: Symmetricom-TRC has undertaken a development effort to produce a prototype chip-scale atomic clock "CSAC". The overall architecture of the CSAC and, in particular, the physics package, must be defined early in the project, prior to the onset of a large-scale engineering effort. Within the constraints imposed by the performance goals of the project, we have recognized two possible schemes for interrogating the ground-state hyperfine frequency of the gaseous atomic ensemble: the conventional double-resonance technique and the coherent population trapping technique. In this paper, we describe a laboratory apparatus, which allows for in situ comparison of the two techniques, without the ambiguities associated with comparing data from disparate experiments. Data are presented comparing the short-term stability resultant of the two techniques, as well as environmental sensitivity to resonance cell temperature, laser intensity, and RF power.

The Chip-Scale Atomic Clock - Coherent Population Trapping Vs. Conventional Interrogation

We investigated the influence of some critical parameters and operating conditions such as cell temperature, laser intensity, and interrogation technique affecting the performances of a gas cell Cs frequency standard based on coherent population trapping (CPT). Thanks to an original experimental setup, the atoms can be trapped in the dark state and interrogated using continuous wave (CW) or pulsed coherent optical radiations. Using a double-lambda scheme, a signal contrast as high as 52% has been measured in the continuous regime for an optimum cell temperature of 35degC. Compared with the conventional continuous CPT interrogation, the pulsed interrogation technique reduces the light shift by a factor of 300 and allowed it to reach high-frequency stability for higher laser intensities. The frequency stability has been measured to be 9 x 10-13 for a 1 s integration time. Main noise contributions limiting the short-term and medium-term frequency stability are reviewed and estimated.

Investigations on continuous and pulsed interrogation for a CPT atomic clock

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.

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

A gas cell atomic frequency standard based on coherent population trapping (CPT) with Ramsey interrogation pulses is developed. The clock resonance is detected in a Cs cell with buffer gas using two phase-locked diode lasers tuned to the Dl line. A preliminary short-term stability of 3.5 10-12t is reported. The CPT specificity allows a new interrogation-detection sequence, i.e., the continuous pulse train, which, with phase modulation, could lead to better short-term stability compared to usual continuous CPT clocks. The parameters optimizing the stability include a high Rabi frequency, a very short measurement time, and an interrogation time equal to the lifetime of the hyperfine coherence

Raman–Ramsey Interaction for Coherent Population Trapping Cs Clock

The authors have developed a miniature atomic clock (MAC) for applications requiring atomic timing accuracy in portable battery-powered applications. Recently, we have completed a pre-production build of 10 devices in order to evaluate unit-to-unit performance variations and to gain statistical confidence in the performance specifications, environmental sensitivity, and manufacturability.

G. Rovera

Papers

Optimization of the Buffer Gas Mixture for Optically Pumped CS Frequency Standards