PASSIVE MODE LOCKING IN THE BLUE SPECTRAL REGION. W. SIBBETT and J.R. TAYLOR

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1 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 ( nm) by employing the saturable absorber 2-(p-dimethylaminostyryl)-pyridylmethyl iodide (DASPI). Fully modulated trains 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 passive mode locking of dye lasers in the green-yellow spectral region [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 passively mode-lock dye lasers from nm, which to date 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 which is appropriate for the blue spectral region. The construction of the dye laser already has been described in detail [3] and only a brief outline is given here. A standard double elliptical cylindrical pumping geometry was used where a maximum of ~ 150 J could be deposited via a triggered ceramic-metal spark gap from 0.75 J1.F capacitors into each of the 127 mm bore length, 100Torr Xe mled flashlamps. The typical dissipated electrical energy was in the range J per lamp. The lasing dye was contained in a 1 litre reservoir and no attempt was made to cool or temperature stabilize the active medium, which was continually circulated through the 3 mm i.d. quartz dye cell 10 cated along the common focal axis of the 127 mm long laser head. An overall cavity length of ~ 3 5 cm was used. This was formed by dielectric mirrors of 100 % reflectivity over the spectral range nm and 90%R ( nm). Tuning was carried out using an intracavity Fabry- Perot etalon with an air gap spacing of 7.2 J1.m. At a central wavelength of 470 nm this gives rise to a free spectral range of 15 nm, which for some of the laser dyes reported here allowed two wavelengths to lase simultaneously for a particular orientation of the tuning element. However, it was possible to prohibit this by further intracavity mtering techniques. A 500 J1.m thick, optically-contracted dye cell was placed on the 100% reflector to contain the saturable absorber dye solutions, which were non circulating. Photochemical stability of the saturable absorber appeared quite high since mode locking was achieved for very many laser firings without the necessity of renewal of the solution. The majority of the work reporte d here was carried out 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 as saturable absorbers in the blue spectral range, but, their peak absorption wavelengths are slightly red shifted to that of DASPI above. The molecular structure of DASPI is shown in fig. 1 together with the absorption prome of the dye in ethanol. A peak molar decadic extinction coefficient of 1.8 X mol-i cm-i (a"'" 6.9 X 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, which provided variable viscosity environments were determine d using the Synchroscan streak camera [4] in 32 o /83/ /$ North-Holland

2 o E ~~ 1.0 uj I- i3 u ~ ol~~ch=ch N -Q-NICHJ)2 I CHJ DASPI 306 ps ---j 100ps la) 1.1cp _ r WAVELENGTH conjunction with the DV excitation pulses (A. ~ 308 nm) from an intracavity frequency-doubled, synchronously-pumped CW dye laser [5]. In fig. 2(a) the f-- Fig. 2. Viscosity dependent fluorescence decay of DASPI in (a) 1.1 cp and (b) 60 cp viscous solutions. (nm) Fig. 1. Molecular structure and absorption profile of the saturable absorber DASPI (see text) in ethanol. 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 of DASPI showed a strong solvent viscosity dependence. An example of this can be seen in fig. 2(b) where the measured fluorescence lifetime increased to 306 ps in an ethanol-glycerol solvent mixture of 60 cp viscosity. Below 10 cp the fluorescence lifetime had a 0.66 power dependence of viscosity of the solution (i.e. T rx ), but above 30 cp 'a saturation [8] of this effect was observed. This feature which has already been exploited [1,9] provides a convenient method of modifying the saturation parameters of the dye to suit the available power density of the laser because an increase in the absorber recovery time effectively reduces the saturation flux. In the passively mode locked dye laser, the recovery time of the saturable absorber, although important does not play a major role in the determination of the ultimate duration of the generated picosecond pulses [10,11]. It should be noted that with all the laser dyes reported here passive mode locking was possible using purely ethanolic solutions of DASPI. However, in the case of coumarin 1, the saturable absorber was dissolved in propylene carbonate (77 "'" 56 cp) as it was found that in this more viscous solvent, since the saturation flux was correspondingly 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 X 10-4 M solutions in ethanol, with the exception of coumarin 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 X 10-5 M and 2 X 10-4 M. Passive mode locking on each dye was achieved without the necessity of an intracavity tuning element. Some typical temporal waveforms are shown in fig. 3, for operation under this condition. The central wavelength and the lasing bandwidth depended on several features such as saturable absorber concentration, or the pump power above threshold, etc. For the oscillograms reproduced in fig. 3 the simultaneously measured central wavelength 33

3 Volume 46, number 1 OPTICS COMMUNICATIONS 1 June 1983 and laser bandwidth characteristics were (a) coumarin 1,462 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 coarse tuning effect was possible by varying the saturable absorber concentration, but more precise tuning was carried out using the intracavity Fabry-Perot etalon. In fig. 3 it can be seen that in some cases quite a long build up time to modulation is evident. This feature depended on the wavelength of operation and on the intracavity power. As the wavelength is decreased the efficiency of the coumarin 1 laser decreases and the peak power at the beginning of laser action is insufficient to saturate the absorber. It was possible to decrease the build up time by using more viscous solutions 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 wavelength range obtained to date by direct passive mode locking of a dye laser. Propylene carbonate solutions of the saturable absorber (DASPI) were used, and typical waveforms of the tuned mode locked'output are shown 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 and the pulse trains lasted ~800 ns. Fig. 4(c) shows that to within the resolution of the scope-diode combination single pulses «500 ps) were generated at 455 nm. Using the tuning element the typicallasing bandwidths were ~0.8 nm. As the free spectral range of the Fabry Perot was ~ 15 nm, for some orientations, lasing occurred at two wavelengths. This was generally overcome by the insertion of an intracavity edge mter to transmit only one wavelength or by the inclusion of an additional saturable absorber which was capable of quenching one of the lasing wavelengths. Tuning of the mode locked LD467 dye was carried out from nm and coumarin 102 was tuned to 510 nm. Fig. 3. Typical temporal profiles of the mode locked pulse trains obtained with no intracavity tuning element for (a) coumarin ns/small division (b) LD ns/s.div. (c) coumarin ns/s.div (d) LD ns/s.div and (3) LD ns/s.div. 34

4 which was comparable to previous reports [12]. The maximum wavelength obtained with LD490 and 515 nm. Quantitative measurements of the duration of the mode-locked pulses were made by employing a Photochron II streak camera with an S20 photocathode in the conventional arrangement which has been described elsewhere [13]. As in previous work [1] the synchronism of the electrical triggering was arranged such that pulses ~ 100 ns from the onset of modulation were examined. Fig. 5 shows the microdensitometer trace of a recorded pair of streak images (pulsewidth TR of 7 ps separated by a calibrated def ps ~ Fig. 5. Microdensitometer trace of streak camera recorded pulsewidth TR of 7ps from the passively mode locked coumarin-i laser operating at 452 nm. Fig. 4. Temporal profiles of the mode locked output of the coumarin-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 time scale of 10ns/s.div. lay of 57 ps), for the coumarin 11aser tuned to operate at 452 nm. Together with the typical waveforms of fig. 4 the results showed that single pulses with low interpulse noise were generated. As a consequence of the 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 an actual laser pulse width of 5 ps. Over the full spectral range covered by all the dyes nm, typical recorded pulsewidths TR were in the range 5-9 ps. For coumarin 1 at 452 nm the peak energy per pulse was ~ 5 pj which corresponds to a maximum peak power of ~ 1 MW. Tuning to longer wavelengths gave greater pulse energies and higher peak powers. Passive mode locking of coumarin 102 was also achieved with the generation of picosecond pulses using 1,3' diethyl-2,2'-quinolyl thiacyanine iodide as saturable absorber. Propylene carbonate solutions of the saturable absorber were used in which it exhibited a fluorescence lifetime of 95 ps [14]. Mode locking of 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 megawatt range can be reproducibly generated through passive mode locking of various dye laser species to wavelengths as short as 452 nm. We have further identified 35

5 Volume 46, number 1 OPTICS COMMUNICATIONS 1 June 1983 and detennined the saturation parameters of additional saturable absorbers for operation at shorter wavelengths than those reported here [14] and should permit the passive mode locking of flashlamp pumped dye lasers down to 410 nm. The overall financial support for this research by the 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) [3] W. Sibbett and J.R. Taylor, Optics Comm. 43 (1982) 50. [4] M.C. Adams, W. Sibbett and D.J. Bradley, Optics Comm. 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. Quantum Elect. 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. 36