This paper reports an investigation of parametric frequency conversion in a vapor of
Rb87 atoms in which nearly resonant optical waves are coupled with microwaves at the ground-state hyperfine-structure (hfs) frequency of
Rb87 by the nonlinear susceptibilities of
Rb87 vapor. These nonlinear susceptibilities are greatly enhanced by optical pumping of the ground-state hfs levels of
Rb87 and by using nearly resonant light. The frequency-conversion process generates optical sideband waves which are separated in frequency from the carrier light waves by the ground-state hfs frequency of
Rb87 (6.835 GHz). These sidebands have been directly observed in the spectral profiles of
Rb85 resonance light which had passed through
Rb87 vapor. The conversion efficiency of power from the carrier waves to the sideband waves in a 45-cm length of vapor was greater than 1%. The observed behavior of the sidebands is in excellent qualitative agreement with the predictions of a semiclassical theory.
A scalar magnetic field sensor based on a millimeter-size Rb87 vapor cell is described. The magnetometer uses nearly copropagating pump and probe laser beams, amplitude modulation of the pump beam, and detection through monitoring the polarization rotation of the detuned probe beam. The circularly polarized pump laser resonantly drives a spin precession in the alkali atoms at the Larmor frequency. A modulation signal on the probe laser polarization is detected with a lock-in amplifier. Since the Larmor precession is driven all-optically, potential cross talk between sensors is minimized. And since the pump light is turned off during most of the precession cycle, large offsets of the resonance, typically present in a single-beam Bell–Bloom scheme, are avoided. At the same time, relatively high sensitivities can be reached even in millimeter-size vapor cells: The magnetometer achieves a sensitivity of 1 pT/Hz1/2 in a sensitive volume of 16 mm3, limited by environmental noise. When a gradiometer configuration is used to cancel the environmental noise, the magnetometer sensitivity reaches 300 fT/Hz1/2. We systematically study the dependence of the magnetometer performance on the optical duty cycles of the pump light and find that better performance is achieved with shorter duty cycles, with the highest values measured at 1.25% duty cycle.
Pump-probe experiments are ubiquitous in atomic, molecular, and optical physics, where a strong pump field induces some change in a quantum system that is probed by a much weaker field. Often, high intensity of the pump field is at a premium with all other characteristics of the strong field subordinate. As a particular example, of relevance to atomic clocks and atomic magnetometers, a strong broad-linewidth laser optically pumps an atomic vapor, which is then probed by a much weaker narrow-linewidth laser. Though the broad-linewidth field suffers laser phase noise (PM) to amplitude noise (AM) conversion, the narrow-linewidth probe by itself produces a much quieter signal. Here, we consider the question of whether the noisy pump field maps its (PM-to-AM) absorption cross-section fluctuations onto the quiet probe field's transmission. Our results show that PM-to-AM noise does not transfer directly: atoms interacting with the pump field are instantaneously distinct from those interacting with the probe field. However, the broad-linewidth laser can influence the quiet field's transmission through the vapor due to optical pumping, resulting in fluctuations in the number density of atoms in the absorbing states. Nevertheless, there is a saving grace for this ``PM-to-AM induced optical pumping'' noise transfer: for very high noisy-field intensities (where optical pumping saturates) this type of noise on the probe field becomes negligible.