
Spectroscopic Techniques: Cavity-Enhanced Methods 43.4 Extensions to Solids and Liquids 639
the photodetector, and the FM modulation index. NICE-
OHMS has exploited cavities having a finesse of 100 000
to achieve sensitivities of 1 × 10
−14
cm
−1
Hz
−1/2
. NICE-
OHMS holds the world record in detection sensitivity of
all cavity enhanced techniques.
In a NICE-OHMS experiment, phase modulation of
the laser produces side bands that are set to equal the
free spectral range of the high finesse resonator. Be-
cause the sidebands are transmitted by the cavity in the
same manner as the carrier, any small wavelength fluc-
tuations in the laser or small optical phase shifts of the
transmitted carrier that contribute to noise in the trans-
mitted intensity will appear identically in the sidebands.
After demodulation, this noise will cancel out. Thus, the
transmitted carrier and sidebands are an accurate repre-
sentation of the carrier and sidebands impinging on the
input mirror of the cavity. The carrier laser frequency is
locked to the peak of the optical cavity mode and tracks
this mode if the cavity length is changed in order to pro-
duce a spectral scan. The sidebands are detected and
demodulated as in conventional FM spectroscopy. The
key to NICE-OHMS is that the noise level can approach
the intrinsic shot noise of the laser at the FSR frequency
(namely hundreds of MHz).
If there is no sample in the cavity, the transmitted
sidebands will cancel after demodulation because they
have opposite phases. No signal will be observed. If there
is a sample in the cavity, the sidebands will experience
different transmission amplitudes, so that demodulation
will produce a signal proportional to the difference in
absorption between the sideband frequencies. In addi-
tion, the absorption feature produces a phase shift that
pulls the carrier cavity mode frequency, so that the side-
bands become detuned from the mode peak and acquire
a phase shift on transmission. This sideband phase shift
contributes to the demodulated signal.
43.4 Extensions to Solids and Liquids
Thus far, the discussion of cavity-enhanced techniques
has addressed traditional optical resonators formed us-
ing high reflectivity dielectric mirrors that encompass
gas samples. Extensions of cavity enhanced methods to
liquid and solid media has required additional innova-
tion, specifically in the optical cavities used.
Evanescent-wave CRDS (EW-CRDS) exploits the
fact that total internal reflection allows probing the sur-
face layer of a sample in contact with a prism. The
simplest configuration places a Brewster prism inside
a linear ring-down cavity. The prism folds the cavity
beam path by 90 degrees, thereby producing one point
in the prism having total internal reflection [43.22]. The
evanescent wave produced by this internal reflection can
be used to probe liquid or solid samples. Another pos-
sible embodiment is a ring cavity having its optical path
inside a multi-faceted polygon, where at each facet, total
internal reflection occurs [43.23]. Light is coupled into
and out from the polygon by photon tunneling – which
effectively controls the overall finesse of the cavity. The
absorbing sample material can then be placed on any
one or more of the polygon facets, and its detection is
done using the polygon’s evanescent waves.
Fiber cavities are attractive because they can extend
the use of CRDS into harsh environments, can probe
liquids as well as gases, and can be used to measure
pressure [43.24]. The high reflectivity mirrors on each
end of the fiber can be either dielectrically coated, as
in a traditional ring-down cavity design, or can con-
sist of Fiber Bragg gratings. Fiber-based CRDS has also
been applied to detection in liquid media via a fiber
loop cavity wherein a liquid sample replaced the index
matching fluid in the gap between fibers at the connec-
tor splice [43.25]. This approach produced a 100-fold
enhancement over linear detection.
More direct approaches to measuring liquid samples
involved confining the liquid samples within a more tra-
ditional RDC. The most direct method involves placing
a liquid directly into the cavity, but this approach re-
sults in very limited sensitivity. By placing a Brewster
cell filled with liquid sample in a RDC, the sensitivity
of the P-CRDS can be slightly improved. By matching
the Brewster angles to the refractive indices of the adja-
cent media (the outside angle matches the air-filled RDC,
while the inner angle matches the index of the liquid), the
sensitivity can be dramatically improved [43.26]. The
peak-to-peak baseline noise level of such a P-CRDS sys-
tem was 1.0×10
−5
absorbance units (AU), rivaling the
best available commercial ultraviolet-visible (UV-VIS)
direct absorption detectors. The performance remained
limited by the excitation of multiple cavity modes
A CW-CRDS system using the same angle-matched
Brewster cell [43.27] improved the peak-to-peak base-
line noise by a factor of 50 to 2 × 10
−7
AU.ThisCRDS
detector outperformed the best commercially available
UV-VIS detector by a factor of 30, again illustrating
the potential for CRDS to replace standard absorption
spectroscopy techniques.
Part C 43.4