There is an increasing interest for application in optical systems which present non classical effects in the spatial domain. Here, we investigate on the quantum aspects of the field emitted by a travelling wave Optical Parametric Amplifier (OPA) . A multimode theory is developed in order to describe the spatial properties of the field emitted by down conversion in a chi2 nonlinear crystal. We have firstly focused our attention on the fluorescence emission of the amplifier. Phase matching conditions determine the angular distribution and the frequency spectrum of the light emitted by spontaneous down conversion. The pair of photons generated in these processes are highly correlated. Close to the degenerate frequency they are emitted symmetrically with respect of the system axis because of momentum conservation. As a consequence, intensity correlations are observable in the OPA far field when one considers symmetrical detection areas; more precisely, it was possible to demonstrate that the fluctuations in the subtracted signal measured by two identical detectors are well below the shot noise level. We also propose a phase-insensitive imaging scheme which is able to produce two copies of an injected image, (they can be called conventionally signal and idler images). In the limit of large amplification, the two images have not only the same intensity , but also present locally synchronized intensity fluctuations. With the help of an appropriate feedback loop, there is therefore the possibility of reducing the noise fluctuations, say, in the idler image, by using the information measured from the signal image. Although more difficult to control experimentally, we show that a similar but phase-sensitive imaging scheme presents the advantage to preserve the signal to noise ratio of the input image locally, on both the signal and the idler images. We also identify the conditions under which the object can be amplified with sufficient spatial resolution, by considering the finite transverse size of the system.
Quantum Images in a Travelling Wave Parametric Optical Amplifier
BRAMBILLA, ENRICO;LUGIATO, LUIGI;
1999-01-01
Abstract
There is an increasing interest for application in optical systems which present non classical effects in the spatial domain. Here, we investigate on the quantum aspects of the field emitted by a travelling wave Optical Parametric Amplifier (OPA) . A multimode theory is developed in order to describe the spatial properties of the field emitted by down conversion in a chi2 nonlinear crystal. We have firstly focused our attention on the fluorescence emission of the amplifier. Phase matching conditions determine the angular distribution and the frequency spectrum of the light emitted by spontaneous down conversion. The pair of photons generated in these processes are highly correlated. Close to the degenerate frequency they are emitted symmetrically with respect of the system axis because of momentum conservation. As a consequence, intensity correlations are observable in the OPA far field when one considers symmetrical detection areas; more precisely, it was possible to demonstrate that the fluctuations in the subtracted signal measured by two identical detectors are well below the shot noise level. We also propose a phase-insensitive imaging scheme which is able to produce two copies of an injected image, (they can be called conventionally signal and idler images). In the limit of large amplification, the two images have not only the same intensity , but also present locally synchronized intensity fluctuations. With the help of an appropriate feedback loop, there is therefore the possibility of reducing the noise fluctuations, say, in the idler image, by using the information measured from the signal image. Although more difficult to control experimentally, we show that a similar but phase-sensitive imaging scheme presents the advantage to preserve the signal to noise ratio of the input image locally, on both the signal and the idler images. We also identify the conditions under which the object can be amplified with sufficient spatial resolution, by considering the finite transverse size of the system.
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