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Sensitivity enhancement of long period gratings for temperature measurement using the long period grating pair technique
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Sensors and Actuators A 141 (2008) 314–320
Sensitivity enhancement of long period gratings for temperature measurement using the long period grating pair technique Samer K. Abi Kaed Bey ∗ , Tong Sun, Kenneth T.V. Grattan Electrical, Electronic, and Information Engineering, School of Engineering and Mathematical Sciences, City University, Northampton Square, London EC1V 0HB, United Kingdom Received 16 November 2006; received in revised form 1 July 2007; accepted 3 October 2007 Available online 16 October 2007
Abstract An approach to enhance the sensitivity achievable with long period grating (LPG) technology for temperature measurement, by using a LPG pair technique to create Mach-Zehnder interferometers written into B–Ge co-doped optical fibres, is presented. The separation of the single LPGs constituting the pair is kept very short with, as a consequence, LPGs being written with high coupling coefficients, implying a significant change in the differential effective group refractive index of the fibre. This allows the temperature-induced wavelength shift of the interference fringes (IFs) of the LPG pair to change at a faster rate than the LPG envelope, due to the consequent increasing phase change of the core modes, with respect to the cladding modes within the grating region as a function of temperature and wavelength variations. A brief theoretical explanation is given and an experimental demonstration is shown by comparing the characteristics of two separate LPG pairs (LPGP1 and LPGP2), where LPGP1 comprises two ‘strong’ LPGs, while LPGP2 comprises two ‘weak’ LPGs. Results obtained have shown that the sensitivity of the change of the IF position in LPGP1 occurs at a faster rate than its envelope, whereas for LPGP2, this shifts at a similar rate to the envelope of the spectrum. A simple mathematical approach is suggested to calculate the wavelength shift based on the phase change variation in LPGP1, using basic Fourier analysis. The sensitivity enhancement obtained in the experimental results from LPGP1 to LPGP2 was determined to be ∼50%, from a phase shift of 2◦ /◦ C for the IF of LPGP2 to a phase shift of 3◦ /◦ C for the IF of LPGP1, with a root-mean-square (rms) deviation of 1.9◦ , corresponding to a rms error in temperature of 0.6 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Long period grating; Mach-Zehnder interferometer; Interference fringe; Group index
1. Introduction 1.1. Sensitivities of single long period gratings for temperature measurement The wavelength-shift of a single long period grating (LPG), induced by an external temperature variation, can vary considerably depending on the fibre host material and upon the order of the stop band (SB) used [1]. Various reported research studies, aiming at improving the sensitivity of a single LPG, have relied upon the use of novel cladding structures, thereby offering a sensitivity of 0.8 nm/◦ C [2], whilst another technique makes use of high-order SBs outside the telecommunication window, to achieve sensitivities of 1.5 nm/◦ C [3], and 2.75 nm/◦ C [4],
∗
Corresponding author. E-mail address: [email protected] (S.K. Abi Kaed Bey).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.10.019
for LPGs fabricated in photosensitive B–Ge co-doped optical fibres. Other techniques have relied upon the surrounding of the LPG with high thermo-optic coefficient materials, achieving a sensitivity of 19.2 nm/◦ C over a limited temperature range of 1.1 ◦ C, or upon etching the fibre to produce ultra-thin cladding, followed which it is surrounded by a liquid crystal material, with this combination offering a sensitivity of 2.1 nm/◦ C [5]. 1.2. Sensitivity enhancement using the long period grating pair technique Recently, the potential of LPG pairs has been realised in various sensor applications. LPG pairs act as interferometric sensors and changes in their resulting interference fringes (IFs), have been studied for measuring bending [6], external refractive index [7], transverse loading [8] and temperature [9–11]. LPG pairs have also been used to improve the sensing resolution achievable from single LPGs, due to the relatively narrow SB bandwidth
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resulting from the formation of IFs. However, the temperature induced wavelength-shift sensitivities of the IFs produced, as reported in the literature [9–11], have shown a similar change in their variation with the LPG envelope. In this work, a new method to enhance sensitivity, based on the LPG pair technique is proposed, to monitor the shift of the IF which is occurring at a rate faster than the LPG envelope; this was achieved by mainly increasing the coupling power of each matched LPG during the fabrication process, and decreasing the separation of the LPGs constituting the pair to a minimum.
fringe spacing cannot be maintained constant. As shown in the denominator terms of Eq. (1), this may occur when the core modes in the grating region are experiencing phase shifts at a different rate, as a function of temperature and wavelength, with respect to the cladding modes. Thus the focus of this work is to exploit the conditions where this phenomenon occurs and thus to use it to increase the sensitivity of the LPG pair, through monitoring the IF wavelength shift as a function of temperature and thereby creating a relatively more sensitive sensor system.
2. Theoretical background and LPG pair technique analysis
3. Experiments
Inscribing two matched LPGs along an optical fibre, where they are separated by a specified distance, creates a Mach-Zehnder interferometer. A schematic diagram of such a Mach-Zehnder interferometer, based on two conventional LPGs (noted as LPGa and LPGb), is shown in Fig. 1. When light from a broadband source passes through the first LPG, part of the propagating core modes couple to the forward propagating cladding modes with which they are in phase. In a conventional single LPG, the coupled cladding mode then is either absorbed or scattered; however in the LPG pair configuration, the coupled core and cladding modes from the first LPG combine at the second matched LPG to form the IFs, where the core and cladding paths in the fibre constitute the ‘arms’ of the MachZehnder interferometer. Generally, the fringe spacing S, whose units are of wavelength, can be expressed by [12]: S=
2π (1) core (λ) − βclad (λ)](L − d) (d/dλ)Φin (λ, d) − (d/dλ)[βout out
where Φin is the sum of the phase shifts of the beam passing core and βclad are the propagathrough both LPG gratings, βout out tion constants of the core and cladding modes outside the LPG region, d is the grating length and L is the centre-to-centre separation between the LPG gratings forming the pair (Fig. 1). A fringe spacing, S, that is approximately constant as a function of increasing temperature may occur when LPGs forming the pair have relatively weak coupling strength (<3 dB) [11]. In this case, the UV-induced average refractive index change between the grating region and in the non-grating region is estimated to be less than 5% [12]. However, under certain circumstances the
Fig. 1. Schematic of the Mach-Zehnder interferometer based on a matched LPG pair.
In this work LPG pairs were fabricated, evaluated and cross-compared with a view to determining their temperature sensitivities, as discussed below. 3.1. Fabrication Two separate LPG pairs (labelled LPGP1 and LPGP2) were fabricated, where LPGP1 comprises two single LPGs with a coupling strength of ∼8 dB separated by a distance (L) of ∼8 mm, while LPGP2 comprises two single LPGs with a coupling strength of ∼2 dB separated by ∼10 mm. The LPG pairs were fabricated in B–Ge single mode optical fibre with a numerical aperture of 0.14 and a cut-off wavelength of 1246 nm. The fabrication process for the LPG pair started by inscribing a single LPG following which the fibre, together with the fibre holders, was moved through the specified distance, L, in a sideways direction to fabricate the second LPG using a translation stage controlled by a computer via a motion controller (Newport-Type MM4006). An ultraviolet KrF excimer laser (Braggstar 500, supplied by Tuilaser) was used with a pulse energy of around 8 mJ and a frequency of 100 Hz to create the LPGs. The laser beam width was measured to be approximately 7.3 ± 0.3 mm, which corresponded approximately to the length of a single LPG created. The amplitude mask used has a period of 400 m and was attached to a measurement gauge base. The distance between the fibre and amplitude mask was set to be 0.2 mm, using the measurement gauge and a high resolution CCD camera (Philips). The transmission spectra of the single LPGs and the LPG pairs fabricated were detected using an Optical Spectrum Analyser (OSA-type HP86140A), when illuminated by a low-power light from a broadband source (LS-1 Tungsten Halogen, supplied by Ocean Optics). A relatively short separation between the LPGs was chosen for LPGP1 (L ∼ 8 mm), as this contributed various advantages. First, with reference to (the denominator of) Eq. (1), the dominant factor affecting the fringe spacing S, as a result of the short separation, is the phase change of the optical modes within the LPG grating region over the non-grating region. This may, as a result, highlight the effect of the group refractive index change arising from the ‘strong’ gratings present and its variation with temperature on the IF wavelength shift. In addition, decreasing the separation between the LPGs helped to minimize the overall sensor dimensions, enabling it to be used in some specific applications where small “hot spots” are to be
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Fig. 2. Schematic of the experimental setup for fibre annealing and experimentation.
measured without affecting the integrity of the IF pattern generated. 3.2. Annealing For stability, gratings were annealed prior to the experiments being carried out and the experimental setup used for annealing and experimentation is shown in Fig. 2. The LPG pairs (LPGP1 and LPGP2) were heated in a calibrated tube oven (Carbolite, MTF 12/38/400), at a temperature of 200 ◦ C for around 15 h to achieve the required performance stability and wavelength shift repeatability over the testing range used from room temperature to 150 ◦ C. 3.2.1. Annealing of LPGP1 It was observed that two IFs occurred in every stop band (SB) of LPGP1, as shown in Fig. 3, but fringes denoted as IF4 -bf1 , IF4 bf2 and IF4 -af1 and IF4 -af2 were shown clearly in the fourth stop band, both before and after annealing, respectively (where the subscript refers to the stop band order and the superscript to the IF number). During the annealing process of LPGP1, the envelope of the LPGP1 was observed to shift to the lower wavelength region at a faster rate than the IFs in the fourth SB. In addition, the transmission spectrum of the fourth SB was observed to experience a power drop of ∼3 dB. Only one of the IF bands (IF4 -af1 ) in the fourth SB was selected for detailed investigation through out this work, due to its location in the most sensitive SB within the telecommunications window under study.
Fig. 3. Transmission spectra of LPGP1 before and after the annealing process, with the interference fringes in the fourth stop band highlighted.
Fig. 4. Transmission spectra of LPGP2 before and after the annealing process, with the interference fringes in the fourth stop band highlighted.
3.2.2. Annealing of LPGP2 The two IFs shown in the fourth SB of LPGP2 are illustrated in Fig. 4 and highlighted as IF14 and IF24 . Both of these IFs from LPGP2 had shifted at the same rate with the envelope during the annealing process. In a similar cooling method used for LPGP1, LPGP2 was cooled slowly to room temperature, at a rate of 0.7 ◦ C/min (over the temperature range from 200 to 100 ◦ C), following which it was cooled at a rate of 0.4 ◦ C/min to reach room temperature. The annealing of LPGP2 resulted in a spectral shift to lower wavelengths with a consequent power loss decrease of around 6 dB at the fourth SB. 4. Results 4.1. Determination of the sensitivity enhancement of LPGP1 The LPG pairs, LPGP1 and LPGP2, were evaluated over the range from room temperature to 150 ◦ C. The sensors were inserted in an oven (Fig. 2), and the temperature was increased in steps of 5 ◦ C at a rate of 1 ◦ C/min; readings were recorded at 10 min after reaching the desired oven temperature to ensure the sensor device was in equilibrium with the oven. With LPGP1, it was noticed that the temperature-induced wavelength-shift of IF4 -af1 had shifted faster than its envelope and IF4 -af2 over the 125 ◦ C dynamic range (as shown in Fig. 5). This is due likely to various core modes within the grating region experiencing different phase shifts with respect to the cladding modes at different wavelengths and as a function of temperature. However, with LPGP2, it was noticed that IF14 and IF24 had shifted at a similar rate with the envelope. To determine the validity of the approach and to determine the sensitivity enhancement of the LPG pair technique as demonstrated by LPGP1, the wavelengthshift response of IF4 -af1 was further studied and compared against the wavelength shift of IF14 in LPGP2. The sensitivities obtained for IF4 -af1 in LPGP1 and IF14 in LPGP2 over the 125 ◦ C dynamic range are calculated to be approximately 0.43 and 0.31 nm/◦ C, respectively. Due to the
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Fig. 5. Comparison between the LPGP1 spectra at 150 ◦ C and room temperature, with the distance travelled by each interference fringe highlighted in the fourth stop band.
profile of IF4 -af1 being unclear at room temperature (Fig. 5), the calculated sensitivity value of 0.43 nm/◦ C shows a higher error than is desired (∼±0.05 nm/◦ C). In order to evaluate the proposed approach used in this work, a mathematical analysis to determine more closely the actual sensitivity enhancement of IF4 -af1 is discussed in the next section. 4.2. Phase change of IF4 -af1 To study the effect of temperature on the phase change of IF4 -af1 in LPGP1 and of IF14 in LPGP2, a Fast Fourier transform was applied to parts of the transmission spectra from 1550 to 1650 nm of the outputs of both sensors (LPGP1 and LPGP2) and the results are cross-compared with the resultant Fourier transform of a single LPG, which was fabricated using the same characteristics for the single LPG as was used in LPGP1 (in terms of coupling strength (8 dB before annealing) and grating length (∼7.3 mm)) as shown in Fig. 6, in order to obtain the frequencies representing the unclear profile of IF4 -af1 and for further comparison with IF14 bands, respectively (Fig. 7). Fol-
Fig. 6. Comparison between the single LPG and LPGP1 spectra.
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Fig. 7. Spectra representations of the single LPG, LPGP1 and LPGP2 in the frequency domain.
lowing this, a procedure that is widely reported in the literature [13,14] and used as an interrogation technique [15] was applied. The frequencies representing the fringes of interest (IF4 -af1 and IF14 ) are isolated and translated to the origin and from the ratio of the real to the imaginary values, the associated phase shift was determined. However, the phase change thus obtained is uncertain within a factor of 2π. The discontinuity of the phase can be corrected using the algorithm reported in [13]. The phase change of IF4 -af1 and IF14 are plotted against the phase change of the single LPG as shown in Fig. 8, where clearly reflects the sensitivity enhancement of LPGP1. As a result, the sensitivity of IF4 -af1 was calculated to be around 3◦ /◦ C whereas as the sensitivity of IF14 in LPGP2 was determined to be around 2◦ /◦ C. This method shows that the sensitivity enhancement achieved using the LPG pair technique is a factor of approximately 1.5. With the use of a linear and cubic fit of the data, the IF4 -af1 phase change shift, Φin , can be expressed in terms of the temperature,
Fig. 8. Comparison of the phase shifts of IF4 -af1 (: square), IF14 (: circle) and the single LPG (♦: rhombus). A linear fit (dotted) and a cubic fit (solid) are shown in the phase shift of IF4 -af1 .
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T, through the following equations, respectively: Φin = −0.0532T + 2.9426 Φin = 2 × 10
and
−6 3
T − 0.0006T 2 − 0.0112 T + 2.0767
(2)
where the correlation coefficients, r, are 0.9986 and 0.9998 for the linear and cubic fit, respectively. The rms deviation in the cubic fit is approximately 1.9◦ , corresponding to an rms error value of ±0.6 ◦ C. 5. Discussion 5.1. Temperature induced variation of the differential effective group refractive index in the LPG pair sensor device As the separation between the LPG pair decreases, according to Eq. (1), L (the centre-to-centre separation) tends to approach the grating length, d. As a result, the dominant factor in the denominator of Eq. (1) will be the phase change implied in the grating region; hence, the separation between the IFs may be written as: S=
2π (d/dλ)Φin (λ, d)
(3)
The phase change of the light passing through both LPG gratings constituting the Mach-Zehnder interferometer sensor Φin is given by [12]: core clad (λ) − βin (λ)]d Φin = −[βin
(4)
By substituting the definition of the propagation constant (β = 2πneff /λ) in Eqs. (3) and (4), then the spacing of the IFs, S, may be written as: λ2 S= min d
(5)
where min is the differential effective group index within the grating region, defined as: m ≡ neff − λ
d neff dλ
Fig. 9. Comparison of the differential effective group index, differential effective refractive index and the differential first-order dispersion of the fibre in the grating region of LPGP1.
envelope. In Fig. 9, the differential effective refractive index shows a slight decrease from 4.1 × 10−3 at room temperature to 4.0 × 10−3 at 150 ◦ C and the differential first-order dispersion of the fibre is positive and increases from 1.9 × 10−3 /m at room temperature up to 3.6 × 10−3 /m at 150 ◦ C. It is believed that the increase of the differential first-order dispersion of the fibre with temperature has contributed to the increase of the group index value and as a result enhancing the sensitivity of the IF4 -af1 in LPGP1. To verify further of the values obtained in Fig. 9, and visualize the variation of the differential effective group index outside the grating region as a function of temperature, a cross-comparison with LPGP2 is made. Since the gratings are weak (∼2 dB), then the fringe spacing can be expressed [10]: S≈
λ2 mout L
(8)
where mout is the differential effective group index outside the grating region, and L is the centre-to-centre separation. The two IFs obtained (IF14 and IF24 ), as shown in Fig. 4, in the fourth
(6)
and clad neff ≡ ncore eff − neff
(7)
By using the spectral data set obtained for the LPGP1 (from room temperature up to 150 ◦ C), the separation between IF4 -af1 and IF4 -af2 can be obtained as a function of the temperature increments. Thus using Eq. (5), the differential effective group refractive index min can be calculated, and from the phase matching condition of the LPG, the differential effective refractive index neff can be obtained. Substituting the values of min and neff in Eq. (6), the differential first-order dispersion of the fibre can be also obtained and plotted against the temperature variation, as shown in Fig. 9. It is shown that the differential effective group index has increased from 7.1 × 10−3 at room temperature to 9.7 × 10−3 at 150 ◦ C, which may explain the reason the IF4 -af1 has shifted at a faster rate than the LPG
Fig. 10. Comparison of the differential effective group index, differential effective refractive index and the differential first-order dispersion of the fibre out side the grating region of LPGP2.
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order SB of LPGP2 have shifted together while the temperature has increased from 25 ◦ C up to 150 ◦ C. Hence, the separation between IF14 and IF24 is constant and measured to be ∼44 nm. Using Eqs. (8) and (6), the values of mout , neff and the differential first-order dispersion of the fibre can be plotted, as shown in Fig. 10. Unlike the case for LPGP1, the differential effective group index in LPGP2 has shown a decrease in value with slope which is similar to that of its differential effective refractive index values, as the temperature has increased. This may imply that there was no major change of the group index of the fibre at the grating region in LPGP2 and as a result, it has changed, having a similar slope to that of the differential effective refractive index as a function of temperature. 6. Conclusion In this work, a novel sensor approach, based on the LPG pair technique is demonstrated with in this preliminary work a sensitivity enhancement of 50% over the use of a single LPG, demonstrated over the dynamic range of 125 ◦ C. This technique is still under investigation to allow for further study of its potential to be applied to any single LPG with strong coupling strength, to observe the IFs which shift at a rate faster than their envelopes. Results obtained have showed the validity of the approach for sensors fabricated without using any coatings or special fibre material or structures. A mathematical method is suggested to separate the IFs in the frequency domain and to allow the monitoring of IFs with non-clear profiles or sharp peaks. Future work will be carried out on investigating in greater depth the effect of the LPG coupling strength on the wavelength shift of the IFs. Acknowledgements The authors are pleased to acknowledge the support of the Engineering & Physical Sciences Research Council through several schemes. References [1] S.W. James, R.P. Tatam, Optical fibre long-period grating sensors: characteristics and application, Meas. Sci. Technol. 14 (2003) R49–R61. [2] A.A. Abramov, A. Hale, R.S. Windeler, T.A. Strasser, Widely tunable longperiod fibre gratings, Electron. Lett. 35 (1999) 81–82. [3] T. Venugopalan, T.L. Yeo, T. Sun, K.T.V. Grattan, High sensitivity longperiod grating-based temperature monitoring using a wide wavelength range to 2.2 m, Opt. Commun. 268 (2007) 42–45. [4] X. Shu, T. Allsop, B. Gwandu, L. Zhang, Bennion, High temperature sensitivity of long-period gratings in B–Ge codoped fibre, IEEE Photon. Technol. Lett. 13 (2001) 818–820. [5] S. Yin, K.-W. Chung, X. Zhu, A highly sensitive long period grating based tunable filter using a unique double-cladding layer structure, Opt. Commun. 188 (2001) 301–305. [6] B.H. Lee, J. Nishii, Bending sensitivity of in-series long-period fibre gratings, Opt. Lett. 23 (1998) 1624–1626. [7] O. Duhem, J.F. Henninot, M. Douay, Study of a fibre Mach-Zehnder interferometer based on two spaced 3 dB long period gratings surrounded by a refractive index higher than that of silica, Opt. Commun. 180 (2000) 255–262.
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[8] Y.G. Han, B.H. Lee, W.T. Han, U.C. Paek, Y. Chung, Fibre optic sensing applications of a pair of long period fibre grating, Meas. Sci. Technol. 12 (2001) 778–781. [9] B.H. Lee, J. Nishii, Self-interference of long period fibre grating and its application as temperature sensor, Electron. Lett. 34 (1998) 2059. [10] B.H. Lee, Y. Chung, W.T. Han, U.C. Paek, Temperature sensor based on self-interference of a single long-period fiber grating, IEICE Trans. Electron. E83-C (2000) 287–292. [11] S.K. Abi Kaed Bey, T. Sun, K.T.V. Grattan, Optimization of a long-period grating-based Mach-Zehnder interferometer for temperature measurement, Opt. Commun. 272 (2007) 15–21. [12] B. Lee, J. Nishii, Dependence of fringe spacing on the grating separation in a long-period fiber grating pair, Appl. Opt. 38 (1999) 3450–3459. [13] M. Takeda, H. Ina, S. Kobayashi, Fourier-transform method of fringepattern analysis for computer-based topography and interferometry, J. Opt. Soc. Am. 72 (1982) 156–159. [14] S.W. Smith, The Scientist and Engineer’s Guide to Digital Signal Processing, 2nd ed., California Technical Publishing, San Diego, 1999, pp. 148–150 (Chapter 8). [15] R.P. Murphy, S.W. James, R.P. Tatam, Multiplexing of fibre optic long period grating based interferometer sensors, in: Proceedings of the Photon06: Optics and Photonics, 2006, p. 28.
Biographies Samer K. Abi Kaed-Bey received his degree of Bachelor of Engineering in Mechanical from the Lebanese American University, Lebanon, in 2001. In the same year he joined the Department of Information Technology at Napier University, Edinburgh, UK, where he received his Master of Science degree in Mechatronics and was awarded the Napier University medal for his outstanding achievements in 2002, where his MSc project is currently on a patent filing process. Previous work experience was obtained from Daewoo Motors and Middle East Airlines, Lebanon. He also worked as an engineer at a recycling company, Envirotech, London, and as a production engineer/CNC operator, setter and programmer at Radial for the designing of customized motorbike products, Kent, UK. He is currently pursuing his studies for the PhD degree at the Measurement and Instrumentation Centre in the School of Engineering and Mathematical Sciences, City University, London. He is a Faraday Associate member in Intersect (Intelligent Sensing Faraday Partnership), and a Member of the Institute of Electrical Engineers. His interests are in Mechatronics and in Opto-Mechatronics. Tong Sun was awarded the degrees of Bachelor of Engineering, Master of Engineering and Doctor of Engineering for work in Mechanical Engineering from the Department of Precision Instrumentation of Harbin Institute of Technology, Harbin, China, in 1990, 1993 and 1996, respectively. She came to City University, London, as an Academic Visitor and latterly a Research Fellow to work in the field of fibre optic temperature measurement using luminescent techniques. She was awarded the degree of Doctor of Philosophy at City University in Applied Physics in 1999. She was an Assistant Professor at Nanyang Technological University in Singapore from 2000 to 2001 and currently a Senior Lecturer at City University, London, since she re-joined in April 2001. Dr. Sun is a Member of the Institute of Physics and the Institution of Electrical Engineers and a Chartered Physicist and a Chartered Engineer in the United Kingdom. Her research interest is in optical fibre sensors, optical communications and laser engineering. She has authored or co-authored some 90 scientific and technical papers in the field. Kenneth T.V. Grattan received his Bachelors degree in Physics (with first class honors) from The Queen’s University, Belfast, in 1974 and completed his PhD studies in 1978, graduating from the same University. In the same year he became a Post-Doctoral Research Assistant at Imperial College, London. His research during that period was on laser systems for photophysical systems investigations, and he and his colleagues constructed some of the first of the then new category of excimer lasers (XeF, KrF) in Europe in 1976. His work in the field continued with research using ultraviolet and vacuum ultraviolet lasers for photolytic laser fusion driver systems and studies on the photophysics of atomic and molecular systems. He joined City University, London in 1983 after 5 years at Imperial
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College, undertaking research in novel optical instrumentation, especially in fibre optic sensor development for physical and chemical sensing. The work has led into several fields including luminescence based thermometry, Bragggrating-based strain sensor systems, white light interferometry, optical system modeling and design and optical sensors for water quality monitoring. The work has been extensively published in the major journals and at international conferences in the field, where regularly he has been an invited speaker, and over
600 papers have been authored to date. He was awarded the degree of Doctor of Science by City University in 1992. Professor Grattan is currently Deputy Dean of Engineering at City University, London, having from 1991 to 2001 been Head of the then Electrical, Electronic and Information Engineering Department. He has been Chairman of the Applied Optics Division of the UK Institute of Physics and was President of the Institute of Measurement and Control in 2000.
Sensors and Actuators A 141 (2008) 314–320
Sensitivity enhancement of long period gratings for temperature measurement using the long period grating pair technique Samer K. Abi Kaed Bey ∗ , Tong Sun, Kenneth T.V. Grattan Electrical, Electronic, and Information Engineering, School of Engineering and Mathematical Sciences, City University, Northampton Square, London EC1V 0HB, United Kingdom Received 16 November 2006; received in revised form 1 July 2007; accepted 3 October 2007 Available online 16 October 2007
Abstract An approach to enhance the sensitivity achievable with long period grating (LPG) technology for temperature measurement, by using a LPG pair technique to create Mach-Zehnder interferometers written into B–Ge co-doped optical fibres, is presented. The separation of the single LPGs constituting the pair is kept very short with, as a consequence, LPGs being written with high coupling coefficients, implying a significant change in the differential effective group refractive index of the fibre. This allows the temperature-induced wavelength shift of the interference fringes (IFs) of the LPG pair to change at a faster rate than the LPG envelope, due to the consequent increasing phase change of the core modes, with respect to the cladding modes within the grating region as a function of temperature and wavelength variations. A brief theoretical explanation is given and an experimental demonstration is shown by comparing the characteristics of two separate LPG pairs (LPGP1 and LPGP2), where LPGP1 comprises two ‘strong’ LPGs, while LPGP2 comprises two ‘weak’ LPGs. Results obtained have shown that the sensitivity of the change of the IF position in LPGP1 occurs at a faster rate than its envelope, whereas for LPGP2, this shifts at a similar rate to the envelope of the spectrum. A simple mathematical approach is suggested to calculate the wavelength shift based on the phase change variation in LPGP1, using basic Fourier analysis. The sensitivity enhancement obtained in the experimental results from LPGP1 to LPGP2 was determined to be ∼50%, from a phase shift of 2◦ /◦ C for the IF of LPGP2 to a phase shift of 3◦ /◦ C for the IF of LPGP1, with a root-mean-square (rms) deviation of 1.9◦ , corresponding to a rms error in temperature of 0.6 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Long period grating; Mach-Zehnder interferometer; Interference fringe; Group index
1. Introduction 1.1. Sensitivities of single long period gratings for temperature measurement The wavelength-shift of a single long period grating (LPG), induced by an external temperature variation, can vary considerably depending on the fibre host material and upon the order of the stop band (SB) used [1]. Various reported research studies, aiming at improving the sensitivity of a single LPG, have relied upon the use of novel cladding structures, thereby offering a sensitivity of 0.8 nm/◦ C [2], whilst another technique makes use of high-order SBs outside the telecommunication window, to achieve sensitivities of 1.5 nm/◦ C [3], and 2.75 nm/◦ C [4],
∗
Corresponding author. E-mail address: [email protected] (S.K. Abi Kaed Bey).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.10.019
for LPGs fabricated in photosensitive B–Ge co-doped optical fibres. Other techniques have relied upon the surrounding of the LPG with high thermo-optic coefficient materials, achieving a sensitivity of 19.2 nm/◦ C over a limited temperature range of 1.1 ◦ C, or upon etching the fibre to produce ultra-thin cladding, followed which it is surrounded by a liquid crystal material, with this combination offering a sensitivity of 2.1 nm/◦ C [5]. 1.2. Sensitivity enhancement using the long period grating pair technique Recently, the potential of LPG pairs has been realised in various sensor applications. LPG pairs act as interferometric sensors and changes in their resulting interference fringes (IFs), have been studied for measuring bending [6], external refractive index [7], transverse loading [8] and temperature [9–11]. LPG pairs have also been used to improve the sensing resolution achievable from single LPGs, due to the relatively narrow SB bandwidth
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resulting from the formation of IFs. However, the temperature induced wavelength-shift sensitivities of the IFs produced, as reported in the literature [9–11], have shown a similar change in their variation with the LPG envelope. In this work, a new method to enhance sensitivity, based on the LPG pair technique is proposed, to monitor the shift of the IF which is occurring at a rate faster than the LPG envelope; this was achieved by mainly increasing the coupling power of each matched LPG during the fabrication process, and decreasing the separation of the LPGs constituting the pair to a minimum.
fringe spacing cannot be maintained constant. As shown in the denominator terms of Eq. (1), this may occur when the core modes in the grating region are experiencing phase shifts at a different rate, as a function of temperature and wavelength, with respect to the cladding modes. Thus the focus of this work is to exploit the conditions where this phenomenon occurs and thus to use it to increase the sensitivity of the LPG pair, through monitoring the IF wavelength shift as a function of temperature and thereby creating a relatively more sensitive sensor system.
2. Theoretical background and LPG pair technique analysis
3. Experiments
Inscribing two matched LPGs along an optical fibre, where they are separated by a specified distance, creates a Mach-Zehnder interferometer. A schematic diagram of such a Mach-Zehnder interferometer, based on two conventional LPGs (noted as LPGa and LPGb), is shown in Fig. 1. When light from a broadband source passes through the first LPG, part of the propagating core modes couple to the forward propagating cladding modes with which they are in phase. In a conventional single LPG, the coupled cladding mode then is either absorbed or scattered; however in the LPG pair configuration, the coupled core and cladding modes from the first LPG combine at the second matched LPG to form the IFs, where the core and cladding paths in the fibre constitute the ‘arms’ of the MachZehnder interferometer. Generally, the fringe spacing S, whose units are of wavelength, can be expressed by [12]: S=
2π (1) core (λ) − βclad (λ)](L − d) (d/dλ)Φin (λ, d) − (d/dλ)[βout out
where Φin is the sum of the phase shifts of the beam passing core and βclad are the propagathrough both LPG gratings, βout out tion constants of the core and cladding modes outside the LPG region, d is the grating length and L is the centre-to-centre separation between the LPG gratings forming the pair (Fig. 1). A fringe spacing, S, that is approximately constant as a function of increasing temperature may occur when LPGs forming the pair have relatively weak coupling strength (<3 dB) [11]. In this case, the UV-induced average refractive index change between the grating region and in the non-grating region is estimated to be less than 5% [12]. However, under certain circumstances the
Fig. 1. Schematic of the Mach-Zehnder interferometer based on a matched LPG pair.
In this work LPG pairs were fabricated, evaluated and cross-compared with a view to determining their temperature sensitivities, as discussed below. 3.1. Fabrication Two separate LPG pairs (labelled LPGP1 and LPGP2) were fabricated, where LPGP1 comprises two single LPGs with a coupling strength of ∼8 dB separated by a distance (L) of ∼8 mm, while LPGP2 comprises two single LPGs with a coupling strength of ∼2 dB separated by ∼10 mm. The LPG pairs were fabricated in B–Ge single mode optical fibre with a numerical aperture of 0.14 and a cut-off wavelength of 1246 nm. The fabrication process for the LPG pair started by inscribing a single LPG following which the fibre, together with the fibre holders, was moved through the specified distance, L, in a sideways direction to fabricate the second LPG using a translation stage controlled by a computer via a motion controller (Newport-Type MM4006). An ultraviolet KrF excimer laser (Braggstar 500, supplied by Tuilaser) was used with a pulse energy of around 8 mJ and a frequency of 100 Hz to create the LPGs. The laser beam width was measured to be approximately 7.3 ± 0.3 mm, which corresponded approximately to the length of a single LPG created. The amplitude mask used has a period of 400 m and was attached to a measurement gauge base. The distance between the fibre and amplitude mask was set to be 0.2 mm, using the measurement gauge and a high resolution CCD camera (Philips). The transmission spectra of the single LPGs and the LPG pairs fabricated were detected using an Optical Spectrum Analyser (OSA-type HP86140A), when illuminated by a low-power light from a broadband source (LS-1 Tungsten Halogen, supplied by Ocean Optics). A relatively short separation between the LPGs was chosen for LPGP1 (L ∼ 8 mm), as this contributed various advantages. First, with reference to (the denominator of) Eq. (1), the dominant factor affecting the fringe spacing S, as a result of the short separation, is the phase change of the optical modes within the LPG grating region over the non-grating region. This may, as a result, highlight the effect of the group refractive index change arising from the ‘strong’ gratings present and its variation with temperature on the IF wavelength shift. In addition, decreasing the separation between the LPGs helped to minimize the overall sensor dimensions, enabling it to be used in some specific applications where small “hot spots” are to be
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Fig. 2. Schematic of the experimental setup for fibre annealing and experimentation.
measured without affecting the integrity of the IF pattern generated. 3.2. Annealing For stability, gratings were annealed prior to the experiments being carried out and the experimental setup used for annealing and experimentation is shown in Fig. 2. The LPG pairs (LPGP1 and LPGP2) were heated in a calibrated tube oven (Carbolite, MTF 12/38/400), at a temperature of 200 ◦ C for around 15 h to achieve the required performance stability and wavelength shift repeatability over the testing range used from room temperature to 150 ◦ C. 3.2.1. Annealing of LPGP1 It was observed that two IFs occurred in every stop band (SB) of LPGP1, as shown in Fig. 3, but fringes denoted as IF4 -bf1 , IF4 bf2 and IF4 -af1 and IF4 -af2 were shown clearly in the fourth stop band, both before and after annealing, respectively (where the subscript refers to the stop band order and the superscript to the IF number). During the annealing process of LPGP1, the envelope of the LPGP1 was observed to shift to the lower wavelength region at a faster rate than the IFs in the fourth SB. In addition, the transmission spectrum of the fourth SB was observed to experience a power drop of ∼3 dB. Only one of the IF bands (IF4 -af1 ) in the fourth SB was selected for detailed investigation through out this work, due to its location in the most sensitive SB within the telecommunications window under study.
Fig. 3. Transmission spectra of LPGP1 before and after the annealing process, with the interference fringes in the fourth stop band highlighted.
Fig. 4. Transmission spectra of LPGP2 before and after the annealing process, with the interference fringes in the fourth stop band highlighted.
3.2.2. Annealing of LPGP2 The two IFs shown in the fourth SB of LPGP2 are illustrated in Fig. 4 and highlighted as IF14 and IF24 . Both of these IFs from LPGP2 had shifted at the same rate with the envelope during the annealing process. In a similar cooling method used for LPGP1, LPGP2 was cooled slowly to room temperature, at a rate of 0.7 ◦ C/min (over the temperature range from 200 to 100 ◦ C), following which it was cooled at a rate of 0.4 ◦ C/min to reach room temperature. The annealing of LPGP2 resulted in a spectral shift to lower wavelengths with a consequent power loss decrease of around 6 dB at the fourth SB. 4. Results 4.1. Determination of the sensitivity enhancement of LPGP1 The LPG pairs, LPGP1 and LPGP2, were evaluated over the range from room temperature to 150 ◦ C. The sensors were inserted in an oven (Fig. 2), and the temperature was increased in steps of 5 ◦ C at a rate of 1 ◦ C/min; readings were recorded at 10 min after reaching the desired oven temperature to ensure the sensor device was in equilibrium with the oven. With LPGP1, it was noticed that the temperature-induced wavelength-shift of IF4 -af1 had shifted faster than its envelope and IF4 -af2 over the 125 ◦ C dynamic range (as shown in Fig. 5). This is due likely to various core modes within the grating region experiencing different phase shifts with respect to the cladding modes at different wavelengths and as a function of temperature. However, with LPGP2, it was noticed that IF14 and IF24 had shifted at a similar rate with the envelope. To determine the validity of the approach and to determine the sensitivity enhancement of the LPG pair technique as demonstrated by LPGP1, the wavelengthshift response of IF4 -af1 was further studied and compared against the wavelength shift of IF14 in LPGP2. The sensitivities obtained for IF4 -af1 in LPGP1 and IF14 in LPGP2 over the 125 ◦ C dynamic range are calculated to be approximately 0.43 and 0.31 nm/◦ C, respectively. Due to the
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Fig. 5. Comparison between the LPGP1 spectra at 150 ◦ C and room temperature, with the distance travelled by each interference fringe highlighted in the fourth stop band.
profile of IF4 -af1 being unclear at room temperature (Fig. 5), the calculated sensitivity value of 0.43 nm/◦ C shows a higher error than is desired (∼±0.05 nm/◦ C). In order to evaluate the proposed approach used in this work, a mathematical analysis to determine more closely the actual sensitivity enhancement of IF4 -af1 is discussed in the next section. 4.2. Phase change of IF4 -af1 To study the effect of temperature on the phase change of IF4 -af1 in LPGP1 and of IF14 in LPGP2, a Fast Fourier transform was applied to parts of the transmission spectra from 1550 to 1650 nm of the outputs of both sensors (LPGP1 and LPGP2) and the results are cross-compared with the resultant Fourier transform of a single LPG, which was fabricated using the same characteristics for the single LPG as was used in LPGP1 (in terms of coupling strength (8 dB before annealing) and grating length (∼7.3 mm)) as shown in Fig. 6, in order to obtain the frequencies representing the unclear profile of IF4 -af1 and for further comparison with IF14 bands, respectively (Fig. 7). Fol-
Fig. 6. Comparison between the single LPG and LPGP1 spectra.
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Fig. 7. Spectra representations of the single LPG, LPGP1 and LPGP2 in the frequency domain.
lowing this, a procedure that is widely reported in the literature [13,14] and used as an interrogation technique [15] was applied. The frequencies representing the fringes of interest (IF4 -af1 and IF14 ) are isolated and translated to the origin and from the ratio of the real to the imaginary values, the associated phase shift was determined. However, the phase change thus obtained is uncertain within a factor of 2π. The discontinuity of the phase can be corrected using the algorithm reported in [13]. The phase change of IF4 -af1 and IF14 are plotted against the phase change of the single LPG as shown in Fig. 8, where clearly reflects the sensitivity enhancement of LPGP1. As a result, the sensitivity of IF4 -af1 was calculated to be around 3◦ /◦ C whereas as the sensitivity of IF14 in LPGP2 was determined to be around 2◦ /◦ C. This method shows that the sensitivity enhancement achieved using the LPG pair technique is a factor of approximately 1.5. With the use of a linear and cubic fit of the data, the IF4 -af1 phase change shift, Φin , can be expressed in terms of the temperature,
Fig. 8. Comparison of the phase shifts of IF4 -af1 (: square), IF14 (: circle) and the single LPG (♦: rhombus). A linear fit (dotted) and a cubic fit (solid) are shown in the phase shift of IF4 -af1 .
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T, through the following equations, respectively: Φin = −0.0532T + 2.9426 Φin = 2 × 10
and
−6 3
T − 0.0006T 2 − 0.0112 T + 2.0767
(2)
where the correlation coefficients, r, are 0.9986 and 0.9998 for the linear and cubic fit, respectively. The rms deviation in the cubic fit is approximately 1.9◦ , corresponding to an rms error value of ±0.6 ◦ C. 5. Discussion 5.1. Temperature induced variation of the differential effective group refractive index in the LPG pair sensor device As the separation between the LPG pair decreases, according to Eq. (1), L (the centre-to-centre separation) tends to approach the grating length, d. As a result, the dominant factor in the denominator of Eq. (1) will be the phase change implied in the grating region; hence, the separation between the IFs may be written as: S=
2π (d/dλ)Φin (λ, d)
(3)
The phase change of the light passing through both LPG gratings constituting the Mach-Zehnder interferometer sensor Φin is given by [12]: core clad (λ) − βin (λ)]d Φin = −[βin
(4)
By substituting the definition of the propagation constant (β = 2πneff /λ) in Eqs. (3) and (4), then the spacing of the IFs, S, may be written as: λ2 S= min d
(5)
where min is the differential effective group index within the grating region, defined as: m ≡ neff − λ
d neff dλ
Fig. 9. Comparison of the differential effective group index, differential effective refractive index and the differential first-order dispersion of the fibre in the grating region of LPGP1.
envelope. In Fig. 9, the differential effective refractive index shows a slight decrease from 4.1 × 10−3 at room temperature to 4.0 × 10−3 at 150 ◦ C and the differential first-order dispersion of the fibre is positive and increases from 1.9 × 10−3 /m at room temperature up to 3.6 × 10−3 /m at 150 ◦ C. It is believed that the increase of the differential first-order dispersion of the fibre with temperature has contributed to the increase of the group index value and as a result enhancing the sensitivity of the IF4 -af1 in LPGP1. To verify further of the values obtained in Fig. 9, and visualize the variation of the differential effective group index outside the grating region as a function of temperature, a cross-comparison with LPGP2 is made. Since the gratings are weak (∼2 dB), then the fringe spacing can be expressed [10]: S≈
λ2 mout L
(8)
where mout is the differential effective group index outside the grating region, and L is the centre-to-centre separation. The two IFs obtained (IF14 and IF24 ), as shown in Fig. 4, in the fourth
(6)
and clad neff ≡ ncore eff − neff
(7)
By using the spectral data set obtained for the LPGP1 (from room temperature up to 150 ◦ C), the separation between IF4 -af1 and IF4 -af2 can be obtained as a function of the temperature increments. Thus using Eq. (5), the differential effective group refractive index min can be calculated, and from the phase matching condition of the LPG, the differential effective refractive index neff can be obtained. Substituting the values of min and neff in Eq. (6), the differential first-order dispersion of the fibre can be also obtained and plotted against the temperature variation, as shown in Fig. 9. It is shown that the differential effective group index has increased from 7.1 × 10−3 at room temperature to 9.7 × 10−3 at 150 ◦ C, which may explain the reason the IF4 -af1 has shifted at a faster rate than the LPG
Fig. 10. Comparison of the differential effective group index, differential effective refractive index and the differential first-order dispersion of the fibre out side the grating region of LPGP2.
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order SB of LPGP2 have shifted together while the temperature has increased from 25 ◦ C up to 150 ◦ C. Hence, the separation between IF14 and IF24 is constant and measured to be ∼44 nm. Using Eqs. (8) and (6), the values of mout , neff and the differential first-order dispersion of the fibre can be plotted, as shown in Fig. 10. Unlike the case for LPGP1, the differential effective group index in LPGP2 has shown a decrease in value with slope which is similar to that of its differential effective refractive index values, as the temperature has increased. This may imply that there was no major change of the group index of the fibre at the grating region in LPGP2 and as a result, it has changed, having a similar slope to that of the differential effective refractive index as a function of temperature. 6. Conclusion In this work, a novel sensor approach, based on the LPG pair technique is demonstrated with in this preliminary work a sensitivity enhancement of 50% over the use of a single LPG, demonstrated over the dynamic range of 125 ◦ C. This technique is still under investigation to allow for further study of its potential to be applied to any single LPG with strong coupling strength, to observe the IFs which shift at a rate faster than their envelopes. Results obtained have showed the validity of the approach for sensors fabricated without using any coatings or special fibre material or structures. A mathematical method is suggested to separate the IFs in the frequency domain and to allow the monitoring of IFs with non-clear profiles or sharp peaks. Future work will be carried out on investigating in greater depth the effect of the LPG coupling strength on the wavelength shift of the IFs. Acknowledgements The authors are pleased to acknowledge the support of the Engineering & Physical Sciences Research Council through several schemes. References [1] S.W. James, R.P. Tatam, Optical fibre long-period grating sensors: characteristics and application, Meas. Sci. Technol. 14 (2003) R49–R61. [2] A.A. Abramov, A. Hale, R.S. Windeler, T.A. Strasser, Widely tunable longperiod fibre gratings, Electron. Lett. 35 (1999) 81–82. [3] T. Venugopalan, T.L. Yeo, T. Sun, K.T.V. Grattan, High sensitivity longperiod grating-based temperature monitoring using a wide wavelength range to 2.2 m, Opt. Commun. 268 (2007) 42–45. [4] X. Shu, T. Allsop, B. Gwandu, L. Zhang, Bennion, High temperature sensitivity of long-period gratings in B–Ge codoped fibre, IEEE Photon. Technol. Lett. 13 (2001) 818–820. [5] S. Yin, K.-W. Chung, X. Zhu, A highly sensitive long period grating based tunable filter using a unique double-cladding layer structure, Opt. Commun. 188 (2001) 301–305. [6] B.H. Lee, J. Nishii, Bending sensitivity of in-series long-period fibre gratings, Opt. Lett. 23 (1998) 1624–1626. [7] O. Duhem, J.F. Henninot, M. Douay, Study of a fibre Mach-Zehnder interferometer based on two spaced 3 dB long period gratings surrounded by a refractive index higher than that of silica, Opt. Commun. 180 (2000) 255–262.
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[8] Y.G. Han, B.H. Lee, W.T. Han, U.C. Paek, Y. Chung, Fibre optic sensing applications of a pair of long period fibre grating, Meas. Sci. Technol. 12 (2001) 778–781. [9] B.H. Lee, J. Nishii, Self-interference of long period fibre grating and its application as temperature sensor, Electron. Lett. 34 (1998) 2059. [10] B.H. Lee, Y. Chung, W.T. Han, U.C. Paek, Temperature sensor based on self-interference of a single long-period fiber grating, IEICE Trans. Electron. E83-C (2000) 287–292. [11] S.K. Abi Kaed Bey, T. Sun, K.T.V. Grattan, Optimization of a long-period grating-based Mach-Zehnder interferometer for temperature measurement, Opt. Commun. 272 (2007) 15–21. [12] B. Lee, J. Nishii, Dependence of fringe spacing on the grating separation in a long-period fiber grating pair, Appl. Opt. 38 (1999) 3450–3459. [13] M. Takeda, H. Ina, S. Kobayashi, Fourier-transform method of fringepattern analysis for computer-based topography and interferometry, J. Opt. Soc. Am. 72 (1982) 156–159. [14] S.W. Smith, The Scientist and Engineer’s Guide to Digital Signal Processing, 2nd ed., California Technical Publishing, San Diego, 1999, pp. 148–150 (Chapter 8). [15] R.P. Murphy, S.W. James, R.P. Tatam, Multiplexing of fibre optic long period grating based interferometer sensors, in: Proceedings of the Photon06: Optics and Photonics, 2006, p. 28.
Biographies Samer K. Abi Kaed-Bey received his degree of Bachelor of Engineering in Mechanical from the Lebanese American University, Lebanon, in 2001. In the same year he joined the Department of Information Technology at Napier University, Edinburgh, UK, where he received his Master of Science degree in Mechatronics and was awarded the Napier University medal for his outstanding achievements in 2002, where his MSc project is currently on a patent filing process. Previous work experience was obtained from Daewoo Motors and Middle East Airlines, Lebanon. He also worked as an engineer at a recycling company, Envirotech, London, and as a production engineer/CNC operator, setter and programmer at Radial for the designing of customized motorbike products, Kent, UK. He is currently pursuing his studies for the PhD degree at the Measurement and Instrumentation Centre in the School of Engineering and Mathematical Sciences, City University, London. He is a Faraday Associate member in Intersect (Intelligent Sensing Faraday Partnership), and a Member of the Institute of Electrical Engineers. His interests are in Mechatronics and in Opto-Mechatronics. Tong Sun was awarded the degrees of Bachelor of Engineering, Master of Engineering and Doctor of Engineering for work in Mechanical Engineering from the Department of Precision Instrumentation of Harbin Institute of Technology, Harbin, China, in 1990, 1993 and 1996, respectively. She came to City University, London, as an Academic Visitor and latterly a Research Fellow to work in the field of fibre optic temperature measurement using luminescent techniques. She was awarded the degree of Doctor of Philosophy at City University in Applied Physics in 1999. She was an Assistant Professor at Nanyang Technological University in Singapore from 2000 to 2001 and currently a Senior Lecturer at City University, London, since she re-joined in April 2001. Dr. Sun is a Member of the Institute of Physics and the Institution of Electrical Engineers and a Chartered Physicist and a Chartered Engineer in the United Kingdom. Her research interest is in optical fibre sensors, optical communications and laser engineering. She has authored or co-authored some 90 scientific and technical papers in the field. Kenneth T.V. Grattan received his Bachelors degree in Physics (with first class honors) from The Queen’s University, Belfast, in 1974 and completed his PhD studies in 1978, graduating from the same University. In the same year he became a Post-Doctoral Research Assistant at Imperial College, London. His research during that period was on laser systems for photophysical systems investigations, and he and his colleagues constructed some of the first of the then new category of excimer lasers (XeF, KrF) in Europe in 1976. His work in the field continued with research using ultraviolet and vacuum ultraviolet lasers for photolytic laser fusion driver systems and studies on the photophysics of atomic and molecular systems. He joined City University, London in 1983 after 5 years at Imperial
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College, undertaking research in novel optical instrumentation, especially in fibre optic sensor development for physical and chemical sensing. The work has led into several fields including luminescence based thermometry, Bragggrating-based strain sensor systems, white light interferometry, optical system modeling and design and optical sensors for water quality monitoring. The work has been extensively published in the major journals and at international conferences in the field, where regularly he has been an invited speaker, and over
600 papers have been authored to date. He was awarded the degree of Doctor of Science by City University in 1992. Professor Grattan is currently Deputy Dean of Engineering at City University, London, having from 1991 to 2001 been Head of the then Electrical, Electronic and Information Engineering Department. He has been Chairman of the Applied Optics Division of the UK Institute of Physics and was President of the Institute of Measurement and Control in 2000.