Optical code division multiple access-based ammonia gas sensor network using modified single mode fiber coated with polyaniline/graphite nanofiber

Optical fiber sensor network has received an increasing attention in recent years. This is due to the fact that it has proven to be a crucial platform for monitoring a wide range of parameters in many fields. Optical fiber sensor network which consolidates optical fiber sensors is still in infancy stage especially its applications for chemicals or gas sensing. Some unique properties of optical signal such as immunity to the EMI, resistance to the corrosive and flammable environments make optical fiber a promising candidate for gas sensing applications. Important influence of the sensing layer morphology towards gas sensing performance leads to the deployment of nanomaterials. Nanomaterials based optical fiber sensors are expected to produce highly sensitive optical gas sensors. The sensors can be integrated as a part of optical fiber sensor network for remote and distributed real time in-situ gas monitoring system. One of the efficient ways to manage the multiple sensing nodes in the optical fiber sensor network is by deploying spectral amplitude coding based optical code division multiple access (SAC-OCDMA). SAC-OCDMA is low cost technique and has the ability to suppress the multiple access noise (MAI). In this project, single mode fibers (SMF) were modified and coated with polyaniline (PANI) nanofiber and PANI/graphite nanofiber (GNF) nanocomposite to produce highly sensitive optical ammonia (NH3) sensors. GNF was reported to have a unique structure where it has virtually open edges and large interlayer spacing, which also believed to be useful for different applications such as supercapacitor and sensing applications. These sensors were tested towards NH3 in the visible and C-band wavelengths ranges which is not yet explored for optical NH3 sensing applications and enables the integration of the sensors with the existing optical fiber communication systems such as fiber to the home (FTTH). NH3 is selected for the project because it is highly dangerous gas and widely used for industrial applications. A novel modified SMF that underwent both etching and tapering processes was developed to produce fiber with rough surfaces and reduced cladding structures. Three NH3 etched-tapered SMF sensors coated with PANI/GNF nanocomposite were multiplexed using SACOCDMA technique to establish a star topology optical fiber sensor network. The SAC-OCDMA technique deployed in the optical fiber sensor network for NH3 sensing is based on Khazani Syed (KS) code. KS code is preferred because it reduces the number of FBG filter and thus, reduces the cost and complexity of the developed system. At device level, the sensor performance was evaluated in terms of response and recovery times, low limit of detection (LOD), sensitivity and repeatability. At the optical fiber sensor network level, the optical signal to noise ratio (OSNR) was investigated for the developed NH3 sensing network. The proposed etched-tapered sensors coated with PANI nanofiber outweight the performance of tapered SMF and etched SMF in terms of sensitivity and response time. The SMF sensors coated with PANI/PGN nanocomposite exhibited superior response as compared to the sensors coated with PANI thin films only towards NH3 in the visible and C-band wavelengths ranges. The response time and sensitivity of SMF sensors coated with PANI/PGN nanocomposite towards NH3 was 58 s, 49 s, 300 and 306.8, respectively in the visible and C-band wavelengths ranges. LOD was found to be approximately 0.04% (400 ppm) at room temperature. These sensors were integrated with the developed optical fiber sensor network and investigated for real time remote sensing with 3 km SMF link using erbium doped fiber amplifier (EDFA). The measured OSNR for SAC-OCDMA based optical fiber sensor network was 02.1 dB when sensors were implied in the network. For remote monitoring with 3 km link only and including the EDFA with 20 dB gain, the OSNR was 19.6 and 31.75 dB, respectively. The use of EDFA improved the OSNR significantly. In summary, different modified SMF sensors coated with nanostructured thin films were successfully developed and investigated towards NH3 gas. Strong optical sensing performance showed by the novel fiber sensors indicate their potential to be multiplexed in optical sensor networks for remote as well as distributed NH3 detection.

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