Research And Design Of Concentrating System For Solar Pumped Laser

With the depletion of non-renewable energy and the increasingly serious climate problem,the development and utilization of green new energy has become an urgent matter.Solar energy can be regarded as one of the most potential green energy because of its huge reserves,wide distribution,no regional restrictions,clean and pollution-free advantages.Solar Pumped Laser(SPL)is a device that uses solar radiation as a pump source to directly convert a wide spectrum of incoherent sunlight into a narrow band coherent laser.As a new solar energy technology,SPL has fewer energy conversion links,high efficiency,no complex power system and simple structure compared with traditional laser.It has great development potential in space energy transmission,free space optical communication,space solar power stations,laser weapons and other applications.Sunlight is a low power density radiation,far from enough to pump laser working medium,need to use solar concentrating system to improve the power density.The solar concentrating system collects and compresses solar radiation and then couples it to the laser working medium.The research on the concentrating system is an important aspect to improve the performance of SPL output laser.By collecting and summarizing the research status at home and abroad,this paper concludes that Solar pumped fiber laser(SPFL)and Solar-pumped solid-crystal laser(SPSL).The two types of lasers are very different in the design of the concentrating system.The current concentrating system also has some problems to be overcome:(1)The coupling efficiency of sunlight and optical fiber is not high.In solar pumped fiber laser(SPFL),the coupling efficiency of sunlight and fiber is the limit of laser output power.Because of its own property,optical fiber can only receive sunlight in a certain Angle range,so it is necessary to consider the matching between the Angle of the outgoing light and the optical fiber when designing the concentrating system.(2)There are shortcomings of traditional condenser.In the current solar pumped solid crystal laser(SPSL),large aperture Fresnel lens and parabolic mirror are widely used as main concentrators.As the sunlight has a wide spectrum and divergence Angle,when using a large aperture Fresnel lens to focus the light,the focal surface spot is very large due to dispersion,which seriously affects the matching degree between the laser working medium and the solar spot,and reduces its absorption efficiency.Because of its reflection characteristics,parabolic mirror has no color difference.However,its focal point is on the same side as the incident sunlight,so the laser crystal near the focal point,temperature control system and other devices inevitably cause the central blocking,which reduces the receiving efficiency of the concentrating system and thus affects the laser output.According to the problems existing in the current concentrating system,this paper puts forward effective solutions.The main research content can be divided into the following two parts:(1)The compound parabolic concentrator is used in the solar pump fiber concentrator system.Based on the analysis of the feasibility and defects of compound parabolic condenser(CPC)used in fiber coupling,an improved CPC is proposed.A solar pump multistage concentrator scheme consisting of a primary parabolic mirror and a secondary improved CPC is completed and compared with the scheme with a common CPC and lens group consisting of a secondary concentrator and a scheme without a secondary concentrator.The ray tracing simulation results show that the improved CPC can overcome the divergence defect of the ordinary CPC outgoing ray,thus improving the coupling efficiency with the fiber.The coupling efficiency is maintained at 93.53%-96.43%within the receiving Angle range of the primary mirror±0.55°,and the output power of the optical fiber bundle meets the requirement of 15 W.This result is better than other schemes above.The solution of the laser rate equation shows that the concentrator system composed of the improved CPC and the main condenser can realize the end-pumping of doped fiber.(2)A novel solar pumped solid crystal laser concentrating system.The design principle of parabolic ring array concentrator(PRAC)is deduced in detail,and the design process and related calculation formula of PRAC are summarized.The influence of design parameters on the concentrating performance of parabolic ring array condenser(PRAC)is studied,and the matters that should be paid attention to before the design are determined to achieve the best concentrating performance.Based on this,a multistage solar concentrating system with parabolic ring array concentrator(PRAC)and small aperture Fresnel lens as primary concentrator and CPC as secondary concentrator is proposed.The simulation analysis of the whole concentrating system shows that the geometric concentrating ratio of the system is2.15×10~4,the optical concentrating ratio can reach 2.05×10~4,the irradiation density at the end face of the crystal is 4.5 W/mm~2,and the end face pumping of Nd:YAG crystal is realized.Compared with large aperture Fresnel lens and parabolic mirror of the same caliber,the average irradiation density of the new concentrator system on the receiving surface is increased by 28.2%and 3.4%,respectively.The maximum laser output power generated by the new concentrator system is 22.7 W,while the maximum output power of large aperture Fresnel lens and parabolic mirror is 12.3W and 19.1W,respectively,increasing by 84.6%and 18.8%,respectively.It overcomes the shortcomings of traditional large aperture Fresnel lens and parabolic mirror in solar-pumped solid-state laser.