Attorney Docket 006545/PCT

TRANSLATIONAL LASER ABSORPTION SPECTROSCOPY APPARATUS AND METHOD

TECHNICAL FIELD

[0001] The present invention is directed toward a method and apparatus for obtaining high spatial resolution spectroscopic measurements.

BACKGROUND OF THE INVENTION

[0002] Absorption spectroscopy, including tunable diode laser absorption spectroscopy (TDLAS) are techniques for measuring the concentration of various species in a gaseous mixture. Absorption spectroscopy techniques are particularly well-suited to achieve very low detection limits. In addition to species concentration, it is possible with absorption spectroscopy to determine other parameters, such as temperature, pressure, velocity and mass flux of certain species of the gas under observation.

[0003] A typical TDLAS apparatus includes a tunable diode laser light source plus transmitting and receiving optics and detectors. The output of the tunable diode laser is tuned over a wavelength range encompassing selected absorption lines of various gas species of interest in the path of the laser beam. Absorption features cause a reduction of measured signal intensity which can be detected and used to determine gas concentration and other properties. The use of TDLAS is described in detail in United States Patent No. 7,248,755 entitled "Method And Apparatus For The Monitoring And Control Of Combustion", which application is incorporated herein by reference in its entirety for the specific teaching of the use of TDLAS to monitor or control a process.

[0004] Tomography is a technique whereby spatial resolution is obtained from line- of-sight measurements over multiple, often intersecting paths or projections, at a variety of selected orientations. Tomography is a well-known technique which has been used extensively in applications such as medical imaging. At each orientation, the transmitted radiation is monitored. Each transmission measurement is an average over the path traversed by the beam. In other words, an individual projection provides no spatial information. Using the transmission results from many projections as inputs allows the use of mathematical transforms to reconstruct what the object must look like in order to produce the measured transmissions. In this way, spatial resolution is obtained from a technique that intrinsically produces a line-of-sight average. High tomographic spatial resolution requires that many

projections be used. Cost considerations may limit the number of possible beam paths thus indirectly limiting the obtainable resolution.

[0005] One method and apparatus for enhancing the tomographic spatial resolution of a laser absorption spectroscopy system is disclosed in pending and commonly assigned U.S. Patent Application 60/940,006 entitled "Binning and Tomography for High Spatial Resolution Temperature and Species Concentration Measurements", which application is incorporated herein by reference in its entirety for the specific teaching of the use of temperature or concentration binning in conjunction with a tomographic TDLAS grid to determine gas temperatures or species concentration along select line-of-sight spectroscopy paths. Thus, the overall tomographic resolution may be enhanced without requiring additional optical components as would be needed to enhance tomographic resolution in a conventional manner. The number of bins which can be calculated along a line-of-sight path according to the 60/940,006 application is limited, however, because each additional bin generally requires at least two new interrogated wavelengths to make a measurement. Thus, certain quantities of gas may be difficult to interrogate at a high level of spatial resolution through binning combined with conventional tomography.

[0006] The present invention is directed toward overcoming one or more of the problems discussed above.

SUMMARY OF THE INVENTION

[0007] One embodiment includes an absorption spectroscopy apparatus having at least one transmitting and receiving optic pair for obtaining absorption spectroscopy data through a quantity of gas. The absorption spectroscopy apparatus further includes means for moving one or both of the transmitting and receiving optics with respect to the quantity of gas while maintaining optical association between the transmitting and receiving optic. Thus, absorption spectroscopy data is obtained along more than one line of sight with the optic pair. [0008] In an alternative embodiment, the absorption spectroscopy apparatus may include multiple transmitting and receiving optic pairs. More than one transmitting optic may be arranged into at least one transmitting array and the corresponding receiving optics may be arranged into a corresponding receiving array. In this embodiment, the entire transmitting array, receiving array or both arrays may be moved with respect to the quantity of gas. Data may thus be obtained from more than one transmitting and receiving optic pair substantially simultaneously.

[0009] In all embodiments, the means for moving an optic or an array may be a carriage that rotates around the perimeter of the quantity of gas of interest. Alternatively, the carriage may translate with respect to the quantity of gas. For example, the carriage may translate along linear segments of a polygon defining the perimeter of the quantity of gas of interest. The quantity of gas may be associated with a combustion zone, for example, the gases in or downstream from the combustion zone of a jet engine. [0010] An alternative embodiment includes a method of absorption spectroscopy featuring the collection of absorption spectroscopy data along the first optical path between a transmitting and receiving optic pair. The method further includes moving one or both of the transmitting optic or receiving optic while maintaining optical association between the pair, and then obtaining spectroscopy data along a second optical path.

[0011] The method may be expanded to include multiple pairs of transmitting and receiving optics which may be arranged in arrays. Thus, spectroscopic data may be obtained along more than one optical path substantially simultaneously.

[0012] Another embodiment is a jet engine test stand including optical components as described above for obtaining spectroscopy data from the measurement zone of a jet engine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Fig. 1 is a schematic diagram of a laser absorption apparatus with stationary optics.

[0014] Fig. 2 is a schematic diagram of a translational laser absorption spectroscopy apparatus featuring translational movement of an optical array.

[0015] Fig. 3 is a schematic diagram of a laser absorption spectroscopy apparatus featuring rotation of an optical array around the perimeter of a quantity of gas.

[0016] Fig. 4 is a schematic diagram of a laser absorption spectroscopy apparatus featuring rotation of an optical array around the perimeter of a quantity of gas.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] Certain apparatus and methods disclosed herein feature spectroscopy along a select number of line-of-sight paths through a quantity of gas which is being probed. The embodiments disclosed herein are not limited to any particular tomographic pattern or number of paths. The techniques described herein can be used with any gas system, including but not limited to combustion systems, jet engines, conventional engines, rockets

and laboratory or industrial gas systems. All embodiments feature absorption spectroscopy the key elements of which are described below.

[0018] The present embodiments feature tunable diode laser absorption spectroscopy

(TDLAS) using techniques known to those skilled in the art of laser spectroscopy. Generally, TDLAS is performed by the transmission of laser light through a target environment, followed by the detection of the absorption of the laser light at specific wavelengths, due to target gases such as carbon monoxide or oxygen. Spectral analysis of the detected light allows identification of the type, temperature and quantity of gas along the laser path. The non-contact nature of laser absorption spectroscopy makes it well-suited for harsh environments such as the measurement zone of a jet engine, defined herein as the jet combustion zone plus any downstream gas flow zones, the combustion zone or downstream gas zones of a coal-fired power plant, or other flammable or toxic environments where conventional probes cannot be used. The use of laser light provides the high brightness necessary to get detectible transmission in the presence of severe attenuation (e.g., greater than 99.9% loss of light) that may be seen in some of these environments. To better withstand the harsh conditions of the target applications, the laser light may be brought in to the target environment through armored optical fiber.

[0019] Effective sensing of temperature or multiple combustion process component gasses requires the performance of TDLAS with multiple widely spaced frequencies of laser light. The frequencies selected must match the absorption lines of the transitions being monitored.

[0020] A TDLAS apparatus 10 is depicted schematically in Fig. 1. The Fig. 1 embodiment features more than one set of transmitting optics 20 and corresponding receiving optics 22 associated with a single gas process 24. A wavelength multiplexed TDLAS probe beam can be routed by a routing device which, as is shown in Fig. 1 , may be an optical switch 25 to each set of transmitting optics 20. Suitable routing devices include optical switches which may be implemented to route the probe beam with minimal attenuation to each set of transmitting/receiving optics in a predetermined sequence or an optical splitter which simultaneously routes a fractional portion of the multiplexed probe beam to each set of optics.

[0021] A similar optical routing device which, in Figure 1, is shown as a multimode optical switch 26 can be employed on the receiving side of the system to route the portion of the multiplexed probe beam received by each receiving optic 22 to the catch side demultiplexer 28. Although the embodiment depicted in Figure 1 shows only two sets of

transmitting and receiving optics, the system can employ any number of transmitting and receiving optical sets. In addition, the multiple sets may be disposed along intersecting optical paths to facilitate tomographic reconstruction. The use of fiber coupling and a (de)multiplexed probe beam on both the transmitting and receiving sides of the system allows multiple sets of transmitting and receiving optics to be implemented with one set of lasers 30 and detectors 32. Without the incorporation of optical multiplexing techniques, separate sets of lasers, detectors and fiber cables, all requiring calibration, would be needed for each transmitter/receiver pair. As discussed in detail below, multiple transmitter/receiver pairs allow the implementation of one or more two dimensional sensing grids over the entire gas process 24 or elsewhere, such as for sensing a downstream gas process. In addition, the fiber-coupled nature of the present invention allows readily available telecommunications components to be used to positive effect. For instance, a fiber-optics switch can be used to route the multiplexed probe beam to different locations for measurement. I X N optical switches with N up to 8 are readily available as off-the-shelf components from a variety of suppliers. Switches with N up to 16 can be custom ordered, and switches with N greater than 16 may be available.

[0022] A switch and multiple pairs of transmitting and receiving optics can be used for serial probing of a gas species at different locations throughout the gas process 24. For situations in which averaged results are sufficient, serial probing of different beam paths is acceptable. However, certain applications require near instantaneous probing of an entire sensing grid. For example, certain combustion process flows exhibits high-frequency fluctuations, or the flow may only exist for a short period of time, e.g. shock tubes or tunnels. In such a case a 1 X N splitter may be used to divide the probe beam into N branches each of which occupies a different position on the grid. Since the entire grid is illuminated simultaneously, a two dimensional analysis can be generated very quickly. However, simultaneous two dimensional analysis may require that each component on the receiving side be reproduced for each beam path including demultiplexers, detectors, electronics such as A/D cards and, to some extent, computers.

[0023] Thus, embodiments featuring switches or splitters facilitate somewhat coarse tomographic reconstruction of two-dimensional cross sections of the probed region. In certain implementations, however, a tomographic reconstruction with higher spatial resolution may be required than is easily produced with a convenient number of switches, splitters or transmitting and receiving optic pairs.

[0024] Fig. 2 is a schematic representation of an apparatus for absorption spectroscopy 200 which is suitable for achieving enhanced tomographic spatial resolution with a limited number of optical components. The embodiment illustrated in Fig. 2 includes two arrays 202A and 202B of individual transmitting optics 204A-n. Each transmitting optic 204 is in optical communication with a receiving optic 206A-n, which receiving optics 206A- n are grouped into corresponding receiving arrays 208 A and 208B. Each of the arrays 202, 208 and the optical components contained therein are associated with a quantity of a gas of interest 210 which may be, but is not necessarily associated with a combustion process chamber 212 such as downstream from the combustion zone of a jet engine. [0025] The apparatus 200 also includes means for moving one or both of selected transmitting and receiving optic pairs (for example, 204A and 206A) with respect to the quantity of gas 210. Alternatively, an entire array, 202 A for example, and/or the corresponding receiving array 208 A may be moved with respect to the quantity of gas 210. [0026] By moving one or more optical components with respect to the quantity of gas, multiple optical paths 214A-n can be defined through the quantity of gas 210. The number of multiple paths 214 defined by the moving optical components is limited only by readily controllable factors such as the utilized speed of translational movement or the frequency at which data is collected from each optical pair. The embodiment illustrated in Fig. 2 features two sets of transversely disposed transmitting arrays 202A and 202B which correspond to transversely disposed receiving arrays 208 A and 208B, thus assuring that intersecting optical paths are defined, as is desirable for tomographic reconstruction. The arrays are each configured to translate or otherwise move along linear segments of a square corresponding to the perimeter of the process chamber 212. As each array and the optical components contained therein moves or translates along the corresponding side of the square, data may be obtained throughout a substantially rectangular, tomographic grid having virtually any desired spatial resolution.

[0027] In certain embodiments, it may be less difficult to maintain proper alignment between transmitting optic and receiving optic pairs if both sides of an optical pair move or translate together, as is shown in Fig. 2. With proper aiming techniques, however, it would be possible to generate high resolution asymmetrical tomographic reconstructions by moving either the transmitting optic or receiving optic with respect to a substantially stationary companion optic.

[0028] The line segments along which the optics and arrays translate in Fig. 2 are shown as a simple square. In addition, a small number of separate arrays and separate optical

pairs are illustrated. The embodiments disclosed herein may be implemented with any number of optical components in conjunction with translation or movement along any shape of perimeter. In practice, the decision of whether to group transmitting optic 204 and receiving optic 206 pairs into arrays will be made based upon the optical requirements of a selected implementation. Similarly, if arrays are utilized, the number of transmitting optics 204 and receiving optics 206 included on each array may be varied to achieve desired design goals. In a simplest embodiment, a single transmitting optic 204 may be paired with a single receiving optic 206 provided that one optic or the other may be moved with respect to the quantity of gas 210 resulting in multiple optical paths 214 which may be used for tomographic reconstruction. A more sophisticated implementation may include multiple arrays having multiple optical pairs. The scope of the embodiments disclosed herein covers both simple and complex implementations. The embodiment of Fig. 2 features arrays which translate back and forth along the line segments which define a square perimeter. As described above, arrays and/or optical components may be caused to move or translate along linear segments of other types of polygons, or through curved or irregular patterns to achieve specific tomographic goals.

[0029] For example, as is shown in Figs. 3 and 4, appropriately configured transmitting and receiving optic pairs 204A, 206A may be rotated around the perimeter of a substantially circular gas process zone. As shown in Fig. 3, substantially transverse optic pairs may be rotated around a perimeter in unison together to fully cover a gas process zone with a tomographic grid using a minimized number of optical components. If a transmitting optic or array and a receiving optic or array pair are rotated around the entire circumference of a substantially circular combustion zone, a full two-dimensional tomographic reconstruction can be made with data collected sequentially with a single transmitting array and a single receiving array.

[0030] The embodiments of Figs. 2 and 3 feature optical paths which are substantially transverse to one another. Alternative embodiments may include optical paths 214 which intersect but are not necessarily transversely disposed. For example, Fig. 4 which illustrates an array of transmit and receive optic pairs (202, 208) which produce generally parallel optical paths 214. The generally parallel optical paths 214 may be made to intersect with radial optical paths 220 by rotating or translating the optics producing optical paths 214. Any other configuration of optics which results in multiple intersecting optical paths upon movement of one or more of the optics is within the scope of this disclosure.

[0031] The arrays 202, 208 and transmitting or receiving optics 204, 206 respectively may be made to move or translate with respect to the quantity of gas of interest 210 by any means known in the mechanical arts. For example, the optical components may be operatively disposed on a frame which moves, translates or rotates around a combustion zone. Alternatively, the optical components may be mounted on carriages or drives which travel along a frame or track which is stationary with respect to the gas process zone. Movement may involve literally moving the optics. Alternatively, movement may be produced by refracting, reflecting or otherwise optically steering a beam. A combination of techniques may be used to move an optic or beam. Data may be collected continuously, or data may be collected only at times when the optical components are stationary, for example, between periods of movement.

[0032] The apparatus and methods disclosed herein have been described with respect to two-dimensional arrays. Translation of pairs of optical elements, or arrays of optical elements along a third axis may also provide for the three-dimensional tomographic probing of a quantity of gas of interest. Movement along a third axis may be accomplished in conjunction with, or separate from the two-dimensional range of movements described above.

[0033] The embodiments described herein are particularly well suited for the implementation of a high tomographic resolution laser absorption spectroscopy jet engine test stand having probe grids defined at one or more locations in the jet engine measurement zone.

[0034] Various embodiments of the disclosure could also include permutations of the various elements recited in the claims as if each dependent claim was multiple dependent claim incorporating the limitations of each of the preceding dependent claims as well as the independent claims. Such permutations are expressly within the scope of this disclosure. In addition, various embodiments disclosed herein can be combined if technically feasible even if disclosed as separate embodiments and such combinations are within the scope of the disclosure and the invention.

[0035] While the invention has been particularly shown and described with reference to a number of embodiments, it would be understood by those skilled in the art that changes in the form and details may be made to the various embodiments disclosed herein without departing from the spirit and scope of the invention and that the various embodiments disclosed herein are not intended to act as limitations on the scope of the claims.