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Annual Meeting Optical Society of America

Study of frequency-modulation effects in a laser-pumped rubidium maser

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

Published Jan 1, 1993• 0 citations

Abstract

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.

DOI: 10.1364/oam.1993.thdd.13

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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.

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}

.

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.

The phenomenon of coherent microwave stimulated emission in a cavity is analyzed for the case of alkali atoms such as Cs and Rb under coherent population trapping (CPT) in a \ensuremath{\Lambda} scheme. The calculations are made in transient and in continuous operation within the framework of a three level model. The coupling of the atoms to the microwave field generated inside the cavity by the atomic ensemble, is fully taken into account; it introduces radiation damping and an imbalance in the populations of the ground-state hyperfine levels, decreasing the intrinsic symmetry of the \ensuremath{\Lambda} scheme. The effect leads also to a broadening and to a frequency shift of the microwave emission profile. A comparison is made with the optically pumped alkali atom maser, allowing a deeper understanding of the CPT stimulated emission phenomenon.

We calculated the oscillation power and total linewidth of the diode-laser-pumped Rb87 maser with spatial variations of the pumping rate and the radiative microwave field in the resonance cell. It is found that the maximum oscillation power P max corresponding to the optimum pumping rate Γ opt does not increase monotonically with the atomic density n 0 and that the short-term frequency stability of the maser corresponding to P max is about a factor of three better than that currently obtained with a lamp-pumped Rb87 maser. Some results are in fair agreement with our previous experiments. We also examined the variation of the oscillation powers vs light intensity in an average mode for three laser-pumping schemes with different frequencies and polarizations of the diode lasers.

The linewidth and frequency associated with the ground state hyperfine energy fo Rb 87 has been studied. A stable single mode semiconductor laser is used to optically pump Rb, in order to obtain a highly collimated, monochromatic light source. The laser is stabilized by locking its optical frequency to the Rb absorption line center. The dependence of the hyperfine frequency and resonance linewidth have been measured as a function of temperature, light intensity and microwave power; and a theoretical description of the linewidth vs. temperature data is presented. Keywords: Optical pumping; Rubidium; Frequency standard; Frequency shift; Light shift; Density matrix; Buffer gas; Linewidth; and Magnetic resonance.

The /sup 87/Rb cell frequency standard, optically pumped by a laser diode, is of great interest in the development of high short-term stability oscillators. We report experiments performed on a rubidium cell prototype aiming at evaluate the transfer of the laser noise to the hyperfine clock transition via the light shift effect, which appears to be the main contribution to the observed short term frequency stability in this kind of standard. Pulsed optical pumping will also be examined in order to strongly reduce the light shift effect and to achieve the theoretical limit never approached up to now despite the strong detected microwave signals.