The femtosecond optical synthesizer as a tool for determination of the refractive index of air in ultra-precise measurement of lengths

In production of semiconductor microchips, nano-components like MEMS, or optical lithography, an even more precise length measuring devices are necessary for a reliable fabrication [1]. Modern high-resolution laser interferometers are considered in this field as these more precise measuring length sensors. They use a wavelength of the laser light like a fundamental length unit [2]. If the interferometers measure distances in atmospheric conditions, the absolute value of the laser wavelength fluctuates in linear form with a refractive index of air [3]. It causes a distortion of the measured values of the length, so that, the repeatability of controlled dimensions can be degraded. On the other hand, if we know the value of the refractive index of air we are able to compensate the measured length value obtained by the laser interferometer easily. But techniques of direct measurement of the index are not solved sufficiently now.