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Journal of the Optical Society of America B

Enhancement of optical pumping efficiency by frequency-modulated lasers

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

Published Sep 1, 1992• 6 citations
Publisher

Abstract

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.

DOI
10.1364/josab.9.001537

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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.
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.
We consider the effects of optical pumping on the conversion of laser-frequency modulation into intensity modulation by an atomic absorption line in a vapor of alkali atoms driven in a ?-configuration. It is found that, due to optical pumping in combination with the excited-state hyperfine structure, the absorption line shape is distorted substantially as the Fourier frequency of the FM is changed. The most significant effect of the distortion is a shift of the apparent line center, which depends on how the frequency of the modulation compares with the optical pumping rate. This shift has implications for locking lasers to atomic transitions and also for FM-AM noise conversion in atomic vapors.
We use a 87Rb^{87}\text{Rb}87Rb atomic vapor, suitable for an optically-pumped magnetometer (OPM) in Earth-field conditions, to study the noise properties of three strategies for generating pulsed optical pumping. We compare a frequency-modulated (FM) laser, amplitude modulation (AM) via an acousto-optic modulator (AOM), and amplitude modulation via a semiconductor optical amplifier (SOA). Pumping the ensemble to operate as a Bell-Bloom OPM, and with an equal degree of spin polarization, the three methods give nearly identical sensitivity, showing that the SOA, despite being an active device, can introduce negligible additional noise. Pumping the ensemble to operate as a free-induction-decay OPM, we observe longer unpumped coherence times with the SOA-AM method than with the FM method. Finally, using the higher power available from the SOA, we demonstrate an environment-limited sensitivity of 80fT/Hz80\text{fT}/\sqrt{\text{Hz}}80fT/Hz​ at 600Hz600\text{Hz}600Hz and 200fT200fT/Hz200\text{fT}/\sqrt{\text{Hz}}200fT/Hz​ at 4kHz4\text{kHz}4kHz, one to two orders of magnitude beyond what was achievable with the other pumping methods.
The stabilizing and shifting of laser frequency are very important for the interaction between the laser and atoms. The modulation transfer spectroscopy for the 87 Rb atom with D2 line transition F = 2 → F′ = 3 is used for stabilizing and shifting the frequency of the external cavity grating feedback diode laser. The resonant phase modulator with electro—optical effect is used to generate frequency sideband to lock the laser frequency. In the locking scheme, circularly polarized pump- and probe-beams are used. By optimizing the temperature of the vapor, the pump- and probe-beam intensity, the laser linewidth of 280 kHz is obtained. Furthermore, the magnetic field generated by a solenoid is added into the system. Therefore the system can achieve the frequency locking at any point in a range of hundreds of megahertz frequency shifting with very low power loss.