Techniques for characterizing gas molecules in atmosphere around airborne vehicle

Abstract

To provide techniques for determining levels of different types of molecules, e.g., of greenhouse gases, in measurement volumes external to an airborne vehicle or internal to a portion of the airborne vehicle.SOLUTION: In one embodiment, dual optical frequency comb (DOFC) systems 102-1, 102-2, 102-3, 102-4 are installed on and / or in an airborne vehicle 101. A first DOFC system 102-1 is mounted on the body 123 of the airborne vehicle. A second DOFC system 102-2 is mounted on a second foil 107 of the airborne vehicle. A third DOFC system 102-3 is mounted on a first foil 103 of the airborne vehicle. A fourth DOFC system 102-4 is mounted on a third foil 109.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This application claims the benefit of U.S. Patent Application No. 63 / 607,912, filed on December 8, 2023, and U.S. Patent Application No. 63 / 609,777, filed on December 13, 2023. The entire contents of each of the foregoing patent applications are hereby incorporated by reference as if fully set forth herein.

Background Art

[0002] Greenhouse gases in the atmosphere cause global climate change. To predict such climate change, it is necessary to monitor greenhouse gases in the atmosphere.

Summary of the Invention

[0003] In some aspects, the technology described herein relates to a method for determining the level of types of molecules within a volume of air inside or around an aircraft. The method includes emitting, from a first region of the internal volume or external portion of the aircraft, two optical frequency combs, at least one of which is transmitted through a measurement volume inside or around an internal portion of the aircraft or through the surrounding area outside the aircraft; after at least one of the two optical frequency combs has passed through the measurement volume, receiving the two optical frequency combs in a first region or a second region of the internal volume or external portion of the aircraft, respectively; using the received two optical frequency combs to generate a heterodyne beat frequency comb; determining which spectral components of the heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude; using the spectral components of the heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within the measurement volume; and using the amplitudes of each spectral component of the heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of one or more types of molecules within the measurement volume.

[0004] In some aspects, the technology described herein relates to an apparatus configured to determine the level of types of molecules within a volume of air inside or around an aircraft, the apparatus including a first dual optical frequency comb spectroscopy (DOFC) system including a first optical frequency comb generator, a second optical frequency comb generator, a first mixer, and a DOFC processing circuit, each of the first and second optical frequency comb generators including a laser, the first optical frequency comb generator being configured to transmit a first optical frequency comb to the first mixer, the second optical frequency comb generator being configured to transmit a second optical frequency comb to the first mixer, at least one of the first and second optical frequency combs being configured to be transmitted through a first measurement volume before being received by the first mixer, using the received first and second optical frequency combs, the first mixer being configured to generate a first heterodyne beat frequency comb, the DOFC processing circuit being configured to receive the first heterodyne beat frequency comb, determine which spectral components of the first heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude, use the spectral components of the first heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within the first measurement volume, and use the amplitude of each spectral component of the first heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of at least one type of molecule within the first measurement volume.

[0005] In some aspects, the technology described herein relates to an apparatus configured to determine the level of types of molecules generated within an engine, the apparatus including a first dual optical frequency comb spectroscopy (DOFC) system including a first optical frequency comb generator, a second optical frequency comb generator, a first mixer, and a DOFC processing circuit, each of the first and second optical frequency comb generators including a laser, the first optical frequency comb generator being configured to transmit a first optical frequency comb to the first mixer, the second optical frequency comb generator being configured to transmit a second optical frequency comb to the first mixer, at least one of the first and second optical frequency combs being configured to be transmitted through a first measurement volume before being received by the first mixer, the first measurement volume being at a first location within the engine, using the received first and second optical frequency combs, the first mixer being configured to generate a first heterodyne beat frequency comb, the DOFC processing circuit being configured to receive the first heterodyne beat frequency comb, to determine which spectral components of the first heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude, to use the spectral components of the first heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within the first measurement volume, and to use the amplitude of each spectral component of the first heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of at least one type of molecule within the first measurement volume, the apparatus further including a second DOFC system including a third optical frequency comb generator, a fourth optical frequency comb generator, and a second mixer, each of the third and fourth optical frequency comb generators including a laser, the third optical frequency comb generator being configured to transmit a third optical frequency comb to the second mixer, the fourth optical frequency comb generator being configured to transmit a fourth optical frequency comb to the second mixer, at least one of the third and fourth optical frequency combs being configured to be transmitted through a second measurement volume before being received by the second mixer, the second measurement volume being at a second location within the engine, using the received third and fourth optical frequency combs, the second mixer isconfigured to generate a second heterodyne beat frequency comb, the DOFC processing circuit receives the second heterodyne beat frequency comb, determines which spectral components of the second heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude, uses the spectral components of the second heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within a second measurement volume, uses the amplitude of each spectral component of the second heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of at least one type of molecule within the second measurement volume, and uses the determined concentrations of at least one type of molecule determined to be within the first measurement volume and the second measurement volume to determine the concentration of at least one type of molecule added or removed between the second measurement volume and the first measurement volume, and is further configured to perform.

Brief Description of the Drawings

[0006] It is understood that the drawings show only exemplary embodiments and should not be regarded as limiting the scope. The exemplary embodiments will be described with further specificity and detail using the accompanying drawings.

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[0007] In accordance with convention, the various features described are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments.

Best Mode for Carrying Out the Invention

[0008] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific exemplary embodiments. However, it should be understood that other embodiments may be utilized and logical, mechanical, and electrical changes may be made. Further, the methods presented in the drawings and the specification should not be construed as limiting the order in which the individual steps may be performed. Accordingly, the following detailed description should not be construed in a limiting sense.

[0009] The embodiments described below include techniques for determining the concentration of different types of molecules of greenhouse gases, for example, within a measurement volume external to an aircraft or within a portion of an aircraft. The embodiments described herein also include techniques for determining the concentration of different types of molecules of greenhouse gases, for example, within a measurement volume generated by an aircraft engine. The measurement volume used herein is a region of the ambient environment external to the aircraft, for example, a region of the atmosphere. In this specification, the aircraft is shown for illustrative purposes. Embodiments of the present invention are applicable to other types of vehicles other than aircraft.

[0010] In some embodiments of the present invention, a pair of optical frequency combs (described elsewhere in this specification) are emitted from a first region of the surface of an aircraft. At least one of the optical frequency combs propagates through one or more measurement volumes (or regions of the environment or atmosphere) surrounding the exterior of the aircraft. Optionally, at least one of the optical frequency combs is reflected from a second region of the surface of the aircraft. The pair of optical frequency combs are received at a first or third region of the surface of the aircraft.

[0011] To do so, one or more dual optical frequency comb spectroscopy (DOFC) systems (or DOFC sensors) are mounted within or on the aircraft. The DOFC system is configured to generate a first optical frequency comb and a second optical frequency comb. Optionally, the first and second optical frequency combs are phase locked. Each optical frequency comb includes spectral components within the optical spectrum. Each pair of adjacent spectral components of each optical frequency comb are separated by a constant frequency. However, such a constant frequency separation is different between the first optical frequency comb and the second optical frequency comb. The constant frequency separation for the first optical frequency comb is the first separation frequency f s . The constant frequency separation of the second optical frequency comb is the second separation frequency f s +Δf. Δf may be referred to as an offset frequency. Optionally, the first separation frequency f s is at least four orders of magnitude greater than the offset frequency Δf.

[0012] The mixer receives two frequency combs after each has propagated through the measurement volume at least once and generates a heterodyne beat frequency comb, for example, in the radio spectrum below the optical spectrum. The measurement volume includes a portion of the atmosphere surrounding the aircraft. Such a portion of the atmosphere contains one or more different types of molecules including molecules of oxygen and greenhouse gases. The DOFC technique is described in Ian Coddington, Nathan Newbury, and William Swann, "Dual-comb spectroscopy", Optica 3, 414 - 426 (2016), which is incorporated herein by reference.

[0013] Greenhouse gas means a type of gas that captures heat when in the Earth's atmosphere. Examples of types of greenhouse gas molecules include, but are not limited to, carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, nitrogen trifluoride, water vapor, and ozone.

[0014] For example, each type of greenhouse gas molecule absorbs electromagnetic energy at one or more frequencies. For example, such absorbed electromagnetic energy is converted to heat by the molecule. Thus, when at least one of the optical frequency combs propagates through a measurement volume containing one or more types of molecules of greenhouse gases, for example, the power level of one or more spectral components of at least one optical frequency comb decreases due to absorption by one or more types of molecules within the measurement volume.

[0015] Each spectral component of the heterodyne beat frequency comb has a first separation frequency f sOnly the frequency is shifted lower. Since the heterodyne frequency comb is in a lower frequency range, it can be processed by electronic signal processing, for example, using a processing system or circuit. Optionally, the electronic signal processing determines how much the power of each spectral component of the heterodyne beat frequency comb is attenuated by absorption by one or more types of molecules within the measurement volume, and adds a first separation frequency f s to the frequency (of each spectral component of the heterodyne frequency comb for which attenuation was detected) and compares it to one or more optical frequencies at which each type of molecule of the greenhouse gas, for example, absorbs energy. Optionally, such reference information for one or more optical frequencies at which each type of molecule of the greenhouse gas, for example, absorbs energy can be stored in a library within the processing system. When it is identified that attenuation occurs in one or more spectral components whose frequency corresponds to where the type of molecule of the greenhouse gas absorbs electromagnetic energy, the level of absorption in each of the one or more spectral components may be used to confirm the concentration of the type of molecule within the measurement volume using conventional spectroscopic measurement techniques.

[0016] Techniques using a DOFC system can be used to characterize the levels of different types of molecules of, for example, greenhouse gases within a measurement volume outside the aircraft or inside a part of the aircraft. First, techniques for characterizing the levels of different types of molecules of, for example, greenhouse gases within a measurement volume outside the aircraft using a DOFC system will be described.

[0017] The DOFC system is mounted within or on the aircraft and is configured to emit a first and a second optical frequency comb through a measurement volume of the atmosphere by or within the aircraft. Both optical frequency combs are emitted from either (a) an inner surface of the inner volume of the aircraft or (b) a first region of the outer surface of the aircraft. The first and second optical frequency combs are phase-locked to each other.

[0018] Optionally, each of the first and second optical frequency combs is reflected by an optical reflective surface of the aircraft. The optical reflective surface is either (a) an inner surface of the inner volume of the aircraft or an inner surface of another inner volume of the aircraft, or (b) a second region of any of the outer surfaces of the aircraft.

[0019] The optical reflective surface may be a metallic surface of metal forming part of the aircraft. Alternatively, the optical reflective surface may be implemented by attaching an optical reflective sticker onto the second region, applying an optical reflective paint to the second region, and / or forming an array of optical reflectors onto the second region. Each outer surface of the aircraft may be an outer surface of the fuselage, a wing, a control surface, an engine, a measuring device, or any other outer surface of the aircraft.

[0020] Optionally, at least one of the first and second optical frequency combs is directly transmitted through a measurement volume by a part of the DOFC system to another part of the DOFC system. Optionally and alternatively, at least one of the first and second optical frequency combs is transmitted through the measurement volume by a part of the DOFC system. Then, at least one of the first and second optical frequency combs is reflected back through the measurement volume to the same or a different part of the DOFC system, for example, by an optical reflective surface. Optionally, when the first and second optical frequency combs travel through the measurement volume, are reflected back through the measurement volume, and both optical frequency combs travel through the measurement volume twice, this is taken into account when determining the molecular concentration. For clarity, a portion of the set of the first and second optical frequency combs incident on an item, such as a reflective surface, an engine, or any other item, may be reflected back to the DOFC system (or a part thereof) that emitted the set.

[0021] FIG. 1 shows an exemplary embodiment of a mounting configuration 100 for one or more DOFC systems 102-1, 102-2, 102-3, 102-4 on and / or within an aircraft 101. Four different positions of the first, second, third, and fourth DOFC systems 102-1, 102-2, 102-3, 102-4 are shown in FIG. 1 for illustrative purposes, but it should be understood that one or more DOFC systems can be used and can be placed at other locations on and / or within the aircraft 101.

[0022] In one embodiment of FIG. 1, the first DOFC system 102-1 is attached to the fuselage 123 of the aircraft 101, which is, for example, the fuselage of the aircraft, and a first set of the first and second optical frequency combs is directed along a first line of sight 112-1 towards a portion 105, which is, for example, a reflective portion, of the first wing 103 of the vehicle, which is, for example, a winglet of the aircraft. In another embodiment, the first DOFC system 102-1 is mounted flush with the outer surface 115 of the fuselage 123 of the aircraft 101. Optionally, the portion 105 is at least partially covered with an optical reflective material 125 (e.g., a reflective sticker, reflective paint, and / or an array of reflectors), and the first set of the first and second optical frequency combs contacts the portion 105, e.g., the surface of the winglet. The optical reflective material 125 can cover a portion or the entire portion 105. The optical reflective material 125 increases the reflectivity of that portion. Optionally, the optical reflective material 125 includes a reflective material that is reflective with respect to the wavelengths of the spectral components of the first and second optical frequency combs of the first set. The portions shown in other figures of this specification can be implemented in the same manner as described above.

[0023] Portion 105 is arranged or oriented such that a portion of each of the first and second optical frequency combs of the first set is reflected back along the first LOS 112-1 to the first DOFC system 102-1 that emitted such an optical frequency comb. A first measurement volume 114-1 for the DOFC system 102-1 is disposed between the first DOFC system 102-1 and the portion 105. The first set of the first and second optical frequency combs is transmitted by the first DOFC system 102-1 along the first LOS 112-1 through the first measurement volume 114-1, and a portion of each of the first and second optical frequency combs of the first set is reflected by the portion 105 and returned to the first DOFC system 102-1. Optionally, the position of the first measurement volume 114-1 can be adjusted to sample a volume of the atmosphere disturbed or undisturbed by the aircraft 101, for example, by the movement of the aircraft 101.

[0024] In another embodiment, a second DOFC system 102-2 (optionally, can be considered as part of the first DOFC system 102-1) is mounted on the second wing 107 of the aircraft 101, such as on the vertical stabilizer of the aircraft. Optionally, the second DOFC system 102-2 is mounted flush with the surface of the second wing 107. Optionally, the second DOFC system 102-2 is configured to transmit a second set of the first and second optical frequency combs along the second LOS 112-2 towards the terminal portion 105 on the first wing 103 of the aircraft 101. Optionally, as shown in FIG. 1, when both the first and second DOFC systems 102-1, 102-2 are used in combination, a portion of the first and second optical frequency combs of the first set (emitted from the first DOFC system 102-1) is reflected from the aircraft 101 along the second LOS 112-2 towards the second DOFC system 102-2, for example, by the portion 105. A portion or the whole of the portion 105 may be covered with an optical reflective material 125 as described elsewhere in this specification.

[0025] The second measurement volume 114-2 characterized by the second DOFC system 102-2 is disposed between the second DOFC system 102-2 and the portion 105. Optionally, the exact position of the second measurement volume 114-2 can be adjusted to sample the volume of the atmosphere disturbed or not disturbed by the aircraft 101, for example, by the movement of the aircraft 101. Optionally, each reflected portion of the first and second optical frequency combs of the second set propagating along the second LOS 112-2 traverses the second measurement volume 114-2.

[0026] Alternatively, the second DOFC system 102-2 is configured to receive each reflected portion of the first and second optical frequency combs of the first set radiated by the first DOFC system 102-1 and reflected by the aircraft 101, such as the optical reflective material 125, along the second LOS 112-2. Thus, the first DOFC system 102-1 operates as a transmitter and the second DOFC system 102-2 operates as a receiver. The first and second DOFC systems 102-1, 102-2 are configured to be used to analyze the first measurement volume 114-1 and / or the second measurement volume 114-2 shown in FIG. 1 through which the first set and / or the reflected portions of the first and second optical frequency combs propagate.

[0027] In a further embodiment, a third DOFC system 102-3 is attached to the first wing 103 of the aircraft 101. Optionally, the third DOFC system 102-3 is attached flush with the surface of the first wing 103. The third DOFC system 102-3 is configured to transmit a third set of the first and second optical frequency combs along a third LOS 112-3 towards a third wing 109 attached to the second wing 107, e.g., a horizontal stabilizer attached to a vertical stabilizer of the aircraft. In a further embodiment, at least a portion of the surface of the third wing 109 includes an optical reflective material 125, described elsewhere herein, disposed at a location where the third set of the first and second optical frequency combs contacts the surface of the third wing 109. A portion of each of the first and second optical frequency combs of the third set is reflected back along the third LOS 112-3 towards the third DOFC system 102-3. A third measurement volume 114-3 for the third DOFC system 102-3 is disposed between the third DOFC system 102-3 and the third wing 109. Optionally, the exact position of the third measurement volume 114-3 can be adjusted to sample a volume of the atmosphere disturbed or undisturbed by the aircraft 101, e.g., by the movement of the aircraft 101.

[0028] In yet another embodiment, a fourth DOFC system 102-4 can also be attached to the third wing 109 and is configured to transmit a fourth set of first and second optical frequency combs along a fourth LOS 112-4 towards, for example, another portion of the first wing 103. Optionally, the optical reflective material 125 described elsewhere herein may be disposed at locations where the fourth set contacts the surface or other portions of the first wing 103. A fourth measurement volume 114-4 for the fourth DOFC system 102-4 is disposed between the fourth DOFC system 102-4 and the horizontal plane of the first wing 103 or other portion 105. A portion of each of the first and second optical frequency combs of the fourth set is reflected back along the fourth LOS 112-4 to the fourth DOFC system 102. Optionally, the exact position of the fourth measurement volume 114-4 can be adjusted to sample an atmospheric volume disturbed or undisturbed by the movement of the aircraft 101.

[0029] Figure 2 shows another embodiment of the mounting configuration 200 of one or more DOFC systems 202-1, 202-2 on an aircraft 201. Although a first DOFC system 202-1 and a second DOFC system 202-2 are shown in Figure 2 for illustrative purposes, it should be understood that one or more DOFC systems can be used and can be located elsewhere on or within the aircraft 201.

[0030] In the exemplary embodiment of FIG. 2, a first DOFC system 202-1 is attached to a portion 205 of a first wing 203 of an aircraft 201, and the first DOFC system 202-1 is configured to transmit two or more sets of first and second optical frequency combs along two different, e.g., two or more LOSs 212-1, each set being transmitted towards different surfaces on the aircraft 101, e.g., surfaces on the fuselage 223 and on the second wing 207, e.g., towards the vertical stabilizer of the aircraft. A portion of each of the first and second optical frequency combs of each set incident at each different location is reflected back along the same LOS along which the reflected set propagated. Optionally, the first DOFC system 202-1 is mounted flush with the surface of the portion 205 of the first wing 203. In another embodiment, at least portions of the surfaces of the fuselage 223 and the second wing 207 include a light reflecting material 225 disposed at locations where each of two or more sets of first and second optical frequency combs contacts the surfaces of the fuselage 223 and the second wing 207, and is configured to reflect back a portion of each set of first and second optical frequency combs incident on the DOFC systems that emitted the incident sets of first and second optical frequency combs. The first, second, and third measurement regions 214-1a, 214-1b, 214-1c of the first DOFC system 202-1 are disposed between the first DOFC system 202-1 and each of the surfaces of the fuselage 223 and the second wing 207 on which the sets of first and second optical frequency combs are incident. Optionally, the exact positions of the first measurement region 214-1a, the second measurement region 214-1b, and the third measurement region 214-1c can be adjusted to sample the volume of the atmosphere that is not or is less disturbed by the aircraft 201, e.g., the movement of the aircraft 201.

[0031] The second DOFC system 202-2 is attached to the first wing 203 of the aircraft 201. Optionally, the second DOFC system 202-2 is attached flush with the surface of the first wing 203 of the aircraft 201. The second DOFC system 202-2 is configured to transmit a second set of the first optical frequency comb and the second optical frequency comb along another LOS 212-2 towards the second wing 207 of the aircraft 201. In another embodiment, the surface of the second wing 207 includes an optical reflective material 225 as described elsewhere herein, where the second set of the first optical frequency comb and the second optical frequency comb contacts the surface of the second wing 207. A fourth measurement volume 214-2 for the other DOFC system 202-2 is disposed between the second DOFC system 202-2 and the second wing 207. Optionally, the exact position of the fourth measurement volume 214-2 can be adjusted to sample the atmosphere that is not or is disturbed by the aircraft 201, for example, by the movement of the aircraft 201.

[0032] FIG. 3 shows yet another embodiment of the mounting configuration 300 of one or more DOFC systems 302-1, 302-2, 302-3 on an aircraft 301. For illustrative purposes, FIG. 3 shows a mounting configuration 300 that directs one or more sets of the first optical frequency comb and the second optical frequency comb towards the engine inlet 311 of an engine 333 of the aircraft 301, e.g., a jet engine. Each portion of each of the first and second optical signals of each set is configured to be reflected from a portion of the engine 333, e.g., a turbine blade, or an inner or outer surface of the engine nacelle, along a respective LOS 312-1, 312-2, 312-3. Although three DOFC systems 302-1, 302-2, 302-3 are shown in FIG. 3 for illustrative purposes, it should be understood that one or more DOFC systems can be used and can be located elsewhere on or within the aircraft 301.

[0033] In the exemplary embodiment of FIG. 3, a first DOFC system 302-1 is mounted on a first wing 303 of an aircraft 301, and the first DOFC system 302-1 is configured to transmit a first set of a first optical frequency comb and a second optical frequency comb along a first LOS 312-1 towards an engine inlet 311 of an engine 333. Optionally, the first DOFC system 302-1 is mounted flush with the surface of the first wing 303 of the aircraft 301. Optionally, the first set of the first optical frequency comb and the second optical frequency comb is transmitted along the first LOS 312-1 so as to contact the surface of the engine inlet 311. In such an embodiment, the surface of the engine inlet 311, e.g., the inner surface, can include an optical reflective material described elsewhere herein that is disposed at a location where the first set of the first and second optical frequency combs 312-1 contacts the surface of the engine inlet 311. A first measurement volume 314-1 for the first DOFC system 302-1 is disposed between the first DOFC system 302-1 and the engine inlet 311. Optionally, the exact position of the first measurement volume 314-1 can be adjusted to sample the atmosphere that is not or is disturbed by the aircraft 301, e.g., by the movement of the aircraft 301.

[0034] The second DOFC system 302-2 is attached to the airframe 323 of the aircraft 301. In one embodiment, the second DOFC system 302-2 is attached flush with the surface of the airframe 323 of the aircraft 301. The second DOFC system 302-2 is configured to transmit a second set of the first optical frequency comb and the second optical frequency comb along the second LOS 312-2 towards the engine inlet 311. Optionally, the second set of the first optical frequency comb and the second optical frequency comb are transmitted to contact the surface within the engine inlet 311. Optionally, the surface of the engine inlet 311 can include an optical reflective material described elsewhere herein that is disposed at a location where the second set of the first optical frequency comb and the second optical frequency comb contact the surface of the engine inlet 311. A second measurement volume 314-2 for the second DOFC system 302-2 is disposed between the second DOFC system 302-2 and the engine inlet 311. Optionally, the exact position of the second measurement volume 314-2 can be adjusted to sample the undisturbed or disturbed atmosphere by the aircraft 301, for example, by the movement of the aircraft 301.

[0035] In one embodiment, the third DOFC system 302-3 can also be mounted on a portion 325 of the first wing 303 or on another component of the aircraft 301. The third DOFC system 302-3 is configured to transmit a third set of the first optical frequency comb and the second optical frequency comb along the third LOS 312-3 towards the engine inlet 311. Optionally, the second set of the first optical frequency comb and the second optical frequency comb is transmitted to contact the surface within the engine inlet 311. Optionally, the surface of the engine inlet can include a light-reflecting material disposed at a location where the third set of the first optical frequency comb and the second optical frequency comb contacts the surface of the engine inlet 311, as described elsewhere in this specification. A third measurement volume 314-3 for the third DOFC system 302-3 is disposed between the third DOFC system 302-3 and the engine inlet 311. Optionally, the exact position of the third measurement volume 314-3 can be adjusted to sample the atmosphere that is not or is disturbed by the aircraft 301, for example, by the movement of the aircraft 301.

[0036] FIG. 4 shows yet another exemplary embodiment of the mounting configuration of one or more DOFC systems 402-1, 402-2 on an aircraft 401. For illustrative purposes, FIG. 4 shows a mounting configuration 400 in which each of two sets of the first optical frequency comb 412-1 and the second optical frequency comb 412-2 is directed along different LOSs 412-1, 412-2 towards different engine inlets 411a, 411b of the aircraft 401, for example, jet engine inlets. A portion of each of the first and second optical frequency combs of each set is reflected back along the corresponding LOS 412-1, 412-2 to the corresponding DOFC systems 402-1, 402-2. It should be understood that two DOFC systems 402-1, 402-2 are shown in FIG. 4 for illustrative purposes, but one or more DOFC systems can be used and can be disposed elsewhere on or within the aircraft 401.

[0037] In the case of an aircraft 401 having engine inlets 411a, 411b disposed below the first wings 403a, 403b, each DOFC system 402-1, 402-1 can be attached to the fuselage 423 of the aircraft 401. Each DOFC system 402-1, 402-2 is configured to transmit a set of a first optical frequency comb and a second optical frequency comb along corresponding LOSs 412-1, 412-2 towards the engine inlets 411a, 411b of the engines 433a, 433b of the aircraft 401. Optionally, each set of the first optical frequency comb 412-1 and the second optical frequency comb 412-2 is transmitted to contact the surface within the engine inlets 411a, 411b. Optionally, the surfaces of the engine inlets 411a, 411b can include a light reflecting material disposed at a location where the sets of the first optical frequency comb 412-1 and the second optical frequency comb 412-2 contact the surfaces of the engine inlets 411a, 411b, as described elsewhere in this specification. The measurement volumes 414-1, 414-2 of each DOFC system 402-1, 402-2 are disposed between the DOFC systems 402-1, 402-2 and the engine inlets 411a, 411b. Optionally, the exact positions of each measurement volume 414-1, 414-2 can be adjusted to sample the atmosphere that is not or is disturbed by the aircraft 401, for example, by the movement of the aircraft 401.

[0038] A technique for characterizing the levels of different types of molecules, such as greenhouse gases, within a measurement volume, for example, within a portion of the interior of an aircraft, using a DOFC system will now be described. FIG. 5A shows an embodiment of the mounting configuration of one or more DOFC systems 502-1, 502-2 within the interior volume 551 of an aircraft 101. The interior volume 551 may be in any part of the aircraft and allows air to flow through the interior volume 551. Optionally, the interior volume 551 may be within the fuselage of the aircraft or through the fuselage, within a measurement system within or on the aircraft, or within an engine nacelle within or on the aircraft. Only one DOFC 502-1, 502-2 can be used within the interior volume 551, but for illustrative purposes, the use of two DOFC systems will be described.

[0039] For illustrative purposes, FIG. 5A shows the mounting configuration 500A of two DOFC systems 502-1, 502-2 within the engine nacelle of an engine of an aircraft 101, for example, a jet engine. The first DOFC system 502-1 is mounted in, for example, a first region (or area) of the inlet 552-1 of the engine nacelle, or around it. The first DOFC system 502-1 transmits a first set of a first optical frequency comb and a second optical frequency comb along a first LOS 512-1 towards an optical reflecting surface of a second region (or area) of the engine nacelle, for example, of the inlet 552-1 of the engine nacelle, or around it. A portion of each of the first and second optical frequency combs of the first set is reflected back to the first DOFC system 502-1 along the first LOS 512-1.

[0040] The second DOFC system 502-2 is attached, for example, to or around a third region (or area) of the outlet 552-2 of the engine nacelle. The second DOFC system 502-2 transmits a second set of a first optical frequency comb and a second optical frequency comb along the second LOS 512-2 towards an optical reflecting surface of a fourth region (or area) of the engine nacelle, for example, of the outlet 552-2 of the engine nacelle or around it. A portion of each of the first and second optical frequency combs of the second set is reflected back to the second DOFC system 502-2 along the second LOS 512-2.

[0041] Optionally, each of the second and fourth regions includes an optical reflecting material 525 arranged at a location where a set of a first optical frequency comb and a second optical frequency comb is incident on each of such regions, as described elsewhere in this specification. The measurement volumes 514-1, 514-2 of each DOFC system 502-1, 502-2 are arranged along each LOS 512-1, 512-2. Optionally, the exact positions of each measurement volume 514-1, 514-2 can be adjusted by the aircraft, for example, to sample the undisturbed or disturbed atmosphere, for example, by the movement of the aircraft.

[0042] Optionally, the first DOFC system 502-1 is configured to measure the concentration of, for example, the type of molecules of greenhouse gases within the first measurement volume 514-1, and the second DOFC system 502-2 is configured to measure the concentration of, for example, the type of molecules of greenhouse gases within the second measurement volume 514-2. Thus, for example, if the first and second measurement volumes are respectively at or around the inlet and outlet of the engine nacelle, the concentration of the type of molecules generated by the engine 556 can be determined by subtracting the concentration of the type of molecules measured in the second measurement volume from the concentration of the type of molecules measured in the first measurement volume. The type of molecule can be a greenhouse gas, water, or the type of molecule used in the fuel consumed by the engine. Optionally, a control system (or circuit) within the aircraft, or the pilot of the aircraft, can utilize the concentration of additional greenhouse gases generated by the engine to modify the engine operation and reduce the greenhouse gases emitted by the engine.

[0043] Optionally, a processing system (or processing circuit) 558 is communicatively coupled to each of the first and second DOFCs 502-1, 502-2. The processing system 558 is configured to obtain from each DOFC, for example, the concentration 555-1 of the type of molecules (e.g., of greenhouse gases) at the input of the engine, and the concentration 555-2 of the type of molecules (e.g., of greenhouse gases) of the greenhouse gases emitted by the engine within the engine nacelle, or to determine these concentrations based on the data provided by each of the first and second DOFCs 502-1, 502-2. Further, the processing system 558 is configured to determine the concentration of additional greenhouse gases generated by the engine 556 by obtaining the difference between both such concentrations 555-1, 555-2.

[0044] FIG. 5B shows another embodiment of the mounting configuration 500B of one or more DOFC systems 502-1, 502-2 within the internal volume 551 of the aircraft 101. FIG. 5B is similar to FIG. 5A except that another DOFC system 502-1-2, 502-2- is used within the engine nacelle, for example at each of the inlet and outlet, or around it. Although only a pair of DOFCs can be used within the internal volume 551, for illustrative purposes, the use of two pairs of DOFC systems will be described.

[0045] As described elsewhere in this specification, a portion of each of the first optical frequency comb and the second optical frequency comb of the first set is reflected along the third LOS 512-1-1 to another DOFC (or a component of the first DOFC) 502-1-2. A portion of each of the first optical frequency comb and the second optical frequency comb of the second set is reflected along the fourth LOS 512-2-1 to another DOFC (or a component of the first DOFC) 502-1-2.

[0046] FIG. 6 shows a block diagram of an embodiment of a DOFC system 602. The illustrated DOFC system 602 includes a first optical frequency comb generator 662-1, a second optical frequency comb generator 662-2, an optional phase-locked loop circuit (PLL) 664, a frequency mixer (or mixer) 665, an optional frequency reference 669, and a DOFC processing system (or circuit) 667. The DOFC processing system 667 includes a library of greenhouse gases and one or more corresponding frequencies and / or frequency bands at which each type of greenhouse gas attenuates to attenuate spectral components within the heterodyne beat frequency comb (discussed elsewhere in this specification).

[0047] Optionally, each optical frequency comb generator 662-1, 662-2 includes a laser coupled to an optical resonator to generate a first optical frequency comb 668A and a second optical frequency comb 668B. Such first and second optical frequency combs 668A, 668B are described elsewhere in this specification. Optionally, the fixed frequency of each optical frequency comb 668A, 668B is about 100 MHz. Optionally, the offset frequency is from about 100 Hz to 1 kHz. Typically, the offset frequency is smaller than the fixed separation frequency. Optionally, each of the first optical frequency comb 668A and the second optical frequency comb 668B is phase-locked to each other to enhance measurement accuracy. This is easy.

[0048] Optionally, each of the first optical frequency comb 668A and the second optical frequency comb 668B is phase-locked to an optional reference signal 669-1 of an optional frequency reference 669 to further enhance measurement accuracy. This is facilitated by an optional frequency reference 669 coupled to an optional PLL 664 coupled to each optical frequency comb generator 662-1, 662-2.

[0049] In the illustrated embodiment, both the first optical frequency comb 668A and the second optical frequency comb 668B are transmitted through the measurement volume 663. However, in other alternative embodiments, the second optical frequency comb 668B is not transmitted through the measurement volume 663. Each optical frequency comb transmitted through a measurement volume 663 containing one or more different types of molecules will be attenuated in one or more frequency ranges where each type of molecule absorbs the electromagnetic spectrum.

[0050] Each of the first and second optical frequency combs 668A, 668B is received by a mixer 665. The mixer 665 is configured to heterodyne mix two incident optical frequency combs (each within the optical spectrum) and emit a heterodyne beat frequency comb 661 (within the radio spectrum) that is the difference between the two incident optical frequency combs. The heterodyne beat frequency comb includes adjacent frequency components separated by the offset frequency.

[0051] The DOFC processing system 667 is configured to receive a heterodyne beat frequency comb. The DOFC processing system 667 is configured to determine, in the measurement volume (through which one or both of the optical frequency combs pass), each type of molecule and the concentration of each such type of molecule, in the manner described elsewhere in this specification.

[0052] FIG. 7 shows a flowchart of one embodiment of a method 770 for verifying a gas concentration. It should be understood that, as long as the method shown in any figure is described as being implemented by any of the apparatuses shown in this specification, other embodiments can be implemented in other ways. Optionally, at least a portion of the method 770 can be implemented by a processing system described herein, such as, for example, the DOFC processing system 667, or by any other type of processing system.

[0053] The blocks of the flowchart are generally sequentially arranged for ease of explanation. However, this arrangement is merely exemplary, and it should be understood that the processes associated with the method (and the blocks shown in the figure) can occur in a different order (e.g., at least some of the processes associated with the blocks are executed in a parallel and / or event-driven manner).

[0054] In block 770-1, two optical frequency combs are radiated from a first region of the aircraft, for example by a DOFC system. At least one of the two optical frequency combs is transmitted through an external environment of the aircraft, for example the environment around its fuselage, for example a measurement volume in the atmosphere. Optionally, the first region of the aircraft is, for example, an external region (e.g., of the fuselage or wing) or an internal region of the internal volume of the aircraft. The first optical frequency comb includes adjacent spectral components separated by a first separation frequency. The second optical frequency comb includes adjacent spectral components separated by a second separation frequency. The two optical frequency combs are further described elsewhere in this specification. Optionally, each of the two optical frequency combs is phase-locked to each other. Optionally, each of the two optical frequency combs is phase-locked to a reference signal.

[0055] In optional block 770-2, at least one of the two optical frequency combs is reflected from a part of the aircraft, for example from an optical reflecting surface. Optionally, the part may be the surface of the fuselage of the aircraft, a part of the engine nacelle, for example the surface of the turbine blade of an engine which is a jet engine of the vehicle, the surface of an internal part of the aircraft, or any other part of the aircraft.

[0056] In block 770-3, after passing through the measurement volume, the two optical frequency combs or one or more parts thereof are received in a first region or a second region of the aircraft, for example on its fuselage or wing. In block 770-4, a heterodyne beat frequency comb within the radio spectrum below the optical spectrum is generated by mixing the two optical frequency combs or one or more parts thereof received in the first or second region of the aircraft.

[0057] In block 770-5, it is determined which spectral components of the heterodyne beat frequency comb have a reduced amplitude as compared to other spectral components of the heterodyne beat frequency comb, for example the spectral component having the maximum amplitude. Further, it is also determined that each amount of such reduced amplitude has been reduced.

[0058] In block 770-6, at least one type of molecule within the measurement volume, e.g., a greenhouse gas, and the concentration of each such type are determined. Each type of molecule within the measurement volume, e.g., a greenhouse gas, is determined by comparing spectral components having a reduced amplitude to a library of frequency spectra reduced by one or more types of molecule types, e.g., greenhouse gas types, and one or more types of molecules, e.g., each greenhouse gas type. The concentration of each type of molecule determined to be within the measurement volume, e.g., a greenhouse gas, is found by, e.g., using the ratio of one or more amplitudes of each spectral component whose amplitude has been reduced by the type of greenhouse gas and the amplitude of at least one spectral component whose amplitude has not been reduced.

[0059] In optional block 770-7, another two optical frequency combs are radiated from a third region of the aircraft, e.g., by a DOFC system. At least one of the another two optical frequency combs is transmitted through another measurement volume outside the aircraft, e.g., in the environment around its fuselage, e.g., in the atmosphere. Optionally, the third region of the aircraft is, e.g., an external region (e.g., of the fuselage or wing) or an internal region of the internal volume of the aircraft. The first optical frequency comb includes adjacent spectral components separated by a first separation frequency. The second optical frequency comb includes adjacent spectral components separated by a second separation frequency. The two optical frequency combs are further described elsewhere in this specification. Optionally, each of the two optical frequency combs is phase-locked to the other. Optionally, each of the two optical frequency combs is phase-locked to a reference signal. Optionally, the two optical frequency combs are radiated from an internal volume of the aircraft, e.g., a part of the engine nacelle, e.g., from an inlet. Optionally, the another two optical frequency combs are radiated from another part of the internal volume, e.g., from an outlet.

[0060] In an optional block 770-8, at least a portion of one of two different optical frequency combs is reflected from another part of the aircraft, for example, from an optical reflecting surface. Optionally, the other part may be the surface of the aircraft fuselage, a portion of the engine nacelle, such as the surface of the turbine blades of an engine that is the jet engine of the vehicle, the surface of an internal part of the aircraft, or any other part of the aircraft.

[0061] In an optional block 770-9, two other optical frequency combs or one or more of their portions are received in a third or fourth region of the aircraft, for example, on its fuselage or wing, after passing through another measurement volume. In block 770-10, by mixing two optical frequency combs or one or more of their portions received on the surface of the aircraft or another surface, another heterodyne beat frequency comb within the radio spectrum below the optical spectrum is generated.

[0062] In an optional block 770-11, which spectral components of another heterodyne beat frequency comb have a reduced amplitude is determined by comparison with other spectral components of the heterodyne beat frequency comb, for example, the spectral component having the maximum amplitude. Further, it is also determined that each amount of such reduced amplitude has decreased.

[0063] In an optional block 770-12, at least one type of molecule within another measurement volume, for example, a greenhouse gas, and the concentration of each such type are determined. In block 770-13, the added or removed concentration of at least one type of molecule, for example, a greenhouse gas and / or a component of the fuel, between the second measurement volume and the first measurement volume is determined. The first measurement volume is a measurement volume and the second measurement volume is another measurement volume or vice versa.

[0064] Each processing system or circuit described herein may be implemented by a processor circuit coupled to a memory circuit. The processor circuits described herein may include one or more microprocessors, microcontrollers, digital signal processing (DSP) elements, application-specific integrated circuits (ASICs), and / or field programmable gate arrays (FPGAs). In this exemplary embodiment, the processor circuit includes, or functions with, software programs, firmware, or other computer-readable instructions for performing the various process tasks, calculations, and control functions used in the methods described herein. These instructions are typically tangibly incorporated into any storage medium (or computer-readable medium) used for storage of computer-readable instructions or data structures.

[0065] The memory circuits described herein can be implemented on any available storage medium (or computer-readable medium) that can be accessed by a general-purpose or special-purpose computer or processor, or any programmable logic device. Suitable computer-readable media may include storage or memory media such as semiconductor, magnetic, and / or optical media. For example, computer-readable media may include conventional hard disks, compact disk-read only memory (CD-ROM), DVDs, random access memory (RAM) (including but not limited to dynamic random access memory (DRAM)), read only memory (ROM), electrically erasable programmable ROM (EEPROM), and / or flash memory, among other volatile or non-volatile media. Combinations of these are also included within the scope of computer-readable media.

[0066] The method of the present invention can be implemented with computer-readable instructions such as program modules or applications, which are part of a processing circuit (optionally a memory circuit) or stored on a computer-readable medium communicatively coupled to the processing circuit and can be executed by the processing circuit, optionally a processor circuit. Generally, program modules or applications include routines, programs, objects, data components, data structures, algorithms, and the like that perform specific tasks or implement specific abstract data types.

[0067] The term "vertical" refers to a direction perpendicular to horizontal. Terms such as "on", "side" (such as "sidewall"), "higher", "lower", "over", "top", and "under" are defined with respect to a conventional plane or working surface that is on the upper surface, regardless of orientation.

[0068] Exemplary embodiments Example 1 includes a method for determining the level of types of molecules within the volume of air inside or around an aircraft. The method includes emitting two optical frequency combs from a first region of the internal volume or external portion of the aircraft, with at least one of the optical frequency combs being transmitted through a measurement volume that is either ambient outside the aircraft or within or around an internal portion of the aircraft; after at least one of the two optical frequency combs has passed through the measurement volume, receiving the two optical frequency combs in a first region or a second region of the internal volume or external portion of the aircraft; using the received two optical frequency combs to generate a heterodyne beat frequency comb; determining which spectral components of the heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude; using the spectral components of the heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within the measurement volume; and using the amplitude of each spectral component of the heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of one or more types of molecules within the measurement volume.

[0069] Example 2 includes the method of Example 1, wherein either one of the two optical frequency combs is phase-locked to the other of the two optical frequency combs, or each of the two optical frequency combs is phase-locked to a reference signal.

[0070] Example 3 includes the method of any one of Examples 1-2, wherein one determined type of molecule includes water, one type of greenhouse gas, or one type of molecule in the fuel used by the aircraft's engine.

[0071] Example 4 includes any one of the methods of Examples 1 - 3 and further includes reflecting portions of each of two optical frequency combs from a portion of an aircraft, wherein receiving the two optical frequency combs includes receiving each reflected portion, and generating a heterodyne beat frequency comb includes generating a heterodyne beat frequency comb using each reflected portion.

[0072] Example 5 includes any one of the methods of Examples 1 - 4 and further includes emitting two other optical frequency combs from a third region of the internal volume or external portion of the aircraft, wherein at least one optical frequency comb is transmitted through another measurement volume surrounding the external or internal portion of the aircraft, receiving the two other optical frequency combs in each of a third region or a fourth region of the internal volume or external portion of the aircraft after at least one of the two other optical frequency combs has passed through the measurement volume, generating another heterodyne beat frequency comb using the received two other optical frequency combs, determining which spectral components of the another heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude, determining each type of molecule within another measurement volume using the spectral components of the another heterodyne beat frequency comb determined to have a reduced amplitude, determining the concentration of one or more types of molecules within another measurement volume using the amplitude of each spectral component of the another heterodyne beat frequency comb determined to have a reduced amplitude, and determining the concentration of at least one type of molecule added or removed between the measurement volume and another measurement volume using the determined concentrations of the one or more types of molecules determined to be within the measurement volume and another measurement volume.

[0073] Example 6 includes the method of Example 5, wherein the measurement volume is at or around the inlet of the engine nacelle. Another measurement volume is at or around the outlet of the engine nacelle.

[0074] Example 7 includes any one of the methods of Examples 5 - 6, and further includes reflecting each part of two optical frequency combs from another part of the aircraft, and receiving two other optical frequency combs includes receiving each reflected part, and generating another heterodyne beat frequency comb includes using each reflected part to generate another heterodyne beat frequency comb.

[0075] Example 8 includes any one of the methods of Examples 5 - 7, and either one of two other optical frequency combs is phase - locked to another optical frequency comb of the two other optical frequency combs, or each of the two other optical frequency combs is phase - locked to a reference signal.

[0076] Example 9 includes an apparatus configured to determine the level of the type of molecules within a volume of air inside or around the aircraft, the apparatus including a first dual - optical - frequency - comb spectroscopy (DOFC) system including a first optical frequency comb generator, a second optical frequency comb generator, a first mixer, and a DOFC processing circuit, each of the first and second optical frequency comb generators including a laser, the first optical frequency comb generator being configured to transmit a first optical frequency comb to the first mixer, the second optical frequency comb generator being configured to transmit a second optical frequency comb to the first mixer, at least one of the first and second optical frequency combs being configured to be transmitted through a first measurement volume before being received by the first mixer, using the received first and second optical frequency combs, the first mixer being configured to generate a first heterodyne beat frequency comb, the DOFC processing circuit being configured to receive the first heterodyne beat frequency comb, determine which spectral components of the first heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude, use the spectral components of the first heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within the first measurement volume, and use the amplitude of each spectral component of the first heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of at least one type of molecule within the first measurement volume.

[0077] Example 10 includes the apparatus of Example 9, and either the first optical frequency comb is phase-locked to the second optical frequency comb or each of the first and second optical frequency combs is phase-locked to a reference signal.

[0078] Example 11 includes the apparatus of any of Examples 9 - 10, and one determined type of molecule includes water, one type of greenhouse gas, or one type of molecule in the fuel used by an aircraft engine.

[0079] Example 12 includes the apparatus of any of Examples 9 - 11, and is a second DOFC system that further includes a third optical frequency comb generator, a fourth optical frequency comb generator, and a second mixer, each of the third and fourth optical frequency comb generators including a laser. The third optical frequency comb generator is configured to transmit a third optical frequency comb to the second mixer, the fourth optical frequency comb generator is configured to transmit a fourth optical frequency comb to the second mixer, at least one of the third and fourth optical frequency combs is configured to be transmitted through a second measurement volume before being received by the second mixer, and using the received third and fourth optical frequency combs, the second mixer is configured to generate a second heterodyne beat frequency comb. The DOFC processing circuit is further configured to receive the second heterodyne beat frequency comb, determine which spectral components of the second heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude, use the spectral components of the second heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within the second measurement volume, use the amplitude of each spectral component of the second heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of at least one type of molecule within the second measurement volume, and use the determined concentrations of at least one type of molecule determined to be within the first and second measurement volumes to determine the concentration of at least one type of molecule added or removed between the second measurement volume and the first measurement volume.

[0080] Example 13 includes the apparatus of Example 12, and the first measurement volume is at or around the inlet of the engine nacelle. The second measurement volume is at or around the outlet of the engine nacelle.

[0081] Example 14 includes the apparatus of any one of Examples 12 to 13, and either the third optical frequency comb is phase-locked to the fourth optical frequency comb or each of the third and fourth optical frequency combs is phase-locked to a reference signal.

[0082] Example 15 includes an apparatus configured to determine the level of types of molecules generated within an engine, the apparatus including a first dual optical frequency comb spectroscopy (DOFC) system including a first optical frequency comb generator, a second optical frequency comb generator, a first mixer, and a DOFC processing circuit, each of the first and second optical frequency comb generators including a laser, the first optical frequency comb generator being configured to transmit a first optical frequency comb to the first mixer, the second optical frequency comb generator being configured to transmit a second optical frequency comb to the first mixer, at least one of the first and second optical frequency combs being configured to be transmitted through a first measurement volume before being received by the first mixer, the first measurement volume being at a first position within the engine, using the received first and second optical frequency combs, the first mixer being configured to generate a first heterodyne beat frequency comb, the DOFC processing circuit being configured to receive the first heterodyne beat frequency comb, determine which spectral components of the first heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude, use the spectral components of the first heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within the first measurement volume, and use the amplitude of each spectral component of the first heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of at least one type of molecule within the first measurement volume, the apparatus further including a second DOFC system including a third optical frequency comb generator, a fourth optical frequency comb generator, and a second mixer, each of the third and fourth optical frequency comb generators including a laser, the third optical frequency comb generator being configured to transmit a third optical frequency comb to the second mixer, the fourth optical frequency comb generator being configured to transmit a fourth optical frequency comb to the second mixer, at least one of the third and fourth optical frequency combs being configured to be transmitted through a second measurement volume before being received by the second mixer, the second measurement volume being at a second position within the engine, using the received third and fourth optical frequency combs, the second mixer being configured toconfigured to generate a second heterodyne beat frequency comb, the DOFC processing circuit is configured to receive the second heterodyne beat frequency comb, determine which spectral components of the second heterodyne beat frequency comb have a reduced amplitude and the amount of each such reduced amplitude, use the spectral components of the second heterodyne beat frequency comb determined to have such a reduced amplitude to determine each type of molecule within the second measurement volume, use the amplitude of each spectral component of the second heterodyne beat frequency comb determined to have a reduced amplitude to determine the concentration of at least one type of molecule within the second measurement volume, and use the determined concentration of at least one type of molecule determined to be within the first measurement volume and the second measurement volume to determine the concentration of at least one type of molecule added or removed between the second measurement volume and the first measurement volume.

[0083] Example 16 includes the apparatus of Example 15, where the first measurement volume is at or around the inlet of the engine nacelle. The second measurement volume is at or around the outlet of the engine nacelle.

[0084] Example 17 includes the apparatus of any of Examples 15 - 16, where either the first optical frequency comb is phase - locked to the second optical frequency comb, or each of the first and second optical frequency combs is phase - locked to a reference signal. Either the third optical frequency comb is phase - locked to the fourth optical frequency comb, or each of the third and fourth optical frequency combs is phase - locked to another reference signal.

[0085] Example 18 includes the apparatus of any of Examples 15 - 17, where determining the concentration of at least one type of molecule added or removed between the second measurement volume and the first measurement volume includes determining the concentration of at least one type of greenhouse gas generated by the engine.

[0086] Example 19 includes the apparatus of any one of Examples 15 to 18, and determining the concentration of at least one type of molecule added or removed between the second measurement volume and the first measurement volume includes determining the concentration of at least one type of molecule in the fuel used by the engine.

[0087] Example 20 includes the apparatus of any one of Examples 15 to 19, and the engine is an aircraft engine.

[0088] Although specific embodiments have been illustrated and described herein, it will be understood by those skilled in the art that any configuration that can be predicted to achieve the same purpose may be used instead of the specific embodiments shown. Therefore, it is clearly intended that the present invention be limited only by the claims and their equivalents.

Claims

1. 1. An apparatus configured to determine a level of a type of molecule within a volume of atmosphere within or around an aircraft, the apparatus comprising: a first dual optical frequency comb spectroscopy (DOFC) system including a first optical frequency comb generator, a second optical frequency comb generator, a first mixer, and a DOFC processing circuit; each of the first and second optical frequency comb generators includes a laser; the first optical frequency comb generator configured to transmit a first optical frequency comb to the first mixer; the second optical frequency comb generator configured to transmit a second optical frequency comb to the first mixer; at least one of the first and second optical frequency combs is configured to be transmitted through a first measurement volume before being received by the first mixer; using the received first and second optical frequency combs, the first mixer is configured to generate a first heterodyne beat frequency comb; The DOFC processing circuit includes: receiving the first heterodyne beat frequency comb; determining which spectral components of the first heterodyne beat frequency comb have reduced amplitudes and amounts of each such reduced amplitude; determining each type of molecule in the first measurement volume using the spectral components of the first heterodyne beat frequency comb determined to have such reduced amplitude; and and determining a concentration of at least one type of molecule in the first measurement volume using the amplitude of each spectral component of the first heterodyne beat frequency comb that is determined to have a reduced amplitude; 11. The apparatus, wherein either the first optical frequency comb is phase-locked to the second optical frequency comb, or each of the first and second optical frequency combs is phase-locked to a reference signal.

2. The apparatus of claim 1 , wherein the one determined type of molecule comprises water, a type of greenhouse gas, or a type of molecule in a fuel used by an engine of the aircraft.

3. a second DOFC system including a third optical frequency comb generator, a fourth optical frequency comb generator, and a second mixer; each of the third and fourth optical frequency comb generators includes a laser; the third optical frequency comb generator configured to transmit a third optical frequency comb to the second mixer; the fourth optical frequency comb generator configured to transmit a fourth optical frequency comb to the second mixer; at least one of the third and fourth optical frequency combs is configured to be transmitted through a second measurement volume before being received by the second mixer; the second mixer is configured to generate a second heterodyne beat frequency comb using the received third and fourth optical frequency combs; The DOFC processing circuit includes: receiving the second heterodyne beat frequency comb; determining which spectral components of the second heterodyne beat frequency comb have reduced amplitudes and amounts of each such reduced amplitude; determining each type of molecule in the second measurement volume using the spectral components of the second heterodyne beat frequency comb determined to have such reduced amplitude; and determining a concentration of at least one type of molecule in the second measurement volume using the amplitude of each spectral component of the second heterodyne beat frequency comb determined to have a reduced amplitude; and determining a concentration of at least one type of molecule added or removed between the second measurement volume and the first measurement volume using the determined concentrations of the at least one type of molecule determined to be in the first measurement volume and the second measurement volume; Optionally, the first measurement volume is at or about an inlet of an engine nacelle and the second measurement volume is at or about an outlet of the engine nacelle.

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