In a spin-exchange relaxation-free (SERF) atomic magnetometer, the pump and probe laser frequencies are tuned to the D 1 transition line of alkali metal atoms to enhance measurement sensitivity. The D 1 line, a non-cycling transition, exhibits a low absorption coefficient. This study theoretically analyzes the influence of different parameters on the absorption coefficient of the D 1 line and proposes a polarization-dependent approach to enhance it. A frequency stabilization method based on modulation transfer spectroscopy (MTS) with an enhanced absorption coefficient is introduced. Compared with the conventional method, optimizing the probe beam’s polarization increases the absorption coefficient, raising the amplitude of the MTS signal. The results show that using a stronger MTS signal for frequency stabilization reduces the laser's frequency noise. Using heterodyne measurement, we characterize the linewidth-narrowing effect of MTS locking and evaluate the frequency stability of the locked laser. The linewidth of each laser is reduced from the free-running 2.2 MHz to 885 kHz after MTS stabilization. The Allan deviation measurements of the beat signal between two locked lasers are 2.6 × 10 −11 / τ . This work provides a stable optical frequency reference for SERF atomic magnetometers and offers potential applications in extracting spatial information from optically pumped atoms.
In a spin-exchange relaxation-free (SERF) atomic magnetometer, the pump and probe laser frequencies are tuned to the D 1 transition line of alkali metal atoms to enhance measurement sensitivity. The D 1 line, a non-cycling transition, exhibits a low absorption coefficient. This study theoretically analyzes the influence of different parameters on the absorption coefficient of the D 1 line and proposes a polarization-dependent approach to enhance it. A frequency stabilization method based on modulation transfer spectroscopy (MTS) with an enhanced absorption coefficient is introduced. Compared with the conventional method, optimizing the probe beam’s polarization increases the absorption coefficient, raising the amplitude of the MTS signal. The results show that using a stronger MTS signal for frequency stabilization reduces the laser's frequency noise. Using heterodyne measurement, we characterize the linewidth-narrowing effect of MTS locking and evaluate the frequency stability of the locked laser. The linewidth of each laser is reduced from the free-running 2.2 MHz to 885 kHz after MTS stabilization. The Allan deviation measurements of the beat signal between two locked lasers are 2.6 × 10 −11 / τ . This work provides a stable optical frequency reference for SERF atomic magnetometers and offers potential applications in extracting spatial information from optically pumped atoms.
Modulation transfer spectroscopy (MTS) is widely used in an optical laboratory for laser frequency stabilization. There have been many theoretical and experimental papers on this spectroscopic method. However, most of the works focus on the cycling transitions like the D2 line of rubidium. In this paper, the MTS for noncycling transitions is investigated. We consider the D1 line of rubidium whose modulation transfer spectrum has been determined and compared to the polarization spectrum. It is found that the slope of the modulation transfer signal is sharper than the polarization spectrum. The performance of laser frequency locking based on the MTS method is better than that of the polarization spectroscopy method. The dependence of the signal amplitude on the temperature of the vapor cell was analyzed, and the high signal-to-noise-ratio spectra are achieved for D1 open transitions. The obtained results provide an effective spectroscopic method for the robust frequency locking of a laser at a wavelength of 795 nm.
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.
Precise laser stabilization is a fundamental aspect of modern metrology, directly influencing the accuracy of fundamental measurements in fields such as high-precision metrology, atomic sensor applications, and quantum technologies. Saturation absorption spectroscopy (SAS) is one of the most effective techniques for achieving high spectral stability, as it enables laser frequencies to be locked to atomic resonances. This technique exploits the nonlinear interaction between a weaker probe beam and a stronger pump beam within a rubidium (Rb) atomic vapor cell. By eliminating Doppler broadening, SAS allows well-defined hyperfine transitions to be resolved, providing a reliable optical frequency reference. This study aims to optimize laser frequency stabilization using the SAS technique by investigating the effects of laser light polarization, atomic vapor cell temperature, and pump-probe power adjustments. Polarization-dependent selection rules influence transition probabilities, thereby affecting spectral contrast and frequency stability. Additionally, atomic density variations, which depend on the vapor cell temperature, influence signal strength and linewidth, necessitating precise temperature control for optimal performance. Furthermore, the pump power must be sufficient to induce saturation without excessive power broadening, while the probe power must remain low enough to prevent spectral line distortion. By optimizing polarization, temperature, and power parameters, the stabilization of the Rb atomic laser can be enhanced, leading to improved reproducibility and consistency in high-precision measurements. SAS-based laser stabilization is particularly crucial for atomic sensor applications and quantum technologies, where highly stable, narrow-linewidth lasers are essential for precision measurements. Applications such as atomic magnetometers, gyroscopes, and inertial navigation systems depend on precisely stabilized lasers for optimal performance. By enhancing the reliability of laser frequency stabilization, this work contributes to the advancement of next-generation atomic sensors with improved sensitivity and stability.
We report laser frequency stabilization with modulation transfer spectroscopy (MTS) on 85 Rb atoms. With both PZT (piezo-electric transducer) slow-loop feedback and current fastloop feedback to the laser head, we get a linewidth narrowing less than 5 kHz simultaneously. Laser injection to a laser diode and frequency beating with another polarization spectroscopy based stabilization setup are also employed to check the narrow linewidth property. With the help of the technique, a linewidth around kHz-level laser is obtained and pave the way for the locking of the lattice laser of ytterbium clock with transfer cavity technique. The setup can be used as a frequency reference for precise frequency control of atomic clock system.
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.
Optical pumping effect on modulation transfer spectroscopy of the D1 line of 85Rb atoms with non-cycling transition
The [Formula: see text] line of [Formula: see text]Rb exhibits non-cycling transitions; therefore, only weak modulation transfer spectroscopy (MTS) signals could be obtained. In this study, however, strong MTS spectra were obtained for the [Formula: see text] line via optical pumping (OP) of the [Formula: see text] line of [Formula: see text]Rb. Four linear parallel (lin ∥ lin) and linear perpendicular (lin⊥lin), [Formula: see text], and [Formula: see text] polarization configurations were investigated using an OP laser with four different polarizations such as linear horizontal (P), linear vertical (S), [Formula: see text], and [Formula: see text] polarizations. Under the linear perpendicular configuration, the MTS signal for the [Formula: see text] transition of the [Formula: see text] line exhibited approximately a 41-fold increase in the signal slope in the presence of a linearly-polarized OP beam, compared with the signal slope obtained without OP. The calculated MTS spectra for the [Formula: see text] configuration in the presence of [Formula: see text]- or [Formula: see text]-polarized OP beam as an example matched well with experimental observations.