International comparison of eight semiconductor lasers stabilized on ¹²⁷I₂ at λ = 633 nm

An international comparison of eight ¹²⁷I₂-stabilized semiconductor laser systems (DLs) has been carried out. Five of the DLs were extended-cavity lasers (ECLs) using extra-cavity saturation spectroscopy; another was a microlens-mounted diode modified to have weak optical feedback, stabilized using the same technique; the seventh ECL was stabilized using frequency-modulated spectroscopy. The final DL was a simple laser diode at 635 nm locked with a digital system on a linear absorption of iodine. The P(33) 6-3 transition of iodine was first used to compare the first seven DLs with a He-Ne laser stabilized on the R(127) 11-5 transition of iodine. The relative frequency stability of these lasers was between 5 parts in 10¹¹ and 7 parts in 10¹² for a sampling time of 1 s, with the best results less than 2 parts in 10¹³ over 1000 s. The frequency repeatability measured during one week was of the order of a few tens of kilohertz. This large fluctuation was caused by poor adjustment of the electronic offset of two of the lasers. For the well-corrected lasers, the repeatability was within a few kilohertz. A study of stabilization on the strong absorption group of transitions R(60) 8-4, R(125) 9-4 and P(54) 8-4, located about -12 GHz from the R(127) 11-5 transition, was also carried out. For the first time, a short-term frequency stability better than that of the classical He-Ne laser around 633 nm has been achieved with a relative frequency stability of 4 parts in 10¹² for 1 s.