We proposed and experimentally demonstrated a compact micro-displacement sensor with high sensitivity based on a long-period fiber grating (LPG) with an air-cavity. The sensor head is obtained by composing an air-cavity with the ends of a LPG and a single mode fiber (SMF). The wavelength shift of the LPG has a linear relationship with the length of the air gap which agrees well with the theoretical analysis. The experimental results show that the sensitivity is ~0.22 nm/µm within the micro-displacement range of 0 to 140 µm.
© 2013 OSA
Displacement measurement is one of the key factors in various fields of nanotechnology systems, such as micro-manufacture, precisely positioning, scanning tunneling microscopy (STM), atomic force microscopy (AFM) and so on. Several types of displacement sensors, such as capacitive sensors and inductive sensors, have been proposed [1, 2]. Compared with displacement sensors based on electronic components, fiber optical displacement sensors have important advantages of electrically-passive operation and immunity to electromagnetic interference. Recently, fiber grating based displacement sensors have been widely studied, which mainly use fiber Bragg grating (FBG) [3–8], tilted fiber Bragg gratings (TFBGs)  and long-period gratings (LPGs) . Most of the proposed FBG displacement sensors are structures that glue a FBG on a cantilevered beam [4–8]. Those sensors need employ relatively complicated force transfer components, leading to a large error introduced to the sensor, so that they are not practical for micro-displacement measurement.
Due to a large period of perturbations along longitudinal direction of optical fiber, LPG couples light from the fundamental core mode to some cladding modes inducing a series of attenuation bands in the transmission spectrum. LPGs have been employed to measure the displacement variation utilizing the property of LPGs offering high bending sensitivity . However, the large volume and complicated fabrication of the LPG displacement sensor is unsuitable for applications in confined spaces such as precision instruments, piezoelectric device etc.
In this paper, a compact micro-displacement sensor with high sensitivity based on a long-period fiber grating with an air-cavity is proposed and experimentally demonstrated. The cavity is composed by the ends of an LPG and a single mode fiber. The transmission spectrum of the LPG is modulated by the air-cavity leading to the dip wavelength of the LPG gradually shift with the length of the air-cavity increasing. By monitoring the dip wavelength shift of the LPG, we can obtain the variation of the length of the air-cavity. Experimental results show that the proposed LPG-based displacement sensor works well, and the sensitivity of the sensor within the measurement range of 0 to 140 μm is about 0.22 nm/μm. Compared with FBG displacement sensors, the proposed displacement sensor based on LPG with an air cavity has a high sensitivity.
2. Experimental setup
Figure 1 shows the proposed micro-displacement sensor that consists of a LPG and a SMF. In the experiment, the LPG and the SMF are packaged inside a fiber capillary with inner diameter, outer diameter and length of 127 μm, 300 μm and 3cm, respectively, which is used to align the two fibers and to protect the sensor from environmental damage. The SMF is glued on the fiber capillary, while the LPG can be moved axially along the capillary. The LPG and the fiber capillary are clamped in two holders which are fixed on two translation stages. The gap between the LPG’s end and the SMF’s end is used as the sensing part, and is controlled within 200 μm by adjusting one translation stage. The transmission spectra are measured by using a superluminescent light-emitting diode light source (SLED) whose wavelength range is from 1200 nm to 1700 nm, and an optical spectrum analyzer (OSA, YOKOGAWA735301) with a spectral resolution of 0.02 nm. The LPG used in the experiment is fabricated by using a high-frequency pulsed CO2-laser (CO2-H10, Han's Laser) with a maximum average output power of 10 W. The detail manufacturing process refers to our team's relavent works . The grating pitch of the LPG is about 530 μm and the grating length is 20.12 mm (40 grating periods).
Figure 2 shows the transmission spectra of the used LPG and the proposed sensor. For the original LPG, a resonant dip can be observed at about 1559 nm wavelength and the depth of the dip is nearly 35 dB which is due to the fundamental core mode coupling to one cladding mode. For the proposed structure, the light from the end of the LPG directly launches into the air cavity and interferes in the air cavity formed by the ends of the LPG and the SMF. It is shown that the output intensity of the proposed sensor in the whole wavelength range is lower than that of the original LPG due to the air cavity induced loss. The resonant wavelength shifts towards a shorter wavelength direction, and the depth becomes smaller. Meanwhile, the interference fringes appear in the range of 1400nm to 1500nm, as shown in the dashed rectangle in Fig. 2.
3. Sensing principle
In the proposed structure, the air cavity is cascaded closely to the LPG’s grating area. Thus, both lights distributed in the core and the cladding at the end of the LPG, which are just called the “core” mode and the “cladding” modes and are formed by mode coupling with phase matching condition, enter into the air cavity and interfere simultaneously. The transmission lights from the interference cavity launch into the SMF (right), then propagate along the SMF. Since large propagation losses of cladding modes, only core mode exists in the SMF after propagating several centimeters. The diagrammatic sketch of the above process is shown in the lower right of Fig. 1.
According to the mode coupling theory, when the light propagates through the LPG by the length of L, the amplitudes of the core mode and the v-th co-propagating cladding mode can be expressed as following [12,13],Eq. (1) can describe the modified power distribution between the core mode and the co-propagating higher order modes at the output end of a LPG. For one given LPG, the amplitudes of the core mode and the cladding mode at one certain wavelength are determinate. Then those modes launch into the air cavity simultaneously, and the interference happens here. Here, the matrix method is used for analysis the function of the air cavity since the air cavity can be considered as a phase shift which applies on all the light inside.
For an air cavity, its matrix can be expressed as 
According to Eq. (9), (10), and (11), the parameters of, , in Eq. (4) are constants. For one given wavelength, the transmission matrix of is only dependent on the parameter of . In long-distance transmission, because of the attenuation of cladding modes, only core mode can be detected at the receiving terminal. So the above equation can be simplified asEquation (13) contains a periodic factor of which makes the a periodic equation of , so has its minimum value where equals to a certain constant . In addition, the consists of an initial value and the displacement variation of . Then Eq. (11) can be expressed asEq. (14) will become bigger and then the sensitivity is enhanced.
4. Experimental results
Figure 3 shows the transmission spectra of the proposed sensor when displacements variation step of 10 μm in a range of 0-140 μm is used in the experiment. Increasing the length of the air cavity causes the resonant wavelength to shift towards a longer wavelength direction. The resonant wavelength of the interference fringe, shown in the dashed rectangle, also shift towards a longer wavelength direction, but the shift of the wavelength is not strong.
With the displacement variation increasing from 0 to 140 μm, about 30 nm wavelength shift is achieved. According to the experiment, we get constant equaling to 29.16. The resonant wavelength responding to displacement variation is plotted in Fig. 4 and shows a good linearity which agrees well with Eq. (14). The sensitivity is just the slope of the curve in blue and as high as ~0.216 nm/μm. Furthermore, the intercept of the ordinate axis in Fig. 4 can be controlled by changing the initial cavity length . Because of the phase modulation factor of , the transmission intensity of the resonant wavelength has certain periodicity. It can also be seen from Fig. 3 that the transmission intensity at resonant wavelength changes with the increasing length of the air cavity, and reaches the minimum value when the displacement variation is about 50 μm. This trend can also be used as intensity demodulation.
For comparison, we also investigated the performance of the air cavity formed only by two SMF ends, as shown in Fig. 5 . The interference extinction ratios are low and about 0.25 dB. As the variation of displacement increases, the interference spacing becomes smaller and the transmission intensity reduces. The interference peak responding to displacement variation is also plotted in Fig. 4 and the sensitivity is ~0.127 nm/μm. Obviously, the proposed LPG with the air cavity has the high extinction ratio and is easy to distinguish the resonant wavelength change. Table 1 illustrates that the proposed sensor has a high sensitivity and is more suitable for micron-dimension-displacement measurement.
In practical applications, the conventional LPG can be replaced by temperature-insensitive one in order to eliminate temperature-induced wavelength shift. Then both the temperature-insensitive LPG and the SMF can be packaged in ZrO2 Ceramic Ferrule Bore which does not exceed 4 cm. After that, utilizing the transmission intensity of the LPG increasing with displacement variation at a fixed wavelength (for example, at 1536 nm in experiment), a low cost micro-displacement sensor can be obtained basing on an intensity measurement technique , in which a single wavelength light source (such as a distributed-feedback-Bragg laser, DFB) and an optical power meter is used. Comparing with FBG-based displacement sensors, the LPG-based micro-displacement sensor has advantages of simple, small and low cost.
In conclusion, we have proposed and demonstrated a compact micro-displacement sensor with high sensitivity based on a long-period fiber grating with an air-cavity. The sensing system can detect the displacement varies with a high sensitivity of 0.2155 nm/μm in the measurement range of 0 to 140 μm. Due to compact dimension of the sensor, the proposed displacement sensor has great applications in confined space.
This work was supported by the National Natural Science Foundation of China under Grant No. 61108058, the National Basic Research Program of China (973 Program) under Grant No. 2010CB327804, the National Key Technology R&D Program 2011BAF06B02 and the Science and Technology Commission of Shanghai Municipality of China under Grant No. 10595812300.
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