Simultaneous measurement of strain and

INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS

J. Opt. A: Pure Appl. Opt. 6 (2004) 553–556

PII: S1464-4258(04)71219-2

Simultaneous measurement of strain and temperature using a Bragg grating structure written in germanosilicate fibres O Fraz˜ao1 and J L Santos1,2 1

Unidade de Optoelectr´onica e Sistemas Electr´onicos, INESC PORTO, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal 2 Departamento de F´ısica da Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal E-mail: [email protected] and [email protected]

Received 30 October 2003, accepted for publication 18 March 2004 Published 23 April 2004 Online at stacks.iop.org/JOptA/6/553 DOI: 10.1088/1464-4258/6/6/010

Abstract A fibre Bragg grating sensing configuration is presented for simultaneous measurement of strain and temperature. The proposed concept relies on writing a single Bragg grating on the splice region of two fibres with different levels of germanium doping. Doing so, a grating structure appears with three resonance peaks, which show distinct temperature sensitivities but similar strain responses. The experimental validation of the concept indicated resolutions of ±1.5 ◦ C Hz−1/2 and ±5.6 µε Hz−1/2 over measurement ranges of 80 ◦ C and 1000 µε for temperature and strain, respectively. Keywords: fibre optic sensors, fibre Bragg gratings, simultaneous measurement

1. Introduction It is nowadays clear that fibre Bragg gratings (FBGs) have enormous potential for strain sensing in a large spectrum of applications [1, 2]. Yet one of the most significant limitations of these sensing structures is their high intrinsic temperature cross-sensitivity. To overcome this problem, various approaches have been suggested and demonstrated, with particular emphasis on those that are based on the simultaneous measurement of strain and temperature [3]. To attain this objective a considerable number of techniques relying on diverse physical principles have been proposed [1]. In particular, the variation of the thermal dependence of the silica refractive index on type and level of doping and FBG fabrication conditions has been the underlying concept of various sensing heads with features that allow temperature/strain discrimination. Examples of such solutions are the inscription of gratings in neighbour fibres with different dopants in their cores [4, 5], and the impression of type I/II gratings in bore-germanium [6], or germanium [7] doped fibres. Other approaches involve sensing head 1464-4258/04/060553+04$30.00 © 2004 IOP Publishing Ltd

architectures supported by FBGs written in fibres with different diameters [8], using dual-wavelength fibre grating sensors [9], considering a single Bragg grating written in highly birefringent fibre [10], or based on a hybrid FBG/LPG (long period grating) structure [11]. In this work we presented a sensing head based on the writing of a single Bragg grating in the splice region of two different optical fibres, one with a low concentration of germanium in the core (Lo-Ge) and another with a high concentration of germanium (Hi-Ge). Such a grating shows a spectrum with three distinct signatures, all with similar strain sensitivity but with different temperature responses.

2. Principle The principle behind the sensing head solutions that allow simultaneous measurement of pairs of quasi-static parameters, in the present case strain and temperature, is the identification of two characteristics of the sensing head structure that change differently under the action of the measurands of the pair. If

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this happens, it is always possible to write a pair of equations that give the change of each of the selected characteristics in the general situation of simultaneous actuation of the measurands, and with a well-conditioned matrix that allows inversion in order to obtain the actual measurand values. The most favourable situation occurs when one of the selected characteristics is not affected by one of the measurands of the pair, but in general it is sufficient to have only one of the two characteristics with different response to the action of one of the measurands. In the present case the selected characteristics are two out of three spectral resonances that appear when a single grating is written in the splice region of two fibres with different levels of core germanium doping. The peak wavelength (λBi ) of these resonances changes with variation of the applied strain (ε) and temperature (T ). Therefore it is possible to write λBi = K Ti T + K εi ε

(1)

where i = 1, 2 addresses the two resonances considered. The thermal sensitivity, K Ti , depends both on the thermal expansion of the fibre and on the thermo-optic coefficient of the fibre material and, in general, will be dependent on the type and degree of core doping, i.e. K T 1 = K T 2 . On the other hand, the core germanium doping should not affect substantially the fibre mechanical properties, and therefore K ε1 = K ε2 . In such conditions, equation (1) can be inverted in order to have an explicit matrix equation that permits us to simultaneously obtain T and ε:


 1 K ε2 λ B1 −K ε1 T (2) = λ B2 ε  −K T 2 K T 1 where  = K T 1 K ε2 − K ε1 K T 2 . The accuracy of the recovered T and ε values increases with the increase of  [12], which in the present case depends on the difference between K T 1 and K T 2 . As will be shown below, the sensing structure proposed in this work permits a favourable performance in the determination of T and ε, comparable or better than those reported in the literature and relative to simultaneous strain and temperature measurement using the technique of different levels of core doping of the sensing head fibres.

3. Experiment and discussion The fibre Bragg gratings used in this experiment were fabricated using the following fibres: SMF28, simple germanosilicate core (diameter 8.2 µm), 3 mol% GeO2 (Lo-Ge), cold-hydrogenated at 100 atm; fibrecore SM1500, germanosilicate core (diameter 4.2 µm) with >20 mol% GeO2 (Hi-Ge). This fibre is not hydrogenated because of its high intrinsic photosensitivity related to the high level of germanium doping. Using a 10 mm length diffractive phase mask ( = 1070 nm) illuminated with a KrF laser operating at 248 nm, a grating structure was written over the splice region of the two fibres. Such a structure was constituted by three Bragg gratings, which appeared simultaneously during the imprint process. From the signatures obtained when the same phase mask was used in individual pieces of these fibres, it turned out that one of these gratings is connected with the fraction 554

Figure 1. Spectral signatures of the fibre grating structure.

Figure 2. Experimental set-up and sensor head geometry (inset: relative core diameters of the two sensing head optical fibres).

of the structure written in the SMF28 fibre and the other with the part impressed in the SM1500 fibre. From the characteristics of the third grating, namely the low reflectivity and the relatively large spectral width, associated with a small length grating, it is interpreted as a grating located just in the splice region and related to the change of the fibre core refractive index due to diffusion of dopants induced by the electric arc [13, 14]. As shown in figure 1, the wavelengths of these gratings were 1548.86 nm (SMF28), 1560.78 nm (SM1500) and 1550.75 nm (splice fibre segment). Having in mind the relation for the Bragg grating resonance wavelength, these differences between the spectral signatures of the three gratings can be translated to differences between the core effective refractive indexes of the host fibre segments, resulting in n eff ∼ = 5.57× 10−3 and n eff ∼ = 8.83× 10−4 (between the SMF28 and the SM1500 fibres and the SMF28 and the fibre segment acted on by the electric arc, respectively). Figure 2 illustrates the experimental set-up for testing this sensing concept. An erbium-doped broadband source (BBOS) was used to illuminate the fibre structure through a standard 3 dB coupler. The sensing head was attached to translation stages and placed over a thermoelectric cooler device (TEC). Cycles of strain were applied to the fibre structure in order to check some possible weakness of the splice region, which was not observed. All measurements were recorded by an optical spectrum analyser (OSA), with a resolution of 0.1 nm, connected to a computer (PC) data

Simultaneous measurement of strain and temperature using a Bragg grating structure written in germanosilicate fibres

Figure 3. Sensor response to temperature.

Figure 5. Applied and recovered temperature and strain values.

microstrain, respectively)

T 1.07 = −500.25 ε −10.92

Figure 4. Sensor response to applied strain.

acquisition system (LabViewTM ) for flexibility in data display, processing and storage. The concept underlying the simultaneous measurement of strain and temperature only requires the monitoring of two grating signatures. Therefore, the two chosen were the ones associated with the SMF28 and SM1500 fibres. The splice-FBG was not considered for this job but the associated strain and temperature sensitivities were measured for completeness, turning out to be 1.04 ± 0.08 pm/µε and 9.51 ± 0.03 pm ◦ C−1 , respectively. These values are not far from those obtained with the grating written in the SMF28 fibre, but with sufficient differentiation in order to sustain the possibility of using this sensing head for threeparameter simultaneous measurement [15]. Figures 3 and 4 show the responses of the two sensing gratings to changes in temperature and applied strain, respectively. As expected, different temperature sensitivities are observed for the two gratings, but they exhibit similar responses to applied strain. Considering the fact that there was no observable effect on the mechanical properties of the fibre, it can be concluded that K T SMF28 = K T SM1500 and K εSMF28 ≈ K εSM1500 . Therefore, a well-conditioned system of two equations for T and ε can be written which, from the slopes indicated in figures 3 and 4, is given by (λSMF28 and λSM1500 in nanometres and T , ε in degrees and

−1.11 9.46




 λSMF28 . λSM1500

The relative difference between the thermal coefficients of the two selected Bragg signatures is 15.4% (referenced to K T SMF28 ). For comparison, reported solutions based on dual gratings written in Ge/Ge + B [4] and in Ge/Er + Yb doped fibres [5] have values for such relative differences of 7.3% and 13.2%, respectively. Therefore, besides its intrinsic fabrication simplicity related to the fact that only a single-step UV exposure is needed (as also happens in the configuration described in [5]), the more favourable thermal behaviour of the sensing head proposed here indicates the potential for a better discrimination between temperature and strain induced effects. The amenability of single or composed sensing heads to multiplexing is always a merit factor. When fibre Bragg gratings are involved, the most natural and advantageous choice is wavelength multiplexing. The sensing concept proposed in this paper permits the serial wavelength multiplexing of some sensors, however its number is limited by two factors: each sensing head demands a relatively large fraction of the spectrum (around 12 nm), and the losses related to splicing two different optical fibres. Combining these two factors, wavelength multiplexing would probably be limited to two or three sensing heads. The system resolution was estimated when temperature and strain variations with amplitudes up to 80 ◦ C and 1000 µε were simultaneously applied to the sensing head. Figure 5 shows an example of the obtained measurand values versus the actual ones for the full measurement ranges. The rms deviations of the measured values relative to the calibration indicate fluctuations up to ±3.3 ◦ C and ±12.5 µε for the recovered temperature and strain data, respectively. Associated to a system bandwidth of 5 Hz, these values translate into normalized resolutions of ±1.5 ◦ C Hz−1/2 and ±5.6 µε Hz−1/2 . This medium performance is related to the decoupling matrix and with the noise level present in the system. The former is a fixed intrinsic characteristic of the technique considered and, therefore, has little margin for improvement. The same does not apply to the noise factor, which can be minimized in order to optimize the readout 555

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parameter resolution. However, it should be emphasized that the achieved resolutions are appropriate for a large spectrum of applications where simultaneous measurement of temperature and strain is required.

4. Conclusion In this work we reported the design, fabrication and characterization of a new sensing head based in one grating structure imprinted in the splice junction of SMF28 and SM1500 fibres. Such fibres have distinct levels of germanium doping in their cores, resulting in different temperature sensitivities but similar responses to strain. The imprinted structure showed three spectral signatures, one related to the SMF28 fibre, another to the SM1500 fibre, and a third one associated with the refractive index periodic modulation in the fibre segment significantly affected by the electric arc. The first two signatures were used to implement a strain and temperature simultaneous measurement configuration, which exhibited measurand reading sensitivity adequate for a significant number of practical applications.

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