Flow cell with hybrid LPG and FBG optical fiber sensor for refractometric measurements F. Baldini 1, M. Brenci 1, F. Chiavaioli 2, R. Falciai 1, C. Trono 1 1 CNR-IFAC, Sesto Fiorentino (FI), Italy; 2 Dept. Information Engineering, Siena, Italy Abstract The combination of a long period grating and a fiber Bragg grating written on the same fiber is presented as method to reduce noticeably the interferences caused by strain and temperature in the measurement of refractive index. The hybrid LPG and FBG optical fiber sensor is manufactured and located in a small volume flow cell. The whole system with its flow cell and the gratings fabrication are described as well as the characterization of the cross-sensitivities. The maximum sensor sensitivity and resolution are 3120 nm/RIU and 2 x RIU, respectively. We believe that this system is suitable for being used as label-free chemical/biochemical sensing. Introduction Thanks to their sensitivity to external refractive index, long period gratings (LPGs) have been proposed for refractometric measurements [1,2]. Because of the influence of temperature, strain and fiber bending in the LPG response, it is crucial to control, correct and stabilize these parameters in order to measure accurately the refractive index changes. Conversely, a fiber Bragg grating (FBG) is insensitive to external refractive index and fiber bending, but it has a well-known response to both temperature and strain [3]. Differently from what already exists in the literature [4,5], the challenge aspect of the proposed sensing system is both the realization of a closed flow cell of relatively low volume where the optical fiber gratings can be fixed securely, and a methodology to control the LPG cross-sensitivities [6]. Figure 1. Longitudinal section (a), cross section (b) and a photo (c) of the optical chip. Materials and methods 1.Flow cell The sketch of the flow cell together with its picture is shown in Figures 1. A thermo electric cooler (TEC) element, composed by a series of three Peltier cells, acts as temperature stabilization section of the flow cell making use of a thermistor as feedback element of TEC, driven by a suitable controller (ILX Lightwave LDC-3722B TEC controller). The volume of the flow channel is 50 μL. A gasket obtained by an opportunely shaped parafilm® sheet is interposed between the two parts in order to assure the water-proof of the flow cell. The temperature of the flow cell is measured by a thermocouple connected to a thermometric measuring unit (Lutron TM-917). Results and discussion Figure 4 and Figure 5 show the strain and temperature characterization of the two gratings, respectively. As for temperature characterization, the cycle shown in Figure 5 was repeated for different refractive indices, in order to take into account the thermo-optic effect acting on the different solutions. Figure 6 and Figure 7 show the sensorgram and the response curve of the proposed LPG-based refractometer, respectively. In the last two figures, the λ RES of the two gratings are reported together with time evolution of the flow cell temperature. The minimum sensitivity is around the refractive index of water (1.1 nm/RIU) and increases when the external refractive index approaches the refractive index of the fiber cladding: 195 nm/RIU around RIU with a resolution of 4 x RIU and 3120 nm/RIU around RIU with a resolution of 2 x RIU. Conclusions A full characterization of a flow cell containing a hybrid LPG-FBG sensor was carried out. The combined use of FBG and LPG written in the same fiber allows to control the following parameters: i) strain: by means of the FBG; ii) fiber bending: by means of the FBG which allows to check that the fiber is under a given strain at the end of the fixing process into the flow cell; iii) temperature: by means of the thermocouple and iv) temperature stabilization of the flow cell: by means of the TEC system. A maximum sensitivity of 3120 nm/RIU and a resolution of 2 x RIU around RIU are achieved. References 1. H. J. Patrick et al., J. Lightwave Technol., 16, 1606–1612 (1998). 2. S. W. James and R. P. Tatam, Meas. Sci. Technol., 14, R49-R61 (2003). 3. A. D. Kersey et al., J. Lightwave Technol., 15, 1442–1463 (1997). 4. P. Pilla et al., Opt. Express, 17, (2009). 5. Y. Zhu et al., Opt. Comm., 229, (2004). 6. C. Trono et al., accepted for publication in Meas. Sci. Technol.. 7. K. O. Hill and G. Meltz, J. Lightwave Technol., 15, 1263–1276 (1997). Figure 6. Sensorgram of the proposed refractometer. Figure 7. Response curve of the LPG- based refractometer. Figure 5. Temperature characteri- zation of the two gratings in water. Figure 3. System block diagram of the optical chip. Figure 4. Strain characterization of the two gratings. 3.Interrogation and fluidic system Figure 3 shows the whole system block diagram of the optical chip, composed of a broad band superluminescent diode (SLD, INPHENIX IPSDD1503) as optical source, an optical spectrum analyzer (OSA) with 0.1 nm optical resolution (Anritsu MS9030A and MS9701B) which acquires the fiber transmission spectrum and automatically calculates the resonance wavelengths (λ RES ) and a peristaltic pump (Gilson MINIPULS 3). The solutions at different refractive index were prepared mixing glycerol and water in different ratio. The refractive index changes from of the pure water up to of the last test solution, and was measured by means of an hand-held refractometer (Atago R5000). The protocol followed to measure the λ RES of the two gratings implies the flow rate of about 0.5 mL/min for approximately 4 minutes, the stop of the pump for about 10 minutes and, finally, the acquisition of FBG and LPG resonance spectra with the minimum values extrapolated by a proper data fitting (Gaussian function for FBG and Lorentzian one for LPG). 2.Gratings fabrication FBGs are inscribed into photosensitive Boron-Germanium co-doped optical fiber (Fibercore PS1250/1500) by irradiating it through a rectangular phase mask ( nm phase mask period) with an Excimer KrF laser (LAMBDA PHYSIC COMPex 110) [7]. LPGs are manufactured by a point to point technique using the same laser source and irradiating the same fiber through an appropriately shaped and focused laser spot. The ad hoc developed fabrication setup is made up of a motorized translation stage (Burleigh 6000) and a control/management program for choosing both the grating period and the number of shots for each step. The transmission spectrum of the two gratings is reported in Figure 2. Figure 2. FBG and LPG transmission, spectrum and characteristics.