Two-photon Ramsey fringes at 30 THz referenced to an H maser/Cs fountain via an optical-frequency comb at the 1-Hz level

Abstract

Index Terms--Carbon dioxide lasers, Frequency stability, SF 6 , Optical frequency standard. I. INTRODUCTIONThe carbon dioxide laser has given to the 30 THz spectral region particular significance in frequency metrology. The current standard at 30THz is provided by the CO 2 laser locked onto a saturated absorption resonance of OsO 4 in a cell, the reference signal having full width half maximum (FWHM) of 20 kHz [1,2]. The same area is particularly rich in molecular spectra and many problems in this area, together with questions of fundamental physics, have been investigated using the related saturation spectroscopy [3,4]. Typical linewidths are 1-100 kHz. This paper presents continuing work on a two-photon Ramsey fringe experiment on a supersonic beam of SF 6 with the objectives, now essentially realized, of resolving the entire complex hyperfine structure over some 50 kHz and establishing absolute frequencies at the 1Hz level. Both aspects represent huge advances over cell saturation techniques. Two experimental developments have led to the recent advances. First, the distance between the absorption zones has been increased to 1m, so that the fringe periodicity is now 200 Hz for pure SF 6 . Data is routinely recorded with a signal-to-noise ratio (SNR) of 20 in a bandwidth of 1 Hz. The implied limit on the resolution of two components within the hyperfine structure of the SF 6 spectrum is only 10Hz. This is indeed found when fringe patterns are fitted. Second the entire system can now be directly related to the frequency comb of a femtosecond laser, itself referenced to a Hydrogen maser. The maser is compared to a Caesium fountain and to the GPS system. This gives a long-term stability and frequency reproducibility limited by the performance of the current electronics, various optical links and, possibly, the reference maser. The fs technology is necessarily operated in a new area of precision, comparable with the best currently in use [5,6]. Figure 1 gives a block outline of the entire experiment.Two related series of experiments are presented. First the central Ramsey fringe is employed as the reference point for a molecular clock; the clock frequency is measured, and its performance established, relative to the H maser. Second, and separately, Ramsey fringes are measured on an absolute frequency scale provided by the H maser. This gives a second measurement of the central fringe and the entire spectrum of Ramsey fringes over 50 kHz is put onto an absolute frequency scale.This work is partly directed at a new frequency standard. Since a two-photon transition is used the absolute frequency depends on the optical power. Given the absolute scale, this small effect can be investigated and results are reported.

High resolution spectroscopy of methyltrioxorhenium: towards the observation of parity violation in chiral molecules

“…The ultra-high resolution laser spectrometer used for the experiment dedicated to the PV observation is based on the combination of two CO 2 lasers which can operate in the 8-12 µm range (already described elsewhere 29,30 ). Frequency stability and tunability are important issues for this experiment.…”

“…Thus to maximise the signal-to noise ratio, the single pass laser power was chosen to be ~2 µW, the smallest possible to be just above the photodetector noise. On Figure 6, three Doppler broadened lines (between 2 and 5 MHz HWHM) respectively centred on (i) 29 187 Re isotopologue (but higher-order effects should be included in the model). The signal-to-noise ratio appears to be rather poor for ~20 min total integration time, but it constitutes an important step towards the observation of molecular PV.…”

“…[25][26][27][28] The experiment is adapted from an existing Ramsey fringes set-up that enabled us to measure SF 6 absolute frequencies with a sub-Hz (10 -14 ) accuracy. 29,30 In particular, it will benefit from the accurate control of the CO 2 laser frequency by comparison to a primary frequency standard. Besides we expect a strong reduction of systematic effects for three reasons: (i) in such a supersonic jet set-up, collisions are mainly due to background pressure, 4 to 5 orders of magnitude lower than in the CHFClBr experiment, and the corresponding shift will be reduced accordingly; (ii) the same set-up will be alternatively fed with right-and left-handed molecules which will cancel out a number of systematic effects; (iii) the probed line width will be of the order of 100 Hz, instead of 60 kHz for the CHFClBr experiment, which will reduce most systematic errors accordingly.…”

Assessing the time constancy of the proton-to-electron mass ratio by precision ro-vibrational spectroscopy of a cold molecular beam

“…24 over the distribution of molecular velocities is needed to obtain the actual signal; this will result in a gradual attenuation of the side fringes. The detection beam, instead, injects a separate Fabry-Perot cavity (with a finesse of a few thousand) to probe, via stimulated emission, the upper level population as induced by the Ramsey excitation and thus proportional to the actual fringe signal [45]. Eventually, a suitable function is fitted to the obtained fringe pattern to extract the periodicity and the center frequency.…”

Absolute frequency measurement of a SF_6 two-photon line by use of a femtosecond optical comb and sum-frequency generation

“…As a first application, a 2-photon line of SF 6 has been measured with a stability of 3×10 -14 at 1 s limited by the CO 2 /SF 6 stability. Further applications include the characterization of the metrological performance of a 2-photon resonance detected in a Ramsey scheme 18 and the test of the possible temporal variation of fundamental constants using such a 2-photon molecular line 19 . …”