KR101212439B1 - optical mode noise averaging device - Google Patents

Abstract

Disclosed is an optical mode noise averaging apparatus 300 comprising a multimode optical fiber 302 and means 308 for averaging mode noise due to signal level changes of light propagating within the multimode optical fiber 302. The device may periodically vary the refractive index of the multimode optical fiber 302 during the selection period and scramble the light dispersion within the multimode optical fiber 302 to average the mode noise induced signal level changes. The refractive index of the multimode optical fiber may be changed periodically by periodically changing the temperature of the multimode optical fiber 302. Alternatively, by manipulating the multimode optical fiber 302 periodically, the refractive index may be modified or light scattering within the multimode optical fiber 302 may be scrambled.

Multimode fiber, mode noise, optical device, averaging, refractive index change, temperature change

Description

Optical mode noise averaging device {OPTICAL MODE NOISE AVERAGING DEVICE}

TECHNICAL FIELD The present invention relates to combustion monitoring, and more particularly, to apparatus and methods for averaging mode noise associated with multimode fiber.

Most of the electricity produced in the United States comes from coal-fired power plants. Similarly worldwide, the placenta of electricity production depends on coal as the main energy source. Coal is likely to remain the main source of energy in the near future given the long-term environmental problems associated with waste generated by nuclear power generation and the inefficiencies associated with solar power generation. In addition, huge global coal reserves are sufficient to produce energy for at least 200 years, even at current rates.

However, there has been a strong need to reduce the emissions of contaminants associated with coal-fired power generation and to increase the overall efficiency of the coal-fired power generation process and will continue. Traditionally, in power plants and other industrial combustion facilities, the efficiency of the combustion process and the degree of pollutant emissions are indirectly measured by measuring a gas sample taken using techniques such as non-dispersive infrared (NDIR) photometry. Determined. Sampling systems are not particularly suitable for closed loop control of the combustion process, as sufficient delay can be introduced between the gas sampling time and the final analysis time. In addition, as the sampling process is generally a one-point measurement, the results are very variable and may or may not represent the actual concentration of the measured material in the dynamic combustion chamber.

Laser-based optical component sensors have recently attracted interest with extraction measurement techniques. Laser-based measurement technology has the advantage of making instant measurements in the field and providing high-speed feedback for dynamic process control. Tunable diode laser absorption spectroscopy (hereinafter referred to as TDLAS) is a very promising technique for measuring combustion parameters such as the composition and temperature of combustion gases. TDLAS is suitable for the control and monitoring of coal combustion processes. TDLAS is equally well suited for monitoring other combustion processes. In particular, the spectroscopic techniques described herein are useful for monitoring and controlling jet aircraft engine combustion parameters. TDLAS typically uses diode lasers operating in the near-infrared and mid-infrared spectral regions. Since lasers suitable for use in the telecommunications industry have been developed extensively, these lasers are readily available for TDLAS applications. Various TDLAS technologies have been developed that are suitable for detecting and controlling combustion processes. Commonly known techniques include wavelength modulation spectroscopy, frequency modulation spectroscopy, and direct absorption spectroscopy. These techniques are based on a constant relationship between the nature and amount of laser light received by the detector after laser light, a characteristic of the gases present in the combustion chamber, passes through the combustion chamber and is absorbed in a specific spectral band. The absorption spectrum received by the detector is used to determine the amount of gas component under analysis in addition to the relevant combustion parameters such as temperature.

A typical coal fired power plant has a combustion chamber of 10-20 meters on one side. The power plant is operated by the combustion of pulverized coal, so that the combustion process is difficult to transmit the laser light due to the high dust load and becomes very luminous. In addition, under these circumstances, various strong disturbances are found. The overall transmission of light through the process chamber will vary dramatically over time as a result of broadband absorption, beam steering due to refractive index variations, or scattering by particles. In addition, strong thermal background radiation from burning coal particles can interfere with the detector signal. The external environment of the power plant boiler also causes problems with the TDLAS detection or the implementation of the control system. For example, electronic components, optical components or other sensitive sensing spectroscopy components may be located away from strong heat or properly shielded and cooled. Although the implementation of the TDLAS system is very difficult under these conditions, TDLAS is best suited for monitoring and controlling coal combustion processes.

As described in detail in International Patent Application No. PCT / US04 / 10048 (Publication Number WO2004 / 090496), filed March 31, 2004, filed "Methods and Apparatus for Monitoring and Control of Combustion," which is hereby incorporated by reference. Fiber optic coupling is particularly desirable for the implementation of TDLAS systems. In a fiber optic coupling system, one or more probe beams of multiplexed light of various associated wavelengths are delivered to a transmit optic and projected into the combustion chamber. The probe beam is received by the receiving optics after traversing the combustion chamber. As described in International Patent Application No. PCT / US04 / 10048, it is preferable to use a multi-mode optical fiber for the receiving optical coupling device. The use of multimode fiber necessarily results in mode noise, which is detected as a result of non-uniform time and tunable light dispersion in the core of the multimode fiber used to collect and transmit light. Let's say change of signal level. The "receive" side mode noise may uncertain the absorption characteristics that must be observed for a valid TDLAS.

The phenomenon of mode noise is not limited to the TDLAS implementation that characterizes the receiving multimode fiber. In contrast, mode noise inevitably occurs in multi-mode optical fibers of substantial length carrying light. Mode noise is inevitable in multimode fiber because multimode fiber of larger cross-sectional diameter allows transmitted light to propagate along numerous optical paths or modes compared to single mode fiber. Some paths or modes may be longer or shorter than others. Thus, constitutive and destructive interference necessarily occurs, resulting in non-uniform time and wavelength varying light dispersion in the core of a multimode optical fiber, which generates a conventional mode noise speckle pattern. Thus, mode noise occurs in computing, telecommunications, or other scientific applications that use multi-mode fiber of substantial length. Whether or not mode noise interferes with the efficiency of any optical system depends on the conditions of the particular system.

The present invention seeks to solve one or more of the problems described above.

The present invention is an optical mode noise averaging apparatus comprising means for averaging modal noise of light propagating within a multimode optical fiber and a multimode optical fiber. The device may periodically change the refractive index of the multimode optical fiber for a selected period of time, scramble the light dispersion in the multimode optical fiber, or both to average the mode noise due to signal level changes in the light. The refractive index of a multimode optical fiber may be periodically modified by periodically modifying the temperature of the multimode optical fiber. By periodically and physically manipulating the multimode fiber, it is possible to modify the refractive index or scramble the light dispersion within the multimode fiber.

The temperature of the multimode optical fiber can be changed by the action of the thermal element and the thermal element positioned to transfer heat. Appropriate devices for use as thermal elements include thermoelectric modules, resistance heaters, infrared heaters, chemical heaters, conventional refrigeration units, chemical coolers, sources of fluid cooled below ambient temperature, or heated above ambient temperature. Including, but not limited to, a fluid source.

The optical device includes 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 another embodiment having the characteristics of an apparatus for periodically manipulating a multimode optical fiber, the manipulation includes twisting, stretching or shaking the multimode optical fiber. Piezoelectric stretchers are used to periodically stretch multimode optical fibers. Alternatively, the motor may be used to periodically twist a portion of the multimode optical fiber alternately clockwise and counterclockwise relative to the longitudinal axis of the optical fiber and the fixed portion of the optical fiber.

The invention also includes coupling light to the input of the multimode optical fiber, periodically modifying the refractive index of the multimode optical fiber, and receiving the averaged light at the output of the multimode optical fiber. It relates to an optical mode noise averaging method in. The mode noise averaging method includes modifying the refractive index by one of periodically modifying the temperature of the multimode optical fiber and periodically operating the multimode optical fiber. The temperature of the multimode optical fiber can be periodically modified by providing a thermal component that allows heat to be transferred with the multimode optical fiber. Alternatively, the multi-mode fiber can be manipulated periodically by twisting, stretching or shaking the multi-mode fiber.

1 is a schematic diagram of a TDLAS sensing device.

2 is a schematic diagram of a TDLAS sensing device having the properties of a remotely located component optically coupled to a component near the combustion chamber.

3 is a schematic diagram of an optical mode noise averaging apparatus compatible with the present invention.

4 is an enlarged view of a temperature based phase shifting device having a fluid source heated or cooled below ambient temperature with a thermal element.

5 is an enlarged view of a temperature based phase shift device using a series of thermoelectric devices as thermal elements.

FIG. 6 is another enlarged view of the temperature based phase shifting device of FIG. 5.

7 is a schematic diagram of an optical mode noise averaging apparatus using a motor for mechanical operation of a multimode optical fiber.

8 is a schematic diagram of an optical mode noise averaging apparatus using a piezoelectric stretcher for the mechanical manipulation of a multimode optical fiber.

9 is a schematic diagram of a source 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 section E below. The optical mode noise averaging device is suitable for averaging the mode noise inherent in the catch-side (or receive-side) multimode fiber associated with a fiber-coupled tunable diode laser absorption spectroscopy (TDLAS) sensing device. It is not limited thereto. International patent application number PCT / US04 / 10048 (published number) entitled "Methods and Devices for Monitoring and Control of Combustion", filed March 31, 2004, in which several embodiments of such sensing devices are incorporated herein by reference. WO2004 / 090496). In addition, an optical fiber coupled TDLAS sensing device is described below. A preferred embodiment of the present invention is suitable for averaging optical mode noise in all optical systems where mode noise is present. In particular, the optical mode noise averaging device may be implemented as a system having the characteristics of a considerable length of multimode fiber for transmitting light in computing, telecommunications, scientific research, or other. The averaging device is useful in any optical system, where the efficiency of the system can be enhanced by averaging the optical mode noise inherent in the light propagating within the multimode fiber.

B. Sensing Device

One embodiment of the present invention relates to a sensing device 10 suitable for sensing, monitoring and controlling the combustion process, as shown in FIG. The sensing device 10 uses a tunable diode laser absorption spectroscopy technique (TDLAS) that uses laser light from a series of tunable diode lasers 12 oscillating at a constant frequency in the near infrared or mid infrared spectrum. The output of each of the tunable diode lasers 12 is coupled to a separate optical fiber, which is a single mode optical fiber 14, and is delivered to a multiplexer 16. As used herein, the expression "coupled", "optically coupled" or "in optical communication with" means that light passes through an intermediate medium or an empty space, It is defined as the functional relationship between corresponding parts that can pass from the first component to the second component without passing through. The partial or full frequency laser light generated by the multiplexing forms a multiplexed probe beam having a complex selection frequency in the multiplexer 16. The multiplexed probe beam is coupled to an outgoing (or transmitting) optical fiber 18 and is a pitch optic 20 collimator operatively coupled to the process chamber, indicated as combustion chamber 22 in FIG. 1. Is delivered.

The transmission optical unit 20 is provided to project the multiplexed probe beam through the combustion chamber 22. A catch optic 24 is positioned across the combustion chamber 22 that is in optical communication with the transmitting optic 20. The receiving optics 24 are preferably installed approximately opposite the outgoing optics 20 and are operatively coupled to the combustion chamber 22. The receiving optical unit 24 is installed to receive the multiplexed probe beam emitted through the combustion chamber 22. The receiving optics 24 are optically coupled to the light receiving side optical fiber 26 which delivers the multiplexed probe beams received by the receiving optics 24 to the demultiplexer 28. The multiplexed probe beams received by the receiving optics 24 are demultiplexed in the demultiplexer 28, and each wavelength of the demultiplexed laser light is coupled to the output optical fiber 30. Each output optical fiber 30 is in turn optically coupled to the detector 32. Each detector 32 is typically a photodetector sensitive to one of the selected frequencies of multiplexed laser light to form a probe beam. Detector 32 generates an electrical signal in accordance with the nature and amount of light transmitted to detector 32 at the detector frequency. The electrical signal of each detector 32 is typically digitized and analyzed in the data processing system 34. As described below, the digitized and analyzed data can be used to recognize physical parameters within the process chamber, including the concentration of various gas components in the combustion chamber 22, the combustion temperature, and the like. The data processing system 34 may send a signal to the combustion control device 38 via the feedback loop 36, thereby actively controlling the selected process parameters. In the case of a combustion process, controlled process parameters may include fuel (eg, pulverized coal) feed rate, oxygen feed rate, and catalyst or chemical addition rate. When the electronic and optical components of the transmitting and receiving sides of the sensing device 10 are combined with optical fibers, sensitive and temperature sensitive devices such as the tunable diode laser 12, the detector 32, and the data processing system 34 are provided. Can be installed in a control room with a stable operating environment. Then, only the transmitting and receiving optics 20 and 24 which are relatively environmentally friendly are installed near the harsh environment of the combustion chamber 22.

2 is a schematic view of the overall component arrangement of a multiplexing detection system 40 coupled with an optical fiber. The sensing system 40 typically includes a system rack 42, a breakout box 44, a transmitter head 46 with pitch optics 48, a receiving optics a receiver head with catch optics 52 and coupling optical fibers. The system rack 42 is preferably installed in a remote control room remote from the combustion chamber 54, for example one kilometer away. The control room usually has a controlled environment. System rack 42 includes a laser 56, a detector 58, a wavelength multiplexer 60, and a wavelength demultiplexer 62. The system rack 42 also includes system electronics and control software (not shown in FIG. 2). System rack 42 may optionally include an alignment light source 64.

The optical fiber that couples the system rack 42 and the breakout box 44 generally uses standard single-mode telecom optical fiber. This type of fiber is inexpensive, easy to use, low loss, and allows the use of laser light in a variety of off-the-shelf communications components such as optical switches, splitters and wavelength division multiplexers. Without the use of optical fibers, the laser light would reach the combustion chamber 54 through the void, making the device very difficult to construct. Sensitive electronic and optical components will also have to be installed very close to the combustion chamber 54.

Also shown in FIG. 2 is a breakout box 44. The breakout box 44 is packaged to have good environmental resistance so that it can be located near the boiler. Breakout box 44 includes optical switches, splitters, and couplers (collectively 66) that are used to cause optical signals to be sent to a plurality of transmit-receiver head pairs, as described below.

As shown in FIG. 2, the third group of system components are transmitter and receiver heads 46, 50. The optical and electronic components of the transmitter and receiver heads 46 and 50 convert the light transmitted to the optical fiber 68 into parallel light, which passes the light accurately through the combustion chamber 54, On the other side it must receive the light beam and have it connected to the optical fiber 70. Optical components that perform this function are selected according to the transmission distance, the disturbance in the combustion region, the transmitted light beam and the core size of the optical fiber 70. The choice of core size is a matter of design choice as dictated by the application. The larger the core, the more laser light it receives, but also more background light. When a coal fired boiler is used, a fiber core diameter of 50 microns gives acceptable results. There are several advantages to using fiber optics on the receiver (receiver) side. In particular, only light transmitted in the same direction at the same position as the laser light is focused on the optical fiber 70. This drastically reduces the amount of background light detected. The light is received by one of several receiver fibers and an optical switch or other optical path can select the light from one optical fiber that forms the path to the detector 58. In FIG. 2 only one receiving optic is shown.

To use fiber coupling on the receiver side, the alignment tolerances on both the transmitter and receiver optics must be maintained correctly (0.5 milliradian or less for both transmitter and receiver pointing). It is desirable that both the transmitting and receiving optics 48, 52 be custom designed and aberration corrected for wavelengths from 660 nm to 1650 nm so that multiplexed 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 well known to those skilled in the art having laser spectroscopy techniques. In general, TDLAS is achieved by transmitting laser light through the target environment and detecting the absorption of the laser light at a particular wavelength due to the target gas, such as carbon monoxide or oxygen. Spectroscopic analysis of the detected light reveals the type and amount of gas along the laser path. Details of direct absorption spectroscopy can be found in Teichert, Fernholz, and Ebert, "Simultaneous Measurement of CO, H ₂ O, and Gas Temperatures in Full-Size Coal-fired Power Plants by Near-Infrared Diode Lasers" (April 20, 2003, Applied Optics, 42 (12): 2043), the contents of which are hereby incorporated by reference in their entirety. The non-contact nature of laser absorption spectroscopy makes them suitable for harsh environments, such as combustion areas of coal-fired power plants, where other probes will be used, or for flammable or toxic environments. Using laser light, it is possible to obtain the high brightness needed to detect the transmission of light in the presence of the severe attenuation seen in these environments (typically greater than 99.9% light loss). In order to better withstand the harsh conditions of the target application, laser light is transmitted to the target environment through the reinforced optical fiber.

Efficient detection of temperature or multiple combustion process constituent gases requires the performance of TDLAS with laser light at multiple wide intervals. The selected frequency should match the absorption line of the transition to be monitored. For example, it is useful to monitor NO ₂ at a wavelength of 670 nm for approximately emitted NO concentrations. It is also very useful to monitor oxygen, steam (temperature) and carbon monoxide in coal fired boilers. The proper absorption line and the appropriate laser frequency are 10 meters in length for the laser probe path through the combustion chamber and the mole fractions of each component are CO (1%), O ₂ (4%), CO ₂ (10%), and H ₂ O (10%). Can be determined under the assumption that To select the frequency, it can be assumed that the process temperature is 1800K slightly higher than the temperature normally observed in coal-fired power plants, and cushions serve as safety factors in the calculations.

For example, three moisture absorption lines that meet the following criteria can be selected as TDLAS:

1. low state energy of ˜1000, 2000, 3000 cm ⁻¹ , respectively;

2. provide a convenient absorbance of 0.1-0.4, which in turn leads to a beam absorption of approximately 20% by resonance;

3. Optimal environment using transitions in the region of 1250-1650 nm where low cost, high output and DFB diode communication laser can be used;

4. Transitions should be well separated for easy multiplexing;

5. The selected wavelength should be effectively diffracted by the existing (de) multiplexer grating;

Appropriate water lines occur at the next wavelength.

Wavelength (nm) Frequency (cm-1) Low level energy (cm-1) Grid order Absorption at 1800K and 10M UNP diffraction efficiency (model) 1349.0849 7412.432 1806.67 6.87 19.7% 81% 1379.4507 7265.062 3381.662 6.73 28.1% 77% 1394.5305 7170.872 1045.058 6.65 6.8% 72%

There is no interference with other combustion gases. The most likely to interfere component, CO _2, is modeled so that there are no strong and interfering lines in the 1.3-1.4 micron region.

Similarly, a suitable carbon monoxide line can be selected based on Ebert's paper described above. Suitable carbon monoxide lines are found at 1559.562 nm using the R (24) line in coal fired boilers. This line selection avoids the interference of moisture and carbon dioxide. Known diffraction gratings are very effective in this wavelength region because they are in the optical communication C band. The absorbance of CO at this wavelength is about 0.7%.

In addition, oxygen can be measured at 760.0932 nm. The preferred (de) multiplexing diffraction grating efficiency is calculated at 40% in this area, but an appropriate laser power must be used to obtain this particular efficiency.

As will be described below, strict alignment of the transmitting and receiving optics is necessary to use fiber coupling on both the transmitting and receiving sides of the TDLAS sensing device. It is preferred that active alignment is performed by the selected alignment wavelength. One of the available alignment wavelengths is 660 nm, since a high output (45 mW) diode is available at this frequency, and 660 nm is near the peak of the 14th order grating operation. Other alignment wavelengths may be determined equally or more appropriately.

A reasonable set of wavelengths selected for multiplexing on the probe beam for the TDLAS of the present invention is summarized in Table 2. Note that this set of wavelengths is for one embodiment of a TDLAS sensing device suitable for the detection and control of coal-fired power plants. Of course, other wavelength sets can be used.

purpose Wavelength (nm) Alignment 660 O ₂ ba band 760.0932 H ₂ O (medium temperature line). 1349.0849 H ₂ O (high temperature line) 1376.4507 H ₂ O (low temperature line) 1394.5305 CO (2.0) over tone R (24) 1559.562

D. Using Multiplexed Beams TDLAS Specific benefits of

A particular advantage of TDLAS using wavelength multiplexed probe beams is that they improve the accuracy of temperature measurements. Accurate concentration measurements with TDLAS require knowing the temperature of the gas being monitored. The strength of molecular absorption is a function of temperature. Therefore, temperature must be known to convert the amplitude of the absorption feature into concentration. Conventional methods for determining the concentration of combustion components, such as CO, have suffered from inaccurate temperature measurements that lead to errors in quantification. This is especially true for diode-laser based ammonia slip monitors, which are typically not compatible with temperature measurements. In the sensing system for 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 combined strengths of the two lines is a function of temperature only (assuming the pressure in the entire system is constant). Thus, in principle, the correct temperature can be obtained with two lines. However, in the case of non-uniform temperature distributions (generally found in industrial combustion processes), only two lines cannot determine the temperature distribution. In the case of this non-uniform temperature distribution, the two lines can only determine the "path-averaged" temperature. Conversely, measuring the integrated amplitude of two or more lines (same component) can detect temperature non-uniformity. Reference is made here in its entirety by Sanders, Wang, Jeffries and Hanson, who have implemented this technique using oxygen as the probe molecule (Application Optics, 40 (24): 4404, Aug. 20, 2001). The preferred technique is based on the fact that the distribution of peak intensities measured along the line of sight does not coincide with the path of average temperature 500K. For example, half of the path is 300K and the other half is 700K.

In addition to the benefits of more accurate temperature measurements, multiplexed probe beams allow simultaneous monitoring of one or more combustion gas components and more precise control of the combustion process.

E. mode noise

Optical coupling devices and similar devices in TDLAS systems that require multiplexed signals from a wide spacing wavelength present many design challenges due to the conflicting design requirements of reduced mode noise and high efficiency of light aggregation. Mode noise is defined here as the change in signal level of detected light resulting from light dispersion that varies in time and wavelength at non-uniformity in the core of the optical fiber used to collect and transmit the light entering or exiting the process chamber to be measured.

In multimode optical fibers, different modes propagate at different speeds as the refractive index changes. Intensity distribution in an optical fiber becomes a speckle pattern by interference of all propagation modes passing through different effective path lengths. If all the light in the speckle pattern is collected and detected, constructive and destructive interference will be accurately offset so that the total transmitted power will not change with wavelength or fiber length. If there is a loss of clipping, vignetting, or the like, the output that is detected is not offset exactly and changes with wavelength and / or time. In the TDLAS sensing system as described above, the change in output due to mode noise is quite problematic. Specific spectroscopic techniques depend on the absorption of certain wavelengths of light by the gas component being studied. Absorption is detected as a decrease in power at the critical wavelength. Thus, the mode noise may be similar to the output drop along the absorption to obscure the data collected by the TDLAS. The general formula of the output detected later in a fiber of length z is:

P = P ₀ + ∑ _ij c _ij E _i E _j cos [2πγ ₀ Δn _ij z / c + △ Φ _ij (T, σ))]

Where P ₀ = average power independent of wavelength

E _i = the amplitude of light in the ith crossing mode

c _ij = overlap integration between the i th and j th traversal modes

Δn _ij = refractive index difference between the i th and j th modes

△ Φ _ij = Phase shift between the i th and j th modes with temperature and stress.

In the case of orthogonal set of modes and lossless, c _ij = 0. However, if there are beam clipping, vignetting, or other mode loss, c _ij ≠ 0. This causes ripples in the average propagation output.

Conventional warp refractive index optical fibers with a 50 micron core have a total refractive index change, Δn of ˜1%, but most modes consume a lot of propagation time when they are close to the center of the fiber core. So, in general, Δn _ij <0.0005. The commonly used fiber optic GIF50 supports roughly 135 modes and is rough enough to produce significant mode noise during wavelength scans at a given reasonably obtainable beam clipping level.

As an example of specific mode noise, consider the simplest system with mode noise: a rectangular waveguide that supports only the lowest mode in one dimension and two lowest modes in normal orthogonal dimensions:

Lowest mode: E ₁ = E ₁ ⁰ [exp i (kz-wt)] cosπx / 2a

Next mode: E ₂ = E ₂ ⁰ [exp i (kz-wt)] sinπx / a

The strength at z along the fiber is:

I (x) = │E ₁ + E ₂ │ ² and the total output is P = ∫│E ₁ + E ₂ │ ² dx

Here, the integral should include the effects of clipping and vignetting.

Without clipping, P ~ E ₁ ² + E ₂ ² and do not depend on wavelength. Adding clipping will change the limit of integration. Clipping is represented by the additional term ~ E ₁ E ₂ cosΔΦ, where ΔΦ = ΔkL = 2πΔnL / λ.

If a single mode fiber is used in the receiving optical coupling device of the system as described above, mode noise is not a problem. However, the optical coupling device on the receiving side of the optical fiber coupling TDLAS system typically requires the use of multimode fiber for two reasons. Firstly, after traversing the static volume (combustion chamber with a measuring path exceeding 10 meters), the initial single-mode (Gaussian spatial distribution) beam is of poor quality. Therefore, the coupling efficiency of combining such a severely distorted beam into a single mode optical fiber is very bad. This is unacceptable as the beam passes through a static volume and is attenuated 3-4 times in size, mainly by being scattered and obscured by soot and fly ash. Additional attenuation caused by using single mode fiber will interfere with the measurement. Second, the refracting beam steering effects in the fireball make the position and direction of the beam unstable. Under such a given effect, it is difficult to "hit" the core of a single mode fiber regularly.

In contrast, the core of the multimode fiber is at least 25 times the target cross-sectional area of the single mode fiber. Thus, the effect of beam steering is significantly reduced. In addition, since the coupling efficiency of the multimode optical fiber is independent of the spatial mode of light, the poor quality beam obtained after passing through the crater is not a problem.

Other types of implementations in computing, telecommunications, or general science may have similar or completely unrelated limitations that require or use the substantial length of multimode fiber. In other implementations, mode noise can also be a problem and represent a significant data set or data transfer task.

Therefore, mode dependent losses occurring in multimode fiber optic coupling devices are a design challenge. Light dispersion from the core of a multimode fiber is a random speckle pattern. In other words, it is a random pattern of bright and dark portions generated by constructive and destructive interference between different modes of optical fibers. If the speckle pattern does not change as a function of time and wavelength, this is not a problem. However, if the beam is clipped somewhere in the multi-mode receiving side optical coupling device as described above, slow changes in the speckle pattern, especially as a function of wavelength, can cause mode noise. This clipping is inevitable and can only be reduced. Therefore, in order to improve the detection sensitivity of the system, additional measurements must be performed to reduce the mode noise.

There are several ways to reduce mode noise. From equation (2) above, the mode noise can be reduced by:

1. Reduce mode dependent losses. That is, reduce clipping to keep c _ij less;

2. Reduce z to increase the period of mode noise to be much larger than the period of the absorption line of interest;

3. Reduce _{Δn ij} by using low dispersion fiber;

4. Change the mode. However, all the mode change techniques do not have the same effect as described below.

The receiving optics of the fiber coupled TDLAS sensing system of the present invention are designed and configured to combine all of the above to reduce mode noise. The receiving optics are designed so that beam clipping occurs at low levels if the alignment of the system is almost complete. Efforts should be made to keep the length of the multimode fiber to a minimum; In some applications, Z must be long to position the control electronics in an environmentally controlled location. The value of _{Δn ij} can be reduced using the highest quality, low dispersion multimode fiber. In addition, the best results can be obtained by periodically varying or mechanically manipulating the refractive index of the receiving multi-mode fiber by averaging the modes and extracting data from the collected average optical signal.

Speckle patterns present in multimode optical fibers change as a function of time and wavelength and also as a function of the mechanical position of the optical fiber. Both transmission time and wavelength are affected by the refractive index of the optical fiber. Flexing the fiber and manipulating it in a specific way can change the speckle pattern. If such mechanical manipulation or periodic change of the refractive index is performed continuously for a certain period of time, the spatial distribution of light emitted from the optical fiber is averaged in a relatively uniform pattern.

Through efficient periodic phase or scrambling of mode noise, the time averaging measurement results in a uniform signal level. The refractive index of an optical fiber can be changed by stretching or twisting the optical fiber or by changing the temperature of the optical fiber. Changing the temperature of the optical fiber causes a change in the refractive index difference _{Δn ij} between the i-th and j-th crossing modes. The change of the refractive index of the optical fiber transfers the mode noise by the function cos (2πγ ₀ Δn _ij z) / c given in Equation 2.

F. Optics mode noise  Averaging

As schematically shown in FIG. 3, periodic phase transition or scrambling of mode noise to form a time averaged survey may be accomplished with the optical device 300. Optical device 300 will include a multi-mode fiber 302 having an input 304 and an output 306. Light is introduced by the input 304 of the multi-mode fiber 302 and propagates through the system in the direction of the arrow shown in FIG. 3, usually with respect to the input 304 and the output 306.

Optical device 300 also includes an averaging component 308 to which the multi-mode fiber 302 is input and coupled. The averaging component 308 may include an apparatus for periodically changing the refractive index of the multi-mode fiber 302 during the selection period. Or in place of the above device, the averaging component 308 may comprise a device for scrambled light dispersion in the multi-mode fiber 302. Changes in refractive index or scrambling of light dispersion by the averaging component 308 may be achieved by periodically changing the temperature of the multimode optical fiber 302, by manipulating the multimode optical fiber 302 periodically, or both. Is achieved.

In embodiments where the averaging component 308 performs periodic manipulation of the multi-mode fiber 302, the averaging component 308 can twist, stretch, or shake the optical fiber 302. In embodiments where the averaging component 308 periodically changes the temperature of the multimode optical fiber 302, several thermal elements or thermal components may be configured to exchange heat with the multimode optical fiber. An apparatus for varying the temperature of the multimode fiber 302 may be included in the averaging component 308. Representative devices that can be used to vary the temperature of the multi-mode fiber 302 include thermoelectric modules, resistance heaters, infrared heaters, chemical heaters, conventional refrigeration devices using compressed fluids and heat exchangers, chemical coolers, sub-ambient temperatures. A fluid source cooled to or a fluid source heated above ambient temperature. Some of these devices are described in detail below.

In embodiments where the averaging component 308 periodically heats or cools the multi-mode fiber 302, a sensor 310 may be installed to control heat transfer with the multi-mode fiber 302. When sensor 310 provides information to controller 312, controller 312 controls averaging component 308 via control line 314.

G. Temperature Based Phase Shifter

The efficiency of the temperature based mode phase transition is directly related to the change in temperature per unit time and the length z of the fiber exposed to the change in temperature. Temperature-based mode transitions are a particularly effective way of dealing with mode noise. This is because the temperature change of the optical fiber changes the refractive index of all the transverse modes and can occur over a significant optical fiber length. Therefore, the change of the refractive index of the optical fiber ensures the transition of all the crossing modes, and no mode can keep the signal "fixed".

Indeed, various types of heating / cooling devices can be installed for heat transfer operations with multimode fibers to purify the temperature of the optical fiber. Electrical resistance heaters, conventional refrigeration devices, heating or cooling fluids, Peltier or other thermoelectric devices, infrared devices, or chemical devices can all be used to change the temperature of the optical fiber.

One embodiment of a mode phase shift device using periodic temperature changes is a fluid based mode phase shift device (fluid device) 400. 4 shows an enlarged view. An embodiment of a fluid apparatus 400 using vortex tubes 402A and 402B to alternately blow hot and cold air around an optical fiber is shown in FIG. 4. Air delivered from a compressed air source (not shown in FIG. 4) is alternately delivered to one of two vortex tubes 402A, 402B. Vortex tubes 402A and 402B are coupled to allow fluid to be delivered to the interior of chamber 404. As shown in FIG. 4, the chamber 404 is formed of an enclosure 406 having sides 408A, 408B, top 410, front 412, and rear access ports 414. The entire enclosure 406 is suitable for mounting in a rack, which is conventional data processing equipment. Although rack-mountable enclosures 406 are particularly suitable, fluid devices may be utilized using enclosures of various shapes, types, or styles suitable for receiving spools 416 that hold the length of the multi-mode fiber 418. 400). Alternatively, a device without enclosure 406 may be used.

Vortex tubes 402A and 402B transfer fluid through the access port 414 to the interior of the enclosure, and the fluid transfers heat to the multi-mode fiber 418 wound on the spool 416. Thus, the multimode optical fiber 418 may be periodically heated and / or cooled by air heated or cooled above or below the ambient temperature supplied by the vortex tubes 402A and 402B.

Suitable vortex tubes 402A, 402B are readily available. For example, EXAIR ^® 3230 vortex tubes are commercially available from EXAIR ^® company. EXAIR ^® 3230 vortices or similar vortices operating at a yield of 30 ft ³ / min can provide air cooled to + 60 ° C or cooled to -20 ° C, depending on the orientation of the tube. In addition, the vortex tube is relatively easy to circulate between heated and cooled air when using the vortex tube. In addition, it is important to note that various devices or methods for circulating and supplying heated or cooled fluid to a multimode optical fiber are suitable for implementation of embodiments of fluidic device 400. Heating and cooling fluids may be air as described above, but water, heating / cooling oils, compressed gases, or other fluids may be used to heat or cool the multimode optical fiber.

As an example and not by way of limitation, during operation, one of the vortex tubes 402A, 402B may deliver heated air until the optical fiber reaches a temperature of about 10 ° C. which is higher than the inlet temperature. The temperature of the optical fiber may be determined through a thermocouple 420 or a temperature sensor inserted in contact with the optical fiber. The temperature control unit (not shown in FIG. 4) receives input from the thermocouple 420 and activates the solenoid switch to allow air to be sent to other vortex tubes 402A and 402B for cooling. Alternatively, the use of a temperature controller is between vortex tubes 402A, 402B between heating and cooling, since the heating air supplied to the multi-mode fiber 418 by the vortices 402A, 402B never reaches a critical level. Can be excluded or replaced with a timed relay to periodically switch

A highly preferred embodiment of a mode phase shift device based on periodically changing the temperature of a multimode fiber is shown in FIGS. 5 and 6. The thermoelectric mode phase shift device (thermoelectric device) 500 includes a spool 502 that fixes the selected length of the multimode fiber 504. One or more thermoelectric heating / cooling modules 506 are positioned to exchange heat with the multi-mode fiber 504. In the embodiment shown in FIG. 5, a plurality of thermoelectric heating / cooling modules 506 are disposed radially around the inside of the spool 502. Heat transfer is facilitated using thermoelectric resin between the outer surface of the thermoelectric module 506 and the coils of the multimode optical fiber 504. 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 facilitate the winding of the multimode optical fiber 504. .

One configuration of positioning the thermoelectric module 506 in contact with the multi-mode fiber 504 and relative to the spool 502 is shown in the enlarged view of FIG. 6.

One or more heat sinks 508 may also be operatively arranged to transfer heat with the thermoelectric module 506. Preferably, heat sink 508 is made of a high heat conducting material such as aluminum or copper and has fins or other devices designed to increase the surface area of each heat sink 508. Fan 510 is operatively positioned to exert or blow air through heat sink 508 to facilitate rapid extraction of heat from thermoelectric module 506 and thus rapid heating or cooling of multi-mode fiber 504. Make it smooth. As shown in FIGS. 5 and 6, the framework 512 with the top ring 514 and the bottom ring 516 are in proper orientation relative to each other without disturbing air leakage above and around the heat sink 508. It can be used to secure the components of the thermoelectric device 500. Preferably, an opening 518 is formed in the framework 512 to ensure free air outflow.

5 and 6 use a thermoelectric module 506 based on the Peltier principle. Direct current is supplied to the thermoelectric module 506 through the leads 520. In the thermoelectric module 506 operating on the Peltier principle, the opposing surface of the thermoelectric module 506 is heated or cooled according to the direction of the supplied DC current. Thus, these modules provide the particular advantage that it is relatively easy to selectively heat or cool the multi-mode fiber 504 by selectively switching the polarity of the direct current provided to the leads 520. However, it should be noted that other types of devices may also be implemented to heat and / or cool the multi-mode fiber 504. For example, direct resistance heaters, conventional refrigeration units, infrared heating units, and / or chemical reaction based heaters and / or coolers may be used to vary the temperature of the multimode optical fiber 504.

In a preferred embodiment, power is delivered from a suitable power source to a thermoelectric module 506 mounted in a cylindrical configuration as shown in FIGS. 5 and 6. Current switching electronics are used to periodically change the polarity of the DC output delivered to each thermoelectric module 506. The mode phase transition occurs at the length of the thermoelectric module 506 and the multimode fiber 504, preferably the premium GIF50 multimode fiber, to which heat is transferred. 55m to 100m was found to be the length of a multimode optical fiber suitable for averaging with the mode phase transition. Other lengths may also be suitable. When 100m of wrapped fiber is used, about 50% of the fiber is in direct contact with the thermoelectric cooler. It should be noted that in direct contact, there is no conductive material between the thermoelectric module 506 and the optical fiber 504, except for the thermoelectric resin. This configuration minimizes the heat of the system. By minimizing heat, the system's temperature response is faster and more efficient mode phase transitions are achieved.

One or more thermocouples 522 or other temperature measuring device may be mounted between the thermoelectric module 506 and the multi-mode fiber 504 to be used to always monitor the temperature of the multi-mode fiber 504. The temperature control unit (not shown in FIGS. 5 and 6) receives the thermocouple crystal temperature and cycles the current direction based on the temperature reading. A temperature range of about 35 ° C. to 50 ° C. is achievable with readily available thermoelectric modules. It is important not to reach the maximum temperature of 85 ° C. at which damage to the optical fiber can occur. Tests are completed at 65 ° C. to 10 ° C. for single side coolers and 65 ° C. to 30 ° C. for double side coolers. The complete period may be a randomly chosen period, but about 25 seconds have been found to be valid.

As described above, heat may be dissipated on the opposite side of the thermoelectric module 506 using the heat sink 508 mounted with the thermoelectric module 56 to transfer heat. Pressurized air travels through the heat sink fins to help dissipate heat equivalent to the input output. Efficient heat dissipation is achieved by passing air through the openings 518 at the bottom of the fins so that air flows out of the top of the unit through the system. Other configurations are also applicable that allow sufficient air outflow. Alternatively, the thermoelectric device 500 can be encased in or otherwise cooled. Preferably, the fan is operated constantly while the device is running. Although a suitable fan or fluid source is used to dissipate heat, it has been found that 300CFM fans are effective at removing heat from the system as shown in FIGS. 5 and 6.

Control of the electronic components involves heating and cooling of the thermoelectric module 506. The feedback control electronics sense the temperature of the optical fiber based on input from thermocouple 518, thermometer or other temperature sensor. In addition, based on the temperature input, the controller redirects the current of the power delivered to the thermoelectric module 506 and adjusts the output level for the heating and cooling cycles (heating is usually more efficient and therefore requires less power). . The controller also adjusts the maximum and minimum temperatures delivered to the optical fiber, and can block the drive circuit in case of overheating.

H. Thermoelectric Module Phase Transition System Test

The test is performed with the thermoelectric device 500 described above. Testing is carried out over four wavelength bands through a free absorption nitrogen purge chamber. In the absence of an absorbing component in the path length, the laser should exhibit a linear wavelength response after being split into a reference signal. The deviation of the linearity of the slope is mainly caused by the mode noise. The expression of the equation used to determine the uncertainty in the measurement is:

σ _x = [1 / N∑ (x _i -f _i (ax + b) ² ) ^1/2

X _i = singal _i / tap _i

f _i (ax + b) = linear fit of x _1-n

The start and end of each wavelength period can be ignored due to time delays due to the travel time of the source and receive heads. These time delays cause significant distortion between the tap and the observed signal at the beginning and end of each cycle.

System performance consists of embodiments of the thermoelectric device 500 using single-sided and double-sided thermoelectric modules 506, multiple averaging times, and different fiber lengths. For all results, mode phase transitions and averaging result in reduced mode nodal signal deviations. Reduction of noise depends on the length of the optical fiber used for the experiment as shown in equation (3); Long fiber has higher frequency mode noise deviation than short fiber. Because of this, the relative resolution enhancement of mode phase transitions is further demonstrated in long fiber optics. The results are presented below in several forms:

uncertainty type Average time
(second) 1349 nm 1376 nm 1394 nm 1560 nm Average
uncertainty  200m of unheated fiber - 8792 19514 6838 13712 11715  30m of unheated fiber - 3156 5174 5311 1231 4547  270m of unheated fiber 10 16837 24417 21989 14533 19444

No mode scramble, TEM turn off

3 1684 2394 3527 2647 2535 10 612 966 3013 0 1530 10 1229 1592 3084 0 1968 10 1434 1581 1823 0 1613

Single-sided TEM, 100 m operating fiber length, 270 m total fiber length, 1912

Temperature range average of 10 ℃ -65 ℃

10 3048 3896 2020 1871 2709 10 4213 2680 1856 3216 2991 10 3317 1742 2257 2838 2537 In turnaround One 4957 1628 1736 2939 2815 Hot to cold One 2712 2490 1208 2345 2189 Hot to cold One 3119 3559 4965 1921 3391 Hot to cold One 2762 3163 3519 1541 2746 Hot to cold One 2468 3388 1928 2887 2668 Hot to cold One 3724 2479 2394 3005 2901 Hot to cold One 2865 2363 2700 1917 2461 Hot to cold One 5187 1924 2327 3633 3268 Hot to cold One 3327 3392 1447 1777 2486 Hot to cold One 4989 1556 2819 3090 3113 Hot to cold One 2812 1985 1542 1642 1995

75m operating multimode fiber, total fiber length of 245m, 2531

Single-sided TEM, temperature range average of 65 ° C-10 ° C

cycle 10 3975 3465 2375 3014 3207 cycle 10 2997 963 1408 2085 1863 Hot to cold One 3111 1841 1538 2982 2368 Hot to cold One 2268 1518 2365 2932 2271

5m operating multimode fiber, total fiber length of 225m, 2167

Double side TEM, temperature range average of 65 ℃ -30 ℃

Devices based on mechanical operation

As mentioned above, mode noise allows to average and planarize various refractive indices by mechanical manipulation of multimode optical fibers and data extracted from the aggregated average optical signal. The above-described temperature based phase shifting device modifies the refractive index of a multimode optical fiber with periodic temperature changes to achieve mode phase shift. As described below, mechanical manipulation of multimode optical fibers can be used to modify the refractive index. In addition, mechanical manipulation of the optical fiber results in averaging and flattening of the signal affected by the mode noise due to the inability of light to fully follow a particular mode in the waveguide. Thus, the averaging and flattening of the mode noise induced speckle pattern within the length of the multi-mode fiber can be achieved with a combination of phase transition and mechanical scrambling.

Certain modes of mechanical fiber optic manipulation are more effective at averaging mode noise than others. In particular, twisting the fiber in the longitudinal (z) axis relative to other fiber points will change the speckle pattern. The major change obtained is the rotation of the speckle pattern about the z axis. Of interest is that the pattern does not rotate so far around the axis as the optical fiber is mechanically rotated. The secondary effect is that the actual light dispersion has changed slightly with rotation. The rotation of the speckle pattern is not primarily due to the stress induced refractive index change of the optical fiber, but may explain the small change of the speckle intensity pattern. Rather, the rotation is due to the inability of the light to fully follow the conduit when operated in torsional motion.

In one embodiment of the present invention, a mechanical mode noise averaging apparatus (mechanical apparatus) 700 is schematically shown in FIG. The mechanism 700 uses a hollow shaft motor 702 that allows the multi-mode fiber 704 to be positioned and fixed. The remote portion of the optical fiber 706 is fixed relative to the shaft position of the motor 702, and the motor is repeatedly shaken through a torsional movement, which is preferably +360 degrees and then -360 degrees. The frequency of this motion is greater than or equal to 10 Hz to enable efficient averaging of the transmitted signal and to significantly reduce the effect of receive mode noise. Although it has been determined that torsion of multimode optical fibers along the longitudinal axis is effective in scrambling mode noise, other mechanical manipulations such as shaking, stretching, or bending may also be used.

Piezoelectric Elongation

Stretching an optical fiber causes changes in both the refractive index and the length of the optical fiber. Multi-mode optical fibers can be stretched with piezoelectric stretchers. Piezoelectric devices are generally used to induce a modulated time delay in a single mode fiber. Multimode fiber is not used in piezoelectric stretching devices because the time delay of multimode fiber is not controllable due to the multiple paths or modes through which light can travel. However, piezoelectric stretching devices are not practical for making time delays, but can be used to induce mode phase transitions.

As the multimode optical fiber is stretched, the stress induced in the optical fiber causes a change in both the refractive index and the optical fiber length. As schematically shown in FIG. 8, the piezoelectric device 800 winds a multimeter optical fiber 802 several meters around the half cylinder 804 and then oscillates the half cylinder 804 at a predetermined oscillation frequency and distance. It works. As the distance between the half cylinders 804 increases and decreases, the stress of the optical fiber 802 oscillates harmoniously. This oscillation causes the refractive index of the optical fiber 802 to vibrate. The effectiveness of the mode transition is a function of both the change in the length (z) of the optical fiber and the change in the refractive index (Δn _ij ) of the optical fiber (Equation (1)).

The mode phase transition can be made by one of two techniques using the piezoelectric device 800. In a first technique, the piezoelectric device 800 is implemented with an optical fiber 802 sufficient to make a large mode change and is configured to induce stress so that a uniform signal level can be achieved by averaging many modes. Alternatively, because the piezoelectric device 800 cycles with even refractive index changes, the piezoelectric device 800 can be operated to oscillate in such a way as to form a mode transition of harmonics of 180 ° with minimum and maximum extension distances. In this way, the mode noise can be reduced by optimizing the stretching characteristics to achieve 180 ° phase transition, rather than time-averaging many mode transitions. Therefore, the mode noise can be averaged as small as one cycle, so that fast data acquisition without mode noise can be obtained.

Originating side  Optical coupling

The outgoing optical coupling device of the fiber coupled TDLAS sensing device also presents significant problems in design due to the need to form a single mode beam of all wavelengths transmitted through the measurement area. If single mode fiber is used throughout the source optical coupling device, mode noise is not a problem. However, the optical fiber acts as a single mode waveguide for a limited wavelength window. Beyond the range of short wavelength cutoffs for a particular fiber, light can be transmitted through the fiber in some higher order spatial modes. These higher-order modes form speckle patterns as light exits the fiber. Speckle patterns vary in time and wavelength. Even a small amount of beam clipping causes noise in the measurement.

Conversely, if an optical fiber with a single mode cutoff is selected that matches the shortest wavelength that needs to be transmitted, longer waveguides suffer significant losses when coupled to the optical fiber and the optical fiber exhibits wider deflection losses for longer waveguides. .

This problem can be severe in the fiber-coupled wavelength multiplexed TDLAS sensing and control devices described above, due to the need to multiplex wavelengths as long as 1.67 microns with wavelengths as short as 760 nm or 670 nm. There is no known commercially available optical fiber that will provide single mode operation, high coupling efficiency and low bending loss over this wide range of wavelengths. Photonic crystal fiber offers a solution to this dilemma in the future, but photonic crystal fiber technology is currently in its infancy.

As shown in FIG. 9, the problem of multiplexing and pitching light of a single mode beam of 670 nm or 760 nm to 1670 nm is a very short multiple that does not allow for higher order spatial mode for shorter wavelengths than single mode cutoff. It can be minimized by using the transmitter of the mode optical fiber 120. Referring to Equation 1, if the length L of the multi-mode optical fiber is short, the mode noise will be minimized. In this case, for example, if 760 nm light is coupled to a short portion of a single mode optical fiber having a cutoff wavelength of 1280 nm (eg, Corning SMF28), 760 nm light remains at least a few meters in single mode. Thus, the solution to source-side noise is to combine 760 nm light into an optical fiber that can be single mode for wavelengths longer than 1280 nm but multimode for 760 nm, with only a short distance to traverse before aiming to be transmitted through the measurement area. .

A schematic of this system is shown in FIGS. 9 and 2. With reference to FIG. 9, multiple diode laser sources 902 firing at widely spaced laser firing frequencies are coupled to individual single mode optical fibers 904A- 904n. Diode lasers emitting at wavelengths between 1349 nm and 1670 nm are multiplexed with a multiplexer 906. The output of the multiplexer 906 is coupled to an outgoing optical fiber 908 that is sized to transmit light in a wavelength in the range of 1349 nm-1670 nm without substantial transmission loss and without inducing mode noise. Suitable optical fibers for these wavelengths are Corning SMF28. However, when the 760nm input is multiplexed and coupled to the SMF28 optical fiber, after a relatively short distance transmission, it is multimodulated. Thus, the output of the 760 nm laser is coupled to a single mode optical fiber for wavelengths below 1280 nm, such as SMF750. The laser light transmitted from the input optical fiber 904n and the multiplexed laser light transmitted from the transmitting optical fiber 908 may be combined near the transmitting optical unit 910. Coupler 912 and outgoing optics 910 are preferably selected such that transmit fiber 914 is coupled without significant loss to transmit both multiplexed wavelengths so that short-range optical connection of transmit fiber 914 is possible. A suitable transmission optical fiber for the system shown in FIG. 9 is Corning SMF28. If the transmission fiber is relatively short, 760 nm laser light coupled to the transmission fiber 914 does not exhibit significant multi-mode operation. For the system and optical fiber shown in FIG. 9, it was determined that the transmission optical fiber should be kept less than 3 meters in length to avoid the introduction of significant multi-mode noise.

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

The object of the invention is realized through the embodiments disclosed herein. Those skilled in the art will appreciate that various forms of the invention may be accomplished in various 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 described in the following claims.

Claims (28)

In an optical device: Multimode fiber; And Means for averaging mode noise resulting from changes in signal levels of light propagating within the multimode optical fiber; and Means for averaging are: Means for periodically varying the refractive index of the multimode optical fiber for a selection period; And And one of the means for scrambled light dispersion in the multi-mode optical fiber. delete The method of claim 1, wherein the means for averaging is: Means for periodically changing a temperature of the multimode optical fiber; And One of the means for periodically manipulating the multimode optical fiber. 4. The means of claim 3, wherein the means for periodically manipulating the multimode fiber is: Twisting the multi-mode fiber; Stretching the multimode optical fiber; Shaking the multi-mode optical fiber An apparatus configured to execute at least one of the following. 4. The apparatus of claim 3, wherein the means for periodically changing the temperature of the multimode optical fiber includes a thermal element through which heat is transferred with the multimode optical fiber, wherein the thermal element is a heater, a cooler, a fluid heated above ambient temperature. At least one of a source of fluid cooled below the source and the ambient temperature. The method of claim 3; A temperature sensor in thermal contact with the multimode optical fiber; And And a controller for receiving an input from said temperature sensor and controlling means for periodically changing the temperature of said multi-mode fiber. In a method of time-averaging mode noise resulting from signal strength variations in a multimode fiber with inputs and outputs: Coupling light to the input of the multi-mode fiber; Periodically changing the refractive index of the multi-mode optical fiber; And Receiving the light at the output of the multi-mode fiber &Lt; / RTI &gt; The method of claim 7, wherein the refractive index of the multi-mode fiber is: Periodically changing a temperature of the multi-mode optical fiber; And Changing to one of the steps of periodically manipulating the multi-mode fiber. 10. The method of claim 8, wherein periodically changing the temperature of the multimode optical fiber comprises providing a thermal component to which heat is transferred to the multimode optical fiber. The method of claim 9; Providing a temperature sensor through which heat is transferred to the multi-mode optical fiber; And Controlling the thermal component with a controller receiving an input from the temperature sensor &Lt; / RTI &gt; 9. The method of claim 8, wherein periodically manipulating the multimode fiber comprises: Twisting the multi-mode fiber; Stretching the multimode optical fiber; Shaking the multi-mode optical fiber How to include at least one of In an optical device for averaging mode noise due to a change in signal level of light: Multimode fiber; A thermal element in thermal contact with the multimode optical fiber; A temperature sensor in thermal contact with the multimode optical fiber; And A controller that receives an input from the temperature sensor and controls the thermal element; And the controller periodically varies the temperature of the multimode optical fiber. delete The method of claim 12; A heat sink in thermal contact with the thermal element; A fan through which the heat sink and the fluid are delivered Optical device further comprising. 13. The optical device of claim 12 further comprising a spool for supporting a selected length of the multimode optical fiber in substantially thermal contact with the thermal element. 16. The optical device of claim 15, wherein the selected length of the multimode optical fiber is between 55 m and 100 m. The method of claim 12, wherein the thermal element comprises at least one of a thermoelectric module, a resistance heater, an infrared heater, a chemical heater, a refrigerating device, a chemical cooler, a fluid source cooled below ambient temperature, and a fluid source heated above ambient temperature. It includes an optical device. In an optical device for averaging mode noise resulting from a change in signal level of light: Multimode fiber; And And an operating device operatively associated with the multimode optical fiber. 19. The multi-mode of claim 18, wherein the operating device includes at least one of extending the selected length of the multi-mode optical fiber, twisting the selected length of the multi-mode optical fiber, and shaking the selected length of the multi-mode optical fiber. Optical device that performs mechanical manipulation of optical fibers. 19. The optical device according to claim 18, wherein the operation device includes a piezoelectric extender. 21. The optical device of claim 20, wherein the piezoelectric stretcher is configured to stretch the multimode optical fiber through an oscillation order selected to effect an optical mode transition of 180 degrees with a minimum and maximum stretching distance. 19. The optical device of claim 18 wherein the operating device comprises a motor. 23. The method of claim 22, wherein the motor periodically rotates the first portion of the multimode optical fiber alternately clockwise and counterclockwise with respect to the longitudinal axis of the multimode optical fiber and the second fixed portion of the multimode optical fiber. Optical device configured to twist. In the combustion detection device constituting the receiving optical system: Multimode fiber; And Means for averaging mode noise resulting from signal level changes of light propagating within the multimode optical fiber, Means for averaging are: Means for changing the refractive index of the multi-mode optical fiber periodically during a selection period; And Means for scrambling light dispersion in the multimode optical fiber; Device comprising one of the. delete The method of claim 24, wherein the means for averaging is: Means for periodically changing a temperature of the multimode optical fiber; And Means for periodically manipulating the multimode optical fiber Device comprising one of the. 27. The apparatus of claim 26, wherein the means for periodically manipulating the multimode fiber is: Twisting the multi-mode fiber; Elongate the multimode optical fiber; Rocking the multi mode optical fiber And a device configured to execute at least one of the devices. 27. The multi-fiber optical fiber of claim 26, wherein the means for periodically changing the temperature of the multi-mode optical fiber comprises at least one of a heater, a cooler, a fluid source heated above ambient temperature and a fluid source cooled below ambient temperature. And a thermal element through which heat is transferred.

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