CN103940354A - Device and method for measuring glass thickness through linear frequency modulation multi-beam laser heterodyne - Google Patents

Abstract

The invention discloses a device and a method for measuring glass thickness through linear frequency modulation multi-beam laser heterodyne and belongs to the technical field of precision glass thickness measurement. The device and the method for measuring the glass thickness through the linear frequency modulation multi-beam laser heterodyne solve the problem that the glass thickness measurement error is large due to the fact that only single to-be-measured parameters are obtained after the existing laser heterodyne signal frequency spectrums are demodulated. Laser emitted from a frequency modulation laser enters into a first reflecting mirror after being reflected through a second reflecting mirror and light beams enter into plain glass after being reflected through the first reelecting mirror; a plurality of beams of reflecting light is obtained after the incident light is reflected by the front surface and the rear surface of the plane glass for a plurality of times; the plurality of beams of reflecting light is converged to the photosensitive surface of a photoelectric detector through a convergent lens after being transmitted through the plain glass; current signals are sent to a filter after being performed photovoltaic conversion through the photoelectric detector, the signals are output to a front arranged amplifier after being filtered through the filter, and the signals are sent to a digital signal processor after being performed conversion through an A/D (Analog to Digital) converter. The device and the method for measuring the glass thickness through the linear frequency modulation multi-beam laser heterodyne are applicable to measurement of the glass thickness.

Description

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

Technical field

The invention belongs to precision glass thickness measurement technique field.

Background technology

Precision glass thickness measure is the problem that engineering field needs in the face of always and solves.Along with scientific and technical development, method for measuring thickness is constantly weeded out the old and bring forth the new, and comprises optical measuring method, interferometry and diffraction approach etc.Utilize these methods generally all can not reach the requirement of pin-point accuracy measurement of angle.

Because the features such as optics thickness measuring is untouchable owing to having, precision is high and simple in structure enjoy people's attention, so use the method for optics thickness measuring to obtain application more and more widely.And in optical 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, the error of measuring thickness of glass is large.

Summary of the invention

The present invention can only obtain single parameter value to be measured after solving the demodulation of existing heterodyne signal spectrum, causes measuring the large problem of error of thickness of glass.The device and method of linear frequency modulation multi-beam laser heterodyne measurement thickness of glass has been proposed.

Linear frequency modulation multi-beam laser heterodyne is measured the device of thickness of glass, and this device comprises digital signal processor, A/D converter, prime amplifier, wave filter, photodetector, convergent lens, sheet glass, catoptron, No. two catoptrons and frequency modulation laser;

The laser of frequency modulation laser transmitting is incident to catoptron No. one after No. two catoptron reflections, and the light beam after a catoptron reflection is incident to flat glass; This incident light obtains multi beam reflected light after the front surface of sheet glass and the rear surface multiple reflections of sheet glass, and this multi beam reflected light converges on the photosurface of photodetector through convergent lens after sheet glass transmission;

Current signal output end after photodetector carries out opto-electronic conversion connects the signal input part of wave filter, after the filtering of wave filter, signal output part connects the signal input part of prime amplifier, the amplifying signal output terminal of prime amplifier connects the input end of analog signal of A/D converter, the signal input part of the digital signal output end linking number word signal processor of A/D converter.

Linear frequency modulation multi-beam laser heterodyne is measured the method for thickness of glass, and the concrete steps of the method are:

Step 1, adopt frequency modulation laser Emission Lasers, laser retreads and is incident upon sheet glass through a catoptron and No. two catoptron two secondary reflection; Writing light beam is slanted to the incidence angle θ of sheet glass simultaneously ₀; And measure frequency modulation laser Emission Lasers to the light path l between incident sheet glass, calculate incident field E (t), the reflection light field E of sheet glass ₁(t) with through the light of sheet glass transmission not in the same time by the light field E of sheet glass multiple reflections ₂(t), E ₃(t) ..., E _m(t)

The mathematic(al) representation of incident field is:

E(t)＝E ₀exp{i(ω ₀t+kt ²)} (1-1)

Wherein, for the rate of change of modulating bandwidth, T is the frequency modulation cycle, and △ F is modulating bandwidth; E ₀for incident field amplitude, t is the time, ω ₀for light field angular frequency; I is imaginary unit;

The reflection light field that constantly arrives sheet glass front surface according to formula (1-1) acquisition t-l/c is:

$E_1\left(t\right)={\mathrm{\α}}_1{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{1}{c})+k{(t-\frac{1}{c})}^2\left]\right\}---(2-1)$

Through the light of sheet glass transmission, not in the same time by sheet glass multiple reflections, its catoptrical expression formula is respectively:

$\left\{\begin{array}{c}{E}_2\left(t\right)={\mathrm{\α}}_2{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ E_3\left(t\right)={\mathrm{\α}}_3{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{4{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ .\\ .\\ .\\ E_j\\ .\\ .\\ .\\ E_{j+p}\\ .\\ .\\ .\\ E_m\left(t\right)={\mathrm{\α}}_m{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2(m-1){\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ .\\ .\\ .\end{array}\right.---(3-1)$

Wherein, m is greater than 2 integer, α ₁=r, α ₂=β β ' r ' ..., α _m=β β ' r ' ^(2m-3)r is the reflectivity of light while injecting plane standard mirror from surrounding medium, transmissivity is β, r ' is the reflectivity of plane standard mirror rear surface, transmissivity when reflected light penetrates plane standard mirror before and after plane standard mirror is β ', and d is sheet glass thickness, and θ is refraction angle, n is sheet glass refractive index, and c is the light velocity; P and j are integer, and 0≤p≤m-1,1≤j≤m-p;

Step 2, laser, after sheet glass refraction transmitting, are assembled to the light-sensitive surface of photodetector through convergent lens, calculate total light field that photodetector receives;

Total light field that photodetector receives is expressed as:

E′(t)＝E ₁(t)+E ₂(t)...+E _m(t)+... (4-1)

The photocurrent of photodetector output is expressed as:

$I=\frac{\mathrm{\ηe}}{\mathrm{h\ν}}\frac{1}{Z}\underset{D}{\∫\∫}\frac{1}{2}[E_1\left(t\right)+E_2\left(t\right)+\·\·\·+E_m\left(t\right)+\·\·\·][E_1\left(t\right)+E_2\left(t\right)+\·\·\·+E_m\left(t\right)+\·\·\·{]}^{*}\mathrm{ds}---(5-1)$

Wherein, e is electron charge, and Z is the intrinsic impedance of detector surface medium, and η is quantum efficiency, and D is the area of detector photosurface, and h is Planck's constant, and v is laser frequency, represents complex conjugate No. *;

Step 3, after the opto-electronic conversion of photodetector, obtain current signal and carry out low-pass filtering through wave filter;

Obtain filtered electric current of intermediate frequency signal:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{2\mathrm{h\ν}}\frac{1}{Z}\underset{D}{\∫\∫}\underset{p=0}{\overset{m-1}{\mathrm{\Σ}}}\underset{j=0}{\overset{m-p}{\mathrm{\Σ}}}({E_j\left(t\right)E_{j+p}}^{*}\left(t\right)+{E_j}^{*}\left(t\right)E_{j+p}\left(t\right))\mathrm{ds}---(6-1)$

By (2-1) formula and (3-1) formula substitution (6-1) formula, result is:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{\mathrm{h\ν}}\frac{\mathrm{\π}}{Z}{E}_0^2\underset{p=0}{\overset{m-1}{\mathrm{\Σ}}}\underset{j=0}{\overset{m-p}{\mathrm{\Σ}}}{\mathrm{\α}}_{j+p}{\mathrm{\α}}_j\cos (\frac{4\mathrm{pknd}\cos \mathrm{\θ}}{c}t-\frac{4\mathrm{pknd}\cos \mathrm{\θ}(l+\mathrm{nd}\cos \mathrm{\θ})}{c^2})---(7-1)$

Wherein, α _j=β β ' r ' ^(2j-3), α _j+p=β β ' r ' ^[(2j+p)-3];

Step 4, the difference on the frequency in the electric current of intermediate frequency signal in step 3 is carried out to Fourier transform, obtain the frequency of difference on the frequency signal

With the scale relation of sheet glass thickness d, and then obtain sheet glass thickness d;

$f_p=\frac{2\mathrm{pknd}\cos \mathrm{\θ}}{\mathrm{\πc}}=K_{p}d---(8-1)$

The Proportional coefficient K that the frequency of difference on the frequency signal is directly proportional to sheet glass thickness _pfor:

$K_p=\frac{2\mathrm{pkn}\cos \mathrm{\θ}}{\mathrm{\πc}}.---(9-1)$

The present invention is by linear frequency modulation technology and heterodyne technology effective integration, sheet glass thickness information to be measured is loaded in the difference on the frequency of heterodyne signal, by Fourier, change and be easy to just to demodulate sheet glass thickness information to be measured, and measuring accuracy is high.The method is a kind of method of good non-cpntact measurement sheet glass thickness, can be applied in 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 adopt the method for the invention when measuring different sheet glass thickness, and measuring error is less than 0.01%.

Accompanying drawing explanation

Fig. 1 is the structural representation of the device of linear frequency modulation double light beam laser heterodyne measurement thickness of glass of the present invention;

Fig. 2 is double light beam laser principle of interference schematic diagram;

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

Frequency spectrum corresponding to the different sheet glass thickness measures of Fig. 4.

Embodiment

Embodiment one, in conjunction with Fig. 1, Fig. 2, present embodiment is described, described in present embodiment, linear frequency modulation multi-beam laser heterodyne is measured the device of thickness of glass, and this device comprises digital signal processor 1, A/D converter 2, prime amplifier 3, wave filter 4, photodetector 5, convergent lens 6, sheet glass 7, catoptron 8, No. two catoptrons 9 and frequency modulation lasers 10;

The laser of frequency modulation laser 10 transmittings is incident to catoptron 8 No. one after No. two catoptron 9 reflections, and the light beam after catoptron 8 reflections is incident to flat glass 7; This incident light obtains multi beam reflected light after the front surface of sheet glass 7 and the rear surface multiple reflections of sheet glass 7, and this multi beam reflected light converges on the photosurface of photodetector 6 through convergent lens 6 after sheet glass 7 transmissions;

Through photodetector 5, carry out the signal input part that current signal output end after opto-electronic conversion connects wave filter 4, after the filtering of wave filter 4, signal output part connects the signal input part of prime amplifier 3, the amplifying signal output terminal of prime amplifier 3 connects the input end of analog signal of A/D converter 2, the signal input part of the digital signal output end linking number word signal processor 1 of A/D converter 2.

First present embodiment opens laser instrument, make linear frequency modulation linearly polarized light through two catoptrons, retread and incide on sheet glass successively, after the light of sheet glass front surface transmission is by the reflection of the rear surface of sheet glass with together with light through sheet glass front surface reflection, be converged lens and converge on detector photosurface, finally by the electric signal after detector opto-electronic conversion, after device, amplifier, A/D converter and digital signal processor, obtain parameter information to be measured after filtering.

After the present invention is directed to traditional heterodyne signal spectrum demodulation, can only obtain single parameter value shortcoming and defect to be measured, propose a kind of linear frequency modulation multi-beam laser heterodyne and measured the apparatus and method of sheet glass thickness, by linear frequency modulation technology and heterodyne technological incorporation, obtained linear frequency modulation multi-beam laser heterodyne signal, in its signal spectrum, comprise a plurality of frequency values simultaneously, each frequency values comprises sheet glass thickness information to be measured, after Fast Fourier Transform (FFT) demodulation, can obtain a plurality of sheet glass one-tenth-value thickness 1/10s to be measured simultaneously, to the multiple parameter values weighted mean obtaining, improved the measuring accuracy of sheet glass thickness to be measured.

Embodiment two, present embodiment are linear frequency modulation multi-beam laser heterodyne described in embodiment one to be measured to the further illustrating of device of thickness of glass, and wave filter 4 is low-pass filters.

Embodiment three, present embodiment are the methods that the linear frequency modulation multi-beam laser heterodyne described in employing embodiment one is measured the measurement device thickness of glass of thickness of glass, and the concrete steps of the method are:

Step 1, adopt frequency modulation laser 10 Emission Lasers, laser retreads and is incident upon sheet glass 7 through a catoptron 8 and 9 liang of secondary reflection of No. two catoptrons; Writing light beam is slanted to the incidence angle θ of sheet glass 7 simultaneously ₀; And measure frequency modulation laser 10 Emission Lasers to the light path l between incident sheet glass 7, calculate incident field E (t), the reflection light field E of sheet glass 7 ₁(t) with through the light of sheet glass transmission not in the same time by the light field E of sheet glass multiple reflections ₂(t), E ₃(t) ..., E _m(t)

The mathematic(al) representation of incident field is:

E(t)＝E ₀exp{i(ω ₀t+kt ²)} (1-1)

Wherein, for the rate of change of modulating bandwidth, T is the frequency modulation cycle, and △ F is modulating bandwidth; E ₀for incident field amplitude, t is the time, ω ₀for light field angular frequency; I is imaginary unit;

The reflection light field that constantly arrives sheet glass front surface according to formula (1-1) acquisition t-l/c is:

$E_1\left(t\right)={\mathrm{\α}}_1{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{1}{c})+k{(t-\frac{1}{c})}^2\left]\right\}---(2-1)$

Through the light of sheet glass transmission, not in the same time by sheet glass multiple reflections, its catoptrical expression formula is respectively:

$\left\{\begin{array}{c}{E}_2\left(t\right)={\mathrm{\α}}_2{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ E_3\left(t\right)={\mathrm{\α}}_3{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{4{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ .\\ .\\ .\\ E_j\\ .\\ .\\ .\\ E_{j+p}\\ .\\ .\\ .\\ E_m\left(t\right)={\mathrm{\α}}_m{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2(m-1){\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ .\\ .\\ .\end{array}\right.---(3-1)$

Wherein, m is greater than 2 nonnegative integer, α ₁=r, α ₂=β β ' r ' ..., α _m=β β ' r ' ^(2m-3)r is the reflectivity of light while injecting plane standard mirror from surrounding medium, transmissivity is β, r ' is the reflectivity of plane standard mirror rear surface, transmissivity when reflected light penetrates plane standard mirror before and after plane standard mirror is β ', and d is sheet glass thickness, and θ is refraction angle, n is sheet glass refractive index, and c is the light velocity; P and j are integer, and 0≤p≤m-1,1≤j≤m-p;

Step 2, laser, after sheet glass 7 refraction transmittings, are assembled to the light-sensitive surface of photodetector 5 through convergent lens 6, calculate total light field that photodetector 5 receives;

Total light field that photodetector receives is expressed as:

E′(t)＝E ₁(t)+E ₂(t)...+E _m(t)+... (4-1)

The photocurrent of photodetector output is expressed as:

$I=\frac{\mathrm{\ηe}}{\mathrm{h\ν}}\frac{1}{Z}\underset{D}{\∫\∫}\frac{1}{2}[E_1\left(t\right)+E_2\left(t\right)+\·\·\·+E_m\left(t\right)+\·\·\·][E_1\left(t\right)+E_2\left(t\right)+\·\·\·+E_m\left(t\right)+\·\·\·{]}^{*}\mathrm{ds}---(5-1)$

Wherein, e is electron charge, and Z is the intrinsic impedance of detector surface medium, and η is quantum efficiency, and D is the area of detector photosurface, and h is Planck's constant, and v is laser frequency, represents complex conjugate No. *;

Step 3, after the opto-electronic conversion of photodetector 5, obtain current signal and carry out low-pass filtering through wave filter 4;

Obtain filtered electric current of intermediate frequency signal:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{2\mathrm{h\ν}}\frac{1}{Z}\underset{D}{\∫\∫}\underset{p=0}{\overset{m-1}{\mathrm{\Σ}}}\underset{j=0}{\overset{m-p}{\mathrm{\Σ}}}({E_j\left(t\right)E_{j+p}}^{*}\left(t\right)+{E_j}^{*}\left(t\right)E_{j+p}\left(t\right))\mathrm{ds}---(6-1)$

By (2-1) formula and (3-1) formula substitution (6-1) formula, result is:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{\mathrm{h\ν}}\frac{\mathrm{\π}}{Z}{E}_0^2\underset{p=0}{\overset{m-1}{\mathrm{\Σ}}}\underset{j=0}{\overset{m-p}{\mathrm{\Σ}}}{\mathrm{\α}}_{j+p}{\mathrm{\α}}_j\cos (\frac{4\mathrm{pknd}\cos \mathrm{\θ}}{c}t-\frac{4\mathrm{pknd}\cos \mathrm{\θ}(l+\mathrm{nd}\cos \mathrm{\θ})}{c^2})---(7-1)$

Wherein, α _j=β β ' r ' ^(2j-3), α _j+p=β β ' r ' ^[(2j+p)-3];

Step 4, the difference on the frequency in the electric current of intermediate frequency signal in step 3 is carried out to Fourier transform, obtain the frequency of difference on the frequency signal

With the scale relation of sheet glass thickness d, and then obtain sheet glass thickness d;

$f_p=\frac{2\mathrm{pknd}\cos \mathrm{\θ}}{\mathrm{\πc}}=K_{p}d---(8-1)$

The Proportional coefficient K that the frequency of difference on the frequency signal is directly proportional to sheet glass thickness _pfor:

$K_p=\frac{2\mathrm{pkn}\cos \mathrm{\θ}}{\mathrm{\πc}}.---(9-1)$

Employing Matlab verifies the feasibility of put forward the methods of the present invention, generally refractive index n=1.493983 of sheet glass; Linear frequency modulation laser wavelength is 1.55 μ m, frequency modulation cycle T=1ms, modulating bandwidth △ F=5GHz.

By emulation, can see, the Fourier transform frequency spectrum of the linear frequency modulation multi-beam laser heterodyne 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 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 signal while measuring slab-thickness d.

As can see from Figure 3, in simulation process, provided the theoretical curve in the situation of normal incidence, object is: in multi-beam laser heterodyne signal spectrum figure, the numerical value of the centre frequency of theoretical curve when the centre frequency of first main peak of linear frequency modulation multi-beam laser heterodyne signal spectrum and normal incidence in the time of can simultaneously obtaining oblique incidence, like this, be easy to the ratio of two centre frequencies obtaining:

ζ＝cosθ (10-1)

In the situation that obtaining centre frequency, by (10-1) formula, can calculate the size of laser refraction angle θ after sheet glass, and then according to refraction law, can obtain the size of incidence angle θ 0, the K finally asking by (9-1) formula _pnumerical value, finally obtain the value of sheet glass thickness d.

Meanwhile, utilize Matlab emulation to obtain different incidence angles θ ₀in situation, 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 thickness, the increase frequency that relative position of frequency spectrum moves along with thickness to high frequency direction increases.Reason is: the in the situation that of sheet glass invariable incident angle, and Proportional coefficient K _pa constant, when thickness increases, due to frequency f _pclosing with sheet glass thickness d is f _p=K _pd, K _pin constant situation, frequency f _plinear with sheet glass thickness d.Therefore, the relative position that when thickness increases, frequency also increases along with the increase frequency spectrum of thickness thereupon 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, so in Fig. 4, the signal to noise ratio (S/N ratio) of heterodyne signal is very high.

Utilize above-mentioned linear frequency modulation multi-beam laser heterodyne 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

Measure number of times	1	2	3	4	5	6	7	8
									d(mm)	1.0	3.0	5.0	7.0	9.0	11.0	13.0	15.0
d i(mm)	1.000112	3.000355	5.000590	7.000822	9.001058	11.001294	13.001530	15.001764

The emulation experiment data of utilizing table 1, the maximum relative error that finally can obtain measured value is less than 0.01%, and the measuring accuracy that can find out the 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 trueness error after Fast Fourier Transform (FFT) (FFT) and the round-off error in computation process.

Simulation result shows, the method is when 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 (3)

1. linear frequency modulation multi-beam laser heterodyne is measured the device of thickness of glass, it is characterized in that, this device comprises digital signal processor (1), A/D converter (2), prime amplifier (3), wave filter (4), photodetector (5), convergent lens (6), sheet glass (7), a catoptron (8), No. two catoptrons (9) and frequency modulation laser (10);

The laser of frequency modulation laser (10) transmitting is incident to a catoptron (8) after No. two catoptrons (9) reflection, and the light beam after a catoptron (8) reflection is incident to flat glass (7); This incident light obtains multi beam reflected light after the front surface of sheet glass (7) and the rear surface multiple reflections of sheet glass (7), and this multi beam reflected light converges on the photosurface of photodetector (6) through convergent lens (6) after sheet glass (7) transmission;

Through photodetector (5), carry out the signal input part that current signal output end after opto-electronic conversion connects wave filter (4), after the filtering of wave filter (4), signal output part connects the signal input part of prime amplifier (3), the amplifying signal output terminal of prime amplifier (3) connects the input end of analog signal of A/D converter (2), the signal input part of the digital signal output end linking number word signal processor (1) of A/D converter (2).

2. linear frequency modulation multi-beam laser heterodyne according to claim 1 is measured the device of thickness of glass, it is characterized in that, wave filter (4) is low-pass filter.

3. linear frequency modulation multi-beam laser heterodyne is measured the method for the measurement device thickness of glass of thickness of glass, it is characterized in that, the concrete steps of the method are:

Step 1, adopt frequency modulation laser (10) Emission Lasers, laser retreads and is incident upon sheet glass (7) through a catoptron (8) and No. two catoptron (9) two secondary reflection; Writing light beam is slanted to the incidence angle θ of sheet glass (7) simultaneously ₀; And measure frequency modulation laser (10) Emission Lasers to the light path l between incident sheet glass (7), calculate incident field E (t), the reflection light field E of sheet glass (7) ₁(t) with through the light of sheet glass transmission not in the same time by the light field E of sheet glass multiple reflections ₂(t), E ₃(t) ..., E _m(t)

The mathematic(al) representation of incident field is:

E(t)＝E ₀exp{i(ω ₀t+kt ²)} (1-1)

Wherein, for the rate of change of modulating bandwidth, T is the frequency modulation cycle, and △ F is modulating bandwidth; E ₀for incident field amplitude, t is the time, ω ₀for light field angular frequency; I is imaginary unit;

The reflection light field that constantly arrives sheet glass front surface according to formula (1-1) acquisition t-l/c is:

$E_1\left(t\right)={\mathrm{\α}}_1{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{1}{c})+k{(t-\frac{1}{c})}^2\left]\right\}---(2-1)$

Through the light of sheet glass transmission, not in the same time by sheet glass multiple reflections, its catoptrical expression formula is respectively:

$\left\{\begin{array}{c}{E}_2\left(t\right)={\mathrm{\α}}_2{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ E_3\left(t\right)={\mathrm{\α}}_3{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+4\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{4{\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ .\\ .\\ .\\ E_j\\ .\\ .\\ .\\ E_{j+p}\\ .\\ .\\ .\\ E_m\left(t\right)={\mathrm{\α}}_m{E}_0\exp \left\{i\right[{\mathrm{\ω}}_0(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})+k{(t-\frac{l+2(m-1)\mathrm{nd}\cos \mathrm{\θ}}{c})}^2+\frac{2(m-1){\mathrm{\ω}}_0\mathrm{nd}\cos \mathrm{\θ}}{c}\left]\right\}\\ .\\ .\\ .\end{array}\right.---(3-1)$

Wherein, m is greater than 2 integer, α ₁=r, α ₂=β β ' r ' ..., α _m=β β ' r ' ^(2m-3)r is the reflectivity of light while injecting plane standard mirror from surrounding medium, transmissivity is β, r ' is the reflectivity of plane standard mirror rear surface, transmissivity when reflected light penetrates plane standard mirror before and after plane standard mirror is β ', and d is sheet glass thickness, and θ is refraction angle, n is sheet glass refractive index, and c is the light velocity; P and j are integer, and 0≤p≤m-1,1≤j≤m-p;

Step 2, laser, after sheet glass (7) refraction transmitting, are assembled to the light-sensitive surface of photodetector (5) through convergent lens (6), calculate total light field that photodetector (5) receives;

Total light field that photodetector receives is expressed as:

E′(t)＝E ₁(t)+E ₂(t)...+E _m(t)+... (4-1)

The photocurrent of photodetector output is expressed as:

$I=\frac{\mathrm{\ηe}}{\mathrm{h\ν}}\frac{1}{Z}\underset{D}{\∫\∫}\frac{1}{2}[E_1\left(t\right)+E_2\left(t\right)+\·\·\·+E_m\left(t\right)+\·\·\·][E_1\left(t\right)+E_2\left(t\right)+\·\·\·+E_m\left(t\right)+\·\·\·{]}^{*}\mathrm{ds}---(5-1)$

Wherein, e is electron charge, and Z is the intrinsic impedance of detector surface medium, and η is quantum efficiency, and D is the area of detector photosurface, and h is Planck's constant, and v is laser frequency, represents complex conjugate No. *;

Step 3, after the opto-electronic conversion of photodetector (5), obtain current signal and carry out low-pass filtering through wave filter (4);

Obtain filtered electric current of intermediate frequency signal:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{2\mathrm{h\ν}}\frac{1}{Z}\underset{D}{\∫\∫}\underset{p=0}{\overset{m-1}{\mathrm{\Σ}}}\underset{j=0}{\overset{m-p}{\mathrm{\Σ}}}({E_j\left(t\right)E_{j+p}}^{*}\left(t\right)+{E_j}^{*}\left(t\right)E_{j+p}\left(t\right))\mathrm{ds}---(6-1)$

By (2-1) formula and (3-1) formula substitution (6-1) formula, result is:

$I_{\mathrm{IF}}=\frac{\mathrm{\ηe}}{\mathrm{h\ν}}\frac{\mathrm{\π}}{Z}{E}_0^2\underset{p=0}{\overset{m-1}{\mathrm{\Σ}}}\underset{j=0}{\overset{m-p}{\mathrm{\Σ}}}{\mathrm{\α}}_{j+p}{\mathrm{\α}}_j\cos (\frac{4\mathrm{pknd}\cos \mathrm{\θ}}{c}t-\frac{4\mathrm{pknd}\cos \mathrm{\θ}(l+\mathrm{nd}\cos \mathrm{\θ})}{c^2})---(7-1)$

Wherein, α _j=β β ' r ' ^(2j-3), α _j+p=β β ' r ' ^[(2j+p)-3];

Step 4, the difference on the frequency in the electric current of intermediate frequency signal in step 3 is carried out to Fourier transform, obtain the frequency and the scale relation of sheet glass thickness d of difference on the frequency signal, and then acquisition sheet glass thickness d;

$f_p=\frac{2\mathrm{pknd}\cos \mathrm{\θ}}{\mathrm{\πc}}=K_{p}d---(8-1)$ The Proportional coefficient K that the frequency of difference on the frequency signal is directly proportional to sheet glass thickness _pfor:

$K_p=\frac{2\mathrm{pkn}\cos \mathrm{\θ}}{\mathrm{\πc}}.---(9-1)$