Research On Parametric Spectrum Of Periodically Poled Crystal

Since quantum mechanics was born at the beginning of the twentieth century, people's view on the structure of matter and its interaction has changed dramatically. Many phenomena which cannot be understood by people have been explained by quantum mechanics. They were also be exactly verified by many experiments. The interdisciplinary research becomes more and more apparent and close along with development of science and technology. The combination of quantum mechanics and information science has brought information science to a new area—quantum information science. The foundation of quantum information is preparation, operation, transmission, storage and measurement of quantum state. Now, people can prepare quantum state in many systems such as linear optics, cavity QED, NMR, iron trap and quantum dot systems. Since optical system use photons as carrier of information, it has many advantages such as easy to manipulate, good quality of transmission and weak decoherence of environment.Entangled photon pairs have many applications in quantum information field, such as quantum key distribution (QKD), quantum teleportation, quantum repeater and test of basic theory of quantum mechanics. Most of the sources used to create photon pairs utilize the process of spontaneous parametric down-conversion (SPDC), which can be realized in several configurations and by different crystals. Usually, in SPDC process, phase matching is required, but the generation rate of traditional bulk crystals is low, so it limits the long distance transmission due to the attenuation in optical fiber. In recent years, many experiments about the generation of photon pairs based on periodically poled crystals or waveguides were reported, because they have higher generation rate by utilizing quasi-phase matching (QPM).In this paper, we present a theoretic and experimental study on the spectrum of parametric down-converted photons generated from PPLN crystal, especially the wavelength and bandwidth. Because PPLN crystal has large working temperature and coefficient of thermal expansion, it is suitable to tune temperature of wavelength in a wide range. The wavelength of parametric down-converted photons can be changed linearly by changing the temperature of crystal. Besides, when the temperature of crystal is fixed, changing the wavelength of pump laser can also lead to the same result. Due to the chromatic dispersion in the fiber, bandwidth of photon pairs is an important factor needed consideration. Through comparison among crystal with different lengths and pump beams with different bandwidths, we demonstrate that the bandwidth of down-converted photons will increase for boarder bandwidth of pump pulse and decrease for longer crystal, but the influence of crystal length will become weaker along with increase of both crystal length and pump pulse bandwidth. Especially under the condition of our experiment, the bandwidth almost does not change for longer crystals when pumped by femtosecond laser. We wish this would be helpful for schemes in which such crystals are used.