The frequency stability of a 1560-nm diode laser, whose second harmonic was locked to 87Rb sub-Doppler lines, was characterized by measuring the beat frequency relative to a 780-nm reference laser that was locked to sub-Doppler lines of another rubidium cell. The square root of the Allan variance reached a minimum value of 7.5 × 10-12 in 1 s, which corresponded to frequency variations of 1.44 kHz for the 1560-nm laser. The frequency reproducibility of the system was ≈1 × 10-9. These values are better than those that can be achieved by locking to Doppler-broadened transitions at the 1550-nm wavelength band.
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