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Quantum Electronics

Optimal detection of partially spatial coherent radiation by using optical quantum amplifiers

G I Kozin, А. P. Kuznetsov

Published Nov 30, 2005• 0 citations
Publisher

Abstract

Conditions for the efficient detection of reflected laser radiation by means of optical quantum amplifiers are considered theoretically. It is shown that in the case of a strongly disturbed spatial coherence, there is no need to take it into account and a simple geometrical consideration is sufficient. The expressions relating the effective radius of the input aperture and the angular magnification of a projecting optical system with the laser radiation divergence and amplifier parameters are obtained. The theoretical results are confirmed by experiments.

DOI
10.1070/qe2005v035n11abeh002860

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An experimental investigation was made of the efficiency of a regenerative optical quantum amplifier in detection of scattered laser radiation. In agreement with theoretical estimates, the experimental results demonstrated that the average energy of the scattered field amplified in this amplifier did not exceed the energy in the spatial coherence area. The relative variance of the output signal, which was distributed exponentially under linear conditions, was close to unity and was independent of the relationship between the correlation radius of the spatially random field and the receiver aperture radius.
We calculate the input-output characteristics of an optical amplifier that consists of an inverted-population medium placed inside a high-Q Fabry-P\'erot cavity. Expressions are derived for the output second- and fourth-order spectral and temporal correlation functions, and more generally for the output characteristic functional, in terms of the corresponding input quantities. The photocount first and second factorial moments are obtained for both homodyne and direct detection of the amplifier input and output. The general results are applied to several cases of practical interest, including inputs that have coherent or chaotic coherence properties. Particular attention is paid to the effects of amplification on specific nonclassical varieties of input light. It is shown that a maximum of only twofold amplification is permitted if any squeezing present in the input is to survive as squeezing in the output light. Similarly, for the preservation of photon antibunching in amplification, we show, by consideration of a kind of free-space number-state input, that only small gains are allowed. The amplifier model treated here provides a detailed example of the limitations imposed by quantum mechanics on the minimum noise generated by a multimode linear amplifier. In particular, we show that minimum noise occurs in a cavity that is asymmetric with respect to mirror reflectivities, for which we derive the corresponding conditions. In addition, we demonstrate the ability of the amplifier to improve upon signal-to-noise ratio, otherwise limited by low detector quantum efficiency.
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A review of basic analytical results on coherent propagation of optical pulses through nondegenerate amplifying media is presented. Original numerical results concerning pulse coherent amplification by degenerate media and, in particular, by the amplifying medium of an iodine laser are described also. Furthermore, some analytical results on coherent effects in the pulsed mode of laser generation are given. We have pointed out that the stochastic mode of laser radiation is related also to the coherency of interaction of a laser field with matter.