Tunable Lasers Handbook

Dye Lasers

F. J. Duarte

Eastman Kodak Company

Rochester, New York

1. INTRODUCTION

Dye lasers are unique sources of tunable coherent radiation that offer unpar-

alleled operational flexibility. Broad tunability, from the near ultraviolet to the

near infrared (Fig. 1), is facilitated by the existence of hundreds of laser dye

molecular species. The tuning range of pulsed narrow-bandwidth emission

achievable with a single dye can be up to 50 nm. Significantly broader tuning

ranges (up to 100 nm) can be obtained with some dyes under pulsed broadband

and continuous wave (cw) operation.

In addition to tunability, an intrinsic feature of dye lasers is their inherent

ability to yield high-pulse energies and high-average powers in the visible [1].

Single-pulse energies in excess of several hundred joules and average powers in

excess of 2.5 kW have been reported [2]. This unparalleled performance is greatly

facilitated by the heat dissipating ability of the gain medium in the liquid phase.

On the other hand, highly stabilized single-longitudinal-mode cw dye lasers

can yield frequency drift rates below 1 Hz/sec [3,4]. In the ultrashort-pulse

regime dye lasers have demonstrated pulses as short as 19 fs [5] and extra cavity

prism-grating compressors have yielded 6-fs pulses [6]. In this regard, dye

lasers have been an important vehicle in the development of many pioneering

Tunable Lasers Handbook

Copyright 9 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.

167

168 F.J. Duarte

Cyanines

Merocyanines

Xanthenes

Coumarins

Stilbenes

Oligophenylenes

Oxadiazoles

FIGURE 1

400 5()0 600 700 800 g00 1000 11 ~)0

Emission Wavelength (nm)

Approximate wavelength coverage achieved with different classes of laser dye molecules.

techniques that have now found wide applicability in lasers in general. Impor-

tant contributions to the laser field first demonstrated in dye lasers include fre-

quency stabilization techniques, pulse compression techniques, dispersive oscil-

lator configurations, and numerous cavity and resonator innovations.

Although dye molecules have been demonstrated to lase in the three states

of matter, it is in the liquid phase that dye lasers have made their most signifi-

cant impact. Recently, however, there has been a resurgence of work in solid-

state dye lasers. Hence a section of this chapter is especially devoted to this

topic.

This chapter is intended to provide an expeditious guide to the performance

and basic features of dye lasers. For an in-depth approach to the subject, the

reader should consult the references provided in the various sections and the fol-

lowing books:

High Power Dye Lasers

[1 ],

Dye Laser Principles

[7],

Dye Lasers

[8], and

Selected Papers on Dye Lasers

[9]. These works should also be con-

suited for a historical perspective on the subject.

1.1 The Molecular Gain Media

Laser dye molecules are large, with atomic weights in the 175- to 1000-amu

range (see the Appendix at the end of this chapter). These molecules exhibit a

wide absorption spectrum with broad and strong absorption maxima correspond-

ing to

So---~S n

electronic transitions (Fig. 2). Here, dye molecules are considered

simply from an energetic perspective with an excitation dynamics involving sev-

eral energy levels. A feature of dye laser molecules is that each electronic state

contains a multitude of overlapping vibrational-rotational levels. This plethora of

closely lying vibrational levels gives origin to the broadband gain and tunability

features that are so characteristic of dye lasers.