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Raman Excited Spin Coherences for Turbulence Imaging

M. S. Shahriar, Shaoul Ezekiel

Published Apr 1, 1999• 0 citations

Abstract

The objective of this project was to explore the feasibility of using Raman excited spin coherence for turbulence imaging. We experimentally demonstrated that it is possible to use degenerate four-wave mixing (DFWM) in Na vapor to correct intracavity high-speed turbulence aberrations. The use of sodium vapor as a phase conjugate mirror (PCM) provides gains greater than unity and an ideal response time of about 16 nsec, even with cw laser pump beams. We demonstrated a cw degenerate phase conjugate resonator (PCR) using Na vapor with a very low lasing threshold of 4.5 W/cm2 of pump intensity. This low power threshold is produced by DFWM based on coherent population trapping (CPT) in Na vapor when operating close to the Dl transition. The intracavity turbulence was created by a He jet from a nozzle that was driven at the nozzle's resonance of 18 kHz. In the intracavity turbulence correction experiment, a typical intensity for the forward and reverse DFWM pump beams is 10W/ cm2. This experiment provided a low-threshold PCR involving a double-A CPT interaction that produced a correction factor of 9.6 for the temporal aberrations caused by an 18 kHz intracavity turbulence with a flow rate of 250 m/s.

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The objective of this project was to explore the feasibility of using Raman excited spin coherence for turbulence imaging. We experimentally demonstrated that it is possible to use degenerate four-wave mixing (DFWM) in Na vapor to correct intracavity high-speed turbulence aberrations. The use of sodium vapor as a phase conjugate mirror (PCM) provides gains greater than unity and an ideal response time of about 16 nsec, even with cw laser pump beams. We demonstrated a cw degenerate phase conjugate resonator (PCR) using Na vapor with a very low lasing threshold of 4.5 W/cm2 of pump intensity. This low power threshold is produced by DFWM based on coherent population trapping (CPT) in Na vapor when operating close to the Dl transition. The intracavity turbulence was created by a He jet from a nozzle that was driven at the nozzle's resonance of 18 kHz. In the intracavity turbulence correction experiment, a typical intensity for the forward and reverse DFWM pump beams is 10W/ cm2. This experiment provided a low-threshold PCR involving a double-A CPT interaction that produced a correction factor of 9.6 for the temporal aberrations caused by an 18 kHz intracavity turbulence with a flow rate of 250 m/s.

The work reported here demonstrates for the first time degenerate four wave mixing (DFWM) phase conjugation in cesium (Cs) atomic vapor. The strong, spectrally narrow transition of cesium at 852 nm gives conjugate reflectivities up to 154% with low pump beam powers. Temperature tuning of the DFWM response over a 30 GHz bandwidth is also demonstrated. An improved experimental design to determine the DFWM angular response shows a field-of-view of 20 mrad at the half response points. Self-focusing effects are observed to be similar to those in sodium vapor with an onset of increased conjugate divergence for reflectivities greater than about 25%.

Resonant degenerate 4-wave mixing (DFWM) has proven to be a powerful technique to generate phase conjugate (PC) wave fronts. In this contribution we consider the situation where the nonlinear medium is placed inside a Fabry-Perot cavity (Fig. 1). In this geometry, the intracavity pump intensity and, as a consequence, also the PC emission are expected to exhibit bistable switching /1,2/. With respect to practical applications, intracavity PC might be of interest, e.g., for probing an optical memory element or for Q-switching a resonator with a phase conjugating mirror. Only recently, however, first experimental studies on bistability in intracavity PC have been performed /3,4/.

Energetic, temporal and spectral characteristics of the process of phase conjugation (PC) at intracavity degenerate four wave mixing (DFWM) of frequency selective pulsed e-beam controlled discharge CO laser radiation have been studied both experimentally and theoretically. Maximum PC reflectivity (PCR) on energy comes up to 1.5% being independent of specific input energy. The dependencies of PCR on energy have quite different behavior for various selected spectral lines. An influence of lasing and PC process in adjacent vibrational bands on PCR for the selected spectral line has been experimentally demonstrated, PCR decreasing at switching on adjacent vibrational bands. The numerical model has been formulated to simulate the temporal behavior of PC process on large scale gain (amplitude) and refraction (phase) gratings formed in the active medium of CO laser. The trend in the calculated temporal behavior of PCR is the same as for the experimental result, the theoretical PCR being higher as compared to experimental one. The resonance (amplitude) mechanism of PC process seems to be a prevailing one for DFWM of CO laser radiation.

Degenerate four-wave mixing in sodium vapor is investigated using a flashlamp-pumped dye laser synchronized on a cw dye laser. At low atomic density, the reflectivity versus the pump intensity and the line shapes are in good agreement with theoretical predictions in the case of both plane waves and Gaussian beams. At high atomic density, reflectivities much higher than unity have been observed. The influence of the pump imbalance has been studied as well as the deformation of the wave front of the conjugated beam introduced by the pump beams. In the high-atomic-density case, the theoretical problem is more complex, and our experimental results exhibit only qualitative agreement with theory.

We have measured the conjugation fidelity produced by four-wave mixing in sodium vapor using weak probe beams that were aberrated by simulated atmospheric turbulence. Near-diffraction-limited (1.3 times the diffraction limit) return beams were obtained for both moderate (D/r(0) approximately 10) and strong (D/r(0) approximately 25) simulated turbulence, down to probe-beam pulse energies of 200 pJ. We believe this to be the first systematic study of conjugation fidelity as a function of probe energy.