Inverse multiplexing filter and method

Abstract

The inverse multiplexed filtering method includes propagating an optical beam from an input optical fiber to a diffraction grating to generate first and second diffracted beams having respective first central wavelengths λ1 and second central wavelengths λ2>λ1 of the optical beam. The first diffracted beam propagates backward toward the input optical fiber at a first diffraction angle that is partially determined by λ1 and the diffraction order m1 of the first diffracted beam. The second diffracted beam propagates backward toward the input optical fiber at a second diffraction angle that is partially determined by λ2 and the diffraction order m2<m1. The method also includes (i) coupling the first diffracted beam to a first optical fiber of a one-dimensional optical fiber array including the input optical fiber, and (ii) coupling the second diffracted beam to a second optical fiber of the one-dimensional optical fiber array.

Description

[Technical Field]

[0001] (Related applications) This application claims priority to U.S. Provisional Patent Application No. 63 / 053,086, filed on 17 July 2020, which is incorporated herein by reference in its entirety. [Background technology]

[0002] Maintaining the efficient operation of combustion equipment such as furnaces and boilers requires accurate measurement of gas species concentration and temperature within the combustion zone of the equipment. Laser spectroscopy is one measurement method in which a light beam from a laser is sensed after it has passed through the combustion zone. In such measurements, high levels of background light are often present due to the presence of high-luminosity emissions from flames (e.g., coal-fired boilers) or arcs or discharges (e.g., electric arc furnaces). The light emissions from these background sources are broadband, typically containing a bandwidth of 1 micrometer or more. After passing through these environments, the light beam, which may be narrowband, is often attenuated at a rate of 70 dB or more, depending on the length of the beam path through the combustion zone. After attenuation, the broadband background light can exceed the power from the probe signal, making measurement extremely difficult or impossible. [Overview of the project]

[0003] The embodiments disclosed herein solve this problem by filtering broadband background light while enabling detection of the probe signal with minimal attenuation.

[0004] In a first embodiment, the demultiplexed filtering method includes propagating an optical beam from an input optical fiber to a diffraction grating to generate a first diffracted beam and a second diffracted beam. The center wavelength of the first diffracted beam is equal to the first center wavelength of the first channel of the optical beam. The center wavelength of the second diffracted beam is equal to the second center wavelength of the second channel of the optical beam, the second center wavelength being greater than the first center wavelength. The first diffracted beam propagates backward toward the input optical fiber with a first diffraction angle partially determined by the first center wavelength and diffraction order m1 of the first diffracted beam. The second diffracted beam propagates backward toward the input optical fiber with a second center wavelength and a second diffraction angle partially determined by the diffraction order m2 of the second diffracted beam, which is less than diffraction order m1. The method also includes (i) coupling the first diffracted beam to a first optical fiber of a one-dimensional optical fiber array including the input optical fiber, and (ii) coupling the second diffracted beam to a second optical fiber of the one-dimensional optical fiber array.

[0005] In a second embodiment, a method for measuring the concentration of a species in a combustion zone includes propagating a multiplexed input probe beam through the combustion zone to generate an output probe beam, and coupling the output probe beam to an input optical fiber. The combustion zone includes (i) a first gas-phase species having an absorption line at a first central wavelength, and (ii) a second gas-phase species having an absorption line at a second central wavelength above the first central wavelength. The method also includes propagating the output probe beam from the input optical fiber to a diffraction grating to generate a first diffraction beam and a second diffraction beam. The central wavelength of the first diffraction beam is equal to the first central wavelength. The central wavelength of the second diffraction beam is equal to the second central wavelength. The first diffraction beam propagates backward toward the input optical fiber at a first diffraction angle, which is partially determined by the first central wavelength and diffraction order m1 of the first diffraction beam. The second diffracted beam propagates backward toward the input optical fiber at a second diffraction angle, partially determined by the second central wavelength of the second diffracted beam and a diffraction order m2 smaller than the diffraction order m1. This method also includes (i) coupling the first diffracted beam to a first optical fiber in a one-dimensional optical fiber array including the input optical fiber, and (ii) coupling the second diffracted beam to a second optical fiber in the one-dimensional optical fiber array. This method also includes (i) measuring the first signal amplitude of the first diffracted beam output from the first optical fiber, and (ii) measuring the second signal amplitude of the second diffracted beam output from the second optical fiber. This method also includes (i) determining the concentrations of a first gaseous species and a second gaseous species from the first signal amplitude.

[0006] In a third embodiment, the inverse multiplexing filter includes an optical fiber array, a diffraction grating, and a lens. The optical fiber array includes (i) an input optical fiber having a fiber optical axis and an input fiber end face, the surface of which defines a fiber end face plane; (ii) a first output optical fiber having a first fiber end face substantially coplanar with the fiber end face plane, and a first optical axis parallel to the fiber optical axis and coplanar with the fiber optical axis; and (iii) a second fiber end face coplanar with the fiber end face plane within tolerance, and a second output optical fiber having a fiber optical axis that is colinear, parallel to the input fiber end face, the first fiber end face, and the second fiber end face, and coplanar with the fiber end face. The diffraction grating faces the fiber end face plane and has a blazed diffractive surface inclined at an angle of inclination that is 0.05 to 0.5 degrees from the blaze angle of the blazed diffractive surface with respect to the fiber optical axis. The lens is positioned along the optical path between the optical fiber array and the diffraction grating, has a lens optical axis perpendicular to the fiber end-face plane, and is configured to form an image of the blazed diffractive surface at a focal plane that is substantially coplanar with the fiber end-face plane. [Brief explanation of the drawing]

[0007] [Figure 1] This is a schematic block diagram of an inverse multiplexing filter operating as part of a monitored combustion system in one embodiment. [Figure 2] This flowchart illustrates a demultiplexed filtering method that can be implemented using the demultiplexed filter shown in Figure 1, according to one embodiment. [Figure 3] This is a schematic cross-sectional view of the demultiplexing filter of Figure 1, shown in more detail than in Figure 1, in one embodiment. [Figure 4] A portion of the spreadsheet used to design the demultiplexing filter embodiment shown in Figure 1 in one embodiment is shown as an example. [Figure 5]This is a schematic cross-sectional view of an inverse multiplexing filter, including a lens designed to fill the fiber positions in the spreadsheet of Figure 4, in one embodiment. [Figure 6] This is a table of exemplary features of the lens of the inverse multiplexing filter shown in Figure 5 in one embodiment. [Figure 7] Figure 4 shows the optical spectra of the diffracted beams coupled to each output optical fiber in the embodiment of the inverse multiplexing filter. [Figure 8] Figure 4 shows the optical spectra of the diffracted beams coupled to each output optical fiber in the embodiment of the inverse multiplexing filter. [Figure 9] Figure 4 shows the optical spectra of the diffracted beams coupled to each output optical fiber in the embodiment of the inverse multiplexing filter. [Modes for carrying out the invention]

[0008] Figure 1 is a schematic block diagram of the demultiplexing filter 100 in an exemplary and non-limiting use scenario as part of a monitored combustion system 190. The demultiplexing filter 100 may be used in other applications requiring optical demultiplexing, such as telecommunications.

[0009] In the combustion system 190, the inverse multiplexing filter 100 is configured to measure the concentration of species within the combustion zone 162 of the combustion apparatus 160. Examples of the combustion apparatus 160 include a heating furnace and a boiler. The cross-section shown in Figure 1 is parallel to a plane hereinafter referred to as the yz plane, which is formed by orthogonal axes 298Y and 298Z, respectively, which are perpendicular to axis 298X. In this specification, the xy plane is formed by orthogonal axes 298X and 298Y, and a plane parallel to the xy plane is referred to as the transverse plane. Unless otherwise specified, the height of an object in this specification refers to the length of the object along axis 298Y. In this specification, references to axes x, y, or z refer to axes 298X, 298Y, and 298Z, respectively. In this specification, the horizontal plane is parallel to the xz plane, the width refers to the length of the object along the z axis, and the vertical refers to the direction along the y axis.

[0010] The monitored combustion system 190 also includes optoelectronic equipment 180, which includes a plurality of lasers 182, a plurality of photodetectors 186, and a multiplexer 184. The lasers 182 include lasers 182(1) and 182(2), and may also include additional lasers 182(3-N), where the number of channels N is an integer greater than or equal to 2. The photodetectors 186 include photodetectors 186(1) and 186(2), and may also include additional photodetectors 186(3-N). The multiplexer 184 mixes the outputs of the lasers 182 to produce a multiplexed probe beam 141. In some embodiments, an inverse multiplexing filter 100 includes at least one of the lasers 182, the multiplexer 184, and the photodetectors 186. In some embodiments, the monitored combustion system 190 includes a computer 192 for processing the outputs of the photodetectors 186.

[0011] Each channel of the multiplexed probe beam 141 corresponds to the respective central wavelength 183(k) of the laser 182(k), where k is the channel number and is an integer in the range of 1 to N. In this specification, λ krepresents the central wavelength 183(k). In several embodiments, the central wavelength 183(k) increases with k, so the central wavelength 183(k+1) is greater than the central wavelength 183(k).

[0012] The inverse multiplexing filter 100 includes an optical fiber array 110, a lens 120, and a diffraction grating 130. The optical fiber array 110 includes an input optical fiber 112 and a plurality of output optical fibers 116. The output optical fibers 116 include output optical fibers 116(1) and 116(2), and may also include an additional output optical fiber 116(3-N). The input optical fiber 112 receives the multiplexed probe beam 141 as an optical beam 142, which is also referred to herein as the output probe beam. The optical beam 142 propagates from the input optical fiber 112 to the diffraction grating 130 to generate diffracted beams 146(1) and 146(2), and in some embodiments, to generate an additional diffracted beam 146(2-N).

[0013] Species present in the combustion zone 162 have absorption lines in the near-infrared portion of the electromagnetic spectrum. In some embodiments, the spectral density of the light beam 142 is different from the spectral density of the multiplexed probe beam 141 corresponding to the concentration of the species. The central wavelength 183 can be 1.3 micrometers to 2.5 micrometers. In some embodiments, at least one central wavelength 183 corresponds to an absorption line of one of water, carbon monoxide, and carbon dioxide. In one such embodiment, each of the central wavelengths 183 corresponds to an absorption line of one of water, carbon monoxide, and carbon dioxide, and the number of channels N is at least 5.

[0014] Multimode fibers have a wider fiber core than single-mode fibers, which facilitates coupling the diffracted beam 146 to the output optical fiber 116 using lens 120. In several embodiments, each output optical fiber 116 operates as a multimode fiber in the wavelength range of 1.3 micrometers to 2.5 micrometers. At any wavelength within this range, the input optical fiber 112 can operate as a single-mode fiber or a multimode optical fiber. In several embodiments, each output optical fiber 116 may have a fiber core diameter greater than 90 micrometers, and the input optical fiber 112 may have a fiber core diameter of 50 micrometers to 200 micrometers to facilitate optical coupling to its input optical fiber.

[0015] In several embodiments, the input optical fiber 112 has a core radius a0 and a numerical aperture NA0, and therefore the V parameter of the optical fiber is 2π(a0 / max(λ). k ))NA0 is 2.405 or greater, and each center wavelength λ k This ensures multimode operation. In some embodiments, each output optical fiber 116(k) has a core radius a k and numerical aperture NA k It has, and therefore the V parameter of its output optical fiber is 2π(a k / λ k )NA0 is 2.405 or greater, and the center wavelength λ k To ensure multi-mode operation.

[0016] Figure 2 is a flowchart illustrating the demultiplexed filtering method 200. In some embodiments, the method 200 is implemented by a demultiplexed filter 100 as part of a monitored combustion system 190. The method 200 includes steps 210 and 220. In some embodiments, the method 200 also includes at least one of steps 230 and 240.

[0017] Step 210 includes propagating an optical beam from an input optical fiber to a diffraction grating to generate a first diffracted beam and a second diffracted beam. The central wavelength of the first diffracted beam is equal to the first central wavelength of the first channel of the optical beam. The central wavelength of the second diffracted beam is equal to the second central wavelength of the second channel of the optical beam. The first diffracted beam propagates backward toward the input optical fiber at a first diffraction angle that is partially determined by the first central wavelength of the first diffracted beam and the diffraction order m1. The second diffracted beam propagates backward toward the input optical fiber at a second diffraction angle that is partially determined by the second central wavelength of the second diffracted beam and a diffraction order m2 that is smaller than the diffraction order m1.

[0018] In an example of step 210, an optical beam 142 propagates from an input optical fiber 112 to a diffraction grating 130 to generate diffracted beams 146(1) and 146(2), and those diffracted beams have respective central wavelengths 183(1) and 183(2). The diffracted beams 146(1) and 146(2) propagate backward toward the input optical fiber 112 at respective diffraction angles 147(.) and 147(2). The angle 147(1) is partially determined by the central wavelength 183(1) of the diffracted beam 146(1) and the diffraction order. The angle 147(2) is partially determined by the central wavelength 183(2) of the diffracted beam 146(2) and the diffraction order. Each angle 147 is in a plane parallel to the y-z plane.

[0019] In multiple embodiments, step 210 generates additional diffracted beams having respective central wavelengths and diffraction orders. For example, step 210 generates a diffracted beam 146(3-N) having respective central wavelengths 183(3-N) and a diffraction order m(3-N), where m k+1 is k smaller than m.

[0020] In several embodiments, step 210 includes step 214. Step 214 includes collimating the light beam using a lens positioned between the tip of the input optical fiber and the diffraction grating. In an example of step 214, the lens 120 collimates the light beam 142 as it propagates from the input optical fiber 112 and the diffraction grating 130.

[0021] Step 220 includes coupling the first diffracted beam to the first optical fiber of a one-dimensional optical fiber array, which includes the input optical fiber, and coupling the second diffracted beam to the second optical fiber of the one-dimensional optical fiber array. In the example of step 220, each diffracted beam 146(k) is coupled to its respective output optical fiber 116(k).

[0022] In several embodiments, each step 220 includes step 225, which includes using a lens to focus the first diffracted beam and the second diffracted beam onto the respective ends of the first and second output optical fibers. In an example of step 225, the lens 120 focuses each diffracted beam 146(k) onto the respective ends of the output optical fiber 116(k).

[0023] Step 230 includes measuring (i) the first signal amplitude of the first diffracted beam output from the first optical fiber, and (ii) the second signal amplitude of the second diffracted beam output from the second optical fiber. In the example of step 230, each photodetector 186(k) measures the respective signal amplitudes of the diffracted beam 146(k).

[0024] Step 240 includes determining (i) the concentration of a first gas-phase species having an absorption line at a first central wavelength from a first signal amplitude, and (ii) the concentration of a second gas-phase species having an absorption line at a second central wavelength from a second signal amplitude. In the example of step 240, for each channel number k, the computer 192 determines the concentration of each gas-phase species having an absorption line at a central wavelength 183(k) from each signal amplitude of the diffracted beam 146(k).

[0025] In several embodiments, Method 200 is part of a method for measuring the concentration of a species in a combustion zone. In such embodiments, Method 200 includes steps 202 and 204. Step 202 includes propagating a multiplexed probe beam through the combustion zone, where the light beam of step 210 is the multiplexed probe beam after it has passed through the combustion zone. In the example of step 202, the multiplexed probe beam 141 propagates through the combustion zone 162, where the light beam 142 is the multiplexed probe beam 141 after it has propagated through the combustion zone 162. Step 204 includes coupling the light beam into an input optical fiber. In the example of step 204, the light beam 142 is coupled to the input optical fiber 112, for example, by a lens.

[0026] Figure 3 is a schematic cross-sectional view of the inverse multiplexing filter 100, shown in more detail than that shown in Figure 1. The input optical fiber 112 has a fiber axis 312A and an input fiber end face 312T. The surface of the input fiber end face 312T defines the fiber end face plane 318. In some embodiments, the fiber axis 312A defines the direction of the axis 298Z, and the fiber end face plane 318 defines the orientation of the xz plane.

[0027] The output optical fiber 116(1) has a fiber end face 316T(1) that is substantially coplanar with the fiber end face plane 318, and a fiber axis 316A(1) that is parallel to and coplanar with the fiber axis 312A in the xz plane. The output optical fiber 116(2) has a fiber end face 316T(2) that is substantially coplanar with the fiber end face plane 318, and a fiber axis 316A(2) that is parallel to and coplanar with the fiber axis 312A. The fiber end faces 312T, 316T(1), and 316T(2) are substantially collinear along a line parallel to the y axis in the fiber end face plane 318. In some embodiments, the fiber end faces 312T, 316T(1), and 316T(2) are substantially collinear when a line in the fiber end face plane 318 intersects with the respective cores of the output optical fibers 112, 116(1), and 116(2). More generally, the leading edges of optical fibers 112 and 116(1-N) are substantially collinear within the fiber end face plane 318 when the lines within the fiber end face plane 318 intersect with the respective cores of optical fibers 112 and 116(1-N).

[0028] The diffraction grating 130 faces the fiber end face plane 318 and, with respect to the fiber axis 312A, also referred to herein as θ tilt It has a blazed diffractive surface 332 that is inclined by a tilt angle 334, which is called the angle. The blazed diffractive surface 332 is characterized by the blaze angle 333, which is also hereafter referred to as θ B It is also expressed as Λ, and the grating period 336 is also expressed as Λ from here on. Since this tilt angle 334 is shifted by 0.05 to 0.5 degrees from the blaze angle 333, the diffraction grating 130 is in an approximate Littrow configuration with respect to the input optical fiber 312. In some embodiments, the diffraction grating 130 is deviated from a true Littrow configuration, so the light beam emitted from the input optical fiber 312 and reflected by the diffraction grating 130 is coupled to one of the output fibers 316, rather than to the input optical fiber 312.

[0029] The lens 120 is located along the optical path between the optical fiber array 110 and the diffraction grating 130. The lens 120 has an optical axis 322 which is at least one of the following: perpendicular to the fiber end face plane 318 and parallel to the fiber optical axis 318A. The lens 120 is configured to form an image of the blazed diffractive surface 332 within a focal plane 328 which is substantially coplanar with the fiber end face plane 318.

[0030] In order of distance from the optical axis 322, Figure 3 shows the output optical fiber 116(2) closest to the optical axis 322, followed by the input optical fiber 112 and the output optical fiber 116(1). Without departing from the scope of the embodiment, the relative distances of the output optical fiber 116 and the input optical fiber 112 with respect to the optical axis 322 may differ from the distances illustrated in Figure 3.

[0031] In several embodiments, the fact that the focal plane 328 is substantially coplanar with the fiber end face plane 318 means that the focal plane 328 is coplanar with the fiber end face plane 318 within the depth of focus 326 of the lens 120. In several embodiments, the depth of focus 326 is equal to twice the Rayleigh range of the focused spot formed by the lens 120 from the collimated beam incident from above. In several embodiments, the fact that the fiber end face 316T is substantially coplanar with the fiber end face plane 318 means that the fiber end face 316T is coplanar with the fiber end face plane 318 within a tolerance range 319. The tolerance range 319 is the distance in the direction parallel to the fiber axis 312A. In several embodiments, the tolerance range 319 is less than or equal to the depth of focus 326.

[0032] The lens 120 has a plane 323 and an effective focal length 324. The main plane 323 is at a distance 325 from the fiber end face plane 318 in the z direction, and thereafter, d 325It is positioned at a distance referred to as . In some embodiments, distance 325 is equal to the effective focal length 324. In some embodiments, distance 325 and the effective focal length 324 are substantially equal, and as a result, the difference between distance 325 and the effective focal length 324 is less than the depth of focus 326. In some embodiments, lens 120 is a composite lens including at least two single lenses. In such embodiments, the principal plane 323 may be positioned between any two of the single lenses of lens 120.

[0033] The operation of the demultiplexing filter 100 in an exemplary usage scenario is described below. Figures 1 and 3 are both most frequently referenced in the following description.

[0034] The input optical fiber 312 emits an optical beam 142, which includes a first optical channel with a central wavelength 183(1) and a second optical channel with a central wavelength 183(2) that is higher than the central wavelength 183(1). For clarity, Figure 3 shows the optical beam 142 as the principal ray 342 of the optical beam 142. The principal ray 342 propagates at a principal ray angle 343, which is also θ with respect to the optical axis 322 thereafter. 343 This is referred to as [the lens]. Lens 120 collimates the light beam 142. As the principal ray, the principal ray 342 propagates through lens 120 without refraction, and as a result, the propagation angle θ mp This is the same on each side of the lens 120. In the fiber end face plane 318, the fiber optical axis 318A is positioned at a distance 313 from the optical axis 322, in which case the distance 313 is d 325 tan(θ 343 ) is equal to. In some embodiments, the distance 313 is f eff tan(θ 343 ) is equal to, where f eff is the effective focal length 324 of lens 120. In this specification, the distance 313 is also given by y as in equation (1). 313 This is also shown by [another source].

number

[0035] The principal ray 342 is incident on the diffraction grating 130 at an incident angle of 143, and its incident angle is also referred to herein as α with respect to the surface normal of the blazed diffracting surface 332. in Therefore, the incident angle 143 can be expressed in relation to the grid tilt angle 334 and the principal ray angle 343, as shown in equation (2).

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[0036] From equation (1) to equation (2) θ 334 Substituting these values, a second expression for an incident angle of 143 is given, as shown in equation (3).

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[0037] The diffraction grating 130 generates a diffracted beam 146(k) from the light beam 142 collimated by the lens 120, and the diffracted beam has a central wavelength of 183(k) and diffraction order m. k It has the following characteristics. In some embodiments, the central wavelength increases with increasing subscript k, but the diffraction order m k This remains constant or decreases with increasing k, facilitating the configuration of the diffraction grating 130 and operating in a near-Littrow configuration with multiple central wavelengths 183. For this purpose, in several embodiments, (i) each diffraction order m greater than 1 k (ii) at least one of (ii) the lattice period 336 of the diffraction grating 130 is greater than four times the maximum value of the central wavelength 183.

[0038] The diffraction grating 130 has a blaze angle of 333, and arctan(m1λ) k When equal to / (2Λ)), it operates in a Littrow configuration at a central wavelength of 183 (k), where Λ is the lattice period of 336. The minimum and maximum wavelengths of the central wavelength of 183 are λ min and λ maxExpressed as follows, the corresponding minimum blaze angle and maximum blaze angle are, Φ Bmin =arctan(m1λ min / (2Λ)) and θ Bmax =arctan(m1λ max / (2Λ)). In some embodiments, the blaze angle 333 is angle Φ min and Φ max Both differ by less than 5 degrees, and as a result, the diffraction grating 130 operates in an almost Littrow configuration for all central wavelengths 183.

[0039] Each diffracted beam 146(k) propagates backward toward the optical fiber array 110 at its respective diffraction angle 147(k), and its diffraction angle is also, as herein defined, β with respect to the surface normal of the diffraction grating 130. k Also shown as . Figure 3 illustrates diffracted beams 146(1) and 146(2), which have diffraction orders m1 and m2, and diffraction angles 147(1) and 147(2), respectively, and are shown herein as β1 and β2, respectively. For each lattice equation in a nearly Littrow configuration, the diffraction angle β k This is expressed by equation (4), where n inc This is the refractive index of the incident medium adjacent to the blazed diffracting surface 332. When the incident medium is air, the set value n is inc =1 is sufficient for most uses.

number

[0040] Therefore, diffraction angle β k The central wavelength is λ k and diffraction order m k It depends on both. The right side of equation (3) is α in Substituting this into the equation yields equation (5).

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[0041] With respect to the optical axis 322, the diffracted beams 146(1) and 146(2) propagate at their respective propagation angles 348(1) and 348(2), where, in this specification, θ out1 and θ out2 It is shown as: angle θ out1 and θ out2 These are (β1-θ axis ) and (β2-θ axis ) is equal to ). More generally, for a diffracted beam k of diffracted beam 1-N, the angle θ outk is, (β k -θ axis ) is equal to. To clarify the explanation, Figure 3 shows the diffracted beam 146(1,2) with the respective principal rays 346(1,2) of the diffracted beam 146(1,2). As principal rays, the principal rays 346 propagate through the lens 120 without refraction, and as a result, the lens 320 does not change the value of the propagation angle 348 of the principal rays 346.

[0042] In the fiber end face plane 318, and with respect to the optical axis 322, each fiber axis 316A(k) is positioned along the y-axis at its respective fiber axis height 314(k). Similarly, each diffracted beam 146(k) intersects the fiber end face plane 318 at its respective beam height 349(k), the height of which is also referred to herein as y k The height is y. k is, d 325 tan(θ outk )) is equal to, or similarly, y k is, d 325 tan(β k -θ axis ) is equal to. In some embodiments, the height y k This satisfies equation (6), where f eff The effective focal length of lens 120 is 324, and β k This is known from equations (4) and (5), respectively.

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[0043] In multiple embodiments of both the inverse multiplexing coupler 100 and the method 200 (for example, in step 220), each fiber axis height 314(k) is substantially equal to the beam height 349(k), and as a result, the lens 120 couples the diffracted beam 146(k) to the output optical fiber 116(k). In multiple embodiments, the fiber axis height 314(k) and the beam height 349(k) are substantially equal when they differ by less than half the core diameter of the output optical fiber 116(k).

[0044] Figure 4 shows a portion of the spreadsheet 400 used to design one embodiment of the inverse multiplexing filter 100. The spreadsheet 400 includes the blaze angle 433, the tilt angle 434, the incident angle 443, and the effective focal length 424, which are examples of the blaze angle 333, the tilt angle 334, the incident angle 143, and the effective focal length 324, respectively. The spreadsheet 400 also includes the grid frequency 436, which is the reciprocal of the grid period 336.

[0045] Spreadsheet 400 also contains channel number 482, where channel number k is an integer in the range of 1 to 10, and channels 3 and 8 are centering channels. Channel number 482 lists the rows in spreadsheet 400, each containing parameters for each channel, such as channel name 411, fiber core diameter 412, fiber cladding 416, channel center wavelength 483, refractive index 402, diffraction order 438, diffraction angle 447, propagation angle 448, ideal diffraction beam height 449, calculated diffraction beam height 414, and height difference 425. For example, channel center wavelength 483(k) is the center wavelength of channel k.

[0046] Each channel name 411 indicates a gaseous species having an absorption line at the respective channel center wavelength 483 for each channel, with the center wavelength being an example of center wavelength 183. Each fiber core diameter 412(k) is the diameter of the output optical fiber 116(k). The refractive index 402(k) is the refractive index of air at the channel center wavelength 483(k). The diffraction order 438(k) is the diffraction order m of the diffracted beam 146(k) with a channel center wavelength of 483(k). k Therefore, the diffraction angle 447(k) is an example of the diffraction angle 147(k), which is calculated from equation (5). Each propagation angle 448(k) is an example of the propagation angle 148(k), which is equal to the tilt angle 434 obtained by subtracting it from the diffraction angle 447(k).

[0047] Except for channel 7 (k=7), each ideal diffraction beam height 449(k) is calculated from equation (6) and is an example of beam height 349(k). Beam height 449(7) corresponds to the distance 313 (from the optical axis 322) of the input optical fiber 112 and is the initial input to spreadsheet 400 used to calculate the incident angle 443 for each equation (2).

[0048] The incident angle 443 is the sum of the propagation angle 448(7) and the inclination angle 434. The propagation angle 448(7) is the inclination angle 334 calculated for each equation (1), where y 313 The beam height is 449(7), and f eff The effective focal length is 424.

[0049] The calculated diffracted beam height 414 is derived from designing a lens 520 optimized to have a beam height corresponding to an effective focal length 424 and an ideal diffracted beam height 449. Figure 5 is a schematic cross-sectional view of the inverse multiplexing filter 500 with the resulting traced rays as calculated by commercially available optical design software. Filter 500 is an embodiment of filter 100 in Figure 1. The inverse multiplexing filter 500 includes a lens 520 and a diffraction grating 530, either of which is based on the parameters of spreadsheet 400. The diffraction grating 530 has a blazed diffraction surface 532, which is an example of a blazed diffracted surface 332. Lens 520 and diffraction grating 530 are examples of lens 120 and diffraction grating 130, respectively. Figure 6 is a table 600 of exemplary features of lens 520. Figures 5 and 6 are most frequently viewed together in the following description.

[0050] Lens 520 includes lenses 521, 522, 523, and 524. Lens 520 also includes coating 541 between lenses 521 and 522, and coating 542 between lenses 523 and 524. Lenses 521-524 have front surfaces 502, 504, 506, and 508, and rear surfaces 503, 505, 507, and 509, respectively. In some embodiments, the inverse multiplexing filter 500 includes a shielding plate 510 between lens 524 and the diffraction grating 530 to block stray light. Table 600 includes a row for each of the aforementioned surfaces, and a column showing the radius of curvature, thickness, and material associated with each surface.

[0051] The thickness values ​​in the rows indicating a specific surface represent the axial distance between that particular surface and the next surface. For example, on the optical axis of lens 520, surfaces 502 and 503 are separated by 10.00 mm, which is the axial thickness of lens 521. Similarly, surfaces 504 and 505 are separated by 7.5 mm, which is the axial thickness of lens 522. Table 600 shows the minimum diameter of each surface that is sufficient for light rays emitted from the input optical fiber 112 to pass through that surface.

[0052] Table 600 also shows the materials of lenses 521-524 and coatings 541 and 542, respectively. Lenses 521 and 523 are made of niobium high refractive index glass, such as glass S-NPH1 from Ohara GmbH. Coatings 541 and 542, respectively, are made of barium low refractive index glass, such as glass S-BAL41 from Ohara GmbH. Lenses 522 and 523, respectively, are made of heavy flint glass, such as glass N-SF4 from Schott AG.

[0053] Figures 7, 8, and 9 show the optical spectra 700, 800, and 900 of the diffracted beams 146 coupled to each output optical fiber 116 in an embodiment of the inverse multiplexing filter 100 described in Figure 4. The center wavelengths of each optical spectrum 700, 800, and 900 correspond to the center wavelengths of channels 1, 6, and 10 in Table 400 of Figure 4.

[0054] The narrow spectral width of each optical spectrum from 700 to 900—approximately 2 nanometers—indicates high inverse linear dispersion (flatness) of the inverse multiplexing filter 100, which in part results from a large ratio (e.g., more than 4 times) of the lattice period 336 to the longest central wavelength 183. In several embodiments, the inverse linear dispersion of the inverse multiplexing filter 100 is approximately 1 nanometer per 100 micrometers for all channels. combination of features

[0055] (A1) The demultiplexed filtering method includes propagating an optical beam from an input optical fiber to a diffraction grating to generate a first diffracted beam and a second diffracted beam. The center wavelength of the first diffracted beam is equal to the first center wavelength of the first channel of the optical beam. The center wavelength of the second diffracted beam is equal to the second center wavelength of the second channel of the optical beam. The second center wavelength is greater than the first center wavelength. The first diffracted beam propagates backward toward the input optical fiber with a first diffraction angle partially determined by the first center wavelength and diffraction order m1 of the first diffracted beam. The second diffracted beam propagates backward toward the input optical fiber with a second center wavelength and a second diffraction angle partially determined by the diffraction order m2 of the second diffracted beam, which is less than diffraction order m1. The method also includes (i) coupling the first diffracted beam to a first optical fiber of a one-dimensional optical fiber array including the input optical fiber, and (ii) coupling the second diffracted beam to a second optical fiber of the one-dimensional optical fiber array.

[0056] (A2) Method (A1) may further include collimating the light beam using a lens positioned between the tip of the input optical fiber and a diffraction grating, wherein the collimation includes focusing the first diffracted beam and the second diffracted beam using the lens.

[0057] (A3) Either method (A1) or (A2) is a coupling of a third diffracted beam into a third optical fiber of a one-dimensional optical fiber array, wherein the propagation of the light beam into the grating also generates a third diffracted beam, the center wavelength of which is equal to the third center wavelength of the third channel of the light beam and greater than the second center wavelength, and the third diffraction angle is partially determined by the third center wavelength and the diffraction order m3 of the third diffracted beam which is less than the diffraction order m2, and the coupling propagates backward toward the input optical fiber.

[0058] (A4) Any one of the methods (A1) to (A3) may further include collimating the light beam using a lens placed between the tip of an input optical fiber and a diffraction grating, wherein the distance between the first optical fiber and the optical axis of the lens is f eff · tan(β out1 -θ axis ) is equal to, and in the formula, f eff β is the effective focal length of the lens, and out1 θ is the angle between the first diffracted beam and the surface normal of the diffraction grating. axis β is the angle between the surface normal and the optical axis in the plane containing the one-dimensional optical fiber array. out1 =arcsin(m1λ1 / Λ-sin[θ axis +arctany mp / f eff )]) and in the formula, λ1 is the first central wavelength, Λ is the period of the diffraction grating, and y mp f is the distance between the input optical fiber and the optical axis, and the distance between the second optical fiber and the optical axis of the lens is f eff · tan(β out2 -θ axis ) and β out2 β is the angle between the first diffraction beam and the surface normal of the diffraction grating. out2 =arcsin(m2λ2 / Λ-sin[θ axis +arctan(y mp / f eff )]) and in the formula, λ2 is the second central wavelength.

[0059] (A5) In any of the methods (A1) to (A4), the diffraction order m2 may be greater than 1.

[0060] (A6) Any of the methods (A1) to (A5) may further include (i) propagating a multiplexed probe beam through a combustion zone, wherein the optical beam is the multiplexed probe beam after it has passed through the combustion zone, and (ii) coupling the optical beam to an input optical fiber.

[0061] (B1) A method for measuring the concentration of a species in a combustion zone includes propagating a multiplexed input probe beam through the combustion zone to generate an output probe beam and coupling the output probe beam to an input optical fiber. The combustion zone includes (i) a first gas-phase species having an absorption line at a first central wavelength, and (ii) a second gas-phase species having an absorption line at a second central wavelength above the first central wavelength. The method also includes propagating the output probe beam from the input optical fiber to a diffraction grating to generate a first diffraction beam and a second diffraction beam. The central wavelength of the first diffraction beam is equal to the first central wavelength. The central wavelength of the second diffraction beam is equal to the second central wavelength. The first diffraction beam propagates backward toward the input optical fiber at a first diffraction angle, which is partially determined by the first central wavelength and diffraction order m1 of the first diffraction beam. The second diffracted beam propagates backward toward the input optical fiber at a second central wavelength of the second diffracted beam and a second diffraction angle partially determined by a diffraction order m2 smaller than diffraction order m1. This method also includes (i) coupling the first diffracted beam to a first optical fiber in a one-dimensional optical fiber array including the input optical fiber, and (ii) coupling the second diffracted beam to a second optical fiber in the one-dimensional optical fiber array. This method also includes (i) measuring the first signal amplitude of the first diffracted beam output from the first optical fiber, and (ii) measuring the second signal amplitude of the second diffracted beam output from the second optical fiber. This method also includes (i) determining the concentrations of a first gaseous species and a second gaseous species from the first signal amplitude.

[0062] (C1) The inverse multiplexing filter includes an optical fiber array, a diffraction grating, and a lens. The optical fiber array includes (i) an input optical fiber having a fiber optical axis and an input fiber end face, the surface of which defines a fiber end face plane; (ii) a first output optical fiber having a first fiber end face substantially coplanar with the fiber end face plane, and a first optical axis parallel to the fiber optical axis and coplanar with the fiber optical axis; and (iii) a second fiber end face coplanar with the fiber end face plane within tolerance, and a second output optical fiber having a fiber optical axis that is colinear, parallel to the input fiber end face, the first fiber end face, and the second fiber end face, and coplanar with the fiber end face. The diffraction grating has a blazed diffractive surface facing the fiber end face plane and inclined at an angle of inclination that is 0.05 to 0.5 degrees from the blaze angle of the blazed diffractive surface with respect to the fiber optical axis. The lens is positioned along the optical path between the optical fiber array and the diffraction grating, has a lens optical axis perpendicular to the fiber end-face plane, and is configured to form an image of the blazed diffractive surface at a focal plane that is substantially coplanar with the fiber end-face plane.

[0063] (C2) In the inverse multiplexing filter (C1), the distance between the fiber end face plane and the principal plane of the lens may be substantially equal to the effective focal length of the lens.

[0064] (C3) In either of the demultiplexing filters (C1) and (C2), at least one of the input optical fiber, the first output optical fiber, and the second output optical fiber can operate as a multimode fiber in the wavelength range of 1.3 micrometers to 2.5 micrometers.

[0065] (C4) In any of the inverse multiplexing filters (C1) to (C3), the input optical fiber emits an optical beam containing a first optical channel having a first center wavelength and a second optical channel having a second center wavelength greater than the first center wavelength. Its lens collimates the optical beam. Its diffraction grating generates a first diffracted beam and a second diffracted beam from the collimated optical beam. The center wavelength of the first diffracted beam is equal to the first center wavelength of the first channel of the optical beam. The center wavelength of the second diffracted beam is equal to the second center wavelength of the second channel of the optical beam. The second center wavelength is greater than the first center wavelength. The first diffracted beam propagates backward toward the input optical fiber with a first diffraction angle partially determined by the first center wavelength and diffraction order m1 of the first diffracted beam. The second diffracted beam propagates backward toward the input optical fiber with a second diffraction angle partially determined by the second center wavelength and diffraction order m2 less than diffraction order m1 of the second diffracted beam.

[0066] (C5) In any inverse multiplexing filter (C4), the first and second center wavelengths may be in the range of 1.3 micrometers to 2.5 micrometers.

[0067] (C6) In either of the inverse multiplexing filters (C4) and (C5), at least one of the first central wavelength and the second central wavelength corresponds to one absorption line among carbon monoxide, water, and carbon dioxide.

[0068] (C7) In any one of the demultiplexing filters (C4) to (C6), the input optical fiber has a core radius a0 and a numerical aperture NA0, and the parameter 2π(a0 / λ2)NA0 is 2.405 or greater, so that multimode operation can be ensured at both the first central wavelength and the second central wavelength indicated by λ2; the first output optical fiber r has a core radius a1 and a numerical aperture NA1, and the parameter 2π(a1 / λ1)NA1 is 2.405 or greater, so that multimode operation can be ensured at the first central wavelength indicated by λ1; and the second optical fiber has a core radius a2 and a numerical aperture NA2, and the parameter 2π(a2 / λ2)NA2 is 2.405 or greater, so that multimode operation can be ensured at the second central wavelength.

[0069] (C8) In any one of the inverse multiplexing filters (C4) to (C7), the diffraction order m2 may be greater than 1.

[0070] (C9) In any one of the inverse multiplexing filters (C4) to (C8), the diffraction grating may have a grating period greater than four times the second central wavelength.

[0071] (C10) Any one of the inverse multiplexing filters (C4) to (C9) may further include multiple lasers optically coupled to the input optical fiber for generating a light beam.

[0072] (C11) In one embodiment of any one of the inverse multiplexing filters (C4) to (C10), the emitted light beam includes a third optical channel having a third central wavelength that exceeds the second central wavelength. The optical fiber array has a third fiber end face that is in the same plane within a tolerance with respect to the fiber end face plane and is on the same straight line as the first and second fiber end faces, and includes a third optical axis that is parallel to the input fiber optical axis and is in the same plane. The diffraction grating generates, from the collimated light beam, a third diffracted beam that propagates backward toward the optical fiber array at a third diffraction angle, where the central wavelength of the third diffracted beam is equal to the third central wavelength and is partially determined by the diffraction order m3 of the third diffracted beam, which is smaller than the third central wavelength and the diffraction order m2.

[0073] (C12) In one embodiment of any one of the inverse multiplexing filters (C4) to (C11), the diffraction grating has ideal grating blaze angles for retro-configuration operation at the first central wavelength and the second central wavelength, which are a period Λ, Φ1 = arctan(m1λ1 / (2Λ)), and Φ2 = arctan(m2λ2 / (2Λ)), where λ1 and λ2 represent the first and second central wavelengths, respectively. The diffraction grating has a grating blaze angle θ that is different from each of the grating blaze angles Φ1 and Φ2 by less than 5 degrees. B It has.

[0074] (C13) In one embodiment of any one of the inverse multiplexing filters (C4) to (C12), the distance between the first output optical fiber and the lens optical axis is f eff ·tan(β out1 - θ axis ), where f eff is the effective focal length of the lens, β out1 is the first diffraction angle, and θ axis is the angle between the surface normal and the lens optical axis in the plane including the optical fiber array. The distance between the second output optical fiber and the lens optical axis is f eff ·tan(β out2 - θ axis ), where βout2 is the second diffraction angle.

[0075] (C14) In an embodiment of the demultiplexing filter (C13), the first diffraction angle β out1 is equal to arcsin(m1λ1 / Λ - sin[θ axis + arctan(y mp / f eff ), where λ1 is the first central wavelength, Λ is the period of the diffraction grating, y mp is the distance between the input optical fiber and the lens optical axis, and the second diffraction angle β out2 is equal to arcsin(m2λ2 / Λ - sin[θ axis + arctan(y mp / f eff ), where λ2 is the second central wavelength).

[0076] (C1) In any embodiment of the demultiplexing filter (C13) or (C14), the emitted light beam includes a third optical channel having a third central wavelength that exceeds the second central wavelength, and the optical fiber array includes a third fiber end face that is in the same plane within the tolerance with respect to the fiber end face plane and is on the same straight line as the first and second fiber end faces, and a third output optical fiber having a third optical axis that is parallel to and in the same plane as the input fiber optical axis, and the diffraction grating is configured to generate, from the collimated light beam, a third diffracted beam that propagates backward toward the optical fiber array at a third diffraction angle that is determined in part by the third central wavelength and a third diffraction order m3 of the third diffracted beam that is less than the diffraction order m2, and the distance between the third output optical fiber and the lens optical axis of the lens is equal to f eff ·tan(β out3 - θ axis ), where β out3 is the third diffraction angle.

[0077] (C16) In an embodiment of the demultiplexing filter (C15), the third diffraction angle β out3is arcsin(m3λ3 / Λ-sin[θ axis +arctan(y mp / f eff )]) is equal to, where λ3 is the third central wavelength.

[0078] (C17) In any embodiment of the demultiplexing filters (C1) to (C15), at least one of the input optical fiber, the first output optical fiber, and the second output optical fiber has a fiber core diameter greater than 50 micrometers.

[0079] Modifications to the methods and systems described above can be made without departing from the scope of these embodiments. Therefore, it should be noted that the subject matter included in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in an restrictive sense. In this specification, unless otherwise specified, the phrase "in some embodiments" is equivalent to the phrase "in a particular embodiment" and does not necessarily refer to all embodiments. The following claims are intended to cover all general and specific features described herein, as well as all statements regarding the scope of the methods and systems of the present invention, and may be said to lie between them in terms of language.

Claims

1. A demultiplexed filtering method performed on multiplexed light beams propagating through a combustion zone, The light beam is propagated from the input optical fiber to the diffraction grating. (i) A first diffraction beam, wherein the center wavelength of the first diffraction beam is equal to the first center wavelength of the first channel of the optical beam, and the first center wavelength and diffraction order m of the first diffraction beam. 1 A first diffracted beam propagates toward the input optical fiber at a first diffraction angle partially determined by, (ii) A second diffraction beam, wherein the center wavelength of the second diffraction beam is equal to the second center wavelength of the second channel of the optical beam and greater than the first center wavelength, and the second center wavelength of the second diffraction beam and the diffraction order m 1 Diffraction order m is smaller than 2 To generate a second diffracted beam that propagates toward the input optical fiber at a second diffraction angle partially determined by, The first diffracted beam is coupled to the first optical fiber of a one-dimensional optical fiber array including the input optical fiber, A demultiplexed filtering method comprising coupling the second diffracted beam to the second optical fiber of the one-dimensional optical fiber array.

2. The optical beam is collimated using a lens positioned between the tip of the input optical fiber and the diffraction grating. The method according to claim 1, further comprising the combination including focusing the first diffracted beam and the second diffracted beam using the lens.

3. Coupling a third diffracted beam to a third optical fiber of the one-dimensional optical fiber array, and propagating the optical beam through the grating, also generates the third diffracted beam, wherein the center wavelength of the third diffracted beam is equal to the third center wavelength of the third channel of the optical beam and greater than the second center wavelength, and the diffraction order m 2 The diffraction order m of the third diffraction beam is smaller than 3 The method according to claim 1, further comprising coupling, which propagates toward the input optical fiber at a third diffraction angle partially determined by the method.

4. The method further includes collimating the light beam using a lens positioned between the tip of the input optical fiber and the diffraction grating, The distance between the first optical fiber and the optical axis of the lens is f eff ·tan(β out1 −θ axis ), where f eff is the effective focal length of the lens, β out1 is the angle between the first diffracted beam and the surface normal of the diffraction grating, θ axis is the angle between the surface normal and the optical axis in the plane containing the one-dimensional optical fiber array, and β out1 = arcsin(m 1 λ 1 / Λ − sin[θ axis + arctan(y mp / f eff )]), where λ 1 is the first central wavelength, Λ is the period of the diffraction grating, y mp is the distance between the input optical fiber and the optical axis, The distance between the second optical fiber and the optical axis of the lens is f eff tan(β) out2 -θ axis ) and β out2 β is the angle between the first diffraction beam and the surface normal of the diffraction grating. out2 = arcsin(m 2 λ 2 / Λ-sin[θ axis +arctan(y mp / f eff ) ] ) and in the formula, λ 2 The method according to claim 1, wherein is a second central wavelength.

5. Diffraction order m 2 The method according to claim 4, wherein the value is greater than 1.

6. The process involves propagating a multiplexed probe beam through a combustion zone, wherein the light beam is the multiplexed probe beam after it has passed through the combustion zone. The method according to claim 1, further comprising coupling the light beam to the input optical fiber.

7. An inverse multiplexing filter for multiplexed light beams propagating through a combustion zone, A fiber optic array, An input optical fiber having a fiber optical axis and an input fiber end face, wherein the surface of the input optical fiber defines the fiber end face plane, A first output optical fiber having a first fiber end face substantially coplanar with the fiber end face plane, and a first optical axis parallel to the fiber optical axis and coplanar with the fiber optical axis, An optical fiber array comprising a second fiber end face that lies in the same plane as the fiber end face plane within tolerance, and a second output optical fiber having a second optical axis parallel to and in the same plane as the fiber optical axis, wherein the input fiber end face, the first fiber end face, and the second fiber end face are on the same straight line, A diffraction grating having a blazed diffractive surface that faces the fiber end face plane and is inclined at an angle that is shifted by 0.05 to 0.5 degrees from the blaze angle of the blazed diffractive surface with respect to the fiber optical axis, An inverse multiplexing filter comprising: a lens having a lens optical axis perpendicular to the fiber end face plane along the optical path between the optical fiber array and the diffraction grating, and configured to form an image of the blazed diffraction surface in a focal plane substantially coplanar with the fiber end face plane.

8. The inverse multiplexing filter according to claim 7, wherein the distance between the fiber end face plane and the main plane of the lens is substantially equal to the effective focal length of the lens.

9. The demultiplexing filter according to claim 7, wherein at least one of the input optical fiber, the first output optical fiber, and the second output optical fiber operates as a multimode fiber in the wavelength range of 1.3 micrometers to 2.5 micrometers.

10. The demultiplexing filter according to claim 7, wherein at least one of the input optical fiber, the first output optical fiber, and the second output optical fiber has a fiber core diameter greater than 50 micrometers.

11. The input optical fiber emits an optical beam including a first optical channel having a first central wavelength and a second optical channel having a second central wavelength exceeding the first central wavelength. The lens collimates the light beam, The diffraction grating receives from the collimated light beam (i) a first diffraction beam, wherein the central wavelength of the first diffraction beam is equal to the first central wavelength, and the first diffraction beam has the first central wavelength and diffraction order m 1 (ii) a first diffraction beam propagating toward the optical fiber array at a first diffraction angle with respect to the surface normal of the diffraction grating determined by (ii), and a second diffraction beam, wherein the center wavelength of the second diffraction beam is equal to the second center wavelength, and the second diffraction beam, the second center wavelength of the second diffraction beam, and the diffraction order m 1 Diffraction order m is smaller than 2 The inverse multiplexing filter according to claim 7, which generates a second diffracted beam that propagates toward the optical fiber array at a second diffraction angle with respect to the surface normal determined by the above.

12. The inverse multiplexing filter according to claim 11, wherein each of the first and second central wavelengths is in the range of 1.3 micrometers to 2.5 micrometers.

13. The inverse multiplexing filter according to claim 11, wherein at least one of the first central wavelength and the second central wavelength corresponds to an absorption line of one of carbon monoxide, water, and carbon dioxide.

14. The input optical fiber has a core radius a 0 and numerical aperture NA 0 It has the parameter 2π(a 0 / λ 2 ) NA 0 The first center wavelength and λ are set so that the value is 2.405 or greater. 2 To ensure multimode operation at both of the second central wavelengths indicated by, The first output optical fiber has a core radius a 1 and numerical aperture NA 1 It has the parameter 2π(a 1 / λ 1 ) NA 2 Make sure that λ is 2.405 or higher. 1 To ensure multimode operation at the first central wavelength indicated by, The second optical fiber has a core radius a 2 and numerical aperture NA 2 It has the parameter 2π(a 2 / λ 2 ) NA 2 The inverse multiplexing filter according to claim 11, wherein the value is set to 2.405 or higher to ensure multimode operation at the second center wavelength.

15. Diffraction order m 2 The inverse multiplexing filter according to claim 11, wherein the value is greater than 1.

16. The inverse multiplexing filter according to claim 11, wherein the diffraction grating has a grating period that is greater than four times the second central wavelength.

17. The inverse multiplexing filter according to claim 11, further comprising a plurality of lasers optically coupled to the input optical fiber for generating the light beam.

18. The emitted light beam includes a third light channel having a third central wavelength that is greater than the second central wavelength, The optical fiber array includes a third optical fiber having a third optical end face that lies on the same plane as the fiber end face plane within tolerance and is collinear with the first and second fiber end faces, and a third optical axis that is parallel to and on the same plane as the input fiber optical axis. The diffraction grating is a third diffraction beam obtained from the collimated light beam, wherein the central wavelength of the third diffraction beam is equal to the third central wavelength, and the diffraction order is m. 2 Diffraction order m is smaller than 3 The inverse multiplexing filter according to claim 11, which generates a third diffracted beam that propagates toward the optical fiber array at a third diffraction angle partially determined by the above.

19. The diffraction grating has periods Λ and Φ respectively. 1 = arctan(m 1 λ 1 / (2Λ)) and Φ 2 = arctan(m 2 λ 2 The first and second central wavelengths are (2Λ) and have an ideal lattice blaze angle for Littrow configuration operation, where λ 1 and λ 2 These represent the first and second central wavelengths, respectively, and the diffraction grating has a grating blaze angle Φ 1 and Φ 2 Each of these has a lattice blaze angle θ that differs by less than 5 degrees. B The inverse multiplexing filter according to claim 11, having the following features.

20. The distance between the first output optical fiber and the lens optical axis is f eff tan(β) out1 -θ axis ) is equal to, and in the formula, f eff β is the effective focal length of the lens, and out1 θ is the first diffraction angle, axis This is the angle between the surface normal and the optical axis of the lens in the plane containing the optical fiber array. The distance between the second output optical fiber and the lens optical axis of the lens is f eff tan(β) out2 -θ axis ) is equal to, in the formula, β out2 The inverse multiplexing filter according to claim 11, wherein is the second diffraction angle.

21. The first diffraction angle β out1 but, arcsin(m 1 λ 1 / Λ-sin[θ axis +arctan(y mp / f eff ) ]) is equal to, in the formula, λ 1 is the first central wavelength, Λ is the period of the diffraction grating, and y mp This is the distance between the input optical fiber and the lens optical axis, The second diffraction angle β out2 but, arcsin(m 2 λ 2 / Λ - sin[θ axis + arctan(y mp / f eff )) and is equal to, where λ 2 is the second center wavelength, the demultiplexing filter according to claim 20.

22. The emitted light beam includes a third light channel having a third central wavelength that is greater than the second central wavelength, The optical fiber array includes a third optical fiber having a third optical end face that lies on the same plane as the fiber end face plane and within the tolerance, and is collinear with the first and second fiber end faces, and a third optical axis that is parallel to and on the same plane as the input fiber optical axis. The diffraction grating is a third diffraction beam obtained from the collimated light beam, wherein the central wavelength of the third diffraction beam is equal to the third central wavelength, and the diffraction order is m. 2 Diffraction order m is smaller than 3 It is configured to generate a third diffracted beam that propagates toward the optical fiber array at a third diffraction angle partially determined by, The distance between the third output optical fiber and the lens optical axis of the lens is f eff tan(β) out3 -θ axis ) is equal to, in the formula, β out3 The inverse multiplexing filter according to claim 20, wherein is the third diffraction angle.

23. the third diffraction angle β out3 is equal to arcsin(m 3 λ 3 / Λ - sin[θ axis + arctan(y mp / f eff )]), where λ 3 is the third central wavelength, the demultiplexing filter according to claim 22.

24. A method for measuring the concentration of species within a combustion zone, The process involves propagating a multiplexed input probe beam through the combustion zone to generate an output probe beam, wherein the combustion zone includes (i) a first gas phase species having an absorption line at a first central wavelength, and (ii) a second gas phase species having an absorption line at a second central wavelength above the first central wavelength. The output probe beam is coupled to the input optical fiber, The output probe beam is propagated from the input optical fiber to the diffraction grating, (i) A first diffraction beam, wherein the central wavelength of the first diffraction beam is equal to the first central wavelength, and the first central wavelength and diffraction order m of the first diffraction beam 1 A first diffracted beam propagates toward the input optical fiber at a first diffraction angle partially determined by, and (ii) A second diffraction beam, wherein the central wavelength of the second diffraction beam is equal to the second central wavelength, and the diffraction order m 1 Smaller diffraction order m 2 To generate a second diffracted beam that propagates toward the input optical fiber at a second diffraction angle partially determined by, The first diffracted beam is coupled to the first optical fiber of a one-dimensional optical fiber array including the input optical fiber, The second diffracted beam is coupled to the second optical fiber of the one-dimensional optical fiber array, (i) measuring the first signal amplitude of the first diffraction beam output from the first optical fiber, and (ii) measuring the second signal amplitude of the second diffraction beam output from the second optical fiber, (i) A method comprising determining the concentrations of the first gas-phase species and the second gas-phase species from the first signal amplitude.

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