An all-fiber twist sensor based on multimode interferometer (MMI) has been proposed and fabricated by splicing both ends of a section of square no-core fiber (NCF) with a single mode fiber. We have investigated the transmission spectral characteristics of the square fiber under different applied twisting angles. Within a torsion angle range of -360°~360°, the wavelength and transmission sensitivities are 1.28615nm/(rad × m−1) and 0.11863%/ (rad × m−1), respectively. Moreover due to the trivial thermal expansion coefficient of pure silica fiber, the proposed twist sensor has a low temperature sensitivity, which is desirable to solve the temperature cross sensitivity.
© 2013 Optical Society of America
Currently, security monitoring in civil engineering applications, such as bridges, buildings, many other civil structures, has been an important and indispensable subject. Generally, the studies on structural health status are almost focused on several physical parameters including bend, axial stress, transverse load, temperature, twist and so on. Twist is one of the most important mechanical parameters for security monitoring of buildings. And accordingly, various twist sensors have been developed before. Traditional twist sensors such as optical encoders and magnetic sensors  always have a large size, which is hardly compatible with building structures. Recently, fiber-based torsion sensors have been widely applied in the civil engineering owing to their excellent advantages such as strong flexible, compact structure, ease of integration, and strong resistance to environmental interferences.
A variety of optical torsion sensors have been reported utilizing different fiber devices, which could be classified into the ones elastic beams or without elastic beams. On the one hand, when a twist is applied to the fiber which is firmly bonded to the surface of elastic beam, the center of rotation shaft is not along the fiber axis, but along the elastic beam axis , and in this case the relationship between resonance wavelength shift Δλ and strain ɛ along the fiber axis could be used for sensing purposes. However, the indirect measurement method would produce a larger error and impose some limit on the angle measurement range. On the other hand, the torsion shaft of twist sensor without elastic beams is along the fiber axis. Conventional fiber-optic torsion sensors based on LPFGs fabricated by UV radiation or corrugated structures [3,4], CO2 radiation and electric arc , and mechanical [6–8] were reported, the unsymmetrical distribution of refractive index change induced by the photoabsorption or thermal damage in the cladding or core region of the optical fiber results in many unique torsion characteristics . However, LPFGs present cross sensitivities between different physical parameters such as temperature, strain, pressure, refractive index, load, and so on . Utilizing the birefringence property of titled fiber Bragg grating (TFBG) and retroreflection capability of FBG, the twist sensors based on tilted fiber gratings  and distributed Bragg reflector (DBR) fiber laser  were reported. Recently, many studies on pohotonic-crystal-fiber-(PCF-) based torsion sensors were also reported. Since fiber twist causes certain circular birefringence, various high-birefringent pohotonic crystal fibers (HB-PCFs) were used in Sagnac loop structure to construct twist sensors [13,14]. Low-birefringent pohotonic crystal fibers (LB-PCFs) have been also utilized in the Sagnac loop structure [15,16]. In addition, twist sensors based on the interference between two linear polarization (LP) modes in HB-PCFs have been achieved . However, as well known, the fabrication of PCFs is costly, which limits their applications to a large extend. Therefore, there is always the demand for the developing fiber-optic twist sensors with simple configuration, performance reliability, and low cost.
Fiber-optic interferometric sensors based on multimode fibers (MMFs) have been extensively investigated in recent years due to their several advantages such as easy of fabrication, low cost, high sensitivity, and capability of multi-parameter measurement, etc [18,19]. Up to date, mostly related works are focused on the circular-core-based MMIs, and HB-rectangular silica fibers had been reported in recent years [20,21], which have ultrahigh sensitivity to refractive index . The MMF with unsymmetrically rectangular transverse fiber geometry is sensitive to tangential and axial strain because of its maldistribution of refractive index when deformed, which provides a possibility to design a compact twist sensor.
In this paper, we have proposed a compact MMI-based all-fiber twist sensor. A section of square NCF is employed instead of the conventional MMF, which is simply spliced between two SMFs. As twist effect is applied the MMI, the transmission spectral with multimode interferential fringes have been investigated. Within a torsion angle range of −360~360°, the wavelength shift and transmission loss both exhibit the sinuous relationship with the change of twist rate for the spectral notches. Furthermore, the NCF interferometer has ultra-low temperature sensitivity. And high twist angle sensitivity without temperature cross-sensitivity has been achieved.
2. Principle and experiment
The schematic diagram of our proposed fiber twist test system is shown in the Fig. 1 (b), when the light is coupled from the lead-in SMF into the square NCF, multitude of low or high-order eigenmodes, i.e. or modes, are excited and the interference between different modes occurs while the light propagates along the square NCF . The electric field distribution at any point in the NCF can be written as :
where the timedependent factor is omitted, and , , and are the excitation coefficient, field profile and the propagation constant of the mode in the square NCF, respectively; and , , and have the same connotations for the mode. As analyzed in reference , the field excitation coefficients and can be estimated using the overlap integral:22]:22]:24]:
Based on the interference theory, intermodal interference condition for spectral notches can be expressed as, and for our proposed device, . Therefore, using Eq. (3), the expression of notch wavelengths can be obtained:
The experiment setup of the proposed twist sensor is shown in Fig. 1 which consists of a supercontinuum broadband source (SBS) and an optical spectrum analyzer (OSA, Yokogawa AQ6370C, operation wavelength ranges from 600nm to 1700nm) with a resolution of 0.5nm. The MMI as sensing element spliced between the SBS and OSA is shown in dotted circle region. The enlarged view of MMI is shown in the Fig. 1(b) The two ends of the square NCF are spliced between two SMFs (SMF-28e, Corning, Inc), the NCF with a cross section size of 90μm × 90μm and length L0 = 1.9cm is a pure silica square rod with air as its cladding. The distance L between the fiber holder and rotator is 37cm. When incident light propagates through the square NCF, a large number of high-order modes would be excited, and multimodeinterference simultaneously occurs.
3. Experimental results and discussion
As twist is applied, multi torsional stress zones were formed along the fiber axis, and thus twist effect would cause the periodic refractive index modulation, which ultimately affects the wavelength shifts and interference strength. Also due to twist effect that has an influence on the polarization state of incident light, the degeneracy state of Emnx or Emny modes with the same orders also changed, which eventually lead to the split of resonant dips. Figure 2 shows the transmission spectrum with different torsion angles ranging from −360°-360°. As shown in Fig. 2, because of the obvious multimode interference effect, complicated interference spectrum appears with a contrast ratio over 20dB. With the increment of torsion angles, resonance wavelength shifts, transmission intensity and spectral split change simultaneously. The transmission dip in the red dotted frame is shown in the inset of Fig. 2. With the increment of torsion angles, a new dip emerges in the shorter wavelength region which is induced by the breaks of degeneracy state of Emnx or Emny modes with the same orders.
Figure 3 shows the wavelength shift and transmission loss change of the dip, respectively. The torsion angle is different for different points along the distance between fiber holder and rotator, and thus it is necessary to use γ = θ/L to quantify the twist rate. As shown in Fig. 3(a), the red line is the nonlinear fitting of a sine function with R2 = 0.99658 for the torsion angle ranging from −360° to 360°, and the wavelength of dip A shifts by 17nm from 1335.7nm to 1352.7nm. The same responses can be seen in Fig. 3(b), in which the red line is also the nonlinear fitting of a sine function with R2 = 0.98412, and the transmission loss variation is 16.9dB, equal to 0.996% in percentage. By linear fitting the particular region, the wavelength shifts and transmission losses reach the maximum sensitivities of 1.28615nm/(rad × m−1) and 0.11863%/ (rad × m−1), respectively, which is in good agreement with our theoretical analysis.
Figure 4 shows the transmission spectral characteristics of the MMI for different temperatures, and the inset corresponds to the same dip for twist effect analysis. Due to the low thermal-expansion coefficient of the pure silica fiber, the NCF interferometer has a very low temperature sensitivity, as shown in the Fig. 5, within a temperature range from room temperature to 95°C, the measured wavelength shift and intensity variation are only 0.8nm and less than 0.2dB, respectively. The environmental temperature has been kept stable to minimize the influence of temperature fluctuation and achieve highly accurate twist angle measurement. Therefore, for a small temperature range, the twist sensor can be considered as temperature-insensitive.
A twist sensor based on multimode interferometer employing the square NCF with ultra- low temperature sensitivity has been proposed and experimentally demonstrated in this paper. Experimental results indicate the wavelength and transmission loss reach the maximum twist sensitivities of 1.28615nm/(rad × m−1) and 0.11863%/ (rad × m−1), respectively, which is in good agreement with our theoretical analysis. Since the MMI is fabricated using pure silica square NCF, the temperature cross-sensitivity issue has been resolved. The proposed twist sensor with several advantages such as compact structure, low cost and immunity to temperature cross-sensitivity is highly desirable for civil engineering applications.
This work was jointly supported by the National Natural Science Foundation of China under Grant Nos. 11204212, 11274182, and 11004110, Science & Technology Support Project of Tianjin under Grant No. 11ZCKFGX01800, Key Natural Science Foundation Project of Tianjin under Grant No. 13JCZDJC26100, China Postdoctoral Science Foundation Funded Project under Grant No. 2012M520024, the National Key Basic Research and Development Program of China under Grant No. 2010CB327605, and the Fundamental Research funds for the Central Universities.
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