Laser spectroscopy stabilizes laser frequency to atomic resonances for cooling, clocks, and interferometry. Modulation Transfer Spectroscopy (MTS), a variant of Saturated Absorption Spectroscopy (SAS), locks a laser by modulating a single beam for precise, stable frequency control.
We characterize MTS on the D₂ lines of ⁸⁵Rb and ⁸⁷Rb using an electro-optic modulator. First, we study power broadening by symmetrically increasing probe and pump intensities. Next, we keep the probe near saturation while increasing pump power to optimize the locking signal [1,2].
Using 20 MHz rather than the conventional 5 MHz places the system in a fast-modulation regime where atoms cannot follow the modulation adiabatically. The MTS signal is dominated by four-wave mixing between the carrier and well-separated sidebands [1,4], which reduces the zero-crossing slope (lower Hz/V sensitivity) and complicates the line shape. Fast modulation can nonetheless improve rejection of low-frequency technical noise, reduce sensitivity to slow system drift, and separate the desired signal from other modulations present in the setup [3,4].
The 20 MHz choice therefore trades slope for noise immunity. We propose controlled power broadening as a practical route to mitigate the complex line-shape effects encountered in fast-modulation MTS [1,3,4].
# References
- D. J. McCarron, S. A. King, S. L. Cornish, Modulation transfer spectroscopy in atomic rubidium, Meas. Sci. Technol. 19, 105601 (2008).
- H.-R. Noh et al., Modulation transfer spectroscopy for 87Rb atoms: theory and experiment, Opt. Express 19, 23444–23452 (2011).
- T. Preuschoff, M. Schlosser, G. Birkl, Optimization strategies for modulation transfer spectroscopy applied to laser stabilization, Opt. Express 26, 24010–24019 (2018).
- E. Jaatinen, Theoretical determination of maximum signal levels obtainable with modulation transfer spectroscopy, Opt. Commun. 120, 91–97 (1995).
