Portable Laser-Pumped Rb Atomic Clock with Digital Circuits

Abstract

Reducing the size and complexity of high-performance timekeeping devices is an ever-growing need for various applications, such as 6G wireless technology, positioning, navigation and timing (PNT), Internet of Things (IoT), and ultrafast spectroscopy. This work presents a distributed feedback (DFB) laser-pumped Rb atomic clock, which features extraordinary frequency stability, small size and low power consumption. The DFB laser head employs a built-in isolator with a linewidth of approximately 1 MHz. For complete optical pumping of the atoms in the absorption cell, the laser beam is expanded to a diameter of 10 mm by using an optical diffuser-based beam expander. The physics package is based on a magnetron microwave cavity and surrounded by two layers of magnetic shielding. The overall volume of the optical system combined with the physics package is 250 cm$^3$. The proposed atomic clock is also designed to operate at a low temperature, whose absorption cell is maintained at 323 K. Benefiting from the lower Rb atom density, the excited atoms present a long population relaxation time of 5.8 ms. The frequency synthesizer and frequency-locked loop are implemented by digital circuits. The short-term stability of the atomic clock is measured to be $1.8\times10^{-12}\tau ^{-1/2}$ (1-100s). Our achievement paves the way for practical application of the laser-pumped Rb atomic clocks.