Optical Microwaves Based on a Difference Frequency Comb

Optical microwaves have the potential for ultra-low phase noise and high frequency signal generation, which becomes increasingly important for a variety of applications, e.g. in radar [1] and communications [2]. We report on the generation of ultra-low noise and long-term stable 9.6 GHz microwaves through optical frequency division of an offset-free comb, utilizing a novel scheme for GPS referencing. Instead of detecting and locking the carrier envelope offset frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(f_{ceo})$</tex>, the comb (DFC CORE 200 +) is based on difference frequency generation (DFG) [3], which cancels the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex> for each optical pulse. Thus, all actuators (temperature, piezo, oscillator) can be used for the stabilization of the comb repetition rate <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(f_{rep})$</tex> without the risk of crosstalk between <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{rep}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{ceo}$</tex>. The error signal for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{rep}$</tex> stabilization is generated by a beat note of one comb mode with a low-phase noise continuous wave (cw) reference laser.