Studies On High-power Self-frequency-doubled Slab Lasers

Visible lasers have been widely used in daily production and life,involving medical,military,entertainment,scientific research,industry,and other aspects.At present,the use of semiconductor lasers to directly obtain visible lasers is the simplest and most efficient method.At present,the technology of blue-green laser with wavelength below 530 nm and red and near-infrared semiconductor laser with wavelength above 620 nm has been mature and commercialized,occupying a large share in the laser market.However,in terms of short-wavelength semiconductor lasers with wavelengths of 530-620 nm,due to the insurmountable difficulties in the preparation of raw materials and device processing of semiconductor materials,the research progress of semiconductor lasers with wavelengths of 530 nm-620 nm is slow and still has not reached the stage of practical application.At present,there are several methods to obtain 530 nm-620 nm visible laser by using nonlinear optical technology:1.The laser emitted by the semiconductor laser whose output band is infrared wavelength passes directly through the frequency-doubled crystal,and the frequency-doubled crystal is used to convert the laser in the infrared wavelength band into visible laser.Although this method is extremely simple and easy to implement,the output visible laser has a wider spectrum,and its wavelength stability is consistent with infrared semiconductor lasers,and the wavelength of semiconductor lasers will drift with temperature changes,so the wavelength stability of visible laser produced by frequency-doubled is also poor,so this kind of laser is not widely used;2.By using a diode laser as the pumping source to pump the laser crystal doped with rare earth ions such as Nd3+,Yb3+,the infrared laser output can be obtained first,and then the green laser can be obtained by using the frequency-doubled crystal.Since the beam quality and stability of the infrared laser are excellent,the spectrum and beam quality of the obtained frequency-doubled laser are very good,but the disadvantage of this method is that the laser system needs at least two crystals to realize it,so the structure is more complicated,the cost is high,and limited by the emission band of rare earth ions,and the number of available visible light laser wavelengths is limited;3.Although a blue laser diode can be used as a pump source to pump Dy3+ or Tb3+ ion-doped crystals to achieve yellow laser output,the current output power of the blue pump source is limited,which limits the output power of this method,so it is still need to use nonlinear optics technology to obtain the yellow laser.Frequency doubling and sum frequency are the two main technologies for producing yellow lasers,but the device to obtain yellow laser requires at least one laser crystal and one nonlinear optical crystal.Therefore,there are still some shortcomings in this laser,such as high cost,complex structure,and difficult adjustment.4.In recent years,it has been reported that red laser,orange laser and green laser can be directly generated by using a blue semiconductor laser to pump a crystal doped with Pr3+ ions.This method is indeed simple and effective,and the output wavelength is also diversified.However,the currently used blue semiconductor laser pump source has low power,so it fundamentally determines the low output power of the visible laser obtained by pumping the Pr3+ laser,which greatly limits its widespread application.5.The simplest and most effective method is to use a crystal that can emit infrared laser and double the frequency of infrared laser,so that the output of self-frequency-doubled visible laser can be directly obtained.This kind of crystal is called self-frequency-doubled crystal.Since only a single crystal is needed in a self-frequency-doubled laser to achieve fundamental frequency laser oscillation and frequency doubled laser output,this kind of laser has many advantages such as simple structure,low cost,convenient adjustment,high integration,high stability and so on.It is also a research hotspot of visible laser.In 2011,our research group used a Nd:GdCOB self-frequency-doubled crystal,which has achieved a self-frequency doubled green laser output of 3.01 W and has been commercialized.However,due to the low thermal conductivity of the crystal itself,poor heat dissipation ability,and the end pumped rod crystal laser structure used in the experiment.When the pump power is increased,the heat inside the crystal cannot be dissipated in time,and the internal heat effect of the crystal is large,so increase the pump power is also difficult to achieve higher power self-frequency-doubled laser output.It is an efficient,high-power,and compact way to obtain yellow laser output directly through self-frequency-doubling crystals.However,for many years,there has not been a crystal that can directly generate yellow laser fundamental wavelength with high power output,so the development is limited.Through the electron phonon coupling theory,our research group has successfully realized the wavelength expansion in Yb:YCOB crystal and obtained the fundamental laser output at 1140 nm.Then we successfully realized the self-frequency-doubled yellow laser output in the crystal by using the frequency-doubled effect of Yb:YCOB crystal.At present,we have obtained the output of 1.71 W self-frequency-doubled yellow laser,which fills the blank of self-frequency-doubled yellow laser.Although low-power self-frequency-doubled lasers have been commercially produced,with the increasing demands of people in production and life in modern society,self-frequency-doubled visible lasers with single output wavelength and watt-level output power are far not meeting people's higher requirements,more and more people yearn for highly integrated and high-power visible lasers.However,high-power,multi-wavelength self-frequency-doubled lasers still face challenges.This is because in the self-frequency-doubled process,the fundamental frequency laser generation and the frequency doubling effect are carried out at the same time,and it is not a simple addition of the laser and the frequency doubling.It is necessary to consider both the effective emission of the fundamental frequency laser and the high-efficiency conversion of the frequency doubling.This requires consideration of the influence of factors such as the best absorption cross-section,the best emission cross-section,and the best phase matching direction.In order to obtain a high-power self-frequency-doubled laser,the influence of the thermal effect of the crystal is crucial.This requires us to clarify the heat generation and temperature distribution of the crystal during the self-frequency-doubled process,so as to optimize the structure,reduce the thermal effect of the crystal,and increase the output power of the self-frequency-doubled laser.To obtain multi-wavelength self-frequency-doubled lasers,it is necessary to deepen the research on the theory of electric-phonon coupling and carry out research on multi-wavelength self-frequency-doubled lasers.Different laser wavelengths correspond to different application fields,and visible lasers with diversified wavelengths are also the current development direction.Visible lasers with high power output should have broad application prospects in many fields such as laser display,medicine,military,and scientific research,so we seek a simple structure,low cost,and high-power output multi-wavelength self-frequency-doubled lasers are imperative.This thesis aims to increase the output power of the self-frequency-doubled laser and expand the output wavelength of the self-frequency-doubled laser.A series of progress has been made in the self-frequency-doubled laser crystal represented by Nd:GdCOB and Yb:YCOB.A theoretical model of self-frequency-doubling laser was established,the heat production and heat distribution during laser operation were analyzed,the technical scheme of slab laser to promote heat dissipation was proposed,and the optimal crystal thickness was calculated.The output of ten watts 545 nm self-frequency-doubled green laser is realized in Nd:GdCOB crystal,which is currently the highest level in the world.At the same time,the electron-phonon coupling theory in the self-frequency-doubled crystal was developed,and the strongly coupled Yb3+ rare earth ion was used as the research object,breaking the limit of the fluorescence spectrum,and the tunability of 1140 and 1260 nm,self-frequency-doubled yellow laser output of 570 nm and self-frequency-doubled red laser output of 613 nm are realized in the Yb:YCOB crystal.The output power of self-frequency-doubled reaches watt level.Related work has enriched laser theory,led the research of all-solid-state lasers,and laid the theoretical foundation and core crystal material foundation for future multi-wavelength,highly integrated visible light all-solid-state lasers.The main works of this paper are as follows:1.Theoretical analysis of the internal thermal effect of the crystal during the self-frequency-doubled process.In the process of self-frequency-doubled laser generation,the heat generation expression and steady-state heat conduction equation in the crystal corresponding to several sources of heat generation in the crystal during the laser process are analyzed,and the heat generation per unit volume in the slab crystal is calculated.According to the expression and the boundary conditions,the analytical expression of the temperature distribution in the slab crystal during the laser process is obtained.2.Study on the performance of end-pumped Nd:GdCOB slab crystal green light self-frequency-doubled laser.The temperature distribution of end-pumped Nd:GdCOB rod crystals and slab crystals are analyzed,and two structures of part end-pumped rod crystals and part end-pumped slab crystals are simulated theoretically using software such as MATLAB,COMSOL and ANSYS.It can be seen from the results that the use of the slab-shaped crystal structure can greatly reduce the thermal effect inside the crystal,which can withstand higher input pump power and increase the laser output power.By using different software to simulate the temperature distribution inside the slab crystals of different thicknesses,the thickness of the slab crystals is determined,and the conclusion is pointed out that the smaller the crystal thickness,the smaller the thermal effect inside the crystal.In combination with the actual situation,the final thickness is determined.It is a 1 mm slab crystal.Since the slab crystal can greatly reduce the thermal effect inside the crystal,it can greatly increase the output power of the Nd:GdCOB crystal with lower thermal conductivity.The phase matching condition of the biaxial crystal is calculated theoretically,combined with the crystal refractive index equation,the best phase matching curve of the 545.5 nm self-frequency-doubled laser in the GdCOB crystal is calculated,and the best phase matching angle is(113°,49°).Through the optimization of the heat sink and the pump wavelength and pump spot in the experiment,the output power of the self-frequency-doubled green laser finally reached 17.91 W,which is the highest international level reported so far.At the same time,the prototype of self-frequency-doubled green laser with output power of 10 W has been fabricated.3.Study on the performance of Yb:YCOB slab crystal self-frequency-doubled laser of Yb:YCOB slab crystalIn the experiment,the absorption spectra of Yb:YCOB crystals with doping concentrations of 15 at.%and 20 at.%at room temperature in the phase matching direction were tested.The Yb:YCOB crystal is cut into slab-shaped along the phase matching direction for experiments.By designing the coating parameters,the pump spot size and crystal size are optimized,and finally a self-frequency-doubled yellow laser output with an output power of 7.02 W and a yellow wavelength of 570 nm is realized.