Synthesis of Optical Frequencies and Ultrastable Femtosecond Pulse Trains from an Optical Reference Oscillator

Abstract

Recently, atomic clocks based on optical frequency standards have been demonstrated [1,2]. A key element in these clocks is a femtosecond laser that downconverts the petahertz oscillation rate into countable ticks at 1 GHz. When compared to current microwave standards, these new optical clocks are expected to yield an improvement in stability and accuracy by roughly a factor of 1000. Furthermore, it is possible that the lowest noise microwave sources will soon be based on atomically-stabilized optical oscillators that have their frequency converted to the microwave domain via a femtosecond laser. Here, we present tests of the ability of femtosecond lasers to transfer stability from an optical oscillator to their repetition rates as well as to the associated broadband frequency comb. In a first experiment, we phase-lock two lasers to a stabilized laser diode and find that the relative timing jitter in their pulse trains can be on the order of 1 femtosecond in a 100 kHz bandwidth. It is important to distinguish this technique from previous work where a femtosecond laser has been stabilized to a microwave standard [3,4] or another femtosecond laser [5]. Furthermore, we extract highly stable microwave signals with a fractional frequency instability of 2×10⁻¹⁴ in 1 s by photodetection of the laser pulse trains. In a second experiment, we similarly phase-lock the femtosecond laser to an optical oscillator with linewidth less than 1 Hz [6]. The precision with which we can make the femtosecond frequency comb track this reference oscillator is then tested by a heterodyne measurement between a second stable optical oscillator and a mode of the frequency comb that is displaced 76 THz from the 1 Hz-wide reference. From this heterodyne signal we place an upper limit of 150 Hz on the linewidth of the elements of the frequency comb, limited by the noise in the measurement itself.