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Optics Letters

Optical phase conjugation of weak signals by using degenerate four-wave mixing

Irwin C. Winkler, Robert S. Mandra

Published Apr 15, 1992• 1 citation
Publisher

Abstract

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.

DOI
10.1364/ol.17.000568

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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.
We present a study of vector phase conjugation based on degenerate four-wave mixing in inverted Nd:YAG. The results demonstrate polarization correction for weak probe strength, with near-unity fidelity for polarization compensation of the returning conjugate. As the probe strength is increased, the fidelity of the polarization state of the conjugate is seen to deteriorate to an average value of 92% when the probe is strongly saturating and is several times stronger than the pump beams. We also show that, for strong probe strengths, an individual transmission grating or reflection grating is more efficient than when both gratings are present.
We present the results of a comprehensive investigation of the quantum-noise properties of a continuous-wave phase-conjugate mirror (PCM) formed using backward-four-wave mixing in potassium vapor. We characterize the quantum-noise properties of the PCM as functions of the vapor density, pump detuning from resonance, and relative frequency detuning of the signal beam from the probe beam. We compare the noise measurements of the PCM with the predicted noise of an ideal quantum-noise-limited PCM and determine the value of the minimum signal that can be used to perform phase conjugation with unity signal-to-noise ratio. For the range of vapor densities studied, we find that the PCM operates nearest the quantum-noise limit and that the value of the minimum signal is lowest under conditions in which the reflectivity is maximized. These results demonstrate that it is possible to perform phase conjugation with signals as weak as 14 fW with near-unity reflectivity. Our measurements are in qualitative agreement with the predictions of a quantum theory of phase conjugation via nearly degenerate four-wave mixing in a two-level system.
Four wave mixing (FWM) performance of sodium vapor was investigated in the strong pump regime (I»I at) necessary to achieve good phase conjugate reflectivity, R. Reflectivities >230% were observed using narrowband CW pump powers less than 1 W. Degenerate FWM spectral response was measured with R as a parameter, and shown to depend on self-focusing effects at higher R. The field of view of the sodium FWM was determined under narrowband high R conditions and found to behave as expected, except for nearly collinear geometries. Faith-ful imaging through a severe optical aberration was demonstrated at moderate R, but experimental observations and analysis indicate potential fidelity problems at large R. Reflect-ivity and field of view were also measured for wideband (2 GHz) laser pumping.
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.