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Journal de Physique II

4He optical pumping with intensity modulated laser light

A. Cassimi, Bernard Cheron, J. Hamel

Published Feb 1, 1991• 10 citations
Publisher

Abstract

$^4$He (2 $^3$S$_1$) metastable atoms are optically pumped with intensity modulated light. The use of a single mode diode-pumped LNA laser as the pumping source, allows the study of resonance signals predicted by the optical pumping theory but not yet observed. Resonances are detected on the transmitted pumping beam at the modulation frequency and its harmonics. Application of this method to high sensitivity magnetometers is suggested.

DOI
10.1051/jp2:1991151

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4^44He (2 3^33S1_11​) metastable atoms are optically pumped with intensity modulated light. The use of a single mode diode-pumped LNA laser as the pumping source, allows the study of resonance signals predicted by the optical pumping theory but not yet observed. Resonances are detected on the transmitted pumping beam at the modulation frequency and its harmonics. Application of this method to high sensitivity magnetometers is suggested.
Recently a distributed Bragg reflection (DBR) laser diode was commercialised by SDL (USA) for helium optical pumping experiments. An interesting feature of the new device is the possibility of frequency modulation of the diode output by modulating the current intensity, leading to a new pumping scheme called optical pumping with frequency modulated light. We develop this method to induce spin precession and obtain new resonances in 4He atomic gas, simply by modulating the frequency of the pumping light at the Larmor frequency of atomic states in a static magnetic field. These resonances are analysed with a simple model taking into account velocity changing collisions.
Diode laser optical pumping to produce a highly spin-polarized metastable He beam to be used in a spin-polarized metastable atom deexcitation spectroscopy experiment on magnetized surfaces is described. Efficient pumping of the beam is performed by means of an SDL-6702 distributed Bragg reflector diode laser which yields 50 mW of output power in a single longitudinal mode at 1083 nm, the resonance wavelength for the 23 S→23 P0,1,2 (D0, D1, and D2) transitions of He*. The light is circularly polarized by a quarter-wave plate, allowing easy change of the sense of atomic polarization. The laser frequency can be locked to the atomic transition for several hours by phase-sensitive detection of the saturated absorption signal in a He discharge cell. Any of the three transitions of the triplet system can be pumped with the laser but the maximum level of atomic polarization of 98.5% is found pumping the D2 line.
Optical pumping is an efficient method for creating nuclear polarization M in a gas of 3He. M is first achieved in the metastable 23S state and then transfered to the ground state. Polarized 3He has numerous applications: quantum fluids, precision magnetometry, nuclear targets, etc. Much progress in achieving high values of M has resulted from the development of efficient IR lasers capable of exciting the 23S–23P3He transition at 1.08 μm. Using a novel lamp pumped LNA laser delivering 5 W cw, we have achieved M >85% for samples in the torr pressure range. To measure M we have developed an accurate technique based on the change as a function of M of the ratio between σ and π polarized light absorbed from a weak; probe beam by the 23S metastable atoms. The probe was delivered by a single mode tunable diode pumped LNA laser that was frequency modulated by using an étalon mounted on a galvoplate. M was also measured by monitoring the circular polarization of the 668 nm line emitted by the dis charge. Experimental and theoretical results on the optimization of the pumping will be presented, and the laser systems will be discussed.
We present both a theoretical and experimental study of the magneto-optical spectra induced with intensity-modulated resonant laser light that synchronously pumps and detects the metastable 4He^{4}\mathrm{He}4He atoms in the presence of a quasistatic magnetic field. We extend previous work and derive complete analytical expressions for the resonance signals by taking into account arbitrary magnetic-field directions and arbitrary light polarizations. The analytical results are derived by solving the Liouville equation using the irreducible tensor formalism on the assumption of low light power. We discuss the potential application of the derived analytical results in constructing and optimizing a dead-zone-free all-optical atomic magnetometer.
Introduction Soon after the development of optical magnetometers based on the radio-optical double resonance method (see Chapter 4), it was realized by Bell and Bloom [1] that an alternative method for optical magnetometry was to modulate the light used for optical pumping at a frequency resonant with the Larmor precession of atomic spins. In a Bell-Bloom optical magnetometer, circularly polarized light resonant with an atomic transition propagates through an atomic vapor along a direction transverse to a magnetic field B . Atomic spins immersed in B precess at the Larmor frequency Ω L , and when the light intensity is modulated at Ω m = Ω L , a resonance in the transmitted light intensity is observed. The essential ideas of the Bell–Bloom optical magnetometer are reviewed in Chapter 1 (Section 1.2), and can be summarized in terms of what Bell and Bloom termed optically driven spin precession: in analogy with a driven harmonic oscillator, in a magnetic field B atomic spins precess at a natural frequency equal to Ω L and the light acts as a driving force oscillating at the modulation frequency Ω m . From another point of view, the Bell-Bloom optical magnetometer can be described in terms of synchronous optical pumping: when Ω m = Ω L , there is a “stroboscopic” resonance in which atoms are optically pumped into a spin state stationary in the frame rotating with Ω L . Depending on the details of the atomic structure, the spin state stationary in the rotating frame can be either a dark state that does not interact with the modulated light or a bright state for which the strength of the light–atom interaction is increased.