| RF intensity modulated lidar,an integration of Lidar and microwave radar,has high spatial resolution and strong resistance to atmospheric turbulence interference and to electromagnetic interferenc,which makes it significant in military detection and civil remote sensing.RF modulated laser is one of the most important components in RF intensity modulated lidar system.RF intensity modulated laser source can be achieved by direct modulation via electro-optic modulator,and modulation frequency can reach hundreds of MHz to dozens of GHz.Direct modulation method is usually used in the field of optical fiber communication,while the power is low and modulation depth is limited.The interference of two beam with frequency difference in the radio frequency band is another important method to produce the radio frequency intensity modulated laser.The dual-frequency laser generates radio frequency intensity modulated laser.Dual-frequency lasers have high output power,realize deep modulation index and large tuning range of modulation frequency.RF intensity modulated lasers have important applications in long-distance detection and underwater detection.In this paper,the semiconductor laser pumped multistage optical fiber MOPA and dualfrequency injection seeding method are used to realize high power continuous and pulse RF intensity modulated laser output.High power RF intensity modulated green light output is achieved by frequency doubling.This dissertation completes the following works:(1)A theoretical model of dual-frequency oscillation in solid-state lasers is established according to the Lamb’s semiclassical theory,the conditions of achieving dual-frequency laser output are given,and the optimization method of dual-frequency lasers is proposed.Using Jones matrix,the principle and tuning range of frequency difference of the dual-frequency laser are presented.A dual-frequency Nd:YAG laser is built by using two quarter wave plates.9.5 m W dual-frequency laser output is obtained.The frequency difference tuning range is 30 MHz 1.5 GHz.The coupling coefficient between the dual-frequency modes is measured,and the value is consistent with the theoretical model.(2)The study of the fiber power amplification of CW dual-frequency laser is carried out.A theoretical model of ytterbium-doped double cladding fiber amplification is established.The optical power of signal and pump light are analyzed.The theoretical optimum length of the gain fiber is obtained by the transmission rule of the power in the optical fiber amplification process.A three-stage fiber power amplification system is built,the dual-frequency laser is used as the seed signal.50 W dual-frequency output power is obtained.The stabilities of output power and beat note frequency before and after amplification are measured,respectively.The amplification process does not bring any deterioration to the stabilities of both the output power and frequency.(3)A theoretical model of dual-frequency laser injection seeding is proposed based on rate equations.The process of dual-frequency laser injection seeding is analyzed.The bandwidth between seed frequency and its nearest axial mode of injection seeding is obtained.Dual-frequency laser injection seeding pulse waveform is numerically simulated.The experiment of dual-frequency laser injection-seeding an electro-optic Q-switched pulse laser is carried out,and maximum peak power of 437 k W of intensity modulated laser output is obtained.The pulse modulation frequency is 224 MHz,which depends on the frequency space of the adjacent axial modes of the passive cavity.The dependence of locking probability on the power of injected signal is studied experimentally.When the injected dual-frequency signal is 40 m W and the frequency difference is 210 MHz.The maximum probability of injection locking is 60%.(4)The CW high power dual-frequency signal is frequency doubled via using a Mg O:PPLN crystal.The frequency up-conversion efficiency is 14.6%.A KTP crystal is used to generate 532 nm intensity modulated pulses from the high power 1064 pulses.The frequency up-conversion efficiency of dual frequency pulse laser is 32.5%. |
