We present a tunable extended-cavity semiconductor laser (ECL) system based on the Littman configuration emitting in the visible region of spectra with the wavelength close to the 633 nm of He-Ne lasers. It has been frequency stabilized to Doppler free hyperfine transitions in molecular iodine. The stability was measured compared to the reference He-Ne-I_2 laser system, the present most commonly used laser primary standard, while 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 arrange a beat frequency counting. A relative stability of 4 X 10~(-12) over a 100 s integration time was achieved. The laser configuration allowed a mode-hop free tuning over a range including a group of strong overlapping transitions R(60) 8-4, R(125) 9-4 and P(54) 8-4 with higher signal-to-noise ratio.
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