GB2514326A

GB2514326A - A fibre optic chemical sensor that is insensitive to the influence of interfering parameters

Info

Publication number: GB2514326A
Application number: GB201305475A
Authority: GB
Inventors: Stephen Wayne James; Ralph Peter Tatam; Serhiy Oleksandrovich Korposh
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-03-26
Filing date: 2013-03-26
Publication date: 2014-11-26
Also published as: GB201305475D0

Abstract

A device comprising two or more optical fibre long period gratings (LPGs), where physically adjacent LPGs are separated by a length of optical fibre that does not contain an LPG, coated with a porous material 10. The device is an optical fibre chemical sensor based upon cascaded LPGs that is insensitive to interfering parameters such as temperature, strain and surrounding refractive index. The transmission spectrum of in-series LPGs is characterised by a channelled spectrum that lays within the envelope of the typical LPG resonance bands. The porous coating over the section of optical fibre separating the LPGs is infused with a functional chemically sensitive material 9, such that only the section of fibre separating the LPGs will be sensitive to the analyte of interest, causing a change in the phase of the channelled spectrum within the resonance band envelope.

Description

i A Fibre Optic Chemical Sensor That Is Insensitive To The Influence Of 2 Interfering Parameters 3 Serhiy Korposh, Stephen W. James, Ralph P. Tatam 4 Department of Engineering Photonics, School of Engineering, Cranfield University, Cranfield, Bedfordshire MK43 GAL UK

7 Description

9 INTRODUCTION

Optical fibre long period grating (LPG) based devices offer interesting opportunities for sensing applications due to their 11 inherent sensitivity to a range of environmental perturbations, temperature, strain, pressure, surrounding refractive 12 index and curvature. Typically, the transmission spectrum of an LPG contains a number of resonance bands, each 13 corresponding to coupling to a different cladding mode and each showing a different sensitivity to environmental 14 perturbation, which has been noted to offer the potential for multi-parameter sensing. The wavelengths at which light is coupled from the core to the cladding modes is governed by the phase matching equation [11 16 = -12 clad (1) 17 where A represents the wavelength at which light is coupled to the LP0 cladding mode, n is the effective refractive 18 index of the mode propagating in the core of the fibre, cx) is the effective index of the LP0 cladding mode and A is t the period of the LPG. The dispersion of an optical fibre is such that the difference between the core and cladding S...

* :* £0: mode effective indices exhibits a turning point where the value is maximum. For an FBG fabricated with a period such *. J. that equation (1) is satisfied at the phase matching turning point (PMTP), It has been shown that the sensitivity of the * S * ** 22 transmission spectrum to perturbation is at its maximum, and that for subsequent decreases in (n.-nd()), the INS's 23 transmission spectrum is characterised by the formation of a broad resonance band that subsequently splits into S. ** Zff4 two[21. Generally, for LPGs of period such that they operate near the PMTP, the transmission spectrum contains * S* p 2 resonance bands corresponding to coupling to one or two adjacent cladding modes, and thus the ability to correct for 26 interfering effects is limited.

27 The formation of an intrinsic fibre optic Mach-Zehnder interferometer (MZI) by cascading two identical long period 28 (LPG5) has been shown to produce a sinusoidal channelled spectrum within the attenuation bands that characterize the 29 transmission spectrum of an LPG, as illustrated in figure 1. [31. The phase (qi)of the channelled spectrum is dependent on the difference in the optical path lengths of the light propagating in the core and cladding modes, according to (4): 31 = (ncore nCzda(X))L (2) 32 where A represents the wavelength, and L is the centre-to-centre distance between two gratings. Perturbation of the 33 section of optical fibre separating the LPGs results in a change in the phase of the channelled spectrum within the 34 resonance band envelope. As the phase and the resonance wavelength are both dependent upon the difference between the core and cladding mode effective indices, when the entire length of the cascaded LPG is perturbed both 36 the resonance band central wavelength and the phase of the channelled spectrum change at the same rate [5]. This has 37 been exploited in a system for the simultaneous measurement of temperature and strain using cascaded LPGs 38 fabricated in a double clad fibre. The fibre was mounted such that the fibre separating the LPGs could be strained, 39 while the entire length of the device experienced temperature changes. The authors exploited coupling to the modes of the inner cladding as they would be insensitive to surrounding refractive index.

41 Here we describe a chemical sensor based on cascaded LPGs that offers the capabilIty to measure a desired analyte 42 using the phase shift of the channelled spectrum while being immune to the undesired effects of interfering 43 parameters such as temperature and refractive index (RI). The key to this is the use of a porous coating that covers the :w LPGs and the section of optical fibre separating them. The coating over the section of optical fibre separating the LPGs * 45 is infused with a functional, chemically sensitive, material. In this way only the section of fibre separating the LPGs will be sensitive to the analyte of interest, with exposure to the analyte causing a change in the phase of the channelled 47 spectrum within the resonance band envelope. Perturbation of the entire length of the device by environmental 48 parameters such as temperature, strain and changes in the surrounding refractive index cause the central wavelength * * * * 49 of the resonance band envelope and the phase of the channelled spectrum to change at the same rate. Thus *.* measurement of the phase of the channelled spectrum with respect to the centre wavelength of the resonance band 51 allows the response to the analyte to be separated from interfering parameters.

52 PaIrs of LPGs, each of length 30 mm, separated by 30 mm, Figure la, were fabricated in boron-germanium co-doped 53 optical fibre (Fibercore P5750) with cut-off wavelength 670 nm in a point-by-point fashion, side-Illuminating the optical 54 fibre by the output from a frequency-quadrupled NdfYAG laser, operating at 266 nm. LPG pairs with different grating periods ranging from 110.3 to 111.5 pm were fabricated in this way. The transmission spectra of the cascaded LPGs 56 were recorded by coupling the output from a tungsten-halogen lamp (Ocean Optics HL-2000) into the fibre, and by 57 analyzing the transmitted light using a fibre coupled CCD spectrometer (Ocean Optics HR4000). The grating periods 58 were selected such that the LPGs operated at the phase matching turning point, which, for coupling to a particular 59 cladding mode (in this case LP021), ensuring optimised sensitivity (6]. The response of the device to perturbation of different sections was investigated. This involved exposing, in turn, the section separating the LPGs, and the whole 61 structure (both LPGs and middle section) to temperature and RI changes. The effect of the grating period on sensor 62 performance was studied. For temperature measurements, the appropriate section of the IPG sensor device was 63 placed into a calibrated tube furnace and exposed to varying temperature. A temperature data logger (D51923 64 temperature/humidity logger iButton, range -20 to +85°C, resolution 0.5°C) was used to monitor real temperature values in the oven that were used to calibrate the LPG sensor. To demonstrate the response to RI, the appropriate 66 sections of the LPG pair was immersed into water and ethanol solution with different concentrations that have 67 different RI values ranging from 1.33 to 1.36. Operation of the sensor in not limited to this range.

69 Figure 1 shows the transmission spectra (TS) of cascaded LPGs with different grating periods; 110.3 lAm, 111 pm 111.5 pm. In this instance, the grating periods were chosen such that the LPG was operating at the PMTP and just after the * :it: PMTP (2]. For the LPG with period 111.5 pm, the resonance band corresponding to coupling to the LP021 cladding mode *::2: just starts to appear (at the PMPT). This band is well developed (=111 pm) and is fully split (=110.3 pm) (after the 73 PMTP), Figure lb. It should be noted that, when the LPG operates at or near the PMTP it is highly sensitive to relatively : small variations of grating period and refractive index (8]. As can be seen in Figure ib, variations in grating period, as * 75 small as 0.5 iint lead to significant changes in the TS.

S I 4

78 Figure 2(a) shows the response of the TS of the cascaded long period grating pair with grating period of 111 i.Im when 79 the central section of the device experiences changes in temperature. The phase of the channeled spectrum changes, whIle the resonance band envelopes remain fixed. This can be seen for the resonance bands corresponding to coupling 81 to the LP0 cladding mode (at the PMTP) and the LI'019 and LP013 resonances. When both LPGs and the middle section 82 were exposed to temperature change the LP0 resonance band operating at the PMPT, forms and splits into two, as 83 shown in Figure 2(b), The phase of the channeled spectrum can be seen to change at the same rate, remaining constant 84 relative to the central wavelength of the resonance band envelope. The lower order cladding mode resonances suffer blue shifts and again the channeled spectrum phase changes at the same rate as the envelope.

86 similar effects were observed when the cascaded LPG was exposed to changes in the surrounding refractive index, as 87 shown in Figure 3, where, on changing the surrounding refractive Index by varying the concentration of a solution of 88 ethanol around the entire device, the resonance band wavelength and the channelled spectrum phase were observed 89 to shift at the same rate. When only the middle section was immersed into ethanol solution the envelope didn't shift while changes of the phase of the channelled spectrum were observed, Figure 3b.

92 Based on these observations, a sensor of the type shown in figure 4 is proposed. The entire length of the cascaded LPG 93 device is coated with a mesoporous coating, of thickness of order 300nm, via the electrostatic self-assembly process[7] 94 or other appropriate technique. The section of optical fibre separating the LPGs Is then Infused with a functional material that changes its properties in response to the analyte of interest, as we have described previously 171. This "6 structure is key to ensuring that the device operates with built-in compensation for Interfering parameters. The * *...* * 0 97 presence of the coating along the entire length is significant as the response of the LPG to changes in environmental **** parameters such as temperature and surrounding refractive index are influenced by the presence of a coating, due to, 99 for example, the thermo-optical and thermal expansion characteristics of the coating and to influence of the iOO. surrounding refractive index on the cladding modes of the fibre and on the wave guiding properties of the coating (if I4I. appropriate), all of which influence the phase matching condition. The infusion into only the central section then 102 ensures a differential response only for the analyte of interest, changing the phase of the channelled spectra, as 103 explained earlier. While there may be an influence of thermally induced changes in the properties of the analyte, and a 104 small change in the refractive Index of the infused region1, it is not anticipated that these effects will be significant, and, as the change in wavelength of the resonance band wavelength contains information on the magnitude of the 106 interfering parameters, these effects, if significant, can be compensated for. Such a system could be designed to 107 operate within either liquid or gaseous environments.

108 It should be noted that it is also possible to configure the sensor such that the functional material is infused into the 109 coating over each LPG, and not Into the coating on the section of fibre separating the LPGs. In this case, the envelope will respond to the analyte and interfering parameters, while the phase of the channelled spectrum will respond only to 111 the Interfering parameters.

113 A theoretical model has been developed to demonstrate the operation of this configuration. Using the method 114 described in (81, the core and cladding mode refractive indices, and their response to changes in the refractive index of a surrounding thin-film coating were first calculated using the approach presented in [9) and then used to determine 116 the transmission spectrum using the matrix method reported in (10). The results are illustrated In figure 5 for a 117 cascaded LPG pair of period 100 lAm, separated by 30 mm where figure 5(a) shows the response of the resonance band 118 corresponding to coupling to the 1P019 mode at the phase matching turning point changes in temperature, from 25°C to 119 100 °C, occurring along the entire length of the coating, while figure 5(b) shows the response of the device under the same thermal conditions as in figure 5(a), but with an independent additional perturbation of the refractive Index of the section of fibre separating the LPGs, from 0-2 x10 refractive index units RlU. In figure 5(a) it is clear that both the 122. central wavelength of the resonance band envelope and the phase of the channelled spectrum change at the same * * 123 rate, while In figure 5(b) the additional perturbation of the central section of the LPG induces a change of the phase of !124 the channelled spectrum with respect to the resonance band envelope. 0 ** * * .

* 1.26 It is known that, for a given period, the LPG shows its highest sensitivity to coatings of thickness such that the operation * S. * 127 is at the mode transition region, which is generally associated with coatings of materials of refractive index higher than 128 that of the cladding. The porosity of the coatings used here results in the refractive index of the coating being lower 129 than that of the material from which it is formed. To ensure that this device operates with optimised sensitivity, it may be possible to deposit a non-porous coating onto the optical fibre of a material of refractive index higher than that of 131 the cladding, of thickness such that the effective IndIces of the cladding modes are such that the device is biased to be 132 at or near the mode transition region. The porous coating would deposit on top of this base coating, and infused as 133 descrIbed previously. * . * * . * ** * ** ** * * * * * * ** * I 7

References 1 V Bhatia Applications of long-period gratings to single and multi-parameter sensing Opt. Express, 4457, 1999 2 X Shu, L Zhang, and I Bennion, "Sensitivity Characteristics of Long-Period Fiber Gratings", J. Lightwave Technol., 20, 255, 2002 3 B. H. Lee and J. Nishil, "Bending sensItivity of In-series long-period gratings," Opt. Lett., 23, pp. 1624, 1998.

4 Y. Ilu, S A. R. Williams, L. Zhang, and I. Bennion, "Phase shifted and cascaded long-period fiber gratings," Opt. Comm., 164, 27, 1999 S M. J. Kim, V. H. Kim, G. Mudhana, and B. H. Lee, 4timultaneous Measurement of Temperature and Strain Based on Double Cladding Fiber Interlerometer Assisted by Fiber Grating Pair", IEEE Photon Technol Lett, 20, 1290, 2008.

6 S.C. Cheung, S.M. Topliss, S.W. James and R.P. Tatam, "Response of fibre optic long period gratings operating near the phase matching turning point to the deposition of nanostructured coatings". Journal of the Optical Society of America B, 25, 902, 2008.

7 S. Korposh, S. W. James, S-W Lee, S. M. Topliss, S. C. Cheung, W. J. Batty, and R. P. Tatam, "Fiber optic long period grating sensors with a nanoassembled mesoporous film of 5102 nanoparticles", Opt. Express 18, 13227, 2010.

85. W. James, S. M. Topliss and R. P. Tatam, "Properties of length-apodized phase-shifted long period gratings operating at the phase matching turning point", J. Lightwave Technol., 30, 2203, 2012.

9 I. D. Villar, I. R.Matias, and F. J. Arregui, "Influence on cladding mode distribution of overlay deposition on long period fiber gratings," J. Opt.Soc. Amer. A, 23, 651, 2006.

V. Liu, J. A. R.Williams, L. Zhang, and I. Bennion, "Phase shifted and cascaded long-period fiber gratings," Opt.

Comm., 164, 27, 1999. *0** * * **** * * . * S S. *S * S * * * * * .5

Figure Descriptions

Figure 1(a): Schematic of diagram of a cascaded LPG Mach Zehnder interferometer Figure 1(b). The transmission spectra (IS) of cascaded LPGs with different grating periods; 110.3 rIm, 111 pm 111.5 pm. LP refer to the cladding modes to which the attenuation bands correspond -operation of this device is not limited to these particular LP modes Figure 2(a). Plot illustrating the temperature response of the transmission spectrum of the LPG when the section of fibre separating the LPGs was heated/cooled. The white and black correspond to transmission values of 100% and 0%, respectively. The colour bar shows the temperature, measured using a commercial temperature data logger and scale bar shows the values of the temperature at a given time.

Figure 2(b). Plot illustrating the temperature response of the transmission spectrum of the LPG when the entire device was heated/cooled. The white and black correspond to transmission values of 100% and 0%, respectively. The colour bar shows the temperature, measured using a commercial temperature data logger and scale bar shows the values of the temperature at a given time.

Figure 3(a). TS of the of the cascaded LPG pair with grating period 110.3 pm, measured when the RI of the material surrounding the entire device was changed. % values refer to the amount of ethanol in water.

Figure 3(b). TS of the of the cascaded LPG pair with grating period 110.3 pm, measured when only the middle section was exposed to different RI values. % values refer to the amount of ethanol in water.

Figure 4. Schematic of the proposed device. The entire length of the CLPG device is coated with a single-or multi-coating of nanospheres silica, titania with other suitable material to form a porous coating. The functional material, shown here as MA, is then infused into the length of fibre separating the LPGs Figure 5(a). Plot of the simulated response of the cascaded LPG spectrum to a linear change in * :° : temperature, from 25 °C to 100 °C, experienced by the entire length of the device. White represents 100% transmission, and black 0%. * * *

Figure 5(b). Plot of the simulated response of the cascaded LPG spectrum to a linear change in temperature, from 25 °C to 100 °C, with the entire device experiencing the same temperature : * ..* change as in figure 5(a) and with the central section experiencing an additional linear increase in * * refractive index, from 0 to 2x10-4 RIU. White represents 100% transmission, and black 0%. * ** * Key.

1. jacket 2 cladding 3. LPG1 4. LPG2 5. Li. 6. L 7. Li

8. PAH poly(allylamine hydrochloride) 9. PM poly(acrylic acid) 10. 5i02 NPs 4** * * * . * * * ** * * * * * *t *