Evaluation of gold layer configuration for plasmonic fiber grating biosensors.

Gold-coated fiber Bragg gratings (FBGs) are nowadays a mature technology for lab-on-fiber sensing based on surface plasmon resonance (SPR) excitation. Tilted FBGs bring valuable assets such as easy light injection, remote operation in very small volumes of analytes and immunity to temperature fluctuations. Different gold configurations have been reported to date, without considering their relative performances in terms of biochemical sensing. In this work, we experimentally study the impact of the gold coating on the cladding mode distribution in the tilted FBG amplitude spectrum and subsequently on its sensitivity to cytokeratins used as biomarkers for cancer diagnosis. Some relevant configurations of gold coatings are produced and tested, relying on both the sputtering and electroless plating (ELP) processes. The obtained results confirm that the coating thickness and its roughness drive the biosensing performances. The experimental limit of detection for cytokeratins 17 sensing reaches 14 fM for the most sensitive configurations.

Magnetic field sensing using standard uniform FBGs.

Magnetic field sensing can be directly (i.e. without requiring magnetic fuilds or magnetostrictive materials) obtained from the estimation of the circular birefringence induced in optical fibers through the so-called Faraday effect. In standard telecommunication-grade optical fiber, the amount of induced circular birefringence is however of the same order of the intrinsic fiber linear birefringence or even below. Hence, whenever uniform fiber Bragg gratings (FBGs) are used to probe this evolution, the resulting accuracy is usually very poor, even in the case of polarization-assisted measurements based on polarization dependent loss (PDL) or differential group delay (DGD). In this work, we demonstrate that the rotation of the diattenuation vector computed from the Mueller matrix of an FBG in transmission mode can be efficiently used as a read-out technique to sense a magnetic field evolution with a resolution of 0.1T.

Self-optimized metal coatings for fiber plasmonics by electroless deposition.

We present a novel method to prepare optimized metal coatings for infrared Surface Plasmon Resonance (SPR) sensors by electroless plating. We show that Tilted Fiber Bragg grating sensors can be used to monitor in real-time the growth of gold nano-films up to 70 nm in thickness and to stop the deposition of the gold at a thickness that maximizes the SPR (near 55 nm for sensors operating in the near infrared at wavelengths around 1550 nm). The deposited films are highly uniform around the fiber circumference and in spite of some nanoscale roughness (RMS surface roughness of 5.17 nm) the underlying gratings show high quality SPR responses in water.

Spectral characterization of differential group delay in uniform fiber Bragg gratings.

In this paper, we completely study the wavelength dependency of differential group delay (DGD) in uniform fiber Bragg gratings (FBG) exhibiting birefringence. An analytical expression of DGD is established. We analyze the impact of grating parameters (physical length, index modulation and apodization profile) on the wavelength dependency of DGD. Experimental results complete the paper. A very good agreement between theory and experience is reported.