North-Holland COM MUN ICATIONS
Measurement of background energy content of a linear cw colliding pulse mode-locked dye laser
W. Biiumler and A. Penzkofer
Naturwissenschc~#liche Fakult~it ll-Physik. Universitdt Regensburg, W-8400 Regensburg. Germany Received 1 February 1991 ; revised manuscript received 20 June 1991
The background energy content of a linear cw CPM rhodamine 6G-DODCI femtosecond dye laser is determined by time- resolved measurement of the Rayleigh scattered signal in a colloidal silicon dioxid suspension with an ungated inverse time- correlated single photon counting system. A constant background intensity signal is observed outside the temporal region of the instrument response function of the detection system. Assuming the same background intensity level within the temporal region of the instrument response function, an average background intensity level of approximately 2.5 x I 0 ~ of the peak pulse intensity, and a background energy content of approximately 2 × 10 3 of the pulse energy are found.
1. Introduction
The background energy content between the pulses o f a m o d e - l o c k e d laser is a measure o f the m o d e - locking quality. It reduces the resolution in time-re- solved spectroscopic m e a s u r e m e n t s . F o r intense pulsed picosecond lasers v a r i o u s techniques have been a p p l i e d to d e t e r m i n e the b a c k g r o u n d energy content [ 1-14 ] ( o v e r e x p o s e d p h o t o d e t e c t o r s [ 1,2 ], p h o t o - c o n d u c t i v i t y o f silicon switches [ 3 , 4 ] , con- trast ratio o f t w o - p h o t o n fluorescence traces [ 2 , 5 ] , efficiency o f t h i r d h a r m o n i c generation [6,7] and t h r e e - p h o t o n fluorescence [ 2 , 8 ] , p a r a m e t r i c four- p h o t o n interaction [ 9,10 ], frequency m i x i n g o f fun- d a m e n t a l and second h a r m o n i c light [11], streak c a m e r a m e a s u r e m e n t s [ 1 2 ] , and saturable absorp- tion [ 13,14 ] ). F o r cw m o d e - l o c k e d p i c o s e c o n d and femto-second lasers [ 1 5 - 1 8 ] no detailed back- ground energy content m e a s u r e m e n t s have been re- p o r t e d yet.
In this p a p e r the t e m p o r a l b a c k g r o u n d intensity d i s t r i b u t i o n and the b a c k g r o u n d energy content o f a linear cw colliding pulse m o d e - l o c k e d f e m t o s e c o n d dye laser [ 1 5 - 1 8 ] is d e t e r m i n e d by t i m e - r e s o l v e d m e a s u r e m e n t o f the signal o f a Rayleigh scatterer ( L u d o x C L - X ) with an ungated inverse time-cor- related single p h o t o n counting system [ 19,20 ].
For the signal analysis the s a m e b a c k g r o u n d in-
tensity level is assumed within the i n s t r u m e n t re- sponse region o f the detection system as in the time- resolved region. W i t h o u t this a s s u m p t i o n an u p p e r b a c k g r o u n d intensity limit within the i n s t r u m e n t re- sponse region is e s t i m a t e d from n o n - c o l l i n e a r sec- ond harmonic generation autocorrelation traces [ 18 ].
2. Experimental
The e x p e r i m e n t a l setup o f the cw linear colliding pulse mode-locked ( C P M ) dye laser system [ 17 ] and o f the inverse t i m e - c o r r e l a t e d single p h o t o n counting system [ 19,20 ] is d e p i c t e d in fig. 1. The C P M laser is p u m p e d by the m u l t i m o d e emission o f a cw argon ion laser including the 488 n m and 514.5 nm lines (Spectra-Physics M o d e l 2016, 3 watt power ap- p l i e d ) . The gain m e d i u m is r h o d a m i n e 6G in,eth- ylene glycol ( 3 X 10 -3 molar, jet thickness 250 ~tm, flow velocity 2.8 m s - ~ ) and the saturable a b s o r b e r m e d i u m is D O D C I ( 3 , 3 ' - d i e t h y l o x a d i c a r b o c y a n i n e i o d i d e ) in ethylene glycol ( 3 . 5 X 1 0 -4 molar, jet thickness 35 ~tm, flow velocity 7 m s - ~ ). The satur- able a b s o r b e r j e t is placed in the center o f the res- o n a t o r to achieve t e m p o r a l overlap o f the counter- propagating pulses in the absorber jet (colliding pulse m o d e - l o c k i n g ) . The r e s o n a t o r length is 2.5 m lead- ing to a pulse s e p a r a t i o n o f 8.3 ns. A parallel prism
Volume 85, number 4 OPTICS COMMUNICATIONS 15 September 1991 WP
[ At-ION LASER p ' - - ' J ' - - ' - -
-~
i I
~ "&"\ i • i. - " - - J ~ - " J ' - - J ' -
I- -
MI p ,,I~..- • ---" A G M2
S AP F
. . . 9 " -+
.cP [-1 INV [
A M P [
C F T D ['
---7,
APD
1 F
START p ~ STOP
I
. . . . e . - - >
I
LED
DELAY
Fig. 1. Experimental setup. WP, 2/2 wave plate. M 1, high reflec- tivity broadband single-stack mirror. M2, single-stack output mirror with reflectivity R=0.97. G, gain cavity with mirror cur- vaturesp= 10 cm, jet thickness 250 tam (position at one quarter of resonator length). A, absorber cavity with mirror curvatures p = 5 cm, jet chickness 35 tam (position exactly at one half of res- onator length ). P, prism pair, AC, intensity autocorrelator [24], SP I, SP3, grating spectrometers. F, interference filter. AP, aper- lure. S, sample containing Ludox CL-X. L, lens. MCP micro- channel plate photomultiplier (Hamamatsu R1564-01, voltage -2900 V). INV, inverting transformer (EG&G IT 100). AMP, 25 dB broadband amplifier (Minicircuits ZL-1042 J, 10 MHz-4.2 GHz). APD avalanche photodiode (Telefunken BPW28), CFTD, constant-fraction time discriminator (Tenne- lec TC454 with fraction module f= 0.2 ). DELAY, coaxial cable delay box (Ortec model 425 A). TAC, biased time to pulse height converter (Ortec model 457) adjusted to 130 channels per ns, PHA, pulse height analyser multichannel analyser system (Ortec Norland 5608 ). PC, personal computer.
The pulse spectra are m o n i t o r e d with a grating s p e c t r o m e t e r SPI and a v i d i c o n system. The laser- wavelength is AL~ 620 nm. The pulse d u r a t i o n s are m e a s u r e d with a rotating m i r r o r intensity autocor- relator a p p l y i n g a n o n c o l l i n e a r p h a s e m a t c h e d sec- o n d h a r m o n i c K D P crystal [ 2 4 ] . Pulse d u r a t i o n s d o w n to 50 fs have been o b t a i n e d by fine a d j u s t m e n t o f the prism p o s i t i o n s and the jet positions. In the e x p e r i m e n t s d e s c r i b e d here the pulse d u r a t i o n s were AtL--~ 90 fs.
The t i m e - c o r r e l a t e d single-photon counting sys- tem is o p e r a t e d in an ungated inverse m o d e (single p h o t o n signals start t i m e - t o - a m p l i t u d e c o n v e r t e r ) . The full 120 M H z pulse repetition rate o f the C P M laser is applied. The c o m p o n e n t s o f the single pho- ton counting system are listed in the c a p t i o n o f fig.
1. The s a m p l e cell contains a colloidal silicon d i o x i d suspension in water ( L u d o x CL-X from D u P o n t , particle size 21 nm, u n d i l u t e d scattering coefficient at 620 nm is 0.2 cm -~ [25] ). In fluorescence decay studies this suspension is used for recording the in- s t r u m e n t response function o f the system. The single p h o t o n d e t e c t o r is a fast m i c r o c h a n n e l plate photo- m u l t i p l i e r tube ( H a m a m a t s u type R1564-01, micro- channel d i a m e t e r s are 12 Jam) and the trigger de- tector is a fast a v a l a n c h e p h o t o d i o d e (Telefunken BPW28 ). The Rayleigh scattered signal S(t) and the p h o t o m u l t i p l i e r noise signal Su ( t ) are m e a s u r e d sep- arately and the noise signal is subtracted to o b t a i n the true Rayleigh scattering signal S o ( t ) = S ( t ) - Su(t). An analysis o f S o ( t ) delivers the b a c k g r o u n d intensity and energy contents.
3. Results
The Rayleigh scattering signal is recorded and an- alysed by a s s u m i n g a constant b a c k g r o u n d light dis- tribution. A d d i t i o n a l l y an u p p e r limit o f the back- ground intensity level within the instrument response width is e s t i m a t e d by analysing n o n c o l l i n e a r phase- m a t c h e d second h a r m o n i c generation autocorrela- tion traces.
pair ( p r i s m distance 30 cm, fused silica Brewster an- gle p r i s m s ) is inserted for c o m p e n s a t i o n o f group ve- locity dispersion and chirped pulse compression [ 2 1 - 231.
3.1. Rayleigh scattering trace
The Rayleigh scattered signal S(t) was a c c u m u - lated over a t i m e p e r i o d o f 3 hours. The n u m b e r o f
counts in the channel of m a x i m u m counts was nearly 2 × 106. The total n u m b e r o f counts s u m m e d over all channels was approximately 6 . 4 5 × 107 , The micro- channel plate photomultiplier noise signal S, (t) was accumulated over the same time period, where half of the measurement was done before and the other half after the Rayleigh scattering measurement. For the noise signal measurement the entrance to the spectrometer SP2 was covered. The noise signal was rather constant over the channels. The average noise signal height was the same in the accumulation be- fore and after the Rayleigh scattering measurement.
The total accumulated average n u m b e r of noise counts per channel was 290.
The dark-noise corrected and normalized Ray- leigh scattering signal
S,(t)=So(t)/So
. . . . is dis- played in fig. 2 [ S o ( t ) = S ( t ) - S u ( t ) ] . It consists of an intense pulse extending from tpb to /pc, two weak pulses at 4, and t,,, and a rather constant pedestalql c
zo 10
_ l o ' I
! J
¢ 10 o
tub
I
4 5 2 lO
10
I
t u e
l O ~ ~ ~tal toe t pt II ta2 I b I
-2 -1 0 ~ 2 s 4 s 6 7 8
TJME T (NS)
Fig. 2. Normalized Rayleigh scattering signal versus time. Signal detection is limited to time separation between adjacent pulses (longest delay between start and stop pulse is pulse separation 7 = 8 . 3 ns ).
from tub tO rue, The intense signal pulse has a full halfwidth o f 180 ps characterizing the instrument re- sponse time. The signal pulse extension from tpb to tp~ is caused mainly by the transit time spread of photoelectrons in the microchannel plate photomul- tiplier ( M C P ) and the timing jitter o f the constant fraction time discriminator ( C F T D ) and the leading edge discriminator ( L E D ) . The true signal duration is equal to the CPM laser pulse duration (AtL~
90 fs). The two weak pulses at t~ and 1,,2 are most likely after-pulses of the MCP photomultiplier. They are not due to satellite pulses of the C P M laser be- cause there are no optical components at appropriate timing positions in the resonator. The pedestal signal is thought to be Rayleigh scattering in the sample caused by the background laser light between the femtosecond laser pulses.
3.2. Background signal analysis j'rom Rayleigh scattering trace
The ratio of background laser energy content 14"b,,r in the time resolved regions t,b to /pb and tp~. to t,~
(5.4 ns) to the laser pulse and background laser en- ergy W,,r in the unresolved time region rob to to,. (2.9 ns) is
• _ d t +
,peS.(t)dt=l.35XlO_
3g]u,- j-'£;S,(t) dt
The total background energy content I4", between the femtosecond laser pulses and the average back- ground laser power Pb or the average background laser intensity [b are estimated in the following by assuming that the background laser power Pb(t) in the unresolved time region from lob to tot is the same as in the resolved time regions from lub to lob and tp~, to tu~. Using this assumption it is
t'pe
f S,(t)
dt=~c[ 14" o + (tp~- tob)/Sbl . ( l )tpb
and
/pb lu¢
. f d,+ f
/ub /pc
=K(lpb--tub+luc
-- toc)lSb , (2)Volume 85, number 4 OPTICS COMMUNICATIONS 15 September 1991 where K is a proportionality constant and Wp is the
laser pulse energy. Eq. (2) determines the average background laser power to
/pb lue
Pb = J 'u"Sn(t) dt+ f'PeSn(t) dt (3) K'( tpb -- tub "~/ue --/pc )
The laser background energy content is
Wb = TPb, (4)
where T is the temporal laser pulse separation (half of the resonator round-trip time). The ratio of back- ground energy content to pulse energy is obtained from eqs. ( 1 ) - ( 4 ) to be
U b = T [ &(t)dt+ &(t)dt
lub /pc
tpe
X {(tpb--tub "]-tue--tpe ) I
Sn(t)dt
tpuy ; ]}
--(tpe--/pb) l
[
S n ( t ) d t + S n ( t ) d t . (5)lub hab
The ratio of average laser background power /3b or intensity 1~ to peak laser power POE or intensity 'COL is
PoL --
loL
- 2 \ l n - f f ~ ] TWp
(6) The relation Wp = 0.5 (z~/ln 2 ) ~/2AtLPoL for gaussian pulses is used in eq. (6).The experimental data of fig. 2 give W b / ~ p 2 × 10 -3 (eq. ( 5 ) ) and [b/loL~2.5× 10 -8 (eq. (6)).
Some contributions of the transit-time spreading of the MCP photomultiplier and of the timing jitter of the discriminators to the pedestal signal in the time range between t~b to tpb and toe to t~ would reduce I4~/Wp and
[b/loL.
It should be emphasized that in the determination of the background energy ratio Wb/Wp and in the average background intensity ratio ~/IoL it was as- sumed that the background intensity level in the un- resolved temporal region from tpb to tpe is the same as in the resolved temporal region. The theoretical understanding of femtosecond pulse formation in a CPM dye laser [ 15-18 ] (background suppression in leading edge region of pulse by saturable absorber,
and background suppression in trailing pulse region by partial fast absorption recovery of saturable ab- sorber [26] combined with gain saturation of am- plifying dye [ 2 3 ] ) supports this assumption. Oth- erwise only an upper limit of the background intensity level in the unresolved temporal region may be de- termined from an analysis of the second harmonic autocorrelation traces.
3.3. Upper limit estimation of background intensity level from autocorrelation trace
In noncollinear phasematched second harmonic intensity autocorrelation measurements the normal- ized autocorrelation signal Sa ( r ) / S a (0) is given by
[18,27]
Sa(T ) f ~ l , ( t ) 1 2 ( t - r ) d t
S,(O) - J;~l, (t) 12(t) dt (7)
where r is the temporal delay between the interacting noncollinear pulses 1 and 2.
With I , ( t ) = 1 2 ( t ) = I ( t ) = ~ + I o ( t ) , eq. (7) gives Sa(r) fbT+21b.fdip(t)dt+f-orlp(t)Ip(t--r)dt Sa(O) - [~T+21bfg'Ip(t) dt+f~IZo(t) dt
(8) Outside the pulse overlap region (f>> AtL) the in- tegral f ~lp ( t ) I o ( t-- r) dt vanishes.
Setting [b =flloL and Iv(t)=loL e x p ( - - t 2 / t o ) (to = 0 . 5 A t L [ l n ( 2 ) ] ~/2) one obtains
Sa( f) Tfl 2 + 2rtJ/2tofl
Sa(0) - Tfl 2 + Dz l /2tofl + 2- ' /2~r '/2to " (9) Solving eq. (9) to the background intensity ratio fl=~/loL results in
+ l - & ( O / & ( o ) (2~)'/-~to) ~ ~ 2'/~2&(0) "
(10)
The accuracy of the intensity autocorrelation traces allows an estimation of Sa ( f ) / S a (0) < 10- 2 leading to fl=~/loL < 3 . 5 X 10 -3.
4. Conclusions
T h e b a c k g r o u n d e n e r g y c o n t e n t o f a l i n e a r cw C P M d y e l a s e r w a s d e t e r m i n e d b y R a y l e i g h s i g n a l a n a l y s i s w i t h a n u n g a t e d i n v e r s e t i m e - c o r r e l a t e d s i n g l e p h o - t o n c o u n t i n g s y s t e m . A s s u m i n g a c o n s t a n t b a c k - g r o u n d level f o r all t i m e s a b a c k g r o u n d t o p e a k p u l s e r a t i o o f [b/lok ~ 2.5 × 10 --8 a n d a b a c k g r o u n d e n e r g y c o n t e n t o f 1 4 b ~ 0 . 0 0 2 14p are o b t a i n e d . T h e d e - s c r i b e d R a y l e i g h s c a t t e r i n g t e c h n i q u e c a n n o t ex- c l u d e t h e p o s s i b i l i t y o f a h i g h e r b a c k g r o u n d level i n t h e u n r e s o l v e d t e m p o r a l r e g i o n o f t h e i n s t r u m e n t a l r e s p o n s e o f t h e d e t e c t i o n s y s t e m t o t h e f e m t o s e c o n d pulses. T h e n o n c o l l i n e a r s e c o n d h a r m o n i c g e n e r a - t i o n i n t e n s i t y a u t o c o r r e l a t i o n t r a c e a n a l y s i s a l l o w s t h e e s t i m a t i o n o f a n u p p e r l i m i t o f t h e b a c k g r o u n d t o p e a k p u l s e r a t i o o f ]b/loL ~< 3 . 5 × l 0 - 3 in t h e u n - r e s o l v e d t e m p o r a l r e g i o n .
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