A simple, low cost and robust method for measurement of the zero-crossing temperature of an ultralow expansion cavity

We present a simple, low cost and robust method to measure zero-crossing temperature (ZCT) of an ultralow expansion (ULE) cavity. Low cost fiber electro-optic modulator (EOM) and free space EOM as a frequency bridge and a frequency ruler respectively are used to precisely measure the frequency of cavity mode relative to atom transition which acts as an absolute reference frequency. Neither an expensive optical frequency comb and another ULE cavity-based stabilized laser for reference frequency, nor Pound-Drever-Hall locking system for coupling laser is required, but still allows easily using data acquisition card to record abundant data of cavity mode frequency in short term. Using our presented method, the ZCT fitting error can reach 0.01 degrees C and the total uncertainty can reach 0.22 degrees C, meaning that our presented method is accuracy and reliable.