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Journal of the Optical Society of America B

Phase-conjugation properties of two-photon-pumped backward-stimulated emission I Experimental studies

Guang S. He, Paras N. Prasad

Published Mar 1, 1998• 17 citations
Publisher

Abstract

Optical phase-conjugation properties of the backward-stimulated radiation from a two-photon-pumped frequency-upconversion lasing medium are studied. The gain medium is a new dye solution pumped with ∼11-ns and ∼1064-nm laser pulses; a highly directional and phase-conjugate backward-stimulated emission at ∼616-nm wavelength could be obtained with a net conversion efficiency of ∼10%. The spectral, temporal, and output–input characteristics of this kind of stimulated emission are presented in detail. The phase-conjugation properties of the backward-stimulated emission have been demonstrated by both the far-field (beam-divergence) measurement and the near-field (image-reconstruction) measurement. The wave-front reconstruction behavior is not perfect, especially under a larger aberration influence.

DOI
10.1364/josab.15.001078

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Journal of the Optical Society of America B•Apr 17, 2007•9 citations
Optical phase-conjugation properties of the backward-stimulated radiation from a two-photon-pumped frequency-upconversion lasing medium are studied. The gain medium is a new dye solution pumped with ∼11-ns and ∼1064-nm laser pulses; a highly directional and phase-conjugate backward-stimulated emission at ∼616-nm wavelength could be obtained with a net conversion efficiency of ∼10%. The spectral, temporal, and output–input characteristics of this kind of stimulated emission are presented in detail. The phase-conjugation properties of the backward-stimulated emission have been demonstrated by both the far-field (beam-divergence) measurement and the near-field (image-reconstruction) measurement. The wave-front reconstruction behavior is not perfect, especially under a larger aberration influence.
Optical phase-conjugation properties of the backward stimulated radiation from a one-photon pumped lasing medium have been studied. The gain medium is a dye solution (pyrromethene 597 in ethanol) or the same dye doped poly(methyl methacrylate) (PMMA) rod pumped with /spl sim/7-ns and 532-nm laser pulses. A highly directional and phase-conjugate backward stimulated emission at /spl sim/573 nm wavelength could be obtained with a conversion efficiency of /spl sim/42% by using a 1-cm-long dye solution sample at the concentration level of d/sub 0/=0.000 15 mol/L. The spectral, temporal, output/input, and phase-conjugation properties of this backward stimulated emission are presented in detail. The induced aberration influence, which is much larger than the divergence angle of the input pump beam, can be basically removed by the backward stimulated emission; however, the fidelity of the near-field detail for the backward stimulated emission is relatively poor. A quasi-collinear holographic interaction model and a mathematical analysis are presented to explain the basic experimental results.
Recent experimental results have shown that the highly directional frequency-upconverted backward-stimulated emission from a two-photon-pumped lasing medium possesses the property of optical phase conjugation. To explain this property, a Gabor’s quasi-collinear holographic model is proposed for a two-photon-pumped gain medium. According to this model, the input IR pump beam (after passing through a phase-distorting medium or an aberration plate) can be assumed to consist of two parts: an undisturbed regular portion and a phase-distorted irregular portion. These two portions interfere with each other and create a volume holographic grating within the pumped region inside the gain medium. While an initial regular backward-stimulated emission (as a reading beam) is passing through this holographic grating, a diffracted wave is created and amplified together with the reading beam. A rigorous mathematical analysis shows that under certain conditions the combination of these two parts (the reading beam plus the diffracted wave) of the backward-stimulated emission can be an approximate phase-conjugate field of the total input-pump field.
A frequency-unshifted and backward stimulated scattering can be efficiently generated in one-photon-absorption free but two-photon absorbing materials. Using a number of novel two-photon absorbing dye solutions as the scattering media and nanosecond pulsed laser as the pump beams, a highly directional backward stimulated scattering at the exact pump wavelength can be readily observed once the pump intensity is higher than a certain threshold level. The spectral and spatial structures as well as the temporal behavior and optical phase-conjugation property of this new type of backward stimulated scattering have been experimentally studied. This stimulated scattering phenomenon can be explained by using a model of two-photon-excitation enhanced standing-wave Bragg grating initially formed by the strong forward pump beam and much weaker backward Rayleigh scattering beam; the partial reflection of the pump beam from this grating provides an positive feedback to the initial backward Rayleigh scattering beam without suffering linear attenuation influence. Comparing to other known stimulated (Raman, Brillouin, Rayleigh-wing, and Kerr) scattering effects, the stimulated Rayleigh-Bragg scattering exhibits the advantages of no frequency-shift, low pump threshold, and low spectral linewidth requirement.
Regular and random lasing properties in a multiphoton active and highly scattering dye solution containing polystyrene microspheres of 366-nm diameter have been experimentally studied. Using 100-fs laser pulses of wavelength 790 nm and 1300 nm as the two- and three-photon excitation sources, the frequency-upconversion regular lasing of ~610 nm wavelength can be generated along both the forward and backward directions, characterized by high directionality, higher spectral intensity, and remarkable spectral narrowing. In the meantime, the random lasing can also be observed along all other directions, characterized by only spectral narrowing but with much lower spectral brightness. The spectral brightness ratio between the forward regular lasing and the random lasing is (10410^{4}104-10510^{5}105):1. Comparing to the input pump beam of ~3 mm diameter and divergence angle ofθ0 ≈ 0.9\theta _{0}~\approx ~0.9θ0​ ≈ 0.9mrad, the forward lasing beam size and divergence angle were ~7 mm and ~(3-4)θ0\theta _{0}θ0​, whereas the corresponding parameters were ~5 mm and∼1.5 θ0\sim 1.5~\theta _{0}∼1.5 θ0​for the backward lasing beam, indicating the phase-conjugation property of the latter.
A frequency-unshifted and backward stimulated Rayleigh scattering can be produced in a linearly transparent but two-photon absorbing medium. Using a novel two-photon active dye solution as the nonlinear medium pumped by 532-nm and ~10-ns laser pulses, a highly directional backward stimulated scattering at the pump wavelength can be readily observed. The experimental results on spectral structure, spatial and temporal behaviors, and output/input relationship of this new type of stimulated scattering are presented. To explain the observed phenomenon and its experimental behaviors, a physical model of feedback mechanism provided by a two-photon-excitation enhanced Bragg grating inside the scattering medium is proposed. Comparing to other types of stimulated scattering, the stimulated Rayleigh-Bragg scattering exhibits the advantages of no-frequency shift, low threshold, and low requirement for pump spectral line-width.
We show that the backward-stimulated Rayleigh-Bragg scattering (SRBS) can be efficiently generated in a three-photon absorbing medium. Compared with all other known stimulated (such as Brillouin and Raman) scattering effects, the observed effect exhibits the following three features: (i) no frequency shift, (ii) a lower pump threshold, and (iii) no critical requirement for pump spectral linewidths within a range of Δv¯≤1cm−1. The specific nonlinear scattering medium is a three-photon absorbing chromophore solution (PRL-OT04 in chloroform), pumped by 1064nm laser pulses of nanoseconds duration but with three different spectral linewidths. The mechanism for generating backward SRBS in a three-photon active medium is the formation of a stationary Bragg grating enhanced by three-photon-absorption-associated refractive index changes. A superior optical phase-conjugation property of the backward SRBS beam has been experimentally demonstrated by employing two different optical setups. In both cases, a specially introduced aberration influence of 4-5 mrad can be basically removed by the backward SRBS beam that retains a much smaller beam divergence of ≤0.4 mrad.