Single-mode source of bright squeezed vacuum

Bright squeezed vacuum (BSV) can be obtained at the output of a high-gain traveling-wave optical parametric amplifier with the vacuum at the input. Without any special efforts, it is multimode both spatially and temporally. This means it comes as not Fourier-limited pulses and not diffractionlimited beams. At the same time, what theory describes well and what is required for most applications is single-mode BSV. Here we demonstrate how a spatially single-mode source can be created. The idea is to make a parametric amplifier of two spatially separated crystals (Fig.1). Because amplification in the second crystal only occurs within the pump beam waist, the angular spectrum gets narrower at larger separation. This leads to the reduction in the number of spatial modes – ideally, to a single-mode beam as the crystals get more and more separated. In order to observe this, one can measure the second-order intensity auto-correlation function g. Its value depends on the number of transverse modes T m