The use of glasses doped with PbS nanocrystals as intracavity saturable absorbers for passive Q-switching and mode locking of c-cut Nd:Gd0.7Y0.3VO4, Nd:YVO4, and Nd:GdVO4 lasers is investigated. Q-switching yields pulses as short as 35 ns with an average output power of 435mW at a repetition rate of 6-12 kHz at a pump power of 5-6W. Mode locking through a combination of PbS nanocrystals and a Kerr lens results in 1.4 ps long pulses with an average output power of 255 mW at a repetition rate of 100 MH

Di Labio, L.

Feurer, T.

Kutovoi, S.

Lüthy, W.

Onushchenko, A.

Shcherbakov, I.

Sirotkin, A.

Vlasov, V.

Zagumennyi, A.

Zavartsev, Yu

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Appl Phys B (2009) 94: 375–379DOI 10.1007/s00340-008-3347-1Mode-locked diode-pumped vanadate lasers operated with PbSquantum dotsA.A. Sirotkin · L. Di Labio · A.I. Zagumennyi ·Yu.D. Zavartsev · S.A. Kutovoi · V.I. Vlasov · W. Lüthy ·T. Feurer · A.A. Onushchenko · I.A. ShcherbakovReceived: 15 July 2008 / Revised version: 29 October 2008 / Published online: 10 January 2009© Springer-Verlag 2009Abstract The use of glasses doped with PbS nanocrystalsas intracavity saturable absorbers for passive Q-switchingand mode locking of c-cut Nd:Gd0.7Y0.3VO4, Nd:YVO4,and Nd:GdVO4 lasers is investigated. Q-switching yieldspulses as short as 35 ns with an average output power of435 mW at a repetition rate of 6–12 kHz at a pump powerof 5–6 W. Mode locking through a combination of PbSnanocrystals and a Kerr lens results in 1.4 ps long pulseswith an average output power of 255 mW at a repetition rateof 100 MHz.PACS 42.55.Xi · 42.60.Fc · 42.70.Hj1 IntroductionGenerating stable short pulses in the infrared spectral rangefrom diode-pumped lasers is still an active area of research.Nano-, pico- and femtosecond laser pulses find applica-tions in telecommunication, biology, medicine, chemistry,and material processing. Passive modulators for mode lock-ing and Q-switching are very favourable due to their com-A.A. Sirotkin · A.I. Zagumennyi · Y.D. Zavartsev · S.A. Kutovoi ·V.I. Vlasov · I.A. ShcherbakovProkhorov General Physics Institute of the Russian Academyof Sciences, Vavilov street 38, 119991 Moscow, RussiaL. Di Labio () · W. Lüthy · T. FeurerInstitute of Applied Physics, University of Bern, Sidlerstr. 5,3012 Bern, Switzerlande-mail: loredana.dilabio@iap.unibe.chFax: +41-31-6313765A.A. OnushchenkoS.I. Vavilov State Optical Institute, 197131 St. Petersburg, Russiapactness, simplicity and low costs. One of the most promis-ing methods to tailor fast saturable absorbers uses nano-structures, such as PbS nanocrystals [1] or single-wall car-bon nanotubes [2, 3] embedded in various host materials.PbS nanocrystals in a silicate or a phosphate glass ma-trix are ideal candidates for the near-infrared region. Thenanometre-sized PbS crystals or quantum dots (QD) giverise to the quantum confinement effect, which is commonfor semiconductor crystals of sizes comparable with the ex-citon Bohr radius. Enhanced quantum confinement mani-fests itself as a blue shift of the energy band gap and astrong nonlinearity in the range of the lowest transition,i.e. the first excitonic resonance. PbS is attractive for QD-doped glass fabrication because it has a large bulk ex-citon Bohr radius of 18 nm and a narrow band gap of0.41 eV [4]. This property allows obtaining a strong quan-tum confinement with relatively large crystals and the ab-sorption resonance can be tuned between 1 and 2 µm.Glasses doped with PbS nanocrystals were used as sat-urable absorbers for mode locking of a continuous-wave-pumped (CW-pumped) Cr:Forsterite laser [5, 6], a Yb:KYWlaser [7] and a Cr4+:YAG laser [8]. Further for Q-switchingof an erbium–ytterbium-glass laser [9], a neodymium laserat 1.3 µm [10] and for mode locking and Q-switching ofNd:KGd(WO4)2 and Nd:YAG lasers [11].Neodymium-doped vanadate crystals, such as Nd:YVO4,Nd:GdVO4, or mixed Nd:YxGd1−xVO4 crystals, are ef-ficient laser media and have a considerable potential fordiode-pumped laser systems. Their laser characteristics andtheir thermal properties are superior to those of Nd:YAGand, therefore, they are promising substitutes for diode-pumped Nd:YAG lasers. For example, an a-cut Nd:GdVO4crystal has a seven times larger absorption cross-section(5.2 · 10−19 cm2, E||c-axis) and a three times larger emis-sion cross-section at 1.06 µm (7.6 · 10−19 cm2, E||c-axis)376 A.A. Sirotkin et al.Fig. 1 Luminescence spectra around the 4F3/2 → 4I11/2 transition for a Nd:Gd0.7Y0.3VO4, b Nd:YVO4 and c Nd:GdVO4. The dotted curves (1)refer to the a-cut and the solid curves (2) to the c-cut crystals, respectivelycompared to Nd:YAG [12]. The room temperature ther-mal conductivity of 11.7 W m−1 K−1 in the direction par-allel to the c-axis for a 1.3 at.% Nd:GdVO4 crystal ex-ceeds the 11.1 W m−1 K−1 of a 0.9 at.% Nd:YAG crys-tal. High-quality 0.5 at.% Nd:YVO4 crystals even featurea thermal conductivity as high as 14.5 W m−1 K−1 alongthe c-axis [13]. Passive mode locking has been demon-strated for an a-cut Nd:GdVO4 laser yielding a pulse du-ration of 2.1 ps [14] and a Kerr-lens mode-locked a-cutNd:Gd0.7Y0.3VO4 laser produced a stable train of pulseswith a minimal pulse duration of 1.7 ps at a repetition rateof 140 MHz and a maximal average output power of 630mW [15].Given the superior lasing characteristics of various vana-date crystals and the ideal properties of PbS nanocrys-tals as saturable absorbers suggests to investigate combina-tions of the two with respect to efficient short-pulse gen-eration. Especially c-cut vanadate crystals have a broademission spectrum [16] and, thus, the potential to produceshort pulses. Here, we report on mode locking and Q-switching of lasers based on various c-cut Nd:vanadate crys-tals (Nd:Gd0.7Y0.3VO4, Nd:YVO4 and Nd:GdVO4) by us-ing PbS nanocrystals, Kerr lensing, or a combination of thetwo as saturable absorbers. The Nd3+ concentration in allcrystals is 0.5 at.%. We demonstrate lasing in a nonselectiveresonator at 1065.5 nm with a c-cut Nd:GdVO4 crystal andat 1066.4 nm with c-cut Nd:YVO4 crystal. Q-switching andmode locking of c-cut vanadates with nanocrystalline PbSabsorbers in combination with a Kerr lens have not been re-ported before.2 Spectroscopic investigations of vanadate crystalsThe vanadate crystals are grown by the Czochralski methodon industrial installations (Kristall-2 and Kristall-3M) at theA.M. Prokhorov General Physics Institute of the RussianAcademy of Sciences. Optical absorption spectra of thecrystals are recorded with a spectrophotometer (ShimadzuUV-3101PC) and the fluorescence spectra are obtained witha spectrometer based on an autocollimator tube (UV-90) anda detector (Toshiba TSD1304JK) under 808 nm diode laserexcitation.While the absorption spectra of a-cut (π -polarisation)and c-cut neodymium-doped vanadate crystals are almostidentical, the luminescence spectra show significant differ-ences. Figure 1 shows the normalised luminescence spec-tra in the wavelength range from 1059 to 1070 nm aroundthe 4F3/2 → 4I11/2 transition. All c-cut vanadate crystalsexhibit a considerable spectral broadening and substruc-tures due to transitions originating from different Stark lev-els. For c-cut Nd:Gd0.7Y0.3VO4 (Fig. 1a) and Nd:GdVO4(Fig. 1c) crystals the emission originates from two Starklevels, and consequently two peaks are observed. The twopeaks of the Nd:Gd0.7Y0.3VO4 crystals almost merge toa single emission feature centred around 1064.5 nm witha full width at half maximum (FWHM) of approximately5.1 nm. For Nd:GdVO4 crystals the intensity of the emis-sion at 1065.5 nm exceeds the one at 1063.2 nm where laseroscillation in a-cut crystals is usually observed. This sug-gests that for a c-cut Nd:GdVO4 crystal lasing at 1065.5 nmcan be obtained even in a nonselective resonator. Figure 1bshows that the luminescence spectrum of a c-cut Nd:YVO4crystal consists of three transitions with the strongest line at1066.4 nm. The three transitions cover a spectral range ofalmost 6 nm. For all three c-cut vanadate crystals we expectthat the broader emission spectra allow for new functional-ities, such as tuning, two-frequency lasing or generation ofsub-picosecond pulses.3 Tuneable lasingThe broad luminescence spectra (Fig. 1) of c-cut vana-date crystals should allow for an increased tuning range inthe continuous wave mode of operation. The experimentalarrangement to measure the tuning curve is shown schemati-cally in Fig. 2.Mode-locked diode-pumped vanadate lasers operated with PbS quantum dots 377Fig. 2 Schematic arrangement of the tuneable CW Nd-doped vana-date laser. 1: pump diode, 2: fibre pigtail, 3: focusing optics, 4: laserresonator, 5: crystal, 6: intracavity Fabry–Pérot etalon, 7: powermeter,8: spectrometer, 9: beam splitterFig. 3 The tuning curves of c-cut Nd:Gd0.7Y0.3VO4, Nd:YVO4 andNd:GdVO4 lasersThe crystal is pumped with a fibre-coupled (2) diodelaser (1) at a wavelength of 808 nm (Limo 30-F200-DL800).Its maximum output power is 30 W in a numerical aper-ture of NA = 0.22. The output from the fibre pigtail is fo-cused with two lenses of focal lengths f = 20 mm andf = 35 mm (3) to the active element resulting in a pumpspot radius of approximately 400 µm. The size of the c-cut Nd3+:Gd0.7Y0.3VO4, Nd:YVO4, and Nd:GdVO4 lasercrystals (5) is 4 mm × 4 mm × 6 mm and both side sur-faces are antireflection (AR) coated for 1064 and 808 nm.The crystals are mounted between two water-cooled heatsinks with indium foil providing an effective and uniformthermal contact. The laser resonator consists of a spheri-cal mirror (4) with a radius of curvature of 250 mm beinghighly reflective (HR) at 1064 nm and highly transmissive(HT) at 808 nm and a flat output coupler with R = 92% at1064 nm. The overall length of the laser cavity is approxi-mately 45 mm. Wavelength tuning is accomplished with anintracavity Fabry–Pérot etalon (6). The etalon is a 100 µmthick uncoated YAG slide.Figure 3 shows the measured tuning curves of the threedifferent laser crystals. All three curves very nicely repro-duce the shape and structure of the corresponding lumines-cence spectra (see Fig. 1) and cover a spectral range between1062 and 1067 nm. The incident pump power for the tuningFig. 4 Small signal absorption spectrum of a 1 mm thick PbS-dopedglass slideexperiments is 5–6 W for Nd:YVO4 or Nd:GdVO4 and 8 Wfor Nd:Gd0.7Y0.3VO4.4 Q-switchingPassive Q-switching of Nd:YVO4, Nd:GdVO4 and Nd:Gd0.7Y0.3VO4 lasers with PbS-doped glasses is demon-strated with an experimental arrangement similar to the oneshown in Fig. 2, except the Fabry–Pérot etalon is replacedby a PbS-doped glass slide. The temporal duration of theQ-switched pulses is measured with a germanium avalanchephotodiode (LFD-2) and recorded with an oscilloscope (Tek-tronix TDS 3052).The PbS nanocrystals are embedded in a 1 mm thickSiO2–Al2O3–NaF–Na2O–ZnO glass matrix and the con-centration is between 0.3 and 0.6 mol.%. The wavelengthdependent small signal absorption spectrum is shown inFig. 4. It features a roughly 100 nm wide peak centred at1050 nm on top of a slowly decreasing background. Fromthe 1 mm thick slides four wedges are prepared with a maxi-mal thickness of 0.8, 0.6, 0.4 and 0.2 mm, respectively. Eachwedge has an angle of ∼1.5° and is in direct contact to a1 mm undoped YAG plate for cooling. With those the smallsignal absorption at 1064 nm can be varied continuously be-tween 0.3 and 14%.Passive Q-switching is obtained for all three crystal ma-terials with pulse durations ranging from 35 to 70 ns. Pulseenergies are typically on the order of 40 µJ. The best per-formance is found for the c-cut Nd:Gd0.7Y0.3VO4 crystal incombination with a 14% saturable absorber. Here, the pulseduration is 35 ns and the average output power amounts to430 mW at repetition rates of 6 to 12 kHz (pump power:5–6 W).378 A.A. Sirotkin et al.Fig. 5 Schematic of the mode-locked Nd:vanadate lasers. LD: laserdiode, F: fibre pigtail, O: focusing optics, AE: active element (vana-date crystals), M1: end mirror, M2 and M3: concave (R = 100 mm)high reflectors, SF57: Kerr-lens medium (5 mm thick), M4: concave(R = 250, 150 or 100 mm) high reflector, PbS QD: saturable absorber,M5: output coupler (T = 4.8% @ 1064 nm), P1, P2: prism pair (SF10)5 Mode lockingThe experimental arrangement for mode locking is shownschematically in Fig. 5. The end mirror of the resonatoris the end surface of the crystal (M1) which has a HRcoating for 1064 nm and an AR coating for 808 nm. Allcrystals have dimensions of 4 mm × 4 mm × 6 mm with aslightly wedged (1.5°) and AR-coated front surface to sup-press etalon effects. The output coupler (M5) is a wedgewith a transmission of 4.8% at 1064 nm. The two curvedfolding mirrors (M2 and M3) with radii of 100 mm producean additional focus at which a glass slide for Kerr lensingcan be inserted. Both mirrors are tilted by ∼16° to partlycompensate for aberrations. The Kerr medium is a 5 mmthick SF57 glass slide with a nonlinear refractive index ofn2 = 4.1 · 10−19 m2/W [17] inserted at Brewster’s angle.Dispersion compensation is achieved through an intracav-ity prism pair consisting of two SF10 prisms (P1 and P2).The main beam waist is located close to the output couplerand has a diameter between 20 and 100 µm with the ex-act value being determined by the curvature of the spheri-cal mirror M4 (R = 250, 150 or 100 mm). One of the PbS-doped glass wedges is positioned close to the output cou-pler M5. Its small signal absorption can be varied from 0.3 to6% at 1064 nm. The temporal duration of the optical pulsesis measured with either a streak camera of 0.7 ps rise time(GPI Photoelectronics Dept. Mod. PN-01/s20) or an auto-correlator based on noncollinear second harmonic genera-tion with a 100 µm thick BBO crystal.Mode locking is performed with either the saturable ab-sorber alone, the Kerr lens medium alone or a combina-tion of the two. No mode locking could be obtained with-out these two elements. Due to the large beam diameter ofabout 300 µm inside the vanadate laser crystal, the intensitywas not sufficient for self-focusing in the gain medium. Pas-sive mode locking with PbS-doped glass alone is studied bymoving the SF57 plate out of the 35 by 60 µm beam waist us-ing it only to compensate for aberrations. Self-starting modelocking is obtained for all three crystals with the PbS-dopedFig. 6 a Intensity autocorrelation and b spectrum for a c-cutNd:Gd0.7Y0.3VO4 crystalglass wedge (5% small signal absorption) positioned 1 to2 mm away from the output coupler M5. In this range thebeam diameter varies from 20 to 45 µm. With a pump powerof between 3 and 5 W the laser produces a stable train ofpulses of duration of about 6 ps FWHM and output powerof 350 mW at a pulse repetition rate of 100 MHz.Passive mode locking with the Kerr-lens medium alone isachieved by moving the SF57 plate back to the beam waistbetween M2 and M3 and by removing the PbS saturable ab-sorber. The long-term stability of passive mode locking withthe Kerr lens depends mostly on the stability of the pumpsource. The duration of laser pulses measured in this regimeis between 2 and 4 ps with an average power of 240 mW(pump power: 5 W).Laser performance is most stable and produces the short-est laser pulses when a hybrid passive mode-locking schemeis used which is based on a combination of the PbS sat-urable absorber and the Kerr-lens medium. For the c-cutNd:Gd0.7Y0.3VO4 crystal the shortest measured autocorre-lation trace has a FWHM of 2.1 ps. Assuming a sech2 in-tensity profile, the pulse duration is on the order of 1.4 ps.For a pump power of 6 W the average output power is mea-sured to be 255 mW, and the repetition rate is 100 MHz.Figure 6 shows a typical intensity autocorrelation trace andthe corresponding optical spectrum with a bandwidth of1.26 nm FWHM. The resulting time-bandwidth product isτν = 0.468, somewhat larger than the transform limit ofτν = 0.32.6 ConclusionsC-cut Nd:Gd0.7Y0.3VO4, Nd:YVO4, and Nd:GdVO4 crys-tals show an amplification bandwidth that is much broaderthan that of the corresponding a-cut crystals, roughly rang-ing from 1062 to 1067 nm. Consequently, the tuning rangeof continuous wave lasers is shown to be considerablywider. The specific shape of the luminescence spectra al-lows for lasing in a nonselective resonator at 1065.5 nm withNd:GdVO4 and at 1066.4 nm with Nd:YVO4. When thesecrystals are combined with a PbS-doped glass Q-switchingproduces pulses between 35 and 70 ns. Specifically, the c-cutMode-locked diode-pumped vanadate lasers operated with PbS quantum dots 379Nd:Gd0.7Y0.3VO4 crystal yields 35 ns long pulses with anaverage output power of 430 mW at a repetition rate of 6–12 kHz at a pump power of 5–6 W. In a resonator suitable formode locking the PbS-doped saturable absorber generates astable train of pulses with a repetition rate of 100 MHz. At apump power of 5 W the individual pulses are 6 ps long andthe average output power is 255 mW. By combining the PbS-doped absorber with Kerr lens pulses as short as 1.4 ps aremeasured. This pulse duration is still larger than the band-width limit and we expect that even shorter pulses can beobtained by further optimising the PbS-doped glass.Acknowledgements This work was supported in part by the SwissNational Science Foundation under contract IB7420-110873/1.References1. A.A. Onushchenko, A.A. Zhilin, G.T. Petrovskiı˘, É.L. Raaben,M.S. Gaponenko, A.M. Malyarevich, K.V. Yumashev, V.V. Gol-ubkov, J. Opt. 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Mode-locked diode-pumped vanadate lasers operated with PbS quantum dots

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Mode-locked diode-pumped vanadate lasers operated with PbS quantum dots

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