Absolute frequency control of a 1560 nm (192 THz) DFB laser locked to a rubidium absorption line using a second-harmonic-generated signal

We demonstrate second-harmonic generation from a DFB laser at 1560 nm in a type I critically phase-matched KNbO₃ crystal. We obtain 2.2 nW at 780 mm with 11.3 mW at 1560 nm incident on the crystal. The conversion efficiency is 17.2 pW/(mW)². The 780 nm beam is used to interrogate a resonance of the ⁸⁷Rb-D₂ line at 780 nm and lock the laser frequency. To characterize the absolute frequency stability, two 1560 nm DFB lasers are respectively stabilized on a Doppler resonance of the ⁸⁷Rb-D₂ line (780.246 nm) and of the ⁸⁵Rb-D₂ line (780.244 nm). The square root of the Allan variance measured from the beat note is around 1.5×10^-9 for averaging times between 3 and 100 s. To improve the precision of the frequency locking, we realize a setup to observe a saturated absorption profile. We use a 780 nm stabilized laser as a pump and the SHG signal as a probe. A saturated absorption profile is observed over the Doppler envelope. Work is under progress to use this saturated resonance for an improved frequency control