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Annual Meeting Optical Society of America

Optical pumping of an atomic vapor with a frequency-modulated laser

F. Tremblay, A. Michaud, Jean Beaubien, Éric Cauchon

Published Jan 1, 1990• 0 citations

Abstract

Rubidium maser and passive frequency standards performances rely on the efficiency of the optical pumping scheme. When using a semiconductor laser as the pumping source in such experiments, one found that the sharpness of the laser spectrum allows only the excitation of a small class of atomic velocities. The use of a frequency-modulated laser has shown great promise by largely increasing the overall pumping efficiency.1 Broadening of the narrow laser spectrum ensures the contribution of all classes of atomic velocities present in the vapor.2 Different modulation waveforms, amplitudes and frequencies have very important effects in overall optical pumping efficiency. We theoretically evaluate the efficiency of a frequency-modulated laser pumping source by studying deterministic as well as random modulation. Important considerations are obtained regarding the optimal conditions concerning the magnitude and the bandwidth of the modulation signal. Moreover a deterministic modulation must be used to lock the laser frequency by interrogating an atomic cell and synchronously detecting the transmitted intensity. Experiments conducted on a rubidium 87 maser optically pumped with a semiconductor laser confirm the predictions. In fact the oscillation of the maser was only possible with the help of the laser frequency modulation.

DOI: 10.1364/oam.1990.wh3

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Rubidium maser and passive frequency standards performances rely on the efficiency of the optical pumping scheme. When using a semiconductor laser as the pumping source in such experiments, one found that the sharpness of the laser spectrum allows only the excitation of a small class of atomic velocities. The use of a frequency-modulated laser has shown great promise by largely increasing the overall pumping efficiency.1 Broadening of the narrow laser spectrum ensures the contribution of all classes of atomic velocities present in the vapor.2 Different modulation waveforms, amplitudes and frequencies have very important effects in overall optical pumping efficiency. We theoretically evaluate the efficiency of a frequency-modulated laser pumping source by studying deterministic as well as random modulation. Important considerations are obtained regarding the optimal conditions concerning the magnitude and the bandwidth of the modulation signal. Moreover a deterministic modulation must be used to lock the laser frequency by interrogating an atomic cell and synchronously detecting the transmitted intensity. Experiments conducted on a rubidium 87 maser optically pumped with a semiconductor laser confirm the predictions. In fact the oscillation of the maser was only possible with the help of the laser frequency modulation.

Optical pumping of an atomic vapor with low-power lasers often results in low pumping efficiency because of the narrow laser linewidth. Broadening of the laser spectrum by frequency modulation (FM) ensures the contribution of atoms from all velocity classes. We numerically evaluate the difference between the population enhancement for a vapor pumped with a modulated laser and that with a nonmodulated laser. The most efficient modulation scheme among those considered is FM with narrow-band Gaussian noise. The achieved optical pumping enhancement depends on laser linewidth and available power.

We report the operation of a

^{87}

Rb maser in the self-oscillating mode, using a laser diode as the optical pumping source. The maser uses a TE

_{021}

cavity surrounding a cell containing the atoms and 11 Torr of N

_{2}

as the buffer gas. The optical pumping is accomplished by using a commercial laser diode frequency-locked on the linear absorption line of an external rubidium cell. The maser output power is maximized when the laser spectrum is spread by modulating its frequency through the variation of its injection current. The maser output power is also presented for various modulation waveforms and frequencies. We find that the spurious sidebands induced by the modulation can be minimized. Finally, the linear dependence between the laser and maser frequencies is shown. We find that the relative maser frequency pulling from the laser is about

4.6 \times 10^{-3}

.

The laser-pumped 87Rb maser showed many new experimental features.1 A precise understanding of the maser behavior is obtained by considering a simple three-level atom, located inside a microwave cavity, interacting with a frequency-modulated laser. This simple model is based on the density operator formalism within a semi-classical approach. We show that moderately rapid modulation of the laser frequency results in sustained large hyperfine coherence, thus high gain, due to reduced light broadening, even for a high intensity pump. This is optimally achieved with a given modulation depth which is proportional to the light intensity. This last result has also been obtained when considering only the maximization of the population inversion.2 On the other hand the modulation greatly decreases the dependence of the maser frequency upon the laser frequency (through the variation of the light shift), therefore reducing the requirements for the mid- and the long-term frequency stability of the pumping laser. Finally the maser frequency suffers a small modulation, relatively insensitive to the modulation depth, which becomes negligible for sufficiently high modulation frequencies.

The paper presents the realisation of a high-performance vapour-cell atomic frequency standard demonstrator based on laser optical pumping. The development of such devices aims to fully exploit the advantages of using a narrow-band diode laser pump light source instead of the discharge lamps commonly used in Rb gas-cell clocks. These advantages include improved control over the pump light spectrum, higher selectivity of the optical pumping, and increased signal contrast, and thus show the potential for the realisation of Rb atomic clocks with improved short-term frequency stability.

A vapor of alkali-metal atoms in the external cavity of a semiconductor laser, pumped with a time-independent injection current, can cause the laser to self-modulate at the "field-independent 0-0 frequency" of the atoms. Push-pull optical pumping by the modulated light drives most of the atoms into a coherent superposition of the two atomic sublevels with an azimuthal quantum number m=0. The atoms modulate the optical loss of the cavity at the sharply defined 0-0 hyperfine frequency. As in a maser, the system is not driven by an external source of microwaves, but a very stable microwave signal can be recovered from the modulated light or from the modulated voltage drop across the laser diode. Potential applications for this new phenomenon include atomic clocks, the production of long-lived coherent atomic states, and the generation of coherent optical combs.

Utilizing the Zeeman Faraday effect in atomic vapor cells, a novel setup is introduced both for laser intensity stabilization and laser intensity modulation. The method is based on the closed loop control of the polarization rotation angle of the laser light in an atomic vapor cell for adjustment of the laser intensity. Characterizing the implemented setup, it is shown that more than 30 dB attenuation of the optical fluctuation is achieved in the frequency range from DC to 1 kHz. Meanwhile, the laser intensity could be efficiently locked to a modulating voltage signal, which results in amplitude modulation of the laser beam. We believe that this simple method could be effectively used in different atomic physics experiments, including the stabilization of the laser intensity in applications where near resonance frequency sweeping is required or applications that contain optical modulation of lasers in lock-in detection schemes.