EVANESCENT-WAVE QUARTZ-ENHANCED PHOTOACOUSTIC SENSOR WITH RESONATOR ELEMENTS
1. An evanescent-wave quartz enhanced microfiber photoacoustic detection device with oscillator and micro-resonator elements for detecting trace gas concentrations, the evanescent-wave quartz enhanced microfiber photoacoustic detection device comprising:
- a light source tuned to a wavelength corresponding to the optical absorption of a gas to be detected;
an optical fiber;
an input coupler connected to the optical fiber;
a fiber-taper to generate an evanescent wave;
a quartz tuning fork having its free arms arranged at the level of the fiber-taper to absorb a mechanical force generated following the optical absorption by the gas, the mechanical force exciting a piezoelectric mode of the quartz tuning fork and generating an electrical current;
micro-resonators positioned near the quartz tuning fork to enhance the mechanical force; and
means for amplifying and detecting a current generated by the quartz tuning fork to determine the concentration of the gas;
wherein the fiber-taper, quartz tuning fork, and micro-resonators are assembled in a sealed gas cell for gas detection.
A novel evanescent-wave quartz-enhanced optical microfiber photoacoustic gas sensor is provided for detecting trace amounts of gas. Both fiber-taper based evanescent field and photoacoustic spectroscopy can be used to exploit the merits of both technologies. The use of a fiber half-taper into the tuning fork and microresonator tubes can result in reduced system size, simplified optical alignment, and high sensitivity. The techniques described can be used in chemical, biological and environmental sensing applications.
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- 1. An evanescent-wave quartz enhanced microfiber photoacoustic detection device with oscillator and micro-resonator elements for detecting trace gas concentrations, the evanescent-wave quartz enhanced microfiber photoacoustic detection device comprising:
a light source tuned to a wavelength corresponding to the optical absorption of a gas to be detected; an optical fiber; an input coupler connected to the optical fiber; a fiber-taper to generate an evanescent wave; a quartz tuning fork having its free arms arranged at the level of the fiber-taper to absorb a mechanical force generated following the optical absorption by the gas, the mechanical force exciting a piezoelectric mode of the quartz tuning fork and generating an electrical current; micro-resonators positioned near the quartz tuning fork to enhance the mechanical force; and means for amplifying and detecting a current generated by the quartz tuning fork to determine the concentration of the gas; wherein the fiber-taper, quartz tuning fork, and micro-resonators are assembled in a sealed gas cell for gas detection.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
This application claims the benefit of U.S. Provisional Application Ser. No. 62/319,899, filed Apr. 8, 2016, the disclosure of which is hereby incorporated by reference in its entirety, including all figures, tables and drawings.
The invention relates to a photoacoustic gas sensor based on the combination of evanescent-wave absorption using an optical microfiber with an acoustic oscillator and resonator elements.
Photoacoustic spectroscopy detection is applied for the analysis of various media including solids, liquids, biological tissues, and gases. Photoacoustic gas sensors operate by detecting acoustic vibrations induced by the modulated optical radiation in an analyzed gas sample. Quartz-enhanced photoacoustic spectroscopy (QEPAS) is one of the most sensitive photoacoustic detection techniques using an oscillator like a quartz tuning fork as the sharply resonant acoustic transducer. This method was first presented by A. Kosterev et al. (Opt. Lett. 27, 1902-1904 (2002) and U.S. Pat. No. 7,245,380 B2). QEPAS detection is less sensitive to environmental noise and has the advantages of small size, low cost, and ease of fabrication compared with traditional photoacoustic gas sensors. Numerous QEPAS-based gas sensors have been developed for various gas sensing applications with a normalized noise equivalent absorption (NNEA) coefficient in the range of 10−10-10−7 cm−1 WhiFiz as reviewed in Sensors 14, 6165 (2014) The previous QEPAS sensors using on-beam or off-beam detection schemes (A. Kosterev et al., Opt. Lett. 27, 1902 (2002); K. Liu et al., Opt. Lett. 34, 1594 (2009)) have mostly adopted the open-path optical configuration. Complicated optical alignment, a precise focusing system, and excitation sources with high spatial radiation quality were required to make the laser beam pass through a gap of 300-μm wide between the prongs of the quartz tuning fork and the micro-resonator tubes without touching any surfaces. An additional visualization system is sometimes required for the sensor setup.
Gas absorption by an evanescent field has been demonstrated with a palladium film deposited at a core-exposed fiber (M. Tabib-Azar el al., Sensor. Actuat. B-Chem. 56, 158 (1999)) or the D-shaped optical fiber (G. Stewart et al., Sensor. Actuat. B-Chem. 38; 42 (1997)). The sensitivity of evanescent-wave sensors is determined by the fraction of the optical power in the evanescent field. In the D-fiber evanescent-wave absorption sensor used for methane detection, due to the inherently low evanescent field, an extra sot-gel process was applied to coat the flat surface of the D-fiber to enhance the sensitivity. In the evanescent-wave hydrogen sensor, by depositing palladium over an exposed core region of a multimode fiber, the hydrogen can be detected based on evanescent field interaction with the palladium coating. However, microfibers made by the mechanical processing or chemical etching methods mentioned above suffer from the issue of fragility, Microfihers can also be obtained using the flame-brushing technique as described in Opt. Lett. 35, 85 (2010). The silica fiber taper fabricated using the flame-brushing method has the advantages of a high evanescent field and good mechanical properties.
The tapered microfibers provide an alternative method for photoacoustic detection with evanescent field interactions. A C2H2 photoacoustic sensor using tapered microfibers has been demonstrated by employing a full-taper optical fiber, However, only a bare quartz tuning fork was used in that sensor without micro-resonators. This caused a much lower sensitivity compared to the traditional open-path QEPAS sensors as presented by Y. Cao et al. in Opt Lett 37, 214-216 (2012).
The subject invention combines the fiber-taper based evanescent field with photoacoustic detection using oscillator and resonator elements and exploits the advantages of both techniques to achieve ultra-sensitive, compact, low-cost evanescent-wave gas sensors.
Embodiments of the subject invention provide microfiber photoacoustic detection devices for detecting the concentration of trace amounts of a target gas. A device can include a laser source tuned to the wavelength corresponding to the optical absorption of the target gas, a single mode fiber coupled with the laser for light delivery, a fiber-taper to generate evanescent waves for acoustic wave generation, an oscillator like quartz tuning fork to detect the acoustic waves, and/or micro-resonators positioned near the quartz tuning fork to enhance the acoustic signal.
The previous evanescent-wave fiber sensor can be employed for gas sensing, but requires a very long microfiber (meter to hundred meters) to achieve sufficient detection sensitivity. Traditional photoacoustic gas sensors using a quartz tuning fork as the acoustic transducer can achieve a compact and low-cost sensor configuration, but require complicated optical alignment and laser sources with high spatial radiation quality. The subject invention merges the fiber-taper based evanescent field and photo-acoustic spectroscopy and exploits the merits of both. The adoption of a fiber taper into the quartz tuning fork and micro-resonators significantly reduces system size, simplifies optical alignment, and achieves high sensitivity. This spectroscopic technique can be widely used in chemical, biological, and environmental sensing applications.
Embodiments of the subject invention provide tnicrofiber photoacoustic detection devices for detecting the concentration of trace amounts of a target gas. A device can include a laser source tuned to the wavelength corresponding to the optical absorption of the target gas, a single mode fiber coupled with the laser for light delivery, a fiber-taper to generate evanescent waves for acoustic wave generation, an oscillator like quartz tuning fork to detect the acoustic waves, and/or micro-resonators positioned near the quartz tuning fork to enhance the acoustic signal.
Optical microfibers can be employed for robust light delivery and realize optical interconnection between optical devices on the microscale or nanoscale, The strong evanescent field generated by microfibers can also lead to high detection sensitivity for optical sensing applications. If an optical fiber without cladding is surrounded by gas that absorbs at the light wavelength, the evanescent wave penetrates into the region outside the fiber and transfers energy into the gas molecules. The use of an evanescent field for gas and liquid sensing has the advantages of low optical loss, easy alignment, and the potential of making integrated devices.
The principle of the photoacoustic gas sensor according to an embodiment of the subject invention, comprising a microfiber, a turning fork, and resonator elements, is illustrated generally at 100 in schematic form in
The following steps are designed to facilitate the assembly of the microfiber with the quartz tuning fork and acoustic resonators. First, a visible laser diode (typical wavelength of 650 nm) can be employed as the light source 110 to connect with the input fiber 115, The visible light leaking out of the fiber taper 125 can be observed. In this way, the position of the fiber-taper 125 can be easily adjusted so that the evanescent field 130 between the two prongs 140 of the quartz tuning fork 135 has the maximum power density. Then, the visible light can be switched to an infrared laser with its emission wavelength coincident with the absorption line of the target gas. Compared with the traditional open-path quartz-enhanced photoacoustic system (U.S. Pat. No. 7,245,380 B2,), the optical windows of the gas cell is eliminated in this technique. The evanescent wave 130 is confined closely around the fiber-taper leading to negligible optical noise, which enhances the detection sensitivity.
A large fraction of power existing in the evanescent field is required in this technique because the photoacoustic signal is proportional to the optical excitation power. The evanescent field 130 is related to the diameter of the fiber taper 125: the thinner the fiber diameter, the stronger the evanescent field. Thus, an ultrathin single mode fiber can be used to achieve high sensitivity.
If a fiber taper is placed between the two prongs of the quartz tuning fork 135 to generate evanescent-wave absorption, in the case of an optically thin gas sample, the detected acoustic signal S can be expressed as:
S=kγαxP0Q/F0, (Eq. 1)
where α (cm−1/(molecule cm−3) is the absorption coefficient of the target species; x (molecular/cm−3) is the species concentration; P0 (W) is the incident optical power; Q and f0 (Hz) are the quality factor and the resonant frequency of the quartz tuning fork, respectively; γ (γ<1) is the attenuation coefficient of the incident optical power associated with evanescent field; and k is a dimensionless coefficient describing the system parameters and acoustic transfer function. Therefore, a fiber taper with larger γ is required to obtain a stronger photoacoustic signal.
In an embodiment, the quartz tuning fork can be integrated with the resonator tube using an alternative configuration. Different from the configuration 100 shown in
In an embodiment of the subject invention, a fiber half-taper with applications in a near-field scanning optical microscope can also be used in microfiber-based photoacoustic detection.
In a particular embodiment, a silica-core single mode fiber 115 of 11-μm core, 125-μm cladding and a typical attenuation of 0.25 dB/m at the optical wavelength of 2.3 μm is selected. The optimized parameters of flame scanning length down to 1 mm and scanning speed of 0.2 mm/s are selected to obtain a fiber half-taper 510 in length of 14 mm and taper angle of 0.25°. The laser radiation 110 is guided through the single mode fiber 115 to the half-taper 510 that is integrated with the quartz tuning fork 135 and micro-resonator tubes 120 for the photoacoustic detection.
A photoacoustic sensor for CO detection is illustrated in
The free-space laser beam can be coupled into the single mode fiber 115 by employing an aspheric lens 620 fixed on a multi-axis translation stage. The single mode fiber 115 can be connected with the fiber half-taper 510 that is placed on a stage 625. In one embodiment, the laser wavelength is 2.3 μm and the fiber-taper diameter is 2.4 μm leading to about 7% of the incident optical power leaking out of the fiber core. In another embodiment, a fiber-coupled laser is employed and such an optical coupling system is not required. The CO molecules surrounding the fiber taper 510 absorb the modulated optical energy of the evanescent field 130 that leads to an acoustic wave, which is amplified by the resonator tubes 120. The generated acoustic wave excites the fundamental piezoelectric mode of the quartz tuning fork 135, thereby generating a weak current that is further amplified by a transimpedance amplifier 635. The pre-amplified signal can subsequently be demodulated at the resonant frequency of the tuning fork to obtain its component using a lock-in amplifier 640. All the data acquisition and signal processing operations can be controlled (e.g., via a LabVIEW program) by a computing device 645, such as a laptop computer.
Advantageously, the evanescent-wave quartz-enhanced photoacoustic sensor of embodiments of the subject invention requires neither a focusing lens and visualization system for optical alignment, nor an optical window,
In one embodiment, the evanescent-wave quartz-enhanced photoacoustic sensor is fixed inside a compact gas cell equipped with a gas inlet and outlet.
In a specific embodiment, water vapor is added into the gas mixture as a relaxation promoter to enhance the vibration-translation relaxation, thereby enhancing the photoacoustic signal. To this end, the testing gas mixture, before being introduced into the gas cell, is passed through a permeation tube that is immersed inside a water circulating bath. Advantageously, a typical relative humidity of 46% maintained in the gas flow constantly enhances the photoacoustic signal by a factor of approximately 3.
All patents, patent applications, provisional applications, and publications referred to or cited herein (including those in the “References” section) are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.
It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application. In addition, any elements or limitations of any invention or embodiment thereof disclosed herein can be combined with any and/or all other elements or limitations (individually or in any combination) or any other invention or embodiment thereof disclosed herein, and all such combinations are contemplated with the scope of the invention without limitation thereto.
- A. Kosterev et al., Opt. Lett. 27, 1902-1904 (2002).
- U.S. Pat. No. 7,245,380 B2.
- K. Liu et al., Opt. Lett. 34, 1594 (2009).
- M. Tabib-Azar et al, (Sensor. Actuat. B-Cheer. 56, 158 (1999)).
- G. Stewart et al., Sensor. Actuat. B-Chem. 38, 42 (1997).
- Opt. Lett. 35, 85 (2010).
- Y. Cao et al., Opt Lett 37, 214-216 (2012).
- U.S. Pat. No. 7,605,922 B2.
- U.S. Pat. No. 8,040,516 B2.
- U.S. Patent Application Publication No. 2011/0088453 A1.
- U.S. Pat. No. 7,446,877 B2.
- A. Kosterev, et al., Rev. Sci. Instrum. 76, 043105 (2005).
- P. Patimisco et al., Sensors 14, 6165 (2014).
- A. Dudus et al., IEEE J. Sel. Topics Quantum Electron, 22, 1 (2016).
- W. Jin et al., Opt. Express 22, 28132 (2014).
- L. Dong et al., Appl. Phys. B 100, 627 (2010).