Appl. Phys. B (2012) 109:359–366 DOI 10.1007/s00340-012-5221-4
Acetylene detection based on diode laser QEPAS: combined wavelength and residual amplitude modulation Y. Cao • W. Jin • H. L. Ho • L. Qi Y. H. Yang
•
Received: 18 May 2012 / Revised: 8 August 2012 / Published online: 22 September 2012 Ó Springer-Verlag 2012
Abstract Quartz-enhanced photoacoustic spectroscopy (QEPAS) is demonstrated for acetylene detection at atmospheric pressure and room temperature with a fiber-coupled distributed feedback (DFB) diode laser operating at *1.53 lm. An efficient approach for gas concentration calibration is demonstrated. The effect of residual amplitude modulation on the performance of wavelength modulated QEPAS is investigated theoretically and experimentally. With optimized spectrophone parameters and modulation depth, a minimum detectable limit (1r) of *2 part-permillion volume (ppmv) was achieved with an 8.44-mW diode laser, which corresponds to a normalized noise equivalent coefficient (1r) of 6.16 9 10-8 cm-1 W/Hz1/2.
1 Introduction Photoacoustic spectroscopy (PAS) has demonstrated outstanding performance among various methods for trace gas detection [1, 2]. PAS with a diode laser possesses several advantages over other approaches including compact size, simplicity of use, wide dynamic range, high sensitivity and selectivity, zero background signals, and so on. In PAS, the absorbed laser energy by target gas is transformed to heat by non-radiative processes, and the heat energy results in
Y. Cao (&) W. Jin H. L. Ho L. Qi Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China e-mail:
[email protected] W. Jin Y. H. Yang Department of Opto-electronics Engineering, School of Instrument Science and Opto-electronics Engineering, BeiHang University, Beijing 100191, China
an increase of localized pressure and temperature in the gas sample, which is known as photoacoustic effect. Periodic optical absorption will generate a periodic acoustic pressure, which propagates in the sample as sound wave and can be detected with an acoustic transducer. Most commonly, a sensitive microphone with a capacitive membrane is used to detect the photoacoustic pressure wave [3–5]. But its sensitivity is limited and sensor size is kind of large. Another approach to detect the photoacoustic pressure wave is to use a micro-cantilever combined with a Michelson interferometer, which has demonstrated a very high sensitivity of *1.4 9 10-10 cm-1 W/Hz1/2 [6, 7]. However, the precise optical path control and alignment requirement of the Michelson interferometric detection makes the system complex. In addition, the influence of environmental and gas flow noises is a big issue for cantilever-based PAS systems [7]. Photoacoustic detection with a tiny quartz tuning fork (QTF) as acoustic transducer was first proposed by Kosterev et al. in 2002 [8], which is also called quartzenhanced photoacoustic spectroscopy (QEPAS). The QTF is commonly used in clock and wrist watch, with a resonant frequency about *32.768 kHz in vacuum and an extremely high Q factor ([10,000 at atmospheric pressure). The photoacoustic energy in QEPAS is accumulated in the sharply resonant QTF, instead of gas cell in traditional PAS, as oscillation, which will generate a current signal by piezoelectric effect. The piezoelectric current signal is then transformed to a voltage signal by a transimpedance amplifier and further demodulated by a lock-in amplifier. Since it is operated at a high frequency, the QEPAS system is immune to environmental noise [9]. With its detection sensitivity comparable to that of traditional microphonebased PAS, QEPAS has been investigated for trace gas detection of several simple molecules using a single
123
360
ro-vibrational line [10–13] and larger molecules with broad, unresolved absorption spectra [14–16] by using open-path detection with both on-beam and off-beam configurations [17–20]. Most recently, QEPAS based on evanescent field photoacoustic excitation with a tapered micro/nano fiber was demonstrated to provide an alternative means for gas sensing [21]. In this paper, we demonstrate the detection of acetylene using QEPAS method with a distributed feedback (DFB) pigtailed diode laser in the near infrared region. Previous PAS systems used wavelength modulation (WM) [3, 22, 23] or amplitude modulation (AM) [14–16] to facilitate PA generation. However, in diode laser systems that use direct current modulation to achieve WM, a residual modulation of the laser output power usually exists. We investigate theoretically and experimentally the effect of such residual amplitude modulation on the performance of the WM systems and study the relationship between the signal profile and the modulation parameters. An efficient approach for calibrating the gas concentration in a gas chamber is also proposed and verified.
2 Experimental setup and sensor characterization 2.1 Setup The experimental setup of our gas sensing scheme is depicted in Fig. 1. A DFB laser with a wavelength around 1.53 lm is employed as the light source. The laser temperature is tuned by a slow varying triangle wave scanning
Fig. 1 Experimental setup for diode laser-based QEPAS. The feedback resistance of transimpedance amplifier is Rf = 10 MX; the supply current of DFB laser is modulated at half of the resonant frequency of QTF; DAQ, data acquisition. The blue line represents single mode fiber (SMF), the red line is the optical beam in open path and the black line is the electrical path
123
Y. Cao et al.
signal from a computer-controlled DAQ device and the laser driving current is modulated simultaneously by a fast varying sinusoidal modulation signal at half of the resonant frequency of QTF. Standard acetylene gas with a concentration of 1 % balanced by nitrogen is injected into the gas chamber as sample gas. Light from the DFB laser is fed into a gas chamber through a single mode fiber (SMF) and focused through the gap between the two prongs of QTF by a collimator. A pair of stainless steel tubes, known as acoustic micro-resonator, is placed along the laser beam and beside the two facets of QTF to enhance the acoustic signal. The inner diameter of the resonant tubes is selected to be 0.5 mm and each tube length is tailored to be 4.4 mm to maximize the acoustic signal as suggested by Ref. [17]. The gap between the tube and QTF facet is about 50 lm to ensure efficient acoustic coupling. A QTF with a high Q factor (*10,000) and resonant frequency *32.75 kHz at atmospheric pressure is used as photoacoustic transducer. The QTF along with the resonant tubes are also called spectrophone. The piezoelectric current from QTF is transformed to voltage signal by a self-designed transimpedance amplifier with a feedback resistor Rf = 10 MX and then demodulated by a lock-in amplifier for second harmonic detection. The time constant of lock-in amplifier is set to 1 s with a filter slope 18 dB/octave, which corresponds to a detection bandwidth of DfBW = 0.094 Hz. The laser light after the spectrophone is refocused into a SMF for reference. 2.2 Gas concentration calibration In laboratorial gas sensing research, the sample gas is usually prepared by injecting a standard gas flow into the gas chamber. For a small gas chamber, the sample gas concentration can reach that of the source gas in seconds, so it can be treated the same as the standard gas with its concentration level provided by its manufactory. However, three-dimensional optical alignment and focusing parts are included in our gas chamber, which requires the gas chamber to have a relatively larger volume. A volume of *11.9 L (34 9 25 9 14 cm) is estimated for our gas chamber. Thus a relatively longer time is needed for the gas concentration to reach that of the source gas. In order to precisely know the instaneous gas concentration in the chamber and save the amount of the source gas, which is to some extent meaningful in laboratory gas sensing research, we developed a simple but efficient method for the gas concentration calibration in the gas chamber. Standard gas with a concentration of C0 is assumed to be injected into a gas chamber, the volume of which is V0 , with a instaneous gas flow rate of Qflow ðtÞ. The gas chamber is full of atmospheric air initially and the gas concentration within the chamber at an arbitrary time is
Acetylene detection based on diode laser QEPAS
361
C ðtÞ with an assumption that the gas in the chamber reaches a homogenous distribution in short time. The time derivative of the gas concentration may be expressed as 0
C ðtÞ ¼ ½C0 CðtÞQflow ðtÞ=V0 ;
ð1Þ
For a constant gas flow rate Qflow , the gas concentration in the chamber is derived as CðtÞ ¼ C0 ½1 expðQflow t=V0 Þ:
ð2Þ
As the photoacoustic signal is proportional to the gas concentration for trace gas detection with weak absorption [9], the acoustic signal S ðtÞshould follow the below expression with a constant flow rate filling process S ðtÞ ¼ S0 C ðtÞ ¼ S0 C0 ½1 expðQflow t=V0 Þ;
ð3Þ
where S0 is a constant describing the sensitivity of the setup. Hence, if the signal S ðtÞ could be measured experimentally, the instaneous gas concentration C ðtÞ ¼ S ðtÞ=S0 and the constant gas flow rate Qflow would be obtained by fitting the measured curve with Eq. (3). With DFB laser wavelength tuned to the peak of the gas absorption line, the signal from QTF was recorded by filling the *1 % acetylene into our gas chamber with a constant flow rate and is shown in Fig. 2 as the blue line. The fitted curve from Eq. (3) is also plotted in Fig. 2 as the green dashed line. They match perfectly with each other, proving the validity of the calibration method. From the fitted curve, we can obtain the value of sensitivity as S0 ¼ 0:489 lV/ ppmv (ppmv, part-per-million volume), and the gas flow rate as Qflow ¼ 673:5 cm3/min, comparable to the value from the gas flow meter. At the end of this recording (*54 min), the gas concentration is estimated to be *0.95 %. The gas filling process is then stopped and the
Fig. 2 Experimental result (blue line) and fitting curve (green line) for photoacoustic signal during the constant flow rate filling process. The sensitivity and gas flow rate were found to be 0.489 lV/ppmv and 673.5 cm3/min for our gas chamber with a volume of *11.9 L
gas inlet and outlet are shut for the consequent measurement, which will be reported in the following sections. 2.3 Photoacoustic signal at different measurement modes To evaluate the characteristics of our sensing system, experiments were performed at three different measurement modes as described below: (a) Laser wavelength scanning mode. In this mode, the central wavelength of laser was scanned across the target absorption line by controlling the laser temperature, which is linearly related to the wavelength. The modulation frequency was fixed at 16.373 kHz (around half the resonant frequency of QTF) and the modulation current was set to 100 mA to ensure a relatively large signal. The second harmonic output signal as function of laser temperature is shown in Fig. 3a. The peak of the signal happens around 21.7 °C, corresponding to the central wavelength of P(9) absorption line in the m1 ? m3 band of C2H2. The peak-to-peak value is about 7.56 mV. The signal in the non-absorption range approaches zero quickly as expected. For comparison, the photoacoustic signal without micro-resonator is also shown in Fig. 3a as the black line with the same acetylene gas concentration. The amplitude of the signal is reduced by over 20 times. However, the signal profile, which is determined by the gas absorption line, agrees well with that of the micro-resonator-enhanced configuration. (b) Central wavelength fixed mode. To evaluate the long term stability of our sensing system, the laser wavelength was fixed at the maximum absorption point by operating the laser temperature at 21.7 °C for both configurations with and without micro-resonator. The frequency and depth of the modulation was kept the same as that for the wavelength scanning mode. The amplitude of photoacoustic signals was recorded continuously with a 1 s step as shown in Fig. 3b. The fluctuation of the signal is found to be comparable to the resolution of our DAQ device (*1.22 lV) for both configurations. (c) Modulation frequency scanning mode. In this mode, the frequency response of the sensing system was tested to find the optimum operation frequency. Similar to (a) and (b), the laser wavelength was fixed at the maximum absorption of target gas with a modulation current of 100 mA. The magnitude of second harmonic signal was recorded for varying modulation frequency from 16.34 to 16.4 kHz and plotted as in Fig. 3c. By fitting the experimental data with a Lorentzian profile, we obtain a Q factor about 4,433 and maximum signal at 16,372.15 Hz. To study the effect of micro-resonator to
123
362
Y. Cao et al.
the resonance of QTF, the resonance curve for the nonmicro-resonator case was also measured and shown in Fig. 3c. The Q factor and peak modulation frequency are calculated to be 8,699 and 16,373.3 Hz. That is to say, the resonant frequency of the system is shifted down by 2.3 Hz and Q factor reduced by almost a half with the introduction of the micro-resonator.
3 Effect of residual amplitude modulation For most reported QEPAS research, the signal is obtained by WM and second harmonic detection method, i.e., the wavelength of the laser source is modulated at half of the resonant frequency of QTF around the target absorption line of the sample gas, which excites a localized acoustic pressure wave with a frequency twice of the modulation by photoacoustic effect and then detected by a piezoelectric QTF transducer for second harmonic demodulation [24]. WM approach can effectively suppress the background noise from spectrally non-selective absorbers, such as resonator walls, tuning fork prongs and gas chamber elements, and is easy for locked-to-line gas concentration sensing which attributes to its maximum value always arising at the absorption line center. For species with a broad absorption spectrum, the WM is not suitable. In this case, AM is usually applied for first harmonic detection [14–16]. But the background noise in this feature can not be removed. For a semiconductor laser, modulation of the current simultaneously results in the modulation of laser wavelength and output power, and the power modulation associated with WM is also called residual amplitude modulation (RAM). In this section, the effect of RAM on WM is theoretically investigated and compared with experimental results. 3.1 Theoretical description When an external sinusoidal current, i ¼ i0 cosðxm tÞ, is applied to the DFB laser, both of the laser wavelength and output power are modulated with a phase shift w between them [24]. m ¼ m dm cosðxm t þ wÞ; ð4Þ Fig. 3 Second harmonic photoacoustic signals at different measurement modes. a Central wavelength scanned across the target absorption line; b central wavelength fixed to the absorption peak; c modulation frequency scanned around half the QTF resonant frequency. (The red lines are the results for micro-resonator-based QEPAS configuration, while the black ones are the results without micro-resonator, the values of which are tenfold amplified for better visibility)
123
PðmÞ ¼ PðmÞ þ DPðdmÞ cosðxm tÞ ¼ P0 ð1 þ pX x þ px m cosðxm tÞÞ;
ð5Þ
where m is the central wavenumber of laser, dm and DP are respectively the magnitudes of wavenumber and laser power modulation. P0 is laser power at gas absorption line center, pX and px are power coefficients for slow ramp
Acetylene detection based on diode laser QEPAS
363
(scanning) at frequency X and fast modulation at frequencyxm , respectively. x ¼ ðm m0 Þ=cline is nondimensional wavenumber deviation from the line center m0 , m ¼ dm=cline is modulation depth, cline , in unit of cm-1, is absorption line width. The absorption coefficient can be expressed as. CN0 S 1 aðmÞ ¼ CN0 Sg ðmÞ ¼ pcline 1 þ ½ðm m0 Þ=cline 2 1 ¼ a0 ; ð6Þ 1 þ x2 where C is gas concentration, N0 is total molecular density, S is absorption line intensity, gðmÞis normalized line shape function, here Lorentzian profile is used for absorption in atmospheric pressure. a0 ¼ CN0 S=pcline is peak absorption coefficient at the line center and x ¼ ðm m0 Þ=cline is nondimensional laser wavenumber. x can be also expressed as x ¼ x m cosðxm t þ wÞ
ð7Þ
So absorption coefficient can be rewritten and expanded into Fourier series as 1 að x Þ ¼ a0 1 þ ðx m cosðxm t þ wÞÞ2 " # 1 X ¼ a0 H0 ð xÞ þ H1 ð xÞ cosðnxm t þ nwÞ ð8Þ n¼1
where 1 H0 ð xÞ ¼ p Hn ð xÞ ¼
2 p
Z
p
1 þ ðx m cosðhÞÞ2
0
Z
p 0
1 cosðnhÞ 1 þ ðx m cosðhÞÞ2
dh;
ð9aÞ
dh;
ð9bÞ
are the harmonic coefficients. In WM-based PAS method, the second harmonic acoustic signal is detected [3, 25], so SPA;2fm ðmÞ ¼ kCcell ½aðmÞPðmÞ2fm 9 8 > = < ð1 þ pX xÞH2 cosð2xm t þ 2wÞ > ¼ kCcell a0 P0 þ0:5px m½H1 cosð2xm t þ wÞ > > ; : þH3 cosð2xm t þ 3wÞ ð10Þ where fm ¼ xm =2pis modulation frequency, kis transform coefficient of the system, Ccell is setup constant of the spectrophone. It is shown in Eq. (10) that the PA signal comprises of the pure second harmonic of the WM, and the first and third harmonics of the RAM, each with a phase delay of w. For convenient data processing, the PA signal can be decomposed into two orthogonal directions from Eq. (10) as
Fig. 4 Second harmonic signal with different modulation depths for a pure WM and b WM combined with RAM
SPA;p ¼ kCCell a0 P0 ½ð1 þ pX xÞH2 cosð2wÞ þ 0:5px mðH1 cosðwÞ þ H3 cosð3wÞÞ;
ð11aÞ
SPA;p ¼ kCCell a0 P0 ½ð1 þ pX xÞH2 sinð2wÞ þ 0:5px mðH1 sinðwÞ þ H3 sinð3wÞÞ;
ð11bÞ
When the PA signal is detected by lock-in amplifier with a reference phase U, the detected signal is given by SPA ðUÞ ¼ SPA;p cosðUÞ þ SPA;q sinðUÞ
ð12Þ
For pure WM, the PA signal is maximized at U ¼ 2w. For our system, the phase delay w was determined to be -25°, as will be shown in Sect. 3.2. The second harmonic acoustic coefficients in the brace of Eq. (10) at detection phase 2w are plotted for both pure WM (Fig. 4a) and
123
364
Y. Cao et al.
combined WM and RAM (Fig. 4b), respectively. It is shown that the second harmonic signal is deformed due to the influence of RAM. The effect is more evident for a larger RAM. However, the amplitude of the signal near the line center isn’t affected much because the first and third harmonics (i.e., H1 and H3) approach zero at the line center. 3.2 Experiments and discussion The combined WM and RAM experiment was carried out by using QEPAS for acetylene detection. The target gas is 0.95 % C2H2 balanced with N2. The P(9) absorption line of C2H2 is selected, corresponding to a line intensity of 1.211 9 10-20 cm-1/(molecule cm-2), line width of 0.082 cm-1 and line center at 6534.3636 cm-1 [26]. The laser power after passing through the tuning fork is measured to be 8.44 mW. The relative magnitude of laser output power modulation was measured to be DP/P0 = 3.64i0, where i0 (in Ampere) is the amplitude of current modulation applied to laser source. The phase delay between WM and RAM was found to be -25° at the modulation frequency of 16.373 kHz by using an interferometric method [27]. With these parameters, the theoretical second harmonic PA signal was calculated and shown in Fig. 5 as the blue line. The second harmonic PA signal was also experimentally measured by tuning the laser wavelength across the gas absorption line and is plotted in Fig. 5 as the green dashed line. The profiles of the experimental and theoretical signals agree perfectly with each other. The asymmetry of the signal profiles is due to RAM as discussed in Sect. 3.1. The dependence of maximum PA signal on modulation depth, regardless of the sign, for combined WM and RAM is theoretically calculated and shown in Fig. 6 as the blue line. The optimal modulation depth is found to be 2.184 (marked as red square), very close to the pure WM value of 2.2 [24]. The slight shift can be explained by the small departure of signal peak from the absorption center due to RAM. For comparison, the experimental results are also plotted in Fig. 6. By fitting the experimental data to the theoretical curve, a relationship between modulation current and the modulation depth was found to be m = 19.5i0. Hence the optimum modulation current for our experimental system can be calculated to be 112 mA. The tendence of them matches well except for large modulations (m [ 3) because the RAM isn’t linearly related to the modulation depth any more. The PA signal profiles for different reference phases of the lock-in amplifier are investigated and shown in Fig. 7. The signal achieves its positive maximum value around -180° while achieves negative maximum value near 0°.
123
Fig. 5 Experimental and theoretical results for second harmonic PA signal around the P(9) absorption line of C2H2
Fig. 6 Normalized maximum second harmonic PA signals for different modulation depth
Regardless of the sign of the signal, the peak value of the second harmonic signal for varying detection phase angle was theoretically calculated and experimentally measured, and the normalized results are shown in Fig. 8. The profile and relative amplitude agree well with each other for experimental and theoretical results. The theoretical PA signal is maximized at a detection phase Umax;theo ¼ 50 þ n 180 , where n ¼ 0; 1; 2; . . ., in consistent with Eq. (10). However, the experimental signal maximums occur at Umax;exp ¼ 4 þ n 180 . Considering the sign of signals, a phase lag of 134° is found between the experiment and the theory. To understand the source of the phase delay, we measured the PA signal with different detection phases for a spectrophone without micro-resonator, and the maximum signal is achieved at -113°, which lags the theoretical result by 63° but leads the result of the micro-resonator-based QEPAS by 71°. Therefore, we may conclude that the 63° phase lag may be resulted from the time
Acetylene detection based on diode laser QEPAS
Fig. 7 Dependence of experimental PA signals on lock-in phase from -180° to 0°
Fig. 8 Normalized peak values of second harmonic PA signal for varying detection phase
delay in photoacoustic process and phase delay of the QTF [28], while the part of 71° mainly comes from the acoustic resonance setup and coupling in the tubes.
365
Fig. 9 Allan deviations for time series measurement of signals for 0.95 % C2H2 in N2 and pure air. The red and green lines are fitting curves with slopes *t and *t-1/2, respectively. A, B are fitting coefficients
of QTF (1 lV) under our experimental condition [29]. This gives a signal-to-noise ratio (SNR) of *4,798 for 0.95 % acetylene, corresponding to a minimum detectable gas concentration of 2 ppmv and a normalized noise equivalent absorption coefficient (1r) of 6.16 9 10-8 cm-1 W/Hz1/2. To evaluate the long term stability of our system, Allan variance analysis was performed and the results are shown in Fig. 9 [30]. The black and blue lines show the Allan deviation of time series measurements for 0.95 % C2H2 in N2 and pure air, respectively. The red line with a slope *t demonstrates the linear drift of the signal due to the gas leakage of the chamber, which can be explained by the imperfectly sealing of the gas chamber. For pure air, there is no gas leakage induced signal variation, so the Allan deviation shows a white noise behavior, as indicated by the green line with a slope *t-1/2. As can be seen, the stability time for our present system is about 10 s, which is mainly determined by the laser temperature stability time and gas chamber stability. The sensor sensitivity could be further improved with a longer integration time of 10 s.
5 Conclusion 4 Sensor performance As shown in Fig. 3c, a peak PA signal of 4.894 mV was obtained for 0.95 % acetylene at a modulation frequency of 16.372 kHz with a modulation current of 112 mA. To evaluate the noise level of our sensing system, the laser wavelength was tuned to a non-absorption region, and the output voltage was recorded with a 1 s time step. A noise level (1r) of 1.02 lV was obtained, consistent with the thermal noise level
Laser diode QEPAS is demonstrated for acetylene detection at atmospheric pressure. An efficient method for calibrating gas concentration level within a relatively large gas chamber is proposed and experimentally demonstrated. Experiments at different measurement modes were carried out to evaluate the performance of our sensing system. The effect of RAM in a WM QEPAS system is theoretically investigated and found to agree very well with experimental results. This provides better understanding of the
123
366
modulation/detection process and would be a useful guidance for optimal parameter selection in actual second harmonic PAS-WM detection systems. With a laser output power of 8.44 mW operating at 1.53 lm, a minimum detectable gas concentration of 2 ppmv was achieved for acetylene detection, corresponding to a normalized noise equivalent coefficient of 6.16 9 10-8 cm-1 W/Hz1/2. Acknowledgments This work is supported by Hong Kong SAR government through a GRF grant PolyU5177/10E, and the Hong Kong Polytechnic University through a studentship and a grant J-BB9 K.
References 1. A. Miklos, P. Hess, Z. Bozoki, Rev. Sci. Instrum. 72, 1937 (2001) 2. A. Elia, P.M. Lugara`, C.D. Franco, V. Spagnolo, Sensors 9, 9616 (2009) 3. M.E. Webber, M. Pushkarsky, C.K.N. Patel, Appl. Opt. 42, 2119 (2003) 4. J.-P. Besson, S. Schilt, L. The´venaz, Spectrochim. Acta A 60, 3449 (2004) 5. S. Barbieri, J.-P. Pellaux, E. Studemann, D. Rosset, Rev. Sci. Instrum. 73, 2458 (2002) 6. T. Laurila, H. Cattaneo, V. Koskinen, J. Kauppinen, R. Hernberg, Opt. Express 13, 2453 (2005) 7. V. Koskinen, J. Fonsen, K. Roth, J. Kauppinen, Vib. Spectrosc. 48, 16 (2008) 8. A.A. Kosterev, Y.A. Bakhirkin, R.F. Curl, F.K. Tittel, Opt. Lett. 27, 1902 (2002) 9. A.A. Kosterev, F.K. Tittel, D.V. Serebryakov, A.L. Malinovsky, I.V. Morozov, Rev. Sci. Instrum. 76, 043105 (2005) 10. A.A. Kosterev, F.K. Tittel, Appl. Opt. 43, 6213 (2004) 11. G. Wysocki, A.A. Kosterev, F.K. Tittel, Appl. Phys. B 85, 301 (2006) 12. A.A. Kosterev, F.K. Tittel, Appl. Opt. 43, 6213 (2004) 13. S. Schilt, A.A. Kosterev, F.K. Tittel, Appl. Phys. B 95, 813 (2009)
123
Y. Cao et al. 14. M.C. Phillips, T.L. Myers, M.D. Wojcik, B.D. Cannon: Opt. Lett.32, 1177 (2007) 15. R. Lewicki, G. Wysocki, A.A. Kosterev, F.K. Tittel, Opt. Express 15, 7357 (2007) 16. H. Yi, K. Liu, W. Chen, T. Tan, L. Wang, X. Gao, Opt. Lett. 36, 481 (2011) 17. L. Dong, A.A. Kosterev, D. Thomazy, F.K. Tittel, Appl. Phys. B 100, 627 (2010) 18. K. Liu, J. Li, L. Wang, T. Tan, W. Zhang, X. Gao, W. Chen, F.K. Tittel, Appl. Phys. B 94, 527 (2009) 19. K. Liu, X. Guo, H. Yi, W. Chen, W. Zhang, X. Gao, Opt. Lett. 34, 1594 (2009) 20. K. Liu, H. Yi, A.A. Kosterev, W. Chen, L. Dong, L. Wang, T. Tan, W. Zhang, F.K. Tittel, X. Gao, Rev. Sci. Instrum. 81, 103103 (2010) 21. Y. Cao, W. Jin, L.H. Ho, Z. Liu, Opt. Lett. 37, 214 (2012) ˚ .M. Lindberg, O. Axner, Spec22. P. Kluczynski, J. Gustafsson, A trochim. Acta B 56, 1277 (2001) 23. J. Li, X. Gao, L. Fang, W. Zhang, H. Cha, Opt. Laser Technol. 39, 1144 (2007) 24. S. Schilt, L. The´venaz, Infrared Phys. Technol. 48, 154 (2006) 25. S. Schilt, L. The´venaz, M. Nikle`s, L. Emmenegger, C. Hu¨glin, Spectrochim. Acta A 60, 3259 (2004) 26. L.S. Rothman, I.E. Gordon, A. Barbe, D.C. Benner, P.F. Bernath, M. Birk, V. Boudon, L.R. Brown, A. Campargue, J.-P. Champion, K. Chance, L.H. Coudert, V. Dana, V.M. Devi, S. Fally, J.M. Flaud, R.R. Gamache, A. Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. Lafferty, J.-Y. Mandin, S.T. Massie, S.N. Mikhailenko, C.E. Miller, N. Moazzen-Ahmadi, O.V. Naumenko, A.V. Nikitin, J. Orphal, V.I. Perevalov, A. Perrin, A. PredoiCross, C.P. Rinsland, M. Rotger, M. Simeckova, M.A.H. Smith, K. Sung, S.A. Tashkun, J. Tennyson, R.A. Toth, A.C. Vandaele, J. Vander Auwera, J. Quant. Spectrosc. Radiat. Transf. 110, 533 (2009) 27. S. Schilt, L. The´venaz, Appl. Opt. 43, 4446 (2004) 28. J.-M. Friedt, E´. Carry, Am. J. Phys. 75, 415 (2007) 29. R.D. Grober, J. Acimovic, J. Schuck, D. Hessman, P.J. Kindlemann, J. Hespanha, A.S. Morse, K. Karrai, I. Tiemann, S. Manus, Rev. Sci. Instrum. 71, 2776 (2000) 30. P. Werle, Appl. Phys. B 102, 313 (2011)
