This application claims the benefit of European Patent Application No. 20154149.7, filed on Jan. 28, 2020, which application is hereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present application relates to a light emitting structure and to a photo-acoustic spectroscopy sensing device comprising the light emitting structure. Further embodiments relate to a method for operating a photo-acoustic spectroscopy sensing device and an apparatus for obtaining information about a target gas. In particular, embodiments of the present disclosure relate to photo-acoustic spectrometry for environmental sensing applications, particularly to a novel apparatus, system and method to sense/detect/measure multiple gasses and particulate matter in space constrained, mobile environmental sensing applications based on photo-acoustic spectrometry. Embodiments relate to a method for operating a photo-acoustic spectroscopy sensing device for efficient and accurate gas detection in acoustically noisy environments.

BACKGROUND

Photo-acoustic spectroscopy (PAS) is the measurement of the effect of absorbed electromagnetic energy, particularly of light, on matter by means of acoustic detection. It is a widely used method for measuring accurately the concentrations of various gasses due to its ability to evaluate subject sample in its in-situ state. A state of the art PAS working setup comprises a heater or a radiator emitting coherent radiation in the IR, the ultraviolet (UV), and the visible spectrum. An optical selection filter only allows radiation of interest (depending on the gas to be detected) and discards the rest of the radiation. The emitted radiation excites a mixture of gasses in a detection chamber. The excitation exerts pressure in an audible range that is to be detected by a detector, for example a microphone.

PAS is most popular for gasses that exhibit receptivity for infrared (IR) radiation due to following facts: Molecules can absorb and emit energy only at certain wavelengths. Further, the typical IR wavelengths of interest are considerably high to the size of the molecule of a gas to be detected. For example, infrared wavelength required for detection of carbon dioxide is 400 times greater than the size of a CO2molecule, where interaction between IR radiation and a gas molecule happens via an interaction of an electric field of the IR radiation with the molecule's electric charge distribution. Additionally, excited molecular states have very long lifetimes compared to an excited electron states—from milliseconds to tenths of seconds, this helps in easy and prominent detection. Further, the mean time between collisions with another molecule is roughly 0.1 μs—so, many collisions and efficient energy transfer further enhance the detectability. Finally, very little scattering occurs, leading to a higher robustness and viability for the detection solution. This also enhances the accuracy of the measurement.

Thus, the above described setup is typically targeting gasses that are well absorbent to IR radiation with molecules which exhibit easy excitation, for example, of bending and rotation and/or vibration modes. Such gasses are finite, as N2and O2, which are significant gasses in environment, are transparent to IR radiation, leaving CO2, CH4, H2O and CO as most popular gasses for PAS based sensors, especially for environmental sensing and air quality monitoring systems.

However, the principle of PAS is also popular in detection of infrared insensitive environment pollutants. Such pollutants comprise other gasses that are sensitive to other specific wave lengths of the electromagnetic spectrum, and particulate matter that is most hazardous for human life. For infrared insensitive pollutants, setups similar to the one described before, with different wavelength selection filters, may be employed.

To detect multiple gasses, different certain wavelengths corresponding to the gas under detection are required to be emitted. For multi-gas detection using PAS, i.e. for detection of multiple gasses using one device/setup/geometry, an optical selection filter may be employed, which may be tuned to a specific absorption wavelength of a gas. Hence, the quality of gas detection in such setups may depend on transmissivity and selectivity of the optical selection filter. That is, for achieving good quality of multi-gas detection using PAS, a tunable optical filter with high transmissivity and selectivity may be required. State of the art examples of tunable optical filters rely on electrical, electromechanical, or a combination of both technologies. Examples are quartz based tunable filters, mechanical rotating wheel filters, micro-electromechanical systems (MEMS) based solutions, Fabry-Perot interferometer (FPI) based solutions, liquid crystal tunable filters, semiconductor/silicon grown IR heater with optical filters, and semiconductor/silicon grown light emitting diodes.

The aim of the present disclosure is to provide for a concept for an excitation of one or more target gasses with electromagnetic radiation, the concept enabling an accurate measurement of a concentration of the target gas via PAS while being compatible to space-constrained applications and cost-efficient manufacturing processes.

SUMMARY

Embodiments of the present disclosure provide a light emitting structure for a PAS sensing device for sensing a target gas. The light emitting structure comprises a light source configured for emitting light of an input wavelength. The light emitting structure further comprises a conversion structure that is configured for absorbing light of the input wavelength, and that is further configured for emitting light of an output wavelength. The output wavelength of the conversion structure is adapted to an absorption wavelength of the target gas. The conversion structure comprises an output conversion layer that comprises a plurality of nanoparticles. The nanoparticles of the output conversion layer are configured for emitting light of the output wavelength.

Further embodiments of the present disclosure provide for a PAS sensing device for sensing a target gas. The sensing device comprises a cavity providing an interaction volume for an optical interaction between the target gas and light, wherein the cavity is accessible for an environmental gas comprising the target gas. The sensing device comprises at least one light emitting structure. The at least one light emitting structure is configured for emitting the light into the cavity, the light having the output wavelength of the at least one light emitting structure, wherein the output wavelength is adapted to an absorption wavelength of the target gas. The sensing device further comprises a receiver configured for receiving an acoustic signal from within the cavity.

Further embodiments of the present disclosure provide a method for operating a PAS sensing device. The method comprises receiving an acoustic reference signal from a cavity accessible for an environmental gas comprising a target gas. The method further comprises analyzing the acoustic reference signal to obtain noise information. The method comprises adjusting a filter function based on the noise information. Further, the method comprises sensing the target gas, wherein the sensing comprises providing the cavity with a light, so that the light is modulated based on the filter function. Sensing the target gas comprises receiving an acoustic signal from the cavity simultaneously to providing the cavity with the light. The method further comprises analyzing the acoustic sensing signal by using the filter function so as to obtain information about the target gas in the cavity.

Further embodiments of the present disclosure provide an apparatus for obtaining an information about a target gas. The apparatus comprises means for receiving a reference signal, the reference signal representing an acoustic reference signal from a cavity accessible for an environmental gas comprising a target gas. The apparatus further comprises means for analyzing the reference signal to obtain noise information, and comprises means for adjusting a filter function based on the noise information. Further, the apparatus comprises means for providing the filter function, means for receiving a sensing signal representing an acoustic sensing signal from the cavity, and means for analyzing the sensing signal by using the filter function so as to obtain the information about the target gas in the cavity.

