Appl Phys B(2011)104:861–866
DOI10.1007/s00340-011-4462-y
High electro-to-optical efficiency180W Q-switched532nm lar with a pulwidth of70ns
S.Zhang·L.Guo·B.Xiong·Y.Liu·W.Hou·X.Lin·
J.Li
Received:6December2010/Revid version:21January2011/Published online:16March2011
©Springer-Verlag2011
Abstract A compact straight cavity with two side-pumped Nd:YAG lar heads and a90°quartz rotator in between is prented.By intracavity-frequency-doubling with a type II LBO crystal in this cavity,an output power of180.2W at 532nm with a repetition rate of10kHz was achieved,cor-responding to an electrical-to-optical efficiency of10.9%. To best of our knowledge,this is the highest electrical-to-optical efficiency of the high power green lars with above 100W output power,ever reported.The pulwidth was 70ns and the peak power was257.4kW.The beam pa-rameter product(beam waist multiplied by ha
lf beam di-vergence angle)was estimated to be4.2mm mrad and the powerfluctuation over2.5hours was calculated to be better than±1.2%.
1Introduction
Diode pumped solid-state green lars(DPSSGL)with an average output power in excess of100W have developed rapidly at recent years.The green lars perform better than IR lars in many applications,such as processing of some materials with high IR reflCu,Au),precision micro solar cells manufacturing),lar ther-apy of benign prostatic he GreenLight S.Zhang( )·L.Guo·B.Xiong·Y.Liu·W.Hou·X.Lin·J.Li Laboratory of All-Solid-State Light Sources,Institute of Semiconductors,Chine Academy of Sciences,P.O.Box912, Beijing100083,China
e-mail:shbzhang@mi.ac
Fax:+86-10-82305052
S.Zhang·B.Xiong
Graduate University of Chine Academy of Sciences,Beijing 100080,China HPS™Lar System fro
m AMS)and underwater commu-nication due to lower absorption in water.Besides,the high power green lars are usually ud to pump Ti:sapphire lar systems and generate UV 355nm[1], 266nm[2,3]).
So far,many DPSSGLs with an average output power in excess of100W have been reported.Kiriyama et al.
[4]and Hirano et al.[5]have obtained an output power of more than130W at532nm,respectively,by extracavity-frequency-doubling with a complex MOPA system.By intracavity-frequency-doubling,Le Garrec et al.[6]have achieved106W at532nm with a Z-shaped cavity,of which the electrical-to-optical efficiency was5.4%;Utilizing a L-shaped cavity,Konno et al.[7]have gotten an average power of138W at532nm with an electrical-to-optical efficiency of7.9%;Bo et al.[8]have also obtained a green lar power of120W corresponding an electrical-to-optical ef-ficiency of6.5%.Although the average power of532nm green lars using diode-side-pumped lar modules have incread rapidly to more than400W[9]during recent years,however,the electrical-to-optical efficiency of the lar systems still maintained at about7.9%[7].
To achieve an average output power in excess of100W with a high electrical-to-optical efficiency,gre
en lars with a compact straight cavity[10]attracted our interest.And with a compact straight cavity we have achieved an aver-age power of69.2W at532nm with an electrical-to-optical efficiency of10.4%[11].High-power high-efficiency side-pumped Nd:YAG lar modules are esntial for the gen-eration of high-power high-efficiency green lars.In ad-dition,the frequency doubler is another key element for high-power high-efficiency green lars.The usually ud high-efficiency frequency-doublers are LiBO3(LBO)and KTiOPO4(KTP).However,gray-tracking problem and the relatively low damage threshold limit the u of KTP for
862S.Zhang et al.
Fig.1Schematic of the straight plano–plano cavity(M1:the plane total reflector,M2,M3: the plane dichroic mirrors, QR:quartz
rotator)
high-power green lar generation.LBO crystal was chon
as the frequency doubler owing to the high damage threshold
(2.5GW/cm2),no gray-tracking problem and the relatively
high nonlinear conversion coefficient(0.85pm/V)[12].
In this paper,a compact straight cavity with two diode-
side-pumped Nd:YAG lar modules is prented,which
can generate an average power of180.2W at532nm with
a repetition rate of10kHz and a pul width of70ns.
The corresponding electrical-to-optical efficiency is about
10.9%,which is believed to be the highest electrical-to-
optical conversion efficiency of the high power green lar
迪卡侬简介
sources with above100W output power ever reported.
2Experimental tup
The straight plano–plano cavity with two Nd:YAG lar
heads is shown schematically in Fig.1.The compact cav-
ity is just630mm long with l1=l2=l=200mm.The short cavity means low diffraction loss,which ensures the
high-efficiency high-power lar output.In the lar head,
the lar diode arrays are distributed in afive-fold symme-
try around the Nd:YAG rod(4mm in diameter,100mm
in length,with0.6%Nd doping)with an available maxi-
mum pump power of approximately510W at808nm when
working at drive current of20.0A with an input electrical
pump power of1030W.The electrical-to-optical efficiency
of the diode arrays is50%.Figure2shows the schematic of
the side-pumped lar module.The Nd:YAG rod is cooled
withflowing water.And the lar diode bars are soldered
to a water-cooled copper sink,which allows utilization of
distilled cooling water.This is a major advantage compared
to micro-channel cooled bars,which require PH-controlled
cooling water.The pumping configuration of the Nd:YAG
lar head was optimized to enhance the electrical-to-IR
efficiency.The distance between diode array and Nd:YAG
rod was adjusted to improve gain distribution uniformity.
Theflow tube and the reflector were AR and HR coated at
808nm,respectively,to increa transfer efficiency.Besides,
an optimum Nd3+concentration was chon for a high ab-
sorption efficiency of the pump radiation.
In a symmetrical plano–plano resonator(300mm long)
with a partial output coupler of T=20%,the output charac-
teristics of the lar head at1064nm was investigated.And
213.0W of CW1064nm was achieved at diode drive cur-
rent of20.0A,corresponding to an optical pump power
of Fig.2Schematic of the side-pumped lar module
about510W.The optical-to-IR efficiency was41.8%and the electrical-to-IR efficiency was20.7%.
A90°quartz rotator was inrted between the two lar heads for compensation of polarization dependent thermal birefringence.The birefringence compensation reduced de-polarization loss and eliminated bifocusing of the rod, which both enhanced the output power and improved the beam quality.Two acousto-optic(AO)Q-switches(QSG40-1Z,by26th Rearch Institute of CETC)were placed or-thogonally to improve the hold-off capacity[8]and driven synchronously by a single power supply,operating at a rep-etition rate of10kHz.
The LBO crystal(4×4×18mm3,type II pha match-ing at100°C),AR coated at532nm&1064nm(T>99.8% @1064nm&T>99.5%@532nm),was mounted in a copper oven.The temperature of the oven was precily controlled with a precision of±0.1°C,which directly con-tributed to the high efficiency high-stability cond har-monic generation(SHG).The oven was placed as clo to the end dichroic mirror M2(0◦R>99.8%@1064nm& T>98.5%@532nm)as possible,where the lar spot size was small,in order to obtain a high power density in the LBO crystal.The cond harmonic wave,generated in two directions,was extracted unidirectionally from the dichroic mirror M2by the total reflection of dichroic mirror M3(0◦T>99.8%@1064nm&R>99.5%@532nm).
High electro-to-optical efficiency180W Q-switched532nm lar with a pulwidth of70ns
863
Fig.3The measured average thermal focal length of Nd:YAG rod
3Thermal stability analysis
广州医科大学是211吗Thermal lensing effect of the lar head was investigated in a symmetric plano–plano resonator with the method intro-duced in[13].When the output power decread rapidly to zero at some optical pump power P in,the average thermal focal length of the lar head(f)at optical pump power P in equaled a quarter of the cavity length(L/4).The measured average thermal focal length versus the optical pump power is given in Fig.3.The average thermal focal length decreas from600mm to about230mm with the input optical pump power increasing from260W to about540W.
With the standard ABCD ray propagation matrix,we cal-culated the stable region of the resonator versus the aver-age thermal focal length f.The focal length of radial po-larization and tangential polarization,f r and fφ,are de-termined by relationships,(1)the ratio of the focal length f r/fφ=1.2,and(2)the definition of average thermal fo-cal length f=(f r+fφ)/2[14].Beam radius of TEM00
听桥mode at the principal plane of the left Nd:YAG rod ver-sus the average thermal focal length is shown
in Fig.4.The solid line reprents the radial polarized component and the dashed line stands for the tangential polarized component. There is a narrow unstable region when the thermal focal length decreas to about500mm corresponding to an opti-cal pump power of290W for each lar head(e Fig.3). In the two stable regions,the beam radius of TEM00mode almost maintains a constant value of0.4mm as the focal length decreas,which contributes to the establishment of a stable multi-mode oscillation.The stable multi-mode oscil-lation will result in a stable output at a constant pump power, or lead to a linear increa of the output power as the pump power increas during the stable zone.And when it comes to the edge of the stable region,the stable multi-mode oscil-lation will be tampered,which will lead to sharpfluctuation and rapid decrea.At thermal focal length of300mm,
the Fig.4Beam radius of TEM00mode at the principal plane of the left Nd:YAG rod versus the average thermal focal length(the red solid line for radial polarization,the blue dashed line for tangential polarization) TEM00mode radius in the LBO was calculated to be about 0.2mm,which would lead to a four times higher power den-sity than that in Nd:YAG rod.
4Experimental results and discussions
哺乳期能怀孕吗The characteristic of the resonator at1064nm was investi-gated by replacing the end dichroic mirror M2with an op-timum output coupler(T=30%@1064nm)and remov-ing the LBO crystal.It was found that only when the two lar heads have the identical thermal effects,namely,the identical input–output relationship under the same condi-tion,a high output power at1064nm with a high efficiency can be obtained from the resonator.Otherwi,the different thermal lens of the lar heads narrow the thermal-stability region.Conquently,the narrowed stable zone limits the pump power as well as the output power to a low level. Accordingly,we carefully cho two lar heads posssing the same input–output relationship for the following experi-ment.The output power at1064nm versus the diode current is shown in Fig.5.When operating at continuous wave(CW) mode,an output power of284.5W at1064nm was achieved at the diode current of16.5A corresponding to a total opti-cal pump power of about840W and an electrical power of 1675W.And when it was Q-switched with a rep
etition rate of10kHz,the output power decread to233.7W with a loss of about18%.The pulwidth at233.7W was84ns, and the peak power was calculated to be278.2kW.And the multi-mode1064nm beam radius at the end mirror was measured to be0.9mm.The decrea of slope efficiency around optical pump power of about600W was due to the fact that it entered the unstable region around the thermal focal length of about500mm shown in Fig.4.The output
864S.Zhang et al.
侵蚀的近义词
Fig.5Output power and pulwidth of1064nm lar versus the optical pump power. The cavity was about630mm long with l=200mm and the transmittance of the output coupler was30%at1064
nm
Fig.6Average power and pulwidth of532nm lar versus the optical pump power. The cavity is about630mm long with l=200
mm
power incread linearly during the two stable regions be-fore it came to the edge.
When coming to cond harmonic generation by intra-cavity-frequency-doubling,the temperature of the oven can-not be controlled under the desired precision as the green output power reached hundred watts at high pump level, and the output power at532nm dropped rapidly when the temperature went out of control.The rapid temperature ri aro from the lf heating effect due to more absorption of both fundamental wave and harmonic wave in compar-ison with that at low pump level[15].Then we improved the configuration of the oven to ensure that the temperature can be controlled with the desired precision not only at low pump level but also at high pump level.Itfinally works very well,and the issue has not emerged again by now.
Figure6shows the average output power of532nm lar as a function of the electrical current.When the diode cur-rent reached16.3A with a total optical pump power of about830W and an electrical power of1655W,a max-imum output power of180.2W at532nm was achieved with a repetition rate of10kHz and a pulwidth of70ns (e Fig.7).The peak power of532nm lar reached as high as257.4kW.The optical-to-optical efficiency and the electrical-to-optical efficiency were21.7%and10.9
%, respectively.To best of our knowledge,this is the high-est electrical-to-optical efficiency of the ever reported c-ond harmonic generation systems with an average output power in excess of100W.It is believed that the high-efficiency lar head and the high-conversion-efficiency fre-quency doubler directly contributed to the high electrical-to-optical efficiency.The ratio of532nm output to the optimum 1064nm output was estimated to be higher than77.1%.Be-sides,the straight cavity configuration was believed to be a third factor for its compactness and low loss compared to the complex folded cavities as we analyzed in[11].
At the average green power of about180W,the green beam diameters at different places were measured with a Lar Beam Analyzer from Spiricon Inc.The beam para-meter product(beam waist multiplied by half beam diver-gence angle)was estimated to be4.2mm mrad with beam quality factor M2≈25by Hyperbolicfitting.The high M2 value esntially resulted from the short cavity(630mm
High electro-to-optical efficiency 180W Q-switched 532nm lar with a pulwidth of 70ns
865
Fig.7Green lar pul at 180W with a repetition rate of 10kHz.The pulwidth was 70
ns
Fig.8Two-dimensional far-field intensity profile of 532nm beam at about 180W
免疫系统紊乱的症状long with l =200mm),which allow a multi-transver-mode oscillation.The diffraction loss of lar cavity be-comes lower as the cavity length decreas,since the Fresnel number is inverly proportional to the cavity length.Lower diffraction loss leads to the oscillation of more high-order transver modes,which results in a high M 2value.How-ever,the low diffraction loss of the short cavity ensures the high-efficiency and high-power lar output.The far-field intensity profile of the green beam (Fig.8)was Gaussian-like intensity profile,which is exactly the desired inten-sity distribution for various actual applications.The green power fluctuation around 180W was measured,which is shown in Fig.9.And the power fluctuation over 2.5hours was calculated to be better than ±1.2%using the equa-tion ± P P =±(P max −P min )/2P
.Additionally,power degrada-
tion was not obrved after hours of continuous opera-tion.
Fig.9Power fluctuations over 2.5hours at an output power of about 180W
5Conclusions
In conclusion,a compact straight plano–plano cavity with two side-pumped Nd:YAG lar heads in ries was in-troduced.With this cavity an average power of 233.7W at 1064nm have been obtained with a peak power of 278.2kW at a repetition rate of 10kHz.Then by intracavity-frequency-doubling with a type II LBO crystal,a maxi-mum output power of 180.2W at 532nm green lar was achieved.The corresponding electrical-to-optical efficiency was 10.9%,which,to best of our knowledge,is the high-est electrical-to-optical efficiency of the high power green lars with above 100W output power ever reported.The pulwidth of Q-switched 532nm lar was 70ns corre-sponding to a peak power of 257.4kW.The beam parameter product was estimated to be 4.2mm mrad (M 2≈25).The green beam had a Gaussian-like intensity distribution,de-sired distribution for various actual applications.The power fluctuation over 2.5hours was calculated to be better than ±1.2%,and none power degradation trend was obrved.
Acknowledgement This work was supported by the National High Technology Rearch and Development Program of China (Grant No.2008AA030116),the National Key Basic Rearch Program of China (Grant No.2010CB933800),and the Project of Development and Transformation of Scientific Equipment from Chine Academy of Sciences (Grant No.YZ200801).
苦瓜炒蛋
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