Frequency Stabilization of Ar+ Lasers at 582 THz Using Expanded Beams in External 127I/2 Cells

The I2 stabilized Ar+ laser is now an internationally accepted frequency or wavelength standard having the wavelength λa3 = 514.673 467 nm. Using an improved stabilization scheme the intrinsic frequency uncertainty is now less than 10-11 𝒱. A Fabry-Perot prestabilization circuit reduces the effective laser linewidth from 2 MHz to 70 kHz. Therefore, the iodine cell can be operated at a lower I2 vapor pressure of 1 Pa yielding narrower I2 reference signals. In order to avoid power shifts, the power density inside the iodine cell was reduced to 0.3 mW/mm2 by expanding the beam diameter. The resulting improved signal-to-noise ratio also allowed to measure the frequency separation of 13 isolated 127I2 hyperfine components of the P(13) 43-0 absorption line with an uncertainty of less than 1 kHz.