CH4 optical sensor using a 1.31 μm DFB laser diode

em

7 ELSEVIER

CHEMICAL

Sensors and Actuators B 24-25 (1995) 603-606

CH4 optical sensor using a 1.31 pm DFB laser diode J.P. Silveira, F. Grasdepot Schlwnberger IndwttiesfGEM,

50 av. Jean Jaures, 92542 Montmuge Ceder, France

Abstract We present a m optical sensor using a 1.31 nm distributed feedback (DFB) InGaAsP laser diode. This sensor uses a new type of signal processing providing auto-calibration to cancel emitter-amplitude variations as well as changes in the optical transmission not due to the gas and enables the concentration of gases to be measured accurately. The realization of an industrial prototype of this methane sensor is reported. Keyr~ordr:Distributed feedback: Laser diodes; Methane sensors

1. Introduction

The measurement of the absorption of a gas along an optical beam is known as a sensitive method for trace-gas detection. The availability of compact laserdiode emitters together with the presence in their commercial wavelength range of gaseous absorption lines has led to various absorption-based sensing methods for the detection of gases. One of the basic problems of absorption-based gas sensors is auto-calibration, which is necessary for Iongterm stability. Auto-calibration enables a gas sensor to be only sensitive to absorption due to the gas and not to the spurious variations of the beam energy, due, for example, to emitter-amplitude variations, screening by dust particles or changes in optical window transmission. In the case of absorption-based gas sensors using a wavelength-modulated laser-diode emitter, the problem is more acute because when wavelength modulated, the emitter itself shows large amplitude variations. As a result, the optical signal in the presence of gas shows a large amplitude modulation due to the emitter together with the amplitude modulation due to the gas absorption. It was recognized early in trace-gas detection that because the large periodic amplitude modulation of the emitter was slightly distorted by gas absorption, it was highly sensitive operating in the frequency domain to extract information on the presence of gas. These methods use, in general, one absorption-signal harmonic strongly affected by the gas as a measurement [l-3]. 0925-4005/95/$09.50 Q 1995Elsevier Science S.A. All rights reserved SSDI 0925-4005(94)01428-K

These methods are in genera1 used for high-sensitivity measurements of spectra1 lines and they provide as output the first or higher-order derivatives of the line shape. They require a preliminary calibration of the set-up to measure the optical losses that are not due to the gas. For concentration measurements, the use of these techniques is not widespread because the calculation of the concentration from the sensing-device output is complex and varying with the laser modulation parameters for a given absorption line. Time-domain signal processing for detecting the optical amplitude modulation due to gas absorption is attractive, since if we except the amplitude modulation of the emitter, the output represents the absolute absorption or line shape of the absorption line [4,5]. Straightforward calculation of the area under the line gives a value directly related and almost proportional to the gas concentration in all the validity domain of the Beer-Lambert law [6]. Operation in the time domain is difficult because one has to define a reliable reference for auto-calibration in order to separate the absorption of the gas from other modulations. Though it is also possible to take a reference by making a preliminary calibration, an idea1 reference would have to be measured simultaneously with the signal. To achieve this, a reference from a beam splitter and its subraction from the signal with gas absorption could be performed. Nevertheless, it is known that the gain between the signal and the reference then has to be equalized continuously (auto-

LP. Silveira, F. Gras&pot

604

I Sensors and Actuators B 24-25 (1995) 603-606

calibration). Although several methods for doing this have been tried, none was found satisfactoq [5]. The purpose of this paper is to present a method that eliminates the need for such an equalization, thus providing auto-calibration.

with k(~)nl < 0.02. The signal and reference at the input of the signal processing are

2, Signal processing The signal processing method presented here can be applied to a laser beam of optical frequency v(t) (Fig. 1) inducing a current 1(t) in a photodetector, with Z(f) and v(t) being periodic functions of time with a frequency equal to J I(r) can be obtained on a detector receiving an optical beam that has encountered gas absorption on the optical path, which will be called IS(t), and on a detector receiving a signal that has not crossed the gas, which will be called 1#). As the optical frequency is also a function of time, we can writeI,[v(t)] andl,[v(t)] without loss of generality. The transmission of the gas cell is given by T(v) and taking 1,[~(t)] as a reference, we can write

4[V(ol=~[4w~~(~)l 4[4f)l =rI41 where a is an unknown factor of proportionality between ZJv(i)] and 1,[+)] in the absence of gas, including all intensity differences between both beams (window transmittance, reflectance-transmittance ratio of beam splitter, amplification gain, etc.). t can e written according to the Beer-Lambert 1,:: )I b

wherep[v(t)] is the absorption due to the gas, k(v) the absorption coefficient per molecule, n the density of molecules of gas and 1 the optical path length. For gas detection, the most interesting region is the low absorption one. In the limit of low absorption, the above expression can be approximated as

I

IO)

LO

L

GAS CELL

The d.c. and a.c. parts of this signal are then extracted, which can be done easily with low-pass and high-pass filters: d.c. =a[1 -p] a.c.(l)=DNl(t)-d.c.=ak-p(t)] where p is the average value of p(t). The a.c. signal is then divided by the d.c. signal in a second divider stage to cancel the factor a, which contains the differences of static absorption between both optical paths, to give On/z(t) = B-PM -

zz~-~(t) with P-C 1

1-j

or as a function of gas concentration DNZ(t) e [tb k(t)]nl The output of the second divider is thus proportional to the absolute optical absorption, to the gas concentration and to the optical path length. This output is independent of the laser emitter energy fluctuations and of all spectrally flat attenuations over the optical path (such as dust particle screening or window transmission variations). In this respect, this signal processing is thus auto-calibrated. In the case where k(t) is the absorption by a gas line, the signal output is proportional to the line shape -k(t), shifted by its average value, i. It is therefore possible for a given concentration to display in real time the line shape from DXi!(t). detection

_++~+,z/+$ PO

respectively. The signal processing (Fig. 2) then consists in a first divider stage where I,[ z@)]is divided by I,[ I] to suppress the laser modulation. The output of this first divider stage is thus

L

division

filtering

division

PD

output

Laser package

DIV2 -

D AM

b(l) I

flit*, !

Fig. 1. Schematic diagram of the experimental apparatus: LD, laser diode; L, lens; PD, photodiode; AM, signal amplifier; AC and DC, high- and low-pass filters, respectively.

amplification

Fig. 2. Architecture

of analog signal processing.

J.P. Sihwira, F. Grasdepot I Sensors and Actuators B 24-25 (1995) 603-606

If the gas concentration, n, is desired, as is required in a calibration set-up for gas sensors, further signal processing is needed on DIVZ(t) to remove its time dependence. As DIE(?) is a time-dependent function with a null average value, it is possible to use rectifying and filtering, r.m.s value calculation or clamping and filtering. All these additional treatments give a set-up output of the form R(n) = ml where different sensitivity values, s, are obtained from the treatment of [k-k(t)]. As the area under the absorption line is related to the line strength S [7], it is advantageous to use the solution of clamping the signal over zero value and filtering to have an estimation of S through s. Anyway it is also advantageous to use the r.m.s. value calculation or the solution of rectifying and filtering, since these operations can be performed by an ordinary voltmeter fed with DZKl(t), to give as well a linear output in gas concentration. The slope factor, s, is in both cases only dependent on experimental parameters. provided these parameters are stable, long-term stability and linearity in gas concentration of the set-up output are ensured. When k( v)nl > 0.02, a straightforward calculation gives for the DICZ(t) output the value Dm(t)=

y

605

The laser-diode was characterized using a monochromator of 0.3 m focal length. The spectra are broadened by the instrument resolution. Emission spectra of the laser were obtained as a function of temperature and current. Fig. 3 shows the emission spectrum at 25 “C for laser-current variation between 15 and 65 mA. The changes in wavelength with current and temperature were obtained from these measurements. They are 0.005 nm m&-l and 0.076 nm ‘C-l. The R9 line of the v2+2v, combination band of methane was used, as it was close enough to the central wavelength of the laser. The chosen absorption line is a feature made of at least four lines that was studied with our laser diode by varying the current (Fig. 4). The set-up is shown in Fig. 1. By tuning the temperature, the laser wavelength is positioned over the absorption line selected and modulated by the laser current. The r.m.s. value of the DWZ(t) output is measured by an ordinary voltmeter. The slope, s, of the output signal is generally a function of experimental parameters, as are the wavelength modulation and mean wavelength of the laser. Nevertheless, when the wavelength modulation of the laser is far superior to the spectral width of the gaseous absorption line of interest, a very low dependence of s on these parameters is obtained. In this mode of

-15mA - - -25 mA ----.35 mA . . . ..45mA

-1

which is related to the transmission T encountered on the optical path. This output signal is no longer linearly dependent on gas concentration. Nevertheless, it is still possible to use this non-linear output as a measurement of gas concentration with additional calibration.

___.__ 65 “,A

0 1312.2

1312.4

1312.6

1312.8

Wavelength (nm)

3. Experimental We applied this new signal processing method to methane detection and built a portable prototype having an optical path length of 30 cm and fitting in a 19” rack. In the near-infrared region, there are many overtones and band combinations of this gas. One of them, the combination band +_+2r+, is around 1.3 pm. Chand et al. [S] and more recently Scott et al. [9] have characterized this band and assigned the main transitions. We used a commercial InGaAsP distributed-feedback (DFB) laser-diode source from Alcatel CIT, packaged with a Peltier element to control the chip temperature. The wavelength of this laser is around 1312 mu. The temperature and current are controlled by a custom laser-diode controller system.

Fig. 3. Laser emission spectra at 25 “C for different injection currents. The spectra are broadened by the instrument resolution.

1 ,

25

35

1

Injection current (mA) Fig. 4. High-resolution protile of the 1312 run absorption line of methane at 1 atm and room temperature, recorded with a DFB laser from Alcatel (rewlution=0.O6 cm-‘).

J.P. Silveim, F. Grasdepot I Sensors and Actuators B 24-25 (19951 603dw

606

0

20

40

60

80

100

METHANE CONCENTRATION (%)

Fig. 5. Response curve of gas-sensor prototype with 30 cm optical path length for methane concentrations from 0 to 100% at 1 atm and mom temperature.

operation, neither temperature stabilization nor current stabilization of the laser wavelength need be accurate. This favours the industrialization of a gas sensor based on this principle. In the present case, the width of the set of at least four lines that forms R9 was too large to operate our laser in the above-mentioned mode. The calibration curve of the prototype was made by maximizing the output signal to achieve the maximum sensitivity and resolution in gas concentration. Operating with 100% CH,, the maximum of s was found when scanning approximately one half of the line. Tbeoretically, this would be obtained by calculating the maximum r.m.s. value of

calibrated signal processing method switch for gasconcentration measurement with a wavelength-modulated laser diode. The output of the sensor is directly related to the absorption line shape. In the low concentration limit, the measurement of this signal with a voltmeter shows a linear dependence on the gas concentration. To realize an industrial prototype of an optical gas sensor, we used the absorption of the methane R9 branch at 1.312 pm and a DFB laser diode as a source. The response curve of this calibration set-up for methane concentrations form 0.2-100% was measured with 0.2% resolution.

Acknowledgements We are grateful to Alcatel CIT (Optronics Division Centre de ViIlarceaux) for supplying the DFB lasers, to A. Bazin for his help with the electronics problems and to F. Stoeckel for his helpful discussions. J.P. Silveira was supported by the Spanish Education and Science Ministry. This work was supported by No. 6374 M3gas ESPRIT program of the EEC.

References 111 J. Reid and D. Labrie, Second-harmonicdetection with tunable

DM@)=

y

-1

for the laser used and the structure of the R9 line. Under the experimental conditions T,,,,,= 24 “C, I ,_= 45 mA, Lv,,,,=S mA r.m.s. for the laser and P= 10’ Pa, T-20 “C!, we obtained a response curve for methane in the range 0.2-100%. The noise level obtained was equivalent to 0.2% CH,. The mixture was made by an Air Liquide mixing system from N, (C or N45) and CH, (N30). We used a cell 0.3 m in length. The signal output as a function of gas concentration is shown in Fig. 5. The saturation of signal at high concentrations comes from the exponential form of the Beer-Lambert law for a 0.3 m optical path in CH.+ In order to improve the lower detection limit, the use of lasers’gt 1.6 ,um and 2.3 pm has to be considered. In any case, the signal processing described above applies as well at these wavelengths. 4. Conclusion We have presented a new CH, optical sensor using a 1.31 pm DFB laser. This sensor uses a new auto-

diode lasers - comparison of experiment and theory, Appl. Phys., 826 (1981) 203-210. I21 N. Goldstein and S.M. AIdler-Golden, Long-atmospheric-path measurements of near-visible absorption lines of 4 isotopes and Hz0 with a prototype AlGaAs laser transceiver system, AppL Opt., 32 (1993) 5849-5855. 131 K. Uehara and T. Tai, Remote detection of methane with a 1.66 pm diode laser, Appl. Opt., 31 (1992) W-814. I41 T. Chen, Wavelength-modulated optical gas sensors, Sensors and Actuators B, 13-14 (1993) 284-287. I51 M.P. Arroyo and R.K. Hanson, Absorption measurement of water-vapor concentration, temperature, and line-shape parameters using a tunable InGaAaP diode laser, AppZ. Opt. 32 (1993) 6104-6116. 161 C.W. Pattenon, in L.J. Radziemski, R.W. Solarz and J.A. Paisner (eds.), Laser Spectrvscopyand ILFAppIicationr, Optical Engineerirg, Vol. 11, Marcel Dekker, New York, 1987, Ch. 1, pp. l-90. r71 L.A. Gross, P.R. Griffiths and J.N.-P. Sun, in J. Wormhoudt (ed.), Infrared Methodr for Gaseous Measurem enlr, Optical Engineering, Vol. 7, Marcel Dekker, New York, 1985, Ch. 3, pp. 81-138. 181 K. Chand, H. Ito and H. Inaba, Absorption measurement of vz+ 2u3 band of CH, at 1.33 gm using an InGaAsP light emitting diode, Apple Opt., 22 (1983) 3802-3804. 191 J.C. Scott, R.A.M. Maddever and A.T. Paton, Spectroscopy of methane using a Nd:YAG laser at 1.34 Mm, Apple Opt., 31 (1992) 815-821.