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摘要
Objective: Compared with the traditional 780 nm saturated absorption spectroscopy, modulation transfer spectroscopy has higher sensitivity and resolution. It is very suitable for laser frequency stabilization. Nowadays, most of the researchers focus their attention on the hyperfine D2 line 52S1/2 (Fg = 2)→52P3/2 (Fe = 3) of Rb87 modulation transfer spectrum frequency stabilization technology. However, there are few reports on the hyperfine D2 line 52S1/2 (Fg=1)→52P3/2 (Fe=0, 1, 2). The main reason is that the saturation absorption peak and error signal amplitude of the latter are much smaller than the former. But the frequency of hyperfine D2 line 52S1/2 (Fg=1)→52P3/2 (Fe=0, 1) cross resonance peak of Rb87, which is suitable for the re-pumping laser needed by cold atomic clock system to cool rubidium atoms, has great research significance. Therefore, it is necessary for us to study it. Methods: By main oscillator power amplifier and efficient frequency doubling of 1560 nm distributed feedback laser with a periodically poled lithium niobate (PPLN) crystal, we have obtained a 780 nm laser about 200 mW. We mix the modulated probe laser with the local signal and get the error signal of modulation transfer spectrum. In order to get better signal-noise ratio (SNR), the signal is amplified and passed by a low-pass-filter. We also change the modulation frequency and modulation depth of electro-optic modulator (EOM) to increase the amplitude of the error signal. The laser frequency is locked to the D2 hyperfine transition 52S1/2(Fg=1)→52P3/2(Fe=0, 1) of Rb87 via proportional-integral-derivative (PID) controller and we get a 20 h laser frequency stability data finally. We change the polarization of probe laser and pump laser by adjusting the optical structure, so as to study the influence of different polarization states on the error signal of modulation transfer spectrum. By changing the absorption length of rubidium cell (25 mm and 50 mm), we study its effect on laser frequency stability. Results and Discussions: We find that the error signal on the hyperfine D2 line 52S1/2 (Fg=1)→ 52P3/2 (Fe=0, 1, 2) of Rb87 can be influenced by different polarization of probe laser and pump laser. Only when the probe laser and pump laser are linearly polarized in the same direction can there be a clear and large single error signal on the D2 hyperfine transition 52S1/2(Fg =1)→52P3/2(Fe=0, 1) of Rb87 (Fig. 3). There will be no locking error on this transition compared with probe laser and pump laser are perpendicular polarized or circular polarization. We finally lock the laser frequency on this transition for 20 h and measure laser frequency is 384234489.5 MHz (the theoretical calculation is 384234490.2 MHz, with a difference of 0.7 MHz). The peak to peak amplitude of frequency fluctuation is 105 kHz in 20 h. The Allan deviation of the frequency stability is 3.7×10-11 when the integration time is 1 s and 4.6×10-12 when the integration time is 10000 s (Fig. 7). The influence of rubidium cell absorption length on frequency stability is studied. We find the amplitude of error signal in 50 mm is higher than that in 25 mm (Fig. 5). The 1 h frequency fluctuation of rubidium cell in 50 mm is 80 kHz, which is less than that in 25 mm (152 kHz). The 1 s frequency stability of rubidium cell in 50 mm is 3.7 × 10-11, which is also less than that in 25 mm (5.4 × 10-11), but 1000 s frequency stability about the same (Fig. 6).The results show that increasing the absorption length of a single rubidium cell will produce a higher SNR, which will be beneficial to the short-term laser frequency stability. However, its effect on the long-term laser frequency stability is not significant. Perhaps the long-term laser frequency stability depends on other factors, such as the temperature control of rubidium cell and EOM. Conclusions: The reason for the difference of modulation transfer spectroscopy (MTS) error signal, which is caused by probe laser and pump laser in different polarization directions, on the hyperfine D2 line 52S1/2 (Fg = 1) → 52P3/2 (Fe= 0, 1, 2) of Rb87 is analyzed theoretically. The influence of the absorption length of rubidium cell on the frequency stability is studied. The laser frequency is locked on the D2 hyperfine transition 52S1/2(Fg =1)→52P3/2(Fe=0, 1) of Rb87 via modulation transfer spectroscopy, The results show that the peak to peak frequency fluctuation is 105 kHz in 20 h. The Allan deviation of the frequency stability is 3.7×10-11 when the integration time is 1 s and 4.6×10-12 when the integration time is 10000 s, which meets the requirement of cold atom platform for cooling atoms.
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