Real Time in Situ Remote Monitoring for Cladding Modified SMF Integrating Nanocomposite Based Ammonia Sensors Deploying EDFA

The increased deployment of optical fiber sensors along with the development of internet of things has increased the demand for real-time in situ remote monitoring systems in the optical sensing industry. Etched-tapered single-mode optical fiber (ETSMF) sensors coated with polyaniline nanofiber/graphite nanofiber (PANI/GNF) nanocomposites have been developed for the real-time in situ remote monitoring of NH3. In this investigation, the sensors with a reel of SMF with 3 km length were integrated with Erbium-doped fiber amplifier (EDFA) with 15 dB gain. Modification performed on SMF significantly enhanced the interaction between the PANI/GNF sensing layer and the evanescent wave due to the light that propagates from the core layer. The response of the modified fiber sensor to NH3 with different concentrations over the C-band (1535–1565 nm) was investigated. The integration of EDFA into SMF sensors extended the remote monitoring distance. In the C band, the developed sensor showed the response, recovery time, and sensitivity of 59 s, 461 s, and 210, respectively. ETSMF sensors coated with nanocomposite of PANI/GNF exhibit successful remote-sensing performances.


I. INTRODUCTION
Optical fibers have played vital roles in the development of optical communication systems and the telecommunication industry over the past few decades. Intensive studies [1], [2] have been conducted on the consolidation and development of optical fiber sensor networks for applications in communications and in other fields. The research, fabrication, and applications of optics, have drastically expanded. In particular, the gas sensors development led to a technological revolution similar to that facilitated by computers during the 1980s. Thus, the first decade of the present century has been termed as the sensor decade [3]. Sensor technology has witnessed a tremendous advance and more developments are yet to come.
The associate editor coordinating the review of this manuscript and approving it for publication was Wenming Cao .
The optical fiber sensors produce fast response in the order of seconds. They also can provide real-time monitoring, that is an important aspect of the industry [4]. The optical sensors have a substantial advantage, which is the ability to be used for in situ real-time remote sensing so that more extensive areas be covered [5]- [7]. Researchers showed intensive focus on modified optical fiber platforms as sensing tools because they have better sensitivity compared to the conventional fibers. Better sensitivity of the modified fiber is due to the evanescent wave which propagates beyond the core area [8].
The implication of optical fiber-based sensors in gas sensing applications has enabled new opportunities for the in-situ detection of various gases like ammonia (NH 3 ) in remote or inaccessible areas. Several fields, such as process control, automotive industry, and medicine, have high demand for the continuous remote monitoring in real-time of certain gases. Ammonia (NH 3 ) is extensively used in industry and other fields [9]. Commonly, severe poisoning and life-threating cases such as respiratory disease may occur because of the exposed NH 3 concentrations in the air [10]- [13]. Highly sensitive NH 3 sensors have been fabricated with the aid of incorporation of the modified optical fiber integrated with nanomaterials as sensing layers [12]- [15]. Polyaniline (PANI)/graphite nanofiber (GNF) is a nanocomposite recently used (by authors) for NH 3 detection with high sensitivity [15].
PANI/GNF nanocomposites have created a growing attention in sensing applications due to the exceptional properties of both PANI and graphite such as optical, and structural properties [16]. They have conjugated electrons [17].
Graphite has a proficient use for polymer composite because of its dispersion the polymer matrix. Graphite is considered as a brilliant material to modify electrical conductivity and optical properties of the polymers. The high surface area as a result of using polymer/graphite nanocomposites allowed exploring of better modifications of electrical conductivities properties comparative to pure polymers [18]. To the authors' information, investigations of PANI/GNFs nanocomposite against NH 3 in the C-band using SMF platforms has not explored yet [19]. Therefore, it is an interesting research topic.
A simple, reliable, and continuous real-time remote monitoring system should be developed to help minimizing the risk of human exposure to hazardous gases like ammonia (NH3) [15], [20]. Such a real-time remote monitoring system will provide early warnings of abnormal situations or accidents in the field to enable suitable responses from a control center. Optical fiber based sensors have been become more popular as practical and highly sensitive devices for the detection of chemicals at lower concentrations [21]. Optical fiber based sensors may enable distributed remote sensing with real-time monitoring over a wide geographical area [5], [7] given their well-established long-distance optical fiber telecommunication networks [22], [23]. A remote monitoring system wherein a collection of optical fiber-based sensors is disseminated over a wide area distant from the control center demands the deployment of multiplexing techniques and optical fiber-based sensor networks [24], [25]. Nevertheless, the involvement of optical fiber-based sensors in gas monitoring through a communication network remains in its infancy. Gas sensor networks could be established by using modified optical fiber-based sensors coated with nanostructures. The demand to integrate gas remote monitoring systems with the existing optical fiber communication network infrastructures is continuously increasing and will simplify network system design and decrease the cost due to use of chemical detection in remote monitoring.
Single-mode optical fiber (SMF) based sensors can be merged with the existing optical fiber-based sensor networks for remote gas sensing to extend coverage to the C-band wavelength range. An interrogation subsystem and suitable amplifiers should be integrated in the design of a remote monitoring network to increase monitoring distance. Fiber laser schemes, such as erbium-doped fiber (EDF), are used to extend remote monitoring distance because of their good signal-to-noise ratio (SNR) practically. These schemes usually comprise Raman amplification or other types of amplification methods that combine Raman amplification with Brillouin [26], [27].
This work aims to design and demonstrate a real-time in situ remote monitoring system that is based on a modified SMF gas sensor coated with polyaniline/graphene nanofiber (PANI/GNF) composite. NH 3 gas is tested because of its adverse effects and extensive industrial application. Erbiumdoped fiber amplifier (EDFA) is incorporated for loss compensation and extending monitoring distance.

II. MATERIALS AND METHODS
A real-time in situ remote monitoring system for etchedtapered single mode optical fiber (ETSMF) NH 3 sensors was designed and developed. The system is shown in Fig. 1. The system is comprised of a 3 km SMF link for ETSMF ammonia sensors (G1-G3) coated with PANI/GNF nanocomposite. The design parameters for sensors G1-G3 are listed in Table 1. A standard single-mode silica fiber (SMF-28) with core/cladding diameter of 9 and 125 µm, respectively, was used as the optical sensing platform for ammonia sensing. The fiber was modified using combination of etching and tapering processes. Three chemically etched SMF platforms with diameters of 50 µm were fabricated using hydrofluoric acid (HF). The etched platforms were tapered by using the Vytran Glass Processing System workstation (GPX-3000, USA) with waist diameters of 15, 20, and 25 µm and up tapering, down tapering, and waist lengths of 2, 2, and 10 mm, respectively. The ETSMF platforms were spray-coated with a thin film of PANI/GNF nanocomposite as a sensing layer. In order to prepare the PANI/GNF solution, a 10 mg of PANI powder, 5 mg of GNF, and 15 mg of camphor sulfonic acid are mixed [15]. This mixture is then dissolved in 8 ml chloroform. Next, the solution is stirred for 1 hour and sonicated for one hour (Hielscher, Ultrasound Technique, UPS2005 sonication) [15]. Using the spray method, the PANI/GNF solution is deposited on the modified area of the SMF and under a fume hood. The solution is heated on a hotplate for thirty minutes up to fifty centigrade before deposition. By this, a uniform coating is obtained since the binding of the nanomaterial is increased. The PANI/ GNF nanocomposite morphology was investigated using scanning electron microscopy (SEM) as depicted in Fig. 2 (a and b) as published previously by authors in [15]. An ETSMF sensor G1-G3 were used in the investigation. The NH 3 sensing performance of the SMF sensors coated with PANI/GNF nanocomposite was remotely monitored and investigated at room temperature over the link of 3 km SMF with the use of EDFA. The measurements are based on the transmission characteristics in the C-band (1535-1565 nm) wavelength range (output optical power in dBm) under exposure to 0.125%-1% NH 3 . The performance  parameters of the SMF sensors including response, recovery times, and sensitivity were calculated.
The modified SMF sensors were tested by using an ASE source (Amonics ALS 18-B-FA) as a broadband light source and an optical spectrum analyzer (OSA Yoqoqawa AQ-6317) as an optical detector over the C-band wavelength range. For real-time in situ remote monitoring, the 3 km SMF was included as a telecommunication route, and EDFA was included to increase optical response power and transmission distance. EDFA compensates for different sources of losses in the system. These losses include connector loss, splice loss, optical fiber attenuation, and nanostructured thin film absorption, which is the demonstrated loss source. The responses of ETSMF sensor upon exposure to a series of different NH 3 concentrations were investigated to obtain the dynamic response of the sensors. Each gas concentration cycle persisted for 8 min, and sensor air regeneration lasted for 15 min.

III. REMOTE SENSING RESULTS
The plot of the responses of the sensors over 3 km in the absence of EDFA is shown in Fig. 3. The output optical power of the sensors decreased upon exposure to different NH 3 concentrations beginning from 0.125%. This response, however, weakened as the waist diameter of the platforms increased. The SMF Sensor G3 with the smallest waist diameter (15 µm) showed the maximum decrease in output optical power in response to increasing NH 3 concentrations. The decrease in output optical power is attributable to the increased absorbance of the PANI/GNF nanocomposite upon interaction with NH 3 molecules. Moreover, the response of G3 is more intense than that of G1 and G2. This difference in response resulted from the interaction between the evanescent wave and PANI/GNF nanocomposite. This interaction is stronger in SMF sensors with small waist diameters than in SMF sensors with large waist diameters. Thus, the light intensity decreased. G1 and G2 exhibit similar responses to different concentrations because the evanescent wave on the surfaces of these sensors is not as strong as those on the surfaces of other sensors (G3) and is thus resulted in o detect slight changes in the PANI/GNF sensing layer.

IV. REMOTE SENSING RESULTS USING EDFA
Longer sensing distances are preferred in remote-sensing applications. The previous subsection showed that optical power declines over a transmission distance of 3 km because of optical fiber attenuation. EDFA (IPG Photonics model EAU-1LT 30 dBm) was used to extend remote monitoring distance. Optical amplification would enhance response magnitude before transmission and enable long-distance monitoring. The ETSMF sensors were remotely monitored over a distance of 3 km, and their NH 3 sensing performance at room temperature in the presence of EDFA was investigated by using the setup shown Fig. 1. Fig. 4 shows the spectrum of the received SMF sensor responses upon exposure to different concentrations of NH 3 at room temperature after amplification with 15 dB by using EDFA. The optical spectrum of the SMF sensors decreases when the NH 3 concentration increased over the range of 0.125% -1%. Notably, power drastically increases in the presence of EDFA, which compensates for the losses inherent to the developed setup. Moreover, the output optical power decreases as the SMF sensor waist diameter decreases. The use of EDFA has enabled the successful real-time remote sensing of NH 3 by the PANI/GNF-coated SMF sensor. As shown in Fig. 2 and 4, the incorporation of EDFA leads to a change in the spectrum of the sensor's response due to the change in EDFA gain with wavelengths shown in Fig. 5. The maximum gain occurs at 1,535.7 and 1,543 nm. Wavelengths below 1550 nm are intensely amplified by EDFA. By contrast, the power of wavelengths above 1550 nm negligibly increases. Fig. 6 represents the dynamic response of G1-G3 over distances of 3 km in the presence of EDFA. The response time, recovery time, and sensitivity parameters have remained clear and measurable. As shown in the figure, the optical power of the system with EDFA has improved compared with that of the system without EDFA. The increased response of the PANI/GNF nanocomposite-coated SMF sensors to increased NH 3 concentrations (0.125%-1%) could be attributed to the increase in the absorbance of the PANI/GNF sensing layer.  There is an inverse proportion between the increase in the NH 3 concentration and response time and an inverse relationship with the recovery time. Based on the definition of response time as the duration it takes to rise 90% of the total magnitude, and recovery time as the duration it takes   Remote-sensing performance parameters are dependent on the waist diameters of G1-G3. The output response of G1 is higher than that of G2 and G3, which have small waist diameters. Upon exposure to 0.125% and 1% NH 3 , G1 exhibits a received optical power of 9.5 and 8.98 dBm, respectively, over a transmission distance of 3 km with 15 dB EDFA. These values are higher than those measured for the same sensor upon exposure to the same NH 3 concentrations in the absence of EDFA. These results indicates that EDFA successfully compensates for losses within the developed setup without affecting the responses of different SMF sensors to different NH 3 concentrations.
To prove the selectivity of the developed sensors towards ammonia, the sensor G3 was investigated toward methane (CH 4 ), hydrogen (H 2 ), and ammonia (NH 3 ) individually, as shown in Fig. 7. The concentrations of these gases range from 0.125% to 1% at room temperature. It can be seen from the figure that the change in the output optical power during NH 3 exposure is higher than that in the response toward CH 4 and H 2 . Thus, the sensors are highly selective towards NH 3 .

V. CONCLUSION
The real-time in situ remote monitoring of NH 3 at room temperature by PANI/GNF-coated ETSMF sensors in the presence of EDFA was successfully developed and investigated. Sensing performance was considered in the C-band (1,535-1,565 nm) and through absorbance measurements. The interaction between NH 3 molecules and the PANI/GNF nanocomposite sensing layer increased with NH 3 concentration resulted in light intensity modulation at the sensor output. The remote-sensing response of ETSMF sensors was dependent on the waist diameter of the ETSMF sensors. The ETSMF sensor with the smallest waist diameter showed stronger and faster response with higher sensitivity than SMF sensors with high waist diameters. The ETSMF could detect NH 3 remotely online. Results verified that ETSMF design may enable the elaboration of SMF-transducing platforms with nanostructured sensitive materials. To the best of the author's knowledge, this work is the first to demonstrate the successful real-time remote monitoring of NH 3 by PANI/GNF-coated SMF sensors in the C-band and in the presence of EDFA.