阅读说明：本技术 光学火焰传感器 (Optical flame sensor ) 是由 查理·卡拉汉 安德鲁·萨佩 特里斯坦·斯特尔茨纳 于 2020-04-13 设计创作，主要内容包括：一种光学火焰传感器,包括光学循环器、光纤腔,和第一光学传感器。光学循环器包括第一端口、第二端口,和第三端口。第一端口被配置成接收光学信号。第二端口被配置成输出在第一端口接收到的光学信号。第三端口被配置成输出第二端口的输入。光纤腔包括光学地耦合到第二端口的腔近端和位于腔远端的镜,使得由光纤腔输出的腔光学信号是第二端口的输入。第一光学传感器光学地耦合到第三端口以量化腔光学信号。(An optical flame sensor includes an optical circulator, a fiber cavity, and a first optical sensor. The optical circulator includes a first port, a second port, and a third port. The first port is configured to receive an optical signal. The second port is configured to output the optical signal received at the first port. The third port is configured to output an input of the second port. The fiber optic lumen includes a lumen proximal end optically coupled to the second port and a mirror at a lumen distal end such that a lumen optical signal output by the fiber optic lumen is an input to the second port. The first optical sensor is optically coupled to the third port to quantify the cavity optical signal.)

1. An optical flame sensor comprising:

an optical circulator comprising a first port, a second port, and a third port,

the first port is configured to receive an optical signal,

the second port is configured to output an optical signal received at the first port, an

The third port is configured to output an input of the second port;

a fiber optic lumen comprising a lumen proximal end optically coupled to the second port and a mirror at a lumen distal end such that a lumen optical signal output by the fiber optic lumen is the input of the second port; and

a first optical sensor optically coupled to the third port and configured to quantify the cavity optical signal.

2. The optical flame sensor of claim 1, further comprising:

a laser configured to generate the optical signal; and

a signal generator electrically coupled to the laser and configured to apply a periodic waveform to the laser.

3. The optical flame sensor of claim 2, the laser being a distributed feedback laser configured to be tuned by the periodic waveform.

4. The optical flame sensor of claim 2, further comprising a plurality of optical flame sensors according to claim 1, each configured to receive the optical signal at its respective first port.

5. The optical flame sensor of claim 1, the fiber cavity having a return loss between 3% and 5% at the cavity proximal end.

6. The optical flame sensor of claim 1, further comprising a first optical fiber optically coupling the fiber cavity to the second port via a flat polished fiber connector.

7. The optical flame sensor of claim 1, the fiber cavity including a metalized fiber.

8. The optical flame sensor of claim 1, the fiber cavity comprising a fiber having an index of refraction n₁The mirror comprises a fiber core formed of a first material having an index of refraction n_R(ii) a reflective surface formed of a second material of (n)_R-n₁)≥0.2。

9. The optical flame sensor of claim 8, the second material being selected from the group consisting of silicon, aluminum oxide, and zinc oxide.

10. The optical flame sensor of claim 1, further comprising:

a reference optical sensor; and

a fiber optic coupler, said fiber optic coupler comprising: (i) a first coupler output that optically couples a first percentage of the optical signal to the first port, and (ii) a second coupler output that optically couples a reference optical signal to the reference optical sensor,

the reference optical signal is a second percentage of the optical signal, the reference optical sensor is configured to quantize the reference optical signal, the first percentage exceeds the second percentage.

11. The optical flame sensor of claim 1, further comprising:

a processor; and

a memory configured to store the quantized cavity optical signal and machine readable instructions that, when executed by the processor, control the processor to:

analyzing the quantified cavity optical signal to determine whether the fiber cavity is heating or cooling according to at least one of: a temporal variation of (i) interference fringes of the cavity optical signal and (ii) a variation of a number of detected interference fringes of the cavity optical signal during a modulation period of the optical signal.

12. The optical flame sensor of claim 11,

the optical signal has a central wavelength λ₀；

The mirror at a central wavelength λ₀Has (i) at a first temperature T₁Refractive index n of₁And a thickness L₁And (ii) at a second temperature T₂Refractive index n of₂And a thickness L₂So thatAnd isq₁And q is₂Is a non-negative integer, and | T₂-T₁|>500K; and

the memory further stores machine-readable instructions that, when executed by the processor, control the processor to:

extracting the carrier signal from the quantized cavity optical signal by applying a low-pass filter, an

The temperature range of the fiber cavity is determined from the shape of the carrier signal, which is determined in part by the temperature dependent reflectivity of the mirror.

13. The optical flame sensor of claim 1, wherein the fiber optic cavity includes an optical fiber and the mirror is a distal end of the optical fiber defining a distal cavity end.

14. An optical flame sensor comprising:

a fiber optic cavity, comprising:

a proximal lumen end configured to be optically coupled to (i) a laser and (ii) an optical sensor, a distal lumen end, and

a scope at the distal end of the lumen; and

a housing configured to load the fiber optic cavity to a flame sensor port of a burner.

15. The optical flame sensor of claim 14, the housing configured to replace a flame based flame straightening flame sensor without modifying the burner.

16. A method for detecting the presence of a flame, comprising:

periodically modulating a wavelength of an optical signal generated by a laser to produce a modulated signal;

detecting a cavity optical signal output by a fiber cavity coupled to the laser, the cavity optical signal including an amplitude modulation determined by when a wavelength of a modulation signal corresponds to a mode of the fiber cavity; and

determining whether the fiber cavity is heating or cooling based on a time dependence of the amplitude modulation.

17. The method of claim 16, further comprising:

extracting a carrier signal from the cavity optical signal by applying a low pass filter; and

determining a temperature range of the fiber cavity based on a shape of the carrier signal.

18. The method of claim 16, the step of determining comprising:

tracking a phase of the amplitude modulated frequency domain representation.

19. The method of claim 16, the step of determining comprising:

converting the amplitude modulation into a binary time sequence; and

tracking when the binary time series transitions from a first discrete value and a second discrete value during a time interval of the binary time series that does not exceed a duration between successive modes of the fiber cavity.

20. The method of claim 16, the fiber cavity being proximate to an igniter tip of a furnace burner, and the method further comprising at least one of: closing a valve that discharges fuel that is burned by the flame, and generating a warning signal.

Background

The burner operates within the furnace to generate thermal energy by combustion of a fuel. Depending on the application, the furnace may include one to more burners. When operating correctly, the furnace produces a flame by continuously combusting the gases exiting the gas valve. To prevent gas leakage and associated hazards, the furnace is configured to close the gas valve when a flame is not present. To this end, the melter burner includes a flame sensor mounted in a flame sensor port of the burner housing. Common flame sensors operate by flame straightening, including a metal rod to which an alternating voltage is applied. See, for example, U.S. patent No.4,427,363 to hammond. The metal rod is grounded at its proximal end, e.g., away from the flame at the burner housing, and is configured such that when a flame is present, the distal end of the rod is in the flame. The alternating voltage ionizes the molecules in the flame. The ionized molecules generate a direct current in the metal rod, which is detected by the flame sensor. Due to the harsh environment in which these flame sensors operate, these flame sensors are susceptible to ground faults, causing them to fail.

Disclosure of Invention

In a first aspect, an optical flame sensor includes an optical circulator, a fiber cavity, and a first optical sensor. The optical circulator includes a first port, a second port, and a third port. The first port is configured to receive an optical signal. The second port is configured to output the optical signal received at the first port. The third port is configured to output an input of the second port. The fiber optic lumen includes a lumen proximal end optically coupled to the second port and a mirror at a lumen distal end such that a lumen optical signal output by the fiber optic lumen is an input to the second port. The first optical sensor is optically coupled to the third port and configured to quantify the cavity optical signal.

In certain embodiments of the first aspect, the laser is configured to generate an optical signal.

In certain embodiments of the first aspect, the signal generator is electrically coupled to the laser and configured to apply a periodic waveform to the laser.

In certain embodiments of the first aspect, the laser is a distributed feedback laser configured to be tuned by a periodic waveform.

In certain embodiments of the first aspect, the fiber cavity has a return loss between 3% and 5% at the cavity proximal end. In certain embodiments of the first aspect, the optical flame sensor further includes a first optical fiber optically coupling the fiber cavity to the second port via an angle-polished fiber optic connector.

In certain embodiments of the first aspect, the fiber cavity comprises a metalized fiber.

In certain embodiments of the first aspect, the fiber cavity comprises a fiber having an index of refraction n₁An optical fiber core formed of the first material of (a). The mirror comprises a lens having a refractive index n_R(ii) a reflective surface formed of a second material of (n)_R-n₁)≥0.2。

In certain embodiments of the first aspect, the second material is selected from the group consisting of silicon, aluminum oxide, and zinc oxide.

In certain embodiments of the first aspect, the optical flame sensor further comprises a reference optical sensor.

In certain embodiments of the first aspect, the optical flame sensor further includes a fiber optic coupler including (i) a first coupler output that optically couples a first percentage of the optical signal to the first port, and (ii) a second coupler output that optically couples the reference optical signal to the reference optical sensor. In certain embodiments of the first aspect, the reference optical signal is a second percentage of the optical signal, the reference optical sensor is configured to quantize the reference optical signal, and the first percentage exceeds the second percentage.

In certain embodiments of the first aspect, the optical flame sensor further comprises a processor; and a memory configured to store the quantized cavity optical signal and machine readable instructions that, when executed by the processor, control the processor to: analyzing the quantified cavity optical signal to determine whether the fiber cavity is heating or cooling according to at least one of: a temporal variation of (i) interference fringes of the cavity optical signal and (ii) a variation of a number of detected interference fringes of the cavity optical signal during a modulation period of the optical signal.

In certain embodiments of the first aspect, the optical signal has a center wavelength λ₀. In certain embodiments of the first aspect, the mirror is at a center wavelength λ₀Has (i) at a first temperature T₁Refractive index n of₁And a thickness L₁And (ii) at a second temperature T₂Refractive index n of₂And a thickness L₂So thatAnd isq₁And q is₂Is a non-negative integer, and | T₂-T₁|>500K。

In certain embodiments of the first aspect, the memory further stores machine-readable instructions that, when executed by the processor, control the processor to: the carrier signal is extracted from the quantized cavity optical signal by applying a low pass filter and the temperature range of the fiber cavity is determined from the shape of the carrier signal, which is determined in part by the temperature dependent reflectivity of the mirror.

In certain embodiments of the first aspect, the fiber optic lumen includes an optical fiber, and the mirror is a distal end of the optical fiber defining a distal end of the lumen.

In a second aspect, an optical flame sensor includes an optical circulator, a fiber cavity, a mirror, and a first optical sensor. The optical circulator comprises a firstA port, a second port, and a third port. The first port is configured to receive a signal having a center wavelength λ₀The optical signal of (1). The second port is configured to output the optical signal received at the first port. The third port is configured to output an input of the second port. The first optical fiber includes (i) a proximal end optically coupled to the second port and (ii) a distal end. A mirror is coupled to the distal end and configured to reflect light emitted from the distal end back to the proximal end. Mirror at center wavelength λ₀Has (i) at a first temperature T₁Refractive index n of_R1And a thickness L₁And (ii) at a second temperature T₂Refractive index n of_R2And a thickness L₂So thatAnd isq₁And q is₂Is a non-negative integer, and | T₂-T₁|>500K. The first optical sensor is optically coupled to the third port and configured to quantify a reflected optical signal output by the distal end.

In a third aspect, an optical flame sensor includes a fiber optic cavity and a housing. The fiber optic lumen includes a distal lumen end, a scope at the distal lumen end, and a proximal lumen end configured to optically couple to (i) the laser and (ii) the optical sensor. The housing is configured to load the fiber optic cavity into a flame sensor port of the burner.

In certain embodiments of the third aspect, the housing is configured to replace the flame rectification-based flame sensor without modifying the burner.

In a fourth aspect, a method for detecting the presence of a flame includes: (i) periodically modulating a wavelength of an optical signal generated by the laser to produce a modulated signal, and (ii) detecting a cavity optical signal output by a fiber cavity coupled to the laser. The cavity optical signal includes an amplitude modulation determined according to when the wavelength of the optical signal corresponds to a mode of the fiber cavity. The method also includes determining whether the fiber cavity is heating or cooling based on a time dependence of the amplitude modulation.

In certain embodiments of the fourth aspect, the method further comprises: extracting a carrier signal from the cavity optical signal by applying a low pass filter; and determining the temperature range of the fiber cavity based on the shape of the carrier signal.

In certain embodiments of the fourth aspect, determining the temperature range of the fiber cavity includes tracking the phase of the amplitude-modulated frequency domain representation. In certain embodiments of the fourth aspect, determining the temperature range of the fiber cavity comprises: converting the amplitude modulation into a binary time sequence; and tracking when the binary time series transitions from the first discrete value and the second discrete value during a time interval of the binary time series that does not exceed a duration between successive modes of the fiber cavity.

In a fifth aspect, a method for detecting the presence of a flame includes coupling an optical signal generated by a laser to a proximal end of an optical fiber having a mirror at a distal end of the optical fiber. The mirror has a predetermined temperature dependent reflectivity. The method further comprises the following steps: detecting the optical power of the output optical signal reflected by the mirror; determining the reflectivity of the mirror from the detected optical power; and determining the temperature of the mirror by mapping the determined reflectivity to a predetermined temperature dependent reflectivity.

Any of the above aspects may be combined together such that one aspect may include one or more components of another aspect. In combination with the method aspects, the system may include a processor and a memory storing machine-readable instructions that, when executed by the processor, implement the steps described in the method aspects.

Drawings

FIG. 1 is a schematic view of an optical flame sensor in one embodiment.

Fig. 2 includes a plot showing data measured by the optical flame sensor of fig. 1, wherein its mirror has 50% reflectivity.

Fig. 3 includes a plot showing data measured by the optical flame sensor of fig. 1, wherein its mirror has 80% reflectivity.

Fig. 4 includes a plot showing data measured by the optical flame sensor of fig. 1, wherein its mirror has 50% reflectivity.

FIG. 5 is a flow diagram illustrating a first method for detecting the presence of a flame in one embodiment.

FIG. 6 is a flow diagram illustrating a second method for detecting the presence of a flame in one embodiment.

FIG. 7 is a graphical representation of signals processed and stored by the optical flame sensor of FIG. 1 in one embodiment.

FIG. 8 is a graphical representation of a filtered signal resulting from filtering the signal of FIG. 7 in one embodiment.

Fig. 9 is a pseudo-color plot representing the time evolution of a binary signal derived from the filtered signal of fig. 8 in an embodiment.

FIG. 10 is a graphical representation of a half sawtooth voltage to modulate a wavelength of a laser of the optical flame sensor of FIG. 1 in one embodiment.

FIG. 11 is a graphical representation of the cavity electrical signal generated by the half sawtooth voltage of the embodiment of FIG. 10.

FIG. 12 is a schematic block diagram of a multi-channel furnace flame monitor in an embodiment that includes a plurality of the optical flame sensors of FIG. 1, each sensor monitoring a respective furnace flame.

Detailed Description

FIG. 1 is a schematic view of an optical flame sensor 100 proximate a burner 185 of a furnace 180. Furnace 180 includes igniter 190, igniter 190 including gas valve 192 and igniter tip 193. Gas valve 192 is configured to control fuel emissions from igniter tip 193. Upon combustion, the discharged fuel forms a flame 194. Where the furnace 180 includes a plurality of burners 185, each burner 185 may have a respective igniter 190. Alternatively, the furnace 180 may include more burners 185 than igniters 190 such that a single igniter 190 is associated with multiple burners 185. The igniter 190 may be attached to the combustor 185, such as to a front plate of the combustor 185. The furnace 180 includes a flame sensor port 186. In various embodiments, the flame sensor port 186 is a port of the burner 185, such as in a back plate thereof. In various embodiments, the flame sensor port 186 is a port of the igniter 190 such that the flame sensor can be inserted into the igniter 190. The flame sensor port 186 can be a conventional flame sensor port configured to receive a non-optical flame sensor, such as those based on flame rectification described above.

The optical flame sensor 100 includes a fiber optic cavity 130. In embodiments, the optical flame sensor 100 further includes at least one of a signal generator 102, a laser 110, an optical circulator 120, an optical sensor 151, and a flame sensor processor 160. In certain embodiments, the optical sensor 151 is an InGaAs photodiode detector. In embodiments, the flame sensor processor 160 is communicatively coupled to the furnace controller 181 of the furnace 180 such that when the flame sensor processor 160 determines that the flame 194 has extinguished, the flame sensor processor 160 transmits a signal 169 to the furnace controller 181. In response to receiving signal 169, furnace controller 181 controls gas valve 192. In embodiments, in addition to or in lieu of closing gas valve 192, furnace controller 181 displays an indication or alarm on display 183 in electrical communication with furnace controller 181 in response to receiving signal 169. Display 183 may be part of a main control panel of furnace 180 or may be wirelessly coupled to furnace controller 181, such as a handheld device used by operators and technicians of furnace 180.

In embodiments, the flame sensor processor 160 is an integral part of the furnace controller 181. The furnace controller 181 may control multiple burners 190 within a given furnace or multiple furnaces depending on the application. Alternatively or additionally, the flame sensor processor 160 may be coupled to another external device (not shown), such as a furnace management system, and configured to transmit an alert to the burner management system indicating the presence or absence of the flame 194.

In embodiments, one or more components of the optical flame sensor 100 are configured to be removable to facilitate replacement. For example, in embodiments, the fiber optic cavity 130 is a separate replaceable component of the optical flame sensor 100, wherein replacing the fiber optic cavity 130 does not require replacing components of the laser 110, the fiber optic couplers 106 and 107, the optical circulator 120, the optical sensors 151, 155, and the flame sensor processor 160.

The optical circulator 120 includes a first port 121, a second port 122, and a third port 133. The first port 121 is configured to receive an optical signal, such as the optical signal 112. The second port 122 is configured to output the optical signal 112 received at the first port 121. The third port 133 is configured to output an input of the second port 122.

The fiber lumen 130 includes a lumen proximal end 131, an optical fiber 135, and a mirror 140 located at a distal end 136 of the optical fiber 135. The lumen proximal end 131 is optically coupled to the second port 122 such that it receives the optical signal 112. The fiber cavity 130 maintains the cavity signal 113 therein. A portion of the lumen signal 113 exits the lumen proximal end 131 as the lumen output signal 114. The cavity output signal 114 output by the fiber cavity 130 is an input to the second port 122. The optical sensor 151 is optically coupled to the third port 123 to detect the cavity output signal 114 and thereby generate the cavity electrical signal 154. A mirror 140 is attached to the fiber 135 and may surround the distal end 136 thereof. An advantage of the mirror 140 being part of the fiber optic cavity 130 (e.g., as a layer deposited thereon) is that it enables the fiber optic cavity to be a modular component of the optical flame sensor 100 that is easily replaceable.

The signal generator 102 is electrically coupled to the laser 110 and is configured to apply a time-varying voltage 104 to the laser 110. The laser 110 is configured to generate an optical signal 111. Optical signal 112 includes at least a portion of optical signal 111.

In embodiments, the laser 110 is a diode laser, such as a distributed feedback laser, a distributed bragg reflector laser, or a vertical cavity surface emitting laser. In embodiments, the laser 110 comprises an InGaAsP laser diode. The optical signal 111 has a center wavelength λ₀It may be a visible wavelength or a near infrared wavelength, such as a wavelength between 1.2 μm and 1.70 μm.

In embodiments, the laser 110 is configured to be tuned by the time-varying voltage 104 such that the time-varying voltage 104 periodically modulates the wavelength of the optical signal 111 at the modulation period 105. The laser 110 is electrically connected to the signal generator 102 such that the time-varying voltage 104 modulation is applied thereto (e.g., to the laser)A photodiode). Increasing the current such that the center wavelength λ₀Increase, and decrease the current such that the center wavelength λ₀And decreases. In embodiments, the time-varying voltage 104 is periodic and has a frequency between 5kHz and 15kHz, such as 10kHz, and may be a sinusoidal or non-sinusoidal wave, such as a sawtooth or triangular waveform. In embodiments, the time-varying voltage 104 implements other frequencies less than 5kHz or greater than 15 kHz. The time-varying voltage 104 has a wavelength sufficient to center the wavelength λ₀Modulating the amplitude of the + -delta lambda. In embodiments where laser 110 is a distributed feedback laser, δ λ may be between 0.2nm and 0.4 nm. The optical fiber 135 and any other optical fiber disclosed herein may be of a center wavelength λ₀A single mode optical fiber.

Fig. 1 shows the reflectivity 132 of light incident thereon from the fiber optic cavity 130 at the cavity proximal end 131. In embodiments, the reflectivity 132 is at the center wavelength λ₀Between 3% and 5%. This reflectivity range far exceeds the loss of physical contact ("PC") fiber coupling and increases the amplitude of the interference maxima (and the contrast of the interference fringes) of the cavity output signal 114 and thereby facilitates determining the presence or absence of the flame 194.

In embodiments, the optical flame sensor 100 includes an optical fiber 125 optically coupling an optical fiber cavity 130 to the second port 122. The optical fiber 125 has a fiber distal end 126.

The fiber distal end 126 and the lumen proximal end 131 may be coupled such that the air gap between them increases the amplitude of the reflectivity 132. In embodiments, the fiber distal end 126 and the cavity proximal end 131 are terminated (e.g., terminated) with an angled polish (e.g., an APC connector) and a non-angled (flat) polish (e.g., a flat, PC, or UPC connector), respectively, which creates the air gap described above.

In embodiments, fiber distal end 126 and cavity proximal end 131 each have flat polished terminations and are coupled without an air gap between them (e.g., PC-to-PC or UPC-to-UPC coupling), which results in a reflectivity 132 that is less than that in the above-described coupling that includes an air gap. To increase the reflectivity 132, the fiber distal end 126 may have a reflective coating thereon, such as a dielectric coating. In embodiments, reflectionCoating at central wavelength lambda₀The reflectivity is between 40% and 60%. An advantage of having the reflective coating on the distal fiber end 126 rather than the proximal cavity end 131 is that replacement of the fiber cavity 130 (necessary due to flame-induced wear) does not require replacement of the reflective coating.

In embodiments, the optical flame sensor 100 includes a housing 109. The optical fiber 125 may be of a center wavelength λ₀A single mode fiber that is long enough to facilitate loading of the fiber optic cavity 130 (e.g., with the housing 109) into the flame sensor port 186 such that the fiber optic cavity 130 is close enough to the gas valve 192 to detect heat generated by the flame 194. The housing 109 may be configured such that the lumen distal end 136 may be secured (e.g., removably secured) within the housing 109. In embodiments, the housing 109 is configured to load the optical flame sensor 100 in the same location as the flame rectification based flame sensor described above. For example, the housing 109 may be configured to load the fiber optic cavity 130 directly within the flame 194, or within a portion of the burner 185 that receives the excess flame 194, such as a flame detector side tube. Thus, the housing 109 is configured such that the optical flame sensor 100 is a modification of the flame sensor based on the above-described rectification.

In embodiments, the optical fiber 135 is a metalized optical fiber. For example, the optical fiber 135 may have a metal coating covering its cladding surface, while the cladding surface of a conventional optical fiber has a polyacrylate coating. The metal may be comprised of a metal selected from the group of metals including aluminum, copper, gold, silver, and any combination thereof. Thus, the metal may have a melting point that may exceed 700 ℃ so as not to melt when near or in the flame 194. In embodiments, for additional protection, at least a portion of the fiber lumen 130 is located within a protective shell, such as a flexible metal tube or a single wire tube (monocoil), which may be formed of steel or other material.

In embodiments, the optical flame sensor 100 includes a fiber coupler 106 and an optical sensor 155, which may be similar or identical to the optical sensor 151. The fiber coupler 106 includes a first coupler output 107 that optically couples the optical signal 112 to a first port 121. In embodiments, the fiber optic coupler further comprises a second coupler output 108 that optically couples the reference optical signal 116 to the optical sensor 155. The optical signal 112 is a first percentage of the optical signal 111. The reference optical signal 116 is a second percentage of the optical signal 111. In embodiments, the first percentage exceeds the second percentage.

For example, the first percentage is ninety percent and the second percentage is ten percent. Other ratios of the first percentage and the second percentage may be implemented without departing from the scope thereof. The optical sensor 155 generates an electrical reference signal 156 from the optical reference signal 116. In embodiments, the optical sensor 155 is optically coupled to the fiber coupler 106 to receive the reference optical signal 116. For example, the respective single mode optical fibers guide the optical signal 112 between the fiber coupler 106 and the first port 121, the cavity output signal 114 between the third port 123 and the optical sensor 151, and the reference optical signal 116 between the fiber coupler 106 and the optical sensor 155.

In embodiments, optical fiber 135 is formed from a first material having a wavelength λ at the center wavelength₀At a first temperature T₁Refractive index n of₁. First temperature T₁May be between 15 c and 30 c and corresponds to a "flameless" temperature in the absence of flame 194. In embodiments, the distal end 136 of the optical fiber 135 is perpendicular to the optical axis of the optical fiber 135. In embodiments, distal end 136 comprises a polished surface.

The reflective properties of mirror 140 may be derived from the interface of the physical end of fiber 135 with the ambient medium (such as air) surrounding the physical end. For example, the mirror 140 can be a polished and/or cleaved surface of the distal end 136 (e.g., perpendicular to the optical axis of the optical fiber 135) such that the mirror 140 is part of the optical fiber 135. In embodiments, distal end 136 has a reflective coating thereon and functions as a mirror. In embodiments, the mirror 140 is a semiconductor layer, such as a silicon layer, that protects the distal end 136 from moisture and other deposits while being sufficiently thin (e.g., ≦ λ₀10) to prevent interference/etalon effects.

In embodiments, mirror 140 is different from optical fiber 135. For example, the mirror 140 may include a layer 142 formed of a second material having a temperature at a first temperatureDegree T₁Refractive index n of_RAnd a thickness L₁. Examples of the second material include silicon, aluminum oxide, and zinc oxide. Refractive index n_RCan satisfy (n)_R-n₁) ≧ 0.2, such that n is partially substituted by_RAnd n₁The determined reflectivity of layer 142 results in cavity output signal 114 having a modulated interference maximum as follows: its amplitude is sufficient to be detected by the optical sensor 151 and characterized by the flame sensor processor 160.

The flame sensor processor 160 includes electronics 162, a processor 164, and a memory 170. The processor 164 is communicatively coupled to the memory 170 and may be communicatively coupled to the electronics 162. The electronics 162 are communicatively coupled to a memory 170. The electronics 162 may include one or more of an operational amplifier and a microcontroller, and may be a data acquisition device.

The memory 170 may be transitory and/or non-transitory, and may include one or both of volatile memory (e.g., SRAM, DRAM, computing RAM, other volatile memory, or any combination thereof) and non-volatile memory (e.g., FLASH, ROM, magnetic media, optical media, other non-volatile memory, or any combination thereof). A portion or all of the memory 170 may be integrated into the processor 164.

The electronics 162 receive the chamber electrical signal 154 and, in embodiments, the reference electrical signal 156 and generate a processed signal 172 therefrom. For example, the electronics 162 generate the processed signal 172 by: the cavity electrical signal 154 is normalized by the reference electrical signal 156 to increase the signal-to-noise ratio of the cavity electrical signal 154. In embodiments, the processed signal 172 is a digital signal, and the electronics 162 include an analog-to-digital converter for converting the reference electrical signal 156 and the cavity electrical signal 154 from analog to digital. In embodiments, the components that perform the analog-to-digital conversion are instead part of the optical sensor 151 and the optical sensor 155. The memory 170 stores the processed signal 172.

The memory 170 includes software 180. Software 180 includes one or both of a streak analyzer 182 and a signal shape analyzer 184. Each of the streak analyzer 182 and the signal shape analyzer 184 includes respective machine readable instructions that are executable by the processor 164 to perform the functions of the optical flame sensor 100 as described herein. When executed by the processor 164, the fringe analyzer 182 controls the processor 164 to analyze the processed signal 172 to determine whether the fiber cavity 130 is heating or cooling based on the temporal variation of the interference fringes of the processed signal 172. When executed by the processor 164, the signal shape analyzer 184 controls the processor 164 to determine the temperature or temperature range of the fiber cavity 130.

Fig. 2 includes plots 250 and 270, each illustrating data measured by an embodiment of optical flame sensor 100. In this embodiment, the time-varying voltage 104 is a 1kHz sawtooth waveform and the laser 110 is centered about a wavelength λ₀1550nm distributed feedback laser. The fiber coupler is used as a 90:10 splitter where 90:10 is the power ratio of the optical signal 112 to the reference optical signal 116. The fiber 135 has a length of 1.0 ± 0.1 meters, the mirror 140 has a reflectivity of 50%, and the reflectivity 132 is about 4%. In embodiments, mirror 140 has a wavelength λ at the center₀A reflectivity between 45% and 55%.

Plot 250 shows a cavity voltage 254 and a reference voltage 256 that are examples of cavity electrical signal 154 and reference electrical signal 156, respectively. Plot 250 represents time intervals 251 and 252. Time intervals 251 and 252 span the modulation period 205 (also referred to herein as τ) of the time-varying voltage 104. Fig. 2 shows time τ/2, which represents the end of time interval 251 and the beginning of time interval 252. Modulation period 205 is an example of modulation period 105.

The time-varying voltage 104 applied to the laser 110 modulates both the wavelength and the power of the optical signal 111 such that the center wavelength λ₀And voltages 254 and 256 increase during time interval 251 and decrease during time interval 252. For example, the center wavelength λ of the optical signal 111₀λ may be from the beginning of time interval 251₁Increasing to lambda at time tau/2₂And decreases back to λ at the end of time interval 252₁。

Plot 270 includes fringe signal 272, which is cavity voltage 254 normalized by reference voltage 256, and is scaledAn example of the processing signal 172. The fringe signal 272 includes a plurality of interference maxima that occur at the resonant time 274 corresponding to the resonant wavelength λ of the fiber cavity 130 at which the laser 110 is tuned_mTime of (d). The vertical dashed lines represent each resonance time 274. For clarity of illustration, not all resonance times 274 are labeled with a reference numeral.

When the temperature of the fiber cavity 130 is constant, the resonance time 274 is stationary with time, e.g., between successive modulation periods 205(τ). Each resonance time 274 of the time interval 251 corresponds to satisfying λ_m＝n₁(T) respective moduli m of L (T)/2m and respective wavelengths lambda_mWherein n is₁(T) and L (T) are the temperature dependent refractive index and temperature dependent length, respectively, of fiber cavity 130. Product n as the temperature of the fiber cavity 130 increases₁(T) L (T) is increased so that each lambda_mAlso increases, which means that the resonance wavelength λ with which the laser 110 emits_mThe corresponding resonance time 274 increases. Thus, when comparing fringe signals 272 of successive modulation periods 205 as the fiber cavity 130 heats up, each resonance time 274 within the time interval 251 moves toward time τ/2. Similarly, each resonance time 274 within time interval 252 is shifted toward time τ/2 because during time interval 252, the center wavelength λ₀From λ₂Reduced to λ₁。

Cooling the fiber cavity 130 has an opposite effect on the resonance time 274. When the fiber cavity 130 cools, the resonance time 274 in time intervals 251 and 252 is away from time τ/2.

In embodiments, the fringe analyzer 182 determines the aforementioned movement or time drift of the resonance time 274 and generates therefrom the temperature change state 176 stored in the memory 170. The signal 169 may include a temperature change status 176. The temperature change condition 176 may be a binary indication of whether a flame 194 is present. In other words, when the fringe analyzer 182 determines that the fiber cavity 130 is heating, the flame 194 must be present, and thus the temperature change condition 176 indicates that a flame is present or heating. Conversely, when the fringe analyzer 182 determines that the fiber cavity 130 is cooling, the flame 194 is not present, and thus the temperature change condition 176 indicates that a flame is not present or is cooling. In embodiments, the temperature change status 176 indicates one of three states: heating, cooling, and steady state temperatures. Alternative or additional techniques for determining the time drift of the resonance time 274 include fringe counting, cross-correlation, and phase recovery methods.

In embodiments, alternatively or additionally, the stripe analyzer 182 determines the temperature change status 176 and/or the measured temperature 178 by counting stripes within time intervals 251 and 252. As the fiber cavity 130 is heated, its length increases such that its free spectral range decreases, which results in tighter resonance times 274, and therefore more resonance times 274 (i.e., more fringes), within the time intervals 251 and 252. The opposite is true when the fiber cavity 130 is cooled. The memory 170 may store calibration data 174. In embodiments, the calibration data 174 includes the number of fringes as a function of temperature, e.g., as a look-up table, such that the flame sensor processor 160 can map the number of fringes in the time interval 251 and/or 252 counted by the fringe analyzer 182 to the temperature of the calibration data 174. The measured temperature 178 may represent a particular temperature value, a range of temperature values, or a binary indicator indicating a "hot" or "cold" state of the combustor 185. In embodiments, "hot" refers to the time when the measured temperature exceeds an upper threshold temperature (e.g., 500℃.) and "cold" refers to the time when the measured temperature is below a lower threshold temperature (e.g., 100℃.).

In embodiments, the stripes are defined by data points in the processed signal 172 that have values that exceed (or are less than) each signal value of a predetermined number of data points. For example, the predetermined number is equal to 10. To prevent double counting of a single stripe, in embodiments, only one of two temporally successive points is eligible for a stripe. In embodiments, the fringe analyzer 182 determines the fringe count as the average number of fringes counted over the plurality of modulation periods 105. In embodiments, the streak analyzer 182 applies a low pass filter, e.g., with a cutoff frequency of 100kHz, to the processed signal 172 to remove noise that may result in inaccurate streak counts.

In one embodiment, the fiber cavity 130 is 30 cm long and the fibers of the optical fiber 135 areThe refractive indices of the core at 25 ℃ and 700 ℃ respectively being n_c1.446 and n_h1.45354. In this embodiment, the fiber cavity 130 has a free spectral range (fringe spacing) of 2.10 picometers at 25 ℃ and 2.09 picometers at 700 ℃. The center wavelength λ of the laser 110 is modulated by the signal generator 102 over a scan range of about 700 pm₀The difference in the number of stripes counted between 25 ℃ and 700 ℃ was 1.65 stripes. This difference is discernable when the fringe counts are averaged over multiple scans.

In embodiments, when the center wavelength λ of the laser 110 is₀Is equal to its maximum value lambda₂Then reduced back to its minimum value λ₁The fringe analyzer 182 determines the measured temperature 178 by counting the number of data points between the predetermined number of fringes and the "u-turn" point. For example, when time interval 251 ends and time interval 252 begins, see time τ/2 in FIG. 2. As the fiber cavity 130 heats up, the number of fringes between the u-turn points increases, such that the number of data points between the u-turn points and the predetermined fringes decreases. Similarly, as the fiber cavity 130 cools, the number of stripes between the u-turn points decreases, such that the number of data points between the u-turn points and the predetermined stripes increases.

In embodiments that include a cross-correlation method, alternatively or additionally, the fringe analyzer 182 determines the temperature change state 176 by determining a respective cross-correlation of temporally successive fringe signals 272(1,2,. eta.) corresponding to temporally successive modulation periods 205(1,2, …). The cross-correlation results in a correlation with a time t relative to the start of the modulation period 205₁，t₂… } corresponding time series of respective correlated maxima. Heating of the fiber cavity 130 corresponds to time t₁，t₂… monotonically increases during time interval 251 and monotonically decreases during time interval 252. Cooling of the fiber cavity 130 corresponds to time t₁，t₂… monotonically decreases during time interval 251 and monotonically increases during time interval 252.

In embodiments, the stripe analyzer 182 alternatively or additionally responds by applying a signal to successive stripe signals 272(1, 2.)The temperature change state 176 is determined using a phase recovery technique and the resulting phase { phi ] is compared₁，φ₂… to determine the time drift of the resonance time 274. The phase recovery technique may include applying a fourier transform (which produces a complex frequency domain signal (including real and imaginary parts)) to the temporally successive fringe signal 272(1, 2. -) and calculating therefrom a composite phase { phi }₁，φ₂… }. The signals that are out of phase in the frequency domain are shifted in time relative to each other in the time domain according to the fourier transform theorem.

Thus, in embodiments, when the phase is synthesized { φ }₁，φ₂…, the streak analyzer 182 updates the temperature change state 176 as it increases or decreases consistently over time. Fringe analyzer 182 evaluates phase difference sgn (Δ φ)_i) Wherein Δ φ_i＝φ_i-φ_i-1. In embodiments, sgn (Δ φ) for a predetermined number N of consecutive periods_i) The sum of (d) exceeds a positive threshold or is less than a negative threshold, the stripe analyzer 182 updates the temperature change status 176. For example, when N is 25, the absolute values of the positive and negative thresholds may each be equal to 10. When sgn (Δ φ)_i) The sum of (d) is between the positive and negative thresholds, the fringe analyzer 182 may output a temperature change status 176 indicative of the steady-state temperature.

In embodiments, the accumulated phase difference Σ Δ φ when a predetermined number N of cycles (consecutive or non-consecutive cycles) is_iAbove the positive threshold or below the negative threshold, the streak analyzer 182 updates the temperature change status 176. For example, a positive accumulated phase difference indicates cooling, while a negative accumulated phase difference indicates heating.

In embodiments, the phase recovery technique may include pre-processing the time-sequential fringe signals 272 to remove temporal frequencies that are not related to the frequency of interest, such as the time intervals between sequential resonance times 274.

In embodiments, alternatively or additionally, the fringe analyzer 182 uses a "binary data transition tracking" technique to determine the temperature change state 176, which includes N successive modulation periods { P } having a duration τ (e.g., modulation period 205) for each₁，P₂，…，P_NCompares the M samples S of the fringe signal 272 over a time segment Δ t₁，S₂，…，S_MWhich may be smaller than the time interval between adjacent resonance times 274 for tracking the resonance time 274 corresponding to the single modulus m. The sample count M may exceed the successive period count N.

Fig. 2 shows a time segment 253 as an example of a time segment Δ t. The start and stop times of the defined time segment at are constant with respect to the start and end of the modulation period τ such that the time segment at corresponds to the same wavelength range of the optical signal 112 during each modulation period P. Sample { S₁，S₂，…，S_MChronologically within a time segment at. In the following example, N and M are equal to 6 and 3, respectively. Also in the following example, sample { S₁，S₂，…，S_M}_iCorresponding to the modulation period P_iWherein the integer i ranges from 1 to N. In embodiments, alternatively or additionally, the streak analyzer 182 operates by, for each sample, { S }₁，S₂，…，S_M}_iThe sign um function is applied to determine the temperature change status 176. The signum function produces binary data samples B_i＝{B₁，B₂，…，B_M}_iWhere each sample value B is one of two values, such as 0, +1 or { -1, +1 }. The streak analyzer 182 determines the time trend of the transition times when the temporally successive sample value B changes from one value to another (e.g., from 0 to 1 or vice versa, or changes in sign). The direction of this time trend (forward or backward in time) indicates the direction of the resonance time 274, and thus indicates whether the fiber cavity 130 is heating or cooling. The absence of a time trend (e.g., the transition time exhibits a time jitter) indicates a steady state temperature. In embodiments, the stripe analyzer 182 updates the temperature change status 176 when the time trend continues in the same direction for a time that exceeds a threshold time. In embodiments, the threshold time is a multiple of the modulation period 105, where the multiple is between 5 and 10.

For example, the fringe analyzer 182 may generate successive samples{+1、+1、+1}₁、{-1,+1,+1}₂、{-1,-1,+1}₃、{-1,-1,-1}₄、{+1,-1,-1}₅And { +1, +1, -1}₆. In these samples, the time position of the sign change moves from an earlier time to a later time, indicating that the resonance time 274 is moving forward in time. This time shift means that the fiber cavity 130 is heating up when the time segment at is within the time interval 251. This time shift means that the fiber cavity 130 is cooling when the time segment at is within the time interval 252. In embodiments, after analyzing the M samples and N modulation cycles, the fiber movement analyzer records the result (whether the cavity 130 is heating or cooling) as the temperature change state 176.

The memory 170 may also include a temperature change history 177, the temperature change history 177 including a time series of previously measured temperature change states 176, each of which may be paired with a respective timestamp. In embodiments, even after the fiber cavity 130 has reached a temperature equilibrium such that its temperature does not change over time, the temperature change history 177 enables a determination of whether the fiber cavity 130 is "hot" (near the flame 194, e.g., T >500℃.) or "cold" (the flame 194 is not present, e.g., T <100℃.). When the memory 170 is a non-volatile memory, the temperature change history 177 may be useful in the event of a power outage or a reboot of the flame sensor processor 160.

In embodiments, software 180 additionally or alternatively determines whether fiber cavity 130 is hot or cold at steady state temperature by quantifying the time jitter in fringe signal 272 that increases with increasing temperature. For example, when the fiber cavity 130 is hot, the fringe signal 272 includes a higher frequency than cold, so that the software 180 can evaluate the spectrum of the fringe signal 272.

Fig. 3 includes a plot 300 illustrating data measured by another embodiment of the optical flame sensor 100. This embodiment of fig. 3 is the same as the embodiment of fig. 2, except that the reflectivity of the mirror 140 is 8%. Plot 300 includes reference voltage 356 and fringe signal 372, which are respective examples of reference electrical signal 156 and processed signal 172. Plot 300 includes a lower curve 310 and an upper curve 330. The lower curve 310 may be a linear fit to the local minima of the fringe signal 372. The lower curve 330 may be a linear fit to the local maxima of the fringe signal 372. Plot 300 also includes a median curve 320, which may be the average of curves 310 and 330, or may be obtained by applying a low-pass filter to fringe signal 372 to remove resonance fringes. Curves 310, 320 and 330 have respective fringe signal amplitudes 312, 322 and 332 between their respective maxima and minima, each of which is dependent on the reflectivity of mirror 140, as shown in fig. 4.

Fig. 4 includes a plot 400 illustrating data measured by another embodiment of the optical flame sensor 100. This embodiment of fig. 4 is the same as the embodiment of fig. 2, except that the mirror 140 includes a layer having 50% reflectivity. Plot 400 includes reference voltage 456, fringe signal 472, which are respective examples of reference electrical signal 156 and processed signal 172. Plot 400 includes a lower curve 410, a median curve 420, and an upper curve 430 derived from a fringe signal 472. Curves 410, 420 and 430 are similar to curves 310, 320 and 330, respectively, of fig. 3. Curves 410, 420, and 430 have respective fringe signal amplitudes 412, 422, and 432 that are similar to and greater than fringe signal amplitudes 312, 322, and 332 of fig. 3, respectively. In embodiments, the calibration data 174 stores a temperature dependence of at least one of the above-described fringe signal amplitudes, such that the flame sensor processor 160 determines the measured temperature 178 by comparing the measured fringe signal amplitudes to those stored in the calibration data 174.

Fig. 3 and 4 illustrate the relationship known from the physics of a fabry-perot optical cavity, i.e., the fringe signal amplitude associated with the fringe signal increases as the reflectivity of the mirror 140 increases. Neither the fringe signal 372 nor the fringe signal 472 is normalized by the reference voltage (e.g., the reference electrical signal 156). Given the temperature-dependent reflectivity of the mirror 140, the fiber cavity 130 can be calibrated such that its temperature can be derived from the processed signal 172, e.g., via one of the curves and/or fringe signal amplitudes of fig. 3 and 4. The temperature-dependent reflectivity (or equivalent, e.g., normalized temperature-dependent reflectivity or fringe signal amplitude, such as those of fig. 3 and 4) of the mirror 140 can be the temperature T₁And T₂In betweenA ray ("one-to-one") function such that each reflectivity value is mapped across T₁And T₂Is and only has one temperature within the range of (a). The memory 170 may store such functions as calibration data 174, which may be a look-up table.

In embodiments, the optical flame sensor 100 is configured such that the processed signal 172 resembles the median curves 320 and 420, which enables the temperature of the fiber cavity 130 to be determined. In such an embodiment (hereinafter referred to as a "nominal cavity" embodiment), the fiber cavity 130 is optically coupled to the second port 122, e.g., via the fiber distal end 126, such that the reflectivity 132 is low enough that the fiber cavity 130 is only nominally a cavity and acts as a fiber mirror. For example, the fiber distal end 126 and the cavity proximal end 131 may be optically coupled such that the reflectivity 132 is less than 0.1% (-30 decibels). In embodiments, mirror 140 includes a layer 142 having a temperature dependent reflectivity in the range of temperature T described above₁("flameless" temperature) and a second temperature T₂Between a maximum and a minimum (and vice versa) which may exceed T₁At least 500 kelvin. For example, at a central wavelength λ₀And a first temperature T₁Layer 142 may have a refractive index n_R1And thicknessWherein q is₁Is a non-negative integer, such that layer 142 acts as a quarter-wave reflective coating (multi-order coating, order equal to q)₁). For example, at a central wavelength λ₀And a second temperature T₂Layer 142 may have a refractive index n_R2And thicknessWherein q is₂Is a non-negative integer, such that layer 142 acts as a half-wave anti-reflection coating (multi-step coating, order equal to q)₂). In embodiments, q₁＝2q₂So thatWhereinIs n_R1And n_R2Which makes layer 142 at T₁And T₂With a one-to-one mapping of temperature to reflectivity in between.

Thickness L₁And L₂Refers to the distance between the top and bottom surfaces 143, 144 of the layer 142, and the top and bottom surfaces 143, 144 may be flat and parallel to each other. Given the material of layer 142, its coefficient of thermal expansion and temperature dependent index of refraction can be used for length L₁And L₂In the above expression, to find a positive integer q₁And q is₂Such that the two expressions are at their respective temperatures T₁And T₂The following is satisfied. Integer q₁And q is₂May be equal. When q is₁And q is₂Both equal zero, or more generally when q is equal to zero₁＝2q₂When layer 142 has the aforementioned one-to-one mapping of temperature to reflectivity.

Thickness L₁The advantageous values of (b) can be found numerically or analytically. Thickness L₁Corresponding to the temperature T₁The thickness of layer 142 (i.e., the "flameless" temperature) and thus represents the temperature at which layer 142 is fabricated. For example, given the coefficient of thermal expansion and the temperature dependent refractive index of layer 142, its reflectivity R is taken as the corresponding thickness L₁And L₂May be (i) at a temperature T₁Is determined to produce R₁(L₁) And (ii) at a temperature T₂Is determined to produce R₂(L₂)。R₁(L₁) And R₂(L₂) Is a thickness L_1,2Because the layer switches between a multi-order quarter wave reflector and a multi-order half wave anti-reflection coating.

An advantageous value of the thickness L1 is obtained by first bringing R₂(L₂) Conversion to R₂(L₁) To find out: by using the coefficient of thermal expansion of layer 142 to convert L₂Value range (at temperature T)₂Lower thickness) to L₁Value range (at temperature T)₁Thickness of) by mixing R₂Value mapping to L₁A range of values. R₂(L₁) And R₁(L₁) The difference, e.g. | R₂(L₁)-R₂(L₁) I is generated at temperature T₁And T₂The difference in reflectivity Δ R (L) of the interlayer 142₁) As temperature T₁Lower candidate thickness L₁As a function of (c). And Δ R (L)₁) Thickness L corresponding to the local maximum of₁Is a thickness L₁Advantageous values of (a).

Fig. 5 is a flow chart illustrating a method 500 for detecting the presence of a flame. The method 500 may be implemented within one or more aspects of the optical flame sensor 100. The method 500 may be implemented by the processor 164 executing computer readable instructions of the software 180. Method 500 includes at least one of steps 510, 520, 530, 540, 550, and 560.

Step 510 includes periodically modulating a wavelength of an optical signal generated by a laser to produce a modulated signal. In the example of step 510, signal generator 102 periodically modulates a center wavelength λ of optical signal 111 generated by laser 110₀。

Step 520 includes detecting a cavity optical signal output by a fiber cavity coupled to the laser. The cavity optical signal includes an amplitude modulation determined by when the wavelength of the modulated optical signal corresponds to a mode of the fiber cavity. In the example of step 520, the optical sensor 151 detects the cavity output signal 114.

Step 530 includes determining whether the fiber cavity is heating or cooling based on the time dependence of the amplitude modulation. In a first example of step 530, the fringe analyzer 182 determines whether the fiber cavity 130 is intended to be heated or cooled based on the time dependence of the one or more resonance times 274 (FIG. 2). In a second example of step 530, the fringe analyzer 182 determines whether the fiber cavity 130 is heating or cooling by counting the number of resonances (each occurring at a particular resonance time 274) during one or both of the time intervals 251 and 252 and matching the counted number of resonances to the temperature stored in the calibration data 174.

Step 530 may include at least one of the techniques described above: fringe counting, cross-correlation, phase recovery, and binary numberAnd tracking according to the conversion. In embodiments, step 530 includes step 532, which includes tracking the phase of the amplitude modulated frequency domain representation. In the example of step 532, the fringe analyzer 182 applies a fourier transform to the fringe signal 272 and tracks the time correlation of the phase of the resulting frequency domain signal. In embodiments, step 530 includes steps 534 and 536. Step 534 includes converting the amplitude modulation to a binary time series. In the example of step 534, the streak analyzer 182 converts the streak signal 182 to include the aforementioned binary data samples { B }₁,B₂,…,B_M}_iIn which the sample value B is_1-MSegment Δ t across time.

Step 536 includes tracking when the binary time series transitions from the first discrete value and the second discrete value during a time interval of the binary time series that does not exceed a duration between successive modes of the fiber cavity. In embodiments, the time-varying voltage 104 determines the duration between the existence of successive modes that resonate within the fiber cavity 130. In the example of step 536, the streak analyzer 182 determines a time trend of the conversion time at which the temporally successive sample values B are converted from a first discrete value (e.g., 0) to a second discrete value (e.g., 1).

Step 540 includes extracting the carrier signal from the cavity optical signal by applying a low pass filter. In a first example of step 540, the signal shape analyzer 184 applies a low pass filter to the streak signal 372 to generate the median curve 320. In a second example of step 540, the signal shape analyzer 184 applies a low pass filter to the streak signal 472 to generate the median curve 420.

Step 550 includes determining a temperature range of the fiber cavity based on the shape of the carrier signal. In a first example of steps, the signal shape analyzer 184 determines a temperature range of the fiber cavity 130 from the fringe signal amplitude 322 and the calibration data 174. In a second example of step 550, the signal shape analyzer 184 determines the temperature or temperature range of the fiber cavity 130 from the fringe signal amplitude 422 and the calibration data 174. The signal shape analyzer 184 may output the temperature or temperature range as the measured temperature 178, which may be part of the signal 169. The method 500 can include a step 560 when the fiber cavity is proximate to an igniter tip of a furnace burner controlled by a furnace controller. Step 560 includes one or both of the following when the fiber cavity is cooling and/or has a temperature below a predetermined threshold temperature: generating a warning signal and controlling a furnace controller to close a valve that discharges fuel burned by the ignition flame. In the example of step 560, the fiber cavity 130 is proximate to the igniter tip 193. The fringe analyzer 182 determines that the fiber optic cavity 130 is cooling (via step 530) and/or via a temperature below a predetermined threshold temperature (via step 550), and in response to receiving the signal 169 from the flame sensor processor 160, the furnace controller 181 generates a warning signal and/or closes the gas valve 192. The furnace controller 181 may display a warning signal on the display 183.

FIG. 6 is a flow chart illustrating a method 600 for detecting the presence of a flame. The method 600 may be implemented in one or more aspects of the optical flame sensor 100, such as one of the aforementioned "nominal cavity" embodiments of the optical flame sensor 100. The method 600 may be implemented by the processor 164 executing computer readable instructions of the software 180. Method 600 includes at least one of steps 610, 620, 630, and 640.

Step 610 includes coupling the optical signal generated by the laser to a proximal end of an optical fiber having a mirror at a distal end thereof, the mirror having a predetermined temperature dependent reflectivity. In the example of step 610, the optical circulator 120 couples the optical signal 112 into the fiber cavity 130, the fiber cavity 130 including a layer 142 having a predetermined temperature-dependent reflectivity stored as calibration data 174.

Step 620 includes detecting the optical power of the output optical signal reflected by the mirror. In the example of step 620, the optical sensor 151 detects the cavity output signal 114. Step 620 may also include detecting the optical power of the reference optical signal. For example, step 620 may include detecting the reference optical signal 116 using the optical sensor 155.

Step 630 includes determining the reflectivity of the mirror based on the detected optical power. In the example of step 630, the signal shape analyzer 184 determines the reflectivity of the layer 142 from the cavity electrical signal 154 and the reference electrical signal 156 via the processed signal 172. In embodiments, the processed signal 172 is the cavity electrical signal 154 divided by the reference electrical signal 156 such that the amplitude of the processed signal 172 or a time average thereof corresponds to or is equal to the reflectivity of the layer 142.

Step 640 includes determining the temperature of the mirror by mapping the determined reflectivity to a predetermined temperature dependent reflectivity. In the example of step 640, the signal shape analyzer 184 determines the temperature of the layer 142 by mapping the amplitude of the processed signal 172 to the temperature using the calibration data 174.

7-9 are graphical representations of measured and processed signals illustrating the steps of the above-described binary data transition tracking technique implemented by an embodiment of the flame sensor processor 160 to determine the temperature change state 176. FIG. 7 includes a graph 700 depicting a signal 772, which signal 772 is an example of the processed signal 172 (FIG. 1) generated by an embodiment of the optical flame sensor 100. In this embodiment, the time-varying voltage 104 is a 50mV 200Hz sawtooth waveform, and the electronics 162 samples the cavity electrical signal 154 at 10 kHz. The amplitude of the signal 772 is modulated by the time-varying voltage 104 and has multiple cavity resonance modulations corresponding to the resonances of the fiber cavity 130 during each cycle of the time-varying voltage 104. Graph 700 includes sampling intervals 752(1) and 752(2) of decreasing amplitude of time-varying voltage 104. Thus, sampling interval 752 is an example of time interval 252 (FIG. 2) during which the center wavelength λ is₀And decreases.

Fig. 8 includes a graph 800 depicting a filtered signal 872 resulting from the flame sensor processor 160(a) demodulating the signal 772 to remove the amplitude modulation imparted by the time-varying voltage 104 and (b) applying a band pass filter to the signal 772. The bandpass filter has a passband centered at a frequency corresponding to the cavity resonance modulation. The filtered signal 872 includes sampled data 801 represented by an open loop 802 measured during sampling intervals 752(1) and 752(2), respectively. The filtered signal 872 further comprises sampled data 803 measured during sampling intervals 752 successively following the sampling intervals 752(2), the sampled data 803 also being represented by an open loop.

The signal generator 102 may be communicatively coupled to the flame sensor processor 160 such that the streak movement analyzer 182 canThe time-varying voltage 104 can be used as a trigger to select the sampled data 801 and 802. Each of the sample data 801-. For sampled data 801, data point 801(k) is the kth data point of sampled data 801, where k is the sample index ranging from 1 to 16. For example, fig. 8 shows data point 801(6) being the sixth data point of the sampled data 801. For example, in steps 534 and 536 of the method 500 of FIG. 5, the stripe analyzer 182 may apply a signum function to the sampled data 801 and 803, which results in the binary sampled data 901 and 903 shown in FIG. 9. Binary sample data 901-903 is the corresponding binary data sample B in the discussion of the binary data transition tracking technique described above_i＝{B₁,B₂,…,B_M}_iEach example of (a).

The binary sample data 901-903 is the first three rows of the data array 900, as shown by the pseudo-color plot in FIG. 9. The data array 900 includes 35 rows and 16 columns. Each of the 35 rows corresponds to a respective sampling interval 752(1-35), with sampling intervals 752(1-3) being shown in FIG. 8. The sampling intervals 752(1-35) correspond to 35 successive periods of the time-varying voltage 104. The fringe analyzer 182 generates 835 the remaining rows, rows 4 through 35, of the data array 900 to the sample data 804, not shown in FIG. 8, to generate 935 the binary sample data 904. For clarity of illustration, FIG. 9 does not include reference numerals for each of the binary sample data 904 and 935. In fig. 9, the dark area of each binary sample data 901 and 935 indicates 0 and the bright area indicates 1.

As can be seen in fig. 9, there is a trend to the left in time, i.e., as index i of sample interval 752 increases from 1 to 35 from the top to the bottom of data array 900, the value of sample index k corresponding to the dark region decreases. Recall that each sampling interval 752 corresponds to a reduction in the amplitude and thus a reduction in the center wavelength λ of the time-varying voltage 104₀Time of (d). The trend to the left indicates that the fringes or resonance times 274 of FIG. 2 are shifted toward the beginning of the sampling interval 752, which indicates heating of the fiber cavity 130, in accordance with the discussion of FIG. 2.

In embodiments, the stripe analyzer 182 evaluates sub-arrays of the data array 900, such as a plurality of columns corresponding to successive sample indices, and tracks transitionsThe temporal trend of the trade time, as described in steps 534, 536 discussing the binary data translation tracking technique and method 500. For example, if the successive sample indices k range from 4 to 6, each of the binary sample data 901(4-6), 902(4-6), …, 935(4-6) is a binary data sample B_i＝{B₄,B₅,B₆}_iWherein i ranges from 1 to 35.

Fig. 10 and 11 are graphical representations of measured signals illustrating example steps of the aforementioned phase recovery method implemented by the flame sensor processor 160 to determine the temperature change condition 176.

Fig. 10 includes a graph 1000 depicting a sawtooth voltage 1004, which is an example of the time-varying voltage 104. Sawtooth voltage 1004 has period 1005, and period 1005 is an example of period 105. The half sawtooth waveform simplifies the portion of the software 180 required for triggering (i.e., detecting the beginning of a successive modulation cycle 105). In embodiments, as part of step 532, software 180 evaluates only the cavity electrical signal 154, rather than the reference electrical signal 156, to determine the start of a new modulation cycle 105.

Fig. 11 includes a graph 1100 depicting the cavity electrical signal 1154, which cavity electrical signal 1154 is an example of the cavity electrical signal 154 and/or the processed signal 172 of fig. 1. The plot 1100 includes a trigger point 1102, which corresponds to a time at which the time derivative of the chamber electrical signal 1154 exceeds a threshold value. FIG. 11 shows a sampling interval 1106 during each cycle 1005, which includes successive signal segments 1172(1-3) of the cavity electrical signal 1154. Each signal segment 1172 is an example of a fringe signal 272 during a time interval 252 during which the center wavelength λ₀And decreases. In embodiments, the fringe analyzer 182 applies a Fourier transform to the signal segments 1172(1,2,3, …) and calculates therefrom the corresponding phases { φ, φ₂,φ₃,...}₁₁₇₂. Phase phi, phi₂,φ₃,...}₁₁₇₂Is the synthetic phase phi, phi described in the phase recovery technique above₂,.. In calculating the phase phi, phi₂,φ₃,...}₁₁₇₂Previously, the fringe analyzer 182 could process the signal segment 1172 to isolate a temporal frequency range corresponding to the cavity resonance of the fiber cavity 130. FIG. 12 is a multiple pass fuseA schematic block diagram of a furnace flame monitor 1200 that includes a plurality of optical flame sensors 100(1,2, …, N), each sensor monitoring a respective flame 194(1,2, …, N) associated with a burner 185(1,2, …, N) of a furnace 1280. Furnace 1280 is an example of furnace 180 and may include furnace controller 181. The multi-channel furnace flame monitor 1200 also includes N optical sensors 151, a flame sensor processor 160, and in embodiments an optical sensor 155.

The integer N represents the number of channels of the multi-channel furnace flame monitor 1200. Here, an element represented by a reference numeral with a parenthesized numeral suffix in the drawing is an example of the element represented by the reference numeral. For example, the elemental optical flame sensor 100(k) is an example of the optical flame sensor 100(k), where the index k is a positive integer in the range of 1 to N. Furnace 1280 may include a plurality of burners 190 such that burner 190(1,2, …, N) refers to up to N different burners 190 and may refer to less than N different furnaces 180. For example, burner 185(1) and burner 185(2) may be different burners within the same furnace 180.

In embodiments, each optical flame sensor 100 receives a respective optical signal 1212 generated by a single laser 110, where fiber coupler 106 includes a beam splitter, such as a fiber beam splitter that splits optical signal 112 into at least N optical signals 1212. The use of a single laser (laser 110) as the light source for multiple optical flame sensors 100 reduces the cost per channel of the multi-channel furnace flame monitor 1200. Each optical sensor 151 receives a respective chamber output signal 114(k) from the optical flame sensor 100(k) and outputs a respective chamber electrical signal 154(k) to the flame sensor processor 160. Based on each chamber electrical signal 154(k), the flame sensor processor 160 generates a corresponding signal 169(k) that is received by the furnace controller 181.

Feature combination

The above-mentioned features as well as the claimed features can be combined in various ways without departing from the scope thereof. The following enumerated examples illustrate some possible non-limiting combinations:

(A1) an optical flame sensor includes an optical circulator, a fiber cavity, and a first optical sensor. The optical circulator includes a first port, a second port, and a third port. The first port is configured to receive an optical signal. The second port is configured to output the optical signal received at the first port. The third port is configured to output an input of the second port. The fiber optic lumen includes a lumen proximal end optically coupled to the second port and a mirror at a lumen distal end such that a lumen optical signal output by the fiber optic lumen is an input to the second port. The first optical sensor is optically coupled to the third port and configured to quantify the cavity optical signal.

(A2) The optical flame sensor of (a1) may further include a laser configured to generate an optical signal; and a signal generator electrically coupled to the laser and configured to apply a periodic waveform to the laser.

(A3) In any of the optical flame sensors (a2), the laser may be one of a distributed feedback laser, a distributed bragg reflector laser, and a vertical cavity surface emitting laser configured to be tuned by a periodic waveform.

(A4) Any of the optical flame sensors (a2) and (A3) may include a plurality of optical flame sensors (a1) - (A3), each configured to receive an optical signal at its respective first port.

(A5) In any of the optical flame sensors (a2) - (a4), the fiber cavity may have a return loss between 3% and 5% at the cavity proximal end.

(A6) Any of the optical flame sensors (a1) - (a5) may also include a first optical fiber optically coupling the fiber cavity to the second port via a flat polished fiber optic connector.

(A7) In any of the optical flame sensors (a1) - (a6), the fiber cavity may include a metalized fiber.

(A8) In any of the optical flame sensors (A1) - (A7), the fiber cavity comprises a fiber having an index of refraction n₁The mirror may comprise a fiber core formed of a first material having an index of refraction n_R(ii) a reflective surface formed of a second material of (n)_R-n₁)≥0.2。

(A9) In the optical flame sensor (A8), the second material may be selected from the group consisting of silicon, aluminum oxide, and zinc oxide.

(A10) Any of the optical flame sensors (a1) - (a9) may also include a reference optical sensor and a fiber optic coupler. The fiber optic coupler includes (i) a first coupler output optically coupling a first percentage of the optical signal to the first port, and (ii) a second coupler output optically coupling the reference optical signal to the reference optical sensor. The reference optical signal is a second percentage of the optical signal. The reference optical sensor is configured to quantize the reference optical signal. The first percentage exceeds the second percentage.

(A11) Any of the optical flame sensors (a1) - (a10) may also include a processor; and a memory. The memory is configured to store the quantified cavity optical signal and machine-readable instructions, which when executed by the processor, control the processor to: analyzing the quantified cavity optical signal to determine whether the fiber cavity is heating or cooling according to at least one of: a temporal variation of (i) interference fringes of the cavity optical signal and (ii) a variation of a number of detected interference fringes of the cavity optical signal during a modulation period of the optical signal.

(A12) In any of the optical flame sensors (A1) - (A11), wherein the optical signal has a center wavelength λ₀(ii) a Mirror at center wavelength λ₀Has (i) at a first temperature T₁Refractive index n of₁And a thickness L₁And (ii) at a second temperature T₂Refractive index n of₂And a thickness L₂So thatAnd isq₁And q is₂Is a non-negative integer, and | T₂-T₁|>500K. The memory may also store machine-readable instructions that, when executed by the processor, control the processor to: extracting a carrier signal from the quantized cavity optical signal by applying a low-pass filter, and a rootThe temperature range of the fiber cavity is determined by the shape of the carrier signal, which is determined in part by the temperature dependent reflectivity of the mirror.

(A13) In any of the optical flame sensors (a1) - (a12), the fiber optic lumen may include an optical fiber, the mirror being a distal end of the optical fiber defining a distal end of the lumen.

(A14) In any one of the optical flame sensors (A1) - (A13), at a center wavelength λ of the optical signal₀The mirror may have a reflectivity of between 45% and 55%.

(A15) In any one of the optical flame sensors (A1) - (A14), the center wavelength λ of the optical signal₀May be between 1.2 μm and 1.70 μm.

(A16) In any of the optical flame sensors (a6) - (a15), the distal end of the first optical fiber may be optically coupled to the proximal end of the optical fiber lumen. The distal end may have a reflective coating thereon at the center wavelength λ of the optical signal₀Has a reflectivity of 40% to 60%.

(B1) An optical flame sensor comprising a fiber optic lumen comprising (a) a proximal lumen end, a (b) distal lumen end, and (c) a mirror at the distal lumen end, the proximal lumen end configured to be optically coupled to (i) a laser and (ii) an optical sensor. The optical flame sensor also includes a housing configured to load the fiber optic cavity to a flame sensor port of the burner.

(B2) In any of the optical flame sensors (B1), the housing may be configured to replace a flame sensor based on flame straightening without modifying the burner.

(B3) In either optical flame sensor (B1) or (B2), the fiber cavity may have a return loss between 3% and 5% at the cavity proximal end.

(B4) In any of the optical flame sensors (B1) - (B3), the fiber cavity may include a metallized fiber.

(B5) In any of the optical flame sensors (B1) - (B4), the fiber cavity may comprise a fiber having an index of refraction n₁The mirror comprises a fiber core formed of a first material having an index of refraction n_R(ii) a reflective surface formed of a second material of (n)_R-n₁)≥0.2。

(B6) In any of the optical flame sensors (B1) - (B5), the fiber optic cavity may include an optical fiber, and the mirror may be a distal end of the optical fiber defining a distal end of the cavity.

(B7) In any one of the optical flame sensors (B1) - (B6), the center wavelength λ of the optical signal output from the optical fiber cavity₀The mirror may have a reflectivity of between 45% and 55%.

(B8) In an optical flame sensor (B7), the central wavelength λ of the optical signal₀May be between 1.2 μm and 1.70 μm.

(C1) A method for detecting the presence of a flame, comprising: the method includes periodically modulating a wavelength of an optical signal generated by a laser to produce a modulated signal, and detecting a cavity optical signal output by a fiber cavity coupled to the laser. The cavity optical signal includes an amplitude modulation determined by when the wavelength of the modulated optical signal corresponds to a mode of the fiber cavity. The method also includes determining whether the fiber cavity is heating or cooling based on a time dependence of the amplitude modulation.

(C2) Either method (C1) may further include extracting the carrier signal from the cavity optical signal by applying a low pass filter; and determining the temperature range of the fiber cavity based on the shape of the carrier signal.

(C3) In any of the methods (C1) and (C2), the determining step may include tracking a phase of the amplitude-modulated frequency-domain representation.

(C4) In any one of the methods (C1) - (C3), the determining step may include: converting the amplitude modulation into a binary time sequence; and tracking when the binary time series transitions from the first discrete value and the second discrete value during a time interval of the binary time series that does not exceed a duration between successive modes of the fiber cavity.

(C5) In any of methods (C1) - (C4), the method may further include when (i) the fiber cavity is proximate to an igniter tip of the furnace burner and (ii) at least one of: (a) cooling and (b) the temperature is below a predetermined threshold temperature, at least one of: closing a valve that discharges fuel that is burned by the flame, and generating a warning signal.

Modifications may be made to the above-described methods and systems without departing from the scope thereof. It is therefore to be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The various embodiments described above may be combined in any way, and certain individual features of each embodiment may or may not be included in these combinations. Herein and unless otherwise indicated, the adjective "exemplary" means serving as an example, instance, or illustration. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system (which, as a matter of language, might be said to be intermediate between these two.