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Coupling Down Converted Light Into Single Mode Fibers

David A. Niemi

Published Jan 1, 2007• 0 citations
OpenAlex

Abstract

We investigate the influence of the pump and collection mode parameters on the collection efficiency of Type I down converted photons into single mode fibers. For best single and coincidence counting rates, we find that the mode sizes should be close to the same size and that the mode waists should be located near the down-conversion crystal. Larger collection waists give higher collection efficiencies, but lower singles counts.

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We investigate the influence of the pump and collection mode parameters on the collection efficiency of Type I down converted photons into single mode fibers. For best single and coincidence counting rates, we find that the mode sizes should be close to the same size and that the mode waists should be located near the down-conversion crystal. Larger collection waists give higher collection efficiencies, but lower singles counts.
Both correlated-mode coupling efficiency and photon pair flux (brightness) of spontaneous parametric down-conversion (SPDC) coupled into single-mode fibers are key parameters for various quantum information science applications. We present an experimental investigation on the optimization of the correlated-mode coupling efficiency and the brightness of frequency-degenerated SPDC photon pairs into single-mode optical fibers. Using photon pairs generated from a type-I phase-matched bulk $\ensuremath{\beta}$-barium borate crystal pumped by a continuous-wave laser, we carefully evaluated both the correlated-mode coupling efficiency and the brightness of SPDC light coupled to a single-mode optical fiber, while continuously changing the focusing of the pump and collection modes, experimenting with Rayleigh lengths from 3 to 280 (for the pump) and 0.2 to 20 (for the collection) times the crystal length. The measured data suggest that the correlated-mode coupling efficiency can be close to unity for any pump focus, however, the optimal Rayleigh length of the collection mode depends on the pump focus parameter, and the brightness can be increased by tightening the pump focus, provided that the Rayleigh length of the collection mode is precisely adjusted. A maximal correlated-mode coupling efficiency of $99.0\ifmmode\pm\else\textpm\fi{}1.3%$ was experimentally achieved. These results will be useful for photonic quantum science and technology using SPDC sources with high heralding efficiencies.
We investigate the coupling efficiency of parametric downconversion light (PDC) into single and multi-mode optical fibers as a function of the pump beam diameter, crystal length and walk-off. We outline two different theoretical models for the preparation and collection of either single-mode or multi-mode PDC light (defined by, for instance, multi-mode fibers or apertures, corresponding to bucket detection). Moreover, we define the mode-matching collection efficiency, important for realizing a single-photon source based on PDC output into a well-defined single spatial mode. We also define a multimode collection efficiency that is useful for single-photon detector calibration applications.
We investigate the coupling efficiency of parametric downconversion light (PDC) into single and multi-mode optical fibers as a function of the pump beam diameter, crystal length and walk-off. We outline two different theoretical models for the preparation and collection of either single-mode or multi-mode PDC light (defined by, for instance, multi-mode fibers or apertures, corresponding to bucket detection). Moreover, we define the mode-matching collection efficiency, important for realizing a single-photon source based on PDC output into a well-defined single spatial mode. We also define a multimode collection efficiency that is useful for single-photon detector calibration applications. 1.
Photon modes have an important role in characterizing the quantum sources of light. Proper coupling of various photon modes obtained in spontaneous parametric down conversion (SPDC) process in optical fibers is essential to generate an effective source of entangled photons. The two main pre-detection factors affecting the biphoton mode coupling in SPDC are the pump beam focusing parameter and the crystal thickness. We present the numerical and experimental results on the effect of pump focusing on conditional down-converted photon modes for a Type-I BBO crystal. We experimentally verify that biphoton coupling efficiency decreases asymptotically with pump beam focusing parameter. We attribute this behaviour to (a) the asymmetry in the spatial distribution of down-converted photons with the pump beam focusing parameter and (b) the ellipticity of biphoton modes introduced due to the focusing of the pump beam. We also show experimentally the ellipticity as well as quantify the asymmetry with the pump focusing parameter. These results may be useful in selecting optimum conditions in the down-conversion process for generating efficient sources of entangled photons for quantum information applications.
We investigate the coupling efficiency of parametric downconversion light (PDC) into single and multimode optical fibers as a function of the pump beam diameter, crystal length, and walk-off. We outline two different theoretical models for the preparation and collection of either single-mode or multimode PDC light (defined by, for instance, multimode fibers or apertures). Moreover, we define the mode-matching collection efficiency, important for realizing a single-photon source based on PDC output into a well-defined single mode. We also define a multimode collection efficiency that is useful for single-photon detector calibration applications.
We develop and apply an effective analytic theory of a noncollinear, broadband type-I parametric down-conversion to study a coupling efficiency of the generated photon pairs into single mode optical fibers. We derive conditions necessary for highly efficient coupling for single and double type-I crystal producing polarization entangled states of light. We compare the obtained approximate analytic expressions with the exact numerical solutions and discuss the results for a case of beta barium borate crystals.