Noise in a dual dye jet hybridly mode-locked near infrared femtosecond laser

Optics Communications91 (1992) 465-473 North-Holland

OPTICS COMMUNICATIONS

Full length article

Noise in a dual dye jet hybridly mode-locked near infrared femtosecond laser P.A. Hal'ten, S.G. Lee, J.P. Sokoloff, J.R. Salcedo ~ a n d N. P e y ~ a m b a r i a n Universityof Arizona, OpticalSciences Center, Tucson, AZ 85 721, USA Received 25 August 1991; revised manuscript received 9 January 1992

The noise properties of a near infrared hybridly mode-lockeddye laser systemoperating in the 100 fs pulse regime are investigated. Two closely spaced cavity length values near the zero mismatch point are identified which produce distinctly different pulses. These pulse trains and the pump laser pulse train are examined in the time and frequency domains for the purpose of determining their usefulnessfor the study of ultrafast physicalprocesses. The differencesin pulse energyfluctuation, pulse width fluctuation, and intensity autocorrelationof the two modes of operation are explained in terms of cavity detuning effects.

1. Introduction Hybridly mode-locked dye lasers have been used extensively in ultrafast spectroscopy experiments. This is mainly due to the larger number of gain and saturable absorber dye combinations available as compared to passively mode-locked systems and the ease of synchronization with amplifiers and other lasers that are synchronously pumped in parallel. Sub 100 fs pulse durations have so far been achieved in the visible and near infrared spectral region [ l ]. The widespread use and the complexity of these systems has led to a number of different theoretical and experimental approaches to characterize them [ 2-5 ]. One of the characteristics of this type of modelocking is its extreme sensitivity to cavity length mismatch between the pump laser and the dye laser. A previous study has established the general pulse width and spectral dependence on the cavity detuning [ 6 ]. We have verified this general behavior for our laser. In addition to that we observe in our experimental set up two cavity length positions close to the optim u m length that yield transform limited pulses with durations of ~ 100 fs while exhibiting quite different scanning intensity autocorrelations. The same bePermanent address: Centro de Fisica, Universidadedo Porto, Praqa Gomes Teixeira, 4000 Porto, Portugal.

havior has been observed in a similar hybridly modelocked femtosecond laser, but to our knowledge this has not been studied in detail [ 7]. These small cavity length detuning regimes are of particular interest, because almost all hybridly mode-locked dye laser applications demand transform limited and stable femtosecond pulse trains. The difference in operating characteristics between these two modes of operation is the object of this study. Autocorrelation measurements are insufficient for the characterization of the laser output in general. In particular, they are not sufficient to uniquely determine a stable femtosecond pulse train. For this reason the random fluctuations of pulse energy and pulse duration are investigated employing the technique of radio frequency spectrum analysis [8]. This technique is based on recording the power spectrum of the laser intensity with the help of a fast photodiode and an electronic spectrum analyzer. The different random contributions to the laser noise are identified by their characteristic spectral appearances. Studies on similar laser systems using this technique have provided valuable insight into the noise properties of those systems [8,9]. The theoretical approach used here follows the development in ref. [ 8 ]. The laser output intensity is described by a function F(t)=[l+A(t)]Fo(t)+TJ(t)

0030-4018/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

(O/Ot)Fo(t),

(1) 465

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where Fo(t) = ~ f ( t + n T )

(2)

is the output intensity of a perfectly mode-locked laser with pulse shape f(t) in the absence of any noise, A (t) describes the rant om amplitude fluctuations of the pulse train, J(t) represents the random deviations from the average r ~petition period T, and the integer n runs from -cx~ to +or. This description is valid for random fluctttations that vary slowly compared to the pulse inteasity f(t). The power spectrum PF(co) of the laser intensity is Pr(og) = e ~ ( c o ) ~ [6(co~) +Pa(co~) + (2nn)2PJ(co~) ] ,

(3)

with the envelope function P~n~(co) = ( 2 x / T ) 2 Ig(o~) I~ ,

(4)

where g(og) denotes the Fourier transform o f f ( t ) , PA(co) is the power spectrul~l o f A ( t ) , Pj(co) is the power spectrum of J(t), and co~=co-21tn/T. Centered around each multiple of the repetition frequency one finds the same amplitude fluctuation power spectrum Pa (co) and .-tided to that the timing jitter noise power spectrum l'j(co) multiplied by the square of the order number r~. For highe r and higher order numbers the spectra will eventually decrease in magnitude due to the limited frequency range of the detection system. The delection system used here delivers useful data up to as least n = 18. Therefore the amplitude noise can be obtained from the lowest order spectra while the jitter noise, which is recognized by its dependence on t] le order number, can be determined from the higher order spectra. The rms deviations of the pulse energy AE and the repetition time AT are related to the l~ower spectra by [8] +cx~

(AE/E)2= J PA(co)dco,

(5)

+oo

(AT/T)2= J Pj(co) dco. ~

(6)

oo

The integrals on the right h m d sides of these equations are evaluated by integ'ating eq. (3) using several approximations. The first one relies on the as466

1 August 1992

sumption that Ig(co)12 is not varying considerably between the n = 1 and n = 18 noise spectra. Then the envelope function p~,v(co) becomes a constant and can be pulled out of the integral. Each spectrum analyzer data point is already a result of an automatic integration over its resolution bandwidth Af~es. Due to this fact we can relate the power spectrum peak value of the nth measured order Pmax.n to the envelope function:

e~nv(co)=Pmax,nAfres.

(7)

Equation (5) may be easily solved with the further assumption that for the n = 1 order the integral over the timing jitter term can be ignored, because it is much smaller than the integral over the amplitude noise. Then an integral S, over PF (co) encompassing the whole n = 1 noise band with the exception of a small region around cot of the size + Af~esyields the desired right hand side of eq. (5) times the envelope function. In short we have +oo

~ PA(CO)dco~-oo

Si P .... t Af~s"

(8)

For high-order numbers n one cannot neglect the timing jitter contribution in eq. (3). In the experimental section of this paper it will be shown that the n = 10 component contains considerable timing noise contributions. In that case we can extract the timing jitter by an additional similar integration as described above for the amplitude fluctuations. This additional integral Sto covers the region around the n = 10 spectral peak in the same manner as before. Then we obtain +oo

1

Szo - S t

Pj(co) dco~- (20~)-------~ P . . . . to Af,~s "

(9)

-oo

The integrations S~ and St0 are numerically evaluated by summing the measured data points over the specified range. The special case of intensity fluctuations A (t) that vary on the timescale of the pulse or faster with (A(t)) = 0 and (A2(t))= 1 is known as the noise burst model [ 10 ]. In this case it is possible to derive an exact expression for the pulse energy fluctuations [8 ] as AE/E= (tn/tp)t/2, where tn is the width of the random pulse substructure autocorrelation, and tp is

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the width of the pulse envelope autocorrelation. It is straightforward to include a timing jitter term in the noise-burst model and then evaluate the measured noise power spectra as summarized above in eq. (4) and eq. (5). We will show, however, that this model is not appropriate for noise analysis in our dye laser with small cavity length detunings, because it leads to a too large amplitude fluctuation value compared with the data.

2. Experiment The laser system studied here is similar to the ones used in refs. [ 1,5 ]. The pump laser is a Spectra Physics 3800 cw mode-locked frequency doubled Nd: YAG laser. The pump pulse train consists of 100 ps full-width-at-half-maximum (fwhm) pulses at 82 MHz repetition rate and 532 nm wavelength. The average pump power at the gain dye jet is 640 mW. The linear, double-Z-shaped, dye laser cavity contains gain dye and saturable absorber dye jets, each surrounded by two focussing mirrors of 10 cm and 5 cm radii of curvature, respectively, which are arranged for astigmatic compensation. It also includes, besides end mirror and output coupler, a dispersion compensation section consisting of two quartz Brewster prisms for double pass operation. All mirrors, with the exception of the output coupler, are high reflectors between 2 = 800 nm and 2 = 900 nm. The output coupler, a 94% reflector from 2 = 840 nm to 2 = 8 8 0 rim, is mounted on a translation stage equipped with a piezoelectric micrometer adapter for cavity length fine tuning. The LDS 821 gain dye is dissolved in 15% propylene carbonate (PC) and 85% ethylene glycol (EG) to give a 2.3X 10 -3 M solution. The IR 140 saturable absorber is first predissolved in PC, then EG is added for a 1.5)< l0 -4 M solution. For the measurements described here, the dye laser puts out ~ 100 fs pulses (fwhm) at the pump laser repetition rate, 870 nm peak wavelength, and 25 mW average power. The dye laser and the pump laser breadboards are directly bolted down onto a 1 ft. thick floating NRC optical table for improved mechanical stability. The autocorrelation measurements are performed using the standard background free second harmonic generation technique with a variable delay line that

1 August 1992

is piezoelectrically scanned at 10 Hz repetition rate. The power spectra are recorded with a 25 ps risetime Antel AR-S3 silicon photodiode feeding a Hewlett Packard 8568B 100 Hz to 1.5 GHz spectrum analyzer. For the dye laser second harmonic noise spectra the photodiode is replaced by a Hamamatsu R212 photomultiplier tube. Both detectors are operated in their linear range of response. It is convenient to label the two distinctly different regimes of operation thht we identify in this paper by cavity length mismatch as measured from some reference value. The optimum dye laser cavity length, which by this definition we will label as AL=0, is assumed to correspond to the cavity length which yields the shortest pulses as measured by their autocorrelation function. This cavity length does not, in general, correspond exactly to the pump laser cavity length, nor does it necessarily coincide with the dye laser cavity length that yields maximum second harmonic generation [2,6]. The first mode of operation, AL=0, is characterized by higher second harmonic, shortest pulses, and a "noisier" autocorrelation. The second mode of operation, AL = - 0 . 7 ~tm, yields slightly longer pulses and a smoother autocorrelation. The autocorrelation functions of the dye laser pulses labelled A L = 0 and AL= - 0 . 7 lsm, as well as the correspor~ding laser output spectra are shown in figs. la, lb, lc and ld, respectively. The time-bandwidth products of 0.15 suggest an asymmetric pulse shape close to a single sided negative exponential. This time-bandwidth product can be reproduced by a pulse shape with a steep sech 2 rising and an exponentially decaying trailing edge. For such pulses the autocorrelation deconvolution factor of 1.8 yields fwhm pulse width estimates of 85 fs and 95 fs for A L = 0 and A L = - 0 . 7 ttm, respectively. Before proceeding le~ us first consider the power spectrum of the pump beam intensity. This permits identification of fluctuations that are simply transferred from the pump laser to the dye laser. Figures 2b and 2c show power spectra of the frequency doubled Nd:YAG output centered at 82 MHz and 822 MHz, respectively. The 0 dB level corresponds to 10 dBm electrical power into the spectrum analyzer for all measured noise power spectra shown in this pal per. Since the pump laser behavior is expected to be strongly dependent on ithe mode-locker, a power spectrum of the mode-10cker synch out signal is also 467

Volume 91, number 5,6

120 s-%

OPTICS COMMUNICATIONS -

40

-

11o

1 August 1992

!

(o) ,u

~1oo

90 Z 8O 0 70

~5o

4oF

S 2o~<

Jl

I I0 ~~ 0 o

_ l_O~d:K)

21111 300

,oo

-

TIME (fs)

c

(c) ~

:, 0

FWHM

=

3.7

nm

0

(d) 61.--0.7#wn

~s o v

z

z

z

z

300

FVatM - 3.0 nm

0 t ~J

u4

N2

z

-200 -1~00 0 100 200 TIME (fs)

/Z !

z

I.--

o hi

|

865 875 ~VELENGTH (nm)

6

885

i

865 875 WAVELENGTH (nm)

885

Fig. 1. Background tree intensity autocorrelations and laser spectra for the regimes described in the text.

included as fig. 2a. This spec rum consists of distinct peaks at multiples of 500 H ~ which are direct consequences of a phase detect( r in the frequency synthesizer Of the mode-lockez driver unit that compares and adjusts the RF oul aut signal to a precision reference oscillator signal at 500 Hz rate [ 11 ]. The peaks result from the periodi = adjustments of the RF signal driving the mode-loc :er. These modulations bring about periodic cavity Q changes in the Nd:YAG laser causing the 2 = 532 nm output power spectra to duplicate the san e sharply peaked structure. We would like to point out that our theoretical approach which follows ref. [ 8 ] is valid for this type of noise. The only restrictio as of this theory are the assumptions of a constant I ,ulse shape and fluctuations which are slowly varyil Lgcompared to the pulse repetition time. The sinuso.dal modulations of the pump laser intensity repres, .'nted by the sharp sidebands in the noise spectra a~e sufficiently low in frequency to satisfy the assuml: tions. Between n = 1 (82 468

MHz) and n = 10 (822 MHz) the noise bands on both sides of the central peak increase strongly indicating the presence of a timing jitter. Figure 3 shows the square dependence of the pump laser jitter noise on the order number n for the 1 kHz sidebands. The excellent agreement of the data points with a quadratic function is emphasized by the very small difference between the 6th order polynomial best fit which has been added for reference (dashed line) and the quadratic best fit (solid line). Using eq. (9), a r m s repetition rate fluctuation of AT/T= O.12% is obtained. At 82 MHz this is equivalent to AT= 15 ps. This result is supported by direct sampling oscilloscope measurements which showed arms timing jitter of 25 ps [ 12 ]. The rms energy fluctuations of the green Nd:YAG pulses are AE/E= 3% according to eq. (8). In summary, we note that the pump!aser carriers considerable timing jitter compared to its pulse duration, while its amplitude fluctuations are small.

Volume 91, number 5,6 -30

50

(o)

-40

fo -, 82 MHz

/

_ _ 6th order polynorniol beet f i t / quodrotic best fit

RF synch out

4O

~.... - 5 0 O3 v

1 A u g u s t 1992

OPTICS COMMUNICATIONS

--60

0 n

-70

e~

-80

0 20

30

O..

FZ -100

-S

,

~- "~ -3 -1 f-fo

3

1 (kHz)

S

lo 0

-40

0

I

I

|

I

4

6

8

10

ORDER NUMBER

fo ,, 82 UHz

(b)

I

2

n

-50

),

-

532 nm

Fig. 3. Increaseof the integrated 1 kHz noise of the pump laser. The vertical scale is linear instead of logarithmic in this graph. The solid line is a quadratic least squares fit to the data points. The dashed line is a 6th order polynomialbest fit for reference showing that the data are already very well explained by a quadratic function.

-50 -70 -80 I

.-, -- 1n~n. . ,

_~,~.~, -.~ --3 --1 1 f-fo (kHz)

~3

S

-40 (c)

fo - 822 g i l l

-50 X -

532 nm

~'-60

O

-70

- 1 ~

'

-5

"

-3

. . . . . -1 1 f-fo (kHZ)

.

3

Fig. 2. Noise power spectra of (a) the mode-locker synch out signal and (b), (c) the i = 5 3 2 nm intensity of the Nd:YAG pump laser. The resolution bandwidth is 30 Hz.

Next, the dye laser noise is examined. Figures 4 and 5 show the dye laser noise for the A L = - 0 . 7 pm and AL = 0 cases, respectively, in the same frequency span of _+5 kHz around the 82 MHz and the 822 MHz peaks that contained all of the pump laser noise components. Figure 4 exhibits an increase of the noise

sidebands for n = 10 similar to the pump laser noise which indicates that the dye laser follows the pump laser in terms of timing jitter. When evaluated in the same way as above, the i rms jitter is A T = 12 ps for the AL = - 0 . 7 ~tm case.i The AL = 0 case presents itself slightly differently as shown in fig. 5 exhibiting a broadband noise pedestal with the familiar narrow sidebands superimposed. Although the 82 MHz and the 822 MHz noise spectra look similar eq. (9) yields a r m s jitter of 14 ps. Within the accuracy of these measurements the dye laser jitter can be attributed to pump laser jitter in both cases. This explanation agrees with previously observed behavior of synchronously pumped dyei lasers [ 13 ]. In order to estimate the energy fluctuations of the dye laser, a larger frequency span has to be chosen. Unlike the pump laser noise, which is confined within a range of _+2.5 kHz from each repetition rate multiple, the dye laser noise covers a much broader frequency range of _ 300 H-Iz. Figure 6 shows a comparison of the amplitude fluctuations of the AL--0 with the AL 0.7 ftm case. Using eq. (8) the pulse energy fluctuations are IAE/E=9%in both cases. Higher order noise spec~a do not exhibit any noise increase signifying no additional timing jitter contributions. As mentioned in the introduction the =

-

469

Volume 91, number 5,6

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1 August 1992

-40 (o)

fo = 82 MHz

(b)

-50

-5O I

~" -60

m~ -60 I

-70

~ -70 I

"10 v

o o_ - 8 0

a.

-90

fo = 822 MHz

-80 I

-901 ~

-100 ' -5

'

i

-3

I

-1 f-fo

i

I

i

I

I

_10OI

i

3

,

-5

5

I

,

-3

=

(kHz)

i

I

i

-1

I

f-fo

(kHz)

I

i

3

5

Fig. 4. Noise power s ectra of the dye laser intensity at AL= -0.7 i~m. The resolution bandwidth is 30 Hz.

-40

-40

-50

(o)

(b)

I fo = 82 MHz

II

~" -60

fo = 822 MHz

-5O I m,~ - 6 0 I

t~ -70

~ -7o I n

-801

-9o I

-1%5

"~'-'1"

;

f-fo

' ~

-10o/5

' s

'

_~3

i

(kHz)

_11

i

I

f-fo

3

5

(kHz)

Fig. 5. Noise pov .=rspectra of the dye laser intensity at AL=0. The resolution bandwidth is 30 Hz.

-40

- -

-40

6J = 0

fo = 82 MHz

&L = -0.7 /,t,m

-50

fo = 8 2 MHz

-50 (b)

(o) -60

o j a.

,.=,

-70

-8o

-90

-

-90

oo

I

!

I

-250

0

250

f-fo

(kHz)

500

' 1----I ~0

!

!

I

-250

0 f-fo

250

5OO

(kHz)

Fig. 6. Lower resolutic I larger frequency span versions of figs. 5a and 4a. The resolution bandwidth is 100 Hz. 470

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Volume 91, number 5,6

We devised a simple model to describe this behaviour as illustrated in fig. 8. Since we observe oscillations of the A L = - 0 . 7 ~tm pulse train directly in the time domain at the i same frequency of approximately 25 kHz, which are not present in the A L = 0 pulse train, we assume b sinusoidal function for the random intensity fluctuation function A (t) as shown in fig. 8a. Fig. 8b shows G~ (t), the autocorrelation ofA (t) and the resultingi power spectrum log [PA (to) ] is plotted in fig. 8c. The shape of log [PA (to) ] agrees very well with the measured shape of fig. 7b identifying a plausible explanation for the AL = - 0.7 lam power spectra. It should be noted that single sided negative exponential asl well as gaussian and triangular envelope functions for A (t) did not give as good an agreement. The good agreement i between the data and our simple model suggests a lsinusoidal modulation A (t) which we attribute to se!f-pulsing [ 14,15 ] that is interrupted by competing noise mechanisms such as dye jet inhomogeneity. Self-pulsing develops when a cavity length mismatch prevents the pulse shaping mechanism provided by the gain and absorber media from fully compensating the temporal walk-off between the dye laser pulse with respect to the pump pulse. Eventually the dye laser pulse moves out of the gain window and collapses. Then a new pulse builds up from amplified spontaneous emission. This periodic phenomenon carriers random fluctuations due its dependence on spontaneous emission. In ref. [ 14 ] Catherall and NeW distinguish between tr, > 0 self-pulsing and tr,< 0 self-pulsing, where tm=

noise burst model predicts pulse energy fluctuations of magnitude AE/E= (t,/tp)'/L In both our cavity detuning cases we observe narrow autocorrelations without "coherence spikes" such that t. ~ tp, yielding approximately 100% pulse energy fluctuations. This result shows that the noise burst model is not adequate to describe our laser at small cavity length detunings. We do not observe shifts between the &parts and the centers of the noise bands in the power spectra that could justify further consideration of the noise burst model. We ascribe the absence of these shifts to the smallness of the cavity length detuning in both of our cases. An interesting difference between the energy fluctuations in the two cases is seen better in an intermediate frequency span as shown in fig. 7. The AL = - 0.7 ~tm spectrum has a double lobed structure with its peaks at about + 25 kHz away from the center and almost no noise frequency components between them. We discussed similar distinct spectral features before in the pump laser power spectra that were narrower and located at different frequencies. In this case the 25 kHz oscillation peaks are broader due to the presence of other noise sources. The side lobes are related to a sinusoidal modulation of the laser output. This can be understood by considering the fact that a sinusoidal amplitude modulation function has a power spectrum with two symmetrically located side peaks, where the distance from the center to each peak equals the oscillation frequency. The detailed structure of these sidepeaks is determined by the envelope of the sinusoidal modulation.

-40 ] -50~

,= &L

-40 0

&L = -0.7 /am

fo - 82 MHz

fo = 82 MHz

-50

!

(b)

(o) ~-50 -70

°~-90 -7ol~- , -80

o"O.

"=' "

-80 -90

-10~-125

-75

-2,5 25 f--fo (kHz)

75

125

-100

I

25

-75

-25 f-fo

!

i

25 ! (kH~)

!

75

125

Fig. 7. Intermediate frequency span versions of figs. 5a and 4a. The resolution bandwidth is 100 Hz.

471

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1.2 (o) A(t)

0.8 <~ 0.4

~ 0.0 ~-0.4 -0.8

i

-!.2 -0.2

'

i

'

!

0.0 0.2 TIMF" ',ms)

0.4

1.2 (b) ~(t) Z 0

O.B 0.4

fluorescent lifetime of our laser dye has been measured to be less than 1 ns [ 16 ] which is small compared to our Tc,v = 12 ns. Therefore, we can exclude the tm> 0 case from consideration. This agrees with the fact that we have to shorten our cavity slightly in order to observe the A L = - 0 . 7 pm mode of operation. Self-pulsing is expected to be perturbed by other noise sources in our laser that vary on the same time scale, e.g. dye jet inhomogeneity. This effectively stops self-pulsing and proves reasonable the assumptions made in our simple model explaining the shape of the observed power spectra. Finally, the pulse duration fluctuations are extracted from noise spectra of the dye laser second harmonic as displayed in fig. 9. Assuming a constant pulse shape the rms pulse duration fluctuations are given by [ 8 ]

(Atp/tp)2= (AE/E)2h - 4 (AE/E)2u,d,

W

1 August 1992

(10)

0.0

~0~-0.4 -0.8 -1"204

. . . .

0

2

c' .0=

'

J 0.2

'

0.4

TIME: ( m s )

(¢) Lo9 o,(~) 1i

A

0.0

:

-4.o IM

=

a,. -8.0

-t2.0 -125

•

-75 -2~, 25 75 FREQU "NCY (kHz)

125

Fig. 8. (a) Sampletime interval o[ modelled random amplitude fluctuation function A(t). (b) At: tocorrelation Ga(t) of A(t). (c) Logarithmof the normalizedp )wer spectrum Pa (t) of A(t). The dashed line has been inserted o mark the central frequency location. To,v- Text is the difference b ,'tween the cavity transit time Tcav and the pump la:~er repetition time Text. They obtain self-pulsing for tm> 0 only when the relaxation time of the laser m =dium is comparable to T~v while this condition doe; not apply to tm< 0. The 472

where the subscripts "sh" and "fund" stand for second harmonic and fundamental frequency, respectively. The results are Atv/tp=21% (AL=O) and Atp/ tp=52% ( A L = - 0 . 7 ~tm). Examination of higher order noise spectra shows no timing jitter contributions on this frequency scale. The pulse duration fluctuations are about twice as large in the A L = - 0 . 7 ~tm case. This agrees with the lower autocorrelation peak: an average over pulses of different duration, but roughly the same pulse energy yields a reduced peak intensity and a broader shape compared to an average over mainly shortest pulses. The larger pulse width fluctuations in the A L = - 0 . 7 lxm case are another piece of evidence for the presence self-pulsing. The calculations of ref. [ 14 ] clearly show such large fluctuations.

3. Conclusion Two distinct modes of operation of a femtosecond synch-pumped dye laser have been identified and characterized. Timing jitter, pulse energy and pulse duration fluctuations have been investigated. The timing jitter has been ascribed to the timing jitter of the pump laser which in turn has been linked to electrical noise in the mode-locker driver. The pulse energy fluctuations arc similar in both modes of operation, whereas the shape of the respective noise

1 August 1992

OPTICS COMMUNICATIONS

Volume 91, number 5,6

-50

-50 &L -

0

fo

=

82

-60

AL =

MHz

-0.7

p.m

fo

=

82

-60

(o)

-70

MHz

(b)

--v -70

he" L/.I

0 -80 O_

-90 t

-90

-100

~ -50

-25

0'

25

50

-'0%0

f - f o (kHz)

' -25

0

2'5

5o

f - f o (kHz)

Fig. 9. Noise power spectra of dye laser second harmonic intensity. The resolution bandwidth is 100 Hz. spectra differs distinctly. The A L = - 0 . 7 ~tm case has distinct noise peaks at + 25 kHz off center a n d shows corresponding oscillations in its pulse train envelope. In a d d i t i o n it exhibits pulse d u r a t i o n fluctuations which are twice as strong as in the AL = 0 case. The noise p o w e r spectrum can be explained b y a simple m o d e l based on c h o p p e d sinusoidal m o d u lations. This evidence points t o w a r d a form o f selfpulsing that is i n t e r r u p t e d b y c o m p e t i n g noise mechanisms such as dye j e t inhomogeneity. C o n t r a r y to a naive interpretation o f the autocorrelation measurements, it seems that the AT. = -- 0.7 lain case is less useful for t i m e resolved ultrafast experiments, because its pulse d u r a t i o n is not well defined. In general, the stability o f this laser system leaves m u c h to be desired, unless sophisticated active stabilization schemes are employed. F o r t u n a t e l y in m a y cases these systems m a y be replaced by much simpler m o d e - l o c k e d Ti: sapphire lasers [ 17 ] which p r o m i s e to deliver m o r e stable femtosecond pulses in the near infrared and, frequency doubled, in the blue spectral region.

Acknowledgements This work was s u p p o r t e d in part by the U.S. National Science F o u n d a t i o n , the A r m y Research Ofrice, JSOP, a n d a N A T O travel grant. One o f us, J.R.S., acknowledges a fellowship from JNICT, Junta N a c i o n a l de I n v e s t i g a ~ o Cientifica e Technol6gica, Portugal.

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