Twin beam in fibers ; its production with a wave-guide type nonlinear crystal

双光束是一对具有相同光的统计特性的光束。双光束只能用量子理论来描述，没有经典的类比。多年来，双光束被用来阐明光的量子性质，如强干涉、光子纠缠、非局域相关（EPR悖论）等。由于双光束是由体非线性晶体的参数下转换产生的，因此存在不可避免的几何问题。在参量下转换中，只要满足相位匹配条件，光子对就以相当大的立体角发射。因此，几乎不可能成对地收集所有光子。另一方面，如果使用波导型非线性晶体作为下变换器，光子对被限制在波导中，很容易将它们耦合到光纤中并检测到所有光子对。我们利用波导型晶体来产生双光束。首先，我们用二次谐波产生（SHG）来表征晶体。我们制作了一个在1.55 μ m范围内振荡的扩展腔激光二极管。通过改变激光二极管的频率和功率，测量了晶体的SHG光谱和效率，并与理论计算结果进行了比较。我们发现实验和计算之间有一个小的差异，但如果我们考虑到晶体的不均匀性，这种差异是可以接受的。不幸的是，由于在1.55 μ m区域缺乏有效的探测器，并且开发这种探测器的时间有限，我们无法在给定的时间内完成我们的项目。然而，我们已经积累了实现光纤双束的基本信息和数据。我们将继续这个项目，并在不久的将来完成它。

Twin beam is a pair of optical beams that have the same statistical properties of light. Twin beam is described only by a Quantum theory and there is no classical analogue to it. In many years, twin beam has been utilized to clarify the Quantum nature of light, such as an intensity interference, photon entanglement, non-local correlation (EPR paradox) etc. Since twin beams were generated by a parametric down-conversion with bulk nonlinear crystals in most of the former works, there were unavoidable difficulties so called a geometrical problem. In the parametric down-conversion, pairs of photons are emitted in a fairly large solid angle as long as a phase match condition is satisfied. Therefore, it is almost impossible to collect all photons in pairs. On the other hand, if one uses a wave-guide type nonlinear crystal as the down-converter, pairs of photons are confined in the wave-guide and it is easy to couple them to optical fibers and detect all pairs of photons.We utilized the wave-guide type crystal to generate the twin beam. First of all, we characterized the crystal by means of a second harmonics generation (SHG). We made a laser diode with an extended cavity oscillating in 1.55 mu m range. We measured SHG spectra and efficiency of the crystal by changing the frequency and power of the laser diode and compared them with theoretical calculations. We found a small discrepancy between experiments and calculations, but the discrepancy was acceptable if we took account of an inhomogeneity of the crystal.Unfortunately, we can not complete our project within a given period for lack of efficient detectors in 1.55 mum region and the limitation of time to develop such detectors. However, we accumulated the basic information and data to realize twin beam in fibers. We will continue the project and complete it in the near future.