GAS LEAK DETECTOR AND DETECTION METHODS

A gas leak detector includes a solar detector and a signal filter. The solar detector generates an electrical response by interfering a light signal with a solar signal and detecting a resultant interference signal. The signal filter is communicatively coupled to the solar detector and filters the electrical response to isolate a beat-note signal. The beat-note signal has an amplitude that is inversely related to a concentration of a species that forms a gaseous plume located along a path of the solar signal.

BACKGROUND

Methane gas is a potent green-house gas due to its strong absorption of infrared (IR) light. Methane gas in the earth's atmosphere serves to absorb sunlight on its way to the earth's surface and light emitted from the earth's surface, e.g., reflected, and emitted blackbody radiation. Methane is a more potent greenhouse gas than carbon dioxide due to its larger absorption of IR light. A main process by which methane is introduced into the atmosphere is by leaks. Methane storage and manufacturing facilities unintentionally release methane gas due to system failures and poor oversight. Detection of methane leaks is crucial to mitigating its unintended release into the environment.

SUMMARY OF EMBODIMENTS

In a first aspect, a gas leak detector includes a solar detector and a signal filter. The solar detector generates an electrical response by interfering a light signal with a solar signal and detecting a resultant interference signal. The signal filter is communicatively coupled to the solar detector and filters the electrical response to isolate a beat-note signal. The beat-note signal has an amplitude that is inversely related to a concentration of a species that forms a gaseous plume located along a path of the solar signal.

In a second aspect, a method for detecting a gas leak includes detecting an interference signal produced from interference of a solar signal with a light signal to generate an electrical response. The method also includes filtering the electrical response to isolate a beat-note signal having an amplitude that is inversely related to a concentration of a species that forms a gaseous plume located along a path of the solar signal.

In a third aspect, a photonic integrated circuit for gaseous leak detection includes a multimode interference coupler, a first grating coupler, a second grating coupler, an output grating coupler, and a detector. The multimode interference coupler has a first input port, a second input port, and an output port. The first grating coupler is coupled to the first input port and couples a solar signal into the multimode interference coupler. The second grating coupler is coupled to the second input port and couples a light signal into the multimode interference coupler. The output grating coupler is coupled to the output port, which outputs an interference signal. The detector is coupled to the output grating coupler, and generates an electrical response to detection of the interference signal

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG.1illustrates a gas leak detector100that determines a concentration and location of a gaseous plume180that includes a species181. Plume180is located at an altitude189above surface194, which may be a terrestrial surface. Gas leak detector100includes a local oscillator110, a solar detector130, and electronics140. Electronics140includes a signal filter144, and may also include at least one of an amplifier142, an RF amplifier145, and a signal detector146.

In an example mode of operation, solar detector130generates an electrical response184by interfering a light signal119with a solar signal182propagating from a source196, which may be the sun. Solar detector130may include at least one of an interferometer (e.g., an optical fiber interferometer), a balanced detector, and a product detector. Signal filter144filters electrical response184to isolate a beat-note signal186, the amplitude of which is inversely related to (e.g., a decreasing function of) a concentration of species181along a path191of solar signal182.

In an embodiment, a local oscillator110generates light signal119with one or more light-signal frequencies associated with a spectral line of species181. Examples of gas-phase species include ozone, carbon dioxide, methane, nitrous oxide, water, and dichlorodifluoromethane (CCl2F2). A linewidth of light signal119may be less than a spectral width of the species181's spectral line. For example, when species181is methane, light signal119may have a free-space wavelength between 1630 nm and 1680 nm. A linewidth of light signal119may be less than 2 MHz.

In an embodiment, gas leak detector100further includes a controller121that sets the one or more light-signal frequencies based at least in part on (a) intensity of solar signal182and/or (b) target-species concentrations along path191. Controller121may for example set the one or more light-signal frequencies to optimize sensitivity or signal strength of gas leak detector100. For example, when species181is methane, and the solar signal182propagates through large target-species concentrations before reaching the solar detector130, light resonant with methane absorption may be completely absorbed and a different light-signal frequency may produce more sensitive methane leak detection.

Electronics140may include a signal detector146that detects beat-note signal186isolated by signal filter144. The primary spectral content of beat-note signal186may be at radio-frequencies, and thus signal detector146may be an RF detector. When detector100includes amplifier145, amplifier145amplifies beat-note signal186to generate an amplified beat-note signal188, which is then detected by signal detector146. In embodiments, detector100does not include amplifier145, and beat-note signal188is identical to beat-note signal186.

In embodiments, at least one of: amplifier145is a low noise RF power amplifier; beat-note signal186and amplified beat-note signal188are RF signals; and solar detector130connects with, or includes, amplifier142. Amplifier142amplifies current183output by solar detector130into usable voltage as electrical response184, which is received by signal filter144. Amplifier142may be a transimpedance amplifier. In embodiments, detector100does not include amplifier142, and electrical response184is identical to current183.

In embodiments, gas leak detector100includes a data processor120. Electronics140outputs an analog signal149, which data processor120processes to determine altitude189. Data processor120includes a processor122and a memory124. Memory124stores non-transitory computer-readable instructions as software125, which includes a lineshape generator126and a lineshape discriminator128. When executed by processor122, software125causes processor122to implement selected functionality of gas leak detector100as described herein. Software125may be, or include, firmware. Controller121may be part of data processor120, for example, as part of software125.

Data processor120is communicatively coupled to electronics140, e.g., to signal detector146, to determine altitude189of a gaseous plume180present along path191of solar signal182. The absorption spectrum plume180is affected by total atmospheric pressure of the methane gas at specific altitude. Thus, a plume180at high altitudes within earth's atmosphere will be spectrally distinct from a plume of identical species located at low altitudes within earth's atmosphere. Data processor120may for example determine an altitude of gaseous plume180by comparing an absorption spectrum derived from beat-note signal186to previously measured absorption spectra at known pressures.

Memory124may store such spectra as a plurality of fitting parameters192. Fitting parameters192may include measured spectra of the same absorption line of species181, and differ in terms of the atmospheric pressure during measurement. Since atmospheric pressure affects line shape, data processor120may use fitting parameters192to determine altitude189by fitting absorption spectrum176to one or more fitting parameters192. When path191traverses multiple gaseous plumes180at different altitudes189, lineshape generator126generates, from analog signal149, absorption spectrum176that includes absorption lines of species181at different altitudes, and hence different atmospheric pressures. Lineshape discriminator128fits absorption spectrum176to multiple fitting parameters192to determine multiple altitudes at which gaseous plumes180are present in addition to background concentrations of species181.

In embodiments, fitting parameters192including broadening coefficients for each of one or more lineshape functions that describe absorption spectrum176. In such embodiments, fitting parameters192may include including broadening coefficients, and not include measured absorption spectra. Example lineshape functions include Gaussian, Lorentzian, and combinations thereof, such as a Voigt profile. For a given species181, fitting parameters192may include a respective broadening coefficient for each of a plurality vibrational and/or rotational modes of species181Each broadening coefficient may be a function of atmospheric pressure, such that lineshape discriminator128determines, based on absorption spectrum176, one or more altitudes at which gaseous plumes180are present.

Solar detector130and signal filter144may be integrated into a PIC108to form a photonic integrated circuit (PIC). See alsoFIG.7. Processor120may receive signals from PIC108and make determinations therefrom such as target-species concentrations, methane plume location, and three-dimensional tomographic datasets.

PIC108may also integrate at least one of local oscillator110, controller121, processor120, amplifier145, signal detector146, and other electronic components of gas leak detector100. Such integration onto PIC108serves several benefits. First, gas leak detector100can be small and lightweight when integrated onto a PIC and therefore may be used in aerial drone applications in sensing methane or other target gases. Second, PIC108may allow solar detector130and amplifier145to be closely-coupled to reduce noise and improve overall sensitivity of gas leak detector100. Third, the size of the solar detector130can be reduced, for example, its diameter may be reduced from approximately three-hundred microns to thirty microns. This size reduction reduces the intrinsic capacitance of solar detector130based on the areal ratio,

by a factor of 100. For high-speed detection (e.g., repetition rates greater than a few hundreds of MHz), the reduced capacitance of solar detector130translates to lower voltage noise, further improving the signal to noise ratio of gas leak detector100. Fourth, the likelihood and severity of optical loss between elements of gas leak detector100are reduced.

Local oscillator110includes a light source116, and may include at least one of a wavelength modulator112, an amplitude modulator114, and a laser driver113. In embodiments, light source116is a scannable single-frequency laser, such as a diode laser, examples of which include a vertical-cavity surface-emitting laser (VCSEL), and a distributed feedback laser.

In embodiments, wavelength modulator112modulates frequency of light signal119, which beneficially allows for lock-in amplification and/or other forms of noise reduction to increase overall sensitivity of gas leak detector100.

Gas leak detector100may include an amplitude modulator114that is for example part of local oscillator110, as shown; though amplitude modulator114may be a separate element communicatively coupled to local oscillator110. In embodiments, amplitude modulator114modulates amplitude of light signal119, which beneficially allows for lock-in amplification and/or other forms of noise reduction to increase overall sensitivity of gas leak detector100. Amplitude modulator114may be an optical chopper that amplitude modulates signal182before reaching solar detector130, which beneficially allows for lock-in amplification and/or other forms of noise reduction to increase the overall sensitivity of gas leak detector100. Optical chopper151may be physically integrated with solar detector130and/or PIC108, or it may be a separate component.

Light source116may be temporally-tuned through its wavelength range at a scanning frequency, which may be between 100 Hz and 1 kHz. For example, laser driver113may be coupled to light source116and operates to temporally tune the center wavelength of light source116via a drive signal, which may be a current or voltage that is time-varying, e.g., a periodic waveform.

In an embodiment, gas leak detector100includes a spectral filter152that transmits solar signal182while blocking unwanted portions of broadband emissions from source196. Spectral filter152may be located along path191of solar signal182, unattached to PIC108, as shown; though spectral filter152may be integrated to or with solar detector130and/or PIC108without departing from the scope hereof.

In an embodiment, gas leak detector100includes collection optics154located along path191. Collection optics154may be one or more separate optical elements, as shown inFIG.1, or may be integrated (e.g., as fiber optics) with PIC108, spectral filter152or solar detector130without departing from the scope hereof light-collection optics154may be a mirror, such as an off-axis parabolic mirror. Collection optics154serve to focus solar signal182into solar detector130to increase efficiency and signal strength, and to reduce noise. Collection optics154may be fiber-coupled to solar detector130.

In the embodiment illustrated inFIG.1, optical chopper151, spectral filter152and collection optics154are positioned so that solar signal182interacts with all three, and in this order; though the order of may vary without departing from the scope hereof. Optical chopper151, spectral filter152, and collection optics154may for example attach to one another or be mounted individually within gas leak detector100.

In an embodiment, gas leak detector100includes an anemometer156, which may be communicatively coupled to data processor120. Anemometer156assists in locating methane leak location, for example the location of gaseous plume180. The measured distance and relative location of gaseous plume180to gas leak detector100varies more dramatically without data from anemometer156. Anemometer156may for example measure wind speed and/or wind direction usable to isolate methane leak location.

As noted above, source196may be the sun, which emits radiation traveling directly to gas leak detector100after passing through the earth's atmosphere. Source196may be the moon, which reflects light emitted from the sun toward, through the earth's atmosphere, and toward gas leak detector100. When the source196is the moon, it may be necessary to use amplifier145to generate amplified beat-note signal188so that gas leak detector100has sufficient sensitivity to detect methane leaks, e.g., gaseous plume180.

FIGS.2A and2Bare schematics of a compound gas leak detector201, which includes an array of gas leak detectors200. Each gas leak detector200is an example of gas leak detector100ofFIG.1.FIGS.2A and2Billustrate the detector201at different times of the day, illustrated by source196(e.g., the sun) shown at different positions in the sky.FIGS.2A and2Bare best viewed together along with the following description. ThoughFIGS.2A and2Billustrates P systems where integer P is at least four, the total number of detectors200of compound gas leak detector201may be two or three without departing from the scope hereof.

In embodiments, compound gas leak detector201includes a data processor220, which receives respective beat-note signals186or188from each of detectors200. Data processor220is an example of data processor120, and determines a respective altitude189(p) from each detector200(p), where index p≤P. Data processor220may be part of any one of detectors200, or alternatively be separate from each of detectors200such that each detector200is communicatively coupled to data processor220.

Gas leak detectors200may be spatially arrayed in one or two dimensions. InFIG.2A, each gas leak detector200receives solar signal282(e.g.,282(1), which is an example of solar signal182); and each gas leak detector200isolate a beat-note signal (e.g., beat-note signal186,FIG.1) from the solar signal282it receives. InFIG.2A, source196emits solar signals282to each gas leak detector200from its present location; but inFIG.2B, source196has moved across the sky so appears at a different location, and hence is designated as source196′. Source196is shown inFIG.2Bin dotted outline to aid in illustrating the motion of the source (196/196′) across the sky. InFIG.2B, given the location of source196′, each gas leak detector200receives solar signal282′ (e.g.,282′(1), which is an example of solar signal182) from the location of source196′. Each pair of solar signals (e.g.,282(1) and282′(1)) are received by each gas leak detector (e.g.,201(1)) at different times of day as earth turns and source196traverses the sky.

Accordingly, the plurality gas leak detectors200, each receiving solar signal282at multiple times during the day, constructs a two-dimensional tomographic dataset of the atmosphere viewed by the plurality of gas leak detectors200. Each point in the two-dimensional tomographic dataset includes a methane concentration present along the path (not shown) of solar signal282from the source196/196′ to the gas leak detector200that absorbs it, thus resolving the three-dimensional location (which may include an altitude) of a methane plume (e.g., plume180,FIG.1).

In embodiments, detector100includes a solar tracker358used to maximize intensity of a solar signal182/182′ into solar detector130. In the embodiment shown inFIG.3, solar tracker358includes a light-collection optic354that directs solar signals182/182′ into the solar detector130as a solar source196moves. light-collection optic354is an example of collection optics154. In another embodiment, solar tracker358moves solar detector130directly (not shown) so that solar detector130is best positioned to receive solar signals182/182′ while source196moves. Thus, solar signal182is emitted from source196at a first time and solar signal182′ is emitted from source196′ a second time; source196and196′ is the same physical object that moves with respect to solar detector130over a given time period.

Operationally, solar tracker358maximizes intensity of solar signals182/182′ reaching the solar detector130using (a) solar data indicating position of source196/196′ of the solar signal182/182′ with respect to solar detector130and/or (b) intensity of solar signal182/182′ reaching solar detector130at a given time. Solar tracker358may for example make use of known astronomical data to position light-collection optic354(in the embodiment illustrated inFIG.3) or solar detector130(in the alternative embodiment) maximize the amount of the solar signal182/182′ into the solar detector130. Additionally, or instead, the solar tracker358may iteratively control the position of solar signal182/182′ into solar detector130using intensity of solar signal182/182′ reaching solar detector130at a given time, meaning it iteratively improves the alignment based on recent measurement and alignment.

FIG.4illustrates a multi-wavelength gas leak detector400that includes a local oscillator410, which generates a plurality of light signals419(1,2, . . . , M) that have a respective one of M center wavelengths. Gas leak detector400and local oscillator410are respective examples of gas leak detector100and local oscillator110. In embodiments, local oscillator410includes at least one light source, such as any combination of single-frequency lasers and tunable lasers, that collectively produce each light signal419. In a first example, local oscillator410may include a single tunable laser that produces each light signal419. In a second example, local oscillator410may include M light sources, e.g., M single-frequency lasers, each of which produces a respective light signal419.

The plurality of light signals419are received by solar detector130, which mixes each of light signals419with solar signal182to generate one of a plurality of electrical responses484(1-M), each containing a respective beat-note signal486to form a plurality of beat-note signals486(1-M). Signal filter144filters each of the plurality of electrical responses484, to isolate its respective beat-note signal486to be recorded by signal detector146. Signal detector146records each beat-note signal486and outputs an analog signal449to processor120. Analog signal449is an example of analog signal149.

For example, local oscillator410generates light signal419(2), which is mixed with solar signal182to generate electrical response484(2) that contains beat-note signal486(2). Signal filter144isolates beat-note signal486(2), which is recorded by signal detector146and transmitted to processor120as analog signal449. When each of the plurality of beat-note signals486is plotted with respect to the light-signal frequency of corresponding light signal419, a spectrum476is generated. Spectrum476is an example of absorption spectrum176.

FIG.5is a schematic of electronics540, which is an example of electronics140,FIG.1. Electronics540includes signal filter544and signal detector546, which are respective examples of signal filter144and signal detector146. Signal filter544includes a plurality of sub-filters547. Signal detector546includes a plurality of sub-detectors548. Each of the plurality of sub-filters547is associated with a respective frequency range to isolate a corresponding frequency-domain portion of an electrical response584, which is an example of electrical response184. For example, sub-filter547(2) isolates a portion of the electrical response584(2).

Each sub-detector548(n) is communicatively coupled to one sub-filter547(N), as shown, where index n≤N. Each of the sub-detectors548(n) records the portion of electrical response584(n) isolated by its corresponding sub-filter547(n). For example, sub-detector548(2) communicatively couples to sub-filter547(2) and thereby is able to detect the corresponding portion of electrical response584(2). Portions of electrical response584recorded by the sub-detectors548, when graphed versus frequency ranges of the corresponding sub-filter547, generates spectrum476,FIG.4.

FIG.6is a flowchart illustrating a gas leak detection method600. In some implementations, one or more process blocks ofFIG.6may be performed by an embodiment of gas leak detector100. In some implementations, one or more process blocks ofFIG.6may be performed by another device, or a group of devices separate from or including gas leak detector100. Additionally, or alternatively, one or more process blocks ofFIG.6may be performed by one or more components of gas leak detector100.

As shown inFIG.6, method600may include detecting an interference signal produced from interference of a solar signal with a light signal to generate an electrical response (block610). In an example of block610, gas leak detector100detects an interference signal produced from interference of solar signal182with a light signal119to generate electrical response184.

As further shown inFIG.6, method600may include filtering the electrical response to isolate a beat-note signal having an amplitude that is inversely related to a concentration of a species that forms a gaseous plume located along a path of the solar signal (block620). In an example of block620, gas leak detector100filters electrical response184to isolate beat-note signal186, which has an amplitude that is inversely related to a concentration of species181.

In a first implementation, method600includes generating, with a local oscillator, the light signal having a light-signal frequency associated with species absorption. For example, local oscillator110generates light signal119. In this implementation, method600may include selecting the light-signal frequency based at least in part on one or more or more of (a) intensity of the solar signal and (b) the concentration of species181.

In a second implementation, method600includes determining a location of a gaseous plume corresponding to the species concentration, where said determining is based at least in part on atmospheric pressure. In a third implementation, method600includes detecting, with a plurality of sub-detectors each communicatively coupled to one of a plurality of sub-filters, a corresponding portion of the electrical response isolated by a corresponding sub-filter, as described inFIG.5for example. In a fourth implementation, method600includes modulating the light signal to allow increased sensitivity.

In a fifth implementation, block610includes detecting a plurality of interference signals produced from interference of the solar signal with a plurality of light signals to generate a plurality of electrical responses, each of the plurality of light signals each having a respective one of a plurality of center frequencies (block612). In an example of block612, gas leak detector400,FIG.4, detects interference signals produced from interference of solar signal182with light signals419to generate electrical responses484.

In the fifth implementation, block620includes filtering each of the plurality of electrical responses to isolate a respective one of a plurality of beat-note signals having a respective amplitude that is inversely related to the concentration of the species (block622). The plurality of interference signals, the plurality of light signals, and the plurality of beat-note signals including the interference signals, the light signal, and the beat-note signal, respectively. In an example of block622, gas leak detector400filters electrical responses484to isolate beat-note signals486, each of which has an amplitude that is inversely related to a concentration of species181.

In embodiments of the fifth implementation, method600includes determining, from the plurality of beat-note signals, an absorption spectrum spanning the plurality of center frequencies (block640). For example, lineshape generator126determines absorption spectrum176from analog signal449received by processor120of gas leak detector400. Such embodiments may also include determining an altitude of the gaseous plume by determining an altitude of the gaseous plume by fitting pressure-dependent lineshape functions to the absorption spectrum (block650). For example, lineshape discriminator128determines altitude189by comparing absorption spectrum176to fitting parameters192.

Such embodiments may include determining an altitude of the gaseous plume by comparing the absorption spectrum to a plurality of reference absorbance spectra of the species at a respective one of plurality of atmospheric pressures. For example, lineshape discriminator128determines altitude189by comparing absorption spectrum176to fitting parameters192, where fitting parameters192include the aforementioned plurality of reference absorbance spectra of the species.

In an eighth implementation, method600includes determining a location of the gaseous plume from the altitude, an elevation angle of a source of the solar signal, and a direction of the source relative to a device that detects the interference signal. In an example of this implementation, memory124stores the direction and elevation angle, and determines the location of gaseous plumes180.

In a ninth implementation, method600includes the eight implementation and also includes generating a three-dimensional tomographic dataset of a plurality of gaseous plumes by, for each of the plurality of gaseous plumes determining a respective location of the gaseous plume by executing the eight implementation of method600.

In a tenth implementation, method600includes the eight implementation and also includes measuring wind velocity, and determining the location comprising determining a location from the altitude, the elevation angle, and the direction. In an example of this implementation, anemometer156determines wind velocity.

AlthoughFIG.6shows example blocks of method600, in some implementations, method600may include additional blocks, fewer blocks, different blocks, or differently arranged blocks than those depicted inFIG.6. Additionally, or alternatively, two or more of the blocks of method600may be performed in parallel.

As noted above, solar detector130and signal filter144may be integrated into a single photonic integrated circuit (PIC) with advantageous benefits. Signals from the PIC may be interpreted and processed (e.g., by processor122,FIG.1) to isolate methane plume location such as in a three-dimensional tomographic dataset.

FIG.7illustrates one such PIC700, but without a local oscillator110and other components, which may be off-chip instead. Components of PIC700reside on an insulating substrate701(e.g., silicon, silicon dioxide (Si, SiO2), though other materials may be suitable including silicon nitride, silicon oxide, sapphire, aluminum nitride, germanium, and silicon germanium alloy). These components include grating couplers702(1,2), input waveguides704, multimode interference coupler706, output waveguides708(1,2), grating couplers710, detectors712(1,2) and a transimpedance amplifier (TIA)720. In embodiments, solar detector130includes multimode interference coupler706and electronics140includes detectors712and TIA720.

The split ratio of multimode interference coupler706may be 50/50. Waveguides704and708may be formed of silicon. In embodiments, detectors712are semiconductor-based photodetectors, where the semiconductor may be indium gallium arsenide. Detectors712may be attached to substrate701via flip-chip bonding. TIA720may be on PIC700, as shown, or off-chip, such as amplifier145,FIG.1.

Grating couplers702(1) and702(2) are spaced apart by a pitch703, which may be between 200 μm and 300 μm, e.g., 250 μm. Grating couplers702(1,2) respectively couple electromagnetic energy from solar signal182and light signal119(from local oscillator110) into PIC700. Electromagnetic energy output from couplers702(1),702(2) travels along input waveguides704(1),704(2) into multimode interference coupler706, so that combined signals are equally half power at output waveguides708(1),708(2) and into respective detectors712(1),712(2) with heterogeneous integration. Output from detectors712couple into a TIA720, thereby facilitating connections to off-chip radio frequency domains. Though two detectors712are shown, one may be used instead depending on signal to noise ratio (SNR). Integrating TIA720on PIC700reduces distance between detector(s)712and TIA720, again reducing noise. In comparison toFIG.1, the remaining RF detection train (RF amplifier145, signal detector146, data processor120) are off of PIC700so as to easily switch between amplifier gains and filter bandwidths. However, as inFIG.1, all components may instead reside on chip (e.g., PIC108).

FIG.8is a schematic of a gas leak detector800, which is an example of gas leak detector100. Gas leak detector800includes local oscillator810, a 2×2 coupler833, a balanced detector835, electronics840, and a data processor820.

Local oscillator810is an example of local oscillator110, and includes a function generator812, a diode laser driver813, and a single mode (SM), fiber-coupled, distributed feedback (DFB) laser816. Local oscillator810may also include a thermoelectric cooler818coupled to DFB816. DFB laser816may have a 2-MHz bandwidth. Data processor820is an example of data processor120, and includes an analog-to-digital converter827. Diode laser driver813and DFB laser816are respective examples of laser driver113and light source116.

Gas leak detector800may also include at least one of an off-axis parabolic mirror854and a 1×2 fiber-optic switch804. Switch804may be remotely operated. Electronics840includes an RF detector846and least one of: a bandpass filter841, a lowpass filter842, a lowpass filter843, an amplifier845, an amplifier847, and an amplifier848. RF detector846is an example of signal detector146. Mirror854is an example of, or may be part of, collection optics154.

The following describes an example mode of operation of gas leak detector800. In this example, species181of gaseous plume180is methane. Light from local oscillator810is mixed in coupler833with a solar signal882sunlight collected into a single-mode using mirror854, which in this example has a 33-mm focal length. Solar signal882is an example of solar signal182. Fiber-optic switch804enables sampling the RF background offset level intermittently, which is affected by the temperature of the RF detector. The RF offset needs to be taken into account to accurately fit the lineshape of a target species of plume180. Both output legs of coupler833provide the dual inputs to balanced detector835.

In embodiments, off-axis parabolic mirror854tracks source196by piggy-backing on a GPS-enabled commercial telescope with an alt/azimuth mount using a micro-controller-based tracking capability. The tracking system automatically follows the known position of source196based on the date, time, and latitude after initial alignment. The direct absorption (DA) signal is collected with the fast sinusoidal modulation voltage set to zero.

Function generator812synthesizes a 200-Hz triangle wave as input to diode laser driver813. The resulting current modulation repetitively scans DFB laser816over approximately the wavelength range from 1665.868 nm to 1666.075 nm covering the CH42ν3overtone Q(6) transitions centered at 1665.956 nm. Light from DFB laser816is combined with light from source196to form an RF beat note or intermediate frequency, IF. Imprinted on the envelope of the beat note is any lined absorption that occurs as sunlight transits the atmosphere. Filters cut off IF frequencies above 225 MHz leaving a narrow range of frequencies centered on the instantaneous wavelength of DFB laser816. DFB laser816is then swept at 200 Hz across the Q(6) features to generate the spectrum of interest. The data is digitized by a 2 MS/sec A/D converter, which is triggered to collect and column average the spectral data synchronously with the LO 200 Hz sweep. The A/D converter may be part of data processor120.

The collected data is fit using a retrieval program, stored as software125, to determine the methane mixing ratio versus altitude189in the following way. The actual atmospheric vertical column is approximated using the U.S. Standard Atmosphere (1826version) up to the top of the stratosphere (50 km). This standard atmosphere is divided into equal average ρiΔZilayers; that is, the average density in layer i multiplied by the vertical depth of layer i is equal for all layers i=1, . . . , N. The number of layers N is arbitrary, but for our data analysis here, we have used N=11 for reasons described below. As a result, the layers closer to the surface are shallower and the layers are deeper at higher altitudes. For example, when there are only two layers (N=2) the bottom layer is 5.6 km and the top layer is 44.4 km in depth. By dividing the vertical column in this manner, the different layers contribute nearly equal changes in integrated methane absorbance for a given change in methane concentration within the layer.

The retrieval algorithm, stored as part of software125, calculates a model of the vertically integrated methane absorption spectrum by summing the absorbance over N (equal ρiΔZi) layers from the surface up to an altitude of 50 km. This model is used to fit the actual, measured POHS methane absorption spectrum using a Levenberg-Marquadt algorithm. The routine varies the fit parameters in order to minimize the (squared) difference between the model and the measured spectrum, and the N layer methane concentrations are fit parameters.

A vertical profile of atmospheric methane results from fitting a given, measured spectrum. The profile is not sensitive to reasonable initial conditions; for results shown here, we assume a methane concentration equal to 1.8 ppm for all layers. The vertical profile of methane resulting from fitting a given measured spectrum remains similar while increasing the number of layers beyond 10 (N>10), although profiles are naturally smoother with increasing N. Because the problem is mathematically under-determined with the small number of spectral features available in the scan range of our laser, we choose to keep the number of layers as small as possible while still reproducing the major features of the profile. We modelled all results using 11 bins.

Balanced detector835, electronics840, and data processor120determine the spectral bandwidth of gas leak detector800. Balanced detector835receives the light from the two output legs of 2×2 coupler833. These optical outputs are intrinsically of opposite phase. The common mode noise is eliminated, and the signal is reinforced by subtracting the photodiode signal from the two inputs. The output bandwidth of the balanced detector835may be 400 MHz. The output of balanced detector835is then bandpass filtered by bandpass filter841(bandwidth is 20 MHz-1 GHz in this example), amplified in RF power amplifier845(+30 dB power gain, 10 MHz-1 GHz, in this example), and then low-pass filtered by lowpass filter842at an adjustable frequency that determines the spectral resolution of gas leak detector800.

In embodiments, the cutoff frequency of lowpass filter842is between 52 MHz and 225 MHz. In this range, the bandwidth of gas leak detector800does not cause broadening of the measured methane spectral profile. Yet, a drawback of lower cutoff frequencies is decreased signal amplitude. The low-pass filtered signal power is converted to a voltage at RF detector846, which may include a zero-bias Schottky diode, a bandwidth of which may span 10 MHz to 2 GHz. The voltage output of RF detector846is then amplified and filtered through amplifier847, lowpass filter843, and amplifier848to yield an output analog signal849, which is an example of analog signal149. In this example, the amplification and filtering yield a 1472× increase in voltage with an output bandwidth that is less than 70 kHz.

Analog signal849is then directed to one of the input channels of an A/D converter of data processor120, in which it is synchronously digitized and column averaged. Operating at 200 Hz and averaging for 1000 scans leads to a 5 second acquisition sequence. InFIG.8, all leads are electrical connections, expect for (a) those between DFB laser816and balanced detector835and (b) those between off-axis parabolic mirror854and balanced detector835, which are optical components/connections. The electrical components dominate, which is one of the main advantages of detector100and will allow the large-scale lab instrument to be miniaturized to a single-board, purpose-built sensor.

Gas leak detector800may also include a digital lock-in amplifier806for collection of the wavelength modulation spectroscopy (WMS) 1f and 2f signals. Amplifier806is communicatively coupled to lowpass filter843and data processor120. In this example, amplifier806produces a fast sinusoidal modulation waveform which is imprinted on the 200-Hz sweep by a bias T circuit in diode laser driver813. Amplifier806also allows phase to be adjusted to maximize the 1f and2fXsignal components. The optimal phase changes as the electronics configuration determining the bandwidth changes but is constant thereafter. Therefore, only a single lock-in measuring theXcomponents of the signal is required. We operate at a modulation voltage of up to 700 mV (78 pm modulation depth) at a frequency of about 45 kHz. The 1f and 2f outputs of amplifier806are directed to channels2and3of A/D converter827where they are likewise digitized and column averaged. When collecting direct absorption data, the fast modulation voltage on amplifier806is set to zero to avoid broadening the methane spectral features.

FIG.9shows a spectrum910(black curve) of naturally occurring background methane which occurs at a ground level concentration of approximately 1.8-2.0 ppm on the CH42ν3Q(6) transitions centered at 1665.956 nm. The spectroscopy of the Q(6) transitions is well known. It consists to two groups of three lines that are closely spaced giving rise to peaks at approximately 1665.948 and 1665.967 nm at our spectral resolution. Fitted spectrum920(gray curve) is a fit to spectrum910using our retrieval algorithm, which is stored as software125.

All six lines are part of the Q-branch of the 2ν3overtone band (2 quanta of asymmetric stretch) and emanate from the vibrationless, nearly degenerate lower state level with E″=219.9 cm−1. Spectrum910is the data acquired by an embodiment of detector100on 22-May 2021 at 10:18:54 AM MDT. It consists of only the background methane signal present at that time and spatial direction which is determined by the position of the sun relative to detector100. Fitted spectrum920is the retrieved spectral fit. As can be seen, the fit is good at the noise level of the spectrum which was acquired by averaging 1,000 scans over 5 seconds. This spectral region was chosen because it consists of two, closely-spaced features at 450 MHz spectral resolution that are nevertheless well resolved at low pressure (high altitude) and blend into a single feature near atmospheric pressure (low altitude). This pressure dependence provides a means to determine the approximate altitude of the methane which, with positional data on the elevation angle of the sun and its direction relative to detector100, allows one to calculate the location of any anomalous sources of methane.

Our efforts to determine better broadening coefficients resulted in the data shown inFIG.10, which includes reference spectra1001-1006of the 2ν3Q(6) lines centered at 1665.956 nm. Each reference spectrum1001-1006is an example of a spectrum of fitting parameters192. In embodiments, fitting parameters192include broadening coefficients determined from spectra1001-1006. The spectra are acquired at 0.12, 0.2, 0.3, 0.5, 0.7, and 1.0 atmosphere total pressure. These pressures roughly correspond to altitudes of 16 km, 12 km, 9 km, 7 km, 3 km, and 0 km, respectively. The scan range for each spectrum is identical—approximately 1666.035 to 1665.879 nm. Each scan's wavelength range is the reverse of that shown inFIG.9—an artifact of the data collection method.

To generateFIG.10, the Q(6) lines were scanned repetitively at pressures from 0.12 to 1.0 atmosphere using a standard TDLAS system and a multi-pass cell in which we can control the composition, temperature, pressure, and path length of the methane sample. To make these measurements, we started with a calibrated bottle of 5% methane in nitrogen and flow diluted the methane with nitrogen using calibrated mass flow controllers until we achieved a 1% methane concentration—so the broadening coefficients that we determined were for nitrogen; not air. The concentration was verified using tunable diode laser measurements with direct detection at the same wavelength (approximately 1666 nm) as the laser heterodyne radiometry (LHR) measurements made by detector100. The laser was double-passed in our spectroscopy cell to achieve a path length of 1.22 meters. The concentration remained constant at 1% for all measurements; only the pressure was varied for the different runs. From these measurements, we learned that it is insufficient to model the Q(6) features as two groups of three very closely spaced peaks even though the triad of peaks is completely unresolved. Instead, we had to model all six features independently with individual positions, and broadening parameters, although the final line broadening parameters were similar to the values of the high-resolution transmission molecular absorption database (HITRAN).

The retrieval algorithm described above analyzed the spectral profile ofFIG.9yielding the methane mixing ratio versus altitude (pressure) shown inFIG.11.FIG.11is a plot of (i) background methane mixing ratio versus altitude (trace1110) and (ii) methane mixing ratio with approximately 18,000 ppm-m of methane at room temperature and local atmospheric pressure injected directly in front of off-axis parabolic mirror854(trace1120). The dramatic difference between the profiles indicates that we can detect small levels of excess methane and approximately determine the altitude of the excess methane. The mixing ratio versus altitude profile that we determine from the retrieval algorithm of the background methane is in line with expectations based on earlier work.

To test the ability of our LHR to see an anomalous methane “leak”, we obtained normal background spectra as inFIG.9and compared them to spectra obtained by inserting a 10-cm spectroscopic cell filled with 5% methane (5000 ppm-m) in nitrogen at room temperature and local atmospheric pressure (0.83 atm.) or by injecting approximately 18,000 ppm-m of methane directly into the path of the LHR collection optic. The concentration of the injected methane is calculated from the flow of methane, 500 standard liters/minute (slm) (21.4 kg/hr), and the diameter of the injection pipe (5 cm) using computational fluid dynamics calculations. This leak level is less than half of the super-emitter definition, >50 kg/hr.

We also detected leaks at the 13 kg/hr (300 slm) level. The resulting spectra without and with the injected methane are shown inFIG.12.FIG.12is a plot of direct absorption (DA) spectra of background methane (trace1210) and with an additional amount of methane equal to about 18,000 ppm-m from direct injection of methane into the detection path of gas leak detector800(trace1220).FIG.12also includes retrieval fits1212and1222to traces1210and1220respectively. If one compares the spectra, it is clear that the additional methane increased the absorbance roughly in line with expectations and also greatly broadened the lineshape.

WMS Laser Heterodyne Radiometry Data

We suspected that the WMS 1f and 2f LHR signals might be advantageous to collect as WMS overcomes certain types of noise allowing smaller signals to be detected with reasonable signal-to-noise ratio. We collected 2f spectra with the methane cell in and out of the path.

FIG.13is a plot of the WMS 2f signal with (signal1310) and without (signal1320) the 10 cm-cell filled with 5% methane at 0.83 atmospheres and room temperature. The modulation depth was low (18 pm for this scan) which preserves the essential features of the lineshape well. Larger modulation depth increases the signal but obscures the details of the lineshape which are particularly relevant for plume localization efforts.

We expected that the peak-to-peak 2f signal would be larger due to the additional methane in the cell. Contrary to our expectations, as shown inFIG.13, the peak-to-peak 2f methane signal (and the wavelength-integrated 2f signal) actually decreases when the methane cell is inserted. This initially surprising result may be understood as follows. The 2f signal is essentially the second derivative of the direct absorption spectrum. Mathematically, the second derivative is a measure of the curvature of the lineshape. Since the added methane exhibits a broad lineshape relative to the integrated atmospheric column, the curvature of the lineshape decreases when the methane cell is inserted leading to a smaller 2f signal.

This would seem to make the 2f and 1f signals useless for detecting and locating methane leaks; however, consider the following. If the direct absorption (DA) signal indicates an increase in absorbance and the 2f peak-to-peak amplitude decreases, it suggests that the additional methane is close to the ground (also indicating that it is close to gas leak detector100. Taking this information along with the altitude and azimuthal angles of the sun, simple trigonometry allows calculation of the approximate location of the leak. If the additional absorbance is accompanied by an increase in the 2f signal, it indicates that the additional methane is at high altitude (far from detector100). This information is supplementary to the retrieval profile and should help in plume localization.

Combination of Features

Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following enumerated examples illustrate some possible, non-limiting combinations:

(A1) A gas leak detector includes: a solar detector that generates an electrical response by interfering a light signal with a solar signal and detecting a resultant interference signal; and a signal filter, communicatively coupled to the solar detector, that filters the electrical response to isolate a beat-note signal having an amplitude that is inversely related to a concentration of a species that forms a gaseous plume located along a path of the solar signal.

(A2) The embodiment (A1) further including a local oscillator that generates the light signal, the light signal having a light-signal frequency corresponding to a resonance absorption of the species.

(A3) Either one of embodiments (A1) or (A2) further including a controller communicatively coupled to the local oscillator that sets the light-signal frequency based at least in part on one or more of (a) intensity of the solar signal and (b) target-species concentration.

(A4) Any one of embodiments (A1)-(A4) further including a light source; and a laser driver that tunes a frequency of the light source such that (i) the local oscillator generates a plurality of light signals each having a respective one of a plurality of center frequencies, (ii) the solar detector generates a respective one of a plurality of electrical responses by mixing each of the plurality of light signals with the solar signal, and (iii) the signal filter filters each of the plurality of electrical responses to isolate a respective one of a plurality of beat-note signals,

(A5) Any one of embodiments (A1)-(A5) further including a signal detector that records each of the plurality of beat-note signals; and a processor communicatively coupled to the signal detector; and a memory storing (i) a plurality of reference absorbance spectra of the species at a respective one of plurality of atmospheric pressures, and (ii) machine readable instructions that when executed by the processor, cause the processor to:

(B1) A gas leak detector comprising: an array of gas leak detectors of embodiment (A4), each of which generates a respective plurality of beat-note signals associated with a respective one of a plurality of gaseous plumes that includes the gaseous plume; a signal detector that records each of the respective pluralities of beat-note signals; and a processor communicatively coupled to the signal detector; a memory storing machine readable instructions that when executed by a processor, cause the processor to generate a three-dimensional tomographic dataset of the plurality of gaseous plumes by, for each of the plurality of gaseous plumes: determine, from the plurality of beat-note signals, an absorption spectrum spanning the plurality of center frequencies; and determine an altitude of the gaseous plume by at least one of (i) fitting pressure-dependent lineshape functions to the absorption spectrum and (ii) comparing the absorption spectrum to a plurality of reference absorbance spectra of the species at a respective one of plurality of atmospheric pressures

(B2) The embodiment (B1) further including a signal detector communicatively coupled to the signal filter, that records the beat-note signal.

(B3) Either one of embodiments (B1) or (B2) further including a photonic integrated circuit that includes the solar detector and signal filter.

(B4) Any one of embodiments (B1)-(B4) further including an anemometer that assists in locating methane leak location.

(C1) A method for detecting a gas leak includes: detecting an interference signal produced from interference of a solar signal with a light signal to generate an electrical response; and filtering the electrical response to isolate a beat-note signal having an amplitude that is inversely related to a concentration of a species that forms a gaseous plume located along a path of the solar signal.

(C2) The embodiment (C1) further including the method includes generating, with a local oscillator, the light signal having a light-signal frequency associated with species absorption.

(C3) Either one of embodiments (C1) or (C2) further including the method includes selecting the light-signal frequency based at least in part on one or more or more of (a) intensity of the solar signal and (b) the concentration of the species.

(C4) Any one of embodiments (C1)-(C4) further including the method includes determining location of a gaseous plume corresponding to the concentration of the species, said determining based at least in part on atmospheric pressure.

(C5) Any one of embodiments (C1)-(C5) further including the method includes detecting, with a plurality of sub-detectors each communicatively coupled to one of a plurality of sub-filters, a corresponding portion of the electrical response isolated by a corresponding sub-filter.

(C6) Any one of embodiments (C1)-(C6) further including the method includes amplitude modulating the light signal to allow increased sensitivity.

(C7) In any one of embodiments (C1)-(C7), the method includes detecting an interference signal comprising: detecting a plurality of interference signals produced from interference of the solar signal with a plurality of light signals to generate a plurality of electrical responses, each of the plurality of light signals each having a respective one of a plurality of center frequencies; and filtering the electrical response comprising: filtering each of the plurality of electrical responses to isolate a respective one of a plurality of beat-note signals having a respective amplitude that is inversely related to the concentration of the species, the plurality of interference signals, the plurality of light signals, and the plurality of beat-note signals including the interference signals, the light signal, and the beat-note signal, respectively.

(C8) Any one of embodiments (C1)-(C8) further including the method includes determining, from the plurality of beat-note signals, an absorption spectrum spanning the plurality of center frequencies; and determining an altitude of the gaseous plume by at least one of (i) fitting pressure-dependent lineshape functions to the absorption spectrum and (ii) comparing the absorption spectrum to a plurality of reference absorbance spectra of the species at a respective one of plurality of atmospheric pressures.

(C9) Any one of embodiments (C1)-(C9) further including the method includes determining a location of the gaseous plume from the altitude, an elevation angle of a source of the solar signal, and a direction of the source relative to a device that detects the interference signal.

(C10) Any one of embodiments (C1)-(C10) further including the method includes generating a three-dimensional tomographic dataset of a plurality of gaseous plumes by, for each of the plurality of gaseous plumes: determining a respective location of the gaseous plume by executing the method of embodiment (C9).

(C11) Any one of embodiments (C1)-(C11) further including the method includes measuring wind velocity; and determining the location comprising determining a location from the altitude, the elevation angle, and the direction.

(D1) A photonic integrated circuit for gaseous leak detection includes: a multimode interference coupler having a first input port, a second input port, and an output port; a first grating coupler, coupled to the first input port, that couples a solar signal into the multimode interference coupler; a second grating coupler, coupled the second input port, that couples a light signal into the multimode interference coupler; an output grating coupler coupled to the output port, which outputs an interference signal; and a detector, coupled to the output grating coupler, that generates an electrical response to detection of the interference signal.

(D2) The embodiment (D1) further including a transimpedance amplifier that amplifies the electrical response.

(D3) Either one of embodiments (D1) or (D2) further including a signal filter that filters the electrical response to isolate a beat-note signal inversely related to a species concentration of a gaseous plume located along a path of the solar signal; a local oscillator that generates the light signal having a light-signal frequency associated with methane absorption; and a signal detector that records the beat-note signal.

(D4) Any one of embodiments (D1)-(D4) further including a lineshape discriminator communicatively coupled to the signal detector to determine altitude of a gaseous plume corresponding to the species concentration.

(D5) Any one of embodiments (D1)-(D5) further including a RF amplifier that amplifies the beat-note signal to generate an amplified beat-note signal.