Dual picosecond dye lasers synchronously pumped by a mode locked cw yag laser

Volume 47, number 4

OPTICS COMMUNICATIONS

15 September 1983

DUAL PICOSECOND DYE LASERS SYNCHRONOUSLY PUMPED BY A MODE LOCKED CW YAG LASER ~ John M. CLEMENS, Jan NAJBAR 1, I. BRONSTEIN-BONTE 2 and R.M. HOCHSTRASSER Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA Received 13 June 1983 The performance of a novel dual dye laser system synchronouslypumped by the frequency doubled output of a modelocked CW-YAGlaser is evaluated in relation to pulsewidth, pulse substructure, pulse spectral width and timing jitter. The behavior of the system is adequately described by a theoretical model which includes the time dependent gain and losses due to frequency bandwidth, cavity length mismatch and output coupler. The jitter is significantlyreduced from that obtained with CW gas laser pumping as a result of the shorter pump pulse (50 ns instead of ~100 ps). A routine operating condition uses 2-plate birefringent filters, 0.8 W pump power at 532 rim, to yield two ca. 2.0 ps pulses having a cross correlation width of 3.8 ps, and 30 mW average power from each laser.

I. Introduction The development of laser systems generating ultrashort light pulses at two different wavelengths and at very high repetition rates has significantly extended the scope and reliability of picosecond spectroscopy [ 1 - 3 ] . The applications range from simple experiments using a pump pulse and a variably delayed probe pulse at a different wavelength, to the experiments using nonlinear interactions of radiation and matter with picosecond time resolution such as time resolved coherent anti-Stokes Raman generation [1,4-8]. The time resolution of these experiments is always determined by the pulse widths and often, such as in pumpprobe experiments, by the timing jitter between the pulses from the two dye lasers. The most common high power continuous modelocked lasers are the argon and krypton ion lasers which generate trains of pulses having 100 ps duration. This research was supported by a grant from the National Science Foundation Division of Materials Research through the NSF - University/IndustryCooperative Program (DMR 8116629). We are grateful to the Polaroid Corporation and to Dr. IE.H.Land for their contribution t 9 the development of this project. t Institute of Chemistry, Jagiellonian University, Cracow, Poland. 2 Polaroid Corporation, Cambridge, Mass.

The dye lasers synchronously pumped by gas ion lasers are capable of producing pulses ~1 ps duration, but more typically pulsewidths of 5 - 8 ps are achieved. Recently synchronous pumping of a single visible dye laser with a frequency doubled cw mode-locked Nd:YAG laser was reported [9,10]. The pump modelocked cw Nd :YAG laser has good stability, short pulsewidth, rugged construction and low maintenance costs. The pulse duration of the second harmonic (532 nm) is approximately 50 ps, narrower than gas laser pulses. In this paper we report the characteristics of a system in which two visible dye lasers are synchronously pumped (rhodamine 6G, I)CM) using the second harmonic of a mode-locked Nd:YAG cw laser. The properties of the synchronously pumped two dye laser system are a result of compromises between pulse duration, output power, the presence at substructure at the dye laser pulses and generation of satellite pulses. Here we are interested in achieving conditions for minimum substructure of the pulses which necessitates accurate cavity matching, moderate pump power and good mode structure of the dye laser pulses. These are not unique operating conditions for synchronous pumping and much attention was already given to them [11-15].

0 030-4018/83/0000-0000/$ 03.00 © 1983 North-Holland

2. Dual dye laser system Our experimental system is shown schematically in 271

Volume 47, number 4 1064n~ mE ~c BS

•

SHG ~A

""T~

OPTICS COMMUNICATIONS

I

a)

Nd:YAG, ML,IOOMHz

L E

~

~

I 0%

B~ ~ BF

Ts

Bs~

~4 o ~"-" 2 2 ps

-~

~--27ps

~

~ I

SBps

OC

PZT

Fig. 1. Schematic diagram of dual-synchronously pumped dye laser system as described in test. BF = birefringent filter, PZT = piezoelectrically driven translation stage, OC = output coupler, TS = translation stage, BS = 50% R, 50% T beam splitter, SHG = second harmonic generating crystal.

fig. 1. A continuous train of pulses each having approximately 100 ps width was obtained from a cw Nd :YAG laser actively mode4ocked at 100 MHz and frequency doubled using a K(TiO)PO 4 (KTP) crystal. The pump laser consists of a Quantronix 116 cw Nd:YAG laser head, a Brewster mode4ocker, and a high-stability 50 MHz frequency synthesizer. The mirrors of the Nd:YAG laser were mounted on an independent frame of three invar rods fixed to the bench at one point, to ensure stability of the cavity length. An independent system was used for cooling the Kr lamp and YAG rod (Neslab HX-200). The mode4ocker was mounted on a specially designed mount with 4 micrometer screws to enable precise adjustment of its position, the Brewster angle and the Bragg angle. Pump pulses at 1064 nm were generated with a repetition rate of 100 MHz, the infrared pulse duration was approximately 100 ps FWHM. In our investigations we worked with an average pump power of 8 W. The power of the second harmonic using a focusing lens (40 nun) was about 1 W. Usually 0.8 W power at 532 nm was used to excite the two dye lasers synchronously. The dye-laser cavities were the standard three mirror type supported on four super invar rods fixed to the table at one point and floating on steel ball supports. Using 400 mW o f 532 nm pump radiation and 10% T output couplers the total output of the dye (R6G) laser was > 1 0 0 mW at 600 nm. With the 2 plate filter and a 40% T output coupler the average power was ~ 3 0 mW. The autocorrelation trace for the 532 nm pump pulses (76.9 ps) and the cross correlation trace (55.5 ps) for the pump and dye laser pulses (2 ps FWHM) were used to determine that the width of the pump 272

15 September 1983

z bJ F--

. . . . .

r

-IO ©

I0

l-

-IO O IO

-IO

T

© IO

t(ps)

Fig. 2. Correlation functions: a) and b) are background-free auto-correlation traces of the pulses produced by two dye lasers and c) is their cross-correlation trace.

pulses are 50 ps (FWHM). In fig. 2 background-free autocorrelation and cross correlation traces of dye laser pulses are given. For a gaussian pulse shape we find a pulse duration of 1.6 ps (2 plate birefringent rflter and 30% output coupler) under the most favorable operating conditions. The FWHM for the cross correlation of a 1.6 ps and a 1.9 ps pulse is 3.8 ps demonstrating the low timing jitter between the two dye lasers.

3. The timing jitter in a dual laser system The fluctuations in the dye laser operation occur over a range of frequencies. The highest frequency fluctuations contribute to pulse substructure and can be inferred from the shapes of the autocorrelation traces. These fluctuations should be confined within the pulse envelope and the jitter due to such noise bursts should be smaller than the pulse width [12]. Here we consider the case where the dye lasers operate under steady state conditions for the reasonably long time of tens of/as. This steady state is usually achieved after ca. 200 round trips of the pulse in the cavity [16] The second harmonic (532 nm) derived from the Nd :YAG cw mode-locked system shows significant intensity modulation in the millisecond and longer timescales. In a dual dye laser system this modulation influence both lasers sL,nultaneously, changing delays between the pump pulse and the dye lasers pulses. The second source o f timing jitter is connected with the

Volume 47, number 4

OPTICS COMMUNICATIONS

detuning of the cavity lengths of the dye lasers. Fluctuations of the cavity optical lengths (for example as a result of the changing thicknesses of the dye jets) which are uncorrelated in two lasers, introduces additional jitter. To evaluate the magnitude of the jitter connected with both the above kinds of fluctuations we used the simple model of a synchronously modelocked cw dye laser developed by Haus [17] and Ausschnitt et al. [18,19]. We have included the effect of a saturable absorber in the model. The equation for the time-dependent electric field pulse envelope o(t) is given by [G(t) - L - B ( t ) + 8 T d/dt + (1/~o 2) d2/dt 2 ] v(t) = 0 ,

(1)

15 September 1983

The gaussian dye laser pulse envelope I(t) = I 0 e x p ( - t 2 / r 2 ) ,

(6)

should give a reasonable approximation to the shape near the peak position of the dye laser pulse and should allow the appropriate estimate of the timing characteristics of the synchronously pumped laser. A solution in the form given by eq. (6) can be obtained if we approximate the gain and loss functions G(t) and B(t) by their series expansion and retain terms to second order in t. Substitution of the gaussian field envelope into the simplified form of eq. (1) yields the characteristic equations: ) - B(0) - L - 1/w2r 2 = 0 , Gs0( 1 - ~x/~I0r 1

(7)

1/r r - I 0 Gs0 + B(0) I0o A - 8T/r 2 = 0 ,

(8)

where G(t) is the round trip gain, L is the constant cavity loss, B(t) is the time dependent loss function due to the presence of a saturable absorber, 6T is the detuning parameter, and w c is the intracavity bandwidth determined by the tuning element. The photon intensity of the dye pulse I(t) is proportional to Iv(t) 12/hvA, where A is the cross-sectional area of the beam. The equation for the gain G(t) is assumed to have simple form

where

dG(t)/dt = dGs(t)/dt - fit) G(t) .

B(0) = OAr/0 exp(--o g ½X/~I0r) ,

(2)

t

Ip(t') d t ' ,

(3)

where G_nt is the maximum available gain and I_(t) is ~, the pump pulse envelope function which is assumed to have the form: Ip(t) = (Ep0/2rp) sech 2 [(t + t0)/rp],

(4)

where rp gives pulse duration and t o the pulse position with zero time chosen as the peak position of dye laser pulse envelope. The loss function due to the saturable absorber is B(t) = oAn(t) where o A is the absorption cross section at the dye laser frequency and n(t) is the population difference for the saturable absorber. The function B(t) is determined by the approximate kinetic equation: dB(t)/dt = -

o A

fit) B(t),

+ B(O) I 2 o 2 / 2 - 1/w2r 4 = 0 ,

Gs0 = Gs(0),

(9)

1/r r = (dGs/dt)0 ,

1/r 2 = - ~ d2Gs/dt 2 ,

and n O is the population difference prior to the arrival of the laser pulse. In the absence of a saturable absorber, eqs. (7), (8) and (9) yield:

The small signal gain function Gs(t ) is given by Gs(t ) = G m f

1/r 2 + I(1/r r - IoGs0)

(5)

which neglects the relaxation of the saturable absorber.

2r4(p/r2)(1

-

X 2 ) X + T2 6T(1

-

X)/~'p

- 28T2/p(l + X) - 2/w 2 = 0 ,

(I0)

I 0 - (1 - X')/rp + 26T/r2pP(1 + X) = 0 ,

(11)

½P(1 + X ) r 2 ( 1 - 1 x / 7 r i o) - L r 2 - 1/~o2 = 0 ,

(12)

where X = tanh(to/rp), P = E0pG m . The numerical solution of eqs. (10)-(12) gives r, I 0 and t o as functions of P, L, rp, coc and 6 T. Some comparisons with experiment of the predictions calculated from eqs. (10)-(12) concerning detuning characteristics (dependence on 6 T) and bandwidth of the wavelength tuning element (dependence on ¢Oc) were already given by Jain et al. [20] and Ausschnitt et al. [18,19]. Here we are interested in the dependences of t o (the delay time between pump pulse and 273

Volume 47, number 4

OPTICS COMMUNICATIONS

dye laser pulse) on small detunings of the cavity length and on the parameter P (= Ep0 Gm). We include also comparison of the calculated and observed pulse widths (dependences on 6% and L) to evaluate the predictions resulting from this model. Due to the assumptions upon which this model is based we should expect reasonable modelling only at sufficiently low pump powers. The results of the numerical solution of eqs. ( 1 0 ) (12) for different values of P are shown in figs. 3a and 3b for a 30% transmission output coupler and a bandwidth of the wavelength tuning element w c = 20 X 1012 s -1 (two-plate birefringent triter). Predictions for two different pump pulse durations, rp = 30 ps (typical of cw-YAG system) and Zp = 60 ps (typical of gas laser) are given. Table 1 shows calculated characteristics for different values of P and L. The parameter 10 -1 (dto/d l n P ) gives the change in t o due to a 10% change in P. dto/ d In P and dt o/d6 T both decrease with increasing P. The smallest value of dto/dlnP is found for 6 T = 0. For evaluation of the relative importance of changes

15 September 1983

a)

\ 5~

' % Idto

[PS)

, dlo I0 i " t° ~ d In~P*

",

:

x! 0

,,

'

I~

'

~

\,

I0

"~

~

02

\

5 [

0

z

0 I

"k ~ x

L-'// (5

1

04

t 35

-

05

-

-

04

~--.~ 05

P

Fig. 3. Comparison of dye laser timing and pulse characteristics under matched cavity conditions with 30% transmission output couplers, 2-plate birefringent filters, and (a) pump laser pulsewidth rp = 30.0 ps and (b) rp = 60 ps. r is the dye laser pulse width and to is the delay between the pump and dye laser pulses.

Table 1 Calculated dye laser characteristics, tp = 30 ps, coc = 20.0 × 1012 s-1, 6 T = 0.

274

--

L

P

r FWHM [psi

t o [ps]

-0.1 ~

[ps]

d"-L-

0.3

0.32 0.34 0.350 0.400 0.450 0.500 0.550 0.600

2.08 1.81 1.75 1.63 1.64 1.73 1.89 2.19

43.0 32.3 28.9 18.3 12.2 8.0 4.8 2.4

24.72 13.04 10.70 6.033 4.458 3.632 3.057 2.462

1.00 0.57 0.48 0.29 0.22 0.19 0.16 0.14

0.4

0.450 0.500 0.550 0.600 0.650 0.700

1.70 1.55 1.51 1.53 1.59 1.68

33.1 22.5 16.4 12.1 8.8 6.1

13.78 7.581 5.508 4.457 3.805 3.334

0.48 0.30 0.22 0.18 0.16 0.14

0.5

0.550 0.600 0.650 0.700 0.750 0.800

1.68 1.50 1.44 1.43 1.45 1.49

36.4 25.8 19.7 15.3 12.0 9.3

16.85 9.121 6.541 5.245 4.457 3.918

0.50 0.30 0.22 0.19 0.16 0.13

m

Volume 47, number 4

OPTICS COMMUNICATIONS

of P (10%) and 5T (-+ 1/am) the derivatives dto/dlnP and dto/dSTare given for different values of P and L for the case of a 2-plate filter and rp = 30 ps. From table 1 we see that the timing jitter due to acoustic range frequencies and lower frequencies can be virtually eliminated by adjusting the two synchronously pumped lasers to the same values of P with close cavity length matching. Fig. 2 and table 1 show that the dye lasers should function far above threshold where dt0/d lnP is not too large and the pulsewidth is a minimum. The timing jitter due to cavity length detuning increases as the pump power and cavity losses decrease. The estimated value of the detuning timing jitter is < 1 ps. Under conditions of high pump power, fluctuations in the pulse envelope (noise bursts) are the major fluctuations in the system. The transient mode operation of the laser produces additional fluctuations which should have frequencies around 1 MHz.

4. Comparison of experimental results and the theoretical model The timing characteristics of different synchronously pumped dual-dye laser systems are compared on fig. 4 which shows the correlation between the smallest measured pulse duration and the smallest calculated pulse durations possible under the given conditions of bandpass of the tuning element, pump pulse duration and transmission of the output (constant

54 3

15 September 1983

loss L). We consider this correlation to be excellent. Fig. 3 allows the evaluation of the timing jitter of the dual pumped dye lasers. A comparison of figs. 3a and 3b shows that when conditions are set to achieve the shortest dye laser pulses (dt0/d lnP) is proportional t o r p . T h e narrowest pulse characteristics obtained for an Ar+ laser pumped dye laser system (2-plate fdter, 20% output couplers) are 3.9 ps for autocorrelation traces and 6.5 ps for cross-correlations [21 ]. The narrowest cross-correlation trace we have obtained so far is 3.8 ps (FWHM) from two pulses having 2.2 ps and 2.7 ps (FWHM) autocorrelation traces. To measure the timing jitter between the two lasers a gaussian pulse shape was assumed to determine their respective pulse widths. The jitter was calculated assuming a gaussian distribution using r 2 c = r 2 +T 2 +T2 , where rcc, rl, r 2 and rj are the full width half maximum of crosscorrelation, first laser, second laser, and the timing jitter respectively. In this manner the timing jitter is approximately 2.9 ps in our system and 5.2 ps for the recent experiments described by Kuhl et al. [21,22] for an Ar ÷ pumped dual dye laser system. In our system for two dye lasers operating at 25 mW average power with 2.7 ps pulses and a cross correlation trace of 6.6 ps, the corresponding timing jitter is 5.2 ps. We found that reducing the pump power by 10% in one laser shifts the cross-correlation trace by - 2 0 ps and broadens the cross-correlation trace to 13.2 ps (corresponding to 12.6 ps tuning jitter). By interchanging the dye laser power mismatch we obtained a +20 ps shift of the cross-correlation trace having 14.8 ps FWHM. These results for shifts and broadenings of the cross-correlation traces agree well with computed characteristics given in fig. 2a.

5. Spectral character of the pulses

rex p (ps)

2 I -0

,)(/ I

2PF I 2

--] 3

4

.5

rcalc (ps) Fig. 4. Comparison of shortest measured pulse durations with smallest calculated pulse widths subject to variations of cavity losses and bandwidth. (+ is from ref. [22], * are present work.)

An independent measure of the quality of the pulses from the Nd:YAG synchronously pumped dye laser system is given by the spectral bandwidth of the laser radiation. Recently, a careful analysis of the pulse shapes and spectral bandwidths for Ar+ synchronously pumped dye lasers was given by McDonald et al. [ 11 ] and Millar et al. [13] taking into account the pulse substructure. The At connected with the pulse sub275

Volume 47, number 4

OPTICS COMMUNICATIONS

structure determines the spectral width Av of the dye laser pulses. The corresponding Av At products give values characteristic of transform limited pulses. The products of the pulse envelope duration and the spectral bandwidth of the dye laser, however, are many times larger than the corresponding transform limited product for these gas laser systems. Experiments for evaluation of the pulsewidth-bandwidth characteristics of the dye laser were done using R6G, a 2-plate birefringent filter, a 30% T output coupler and ~ 3 0 mW average power. Adjustment of the dye laser cavity length to give maximum second harmonic generation with a KDP crystal results in pulses characterized by Av At twice that for the transform limited product assuming gaussian pulse shapes. Shortening the cavity by ~10/am results in a shorter pulse duration and a reduction of the Av At product to ~0.4 which is in the range of the transform limited value (0.44 for gaussian, 0.22 for lorentzian). For a dye laser operating in the high power regime (~150 mW output power, 30% T output coupler), the autocorrelation traces reveal remarkable substructure with similar characteristics to those described by Millar et al. [13] for an Ar + pumped dye laser system.

15 September 1983

A preliminary application to demonstrate the capabilities of the dual dye laser system was to measure the vibrational dephasing at room temperature for the neat liquids benzene and toluene. The two lasers were tuned to frequencies w 1 and w 2 such that 6oI - w 2 is resonant with the Raman vibrational transition. The w 1 beam is split into two beams: w 1 , coincident in time with ~2, and e 1 variably delayed. The time resolved CARS for the 992 cm-1 mode of benzene and the 1002 c m - 1 mode of toluene at ambient temperature exhibited exponential decay in both cases yielding T2/2 values of 2.6 + 0.2 ps, and 2.8 + 0.2 ps, respectively. These results are in agreement with previous measurements [21,23 ]. We have demonstrated a robust dual dye laser system pumped by a cw mode locked YAG laser. Significant improvements over the mode locked gas laser systems have been found in regard to pulsewidth in agreement with recent reports by Mourou et al. [9]. In addition we have shown that the jitter is considerably reduced in the YAG based system as a result of the shorter pump pulses. It is expected that the jitter can be reduced even more by stabilizing the dye pump and adding saturable absorbers therefore making it worthwhile to contemplate femtosecond dual laser pump-probe experiments. t

6. Summary and applications to CARS processes Numerical calculations show that the pump power instability is a more important source of timing jitter than mechanical instabilities of the dye jet or the laser cavity. The parameter P used here is the product of the peak intensity of the pump pulse Ep0 and the maximum gain G m. Fluctuations of the gain parameter G m introduce fluctuations in the parameter P which are uncorrelated in two lasers, and cannot be reduced by adjustment of the lasers. The use of shorter pump pulses (smaller rp) results in a reduction of the dye pulse duration, and the jitter. Reduction of the pump pulse duration in synchronous pumping increases the output power of the dye laser. However, the model used here seems to be more suitable for predicting the timing characteristics of the dye laser than the intensities of the dye laser pulses and their second harmonics. The model predicts that 500 fs pulses should be obtained using a 50% transmission output coupler and tuning element bandpass equal to 160 × 1012 s -1. This corresponds 1/2 of the thinnest conventional birefringent f'dter. 276

References [ 1] G.R. Holtom, R.M. Hochstrasser, M.R. Topp and J.C. Pruett, Laser studies of molecular relaxation and dynamics in advances in laser spectroscopy. Vol. 1, eds. B. Ganetr and Y. Lombardi (Hayden). [2] E.P. lppen and C.V. Shank, in: Ultrashort light pulses, Topics in applied physics, Vol. 18, ed. S.L. Shapiro (Springer-Verlag, Berlin, 1977). [3] G.R. Fleming, in: Advancesin chemical physics, Vol. 49 (J. Wiley, New York, 1982) p. 1. [4] B.I. Greene, R.M. Hochstrasser and R.B. Weisman, J. Chem. Phys. 70 (1979) 1247. [5] I.I. Abram and R.M. Hochstrasser, J. Chem. Phys. 72 (1980) 3617. [6] F. Ho, W.-S. Tsay, J. Trout and R.M. Hochstrasser, Chem. Phys. Lett. 83 (1981) 5. [7] M.G. Sceats, F. Kamga and D. Podelski, in: Picosecond phenomena II, eds. R.M. Hichstrasser, W. Kaiser and C.V. Shank (Springer-Verlag, Berlin, 1980) p. 348. [8] G.R. Fleming and D. Waldeck, Nuovo Cimento 63B (1981) 151. [9] G.A. Mourour and T. Sizer II, Optics Comm. 41 (1982) 47.

Volume 47, number 4

OPTICS COMMUNICATIONS

[10] T. Sizer II, G.D. Kafka, A. Krisiloff and G. Mourou, Optics Comm. 39 (1981) 259. [11] D.B. McDonald, J.L. Rossel and G.R. Fleming, IEEE J. Quantum Electron. QE-17 (1981) 1134. [12] H.A. Pike and M. Hercher, J. Appl. Phys. 41 (1970) 4562. [13] D.P. Millar and A.H. Zewail, Chem. Phys. 72 (1982) 381. [14] Z.A. Yasa, Appl. Phys. B30 (1983) 135. [15] D.B. McDonnald, D. Waldeck and G.R. Fleming, Optics Comm. 34 (1980) 127. [16] J. Herrmann and U. Motschmann, Optics Comm. 40 (1982) 379. [17] H.A. Haus, IEEE J. Quantum Electron. QE-11 (1975) 736. [18] C.P. Ausschnitt, R.K. Jain and J.P. Heritage, IEEE J. Quantum Electron. QE-15 (1979) 912.

15 September 1983

[19] C.P. Ausschnitt and R.K. Jain, Appl. Phys. Lett. 32 (1978) 727. [20] R.K. Jain and J.P. Heritage, Appl. Phys. Lett. 32 (1978) 41. [21] J. Kuhl and D. yon der Linde, in: Picosecond phenomena III, Proc. Third Intern. Conf. on Picosecond phenomena, Garmish-Partenkirchen, June 1982, eds. K.B. Eisenthal, R.M. Hochstrasser, W. Kaiser and A. Laubereau (Springer-Verlag, Berlin, Heidelberg, New York, 1982) p. 201. [22] T. Kuhl, H. Klingenberg and D. vonder Linde, Appl. Phys. 18 (1979) 279. [23] A. Laubereau and W. Kaiser, Rev. Mod. Phys. 50 (1978) 607.

277