CN103940353A - Linear frequency modulation multi-beam laser heterodyne second harmonic method based glass thickness measuring device and method - Google Patents

Abstract

The invention discloses a linear frequency modulation multi-beam laser heterodyne second harmonic method based glass thickness measuring device and method and belongs to the technical field of optical measurement. The linear frequency modulation multi-beam laser heterodyne second harmonic method based glass thickness measuring device and method aims at solving the problem that the measuring result is large in error due to the fact that only single to-be-measured parameter values are obtained through the existing glass thickness measuring method. The linear frequency modulation multi-beam laser heterodyne second harmonic method based glass thickness measuring device comprises to-be-measured sheet glass, a linear frequency modulation laser, a first planar mirror, a second planar mirror, a convergent lens, a photoelectric detector and a signal processing system. The linear frequency modulation multi-beam laser heterodyne second harmonic method glass thickness measuring method comprises opening the linear frequency modulation laser, enabling the linear frequency modulation laser to emit linearly polarized light, enabling the photoelectric detector to receive light beam signals and a DSP (Digital Signal Processor) to continuously collect electric signals output from the photoelectric detector and process collected difference frequency signals and obtaining the thickness of the to-be-measured sheet glass according to the relationship between the frequency and the thickness. The linear frequency modulation multi-beam laser heterodyne second harmonic method based glass thickness measuring device and method is used for measuring the glass thickness.

Description

Linear frequency modulation multi-beam laser heterodyne second harmonic method is measured the device and method of thickness of glass

Technical field

The present invention relates to linear frequency modulation multi-beam laser heterodyne second harmonic method and measure the device and method of thickness of glass, belong to field of optical measuring technologies.

Background technology

Precision glass thickness measure is the problem that engineering field is being faced always and wish solves.Along with scientific and technical development, method for measuring thickness is constantly weeded out the old and bring forth the new, comprising optical measuring method, interferometry and diffraction approach etc.But the measuring accuracy of these methods is limited, be difficult to meet the demand that high precision thickness is measured.

The features such as optics thickness measuring is untouchable owing to having, precision is high and simple in structure enjoy people's attention, use optical means to carry out the application more and more widely that measured of thickness.In measuring method, laser heterodyne measurement technology has been inherited the plurality of advantages of heterodyne technology, be one of current superhigh precision measuring method, but can only obtain single parameter value to be measured after traditional heterodyne signal spectrum demodulation, make the measuring result error of thickness of glass large.

Summary of the invention

The present invention seeks to the measuring method in order to solve existing thickness of glass, owing to can only obtaining single parameter value to be measured, cause measuring result error large problem, provide a kind of linear frequency modulation multi-beam laser heterodyne second harmonic method to measure the device and method of thickness of glass.

Linear frequency modulation multi-beam laser heterodyne second harmonic method of the present invention is measured the device of thickness of glass, it comprises sheet glass to be measured, it also comprises linear frequency modulation laser instrument, the first plane mirror, the second plane mirror, convergent lens, photodetector and signal processing system

The linearly polarized light that linear frequency modulation laser instrument sends is after the first plane mirror and the reflection of the second plane mirror, oblique incidence is to the front surface of sheet glass to be measured, after the light beam of sheet glass front surface to be measured transmission is reflected by the rear surface of sheet glass to be measured, jointly be converged on the photosurface that lens converge to photodetector with the light beam through sheet glass front surface reflection to be measured, photodetector will obtain electric signal after the beam signal conversion receiving, photodetector output electrical signals is to signal processing system, signal processing system is for processing the electric signal receiving, obtain the thickness d of sheet glass to be measured.

Described signal processing system is made up of wave filter, prime amplifier, analog to digital converter and digital signal processor DSP,

Photodetector output electrical signals is to the wave filter of signal processing system, after filter filtering, send prime amplifier to, after prime amplifier amplifies the signal receiving, outputting analog signal is to analog to digital converter, analog to digital converter sends to digital signal processor DSP after simulating signal is converted to digital signal, digital signal processor DSP is processed the digital signal receiving, and obtains the thickness d of sheet glass to be measured.

The device that adopts above-mentioned linear frequency modulation multi-beam laser heterodyne second harmonic method to measure thickness of glass is realized the method for linear frequency modulation multi-beam laser heterodyne second harmonic method measurement thickness of glass, and the process of the method is:

First, open linear frequency modulation laser instrument, make it send linearly polarized light, and make photodetector start receiving beam signal, the electric signal of digital signal processor DSP continuous acquisition photodetector output, and the difference frequency signal collecting is processed to the relation according to frequency and thickness:

f＝Kd，

Obtain the thickness d of sheet glass to be measured:

d＝f/K，

The frequency that in formula, f is heterodyne signal, K is scale-up factor.

The detailed process that the thickness d of sheet glass to be measured obtains is:

Set light beam after the second plane mirror reflection with incidence angle θ ₀oblique incidence is the front surface of sheet glass to be measured extremely, total light field E of the beam signal that now photodetector receives _Σ(t) be:

E _Σ(t)=E ₁(t)+E ₂(t)+...+E _m(t), m is greater than 1 positive integer;

E in formula ₁(t), for the t-l/c moment arrives sheet glass front surface to be measured and by the catoptrical light field of this front surface reflection, obtain as follows:

$E_1\left(t\right)={\mathrm{\α}}_1{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l}{c})+k{(t-\frac{l}{c})}^2\left]\right\},$

α in formula ₁for coefficient, α ₁=γ, γ is the reflectivity of light while injecting sheet glass to be measured from surrounding medium; E ₀for incident field amplitude, i represents imaginary number, ω ₀for incident field angular frequency, t is the time, and l is the light path that the light beam after the second plane mirror reflection arrives the front surface of sheet glass to be measured, and c is the light velocity; K is the rate of change of modulating bandwidth:

$k=\frac{\mathrm{\ΔF}}{T},$

Wherein T is the frequency modulation cycle, and △ F is modulating bandwidth;

E ₂(t) ..., E _m(t) the catoptrical reflection light field of multi beam for obtaining after the rear surface at sheet glass to be measured and front surface multiple reflections:

$\left\{\begin{array}{c}{E}_2\left(t\right)={\mathrm{\α}}_2{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ E_3\left(t\right)={\mathrm{\α}}_3{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{4{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ ...\\ E_m\left(t\right)={\mathrm{\α}}_m{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\end{array}\right.,$

In formula, α ₂, α ₃..., α _mbe coefficient, and α ₂=β β ' γ ', α ₃=β β ' (γ ') ³..., α _m=β β ' (γ ') ^(2m-3); β in formula is the transmissivity of light while injecting sheet glass to be measured from surrounding medium, transmissivity when β ' penetrates sheet glass to be measured for sheet glass front and rear surfaces multiple reflections light to be measured, and γ ' is the reflectivity of sheet glass to be measured rear surface; θ is the refraction angle of light beam while being incident to sheet glass front surface to be measured from surrounding medium, the refractive index that n is sheet glass to be measured;

The photocurrent I of photodetector output is:

$I=\frac{\mathrm{\ηe}}{\mathrm{hv}}\frac{1}{Z}\underset{D}{\∫\∫}\frac{1}{2}{[E_1\left(t\right)+E_2\left(t\right)+...+E_m\left(t\right)][E_1\left(t\right)+E_2\left(t\right)+...+E_m\left(t\right)]}^{*}\mathrm{ds},$

In formula, η is quantum efficiency, and e is electron charge, and h is Planck's constant, and v is the linear polarization light frequency that linear frequency modulation laser instrument sends, and Z is the intrinsic impedance of photodetector surfaces medium, and D is the area of photodetector photosurface, represents complex conjugate No. *;

Photocurrent I is through processing, and after filtering DC terms, the interchange item of acquisition is electric current of intermediate frequency I _iF, electric current of intermediate frequency I _iFfor:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{\mathrm{hv}}\frac{\mathrm{\π}}{Z}{E}_0^2\underset{p=2}{\overset{m-2}{\mathrm{\Σ}}}{\mathrm{\α}}_{p+2}{\mathrm{\α}}_p\cos (\frac{8\mathrm{knd}\cos \mathrm{\θ}}{c}t-\frac{8\mathrm{knd}\cos \mathrm{\θ}(l+\mathrm{nd}\cos \mathrm{\θ})}{c^2}),$

Wherein, p=2,3 ..., m-2;

To intermediate frequency electric current I _iFcarry out Fourier transform, obtain the frequency f of its heterodyne signal:

$f=\frac{8\mathrm{knd}\cos \mathrm{\θ}}{2\mathrm{\πc}}=\frac{4\mathrm{knd}\cos \mathrm{\θ}}{\mathrm{\πc}}=\mathrm{Kd},$

$K=\frac{4\mathrm{kn}\cos \mathrm{\θ}}{\mathrm{\πc}},$

Above formula is calculated, obtain the thickness d of sheet glass to be measured.

The result of cos θ obtains by the ratio ζ of two spectrum curve centre frequencies in multi-beam laser heterodyne second harmonic signal spectrum:

ζ＝cosθ。

Advantage of the present invention: the present invention is based on linear frequency modulation technology and heterodyne technology, by linear frequency modulation technology, parameter information to be measured is modulated in heterodyne signal second harmonic, by can accurately obtaining thickness of glass information to be measured to the demodulation of heterodyne second harmonic, it can collect good laser difference frequency signal and improve the arithmetic speed of signal processing, verify through experiment simulation, the thickness of glass measuring error that the present invention obtains is less than 0.01%, can realize broad ranges of thicknesses and detect in meeting measuring accuracy.

Brief description of the drawings

Fig. 1 is the light path schematic diagram that linear frequency modulation multi-beam laser heterodyne second harmonic method of the present invention is measured the device of thickness of glass;

Fig. 2 is multi-beam laser principle of interference schematic diagram;

Fig. 3 is the Fourier transform spectrogram of multi-beam laser heterodyne second harmonic signal;

Fig. 4 is spectrogram corresponding to different sheet glass thickness measures in table 1.

Embodiment

Embodiment one: present embodiment is described below in conjunction with Fig. 1, described in present embodiment, linear frequency modulation multi-beam laser heterodyne second harmonic method is measured the device of thickness of glass, it comprises sheet glass 1 to be measured, it is characterized in that, it also comprises linear frequency modulation laser instrument 2, the first plane mirror 3, the second plane mirror 4, convergent lens 5, photodetector 6 and signal processing system 7

The linearly polarized light that linear frequency modulation laser instrument 2 sends is after the first plane mirror 3 and the second plane mirror 4 reflections, oblique incidence is to the front surface of sheet glass 1 to be measured, after the light beam of sheet glass 1 front surface transmission to be measured is reflected by the rear surface of sheet glass 1 to be measured, with be jointly converged through the light beam of sheet glass 1 front surface reflection to be measured on the photosurface that lens 5 converge to photodetector 6, photodetector 6 will obtain electric signal after the beam signal conversion receiving, photodetector 6 output electrical signals are to signal processing system 7, signal processing system 7 is for processing the electric signal receiving, obtain the thickness d of sheet glass 1 to be measured.

Embodiment two: present embodiment is described below in conjunction with Fig. 1, present embodiment is described further embodiment one, described in present embodiment, signal processing system 7 is made up of wave filter 7-1, prime amplifier 7-2, analog to digital converter 7-3 and digital signal processor DSP 7-4

Photodetector 6 output electrical signals are to the wave filter 7-1 of signal processing system 7, after wave filter 7-1 filtering, send prime amplifier 7-2 to, after prime amplifier 7-2 amplifies the signal receiving, outputting analog signal is to analog to digital converter 7-3, analog to digital converter 7-3 sends to digital signal processor DSP 7-4 after simulating signal is converted to digital signal, digital signal processor DSP 7-4 processes the digital signal receiving, and obtains the thickness d of sheet glass 1 to be measured.

Present embodiment median filter 7-1 is low-pass filter.

Embodiment three: present embodiment is described below in conjunction with Fig. 1 and Fig. 2, present embodiment is to adopt the device of above-mentioned linear frequency modulation multi-beam laser heterodyne second harmonic method measurement thickness of glass to realize the method for linear frequency modulation multi-beam laser heterodyne second harmonic method measurement thickness of glass, and the process of the method is:

First, open linear frequency modulation laser instrument 2, make it send linearly polarized light, and make photodetector 6 start receiving beam signal, the electric signal that digital signal processor DSP 7-4 continuous acquisition photodetector 6 is exported, and the difference frequency signal collecting is processed to the relation according to frequency and thickness:

f＝Kd，

Obtain the thickness d of sheet glass 1 to be measured:

d＝f/K，

The frequency that in formula, f is heterodyne signal, K is scale-up factor.

Embodiment four: below in conjunction with Fig. 1 to Fig. 3, present embodiment is described, present embodiment is described further embodiment three, the detailed process that the thickness d of sheet glass 1 to be measured obtains described in present embodiment is:

Set light beam after the second plane mirror 4 reflection with incidence angle θ ₀oblique incidence is to the front surface of sheet glass 1 to be measured, total light field E of the beam signal that now photodetector 6 receives _Σ(t) be:

E _Σ(t)=E ₁(t)+E ₂(t)+...+E _m(t), m is greater than 1 positive integer;

E in formula ₁(t), for the t-l/c moment arrives sheet glass 1 front surface to be measured and by the catoptrical light field of this front surface reflection, obtain as follows:

$E_1\left(t\right)={\mathrm{\α}}_1{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l}{c})+k{(t-\frac{l}{c})}^2\left]\right\},$

α in formula ₁for coefficient, α ₁=γ, γ is the reflectivity of light while injecting sheet glass 1 to be measured from surrounding medium; E ₀for incident field amplitude, i represents imaginary number, ω ₀for incident field angular frequency, t is the time, and l is the light path that the light beam after the second plane mirror 4 reflections arrives the front surface of sheet glass 1 to be measured, and c is the light velocity; K is the rate of change of modulating bandwidth:

$k=\frac{\mathrm{\ΔF}}{T},$

Wherein T is the frequency modulation cycle, and △ F is modulating bandwidth;

E ₂(t) ..., E _m(t) the catoptrical reflection light field of multi beam for obtaining after the rear surface at sheet glass 1 to be measured and front surface multiple reflections:

$\left\{\begin{array}{c}{E}_2\left(t\right)={\mathrm{\α}}_2{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ E_3\left(t\right)={\mathrm{\α}}_3{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{4{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ ...\\ E_m\left(t\right)={\mathrm{\α}}_m{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\end{array}\right.,$

In formula, α ₂, α ₃..., α _mbe coefficient, and α ₂=β β ' γ ', α ₃=β β ' (γ ') ³..., α _m=β β ' (γ ') ^(2m-3); β in formula is the transmissivity of light while injecting sheet glass 1 to be measured from surrounding medium, transmissivity when β ' penetrates sheet glass 1 to be measured for sheet glass 1 front and rear surfaces multiple reflections light to be measured, and γ ' is the reflectivity of sheet glass to be measured 1 rear surface; θ is the refraction angle of light beam while being incident to sheet glass 1 front surface to be measured from surrounding medium, and n is the refractive index of sheet glass 1 to be measured;

The photocurrent I that photodetector 6 is exported is:

$I=\frac{\mathrm{\ηe}}{\mathrm{hv}}\frac{1}{Z}\underset{D}{\∫\∫}\frac{1}{2}{[E_1\left(t\right)+E_2\left(t\right)+...+E_m\left(t\right)][E_1\left(t\right)+E_2\left(t\right)+...+E_m\left(t\right)]}^{*}\mathrm{ds},$

In formula, η is quantum efficiency, and e is electron charge, and h is Planck's constant, and v is the linear polarization light frequency that linear frequency modulation laser instrument 2 sends, and Z is the intrinsic impedance of photodetector 6 surface dielectrics, and D is the area of photodetector 6 photosurfaces, represents complex conjugate No. *;

Photocurrent I is through processing, and after filtering DC terms, the interchange item of acquisition is electric current of intermediate frequency I _iF, electric current of intermediate frequency I _iFfor:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{\mathrm{hv}}\frac{\mathrm{\π}}{Z}{E}_0^2\underset{p=2}{\overset{m-2}{\mathrm{\Σ}}}{\mathrm{\α}}_{p+2}{\mathrm{\α}}_p\cos (\frac{8\mathrm{knd}\cos \mathrm{\θ}}{c}t-\frac{8\mathrm{knd}\cos \mathrm{\θ}(l+\mathrm{nd}\cos \mathrm{\θ})}{c^2}),$

Wherein, p=2,3 ..., m-2;

To intermediate frequency electric current I _iFcarry out Fourier transform, obtain the frequency f of its heterodyne signal:

$f=\frac{8\mathrm{knd}\cos \mathrm{\θ}}{2\mathrm{\πc}}=\frac{4\mathrm{knd}\cos \mathrm{\θ}}{\mathrm{\πc}}=\mathrm{Kd},$

$K=\frac{4\mathrm{kn}\cos \mathrm{\θ}}{\mathrm{\πc}},$

Above formula is calculated, obtain the thickness d of sheet glass 1 to be measured.

In present embodiment, as shown in Figure 2, because light beam can constantly reflect and reflect between the front and rear surfaces of sheet glass 1 to be measured, and this reflection and refraction for reflected light and transmitted light, the interference at infinity or on lens focal plane has contribution, so in the time that interference is discussed, must consider multiple reflections and refraction effect, multi-beam laser should be discussed and interfere.

But, because laser transmits the optical mixing of sheet glass front surface after reflected light two secondary reflections adjacent with sheet glass rear surface of sheet glass front surface, the amplitude of two difference frequency signals that produce differs 2～3 orders of magnitude, after Fourier transform, in order to collect good laser difference frequency signal and to improve the arithmetic speed of signal processing, only consider the E of the rear surface p-1 secondary reflection detecting _p-1(t) E and after the p+1 secondary reflection of rear surface _p+1(t) the humorous frequency difference of secondary that optical mixing produces.

When the light beam after the second plane mirror 4 reflections is with incidence angle θ ₀oblique incidence is during to the front surface of sheet glass 1 to be measured, and the mathematic(al) representation of incident field is:

E(t)＝E ₀exp{i(ω ₀t+kt ²)}。

Total light field E of the beam signal that this E (t) and photodetector 6 receive _Σ(t) equal in theory.

Only consider E _p-1and E (t) _p+1(t) difference frequency signal that optical mixing produces, DC terms can filtering after low-pass filter, therefore, only considers to exchange, and this exchanges and is commonly referred to electric current of intermediate frequency, and arrangement can obtain electric current of intermediate frequency I _iF:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{2\mathrm{hv}}\frac{1}{Z}\underset{D}{\∫\∫}\underset{p=2}{\overset{m-2}{\mathrm{\Σ}}}\underset{j=p+2}{\overset{m-2}{\mathrm{\Σ}}}(E_p\left(t\right){E_j}^{*}\left(t\right)+{E_p}^{*}\left(t\right)E_j\left(t\right))\mathrm{ds},$

Wherein, p and j are nonnegative integer.

Obtain through arranging:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{\mathrm{hv}}\frac{\mathrm{\π}}{Z}{E}_0^2\underset{p=2}{\overset{m-2}{\mathrm{\Σ}}}{\mathrm{\α}}_{p+2}{\mathrm{\α}}_p\cos (\frac{8\mathrm{knd}\cos \mathrm{\θ}}{c}t-\frac{8\mathrm{knd}\cos \mathrm{\θ}(l+\mathrm{nd}\cos \mathrm{\θ})}{c^2}),$

In above formula, need the thickness d information of master plate glass 1, analyze for intermediate frequency item intermediate frequency rate variance, adopt Fourier transform to be easy to realize frequency measurement.Now, the frequency of heterodyne signal is designated as:

$f=\frac{8\mathrm{knd}\cos \mathrm{\θ}}{2\mathrm{\πc}}=\frac{4\mathrm{knd}\cos \mathrm{\θ}}{\mathrm{\πc}}=\mathrm{Kd},$

The frequency of learning thus heterodyne signal is directly proportional to sheet glass thickness, and its scale-up factor is:

$K=\frac{4\mathrm{kn}\cos \mathrm{\θ}}{\mathrm{\πc}},$

Rate of change k and the light velocity c of the refraction angle θ of this scale-up factor and sheet glass, refractive index n, modulating bandwidth are relevant.

Embodiment five: present embodiment is described below in conjunction with Fig. 1 to Fig. 4, present embodiment is described further embodiment four, and the result of cos θ obtains by the ratio ζ of two spectrum curve centre frequencies in multi-beam laser heterodyne second harmonic signal spectrum described in present embodiment:

ζ＝cosθ。

Obtain θ according to refraction law ₀:

θ ₀＝arcsin(nsinθ)。

Numerical simulation of the present invention and interpretation of result:

Application Matlab verifies the feasibility of the inventive method, refractive index n=1.493983 of sheet glass under normal circumstances; Linear frequency modulation laser wavelength is 1.55 μ m, frequency modulation cycle T=1ms, modulating bandwidth △ F=5GHz.

The inventive method is carried out to emulation, the Fourier transform frequency spectrum of the linear frequency modulation multi-beam laser heterodyne second harmonic signal obtaining through signal processing as shown in Figure 3, wherein solid line is in laser oblique incidence situation, the Fourier transform frequency spectrum of corresponding linear frequency modulation multi-beam laser heterodyne second harmonic signal while measuring sheet glass thickness d; Dotted line is in laser normal incidence situation, the Fourier transform frequency spectrum of corresponding linear frequency modulation multi-beam laser heterodyne second harmonic signal when detect thickness d.

As can see from Figure 3, in experiment, provide the theoretical curve in the situation of normal incidence, object is: in Linear Frequency Modulation multi-beam laser heterodyne second harmonic signal spectrum figure, the numerical value of the centre frequency of theoretical curve when the centre frequency of first main peak of multi-beam laser heterodyne second harmonic signal spectrum and normal incidence can simultaneously obtain oblique incidence time, like this, be easy to the ratio ζ of two centre frequencies that obtain:

ζ＝cosθ，

In the situation that obtaining centre frequency, can be calculated the size of laser refraction angle θ after sheet glass by above formula, and then can obtain incidence angle θ according to refraction law ₀size, finally try to achieve the numerical value of K, finally obtain the value of sheet glass thickness d.

Meanwhile, utilize Matlab emulation to obtain different incidence angles θ ₀in situation, linear frequency modulation multi-beam laser heterodyne is measured multi-beam laser heterodyne signal Fourier transform frequency spectrum that sheet glass thickness is corresponding as shown in Figure 4, as can be seen from Figure 4, along with the increase of sheet glass thickness, the relative position of frequency spectrum moves the i.e. increase frequency along with thickness to high frequency direction to be increased.Reason is: the in the situation that of sheet glass invariable incident angle, Proportional coefficient K is a constant, and in the time that thickness increases, closing due to frequency f and sheet glass thickness d is f=Kd, and in the constant situation of K, frequency f and sheet glass thickness d are linear.Therefore, when thickness increases, frequency also increases the increase along with thickness thereupon, and the relative position of frequency spectrum moves to high frequency direction, and Fig. 4 has verified the correctness of theoretical analysis above well.It should be noted that, because heterodyne detection is a kind of detection mode of nearly diffraction limit, detection sensitivity is high, and therefore in Fig. 4, the signal to noise ratio (S/N ratio) of heterodyne signal is very high.

Utilize linear frequency modulation multi-beam laser heterodyne second harmonic mensuration, eight groups of data of continuous coverage, have obtained the simulated measurement result of different sheet glass thickness, as shown in table 1:

The actual value d of the different sheet glass thickness of table 1 and simulated measurement value d _i

The emulation experiment data of utilizing table 1, the maximum relative error that finally can obtain measured value is less than 0.01%, can find out that the measuring accuracy of the inventive method is very high.Meanwhile, analyze data and it can also be seen that, the systematic error that environment brings and reading error are negligible in emulation, and the error in emulation experiment mainly comes from the round-off error in trueness error and the computation process after Fast Fourier Transform (FFT) FFT.

According to the needs of engineering field high-acruracy survey sheet glass thickness, the present invention is by linear frequency modulation technology and the effective combination of heterodyne technology, propose a kind of linear frequency modulation multi-beam laser heterodyne second harmonic and measured the method for sheet glass thickness, by linear frequency modulation technology, the light that does not incide in the same time its front surface is carried out to frequency modulation (PFM), sheet glass thickness information to be measured is loaded in the difference on the frequency of heterodyne signal second harmonic, change and be easy to just can demodulate sheet glass thickness information to be measured by Fourier, and measuring accuracy is high, can collect good laser difference frequency signal simultaneously, make the modulation and demodulation of signal simple, improve the arithmetic speed of signal processing.The method is a kind of method of good non-cpntact measurement sheet glass thickness, can be applied under severe measurement environment.The advantages such as it is high that adopting said method has precision while measuring sheet glass thickness, and Linearity is good, and measuring speed is fast.

Simulation result shows, the method is in the time measuring different sheet glass thickness, measuring error is less than 0.01%, illustrate that the method application is feasible, reliable, can meet the requirement that small thickness of glass is measured, for many engineering fields provide good measurement means, can be widely used in laser radar, machinery, instrument and meter and electronics product manufacturing industry, there is good application prospect and value.

Claims (5)

1. the device of a linear frequency modulation multi-beam laser heterodyne second harmonic method measurement thickness of glass, it comprises sheet glass to be measured (1), it is characterized in that, it also comprises linear frequency modulation laser instrument (2), the first plane mirror (3), the second plane mirror (4), convergent lens (5), photodetector (6) and signal processing system (7)

The linearly polarized light that linear frequency modulation laser instrument (2) sends is after the first plane mirror (3) and the second plane mirror (4) reflection, oblique incidence is to the front surface of sheet glass to be measured (1), after the light beam of sheet glass to be measured (1) front surface transmission is reflected by the rear surface of sheet glass to be measured (1), with be jointly converged lens (5) through the light beam of sheet glass to be measured (1) front surface reflection and converge on the photosurface of photodetector (6), photodetector (6) will obtain electric signal after the beam signal conversion receiving, photodetector (6) output electrical signals is to signal processing system (7), signal processing system (7) is for processing the electric signal receiving, obtain the thickness d of sheet glass to be measured (1).

2. linear frequency modulation multi-beam laser heterodyne second harmonic method according to claim 1 is measured the device of thickness of glass, it is characterized in that, described signal processing system (7) is made up of wave filter (7-1), prime amplifier (7-2), analog to digital converter (7-3) and digital signal processor DSP (7-4)

Photodetector (6) output electrical signals is to the wave filter (7-1) of signal processing system (7), after wave filter (7-1) filtering, send prime amplifier (7-2) to, after prime amplifier (7-2) amplifies the signal receiving, outputting analog signal is to analog to digital converter (7-3), analog to digital converter (7-3) sends to digital signal processor DSP (7-4) after simulating signal is converted to digital signal, digital signal processor DSP (7-4) is processed the digital signal receiving, obtain the thickness d of sheet glass to be measured (1).

3. the device that adopts linear frequency modulation multi-beam laser heterodyne second harmonic method described in claim 2 to measure thickness of glass is realized linear frequency modulation multi-beam laser heterodyne second harmonic method and measures the method for thickness of glass, it is characterized in that, the process of the method is:

First, open linear frequency modulation laser instrument (2), make it send linearly polarized light, and make photodetector (6) start receiving beam signal, the electric signal of digital signal processor DSP (7-4) continuous acquisition photodetector (6) output, and the difference frequency signal collecting is processed to the relation according to frequency and thickness:

f＝Kd，

Obtain the thickness d of sheet glass to be measured (1):

d＝f/K，

The frequency that in formula, f is heterodyne signal, K is scale-up factor.

4. linear frequency modulation multi-beam laser heterodyne second harmonic method according to claim 3 is measured the method for thickness of glass, it is characterized in that,

The detailed process that the thickness d of sheet glass to be measured (1) obtains is:

Set light beam after the second plane mirror (4) reflection with incidence angle θ ₀oblique incidence is the front surface of sheet glass to be measured (1) extremely, total light field E of the beam signal that now photodetector (6) receives _Σ(t) be:

E _Σ(t)=E ₁(t)+E ₂(t)+...+E _m(t), m is greater than 1 positive integer;

E in formula ₁(t), for the t-l/c moment arrives sheet glass to be measured (1) front surface and by the catoptrical light field of this front surface reflection, obtain as follows:

$E_1\left(t\right)={\mathrm{\α}}_1{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l}{c})+k{(t-\frac{l}{c})}^2\left]\right\},$

α in formula ₁for coefficient, α ₁=γ, γ is the reflectivity of light while injecting sheet glass to be measured (1) from surrounding medium; E ₀for incident field amplitude, i represents imaginary number, ω ₀for incident field angular frequency, t is the time, and l is the light path that the light beam after the second plane mirror (4) reflection arrives the front surface of sheet glass to be measured (1), and c is the light velocity; K is the rate of change of modulating bandwidth:

$k=\frac{\mathrm{\ΔF}}{T},$

Wherein T is the frequency modulation cycle, and △ F is modulating bandwidth;

E ₂(t) ..., E _m(t) the catoptrical reflection light field of multi beam for obtaining after the rear surface at sheet glass to be measured (1) and front surface multiple reflections:

$\left\{\begin{array}{c}{E}_2\left(t\right)={\mathrm{\α}}_2{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ E_3\left(t\right)={\mathrm{\α}}_3{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{4{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ ...\\ E_m\left(t\right)={\mathrm{\α}}_m{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\end{array}\right.,$

In formula, α ₂, α ₃..., α _mbe coefficient, and α ₂=β β ' γ ', α ₃=β β ' (γ ') ³..., α _m=β β ' (γ ') ^(2m-3); β in formula is the transmissivity of light while injecting sheet glass to be measured (1) from surrounding medium, transmissivity when β ' penetrates sheet glass to be measured (1) for sheet glass to be measured (1) front and rear surfaces multiple reflections light, γ ' is the reflectivity of sheet glass to be measured (1) rear surface; θ is the refraction angle of light beam while being incident to sheet glass to be measured (1) front surface from surrounding medium, and n is the refractive index of sheet glass to be measured (1);

The photocurrent I of photodetector (6) output is:

$I=\frac{\mathrm{\ηe}}{\mathrm{hv}}\frac{1}{Z}\underset{D}{\∫\∫}\frac{1}{2}{[E_1\left(t\right)+E_2\left(t\right)+...+E_m\left(t\right)][E_1\left(t\right)+E_2\left(t\right)+...+E_m\left(t\right)]}^{*}\mathrm{ds},$

In formula, η is quantum efficiency, e is electron charge, h is Planck's constant, v is the linear polarization light frequency that linear frequency modulation laser instrument (2) sends, Z is the intrinsic impedance of photodetector (6) surface dielectric, D is the area of photodetector (6) photosurface, represents complex conjugate No. *;

Photocurrent I is through processing, and after filtering DC terms, the interchange item of acquisition is electric current of intermediate frequency I _iF, electric current of intermediate frequency I _iFfor:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{\mathrm{hv}}\frac{\mathrm{\π}}{Z}{E}_0^2\underset{p=2}{\overset{m-2}{\mathrm{\Σ}}}{\mathrm{\α}}_{p+2}{\mathrm{\α}}_p\cos (\frac{8\mathrm{knd}\cos \mathrm{\θ}}{c}t-\frac{8\mathrm{knd}\cos \mathrm{\θ}(l+\mathrm{nd}\cos \mathrm{\θ})}{c^2}),$

Wherein, p=2,3 ..., m-2;

To intermediate frequency electric current I _iFcarry out Fourier transform, obtain the frequency f of its heterodyne signal:

$f=\frac{8\mathrm{knd}\cos \mathrm{\θ}}{2\mathrm{\πc}}=\frac{4\mathrm{knd}\cos \mathrm{\θ}}{\mathrm{\πc}}=\mathrm{Kd},$

$K=\frac{4\mathrm{kn}\cos \mathrm{\θ}}{\mathrm{\πc}},$

Above formula is calculated, obtain the thickness d of sheet glass to be measured (1).

5. linear frequency modulation multi-beam laser heterodyne second harmonic method according to claim 4 is measured the method for thickness of glass, it is characterized in that, the result of cos θ obtains by the ratio ζ of two spectrum curve centre frequencies in multi-beam laser heterodyne second harmonic signal spectrum:

ζ＝cosθ。