CN120008806B - A backscattering measurement system and method based on low-coherence interferometry - Google Patents
A backscattering measurement system and method based on low-coherence interferometryInfo
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- CN120008806B CN120008806B CN202510175314.3A CN202510175314A CN120008806B CN 120008806 B CN120008806 B CN 120008806B CN 202510175314 A CN202510175314 A CN 202510175314A CN 120008806 B CN120008806 B CN 120008806B
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Abstract
The invention provides a back scattering measurement system and method based on low coherence interference, when in measurement, firstly, a system is built, and for back scattering light of each back reflection angle, a retroreflector is moved at a constant speed to provide continuously-variable optical path difference; the invention utilizes the characteristics of wide spectrum and short coherence length of low coherence light source to screen out the concerned stray light source and remove the influence of other non-target stray light to distinguish different stray light sources, in addition, the invention uses the interference method to measure, is more similar to the actual working condition of telescope, and improves the sensitivity and accuracy of measurement.
Description
Technical Field
The invention relates to the technical field of optical precision measurement, in particular to a backward scattering measurement system and method based on low coherence interferometry.
Background
The telescope is used as one of the key loads of the inter-satellite laser interferometry of the Tianqin, and is mainly used for improving the transmission efficiency of light beams among satellites. In order to realize detection of the power wave signals in the frequency range of 0.1 Hz-1 Hz, the inter-satellite displacement measurement accuracy needs to reach pm/Hz 1/2 orders aiming at the inter-satellite laser interference arm length of 17 ten thousand kilometers. Because the inter-satellite laser interferometry system adopts a laser receiving and transmitting multiplexing design, the receiving power of 4W transmitting laser reaching a remote satellite is 10nW magnitude, and therefore the backscattering of a telescope at a receiving end can have great influence on a final beat frequency signal. The existing research shows that the total back scattering light power is smaller than 9× -10 W to meet the requirement, which makes the design and processing of the telescope challenging, and makes the design of the stray light test system higher. Therefore, the research and construction of the test system for ultra-low back scattering stray light has important significance.
Conventional laser coherence measurement techniques, such as single-mode lasers, have the advantage of high frequency stability, but have a narrow spectral width, which results in a single emitted light in frequency. Meanwhile, because the coherence length is long, interference patterns can be formed in the whole measurement range, and if the interference patterns are applied to inter-satellite interferometry, the sources of stray light are difficult to accurately position. Secondly, limited by the physical dimensions of the laser and detector, traditional measurement systems employ a spectroscopic plate to separate the measurement light from the reference light, which can easily collect stray light from other sources due to imperfections in the test system itself, thereby introducing additional measurement errors. In addition, the direct detection measurement method cannot accurately position the sources of different stray light, and cannot know the contribution of the stray light of each optical element in the optical system, so that the guidance of the design of the optical system is further affected.
Disclosure of Invention
The invention provides a backward scattering measurement system and a backward scattering measurement method based on low coherence interference, which are used for overcoming the problem that the prior art cannot accurately position the sources of different stray light, realizing screening and measurement of light in a specific path based on a broadband light source used by the low coherence interference technology, accurately distinguishing the contributions of different stray light sources and providing guidance for the design of a telescope optical system.
In order to solve the technical problems, the technical scheme of the invention is as follows:
A back scattering measurement system based on low coherence interference comprises a low coherence light source module, a first beam splitter, a second beam splitter, a first reflecting mirror, a second reflecting mirror, a third reflecting mirror, a retroreflector, a balance detector module and a central processing unit module;
the low-coherence light source module emits a light beam with the spectral width meeting the preset condition, and the light beam passes through the first beam splitter to obtain a first beam splitting and a second beam splitting which are perpendicular to each other;
The first light beam is incident to a preset sample, and the back scattered light on the sample is reflected to a first beam splitter along an incident light path of the first light beam;
The second light beam is directly incident to the retroreflector, the retroreflector retroreflects the second light beam to the second reflector, and the second light beam refracted by the second reflector is incident to the second beam splitter;
Taking the back scattered light as measurement light, taking the second light beam as reference light, and obtaining a third light beam and a fourth light beam which are perpendicular to each other after the measurement light passes through a second beam splitter;
The fourth light beam and the fifth light beam are used as a group of interference light beams which interfere with each other to be commonly incident to the balance detector module, and the third light beam and the sixth light beam are used as another group of interference light beams which interfere with each other to be commonly incident to the third reflector and then to be incident to the balance detector module;
The balance detector module is electrically connected with the central processing unit module, performs photoelectric conversion on the two groups of interference light respectively, and inputs the acquired electric signals into the central processing unit module for analysis and processing;
the sample in the present invention may be a mirror or a lens, etc., and the influence of stray light can be evaluated by the present system as long as the reflected light can be back-scattered.
Preferably, the low coherence light source module comprises a super-radiation light emitting diode and a collimator;
and the light emitted by the super-radiation light-emitting diode passes through the collimator to obtain a light beam with the spectral width meeting the preset condition.
Preferably, the sample is arranged on a rotatable turntable for driving the sample to rotate, thereby changing the angle of retroreflection of the backscattered light.
Preferably, the retroreflector is disposed on a movable displacement platform;
the control end of the displacement platform is electrically connected with the central processing unit module, and the central processing unit module controls the displacement platform to move at a constant speed and drives the retroreflector to move at a constant speed, so that the optical path difference between the measuring light and the reference light is changed.
Preferably, the first mirror, the second mirror and the third mirror are all piezoelectric driven mirrors.
Preferably, the balance detector module comprises a first photoelectric detector, a second photoelectric detector and a differential amplifying circuit;
the first photoelectric detector and the second photoelectric detector are used for respectively receiving two groups of interference light and performing photoelectric conversion, respectively obtaining photocurrents and inputting the photocurrents into the differential amplifying circuit;
the differential amplifying circuit is electrically connected with the central processing unit module and is used for amplifying the photocurrent, acquiring an electric signal and inputting the electric signal into the central processing unit module.
Preferably, the system further comprises a first linear polarizer, a second linear polarizer and a third linear polarizer;
the light beam emitted by the low-coherence light source module passes through the first linear polarizer and then enters the first beam splitter;
The two groups of interference light respectively pass through the second linear polarizer and the third linear polarizer and then are incident to the balance detector module.
The invention also provides a back scattering measurement method based on low coherence interference, which is based on the back scattering measurement system based on low coherence interference, and comprises the following steps:
Building the system, for each retroreflective angle of backscattered light, moving the retroreflector at a constant speed to provide a continuously varying optical path difference;
Respectively carrying out real-time photoelectric conversion on two groups of interference light by utilizing a balance detector module, calculating output voltage V by utilizing a preset back scattering alternating current signal mathematical model, and acquiring the electric signal in real time;
And inputting the electric signals into a central processing unit module in real time, analyzing and processing the electric signals, and completing the measurement of the back scattered light.
Preferably, the preset mathematical model of the back-scattering alternating-current signal is specifically:
Wherein V is the output voltage of the balanced detector module, G is the gain coefficient of the balanced detector module, Γ is a preset global transmission coefficient, S (f) is the response function of the photoelectric detector, P (f) is the spectral function of the light source, alpha is the balance factor, f is the frequency of the light emitted by the low-coherence light source module, eta a is the overlapping factor between the light emitted by the low-coherence light source module and the photoelectric detector, ρ (f 0) is the coherence coefficient of back scattering, f 0 is the optical frequency of the central wavelength, P 0 is the optical power of the low-coherence light source, delta is the phase change introduced by the wavefront error, deltaL is the optical path difference between the reference beam and the measuring beam, T ref,1 is the complex transmission coefficient of the third light beam, T ref,2 is the complex transmission coefficient of the fourth light beam, T sig,1 is the complex transmission coefficient of the sixth light beam, and T sig,2 is the complex transmission coefficient of the fifth light beam.
Preferably, the central processor module performs fourier transform on the electrical signal, and the electrical signal FFTV after fourier transform is expressed as:
The peak frequency of the electric signal FFTV after Fourier transformation is obtained, filtering is carried out by taking the peak frequency as the center, the signal to noise ratio of the signal is improved, and the back scattered light is measured according to the electric signal after filtering.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
The invention provides a back scattering measurement system and method based on low coherence interference, when in measurement, firstly, a system is built, and for back scattering light of each back reflection angle, a retroreflector is moved at a constant speed to provide continuously-variable optical path difference; the method comprises the steps of respectively carrying out real-time photoelectric conversion on two groups of interference light by utilizing a balance detector module, calculating output voltage V by utilizing a preset back scattering alternating current signal mathematical model, and acquiring an electric signal in real time;
In addition, the invention uses the interference method to measure, is closer to the actual working condition of the telescope, improves the sensitivity and accuracy of measurement, can improve the detection level to 10 -10 orders of magnitude (the order of magnitude is the theoretical measurement limit calculated by considering the shot noise, dark current noise and relative intensity noise of the used balance detector), provides more accurate data for gravitational wave measurement, and effectively improves the accuracy and reliability of gravitational wave detection.
Drawings
Fig. 1 is a schematic diagram of a low coherence michelson interferometer provided in an embodiment.
Fig. 2 is an interferogram provided in the present embodiment in the presence of four backscatter sources.
Fig. 3 is a block diagram of a backscatter measurement system based on low coherence interferometry provided in example 1.
Fig. 4 is a flow chart of a backscatter measurement method based on low coherence interferometry provided in example 2.
Fig. 5 is a schematic representation of multiple scattering as provided in example 2.
Fig. 6 is a spectrum plot after discrete fourier transform provided in example 2.
Fig. 7 is a spectrum plot after the continuous fourier transform provided in example 2.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the present patent;
For the purpose of better illustrating the embodiments, certain elements of the drawings may be omitted, enlarged or reduced and do not represent the actual product dimensions;
it will be appreciated by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The technical scheme of the invention is further described below with reference to the accompanying drawings and examples.
The low coherence interferometry is mainly different from the traditional laser coherence measurement technology in light source aspect in terms of spectrum width and coherence length, the two parameters directly influence the formation and measurement result of interference patterns, the low coherence interferometry generally selects a light source with the spectrum width of tens of nanometers, the generated interference patterns only generate obvious interference when light Cheng Chaxiao is in the coherence length, the measurement range is effectively limited, the measurement selectivity is improved, light of a specific path can be screened out, and therefore the low coherence interferometry is particularly suitable for application scenes needing to distinguish contributions of different stray light sources, such as stray light measurement of an Tianqin telescope, and the selective measurement capability of the technology provides new possibility for precise measurement and imaging.
The specific theoretical analysis on which the invention depends is as follows:
In view of the practical situation, the present invention assumes that the spectral distribution of a low coherence light source is gaussian, and thus its intensity distribution can be expressed by the following formula:
Wherein k 0＝2π/λ0 is the wave number corresponding to the center wavelength lambda 0, G 0 is the intensity of the spectrum at the center wavelength, ζ is the spectral coefficient, Is the coherence length of the light source, Δλ is the half-width of the light source;
The invention takes a low coherence Michelson interferometer as an example to illustrate the principle of measuring back scattered light, a schematic diagram of the low coherence Michelson interferometer is shown in figure 1, light emitted by a light source is divided into two beams by a beam splitter, one path of light incident on a fixed total reflection mirror is used as measuring light, one path of light incident on a scanning total reflection mirror is used as reference light, and the scanning total reflection mirror moves at a constant speed v to provide a continuously variable optical path difference;
the relationship between the intensity of light received by the detector and the optical path difference can be expressed by the following equation:
Wherein R is the reflectivity of the fixed total reflection mirror and the scanning total reflection mirror, and let k' =k-k 0, the integral of the whole spectrum can be obtained:
In the above formula, the backscattering information which is obtained by the invention is all in the amplitude part, and the backscattering can be obtained by resolving the signal;
If there are multiple back scattering sources, i.e. there are multiple measuring light paths, the interference pattern will have multiple peaks, for example, fig. 2 is the interference pattern when there are four back scattering sources, the abscissa is the scanning distance, and the ordinate is the voltage after photoelectric conversion, therefore, the method can isolate the mixed influence of other stray light sources on detection, i.e. the peak at a certain moment is caused by the stray light of the optical path difference, at the same time, the method can screen the stray light of different paths, and for the space organ telescope, the contribution of each mirror to the stray light can be clarified.
Based on the above basic principle, the present invention provides the following embodiments:
example 1
As shown in fig. 3, the present embodiment provides a backscatter measurement system based on low coherence interferometry, which includes a low coherence light source module, a first beam splitter BS1, a second beam splitter BS2, a first mirror M1, a second mirror M2, a third mirror M3, a retroreflector M4, a balanced detector module, and a central processor module;
The low-coherence light source module emits a light beam with the spectral width meeting the preset condition, and the light beam passes through the first beam splitter BS1 to obtain a first beam splitting and a second beam splitting which are perpendicular to each other;
the first light beam is incident to a preset sample, and the back scattered light on the sample is reflected to a first beam splitter BS1 along an incident light path of the first light beam, and the back scattered light is refracted by a first reflector M1 and then is incident to a second beam splitter BS2;
The second light beam is directly incident on the retroreflector M4, the retroreflector M4 retroreflects the second light beam to the second mirror M2, and the second light beam refracted by the second mirror M2 is incident on the second beam splitter BS2;
Taking the back scattered light as measurement light, taking the second light beam as reference light, and obtaining a third light beam and a fourth light beam which are perpendicular to each other after the measurement light passes through a second beam splitter BS2, wherein the reference light beam obtains a fifth light beam and a sixth light beam which are perpendicular to each other after the reference light passes through the second beam splitter BS 2;
The fourth light beam and the fifth light beam are used as a group of interference light beams which interfere with each other to be commonly incident to the balance detector module, and the third light beam and the sixth light beam are used as another group of interference light beams which interfere with each other to be commonly incident to the third reflector M3 and then to be incident to the balance detector module;
The balance detector module is electrically connected with the central processing unit module, performs photoelectric conversion on the two groups of interference light respectively, and inputs the acquired electric signals into the central processing unit module for analysis and processing;
In this embodiment, for the sake of description uniformity, after the light beam passes through the beam splitter, the transmission direction is changed to be named in the singular (first, third and fifth split), and the transmission is continued in the original transmission direction to be named in the even number (second, fourth and sixth split);
The low-coherence light source module comprises a super-radiation light-emitting diode (SLD) and a collimator (RC);
The light emitted by the super-radiation light-emitting diode SLD passes through a collimator RC to obtain a light beam with the spectral width meeting the preset condition;
the sample is arranged on a rotatable turntable, and the turntable is used for driving the sample to rotate so as to change the retroflection angle of the backscattered light;
the retroreflector M4 is disposed on a movable displacement platform;
The control end of the displacement platform is electrically connected with the central processing unit module, and the central processing unit module controls the displacement platform to move at a constant speed and drives the retroreflector M4 to move at a constant speed so as to change the optical path difference of the measuring light and the reference light;
the first reflecting mirror M1, the second reflecting mirror M2 and the third reflecting mirror M3 are piezoelectric driving reflecting mirrors;
the balance detector module comprises a first photoelectric detector PD1, a second photoelectric detector PD2 and a differential amplification circuit;
the first photodetector PD1 and the second photodetector PD2 are configured to receive two sets of interference light respectively, perform photoelectric conversion, acquire photocurrents respectively, and input into a differential amplifying circuit;
the differential amplifying circuit is electrically connected with the central processing unit module and is used for amplifying the photocurrent, acquiring an electric signal and inputting the electric signal into the central processing unit module;
The system further includes a first linear polarizer LP1, a second linear polarizer LP2, and a third linear polarizer LP3;
The light beam emitted by the low-coherence light source module passes through the first linear polarizer LP1 and then enters the first beam splitter BS1;
the two groups of interference light respectively pass through the second linear polarizer LP2 and the third linear polarizer LP3 and then are incident to the balance detector module.
In the specific implementation process, light emitted by the super-radiation light-emitting diode SLD passes through the collimator RC to obtain a light beam with a wide spectrum and a short coherence length, the light beam passes through the first linear polarizer LP1 and then enters the first beam splitter BS1 (50:50), and the light beam is subjected to light splitting treatment to achieve the required characteristics to obtain a first light splitting and a second light splitting which are perpendicular to each other;
the first beam splitter reaches the rotary table, a sample is placed on the rotary table, the rotary table is rotated, so that backward scattered light (measuring light) with different angles on the sample returns along an original light path and is transmitted through the first beam splitter BS1, the backward scattered light is reflected by the first reflecting mirror M1 and is split by the second beam splitter BS2 to obtain third and fourth beams respectively, and the third and fourth beams respectively enter two photodetectors of the balanced detector module;
The second beam (reference beam) is incident on a hollow retroreflector M4 arranged on a displacement platform, and the displacement platform is used for changing the optical path difference between the reference beam and the measuring beam, and then the beam reflected by the retroreflector M4 is also divided into two beams (fifth and sixth beams) by a second beam splitter BS2 to be incident on a balanced detector module, and the two beams interfere with the fourth beam and the third beam respectively to obtain two groups of interference light;
the difference between the photocurrents of the two photodetectors is amplified and then collected and output to a central processing unit module, which is a computer in the embodiment;
In this embodiment, the piezoelectric driving mirror M2 can be used for fine tuning the alignment between the reference light and the signal light, and in addition, a first linear polarizer LP1 is arranged behind the collimator RC, and although the power is additionally reduced, a second linear polarizer LP2 and a third linear polarizer LP3 can be respectively arranged in front of the two photodetectors, so as to fine tune the balance of the two photodetectors;
in the whole process, each module needs to be matched precisely so as to ensure that a measurement system can work normally and obtain a reliable measurement result;
Finally, the first photoelectric detector PD1 and the second photoelectric detector PD2 respectively receive two groups of interference light and perform photoelectric conversion, and respectively acquire photocurrents and input the photocurrents into a differential amplifying circuit;
The system utilizes the characteristics of wide spectrum and short coherence length of the low coherence light source, and utilizes optical path difference control to screen out the concerned stray light source, remove the influence of other non-target stray light and distinguish different stray light sources.
Example 2
As shown in fig. 4, the present embodiment provides a low-coherence interference-based backscatter measurement method, based on the low-coherence interference-based backscatter measurement system described in embodiment 1, comprising the steps of:
S1, building the system, and for the backward scattered light of each retroreflection angle, uniformly moving the retroreflector to provide a continuously-variable optical path difference;
s2, respectively carrying out real-time photoelectric conversion on two groups of interference light by using a balance detector module, calculating output voltage V by using a preset back scattering alternating current signal mathematical model, and acquiring the electric signal in real time;
the preset back scattering alternating current signal mathematical model specifically comprises the following steps:
Wherein V is the output voltage of the balanced detector module, G is the gain coefficient of the balanced detector module, Γ is a preset global transmission coefficient, S (f) is the response function of the photoelectric detector, P (f) is the spectral function of the light source, alpha is a balance factor, f is the frequency of light emitted by the low-coherence light source module, eta a is the overlapping factor between the light beam emitted by the low-coherence light source module and the photoelectric detector, ρ (f 0) is the coherence coefficient of back scattering, f 0 is the optical frequency of the central wavelength, P 0 is the optical power of the low-coherence light source, delta is the phase change introduced by the wavefront error, deltaL is the optical path difference between the reference beam and the measuring beam, T ref,1 is the complex transmission coefficient of the third light beam, T ref,2 is the complex transmission coefficient of the fourth light beam, T sig,1 is the complex transmission coefficient of the sixth light beam, and T sig,2 is the complex transmission coefficient of the fifth light beam;
s3, inputting the electric signals into a central processing unit module in real time, analyzing and processing the electric signals, and completing the measurement of the back scattered light;
The central processing unit module performs fourier transform on the electrical signal, and the electrical signal FFTV after fourier transform is expressed as:
The peak frequency of the electric signal FFTV after Fourier transformation is obtained, filtering is carried out by taking the peak frequency as the center, the signal to noise ratio of the signal is improved, and the back scattered light is measured according to the electric signal after filtering.
In the specific implementation process, firstly, building a whole system, and for the backward scattered light of each retroreflection angle, uniformly moving the retroreflector to provide continuously-changed optical path difference;
Respectively carrying out real-time photoelectric conversion on two groups of interference light by utilizing a balance detector module, calculating output voltage V by utilizing a preset back scattering alternating current signal mathematical model, and acquiring the electric signal in real time;
the optical fields of the reference beam and the measuring beam received by the photodetector are represented by the following formulas, j=1, 2 representing the serial number of the photodetector:
Where ε i (f) is the amplitude of the incident light at frequency f, L ref(Lsig) is the length that the reference beam (measuring beam) passes through in physical space, Is the complex transmission coefficient of the reference beam (measuring beam);
Considering the phenomenon of multiple scattering, as shown in fig. 5, which shows the incident laser light, and the back scattering of the first (m=1) and second (m=2) steps, r (f) can be expressed as:
n and d are the refractive index and thickness, respectively, of the medium, r m being defined as the scattering coefficient of the m-order scattered light, which has been expressed as BRDF multiplied by the solid angle, and can be expressed as:
Wherein r, t and ρ are respectively the reflection coefficient, the transmission coefficient and the scattering coefficient of the surface, the reflectivity of the coating can be more than 99 percent, the transmissivity is less than 1 percent, and the secondary scattering is more than two orders of magnitude less than the power of the single scattering, so that only the single scattering part can be considered;
According to the measurement scheme design in this embodiment, the complex transmission coefficients of the two beams can be expressed as:
Tref,1＝TLP0TBS1RM2RM3TBS2RM4TLP1
Φref,1＝τBS1+ρM2+ρM3+τBS2+ρM4+2k(nBS-1)eBS
Tref,2＝TLP0TBS1RM2RM3RBS2TLP2
Φref,2＝τBS1+ρM2+ρM3+ρBS2+2k(nBS-1)eBS
Tsig,1＝TtP0RBS1TBS1RM3RBS2RM4TLP1
Φsig,1＝ρBS1+τBS1+ρM3+ρBS2+ρM4+2k(nBS-1)eBS
Tsig,2＝TLP0RBS1TBS1RM3TBS2TLP2
Φsig,2＝ρBS1+τBS1+ρM3+τBS2+2k(nBS-1)eBS
Where n BS is the reflectivity of the beam splitter, e BS is the thickness of the beam splitter, ρ (τ) represents the phase change due to reflection (transmission), and thus the received light intensity at each detector is:
εj(f)＝εsig,j(f)+εref,j(f)
the integration is carried out within the photosensitive area of the detector, and the direct current part and the alternating current part in the current generated by the detector can be obtained as follows:
where S (f) is the response function of the detector, P (f) is the spectral function of the light source, η a is the overlap factor between the gaussian beam and the detector, expressed by:
Wherein a is the radius of the detector, w d is the beam waist radius of the Gaussian beam at the detector position, and the output voltage V of the balanced detector is obtained by differential amplification of the currents generated by the two photodiodes, namely:
where G is the gain factor of the balanced detector and Γ is the artificially defined global transmission factor given by:
wherein α is a balancing factor introduced by LP1 and LP2 to balance the dc components of the two beams;
in summary, the signal of the ac term can be expressed as:
Finally, a mathematical expression of the back scattering in a measuring system based on low coherence interferometry is obtained, and then the data are processed on the basis of the mathematical model, so that parameters such as the intensity, the scattering rate and the like of the back scattering light can be obtained;
it should be noted that, in the above calculation process, the scattering coefficient is a parameter for the light field amplitude, and the scattering rate for representing the light intensity change should be ρ 2;
Finally, carrying out Fourier transform on the measurement signals, and improving the signal to noise ratio of the signals by using a band-pass filtering mode, wherein the expression of the measurement signals after Fourier transform is shown as follows:
The method has the advantages that the method can obviously show that two peak frequencies (only one if only a single-side spectrum is considered), namely Doppler frequency, are used for filtering by taking the frequency as the center, and can filter noise in other frequency bands to effectively improve the signal-to-noise ratio;
However, in actual measurement, the measurement signal obtained by the method is discrete, and in order to study the influence of the discretized data on the measurement result, the embodiment also shows a frequency domain diagram after discrete Fourier transform thereof as shown in FIG. 6;
The results of the discrete Fourier transform and the continuous Fourier transform are compared and analyzed, and the peak frequency and the peak amplitude error of the discrete Fourier transform and the continuous Fourier transform are less than 1%, so that the feasibility of performing signal processing on the measured time domain signal is proved;
In addition, the method uses an interference method to measure, is closer to the actual working condition of a telescope, improves the sensitivity and accuracy of measurement, can improve the detection level to 10 -10 orders of magnitude, provides more accurate data for gravitational wave measurement, and effectively improves the accuracy and reliability of gravitational wave detection.
The same or similar reference numerals correspond to the same or similar components;
The terms describing the positional relationship in the drawings are merely illustrative, and are not to be construed as limiting the present patent;
It is to be understood that the above examples of the present invention are provided by way of illustration only and not by way of limitation of the embodiments of the present invention. Other variations or modifications of the above teachings will be apparent to those of ordinary skill in the art. It is not necessary here nor is it exhaustive of all embodiments. Any modification, equivalent replacement, improvement, etc. which come within the spirit and principles of the invention are desired to be protected by the following claims.
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