CN115325943A - High-resolution displacement measuring device and measuring method - Google Patents
High-resolution displacement measuring device and measuring method Download PDFInfo
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- CN115325943A CN115325943A CN202210994259.7A CN202210994259A CN115325943A CN 115325943 A CN115325943 A CN 115325943A CN 202210994259 A CN202210994259 A CN 202210994259A CN 115325943 A CN115325943 A CN 115325943A
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- G01—MEASURING; TESTING
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Abstract
The invention provides a high-resolution displacement measuring device and a measuring method. Measuring the frequency change of the laser double longitudinal modes by a high-frequency photoelectric detector in the moving process of the wedge-shaped glass sheet; and carrying out light splitting treatment on the double longitudinal modes through a light splitting prism, respectively measuring the light intensity of the first light beam and the second light beam through the first photoelectric detector and the second photoelectric detector, and calculating the displacement of the moving target connected with the wedge-shaped glass sheet based on the positive and negative values of the light intensity change and the frequency change value. The device and the method relate the frequency difference of the laser oscillation longitudinal mode with the displacement of the target, and can fully utilize the high precision and high resolution of frequency measurement, so that the displacement measurement reaches the high resolution of sub-nanometer, and the device and the method are favorable for being applied to the field of micro-displacement measurement.
Description
Technical Field
The invention relates to the technical field of optical measurement, in particular to a high-resolution displacement measuring device and a measuring method.
Background
The nanometer displacement measurement is one of the main research directions in the field of precision measurement, and has important application in ultra-precision and ultra-micro machining. Methods for performing nanoscale displacement measurements generally include: microscopic, electrical and optical methods. The microscopic technology comprises a scanning tunnel microscope, an atomic force microscope, a scanning electron microscope and the like, the measurement resolution of displacement or micro fluctuation can reach 1nm or below, three-dimensional shape imaging can be realized, and the defects that the measurement range is limited and the linearity of displacement measurement is not high are overcome. The electrical method generally measures target displacement through electrical quantities such as capacitance, inductance, eddy current and the like, has simple principle and structure, is convenient for electronic subdivision, has displacement measurement resolution reaching the nm level, but also has larger nonlinear error, and needs to be compensated or corrected through an optical measurement technology and the like. The optical method is used for micro-displacement measurement, and has the advantages of high measurement precision and resolution up to sub-nanometer by adopting the optical interference principle, the grating and the Fabry-Perot etalon, wherein the interference measurement can realize a large measurement range, has good measurement linearity, and can be used for calibrating the microscopic technology and the electrical measurement method. The disadvantages are that the optical resolution of the optical measurement is usually half wavelength or quarter wavelength, and to achieve the nano-scale displacement measurement, a complex electronic subdivision and direction finding circuit is required, and the wavelength (i.e. frequency) of light is difficult to trace in an optical stripe. Therefore, it is necessary to research a new displacement measurement technology, which can simultaneously satisfy the requirements of resolution and measurement accuracy reaching the nanometer level, and can trace to the laser wavelength or frequency without electronic subdivision.
Disclosure of Invention
The invention aims to overcome the defects of the displacement measuring method of the optical system in the prior art and provides a high-resolution displacement measuring device and a measuring method.
In order to achieve the above object, the present invention firstly provides a high resolution displacement measuring device, which adopts the technical scheme that:
a high resolution displacement measurement device comprising:
a laser gain tube: the first end of the reflecting mirror is provided with a fixed reflecting mirror, and the second end of the reflecting mirror is provided with an anti-reflection window sheet;
independent mirrors: the double-longitudinal-mode oscillation laser is arranged at the second end side of the gain tube, is positioned at the outer side of the anti-reflection window sheet and is combined with the laser gain tube and the fixed reflector to emit double-longitudinal-mode oscillation laser;
wedge-shaped glass sheet: the reflection reducing window is arranged between the independent reflector and the anti-reflection window sheet, one surface of the reflection reducing window sheet positioned on the light path is vertical to the laser axis, and the other surface of the reflection reducing window sheet is inclined to the laser axis;
spectroscope: is arranged at the outer side of the fixed reflector;
polarizing plate: the reflecting light path is arranged on one side of the spectroscope;
high-frequency photoelectric detector: the device is arranged at the light ray output end of the polaroid and used for receiving beat frequency signals of two longitudinal modes;
a frequency meter: connecting the electric signal output by the high-frequency photoelectric detector to measure the frequency difference of two longitudinal modes;
a beam splitter prism: the two longitudinal modes are arranged on one side of a transmission light path of the spectroscope and are divided into a first light beam and a second light beam according to orthogonal polarization states;
a photoelectric detector: the system comprises a first photoelectric detector and a second photoelectric detector which are respectively arranged on a propagation light path of a first light beam and a propagation light path of a second light beam and used for detecting the light intensity of two longitudinal modes;
a subtracter: the output signal of the first photoelectric detector and the output signal of the second photoelectric detector are respectively input into a subtracter so as to judge the positive and negative values of the light intensity difference of the two longitudinal modes;
a target object: the wedge-shaped glass sheet is installed and can be driven to synchronously move;
a data processing unit: and receiving the positive and negative values of the light intensity difference obtained by the calculation of the subtracter and the frequency difference of the two longitudinal modes measured by the frequency meter, and combining the positive and negative values of the frequency difference and the light intensity difference to calculate the displacement of the wedge-shaped glass sheet.
In some embodiments of the present invention, the light transmission direction of the polarizing plate is at an angle of 45 ° to the orthogonal polarization direction of the oscillation laser light.
In some embodiments of the invention, antireflection films are plated on both surfaces of the wedge-shaped glass sheet in the light path.
In some embodiments of the invention, the wedge-shaped glass sheet is connected to the displacement measuring rod through a fixed frame, and the fixed frame is movably arranged on the guide rail and can move along the guide rail; the displacement measuring rod is contacted with a measured target object, so that the wedge-shaped glass sheet is synchronously driven to displace along the laser axis through the displacement measuring rod in the moving process of the target object.
Some embodiments of the invention comprise the steps of:
a double longitudinal mode laser is adopted as a light source;
moving the wedge-shaped glass sheet, measuring the frequency difference of two longitudinal modes output by laser, and recording the minimum value delta of the frequency difference of the two longitudinal modes min And maximum value of max Recorded as extreme points;
measuring the light intensity I of the laser output double longitudinal modes // And I ⊥ Positive and negative values of the light intensity difference of (a):
s＝sign(I ⊥ -I // )；
in the measuring process, the measured target and the wedge-shaped glass sheet connected with the measured target synchronously move;
recording the frequency difference value of the two longitudinal mode lasers at the beginning of the displacement as delta v 0 After the displacement is finished, the frequency difference of the two longitudinal mode lasers is delta v;
and the difference value of the laser frequencies of the two longitudinal modes in the displacement process is delta v when reaching an extreme point for the first time 1 When the difference value of the laser frequencies of the two longitudinal modes passes the extreme point for the last time before the displacement is finished, the difference value is delta v 2 The total times that the frequency difference value of the two longitudinal mode lasers passes through the extreme point in the displacement process is m, and the light intensity I of the two longitudinal mode lasers is recorded in real time // And I ⊥ ；
Based on the frequency difference and the positive and negative of the light intensity difference of the double longitudinal mode oscillation laser, calculating the decimal part delta l of the single displacement of the measured target object:
wherein λ is the laser wavelength;
if the frequency difference value of the two longitudinal modes does not reach an extreme value point in the displacement process, then delta v 1 ＝Δv 2 =0, the displacement of the target is Δ L = Δ L;
if the frequency difference value of the two longitudinal modes reaches an extreme point in the displacement process, judging the displacement direction according to the sign of delta l, and calculating the displacement of a target object comprising an integer part as follows:
wherein m is an integer.
In some embodiments of the present invention, the m obtaining method includes: and recording the change of the frequency difference delta v of the two longitudinal modes of laser in the displacement adjusting process, wherein when the frequency difference delta v of the two longitudinal modes of laser changes from the maximum value to the minimum value or changes from the minimum value to the maximum value, the displacement moves by one quarter wavelength period, and the accumulated integer m is increased by 1.
Compared with the prior art, the invention has the advantages and positive effects that:
1. the frequency difference of the laser oscillation longitudinal mode is associated with the displacement of the target object, so that the high precision and high resolution of frequency measurement can be fully utilized, the displacement measurement reaches the high resolution of sub-nanometer, and the method is favorable for being applied to the field of micro-displacement measurement.
2. The target displacement changes the half wavelength and changes a period corresponding to the frequency difference of two longitudinal modes of the laser, so that the displacement measurement does not need other reference standards and can be traced to the natural reference of the laser wavelength.
3. The measuring method can carry out micro-displacement measurement direction judgment, does not need complex subdivision and direction judgment circuits, and has simpler structure and high precision.
Drawings
FIG. 1 is a schematic diagram illustrating the coupling principle of reflected light from a wedge-shaped glass plate in a resonant cavity and oscillation laser;
FIG. 2 is a schematic diagram of polarization state and frequency difference of two longitudinal oscillation modes of laser;
FIG. 3 is a schematic diagram of the tuning of the light intensity and frequency difference of the longitudinal laser mode caused by the displacement of the glass sheet in the cavity;
FIG. 4 is a schematic view of a high resolution displacement measuring device according to the present invention.
In the above figures:
1-laser gain tube;
2-a fixed mirror;
3-anti-reflection window sheets;
4-independent mirror;
5-a glass sheet;
601-a fixed frame, 602-a displacement measuring rod;
a 7-spectroscope;
8-a polarizer;
9-high frequency photodetectors;
10-a frequency meter;
11-a beam splitting prism;
1201-a first photodetector, 1202-a second photodetector;
13-a subtractor;
14-a data processing unit;
15-target.
Detailed Description
The invention is described in detail below by way of exemplary embodiments. It should be understood, however, that elements, structures and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.
In the description of the present invention, it should be noted that the terms "upper", "lower", "front", "rear", and the like indicate orientations or positional relationships based on positional relationships shown in the drawings, and are only for convenience in describing the present invention and simplifying the description, but do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and thus, should not be construed as limiting the present invention.
It will be understood that when an element is referred to as being "disposed on," "connected to," or "secured to" another element, it can be directly on the other element or be indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or be indirectly connected to the other element. Furthermore, the terms "first," "second," and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
The invention provides a high-resolution displacement measurement principle and a measurement device, which can realize the measurement precision of displacement with the precision of 0.1nm based on the optical measurement principle.
First, the principle of the inventive concept is explained.
Referring to fig. 1, the half external cavity laser is a laser resonant cavity formed by a fixed mirror 2 and an independent mirror 4, and gain is obtained through a laser gain tube 1 between the fixed mirror 2 and an anti-reflection window 3 to form oscillation. Let the reflection coefficients of the fixed mirror 2 and the independent mirror 4 be r 1 And r 2 The transmission coefficients of both surfaces of the antireflection window 3 are t 3 And t 4 . Putting a wedge-shaped glass sheet 5 in the open part of the resonant cavity, adjusting to make one surface of the wedge-shaped glass sheet perpendicular to the laser axis, wherein the corresponding reflection coefficient and transmission coefficient are r 6 And t 6 While the other surface is inclined to the laser axis, corresponding to a transmission coefficient r 5 。
The surface of the wedge-shaped glass sheet 5 refers to two surfaces of the wedge-shaped glass sheet on a light path, wherein one surface is a vertical surface and the other surface is an inclined surface; wherein, the vertical face is perpendicular to the light propagation direction, and the inclined plane is at a certain angle with the light propagation direction. The direction of the inclined surface is not limited, and the inclined surface may be directed to the fixed mirror 2 side or the independent mirror 4 side, and the effects of the present invention can be achieved. The following explanation of the principle and embodiments of the present invention will be given by taking the case where the inclined surface of the wedge-shaped glass plate 5 faces the fixed mirror 2.
Referring to fig. 2, the half external cavity laser operates in the dual longitudinal modes, and when the two longitudinal modes are located at left-right symmetrical positions of the light-emitting band relative to the center frequency, the respective light intensities are equal. Meanwhile, due to mode competition, the polarization states of the two longitudinal modes are orthogonal (namely vertical) and are respectively expressed as a polarization state of ^ and/or a polarization state. Let the longitudinal mode number of the perpendicular polarized light be q, let the longitudinal mode number of the// polarized light be q +1, and the frequency difference between them be Δ v = v q+1 -v q . If the influence of the wedge-shaped glass sheet 5 placed in the resonant cavity is not considered, the light intensity and the frequency of the two longitudinal oscillation modes are determined only by the gain medium and the resonant cavity parameters. When the laser is in a stable working state, the frequency difference of the two output frequencies is unchanged. When the wedge-shaped glass sheet 5 is placed in the resonant cavity, the reflected light on the surface of the wedge-shaped glass sheet and the oscillation laser are mutually coupled to jointly meet the laser resonance condition, so that the wedge-shaped glass sheet can be used for ensuring that the wedge-shaped glass sheet is in a stable stateThe light intensity and frequency of the two longitudinal modes vary. More importantly, for the standing wave laser, the optical field distribution in the cavity is periodically changed along the axial direction, and the period is half wavelength. When the wedge-shaped glass sheet 5 changes position (i.e. shifts) in the resonant cavity along the laser axis, the effect on the light intensity and frequency of the two longitudinal modes is also periodically changed, and a tuning effect with a certain change amplitude is generated.
Referring to FIG. 1, let E be the complex amplitudes of the electric field vectors of the initial light wave emitted from the fixed mirror 2 in the resonant cavity of the two longitudinal modes 0 ⊥ ，E 0 // After the light beam propagates to the right and is reflected by the independent mirror 4, the formed light beam is in the positive direction. The light wave propagates to the right, is reflected by the independent mirror 4 and then propagates to the left to form a main electric field vector E 1 ⊥，// (ii) a At the same time E 0 ⊥，// The electric field vector which is formed by the weak reflection of the surface of the wedge-shaped glass sheet 5 and then propagates in the right direction is E 2 ⊥，// ；E 1 ⊥，// After being weakly reflected by the surface of the wedge-shaped glass sheet 5, propagating to the right and reflected by the independent mirror 4, an electric field vector E propagating to the left is formed through the wedge-shaped glass sheet 5 3 ⊥，// . There are also multiple reflections between the wedge-shaped glass plate 5 and the fixed mirror 2 and the separate mirror 4 to form a high order folded beam. Because the anti-reflection film is plated on the surface of the wedge-shaped glass sheet, the high-order turn-back light intensity formed by multiple reflections is weakened to more than two orders of magnitude compared with the light beam, and the influence of the high-order turn-back light intensity on the longitudinal mode of the oscillating laser can be ignored. Each light beam returns to the initial point after going back and forth for a circle in the resonant cavity, and the electric field vector satisfies the following conditions:
wherein gL is a And L is the length of the resonant cavity, L is the distance between the surface of one side of the wedge-shaped glass sheet 5, which is vertical to the laser, and the independent reflector 4, d is the thickness of the wedge-shaped glass sheet 5 passing through the laser position, and n is the refractive index of the wedge-shaped glass sheet 5. Then according to the self-consistent condition satisfied by the intracavity light field, i.e. the light wave is at resonanceAfter one round of round trip in the cavity, the electric field vector E is not changed, and the following can be obtained:
the oscillation mode within the laser cavity satisfies the above equation. Simplifying can obtain that the respective light intensity and frequency of two longitudinal modes satisfy the relational expression:
wherein k is 1 ＝2πv q /c，k 2 ＝2πv q+1 And c is the wave number corresponding to the two longitudinal modes, and c is the speed of light in vacuum. Solving the above formula can obtain the light intensity I of two orthogonal polarization longitudinal modes ⊥ And I // And a frequency v q And v q+1 。
Since the laser works in the visible light band, the frequency of the laser cannot be directly measured, and actually the frequency difference between two longitudinal modes, namely the longitudinal mode interval, can be measured. Setting the output light intensity and the longitudinal mode interval as I without considering the tuning effect of the wedge-shaped glass sheet 5 on the longitudinal mode 0 ，Δ 0 . When the wedge-shaped glass sheet 5 in the cavity of the resonant cavity is displaced along the laser axis, i.e. the distance l from the glass sheet to the independent reflector 4 in the attached figure 1 is changed, the relative light intensity tuning curve I of the two longitudinal modes is obtained by calculation according to the formulas (2) and (3) ⊥，// /I 0 And relative frequency difference tuning curve delta/delta 0 As shown in fig. 3.
It can be seen that under the determined parameters of the laser resonant cavity and the coating parameters of the wedge-shaped glass sheet 5, the light intensity and the frequency difference of two oscillating longitudinal modes of the laser are respectively modulated along with l, and each half-wavelength tuning curve changes for a period; the frequency difference between the two longitudinal modes varies sinusoidally between a maximum and a minimum, the modulation amplitude and the maximum delta max And a minimum value Δ min Is about Δ v max =15MHz; the extreme point of the frequency difference tuning curve corresponds to a position where the light intensities of the two longitudinal modes are equal, i.e. the equal light intensity point of the light intensity tuning curve.
According to the calculation result, when the wedge-shaped glass sheet 5 placed in the resonant cavity is translated along the laser axis, the frequency difference of the laser double longitudinal modes is correspondingly changed. And the frequency difference of the longitudinal mode of the laser changes from a minimum value to a maximum value or vice versa for each shift of the glass sheet by a quarter wavelength (for the He-Ne laser used, its operating wavelength is λ =632.8 nm). Since the frequency can be measured accurately, if the measurement resolution of the laser frequency difference is dv =10kHz, the displacement measurement resolution of the corresponding wedge-shaped glass sheet 5 is about:
sub-nanometer resolution measurement of the displacement of the wedge-shaped glass plate 5 within the cavity can thus be achieved. Since the frequency difference between the two longitudinal modes is a periodic variation, in order to determine the displacement of the glass sheet, a tuning period can be divided into two intervals, i.e., a interval a and a interval B, according to the intensity of the two polarized lights, as shown in fig. 3. Wherein, the interval A corresponds to the light intensity I of two polarized lights ⊥ ≥I // The B interval corresponds to I // ≥I ⊥ . The sign of displacement can be represented by I ⊥ -I // And obtaining the result, namely setting the interval A as a positive area and the interval B as a negative area. If the laser frequency difference is increased when the wedge-shaped glass sheet is displaced along the laser axis, I is simultaneously carried out ⊥ -I // Positive, the displacement direction of the glass sheet is the direction in which l increases (set as positive), i.e. the positive direction of the A area along the abscissa; if the laser frequency difference becomes small, while I ⊥ -I // And a negative sign, it also represents that the direction of displacement of the glass sheet is in the direction of increase of i, i.e., the zone B is in the positive direction along the abscissa. On the other hand, it means that the displacement direction of the glass sheet is the direction in which l decreases (set to be opposite), that is, the A and B regions are in the negative direction of the abscissa. In summary, the sign of the light intensity difference between two longitudinal modes can be used as the basis for the displacement direction determination, i.e. sign (I) is combined ⊥ -I // ) And the change amount of the frequency difference is summed, so that the displacement measurement with the resolution ratio of 0.1nm and the translation direction capable of being distinguished is realized on the glass sheet in the cavity.
Referring to FIG. 3, when the wedge-shaped glass sheet is in contact with the glass substrateThe shift of (2) spans the regions a and B, i.e. besides the fractional part of the quarter wavelength, there is also the integer part passing through the frequency difference extreme point, then it can be discriminated as follows: setting the frequency difference value of two longitudinal mode lasers as delta v when displacement measurement begins 0 Thus, when the frequency difference reaches the extreme point for the first time, the extreme value of the frequency difference is recorded as Deltav 1 (Δv 1 ＝Δ max Or Δ min ) The fractional part corresponding to the displacement of the start segment can be calculated according to the attached figure 3 as follows:
wherein λ is the laser wavelength; Δ l 1 The sign of (a) determines the positive or negative direction of displacement of the wedge-shaped glass sheet. After the frequency difference value of the two longitudinal modes crosses the extreme point, it changes from the maximum value to the minimum value or from the minimum value to the maximum value every time, i.e. it represents that the displacement goes through half of the frequency difference tuning curve in fig. 3, i.e. a quarter wavelength cycle. If the frequency difference value is increased by 1 every time the recording integer m reaches the extreme point, the number of the whole cycles of the passing displacement is: (-1). When the displacement is finished, the frequency difference of the two longitudinal mode lasers is delta v, and the frequency difference of the last passing of the extreme point is delta v 2 (Δv 2 ＝Δ max Or Δ min ) Then the fractional part corresponding to the displacement from the extreme point to the stop position is:
according to the recorded frequency difference and light intensity difference (I) of the two longitudinal mode lasers ⊥ -I // ) Can calculate the decimal part delta l of the displacement of the measured target object:
so far, the measurement of the total displacement of the wedge-shaped glass sheet can be divided into two cases:
(1) When the displacement is small and the frequency difference between two longitudinal modes does not reach an extreme point, delta v 1 And Δ v 2 If not, it can be set to 0, and the total displacement is, according to equations (5) and (6):
(2) When the displacement is larger, the two longitudinal modes reach the extreme point for multiple times, the total number of times is m, and the corresponding integer part subjected to the displacement is
The total displacement then contains the fractional part al 1 +Δl 2 And an integer portion, wherein the sign of the displacement of the integer portion can be distinguished by the direction of the displacement of the fractional portion, and the total displacement of the corresponding wedge-shaped glass sheet is:
wherein Δ l is calculated according to formula (7). Based on the above high-resolution displacement measurement principle, the following high-resolution displacement measurement device is provided.
A first embodiment of the present invention provides a high-resolution displacement measuring device.
In order to achieve the above object, the present invention firstly provides a high resolution displacement measuring device, which adopts the technical scheme that:
the structure of the high-resolution displacement measuring device is shown in fig. 4, and the specific composition structure is as follows.
Laser gain tube 1: the first end of the reflecting mirror is provided with a fixed reflecting mirror 2, and the second end is provided with an anti-reflection window sheet 3.
Independent mirror 4: and the double-longitudinal mode oscillation laser is arranged at the second end side of the laser gain tube 1, is positioned at the outer side of the anti-reflection window 3 and is combined with the laser gain tube 1 and the fixed reflector 2 to emit double-longitudinal mode oscillation laser.
Wedge-shaped glass sheet 5: the reflection reducing window is arranged between the independent reflector 4 and the anti-reflection window 3, one surface of the reflection reducing window is vertical to the laser axis, and the other surface of the reflection reducing window is inclined to the laser axis; in some embodiments of the present invention, antireflection coating is coated on both surfaces of wedge-shaped glass sheet 5 located on the light path.
The main structure is the same as that disclosed in fig. 1, and the transmission principle of the optical path is the same. In order to measure the displacement, the problems of longitudinal mode frequency difference and detection of positive and negative directions of light intensity need to be solved.
In order to realize the displacement measurement, the measuring apparatus further includes the following structure.
The spectroscope 7: the laser beam splitter is arranged outside the fixed reflector 2, is positioned on an emergent light path of the fixed reflector 2, and splits the laser beam into reflected light and transmitted light.
Polarizing plate 8: is arranged at one side of the reflection light path of the spectroscope 7; the light-passing direction of the polarizing plate 8 forms an angle of 45 ° with the direction of the bisector of the orthogonal polarization direction angles of the two oscillation modes, that is, the light-passing direction of the polarizing plate 8 forms an angle of 45 ° with the polarization direction of the oscillation laser.
High-frequency photodetector 9: the beat frequency signal is arranged at the light ray output end of the polaroid 8 and used for receiving beat frequency signals of two longitudinal modes; the two longitudinal modes form a light beat after passing through a polaroid 8, and the frequency of the light beat is detected by a high-speed high-frequency photoelectric detector 9. Since the beat signal formed by the two longitudinal modes is a high frequency signal, which can reach several hundred MHz in general, the high frequency photodetector 9 is used here.
The frequency meter 10: the electric signal output by the high-frequency photoelectric detector 9 is accessed to measure the frequency difference of two longitudinal modes; the electrical signals of two longitudinal mode frequencies of the high-frequency photoelectric detector 9 are input into a frequency meter 10, and the frequency meter 10 automatically calculates the frequency difference value.
Beam splitter prism 7: and the two longitudinal modes are arranged on one side of the transmission light path of the spectroscope 7 and are divided into a first light beam and a second light beam according to orthogonal polarization states.
A photoelectric detector: the device comprises a first photoelectric detector 1201 and a second photoelectric detector 1202, which are respectively arranged on a propagation light path of a first light beam and a propagation light path of a second light beam; two photodetectors measure the light intensity of the first and second beams.
A subtractor 13: the output signal of the first photodetector 1201 and the output signal of the second photodetector 1202 are input to a subtractor respectively to determine the positive and negative values of the light intensity difference between the two longitudinal modes. Specifically, the subtractor 13 is a voltage comparator, and converts the light intensity values of the light in the two polarization states recorded by the first photodetector 1201 and the second photodetector 1202 into voltage values, and calculates and compares the positive and negative values.
Target 15: mounted with the wedge-shaped glass sheets 5, the wedge-shaped glass sheets 5 can be driven to move synchronously.
The data processing unit 14: the positive and negative values of the light intensity difference obtained by the calculation of the receiving subtracter 13 and the frequency difference of the two longitudinal modes measured by the frequency meter 10 are combined to calculate the displacement of the glass sheet 5. Since the target 15 moves in synchronism with the wedge-shaped glass sheet 5, the measured displacement of the wedge-shaped glass sheet 5 is the displacement of the target.
Based on the frequency value measured by the high frequency photodetector 9 and the light intensity values measured by the first photodetector 1201 and the second photodetector 1202, a curve corresponding to the displacement amount and the light intensity change of the wedge-shaped glass sheet 5 shown in fig. 3 can be plotted.
Obtaining the curve shown in fig. 3, wherein the curve located at the upper part is a frequency difference fluctuation curve, and the curve located at the lower part corresponds to a frequency curve of two longitudinal modes, and acquiring and storing the following data through the curve shown in fig. 3:
Δv max : the maximum value of the fluctuation curve of the frequency difference change of the two longitudinal modes, namely the difference between the maximum value and the minimum value of the fluctuation curve, corresponds to the difference between the peak value and the valley value of the frequency difference fluctuation curve at the upper part;
Δ min : the minimum value of the fluctuation curve of the frequency difference change of the two longitudinal modes corresponds to the frequency minimum value of the frequency difference fluctuation curve at the upper part, namely the wave valley value.
In practical applications, the above measuring device can be adapted to be disposed in an optical system to perform synchronous measurement of the displacement of the target 15.
In order to synchronize the movement of the target 15 and the wedge-shaped glass sheet 5, in some embodiments of the invention, the wedge-shaped glass sheet 5 is connected to the displacement measuring rod 602 by a fixed mount 601, with a guide rail below the fixed mount 601, on which the fixed mount 601 is movably mounted and can move along the guide rail. The displacement measuring rod 602 is contacted with a measured target object, so that the wedge-shaped glass sheet 5 is synchronously driven to displace along the laser axis by the displacement measuring rod in the movement process of the target object. The movement of the wedge-shaped glass sheet 5 reflects the movement of the target 15.
A second embodiment of the present invention provides a high resolution displacement measurement method.
The method comprises the following steps:
s1: a double longitudinal mode oscillation laser is adopted as a light source;
s2: moving the wedge-shaped glass sheet, measuring the frequency difference of two longitudinal modes output by laser, and recording the minimum value delta of the frequency difference of the two longitudinal modes min And maximum value Δ max Recorded as extreme points;
measuring the light intensity I of the double longitudinal modes of the laser output // And I ⊥ Positive and negative values of the light intensity difference of (a):
s＝sign(I ⊥ -I // )；
in the measuring process, the measured target and the wedge-shaped glass sheet connected with the measured target synchronously move;
recording the frequency difference value of the two longitudinal mode lasers at the beginning of the displacement as delta v 0 After the displacement is finished, the frequency difference of the two longitudinal mode lasers is delta v;
and the difference value of the laser frequencies of the two longitudinal modes in the displacement process is delta v when reaching an extreme point for the first time 1 When the difference value of the laser frequencies of the two longitudinal modes passes the extreme point for the last time before the displacement is finished, the difference value is delta v 2 The total times that two longitudinal modes pass through the extreme point in the displacement process is M, and the light intensity I of the laser of the two longitudinal modes is recorded in real time // And I ⊥ ；
Based on the frequency difference and the positive and negative of the light intensity difference of the double longitudinal mode oscillation laser, calculating the decimal part delta l of the single displacement of the measured target object:
wherein λ is the laser wavelength;
if the frequency difference value of the two longitudinal modes does not reach an extreme value point in the displacement process, the delta v is calculated 1 ＝Δv 2 =0, the total displacement is Δ L = Δ L;
if the frequency difference value of two longitudinal modes reaches an extreme point in the displacement process, judging the displacement direction according to the sign of delta l, and calculating the total displacement including an integer part as follows:
wherein: m is an integer,. DELTA. min Is the minimum value of the frequency difference between two longitudinal modes of the laser, delta v max The maximum value of the frequency difference of two longitudinal modes of the laser is lambda, and the lambda is the laser wavelength.
The m acquisition method comprises the following steps:
the m acquisition method comprises the following steps: and recording the change of the frequency difference delta v of the two longitudinal mode lasers in the displacement adjusting process, as shown in fig. 3, when the frequency difference delta v of the two longitudinal mode lasers changes from a maximum value to a minimum value or from the minimum value to the maximum value, the displacement is moved by one quarter wavelength period, and the accumulated integer m is increased by 1.
Considering the displacement judgment direction, in practical application, the displacement of the target object is stopped from the beginning of the displacement to the end of the displacement as a single displacement measurement, and the displacement measurement result is accumulated according to the process when the target object continues to move. If the target object moves back and forth for a plurality of times in a range, the start and stop of the times in the back and forth movement are regarded as the combination of a plurality of single displacement measurements and are not regarded as the accumulated value m of the single displacement measurements, each time as long as the target object moves and stops, the single displacement measurement is finished and a measurement result is given, and when the displacement starts again, the next single displacement measurement is continued and the displacement is accumulated.
According to the displacement metering method of the wedge-shaped glass sheet 5, in the displacement measuring process, if the displacement variation is within a quarter wavelength range, the decimal part of the displacement is calculated by measuring delta v; if the variation exceeds the quarter wavelength range, I is passed once // ＝I ⊥ The integral value of the recording displacement is accumulated at the position point of (2).
Since the target 15 and the wedge glass 5 are synchronously movable, the displacement of the wedge glass 5 obtained by measurement and calculation is reflected as the displacement of the target 15.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention.
Claims (6)
1. A high resolution displacement measurement device, comprising:
laser gain tube: the first end of the reflecting mirror is provided with a fixed reflecting mirror, and the second end of the reflecting mirror is provided with an anti-reflection window sheet;
independent mirrors: the double-longitudinal-mode oscillation laser is arranged at the second end side of the gain tube, is positioned at the outer side of the anti-reflection window sheet and is combined with the laser gain tube and the fixed reflector to emit double-longitudinal-mode oscillation laser;
wedge-shaped glass sheet: the reflecting mirror is arranged between the independent reflecting mirror and the anti-reflection window sheet, one surface of the reflecting mirror positioned on the light path is vertical to the laser axis, and the other surface of the reflecting mirror positioned on the light path is inclined to the laser axis;
spectroscope: is arranged at the outer side of the fixed reflector;
polarizing plate: the reflecting light path is arranged on one side of the spectroscope;
high-frequency photoelectric detector: the light output end is arranged on the polaroid and is used for receiving beat frequency signals of two longitudinal modes;
frequency meter: connecting the electric signal output by the high-frequency photoelectric detector to measure the frequency difference of two longitudinal modes;
a beam splitter prism: the two longitudinal modes are arranged on one side of a transmission light path of the spectroscope and are divided into a first light beam and a second light beam according to orthogonal polarization states;
a photoelectric detector: the system comprises a first photoelectric detector and a second photoelectric detector which are respectively arranged on a propagation light path of a first light beam and a propagation light path of a second light beam and used for detecting the light intensity of two longitudinal modes;
a subtracter: the output signal of the first photoelectric detector and the output signal of the second photoelectric detector are respectively input into the subtracter to judge the positive and negative values of the light intensity difference of the two longitudinal modes;
a target object: the wedge-shaped glass sheet is arranged and can be driven to synchronously move;
a data processing unit: and receiving the positive and negative values of the light intensity difference obtained by the calculation of the subtracter and the frequency difference of the two longitudinal modes measured by the frequency meter, and calculating the displacement of the wedge-shaped glass sheet by combining the positive and negative values of the frequency difference and the light intensity difference.
2. The high-resolution displacement measuring device according to claim 1, wherein a light-passing direction of the polarizing plate makes an angle of 45 ° with an orthogonal polarization direction of the oscillation laser light.
3. The high resolution displacement measurement device according to claim 1, wherein both surfaces of the wedge-shaped glass plate in the optical path are coated with an antireflection coating.
4. The high resolution displacement measuring device according to claim 1, wherein the wedge-shaped glass sheet is connected to the displacement measuring rod by a mount, the mount being movably mounted on the guide rail and movable along the guide rail; the displacement measuring rod is contacted with a measured target object, so that the displacement of the wedge-shaped glass sheet along the laser axis is synchronously driven by the displacement measuring rod in the moving process of the target object.
5. The high resolution displacement measurement method according to claim 1, using the measurement device according to any one of claims 1 to 4, comprising the steps of:
a double longitudinal mode laser is adopted as a light source;
moving the wedge-shaped glass sheet, measuring the frequency difference of two longitudinal modes output by laser, and recording the minimum value delta of the frequency difference of the two longitudinal modes min And maximum value Δ max Recorded as extreme points;
measuring the light intensity I of the double longitudinal modes of the laser output // And I ⊥ Positive and negative values of the light intensity difference of (a):
s＝sign(I ⊥ -I // )；
in the measuring process, the measured target and the wedge-shaped glass sheet connected with the measured target synchronously move;
recording the frequency difference value of the two longitudinal mode lasers at the beginning of the displacement as delta v 0 After the displacement is finished, the frequency difference of the two longitudinal mode lasers is delta v;
and the difference value of the laser frequencies of the two longitudinal modes in the displacement process is delta v when reaching an extreme point for the first time 1 When the difference value of the laser frequencies of the two longitudinal modes passes the extreme point for the last time before the displacement is finished, the difference value is delta v 2 The total times that the frequency difference value of the two longitudinal mode lasers passes through the extreme point in the displacement process is m, and the light intensity I of the two longitudinal mode lasers is recorded in real time // And I ⊥ ；
Based on the frequency difference and the positive and negative of the light intensity difference of the double longitudinal mode oscillation laser, calculating the decimal part delta l of the single displacement of the measured target object:
wherein λ is the laser wavelength;
if the frequency difference value of the two longitudinal modes does not reach an extreme value point in the displacement process, then delta v 1 ＝Δv 2 =0, the displacement of the target is Δ L = Δ L;
if the frequency difference value of the two longitudinal modes reaches an extreme point in the displacement process, judging the displacement direction according to the sign of delta l, and calculating the displacement of a target object comprising an integer part as follows:
wherein m is an integer.
6. The high resolution displacement measuring device according to claim 5, wherein the m obtaining method comprises: and recording the change of the frequency difference delta v of the two longitudinal modes of laser in the displacement adjusting process, wherein when the frequency difference delta v of the two longitudinal modes of laser changes from the maximum value to the minimum value or changes from the minimum value to the maximum value, the displacement moves by one quarter wavelength period, and the accumulated integer m is increased by 1.
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