Instrumentation for multistep excitation of lithium atoms to Rydberg states

We have developed a diode laser apparatus to excite Li from its ground 2S state, through 2P and 3S, to its Rydberg states with three cw diode lasers operating at λ = 671 nm, 813 nm, and 630–635 nm. A He–Ne laser at λ = 633 is sometimes used in place of the 635-nm diode laser for the last step. The output power of each of these lasers was ~1 mW. We describe our technique of locking the first two lasers on Li resonance lines by obtaining a fluorescent signal from the second decay (3S → 2P) that is normally overpowered by a strong background of fluorescent light from the first decay (2P → 2S). We used two balanced photodiodes to reject the strong fluorescent light without loss of collection efficiency. A rejection ratio as high as 100 has been obtained.