Here, a comprehensive analysis of the impact of the multimode fiber (MMF) lengths on different sensitivities of single mode-multi mode-single mode (SMS) fiber segment sensor probe under different external perturbations such as te. Here, a comprehensive analysis of the impact of the multimode fiber (MMF) lengths on different sensitivities of single mode-multi mode-single mode (SMS) fiber segment sensor probe under different external perturbations such as temperature, strain and refractive index has been propounded and studied thoroughly. In addition, different phenomenal characteristics of the SMS segment namely fast Fourier transform, fringe visibility, the free spectral range of the observed transmission spectrum and most importantly the self-imaging phenomena have been investigated meticulously. These aforementioned characteristics are also verified theoretically. The experimental results show that the sensitivities increase with an increase of the MMF length and are restricted to 12 cm length due to the supe. ••We proposed the impact of MMF length of SMS on temp., strain and RI sensitivity.••Different spectral characteristics of SMS are analysed and verified thoroughly.••The self-imaging phenomena of the SMS is demonstrated and verified theoretically.••Our proposed sensing approach offers higher intrinsic sensitivities and resolutions.••Multimode interferenceSMS fiber structureTemperatureStrainOptical fiber sensors have been promising and popular sensing devices owing to their distinct features. Numerous captivating fiber configurations have been propounded and demonstrated to measure different perturbations like temperature, strain and refractive index (RI). Sensing arrangements like fiber Bragg grating (FBG),,, long-period grating (LPG),, Fabry-Perot interferometers (FPI),, in-fiber interferometers, are the most popular and fascinating methods for monitoring the aforementioned parameters.Cascading different fiber structures is one of the easiest and most clever ways to overcome the disadvantages such as employing costly equipment, complicated demodulation techniques, controlling the precise cavity length etc. of the aforemen. 2.1. Multimode interference and the mode couplingThe schematic of the SMS fiber structure is shown in Fig. 4 (a). The basic underlying principle behind the SMS structure is the multimode interference (MMI) phenomenon. The interaction between the core and the cladding modes of the uncoated MMF is responsible for the phenomena. The lead-in SMF (SMF1) works as a mode combiner whereas the lead-out SMF (SMF2) works as a mode splitter. At the SMF-MMF junction, a part of the light transmits through the core of the MMF and the remaining part will be coupled to the cladding of the MMF. At the MMF-SMF junction, the cladding modes of the MMF will be coupled back to the core of the lead-out SMF. Due to the differences in t.