International Intercomparison of Iodine-Stabilized Helium-Neon Lasers at 633 nm Involving Ten Standards Laboratories

An intercomparison has been made of the frequencies of He-Ne lasers stabilized using intracavity saturated absorption in 127I2 at 633 nm. Lasers at ten standards laboratories (PEL (New Zealand), NRC (Canada), NBS (USA), NPL (UK), PTB (FRG), NPL (India), NIM (China), NRLM (Japan), KSRI (South Korea), and NML (Australia)) were each compared in turn with a portable laser from PEL during March to May 1984. The optical frequencies of all participating lasers exhibited a standard deviation of 23 kHz (5 parts in 1011) with a range of ± 40 kHz. This standard deviation is a factor of seven smaller than the standard deviation of the absolute value recommended for the optical frequency of these lasers. If allowance is made for one iodine cell believed to be contaminated, the standard deviation is reduced to 18 kHz. Power-shift coefficients were measured, and ranged from + 1.4 kHz/mW to - 6 kHz/mW, referred to intracavity power. It is concluded from the measured frequency offsets and power shifts that the best frequency reproducibility amongst different designs of laser is likely to be obtained at low levels of intracavity beam power (5 - 10 mW). The frequency reproducibility was not improved by extrapolating to zero power.