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Journal of the Optical Society of America B

Laser frequency locking with subnatural linewidth spectroscopy

Guoqing Yang, Xu‐xing Geng, Qi Yu, Shangqing Liang, Yifu Zhu, Guangming Huang, Gao‐xiang Li

Published Jan 3, 2020• 7 citations
Publisher

Abstract

We demonstrate a simple technique to lock the frequency of a laser to an atomic transition with a new technique of subnatural linewidth spectroscopy. The coupled rubidium system exhibits an enhanced absorption spectrum of the probe beam with a subnatural linewidth. Compared with other techniques with an additional laser, our scheme only utilizes a single laser, which will be locked. The observed spectroscopic linewidth is down to about 3.5 MHz. The laser frequency fluctuation is about 226 kHz after locking on. The demonstrated scheme offers a convenient and low-cost technique for laser frequency stabilization and related precision measurements.

DOI
10.1364/josab.380337

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We demonstrate a simple technique to lock the frequency of a laser to an atomic transition with a new technique of subnatural linewidth spectroscopy. The coupled rubidium system exhibits an enhanced absorption spectrum of the probe beam with a subnatural linewidth. Compared with other techniques with an additional laser, our scheme only utilizes a single laser, which will be locked. The observed spectroscopic linewidth is down to about 3.5 MHz. The laser frequency fluctuation is about 226 kHz after locking on. The demonstrated scheme offers a convenient and low-cost technique for laser frequency stabilization and related precision measurements.
The utilization of atomic or molecular spectroscopy for frequency locking of single-frequency laser to improve laser frequency stability plays an important role in the experimental investigation of optically pumped atomic magnetometers, atomic clocks, laser cooling and trapping of atoms, etc. We have experimentally demonstrated a technique for frequency stabilization of a single-frequency laser employing the bright state spectroscopy (BSS) with a rubidium atomic vapor cell. By utilizing the counter-propagating dual-frequency 795 nm laser beams with mutually orthogonal linear polarization and a frequency difference of 6.834 GHz, which is equal to the hyperfine splitting of rubidium-87 ground state 5S1/2, an absorption-enhanced signal with narrow linewidth at the center of Doppler-broadened transmission spectroscopy is observed when continuous scanning the laser frequency over rubidium-87 D1 transition. This is the so-called BSS. Amplitude of the absorption-enhanced signal in the BSS is much larger compared with the conventional saturation absorption spectroscopy (SAS). The relationship between linewidth and amplitude of the BSS signal and laser beam intensity has been investigated. This high-contrast absorption-enhanced BSS signal has been employed for the laser frequency stabilization. The experimental results show that the frequency stability is 4.4×10−11 with an integration time of 40 s, near one order of magnitude better than that for using the SAS.
The stabilizing and shifting of laser frequency are very important for the interaction between the laser and atoms. The modulation transfer spectroscopy for the 87 Rb atom with D2 line transition F = 2 → F′ = 3 is used for stabilizing and shifting the frequency of the external cavity grating feedback diode laser. The resonant phase modulator with electro—optical effect is used to generate frequency sideband to lock the laser frequency. In the locking scheme, circularly polarized pump- and probe-beams are used. By optimizing the temperature of the vapor, the pump- and probe-beam intensity, the laser linewidth of 280 kHz is obtained. Furthermore, the magnetic field generated by a solenoid is added into the system. Therefore the system can achieve the frequency locking at any point in a range of hundreds of megahertz frequency shifting with very low power loss.
We make a detailed comparison of frequency modulation spectroscopy (FMS) and modulation transfer spectroscopy (MTS) and of their applications in stabilizing laser frequency. In both FMS and MTS, intensity modulations of probe beam are observed in saturated absorption spectroscopy of 87Rb atoms. However, the two modulation profiles appear different due to their distinct origins. Demodulation gives rise to frequency discrimination spectra, which can serve as error signals for frequency locking. The optimal error signal of FMS has a larger linear slope but also a slanted baseline, in contrast to that of MTS. We apply FMS and MTS to stabilize two 780 nm external cavity diode lasers to D2 lines of 87Rb atoms. We measure laser linewidth and stability using beat notes of two independently locked lasers. A laser linewidth as narrow as 12.9 kHz is achieved in FMS locking, compared to 70.8 kHz in MTS locking. A laser stability at the level of 10−12 is reached in both schemes. Nevertheless, FMS locking offers a better short-term stability, whereas MTS locking yields a smaller long-term drift. Our investigation of FMS and MTS would be valuable for making a choice of these spectroscopy techniques for laser stabilization as well as other relevant applications.
We present two techniques to lock a laser frequency on an atomic transition line by using an electromagnetically induced transparency (EIT) signals, which give optical and electric feedback. We employed these methods to the D2 line of Rb87 atoms. Spectral characteristics of subnatural linewidth EIT allow us to improve frequency stability. By optical feedback of EIT signals, we were able to obtain locking bandwidths of 12 MHz, while frequency stability was 5×10−13 at best after 100 s of integration using a frequency modulation technique.
Recently, Narducci et al.1 studied V-configuration three-level atoms and predicted that quantum fluctuations associated with a "strong" optical transition driven by a coherent field can be stabilized through excitation of a coupled "weak" transition. This stabilization is manifest in a narrowing, to subnatural values, of the fluorescence linewidths associated with the strong transition as the weak transition is driven more and more strongly. We have experimentally investigated this problem and provide the first demonstration of the effect. In the experiment, a beam of Ba atoms was intercepted by two copropagating cw laser beams. One of the beams, of Rabi frequency Ω1, was tuned into resonance with the strong, 553.5 nm, 1S0→3P1 transition, while the other beam, of Rabi frequency Ω2, was tuned into resonance with the weak, 791.1 nm, 1S0→3P1 transition. The fluorescence spectrum of the strong transition was recorded for the case when Ω1≈35 MHz and Q2≈45 MHz and, it was found that the spectrum was three-peaked. Most interestingly, the measured linewidth (FWHM) for the central peak was 16 MHz, or 2 MHz less than the 18 MHz natural width of the strong transition even before the substantial instrumental broadening (≈8 MHz) of the peak was deconvolved.
We demonstrate a spectroscopy method that can be applied in an atomic beam, light-pressure-induced spectroscopy (LiPS). A simple pump and probe experiment yields a dispersivelike spectroscopy signal that can be utilized for laser frequency stabilization. The underlying principles are discussed and compared to Monte Carlo simulations. The shape of the dispersive signal is well described by the simulations. The zero crossing point of the signal is redshifted with respect to the atomic resonance by $\ensuremath{\sim}3\text{ }\text{MHz}$ (one-tenth of the natural linewidth) and depends slightly on the laser intensity. The shift of the zero crossing is determined by laser-induced fluorescence and compared with results of Doppler-free saturation spectroscopy. To check the potential of LiPS for frequency stabilization purposes, it was used to frequency-stabilize the laser for 400 min. Frequency fluctuations of only 0.12 MHz were measured.