Next-generation optical atomic clocks and quantum sensors are currently being investigated for positioning, navigation, and timing (PNT) applications such as navigation in GPS-denied environments and multi-static synthetic aperture radar (SAR) as well as commercial applications in 5G-and-beyond wireless communication, satellite synchronization, and geodetic sensing. These sensors have optical, electrical, and mechanical requirements for field deployability that are more challenging than those of 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 require unacceptably large size, weight, and power designs and are sensitive to thermal and acoustic fluctuations. In this effort, we focus on an optical clockwork that will facilitate both civilian and military applications on a path to eventual deployment in GPS-denied environments. 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 developed a radiation-hardened-by-design optical frequency comb and is miniaturizing and ruggedizing these comb systems to eventually be deployed on satellites. The Technology Readiness Level of these OFCs has been tested at level 6 without any appreciable performance degradation and will be discussed. A summary of how OFC and UNL systems can be integrated into potential optical atomic clock systems will be presented.
Vescent has developed a prototype ultra-stable microwave photonic oscillator capable of advancing the dual DoD and non-DoD needs for alternative positioning, navigation and timing (aPNT), multi-static synthetic aperture radar (SAR), 5G-and-beyond wireless communication, satellite synchronization, and geodetic sensing. Due to shortcomings in sensitivity, dynamic range, and/or resolution, current microwave oscillators for radar limit the identification and tracking of objects with small radar cross sections, including slow-moving objects such as drones. These limitations are dominated by the microwave oscillator phase noise and/or instability. Vescent’s photonic microwave source exploits the method of optical frequency division to transfer the pristine phase noise properties of an ultranarrow linewidth optical laser to microwaves in the L-, C-, or X-band for sensing and imaging. Efforts to improve the long-term frequency stability required in communications and timing synchronization will be discussed. The environmental performance of several key subsystems will also be considered with pathways to reduced size, weight, and power (SWaP). Finally, performance improvements related to the long-term stability of this system will be discussed to simultaneously provide both ultralow phase noise comparable to the best deployable microwave oscillators available and low frequency instability for communication and timing synchronization at a drastically reduced SWaP and environmental susceptibility.
KEYWORDS: Sensors, Laser stabilization, Frequency combs, Laser optics, Laser applications, Clocks, Photonics, Near infrared, Laser development, Global Positioning System
Next-generation quantum sensors are currently being investigated in laboratories for a variety of applications. One application area that will benefit from increased precision in sensors is positioning, navigation, and timing (PNT). Current laser technologies are not deployable and are generally constrained to the lab due to sensitivities to thermal and acoustic perturbations. In this effort, we focus on an optical clockwork that will aid both civilian and military applications including improved GPS instabilities and navigation in GPS-denied environments.
Optical fiber sensors must compete in performance with traditional electronic sensors, such as quartz crystal pressure and temperature monitors. The precision of commercial electronic sensors can reach the parts-per-billion (ppb) level. To test the precision of a laser based spectrometer system, repeated measurements of an absorption line of a molecular gas cell were made. The Allan deviation is computed, and it is shown that the laser interrogation system, built completely out of commercially available components, can achieve precision at the 10-ppb level.
We describe in detail an optical clockwork based on a 1 GHz repetition rate femtosecond laser and silica microstructure optical fiber. This system has recently been used for the absolute frequency measurements of the Ca and Hg+ optical standards at the National Institute of Standards and Technology (NIST). The simplicity of the system makes it an ideal clockwork for dividing down high optical frequencies to the radio frequency domain where they can readily be counted and compared to the existing cesium frequency standard.
In a ceramic vapor cell we have created a robust Sr magneto- optical trap that stores about 108 atoms with lifetimes > 200 ms. We eliminate the 5p 1P1 yields 4d1D2 yields 5p 3P2 leak and achieve a 10-fold improvement in trap lifetime by re-pumping the 5p 3P0,2 dark states with 679 nm and 707 nm light. The observed lifetime is now limited by cold collision losses, and we have preliminary measurements of the 2-body loss rate. Direct readout of the trap velocity distribution is possible using the narrow 5s21S0 yields 5p 3P1 intercombination line at 689 nm. We can also cool with this narrow transition and have achieved a 40-fold 1D velocity compression for about 5 percent of the trapped atoms by applying this second-stage cooling.
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