Improved laser locking techniques for realizing an enhanced measurement of the helium 2 triplet P fine structure

The atomic fine-structure constant is a fundamental parameter that describes the coupling strength between charged particles and electromagnetic fields. Our group's objective is to measure this constant to an accuracy of 0.5 ppb based on precise measurements of the 23PJ fine-structure splittings of atomic helium. To measure the fine-structure splittings, we utilize an excitation scheme which requires two lasers locked to atomic resonances at wavelengths of 1083 nm and 707 nm. The accuracy of the fine-structure splitting measurement requires both laser frequencies to be stably locked to high precision. To achieve the desired accuracy of these laser locks, we have incorporated a saturated absorption setup in a dichroic atomic vapour laser locking (DAVLL) scheme. Using this locking technique, we have obtained a lock stability of ±40 kHz for the 1083-nm laser and ±140 kHz for the 707-nm laser, with the lasers remaining locked for a time scale of several weeks.