CN100538397C - Birefringence External Cavity Feedback Displacement Measurement System - Google Patents

Abstract

The invention belongs to the technical field of laser measurement, and is characterized in that a birefringent external cavity feedback displacement measurement system is formed by using a single-frequency microchip Nd:YAG laser pumped by LD, a birefringent external cavity feedback and a measurement circuit. When the measured object moves along the laser axis, two cosine signals with a phase difference of 90 degrees are obtained in the X and Y directions of the output. Every time the measured object moves by half the wavelength, the output light intensity of the laser changes by one fringe. After the threshold light intensity is introduced, the laser light intensity of one cycle is divided into four polarization regions, each region corresponds to the displacement of one-eighth of the light wavelength of the measured object, and the displacement of the measured object can be obtained by detecting the number of polarization regions; when the measured When the moving direction of the object changes, the order in which the four polarization regions appear in each cycle also changes, so that the moving direction of the object can be judged. The device has the characteristics of compact structure, high cost performance, easy direction determination and small volume.

Description

Birefringence External Cavity Feedback Displacement Measurement System

technical field

The invention belongs to the technical field of laser measurement.

Background technique

Laser feedback means that in the laser application system, after the output light of the laser is reflected by an external object, part of the light is fed back to the laser resonator. The feedback light carries the information of the external object and interacts with the light in the cavity to modulate the output of the laser. By demodulating the output light intensity of the laser, the information of the external measured object is obtained. Laser feedback technology, also known as self-mixing interferometry, has the same phase sensitivity and depth as traditional two-beam interferometry. However, the measurement system based on laser feedback technology has only one optical channel, which has the advantages of simple structure, compactness, easy alignment, and high cost performance. It can be applied to the measurement of displacement, velocity, absolute distance, vibration, angle, surface topography of finely machined parts, three-dimensional shape of cells, etc. It can also be used for analysis and flaw detection of molds, reconstruction of three-dimensional image data, etc. The surface is very wide.

The research on laser feedback displacement measurement technology started in the 1980s, and most of the research focused on the feedback research in semiconductor lasers. The optical path system of this displacement measuring device has only one laser and one external reflector. The light output by the laser is reflected or scattered and then returns to the laser resonator cavity. It mixes with the light in the cavity to cause the power change of the laser. When the external mirror moves half the wavelength of the light, the power of the laser changes by one stripe. The fluctuation depth of the stripe is the same as that of the traditional double Beam interferometric systems are comparable. The interference fringes can be directly used for counting to achieve a displacement measurement with a resolution of half the wavelength of light. However, the following three problems exist in the semiconductor laser feedback phenomenon used for displacement measurement: First, the semiconductor laser has a large divergence angle, and collimating and focusing optical elements must be added to the external cavity, resulting in a complex structure, and it is easy to generate multiple feedbacks, resulting in counting errors. Secondly, the semiconductor laser feedback cannot identify the displacement direction of the measured object. Although some studies have found that semiconductor lasers work under weak and medium light feedback, the feedback signal is an asymmetrical sawtooth-like shape, and the inclination direction of the sawtooth wave is related to the moving direction of the external cavity reflector. Some researchers try to use this phenomenon to solve the direction judgment problem of semiconductor laser feedback displacement measurement. But at this time, the resolution of the feedback system is only half of the light wavelength, which is not conducive to subdivision, and the sawtooth wave stripes will generate counting errors when commutating. Finally, the feedback phenomenon of semiconductor lasers is more complicated. According to the different feedback levels (strong feedback, medium feedback, weak feedback), the feedback signals are different, which is easy to cause counting errors.

Some scholars also proposed to use He-Ne laser as the light source of the optical feedback displacement measurement system. Although the He-Ne laser has good collimation and can realize large-scale measurement without external cavity collimation components, the size of the He-Ne laser is huge, and the resonant cavity is hundreds or even thousands of millimeters long, and the discharge tube of the He-Ne laser It is a large heat source, which affects the stability of laser power, leads to errors in feedback signals, and causes counting errors.

Contents of the invention

The purpose of the present invention is to provide a compact structure, high cost-effective, high-resolution optical feedback displacement measurement system that can identify the displacement direction of the measured object, which can effectively solve the above problems.

The present invention is characterized in that it contains:

1. Birefringent external cavity feedback displacement measurement system, characterized in that it contains:

Microchip Nd:YAG single-frequency linearly polarized laser, birefringent external cavity feedback, and signal detection and processing are three parts, of which:

A: Microchip Nd:YAG single-frequency linearly polarized laser, containing:

LD pumping source 1 with a pigtail to generate pumping light;

Collimating and focusing lens group 2;

The full-cavity Nd:YAG crystal 3, its left and right surfaces form a laser resonator, and the collimating and focusing lens group 2 converges the pump light output by the LD pump source 1 with pigtails on the Nd:YAG Crystal surface to generate single-frequency linearly polarized laser light;

B: Birefringent external cavity feedback, the birefringent external cavity feedback contains:

The beam splitter 4 divides the light output by the microchip Nd:YAG single-frequency linearly polarized laser into two parts: one part is used for feedback, and the other part is used for light intensity detection;

45 degree wave plate 5, the fast and slow axes of this wave plate form an angle of 45 degrees with the single-frequency linearly polarized light output for feedback of the microchip Nd:YAG single-frequency linearly polarized laser output respectively;

Attenuator 6 receives and attenuates the output light of the 45-degree wave plate 5, to control the feedback level of the microchip Nd:YAG single-frequency linearly polarized laser, preventing the generation of mode hopping;

The external cavity feedback mirror 7 is used as an external reflector, receives the output light of the attenuator 6, and feeds the laser back to the laser resonator;

The piezoelectric ceramic PZT 8 is fixed on the outer side of the external cavity feedback mirror 7 along the direction of the input light, so that under the action of the input voltage, the piezoelectric ceramic PZT 8 pushes the external cavity feedback mirror 7 to the left along the direction of the laser axis , move right;

C: Signal detection and processing part, including:

The polarizing beam splitter 9 spatially divides the polarized light output by the beam splitter 4 into two X-direction and Y-direction light intensity cosine components with a phase difference of 90 degrees;

There are two photodetectors 10 and 11, respectively detecting the two light intensity cosine components of the X direction and the Y direction output by the polarization beam splitter prism 9;

Amplifying and filtering circuit 12, two input ends are respectively connected with the output ends of described photodetectors 10, 11, the signal received by described photodetector is amplified and filtered;

A voltage comparison circuit 13, the two input terminals are respectively connected to the two output terminals of the amplification and filtering circuit 12, and a threshold voltage corresponding to the threshold light intensity is preset in the voltage comparison circuit 13 to control the two input signals, In order to move the displacement of one-half of the light wavelength in the external cavity feedback mirror 7, so that the length of the external cavity changes within one cycle of one-half of the light wavelength, the following four different signals are obtained after the combination of the two signals Polarization area: X-direction polarized light, X-direction and Y-direction polarized light coexistence area, Y-direction polarized light and no light area, each of the four polarization areas corresponds to one-eighth of the light of the measured external reflector wavelength shift;

The remaining processing circuits 14 include interrelated logical direction determination, counting and digital display circuits in sequence. When the external cavity feedback mirror 7 moves forward or reverse, the order in which the four polarization regions appear in the two-way signal combination Different; the size and direction of the motion displacement can be given by judging the number of polarization regions and the order of their appearance.

The present invention is also characterized in that the 45-degree wave plate 5 is a crystal quartz wedge with a retardation of 45 degrees, or the 45-degree wave plate 5 consists of a glass plate and a glass plate clamped at the upper and lower ends of the glass plate. Mechanical stress generator constitutes.

The invention provides a birefringence external cavity feedback displacement measurement system, the resolution can reach one-eighth wavelength, and for the 1.064μm Nd:YAG laser, the system resolution is 133nm. The system has the characteristics of compact structure, high cost performance, easy recognition of displacement direction and high resolution.

Description of drawings

Figure 1: One of the implementation examples of the birefringent external cavity feedback displacement measurement system of the present invention.

Figure 2: The second implementation example of the birefringent external cavity feedback displacement measurement system of the present invention.

Figure 3: The third implementation example of the birefringent external cavity feedback displacement measurement system of the present invention.

Figure 4: Schematic diagram of the coordinate system of the birefringent external cavity feedback displacement measurement system.

Figure 5: Numerically simulated feedback light intensity curves for X- and Y-polarized light.

Figure 6: The feedback light intensity curves of X-direction and Y-direction polarized light during the feedback process obtained from the experiment.

Figure 7: Schematic representation of the four polarization regions created after introducing threshold light intensities.

Detailed ways

The experimental device (example one) of the present invention is shown in Figure 1. In Fig. 1, 1 is a semiconductor laser with pigtail output, which is used as a pump source; 2 is a collimating and focusing lens group, which focuses the pump light on the surface of 3; 3 is a laser gain medium—Nd:YAG crystal, 3 The two surfaces of 2 constitute the laser resonator; 4 is a beam splitter, which divides the output light of the laser into two parts, one part is used for feedback, and the other part is used for signal detection; 5 is a wave plate with a phase delay of 45 degrees, its The included angle between the fast and slow axes and the polarization direction of the laser output light is 45 degrees; 6 is the attenuator, used to control the feedback level and prevent the occurrence of mode hopping; 7 is the external cavity feedback mirror with a reflectivity of 50%; 8 is Piezoelectric ceramic PZT, which is fixed on the external cavity feedback mirror 7, under the action of the input voltage, it pushes the external cavity feedback mirror 7 to move left and right along the laser axis direction; 4, 5, 6, 7 and the output surface of the Nd:YAG crystal 3 together form a birefringent external cavity feedback; 9 is a polarization beam splitter (Wollaston prism); 10 and 11 are two photodetectors; 9, 10 and 11 constitute a signal receiving device , the detected signal is input to the amplification and filtering circuit 12, the signal is amplified and filtered and then input to the voltage comparison circuit 13, and the threshold voltage is introduced to obtain two signals, and the combination of the two signals constitutes four These two signals are input to the rest of the processing circuit 14, including logic direction determination, counting and digital display circuits, which give the magnitude and direction of the motion displacement of the measured object.

Principle of the present invention is as follows:

Establish a coordinate system as shown in Figure 4, and the Z axis is the propagation direction of the output light of the Nd:YAG laser. The electric field direction E of the laser is 45 degrees to the X axis and the Y axis respectively. The fast and slow axes (ie, o-axis and e-axis) of the wave plate 5 in the birefringent external cavity feedback coincide with the X-axis and the Y-axis respectively. The two optical axes of the polarizing beam splitter prism 9 coincide with the X-axis and the Y-axis respectively.

Under the optical feedback of the single-mode Nd:YAG laser, the variation Δg of the threshold gain is:

$\mathrm{<span\; class="notranslate">\; \&Delta;\; g</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; g</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula, g is the threshold gain when there is optical feedback, g ₀ is the threshold gain when there is no optical feedback, ζ is the optical feedback factor proportional to the reflection coefficient of the external cavity mirror, n is the refractive index of the Nd:YAG crystal, d is the thickness of the Nd:YAG crystal, ω is the angular frequency of the laser, c is the speed of light in vacuum, and L is the length of the external cavity of the laser.

The electric field E is along the wave plate in the birefringent external cavity. The axial and e-axis directions are decomposed into E _o , E _e . The E _{o and} E _e that are fed back to the laser resonator by the external cavity mirror interact with the X-direction and Y-direction components E _X and E _Y of the electric field E in the cavity respectively, and modulate the threshold gain of the laser in the X and Y directions respectively. as follows:

$\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; Y</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; nd</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

In the formula, δ is the additional phase difference between E _o and E _e caused by the wave plate in the birefringent external cavity.

Since the change of laser intensity during feedback is proportional to the change of threshold gain, that is

I=I ₀ -kΔg, (4)

In the formula, I ₀ is the initial light intensity without feedback, and k is a constant.

Then under optical feedback, the light intensity components of the laser in the X and Y directions are:

I _X =I _X0 +ζk/2nd·cos(2ωL/c), (5)

I _Y =I _Y0 +ζk/2nd·cos(2ωL/c+2δ), (6)

In the formula, I _X and I _Y are the light intensities in the X and Y directions when the light is fed back, and I _X0 and I _Y0 are the initial light intensities in the X and Y directions when there is no light feedback.

For the optical feedback system, assuming that the displacement change of the measured object is ΔL, then the phase change caused by the displacement of the external object is:

It can be known from formula (7) that the laser intensity fluctuates for one period every time the object changes the displacement of one-half of the light wavelength.

When the wave plate delay in the external cavity feedback is 45 degrees, the expressions of the light intensity in the X direction and Y direction in the birefringent external cavity feedback displacement measurement system are:

I _X ＝I _X0 +ζk/2nd·cos(2ωΔL/c) (8)

I _Y ＝I _Y0 +ζk/2nd·cos(2ωΔL/c+π/2) (9)

From the above two formulas, it can be seen that for every half of the light wavelength shifted by the external cavity feedback mirror, the light intensity of the polarized light in the X and Y directions changes by a cosine fringe, and there is a phase difference π/2 between the two fringes. The polarized light feedback curve in the Y direction is ahead of the X direction when the external cavity is extended, and the polarized light feedback curve in the Y direction is behind the X direction when the external cavity is shortened. The simulation analysis is carried out according to formulas (8) and (9), and the feedback light intensity curves of polarized light in the X and Y directions during the light feedback process are obtained through numerical simulation, as shown in Figure 5.

When the external cavity feedback mirror 7 is driven by the piezoelectric ceramic PZT 8 and moves left and right along the laser axis, the length of the external cavity is continuously changed, and the laser intensity curves obtained by the photoelectric receivers 10 and 11 are shown in Figure 6. Among them, the solid dot line is the light feedback laser intensity curve in the X direction, the circled line is the light feedback laser intensity curve in the Y direction, and the triangular wave is the driving voltage curve of the piezoelectric ceramic PZT driving the measured object. Due to the different moving directions of the external cavity mirror, the polarized light feedback curves in the two directions have a leading or lagging relationship, which can realize direction judgment. The experimental results in Figure 6 are in good agreement with the numerical simulation results in Figure 5.

A threshold light intensity I _th is introduced and implemented on the circuit through the voltage comparison circuit 13, then the part of the polarized light in the X and Y directions smaller than I _th can be ignored, and the signal shown in FIG. 7 is obtained. In this way, within a period when the length of the external cavity changes by one-half of the light wavelength, the feedback curves of polarized light in the X and Y directions are equally divided into four regions: X-directed polarized light, X-directed and Y-directed polarized light coexistence area, and Y-directed polarized light Light and no-light areas, each area corresponds to one-eighth of the light wavelength displacement of the measured object. When the piezoelectric ceramic PZT voltage increases, that is, when the measured object moves to the direction in which the length of the feedback cavity decreases, the order of the four areas appearing is no light area, X-direction polarized light, X-direction and Y-direction polarized light coexistence area, Y When the piezoelectric ceramic PZT voltage decreases, that is, when the measured object moves to the direction in which the length of the feedback cavity increases, the order of the four areas appearing is the no-light area, Y-directed polarized light, X-directed and Y-polarized light coexistence area, X-ray area, and no-light area; when the displacement direction of the measured object is different, the four areas appear in different orders, and the identification of the object displacement direction can be easily realized through signal processing.

The schematic structural diagram of the second example of the present invention is shown in FIG. 3 . The structures shown in Fig. 3 and Fig. 1 are basically the same, and the fourteen elements from 1 to 14 are the same as those in Fig. 1 except 5, which will not be repeated here. 5 is a crystal quartz wedge; due to the birefringence effect of crystal quartz, this quartz wedge produces a 45-degree phase difference in the X and Y directions, and the output surfaces of 4, 5, 6, 7 and Nd:YAG crystal 3 together form a birefringence External cavity feedback. The resolution of the system is still one-eighth of the light wavelength, which can identify the displacement direction of the measured object.

The schematic structural diagram of the third example of the present invention is shown in FIG. 3 . The structures shown in Fig. 3 and Fig. 1 are basically the same, and the fourteen elements from 1 to 14 are the same as those in Fig. 1 except 5, which will not be repeated here. 5 is a piece of glass;

15 is a stress applying device, which applies a stress to the glass sheet 5 along the axial direction perpendicular to the laser. Due to the stress-birefringence effect, the glass sheet produces a phase difference in the X and Y directions, which is equivalent to a birefringent element. The output surfaces of 4, 5, 6, 7, 15 and the Nd:YAG crystal 3 together form a birefringent external cavity feedback.

The invention provides a birefringent external cavity feedback displacement measurement system, the device uses LD-pumped single-frequency microchip Nd:YAG laser, attenuator, 45-degree wave plate and external mirror to form a birefringent external cavity feedback displacement measurement system. The external reflector is fixed on the measured object. When the measured object moves along the laser axis, two cosine signals with a phase difference of 90 degrees are obtained in the X and Y directions of the output end. Every time the measured object moves by half the wavelength, the output light intensity of the laser changes by one fringe. After introducing the threshold light intensity, the laser light intensity of one cycle is divided into four polarization areas: X-direction polarized light, X-direction and Y-direction polarized light coexistence area, Y-direction polarized light and no light area, each area corresponds to eight One-half of the wavelength of light, that is, the displacement of 133nm, the displacement of the measured object can be obtained by detecting the number of polarization regions; when the moving direction of the measured object changes, the order of the four polarization regions in each cycle also changes, by This can determine the direction of object displacement. The system has the characteristics of compact structure, high cost performance, easy recognition of displacement direction and high resolution.

Claims (3)

1. double-refraction external cavity displacement measuring system is characterized in that, contains: microplate Nd:YAG single-frequency linearly polarized laser device, double-refraction external cavity feedback and acquisition of signal and three parts of processing, wherein:

Microplate Nd:YAG single-frequency linearly polarized laser device, contain:

The LD pumping source (1) of magnetic tape trailer fibre, the LD pumping source (1) of described magnetic tape trailer fibre produces pump light;

Collimation focus lens group (2);

And full inner chamber Nd:YAG crystal (3), its left and right two surfaces constitute laserresonator, described collimation focus lens group (2) converges at the Nd:YAG plane of crystal to the pump light of the LD pumping source (1) of magnetic tape trailer fibre output, makes it to produce the single-frequency linearly polarized laser;

The double-refraction external cavity feedback, contain:

Beam splitter (4), the light separated into two parts that described beam splitter (4) is exported described microplate Nd:YAG single-frequency linearly polarized laser device: a part is used for feedback, and another part is used for light intensity and surveys;

45 degree wave plates (5), fast, the slow axis of this wave plate respectively with the single-frequency linearly polarized light that is used for feedback of described microplate Nd:YAG single-frequency linearly polarized laser device output in angle of 45 degrees;

Attenuator (6), described attenuator (6) receive and decay described 45 the degree wave plates (5) output light, to control the feedback level of described microplate Nd:YAG single-frequency linearly polarized laser device, prevent the generation of mode jump;

Exocoel feedback catoptron (7), described exocoel feedback catoptron (7) is made external reflection thing usefulness, receives the output light of described attenuator (6), laser feedback to described laserresonator;

And piezoelectric ceramics PZT (8), described piezoelectric ceramics PZT (8) is fixed on the outside along the input light direction of described exocoel feedback catoptron (7), so that under the effect of input voltage, this piezoelectric ceramics PZT (8) promotes exocoel feedback catoptron (7) and moves along the laser axis direction is left and right;

Acquisition of signal and processing section, contain:

Polarization splitting prism (9), described polarization splitting prism (9) the polarized light of beam splitter (4) output spatially be divided into X that two-way has 90 degree phasic differences to, Y to the light intensity cosine component;

Photodetector (10,11), totally two, the X that described two photodetectors (10,11) are surveyed described polarization splitting prism (9) output respectively to Y to two light intensity cosine components;

Amplify and filtering circuit (12), two input ends of described amplification and filtering circuit (12) link to each other with the output terminal of described two photodetectors respectively, and the signal that described photodetector is received amplifies and filtering;

Voltage comparator circuit (13), two input ends of described voltage comparator circuit (13) link to each other with two output terminals of described amplification and filtering circuit (12) respectively, by default threshold voltage in voltage comparator circuit (13) corresponding to the threshold value light intensity, control two-way input signal, so that move the displacement of 1/2nd optical wavelength at exocoel feedback catoptron (7), make in the one-period of the long change of exocoel 1/2nd optical wavelength, obtain following four mutually different polarized regions: X altogether to the polarized light district after the described two paths of signals combination, X to polarized light and Y to the polarized light coexistence, Y is to the polarized light district, and no light zone, 1/8th optical wavelength displacements of each the corresponding tested external reflection thing in zone in described four polarized regions;

And all the other treatment circuits (14), described all the other treatment circuits (14) comprise that successively the logic that is mutually related declares to, counting and digital display circuit, when exocoel feedback catoptron (7) forward or reverse when mobile, the order difference that described four polarized regions occur; Change the size and Orientation that can provide moving displacement by the order of differentiating polarized regions number and appearance thereof.

2. double-refraction external cavity displacement measuring system according to claim 1 is characterized in that, described 45 degree wave plates (5) are that a retardation is the crystalline quartz wedge of 45 degree.

3. double-refraction external cavity displacement measuring system according to claim 1 is characterized in that, described 45 degree wave plates (5) by a glass sheet and be clipped in this glass sheet up and down a mechanical stress generator at two ends constitute.

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