Evaluation of thermal expansion coefficient of Fabry-Perot cavity using an optical frequency comb

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In construction of highly mechanically stable measuring devices like AFM microscopes or nano-comparators the use of low expansion materials is very necessary. We can find Zerodur ceramics or ULE glasses used as a frame or basement of these devices. The expansion coefficient of such low-expansion materials is lower than 0.01 x 10(-6) m . K-1. For example in case of a frame or basement 20 cm long it leads to a dilatation approximately 4 nm per 1 K. For calculation of the total uncertainty of the mentioned measuring devices the knowledge of the thermal expansion coefficient of the frame or basement is necessary. In this work we present a method, where small distance changes are transformed into rf-frequency signal. The frequency of this signal is detected by a counter which measures the value of the frequency with respect to an ultra-stable time-base. This method uses an Fabry-Perot cavity as a distance measuring tool. The spacer of the optical resonator is made from the investigated low-expansion material. It is placed into a vacuum chamber where the inside temperature is controlled. A selected mode of the femtosecond frequency comb is locked to a certain mode of the optical resonator. We monitor the changes of the repetition frequency of the femtosecond comb which represent the distance changes of the optical resonator. The frequency is measured by the rf-counter which is synchronized by a time-base signal from an atomic clock. The first results show the resolution of the method in the 0.1 nm order. Therefore the method has a potential in characterisation of materials in the nanoworld.

length standardfemtosecond laserstabilizationFabry-Perot cavity

OULEHLA, J.; ŠMÍD, R.; BUCHTA, Z.; ČÍŽEK, M.; MIKEL, B.; JEDLIČKA, P.; LAZAR, J.; ČÍP, O.

1. 1. 2011

978-0-8194-8678-3

Proceedings of SPIE