10 GHz generation with ultra-low phase noise via the transfer oscillator technique

Abstract

Coherent frequency division of high-stability optical sources permits the extraction of microwave signals with ultra-low phase noise, enabling their application to systems with stringent timing precision. To date, the highest performance systems have required tight phase stabilization of laboratory grade optical frequency combs to Fabry–Pérot optical reference cavities for faithful optical-to-microwave frequency division. This requirement limits the technology to highly controlled laboratory environments. Here, we employ a transfer oscillator technique, which employs digital and RF analog electronics to coherently suppress additive optical frequency comb noise. This relaxes the stabilization requirements and allows for the extraction of multiple independent microwave outputs from a single comb, while at the same time, permitting low-noise microwave generation from combs with higher noise profiles. Using this method, we transferred the phase stability of two high-finesse optical sources at 1157 and 1070 nm to two independent 10 GHz signals using a single frequency comb. We demonstrated absolute phase noise below −106 dBc/Hz at 1 Hz from the carrier with corresponding 1 s fractional frequency instability below 2 × 10−15. Finally, the latter phase noise levels were attainable for comb linewidths broadened up to 2 MHz, demonstrating the potential for out-of-lab use with low SWaP lasers.

Optical and Microwave Metrology at the 10−18 Level with an Er/Yb:glass Frequency Comb

“…At NIST, optical frequency combs are used to characterize and compare the 171 Yb and 87 Sr neutral lattice clocks [56,62] and the 27 Al + quantum logic clock. [61] While the clocks operate on transition frequencies of 518 THz, 489 THz, and 1.12 PHz, respectively, OFCs measure the clock local oscillator frequencies of 259 THz (1157 nm), 195 THz (1542 nm), and 280 THz (1070 nm).…”

“…In this work, we describe a more recently developed OFC based on a 500 MHz free‐space laser with an Er/Yb co‐doped phosphate glass gain medium [ 27 ] that is designed to address the issues mentioned above. Due to the choice of gain medium, with an emission bandwidth near 1550 nm, the laser benefits from the same 980 nm pump diodes used in the Er:fibre laser.…”

“…Optical beat note between two 100 m long 1070 nm fibre links (blue trace). The current best inter-species optical clock comparison[70] between the27 Al+ ion clock and 171 Yb optical lattice clock using differential spectroscopy (red points) fit to a white frequency noise trend line (1.97 × 10 −16 𝜏 −1∕2 ), shown with red dashed line. The purple squares represent a high-stability, cryogenic Si optical reference cavity [48].…”

Phase noise mitigation of the microwave-to-photonic conversion process using feedback on the laser current

“…The purest microwave signals have been achieved by frequencydividing the optical wave of ultra-narrow linewidth cavitystabilized lasers to the microwaves domain using optical frequency combs [18][19][20]. Best systems offer microwave signals with phase noise levels of −103 and −180 dBrad 2 /Hz at 1 Hz and 100 kHz offset frequencies, respectively [18][19][20][21]. Some substantial efforts have also been pursued towards the use of the transfer oscillator technique [21,22], the use of monolithic freerunning combs [23] or the development of chip-scale microresonator combs [24,25].…”

“…Best systems offer microwave signals with phase noise levels of −103 and −180 dBrad 2 /Hz at 1 Hz and 100 kHz offset frequencies, respectively [18][19][20][21]. Some substantial efforts have also been pursued towards the use of the transfer oscillator technique [21,22], the use of monolithic freerunning combs [23] or the development of chip-scale microresonator combs [24,25]. Once generated, ultra-stable microwave signals have to be disseminated for end users.…”