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Lithuanian Journal of Physics

Spatial and temporal coherence in optical parametric devices pumped with multimode beams

Valdas Pašiškevičius, V. Smilgevičius, Rytis Butkus, Riaan Coetzee, Fredrik Laurell

Published Mar 28, 2018• 0 citations
Publisher

Abstract

We investigate the emergence of spatial and temporal coherence for the fields in the noncritical nondegenerate parametric second-order down-conversion process pumped with low spatial and temporal coherence beams. It is shown that in this scenario, which is of considerable practical importance, the parametric gain in the near field breaks down into an ensemble of mutually incoherent beamlets containing parametric waves. The field generated in a single beamlet is fully spatially coherent. The size of such coherent parametric gain regions is governed by the near-field spatial coherence radius of the pump, which also acts as a parameter, restraining the linear diffraction of the parametric waves generated in the nonlinear interaction. Furthermore, we experimentally demonstrate how the spatial and temporal coherence can be substantially enhanced by manipulating the spatial field correlation of the multilongitudinal and multi-transversal mode pump.

DOI
10.3952/physics.v58i1.3652

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We investigate the amplitude and intensity spatiotemporal coherence and the photon coincidence rates of far-field down-converted light resulting from the interaction of a pump wave of finite spectral width with a nonlinear crystal of finite spatial width. We examine the interplay between energy and phase mismatches and their effects on the intrabeam and interbeam spatial and temporal coherence of the down-converted beams. We show that the down-converted light is spatially incoherent in the second order, and that the signal-idler fourth-order coherence extends over finite entanglement angles. We also study the effect of the apertures through which the down-converted light is collected, when they are centered at phase-matched or misaligned directions.
Temporally incoherent twin beams of light are generated by means of an optical parametric amplifier that is pumped by the second harmonic of a Q-switched multi-longitudinal-mode Nd:YAG laser. For a parametric gain of 1·12, the intensity noise levels on the direct-detected signal and idler twin beams are found to be correlated by 1·4 dB, whereas the subtracted noise falls below the quantum limit by 0·7 ± 0·1 dB (15%). Although no direct coherence experiment is performed on the twin beams themselves, their temporal incoherence is manifested by the stringent path-length matching requirement between the pump and input signal beams.
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We examine experimentally how the degree of position-momentum entanglement of photon pairs depends on the transverse coherence of the pump beam that excites them in a process of spontaneous parametric down-conversion. Using spatially incoherent light from a light-emitting diode, we obtain strong position correlation of the photons, but we find that transverse momentum correlation, and thus entanglement, is entirely absent. When we continuously vary the degree of spatial coherence on the pump beam, we observe the emergence of stronger momentum correlations and entanglement. We present theoretical arguments that explain our experimental results. Our results shed light on entanglement generation and can be applied to control entanglement for quantum information applications.
Using the model of parametric interaction based on the spatiospectral Schmidt modes and generalized parametric approximation, we analyze coherence and the mode structure of ultraintense twin beams generated in the regime with pump depletion. We show that the increase of spatial and spectral coherence with the increasing pump power observed for moderate powers is replaced by a decrease for the pump powers at which pump depletion occurs. This behavior of coherence is opposed to that exhibited by a number of spatiospectral modes effectively constituting the twin beam. The conditions for maximal coherence are analyzed by considering pump-beam parameters (spectral width and transverse radius). The existence of additional coherence maxima occurring at even higher pump powers is predicted and explained by the oscillatory evolution of the modes' populations.