Cross-Spectrum Phase Noise Measurements of Ultrastable Photonic Microwave Oscillators

Presently, continuous-wave (CW) lasers stabilized by high-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$</tex-math> </inline-formula> Fabry–Pérot optical cavities represent the most stable human-engineered frequency oscillators. For optical signals at 194.4 THz (1.5-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula>m wavelength), cutting-edge systems exhibit fractional frequency instabilities as low as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3 \times 10^{-17}$</tex-math> </inline-formula> over a 1-s averaging time. The lowermost phase noise on microwave (MW) signals can be obtained via phase-coherent optical frequency division (OFD) of this class of optical reference signals. Characterizing this level of phase stability with conventional instrumentation is extremely challenging. Digital electronics-based cross correlation techniques allow the inference of MW signals’ phase noise and amplitude noise via downconversion with independent pairs of internal References, generating two independent measurement channels to be cross-correlated. We present in this article the synthesis of ultrastable MW signals via phase-coherent OFD and their characterization by using a commercial cross correlation-based phase noise analyzer and two auxiliary commercial photonic MW oscillators. These X-band frequency sources exhibit a fractional frequency stability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lt\!4\times 10^{-15}$</tex-math> </inline-formula> at 1 s, with phase noise as low as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lt\!-90$</tex-math> </inline-formula> dBc/Hz at 1 Hz and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lt\!-150$</tex-math> </inline-formula> dBc/Hz at 1 kHz offset from a 10-GHz carrier. Conventional instruments typically do not permit the reveal of this level of fidelity. We show the use of a digital system and two auxiliary independent and free-running ultrastable sources at >1-MHz offset from the DUT to enable cross correlation measurements within minutes for this level of spectral purity and frequency stability.