CMOS Saturation Spectrometer With a Miniaturized Fabry–Pérot Cavity for the Lamb-Dip Interrogation of OCS Molecules to Enhance the <i>Q</i> of CSMC

Chip-scale molecular clock (CSMC), as a portable, “atomic”-grade time standard, locks to the sub-THz rotational spectra of the polar gaseous molecules (e.g., carbonyl sulfide, OCS). It features a low cost, a short start-up time, and environmental robustness. The quality factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(Q)$</tex-math> </inline-formula> of the unsaturated rotational spectra of OCS (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q = 10^{5} \sim 10^{6}$</tex-math> </inline-formula>) is lower than that of the coherent population trapping (CPT) in alkaline atoms (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q = 10^{6} \sim 10^{7}$</tex-math> </inline-formula>) used in chip-scale atomic clocks (CSACs). Thanks to its higher spectra detection signal-to-noise ratio (SNR), the CSMCs demonstrate highly competitive short-to-medium term stability (averaging time <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau \lt 10^{3}$</tex-math> </inline-formula> s), expressed as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma _{y}~\propto $</tex-math> </inline-formula> (SNR<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $</tex-math> </inline-formula>Q<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot \sqrt {\tau }$</tex-math> </inline-formula>)<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>, compared to CSACs. Nevertheless, its long-term stability is limited by spectral line broadening and baseline drift, which can be mitigated by enhancing the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i>-factor. In this article, a CMOS saturation spectrometer integrated with a miniaturized Fabry-Pérot (FP) cavity is presented. The FP cavity, featuring pinhole coupling and a compact volume of ~0.36 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>, achieves a measured cavity <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.9 \times 10^{4}$</tex-math> </inline-formula>, generating sufficiently strong electromagnetic (EM) fields. By selectively “burning out” zero-velocity OCS molecules at a pressure of 2.5 Pa within the FP cavity, the 231.061-GHz sub-Doppler Lamb-dip spectra of OCS demonstrates a measured spectral line <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.9 \times 10^{5}$</tex-math> </inline-formula> using a laboratory-scale spectrometer, achieving a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.9\times Q$</tex-math> </inline-formula> enhancement compared to the unsaturated case. The CMOS spectrometer chipset integrates a 216–233-GHz fractional-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> transmitter (TX) with digital wavelength modulation, a heterodyne receiver (RX) with a local oscillator (LO) leakage-suppressed subharmonic mixer (SHM), and an inductor-less analog baseband with an LO synthesizer. At a pressure of 7.5 Pa, the CMOS spectrometer generates Lamb-dip spectra with a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i>-factor of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$= 3.55 \times 10^{5}$</tex-math> </inline-formula>, which is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.1\times $</tex-math> </inline-formula> higher than the unsaturated spectral line. With an RX noise equivalent power (NEP) of 263 fW/Hz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sup>, it achieves a measured SNR of 42 dB.