Proceedings of the ... annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting/Proceedings of a meeting - Precise Time and Time Interval (PTTI) Systems and Applications Meeting
A Versatile Testbed for CubeSat Atomic Clock Development: EOM vs Laser Current Modulation
Zachary Warren, Michael C. Y. Huang, Hunter Kettering, Andrew Stapleton, James C. Camparo
Atomic clock applications for CubeSat missions require low size, weight, and power (SWaP), while maintaining high stability and accuracy. This puts “large” microwave cavity clocks at a disadvantage, and encourages all-optical schemes of clock signal generation such as coherent population trapping (CPT). Two alternatives for optical pumping in compact atomic clocks are 1) electro-optic modulation (EOM) or 2) injection-current modulation (ICM) of the pump laser. CPT contrast and signal-to-noise ratio (SNR) are both influenced by the efficiency of sideband generation and suppression of the carrier frequency, which can vary between EOM and ICM methods. Light shift effects in CPT are also increased by asymmetry in the sideband intensities, a feature of ICM that can impair clock performance. The testbed we are developing enables reconfiguration of components in order to quantify compact-clock signal generation techniques and sensitivities, and their effects on CPT. This paper describes preliminary results from our testbed analysis of the EOM-based and ICM-based optical sideband generation schemes for compact CubeSat clocks.
Atomic clock applications for CubeSat missions require low size, weight, and power (SWaP), while maintaining high stability and accuracy. This puts “large” microwave cavity clocks at a disadvantage, and encourages all-optical schemes of clock signal generation such as coherent population trapping (CPT). Two alternatives for optical pumping in compact atomic clocks are 1) electro-optic modulation (EOM) or 2) injection-current modulation (ICM) of the pump laser. CPT contrast and signal-to-noise ratio (SNR) are both influenced by the efficiency of sideband generation and suppression of the carrier frequency, which can vary between EOM and ICM methods. Light shift effects in CPT are also increased by asymmetry in the sideband intensities, a feature of ICM that can impair clock performance. The testbed we are developing enables reconfiguration of components in order to quantify compact-clock signal generation techniques and sensitivities, and their effects on CPT. This paper describes preliminary results from our testbed analysis of the EOM-based and ICM-based optical sideband generation schemes for compact CubeSat clocks.
Atomic clocks for compact satellite missions have limitations on size, weight, and power (SWaP), while requiring ever greater levels of frequency stability and accuracy. To aid in the development of next-generation low SWaP Coherent Population Trapping (CPT) clocks, we are developing a CubeSat Atomic Clock Testbed (CuSACT). This testbed will give us the capability to vary single aspects of CPT-clock design while keeping all others constant. In this presentation, we will provide an overview of CuSACT, and discuss our preliminary results comparing direct injection-current modulation of a VCSEL with electro-optic phase modulation of the VCSEL.
More and more, precise timekeeping is becoming a crucial element of space-system operations. Not only are atomic clocks required at mission control ground stations, but they are expected to fly on spacecraft ranging in size from the “large” to CubeSats. This, of course, requires that the size, weight, and power (SWaP) of the clock cannot be sacrificed for frequency stability, and that the frequency stability of the device cannot be sacrificed for SWaP. Traditional space clocks (e.g. Rb clocks, Cs clocks, and hydrogen masers) require microwave cavities, rf-discharge lamps, alkali ovens, hydrogen dissociators, filter vapor-cells, or state-selecting magnets. In contrast, Coherent-Population-Trapping (CPT) clocks can have very low SWaP and have led to the development of the chip-scale atomic-clock (CSAC) – perfect for CubeSat missions. In the standard CPT clock, a narrow atomic resonance is formed by a bichromatic optical field. Unfortunately, in the standard CPT clock design the atomic signal suffers a low signal-to-noise ratio (SNR), due to atoms becoming systemically stuck in “trap” states inaccessible to the laser light. At The Aerospace Corporation’s Physical Sciences Laboratories, we have constructed a CPT atomic clock testbed, with the goal of accelerating CSAC development for next-generation space missions. Specifically, our plan is to investigate atomic physics issues that influence CSAC performance and SWaP, and prioritizing issues not being addressed in academic or industrial laboratories. In this presentation, we will discuss our CPT testbed, and recent work investigating CPT-enhancement strategies for improved clock performance that recover atoms from trap states and improve the SNR. Additionally, we will discuss our studies of the physical processes taking place in these CPT-enhancement strategies, and how they can affect advanced-CSAC functioning and design.
We demonstrate the development of an atomic clock using a diode laser and a vapor cell of Cs atoms. We use the technique of coherent population trapping (CPT). The required phase coherence between the probe and control beams, whose frequency difference is the clock frequency, is obtained by frequency modulating the laser with an electro-optic modulator (EOM). The EOM is fiber coupled and hence does not require alignment, which is advantageous for space applications. The atoms are contained in a vapor cell, which is again useful in satellite launches.
A scheme for a Ramsey-coherent population trapping (CPT) atomic clock that eliminates the acousto-optic modulator (AOM) is proposed and experimentally studied. Driven by a periodically microwave modulated current, the vertical-cavity surface-emitting laser emits a continuous beam that switches between monochromatic and multichromatic modes. Ramsey-CPT interference has been studied with this mode-switching beam. In eliminating the AOM, which is used to generate pulsed laser in conventional Ramsey-CPT atomic clock, the physics package of the proposed scheme is virtually the same as that of a conventional compact CPT atomic clock, although the resource budget for the electronics will slightly increase as a microwave switch should be added. By evaluating and comparing experimentally recorded signals from the two Ramsey-CPT schemes, the short-term frequency stability of the proposed scheme was found to be 46% better than the scheme with AOM. The experimental results suggest that the implementation of a compact Ramsey-CPT atomic clock promises better frequency stability.
Quantum sensors, such as optical atomic clocks, are increasingly used in out-of-the-lab environments. This development necessitates a high degree of integration and robustness across all components of these systems. In this context, we present the status of our efforts to develop and realize compact laser distribution modules for the implementation of low-SWaP (size, weight, and power) strontium-based optical clocks. These modules are designed to control and manipulate the properties of laser light, such as frequency, phase or intensity, prior to the light’s interaction with the atoms of the clock. By integrating miniaturized free-space acousto- and electro-optic modulators into these modules, they enable applications such as laser power or fiber path length stabilization, as well as frequency-modulation spectroscopy. In this paper, we present a distribution module designed for 461 nm that was assembled using our hybrid micro-integration technology and will be integrated into a compact optical clock demonstrator. Additionally, we provide an outlook on other distribution modules currently under development.
Observation of CPT for the ground hyperfine interval in 133Cs
We use the technique of coherent population trapping (CPT) to access the ground hyperfine interval (clock transition) in 133Cs. The probe and control beams required for CPT are obtained from a single compact diode laser system. The phase coherence between the beams, whose frequency difference is the clock frequency, is obtained by frequency modulating the laser with an electro-optic modulator (EOM). The EOM is fiber coupled and hence does not require alignment, and the atoms are contained in a vapor cell. Both of these should prove advantageous for potential use as atomic clocks in satellites.