PNNL-13501 - Pacific Northwest National Laboratory

Cavity-Enhanced Infrared Absorption with Quantum Cascade Lasers
Tanya L. Myers, Richard M. Williams, Matthew S. Taubman, Steven W. Sharpe, Robert L. Sams, James F. Kelly
Study Control Number: PN00018/1425
Laser spectrometers can detect and characterize chemical compounds in the atmosphere. Signals from tunable laser
absorption spectrometers can be enhanced with the use of an optical cavity, providing improved sensitivity to trace
amounts of key compounds critical to atmospheric processes. An apparatus has been developed under this project and
design configurations specific to infrared optical detection are being evaluated.
Project Description
The purpose of this project is to build and test a novel
cavity-enhanced, infrared laser absorption spectrometer
used to detect molecular compounds in the atmosphere
that are relevant to key environmental processes. As our
level of understanding about the chemistry of the
atmosphere increases, new high-sensitivity instruments
are needed to reveal even more subtle details. The
instrument under development offers the potential for
high-sensitivity and field-portability; which are two key
aspects required for next-generation sensors. We built a
working in situ measurement instrument and obtained
promising initial results. During this initial testing, we
found that our instrument yields comparable path-length
results to commercially available alternatives (optical
multi-path cells). Path-length enhancement provides for a
longer interaction length between the interrogating laser
and sample under study, thereby improving absolute
sensitivity. We have also identified technical issues
limiting our present sensitivity. We expect that our
instrument will be several times more sensitive than
conventional off-the-shelf alternatives. Additionally, we
have observed that our instrument benefits from
mechanical vibrations; this is truly exciting as it supports
instrument deployment on airplanes.
Introduction
Infrared absorption spectroscopy is recognized as an
extremely valuable method for chemical detection and
identification efforts. In the mid- (3 to 5 micron) and
long-wave (8 to 14 micron) infrared regions, molecules
exhibit unique absorption features, commonly referred to
as molecular finger-print bands, which allow for accurate
identification and quantification. Recently, a new
infrared laser source, the quantum cascade laser (Capasso
et al. 1999), was developed at Bell Laboratories (Lucent
Technologies). The quantum cascade laser allows
tremendous advances in the field of atmospheric
384 FY 2000 Laboratory Directed Research and Development Annual Report
monitoring. Our Laboratory has a strong collaborative
arrangement with Bell Labs, and we are developing
instrumentation under this project that incorporates the
quantum cascade laser.
A tunable laser absorption spectrometer obtains
concentration information about specific compounds
within a sample cell by measuring the amount of
attenuation a laser experiences as it is passed through the
sample cell. If very little of the compound is present, then
very little of the laser light is attenuated because of
absorption by the vapor sample. To observe higher levels
of attenuation producing greater signal-to-noise and
higher sensitivity, the physical cell is made longer or the
laser is passed many times back-and-forth through the
cell. Using this technique, path lengths of tens of meters
are possible from a cell which is 40 cm in length. An
alternative to multi-path cells is the use of opticalresonator-cavities
to capture and “store” the light inside
the cavity (Engeln et al. 1998; O’Keefe et al. 1999).
These new cavity-enhanced techniques can produce
effective path lengths in the hundreds to thousands of
meters in a cell 40 cm in length. Our goal is to produce a
cavity-enhanced spectrometer, operating in the infrared
spectral region, with an effective path length of 1 km
(roughly an order of magnitude improvement over
conventional technology).
Results and Accomplishments
This project is aimed at integrating the necessary
components to build an infrared laser absorption
spectrometer uses cavity enhancement. The main thrust
of the first year has been the successful assembly of a
complete apparatus and initial experimentation.
A schematic layout of the experimental apparatus is
shown in Figure 1. The experiment consists of an
8.5-micron single-mode quantum cascade laser operating
at liquid nitrogen temperatures (77 K), a reference cell