摘要： Helium-neon lasers at 633 nm are still widely used in interferometers and, stabilized to iodine, serve as secondary frequency standards for the practical realisation of the meter [1]. However, the technical know-how for building and maintaining helium-neon laser tubes is slowly disappearing. Diode lasers are an attractive alternative with respect to availability, cost, output power, size and power consumption. The emission wavelength of 633 nm allows new systems to be easily integrated into existing setups. For replacing the helium-neon laser and enhancing the stability and accuracy, we have investigated an extended-cavity diode laser (ECDL) stabilized on an external cavity containing an iodine cell using Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) at 633 nm. The cavity provides a well-defined, long-term stable geometry, resulting in a robust system. In addition, the increased intra-cavity power and the increased effective absorption length result in absorption signals with high signal-to-noise ratio [2]. NICE-OHMS has been widely used for ultra-sensitive detection of molecular absorption lines [3]. However, for laser stabilization it has only been applied in the infrared regime so far [4,5]. Fig. 1 shows the Allan deviations of a diode laser stabilized to iodine using NICE-OHMS (red) compared to a PTB iodine-stabilized helium-neon laser (black). The diode laser reaches a frequency instability of 3.2(cid:117)10-12 at 1 s, which is lower than for the helium-neon laser (4.1(cid:117)10-12 at 1 s), while the instability of the diode laser starts to rise at averaging times longer than 100 s. Using a NICE-OHMS stabilized diode laser at 633 nm, we have – to the best of our knowledge – reached for the first time a short time frequency instability below that of a state-of-the-art iodine stabilized helium-neon laser. This system has good prospects to serve as a frequency standard at 633 nm. To reach this goal, further investigations of the long-term instability, of the reproducibility and of the sensitivities of the absolute frequency on various parameters are underway.