From relative to absolute: Optical phase measurement and control in ultrashort pulses

Precision phase control of femtosecond laser-based optical frequency combs has produced remarkable and unexpected progresses in the areas of precision spectroscopy, optical frequency metrology, and ultrafast optics. A phase stabilized frequency comb spanning an entire optical octave is established, leading to a single step, phase coherent connection between the optical and radio-frequency spectral domains. This paper presents one of the systems based on an optical transition of iodine molecules, providing an rf clock signal with a frequency stability comparable to that of an optical standard, and that is superior to almost all conventional rf sources.