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European Frequency and Time Forum

FM noise reduction limits of a laser diode frequency stabilized on an atomic vapor

Messaoud J. Bahoura

Published Jan 1, 1996• 0 citations
Publisher

Abstract

We have studied the properties of a noisy laser diode light tuned near the D/sub 2/ line of cesium resonance transition after passing through an atomic-cesium vapour cell. We have found that the transmitted beam acquires excess noise. This noise depends on the laser linewidth, laser detuning, intensity, and Fourier frequency.
DOI
10.1049/cp:19960056

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We report on oscillating complex noise spectra obtained when a diode-laser beam passes through a resonant dense Doppler-broadened cesium-vapor cell. Atomic coherence converts the laser phase noise into amplitude noise in the transmitted beam. We have found that the level of amplitude noise is orders of magnitude above the intrinsic laser noise. As a function of laser detuning, this noise extends over several inhomogeneous widths, depending on the spectral frequency. Numerical calculations based on a simple theory remarkably mimic the details of the experimental noise spectra.
There has been much interest in the influence of the laser noise on the atomic system. So far theoretical works have been performed on calculation of the mean values and the variance of the fluctuated atomic populations, and verified through observation of the fluctuating resonance fluorescence from the atomic vapors. Yabuzaki et al. have observed the excess intensity noise of the resonant light from a diode laser propagated through alkali atomic vapor. They have measured the intensity power spectrum of the light when the laser was tuned near the D2 line of Cs and observed all corresponding hyperfine-splitting resonances in the excited state and Zeeman splitting resonances within a hyperfine level of the ground state. Walser et al. have theoretically studied the intensity fluctuation of laser light propagated through a weakly absorbing medium. The calculated power spectrum of a phase-diffusing field has been found qualitative agreement with the experimental results by Yabuzaki et al. on hyperfine-splitting resonances. Recently, Mitsui has reported a new type of noise spectroscopy with a highly stabilized diode laser. He has investigated the spontaneous noise appeared in optical detection of magnetic resonance of Rb atoms. In this note we report the first observation of hyperfine spectra in the ground state of K and K appearing as the intensity fluctuation at hyperfine splittings of 462MHz and 254MHz of circularly polarized light from a diode laser transmitted through a sample cell. We have also observed the Zeeman hyperfine spectra of K in a static magnetic field. We show that the spectra are strongly dependent of the direction of the static magnetic field. Their splittings as functions of the strength of static magnetic field are compared with the Breit–Rabi formula. The experimental apparatus is quite simple as shown in Fig. 1. The diode laser used was an ordinary Fabry–Perot type with output power of 5mW, operating near the D1 line (769.9 nm) of K atoms. The output of the laser was circularly polarized and applied to a cell containing K vapor at a temperature 90 C, corresponding atomic density to be about 1:15 10 cm 3. The transmitted light was detected by an avalanche photo-diode having frequency response up to 1GHz. The output of the photo-diode was applied to a rf frequency analyzer to obtain the power spectrum of the fluctuation of the transmitted light intensity. A static magnetic field is provided by two pairs of Helmholtz coils mutually perpendicular and about 90 cm in diameter. The strength of the magnetic field is calibrated by using magnetic resonance signals of optically pumped Rb atoms. Figure 2 shows the frequency spectrum of the intensity fluctuation of transmitted light observed when the laser was tuned to the transition from the state F 1⁄4 1 in the ground state to the state F0 1⁄4 1 in the excited state by adjusting the driving current of the laser diode. Sharp resonance signals can be seen at the hyperfine splittings of K and K, 462 and 254MHz, respectively, on the background of broad noise spectrum. When a weak static magnetic field H0 is applied to the K vapor, the hyperfine spectra of K splits into several lines (the effective gyro-magnetic ratio of K is known to be 1⁄4 2 700 kHz/G). Figures 3(a) and 3(b) show the observed Zeeman hyperfine spectra of K for
In order to clarify the mechanism of increased laser relative intensity noise (RIN) in the diode laser pumped gas-cell atomic clock, we have performed experiments examining the influence of laser linewidth, /spl Delta/v/sub L/, on RIN. Our measurements are consistent with the phase noise (PM) to intensity noise (AM) conversion theory: laser phase fluctuations induce fluctuations in the absorption cross section of the atomic vapor and these yield fluctuations in the transmitted intensity of the laser. In the experiments, transmitted RIN of a beam from a 100 kHz linewidth diode laser was compared with that from a 60 MHz linewidth diode laser after each was passed through a cell containing natural rubidium vapor. We present some results of computational theory showing that for laser linewidths less than the atomic dephasing rate of the atomic vapor, RIN is proportional to /spl radic/(/spl Delta/v/sub L/); however, for laser linewidths greater than the atomic dephasing rate RIN is a decreasing function of /spl Delta/v/sub L/.
We have found that an intense shot-noise-limited laser beam tuned near the 4 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​\ensuremath{\leftrightarrows}4 2^{2}2P3/2{\mathit{P}}_{3/2}P3/2​ potassium resonance transition acquires excess noise after passing through an atomic-potassium vapor cell. The noise is maximum for laser detunings of approximately \ifmmode\pm\else\textpm\fi{}1 GHz and falls to nearly the shot-noise limit for detunings greater than \ifmmode\pm\else\textpm\fi{}3 GHz. We describe the production of this noise in terms of a forward four-wave mixing process involving the laser field and its side modes, which are initially in the vacuum state. We present a fully quantum-mechanical theory of forward four-wave mixing in a system of two-level atoms and use it to predict the noise properties of the transmitted laser beam. The predictions of this theory are in good agreement with the experimental data.
In this work we investigate the transmission in an optical cell containing an atomic vapor, when a laser is tuned to the wings of the Doppler line, presenting inhomogeneous broadening. The experiment is designed to detect the transmission of a laser beam through a an atomic cesium vapor within a quartz cell. Due to the high atomic density in the 1 mm cell, the laser beam is totally absorbed at resonance. On the other hand, when the detuning by few GHz from the center of the line an oscillatory signal is observed as a function of the laser frequency. The amplitude of the transmission signal varies linearly with the laser intensity. These oscillations exhibit diferent behavior as we change some parameters of the system, and the rate of the oscillations vary with laser detuning and on the atomic density. The phase of the transmited field is also sensitive to the angle of incidence of the laser beam in the cell, which allows to vary the lineshape of the signal without changing the parameters of the laser (intensity, frequency, ...) or atomic vapor. Out of resonance with the atomic vapor, the parallel windows of commercial optical cell cavity works as a low finesse Fabry-Pérot with a free spectral range of 150 GHz. However, in resonance, the optical path of the cavity changes varies with frequency due to the dispersive response of the atomic vapor. This study experimentally characterized the transmission signal through a cavity containing resonant vapor as a function of intensity and frequency of the laser, as well as its dependence on the angle of incidence of the beam. A numerical model that allows to determine the collisional broadening of the atomic vapor adjusting the spectral curves of the transmission signal was developed. As an application, exploring the oscillations in the wings of the resonance, we can lock the laser frequency at a few GHz off the center of the atomic resonance. The lock was performed in the frequency range between 0,8 GHz and 2,4 GHz from the center of the D2 line of cesium. To characterize the locking stability we measure the optical beat signal between two similar experiments, using the same locking technique.
Summary form only given. Efficient conversion of laser frequency noise to amplitude noise takes place when a laser propagates through a resonant medium. This is demonstrated in the case of a solitary diode laser having the usual frequency or phase noise with a bandwidth of /spl sim/30 MHz but very low amplitude noise, propagating through a Doppler-broadened K vapor cell. The lineshape of the noise as a function of laser frequency near the K resonance has the characteristic M shape of FM spectroscopy. Measurements of the RF noise spectrum of the emergent beam in the case where the diode laser is near the 770 nm resonance of K reveals the ground state hyperfine structure of the K atoms at 460 MHz.