CN110319940B - Laser fiber interferometer diagnostic system for high density plasma density measurement - Google Patents

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

Laser fiber interferometer diagnostic system for high-density plasma density measurement

Technical Field

The invention relates to the technical field of plasma density diagnosis, and more specifically to a laser fiber interferometer diagnosis system for high-density plasma density measurement.

Background technique

Interferometry is a very mature method for measuring plasma density in a “non-invasive” manner. There are many types of interferometers for measuring plasma density, which can be roughly divided into two categories: spatially resolved interferometers and time-resolved interferometers.

The interferometers currently used to measure plasma density are usually constructed as open beam systems, that is, the coherent light in the system propagates in free space, and they mainly have devices such as mirrors, beam splitters and detectors.

However, in many cases, it is very difficult for an open interferometer to accurately record the evolution of plasma density, especially when the plasma density changes very little and the fringe offset is very small, which means that the interferometer needs to be very stable on a time scale that is much longer than the time under study to eliminate background noise, such as mechanical vibrations of external support structures and interference from airflow in the beam propagation path. In order to eliminate these interferences, the open beam interferometer system needs to be fixed on a large optical platform that eliminates vibrations, and even the optical path needs to be placed in a pipe that is isolated from the surrounding air.

In addition, the laser used to measure high-density plasma is in the visible light and near-infrared bands. Traditionally, open beam interferometer systems use gas lasers, such as He-Ne lasers, for interference experiments. Since the wavelength broadening of the laser mainly comes from the subtle changes in energy levels during the laser excitation process, which is closely related to the thermal noise of the energy level system, the thermal noise of traditional gas lasers is very large, resulting in a wide frequency (wavelength) broadening of the emitted laser, usually reaching more than 100 MHz, making it impossible to perform zero-beat measurement of plasma density. In order to reduce laser broadening, an additional precise Fabry-Perot system is required for filtering.

In addition, the technology of using heterodyne method to measure plasma density can reduce the broadening requirements of laser source, but it is necessary to add an acousto-optic modulator with very narrow frequency broadening (generally the modulation frequency is 40MHz-80MHz, and the broadening is less than 100Hz). The reference light and the light to be measured enter the mixer for mixing, and then pass through the I-Q phase detector for phase demodulation. The heterodyne method has a complex structure and is expensive, which in turn leads to a complex structure and expensive price of the interferometer for measuring plasma density.

Summary of the invention

In view of this, in order to solve the above problems, the present invention provides a laser fiber interferometer diagnostic system for high-density plasma density measurement, and the technical solution is as follows:

A laser fiber interferometer diagnostic system for high-density plasma density measurement, the laser fiber interferometer diagnostic system comprising: a laser, a first fiber coupler, an attenuator, a first optical power meter, a second optical power meter, a second fiber coupler, first to third photoelectric detectors and a signal processing device;

Wherein, the laser is used to output coherent laser;

The first optical fiber coupler is used to split the coherent laser into a signal beam and a reference beam;

The signal light beam is used to sequentially pass through the plasma region to be measured, the first optical power meter and the second optical fiber coupler;

The reference beam is used to sequentially pass through the attenuator, the second optical power meter and the second optical fiber coupler;

The second optical fiber coupler is used to connect the three output ends to the first to third photodetectors respectively;

The signal processing device is used to analyze the output voltage signals of the first to third photodetectors to obtain the density parameter of the plasma to be measured;

The attenuator is adjusted to make the light intensity of the signal light beam entering the second optical fiber coupler equal to the light intensity of the reference light beam.

Preferably, in the above-mentioned laser fiber interferometer diagnostic system, the laser is an ultra-narrow linewidth laser;

The ultra-narrow linewidth laser is used to output the coherent laser from a single-mode optical fiber.

Preferably, in the above-mentioned laser fiber interferometer diagnostic system, the linewidth of the ultra-narrow linewidth laser is less than 3 kHz.

Preferably, in the above-mentioned laser fiber interferometer diagnostic system, the first fiber coupler is a 2×2 fiber coupler.

Preferably, in the above-mentioned laser fiber interferometer diagnostic system, the second fiber coupler is a 3×3 fiber coupler.

Preferably, in the above-mentioned laser fiber interferometer diagnostic system, the three output signals of the three output ends of the 3×3 fiber coupler differ by 120°.

Preferably, in the above-mentioned laser fiber interferometer diagnostic system, the first to third photodetectors are high-speed and low-noise InGaAs photodetectors.

Preferably, in the above-mentioned laser fiber interferometer diagnostic system, the laser fiber interferometer diagnostic system further comprises: a collimating lens;

Wherein, after being processed by the collimating lens, the signal light beam passes through the plasma region to be measured, the first optical power meter and the second optical fiber coupler in sequence.

Compared with the prior art, the present invention has the following beneficial effects:

The laser fiber interferometer diagnostic system adopts a fiber coupler, which reduces the use of optical platforms and other optical devices in an open beam system, thereby having a simple structure and low cost.

In addition, the laser fiber interferometer diagnostic system utilizes the bendability and extensibility of optical fiber to enable the photodetector to be far enough away from the plasma region to be measured, thereby avoiding the influence of electromagnetic interference in the plasma generation process on the measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

FIG1 is a schematic structural diagram of a laser fiber interferometer diagnostic system for measuring high-density plasma density provided by an embodiment of the present invention;

FIG2 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;

FIG3 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 schematic diagram of signal comparison of lasers with different line widths provided by an embodiment of the present invention.

Detailed ways

The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

Refer to FIG. 1 , which is a schematic diagram of the structure of a laser fiber interferometer diagnostic system for high-density plasma density measurement provided by an embodiment of the present invention.

The laser fiber interferometer diagnostic system comprises: a laser 11, a first fiber coupler 12, an attenuator 13, a first optical power meter 14, a second optical power meter 15, a second fiber coupler 16, first to third photodetectors 17 and a signal processing device 18;

Wherein, the laser 11 is used to output coherent laser;

The first optical fiber coupler 12 is used to split the coherent laser into a signal beam and a reference beam;

The signal light beam is used to sequentially pass through the plasma region to be measured 19, the first optical power meter 14 and the second optical fiber coupler 16;

The reference beam is used to sequentially pass 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 to connect the three output ends to the first to third photodetectors 17 respectively;

The signal processing device 18 is used to analyze the output voltage signals of the first to third photodetectors 17 to obtain the density parameters of the plasma to be measured;

The attenuator 13 is adjusted to make the light intensity of the signal light beam entering the second optical fiber coupler 16 equal to the light intensity of the reference light beam.

In this embodiment, the output voltage signal of the photodetector is proportional to the power of light. The power-to-voltage coefficients of the three photodetectors can be calibrated in advance to determine their relative sizes. The interferometer phase difference Δφ(t) is obtained by analyzing the output voltage signals of the first to third photodetectors, and then the chord average density of the plasma to be measured is obtained.

Among them, the phase difference and the average chord density satisfy the following relationship:

The output voltage signals P1, P2 and P3 and the phase difference of the first to third photodetectors satisfy the following relationship:

Where λ is the incident laser wavelength;

L is the thickness of the plasma penetrated by electromagnetic waves;

_re is the classical radius of electron 2.82×10 ^-15 m.

As shown in Figures 2 and 3, Figure 2 is a schematic diagram of the original voltage signal given by the laser interferometer when the measured plasma density changes according to an embodiment of the present invention, and Figure 3 is a schematic diagram of the phase difference calculated by the phase difference formula according to an embodiment of the present invention, which is equivalent to the actual structure, indicating the feasibility of the laser fiber interferometer diagnostic system in measuring plasma density.

In addition, the laser fiber interferometer diagnostic system adopts a fiber coupler, which reduces the use of optical platforms and other optical devices in an open beam system, thereby simplifying the structure and reducing the cost.

Furthermore, the laser fiber interferometer diagnostic system utilizes the bendability and extensibility of optical fiber to place the photodetector far enough away from the plasma region to be measured, thereby avoiding the influence of electromagnetic interference in the plasma generation process on the measurement.

Further, based on the above embodiment of the present invention, the laser is an ultra-narrow linewidth laser;

The ultra-narrow linewidth laser is used to output the coherent laser from a single-mode optical fiber.

The line width of the ultra-narrow line width laser is less than 3 kHz.

In this embodiment, the optical fibers used for optical path connection are all single-mode optical fibers, and the wavelength of the laser is 1550nm. At this wavelength, the loss of the optical fiber is minimal, and compared with the 532nm or 652nm laser used in the open beam system, the detection sensitivity is higher.

In addition, referring to FIG4 , FIG4 is a schematic diagram of signal comparison of lasers with different line widths provided in an embodiment of the present invention. An ultra-narrow line width laser with a line width less than 3 kHz has a lower phase noise level than a laser with a line width of 3 MHz, which is several orders of magnitude lower, i.e., it has a higher resolution capability for density changes, which is also the key to obtaining a high signal-to-noise ratio density signal in the present invention.

Further, based on the above embodiment of the present invention, the second optical fiber coupler is a 3×3 optical fiber coupler.

The three output signals at the three output ends of the 3×3 optical fiber coupler have a phase difference of 120°.

In this embodiment, a 3×3 fiber coupler is used for phase difference detection, which is one of the keys to the entire system. The present invention uses zero-beat measurement. Conventional interferometer zero-beat measurement cannot determine the direction of density change. Therefore, a 3×3 fiber coupler is used in the present invention to provide an orthogonal output of the interference phase difference, that is, it can determine whether the phase difference increases or decreases within any time period, and solves the problem of low phase detection accuracy in the zero-phase annex of conventional zero-beat measurement, providing higher phase detection accuracy.

In addition, the laser wavelength of 1550nm falls within the wavelength range of the photodetector. Since the optical power reaches more than 30mW, the photodetector does not need to add an amplification function, further reducing the complexity of the system structure. The 3×3 fiber coupler is optimized to achieve a final minimum measurement density of about 10 ¹⁸ m ^-3 .

Further, based on the above embodiment of the present invention, the first optical fiber coupler is a 2×2 optical fiber coupler.

Further, based on the above embodiment of the present invention, the first to third photodetectors are high-speed and low-noise InGaAs photodetectors.

Further, based on the above-mentioned embodiment of the present invention, the laser fiber interferometer diagnostic system further includes: a collimating lens;

Wherein, after being processed by the collimating lens, the signal light beam passes through the plasma region to be measured, the first optical power meter and the second optical fiber coupler in sequence.

The following is an example of a specific hardware implementation method:

The model of the laser is UNL-1550-50-FC/APC-B-SM, and the line width is less than 3kHz.

The model of the 2×2 fiber optic coupler is 1310/1550-BWC-2×2.

The model of the 3×3 fiber coupler is 1550-SSC-3×3.

The model of the attenuator is VOA.

The first optical power meter and the second optical power meter are of the same model, PM20CH.

The first to third photodetectors are of the same model, namely DET01CFC/M.

The model of the optical fiber is SMF-28/FC-APC.

The model of the collimating lens is PAF2-2C.

The above is a detailed introduction to the laser fiber interferometer diagnostic system for measuring high-density plasma density provided by the present invention. Specific examples are used in this article to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; at the same time, for general technicians in this field, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as a limitation on the present invention.

It should be noted that each embodiment in this specification is described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same or similar parts between the embodiments can 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 parts can be referred to the method part description.

It should also be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device that includes a series of elements is inherent to the elements, or also includes elements inherent to these processes, methods, articles or devices. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the presence of other identical elements in the process, method, article or device that includes the elements.

The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to 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, characterized in that the laser fiber interferometer diagnostic system comprises: a laser, a first fiber coupler, an attenuator, a first optical power meter, a second optical power meter, a second fiber coupler, first to third photodetectors and a signal processing device, and a collimating lens; Wherein, the laser is used to output coherent laser, and the laser is an ultra-narrow linewidth laser; The first optical fiber coupler is used to split the coherent laser into a signal beam and a reference beam; The signal light beam is used to sequentially pass through the plasma region to be measured, the first optical power meter and the second optical fiber coupler, wherein the signal light beam is processed by the collimating lens and then sequentially passes through the plasma region to be measured, the first optical power meter and the second optical fiber coupler; The reference beam is used to sequentially pass through the attenuator, the second optical power meter and the second optical fiber coupler; The second optical fiber coupler is used to connect the three output ends to the first to third photodetectors respectively; The signal processing device is used to analyze the output voltage signals of the first to third photodetectors to obtain the density parameter of the plasma to be measured; The attenuator is adjusted to make the light intensity of the signal light beam entering the second optical fiber coupler equal to the light intensity of the reference light beam. 2 . The laser fiber interferometer diagnostic system according to claim 1 , wherein the line width of the ultra-narrow line width laser is less than 3 kHz. 3 . The laser fiber interferometer diagnostic system according to claim 1 , wherein the first fiber coupler is a 2×2 fiber coupler. 4 . The laser fiber interferometer diagnostic system according to claim 1 , wherein the second fiber coupler is a 3×3 fiber coupler. 5 . The laser fiber interferometer diagnostic system according to claim 4 , wherein the three output signals of the three output ends of the 3×3 fiber coupler differ by 120°. 6 . The laser fiber interferometer diagnostic system according to claim 1 , wherein the first to third photodetectors are high-speed, low-noise InGaAs photodetectors.