CN110319940B - Laser fiber interferometer diagnostic system for high density plasma density measurement - Google Patents
Laser fiber interferometer diagnostic system for high density plasma density measurement Download PDFInfo
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- CN110319940B CN110319940B CN201910730360.XA CN201910730360A CN110319940B CN 110319940 B CN110319940 B CN 110319940B CN 201910730360 A CN201910730360 A CN 201910730360A CN 110319940 B CN110319940 B CN 110319940B
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- 239000000835 fiber Substances 0.000 title claims abstract description 54
- 238000001739 density measurement Methods 0.000 title claims abstract description 10
- 239000013307 optical fiber Substances 0.000 claims abstract description 35
- 230000003287 optical effect Effects 0.000 claims abstract description 29
- 230000001427 coherent effect Effects 0.000 claims description 9
- 238000012545 processing Methods 0.000 claims description 6
- 229910000530 Gallium indium arsenide Inorganic materials 0.000 claims description 3
- 238000000034 method Methods 0.000 abstract description 15
- 238000003745 diagnosis Methods 0.000 abstract description 9
- 238000005259 measurement Methods 0.000 abstract description 8
- 230000008569 process Effects 0.000 abstract description 7
- 238000010586 diagram Methods 0.000 description 8
- 238000001514 detection method Methods 0.000 description 4
- 230000009471 action Effects 0.000 description 3
- 230000008859 change Effects 0.000 description 2
- 238000005305 interferometry Methods 0.000 description 2
- 230000009286 beneficial effect Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000005284 excitation Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 238000001914 filtration Methods 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000000149 penetrating effect Effects 0.000 description 1
- 230000000750 progressive effect Effects 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N9/00—Investigating density or specific gravity of materials; Analysing materials by determining density or specific gravity
- G01N9/24—Investigating density or specific gravity of materials; Analysing materials by determining density or specific gravity by observing the transmission of wave or particle radiation through the material
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
- G01J2009/0226—Fibres
- G01J2009/023—Fibres of the integrated optical type
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Abstract
The invention provides a laser fiber interferometer diagnosis system for high-density plasma density measurement, which adopts a fiber coupler, reduces the use of an optical platform and other optical devices in an open beam system, and has simple structure and low cost. In addition, the laser fiber interferometer diagnosis system utilizes the bendable and easily expandable characteristics of the optical fiber, so that the photoelectric detector is far away from the plasma region to be detected, and the influence of electromagnetic interference in the plasma generation process on measurement is avoided.
Description
Technical Field
The invention relates to the technical field of plasma density diagnosis, in particular to a laser fiber interferometer diagnosis system for high-density plasma density measurement.
Background
Interferometry is well established as a "non-invasive" method of measuring plasma density, and interferometry has a number of types of interferometers for measuring plasma density, which can be broadly divided into two categories: spatially resolved interferometers and time resolved interferometers.
Interferometers currently used for measuring plasma density are typically constructed as open beam systems, i.e. systems in which coherent light propagates in free space, essentially with mirrors, beam splitters and detectors.
In many cases, however, accurate recording of the plasma density evolution by an open interferometer is difficult, especially when the magnitude of the plasma density variation is small, and the fringe offset is very small, which means that the interferometer needs to be very stable on a time scale, which is much longer than the time under investigation, for eliminating background noise, such as mechanical vibrations of the external support structure and disturbances of the gas flow in the beam propagation path. To eliminate these disturbances, the open beam interferometer system needs to be fixed on a large optical platform that eliminates the vibrations, and even the optical path needs to be placed in a duct that insulates the surrounding air.
In addition, the laser used for measuring the high-density plasma is located in the visible light and near infrared bands, the conventional open beam interferometer system uses a gas laser, such as a He-Ne laser, for interference experiments, because the wavelength broadening of the laser mainly comes from the subtle energy level change in the excitation process of the laser, namely, the thermal noise of the conventional gas laser is closely related to the thermal noise of the energy level system, the thermal noise of the conventional gas laser is very large, so that the frequency (wavelength) broadening of the emergent laser is wider, generally can reach more than 100MHz, the zero beat measurement of the plasma density cannot be performed, and a precise Fabry-Perot system is additionally added for filtering in order to reduce the laser broadening.
In addition, the technology of performing plasma density measurement by heterodyne method can reduce the requirement of broadening laser source, but needs to add an acousto-optic modulator with very narrow frequency broadening (generally, the modulation frequency is 40MHz-80MHz, the broadening is less than 100 Hz), the reference light and the light to be measured enter a mixer for mixing, and then the reference light and the light to be measured enter an I-Q phase discriminator for phase demodulation.
Disclosure of Invention
In view of the above, the present invention provides a laser fiber interferometer diagnostic system for high density plasma density measurement, which has the following technical scheme:
A laser fiber interferometer diagnostic system for high density plasma density measurement, the laser fiber interferometer diagnostic system comprising: the device comprises a laser, a first optical fiber coupler, an attenuator, a first optical power meter, a second optical fiber coupler, first to third photodetectors and a signal processing device;
wherein the laser is used for outputting coherent laser;
the first optical fiber coupler is used for splitting the coherent laser beam into a signal beam and a reference beam;
The signal light beam is used for sequentially passing through a plasma region to be detected, the first optical power meter and the second optical fiber coupler;
the reference beam is used for sequentially passing through the attenuator, the second optical power meter and the second optical fiber coupler;
The second optical fiber coupler is used for connecting three paths of output ends to the first to third photodetectors respectively;
The signal processing device is used for analyzing the output voltage signals of the first to third photodetectors to obtain the density parameters of the plasma to be detected;
wherein the optical intensity of the signal beam entering the second optical fiber coupler and the optical intensity of the reference beam are equalized by adjusting the attenuator.
Preferably, in the above laser fiber interferometer diagnostic system, the laser is an ultra-narrow linewidth laser;
the ultra-narrow linewidth laser is used for outputting the coherent laser from a single-mode fiber.
Preferably, in the above laser fiber interferometer diagnostic system, the linewidth of the ultra-narrow linewidth laser is less than 3kHz.
Preferably, in the above laser fiber interferometer diagnostic system, the first fiber coupler is a2×2 fiber coupler.
Preferably, in the above laser fiber interferometer diagnostic system, the second fiber coupler is a 3×3 fiber coupler.
Preferably, in the laser fiber interferometer diagnostic system, three output signals of three output ends of the 3×3 fiber coupler are different by 120 °.
Preferably, in the laser fiber interferometer diagnostic system, the first to third photodetectors are InGaAs photodetectors with high speed and low noise.
Preferably, in the above laser fiber interferometer diagnostic system, the laser fiber interferometer diagnostic system further includes: a collimating lens;
The signal light beam passes through the collimating lens, and then sequentially passes through the plasma region to be detected, the first optical power meter and the second optical fiber coupler.
Compared with the prior art, the invention has the following beneficial effects:
the laser fiber interferometer diagnosis system adopts the fiber coupler, reduces the use of an optical platform and other optical devices in an open beam system, and further has simple structure and low cost.
In addition, the laser fiber interferometer diagnosis system utilizes the bendable and easily expandable characteristics of the optical fiber, so that the photoelectric detector is far away from the plasma region to be detected, and the influence of electromagnetic interference in the plasma generation process on measurement is avoided.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only embodiments of the present invention, and that other drawings can be obtained according to the provided drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic diagram of a system for diagnosing a laser fiber interferometer for measuring density of high density plasma according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of an original voltage signal provided by a laser interferometer when the density of a plasma to be measured changes according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a phase difference calculated by a phase difference formula according to an embodiment of the present invention;
Fig. 4 is a signal comparison diagram of lasers with different linewidths according to an embodiment of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
In order that the above-recited objects, features and advantages of the present invention will become more readily apparent, a more particular description of the invention will be rendered by reference to the appended drawings and appended detailed description.
Referring to fig. 1, fig. 1 is a schematic structural diagram of a laser fiber interferometer diagnostic system for high density plasma density measurement according to an embodiment of the present invention.
The laser fiber interferometer diagnostic system includes: a laser 11, a first optical fiber coupler 12, an attenuator 13, a first optical power meter 14, a second optical power meter 15, a second optical fiber coupler 16, first to third photodetectors 17, and a signal processing device 18;
wherein the laser 11 is used for outputting coherent laser light;
the first fiber coupler 12 is used for splitting the coherent laser beam into a signal beam and a reference beam;
The signal beam is used for sequentially passing through a plasma region 19 to be measured, the first optical power meter 14 and the second optical fiber coupler 16;
the reference beam is used for sequentially passing through the attenuator 13, the second optical power meter 15 and the second optical fiber coupler 16;
the second optical fiber coupler 16 is used for connecting three output ends to the first to third photodetectors 17 respectively;
the signal processing device 18 is configured to parse the output voltage signals of the first to third photodetectors 17 to obtain a density parameter of the plasma to be measured;
wherein the intensity of the signal beam and the intensity of the reference beam entering the second fiber coupler 16 are equalized by adjusting the attenuator 13.
In this embodiment, the output voltage signals of the photodetectors are proportional to the power of the light, the power-to-voltage coefficients of the three photodetectors may be calibrated to be relatively large in advance, and the interferometer phase difference Δφ (t) is obtained by analyzing the output voltage signals of the first to third photodetectors, thereby obtaining the chord-average density of the plasma to be measured
Wherein the phase difference and the chord average density satisfy the following relationship:
the output voltage signals P1, P2, and P3 of the first to third photodetectors and the phase difference satisfy the following relationship:
wherein lambda is the wavelength of the incident laser;
l is the thickness of electromagnetic wave penetrating through the plasma;
r e is electron classical radius 2.82× -15 m.
As shown in fig. 2 and fig. 3, fig. 2 is a schematic diagram of an original voltage signal provided by the laser interferometer when the density of the plasma to be measured changes in the embodiment of the present invention, and fig. 3 is a schematic diagram of a phase difference calculated by a phase difference formula in the embodiment of the present invention, which is equivalent to an actual structure, that is, shows the feasibility of measuring the density of the plasma by the diagnostic system of the laser fiber interferometer.
In addition, the diagnosis system of the laser fiber interferometer adopts a fiber coupler, reduces the use of an optical platform and other optical devices in an open beam system, and further has simple structure and low cost.
And the laser fiber interferometer diagnosis system can make the photoelectric detector far away from the plasma region to be measured by utilizing the flexible and expandable characteristics of the optical fiber, so that the influence of electromagnetic interference in the plasma generation process on measurement is avoided.
Further, according to the above embodiment of the present invention, the laser is an ultra-narrow linewidth laser;
the ultra-narrow linewidth laser is used for outputting the coherent laser from a single-mode fiber.
The linewidth of the ultra-narrow linewidth laser is less than 3kHz.
In this embodiment, the optical fibers for optical connection are all single-mode optical fibers, the wavelength of the laser is 1550nm, the loss of the optical fibers is minimum at the wavelength, and the detection sensitivity is higher than that of the 532nm or 652nm laser used in the open beam system.
Moreover, referring to fig. 4, fig. 4 is a signal comparison schematic diagram of a laser with different linewidths provided in the embodiment of the present invention, where an ultra-narrow linewidth laser with a linewidth smaller than 3kHz has a lower phase noise level than a laser with a linewidth of 3MHz, and is lower by a multiple order, that is, has a higher resolution for density variation, which is also a key of obtaining a high signal-to-noise ratio density signal in the present invention.
Further, according to the above embodiment of the present invention, the second fiber coupler is a 3×3 fiber coupler.
Three output signals of three output ends of the 3×3 optical fiber coupler are different by 120 °.
In the embodiment, the 3×3 optical fiber coupler is used for phase difference detection, which is one of the keys of the whole system, the homodyne measurement is adopted in the invention, and the traditional interferometer homodyne measurement cannot judge the density change direction, so that the 3×3 optical fiber coupler is used in the invention to give quadrature output of interference phase difference, namely, whether the phase difference is increased or decreased can be determined in any time period, the problem that the traditional homodyne measurement has low phase detection precision at a zero phase accessory is solved, and higher phase detection precision is provided.
In addition, the laser wavelength 1550nm falls in the wavelength range of the photoelectric detector, and the photoelectric detector does not need to be added with an amplifying function because the optical power reaches more than 30mW, so that the complexity of a system structure is further reduced, and the 3×3 optical fiber coupler is optimized, so that the final minimum measurement density is about 10 18m-3.
Further, according to the above embodiment of the present invention, the first fiber coupler is a2×2 fiber coupler.
Further, according to the above embodiment of the present invention, the first to third photodetectors are InGaAs photodetectors with high speed and low noise.
Further, according to the above embodiment of the present invention, the laser fiber interferometer diagnosis system further includes: a collimating lens;
The signal light beam passes through the collimating lens, and then sequentially passes through the plasma region to be detected, the first optical power meter and the second optical fiber coupler.
A specific hardware implementation thereof is illustrated below:
the model of the laser is UNL-1550-50-FC/APC-B-SM, and the line width is smaller than 3kHz.
The model of the 2X 2 optical fiber coupler is 1310/1550-BWC-2X 2.
The model of the 3X 3 optical fiber coupler is 1550-SSC-3X 3.
The model of the attenuator is VOA.
The first optical power meter and the second optical power meter are the same in model number and are PM20CH.
The first to third photodetectors are identical in model number and are DET01CFC/M.
The model of the optical fiber is SMF-28/FC-APC.
The model of the collimating lens is PAF2-2C.
The foregoing has described in detail a laser fiber interferometer diagnostic system for high density plasma density measurement provided by the present invention, and specific examples have been presented herein to illustrate the principles and embodiments of the present invention, the above examples being provided only to assist in understanding the method of the present invention and its core ideas; meanwhile, as those skilled in the art will have variations in the specific embodiments and application scope in accordance with the ideas of the present invention, the present description should not be construed as limiting the present invention in view of the above.
It should be noted that, in the present specification, each embodiment is described in a progressive manner, and each embodiment is mainly described as different from other embodiments, and identical and similar parts between the embodiments are all enough to be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant points refer to the description of the method section.
It is further noted that relational terms such as first and second, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include, or is intended to include, elements inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (6)
1. A laser fiber interferometer diagnostic system for high density plasma density measurement, the laser fiber interferometer diagnostic system comprising: the device comprises a laser, a first optical fiber coupler, an attenuator, a first optical power meter, a second optical fiber coupler, first to third photodetectors, a signal processing device and a collimating lens;
the laser is used for outputting coherent laser, and is an ultra-narrow linewidth laser;
the first optical fiber coupler is used for splitting the coherent laser beam into a signal beam and a reference beam;
The signal light beam is used for sequentially passing through the plasma region to be detected, the first optical power meter and the second optical fiber coupler, wherein the signal light beam passes through the collimating lens and then sequentially passes through the plasma region to be detected, the first optical power meter and the second optical fiber coupler;
the reference beam is used for sequentially passing through the attenuator, the second optical power meter and the second optical fiber coupler;
The second optical fiber coupler is used for connecting three paths of output ends to the first to third photodetectors respectively;
The signal processing device is used for analyzing the output voltage signals of the first to third photodetectors to obtain the density parameters of the plasma to be detected;
wherein the optical intensity of the signal beam entering the second optical fiber coupler and the optical intensity of the reference beam are equalized by adjusting the attenuator.
2. The laser fiber interferometer diagnostic system of claim 1, wherein the linewidth of the ultra-narrow linewidth laser is less than 3kHz.
3. The laser fiber interferometer diagnostic system of claim 1, wherein the first fiber coupler is a2 x 2 fiber coupler.
4. The laser fiber interferometer diagnostic system of claim 1, wherein the second fiber coupler is a3 x3 fiber coupler.
5. The laser fiber interferometer diagnostic system of claim 4, wherein the three output signals of the three output ends of the 3 x 3 fiber coupler differ by 120 °.
6. The laser fiber interferometer diagnostic system of claim 1, wherein the first to third photodetectors are high speed low noise InGaAs photodetectors.
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| CN108827601A (en) * | 2018-06-01 | 2018-11-16 | 苏州维创度信息科技有限公司 | A kind of measuring device of fibre optic interferometer arm length difference |
| CN109342362A (en) * | 2018-09-28 | 2019-02-15 | 西安工业大学 | A kind of micro- priming system plasma refraction rate distributed in three dimensions test device |
| CN210221312U (en) * | 2019-08-08 | 2020-03-31 | 中国科学技术大学 | Laser fiber interferometer diagnosis system for high-density plasma density measurement |
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| CN108827601A (en) * | 2018-06-01 | 2018-11-16 | 苏州维创度信息科技有限公司 | A kind of measuring device of fibre optic interferometer arm length difference |
| CN109342362A (en) * | 2018-09-28 | 2019-02-15 | 西安工业大学 | A kind of micro- priming system plasma refraction rate distributed in three dimensions test device |
| CN210221312U (en) * | 2019-08-08 | 2020-03-31 | 中国科学技术大学 | Laser fiber interferometer diagnosis system for high-density plasma density measurement |
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| 基于光纤光栅法布里-珀罗干涉仪的液体介质声波检测技术研究;张伟超;赵洪;刘通;王国利;李锐海;;光学学报(09);全文 * |
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