Zero-field spin-noise spectrum of an alkali vapor with strong spin-exchange coupling

We present a theoretical and experimental paper of the zero-field optical spin noise (OSN) spectrum for a thermal state alkali vapor in the strong spin-exchange coupling regime. We show that the OSN spectrum consists of two components corresponding to a positive and a negative hyperfine spin correlation (HSC). We quantify the power ratio of each HSC component to the total OSN as a function of probe's detuning. At some special detunings, the OSN spectrum contains only one kind of HSC. At far detunings, the negative HSC component holds more power than the positive one for alkali isotopes with nuclear-spin-number I > 1. We also briefly discuss the effect of the negative HSC component on spin squeezing experiments.