Iodine-stabilized VECSEL at 689 nm with 3 × 10^-13 stability

We report frequency stabilization of a 689 nm AlGaInP-based vertical-external-cavity surface-emitting laser (VECSEL) directly and exclusively to molecular iodine. Sub-Doppler saturation spectroscopy of iodine around the 689 nm wavelength region was performed to identify suitable absorption features for laser locking. Four spectral features were identified within ±2 GHz of the absolute frequency of the ¹S₀ → ³P₁ transition of neutral strontium-88 atoms (434.829 THz). The VECSEL was subsequently directly locked to a transition at +1.77 GHz relative to the ¹S₀ → ³P₁ target. The resulting iodine-locked system exhibited a linewidth of 6.2(1) kHz (4 s averaging time) as determined via beat note measurement to a reference VECSEL (integrated linewidth of ∼400 Hz). Using the Allan deviation, minimum instability of 2.7 x 10^-13 at 4 s averaging time was identified. Over a 20-minute period, the oscillation frequency of the iodine-locked system was observed to drift by only 18 kHz, compared with 88 kHz when using the conventional technique of locking to a reference cavity (finesse = 1k). By stabilizing exclusively to molecular iodine - a species compatible with compact glass-blown or micro-machined cell embodiments and usage at room temperatures - the VECSEL offers potentially significant reductions in complexity for quantum technology applications reliant on addressing narrow transitions of neutral Sr atoms. Enhancement of spectroscopy signals presents future opportunities to lock more closely to the ¹S₀ → ³P₁ target, within the range of a standard electronic modulator.