Femtosecond Laser Birefringent Crystal Damage Mechanism

Because it has good optical,thermotics nature and chemical stability and has anti-mechanical stress and pull,double refracting crystal is widely used in physics, chemistry laser technology, information handing system and biology technology and so on. Traditional double refracting crystal can satisfy the need in low energy density system. But with the development of the laser technology, high energy laser system has come out, especially femtosecond laser has been produced. The device is made of double refracting crystal materials should not be overlooked. Improving damaged threshold of device which is made of double refracting crystal materials is a more significance in use of high energy optical system.In recent years, with the progress of the study of damage of optical materials, the way to damage the crystal is becoming more and more variety. From the mechanism of damage, there are the main views as follows:1, In the process of crystal, as the polishing or exposed for too long, it is caused surface of the pit, score, impurities, and all kinds of flaw and score in the crystal.2, Multi-photon absorption ionization damage,3, Damage suffered by light pressure from crystal surface.4, Avalanche breakdown of crystal material.5, Damage of strong laser nonlinear effect self-focusing. At last it is brillouin scattering inspire the damage caused by ultrasound. It is very difficult to differ the mechanism two and mechanism three, so they are studied by combined the two mechanism.According to the optic axis of crystal, double refracting crystal can be divided into single axis crystal and double axis crystal; According to propagation velocity of light in crystal, double refracting crystal can be divided into positive and negative crystal. These crystals can be made into many kinds of application devices, and there are broad application prospects in laser technology development in the future. In order to adapt the need of new laser technology development, research of damage threshold of optical materials has obtained encouraging results in recent years at home and abroad. Film and photonic crystals have a more in-depth injury study. Research on damage morphology and mechanism of tradition crystal, such as Iceland crystal, silicon and mica, has few report. Research of traditional double refracting crystal damage morphology and the mechanism has also a certain reference value to improve the damage threshold of crystals and crystal optical element.There are several components thesis content as follows. Chapter one is introduction, it introduces Optical Material Damage of foreign and domestic status on the current, describes the source of the subject and briefly elaborates the necessity of the research.Chapter two introduces type of laser damage of crystal from the common damage mechanism, and dicusses a variety of possible effect factors of laser damage.From the basic theory chapter three discuss the commonly used experimental device system. In theory, it discusses commonly used method and detection methods of damage to the crystal in detail, and carefully analyses uncertainty during process of the damage and the analysis process.Chapter four mainly introduces the structure and principle of laser, introduces generally research of damage to optical materials of the femtosecond laser, and application in femtosecond laser micro-processing applications.Chapter five is the most important one. According to damage to crystal in different energy damage experiment, from two aspects it dicusses the crystal damage morphology, the phenomenon in process of experiment and finally come to the conclusion consistent with theory and experiment.There are two aspects of innovation of this thesis as follows.1. It analyzed internal factors to cause the crystal damage. From the uneven distribution of energy of femtosecond laser beam, it analyses the character of crystal damage morphology, describes the phenomenon which arise in process of experiment in detail. It has guidance and reference significant in the traditional damage to double refracting crystals in strong laser system.2. Through the analysis model of self-focusing damage and multi-electron ionization absorption, it supplies the theory reference for optical materials at different energies to determine the corresponding mechanism of damage.