A Self-Calibrated Frequency Doubled Nd:YAG Laser. Application to the French Kibble Balance

We present a new method to check the frequency stability and absolute frequency value of a frequency doubled Nd:YAG laser at 563,5 THz locked to hyperfine components of an I2 line. This method consists in producing a second laser beam the frequency of which is shifted by an acousto-optic modulator driven by a voltage controlled oscillator. This second beam passes through the I
₂
cell parallel to the main beam. Using two independent lock-in amplifiers we lock the frequency of each laser beam to two different, closely spaced, hyperfine components of the same I
₂
line. The difference between the two laser frequencies is then measured by a heterodyne technique with an accuracy of a few kilohertz, thereby ensuring that the system remains locked robustly at the correct frequency. In this way, the long term smooth running of the French Kibble balance is guaranteed.