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A cavity-enhanced absorption spectrometer was used to saturate several lines of ammonia in the 1510 nm - 1560 nm region. Analysis of power broadening of the saturated absorption feature for one of the ammonia lines yielded a dipole moment value comparable to that of the lines in the nu(1)+nu(3) band in acetylene. Highly reproducible frequency measurements of four ammonia line centres were carried out using a frequency comb generated by a mode-locked Cr(4+):YAG laser. These results demonstrate the possible application of ammonia saturated absorption lines for frequency metrology and calibration in a spectral region lacking strong absorbers. To our knowledge, this is the first frequency measurement of saturated absorption lines in ammonia at near infrared frequencies and the first reported observation of saturated absorption lines in the nu(1)+2nu(4) band.

a b s t r a c tWe report on a detailed analysis of frequency noise and intensity dynamics of a free-running femtosecond Ti:sapphire optical frequency comb (OFC) pumped by a multi-mode 532 nm laser. We have used these results for phase locking the OFC to a Hz-wide, cavity stabilized, 698 nm semiconductor laser. An analysis of the in-loop error signals and an estimation of Allan deviation indicates a lower limit for the frequency stability of the OFC at the level of $ 5:3 Â 10 À14 for 125 ms integration time comparable with the clock laser frequency stability over the entire OFC spectrum. The noise transfer processes between the pump laser and the Ti:sapphire laser have been studied in detail, comparing the resulting frequency noise of the OFC output spectrum with both a single-mode and a multi-mode pump lasers. In the latter case, we employed a pump beam steering control to maintain an OFC regime with a minimum impact of pump laser amplitude noise on the carrier-envelope offset frequency.

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