Proceeding of International Conference On Research, Implementation And Education Of Mathematics And Sciences 2014, Yogyakarta State University, 18-20 May 2014

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QCL BASED INTEGRATED CAVITY OUTPUT SPECTROSCOPY FOR CO GAS DETECTION

R. Widiatmono1,2, J. Mandon3, F.J.M. Harren3, Kusminarto4, M.A.J. Wasono4, Mitrayana4 email: restu@uny.ac.id

1. Department of Physics Education, Universitas Negeri Yogyakarta, Yogyakarta, Indonesia 2. Physics Doctoral Program, Universitas Gadjah Mada, Yogyakarta, Indonesia
3. Life Science Trace Gas Research Group, Molecular and Laser Physics Department, Radboud University Nijmegen, The Netherlands
4. Physics Department, Universitas Gadjah Mada, Yogyakarta, Indonesia

Abstract

A sensitive gas detection system using a distributed feedback CW Quantum Cascade Laser (QCL) centered at 4610 nm as a radiation source has been developed. QCL was used in combination with an off-axis integrated cavity output spectroscopy for Carbon Monoxide (CO) gas detection in a low gas pressure. a 15 cm long high-finesse cavity with an effective optical path length of 400 m has been used as a sample gas cell. We demonstrated detection limits of an up to 1 ppb level with less than 2 second averaging time. The result clearly showed that The QCL based ICOS system is a robust technique that suitable for fast and sensitive gas detection.

Keywords: QCL, ICOS, Absorption Spectroscopy, CO gas detection

INTRODUCTION

Quantum cascade laser (QCL) has proven as an effective source for laser absorption spectrometry (LAS) in fundamental vibrational molecular absorption bands in the mid-infrared region or in a very wide range of wavelengths from ∼3 µm to ∼24 µm [1]. LAS molecular trace gases detection and quantification ranging from ppmv, ppbv, to pptv levels using various gas species have been developed and demonstrated in many applicable methods such as multipass absorption spectroscopy [1,2], cavity ring down spectroscopy (CRDS) [1,3], wavelength modulation spectroscopy [4], integrated cavity output spectroscopy (ICOS) [1,5], and quartz-enhanced photoacoustic spectroscopy (QEPAS) [1,6]. Room temperature continuous-wave QCL also became commercially available and better performance in measurements could be achieved [7,8].
Continuous-wave (CW) distributed feedback (DFB) QCL with thermoelectric cooling (TEC) has been used for CO detection with various methods namely CRDS, ICOS, and QEPAS [6,9]. Several methods have been successfully employed for CO detection in many application from atmospheric monitoring [10], combustion environment [11], to exhaled human breath analysis [12]. Recent work in exhaled human breath detection was reported that QCL-based sensor employing ICOS is proven as a suitable method on online exhaled gas detection [13].

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Widiatmono et.al. / QCL Based Integrated …	ISBN 978 – 979 – 99314 – 8 – 1

Tracing gases in exhaled breath is a difficult task due to their varying concentrations (from ppmv, ppbv to pptv) and high possibility to interference with other molecules that can be found in human breath [12]. Exhaled human breath contains more than a thousand of different gas compound, not only oxygen, nitrogen, carbon dioxide, and water vapor, originated by endogenous and exogenous reasons [14]. Concentration of molecules that exist in an exhaled human breath can be use as a biomarker to determine pulmonary pathogenic or a person healthy level [15].
Zhou et.al. already compiled several elements that used as a biomarker, such as nitric oxide (NO), carbon monoxide (CO), hydrocarbon, hydrocarbon peroxide, and even carbon disulfide that is not found on a healthy human exhaled breath [16]. The CO gas can be use as a biomarker of a pulmonary disease such as indication of liver problem, oxidative stress, respiratory infections, and anemia [17]. More known as a hazardous air pollutant, CO actually also produced endogenously inside human body and exhaled via the lungs [18]. The concentration of CO in exhaled breath of a healthy person is around 1 to 5 ppm [19].
There are many number of techniques can already detect exhaled CO with their respective detection limits. Besides those laboratory scale methods, several commercial sensors are actually available employing an electrochemical CO sensor that are both selective and inexpensive. However, these sensors are sensitive to interference with other substances [20]. Another instrument capable to measured exhaled CO is near-infrared CO analyzers, which is commonly used to monitor atmospheric CO. This instrument have proven to be both sensitive and stable, but is limited by its large size, sensitivity to water and CO2 concentrations, and the need of large sample volume [21]. QCL-based systems, in conjunction with highly selective optical spectroscopy principles, is a trustable and accurate technique and suitable for multiple flow rates analysis compared to chemiluminescence analyzers or electrochemical sensors [22].
Nowadays, the investigation of breath analysis application for biomedical diagnostics become more intensive. Breath analysis using highly selective and sensitive laser absorption method have many advantages such as noninvasive operation, easy to use, reuse possibility, real time/online measurement, comfortable level of patients, and the ability to detect changes at molecular level [23]. Thus, the availability of highly sensitive exhaled gas sensor which is capable to measure and monitor human breath component with its respective detection limit is absolutely necessary.
In order to provide an ultra sensitive sensor to detect an online exhaled CO gas, a system that is capable to detect CO in a complex gas mixture with high sensitivity and selectivity was developed. In this paper, the performance of sub-ppbv measurement from a recent development of high power 4.61 µm CW-DFB-QCL in combination with integrated cavity output spectroscopy will be demonstrated for the detection of CO gas.

METHOD

ICOS sometimes referred as another term of Off-axis cavity enhanced absorption spectroscopy (CEAS) is an important variant of CRDS and first reported in 1998. In the ICOS approach, light is coupled into a optical cavity and the absorption spectra can be obtained by the integration of the total light signal transmitted through the high finesse or highly reflective optical cavity [24]. A high finesse optical cavity also behave like a spectral filter and produce narrow band light which passed through it. In addition, CW-ICOS approach enable easier technique to coupling energy into a high finesse optical cavity [25].
Off-axis (OA) cavity alignment geometry approach of a laser light injection into the

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Proceeding of International Conference On Research, Implementation And Education Of Mathematics And Sciences 2014, Yogyakarta State University, 18-20 May 2014

cavity can systematically eliminate the resonances associated with optical cavities, but still maintain the absorption signal amplifying properties [26]. Therefore, OA-ICOS can effectively lowering the free spectral range (FSR) of the cavity and producing a dense mode structure. It provides the opportunity to make an ultra-sensitive measurements [27,28]. A successful off-axis alignment yield better cavity mode suppression. Thus, the laser linewidth should be broad related to the FSR of the cavity, but narrow related to the molecular line. Narrowband lasers couple to many cavity modes simultaneously and provide alignment robustness, but at the cost of cavity throughput power [28].
Laser power transmitted through the cavity in an off-axis alignment is given by [29]

=𝐼𝐼(

)

)	]

(1)

[(
Where I0 is the incident laser power, R is the mirror reflectivity, Cp is a spatial coupling parameter (with usual value is between 0 and 1), and k is the absorption coefficient. When Reff is defined as the effective mirror reflectivity. The absorption in the cavity is then given by

(	)
=𝐴	                                                             (2)

A schematic diagram of the QCL-based ICOS sensor is shown in Figure 1.

Fig. 1. Schematic diagram of the QCLbased off-axis ICOS sensor for CO detection at 4.61 µm.

A thermoelectrically cooled CW-DFB-QCL type M9501-modified from Maxion Inc. is a single mode laser, operating at wavelength 2169.2 cm-1 or 4.61 µm, generates an average output power of  40mW. The laser is mounted on a QCL housing attached with a 5-mm diameter collimating lens and a water cooled Peltier element. Overall tuning range of the QCL is 3 cm-1, based on Maxion’s laser specifications details. To scan the transition at 4.61 µm, the temperature of the laser is set to 20 C, with a current value of 418 mA. At this fixed temperature, the frequency of the laser output is fine-tuned by modulating the QCL current with a 12 kHz triangular signal. The fine-tuning range is about 0.25 cm-1.

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Laser beam is sent to a high finesse cavity (F ∼4,500), formed by two 1’’ (∼2.54 cm) concave mirrors from CRD Optics Inc. with 1 m radius of curvature. Mirror reflectivity R is

∼99.98 % at center wavelength 4.6 µm. The self builded optical cavity spesification are 15 cm long aluminum tube, with approximate volume of 170 ml. Figure 2 is shown the high finesse cavity which also functions as an absorption cell. The effective optical path length inside the cell is about 400 m. The cavity is aligned off-axis with respect to the incident laser beam.

Gas out

15 cm

Gas in

Fig. 2. 15 cm long alumunium high finesse cavity which also function as a sample gas cell and produce 400 m optical path length.

A pressure inside the absorption cell is maintained at 80 mbar by a vacuum pump equipped with manually adjusted valves. The flow rate through the gas cell is set to 30 l/h, producing a refresh time for the cell of approximately 1 s. The output beam is focused by means of a 25 mm BaF2 lens (5 cm focus distance) on a 4 stages thermoelectrically cooled (TEC) HgCd Te infrared detector with a built-in preamplifier (PVI-4TE, D* = 4.01011 cmHz1/2/W, t=20 ns) by Vigo System. The signal is amplified by 20 dB (Femto HVA-S, Germany) and acquired via a data acquisition card (GaGe CompuScope 14200, USA) for computer analysis using LabView software.
To carry out gas concentration measurement in absorption cell and monitor laser drifting simultaneously, a small reference cell filled with 1‰ CO in 1 atm pressure is placed on the system setup. A beam splitter with 80% reflection and 20% transmission properties is used to provide laser beam for both of the cell. The QCL-based sensor is calibrated with a reference mixture of 100±3 ppmv of CO in N2 (VSLNational Dutch Metrology Institute). A N2 gas cylinder is used to provide a gas reference free of CO.
In the spectral region of the QCL, there two absorption lines from CO2 and CO are present (4,610.6 and 4,610 nm, respectively). Figure 3 reveals simulated spectra of 5% H2O, 5 % CO2, 2 ppmv CH4, and 10 ppbv CO which represent typical human breath concentration at working conditions (pressure 100 mbar, temperature 293 K)[23]. The spectra lines shown in figure 3 are simulated based on HITRAN 2008 database [30].

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Proceeding of International Conference On Research, Implementation And Education Of Mathematics And Sciences 2014, Yogyakarta State University, 18-20 May 2014

Fig. 3. Simulated spectra of typical breath gas concentration within QCL 4.61 µm wavelength range.

RESULTAND DISCUSSION

The aim of the measurement was to assess the performance of the develop QCL based ICOS system to monitor CO gas in typical breath concentration with sub-ppb sensitivity. In order to meet the requirement several measurement was conducted using several concentration of CO in N2. In this paper we reported results from measurements which represent two CO conditions. First, for 1 ppmv concentration of CO which represent a normal concentraton from healthy exhaled human breath. Second, for 0.24 ppmv concentration which represent low level concentration of CO.
Figure 4 is shown concentration measurement result for 1 ppmv concentration of CO.

Fig. 4. 1 ppmv CO concentration monitoring result from the developed LabView data acquisition program beyond two hour measurement

System was running for more than two hour (approximately 7,300 s) and measurement was conducted simultaneously. System is able to present concentration measurement visually, while in addition measurement raw data is saved into a text file. Figure 5 represent concentration measurement result for 0.24 ppmv concentration of CO. Both of fig. 4 and fig. 5 are indicate that CO concentration is stable in their respective concentration.

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Fig. 5. 0.24 ppmv CO concentration monitoring result from the developed LabView data acquisition program beyond one hour measurement

In order to estimate the detection limit value of the QCL-based sensor, which will give us a quantitative term of stability, data is analyzed using Allan varriance standard deviation. The Allan variance could determined system stability by presenting noise level of the signal as a function of the integration time of the measurement [31]. Figure 6 is represent the Allan deviation of 1 ppmv concentration of CO. The detection limit is defined as 1 ppbv of CO in less than 2 s average. A better sensitivity is obtained using longer integration times, e.g., 0.4 ppbv for 115 s.

Fig. 6. Allan variance represents QCL-based ICOS detection limit for 1 ppmv concentration of CO. A detection limit of 1 ppbv in less than 2 s average is achieved.

Figure 7 is represent the Allan deviation of 0.24 ppmv concentration of CO. The detection limit of 1 ppbv in less than 2 s average is achieved. The best detection limit is obtained at 0.2 ppbv for a 62 s acquisition time. Both of two measurement condition results are in good agreement that the QCL based ICOS detector system proved to be suitable for monitoring CO gas in a low gas pressure condition with sub-ppb sensitivity.

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Proceeding of International Conference On Research, Implementation And Education Of Mathematics And Sciences 2014, Yogyakarta State University, 18-20 May 2014

Fig. 7. Allan variance represents QCL-based ICOS detection limit for 0,24 ppmv concentration of CO. A detection limit of 1 ppbv in less than 2 s average is achieved.

CONCLUSION

The development of a QCL-based sensor has been reported, employing integrated cavity output spectroscopy as a detection method. The results clearly show that the QCL-based sensor employing integrated cavity output spectroscopy is a robust technique, suitable for fast and sensitive detection of CO traces. The developed sensor can be easily applied to exhaled CO gas measurement and monitoring. The system also has a great possibilities to be extended to detection of other gases and can be applied for investigations that involve biomedical background.

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