IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 13, JULY 1, 2011
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Optical Fiber Cladding Ring Magnetic Field Sensor Paul Childs, Alessandro Candiani, and Stavros Pissadakis
Abstract—A magnetic field sensor utilizing a ferrofluid encapsulated fiber with a ring resonance coupling to and from the cladding modes is demonstrated. The sensor responds to magnetic fields aligned in a certain direction by changes to the visibility of fringes T over of its interference spectrum. An accuracy of the range of 0.03–0.14 T is achieved.
Fig. 1. Design of the cladding ring sensor showing propagation for the Fabry–Pérot (Bragg) and ring (ghost mode) resonances [8].
Index Terms—Blazed fiber Bragg gratings, ferrofluids, fiber sensors, harmonic analysis, magnetic field detection.
II. DESIGN
I. INTRODUCTION
O
PTICAL fiber sensors have the advantage over other conventional sensors in that they are noninvasive, have excellent long term stability, can operate in harsh environments and are easily multiplexed along a single fiber. Optical fibers have already been used in the domain of electromagnetic field detection, and specifically as magnetometers. For sensing magnetic fields there are two options. The direct option is to employ a specialized magnetically susceptible fiber and measure the Faraday rotation of the polarization [1]. The indirect option is to use a optical fiber sensor that is sensitive to a different parameter (such as refractive index or strain) in conjunction with a magnetic material that responds in such a fashion (e.g., ferrofluids [2], [3] or magnetostrictive materials [4], [5]). A sensor utilizing refractive index based measurements allows a great deal of flexibility in tailoring the response by varying the effective refractive index of the fiber and the surrounding ferrofluid. It also provides good sensitivity with only a short length of fiber needed, thus retaining the noninvasive advantage of optical fiber sensors. Refractive index sensing in optical fibers involves evanescent coupling. This can be achieved either using a tapered fiber to expand the core mode so as to be able to interact with the surrounding medium or to use coupling to the cladding modes with a long-period [6] or blazed grating [7]. Tapered fibers tend to be fragile and long period gratings (though offering high sensitivities) generally have high temperature cross sensitivities. In this letter we investigate the use of a refractive index sensor based on two blazed gratings [8] and modify it further by surrounding it with a colloid of paramagnetic nanoparticles to allow sensitivity to an applied magnetic field. Manuscript received November 24, 2010; revised January 27, 2011; accepted March 14, 2011. Date of publication April 19, 2011; date of current version June 15, 2011. This work was supported in part by the European Commission Grant Agreement 232479 (IASiS) and in part by a Marie Curie Transfer of Knowledge Grant (contract MTKD-CT-2006-042459). P. Childs and S. Pissadakis are with the Institute of Electronic Structure and Laser, Foundation of Research and Technology–Hellas, 711-10 Crete, Greece (e-mail:
[email protected]). A. Candiani is with the Institute of Electronic Structure and Laser, Foundation of Research and Technology–Hellas, 711-10 Crete, Greece, and also with the Department of Information Engineering, University of Parma, 43100 Parma, Italy. Digital Object Identifier 10.1109/LPT.2011.2143397
The cladding ring cavity sensor [8] shown in Fig. 1 consists of two identical tilted fiber Bragg gratings separated by a distance . Light incident on the gratings is back-reflected into the core and cladding modes. With the two gratings forming a cavity, two interferences are created. At the Bragg wavelength there is the Fabry–Pérot interference between the core mode to core mode reflections. At a slightly lower wavelength (corresponding to coupling to a highly resonant collection of cladding modes called the ghost mode [9]) there is a ring resonance of reflections from the core mode to the cladding modes then back into the original core mode. This structure differs from that of Guo et al. where recoupling is achieved either by an offset splice [10] or taper [11] to couple the reflected cladding modes to the copropagating core mode. Such an architecture is that of a Sagnac interferometer (though the interferometric structure is not made use of for their application; the optical phase difference between the two paths being zero gives no fringes). The device works as a sensor in that losses of the cladding mode to the surrounding ferrofluid will reduce the quality factor of the ring cavity as evidenced by a reduction of the fringe visibility of the interference on the ghost mode. These losses can come from a number of sources. Firstly the amount of light leakage from the cladding is dependent on the difference of the effective index of the cladding modes and the refractive index of the ferrofluid. Secondly the attenuation of the ferrofluid causes loss to the evanescent field of the cladding mode that overlaps it. As an applied magnetic field alters both the attenuation and refractive index of the ferrofluid suspension (both directly [12] and due to changing its spatial distribution) there is expected to be contributions from both effects in the sensors spectral response to the magnetic field. The visibility of fringes can be approximated from the Fourier domain as:
F F
(1)
is the transmission spectrum of the sensor, is the where location of the first interference peak in the inverse wavelength and are half widths in the Fourier domain domain , and to allow full windowing of the respective DC and first interference peaks without overlapping each other. As can be seen, due
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 13, JULY 1, 2011
to the sum over the DC peak, the equation is self referencing against power fluctuations. Being based on the magnitude spectrum only, it is also immune to wavelength shift fluctuations. The Bragg mode interference, being localized to the core, is immune to the magneto-optic and thermo-optic effects occurring in the surrounding ferrofluid. As such, shifts in the Bragg mode can be used to track effects occurring within the fiber itself such as changes in temperature or strain. If packaged with some strain relief, the more useful parameter of temperature can be isolated. Due to the core-to-core Fabry–Pérot interference being of much higher order than the Bragg scattering, greater sensitivity is achievable by monitoring the phase shift of the interference fringes than that of the Bragg envelope. The strain or temperature can be reconstructed also from the Fourier domain as:
F
(2)
is the Bragg wavelength and is the strain/ where temperature conversion coefficient given as 0.78 for strain and 8.66 K for temperature. The sensor can also be used as a full three parameter sensor. However, considering the similar strain and temperature responses of the Bragg and ghost mode envelopes, the determinant of the transfer matrix is close to zero and inversion creates large errors. As such discrimination between strain and temperature is still better assessed using other means. Compared with ferrofluid based microstructured optical fiber devices [3], where small amounts of ferrofluid are infiltrated into the air holes near a grating, the external medium design of the cladding ring sensor has the advantage of simpler grating fabrication and greater versatility in the choice of ferrofluids (wettability, viscosity and particle size parameters have to be carefully selected for infiltration in microstructured optical fiber). As the ferrofluid is external to the fiber maintenance operations (such as changing the nanoparticle solution) are more easily performed. III. EXPERIMENT Two identical 5 mm long blazed gratings at a separation of 12 mm and a tilt angle of 3.2 were inscribed in photosensitive fiber (GF1B, Thorlabs) using 193 nm radiation and a phase mask. Initially the ghost and Bragg modes grew at the same rate, but due to photo-bleaching of the cladding the growth of the Bragg mode slowed giving a stronger ghost mode. As inscription of the gratings was performed using 193 nm light, cladding mode resonances were small allowing this region of the spectrum to be potentially reused. Spectral interrogation was achieved by launching light into one end of the device using an unpolarized superluminescent diode and using an Optical Spectrum Analyzer (OSA) attached to the other end to obtain transmission measurements. As blazed gratings are polarization sensitive, greater fringe visibility is possible using a polarized light source; however, in this case only a minor improvement is possible (for gratings with a blaze of the Polarization Dependent Loss is dB[13]) and it was decided that the improvement in stability and decrease in complexity obtained by using an unpolarized source were more important. A spectral resolution of 10 pm was chosen for the OSA. This was done to ensure
Fig. 2. Spectrum of the cladding ring sensor following encapsulation.
it was fine compared to the period of interference ( pm), so that the Nyquist frequency is high enough that depletion of the visibility is minor, yet not too fine that the Signal to Noise Ratio would be reduced more than necessary. The grating pair was then encapsulated in a 2 mm diameter silica capillary, filled with a water based ferrofluid colloid (EMG 605, Ferrotec) and sealed using silicone. A water based ferrofluid was used as its measured refractive index of was below that of silica (so as to allow guiding of the ghost mode) and within the established sensitivity range for this sensor design. The colloid has a viscosity of 5 mPas, a saturation magnetization of 20 mT and a volume content of 3.6% 10 nm Fe O particles coated with a cationic surfactant. The nanoparticles of water based ferrofluids tend to agglomerate, so sonication for half an hour was applied to help break up the clusters. The spectrum is shown in Fig. 2. A 40 20 mm magnet was mounted on a linear stage and the packaged sensor was mounted through the center axis of a rotation stage with this axis being perpendicular to the applied magnetic field. A hall effect probe was placed in close proximity to the sensor to provide reference readings of the magnetic field. By adjusting the position of the magnet, measurements of the spectral response to transverse magnetic field were taken. The response to the direction of the transverse field was tested by rotating the sensor about its axis with the magnet at a fixed position with a field strength of 0.08 T (roughly half of the value before which the sensors response starts to saturate). Each spectral measurement was analyzed by selecting just the ghost mode region of the spectrum (1550.122–1550.725 nm) and using eqn. (1) to calculate the visibility ( nm , nm and nm ). The results are presented in Figs. 3 and 4 for the case of transverse magnetic fields of varying strengths and applied angles respectively. Above 0.15 T the response saturated. IV. DISCUSSION In Fig. 3 a clear increase of the visibility of fringes with applied magnetic field can be seen. This corresponds to a decrease with magnetic field in the refractive index and/or attenuation of the region surrounding the fiber, potentially due to migration of the nanoparticles away from this region and toward the magnet as well as increases in density and viscosity of the nanoparticles due to increasing interparticle forces as the nanoparticles align to the field [14]. The results show a near linear change in the visibility over the range of 0.03 to 0.14 T. A linear regression over this region gives a response of 1.27 T . Short term stability measurements in the Fourier domain give a standard deviation of for the visibility, which gives an accuracy
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of the ferrofluid has not caused the fiber to be bent or strained and that its response is purely due to refractive index and attenuation changes. The ghost peak, however, varied by 48 pm over the measured range, but was unreliable above 0.085 T where large fluctuations were seen. Below this range the sensitivity was calculated as 0.4 nmT giving a potential accuracy of 2.5 T (provided a measurement system with an accuracy as good as pm is used). This coarser accuracy and the more limited range show the advantage of making use of visibility of fringes measurements; made possible due to the ring cavity structure. Fig. 3. Visibility of fringes response to an applied static transverse magnetic field.
V. CONCLUSION Sensitivity of a blazed fiber grating ring cavity to the magnetic field induced changes in a ferrofluid medium is demonstrated. The device shows good linearity over the range of 0.03 to 0.14 T and strong azimuthal dependence on the applied field. A transverse field response of 1.27 T based on visibility of fringes measurements is measured. An accuracy of 1.4 T was achieved. REFERENCES
Fig. 4. Polar plot of the visibility of fringes response of the sensor with respect to the azimuthal direction of the applied static transverse magnetic field.
of 1.4 T. Long term drift and repeatability will depend heavily on the particular ferrofluid and its history. Agglomeration increases over time and causes a reduction in the response of the device. In order to maintain performance sonication can be applied to temporarily counteract this effect or the ferrofluid can be replaced. In examining Fig. 4 an offset from the center (by in the direction of 18 ) is seen that can most likely attributed to the fiber rotating about a line offset from its axis and thus moving towards and away from the magnet as it is rotated. As this effect gives a dipole moment to the direction, whereas the expected response has a quadrupole moment, the two can readily be distinguished. Directional dependence of the field is thus evidenced by the eccentricity of the response, , calculated by performing a nonlinear regression. The minima of the response is also seen to be indistinguishable from the case of no applied magnetic field; implying a strong directional dependence is present. Wavelength shift measurements of the ghost and Bragg mode peaks based on eqn. (2) were within error ( pm for this system) of showing no Bragg shift until 0.145 T. This shows that the effect of the magnetic force and hydrodynamic motion
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