Abstract: Embodiments herein describe spectroscopy systems that use an unmodulated reference optical signal to mitigate noise, or for other advantages. In one embodiment, the unmodulated reference optical signal is transmitted through the same vapor cell as a modulated pump optical signal. As such, the unmodulated reference optical signal experiences absorption by the vapor, which converts laser phase noise to amplitude noise like the other optical signals passing through the vapor cell. In one embodiment, the unmodulated reference optical signal has an optical path in the gas cell that is offset (or non-crossing) from the optical path of the modulated pump optical signal. The unmodulated reference optical signal allows removal or mitigation of the noise on the other optical signal.