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Acta Sinica Quantum Optica

The Spectrum Btribution Property in Type-II BBO Crystal Down-conversion with a Femtosecond Pulse Laser Pump

Jiangrui Gao

Published Jan 1, 2010• 0 citations
OpenAlex

Abstract

Spectrum distribution of spontaneous parametric down-conversion is discussed in type-Ⅱ phase matching process with a Femto-second pulse laser pump.The spectrum distribution of signal and idle beams are different from each other and the differences vary with the width of Femta second pulse laser pump.We have also measured the spectrum distribution of type-Ⅱ phase matching BBO crystal's spontaneous parametric down-conversion beams.The results coincide with theorctical analysis.They are useful when measuring the light non-classical property with Femto-second pulse balance homodyne detecting system.

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 7, 2015•0 citations
Spectrum distribution of spontaneous parametric down-conversion is discussed in type-Ⅱ phase matching process with a Femto-second pulse laser pump.The spectrum distribution of signal and idle beams are different from each other and the differences vary with the width of Femta second pulse laser pump.We have also measured the spectrum distribution of type-Ⅱ phase matching BBO crystal's spontaneous parametric down-conversion beams.The results coincide with theorctical analysis.They are useful when measuring the light non-classical property with Femto-second pulse balance homodyne detecting system.
Spontaneous parametric upconversion (SPUC) and spontaneous parametric downconversion (SPDC) have been observed in the β-barium borate (BBO) crystal, and their mechanisms are experimentally and theoretically investigated. SPUC, tuned from 530 to 600 nm with a FWHM of about 25 nm, can be attributed to the sum frequency between the quantum noise and the fundamental laser pulse in the type II phase matching condition (e+o→e). SPDC, tuned from 480 to 520 nm with a FWHM of about 15 nm, can be attributed to the difference frequency between the quantum noise and the fundamental laser pulse in the type I phase matching condition (e→o+o).
There are several problems that must be solved in order to increase the distance of quantum communication protocols based on photons as an information carriers. One of them is the dispersion, whose effects can be minimized by engineering spectral properties of transmitted photons. In particular, it is expected that positively correlated photon pairs can be very useful. We present the full characterization of a source of single photon pairs at a telecom wavelength based on type II spontaneous parametric down conversion (SPDC) process in a beta-barium borate (BBO) crystal. In the type II process, a pump photon, which is polarized extraordinarily, splits in a nonlinear medium into signal and idler photons, which are polarized perpendicularly to each other. In order for the process to be efficient a phase matching condition must be fulfilled. These conditions originate from momentum and energy conservation rules and put severe restrictions on source parameters. Seemingly, these conditions force the photon pair to be negatively correlated in their spectral domain. However, it is possible to achieve positive correlation for pulsed pumping. The experimentally available degrees of freedom of a source are the width of the pumping beam, the collected modes’ widths, the length of the nonlinear crystal and the duration of the pumping pulse. In our numerical model we use the following figures of merit: the pair production rate, the efficiency of photon coupling into a single mode fiber, the spectral correlation of the coupled photon pair. The last one is defined as the Pearson correlation parameter for a joint spectral distribution. The aim here is to find the largest positive spectral correlation and the highest coupling efficiency. By resorting to the numerical model Ref. [1] we showed in Ref. [2], that by careful adjustment of the pump’s and the collected modes’ characteristics, one can optimize any of the source's parameters. Our numerical outcomes conform to the experimental results presented in Refs [3,4]. Here we investigate typical, experimentally available source parameters: the widths of the pump beam and collected modes ranging from 20μm to 500m, the crystal length ranging from 1mm to 7.5mm while the pulse duration is set to 50fs, 100fs or 150fs. We achieve the correlation coefficient value as high as approximately 0.8, or – for different values of parameters – coupling efficiency equal to 0.76.
Abstract Spontaneous parametric down-conversion (SPDC) is the most widely used process for generating photon pairs entangled in various degrees of freedom such as polarization, time-energy, position-transverse momentum, and angle-orbital angular momentum (OAM). In SPDC, a pump photon interacts with a non-linear optical crystal and splits into two entangled photons called the signal and the idler photons. The SPDC process has been studied extensively in the last few decades for various pump and crystal configurations, and the entangled photon pairs produced by SPDC have been used in numerous experimental studies on quantum entanglement and entanglement-based real-world quantum-information applications. In this tutorial article, we present a thorough study of phase matching in BBO crystals for spontaneous parametric down-conversion and thereby also investigate the generation of entangled photons in such crystals. First, we present a theoretical derivation of two-photon wavefunction produced by SPDC in the frequency and transverse momentum bases. We then discuss in detail the effects due to various crystal and pump parameters including the length of the crystal, the angle between the optic axis and the pump propagation direction, the pump incidence angle on the crystal surface, the refraction at the crystal surfaces, and the pump propagation direction inside the crystal. These effects are extremely relevant in experimental situations. We then present our numerical and experimental results in order to illustrate how various experimental parameters affect the phase matching and thus the generation of entangled photons. Finally, using the two-photon wavefunction in the transverse wave-vector basis, we show how to derive the two-photon wavefunction in the OAM basis and thereby calculate the two-photon angular Schmidt spectrum. We expect this article to be useful for researchers working in various capacities with entangled photons generated by SPDC in BBO crystals.
We observed the spectral coherence structure of twin beam states generated by spontaneous parametric down-conversion by using a simple fiber spectrometer synchronized with a pulsed laser source. The recorded single-shot spectra exhibit a well-defined peak structure, where the peak wavelengths change values from shot to shot. We studied the number and the width of the peaks as a function of the different parameters in the experimental setup (pump power, iris size, iris distance from the BBO crystal). Moreover, we evaluated the number of modes in the intensity distribution of the light in different portions of the spectrum. The experimental results indicate that the number of modes from statistics and the number and width of the peaks evolve differently with the parameters.