Exalite 392E: a new laser dye for efficient cw operation between 373 and 408 nm Frank P. Tully and Joseph L. Durant, Jr.

Sandia National Laboratories, Combustion Research Fa­cility, Livermore, California 94550. Received 7 March 1988. Sponsored by Richard K. Chang, Yale University. 0003-6935/88/112096-02$02.00/0. © 1988 Optical Society of America. Continuous wave dye lasers operating from the near ultra­

violet through the blue spectral region have been developed during the past fifteen years.1-6 However, excluding fre­quency-doubling techniques, cw dye-laser operation below 392 nm has been achieved only using the dye polyphenyl 1. Huffer et α/,4 reported cw operation of this dye throughout the 362-412-nm range with a maximum output power of 70 mW at 383 nm. Polyphenyl 1, however, is both very expen­sive and only reluctantly soluble in hot (160°C) ethylene glycol, and it absorbs the 351- and 363-nm Ar+ laser lines inefficiently.

Recently, we began to study CN-radical chemical kinetics in pulsed-laser photolysis/cw laser-induced fluorescence (LP-LIF) experiments. Our initial studies utilized poly­phenyl 1 tuned near 387 nm. While LIF signal levels were sufficient to perform kinetics studies, we found the poly­phenyl 1 dye laser difficult to maintain over an extended period. Shortly after our experiments began, Exciton Chemical Corp. announced their new Exalite series of dyes for ultraviolet pulsed-laser operation. In subsequent dis­cussions with Exciton Chemical Corp., we learned of an experimental dye, Exalite 392E, that might be suitable for UV cw-laser operation. In this paper, we report successful tests of this new dye.

We determined the absorption coefficient ε for Exalite 392E as a function of wavelength using a Perkin-Elmer Lambda 9 spectrophotometer. We observe two absorption bands of comparable strength (εmax ≈ 4 × 104 liter/mole • cm) centered at 214 and 336 nm. Figure 1 displays those results that are relevant to an all-lines UV Ar+-laser pump source. All ultraviolet Ar+-laser pump wavelengths are strongly ab­sorbed by the Exalite 392E dye. Between 365 and 380 nm, ε decreases precipitously (by a factor of ≈30) to a value of 130 liter/mole • cm. Above 380 nm, ε diminishes gradually to a value of 6 liter/mole • cm at 450 nm. Exalite 392E does not absorb significantly throughout the remainder of the visible and the near-IR spectral regions.

We utilized commercially available equipment in all of our laser experiments. The U V output (333-364 nm) of a Coher­ent CR-18 Ar+ laser, equipped with an INNOVA-20 plasma tube, pumped a Spectra-Physics 375 dye laser containing a three-plate birefringent filter as the only tuning element.

2096 APPLIED OPTICS / Vol. 27, No. 11 / 1 June 1988

Fig. 1. Decadic molar absorption coefficient ε vs wavelength for laser dye Exalite 392E dissolved in ethylene glycol. The ultraviolet

Ar+-laser pump wavelengths are indicated.

Fig. 2. Dye-laser output spectrum using laser dye Exalite 392E.

Fig. 3. Dye-laser output power at 392 nm vs Ar+-laser pump power.

Exalite 392E dissolved in ethylene glycol in a few minutes at room temperature to produce a 1.6-mM dye solution.. We monitored ion and dye-laser output powers using Coherent 210 and/or Molectron PR200 power meters. We measured dye-laser output wavelengths using a Burleigh wavemeter.

Figure 2 displays the dye-laser tuning curve for Exalite 392E. In this experiment, we pumped the Exalite 392E dye laser with 3.5 W all-lines UV from the Ar+ laser. We utilized an output coupler that is 7% transmitting at 387 nm. The laser output is >250 mW at 392 nm, and it significantly exceeds that from a polyphenyl 1 dye laser throughout the 373-408-nm tuning range. While we have not performed quantitative dye-lifetime studies, the dye-laser output pow­er did decrease by ~10% after 20-W • h Ar+-laser irradiation.

In Fig. 3 we plot dye-laser output power vs Ar+-laser pump power utilizing the 7% transmitting output coupler in the Exalite 392E dye laser. The lasing threshold appears at 850-mW Ar+-laser pump power. We found this threshold to be

reduced to 750-mW pump power when a 2% transmitting output coupler is used. Based on the data plotted in Fig. 3, we compute a slope efficiency equal to 10%.

We have performed only a few tests aimed at optimizing Exalite 392E dye-laser performance. We measured the frac­tional absorption of pump-beam radiation in the dye jet to be 0.45. Presumably, one could obtain higher dye-laser output powers by increasing the dye concentration above 1.6 mM. However, given the steep dependence of ε on wavelength between 365 and 380 nm, we would expect such an increase to be accompanied by a red shift of the dye-laser tuning curve. We tested a variety of output couplers in the dye laser and found optimum performance at 392 nm for those transmit­ting 4-7% of the intracavity power. Finally, we found that the dye-laser output power is quite sensitive to the dye-nozzle pressure, increasing by 30% as the pressure is raised from 75 to 110 psi. Closed-cycle refrigerated cooling com­bined with higher dye-nozzle pressures appears to be a po­tentially promising improvement.

Exalite 392E renders polyphenyl 1 obsolete in our chemi­cal kinetics studies. It combines higher dye-laser output powers with a significant improvement in the dye-solution preparation process. The tuning curve of the Exalite 392E dye laser spans nearly the entire wavelength range of the four pulsed-laser Exalite dyes. Further investigation of Exalite dyes in both standing-wave and ring dye laser configurations is indicated.

This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy. We gratefully acknowledge Richard Steppel, Larry Knaak, and Tom Mazer of Exciton Chemical Corp. for developing and supplying the new laser dye Exalite 392E. We also thank George Wilkerson for technical support.

References 1. S. A. Tuccio, K. H. Drexhage, and G. A. Reynolds, "CW Laser

Emission from Coumarin Dyes in the Blue and Green," Opt. Commun. 7, 248 (1973).

2. J. Kuhl, H. Telle, R. Schieder, and U. Brinkman, "New Efficient and Stable Laser Dyes for CW Operation in the Blue and Violet Spectral Region," Opt. Commun. 24, 251 (1978).

3. W. Huffer, R. Schieder, H. Telle, R. Raue, and W. Brinkwerth, "CW Dye Laser Emission Down to the Near UV," Opt. Commun. 28, 353 (1979).

4. W. Huffer, R. Schieder, H. Telle, R. Raue, and W. Brinkwerth, "New Efficient Laser Dye for Pulsed and CW Operation in the UV," Opt. Commun. 33, 85 (1980).

5. J. M. Yarborough, "CW Dye Laser Emission Spanning the Visible Spectrum," Appl. Phys. Lett. 24, 629 (1974).

6. T. F. Johnston, Jr., R. H. Brady, and W. Profitt, "Powerful Single-Frequency Ring Dye Laser Spanning the Visible Spec­trum," Appl. Opt. 21, 2307 (1982).

1 June 1988 / Vol. 27, No. 11 / APPLIED OPTICS 2097