76 Monday Tuesday Wednesday Thursday Friday 2010 Conference on Precision Electromagnetic Measurements June 13-18, 2010, Daejeon Convention Center, Daejeon, Korea PROGRESS IN BUILDING OF CESIUM FOUNTAIN FREQUENCY STANDARD AT NPL, INDIA Amitava Sen Gupta, Ashish Agarwal, Poonam Arora and Kavindra Pant Time and Frequency Section, National Physical Laboratory India, CSIR Dr. K. S. Krishnan Road, New Delhi 110012, India Abstract We describe the current state of progress of the Cesium fountain frequency standard development at the National Physical Laboratory India (NPLI). The optical set-up of the fountain needed to capture, cool, manipulate and detect the cesium atoms is discussed in detail. The concept and design of physical structure of the fountain is also described. In addition, some of the recent results on cooling and trapping of cesium atoms are reported. Introduction Atomic fountain clocks based on laser cooling and trapping of atoms have been reviewed by Wynands and Weyers [1]. The accuracy level achieved by cesium fountain frequency standard is highest among any other measurement device currently available. Such atomic fountains are being developed at several laboratories around the world [2-3] and a few of them operate as primary frequency standards. At NPLI we have started developing a cesium fountain clock a few years back. The aim of the entire activity is to build a primary frequency standard with a relative uncertainty at about 1 x 10 -15 . The NPLI Cesium Fountain Clock In this paper, the current status of the progress made in building NPLI’s primary frequency standard is reported. The concept and design of various parts of the fountain physical structure are discussed in detail. Optical system Generation of atom cooling, repump, launch and detection beams, and finally coupling them onto 8 optical fibers is done on an environmentally controlled vibration free optical table of dimensions 1.04m by 1.88m. An Extended cavity diode laser (ECDL) in Littrow mode is frequency locked to a caesium D 2 line [crossover peak of 133 Cs 6 2 S 1/2 (F=4) o 6 2 P 3/2 (Fc= 4 and 5) at 852nm], generated by high resolution saturated absorption spectroscopy using the 110 MHz frequency shifted beam from an acousto-optic modulator (AOM). A tapered amplifier is used to amplify the frequency locked laser beam. For polarization gradient cooling, a double pass AOM was optimized to ramp down the intensity of the laser within 2 ms to about 0.5 mW cm í2 , together with a simultaneous increase of the detuning. Double pass bandwidth of 40 MHz was achieved in order to implement the polarization gradient cooling. This beam was amplified by a Tapered Amplifier system to obtain all the cooling and detection beams required in a fountain frequency standard. More than 400 mW of frequency locked optical power is generated by this system. The beam was split and directed to three cat’s eye double pass arrangements of AOM [2]. The output of first AOM is further split into six separate beams, namely the four horizontal cooling beams X 1 , X 2 , Y 1 and Y 2 (in x and y directions) and two detection beams (D 1 , D 2 ). The other two AOMs gives two vertical beams Z 1 and Z 2 . The atoms will be launched by upward directed beam tuned to a frequency Ȟ c + įȞ and the downward-directed beam to Ȟ c í įȞ. In addition, home-built, fast mechanical shutters were installed at appropriate places to eliminate residual scattering of resonant light during the Ramsey time. In Cs, while the cooling laser beams addresses the cyclic transition 6 2 S 1/2 F=4 ĺ 6 2 P 3/2 Fc =5, non- resonant spontaneous emission populates 6 2 S 1/2 F=3 level eventually. Since this level is not optically coupled to the cooling transition, laser-cooling process comes to a halt in due course. To avoid such a population trapping in 6 2 S 1/2 F=3 level, a second (re-pumping) laser is tuned and locked to the transition 133 Cs 6 2 S 1/2 (F=3, M f = 0) o 6 2 P 3/2 (Fc= 4, M f = 0). The output beam is split and mixed with one of the cooling beams,Y 2 , and a detection beam, D 2 . Single mode polarization-maintaining optical fibers (PMFs) transfer the 6 cooling and repump beams and 2 detection beams to the Physics Package from the optical table. Transmission efficiency of around 50% is obtained for all eight fibers. At the output end of the fibers, the beams are collimated with home made beam expanders and give out the desired beam size and polarization.