Research On Thermal Distortion In Thin-Disk Laser Gain Mirrors

Thin-disk laser gain mirror is composed of a laser crystal with large radius-thickness ratio bonded on high thermal conductive metal or nonmetal heatsink,which is the key component of a laser device to produce light amplification by stimulated emission of radiation.Due to end uniform pumping and high efficient cooling on disk back side the direction of heat flux is parallel to the disk axis.The heat sink can also restrain the thermal deformation of thin-disk crystal.Thus,thin-disk laser allows realization of lasers with high output power and good beam quality,simultaneously.Relative to slab and fiber laser gain medium,thin-disk laser gain mirror have the characteristics of large laser spot area,low laser peak power density,thin-disk laser gain medium,which make the laser pulse suffer very low nonlinear effect in the thin-disk gain medium.Therefore,thin-disk laser is the best choice to obtain laser output with high pulse energy,short pulse width,high average power and high beam quality.It is mastering the thermal distortion law of the disk laser gain mirror that is the basis for designing and fabricating the thin-disk laser gain mirror.Systematic research on the generation mechanism and control method of spherical and aspherical thermal distortion of thin-disk laser gain mirror is carried out.The main content includes:(1)Based on comprehensive analysis of the generation mechanism of the thermal distortion the thin-disk laser gain mirror,thermal distortion can be divided into spherical thermal distortion and aspherical thermal distortion.The spherical thermal distortion is characterized by specific dioptric power,and the aspherical thermal distortion is characterized by TEM₀₀ mode diffraction loss.Research results show that the thermal distortion mainly comes from the thermo-optic effect,the thermo-axial strain effect,the thermo-elastic optic effect and the thermal deformation of the back surface of the thin-disk medium.(2)Physical models and numerical analysis methods are used to research the influence of characteristics of optical devices(initial surface profile,thickness of thin-disk medium,highly reflective coating and end cap),package characteristics(solder and heat sink)and characteristics of laser distribution(pumping laser and oscillation laser)on the spherical thermal distortion and the aspherical thermal distortion.The numerical calculation results show that the change of the initial spherical surface on the variation of specific dioptric power of the thin-disk laser gain mirror is less than 1%.Compared to other factors,the variation of spherical distortion and aspherical thermal distortion caused by different initial spherical surface is very small.The influence of the initial spherical surface on the thermal distortion of the thin-disk laser gain mirror can be ignored.When the size of the pump spot on the thin-disk laser gain mirror is small,increasing the thickness of the thin-disk medium and the thermal resistance of highly reflective coating can correct the negative lens spherical thermal distortion,but the aspherical thermal distortion will increase.Using yttrium aluminum garnet(YAG)end cap,silicon carbide(Si C)end cap and diamond end cap,the negative lens spherical thermal distortion can be effectively corrected in case of large pump spot radius(2?12 mm).However,the YAG end cap will increase the aspherical thermal distortion in the pumped region,Si C end cap will increase the aspherical thermal distortion affected zone at the edge of the pump spot.Si C end cap can reduce the aspherical thermal distortion in the not-pumped region.Diamond end cap can reduce the aspherical thermal distortion in the pumped region and the not-pumped region simultaneously.The thermal distortion of thin-disk laser gain mirror is not affected by the prestress produced by Au Sn soldering process.Using heat sink with high thermal conduction,low thermal expansion and high bending resistance,the negative lens spherical thermal distortion of thin-disk laser gain mirror can be corrected,the aspherical thermal distortion in the not-pumped region will also be reduced.When the pumping distribution is high order super-Gaussian profile,reducing the size of the pump spot can correct the negative lens spherical thermal distortion of thin-disk laser gain mirror.Using pumping distribution with low order super-Gaussian or truncated-Gaussian profile,the negative lens spherical thermal distortion of thin-disk laser gain mirror can be corrected.When the negative lens spherical thermal distortion is corrected completely,the aspherical thermal distortion will be reduced.Increasing the transmissivity of output coupler of resonator can correct the negative lens spherical thermal distortion of thin-disk laser gain mirror.When intracavity mode changes from TEM₀₁ mode to TEM₀₀ mode,the negative lens spherical thermal distortion will increase.(3)The spherical thermal distortion can be determined by measuring the change of propagation characteristics of a laser beam after reflection on thin-disk laser gain mirror.This method is also calibrated by the discriminant method according to mode transformation with translation of the stability zone.Space carrier interferometry is used to measure the total thermal distortion and the aspherical thermal distortion of the thin-disk laser gain mirror.The above experimental data are in agreement with the simulation results.Thus,the validity and correctness of the thermal distortion correction theory of the thin-disk laser gain mirror are established in this paper.(4)We propose that through the comprehensive optimization design of thickness of thin-disk medium,material of end cap,thickness of end cap,material of heatsink and pumping distribution,the paraxial spherical thermal distortion can be corrected completely.Considering three cases of super-Gaussian pumping profile without end cap,truncated-Gaussian pumping profile without end cap and super-Gaussian pumping profile with end cap,we design and fabricate thin-disk laser gain mirror with ideal spherical thermal distortion correction effect.When the pumping distribution is a high order super-Gaussian profile with a half width of 2 mm,the change rate of dioptric power with pump power density of the thin-disk laser gain mirror made by our group is-0.055m^-1/(k W/cm²),the paraxial spherical thermal distortion is smaller than that of similar products of D+G company in Germany.