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Journal of the Optical Society of America B

Modulation transfer spectroscopy for D1 transition line of rubidium

Bin Wu, Yin Zhou, Kanxing Weng, Dong Zhu, Zhijie Fu, Bing Cheng, Xiaolong Wang, Qiang Lin

Published Oct 8, 2018• 30 citations
Publisher

Abstract

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.

DOI
10.1364/josab.35.002705

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Optical pumping effect on modulation transfer spectroscopy of the $$D_1$$ line of $$^{85}$$Rb atoms with non-cycling transition

Shabraz Khan, Heung‐Ryoul Noh, Jin‐Tae Kim

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.

Scientific Reports•Mar 11, 2026•0 citations
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.
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.
The modulation transfer spectroscopy (MTS) technique used to stabilize two different transition lines of rubidium, required for laser cooling of atoms, is investigated. The laser source is based on a 1560 nm semiconductor laser diode that is amplified by an erbium doped fiber amplifier (EDFA). The laser output is frequency doubled with a fiber coupled periodically poled Lithium Niobate (PPLN) crystal to 780 nm. To address the weak spectral features of repumping laser, a theoretical study is conducted followed by experimental verification. The 87Rb D2 line F = 1 → F' = 0 transition temperature-enhanced MTS signal is observed, which provides a choice to stabilize the repumping laser frquency on the F = 1 → F ' = 2 transition using frequency shifting with acousto-optic modulators (AOMs). Compared with the orignal MTS signal, this method is shown to be more stable and accurate for laser frequency locking. Laer frequency locking is realized by controlling the driving current of the seed laser using digital PID feedback. After locking, the laser linewidth is 35.36 kHz.
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.
The D_1 line of ^{85}Rb exhibits non-cycling transitions; therefore, only weak modulation transfer spectroscopy (MTS) signals could be obtained \cite{Khan2024}. In this study, however, strong MTS spectra were obtained for the D_1 line via optical pumping (OP) of the D_2 line of 85^{85}85Rb. Four linear parallel (lin \parallel lin) and linear perpendicular (lin\perplin), \sigma^+ - \sigma^+, and \sigma^+ - \sigma^- polarization configurations were investigated using an OP laser with four different polarizations such as linear horizontal (P), linear vertical (S), \sigma^+, and \sigma^- polarizations. Under the linear perpendicular configuration, the MTS signal for the F_g=3 \rightarrow F_e=2 transition of the D_1 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 \sigma^+ \sigma^- configuration in the presence of \sigma^+- or \sigma^--polarized OP beam as an example matched well with experimental observations.
The D_1 line of ^{85}Rb exhibits non-cycling transitions; therefore, only weak modulation transfer spectroscopy (MTS) signals could be obtained \cite{Khan2024}. In this study, however, strong MTS spectra were obtained for the D_1 line via optical pumping (OP) of the D_2 line of 85^{85}85Rb. Four linear parallel (lin \parallel lin) and linear perpendicular (lin\perplin), \sigma^+ - \sigma^+, and \sigma^+ - \sigma^- polarization configurations were investigated using an OP laser with four different polarizations such as linear horizontal (P), linear vertical (S), \sigma^+, and \sigma^- polarizations. Under the linear perpendicular configuration, the MTS signal for the F_g=3 \rightarrow F_e=2 transition of the D_1 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 \sigma^+ \sigma^- configuration in the presence of \sigma^+- or \sigma^--polarized OP beam as an example matched well with experimental observations.
Optical pumping effect on modulation transfer spectroscopy of the D1D_1D1​ line of 85^{85}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.