CN108037143B - Method and device for measuring refractive index of gas - Google Patents
Method and device for measuring refractive index of gas Download PDFInfo
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- CN108037143B CN108037143B CN201711320790.1A CN201711320790A CN108037143B CN 108037143 B CN108037143 B CN 108037143B CN 201711320790 A CN201711320790 A CN 201711320790A CN 108037143 B CN108037143 B CN 108037143B
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- G01N22/00—Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more
Abstract
The invention discloses a method and a device for measuring gas refractive index, wherein the method comprises the following steps: filling gas to be measured into a gas measuring cell; the optical frequency comb enters a first photoelectric detector after passing through the gas measuring cell; measuring an electric signal output by the first photoelectric detector through the frequency spectrograph and collecting a measuring result of the frequency spectrograph; calculating the refractive index of the gas to be measured, wherein the refractive index formula is as follows:
wherein c is the speed of light, Δ f is the frequency interval between two adjacent wave crests or wave troughs of the electrical signal obtained by the spectrometer, and d is the length of the light passing through the gas to be measured in the gas measuring cell. The invention uses the optical frequency comb to realize the measurement of the gas refractive index, converts the refractive index information to the microwave signal through phase, adopts a mature microwave instrument to realize the measurement of the gas refractive index to be measured, and has the characteristics of adjustable measurement sensitivity, stable system and the like compared with an optical measurement means.
Description
Technical Field
The invention relates to the field of optical measurement, in particular to a method and a device for measuring a gas refractive index.
Background
The refractive index is a basic physical quantity characterizing the optical properties of a substance, and this parameter is an important condition for determining the synthesis, manufacture and application of the substance in various fields.
The current methods for measuring the refractive index of gas include optical interferometry, surface plasmon resonance, optical fiber sensing and critical angle methods. These measurement methods are either not accurate enough or the operation flow is complicated.
Therefore, there is a need for a method and apparatus for measuring refractive index of gas with accurate measurement results and simple operation.
Disclosure of Invention
The invention provides a method and a device for measuring the refractive index of gas, which have accurate measurement results and simple operation.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a method of measuring the refractive index of a gas, comprising: filling gas to be measured into a gas measuring cell; the light frequency comb is normally incident into the gas measuring cell and then enters a first photoelectric detector; measuring an electric signal output by the first photoelectric detector through the frequency spectrograph and collecting a measuring result of the frequency spectrograph; calculating the refractive index of the gas to be measured, wherein the refractive index formula is as follows:
wherein c is the speed of light, Δ f is the frequency interval between two adjacent wave crests or wave troughs of the electrical signal, and d is the length of light passing through the gas to be measured in the gas measuring cell.
In one embodiment, the method for forming the optical frequency comb includes: outputting continuous light waves by a narrow linewidth laser with the frequency f; the light wave is divided into two paths of light signals through a polarization maintaining optical fiber coupler; the light of the beam path 1 passes through a first phase modulator to which a modulation signal of a frequency f is appliedmThe first phase modulator output spectrum will appear at a modulation frequency fmA series of sidebands of integer multiples; the light of the beam path 2 passes through a second phase modulator to which a modulation signal of frequency f is appliedm+f0The second phase modulator output spectrum will appear at the modulation frequency fm+f0A series of sidebands that are integer multiples; the light output by the second phase modulator passes through an acousto-optic frequency shifter, and the frequency of the modulation signal loaded on the frequency shifter is fAOMThen, after passing through the frequency shifter, the frequency of the optical carrier in the optical path 2 is f + fAOM(ii) a The optical path 1 and the optical path 2 generate beat frequency signals after passing through the single-mode fiber coupler to form an optical frequency comb, and the frequency interval of the optical frequency comb is f0。
In one embodiment, the optical frequency comb is divided into two beams of light after passing through the single-mode fiber coupler, one beam of light enters the first photoelectric detector after passing through the gas measurement cell, and the other beam of light directly enters the second photoelectric detector as a reference signal.
The invention also provides a device for measuring the refractive index of gas, which comprises a light frequency comb generation module, a gas measurement pool, a first photoelectric detector, a frequency spectrograph and a signal acquisition and processing module, wherein the light frequency comb generated by the light frequency comb generation module enters the first photoelectric detector after entering the gas measurement pool filled with the gas to be measured, the first photoelectric detector generates an electric signal, the frequency spectrograph measures the electric signal output by the first photoelectric detector and acquires the measurement result of the frequency spectrograph through the signal acquisition and processing module to calculate the refractive index of the gas to be measured, and the refractive index formula is as follows:
wherein c is the speed of light, Δ f is the frequency interval between two adjacent wave crests or wave troughs of the electrical signal, and d is the length of light passing through the gas to be measured in the gas measuring cell.
In one embodiment, the optical frequency comb generating module includes a narrow linewidth laser, a polarization maintaining fiber coupler, a first phase modulator, a second phase modulator, a frequency shifter, and a single-mode fiber coupler, and the narrow linewidth laser with frequency f outputs a continuous light wave; the light wave is divided into two paths of light signals through a polarization maintaining optical fiber coupler; the light of the beam path 1 passes through a first phase modulator to which a modulation signal of a frequency f is appliedmThe first phase modulator output spectrum will appear at a modulation frequency fmA series of sidebands of integer multiples; the light of the beam path 2 passes through a second phase modulator to which a modulation signal of frequency f is appliedm+f0The second phase modulator output spectrum will appear at the modulation frequency fm+f0A series of sidebands that are integer multiples; the light output by the second phase modulator passes through an acousto-optic frequency shifter, and the frequency of the modulation signal loaded on the frequency shifter is fAOMThen, after passing through the frequency shifter, the frequency of the optical carrier in the optical path 2 is f + fAOM(ii) a The optical path 1 and the optical path 2 pass throughThe single-mode fiber coupler generates beat frequency signal to form optical frequency comb with frequency interval of f0After the optical frequency comb passes through the photoelectric detector, an electrical frequency comb signal can be formed.
In one embodiment, a polarization state controller is further connected in series in the optical path 1 for adjusting the polarization state of the optical path 1 so as to optimize the interference contrast generated by the optical path 1 and the optical path 2.
In one embodiment, the measuring device further includes a second photodetector, the optical frequency comb is divided into two beams of light after passing through the single-mode fiber coupler, one beam of light enters the first photodetector after passing through the gas measuring cell, and the other beam of light directly enters the second photodetector as a reference signal, and the function of the second photodetector is to monitor the frequency interval of the frequency comb so as to adjust the measurement accuracy of the system.
The invention has the beneficial effects that: the invention uses the optical frequency comb to realize the measurement of the gas refractive index, converts the refractive index information to the microwave signal through phase, adopts a mature microwave instrument to realize the measurement of the gas refractive index to be measured, and has the characteristics of adjustable measurement sensitivity, stable system and the like compared with an optical measurement means.
Drawings
FIG. 1 is a diagram of an embodiment of the present invention.
FIG. 2 is a diagram illustrating an embodiment of the present invention.
In the figure, 101: a narrow linewidth laser; 102: a polarization maintaining fiber coupler; 103: a first phase modulator; 104: a polarization state controller; 105: a second phase modulator; 106: an acousto-optic frequency shifter; 107: a single mode fiber coupler; 108: a gas measurement cell; 109: a first photodetector; 201: a second photodetector; 202: a radio frequency switch; 203: a frequency spectrograph; 204: a signal acquisition and processing module; 205: a first microwave signal generator; 206: a second microwave signal generator; 207: a third microwave signal generator.
Detailed Description
The invention is further described below with reference to the accompanying drawings and examples.
The embodiment of the invention provides a method for measuring the refractive index of gas by using an optical frequency comb, which converts refractive index information to a microwave signal through phase, adopts a mature microwave instrument to measure the refractive index of the gas to be measured, and has the characteristics of adjustable measurement sensitivity, stable system and the like compared with an optical measurement means.
In this embodiment, as shown in fig. 1, the apparatus for measuring the refractive index of a gas includes a light frequency comb generating module, a gas measuring cell 108, a first photodetector 109, a second photodetector 201, a radio frequency switch 202, a spectrometer 203, and a signal collecting and processing module 204. The optical frequency comb generation module comprises a narrow linewidth laser, a polarization-maintaining optical fiber coupler, a first phase modulator, a second phase modulator, a frequency shifter and a single-mode optical fiber coupler.
The continuous wave output by the narrow linewidth laser 101 with frequency f is split into two optical signals by the polarization maintaining fiber coupler 102. The light of the beam path 1 passes through a first phase modulator 103, to which a modulation signal of frequency f is appliedmThe phase modulator output spectrum will appear at a modulation frequency fmA series of sidebands of integer multiples. Similarly, the light of beam path 2 passes through a phase modulator 105, to which a modulation signal of frequency f is appliedm+f0The phase modulator output spectrum will appear at the modulation frequency fm+f0A series of sidebands that are integer multiples. The light output from the second phase modulator 105 passes through an acousto-optic frequency shifter 106, where the frequency of the modulation signal loaded on the frequency shifter is fAOMThen, after passing through the frequency shifter, the frequency of the optical carrier in the optical path 2 is f + fAOM. After the optical paths 1 and 2 pass through the single-mode fiber coupler 107, beat signals are generated to form an optical frequency comb, and the frequency interval of the optical frequency comb is f0. If the optical frequency comb is directly detected by a high-speed photoelectric detector, the radio frequency comb is observed on a frequency spectrograph. The polarization state controller 104 in the optical path 1 is used for adjusting the polarization state of the optical path 1 to optimize the interference contrast generated by the optical path 1 and the optical path 2, thereby improving the beat frequency effect of the two paths of signals. The modulation signal sources of the first phase modulators 103 and 105 in the measurement system are a first microwave signal generator 205 and a second microwave signal generator, respectively206, the modulation signal source of the acousto-optic frequency shifter is a third microwave signal generator 207. The formation of the rf comb is illustrated in fig. 2.
The generated frequency comb is divided into two beams of light after passing through the single-mode fiber coupler 107, one beam of light enters the first photoelectric detector 109 after passing through the gas measuring cell 108, and the other beam of light directly enters the second photoelectric detector 201 as a reference signal. The electrical signals output by the two photodetectors pass through the radio frequency switch 202 and then enter the spectrometer 203 for signal measurement, and signal acquisition and result display are performed through the signal acquisition and processing module 204, and the module 204 analyzes the acquired electrical signals and calculates the refractive index of the gas to be measured. The gas measuring cell 108 may be a conventional cuvette, and two light-transmitting inner surfaces of the cuvette are coated with reflective films, so that the cuvette becomes a fabry-perot interferometer.
The measurement principle of the invention is similar to the Fabry-Perot measurement spectrum. The frequency comb of light having a plurality of frequency components passes through a cuvette of a certain thickness, which is equivalent to passing quasi-monochromatic light or white light through an etalon. The emergent light intensity of the light frequency comb after passing through the gas absorption cell can be expressed as follows:
in the above formula I0For the incident light intensity, F is the fresnel coefficient, and Φ (F) is the phase difference between two adjacent interfering light beams, which can be expressed as:
in the above formula, d is the length of light passing through the gas to be measured in the gas measurement cell, n is the refractive index of the gas to be measured, c is the speed of light, and θ is the refraction angle of the light in the gas to be measured. If the optical frequency comb is a normal incidence gas measurement cell, the photoelectric detector receives the light output by the gas measurement cell and converts the light into an electric signal, the electric signal is observed by the spectrometer 203, a cosine microwave signal can be obtained, and the frequency interval between two adjacent wave crests or wave troughs of the signal is delta f:
the refractive index of the gas to be measured can be expressed as:
according to the formula, according to the light velocity constant c, the refractive index of the gas to be measured can be obtained through the length d of the gas to be measured in the gas measurement cell and the signal frequency range delta f in one period obtained by the spectrometer. If the gas measurement cell is a cuvette, the light transmission length between two inner walls of the cuvette is 5mm, and when the gas measurement cell is air, the frequency interval between two adjacent wave crests of the microwave signal output by the photoelectric detector is 30 GHz. When d is not changed, the modulation frequency of the phase modulator is reduced, so that the repetition frequency of the optical frequency comb can be reduced, the radio frequency interval in the optical frequency comb is smaller, effective data points obtained by the frequency spectrograph in one period are more, and the precision of the measurement result can be improved. If d is large, the modulation frequency of the phase modulator needs to be increased. Likewise, the present invention can be used to measure the thickness of a transparent object of known refractive index.
The second photodetector 201 in the present invention is used to monitor the frequency interval of the optical frequency comb and adjust the measurement accuracy of the system in real time.
The gas refractive index measuring system provided by the invention has simple working process, after the system is electrified, gas to be measured is filled into the gas measuring cell, the frequency interval delta f of the output signal of the frequency spectrograph in one period is recorded, and the result to be measured can be obtained according to the formula (4).
Claims (5)
1. A method of measuring the refractive index of a gas, comprising: filling gas to be measured into a gas measuring cell; the light frequency comb is normally incident into the gas measuring cell and then enters a first photoelectric detector; measuring an electric signal output by the first photoelectric detector through the frequency spectrograph and collecting a measuring result of the frequency spectrograph; calculating the refractive index of the gas to be measured, wherein the refractive index formula is as follows:
wherein c is the speed of light, Δ f is the frequency interval between two adjacent wave crests or wave troughs of the electrical signal obtained by the frequency spectrograph, and d is the length of the light passing through the gas to be measured in the gas measuring cell;
the forming method of the optical frequency comb comprises the following steps: outputting continuous light waves by a narrow linewidth laser with the frequency f; the light wave is divided into two paths of light signals through a polarization maintaining optical fiber coupler; the light of the beam path 1 passes through a first phase modulator to which a modulation signal of a frequency f is appliedmThe first phase modulator output spectrum will appear at a modulation frequency fmA series of sidebands of integer multiples; the light of the beam path 2 passes through a second phase modulator to which a modulation signal of frequency f is appliedm+f0The second phase modulator output spectrum will appear at the modulation frequency fm+f0A series of sidebands that are integer multiples; the light output by the second phase modulator passes through an acousto-optic frequency shifter, and the frequency of the modulation signal loaded on the frequency shifter is fAOMThen, after passing through the frequency shifter, the frequency of the optical carrier in the optical path 2 is f + fAOM(ii) a The optical path 1 and the optical path 2 generate beat frequency signals after passing through the single-mode fiber coupler to form an optical frequency comb, and the frequency interval of the optical frequency comb is f0。
2. The measurement method according to claim 1, wherein the optical frequency comb is divided into two beams after passing through a single-mode fiber coupler, one beam enters the first photodetector after passing through the gas measurement cell, and the other beam directly enters the second photodetector as a reference signal.
3. The utility model provides a measuring device of gaseous refracting index, its characterized in that, includes that optical frequency comb produces module, gaseous measuring cell, first photoelectric detector, spectrometer, signal acquisition and processing module, the optical frequency comb that optical frequency comb produced the module, the normal incidence gets into first photoelectric detector after having filled into the gaseous gas measuring cell that awaits measuring, and first photoelectric detector produces the signal of telecommunication, and the spectrometer measures the signal of telecommunication of first photoelectric detector output and gathers the measuring result of spectrometer through signal acquisition and processing module, calculates the gaseous refracting index that awaits measuring, and the refracting index formula is:
wherein c is the speed of light, Δ f is the frequency interval between two adjacent wave crests or wave troughs of the electrical signal, and d is the length of the light passing through the gas to be measured in the gas measuring cell;
the optical frequency comb generation module comprises a narrow linewidth laser, a polarization-maintaining optical fiber coupler, a first phase modulator, a second phase modulator, a frequency shifter and a single-mode optical fiber coupler, wherein the narrow linewidth laser with the frequency of f outputs continuous light waves; the light wave is divided into two paths of light signals through a polarization maintaining optical fiber coupler; the light of the beam path 1 passes through a first phase modulator to which a modulation signal of a frequency f is appliedmThe first phase modulator output spectrum will appear at a modulation frequency fmA series of sidebands of integer multiples; the light of the beam path 2 passes through a second phase modulator to which a modulation signal of frequency f is appliedm+f0The second phase modulator output spectrum will appear at the modulation frequency fm+f0A series of sidebands that are integer multiples; the light output by the second phase modulator passes through an acousto-optic frequency shifter, and the frequency of the modulation signal loaded on the frequency shifter is fAOMThen, after passing through the frequency shifter, the frequency of the optical carrier in the optical path 2 is f + fAOM(ii) a The optical path 1 and the optical path 2 generate beat frequency signals after passing through the single-mode fiber coupler to form an optical frequency comb, and the frequency interval of the optical frequency comb is f0After the optical frequency comb passes through the photoelectric detector, an electrical frequency comb signal can be formed.
4. The measuring apparatus according to claim 3, wherein a polarization state controller is further connected in series in the optical path 1 for adjusting the polarization state of the optical path 1 to optimize the interference contrast generated by the optical path 1 and the optical path 2.
5. The measuring device of claim 3, wherein the measuring device further comprises a second photodetector, the optical frequency comb is divided into two beams of light after passing through the single mode fiber coupler, one beam of light enters the first photodetector after passing through the gas measuring cell, and the other beam of light directly enters the second photodetector as a reference signal, and the function of the second photodetector is to monitor the frequency interval of the frequency comb so as to adjust the measuring accuracy of the system.
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| CN101482432A (en) * | 2008-01-09 | 2009-07-15 | 中国科学院电子学研究所 | Timer resolved dual-grating interferometer |
| CN102183486A (en) * | 2011-01-28 | 2011-09-14 | 清华大学 | Gas refractive index measurer and measuring method thereof based on optical frequency comb |
| CN102183234A (en) * | 2011-03-21 | 2011-09-14 | 清华大学 | Method and device for measuring frequency scanning absolute distance based on femtosecond optical frequency comb |
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| CN101482432A (en) * | 2008-01-09 | 2009-07-15 | 中国科学院电子学研究所 | Timer resolved dual-grating interferometer |
| CN102183486A (en) * | 2011-01-28 | 2011-09-14 | 清华大学 | Gas refractive index measurer and measuring method thereof based on optical frequency comb |
| CN102183234A (en) * | 2011-03-21 | 2011-09-14 | 清华大学 | Method and device for measuring frequency scanning absolute distance based on femtosecond optical frequency comb |
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