**Optical Fibre Sensors Based on UV Inscribed Excessively Tilted Fibre Grating**

Chengbo Mou, Zhijun Yan, Kaiming Zhou and Lin Zhang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/57146

#### **1. Introduction**

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22 Optical Sensors - New Developments and Practical Applications

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tember 2013) (accessed 4 September 2013).

In-fibre inscription of grating structures in the core of an optical fibre was firstly reported in 1978 by Hill [1]. Being a promising device as narrow band reflector, the fibre Bragg gratings (FBGs) have drawn a lot of attentions in the field of optical communication at that time. However, the functionality of FBGs as sensors has been only recognised after a decade of the invention of the device which can inscribe FBG with resonant wavelength independent of the writing laser wavelength [2]. Since then, the research of FBG based sensors has grown tremendously [3]. The techniques using diffractive optical element to fabricate FBG have put the field into a more commercial way as reproductive of identical FBGs is possible [4]. FBGs are then found a range of applications in sensing field such as strain, temperature, curvature, loading, displacement etc. In 1996, a new type of fibre grating device which is called long period fibre grating (LPG) was demonstrated which has superior temperature sensitivity while possessing refractive index (RI) responsivity [5]. Both FBGs and LPGs have shown significant role in the optical sensing domain. They have been utilised directly or functionalised or integrated with other structures to show functionality in various sensing applications.

Another class of in-fibre gratings is the grating structure with tilted grating planes which called tilted fibre gratings (TFGs). Such a type of gratings is capable of couple the core propagating mode into strong cladding modes. In terms of the tilted angles, such gratings can be divided into three types namely small angle (<45°) TFG, 45° TFG and excessively (>45°) TFG (ETFG). The small angle TFGs were originally used as mode coupler which taps the light out from the fibre core area [6]. Recently, such gratings have shown strong potential in sensing field [7-10]. When incorporated with metal coating, such gratings also exhibit great potential for refractive index sensing based on surface plasmon resonance [11]. 45°-TFG was initially demonstrated

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as a polarisation dependent loss equaliser [12] and later as an in-fibre polariser [13]. The ETFG is a new class of fibre gratings which was first demonstrated in 2006 by Zhou *et al* [14]. Since then, the ETFGs have shown great capability as a novel kind of fibre sensors. This chapter will review the recent development of ETFGs as various optical sensors. The chapter will be organised in three main parts: first part (sections 2) gives a general introduction and funda‐ mental background on fibre gratings with a particular emphasis on the ETFG; second part (section 3) describes the inscription and characterisation of ETFG; third part (sections 4-9) discusses ETFG based sensors and fibre laser sensing systems including strain, twist [15, 16], loading[17], refractive index (RI) and liquid level sensing [18].

#### **2. Background of fibre gratings**

Light coupling in a non-tilted fibre grating can be well illustrated by ray tracing as shown in Figure 1. For an FBG, the mode coupling occurs at resonant wavelength where the forward propagating mode reflects into an identical backward propagating mode (Figure 1a). While for an LPG, the mode coupling occurs close to wavelength at which a forward propagating core mode is strongly coupled into co-propagating cladding modes (Figure 1b). For TFGs, the mechanism of light coupling can also be described by ray tracing method as shown in Figure 2.

which is commonly regarded as the phase matching condition*Κout* = *Kin* + *ΚG*. All *K* described

the grating vector. The phase matching condition of a fibre grating can then be described in a vectorial plane in Figure 3 and Figure 4. For the case of FBG mode coupling, as shown in Figure

a forward propagating core mode into an identical backward propagating core mode. For the case of LPG mode coupling, as shown in Figure 3b, the grating can couple the incident light

For TFGs, as the grating has the ability to couple the forward propagating core mode into

*<sup>λ</sup>* <sup>⋅</sup>*nco*is the wave vector of the incident light and *Κ<sup>G</sup>* <sup>=</sup> <sup>2</sup>⋅*<sup>π</sup>*

Optical Fibre Sensors Based on UV Inscribed Excessively Tilted Fibre Grating

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25

*<sup>λ</sup>* <sup>⋅</sup>*nco* applies as an FBG structure will couple the light from

*<sup>λ</sup>* <sup>⋅</sup>*nclad*which is similar to LPG. With the condition

*<sup>λ</sup>* <sup>⋅</sup>*ncl*

*ΛG* is

indicating the cladding modes.

in this section are vectors. *Κin* <sup>=</sup> <sup>2</sup>*<sup>π</sup>*

3a, the relationship *Kout* <sup>=</sup> *Kin* <sup>=</sup> <sup>2</sup>⋅*<sup>π</sup>*

into forward propagating cladding modes with *Κout* <sup>=</sup> <sup>2</sup>*<sup>π</sup>*

**Figure 2.** Schematic of light coupling of TFGs with tilted angle (a)< 45°; (b)=45°;(c)>45°.

**Figure 3.** Vectorial descriptions of phase matching conditions of (a) FBG and (b) LPG.

radiation mode, it is hence to have *Kout* <sup>=</sup> <sup>2</sup>⋅*<sup>π</sup>*

**Figure 1.** Schematic of light coupling of (a) a FBG showing light coupled from forward propagating core mode to backward propagating cladding mode; (b) a LPG showing light coupled from forward propagating core mode to for‐ ward propagating cladding modes.

As can be seen from Figure 2, when the grating tilted angle is smaller than 45°, the grating is capable of coupling forward propagating core mode into backward propagating cladding modes (Figure 2a). At 45°, as a unique case, the core mode will be coupled into radiation mode normal to the fibre axis (Figure 2b). When the tilted angle is larger than 45°, like LPGs, the ETFGs are capable of coupling the forward propagating core mode into forward propagating cladding modes, but to the high order ones (Figure 2c). The strongest light coupling occurs at the resonant wavelength where the phase matching condition λco-cl=(nc0±ncl,m) Λ/cosθ is satisfied, where *nc0* and *ncl,m* are the effective mode refractive indices of the core mode and the *m*th cladding mode, *Λ* is the grating period and *θ* is the tilted angle of the grating structure. The mode coupling mechanism can be well understood by the phase matching condition. We hereby define the following wave vector relationship for mode coupling in a fibre grating Optical Fibre Sensors Based on UV Inscribed Excessively Tilted Fibre Grating http://dx.doi.org/10.5772/57146 25

**Figure 2.** Schematic of light coupling of TFGs with tilted angle (a)< 45°; (b)=45°;(c)>45°.

as a polarisation dependent loss equaliser [12] and later as an in-fibre polariser [13]. The ETFG is a new class of fibre gratings which was first demonstrated in 2006 by Zhou *et al* [14]. Since then, the ETFGs have shown great capability as a novel kind of fibre sensors. This chapter will review the recent development of ETFGs as various optical sensors. The chapter will be organised in three main parts: first part (sections 2) gives a general introduction and funda‐ mental background on fibre gratings with a particular emphasis on the ETFG; second part (section 3) describes the inscription and characterisation of ETFG; third part (sections 4-9) discusses ETFG based sensors and fibre laser sensing systems including strain, twist [15, 16],

Light coupling in a non-tilted fibre grating can be well illustrated by ray tracing as shown in Figure 1. For an FBG, the mode coupling occurs at resonant wavelength where the forward propagating mode reflects into an identical backward propagating mode (Figure 1a). While for an LPG, the mode coupling occurs close to wavelength at which a forward propagating core mode is strongly coupled into co-propagating cladding modes (Figure 1b). For TFGs, the mechanism of light coupling can also be described by ray tracing method as shown in Figure 2.

**Figure 1.** Schematic of light coupling of (a) a FBG showing light coupled from forward propagating core mode to backward propagating cladding mode; (b) a LPG showing light coupled from forward propagating core mode to for‐

As can be seen from Figure 2, when the grating tilted angle is smaller than 45°, the grating is capable of coupling forward propagating core mode into backward propagating cladding modes (Figure 2a). At 45°, as a unique case, the core mode will be coupled into radiation mode normal to the fibre axis (Figure 2b). When the tilted angle is larger than 45°, like LPGs, the ETFGs are capable of coupling the forward propagating core mode into forward propagating cladding modes, but to the high order ones (Figure 2c). The strongest light coupling occurs at the resonant wavelength where the phase matching condition λco-cl=(nc0±ncl,m) Λ/cosθ is satisfied, where *nc0* and *ncl,m* are the effective mode refractive indices of the core mode and the *m*th cladding mode, *Λ* is the grating period and *θ* is the tilted angle of the grating structure. The mode coupling mechanism can be well understood by the phase matching condition. We hereby define the following wave vector relationship for mode coupling in a fibre grating

loading[17], refractive index (RI) and liquid level sensing [18].

**2. Background of fibre gratings**

24 Optical Sensors - New Developments and Practical Applications

ward propagating cladding modes.

which is commonly regarded as the phase matching condition*Κout* = *Kin* + *ΚG*. All *K* described in this section are vectors. *Κin* <sup>=</sup> <sup>2</sup>*<sup>π</sup> <sup>λ</sup>* <sup>⋅</sup>*nco*is the wave vector of the incident light and *Κ<sup>G</sup>* <sup>=</sup> <sup>2</sup>⋅*<sup>π</sup> ΛG* is the grating vector. The phase matching condition of a fibre grating can then be described in a vectorial plane in Figure 3 and Figure 4. For the case of FBG mode coupling, as shown in Figure 3a, the relationship *Kout* <sup>=</sup> *Kin* <sup>=</sup> <sup>2</sup>⋅*<sup>π</sup> <sup>λ</sup>* <sup>⋅</sup>*nco* applies as an FBG structure will couple the light from a forward propagating core mode into an identical backward propagating core mode. For the case of LPG mode coupling, as shown in Figure 3b, the grating can couple the incident light into forward propagating cladding modes with *Κout* <sup>=</sup> <sup>2</sup>*<sup>π</sup> <sup>λ</sup>* <sup>⋅</sup>*ncl* indicating the cladding modes.

**Figure 3.** Vectorial descriptions of phase matching conditions of (a) FBG and (b) LPG.

For TFGs, as the grating has the ability to couple the forward propagating core mode into radiation mode, it is hence to have *Kout* <sup>=</sup> <sup>2</sup>⋅*<sup>π</sup> <sup>λ</sup>* <sup>⋅</sup>*nclad*which is similar to LPG. With the condition *nco* ≅*nclad* , the following relationship *Kin* ≅ *Kout*therefore applies. Hence, the phase matching condition of TFGs can be depicted in the vector plane which is shown in Figure 4, where *θ* indicates the tilted angle of the grating with respect to the fibre axis. In Figure 4a, we can simply infer that when the tilted angle is minimised to zero, the phase matching illustration evolves into the standard FBG condition from which a forward propagating mode has been coupled into an identical backward propagating mode via Bragg diffraction. Figure 4b shows the special case of 45°-TFG which is capable of coupling out light perpendicular to the fibre axis or incident beam propagation direction. While Figure 4c shows the mechanism of an incident beam couples into a forward propagating mode through an excessively titled grating structure. Although the phase matching condition gives very good approximation for interpretation of mode coupling mechanism inside the TFGs, it does not involve the polarisation effect which is actually one of the key properties of the TFGs.

**Figure 5.** Schematic illustration of an ETFG structure with two assigned orthogonal polarisation axes.

ETFG is shown in Figure 7b demonstrating the slanted grating fringes at ~78°.

As illustrated in Figure 6, a TFG can be inscribed either by tilting the mask with respect to the fibre axis (Figure 6a), or by using a mask with tilted pitches (Figure 6b). As an alternative approach, one can inscribe such gratings by tilting the fibre about its axis orthogonal to the plane defined by the two interfering UV beams in a two-beam holographic fabrication system (Figure 6c). A commercial argon ion UV laser is employed to inscribe ETFG in hydrogenated standard telecom fibre (SMF28). Similar to standard FBG fabrication, we have adopted mask scanning technique for ETFG inscription due to high reproducibility and fine control of the grating devices. A commercial amplitude mask with 6.6 µm period was purchased for ETFG inscription ensuring the spectral response residing within a broad range from 1200 to 1700 nm. The schematic UV inscription setup is shown in Figure 7a. A typical microscopic image of an

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A broadband light source (BBS), a polariser and a polarisation controller (PC) are utilised to examine the spectral properties of ETFG through an optical spectrum analyser (OSA). A typical

Figure 9a shows the optical spectrum of a typical ETFG from 1200 to 1700 nm, exhibiting unique paired loss peaks due to polarisation mode splitting, when probed using unpolarised BBS. When polarised light with proper polarisation state is launched as a probe, only one set of split modes will be excited and the other set disappears. As can be seen from Figure 9b, either the equivalent fast- (blue dash-dotted line) or the slow-axis (red-dashed line) mode can be fully

**3. Fabrication and spectral properties of ETFG**

measurement schematic setup is illustrated in Figure 8.

excited or eliminated with polarised light.

**Figure 4.** Vectorial description of phase matching conditions for TFGs with titled angles at (a) < 45°, (b) = 45°and (c) > 45°.

Due to their large tilted angle induced strong asymmetry to the fibre geometry, ETFGs exhibit polarisation dependent mode splitting which features with pairs of peaks corresponding to two orthogonal polarisation modes. We can therefore identify an equivalent fast-axis and slowaxis similar to the conventional polarisation maintaining (PM) fibre structure as shown in Figure 5. It is this distinctive polarisation mode splitting mechanism makes ETFGs as ideal loading [17] and twisting sensors [16] based on their polarisation property and as refractive index sensors utilising intrinsic sensitivity of the high order modes to surrounding medium.

**Figure 5.** Schematic illustration of an ETFG structure with two assigned orthogonal polarisation axes.
