We designed and demonstrated two new schemes to improve the performance of stimulated Brillouin scattering (SBS) cells for a high average power laser. By introducing a lamellar flow cell, thermallyinduced defocusing and convection via laser light absorption were removed. By inserting a rotational wedge plate,the spot was rotated in time and locate heat sources were diffused. As a result, the SBS reflectivity and fidelity were improved dramatically with acetone. Using the cell, the operational power can be improved by as much as several hundred times the closed-type. If a medium with a small absorption coefficient such asheavy fluorocarbons is used, the phase conjugate performance enables the incident power to reach a kW level.

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The long-term performance of two active hydrogen masers developed at the National Measurement Laboratory of the CSIRO is presented. Six years of comparisons between the masers, the United States Naval Observatory (USNO) and the Bureau International des Poids et Mesures (BIPM) are summarized. Operating as independent clocks the masers showed outstanding long-term performance and consistently high weightings in the coordination of major time-keeping centres carried out by the BIPM.