| Elastic modulus is one of the key parameters to characterize the mechanical properties of biological tissues and materials.It is an emerging research field to study the mechanical properties of biological tissues by stimulated Brillouin scattering method,which is of great significance for the study of light scattering physics theory and biomedical application research.At present,the stimulated Brillouin scattering material elastic modulus measurement system mainly uses a single-frequency green pulse laser as a light source,and it is difficult to extract the stimulated Brillouin scattering signal.Therefore,this paper designs an orthogonal linear polarization dual-frequency tunable pulsed green laser for the biological tissue elastic modulus measurement system,which provides a new way for the measurement of biological tissue elastic modulus.The main contents of this paper include the following aspects:Firstly,the research status and development trend of pulsed laser technology at home and abroad are introduced.The principle of stimulated Brillouin scattering and the measurement method of biological tissue elastic modulus are presented.The principle of the dual-frequency stimulated Brillouin scattering biological tissue elastic modulus measurement system is discussed.Second,the basic composition and working characteristics of solid-state lasers are discussed.The design scheme of dual-frequency pulsed green laser for material elastic modulus measurement is introduced.The laser uses PBS to divide the resonant cavity into linear cavity and right-angle cavity,and adopts Cr4+:YAG passive Q-switching method,KTP intracavity frequency doubling,birefrngent filter composed of PBS and HWP for mode selection,etc.Thirdly,based on the passive Q-switching rate equationthe intracavity frequency-doubled passive Q-rate equation was established.The rate equation of the intracavity frequency doubling passive Q-switching is numerically simulated.The effects of pumping rate,initial transmission rate of Cr4+:YAG and output mirror reflectivity on the pulse characteristics are simulated.Fourthly,a single-frequency pulsed green laser experimental system was established.When the threshold pump power was 1.31W,a pulsed green laser output with a pulse width of 22 ns and a repetition rate of 1.35 kHz was obtained.The power characteristics of 532 nm continuous light and pulsed light and the modelability of the birefringent filter were investigated.The optimal placement azimuth of KTP was measured.The relationship between the output pulse characteristics and the incident pump power and the relationship between the output pulse characteristics and the cavity length are experimentally studied.As the incident pump power and cavity length increase,the repetition frequency gradually decreases and the pulse width increases accordingly.The experiment is consistent with theory.The variation of laser output power and polarization state when HWP rotates around the cavity axis was measured.As the HWP rotates,the output power changes as a sinusoidal function with a period of ?.When the HWP is rotated by ?°,the output laser polarization state changes by 2?°.Fifthly,an experimental system of dual-cavity dual-frequency pulse green laser was established,and an orthogonal linear polarization tunable dual-frequency pulse green laser output was obtained.When the threshold pump power is 2.01W,the linear cavity and the right-angle cavity respectively obtain a dual-frequency pulsed green laser output with a pulse width of 40 ns,42 ns,and a repetition rate of 1.72 kHz and 1.56 kHz.With the increase of pump power,the repetition frequency of linear cavity and right-angle cavity varies between 1.74kHz?3.75kHz and 1.56kHz?3.85kHz,and it shows an increasing trend.The pulse width varies from 40 ns to 35.7 ns and 42 ns to 36.9 ns,showing a decreasing trend.By changing the tilt angles of HWP1 and HWP2,the frequency tuning range obtained by the linear cavity and the right-angle cavity is 0?39.3GHz and 0?26.1GHz,respectively.The laser has a simple structure and convenient tuning,and can meet the measurement requirements of the elastic modulus of the material based on stimulated Brillouin scattering. |
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