Kilohertz-Resolution Spectroscopy of Cold Atoms with an Optical Frequency Comb

Abstract

We have performed sub-Doppler spectroscopy on the narrow intercombination line of cold calcium atoms using the amplified output of a femtosecond laser frequency comb. Injection locking of a 657-nm diode laser with a femtosecond comb allows for two regimes of amplification, one in which many lines of the comb are amplified, and one where a single line is predominantly amplified. The output of the laser in both regimes was used to perform kilohertz-level spectroscopy. This experiment demonstrates the potential for high-resolution absolute-frequency spectroscopy over the entire spectrum of the frequency comb output using a single high-finesse optical reference cavity. PACS numbers: Valid PACS appear hereThe coherent optical bandwidth provided from frequency combs based on modelocked femtosecond lasers provide the optical-to-microwave division necessary to directly count optical frequencies, which has lead to significant advances in optical frequency measurement [1,2]. With the development of ultra-broadband titaniumdoped sapphire (Ti:S) lasers [3,4,5] and the coherent broadening possible with nonlinear optical fibers,[6] the optical bandwidth of comb generators extends from near infrared into visible frequencies. The subsequent stabilization of such frequency combs allows for absolute optical measurements over hundreds of terahertz making them an ideal tool for spectroscopy. Already, frequency combs have been used for direct spectroscopy of allowed two-photon and one-photon transitions [7,8,9,10,11,12]. Previous measurements with the direct output of a frequency comb have been performed by stabilization of the comb to microwave references and studying transitions with megahertz linewidths. A greater potential can be realized by transferring the stability of a narrow optical reference to the comb.A new paradigm is attained by combining the extremely precise optical synthesis of the comb with cold atomic samples that allow long interaction times and permit for nonlinear spectroscopy of very narrow optical transitions with low optical powers. Here we work with a forbidden intercombination transition in cold neutral calcium (Ca) and resolve features as narrow as 1.2 kHz. In order compensate for the relatively high residual temperatures of our atoms,we amplifying the teeth centered at the transition wavelength in a laser diode [14]. Additionally, we demonstrate the comb's wavelength versatility by stabilizing the comb relative to a high-finesse cavity at 534 nm, but perform high-resolution spectroscopy at 657 nm. The application of direct optical measurements with an optical frequency comb to optical clocks that rely on both very cold atoms and optical frequency combs. Direct spectroscopy with an optical comb could potentially yield access to transition frequencies that would be difficult to reach otherwise.The laser that is used in the experiments is a ring cav-Experimental depiction for high-precision optical spectroscopy of cold Ca atoms using a femtosecond laser frequency comb. Acousto-optic modulators (AOM) switch ...

Synthetic frequency protocol for Ramsey spectroscopy of clock transitions

“…Moreover, for ultranarrow transitions (e.g., electric octupole [23] and twophoton transitions [24,25]) the ac-Stark shift can be so large in some cases to rule out high accuracy clock performance at all. A similar limitation exists for clocks based on direct frequency comb spectroscopy [26,27] due to ac-Stark shifts induced by large numbers of off-resonant * Electronic address: viyudin@mail.ru laser modes.Unconventional solution to this important problem was proposed in the paper [28], in which so-called hyperRamsey method has been developed. Soon this approach was successfully realized in [29], where the huge suppression (by four orders of magnitude) of probe-induced shifts was experimentally demonstrated (see also [9]).…”

“…Moreover, for ultranarrow transitions (e.g., electric octupole [23] and twophoton transitions [24,25]) the ac-Stark shift can be so large in some cases to rule out high accuracy clock performance at all. A similar limitation exists for clocks based on direct frequency comb spectroscopy [26,27] due to ac-Stark shifts induced by large numbers of off-resonant * Electronic address: viyudin@mail.ru laser modes.…”

Direct frequency-comb spectroscopy of a dipole-forbidden clock transition in trapped Ca^+40 ions

“…For example, such a "clock" transition could be used as an in situ frequency reference for the calibration of other transitions in ions or as a probe for external fields in the ion trap. The concept of DFCS on narrow transitions has been investigated for cold neutral calcium atoms on a transition with a linewidth of 374 Hz, using the amplified modes of an FC [12]. In this Letter, we show that DFCS can be performed on significantly weaker transitions, using an ion trap and even without amplification of the FC laser.…”

Atmospheric Clock Transfer Based on Femtosecond Frequency Combs

“…This establishes a grid of welldefined spectral markers through the simple relation of mC E O R f fm f   , where R f and www.intechopen.com CEO f are the repetition rate and the CEO frequency, respectively, and m is the index number of the comb line in concern. Unknown spectral features within the span of the supercontinuum can be compared with these comb lines through interference and their exact frequencies can be determined with kilohertz-level resolutions or even better (Fortier et al, 2006). Locking R f and CEO f , or any two independent combinations of them to atomic clocks will transfer the frequency accuracy of the clocks onto the entire spectrum.…”