In the December issue of Optics and Photonics News, the Optical Society of America highlighted exciting and significant research findings that characterized the year 2011 in Optics and Photonics. Among these annual highlights (see video), the work of the FORTH-IESL, research group of Dr Stavros Pissadakis, focused on the development of Magnetofluidically Tunable Microstructured Optical Fiber Grating Devices was included.

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Magnetofluidically Tunable Microstructured Optical Fiber Grating Devices

Stavros Pissadakis, Alessandro Candiani, Maria Konstantaki, Carola Sterner and Walter Margulis

Microstructured optical fibers (MOFs)¹ constitute a versatile platform for developing novel optofluidic², sensing and actuating devices, fusing together photonics and microfluidics. We have infiltrated ferrofluids inside MOF Bragg gratings for the development of "in-fiber" magnetofluidic photonic devices, tuning their spectral response by applying external magnetic fields³. Ferrofluids are colloidal liquid suspensions of magnetic nanoparticles with applications from space to biology; they undergo significant changes related to their viscosity and refractive index when stimulated by a magnetic field, allowing spatial fluid translation or confinement following the dynamic lines of the field applied. The infiltration and manipulation of such highly viscous, non-transparent magnetic liquids inside a MOF can lead to unique optical and interrogation capabilities.

The ferrofluid infiltrated inside the capillaries of a MOF induces a strong loss and phase perturbation prompted by its extreme absorption loss and a refractive index greater than silica; this affects the spectrum of the infiltrated MOF Bragg grating. The tailoring of the grating characteristics and those of the ferrofluid, together with the optogeometrical structure and the "wettability" of the MOF, define the operational characteristics and performance of the magnetofluidic device. We have developed a number of such devices of different sensing or actuating functionalities using "grapefruit" geometry MOF and commercial ferrofluids. We' ve exemplified tunable phase-shifted Bragg reflectors4 and a vectorial magnetometer⁵.

In the case of phase-shifted Bragg reflectors, we inserted short lengths of ferrofluids inside uniform and chirped MOF Bragg gratings, leading to the formation of lossy Fabry-Perot resonators between the opposite grating sides. By translating the ferrofluidic socket using an external magnetic field within the grating length, we modified the spectral characteristics of such fluidic Fabry-Perot resonator. Upon uniform or chirped grating periodicity, one can observe the generation of a parasitic spectral notch in reflection, or tune the bandwidth and strength of the spectral peak, respectively⁴. Similarly, by infiltrating and pneumatically immobilizing a short ferrofluidic length inside a uniform MOF Bragg grating, a miniature magnetometer is realized. Once the magnetic field is applied, the ferrofluidic defect "scans" the MOF Bragg grating length, changing the visibility of the parasitic Fabry-Perot spectral notch and allowing straightforward correlation to the strength and direction of the applied field⁵.

Our future plans for the infiltration of MOF gratings using ferrofluids include the development of magnetically tunable DBR fiber lasers and shear stress sensing smart pads.

Stavros Pissadakis, Alessandro Candiani and Maria Konstantaki are with the Foundation for Research and Technology-Hellas, Institute of Electronic Structure and Laser, Heraklion, Greece. Carola Sterner and Walter Margulis are with the department of fiber photonics, Uppsala University, Stockholm, Sweden. References and Resources
1. P.S.J. Russell. J. Lightwave Technol. 24, 4729 (2006).
2. D. Psaltis et al. Nature 442, 381 (2006).
3. A. Candiani et al. Opt. Express 18, 24654 (2010).
4. A. Candiani et al. Opt. Lett. 36, 2548 (2011).
5. A. Candiani et al. in CLEO/Europe and EQEC 2011 Conference Digest (OSA, 2011), CH 6.3.

See also: Optics and Photonics News