Absolute frequency measurement of an iodine-stabilized 543-nm He-Ne laser with modulation-synchronized phase-locked loop for improved frequency counting

An iodine-stabilized 543-nm He-Ne laser is an important laser radiation for the realization of the definition of the meter. These secondary standards should be absolutely measured for traceability to the SI-second. Due to the low power of 543-nm He-Ne lasers, a method to improve the signal-to-noise ratio of this measurement is needed. We use a modulation-synchronized phase-locked loop to lock a slave laser without transferring modulation, thus improving the signal-to-noise ratio through reduced bandwidth and higher optical power. The absolute frequency value thus obtained is in good agreement with the CIPM value.