Simultaneous Measurement of Strain and Temperature Incorporating
a Long-Period Fiber Grating Inscribed on a Polarization-Maintaining Fiber
Abstract—Simultaneous measurement of strain and temperature was demonstrated by using a long-period fiber grating inscribed on a polarization-maintaining fiber (PM-LPG). The spectral sensitivities of the two adjacent resonant dips of the PM-LPG were measured with respect to strain and temperature.The sensitivities of each resonant dip for strain and temperature were different in magnitude and sign, which makes this PM-LPG have a capability of simultaneously sensing strain and temperature.Experimental results show root mean square deviations of 15.6 and 0.7 C for strain and temperature, respectively.Index Terms—Long-period fiber grating (LPG), optical fiber sensor, polarization-maintaining fiber (PMF), strain, temperature.
I. INTRODUCTION
FIBER GRATING sensors have yielded great attention due to their significant advantages such as wavelength domain response, electromagnetic noise immunity, high sensitivity, compactness, simplicity of fabrication, etc. They have been used for health monitoring and spatial analysis of engineering structures. One of the most significant limitations of fiber grating sensors is their dual sensitivity to strain and temperature. Therefore, a great many efforts have been made to discriminate the wavelength shift produced by strain from that produced by temperature [1]–[4], among which are the two fiber Bragg gratings (FBGs) embedded in a glass tube [3] and a single FBG written in an erbium : ytterbium-doped fiber [4]. Recently, Chen et al. [5] reported that the FBG written on a high birefringence fiber can be incorporated for the simultaneous measurement of strain and temperature. They simultaneously measured strain and temperature by measuring wavelength shifts of two Bragg wavelengths in the FBG which have different responses to strain and temperature with respect to the two principal input polarizations aligned along the fast and slow axes of the fiber. But the differences between each response of the FBG to strain and temperature at each Bragg wavelength were not so large, which might restrict the accuracy of the sensor. In this letter, we demonstrate simultaneous measurement of the strain and temperature with high accuracy compared with the previous work using FBGs reported in [5],based on a long-period fiber grating (LPG) inscribed on a polarization-maintaining fiber (PMF). To our best knowledge it is the first time the LPG inscribed on the PMF (PM-LPG) is applied for simultaneous sensing of strain and temperature,though the PM-LPG by itself is not a new concept.
In general, because the LPG resonances have different responses to strain and temperature depending on their cladding mode order [7], it is well known that these properties makeLPG useful for the simultaneous measurement of strain and temperature [8]–[10]. Particularly, the more the responses of the two dips used in the sensor system are different, the better accuracy can be achieved. In addition, resonant dips of an LPG exist in the several hundreds nanometer spectral range, so extremely broad-band light sources are required to monitor the spectral changes. In this letter, we show that it is possible to simultaneously measure strain and temperature without a broadband light source over 100-nm range, by using the PM-LPG whose two resonant dips, resulting from two orthogonal input polarizations aligned along the principal axes of the PMF, have different cladding-mode-order coupling within narrow spectral range ( <~50 nm).
II. DISCRIMINATION BETWEEN STRAIN AND TEMPERATURE
LPGs are photoinduced fiber devices that yield codirectional mode coupling between the fundamental core mode and forward propagating cladding modes. The resonant wavelength in the LPG can be obtained through the following expression(公式)(1) where n and n are the effective refractive indexes of core and cladding modes, respectively, and is the grating pitch. In the case of a PMF, the resonant wavelengths for which coupling condition (1) is satisfied are different depending on two principal input polarizations (two orthogonal polarizations aligned along the principal axes of the PMF) because the birefringence and asymmetric cladding structure of the PMF make each input polarization corresponding to each axis (slow or fast axis) have different effective refractive indexes of core and cladding modes ( and ). This can result in a splitting ofwavelength-dependent loss band and make the codirectional mode coupling condition (of the fundamental core mode into cladding modes) be different at two principal input polarizations.In a PM-LPG fabricated with a PMF having relatively small birefringence , the adjacent resonant dips (slow-axis and fast-axis resonant dips) resulting from the two principal input polarizations have identical cladding mode order and the wavelength separation between them is several nanometers
( 3.5 nm) [6]. Unlike the case of the PMF with small birefringence,the transmission spectra of the PM-LPG fabricated using the PMF with high birefringence and air hole structure in the cladding region exhibited a strong splitting ( 43 nm) of the two resonant dips (slow-axis and fast-axis resonant dips) with same cladding mode order [6]. This increment of spectral separation of the two resonant dips with same cladding mode order tells us that if the birefringence of the PMF is increased and the cladding structure of PMF is suitably chosen,we can make two resonant dips (slow-axis and fast-axis resonantdips) with different cladding mode order get closer than those with same cladding mode order. The PMF chosen in our proposed sensor system is PANDA type PMF (Sumitomo) which has high birefringence and has cladding glass element to play the role of applying strain to fiber core in the cladding region. By inscribing the LPG on this PMF, two resonant dips (slow-axis and fast-axis resonant dips) with different cladding modes could be made to be closely located within narrow wavelength span ( 50 nm) and to be selected for the wavelength of interest. It is already reported that wavelength shifts of resonance dips of an LPG due to the applied strain or temperature are different depending on their cladding mode orders [7]. Hence, temperature and strain responses in the lower dip (i.e., the dip of the shorter resonant wavelength) of the fabricated PM-LPG can be different from those in the upper dip (longer resonant wavelength) because of the difference in the cladding mode order. Especially if the two resonant dips are shifted linearly by applied strain and temperature, simultaneous measurement of strain and temperature can be achieved through following equations两个公式) where and are, respectively, the wavelength shift of lower and upper resonant dips due to applied temperature variation and strain variation , and and are temperature coefficients of the lower and upper resonant dips, respectively. and are strain coefficients of the lower and upper resonant dips, respectively.
III. EXPERIMENT AND DISCUSSION
Fig. 1 shows the transmission spectrum of the fabricated PM-LPG. As can be seen from the figure, two resonant wavelengths of 1565.93 (lower resonant dip) and 1602.05 nm (upper resonant dip) can be found at the orthogonal polarization conditions of the rotatable linear polarizer (RLP). The PM-LPG was fabricated by inscribing the LPG on a PMF (birefringence:
~5.05*10负4方, B–Ge codoped). The KrF excimer laser beam at 248 nm was illuminated, via 480- μm pitch amplitude mask of the longitudinal length of 30 mm, on the 40-mm-long PMF which had been H2-loaded at 100 ℃ at 100 bar for 7 days. The schematic diagram of the experimental setup for the simultaneous measurement is given in Fig. 2. We used two horizontally separated translation stages to axially strain the PM-LPG while using a temperature chamber placed between the stages to heat the grating independently. While applying strain and temperature to the PM-LPG, each wavelength shift of each resonant dip (lower and upper one) in the transmission spectrum was monitored by an optical spectrum analyzer through the adjustment of the RLP.Fig. 3(a) and (b) shows measured wavelength shifts of lower and upper resonant dips by applied temperature under zero axial strain and applied strain under the fixed temperature of 28 ℃,respectively. As can be seen from the figure, strain and temperature responses of two resonant dips show good linearity over ranges 0 ~1200μεof and 35℃~90℃ respectively. From the measurement results, the coefficients A , B, C, and
D
were measured as -36.6pm /℃, 1.36 pm /με, 129.12 pm /℃, and pm , respectively. With these coefficients and (3), we can simultaneously estimate applied temperature and strain by measuring and . In these coefficients and in Fig. 3, we can see that the lower resonant dip moves toward a shorter wavelength region by the temperature and toward a longer wavelength one by the strain. On the contrary, the upper resonant dip oppositely responds to the strain and temperature compared with the case of the lower resonant dip. These phenomena are expected to be caused from the fact that a PMF has many input polarization-dependent elements such as core and cladding effective indexes, waveguide structure, and thermooptic or strain-optic coefficients. The above temperature and strain coefficients, which have different magnitudes and signs,allow better sensing accuracy compared with those of previously reported works [5], [11].
In order to investigate the capability of the simultaneous measurement,
we heated the PM-LPG up to 90 ℃ while the strain was randomly applied in the range of 0~1200με . Fig. 4 shows the comparison between measured and applied parameters under the condition of random strain and temperature. The root mean square (rms) deviations of the measured strain and temperature were 15.6μεand 0.7
℃, over the measurement ranges of 0~1200μεand35
℃
~ 90
℃, respectively.
IV. CONCLUSION
Based on a single PM-LPG, simultaneous measurement of strain and temperature has been demonstrated by measuring wavelength shifts of two resonant dips resulting from two orthogonal input polarizations aligned along slow- and fast-axis of the PMF. The experimental result shows that the proposed sensor can simultaneously measure strain and temperature with rms deviation less than 15.6μεand 0.7
℃, over t ranges of 0~1200μεand35
℃
~ 90
℃, respectively.
发表于 2009-5-6 15:09:10
两个公式) where and are, respectively, the wavelength shift of lower and upper resonant dips due to applied temperature variation and strain variation , and and are temperature coefficients of the lower and upper resonant dips, respectively. and are strain coefficients of the lower and upper resonant dips, respectively.