Optimization strategies for modulation transfer spectroscopy applied to laser stabilization

Accurate measurement of the frequency offset of the laser based on electromagnetically induced transparency

We propose a method using electromagnetically induced transparency (EIT) to measure the frequency offset of the laser relative to a cavity's resonance frequency, thereby reducing the laser detuning when preparing Rydberg atoms. Laser reflection by the vapor cell enables observation of two EIT peaks corresponding to the co-propagating and counter-propagating beams, and the peaks' position is related to laser detuning, allowing us to estimate the frequency offset of the probe and coupling lasers. The method reduces the measurement uncertainty compared to directly observing saturated absorption spectroscopy (SAS) and EIT, making it suitable for applications that require strict control over laser detuning.

Magnetic-field enhanced modulation transfer spectroscopy: theory and experiment

We herein present a theoretical and experimental study on magnetic-field enhanced modulation transfer spectroscopy (MTS) for the 5S_1/2 (F = 1) → 5P_3/2 (F' = 0, 1, and 2) transitions of ⁸⁷Rb atoms. The density matrix equations are solved numerically to obtain the MTS spectra and an excellent agreement is found between the experimental and calculated results. In particular, the enhancement of the MTS signal for the F = 1 → F' = 0 transition in the presence of the magnetic field is directly verified based on the comparison of the results calculated by neglecting with those calculated including the Zeeman coherences in the F = 1 ground state. The unexpected behaviors of the F = 1 → F' = 1 transition are also examined.

Digital laser frequency and intensity stabilization based on the STEMlab platform (originally Red Pitaya)

We report on the development, implementation, and characterization of digital controllers for laser frequency stabilization as well as intensity stabilization and control. Our design is based on the STEMlab (originally Red Pitaya) platform. The presented analog hardware interfaces provide all necessary functionalities for the designated applications and can be integrated in standard 19-in. rack mount units. Printed circuit board layouts are made available as an open-source project (T. Preuschoff et al., https://github.com/TU-Darmstadt-APQ/RedPitaya-Lockbox, 2020 and T. Preuschoff et al., https://github.com/TU-Darmstadt-APQ/RedPitaya-IntStab, 2020). A detailed characterization shows that the bandwidth (1.25 MHz) and the noise performance of the controllers are limited by the STEMlab system and not affected by the supplementary hardware. Frequency stabilization of a diode laser system resulting in a linewidth of 52(1) kHz (FWHM) is demonstrated. Intensity control to the 1 × 10^-3 level with sub-microsecond rise and fall times based on an acousto-optic modulator as actuator is achieved.

Large optical depth frequency modulation spectroscopy

Band-resolved frequency modulation spectroscopy is a common method to measure weak signals of radiative ensembles. When the optical depth of the medium is large, the signal drops exponentially and the technique becomes ineffective. In this situation, we show that a signal can be recovered when a larger modulation index is applied. Noticeably, this signal can be dominated by the natural linewidth of the resonance, regardless of the presence of inhomogeneous line broadening. We implement this technique on a cesium vapor, and then explore its main spectroscopic features. This work opens the road towards measurement of cooperative emission effects in bulk atomic ensemble.