| Dual-frequency laser(DFL)/dual-wavelength laser(DWL)signals with frequency separation between 0 and 10 THz have potential applications in many fields,such as photo-generated microwave/photo-generated millimeter waves,doppler lidars,and photo-generated terahertz waves.Neodymium-doped laser gain medium is an ideal gain medium material for DWLs due to its large Emission Cross Section(ECS)and wide-band pumped light absorption characteristics,which makes it an important research object in the field of DWLs in recent years.In this paper,a dual-wavelength laser based on Nd:YV04/Nd:GdVO4 combined crystal is presented.The thermal effect of the DWL and the frequency-separation tuning mechanism of the DWL are investigated.The details are as follows:(1)The development trend of tunable DFLs/DWLs at home and abroad is reviewed,and the current research status of tunable DFLs/DWLs at home and abroad is briefly summarized.(2)The system composition and operating conditions of the DWL are studied and discussed,and the steady-state rate equation of the combined crystal DWL is derived.Based on this,the output characteristics of the DWL are further analyzed,which provides a theory for the output beam quality.In addition,the frequency difference tuning mechanism of the DWL is discussed.(3)The thermal frequency difference tuning theory of Nd:YVO4/Nd:GdVO4 combined crystal is studied.Firstly,the cause and effect of thermal effect are analyzed,and the formula of thermal wavelength drift is derived and proved.Then the temperature field of the Nd:YVO4/Nd:GdVO4 combined crystal was modeled and analyzed by finite element analysis.The simulation results show that the central temperature of Nd:YV04/Nd:GdVO4 combined crystal has a positive linear relation with LD pump power and temperature control temperature,and a nonlinear relation with heat conduction coefficient.According to the LD pump power of 2.23 W,heat conduction coefficient of 5000 W/m2,the temperature difference between the central temperature of the Nd:YV04 crystal and the central temperature of the Nd:GdVO4 crystal is stable at 28.3?,the temperature difference between the central temperature of the Nd:YV04 crystal and the temperature of the heat sink is stable at 52?,and the temperature difference between the central temperature of the Nd:GdV04 crystal and the temperature of the heat sink is stable at 23.7?.(4)Based on the thermal effect simulation results of the Nd:YV04/Nd:GdVO4 combined crystal,the frequency separation tuning mechanism of the DWL are further explored.In the experiments,with the fixed LD pump power,the tunable DWL signal with large frequency separation above 310 GHz is achieved by adjusting the temperature of heat sink to control the temperature,the frequency separation decreasing rate is measured as-0.95 GHz/?,and negatively correlated to the temperature of the heat sink temperature control.For the experimental results,it is found that the wavelength shifting of the DWL signal is mainly caused by the ECS spectra shifting of the laser crystals with the temperature varying,and the change of the frequency separation is caused by different ECS spectral shifting rates of the crystals.The analysis results are in good agreement with the experimental results.It is verified that the thermal drift characteristics of the Nd:YVO4/Nd:GdVO4 combined crystal ECS is the most direct factor for achieving the frequency difference tuning function. |
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