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Tesi etd-11232011-175011

Thesis type
Tesi di dottorato di ricerca
Development of a stabilized Ti:Sa frequency comb for frequency comparisons at high stability in the optical region
Settore scientifico disciplinare
Corso di studi
tutor Prof. Beverini, Nicolò
correlatore Prof. Tino, Guglielmo Maria
Parole chiave
  • precision frequency measurements
  • fixed point formalism
  • frequency noise
  • femtosecond laser cavity
  • f-2f interferometer
  • comb tooth linewidth
  • ti:sapphire
Data inizio appello
Riassunto analitico
This dissertation describes the development of a self-referenced optical frequency comb (OFC) based on a Ti:Sa femtosecond (fs) laser, to be employed in frequency comparisons between a strontium optical lattice clock and other frequency references, both in the radio-frequency (RF) and in the optical domain. <br><br>The Ti:Sa mode-locked laser, which employs external fiber broadening (EB) for the generation of octave-spanning spectrum, has been stabilized by locking an OFC tooth to a clock laser with high spectral purity, operating at 698 nm and resonant with the clock transition $^1$S$_0$-$^3$P$_0$ in neutral strontium atoms. <br><br>The frequency stability of this EB OFC has been tested both in the RF domain by comparison with a high-quality quartz oscillator slaved to the global positioning system (GPS) signal, and in the optical domain with a second stabilized diode laser at 689 nm slaved at long term to the intercombination transition $^1$S$_0$-$^3$P$_1$ in atomic strontium.<br><br>We perform a frequency noise and intensity-related dynamics characterization of the free-running fs Ti:Sa EB OFC and implement these results for optimizing the phase--lock of the OFC to a Hz-wide 698 nm semiconductor laser. Based on the frequency noise of the beatnote between the clock laser and corresponding EB OFC tooth $f_{b698}$ we expect that the short term frequency stability of the 698 nm clock laser is then transferred to each tooth of the octave-spanning EB OFC. <br><br>Moreover, the noise transfer processes between the pump laser and the Ti:Sa laser have been studied in detail, both comparing the resulting frequency noise of the EB OFC output spectrum with a single-mode Coherent Verdi V5 and a multi-mode Spectra Physics Millennia Xs 532 nm pump lasers. In particular, in the latter case we demonstrate that the implementation of an additional control loop for the stabilization of carrier-envelope offset (CEO) frequency $f_{CEO}$ allowed us to stabilize this signal at mHz level, that is compatible with $f_{CEO}$ stabilization results with the single-mode pump laser case. <br><br>Moreover, we show that, with our optical standard operated at a wavelength 698 nm the impact of $f_{CEO}$ frequency noise on the frequency noise of any EB OFC tooth is negligible when compared with the frequency noise of the $f_{b698}$. <br><br>Despite the OFCs are used typically for precision frequency measurements, we demonstrate an approach to perform an absolute frequency measurement of unstable frequency by the EB OFC.<br><br>Most of this thesis is devoted to the EB OFC. However, I also present characteristics and our first stabilization results of the OFC working at quasi octave-spanning regime.