REAL-TIME TRACE GAS SENSOR USING A MULTI-MODE DIODE LASER AND MULTIPLE LINE INTEGRATED CAVITY ENHANCED ABSORPTION SPECTROSCOPY

Abstract

A highly sensitive trace gas sensor based on a Fabry-Perot semiconductor laser and cavity enhanced absorption spectroscopy is designed to be capable of measuring sub-ppb concentrations of trace gases in real time. The broad frequency range of the multi-mode Fabry-Perot semiconductor laser spans a large number of absorption lines of the species of interest enabling multiple line integrated absorption spectroscopy which improves the sensitivity of detection. Additionally, the broad wavelength range of the laser excites a large number of cavity modes simultaneously, thereby reducing the sensor's susceptibility to vibration and thermal fluctuations making it suitable for field based monitoring applications. Using a high finesse optical cavity also enhances the sensitivity of the sensor by providing large path lengths, on the order of kilometers, in a small volume. Relatively high laser power is used to compensate for the low coupling efficiency of a broad linewidth laser to the optical cavity.

Claims

1. A method for detecting trace gases in a gas sample using cavity enhanced absorption spectroscopy, comprising the steps of: generating a continuous multi-mode laser beam with a Fabry-Perot semiconductor laser; passing said laser beam into a high finesse optical cavity cell in which the sample gas is located, whereby the laser beam bounces back and forth in the cavity cell a number of times and exits the cavity cell; and detecting integrated absorption in the laser beam exiting the cavity cell due to rovibronic and/or rovibrational transitions of the molecular species as it interacts with the laser beam bouncing in the cavity.

2. The method of claim 1 wherein the laser beam is of relatively high power to compensate for low cavity throughput.

3. The method of claim 1 wherein the broadband multi-mode laser beam excites a large number of cavity modes, thereby making the apparatus insensitive to vibration.

4. Apparatus for detecting trace gas species in a gas sample using cavity enhanced absorption spectroscopy, comprising: a multi-mode semiconductor laser source that provides a continuous laser beam with a broad frequency bandwidth; a high finesse optical cavity cell in which the sample gas is located, said cell having high reflectivity mirrors at the wavelength corresponding to the absorption features of the trace species; and a detector for detecting the laser beam after it exits the cell

5. The apparatus of claim 4 wherein the laser is a Fabry-Perot semiconductor laser.

6. The apparatus of claim 4 wherein the cell has an entrance through which the sample gas enters the cell at one end, and an exit from which the sample gas exits at the other end.

7. The apparatus of claim 4 wherein the laser beam is of relatively high power.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of an illustrative embodiment of the invention in which:

[0022] FIG. 1 is a schematic diagram of apparatus for carrying out the demonstration of the present invention;

[0023] FIG. 2 illustrates the spectrum of a multi-mode semiconductor laser beam used in an embodiment of the present invention;

[0024] FIG. 3 shows the NO.sub.2 absorption at 298.5 K and atmospheric pressure over the laser's wavelength range;

[0025] FIG. 4 is a plot of a CEAS absorption signal vs concentration for a 5 second integration time;

[0026] FIG. 5A illustrates a CEAS signal recorded for a period of 10 minutes using an integration time of 20 sec with Zero Air flowing through the cell at 1 liter/min, FIG. 5B uses an integration time of 5 sec, and FIG. 5C uses an integration time of 50 ms;

[0027] FIG. 6 is a log-log plot of standard deviation of a CEAS signal vs sample averaging time; and

[0028] FIG. 7 is a graph of deviation from a baseline signal during a long term stability test.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT OF THE INVENTION

[0029] A new trace gas detection technique and its applications are discussed herein. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details.

[0030] The present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated by the figures or description below. More specifically, some of the details provided below include a demonstration of the invention to detect NO.sub.2. The details specific to NO.sub.2 detection (for example the use of a multi-mode diode laser emitting near 405 nm), pertain to this demonstration and are not intended to limit the invention to this specific laser, wavelength or molecular species.

[0031] FIG. 1 shows apparatus as configured for demonstrating Cavity Enhanced Absorption Spectroscopy (CEAS) using a multi-mode diode laser by measuring trace concentrations of NO.sub.2. The apparatus includes a diode laser 12 whose operation is directed by a computer control and data acquisition system 10. The beam from laser 12 passes through optics which include a polarizing beam splitter 11 and a quarter wave plate 13 that provide optical isolation from the back reflection of the optical cavity. The optical elements also include an anamorphic prism 14 that is used to shape the asymmetric diode laser beam.

[0032] The beam from the prism 14 is directed by mirrors so it enters a High Finesse Optical Cavity 15. In the cavity it encounters the sample gas which flows through the cavity from a gas sample input 17 to a gas sample output 19. The optical output of the cavity is reflected by a mirror through focusing optics 18 to a detector 16. Detector 16 converts the optical signal into an electrical signal that is input to the data acquisition portion of computer 10.

[0033] There are two main factors that needed to be considered for the selection of a spectral region for investigation: 1) A region with strong absorption lines; and 2) A region free from interference due to other species in the atmosphere (especially water vapor and other gases). Some of the strongest NO.sub.2 rovibronic transitions are in the region accessible using 405 nm diode lasers. S. Voigt, J. Orphal and J. P. Burrows, “The temperature and pressure dependence of the absorption cross-sections of NO.sub.2 in the 250-800 nm region measured by Fourier-transform spectroscopy,” J. Photochem. Photobiol. A: Chem., vol. 149, pp. 1-7, 2002 (“Voigt”), which is incorporated herein by reference in its entirety. A review of the spectra of the main atmospheric components, shows that there are no interfering species within 5 nm on either side of the laser line at 405 nm. See L. S. Rothman et al., “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transfer, vol. 110, pp. 533-572, 2009 (“Rothman”); C. N. Mikhailenko, Y. L. Babikov and V. F. Golovko, “Information-calculating system Spectroscopy of Atmospheric Gases. The structure and main functions.,” Atmos. Oceanic Opt., vol. 18, pp. 685-695, 2005 (“Mikhailenko”); and NASA, “Atmosphere, Earth Fact Sheet—Terrestrial,” [Online]. Available at: http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html (“NASA”), all of which are incorporated herein by reference in their entirety.

[0034] In an embodiment of the apparatus of FIG. 1, the laser source is a high-power violet diode laser operated in CW mode with an injection current of 500 mA at a temperature of 25° C. A clean current source is used to drive the laser; a temperature controller is used to drive a thermo-electric cooler to maintain a stable, constant temperature (ΔT˜0.01° C.). The spectrum of the laser's multi-mode output is recorded using a monochromator, and contains approximately 50 modes in a Gaussian-like envelope centered at 407 nm (see FIG. 2). The mode distribution had a FWHM of approximately 1 nm and each individual mode had a FWHM of approximately 0.01 nm (˜15 GHz)

[0035] The experimental cell is a high-finesse optical cavity that is 50 cm long, and has mirrors with a reflectivity of ˜99.98% at 405 nm and a radius of curvature of 1 meter. It is important to note that the invention may use mirrors with other similar reflectivities and different radii of curvature (the ones used for the demonstration were selected in part due to the fact that they were commercially available at that time). The free spectral range (FSR) of the cavity was 300 MHz, and its resonance width was approximately 10 kHz.

[0036] Due to the cavity parameters described above and the broad frequency range of the laser source, a very small fraction of the incident laser light is coupled to the cavity. As discussed above, this is because the laser's power is spread across multiple modes, where each individual mode may be many GHz wide (in the case of the present embodiment, there are fifty modes, corresponding to a frequency range of approximately 1500 GHz, which is hundreds of FSR). The low coupling efficiency to the optical cavity is therefore primarily due to the low transmission of light at frequencies between the cavity resonances. To compensate for the low coupling efficiency, the invention employs a laser that emits at a relatively high output power (˜400 mW). Despite this high power, the manufacturer indicates that the laser is expected to have a long life (lifetime >10,000 hours), and it has been observed to have an output spectrum that is both repeatable and stable.

[0037] The cell or cavity 15 of FIG. 1 has input and output valves 17, 19 allowing test gas mixtures to flow through it at a constant rate. It is important to note that the choice of a silicon photodiode was due to its suitability for detecting light at the wavelength used in the demonstration using NO.sub.2. When a laser of significantly different wavelength is used in the invention to detect a different gas, a different low noise detector would be used. The detector 16 output is fed to a commercial data acquisition (DAQ) interface for analysis in a computer (e.g., a laptop) 10. The signal analysis is conducted using software created using LabView for Windows. To demonstrate real-time data acquisition, data was recorded using three different integration times: 20 s, 5 s and 50 ms.

[0038] The NO.sub.2 concentration is determined using Beer's Law, see Eq. (3) and Eq. (4). In doing so, I (v) is chosen to be the CEAS signal when only Zero Air is flowing through the cell at 1 liter/min. As a result, this signal contained loss and noise contributions from all components of the setup, and provided a baseline signal for the absorption measurements. The absorption cross-section could be treated as having a constant value of ˜5×10.sup.−19 cm.sup.2 over the laser's wavelength range for the following reasons: 1) The close spacing of the energy levels in NO.sub.2 and the large width of the absorption features at one atmosphere result in very broad, overlapping absorption features; (See FIG. 3); 2) Spectra recorded for the laser over the course of several hours using a monochromator shows no noticeable drift when compared with the broad absorption features; 3) The ambient (i.e., sample cell and gas) temperature is constant during runs of the equipment. A. C. Vandaele, C. Hermans, P. C. Simon, M. Carleer, R. Colins, S. Fally, M. F. Mérienne, A. Jenouvrier and B. Coquart, “Measurements of the NO.sub.2 absorption cross-sections from 42000 cm-1 to 10000 cm-1 (238-1000 nm) at 220 K and 294 K,” J. Quant. Spectrosc. Radiat. Transfer, vol. 59, pp. 171-184, 1998 (“Vandaele”), which is incorporated herein by reference in its entirety.

[0039] The apparatus of FIG. 1 was used to detect several concentrations of NO.sub.2 (25, 50, 75 and 100 ppb), using CEAS and a multi-mode diode laser. To demonstrate the real-time measurement capabilities, data sets were recorded using integration times of 50 milliseconds, 5 seconds and 20 seconds. The absorption signal [I(v)−I′(v)] was plotted as a function of known NO.sub.2 concentration. FIG. 4 shows a plot of the absorption signal vs. concentration for a 5 sec. integration time, as well as a weighted linear least-squares fit of this data. The horizontal error bars represent the uncertainty in the gas mixing apparatus (±3 ppb).

[0040] The instrument's sensitivity was calculated by determining the noise level in the CEAS signal. This was accomplished by flowing Zero Air through the cell at 1 liter/min, and recording data for 10 minutes (see FIG. 5A). The standard deviation of the CEAS signal with a 20 second integration time was found to be 0.0076%. The minimum detectable concentration (at the 1σ level) is found by dividing the voltage level of the standard deviation (0.68 mV) by the slope of the weighted linear least-squares fit of the data recorded from the NO.sub.2 concentrations (10.2 mV/ppb). The slope is used since it incorporates uncertainties from all aspects of the measurements (e.g., repeatability of the measurement with different concentrations of NO.sub.2). Using this data it was determined that the sensitivity of the apparatus using a 20 second integration time was approximately 65 ppt. Following the same procedure, the standard deviation for CEAS using a 5 sec. integration time was found to be 0.013% (1.12 mV), and the sensitivity was determined to be 110 ppt. Using a 50 ms integration time the standard deviation was found to be 0.079% (7.03 mV), and the sensitivity was determined to be 750 ppt. This result is comparable to both the sampling time and sensitivity achieved by Courtillot, using optical-feedback CEAS. The results in this demonstration, however, were obtained using a design that is significantly less complicated and less expensive than that of Courtillot.

[0041] To analyze the stability of the sensor, the signal was recorded using several different averaging settings (30, 20, 10, 5, 2, 1, 0.5, 0.2, 0.1, and 0.05 sec), with 1 liter per minute of Zero Air flowing through the cell. Data was recorded for ten minutes for each setting, and the standard deviation was calculated. A log-log plot of the standard deviation vs. avg. time (FIG. 6) shows that the optimal short-term sensitivity occurs with 20 second averaging.

[0042] The present invention thus is a highly sensitive, real-time trace gas sensor using a multi-mode semiconductor laser and MLIAS coupled with cavity enhanced absorption spectroscopy. The relatively broad frequency spread of this type of laser (on the order of 1500 GHz, or 1 nm) spans a large number of absorption lines, thereby removing the need for a tunable laser source. Its frequency spread, however, is still narrow enough to maintain the specificity necessary for trace gas detection without the need for a spectrometer. CEAS enhances the sensitivity of detection by providing a path length on the order of 1 km in a small-volume cell. The broad-band source excites a large number of cavity modes, thereby minimizing effects of vibration on the signal from the optical cavity. The use of MLIAS further enhances the sensor's sensitivity and is well suited for measurements at atmospheric pressure. Though the use of a relatively broadband source results in a low coupling efficiency of the laser source to the cavity, it is addressed simply by the use of a readily available, high power semiconductor laser.

[0043] The technique demonstrated via the construction of a sensor to detect trace quantities of NO.sub.2 in Zero Air, and sensitivities of 65 ppt, 110 ppt and 750 ppt were achieved using integration times of 20 sec., 5 sec., and 50 ms. These results are comparable to some of the most sensitive results reported. See the Fuchs and Courtillot articles as well as G. N. Rao and A. Karpf, “Extremely sensitive detection of NO2 employing off-axis integrated cavity output spectroscopy coupled with multiple-line integrated absorption spectroscopy,” Appl. Opt., vol. 50, pp. 1915-1924, 2011(“Rao 4”), which is incorporated herein by reference in its entirety. Nevertheless, the present invention makes use of a design that is simpler and significantly less expensive than other reported devices. Although the illustrated embodiment uses a 407 nm multi-mode diode laser and NO.sub.2, the invention could be carried out using different Fabry-Perot diode lasers or Fabry-Perot quantum cascade lasers to detect other species.

[0044] While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.