Nowadays embedding smart devices into various structures is making great strides: from skyscrapers and bridges to lab-on- a-chip and many biomedical fields. Out of a wide variety of sensors, this study aims to design, simulate, and analyze a mechanical elongation one. Classical elongation sensors tend to rely on Bragg peak wavelength variations, which in many cases are reported with respect to strain variation. In this paper, using a 2D triangular photonic crystal structure embedded perpendicularly into a fiber optic core, we rely on a different sensing mechanism. The optimized 2D photonic crystal structure was simulated using EM Explorer, a specialized electromagnetic (EM) wavelength propagation equations solver. The simulations performed have shown that measuring the phase variations of the EM components can provide very accurate information of ultra-small mechanical deformations. In contrast, other sensors have been relying on amplitude variations for such measurements. The EM propagation through the photonic crystal embedded into a fiber optic core has been simulated at ten different elongation forces, spanning from 1 N to 10 N. The applied force changes the geometry of the lattice structure. The geometric coordinates of the photonic crystal structure are used as input values for EM Explorer, which solves the EM wavelength propagation. The simulation results have shown that the phase of the electrical field exhibits a steep change (highly non-linear) when a particular elongation force is applied. The phase of the electrical field varied from 3.089 to –3.058 radians for a force variation of 1 N. This means that the proposed mechanical deformation sensor had a phase variation of Δθ = 6.14 radians for a mechanical deformation of ΔL = 3.55 nm (1 N loading force). Hence, the sensitivity of the sensor is about 10 pm/°.