CPT spectroscopy on low-temperature sealed MEMS rubidium vapour cells
In recent years there has been a strong effort to reduce the size and power consumption of vapour cell atomic clocks [1,2]. The progress in this direction is driven by several factors such as the use low power laser diodes (VCSEL), Coherent Population Trapping resonances (CPT), and micro-fabricated (MEMS) alkali-vapour cells. Here the micro-fabrication of vapour cells has proven a challenging task. All results reported on this task use anodic bonding at high-temperatures (>300°C) to seal the cell . However, the low melting point and high vapour pressure of the alkali-metal combined with long bonding-times (>1hour) complicate this process. We have recently developed a low temperature ( 150°C) sealing technique with fast process time (<1min) based on soldering . We report here on the measurement of 85Rb σ+ CPT resonance in low temperature sealed MEMS-fabricated vapour cells containing natural rubidium and buffer gas. The resonance is recorded on the rubidium D1-line (795nm) using a circular polarized and current-modulated VCSEL. We record the resonance shift, linewidth and amplitude as function of several experimental parameters such as light intensity, cell-temperature, and buffer gas pressure- and mixture. In addition we perform noise measurements on the resonance signal to characterize the cell for clock-applications. Preliminary results show a contrast of 1.7% and linewidth of 900Hz for a 4mm long cell with 70mbar of nitrogen buffer gas. Finally we present and characterize two problems related to the application of 85Rb resonance in clock-applications. First, the low modulation frequency of the VCSEL (1.5GHz) leads to a strong asymmetry in the first order sideband spectrum due to the combined effect of AM- and FM modulation. Second, the buffer gas broadening of the absorption spectrum combined with the small separation between VCSEL carrier and sideband reduces the CPT contrast due to off-resonant absorption. We demonstrate that the impact of both these effects can be reduced by modulating the VCSEL at 3GHz and probing the CPT resonance with the carrier and first order sideband. We acknowledge support from the European Space Agency ESA (ESTEC contract number 20794/07/NL/GLC), the Conference Universitaire Suisse CUS (project CIMENT), the Swiss Space Office SSO, and SpectraTime SA (Neuchâtel, Switzerland).
Record created on 2010-11-11, modified on 2016-08-08