**5.4 Ammonia sensing performance of modified SMF coated with PANI nanofiber**

The setup used to investigate the optical response of the modified SMF sensors towards NH3 is outlined in **Figure 7**. This setup is used to prove the behavior of the sensors in the visible wavelength range (600–750 nm) and C-band wavelength range (1535–1565 nm). Based on the setup, the modified SMF sensor is placed inside a gas chamber which contains a gas inlet and outlet as well as FC/PC connecting adapters to fit the sensors. The sensor is connected to a light source (tungstenhalogen lamp (Ocean Optics HL2000) for visible and C-band (Ammonics) for C-band) and the other end is connected to a detection system spectrophotometer (Ocean Optics USB4000) for visible and OSA for C-band). A proprietary software is used to record and measure the responses from the detection system. A gas calibration system (AALBORG) is deployed to vary the gas concentrations and purging time, automatically. NH3 of 1% concentrations in 99% synthetic air is purged into the chamber via the MFCs. Another pure synthetic air is used as the reference gas. The gas flow is fixed at a rate of 200 sccm. This is completely

**57**

length range [11].

**Figure 7.**

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

computerized using Labview control program. Certified NH3 and synthetic air gas cylinders (Linde, Malaysia-Singapore Sdn Bhd) were used in the mixing and purging of the gases into the chamber. The mixing was done for the purpose of changing the concentration of ammonia in the range of 0.125–1%. The dynamic response of the SMF sensors was investigated upon exposure to NH3 with different concentrations at room temperature. This was carried out through measuring cumulative absorbance of the sensor as it exposed to a NH3 at abovementioned concentrations. Each gas concentration cycle was persisted for 8 minutes while the

*Experimental setup for modified SMF ammonia sensing coated with PANI thin film [2, 11].*

In order to verify the compatibility of modified SMF sensors for gas sensing, many

**Figure 8** demonstrates the dynamic responses of the etched-tapered SMF (S1-S4) while **Figure 9** represents that of tapered and etched SMF (S5 and S6) sensors against different concentrations of ammonia, respectively. SMF sensors with different modifications proved proportional increase in the output optical power against NH3 concentrations. The etched-tapered sensors exhibited superior response magnitude over that of the sensors with other modifications. The etched-tapered sensors (S1-S4) shows response of 1.6, 1.5, 1.39, and 1.29 dBm, respectively, when exposed to 1% of ammonia while the tapered and etched sensors response is 0.84 and 0.68 dBm, respectively. Lower increases are noted at lower NH3 concentrations. As exposed to 0.125% NH3 concentration, the modified sensors (S1-S6) exhibited a change

experiments were carried out using these sensors towards NH3. The sensors were coated with sprayed PANI nanostructure thin films as a sensing layer. The sensing performances were investigated and analyzed in both visible and C-band wavelength ranges. The details of morphology and thickness the of the PANI sensing layer was introduced in Section 5.3. Different modified SMF, namely etched, tapered and ETSMFs were investigated towards NH3 gas with different concentrations. The design parameters for the fabricated sensors used are summarized in **Table 1**. These platforms' dynamic response was investigated towards NH3 in the C band wave-

sensor air regeneration lasted for 15 minutes [2, 11].

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

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

**Figure 7.** *Experimental setup for modified SMF ammonia sensing coated with PANI thin film [2, 11].*

computerized using Labview control program. Certified NH3 and synthetic air gas cylinders (Linde, Malaysia-Singapore Sdn Bhd) were used in the mixing and purging of the gases into the chamber. The mixing was done for the purpose of changing the concentration of ammonia in the range of 0.125–1%. The dynamic response of the SMF sensors was investigated upon exposure to NH3 with different concentrations at room temperature. This was carried out through measuring cumulative absorbance of the sensor as it exposed to a NH3 at abovementioned concentrations. Each gas concentration cycle was persisted for 8 minutes while the sensor air regeneration lasted for 15 minutes [2, 11].

In order to verify the compatibility of modified SMF sensors for gas sensing, many experiments were carried out using these sensors towards NH3. The sensors were coated with sprayed PANI nanostructure thin films as a sensing layer. The sensing performances were investigated and analyzed in both visible and C-band wavelength ranges. The details of morphology and thickness the of the PANI sensing layer was introduced in Section 5.3. Different modified SMF, namely etched, tapered and ETSMFs were investigated towards NH3 gas with different concentrations. The design parameters for the fabricated sensors used are summarized in **Table 1**. These platforms' dynamic response was investigated towards NH3 in the C band wavelength range [11].

**Figure 8** demonstrates the dynamic responses of the etched-tapered SMF (S1-S4) while **Figure 9** represents that of tapered and etched SMF (S5 and S6) sensors against different concentrations of ammonia, respectively. SMF sensors with different modifications proved proportional increase in the output optical power against NH3 concentrations. The etched-tapered sensors exhibited superior response magnitude over that of the sensors with other modifications. The etched-tapered sensors (S1-S4) shows response of 1.6, 1.5, 1.39, and 1.29 dBm, respectively, when exposed to 1% of ammonia while the tapered and etched sensors response is 0.84 and 0.68 dBm, respectively. Lower increases are noted at lower NH3 concentrations. As exposed to 0.125% NH3 concentration, the modified sensors (S1-S6) exhibited a change

*Application of Optical Fiber in Engineering*

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.

*SEM images of (a) PANI nanofibers on glass, (b) etched-tapered SMF transducer coated with PANI nanofibers and (c) 3D AFM image of uncoated and PANI coated areas on a glass substrate [2, 11].*

The setup used to investigate the optical response of the modified SMF sensors towards NH3 is outlined in **Figure 7**. This setup is used to prove the behavior of the sensors in the visible wavelength range (600–750 nm) and C-band wavelength range (1535–1565 nm). Based on the setup, the modified SMF sensor is placed inside a gas chamber which contains a gas inlet and outlet as well as FC/PC connecting adapters to fit the sensors. The sensor is connected to a light source (tungstenhalogen lamp (Ocean Optics HL2000) for visible and C-band (Ammonics) for C-band) and the other end is connected to a detection system spectrophotometer (Ocean Optics USB4000) for visible and OSA for C-band). A proprietary software is used to record and measure the responses from the detection system. A gas calibration system (AALBORG) is deployed to vary the gas concentrations and purging time, automatically. NH3 of 1% concentrations in 99% synthetic air is purged into the chamber via the MFCs. Another pure synthetic air is used as the reference gas. The gas flow is fixed at a rate of 200 sccm. This is completely

**5.4 Ammonia sensing performance of modified SMF coated with PANI** 

Consequently, increases the sensor sensitivity [11].

**56**

**nanofiber**

**Figure 6.**

**Figure 8.**

*Dynamic response of the etched-tapered SMF sensors (S1-S4) exposed to different NH3 concentrations in the C-band wavelength range [2].*

#### **Figure 9.**

*Dynamic responses for tapered and etched SMF sensors (S5 and S6) when exposed to NH3 with different concentrations [2].*

of 0.96, 0.86, 0.68, 0.55, 0.36, and 0.29 dBm, respectively [2, 11]. Generally, SMF sensors with different modifications proved different responses and recovery times are. Moreover, the response time is inversely proportional to the ammonia concentrations while the recovery time is linearly proportional to it for all SMF sensors with different modifications investigated in the C-band range. The core-to-cladding ratio due to etching and tapering processes of the modified SMF sensors affects the response and recovery times as shown in **Figures 8** and **9** [2, 11].

The average response time for the modified sensors (S1-S6) against ammonia are 58–71 s, 79 s, and 92 s, respectively. The average recovery time designates the opposite performance for the sensors with different modification techniques. The values for modified sensors (S1-S6) against ammonia are 466–453 s, 380 s, and 360 s, respectively. Different sensing performances for different types of modified SMF sensors is attributed to different rates of ammonia molecules adsorption/

**59**

**Figure 10.**

*wavelengths range [2, 11].*

OSHA [11].

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

response and recovery times of 180 s and 480 s, respectively [2, 11].

The normalized cumulative Δ optical power of the modified SMF sensors coated with PANI thin film against ammonia is depicted in **Figure 10**. The etched-tapered SMF sensors (S1–S4) proved significant enhancement in response compared with other modifications. On the contrary, the etched-only sensor exhibited the lowest response. The etched-tapered SMF response as exposed to ammonia is improved when the SMF cladding thickness is reduced due to etching process. For example, sensor (S1) with smallest etching diameter shows the strongest response among the etched-tapered sensors. This result principally is a consequence of the strong evanescent field energy propagating out of the core physical dimensions for the modified optical fiber sensors into the sensing layer of PANI. Additionally, the modified SMF sensors integrated with PANI show a variation in response that is proportional to the ammonia concentrations. The normalized cumulative optical powers for the sensors (S1–S6) are 17.7%, 14.6%, 10.8%, 9.5%, 6.3%, and 1.9% at the limit of detection of the sensors [2, 11]. The practical of the gas system used in the work, the limit of detection for the SMF sensors is found to be 0.04% or 400 ppm. Based on established technique introduced by Mola et al. [11, 56], the limit of detection for the etched-tapered SMF Sensor S1 is 0.0025%, which is equal to 25 ppm. Accordingly, the established PANI coated SMF sensor can detect ammonia gas concentration below the ammonia lowest tolerable exposure limit reported by

The sensitivities for the SMF sensors (S1-S6) are found to be 231.5 (S1), 209.7 (S2), 172.1 (S3), 146.6 (S4), 100.4 (S5) and 81.2 (S6). The ETSMF sensors (S1-S4) proved considerably higher sensitivity towards NH3 compared to the tapered only (S5) and etched only sensors (S6). The sensitivity of the ETSMF sensor (S1-S4) are 2.8, 2.3,1.7 and 1.5 times the sensitivity of the tapered only SMF sensor (S5)

*Normalized* Δ *power versus NH3 concentrations for the modified SMF sensors (S1-S6) in the C-band* 

desorption on each kind of the sensors surfaces. For example, fastest response but slowest recovery is observed for the etched-tapered sensors. Contrary, the etched-only sensor shows the reverse time responses. The response of the modified SMF sensors presented here and investigated in the C-band wavelength range were significantly improved compared with the tapered MMF sensors described in a previous study investigated in the visible wavelength range [54]. The response time of etched-tapered SMF sensors (62 s) was more than twice that of the tapered MMF sensors (2.27 minutes or 136.2 s) [11, 54]. The recovery time of the SMF and the MMF sensors were 453 and 583.8 s, respectively [54]. The sensor introduced by Airoudj et al. [55] investigated in the visible to near-infrared wavelengths (632.8–980 nm) based on the single mode planar polymer waveguide coated with PANI exhibited

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

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

desorption on each kind of the sensors surfaces. For example, fastest response but slowest recovery is observed for the etched-tapered sensors. Contrary, the etched-only sensor shows the reverse time responses. The response of the modified SMF sensors presented here and investigated in the C-band wavelength range were significantly improved compared with the tapered MMF sensors described in a previous study investigated in the visible wavelength range [54]. The response time of etched-tapered SMF sensors (62 s) was more than twice that of the tapered MMF sensors (2.27 minutes or 136.2 s) [11, 54]. The recovery time of the SMF and the MMF sensors were 453 and 583.8 s, respectively [54]. The sensor introduced by Airoudj et al. [55] investigated in the visible to near-infrared wavelengths (632.8–980 nm) based on the single mode planar polymer waveguide coated with PANI exhibited response and recovery times of 180 s and 480 s, respectively [2, 11].

The normalized cumulative Δ optical power of the modified SMF sensors coated with PANI thin film against ammonia is depicted in **Figure 10**. The etched-tapered SMF sensors (S1–S4) proved significant enhancement in response compared with other modifications. On the contrary, the etched-only sensor exhibited the lowest response. The etched-tapered SMF response as exposed to ammonia is improved when the SMF cladding thickness is reduced due to etching process. For example, sensor (S1) with smallest etching diameter shows the strongest response among the etched-tapered sensors. This result principally is a consequence of the strong evanescent field energy propagating out of the core physical dimensions for the modified optical fiber sensors into the sensing layer of PANI. Additionally, the modified SMF sensors integrated with PANI show a variation in response that is proportional to the ammonia concentrations. The normalized cumulative optical powers for the sensors (S1–S6) are 17.7%, 14.6%, 10.8%, 9.5%, 6.3%, and 1.9% at the limit of detection of the sensors [2, 11]. The practical of the gas system used in the work, the limit of detection for the SMF sensors is found to be 0.04% or 400 ppm. Based on established technique introduced by Mola et al. [11, 56], the limit of detection for the etched-tapered SMF Sensor S1 is 0.0025%, which is equal to 25 ppm. Accordingly, the established PANI coated SMF sensor can detect ammonia gas concentration below the ammonia lowest tolerable exposure limit reported by OSHA [11].

The sensitivities for the SMF sensors (S1-S6) are found to be 231.5 (S1), 209.7 (S2), 172.1 (S3), 146.6 (S4), 100.4 (S5) and 81.2 (S6). The ETSMF sensors (S1-S4) proved considerably higher sensitivity towards NH3 compared to the tapered only (S5) and etched only sensors (S6). The sensitivity of the ETSMF sensor (S1-S4) are 2.8, 2.3,1.7 and 1.5 times the sensitivity of the tapered only SMF sensor (S5)

#### **Figure 10.**

*Normalized* Δ *power versus NH3 concentrations for the modified SMF sensors (S1-S6) in the C-band wavelengths range [2, 11].*

*Application of Optical Fiber in Engineering*

of 0.96, 0.86, 0.68, 0.55, 0.36, and 0.29 dBm, respectively [2, 11]. Generally, SMF sensors with different modifications proved different responses and recovery times are. Moreover, the response time is inversely proportional to the ammonia concentrations while the recovery time is linearly proportional to it for all SMF sensors with different modifications investigated in the C-band range. The core-to-cladding ratio due to etching and tapering processes of the modified SMF sensors affects the

*Dynamic responses for tapered and etched SMF sensors (S5 and S6) when exposed to NH3 with different* 

*Dynamic response of the etched-tapered SMF sensors (S1-S4) exposed to different NH3 concentrations in the* 

The average response time for the modified sensors (S1-S6) against ammonia are 58–71 s, 79 s, and 92 s, respectively. The average recovery time designates the opposite performance for the sensors with different modification techniques. The values for modified sensors (S1-S6) against ammonia are 466–453 s, 380 s, and 360 s, respectively. Different sensing performances for different types of modified SMF sensors is attributed to different rates of ammonia molecules adsorption/

response and recovery times as shown in **Figures 8** and **9** [2, 11].

**58**

**Figure 8.**

**Figure 9.**

*concentrations [2].*

*C-band wavelength range [2].*

**Figure 11.**

*Repeatability for the ETSMF sensor S1 coated with PANI nanostructured thin film as exposed to 1% NH3 for three cycles [2, 11].*

**Figure 12.** *Optical response for ETSMF sensor S1 coated with PANI nanostructured thin film towards CH4, H2 and NH3 [2].*

and 9.3, 7.7, 5.7 and 5 times that of the etched only SMF sensor (S6) [2, 11]. Based on **Figure 10**, the sensitivities for the SMF sensors (S1–S6) are found to be 231.5, 209.7, 172.1, 146.6, 100.4, and 81.2, respectively [2, 11]. As compared with the tapered and etched sensors, the etched-tapered SMF sensors have higher sensitivity towards NH3 [2].

**Figure 11** shows The repeatability and reversibility of etched-tapered Sensor S1 against 1% ammonia concentration for three cycles each lasts for 8 minutes of ammonia, followed by purging purified air for 15 minutes. These three cycles in the figure exhibits slight difference when exposed towards 1% ammonia. Furthermore, the base time experienced slight shift as a result of incomplete elimination of ammonia from the PANI sensing layer when the air was purged for 15 min. The etched-tapered SMF sensor shows good repeatability and reversibility as demonstrated in the figure.

The etched-tapered Sensor S1 was examined towards methane (CH4), hydrogen (H2), and ammonia (NH3) to prove its selectivity towards ammonia. The range of concentrations of these gases utilized in this test was from 0.125–1% at room temperature. The sensor exhibits superior response towards ammonia compared to that due to methane and hydrogen as depicted in **Figure 12**. Accordingly, the sensor can be described to be highly selective towards NH3.

### **6. Conclusion**

Simple and low-cost modified SMF platforms were successfully designed, developed and investigated for optical sensing towards NH3 with low concentrations.

**61**

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

The modification processes performed on the SMFs were etching, tapering and combination of etching-tapering. The sensing performance of the modified SMF sensors coated with PANI nanofibers thin films was investigated towards NH3 at room temperature for C-band wavelengths range (1535–1565 nm). The investigation showed that the principle of the gas sensors used is the change in the light characteristics because of the interaction between the NH3 molecules and PANI sensing layer. The interaction between the NH3 molecules and the PANI sensing layer reduces the absorbance in the C-band range and inversely proportional to the NH3 concentrations. Consequently, the transmitted optical power increased. These PANI nanofiber thin films show high sensitivity towards NH3 with low concentration as well as good repeatability indication. The performance of the modified SMF sensors was found to be dependent on the modification technique used in the fabrication of the SMF platform as well as the thickness of the cladding layer after modification (core/cladding ratio). The investigation on the NH3 optical sensing performance using absorbance measurement proved that the ETSMF showed superior response than the other modified fibers and thus, highly potential for novel optical transducer. The ability of the ETSMF coated with PANI thin films operates at room temperature indicates its promising candidate for NH3 sensing applications such as chemical plant leakage remote sensor. Particularly, its response in the C-band wavelength range allows easy integration with the existing all optical fiber communication networks infrastructures. The modification technique used in the fabrication of the SMF platform namely etching, tapering or combination of both strongly affects the performance of the modified SMF sensors. Furthermore, the performance of the modified SMF sensors is also dependent on the thickness of the cladding layer after modification and core/cladding ratio. The investigation on the ammonia optical sensing performance demonstrated that the response of etched-tapered SMF is the stronger among that of other modified sensors. Thus, the developed etched-tapered SMF sensors show high potential to be a novel optical transducer. The ability of the etched-tapered SMF coated with PANI to perform at room temperature makes it a good candidate for ammonia sensing and remote monitoring applications. Particularly, its strong response in the C-band wavelength range makes it is easy to be integrated with all well-established optical fiber communication network infrastructures such as fiber to the home. The developed sensor exhibits good repeatability and reversibility. The limit of detection of the

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

modified SMF sensor was 0.0025% (25 ppm).

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

The modification processes performed on the SMFs were etching, tapering and combination of etching-tapering. The sensing performance of the modified SMF sensors coated with PANI nanofibers thin films was investigated towards NH3 at room temperature for C-band wavelengths range (1535–1565 nm). The investigation showed that the principle of the gas sensors used is the change in the light characteristics because of the interaction between the NH3 molecules and PANI sensing layer. The interaction between the NH3 molecules and the PANI sensing layer reduces the absorbance in the C-band range and inversely proportional to the NH3 concentrations. Consequently, the transmitted optical power increased. These PANI nanofiber thin films show high sensitivity towards NH3 with low concentration as well as good repeatability indication. The performance of the modified SMF sensors was found to be dependent on the modification technique used in the fabrication of the SMF platform as well as the thickness of the cladding layer after modification (core/cladding ratio). The investigation on the NH3 optical sensing performance using absorbance measurement proved that the ETSMF showed superior response than the other modified fibers and thus, highly potential for novel optical transducer. The ability of the ETSMF coated with PANI thin films operates at room temperature indicates its promising candidate for NH3 sensing applications such as chemical plant leakage remote sensor. Particularly, its response in the C-band wavelength range allows easy integration with the existing all optical fiber communication networks infrastructures. The modification technique used in the fabrication of the SMF platform namely etching, tapering or combination of both strongly affects the performance of the modified SMF sensors. Furthermore, the performance of the modified SMF sensors is also dependent on the thickness of the cladding layer after modification and core/cladding ratio. The investigation on the ammonia optical sensing performance demonstrated that the response of etched-tapered SMF is the stronger among that of other modified sensors. Thus, the developed etched-tapered SMF sensors show high potential to be a novel optical transducer. The ability of the etched-tapered SMF coated with PANI to perform at room temperature makes it a good candidate for ammonia sensing and remote monitoring applications. Particularly, its strong response in the C-band wavelength range makes it is easy to be integrated with all well-established optical fiber communication network infrastructures such as fiber to the home. The developed sensor exhibits good repeatability and reversibility. The limit of detection of the modified SMF sensor was 0.0025% (25 ppm).

*Application of Optical Fiber in Engineering*

**60**

**Figure 12.**

**Figure 11.**

*three cycles [2, 11].*

towards NH3 [2].

strated in the figure.

**6. Conclusion**

*Optical response for ETSMF sensor S1 coated with PANI nanostructured thin film towards CH4, H2 and NH3 [2].*

*Repeatability for the ETSMF sensor S1 coated with PANI nanostructured thin film as exposed to 1% NH3 for* 

and 9.3, 7.7, 5.7 and 5 times that of the etched only SMF sensor (S6) [2, 11]. Based on **Figure 10**, the sensitivities for the SMF sensors (S1–S6) are found to be 231.5, 209.7, 172.1, 146.6, 100.4, and 81.2, respectively [2, 11]. As compared with the tapered and etched sensors, the etched-tapered SMF sensors have higher sensitivity

**Figure 11** shows The repeatability and reversibility of etched-tapered Sensor S1 against 1% ammonia concentration for three cycles each lasts for 8 minutes of ammonia, followed by purging purified air for 15 minutes. These three cycles in the figure exhibits slight difference when exposed towards 1% ammonia. Furthermore, the base time experienced slight shift as a result of incomplete elimination of ammonia from the PANI sensing layer when the air was purged for 15 min. The etched-tapered SMF sensor shows good repeatability and reversibility as demon-

The etched-tapered Sensor S1 was examined towards methane (CH4), hydrogen

Simple and low-cost modified SMF platforms were successfully designed, developed and investigated for optical sensing towards NH3 with low concentrations.

(H2), and ammonia (NH3) to prove its selectivity towards ammonia. The range of concentrations of these gases utilized in this test was from 0.125–1% at room temperature. The sensor exhibits superior response towards ammonia compared to that due to methane and hydrogen as depicted in **Figure 12**. Accordingly, the sensor

can be described to be highly selective towards NH3.

*Application of Optical Fiber in Engineering*
