A double beam Fabry-Perot interferometer for measuring laser wavelength

A modified Fabry-Perot interferometer has been used for measuring the laser wavelength. Two laser beams are directed to the Fabry-Perot interferometer. The first laser, i.e. a HeNe laser, is used as a reference, and the second laser is the measured laser. A driving mechanism is coupled to the interferometer to move the mirror. During the scanning the resulting fringes are monitored using two light sensors. A fast Fourier transform is used to determine the frequency of the fringe pattern shift of both lasers. The wavelength of the unknown laser is then calculated using the ratio of the frequency of the fringe pattern shift and the known reference laser's wavelength. The performance of the system has been tested by determining the wavelength of three pointer lasers. The accuracy of this system can reach 3 nm, which is appropriate for undergraduate experiments.