Dark-line Resonances Research Articles

Self-induced transparency and coherent population trapping of ^87Rb vapor in a mode-locked laser

Simultaneous self-induced transparency and a dark line resonance are observed inside a mode-locked laser. The circulating pulse, tuned to the 795-nm optical resonance of rubidium, has sufficient intensity to create at each passage a population inversion-return to ground state, typical of self-induced transparency. A drop in fluorescence (dark line resonance), is observed as the repetition rate is tuned to a submultiple of the hyperfine ground-state splitting.

Coherent population trapping resonances in Cs–Ne vapor microcells for miniature clocks applications

We report the characterization of dark line resonances observed in Cs vapor microcells filled with a unique neon (Ne) buffer gas. The impact on the coherent population trapping (CPT) resonance of some critical external parameters such as laser intensity, cell temperature, and microwave power is studied. We show the suppression of the first-order light shift by proper choice of the microwave power. The temperature dependence of the Cs ground state hyperfine resonance frequency is shown to be canceled in the 77–80 °C range for various Ne buffer gas pressures. The necessity to adjust the Ne buffer gas pressure or the cell dimensions to optimize the CPT signal height at the frequency inversion temperature is pointed out. Based on such Cs–Ne microcells, we preliminary demonstrate a 852 nm vertical cavity surface emitted laser (VCSEL)-modulated based CPT atomic clock exhibiting a short term fractional frequency instability σy(τ)=1.5×10−10τ−1/2 until 30 s. These results, similar to those published in the literature by others groups, prove the potential of our original microcell technology in view of the development of high-performance chip scale atomic clocks.

Mode-locked lasers applied to coherent interactions and phase measurements

Coherent interaction of mode locked lasers with 87Rb vapour is studied, for both two- and single-photon transitions. A ‘dark line’ is observed when the repetition rate of the laser is a submultiple of the ground state hyperfine splitting. The dark line resonance can be used for stabilization of mode locked lasers. Such stabilization has an impact on the development of sensors of extreme sensitivity. A new type of intracavity phase sensors is introduced: a purely optical accelerometer.

Repetition rate spectroscopy of the dark line resonance in rubidium

Spectroscopy of a Λ structure in 87Rb is performed with a mode locked laser. A dark line, resulting from population trapping between the hyperfine levels, is observed when the repetition rate is 1/57th of the hyperfine splitting. Simulation of coherent interaction of Rb atoms with laser pulses shows darklines corresponding to different submultiple of the hyperfine splitting. It is demonstrated experimentally and theoretically that the resonance width is insensitive to fluctuations in optical frequency.

Dark-line atomic resonances in submillimeter structures.

We present measurements of dark-line resonances excited in cesium atoms confined in submillimeter cells with a buffer gas. The width and contrast of the resonances were measured for cell lengths as low as 100 microm. The measured atomic Q factors are reduced in small cells because of frequent collisions of atoms with the cell walls. However, the contrast of coherent population trapping resonances measured in the small cells is similar in magnitude to that obtained in centimeter-sized cells, but substantially more laser intensity is needed to excite the resonance fully when increased buffer-gas pressure is used. The effect of the higher intensity on the linewidth is reduced because the intensity broadening rate decreases with buffer-gas pressure.

Temperature dependence of coherent population trapping resonances

We measure the properties of coherent population trapping (CPT) resonances in Cs vapor cells as a function of temperature. We expected the CPT signal to increase with higher vapor density, but instead the signal fades away above a certain density. Two possible density-dependent explanations are discussed: spin-exchange collisions, which are found to give no relevant contribution at the temperatures considered here, and increased absorption due to the optical thickness of the vapor. The dependence of the dark-line resonance amplitude as a function of cell temperature can be well represented by a simple model based on the optical thickness of the vapor as a function of temperature.

Characterization of coherent population-trapping resonances as atomic frequency references

A low-cost, potentially compact and robust microwave frequency reference can be constructed by use of vertical-cavity surface-emitting lasers and coherent population-trapping resonances in Cs vapor cells. Fractional frequency instabilities of 2×10-11/τ/s have been achieved with a minimum of 7×10-13 at τ=1000 s. The performance of this device as a function of external parameters such as light intensity, optical detuning, and cell temperature is discussed. The dependence of the dark-line resonance signal on these parameters can be understood largely by means of a simple, three-level model. The short-term stability depends critically on the optical detuning, whereas the long-term stability is limited currently by line shifts due to drifts in cell temperature.

A microwave frequency reference based on VCSEL-driven dark line resonances in Cs vapor

Dark line resonances, narrowed with a buffer gas to less than 100 Hz width, are observed in a Cs vapor cell using a directly modulated vertical-cavity surface-emitting laser (VCSEL). An external oscillator locked to one of these resonances exhibits a short-term stability of /spl sigma//sub y/(/spl tau/)=9.3/spl times/10/sup -12///spl radic//spl tau/, which drops to 1.6/spl times/10/sup -12/ at 100 s. A physics package for a frequency-reference based on this design could be compact, low-power, and simple to implement.