In this article that first appeared in Spectroscopy Magazine, Dr. Steve Buckley shares his insights on the evolution and application of optical parametric oscillators (OPOs).
Abstract
Fundamental research often requires narrowband light to accurately map out fluorescence spectra, reaction energetics, or other key questions in chemical, biological, and physical systems. Tunable narrowband sources are often key to this endeavor. Pulsed or continuous wave, the optical parametric oscillator (OPO) has come a long way, dropping in price while improving in performance. ocean Insight explores the landscape and some common uses of this essential tool for spectroscopists.
Introduction
When it comes to spectroscopy, the combination of narrowband light sources with broadband detectors (or the converse, narrowband detection with broadband light sources) are the equivalent of the surgeon’s scalpel. They allow us to see deeply into the spectroscopic fine structure, providing experimental data to feed models, probing atomic and molecular structure, and allowing the science of spectroscopy to move forward.
For nearly a century before the advent of the laser, spectroscopists only had narrowband detectors. Kirchhoff and Bunsen’s work with their “spectroscope” led to the discovery of rubidium and cesium, and the mapping out of many of the fundamental emission lines in atomic spectroscopy, as the pair feverishly fed element after element into their “Bunsen Burner” and recorded spectra. Of course, Kirchhoff would go on to use the spectroscope to study radiative equilibrium (leading to the eponymous “Kirchoff’s Laws”) and make contributions across the landscape of physics, including thermodynamics and fluid mechanics.
Even then, while Kirchhoff and Bunsen could get reasonable resolution and sensitivity on their detectors – Kirchhoff dispersed light from the sun over nearly three meters, for example, and at least for steady-state experiments in flames or studying solar emission, photographic plates could be exposed for long periods – it was hard to obtain sufficient intensity of monochromatic light to do the inverse experiment. In other parlance, while it was easy to obtain fluorescence or absorption spectra, it was difficult to obtain excitation spectra.
This situation changed with the advent of the laser. Early solid state and gas lasers emitted one or more narrowband lines, which had some limited spectroscopic usefulness. It was the development of the organic dye laser in 1966 that provided revolutionary access to narrowband light in the visible and near-infrared. This resulted in an explosion of scientific and spectroscopic work, as nicely summarized by Frank Duarte in 2003.[1] However, as Duarte notes, “…over the years dye lasers have acquired a reputation in some quarters as being ‘user unfriendly.’”
Duarte does a good job of defending the many contributions and the unique attributes of dye lasers, including the available high pulse energy, femtosecond, and narrow linewidth, among several. However, the reputational aspect of dye lasers is hardly addressed, except though technological advances, and your author’s opinion and experience is that while dye lasers are a certain amount of “fun” and great tools for certain problems, they are also more difficult to maintain and operate than might be optimal.