SE541185C2

SE541185C2 - High Spectral Resolution Scheimpflug Lidar

Info

Publication number: SE541185C2
Application number: SE1830093A
Authority: SE
Inventors: Mikkel Brydegaard
Original assignee: Neolund Ab
Priority date: 2018-03-21
Filing date: 2018-03-21
Publication date: 2019-04-23
Also published as: SE1830093A1

Abstract

A method is provided for detecting a property of a gas comprising: emitting a light, comprising a plurality of wavelengths covering a plurality of absorption lines of the gas, along a first axis, the light being scattered by particles of the gas resulting in a scattered light, generating a sensor image using a detection arrangement configured to receive the scattered light and comprising: an optical arrangement having an optical plane and being configured to direct the scattered light on to a light sensor, the light sensor having at least one pixel columns, wherein the pixel columns are aligned to an image plane and configured to output a sensor image, wherein the first axis, the optical plane, and the image plane intersect such that a Scheimpflug condition is achieved, determining, from the sensor image, properties of the gas at a plurality of positions along the first axis.

Description

High Spectral Resolution Scheimpflug Lidar Technical Field The present disclosure relates to laser projection systems and more particularly to Scheimpflug LIDAR systems and methods.

Background Art A lidar or laser radar is an optical device for detection and ranging with applications in a very broad range of environments, from industrial combustion furnaces to ecosystem monitoring. In contrast to the now wide-spread topographical lidar systems which detect and range hard targets, atmospheric lidars have sufficient sensitivity to retrieve a continuous molecular echo from entirely clean air.

Atmospheric lidars have been around for several decades and they have been extensively applied to vertical profiles of aerosols in the troposphere. They are typically implemented in containers or trucks with systems weights of several tons. Smaller commercial systems of several hundreds of kg have also been developed. The cost is approximately 1 Euro per gram of equipment. The conventional method relies on Time-of-Flight (ToF) with expensive and bulky pulsed neodymium-doped yttrium aluminium garnet lasers of several hundred kg, the laser provide high peak powers (-100 MW) but the lidar resolution is poor in time and space (approx. -1 minute and -50 m). Elastic aerosol lidar systems are exceedingly challenging to calibrate and yield very little specificity for the aerosols sensed unless they are expanded by N2and O2Raman channels. Raman channels only work during night time, are very noisy and require even longer, averaging typically 10 minutes to produce a usable signal.

A highly specific atmospheric lidar method is the Differential Absorption Lidar (DIAL). In this method a pulsed tuneable laser targets specific molecular absorption lines and concentration profiles of a gas can be acquired. In practice, high peak powers (MW), short pulses (ns), narrow bands ( There exists a need of a method sufficiently sensitive to profile, map and visualize the exhausted O2-hole from a single breath, e.g., with a resolution of millimetres and not meters, and capable of operating within milliseconds and not minutes in order to capture the exhaust plume as it flies by, and to determine the exhaust temperature.

In fact, the applicant has already reduced atmospheric lidarto a tenth of the ToF lidar cost and weight while improving the resolution thousand times. This is accomplished by the implementation of inexpensive Continuous Wave (CW) Laser Diodes (LDs) and linear 1D CMOS detectors in so called Scheimpflug condition described in Swedish patent application 1730251-4.

Laser diodes are readily tuneable by temperature or current, a feature exploited in the technique Tuneable Diode Laser Absorption Spectroscopy (TDLAS) where a single mode laser diode can be scanned over a molecular gas absorption line. However, single-mode lasers emit just one percent of the power compared to multimode diodes and they are too week for daytime operation of Scheimpflug lidar. Single mode diodes can be amplified a factor of hundred by tapered amplifiers and fibres, this is expensive and subject to current research and no off-the-shelf amplified sources are available. Further, both TDLAS and conventional DIAL have a significant drawback: It takes time to scan the wavelength and in the meanwhile the atmosphere is changing and the static atmosphere assumptions ruined, yielding poor signal certainty.

The applicant expanded the Scheimpflug lidar concept to inelastic hyperspectral lidar of broadband features by using a 2D detector and dispersive optics. This system was employed for fluorescence and Raman sensing in aquatic environments. The system was constructed for a couple of thousands Euros and with a weight of a couple of kg. It was so cheap and small that a replicate was placed on a drone for hyperspectral scanning of vegetation structure resolving the tiniest vegetation structures with unpreceded spatial, temporal and spectral resolution.

The aquatic and vegetation inelastic hyperspectral lidar was tailored with 60 spectral bands of 5 nm resolution to cover the range 430 nm to 750 nm which spans over the large Stokes shift of Chlorophyll fluorescence. The elastic band had to be attenuated a factor hundred to match the magnitude of the inelastic bands and to avoid sensor saturation.

Summary It is an objective of the disclosure to at least partly overcome one or more of the above-identified limitations of the prior art. One or more of these objectives, as well as further objectives that may appear from the description below, are at least partly achieved by means of a method for data processing, a computer readable medium, devices for data processing, and an optical apparatus according to the independent claims, embodiments thereof being defined by the dependent claims.

A first embodiment provides a method for detecting a property of a gas comprising: emitting a light covering a plurality of spectral bands along a first axis 30, the light being scattered by particles of the gas resulting in a scattered light, generating a sensor image using a detection arrangement 40 configured to receive the scattered light and comprising: an optical arrangement 50 having an optical plane 60 and being configured to direct the scattered light on to a light sensor 70, the light sensor 70 having a plurality of pixel columns, wherein the pixel columns are aligned to an image plane 80 and configured to output a sensor image 75, wherein the first axis 30, the optical plane 60, and the image plane 80 intersect such that a Scheimpflug condition is achieved, determining, from the sensor image, properties of the gas at a plurality of positions along the first axis. Preferably, the scattered light is distributed across light sensor 70 such that the photons of the scattered light are distributed along the pixel columns of the light sensor according to a position along the first axis where the photons were scattered. In one embodiment, the optical arrangement 50 is configured to spectrally distribute the scattered light across light sensor 70. In some configurations, the scattered light is distributed according to photon energy. In other configurations, the scattered light is distributed according to a Fourier or frequency distribution, e.g. an interferogram. In an embodiment in which the light sensor comprises a plurality of columns and pixel rows, the scattered light is distributed across light sensor 70 such that the photons of the scattered light are distributed along the pixel rows of the light sensor according to the wavelengths of the photons. The scattered light may be distributed across light sensor 70 via a spectrometer or interferometer, e.g. a dispersive spectrometer, a virtually imaged phase array, or a Fabry-Perot cavity. Preferably, the optical arrangement 50 further comprising a band pass filter and/or a slit to remove ambient noise (stray light) or to provide high-quality spectrally sensitive data. Also preferably, the optical arrangement 50 is spectrally sensitive and further comprising a lens and a slit and wherein the lens is configured to focus the scattered light onto the slit. In one embodiment, optical arrangement 50 comprises a first lens to focus the scattered light onto the slit, then the light from the slit is collimated by a second lens, and then passed through a grating, a dispersive element, or an interferometer. A third lens then focusses the light from the grating, a dispersive element, or an interferometer onto the light sensor 70. One embodiment of this is shown in figure 4 and described below. Preferably, the arrangement is 1:1 imaging, meaning that if the slit is placed at the Brewster angle with respect to the axis of the received light, the sensor will end up at a Brewster angle to the same axis. Arranging the slit at the Brewster angle advantageously allows the reduction of ghosting and optimization of the amount of detected light, i.e., more fraction of the light will hit the sensor instead of being reflected form the surface. In some embodiments, the tilted slit is between 20 and 200 microns.

In one embodiment, a concentration of the gas is determined at one or more positions along the first axis from an attenuation of at least one spectral band of the scattered light received at the light sensor. This may be achieved by determining an integral of the attenuation of at least one spectral band with respect to the position along the first axis. Alternatively, a concentration of the gas is determined according to a differential absorption lidar. Furthermore, a temperature of the gas may be determined at one or more positions along the first axis from a ratio between an attenuation of at least two spectral bands i.e. absorption line imprints (the signal attenuated by an absorption line) of the scattered light received at the light sensor, e.g. where temperature is high, ratio between two absorption lines may be greater than when temperature is low. Alternatively or in combination with this method, a temperature and/or pressure of the gas may be determined based on a fitting of a gas absorption profile to at least one spectral band of the scattered light received at the light sensor, e.g. fitting based on height, width, and shape of gas absorption line.

In some embodiments, the emitted light comprises photons having a wavelength in the range 760 nm to 762 nm, and wherein the method is used to determine O2levels in the gas. Alternatively, the emitted light comprises photons having a wavelength in the range 934 nm to 936 nm, and wherein the method is used to determine H2O levels in the gas. Other wavelength ranges suitable to determine the characteristics of other gas particle are known to the skilled man. The emitted light may be comprised of to cover between 2 and 800 elastic spectral bands.

In one embodiment, a light source used to generate the emitted light is a multimode continuous wave laser diode. In this embodiment, the spectral bands are covered by the emitted light output simultaneously. In this embodiment, the scattered light is spectrally distributed across light sensor 70 in one dimension and distributed across light sensor 70 in a second dimension according to a position along the first axis where the photons of the scattered light were scattered.

In an alternative embodiment to the above, a light source for the emitted light is a single mode laser diode configured to generate a spectrally narrow emission which is scanned across the absorption lines over the desired wavelength range i.e. the absorption line wavelengths of the target gas. In this embodiment, the scattered light is distributed across light sensor 70 in a one dimension according to a position along the first axis where the photons of the scattered light were scattered and spectral distribution of the scattered light is determined with respect to time.

Another embodiment provides a device 100 for detecting a property of a gas comprising: a light source 20 configured to emit spectrally broad light covering a plurality of spectral bands along a first axis 30, the light being scattered by particles of the gas resulting in a scattered light, a light detection arrangement 40 comprising: an optical arrangement 50 having an optical plane 60 and being configured to direct the scattered light on to a light sensor 70, the light sensor 70 having a plurality of pixel columns, wherein the pixel columns are aligned to an image plane 80 and configured to output a sensor image 75, wherein the first axis 30, the optical plane 60, and the image plane 80 intersect such that a Scheimpflug condition is achieved, the device being configured to determine, from the sensor image, properties of the gas at a plurality of positions along the first axis.

Brief Description of Drawings These and other aspects, features and advantages of which examples of the invention are capable of will be apparent and elucidated from the following description of examples of the present invention, reference being made to the accompanying drawings, in which; Figure 1 displays O2lines at different temperature superimposed on the emission of a multimode laser diode.

Figure 2 shows an apparatus according to an embodiment of the disclosure. Figure 3 shows a schematic of an embodiment of the disclosure.

Figure 4 shows an embodiment of the optical arrangement.

Figure 5 shows a schematic of an embodiment of the disclosure.

Detailed Description of Example Embodiments In the following, embodiments of the present invention will be presented for a specific example of a gas analysing apparatus.

Embodiments are presented for an elastic hyperspectral lidar for pursuing differential absorption lidar on atmospheric gasses. A multimode 10W, 761 nm, 2nm FWFIM (Full width at half maximum) CW (continuous wave) laser diode may be employed. The acquisition of some 400 elastic spectral bands in the range 760 nm to 762 nm is performed. This will allow the resolving of a large number of O2absorption lines, as shown in figure 1. The absorption lines provide information on concentration, pressure and temperature of the air. Generally O2concentration is 21%, but local exhausts after metabolism or combustion can produce O2holes. The drop in O2corresponds to the rise in CO2and H2O and the drop in O2provides information on, e.g., the amount of metabolism present or the amount of fuel providing a mean for normalizing aerosol emissions and assessing engine quality. This technique allows indirect assessment of profiling of CO2, pressure and temperature. The plot of figure 1 displays O2lines at different temperatures superimposed on the emission of a multimode laser diode. Temperature and pressure are key parameters which are difficult to determine in meteorology and industrial processes.

The TDLAS community has developed their techniques over several decades. TDLAS requires great source stability and the accuracy result from the assumption of a static and spectrally flat baseline. In practice the majority of uncertainties in TDLAS arise from source noise, speckles or interference fringes, this noise source scales with source power and stronger sources does not help. However, the described employment of multimode diodes in Scheimpflug DIAL is made possible by two facts: 1) Absorption in DIAL does not rely on a baseline and a stable, fringe-free source because absorption is derived from the range derivative in Beer-Lamberts law and this is unrelated to laser stability. 2) All the spectral bands and on- and off-resonance lines are illuminated and exposed simultaneously. In other words, it will be a hyperspectral snapshot of the same atmosphere in space and time. A multimode Scheimpflug DIAL has light speed synchronization and self-calibration.

Gas temperature is a key parameter for many lidar applications. Temperature is challenging in TDLAS because it requires the single mode laser diode to scan broadly over at least one absorption line without mode hopping. In ToF DIAL, apart from bulkiness, temperature is challenging because of low laser repetition rate and a trade-off between number of bands and static atmosphere assumptions. As can be seen in figure 1, a single high power multimode laser diode is capable of illuminating a large number of absorption lines, and some of these lines change substantially over ambient temperatures. In other words, a Scheimpflug O2DIAL system can be thought of as thousands of light-speed thermometers which can profile vertical gradients in ecosystems or be swept over an area to produce thermal air maps.

Dispersing light over 2 nm span into hundreds of spectral bands on a CMOS detector is not a trivial task. Never-the-less this has already been accomplished for other remote sensing instrument for Solar Induced Fluoresce (SIF) on vegetation such as NASAs carbon observatory. It can be done by brute force and employing a dispersive spectrometer of roughly one meters focal length. High Spectral Resolution Lidar can also be pursued by clever approaches with devices such as VIPAs (Virtually Imagined Phase Arrays) or Fabry-Perot cavities.

Figure 2 shows an embodiment of the apparatus 100. Flardware processor 10 drives light source 20 to emit light along a first axis 30. The light travels along axis 30 until being scattered back towards light detection arrangement 40 by a particle 90. Light detection arrangement 40 comprises an optical arrangement 50 having an optical plane 60 and being configured to direct the light scattered by the scattering particle to a light sensor 70. Light sensor has a pixel column aligned to an image plane 80 and configured to output a sensor signal 75 to the hardware processor. The first axis, the lens plane, and the image plane intersect such that a Scheimpflug condition is achieved. Furthermore, a displaced image plane 82, a front focal plane 62 of the lens arrangement, and a relationship between the light source and the light detection arrangement fulfil the Hinge rule at intersection 63. Hardware processor 10 processes the sensor signal to determine a pixel signal for one or more pixels of the light sensor.

Figure 3 shows a schematic of the apparatus according to an embodiment of the invention. A high power broad band multimode diode is temperature-tuned to the O2A-band around 761 nm and transmitted to the atmosphere. The collected atmospheric echo is filtered, dispersed by a 4 cm Fabry-Perot etalon and projected onto a 2D detector array. A strobe modulates the laser diode for background subtraction.

In certain embodiments, the transmitter-receiver baseline separation may be in the order 10-20 cm, the emitted beam may be ~?5 mm, the receiver may be ~?75 mm. Receiver focal length may be -20 cm. The tilted slit may be 50 ?m and dispersed on -400 spectral bands on a 20x30 mm 2D Si-CMOS array. The instrument may contain three linear micro actuators for adaptive alignment. These actuators control beam divergence, receiver focus and the overlap between beam and field-of-view. These actuators work on a slow time scale to optimize the signals in a closed-loop manner where the signal quality is continuously evaluated (signal strength and focus). The entire instrument including the transmitter, receiver and all electronics may be contained in a large extruded aluminium rectangular tube with dimensions smaller than 12x30x80 cm outer dimension and weigh approximately 15 kg. The instrument may be hermitically sealed and weather proof.

A typical range for the embodiment described above is 200 m, but if necessary the method could be scaled up to profile the entire troposphere by increasing receiver optics (the signal is not subject to 1/r<2>attenuation in Scheimpflug lidar). Spatial resolution is better than 1% of the range. Typical time resolution may be less than 1 second. Concentration precision below 1 per mille O2and temperature better than 1° C may be determined. The fact that an O2instrument covers a broad range of 2 nm including many absorption lines, some of which first saturates and later broaden with increasing range, implies that a high precision can be verified. There are various options for spatial and temporal averaging, exposure time and on-chip binning which could optimize precisions in various domains when necessary.

Figure 4 shows an embodiment of optical arrangement 50. In this embodiment, scattered light 220 is received by lens 210. Lens 210 focusses light onto slit 230. An axis 235 of slit 230 and an axis 225 of lens 210 cross at a first Scheimpflug intersect point 240. The light from the slit is then directed onto collimating lens 250, which collimates the light and directs it through component 260. Component 260 may be a spectrometer or interferometer, e.g. a dispersive spectrometer, a virtually imaged phase array, or a Fabry-Perot cavity. Lens 270 then focuses the light onto sensor 70. The axis 80 of light sensor 70 and the axis 235 of slit 230 cross at a second Scheimpflug intersect point 280. The angle between axis 80 of light sensor 70 and the axis 265 along component 260 is the same as the angle between axis 235 of slit 230 and the axis 265 along component 260.

Figure 5 shows an embodiment of the invention configured with a single mode laser diode. This embodiment may be used to perform gas analysis via scanning and/or DIAL functions.

Some embodiments described above may be spectrally calibrated to ensure that the sensor signal outputs received wavelengths that match reality. Calibration may be performed according to at least the following methods. In a first embodiment wherein the apparatus comprises a single mode tuneable diode laser, the laser is configured to output a known wavelength. The sensor signal is then calibrated using the known wavelength. This may be repeated for a plurality of wavelengths to ensure calibration across the spectrum. In a second embodiment wherein the apparatus comprises a multi-mode diode laser configured to output a plurality of spectral bands, wavelength calibration is performed through matching of the intrinsic spectral positions of the absorption line profiles of a known target gas to the absorption lines indicated in the sensor signal.

Claims

1. A method for detecting a property of a gas comprising: emitting a light, comprising a plurality of wavelengths covering a plurality of absorption lines of the gas, along a first axis (30), the light being scattered by particles of the gas resulting in a scattered light, generating a sensor image using a detection arrangement (40) configured to receive the scattered light and comprising: an optical arrangement (50) having an optical plane (60) and being configured to direct the scattered light on to a light sensor (70), the light sensor (70) having at least one pixel columns, wherein the pixel columns are aligned to an image plane (80) and configured to output a sensor image (75), wherein the first axis (30), the optical plane (60), and the image plane (80) intersect such that a Scheimpflug condition is achieved, determining, from the sensor image, properties of the gas at a plurality of positions along the first axis.

2. The method of claim 1, wherein the scattered light is distributed across light sensor (70) such that the photons of the scattered light are distributed along the pixel columns of the light sensor according to a position along the first axis where the photons were scattered.

3. The method of any proceeding claim, wherein the optical arrangement (50) is configured to spectrally distribute the scattered light across light sensor (70).

4. The method of any proceeding claim, the light sensor (70) having a plurality of pixel rows and wherein the scattered light is distributed across light sensor (70) such that the photons of the scattered light are distributed along the pixel rows of the light sensor according to a wavelength of the photons.

5. The method of claim 4, wherein the scattered light is distributed across light sensor (70) via a spectrometer or interferometer, e.g. a dispersive spectrometer, a virtually imaged phase array, or a Fabry-Perot cavity.

6. The method of any proceeding claim, wherein the optical arrangement (50) further comprising a band pass filter to remove ambient noise.

7. The method of any proceeding claim, wherein the optical arrangement (50) further comprising a lens and a slit and wherein the lens is configured to focus the scattered light onto the slit.

8. The method of claim 7, wherein the slit and image sensor are arranged at a Brewster angle relative to an axis of the received scattered light.

9. The method of claim 7 or 8, wherein the tilted slit is between 20 and 200 microns.

10. The method of any proceeding claim, wherein a concentration of the gas is determined at one or more positions along the first axis from an attenuation of at least one spectral band of the scattered light received at the light sensor.

11. The method of claim 10, wherein a concentration of the gas is determined from an integral of the attenuation of at least one spectral band with respect to the position along the first axis.

12. The method of claim 10, wherein a concentration of the gas is determined at one or more positions along the first axis from a ratio between an attenuation of at least two absorption lines of the scattered light received at the light sensor.

13. The method of any proceeding claim, wherein a temperature of the gas is determined at one or more positions along the first axis from a ratio between an attenuation of at least two absorption lines of the scattered light received at the light sensor.

14. The method of any proceeding claim, wherein a temperature and/or pressure of the gas is determined based on a fitting of a gas absorption profile to at least two spectral bands of the scattered light received at the light sensor.

15. The method of any proceeding claim, wherein the concentration, temperature or pressure is determined based on a ratio of at least two spectral bands and a derivative with respect to the position along the first axis.

16. The method of any proceeding claim, wherein the emitted light comprises photons having a wavelength in the range 760 nm to 762 nm, and wherein the method is used to determine O2levels in the gas.

17. The method of any proceeding claim, wherein the emitted light comprises photons having a wavelength in the range 934 nm to 936 nm, and wherein the method is used to determine H2O levels in the gas.

18. The method of any proceeding claim, wherein emitted light is spectrally comprised to cover between 2 and 800 elastic spectral bands.

19. The method of any proceeding claim, wherein a light source for the emitted light is a multimode continuous wave laser diode configured to generate the spectral bands simultaneously.

20. The method of claim 1 , wherein a light source for the emitted light is a single mode laser diode configured to generate the spectral bands by spectral scanning.

21. A device (100) for detecting a property of a gas comprising: a light source (20) configured to emit a light comprising a plurality of spectral bands along a first axis (30), the light being scattered by particles of the gas resulting in a scattered light, a light detection arrangement (40) comprising: an optical arrangement (50) having an optical plane (60) and being configured to direct the scattered light on to a light sensor (70), the light sensor (70) having a plurality of pixel columns, wherein the pixel columns are aligned to an image plane (80) and configured to output a sensor image (75), wherein the first axis (30), the optical plane (60), and the image plane (80) intersect such that a Scheimpflug condition is achieved, the device being configured to determine, from the sensor image, properties of the gas at a plurality of positions along the first axis.

22. The device of claim 21, wherein the light source is a single mode tuneable diode laser, and wherein the sensor image is calibrated by: emitting a light having a first wavelength, calibrating the sensor image according to the first wavelength.

23. The device of claim 21, wherein the light source is a multi-mode diode laser, and wherein the sensor image is calibrated by: emitting a light having a plurality of spectral bands, calibrating the sensor image by matching a plurality of intrinsic spectral positions of absorption line profiles ofthegasto the absorption lines indicated in the sensor signal.