**1. Introduction**

The conventional ammonia (NH3) sensors are electrical type. The electrical sensors have simple structure and low cost but they have poor selectivity as they respond to other gases. Moreover, electrical sensors are susceptible towards electromagnetic interference. Because of signal ignition opportunity, the electrical sensors are not appropriate to be used in oil and gas volatile environment [1]. There is a critical demand to develop an alternate type of sensors to avoid disasters resulted from ammonia leakages or drawbacks related to electrical signal based sensors {Mohammed, 2019 #3943}. The optical fiber sensor is an outstanding alternate [2]. Mostly, modified multimode optical fiber (MMF) is deployed to fabricate current NH3 optical fiber sensors. Generally, the MMF sensors have lower sensitivity than the single optical fiber SMF sensors that not extensively explored for NH3 sensing [2].

Researchers showed intensive focus on modified optical fiber platforms as sensing tools since they are more sensitive compared to the conventional fibers. Cladding modified SMF sensors with high sensitivity integrated with nanostructured thin films against ammonia can be deployed to avoid crises resulted from gas leakage such as ammonia [2]. These sensors have been gained popularity as practical tool to detect chemicals with low concentrations such as gases. By utilizing these configurations, it is expected to fabricate sensors with high sensitivity and fast response.

The aim of this chapter is to design and demonstrate an etched-tapered SMF optical fiber gas sensor for remote monitoring application. The gas under testing is ammonia due to its high severity and deployment in the industry. The objectives to achieve this research project are as follows:


The next section presents a description of the modified optical fiber platforms as sensing tools since they are more sensitive compared to the conventional fibers. The etched, tapered, etched-tapered platforms as modified optical fiber platforms will be elaborated. After that, the Polyaniline nanofiber employed as a sensing layer is introduced in details. The properties of PANI thin films will be discussed by highlighting its attraction and factors that influence the sensing performance. Later, a detailed review on previous works that use PANI as a sensing layer for ammonia sensors will be presented. Moreover, PANI nanostructured thin film preparation and deposition onto the SMF transducing platforms will be highlighted. Several micro-characterizations of the fabricated nanostructured thin films were carried out to investigate sensing layer morphology and thickness of the nanostructured thin film. These parameters affect gas sensing performance. The sensing performance of the modified SMF sensors including etched, tapered and etched-tapered sensors coated with sprayed PANI nanofibers will be investigated and analyzed when exposed to ammonia with different concentrations. The investigation performed in the range of C-band wavelengths at room temperature. Based on author's knowledge, the investigation of the SMF sensors coated with PANI in C-band wavelengths ranges is not explored yet. Finally, chapter conclusions will be summarizing the performance properties behind the deployment of modified SMF sensing platforms Integrated with PANI nanofibers at room temperature.
