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output

CThL2 Fig. 1. Schematics of the erbiumdoped fiber laser. Notations: HR= high reflector,SA = saturable absorber, TPA = two-photonabsorption material (placed either on SA or onopposite HR). Ref. = reference beam used tomeasure the reflection fromSA, BP = beam split-ter. EDF = erbium doped fiber, WDM = wave-length division multiplexer, FR = Faraday rota-tor,WP = wave plate.

-2W -lW 0

Record Time (Microseconds)

CThL2 Fig. 2. Illustration of the start -up pro-cess of the erbium doped fiber. The signalstrength is normalized to that of CW mode-locked signal. Inset: Illustrative trace of a typicalQ-switched envelope with mode-locked pulses.

minimize the amount of two-photon absorp-

tion.We report here a new mode-locking

mechanism, where the use of a two-photonabsorber in addition to a saturable absorberallows for CW mode-locking o reliably evolvefrom Q-switched mode locking. An optimaldesign of the two-photon absorber limits thepeak power of the Q-switched pulses via opti -cal limiting and thus aids in preventing dam-age to the saturable absorber. Moreover, thethreshold for optical limiting is chosen to setan optimum stability point for CW mode-locked operation.

The environmentallystable fiber laser3 con-figuration is sketched in Fig. 1. When mode-locked, the laser provides -350 fs pulses at awavelength of -1.56 pm and -5 0 MHz rep-etition rate. Pumped with a single-mode laserdiode operating at 980 nm, the output powercan be varied between 3-15 mW dependingonthe pump level and on the output coupling,which may be adjusted with the orientation ofthe waveplate. The saturable absorbers used

for modelocking are InGaAsP epilayers withan insertion loss that can vary from 20 to 80%.

The carrier lifetime is -5 ps.The start-up process of the fiber laser was

recorded using a gigahertz detector and oscil-loscope. As shown in Fig. 2, the fiber laser firstgenerates a seriesofQ-switched, mode-lockedspikes. This behavior persists for several milli-seconds. The laser makes a transition to CWmode-locking after a couple of relatively small,but broader Q-switched, mode-locked spikes(Fig. 2). The Q-switched envelopes have awidth of approx. 1 ps,and contain a number ofshort mode-locked pulses as shown in the inset

TPALoss"I L 4OUOOl 0.01 0.1

I _ _ _ _ - -- -_I lo loo

Intensity (GW/C~')

CThL2 Fig. 3. Intensity dependence of thenonlinear losses due to saturable absorber, two-photon absorption and the sum of the two com-ponents.

of Fig. 2. Using an intensity autocorrelation,the pulse width of the Q-switched, mode-locked pulses was measured to be less than3 ps. This data indicates that the initial pulse

shortening process i s completed in very fewround trips. After the transition to CW mode-locking, the pulses then shorten further to300 fs, requiring several psec to do so.Duringmodelocked operation, the absorber is drivenheavily into saturation.

An accurate theoretical description for theabove phenomenon is outside the range ofstandard laser stability models based on per-turbation theory because of the presence of thelarge, heavily saturated gain in the cavity(20 dB). By measuring the relevant laser pa-rameters, conventional perturbation theories1.4 predict the laser to be well into theQ-switched,mode-locked region. This predic-tion qualitatively agrees with the observedQ-switched, mode-locked pulses. A descrip-tion for the transition from Q-switched,mode-locking to CW mode-locking, however,remains to be provided.

In the present laser, the transition fromQ-switched mode-locking to CW mode-locking is facilitated by the incorporation of abulk InP two-photon absorber5 nto the cavity.Two-photon absorption in the InP provides anoptical limiting mechanism, which suppressesthe Q-switched, mode-locked pulse intensitiesand stretches the Q-switch pulse envelopes toeventuallyachieve CW mode-locking. The InPtwo-photon absorber is designed to have asmall insertion loss (-5%) at CW mode-locked power levels but to have signif icant ab-sorption (>50%) at the power levels of theQ-switched, mode-locked pulses. The two-photon absorber material can be placedon the

saturable absorber or at the opposite end of thecavity.

With the saturable absorber/two-photonabsorber combination, the total nonlinear losscan be self-adjusted to provide an optimumstability egion for CW mode-locking. (Fig. 3).As experimentally verified, the insertion of atwo-photon absorber has no measurable det-rimental effects on the attainable output powerand achievable pulse widths from the fiber la-ser cavity. Rather, the power limitation fromthe saturable absorber allows tighter focusingon the saturable absorber without incurringoptical damage, allowing the exploitation of

larger nonlinear lossneeded for the most rapidself-starting.

In conclusion we present here a mode-locked laser which self starts by a means differ-ent from conventional understanding. Presenttheories do not predict self-starting throughQ-switch mode-locking. Differences with per-

turbative theories are understandable. Thesepredicted conditions for self-starting of CWmode-locking use static numbers, while thekey parameters for stable modelocking are

,varying dynamically during the Q-switchedmode-locking of this laser. It is very conceiv-able that these numbers vary appropriatelysothat CW modelocking can initiate. More spe-cifically, he laser makes a transition from fastsaturable absorber modelocking duringQ-switching to slow saturable absorber mod-elocking during CW mode-locked operation.A more complete dynamical analysis is re-quired to understand these transitions.

1. H.A. Haus et al., J. Quantum Electronics,

2. A.T. Obeidat, W.H. Knoxand J.B. Khur-gin, Conf. on Lasers and Electro-Optics,CLEO, Baltimore, paper CtuP28 (1997).M.E. Fermann, L.-M. Yang, M.L. Stockand M.J. Andrejco, Opt. Lett., 19, 43

(1994).F. Kartner, L. Brovelli, D. Kopf, M. Kamp,

I. Calasso, U. Keller, Opt. Eng., 34, 2024

(1995).A. Agnesi et al., Opt. Lett., 18,637 (1993).

QE-12,169, (1976).

3.

4.

5.

CThW 3:OO pm

Saturable absorber modelocking uslngnonepitaxially grown serniconductor-doped films

I.P. Bilinsky, R.P. Prasankumar,

J.N.Walpole,*L.J. Missaggia,, J.G. Fujimoto,Department of Physics, Departm ent of

Electrical Engineerin g and Com pute r Scienceand Research Laboratory of Electronics,Massachusetts Institute of Technology,Cambridge, Masachusetts 02139 USA

Semiconductor saturable absorbers are usefulfor many applications in ultrafast optics, in-cluding the generation of femtosecond pulsesin solid state The most common satu-rable absorber technologiesare semiconductorsaturable absorber mirrors3 and saturableBragg reflectors,4 which have been extensivelyused for both saturable absorber modelockingand initiation of Kerr lens modelocking(KLM). However, these devices require epi-taxial growth, which imposes lattice-matching

constraintson the absorber materials and alsorequires complex and expensive systems.

We demonstrate novel, non-epitaxiallygrown semiconductor saturable absorber de-vices for laser modelocking.These devices con-sist of semiconductor nanocrystallites dopedinto silica films using RF ~puttering.~hesefilms can be deposited on virtually any sub-strate, including oxides such as glass and di-electric coatings as well as metal mirrors. Byvarying he doping density of the semiconduc-tor quantum dots, one can adjust the absorp-tion coefficient of the device. A wide range ofsemiconductor materials can be doped in to the

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I

a) asgrownb) 600 C 60 ecc) 750 "C 60 ec

I 1 , . 11

0 I , I " " I " " I . '

0 500 1000 15W

Time [IS]

CThL3 Fig. 1. Pump-probe traces for an-nealed and as-grown InAs-doped silica films. Thelaser wavelength was 800 nm, pulse duration 30fs, repetition rate 80 MHz. RTA in nitrogen wasused to modify absorption saturation dynamics.

~2 SA M3

CThL3 Fig. 2. SchematicdiagramofTi:AI,O,laser resonator. OC-2% output coupler; SA-saturable absorber;M1-10 cm radius of curvature(ROC) mirrors; M2-7.5 cm ROC mirror; M3-5cm ROC mirror.

silica films. Therefore, appropriate choice ofthe semiconductor and knowledge of quan-tum confinement effects allow one to controlthe operating wavelength and absorption edgeof the device.

The films used in our experiments werefabricated using RF sputteringand consisted ofInAs nanocrystallites doped into an SiO, ma-trix. The structural and optical properties ofthese films were comprehensively character-ized. We found the films to contain a broadsize distribution of I d s nanocrystallites up to-80 A, resulting in a smooth absorption edgeat- .2 p m compared to 3.5 p m for bulk InAs.We used rapid thermal annealing (RTA) innitrogen to modify the absorption saturationdynamics of the films; the results of pump-probe experiments at 800 nm are shown inFig. 1.

Films used for TI:Al,O, laser modelockingwere 3008, hick, with absorption of -2% perpass, and were deposited onto 3 mm thicksapphire substrates. The substrates were singlecrystal and 0-degree-oriented to avoid bire-fringence effects. The films were annealed at

either 600 or 750°C for 60 seconds. The deviceswere used in a standard argon-pumpedz-cavity Ti:Al,O, laser (Fig. 2) with an addi-tional fold for focusing onto the absorber. Thesaturable absorber or a blank sapphire sub-strate (for KLM alignment) was positionedat the focus of the additional fold and orientedat Brewster's angle. The laser spot size on thesaturable absorber was estimated to be-25 pm.

With a saturable absorber in the cavity, self-starting saturable absorber assisted KLM wasobtained, with pulses as short as 25 fs and aFWHM bandwidth of 53 nm (Fig. 3), giving a

8

-m

5 4

Lo

C

+-0

-100 0 100

Time [fs]

(a)

780 800 820 840 860 880 900

Wavelength [nm]

(b)

CThL3 Fig. 3. (a) Interferometric autocorre-lation of self-starting saturable-absorber-assistedKLM pulses. (b) Normalized pulse spectra ofmodelocked pulses. The center wavelength couldbe tuned over 80nm while sustaining self-startingmodelocking operation.

time-bandwidth product of 0.59. Self-phasemodulation in the sapphire substrate may haveprevented the pulses from being transform-limited. The laser wavelength was tunablefrom 800 to 880 nm while sustaining self-starting operation.

We are currently developing devices thatcombine the semiconductor doped silica films

with metal and dielectric mirrors for use in a

reflective geometry, making cavity designsmore compact an d flexible. We are also inves-tigating extension of this technique to otherlaser materials, including Cr:Forsterite(1.3 p m) and Cr:YAG (1.5 pm) . In conclusion,we have developed non-epitaxially grownsemiconductor saturable absorbers that, inconjunction with KLM, resulted in self-starting 25 fs pulses and a tuning range of 80nm. These devices are a versatile and inexpen-sive alternative technology to epitaxially

grown saturable absorbers and could thereforehave a significant impact on ultrashort pulsegeneration.

*Lincoln Laboratory, Massachusetts Inst itute of

Technology, Lexington, Massachusetts 02420

USA;E-mail: rpprasan@mit.edu 

1. U. Keller, D.A.B. Miller, G.D. Boyd, T.H.Chiu,-J.F. Ferguson, M.T. Asom, Opt. Lett.17,505 (1992).R. Mellish, P.M.W. French, J.R. Taylor,P.J. Delfyett, and L.T. Florez, Electron.Lett. 29,894 (1993).U. Keller, K.J. Weingarten, F.X. Kartner,D. Kopf, B. Braun, I.D. Jung, R. Fluck, C.Honninger, N. Matuschek, and J. Aus derAu, IEEE J. Select. Top. Quantum Elec-tron. 2,435 (1996).S. Tsuda, W.H. Knox, S.T. Cundiff, W.Y.

Jan, and J.E. Cunningham, IEEE J. Select.Top. Quan tum Electron. 2, 454 (1996).

2.

3.

4.

5. K. Tsunetomo, H. Nasu, H. Kitayama,A.

Kawabuchi, Y. Osaka, and K. Takiyama,Jap. J. Appl. Phys. 28, 1928 (1989).

CThL4 3:15 pm

Mode-locking of a femtosecond

Cr:forsterite laser with negativenonlinear phase shifts generated byx(*):x(*)ascading

X. Liu, L.J. Qian, and F.W. Wise, Department

of Applied Physics, Cornel1 University, Ithaca,

New York 14853 USA;E-mail:XL23@Cornell.edu 

Recently, there has been a resurgence of inter-est in the effective third-order nonlinearitythat arises from the cascading of x(') pro-cesses.' Applications of the cascade nonlinear-ity include the modelocking of lasers, and thishas been proposed and demonstrated with pi-cosecond pulses.2 Previous workers 3,4 notedthat the phase shift produced by the cascadeprocess is modulated significantly owing to

group-velocity mismatch (GVM) and satura-tion. Here we show that undistorted frequencychirps can be produced in the cascade process,and describe the first KLM laser operating withan overall negative nonlinear phase shift.

We numerically solved the coupled waveequations for the femtosecond pulse propaga-tion in the cascade processes. The solutionsshow that the temporal evolution of AaNL sgenerally distorted, so the frequency chirpacross the pulse is nonlinear. However, for AkLlarger than the drive of second harmonic gen-eration (SHG), the phase follows the pulse in-tensity envelope nearly ideally, similar to theKerr nonlineari ty. This is illustrated in Fig. 1.Large undistorted negative nonlinear phaseshifts (- 1 rad) can be generated with pulsedurations similar to the GVM wall-off time,

which suggests the use of Kerr-lens modelock-ing with positive group-velocity dispersion(GVD).

Figure 2 shows the schematic of a laser de-signed to operate with AQNL <0.The gain endof an ordinary Cr:forsterite laser cavity is im-aged ont o a second fold for a lithium triborate

10050F--T--T

-150

-4 -2 0 2 4

UT

CThL4 Fig. 1. Frequencysweeps generated inthe cascade process with GVM equal to the pulseduration andAkL = 2a dashed) and 1 ~ (solid).The magnitude of the phase shift produced withAkL = 2a s -2.5 times as large as that producedwith AkL = 1la. he fine solid line correspondsto an instantaneous nonlinear indexofrefraction.