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Electronics and Communications in Japan (Part II Electronics)

Acceptance angles of four‐wave parametric upconverters

Kojiro Koyanagi, Teruhito Mishima, Ichiro Sakuraba

Published Jan 1, 1986• 0 citations
Publisher

Abstract

Abstract In the four‐wave parametric upconverter based on isotropic alkali‐metal vapor which is considered useful as an infrared detector, the acceptance angle characteristics of the infrared ray have been studied theoretically in the case of angle phase matching (or pump phase matching) in which a phase matching is taken by way of small angular difference of two pumping light beams. It is found that various acceptance angle characteristics such as single‐peak, double‐peak, and multi‐peak characteristics can be obtained by adjustment of two pumping light angles, if the collinear phase mismatch is negative due to anomalous dispersion of the metal vapor. The collinear phase mismatch is defined as the magnitude of wave vector of the sum frequency minus the sum of the magnitudes of the wave vectors of two pumping beams and the infrared beam. A flat and broad acceptance angle is obtained in the case of tangential phase matching. It is also found that an optimum state as an image upconverter with a high conversion efficiency and a large acceptance angle exists in the four‐wave parametric upconverter of the two‐photon resonance excitation type in which a large conversion efficiency has been expected.

DOI
10.1002/ecjb.4420690809

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Abstract In the four‐wave parametric upconverter based on isotropic alkali‐metal vapor which is considered useful as an infrared detector, the acceptance angle characteristics of the infrared ray have been studied theoretically in the case of angle phase matching (or pump phase matching) in which a phase matching is taken by way of small angular difference of two pumping light beams. It is found that various acceptance angle characteristics such as single‐peak, double‐peak, and multi‐peak characteristics can be obtained by adjustment of two pumping light angles, if the collinear phase mismatch is negative due to anomalous dispersion of the metal vapor. The collinear phase mismatch is defined as the magnitude of wave vector of the sum frequency minus the sum of the magnitudes of the wave vectors of two pumping beams and the infrared beam. A flat and broad acceptance angle is obtained in the case of tangential phase matching. It is also found that an optimum state as an image upconverter with a high conversion efficiency and a large acceptance angle exists in the four‐wave parametric upconverter of the two‐photon resonance excitation type in which a large conversion efficiency has been expected.
We present a theoretical and experimental study of IR image upconversion using two-photon resonant optical four-wave mixing in alkali metal vapors. A closed form solution for the upconversion efficiency which includes Doppler broadening of the two-photon transition and a finite bandwidth pump laser is given. Experimental results for two types of image upconverters using Cs are presented. The first upconverts 2.94 μm to 4558 Å with a power efficiency of 32 percent and1.6 \times 10^{4}resolvable spots. The second is IR tunable, but of lower efficiency and resolution. Finally, the optimized characteristics of the Cs upconverter are compared to crystal upconverters and to the requirements of various applications.
A systematic analysis was made of the spectral characteristics of resonance four-photon parametric conversion of infrared radiation as a result of two-photon resonance pumping of the 3S–3D and 3S–5S transitions in sodium and the influence of these characteristics on the threshold sensitivity of a parametric conversion detector was investigated. An experimental study was made of the characteristics of the noise radiation generated as a result of hyperparametric scattering. The results obtained can be used to select the optimal parameters of high-sensitivity detectors of weak infrared signals by parametric conversion in alkali metal vapors.
A study was made of the dependences of the efficiency of resonant parametric frequency conversion in sodium vapor on the infrared radiation wavelength, vapor density, and detuning from two-photon pump resonance. It was found in this conversion system that for each infrared wave the conversion efficiency had three maxima on tuning the frequency of two-photon pumping over the region of a forbidden transition. The principal maximum was realized for accurate two-photon resonance, and the two other maxima resulted from a rise in the cubic susceptibility close to single-photon sum-frequency resonances. A quantum efficiency of 35% was achieved for a 9.26 μ infrared wave.
Four-photon parametric up-conversion of a weak infrared signal under conditions of two-photon resonance of a constant nonmonochromatic pump field is analyzed allowing for population migration, transient response of the medium, Stark shift of the levels, and width of the pump frequency spectrum. It is found that under steady-state conversion conditions in a nonmonochromatic pump field, it is possible to achieve an average conversion efficiency differing only negligibly from the maximum conversion efficiency in a monochromatic pump field. An analysis is made of the influence of optically induced relaxation on the average conversion efficiency in the transient regime. Numerical estimates are made of the transition parameters for an alkali metal vapor in a frequency-degenerate pump field exciting a two-photon transition.
Abstract A new method is proposed for measurement of the index of refraction of gas and oscillator strength. In four‐wave sum‐mixing in such nonlinear media as metal vapor or rare gas using the third‐order nonlinear optical effect, the refractive index of a nonlinear medium at the sum frequency in the vacuum ultraviolet range is found from the angle phase‐matching condition for the maximum conversion efficiency, and the oscillator strength is determined from the measurement of refractive index by the phase matching in terms of the sum frequency. The present method in which the refractive index and the oscillator strength are determined from the incident angle of each wave satisfying the angle phase‐matching has the advantage of fewer measurement parameters compared to the conventional methods in which a phase‐matching is accomplished with a mixture of gases with ordinary and extraordinary dispersions. This is because no buffer gas is mixed. In addition, the measurement accuracy is found theoretically to improve in the region with a higher atomic density.