Long period and fiber Bragg gratings written within the same fiber for sensing purposes

Long period and fiber Bragg gratings written within the same fiber for sensing purposes Francesco Baldini, Massimo Brenci, Ambra Giannetti, Cosimo Trono Institute of Applied Physics, National Research Council, Florence – Italy Francesco Chiavaioli, Marco Mugnaini Dept. Information Engineering, Univ. of Siena, Italy

Outline • introduction • experimental setup • grating manufacture • the flow cell • grating characterisation • experimental results • conclusions

Introduction • Measurements of refractive index in biological fluids are being used since many years for the quantitative measurements of analytes, by means of the use of chemical/biochemical recognition layers deposited on suitable substrates • surface plasmon resonance • interferometric configurations • resonating structures • Long period gratings have been recently proposed for chemical and biochemical sensing. These sensors have high sensitivity to external refractive index in addition to all the other benefits offered by the optical fibre sensors but they are also sensitive to temperature and strain

Experimental setup

Grating manufacture (LPG) • fiber: Photosensitive B/Ge co-doped optical fiber (Fibercore PS1250/1500) • source: UV excimer laser source (KrF @ 248 nm) • LPG writing by point-to-point technique • fibre mounted on a motorized translation stage and irradiated by a laser spot appropriately shaped and focused by means of a variable micrometric slit and a cylindrical lens. • possibility to choose the grating period, the number of shots for each step and the total number of step (grating length) thanks to an ad-hoc software which drives all the process

Grating manufacture (FBG) • FBG are written by the phase-mask technique • Fiber: photosensitive B/Ge co-doped optical fiber (Fibercore PS1250/1500) • Source: UV excimer laser source (KrF @ 248 nm) • Phase mask pitch: 1059.9 nm

Transmission spectrum FBG Length: 1 cm Period: 530 nm λres: 1534 nm FWHM: 0.26 nm transmission loss: 10 dB LPG Length: 2.46 cm Period: 615 μm λres: 1567 nm FWHM: 6.5 nm transmission loss: 15 dB

The flow-cell

Grating characterisation - strain

Grating characterisation - temperature

Refractive index measurements • mixture of glycerol and water • • flow rate of about 0.5 mL/min for approximately 4 minutes • • stopped pump and acquisition of FBG and LPG minima extrapolated by the Gaussian (FBG) and Lorentzian (LPG) fit averaged on a period of about 10 minutes

Refractive index measurements

First results on IgG/anti- IgG bioassay • deposition of an Eudragit layer on the LPG surface • activation of the surface by EDC/NHS • covalent binding of the IgG antibody (1 mg/mL) • passivation of the surface with BSA (3% in PBS) • interaction with the anti-IgG antibody • non specific interaction anti-PSA antibody (10 μg/mL)

Conclusions • Realization of cascaded LPG and FBG gratings • Design and realization of a stabilised flow-cell • Refractive index measurement with a resolution of the order of 10-5 RIU • Preliminary IgG/anti-IgG assay

Long period and fiber Bragg gratings written within the same fiber for sensing purposes