Analysis of noise mechanisms limiting frequency stability of microwave signals generated with a femtosecond laser

The femtosecond laser is a key element of a coherent link between optical and microwave domains. Such a link has already enabled measurements of optical frequencies with an accuracy of a current time standard, the caesium beam microwave ´clock´ (S.A. Diddams et al., Phys. Rev. Lett. vol. 84, pp. 5102-5105, 2000; T. Udem et al., ibid., vol. 86, p. 4996, 2001). Making use of a femtosecond laser, it is also possible to transfer the frequency stability of an optical ´clock´ to radio frequencies. This may result in redefinition of a ´second´ when optical frequency standards supersede their microwave counterparts in terms of accuracy (J.L. Hall et al., IEEE J. Quantum Electron., vol. 37, no. 12, pp. 1482-1492, 2001; L. Hollberg et al., ibid., vol. 37, no. 12, pp. 1502-1513, 2001). In this work we discuss the basic noise mechanisms affecting the frequency stability of a microwave signal produced with a femtosecond laser, including power-to-phase conversion in the photodetector and laser beam-pointing fluctuations.