Comparison of single and double cladding long period fiber grating sensor using an intensity modulation interrogation system

(1)

Comparison of single and double cladding long period

ﬁber grating sensor using an intensity

modulation interrogation system

Chow-Shing Shin

a,*

, Chia-Chin Chiang

a

, Shien-Kuei Liaw

b

a_{Department of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan} b_{Department of Electron Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4,}

Keelung Road, Taipei 10617, Taiwan

Received 26 April 2005; received in revised form 23 June 2005; accepted 20 July 2005

Abstract

The performance of long period ﬁber grating (LPFG) sensors written in single cladding and double cladding ﬁbers have been compared by using a fast responding interrogation system based on intensity modulation. Temperature and dynamic strain monitoring using this system have been demonstrated. This system is capable of resolving strain to 0.2 and 0.4 le at a loading frequency of 20 Hz, and temperature resolution to 0.02 and 0.19°C by using LPFG in the single cladding (SC-LPFG) and double cladding (DC-LPFG), respectively.

Keywords: Fiber Bragg gratings; Long period ﬁber grating; Optical ﬁber sensor; Intensity modulation

1. Introduction

Long period ﬁber grating (LPFG) has found increasing applications in optical communication and sensing systems. LPFG promotes the coupling between the propagating core modes and co-propagating cladding modes. In optical communi-cations, LPFG devices have been demonstrated for applications such as in band-rejection ﬁlters

[1], gain-flattening filters [2]. In the sensing appli-cations, LPFG plays an important role as in sens-ing for temperature [3–6], strain [4,5], and refractive index [7,8]. These sensors possess a number of advantages over conventional sensors. For example, they are light and small in diameter yet they have good sensitivity and good long-term stability. They are also relatively free from corro-sion attack [9], and electromagnetic interferences [4,5] that seriously affect many conventional sen-sors. Physical quantity changes are reflected as wavelength shift in the characteristic resonant

*

Corresponding author. Tel./fax: +886 2 2362 2160. E-mail address:[email protected](C.-S. Shin).

Optics Communications 258 (2006) 23–29

(2)

the SC-LPFG and DC-LPFG. For the SC-LPFG shows in Fig. 5(a), the signal exhibited occa-sional spikes and ﬂuctuation in the peaks and valleys. However, the output from the strain gauge remained fairly constant, indicating that the applied strain was quite steady and free from spikes. The discrepancy may be due to the high temperature-sensitivity of the SC-LPFG. On the other hand, Fig. 5(b) shows that the measured results for the DC-LPFG is virtually the same as that given by the strain gauge.

3.4. Comparison of SC-LPFG and DC-LPFG sensors

Table 1 shows the strain and temperature coefficients of SC-LPFG and the DC-LPFG. The temperature coefficient of the SC-LPFG is negative and is about an order of magnitude lar-ger than that of the DC-LPFG. Its strain coeffi-cient is double that of the DC-LPFG. Such large temperature coefficient is advantages in provid-ing a good temperature resolution but on the down side, it limits the applicable temperature measurement range. The DC-LPFG has a good balance between the temperature sensitivity and measurement range. Though the DC-LPFG has only half the strain sensitivity than that of the SC-LPFG, it gives more stable performance in the monitoring of dynamic strain variations.

4. Conclusions

A hybrid FBG-LPFG arrangement to achieve intensity modulation has been developed for inter-rogating wavelength shift in LPFG housed in sin-gle and double cladding ﬁbers. It is capable of resolving strain to 0.2 and 0.4 le at a loading fre-quency of 20 Hz, and temperature resolution to

0.02 and 0.19°C, by using the SC-LPFG and DC-LPFG, respectively. The SC-LPFG has a large temperature coefficient (284 pm/°C) and is better fitted for monitoring a small temperature variation over a narrow range. The DC-LPFG has a good balance between temperature sensitiv-ity (29.7 pm/°C) and the measurement range. Though the DC-LPFG has only half of the strain sensitivity (0.8 pm/le) than that of the SC-LPFG did, it gives a more stable performance in monitor-ing the dynamic strain variations. Moreover, the SC-LPFG will lose its characteristic resonant dip spectrum when it is embedded in a carbon fiber epoxy composite while the dip spectrum of DC-LPFG is not affected by the embedment. Thus, the DC-LPFG is better suited for applications such as structural integrity monitoring that involves embedment of fiber in the structure and dynamic strain variations.

Acknowledgments

The work is supported by projects NSC 90-2212-E-002-175 and NSC 91-2212-E-002-056, Tai-wan. We thank the optoelectronic laboratory of NTUST for supporting the KrF Excimer laser.

References

[1] A.M. Vengsarkar, P.J. Lemaire, J.B. Judkins, V. Bhatia, T. Erdogan, J.E. Sipe, Journal of Lightwave Technology 14 (1996) 58.

[2] A.M. Vengsarkar, P.J. Lemaire, G. Jacobovitz-Veselka, V. Bhatia, J.B. Judkins, in: Proceedings of IOOCÕ95, PD1–2, Hong Kong, June, 1995.

[3] B.H. Lee, J. Nishii, Proceedings of SPIE – The International Society for Optical Engineering 3746 (1999) 418.

[4] J.A. Besley, T. Wang, L. Reekie, Journal of Lightwave Technology 21 (3) (2003) 848.

[5] S.W. James, R.P. Tatam, Measurement Science and Technology 14 (5) (2003) 49.

[6] K.J. Han, Y.W. Lee, J. Kwon, S. Roh, J. Jung, B. Lee, IEEE Photonics Technology Letters 16 (9) (2004) 2114.

[7] S. Khaliq, S. James, W. Stephen, R.P. Tatam, Optics Letters 26 (16) (2001) 1224.

[8] M.N. Ng, Z. Chen, K.S. Chiang, IEEE Photonics Tech-nology Letters 14 (3) (2002) 361.

Table 1

Temperature and strain coeﬃcients of diﬀerent LPFG Fiber type housing

the LPFG Temperature coeﬃcient (pm/°C) Strain coeﬃcient (pm/le) Single cladding 284 1.59 Double cladding 29.7 0.8

(3)

[9] K.R. Cooper, J. Elster, M. Jones, R.G. Kelly, in: IEEE Systems Readiness Technology Conference, 2001, p. 847.

[10] H.J. Patrick, A.D. Kersey, F. Bucholtz, Journal of Lightwave Technology 16 (1998) 1606.

[11] B.A.L. Gwandu, X. Shu, T.D.P. Allsop, W. Zhang, L. Zhang, I. Bennion, Electronics Letters 38 (2002) 695. [12] T. Allsop, D.J. Webb, I. Bennion, Journal of Lightwave

Technology 21 (2003) 264.

[13] T. Allsop, D.J. Webb, I. Bennion, Electronics Letters 39 (2003) 508.

[14] T. Allsop, D.J. Webb, I. Bennion, Optical Fibre Technique 9 (2003) 210.

[15] D3039-76, ASTM Standards and Literature References for Composite Materials, American Society for Testing and Materials, Philadelphia, PA, 1990.

[16] R.W. Fallon, L. Zhang, L.A. Everall, I. Bennion, Mea-surement Science & Technology 9 (1998) 1969.

[17] C.S. Shin, C.C. Chiang, Journal of the Chinese Institute of Engineers (in press).

[18] A.D. Kersey, W.W. Morey, Electronics Letters 29 (1993) 112.