Laser Frequency Stabilization for Aerospace Applications

Long term frequency stability of lasers is a key parameter in many research areas ranging from dimensional metrology to fundamental physics. Many solutions have been proposed by the scientific community and lasers whose frequency is referenced to molecular transitions are commonly used when long term stability and wavelength accuracy are needed. Our goal is the development of a frequency stabilization system with a relative frequency stability (Allan variance) of 1E-12 over 1 second integration time which will be suitable for aerospace applications. The frequency reference is a 10 cm long Fabry-Perot optical resonator which has been designed in order to withstand the huge launch loads (up to 30g at 1 kHz). As the stability of the resonant frequency is determined by the long term stability of the cavity length, a low CTE material (ULE) was adopted as the cavity spacer. During the laboratory tests the cavity has been housed inside a mechanical insulating vacuum chamber, thermally controlled by a digital system. The laser is referenced to one of the cavity resonances by means of a digital implementation of the Pound-Drever-Hall technique. An FPGA equipped with a small number of auxiliary components provides a flexible and reconfigurable digital system. A laser was locked to an iodine transition using the “digital PDH” and a classical analog system; the same frequency stability (parts in 1E13 over a 1 second integration time) was measured. Currently we are working on the implementation of the thermostat in order to properly length-stabilize the Fabry-Perot resonator and to characterize the coefficient of thermal expansion of the ULE spacer we are using.