JP2011145680A - Optical mode noise averaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical mode noise averaging device (300) that includes a multi-mode optical fiber (302), and a means (308) for averaging variations of signal level in an optical level of an optical signal. <P>SOLUTION: The device averages the variations of the signal level induced by mode noise by periodically changing a refractive index of the multi-mode optical fiber (302) for a selected period or scrambling optical distribution in the multi-mode optical fiber (302), or by both of them. The refractive index of the multi mode optical fiber is periodically changed by periodically changing temperature of the multi-mode optical fiber (302). Alternatively, cyclically manipulation of the multi-mode optical fiber (302) enables the refractive index to be changed, or enables the optical distribution in the multi-mode optical fiber to be scrambled. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a combustion monitoring method, and more particularly to an apparatus and method for averaging mode noise associated with a multimode optical fiber.

  Most of the electricity produced in the United States is generated at coal-fired power plants. Similarly, the majority of power generation around the world relies on coal as a primary energy source. Taking into account long-term environmental issues related to the storage of waste from nuclear energy production operations and inefficiencies associated with solar power generation, coal is likely to remain a primary energy source for the time being . In addition to this, there are huge reserves of coal all over the world that are sufficient for energy production over the past 200 years.

  However, there is a continuing and continuing need to reduce pollutant emissions associated with coal-fired power generation and to improve the overall efficiency of the coal-fired power generation process. Traditionally, in power plants and other industrial combustion environments, the efficiency of the combustion process and the level of pollutant emissions have been obtained from extracted gas samples using techniques such as non-dispersive infrared (NDIR) photometry. It has been indirectly determined by measurements. Extraction sampling systems are not particularly well suited for closed-loop control of combustion processes because large delays can occur between gas extraction time points and final analysis time points. Furthermore, the extraction process generally results in a single point measurement that may or may not represent the actual concentration of species measured in the combustion process chamber, which can be made highly variable and dynamic.

  Recently, laser-based optical molecular species sensors have been implemented to address the problems associated with extraction measurement techniques. Laser-based measurement techniques can be implemented “in situ” and can provide the additional advantage of fast feedback suitable for dynamic process control. A particularly promising technique for measuring combustion gas composition, temperature and other combustion parameters is tunable diode laser absorption spectroscopy (TDLAS). TDLAS is suitable for controlling and monitoring the combustion process by coal combustion. TDLAS is also equally suitable for monitoring other combustion processes. In particular, the spectroscopic techniques described herein are suitable for monitoring and controlling jet engine combustion parameters. TDLAS is typically realized by a diode laser operating in the near-infrared and mid-infrared spectral regions. Suitable lasers are readily available for TDLAS applications since they have been extensively developed for use in the telecommunications industry. Various TDLAS technologies have been developed that are somewhat suitable for detection and control of combustion processes. Common known techniques are wavelength modulation spectroscopy, frequency modulation spectroscopy, and direct absorption spectroscopy. Each of these techniques is a laser beam where light is transmitted through a combustion process chamber and received by a detector after being absorbed in a specific spectral band specific to the gas present in the process chamber or chamber. Is based on a predetermined relationship between the quantity and nature of The absorption spectrum received by the detector is used to determine the amount of gas species to be analyzed and combustion parameters such as the associated temperature.

  A typical coal fired power plant has a combustion chamber size of 10-20 meters on a side. The power plant is refueled with pulverized coal, resulting in an extremely bright combustion process that blocks the transmission of laser light due to the high dust load. The environment is also highly turbulent. The overall transmission of light passing through the process chamber will dramatically increase over time as a result of broadband absorption, scattering by particulates, or beam direction changes due to refractive index variations. fluctuate. In addition, there is intense thermal background radiation from the burning coal particulates that can interfere with the detector signal. The environment outside the power plant boiler also makes it difficult to implement a TDLAS detection or control system. For example, any electronics, optical components, or other sensitive spectral components must be spaced from intense heat or properly shielded and cooled. Although it is extremely difficult to implement a TDLAS system under these conditions, TDLAS is particularly suitable for monitoring and controlling a coal combustion process.

  International patent application filed on March 31, 2004, the entire disclosure of which is incorporated herein by reference and referred to as "Method and Apparatus for Combustion Monitoring and Control" As discussed in detail in PCT / US04 / 10048 (publication number WO2004 / 090496), fiber optic coupling is particularly suitable for the implementation of TDLAS systems. In a fiber coupling system, one or more probe beams, which may consist of multiple light beams of various suitable wavelengths, are delivered to the pitch side (transmit) optics and emitted into the combustion chamber. The probe beam is received by the catch side (receiving) optical device after traversing the combustion chamber. As described in detail in the above-mentioned International Patent Application No. PCT / US04 / 10048, it is preferable to use a multimode optical fiber in the catch-side optical train. However, the use of multimode fiber inevitably results in a detected light signal resulting from a non-uniform light distribution that varies in time and wavelength in the core of the multimode fiber used for light collection and transmission. This results in mode noise that is a level change. Mode noise on the “catch” side can obscure the absorption characteristics to be observed for effective TDLAS.

  The phenomenon of mode noise is not limited to or caused by the TDLAS implementation scheme that characterizes the catch-side multimode optical fiber. Conversely, mode noise inevitably occurs in any multimode optical fiber of considerable length that transmits light. Mode noise is inevitable in multimode fibers because the cross-sectional diameter of multimode fibers is large compared to single mode fibers, so that the transmitted light propagates in many optical paths or modes. Because it can. Some paths or modes are longer or shorter than other paths or modes. Therefore, inevitable constructive or destructive interference results, resulting in a non-uniform light distribution that varies in time and wavelength in the core of the multimode fiber, causing a typical mode noise speckle pattern. Therefore, mode noise occurs in computer processing, telecommunications, or other scientific applications that utilize significant lengths of multimode fiber. Whether mode noise hinders the efficiency of a given optical system depends on the requirements of the particular system.

  The present invention is directed to a method for overcoming one or more of the problems discussed above.

  The present invention relates to an optical mode noise average comprising a multimode optical fiber and means for averaging signal level variations of light propagating in the multimode optical fiber and induced by mode noise. Device. The device may cause modal noise by cyclically changing the refractive index of the multimode optical fiber over a selected period, scrambling the light distribution in the multimode optical fiber, or both. Induced signal level variations can be averaged. The refractive index of the multimode optical fiber can be cyclically changed by cyclically changing the temperature of the multimode optical fiber. By manipulating the multimode optical fiber cyclically and physically, the refractive index can be changed, or the light distribution within the multimode optical fiber can be scrambled.

  The temperature of the multimode optical fiber can be changed by the action of a thermal element mounted in thermal communication with the multimode optical fiber. Suitable devices for use as thermal elements include, but are not limited to, thermoelectric modules, resistance heaters, infrared heaters, chemical heaters, customary cooling devices, chemical coolers, ambient temperature A source of fluid that has been cooled low, or a source of fluid that has been heated above ambient temperature.

  The optical device may include a temperature sensor, such as a thermocouple, in thermal contact with the multimode optical fiber, and a controller that receives input from the temperature sensor and controls the thermal element.

  In an alternative embodiment featuring an apparatus for cyclically manipulating the multimode optical fiber, the manipulation includes twisting, stretching or shaking of the multimode optical fiber. Piezo stretchers can be used to achieve the cyclic stretching of the multimode optical fiber. Alternatively, to cyclically twist a portion of the multimode optical fiber with respect to the longitudinal axis of the multimode optical fiber and alternately clockwise and counterclockwise with respect to a fixed portion of the multimode optical fiber. A motor can be used.

  The present invention further provides a method of averaging optical mode noise in a multimode optical fiber, the step of coupling light to the input of the multimode optical fiber, and the refractive index of the multimode optical fiber. The method includes the steps of cyclically changing and receiving the averaged light at the output of the multimode optical fiber. The mode noise averaging method may include changing the refractive index by one of cyclically changing the temperature of the multimode optical fiber and cyclically operating the multimode optical fiber. . The temperature of the multimode optical fiber can be cyclically varied by providing a thermal component in thermal communication with the multimode optical fiber. Alternatively, the multimode optical fiber can be operated cyclically by twisting, stretching, or shaking the multimode optical fiber.

FIG. 1 is a schematic diagram of a TDLAS detection apparatus of the present invention. FIG. 2 is a schematic diagram of a TDLAS detection device featuring remotely located components that are optically coupled to components near the combustion chamber. FIG. 3 is a schematic diagram of an optical mode noise averaging device according to the present invention. FIG. 4 is an exploded view of a thermal phase shift apparatus with a fluid source that is heated above ambient temperature or cooled below ambient temperature as a thermal element. FIG. 5 is an exploded view of a thermal phase shift apparatus that utilizes a series of thermoelectric devices as thermal elements. FIG. 6 is an alternative exploded view of the thermal phase shifter of FIG. FIG. 7 is a schematic diagram of an optical mode noise averaging device that uses a motor for mechanical manipulation of multimode optical fibers. FIG. 8 is a schematic diagram of an optical mode noise averaging device that uses a piezo stretcher for mechanical manipulation of multimode optical fibers. FIG. 9 is a schematic diagram of a pitch-side mode noise reduction device.

A. Overview A preferred embodiment of the present invention is an optical mode noise averaging device. The optical mode noise averaging device is described in detail in paragraph E below. The optical mode noise averaging device is not limited and is specific to a catch-side (ie, receive-side) multimode optical fiber associated with a fiber coupled tunable diode laser absorption spectroscopy (TDLAS) detector This is particularly suitable for the mode noise averaging method. Several embodiments of such a detection device are filed on March 31, 2004, the entire disclosure of which is incorporated herein by reference for “for combustion monitoring and control. It is discussed in detail in International Patent Application No. PCT / US04 / 10048 (Publication No. WO2004 / 090496) entitled “Methods and Apparatus”. In addition, a fiber coupled TDLAS detector is described below. The preferred embodiment of the present invention is suitable for averaging optical mode noise in any optical system where mode noise is present. In particular, the optical mode noise averaging device can be implemented in any computer processing, telecommunications, scientific research or other system featuring a length of multimode optical fiber that transmits light. The averaging device is useful in any optical system where the efficiency of the optical system can be enhanced by averaging the optical mode noise specific to light propagating in a multimode optical fiber.

B. The detection device FIG. 1, the detection of the combustion process, embodiments of the detection device 10 suitable for the monitoring and detection is shown. The detection apparatus 10 performs tunable diode laser absorption spectroscopy (TDLAS) by using laser light from a series of tunable diode lasers 12 that oscillate at a selected frequency in the near-infrared or mid-infrared spectrum. Do. Each output of the tunable diode laser 12 is coupled to a separate optical fiber, which can be a single mode optical fiber 14 and routed to a multiplexer 16. As used herein, the expression “coupled”, “optically coupled”, or “in optical communication with” refers to optical Is defined as the functional relationship between the counterparts in which can proceed from a first component to a second component with or without passing through an intermediate component or free space. In the multiplexer 16, a part or all of the generated laser light is multiplexed to form a multiplexed probe beam having a plurality of selected frequencies. This multiplexed probe beam is coupled to a pitch-side (ie, transmitting) optical fiber 18 and is operatively associated with a process chamber, shown as a combustion chamber 22 in FIG. That is, it is transmitted to the collimator.

  The pitch optic 20 is oriented to emit a multiplexed probe beam through the combustion chamber 22. The catch optical component 24 is in optical communication with the pitch optical component 20 via the combustion chamber 22. The catch optic 24 is preferably located substantially opposite the pitch optic 20 and is operatively associated with the combustion chamber 22. The catch optic 24 is positioned and oriented to receive the multiplexed probe beam emitted through the combustion chamber 22. The catch optic 24 is optically coupled to a catch-side optical fiber 26 that transmits a portion of the multiplexed probe beam received by the catch optic 24 to the demultiplexer 28. Within the demultiplexer 28, a portion of the multiplexed probe beam received by the catch optic 24 is demultiplexed and each wavelength of the demultiplexed laser light is coupled to the output optical fiber 30. On the other hand, each output optical fiber 30 is optically coupled to a detector 32, which typically has a selected frequency of the laser light generated and multiplexed to form its probe beam. This is a photodetector having sensitivity to one of the frequencies. The detector 32 generates an electrical signal based on the nature and amount of light transmitted to the detector 32 at the detector frequency. The electrical signal from each detector 32 is typically digitized and analyzed within the data processing system 34. As will be described in more detail below, this digitized and analyzed data is not limiting and includes physical parameters within the process chamber, including the concentration and combustion temperature of various gas species within the combustion chamber 22. Can be used to detect. Data processing system 34 may further be used to actively control selected process parameters by sending signals to combustion controller 38 through feedback loop 36. For a combustion process, the controlled process parameters may include fuel (eg, pulverized coal) feed rate, oxygen feed rate, and catalyst or chemical addition rate. The use of optical fiber coupling of electronic and optical components on both the pitch and catch sides of the detection device 10 will affect the elaborate and temperature effects such as the tunable diode laser 12, the detector 32 and the data processing system 34. Can be placed in a control room with a stable operating environment. Therefore, only the relatively robust pitch optic 20 and catch optic 24 need be placed near the harsh environment of the combustion chamber 22.

  FIG. 2 schematically illustrates the overall component arrangement of the fiber coupled multiplexed detection system 40. The detection system 40 generally consists of a system rack 42, a breakout box 44, a transmitter head 46 having a pitch optic 48, a receiver head having a catch optic 52, and a connecting optical fiber. The system rack 42 is preferably located in a remote control room located at a distance such as 1 kilometer from the combustion chamber 54. The control room typically has a moderately controlled environment. The system rack 42 includes a laser 56, a detector 58, a wavelength multiplexer 60, and a wavelength demultiplexer 62. The system rack 42 also contains system electronics and control software (not shown in FIG. 2). The system rack 42 may optionally contain an alignment light source 64.

  The optical fiber connecting the system rack 42 to the breakout box 44 is typically a standard single mode telecommunications fiber. This type of fiber is inexpensive, readily available, has low loss, and is compatible with various standards for telecommunications that operate light, such as optical switches, optical splitters, and wavelength division multiplexers. Can direct the laser beam. Without fiber optic coupling, the laser light must always be directed through free space to the combustion chamber 54, which is very difficult to achieve or, alternatively, sensitive electronics and optical components It must be placed in the immediate vicinity of the combustion chamber 54.

  A breakout box 44 is also shown in FIG. The breakout box 44 is a robust hermetic container located near the boiler. Breakout box 44 houses optical switches, optical splitters and optical couplers (collectively 66) that can be used as described below to send optical signals to a plurality of transmitter / receiver head pairs.

  A third group of system components shown in FIG. 2 is a transmitter head 46 and a receiver head 50. The optics and electronics in the transmitter head 46 and receiver head 50 convert the light in the fiber 68 into a collimated beam that is directed accurately through the combustion chamber 54 and on the distal side of the combustion chamber 54. The beam must be captured and coupled into the fiber 70. The choice of optical components to achieve this is determined by the transmission distance, the turbulence in the combustion zone, the effect of this turbulence on the quality of the transmitted beam, and the core size of the fiber 70. The choice of core size is a design choice that depends on the application. Larger cores capture more background light as well as more laser light. When used with a coal fired boiler, a 50 micron fiber core diameter provided acceptable results. Fiber coupling on the catch (receiver) side has several advantages. In particular, only light that is in the same location as the laser light and travels in the same direction is focused into the fiber 70. This dramatically reduces the amount of background light detected. Light can be captured into one of several receiver fibers and an optical switch or other optical routing device redirects light from one fiber for routing to detector 58. You can choose. In FIG. 2, only one catch optic is shown.

  The use of fiber coupling on the catch side requires that the alignment tolerances of both the transmitter and receiver optics be accurately maintained (0. 0 for both transmitter aiming and receiver aiming). Less than 5 milliradians). Preferably, both pitch optic 48 and catch optic 52 are adaptively designed and aberration corrected for wavelengths from 660 nm to 1650 nm so that multiple laser signals can be efficiently transmitted and received simultaneously.

C. Tunable Diode Laser Absorption Spectroscopy Tunable Diode Laser Absorption Spectroscopy (TDLAS) is performed using techniques known to those skilled in the art of laser spectroscopy. In general, TDLAS is performed by transmission of laser light through a target environment, followed by detection of absorption of laser light at a specific wavelength by a target gas such as carbon monoxide or oxygen. Spectral analysis of the detected light can identify the type and amount of gas along the laser path. Details of direct absorption spectroscopy can be found in the “Simultaneous in situ Measurement of CO, H ₂ O, and Gas Temperature in a Full of Teichert, Fernholz and Ebert, incorporated herein by reference in their entirety. -Sized, Coal-Fired Power Plant by Near-Infrared Diode Lasers, "(Applied Optics, 42 (12): 2043, 20 April 2003). Due to the non-contact nature of laser absorption spectroscopy, the laser absorption spectroscopy described above can be used in harsh environments such as combustion areas of coal-fired power plants or flammable or toxic environments where other probes cannot be used. Is adapted to. Using laser light, the high required to receive a detectable transmission in the presence of extreme attenuation (typically greater than 99.9% light loss) that can be encountered in some of these environments. Brightness is provided. In order to better withstand the harsh conditions of the target application, the laser light can be brought into the target environment through a protected optical fiber.

Effective detection of temperature or multiple combustion process component gases requires the implementation of TDLAS using widely spaced laser beams of multiple frequencies. The selected frequency must match the absorption line of the transition being monitored. For example, it is useful to monitor NO ₂ at a wavelength of 670 nm to approximate the emitted NO concentration. Furthermore, it is extremely useful to monitor oxygen, water (temperature) and carbon monoxide in a coal-fired utility boiler. Appropriate absorption lines, and hence the appropriate lasing frequency, have a laser probe path length through the combustion chamber equal to 10 meters, and the molar fraction of each species is CO (1%), O ₂ (4%), CO ₂ (10%), and H ₂ O (10%). For frequency selection, the process temperature can be assumed to be 1800K, which is slightly higher than the temperature typically found in coal-fired power plants, but this margin serves as a safety factor in the calculation.

For example, three water absorption lines that meet the following criteria may be selected for TDLAS.
1. Lower state energies of ˜1000, 2000, and 3000 cm ⁻¹ , respectively.
2. Providing a suitable absorbance of about 0.1-0.4 resulting in about 20% beam absorption at resonance.
3. The optimal situation is to use transitions in the 1250 nm to 1650 nm range where cheap, high power DFB diode telecommunications lasers can be used.
4). Transitions should be properly separated to allow easy multiplexing.
5. The selected wavelength should be efficiently diffracted by the existing (de) multiplexer grating.

Appropriate water absorption occurs at the following wavelengths:

Interference from any other combustion gases is not assumed. CO ₂ , which is the most likely species of interference, is modeled, and there are no strong interference lines in the 1.3-1.4 micron range.

  Similarly, a suitable carbon monoxide line may be selected based on the above-mentioned Ebert study mentioned and incorporated. A suitable carbon monoxide line is found at 1559.562 nm using the R (24) line in a coal fired utility boiler. Selecting this line avoids interference from water and carbon dioxide. Since this wavelength range is in the optical communication C band, the known grating is very efficient within this wavelength range. The absorbance of CO at this wavelength is expected to be 0.7%.

  Furthermore, oxygen can be measured at 760.0932 nm. The preferred (inverse) multiplexing grating efficiency is calculated to be only 40% within this range, but appropriate laser power should be available for reasonable measurement efficiency.

  As discussed herein, the use of fiber coupling on both the pitch side and the catch side of the TDLAS detector requires a tight alignment of the pitch and catch optics. Active alignment is preferably performed using a selected alignment wavelength. One possible alignment wavelength is 660 nm because high power (45 mW) diodes are available at this frequency and 660 nm is near the peak of 14th order grating operation. Other alignment wavelengths that are equally suitable or even more suitable may be determined.

In summary, the appropriate wavelength groups selected to multiplex into the probe beam for the TDLAS embodied in the present invention are shown in Table 2. It should be noted that this wavelength group is for one embodiment of a TDLAS detector suitable for detection and control in a coal-fired power plant. Other wavelength groups may be equally suitable.

D. Unique Advantage of TDLAS Using Multiplexed Beam A specific advantage of TDLAS using a wavelength multiplexed probe beam is improved accuracy of temperature measurements. In order to perform accurate concentration measurements using TDLAS, the temperature of the monitored gas must be known. The intensity of molecular absorption is a function of temperature. Thus, in order to convert the absorption feature amplitude into concentration, the temperature must be known. Several previous attempts to measure the concentration of combustion species such as CO have the disadvantage of insufficiently accurate temperature measurements that cause errors in quantification. This is especially true in the case of ammonia slip monitoring based on diode lasers, which do not include any temperature measurements by convention. In the detection system of the present invention, the temperature can be determined by measuring the ratio of the intensity of two or more molecular water lines. The ratio of the integrated intensities of these two lines is a function of temperature only (assuming that the overall system pressure is constant). Thus, in principle, the two lines give the correct temperature. However, in the case of a non-uniform temperature distribution (as typically seen in industrial combustion processes), the two lines are insufficient to determine the temperature distribution. In the case of such a non-uniform temperature distribution, the two lines can only determine the “path-averaged” temperature. In contrast, measuring the integrated amplitude of more than two lines (of the same species) can investigate temperature non-uniformities. An example of this technique is illustrated by the use of oxygen as a probe molecule by Sanders, Wang, Jeffries and Hanson, the entire disclosure of which is incorporated herein by reference (Applied Optics, 40 (24): 4404, 20 August 2001). This preferred technique is identical to the distribution of peak intensities measured along the line of sight for a path at an average temperature of 500K, for example when one half of the path is 300K and the other half is 700K. Based on the fact that it is not.

  In addition to the benefits of more accurate temperature measurement, the use of multiplexed probe beams allows for more precise control throughout the combustion process by allowing simultaneous monitoring of two or more combustion gas species.

E. The optical train of a mode noise TDLAS system and similar implementations that require signals multiplexed from wavelengths that are widely separated from each other are conflicting designs of mode noise reduction and high efficiency light collection. There are many design challenges depending on the requirements. Mode noise herein varies with time and wavelength in the core of the fiber used to collect light from the process chamber being measured and to transmit light to the process chamber. Defined as a change in the signal level of the detected light resulting from a non-uniform light distribution.

In multimode fiber, different modes propagate at different speeds due to refractive index variations. In that case, the intensity distribution in the fiber is a speckle pattern resulting from the interference of all propagation modes through different effective path lengths. If all the light of this speckle pattern is collected and detected, constructive and destructive interference cancel each other out exactly, and the total transmitted power is independent of wavelength or fiber length. However, if clipping, ambient dimming or other losses are introduced, accurate cancellation is not performed and the detected output varies with wavelength and / or time. In the TDLAS detection system as described above, the output change resulting from mode noise is very problematic. Some spectroscopic techniques are based on the absorption of specific wavelengths of light by the gas species considered. The absorption is detected by a decrease in power at a critical wavelength. Thus, mode noise can exhibit power loss due to absorption and can obscure data collected via TDLAS. The general formula for the output detected after the fiber length z is
P = P ₀ + Σ _ij c _ij E _i E _j cos [(2πν ₀ Δn _ij z / c + Δφ _ij (T, σ))]
(1)
Where
P ₀ = wavelength independent average output,
E _i = amplitude of light in the _i -th transverse mode,
c _ij = overlap integral between i-th and j-th transverse modes,
Δn _ij = difference of refractive index between i-th and j-th modes,
Δφ _ij = phase shift between the i-th mode and the j-th mode due to temperature and stress.

For the orthonormal group mode and lossless, c _ij = 0. However, any beam clipping or marginal dimming or any other mode dependent loss will cause some c _ij ≠ 0. This results in a ripple in the average transmission output.

For a typical graded-index fiber with a 50 micron core, the total index change Δn is ˜1%, but most modes have large transmission times near the center of the fiber core. Expendily, and therefore roughly Δn _ij ≦ 0.0005. The commonly available fiber GIF 50 supports about 135 modes, which is sufficient to produce significant mode noise during wavelength scanning, assuming a reasonably achievable beam clipping level. As rough as possible.

As an example of mode noise, consider the simplest feasible system that exhibits mode noise, ie, a rectangular waveguide that supports only the lowest mode in one dimension and only the two lowest modes in the orthogonal dimension. Can be:
Lowest mode: E ₁ = E ₁ ⁰ [expi (kz−ωt)] cos πx / 2a
Next mode: E ₂ = E ₂ ⁰ [expi (kz−ωt)] sin πx / a
It is.
The intensity at point z along the fiber is
I (x) = | E ₁ + E ₂ | ² and the total output is P = ∫ | E ₁ + E ₂ | ² dx (2)
Where the integral must include the effects of clipping and ambient dimming.

Without clipping, P to E ₁ ² + E ₂ ² and there is no wavelength dependence. Adding clipping will change the limits of that integration. It can be shown that clipping results in an additional term ~ E ₁ E ₂ cos ΔΦ, where ΔΦ = ΔkL = 2πΔnL / λ.

  If single mode fiber can be used in the optical train of the system described above, mode noise is not an issue. However, multimode fiber typically must be used in the catch-side optical train of a fiber coupled TDLAS system for two reasons. First, after traversing the measurement volume (combustion chamber with a measurement path exceeding 10 meters), the beam that is initially single mode (Gaussian spatial distribution) is significantly degraded. Therefore, the efficiency of coupling this highly distorted beam into a single mode fiber is very low. This is an unacceptable situation because the beam is attenuated 3-4 orders of magnitude when passing through the measurement volume, mainly due to scattering and occultation by hail and fly ash. . Increased attenuation resulting from the use of a single mode fiber will hinder measurement. Second, the refractive beam steering effect in the fireball makes the beam position and aim unstable. Given this effect, it would be difficult to “hit” the single-mode fiber core with some regularity.

  On the other hand, the core of a multimode fiber has a target cross-sectional area that is more than 25 times the target cross-sectional area of a single mode fiber. Hence, the beam directing effect can be significantly reduced. Furthermore, since the coupling efficiency into a multimode fiber is independent of the spatial mode of light, the low beam quality obtained after passing through a fireball is not a problem.

  Computer processing, telecommunications or other types of implementation in the general scientific field have other similar or completely unrelated limitations that require or favor the use of significant lengths of multimode optical fiber. Can have. In other implementations, mode noise is a problem and can present considerable challenges for data collection or data transmission.

  Therefore, the mode dependent loss that occurs in a multimode fiber train is a major design challenge. The light distribution emanating from the core of a multimode fiber exhibits a random speckle pattern, i.e., random in bright and dark areas due to constructive and destructive interference between different modes of the fiber. Indicates a pattern. If this speckle pattern is generally constant as a function of time and wavelength, this speckle pattern will not present a problem. However, as mentioned above, if the beam is clipped somewhere in the multimode catch-side optical train, a slow change in speckle pattern, especially as a function of wavelength, can cause mode noise. It is impossible to avoid this clipping and it can only be reduced. Therefore, in order to improve the detection sensitivity of the system, additional measures must be taken to reduce mode noise.

There are several ways to mitigate mode noise. From the above equation (1), the mode noise is
1. Reduce mode dependent loss, ie keep c _ij small by reducing clipping,
2. By reducing z, the period of mode noise is increased so that it is significantly larger than the absorption line to be examined,
3. △ n _ij is reduced by using low dispersion fiber,
4). Scrambling or phase shifting modes (but not all mode scrambling or phase shifting techniques are equally effective as described below)
Can be reduced.

Preferably, the catch optics of the fiber coupled TDLAS detection system is designed and implemented to include all of the above to reduce mode noise. These optical components are designed so that any beam clipping occurs at a low level, assuming a nearly perfect alignment of the system. Efforts should be made to keep the length of the multimode fiber to a minimum, but depending on the application, z must be long in order to have the control electronics in a conditioned area. The value of Δn _ij can be reduced by using very high quality, low dispersion multimode fiber. In addition, each mode is averaged by cyclically changing the refractive index of the catch-side multimode fiber or mechanically manipulating the fiber, and data is collected from the averaged optical signal collected. By extracting, excellent results can be achieved.

  The speckle pattern exhibited by a multimode fiber varies as a function of time and wavelength and also as a function of the physical position of the fiber. Both transmission time and wavelength are affected by the refractive index of the fiber. As the fiber is bent and the fiber is manipulated in a particular fashion, the speckle pattern can be changed. If this mechanical manipulation or cyclical change in refractive index occurs continuously over a period of time, the spatial distribution of light emitted from the fiber is averaged into a relatively uniform pattern.

A time averaged measurement by effectively cyclically phase shifting or scrambling the mode noise produces a uniform signal level. The refractive index of a fiber can be changed by stretching or twisting the fiber, or by changing the temperature of the fiber. Changing the temperature of the fiber causes a change in the refractive index difference Δn _ij between the i-th and j-th transverse modes. This change in the refractive index of the fiber shifts the mode noise due to the function cos (2πν ₀ Δn _ij z) / c given in Equation 2.

F. Optical Mode Noise Averaging As shown schematically in FIG. 3, cyclic phase shifting or scrambling of mode noise to produce a time averaged measurement can be achieved by the optical device 300. The optical device 300 includes a multimode optical fiber 302 having an input unit 304 and an output unit 306. Light may be coupled to the input 304 of the multimode optical fiber 302 and may propagate through the system in the direction of the arrows shown in FIG. 3 in general relation to the input 304 and output 306.

  The optical device 300 also includes an averaging component 308 operatively associated with the multimode fiber 302. The averaging component 308 may include a device that cyclically changes the refractive index of the multimode optical fiber 302 over a selected period of time. Alternatively, the averaging component 308 can include an apparatus for scrambling the light distribution within the multimode optical fiber 302. Refractive index change or light distribution scrambling may be achieved by averaging component 308 by cyclical changes in temperature of multimode optical fiber 302, cyclic manipulation of multimode optical fiber 302, or both. .

  In embodiments where the averaging component 308 performs a cyclic operation of the multimode optical fiber 302, the averaging component 308 can twist, stretch or vibrate the multimode optical fiber 302. In embodiments where the averaging component 308 cyclically changes the temperature of the multimode optical fiber 302, various thermal or thermal components in thermal communication with the multimode optical fiber can be deployed. Any device that affects the temperature of the multimode optical fiber 302 may be included in the averaging component 308. Typical devices that can be used to affect the temperature of the multimode optical fiber 302 include conventional cooling devices that utilize thermoelectric modules, resistance heaters, infrared heaters, chemical heaters, compressed fluids and heat exchangers. Chemical coolers, sources of fluid cooled below ambient temperature, and sources of fluid heated above ambient temperature. Some of these devices are discussed in detail below.

  In embodiments where the averaging component 308 causes cyclic heating or cooling of the multimode fiber 302, a sensor 310 that is in thermal communication with the multimode optical fiber 302 may also be mounted. Sensor 310 may provide information to controller 312 that may control averaging component 308 via control line 314.

G. Thermal Phase Shifter The effectiveness of the thermal mode phase shift is directly related to the change in temperature per unit time and the length z of the fiber exposed to the temperature change. Thermal mode shifting is a particularly effective method of dealing with mode noise, because changing the fiber temperature changes the refractive index of all transverse modes and the temperature change is a significant length of the fiber. This is because it can occur in Thus, by changing the refractive index of the fiber, all transverse modes can be reliably shifted and no mode can remain “fixed” with the signal.

  Virtually any type of heating / cooling system can be placed in thermal communication with the multimode fiber to circulate the temperature of the fiber. Electrical resistance heaters, custom cooling devices, heated or cooled fluids, Peltier or other thermoelectric devices, infrared devices or chemical devices can all be used to affect the temperature of the fiber.

  One example of a mode phase shift device that utilizes a cyclic temperature change is a fluid mode phase shift device (fluid device) 400. FIG. 4 shows an exploded view. FIG. 4 shows an embodiment of a fluidic device 400 that uses vortex air tubes 402A, 402B that alternately deliver hot and cold air from one end of the fiber to the other and around the fiber. Air supplied from a compressed air source (not shown in FIG. 4) is alternately supplied to one of the two vortex tubes 402A and 402B. The vortex tubes 402A, 402B are coupled in fluid communication with the chamber 404. As shown in FIG. 4, the chamber 404 may be formed by a sealed container 406 having sides 408A, 408B, a top plate 410, a front 412 and a rear access port 414. The entire sealed container 406 is suitable for mounting to a typical data processing equipment rack. Although a rack mountable enclosure 406 is particularly suitable, any enclosure shape, type, or model that can receive a spool 416 that holds a length of multimode optical fiber 418 to implement the fluidic device 400 is acceptable. Can be used. Alternatively, devices that do not include a sealed container 406 can be used.

  Since the vortex tubes 402A and 402B are in fluid communication with the interior of the sealed container via the access port 414, the vortex tubes 402A and 402B are in fluid communication and thermal communication with the multimode optical fiber 418 wound on the spool 416. Thus, the multimode optical fiber 418 can be heated and / or cooled cyclically by applying air heated above or below ambient temperature and cooled by vortex tubes 402A, 402B.

Suitable vortex tubes 402A, 402B are readily available. For example, EXAIR® 3230 vortex tube is available from EXAIR®. These or similar vortex tubes operating at a throughput of 30 ft ³ / min can provide air heated to + 60 ° C. or cooled to −20 ° C., depending on the tube orientation. In addition, when using a vortex tube, it is relatively easy to circulate between heated and cooled air. However, it should be noted that any device or method that provides a heated or cooled fluid in thermal communication with the multi-mode optical fiber 418 in a circulating manner to implement an embodiment of the fluidic device 400 is appropriate. This is very important. The heating and cooling fluid may be air as discussed above, but water, heating / cooling oil, compressed gas or other fluids may be used to heat or cool the multimode fiber.

  By way of example and not limitation, during operation, one of the vortex tubes 402A, 402B may deliver heated air until the fiber reaches a temperature that is about 10 ° C. above the inlet temperature. The temperature of the fiber may be determined by a thermocouple 420 or other temperature sensor incorporated in contact with the fiber. The temperature control unit (not shown in FIG. 4) receives input from the thermocouple 420 and triggers the solenoid switch so that air is sent to the other of the vortex tubes 402A, 402B for cooling. obtain. Alternatively, since the heated air provided to the multimode optical fiber 418 by the vortex tubes 402A, 402B never reaches a critical level, the use of a temperature controller is eliminated and the vortex tube between heating and cooling is eliminated. It can be replaced by a timing relay that cycles between 402A and 402B. The circulation between each temperature is preferably continuous during operation of the device.

  5 and 6 show a highly preferred embodiment of a mode phase shift device based on a cyclic change in the temperature of a multimode fiber. A thermoelectric mode phase shift device (thermoelectric device) 500 includes a spool 502 that holds a multimode optical fiber 504 of a selected length. One or more thermoelectric heating / cooling modules 506 are placed in thermal communication with the multimode optical fiber 504. In the embodiment shown in FIG. 5, a plurality of thermoelectric heating / cooling modules 506 are arranged radially around the inner portion of the spool 502. Thermal communication is promoted between the outer surface of the thermoelectric module 506 and the coil of the multimode optical fiber 504 by using thermoelectric grease. In the embodiment shown in FIG. 5, the spool 502 is configured with an open edge to facilitate contact between the thermoelectric module 506 and the multimode optical fiber 504 and to wrap the multimode optical fiber 504. Promote.

  The exploded view of FIG. 6 shows one arrangement for positioning the thermoelectric module 506 relative to the spool 502 in contact with the multimode fiber 504.

  One or more heat sinks 508 may also be operatively disposed for the thermoelectric module 506. Preferably, the heat sinks 508 are made from a highly thermally conductive material, such as aluminum or copper, and have fins or other features designed to increase the surface area of each heat sink 508. The fan 510 is operatively positioned to energize or draw air across the heat sink 508 so that heat can be quickly extracted from the thermoelectric module 506 and hence the multimode fiber 504 can be heated quickly or Cooling is promoted. As shown in FIGS. 5 and 6, the top ring is used to hold the components of the thermoelectric device 500 in proper orientation with respect to each other without disturbing the air flow around and around the heat sink 508. A frame structure 512 having 514 and a bottom ring 516 may be employed. Preferably, an opening 518 is formed in the frame structure 512 to ensure a free air flow.

  The embodiment shown in FIGS. 5 and 6 uses a thermoelectric module 506 based on the Peltier principle. Direct current is provided to the thermoelectric module 506 via the lead wire 520. According to the thermoelectric module 506 operating on the Peltier principle, both sides of the thermoelectric module 506 are heated or cooled depending on the direction of the DC current provided. Thus, these modules provide certain advantages, because the multimode optical fiber 504 is selectively heated or cooled by selectively switching the polarity of the direct current provided to the lead 520. Because it is relatively easy. However, it is important to note that other types of devices can be implemented to heat and / or cool the multimode optical fiber 504. For example, direct resistance heaters, conventional cooling devices, infrared heating devices, and / or chemical reaction based heaters and / or coolers may be used to change the temperature of the multimode fiber 504.

  In the preferred embodiment as shown in FIGS. 5 and 6, power can be delivered from a suitable power source to the thermoelectric module 506 mounted in a cylindrical configuration. Current switching electronics can be used to cyclically reverse the polarity of the DC power delivered to each thermoelectric module 506. A mode phase shift occurs in a length of multimode fiber 504, preferably a Premium GIF 50 multimode fiber, which is in thermal communication with thermoelectric module 506. In order to achieve mode phase shift and averaging, it has been found that 55-100 m is a suitable length for multimode fiber. Other lengths may be appropriate. If a wound 100 m fiber is used, about 50% of the fiber is in direct contact with the thermoelectric cooler. Note that with direct contact, there is no conductive material between the thermoelectric module 506 and the fiber 504 other than the thermoelectric grease. With this configuration, the thermal mass of the system is minimized. By minimizing the thermal mass, the temperature response of the system is quick and a more efficient mode phase shift is performed.

  One or more thermocouples 522 or other temperature measurement devices are mounted between the thermoelectric module 506 and the multimode optical fiber 504 and can be used to constantly monitor the temperature of the multimode optical fiber 504. A temperature control unit (not shown in FIGS. 5 and 6) may receive the temperature determined by the thermocouple and cycle the current direction based on the temperature reading. With a readily available thermoelectric module, a temperature range of 35 ° C. to 50 ° C. can be achieved. It is important that the fiber does not exceed a maximum temperature of 85 ° C or damage to the fiber does not occur. Tests were completed with temperature changes from 65 ° C. to 10 ° C. with a single-sided cooler and 65 ° C. to 30 ° C. with a double-sided cooler. Although a full cycle can be any selected lifetime, a cycle of about 25 seconds has been found to be effective.

  As described above, heat may be dissipated to the opposing surface of the thermoelectric module 506 by a heat sink 508 mounted in thermal communication with the thermoelectric module 506. To help dissipate heat equal to the input power, energized air can be delivered or aspirated through the fins of the heat sink. Efficient heat dissipation can be achieved by energizing air through openings 518 in the bottom of the fins so that air flows through the system and out of the top of the unit. Other configurations that allow sufficient airflow are also suitable. Alternatively, the thermoelectric device 500 can be immersed in the cooling fluid or otherwise cooled. Preferably, the fan is in steady operation while the device is operating. Although any suitable fan or fluid source may be utilized to dissipate heat, a 300 CFM fan has been found effective to remove heat from the systems shown in FIGS. It was done.

  Control electronics can be combined for heating and cooling of the thermoelectric module 506. The feedback control electronics may detect the temperature of the fiber based on input from a thermocouple 518, thermometer or other temperature sensor. In addition, based on the temperature input, the controller can switch the current direction of power delivered to the thermoelectric module 506 and adjust the power level for heating and cooling cycles (typically heating is more efficient). So the required power can be further reduced). Similarly, the controller can control the maximum and minimum temperatures applied to the fiber optic and can shut off the drive circuitry in the event of overheating.

H. Test of Phase Shift System with Thermoelectric Module A test was performed with the thermoelectric device 500 described above. Tests were performed by throwing four wavelength bands through a chamber that had no absorption and was purged with nitrogen. Without absorbing species in the path, the laser must exhibit a linear wavelength response after being divided by the reference signal. The gradient linearity deviation is mainly caused by mode noise. A schematic representation of the equation used to determine the uncertainty in the measurement is given by:
σ _x = [1 / N Σ (x _i −f _i (ax + b)) ² ] ^1/2 (3)
Where
x _i = signal _i / tap _i
f _i (ax + b) = “linear fit of x _1−n ”.

  The start and end of each wavelength cycle can be ignored due to the time delay caused by the pitch head and receive head traversal times. These time delays cause considerable variation between the tap and the observed signal at the beginning and end of each cycle.

System performance tests were performed with examples of thermoelectric devices 500 using single-sided and double-sided thermoelectric modules 506, multiple averaging times and different fiber lengths. For all results, reduced mode noise signal deviation was generated by mode phase shifting and averaging. The noise reduction depends on the length of the fiber used in the experiment as given in equation (3), where the long fiber has a more frequent mode noise deviation than the short fiber. Therefore, the relative resolution increase due to mode phase shift is further clarified in long fibers. The results are shown below for various configurations.

A device based on mechanical manipulation As discussed above, mode noise can be obtained by cyclically changing the refractive index of a multimode fiber or by mechanically manipulating the multimode fiber and collecting data from the average optical signal collected. Can be averaged and smoothed. The thermal phase shift apparatus discussed above achieves a mode phase shift by changing the refractive index of the multimode fiber via a cyclic temperature change. As discussed below, mechanical manipulation of multimode fibers can also be employed to change the refractive index. In addition, mechanical manipulation allows the light affected by the mode noise to be averaged and smoothed because the light cannot completely follow a particular mode in the waveguide when the fiber is manipulated. Result. Thus, averaging and smoothing of speckle patterns caused by mode noise in a given length of multimode fiber can be achieved by a combination of phase shifting and mechanical scrambling.

  Some specific modes of mechanical fiber operation are more effective than others in averaging mode noise. In particular, twisting the fiber about the longitudinal (z) axis of the fiber relative to some other point on the fiber causes a change in the speckle pattern. The dominant change obtained is the rotation of the speckle pattern around the z-axis. The fact that the pattern does not rotate about the axis is as important as the fiber is mechanically rotated. A secondary effect is that the actual light dispersion is somewhat altered by the rotation. Although the rotation of the speckle pattern is not primarily due to stress-induced changes in the refractive index in the fiber, this refractive index change can account for slight changes in the speckle intensity pattern. Rather, the rotation is due to the fact that light cannot follow the waveguide completely when the waveguide is manipulated in the form of a twisting motion.

  FIG. 7 schematically illustrates one embodiment of the present invention, a mechanical mode noise averaging device (mechanical device) 700. The mechanical device 700 uses a hollow shaft motor 702 through which a multimode fiber 704 is mounted and clamped. The remote section of fiber 706 is held firmly with respect to the shaft position of the motor 702, and the motor is repeatedly swept by a twisting action, preferably an action of + 360 ° and then -360 °. Is done. The frequency of this operation is preferably 10 Hz or more, so that the transmitted signal can be efficiently averaged and the mode noise on the catch side can be considerably reduced. Although it has been determined that twisting the fiber along the longitudinal axis of the multimode fiber is effective for modal noise scrambling, other mechanical manipulations such as shaking, stretching or bending may also be employed.

Stretching a piezo stretcher optical fiber results in changes in both the refractive index and the length of the fiber. Multimode fiber can be stretched by a piezo stretcher. Piezo devices are generally used to introduce a modulated time delay into a single mode fiber. Multimode fiber is not used in piezo stretching devices because the time delay in multimode fiber cannot be controlled because of the multiple paths or modes that light can traverse. However, a piezo stretching device can be used to introduce a mode phase shift, even if it is impractical to generate a time delay.

When a multimode fiber is stretched, the stress introduced to the fiber causes changes in both the refractive index and the length of the fiber. As schematically shown in FIG. 8, the piezo device 800 has several meters of multimode optical fiber 802 wound around each half cylinder 804 and then each half cylinder at a predetermined vibration frequency and distance. It works by vibrating 804. As the distance between each semi-cylindrical body 804 expands and contracts, the stress in the fiber 802 harmonizes. Due to this vibration, the refractive index of the fiber 802 is changed. The effectiveness of the mode shift is a function of both the change in the fiber length (z) and the change in the fiber refractive index (Δn _ij ) (Equation (1)).

  The mode phase shift can be achieved by one of two techniques using the piezo device 800. For the first technique, the piezo device 800 should be implemented with sufficient fiber 802 and introduce sufficient stress so that a uniform signal level is achieved by averaging many modes. By constructing, a large degree of mode change is generated. Alternatively, since the piezo device 800 repeats a constant refractive index change, the piezo device 800 potentially produces a 180 ° harmonic mode shift at the minimum and maximum extension distances. Can be manipulated to vibrate in a manner. With this method, mode noise can be reduced by optimizing the stretch characteristics so that a 180 ° phase shift is achieved, rather than by time averaging multiple mode shifts. Therefore, in that case, the mode noise is averaged in only one cycle, so that quick data acquisition without the mode noise can be allowed.

Since single mode beam generation is required for all wavelengths to be transmitted through the pitch side optical train measurement region, the pitch side optical train of fiber coupled TDLAS detectors also presents significant design challenges. If single mode fiber is available in the entire pitch side optical train, mode noise is not a problem. However, the fiber operates as a single mode waveguide only for a limited wavelength window. Beyond the short wavelength cutoff for a particular fiber, light can be transmitted through the fiber in several higher order spatial modes. When light exits the fiber, these higher order modes interfere to produce a speckle pattern. This speckle pattern varies with time and wavelength. In that case, even a small amount of beam clipping causes noise in the measurement.

  Conversely, if a fiber with a single mode cut-off that matches the shortest wavelength that needs to be transmitted is selected, the additional longer wavelengths will suffer considerable loss when coupled into the fiber. The fiber exhibits a large bending loss for further longer wavelengths.

  This problem is a serious problem in the above-described fiber-coupled wavelength multiplexing TDLAS detection / control apparatus because it is necessary to multiplex a wavelength as long as 1.67 microns with a wavelength as short as 760 nm or 670 nm. It is. There is no known single commercial fiber that achieves single mode operation, high coupling efficiency, and low bending loss over such a wide range of wavelengths. In the future, photonic crystal fibers may provide a solution to this challenge, but photonic crystal fiber technology is currently in an early stage.

  As shown in FIG. 9, the problem of light multiplexing and pitching in single mode beams from 670 nm or 760 nm to 1670 nm is due to higher order spatial modes for wavelengths shorter than single mode cutoff. It can be minimized by utilizing a very short transmission section of multimode fiber 120 that does not allow progress. Referring to equation (1), mode noise will be minimized if the length L of the multimode fiber is short. In this case, even if 760 nm light is coupled to a short section of a single mode fiber (eg Corning SMF28) having a cutoff wavelength of 1280 nm, the 760 nm light is single over at least a few meters. Remain in mode. Therefore, the solution to pitch-side mode noise is that 760 nm light is transmitted through the measurement area in a fiber that can be single mode for wavelengths longer than 1280 nm but multimode for 760 nm. The 760 nm light is coupled only to a short distance to be traversed before being collimated as much as possible.

  A schematic diagram of such a system is shown in FIGS. Referring to FIG. 9, a plurality of diode laser sources 902 that oscillate at widely spaced lasing frequencies are coupled to individual single mode optical fibers 904A-904n. Diode lasers that oscillate at wavelengths between 1349 nm and 1670 nm are multiplexed by multiplexer 906. The output of the multiplexer 906 is coupled to a pitch-side optical fiber 908 having dimensions suitable for transmitting light having a wavelength in the range of 1349 nm to 1670 nm without significant transmission loss and without mode noise. A suitable optical fiber for these wavelengths is Corning SMF28. However, if the 760 nm input is multiplexed and coupled into an SMF28 optical fiber, it becomes multimodal after transmission over a relatively short distance. Thus, the output of the 760 nm laser is coupled to a fiber such as SMF750 that is single mode for wavelengths below 1280 nm. Laser light transmitted through the input fiber 904n and multiplexed laser light transmitted through the optical fiber 908 on the pitch side can be combined in the vicinity of the pitch optical component 910. Coupler 912 and pitch optics 910 are preferably optically connected by a short transmission optical fiber 914 that is selected to transmit all of the combined and multiplexed wavelengths without significant loss. A suitable transmission optical fiber for the system shown in FIG. If the transmission optical fiber is relatively short, the 760 nm laser light coupled to the transmission optical fiber 914 does not show that much multimode behavior. For the system and fiber shown in FIG. 9, it has been determined that the transmission optical fiber should be maintained at a length of 3 meters or less to avoid significant multimode noise.

  A similar system is shown in FIG. 2, where the coupler 134 receives input from both a 760 mm diode laser and a multiplexed beam from a diode laser having a fairly long wavelength.

  The objectives of the present invention have been fully realized by the embodiments disclosed herein. Those skilled in the art will appreciate that various aspects of the present invention can be achieved through different embodiments without departing from the essential function of the invention. The specific embodiments are illustrative and are not intended to limit the scope of the invention as set forth in the appended claims.

Claims (10)

A multimode optical fiber;
Means for averaging signal level fluctuations of light propagating in the multimode optical fiber and induced by mode noise, the averaging means being in thermal communication with the multimode optical fiber Comprising a thermal element, the thermal element comprising at least one of a heater, a cooler, a source of fluid heated above ambient temperature, and a source of fluid cooled below ambient temperature, Averaging means;
A temperature sensor in thermal contact with the multimode optical fiber;
A controller that receives input from the temperature sensor and controls the thermal element to periodically change the temperature of the multimode optical fiber;
An optical device comprising: A method of time averaging signal intensity fluctuations induced by mode noise in a multimode optical fiber having an input part and an output part,
Coupling light to the input of the multimode optical fiber;
Periodically changing the temperature of the multimode optical fiber using a thermal component in thermal communication with the multimode optical fiber;
Comprising a method. Providing a temperature sensor in thermal communication with the multimode optical fiber;
Controlling the thermal component with a controller that receives input from the temperature sensor;
The method of claim 2, further comprising: An optical device for averaging signal level fluctuations of light induced by mode noise,
A multimode optical fiber;
A thermal element in thermal contact with the multimode optical fiber;
A controller configured to periodically change the temperature of the thermal element;
An optical device comprising: A temperature sensor in thermal contact with the multimode optical fiber;
Further comprising
The controller receives input from the temperature sensor to control the thermal element;
The optical device according to claim 4. A heat sink in thermal contact with the thermal element;
A fan in fluid communication with the heat sink;
The optical device according to claim 4, further comprising:   The optical device of claim 4, further comprising a spool that supports the multimode optical fiber of a selected length in substantial thermal contact with the thermal element.   The optical device according to claim 4, wherein the selected length of the multimode optical fiber is 55m to 100m.   The thermal element is a thermoelectric module, resistance heater, infrared heater, chemical heater, cooling device, chemical cooler, source of fluid cooled below ambient temperature, and heated above ambient temperature The optical device according to claim 4, comprising at least one of a source of fluid. A combustion detection device comprising a catch side optical system,
A multimode optical fiber;
Means for averaging signal level fluctuations of light propagating in the multimode optical fiber and induced by mode noise, the averaging means being in thermal communication with the multimode optical fiber Comprising a thermal element, the thermal element comprising at least one of a heater, a cooler, a source of fluid heated above ambient temperature, and a source of fluid cooled below ambient temperature; Averaging means;
The averaging means controls the thermal element to periodically change the temperature of the multimode optical fiber;
Combustion detection device.

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