Efficient generation of mode-locked

发布时间：2024-10-11

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Efﬁcient generation of mode-locked

pulses in Nd:YVO4with a pulse duration

adjustable between34ps and1ns

Markus L¨uhrmann,Christian Theobald,Richard Wallenstein and

Johannes A.L’huillier

University of Kaiserslautern,Physics department,Erwin-Schroedinger-Str.46,67663

Kaiserslautern,Germany

luehrmann@physik.uni-kl.de

Abstract:We report on the generation of highly stable active continuous

mode-locked pulses in diode pumped Nd:YVO4with an adjustable pulse

duration between34ps and1ns.With this laser an average output power of

up to7.3W with an excellent stability and beam quality with a M2-value

of<1.1is obtained.For all pulse durations the pulses were within a factor

of1.15above the Fourier limit.Due to these characteristics the presented

system is an attractive oscillator for OPCPA concepts.

OCIS codes:(140.3480)Lasers,diode-pumped;(140.3530)Lasers,neodymium;(140.3580)

Lasers,solid-state;(140.4050)Mode-locked lasers.

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1.Introduction

Today the generation of mode-locked pulses with pulse durations of a few10picoseconds down to a few femtoseconds is well developed[1,2]and widely used in science and technology. Similarly Q-switched lasers,which provide pulses with durations typically longer than5ns, are well established.However a source which provide pulses with durations of some hundred ps is hardly found today.On one hand Q-switching is too slow to provide pulse durations well below1ns and on the other hand it is difﬁcult to maintain stable mode-locking with pulse durations signiﬁcantly longer than100ps.

Nevertheless pulses with durations between100ps and1ns are interesting for numerous ap-plications,since they allow for a moderate peak power and simultaneously a high pulse energy. Hence nonlinear effects such as self phase modulation(SPM)or a destruction of the compo-nents in the setup can be avoided even for a high average output power[3].This is in particular interesting for chirped pulse ampliﬁcation(CPA)of fs-pulses in an optical parametric ampliﬁer (OPA),for instance[4,5].OPAs are attractive ampliﬁers for CPA,due to the extreme broad ampliﬁcation bandwidth,the high gain as well as the excellent output-beam quality and inten-sity contrast ratio[6,7].In order to achieve an optimal overlap between the pump pulse and the stretched fs-pulse the pulse durations of both pulses have to be very similar.The shortest possible pulse duration of the stretched pulses is often limited by the damage threshold of the used components,because of nonlinear effects,such as small-scale self-focusing which leads to beamﬁlamentation[8],and is typically in the order of some hundred picoseconds.Moreover pump and signal pulses have to be stable and precisely synchronized.Otherwise the result is a poor conversion efﬁciency due to a deﬁcient temporal overlap and aﬂuctuation in the OPA gain and bandwidth.Further requirements for this application are a high pump pulse energy and an excellent beam quality of the pump light to ensure an efﬁcient ampliﬁcation in the OPA. Ultrashort pulse lasers with pulse durations from some100ps up to1ns were demonstrated using different principles.Among others regenerative ampliﬁers with frequency selective ele-ments[9]or stacked pulses[10,11]were used.Furthermore different lamp-pumped oscillators were reported which utilized frequency selective elements for long pulse durations[12–16]. These oscillators utilized nearly exclusively Q-switch mode-locking to reach directly high pulse energies,but the usedﬂash lamps limited the repetition rate to a few Q-switched pulse trains per second.A continuous diode pumped solid state laser in contrast is able to produce a inﬁnite #106911 - $15.00 USD Received 28 Jan 2009; revised 17 Mar 2009; accepted 19 Mar 2009; published 1 Apr 2009 (C) 2009 OSA13 April 2009 / Vol. 17, No. 8 / OPTICS EXPRESS 6178

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pulse train with constant pulse amplitude and therefore makes a much higher repetition rate

possible.Furthermore a diode pumped solid state laser is more efﬁcient,compact and reliable

compared to lamp pumped oscillators.An attractive concept for this purpose is a frequency

doubled diode pumped and mode-locked solid-state laser based on Nd:YVO4.This laser con-

cept is proven to provide stable pulses with pulse durations around10ps and an excellent beam

quality[17–19].To obtain the required pulse energy,regenerative ampliﬁer systems are com-

monly used.However the pulse durations obtained from conventional Nd:YVO4oscillators are

to short for high power CPA.A suitable way to increase the pulse duration is gain narrowing

by a resonator internal etalon,for instance.But this may result in perturbations of the mode-

locking process which lead to severe instabilities.The major task which has to be solved in

order to develop a suitable pump source for CPA is to develop an oscillator which provides

simultaneously stable mode-locking and long pulse durations up to the ns range.

During the last years it has turned out that passive mode-locking using a saturable absorber

provides very stable and reliable mode-locking of Nd:YVO4oscillators.But the stability of this

method strongly depends on theﬂuence on the saturable absorber and hence on the pulse energy,

duration and beam diameter.Thus a changed pulse duration always require a new resonator

design with other beam diameters or a different saturable absorber with adjusted properties for

stable mode-locking.With a saturable absorber setup we achieved from14ps up to120ps

with one type of resonator and absorber but a continuous tuning or a broader tuning range was

not possible.Neither the achieved pulse duration nor the achieved tuning range of the pulse

duration is sufﬁcient for the planed application.Furthermore it is difﬁcult to synchronize the

stretched fs-pulse and the pump pulse generated by passive mode-locking,which is required

for efﬁcient CPA.

Thus we turned back to active mode-locking,which solves many of these problems.In

general an active mode-locking technique provides longer pulses compared to passive mode-

locking,which is an advantage in this case.Moreover the active mode-lock process is inde-

pendent of theﬂuence on nonlinear components.Therefore stable mode-locking is achieved

on a wide range of intracavity losses.This allows for a wide variation of the pulse durations

by changing the intracavity etalons.Furthermore the possibility of a external variation of the

modulation permits a continuous tuning of pulse durations.Finally the typical drawback of

instability in actively mode-locked lasers can be avoided with long pulse durations.

Thus we report on our experimental results on an actively mode-locked Nd:YVO4oscillator,

which provides stable mode-locked pulses with pulse durations adjustable between34ps and

well above1ns.

2.Experimental setup

Figure1shows the experimental setup of the laser oscillator,which consists of a six mirror

cavity in double z conﬁguration,the Nd:YVO4crystal and an acousto-optic modulator(AOM).

The cavity is formed by3plane mirrors(the output coupler M1,M4and M6)and three curved

mirrors M2(R=500mm),M3and M5(R=400mm).In this setup,the curvature and the dis-

tances between mirrors have been optimized with respect to stability and mode matching of

the laser and pump beam.The Nd:YVO4crystal is pumped through mirror M4by aﬁber cou-

pled pump diode,which provides15W at808nm from aﬁber with a diameter of800μm.In continuous-wave(cw)operation the laser delivers an output power of7.6W into a diffraction

limited beam(M2<1.1)for an output coupler with a transmission of9.5%.Active mode-locking of this system has been obtained by amplitude modulation(AM)with an AOM.The

laser repetition rate was108MHz,corresponding to the AOM frequency of54MHz.

Active mode-locked systems are typically very sensitive to detuning of the mode-lock fre-

quency and the cavity round trip time.Therefore these parameter are mostly actively synchro-

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nized in order to stabilize the systems.The stability of externally driven mode-locking with respect to detuning depends on the phase dispersion ∂Φ/∂νbetween phase shift and round trip frequency.In the past mostly the shortest possible pulse duration was interesting,which is in the order of some 10ps for AOM mode-locked lasers.In our case the desired pulse duration is signiﬁcant longer.Due to [20]

∂Φ/∂ν∼1

τ2(1)

a 10times longer pulse durations result into a reduction of the sensibility to detuning by a factor of 100.Therefore it was not necessary to actively stabilize the system during our experiments.This setup without a resonator internal etalon provided stable active mode-locking with an average output power of 6.4W and good beam quality with an M 2factor of less than 1.15.The pulses emitted in active mode-locked operation were characterized by an autocorrelator and a scanning Fabry-Perot interferometer (SFPI).The measured autocorrelation and spectrum were well ﬁtted by a Gaussian function,as assumed in the theory of active mode-locking.From this ﬁt a pulse duration of 33.7ps and a spectral full width at half maximum (FWHM)of 15.0GHz were obtained.This results into a time-bandwidth product of 0.506,which is 1.15times above the Fourier limit of 0.441.

In order to extend the pulse duration of these pulses obtained from the oscillator,the overall gain bandwidth in the cavity has to be artiﬁcially reduced.The pulse duration,which can be expected from an active frequency modulation (FM)mode-locked oscillator with additional fre-quency selective elements within the cavity,can be calculated for pulse durations much shorter than the modulation period t m [21].A very similar relation can be obtained for AM http://www.77cn.com.cnpared to the result for FM mode-locking the pulse duration is shorter by a factor of √2:

τAM = √2ln2π4 δ t m ΔνFWHM ,2

(2)with δthe losses caused by the AOM for a cw-beam.

For a systematic reduction of the bandwidth we used different etalons between the curved mirror M5and the AOM.The thickness t ,the reﬂectivity R per side,the FWHM in double pass of the etalon transmission ΔνFWHM ,2[24]and free spectral range (FSR)ΔνFSR are summarized in Table 1for the different etalons,respectively.Of particular interest is ΔνFWHM ,2,since this parameter determines the effective gain bandwidth within the cavity and thus the shortest pos-sible pulse duration.However the FSR has to be sufﬁciently large,to ensure that only in one

Fig.1.Schematic of the experimental setup;Gain in the Middle cavity,acousto-optically

mode-locked.

#106911 - $15.00 USD Received 28 Jan 2009; revised 17 Mar 2009; accepted 19 Mar 2009; published 1 Apr 2009

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Table1.Parameters of the used etalons.

FWHM,2FSR

10.24508517

20.54203207

30.1501501034

40.325106345

50.25074.9517

60.35050.0345

732212.434.5

8 6.3522 5.8616.3

91022 3.7010.3 transmission maximum of the etalon the laser reaches the threshold.Otherwise severe instabili-ties in the laser process are expected.From a conservative estimation we expect that the FSR of the etalon has to be at least as wide as ampliﬁcation bandwidth of the laser.In our experiment we found that an etalon FSR of35GHz is sufﬁcient to suppress radiation in adjacent etalon transmission maxima for a laser based on Nd:YVO4with a gain bandwidth of210GHz[22]. Figure2(a)shows the calculated transmission of etalon no.8and Fig.2(b)those of etalon no.5.Number8provides aΔνFWHM,2of5.86GHz which allows for sufﬁcient narrowing of the gain bandwidth.However due to the small FSR of16.3GHz a lot of close adjacent transmission maxima are located within the gain bandwidth of Nd:YVO4.In contrast to etalon no.8only one transmission maxima of etalon no.5is within the gain bandwidth,butΔνFWHM,2is comparable to the gain bandwidth of Nd:YVO4.Hence no signiﬁcant reduction in gain bandwidth and therefore pulse elongation can be achieved.So neither etalon no.5nor no.8are sufﬁcient for the experiments.To assume ideal etalons a narrow transmission bandwidth comparable with etalon no.8together with a large FSR like etalon no.5and a transmittance of1can be obtained with one single etalon.But a real etalon lacks perfect plane-parallel surfaces which cause a lower limit for the transmission bandwidth and reduce the transmittance signiﬁcantly for reﬂectivities near unity.Therefore often etalons with different properties are combined[24].

In order to better understand the interaction of the transmission of both etalons and the gain of Nd:YVO4the combined transmission of the two etalons in the cavity and the transmission of both etalons together with the gain of the laser medium are illustrated in Figs.2(c)and2(d), respectively.The dashed curve in(d)represents the gain bandwidth of Nd:YVO4,which is supposed to be a Lorentzian function and normalized to one.The solid curve gives the effective gain with etalon no.5and no.8in the cavity for the case where the maxima of the etalon transmission and the maxima of the gain bandwidth are at the same frequency.The coincidence between all these maxima promises the lowest losses in the laser and therefore the highest output power.

To adjust the position of the etalon transmission to the center of the gain bandwidth the optical path length in the etalon can be adjusted by tilting the etalons.Furthermore the tilting of the etalon avoids the operation in a coupled multi-resonator conﬁguration and hence instabilities in the mode-locking process.But tilting the etalons results in walk off losses,which increases with the tilt angle,etalon thickness and reﬂectivity[25].For this reason the tilt angle should be as small as possible.For relatively slim etalons with a FSR larger or in the order of the gain bandwidth the optimal tilt angle is given by the coincidence of etalon transmission and maximum laser gain.For thick etalons with a FSR much narrower than the gain bandwidth,the losses,because of a little off-center laser wavelength operation,can be neglected.In this case the tilt angle has to be chosen in order to to avoid coupled multi-resonator conﬁgurations and #106911 - $15.00 USD Received 28 Jan 2009; revised 17 Mar 2009; accepted 19 Mar 2009; published 1 Apr 2009 (C) 2009 OSA13 April 2009 / Vol. 17, No. 8 / OPTICS EXPRESS 6181

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Fig.2.Transmission of etalon no.8(a),no.5(b)and no.8together with no.5(c).Gain of

the laser without etalons(dashed line)and with the etalons of the previous sub-ﬁgure(solid

line)(d).

to minimize walk off losses.

3.Experimental results

3.1.Temporal and spectral behavior

The etalon transmission bandwidth was systematically varied between3.70GHz and508GHz by etalon exchange.Due to our results for a sufﬁcient suppression of radiation in adjacent etalon transmission maxima only etalon no.8and no.9,which have aΔνFSR<35GHz,were used in combination with an other etalon(no.5).TheΔνFWHM,2of those etalon combinations differ of

less than1%compared to theΔνFWHM,2without etalon no.5.For this reason the inﬂuence of

etalon no.5toΔνFWHM,2can be neglected.

For each transmission bandwidth the pulse duration has been measured with a fast photo-diode and a sampling oscilloscope.The measured response time of this setup was19.3ps.The spectrum has been measured with different SFPI with an adequate FSR between2GHz and 75GHz for the particular pulse duration.The results of these measurements of the temporal and spectral intensity distributions are shown exemplary for etalon no.5together with no.8 in Figs.3(a)and3(b),respectively.As expected the temporal intensity distribution and the envelope of the spectral distribution are wellﬁtted by a Gaussian function.The spectrum shows equidistant peaks with a separation determined by the repetition rate108MHz.For an inﬁnite

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Fig.3.(a)Temporal intensity distribution for mode-locked pulses with etalon no.8and

no.5with a pulse duration of493ps.The measured data isﬁtted by a Gauss.(b)Spectral

intensity distribution for mode-locked pulses with etalon no.8and no.5with a spectral

bandwidth of0.896GHz.The envelope isﬁtted by a Gauss.

undisturbed pulse train the width of these peaks is theoretically inﬁnitesimally small.However in the measurement the width is limited by the resolution of the SFPI.Assuming a constant repetition rate the position of the maxima is given by the carrier envelope offset frequency of the pulse train[23].

Figure4shows the pulse duration4(a)and the time-bandwidth product4(b)as function of ΔνFWHM,2for a high and a low value ofδrespectively.From Fig.4(a)it is seen that a larger modulation result in shorter pulses due to the higher losses in the AOM.The experimental data corresponds very well with the solid curves calculated from Eq.2forδ=39%andδ=11%, respectively.This behavior provides an easy method for aﬁne tuning of the pulse duration in the order of10-30%,but with the drawback of a slightly lower stability at low modulations. However large tuning of the pulse duration has to be done by changing the etalons.Finally it is obvious from Fig.4(a)that it is possible to tune the laser with different etalons and changing of the modulation from pulse durations of33.7ps to well above1ns.

Figure4(b)conﬁrms the high temporal quality of the pulses.It is seen that the time-bandwidth product is less than11%above the Fourier-limit for all pulses.These values are even lower compared to the case without etalons.The lowest values are obtained for an etalon transmission bandwidth between5and100GHz and hence pulse durations between150ps and 700ps.

3.2.Output power

Figure4(c)shows the obtained output power as function of the effective etalon transmission bandwidthΔνFWHM,2.For narrow bandwidths the output power strongly decreases,due to high walk-off losses in the thick etalons.Some of the other etalons can be applied with nearly no out-put power reduction compared to the laser output power of6.4W without etalons.As expected the output power for a low modulation depth is higher,due to smaller overall losses in the AOM compared to the high modulation depth.Even with the highest pulse durations of1040ps the output power achieved nearly half of the output power without pulse elongation.The M2factor for all measurements was lower1.1comparable with the mode-locked laser without etalons.

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Fig.4.Pulse duration(a),time-bandwidth product(b)and output power(c)as function of

theΔνFWHM,2from different etalons for high and low values of modulationδby the AOM.

3.3.Stability

Already the visibility of the narrow equidistant maxima Fig.3(b)is a hint for a stable mode-locked operation during the sweep time of the SFPI of about100ms.In addition the radio frequency(RF)spectrum in Fig.5shows a suppression of side bands exceeding60dB which also supports the assumption of stable mode-locked operation.

In order to conﬁrm the long-term stability a measurement with a GaAsP photo diode,which is only sensitive to2-photon processes at1064nm has been performed.The measurement with a pulse duration of350ps,for instance,in a time interval of three hours showed a standard deviation of0.8%and a peak-to-valley less than7%.These values illustrate a high long term stability in output power and pulse duration comparable with typical semiconductor saturable absorber mirror passively mode-locked lasers.

Thus it is instructive to compare the stability of the actively mode-locked system with a pulse duration elongated to several hundred picoseconds and without pulse elongation(Fig.6).Again the measurement was done with a GaAsP photo diode.The cavity detuning was achieved by mounting M6on a piezo and apply a delta voltage to the piezo.In the measurement for34ps the mode-locking is only stable in a window of6μm below the top on the rising edge of the signal.Outside this window the measured values areﬂuctuating,indicated by the light gray envelope,and the mode-locking is instable.Whereas the system with215ps and340ps shows over the hole detuning a stable mode-locking.The declension in the signal for215ps results #106911 - $15.00 USD Received 28 Jan 2009; revised 17 Mar 2009; accepted 19 Mar 2009; published 1 Apr 2009 (C) 2009 OSA13 April 2009 / Vol. 17, No. 8 / OPTICS EXPRESS 6184

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Fig.5.RF spectrum of the mode-locked laser measured at a pulse duration of about350ps.

Fig.6.Stability measurement of mode-locking by a GaAsP photo diode.Diode signal de-

pending on the cavity length detuning with a piezo in the case of active mode-locking for

34ps,215ps and340ps pulse duration.

from a loss of output power and a slightly increasing of pulse duration,which is not observed for a pulse duration of340ps.

To characterize further the stability against cavity length variations for long pulse durations, mirror M6has been moved with a linear stage.For each position the pulse duration as well as the output power was measured.Figure7shows the experimental results for a cavity length detuning with an initial pulse duration of350ps.The experiments showed stable mode-locked operation over the whole detuning range.The pulse duration is nearly constant for a cavity length detuning over1mm in the vicinity of the optimum cavity length in respect to the output power.In this detuning range the output power drops less then20%compared to the optimum cavity length.

Compared to a stability range of6μm in the case of ps pulses without pulse elongation this

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Fig.7.Pulse duration and output power depending on the cavity length detuning with a

linear stage in the case of active mode-locking with a3mm thick etalon and pulse durations

of350ps.

is an improvement by more than two orders of magnitude,as expected from Eq.1.

4.Conclusion

In conclusion we have demonstrated an actively mode-locked picosecond Nd:YVO4oscillator, with adjustable pulse durations between34ps and1ns.This is to the best of our knowledge the ﬁrst diode pumped oscillator,which simultaneously provides stable continuously mode-locked pulses and adjustable pulse durations of some hundred picoseconds.The beam quality as well as temporal and spectral quality are very good.

Due to the demonstrated properties this system is an excellent base for an oscillator-ampliﬁer-system designed for optical parametric chirped pulse ampliﬁcation of fs-pulses.The develop-ment of a suitable ampliﬁer is currently in progress.

Acknowledgments

We grateful acknowledge support by the German ministry of education and research(BMBF) under contract number13N9030.

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