Design and Preparation of Ultra-Steep Dichroic Coating for Continuous Laser Beam Combination
Objective The high-power continuous laser is in urgent demand in military and industrial fields,and it is an important technical support for national defense and economic development.Because it is limited by nonlinear effects,mode instability,and other problems,the output power of single-mode fiber lasers has a specific limitation.To improve the output power of the laser,researchers have proposed a series of laser power enhancement technology routes,such as spectral combination and dichromatic combination.By combining multiple single-path lasers to output laser,higher laser power can be obtained while maintaining good beam quality.Therefore,the combination scheme becomes the key to realize ultra-high laser power output.The dichroic mirror combination technology scheme based on the spectral characteristics of the dichroic film has become one of the main technical routes for high-power continuous laser synthesis output owing to its advantages of stability,high combination efficiency,and easy integration at low cost.Because of the spectral characteristics of the dichromic film,it can simultaneously transmit and reflect two beams with different wavelengths,and by adjusting the incidence angles of the two beams,it can realize the coincidence of the transmitted and reflected beams to achieve the combining effect of two beams.In addition,the higher the transmittance and reflectivity at the transmission and reflection bands of the dichromic film,the smaller is the loss of laser energy in the beam combination process,and the higher is the achieved beam combination efficiency.Methods The differences in spectral properties,such as spectral transmittance,reflectance,and steepness,and the electric field distribution of the film layer between the Fabry-Perot cavity superposition and the long-wavelength-pass basic film structures are compared.The long-wavelength-pass film structure is more suitable for the high-power continuous laser low-absorption transmission.Based on the dual ion beam sputtering thin film deposition equipment,the preparation is carried out using the weighted broad spectrum monitoring method,and the prepared samples are tested using a spectrophotometer.The errors are analyzed based on the test results.Results and Discussions Figure 7 shows the spectral matching of the transmittance spectra of the dichromic film using the layer-by-layer inversion function of the OptiRE software.Among them,the transmittance spectrum of the first film layer is poorly matched because the spectral transmittance curve contains limited information.Then,because of the self-compensating effect of the broad spectrum monitoring,the target is gradually corrected during the deposition process of the film layer,so that the matching of the measured and theoretical design spectra improves.In addition,as shown in Fig.8,the thickness of the high refractive index material is generally positively shifted,and the thickness of the low refractive index material is are generally negatively shifted.The main reason for the formation of this phenomenon is the self-compensating effect of the broad spectrum monitoring.Aiming at the design transmittance spectrum,the thickness errors of high and low refractive index films are staggered with each other when the film is adjusted repeatedly.At the same time,according to the inversed deposition rate distributions of the high and low refractive index materials,the deposition rates from layer 13 to layer 46 are stable.The rate gradients of the first few film layers are from the state-stabilizing process at the beginning of ion beam output of double ion beam sputtering equipment.In layer 47,the film layer sensitivity is increased.At this time,to achieve the spectral matching,the broad band spectral monitoring is more active,the overall spectral changes are concentrated near the transition band,and the overall rate fluctuation becomes larger.Conclusions The optimized design of the coating structure and the strategy of precision preparation are systematically analyzed from the perspectives of design,precision preparation,and test characterization.This allows the achievement of higher steepness,reflectivity,transmittance,absorption,and other properties of the high-steepness dichromatic coating.The long-wavelength-pass film structure is more suitable for the high-power continuous laser low-absorption transmission.The steepness of the designed and prepared high-steepness dichromatic film is 8 nm;the 1060 nm and 1080 nm laser beam combining efficiency reaches more than 99％;and the temperature rise of the dichromatic film is less than 4 ℃ when the combined continuous laser output power reaches 7.16 kW in 180 s.This is of great significance for the design and precision preparation of complex film systems represented by high-steepness dichromatic films.
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