A hybrid fiber-optic sensor consisting of a long-period fiber grating (LPFG) and a micro extrinsic Fabry-Perot (F-P) interferometric (MEFPI) sensor is proposed and demonstrated for simultaneous measurement of high-temperature and strain. The LPFG written by using high-frequency CO2 laser pulses is used for high-temperature measurement while the MEFPI sensor fabricated by using 157nm F2 laser pulses is used for strain measurement under high temperature. The distinguishing feature of such a hybrid fiber-optic sensor is that it can stand for high temperature of up to 650°C and achieve precise measurement of strain under high temperature conditions simultaneously.
©2007 Optical Society of America
Simultaneous measurement of temperature and strain based on different combinations of in-fiber Bragg grating (FBG) sensors or/and fiber-optic extrinsic Fabry-Perot interferometric (EFPI) sensors has attracted intensive interests of researchers [1–5], as the thermal strain induced by temperature change for the strain sensor can be directly compensated via the temperature sensor nearby. These hybrid temperature-strain sensors can find wide applications in the field of structural health monitoring, such as composite materials, large structures (bridges, tunnels, etc), aerospace vehicles, and aircrafts, to achieve so-called smart structures . But, the schemes reported to date can not be employed under high temperature(e.g. 650°C) environments in general. This is because both of the FBG written by UV laser exposures and the EFPI sensor formed by inserting two optical fibers into a glass capillary manually can not stand for such a high temperature. As we know, precise strain measurement is essential for many high-temperature applications, such as health monitoring of engines, aeronautics test of aircrafts and airplanes, and production process monitoring of composite materials, etc. In this paper, we describe a novel hybrid fiber-optic sensor formed by cascading a long-period fiber grating (LPFG) and an in-line micro EFPI (MEFPI) sensor. The beauty of such a sensor is that as both the LPFG and the MEFPI sensor are fabricated by laser pulses it can stand for high temperatures of up to 650°C and hence achieve precise measurement of strain under high temperature environments. The LPFG used for the high-temperature measurement is written by a high frequency CO2 laser based on the permanent refractive index change on the fiber, induced by the thermal shock effect of the high frequency CO2 pulses, hence leading to a much better thermal capability over the FBG written by UV laser pulses . The MEFPI sensor used for the strain measurement is fabricated by a 157nm F2-excimer laser directly on the cleaved end of a single-mode optical fiber. For the last decade, excimer lasers have been widely utilized to many micromachining applications due to high fabrication accuracy and good surface quality resulting from cold machining of this type of lasers [8–10]. Therefore, such a laser micromachining technique can offer significant advantages over traditional ways of making EFPI sensors in terms of precision, operation, cost and repeatability during manufacturing. More importantly, the long-lasting problem with the conventional EFPI sensor, i.e. the bonding strength is not good enough or the bonding adhesive could become aged quickly at high temperature, which would make the sensor unstable and even unusable, can be overcome by using the all-fiber MEFPI sensor that is self-enclosed inside the fiber.
2. Operating principle
The measurement principle of the hybrid LPFG/MEFPI sensor for simultaneous measurement of high-temperature and strain is based on the sensing properties of the LPFG and the EFPI sensor, respectively. According to phase-matching condition and mode-coupling theory, the drift of the resonant wavelength has a linear relationship with the temperature change approximately .
For the MEFPI sensor, according to the interferometry theory, the constructive interference occurs in the condition that a phase shift between successive light beams meets:
where l is the initial length of the MEFPI cavity, λp is the light wavelength of the peak value, n 0 is the refractive index of air. When a strain is applied to the MEFPI cavity, the length of the cavity will change, resulting in a phase shift of the interference fringes. If this shift is so small that the phase shift is <<2π, we could consider that the original wavelength and the new wavelength are of the same fringe of m-th, i.e.
where KlF-P is strain amplification factor because the effective section area of the F-P cavity is less than that of the fiber without being ablated, and K (F-P)ε is the wavelength-strain sensitivity of the MEFPI.
As the LPFG is encapsulated in a quartz tube with a strain-free state in practical applications, we can obtain ΔT and ε as follows:
where K (LPFG)T, K (F?P)T are the wavelength-temperature sensitivities of the LPFG and MEFPI cavity, respectively. K (LPFG)T, K (F?P)T and K (F-P)ε can be obtained by experiment, respectively.
3. Fabrication of the MEFPI sensor
The LPFG, written in a standard single-mode fiber by using the high frequency CO2 laser pulses exposure method, was inserted into a 60mm long quartz tube whose inner and outer diameters are 128um and 300um, respectively. The LPFG was fixed inside the quartz tube by welding to keep the LPFG in a strain-free state. The MEFPI cavity was fabricated onto the end of a single-mode fiber (SMF-128) by a 157nm F2-excimer laser micromachining system which is similar to that used in reference . The energy density of the laser beam delivered onto the fiber end was 12J/cm2 at a frequency of 20Hz. The fiber absorbed such a strong ultraviolet radiation at 157nm, resulting in a square hole with a smooth bottom and a depth of ~30um after 160 pulses hit the fiber end, as shown in Fig. 1. By splicing the ablated fiber to another fiber cleaved, the square hole was self-enclosed inside the fiber, using a commercial arc-fusion splicing machine (S182A, Fitel, Japan), to generate a MEFPI sensor automatically.
The transverse view of such a micro cavity was obtained from the splicing machine, as shown in Fig. 2.
4. Experimental results
The experimental set-up employed for simultaneous measurement of high-temperature and strain is shown in Fig. 3. Both reflection and transmission measurements were performed to monitor the outputs of the MEFPI sensor and the LPFG, respectively, by using a high-accuracy optical spectrum analyzer (OSA) (Si720, Micron Optics, USA) with a wavelength scanning range of 1510nm~1590nm and a wavelength resolution and accuracy of 2.5pm. Light from the sweeping laser source in Si720 was lunched into the hybrid sensor via a circulator. The reflective spectrum of the MEFPI cavity was displayed in Fig. 4. It can be seen that an excellent visibility of ~30dB was achieved. The transmitted light passed through the MEFPI and then entered into the LPFG. The mixed spectrum of the MEFPI cavity and the LPFG is shown in Fig. 5. It can be seen that the signal of the LPFG is much stronger than that of the MEFPI as the LPFG only works at transmission.
The MEFPI sensor and the LPFG were put into a ceramic tube of a high temperature furnace (Lenton, UK). Two high-accuracy translation stages (Newport 561D), fixed on both sides of the high temperature furnace with a spatial distance of 1m, were used for applying strains to the sensor by stretching it from one end of the fiber, as shown in Fig. 3. The temperature resolution of the furnace is 1°C. The test was started from 100oC with a temperature interval of 50°C, and stopped at 650°C. In fact, the maximum temperature this sensor can sustain would be as high as ~800°C as the softening point of silica is around 1000°C, and this also depends on the metal film that is required to be coated on the fiber for sensor protection in practical applications.
When the temperature of the furnace was hold at a specified value, strains were then applied to the sensor by adjusting the distance between the two fixed ends of the fiber from 0um to 500um with an interval of 25um.
Figure 6 displays the high-temperature response of the LPFG and it can be seen that the resonant wavelength of the LPFG has a very good linear relationship with temperature change. The wavelength-temperature sensitivity K (LPFG)T of the LPFG is 0.1142nm/°C, indicating the LPFG is sensitive to temperature change. Figure 7 gives the high-temperature response of the MEFPI under a strain-free state and its strain response at 500°C is shown in Fig. 8. The wavelength-temperature and wavelength-strain sensitivities, K (F-P)T and K (F-P)ε, of the MEFPI are 0.0009nm/°C and 0.0052nm/με, respectively. Hence, the achieved temperature and strain resolutions for the hybrid LPFG/MEFPI sensor were ~0.02°C and ~0.5με, respectively, when the wavelength resolution of the OSA was set at 2.5pm.
In this paper, we report a novel hybrid fiber-optic sensor consisting of a long-period fiber grating (LPFG) and a micro extrinsic fiber-optic interferometric Fabry-Perot (MEFPI) sensor, based on the complimentary properties of the LPFG, which is sensitive to temperature but not to strain, and the MEFPI sensor, which is sensitive to strain but not to temperature, to achieve simultaneous measurement of high-temperature and strain. As the LPFG and the MEFPI sensor are both fabricated by using laser pulses of different types, they can stand for high temperature of up to 650°C and achieve precise measurement of strain under high temperature, this is verified by the experiments. This work represents a breakthrough towards realizing a new generation of high-temperature fiber-optic sensors which can be fabricated by using laser micromachining.
This work is supported by the Key Project of Natural Science Foundation of China (Grant 60537040).
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