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Differential absorption spectroscopy for gas monitoring
at sub-millimeter wavelengths
P. Y. Han*
Physics Department, Rensselaer Polytechnic Institute, Troy, NY 12180
G. Sucha, A. Galvanauskas, D. Harter
IMRA America Inc., Ann Arbor, Michigan 48105
M. Li**, X.-C. Zhang
Physics Department, Rensselaer Polytechnic Institute, Troy, NY 12180
Abstract
A differential absorption spectroscopic system is developed for gas monitoring at sub-millimeter
wavelengths. Tunable, narrowband terahertz (THz) pulse trains are generated using optical pulse
trains obtained by an engineerable pulse shaping technique. By tuning the THz frequency at or
away from the resonant frequency, differential data is obtained to determine the water vapor
concentration. After the time delay between the THz pulse trains and optical pulse trains is
properly set, real-time measurement can be implemented without scanning temporal THz
waveforms.
To be submitted to Applied Physics Letters
*
Current address: Corning Inc. Somerset, NJ 08873
Current address: AMAT Inc. Santa Clara, CA 95050
**
1
Differential absorption spectroscopy (DIAS) has been widely used for the detection of
atmospheric trace gases from the ultraviolet (UV) to near-infrared (IR) spectral ranges.1
However, due to the lack of a bright radiation source and the complexity of cryogenic detection,
it is difficult to apply differential absorption spectroscopy to sub-millimeter region where many
molecular rotational lines fall. With the rapid progress in coherent pulsed THz wave generation
and detection, THz wave sensing of gas has recently attracted considerable attention.2 For
example, real-time chemical recognition of gases3 and gas filter correlation4 are two of the most
recent developments.
In this letter, we report the preliminary results of differential absorption spectroscopy for
monitoring moisture concentration at sub-millimeter wavelengths. The basic concept of THz
wave differential absorption spectroscopy is to monitor the sub-millimeter wave absorption of a
target gas at two different THz wavelengths (on- and off-resonance), and then calculate gas
parameters, such as gas concentration.
In general, the total absorption spectrum of certain gas species can be divided into
spectral components that vary rapidly or slowly with wavelength. The rapid variation normally
happens at molecular rotational lines, while the slowly-varying component is due to the
absorption tails of other trace gases and various kinds of scattering. In a differential absorption
measurement, the gas sample is probed by radiation at two closely spaced wavelengths, the first
being on-resonance, and the second being off-resonance. At a resonant wavelength, the total
absorption has contributions from both the slowly- and rapidly-varying components. For the
second wavelength, slightly away from resonance, the absorption is mainly from the slow
component. Since the absorption from the slow component is virtually the same for these two
closely spaced wavelengths, the ratio of these two absorptions at different wavelengths gives the
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contribution from the rapidly varying component (molecular lines). If the absorption crosssection is known then the gas concentration can be derived by using the Beer-Lambert law.1
To detect water vapor, which has narrow absorption lines at the THz frequency range, a
narrowband THz wave source is desired. Generation of narrowband THz pulse trains has been
demonstrated by the optical rectification of electro-optic (EO) materials,5-7 and the excitation of
the biased photo-conductors8,9 with femtosecond optical pulse trains. An optical pulse train with
a THz repetition rate has been derived through the interference of a pair of chirped optical
pulses,8 the femtosecond pulse shaping,9 and the optical multiplexing.5-7 In this experiment,
second-harmonic (SH) generation (at 780 nm) with a Fourier synthetic quasi-phase-matched
nonlinear crystal (periodically poled Lithium Niobate) is used to generate femtosecond optical
pulse trains at THz repetition rates.10 The PPLN is fabricated in a single LiNbO3 chip by
electrical poling using a suitable mask. The repetition rate of the optical pulse train is tunable
from 0.84 THz to 1.3 THz by varying the period of the grating segments.
Figure 1 shows the schematic experimental setup. 600-fs optical pulses centered at 1560
nm are generated by a chirped pulse amplified fiber laser with a pulse energy of 0.1 J at a
repetition rate of 580 KHz.11 The laser beam is loosely focused through the PPLN to generate
optical second-harmonic pulse trains with average power up to 8 mW. Each fundamental optical
pulse generates a set of second-harmonic optical pulses to form a pulse train. The number of
grating segments determines the number of pulses in the pulse train. The second-harmonic
optical repetition rate can be tuned by simply translating the PPLN in the plane perpendicular to
the optical beam. The second-harmonic beam is separated from the fundamental beam by a
polarizing beamsplitting cube, which is designed for the second-harmonic wavelength (780 nm).
Most of the fundamental beam, which is left after frequency doubling, transmits through the
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beam splitter, while the splitting ratio of 780 nm beam can be adjusted by using a half-wave plate
(for 780 nm). The remaining transmitted fundamental beam is used to generate a single secondharmonic pulse in a standard nonlinear crystal. This single second-harmonic pulse is sent through
an adjustable delay consisting of a retroreflector on a shaker. Accurate measurement of the pulse
train is achieved by cross correlating it with the single 780 nm pulse in a BBO crystal. The crosscorrelation waveform is monitored in real time while the multi-zoned PPLN is translated in the
beam to produce an optimum pulse train, in terms of pulse spacing, efficiency, and uniformity.
The results can be either displayed on an oscilloscope using the shaker or collected by a
computer using a motorized translation stage.
After optimization, the second harmonic optical pulse train is used as the excitation
source for the THz emitter, and the second harmonic single pulse serves as the gating pulse for
the THz sensor. A THz pulse train is generated either from a biased photoconductive antenna (by
way of a current surge) or from a 2-mm-thick ZnTe crystal via optical rectification. The THz
wave detector is a 2-mm-thick ZnTe sensor.12 By removing the nonlinear crystal out of the
optical beam and adjusting the 780 nm half-wave plate so that part of the second-harmonic pulse
train is transmitted and the rest is reflected, auto-correlation of the optical pulse train can be
obtained by using the same path as the cross-correlation. More importantly, when the optical
pulse train is used to probe the THz pulse train in the THz spectroscopy setup, single shot
measurement can be achieved to monitor the molecular absorption after the timing between the
trains is optimized. A 25-cm gas cell is placed in the THz beam path. The humidity in the cell is
varied between 0% and 100%, calibrated by a hygrometer. The whole THz system is purged by
nitrogen gas.
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Figure 2(a) shows a typical cross-correlation waveform between an optical pulse train
with 12 pulses and a single optical pulse, obtained by scanning the time delay between them
using a translation stage. The repetition rate in this figure is 0.84 THz. The optical pulses are
optimized by adjusting the dispersion both in and outside the laser cavity and monitoring the
waveform in real-time on an oscilloscope. A slight pre-chirping of the laser pulse is necessary to
obtain short and uniform second-harmonic pulse trains.
The optical pulse train is used to generate a THz burst and the single optical pulse is used
to probe the generated THz waveform. Both optical rectification and the photoconductive
antennas are used to generate the THz burst. When generating a single-cycle THz pulse, the THz
radiation from optical rectification shows a higher bandwidth than that from the photoconductive
antenna. However, the THz amplitude from the photoconductive antenna is much higher than
that from optical rectification. Therefore, a photoconductive antenna was chosen as the THz
emitter.
A typical THz pulse train generated from a photoconductive stripline antenna and probed
by a single optical pulse is shown in Figure 2(b). The THz train is composed of sinusoidal
waveforms with a repetition rate of 0.84 THz. Since a single THz pulse in the train has a period
of about 1.5 ps, which is longer than the interval of the pulse train, adjacent pulses are temporally
overlapped with each other. A large spike at later time-delay is from the last pulse, which is not
overlapped by other pulses. As illustrated in Figure 2(c), the THz spectrum is composed of a
sharp peak at the repetition frequency of 0.84 THz and some components at lower frequencies.
The second harmonic at 1.68 THz is also observed, due to anharmonicity of the THz waveform.
THz bandwidth at 0.84 THz is narrowed down to about 70 GHz by the multi-pulse sequence.
5
Figure 2(b) and (c) are obtained at a room humidity concentration of 50%. Since no water
vapor line lies at 0.84 THz, little difference is observed when a THz waveform is taken under a
different humidity condition. However, when the repetition rate of the second harmonic pulse
train was tuned to 1.1 THz, which is near the water vapor absorption lines of 1.098 and 1.113
THz, a strong pulse reshaping is observed as illustrated in Figure 3(a). The humidity in this case
is also 50%. The oscillations and beating are caused by free induction decay from the absorption
lines at 1.098 and 1.113 THz. These absorption lines are easily found by Fourier transforming
the data of Fig 3(a) and are shown in Fig. 3(b). Although these absorption lines are narrower
than the THz emission line, they nevertheless have a significant effect on the time-domain data.
Therefore, it is possible to make use of the different behaviors of the THz pulse train at various
frequencies for gas sensing applications.
This narrowband THz system is an excellent source for differential absorption
measurements. In order to use the multi-pulse excitation and detection, the nonlinear crystal is
removed from the optical path and the half-wave plate is rotated, so that 20% of the secondharmonic pulse train transmits the beamsplitter as a probe beam. A biased antenna is used to
generate a THz pulse train with a high signal-to-noise ratio (SNR). Since both the THz beam and
optical probe beam are the second harmonic pulse trains, their cross-correlation waveform gives
a pulse sequence having a repetition rate of 0.84 THz, with a triangular envelope, as shown in
Figure 4(a). Use of the optical pulse train as a probe has the effect of adding another filter
(matched to the first) and the bandwidth is further reduced. If the time delay between the two
pulse trains is properly set at the peak, the use of the probe pulse train with 12 pulses will have
12 times more averaging when detecting the amplitude change at the THz frequency of the
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repetition rate, in comparison with detection using a single pulse. This makes it possible to detect
the amplitude change of gas absorption in real-time.
While the data shown in Fig. 4(a) were obtained by time scanning, ultimately it is
possible to measure the effects of the vapor absorption line without any time delay scanning. To
maximize sensitivity, the time-delay is adjusted to sit at the peak of the THz cross-correlation
waveform (near T=34 ps) in Fig. 4(a). The amplitude of the cross-correlation peak as a function
of humidity is obtained by changing the cell humidity and then reading the output from a lock-in
amplifier without any scanning. Two repetition rates of 0.84 THz and 1.095 THz are used,
corresponding to the spectral positions off- and on-resonance respectively. The THz differential
absorption spectroscopy figure is derived by dividing the cross-correlation amplitude at 1.1 THz
by the amplitude at 0.84 THz and plotting the results as a function of the cell humidity, as shown
in Figure 4(b). Hence, the contribution from the slowly-wavelength-varying component is
cancelled out, leaving only the contribution from the rapidly varying component at the vapor
resonance. The data is fitted by a polynomial function as shown by the solid line. After the
calibration line is determined, the humidity in a gas cell can be derived by first measuring the
relative amplitude at 0.84 and 1.095 THz and next by finding the respective humidity from the
calibrated curve. As an example, a separate measurement is made at 25% humidity. Fitted data
from the curve agrees reasonably well with the value of humidity measured independently by a
humidity probe inserted into the gas cell.
As mentioned earlier, the bandwidth of the source is broader than the actual vapor
linewidth. Thus, the amplitude change is induced by the average absorption within the vapor line
instead of by the peak position only. However, the change is strong enough to be detected with
the help of an optical probe pulse train as a filter, which greatly enhances the sensitivity.
7
In conclusion, the feasibility of a system for the detection of gas concentration by
differential absorption spectroscopy at a sub-millimeter wavelength is demonstrated. Further
improvements of accuracy and sensitivity can be achieved by choosing a THz source with a
narrower spectral line width, a stronger source, and a multipass cell.
The Rensselaer group acknowledges the partial support from the U.S. National Science
Foundation and the U.S. Army Research Office.
8
References:
1.
U. Platt, in Air Monitoring by spectroscopic Techniques, and references therein, M. W.
Sigrist, ed. (Wiley, New York, 1994), p. 27-84.
2.
M. van Exter, C. Fattinger and D. Grischkowsky, Opt. Lett. 14, 1128 (1989).
3.
R. H. Jacobsen, D. M. Mittleman, and M. C. Nuss, Opt. Lett. 21, 2011 (1996).
4.
G. Mouret, W. Chen, D. Boucher, R. Bocquet, P. Mounaix and D. Lippens, Opt. Lett. 24,
351 (1999).
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C. Messner, M. Sailer, H. Kostner and R. A. Hopfel, Appl. Phys. B 64, 619 (1997).
6.
C. W. Siders, J. L. W. Siders, A. J. Taylor, S-G. Park, A. M. Weiner, App. Opt. 37, 5302,
(1998).
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Y.-S. Lee, T. Meade, V. Perlin, H. Winful, T. B. Norris, A. Galvanauskas, Appl. Phys.
Lett. 76, 2505 (2000).
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A. S. Weiling, B. B. Hu, N. M. Froberg and D. H. Auston, Appl. Phys. Lett. 64, 137
(1994).
9.
Y. Liu, S.-G. Park and A. M. Weiner, Opt. Lett. 21, 1762 (1996).
10.
G. Imeshev, A. Galvanauskas, D. Harter, M. A. Arbore, M. Proctor, M. M. Fejer, Opt.
Lett. 23, 864 (1998).
11.
A. Galvanauskas, A. Hariharan, D. Harter, M. A. Arbore, M. M. Fejer, Opt. Lett. 23, 210
(1998).
12.
A. Nahata, A. S. Weling, T. F. Heinz, Appl. Phys. Lett. 69, 2321 (1996).
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Figure captions:
Figure 1:
Experimental schematic setup. F: flipper; TS: translation stage.
Figure 2:
(a) A cross-correlation waveform between an optical pulse train and a single
optical pulse; (b) A THz pulse train generated by a photoconductive stripline
antenna at a repetition rate of 0.84 THz and a humidity of 50%; (c) the spectrum
of the waveform in Fig. 2(b).
Figure 3:
(2) A THz pulse train at a repetition rate of 1.1 THz at 50% humidity; (b) the
spectra of the waveforms in Fig. 3(a).
Figure 4:
(a) A cross-correlation waveform between a THz pulse train and an optical probe
pulse train at a repetition rate of 0.84 THz; (b) The peak amplitude of the crosscorrelation waveform as a function of humidity when the repetition rate is set to
1.095 THz. The amplitude is normalized with the peak amplitude at 0.84 THz
repetition rate. The line is the fitted polynomial function curve.
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Lock-in
Sensor
Emitter
Chopper
F
FCPA
laser
F
Grating
F
PPLN
/2
PMT
TS
Detectors
BBO
Shaker
BS
Fig. 1: Han, et. al. “Differential absorption spectroscopy…”
11
Oscilloscope
Gas Cell
Computer
(a)
Amplitude (a. u.)
0.84 THz
optical train
5
10
15
20
25
Time (ps)
Fig. 2(a): Han, et. al. “Differential absorption spectroscopy…”
12
30
35
(b)
Amplitude (a. u.)
THz pulse train
from antenna
(a)
)
0
10
20
Time (ps)
30
Fig. 2(b): Han, et. al. “Differential absorption spectroscopy…”
13
40
(c)
Amplitude (a. u.)
0.84 THz
1.68 THz
0.5
1.0
1.5
Frequency (THz)
Fig. 2(c): Han, et. al. “Differential absorption spectrscocopy…”
14
Amplitude (a. u.)
(a)
0
10
20
30
40
Time (ps)
Fig. 3(a): Han, et. al. “Differential absorption spectrscocopy…”
15
50
60
Amplitude (a. u.)
(b)
1.098
1.113
0.0
0.5
1.0
1.5
Frequency (THz)
Fig. 3(b): Han, et. al. “Differential absorption spectrscocopy…”
16
2.0
Amplitude (a. u.)
(a)
20
30
40
Time (ps)
50
Fig. 4(a): Han, et. al. “Differential absorption spectrscocopy…”
17
60
Amplitude (a. u.)
(b)
1.0
0.9
0.8
0.7
0.6
0
10
20
30
Humidity (%)
Fig. 4(b): Han, et. al. “Differential absorption spectroscopy…”
18
40
50