ULTRA-SENSITIVE, REAL-TIME TRACE GAS DETECTION USING A HIGH-POWER, MULTI-MODE SEMICONDUCTOR LASER AND CAVITY RINGDOWN SPECTROSCOPY

Abstract

A highly sensitive trace gas sensor based on Cavity Ring-down Spectroscopy (CRDS) makes use of a high power, multi-mode Fabry-Perot (FP) semiconductor laser with a broad wavelength range to excite a large number of cavity modes and multiple molecular transitions, thereby reducing the detector's susceptibility to vibration and making it well suited for field deployment. The laser beam is aligned on-axis to the cavity, improving the signal-noise-ratio while maintaining its vibration insensitivity. The use of a FP semiconductor laser has the added advantages of being inexpensive, compact and insensitive to vibration. The technique is demonstrated using a laser with an output power of at least 200 mW, preferably over 1.0 Watt, (=400 nm) to measure low concentrations of Nitrogen Dioxide (NO.sub.2) in zero air. For single-shot detection, 530 ppt sensitivity is demonstrated with a measurement time of 60 s which allows for sensitive measurements with high temporal resolution.

Claims

1. A method for detecting trace gases in a gas sample using cavity ringdown spectroscopy, comprising the steps of: generating a laser beam with a high power, multimode Fabry-Perot semiconductor laser; passing said laser beam on axis into a high finesse optical cavity cell in which the sample gas is located; terminating the beam; detecting the decay of the light exiting the cavity upon termination of the laser beam; and using the time constant of the decay to determine a concentration of a target molecular species in the gas sample.

2. The method of claim 1 wherein the wavelength range of the diode laser is about 0.6 nm.

3. The method of claim 1 wherein the power of the diode laser is greater than about 200 mW.

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

5. The method of claim 4 wherein the molecular species is NO2, the high power FP semiconductor laser power is greater than 1 W, the wavelength of the laser is about 400 nm and about 512 decays are averaged.

6. Apparatus for detecting trace gas species in a gas sample using cavity ringdown spectroscopy, comprising: a high power, multimode Fabry-Perot semiconductor laser system with a broad wavelength range generates at least one laser beam pulse; a high finesse optical cavity cell in which the sample gas is located; a reflector for directing the laser beam on axis into the cavity cell; a photodetector for detecting the decay of the light pulse exiting the cavity upon termination of each pulse; and a processor for using the time constant of the decay to determine a concentration of a target molecular species in the gas sample.

7. 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.

8. The apparatus of claim 4 wherein a narrow bandpass filter is used to block fluorescence originating from the interaction of the high-power laser light and the glass substrates of the optical elements (e.g., the cavity ringdown mirrors), from interfering with the transmitted signal from the high finesse optical cavity, as necessary.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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:

[0026] FIG. 1 is a schematic diagram of an exemplary embodiment of apparatus for carrying out the demonstration of the present invention;

[0027] FIG. 2 shows the emission spectrum of the multi-mode Ushio model HL40033G diode laser and the NO.sub.2 absorption spectrum;

[0028] FIG. 3 illustrates Cavity Ringdown decays recorded for Zero Air and two different concentrations of NO.sub.2;

[0029] FIG. 4 shows a plot of [(1/)(1/.sub.0)] vs. measured concentration of NO.sub.2 in Zero Air;

[0030] FIG. 5A shows the Cavity ringdown times recorded with Zero Air flowing through the cell at 0.5 liter/min with a single shot, FIG. 5B shows ringdown times with 32 decays averaged and FIG. 5C shows ringdown times with 512 decays averaged; and

[0031] FIG. 6 is a log-log plot of standard deviation of CRDS signal vs Number of averages.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT OF THE INVENTION

[0032] 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.

[0033] The present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the invention to the specific embodiments illustrated by the figures or description below. More specifically, some of the details provided below include the 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 the embodiment described and are not intended to limit the invention to this specific laser, wavelength, molecular species or any other particulars of the embodiment. The invention may be implemented to detect other molecular species using a FP semiconductor laser emitting at the appropriate wavelength (e.g., FP diode lasers or FP quantum cascade lasers provide access to large regions in the visible, near-infrared and mid-infrared, allowing one to detect a large number of different trace gases). Instead, the invention is intended to be limited only by the appended claims.

[0034] According to the present invention, trace concentrations of a gas, e.g., NO.sub.2, are measured by cavity ringdown spectroscopy (CRDS) using a high power Fabry-Perot (FP) diode laser, i.e., 200 mW and above. There were two main factors that needed to be considered for the selection of a wavelength region for this work: 1) Select a region with strong absorption lines; and 2) Select a region that is free from interference due to other species (especially water vapor). Some of the strongest NO.sub.2 rovibronic transitions are in the region accessible using 405 nm diode lasers See Voigt. A review of the spectra of the main atmospheric components 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); NASA, Atmosphere, Earth Fact SheetTerrestrial, [Online at: http://nssdc.gsfc.nasa.gov/planetary/factsheetearthfact.html (NASA) which are incorporated herein by reference in their entirety, show that there are no interfering species (including water vapor) within several nm on either side of the laser line (405 nm). Thus despite the relatively broad linewidth of the laser (.sub.laser0.6 nm), with some care in choosing an appropriate spectral window a high sensitivity sensor for the molecular species can be realized with minimal interferences from other trace species and water vapor.

[0035] FIG. 1 shows an embodiment of apparatus configured for demonstrating CRDS using a high power, multi-mode diode laser as a means for measuring trace concentrations of NO.sub.2. The apparatus includes a diode laser 11 whose operation is directed by a computer control and data acquisition system 10. The beam from laser 11 passes through optics, which include a polarizing beam splitter 12 and a quarter wave plate 13 that provide optical isolation from the back reflection of the optical cavity. Useful in practicing the present invention is an Ushio model HL40033G multi-mode diode laser. Its light output is on the order of 1 W, its wavelength range is 0.6 nm and it has approximately 50 modes (each mode's width is much larger than the cavity's FSR).

[0036] The optical system also includes an anamorphic prism 14 that is used to shape the asymmetric diode laser beam. The beam from the prism 14 is directed by mirrors 15 so it enters a High Finesse Optical Cavity 16 on axis. In the cavity it encounters the sample gas which flows through the cavity from an input 17 to an output 19. The output of the cavity is reflected by a mirror 20 through focusing optics (lens 21, filter 22) to a detector 24. Detector 24 converts the optical signal into an electrical signal that is input to the data acquisition portion of computer 10.

[0037] Using the apparatus of FIG. 1 CRDS was conducted on several NO.sub.2 concentrations (20, 40, 60, and 80 ppb) fed through the cell at 0.5 liter/min. The Cavity Ring-Down Cell was constructed using components and mirrors purchased from CRD-Optics, Inc. The Cavity Ring-Down cell is 50 cm long. The mirrors have a radius of curvature of 6 meters, and a reflectivity of 99.97% at 400 nm.

[0038] The diode laser 11 of FIG. 1 was operated in pulsed mode at a frequency of 4 kHz using a Newport LDP-3840B pulse driver. The duty cycle was 10%, resulting in a pulse duration of 25 s. The laser pulse width was chosen such that about four cavity ring-down times are covered. The laser pulse rise and fall times are approximately 50 ns. The diode laser's modes were contained in a Gaussian-like envelope centered at 399.8 nm with a FWHM of approximately 0.6 nm. The close spacing of the energy levels in NO.sub.2, and the large width of the absorption features at 1 atmosphere resulted in very broad absorption features. See the Karpf 2 article. As a result, the absorption features did not vary significantly over the wavelength range of the laser. The effective absorption coefficient was calculated to be .sub.eff6.410.sup.19 cm.sup.2 by taking a weighted average of the absorption cross-section across the laser profile.

[0039] FIG. 2 displays the multi-mode diode laser spectrum as well as the absorption spectrum of NO.sub.2 in the region of interest. The injection current for the laser was 900 mA, the temperature was 25 C., and the spectrum was recorded using a SPEX 1000M monochromator. It should be noted that this spectrum was recorded over several seconds and thus was comprised of many thousands of laser pulses. Small deviations in the mode structure in each pulse washed out the mode structure seen in the figure, resulting in the relatively smooth spectrum seen in FIG. 2. Previous spectra were recorded using a similar model Ushio laser in cw-mode and that spectra exhibited a well-defined mode structure. See, A. Karpf and G. N. Rao, Real-time trace gas sensor using a multimode diode laser and multiple-line integrated cavity enhanced absorption spectroscopy, Appl. Opt., vol. 54, pp. 6085-6092, 2015 (Karpf 2), which is incorporated herein by reference in its entirety. The NO.sub.2 absorption spectrum shown in FIG. 2 is for room temperature (298.5 K) and at atmospheric pressure over the laser's wavelength range. See, 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. At atmospheric pressure, the dense ro-vibronic spectrum of NO.sub.2 results in very broad, overlapping absorption features.

[0040] In FIG. 1, light exiting the cavity 16 was focused on the detector 24 using a large diameter, short focal length lens 21. The ring-down decays were detected using an avalanche photodiode (Advanced Photonix model SD 197-70-74-661) as the detector. Its output was fed to a Tektronix DP03034 digitizing oscilloscope with a 300 MHz bandwidth and 2.5 GS/s sample rate used as part of the computer control and data acquisition system 10. Averaging of multiple decays was accomplished using the oscilloscope's onboard processing circuits. The oscilloscope output was fed to a personal computer (PC), which was also part of system 10 via USB connection. Additional averaging (when necessary) as well as curve fitting were done using a virtual instrument programmed using the LabView program.

[0041] The high finesse optical cell or cavity 16 had input and output valves 17, 19 allowing test gas mixtures to flow through the cavity at a constant rate. Mixtures of 20, 40, 60 and 80 ppb of NO.sub.2 were passed through the cell at 0.5 liter/min for the test of the embodiment. The gas mixtures were prepared by diluting a pre-calibrated 1 ppm mixture of NO.sub.2 in Zero Air (a mix of 20.9% 02 and 79.1% N.sub.2) with additional Zero Air.

[0042] A brief demonstration of the apparatus' reduced sensitivity to vibration was tested by mounting an unbalanced electrical motor to the breadboard immediately next to the cell. The motor rotated an off-center wheel that introduced relatively low frequency (<100 Hz) vibrations into the apparatus. The vibrations could easily be felt when touching the cell. The susceptibility of the apparatus to small shocks and higher frequency vibrations was tested by repeatedly, sharply striking the breadboard with a wrench (with mild force). The quick strikes likely also contained higher frequency vibration components. Although the detailed profile of the vibrations introduced to the system was not measured, the test demonstrated the basic reduced susceptibility to vibration of the system.

[0043] The 400 nm laser beam incident on the optical cavity caused the fused silica substrate of the cavity mirrors, as well as the collimating lens and other optical elements, to fluoresce in the 450 nm to 550 nm range. The transmission of the cavity mirror coatings at these wavelengths was orders of magnitude higher than the transmission at 400 nm. As a result, the intensity of the fluorescence incident on the sensor was significantly higher than the low power levels of 400 nm light exiting the cavity (2 W), and thus distorted the desired signal. To block this interference, a narrow band-pass filter 23, with a 40 nm bandwidth centered at 400 nm, was placed before the detector 24. This use of a narrow band-pass filter is important to successfully use a high-power semiconductor laser to detect NO.sub.2 using multi-mode CRDS.

[0044] In FIG. 3 the Cavity Ringdown decays are recorded for Zero Air and two different concentrations of NO.sub.2. Samples were passed through the cell at 0.5 liter/min and 512 decays were averaged for each data set. Cavity ringdown times were calculated by using an iterative general Least Square method and the Levenberg-Marquardt method to fit 40 is of data from each decay to an exponential curve of the form (Ae.sup.bx+c). It should be noted that the initial 100 ns from each decay was omitted from the fit in order to avoid distortion to the fit due to light still entering the cavity as the incident laser pulse ended.

[0045] Concentrations of the test gases were calculated using Eq. 4. A plot of ((1/)(1/.sub.0)) vs. measured NO.sub.2 concentration shows the expected linear relationship. See FIG. 4. In FIG. 4 the horizontal error bars represent the uncertainty in preparing the gas mixtures (i.e., mixtures could only be generated with a precision of 3 ppb). It should be noted that the measured values of the NO.sub.2 concentration were found to be approximately 60% of that specified by the mixture. This difference is not unexpected since the pre-calibrated 1 ppm cylinder of NO.sub.2 was over 2 years old, which is over a year beyond its expiration date (the age of the cylinder lowers its expected concentration). It should be noted that the measured NO.sub.2 concentrations are in agreement with previous measurements using Cavity Enhanced Absorption Spectroscopy and a variation of this apparatus, see Karpf 2.

[0046] In order to determine the sensitivity of the detector system, the signal was recorded by averaging different numbers of decays (2048, 1024, 512, 256, 128, 64, 32, 16, 8, 4, 2 and 1), with 0.5 liter/min of zero air flowing through the cell. Fifty data sets were recorded for each specified number of averages, and the standard deviation was calculated. FIG. 5 shows the reduction in the fluctuations in the measured CRD time obtained by averaging multiple decays. Specifically, these figures illustrate the magnitude of fluctuations (and thus the standard deviation) in the CRD times with different numbers of decays averaged. FIG. 5A shows the most fluctuations for the single shot case. FIG. 5B is for the average of 32 decays, and FIG. 5C is for the average of 512 decays. The reduced fluctuations with increased average decays result in a smaller uncertainty in determining the CRD time (i.e., ), and thus via Eq. (4) results in improved sensitivity. A log-log plot of the standard deviation vs. number of averages shows that the optimal sensitivity occurs with the averaging of 512 decays (see FIG. 6).

[0047] The sensitivity of the apparatus was found by rewriting Eq. (4), such that:

[00008]$\begin{array}{cc}{\left[N\right]}_{\min }=\frac{1}{c.Math..Math.{}_{\mathrm{eff}}.Math.}.Math.\frac{}{}.& \left(8\right)\end{array}$

where [N].sub.min is the sensitivity in ppb, is the cavity ringdown time with a 0.5 liter/min zero air flow, is the standard deviation in the measurement of , and .sub.eff is the effective absorption cross-section equal to 6.410.sup.19 cm.sup.2. With averaging of 512 decays, / was measured to be 0.01%, corresponding to a sensitivity of 38 ppt of NO.sub.2 in Zero Air with an integration time of 128 ms. Using single shot detection (i.e., no averaging), / was measured to be 0.15%, corresponding to a sensitivity of 530 ppt of NO.sub.2 in Zero Air with a measurement time of only 60 s.

[0048] A more general view of this sensitivity may be seen in terms of the device's Noise Equivalent Absorption Coefficient of 1.510.sup.9 cm.sup.1 Hz.sup.1/2. The dashed line in the Allen plot in FIG. 6, shows the percent standard deviation (/) expected for random noise. The fact that the standard deviation obtained for larger numbers of averages leaves this dashed line results from signal fluctuations that are not due to white noise but possibly from the slower, temperature-based fluctuations in the avalanche photodiode output. The same test was conducted with vibrations introduced into the apparatus (using the electric motor described above). With averaging 512 decays, / was measured to be 0.01%, indicating that vibrations had no significant effect on the sensitivity of detection. In addition, a qualitative test was conducted where the base of the apparatus was repeatedly struck with a wrench (also described above), and no change was observed in the CRD signal on the oscilloscope.

[0049] It is important to note, however, that the sensitivity of the embodiment of the invention described herein is limited in part due to a temporary lack of commercially available CRD mirrors whose coatings were matched to the 400 nm laser used. Specifically, several pairs of mirrors were tested with coatings specified to have maximum reflectivity in the 405 to 415 nm range. The best results were obtained using a pair of mirrors from CRD Optics whose specifications indicated that they had a reflectivity of 99.995% at 410 nm. It is expected that if mirrors with acceptable coatings at 400 nm were available, or similarly if high power HL40033G lasers were available in a wavelength near 410 nm, that even better results would be obtained. Using light that was 10 nm away from the optimal mirror wavelength resulted in reduced reflectivity. These mirrors had a reflectivity at 400 nm of 99.97%. As a result, the pathlength used to obtain the present results was only 1700 m (i.e., a factor of 10 shorter than what would be achieved using 99.995% reflective mirrors). It would therefore be reasonable to expect a significant (factor of 10+) improvement to the sensitivity is achievable using the invention with a better match between the laser and mirrors.

[0050] Despite the limitation described above, the reported results with the present invention compare favorably with other CRDS measurements of NO.sub.2 concentrations. For example: A sensitivity of 80 ppt of NO.sub.2 with a sample time of 50 seconds was reported using an external cavity diode laser system. R. Wada and A. J. Orr-Ewing, Continuous wave cavity ring-down spectroscopy measurement of NO.sub.2 mixing ratios in ambient air, Analyst, vol. 130, pp. 1595-1600, 2005 (Wada) which is incorporated herein by reference in its entirety. A 40 ppt sensitivity with a measurement time of 1 second was achieved using a pulsed Nd:YAG laser at 532 nm. H. D. Osthoff, S. S. Brown, T. B. Ryerson, T. J. Fortin, B. M. Lerner, E. J. Williams, A. Pettersson, T. Baynard, W. P. Dube, S. J. Ciciora and A. R. Ravishankara, Measurement of atmospheric NO.sub.2 by pulsed cavity ring-down spectroscopy, Jrnl. Geophys. Research, vol. 111, pp. D12305 1-10, 2006 (Osthoff) which is incorporated herein by reference in its entirety. A sensitivity of 60 ppt with an integration time of 60 seconds was reported using a modified commercial, diode-laser based CRD detector. P. Castellanos, W. T. Luke, P. Kelley, J. W. Stehr, S. H. Ehrman and R. R. Dickerson, Modification of a commercial cavity ring-down spectroscopy NO2 detector for enhanced sensitivity, Rev. Sci. Inst., vol. 80, pp. 113107-1-113107-6, 2009 (Castellanos) which is incorporated herein by reference in its entirety. Further, a sensitivity of 80 ppt with an integration time of 60 s was achieved using a light emitting diode based commercial CRD detector. L. C. Brent, et. al., Evaluation of the use of a commercially available cavity ringdown absorption spectrometer for measuring NO.sub.2 in flight, and observations over the Mid-Atlantic States, during DISCOVER-AQ, Jrnl. Atm. Chem., vol. 72, pp. 1-19, 2013 (Brent).

[0051] Fuchs, et. al., used a low power (40 mW), FP diode laser (404 nm) to conduct CRDS and achieved a sensitivity of 22 ppt of NO.sub.2, which is comparable to that of the present invention, but with a measurement time of 1 second, which is approximately an order of magnitude longer than the present invention. It is worth noting, however, that this result was achieved using mirrors that were well matched to their laser (i.e., R=99.9965%) and a cell that was nearly twice the length of that in the present invention. As a result, the pathlength used to achieve the result reported by Fuchs was over 16 times greater (27 km) than that of the present invention. This suggests that in addition to the temporal improvement achieved using the present invention reported approach, the use of a high-power FP laser should result in an order of magnitude improvement in sensitivity when conducted with mirrors that are well matched to the laser source. This improvement would be expected considering the factor of 25 difference in power between the laser sources used by Fuchs, et. al., and that used by the present invention.

[0052] 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.