Embodiment
Describe displacement measurement system provided by the invention and measuring method thereof in detail below with reference to accompanying drawing.
See also Fig. 1, first embodiment of the invention provides a kind of displacement measurement system, and described displacement measurement system comprises a laser instrument 20, one feedback units 30 and a data acquisition and processing unit 40.Enter described feedback unit 30 from the laser of described laser instrument 20 outputs, after feedback unit 30 feedbacks reflection, return described laser instrument 20, and enter data acquisition and processing unit 40.
Described laser instrument 20 is used for the output double-frequency laser; May be selected to be and be gas laser, solid state laser or semiconductor laser etc.In the present embodiment, described laser instrument 20 is a helium-neon laser.Described laser instrument 20 comprises a shell 15, and is arranged at one first inner chamber catoptron 7, a gain tube 6, an anti-reflection window 5, a birefringence element 4 and one second inner chamber catoptron 3 in the shell 15.Described shell 15 helps to keep temperature and the thermal equilibrium of laser instrument 20 inside.Described gain tube 6 can be fixed in the described shell 15 by a bracing frame 14.Be appreciated that support frame as described above 14, shell 15 are an optional structure.Described the first inner chamber catoptron 7, gain tube 6, anti-reflection window 5, birefringence element 4 and the second inner chamber catoptron 3 are along the successively coaxial setting of axis of described laser instrument 20 Output of lasers, and the inner chamber of formation laser instrument.Described birefringence element 4 makes described laser instrument 20 output double-frequency lasers for generation of frequency splitting, and the material of described birefringence element 4 can be quartz crystal, bilingual stone etc., also can produce for other material of frequency splitting.Described birefringence element 4 and arranges with described anti-reflection window 5 and described the second inner chamber catoptron 3 intervals between described anti-reflection window 5 and described the second inner chamber catoptron 3.Described birefringence element 4 has two relative planes at the Output of laser axis direction, and anti-reflection film is all plated on described two planes.The chamber length of the inner chamber of described laser instrument 20 can be 180mm~200mm.
The interference laser that described data acquisition and processing unit 40 are exported from described laser instrument 20 first inner chamber catoptrons 7 in order to reception, and carry out data and process and calculate umber of pulse.Described data acquisition and processing unit 40 comprise an Amici prism 8, the first photodetector 9, the second photodetector 10, a filter amplification circuit 11, a signal processing unit 12 and a display device 13.Described Amici prism 8 arranges near the first inner chamber catoptron 7 of described laser instruments 20, receiving the laser of the first inner chamber catoptron 7 outputs from laser instrument 20, and the laser of output is divided into o light, the e light component that two-way has phasic difference in the space.Described the first photodetector 9, the second photodetector 10 be in order to surveying respectively o light and the e light component by Amici prism 8 outputs, and be converted into two path signal.Described filter amplification circuit 11 is electrically connected with described the first photodetector 9 and the second photodetector 10, and two path signal is carried out current/voltage-converted, amplification and filtering process.Described signal processing unit 12 is used for counting is processed and realized to the two path signal of filter amplification circuit 11 outputs, the umber of pulse N that produces during with the calculating change in optical path length, and the direction of motion of or lag behind judgement testee relatively leading according to the position of two path signal.Further, can be electrically connected with described signal processing unit 12 by described display device 13, be used for number of pulses N and show.Be appreciated that described display device 13 only is an optional structure, described umber of pulse N also can obtain by other counting elements.
See also Fig. 2 and Fig. 3, described feedback unit 30 comprises one first catoptron 1 and the second catoptron 2 and the 1 relative and interval setting of described the first catoptron, and its spacing d can be more than or equal to 1 millimeter less than or equal to 1000 millimeters, such as 2 millimeters, 5 millimeters, 10 millimeters, 20 millimeters etc.In the situation that other parameter constants, described spacing is less, and then the minute mark rate of described displacement measurement system is higher.Described feedback unit 30 consists of the FP feedback exocoel of described laser instrument 20.The material of described the first catoptron 1 and the second catoptron 2 can be glass, also can be other transparent solid materials.Described the first catoptron 1 has a first surface 101 towards described laser instrument 20, makes Output of laser can be directly incident on described first surface 101.Described first surface 101 is the plane, and the surface of described first surface 101 is coated with reflectance coating with the laser of reflection incident.The normal of described first surface 101 and described incident laser form an angle α, and less than 90 degree, preferred, described α can be more than or equal to 1 degree less than or equal to 10 degree greater than zero degree for described α.In the situation that it is constant to keep α, described the first catoptron 1 can move along described laser instrument 20 Output of laser axis directions, to change the spacing between the first catoptron 1 and the second catoptron 2.In the present embodiment, the material of described the first catoptron 1 is glass, and thickness is 3 millimeters, and described α is 4 degree.
Described the second catoptron 2 comprise one in described the first catoptron 1 first surface 101 and parallel in the 3rd surface 201 of first surface 101, and four surface 202 and five surface 203 relative with the described the 3rd surperficial 201, the 202 and the 5th surface 203, described the 4th surface arranges back to described first surface 101.Described the 4th surface 202 can be a plane, curved surface, folding face etc., in the present embodiment, described the 4th surface 202 be one with 201 parallel planes, described the 3rd surface, the distance between described the 4th surface 202 and described the 3rd surface 201 is the thickness d of described the second catoptron 2.Described the 5th surface 203 is a plane, is connected the junction formation one handing-over line on described the 5th surface 203 with described the 4th surface 202 with described the 4th surface 202.Described the 4th surface 202 is near the shoot laser setting of described laser instrument 20, and described the 5th surface 203 is away from the shoot laser setting of described laser instrument 20.Described the 5th surface 203 forms an angle β with described the 3rd surface 201.Described the 5th surface 203 on away from described Output of laser direction gradually near described the 3rd surface 201.Described the 5th surface 203 begins along being the width l that maximum development length on the β angular direction is defined as described the 5th surface 203 with the 4th surface 102 from described boundary line
2, described l
2Can be more than or equal to 10 millimeters.In the present embodiment, the width l on described the 5th surface 203
2Equal 10 millimeters.
Described the 3rd surface 201 is α with the formed angle of described Laser output direction, can define a reflector space 201a and a regional transmission 201b away from described incident laser who has near described incident laser in described the 3rd surface 201 according to the function difference.The surface of described reflector space 201a is coated with reflectance coating, be used for reflection from the laser of first surface 101 reflections, the surface of described regional transmission 201b is coated with anti-reflection film, is used for transmission from the laser of described first surface 101 reflections, makes described laser enter to inject described the 5th surface 203.After being mapped to described first surface 101 from laser instrument 20 import and exports, between first surface 101 and reflector space 201a, through Multi reflection, incide described the 5th surface 203 from described regional transmission 201b.Particularly, the width of described reflector space 201a can be 50 millimeters to 100 millimeters, the width l of described regional transmission 201b
1Be defined as described regional transmission 201b at the maximum span that is along the shoot laser with laser instrument 20 on the α angle direction.Preferably, the width l of described regional transmission 201b
1More than or equal to 10 millimeters.In the present embodiment, the width l of described regional transmission 201b
1Equal 10 millimeters.
Further, the width l on described the 5th surface 203
2Width l with described regional transmission 201b
1Satisfy following relation:
Wherein, d is the distance between the 201 and the 4th surface 202, the 3rd surface in the second catoptron 2, to guarantee that the laser that incides feedback unit 30 from laser instrument 20 can incide described the 5th surface 203 behind Multi reflection, and after the reflectance coating reflection on the 5th surface 203, be reflected back in the described laser instrument 20 along former road again.Preferably, the width l on described the 5th surface 203
2Width l greater than described regional transmission 201b
1Thereby, make from the laser of first surface 101 reflections and can incide described the 5th surface 203.
See also Fig. 4 and Fig. 5, described laser instrument 20 emitting lasers incide after the first catoptron 1, reflex to the reflector space 201a on described the second catoptron 2 described the 3rd surfaces 201, and after round n time (feedback order), incide the 5th surface 203 of described the second catoptron 2 from the regional transmission 201b on described the 3rd surface 201.The refractive index n of described α, β and described the second catoptron 2
2Satisfy following relation:
So that after inciding the 5th surface 203 of the second catoptron 2 from the laser of described regional transmission 201b incident, can be from original optical path return laser light device 20.
The principle of described displacement measurement system is as follows.Described the first catoptron 1 is fixed in object under test one surface.Be appreciated that described the first catoptron 1 can omit when described object under test surface itself namely has a plane of reflection.Two the crossed polarized light o light that is produced by birefringence element 4 division and e light, in the situation that the light feedback, light field can be divided into two parts.Reflector space is the inner chamber propagation field of light beam after laser instrument 20 inner chambers come and go a week, and regional transmission is light beam turns back to laser instrument 20 inner chambers again along former road after 30 interior round n weeks (being the feedback order) at feedback unit propagation field.The inner chamber propagation field forms self-mixed interference with the propagation field stack that turns back to inner chamber at feedback unit 30 round n after week, and the output intensity of o light and e light can be expressed as under low light level feedback condition:
In the formula: I
O0And I
E0The output intensity of two crossed polarized lights when being unglazed feedback, ζ
oAnd ζ
eThe laser feedback factor, ν
oAnd ν
eBe the optical frequency of o light and e light, l is that the chamber of feedback unit 30 is long.(1) formula shows that when the light feedback was arranged, the output intensity of two crossed polarized lights was all modulated, and waveform is similar to cosine curve.The fringe density of the feedback fringe that forms in the display device 13 especially, is mainly determined by feedback order n.Wherein, the geometry that the size of feedback order n can be by feedback unit 30 is by calculating.
When the first catoptron 1 of feedback unit 30 moved right Δ b, the total light path increment that then produces comprised two parts:
Reflector space is to increase to 2m * Δ b by the light path that 1 displacement of the first catoptron causes, wherein m is the round number of times (laser from first catoptron incide second catoptron, return first catoptron for once come and go again) of incident laser in feedback unit 30;
Regional transmission is that laser beam is when moving on the surface of the first catoptron 1 and the second catoptron 2, cause the light path of last light beam in the second catoptron 2 described in the feedback unit 30 to change, and produce additional optical distance-AC(because when moving right, light path in the second catoptron 2 reduces, so get negative sign);
So when the first catoptron 1 moved right Δ b, total light path increment Delta l of generation was
Δl=2m×Δb-AC。Wherein, additional optical distance AC can obtain as follows:
When the first catoptron 1 moves right Δ b, each reflection light point amount of movement from the teeth outwards is KT=2sin α * Δ b on the second catoptron 2, so the feedback light beam is behind the m secondary reflection, last luminous point is AB=2msin α * Δ b at the second catoptron 2 lip-deep amount of movements;
In right-angle triangle ABC (C is the summit, right angle), additional optical distance AC=AB * sin β * n
2So, AC=2mn
2Sin α sin β * Δ b.
So when the first catoptron 1 moved right Δ b, total light path increment Delta l was
Δl=2m×Δb-2mn
2sinαsinβ×Δb。(2)
The umber of pulse N that total light path increment produces is
Can get the move right expression formula of Δ b of the first catoptron 1 by formula (2) and (3)
By (4) formula as can be known, the resolution δ of measuring system (being resolution) is
Wherein, α is incident angle, and it is constant, and λ is optical maser wavelength.Can find out from formula (5), the resolution of measuring system is only relevant with the round number of times m of laser in the feedback unit 30, namely relevant with the reflected light on the first catoptron 1 surface q that counts, coming and going the count pass of q of number of times m and reflected light is, m=2 * q-1), as long as so calculate the number of reflection light point in the feedback unit 30, just can obtain exactly the optical resolution of measuring system, and no longer need other benchmark instrument to demarcate.The resolution of described displacement measurement system can be less than 10 nanometers, as less than 8 nanometers, less than 5 nanometers, less than 2 nanometers.
Shown in Fig. 6 A and Fig. 6 B, when Fig. 6 B is displacement measurement system measurement of the present invention, the first catoptron 1 moves along the laser axis, the laser intensity curve that the first photodetector 9, the second photodetector 10 obtain, being very close high-order frequency multiplication striped, is tens times of the weak feedback of tradition shown in Fig. 6 A.See also Fig. 7 A, Fig. 7 B, Fig. 7 C, provided the intensity modulation curve of feedback unit 30 under different feedback order conditions.Can find out from Fig. 7 A, it is 3 that the reflected light in feedback unit 30 is counted, and namely the feedback order is 7 o'clock, and the density of intensity modulation curve is about 7 times of the weak feedback of tradition; As can be seen from Figure 7B, it is 11 that the reflected light in feedback unit 30 is counted, and namely the feedback order is 21 o'clock, and the density of intensity modulation curve is about 21 times of the weak feedback of tradition; Can find out from Fig. 7 C, it is 17 that the reflected light in feedback unit 30 is counted, and namely the feedback order is 33 o'clock, and the density of intensity modulation curve is about 33 times of the weak feedback of tradition, and this moment, corresponding optical resolution was 9.6nm; And, go back presence bit between the feedback fringe of two polarization state quadratures and differ.After the first photodetector 9, the second photodetector 10 detect this feedback fringe, utilize 11 pairs of signals of filter amplification circuit to carry out filter amplifying processing; Then signal processing unit 12, realize the digital conversion of signal and further shaping, filtering, and umber of pulse is processed and calculated at the edge of the two paths of signals of the first photodetector 9,10 outputs of the second photodetector; Simultaneously, the direction of motion of or hysteresis judgment object relatively leading according to the position of two paths of signals, and direction and umber of pulse be presented on the display device 13.
The present invention is based on the displacement measurement system of FP feedback exocoel, utilize the weak feedback effect of laser substance high-order, not only have the high-order frequency-doubled effect, reach nano level displacement measurement resolution; But also the further electronic fine-grained Measurement Resolution of Subnano-class and the direction of motion of recognition object of reaching; In addition, the method also has advantages of optical resolution from demarcating, can according to the reflected light of FP feedback chamber mirror count the system that obtains resolution and not other benchmark instruments of needs demarcate.Described displacement measurement system have resolution high, simple in structure, can be traceable to the characteristics such as optical maser wavelength.
In addition, those skilled in the art also can do other and change in spirit of the present invention, and certainly these variations of doing according to spirit of the present invention all should be included in the present invention's scope required for protection.