Femtosecond Laser Micromachining of Fabry-Pérot Interferometers for Magnetic Field Sensing

Abstract

Fs-laser micromachining is a high precision fabrication technique that can be used to write novel three-dimensional structures, depending on the nature of light-matter interaction. In fused silica, the material modification can lead to (i) an increase of the refractive index around the focal volume, resulting in the formation of optical circuits, or (ii) an enhancement of the etch rate of the laser-affected zones relative to the pristine material, leading to a selective and anisotropic etching reaction that enables fabrication of microfluidic systems. Here, both effects are combined to fabricate a Fabry-Pérot interferometer, where optical waveguides and microfluidic channels are integrated monolithically in a fused silica chip. By filling the channel with a magnetic fluid whose refractive index changes with an external magnetic field, the device can be used as a magnetic field sensor. A linear sensitivity of -0.12 nm/mT is obtained in the 5.0±0.5 to 33.0±0.5 mT range, with the field being applied parallel to the light propagation direction.

Magnetic field sensors in fused silica fabricated by femtosecond laser micromachining

“…Higher fields were not tested due to hardware limitations. Compared to [17], the sensitivity increases from −0.12 to −0.25 nm/mT. This is probably due to the fact that, in [17], the channel was not treated in KOH, which causes the nanoparticles to adhere to its surface, thereby reducing the concentration of nanoparticles dispersed in the carrier liquid.…”

“…Compared to [17], the sensitivity increases from −0.12 to −0.25 nm/mT. This is probably due to the fact that, in [17], the channel was not treated in KOH, which causes the nanoparticles to adhere to its surface, thereby reducing the concentration of nanoparticles dispersed in the carrier liquid. In turn, for the same field magnitude, less magnetic columns are formed, leading to a smaller index variation and sensitivity in [17].…”

“…This is probably due to the fact that, in [17], the channel was not treated in KOH, which causes the nanoparticles to adhere to its surface, thereby reducing the concentration of nanoparticles dispersed in the carrier liquid. In turn, for the same field magnitude, less magnetic columns are formed, leading to a smaller index variation and sensitivity in [17]. In similar fiber-based Fabry-Pérot interferometers, sensitivities of 1.21 nm/mT in the 6 to 18 mT range with a 3.9% volume concentration of magnetic nanoparticles [27], and 2.3 nm/mT in the 0-12 mT range with a 17.7% concentration [9] were obtained.…”

“…The fs-laser-induced modification can be used to increase the refractive index around the focal volume, resulting in the formation of optical circuits [15], and/or to enhance the etch rate of laser-affected zones relative to the bulk material, leading to a highly-selective and anisotropic etching reaction that enables the fabrication of three-dimensional microfluidic devices [16]. Using this technique we fabricated a Fabry-Pérot interferometer inside a fused silica substrate [17], and briefly explored its potential as a magnetic field sensor, by injecting a ferrofluid inside the cavity. Here, we fabricate a similar device and further study its response.…”