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The development of an optical clock with ultimate accuracy and stabilityrequires lasers with very narrow linewidth. We present two ultrastable lasersystems based on 48 cm long Fabry-Perot cavities made of ULE glass inhorizontal and vertical configurations operating at 698 nm. Fractionalfrequency instability of the beat signal between two lasers reaches 1.6e-15 atthe averaging time of 1 s. We experimentally characterized the contribution ofthe different noise sources (power fluctuations, residual amplitude modulation,the Doppler noise, sensitivity to the shock impact) and found that in our casethe laser frequency instability to a large extent is determined by anoptoelectronic feedback loop. Although the vertical configuration proved to beeasier to manufacture and to transport, it turned out that it is much moresensitive to acoustics and horizontal accelerations compared to the horizontalone. Both laser systems were transported over a 60 km distance from the LebedevPhysical Institute to the Russian National Metrology Institute (VNIIFTRI) wherethey serve as local oscillators for spectroscopy of the clock transition in therecently developed strontium optical clock.

Long 48-cm ULE cavities in vertical and horizontal orientations for Sr optical clock