This paper, focuses on the design and simulation of sensors capable of measuring carbon dioxide concentrations between 0.1 to 500 ppm. Carbon dioxide information at a temperature of 298 ° K and a pressure of 1 atmosphere is imported to Lumerical software for simulation. Then the sensor structure is designed based on vernier effect to select specific wavelength in the absorption spectrum of carbon dioxide. The simulated output is developed inside Lumerical using Finite-difference time-domain (FDTD) method. Sensor structure optimization and sensitivity improvement were achieved by using two distinct wavelengths of absorption for the gas, and Mach-Zehnder interferometer. The outputs were combined and prepared to enter the Detector section.