February 15, 2015 / Vol. 40, No. 4 / OPTICS LETTERS
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Fiber optic stress-independent helical torsion sensor Luís A. Fernandes,1,2,* Jason R. Grenier,1 J. Stewart Aitchison,1 and Peter R. Herman1 1
Institute for Optical Sciences, and the Department of Electrical and Computer Engineering University of Toronto, 10 Kings College Rd., Toronto, Ontario M5 S 3G4, Canada 2 Currently with OZ Optics Ltd., 219 Westbrook Rd, Ottawa, Ontario K0A 1L0, Canada *Corresponding author:
[email protected] Received November 11, 2014; revised January 11, 2015; accepted January 13, 2015; posted January 14, 2015 (Doc. ID 226513); published February 13, 2015 Femtosecond laser-fabricated waveguides have been formed into helical paths throughout the cladding of singlemode optical fibers to demonstrate a strain-independent fiber torsion sensor. A comparison between a Bragg grating sensor and a Mach–Zehnder based on helical waveguides (HWs) showed a much weaker twist sensitivity of 1.5 pm/(rad/m) for the grating in contrast with a value of 261 pm/(rad/m) for the interferometer. The HW geometry provided an unambiguous determination of the rotational direction of the twist while facilitating a convenient and efficient means for optical coupling into the single-mode core of the fiber. The flexible three-dimensional writing by the femtosecond laser fabrication method enabled the direct inscription of compact and robust optical cladding devices without the need for combining or splicing multiple-fiber segments. © 2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (060.2370) Fiber optics sensors; (060.3735) Fiber Bragg gratings. http://dx.doi.org/10.1364/OL.40.000657
Torsion sensing has been demonstrated in optical fibers using a number of different effects, including twistinduced wavelength shifts in long-period gratings [1], interferometric measurements in high birefringent photonic crystal and suspended twin-core fibers [2,3], and polarization state measurements of single-mode fibers (SMFs) coupled with polarization-maintaining fibers [4]. These methods generally provide an absolute measure of the twist; however, their inherent symmetry prevents an absolute determination of the rotational direction. The right- and left-handed symmetry of spiral structures, on the other hand, break such symmetry. For example, a helix with a right-handed chirality is completely distinguishable from a helix with left-handed chirality, regardless of the helix’s orientation. As a result, they respond differently depending on the direction of the twist applied. Helical structures have been used in optical fibers to demonstrate tunable filters [5], twist sensors [6–8], and to produce microfluidic channels in glass [9]. In this Letter, we report on the use of helical waveguides (HWs) in the cladding of optical fibers to form torsion sensors based on a Mach–Zehnder interferometer and Bragg gratings. All cladding waveguides were coupled with the core waveguide of the optical fiber to provide a convenient means of in-fiber sensing using conventional fiber diagnostic tools. The HWs and helical Bragg grating waveguides (HBGWs) were fabricated in the cladding of single-mode optical fibers (Corning SMF-28) by three-dimensional direct writing with frequency-doubled light from a Yb-doped, chirped pulse-amplified fiber laser (IMRA America μJewel D-400-VR) with a center wavelength of 522 nm, and a pulse duration of 250 fs (Lorentzian FWHM). An acousto-optic modulator was used in the laser path to select burst trains of pulses and to produce a periodic variation of the refractive index during scanning [10]. Modulation frequencies from 496 to 506 Hz, with a scan speed of 0.27 mm/s, generated Bragg gratings with resonance wavelengths from 1533 to 1565 nm, providing either Bragg grating waveguides (BGWs) in the fiber cladding or fiber Bragg gratings (FBGs) directly in the fiber core. The laser beam was focused and precisely 0146-9592/15/040657-04$15.00/0
aligned to an accuracy of 1 μm inside the SMFs using a high numerical aperture oil-immersion lens to minimize spherical and cylindrical aberration. The fiber was mounted under tension in a free-standing holder and positioned by an air-bearing three-dimensional motion stage (Aerotech ABL1000) [11–13]. Pulse energies of 150 nJ were used to fabricate single-mode waveguides and BGWs in the fiber cladding, with mode field diameters of 9 μm by 11 μm at a wavelength of 1550 nm. The estimated propagation loss of the waveguides was 0.5 dB/cm. A lower pulse energy of 10 nJ was used to generate a strong FBG response in the fiber core [13]. The BGWs and the FBGs also provided a measure of the waveguide propagation constants and birefringence [10,11,14]. The HWs were designed to have a full rotation over a period p 15 mm. This period was positioned concentric with the core at a helical radius of r 40 μm, far enough from the edge of the cladding surface to avoid evanescence field interaction with the air. This configuration yielded a radius of curvature of ≈140 mm that did not induce a notable bending loss. The helical structures were optically coupled with the fiber core waveguide through an S-bend waveguide section written with a radius of curvature of 35 mm. More detailed information on the methods can be found in [15]. Figure 1 describes the designs used for the fabrication of the fiber sensors based on the HBGWs [Fig. 1(a)], and the helical Mach–Zehnder interferometers (HMZIs), [Fig. 1(b)]. For the fiber characterization, the in-line Bragg and Mach–Zehnder sensors were held vertically and slightly weighted from below the sensor while being probed optically with a broadband source (ASE-FL7002). Real-time recordings of reflection and transmission spectra were made by collecting light into an optical spectrum analyzer (Ando 6317B) as the fiber was twisted to a maximum of 20 turns over a length of 48 cm in either a clockwise (CW) or a counter clockwise (CCW) direction, as defined in Fig. 1. This twist-induced opposing rotation strains according to the right-handed (RH) or left-handed (LH) chirality of each of the HW sections. For example, following Fig. 1(a), a CW rotation would increase the coil of the LH helix (decreasing helical periodicity) while © 2015 Optical Society of America
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Fig. 3. Axial strain-induced wavelength shift of a grating embedded in the fiber core in comparison with the shifts measured in the helical (RH and LH) Bragg gratings, all showing similar sensitivity to the axial strain. Fig. 1. (a) Laser-written helical BGWs emerging from S-bend couplers at the fiber core with RH and LH chirality. (b) A Mach– Zehnder interferometer with a right-handed helical waveguide arm coupled to a core waveguide arm via S-bends.
simultaneously uncoiling the RH helix (increasing helical periodicity). The center fiber core was expected to remain unaffected by torsion, and therefore served as an independent axial strain sensor. The reflection spectrum of the Bragg grating fiber sensor represented in Fig. 1(a) is shown in Fig. 2 simultaneously capturing the Bragg resonances in all three arms. The values obtained are 1534 nm for the RH HBGW, 1550 nm for the core FBG, and 1564 nm for the LH HBGW. The slightly stronger (−4.4 dB) FBG resonance over the (−17 dB, −22 dB) reflection contrast in the HBGWs manifests from a slightly unbalanced 3∶1 splitting in the S-bend couplers, as well as from the higher propagation loss in the BGW side arms when compared with the low-loss FBG in the core. The dependency of the FBGs and HBGWs with the applied axial strain up to 3600 μϵ is seen in the Bragg resonance wavelength shift in Fig. 3. To improve the precision in measuring the wavelength shift, the spectral
Fig. 2. Reflection spectrum obtained from the helical BGW sensor design of Fig. 1(a), showing Bragg-resonant wavelengths simultaneously collected from the core waveguide (green), and the two counter-rotating helical BGWs with RL (blue) and LH (red) chirality.
separation of the Bragg resonances was directly measured from each of the twisted and non-twisted fiber conditions by an overlap minimization numerical method [14]. The slope of all three plots yields similar strain responses of 846, 838, and 849 pm∕μϵ from each of the FBG, RH HBGW, and LH HBGW devices, respectively. In contrast, when the fiber was twisted with uniformly applied torsion and axial strain, the Bragg responses of the HBGWs shifted symmetrically according to their RH and LH chirality (Fig. 4). Meanwhile, the FBG served as an independent axial strain gauge, with the resonant wavelength remaining unchanged (within an uncertainty of 0.02 nm) over a twisting range from −265 rad∕m to 265 rad∕m. A CW rotation, as labeled in Fig. 1, was defined here by a positive twist, while a CCW rotation produced a negative twist. This yielded a Bragg-shift twist sensitivity of 1.57 pm/(rad/m) and −1.45 pm∕rad∕m for the respective RH and LH HBGW. The small 0.12 pm/(rad/m) difference cannot be accounted for by the 0.05 nm uncertainty in determining the peak positions in the present spectra (Fig. 2), and may therefore arise from an imperfect centering of the helical structures by 1 μm with respect to the fiber’s center axis.
Fig. 4. Torsion-induced shift of the Bragg resonance wavelength measured as a function of the fiber twist under both CW and CCW fiber rotations for the LH HBGW and the RH HBGW. The values are compared with the central FBG. Large error bars in the HBGW data arise partially due to birefringence (10−5 to 10−4 ) in the BGW.
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Nevertheless, this combination of HBGWs and FBGs demonstrates complimentary and independent measures of the fiber axial and rotational stresses. The interferometric transmission spectrum of the HMZI [Fig. 1(b)] shown in Fig. 5 was dominated by the optical path difference created by the asymmetric index between the core and the cladding waveguides [15]. The S-bend coupler design was tuned to balance against the extra waveguide propagation losses present in the HW arm, in order to maximize the visibility of the interference spectrum in Fig. 5 over the 1350 to 1600 nm wavelength range. This resulted in modulations as deep as 10 dB at 1450 nm, with a free spectral range of 28 nm at the 1500 nm wavelength. The position of the local minimum at 1500 nm was monitored as a function of the fiber twist (Fig. 6), and yielded a sensitivity of 261 pm/(rad/m). This response was more than 200 times larger than the sensitivity shown for the HBGWs. The inset of Fig. 6 shows the wavelength shift in the 1500 nm interference minimum for three different twist conditions. An alternative and more dynamic means of tracking the fiber twist is to
Fig. 5. Interference spectrum recorded through the helical Mach–Zehnder interferometer design of Fig. 1(b), consisting of one full 360° period rotation of the laser-formed waveguide arm over a total length of 15 mm at a 40 μm radial offset from the fiber core.
Fig. 6. Wavelength shift measured at a transmission minimum (1500 nm) of the Mach–Zehnder interferometric spectrum as a function of the fiber twist. Inset: transmission minima of the spectrum recorded for select values of the fiber twist, as labeled.
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follow the transmission value at a single wavelength. This simplified method may be very practical, as the fiber interferometer is intrinsically stable due to both waveguide arms being present in the same fiber. By following the transmitted power of the interferometer at the quadrature point (half power) of the sinusoidal spectrum, the power will also follow a sinusoidal response with a fiber twist. A linear response to the fiber twist is possible but only in a dynamic sensing range limited to approximately one-third of the free-spectral range, as shown in Fig. 7, with a maximum sensitivity following a straight line at this quadrature point. An experimental demonstration of a torsion pendulum is shown in Media 1 for the Mach– Zehnder sensor. The transmission power was measured while simultaneously recording video images of the weighted fiber under the twisting action of a pendulum. The power as a function of the time for this recorded experiment is shown in the inset of Fig. 7. By choice, the center (quadrature) point was also the point of equilibrium (Twist 0 rad∕m). However, any other point can be chosen in order to access the highest sensitivity at the desired rotation range. The HBGWs provided an absolute determination of the fiber rotation and direction. However, the resulting sensitivity of ≈1.5 pm∕rad∕m was very limited. The HMZI provided a much larger 261 1 pm∕rad∕m response for the torsion determination. By measuring the transmission power at a given wavelength, a maximum ratio sensitivity of 3%/(rad/m) was achieved. However, it was limited in range by the periodic nature of the interferometric measurement. The appropriate calibration directly against a FBG demonstrated the helical Bragg grating torsion response to be strain-insensitive. Since the devices presented were all inscribed in the same fiber and can be expected to follow a similar temperature-induced strain response, one anticipates the torsion sensing response of the HBGW to also be temperature-independent. The devices could be considered for scaling to multiple sensing points, distributed over longer lengths [16] with the appropriate design of couplers to optimize the splitting ratio against device losses [15], and possibly even integration with cladding
Fig. 7. Transmitted power at 1507 nm measured through the Mach–Zehnder interferometer as a function of the fiber twist, with a maximum 0.03/(rad/m) slope response. Inset: torsion pendulum recording of the damped oscillation of the power transmitted at 1507 nm, over the torsion range of 20 rad∕m (Media 1).
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microfluidics [17]. The interferometric results, on the other hand, showed a much higher sensitivity when tracking wavelength shifts. Such an approach may be suitable for high-resolution applications where only small twists are expected. Power measurements on such an interferometric design were also shown to be possible when a practical application was demonstrated with a rotation pendulum. The HBGW reflection peaks had a line-width of ≈2 nm, which was smaller than the estimated 10 nm bandwidths of the transmission peaks of the HMZI. This narrow resonance may contribute to a higher resolution of the HBGW design, despite it being less sensitive than the interferometric sensor. The HBGW and HMZI devices presented in this Letter are flexible in terms of varying helical periodicity, the core-to-cladding coupling coefficient, the number of helical periods, and the Bragg grating periodicity. A shorter helical period would be desirable to improve the sensitivity of the Bragg gratings. Similarly, a shorter helical periodicity may also increase the sensitivity of the HMZI, with the higher bending losses here mostly affecting the power balance in the two waveguide arms and possibly reducing the interference contrast. The results shown demonstrate the use of helical shaped, femtosecond laser-fabricated waveguides in the cladding of optical fibers to obtain strain-independent fiber torsion sensing with an unambiguous determination of the twist direction. The two methods explored were based on Bragg gratings and on a Mach–Zehnder interferometer, and both performed according to the original hypothesis that the mirror symmetry of the helical patterns can create optical responses that are dependent on the twist direction. The methods explored are expected to be readily extensible to different types of optical fibers with applications in distributed shape sensing, with the natural advantages provided by having the devices inscribed directly in the target fibers. An integrated approach of cladding microfluidics with straight and helical gratings, as well as interferometric
devices, can offer a three-dimensional shape and twisting sensing combined with temperature, strain, and chemical sensing to further increase the measurement capabilities of fiber-cladding photonic devices. References 1. C.-Y. Lin, L. A. Wang, and G.-W. Chern, J. Lightwave Technol. 19, 1159 (2001). 2. O. Frazao, C. Jesus, J. M. Baptista, J. L. Santos, and P. Roy, IEEE Photon. Technol. Lett. 21, 1277 (2009). 3. O. Frazao, R. M. Silva, J. Kobelke, and K. Schuster, Opt. Lett. 35, 2777 (2010). 4. D. Lesnik and D. Donlagic, Opt. Lett. 38, 1494 (2013). 5. W. Shin, B.-A. Yu, Y.-C. Noh, J. Lee, D.-K. Ko, and K. Oh, Opt. Lett. 32, 1214 (2007). 6. S. Oh, K. R. Lee, U.-C. Paek, and Y. Chung, Opt. Lett. 29, 1464 (2004). 7. R. Gao, Y. Jiang, and L. Jiang, Opt. Express 22, 15697 (2014). 8. L. Xian, P. Wang, and H. Li, Opt. Express 22, 20260 (2014). 9. Y. Li and S.-l. Qu, Mater. Lett. 64, 1427 (2010). 10. H. Zhang, S. M. Eaton, and P. R. Herman, Opt. Lett. 32, 2559 (2007). 11. J. R. Grenier, L. A. Fernandes, P. V. Marques, J. S. Aitchison, and P. R. Herman, in CLEO: 2011—Laser Applications to Photonic Applications (2011), paper CMZ1. 12. J. R. Grenier, L. A. Fernandes, and P. R. Herman, Opt. Express 21, 4493 (2013). 13. L. A. Fernandes, J. R. Grenier, P. V. S. Marques, J. S. Aitchison, and P. R. Herman, J. Lightwave Technol. 31, 3563 (2013). 14. L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, Proc. SPIE 8247, 82470M (2012). 15. J. R. Grenier, M. Haque, L. A. Fernandes, K. K. C. Lee, and P. R. Herman, in Planar Waveguides and other Confined Geometries, G. Marowsky, ed., Optical Sciences in Springer Series (Springer, 2015), Vol. 189. 16. K. Lee, A. Mariampillai, M. Haque, B. Standish, V. X. Yang, and P. R. Herman, Opt. Express 21, 24076 (2013). 17. M. Haque, K. Lee, S. Ho, L. A. Fernandes, and P. R. Herman, Lab Chip 14, 3817 (2014).
