186
K. A. S. Fessler et al.
calibration is generated by assigning the discrete N 2 O line features in the reference spectrum to the known rotational frequencies and interpolating the intermediate
frequencies. The reference beam is generated when the primary quantum cascade
laser beam is passed through a zinc selenide (ZnSe) window (WG70530-1G, ThorLabs) held at 45
o with respect to the beam line. The first surface of the ZnSe window
is uncoated and the second surface is anti-reflection (AR) coated. This window splits
the primary beam into a reflected reference beam with approximately 10% of the
primary beam power and a displaced sample beam (~90% power). The reference
cell is 316 stainless steel construction with a 50 mm pathlength, ZnSe windows,
and Kalrez 6375 o-rings. The first window is AR coated on both sides (WG70530-G,
ThorLabs) and the second is uncoated (WG70530, ThorLabs). The uncoated window
acts as the low-finesse etalon cavity. The intensity of the reference beam is measured
by focusing it onto a mercury cadmium telluride (MCT) detector (PV1-3TE, 10.6
PIP-DC-FM4, Vigo Systems) with a 25.4 mm focal length lens (LA7542-G, ThorLabs). The Vigo Systems MCT detector is a low-noise photovoltaic device with MHz
frequency response.
The sample beam passes through a low-volume folded-path gas cell. The sample
cell is 316 stainless steel construction with ZnSe windows(WG71050-G, ThorLabs),
and Kalrez 6375o-rings. The sample cell cavity is 100 mm long with a rectangular
cross-section 3 mm wide by 20 mm tall and is tilted with respect to the beam at an
angle α (0.54
o ). The folded path is created by discrete gold micro-mirrors deposited
on the ZnSe windows to guarantee the beam is restricted to a single path. The gold
micro-mirrors were deposited on the windows by plasma deposition using a mask
manufactured by Photo Sciences Inc. The micro-mirrors are 0.5 mm tall and 3 mm
wide. For a large number of folds (N p > 5), beam divergence could allow multiple
paths which exit the cell coincidently, causing interference, leading to large and
complex etaloning effects. Multiple paths can be prevented if the number of folds is
restricted to a prime number.
The quantum cascade laser beam enters the sample cell through the first ZnSe
window at the bottom of the cavity and is reflected at the second ZnSe window by
its first micro-mirror at (180
o – 2α). The beam is reflected a second time at the first
window by the first micro-mirror at 0
o . Beam angles reflected from the first window
are always parallel to the primary beam, and beam angles reflected from the second
window are always (180
o – 2α). For an odd number of passes through the sample
cell, the exiting beam is parallel with the primary beam but displaced by distance H.
The beam displacement and cell tilt angle are related to the physical parameters of
the cell through the following equations:
α = ArcTan
D/
L ∗ N p
(1)
H = D ∗ cos(α)
N p − 1
/N p
(2)
K. A. S. Fessler et al.
calibration is generated by assigning the discrete N 2 O line features in the reference spectrum to the known rotational frequencies and interpolating the intermediate
frequencies. The reference beam is generated when the primary quantum cascade
laser beam is passed through a zinc selenide (ZnSe) window (WG70530-1G, ThorLabs) held at 45
o with respect to the beam line. The first surface of the ZnSe window
is uncoated and the second surface is anti-reflection (AR) coated. This window splits
the primary beam into a reflected reference beam with approximately 10% of the
primary beam power and a displaced sample beam (~90% power). The reference
cell is 316 stainless steel construction with a 50 mm pathlength, ZnSe windows,
and Kalrez 6375 o-rings. The first window is AR coated on both sides (WG70530-G,
ThorLabs) and the second is uncoated (WG70530, ThorLabs). The uncoated window
acts as the low-finesse etalon cavity. The intensity of the reference beam is measured
by focusing it onto a mercury cadmium telluride (MCT) detector (PV1-3TE, 10.6
PIP-DC-FM4, Vigo Systems) with a 25.4 mm focal length lens (LA7542-G, ThorLabs). The Vigo Systems MCT detector is a low-noise photovoltaic device with MHz
frequency response.
The sample beam passes through a low-volume folded-path gas cell. The sample
cell is 316 stainless steel construction with ZnSe windows(WG71050-G, ThorLabs),
and Kalrez 6375o-rings. The sample cell cavity is 100 mm long with a rectangular
cross-section 3 mm wide by 20 mm tall and is tilted with respect to the beam at an
angle α (0.54
o ). The folded path is created by discrete gold micro-mirrors deposited
on the ZnSe windows to guarantee the beam is restricted to a single path. The gold
micro-mirrors were deposited on the windows by plasma deposition using a mask
manufactured by Photo Sciences Inc. The micro-mirrors are 0.5 mm tall and 3 mm
wide. For a large number of folds (N p > 5), beam divergence could allow multiple
paths which exit the cell coincidently, causing interference, leading to large and
complex etaloning effects. Multiple paths can be prevented if the number of folds is
restricted to a prime number.
The quantum cascade laser beam enters the sample cell through the first ZnSe
window at the bottom of the cavity and is reflected at the second ZnSe window by
its first micro-mirror at (180
o – 2α). The beam is reflected a second time at the first
window by the first micro-mirror at 0
o . Beam angles reflected from the first window
are always parallel to the primary beam, and beam angles reflected from the second
window are always (180
o – 2α). For an odd number of passes through the sample
cell, the exiting beam is parallel with the primary beam but displaced by distance H.
The beam displacement and cell tilt angle are related to the physical parameters of
the cell through the following equations:
α = ArcTan
D/
L ∗ N p
(1)
H = D ∗ cos(α)
N p − 1
/N p
(2)