Reducing PM-to-AM Noise Conversion in Atomic Vapors

Abstract

Laser phase noise (PM) to transmitted intensity noise (AM) conversion is a fundamental aspect of the field/matter interaction, and a major noise process that limits the frequency stability of next-generation atomic clocks. It arises from the absorption cross section’s dependence on laser phase and is especially pernicious for vertical-cavity surface-emitting lasers (VCSELs) given their relatively broad singlemode linewidths (e.g., 40 MHz). This is unfortunate, since VCSELs (with their very low operating power and low probability for mode hops) are otherwise ideal for quantum devices. Here, we demonstrate that atomic spectroscopy with N VCSELs reduces the total PM-to-AM noise by a factor of ~ ${\mathrm {N}}^{\mathrm {1/2}}$ . Consequently, atomic clocks with (for example) five VCSELs would have a signal-to-noise ratio (SNR5) approximately 120% higher than the same device operating with one VCSEL (SNR1) at the same total power: $\mathrm{SNR}_5 / \mathrm{SNR}_1 \cong 2.2$ .