © 2008 Optical Society of America OCIS codes: 060.2370, 120.3180, 230.1150.
The interferometric fiber-optic sensors based on Mach–Zehnder, Michelson, and Fabry–Perot (FP) interferometers have been developed for various physical and chemical sensings [1–3]. These sensors have lots of advantages including high resolution, simple configuration, good electromagnetic interference immunity, and low cost. Among them, the FP interferometric sensor is particularly attractive owing to its small cross sensitivity and extremely small form factor. The fiber FP interferometric sensors can be classified into two big categories: extrinsic and intrinsic. The extrinsic FP sensor utilizes two fiber pieces physically separated but bonded with an external housing material [4,5]. Although the extrinsic one has found various applications, it has some disadvantages of limitation in the sensing cavity length owing to high coupling loss and difficulty in bonding. The intrinsic FP sensor, however, has the sensing element in the fiber itself and thus can overcome many of the problems of the extrinsic ones [6,7]. However, it has extremely low reflectivity owing to small refractive index difference along the cavity structure. Most recently, hybrid structured FP interferometric sensors have been introduced for temperature, strain, and refractive index measurements [8,9]. Some of the reported schemes have used a combination of a longperiod fiber grating and a microair cavity, or sophisticated multicavities; however, they needed complicate fabrication processes such as chemical etching and/or laser micromachining. In this Letter, we report the hybrid structured fiber FP sensor implemented by cascading a photonic crystal fiber (PCF), a hollow optical fiber (HOF), and a single-mode fiber (SMF). The reflection spectra of the proposed FP interferometer are measured at several temperatures up to 1000° C and analyzed in the spa0146-9592/08/212455-3/$15.00
tial frequency domain by taking the fast Fourier transform (FFT). The schematic of the proposed sensor system is presented in Fig. 1. The detailed structure of the sensor head and the microscope photograph of the implemented one are also included. To implement the sensor head, at first, the HOF was cleaved and spliced with the PCF. After cleaving the HOF side of the PCF–HOF structure at a desired HOF length, the SMF was spliced. Finally, the SMF side of the PCF– HOF–SMF structure was cleaved again to have a desired SMF length. In this scheme, owing to the interface with the air core of the HOF, the SMF segment can behave as a FP cavity, the main sensing cavity. At the same time, the micro-air-gap of the HOF segment itself forms an additional FP cavity behaving as an auxiliary one. A commercial PCF (LMA-10, Crystal Fibre Co.) was used as a guiding fiber. The HOF was fabricated by drawing a silica tube with a standard fiber drawing tower to have a 40 m center hole and
Fig. 1. (Color online) Schematic of the sensor system (top) and the detail structure of the proposed sensor head (bottom). The inset in the middle is the microscope photograph of a fabricated sensor head. © 2008 Optical Society of America
2456
OPTICS LETTERS / Vol. 33, No. 21 / November 1, 2008
the free spectral range of the cavity [9]. However, in our case, since the system is composed of double cavities, a little more attention is necessary to explain the spatial frequency spectrum. The dc peak in Fig. 2(b) is from the source spectrum, and the one just next to it is from the HOF cavity. The left peak in the middle group is from the SMF cavity, and the nearby peak of it is from the long cavity formed by combining both cavities. The other two small peaks are thought to be resulted from the double passes along the SMF cavity and the combined cavity, respectively. To demonstrate the feasibility as a high temperature sensor, a series of reflection spectra were measured at temperatures varying from 50° C to 1000° C with a step of 50° C. The temperature-induced variation of each noticeable spatial frequency peak was calculated and presented with Fig. 3. Interestingly, the first peak marked in Fig. 3(a) was not affected by temperature as shown in Fig. 3(b). The two peaks in the middle, Figs. 3(c) and 3(d), were shifted toward the high frequency direction with similar amounts. The other two small peaks in the right, Figs. 3(e) and 3(f), were also shifted to the same direction but with almost twice the amounts. In general, the round-trip optical path length (OPL) of an FP cavity are simply given by lOPL = 2nl,
Fig. 2. (Color online) (a) Reflection spectrum of the proposed sensor and (b) its spatial frequency spectrum obtained by taking the FFT. The inset of the wavelength spectrum is the close-up of the middle part marked with a dotted square box.
125 m outer diameters. In the experiment, the lengths of the HOF cavity 共l1兲 and the SMF cavity 共l2兲 were designed to be 70 and 510 m, respectively. To get the desired cleaving length with a high resolution, a specially designed microtranslation stage system was used. The fusion splicing was performed with a commercial fusion splicer (S183PM, FITEL Co.). The optimum arc power and arc time for getting good interference visibility were empirically found to be 100 units and 300 ms, respectively. With these efforts the total length of the sensor head, defined by the sum of both cavity lengths, could be kept as short as 580 m. Figure 2(a) with the inset figure for detail observation shows the measured reflection spectrum of the proposed FP interferometer; a fast varying fine fringe pattern is modulated with a slow varying coarse one. Of course, the overall envelope of the spectrum is the power spectrum of the light source itself. We believe that the fine fringe pattern results from the long SMF cavity and the coarse one is due to the short HOF cavity. The spatial frequency spectrum of Fig. 2(b) was obtained by taking the FFT on the wavelength spectrum of Fig. 2(a). In a single FP cavity system, the spatial frequency is well explained with
共1兲
where n is the refractive index and l is the physical length of the cavity. The OPL depends on temperature through the thermal expansion and/or thermooptics effect:
Fig. 3. (Color online) (a) Spatial frequency spectra obtained from the reflection spectra measured at several temperatures, and the detail of the frequency shift magnified at peaks (b) 1, (c) 2, (d) 3, (e) 4, and (f) 5, respectively.
November 1, 2008 / Vol. 33, No. 21 / OPTICS LETTERS
Fig. 4. (Color online) Temperature-induced shifts of all appreciable spatial frequency peaks in Fig. 3. The shifts happened in three distinct groups.
⌬lOPL = 2
冉
dn dT
dl l+n
dT
冊
⌬T ⬅ lOPL共T + ␣T兲⌬T, 共2兲
where T is the thermo-optics coefficient and ␣T is the thermal expansion coefficient. For the HOF cavity, since the cavity is empty (air in precisely), we can ignore the thermo-optics effect, but for the SMF cavity, both terms should be considered, which gives ⌬lOPL1 ⯝ lOPL1␣T1⌬T,
⌬lOPL2 = lOPL2共T2 + ␣T2兲⌬T, 共3兲
where the subscripts 1 and 2 represent the HOF cavity and the SMF cavity, respectively. The spatial frequency of a wavelength spectrum is approximately given as [10,11]
=
1 1 2
lOPL ,
共4兲
where 1 and 2 are the wavelengths of two adjacent interference fringe peaks. The equation says that the spatial frequency shifts in Fig. 3 can be explained with the temperature-induced variation of the OPL. In Fig. 3(b) the peak produced by the HOF cavity stayed practically constant over the entire temperature range, which means the thermal expansion of the HOF cavity was small enough to be neglected. However, the peak by the SMF cavity was appreciably shifted, as shown in Fig. 3(c), which means that the thermo-optic effect was dominant. We note that the cladding—the majority part—of the SMF is made of fused silica, thus the SMF has a similar thermal expansion coefficient with the HOF. Therefore, the two shifts in Figs. 3(e) and 3(f) are smoothly explained by the double round-trip passes in the SMF cavity. In Fig. 4 the shifts of all spatial frequency peaks are plotted together for mutual comparison. We can
2457
see that the data points are aggregated into three groups. The response of peak 1, which forms the bottom group, is negligible. Peaks 2 and 3 forming the middle group have the moderate sensitivities. Finally, peaks 4 and 5 in the top group have almost two times higher sensitivities than the ones in the middle group. This measurement confirms the analysis related with Fig. 3. The OPL of the bottom group is not related with the SMF cavity. The OPLs of the middle group include only a single round trip along the SMF cavity, and the ones in the top group result from two round trips. The doping material of the SMF has a relatively high thermo-optic coefficient and thus allows high temperature sensitivity. As the guiding fiber the used PCF, which did not have a doped core, had negligible temperature sensitivity. In conclusion, we have demonstrated a compact FP-type fiber sensor suitable for high-temperature measurements. A tiny sensor head was made at the end of a PCF by fusion splicing a 70 m segment of HOF and a 510 m long SMF segment in series. The temperature response of the proposed sensor was measured up to 1000° C and analyzed in the spatial frequency domain of the reflection spectrum. The sensing sensitivity was mainly resulted from the thermo-optic coefficient of the SMF core, but the thermal expansions of the fibers and fiber structures were negligible. It is expected that the proposed device would be used in realizing an ultracompact sensor for biomedical or chemical sensing applications. This work was supported in part by the BK21 project at the Gwangju Institute of Science and Technology (GIST) and by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean Ministry of Science and Technology (MOST) (R01-2007-000-20821-0). References 1. H. Y. Choi, K. S. Park, and B. H. Lee, Opt. Lett. 33, 812 (2008). 2. L. Yuan, J. Yang, Z. Liu, and J. Sun, Opt. Lett. 31, 2692 (2006). 3. Y. Chen and H. F. Taylor, Opt. Lett. 27, 903 (2002). 4. B. Yu, G. Pickrell, and A. Wang, IEEE Photon. Technol. Lett. 16, 2296 (2004). 5. Y.-J. Rao, Opt. Fiber Technol. 12, 227 (2006). 6. W.-H. Tsai and C.-J. Lin, J. Mosc. Phys. Soc. 19, 682 (2001). 7. Z. Huang, Y. Zhu, X. Chen, and A. Wang, IEEE Photon. Technol. Lett. 17, 2403 (2005). 8. Y.-J. Rao, Z.-L. Ran, X. Liao, and H.-Y. Deng, Opt. Express 15, 14936 (2007). 9. Y. Zhang, X. Chen, Y. Wang, K. L. Cooper, and A. Wang, J. Lightwave Technol. 25, 1797 (2007). 10. H. Y. Choi, M. J. Kim, and B. H. Lee, Opt. Express 15, 5711 (2007). 11. J. Xu, X. Wang, K. L. Cooper, and A. Wang, Opt. Lett. 30, 3269 (2005).
