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Physical Review A

Spatial structures in optical parametric amplification

Audrius Berzanskis, Walter Chinaglia, Luigi A. Lugiato, K.‐H. Feller, P. Di Trapani

Published Aug 1, 1999• 24 citations
Publisher

Abstract

We analyze the spatial aspects of the down-converted field emitted by a cavityless optical parametric amplifier, both from theoretical/numerical and experimental viewpoints. Our model neglects pump depletion and includes a classical description of the noise which initiates the process. The ringshaped angular distribution in the far field is determined basically by the phase-matching conditions. In the near field, the configuration arises from the superposition of contributions from a broad frequency band; it is shown that the finite duration of the pump pulse introduces a correlation among neighboring frequencies that allows for the appearance of a spotlike spatial modulation. When the spatial configuration of the pump field is changed from a plane wave to a narrow Gaussian, the near-field pattern assumes the form of a roll structure. The experimental observations obtained with a lithium triborate crystal agree qualitatively well with the theoretical picture.

DOI
10.1103/physreva.60.1626

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We analyze the spatial aspects of the down-converted field emitted by a cavityless optical parametric amplifier, both from theoretical/numerical and experimental viewpoints. Our model neglects pump depletion and includes a classical description of the noise which initiates the process. The ringshaped angular distribution in the far field is determined basically by the phase-matching conditions. In the near field, the configuration arises from the superposition of contributions from a broad frequency band; it is shown that the finite duration of the pump pulse introduces a correlation among neighboring frequencies that allows for the appearance of a spotlike spatial modulation. When the spatial configuration of the pump field is changed from a plane wave to a narrow Gaussian, the near-field pattern assumes the form of a roll structure. The experimental observations obtained with a lithium triborate crystal agree qualitatively well with the theoretical picture.
We study the spatial correlations of quantum fluctuations that can be observed in multimode parametric down-conversion in the regime of high gain. We investigate both a type-I and a type-II phase-matching configuration: in the latter case spatial correlations at the quantum level are shown to exist both in the near-field and in the far-field zones of the down-converted light. In the stationary and plane-wave approximation we treat the problem analytically. A stochastic model is solved numerically to obtain quantitative results beyond this approximation. The finite transverse size and pulse duration of the pump beam and other features of the system, such as spatial walk-off and diffraction are taken into account, and we show that correlations beyond the standard quantum limit exist for values of parameters consistent with realistic experiments.
An optical parametric amplifier (OPA) has been demonstrated in bulk, periodically poled lithium niobate and is shown to operate with a noise figure well below the classical limit. In contrast to conventional OPA's, this device uses quasi-phase matching to provide the coupling between the pump and the signal. Comparison of the measured performance with that of a theoretical model reveals that the main intrinsic contribution to the output noise is due to spatial and temporal mode mixing, which arises as a consequence of tight focusing of the incident beams. Factors that affect the performance of this amplifier are identified theoretically and their relative importance investigated for both amplification and squeezing.
We analyze the spatial-frequency dependence of the gain of phase-insensitive and phase-sensitive optical parametric amplifiers with plane-wave pumping. We discuss the dependence of their spatial bandwidths on pump power and crystal length, L, and observe that the well-known (k s/L)1/2 approximation of spatial bandwidth is not very accurate (here k s is the signal beam's wavevector magnitude). We derive an alternative approximation that is highly accurate at large gains and moderately accurate at low gains. The differences between the phase-insensitive and phase-sensitive amplifier bandwidths are shown to be insignificant for gains above several dB. Maximum phase-sensitive gain and bandwidth are realized by imposing an optimum phase profile onto the input signal's spatial spectrum, which has nearly parabolic shape with added series of π/2 phase discontinuities at spatial frequencies outside the main bandwidth of the amplifier. We show that, apart from the discontinuities, such a profile can be closely approximated by placing the image plane into the middle of the nonlinear crystal (converging input beam). The phase discontinuities contribute an additional narrow component to the point-spread function of the phase-sensitive amplifier.
Optical parametric amplifiers (OPAs) configured as phase-sensitive amplifiers (PSAs) can be used for noiseless optical image amplification, generation of non-classical states of light, and, in particular, for multimode squeezed light generation. For all these applications and for effective use of the quantum properties of the multimode PSA, we need to know its independently squeezed (or amplified) eigenmodes. First we present the quantum theory of a spatially multimode traveling-wave PSA pumped by a high-power pump beam with arbitrary spatial profile. To quantitatively identify the Green's function of the PSA, we have developed a semi-analytical coupled-mode-theory of the PSA using 2D Hermite-Gaussian mode expansions of the signal- and pump-beam spatial distributions. By using Green's functions of the classical OPA, we calculate the normallyordered quadrature correlators at its output, which provide complete quantum description of the phase-sensitive OPA and enable determination of its independently squeezed eigenmodes. We find the number of the supported eigenmodes and their shapes for a spatially broadband frequency-degenerate optical parametric amplifier with elliptical Gaussian pump. We conclude by discussing our recent extensions of the coupled-mode-theory to study higher-order pump modes, compact representation of the PSA eigenmodes (especially convenient for experiments), and effect of non-zero phase mismatch on the PSA eigenmodes. We expect that the results from our model can be used for optimum mode matching in phase-sensitive image amplification and multimode squeezing generation.
Summary form only given. Optical imaging is one of the important potential applications for nonclassic states of light. It has been shown that spatially broadband squeezed light can be used to image faint objects with sub-shot-noise sensitivity. Broad spatial bandwidth of an optical parametric amplifier (OPA), determined by phase-matching conditions, makes it a good choice for studying the feasibility of imaging with nonclassic states. Here, we report, to our knowledge, the first observation of noiseless amplification of an image by means of phase-sensitive parametric amplification.
In the presence of a nonzero idler beam or, equivalently, in the collinear degenerate configuration, optical parametric amplification (OPA) can be phase sensitive. When the photon deamplification mode is selected in the OPA process, amplification of the optical phase occurs instead of the usual amplification of photon number. We demonstrate for the first time, to the best of our knowledge, that the operation of a spatially multimodal optical parametric amplifier in the phase-sensitive mode can result in a significant redistribution of spatial frequencies and near-ideal beam angle amplification. A numerical model is presented for phase-sensitive parametric amplification via three-wave mixing. The model is applied to monochromatic beams and finite beam apertures. The sensitivity of gain to the incident phase, noncollinear angle, and aperture size is studied in particular. Simple conditions are derived for the operation of the optical parametric amplifier in the phase-sensitive mode. Potential applications of the concept are discussed.