*5.1.2 Tapered SMF sensor*

Tapering the SMF is a critical part in the work. Vytran Glass Processing System workstation (GPX-3000, USA) was used to taper transducing platforms. This

**53**

**Figure 4.**

*Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films*

*5.1.3 Fabrication of etched-tapered SMF transducing platform*

Processing System workstation GPX- 3000 [11].

other modified fibers [11].

*Etched-tapered sensing platform schematic diagram [2, 11].*

system implies a heating element which is a filament of graphite and two movable fiber holders blocks as a part of tapering control process to generate the recorded profile parameters. The tapered SMF was fabricated using heat-pulling technique which suggests that the fiber diameter changes smoothly and is adiabatically slow as a function of fiber length. Before tapering, the plastic jacket and polymer coating of the fiber are removed approximately 8 cm length. The machine was controlled using a proprietary software on a computer, where the desired tapered fiber profile (waist diameter, waist length, down taper length, and up taper length) can be specified. The waist length, up and down transitions setting for tapering SMF platforms are 10, 2 and 2 mm, respectively. The dimensions of the modified SMF transducing platforms are verified using the CCD camera of Vytran workstation [2, 11].

Fabrication of the etched-tapered SMF (ETSMF) sensing platform was done by combining the two processes; etching and tapering. Firstly, the SMF was etched using HF acid in the same manners described in SubSection 5.1.1 Afterward, the etched SMF was tapered using Vytran workstation according to the proposed settings used in Section 5.1.2 The ETSMF platform is shown in **Figure 4**. Tapering the etched SMF is critical and more challenging than tapering the standard fiber. The reduction in the fiber diameter and the weaken fiber structure due to the etching process increases the difficulty of fragility the modified fiber. Thus, customized holder that fixed the etched fiber during the tapering process was used. The diameters of the optical fibers used in the research fabricated by both etching and tapering were confirmed using the Vytran Glass

In order to verify the compatibility of modified SMF platforms for gas sensing, many experiments were carried out using these sensors coated with PANI nanostructures thin films as sensing layer for NH3. **Table 1** summarizes the design parameters for the fabricated sensors used in this PhD project. Sensors S1-S4 are ETSMF sensors while Sensors S5 and S6 are the tapered only and etched only sensors. Referring to **Table 1**, it is observed that the core to cladding ratio is found to be varied according to the different modification techniques. It is noted from **Table 1** that in the etched only sensing platform (S6), the original core diameter is unchanged at 9 μm [2, 11]. During the etching process, the cladding layer was dissolved to produce 6 μm thickness diameter. The core to cladding ratio is 0.6. On the other hand, the core diameter is modified as well as the cladding diameter in the case of tapered only sensing platform (S2). The core diameter after tapering became 1.08 μm to give core-cladding ratio as the lowest (0.07) for the tapered only platform that is the lowest relative to

The ETSMF sensing platforms (S1-S4) possess unique core-cladding ratio as compared to the etched only and tapered only platforms. During the etching process, some of the cladding was removed and hence the core becomes more

*DOI: http://dx.doi.org/10.5772/intechopen.94001*

**Figure 3.** *Setup for etching optical fiber using HF acid [2, 11].*

#### *Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94001*

system implies a heating element which is a filament of graphite and two movable fiber holders blocks as a part of tapering control process to generate the recorded profile parameters. The tapered SMF was fabricated using heat-pulling technique which suggests that the fiber diameter changes smoothly and is adiabatically slow as a function of fiber length. Before tapering, the plastic jacket and polymer coating of the fiber are removed approximately 8 cm length. The machine was controlled using a proprietary software on a computer, where the desired tapered fiber profile (waist diameter, waist length, down taper length, and up taper length) can be specified. The waist length, up and down transitions setting for tapering SMF platforms are 10, 2 and 2 mm, respectively. The dimensions of the modified SMF transducing platforms are verified using the CCD camera of Vytran workstation [2, 11].

#### *5.1.3 Fabrication of etched-tapered SMF transducing platform*

Fabrication of the etched-tapered SMF (ETSMF) sensing platform was done by combining the two processes; etching and tapering. Firstly, the SMF was etched using HF acid in the same manners described in SubSection 5.1.1 Afterward, the etched SMF was tapered using Vytran workstation according to the proposed settings used in Section 5.1.2 The ETSMF platform is shown in **Figure 4**. Tapering the etched SMF is critical and more challenging than tapering the standard fiber. The reduction in the fiber diameter and the weaken fiber structure due to the etching process increases the difficulty of fragility the modified fiber. Thus, customized holder that fixed the etched fiber during the tapering process was used. The diameters of the optical fibers used in the research fabricated by both etching and tapering were confirmed using the Vytran Glass Processing System workstation GPX- 3000 [11].

In order to verify the compatibility of modified SMF platforms for gas sensing, many experiments were carried out using these sensors coated with PANI nanostructures thin films as sensing layer for NH3. **Table 1** summarizes the design parameters for the fabricated sensors used in this PhD project. Sensors S1-S4 are ETSMF sensors while Sensors S5 and S6 are the tapered only and etched only sensors. Referring to **Table 1**, it is observed that the core to cladding ratio is found to be varied according to the different modification techniques. It is noted from **Table 1** that in the etched only sensing platform (S6), the original core diameter is unchanged at 9 μm [2, 11]. During the etching process, the cladding layer was dissolved to produce 6 μm thickness diameter. The core to cladding ratio is 0.6. On the other hand, the core diameter is modified as well as the cladding diameter in the case of tapered only sensing platform (S2). The core diameter after tapering became 1.08 μm to give core-cladding ratio as the lowest (0.07) for the tapered only platform that is the lowest relative to other modified fibers [11].

The ETSMF sensing platforms (S1-S4) possess unique core-cladding ratio as compared to the etched only and tapered only platforms. During the etching process, some of the cladding was removed and hence the core becomes more

**Figure 4.** *Etched-tapered sensing platform schematic diagram [2, 11].*

*Application of Optical Fiber in Engineering*

will be discussed extensively here.

*5.1.1 Etched SMF sensor*

*5.1.2 Tapered SMF sensor*

**5.1 Modified SMF fabrication and characterizations**

prevent further etching due to residual HF [2, 11].

*Setup for etching optical fiber using HF acid [2, 11].*

nanostructured thin film. These parameters affect gas sensing performance which

Three types of modified SMF platforms including etched, tapered and etchedtapered platforms were developed and investigated towards ammonia at room temperature. A standard SMF-28 single mode silica fiber (Lucent All-Wave Fiber, 9/125 μm core/cladding diameter ratio) is modified as the optical transducing platforms for NH3 sensing applications. Each transducing platform was made of 1 m length of SMF. The Tafzel ® polymer jacket enfolding the SMF was removed mechanically over 8 cm by a fiber a stripper. The fabrication processes for the three types of modified SMF platform will be elaborated in the following subsections [2, 11].

A 48% hydrofluoric acid (HF) (Sigma Aldrich) was used as an agent for chemically etching the SMF. For better holding of the optical fiber platform, both ends of the fiber are fixed using a metal racks so the fiber dangle into the vessel containing the acid used for the etching as shown in **Figure 3**. This is fixed inside a fume hood to prevent direct exposure to HF vapor and creation of aerosols. The etching process was started by filling 100 μl of HF acid in a container using a Pasteur pipette. To fabricate the etched optical fiber transducer, the stripped fiber is fixed as depicted in the figure to control its emersion in HF acid. The fiber is etched in two stages process to control its modification. The first stage includes immersing the stripped area in 48% HF acid at a specific time to produce different etched diameters. After that, the fiber was taken out and cleaned with deionized water for 30 minutes to remove the HF acid residual. The etched fiber is left to dry at room temperature. In the second stage, the fiber was immersed in HF with less concentrations of 12% to reduce the etching rate and hence, more control on the etching dimensions. A variety of parameters affect the etching rate of the cladding such as acid concentration, humidity and temperature. The humidity and temperature are fixed at 67% and 25 C, respectively. As the second stage of etching is finished, the fiber is immersed in deionized water for 30 minutes to remove the remaining of the HF acid as well as to

Tapering the SMF is a critical part in the work. Vytran Glass Processing System

workstation (GPX-3000, USA) was used to taper transducing platforms. This

**52**

**Figure 3.**


#### **Table 1.**

*Design parameters of the modified SMF sensors [2, 11].*

sensitive to the surroundings and the ratio of core-cladding is modified. Afterward, the core diameter is reduced when tapered to the specified settings. The tapering process does not change the core-cladding ratio of the sensing platforms which is equal to the previous etched fiber. For example, S1 sensing platform with 30 μm etched diameter has a core diameter of 9 μm. By tapering process, the cladding and core diameters are changed so that the core diameter is about 4.5 μm core diameter. Consequently, combination of etching and tapering processes yields to a reduction in cladding layer thickness surrounding the core allowing the latter to be more sensitive to the variations in environmental parameters. It is found that the core to cladding ratio for sensor S1 is 0.3 which the highest ratio for the ETSMF platforms [2, 11]. The core cladding ratio for the ETSMF sensing platforms S2-S4 are found to be 0.2 (S2), 0.16 (S3) and 0.15 (S4) as listed in **Table 1**. Stronger response is expected from the ETSMF sensing platforms as a consequence of both surface area and evanescent field enhancement via combination of etching-tapering processes. These three different groups of optical fiber platform were characterized in terms of their optical transmission in terms of the output optical power.

### **5.2 Characterization of modified SMF transduction platforms**

The surface investigation of the unprocessed and modified single mode optical fibers is shown in **Figure 5** based on scanning electron microscopy (SEM - Hitachi SU1510, Japan). The unprocessed single mode optical fiber with a diameter of 125 μm is shown in **Figure 5(a)**. **Figure 5b** depicts the etched SMF using hydrofluoric acid with total fiber diameter of 9.7 μm. **Figure 5(c)** and **(d)** shows the etchedtapered with a diameter of 77.7 μm after etching and waist diameter of 15 μm after tapering. The tapered part of the etched-tapered fiber is shown in **Figure 5(c)**. The fiber exhibited a uniform transition with surface roughness. The downward transition of the etched-tapered SMF with 15 μm waist diameter is shown in **Figure 5(d)**. it can be noted that the unprocessed SMF has a smooth surface. On the other hand, the etched fiber exhibited a rough surface due to its processing with hydrofluoric acid. The surface roughness of the modified SMF is superior to enhance the surface area of sensing layer deposited onto it which allows stronger interaction between sensing layer molecules and the gas molecules [2, 11].

## **5.3 Synthesis, deposition and characterization of polyaniline (PANI) nanofiber thin film sensing layer**

**55**

**Figure 5.**

*Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films*

micro-characterization of these nanostructured thin films will be presented. The modified SMF transducers were coated with PANI nanofiber thin films as a sensing layer towards NH3. PANI solution was prepared by dissolving and dispersing a 15 mg PANI powder with 15 mg camphor sulfonic acid (Sigma Aldrich) in 8 ml of chloroform (CHCl3). This resulted in CSA-doped PANI nanofiber of green color solution with concentrations of 3.75 mg/ml [11]. The camphor sulfonic acid was implied to boost the ability of solving PANI in chloroform. The resulted solution

*SEM images of modified SMF platforms, (a) original SMF, (b) the etched SMF, (c) the etched-tapered SMF and (d) the down ward transition for etched-tapered SMF in (c) and microscopic images of modified SMF* 

To generate a homogeneous solution to be used to deposited on the SMF transducers and glass substrates, the solvent was sonicated for 1 hour at room temperature using Hielcher, Ultrasound Technique, UPS2005 ultra sound processor. The fabricated PANI-CSA solution had dark green color. This indicates the doping with CSA was carried out successfully. Glass is chosen as one of the substrates for PANI micro-characterization. The optical fibers and glass substrates were heated up to 50°C for 30 minutes prior to deposition of PANI using hotplate. The heating is important to increase the binding of the nanomaterial and generating uniform films. The prepared samples were left to dry for 1 hour at room temperature. The

The polyaniline coated on the glass substrate was investigated using scanning electron microscopy as described in **Figure 6(a)** to prove its morphology. As can be noted from the figure, PANI deposited on the glass exhibits a random distribution over the substrate surface in cluster forms with different sizes. As can be noted from the **Figure 6(b)**, the non-uniform nanofibers agglomerated to produce the cluster morphology. These PANI nanofibers exhibits a typical length of 2.5–3.5 μm with diameter in the range of 180–200 nm. **Figure 6(c)** presents the image of the PANI

*DOI: http://dx.doi.org/10.5772/intechopen.94001*

was stirred for 1 hour using magnetic bar.

*platforms (e) original SMF and (f) a 77.7* μ*m etched SMF [2, 11].*

coating process was done using a fume hood [2, 11].

PANI nanofiber was deposited on modified SMF transducing platforms as sensing layers towards NH3. In the next subsections, the synthesis and

*Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94001*

#### **Figure 5.**

*Application of Optical Fiber in Engineering*

**diameter (**μ**m)**

*Design parameters of the modified SMF sensors [2, 11].*

**Label Etched fiber** 

**Table 1.**

sensitive to the surroundings and the ratio of core-cladding is modified. Afterward, the core diameter is reduced when tapered to the specified settings. The tapering process does not change the core-cladding ratio of the sensing platforms which is equal to the previous etched fiber. For example, S1 sensing platform with 30 μm etched diameter has a core diameter of 9 μm. By tapering process, the cladding and core diameters are changed so that the core diameter is about 4.5 μm core diameter. Consequently, combination of etching and tapering processes yields to a reduction in cladding layer thickness surrounding the core allowing the latter to be more sensitive to the variations in environmental parameters. It is found that the core to cladding ratio for sensor S1 is 0.3 which the highest ratio for the ETSMF platforms [2, 11]. The core cladding ratio for the ETSMF sensing platforms S2-S4 are found to be 0.2 (S2), 0.16 (S3) and 0.15 (S4) as listed in **Table 1**. Stronger response is expected from the ETSMF sensing platforms as a consequence of both surface area and evanescent field enhancement via combination of etching-tapering processes. These three different groups of optical fiber platform were characterized in terms of

**Tapered fiber diameter (**μ**m)**

S1 30 15 4.5 0.3 S2 40 15 2.95 0.2 S3 50 15 2.44 0.16 S4 60 15 2.01 0.15 S5 No 15 1.08 0.07 S6 15 No 9 0.6

**Fiber core diameter after tapering (**μ**m)**

**Core to cladding ratio**

The surface investigation of the unprocessed and modified single mode optical fibers is shown in **Figure 5** based on scanning electron microscopy (SEM - Hitachi SU1510, Japan). The unprocessed single mode optical fiber with a diameter of 125 μm is shown in **Figure 5(a)**. **Figure 5b** depicts the etched SMF using hydrofluoric acid with total fiber diameter of 9.7 μm. **Figure 5(c)** and **(d)** shows the etchedtapered with a diameter of 77.7 μm after etching and waist diameter of 15 μm after tapering. The tapered part of the etched-tapered fiber is shown in **Figure 5(c)**. The fiber exhibited a uniform transition with surface roughness. The downward transition of the etched-tapered SMF with 15 μm waist diameter is shown in **Figure 5(d)**. it can be noted that the unprocessed SMF has a smooth surface. On the other hand, the etched fiber exhibited a rough surface due to its processing with hydrofluoric acid. The surface roughness of the modified SMF is superior to enhance the surface area of sensing layer deposited onto it which allows stronger interaction between

**5.3 Synthesis, deposition and characterization of polyaniline (PANI) nanofiber** 

PANI nanofiber was deposited on modified SMF transducing platforms as sensing layers towards NH3. In the next subsections, the synthesis and

their optical transmission in terms of the output optical power.

**5.2 Characterization of modified SMF transduction platforms**

sensing layer molecules and the gas molecules [2, 11].

**thin film sensing layer**

**54**

*SEM images of modified SMF platforms, (a) original SMF, (b) the etched SMF, (c) the etched-tapered SMF and (d) the down ward transition for etched-tapered SMF in (c) and microscopic images of modified SMF platforms (e) original SMF and (f) a 77.7* μ*m etched SMF [2, 11].*

micro-characterization of these nanostructured thin films will be presented. The modified SMF transducers were coated with PANI nanofiber thin films as a sensing layer towards NH3. PANI solution was prepared by dissolving and dispersing a 15 mg PANI powder with 15 mg camphor sulfonic acid (Sigma Aldrich) in 8 ml of chloroform (CHCl3). This resulted in CSA-doped PANI nanofiber of green color solution with concentrations of 3.75 mg/ml [11]. The camphor sulfonic acid was implied to boost the ability of solving PANI in chloroform. The resulted solution was stirred for 1 hour using magnetic bar.

To generate a homogeneous solution to be used to deposited on the SMF transducers and glass substrates, the solvent was sonicated for 1 hour at room temperature using Hielcher, Ultrasound Technique, UPS2005 ultra sound processor. The fabricated PANI-CSA solution had dark green color. This indicates the doping with CSA was carried out successfully. Glass is chosen as one of the substrates for PANI micro-characterization. The optical fibers and glass substrates were heated up to 50°C for 30 minutes prior to deposition of PANI using hotplate. The heating is important to increase the binding of the nanomaterial and generating uniform films. The prepared samples were left to dry for 1 hour at room temperature. The coating process was done using a fume hood [2, 11].

The polyaniline coated on the glass substrate was investigated using scanning electron microscopy as described in **Figure 6(a)** to prove its morphology. As can be noted from the figure, PANI deposited on the glass exhibits a random distribution over the substrate surface in cluster forms with different sizes. As can be noted from the **Figure 6(b)**, the non-uniform nanofibers agglomerated to produce the cluster morphology. These PANI nanofibers exhibits a typical length of 2.5–3.5 μm with diameter in the range of 180–200 nm. **Figure 6(c)** presents the image of the PANI

thin film taken with the aid of an atomic force microscope (NT-MDT Solver NEXT AFM). The average thickness of PANI thin film was found to be 400 nm while the its surface roughness was approximately 228.2 nm [11]. Surface roughness is significant in the applications of gas sensing as it enhances the surface area which rises the active interaction sites between the gas molecules and the sensing layer. Consequently, increases the sensor sensitivity [11].
