CN1938621B - Optical mode noise averaging device - Google Patents

Abstract

An optical mode noise averaging device (304), comprising: a multimode optical fiber (302) and an averaging device (308), used for averaging signal intensity variations induced by optical mode noise propagating in the multimode optical fiber (302). The averaging means may average modal noise induced signal strength variations by periodically varying the refractive index of the multimode fiber (302), perturbing the light distribution within the multimode fiber (302), or both, over a selected period of time. By periodically changing the temperature of the multimode fiber (302), the refractive index of the multimode fiber can be changed periodically. Alternatively, by periodically manipulating the multimode fiber (302), the refractive index can be changed or the light distribution within the multimode fiber can be disturbed.

Description

Optical Pattern Noise Averaging Device

technical field

The present invention relates to combustion monitoring operations, and more particularly to apparatus and methods for averaging modal noise associated with multimode optical fibers.

Background technique

Most of the electrical power generated in the United States is produced at coal-fired thermal power plants. Most global electricity production also relies on coal as the primary source of energy. As long as the long-term environmental impacts are from waste storage at nuclear power plants and inefficiencies with solar power plants, coal is likely to remain the primary source of energy for the foreseeable future. Furthermore, the huge global coal stockpiles are sufficient to last at least 200 years at current consumption levels.

However, there are still high demands now and in the future to reduce the pollutant emissions associated with coal-fired power generation and to increase the overall efficiency of the coal-fired power generation process. Traditionally, in thermal power plants and other industrial combustion facilities, the efficiency of the combustion process and the level of pollutant emissions have been determined indirectly through measurements taken from gas samples using techniques such as non-dispersive infrared (NDIR) photometry. Extractive sampling systems are not particularly well suited for closed-loop control of combustion processes, as significant time delays can be introduced between the time of gas extraction and final analysis. In addition, the extraction process typically results in a single-point measurement that may or may not represent the actual concentration of the species being measured within the highly variable and dynamic combustion process chamber.

More recently, laser-based photomolecular species sensors have been used to solve problems associated with extraction measurement techniques. Another advantage of laser-based measurement technology, which can be implemented in the field, is high-speed feedback suitable for dynamic process control. One promising technique for measuring combustion gas composition, temperature and other combustion parameters is tunable diode laser absorption spectroscopy (TDLAS). TDLAS is well suited for controlling and monitoring coal-fired combustion processes. TDLAS is also suitable for monitoring other combustion processes. Specifically, the spectroscopy described here is suitable for monitoring and controlling combustion processes in jet aircraft engines. TDLAS is usually implemented using diode lasers operating in the near-infrared and mid-infrared spectral regions. Suitable lasers are widely used in the telecommunications industry and are therefore readily available in TDLAS applications. Various TDLAS techniques have been developed which are more or less suitable for monitoring and controlling combustion processes. Well known techniques are wavelength modulation spectroscopy, frequency modulation spectroscopy and direct absorption spectroscopy. Each of these techniques is based on a predetermined relationship between the amount and nature of the laser light received by the detector, which occurs after the light is transmitted through the combustion process chamber and is absorbed in a specific spectral band that occurs in the process or combustion process. The characteristic spectral bands of the gas in the chamber. The absorption spectrum received by the detector is used to determine the quantity of the gas species analyzed and the associated combustion parameters, eg temperature.

The size of one side of the combustion chamber in a typical coal-fired thermal power plant is 10-20 meters. The fuel of the thermal power plant is pulverized coal, and the combustion process produced by it is blocked by the transmission of the laser due to the high degree of dust, and it also emits strong light. Its environment is also highly turbulent. The overall transmission rate of light through the combustion process chamber fluctuates wildly with time due to broadband absorption by particles, scattering or beam steering due to refractive index fluctuations. A strong thermal background radiation is also generated from the coal burning particles, which can interfere with the signal of the detector. Environments other than thermal power plant boilers also pose problems for the implementation of TDLAS detection or control systems. For example, any electronic components, optical systems or other sensitive spectroscopy components must be placed away from intense heat sources, or be suitably shielded and cooled. Even though the implementation of TDLAS systems under these conditions is extremely difficult, TDLAS is still particularly suitable for monitoring and controlling the coal combustion process.

As discussed in detail in International Patent Application Serial Number PCT/US04/10048 (Publication Number WO2004/090496), filed March 31, 2004, entitled "METHOD ANDAPPARATUS FOR THE MONITORING AND CONTROL OF COMBUSION", which application is incorporated in its entirety For reference herein, fiber optic coupling is particularly advantageous for implementing TDLAS systems. In a fiber-coupled system, one or more probe beams, which can be composed of multiplexed beams of various relevant wavelengths, are transmitted to the launch optics and projected into the combustion chamber. After passing through the combustion chamber, the probe beam is received in the receiver optics. As discussed in detail in International Patent Application Serial Number PCT/US04/10048, it is advantageous to use multimode fiber in the receiver optical system. The use of multimode fiber inevitably produces modal noise, which is the signal intensity variation of the detected light, which is generated by the non-uniform time and wavelength-varying light distribution in the multimode fiber core that collects and transmits light. Receive-side mode noise can interfere with the absorption characteristics that must be observed for effective TDLAS.

The phenomenon of modal noise is not limited to TDLAS or is caused by TDLAS devices, which are characterized by multimode fibers at the receiving end. In contrast, modal noise inevitably occurs in any multimode fiber of sufficient length to transmit light. Mode noise is unavoidable in multimode fiber, because multimode fiber has a large cross-sectional diameter compared with single-mode fiber, which allows light to propagate along multiple optical paths or in multiple modes. Some pathlengths or modes are longer or shorter than others. Therefore, constructive interference and destructive interference must occur, resulting in a non-uniform time- and wavelength-varying light distribution in the core of the multimode fiber, which causes the typical mode noise speckle pattern. Thus, modal noise occurs in computing, telecommunications, or other scientific equipment that utilizes multimode fiber of sufficient length. Whether modal noise affects the efficiency of a given optical system depends on the requirements of the particular system.

It is an object of the present invention to overcome one or more of the problems discussed above.

Contents of the invention

The invention is an optical mode noise averaging device, comprising: a multimode optical fiber and an averaging device, which is used for averaging the signal intensity variation induced by the optical mode noise propagating in the multimode optical fiber. The averaging means may average modal noise induced signal strength variations by periodically varying the refractive index of the multimode fiber, perturbing the light distribution of the multimode fiber, or both over a selected period of time. By periodically changing the temperature of the multimode fiber, the refractive index of the multimode fiber can be changed periodically. By periodically and physically manipulating a multimode fiber, the refractive index can be changed or the light distribution within the multimode fiber can be disturbed.

The temperature of the multimode fiber can be changed through the heat exchange between the heat element and the multimode fiber. Suitable devices for use as thermal elements include, but are not limited to, thermoelectric modules, resistive heaters, infrared heaters, chemical heaters, general cooling devices, fluid sources cooled below ambient temperature, or fluid sources heated above ambient temperature .

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

In another embodiment described for periodically manipulating a multimode fiber, the manipulating may include twisting the multimode fiber, stretching the multimode fiber, or shaking the multimode fiber. Piezoelectric stretchers can be used to complete the periodic stretching of multimode fibers. Alternatively, the motor may periodically twist portions of the multimode fiber in alternating clockwise and counterclockwise directions relative to the longitudinal axis of the fiber and relative to the fixed portion of the fiber.

The invention also relates to a method of averaging optical mode noise in a multimode fiber, comprising: coupling light to the input of the multimode fiber, periodically changing the refractive index of the multimode fiber, and receiving the averaged noise at the output of the multimode fiber of light. The modal noise averaging method may include changing the index of refraction by one of two methods of periodically changing the temperature of the multimode fiber and periodically operating the multimode fiber. The temperature of the multimode fiber can be periodically varied by providing a thermal element in thermal communication with the multimode fiber. Alternatively, the multimode fiber can be manipulated periodically by twisting the multimode fiber, stretching the multimode fiber, or shaking the multimode fiber.

Description of drawings

Fig. 1 is the schematic diagram of TDLAS detection equipment;

Figure 2 is a schematic diagram of the TDLAS detection `device, which depicts remote components optically coupled to components near the combustion chamber;

3 is a schematic diagram of an optical mode noise averaging device according to the present invention;

Figure 4 is an exploded view of a temperature-based phase-shifting device with a fluid source heated above ambient temperature or cooled below ambient temperature as the thermal element;

Figure 5 is an exploded view of a temperature-based phase shifting device utilizing a series of thermoelectric devices as thermal elements;

Figure 6 is another exploded view of the temperature-based phase shifting device shown in Figure 5;

Figure 7 is a schematic diagram of an optical mode noise averaging device using a motor for mechanically manipulating a multimode optical fiber;

Figure 8 is a schematic diagram of an optical mode noise averaging apparatus utilizing a piezoelectric stretcher for mechanically manipulating a multimode optical fiber; and

FIG. 9 is a schematic diagram of a transmitter mode noise reduction device.

Detailed ways

A. Overview

A preferred embodiment of the invention is an optical pattern noise averaging device. In Section E below, we describe the optical pattern noise averaging device in detail. The optical mode noise averaging device is particularly suitable for, but not limited to, averaging mode noise inherent in receive-end multimode fibers associated with fiber-coupled tunable diode laser absorption spectroscopy (TDLAS) detection devices. Multiple embodiments of such detection devices are discussed in detail in International Patent Application Serial Number PCT/US04/10048 (Publication Number WO2004/090496), filed March 31, 2004, entitled "METHOD AND APPARATUS FOR THE MONITORING AND CONTROL OF COMBUSION", the entire text of which is hereby incorporated by reference. In addition, fiber-coupled TDLAS detection equipment is described below. Preferred embodiments of the present invention are applicable to averaging optical pattern noise in any optical system having pattern noise. Specifically, the optical mode noise averaging device can be implemented in any computing, telecommunications, scientific research, or other system that has a sufficient length of multimode optical fiber to transmit light. The averaging device can be used in any optical system in which the efficiency of the system can be increased by averaging the optical mode noise inherent in light propagating within a multimode fiber.

B. Testing equipment

Figure 1 shows an embodiment of a detection apparatus 10 suitable for monitoring, monitoring and controlling the combustion process. The detection device 10 performs Tunable Diode Laser Absorption Spectroscopy (TDLAS) using a series of laser light emitted by a tunable diode laser 12 at selected frequencies in the near-infrared or mid-infrared spectrum. The output of each tunable diode laser 12 is coupled to a single optical fiber, which may be a single-mode optical fiber 14 , and routed to a multiplexer 16 . As used herein, the terms "coupling", "optical coupling" or "optical communication" are defined as a functional relationship between individual elements in which light can be transmitted from a first element to a second with or without intermediate elements or free space. element. Some or all of the frequencies of laser light generated within multiplexer 16 are multiplexed into multiplexed probe beams having a plurality of selected frequencies. The multiplexed probe beam is coupled to launch fiber 18 and transmitted to launch optics 20 or collimator, which is operatively associated with combustion chamber 22 shown in FIG. 1 .

Transmit end optics 20 are oriented to project a multiplexed probe beam through combustion chamber 22 . In the combustion chamber 22 , the receiving optical element 24 is in optical communication with the transmitting optical element 20 . Preferably, receiver optics 24 are substantially opposite transmitter optics 20 and operate in relation to combustion chamber 22 . Receiver optics 24 are positioned and oriented to receive the multiplexed probe beam projected through combustion chamber 22 . The receiving optical element 24 is optically coupled to the receiving optical fiber 26 , and the multiplexed probe beam received by the receiving optical element 24 of the optical fiber transmission section is sent to the demultiplexer 28 . In the demultiplexer 28 , part of the multiplexed probe beam received by the optical element 24 at the receiving end is demultiplexed, and the demultiplexed laser light of each wavelength is coupled to the output optical fiber 30 . Each output fiber 30 is in turn optically coupled to a detector 32, typically a photodetector sensitive to a selected laser frequency, which is multiplexed into the probe beam. Detector 32 generates an electrical signal based on the nature and amount of light transmitted to detector 32 at the detector frequency. The electrical signal from each detector 32 is typically digitized and analyzed in a data processing system 34 . As discussed in detail below, the digitized and analyzed data can be used to monitor physical parameters within the combustion chamber including, but not limited to, concentrations of various gas species within the combustion chamber 22 and combustion temperatures. Data processing system 34 may also be used to actively control selected process parameters by sending signals through feedback loop 36 to combustion control device 38 . In the case of a combustion process, process control parameters may include: fuel (eg, pulverized coal) feed rate; oxygen feed rate, and catalyst or chemical reagent feed rate. The use of fiber optics to couple electronic and optical components to the input and output of the detection device 10 allows precision and temperature sensitive equipment to be placed in a control room with a stable operating environment, such as the tunable diode laser 12, the detector 32 and the data processing system 34. Therefore, only the relatively robust transmit-end optics 20 and receive-end optics 24 need be placed near the hostile environment of the combustion chamber 22 .

FIG. 2 shows a schematic diagram of the overall component arrangement of the fiber-coupled multiplexing detection system 40 . The detection system 40 generally includes: a system rack 42, an intermediate box 44, a transmitter 46 with a transmitter optics 48, a receiver 52 with a receiver optics, and connecting optical fibers. The system rack 42 is preferably located in a remote control room, eg, 1 km from the combustion chamber 54 . A remote control room usually has a suitable control environment. System rack 42 contains laser 56 , detector 58 , wavelength multiplexer 60 and wavelength demultiplexer 62 . System rack 42 also houses system electronics and control software (not shown in Figure 2). Optionally, the system chassis 42 contains an alignment light source 64 .

The optical fiber connecting the system rack 42 to the distribution box 44 is typically a standard single-mode telecommunications optical fiber. This type of fiber is an inexpensive, readily available, and low-loss fiber, and allows laser light to be directed to off-the-shelf telecommunications components for manipulating light, such as optical switches, beam splitters, and wavelength-division multiplexers. Without fiber coupling, the laser light must be directed directly through free space up to the combustion chamber 54, which is difficult to achieve, or sensitive electronic and optical components must be very close to the combustion chamber 54.

Also shown in Figure 2 is a breakout box 44. The split box 44 is a solid seal placed near the boiler. Splitter box 44 contains optical switches, beam splitters, and couplers (generally designated 66), which may be used to direct optical signals to multiple transmitter-receiver head pairs, as discussed below.

A third group of system elements shown in FIG. 2 is transmitter head 46 and receiver head 50 . The optics and electronics in the transmitter head 46 and receiver head 50 must convert the light in the fiber optic 68 into a collimated beam that is precisely directed through the combustion chamber 54 where it is captured at the far end of the combustion chamber 54 , and couple the beam into the fiber 70. The optics to do this are determined by the transmission distance, the turbulence in the burn zone, its effect on the quality of the transmitted beam, and the fiber 70 core size. The core size is selected based on design criteria determined by the application. A larger core traps more laser light, but also traps more background light. A core diameter of 50 microns gives acceptable results when used in coal fired boilers. Fiber-optic coupling at the receiving (receiver) end has several advantages. Specifically, only light traveling in the same position and in the same direction as the laser light is focused into the optical fiber 70 . This can greatly reduce the amount of background light being detected. Light can be captured into one of several receiver fibers, while an optical switch or other light routing device can select the light routed to one of the detector fibers. Only one receiver optical element is shown in FIG. 2 .

Utilizing fiber optic coupling at the receive end requires precise maintenance of alignment tolerances of the transmitter and receiver optics (less than 0.5 mrad in the direction of the transmitter and receiver). Preferably, the optical element 48 at the transmitting end and the optical element 52 at the receiving end are specially designed and corrected for aberrations according to the wavelength from 660nm to 1650nm, so multiple laser signals can be transmitted and received with high efficiency at the same time.

C. Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) can be accomplished using techniques well known to those skilled in the art of laser spectroscopy. In general, TDLAS is accomplished by transmitting laser light through the target environment, after which it detects the absorption of the laser light at specific wavelengths, for example, the absorption of target gases such as carbon monoxide or oxygen. Spectral analysis of the detected laser light can identify the type and amount of gas along the path of the laser light. In Teichert, Fernholz, and Ebert: "Simultaneous in situ Measurement of CO, H ₂ O, and GasTemperature in a Full-Sized, Coal-Fired, Power Plant by Near-Infrared Diode Laser" (Applied Optics, 42(12): 2043 Details of direct spectroscopy are discussed in , 20 April 2003), which is hereby incorporated by reference in its entirety. The non-contact nature of laser absorption spectroscopy is well suited for harsh environments, such as the combustion zone of coal-fired thermal power plants, or flammable or toxic environments, where other detection methods cannot be utilized. The use of lasers can provide high brightness, which is necessary to receive detectable transmissions under severe attenuation (typically greater than 99.9% loss of light), which can be seen in some environments. To better withstand the harsh conditions of the target application, laser light can be directed into the target environment through armored optical fibers.

Efficient detection of temperature or multiple combustion process constituent gases requires TDLAS with the capability of multiple widely spaced frequency lasers. The selected frequency must match the absorption line of the transition being monitored. For example, NO can be monitored at a wavelength of 670 nm to simulate _the emitted NO concentration. Oxygen, water vapor (temperature), and carbon monoxide can also be monitored in coal-fired boilers. Based on the assumption that the laser detection path length through the combustion chamber is equal to 10 m and the mole fractions of each gas species are CO (1%), O ₂ (4%), CO ₂ (10%), and H ₂ O (10%), you can choose a suitable absorption line, and a suitable laser frequency. In choosing the frequency, it can be assumed that the process temperature is 1800K, which is slightly higher than typically observed in coal-fired power plants, but this cushion is used in the calculations as a safety factor.

For example, three water absorption lines for TDLAS can be chosen to meet the following criteria:

1. Low state energies are ~1000, 2000, and 3000cm ^-1 respectively;

2. The provided absorption coefficient is between 0.1-0.4, which can lead to about 20% of beam resonance absorption;

3. The best case is to use the transition in the range of 1250nm to 1650nm, where cheap, high power, DFB diode telecom lasers are available;

4. The individual transitions must be separated sufficiently to enable multiplexed operations; and

5. The selected wavelength must be efficiently diffracted by the existing multiplexing/demultiplexing grating.

Suitable water lines occur at the following wavelengths:

Table 1

wavelength(nm) Wavenumber(cm ^-1 ) Low state energy (cm ^-1 ) Grating order Absorption at 1800K and 10M UNP grating efficiency (model) 1349.0849 7412.432 1806.67 6.87 19.7% 81% 1376.4507 7265.062 3381.662 6.73 28.1% 77% 1394.5305 7170.872 1045.058 6.65 6.8% 72%

No interference from any other combustion gases can be expected. Assuming that the species most likely to interfere is CO ₂ , it has no strong interference lines in the 1.3 micron-1.4 micron region.

Similarly, an appropriate carbon monoxide line can be selected based on the Ebert work referenced above. Using the R(24) spectral line in a coal-fired boiler, a suitable carbon monoxide spectral line can be found at 1559.562nm. This line was chosen to avoid interference from water and carbon monoxide. Known gratings are very effective in this wavelength region, since it is in the C-band of optical communication. The absorption coefficient of CO at this wavelength is expected to be 0.7%.

Additionally, oxygen can be measured at 760.0932nm. The preferred mux/demux grating efficiency calculated in this region is 40%, however, at a reasonable measured efficiency there should be a suitable laser power.

As discussed here, the use of fiber optic coupling at the transmit and receive ends of a TDLAS inspection system requires precise alignment of the transmit and receive optics. A possible alignment wavelength is 660nm, since there are high power (45mW) diodes at this frequency, and 660nm is near the peak of the 14th order grating operation. It may be determined that other alignment wavelengths are equally or more suitable.

In summary, Table 2 shows a set of reasonable wavelengths selected for multiplexing as probe beams for TDLAS according to an embodiment of the present invention. It should be noted that this set of wavelengths is an embodiment of TDLAS detection equipment, which is suitable for detection and control of coal-fired thermal power plants. Other sets of wavelengths are equally suitable.

Table 2

Purpose wavelength(nm) alignment 660 O ₂ ba band 760.0932 H ₂ O (medium temperature spectrum) 1349.0849 H ₂ O (high temperature line) 1376.4507 H ₂ O (low temperature line) 1394.5305 CO R(24) of harmonics of R(2,0) 1559.562

D. TDLAS utilizes specific advantages of multiplexed beams

A specific advantage of TDLAS using wavelength multiplexed probe beams is that it can improve the accuracy of temperature measurements. In order to accurately measure concentrations using TDLAS, the temperature of the gas being monitored must be known. The strength of molecular absorption is a function of temperature. Therefore, in order to convert the magnitude of the absorption feature to concentration, the temperature must be known. Some previous attempts to measure the concentration of combustion species, eg, measuring CO, have had difficulty measuring temperature with sufficient accuracy, leading to errors in quantitative measurements. This is especially true for diode laser based ammonia slip monitors, which traditionally cannot measure temperature at all. In the detection system of the present invention, temperature can be determined by measuring the ratio of the intensities of two or more molecular water lines. The ratio of the integrated intensities of the two lines is a function of temperature only (assuming a constant total system pressure). Therefore, in principle, two spectral lines provide accurate temperature. However, in the case of a non-uniform temperature distribution, as is usually the case in industrial combustion processes, two spectral lines are not sufficient to determine the temperature distribution. In the case of such a non-uniform temperature distribution, the two spectral lines can only determine the "path-averaged" temperature. In contrast, measuring the integrated magnitude of more than two (of the same species) spectral lines can detect temperature non-uniformities. An example of this technique is described by Sanders, Wang, Jeffries and Hanson using oxygen as a probe molecule (Applied Optics, 40(24):4404, 20 August 2001 ), which is hereby incorporated by reference in its entirety. The preferred technique relies on the fact that the peak intensity distribution measured along the line of sight differs from the distribution over the optical path at an average temperature of 500K, for example 300K over half the optical path and 700K over the other half.

In addition to the advantage of more accurate temperature measurements, the use of multiplexed probe beams enables simultaneous monitoring of multiple combustion gas species, allowing for more precise control of the combustion process.

E. Pattern noise

Optical systems for TDLAS systems and similar devices requiring multiplexing of widely spaced wavelength signals suffer from many design challenges due to the opposing design requirements of reduced pattern noise and high efficiency light collection. Here, modal noise is defined as the signal intensity variation of the detected signal, which is the result of non-uniform time and wavelength-altered light distribution in the core of the fiber used to collect and transmit the light to and from the process chamber being measured. Light.

In multimode fibers, different modes propagate at different speeds due to changes in the refractive index. The intensity distribution in an optical fiber is a speckle pattern that is the result of the interference of all propagating modes that experience different effective optical path lengths. If we collect and detect all the light in the speckle pattern, the constructive and destructive interference exactly cancel each other out, so the total transmitted power is independent of the wavelength or the length of the fiber. Accurate cancellation cannot be achieved if clipping, vignetting or other losses are introduced, so the detected power is wavelength and/or time dependent. In the TDLAS detection system described above, power variation due to pattern noise is problematic. Some spectroscopy relies on the absorption of specific wavelengths of light by the species of gas being studied. Detection of absorption utilizes a reduction in power at a critical wavelength. Pattern noise can thus mimic absorption-related power drops, confounding data collected via TDLAS. The general formula for the detected power after transmitting a length z of fiber is:

P＝P ₀ +∑ _ij c _ij E _i E _j cos[(2πv ₀ Δn _ij z/c+ΔΦ _ij (T,σ))] (1)

where P ₀ = average power independent of wavelength

E _i = light amplitude of the nth transverse mode

C _ij = overlap integral between the i-th and j-th transverse modes

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

ΔΦ _ij = phase shift between the i-th and j-th transverse modes due to temperature and stress

In the case of orthogonal mode sets and no loss, C _ij =0. However, it makes some C _ij ≠0 if there is beam clipping or vignetting or other mode related losses. This can cause ripples in the average transmit power.

In a typical graded-index fiber with a 50-micron core, the total index change, Δn, is about 1%, but most modes spend the majority of their transit time near the center of the fiber core, so, generally speaking, Δn _ij <0.0005. Existing fiber GIF50 supports approximately 135 modes, which is sufficient to generate significant mode noise during wavelength scanning, assuming reasonably achievable levels of beam clipping.

As a concrete modal noise example, we can consider the simplest system with modal noise: a rectangular waveguide that supports only the lowest mode in one direction and the two lowest modes in its orthogonal direction:

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

Second lowest mode: E ₂ =E ₂ ⁰ [exp i(kz-ωt)] sinπx/a

The intensity at point z along the fiber direction is:

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

Where the integral must include clipping and vignetting effects.

In the absence of clipping, P ~ E ₁ ² +E ₂ ² , therefore, it is independent of wavelength. Adding clipping is equivalent to changing the upper and lower bounds of the integral. It can be shown that an additional term ~E ₁ E ₂ cosΔΦ is obtained after clipping, where ΔΦ=ΔKL=2πΔnL/λ.

If single-mode fiber is used in the receiver optics, as described above, then modal noise is not an issue. However, multimode fiber must generally be used in the receiver optics of fiber-coupled TDLAS systems for two reasons. First, the quality of the initial single-mode (Gaussian spatial distribution) beam is severely degraded after transmission through the measurement volume (combustion chamber with a measurement distance of more than 10 m). Therefore, the coupling efficiency of this severely distorted beam into the single-mode fiber is very low. This is an unacceptable situation because the beam is attenuated by 3-4 orders of magnitude after traveling through the measurement volume, mainly due to scattering and obscuration by soot and flying dust. The additional attenuation caused by the use of single-mode fiber can hinder the measurement. Second, the steering effect of the refracted beam in the fireball causes the position and pointing of the beam to be unstable. Under these effects, it is difficult to regularly "hit" the core of a single-mode fiber.

On the other hand, the core of multimode fiber is at least 25 times the target cross-sectional area of singlemode fiber. Thus, the effects of beam steering can be greatly reduced. Furthermore, since the coupling efficiency into the multimode fiber is independent of the spatial mode of the light, the resulting low beam quality after transmission through the fireball is not an issue.

Other types of installations in computing, telecommunications, or general science and technology may have other similar or entirely unrelated constraints that require or facilitate the use of very long multimode fibers. In other installations, pattern noise can also be problematic, presenting significant data collection or data transmission challenges.

Therefore, the mode-related losses that occur in multimode fiber systems are a major design challenge. The light distribution emerging from the core of a multimode fiber has a random speckle pattern, ie, a random pattern of bright and dark regions caused by constructive and destructive interference between different fiber modes. It is not a problem if the speckle pattern is completely invariant as a function of time and wavelength. However, if the beam is clipped anywhere in the multimode receiver optics, the slow variation of the speckle pattern as a function of wavelength can generate mode noise. This clipping cannot be avoided; it can only be reduced. Therefore, other measures to reduce pattern noise must be taken to improve the detection sensitivity of the system.

There can be several ways to mitigate pattern noise. According to the above formula (2), the mode noise can be reduced by the following methods:

1. Reduce mode-dependent losses, i.e. reduce clipping to keep C _ij small;

2. Reduce z, thereby increasing the period of the mode noise, making it much larger than the relevant absorption line;

3. Use low dispersion fiber to reduce Δn _ij ;

4. Scrambling or phase-shifting patterns; but not all pattern-scrambling or phase-shifting techniques are equally effective, as described below.

Preferably, the receiver optics of the fiber-coupled TDLAS detection system are designed and fabricated to incorporate all of the above methods to reduce modal noise. The optical system is designed such that, given almost perfect system alignment, any beam clipping should occur at a low level. Efforts should be made to keep multimode fibers to a minimum length; however, in some applications z must be long enough to have control circuits in environmentally controlled areas. The value of Δn _ij can be reduced by using the best low dispersion multimode fiber. Furthermore, by periodically varying the refractive index or mechanical manipulation of the multimode fiber at the receiving end and extracting data from the averaged optical signal collected, the individual modes can be averaged for optimal results.

The speckle pattern in multimode fiber varies as a function of time and wavelength, but also as a function of the mechanical position of the fiber. Transit time and wavelength are affected by the refractive index of the fiber. Bending and manipulating the fiber in certain ways can cause changes in the speckle pattern. If these mechanical operations or periodic changes in the refractive index are completed continuously within a period of time, the spatial distribution of light emitted from the optical fiber can be averaged into a relatively uniform pattern.

Time-averaged measurements yield uniform signal strengths through effective periodic phase shifting or scrambling pattern noise. The refractive index of a fiber can be changed by stretching or twisting the fiber or changing the temperature of the fiber. Changing the temperature of the fiber changes the refractive index difference Δn _ij between the i-th transverse mode and the j-th transverse mode. This change in fiber refractive index can phase-shift the mode noise according to the function cos(2πv ₀ Δn _ij z)/c given by equation (2).

F. Optical Pattern Noise Averaging

As shown in FIG. 3, periodic phase shifting or scrambling pattern noise can be accomplished using optical arrangement 300 to produce time-averaged measurements. Optical device 300 may include a multimode optical fiber 302 having an input end 304 and an output end 306 . Light can be coupled to input end 304 of multimode fiber 302 and propagate through the system generally in the direction of the arrow shown in FIG. 3 , which connects input end 304 and output end 306 .

The optical device 300 also includes an averaging element 308 associated with the multimode fiber 302 . Averaging element 308 may comprise a device that periodically varies the index of refraction of multimode fiber 302 over a selected period of time. Alternatively, averaging element 308 may comprise a device for perturbing the distribution of light within multimode fiber 302 . By means of the averaging element 308, by periodically changing the temperature of the multimode fiber 302, by periodically operating the multimode fiber 302, or both, the refractive index can be changed or the light distribution disturbed.

In embodiments where the averaging element 308 performs periodic operation of the multimode fiber 302 , the averaging element 308 may twist, stretch or vibrate the multimode fiber 302 . In embodiments where the averaging element 308 periodically varies the temperature of the multimode fiber 302, various thermal elements may be provided that are in thermal communication with the multimode fiber. Any device that affects the temperature of multimode fiber 302 may be included in averaging element 308 . Representative devices that can be used to affect the temperature of the multimode fiber 302 include: thermoelectric modules, resistive heaters, chemical heaters, conventional cooling devices utilizing compressed fluid and heat exchangers, chemical coolers, fluid sources cooled to below ambient temperature, and fluid sources heated above ambient temperature. Some of these devices are discussed in detail below.

In embodiments where the averaging element 308 causes the multimode fiber 302 to periodically heat or cool, the sensor 310 may also be placed in thermal communication with the multimode fiber 302 . Sensor 310 can provide information to controller 312 , and controller 312 can in turn control averaging element 308 via control line 314 .

G. Temperature-Based Phase Shift Devices

The effectiveness of the temperature-based mode phase shift is directly related to the temperature change per unit time and the fiber length z exposed to the temperature change. Temperature-based mode phase shifting is a particularly effective way to deal with modal noise because a change in fiber temperature changes the refractive indices of all transverse modes, and temperature changes can occur over a considerable length of fiber. Therefore, by changing the refractive index of the fiber, we can ensure that all transverse modes are phase shifted and no mode can remain "frozen" with the signal.

Virtually any type of heating/cooling system can be placed in heat exchange with the multimode fiber 302 to cause periodic changes in the temperature of the fiber. Resistive heaters, conventional cooling devices, heated or cooled fluids, Peltier or other thermoelectric devices, infrared devices, or chemical devices can be used to affect the temperature of the fiber.

One embodiment of a mode-shifting device that utilizes periodic temperature changes is a fluid-based mode-shifting device (fluidic device) 400 . Figure 4 shows an exploded view of the fluidic device. Figure 4 shows an embodiment of a fluidic device 400 utilizing vortex air tubes 402A, 402B to alternately blow hot and cold air to and around the optical fiber. Air delivered from a compressed air source (not shown in Figure 4) is delivered alternately to one of the two vortex tubes 402A, 402B. Vortex tubes 402A, 402B are coupled in fluid communication with the interior of vessel 404 . As shown in FIG. 4 , the container 404 may be formed from an enclosure 406 having side panels 408A, 408B, a top panel 410 , a front inlet 412 , and a rear inlet 414 . The entire enclosure 406 may be suitable for mounting in a typical data processing equipment rack. While a rack mountable housing 406 is particularly convenient, any housing shape, type or pattern suitable for housing a reel 416 on which a length of multimode optical fiber 418 is wound may be used to form fluidic device 400 . Alternatively, a device without housing 406 may be used.

The vortex tubes 402A, 402B are in fluid communication with the interior of the housing through the rear inlet 414 , and thus, are in fluid and heat communication with the multimode optical fiber 418 wound on the reel 416 . Thus, the multimode fiber 418 may be periodically heated and/or cooled by using the vortex tubes 402A, 402B to bring heated air above ambient temperature or cooled air below ambient temperature.

Corporation购买

3230涡流管。这些或类似的涡流管工作在30ft³/minute的生产率下，它提供加热到+60°或冷却到-20°的空气，这与涡流管的取向有关。此外，利用涡流管可以相对容易地在加热的空气与冷却的空气之间循环。然而，重要的是注意到，周期性提供加热或冷却的流体与多模光纤418热交换的任何设备或方法适合于实现流体装置400的实施例。然而，加热和冷却的流体可以是以上讨论的空气，水，加热/冷却油，压缩气体，或可用于加热或冷却多模光纤的其他流体。Suitable vortex tubes 402A, 402B can be readily purchased. For example, from

Corporation purchase

3230 Vortex Tube. These or similar vortex tubes operate at a production rate of ^30ft3 /minute, which provides air heated to +60° or cooled to -20°, depending on the orientation of the vortex tubes. Additionally, cycling between heated and cooled air is relatively easy with a vortex tube. However, it is important to note that any device or method that periodically provides a heated or cooled fluid in heat exchange with multimode optical fiber 418 is suitable for implementing the fluidic device 400 embodiment. However, the heating and cooling fluid can be air as discussed above, water, heating/cooling oil, compressed gas, or other fluids that can be used to heat or cool multimode optical fibers.

By way of example, and not limitation, during operation, one vortex tube 402A, 402B may deliver heated air until the fiber reaches a temperature about 10°C above the inlet temperature. The temperature of the fiber can be determined by a thermocouple 420 or other temperature sensor embedded in contact with the fiber. A temperature control unit (not shown in Figure 4) may receive input from the thermocouple 420 and trigger a solenoid switch, sending air to another vortex tube 402A, 402B for cooling. Alternatively, since the heated air supplied to the multimode fiber 418 by the vortex tubes 402A, 402B will never reach a critical temperature, the temperature controller may be omitted and replaced by a timed repeater for periodically exchanging between heating and cooling. Between the vortex tubes 402A, 402B. The cycling between temperatures is preferably continuous during operation of the device.

Figures 5 and 6 show a preferred embodiment of the mode phase shifting device, which is based on periodically varying the temperature of the multimode fiber. Thermoelectric mode phase shifting device (thermoelectric device) 500 includes a reel 502 for winding a selected length of multimode optical fiber 504 . 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 interior of the reel 502 . Heat exchange is formed by thermoelectric grease between the outer surface of the thermoelectric module 506 and the coil of the multimode optical fiber 504 . In the embodiment shown in FIG. 5 , the reel 502 is configured with an open edge, which facilitates contact between the thermoelectric module 506 and the multimode fiber 504 and facilitates winding of the multimode fiber 504 .

FIG. 6 shows an exploded view of the device with thermoelectric module 506 placed in contact with multimode optical fiber 504 and relative to reel 502 .

One or more thermal banks 508 may also be provided in heat exchange with the thermoelectric modules 506 . Preferably, thermal banks 508 are made of a high thermal conductivity material, such as aluminum or copper, with fins or other devices designed to increase the surface area of each thermal bank 508 . The location of the fan 510 can force or draw air through the thermal reservoir 508 to facilitate rapid extraction of heat from the thermoelectric module 506 for rapid heating or cooling of the multimode fiber 504 . As shown in FIGS. 5 and 6 , a frame 512 may be utilized with a top ring 514 and a bottom ring 516 that maintain proper orientation between the various elements of the thermoelectric device 500 without impeding air flow through the thermal store 508 . Preferably, a plurality of openings are formed in the frame 512 to ensure free flow of air.

The embodiment shown in Figures 5 and 6 utilizes a thermoelectric module 506 based on the Peltier principle. Direct current is provided to the thermoelectric module 506 through the leads 520 . With the thermoelectric module 506 operating under the Peltier principle, two opposing surfaces of the thermoelectric module 506 are heated or cooled, depending on the direction of the supplied direct current. Accordingly, these thermoelectric modules have certain advantages in that the multimode fiber 504 can be selectively heated or cooled relatively easily by selectively exchanging the polarity of the DC electricity provided to the lead wire 520 . It is important to note, however, that heating and/or cooling of multimode fiber 504 may be accomplished using other types of devices. For example, resistive heaters, conventional cooling devices, infrared heating devices, and/or chemical reaction based heaters and/or coolers may be used to vary the temperature of the multimode fiber 504 .

In this preferred embodiment, as shown in Figures 5 and 6, power may be delivered from a suitable power source to a thermoelectric module 506 mounted on a cylindrical device. A current switching circuit may be used to periodically reverse the polarity of the DC source delivered to each thermoelectric module 506 . Mode phase shifting occurs on a length of multimode fiber 504 , preferably Premium GIF50 multimode fiber, which is thermally exchanged with a thermoelectric module 506 . We have found that 55m to 100m is an appropriate length of multimode fiber for mode phase shifting and averaging. Other lengths of multimode fiber are also suitable. If 100m of cladding fiber is used, about 50% of the fiber is in direct contact with the thermoelectric cooler. It should be noted that with direct contact, there is no conductive material between thermoelectric module 506 and multimode fiber 504 other than thermoelectric grease. This configuration can reduce the thermal mass of the system. By reducing the thermal mass of the system, the temperature response of the system is fast and results in a more efficient mode phase shift.

One or more thermocouples 522 or other temperature measuring devices may be installed between the thermoelectric module 506 and the multimode fiber 504 and monitor the temperature of the multimode fiber 504 at all times. A temperature control unit (not shown in Figures 5 and 6) can receive the temperature measured by the thermocouple and change the direction of the current flow based on the temperature reading. With existing thermoelectric modules, a suitable range of about 35°C to 50°C can be achieved. It is important that the fiber does not exceed the maximum temperature of 85°C, otherwise the fiber may be damaged. The test operation was done using a single side cooler with a temperature change from 65°C to 10°C and a double side cooler with a temperature change from 65°C to 30°C. However, a full cycle can be of any chosen duration, we have found a cycle of about 25 seconds to be effective.

As mentioned above, heat may be dissipated on the opposite side of the thermoelectric module 506 by means of the thermal store 508 mounted in heat exchange with the thermoelectric module 506 . Air can be forced into or drawn through a heat sink that helps dissipate as much heat as the input power. Effective heat dissipation can be accomplished by forcing air through the openings 518 in the bottom of the heat sink so that the air flows through the system and out the top of the unit. Other configurations that generate sufficient air flow are also suitable. Alternatively, the thermoelectric device 500 can be placed in a cooling fluid, or cooled using other methods. Preferably, the fan operates continuously while the device is in operation. The heat can be dissipated using any suitable fan or fluid source, however, a 300 CFM fan is effective for removing heat from the system shown in FIGS. 5 and 6 .

Control circuitry may be associated with heating and cooling of thermoelectric module 506 . Based on input from a thermocouple 518, thermometer, or other temperature sensor, the feedback control circuit can sense the temperature of the fiber. Additionally, based on the temperature input, the controller can swap the current direction of power delivered to the thermoelectric modules 506 and adjust the power levels of the heating and cooling cycles (heating is generally more efficient, it requires less power). In addition, the controller can control the maximum and minimum power delivered to the fiber optic system and shut down the drive circuit in case of overheating.

H. Thermoelectric module phase shift system test

Testing was done using the thermoelectric device 500 described above. Testing is done by transmitting four bands through a non-absorbing nitrogen purge chamber. With no absorbing species in the optical path, the laser should exhibit a linear wavelength response after division by the reference signal. Deviations from slope linearity are mainly caused by pattern noise. A general formula for determining measurement inaccuracy is given by:

σ _x ＝[1/N∑(x _i -f _i (ax+b)) ² ] ^1/2 (3)

where x _i =signal _i /tap _i

Linear fitting of f _i (ax+b)=x _1-n

The start and end of each wavelength cycle can be ignored due to time delays due to propagation times at the transmitter and receiver. These time delays cause significant changes between the tapped and observed signals at the beginning and end of each cycle.

Testing of system performance was done with an embodiment of the thermoelectric device 500 utilizing single-sided and double-sided thermoelectric modules 506, multiple averaging times, and different fiber lengths. In all results, pattern phase shifting and averaging operations yield reduced pattern noise signal bias. The reduction in noise depends on the fiber length used in the experiment, as given in equation (3); longer fibers have higher frequency mode noise deviations relative to shorter fibers. Consequently, relative resolution enhancement of mode phase shifts is seen in longer fibers. The results for various configurations are given below:

table 3

No mode perturbation, turn off TEM

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, 100m working fiber length, 270m total fiber length, 191210℃-65℃ temperature range

average

10 3048 3896 2020 1871 2709 10 4213 2680 1856 3216 2991 10 3317 1742 2257 2838 2538 at the transition point 1 4957 1628 1736 2939 2815 cold to hot 1 2712 2490 1208 2345 2189 cold to hot 1 3119 3559 4965 1921 3391 cold to hot 1 2762 3163 3519 1541 2746 cold to hot 1 2468 3388 1928 2887 2668 cold to hot 1 3724 2479 2394 3005 2901 cold to hot 1 2865 2363 2700 1917 2461 cold to hot 1 5187 1924 2327 3633 3268 cold to hot 1 3327 3392 1447 1777 2486 cold to hot 1 4989 1556 2819 3090 3113 cold to hot 1 2812 1985 1542 1642 1995

75m workable multimode fiber, 245m total fiber length, single-sided TEM, 253165°C-10°C temperature range

average

cycle 10 3975 3465 2375 3014 3207 cycle 10 2997 963 1408 2085 1863 cold to hot 1 3111 1841 1538 2982 2368 cold to hot 1 2268 1518 2365 2932 2271

55m workable multimode fiber, 225m total fiber length, double-sided TEM, 216765°C-30°C temperature range

average

Mechanical operation base equipment

As discussed above, modal noise can be averaged and smoothed by periodically varying the refractive index or mechanically manipulating the multimode fiber and extracting data from the averaged signal collected. The temperature-based phase shifting devices discussed above accomplish mode phase shifting by varying the refractive index of a multimode fiber with periodic temperature changes. As discussed below, mechanical manipulation of multimode fibers can also be used to change the refractive index. Furthermore, mechanical manipulation can result in averaging and smoothing the signal affected by mode noise, because light is not able to follow a specific mode perfectly within the waveguide of the manipulated fiber. Thus, the mode noise-induced speckle pattern can be averaged and smoothed over a section of multimode fiber by a combination of phase shifting and mechanical perturbation.

Some specific modes of mechanical fiber manipulation are more effective at averaging mode noise than others. Specifically, twisting the fiber about the longitudinal axis (z) relative to some other point on the fiber can cause the speckle pattern to change. The main change obtained is the rotation of the speckle pattern around the z-axis. Importantly, the rotation of the speckle pattern around the z-axis is different when the optical fiber is rotated mechanically. The secondary effect is that the actual light distribution changes somewhat due to the rotation. The rotation of the speckle pattern is not primarily due to stress-induced changes in the refractive index of the fiber, although this could explain the small change in the speckle intensity pattern. Conversely, during twisting motion of the fiber, the speckle pattern rotates due to the inability of the light to follow the waveguide completely.

FIG. 7 shows a schematic diagram of a mechanical mode noise averaging apparatus (mechanical device) 700 according to one embodiment of the present invention. The mechanical device 700 utilizes a hollow shaft motor 702 to place and hold a multimode fiber 704 . The distal portion 706 of the optical fiber is fixed in position relative to the axis of the motor 702, which repeatedly undergoes twisting motion from +360 degrees to -360 degrees. The frequency of this motion is preferably greater than or equal to 10 Hz, so it effectively averages the transmitted signal and greatly reduces the effect of pattern noise at the receiver. While twisting of a multimode fiber along its longitudinal axis is effective for perturbing modal noise, other mechanical manipulations such as shaking, stretching, or bending can also be used.

piezoelectric stretcher

Stretching the fiber causes changes in the fiber's refractive index and length. Multimode fibers can be stretched using piezoelectric stretchers. Piezoelectric devices are commonly used to introduce modulation time delays in single-mode fibers. Multimode fibers are not used in piezoelectric stretching devices because the time delay of multimode fibers is not controllable due to the fact that light can travel multiple optical paths or in multiple modes. However, while it is impractical to create time delays, piezoelectric stretching devices can be used to introduce mode phase shifts.

When a multimode fiber is drawn, the stress introduced on the fiber can cause changes in the fiber's refractive index and length. As shown in Figure 8, the piezoelectric device 800 works by winding a few meters of multimode fiber 802 around a half cylinder 804, and then vibrating the half cylinder 804 at a predetermined oscillation frequency and distance. As the distance between the two half cylinders 804 expands and contracts, the stress in the multimode fiber 802 resonates. This resonance causes the refractive index of the fiber 802 to fluctuate. The effectiveness of the mode phase shift is a function of the change in fiber length (z) and the change in the refractive index (Δn _ij ) in the fiber (Equation 1).

Mode phase shifting can be accomplished using one of two techniques using piezoelectric device 800 . In the first technique, the piezoelectric device 800 has enough fibers 802 and is configured to introduce enough stress to produce large mode changes, so that uniform signal strength can be obtained by averaging many modes. Alternatively, because the piezoelectric device 800 cycles through a steady refractive index change, the piezoelectric device 800 can oscillate in such a way that it produces a 180° harmonic modal phase shift at minimum and maximum stretch distances. Using this approach, mode noise can be reduced by not temporally averaging the phase shifts of many modes, but by optimizing the stretch profile to achieve a 180° phase shift. Thus, pattern noise can be averaged in as little as one cycle, enabling fast data acquisition with reduced pattern noise.

Transmitter optical system (Pitch-Side Optical Train)

The optics at the launch end of a fiber-coupled TDLAS detection device also suffers from significant design challenges due to the need to generate a single-mode beam at all wavelengths transmitted through the measurement region. Mode noise is not an issue if single-mode fiber can be used throughout the transmitter optics. However, optical fibers can only operate as single-mode waveguides over a limited wavelength window. Beyond the short wavelength cutoff of a particular fiber, light can have several higher order spatial modes propagating through that fiber. The interference of these higher order modes can produce a speckle pattern when exiting the fiber. Speckle patterns vary with time and wavelength. Even small amounts of beam clipping can introduce noise in the measurement.

In contrast, if a fiber is selected with a single-mode cutoff that matches the shortest wavelength that needs to be transmitted, longer wavelengths suffer significant losses when coupled into the fiber, and the fiber has significant bend losses at long wavelengths .

This problem can be severe in the fiber-coupled wavelength-multiplexed TDLAS detection and control setup described above, because the wavelengths we need to multiplex can be as long as 1.67 μm and as short as 760 nm or 670 nm. There is no commercially available fiber that provides single-mode operation, high coupling efficiency, and low bend loss over such a broad wavelength range. In the future, photonic crystal fibers can solve this problem, but crystal fiber technology is still in its infancy.

As shown in Figure 9, the problem of multiplexing and launching light in a single-mode beam from 670nm or 760nm to 1670nm can be alleviated by using a very short transmission section of multimode fiber 120 that does not allow for higher spatial modes of wave The length is shorter than the cut-off wavelength of single-mode fiber. Referring to the above formula (1), if the length L of the multimode fiber is short, the modal noise can be reduced. In this case, for example, if 760nm light is coupled into a short single-mode fiber with a cutoff wavelength of 1280nm (for example, Corning SMF28), the 760nm light remains single-mode at least over distances of several meters. So, the solution to the problem of mode noise at the transmitter is to couple 760nm light into a single-mode fiber, which has a wavelength greater than 1280nm, but can also be a multimode fiber at 760nm, where the collimated light travels only a short distance before passing through the measurement region .

Figures 9 and 2 show schematic diagrams of such a system. Referring to FIG. 9, a plurality of diode laser sources 902 emitting at widely spaced laser frequencies are coupled to discrete single-mode optical fibers 904A-904n. Laser light emitted by diode lasers at wavelengths between 1349 nm and 1670 nm is multiplexed by multiplexer 906 . The output of the multiplexer 906 is coupled to the fiber 908 at the transmitting end with a suitable size, and the wavelength range of the emitted laser is from 1349nm to 1670nm, neither of which has a large transmission loss and does not introduce mode noise. A suitable fiber at these wavelengths is Corning SMF28. However, if multiplexed and coupled into SMF28 fiber, the 760nm input becomes multimode after transmission over a relatively short distance. Therefore, the output of the 760nm laser is coupled to a single-mode fiber whose wavelength is less than 1280nm, for example, SMF750. The laser light transmitted in the input fiber 904n and the multiplexed laser light transmitted in the transmit fiber 908 may be coupled in the vicinity of the transmit optical element 910 . The coupler 912 and the transmitting optical element 910 are preferably connected by a short transmission fiber 914, which is selected to transmit all coupled and multiplexed wavelengths without loss. A suitable delivery fiber for the system shown in Figure 9 is Corning SMF28. As long as the delivery fiber is relatively short, the 760nm laser coupled to the delivery fiber 914 has no serious multimode performance. In the system and optical fiber shown in Figure 9, in order to avoid the introduction of severe multimode noise, the length of the transmission optical fiber must be kept equal to or less than 3 meters.

In a similar system to that shown in Figure 2, coupler 134 receives the input from a 760nm diode laser and receives the multiplexed beam from a diode laser with a much longer wavelength.

Through the embodiments disclosed herein, we have fully understood the purpose of the present invention. Those skilled in the art should understand that various features of the present invention can be implemented through different embodiments without departing from the essential functions of the present invention. These specific examples are illustrative only, and are not intended to limit the scope of the invention, which is defined by the following claims.

Claims (15)

1. optical devices, comprising:

Multimode optical fiber; With

Equilibration device, the change in signal strength that the modal noise for the light propagated in this multimode optical fiber average brings out,

Wherein equilibration device comprises periodically-varied device, and the time period periodically for choosing at changes the refractive index of multimode optical fiber,

Wherein equilibration device comprises:

For the device of periodically-varied multimode optical fiber temperature.

2. according to the optical devices of claim 1, the device wherein for periodically-varied multimode optical fiber temperature comprises: with the thermal element of multimode optical fiber heat interchange, this thermal element at least comprises with one of lower device: well heater and refrigeratory.

3., according to the optical devices of claim 2, wherein this well heater is the fluid source being heated to more than environment temperature, and this refrigeratory is the fluid source being cooled to below environment temperature.

4., according to the optical devices of claim 1, also comprise:

With the temperature sensor of multimode optical fiber thermo-contact; With

Controller, inputs and the device controlled for periodically-varied multimode optical fiber temperature for receiving from temperature sensor.

5. a method for the change in signal strength of bringing out for time average modal noise in the multimode optical fiber having input end and output terminal, the method comprises:

Couple the light into the input end of multimode optical fiber;

The refractive index of periodically-varied multimode optical fiber; With

Receive the light of this multimode optical fiber output terminal,

Wherein by the temperature of periodically-varied multimode optical fiber, change the refractive index of multimode optical fiber.

6. optical devices for the change in signal strength of bringing out for the modal noise of average light, comprising:

Multimode optical fiber;

With the thermal element of this multimode optical fiber thermo-contact, for changing the temperature of this multimode optical fiber in the time period periodically chosen.

7., according to the optical devices of claim 6, also comprise:

With the temperature sensor of this multimode optical fiber thermo-contact; With

Controller, inputs for receiving from this temperature sensor and controls described thermal element.

8., according to the optical devices of claim 6, also comprise:

With the heat reservoir of described thermal element thermo-contact; With

The fan of fluid communication is carried out with this heat reservoir.

9. according to the optical devices of claim 6, also comprise: reel, for being wound around the multimode optical fiber that is chosen length, this multimode optical fiber and described thermal element thermo-contact.

10., according to the optical devices of claim 9, wherein the length of choosing of this multimode optical fiber is between 55m and 100m.

11. according to the optical devices of claim 6, wherein said thermal element at least comprises with one of lower device: electrothermal module, infrared heater, chemical heater, cooling device, with the fluid source being heated to more than environment temperature, for changing the temperature of this multimode optical fiber in the time period periodically chosen.

12. according to the optical devices of claim 11, and wherein electrothermal module is electric resistance heater, and cooling device is chemical refrigeratory or the fluid source being cooled to below environment temperature.

13. 1 kinds of combustion sensing apparatus comprising receiving end optical system, comprising:

Multimode optical fiber; With

Equilibration device, the change in signal strength that the modal noise for the light propagated in this multimode optical fiber average brings out,

Wherein equilibration device comprises periodically-varied device, and the time period periodically for choosing at changes the refractive index of multimode optical fiber,

Wherein equilibration device comprises:

For the device of periodically-varied multimode optical fiber temperature.

14. according to the combustion sensing apparatus of claim 13, and the device wherein for periodically-varied multimode optical fiber temperature comprises: with the thermal element of this multimode optical fiber heat interchange, this thermal element at least comprises with one of lower device: well heater and refrigeratory.

15. according to the combustion sensing apparatus of claim 14, and wherein this well heater is the fluid source being heated to more than environment temperature, and this refrigeratory is the fluid source being cooled to below environment temperature.

Images