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Journal of the Optical Society of America B

Photon amplification by parametric downconversion

Z. Y. Ou, L. J. Wang, L. Mandeļ

Published Feb 1, 1990• 12 citations
Publisher

Abstract

We investigate the theory of the process in which an idler photon emitted spontaneously from a nonlinear crystal in the process of parametric downconversion serves as idler input to a second downconverter. The signal photon from this second crystal is then detected in coincidence with the signal photon from the first crystal, which provides the time reference. We show that the first idler photon can induce a stimulated downconversion in the second crystal, which serves as a photon amplifier, and we calculate the ratio of the stimulated to the spontaneous emission probability.

DOI
10.1364/josab.7.000211

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We investigate the theory of the process in which an idler photon emitted spontaneously from a nonlinear crystal in the process of parametric downconversion serves as idler input to a second downconverter. The signal photon from this second crystal is then detected in coincidence with the signal photon from the first crystal, which provides the time reference. We show that the first idler photon can induce a stimulated downconversion in the second crystal, which serves as a photon amplifier, and we calculate the ratio of the stimulated to the spontaneous emission probability.
Measurements have been made of the rate at which signal photons are produced in a nonlinear crystal in the process of parametric downconversion, when a coherent light beam from a He:Ne laser is injected into the crystal in the direction of the conjugate-idler photons. The latter provides a stimulating field for the downconversion. When the phase matching condition inside the nonlinear medium is satisfied, the rate of downconversion is enhanced by the presence of the He–Ne laser beam by a factor of the form 1 + βIi, where li is the intensity of the He:Ne laser beam. The effect of defocusing either the pump beam or the He–Ne idler beam is investigated. Possible use of this phenomenon to study the photon amplifier with single-photon input will be discussed.
In the process of spontaneous parametric downconversion, photons from a pump field are converted to signal and idler photon pairs in a nonlinear crystal. The reversed process, or upconversion of these pairs back to single photons in a second crystal, is also possible. Here, we present experimental measurements of the upconversion rate with a controlled time delay introduced between the signal and idler photons. As a function of delay, this rate presents a full width at half maximum of 27.9 fs under our experimental conditions[1].
When a laser beam interacts with a nonlinear crystal, some of the incident photons fission into two lower frequency signal and idler photons through the process of spontaneous parametric downconversion. It is shown that, as a result of the strong positive correlation between signal and idler photons, the normalized intensity cross-correlation function λ12(τ) should be inversely proportional to the downconverted light intensity and ideally should be given by the inverse of the mean number of photons counted in the resolving time. This unusual relationship, which is a reflection of the quantum mechanical character of the downconverted optical field, has been confirmed in photoelectric counting measurements. The experiment makes use of the light from an argon-ion laser oscillating at 351.1 nm interacting with an 8-cm long crystal of KDP. The downconverted light is extracted at 633.5 and 788.0 nm in two directions that satisfy the phase-matching condition and is allowed to fall on two counting photomultipliers. The output pulses are fed to a time-to-digital converter that yields the cross-correlation function.
In this paper we study photon pairs generated in a nonlinear cavity composed of a nonlinear crystal surrounded by mirrors, by the process of spotaneous parametric downconversion. We analyze two different regimes: singly-resonant cavities where the signal and idler modes are resonant and doubly-resonant cavities where the pump mode is also resonant. We present analytic expressions for the joint spectral amplitude in these two cases and study the reduction in emission bandwidth as the cavity coefficient finesse is increased. We also study the enhancement of the source brightness from the presence of the cavity.
Photons emitted in the process of spontaneous parametric downconversion of pump light in a nonlinear crystal are made the basis of a new optical communication scheme, which depends on the fact that two downconverted photons are always produced simultaneously. If the pump light is modulated and the downconverted photons are transmitted in two separate beams which fall on two separate detectors, registering their simultaneous arrival allows great discrimination against background and noise to be achieved. Communication is possible with relatively few photons and even when the signal is ordinarily lost in the noise, so that the communication channel can be made virtually interception proof. The feasibility of the scheme is demonstrated by a pilot experiment.