Mechanically induced long-period fiber grating in side-hole single-mode fiber for temperature and refractive sensing

doi:10.1016/j.optcom.2009.11.057

Optics Communications

Volume 283, Issue 7, 1 April 2010, Pages 1303-1306

https://doi.org/10.1016/j.optcom.2009.11.057 Get rights and content

Abstract

The inscription of a long-period grating in a side-hole single-mode fiber for sensing application is demonstrated. The grating is made by pressing a plate with a periodic groove against a short section of length of a side-hole single-mode fiber. Its strength and wavelength of the cladding-mode resonant can be tuned over 15 dB and 6.2 nm, respectively, by varying the ambient temperature of the grating. The refractive index sensitivity and the polarization dependence of the MPLG are also investigated.

Introduction

Long-period grating (LPG), which can couple light power between the fundamental guided mode and a set of forward-propagating cladding modes, used for a wide variety of applications, such as gain equalizers in fiber amplifiers [1], strain, refractive index, bending, and temperature sensors [2], [3]. The characteristics of sensors based on LPG have been widely investigated over the past decade [4], [5], [6], [7]. The LPG has been conventionally fabricated in an optical fiber by inducing periodic modulation on the fiber’s core refractive index with a period of several hundred micrometers. So far, there is a variety of methods to fabricate it by using ultraviolet exposure through an amplitude mask [3], CO₂ laser with a point-by-point technique [4], electrical discharges [8] and mechanical pressure [9], [10], [11], [12]. In particular, the mechanical pressure type long-period grating (MLPG) is more attractive since it can be fabricated in almost any type of fiber, such as any kind of conventional single-mode fiber (SMF) and photonic crystal fiber (PCF) [9], [13], [14]. Moreover, the mode coupling strength and the resonant wavelength of the MLPG can be tuned with high repeatability.

Owing to its simple structure and excellent characteristics, MPLG can be used for many applications, such as dynamic gain-flattening filter in fiber lasers or fiber amplifiers [15], fiber Mach–Zehnder interferometers [16], and temperature sensors [13], [17].

In this paper, a MLPG in a side-hole single-mode fiber for sensing application is first demonstrated. The grating design and the sensing principle are presented in Section 2. The experiment results and discussion are shown in Section 3 and a conclusion is given in Section 4.

Section snippets

Working principle and sensor fabrication

The working principle of a LPG is that it can couple the light power between the fundamental guided mode and forward-propagating cladding modes inside the fiber. As a result, several of resonant modes are appeared in the corresponding transmission spectrum. The resonant wavelength of m-th order mode $λ_{m}$ is defined by the phase-matching condition: $λ_{m} = (n_{core} - n_{clad}^{m}) Λ$ where Λ is the grating period, n_core and $n_{clad}^{m}$ are the effective indices of the fundamental guided mode and the m-th order cladding

Results and discussions

The dips of the resonant wavelength in the transmission spectrum of the MPLG which were induced by the increasing of the applied pressure over the side-hole SMF are shown in Fig. 2. The pressure, applied on the grooved plate, induced a periodical strain on the fiber. From the photoelastic effect on the fiber, the strain induced a periodical index variation on the fiber core, which caused mode coupling between the guided mode and the forward-propagating cladding modes. Higher pressure induced

Conclusion

In this paper, a first mechanically induced LPG in a side-hole singlemode is presented. It is formed by pressing a side-hole SMF which is placed between a periodically grooved plate and a flat plate. The resonant wavelength of 1588.6 nm has a total blue shift of approximately 12.2 nm for refractive index ranging from 1.35 to 1.43, and a red shift of 6.2 nm for the temperature ranging from 20 °C to 100 °C, with a sensitivity of 77.5 pm/°C. The resonant wavelength of the MLPG for the two incident

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SHF-based fiber Bragg gratings have been demonstrated for simultaneous measurement of temperature and either pressure or strain, or torsion [14–16]. SHF-based long period gratings have also been characterized and used for temperature and refractive sensing [17,18]. Recently, optic fiber interferometer based on SHF or PCF filled with metals, liquids or polymer, which their properties is sensitive to temperature, have especially attracted great interest in temperature application [19–29].
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Mechanically induced long-period fiber grating in side-hole single-mode fiber for temperature and refractive sensing

Abstract

Introduction

Section snippets

Working principle and sensor fabrication

Results and discussions

Conclusion

Opt. Commun.

Opt. Lett.

Opt. Commun.

Opt. Commun.

Opt. Lett.

J. Lightwave Technol.

Opt. Lett.

J. Lightwave Technol.

Appl. Opt.

Opt. Lett.

J. Opt. Adv. Mat.