Michelson Interferometer Instrument For Thin Film Measurement Of Reflectivity

Optical thin film has already become an indispensable part of various optical equipment and apparatus since long time ago. When some light wave impinges on the film, some important optical characters of the light beam, such as intensity, phase and polarization, will have some changes, within which, reflectivity is an important one. Scholars make the accurate measurement of reflectivity in different methods which include the spectrophotometer, the single/multiple-reflection and the resonant cavity. In this paper Michelson Interferometer which has its own merits has been taken to measure reflectivity of thin film.Michelson Interferometer (MI) is a typical equipment of two-beam interferometer which has been used in different purposes such as to observe interference phenomenon, to measure micro-length and light source wavelength. It has two separate light beams and the optical path length or phase of each beam can be changed. However, free space interferometer is too big and too complex to be set up and adjusted. It is sensitive to temperature and vibration, too. All this inconvenience requires a solution. With the rapid development of the fiber and fiber optic sensor technology, there comes more developing space for the traditional optic interferometer. The new fiber optic interferometer appears. It has a compact structure;it is easy to be connected and adjusted;also it has high sensitivity and better anti-interference performances. These characters make it excel the traditional one greatly.In the Michelson Interferometer system which is used for measuring the reflectivity of thin film, the light beam is evenly split at the coupler between the sample and reference arms, so that the light wave in each arm can be controlled respectively. The optical delay line in reference arm can provide the optical path difference between the reference and sample arms. Meanwhile, the optical delay line and phase modulator in the reference arm are usedto control group velocity dispersion and phase velocity dispersion respectively, so that it will get the perfect interferogram. It combines the advantage of low light source and heterodyne detection. Then, by using the method of Short Time Fourier Transform and Hilbert Transform of interferogram, we can get the reflectivity of thin film.The paper has five chapters. The traditional methods review for reflectivity is listed first. The second part shows the composition of the Michelson Interferometer system, with the theory and parameters inside. Chapter three gives the experiment equipment as well as the derivation of measuring theory and the method for data processing. The last fourth and fifth chapters offer the experiment results and the analysis on both the results and the measuring error. After all this above, some proposals are put forward to improve the measuring method at the end of this paper.