Tunable extended-cavity diode laser stabilized on iodine at lambda = 633 nm

We present a tunable extended-cavity semiconductor laser, based on the Littman configuration, which has been frequency-stabilized to Doppler-free hyperfine transitions in I2. The stability was measured compared with the reference He-Ne-I2 laser system, whereas the semiconductor laser was locked on components of the P(33) 6-3 transition close enough to the reference R(127) 11-5 line to allow beat frequency counting. A relative stability of 4 x 10(-12) over a 100-s integration time was achieved. The laser configuration allowed mode-hop-free tuning over a range including the P(33) 6-3 transition and the group of strong overlapping transitions R(60) 8-4, R(125) 9-4, and P(54) 8-4 with higher signal-to-noise ratio than the P(33) 6-3 located approximately 13 GHz toward lower optical frequencies.