29.4 Ultra-Low-Power Atomic Clock for Satellite Constellation with 2.2×10-12 Long-Term Allan Deviation Using Cesium Coherent Population Trapping

Nano/micro satellites in low earth orbit (LEO), and unmanned -aerial-vehicle base stations (UAV-BS) in the stratosphere are being considered to be used for increasing the coverage and provision of on-demand high data rates of mobile communication networks all over the globe as beyond 5G technology. One of the most important key technologies for such high-speed and long-distance communication is a very accurate time standard, especially for the LEO satellites constellation [1]. Presently, the best time accuracy can be acquired from atomic clocks. Atomic clock assisted GEO satellites such as GPS can be a primary reference, but they suffer from large path loss and delay, degrading the clock accuracy to 10
^-6
in the receiver part. In addition, GPS is not always available in the space, while the conventional atomic clock has deployment difficulties in the large array due to large volume and huge power consumption. For example, due to the special condition of the atomic cell required for reference frequency locking and probing, even a compact atomic clock ranges from 150cm
³
to 775cm
³
in size and consumes 1.2W-to-l0W of power. Thus, a miniaturized, low power and low cost time standard is required for each LEO satellite. Recent developments in photonics and MEMS processes show the potential to realize low-power and small-volume quantum package atomic clock based on a coherent population trapping (CPT) method [2]. With the reference frequency locking and probing techniques realized by advanced CMOS integrated circuits, it is now possible to manufacture a small form-factor atomic clock. This paper presents a complete ultra-low-power and miniaturized atomic clock (ULPAC) system with a cesium-133 gas cell, vertical-cavity surface-emitting laser (VCSEL), temperature/magnetic controllers inside a quantum package and the driving/controlling circuitry required for complete atomic clock operation. The prototype of ULPAC achieves a long-term Allan deviation of 2.2×10
¹²
at τ =10
⁵
S 15.4cm
³
volume.