CLOCK LASER SYSTEM FOR STRONTIUM LATTICE CLOCK T. Legero, Ch. Lisdat, J.S.R. Vellore Winfred, H. Schnatz, G. Grosche, F. Riehle and U. Sterr Physikalisch-Technische Bundesanstalt Braunschweig Abstract We report on the setup and characterization of a 698 nm master-slave diode laser system to probe the 1 S 0 - 3 P 0 clock transition of strontium atoms confined in a 1D optical lattice. The instability and the line- width of the laser system have been measured with respect to an ultrastable 657 nm diode laser with 1 Hz linewidth [1] utilizing the transfer oscillator principle [2] with a femtosecond fiber comb. The magnetic- field-induced clock transition of 88 Sr can be used to investigate the instability of the 698 nm clock laser system at intermediate averaging times . Introduction Optical lattice clocks with strontium are approaching relative uncertainties below 10 -15 [3-5]. Ultimately, their instability is limited by the quantum projection noise which for 10 6 atoms and a Fourier-limited linewidth of 1 Hz results in an Allan deviation of the relative frequency fluctuations of σ y = 10 -18 /τ -1/2 . However, to reach this short term instability an even better instability of the clock laser is needed. Therefore the improvement of optical sources which are phase stable during the required interrogation time of a few seconds is a key technology for optical frequency metrology. Until a few years ago, the investigation of the instability of a laser system was only possible by comparison with a similar system operating at the same wavelength. With the invention of the frequency comb technology it is now possible to measure the instability of an ultra narrow laser by comparison with a microwave frequency standard or a laser system operating at a different wavelength. After the description of the setup of a clock laser system for 698 nm we discuss the characterization of the laser by comparing its frequency with that of a 1 Hz-linewidth laser at 657 nm used in a Ca-clock. Setup of the clock laser system For probing the 1 S 0 - 3 P 0 strontium clock transition we have set up a 698 nm master-slave diode laser system as shown in Fig. 1. The master laser is an extended cavity diode laser (ECDL) in Littman configuration operated at a diode temperature of 44 °C and locked to a high finesse optical cavity by the Pound-Drever- Hall stabilization technique. The cavity is made of a 100 mm long ULE spacer. Its optical axis is oriented horizontally. To minimize the sensitivity against vertical vibrations, the cavity is supported at four points near to its horizontal symmetry plane [6]. The positions of the supporting points were determined by a finite elements simulation. The cavity shows a finesse of 330 000, corresponding to a linewidth of about 4.5 kHz. It is mounted in a temperature stabilized vacuum chamber at a residual pressure of 10 -7 mbar. The whole cavity setup is placed on a vibration isolation platform to further reduce the coupling to seismic vibrations. The injection-locked slave laser delivers an output power of 23 mW. Its light is sent to the strontium atoms and to the femtosecond fiber-laser comb by two actively noise-cancelled optical fibers [7]. faraday- isolator reference cavity PDH- detector polarization maintaining singlemode optical fiber vibration isolation platform AOM λ/4 master laser- diode PBS grating 1200/mm piezo- mirror local oscillator 14.3 MHz ~ faraday- isolator vacuum chamber phase modulator slave faraday- isolator to strontium atoms and femtosecond fiber comb laser- diode servo electronics Fig. 1. Master-slave laser setup for a strontium lattice clock. Characterization of the clock laser system A commercial femtosecond fiber-laser comb was used to characterize the laser system. We used the transfer oscillator principle [2,8] as shown in Fig. 2 to compare the optical frequencies ν 1 and ν 2 of the 657 nm and 698 nm laser. For both laser frequencies the beat Δ 1 and Δ 2 with its next comb tooth is measured. 1-4244-2399-6/08/$20.00 ©2008 IEEE