operate the OPO as a singly resonant oscillator, where only one of the three interacting wavelengths is resonant. This allows straightforward monotonic tuning.
Lockheed Martin Aculight’s Argos,launched in 2007, is the first truly singlyresonant CW OPO product. The Argossystems provide multiwatt output powersat both signal and idler wavelengths,linewidth of <1 MHz and mode-hop-freetunability of > 50 GHz. They are configured as quasi-monolithic resonant cavities(Figure 2) with a keyed fiber input collimator acting as a connector to a pre-aligned OPO module. Three interchangeable OPO modules tune from 1.46 to 3. 9μm. The fiber-coupled pump source andmonolithic OPO design make the systemturnkey, so no “tweaking” is required tomaintain alignment. As a result, theseOPO systems are finding wide applicationin major research laboratories around theworld for a variety of spectroscopic investigations.
Applications in chemistry
At the University of Georgia in Athens,
Gary Douberly’s research group uses high-
resolution infrared laser spectroscopy to
study novel molecular species isolated in
cold 0.4-K helium nanodroplets. Low-
temperature helium droplets provide a
unique environment in which to probe the
structural properties of the isolated mole-
cules. The group is using an Aculight
Argos CW OPO system in combination
with a helium nanodroplet instrument to
make the spectroscopic measurements.
The researchers have added additional
controls to a standard OPO system, allow-
ing them to collect wide-range spectra.
Synchronizing the fine- and coarse-tuning
control mechanisms via a PC allows the
OPO to collect high-resolution spectra
over hundreds of nanometers (Figure 3).
These wide scans allow the group toinvestigate the evolution of the infraredspectra of hydrocarbon molecules afterthey undergo reactions with various metalclusters within the helium droplets. Reactions between metal clusters and molecular species such as methane, ethylene andacetylene show up as qualitative changesin the infrared spectra. Full automation ofthe OPO’s tuning enables exceptionallywide, high-resolution measurement of theentire C-H stretching range (approximately 300 nm) characteristic of hydrocarbon molecules in about 3 h, with nouser intervention.
Figure 3. Shown is a depletionspectrum of methanol,recorded using a helium nanodroplet spectrometer in combination with a CW OPO. Theinset shows the wavelengthmeter reading for part of thecomputer-controlled scan,made up of overlapping 50-GHz mode-hop-free scans.After acquisition, data is reconstructed and plotted via aLabView program. Courtesy ofthe University of Georgia.
Figure 4. Shown is an ÉcolePolytechnique Fédérale deLausanne (EPFL) surface science apparatus. The molecular beam exits the aperture(left), traverses the alignmenttool (center), where laser excitation takes place, and collides with the single crystalmetal surface mounted inthe center of the sampleholder (right).