Precise Control Of Laser Frequency By Direct Digital Synthesizer System In An Atom Interferometer

The gravity acceleration g is a key parameter in gravity field. High precision gravity measurement has been widely used in geophysics and fundamental physics. Therefore, it has very important scientific significance to develop sub-micro Gal level atom gravimeter.In our atom gravimeter, the home made DDS is used, which are the key elements used to precise control of the laser frequency both in cold atom fountain and Raman transitions process in our atom gravimeter. We designed and realized the compact and robust DDS signal source to meet the requirements in our moving atom gravimeter. In order to realize the DDS human-machine interface, we achieve the communication between PC and MCU by Lab VIEW software.During the cold atom fountain preparation, DDS is used as the AOM driver, whose frequency directly determines the atoms' velocity and also affects the atoms' temperature. We use the direct frequency measurement method to evaluate the frequency accuracy and stability of our DDS. The measurement shows that our DDS worked at 110 MHz has a relative frequency instability of 121.0 10-× in 100 s, the frequency accuracy is at relative level of 104.5 10-×, and also has a step resolution of several ?Hz for frequency tuning. DDS which is frequency adjustable from DC to 135 MHz is used in our atom fountain to prepare the cold atoms and launch them.In the atom interferometry stage, an OPLL is employed to achieve the low phase noise Raman lasers by phase locking the slave laser and the master laser. We put the home-made DDS inside our OPLL as the local reference. The effective Raman frequency is chirped linearly to compensate the gravity induced Doppler frequency shift during the atom's free fall. This frequency chirp is realized by sweeping the DDS frequency. By figuring out the central fringe of this interferometer, we can get the gravity acceleration /effg =?k. So any noise in the chirp rate ? will contribute to the g measurements. In this work we present a beat-frequency measurement that evaluates the frequency chirp rate stability of DDS. The relative instability of the frequency chirp rate was measured to be 11/ 5.7 10?s ?-= × in 1 s, which is small enough and can be neglected in present atom gravimeter., which is small enough and can be neglected in present atom gravimeter.