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We report the design of two novel triple-pass acousto-optic modulator systems. These designs are extensions of the well known acousto-optic modulator (AOM) double-pass configuration, which eliminates the angle dependence of the diffracted beam with respect to the modulation frequency. In a triple-pass system, however, the frequency dependence of the angle does not disappear but the frequency shift is larger, spanning 3 times the AOM central frequency. In some applications, such as optically pumped Cesium-beam frequency standards, the frequencies of the two laser beams remain fixed and a triple-pass optical system can be used to reduce to one the number of lasers used in such atomic clocks. The two triple-pass configurations use either a retro-reflecting mirror, or a right angle prism to pass for third time the laser beam through the AOM, obtaining diffraction efficiencies of about 27% and 44%, respectively.

Time measurement is of great importance for science, technology, and commerce. Among the seven base units of the International System of Units, the second can be realized with the smallest uncertainty, currently reaching parts in 10 16 . Keeping track of the continuous accumulation of seconds allows the formation of time scales that serve as references for applications that require synchronization to national and international standards. This paper presents and discusses a multinational time scale developed for the Sistema Interamericano de Metrologia (SIM). This time scale, known as the SIM time scale, or SIMT, was developed to complement the official world time scale, Coordinated Universal Time, by providing real-time support to the operational timing systems within the SIM region. SIMT is generated from automated comparisons of time standards in North, Central, and South America, and is believed to be the first operational multinational time scale whose results are continuously published in real time via the Internet.

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