Francesco Chiavaioli - Università degli Studi di Siena

Flow cell for strain- and temperature-compensated refractometric
measurements by means of cascaded optical fiber long period
and Bragg gratings as promising label-free biosensing system
Francesco Chiavaioli
Università degli Studi di Siena, Via Roma 56, 53100 Siena, Italy
e-mail: chiavaioli@dii.unisi.it
Tutor: Prof. Valerio Vignoli; Tutor at IFAC-CNR: Prof. Massimo Brenci
[joint work with “Nello Carrara” Institute of Applied Physics – National Research Council]
Introduction
Refractometric measurements in biological fluids have been used for many years in the
quantitative measurements of analytes, by employing chemical/biochemical recognition
layers deposited on suitable substrates. Chemical/biochemical interactions with these layers
lead to changes in the refractive index of the layer which can be detected by means of optical
methods, and which depends on the concentration of the interacting analyte. This approach is
known as the label-free approach [1, 2], in contrast with the methodology that makes use of
luminescent markers chemically bound to recognition elements. Within the optical approach,
optical fiber long period gratings (LPG) have been recently proposed for chemical
/biochemical sensing [3]. These sensors show a high sensitivity to the refractive index of the
medium surrounding the fiber and thus a number of refractometric measurement systems
have been proposed in the past [4-7]. However, LPGs have great sensitivity not only to the
external refractive index, but also to temperature, strain and fiber bending. A number of
techniques have been proposed in order to get rid of the influence of these cross-sensitivities
[8, 9], which can be critical when an accurate refractometric measurement is carried out with
the investigated sample flowing within a flow cell, as generally occurs in the chemical
/biochemical sensing. In this scenario our goal is to describe the design and characterization
of a thermo-stabilized flow cell of low volume (tens of µL) for accurate refractometric
measurements using LPG and a methodology for measuring and correcting all the LPG
cross-sensitivity, by means of a fiber Bragg grating (FBG) written on the same fiber and an
accurate temperature measurement system [10-13].
Measurements
Strain characterization
Theory and methodology
Definition: an optical fiber grating is a permanent periodic modulation of the fiber core refractive index.
Classification (according to the grating period Λ, which sets out a specific coupling of light):
Short period or fiber Bragg gratings (FBGs):
 Grating period in the range of hundreds of
nanometers;
 Coupling between the fundamental core mode
and its respective counter-propagating mode;
 Characteristic equation [14]:
2Λ
,
Long period gratings (LPGs):
 Grating period in the range of hundreds of
micrometers;
 Coupling between the fundamental core mode and a
discrete set of forward-propagating cladding modes;
 Characteristic equation [15]:
,
Λ
,
,
The specific coupling
between the modes
verifying the phasematching condition
generates in the LPG
transmission spectrum a
series of attenuation
bands centered at discrete
wavelengths.
Methodology:
 Theoretical expressions:
Δ
(1)
Δ
⁄
(2)
 Once the four coefficients in the foregoing equations are determined experimentally, and by measuring the
⁄
can be
wavelength shift of the two gratings and the temperature, the value of the nonlinear function
attained.
Experimental setup
1.
Flow cell: the sketch of the flow cell, its picture, and the block diagram of the experimental setup are shown in the
three figures below.
2.
Fabrication of the gratings
Temperature characterization
Refractive index measurements
 FBG: by irradiating a photosensitive B-Ge co-doped
optical fiber (Fibercore PS1250/1500) through a
rectangular phase mask (1059.9 nm period) with an
Excimer KrF laser [14];
 LPG: by 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 and a control/management
program for choosing both the grating period and the
number of shots for each step (point-to-point
technique).
The maximum sensitivity and resolution (3120 nm/RIU and 2*10-5 RIU) are observed when
the refractive index of the solution is close to that of the fiber cladding. To summarize, the
proposed system makes it possible to accurately measure the refractive index and to control
the following parameters:
 strain: by means of the FBG, with a resolution of 15 pm corresponding to ±3.8 pm of
∆
;
 temperature: by means of the thermocouple, with a resolution of 0.01 °C corresponding
to ±3.6 pm of ∆
;
 temperature stabilization: by means of the TEC system (tenth of degree).
3.
4.
Note: interference problem of the
attenuation bands.
Interrogation system: for each measurement step, the software routine centers at the resonance wavelength,
fixes the λ-span (LPG: 20 nm; FBG: 2 nm), acquires the spectrum and extrapolates the minimum wavelength by
means of a Lorentzian data fitting (first for the LPG and then for the FBG).
Fluidics system and chemicals
 Solutions at a different refractive index (1.334 – 1.467) were prepared by mixing glycerol and water in different
ratios, and the refractive index was measured by means of a hand-held refractometer.
 Measurement protocol: flow rate of about 0.5 mL/min for 4 minutes, afterwards halting of the pump and
acquisition of FBG and LPG minima for 10 minutes.
References
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[10] F. Baldini, M. Brenci, R. Falciai, A. Giannetti, C. Trono, F. Chiavaioli, and M. Mugnaini, Long period and fiber Bragg gratings written within the same fiber for sensing purposes, Proceedings of SPIE Photonics
West, vol. 7941, paper [7941-40], San Francisco California, USA, 2011.
[11] C. Trono, F. Baldini, M. Brenci, F. Chiavaioli, and R. Falciai, Flow cell with hybrid LPF and FBG optical fiber sensor for refractometric measurements, Submitted to 21st International Conference on Optical Fiber
Sensors, OFS-21, 15-19 May, Ottawa, Canada, 2011.
[12] C. Trono, F. Baldini, M. Brenci, F. Chiavaioli, R. Falciai, and A. Giannetti, Cascaded optical fibre long period and Bragg gratings for strain and temperature cross-sensitivities compensation in refractive index
measurements, Submitted to SPIE Optics + Optoelectronics Conference, 18-21 April, Prague, Czech Republic, 2011.
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gratings, Submitted to Meas. Sci. Technol..
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