Passively Q-switching Self-frequency Doubling Laser Performances Of Nd:YCOB,Nd:GdCOB And Nd:CNGS Crystals

Pulsed lasers in the visible range,especially high-power pulsed green light,have important applications in many fields.At present,laser diode(LD)pumped self-frequency-doubling(SFD)passive Q-switching(PQS)technology is one of the main ways to obtain high-power pulsed green light.It has the advantages of simple and compact device,low cost and high pulse peak power.The self-frequency-doubling laser crystal and saturable absorber are two vital and necessary components in SFD PQS technology.Nd:YCOB,Nd:GdCOB and Nd:CNGS crystals have excellent comprehensive self-frequency-doubling laser performance.High-power pulsed green light can be realized by inserting a suitable saturable absorber into the cavity.In 2004,the successful exfoliation of graphene opened the door to two-dimensional materials,and more and more two-dimensional materials have entered the eyes of the world.Since the two-dimensional materials have many unique properties different from the bulk material,such as large band gap,layer-related properties,broadband saturable absorption characteristics,and large modulation depth.In 2009,as a saturable absorber,the graphene film had sucessfully applied in the output of passive mode-locked pulses in fiber lasers,which brings new hopes and opportunities to the development of slow-moving saturable absorber materials.Then a large number of two-dimensional materials have been used in passive Q-switched experiments in the 1?3 pm wavebands successively and some results comparable to conventional saturable absorbers have been acquired.However,continuous self-frequency doubling laser output has a natural disadvantage compared to continuous fundamental frequency laser output,resulting in few reports about self-frequency-doubling passive Q-switching experiments with a saturable absorber based on two-dimensional materials.In response to this problem,based on the sufficient investigation of their nonlinear properties,this paper chooses to combine the laser crystal with excellent self-frequency-doubling laser output capability and the saturable absorber based on two-dimensional material.Several new SFD passive Q-switching operations are realized.The following is the main content of this paper:(1)Nonlinear properties of Nd:YCOB,Nd:GdCOB and Nd:CNGS crystalsWe studied the frequency-doubling phase matching characteristics of Nd:YCOB,Nd:GdCOB and Nd:CNGS crystals,and calculated the effective nonlinear coefficients along the phase matching direction and the full-space direction of Nd:YCOB and Nd:GdCOB crystals.The effects of independent components of nonlinear coefficients on the values and distribution of the global effective nonlinear coefficients are studied.The results show that the phase matching of Nd:YCOB and Nd:GdCOB crystals are symmetric in the first and second octants.And their maximum effective nonlinear coefficients of type_? frequency-doubling at 1064 nm corresponding to the emission peak of Nd3+ ions appear in the second octant,their values are 1.4 pm/V and 1.3 pm/V,respectively.The corresponding angles are(113°,37°)and(113°,47.6°).The maximum effective nonlinear coefficient of type_?frequency-doubling of Nd:YCOB crystal also appears in the second octant,and the corresponding maximum effective nonlinear coefficient is 0.32 pm/V,and the corresponding phase matching angle is(1 12°,81°).Nd:GdCOB crystals do not have type_? phase matching at 1064 nm.The distribution of spatially effective nonlinear coefficient in the first and second octants of Nd:YCOB and Nd:GdCOB crystals are composed of four peaks.Their area distribution and relative values are basically the same,but there is a difference in values of |deff|.In the Nd:YCOB crystal,the variation of the d13,and d32 coefficient values has the greatest influence on the distribution of the whole |deff|,and the distribution area of the peak and the value of the |deff| change significantly with their changes.The change in wavelength has little effect on the |deff| distribution and it can be negligible.The distribution of Nd:YCOB and Nd:GdCOB crystals in the whole three-dimensional space are all composed of peaks of two different clover shapes,and their maximum effective nonlinear coefficients are larger than the maximum effective nonlinear coefficient in the phase matching direction.The phase-matching angles of the type_?and type_? of the positive uniaxial crystal Nd:CNGS crystal at the wavelength of 1064 nm are 36.3° and 55.2°,respectively.Corresponding effective nonlinear coefficient values wre 0.47 pm/V and 0.41 pm/V at 1064 nm.(2)PQS SFD laser performance of Nd:YCOB crystalThe 5 at.%Nd:YCOB crystal with the best laser performance was selected,and the continuous laser output at 1064 nm was obtained by LD pumping.The crystal length was 10 mm,and the two sides were coated.Acquired maximum average output power was 315 mW.The SFD PQS output of Nd:YCOB crystal is realized by the traditional 97.5%Cr4+:YAG crystal.When the incident power is 4.1 W,the average output power of pulsed green light is 16.2 mW,and the corresponding pulse repetition frequency,pulse width,calculated single pulse energy and peak power are 3.5 kHz,60.2 ns,4.6 ?J and 75.8 W,respectively.The graphene film prepared by the combination of liquid phase separation and drop method has a wide spectral transmission range,and a significant absorption peak exists in the wavelength range of 900?1100 nm.When the incident optical power density is 18.3 MW/cm2,the actual modulation depth of the graphene film was 3%.A maximum average output power of up to 13.8 mW was obtained at the incident pump power of 2,84 W.At the same time,the repetition frequency of the Q-switched pulse is increased from 13.1 kHz to 14.4 kHz,and the pulse width is shortened from 283.2 ns to 181.3 ns.The calculated single pulse energy is increased from 0.5 ?J to 0.96 ?J and the obtained maximum peak power is 5.3 W.The saturable absorber based on layered graphene has a lower laser threshold and the stable pulse sequence is acquired at lower incident pump power.As the thickness of the graphene SAs material increases,the pulse width of the Q-switched pulse decreases,the pulse repetition frequency gradually increases,and the maximum single-pulse energy obtained gradually decreases,but the peak power does increase gradually.The SnSe2-PVA film prepared by the combination of liquid phase separation and drop method has a linear transmittance of 77%in the wavelength range of 700 nm-1600 nm.From 700 nm to 200 nm,the transmittance gradually reduces.When the incident light intensity is 16 MW/cm2,the actual modulation depth is 2.3%.A maximum average output power of 19.6 mW was obtained at incident pump power of 4.13 W.The shortest pulse width of 91.9 ns and corresponding repetition rate of 17.6 kHz were obtained under the condition of maximum average output power.The calculated single pulse energy and peak power are 1.1 ?J and 12.1 W,respectively.In the wavelength range of 650?1600 nm,the linear transmittance of the tin disulfide film is above 74.5%,and the transmittance gradually reduces from 650 nm to 200 nm.A maximum average output power of 14.5 mW is achieved at incident pump power of 3.7 W.The shortest pulse width of 124.3 ns and the repetition rate of 2.3 kHz were obtained under the condition of maximum average output power.The corresponding single pulse energy and peak power are 0.68 ?J and 5.5 W,respectively.The monolayer molybdenum disulfide film achieved a maximum average output power of 18.1 mW at incident pump power of 4.38 W,which corresponds to a minimum pulse width of 210.4 ns and a repetition rate of 57.0 kHz.At the same time,calculated single pulse energy and peak power are 0.32 ?J and 1.5 W,respectively.In the wavelength range from 250 nm to 1600 nm,the linear transmittance of CN film is above 62.5%.The transmittance edge extends to below 200 nm in the ultraviolet band and there is no obvious absorption peak.The maximum average output power of 9.6 mW is obtained at incident pump power of 3.7 W,which corresponds to the shortest pulse width of 168.0 ns and the repetition rate of 24.5 kHz.The calculated single pulse energy and peak power are 0.39 ?J and 2.3 W,respectively.(3)PQS SFD laser performance of Nd:GdCOB crystalThe 5 at.%Nd:GdCOB crystal with the best laser performance was selected,and the continuous laser output at 1064 nm was obtained by LD pumping.The crystal length was 10 mm,and the two sides were coated.Acquired maximum average output power is 181.3 mW.The SFD PQS output of Nd:GdCOB crystal is realized via the 97.5%Cr4+:YAG crystal.When the incident pump power is 3.27 W,the maximum pulse peak power of 159.7 W is obtained,and the corresponding pulse width,the repetition frequency and single pulse energy are 19.2 ns,10.6 kHz,and 3.1 ?J,respectively.The graphene film and 3-layer graphene sample were used as the saturable absorber to realize SFD PQS output of the Nd:GdCOB crystal.For the graphene film,the maximum Q-switched average output power of 10.4 mW is obtained at incident pump power of 3.7 W.While the repetition frequency of the Q-switched pulse increases from 7.93 kHz to 10.2 kHz,the pulse width shortens from 392.4 ns to 296.6 ns.The calculated single pulse energy increases from 0.49 ?J to 1.01 ?J,and acquired maximum peak power is 3.4 W.For the 3-layer graphene sample,the maximum average output power of the Q-switched green light is 12.7 mW.The corresponding repetition frequency and pulse width are 7.0 kHz and 437.3 ns,respectively.The calculated single pulse energy and peak power are 1.8 ?J and 4.2 W,respectively.Using SnSe2-PVA film as laser modulation component,the maximum average output power of 14.5 mW is obtained at incident pump power of 3.3 W.The corresponding repetition frequency and pulse width are 22.1 kHz and 48.7 ns,respectively.The calculated single pulse energy and peak power calculated are 0.66?J and 13.5 W,respectively.When a single-layer molybdenum disulfide film was inserted into the self-doubled laser cavity of Nd:GdCOB crystal,the SFD PQS operation realizes and the maximum average output power of 13.7 mW was obtained at incident pump power of 4.55 W.The corresponding repetition frequency and pulse width are 100.0 kHz and 105.4 ns,respectively.The calculated single pulse energy and peak power are 0.14 ?J and 1.3 W,respectively.Using the tungsten selenide-molybdenum composite film material as the saturable absorber,we also successfully realize the passive Q-switched pulsed green light output of Nd:GdCOB crystal,and obtain maximum average output power of 5.6 mW at incident pump power of 3.53 W.The corresponding repetition frequency and pulse width are 186.6 kHz and 376.5 ns,respectively.The calculated single pulse energy and peak power are 0.03 ?J and 0.08 W,respectively.(4)PQS SFD laser performance of Nd:CNGS crystalWe obtained self-frequency doubling green light output using a flat-flat cavity structure.Its laser threshold is 0.8 W and we obtain 62.5 mW maximum green output at incident pump power of 4.3 W.The SFD PQS operation of Nd:CNGS crystal is realized via graphene film.When the incident pump power is 3.7 W,the Q-switched green light output power of 10.5 mW is obtained.The corresponding pulse repetition frequency is 167.7 kHz and pulse width is 340.6 ns.The calculated single pulse energy and pulse peak power are 0.06?J and 0.18 W,respectively.The SFD PQS output of Nd:CNGS crystal is also realized by using tin telluride film as saturable absorber.The obtained maximum average output power is 9.5 mW at incident pump power of 2.84 W.The corresponding pulse repetition frequency and pulse width are 174.5 kHz and 129.2 ns,respectively.The calculated single pulse energy and peak power are 0.05 ?J and 0.42 W,respectively.