Fiber Grating Devices
Biosensors made of an optical fiber section coated with a thin noble metal layer constitute a miniaturized counterpart to the Kretschmann-Raether prism configuration. They allow easy light injection and offer remote operation in very small volumes of analyte. They are perfectly suited to yield in situ (or even possibly in vivo) molecular detection. Usually, such biosensors are obtained from a gold-coated fiber segment for which the core-guided light is outcoupled and brought into contact with the surrounding medium, either by reducing the cladding diameter (through etching or side-polishing) or by using grating coupling. In the latter case, a refractive index modulation is photo-imprinted in the fiber core. Roughly 10 years ago, surface plasmon resonance (SPR) excitation was reported with gold-coated tilted fiber Bragg gratings (TFBGs). TFBGs are short-period gratings whose refractive index modulation is slightly angled with respect to the perpendicular to the optical fiber propagation axis. These devices probe the surrounding medium with narrowband (∼200 pm) cladding mode resonances, which is compatible with the use of high-resolution interrogators as a read-out technique. These gratings remain the single configuration able to probe all the fiber cladding modes individually, with high Q-factors. These unique spectral features are used to sense various analytes, such as proteins and cells. Impressive limit of detection (LOD) and sensitivity are reported, which paves the way to the practical use of such immunosensors, in very small volumes of analytes or even possibly in vivo.