## 1 Introduction  
Considering the living conditions at present occasions, various procedures are being created and explored to check these illnesses [1; 2; 3]. Since the pivotal centre of attention is human life, the inquest stays focused on different aspects [4; 5; 6; 7]. One of them is the air that is being breathed out of the human lungs. Using the information from this air, data in regards to various sicknesses can be found out. An important tool in this connection that can be utilized is spectroscopy, which helps in the examination of a specific material, potentially recognized as Volatile Organic Compounds (VOCs) nicknamed _biomarkers_) [8; 9; 10; 11]. This in turn guides us to critical analysis regarding research in environmental and organic disciplines [5; 12; 13; 14], with substantially tremendous capacity in the future.  
The current research originates from the hypothesis of intracavity absorption spectroscopy in which a material is put within the cavity of the laser [15; 16; 17]. The rudimentary difference between this method and the conventional intercavity absorption spectroscopy is that the latter method investigates the material of interest when it is placed outside the laser cavity [7; 18]. However, the former technique is based on placing the substance of interest inside the laser cavity [19]. On account of this fact, investigation hereby reveals that this technique is extremely susceptible even to little changes in the material [20; 21], in contrast with intercavity spectroscopy, as said material is put within the cavity. This phenomenon can be used to understand minute variations in said substance, after being placed inside the laser cavity [22; 23]. This is a very tempting behaviour which can be extrapolated in a biomedical sensor. As a result, the technique of intracavity absorption spectroscopy throngs attention from the investigators [24; 25]. Therefore, this is the foundation of our laser based system. It has been recently reported that multiple modes can be confined in a nanocavity using metamaterials, and the strength of their electric field is very high [26; 27; 28], which motivates us to use more than one modes inside our proposed sensor setup. Our endeavour is to utilize two modes (frequencies) which makes the measurements more sensitive. Initially, the utilization of single mode does not provide effective outcomes. Thus, a dual mode framework provides more sensitive and accurate results. Afterwards, utilizing many modes is prone to increase the price and complexity of the anticipated biomedical application, which should not be helpful for the pragmatic viewpoint. Based on these arguments, this work attempts to utilize a dual mode framework, which is the first of its kind until now [15; 29; 30].  
The principle of Fibre Bragg Gratings (FBGs) and controlling their wavelengths have been continuously used in the past for various applications in sensors [31; 32]. The efficiency of a setup with FBGs can be improved by governing the devices with the aid of an automated mechanism which is helpful in saving much time and effort [33]. During the design of long gratings, selected etching techniques can be combined with fibre cladding to precise the amount of light which is being emitted from their respective wavelengths [34]. This work at the miniaturized level has been done since the aberration techniques became technically feasible [35], and was enhanced to high resolution images to explore samples in the laboratories [36]. Recent advancement in FBGs extends their applications to holograms that are currently being manufactured, tested and characterized in various forms [37]. On account of these techniques, the FBGs have been extensively used to detect various physical, chemical and biological parameters that might not be possible otherwise, without supplemental gadgets, which is supposedly not desirable for any sensors in the current world [32], in particular the health industry [38].