Abstract
The workhorse technique for generating correlated pairs of photons is based on spontaneous parametric downconversion in nonlinear crystals. These photon pair sources are usually designed with relatively short crystal lengths, in the belief that this is necessary to attain good performance. We show, contrary to common practice, that concurrent high brightness and efficiency are also available to longer crystals. We present comprehensive measurement data on the pump and collection beam parameters necessary to achieve high collection efficiency (89.0%±1.7% and 81.9%±3.7% for signal and idler) together with high brightness when a single thick β-barium borate crystal (15.76 mm) is pumped with a narrow linewidth laser. Spectral analysis of the collinear, nondegenerate photons suggests that the effective interaction length within the crystal is dominated by the collection beam mode, and the use of longer crystals with increased spatial walk-off does not necessarily lead to reduced collection efficiency. This surprising result is an important consideration for optical designers who seek to develop practical photon pair sources.
Highlights
We present comprehensive measurement data on the pump and collection beam parameters necessary to achieve high collection efficiency (89.0 ± 1.7 %) together with high brightness when a single β-Barium Borate crystal is operated in the thick-crystal regime and pumped with a narrow linewidth laser source
Spectral analysis of the collinear, non-degenerate photons suggest that the effective interaction length within the crystal is dominated by the collection beam mode and the use of longer crystals with increased spatial walk-off does not necessarily lead to a reduced collection efficiency
Relying on long crystals (15.76 mm), the reported design was clearly operating in the thick-crystal regime of spontaneous parametric downconversion (SPDC) [16], where the interaction length is defined only by the overlap of the optical beams
Summary
The measured spectrum (using a grating spectrometer with 1 nm resolution) is much broader, corresponding to the emission from a 5.35 mm interaction length To explain this discrepancy, we note that the heat map in Fig. 2(a) neglects the effect of spatial walk-off. The observed photon pair rate at this value was 29, 200 ± 224 pairs per second per mW of pump power, with the error dominated by laser power fluctuations of 0.8 % These values are consistent with previously reported pair sources [14], with additional data regarding scaling of brightness and efficiency against beam parameters. The collection of idler photons is not affected (see Supplementary Material for a detailed treatment) Taking this into account, the overlap of the signal photon emission with the collection mode of the fiber is estimated to be nearly 100%. It should be possible to alter the source to move SPDC emission outside the filter cross-over region, enabling direct observation of this high degree of mode overlap
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