Q-switched mode-locking with Cr4+ YAG

DOI:10.1007/s00340-004-1410-0Appl.Phys.B (2004)Lasers and OpticsApplied Physics Bs.zhang e.wu h.pan h.zeng uQ-switched mode-locking with Cr 4+:YAG in a diode pumped Nd :GdVO 4laserKey Laboratory of Optical and Magnetic Resonance Spectroscopy,and Department of Physics,East China Normal University,Shanghai 200062,P.R.ChinaReceived:14July 2003/Revised version:20November 2003Published online:13February 2004•©Springer-Verlag 2004ABSTRACT We have demonstrated passively Q-switched mode-locking in a laser-diode-pumped Nd :GdVO 4laser with an intracavity Cr 4+:YAG saturable absorber.At a pump power of 19.4W,the Q-switched mode-locked laser produced an aver-age output power of 3.22W,and the repetition rates of the Q-switched envelope and the mode-locked laser pulse were 227kHz and 144MHz,respectively.The duration of the mode-locked pulse was within the sub-nanosecond range.PACS 42.55.Xi;42.60.Fc;42.60.Gd;42.55.RzNd :GdVO 4has recently attracted great interest as a promis-ing laser active material due to its excellent physical,opticaland mechanical properties.As compared with its isomorph Nd :YVO 4,Nd :GdVO 4exhibits a larger absorption cross section [1].Its broad absorption band matches well with the emission band of the GaAlAs laser diode,making it quite suitable to take the full advantages of diode pumping for com-pact all-solid lasers.Most importantly,Nd :GdVO 4possesses a much higher thermal conductivity than Nd :YVO 4[2],which provides the desirable advantage for high-power lasers.Various laser operations have been achieved with Nd :GdVO 4such as those in the infrared and the corresponding frequency-doubled regions to establish red,green,and blue lasers [3–6].Both actively and passively Q-switched Nd :GdVO 4lasers have also been achieved [7,8].Compared with Nd :YVO 4lasers,Nd :GdVO 4lasers have higher slope efficiency and,consequently,output higher laser powers at the same pump power.All the experimental results to date have indicated that Nd :GdVO 4is potentially more competitive than Nd :YVO 4in diode pumped solid-state lasers.In this letter,we re-port on a Q-switched mode-locked (QML)diode-pumped Nd :GdVO 4laser with a Cr 4+:YAG saturable absorber,which is,to the best of our knowledge,the first time a mode-locked pulse train from a Nd :GdVO 4laser has been obtained.Cr 4+:YAG crystal has been used widely as saturable ab-sorber to generate nanosecond Q-switched and picosecond mode-locked laser pulses in the near infrared region,espe-cially in the Nd 3+doped materials that lase at 1.06µm .Diode-u Fax:+86-21/6223-2056,E-mail:hpzeng@pumped QML Nd :YVO 4laser has been achieved by using a Cr 4+:YAG crystal as an intracavity saturable absorber [9].By a direct analysis of the coupled rate equations,one can readily get the criterion for a good passive Q-switching [10]ln (1/T 20)ln (1/T 20)+ln (1/R )+L σgs σA A sγ1−β(1)in a laser cavity with the given parameters such as the reflec-tivity R of the output mirror,the stimulated emission cross section σof the gain medium,the nonsaturable intracavity round-trip dissipative optical loss L ,the initial transmission T 0and the ground-state absorption cross section σgs of the saturable absorber,the ratio A /A s of the effective area in the gain medium to that in the saturable absorber,the in-version reduction factor γ(γ=1and γ=2correspond to four-level and three-level systems),and the ratio βof the excited-state absorption cross section to that of the ground-state absorption in the saturable absorber.Since the strict criterion deduced from the second threshold condition re-quires saturation of the absorber before the gain saturation in the laser crystal,Nd :GdVO 4laser,as limited by the large gain cross section of Nd :GdVO 4,is usually difficult to be passively Q-switched with Cr 4+:YAG in comparison with Nd:YAG and Nd :YVO 4lasers.On the other hand,the ex-cited state absorption (ESA)in Cr 4+:YAG ,which excites Cr 4+ions from the first excited state to the higher-lying lev-els,plays an important role in the pulsed laser performance.At a low intracavity laser intensity,most of Cr 4+ions in Cr 4+:YAG are populated in the ground state,the ESA is negligible,and Cr 4+:YAG crystal functions as an effective intracavity saturable absorber only for Q-switching.While at a sufficiently high intracavity laser intensity,all the Cr 4+ions are quickly excited to the first excited state,from which fur-ther excitation by the strong ESA promotes an accumulation of the Cr 4+ions in the higher-lying levels,leading to satura-tion of the ESA.Since the relaxation time of the ESA is in the subnanosecond region,it is possible to achieve a passive mode-locking operation with Cr 4+:YAG saturable absorber if the intracavity laser intensity is large enough to saturate the ESA [9].As for an Nd :GdVO 4laser,the large absorption and emission cross sections of Nd :GdVO 4enable efficient diode pumping to achieve high-power laser operation with high lasing efficiency,which makes it relatively easy to ob-Applied Physics B–Lasers andOpticsFIGURE1The cavity configuration and transversal mode of the output laser beam of the passively QML Nd:GdVO4lasertain sufficient intracavity laser intensity to make the ESA of Cr4+:YAG saturable for a mode-locking operation.For this purpose,high pump power must be cast to the laser crystal from the pumping laser diode.As a result,the laser crystal will be loaded with enormous heat.Fortunately,one of the remarkable properties of Nd:GdVO4is its large thermal con-ductivity,which makes it competent for high-power operation at high-power pumping while accompanied with relatively low thermal detriments.When pumped with sufficiently high power,Nd:GdVO4with suitable intracavity Cr4+:YAG sat-urable absorber can,therefore,operate in the QML perform-ance in a cavity with a smaller beam area in the Cr4+:YAG crystal than in Nd:GdVO4,which consequently enables the saturation of the absorber before the gain saturation in the laser crystal.Our experiment employed a laser setup as sketched in Fig.1.The pump power was supplied by a20-Wfiber-coupled diode-laser array with the emission wavelength at808nm controlled by a thermal regulation.Thefiber core had a diam-eter of400µm,and a numerical aperture of0.22.The pump laser beam was focused by a series of lenses on a piece of Nd:GdVO4crystal with a focusing diameter about600µm. The laser cavity consisted of three mirrors:an input con-cave mirror M1(R=100mm)with high transmission at 808nm and high reflection at1064nm,a folded concave mir-ror M2(R=500mm)with high reflection at1064nm,and an output coupler(OC)flat mirror.The length between M1 and M2was about600mm,and the total cavity length was about1042mm.A piece of3mm-long a-cut Nd:GdVO4 crystal with1at.%Nd3+doping was used as the laser ac-tive material.It was wrapped with indium foil andfixed in a water-cooled copper heat sink.And the saturable absorber Cr4+:YAG with an appropriate initial transmission T0was mounted on a heat sink and was placed near the OC mirror. We designed the laser cavity with great care to allow mode matching of the laser mode in the Nd:GdVO4crystal with the pump beam and to provide the proper spot size in the saturable absorber.The temporal shape of the output laser pulse was recorded by a fast InGaAs PIN photodiode(Newport818-BB-30) with a rising time of∼200ps,which was connected to a500MHz-bandwidth digital oscilloscope(Hewlett Packard 54616C).The Q-switched pulse train and mode-locked pulse train within the Q-switched pulse envelope were displayed in Fig.2.As normally occurs in a typical passively Q-switched laser,a pulse-to-pulse instability occurred in the Q-switched pulse train of the QML Nd:GdVO4laser,which could be seen in the upper inset of Fig.2.With the augment of pump power,200 ns/div50 us/divFIGURE2The temporal traces of the Q-switched and QML laser pulses in the passively Q-switched and mode-locked Nd:GdVO4lasersthe pulse-to-pulse variation and instability of repetition rate became more evident.This instability is actually the intrinsic vulnerability of all passively Q-switched or passively QML lasers.However,the instability of the repetition rate could be suppressed by modulating the pump power[11].As could be seen in the lower inset of Fig.2,the mode-locked pulse train was enveloped within a∼300-ns Q-switched pulse.The mode-locked pulses had a repetition rate of∼144MHz.Due to the resolution limit of the oscilloscope used in our measure-ments,the recorded temporal trace of the mode-locked pulse appeared as an ns-scale train.Nevertheless,the duration of the mode-locked pulse was believed to be within the subnanosec-ond region,since the relaxation of the ESA of Cr4+ions in the Cr4+:YAG crystal and accordingly,the saturation of the ESA under intense intracavity laser irradiation to achieve a pas-sive mode-locking operation,took place in the subnanosec-ond duration[9].OCs with different transmission(T=20%, T=30%,and T=40%)and Cr4+:YAG crystals with differ-ent initial transmission(T0=90%and T0=85%)were used to optimize the output performance.The dependence of aver-age output power on the incident pump power is shown in Fig.3for QML lasers with the T0=90%Cr4+:YAG crys-tal.When the OC transmission was T=20%,T=30%,and T=40%,the mode-locked pulse train appeared within the Q-switched pulse envelope at the threshold pumping power of 1.7,3.7and4.4W,and the corresponding slope efficiencies at the QML operation were12%,21%,and17.6%,respectively. Within a laser configuration with a given gain medium and saturation absorber atfixed positions as considered here,there existed an OC transmission to optimize the passively QML operation.From Fig.3,one can clearly see that the highest slope efficiency and the largest output power were achieved when the OC transmission was selected as T=30%,from which a maximum average output power of3.22W was ob-tained when the incident pump power was19.4W.Under this circumstance,the repetition rate of Q-switched envelope was227kHz.With the T=20%OC mirror,the output power began to drop when pump power exceeded10.8W.Increas-ing the pump power,the QML pulse turned into continuous-ZHANG et al.Q-switched mode-locking with Cr 4+:YAG in a diode pumped Nd :GdVO 4laserO u t p u t p o w e r (W )Pump power (W)FIGURE 3Dependence of the averageoutput power on the incident pumppower in the passively QML Nd :GdVO 4laser with various output couplers0R e p e t i t i o n r a t e (k H z )Pump power (W)FIGURE 4Dependence of repetition rate of the Q-switched pulse train onthe incident pump power in the passively QML Nd :GdVO 4laser with vari-ous output couplerswave (cw)instead of cw mode-locking,and the output power continued to increase after a little drop,which can be seen distinctly from the dotted curve in Fig.3.This phenomenon was originated from the bleaching of the Cr 4+:YAG crystal due to the high intracavity intensity,and it indicated that it was impossible to achieve pure mode-locking by increasing the intensity on Cr 4+:YAG crystal.Due to the large thermal conductivity of Nd :GdVO 4,the thermal lensing effect was inconspicuous and the QML operation could be maintained to output single-transversal-mode laser beam.The transversal profile of the output laser beam was recorded by a CCD cam-era and was displayed in the inset of in Fig.1.The dependence of the repetition rate of the Q-switched pulses on the pump-ing power was shown in Fig.4.For the QML Nd :GdVO 4laser with a T =20%OC mirror,no repetition rates could be measuredwhen the pump power exceeded 10.8W since QML pulses became cw output.Figure 5shows the out-put power of two different Cr 4+:YAG crystals with the ini-tial transmission of T 0=90%and T 0=85%,respectively,and the transmission of OC mirror employed here was 30%.O u t p u t p o w e r (W )Pump power (W)FIGURE 5Dependence of the average output power on the incident pump power with Cr 4+:YAG crystals of the initial transmission T 0=90%and T 0=85%,respectivelyFIGURE 6Temporal traces of QML laser pulses obtained with saturableabsorber of different initial transmission T 0=90%and T 0=85%,respec-tivelyThe Q-switched pulse could be achieved at a lower pump-ing threshold to output higher laser power with the T 0=90%Cr 4+:YAG crystal than that with T 0=85%,because the Cr 4+:YAG with T 0=90%caused a lower intracavity loss.When saturable absorbers with various initial transmissions were employed,different Q-switched pulse duration and dif-ferent modulation depth were obtained.These phenomena were also observed in the QML Nd :YVO 4laser [9].In our experiment,we tested two Cr 4+:YAG crystals with initial transmissions of 90%and 85%,respectively.Figure 6dis-play the temporal traces in the condition of the OC mirror T =20%and the output powers were both 0.37W .At the initial transmission T 0=85%,as shown in Fig.6a,the mode-locked modulation depth was 79%,and the full-width at the half-maximum (FWHM)duration of the Q-switched pulse en-velope was 280ns .While at the initial transmission T 0=90%,as shown in Fig.6b for comparison,the Q-switched laser pulse exhibited a modulation depth and FWHM pulse dura-tion of 70%and 360ns ,respectively.The result shows that for the saturable absorber (Cr 4+:YAG crystal)tested in ourApplied Physics B–Lasers and Opticsmeasurements,shorter pulse duration and higher modula-tion depth could be achieved by using a lower initial trans-mission.This can be qualitatively understood as follows. With a lower initial transmission,the saturable absorber has a larger density of Cr4+ions involved in the saturable ab-sorption,and the intracavity loss is larger.And accordingly, a shorter Q-switched laser pulse duration was achieved.More-over,Cr4+:YAG crystal with a lower initial transmission required a higher energy to saturate ESA,leading to the gen-eration of a mode-locked pulse train with a higher modulation depth.In summary,a QML was demonstrated in a diode-pumped Nd:GdVO4laser by using Cr4+:YAG as the intracavity sat-urable absorber.A144MHz mode-locked pulse train was enveloped in a∼300ns Q-switched laser pulse.The max-imum energy of a single Q-switched pulse was15.8µJ when the pump power was12.3W.At a pump power of19.4W, the QML laser produced the maximum average output power of3.22W,and the repetition rate of the Q-switched envelope and the mode-locked laser pulse were227kHz and144MHz,respectively.The result in this work confirmed again the ex-cellent laser performance of Nd:GdVO4.REFERENCES1A.I.Zagumennyi,V.G.Ostroumov,I.A.Shcherbakov,T.Jensen, J.P.Meyen,G.Huber:Sov.J.Quantum Electron.22,1072(1992)2P.A.Studennikin, A.I.Zagumennyi,Y.D.Zavartsev,P.A.Popov,I.A.Shcherbakov:Quantum Electron.25,1162(1995)3H.Zhang,X.Meng,L.Zhun,J.Liu,C.Wang,Z.Shao:ser Technol.31,279(1999)4L.Qin,X.Meng,C.Du,L.Zhu,Z.Shao,B.Xu:ser Technol.35, 257(2003)5D.Y.Shen,H.R.Yang,J.G.Liu,S.C.Tam,m,W.J.Xie,J.H.Gu, K.Ueda:Appl.Phys.B72,263(2001)6C.Czeranowsky,M.Schmidt,E.Heumann,G.Huber,S.Kutovoi, Y.Zavartsev:mun.205,361(2002)7J.Liu,C.Wang,C.Du,L.Zhu,H.Zhang X.Meng,J.Wang,Z.Shao, M.Jiang:mun.188,155(2001)8J.Liu,J.Yang,J.He:mun.219,317(2003)9Y.Chen,S.W.Tsai,S.C.Wang:Opt.Lett.25,1442(2000)10G.Xiao,M.Bass:IEEE J.Quantum Electron.QE-33,41(1997)i,M.Brunel,F.Bretenaker,A.Le Floch:Appl.Phys.B79,1073 (2001)。