(Q-switched picosecond dye laser pumped by an excimer laser Janos Hebling

JATE University, Department of Experimental Physics, H-6720 Szeged, Hungary. Received 5 April 1988. 0003-6935/89/030417-03$02.00/0. © 1989 Optical Society of America.

The investigation of fast nonlinear optical phenomena needs high-power picosecond light pulses. In general, how­ever, only low-power picosecond pulses can be created direct­ly, so it is necessary to amplify them. The high-power pico­second light creating system as a whole becomes simpler if the device which emits the short pulses and the amplifier is pumped by the same laser. This is why the possibility of generating picosecond pulses by dye lasers which are pumped by excimer-, N2-, or Q-switched Nd:YAG lasers— the usual amplifier pumping lasers—has been extensively studied.

The pulse duration of these pumping lasers is ~10 ns. This is too short to use the mode-locking technique. The main procedures for generating picosecond dye laser pulses using a few nanosecond long excitation are the controlled resonator transients,1-3 the self Q-switching effect in distrib­uted feedback dye lasers.4,5 and the quenching of oscillation in a double-cavity laser.5-9 The pulses obtained by the reso­nator transient technique are typically 5-20 times shorter than the pump pulse duration. The pulse shortening factor, for distributed feedback dye lasers and quenched double-cavity lasers, reaches a value of 100. Even higher shortening factors can be achieved by cascade arrangements3,8 or using simultaneously different pulse shortening techniques.5

The Q-switching of solid-state lasers by the saturable ab-sorber10 makes it possible to achieve pulse shortening as high as a few times 104. In spite of this, until very recently only one paper11 dealing with the Q-switching of dye laser had been published, and even this paper described a sophisticat­ed arrangement. We found that the reason for this is that the satisfaction of two conditions usually given for Q-switch-ing10,11 is not so obvious in the case of dye lasers as it is in the case of solid-state lasers.

One of the conditions requires a shorter pumping duration than the fluorescence lifetime τƒ of the amplifier medium. This condition is actually not satisfied at all for dye lasers with a pumping pulse longer than a few nanoseconds. How­ever, we should keep in mind that this condition was to gain most of the pumping energy in a short giant pulse and to obtain a single short pulse, not a multiple one,11. In an oscillator-amplifier system, however, the efficiency is main­ly determined by the amplifier; thus the efficiency of the oscillator which is planned to give a short pulse is not too important. Generation of a single pulse is possible, even if the mentioned condition does not hold, by controlling the pump power (similar to the controlled resonator transient and the distributed feedback dye laser technique). For 9-ns long pumping and ΤƑ = 4 ns the pump power can be regarded as being controlled12 if its fluctuation is smaller than ±20%. Since the power fluctuation of the usual pumping lasers is not higher than ±1 (Cu vapor laser) or ±3% (excimer, Q-switched Nd:YAG SH), the possibility of an effective Q-switching exists, even if the above-mentioned condition does not hold.

The other condition for short pulse generation by Q-switching is a higher absorption cross section σa of the satura­ble absorber than the emission cross-section σe of the active

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Fig. 1. Streak camera record of a Q-switched dye laser pulse. Time scale: 4.7 ps/channel.

material. In solid-state lasers which are Q-switched by satu­rable dyes the σa/σe ratio is a few orders of magnitude. For dye lasers it is possible to choose an active material-saturable absorber dye pair having σa/σe ≤ 5. Although this ratio is much smaller than it is for solid-state lasers, it is high enough10 for Q-switching. In conclusion, we can say that Q-switching of dye lasers by a saturable absorber dye seems to be possible.

Very recently, Q-switching of distributed feedback dye lasers by a saturable dye was observed.13,14 In Ref. 15 an undamped relaxation oscillation was obtained in a dye laser having a dye mixture as an active material and saturable absorber. Single-pulse generation was not achieved because the σa/σe ratio was smaller than 1.

In this Letter single-short-pulse generation by Q-switch­ing of a simple dye laser is demonstrated for the first time.

The dye laser was very simple; it consisted of only a cylin­drical lens (ƒ = 15 cm) and a commerical dye cell having a 2-mm inner and 4.5-mm outer thickness. The dye cell con­tained a dye mixture of 1.3 × 10-2-mol/liter coumarin 153 + 0.8 × 10-4-mol/liter rhodamine 6G + 0.4 × 10-4-mol/liter rhodamine B solved in ethanol. The coumarin dye acted as a lasing material, while the rhodamine dyes acted as a satura­ble absorber. This dye solution was transversally excited by 1/15 part of the beam of a homemade XeCl excimer laser. The excimer laser ran at 2 Hz, its pulse energy was 30 mJ, and its pulse duration 12 ns. The cavity was created by the cell walls.

The dye solution emitted pulses at λ = 550 nm and λ = 584 nm. The yellow light was obviously emitted by the rhoda­mine dyes, which means that the saturable absorber also lased.15 The temporal behavior of the pulses was investigat­ed by a C 979 Hamamatsu streak camera-TV camera-multi­channel analyzer system. It was determined that the green light was emitted earlier and it was significantly shorter than the yellow one.

Figure 1 shows the shortest recorded pulse. This was 42 ps long, while a 62-ps typical pulse duration having a ±8-ps deviation was measured. These results mean that a typical 200-fold pulse shortening was reached by the Q-switched dye laser.

From the calculations,12 however, even shorter (10-ps) pulse duration is expected. The reason of this quantitative difference between the results of the calculations and the experiment may be the existence of the nonradiative Forster energy transfer15 from the coumarin molecules to the absorb­

er molecules. This process was not taken into account in the calculation. The significance of the energy transfer can be decreased by using a lower loss cavity.

Q-switching was not observed if only one of the rhodamine dyes was used, because the emission spectrum of the coumar-ine 153 dye is wide. Using one absorber dye alone, the lasing wavelength of the coumarin changes, compared with the lasing wavelength without absorber, so that the lasing wave­length will not coincide with the absorption peak of the absorber. Thus at that wavelength the σa/σe ratio is not high enough for efficient Q-switching. In the case of a Q-switched distributed feedback dye laser,14 there was not a problem of this kind because the feedback was selective.

It is expected that by using dichroic coatings on the walls of the dye cell having 20-30 and <5% reflectivity at the lasing wavelength region and at the other regions, respectively, both above mentioned problems could be solved. This laser would have a smaller loss at the lasing wavelength; thus the significance of the Forster transfer would decrease, and the lasing wavelength would be locked at the desired range, where σa/σe is large because of the wavelength selectivity. This device is expected to make possible even a 1000-fold pulse shortening.

Using the Q-switched dye laser for pumping a short-cavity dye laser16 in a cascade arrangement,3 1-5-ps long pulses without mode-locking are expected to be obtained.

This work was supported by the OTKA Foundation of the Hungarian Academy of Sciences.

References 1. C. Lin, "Studies of Relaxation Oscillations in Organic Dye La­

sers," IEEE J. Quantum Electron. QE-11, 602 (1975). 2. K. Yamada, K. Miyazaki, T. Hasama, and T. Sato, "Generation

of Single Short Tunable UV Pulses Using a Simple Short-Cavity Dye Laser," Appl. Opt. 25, 634 (1986).

3. P. H. Chiu, S. Hsu, S. J. C. Box and H-S. Kwok, "A Cascade Pumped Picosecond Dye Laser System," IEEE J. Quantum Electron. QE-20, 652 (1984).

4. Zs. Bor, "Tunable Picosecond Pulse Generation by an N2 Laser Pumped Self Q-Switched Distributed Feedback Dye Laser," IEEE J. Quantum Electron. QE-16, 517 (1980).

5. J. Hebling, "Excimer Laser Pumped Distributed Feedback Dye Laser," Opt. Commun. 64, 539 (1987).

6. A. Eranian, P. Dezauzier, and 0. De Witte, "2 ns Pulses from Double Cavity Dye Laser," Opt. Commun. 7, 150 (1973).

7. F. P. Schafer, L. Wenchong, and S. Szatmari, "Short UV Laser Pulse Generation by Quenching of Resonator Transients," Appl. Phys. B 32, 123 (1983).

8. S. Szatmari and F. P. Schafer, "Simple Generation of High-Power, Picosecond, Tunable Excimer Laser Pulses," Opt. Com­mun. 48, 279 (1983).

9. Zs. Bor and B. Racz, "Picosecond Dye Laser Pumped by an Excimer Laser," Appl. Opt. 24, 1910 (1985).

10. A. Szabo and R. A. Stein, "Theory of Laser Giant Pulsing by a Saturable Absorber," J. Appl. Phys. 36, 1562 (1965).

11. L. W. Braverman, "Controlled Passive Q-Switch for the N2 Laser Pumped Dye Laser," Appl. Phys. Lett. 27, 602 (1975).

12. J. Hebling, J. Seres, Zs. Bor, and B. Rácz, "Dye Laser Pulse Shortening and Stabilization by Q-Switching," in preparation.

13. J. Hebling and Zs. Bor, "Shortening and Stabilization of Distrib­uted Feedback Laser Pulses by Using a Saturable Absorber," in Technical Digest, Fourth Conference on Luminescence, Szeged (1982), p. 331.

14. J. Hebling, "20 ps Pulse Generation by an Excimer Laser Pumped Double Self-Q-Switched Distributed Feedback Dye Laser," Appl. Phys. B 47, 267 (1988).

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15. N. P. Ernsting and B. Nikolaus, "Undamped Relaxation Oscilla­tions Due to Self-Gain-Switching of Laser Dye Mixtures," Appl. Phys. B 41, 25 (1986).

16. A. J. Cox and G. W. Scott, "Short-Cavity Picosecond Dye Laser Design," Appl. Opt. 18, 532 (1979).

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