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Optics Letters

Sodium Raman laser: direct measurements of the narrow-band Raman gain

M. Poelker, Prem Kumar

Published Mar 15, 1992• 39 citations
Publisher

Abstract

Using a sodium Raman laser that is pumped by a single-frequency cw dye laser, we have measured the narrow-band Raman gain that exists in the vicinity of the sodium D(1) line. Large gains (>10 cm(-1)) are measured at moderate pump intensities and sodium densities. The high gain allows us to detect directly a 1.772-GHz beat on the pump beam after it makes only a single pass through the sodium cell. By scanning the frequency of the sodium Raman laser while keeping the pump intensity and frequency constant, we are able to map out the Raman gain profile. The narrowness of the Raman gain is consistent with the narrow beat signal between the Raman-laser beam and the pump beam that was reported earlier [Opt. Lett. 16, 1853 (1991)].

DOI
10.1364/ol.17.000399

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Mar 29, 1996•2 citations
Using a sodium Raman laser that is pumped by a single-frequency cw dye laser, we have measured the narrow-band Raman gain that exists in the vicinity of the sodium D(1) line. Large gains (>10 cm(-1)) are measured at moderate pump intensities and sodium densities. The high gain allows us to detect directly a 1.772-GHz beat on the pump beam after it makes only a single pass through the sodium cell. By scanning the frequency of the sodium Raman laser while keeping the pump intensity and frequency constant, we are able to map out the Raman gain profile. The narrowness of the Raman gain is consistent with the narrow beat signal between the Raman-laser beam and the pump beam that was reported earlier [Opt. Lett. 16, 1853 (1991)].
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.
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.
We have used a two-wave mixing gain process in sodium vapor to construct a self-pumped phase conjugator. This was done by forming a resonator around a quartz sodium cell equipped with Brewster angle windows. A pump beam is then directed into the sodium cell at a small angle to the longitudinal axis of the resonator. At sufficiently high cw pump intensities, the ac-Stark effect pro- vides sufficient gain to achieve oscillation. Although this gain is unidirectional it occurs on the wings of the sodium resonance so that the backward traveling optical field in the resonator does not experience loss due to sodium absorption. The counterpropagating optical fields in the resonator interact with the pump in an internal four- wave mixing process to produce a fourth optical field (phase conjugate to the pump field). Our experiment was conducted at the sodium D2 resonance. The pump laser was a 699 autoscan single frequency ring dye laser providing pump intensities up to several hundred watts per square centimeter.
We report a steady-state Raman gain measurement of the Q(1)(0) transition (v = 1 ? 0, J = 0 ? 0) in solid parahydrogen. We carry out measurements by pumping with a continuous-wave frequency-doubled YAG laser at 532 nm and observing the direct amplification of a probe-laser beam for the first Stokes transition at 683 nm. A large single-pass amplification coefficient of 2.3 +/- 0.2 is obtained at a pump intensity of 46 kW/cm(2), with an interaction length of 1 cm, giving a steady-state Raman gain coefficient of 18 +/- 3 cm/MW.
The highly selective Na3 + Br reaction creates a continuous electronic population inversion on Na2 transitions which demonstrates optical gain through stimulated emission in regions close to 527, 492, and 460 nm ((alpha) equals 8 X 10-3 cm-1 for an individual rotational level at approximately 527 nm). With a focus to increasing amplifier gain length and amplifying medium concentration (conversion to oscillator), we extend these initial studies employing the intersection of sodium and bromine flows formed from slit source expansions to create an extended reaction-amplification zone. Initial results with this extended reaction zone provide the first example of chemically induced Raman pumping. Unique Raman signals are induced by and correlate with emission from the NaD-line components (D1, D2) formed in the chemical reaction zone primarily as a result of the Na2 + Br yields Na* (3P) + NaBr reaction. The NaD-line emitters interact with cooled sodium dimers in a resonance Raman scattering process, for which computer simulation of the experimentally observed spectra predict an unusually large scattering linewidth close to 4 cm-1. We describe initial laser based studies on a pure sodium expansion to probe the mechanism of the unusually strong Raman scattering and to understand the origin of the large scattering linewidth. These experiments demonstrate that the Raman process can be induced only upon excitation of the NaD-line components, the Raman scattering being insufficient or nonexistent for wavelength shifts (Delta) (lambda) > 1 nm from the D lines. The scattering linewidth for the reactive process is not readily correlated with the line-broadening of the NaD-line and may result from the nature of the Na- Na2 interaction in the reactive environment. Initial double pass measurements indicate the possibility of optical gain associated with the Raman process.