Experimental Study On Compact Nd:YVO₄ Self-mode-locked Laser With Intracavity Etalon

Ultra-short pulse laser is widely used in medical,industrial,information,military and physical science research and other important fields because of its superior characteristics such as high time resolution,high spatial resolution,high peak power and high bandwidth.Self-mode-locking technology is an important means to obtain such ultra-short pulse laser.In order to deeply study the mode locking mechanism,this paper aims to carry out relevant theoretical and experimental research on the longitudinal mode distribution of the spectral system,and explore the factors that affect the mode distribution and the methods to change the mode distribution from the mode selection based on the canonical effect.Finally,the Nd:YVO₄ self-mode-locked laser with flexible cavity length and adjustable mode spacing and the Nd:YVO₄ self-mode-locked laser with flexible reflectivity and tilt ed mirror with controllable mode distribution were obtained.Firstly,the research status and development trend of compact cavity type and self-mode-locked laser with Fabry-Perot Etalons at home and abroad are briefly summarized.The generation principle of mode-locked laser is deduced from the angle of electric field superposition,and the working principles of three kinds of mode-locked pulse forms-single pulse mode-locked,harmonic mode-locked and multi-pulse mode-locked are analyzed in detail.In addition,as an important method of mode selection,the principle of etalon mode selection is introduced in detail to lay a theoretical foundation for subsequent experimental research.In order to explore the factors that affect the mode distribution,this paper proposes three tunable factors:cavity length,output mirror reflectivity and tilt angle.The method to change the pattern distribution is as follows:For the flexible cavity length,this paper points out that in the Nd:YVO₄ self-mode-locked laser,the crystal with single-end coating on the laser wavelength can form the intracavity etalon.The mode distribution and corresponding time sequence diagram of the spectrum under different cavity lengths are analyzed theoretically.When the ratio of the thickness of the etalon to the optical length of the resonator length is a simple fraction of b∕q,the longitudinal mode spacing can be modified to the original q times,and the qth harmonic mode-locked laser can be output.In the experiment,41.4 GHz high repetition rate pulse laser output was obtained when the ratio of the thickness of the etalon to the optical length of the cavity length was adjusted to 3∕5 under a short cavity of 12.2 mm.When the pump power is 6.5 W,the average output power is more than 2.0 W,and the corresponding slope efficiency and optical-to-optical efficiency are 31.2%and 30.7%.For the flexible reflectivity and tilted mirror,it is pointed out that in the Nd:YVO₄ self-mode-locked laser,the intracavity etalon can be formed between the crystal pump surface and the input mirror.The mode distribution of spectrum under different reflectivity and tilt angle is analyzed theoretically.It is found that when the output mirror reflectivity decreases from 95%to 81%and 60%,the mode distribution of the spectrum changes from 7 longitudinal modes equally spaced to 4longitudinal modes equally spaced and 4 longitudinal modes unequally spaced to form 3 groups.In addition,the form of mode-locked pulse changed from single-pulse mode-locked to multi-pulse mode-locked,and the amplitude envelope fluctuation decreased from 22.7%to 5.1%and increased to 44.3%.When the tilt angle of the output mirror is adjusted from 0°to-0.03°and+0.03°,the mode distribution of the spectrum has a blue shift of 0.02 nm and a red shift of 0.02 nm.In this paper,based on the etalon effect,the theoretical analysis and experimental research of Nd:YVO₄ self-mode-locked laser are carried out from the three factors of flexible cavity length,flexible reflectivity and tilted mirror,and the laser with controllable mode is successfully obtained,which provides a reference basis for the research of laser related mode distribution.In the future experiments,the mode-locked pulse sequence will be optimized from quantitative pulse width,laser transverse mode and other aspects,providing a new method for better control of mode distribution.