Mid-Infrared Tunable Lasers
high power of the OPO allow vibrational
transitions to be excited into saturation
(Figure 4) where approximately 50 percent
of the molecules are in a single quantum
state. The wide tunability of the OPO
makes it a highly versatile source for this
study, and the researchers anticipate using
it in the future to extend their studies to a
variety of molecular species and absorption features.
These same attributes also make OPOs
promising for in-the-field applications in
industry, medicine and sensing.
Meet the author
Angus Henderson is a principal scientist at
Lockheed Martin Aculight. He can be reached
1. F.J. Duarte, ed. (1995). Tunable Lasers
Handbook. Academic Press, pp. 167-291.
2. E. Takeuchi et al. (January 2009). Quantum-cascade lasers: applications multiply for external-cavity QCLs. Laser Focus World, pp.
3. G. Wysocki et al. (Nov. 20, 2007). Dual interband cascade laser-based trace-gas sensor
for environmental monitoring. Applied Optics, pp. 8202-8210.
4. Scherer, James J. (Oct. 1, 2007). Nonlinear
optics: quasi-phase-matched DFG lasers for
sensing enter the market. Laser Focus World,
Applications in chemical sensing
Professor Frans Harren leads the Life
Science Trace Gas Facility at Radboud
University Nijmegen in the Netherlands.
The group has developed state-of-the-art
laser-based trace gas detectors that are
used for research in areas from plant physiology to medicine and human health. Harren and his colleagues use a variety of
lasers combination with sensitive detection
schemes, including photoacoustic spectroscopy and cavity ring-down spectroscopy.
The team’s recent experiments have
challenged the findings of a published
study that asserted that emission of significant amounts of methane from plants may
play a significant role in the global atmospheric concentration of this powerful
By growing plant samples in the presence of carbon dioxide molecules based on
the stable isotope of carbon 13C, any
methane emitted by the plants could be
identified by the presence of this isotope in
the methane molecule. Researchers performed trace methane detection with the
photoacoustic technique using a singly resonant CW OPO pumped by a diode-pumped solid-state laser operating at approximately 3. 24 μm (Figure 5). The
OPO’s high power level allowed a 13CH4
detection limit of three parts per billion to
be attained using the OPO spectrometer.
This is well below the naturally occurring
level of 13CH4 in the atmosphere. Monitoring the 13CH4 level showed that there was
not a statistically significant level of
methane emission from the plants, contradicting the original published finding.
It appears likely that, with time, new
types of semiconductor lasers may emerge
as the lowest cost pathway for laser emission in the 3- to 4-μm region. However,
the high power, wide tuning and robust
physical characteristics of OPOs, especially those pumped by fiber lasers, make
these sources important spectroscopic
tools in today’s research laboratories.