Ch. Chardonnet

CO/sub 2/ laser stabilization to 0.1-Hz level using external electrooptic modulation

We have developed a frequency stabilization scheme for CO/sub 2/ lasers using only external modulation via an electrooptic modulator (EOM). One of the two laser sidebands which are generated by the EOM and frequency-modulated is set in resonance with a Fabry-Perot cavity, itself filled with OsO/sub 4/ as an absorber. The saturation signal of an OsO/sub 4/ line detected in transmission of the Fabry-Perot cavity is used for stabilization. We obtained a stability of 0.1 Hz (/spl Delta//spl nu///spl nu/=3.5 10/sup -15/) on a 100-s time scale, and a reproducibility up to 10 Hz with the strongest OsO/sub 4/ reference lines. These results largely improve the performance of our previous setup for which modulation was applied through piezoelectric transducers. Further, the stabilized laser is not frequency-modulated and is easily tunable.

Ultrahigh-resolution saturation spectroscopy using slow molecules in an external cell

We present the narrowest molecular lines so far recorded in the 10 µm spectral region. A linewidth of 80 Hz (HWHM) has been obtained for theP(39)A13(−) line of OsO4, by selecting slow molecules (Teff=0.6 K) in saturation spectroscopy at low laser fields (30 nW) and low pressures (2 × 10−6 Torr). In these conditions, the contribution of the fast molecules is greatly reduced because of the finite size of the beam. This method, applied previously to methane at 3.39 µm, is used for the first time in an external cell and improves by a factor 8 the best resolution of our spectrometer. Heterodyne detection and double frequency modulation have been necessary to extract a signal at a contrast of only 10−6. The physical ideas concerning this regime are described and a detailed analysis of the line shape is given.

Absolute frequency determination of super-narrow CO2 saturation peaks observed in an external absorption cell

Abstract We have succeeded in observing very narrow saturation resonances (hwhm=2.1 kHz) of room temperature CO2 by monitoring directly the absorption of a CO2 laser beam in an external absorption cell. Through successive recordings of the CO2 lines and OsO4 lines of known absolute frequency, we have been able to perform a preliminary determination of the frequencies of three CO2 lines (respectively P(14) and R(10) of the 10.4 μm band) with an accuracy of the order of one kilohertz. A good agreement is found with previous measurements based on the saturated 4.3 μm fluorescence method. The main interest of this new method is to provide values for the frequencies in the free-flight regime.

Strong-Field Saturation Spectroscopy of Weak Hyperfine Crossover Resonances

We show that, thanks to a controlled use of strong laser fields in hyperfine spectroscopy of molecules, it is possible to record the crossover resonances between allowed ΔF = ΔJ lines and ΔF ≠ ΔJ lines which are very weak for high J lines. A demonstration experiment on the Q(45)A22 line of the ν3 band of SF6 yields very accurate values of the spin-rotation and spin-vibration constants, ca and A, whose ratio denotes the importance of purely vibrational effects.

CO2 laser stabilized on narrow saturated absorption resonances of CO2, improved absolute frequency measurements

We have stabilized a CO2 laser on narrow saturated absorption resonances of CO2 detected on the transmitted beam of an external Fabry-Perot cavity in the 30 THz spectral region. By using another CO2 laser stabilized on a close OsO4 line whose absolute frequency is precisely known, and measuring the beat frequency 4 with a precision of 100 Hz, we determined the absolute frequency of twenty CO2 laser lines, with uncertainties between 100 Hz and 1 kHz mainly limited by the accuracy of the OsO4 grid. The set of measurements reveal some discrepancies with data based on measurements performed with the widely used saturated fluorescence technique; we attribute these to pressure shifts of several kilohertz under the conditions of this last method.

Slow molecule detection or Ramsey fringes in two-photon spectroscopy : which is better for high resolution spectroscopy and metrology ?

The CO2 laser locked onto a saturated absorption resonance of OsO4 provides a secondary frequency standard in the 10 mm region, with an accuracy of 50 Hz to 1 kHz. For averaging times less than 100 s its stability performance is better than the Hydrogen maser. This paper deals with the present attempt to increase this performance by using a two-photon molecular resonance as a reference. We begin with some preliminary and promising results on a two-photon line of SF6 leading to characteristics similar to those obtained with a saturation line of OsO4 . Then two alternative methods to increase the resolution are presented : optical detection of slow molecules and a new development of the well-known Ramsey fringes. Metrological features are analyzed for both methods.

Internal Dynamics of Simple Molecules Revealed by the Superfine and Hyperfine Structures of Their Infrared Spectra

This paper reports some of the progress achieved with our 10 µm saturation spectrometer since our 1979 FICOLS presentation [1] with a special emphasis on recent results in molecular physics.

Effects of Curvature in Laser Spectroscopy with Strong Fields

This paper deals with the interaction of two-level systems either with a single traveling wave or with two counterpropagating beams, for systems with lifetimes longer than the transit time (τ=w/vx) and hence for which, the curvature of wavefronts plays a dominant role. In the case of a single traveling wave, one of us has demonstrated, first theoretically and then experimentally [1], that the Rabi nutation [2] was replaced by adiaba-tic fast passage (Fig. 1). A detailed theoretical presentation of this phenomenon can be found in reference [3] from which Fig. 1 and 2 are reproduced. In the case of two counterpropagating beams new consequences of curvature have been recently discovered through the catastrophic distortions of the saturated absorption lineshape reported here for the first time. But we shall first outline some of the results obtained for the single wave case. We shall focus on the final angle (Fig. 2) of the pseudo-spin as a function of three parameters respectively proportional to the Rabi pulsation |A|=Ωba w0 /vx , to the distance to the waist B=4z/b (confocal parameter b) and to the detuning C=(ω +ω+kvz)w/vx .The equation for the two-level spinor is written with Pauli matrices as (Math) in the suitably rotated frame and equivalent equations in other rotating frames. Different theoretical approaches have been used in the low, intermediate and high field domains.

10 μm frequency-referenced saturation spectroscopy using a single carbon dioxide laser

Abstract We report a new and simple method to obtain saturation spectra at extreme resolution. A single carbon dioxide laser and a pair of acousto-optic modulators are used to provide two μm beams from a single primary source. One of these is used to lock to a molecular transition as a reference while the other is used to interrogate the same transition at a spectral linewidth of a few kHz. Examples of hyperfine structure in PH 3 are presented. The experimental complexity usually associated with such high resolution is greatly simplified, the tuning range of the laser is increased, and the technique obviously has very wide applicability.

Progress on an optical link for ultra-stable frequency dissemination using a public telecommunication network

We transfer the frequency of an ultra-stable laser over a 470-km optical link comprising 400 km of public telecommunication fiber network, simultaneously carrying data traffic. A dense wavelength division multiplexing scheme is used. The ultrastable signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The link passes through two important nodes of the telecommunication network and an amplification stage. The phase noise on the optical link is compensated with the usual round-trip technique. The 470-km multiplexed link shows an Allan deviation of 5×10×15 at one second and 1.3×10×18 after 1 hour. This work paves the way to a wide dissemination of ultra stable optical clock signals between distant laboratories via the Internet network.