High-resolution optical frequency metrology with stabilized femtosecond lasers

The merging of such seemingly disparate fields as optical frequency metrology and ultrafast physics over the past few years has had a revolutionary impact on both fields. Extensive research over the past several decades has focused on stabilizing cw lasers to atomic and molecular transitions. These transitions in the optical and near-infrared regimes provide some of the highest Q's accessible in spectroscopy due to their high resonant frequencies (Q ≡ ν _o/δν). Modern experiments have enjoyed increasing levels of precision and accuracy due to such stabilized laser systems. A long standing problem in optical frequency metrology, however, is the difficulty to perform direct frequency measurements in the optical spectrum. Traditional optical frequency chains are complex, costly, and lack flexibility. Recent experiments based on mode-locked femtosecond (fs) lasers promise to eliminate this problem and make optical frequency measurements accessible as a general laboratory tool. The use of fs lasers now enables the direct measurement of optical transitions by simply linking these frequencies to the repetition rate of the fs laser.; The ability of the femtosecond laser to link the optical and radio frequency regimes is ultimately limited by its stability. In this dissertation, we present a novel stabilization scheme in which the frequency, phase, and repetition rate of a Kerr-lens mode-locked (KLM) ti:sapphire laser are locked to that of an ultra-stable Fabry-Perot reference cavity. The large signal to noise ratio of the recovered cavity resonance allows the superb short term stability (τ < 1 second) of the passive reference cavity to be transferred to the femtosecond laser. This technique may find future application in any experiment involving the use of femtosecond pulses in which a resonant cavity is employed, such as intracavity studies of light-matter interactions with ultra-short pulses.; The short term instability of the cavity stabilized femtosecond laser is analyzed in this work. A fractional frequency instability below 4 × 10−13 in only 100 milliseconds of averaging time was measured between the fs comb and the reference cavity. These initial results show great potential as higher finesse reference cavities and increased servo bandwidths can be used to further reduce the short term instability of the laser to unprecedented levels.; Two experiments demonstrating the unique properties of the cavity stabilized mode-locked laser are performed. First., the equally spaced modes of the KLM laser are used as a, “frequency ruler” to characterize the dispersion in the Fabry-Perot reference cavity as a result of frequency dependent phase shifts in the cavity mirrors. This technique is then applied to measure the dispersion of air by characterizing the reference cavity dispersion in vacuum and at atmospheric pressure.; The fs laser is also used to directly measure optical frequencies. The absolute optical frequency of the reference cavity modes were measured with a precision of better than 1 kHz (∼2 parts in 1012) for averaging times less than one second, limited by instabilities in the radio frequency counters used. A two-photon optical transition frequency in atomic rubidium was also measured directly with the mode-locked laser.; The measurement demonstrates the high short term stability and potential accuracy of optical frequency measurements based on the cavity stabilized femtosecond laser. The stability of this measurement (∼10−11 ) was limited primarily by that of the cw laser locked to the atomic transition.