Ultra Long Period Reversible Fiber Gratings as a Pressure Sensor

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Ultra Long Period Reversible Fiber Gratings as a Pressure Sensor Sunita Pandit Ugale1, Vivekanand Mishra1, and Angela Amphawan2 1

S.V. National Institute of Technology, Surat, Gujarat India [email protected], [email protected] 2 School of Computing, Universiti Utara Malaysia, 06010 Sintok, Malaysia

Abstract. We report here for the first time the fabrication and characterization of mechanically induced ultralong period fiber gratings (MULPFG) with period size up to several millimeters. In these gratings the coupling of the fundamental guided core mode takes place with cladding modes of high diffraction orders. The transmission characteristic of grating with different external applied pressure has been experimentally verified. Index Terms: MULPFG, reversible grating, pressure sensing.



The optical fiber grating is one of the key elements in the established and emerging fields of optical communication systems. Fiber grating is an infiber spectrally selective component and have low losses, high stability, and small size compatible with fiber sizes and low cost. Their applications also spread into the area of optical fiber sensing. Long period fiber grating (LPFG) is the special case of FBG. It was first suggested by Vengsarkar and coworkers in 1996. Their spectral properties i. e. resonance wavelength, bandwidth etc. can be varied in a wide range. All these features make LPFG as an important component in a variety of light wave applications such as band rejection filter[1], wavelength selective attenuator, dispersion compensator, multichannel filters in WDM applications and gain flatteners for Erbium doped fiber amplifiers[2]. These gratings are also very suitable for various sensing applications. Sharp filtering characteristics, ease of fabrication, direct connectivity to fiber, high sensitivity to external parameters, and easiness in adjusting the resonant wavelength by simply adjusting the grating period are the strong points to push towards the detail study of this device. The period of a typical long period fiber grating (LPFG) ranges from 100µm to 1000µm. The LPFGs with periods exceeding one millimeter are called ultralong period fiber gratings (ULPFG). The long period size makes fabrication of ULPFG very easy as well as cheap. ULPFG can be induced optically or mechanically [3] - [5]. Optically induced gratings are permanent, whereas mechanically induced gratings are reversible. S. Unnikrishnan, S. Surve, and D. Bhoir (Eds.): ICAC3 2013, CCIS 361, pp. 439–443, 2013. © Springer-Verlag Berlin Heidelberg 2013


S.P. Ugale, V. Mishra, and A. Amphawan

Xuewen Shu et. al. reported fabrication and characterization of ULPFG for the first time in 2002 by using point-by-point writing technique with 244nm UV beam from a frequency doubled Argon ion laser [3]. An ultralong period fiber grating with periodic groove structure(G-ULPFG) fabricated by using an edge-written method with highfrequency CO2 laser pulses is reported by Tao Zhu and co-workers in 2009[4]. We report here, for the first time to our knowledge, the fabrication and characterization of mechanically induced ULPFGs (MULPFG) with periods up to several millimeters. Mechanically induced long period fiber gratings (MLPFG) and MULPFG induced by pressure need neither a special fiber nor an expensive writing device for fabrication. These gratings also offer advantages of being simple, inexpensive, erasable, and reconfigurable and also gives flexible control of transmission spectrum,



ULPFG is a special case of LPFG. In LPFG the core LP01 mode is coupled with cladding modes having same symmetry, namely LP0m modes [6]. Whereas in ULPFG the coupling of the fundamental guided core mode to the cladding modes of high diffraction orders takes place [7]. The phase matching condition for a high diffraction order grating is given by (1). co cl , m − neff λres = ( neff )



co cl , m and n eff are effective indexes of Where λ res is the resonant wavelength, neff

fundamental core mode and mth cladding mode of Nth diffraction order respectively. Λ is the grating period and N is the diffraction order. N=1 for LPFG. The resonant wavelength with the variation in the effective indexes of the core and cladding ignoring the dispersion effect is given by (2).


' res

= (n

co eff


cl , m eff

d λ res  co cl , m − δ n eff (δ n eff )× Λ  dΛ ) N × 1 + co cl , m 2 ( n eff ) − n eff  

    


Where λ ' res is the resonant wavelength with variation in the effective indexes of core co and cl , m are the effective index changes of the fundamental core and cladding, δ neff δ n eff th mode and m cladding mode of the Nth diffraction order



Reversible MLPFG and MULPFG of different periods ranging from several hundred microns to several millimeters were induced and characterized.

Ultra Long Period Reversible Fiber Gratings as a Pressure Sensor


The gratings were induced in single mode fiber SMF28 with core diameter of 9µm and cladding diameter of 125µm. A special probe with unjacketed fiber of 10cm length at the center and APC connectors at both ends was prepared. The experimental set up as shown in fig. 1 was prepared. The light from broadband superluminacent LED with output power of -8dBm, center wavelength 1530nm and bandwidth of 69nm was passed through the grating under test and the transmitted signal was analyzed with the help of optical spectrum analyzer covering the wavelength range from 1250nm to 1650nm. MLPFG with a period 1800µm was induced and characterize. The response of this grating to external applied pressure was also studied.

Fig. 1. Experimental set up for inducing MULPFG and its characterization



The transmission characteristic for MLPFG with period 1800µm is shown in Fig. 2. and the results are summarized in table I. In MULPFG the transmission loss increases with applied pressure.

Fig. 2. Transmission loss to external applied pressure for grating having period of 1800μm


S.P. Ugale, V. Mishra, and A. Amphawan Table 1. Transmission loss to applied pressure for grating 3 External applied pressure in kg

Transmission loss in dBm for grating with (period Λ=1800μm )

Resonance wavelength in nm

2.0 2.5 3 3.5 4.0

53.8 55.0 56.2 57.2 58.4

1534 1538 1546 1550 1555

Fig. 3. Relative change in resonance wavelength with change in external applied pressure



MULPFG with 1800μm period was induced successfully in SMF28 fiber. we observed significant shift of 20nm in resonance wavelength, thus MULPFG with greater period is more sensitive to external applied pressure. Thus MLPFG can be used as band rejection filter, wavelength selective attenuator, gain flatterers for optical amplifier and MULPFG can be used as a sensor for pressure, temperature or RI.

References [1] Vengsarkar, A.M., Lemaire, P.J., Judkins, J.B., Bhatia, V., Erdogan, T., Sipe, J.E.: Longperiod fiber gratings as band-rejection filters. Journal of Lightwave Technology 14(1), 58–65 [2] Sohn, Song, J.: Gain flattened and improved double pass two stage EDFA using microbending longperiod fiber gratings. Optics Communication 236(1-3), 141–144 (2004)

Ultra Long Period Reversible Fiber Gratings as a Pressure Sensor


[3] Shu, X., Zhang, L., Bennion, I.: Fabrication and characterization of ultralong period fiber gratings. Optics Communication 203, 277–281 (2002) [4] Zhu, T., Song, Y., Rao, Y., Zhu, Y.: Highly sensitive optical refractometer based on edgewritten ultralong period fiber grating formed by periodic grooves. IEEE Sensors Journal 9(6) (2009) [5] Savin, S., Digonnet, M., Kino, G., Shaw, H.: Tunable mechanically induced long-period fiber gratings. Optics Letters 25(10), 710–712 (2000) [6] Erdogan, T.: Fiber grating spectra. Journal of Lightwave Technology 15(8) (1997) [7] Zhu, T., Rao, Y.J., Mo, Q.: Simultaneous measurement of refractive index and temperature using a single ultralong period fiber grating. IEEE Photonics Technology Letters 17(12) (2005)

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