Session P1a: Activities at National Metrology Institutes

Frequency Division From Optical to Radio Frequency with Instability Less Than 1E-15
Archita Hati, National Institute of Standards and Technology (NIST), Marco Pomponio, NIST, University of Colorado, Boulder, Nick Nardelli, and Craig W. Nelson, NIST
Location: Seaview A/B
Date/Time: Tuesday, Jan. 28, 2:58 p.m.

Frequency synthesis is an important technique required to convert the interrogation frequency of optical atomic clocks to usable microwave and radio frequencies. The synthesis is typically performed with optical frequency dividers, direct digital synthesizers, analog and digital frequency dividers. To transfer the pristine frequency stability of the optical clocks to usable output frequencies, these synthesis components must have residual instability of the same order or better (Nakamura et al., 2020). New optical clocks exhibit frequency stability that is so low that it is challenging to design and measure suitable synthesis components at these levels, particularly at low radio frequency outputs.
In this study, first, we evaluated a microwave prescaler that exhibits a residual instability of less than 1E-15 at 100 MHz output frequency and cannot be measured using current state-of-the-art commercially available digital measurement systems. Consequently, we implemented a carrier-suppression technique (Ivanov et al., 1998; Sann, 1968) to enhance the sensitivity of time-domain residual measurements. We applied this method to evaluate a pair of prescalers for a 10 GHz input signal and achieved residual instability, sigma(1 s) = 5 × 1E-16 at the 100 MHz output frequency.
In addition to the residual measurement, we plan to implement frequency division from optical to radio frequencies and expect to achieve absolute frequency stability of less than 1E-15 at 100 MHz. For this test, we will use two 10 GHz signals from optical frequency combs each locked to independent cavity stabilized lasers to further synthesize a pair of 100 MHz signals using the same microwave prescalers with a division factor of 100. Measurement of instability, with sigma(1 s) < 1E-15 at radio frequencies, is difficult. The carrier-suppression technique necessitates phase locking the two signals for absolute measurements, thereby suppressing frequency fluctuations within the locking bandwidth, rendering it unsuitable for long-term frequency instabilities measurements. This method, on the other hand, can precisely measure short-term absolute frequency instabilities.
At NIST, we have also developed a state-of-the-art custom digital measurement system with frequency instability noise floor almost an order of magnitude better than any existing commercial measurement systems. We will report measurement of absolute instability below 1E-15 at 1 second for a 100 MHz carrier using our new system. The phase noise results at 10 MHz and 100 MHz will also be presented.