Keywords

1 Introduction

An optical sensor has many advantages over electrical sensors such as high sensitivity, small size, no electromagnetic interference, resistance to corrosion, and its light weight [1]. Fibre Bragg Grating (FBG) sensors are mainly used for sensing temperature and strain with the additional advantages of low weight and small size as well as the capability of having multiple sensors in one line. The FBG sensor operates on the principle of Fresnel reflection wherein it uses the reflection and transmission of light when incident on an optical media having different refractive indices [1]. The FBGs or FBG arrays have been widely applied in the measurement of physical, chemical, biomedical, and electrical parameters where the information is usually encoded by the Bragg wavelength shift of FBGs [2,3,4,5].The basic principle of FBG sensors is the measurement of an induced shift in the wavelength of an optical source due to a measurand, such as strain or temperature. A broadband light source i.e. optical spectrum given by a white light source is used to interrogate the grating, from which a narrowband slice is reflected and rest is transmitted [6]. White light source is a broadband light source which contains all the wavelengths [6]. When such a light is given as an input to the FBG sensor, any change in temperature with respect to the reference temperature will lead to reflect back the centre Bragg wavelength and pass the rest of the wavelengths. An FBG sensor thus, is said to work upon wavelength modulation. A basic block diagram of FBG sensor is given in Fig. 1.

Fig. 1.
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Basic block diagram of FBG sensor

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The sensed information is transmitted and subsequently received over an analog communication link which is realized by Optical Fiber Cable (OFC) and Free Space Optic (FSO) communication in this paper. The analog optical link has three sections: transmitter, optical fiber and receiver [7]. The transmitter basically provides electro-optic conversion; here we have used the sensed information of FBG sensor as an input to the OFC channel. The optical fiber takes the light beam to the destination. The receiver carries out the optoelectronic conversion by using photodiodes/photo detectors such as PIN photodiode and Avalanche photodiode [7, 8]. An FSO channel also has been used to transmit the sensed measurand to the receiver. The working of an FSO is similar to OFC with the only difference that the optical signal is sent through free air in the absence of optical cable [9, 10]. Section 2 describes about the FBG sensor system modeling. The results of experimental layouts for proposed FBG based system are shown in Sect. 3, the results and analysis of experimental arrangements are presented in Sect. 4. Section 5 provides a conclusion of the experimental results obtained.

2 System Model

An FBG is created by engraving periodic patterns of refractive index change inside the core of a single mode fiber. This change in refractive index is typically created by showing the fiber core to an intense interference pattern of UV energy [11]. The exposure produces a permanent increase in the refractive index of the fiber’s core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating [12]. A small amount of light is reflected at each grating period. All the reflected light signals combine consistently to one large reflection at a particular wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength [12, 13].Only those wavelength are reflected that satisfy the Bragg condition and this wavelength has maximum efficiency [13].

When light from an optical white light source i.e. an optical spectrum is incident, only a specific wavelength which satisfies Bragg condition will be reflected while the remaining wavelengths are transmitted, the wavelength for which the incident light is reflected with maximum efficiency is called the Bragg wavelength. In optical fiber gratings, the phase matching condition is given by [14, 15].

$$ \beta_{1} - \beta_{2} = \Delta \beta = \frac{2\pi }{\lambda } $$
(1)

Where \(\beta_{1}\) and \(\beta_{2}\) are the propagation constants of the modes being coupled and \(\Lambda\) is the grating period. In the case of FBGs, the forward propagating core mode couples to the reverse propagating core mode, it means the propagation constants remain the same but with a negative sign.

$$ \beta_{2} = - \beta_{1} = \beta $$
(2)

Then the phase matching condition is given by,

$$ \beta - \left( { - \beta } \right) = 2\beta = \frac{2\pi }{\Lambda } $$
(3)

But the propagation constant \(\beta\) is given by,

$$ \beta = \frac{2\pi }{\lambda }n_{eff} $$
(4)

Where \(n_{eff}\) is the effective refractive index of fiber core, so Eq. (3) becomes

$$ 2\left( {\frac{2\pi }{\lambda }n_{eff} } \right) = \frac{2\pi }{\Lambda } $$
(5)

So the Bragg wavelength is given by [16, 17],

$$ \lambda_{B} = 2n_{eff} \Lambda $$
(6)

From Eq. (6) we can see that Bragg wavelength will shift with respect to any change in effective refractive index or grating period. The effective refractive index has a value as average of all the periodic refractive indices in the optical fiber. The Bragg wavelength will shift with any change in the environmental conditions. As the Bragg wavelength depends upon effective refractive index \(n_{eff}\), any change in effective refractive index will shift the Bragg wavelength. Any changes in the temperature of the surrounding from a reference level will lead to change in refractive index in the material; it means the effective refractive index will change. The change in effective refractive index is given by [19, 20],

$$ n_{eff} = \xi n\Delta T $$
(7)

Where \(\xi\) is the thermo-optic coefficient, n is the refractive index of fiber, and \(\Delta T\) is the change in temperature from the reference temperature. Equation (7) gives the change in temperature that will change the \(n_{eff}\) which leads to a shift in the Bragg wavelength.

3 Proposed System Using FBG

The layout of the proposed system to measure temperature using an FBG sensor system is shown in Fig. 2. Here the white light source (WLS) is given as an input to FBG sensor. In increasing or decreasing the temperature surrounding the grating from a reference temperature level, the refractive index of the fiber will experience a change as given in Eq. (7). Those wavelengths that satisfy the Bragg condition are reflected and rest is transmitted. The output spectrum of WLS, transmission and reflection can be seen through WDM FBG Sensor Interrogator as shown in Fig. 3(a), Fig. 3(b), and Fig. 3(c) respectively. If we increase or decrease the temperature the grating center wavelength will shift accordingly.

Fig. 2.
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Proposed system layout using an FBG sensor

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Fig. 3.
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source signal spectrum; (b) Transmission signal spectrum; (c) Reflection signal spectrum

(a) White light

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We need to sense the temperature at the receiver through both the OFC and FSO link. The FBG output is transmitted over an analog link. In order to cover a large distance some suitable analog or digital modulation schemes may be employed. We have used amplitude modulation (AM) in our work here. The layouts of an OFC based link without and with AM are shown in Figs. 4 and 6 respectively. Similar schematic diagrams for an FSO based analog link are illustrated in Figs. 5 and 7 respectively.

Fig. 4.
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Layout of FBG sensor system data sent through an OFC channel

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Fig. 5.
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Layout of FBG sensor system data sent through an FSO channel

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Fig. 6.
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Layout of FBG sensor system data sent via an analog link through a OFC channel

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Fig. 7.
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Layout of FBG sensor system data sent via an analog link through a OFC channel

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4 Results and Analysis

All the experimental block schematics are implemented in OptiSystem 16.0. The change in temperature varies up to a certain range according to the Thermo-optic coefficient shown in Table 1.

Table 1. Range of temperature vs. thermo-optic coefficient
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The Thermo-optic coefficient (\(\xi\)) is kept to 0.0001/°C, reference temperature is 0°C, effective refractive index \(n_{eff}\) is 1.45, and the temperature surrounding the grating has been changed. There will be a shift in center Bragg wavelength (i.e. 1550 nm) with change in temperature in transmission and reflection of FBG sensor is shown in Fig. 8(a) and Fig. 8(b) respectively. Table 2 shows the colour summary of Fig. 8 w.r.t temperature. It can be observed that at the change of temperature in 50 °C with \(\xi\) value 0.0001, the FBG sensor is unable to reflect the center Bragg wavelength. If we increase the thermo-optic coefficient to 0.00001/°C the change of temperature in 50 °C can reflect the center wavelength.

Fig. 8.
figure 8

(a) Transmission signal spectrum shift w.r.t change in temperature; (b) Reflection signal spectrum shift w.r.t change in temperature

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Table 2. Colour summary of Fig. 8 w.r.t temperature
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The Bragg wavelengths will be shifted if we increase or decrease the temperature in the FBG sensor and send through OFC and FSO channel. The shift in Bragg wavelengths sensed by both OFC and FSO channel for layouts as shown in Figs. 4 and 5 for a length of 1 km with channel attenuation of value 0.2 dB/Km is compared and shown in Table 3. Table 4 shows the difference in Bragg wavelength shift with respect to center Bragg wavelength (i.e. 1.54999 µm) for reference temperature 0 °C, keeping \(\xi\) value 0.0001 and the temperature is changed according to the Table 3.

Table 3. Comparative summary of Bragg wavelength sensed through OFC and FSO channel
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Table 4. Comparative summary of shift in Bragg wavelength sensed through OFC and FSO channel with respect to center Bragg wavelength at reference temperature 0 °C
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The change in wavelength w.r.t temperature is given by [21], the sample observation of theoretical and experimental wavelength shift is shown in Table 5.

$$ \left( {\Delta \lambda_{B} } \right)_{temp} = \lambda_{B} \left( {1 + \xi } \right)\Delta T $$
(8)
Table 5. Summary of theoretical and practical wavelength shift w.r.t temperature
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Figure 9 shows the comparative plot for temperature verses Bragg wavelength shift through OFC and FSO channels with respect to reference temperature 0 °C and \(\lambda_{B}\) = 1.54999 µm (i.e. the center Bragg wavelength found experimentally).

Fig. 9.
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Comparative plot of temperature vs. Bragg wavelength shift for OFC and FSO channel

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Figures 10(a) and 10(b) shows the attenuation verses distance to sense the transmitted data through OFC and FSO channel respectively.

Fig. 10.
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(a) Attenuation verses distance plot to sense the transmitted data through OFC channel; (b) Attenuation verses distance plot to sense the transmitted data through FSO channel

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5 Conclusion

The proposed layouts are implemented in OptiSystem 16.0 and the graphs are plotted using MATLAB R2016b. We have obtained the characteristics of FBG sensor by changing the temperature from −5 ºC to 40 ºC. Thermo-optic coefficients for different range of temperatures to sense the Bragg wavelength have been determined experimentally. The sensed data from FBG Sensor has been sent through wired and wireless channel (i.e. through OFC and FSO) to assess their suitability as transmission media for low data rate applications like temperature. It is found that the difference between Bragg wavelength shifts in OFC and FSO is not very significant. This implies it is possible to send the sensed data of a measurand wirelessly in order to increase the data transmission rate and to avoid the presence of fiber cable. Experimentally, it has also been found that the sensed data of an FBG sensor can be retrieved reliably for longer distances up to 220 Km for attenuation of 0.1 dB/Km through an OFC cable whereas the coverage is limited to 6 Km distance only for the same value of attenuation through an FSO channel. The use of multiple FBG sensor based multiplexed system is currently under investigation.