25E07
High Sensitive Gas Concentration Measurement using the Infrared Absorption
Spectroscopy
赤外吸収分光技術を用いた高感度ガス濃度計測
Takayoshi Yumii1), 2) and Noriaki Kimura1), 2)
弓井孝佳1), 2), 木村憲明1), 2)
1) Mitsui Engineering & Shipbuilding Co., Ltd., 3-16-1, Tamahara, Tamano, Okayama 706-0014, Japan2) Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan
1) 三井造船株式会社 〒706-0014 岡山県玉野市玉原3-16-1
2) 大阪大学 〒565-0871 大阪府吹田市山田丘2-1
The amplitude-to-time conversion (ATT) technique is proposed as a detection method which can achieve awide dynamic range and a high signal-to-noise ratio. Laser absorption measurement is known for its highsensitivity for a gas concentration. To enhance the sensitivity for detection, it is required that the minuteattenuation in a direct signal is amplified. In order to verify the principle and evaluate the performance, agas concentration measurement system was developed with the quantum cascade laser and the ATT circuit.
As a result of the test, the detection limit of NO2 has been improved by about fifty times in comparison
with measurement of amplitude.

1. Introduction
Nitrogen oxides (NOx) and sulfur oxides (SOx)are contained in diesel exhaust gas of ships andvehicles. They are known to cause acid rain andair pollution. Although a catalyst system is often
used to reduce these substances, there is aproblem in terms of energy efficiency. Therefore,
the catalyst-free system with the plasma isexpected to solve this problem[1]. In order torealize an efficient exhaust gas treatment, it isimportant to measure a gas concentration in thefield of plasma reaction.
The most commonly used optical method fortrace-gas sensing is tunable diode laser absorption
spectroscopy (TDLAS)[2]. This is a highly
sensitive method to detect specific molecules
using a laser with high wavelength accuracy.
Recently, quantum cascade lasers (QCLs)[3,4]had been developed. This laser can be directly
emitting a light in the infrared band (3-10µm).
In order to further increase the sensitivity of suchmeasurements, multiple reflection optics is usedtypically. In this case, it is possible to measure
only an average gas concentration in the opticalcell. It is necessary to improve the sensitivity byanother method, when a plasma reactor isminiaturized or the measuring area is localized.
The
amplitude-to-time(ATT)
conversion
technique is proposed as a detection method
which can achieve a wide dynamic range and ahigh signal- to-noise ratio (SNR).

converted to delay time. The purpose of theconversion is to improve the SNR by accumulating
and averaging on a time axis. Conceptual diagramof ATT was shown in Fig. 1. The waveform wasshown in Fig. 2. Since the pulsed light emission isrepeated, the delay timeΔt is amplified. Random
noise in the signal is reduced according to 1 / N ,when N is repeat number.

2. Principle of amplitude-to-time conversion
In this technique, the information on amplitude is

T1=(τ+=(τ+Δt
τ+Δt1)・3１

Trigger
Signal３

Waveform
Regenerator

Amplitude-Delay
Converter２

Laser

PlasmaReference
Voltage

Detector

Fig.1. Conceptual diagram of amplitude-to-time
conversion circuit①Trigger Signal②Photo Detector
Reference
VoltageτΔt1③Waveform Regenerator

τΔt1

τ

Δt1

Fig.2. Waveform in the conceptual diagramPowerCombinerDelay LinePower Splitter

Nanosecond
Pulse Generator

Waveform
Shaping Circuit
Reference Voltage
Control Circuit
Directional
Coupler (A)TimingController

CPLreset
Gas Cell(path length: 10cm)

DFB-QCL
DFB-QCL(λ=6.1
λ=6.1μ6.1μm)

InSb Infrared
Detector

Directional
Coupler (B)

24bitCounter

CPL

Peltier Temperature
Controller

A/D converter ・ Computer for signal processing

difference in delay time [ns]Fig.3. Block diagram of a gas concentration measurement system with the ATT circuitchange of the characteristic shape of a spectrum
was not observed at this time. There is a possibilityof evaluating the specified gas concentration
quantitatively through analysis of the spectral shapechanges.

100

1.7ppm013ppm- 10060976098

6099

wave length [nm]Fig.4. NO2 Spectra at different gas concentration
3. Experimental System
Block diagram of the experimental system wasshown in Fig. 3. The mid-infrared light source wasprovided by a thermoelectrically cooled distributed
feedback QCL. The center wavelength of the laseris 6.1µm (for NO2). The laser beam was focused onan InSb photoconductive detector with apreamplifier. In order to achieve the ATT
conversion, high-speed devices were required.
Therefore, emitter-coupled logic (ECL) devices
such as high-speed comparators and ripple counters
had been implemented.
4. Results
The absorption spectra were shown in Fig. 4.
When the concentration of NO2 was decreased, thespectral variation has been reduced. However,

5. Conclusions
Amplitude-to-time conversion technique wasproposed for highly sensitive measurement with an
infrared absorption spectroscopy. This system issuitable for local and in-situ measurement.
Furthermore this technique can be applied toanother measurements such as microwave ormagnetism.
Acknowledgments
This research was carried out at the Laboratoryfor Osaka University-Mitsui Engineering &Shipbuilding Joint Research Chair (Plasma
Technology and Engineering). I would like to thankProfessor Satoshi Hamaguchi for his useful
discussion on this study.
References[1] A. Fridman: Plasma Chemistry (Cambridge, NewYork, 2008), Chap. 11, p.817.
[2] P. Werle, F. Slemr, K. Maurer, R. Kormann, R.
Mücke and B. Jänker: Opt. Laser Eng. 37 (2002)
101.
[3] J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L.
Hutchinson and A.Y. Cho: Science 264 (1994) 553.
[4] R.F. Curl, F. Capasso, C. Gmachl, A.A. Kosterev,B. McManus, R. Lewicki, M. Pusharsky, G. Wysocki
and F.K. Tittel: Chem. Phys. Lett. 487 (2010) 1.