A diode-pumped Er:Yb:glass miniature laser has been passively mode-locked to generate transform-limited 4.3-ps pulses with a 40-GHz repetition rate and 18-mW average powe

Keller, U.

Krainer, L.

Paschotta, R.

Spühler, G.J.

Weingarten, K.J.

Zeller, S.C.

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DOI: 10.1007/s00340-003-1181-zAppl. Phys. B 76, 787–788 (2003)RapidcommunicationLasers and OpticsApplied Physics Bs.c. zeller1,l. krainer2g.j. spu¨hler2k.j. weingarten2r. paschotta1u. keller1Passively mode-locked 40-GHzEr:Yb:glass laser1 Ultrafast Laser Physics, Institute of Quantum Electronics, Swiss Federal Institute of Technology,ETH Hönggerberg – HPT E16, 8093 Zürich, Switzerland2 GigaTera, Inc., Lerzenstrasse 16, 8953 Dietikon, SwitzerlandReceived: 17 April 2003Published online: 6 June 2003 • © Springer-Verlag 2003ABSTRACT A diode-pumped Er:Yb:glass miniature laser has been passively mode-locked to generate transform-limited 4.3-ps pulses with a 40-GHz repetition rate and18-mW average power.PACS 42.55.Rz; 42.60.Fe1 IntroductionFuture data transmission sys-tems will operate at 40 Gbit per second.Current systems operating at, for ex-ample, 2.5 Gbit/s are normally basedon a continuous-wave laser source incombination with a modulator, whereasmuch higher data rates will greatlybenefit from a pulse-generating laserwith a pulse repetition rate of, for ex-ample, 40 GHz. This approach avoidsextremely stringent demands on themodulator, which then only has tochange its state between the pulses. Par-ticularly with a passively mode-lockedlaser, one can also significantly reducethe complexity of the system archi-tecture, as all of the multi-GHz driveelectronics are eliminated. The 40-GHzlaser should generate transform-limitedpulses with a good extinction ratio andhigh average power, apart from havinga compact and stable setup. Here wedemonstrate a passively mode-lockedEr:Yb:glass laser fulfilling these crite-ria. Previously, the maximum repetitionrate of passively mode-locked 1.5-mmlasers was 10 GHz [1] and more recently25 GHz [2], while high-quality pulseswith 77-GHz repetition rate have been Fax: +41-1/633-1059, E-mail: zellers@phys.ethz.chgenerated at a 1064-nm wavelength withNd:YVO4 [3].2 Experimental setupOur approach is not har-monic mode locking (for which stableoperation requires sophisticated means)but fundamental mode locking, i.e., witha single pulse circulating in the lasercavity. For 40 GHz, this leads to an ex-tremely short optical cavity length of3.75 mm for a linear standing-wave cav-ity. By using very small and stronglycurved mirrors, we realized such a cav-ity (Fig. 1) in a folded geometry, anal-ogous to an earlier 10-GHz cavity [1].The output coupler has 1% transmissionat the laser wavelength and 86% trans-mission for the pump wave at 976 nm.pumpoutputSESAM dichroicmirroroutputcouplerEr:Yb:glassFIGURE 1 Cavity setup of the 40-GHz minia-ture Er:Yb:glass laser (SESAM: semiconductorsaturable absorber mirror)The pump light with up to 230-mWpower is generated with a fiber-coupled(single-mode, polarization maintaining)telecom-grade laser diode. A dielec-tric mirror is used to separate the out-put pulses from the beam. The gainmedium is a thin Er:Yb:glass plate(QX/Er from Kigre), mounted underBrewster’s angle. Passive mode lockingis achieved with a semiconductor sat-urable absorber mirror (SESAM, [4, 5]),having a modulation depth below 1%.The mode radii on the SESAM andin the gain medium are optimized toachieve a low Q-switched mode-lockingthreshold.3 ResultsThe autocorrelation (Fig. 2)for the maximum average output powerof 18 mW shows that the laser gener-ates clean Gaussian-shaped pulses with4.3-ps duration and a good extinctionratio. The optical spectrum (Fig. 3),recorded with an Ando AQ3617C spec-1.00.50.0Autocorr.signal (a.u.)-20 0 20Time delay (ps)FIGURE 2 Autocorrelation trace of the 40-GHzpulse train at full average output power (18 mW),together with a fit (dotted) for 4.3-ps Gaussianpulses788 Applied Physics B – Lasers and Optics-60-40-200Spectral power (dB)153615341532Wavelength (nm)1.00.80.60.40.20.0Spectral power (a.u.)153615341532FIGURE 3 Optical spectrum of the pulse trainat full average output power (18 mW), taken witha resolution bandwidth of 0.01 nm and displayedwith a linear scale (upper graph) and a logarith-mic scale (lower graph). The full width at halfmaximum is 0.8 nm (determined for a Gaussianenvelope), and the time–bandwidth product is0.44trum analyzer (0.01-nm resolution), hasa Gaussian shape with a full widthat half maximum of 0.8 nm. This re--100-80-60-40-20Spectral Power (dB)40.00240.00039.998Microwave frequency (GHz)Span 4 MHzRes. BW 30 kHz-100-5040200(GHz)FIGURE 4 The microwave spectrum, demon-strating the absence of Q-switching instabilitiesand higher order transverse modes. The width ofthe peak at 39.9998 GHz is limited by the reso-lution bandwidthsults in a time–bandwidth product of0.44, which is exactly as expected fortransform-limited Gaussian pulses. Themicrowave spectrum (Fig. 4), recordedwith a 45-GHz photodiode and an Agi-lent 8565EC frequency analyzer, showsa single peak at 39.9998 GHz and noside peaks. At a lower pump power(below 16 mW of output power), forwhich Q-switched mode locking occurs,such peaks are observed (but not shownhere) with a 766-kHz spacing. The rep-etition rate can be tuned from 38.5 to40.8 GHz. The operation is stable overhours.4 ConclusionsWe have demonstrated a pas-sively fundamentally mode-lockedEr:Yb:glass laser operating at the ex-tremely high repetition rate of 40 GHz.The combination of its high outputpower and good pulse quality with thecompactness and simplicity of the setupmakes it very competitive against ac-tively harmonically mode-locked fiberlasers [6] and semiconductor lasers [7].REFERENCES1 L. Krainer, R. Paschotta, G.J. Spühler,I. Klimov, C.Y. Teisset, K.J. Weingarten,U. Keller: Electron. Lett. 38, 225 (2002)2 G.J. Spühler, P.S. Golding, L. Krainer,I.J. Kilburn, P.A. Crosby, M. Brownell,K.J. Weingarten, R. Paschotta, M. Haiml,R. Grange, U. Keller: to appear in Electron.Lett. (2003)3 L. Krainer, R. Paschotta, M. Moser, U. Keller:Electron. Lett. 36, 1846 (2000)4 U. Keller, D.A.B. Miller, G.D. Boyd,T.H. Chiu, J.F. Ferguson, M.T. Asom: Opt.Lett. 17, 505 (1992)5 U. Keller, K.J. Weingarten, F.X. Kärtner, D.Kopf, B. Braun, I.D. Jung, R. Fluck, C. Hön-ninger, N. Matuschek, J. Aus der Au: IEEE J.Sel. Top. Quantum Electron. 2, 435 (1996)6 A.D. Ellis, R.J. Manning, I.D. Phillips, D.Nesset: Electron. Lett. 35, 645 (1999)7 K. Sato: IEEE J. Lightwave Technol. 20,2035 (2002)

Passively mode-locked 40-GHz Er:Yb:glass laser

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Passively mode-locked 40-GHz Er:Yb:glass laser

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