Objective Because of the important applications of Rb atoms in precision measurement, the linearly polarized, narrow-linewidth, and frequency-stable 780 nm laser source which matches the D2 transition line of Rb atoms has recently begun to attract more attention. The linewidth of the 780 nm laser diode and the 1560 nm laser diode used for the frequency doubling of periodically-poled Lithium Niobate (PPLN) crystal both are in the order of MHz. Although the linewidth of the Ti∶Sapphire laser is as narrow as kHz, it presents its own problems, such as large size, high cost, and difficulty of maintenance and transport, which greatly limit the practical use of high-precision atom interferometers. The 780 nm laser source-realized by the combination of techniques including power amplification of the narrow linewidth 1560 nm fiber by the erbium doped fiber amplifier (EDFA), frequency doubling by the PPLN crystal, and sideband locking, is currently the most promising candidate; however, its maximum power is only 1.2 W, presenting difficulty in meeting the requirements of the above applications. In this paper, a high-power 780 nm laser source prototype of our own design and development, with stable frequency, narrow linewidth, and high linear polarization, is presented. In addition, the cooling, repumping, and atomic interference coherent operation lasers can be simultaneously obtained by this prototype, which is convenient for precise measurement based on super-cold Rb atoms.

Methods The 780 nm laser source uses a single 1560 nm fiber laser with linear polarization, narrow linewidth, and broadband tunable frequency as its seed source. After the 1560 nm laser's power is boosted by the EDFA, the polarization-maintaining fiber coupler divides it into two parts. One part is frequency doubled by the waveguide-type PPLN crystal to stabilize the 1560 nm laser's frequency via saturation absorption spectroscopy; by using sideband-locking technology, the sideband frequency of the 780 nm laser is locked to the hyperfine transition of the ⁸⁵Rb atoms, and a wide range of frequency tuning can therefore be obtained. The other part is used as the signal light of the cladding-pumped erbium-ytterbium co-doped double-clad fiber (EY-DCF) amplifier used for power boost. The ytterbium band amplified spontaneous emission(Yb-ASE)during the amplification process can be effectively suppressed by dual-wavelength auxiliary signal injection technology, which subsequently increases the 1560 nm laser output power of the fiber amplifier. Finally, a high-power 780 nm laser output can be obtained due to the relatively high fundamental frequency optical power.

Results The saturation absorption spectroscopy of the Rb atom (Fig. 2) can be scanned by precisely adjusting the control temperature and PZT voltage of the seed laser. When the +1 order modulation sideband of the 780 nm laser is locked to the ⁸⁵Rb saturated absorption cross-resonant peak F=3→F'=CO3-4, the frequency of the 780 nm laser can be stabilized within 150 kHz (Fig. 3(a)) for a long time. By using sideband-locking technology, the frequency of the 780 nm laser can also be precisely tuned in a tuning range of 1.2 GHz (Fig. 4(b)). The 780 nm laser power generated by the PPLN crystal is as high as 2.25 W, benefiting from the improvement of the fundamental frequency light power. The signal-to-noise ratio of the 780 nm laser is as high as 60 dB (Fig. 3(b)), the linewidth is expected to be less than 80 kHz, and the measured linear polarization degree is as high as 23 dB.

Conclusions A sideband-locked high-power 780 nm laser source prototype designed and developed for Rb atom precision measurement applications is reported. The seed laser is a homemade single 1560 nm linearly polarized DBR fiber laser. Dual-wavelength auxiliary signal injection technology is used to suppress the influence of the Yb-ASE, effectively improving the amplification effect of the cladding-pumped EY-DCF amplifier on the 1560 nm laser and making the 780 nm laser power up to 2.25 W after the 1560 nm laser has its frequency doubled by the PPLN crystal. By locking the sideband of the 780 nm laser to the saturated absorption cross-resonant peak of the ⁸⁵Rb atom, the frequency fluctuation of the 780 nm laser reaches about 150 kHz within 15 minutes, with linewidth less than 80 kHz and linear polarization degree as high as 23 dB. In addition, the frequency of the high-power 780 nm laser can be precisely tuned in a tuning range of 1.2 GHz, so the cooling, repumping, and atomic interference coherent operation lasers can be obtained simultaneously by a single laser source. The 780 nm high-power laser source has been integrated in a 4U standard box, which is convenient for handle and transportation, thus it is highly suitable for precise measurement based on super-cold rubidium atoms.