builder:Kohan Chen
The experiment is proposed to become the first demonstration of the absolute power standard. The core technology is to precisely measure the frequency shift of the atomic resonant frequency via the power changes. The ultimate resolution of the atomic spectrum will determine the minimum number of digits of the optical power. Since the quantum efficiency of the power detector is inconsistent with each other, it is hard to define the “international-recognized” optical power. Regarding being a power reference, we believe the cesium atom can be the candidate. The transition frequency of cesium is a digitized value, and the inconsistent quantum efficiency is not a problem for a pure atom. For atomic spectroscopy, cesium has only one electron in the outer shell and has no isotope in nature. For 1079 nm 6S-7S two-photon absorption, there is no linear Zeeman effect. With appropriate care, we could control all the frequency shift effects and pay concentration on the light shift.
In this figure, ECDL1 and ECDL2 are respectively used for frequency scanning and stabilizing. The wavelength of the two lasers is 1079 nm. ECDL2 serves as a laser frequency reference stabilized on Iodine transition lines by frequency modulation spectroscopy. We use a nonlinear crystal to double the frequency from the infrared to the green region. With saturation absorption spectroscopy, we obtain the iodine hyperfine structure with the narrow linewidth from the wide Doppler linewidth. Besides, there is no light shift for one-photon absorption, namely, ECDL2 is a power-insensitive laser system. For increasing the measuring resolution, we narrow the laser linewidth with an ultra-high finesse cavity by the PDH technique.
The ECDL1 frequency refers to the ECDL2’s by offset locking technology which makes the stability of the ECDL1 as same as the ECDL2’s. Besides, the ECDL1 frequency has an offset, generated by a synthesizer, against the ECDL2. With a phase lock loop, we could tune the offset frequency for frequency scanning. This method is proposed to scan the cesium spectrum step by step and derive the spectrum with high precision. The light shift is proportional to the square of the intensity. Thus, we could increase the laser power to increase not only the light effect but the signal-to-noise ratio of the Cs spectrum. For deriving the value of laser power, we build a Fabry-Perot cavity to define the propagation distribution of the laser beam. As long as we have the laser intensity from the Cs spectrum and light-atom cross-section from cavity-enhanced spectroscopy, we could obtain a precise value of laser power!