High power diode-pum ped passivelymode-locked Nd:YVO4 laser at repetition rateof 3.2GHz*

Meng-Yao Cheng(程夢堯),Zhao-HuaWang(王兆華),Yan-Fang Cao(曹艷芳),Xiang-Hao Meng(孟祥昊),Jiang-Feng Zhu(朱江峰),Jun-LiWang(王軍利),and Zhi-YiWei(魏志義),3,4,?

1SchoolofPhysicsand Optoelectronic Engineering,Xidian University,Xi’an 710071,China

2Beijing National Laboratory forCondensed MatterPhysicsand Institute ofPhysics,Chinese Academy ofSciences,Beijing 100190,China

3University ofChinese Academy ofSciences,Beijing 100049,China

4Songshan LakeMaterials Laboratory,Dongguan 523808,China

(Received 9 January 2019;revisedmanuscript received 25 January 2019;published online19April2019)

Keywords:all-solid-state laser,high repetition rate,Nd:YVO4 laser,mode-locked laser

1.Introduction

Ultrashort laser pulses with high-repetition-rate are needed in many applications,such as biological image,[1]high-speed optical communication,[2]and high-repetition-rate optical frequency comb.[3]Laserdiode(LD)pumped all-solidstate lasers have proven to be eff icient and compact laser sources,and Nd:YVO4crystal with large em ission crosssection[4]isan idealgainmedium to achievehigh-gain amplif ication.Shortcavitywith asemiconductorsaturableabsorber mirror(SESAM)is themostcommonway to achievehigh repetition rate picosecond lasers.To realize continuewavemode locking instead ofQ-sw itchedmode locking(QML),the intracavity pulse energy Epshould exceed a threshold given by[5]where Esat,Land Esat,Aare the saturation energy of the gain medium and the SESAM,respectively,andΔR is the modulation depth of the SESAM.The saturation energy of the gain medium Esat,Lis determined only by the type of gain medium,and the saturation energy of the SESAM Esat,Ais hardly to change for the commercial available SESAM s.For high-repetition-rate picosecond lasers,the intra-cavity pulse energy Epis relatively low.As a result,an SESAM with a smallmodulation depthΔR and an output couplerwith small transm ission is required in the cavity.For a short cavity with low transmission of the outputcoupler,it isdiff icult to realize high-repetition-rate and high-average-power continuouswave mode locking at thesame timeonly by an oscillator,so a laser system composed of an oscillator and an amplif ier is necessary.

Using an Nd:YVO4crystal and a short cavity,Krainer et al.f irst achieved a diode-pumped mode-locked laser with a repetition rate of 10 GHz and an average power of 2 W.[6]They also reported a 29-GHz 81-mW[7]mode-locked laser outputby using an Nd:YVO4crystal,one of the surfaces had a curvature and coated with a partial ref lectivity,serving as an outputm irror.By reducing the length of the Nd:YVO4crystal and using a high brightness Ti:sapphire laser as the pump source,they f inally achieved a laser with a repetition rate of 160 GHz and an average power of 110mW.[6]Using this structure,Lecom te etal.realized a diode-pumpedmodelocked laserwith a repetition rate of 40 GHz and an average power of 288mW.[8]Otherapproaches can also realize highrepetition-rate picosecond lasers.Liang etal.demonstrated a Kerr-lensmode-locked diode-pumped Nd:YVO4laserwith a repetition rate of 6 GHz and an average power of 0.8W.[9]Other kinds of gain media have also been demonstrated to beused to generate thehigh-repetition-rate picosecond lasers.Using an Nd:GdVO4crystal,Krainer et al.achieved a compactdiode-pumpedmode-locked laserwith an average power of 0.5W and a repetition rate of 10 GHz,[10]and Agnesi et al.reported a 2.5 GHz–2.7 GHz diode-pumpedmode-locked laser with an average power of 250 mW.[11]Using a semiconductor as a gain medium,Haring et al.realized a 6-GHz passively mode-locked semiconductor laser with an average powerof950mW.[12]

In thispaper,we reporta high repetition rate picosecond Nd:YVO4laserwith a high average power output.The laser system is comprised of an oscillator and an amplif ier.Two f iber-coupled LDsareused to pump theoscillatorand theamplif ier separately.The oscillator is a short cavity including a 2-mm long Nd:YVO4crystal and as SESAM.Under 6W of the pump power,we achieve a 1.38-W laser outputwith a repetition rate of 3.24 GHz and pulse duration of 11.4 ps at 1064 nm.A special pumping geometry in the oscillator is designed to avoid direct pump-light incidence on the SESAM,and a long-term stable operation isachieved.Using a stageof amplif ierbased on a 10-mm-long Nd:YVO4crystal,f inaloutputpowerof 11.3W isobtained with pump powerof 29.6W.The totaloptical-to-opticaleff iciency is about32%.Both oscillator and amplif ier are placed in an alum inum case whose footprint isonly 456mm×206mm,and their power stability within 24 hours is less than 0.6%(RMS).

2.Experimentalsetup

Figure 1(a)shows the schematic diagram of the experimentalsetup.Theoscillatorwith a shortcavity provided astablemode-locked laser to achieve high repetition rate,and the amplif ierwith a longer crystal achieved high average power by amplifying the laser from the oscillator.Two f iber-coupled laser diodeswith amaximum powerof 30W at808 nm were used as pump sources for the oscillator and the amplif ier respectively.Forboth oscillator and amplif ier,the coupled f iber foreach of the laserdiodeshasa core diameterof200μm and a numericalaperture of 0.22.The pump beam delivered from the f iberwas focused into the laser crystalsby a coupling system with amagnif ication of 1×,and the focused beam waist in the crystalwas about 200μm.The laser crystals in both oscillator and amplif ier were w rapped with indium foil and mounted tightly in a copperwater-cooled heatsink whichwas maintained at 14°C.To avoid Fabry–Perot etalon effect,all surfaces of the crystals had antiref lection coating at 808 nm and 1064 nm.As shown in Fig.1(b),for a long-term stable operation,all the lasersystem wasplaced in analuminum case whose footprintwasonly 456mm×206mm.

In theoscillator,ashortcavitywithan SESAM,adichroic m irror(DM 1),a gain medium(Nd:YVO41),and an output coupler(OC)were used to achieve a high-repetitionrate output.In previous high-repetition-rate experiments,the pump beam was injected directly into the crystaland SESAM through the OC.[6]The unabsorbed pump laser heated the SESAM and them irrorholderwhich led to a long-term instability.In our experiment,a plane 45°dichroicmirror(DM 1)with antiref lection coating at 808 nm and high ref lectance coating at 1064 nm was used to avoid pump beam directly entering into the SESAM for a long-term stable operation.A 2-mm long,1-at.%a-cutNd-doped YVO4(Nd:YVO41)crystalwith a cross-section of 3mm×5mm wasused as the laser medium of the oscillator.A commercially available SESAM with a lowermodulation depth of 0.4%,non-saturable lossof 0.3%,saturation f luence of 130μJ/cm2,and recovery time of 1 pswasused as an endm irror in the cavity to achievemodelocking operation.Attheotherend of the cavity,aconcaveOC with a curvature of 80mm and 2%transmission at1064 nm was used as the OC.The distance between the SESAM and the crystalwas about35mm,and the total length of the cavity wasabout45mm,indicating the repetition ratewasabout 3.2GHz.The ABCDmatrix calculation shows that the radius of the laser beam waist in the crystalwasabout120μm.After the OC,a planar-convex lens(f1)with a focal length of 100mm wasused to collimate theoutputbeam.

Fig.1.(a)Experimental setup of passively mode-locked Nd:YVO4 laserand amplif ier,and(b)hotograph of thewhole laser system.Footprintof the case isonly 456mm×206mm.

Two 45°high-ref lectionmirrors(HR1,HR2)were used to guide the laser from the oscillator into the amplif ier.The amplif ier stage was an end-pumped single-pass amplif ier including two planar-convex lenses(f2,f3),a45°high ref lection mirror(HR3),acurvature-free45°dichroicm irror(DM 2),and a gain medium(Nd:YVO42).Before the amplif ier stage,a half-wave plate was used to f inely adjust the polarization directionof the input laser forachieving highereff iciency amplif ication.Thegainmedium of theamplif ierwasa10-mm-long,0.3-at.%a-cut Nd-doped YVO4(Nd:YVO42)crystalwith a cross-section of 3 mm×3 mm,and the longer crystal could reduce the inf luence of the heat[13,14]and realize higheraverage powerand betterbeam quality.Two planar-convex lenses(f2,f3)eachwith a focal length of 200mm and antiref lection coating at1064 nm were used to achievemode-matching and alignmentof the amplif ied laser.

3.Experim ental results

After carefully adjusting the OC and the SESAM in the oscillator,thehigh repetition rate lasercan beobserved behind the OC.W ith the help of the laser from the oscillator,the adjustmentof theamplif ierand high averagepoweroutputcan be achieved.Figures2(a)and 2(b)show themode-locked power as a function of the pump power in oscillator and amplif ier,respectively,and the average power ismeasured by a power meter(Coherent PM 150)whosemaximum available power is about150W.

Fig.2.(a)Mode-locked outputpower versuspump power in the oscillator,and(b)f inaloutputpower versuspump power in the amplif ier.

In theoscillator,the lasing threshold isbelow 100mW of the pump power.The Q-sw itching instabilities are observed before themode-locking threshold.And thestableself-staring continuouswave(CW)mode-locking is obtained at1.5W of the pump power.The stable CW mode-locking with average outputpowerof 1.38W isachieved under the pump powerof 6W,corresponding to an optical-to-opticaleff iciency ofabout 23%.For higher pump power,the stable CW mode-locking is broken.In the amplif ier,the average input power is 1.34W due to the loss of the optical components between the oscillator and the amplif ier.At themax pump power of 29.6W,the input laser is amplif ied to 11.34W.The optical-to-optical eff iciency in the amplif ier isabout34%and the totalopticalto-opticaleff iciency isabout32%.

By an ultrafastphotodiode(UPD-70-UVIR-D,ALPHALAS GmbH)and a spectrum analyzer(FSW 26,R&S),we measured the RF spectrum of the laser at full output power(11.34W)as shown in Fig.3.The RF spectrum shows the signal-to-noise ratio of the laser ismore than 55 dB,indicating a cleanmode-locked pulse train at a repetition rate of 3.236 GHz.Due to the lim ited bandw idth of the ultrafast photodiodeand bayonetnutconnector(BNC),thesecond harmonic of the laser signal isnot recorded.

Fig.3.RF spectrum of the output laser after amplif ier at full output power(11.34 W),with inset show ing RF spectrum with 7-GHz frequency span.

Figure 4 shows the autocorrelation trace at full output power(11.34W),obtained by an auto-correlator(pluseCheck,APE).Themaximum delay of the auto-correlatorwas about 50 ps.The pulse duration is 11.4 ps(FWHM)under the assumption of a sech2temporal intensity prof ile.Ow ing to the lower pulse energy and peak powerof the high repetition rate picosecond pulses,the signal-to-noise ratio of auto-correlator is lowerand theautocorrelation trace isnotsymmetrical.

Fig.4.Autocorrelation with sech2 f itof output laser after amplif ier at fulloutputpower(11.34W).

In this work,a special pumping geometry and an alum inum caseareused to realize long-term stableoperation,and wemeasured the power stability of the laser for 24 hours as shown in Fig.5.The RMS value of the laser in 24 hours is less than 0.6%,indicating that the laser can support the longterm stableoperation.

Fig.5.Power stability(RMS)of laserat fulloutputpower(11.34W).

4.Summary and outlook

In this work,we demonstrate a high-average-power picosecond Nd:YVO4laser at 3.24 GHz.It is comprised of an oscillator and amplif ier.Two f iber-coupled laser diodes are used to pump theoscillatorand theamplif ier,respectively.The oscillatorhad a shortcavity including a2-mm-long Nd:YVO4crystal and an SESAM.Under 6W of the pump power,we achieve a laser outputwith an average power of 1.38W,a repetition rate of 3.24 GHz,and pulse duration of 11.4 ps at 1064 nm from the oscillator.To avoid direct pump light incidence on the SESAM,a special pumping structure is used in the oscillator.Further amplify the laser based on a 10-mm-long Nd:YVO4crystal,then the output laser pulse w ill increase to an average power of 11.34W under pump power of29.6W.The totaloptical-to-opticaleff iciency isabout32%.Both oscillator and amplif ier are placed in an aluminum case whose footprint is only 456mm×206mm for long-term stableoperation.TheRMSvalueofpower f luctuation in 24 hours is less than 0.6%.Furthermore,a shorter cavity can be used in the oscillator to realize even higher repetition rate and more stages in the amplif ier for even higher average power.Our resultsmay supply thew ide applications inmany f ields suchashigh-repetition-ratesecond harmonicgeneration,highrepetition-rateopticalparametric oscillator,etc.

Acknow ledgment

We thank Hao Teng,Shaobo Fang,Hainian Han,and Dehua Li for theirhelpfuldiscussion.