A field-portable, laser-diode spectrometer for the ultra ...

journal of modern optics, 2002, vol. 49, no. 5/6, 769 ±776

A ®eld-portable, laser-diode spectrometer for the ultra-sensitive detection of hydrocarbon gases GRAHAM GIBSON, STEPHEN D. MONK and MILES PADGETT Department of Physics and Astronomy, Kelvin Building, University of Glasgow, Glasgow G12 8QQ, UK (Received 5 September 2001 ) Abstract. We have developed a ®eld-portable optical gas sensor for the ultra-sensitive detection of ethane. The system is based on an adaptation of a commercially available system, which uses a cryogenically cooled, lead-salt laser diode at 3.34 mm and a multi-pass astigmatic Herriott sample cell. We have adapted this system to a second derivative wavelength modulation scheme giving a lower detection limit of less than 100 parts per trillion for a one second measurement time. Our custom-designed software controls every aspect of the instrument operation from spectral scanning of the laser diode, to automatic calibration, optical alignment, spectral analysis and complete data logging.

1.

Introduction Spectroscopy using tunable laser diodes is widely used as a technique for the detection of trace atmospheric gases. Although requiring cryogenic cooling, leadsalt laser diodes are a well-behaved optical source of narrow linewidth radiation in the mid-infrared spectral region allowing the detection of a wide variety of gases such as methane, ethane, carbon dioxide, carbon monoxide and nitrous oxides. Our goal has been to make an ultra-sensitive, ®eld-portable ethane detector. Given that the detection of ethane at low concentration is the prime motivation of this work, then a number of potential approaches exist, for example in situ gas chromatography [1], photo acoustic spectrometry [2], mass spectrometry [3], tunable diode laser absorption spectroscopy [4] and cavity leak-out spectroscopy [5]. Of these possibilities we selected a spectroscopic approach based on laser diodes primarily because this technique has a proven track record as the basis for rugged ®eld portable instruments and such systems are commercially available. As the basis of our own instrument we purchased a complete instrument from Aerodyne, who themselves have produced a number of instruments for a variety of applications including high-precision methane monitoring [6], nitric acid monitoring [6], the remote sensing of on-road vehicle emissions [7] and multiple component analysis of cigarette combustion gases [8]. The standard con®guration favoured by Aerodyne is to use rapid scanning of the laser diode over the wavelength region of interest acquiring a transmission spectrum to which various ®tting techniques can be applied to give the concentration of various gases. Such an approach is well suited to complex spectra where the concentration of a number Journal of Modern Optics ISSN 0950±0340 print/ISSN 1362±3044 online # 2002 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/09500340110108639

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of diÄerent gases may be simultaneously required to high accuracy. In our application, rather than precision, it is the minimum detectable ethane concentration that is of prime importance and hence we are willing to compromise precision for absolute sensitivity. Although the design is highly re®ned, the operation of such instruments has been highly specialized, typically requiring the devotion and intervention of highly quali®ed personnel. A major objective of our work had been to modify the instrument so that all aspects of its operation can be placed under the control of a single laptop computer. This makes it possible for the system to be run by a non-specialist and for it to self-calibrate, monitor its own alignment and data log for many hours without intervention.

2.

The optical and gas-handling systems The optical layout is basically unchanged from that of a standard Aerodyne instrument. A lead-salt laser diode (Laser Components GmbH) mounted within a liquid nitrogen dewar, is driven by a controller (MuÈtek TLS100). The general wavelength of the operation is selected by setting the operation temperature of the laser diode and wavelength tuning over approximately one wavenumber achieved by direct control of the laser drive current. The laser output is collected using a 15£ .4NA re¯ective microscope objective that focuses the beam to a relocatable alignment pinhole, positioned in the back focal plane. A subsequent beam splitter divides the beam between a reference channel containing a 10 mm long ethane cell and the signal channel, which is based on an astigmatic Herriott cell [9] with an eÄective path length of 208 m. Both the reference and signal channels are then focused using 200 mm radius of curvature mirrors onto their respective detectors mounted within the same nitrogen dewar as the laser source. An important modi®cation that we have incorporated are three piezo-electrically actuated micro-positioning stages, which enable the alignment of the microscope objective to be precisely controlled and a piezo-actuated mirror mount controlling the ®nal mirror which couples the light both into and out of the sample cell. Both these actuator types are of a novel design (New Focus Picomotors) whereby when powered oÄ they remain in their existing position and possess stability comparable to standard, high-quality, mirror mounts. Air is drawn into the Herriott cell via two in-line PTFE dust ®lters and the cell pressure is maintained at 30 torr using an oil-free, scroll pump (BOC Edwards XDS10C). Sampling is continuous, the pump speed and system volume resulting in a time constant for the gas mix in the cell of approximately 2 s. A toggle valve on the inlet allows the input to be remotely switched from atmosphere to a clean nitrogen source enabling an accurate zero reference to be maintained.

3.

The spectroscopic technique As is well known, second derivative modulation spectroscopy oÄers a number of advantages over straight absorption techniques [10]. Speci®cally, providing that the modulation depth is set correctly, at a frequency corresponding to twice that of the modulation, the detected signal is zero in the absence of any absorption and maximum at line centre. A Taylor expansion of the resulting signal shows that the recorded trace approximates to the second derivative of the absorption spectrum. This lack of any background spectrum makes this approach particularly appealing

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if the instrument is to be used routinely near its maximum sensitivity. Two potential drawbacks of the second derivative technique are: ®rst, that the observed line shape depends critically upon the ratio between modulation depth and the width of the spectral feature, which itself depends upon sample pressure; and second, that there is no direct measure of the laser power. Both of these mean that the absolute accuracy of the instrument can be diÅcult to maintain, a signi®cant problem if the desired information is a small change in a large background concentration. As mentioned earlier, scanning the laser wavelength over the spectral region of interest is achieved by ramping the drive current. Typically, we scan the laser over a 0.4 wavenumber spectral range once per second. At a sample cell pressure of 30 torr the Doppler and pressure broadening contributions to the ethane transitions are approximately equal in magnitude and the resulting Voigt pro®le is approximately 0.04 wavenumbers wide. Superimposed on the ramp current is an 8088 Hz modulation. The resulting signal from the sample cell detector is demodulated using a high performance digital phase sensitive detector (PSD) (Stanford Research SR830), operating in its 2f mode with an output time constant of 3 ms. The reference channel signal is much larger in size and a simple, single board, analogue PSD is suÅcient to recover the corresponding second derivative spectrum of the reference cell.

4.

The computer control As discussed, a key objective of this work was to simplify the operation of the complete instrument. Working within the LabView programming environment, we have endeavoured to bring all aspects of the instrument operation under direct

Figure 1.

Typical desktop display.

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computer control. Figure 1 shows a typical desktop display from the laptop whilst the program is running; all aspects of the software are controlled using a track ball. The laser diode controller, PSD and position controller are all interfaced using the IEEE bus allowing complete control of their settings and some degree of data transfer. The PSD itself has a number of digital and analogue, input and output channels, which allow control of the inlet selector valve and reading of the pressure gauge, again via the IEEE bus. In addition to a PC card IEEE controller, the laptop has a PC card DAC. Two analogue input channels are used to read the signal and reference PSD analogue outputs and two further input channels are used to monitor the output from the signal and reference channels directly. The ramp current, which comprises 128 steps, is calculated by the software and output on an analogue channel where it is summed with the modulation obtained directly from the high-quality sine-wave generator within the PSD. The data acquisition from the PSD is synchronized to the steps in the ramp current and therefore also comprises 128 data points per scan. Alternatively, the scan can be suspended and a square wave modulation applied to the diode to switch it below and above threshold. The resulting signal from the signal and reference detectors is a measure of the detected power and hence of the system alignment. At start-up, after ®lling the dewar with liquid nitrogen, the software monitors the diode temperature only switching the diode on as the target temperature is approached. The system is then set to scan over one wavenumber and a simple convolution algorithm is applied identifying the location of the main ethane transition from the reference channel. The base current of the laser diode is continuously adjusted to maintain this ethane transition at 2990 cm ¡1 in the centre of the scan. Once the temperature is deemed stable, the software switches into measurement mode, the power level of both sample and reference channel is measured, the sample cell is ®lled with nitrogen and a scan of the background spectrum recorded. The inlet valve is then switched back to air and scanning is resumed. After each scan, the background scan is subtracted and a least-squares ®tting algorithm is used to calculate the gas concentration of ethane and methane, the transition of which falls 0.1 wavenumbers to one side. After a set period of time, typically 5 min, the power levels are reassessed, the sample cell is ®lled with nitrogen and the background spectrum is re-acquired. The whole re-calibration takes less than 10 s. The pure nitrogen can be drawn either from a suitable gas cylinder or from the gas boil-oÄ from our liquid nitrogen reservoir. In addition to monitoring the optical alignment, the software has an option to make adjustments to the alignment to optimize the signal levels. A simple iterative algorithm can be applied to the translation of the microscope objective and the angle of the ®nal beam-steering mirror to optimize the reference and signal channels respectively. Although steps are taken to thermally control and isolate the system from its environment, some degree of mechanical instability is inevitable. Speci®cally, the background spectrum we record with a nitrogen-®lled sample cell consists of fringes from the residual re¯ection between the diode facet and the microscope objective (see ®gure 2). It is the movement of these fringes that represents the limit on the time that the instrument can be run without re-calibration.


Figure 2.

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Typical background spectrum showing fringes. The size of the fringes correspond to π 2 ppb ethane.

Instrument performance The basic Aerodyne instrument complete with all our modi®cations ®ts within a single, thermally isolated ¯ight box, which can be carried in the back of any suitable van. The only components out of this box are the control laptop, the vacuum pump, the nitrogen reference cylinder (if required) and the liquid nitrogen reservoir. The complete system can be run from a suitably isolated 2 kW diesel generator or from the vehicle generator, as available. The mechanical stability coupled with the ease of optimization of the optical alignment means that the whole system is readily transportable, typically running within 20 min after arrival at a new site. Figure 3 shows a typical scan, with background subtracted, including the best ®t in which the 2f signal arising from both the ethane and methane can be readily distinguished from the background. Although a new scan is acquired every second, we are typically interested in the underlying concentration and we average our data into 1 min bins. Conveniently, this allows us regularly to re-calibrate the instrument without signi®cant loss of data. Figure 4 shows the second-by-second gas concentration as measured by our instrument as the inlet to the sample cell is repeatedly switched between air and pure nitrogen; the gaps in the trace correspond to the instrument entering its re-calibration mode. From the ®gure we see that the 1 s measurement for ethane detection is typically 100 ppt. When averaged over 1 min this performance improves to 50 ppt. For each minute, the software calculates the average and standard deviation of the gas concentration; additionally the software can invalidate the data if the

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Figure 3.

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Recorded and ®tted trace obtained by sampling air from outside the laboratory.

received power level, temperature stability of the diode or quality of the ®t falls outside a prede®ned range. All this data can be output via the Comm port for communication to a data logger where it can be combined with time, wind data or other information. To demonstrate the utility of the system, ®gure 5 shows data taken over a 3-day period at our laboratory in Glasgow, Scotland. The validated 1-min gas averages are plotted against the corresponding wind direction, as measured using an ultrasonic anemometer. Another potential application for our instrument is the real-time monitoring of ethane in human breath which is considered to be a marker of cell damage in the

Figure 4.

Gas concentration measured on a 1 s update interval.


Figure 5.

Figure 6.

1 min gas concentration averages plotted against wind direction.

Ethane concentrations, measured over 50 s, from 4 volunteers.

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human body. Dahnke et al. [11] reported the real-time monitoring of ethane in human breath, using mid-infrared cavity leak-out spectroscopy [11], and demonstrated the decaying ethane fraction in exhaled breath after smoking a cigarette. To demonstrate the suitability of our instrument in the real-time monitoring of ethane in breath, we have recorded the ethane concentrations in breath samples taken from our four volunteers, two of which are regular smokers. Figure 6 shows the ethane concentrations recorded when volunteers blow down a sample tube, connected directly to the instrument, for a period of approximately 15 s.

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