Roland Lammegger

Quantitative study of the destructive quantum-interference effect on coherent population trapping

1 line due to the interaction with linearly polarized laser light linlin CPT. In this configuration, the coherence is strongly influenced by the structure of the excited state; consequently, the quantum interference between dark states is an essential feature of this interaction scheme. We study the linlin CPT resonance as a function of the laser optical detuning. The comparison between experimental and theoretical results allows us to quantify the contribution from different dark states to the total signal. Based on these results, we investigate the signal depending on both the pressure broadening of the optical transition and the laser linewidth, and we find in which conditions the laser linewidth does not degrade the linlin CPT resonance.

Enable the inherent omni-directionality of an absolute coupled dark state magnetometer for e.g. scientific space applications

The Coupled Dark State Magnetometer (CDSM) is an optically-pumped absolute scalar magnetometer which is based on two-photon spectroscopy of free alkali atoms. Several magnetic field dependent resonances occur in the presence of an external magnetic field, reaching their maximal strengths at different angles between the magnetic field direction and the reference axis of the sensor defined by the optical path of the laser excitation field. Dependent on this angle the strongest available resonances are selected. This enables an omni-directional dead zone free measurement. The paper discusses the measurement principle, describes the technical realization of the resonance switching, investigates the influence of this transition on the magnetic field measurement and proposes a method to detect the angle of the magnetic field direction in relation to the optical axis. The results presented within this paper show that dead zone free magnetic field measurements are possible with the CDSM without the need of a complex sensor design, e.g. additional moving parts or excitation coils.

Control loops for a Coupled Dark State Magnetometer

The Coupled Dark State Magnetometer (CDSM) is a new type of scalar magnetometer, which is based on two-photon spectroscopy of free alkali atoms. CDSM measurements do not depend on the sensor temperature. Furthermore, it has no dead zones, no moving parts and no excitation coils at the sensor. This paper deals with the control loops which are developed for tracking the magnetic field dependent resonances and for avoiding drifts of the carrier frequency of the laser diode used as excitation light source. It is the first time that a magnetometer of this type is operated in a control loop. The implementation of several functional blocks in a Field Programmable Gate Array (FPGA) is a first step towards miniaturization for a future space application. The magnetic field magnitude is measured by a frequency. Therefore, the Direct Digital Synthesis (DDS) principle is used for accurate signal generation. A noise floor measurement has shown no 1/f dependence near DC and 70pT/√Hz up to the corner frequency of 3Hz for which the CDSM is designed.

Influence of laser sources with different spectral properties on the performance of vapor cell atomic clocks based on lin‖lin CPT

We evaluate the influence of 2 types of laser sources with different spectral profiles on the performance of vapor cell atomic clocks based on lin||lin coherent population trapping (CPT) resonances. We show that a short-term stability of 1-2 · 10-11¿-1/2 may be reached in a compact system using a modulated vertical cavity surface-emitting laser. Here the stability is limited by the detection noise level and can be improved up to a factor of 4 by increasing the lock-in detection frequency to several tens of kilohertz, which is not possible in standard double resonance atomic clocks. We compare these results with CPT prepared under the same experimental conditions, using 2 phase-locked extended cavity diode lasers, with which we predict a challenging short-term stability of 1-3·10-13¿-1/2, comparable to the state-of-the-art laser-pumped Rb-clocks.

Study of the laser linewidth influence on "lin || lin" coherent population trapping

We investigated, theoretically and experimentally, the influence of the laser spectrum (in particular the linewidth) on the coherent population trapping (CPT) resonance in 87Rb buffer gas cell. We investigated the case of linearly polarized laser fields. The experiments have been performed by using either a current-modulated vertical cavity surface emitting laser (VCSEL) or a phase-locked-laser (PLL) system. A model based on the density matrix approach has been used for the data interpretation. We found that, under experimental conditions suitable for compact clock applications, the fact that VCSELs have a broader spectrum than PLLs does not influence significantly the CPT signal amplitude. We demonstrate that a not negligible role is played by the destructive interference between different CPT excitation channels and propose further investigations to evaluate this contribution with respect to the other causes. Finally, we discuss the application of this scheme in compact atomic clocks.

Long-term vacuum tests of single-mode vertical cavity surface emitting laser diodes used for a scalar magnetometer

Scalar magnetometers measure the magnitude of the magnetic field, while vector magnetometers (mostly fluxgate magnetometers) produce three-component outputs proportional to the magnitude and the direction of the magnetic field. While scalar magnetometers have a high accuracy, vector magnetometers suffer from parameter drifts and need to be calibrated during flight. In some cases, full science return can only be achieved by a combination of vector and scalar magnetometers.

Lin || lin coherent population trapping and its application for vapor-cell-atomic-clocks

The authors presented investigations on the application of CPT effect as alternative to the traditional optical microwave double resonance in small atomic clocks. An important factor which may limit the performances of CPT-based atomic clocks is the reduced signal contrast as compared to optical pumping. To overcome this limit, several excitation schemes have been proposed in the last years. Particularly, a significant enhancement of the CPT contrast can be obtained when co-propagating laser waves with parallel linear polarization (lin | | lin CPT) are resonant with the excited state Fe=l of alkali atoms.