CN1938621B

CN1938621B - Optical mode noise averaging device

Info

Publication number: CN1938621B
Application number: CN200580010448.0A
Authority: CN
Inventors: B.·P.·马斯特森; 艾利克·C.·休尔森; 伊恩·S.·史密斯
Original assignee: Zolo Technologies Inc
Current assignee: Zhengdian Technology Co., Ltd
Priority date: 2004-03-31
Filing date: 2005-02-02
Publication date: 2015-04-01
Anticipated expiration: 2025-02-02
Also published as: CA2561711C; WO2005103781A1; CA2561711A1; KR20060135850A; EP1730563A1; EP1730563A4; CN1938621A; AU2005236385A1; KR101212439B1; JP2007534983A

Abstract

An optical mode noise averaging device (300) including a multimode optical fiber (302) and means (308) for averaging a modal noise induced signal level variation of light propagating within the multimode optical fiber (302). The device may average modal noise induced signal level variations by cyclically varying an index of refraction of the multimode optical fiber (302) over a select period of time, scrambling a light distribution within the multimode optical fiber (302), or both. The index of refraction of the multimode optical fiber may be cyclically varied by cyclically varying the temperature of the multimode optical fiber (302). Alternatively, the index for refraction may be varied or the light distribution within the multimode optical fiber may be scrambled by cyclically manipulating the multimode optical fiber (302).

Description

Optical mode noise averaging device

Technical field

The present invention relates to combustion monitoring operation, be specifically related to the equipment for average relevant to multimode optical fiber modal noise and method.

Background technology

The most of electric power produced in the U.S. is produced in coal-fired station.Most of global electricity productioin similarly relies on coal as the main energy.As long as long-term environmental impact is from the waste storage of nuclear power generation plants and the poor efficiency with solar power plant, in predictable future, coal still may as the main energy.In addition, huge global coal storage capacity is enough to maintain at least 200 years under current consumption level.

But the present and the future still has very high requirement for the whole efficiency reducing the pollutant emission relevant to coal fired power generation and increase coal fired generation process.Traditionally, in fuel-burning power plant and other industrial combustion equipments, the efficiency of combustion process and the level of pollutant emission utilize the technology of such as non-dispersive type infrared ray (NDIR) photometric method indirectly to determine by extracting the measurement of gas sample.Extract sampling system is not be particularly suitable for closed-loop control combustion process, because can introduce very large time delay between gas extraction and the time of final analysis.In addition, leaching process obtains the measurement result of single-point usually, and in height change and dynamic combustion process indoor, this measurement result maybe may can not represent the actual concentrations measuring kind.

Recently, utilize laser-based optical molecule kind sensor to solve the problem relevant to extracting measuring technique always.Laser based measurement techniques can realize at the scene, and its another advantage is the high-speed feedback being suitable for dynamic process control.A kind of for measuring combustion gas composition, the prospect technology that has of temperature and other combustion parameters is tunable diode laser absorption spectroscopy art (TDLAS).TDLAS is very suitable for the combustion process controlled and monitoring is coal-fired.TDLAS is suitable for the combustion process of monitoring other equally.Specifically, spectroscopy described herein is applicable to monitor and forecast aerojet engine combustion process.TDLAS normally utilizes the diode laser worked near infrared and middle infrared spectral region to realize.Suitable laser instrument is widely used in telecommunications industry, so, be easily obtain in TDLAS application.People have developed the various TDLAS technology being more or less suitable for detection and control combustion process.Well-known technology is Wavelength modulation spectroscopy art, FM Spectroscopy Measurement art and direct absorption spectrometry.Often kind of technology in these technology be based on detecting device receive laser quantity and character between predetermined relationship, this occurs in optical transport by combustion process room and after specific light bands of a spectrum are absorbed, band is the characteristic frequency spectrum band of gas in this process or firing chamber.Detecting device receive absorption spectra for determining the quantity of analyzed gas species and relevant combustion parameter, such as, temperature.

In typical coal-fired station, the side size of firing chamber is 10-20 rice.The fuel of fuel-burning power plant is fine coal, and the combustion process that it produces makes the transmission of laser be obstructed because of height dust, but also sends high light.Its environment is also high turbulence.Due to the wide band absorption of particle, scattering or the light beam caused due to refractive index fluctuation turn to, and light is violent in time fluctuating by the overall transmission rate of combustion process room.From coal-burning particle, also produce strong thermal background radiation, this radiation can the signal of interference detector.Environment beyond heat power plant boiler also makes TDLAS detect or the enforcement of control system has problems.Such as, any electronic component, optical system or other sensitive spectroscopy art element must be placed on away from strong thermal source, or take suitable shielding and cooling provision.Even if the enforcement under these conditions of TDLAS system is extremely difficult, TDLAS remains and is particularly suitable for monitor and forecast coal combustion process.

As what discuss in detail in International PatentApplication Serial Number PCT/US04/10048 (Publication NumberWO2004/090496) that applies on March 31st, 2004, its title is " METHOD ANDAPPARATUS FOR THE MONITORING AND CONTROL OFCOMBUSION ", this application is incorporated in full that this is for reference, and coupling fiber is particularly advantageous for enforcement TDLAS system.In fiber coupling system, one or more detecting light beam is transferred to transmitting terminal optical device and projection enters firing chamber, and this light beam can be made up of the multiplexed beam of various relevant wavelength.After being conveyed through firing chamber, detecting light beam is received in receiving end optical device.As in International Patent Application SerialNumber PCT/US04/10048 discuss in detail, in receiving end optical system, use multimode optical fiber to be favourable.Use multimode optical fiber to certainly lead to modal noise, modal noise is the change in signal strength of detected light, it be collect and transmission light multimode optical fiber core in the Light distribation generation of time heterogeneous and wavelength variations.The Absorption Characteristics that receiving end modal noise can hinder effective TDLAS to observe.

The phenomenon of modal noise is not limited to TDLAS or is caused by TDLAS device, and the feature of TDLAS device is receiving end multimode optical fiber.In contrast, modal noise inevitably occurs in sufficient length in any multimode optical fiber of transmission light.Modal noise is inevitable in multimode optical fiber, because multimode optical fiber compares with single-mode fiber very large diameter of section, it can make light along many light paths or with multiple mode propagation.Some light paths or pattern than other light path or pattern long or short.Therefore, constructive interference and destructive interference must occur, thus produce the Light distribation of time heterogeneous and wavelength variations in the fibre core of multimode optical fiber, it causes typical modal noise speckle figure.Therefore, modal noise occurs in the calculating utilizing sufficient length multimode optical fiber, telecommunications, or in other scientific equipment.The efficiency whether modal noise affects given optical system depends on the requirement of particular system.

The object of the invention is to overcome one or more problem discussed above.

Summary of the invention

The present invention is a kind of optical mode noise averaging device, comprising: multimode optical fiber and equilibration device, for the noise-induced change in signal strength of optical mode propagated in this multimode optical fiber average.Within a time period chosen, by the refractive index of periodically-varied multimode optical fiber, upset the Light distribation of multimode optical fiber, or the two, equilibration device can the averaging modal noise change in signal strength of bringing out.By the temperature of periodically-varied multimode optical fiber, can the refractive index of this multimode optical fiber of periodically-varied.By periodically and practical operation multimode optical fiber, refractive index can be changed and maybe can upset Light distribation in this multimode optical fiber.

By the heat exchange action of thermal element and multimode optical fiber, the temperature of this multimode optical fiber can be changed.Appropriate device as thermal element includes, but not limited to electrothermal module, electric resistance heater, infrared heater, chemical heater, common cooling device, is cooled to the fluid source of below environment temperature, or is heated to the fluid source of more than environment temperature.

Optical devices can comprise: with the temperature sensor of multimode optical fiber thermo-contact, such as, thermopair, and controller, input for receiving from temperature sensor and control thermal element.

Describing in another embodiment being used for periodical operation multimode optical fiber, this operation can comprise: distortion multimode optical fiber, stretching multimode optical fiber, or vibrations multimode optical fiber.Piezoelectric stretcher can be used for the cyclic stretching of multimode optical fiber.Or, motor can relative to the longitudinal axis of optical fiber and relative to the fixed part of this optical fiber with the multimode optical fiber of the clockwise direction replaced and counterclockwise periodically wreath piece.

The invention still further relates to a kind of method of average light modal noise in multimode optical fiber, comprising: the input end coupling the light into multimode optical fiber, the refractive index of periodically-varied multimode optical fiber, and receive average light at the output terminal of multimode optical fiber.Modal noise averaging method can comprise: change refractive index by one of the temperature of periodically-varied multimode optical fiber and two kinds of methods of periodical operation multimode optical fiber.By providing the thermal element with multimode optical fiber heat interchange, can the temperature of periodically-varied multimode optical fiber.Or by distortion multimode optical fiber, stretching multimode optical fiber or vibrations multimode optical fiber, can this multimode optical fiber of periodical operation.

Accompanying drawing explanation

Fig. 1 is the schematic diagram that TDLAS detects ` equipment;

Fig. 2 is the schematic diagram that TDLAS detects ` equipment, and its describes and the remote element of element optically-coupled near firing chamber;

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

Fig. 4 is the exploded view of temperature base phase shift apparatus, is wherein heated to more than environment temperature or is cooled to the fluid source of below environment temperature as thermal element;

Fig. 5 utilizes a series of thermoelectric device as the exploded view of the temperature base phase shift apparatus of thermal element;

Fig. 6 is another exploded view of Fig. 5 institute temp. displaying function base phase shift apparatus;

Fig. 7 is the optical mode noise averaging device schematic diagram utilizing motor, and this motor is used for mechanically actuated multimode optical fiber;

Fig. 8 is the optical mode noise averaging device schematic diagram utilizing piezoelectric stretcher, and this stretcher is used for mechanically actuated multimode optical fiber; With

Fig. 9 is the schematic diagram that transmitting terminal modal noise reduces device.

Embodiment

a. summarize

The preferred embodiments of the present invention are a kind of optical mode noise averaging equipment.Save at following E, we describe this optical mode noise averaging equipment in detail.Optical mode noise averaging equipment is particularly suitable for, but is not limited to, averaging modal noise, and this noise is intrinsic in the receiving end multimode optical fiber that tunable diode laser absorption spectroscopy art (TDLAS) checkout equipment to coupling fiber is relevant.Multiple embodiments of this checkout equipment are discussed in detail in International PatentApplication Serial Number PCT/US04/10048 (Publication NumberWO2004/090496) of application on March 31st, 2004, its title is " METHOD AND APPARATUS FOR THE MONITORING ANDCONTROL OF COMBUSION ", is incorporated in full that this is for reference.In addition, the TDLAS checkout equipment of coupling fiber is below described.The preferred embodiments of the present invention are applicable to average light modal noise in any optical system having modal noise.Specifically, optical mode noise averaging equipment can in any calculating, telecommunications, scientific research, or have in the other system of the multimode optical fiber transmission light of sufficient length and realize.This par devices can be used in any optical system, and optical mode noise intrinsic in the light wherein by transmission in average multimode optical fiber, can improve the efficiency of this system.

b. checkout equipment

Fig. 1 represents the embodiment of a checkout equipment 10, and it is suitable for detecting, monitor and forecast combustion process.Utilize a series of laser that under selecting frequency, tunable diode laser 12 is launched near infrared or middle infrared spectrum, checkout equipment 10 completes tunable diode laser absorption spectroscopy art (TDLAS).The output terminal of each tunable diode laser 12 is coupled to single optical fiber, and is routed to multiplexer 16, and this optical fiber can be single-mode fiber 14.Term as used herein, " coupling ", " optically-coupled " or " optical communication " is defined as the functional relationship between each element, wherein light can by or by intermediary element or free space from the first element transmission to the second element.The laser of the some or all of frequencies produced in multiplexer 16 is multiplexed into the multiplexed probe beam of multiple selecting frequency.Multiplexed probe beam is coupled to transmitting terminal optical fiber 18 and is transferred to transmitting terminal optical element 20 or collimating apparatus, and the firing chamber 22 shown in it to Fig. 1 is operationally relevant.

Transmitting terminal optical element 20 is orientated to projection multiplexed probe beam by firing chamber 22.In firing chamber 22, receiving end optical element 24 and transmitting terminal optical element 20 carry out optical communication.Preferably, receiving end optical element 24 is substantially contrary with transmitting terminal optical element 20, and its work is relevant to firing chamber 22.The position of receiving end optical element 24 and orientation are to receive the multiplexed probe beam projected by firing chamber 22.Receiving end optical element 24 couples light to receiving end optical fiber 26, and the multiplexed probe beam of receiving end optical element 24 reception of this fibre transmission portions is to demultiplexer 28.In demultiplexer 28, the fractional reuse detecting light beam that receiving end optical element 24 receives is by demultiplexing, and the demultiplexing laser of each wavelength is coupled to output optical fibre 30.Each output optical fibre 30 is optically coupled to detecting device 32 again, this detecting device normally one to the photoelectric detector choosing laser frequency sensitivity, it is multiplexed into detecting light beam.Based on the light property and the quantity that are transferred to detecting device 32 under detecting device frequency, detecting device 32 produces an electric signal.From the electric signal normally digitized signal of each detecting device 32, and analyze in data handling system 34.As discussed in detail below, digitizing and analyzed data can be used for detecting the physical parameter in firing chamber, and it includes, but not limited to concentration and the temperature of combustion of various gas species in firing chamber 22.Data handling system 34 also can be used for transmitting a signal to combustion control equipment 38 by backfeed loop 36, thus actively controls the procedure parameter chosen.In the case of a combustion process, process control parameters can comprise: fuel (such as, fine coal) delivery rate; Oxygen delivery rate and catalyzer or chemical reagent delivery rate.Utilize coupling fiber electronics and optical element equipment that is accurate and temperature sensitive can be made to be placed in the pulpit of stable operation environment to the input end of checkout equipment 10 and output terminal, such as, tunable diode laser 12, detecting device 32 and data handling system 34.Therefore, only relatively healthy and strong transmitting terminal optical element 20 and receiving end optical element 24 need to be placed near the hostile environment of firing chamber 22.

Fig. 2 represents that the integral member of the multiplexing detection system 40 of coupling fiber arranges schematic diagram.Detection system 40 generally includes: system chassis 42, in point box 44, have the transmitter 46 of transmitting terminal optical element 48, have the receiver 52 of receiving end optical element, and connecting fiber.System chassis 42 is preferably placed on Long-distance Control indoor, such as, with firing chamber 54 at a distance of 1 kilometer.Remote control room has suitable controling environment usually.System chassis 42 comprises laser instrument 56, detecting device 58, wavelength multiplexer 60 and wavelength demultiplexer 62.System chassis 42 goes back installation system electronic component and control software design (not drawing in Fig. 2).Optionally, system chassis 42 comprises alignment light source 64.

Connected system frame 42 with in the single mode telecommunication optical fiber of optical fiber normally standard of point box 44.Such optical fiber is not expensive, easily buys, and low-loss optical fiber, and allows laser to be directed into ready-made telecommunication element for operational light, such as, and photoswitch, beam splitter, and wavelength division multiplexer.If do not have coupling fiber, then laser must be directed directly by free space until firing chamber 54, and this is difficult to realize, or sensitive electronics and optical element must closely firing chambers 54.

Point box (breakout box) 44 in also drawing in Fig. 2.In a point box 44 are the solid packoffs be placed near boiler.In a point box 44 comprise photoswitch, beam splitter and coupling mechanism (collectivity mark is 66), as discussed below, it can be used for guiding light signal to multiple transmitter-receiver head pair.

The 3rd group system element shown in Fig. 2 is transmitter head 46 and receptor end 50.Optics in transmitter head 46 and receptor end 50 and electronic component must be transformed into collimated light beam the light in optical fiber 68, accurately guide this collimated light beam by firing chamber 54, catch this light beam at the far-end of firing chamber 54, and this light beam that is coupled enters optical fiber 70.The optical element completing these work is by transmission range, the turbulent flow of combustion zone, and it is on the impact of transmitting beam quality, and the core size of optical fiber 70 is determined.Choosing of core size is according to the determined design standards of application.Larger fibre core can catch more laser, but also catches more bias light simultaneously.When for coal-burning boiler, the core diameter of 50 microns can obtain acceptable result.The coupling fiber received on (receiver) end has several advantage.Specifically, be only focused with laser same position and along the light that equidirectional is propagated and enter optical fiber 70.This can reduce detected background light output greatly.Light can be captured the optical fiber entered in several receiver optical fiber, and photoswitch or other optical routing device can choose the light being routed to an optical fiber in detecting device 58.A receiving end optical element is only drawn in Fig. 2.

The coupling fiber of receiving end is utilized to require accurately to keep the alignment tolerance of transmitter and receiver optical element (being less than 0.5 milliradian on the direction of transmitter and receiver).Preferably, transmitting terminal optical element 48 and receiving end optical element 52 are according to the wavelength specialized designs from 660nm to 1650nm and correct aberrations, therefore, high-level efficiency can transmit and receive multiple laser signal simultaneously.

c. tunable diode laser absorption spectroscopy art

Utilize the technology that in laser spectrum art field, professional knows, tunable diode laser absorption spectroscopy art (TDLAS) can be completed.In general, TDLAS allows Laser Transmission by targeted environment, after this, is completed by the absorption of laser under detection specific wavelength, such as, and the absorption of the object gas of carbon monoxide or oxygen.The laser that spectrum analysis is detected can identify gas type along laser path and quantity.At Teichert, Fernholz, and Ebert: " Simultaneous in situ Measurement of CO, H ₂o, and GasTemperature in a Full-Sized, Coal-Fired, Power Plant byNear-Infrared Diode Laser " details of Spectrographic art is discussed in (Applied Optics; 42 (12): 2043; 20April2003), be incorporated in that this is for reference in full.The noncontact character of laser absorption spectrum art is very suitable for rugged environment, and such as, the combustion zone of coal-fired station, or flammable or toxic environment, wherein can not utilize other detection method.Use laser can provide high brightness, this be in deep fades situation (normally the loss of light is greater than 99.9%) receive that can to detect transmission necessary, this decay can be seen under such some environment.In order to bear the mal-condition of intended application better, laser can be made by armored fiber optic target approach environment.

Effective detected temperatures or multiple combustion process component demand TDLAS have the performance of multiple wide interval frequency laser.The frequency chosen must mate the absorption line with monitored transition.Such as, NO can be monitored under 670nm wavelength ₂with the NO concentration of analog transmissions.Oxygen can also be monitored, water vapour (temperature), and carbon monoxide in coal-burning boiler.Based on such hypothesis, equaling 10 meters with the molal fraction of each gas species by the laser probe path length of firing chamber is CO (1%), O ₂(4%), CO ₂, and H (10%) ₂o (10%), can choose suitable absorption line, and suitable laser frequency.When selecting frequency, can suppose that process temperature is 1800K, this temperature is slightly higher than the temperature usually observed in coal-fired power plant, but this buffering (cushion) is in the calculation as factor of safety.

Such as, three the water absorption lines can chosen for TDLAS meet following criterion:

1. lower state energies be respectively ~ 1000,2000, and 3000cm ^-1;

2. the absorption coefficient provided is between 0.1-0.4, and it can cause the light beam resonance absorption of about 20%;

3. the situation of the best is the transition utilized in 1250nm to 1650nm scope, can obtain cheapness, high power, DFB diode telecommunications lasers within the scope of this;

4. must separate each transition so that realize multiplexing operation fully; With

5. the wavelength chosen must can by existing multiplexing/demultiplexing grating diffraction effectively.

Suitable water spectral line occurs in following wavelength:

table 1

Wavelength (nm)	Wave number (cm ^-1)	Lower state energies (cm ^-1)	Raster stage	Absorption under 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％

It is expected to not from the interference of any other burning gases.Suppose that the kind that most probable is interfered is CO ₂, it does not have strong interference spectrum in 1.3 microns of-1.4 microns of districts.

Similarly, the Ebert based on above reference works, and can choose suitable carbon monoxide line.Utilize R (24) spectral line in coal-burning boiler, suitable carbon monoxide line can be found at 1559.562nm.Choose the interference that this spectral line can be avoided from water and carbon monoxide.Known grating is highly effective in this wavelength zone, because it is the C frequency band in optical communication.The absorption coefficient of expection CO under this wavelength is 0.7%.

In addition, oxygen can be measured under 760.0932nm.Calculate in this district preferably multiplexing/demultiplexing grating efficiency is 40%, but, suitable laser power should be had reasonably measuring under efficiency.

As discussed here, coupling fiber is used to require accurately to aim at the optical system of transmitting terminal and receiving end at the transmitting terminal of TDLAS detection system and receiving end.A possible alignment wavelengths is 660nm, because there is high power (45mW) diode under this frequency, and 660nm is near the peak value of the 14th grade of grating operation.Can determine that other alignment wavelengths is suitable equally or more suitably.

In a word, table 2 represents according to the embodiment of the present invention for the detecting light beam of the multiplexing reasonable wavelength set chosen as TDLAS.It should be noted that this wavelength set is an embodiment of TDLAS checkout equipment, it is suitable for detection and control coal-fired station.Other wavelength set is also suitable equally.

table 2

Object	Wavelength (nm)
		Aim at	660
O ₂B-a frequency band	760.0932
		H ₂O (middle temperature spectral line)	1349.0849
H ₂O (high-temperature spectral line)	1376.4507
		H ₂O (low temperature spectral line)	1394.5305
The CO R (24) of R (2,0) harmonic wave	1559.562

d.TDLAS utilizes the concrete advantage of multiplexed beam

TDLAS utilizes the concrete advantage of wavelength-multiplexed probe light beam to be to improve thermometric accuracy.In order to utilize TDLAS Measurement accuracy concentration, the temperature of monitored gas must be known.The intensity of molecule absorption is the function of temperature.Therefore, in order to the amplitude Absorption Characteristics converts concentration to, this temperature must be known.Attempt before measuring some of burning kind concentration, such as, measure CO, running into can not the difficulty of measuring tempeature enough exactly, thus causes the error of quantitative measurment.This is correct especially for diode laser base ammonia slip (slip) monitor, and this monitor traditionally completely can not measuring tempeature.In detection system of the present invention, by measuring the ratio of two or more pieces molecular water line strength, temperature can be determined.Article two, the integrated intensity ratio rate of spectral line is only the function (supposing constant total system pressure) of temperature.Therefore, in principle, two spectral lines provide temperature accurately.But in the case of a non-uniform temperature distribution (situation normally in industrial combustion process), two spectral lines are not enough to determine Temperature Distribution.In such a non-uniform temperature distribution, two spectral lines only can determine the temperature of " light path is average ".Contrast therewith, the integrated amplitude measured more than two (same breed) spectral lines can the heterogeneity of detecting temperature.The example of this technology is (AppliedOptics, 40 (24): 4404,20August2001) that Sanders, Wang, Jeffries and Hanson utilizes oxygen and illustrates as molecular detection, is incorporated in that this is for reference in full.Preferred technology relies on such fact, and the peak strength distribution along visual line measurement is different from and distributes on the light path under medial temperature 500K, such as, its half light path is 300K, and is 700K on second half light path.

Except having more accurately except thermometric advantage, use multiplexed probe beam can monitor the kind of multiple burning gases simultaneously, thus can control combustion process more accurately.

e. modal noise

The optical system of TDLAS system and require that the similar device of multiplexing wide-space wavelength signals meets with the difficult problem in many designs, this is owing to there is the contradictory design requirement reducing modal noise and high-level efficiency light and collect.Herein, modal noise is defined as the change in signal strength of detected signal, and it is due to time heterogeneous in fiber core and the photodistributed result of wavelength shift, and this optical fiber is for collecting and transmitting the light commuted in measured process room.

In multimode optical fiber, different patterns has different speed to propagate because of variations in refractive index.Intensity distributions in optical fiber is speckle figure, and it is the result that all communication modes are interfered, and these communication modes experienced by different effective optical lengths.If we collect and detect all light in speckle figure, then constructive interference and destructive interference cancel each other exactly, and therefore, total through-put power has nothing to do with the length of wavelength or optical fiber.If introducing slicing, vignetting or other loss, then can not realize offsetting accurately, and therefore, detected power changes with wavelength and/or time.In above-mentioned TDLAS detection system, the changed power that modal noise causes is problematic.Some spectroscopy relies on studied gas species to the absorption of specific wavelength of light.Detecting absorption is the reduction utilizing power in critical wavelength.Therefore, modal noise can imitate and absorb relevant power drop, thus obscures the data of being collected by TDLAS.The detected general formula of power after transmission one section of fiber lengths z is:

P＝P ₀+∑ _ijc _ijE _iE _jcos[(2πv ₀Δn _ijz/c+ΔΦ _ij(T，σ))]　　　　　　　　　　　(1)

Wherein P ₀=the average power irrelevant with wavelength

E _ithe light amplitude of the=the n-th transverse mode

C _ijoverlap integral between=the i-th and j the transverse mode

Δ n _ijrefringence between=the i-th and j the transverse mode

ΔΦ _ijthe phase shift that between=the i-th and j the transverse mode, Yin Wendu and stress produce

In orthogonal modes set with under not having lossy situation, C _ij=0.But if there is the relevant loss of light beam slicing or vignetting or other patterns, then it makes some C _ij≠ 0.This can cause occurring ripple in average transmit power.

In the typical gradation type index fiber of 50 micron core, total refractive index change delta n is about 1%, but the prevailing transmission time of most of pattern cost is the immediate vicinity at fiber core, so, in general, Δ n _ij<0.0005.Existing optical fiber GIF50 about supports 135 patterns, assuming that rational attainable light beam clip level, during length scanning, it is enough to produce significant modal noise.

As concrete modal noise example, we can consider a most single system with modal noise: rectangular waveguide, and it only supports minimum pattern in one direction, and on its orthogonal directions, only support two minimum patterns:

Minimum pattern: E ₁=E ₁ ⁰[exp i (kz-ω t)] cos π x/2a

Secondary low pattern: E ₂=E ₂ ⁰[exp i (kz-ω t)] sin π x/a

Along the intensity of z point on optical fiber direction be:

I (x)=| E ₁+ E ₂| ²p=∫ with general power | E ₁+ E ₂| ²dx (2)

Wherein this integration must comprise slicing effect and vignetting effect.

When there is no slicing, P ~ E ₁ ²+ E ₂ ², therefore, it and wavelength have nothing to do.Add the bound that slicing is equivalent to change integration.Can prove, after slicing, obtain an additive term ~ E ₁e ₂cos ΔΦ, wherein ΔΦ=Δ KL=2 π Δ nL/ λ.

If single-mode fiber can be used in receiving end optical system, as mentioned above, then modal noise does not just become problem.But multimode optical fiber must be used in the receiving end optical system of coupling fiber TDLAS system usually, and it has two reasons.The first, after being conveyed through measurement volumes (measuring the firing chamber of distance more than 10 meters), the quality serious degradation of initial single mode (Gaussian spatial distribution) light beam.Therefore, the coupling efficiency that this serious distortion light beam enters single-mode fiber is very low.This is a unacceptable situation, 3-4 the order of magnitude because light beam is decayed after being conveyed through measurement volumes, and chief reason is the scattering that causes of coal ash and flying dust and dim.The additional attenuation utilizing single-mode fiber to cause can the carrying out of hamper measurements.The second, in fireball, the steering effect of deflecting light beams causes the position of light beam and points to unstable.Under these effects, the fibre core of hit single-mode fiber is very difficult regularly.

On the other hand, the fibre core of multimode optical fiber is at least 25 times that single-mode fiber object cross section is amassed.Therefore, the effect that light beam turns to can greatly be reduced.In addition, because the coupling efficiency entering multimode optical fiber has nothing to do with the spatial model of light, the low beam quality obtained after being conveyed through fireball is not just problem.

In calculating, telecommunications, or the other types device in general science and technology may have the similar of other or completely irrelevant restriction, and it requires or is conducive to using very long multimode optical fiber.In other device, modal noise also may be problematic, and it proposes a difficult problem for very large Data Collection or data transmission.

Therefore, the pattern occurred in multimode optical fiber systems is very large design challenges about losing.The Light distribation penetrated from the fibre core of multimode optical fiber has random speckle figure, that is, the bright district caused by constructive interference and destructive interference between different fiber pattern and the random figure of dark region.If speckle figure is completely constant as the function of time and wavelength, then it is not just problem.But if light beam is at the clipped wave Anywhere of multimode receiving end optical system, then speckle figure occurs to change slowly and can produce modal noise as the function of wavelength.This slicing can not be avoided; Only slicing can be reduced.So, other measures reducing modal noise must be taked to improve the detection sensitivity of system.

Several method alleviating modal noise can be had.According to above formula (2), modal noise can be reduced by following method:

1. reduce the relevant loss of pattern, that is, reduce slicing to keep C _ijvery little;

2. reduce z, thus increase the cycle of modal noise, make it be far longer than relevant absorption line;

3. utilize low dispersion fiber to reduce Δ n _ij;

4. upset or phase shifting modes; But not every pattern upsets or phase-shifting technique is same effective, as described below.

Preferably, the receiving end Optical System Design of coupling fiber TDLAS detection system comprises all above methods to reduce modal noise with being made into.This Optical System Design becomes like this, and provide almost ideal system alignment, any light beam slicing should occur in low-level.Should make great efforts to keep multimode optical fiber to have minimum length; But in some applications, z is sufficiently long, in order that have control circuit in the region of environmental Kuznets Curves.Utilize best low dispersion multimode optical fiber, Δ n can be reduced _ijnumerical value.In addition, by refractive index or the mechanically actuated of periodically-varied receiving end multimode optical fiber, and from collect average optical signal extract data, can on average each pattern to obtain optimum.

Speckle figure in multimode optical fiber is function as time and wavelength and changes, and as the function of this mechanical fiber optic position.Transmission time and wavelength are the impacts by optical fibre refractivity.Bending according to ad hoc fashion and operating optical fiber to make speckle figure change.If the cyclical variation of these mechanically actuateds or refractive index completes in a period of time continuously, then the light space distribution penetrated from optical fiber can on average become relatively uniform figure.

By efficient periodic phase shift or upset modal noise, time averaging measurement result produces uniform signal intensity.By stretching or twisted optical fiber or change the temperature of optical fiber, the refractive index of optical fiber can be changed.The temperature of change optical fiber makes the refractive indices n between i-th transverse mode and a jth transverse mode _ijchange.Function cos (the 2 π v that the change of this optical fibre refractivity can provide according to formula (2) ₀Δ n _ijz)/c phase shifting modes noise.

f. optical mode noise averaging

As shown in Figure 3, utilize optical devices 300 can execution cycle property phase shift or upset modal noise with the average measurement result of generation time.Optical devices 300 can include the multimode optical fiber 302 of input end 304 and output terminal 306.Light can be coupled to the input end 304 of multimode optical fiber 302, and roughly propagates through this system along the direction of arrow shown in Fig. 3, this arrow contact input end 304 and output terminal 306.

Optical devices 300 also comprise the average element 308 relevant to multimode optical fiber 302.Average element 308 can comprise such equipment, and it changes multimode optical fiber 302 refractive index in the time period periodically that is chosen.Or average element 308 can comprise for upsetting a photodistributed equipment in multimode optical fiber 302.By means of average element 308, by the temperature of periodically-varied multimode optical fiber 302, periodical operation multimode optical fiber 302, or the two, refractive index can be changed or upset Light distribation.

In the embodiment of average element 308 execution cycle property operation multimode optical fiber 302, average element 308 can distortion, stretching or vibrations multimode optical fiber 302.In the embodiment of average element 308 periodically-varied multimode optical fiber 302 temperature, the various thermal elements with multimode optical fiber heat interchange can be provided.Any equipment affecting multimode optical fiber 302 temperature can be included in average element 308.The representative device that can be used for affecting multimode optical fiber 302 temperature comprises: electrothermal module, electric resistance heater, chemical heater, utilize the common cooling device of compressed fluid and heat exchanger, chemistry refrigeratory, is cooled to the fluid source of below environment temperature, and is heated to the fluid source of more than environment temperature.Below discuss some devices in these devices in detail.

In the embodiment that average element 308 causes multimode optical fiber 302 periodic heat or cooling, sensor 310 also can be placed to and multimode optical fiber 302 heat interchange.Sensor 310 can supply information to controller 312, and controller 312 can control average element 308 again by control line 314.

g. temperature base phase shift apparatus

The validity of temperature basic mode formula phase shift is directly relevant with the fiber lengths z being exposed to temperature variation with the temperature variation of time per unit.The phase shift of temperature basic mode formula is the special effective method of a kind of tupe noise, because the change of fiber optic temperature just changes the refractive index of all transverse modes, and temperature variation can occur on the optical fiber of one section quite long.Therefore, by changing the refractive index of optical fiber, we can guarantee all transverse mode generation phase shifts, do not have pattern to keep with signal " freezing ".

In fact, the heating/cooling system of any type can be placed to and multimode optical fiber 302 heat interchange, thus the temperature generating period of optical fiber is changed.Electric resistance heater, common cooling device, the fluid of heating or cooling, Peltier or other thermoelectric device, infrared facility, or chemical devices all can be used for the temperature affecting optical fiber.

The pattern phase changer embodiment utilizing periodic temperature to change is fluid-based pattern phase changer (fluid means) 400.Fig. 4 represents the exploded view of fluid means.Fig. 4 represents and utilizes

vortex air pipe

402A, and 402B alternately blows hot-air and cold air to optical fiber and fluid means 400 embodiment around thereof.The air transmitted from compressed air source (not drawing Fig. 4) is alternately sent to two vortex tube 402A, a vortex tube in

402B.Vortex tube

402A, 402B are coupled into and carry out fluid communication with the inside of container 404.As shown in Figure 4, container 404 can be made up of casing 406, and casing 406 has

side plate

408A, 408B, top board 410, front entrance 412, and rear entrance 414.Whole casing 406 can be adapted to be mounted within typical data processing equipment frame.Although can the casing 406 of installation frame be can use any casing shape being suitable for placing reel 416 easily especially, type or pattern to form fluid means 400, wherein winding one section of multimode optical fiber 418 on reel 416.Or, the device not having casing 406 can be used.

Vortex tube

402A, 402B carry out fluid communication by rear entrance 414 and the inside of casing, and therefore, the multimode optical fiber 418 that it and reel 416 are wound around carries out fluid and heat interchange.So utilize

vortex tube

402A, 402B makes the air of more than the air of heating to environment temperature or cooling below environment temperature, can periodic heat and/or cooling multimode optical fiber 418.

Easily can buy suitable vortex tube 402A, 402B.Such as, Ke Yicong

Figure S05810448020061011D00016181419QIETU

corporation buys

Figure S05810448020061011D00016181430QIETU

3230 vortex tubes.These or similar vortex tube are operated in 30ft ³under the throughput rate of/minute, it provides the air being heated to+60 ° or being cooled to-20 °, and this is relevant with the orientation of vortex tube.In addition, vortex tube is utilized relatively easily to circulate between the air and the air of cooling of heating.But, be important to note that, periodically provide the fluid of heating or cooling to be suitable for realizing the embodiment of fluid means 400 with any equipment of multimode optical fiber 418 heat interchange or method.But the fluid of heating and cooling can be air discussed above, water, heating/cold oil, pressure gas, or can be used for other fluids heating or cool multimode optical fiber.

As an example, instead of restriction, during operation, a

vortex tube

402A, 402B can transmit the air of heating, until optical fiber arrives the temperature higher than temperature in about 10 DEG C.The temperature of optical fiber can be determined by the thermopair 420 imbedded with this optical fiber contact or other temperature sensors.Temperature control unit (not drawing in Fig. 4) can receive and inputs and trigger solenoid switch from thermopair 420, makes air be sent to another vortex tube 402A for cooling, 402B.Or, because being supplied to adding hot-air and can never reaching critical temperature of multimode optical fiber 418 by

vortex tube

402A, 402B, can not serviceability temperature controller, but replace with timing repeater, for the vortex tube 402A between periodical exchange heating and cooling, 402B.At this device duration of work, the circulation between temperature is preferably carried out continuously.

Fig. 5 and Fig. 6 represents the pattern phase changer of a preferred embodiment, and it is the temperature based on periodically-varied multimode optical fiber.Thermoelectricity pattern phase changer (thermoelectric device) 500 comprises reel 502, for being wound around the multimode optical fiber 504 that is chosen length.One or more thermoelectricity heating/refrigerating module 506 is placed to and multimode optical fiber 504 heat interchange.In the embodiment shown in fig. 5, multiple thermoelectricity heating/refrigerating module 506 arranges along the inner radial around reel 502.Thermoelectricity grease between the outside surface of electrothermal module 506 and the coil of multimode optical fiber 504 is utilized to form heat interchange.In the embodiment shown in fig. 5, the structure of reel 502 is the edges having opening, and it is convenient to the contact between electrothermal module 506 and multimode optical fiber 504, and is convenient to the winding of multimode optical fiber 504.

Fig. 6 represents that electrothermal module 506 is placed to and contacts and the device exploded view placed relative to reel 502 with multimode optical fiber 504.

One or more heat reservoir 508 also can be arranged to and electrothermal module 506 heat interchange.Preferably, heat reservoir 508 is made up of high thermal conducting material, such as, and aluminium or copper, and have the heat radiator or other equipment that are designed to increase each heat reservoir 508 surface area.The position of fan 510 can force or attract air by heat reservoir 508, thus impels rapid extraction heat from electrothermal module 506, so that Fast Heating or cooling multimode optical fiber 504.As shown in Figure 5 and Figure 6, can utilize the framework 512 of apical ring 514 and base ring 516, it makes to keep suitable orientation between each element of thermoelectric device 500, and does not hinder air to flow through heat reservoir 508.Preferably, in framework 512, multiple perforate is formed to guarantee flowing freely of air.

Embodiment shown in Fig. 5 and Fig. 6 utilizes the electrothermal module 506 based on Peltier principle.By go between 520 give electrothermal module 506 direct current is provided.Electrothermal module 506 under utilizing work to be Peltier principle, two apparent surface's heating of electrothermal module 506 or cooling, it depends on the direct current direction provided.Therefore, these electrothermal modules have some advantage, because by exchanging the direct current polarity being supplied to lead-in wire 520 selectively, can quite easily heat selectively or cooling multimode optical fiber 504.But, be important to note that, the device of other types can be utilized to realize heating and/or cooling multimode optical fiber 504.Such as, electric resistance heater, common cooling device, infrared heating device, and/or chemical reaction base well heater and/or refrigeratory can be used for the temperature changing multimode optical fiber 504.

In this preferred embodiment, as shown in Figure 5 and Figure 6, can from suitable power source delivering power to the electrothermal module 506 be arranged on cylindrical device.Exchanging electrical current circuit can be used for the DC source polarity that periodic inversion sends each electrothermal module 506 to.Pattern phase shift occurs on one section of multimode optical fiber 504, preferably Premium GIF50 multimode optical fiber, it and electrothermal module 506 heat interchange.We find that 55m to 100m has been the appropriate length multimode optical fiber of pattern phase shift and average operation.The multimode optical fiber of other length is also suitable.If use the cladded-fiber of 100m, then the optical fiber of about 50% directly contacts with thermoelectric (al) cooler.It should be noted that by direct contact, except thermoelectricity grease, between electrothermal module 506 and multimode optical fiber 504, there is no conductive material.This configuration can reduce the thermal mass of this system.By reducing the thermal mass of system, the temperature-responsive of this system is fast and produces more effective pattern phase shift.

One or more thermopair 522 or other temperature measuring equipment can be arranged between electrothermal module 506 and multimode optical fiber 504, and in the temperature of all time supervision multimode optical fibers 504.Temperature control unit (not drawing in Fig. 5 and Fig. 6) can receive the temperature of thermocouple measurement, and changes sense of current based on temperature reading.Utilize existing electrothermal module, the OK range of about 35 DEG C to 50 DEG C can be realized.Importantly, optical fiber can not exceed the highest 85 DEG C, otherwise may make optical fibre damage.Utilize and complete test operation from 65 DEG C to the one-sided refrigeratory of 10 DEG C of temperature variation and 65 DEG C to the double-sided cooled device of 30 DEG C of temperature variation.But one the cycle can be any duration chosen completely, we find that the cycle of about 25 seconds is effective.

As mentioned above, by means of being mounted to the heat reservoir 508 with electrothermal module 506 heat interchange, heat can be dissipated on the opposing face of electrothermal module 506.Air can be forced to enter or attract the heat radiator by heat reservoir, it contributes to the heat suitable with power input that dissipate.Force air by the perforate 518 bottom heat radiator, thus make air flow through this system and flow out from the top of this unit, effective heat dissipation can be completed.Other configurations producing enough air stream are also suitable.Or thermoelectric device 500 can be put in cooling fluid, or other method is utilized to cool.Preferably, fan works continuously while plant running.Can utilize any suitable fan or fluid source heat dissipation, but the fan of 300CFM is effective for removing heat from the system shown in Fig. 5 and Fig. 6.

Control circuit can be relevant to the heating and cooling of electrothermal module 506.Based on from thermopair 518, thermometer, or the input of other temperature sensors, feedback control circuit can the temperature of detection fiber.In addition, based on temperature input, controller can exchange the direction of current that power is sent to electrothermal module 506, and adjusts the power level (heating is normally more effective, and it needs less power) of heating and cooling circulation.In addition, controller can control the minimum and maximum power being sent to fiber optic system, and turns off driving circuit when overheated.

h. electrothermal module Phase Shifting System test

Above-mentioned thermoelectric device 500 is utilized to complete test.Test is by completing without the purging with nitrogen gas room absorbed by means of transmitting four wave bands.Light path does not absorb kind, and laser should show linear wavelength response after referenced signal segmentation.The linear deviation of slope is mainly caused by modal noise.Determine that the general formula measuring inaccuracy is provided by following formula:

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

Wherein x _i=signal _i/ tap _i

F _i(ax+b)=x _1-nlinear fit

Due to the time delay that transmitting terminal and receiving end travel-time cause, the macrocyclic beginning of each ripple and end can be ignored.These time delays cause each circulation to start and at the end of great change between tap and observation signal.

The test of system performance completes by means of the embodiment of thermoelectric device 500, wherein utilizes one-sided and bilateral electrothermal module 506, multiple averaging time, and different fiber lengths.In all results, pattern phase shift and average operation produce the mode noise signals deviation reduced.Fiber lengths used in experiment is depended in the reduction of noise, as given in formula (3); Compared with long optical fibers relative to there being higher frequency mode noise bias compared with short fiber.Therefore, can see that the relative resolution of pattern phase shift strengthens in compared with long optical fibers.Below provide the result of various configuration:

table 3

Do not have pattern to upset, 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

One-sided TEM, 100m can working optical fibre length, 270m optical fiber total length, 191210 DEG C of-65 DEG C of temperature ranges

On average

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

75m can work multimode optical fiber, 245m optical fiber total length, one-sided TEM, 253165 DEG C of-10 DEG C of temperature ranges

On average

Circulation	10	3975	3465	2375	3014	3207
							Circulation	10	2997	963	1408	2085	1863
Be as cold as heat	1	3111	1841	1538	2982	2368
							Be as cold as heat	1	2268	1518	2365	2932	2271

55m can work multimode optical fiber, 225m optical fiber total length, bilateral TEM, 216765 DEG C of-30 DEG C of temperature ranges

On average

mechanically actuated based device

As discussed above, from the average signal collected, extract data by periodically-varied refractive index or mechanically actuated multimode optical fiber, can on average and smooth mode noise.By means of periodic temperature change, the refractive index of multimode optical fiber is changed, temperature base phase shift apparatus discussed above completes pattern phase shift.As discussed below, mechanically actuated multimode optical fiber also can be utilized to change refractive index.In addition, mechanically actuated can cause signal that is average and smooth mode noise effect, because light can not follow specific pattern completely being operated in the waveguide of optical fiber.Therefore, by the combination of phase shift and mechanically disturb, can in one section of multimode optical fiber the average and noise-induced speckle figure of smooth mode.

It is more effective than other method that the mechanical optical fiber of some AD HOC operates in averaging modal noise aspect.Specifically, along speckle figure being made relative to some other points on optical fiber to change around the longitudinal axis (z) twisted optical fiber.The Main change obtained is the rotation of speckle figure around z-axis.Importantly, when utilizing mechanical system to rotate optical fiber, speckle figure is different around the rotation of z-axis.Quadratic effect is that some changes occur because of rotation actual Light distribation.The variations in refractive index of the rotation master of speckle figure except for stress-induced optical fiber, although this point can explain the subtle change of speckle intensity pattern.On the contrary, when optical fiber is distorted motion, the rotation of speckle figure is because light fully can not follow waveguide.

Fig. 7 represents the schematic diagram of the mechanical mode noise par devices (mechanical hook-up) 700 according to one embodiment of the invention.Mechanical hook-up 700 utilizes hollow shaft motor 702 to place and fixing multimode optical fiber 704.The distal portions 706 of optical fiber is firmly fixed relative to the shaft position of motor 702, and motor repeatedly experiences the torsional motion from+360 degree to-360 degree.Frequency of this motion is preferably greater than or equal to 10Hz, and therefore, it can the signal of average transmission effectively, and greatly reduces the effect of receiving end modal noise.Although multimode optical fiber is effective along the distortion of its longitudinal axis for upset modal noise, also can utilize other mechanically actuated, such as, vibrations, stretch, or bending.

piezoelectric stretcher

Stretching optical fiber causes the change of optical fibre refractivity and length.Utilize piezoelectric stretcher can to stretch multimode optical fiber.Piezo-electric device is often used in introducing modulating time in single-mode fiber to postpone.Multimode optical fiber not be used in piezoelectricity stretching device, because the time delay of multimode optical fiber is uncontrollable, this can transmit multiple light path or with multiple mode transfer due to light.But although generation time postpones to be unpractical, piezoelectricity stretching device can be used for the phase shift of introducing pattern.

When stretching multimode optical fiber, the stress that this optical fiber is introduced can make optical fibre refractivity and length change.As shown in Figure 8, the work of piezo-electric device 800 is wound around several meters of multimode optical fibers 802 around semicylinder 804, then with predetermined oscillation frequency and distance vibration semicylinder 804.When distance between two semicylinders 804 expands and shrinks, the stress generation resonance in multimode optical fiber 802.This resonance makes the refractive index of optical fiber 802 produce fluctuating.The validity of pattern phase shift is that fiber lengths (z) changes and variations in refractive index (Δ n in optical fiber _ij) function (formula 1).

Utilize one of two of piezo-electric device 800 technology can complete pattern phase shift.In first technology, piezo-electric device 800 has enough optical fiber 802, and is configured to introduce enough stress to produce very large patterns of change, therefore, can obtain uniform signal intensity by average many patterns.Or because piezo-electric device 800 cycles through stable variations in refractive index, piezo-electric device 800 can vibrate in such a manner, it is in the minimum pattern phase shift with generation 180 ° of harmonic waves in maximum tension distance.Profit in this way, can reduce modal noise, and it is not by the many pattern phase shifts of time average, but optimizes stretching special-shaped to realize 180 ° of phase shifts.Therefore, few to averaging modal noise in a circulation, thus data acquisition fast can be realized when reducing modal noise.

transmitting terminal optical system (Pitch-Side Optical Train)

The transmitting terminal optical system of the TDLAS checkout equipment of coupling fiber also runs into great design challenges, and this is owing to needing to produce single-mode beams on all wavelengths being conveyed through measurement zone.If can use single-mode fiber in whole transmitting terminal optical system, then modal noise is not just a problem.But optical fiber only can as single mode waveguide work on limited wavelength window.Beyond short wavelength's cut-off of particular fiber, light can have several comparatively advanced space mode transfer by this optical fiber.When penetrating from optical fiber, the interference of these fine modes can produce speckle figure.Speckle figure is in time and wavelength variations.Even if a small amount of light beam slicing can produce noise in the measurements.

In contrast, the optical fiber having single mode to end if choose, it mates with needing the minimal wave length transmitted, then longer wavelength suffers great loss when being coupled into optical fiber, and this optical fiber has very large bending loses under long wavelength.

This problem can be serious in the above-mentioned wavelength multiplexing TDLAS detection and control device of coupling fiber, because we need multiplexing wavelength can reach 1.67 μm and be as short as 760nm or 670nm.Also do not have such available fibre now, it can provide single mode to run in this wide wavelength coverage, high coupling efficiency and low bending loses.In future, photonic crystal fiber can solve this difficult problem, but crystal optical fibre technology is still in its primary stage at present.

As shown in Figure 9, utilize the very short transmission part of multimode optical fiber 120, can alleviate multiplexing from the single-mode beams of 670nm or 760nm to 1670nm and radiative problem, this multimode optical fiber does not allow the wavelength of advanced space pattern to be shorter than single-mode fiber cutoff wavelength.With reference to above formula (1), if the length L of multimode optical fiber is shorter, then modal noise can be reduced.In this case, such as, if the cutoff wavelength that couples light to of 760nm is (such as, Corning SMF28) in the short single-mode fiber of 1280nm, then the light of 760nm at least still keeps single mode in several meters of distances.So the problem solving transmitting terminal modal noise is that the light of coupling 760nm enters single-mode fiber, and its wavelength is greater than 1280nm, but can be also the multimode optical fiber of 760nm, only transmits very short distance before collimated light is conveyed through measurement zone.

Fig. 9 and Fig. 2 represents the schematic diagram of this system.With reference to Fig. 9, discrete single-mode fiber 904A-904n is coupled in multiple diode laser sources 902 of launching under wide interval laser frequency.The laser of the diode laser emission between wavelength 1349nm and the 1670nm device 906 that is re-used is multiplexing.The transmitting terminal optical fiber 908 of suitable size is coupled in the output of multiplexer 906, and the wavelength coverage of Emission Lasers is from 1349nm-1670nm, and the two does not all have very large transmission loss and does not introduce modal noise.Appropriate optical fibers under these wavelength is Corning SMF28.But if be re-used and be coupled to SMF28 optical fiber, then, after being conveyed through relatively short distance, the input of 760nm just becomes multimode.Therefore, the single-mode fiber that its wavelength is less than 1280nm is coupled in the output of 760nm laser, such as, and SMF750.The multiplexed laser of the laser transmitted in input optical fibre 904n and transmission in transmitting terminal optical fiber 908 can be coupled near transmitting terminal optical element 910.Coupling mechanism 912 and transmitting terminal optical element 910 are preferably connected by the Transmission Fibers 914 of a section short, and the Transmission Fibers 914 chosen can not transmit all couplings and multiplexing wavelength with losing.The adequate transmission optical fiber of system shown in Figure 9 is Corning SMF28.As long as Transmission Fibers is relatively short, the 760nm laser being coupled to Transmission Fibers 914 does not just have serious multimode behaviour.In the system shown in Fig. 9 and optical fiber, in order to avoid introducing serious Multimodal noise, the length of Transmission Fibers must remain on and be equal to or less than 3 meters.

In the similar system shown in Fig. 2, coupling mechanism 134 from the diode laser of 760nm, receiving input and receiving multiplexed beam from having the diode laser of very long wavelength.

By embodiment disclosed herein, we fully understand object of the present invention.Those skilled in the art will appreciate that, under the condition not departing from critical function of the present invention, various feature of the present invention can be realized by different embodiments.These specific embodiments are only illustrative, instead of for limiting the scope of the invention of following claims defined.

Claims

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

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.