Broadband biphotons in a single spatial mode

We demonstrate the experimental technique for generating a spatial single-mode broadband biphoton field. The method is based on a dispersive optical element which precisely tailors the structure of the type-I spontaneous parametric down-conversion (SPDC) frequency angular spectrum in order to shift different spectral components to a single angular mode. Spatial mode filtering is realized by coupling biphotons into a single-mode optical fiber.

Figure 1

The type-I collinear degenerate frequency-angular SPDC spectra, calculated for the 2 mm BBO crystal (zero on the frequency scale corresponds to the central frequency ${\nu }_o={\omega }_p/4\pi )$. (a) The initial spectrum. (b) The typical view of the spectrum for usual broadening techniques. (c) The spectrum, transformed by an angular dispersion. The frequency bandwidth $\mathrm{\Delta }\nu$ corresponds to the collection angular range $\mathrm{\Delta }\theta$.

Figure 2

The principal scheme of the experimental setup.

Figure 3

The pump, signal, and idler Gaussian modes.

Figure 4

Biphoton frequency spectra. The value $\nu =0$ corresponds to the central frequency ${\omega }_p/4\pi$. Solid lines correspond to ideal calculation, dashed line corresponds to exact calculation, taking into account the diffraction efficiency, and circles correspond to experimental data. Black thick lines and circles refer to transformed spectrum and the red thin line to the initial one.

Figure 5

Biphoton correlation functions $G^{\left(2\right)}$ (12) calculated from the spectra plotted in Fig. 4. Solid lines correspond to ideal calculation, dashed line corresponds to exact calculation, taking into account the diffraction efficiency, and circles correspond to experimental data. Black thick lines and circles refer to transformed spectrum and the red thin line to the initial one.

Figure 6

The frequency bandwidth (11) dependence on the beam waist. Straight line: The ideal case. Dashed line: Taking into account the spectral dependence of the grating efficiency. Vertical line: The waist value $W=48$ $\mu \text{m}$ used in the calculations and in the experiment.

Figure 7

The scheme of the experimental setup.