Devices and method for photo acoustically detecting a gas are disclosed. In one illustrative embodiment, a gas sensor includes an optical cavity defined by one or more optical segments separating at least two mirrors. A photo acoustic cell, configured to receive a gas from the surrounding environment, is provided at least partially within one or more of the optical segments of the optical cavity. One of the at least two mirrors is configured to couple electromagnetic radiation into the optical cavity and to interact with the gas. A detector is acoustically coupled to the photo acoustic cell to detect absorption of the electromagnetic radiation by the gas.