Miniaturized module for laser frequency stabilization at 778 nm via rubidium two-photon transition

We present a miniaturized laser frequency-stabilization module based on the 87Rb 5S1/2 → 5D5/2 two-photon transition. A 778 nm external-cavity diode laser excites the atoms in a small vapor cell, and the generated 420 nm fluorescence provides the stabilization for closed-loop feedback. The optical system—including micro-optics, a vapor cell, and a heater—is fully integrated on a 3D-printed nylon substrate with a total volume of 20 mL and a weight of 10.4 g. By optimizing the structure and stabilization scheme, we obtain a high-SNR fluorescence signal and error signal. The stabilized laser reaches fractional frequency stability of 1.1×10−11 at 1 s and 2.89×10−13 at 100 s, improving performance by an order of magnitude over the free-running case.