A Terahertz Molecular Clock on CMOS Using High-Harmonic-Order Interrogation of Rotational Transition for Medium-/Long-Term Stability Enhancement

Abstract

Chip-scale molecular clocks (CSMCs) perform frequency stabilization by referencing to the rotational spectra of polar gaseous molecules. With potentially the "atomic" clock grade stability, cm 3 -level volume and <100 mW DC power, CSMCs are highly-attractive for the synchronization of high speed radio access network (RAN), precise positioning and distributed array sensing. However, the medium/long-term stability of CSMCs is hindered by the transmission baseline tilting due to the uneven frequency response of the spectroscopic system and the molecular cell. To enhance the medium/long-term stability, this paper presents a CSMC architecture locking to the high-oddorder dispersion curve of the 231.061 GHz rotational spectral line of carbonyl sulfide (OCS) molecules, which is selected as the clock reference. A monolithic THz transceiver generates a high-precision, wavelength-modulated probing signal. Then, the wave-molecule interaction inside the molecular cell translates the frequency error between the probing signal and the spectral line center to the periodic intensity fluctuation. Finally, the CSMC locks to the 3 rd -order dispersion curve after a phase-sensitive lock-in detection. In addition, a pair of slot array couplers is employed as an effective chip-to-molecular cell interface. It leads to not only a higher SNR, but also a significantly simplified CSMC package. Implemented on a 65 nm CMOS process, the high-order CSMC presents a measured Allan deviation of 4.3×10 −11 under an averaging time of τ =10 3 s, while consuming 70.4 mW DC power.

A 76-Gbit/s 265-GHz CMOS Receiver With WR-3.4 Waveguide Interface

“…The simulated peak |S 21 | was −4 dB. The loss is comparable to that of the chip-to-waveguide coupler presented in [45].…”

Emerging Terahertz Integrated Systems in Silicon

“…8b) of the OCS transition line. The study on this clock prototype shows that its stability is affected by the non-even transmission baseline of the gas cell, which is estimated to be ∼0.1 dB/GHz [82] and leads to a medium/long-term relative frequency drift of 10 −9 . To address this issue, a 2 nd -generation clock locking to the high odd-order (N = 3 in Fig.…”

“…To address this issue, a 2 nd -generation clock locking to the high odd-order (N = 3 in Fig. 8b) curvature of the OCS line is developed [82], as shown in Fig. 11.…”

“…That, however, requires substrate thinning to tens of μm and placement of the chip inside the waveguide, which limits the chip size and other aspects of the package. In [82], a chip-integrated, 2 × 2 slot array structure is presented, which radiates the THz wave through the silicon substrate, and then into a WR-3 metal waveguide opening attached at the backside of the chip. The design delivers ∼5 dB insertion loss, 21% fractional 3-dB bandwidth, and greatly simplifies the complexity of the package.…”

Brillouin laser-driven terahertz oscillator up to 3 THz with femtosecond jitter

“…Applications such as wireless communications and molecular spectroscopy rely on continuous wave (CW) sources with a high degree of monochromaticity, and their performance metrics degrade with increasing source noise. For example, the stability of molecular clocks that use modulation spectroscopy is currently limited by phase noise [11], highlighting the need for sources with lower phase noise.…”