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Photoacoustic spectrometer based on a DFB-diode laser
Infrared Physics & Technology 41 (2000) 283±286
www.elsevier.com/locate/infrared
Photoacoustic spectrometer based on a DFB-diode laser M. Wol, H. Harde * Universit at der Bundeswehr Hamburg, Holstenhofweg 85, 22043 Hamburg, Germany Received 18 February 2000
Abstract We have developed a photoacoustic spectrometer based on a distributed feedback diode laser. The laser can be tuned continuously over 700 GHz enabling both the precise determinations of absorption line parameters such as the pressure broadening coecient and pressure shift as well as sensitive concentration measurements. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 82.80.Kq; 07.57.Ty; 33.20.Vq; 33.20.Ea Keywords: HF-sensor; Monitor; Line-shape; Pressure broadening; Pressure shift
1. Introduction An increasing perception of environmental problems has initiated the development of sensitive detectors for monitoring pollutants in the atmosphere. Particularly, sensors based on the photoacoustic eect in conjunction with lasers have the potential for monitoring trace gases at concentrations of a few ppb or less [1,2]. Photoacoustic detection utilizes the fact that the excitation energy of light absorbing molecules is essentially transferred into kinetic energy of the surrounding molecules via inelastic collisions. This causes an increase in the local pressure of the absorbing gas. If the excitation source is modulated, a sound wave is generated that can be detected by
* Corresponding author. Tel.: +49-40-6541-2756; fax: +4940-6541-2640. E-mail address: [email protected] (H. Harde).
a microphone. This signal is directly proportional to the concentration of the absorbing molecules in the sample. Therefore, a calibrated setup allows one to measure directly the absolute concentration of a gas [3]. Photoacoustic spectroscopy (PAS) has the advantage of producing a signal, only when light is absorbed. Hence, contrary to the transmission spectroscopy, PAS is an oset-free technique, and it is possible to replace relatively long absorption cells with much smaller ones. In addition, the potential exists when using acoustic cell resonances, which enhance the signal and thereby increase the detection sensitivity. The PAS technique is primarily used in conjunction with high power gas lasers, since the photoacoustic signal is proportional to the intensity of the light [4]. The disadvantage of these lasers is their non-tunability and therefore the requirement of a coincidence with an absorption line. Usually, the modulation of these lasers is performed with a mechanical chopper, which may cause coherent noise by the rotating
1350-4495/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 4 9 5 ( 0 0 ) 0 0 0 4 1 - 4
284
M. Wol, H. Harde / Infrared Physics & Technology 41 (2000) 283±286
blade and an additional photoacoustic signal, by absorptions of the cell windows. This deteriorates the sensitivity of a photoacoustic detector. We have developed a photoacoustic spectrometer, which is based on the use of a single frequency diode laser for the selective excitation of trace gases and eliminates the main disadvantages of conventional PA detectors. Our experiments were performed on the rotational line P2 of the vibrational transition 2±0 (overtone) of hydrogen ¯uoride (HF) at 1304.53 nm (vacuum wavelength).
2. Experimental setup Fig. 1 shows the experimental setup of the photoacoustic spectrometer. A room temperature distributed feedback (DFB) diode laser serves as the excitation source. The laser is mounted on a Peltier cooler in order to tune its operational temperature. The radiation of the laser passes a cylindrical sample cell (aluminum: é 31:2 mm; l 84 mm). This cell is used in conjunction with a sensitive high voltage microphone (é 23:77 mm, transmission factor: f 45:7 mV=Pa). The microphone signal is phase-sensitively detected with a time constant of s 300 ms and normalized according to the average optical power of the laser.
Fig. 1. Experimental setup of the photoacoustic spectrometer.
The single longitudinal mode of the laser has a width of 15 MHz (FWHM). By controlling the temperature and injection current the laser frequency can continuously be tuned over a range of 700 GHz. For line-shape studies and pressure broadening measurements the laser output is modulated by a mechanical chopper. For an application as sensitive HF detector, the injection current is modulated. At a bias of 70 mA and an operational temperature of 16°C, the laser has an output power of 14 mW. 3. Line-shape measurements The wide tunability of the laser and its narrow line width allowed for high precision measurements of absorption line parameters. For these investigations, we applied a mechanical modulation technique with a chopper frequency of 333 Hz, while the laser was operated with a constant injection current of 72 mA. All the measurements were performed using a concentration of 0.1% HF in N2 . A wavelength scan over the P2 absorption line of HF gas is achieved by tuning the temperature of the laser from 13.15°C to 17.54°C. This results in a mode-hop free scan from 1304.4 to 1304.7 nm (53 GHz). The scans, taken over a pressure range of 1±1000 hPa at 296 K, are shown in Fig. 2. For direct comparability, the signals are normalized. From these spectra the pressure broadening and pressure shift of the P2 line can be determined. Using the Pade approximation according to [5] the broadening coecient due to elastic collisions with N2 molecules is found to be 5:92 0:04 MHz=hPa at 296 K. The Doppler line width turns out to be 630 25 MHz at 296 K. Fig. 3 displays the mea-
Fig. 2. Photoacoustic spectra of the P2 line for 0.1% HF in N2 at pressures of 1±1000 hPa and 296 K (chopper modulation).
M. Wol, H. Harde / Infrared Physics & Technology 41 (2000) 283±286
Fig. 3. Linewidth (FWHM) of the P2 line as a function of pressure.
Fig. 4. Pressure shift of the P2 line as a function of pressure.
sured line width (FWHM) of the P2 transition as a function of the pressure, indicating the strong broadening due to collisions at high pressure and the dominant Doppler broadening at low pressure. The pressure shift of the P2 absorption line for N2 gas is 540 40 kHz=hPa (Fig. 4) and to our knowledge was measured for the ®rst time. At 1000 hPa and 296 K, the center wavelength of the P2 line is 1304.534 nm (vacuum) and its line width is 0.034 nm (5.98 GHz, FWHM). 4. HF detection Both in solution and gas form, HF is an extremely hazardous poison that is widely used in industrial processes as a chemical intermediary for puri®cation, cleaning, and synthesis. Garbage incinerators, metalworking and other industries release HF into the atmosphere [6,7]. However,
285
owing to the high toxicity of this gas very small concentrations must be monitored by detectors. HF has a threshold limit value of 3 ppm by volume and is generally measured with expensive, electrochemical sensors such as chromatographs or potentiometers [8]. The photoacoustic spectrometer described here represents an attractive alternative to the conventional monitors, as the photoacoustic cell together with the semiconductor laser forms a very compact and relatively cheap gas detector. The sensitivity of the detector is considerably improved by taking advantage of an acoustic cell resonance, i.e., the laser radiation is modulated at a frequency equivalent to an acoustic mode of the cell. We made use of the ®rst azimuthal resonance of the cell at m010 6158 HZ (1000 hPa) with a quality factor of Q 27. Further, we replaced the mechanical chopper by a modulation of the laser injection current. Modulation of the current causes an essential modulation of the laser wavelength superimposed by a small amplitude modulation. A modulation depth of 1% of the average current contributes already to a frequency shift of 1.3 GHz. This technique avoids coherent noise originating from a chopper. In addition, frequency shifts that are small compared to the absorption bandwidth of a solid, exclude any kind of deteriorating signal from the cell windows [9]. Spectra were obtained with the laser frequency scanned across the absorption line by means of temperature tuning. The detected photoacoustic signal at the lock-in represents the derivative of the absorption line shape. A measurement of this type is shown in Fig. 5. It was taken at a laminar ¯ow (0.1 l=min) of 80 ppm HF in N2 (1000 hPa, 296 K). Applying a modulation depth of 15% a maximum signal-to-noise ratio of S=N 990 is achieved. This results in an idealized detection limit of 80 ppb for S=N 1. 5. Conclusion These experiments demonstrate the applicability of the photoacoustic diode laser spectrometer as a compact detector for sensitive monitoring of
286
M. Wol, H. Harde / Infrared Physics & Technology 41 (2000) 283±286
agreement with the parameters of the literature and the pressure shift is of a reasonable order [10].
References
Fig. 5. Photoacoustic spectrum of the P2 line for 80 ppm HF in N2 at 1000 hPa and 296 K (modulation of injection current).
hydrogen ¯uoride at atmospheric pressure. The modulation of the injection current allows us to generate a photoacoustic signal at the high resonance frequency of the cell and to avoid coherent noise from a chopper or photoacoustic signals from the cell windows. These advantages compensate for the relatively low power of the device and the small absorption cross-section of the overtone. The threshold limit value for HF can be easily measured with this tool. Moreover, the absence of any moving parts makes it very robust, and due to the frequency modulation with a single mode laser any crosssensitivity is excluded. The measured broadening coecient due to collisions with N2 was found to be in good
[1] W. Demtr oder, Laser Spectroscopy, Springer, Berlin, 1996. [2] V.P. Zharov, V.S. Letokhov, Laser Optoacoustic Spectroscopy, Springer Ser. Opt. Sci. vol. 37, Springer, Heidelberg, 1986. [3] P. Hess, J. Pelzl (Eds.), Photoacoustic and Photothermal Phenomena, Springer Ser. Opt. Sci. vol. 58, Springer, Heidelberg, 1986. [4] M.W. Sigrist, A. Thony, Infrar. Phys. Tech. 36 (1995) 585. [5] P. Minguzzi, A. Di Lieto, J. Mol. Spectrosc. 109 (1985) 388. [6] Health and Safety Executive (HSE), A Review of the Manufacture, Uses, Incidents and Hazard Models for Hydrogen Fluoride, HSE, London, ISBN 0717609839, 1995. [7] Hydro¯uoric Acid, AP-42 Section 5.8, Background Report, U.S. Environmental Protection Agency, Oce of Air Quality Planning and Standards, 1998. [8] Threshold limit values for chemical substances and physical agents and biological exposure indices, American Conference of Governmental and Industrial Hygenists, ISBN 1882417321, 1999. [9] M. Feher, Y. Jiang, J.P. Maier, Optoacoustic trace-gas monitoring with near-infrared diode lasers, Appl. Opt. 33 (9) (1994) 1655. [10] L.S. Rothman, et al., Proc. SPIE 3375 (1998) 123.
www.elsevier.com/locate/infrared
Photoacoustic spectrometer based on a DFB-diode laser M. Wol, H. Harde * Universit at der Bundeswehr Hamburg, Holstenhofweg 85, 22043 Hamburg, Germany Received 18 February 2000
Abstract We have developed a photoacoustic spectrometer based on a distributed feedback diode laser. The laser can be tuned continuously over 700 GHz enabling both the precise determinations of absorption line parameters such as the pressure broadening coecient and pressure shift as well as sensitive concentration measurements. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 82.80.Kq; 07.57.Ty; 33.20.Vq; 33.20.Ea Keywords: HF-sensor; Monitor; Line-shape; Pressure broadening; Pressure shift
1. Introduction An increasing perception of environmental problems has initiated the development of sensitive detectors for monitoring pollutants in the atmosphere. Particularly, sensors based on the photoacoustic eect in conjunction with lasers have the potential for monitoring trace gases at concentrations of a few ppb or less [1,2]. Photoacoustic detection utilizes the fact that the excitation energy of light absorbing molecules is essentially transferred into kinetic energy of the surrounding molecules via inelastic collisions. This causes an increase in the local pressure of the absorbing gas. If the excitation source is modulated, a sound wave is generated that can be detected by
* Corresponding author. Tel.: +49-40-6541-2756; fax: +4940-6541-2640. E-mail address: [email protected] (H. Harde).
a microphone. This signal is directly proportional to the concentration of the absorbing molecules in the sample. Therefore, a calibrated setup allows one to measure directly the absolute concentration of a gas [3]. Photoacoustic spectroscopy (PAS) has the advantage of producing a signal, only when light is absorbed. Hence, contrary to the transmission spectroscopy, PAS is an oset-free technique, and it is possible to replace relatively long absorption cells with much smaller ones. In addition, the potential exists when using acoustic cell resonances, which enhance the signal and thereby increase the detection sensitivity. The PAS technique is primarily used in conjunction with high power gas lasers, since the photoacoustic signal is proportional to the intensity of the light [4]. The disadvantage of these lasers is their non-tunability and therefore the requirement of a coincidence with an absorption line. Usually, the modulation of these lasers is performed with a mechanical chopper, which may cause coherent noise by the rotating
1350-4495/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 4 9 5 ( 0 0 ) 0 0 0 4 1 - 4
284
M. Wol, H. Harde / Infrared Physics & Technology 41 (2000) 283±286
blade and an additional photoacoustic signal, by absorptions of the cell windows. This deteriorates the sensitivity of a photoacoustic detector. We have developed a photoacoustic spectrometer, which is based on the use of a single frequency diode laser for the selective excitation of trace gases and eliminates the main disadvantages of conventional PA detectors. Our experiments were performed on the rotational line P2 of the vibrational transition 2±0 (overtone) of hydrogen ¯uoride (HF) at 1304.53 nm (vacuum wavelength).
2. Experimental setup Fig. 1 shows the experimental setup of the photoacoustic spectrometer. A room temperature distributed feedback (DFB) diode laser serves as the excitation source. The laser is mounted on a Peltier cooler in order to tune its operational temperature. The radiation of the laser passes a cylindrical sample cell (aluminum: é 31:2 mm; l 84 mm). This cell is used in conjunction with a sensitive high voltage microphone (é 23:77 mm, transmission factor: f 45:7 mV=Pa). The microphone signal is phase-sensitively detected with a time constant of s 300 ms and normalized according to the average optical power of the laser.
Fig. 1. Experimental setup of the photoacoustic spectrometer.
The single longitudinal mode of the laser has a width of 15 MHz (FWHM). By controlling the temperature and injection current the laser frequency can continuously be tuned over a range of 700 GHz. For line-shape studies and pressure broadening measurements the laser output is modulated by a mechanical chopper. For an application as sensitive HF detector, the injection current is modulated. At a bias of 70 mA and an operational temperature of 16°C, the laser has an output power of 14 mW. 3. Line-shape measurements The wide tunability of the laser and its narrow line width allowed for high precision measurements of absorption line parameters. For these investigations, we applied a mechanical modulation technique with a chopper frequency of 333 Hz, while the laser was operated with a constant injection current of 72 mA. All the measurements were performed using a concentration of 0.1% HF in N2 . A wavelength scan over the P2 absorption line of HF gas is achieved by tuning the temperature of the laser from 13.15°C to 17.54°C. This results in a mode-hop free scan from 1304.4 to 1304.7 nm (53 GHz). The scans, taken over a pressure range of 1±1000 hPa at 296 K, are shown in Fig. 2. For direct comparability, the signals are normalized. From these spectra the pressure broadening and pressure shift of the P2 line can be determined. Using the Pade approximation according to [5] the broadening coecient due to elastic collisions with N2 molecules is found to be 5:92 0:04 MHz=hPa at 296 K. The Doppler line width turns out to be 630 25 MHz at 296 K. Fig. 3 displays the mea-
Fig. 2. Photoacoustic spectra of the P2 line for 0.1% HF in N2 at pressures of 1±1000 hPa and 296 K (chopper modulation).
M. Wol, H. Harde / Infrared Physics & Technology 41 (2000) 283±286
Fig. 3. Linewidth (FWHM) of the P2 line as a function of pressure.
Fig. 4. Pressure shift of the P2 line as a function of pressure.
sured line width (FWHM) of the P2 transition as a function of the pressure, indicating the strong broadening due to collisions at high pressure and the dominant Doppler broadening at low pressure. The pressure shift of the P2 absorption line for N2 gas is 540 40 kHz=hPa (Fig. 4) and to our knowledge was measured for the ®rst time. At 1000 hPa and 296 K, the center wavelength of the P2 line is 1304.534 nm (vacuum) and its line width is 0.034 nm (5.98 GHz, FWHM). 4. HF detection Both in solution and gas form, HF is an extremely hazardous poison that is widely used in industrial processes as a chemical intermediary for puri®cation, cleaning, and synthesis. Garbage incinerators, metalworking and other industries release HF into the atmosphere [6,7]. However,
285
owing to the high toxicity of this gas very small concentrations must be monitored by detectors. HF has a threshold limit value of 3 ppm by volume and is generally measured with expensive, electrochemical sensors such as chromatographs or potentiometers [8]. The photoacoustic spectrometer described here represents an attractive alternative to the conventional monitors, as the photoacoustic cell together with the semiconductor laser forms a very compact and relatively cheap gas detector. The sensitivity of the detector is considerably improved by taking advantage of an acoustic cell resonance, i.e., the laser radiation is modulated at a frequency equivalent to an acoustic mode of the cell. We made use of the ®rst azimuthal resonance of the cell at m010 6158 HZ (1000 hPa) with a quality factor of Q 27. Further, we replaced the mechanical chopper by a modulation of the laser injection current. Modulation of the current causes an essential modulation of the laser wavelength superimposed by a small amplitude modulation. A modulation depth of 1% of the average current contributes already to a frequency shift of 1.3 GHz. This technique avoids coherent noise originating from a chopper. In addition, frequency shifts that are small compared to the absorption bandwidth of a solid, exclude any kind of deteriorating signal from the cell windows [9]. Spectra were obtained with the laser frequency scanned across the absorption line by means of temperature tuning. The detected photoacoustic signal at the lock-in represents the derivative of the absorption line shape. A measurement of this type is shown in Fig. 5. It was taken at a laminar ¯ow (0.1 l=min) of 80 ppm HF in N2 (1000 hPa, 296 K). Applying a modulation depth of 15% a maximum signal-to-noise ratio of S=N 990 is achieved. This results in an idealized detection limit of 80 ppb for S=N 1. 5. Conclusion These experiments demonstrate the applicability of the photoacoustic diode laser spectrometer as a compact detector for sensitive monitoring of
286
M. Wol, H. Harde / Infrared Physics & Technology 41 (2000) 283±286
agreement with the parameters of the literature and the pressure shift is of a reasonable order [10].
References
Fig. 5. Photoacoustic spectrum of the P2 line for 80 ppm HF in N2 at 1000 hPa and 296 K (modulation of injection current).
hydrogen ¯uoride at atmospheric pressure. The modulation of the injection current allows us to generate a photoacoustic signal at the high resonance frequency of the cell and to avoid coherent noise from a chopper or photoacoustic signals from the cell windows. These advantages compensate for the relatively low power of the device and the small absorption cross-section of the overtone. The threshold limit value for HF can be easily measured with this tool. Moreover, the absence of any moving parts makes it very robust, and due to the frequency modulation with a single mode laser any crosssensitivity is excluded. The measured broadening coecient due to collisions with N2 was found to be in good
[1] W. Demtr oder, Laser Spectroscopy, Springer, Berlin, 1996. [2] V.P. Zharov, V.S. Letokhov, Laser Optoacoustic Spectroscopy, Springer Ser. Opt. Sci. vol. 37, Springer, Heidelberg, 1986. [3] P. Hess, J. Pelzl (Eds.), Photoacoustic and Photothermal Phenomena, Springer Ser. Opt. Sci. vol. 58, Springer, Heidelberg, 1986. [4] M.W. Sigrist, A. Thony, Infrar. Phys. Tech. 36 (1995) 585. [5] P. Minguzzi, A. Di Lieto, J. Mol. Spectrosc. 109 (1985) 388. [6] Health and Safety Executive (HSE), A Review of the Manufacture, Uses, Incidents and Hazard Models for Hydrogen Fluoride, HSE, London, ISBN 0717609839, 1995. [7] Hydro¯uoric Acid, AP-42 Section 5.8, Background Report, U.S. Environmental Protection Agency, Oce of Air Quality Planning and Standards, 1998. [8] Threshold limit values for chemical substances and physical agents and biological exposure indices, American Conference of Governmental and Industrial Hygenists, ISBN 1882417321, 1999. [9] M. Feher, Y. Jiang, J.P. Maier, Optoacoustic trace-gas monitoring with near-infrared diode lasers, Appl. Opt. 33 (9) (1994) 1655. [10] L.S. Rothman, et al., Proc. SPIE 3375 (1998) 123.