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Applied Sciences

Dark State Magnetometer Based on Enhanced Acousto-Optics Modulator

Dastan Khalid

Published Nov 8, 2022• 1 citation
Publisher

Abstract

We report on detailed experimental setup and the results of an enhanced acousto-optics modulator (AOM) setup for observation of a dark state magnetometer. A Λ-type dark state based on D1 line of 87Rb:F=2→F′=1 Zeeman sub-levels with neon (Ne) buffer gas was created using a single laser and a scanning acousto-optics modulator. The technical challenges in using this method and how to overcome these difficulties are discussed, and we report on the observation of a dark state resonance with linewidth of 168 Hz and a detectable magnetic field of 9 nT. This method offers many advantages, including the creation of mutually coherent beams outside an external cavity diode laser (ECDL), where the beams are equally affected by external perturbations to the ECDL. Only factors related to the AOM dictates the difference between the two beams.

DOI
10.3390/app122211324

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We report on detailed experimental setup and the results of an enhanced acousto-optics modulator (AOM) setup for observation of a dark state magnetometer. A Λ-type dark state based on D1 line of 87Rb:F=2→F′=1 Zeeman sub-levels with neon (Ne) buffer gas was created using a single laser and a scanning acousto-optics modulator. The technical challenges in using this method and how to overcome these difficulties are discussed, and we report on the observation of a dark state resonance with linewidth of 168 Hz and a detectable magnetic field of 9 nT. This method offers many advantages, including the creation of mutually coherent beams outside an external cavity diode laser (ECDL), where the beams are equally affected by external perturbations to the ECDL. Only factors related to the AOM dictates the difference between the two beams.
We present an experimental setup of optimized acousto-optic modulator designed for observing a V-type electromagnetically induced transparency resonance in the D 1 line of rubidium-87, with the full width at half maximum being equal to 2.7 MHz. Possible artefacts and undesirable influences on the signal resulting from this method are discussed in detail.The solutions to such technical challenges as beam steering and intensity fluctuations are found in optimization of optical setup.The reported method offers a number of advantages such a single external-cavity diode laser to create a bichromatic optical field outside the laser, whereby both the coupling and probe beams have identical properties.Moreover, the both beams are equally affected by environmental perturbations to the laser.Subsequently, the influence of mode hopping, beam-power fluctuation and frequency drift on the resonance can be eliminated.This negates a need in complicated and expensive temperature and current stabilizations, as well as in a locking unit.The simplicity and stability of the improved setup make our method suitable for many applications based upon electromagnetically induced transparency, especially those in the field of precision measurements.
We report the first observation of sub-natural-width dark resonances of enhanced transparency on the D2 line of potassium, contained in an optical cell with reduced (8mm) dimensions and buffered by 30Torr of Ne. The Potassium vapours are irradiated by a frequency modulated DFB diode laser light (with λ = 766.7nm and 2MHz bandwidth), and are placed in orthogonal to the light magnetic field scanned around zero value. Very good signal-to-noise ratio and narrow (∼20kHz) dark resonances are observed and studied in unshielded laboratory environment. The dark resonance formation is related to coherent superposition of the ground-state Zeeman sublevels by respective components of the frequency-modulated light.
Magnetic field and its gradient measurements based on coherent population trapping resonances at the D1 line of 87 Rb are performed by means of coupling two ground-state Zeeman sublevels belonging to different hyperfine levels to a common excited state.Two coherent laser fields with frequency difference of 6.8 GHz are used.They are produced by direct current modulation of a diode laser.The resonance splitting and broadening caused by the applied magnetic field is measured by scanning the laser modulation frequency around the ground-state hyperfine frequency.
The results of frequency-modulation (FM) spectroscopy of coherent dark resonances from the Zeeman sublevels of the transition F=2 ↔ F=1 of D1 line in absorption of 87Rb atoms are presented and discussed in detail. By contrast with the conventional spectroscopy of coherent dark resonances employing two laser beams, relative frequency of which can be varied, these data has been obtained with the help of a single frequency-modulated laser field. Variation of the modulation frequency plays then similar role with variation the relative frequency in conventional spectroscopy. Experimental data are fit to the theoretical calculations, which are based on the theory of FM spectroscopy of coherent dark resonances recently developed by us. Feasibility of using such experimental technique for accurate measurements of magnetic fields is also discussed.
24 In recent years interest has been growing into the fascinating properties of dark resonances. Most of the experiments have been carried out in alkali atoms due to their favourable level structure and the commercially available diode lasers, resulting in numerous applications. Teams from Siena and Sofia developed an all-optical magnetometer based on such dark resonances. The dark-state effect was discovered in 1976 by G. Alzetta, A. Gozzini, L. Moi and G. Orriols [1], while they were working on optical-pumping experiments with a multimode dye laser. They wanted to make the observation of multi-photon radio-frequency resonances in sodium atoms easy, i.e., observable by the naked eye. To this end, they introduced a magnetic field (MF) that was constant in time but spatially inhomogeneous along the laser beam, so as to produce Zeeman splitting of groundstate hyperfine levels (Fig.1a, left). In this way, an applied radio-frequency field was resonant only if it matched the Zeeman splitting. As a result, a bright radio-frequency resonance appeared (Fig.1a, right). During these experiments, besides the bright resonances, a new “dark” resonance was observed, which does not require any radio-frequency field. In the introductory illustration only the dark resonance is shown. The new dark state (DS) effect appears in a position along the laser beam where the frequency difference (ω10-ω20) between two optical frequencies is equal to the frequency splitting ω12 between two Zeeman sublevels (Fig.1b, left). This is the resonant condition for the stimulated Raman transitions between the two longliving ground levels. Theoretical studies described the dark resonance as a coherent superposition of two long-living ground levels prepared by the bi-chromatic laser field, and ‘atomic population trapping’ was introduced, leading to the term Coherent Population Trapping (CPT) [2]. l Luigi Moi1, Stefka Cartaleva2 DOI: 10.1051/epn/2012603
We have set up a coherent population trapping (CPT)-based magnetometer prototype with the D1 line of ⁸⁷Rb atoms. The dichromatic light field is derived from a fiber electro-optic modulator (FEOM) connected to an external cavity laser diode. A CPT resonance signal with a 516 Hz linewidth is observed. By feeding back the derivative of the resonance curve to the FEOM with a proportional integral controller, of which the voltage output is directly converted to the measured magnetic field intensity, the resonance peak is locked to the environmental magnetic field. The measurement data we have achieved are well matched with the data measured by a commercial fluxgate magnetometer within 2 nT, and the sensitivity is better than 8 pT/√Hz in a parallel B field.