Volume 46, number I OPTICS COMMUNICATIONS I June 1983

PASSIVE MODE LOCKING IN THE BLUE SPECTRAL REGION

W. SIBBETT and J.R. TAYLOR

Laser Optics Group, Physics Department, Imperial College, London SW72BZ, UK

Received 9 March 1983

The spectral range for passive mode-locking of flashlamp pumped dye lasers has been extended towards the blue (452­

515 nm) by employing the saturable absorber 2-(p-dimethylaminostyryl)-pyridylmethyl iodide (DASPI). Fully modulatedtrains of picosecond pulses with megawatt peak powers have been obtained.

We have previously reported on the use of 2-(p­dimethylaminostyry1)-benzthiazolyl ethyl iodide(DASBTI) as a saturable absorber for the passivemode locking of dye lasers in the green-yellow spectralregion [1,2]. In this letter we now report on the ap­plication of a substituted styryl dye with peak absorp­tion in the blue which has been used to passivelymode-lock dye lasers from 452-515 nm, which todate covers the shortest wavelength region to be ob­tained directly by passive mode locking of a dye laser.The relevant relaxation parameters of DASPI are pre­sented together with another saturable absorber whichis appropriate for the blue spectral region.

The construction of the dye laser already has beendescribed in detail [3] and only a brief outline is givenhere. A standard double elliptical cylindrical pumpinggeometry was used where a maximum of ~ 150 J could

be deposited via a triggered ceramic-metal spark gapfrom 0.75 J1.F capacitors into each of the 127 mmbore length, 100Torr Xe mled flashlamps. The typicaldissipated electrical energy was in the range 90-110 Jper lamp. The lasing dye was contained in a 1 litre re­

servoir and no attempt was made to cool or tempera­ture stabilize the active medium, which was continuallycirculated through the 3 mm i.d. quartz dye cell 10­cated along the common focal axis of the 127 mm

long laser head. An overall cavity length of ~ 3 5 cmwas used. This was formed by dielectric mirrors of100 % reflectivity over the spectral range 400-525nm and 90%R (450-500 nm).

Tuning was carried out using an intracavity Fabry-

Perot etalon with an air gap spacing of 7.2 J1.m. At acentral wavelength of 470 nm this gives rise to a freespectral range of 15 nm, which for some of the laserdyes reported here allowed two wavelengths to lasesimultaneously for a particular orientation of thetuning element. However, it was possible to prohibitthis by further intracavity mtering techniques. A500 J1.m thick, optically-contracted dye cell was placedon the 100% reflector to contain the saturable absor­

ber dye solutions, which were non circulating. Photo­chemical stability of the saturable absorber appearedquite high since mode locking was achieved for verymany laser firings without the necessity of renewal ofthe solution.

The majority of the work reporte d here was carriedout using the saturable absorber 2-(p-dimethylamino­styry1)-pyridylmethyl iodide (DASPI). Ethyl substi­tuents of this dye both in the I-position on the pyri­dine ring and in the amino radical also operate assaturable absorbers in the blue spectral range, but,their peak absorption wavelengths are slightly redshifted to that of DASPI above. The molecular struc­

ture of DASPI is shown in fig. 1 together with the ab­sorption prome of the dye in ethanol. A peak molardecadic extinction coefficient € of 1.8 X 104 1 mol-I

cm-I (a"'" 6.9 X 10-17 cm2) at 465 nm was measur­ed for ethanolic solutions of the dye.

The fluorescence recovery times of DASPI in vari­ous solvent mixtures of ethanol and glycerol, whichprovided variable viscosity environments were deter­mine d using the Synchroscan streak camera [4] in

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

Volume 46, number I OPTICS COMMUNICATIONS I June 1983

306 ps

la) 1.1cp

fluorescence decay of a 5 X 10-5 M ethanolic solu­tion (77 = 1.1 cp) ofDASPI is shown. This exhibited a

fluorescence lifetime of 62 ps (TIle to the lie peak in­tensity point). Similar to many polymethine dyes [6]and to DASBTI [7], the fluorescence lifetime ofDASPI showed a strong solvent viscosity dependence.An example of this can be seen in fig. 2(b) where themeasured fluorescence lifetime increased to 306 ps inan ethanol-glycerol solvent mixture of 60 cp viscosity.Below 10 cp the fluorescence lifetime had a 0.66power dependence of viscosity of the solution (i.e.T rx 770.66), but above 30 cp 'a saturation [8] of thiseffect was observed. This feature which has alreadybeen exploited [1,9] provides a convenient methodof modifying the saturation parameters of the dye tosuit the available power density of the laser becausean increase in the absorber recovery time effectivelyreduces the saturation flux. In the passively modelocked dye laser, the recovery time of the saturableabsorber, although important does not play a majorrole in the determination of the ultimate duration of

the generated picosecond pulses [10,11]. It should benoted that with all the laser dyes reported here passivemode locking was possible using purely ethanolicsolutions of DASPI. However, in the case of coumarin

1, the saturable absorber was dissolved in propylenecarbonate (77 "'" 56 cp) as it was found that in thismore viscous solvent, since the saturation flux wascorrespondingly reduced, the build-up time to modu­lation was shortened and the waveforms of the mode

locked trains were more reproducible.The lasing dyes examined were coumarin 1 (Kodak)

coumarin 102 (Lambda), LD466, LD473 and LD490(Exciton Chemical Co.). All were prepared as 3 X10-4 M solutions in ethanol, with the exception ofcoumarin 102 which was 4 X 10-4 M. Various con­centrations of DASPI were used to mode lock these

dyes depending on the wavelength range of operation,but typically the concentrations were between 5 X10-5 M and 2 X 10-4 M. Passive mode locking oneach dye was achieved without the necessity of an in­tracavity tuning element. Some typical temporalwaveforms are shown in fig. 3, for operation underthis condition. The central wavelength and the lasingbandwidth depended on several features such as satur­

able absorber concentration, or the pump power abovethreshold, etc. For the oscillograms reproduced in fig.3 the simultaneously measured central wavelength

o -Q-NICHJ)2 r­l~~CH=CH _N

I

CHJ

350

3.0

2.0

uJ

I-i3u~ 0.5

8

DASPI

---j 100ps f--

oE

~~ 1.0

Fig. 2. Viscosity dependent fluorescence decay of DASPI in

(a) 1.1 cp and (b) 60 cp viscous solutions.

conjunction with the DV excitation pulses (A. ~ 308nm) from an intracavity frequency-doubled, synchro­nously-pumped CW dye laser [5]. In fig. 2(a) the

400 450 500 550 600 650

WAVELENGTH (nm)

Fig. 1. Molecular structure and absorption profile of the

saturable absorber DASPI (see text) in ethanol.

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Volume 46, number 1 OPTICS COMMUNICATIONS 1 June 1983

34

and laser bandwidth characteristics were (a) coumarin1,462 nm (5 nm), (b) LD466, 467 nm (4 nm), (c)coumarin 102,488 nm (15 nm), (d) LD473, 483 nm(3 nm) and (e) LD490, 497 nm (7 nm). A coarsetuning effect was possible by varying the saturable ab­sorber concentration, but more precise tuning wascarried out using the intracavity Fabry-Perot etalon.

In fig. 3 it can be seen that in some cases quite along build up time to modulation is evident. This fea­ture depended on the wavelength of operation and onthe intracavity power. As the wavelength is decreasedthe efficiency of the coumarin 1 laser decreases andthe peak power at the beginning of laser action is in­sufficient to saturate the absorber. It was possible todecrease the build up time by using more viscoussolutions to those of the ethanolic ones shown in fig.3.

With the Fabry-Perot etalon, tuning of the couma­rin 1 dye laser was possible from 452 nm to 470 nm.This represents the shortest operating wavelengthrange obtained to date by direct passive mode lockingof a dye laser. Propylene carbonate solutions of thesaturable absorber (DASPI) were used, and typicalwaveforms of the tuned mode locked'output areshown in fig. 4(a) at 455 nm where a build up time of~ 150 ns was evident and a fully modulated period of~ 300 ns. At 452 nm the general features of the wave­form were similar to those at 455 nm, while at 465 nm(fig. 4(b)) the build up time was less than 100 ns andthe pulse trains lasted ~800 ns. Fig. 4(c) shows thatto within the resolution of the scope-diode combina­tion single pulses « 500 ps) were generated at 455nm. Using the tuning element the typicallasing band­widths were ~0.8 nm. As the free spectral range ofthe Fabry Perot was ~ 15 nm, for some orientations,lasing occurred at two wavelengths. This was generallyovercome by the insertion of an intracavity edge mterto transmit only one wavelength or by the inclusionof an additional saturable absorber which was capableof quenching one of the lasing wavelengths. Tuning ofthe mode locked LD467 dye was carried out from460-479 nm and coumarin 102 was tuned to 510 nm

.••• Fig. 3. Typical temporal profiles of the mode locked pulsetrains obtained with no intracavity tuning element for (a)coumarin-1 200 ns/small division (b) LD466 50 ns/s.div.(c) coumarin-102 100 ns/s.div (d) LD473 50 ns/s.div and(3) LD490 50 ns/s.div.

Volume 46, number I OPTICS COMMUNICATIONS I June 1983

Fig. 4. Temporal profiles of the mode locked output of thecoumarin-l dye laser tuned to Ca)455 nm, 100 ns/s.div Cb)465 m, 200ns/s.div and Cc)as in Ca)only on expanded timescale of 10ns/s.div.

which was comparable to previous reports [12]. Themaximum wavelength obtained with LD490 and 515nm.

Quantitative measurements of the duration of the

mode-locked pulses were made by employing aPhotochron II streak camera with an S20 photo­cathode in the conventional arrangement which hasbeen described elsewhere [13]. As in previous work[1] the synchronism of the electrical triggering wasarranged such that pulses ~ 100 ns from the onset ofmodulation were examined. Fig. 5 shows the micro­densitometer trace of a recorded pair of streak images

(pulsewidth TR of 7 ps separated by a calibrated de-

f---- 57 ps ~

Fig. 5. Microdensitometer trace of streak camera recorded

pulsewidth TR of 7ps from the passively mode lockedcoumarin-I laser operating at 452 nm.

lay of 57 ps), for the coumarin 11aser tuned to ope­rate at 452 nm. Together with the typical waveformsof fig. 4 the results showed that single pulses with lowinterpulse noise were generated. As a consequence ofthe relatively large energy spread of the photoelec­trons liberated by incident radiation in the blue spec­tral region, the overall streak camera resolution is esti­mated to be ~ 5 ps at 450 nm. Deconvolving this re­solution time from the recorded pulsewidth infers anactual laser pulse width of 5 ps. Over the full spectralrange covered by all the dyes 452-515 nm, typicalrecorded pulsewidths TR were in the range 5-9 ps.For coumarin 1 at 452 nm the peak energy per pulsewas ~ 5 pJ which corresponds to a maximum peakpower of ~ 1 MW. Tuning to longer wavelengths gavegreater pulse energies and higher peak powers.

Passive mode locking of coumarin 102 was alsoachieved with the generation of picosecond pulsesusing 1,3'·diethyl-2,2'-quinolyl thiacyanine iodide assaturable absorber. Propylene carbonate solutions ofthe saturable absorber were used in which it exhibited

a fluorescence lifetime of 95 ps [14]. Mode lockingof LD473 was also possible using this dye and in both

cases no intracavity tuning element was required.In conclusion we have demonstrated that using the

dye DASPI pulses ~ 5 ps with powers in the megawattrange can be reproducibly generated through passivemode locking of various dye laser species to wave­lengths as short as 452 nm. We have further identified

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Volume 46, number 1 OPTICS COMMUNICATIONS 1 June 1983

and detennined the saturation parameters of addi­tional saturable absorbers for operation at shorterwavelengths than those reported here [14] and shouldpermit the passive mode locking of flashlamp pumpeddye lasers down to 410 nm.

The overall financial support for this research bythe SERC is gratefully acknowledged.

References

[1] W. Sibbett and J.R. Taylor, Appl. Phys. 29B (1982)19l.

[2] W. Sibbett and J.R. Taylor, IEEE J. Quant. Elect. QE18(1982) 1994.

[3] W. Sibbett and J.R. Taylor, Optics Comm. 43 (1982)50.

36

[4] M.C. Adams, W. Sibbett and D.J. Bradley, OpticsComm. 26 (1978) 273.

[5] D. Welford, W. Sibbett and J.R. Taylor, Optics Comm.35 (1980) 283.

[6] W. Sibbett, J.R. Taylor and D. Welford, IEEE 1. QuantumElect. QE17 (1981) 500.

[7] W. Sibbett and l.R. Taylor, Optics Comm. 44 (1982)12l.

[8] B. Wilhelmi, Chem. Phys. 66 (1982) 351.[9] J.C. Mialocq and P. Goujon, Appl. Phys. Lett. 33

(1978) 819.[10] G.H.C. New, IEEE 1. Quantum Elect. QEI0 (1974) 115.[11] E.G. Arthurs, D.l. Bradley, P.N. Puntambekar, I.S.

Ruddock and T.l. Glynn, Optics Comm. 12 (1974) 360.[12] R. Wyatt, Optics Comm. 38 (1981) 64.[13] DJ. Bradley, B. Liddy, A.G. Roddie, W. Sibbett and

W.E. Sleat, Optics Comm. 3 (1971) 426.[14] W. Sibbett and l.R. Taylor, unpublished.