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Annual Meeting Optical Society of America

Diode-laser pumped Raman-shifted oscillation in cesium vapor

Matt Poelker, Prem Kumar, Philip Hemmer

Published Jan 1, 1992• 0 citations
Publisher

Abstract

We report observing Raman-shifted oscillation within a ring cavity containing cesium vapor and pumped with the output of a single-frequency diode laser tuned near the D2 line. The oscillation is shifted from the diode-laser pump frequency by ≃9 GHz, the hyperfine splitting of the 6S1/2 ground state of cesium. Many different transverse modes of oscillation are observed for slight variations in the cavity alignment. In addition, the oscillation is observed to turn on and off at 60 Hz as a result of the pump-frequency fluctuations associated with the line noise on the diode-laser drive current. The pump-frequency fluctuations (≃10 MHz) are small compared with the free spectral range (175 MHz) of the ring cavity. These characteristics indicate that the Raman oscillation in cesium vapor is narrow-band, similar to the behavior observed in sodium vapor.1–3 As little as 30 mW of pump power is required for oscillation when highly reflective cavity optics are used. We have obtained 0.5 mW of Raman-shifted output with an appropriate output coupler at 65 mW of pump power.

DOI
10.1364/oam.1992.thn5

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Oct 8, 2010•1 citation
We report observing Raman-shifted oscillation within a ring cavity containing cesium vapor and pumped with the output of a single-frequency diode laser tuned near the D2 line. The oscillation is shifted from the diode-laser pump frequency by ≃9 GHz, the hyperfine splitting of the 6S1/2 ground state of cesium. Many different transverse modes of oscillation are observed for slight variations in the cavity alignment. In addition, the oscillation is observed to turn on and off at 60 Hz as a result of the pump-frequency fluctuations associated with the line noise on the diode-laser drive current. The pump-frequency fluctuations (≃10 MHz) are small compared with the free spectral range (175 MHz) of the ring cavity. These characteristics indicate that the Raman oscillation in cesium vapor is narrow-band, similar to the behavior observed in sodium vapor.1–3 As little as 30 mW of pump power is required for oscillation when highly reflective cavity optics are used. We have obtained 0.5 mW of Raman-shifted output with an appropriate output coupler at 65 mW of pump power.
Generation of frequency correlated laser beams is of importance in various applications, in particular, for extending the frequency of the laser Raman microwave clock1 into the millimeter region where acoustooptic methods are not available. We propose and demonstrate an all-optical technique for such a purpose. The technique is based on the recently observed Raman-shifted oscillation2 that occurs when a three-level atomic system of Λ type is optically pumped and placed in a ring cavity. Our initial experiments are performed in sodium vapor pumped by a cw ring dye laser. Raman-shifted oscillation, detuned from the pump laser by 1.772 GHz which is the ground state hyperfine splitting of sodium, occurs in the wings of the Doppler broadened D1 line. This Raman-shifted oscillation is demonstrated to be highly frequency correlated with the pumping laser when the two are heterodyned on a fast photodetector. In our experiments the heterodyne beat signal at 1.772 GHz had a linewidth as low as 17 kHz, whereas the pumping laser has a rms frequency fluctuation of 300 kHz. The beat signal stays narrowband when the pumping laser is unlocked from the reference cavity increasing its rms frequency fluctation to ~10MHz.
When sodium vapor is irradiated with a single-frequency dye laser (pump) tuned in the vicinity of either of the Doppler-broadened D lines and enclosed in an optical cavity, Raman shifted oscillation builds up, which is blue shifted for a blue-detuned pump and red shifted for a red-detuned pump by 1.772 GHz (sodium ground-state hyperfine splitting).1 The gain mechanism responsible for this Raman-laser action can be understood by modeling sodium as a three-level atom. The Raman laser frequency is highly correlated with that of the pump as demonstrated earlier by observing a beat frequency bandwidth between the two lasers of as low as 17 kHz.2 (The pump had rms frequency fluctuation of the order of 10 MHz.) For spectroscopic applications, for example, frequency correlated laser beams are commonly generated using acoustooptic techniques which are difficult to implement beyond the gigahertz region. The Raman laser technique is extendable, in principle, even to the millimeter range. Here we present measurements of the sodium-density and pump-intensity dependence of the Raman-laser output and compare the results with those predicted by the three-level-atom model.
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
By sweeping the frequency of a diode laser beam near the cesium resonance frequency, followed by propagation through a short length of cesium vapor, we obtained periodic peaks with peak-to-average ratios near 5 at pulse repetition rates up to a gigahertz. Light in the wings of a strong atomic absorption line of an alkali metal vapor encounters large group velocity dispersion. A frequency modulation in an off-resonance cw beam passing through the vapor consequently develops an amplitude modulation. For certain combinations of laser and vapor parameters, a sinusoidally frequency modulated beam can be made to develop peaks. We generated pulse trains in a frequency modulated single mode diode laser tuned to the cesium absorption line near 852 nm, which we observed by using a fast p-i-n photodiode and a sampling oscilloscope. The frequency of a diode laser was sinusoidally modulated with a sweep of about a gigahertz and a period of a few nansoseconds by introducing a small modulation into the laser pump current. We observed the formation of sharp peaks with a width of about one-tenth of the modulation period and a peak-to-background ratio of approximately 5.
The spectra of cesium 6P3/2 - 8S1/2 excited-state transition have been obtained using double resonance optical-pumping (DROP) technique in a room-temperature vapor cell, and have shown a much better signal-to-noise ratio (SNR) compared with that using the traditional optical-optical double resonance (OODR) method. Furthermore, the line-width of DROP spectra is obviously narrowed by electromagnetically-induced transparency (EIT) effect in cesium 6S1/2 F=4 - 6P3/2 F'=5 - 8S1/2 F''=4 transitions. Finally, such DROP spectrum of 6P3/2 F'=5 - 8S1/2 F''=4 transition with a high SNR and a narrow line-width is applied into frequency stabilization of a 795 nm external-cavity diode laser, and the residual frequency fluctuation is ~ 600 kHz within 500 s.