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Abstract
An intercomparison of two 127I2 stabilized He-Ne lasers at λ ≈ 543 nm, recommended as length standards, has been made between the DFM and the PTB, and is the first international comparison at this wavelength reported so far. Both lasers use saturated absorption of iodine in an external cell, one (PTB) using frequency-modulation spectroscopy, the other (DFM) using the first or third harmonic detection techniques. The range of the frequency differences between the two lasers was from 11,8 kHz to 18,4 kHz, with a mean of Δf = 14,2 kHz and a standard uncertainty of 1,9 kHz. A minimum value for the two-probe relative standard uncertainty of 2,5 × 10-13 (140 Hz) is reached after 5 000 s. The hyperfine line splittings were measured by the beat-frequency technique, and good agreement was found between the two completely different laser systems. Measurements of frequency dependence on modulation amplitude, iodine pressure, and pump power are also reported.
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