Henry Timmers, Bennett Sodergren, Andrew Attar, Star Fassler, Cole Smith, Jason Pinon, Kurt Vogel, and Kevin Knabe, Vescent Photonics

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Next-generation quantum sensors are currently being investigated for positioning, navigation, and timing (PNT) applications such as navigation in GPS-denied environments and the deployment of 5G (and more advanced) communication networks. These sensors have optical, electrical, and mechanical requirements for field deployability that are more challenging than prior industrial laser developments. These challenges can include broad optical spectral coverage and/or challenging narrow linewidth requirements of laser sources, low noise laser driver and feedback electronics, high bandwidth microwave detection and generation, thermal management and precision temperature control, and environmental ruggedness including passive and active vibration suppression. The laser systems used in current experiments have large size, weight, and power (SWaP) and are sensitive to thermal and acoustic fluctuations, preventing these experiments from leaving the laboratory. In this effort, we focus on an optical clockwork that will aid both civilian and military applications through the development of high-performance optical clocks for eventual deployment in GPS-denied environments. Vescent has a proven track record of converting technologies developed under government funding into commercial off the shelf products built by our production team. Vescent will continue to leverage this history of commercialization by utilizing design for manufacture principals and robust generational engineering design. Two key optical subsystems necessary for next-generation field-deployed timekeepers include optical frequency combs (OFCs) and ultranarrow linewidth (UNL) lasers that are suitable for the interrogation of ultranarrow clock transitions. Vescent has recently miniaturized an OFC into a ruggedized module and will discuss SWaP and performance results under extreme environmental conditions. Additionally, a general stabilization scheme is discussed to transfer the stability of a near-infrared (NIR) UNL laser to lasers in the visible domain through the frequency comb (that could in turn be steered by an atomic correction signals). Optical atomic clocks typically rely on multiple lasers at various wavelengths for cooling, trapping, state preparation, and interrogation of (ultra)narrow spectroscopic transitions. The wavelength and linewidth of the clock transitions can vary over many orders of magnitude depending on the atomic species and implementation of the physics package. The wide variety of laser wavelengths required for these systems can range from the visible to the NIR; no single laser gain medium or family of fiber components can cover this range. OFCs can be used as stability transfer instruments in addition to their core function as microwave clock readouts. Here we offer a stabilization scheme that allows the stability of a NIR UNL laser transferred to a visible laser via efficient nonlinear frequency conversion of comb light. The optical frequency comb is fully stabilized through stabilization of the carrier-envelope offset frequency (????????????????) as well as an optical comb tooth to a UNL laser (which dramatically narrows the entire spectrum of optical comb teeth) after which a visible or NIR clock laser can be phase-locked to the comb. Subsequent interrogation of atoms can generate a correction signal through which either the UNL or the OFC can have their stabilization parameters adjusted to generate ultra-low instability microwave signals. The OFC presented in this work is similar to systems previously developed at NIST [1], but with improved noise performance and ruggedization from over 5 years of internally and government funded efforts. The current performance of Vescent’s OFC modules have f CEO linewidths < 200 kHz and can be stabilized to under 1.5 rad of integrated phase noise (500 Hz –5 MHz). Optical locks between the comb and clock lasers typically show integrated phase noise below 0.3 rad over the same integration range (and depend on the noise characteristics of the clock lasers while optical comb teeth have been shown to exhibit linewidths below 1 kHz). In-loop characterization of these stabilized parameters show that the OFC can support optical clocks with instabilities below 1×10 -16 /???? @ 1 s (through ????-counter data acquisition). Performance of the comb is also discussed when the system is subjected to large environmental temperature changes as well as intense shock and vibration profiles. Performance of a 1556 nm UNL laser designed under recent government funding has also been characterized and exhibits a frequency instability of 2×10-13 @ 1 ms, a linewidth of less than 30 Hz in a 100 ms scan, and an optical frequency drift rate less than 10 kHz/s. Efforts to improve environmental robustness as well as the linear optical drift rate will be discussed. Finally, prospects for generation of UNL lasers at alternate wavelengths will be summarized. A ruggedized optical clockwork based on an OFC and UNL laser developed at Vescent Photonics will be presented. Performance and SWaP of both laser systems will be reviewed in both laboratory and challenging environmental conditions while the benefits to a stability-transfer scheme are summarized. This material is based in part upon work supported by the Office of Naval Research under Contract No. N00014-22-C-1045. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research. [1] L. Sinclair, I. Coddington, W. Swann, G. Rieker, A. Hati, K. Iwakuni and N. Newbury, "Operation of an optically coherent frequency comb outside the metrology lab," Optics Express, vol. 22, no. 6, pp. 6996-7006, 2014.