Photonic Generation of Millimeter-Waves Disciplined by Molecular Rotational Spectroscopy

TL;DR

This paper demonstrates a novel method for generating and stabilizing millimeter-wave signals by locking optical heterodyne sources to molecular rotational transitions, achieving high phase noise performance and long-term stability.

Contribution

It introduces a technique to stabilize millimeter-wave oscillators using molecular spectroscopy, combining optical heterodyne generation with molecular reference locking.

Findings

Achieved millimeter-wave generation with low phase noise.

Demonstrated long-term frequency stability of 4×10⁻¹² at 10,000 s.

Locked the oscillator to a gaseous N₂O rotational transition.

Abstract

Optical generation of millimeter-waves (mm-wave) is made possible by an optical heterodyne of two diode lasers on a uni-traveling-carrier photodiode (UTC-PD). We utilized this technique to produce a mm-wave oscillator with desirable phase-noise characteristics, which were inherited from a pair of narrow-linewidth diode lasers. We present the long-term stabilization of our oscillator, achieved by referencing it to a rotational transition of gaseous nitrous oxide (N2O). Direct frequency modulation spectroscopy at 301.442 GHz (J=11) generated an error signal that disciplined the frequency difference between the diode lasers and thus, locked the millimeter-wave radiation to the molecular rotational line. The mm-wave frequency was down-converted using an electro-optic (EO) comb, and recorded by a frequency counter referenced to a Rubidium (Rb) clock. This resulted in short-term fractional frequency stability of $1.5 \times 10^{-11}/\sqrt{\tau}$ and a long term-stability of $4\times 10^{-12}$ at 10,000 s averaging time.