Since their first demonstration, microstructured optical fibers (MOFs) have paved the way toward the development of new kinds of optical fiber sensors and actuators by integrating tailored guiding and microfluidic properties into a photonic configuration based on cylindrical geometry.1 Light guided by the core of an MOF overlaps with the surrounding microcapillary (tube) structure. The field of the guiding mode interacts with the medium in the microcapillaries, making it possible to probe the optical, chemical, and structural changes taking place inside the tubes. In more advanced schemes, diffractive structures such as Bragg and long-period gratings can be inscribed into the MOF for more efficient interrogation in reflection or transmission mode.
There have been several examples of ‘in-fiber’ optofluidic2 MOF Bragg and long-period grating3 devices that provide a tunable spectral response induced by thermal or pneumatic actuation. Alternatively, we recently infiltrated ferrofluids inside MOF Bragg gratings to produce photonic devices whose spectral behavior is stimulated by external magnetic fields: see Figure 1(a).4 Ferrofluids are stable colloidal suspensions of subdomain magnetic micro- or nanoparticles dispersed inside a liquid carrier. In the presence of a magnetic field, the magnetic moments of the particles orient along the field lines, resulting in magnetization of the fluid.5 The infiltration and manipulation of such a highly viscous, nontransparent (optical loss α>5μm−1) magnetic liquid inside an MOF, as well as its interaction with the guiding mode, can lead to unique optical and material effects as well as probe capabilities.
Figure 1. (a) Photograph of a ferrofluid infiltrated inside a microstructured optical fiber (MOF), placed close to a pin-shaped actuating electromagnetic coil. (b) Sketch of an MOF Bragg reflector with a ferrofluidic defect. Evolution of the reflection spectra of (c) uniform and (d) chirped gratings for different locations of the ferrofluidic defect along the grating length. FBG: Fiber Bragg grating. A: Grating edge. O: Ferrofluid edge.
Most ferrofluids have a refractive index greater than that of silica and show extreme optical absorption. These properties lead to attenuation and phase perturbation in the MOF-guiding mode, which in turn affect the scattering characteristics of the infiltrated MOF Bragg grating.4 We tailored the Bragg grating characteristics and those of the ferrofluid, together with the opto-geometrical structure and wettability properties of the MOF, to define the operational characteristics and performance of our magnetofluidic device. We have developed several magnetofluidic in-fiber devices possessing different sensing or actuating functionalities, which illustrates the design versatility of the infiltration approach. These systems use commercial oil-based ferrofluids and suitable functionalization processing for controlling the wettability properties of the MOF capillaries. In particular, phase-shifted Bragg reflectors6 and a vectorial magnetometer7 feature spectral tunability and magnetic field sensing, respectively. Both of these primitive MOF devices share common photonic design parameters and infiltration protocols. However, modification of the grating topology or the fluidic configuration leads to different operational behavior.
Figure 2. Visibility changes of the parasitic spectral notch versus magnetic field applied along the fiber length for an in-fiber optofluidic magnetometer based on an MOF Bragg grating infiltrated by a ferrofluid.
In the first design, we infiltrated uniform and chirped Bragg gratings using ferrofluidic lengths 10 times shorter than that of the Bragg reflector. In a uniform Bragg grating structure, this short-length ferrofluid forms a lossy Fabry-Perot cavity inside the capillaries of the MOF: see Figure 1(b). By translating the ferrofluidic defect along the grating length using an external magnetic field, parasitic notches appear in the reflection spectrum as a result of light oscillation within this magnetofluidic in-fiber cavity: see Figure 1(c).6 However, more interesting results are obtained when the same ferrofluidic defect is infiltrated inside an MOF with an inscribed chirped Bragg grating. Rather than forming a Fabry-Perot cavity (as is the case of the uniform grating), the ferrofluidic defect splits the chirped grating into two spectrally distinct regions associated with different resonant period ranges. This, in conjunction with the opaque nature of the ferrofluid, leads to reflectivity strength chopping and bandwidth narrowing of almost 100% depending on the relative position of the ferrofluid within the grating: see Figure 1(d).
Similarly, by infiltrating and spatially immobilizing a short ferrofluidic length (1mm) inside a strong, uniform, short-length (∼10mm) MOF Bragg grating, we devised a miniature in-fiber magnetometer. We chose the optimal position for immobilizing the ferrofluidic defect along the Bragg grating to maximize the depth of the parasitic spectral notch observed in reflection. Defect immobilization is achieved pneumatically by sealing one end of the microstructured fiber and creating a closed air cavity. This allows the ferrofluid to move somewhat about its optimum immobilization point yet to return to its initial position in response to hydrostatic pressure. Under the application of a magnetic field along the axis of the MOF, the ferrofluidic defect moves along the Bragg grating length, causing significant changes to the parasitic Fabry-Perot spectral notch observed in reflection. By modifying the visibility of the parasitic notch, we were able to probe magnetic fields parallel to the fiber longitudinal axis between ∼700 and ∼2200 Gauss (see Figure 2). Moreover, monitoring the sign of these visibility changes makes it possible to resolve the direction of the magnetic field stimulus.7
We are working toward further improving the MOF Bragg grating magnetometer by increasing its sensitivity and expanding its dynamic range for applications related to space and medicine. Future plans for infiltrating MOF gratings using ferrofluids include developing magnetically tunable distributed Bragg reflector fiber lasers and shear stress-sensing smart pads for medical applications. At the same time, we are working on infiltrating iron oxide nanoclusters inside MOFs to develop grating-less, in-fiber magnetometers.
This work was partially supported by the European Commission project SP4-Capacities IASIS, contract 232479.
Stavros Pissadakis, Alessandro Candiani, Maria Konstantaki, Carola Sterner
Institute of Electronic Structure and Laser
Foundation for Research and Technology—Hellas, Greece
Carola Sterner, Walter Margulis
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