We report an ultranarrow-linewidth laser spectrometer at 657 nm, consisting of a diode laser locked in a single stage to a stable high-finesse reference cavity. The system is characterized by comparison with a second independent system. From beat frequency measurements a linewidth below 1.5 Hz (FWHM) and a fractional instability of less than 2 x 10(-15) for 1 s of averaging time are observed.

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Three independent iodine stabilized laser systems have been set-up at the Physikalisch-Technische Bundesanstalt (PTB) for applications in frequency-and dimensional metrology. The lasers use saturated absorption of the P(54) 32 -0 iodine transition in an external iodine cell as a frequency reference. The signals for stabilizing the lasers are obtained by using wavelength modulation spectroscopy and frequency modulation spectroscopy, respectively. With the different setups we have achieved instabilities of < 2⋅10 -13 for an integration time of 1 s and an agreement of the stabilized laser frequencies of the individual systems to better than 3 kHz. Together with absolute frequency measurements performed recently [ 1,2 ] these lasers can compete with optical frequency standards based on atoms or ions. The drawbacks and advantages of the individual set-ups are discussed with respect to frequency stability, reproducibility and uncertainty.Besides the reproducibility, impurities of the iodine cells limit the uncertainty of the stabilized frequency. We have used one of our systems to investigate a random sample of iodine cells and compared these results with the ones obtained with an iodine-stabilized HeNe laser.

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