An advantage of some aspects of the invention is to provide a light source that can control its output wavelength and optical output individually. Additionally, another advantage of some aspects of the invention is to provide an atomic oscillator including the above light source. The light source includes an optical oscillation layer having a first reflective layer, an active layer, and a second reflective layer laminated therein in this order; an electrical field absorption layer having a first semiconductor layer, a quantum well layer, and a second semiconductor layer laminated therein in this order; and a heat insulating layer that is disposed between the optical oscillation layer and the electrical field absorption layer and has a lower thermal conductivity than that of the second reflective layer.